Steel plate excellent in machineability and in toughness and weldability and method of production of the same

Abstract

The present invention provides steel plate excellent in machineability and in toughness and weldability and a method of production of the same. Steel with C, Si, Mn, P, S, Al, and N limited to predetermined ranges and further containing, as necessary, Mo, Cr, Nb, Ti, V, Cu, Ni, B, REM, Ca, Zr, or Mg is further strictly defined in the balance of steel ingredients and strictly controlled in the conditions of rolling, water cooling, etc. in the method of production to obtain steel plate where, when the plate thickness is 4 mm to less than 10 mm, the ferrite fraction of locations exactly ¼ and ¾ of the plate thickness inside from a top surface of the steel plate is 30% to 90% and a ferrite fraction of a location exactly ½ of the plate thickness inside from a top surface of the steel plate surface is 20% to 90% and, when the plate thickness is 10 mm to 100 mm, a ferrite fraction of a location 2 mm inside from a front and rear surface of the steel plate is 30% to 90% and a ferrite fraction of locations exactly ¼, ½, and ¾ of the plate thickness inside from a top surface of the steel plate surface is 20% to 90%.

Description

TECHNICAL FIELD

The present invention relates to steel plate excellent in machineability and in toughness and weldability, in particular relates to steel plate having a plate thickness of 4 to 100 mm or so and a tensile strength of 570 to 720 MPa or so and a method of production of the same. The steel plate produced by this method of production can be used for ships, bridges, buildings, marine structures, pressure vessels, line pipe, and other welded structures in general, in particular is effective in use if fields requiring drilling, surface machining, and other machining work when fabricating a structure.

BACKGROUND ART

As steel plate used for a welded structure, high strength and also, as weldability, the suppression of weld fractures and a high weld heat affected zone toughness are usually required. In a steel material with a tensile strength of 570 MPa or more, it has been possible to achieve both high strength and weldability by keeping the amount of addition of alloy elements to a minimum and converting the main structure forming the steel bainite or martensite. However, when fabricating a building, bridge, ship, or other structure, drilling, surface machining, and other machining processes are involved. When bainite or martensite forms the main structure, the productivity of the work falls due to the increased frequency of replacement or resharpening of tools along with tool life, the drop in the machining speed due to the increase in machining resistance, etc. and, as a result, the structure increases in fabrication costs. For example, Japanese Unexamined Patent Publication No. 9-310117 tries to achieve both a high strength and weldability with a relatively low alloy elements by making the structure mainly bainite. However, since the steel has a structure mainly comprised of hard bainite, the machineability is poor and the cost required for machining work is high.

In a welded structure, the steel plate used for the main parts, for example, the webs and flanges, are welded and are subject to relatively large stress at the time of use, so require excellent weldability and toughness. On the other hand, there are parts which are not welded and parts where a high toughness is not required. For example, the tie plates etc. used when bolting main structures of bridges desirably have a toughness of a performance of an extent satisfying the standards, but do not have to have a level of the same extent as the main structures. If using steel plate having bainite or martensite as the main structures for these parts, since the machineability is poor, the machining work takes time so the structure greatly increases in fabrication costs.

To improve the machineability, in particular to extend the tool life and reduce the machining resistance, it is known that addition of S is effective. However, when adding a large amount of S, the matrix toughness falls and the weldability falls. As opposed to this, a technique for achieving both an improvement of the machineability by the addition of S and securing of the weldability is disclosed in Japanese Unexamined Patent Publication No. 6-184695. However, the weldability secured here only eliminates preheating and suppresses weld fractures. The welded zone toughness and the matrix toughness are low. Therefore, the use of such steel for welded structures is not possible. Further, a technique for achieving both machineability and matrix toughness is disclosed in Japanese Unexamined Patent Publication No. 2000-87179. By controlling the form of the sulfides by addition of Ca and Mg, the anisotropy of the matrix toughness is improved, but the absolute value is low and further the weldability is poor, so the use of such steel for a welded structure is not possible.

The machineability also depends on the configuration of the microstructure. It is known that rather than a structure mainly comprised of bainite or martensite, a ferrite and pearlite or a ferrite and bainite structure is excellent. For example, Japanese Unexamined Patent Publication No. 7-54100, Japanese Unexamined Patent Publication No. 7-109518, and Japanese Unexamined Patent Publication No. 7-166235 disclose steels with ferrite and bainite structures. Further, Japanese Unexamined Patent Publication No. 2000-63988, Japanese Unexamined Patent Publication No. 2000-63989, Japanese Unexamined Patent Publication No. 2000-282172, and Japanese Unexamined Patent Publication No. 2001-214241 disclose steels with prescribed fractions of ferrite. Steel plate with microstructures of ferrite and bainite and steel plates securing certain ferrite fractions are qualitatively excellent in machineability to steel mainly comprised of bainite or martensite, but the absolute margin of improvement cannot be said to be sufficient enough to improve the productivity in drilling or surface machining in the process of fabrication of a welded structure. Further, said art all call for large amounts of addition of alloy elements, so the toughness and weldability are low and the use of such steel for welded structures is not possible. From the above, it is not possible with the current art to produce steel plate having a 570 MPa or higher tensile strength and a high toughness, weldability, and machineability.

DISCLOSURE OF THE INVENTION

The present invention provides steel plate excellent in machineability and in toughness and weldability or extremely excellent in machineability and having a toughness of an extent enabling application to welded structures, having a plate thickness of 4 to 100 mm or so, and having a tensile strength of a level of 570 to 720 MPa or so and a method of production of the same. It has as its gist the following:

(1) Steel plate excellent in machineability and in toughness and weldability characterized in that the steel comprises, by mass %, a steel composition comprising:

• • C, 0.005 to 0.2%,
  • Si: 0.01 to 1%,
  • Mn: 0.01 to 2%,
  • P: 0.02% or less,
  • S: 0.035% or less
  • Al: 0.001 to 0.1%
  • N, 0.01% or less, and
  • the balance of iron and unavoidable impurities,
  • X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B is 0.24 or less,
  • when the plate thickness is 4 mm to less than 10 mm, a ferrite fraction of locations exactly ¼ and ¾ of the plate thickness inside from a top surface of the steel plate is 30% to 90% and a ferrite fraction of a location exactly ½ of the plate thickness inside from a top surface of the steel plate surface is 20% to 90%, and
  • when the plate thickness is 10 mm to less than 100 mm, a ferrite fraction of a location 2 mm inside from a top and rear surface of the steel plate is 30% to 90% and a ferrite fraction of locations exactly ¼, ½, and ¾ of the plate thickness inside from a top surface of the steel plate surface is 20% to 90%.

(2) Steel plate excellent in machineability and in toughness and weldability, characterized in that the steel comprises, by mass %, a steel composition as set forth in (1) wherein:

• • Mn: 0.01 to 1.4%,
  • S: 0.01% or less,

X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B is 0.24 or less,

• • X2 expressed by X2=(Si/5+Mo+Cr/2)/Mn is 0.15 to 10.0,
  • the structure forming the steel is a structure having a ferrite fraction of 30% to 90% and the balance of a hard structure mainly comprised of bainite and martensite or a structure having a ratio with a micro Vickers hardness of 190 HV or less of 20% or more, and
  • the steel has a Vickers hardness of 165 HV to 300 HV.

(3) Steel plate excellent in machineability and in toughness and weldability, characterized in that the steel comprises, by mass %, a steel composition as set forth in (1) wherein:

• • Mn: 0.01 to 1.4%,
  • S: over 0.01% to 0.035%,
  • X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B is 0.24 or less,
  • X2 expressed by X2=(Si/5+Mo+Cr/2)/Mn is 0.15 to 10.0,
  • the structure forming the steel is a structure having a ferrite fraction of 30% to 90% and the balance of a hard structure mainly comprised of bainite and martensite or a structure having a ratio with a micro Vickers hardness of 190 HV or less of 20% or more, and
  • the steel has a Vickers hardness of 165 HV to 300 HV.

(4) Steel plate excellent in machineability and in toughness and weldability as set forth in any of (1) to (3), characterized in that said steel further comprises, by mass %, one or more of:

• • Mo: 0.01 to 1%,
  • Cr: 0.01 to 1%,
  • Nb: 0.001 to 0.1%,
  • Ti: 0.001 to 0.1%,
  • V: 0.001 to 0.1%,
  • Cu: 0.005 to 1%,
  • Ni: 0.01 to 2%,
  • B: 0.0002 to 0.005%,
  • REM: 0.0005 to 0.1%,
  • Ca: 0.0005 to 0.02%,
  • Zr: 0.0005 to 0.02%, and
  • Mg: 0.0005 to 0.02%

(5) A method of production of steel plate excellent in machineability and in toughness and weldability characterized by heating a steel slab or cast slab having a steel composition as described in (1) and having an X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V /10+5×B of 0.24 or less, then rough rolling it by a total reduction rate of 30% to 95%, then finish rolling it by a first pass bite temperature of a T1(° C.) expressed by T1=35 ln(X2/2)−25√t+1070, X2=(Si/5+Mo+Cr/2)/Mn, to 720° C. and a total reduction rate of 30% to 95%, starting water cooling after the end of the rolling at a flow rate of 0.2 m³/m²·min to 5.0 m³/m²·min, and ending the water cooling at 600° C. or less, where t is the plate thickness.

(6) A method of production of steel plate excellent in machineability and in toughness and weldability as set forth in (5), characterized in that in the water cooling started after the end of the rolling, an average cooling rate from a water cooling start temperature to over 650° C. is 1° C./s to 5° C./s and an average cooling rate from 650° C. to a water cooling stop temperature is 10° C./s to 100° C./s.

(7) A method of production of steel plate excellent in machineability and in toughness and weldability characterized by heating a steel slab or cast slab having a steel composition as described in (1) and having an X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B of 0.24 or less, then rough rolling it by a total reduction rate of 30% to 95%, finish rolling it by a total reduction rate of 30% to 95%, then air cooling it, starting water cooling when the steel plate surface temperature is T2(° C.) expressed by T2=910−310×C−80×Mn−20×Cu−15×Cr−55×Ni−80×Mo+0.0006t²−0.56t−8.6 to 650° C. by a flow rate of 0=2 m³/m²·min to 5.0 m³/m²·min, and ending the water cooling at 500° C. or less, where t is the plate thickness.

(8) A method of production of steel plate excellent in machineability and in toughness and weldability characterized by heating a steel slab or cast slab having a steel composition as described in (1) and having an X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B of 0.24 or less, then rough rolling it by a total reduction rate of 30% to 95%, finish rolling it by a total reduction rate of 30% to 95%, air cooling to 500° C. or less, reheating the steel plate to a T3(° C.) expressed by T3=0.0017t²+0.17t+730 to 850° C., then starting the water cooling, and ending the water cooling at 500° C. or less, where t is the plate thickness.

(9) A method of production of steel plate excellent in machineability and in toughness and weldability characterized by heating a steel slab or cast slab having a steel composition as described in (2) and having an X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B of 0.24 or less and an X2 expressed by X2=(Si/5+Mo+Cr/2)/Mn of 0.15 to 10.0, then rough rolling it by a total reduction rate of 30% to 95%, then finish rolling it by a first pass bite temperature of a T4(° C.) expressed by T4=35 ln(X2/2)-251t+1100 to an Ar₃ point by a total reduction rate of 30% to 95%, then speedily starting water cooling after the end of the rolling at a flow rate of 0.2 m³/m²·min to 5.0 m³/m²·min, and ending the water cooling at 600° C. or less, where t is the plate thickness.

(10) A method of production of steel plate excellent in machineability and in toughness and weldability as set forth in (9), characterized in that in the water cooling started after the end of the rolling, an average cooling rate from a water cooling start temperature to over 650° C. is 1° C./s to 5° C./s and an average cooling rate from 650° C. to a water cooling stop temperature is 10° C./s to 100° C./s.

(11) A method of production of steel plate excellent in machineability and in toughness and weldability characterized by heating a steel slab or cast slab having a steel composition as described in (2) and having an X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B of 0.24 or less and an X2 expressed by X2=(Si/5+Mo+Cr/2)/Mn of 0.15 to 10.0, rolling it, cooling it to 500° C. or less, reheating the steel plate to 900° C. to 1050° C., water cooling it by an average cooling rate of 1° C./s to 100° C./s, and ending the water cooling at 500° C. or less.

(12) A method of production of steel plate excellent in machineability and in toughness and weldability characterized by heating a steel slab or cast slab having a steel composition as described in (2) and having an X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B of 0.24 or less and an X2 expressed by X2=(Si/5+Mo+Cr/2)/Mn of 0.15 to 10.0, rough rolling it by a total reduction rate of 30% to 95%, finish rolling it by a total reduction rate of 30% to 95%, then air cooling it to an Ar₃ point to a temperature lower than the Ar₃ point by 150° C., starting water cooling at a flow rate of 0.2 m³/m²·min to 5.0 m³/m²·min, and ending the water cooling at 500° C. or less.

(13) A method of production of steel plate excellent in machineability and in toughness and weldability characterized by heating a steel slab or cast slab having a steel composition as described in (2) and having an X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B of 0.24 or Less and an X2 expressed by X2=(Si/5+Mo+Cr/2)/Mn of 0.15 to 10.0, then rough rolling it by a total reduction rate of 30% to 95%, finish rolling it by a total reduction rate of 30% to 95%, then air cooling it to 500° C. or less, reheating the steel plate to 730° C. to less than 900° C., then water cooling it, and ending the water cooling to 500° C. or less.

(14) A method of production of steel plate excellent in machineability and in toughness and weldability characterized by heating a steel slab or cast slab having a steel composition as described in (3) and having an X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B of 0.24 or less and an X2 expressed by X2=(Si/5+Mo+Cr/2)/Mn of 0.15 to 10.0, then rough rolling it by a total reduction rate of 30% to 95%, then finish rolling it by a first pass bite temperature of a T4(° C.) expressed by T4=35 ln(X2/2)−25√t+1100 to an Ar₃ point by a total reduction rate of 30% to 95%, then speedily starting water cooling at a flow rate of 0.2 m³/m²·min to 5.0 m³/m²·min, and ending the water cooling at 600° C. or less, where t is the plate thickness.

(15) A method of production of steel plate excellent in machineability and in toughness and weldability as set forth in (14), characterized in that in the water cooling started after the end of the rolling, an average cooling rate from a water cooling start temperature to over 650° C. is 1° C./s to 5° C./s and an average cooling rate from 650° C. to a water cooling stop temperature is 10° C./s to 100° C./s.

(16) A method of production of steel plate excellent in machineability and in toughness and weldability characterized by heating a steel slab or cast slab having a steel composition as described in (3) and having an X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B of 0.24 or less and an X2 expressed by X2=(Si/5+Mo+Cr/2)/Mn of 0.15 to 10.0, rolling it, cooling it to 500° C. or less, reheating the steel plate to 900° C. to 1050° C., water cooling it by an average cooling rate of 1° C./s to 100° C./s, and ending the water cooling at 500° C. or less.

(17) A method of production of steel plate excellent in machineability and in toughness and weldability characterized by heating a steel slab or cast slab having a steel composition as described in (3) and having an X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B of 0.24 or less and an X2 expressed by X2=(Si/5+Mo+Cr/2)/Mn of 0.15 to 10.0, rough rolling it by a total reduction rate of 30% to 95%, finish rolling it by a total reduction rate of 30% to 95%, then air cooling it to an Ar₃ point to a temperature lower than the Ar₃ point by 150° C., starting water cooling at a flow rate of 0.2 m³/m²·min to 5.0 m³/m²·min, and ending the water cooling at 500° C. or less.

(18) A method of production of steel plate excellent in machineability and in toughness and weldability characterized by heating a steel slab or cast slab having a steel composition as described in (3) and having an X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B of 0.24 or less and an X2 expressed by X2=(Si/5+Mo+Cr/2)/Mn of 0.15 to 10.0, rough rolling it by a total reduction rate of 30% to 95%, finish rolling it by a total reduction rate of 30% to 95%, then cooling it to 500° C. or less, reheating the steel plate to 730° C. to 900° C., water cooling it, and ending the water cooling at 500° C. or less.

(19) A method of production of steel plate excellent in machineability and in toughness and weldability as set forth in any one of (5) to (18) characterized in that said steel slab or cast slab further contains, by mass %, one or more of:

• • Mo: 0.01 to 1%,
  • Cr: 0.01 to 1%,
  • Nb: 0.001 to 0.1%,
  • Ti: 0.001 to 0.1%,
  • V: 0.001 to 0.1%,
  • Cu: 0.005 to 1%,
  • Ni: 0.01 to 2%,
  • B: 0.0002 to 0.005%,
  • REM: 0.0005 to 0.1%,
  • Ca: 0.0005 to 0.02%,
  • Zr: 0.0005 to 0.02%, and
  • Mg: 0.0005 to 0.02%

According to the present invention, by making the structure forming the steel a composite structure of soft ferrite and hard bainite and martensite and [1] strictly defining the ferrite fraction of the front and rear surfaces of the steel plate having a particularly great effect on tool wear, [2] adjusting the balance of steel ingredients so as to enable a great reduction of the machining resistance occurring at the time of machining at a high temperature, or [3] further raising the amount of addition of S in a range not causing a drop in toughness, it becomes possible to provide steel plate provided with a machineability of a high level not achievable with steel plate for welded structures in the past, excellent in strength, toughness, and weldability, and having a tensile strength of 570 to 720 MPa or so and a method of production of the same. The invention has high industrial value.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view for explaining the locations for measurement of the micro Vickers hardness prescribed in the present invention.

BEST MODE FOR WORKING THE INVENTION

The present invention will be explained in detail below.

The inventors engaged in intensive studies on a method of production of steel plate described in (1) of the present invention having a plate thickness of 4 to 100 mm or so, a matrix strength of 570 to 720 MPa or so, and excellent in matrix toughness, weldability, and machineability. As a result, they discovered that by making a composite structure of a soft structure mainly comprised of ferrite and a hard structure mainly comprised of bainite and martensite the main structure of the steel, strictly defining the ferrite fraction of the front and rear surfaces of the steel plate having a large effect on tool wear since it corresponds to the start and end point of the machining work, strictly defining the method of production requiring water cooling, etc., the machineability is greatly improved while securing the strength and matrix toughness and weldability.

Note that the “weldability” referred to in present invention indicates both weld fractures and weld heat affected zone toughness. The more difficult the occurrence of weld fractures or the higher the weld heat affected zone toughness, the better the weldability. On the other hand, the “machineability” indicates the tool life, machining resistance, and chip control. The longer the tool life, the lower the machining resistance, and the easier the chip control, the better the machineability.

The most important thing in the steel plate described in (1) of the present invention is the formation of a large amount of ferrite near the surface of the steel plate. In the machining process, the surfaces of the steel plate correspond to the start and end points. A large load is applied to the tool, so the machining in these regions has an extremely large effect on the tool life or machining resistance and chip control at the subsequent machining. That is, by making the structure at the top and rear surfaces of the steel plate a composite structure of a soft structure and hard structure, the soft part easily deforms at the time of starting and time of ending the machining of the workpiece by the tool, while stress concentrates near the interface between the soft part and hard part. As a result, the machining is started and ended by extremely small plastic deformation. Due to this, the tool life becomes longer, the machining resistance falls, and chip control becomes easy. The inventors evaluated the machineability for various steel plate with different distributions of ferrite in the plate thickness direction and discovered that it is desirable to define as values representing the distribution of ferrite in the plate thickness direction, in the case of steel plate having a plate thickness of 4 mm to less than 10 mm, the three locations of the locations exactly ¼, ½, and ¾ of the plate thickness inside the steel plate from the top surface of the steel plate (hereinafter called the “t/4 part”, “t/2 part”, and “3t/4 part”) and, in the case of steel plate having a plate thickness of 10 mm to 100 mm, the five locations of the locations of the above three locations plus the locations exactly 2 mm inside the steel plate from the top surface and rear surface of the steel plate surface (hereinafter called the “top surface 2 mm part” and “rear surface 2 mm part”).

In the case of steel plate having a plate thickness of 4 mm to less than 10 mm, the inventors discovered that when the t/4 part and 3t/4 part have a ferrite fraction of 30% or more and the t/2 part has a ferrite fraction of 20% or more, the machineability becomes good. On the other hand, when even one of the t/4 part, t/2 part, and 3t/4 part has a ferrite fraction over 90%, the strength greatly falls. From this, the inventors defined the ferrite fraction for the t/4 part and 3t/4 part as 30% to 90% and the ferrite fraction for the t/2 part as 20% to 90% for steel plate having a plate thickness of 4 mm to less than 10 mm. Note that when the t/4 part, t/2 part, and 3t/4 part have ferrite fractions of 50% or more, the machineability is greatly improved, so preferably the t/4 part, t/2 part, and 3t/4 part have ferrite fractions defined as 50% to 90%.

In the case of steel plate having a plate thickness of 10 mm to less than 100 mm, when the top surface 2 mm part and the rear surface 2 mm part have ferrite fractions of 30% or more and the t/4 part, t/2 part, and 3t/4 part have ferrite fractions of 20% or more, the machineability becomes good. On the other hand, if even one of the top surface 2 mm part, rear surface 2 mm part, t/4 part, t/2 part, or 3t/4 part has a ferrite fraction over 90%, the strength greatly falls. From this, the inventors defined the ferrite fraction of the top surface 2 mm part and rear surface 2 mm part as 30% to 90% or less and the ferrite fraction of the t/4 part, t/2 part, and 3t/4 part as 20% to 90% for steel plate having a plate thickness of 10 mm to less than 100 mm. Note that when the top surface 2 mm part, rear surface 2 mm part, t/4 part, t/2 part, and 3t/4 part have a ferrite fraction of 50% or more, the machineability is greatly improved, so preferably the top surface 2 mm part, rear surface 2 mm part, t/4 part, t/2 part, and 3t/4 part have a ferrite fraction of 50% to 90%.

Here, the method of measurement of the ferrite fraction is defined. The measurement is conducted for the plane parallel to both the rolling direction and plate thickness direction (hereinafter called the “L plane”). Avoiding the ends of the steel plate in the width direction and the spans of locations inside the ends in the width directions by exactly the lengths corresponding to the plate thickness, test pieces of the entire thicknesses were taken from locations near the center part of the steel plate in the width direction as much as possible, polished at the L planes, and etched by Nital. After this, the L planes were observed under an optical microscope. A magnification of 500× is preferable. Observation was performed using an eyepiece with a net pattern. The number of lattice points corresponding to ferrite were counted. The fraction (percentage) of lattice points corresponding to ferrite in all of the Lattice points is defined as the ferrite fraction. The measurement was conducted for a minimum of 10 fields for each location. The amount of displacement from one field to the next field was held constant. Here, when a location was difficult to judge as being ferrite or another phase, it was counted as 0.5. Note that regarding the criteria for judgment of ferrite at the time of measurement, the “ferrite” referred to in the present invention generally indicates ferrite called bulk ferrite, polygonal ferrite, equiaxial ferrite, etc. and does not include acicular ferrite formed at a lower temperature. However, even with bulk ferrite, depending on the control of the austenite before transformation, sometimes anisotropy occurs in the growth direction and bulk ferrite having a form long in the rolling direction is produced. This is included in the ferrite in the present invention.

Further, to obtain an excellent weldability and toughness, the amounts of addition of the alloy elements have to be adjusted. When X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B is 0.24 or less, the weld fractures can be greatly reduced. Not only this, the toughness and weld heat affected zone toughness also become excellent. Therefore, X1 is defined as 0.24 or less. Note that when X1 is 0.21 or less, this effect appears more remarkably, so preferably X1 is made 0.21 or less. Note that the C, Mn, Cu, Cr, Si, Ni, Mo, V, and B when calculating the X1 are all amounts of addition expressed by mass %.

Further, the inventors engaged in intensive studies on the method of production of steel plate described in (2) of the present invention having a plate thickness of 4 to 100 mm or so, a matrix tensile strength of 570 to 720 MPa or so, and excellent in all of machineability, matrix toughness, and weldability. As a result, they discovered that by making a composite structure of a soft structure mainly comprised of ferrite and a hard structure mainly comprised of bainite and martensite the main structure of the steel, by strictly defining the balance of the amounts of addition of Si, Cr, Mo, and Mn in the steel ingredients, by strictly defining the method of production mainly comprised of temperature control in a method of production requiring water cooling, etc., the machineability is improved while securing the strength, matrix toughness, and weldability.

Further, the inventors engaged in intensive studies on the method of production of steel plate described in (3) having a plate thickness of 4 to 100 mm or so and a matrix strength of 570 to 720 MPa or so, extremely excellent in machineability, having a toughness of an extent enabling use for welded structures. As a result, they discovered that by making a composite structure of a soft structure mainly comprised of ferrite and a hard structure mainly comprised of bainite and martensite the main structure of the steel, strictly defining the balance of the amounts of addition of Si, Cr, Mo, and Mn among the steel ingredients, increasing the amount of addition of S in a range not greatly reducing the toughness, strictly defining the method of production mainly comprised of temperature control in a method of production requiring water cooling, etc., the machineability is greatly improved while securing the strength and matrix toughness.

Note that the “weldability” referred to in the present invention indicates both weld fractures and weld heat affected zone toughness. The more difficult the occurrence of weld fractures or the higher the weld heat affected zone toughness; the better the weldability. On the other hand, the “machineability” indicates the tool life, machining resistance, and chip control. The longer the tool life, the lower the machining resistance, and the easier the chip control, the better the machineability.

The most important requirements for realizing excellent machineability in the present invention are the following two points in the steel plates described in (2) and (3) of the present invention, but in the steel plate described in (3) of the present invention, there is further the later explained point (third point). First, the first point is to make the structure of the steel plate a composite structure mainly comprised of soft ferrite and hard bainite and martensite. In particular, since steel plate having a plate thickness of 4 to 100 mm or so is covered, it is important that broad locations in the plate thickness direction become a soft and hard composite structure. By controlling the structure in this way, the soft part easily deforms at the time of machining, while stress concentrates near the interface of the soft parts and hard parts and thereby ductile fracture is promoted and as a result machining proceeds with extremely small plastic deformation. Due to this, the tool life becomes longer, the machining resistance falls, and chip control becomes easy. Even in the case of a composite structure mainly comprised of soft ferrite and hard bainite and martensite, if the soft ferrite fraction becomes lower than 30%, the machineability greatly falls, while if over 90%, the strength becomes insufficient, so the ferrite fraction is defined as 30% to 90% and the balance is defined as being mainly comprised of bainite and martensite. Further, when the ferrite fraction is 45% or more, the machineability is further excellent, so preferably the ferrite fraction is defined as 45% to 90% and the balance is defined as being mainly bainite and martensite. Further, when the ferrite fraction is 60% or more, the machineability is remarkably excellent, so more preferably the ferrite fraction is defined as 60% to 90% and the balance is defined as being mainly bainite and martensite. Note that the hard structure is mainly comprised of bainite and martensite, but even when partially including pearlite, acicular ferrite, and other inclusions, in the range defined by the present invention, the machineability does not deteriorate and remains excellent.

The ferrite fraction defined above is measured by observation of the microstructure under an optical microscope. The measurement plane is made the plane formed by the rolling direction and plate thickness direction (hereinafter called the “L plane”). The measurement locations in the plate thickness direction were made, in the case of a plate thickness of 8 mm or less, the three locations of the locations exactly ¼, ½, and ¾ of the plate thickness inside the steel plate from the top surface of the steel plate (hereinafter called the “t/4 part”, “t/2 part”, and “3t/4 part”) and, in the case of a plate thickness of over 8 mm, the three locations of the t/4 part, t/2 part, and 3t/4 part of the plate thickness direction plus the locations exactly 2 mm inside the steel plate from the top surface and rear surface of the steel plate surface (hereinafter called the “top surface 2 mm part” and “rear surface 2 mm part”). The line segments connecting the measurement points are designed to be parallel to the plate thickness direction. The measurement locations in the width direction avoid the ends of the steel plate in the width direction and the spans of locations inside the ends in the width directions by exactly the lengths corresponding to the plate thickness. The measurements are made at locations close to the center part in the width direction as much as possible. The measurements are preferably performed at magnifications of 100× to 500×. An eyepiece with a lattice is used for measurement by the point count method. The average value of the ferrite fractions at all of the measurement locations is used as the ferrite fraction in the present invention. Note that regarding the criteria for judgment of ferrite at the time of measurement, the “ferrite” referred to in the present invention generally indicates ferrite called bulk ferrite, polygonal ferrite, equiaxial ferrite, etc. and does not include acicular ferrite formed at a lower temperature. However, even with bulk ferrite, depending on the control of the austenite before transformation, sometimes anisotropy occurs in the growth direction and bulk ferrite having a form long in the rolling direction is produced. This is included in the ferrite in the present invention.

Next, the second point of the requirements for realizing excellent machineability is as follows: A composite structure of a soft structure mainly comprised of ferrite and a hard structure mainly comprised of bainite and martensite is, as explained above, excellent in machineability, but with this alone, the machineability is not necessarily sufficient for the drilling or surface machining etc. in fabrication of welded structures. It is necessary to optimize the ratio of the amounts of addition of specific alloy elements assuming a composite structure of a soft structure and a hard structure. Specifically, the rates of addition of the amount of Mn, the amount of Si, the amount of Cr, and the amount of Mo are strictly defined. Boring, surface machining, and other machining are in a sense fracture phenomenon of cut materials by tools at a high temperature and high strain rate. How much the energy required for this can be reduced is important, so it is necessary to make the difference in strength between the soft part and hard part at a high temperature large. If the amount of addition of Mn is large, the amount of solution strengthening of the soft ferrite becomes large and the difference in strength of the hard part and soft part is reduced, so the amount of addition of Mn is preferably low. On the other hand, increases in the amounts of addition of Si, Cr, and Mo contribute to the increase in the ordinary temperature strength of the hard part mainly comprised of bainite and martensite and raise the resistance of the hard part to a drop in strength at a high temperature, so have the effect of increasing the difference in strength between the soft part and hard part. The inventors used steel ingots of ingredients with amounts of addition of Mn, Si, Cr, and Mo changed in various ways to produce steels of composite structures of soft structures and hard structures and studied their machineability and ingredient balance and as a result discovered that if the X2 expressed by X2=(Si/5+Mo+Cr/2)/Mn is less than 0.15, the absolute level of the machineability is insufficient while conversely if the X2 is over 10.0, the weldability greatly falls. Accordingly, in the present invention, the X2 expressed by X2=(Si/5+Mo+Cr/2)/Mn is defined as 0.15 to 10.0. Note that when the value of X2 is 0.3 or more, the machineability is further improved, so preferably X2 is made 0.3 to 10.0. Further, when the value of X2 is 0.4 or more, the machineability is improved more remarkably, so more preferably X2 is made 0.4 to 10.0. Note that the Si, Mo, Cr, and Mn when calculating X2 are all amounts of addition expressed by mass %. In the present invention, Cr and Mo are important elements, but are added as need after consideration of the alloy costs. When Cr and Mo are not added, the value of said X2 is calculated from the amounts of Si and Mn.

In addition to the above most important two requirements for achieving excellent machineability in the steel plate described in (3) of the present invention as explained above, the third point in the requirements for achieving further improved machineability is that it is important to add as large an amount of S as possible to a range riot causing the weldability and toughness to greatly fall. S has the function of reducing the machining resistance and extending the tool life by the effect of MnS as a source of stress concentration. If the amount of addition is over 0.01%, the improvement in the machineability becomes remarkable, while if over 0.035%, the toughness and weldability both fall, so the amount of S is defined as over 0.01% to 0.035%.

The above summarizes the most important requirements for achieving excellent machineability in steel plates described in (2) and (3) of the present invention, but regarding the first point, that is, when the structure is complicated or an extremely fine grain structure, it is sometimes difficult to define a composite structure of a soft structure and a hard structure from observation under an optical microscope. In the present invention, instead, the method of judgment of a composite structure by the micro Vickers hardness is defined in combination. The micro Vickers hardness requires a smaller measurement area than the Vickers hardness, so in the case of a composite structure, the measurement value greatly fluctuates depending on the structure. In particular, a region mostly comprised of ferrite becomes low in hardness. It is possible to use the ratio of number of measurement points with a low hardness to define a composite structure of a soft structure and a hard structure. The inventors ran various structures through micro Vickers hardness tests and clarified the range of micro Vickers hardness giving an excellent machineability. As a result, when the ratio of micro Vickers hardness of 190 HV or less is 20% or more, the machineability is excellent, so the ratio of micro Vickers hardness of 190 HV or less is made 20% or more. Further, when the ratio of micro Vickers hardness of 180 HV or less is 20% or more, the machineability is excellent, so preferably the ratio of micro Vickers hardness of 180 HV or less is made 20% or more. Further, when the ratio of micro Vickers hardness of 170 HV or less is 20% or more, the machineability is further excellent, so more preferably the ratio of micro Vickers hardness of 170 HV or less is made 20% or more. Note that when the ratio of micro Vickers hardness of 170 HV or less is 40% or more, the machineability is further improved, so more preferably the ratio of micro Vickers hardness of 170 HV or less is made 40% or more.

The “micro Vickers hardness” referred to in the present invention is the value measured by the method defined in JIS Z 2244. Another method of measurement besides that defined in the standards will be explained in detail here. The test force is made 0.09807N. The measurement plane is made the L plane. The measurement locations in the plate thickness direction are made, in the case of a plate thickness of 8 mm or less, the three locations of the t/4 part, t/2 part, and 3t/4 part, and, in the case of a plate thickness of over 8 mm, five locations including also the top surface 2 mm part and rear surface 2 mm part. The line segments connecting the measurement points are designed to become parallel to the plate thickness direction. The measurement locations in the width direction avoid the ends of the steel plate in the width direction and the spans of locations inside the ends in the width directions by exactly the lengths corresponding to the plate thickness. The measurements are made at locations close to the center part in the width direction as much as possible. The measurements are conducted at intervals of 100 μm as shown in FIG. 1. The number of measurement points is made 121 points. The ratio of the number of points among the 121 points having a micro Vickers hardness of 190 HV or less is measured. When the plate thickness is 8 mm or less, the average value of three locations is calculated, while when the plate thickness is over 8 mm, the average value of five points is calculated. This value is used as the ratio of micro Vickers hardness of 190 HV or less. The ratio of micro Vickers hardness of 180 HV or less and the ratio of 170 HV or less are measured by the same technique.

The above definition is important for improving the machineability, but to secure the strength, toughness, and weldability, the following definitions become necessary.

First, to secure a tensile strength of 570 MPa or more, the Vickers hardness has to be defined. If the Vickers hardness drops below 165 HV, securing a tensile strength of 570 MPa or more becomes difficult, while if over 300 HV, the weldability greatly falls, so the Vickers hardness is defined as 165 HV to 300 HV.

The Vickers hardness referred to in the present invention is the value measured by a method defined in JIS Z 2244. Another method of measurement besides that defined in the standards will be explained in detail here. The test force is made 98.07N. The measurement plane is made the L plane. The measurement locations in the plate thickness direction are made, in the case of a plate thickness of 8 mm or less, the three locations of the t/4 part, t/2 part, and 3t/4 part, and, in the case of a plate thickness of over 8 mm, five locations including also the top surface 2 mm part and rear surface 2 mm part. The line segments connecting the measurement points are designed to become parallel to the plate thickness direction. The measurement locations in the width direction avoid the ends of the steel plate in the width direction and the spans of locations inside the ends in the width directions by exactly the lengths corresponding to the plate thickness. The measurements are made at locations close to the center part in the width direction as much as possible. The measurements are conducted at five points or more for each location and the average value of the locations is calculated. In the case of a plate thickness of 8 mm or less, the average value of three locations is calculated, while in the case of a plate thickness of over 8 mm, the average value of five points is calculated. This is used as the Vickers hardness.

Further, to obtain an excellent weldability and toughness, the amounts of addition of the alloy elements have to be adjusted. When the X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B is 0.24 or less, not only are weld fractures greatly reduced, but also the toughness and weld heat affected zone toughness become excellent, so X1 is defined as 0.24 or less. Note that when X1 is 0.21 or less, this effect appears more remarkably, so preferably X1 is made 0.21 or less. Note that the C, Mn, Cu, Cr, Si, Ni, Mo, V, and B when calculating X1 are all amounts of addition expressed by mass %.

Below, the ranges of the alloy elements of the steel plate of the present invention will be defined.

The following, unless particularly indicated to the contrary, is common for the steel plates described in (1), (2), and (3) of the present invention.

C is an element required for securing strength, so the amount of addition is made 0.005% or more. However, on the other hand, an increase in the amount of C invites a drop in matrix toughness and a drop in weldability due to the formation of coarse precipitates, so the upper limit is made 0.2%. Note that if the amount of C is 0.07% or more, securing a tensile strength of 570 MPa or more becomes easy, while if 0.14% or less, the toughness and weldability become more excellent, so preferably the amount of C is made 0.07% to 0.14%.

Si is an extremely important element in the present invention. It is an element effective for increasing the strength and at the same time improving the machineability and for causing the formation of ferrite in a broad range of plate thickness in a method of production predicated on water cooling after rolling and then transforming the balance to mainly bainite and martensite to obtain a composite structure mainly comprised of soft ferrite and hard bainite and martensite. To achieve this effect, addition of 0.01% or more is necessary. Addition of over 1% causes the weldability to drop, so the amount of addition is made 0.01% to 1%. Note that to achieve the above effect more remarkably, addition of 0.2% or more is effective. On the other hand, with 0.55% or less, the weldability becomes extremely excellent, so preferably the amount is made 0.2% to 0.55%.

Mn is an element effective for increasing strength. To achieve the tensile strength of 570 MPa or more covered by the present invention, even at a minimum, 0.01% or more must be added, but conversely if adding over 2%, the weldability falls. Therefore, the amount is defined as 0.01% to 2%. Further, if adding Mn in an amount over 1.4%, the machineability falls, so from the viewpoint of the machineability, 1.4% or less is preferable. Therefore, in the steel plate described in (2) and (3) of the present invention, the amount is defined as 0.01% to 1.4%.

P is an impurity element and preferably has a low amount of addition. Addition over 0.02% causes a drop in the matrix ductility, toughness, and weldability, so the amount is made 0.02% or less.

S is an important element in the present invention and is positively added to improve machineability.

Addition of S causes formation of MnS which acts as a local source of stress concentration so additionally improves the machineability. This effect becomes greater the greater the amount of addition of S, but addition over 0.035% causes the matrix toughness to drop sharply, so the upper limit is defined as 0.035%.

Note that when reducing the amount of addition of S, the effect of improvement of machineability by S becomes smaller, but the matrix toughness and weldability are improved. Therefore, the amount of addition of S is preferably made large when stressing machineability and made small when conversely stressing matrix toughness and weldability.

That is, in the steel plate described in (2) of the present invention, the addition of S has no bearing in the mechanism of improvement of machineability, so a lower amount of addition is preferable. Addition over 0.01% causes the matrix toughness to drop due to the formation of MnS, so 0.01% or less is defined. Note that if the amount of S is 0.006% or less, the matrix toughness is more improved, so preferably the amount of S is defined as 0.006% or less.

Further, in the steel plate described in (3) of the present invention, addition of S functions to reduce the machining resistance and extend the tool life due to the effect of MnS as a source of stress concentration. If the amount of addition is over 0.01%, the improvement in the machineability becomes remarkable, while if over 0.035%, the toughness and weldability both fall, so the amount of S is made over 0.01% to 0.035%.

Al is an element effective as a deoxidizing material. The amount of addition is made 0.001% or more. However, on the other hand, an increase of the amount of Al invites a drop in the matrix toughness, so the upper limit is made 0.1%.

N is an impurity element. Addition over 0.01% causes the matrix toughness and weldability to drop, so 0.01% or less is defined.

Mo is an extremely important element in the present invention and can be added as needed after considering the cost. It is an element effective for increasing the strength and at the same time improving the machineability and for causing the formation of ferrite in a broad range of plate thickness in a method of production predicated on water cooling after rolling and then transforming the balance to mainly bainite and martensite to obtain a composite structure mainly comprised of soft ferrite and hard bainite and martensite. To achieve this effect, addition of 0.01% or more is necessary. Addition of over 1% causes the weldability to drop, so the amount of addition is made 0.01% to 1%. Note that to achieve the above effect more remarkably, addition of 0.1% or more is effective, so preferably the amount is made 0.1% to 1.0%.

Cr is an extremely important element in the present invention and may be added as needed after consideration of the cost. It is an element effective for increasing the strength and at the same time improving the machineability and for causing the formation of ferrite in a broad range of plate thickness in a method of production predicated on water cooling after rolling and then changing the balance to mainly bainite and martensite to obtain a composite structure mainly comprised of soft ferrite and hard bainite and martensite. To achieve this effect, addition of 0.01% or more is necessary. Addition of over 1% causes the weldability to drop, so the amount of addition is made 0.01% to 1%. Note that to achieve the above effect more remarkably, addition of 0.1% or more is effective, so preferably the amount is made 0.1% to 1.0%.

In the present invention, Nb, Ti, and V are also important elements. Nb, Ti, and V are elements effective for increasing the strength by precipitation strengthening etc. and for improving the toughness by making the structure finer and are added as needed from the viewpoint of securing the strength and toughness. The inventors evaluated the tool life when drilling steel plate comprised of a soft and hard composite structure strengthened by these elements. As a result, they discovered that even with a soft and hard composite structure, if the amount of precipitation strengthening is large, the difference in hardness of the soft part and hard part is reduced and the drill life falls. If any of the amounts of addition of Nb, Ti, and V exceeds 0.1%, the machineability remarkably falls. On the other hand, with addition of less than 0.001%, the effect of increase of strength is not obtained. Therefore, the amounts of addition of Nb, Ti, and V are made 0.001% to 0.1%. Note that when the amounts of addition of Nb, Ti, and V are 0.05% or less, 0.04% or less, and 0.05% or less, the drop in machineability accompanying the increase in strength is particularly small, so preferably the amounts of addition of Nb, Ti, and V are made 0.05% or less, 0.04% or less, and 0.05% or less.

Cu, Ni, and B are added as needed from the viewpoint of securing the strength. Cu is an element effective for securing strength. With addition of less than 0.005%, the effect is small, while addition of over 1% causes the weldability to drop, so the range is made 0.005 to 1%. Ni is added as needed to secure strength. With addition of less than 0.01%, the effect is small, while addition of over 2% causes the weldability to drop, to the range is made 0.01 to 2%. B is an element effective for increasing hardenability. The amount of addition is made 0.0002% or more. However, on the other hand, an increase of the amount of B invites a drop in the matrix toughness due to formation of coarse precipitates, so the upper limit is made 0.005%.

Addition of one or more of a REM, Ca, Zr, and Mg can improve the matrix toughness and weld heat affected zone toughness through control of matrix inclusions, increased fineness of the heated austenite of the weld heat affected zone, and formation of transformation nuclei from inside the grains, so these are added as necessary. To achieve this effect, addition of 0.0005% or more of any or REM, Ca, Zr, or Mg is necessary. On the other hand, if overly added, the sulfides and oxides become coarser and the matrix toughness and ductility are lowered, so the upper limit value is made 0.1% for REM and 0.02% for Ca, Zr, and Mg.

Note that in melting the steel of the present invention, the present invention is not impaired in effect in any way even if the O, Zn, Sn, Sb, Te, Ta, W, Pb, Bi, etc. which may dissolve out from the materials used including the added alloys or the furnace materials during melting and enter the steel as unavoidable impurities is 0.005% or less.

Next, the methods of production of steel plate described in (1) of the present invention will be explained. Roughly classified, there are three methods of production. The first method of production (method of production 1) is a method performing rolling at a relatively low temperature, then speedily water cooling, the second method of production (method of production 2) is the method of air cooling until the formation of ferrite after rolling and then continuing with water cooling, and the third method of production (method of production 3) is the method of heating again when the temperature of the rolled steel plate falls, then water cooling. In each case, to form ferrite in a broad range in the plate thickness direction and secure a high ferrite fraction near the steel plate surface, it is necessary to strictly control the temperature in accordance with the plate thickness.

Note that when producing the steel plate described in (1), a steel slab or cast slab having the steel composition described in (1) of the present invention, that is, C, 0.005 to 0.2%, Si: 0.01 to 1%, Mn: 0.01 to 2%, P: 0.02% or less, S: 0.035% or less, Al: 0.001 to 0.1%, N, 0.01% or less, and the balance of iron and unavoidable impurities, where X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B is 0.24 or less, can be used.

First, the first method of production (method of production 1), that is, the method of rolling at a relatively low temperature, then speedily water cooling and the method of production described in (5) of the present invention will be explained. In this method of production, strict definition of the rough rolling, finish rolling, and water cooling are important.

The rough rolling is important for making the austenite finer by recrystallization and stably forming ferrite and as a result improving the machineability. If the total reduction rate of the rough rolling becomes less than 30%, the ferrite is not stably formed, while if over 95%, the productivity greatly falls, so the total reduction rate of the rough rolling is defined as 30% to 95%. Note that when the total reduction rate of the rough rolling is 50% or more, the machineability is improved more, so preferably the total reduction rate of the rough rolling is made 50% to 95%. Further, when the total reduction rate of the rough rolling is 80% or more, the machineability is improved still more, so more preferably the total reduction rate of the rough rolling is made 80% to 95%. The temperature for the rough rolling may be freely set so long as the condition of the finish rolling temperature is satisfied. Note that the “total reduction rate of the rough rolling” is the plate thickness before rough rolling minus the plate thickness after rough rolling divided by the plate thickness before rough rolling expressed as a percentage.

The finish rolling is important for actively utilizing the dislocations introduced in the unrecrystallization temperature range in various manners so as to promote the formation of ferrite and make it finer and as a result improve the machineability, toughness, and weldability. The inventors produced steel plates of various alloy ingredients and plate thicknesses by this method of production and evaluated them for machineability, weldability, and matrix toughness. As a result, they confirmed that when making the first pass bite temperature of the finish rolling a temperature of not more than T1 (° C.) expressed by T1=35 ln(X2/2)−25√t+1070, X2=(Si/5+Mo+Cr/2)/Mn, ferrite is formed in a broad plate thickness direction and the machineability, weldability, and toughness are all excellent. Accordingly, the first pass bite temperature of the finish rolling is defined as not more than T1 (° C.) expressed by T1=35 ln(X2/2)−25√t+1070, X2=(Si/5+Mo+Cr/2)/Mn. Note that Si, Mo, Cr, and Mn indicate amounts of addition expressed by mass %, while t is the thickness (mm) of the steel plate. If the first pass bite temperature of the finish rolling is less than 720° C., working of the ferrite will cause the matrix toughness and machineability to greatly fall, so the first pass bite temperature of the rolling is given a lower limit of 720° C. Note that if making the first pass bite temperature of the finish rolling 40° C. lower than T1, the machineability is improved more remarkably, so preferably the first pass bite temperature of the finish rolling is made a temperature 40° C. lower than T1. Further, if making the first pass bite temperature of the finish rolling 80° C. lower than T1, the machineability is improved still more remarkably, so more preferably the first pass bite temperature of the finish rolling is made a temperature 80° C. lower than T1. If the final pass bite temperature of the finish rolling is less than 700° C., working of the ferrite will cause the matrix toughness and machineability to greatly fall, while if over T1 (° C.), the ferrite will not be produced in a broad range in the plate thickness direction, so the final pass bite temperature of the finish rolling preferably has a lower limit of 700° C. and an upper limit of T1 (° C.). The total reduction rate is also important for the finish rolling. If less than 30%, ferrite is not formed over a broad range of the plate thickness, while conversely if over 95%, the productivity greatly falls, so the total reduction rate of the finish rolling is defined as 30% to 95%. Further, if the total reduction rate of the finish rolling is 60% or more, the ferrite is formed more stably and the machineability is improved, so preferably the total reduction rate of the finish rolling is made 60% to 95% or less.

Note that in the present invention, the rolling performed by a rough rolling machine is deemed rough rolling, while the rolling performed by a finish rolling machine is deemed finish rolling. If performing rough rolling and finish rolling by the same rolling machine, when there is a clear temperature setting dividing the rolling into a first half and a second half, the first half of rolling is deemed the rough rolling and the second half of rolling is deemed the finish rolling. When there is no clear temperature setting or when there are two or more temperature settings, all of the rolling passes after and including the rolling pass where the temperature of the steel plate surface before the start of that rolling pass became 950° C. or less are deemed the finish rolling. The “first pass bite temperature of the finish rolling” indicates the temperature measured at the surface of the steel plate before the first reduction by the finish rolling. The final pass bite temperature of the finish rolling indicates the temperature measured at the surface of the steel plate before the final reduction by the finish rolling. Note that steel plate surface temperature can be measured for example by using a radiant thermometer.

Next, the water cooling will be explained. Water cooling is effective for securing 570 to 720 MPa or so of tensile strength, securing strength with low alloy content and thereby improving the weldability, and further making the structure finer and thereby improving the matrix toughness. If the flow rate at the time of water cooling falls below 0.2 m³/m²·min, the strength falls, while if over 5.0 m³/m²·min, the ferrite is no longer stably formed in a broad range in the plate thickness direction and the machineability falls, so the flow rate at the time of water cooling is defined as 0.2 m³/m²·min to 5.0 m³/m²·min. If the end temperature of the water cooling exceeds 600° C., the residual austenite after ferrite formation will not transform at a low temperature and the strength will fall, so the end temperature of the water cooling is made 600° C. or less. Here, the “end temperature of the water cooling” means the maximum value of the steel plate surface temperature measured after waiting for heat recovery after water cooling. After water cooling, air cooling is performed.

In the water cooling performed after the end of rolling, by changing the first half and second half cooling rates, it is possible to form the ferrite more stably, so this technique may be adopted if necessary. By making the cooling rate of the first half defined by the water cooling start temperature to over 650° C. 1° C./s to 5° C./s and making the second half cooling rate of the second half defined by 650° C. to the water cooling end temperature 10° C./s to 100° C./s, steel plate with an even more excellent machineability and an equal or better strength can be produced. The cooling rate in the first half of the cooling is lowered so as to increase the amount of production of ferrite and make the C in the untransformed austenite more concentrated and thereby lower the transformation temperature of the bainite etc. formed in the second half cooling, while the cooling rate in the latter half is raised so as to make the transformation temperature of the untransformed austenite as low as possible. Note that the temperatures and cooling rates in this two-stage cooling are made the temperatures measured at the steel plate t/4 part and the average cooling rates calculated based on those values and can be measured using a spare sample of a steel plate in which a thermocouple is embedded and performing water cooling simulating actual water cooling.

Below, other preferable conditions in the first method of production (method of production 1) will be explained. Before the rough rolling and finish rolling, the steel slab or cast slab is heated. If the heating temperature is less than 900° C., part of the structure from before the heating will remain untransformed, so the material will become nonhomogeneous, while if the heating temperature exceeds 1350° C., the austenite will become coarser, the final structure will also become coarser, the matrix toughness will greatly fall, and also the formation of ferrite will be suppressed and the machineability will fall, so the heating temperature is preferably made 900° C. to 1350° C. The water cooling is to be started very speedily after the end of the finish rolling. For example, it is preferably started within 200 from the end of the finish rolling. This is because if the time until the start of the water cooling exceeds 200 s, the dislocations introduced by the rolling will be reduced by recovery, ferrite will not be stably formed in a broad range in the plate thickness direction, and the machineability will fall. Here, the “end of the finish rolling” means the point of time when the frontmost part of the steel plate is reduced at the final pass of the finish rolling, while the “start of the water cooling” means the point of time when the frontmost part of the steel plate reaches the water cooling facility and the water cooling starts. Further, the water cooled, then air cooled steel plate may be heat treated if necessary. For example, it may be tempered from the viewpoint of improving the matrix toughness.

Next, the second method of production (method of production 2), that is, the method of air cooling until formation of ferrite after rolling, then water cooling, and the method of production described in (7) of the present invention will be defined. The heating is similar to the first method of production. The temperature of the rough rolling can be freely set, but if the total reduction rate of the rough rolling is less than 30%, the toughness greatly falls, while if over 95%, the productivity greatly falls, so the total reduction rate of the rough rolling is defined as 30% to 95%. The temperature of the finish rolling is not defined like in the first method of production and can be any condition. If the total reduction rate of the finish rolling is less than 30%, the toughness greatly falls, while if over 95%, the productivity greatly falls, so the total reduction rate of the finish rolling is defined as 30% to 95%. After the heating, rough rolling, and finish rolling end, air cooling is performed. During the air cooling, ferrite is formed, then water cooling is performed. The inventors investigated steels of various ingredients by changing the steel plate surface temperature when shifting from air cooling after finish rolling to water cooling in various ways and discovered that when the steel plate surface temperature when shifting to water cooling is T2 (° C.) expressed by T2=910−310×C−80×Mn−20×Cu−15×Cr−55×Ni−80×Mo+0.0006t²−0.56t−8.6 or less, ferrite is formed in a broad range in the plate thickness direction and the machineability is improved, while when the steel plate surface temperature falls below 650° C., the strength greatly falls. Accordingly, the steel plate surface temperature when shifting to water cooling is defined as T2 (° C.) expressed by T2=910−310×C−80×Mn−20×Cu−15×Cr−55×N−80×Mo+0.0006t²−0.56t−8.6 to 650° C. Here, the “steel plate surface temperature when shifting to water cooling” means the steel plate surface temperature measured before water cooling. C, Mn, Cu, Cr, Ni, and Mo indicate the amounts of addition of the corresponding elements (mass %), while t indicates the plate thickness (mm). If the flow rate at the time of water cooling falls below 0.2 m³/m²·min, the strength falls, while if over 5.0 m³/m²·min, the ferrite is no longer stably formed in a broad range in the plate thickness direction and the machineability falls, so the flow rate at the time of water cooling is defined as 0.2 m³/m²·min to 5.0 m³/m²·min. If the end temperature of the water cooling exceeds 500° C., the residual austenite after formation of ferrite will not transform at a low temperature and the strength will falls, so the end temperature of the water cooling is made 500° C. or less. Here, the “end temperature of water cooling” means the maximum value of the steel plate surface temperature measured after waiting for heat recovery after water cooling. After water cooling, air cooling is performed. Further, the water cooled, then air cooled steel plate may be heat treated if necessary. For example, it may be tempered from the viewpoint of improving the matrix toughness.

Next, the third method of production (method of production 3), that is, the method of heating again after the temperature of the steel plate falls after rolling will be defined. The heating before the rolling is the same as the first method of production. The temperature of the rough rolling may be freely set, but if the total reduction rate of the finish rolling is less than 30%, the toughness will greatly fall, while if it exceeds 95%, the productivity greatly falls. Accordingly, the total reduction rate of the finish rolling is defined as 30% to 95%. The temperature of the finish rolling is not defined as in the first method of production and can be made any condition. If the total reduction rate of the finish rolling is less than 30%, the toughness will greatly fall, while if over 95%, the productivity greatly falls, so the total reduction rate of the finish rolling is defined as 30% to 95%. After the heating, rough rolling, and finish rolling, the steel plate is cooled to 500° C. or less by any means, then is again heated. The inventors investigated the reheating temperature by changing it in various ways. If the reheating temperature is less than T3 (° C.) expressed by T3=0.0017t²+0.17t+730 or if it is over 850° C., sufficient strength cannot be obtained, so the reheating temperature is defined as T3 (° C.) expressed by T3=0.0017t²+0.17t+730 to 850° C. After reheating, water cooling is performed. If the flow rate at the time of water cooling falls below 0.2 m³/m²·min, the strength falls, while if over 5.0 m³/m²·min, the ferrite is no longer stably formed in a broad range and the machineability falls, so the flow rate at the time of water cooling is defined as 0.2 m³/m²·min to 5.0 m³/m²·min. If the end temperature of the water cooling exceeds 500° C., the residual austenite after ferrite formation will not transform at a low temperature and the strength will fall, so the end temperature of the water cooling is made 500° C. or less. Here, the “end temperature of the water cooling” means the maximum value of the steel plate surface temperature measured after waiting for heat recovery after water cooling. After water cooling, air cooling is performed.

Next, the methods of production of steel plate of (2) and (3) of the present invention will be explained. Roughly classified, there are four methods of production. Methods of production 4 to 7 and 4′ to 7′ will be described for the steel plates. Further, the methods of production of steel plate described in (2) and (3) of the present invention are the same in all four methods of production in all conditions other than the steel compositions of the steel slabs or cast slabs, so in the following explanations, the explanations will be given for the steel plates described in (2) and (3). The first method of production (methods of production 4, 4′) is the method of water cooling speedily after rolling, the second method of production (methods of production 5, 5′) is the method of heating again after the temperature of the steel plate falls after rolling, then water cooling, the third method of production (methods of production 6, 6′) is the method of air cooling until the formation of ferrite after rolling, then water cooling, and the fourth method of production (methods of production 7, 7′) is the method of heating again to the two-phase region after the temperature of the steel plate falls after rolling, then water cooling.

Note that when producing the steel plate described in (2) of the present invention, a steel slab or cast slab having the steel composition described in (2) of the present invention, that is, having the steel composition described in (1) wherein Mn: 0.1% to 1.4%, S: 0.01% or less, having X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B of 0.24 or less, and having X2 expressed by X2=(Si/5+Mo+Cr/2)/Mn of 0.15 to 10.0 is used, further, when producing the steel plate described in (3) of the present invention, a steel slab or cast slab having the steel composition described in (3) of the present invention, that is, having the steel composition described in (1) wherein Mn: 0.1% to 1.4%, S: over 0.01% to 0.035%, having X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B of 0.24 or less, and having X2 expressed by X2=(Si/5+Mo+Cr/2)/Mn of 0.15 to 10.0 is used,

First, the first method of production (methods of production 4, 4′) among the methods of production of steel plate described in (2) and (3), that is, the method of starting water cooling speedily after rolling, and the methods of production described in (9) and (14) of the present invention will be explained. In this method of production, the rough rolling, finish rolling, and water cooling become important.

First, the rough rolling will be explained. Rough rolling is important from the viewpoint of increasing the fineness of the austenite through recrystallization and thereby promoting the formation of ferrite. If the total reduction rate of the rough rolling is less than 30%, the ferrite is not stably formed, while if over 95%, the productivity greatly falls, so the total reduction rate of the rough rolling is defined as 30% to 95%. Further, if making the total reduction rate of the rough rolling 50% or more, the ferrite is formed more stably and the machineability is further improved, so preferably the reduction rate in the rough rolling is made 50% to 95% Further, if making the total reduction rate of the rough rolling 80% or more, the ferrite is still more stably formed and the machineability still more improved, so more preferably the reduction rate of the rough rolling is made 80% to 95%. The rough rolling bite temperature and the steel plate surface temperature before the final pass may be freely set so long as the condition of the finish rolling temperature is satisfied. Note that the “total reduction rate of the rough rolling” is the plate thickness before rough rolling minus the plate thickness after rough rolling divided by the plate thickness before rough rolling expressed as a percentage.

The finish rolling is important for stably forming ferrite in the method of production using water cooling. The lower the temperature of the rolling, the higher the density of dislocations introduced per unit reduction rate and the more recovery of dislocations in the middle of transport between rolling passes or from the rolling machine to the water cooling facility is suppressed, so formation of ferrite can be promoted. The behavior of formation of ferrite is heavily influenced by the alloy ingredients, so the finish rolling temperature has to be defined in relation to the ingredients. In general, in thick gauge steel plate having a tensile strength of 570 MPa or more, with the method of production of water cooling speedily after rolling, formation of ferrite is difficult. To achieve this, an extremely low finish rolling temperature becomes necessary and the productivity falls, but in the present invention, it is newly discovered that production is possible without a drop in productivity in a range of ratios of ingredients of Si, Mn, Mo, and Cr defined for improving the machineability. The inventors studied the optimal first pass bite temperature of the finish rolling for steels of various ingredients and discovered that when the first pass bite temperature of the finish rolling is T4 (° C.) expressed by T4=35 ln(X2/2)−25√t−1100 or less, ferrite is stably formed. Accordingly, the first pass bite temperature of the finish rolling is defined as T4 (° C.) or less. Here, X2, as already shown, is the value calculated by X2=(Si/5+Mo+Cr/2)/Mn, and t is the plate thickness (mm). The formula of T4 includes the item of plate thickness because the greater the final plate thickness, the more the reduction rate at the rolling generally falls, so the increasing coarseness of the recrystallization grain size and drop in density of remaining dislocations cause the formation of ferrite to be suppressed and thereby low temperature rolling to become necessary. Note that if the first pass bite temperature of the finish rolling is set to 40° C. lower than T4, the machineability is improved more remarkably, so preferably the first-pass bite temperature of the finish rolling is made a temperature 40° C. lower than T4 or less. Further, if making the first pass bite temperature of the finish rolling 80° C. lower than T4, the machineability is improved still more remarkably, so more preferably the first pass bite temperature of the finish rolling is made a temperature 80° C. lower than T4 or less. Note that if the first pass bite temperature of the finish rolling is lower than the Ar₃ point, the increase in hardness due to working of the ferrite will cause the machineability to fall, so the lower limit of the first pass bite temperature of the finish rolling is defined as the Ar₃ point. The final pass bite temperature of the finish rolling is preferably given a lower limit of a temperature 100° C. lower than the Ar₃ point or more and an upper limit of T4+50 (° C.) from the viewpoint of greatly suppressing any drop in machineability accompanying working of the ferrite.

Note that in the present invention, the rolling performed by a rough rolling machine is deemed rough rolling, while the rolling performed by a finish rolling machine is deemed finish rolling. If performing rough rolling and finish rolling by the same rolling machine, when there is a clear temperature setting dividing the rolling into a first half and a second half, the first half of rolling is deemed the rough rolling and the second half of rolling is deemed the finish rolling. When there is no clear temperature setting or when there are two or more temperature settings, all of the rolling passes after and including the rolling pass where the temperature of the steel plate surface before the start of that rolling pass became 950° C. or less are deemed the finish rolling. The “first pass bite temperature of the finish rolling” indicates the temperature measured at the surface of the steel plate before the first reduction by the finish rolling. The “final pass bite temperature of the finish rolling” indicates the temperature measured at the surface of the steel plate before the final reduction by the finish rolling. The Ar₃ point cannot be directly measured, but can be estimated by thermo-mechanical treatment simulating actual production conditions while measuring the expansion curve. Note that steel plate surface temperature can be measured for example by using a radiant thermometer.

The total reduction rate of the finish rolling is important from the viewpoint of the stable formation of ferrite. If the total reduction rate of the finish rolling is 30% or more, the ferrite is stably formed and thereby the machineability is improved. On the other hand, if the total reduction rate of the finish rolling exceeds 95%, the productivity greatly falls. Accordingly, the total reduction rate of the finish rolling is defined as 30% to 95%. Note that making the total reduction rate of the finish rolling 60% or more improves the machineability more, so preferably the total reduction rate of the finish rolling is made 60% to 95% or less. Note that the “total reduction rate of the finish rolling” is the plate thickness before finish rolling minus the plate thickness after finish rolling divided by the plate thickness before finish rolling expressed as a percentage.

Next, the conditions of the water cooling will be explained. Water cooling is important for simultaneously achieving an improvement in the machineability through stable formation of ferrite and formation of a structure with a balance of mostly bainite and martensite by low temperature transformation, an improvement of matrix toughness by increasing the fineness of the grain size, and improvement of the weldability through securing strength with a low alloy content. If the flow rate at the time of water cooling falls below 0.2 m³/m²·min, the strength falls, while if over 5.0 m³/m²·min, the ferrite is no longer stably formed and the machineability falls, so the flow rate at the time of water cooling is defined as 0.2 m³/m²·min to 5.0 m³/m²·min. If the end temperature of the water cooling exceeds 600° C., the residual austenite after ferrite formation will not transform at a low temperature and the strength will fall, so the end temperature of the water cooling is made 600° C. or less. Here, the “end temperature of the water cooling” means the maximum value of the steel plate surface temperature measured after waiting for heat recovery after water cooling. After water cooling, air cooling is performed.

Further, the water cooling is preferably started speedily after the end of the finish rolling. This is because if the time from the end of the finish rolling to the start of the water cooling becomes long, the dislocations introduced by the rolling will be reduced by recovery, ferrite will not be stably formed, and the machineability will fall. Note that specifically the water cooling is preferably started within 200 s from the end of the finish rolling. Here, the “end of the finish rolling” means the point of time when the frontmost part of the steel plate is reduced at the final pass of the finish rolling, while the “start of the water cooling” means the point of time when the frontmost part of the steel plate reaches the water cooling facility and the water cooling starts.

In the water cooling performed after the end of rolling, by changing the first half and second half cooling rates, it is possible to form the ferrite more stably, so this technique may be adopted if necessary. By making the cooling rate of the first half defined by the water cooling start temperature to over 650° C. 1° C./s to 5° C./s and making the second half cooling rate of the second half defined by 650° C. to the water cooling end temperature 10° C./s to 100° C./s, steel plate with an even further improved machineability and an equal or better strength can be produced. The cooling rate in the first half of the cooling is lowered so as to increase the amount of production of ferrite and make the C in the untransformed austenite more concentrated and thereby lower the transformation temperature of the bainite etc. formed in the second half cooling, while the cooling rate in the latter half is raised so as to make the transformation temperature of the untransformed austenite as low as possible. Note that the temperatures and cooling rates in this two-stage cooling are made the temperatures measured at the steel plate t/4 part and the average cooling rates calculated based on those values and can be measured using a spare sample of a steel plate in which a thermocouple is embedded and performing water cooling simulating actual water cooling.

Below, other preferable conditions in the method of production will be explained. Before the rolling, the steel slab or cast slab is heated. If the heating temperature is less than 900° C., part of the structure from before the heating will remain untransformed, so the material will become nonhomogeneous, while if the heating temperature exceeds 1350° C., the austenite will become coarser, the final structure will also become coarser, the matrix toughness will greatly fall, and also the formation of ferrite will be suppressed and the machineability will fall, so the heating temperature is preferably made 900° C. to 1350° C. Further, the water cooled, then air cooled steel plate may be heat treated if necessary. For example, it may be tempered from the viewpoint of improving the matrix toughness.

Next, the second method of production (methods of production 5, 5′) among the methods of production of steel plate described in (2) and (3) of the present invention, that is, the method of heating again when the temperature of the steel plate falls after rolling, then water cooling, and the methods of production described in (11) and (16) of the present invention will be defined. The heating, rough rolling, and finish rolling may be performed under any conditions. After the end of the heating, rough rolling, and finish rolling, the steel plate is cooled to 500° C. or less by any means, then is again heated to 900° C. to 1050° C. After the heating, it is water cooled by a cooling rate of 1° C./s to 100° C./s. The end temperature of the water cooling is made 500° C. or less. After the water cooling, air cooling is performed.

Due to the reheating after rolling, fine austenite is obtained and ferrite can be stably formed. If the reheating temperature is less than 900° C., austenite is formed with a high concentration of C. This becomes martensite after transformation and causes the matrix toughness to greatly fall. Further, if the reheating temperature is over 1050° C., ferrite is not stably produced, and the machineability falls. Therefore, the reheating temperature is defined as 900° C. to 1050° C. If the cooling rate after reheating is less than 1° C./s, the residual austenite after formation of ferrite will not transform at a low temperature and the strength will fall, while if the cooling rate exceeds 100° C./s, ferrite will not be stably formed, so the cooling rate after reheating is defined as 1° C./s to 100° C./s. If the end temperature of the water cooling exceeds 500° C., the residual austenite after formation of ferrite will not transform at a low temperature and the strength will fall, so the end temperature of the water cooling is defined as 500° C. or less. Further, the water cooled, then air cooled steel plate may be heat treated if necessary. For example, it may be tempered from the viewpoint of improving the matrix toughness.

Here, the “end temperature of water cooling” means the maximum value of the steel plate surface temperature measured speedily after heat recovery after water cooling. The cooling rate of the water cooling is made the average cooling rate calculated based on the temperatures measured at the steel plate t/4 part and can be estimated by using a spare sample of a steel plate in which a thermocouple is embedded and performing water cooling simulating actual water cooling.

Next, the third method of production (methods of production 6, 6′) among the methods of production of steel plate described in (2) and (3) of the present invention, that is, the method of air cooling until the start of formation of ferrite after rolling, then water cooling, and the methods of production described in (12) and (17) of the present invention will be defined. The heating is the same as said first method of production (method of production 4). In the rough rolling, if the total reduction rate is less than 30%, the toughness falls, while if over 95%, the productivity greatly falls, so the total reduction rate of the rough rolling is defined as 30% to 95%. The finish rolling is not defined as to temperature like with the first method and may be performed under any conditions. If the total reduction rate of the finish rolling is less than 30%, the toughness falls, while if over 95%, the productivity greatly falls, so the total reduction rate of the finish rolling is defined as 30% to 95%. After the heating, rough rolling, and finish rolling end, air cooling is performed. During the air cooling, ferrite starts to form, then water cooling is performed. When the temperature for starting the water cooling exceeds the Ar₃ point, the ferrite is not stably formed and the machineability falls, while if lower than a temperature lower than the Ar₃ point by 150° C., the strength falls, so the water cooling start temperature is defined as the Ar₃ point to a temperature lower than the Ar₃ point by 150° C. or higher. Here, the “water cooling start temperature” means the steel plate surface temperature measured before water cooling. The Ar₃ point can be estimated by thermo-mechanical treatment simulating actual production conditions while measuring the expansion curve. If the flow rate at the time of water cooling falls below 0.2 m³/m²·min, the strength will fall, while if over 5.0 m³/m²·min, the productivity will fall, so the flow rate at the time of water cooling is defined as 0.2 m³/m²·min to 5.0 m³/m²·min. If the end temperature of the water cooling exceeds 500° C., the residual austenite after formation of ferrite will not transform at a low temperature and the strength will fall, so the end temperature of the water cooling is made 500° C. or less. Here, the “end temperature of water cooling” means the maximum value of the steel plate surface temperature measured speedily after heat recovery after water cooling. After the water cooling, air cooling is performed. Further, the water cooled, then air cooled steel plate may be heat treated if necessary. For example, it may be tempered from the viewpoint of improving the matrix toughness.

Next, the fourth method of production (methods of production 7, 7′) among the methods of production of steel plate described in (2) and (3) of the present invention, that is, the method of heating again to the two-phase region when the temperature of the steel plate falls after rolling, and the methods of production described in (13) and (18) of the present invention will be defined. The heating, rough rolling, and finish rolling are similar to said third method of production (methods of production 6, 6′). After the heating, rough rolling, and finish rolling end, the steel plate is cooled to 500° C. or less by any means, then is again heated. If the reheating temperature is less than 730° C., the machineability falls, while if 900° C. or more, the strength falls, so the reheating temperature is defined as 730° C. to less than 900° C. After reheating, the plate may be water cooled by any method. If the end temperature of water cooling is over 500° C., the residual austenite after formation of ferrite will not transform at a low temperature and the strength will fall, so the end temperature of water cooling is made 500° C. or less. After the water cooling, air cooling is performed. Further, the cooled steel plate may be heat treated if necessary. For example, it may be tempered from the viewpoint of improving the matrix toughness.

EXAMPLE 1

Examples of the steel plate described in (1) of the present invention will be explained next.

Test steel materials of various chemical ingredients were used to produce steel plates of plate thicknesses of 6, 18, 40, and 100 mm under various production conditions. Their properties were evaluated. The evaluated items were, as strength, the yield stress and tensile strength, as toughness, the Charpy impact absorption energy, as the weld heat affected zone toughness in weldability, the Charpy impact absorption energy of the weld joint, and as the machineability, the drilling property. The chemical ingredients, plate thickness, X1, and ferrite fraction measured at various locations of each of the steel plates are shown in Table 1 to Table 6, the production conditions (methods of production 1 to 3) are shown in Table 7 to Table 9, and the results of evaluation of properties are shown in Table 10 to Table 12.

The yield stress and the tensile strength were measured by the metal material tensile test method described in JIS Z 2241. The test piece was a metal material test piece described in JIS Z 2201. No. 5 test pieces taken from steel plates having plate thicknesses of 6 mm and 18 mm, while No. 10 test pieces taken from the t/4 parts of steel plates having plate thicknesses of 40 mm and 100 mm were used. The test pieces were taken so that their longitudinal directions became vertical to the rolling direction. The yield stress was made 0.2% of the yield strength calculated by the lower yield stress or offset method. Two tests were conducted at ordinary temperature and the average values were used.

The matrix toughness was measured by the metal material impact test method described in JIS Z 2242. The test pieces used were the metal material impact test pieces described in JIS Z 2202. For steel plates having thicknesses of 6 mm, subsize test pieces of widths of 5 mm were taken from the plate thickness center parts, for steel plates having thicknesses of 18 mm, test pieces of widths of 10 mm were taken from the plate thickness center parts, while for steel plates having thicknesses of 40 mm and 100 mm, test pieces of widths of 10 mm were taken from the t/4 parts. The shapes were all made V-notch test pieces. The test pieces were taken so that the lines formed by the notch bottoms became parallel to the plate thickness direction and so that the longitudinal directions of the test piece became perpendicular to the rolling direction. The test temperature was made −5° C. The average value of three tests was employed.

For the weld heat affected zone toughness, Charpy test pieces were taken from weld joints prepared by CO₂ gas shield arc welding and submerge arc welding and measured for absorption energy at −5° C. The welding heat input was made 2 to 3 kJ/mm in the case of CO₂ gas shield arc welding and was made 3 kJ/mm for plate thickness 6 mm materials, 5 kJ/mm for plate thickness 18 mm materials, and 7 kJ/mm for plate thickness 40 mm materials and 100 mm materials in the case of submerge arc welding. The test nieces were taken so that the locations 0.5 mm from the weld bond part corresponded to the Charpy test piece notch positions. The average value of three impact absorption energies was used.

The machineability was evaluated by a drilling test using a drilling machine and a high speed drill. The drilling distance was 42 mm in the case of plate thickness 6 mm steel plates piled up in seven layers, 36 mm in the case of plate thickness 18 mm steel plates piled up in two layers, 40 mm in the case of one plate thickness 40 mm steel plate, and 100 mm in the case of one plate thickness 100 mm steel plate for the test. The drill was used to make a through hole using a 6 mmφ diameter high speed drill SKH51. The rotational speed was 1610 rpm, the feed speed was 190 mm/min, and the machining oil was a water soluble machining oil. Under the above conditions, drilling was performed until drilling was no longer possible. The number of holes bored until the limit was reached was measured.

TABLE 1
Plate Thickness, Chemical Ingredients, X1, Ferrite Fraction (Method of Production 1)
	Final plate thick.	C	Si	Mn	P	S	Al	N	Mo	Cr	Nb	Ti	V
	mm	mass %
Inv. Ex. 1-1	6	0.13	0.25	0.95	0.0082	0.0043	0.032	0.0035
Comp. Ex. 1-1	6	0.22	0.26	0.55	0.0085	0.0042	0.033	0.0036
Inv. Ex. 1-2	18	0.13	0.55	1.15	0.0038	0.0027	0.036	0.0042			0.008	0.010
Comp. Ex. 1-2	18	0.12	1.20	1.16	0.0048	0.0031	0.032	0.0045			0.008	0.009
Inv. Ex. 1-3	40	0.09	0.75	1.25	0.0054	0.0230	0.033	0.0028		0.15			0.023
Comp. Ex. 1-3	40	0.09	0.74	1.23	0.0051	0.0250	0.033	0.0029		0.15			0.022
Inv. Ex. 1-4	100	0.11	0.44	0.85	0.0055	0.0028	0.025	0.0056	0.23	0.23
Comp. Ex. 1-4	100	0.06	0.42	1.37	0.0068	0.0032	0.033	0.0055	0.19	0.23
Inv. Ex. 1-5	6	0.07	0.46	0.75	0.0083	0.0033	0.023	0.0042	0.25	0.23	0.008		0.018
Comp. Ex. 1-5	6	0.07	0.45	0.78	0.0250	0.0033	0.025	0.0045	0.24	0.22	0.007		0.019
Inv. Ex. 1-6	18	0.06	0.75	0.86	0.0056	0.0043	0.027	0.0033	0.28	0.31		0.015
Comp. Ex. 1-6	18	0.06	0.76	0.45	0.0063	0.0042	0.031	0.0032	1.15	0.32		0.016
Inv. Ex. 1-7	40	0.12	0.15	1.12	0.0058	0.0042	0.035	0.0038		0.35	0.022
Comp. Ex. 1-7	40	0.12	0.16	1.11	0.0062	0.0045	0.041	0.0035		0.34	0.021
Inv. Ex. 1-8	100	0.07	0.65	1.45	0.0035	0.0028	0.026	0.0028	0.05	0.21
Comp. Ex. 1-8	100	0.07	0.66	2.15	0.0040	0.0031	0.032	0.0029	0.01	0.36
Inv. Ex. 1-9	6	0.12	0.32	0.65	0.0085	0.0025	0.018	0.0021	0.15	0.15	0.015
Comp. Ex. 1-9	6	0.12	0.32	0.66	0.0091	0.0031	0.022	0.0120	0.16	0.14	0.016
Inv. Ex. 1-10	18	0.06	0.75	0.95	0.0048	0.0028	0.019	0.0042	0.23	0.12			0.023
Comp. Ex. 1-10	18	0.06	0.76	0.96	0.0058	0.0350	0.022	0.0043	0.23	0.11			0.024
Inv. Ex. 1-11	40	0.08	0.45	1.35	0.0056	0.0025	0.035	0.0029		0.21
Comp. Ex. 1-11	40	0.06	0.44	1.34	0.0061	0.0031	0.120	0.0028		1.15

TABLE 2
(Continuation of Table 1)
	Cu	Ni	B	REM	Ca	Zr	Mg		Ferrite fraction (%)
	mass %	X1	Top 2 mm	Rear 2 mm	t/4	3t/4	t/2
Inv. Ex. 1-1								0.188			38	36	39
Comp. Ex. 1-1								0.251			34	30	35
Inv. Ex. 1-2								0.201	41	40	42	43	45
Comp. Ex. 1-2								0.222	43	45	48	51	56
Inv. Ex. 1-3								0.186	37	38	40	40	43
Comp. Ex. 1-3								0.187	0	0	0	0	0
Inv. Ex. 1-4	0.5	0.5						0.222	32	31	33	34	36
Comp. Ex. 1-4	0.5	0.6						0.197	0	0	0	0	0
Inv. Ex. 1-5			0.0010					0.160			43	45	52
Comp. Ex. 1-5			0.0011					0.160			42	46	48
Inv. Ex. 1-6				0.0012	0.0013			0.157	48	48	51	52	56
Comp. Ex. 1-6				0.0011	0.0013			0.197	36	34	35	38	39
Inv. Ex. 1-7	0.3	0.4				0.0021		0.215	48	50	52	54	60
Comp. Ex. 1-7	0.3	0.5				0.0022		0.217	25	24	26	28	30
Inv. Ex. 1-8	0.5	0.5	0.0008					0.213	38	37	40	42	45
Comp. Ex. 1-8	0.5	0.5	0.0009					0.253	36	35	37	37	40
Inv. Ex. 1-9								0.176			58	60	72
Comp. Ex. 1-9								0.177			60	63	68
Inv. Ex. 1-10							0.0018	0.151	72	73	75	75	76
Comp. Ex. 1-10							0.0015	0.153	70	70	71	72	72
Inv. Ex. 1-11	0.5	0.5						0.201	71	72	63	64	65
Comp. Ex. 1-11	0.5	0.5						0.228	0	0	12	12	23

TABLE 3
Plate Thickness, Chemical Ingredients, X1, Ferrite Fraction (Method of Production 2)
	Final plate thick.	C	Si	Mn	P	S	Al	N	Mo	Cr	Nb	Ti	V
	mm	mass %
Inv. Ex. 1-12	6	0.09	0.45	0.88	0.0089	0.0032	0.025	0.0038					0.045
Comp. Ex. 1-12	6	0.09	0.46	0.89	0.0091	0.0035	0.032	0.0036					0.112
Inv. Ex. 1-13	18	0.13	0.25	1.25	0.0055	0.0025	0.018	0.0048			0.012	0.010
Comp. Ex. 1-13	18	0.12	0.24	1.16	0.0048	0.0380	0.032	0.0045			0.012	0.011
Inv. Ex. 1-14	40	0.08	0.76	1.15	0.0045	0.0050	0.025	0.0029	0.15	0.25	0.018
Comp. Ex. 1-14	40	0.08	0.75	1.14	0.0050	0.0060	0.033	0.0029	0.15	0.24	0.019
Inv. Ex. 1-15	100	0.11	0.45	0.83	0.0055	0.0025	0.032	0.0045	0.35	0.23	0.008		0.021
Comp. Ex. 1-15	100	0.11	0.43	0.86	0.0056	0.0022	0.023	0.0048	0.34	0.22	0.115		0.022
Inv. Ex. 1-16	6	0.09	0.65	1.56	0.0035	0.0023	0.031	0.0035
Comp. Ex. 1-16	6	0.09	0.66	1.58	0.0041	0.0021	0.032	0.0036
Inv. Ex. 1-17	18	0.12	0.22	0.85	0.0055	0.0042	0.028	0.0021	0.15	0.36	0.011	0.010	0.008
Comp. Ex. 1-17	18	0.12	0.21	0.86	0.0043	0.0042	0.031	0.0022	0.16	0.37	0.012	0.125	0.009

TABLE 4
(Table 3 Continuation)
	Cu	Ni	B	REM	Ca	Zr	Mg		Ferrite fraction (%)
	mass %	X1	Top 2 mm	Rear 2 mm	t/4	3t/4	t/2
Inv. Ex. 1-12	0.5	0.5						0.186			37	38	40
Comp. Ex. 1-12	0.5	0.5						0.192			63	62	66
Inv. Ex. 1-13								0.202	47	46	50	51	53
Comp. Ex. 1-13								0.190	49	50	55	57	58
Inv. Ex. 1-14				0.0015	0.0018			0.180	72	72	65	60	68
Comp. Ex. 1-14				0.0015	0.0020			0.180	25	26	23	28	32
Inv. Ex. 1-15								0.199	36	37	25	23	28
Comp. Ex. 1-15								0.198	33	30	23	21	25
Inv. Ex. 1-16						0.0018	0.0012	0.185			46	48	51
Comp. Ex. 1-16						0.0018	0.0250	0.187			44	43	50
Inv. Ex. 1-17								0.202	46	43	50	51	55
Comp. Ex. 1-17								0.202	40	38	46	46	49

TABLE 5
Plate Thickness, Chemical Ingredients, X1, Ferrite Fraction (Method of Production 3)
	Final plate thick.	C	Si	Mn	P	S	Al	N	Mo	Cr	Nb	Ti	V
	mm	mass %
Inv. Ex. 1-18	18	0.11	0.25	1.12	0.0038	0.0035	0.032	0.0045	0.15		0.018
Comp. Ex. 1-18	18	0.11	0.26	1.11	0.0041	0.0043	0.032	0.0048	0.16		0.017
Inv. Ex. 1-19	40	0.08	0.55	0.65	0.0035	0.0041	0.032	0.0035		0.55			0.045
Comp. Ex. 1-19	40	0.08	0.56	0.68	0.0041	0.0041	0.032	0.0036		0.56			0.046
Inv. Ex. 1-20	6	0.12	0.76	0.85	0.0061	0.0042	0.031	0.0035			0.012	0.012
Comp. Ex. 1-20	6	0.18	0.76	0.95	0.0061	0.0041	0.032	0.0035			0.012	0.018
Inv. Ex. 1-21	100	0.09	0.25	1.15	0.0023	0.0025	0.035	0.0035	0.23	0.35
Comp. Ex. 1-21	100	0.06	0.26	1.16	0.0023	0.0032	0.033	0.0036	0.23	0.36

TABLE 6
(Table 5 Continuation)
	Cu	Ni	B	REM	Ca	Zr	Mg		Ferrite fraction (%)
	mass %	X1	Top 2 mm	Rear 2 mm	t/4	3t/4	t/2
Inv. Ex. 1-18	0.2	0.3						0.201	43	46	48	51	55
Comp. Ex. 1-18	1.5	0.3						0.266	41	42	45	48	47
Inv. Ex. 1-19					0.0015	0.0015		0.158	71	72	73	73	74
Comp. Ex. 1-19					0.0230	0.0015		0.161	72	71	68	72	75
Inv. Ex. 1-20								0.190			53	51	54
Comp. Ex. 1-20								0.251			58	58	56
Inv. Ex. 1-21	0.5	0.5						0.221	36	38	39	40	43
Comp. Ex. 1-21	0.5	2.5						0.223	44	45	48	49	52

TABLE 7
Production Conditions (Method of Production 1)
				Rough
				rolling
		Final		total
	Slab	plate	Heating	reduction			T1-
	thickness	thickness	temperature	rate	X2	T1	40	T1-80
	mm	mm	° C.	%	—	° C.	° C.	° C.
Inv. Ex. 1-1	240	6	1150	94	0.053	881	841	801
Comp. Ex. 1-1	240	6	1150	94	0.095	902	862	822
Inv. Ex. 1-2	240	18	1200	81	0.096	858	818	778
Comp. Ex. 1-2	240	18	1200	81	0.207	885	845	805
Inv. Ex. 1-3	240	40	1100	58	0.180	828	788	748
Comp. Ex. 1-3	240	40	1100	25	0.181	828	788	748
Inv. Ex. 1-4	400	100	1100	50	0.509	772	732	692
Comp. Ex. 1-4	400	100	1100	50	0.284	752	712	672
Inv. Ex. 1-5	240	6	1180	92	0.609	967	927	887
Comp. Ex. 1-5	240	6	1180	92	0.564	964	924	884
Inv. Ex. 1-6	240	18	1150	85	0.680	926	886	846
Comp. Ex. 1-6	240	18	1150	85	3.249	981	941	901
Inv. Ex. 1-7	240	40	1250	67	0.183	828	788	748
Comp. Ex. 1-7	240	40	1250	77	0.182	828	788	748
Inv. Ex. 1-8	400	100	1130	50	0.197	739	699	659
Comp. Ex. 1-8	400	100	1130	50	0.150	729	689	649
Inv. Ex. 1-9	240	6	1200	94	0.445	956	916	876
Comp. Ex. 1-9	240	6	1200	94	0.445	956	916	876
Inv. Ex. 1-10	240	18	1100	81	0.463	913	873	833
Comp. Ex. 1-10	240	18	1100	81	0.455	912	872	832
Inv. Ex. 1-11	240	40	1150	58	0.144	820	780	740
Comp. Ex. 1-11	240	40	1150	58	0.495	863	823	783
	Finish	Water cooling
	rolling		1st	2nd
	1st	Total			half	half
	pass	reduction	Flow	End	cooling	cooling	Tempering
	bite	rate	rate	temperature	rate	rate	temperature
	° C.	%	m3/m2 · min	° C.	° C./s	° C./s	° C.
Inv. Ex. 1-1	862	60	0.7	20	—	—	450
Comp. Ex. 1-1	865	60	0.7	20	—	—	450
Inv. Ex. 1-2	820	60	1.0	212	—	—	550
Comp. Ex. 1-2	850	60	1.0	216	—	—	550
Inv. Ex. 1-3	800	60	1.5	156	—	—	600
Comp. Ex. 1-3	805	78	1.5	151	—	—	600
Inv. Ex. 1-4	735	50	2.0	256	—	—	480
Comp. Ex. 1-4	758	50	2.0	262	—	—	480
Inv. Ex. 1-5	902	70	0.4	150	5	50	630
Comp. Ex. 1-5	904	70	0.4	152	5	50	630
Inv. Ex. 1-6	853	50	2.0	20	—	—	500
Comp. Ex. 1-6	705	50	2.0	20	—	—	500
Inv. Ex. 1-7	752	50	1.5	20	—	—	480
Comp. Ex. 1-7	755	29	1.5	20	—	—	480
Inv. Ex. 1-8	730	50	1.5	375	—	—	—
Comp. Ex. 1-8	722	50	1.5	345	—	—	—
Inv. Ex. 1-9	850	60	0.4	455	—	—	—
Comp. Ex. 1-9	854	60	0.4	625	—	—	—
Inv. Ex. 1-10	800	60	4.0	210	—	—	480
Comp. Ex. 1-10	800	60	0.2	218	—	—	480
Inv. Ex. 1-11	730	60	1.0	205	—	—	630
Comp. Ex. 1-11	755	60	6.0	203	—	—	630

TABLE 8
Production Conditions (Method of Production 2)
			Rough
	Final		rolling	Finish rolling
	Slab	plate		total	1st pass	Total		Water cooling
	thick-	thick-	Heating	reduction	bite	reduction		Start	Flow	End	Tempering
	ness	ness	temperature	rate	temperature	rate	T2	temperature	rate	temperature	temperature
	mm	mm	° C.	%	° C.	%	° C.	° C.	m3/m2 · min	° C.	° C.
Inv. Ex. 1-12	240	6	1150	94	820	60	763	755	1.0	200	550
Comp. Ex. 1-12	240	6	1150	94	818	60	762	640	1.0	201	550
Inv. Ex. 1-13	240	18	1200	85	860	50	751	745	1.5	455	—
Comp. Ex. 1-13	240	18	1200	85	865	50	760	745	1.5	523	—
Inv. Ex. 1-14	240	40	1250	67	912	50	749	720	2.0	20	480
Comp. Ex. 1-14	240	40	1250	67	915	50	750	765	2.0	20	480
Inv. Ex. 1-15	400	100	1250	50	925	50	721	700	1.0	480	—
Comp. Ex. 1-15	400	100	1250	67	930	25	720	705	1.0	450	—
Inv. Ex. 1-16	240	18	1100	81	905	60	747	735	1.5	200	530
Comp. Ex. 1-16	240	18	1100	81	900	60	745	730	0.2	195	530
Inv. Ex. 1-17	240	40	1200	58	875	60	768	745	1.0	20	550
Comp. Ex. 1-17	240	40	1200	25	878	78	767	740	1.0	20	550

TABLE 9
Production Conditions (Method of Production 3)
	Rough
	rolling	Finish rolling
	Slab	Final		total	1st pass	Total		Water cooling
	thick-	plate	Heating	reduction	bite	reduction		Reheating	Cooling	End	Tempering
	ness	thickness	temperature	rate	temperature	rate	T3	temperature	rate	temperature	temperature
	mm	mm	° C.	%	° C.	%	° C.	° C.	° C./s	° C.	° C.
Inv. Ex. 1-18	240	18	1120	83	905	55	734	755	15	30	450
Comp. Ex. 1-18	240	18	1120	83	915	55	734	870	15	30	450
Inv. Ex. 1-19	240	40	1200	67	860	50	740	760	5	215	450
Comp. Ex. 1-19	240	40	1200	67	870	50	740	710	5	211	450
Inv. Ex. 1-20	100	6	1250	85	915	60	731	760	20	210	500
Comp. Ex. 1-20	100	6	1250	91	918	33	731	755	20	195	500
Inv. Ex. 1-21	400	100	1250	50	885	50	764	780	5	450	—
Comp. Ex. 1-21	400	100	1250	25	875	67	764	780	5	520	—

TABLE 10
Results of Evaluation of Properties (Method of Production 1)
				Weld heat affected zone	Weld heat affected zone
		Tensile	Matrix toughness	toughness (vE-5)	toughness (vE-5)	Number of
	Yield stress	strength	(vE-5)	(CO2 arc welding)	(submerge arc welding)	holes
	MPa	MPa	J	J	J	No.
Inv. Ex. 1-1	504	655	113	94	74	228
Comp. Ex. 1-1	554	698	22	11	5	187
Inv. Ex. 1-2	480	623	175	123	89	278
Comp. Ex. 1-2	514	668	28	23	11	178
Inv. Ex. 1-3	494	625	100	78	65	315
Comp. Ex. 1-3	537	655	82	75	64	15
Inv. Ex. 1-4	478	605	138	88	75	105
Comp. Ex. 1-4	510	615	155	92	69	8
Inv. Ex. 1-5	467	605	101	65	58	423
Comp. Ex. 1-5	487	611	22	16	26	185
Inv. Ex. 1-6	505	623	156	108	100	480
Comp. Ex. 1-6	605	735	25	18	7	26
Inv. Ex. 1-7	515	638	155	123	88	512
Comp. Ex. 1-7	523	655	122	135	75	8
Inv. Ex. 1-8	467	588	215	101	88	111
Comp. Ex. 1-8	550	666	187	5	8	100
Inv. Ex. 1-9	515	630	121	87	61	625
Comp. Ex. 1-9	427	555	17	10	7	380
Inv. Ex. 1-10	489	635	223	208	156	705
Comp. Ex. 1-10	405	552	15	5	8	505
Inv. Ex. 1-11	498	630	123	105	89	380
Comp. Ex. 1-11	586	715	23	32	12	13

TABLE 11
Results of Evaluation of Properties (Method of Production 2)
				Weld heat affected zone	Weld heat affected zone
			Matrix toughness	toughness (vE-5)	toughness (vE-5)	No. of
	Yield stress	Tensile strength	(vE-5)	(CO2 arc welding)	(submerge arc welding)	holes
	MPa	MPa	J	J	J	No.
Inv. Ex. 1-12	473	623	81	61	55	203
Comp. Ex. 1-12	423	556	15	13	5	23
Inv. Ex. 1-13	462	608	150	80	72	321
Comp. Ex. 1-13	426	560	21	15	11	418
Inv. Ex. 1-14	476	610	189	89	88	555
Comp. Ex. 1-14	478	625	121	67	78	11
Inv. Ex. 1-15	456	585	156	120	89	191
Comp. Ex. 1-15	535	655	20	21	10	23
Inv. Ex. 1-16	515	625	151	156	123	385
Comp. Ex. 1-16	432	556	25	65	65	156
Inv. Ex. 1-17	608	713	113	78	65	211
Comp. Ex. 1-17	612	725	25	13	12	11

TABLE 12
Results of Evaluation of Properties (Method of Production 3)
				Weld heat affected zone	Weld heat affected zone
			Matrix toughness	toughness (vE-5)	toughness (vE-5)	No. of
	Yield stress	Tensile strength	(vE-5)	(CO2 arc welding)	(submerge arc welding)	holes
	MPa	MPa	J	J	J	No.
Inv. Ex. 1-18	512	598	188	155	120	235
Comp. Ex. 1-18	466	581	18	25	15	105
Inv. Ex. 1-19	480	623	135	122	101	565
Comp. Ex. 1-19	475	598	23	82	65	200
Inv. Ex. 1-20	522	628	92	55	51	318
Comp. Ex. 1-20	535	632	21	15	14	158
Inv. Ex. 1-21	465	588	123	89	73	126
Comp. Ex. 1-21	408	545	26	15	18	98

Invention Examples 1-1 to 1-11 are steel plates produced by first method of production (method of production 1), that is, the method of speedily water cooling after rolling (the method of production described in (5) of the present invention). Along with these, Comparative Examples 1-1 to 1-11 are also shown.

Invention Example 1-1 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling and water cooling conditions, etc. to produce plate thickness 6 mm steel plate. Ferrite is stably formed in the entire plate thickness range, in particular near the steel plate surface, so this has a tensile strength of 570 MPa or more and is excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 1-1 is similar to Invention Example 1-1 in ingredients and method of production, but the amount of C and X1 are outside the range of the present invention, so the matrix toughness and weld heat affected zone toughness are inferior.

Invention Example 1-2 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling and water cooling conditions, etc. to produce plate thickness 18 ace steel plate. Ferrite is stably formed in the entire plate thickness range, in particular near the steel plate surface, so this has a tensile strength of 570 MPa or more and is excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 1-2 is similar to Invention Example 1-2 in ingredients and method of production, but the Si is outside the range of the present invention, so the matrix toughness and weld heat affected zone toughness are inferior.

Invention Example 1-3 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling and water cooling conditions, etc. to produce plate thickness 40 mm steel plate. Ferrite is stably formed in the entire plate thickness range, in particular near the steel plate surface, so this has a tensile strength of 570 MPa or more and is excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 1-3 is similar to Invention Example 1-3 in ingredients and method of production, but the total reduction rate of the rough rolling and ferrite fraction are outside the range of the present invention, so the machineability is inferior.

Invention Example 1-4 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling and water cooling conditions, etc. to produce plate thickness 100 mm steel plate. Ferrite is stably formed in the entire plate thickness range, in particular near the steel plate surface, so this has a tensile strength of 570 MPa or more and is excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 1-4 is similar to Invention Example 1-4 in ingredients and method of production, but the finish rolling first pass bite temperature and ferrite fraction are outside the range of the present invention, so the machineability is inferior.

Invention Example 1-5 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling and water cooling conditions, etc. to produce plate thickness 6 mm steel plate. Ferrite is stably formed in the entire plate thickness range, in particular near the steel plate surface, so this has a tensile strength of 570 MPa or more and is excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 1-5 is similar to Invention Example 1-5 in ingredients and method of production, but the amount of P is outside the range of the present invention, so the matrix toughness and weld heat affected zone toughness are inferior.

Invention Example 1-6 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling and water cooling conditions, etc. to produce plate thickness 18 mm steel plate. Ferrite is stably formed in the entire plate thickness range, in particular near the steel plate surface, so this has a tensile strength of 570 MPa or more and is excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 1-6 is similar to Invention Example 1-6 in ingredients and method of production, but the amount of Mo and the finish rolling first pass bite temperature are outside the range of the present invention, so the matrix toughness, weld heat affected zone toughness, and machineability are inferior.

Invention Example 1-7 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling and water cooling conditions, etc. to produce plate thickness 40 mm steel plate. Ferrite is stably formed in the entire plate thickness range, in particular near the steel plate surface, so this has a tensile strength of 570 MPa or more and is excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 1-7 is similar to Invention Example 1-7 in ingredients and method of production, but the total reduction rate of the finish rolling and ferrite fraction are outside the range of the present invention, so the machineability is inferior.

Invention Example 1-8 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling and water cooling conditions, etc. to produce plate thickness 100 mm steel plate. Ferrite is stably formed in the entire plate thickness range, in particular near the steel plate surface, so this has a tensile strength of 570 MPa or more and is excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 1-8 is similar to Invention Example 1-8 in ingredients and method of production, but the amount of Mn and X1 are outside the range of the present invention, so the weld heat affected zone toughness is inferior.

Invention Example 1-9 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling and water cooling conditions, etc. to produce plate thickness 6 mm steel plate. Ferrite is stably formed in the entire plate thickness range, in particular near the steel plate surface, so this has a tensile strength of 570 MPa or more and is excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 1-9 is similar to Invention Example 1-9 in ingredients and method of production, but the amount of N and water cooling end temperature are outside the range of the present invention, so the strength, toughness, and weld heat affected zone toughness are inferior.

Invention Example 1-10 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling and water cooling conditions, etc. to produce plate thickness 18 mm steel plate. Ferrite is stably formed in the entire plate thickness range, in particular near the steel plate surface, so this has a tensile strength of 570 MPa or more and is excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 1-10 is similar to Invention Example 1-10 in ingredients and method of production, but the amount of S and flow rate are outside the range of the present invention, so the strength, toughness, and weld heat affected zone toughness are inferior.

Invention Example 1-11 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling and water cooling conditions, etc. to produce plate thickness 40 mm steel plate. Ferrite is stably formed in the entire plate thickness range, in particular near the steel plate surface, so this has a tensile strength of 570 MPa or more and is excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 1-11 is similar to Invention Example 1-11 in ingredients and method of production, but the amount of Al, the amount of Cr, ferrite fraction, and flow rate are outside the range of the present invention, so the toughness, weld heat affected zone toughness, and machineability are inferior.

Invention Examples 1-12 to 1-17 are steel plates produced by the second method of production (method of production 2), that is, the method of air cooling until starting to form ferrite after rolling, then water cooling, and the method of production described in (7) of the present invention. Along with these, Comparative Examples 1-12 to 1-17 are also shown.

Invention Example 1-12 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling and water cooling conditions, etc. to produce plate thickness 6 mm steel plate. Ferrite is stably formed in the entire plate thickness range, in particular near the steel plate surface, so this has a tensile strength of 570 MPa or more and is excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 1-12 is similar to Invention Example 1-12 in ingredients and method of production, but the amount of V and the water cooling start temperature are outside the range of the present invention, so the strength, toughness, weld heat affected zone toughness, and machineability are inferior.

Invention Example 1-13 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling and water cooling conditions, etc. to produce plate thickness 18 mm steel plate. Ferrite is stably formed in the entire plate thickness range, in particular near the steel plate surface, so this has a tensile strength of 570 MPa or more and is excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 1-13 is similar to Invention Example 1-13 in ingredients and method of production, but the water cooling end temperature and the amount of S are outside the range of the present invention, so the strength, toughness, and weld heat affected zone toughness are inferior.

Invention Example 1-14 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling and water cooling conditions, etc. to produce plate thickness 40 mm steel plate. Ferrite is stably formed in the entire plate thickness range, in particular near the steel plate surface, so this has a tensile strength of 570 MPa or more and is excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 1-14 is similar to Invention Example 1-14 in ingredients and method of production, but the water cooling start temperature and ferrite fraction are outside the range of the present invention, so the machineability is inferior.

Invention Example 1-15 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling and water cooling conditions, etc. to produce plate thickness 100 mm steel plate. Ferrite is stably formed in the entire plate thickness range, in particular near the steel plate surface, so this has a tensile strength of 570 MPa or more and is excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 1-15 is similar to Invention Example 1-15 in ingredients and method of production, but the amount of Nb and the total reduction rate of the finish rolling are outside the range of the present invention, so the toughness, weld heat affected zone toughness, and machineability are inferior.

Invention Example 1-16 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling and water cooling conditions, etc. to produce plate thickness 6 mm steel plate. Ferrite is stably formed in the entire plate thickness range, in particular near the steel plate surface, so this has a tensile strength of 570 MPa or more and is excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 1-16 is similar to Invention Example 1-16 in ingredients and method of production, but the amount of Mg and the flow rate are outside the range of the present invention, so the strength and toughness are inferior.

Invention Example 1-17 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling and water cooling conditions, etc. to produce plate thickness 18 mm steel plate. Ferrite is stably formed in the entire plate thickness range, in particular near the steel plate surface, so this has a tensile strength of 570 MPa or more and is excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 1-17 is similar to Invention Example 1-17 in ingredients and method of production, but the amount of Ti and the total reduction rate of the rough rolling are outside the range of the present invention, so the toughness, weld heat affected zone toughness, and machineability are inferior.

Invention Examples 1-18 to 1-21 are steel plates produced by the third method of production (method of production 3), that is, the method of heating until the two-phase region again after the temperature the steel plate falls after rolling (the method of production described in (8) of the present invention). Along with these, Comparative Example 1-18 to 1-21 are also shown.

Invention Example 1-18 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling and water cooling conditions, etc. to produce plate thickness 18 mm steel plate. Ferrite is stably formed in the entire plate thickness range, in particular near the steel plate surface, so this has a tensile strength of 570 MPa or more and is excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 1-18 is similar to Invention Example 1-18 in ingredients and method of production, but the amount of Cu, X1, and reheating temperature are outside the range of the present invention, so the toughness and weld heat affected zone toughness are inferior.

Invention Example 1-19 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling and water cooling conditions, etc. to produce plate thickness 40 mm steel plate. Ferrite is stably formed in the entire plate thickness range, in particular near the steel plate surface, so this has a tensile strength of 570 MPa or more and is excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 1-19 is similar to Invention Example 1-19 in ingredients and method of production, but the amount of Ca and reheating temperature are outside the range of the present invention, so the toughness is inferior.

Invention Example 1-20 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling and water cooling conditions, etc. to produce plate thickness 6 mm steel plate. Ferrite is stably formed in the entire plate thickness range, in particular near the steel plate surface, so this has a tensile strength of 570 MPa or more and is excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 1-20 is similar to Invention Example 1-20 in ingredients and method of production, but the X1 and the total reduction rate of the finish rolling are outside the range of the present invention, so the toughness and weld heat affected zone toughness are inferior.

Invention Example 1-21 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling and water cooling conditions, etc. to produce plate thickness 100 mm steel plate. Ferrite is stably formed in the entire plate thickness range, in particular near the steel plate surface, so this has a tensile strength of 570 MPa or more and is excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 1-21 is similar to Invention Example 1-21 in ingredients and method of production, but the amount of Ni, total reduction rate of the rough rolling, and water cooling end temperature are outside the range of the present invention, so the strength, toughness, and weld heat affected zone toughness are inferior.

From the above examples, it is clear that the steel materials produced by the present invention, that is, the steel plates of Invention Examples 1-1 to 1-21, are steel materials having tensile strengths of 570 to 720 MPa or so, high in matrix toughness, high in weld heat affected zone toughness, and excellent in machineability.

EXAMPLE 2

Examples of the steel plate described in (2) of the present invention will be explained next.

Test steel materials of various chemical ingredients were used to produce steel plates of plate thicknesses of 6, 20, 40, and 100 mm under various production conditions. These were evaluated for, as strength, the yield stress and tensile strength of the matrix, as toughness, the Charpy impact absorption energy of the matrix, as the weld heat affected zone toughness in weldability, the Charpy impact absorption energy of the weld joint, and as the machineability, the drilling property. The chemical ingredients, plate thickness, X1, X2, ferrite fraction, ratio of micro Vickers hardness in a specific range, and Vlckers hardness of each of the steel plates are shown in Table 13 to Table 22, the production conditions (methods of production 4 to 7) are shown in Table 23 to Table 27, and the results of evaluation of properties are shown in Table 28 to Table 32.

The yield stress and the tensile strength were measured by the metal material tensile test method described in JIS Z 2241. The test piece was a metal material test piece described in JIS Z 2201. No. 5 test pieces taken from steel plates having plate thicknesses of 6 mm and 20 mm, while No. 10 test pieces taken from the t/4 parts of steel plates having thicknesses of 40 mm and 100 mm were used. The test pieces were taken so that their longitudinal directions became vertical to the rolling direction. The yield stress was made 0.2% of the yield strength calculated by the lower yield stress or offset method. Two tests were conducted at ordinary temperature and the average values were used.

The matrix toughness was measured by the metal material impact test method described in JIS Z 2242. The test pieces used were the metal material impact test pieces described in JIS Z 2202. For steel plates having thicknesses of 6 mm, subsize test pieces of widths of 5 mm were taken from the plate thickness center parts, for steel plates having thicknesses of 18 mm, test pieces of widths of 10 mm were taken from the plate thickness center parts, while for steel plates having thicknesses of 40 mm and 100 mm, test pieces of widths of 10 mm were taken from the t/4 parts. The shapes were all made V-notch test pieces. The test pieces were taken so that the lines formed by the notch bottoms became parallel to the plate thickness direction and so that the longitudinal directions of the test piece became perpendicular to the rolling direction. The test temperature was made −5° C. The average value of three tests was employed.

For the weld heat affected zone toughness, Charpy test pieces were taken from weld joints prepared by CO₂ gas shield arc welding and submerge arc welding and measured for absorption energy at −5° C. The welding heat input was made 2 to 3 kJ/mm in the case of CO₂ gas shield arc welding and was made 3 kJ/mm for plate thickness 6 mm materials, 5 kJ/mm for plate thickness 20 mm materials, and 7 kJ/mm for plate thickness 40 mm materials and 100 mm materials in the case of submerge arc welding. The test pieces were taken so that the locations 0.5 mm from the weld bond part corresponded to the Charpy test piece notch positions. The average value of three impact absorption energies was used.

The machineability was evaluated by a drilling test using a drilling machine and a high speed drill. The drilling distance was 42 mm in the case of plate thickness 6 mm steel plates piled up in seven layers, 40 mm in the case of plate thickness 20 mm steel plates piled up in two layers, 40 mm in the case of one plate thickness 40 mm steel plate, and 100 mm in the case of one plate thickness 100 mm steel plate for the test. The drill was used to make a through hole using a 6 mmφ diameter high speed drill SKH51. The rotational speed was 1610 rpm, the feed speed was 190 mm/min, and the machining oil was a water soluble machining oil. Under the above conditions, drilling was performed until drilling was no longer possible. The number of holes bored until the limit was reached was measured.

TABLE 13
Plate Thickness, Chemical Ingredients, Etc. of Steel Plate (Method of Production 4)
	Final
	plate
	thickness	C	Si	Mn	P	S	Al	N	Mo	Cr	Nb	Ti	V
	mm	mass %
Inv. Ex. 2-1	6	0.09	0.85	1.12	0.0081	0.0048	0.031	0.0032
Comp. Ex. 2-1	6	0.09	0.84	1.25	0.0081	0.0050	0.033	0.0033
Inv. Ex. 2-2	6	0.13	0.35	0.85	0.0050	0.0032	0.028	0.0035	0.12	0.18	0.008
Comp. Ex. 2-2	6	0.11	1.20	0.85	0.0051	0.0033	0.029	0.0036	0.11	0.17	0.008
Inv. Ex. 2-3	6	0.10	0.18	0.95	0.0033	0.0029	0.031	0.0028	0.22	0.15			0.020
Comp. Ex. 2-3	6	0.08	0.17	1.55	0.0034	0.0030	0.032	0.0027	0.16	0.15			0.020
Inv. Ex. 2-4	6	0.07	0.31	0.75	0.0066	0.0042	0.015	0.0028	0.30	0.31
Comp. Ex. 2-4	6	0.07	0.32	0.74	0.0065	0.0041	0.015	0.0027	0.31	0.32
Inv. Ex. 2-5	20	0.04	0.43	1.38	0.0039	0.0029	0.035	0.0041	0.05	0.16	0.008
Comp. Ex. 2-5	20	0.04	0.41	1.36	0.0038	0.0028	0.120	0.0042	0.03	0.15	0.007
Inv. Ex. 2-6	20	0.09	0.06	0.85	0.0045	0.0085	0.031	0.0038		0.61
Comp. Ex. 2-6	20	0.09	0.05	0.81	0.0043	0.0110	0.032	0.0039		0.62
Inv. Ex. 2-7	20	0.10	0.35	1.05	0.0065	0.0038	0.011	0.0034	0.25	0.15		0.010
Comp. Ex. 2-7	20	0.10	0.34	1.04	0.0221	0.0036	0.012	0.0035	0.28	0.15		0.010
Inv. Ex. 2-8	20	0.13	0.35	0.71	0.0066	0.0048	0.031	0.0035	0.15	0.20	0.015
Comp. Ex. 2-8	20	0.13	0.36	0.72	0.0067	0.0049	0.032	0.0038	0.16	0.21	0.016
Inv. Ex. 2-9	40	0.16	0.33	1.15	0.0053	0.0029	0.031	0.0029	0.12		0.015
Comp. Ex. 2-9	40	0.16	0.32	1.16	0.0055	0.0030	0.032	0.0030	1.15		0.015
Inv. Ex. 2-10	40	0.08	0.95	1.15	0.0068	0.0045	0.045	0.0045		0.35
Comp. Ex. 2-10	40	0.08	0.96	1.14	0.0067	0.0046	0.046	0.0046		1.13

TABLE 14
(Table 13 Continuation 1)
										Ratio of micro Vickers
									Ferrite	hardness in specific	Vickers
	Cu	Ni	B	REM	Ca	Zr	Mg		fraction	range (%)	hardness
	mass %	X1	X2	%	≦190HV	≦180HV	≦170HV	HV
Inv. Ex. 2-1								0.169	0.152	35				195
Comp. Ex. 2-1								0.179	0.134	31				196
Inv. Ex. 2-2								0.202	0.329	50				205
Comp. Ex. 2-2								0.210	0.512	55				203
Inv. Ex. 2-3	0.1	0.2						0.181	0.348		53	38	22	190
Comp. Ex. 2-3	0.2	0.2						0.195	0.174		48	29	18	210
Inv. Ex. 2-4				0.0020				0.148	0.689	70				202
Comp. Ex. 2-4				0.0019				0.152	0.722	0				205
Inv. Ex. 2-5								0.130	0.157		32	15	0	203
Comp. Ex. 2-5								0.129	0.138		30	13	0	200
Inv. Ex. 2-6	0.1	0.1						0.168	0.373	51				192
Comp. Ex. 2-6	0.1	0.1						0.165	0.395	50				191
Inv. Ex. 2-7					0.0015			0.183	0.376	63				196
Comp. Ex. 2-7					0.0018			0.187	0.407	65				198
Inv. Ex. 2-8								0.197	0.451		58	48	41	197
Comp. Ex. 2-8								0.201	0.468		18	0	0	186
Inv. Ex. 2-9								0.233	0.162	46				195
Comp. Ex. 2-9								0.300	1.047	32				254
Inv. Ex. 2-10								0.189	0.317	48				201
Comp. Ex. 2-10								0.227	0.664	46				235

TABLE 15
(Table 13 Continuation 2)
	Final
	plate
	thickness	C	Si	Mn	P	S	Al	N	Mo	Cr	Nb	Ti	V
	mm	mass %
Inv. Ex. 2-11	40	0.07	0.38	0.95	0.0055	0.0029	0.032	0.0038	0.32	0.35	0.012	0.008
Comp. Ex. 2-11	40	0.07	0.37	0.94	0.0054	0.0028	0.031	0.0037	0.31	0.36	0.012	0.009
Inv. Ex. 2-12	40	0.13	0.48	0.65	0.0088	0.0035	0.041	0.0045	0.28	0.25		0.010
Comp. Ex. 2-12	40	0.12	0.47	0.66	0.0089	0.0036	0.042	0.0115	0.29	0.26		0.010
Inv. Ex. 2-13	100	0.07	0.45	1.35	0.0055	0.0029	0.015	0.0055		0.51	0.055		0.050
Comp. Ex. 2-13	100	0.07	0.44	1.34	0.0056	0.0028	0.015	0.0055		0.52	0.055		0.051
Inv. Ex. 2-14	100	0.13	0.55	1.15	0.0078	0.0062	0.039	0.0031	0.25	0.15
Comp. Ex. 2-14	100	0.12	0.54	1.14	0.0078	0.0061	0.038	0.0032	0.24	0.16
Inv. Ex. 2-15	100	0.11	0.85	1.05	0.0065	0.0038	0.031	0.0045	0.08	0.25	0.008	0.010
Comp. Ex. 2-15	100	0.11	0.84	1.04	0.0066	0.0039	0.031	0.0046	0.07	0.24	0.012	0.010
Inv. Ex. 2-16	100	0.09	0.37	0.25	0.0055	0.0029	0.025	0.0031	0.45	0.55	0.008	0.015	0.040
Comp. Ex. 2-16	100	0.09	0.37	0.08	0.0054	0.0028	0.026	0.0030	0.55	0.61	0.008	0.014	0.040
Inv. Ex. 2-17	20	0.05	0.44	1.32	0.0038	0.0028	0.035	0.0042	0.07	0.35	0.011
Comp. Ex. 2-17	20	0.04	0.43	1.31	0.0039	0.0027	0.035	0.0041	0.06	0.36	0.012
Inv. Ex. 2-18	20	0.09	0.14	0.85	0.0044	0.0084	0.032	0.0037		0.58
Comp. Ex. 2-18	20	0.08	0.13	0.82	0.0046	0.0085	0.032	0.0038		0.62
Inv. Ex. 2-19	20	0.16	0.35	0.65	0.0065	0.0037	0.035	0.0035	0.18	0.15		0.012
Comp. Ex. 2-19	20	0.21	0.32	0.61	0.0064	0.0036	0.042	0.0036	0.16	0.14		0.010

TABLE 16
(Table 13 Continuation 3)
										Ratio of micro Vickers
									Ferrite	hardness in specific
	Cu	Ni	B	REM	Ca	Zr	Mg		fraction	range (%)	Vickers hardness
	mass %	X1	X2	%	≦190HV	≦180HV	≦170HV	HV
Inv. Ex. 2-11							0.0015	0.164	0.601	55				205
Comp. Ex. 2-11							0.0016	0.164	0.600	58				163
Inv. Ex. 2-12								0.205	0.771	70				198
Comp. Ex. 2-12								0.205	0.779	71				202
Inv. Ex. 2-13	0.5	0.5						0.211	0.256		28	18	5	186
Comp. Ex. 2-13	0.5	0.6						0.214	0.260		0	0	0	188
Inv. Ex. 2-14			0.0010					0.230	0.378	48				192
Comp. Ex. 2-14			0.0075					0.261	0.375	46				195
Inv. Ex. 2-15								0.204	0.357	55				188
Comp. Ex. 2-15								0.203	0.344	13				187
Inv. Ex. 2-16						0.0015	0.0015	0.171	3.196	74				185
Comp. Ex. 2-16						0.0014	0.0013	0.173	11.613	70				184
Inv. Ex. 2-17								0.148	0.252	40				188
Comp. Ex. 2-17								0.146	0.249	42				160
Inv. Ex. 2-18	0.2	0.5						0.180	0.374	53				190
Comp. Ex. 2-18	0.1	0.1						0.167	0.410	0				211
Inv. Ex. 2-19								0.224	0.500		48	34	14	185
Comp. Ex. 2-19								0.269	0.482		47	33	12	210

TABLE 17
Plate Thickness, Chemical Ingredients, Etc. of Steel Plate (Method of Production 5)
	Final
	plate thickness	C	Si	Mn	P	S	Al	N	Mo	Cr	Nb	Ti	V
	mm	mass %
Inv. Ex. 2-20	6	0.09	0.95	1.20	0.0080	0.0035	0.031	0.0032			0.023
Comp. Ex. 2-20	6	0.08	0.94	1.18	0.0087	0.0038	0.032	0.0033			0.025
Inv. Ex. 2-21	20	0.14	0.65	0.85	0.0075	0.0032	0.033	0.0035	0.11	0.15
Comp. Ex. 2-21	20	0.13	0.66	0.84	0.0073	0.0033	0.035	0.0036	0.11	0.16
Inv. Ex. 2-22	40	0.10	0.35	0.85	0.0053	0.0056	0.034	0.0028	0.25	0.15	0.015
Comp. Ex. 2-22	40	0.10	0.36	0.84	0.0055	0.0055	0.034	0.0027	0.24	0.16	0.110
Inv. Ex. 2-23	100	0.09	0.66	0.95	0.0043	0.0043	0.033	0.0028	0.56			0.018
Comp. Ex. 2-23	100	0.09	0.65	0.94	0.0042	0.0042	0.031	0.0027	0.55			0.112

TABLE 18
(Table 17 Continuation)
									Ferrite	Ratio of micro Vickers
	Cu	Ni	B	REM	Ca	Zr	Mg		fraction	hardness in specific range (%)	Vickers hardness
	mass %	X1	X2	%	≦190HV	≦180HV	≦170HV	HV
Inv. Ex. 2-20				0.177	0.158	40				182
Comp. Ex. 2-20				0.174	0.159	20				185
Inv. Ex. 2-21	0.2	0.2		0.227	0.371		35	18	0	173
Comp. Ex. 2-21	0.2	0.3		0.228	0.383		38	25	5	162
Inv. Ex. 2-22				0.180	0.465	53				188
Comp. Ex. 2-22				0.179	0.467	55				189
Inv. Ex. 2-23				0.192	0.728	58				185
Comp. Ex. 2-23				0.190	0.723	66				162

TABLE 19
Plate Thickness, Chemical Ingredients, Etc. of Steel Plate (Method of Production 6)
	Final
	plate
	thickness	C	Si	Mn	P	S	Al	N	Mo	Cr	Nb	Ti	V
	mm	mass %
Inv. Ex. 2-24	6	0.14	0.85	1.05	0.0085	0.0035	0.031	0.0030
Comp. Ex. 2-24	6	0.13	0.84	1.06	0.0086	0.0036	0.033	0.0031
Inv. Ex. 2-25	20	0.11	0.35	1.05	0.0074	0.0034	0.013	0.0035	0.25		0.023
Comp. Ex. 2-25	20	0.11	0.36	1.04	0.0075	0.0033	0.013	0.0034	0.24		0.024
Inv. Ex. 2-26	40	0.07	0.38	0.85	0.0065	0.0028	0.035	0.0032		0.75			0.020
Comp. Ex. 2-26	40	0.06	0.39	0.85	0.0064	0.0025	0.034	0.0033		0.74			0.120
Inv. Ex. 2-27	100	0.12	0.68	0.88	0.0053	0.0055	0.041	0.0045	0.21	0.25
Comp. Ex. 2-27	100	0.12	0.67	0.87	0.0054	0.0054	0.042	0.0043	0.22	0.24
Inv. Ex. 2-28	20	0.09	0.15	0.65	0.0032	0.0035	0.045	0.0023	0.53
Comp. Ex. 2-28	20	0.09	0.14	0.66	0.0035	0.0032	0.052	0.0025	0.54
Inv. Ex. 2-29	100	0.07	0.25	0.65	0.0065	0.0033	0.035	0.0033	0.35	0.75
Comp. Ex. 2-29	100	0.06	0.24	0.64	0.0066	0.0034	0.034	0.0035	0.36	0.74

TABLE 20
(Table 19 Continuation)
									Ferrite	Ratio of micro Vickers
	Cu	Ni	B	REM	Ca	Zr	Mg		fraction	hardness in specific range (%)	Vickers hardness
	mass %	X1	X2	%	≦190HV	≦180HV	≦170HV	HV
Inv. Ex. 2-24						0.216	0.162		25	10	0	180
Comp. Ex. 2-24						0.215	0.158		0	0	0	182
Inv. Ex. 2-25				0.0015		0.191	0.305	55				185
Comp. Ex. 2-25				0.0205		0.191	0.300	60				162
Inv. Ex. 2-26						0.160	0.531	62				199
Comp. Ex. 2-26						0.169	0.527	61				164
Inv. Ex. 2-27	0.2	0.3				0.229	0.535	66				201
Comp. Ex. 2-27	0.2	2.1				0.260	0.545	48				227
Inv. Ex. 2-28						0.161	0.862	55				185
Comp. Ex. 2-28						0.160	0.861	67				160
Inv. Ex. 2-29	0.3					0.182	1.192	72				182
Comp. Ex. 2-29	1.1					0.220	1.216	73				191

TABLE 21
Plate Thickness, Chemical Ingredients, Etc. of Steel Plate (Method of Production 7)
	Final plate
	thickness	C	Si	Mn	P	S	Al	N	Mo	Cr	Nb	Ti	V
	mm	mass %
Inv. Ex. 2-30	6	0.13	0.25	0.95	0.0067	0.0055	0.031	0.0035		0.25
Comp. Ex. 2-30	6	0.13	0.24	0.94	0.0066	0.0054	0.032	0.0038		0.24
Inv. Ex. 2-31	20	0.11	0.33	0.85	0.0055	0.0033	0.033	0.0045	0.31
Comp. Ex. 2-31	20	0.10	0.32	0.84	0.0054	0.0032	0.033	0.0042	0.32
Inv. Ex. 2-32	40	0.08	0.35	0.55	0.0087	0.0044	0.034	0.0043	0.42	0.43	0.008	0.010
Comp. Ex. 2-32	40	0.08	0.34	0.54	0.0086	0.0045	0.032	0.0041	0.41	0.42	0.008	0.011
Inv. Ex. 2-33	100	0.12	0.33	0.98	0.0077	0.0043	0.033	0.0042	0.35	0.15			0.040
Comp. Ex. 2-33	100	0.11	0.32	0.99	0.0075	0.0042	0.031	0.0040	0.34	0.14			0.112
Inv. Ex. 2-34	100	0.08	0.25	0.75	0.0077	0.0031	0.045	0.0035	0.85		0.035
Comp. Ex. 2-34	100	0.08	0.24	0.74	0.0074	0.0030	0.052	0.0030	0.84		0.110

TABLE 22
(Table 21 Continuation)
										Ferrite	Ratio of micro Vickers	Vickers
	Cu	Ni	B	REM	Ca	Zr	Mg			fraction	hardness in specific range (%)	hardness
	mass %	X1	X2	%	≦190HV	≦180HV	≦170HV	HV
Inv. Ex. 2-30	0.0022				0.200	0.184		25	0	0	185
Comp. Ex. 2-30	0.1210				0.198	0.179		25	0	0	183
Inv. Ex. 2-31		0.0023			0.179	0.442	55				182
Comp. Ex. 2-31		0.0250			0.178	0.457	54				164
Inv. Ex. 2-32				0.0020	0.171	1.282	61				185
Comp. Ex. 2-32				0.0215	0.168	1.274	68				163
Inv. Ex. 2-33					0.210	0.501	58				190
Comp. Ex. 2-33					0.215	0.479	56				191
Inv. Ex. 2-34					0.180	1.200	61				186
Comp. Ex. 2-34					0.177	1.200	63				187

TABLE 23
Production Conditions (Method of Production 4)
				Rough
				rolling
		Final		total
	Slab	plate	Heating	reduction
	thickness	thickness	temperature	rate	X2	T4	T4 - 40	T4 - 80
	mm	mm	° C.	%	—	° C.	° C.	° C.
Inv. Ex. 2-1	240	6	1100	88	0.152	949	909	869
Comp. Ex. 2-1	240	6	1100	88	0.134	944	904	864
Inv. Ex. 2-2	240	6	1150	88	0.329	976	936	896
Comp. Ex. 2-2	240	6	1150	88	0.512	991	951	911
Inv. Ex. 2-3	240	6	1150	88	0.348	978	938	898
Comp. Ex. 2-3	240	6	1150	88	0.174	953	913	873
Inv. Ex. 2-4	240	6	1200	88	0.689	1001	961	921
Comp. Ex. 2-4	240	6	1200	88	0.722	1003	963	923
Inv. Ex. 2-5	240	20	1180	83	0.157	899	859	819
Comp. Ex. 2-5	240	20	1180	83	0.138	894	854	814
Inv. Ex. 2-6	240	20	1150	81	0.373	929	889	849
Comp. Ex. 2-6	240	20	1150	81	0.395	931	891	851
Inv. Ex. 2-7	240	20	1100	79	0.376	930	890	850
Comp. Ex. 2-7	240	20	1100	79	0.407	932	892	852
Inv. Ex. 2-8	240	20	1100	79	0.451	936	896	856
Comp. Ex. 2-8	240	20	1100	79	0.468	937	897	857
Inv. Ex. 2-9	240	40	1200	58	0.162	854	814	774
Comp. Ex. 2-9	240	40	1200	58	1.047	919	879	839
Inv. Ex. 2-10	240	40	1100	67	0.317	877	837	797
Comp. Ex. 2-	240	40	1100	67	0.664	903	863	823
10
	Finish	water cooling
	rolling		1st	2nd
		Total			half	half
	1st pass	reduction	Flow	End	cooling	cooling	Tempering
	bite	rate	rate	temperature	rate	rate	temperature
	° C.	%	m3/m2 · min	° C.	° C./s	° C./s	° C.
Inv. Ex. 2-1	924	80	1	215	—	—	500
Comp. Ex. 2-1	926	80	1	210	—	—	500
Inv. Ex. 2-2	905	80	1	188	—	—	550
Comp. Ex. 2-2	915	80	1	195	—	—	550
Inv. Ex. 2-3	876	80	0.3	188	—	—	450
Comp. Ex. 2-3	870	80	0.3	201	—	—	450
Inv. Ex. 2-4	905	80	2	156	—	—	600
Comp. Ex. 2-4	1015	80	2	162	—	—	600
Inv. Ex. 2-5	890	50	2	211	—	—	550
Comp. Ex. 2-5	891	50	2	215	—	—	550
Inv. Ex. 2-6	865	56	1	455	—	—	—
Comp. Ex. 2-6	860	56	1	460	—	—	—
Inv. Ex. 2-7	838	60	3	103	—	—	600
Comp. Ex. 2-7	852	60	3	100	—	—	600
Inv. Ex. 2-8	850	60	1	212	—	—	550
Comp. Ex. 2-8	720	60	1	195	—	—	550
Inv. Ex. 2-9	822	60	1	155	—	—	550
Comp. Ex. 2-9	885	60	1	150	—	—	550
Inv. Ex. 2-10	791	50	2	485	—	—	—
Comp. Ex. 2-	802	50	2	475	—	—	—
10

TABLE 24
(Table 23 Continuation)
				Rough
				rolling
		Final		total
	Slab	plate	Heating	reduction
	thickness	thickness	temperature	rate	X2	T4	T4 - 40	T4 - 80
	mm	mm	° C.	%	—	° C.	° C.	° C.
Inv. Ex. 2-11	240	40	1150	71	0.601	900	860	820
Comp. Ex. 2-11	240	40	1150	71	0.600	900	860	820
Inv. Ex. 2-12	240	40	1000	45	0.771	909	869	829
Comp. Ex. 2-12	240	40	1000	45	0.779	909	869	829
Inv. Ex. 2-13	400	100	1100	50	0.256	778	738	698
Comp. Ex. 2-13	400	100	1100	66	0.260	779	739	699
Inv. Ex. 2-14	400	100	1200	50	0.378	792	752	712
Comp. Ex. 2-14	400	100	1200	50	0.375	791	751	711
Inv. Ex. 2-15	400	100	1200	50	0.357	790	750	710
Comp. Ex. 2-15	400	100	1200	25	0.344	788	748	708
Inv. Ex. 2-16	400	100	1220	50	3.196	866	826	786
Comp. Ex. 2-16	400	100	1220	50	11.613	912	872	832
Inv. Ex. 2-17	240	20	1150	83	0.252	916	876	836
Comp. Ex. 2-17	240	20	1150	83	0.249	915	875	835
Inv. Ex. 2-18	240	20	1150	81	0.374	930	890	850
Comp. Ex. 2-18	240	20	1150	81	0.410	933	893	853
Inv. Ex. 2-19	240	20	1180	79	0.500	940	900	860
Comp. Ex. 2-19	240	20	1180	79	0.482	938	898	858
	Finish	Water cooling
	rolling		1st	2nd
		Total			half	half
	1st pass	reduction	Flow	End	cooling	cooling	Tempering
	bite	rate	rate	temperature	rate	rate	temperature
	° C.	%	m3/m2 · min	° C.	° C./s	° C./s	° C.
Inv. Ex. 2-11	825	43	3	210	—	—	580
Comp. Ex. 2-11	822	43	0.1	200	—	—	580
Inv. Ex. 2-12	818	70	0.5/2	155	5	25	500
Comp. Ex. 2-12	820	70	0.5/2	156	5	25	500
Inv. Ex. 2-13	768	50	1	218	—	—	450
Comp. Ex. 2-13	760	26	1	225	—	—	450
Inv. Ex. 2-14	780	50	1	105	—	—	450
Comp. Ex. 2-14	785	50	1	108	—	—	450
Inv. Ex. 2-15	740	50	2	201	—	—	450
Comp. Ex. 2-15	743	67	2	202	—	—	450
Inv. Ex. 2-16	775	50	1	156	—	—	500
Comp. Ex. 2-16	825	50	1	134	—	—	500
Inv. Ex. 2-17	910	50	1	585	—	—	—
Comp. Ex. 2-17	911	50	1	612	—	—	—
Inv. Ex. 2-18	862	56	1	20	—	—	450
Comp. Ex. 2-18	860	56	6	20	—	—	450
Inv. Ex. 2-19	830	60	1	103	—	—	500
Comp. Ex. 2-19	840	60	1	100	—	—	500

TABLE 25
Production Conditions (Method of Production 5)
		Final	Reheating		Cooling	Tempering
	Slab thickness	plate thickness	temperature	Cooling rate	end temperature	temperature
	mm	mm	° C.	° C./s	° C.	° C.
Inv. Ex. 2-20	240	6	920	30	150	480
Comp. Ex. 2-20	240	6	1080	30	155	480
Inv. Ex. 2-21	240	20	950	20	475	450
Comp. Ex. 2-21	240	20	950	20	523	450
Inv. Ex. 2-22	240	40	1000	15	188	550
Comp. Ex. 2-22	240	40	880	15	158	550
Inv. Ex. 2-23	240	100	930	3	455	—
Comp. Ex. 2-23	240	100	930	0.5	458	—

TABLE 26
Production Conditions (Method of Production 6)
	Rough	Finish rolling	Water cooling
	rolling	1st pass	Total		Flow
	Slab	Final plate	Heating	total	bite	reduction	Start	rate	End	Tempering
	thickness	thickness	temperature	reduction	temperature	rate	temperature	m3/	temperature	temperature
	mm	mm	° C.	rate %	° C.	%	° C.	m2 · min	° C.	° C.
Inv. Ex. 2-24	240	6	1050	88	865	80	715	1	201	550
Comp. Ex. 2-24	240	6	1050	88	860	80	780	1	222	550
Inv. Ex. 2-25	240	20	1150	83	912	50	725	1.5	480	—
Comp. Ex. 2-25	240	20	1150	83	915	50	728	1.5	550	—
Inv. Ex. 2-26	240	40	1250	67	880	50	721	2	55	550
Comp. Ex. 2-26	240	40	1250	67	870	50	650	2	78	550
Inv. Ex. 2-27	400	100	1280	50	878	50	743	1	20	500
Comp. Ex. 2-27	400	100	1280	65	895	29	742	1	20	500
Inv. Ex. 2-28	240	20	1150	75	911	67	733	3	20	480
Comp. Ex. 2-28	240	20	1150	75	910	67	732	0.1	20	480
Inv. Ex. 2-29	400	100	1180	50	850	50	745	1.5	20	450
Comp. Ex. 2-29	400	100	1180	25	845	67	742	1.5	20	450

TABLE 27
Production Conditions (Method of Production 7)
	Rough	Finish rolling
	Final		rolling	1st pass	Total		Water cooling
	Slab	plate	Heating	total	bite	reduction	Reheating		End	Tempering
	thickness	thickness	temperature	reduction	temperature	rate	temperature	Cooling	temperature	temperature
	mm	mm	° C.	rate %	° C.	%	° C.	rate ° C./s	° C.	° C.
Inv. Ex. 2-30	240	6	1120	92	925	70	770	40	125	620
Comp. Ex. 2-30	240	6	1120	92	920	70	680	40	105	620
Inv. Ex. 2-31	240	20	1150	79	885	60	780	25	455	—
Comp. Ex. 2-31	240	20	1150	79	886	60	785	25	535	—
Inv. Ex. 2-32	240	40	1200	50	912	67	820	5	20	550
Comp. Ex. 2-32	240	40	1200	50	911	67	915	0.5	20	550
Inv. Ex. 2-33	400	100	1150	50	868	50	750	5	395	—
Comp. Ex. 2-33	400	100	1150	65	870	29	750	5	425	—
Inv. Ex. 2-34	400	100	1200	50	858	50	800	3	20	530
Comp. Ex. 2-34	400	100	1200	25	860	67	800	3	20	530

TABLE 28
Results of Evaluation of Properties (Method of Production 4)
				Weld heat affected zone	Weld heat affected zone
			Matrix toughness	toughness (vE-5)	toughness (vE-5)
	Yield stress	Tensile strength	(vE-5)	(CO2 arc welding)	(submerge arc welding)	No. of holes
	MPa	MPa	J	J	J	No.
Inv. Ex. 2-1	491	638	107	73	63	235
Comp. Ex. 2-1	522	645	90	61	53	32
Inv. Ex. 2-2	504	655	128	61	55	413
Comp. Ex. 2-2	525	648	11	9	4	315
Inv. Ex. 2-3	475	589	96	73	63	678
Comp. Ex. 2-3	567	689	86	13	11	205
Inv. Ex. 2-4	501	638	97	76	56	845
Comp. Ex. 2-4	529	645	92	68	50	18
Inv. Ex. 2-5	495	643	186	125	103	255
Comp. Ex. 2-5	497	645	20	68	55	10
Inv. Ex. 2-6	472	613	123	85	78	413
Comp. Ex. 2-6	508	620	23	18	15	455
Inv. Ex. 2-7	502	635	256	178	138	635
Comp. Ex. 2-7	518	632	41	18	9	545
Inv. Ex. 2-8	505	623	245	168	105	898
Comp. Ex. 2-8	468	583	205	138	76	23
Inv. Ex. 2-9	488	618	178	89	88	225
Comp. Ex. 2-9	689	811	135	22	23	232
Inv. Ex. 2-10	480	608	189	123	115	413
Comp. Ex. 2-10	643	756	168	27	33	400

TABLE 29
(Table 28 Continuation)
				Weld heat affected zone	Weld heat affected zone
			Matrix toughness	toughness (vE-5)	toughness (vE-5)
	Yield stress	Tensile strength	(vE-5)	(CO2 arc welding)	(submerge arc welding)	No. of holes
	MPa	MPa	J	J	J	No.
Inv. Ex. 2-11	478	605	168	108	88	623
Comp. Ex. 2-11	425	535	155	98	75	555
Inv. Ex. 2-12	484	613	289	105	88	815
Comp. Ex. 2-12	505	623	20	23	28	612
Inv. Ex. 2-13	472	598	125	103	88	111
Comp. Ex. 2-13	488	588	88	78	77	8
Inv. Ex. 2-14	480	608	178	79	65	213
Comp. Ex. 2-14	505	623	13	13	8	159
Inv. Ex. 2-15	493	632	168	133	78	312
Comp. Ex. 2-15	485	631	135	120	66	13
Inv. Ex. 2-16	463	593	232	205	166	423
Comp. Ex. 2-16	459	589	198	25	13	310
Inv. Ex. 2-17	485	572	212	138	113	124
Comp. Ex. 2-17	420	535	178	121	98	101
Inv. Ex. 2-18	485	620	138	102	79	213
Comp. Ex. 2-18	525	670	130	100	68	10
Inv. Ex. 2-19	501	630	120	85	88	355
Comp. Ex. 2-19	525	660	18	27	25	273

TABLE 30
Results of Evaluation of Properties (Method of Production 5)
				Weld heat affected	Weld heat affected
				zone	zone
			Matrix toughness	toughness (vE-5)	toughness (vE-5)
	Yield stress	Tensile strength	(vE-5)	(CO2 arc welding)	(submerge arc welding)	No. of holes
	MPa	MPa	J	J	J	No.
Inv. Ex. 2-20	481	625	80	50	45	205
Comp. Ex. 2-20	511	628	83	46	43	13
Inv. Ex. 2-21	454	589	235	89	70	413
Comp. Ex. 2-21	420	555	201	80	68	315
Inv. Ex. 2-22	450	585	188	218	155	432
Comp. Ex. 2-22	479	622	18	15	14	18
Inv. Ex. 2-23	476	588	180	105	88	257
Comp. Ex. 2-23	460	558	15	18	13	14

TABLE 31
Results of Evaluation of Properties (Method of Production 6)
				Weld heat affected zone	Weld heat affected zone
			Matrix toughness	toughness (vE-5)	toughness (vE-5)
	Yield stress	Tensile strength	(vE-5)	(CO2 arc welding)	(submerge arc welding)	No. of holes
	MPa	MPa	J	J	J	No.
Inv. Ex. 2-24	466	613	76	50	36	203
Comp. Ex. 2-24	483	620	93	45	32	29
Inv. Ex. 2-25	464	611	212	156	130	411
Comp. Ex. 2-25	412	538	23	88	68	356
Inv. Ex. 2-26	493	632	193	120	88	611
Comp. Ex. 2-26	468	543	23	18	15	8
Inv. Ex. 2-27	493	632	188	156	110	407
Comp. Ex. 2-27	613	718	26	25	20	306
Inv. Ex. 2-28	515	645	211	110	88	615
Comp. Ex. 2-28	414	520	156	105	66	380
Inv. Ex. 2-29	467	586	158	123	77	406
Comp. Ex. 2-29	512	638	32	22	15	358

TABLE 32
Results of Evaluation of Properties (Method of Production 7)
				Weld heat affected zone	Weld heat affected zone
			Matrix toughness	toughness (vE-5)	toughness (vE-5)
	Yield stress	Tensile strength	(VE-5)	(CO2 arc welding)	(submerge arc welding)	No. of holes
	MPa	MPa	J	J	J	No.
Inv. Ex. 2-30	497	645	88	46	41	223
Comp. Ex. 2-30	488	630	17	46	41	21
Inv. Ex. 2-31	481	625	125	88	69	411
Comp. Ex. 2-31	423	550	18	67	75	310
Inv. Ex. 2-32	460	598	185	158	128	612
Comp. Ex. 2-32	420	555	23	101	95	312
Inv. Ex. 2-33	476	618	168	98	69	209
Comp. Ex. 2-33	532	678	23	23	30	5
Inv. Ex. 2-34	478	608	210	153	110	309
Comp. Ex. 2-34	540	656	29	18	15	8

Invention Examples 2-1 to 2-19 are steel plates produced by first method of production of steel plates described in (2) of the present invention (method of production 4), that is, the method of speedily water cooling after rolling, and the method of production described in (9) of the present invention. Along with these, Comparative Examples 2-1 to 2-19 are also shown.

Invention Example 2-1 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 6 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-1 is similar to Invention Example 2-1 in ingredients and method of production, but X2 is outside the range of the present invention, so the machineability is inferior.

Invention Example 2-2 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 6 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-2 is similar to Invention Example 2-2 in ingredients and method of production, but the amount of Si is outside the range of the present invention, so the toughness and weld heat affected zone toughness are inferior.

Invention Example 2-3 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 6 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-3 is similar to Invention Example 2-3 in ingredients and method of production, the amount of Mn is outside the range of the present invention, so the weld heat affected zone toughness is inferior.

Invention Example 2-4 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 6 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-4 is similar to Invention Example 2-4 in ingredients and method of production, but the finish rolling first pass bite temperature and ferrite fraction are outside the range of the present invention, so the machineability is inferior.

Invention Example 2-5 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 20 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-5 is similar to Invention Example 2-5 in ingredients and method of production, but the amount of A1 and X2 are outside the range of the present invention, so the toughness is inferior.

Invention Example 2-6 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 20 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-6 is similar to Invention Example 2-6 in ingredients and method of production, but the amount of S is outside the range of the present invention, so the toughness and weld heat affected zone toughness are inferior.

Invention Example 2-7 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 20 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-7 is similar to Invention Example 2-7 in ingredients and method of production, but the amount of P is outside the range of the present invention, so the toughness and weld heat affected zone toughness are inferior.

Invention Example 2-8 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 20 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-8 is similar to Invention Example 2-8 in ingredients and method of production, but the finish rolling first pass bite temperature and the micro Vickers hardness are outside the range of the present invention, so the machineability is inferior.

Invention Example 2-9 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 40 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-9 is similar to Invention Example 2-9 in ingredients and method of production, but the amount of Mo and the X1 are outside the range of the present invention, so the weld heat affected zone toughness is inferior.

Invention Example 2-10 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 40 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-10 is similar to Invention Example 2-10 in ingredients and method of production, but the amount of Cr is outside the range of the present invention, so the weld heat affected zone toughness is inferior.

Invention Example 2-11 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 40 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-11 is similar to Invention Example 2-11 in ingredients and method of production, but the Vickers hardness and flow rate are outside the range of the present invention, so the strength is inferior.

Invention Example 2-12 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 40 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-12 is similar to Invention Example 2-12 in ingredients and method of production, but the amount of N is outside the range of the present invention, so the toughness and weld heat affected zone toughness are inferior.

Invention Example 2-13 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 100 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-13 is similar to Invention Example 2-13 in ingredients and method of production, but the total reduction rate of the finish rolling and the micro Vickers hardness are outside the range of the present invention, so the machineability is inferior.

Invention Example 2-14 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 100 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-14 is similar to Invention Example 2-14 in ingredients and method of production, but the amount of B and the X1 are outside the range of the present invention, so the toughness is inferior.

Invention Example 2-15 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 100 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-15 is similar to Invention Example 2-15 in ingredients and method of production, but the ferrite fraction and the total reduction rate of the rough rolling are outside the range of the present invention, so the machineability is inferior.

Invention Example 2-16 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 100 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-16 is similar to Invention Example 2-16 in ingredients and method of production, but the X2 is outside the range of the present invention, so the weld heat affected zone toughness is inferior.

Invention Example 2-17 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 20 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-17 is similar to Invention Example 2-17 in ingredients and method of production, but the water cooling end temperature and the Vickers hardness are outside the range of the present invention, so the strength is inferior.

Invention Example 2-18 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 20 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-18 is similar to Invention Example 2-18 in ingredients and method of production, but the flow rate and ferrite fraction are outside the range of the present invention, so the machineability is inferior.

Invention Example 2-19 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 20 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-19 is similar to Invention Example 2-19 in ingredients and method of production, but the amount of C and the X1 are outside the range of the present invention, so the toughness and the weld heat affected zone toughness are inferior.

Invention Examples 2-20 to 2-23 are steel plates produced by the second method of production of steel plates described in (2) of the present invention (method of production 5), that is, the method of reheating after the temperature of the steel plate falls after rolling, then water cooling, and the method of production described in (11) of the present invention. Along with these, Comparative Examples 2-20 to 2-23 are also shown.

Invention Example 2-20 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, water cooling conditions, etc. to produce plate thickness 6 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-20 is similar to Invention Example 2-20 in ingredients and method of production, but the reheating temperature and ferrite fraction are outside the range of the present invention, so the machineability is inferior.

Invention Example 2-21 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, water cooling conditions, etc. to produce plate thickness 20 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-21 is similar to Invention Example 2-21 in ingredients and method of production, but the water cooling end temperature after reheating and the Vickers hardness are outside the range of the present invention, so the strength is inferior.

Invention Example 2-22 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, water cooling conditions, etc. to produce plate thickness 40 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-22 is similar to Invention Example 2-22 in ingredients and method of production, but the amount of Nb and the reheating temperature are outside the range of the present invention, so the toughness, weld heat affected zone toughness, and machineability are inferior.

Invention Example 2-23 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, water cooling conditions, etc. to produce plate thickness 100 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-23 is similar to Invention Example 2-23 in ingredients and method of production, but the amount of Ti, cooling rate after reheating, and Vickers hardness are outside the range of the present invention, so the strength, toughness, weld heat affected zone toughness, and machineability are inferior.

Invention Examples 2-24 to 2-29 are steel plates produced by the third method of production of steel plates described in (2) of the present invention (method of production 6), that is, the method of air cooling until starting to form ferrite after rolling, then water cooling, and the method of production described in (12) of the present invention. Along with these, Comparative Examples 2-24 to 2-29 are also shown.

Invention Example 2-24 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 6 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-24 is similar to Invention Example 2-24 in ingredients and method of production, but the water cooling start temperature and the micro Vickers hardness are outside the range of the present invention, so the machineability is inferior.

Invention Example 2-25 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 20 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-25 is similar to Invention Example 2-25 in ingredients and method of production, but the amount of Zr, water cooling end temperature, and Vickers hardness are outside the range of the present invention, so the strength and toughness are inferior.

Invention Example 2-26 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability arc achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 40 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-26 is similar to Invention Example 2-26 in ingredients and method of production, but the amount of V, water cooling start temperature, and Vickers hardness are outside the range of the present invention, so the strength, toughness, weld heat affected zone toughness, and machineability are inferior.

Invention Example 2-27 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 100 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-27 is similar to Invention Example 2-27 in ingredients and method of production, but the amount of Ni, X1, and the total reduction rate of the finish rolling are outside the range of the present invention, so the toughness and weld heat affected zone toughness are inferior.

Invention Example 2-28 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 20 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-28 is similar to Invention Example 2-28 in ingredients and method of production, but the flow rate and Vickers hardness are outside the range of the present invention, so the strength is inferior.

Invention Example 2-29 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 100 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-29 is similar to Invention Example 2-29 in ingredients and method of production, but the amount of Cu and the total reduction rate of the rough rolling are outside the range of the present invention, so the toughness and weld heat affected zone toughness are inferior.

Invention Examples 2-30 to 2-34 are steel plates produced by the fourth method of production of steel plates described in (2) of the present invention (method of production 7), that is, the method of heating until the two-phase region again after the temperature the steel plate falls after rolling and the method of production described in (13) of the present invention. Along with these, Comparative Examples 2-30 to 2-34 are also shown.

Invention Example 2-30 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 6 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-30 is similar to Invention Example 2-30 in ingredients and method of production, but the amount of RPM and reheating temperature are outside the range of the present invention, so the toughness and the machineability are inferior.

Invention Example 2-31 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 20 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-31 is similar to Invention Example 2-31 in ingredients and method of production, but the amount of Ca, the water cooling end temperature after reheating, and the Vickers hardness are outside the range of the present invention, so the strength and toughness are inferior.

Invention Example 2-32 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 40 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-32 is similar to Invention Example 2-32 in ingredients and method of production, but the amount of Mg, reheating temperature, and Vickers hardness are outside the range of the present invention, so the strength and toughness are inferior.

Invention Example 2-33 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 100 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-33 is similar to Invention Example 2-33 in ingredients and method of production, but the amount of V and the total reduction rate of the finish rolling are outside the range of the present invention, so the toughness, weld heat affected zone toughness, and machineability are inferior.

Invention Example 2-34 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 100 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-34 is similar to Invention Example 2-34 in ingredients and method of production, but the amount of Nb and the total reduction rate of the rough rolling are outside the range of the present invention, so the toughness, weld heat affected zone toughness, and machineability are inferior.

From the above examples, it is clear that the steel materials produced by the present invention, that is, the steel plates of Invention Examples 2-1 to 2-34, are steel materials having tensile strengths of 570 to 720 MPa or so and excellent in all of toughness, weldability, and machineability.

EXAMPLE 3

Examples of the steel plate described in (3) of the present invention will be explained next.

Test steel materials of various chemical ingredients were used to produce steel plates of plate thicknesses of 6, 18, 40, and 100 mm under various production conditions. These were evaluated for, as strength, the yield stress and tensile strength of the matrix, as toughness, the Charpy impact absorption energy of the matrix, and as the machineability, the drilling property. The chemical ingredients, plate thickness, X1, X2, ferrite fraction, Vickers hardness, and ratio of micro Vickers hardness in a specific range of each of the steel plates are shown in Table 33 to Table 40, the production conditions (methods of production 4′ to 7′) are shown in Table 41 to Table 44, and the results of evaluation of properties are shown in Table 45 to Table 48.

The yield stress and the tensile strength were measured by the metal material tensile test method described in JIS Z 2241. The test piece was a metal material test piece described in JIS Z 2201. No. 5 test pieces taken from steel plates having thicknesses of 6 mm and 18 mm, while No. 10 test pieces taken from the t/4 parts of steel plates having thicknesses of 40 mm and 100 mm were used. The test pieces were taken so that their longitudinal directions became vertical to the rolling direction. The yield stress was made 0.2% of the yield strength calculated by the lower yield stress or offset method. Two tests were conducted at ordinary temperature and the average values were used.

The matrix toughness was measured by the metal material impact test method described in JIS Z 2242. The test pieces used were the metal material impact test pieces described in JIS Z 2202. For steel plates having thicknesses of 6 mm, subsize test pieces of widths of 5 mm were taken from the plate thickness center parts, for steel plates having thicknesses of 18 mm, test pieces of widths of 10 mm were taken from the plate thickness center parts, while for steel plates having thicknesses of 40 mm and 100 mm, test pieces of widths of 10 mm were taken from the t/4 parts. The shapes were all made V-notch test pieces. The test pieces were taken so that the lines formed by the notch bottoms became parallel to the plate thickness direction and so that the longitudinal directions of the test piece became perpendicular to the rolling direction. The test temperature was made −5° C. The average value of three tests was employed.

The machineability was evaluated by a drilling test using a drilling machine and a high speed drill. The drilling distance was 42 mm in the case of plate thickness 6 mm steel plates piled up in seven layers, 36 mm in the case of plate thickness 18 mm steel plates piled up in two layers, 40 mm in the case of one plate thickness 40 mm steel plate, and 100 mm in the case of one plate thickness 100 mm steel plate for the test. The drill was used to make a through hole using a 6 mmφ diameter high speed drill SKH51. The rotational speed was 1610 rpm, the feed speed was 190 mm/min, and the machining oil was a water soluble machining oil. Under the above conditions, drilling was performed until drilling was no longer possible. The number of holes bored until the limit was reached was measured.

TABLE 33
Plate Thickness, Chemical Ingredients, Etc. of Steel Plate (Method of Production 4′)
	Final
	plate
	thickness	C	Si	Mn	P	S	Al	N	Mo	Cr	Nb	Ti	V
	mm	mass %
Inv. Ex. 3-1	18	0.18	0.58	0.72	0.0068	0.018	0.031	0.0035
Comp. Ex. 3-1	18	0.22	0.56	0.71	0.0069	0.017	0.032	0.0036
Inv. Ex. 3-2	18	0.12	0.15	0.85	0.0075	0.031	0.033	0.0045	0.23
Comp. Ex. 3-2	18	0.12	0.14	0.84	0.0076	0.032	0.033	0.0043	0.22
Inv. Ex. 3-3	18	0.13	0.35	0.71	0.0080	0.025	0.031	0.0029	0.16	0.22	0.016
Comp. Ex. 3-3	18	0.13	0.36	1.35	0.0081	0.026	0.032	0.0028			0.015
Inv. Ex. 3-4	6	0.06	0.75	1.12	0.0045	0.031	0.018	0.0051		0.25
Comp. Ex. 3-4	6	0.06	1.15	1.15	0.0043	0.033	0.021	0.0050		0.21
Inv. Ex. 3-5	6	0.14	0.35	0.88	0.0038	0.019	0.028	0.0030	0.07	0.30
Comp. Ex. 3-5	6	0.14	0.34	0.89	0.0036	0.019	0.032	0.0031	0.07	0.31
Inv. Ex. 3-6	6	0.12	0.40	0.65	0.0028	0.012	0.025	0.0025	0.12	0.22		0.015
Comp. Ex. 3-6	6	0.11	0.41	0.68	0.0056	0.013	0.033	0.0026	0.12	0.21		0.014
Inv. Ex. 3-7	40	0.15	0.88	1.12	0.0045	0.033	0.018	0.0021					0.018
Comp. Ex. 3-7	40	0.13	0.85	1.55	0.0046	0.032	0.019	0.0022					0.020
Inv. Ex. 3-8	40	0.09	0.15	0.90	0.0035	0.012	0.014	0.0021	0.45
Comp. Ex. 3-8	40	0.09	0.16	0.88	0.0036	0.013	0.012	0.0025	1.10
Inv. Ex. 3-9	40	0.12	0.33	0.81	0.0066	0.021	0.033	0.0045	0.21	0.15	0.023
Comp. Ex. 3-9	40	0.12	0.32	0.80	0.0256	0.023	0.035	0.0046	0.21	0.14	0.025
Inv. Ex. 3-10	100	0.14	0.05	0.40	0.0045	0.018	0.032	0.0035	0.43	0.45	0.025	0.008
Comp. Ex. 3-10	100	0.14	0.04	0.42	0.0046	0.019	0.032	0.0036	0.43	0.44	0.026	0.007
Inv. Ex. 3-11	100	0.10	0.35	0.65	0.0055	0.031	0.025	0.0035	0.40	0.45			0.025
Comp. Ex. 3-11	100	0.10	0.35	0.66	0.0056	0.042	0.033	0.0036	0.41	0.46			0.026
Inv. Ex. 3-12	100	0.08	0.45	0.55	0.0035	0.028	0.031	0.0037	0.68	0.45	0.056
Comp. Ex. 3-12	100	0.08	0.46	0.56	0.0036	0.029	0.120	0.0038	0.67	0.44	0.057
Inv. Ex. 3-13	6	0.08	0.35	0.85	0.0055	0.015	0.032	0.0039	0.22	0.23
Comp. Ex. 3-13	6	0.08	0.34	0.84	0.0054	0.016	0.032	0.0038	0.21	0.24
Inv. Ex. 3-14	6	0.13	0.25	1.21	0.0038	0.022	0.035	0.0036	0.15
Comp. Ex. 3-14	6	0.13	0.25	1.22	0.0039	0.023	0.036	0.0035	0.16

TABLE 34
(Table 33 Continuation)
		Ratio of micro Vickers	Vickers
	Ferrite	hardness in specific	hard-
	Cu	Ni	B	REM	Ca	Zr	Mg		fraction	range (%)	ness
	mass %	X1	X2	%	≦190 HV	≦180 HV	≦170 HV	HV
Inv. Ex. 3-1								0.235	0.161		32	14	0	192
Comp. Ex. 3-1								0.274	0.158		29	13	0	205
Inv. Ex. 3-2	0.35	0.45						0.208	0.306	55				185
Comp. Ex. 3-2	0.34	0.46						0.206	0.295	0				188
Inv. Ex. 3-3								0.199	0.479	68				195
Comp. Ex. 3-3								0.210	0.053	33				189
Inv. Ex. 3-4				0.0025	0.0015			0.154	0.246	40				201
Comp. Ex. 3-4				0.0024	0.0018			0.166	0.291	48				215
Inv. Ex. 3-5								0.215	0.330		55	28	12	205
Comp. Ex. 3-5								0.216	0.329		19	10	0	215
Inv. Ex. 3-6								0.185	0.477	75				190
Comp. Ex. 3-6								0.176	0.451	73				163
Inv. Ex. 3-7								0.232	0.157		28	5	0	185
Comp. Ex. 3-7								0.233	0.110		21	3	0	196
Inv. Ex. 3-8						0.0023		0.170	0.533	55				188
Comp. Ex. 3-8						0.0025		0.213	1.286	31				220
Inv. Ex. 3-9								0.193	0.433	77				185
Comp. Ex. 3-9								0.192	0.430	75				182
Inv. Ex. 3-10			0.0008					0.217	1.663	55				185
Comp. Ex. 3-10			0.0007					0.217	1.567	20				182
Inv. Ex. 3-11	0.23	0.22					0.0023	0.211	1.069		35	16	3	178
Comp. Ex. 3-11	0.22	0.23						0.212	1.076		32	14	3	179
Inv. Ex. 3-12								0.190	1.809	67				188
Comp. Ex. 3-12								0.190	1.754	0				185
Inv. Ex. 3-13								0.155	0.476	72				188
Comp. Ex. 3-13								0.155	0.474	75				162
Inv. Ex. 3-14								0.211	0.165	50				185
Inv. Ex. 3-14								0.211	0.172	0				193

TABLE 35
Plate Thickness, Chemical Ingredients, Etc. of Steel Plate (Method of Production 5′)
	Final
	plate
	thickness	C	Si	Mn	P	S	Al	N	Mo	Cr	Nb	Ti	V
	mm	mass %
Inv. Ex. 3-15	6	0.03	0.25	0.85	0.0038	0.015	0.033	0.0045		0.68
Comp. Ex. 3-15	6	0.03	0.24	0.86	0.0037	0.016	0.032	0.0115		0.66
Inv. Ex. 3-16	18	0.05	0.11	1.31	0.0025	0.021	0.031	0.0035	0.25	0.15		0.023
Comp. Ex. 3-16	18	0.003	0.12	1.32	0.0026	0.023	0.033	0.0036	0.26	0.15		0.024
Inv. Ex. 3-17	40	0.13	0.04	0.92	0.0066	0.013	0.025	0.0051	0.05	0.45	0.007
Comp. Ex. 3-17	40	0.13	0.03	0.91	0.0065	0.014	0.032	0.0050	0.05	1.15	0.008
Inv. Ex. 3-18	100	0.10	0.35	0.68	0.0055	0.020	0.033	0.0035	0.55	0.35
Comp. Ex. 3-18	100	0.10	0.34	0.67	0.0055	0.021	0.032	0.0036	0.54	0.36
Inv. Ex. 3-19	18	0.12	0.23	0.85	0.0063	0.031	0.023	0.0053	0.23	0.21
Comp. Ex. 3-19	18	0.12	0.22	0.84	0.0061	0.042	0.021	0.0051	0.22	0.22

TABLE 36
(Table 35 Continuation)
		Ratio of micro Vickers
	Ferrite	hardness in specific	Vickers
	Cu	Ni	B	REM	Ca	Zr	Mg		fraction	range (%)	hardness
	mass %	X1	X2	%	≦190 HV	≦180 HV	≦170 HV	HV
Inv. Ex. 3-15								0.115	0.459		55	35	25	201
Comp. Ex. 3-15								0.114	0.440		0	0	0	205
Inv. Ex. 3-16								0.143	0.265	43				178
Comp. Ex. 3-16								0.098	0.272	65				139
Inv. Ex. 3-17								0.203	0.308	52				175
Comp. Ex. 3-17								0.237	0.693	31				162
Inv. Ex. 3-18								0.200	1.169	50				188
Comp. Ex. 3-18								0.199	1.176	25				195
Inv. Ex. 3-19								0.196	0.448	40				188
Comp. Ex. 3-19								0.195	0.445	44				187

TABLE 37
Plate Thickness, Chemical Ingredients, Etc. of Steel Plate (Method of Production 6′)
	Final
	plate
	thickness	C	Si	Mn	P	S	Al	N	Mo	Cr	Nb	Ti	V
	mm	mass %
Inv. Ex. 3-20	6	0.13	0.45	0.75	0.0068	0.015	0.033	0.0048		0.23		0.012
Comp. Ex. 3-20	6	0.13	0.44	0.76	0.0069	0.016	0.033	0.0049		0.24		0.011
Inv. Ex. 3-21	18	0.12	0.35	0.32	0.0035	0.022	0.016	0.0035	0.35	0.25	0.015
Comp. Ex. 3-21	18	0.12	0.36	0.31	0.0036	0.021	0.023	0.0036	0.36	0.24	0.015
Inv. Ex. 3-22	40	0.08	0.15	0.35	0.0050	0.018	0.030	0.0030	0.45	0.35			0.045
Comp. Ex. 3-22	40	0.08	0.16	0.36	0.0051	0.019	0.032	0.0031	0.46	0.36			0.105
Inv. Ex. 3-23	100	0.13	0.35	0.78	0.0045	0.025	0.031	0.0035	0.28	0.35	0.015	0.018
Comp. Ex. 3-23	100	0.13	0.36	0.77	0.0045	0.026	0.031	0.0036	0.28	0.34	0.015	0.017
Inv. Ex. 3-24	18	0.11	0.25	1.25	0.0032	0.022	0.031	0.0035	0.05	0.21
Comp. Ex. 3-24	18	0.11	0.24	1.24	0.0031	0.021	0.031	0.0036	0.05	0.22

TABLE 39
Plate Thickness, Chemical Ingredients, Etc. of Steel Plate (Method of Production 7′)
	Final
	plate
	thickness	C	Si	Mn	P	S	Al	N	Mo	Cr	Nb	Ti	V
	mm	mass %
Inv. Ex. 3-25	6	0.13	0.25	0.70	0.0035	0.018	0.031	0.0035	0.25
Comp. Ex. 3-25	6	0.13	0.26	1.10	0.0036	0.019	0.032	0.0036
Inv. Ex. 3-26	18	0.08	0.22	0.85	0.0035	0.025	0.031	0.0034	0.21	0.25	0.008
Comp. Ex. 3-26	18	0.08	0.21	0.86	0.0036	0.026	0.031	0.0035	0.22	0.26	0.008
Inv. Ex. 3-27	40	0.11	0.75	0.99	0.0045	0.023	0.025	0.0036	0.22			0.015
Comp. Ex. 3-27	40	0.11	0.76	0.99	0.0043	0.022	0.025	0.0036	0.21			0.016
Inv. Ex. 3-28	100	0.13	0.56	1.12	0.0044	0.032	0.024	0.0042	0.35				0.035
Comp. Ex. 3-28	100	0.13	0.55	1.11	0.0042	0.032	0.024	0.0043	0.34				0.110
Inv. Ex. 3-29	100	0.08	0.35	0.95	0.0035	0.031	0.031	0.0055	0.33	0.48
Comp. Ex. 3-29	100	0.08	0.35	0.96	0.0036	0.032	0.032	0.0053	0.32	0.46

TABLE 40
(Table 39 Continuation)
	Ferrite	Ratio of micro Vickers hardness	Vickers
	Cu	Ni	B	REM	Ca	Zr	Mg		fraction	in specific range (%)	hardness
	mass %	X1	X2	%	≦190 HV	≦180 HV	≦170 HV	HV
Inv. Ex. 3-25								0.190	0.429	50				178
Comp. Ex. 3-25								0.194	0.047	0				181
Inv. Ex. 3-26								0.156	0.446	62				185
Comp. Ex. 3-26								0.158	0.456	65				163
Inv. Ex. 3-27								0.199	0.374	45				182
Comp. Ex. 3-27								0.199	0.366	43				160
Inv. Ex. 3-28								0.232	0.413	55				178
Comp. Ex. 3-28								0.238	0.405	55				176
Inv. Ex. 3-29			0.0010					0.190	0.674	56				177
Comp. Ex. 3-29			0.0060					0.214	0.646	61				175

TABLE 41
Production Conditions (Method of Production 4′)
			Rough
			rolling
	Final		total		Finish rolling
	Slab	plate	Heating	reduction			T4-	T4-	1st pass	Total
	thickness	thickness	temperature	rate	X2	T4	40	80	bite	reduction rate
	mm	mm	° C.	%	—	° C.	° C.	° C.	° C.	%
Inv. Ex. 3-1	240	18	1020	85	0.161	906	866	826	875	50
Comp. Ex. 3-1	240	18	1020	85	0.158	905	865	825	878	50
Inv. Ex. 3-2	240	18	1100	85	0.306	928	888	848	855	50
Comp. Ex. 3-2	240	18	1100	85	0.295	927	887	847	945	50
Inv. Ex. 3-3	240	18	1150	81	0.479	944	904	864	861	60
Comp. Ex. 3-3	240	18	1150	81	0.053	867	827	787	860	60
Inv. Ex. 3-4	240	6	1120	88	0.246	965	925	885	952	80
Comp. Ex. 3-4	240	6	1120	88	0.291	971	931	891	950	80
Inv. Ex. 3-5	240	6	1000	88	0.330	976	936	896	900	80
Comp. Ex. 3-5	240	6	1000	88	0.329	976	936	896	705	80
Inv. Ex. 3-6	120	6	1150	83	0.477	989	949	909	880	70
Comp. Ex. 3-6	120	6	1150	83	0.203	959	919	879	865	70
Inv. Ex. 3-7	240	40	1080	67	0.157	853	813	773	825	50
Comp. Ex. 3-7	240	40	1080	67	0.110	840	800	760	828	50
Inv. Ex. 3-8	240	40	1200	67	0.533	896	856	816	830	50
Comp. Ex. 3-8	240	40	1200	67	1.286	926	886	846	850	50
Inv. Ex. 3-9	240	40	1230	67	0.433	888	848	808	795	50
Comp. Ex. 3-9	240	40	1230	67	0.430	888	848	808	792	50
Inv. Ex. 3-10	400	100	1180	50	1.313	835	795	755	810	50
Comp. Ex. 3-10	400	100	1180	67	1.245	833	793	753	815	25
Inv. Ex. 3-11	400	100	1000	50	1.069	828	788	748	765	50
Comp. Ex. 3-11	400	100	1000	50	1.076	828	788	748	768	50
Inv. Ex. 3-12	400	100	1250	38	1.809	846	806	766	750	60
Comp. Ex. 3-12	400	100	1250	25	1.754	845	805	765	755	67
Inv. Ex. 3-13	240	6	1100	79	0.476	989	949	909	895	88
Comp. Ex. 3-13	240	6	1100	79	0.474	988	948	908	898	88
Inv. Ex. 3-14	240	6	1150	75	0.165	952	912	872	943	90
Comp. Ex. 3-14	240	6	1150	75	0.172	953	913	873	942	90
	Water cooling
				1st
				half	2nd
		Flow	End	cooling	half cooling	Tempering
		rate	temperature	rate	rate	temperature
		m3/m2 · min	° C.	° C./s	° C./s	° C.
	Inv. Ex. 3-1	1.0	451	—	—	—
	Comp. Ex. 3-1	1.0	455	—	—	—
	Inv. Ex. 3-2	0.3/1.5	213	4	25	580
	Comp. Ex. 3-2	0.3/1.5	211	4	25	580
	Inv. Ex. 3-3	0.7	120	—	—	550
	Comp. Ex. 3-3	0.7	125	—	—	550
	Inv. Ex. 3-4	1.0	105	—	—	600
	Comp. Ex. 3-4	1.0	120	—	—	600
	Inv. Ex. 3-5	0.5	222	—	—	450
	Comp. Ex. 3-5	0.5	225	—	—	450
	Inv. Ex. 3-6	0.5	550	—	—	—
	Comp. Ex. 3-6	0.5	620	—	—	—
	Inv. Ex. 3-7	1	235	—	—	580
	Comp. Ex. 3-7	1	221	—	—	580
	Inv. Ex. 3-8	1	105	—	—	480
	Comp. Ex. 3-8	1	115	—	—	480
	Inv. Ex. 3-9	2	465	—	—	—
	Comp. Ex. 3-9	2	450	—	—	—
	Inv. Ex. 3-10	2	205	—	—	450
	Comp. Ex. 3-10	2	210	—	—	450
	Inv. Ex. 3-11	1	20	—	—	480
	Comp. Ex. 3-11	1	20	—	—	480
	Inv. Ex. 3-12	3	20	—	—	550
	Comp. Ex. 3-12	3	20	—	—	550
	Inv. Ex. 3-13	1	150	—	—	500
	Comp. Ex. 3-13	0.1	130	—	—	500
	Inv. Ex. 3-14	1	160	—	—	500
	Comp. Ex. 3-14	6	155	—	—	500

TABLE 42
Production Conditions (Method of Production 5′)
		Final			Cooling	Tempering
	Slab thickness	plate thickness	Reheating temperature	Cooling rate	end temperature	temperature
	mm	mm	° C.	° C./s	° C.	° C.
Inv. Ex. 3-15	240	6	950	50	150	480
Comp. Ex. 3-15	240	6	950	105	155	480
Inv. Ex. 3-16	240	18	950	30	450	—
Comp. Ex. 3-16	240	18	950	0.8	465	—
Inv. Ex. 3-17	240	40	1000	20	450	—
Comp. EX. 3-17	240	40	1000	20	550	—
Inv. Ex. 3-18	400	100	930	3	135	450
Comp. Ex. 3-18	400	100	1100	3	155	450
Inv. Ex. 3-19	240	18	1000	15	205	500
Comp. Ex. 3-19	240	18	880	15	220	500

TABLE 43
Production Conditions (Method of Production 6′)
	Rough
	rolling	Finish rolling
	Final		total	1st pass	Total	Water cooling
	Slab	plate	Heating	reduction	bite	reduction	Start	Flow rate	End	Tempering
	thickness	thickness	temperature	rate	temperature	rate	temperature	m3/	temperature	temperature
	mm	mm	° C.	%	° C.	%	° C.	m2 · min	° C.	° C.
Inv. Ex. 3-20	240	6	1100	88	885	60	720	1	201	550
Comp. Ex. 3-20	240	6	1100	88	875	60	640	1	222	550
Inv. Ex. 3-21	240	18	1150	81	860	60	713	1.5	425	—
Comp. Ex. 3-21	240	18	1150	81	855	60	711	0.1	435	—
Inv. Ex. 3-22	240	40	1120	58	908	60	721	2	55	550
Comp. Ex. 3-22	240	40	1120	79	910	20	735	2	78	550
Inv. Ex. 3-23	400	100	1180	50	753	50	711	1	405	—
Comp. Ex. 3-23	400	100	1180	25	755	67	732	1	512	—
Inv. Ex. 3-24	240	18	1100	81	925	60	715	0.7	215	500
Comp. Ex. 3-24	240	18	1100	81	920	60	790	0.7	210	500

TABLE 44
Production Conditions (Method of Production 7′)
	Finish rolling
	Final		Rough rolling	1st pass	Total		Water cooling	Tempering
	Slab	plate	Heating	total reduction	bite	reduction	Reheating	Cooling	End	tempera-
	thickness	thickness	temperature	rate	temperature	rate	temperature	rate	temperature	ture
	mm	mm	° C.	%	° C.	%	° C.	° C./s	° C.	° C.
Inv. Ex. 3-25	240	6	1100	88	880	80	750	30	20	500
Comp. Ex. 3-25	240	6	1100	88	885	80	710	30	20	500
Inv. Ex. 3-26	240	18	1150	85	912	50	770	0.5	460	—
Comp. Ex. 3-26	240	18	1150	85	910	50	920	0.5	475	—
Inv. Ex. 3-27	240	40	1200	67	945	50	750	10	450	—
Comp. Ex. 3-27	240	40	1200	67	955	50	750	10	550	—
Inv. Ex. 3-28	400	100	1150	50	865	50	740	2	20	450
Comp. Ex. 3-28	400	100	1150	68	862	23	740	2	20	450
Inv. Ex. 3-29	400	100	1100	50	811	50	770	5	20	500
Comp. Ex. 3-29	400	100	1100	25	810	67	770	5	20	500

TABLE 45
Results of Evaluation of Properties (Method of Production 4′)
		Tensile		No.
	Yield stress	strength	Matrix toughness	of holes
	MPa	MPa	(vE-5) J	No.
Inv. Ex. 3-1	475	612	71	345
Comp. Ex. 3-1	535	665	15	308
Inv. Ex. 3-2	468	613	88	665
Comp. Ex. 3-2	470	622	84	2
Inv. Ex. 3-3	505	635	135	845
Comp. Ex. 3-3	495	613	125	5
Inv. Ex. 3-4	482	623	41	322
Comp. Ex. 3-4	525	656	5	256
Inv. Ex. 3-5	515	656	48	655
Comp. Ex. 3-5	535	648	6	20
Inv. Ex. 3-6	475	595	94	885
Comp. Ex. 3-6	425	540	83	678
Inv. Ex. 3-7	485	601	70	302
Comp. Ex. 3-7	525	648	77	11
Inv. Ex. 3-8	468	596	100	525
Comp. Ex. 3-8	535	710	17	208
Inv. Ex. 3-9	505	595	185	972
Comp. Ex. 3-9	502	613	5	450
Inv. Ex. 3-10	475	585	87	225
Comp. Ex. 3-10	480	595	65	10
Inv. Ex. 3-11	465	578	78	178
Comp. Ex. 3-11	470	585	7	139
Inv. Ex. 3-12	505	603	88	261
Comp. Ex. 3-12	503	623	5	2
Inv. Ex. 3-13	525	635	92	683
Comp. Ex. 3-13	415	528	68	495
Inv. Ex. 3-14	475	623	78	256
Comp. Ex. 3-14	535	678	85	12

TABLE 46
Results of Evaluation of Properties (Method of Production 5′)
	Yield		Matrix toughness
	stress	Tensile	(vE-5)	No. of holes
	MPa	strength MPa	J	No.
Inv. Ex. 3-15	525	638	165	889
Comp. Ex. 3-15	523	610	10	5
Inv. Ex. 3-16	468	608	105	255
Comp. Ex. 3-16	329	445	88	189
Inv. Ex. 3-17	450	585	125	450
Comp. Ex. 3-17	417	555	15	208
Inv. Ex. 3-18	469	578	88	678
Comp. EX. 3-18	473	615	65	23
Inv. Ex. 3-19	512	623	78	315
Comp. Ex. 3-19	516	615	15	300

TABLE 47
Results of Evaluation of Properties (Method of Production 6′)
	Yield		Matrix toughness
	stress	Tensile	(vE-5)	No. of holes
	MPa	strength MPa	J	No.
Inv. Ex. 3-20	465	613	108	355
Comp. Ex. 3-20	413	556	83	218
Inv. Ex. 3-21	464	611	136	455
Comp. Ex. 3-21	395	535	125	389
Inv. Ex. 3-22	492	632	87	925
Comp. Ex. 3-22	512	625	25	15
Inv. Ex. 3-23	448	575	105	312
Comp. Ex. 3-23	420	555	20	256
Inv. Ex. 3-24	463	585	78	222
Comp. Ex. 3-24	462	611	85	11

TABLE 48
Results of Evaluation of Properties (Method of Production 7′)
	Yield		Matrix toughness
	stress	Tensile	(vE-5)	No. of holes
	MPa	strength MPa	J	No.
Inv. Ex. 3-25	480	612	78	315
Comp. Ex. 3-25	485	615	80	10
Inv. Ex. 3-26	463	573	80	450
Comp. Ex. 3-26	432	551	78	350
Inv. Ex. 3-27	505	605	66	380
Comp. Ex. 3-27	425	538	88	250
Inv. Ex. 3-28	511	638	68	350
Comp. Ex. 3-28	512	625	10	12
Inv. Ex. 3-29	476	595	90	225
Comp. Ex. 3-29	485	588	13	228

Invention Examples 3-1 to 3-14 are steel plates produced by the first method of production of steel plate described in (3) of the present invention (method of production 4′), that is, the method of water cooling speedily after rolling, and the method of production described in (14) of the present invention. Comparative Example 3-1 to 3-14 are also shown.

Invention Example 3-1 is plate thickness 18 mm steel plate produced by the first method of production (method of production 4′). It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-1 is similar to Invention Example 3-1 in ingredients and method of production, but the amount of C and X1 are outside the range of the present invention, so the toughness is extremely low.

Invention Example 3-2 is plate thickness 18 mm steel plate produced by the first method of production (method of production 4′). It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-2 is similar to Invention Example 3-2 in ingredients and method of production, but the finish rolling first pass bite temperature and the ferrite fraction are outside the range of the present invention, so the machineability is extremely low.

Invention Example 3-3 is plate thickness 18 mm steel plate produced by the first method of production (method of production 4′). It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-3 is similar to Invention Example 3-3 in ingredients and method of production, but the X2 is outside the range of the present invention, so the machineability is extremely low.

Invention Example 3-4 is a plate thickness 6 mm steel plate produced by the first method of production (method of production 4′). It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-4 is similar in ingredients and method of production with Invention Example 3-4, but the amount of Si is outside the range of the present invention, so the toughness is extremely low.

Invention Example 3-5 is a plate thickness 6 mm steel plate produced by the first method of production (method of production 4′). It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-5 is similar in ingredients and method of production with Invention Example 3-5, but the finish rolling first pass bite temperature and the micro Vickers hardness are outside the range of the present invention, so the toughness and machineability are extremely low.

Invention Example 3-6 is a plate thickness 6 mm steel plate produced by the first method of production (method of production 4′). It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-6 is similar to Invention Example 3-6 in ingredients and method of production, but the water cooling end temperature is outside the range of the present invention, so the strength is extremely low.

Invention Example 3-7 is plate thickness 40 mm steel plate produced by the first method of production (method of production 4′). It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-7 is similar to Invention Example 3-7 in ingredients and method of production, but the amount of Mn and X2 are outside the range of the present invention, so the machineability is extremely low.

Invention Example 3-8 is plate thickness 40 mm steel plate produced by the first method of production (method of production 4′). It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-8 is similar to Invention Example 3-8 in ingredients and method of production, but the amount of Mo is outside the range of the present invention, so the toughness is extremely low.

Invention Example 3-9 is plate thickness 40 mm steel plate produced by the first method of production (method of production 4′). It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-9 is similar to Invention Example 3-9 in ingredients and method of production, but the amount of P is outside the range of the present invention, so the toughness is extremely low.

Invention Example 3-10 is plate thickness 100 mm steel plate produced by the first method of production (method of production 4′). It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-10 is similar to Invention Example 3-10 in ingredients and method of production, but the total reduction rate of the finish rolling and the ferrite fraction are outside the range of the present invention, so the machineability is extremely low.

Invention Example 3-11 is plate thickness 100 mm steel plate produced by the first method of production (method of production 4′). It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-11 is similar to Invention Example 3-11 in ingredients and method of production, but the amount of S is outside the range of the present invention, so the toughness is extremely low.

Invention Example 3-12 is plate thickness 100 mm steel plate produced by the first method of production (method of production 4′). It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-12 is similar to Invention Example 3-12 in ingredients and method of production, but the amount of Al, total reduction rate of the rough rolling, and ferrite fraction are outside the range of the present invention, so the machineability and toughness are extremely low.

Invention Example 3-13 is a plate thickness 6 mm steel plate produced by the first method of production (method of production 4′). It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-13 is similar to Invention Example 3-13 in ingredients and method of production, but the flow rate and Vickers hardness are outside the range of the present invention, so the strength is extremely low.

Invention Example 3-14 is a plate thickness 6 mm steel plate produced by the first method of production (method of production 4′). It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-14 is similar to Invention Example 3-14 in ingredients and method of production, but the flow rate and ferrite fraction are outside the range of the present invention, so the machineability is extremely low.

Invention Examples 3-15 to 3-19 are steel plates produced by the second method of production of steel plate described in (3) of the present invention (method of production 5′), that is, the method of reheating when the temperature of the steel plate falls after rolling, then water cooling, and the method of production described in (16) of the present invention. Comparative Examples 3-15 to 3-19 are also shown.

Invention Example 3-15 is plate thickness 6 mm steel plate produced by the second method of production (method of production 5′). It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-15 is similar to Invention Example 3-15 in ingredients and method of production, but the amount of N, the cooling rate of water cooling after reheating, and the micro Vickers hardness are outside the range of the present invention, so the machineability and toughness are extremely low.

Invention Example 3-16 is plate thickness 18 mm steel plate produced by the second method of production (method of production 5′). It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-16 is similar to Invention Example 3-16 in ingredients and method of production, but the amount of C, cooling rate of water cooling after reheating, and Vickers hardness are outside the range of the present invention, so the strength is extremely low.

Invention Example 3-17 is plate thickness 40 mm steel plate produced by the second method of production (method of production 5′). It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-17 is similar to Invention Example 3-17 in ingredients and method of production, but the amount of Cr, the end temperature of water cooling after reheating, and the Vickers hardness are outside the range of the present invention, so the strength and toughness are extremely low.

Invention Example 3-18 is plate thickness 100 mm steel plate produced by the second method of production (method of production 5′). It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-18 is similar to Invention Example 3-18 in ingredients and method of production, but the reheating temperature and ferrite fraction are outside the range of the present invention, so the machineability is extremely low.

Invention Example 3-19 is plate thickness 18 mm steel plate produced by the second method of production (method of production 5′). It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-19 is similar to Invention Example 3-19 in ingredients and method of production, but the amount of S and reheating temperature are outside the range of the present invention, so the toughness is extremely low.

Invention Examples 3-20 to 3-24 are steel plate produced by the third method of production of steel plate described in (3) of the present invention (method of production 6′), that is, the method of air cooling until the start of formation of ferrite after rolling, then water cooling, and the method of production described in (17) of the present invention. Comparative Examples 3-20 to 3-24 are also shown.

Invention Example 3-20 is plate thickness 6 mm steel plate produced by the third method of production (method of production 6′). It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-20 is similar to Invention Example 3-20 in ingredients and method of production, but the water cooling start temperature and Vickers hardness are outside the range of the present invention, so the strength is extremely low.

Invention Example 3-21 is plate thickness 18 mm steel plate produced by the third method of production (method of production 6′). It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-21 is similar to Invention Example 3-21 in ingredients and method of production, but the flow rate and Vickers hardness are outside the range of the present invention, so the strength is extremely low.

Invention Example 3-22 is plate thickness 40 mm steel plate produced by the third method of production (method of production 6′) It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-22 is similar to Invention Example 3-22 in ingredients and method of production, but the amount of V and the total reduction rate in the finish rolling are outside the range of the present invention, so the toughness and machineability are extremely low.

Invention Example 3-23 is plate thickness 100 mm steel plate produced by the third method of production (method of production 6′). It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-23 is similar to Invention Example 3-23 in ingredients and method of production, but the total reduction rate in the rough rolling, the water cooling end temperature, and the Vickers hardness are outside the range of the present invention, so the strength and toughness are extremely low.

Invention Example 3-24 is plate thickness 18 mm steel plate produced by the third method of production (method of production 6′). It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-24 is similar to Invention Example 3-24 in ingredients and method of production, but the water cooling start temperature and ferrite fraction are outside the range of the present invention, so the machineability is extremely low.

Invention Examples 3-25 to 3-29 are steel plates produced by the fourth method of production of steel plate described in (3) of the present invention (method of production 7′), that is, the method of heating again to the two-phase region after the temperature of the steel plate falls after rolling, and the method of production described in (18) of the present invention. Comparative Examples 3-25 to 3-29 are also shown.

Invention Example 3-25 is plate thickness 6 mm steel plate produced by the fourth method of production (method of production 7′). It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-25 is similar to Invention Example 3-25 in ingredients and method of production, but the X2, reheating temperature, and ferrite fraction are outside the range of the present invention, so the machineability is extremely low.

Invention Example 3-26 is plate thickness 18 mm steel plate produced by the fourth method of production (method of production 7′). It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-26 is similar to Invention Example 3-26 in ingredients and method of production, but the Vickers hardness and reheating temperature are outside the range of the present invention, so the strength is extremely low.

Invention Example 3-27 is plate thickness 40 mm steel plate produced by the fourth method of production (method of production 7′). It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-27 is similar to Invention Example 3-27 in ingredients and method of production, but the water cooling end temperature and the Vickers hardness are outside the range of the present invention, so the strength is extremely low.

Invention Example 3-28 is plate thickness 100 mm steel plate produced by the fourth method of production (method of production 7′). It satisfies all of the requirements defined in the present invention, so has 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-28 is similar to Invention Example 3-28 in ingredients and method of production, but the amount of V and the total reduction rate of the finish rolling are outside the range of the present invention, so the toughness and machineability are extremely low.

Invention Example 3-29 is plate thickness 100 mm steel plate produced by the fourth method of production (method of production 7′). It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-29 is similar to Invention Example 3-29 in ingredients and method of production, but the amount of B and the total reduction rate of the rough rolling are outside the range of the present invention, so the toughness is extremely low.

From the above examples, it is clear that the steel materials produced by the present invention, that is, the steel plates of Invention Examples 3-1 to 3-29, are steel materials having a tensile strength of 570 to 720 MPa or so, a high matrix toughness, and an excellent machineability.

Claims

1. Steel plate excellent in machineability and in toughness and weldability characterized in that the steel comprises, by mass %, a steel composition comprising:

C: 0.005 to 0.2%,

Si: 0.01 to 1%,

Mn: 0.01 to 2%,

P: 0.02% or less,

S: 0.035% or less

Al: 0.001 to 0.1%

N, 0.01% or less, and

the balance of iron and unavoidable impurities,

X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B is 0.24 or less,

when the plate thickness is 4 mm to less than 10 mm, a ferrite fraction of locations exactly ¼ and ¾ of the plate thickness inside from a top surface of the steel plate is 30% to 90% and a ferrite fraction of a location exactly ½ of the plate thickness inside from a top surface of the steel plate surface is 20% to 90%, and

when the plate thickness is 10 mm to less than 100 mm, a ferrite fraction of a location 2 mm inside from a top and rear surface of the steel plate is 30% to 90% and a ferrite fraction of locations exactly ¼, ½, and ¾ of the plate thickness inside from a top surface of the steel plate surface is 20% to 90%.

2. Steel plate excellent in machineability and in toughness and weldability, characterized in that the steel comprises, by mass %, a steel composition as set forth in claim 1 wherein:

Mn: 0.01 to 1.4%,

S: 0.01% or less,

X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B is 0.24 or less,

X2 expressed by X2=(Si/5+Mo+Cr/2)/Mn is 0.15 to 10.0,

the structure forming the steel is a structure having a ferrite fraction of 30% to 90% and the balance of a hard structure mainly comprised of bainite and martensite or a structure having a ratio with a micro Vickers hardness of 190 HV or less of 20% or more, and

the steel has a Vickers hardness of 165 HV to 300 HV.

3. Steel plate excellent in machineability and in toughness and weldability, characterized in that the steel comprises, by mass %, a steel composition as set forth in claim 1 wherein:

Mn: 0.01 to 1.4%,

S: over 0.01% to 0.035%,

X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B is 0.24 or less,

X2 expressed by X2=(Si/5+Mo+Cr/2)/Mn is 0.15 to 10.0,

the structure forming the steel is a structure having a ferrite fraction of 30% to 90% and the balance of a hard structure mainly comprised of bainite and martensite or a structure having a ratio with a micro Vickers hardness of 190 HV or less of 20% or more, and

the steel has a Vickers hardness of 165 HV to 300 HV.

4. Steel plate excellent in machineability and in toughness and weldability as set forth in claim 1, characterized in that said steel further comprises, by mass %, one or more of:

Mo: 0.01 to 1%,

Cr: 0.01 to 1%,

Nb: 0.001 to 0.1%,

Ti: 0.001 to 0.1%,

V: 0.001 to 0.1%,

Cu: 0.005 to 1%,

Ni: 0.01 to 2%,

B: 0.0002 to 0.005%,

REM: 0.0005 to 0.1%,

Ca: 0.0005 to 0.02%,

Zr: 0.0005 to 0.02%, and

Mg: 0.0005 to 0.02%

5. A method of production of steel plate excellent in machineability and in toughness and weldability characterized by heating a steel slab or cast slab having a steel composition as described in claim 1 and having an X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B of 0.24 or less, then rough rolling it by a total reduction rate of 30% to 95%, then finish rolling it by a first pass bite temperature of a T1(° C.) expressed by T1=35 ln(X2/2)-25√t+1070, X2=(Si/5+Mo+Cr/2)/Mn, to 720° C. and a total reduction rate of 30% to 95%, starting water cooling after the end of the rolling at a flow rate of 0.2 m3/m2·min to 5.0 m3/m2·min, and ending the water cooling at 600° C. or less, where t is the plate thickness.

6. A method of production of steel plate excellent in machineability and in toughness and weldability as set forth in claim 5, characterized in that in the water cooling started after the end of the rolling, an average cooling rate from a water cooling start temperature to over 650° C. is 1° C./s to 5° C./s and an average cooling rate from 650° C. to a water cooling stop temperature is 10° C./s to 100° C./s.

7. A method of production of steel plate excellent in machineability and in toughness and weldability characterized by heating a steel slab or cast slab having a steel composition as described in claim 1 and having an X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B of 0.24 or less, then rough rolling it by a total reduction rate of 30% to 95%, finish rolling it by a total reduction rate of 30% to 95%, then air cooling it, starting water cooling when the steel plate surface temperature is T2(° C.) expressed by T2=910−310×C−80×Mn−20×Cu−15×Cr−55×Ni−80×Mo+0.0006t2−0.56t−8.6 to 650° C. by a flow rate of 0.2 m3/m2·min to 5.0 m3/m2·min, and ending the water cooling at 500° C. or less, where t is the plate thickness.

8. A method of production of steel plate excellent in machineability and in toughness and weldability characterized by heating a steel slab or cast slab having a steel composition as described in claim 1 and having an X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B of 0.24 or less, then rough rolling it by a total reduction rate of 30% to 95%, finish rolling it by a total reduction rate of 30% to 95%, air cooling to 500° C. or less, reheating the steel plate to a T3(° C.) expressed by T3=0.0017t2+0.17t+730 to 850° C., then starting the water cooling, and ending the water cooling at 500° C. or less, where t is the plate thickness.

9. A method of production of steel plate excellent in machineability and in toughness and weldability characterized by heating a steel slab or cast slab having a steel composition as described in claim 2 and having an X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B of 0.24 or less and an X2 expressed by X2=(Si/5+Mo+Cr/2)/Mn of 0.15 to 10.0, then rough rolling it by a total reduction rate of 30% to 95%, then finish rolling it by a first pass bite temperature of a T4(° C.) expressed by T4=35 ln(X2/2)-25t+1100 to an Ar3 point by a total reduction rate of 30% to 95%, then speedily starting water cooling after the end of the rolling at a flow rate of 0.2 m3/m2·min to 5.0 m3/m2·min, and ending the water cooling at 600° C. or less, where t is the plate thickness.

10. A method of production of steel plate excellent in machineability and in toughness and weldability as set forth in claim 9, characterized in that in the water cooling started after the end of the rolling, an average cooling rate from a water cooling start temperature to over 650° C. is 1° C./s to 5° C./s and an average cooling rate from 650° C. to a water cooling stop temperature is 10° C./s to 100° C./s.

11. A method of production of steel plate excellent in machineability and in toughness and weldability characterized by heating a steel slab or cast slab having a steel composition as described in claim 2 and having an X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B of 0.24 or less and an X2 expressed by X2=(Si/5+Mo+Cr/2)/Mn of 0.15 to 10.0, rolling it, cooling it to 500° C. or less, reheating the steel plate to 900° C. to 1050° C., water cooling it by an average cooling rate of 1° C./s to 100° C./s, and ending the water cooling at 500° C. or less.

12. A method of production of steel plate excellent in machineability and in toughness and weldability characterized by heating a steel slab or cast slab having a steel composition as described in claim 2 and having an X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B of 0.24 or less and an X2 expressed by X2=(Si/5+Mo+Cr/2)/Mn of 0.15 to 10.0, rough rolling it by a total reduction rate of 30% to 95%, finish rolling it by a total reduction rate of 30% to 95%, then air cooling it to an Ar3 point to a temperature lower than the Ar3 point by 150° C., starting water cooling at a flow rate of 0.2 m3/m2·min to 5.0 m3/m2·min, and ending the water cooling at 500° C. or less.

13. A method of production of steel plate excellent in machineability and in toughness and weldability characterized by heating a steel slab or cast slab having a steel composition as described in claim 2 and having an X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B of 0.24 or less and an X2 expressed by X2=(Si/5+Mo+Cr/2)/Mn of 0.15 to 10.0, then rough rolling it by a total reduction rate of 30% to 95%, finish rolling it by a total reduction rate of 30% to 95%, then air cooling it to 500° C. or less, reheating the steel plate to 730° C. to less than 900° C., then water cooling it, and ending the water cooling to 500° C. or less.

14. A method of production of steel plate excellent in machineability and in toughness and weldability characterized by heating a steel slab or cast slab having a steel composition as described in claim 3 and having an X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B of 0.24 or less and an X2 expressed by X2=(Si/5+Mo+Cr/2)/Mn of 0.15 to 10.0, then rough rolling it by a total reduction rate of 30% to 95%, then finish rolling it by a first pass bite temperature of a T4(° C.) expressed by T4=35 ln(X2/2)−25√t+1100 to an Ar3 point by a total reduction rate of 30% to 95%, then speedily starting water cooling at a flow rate of 0.2 m3/m2·min to 5.0 m3/m2·min, and ending the water cooling at 600° C. or less, where t is the plate thickness.

15. A method of production of steel plate excellent in machineability and in toughness and weldability as set forth in claim 14, characterized in that in the water cooling started after the end of the rolling, an average cooling rate from a water cooling start temperature to over 650° C. is 1° C./s to 5° C./s and an average cooling rate from 650° C. to a water cooling stop temperature is 10° C./s to 100° C./s.

16. A method of production of steel plate excellent in machineability and in toughness and weldability characterized by heating a steel slab or cast slab having a steel composition as described in claim 3 and having an X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B of 0.24 or less and an X2 expressed by X2=(Si/5+Mo+Cr/2)/Mn of 0.15 to 10.0, rolling it, cooling it to 500° C. or less, reheating the steel plate to 900° C. to 1050° C., water cooling it by an average cooling rate of 1° C./s to 100° C./s, and ending the water cooling at 500° C. or less.

17. A method of production of steel plate excellent in machineability and in toughness and weldability characterized by heating a steel slab or cast slab having a steel composition as described in claim 3 and having an X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B of 0.24 or less and an X2 expressed by X2=(Si/5+Mo+Cr/2)/Mn of 0.15 to 10.0, rough rolling it by a total reduction rate of 30% to 95%, finish rolling it by a total reduction rate of 30% to 95%, then air cooling it to an Ar3 point to a temperature lower than the Ar3 point by 150° C., starting water cooling at a flow rate of 0.2 m3/m2·min to 5.0 m3/m2·min, and ending the water cooling at 500° C. or less.

18. A method of production of steel plate excellent in machineability and in toughness and weldability characterized by heating a steel slab or cast slab having a steel composition as described in claim 3 and having an X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B of 0.24 or less and an X2 expressed by X2=(Si/5+Mo+Cr/2)/Mn of 0.15 to 10.0, rough rolling it by a total reduction rate of 30% to 95%, finish rolling it by a total reduction rate of 30% to 95%, then cooling it to 500° C. or less, reheating the steel plate to 730° C. to 900° C., water cooling it, and ending the water cooling at 500° C. or less.

19. A method of production of steel plate excellent in machineability and in toughness and weldability as set forth in claim 5, characterized in that said steel slab or cast slab further contains, by mass %, one or more of:

Mo: 0.01 to 1%,

Cr: 0.01 to 1%,

Nb: 0.001 to 0.1%,

Ti: 0.001 to 0.1%,

V: 0.001 to 0.1%,

Cu: 0.005 to 1%,

Ni: 0.01 to 2%,

B: 0.0002 to 0.005%,

REM: 0.0005 to 0.1%,

Ca: 0.0005 to 0.02%,

Zr: 0.0005 to 0.02%, and

Mg: 0.0005 to 0.02%