HIGH STRENGTH HOT ROLLED STEEL SHEET AND METHOD FOR MANUFACTURING THE SAME (AS AMENDED)

- JFE STEEL CORPORATION

A high strength hot rolled steel sheet having a tensile strength TS of 980 MPa or more, has a composition containing, on a percent by mass basis, C: 0.05% or more and 0.18% or less, Si: 1.0% or less, Mn: 1.0% or more and 3.5% or less, P: 0.04% or less, S: 0.006% or less, Al: 0.10% or less, N: 0.008% or less, Ti: 0.05% or more and 0.20% or less, V: more than 0.1% and 0.3% or less, and the balance being Fe and incidental impurities, and has a microstructure including a primary phase and a secondary phase, the primary phase being a bainite phase having an area fraction of more than 85%, the secondary phase being at least one of ferrite phase, martensite phase, and retained austenite phase, the secondary phase having an area fraction of 0% or more and less than 15% in total, the bainite phase having an average lath interval of laths of 400 nm or less, and the laths having an average long axis length of 5.0 μm or less.

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Description
CROSS REFERENCE TO RELATED APPLICATIONS

This is the U.S. National Phase application of PCT/JP2014/001509, filed Mar. 17, 2014, which claims priority to Japanese Patent Application No. 2013-084448, filed Apr. 15, 2013 and Japanese Patent Application No. 2013-084450, filed Apr. 15, 2013, the disclosures of each of these applications being incorporated herein by reference in their entireties for all purposes.

FIELD OF THE INVENTION

The present invention relates to a high strength hot rolled steel sheet having a tensile strength of 980 MPa or more, which is suitable for a material for structural parts and frameworks of automobiles, frames of trucks, steel pipes, and the like.

BACKGROUND OF THE INVENTION

In recent years, automobile exhaust gas regulations have been tightened from the viewpoint of global environmental conservation. Under such circumstances, an improvement of fuel efficiency of automobiles, e.g., trucks, has been an important issue and enhancement of strength and reduction in thickness of the material employed have been further required. Along with this, in particular, high strength hot rolled steel sheets have been actively applied to materials for automobile parts.

Also, in accordance with a demand for further reduction in the construction cost of pipeline, reduction in the material cost of steel pipes have been required. Consequently, instead of UOE steel pipes formed from steel plate, high strength welded steel pipes produced from coil-shaped hot rolled steel sheets with low-price and high productivity, have been noted as transport pipes.

As described above, demands for high strength hot rolled steel sheets having predetermined strength as materials for automotive parts and materials for steel pipes have increased year after year. In particular, the high strength hot rolled steel sheet having tensile strength: 980 MPa or more is highly expected to serve as a material capable of improving fuel efficiency of automobile by leaps and bounds or a material capable of reducing the construction cost of pipeline to a large extent.

However, as the strength of the steel sheet increases, the toughness is degraded in general. Therefore, in order to provide the toughness required of automotive parts and steel pipes to the high strength hot rolled steel sheet, various studies have been conducted on the improvement of toughness. In addition, various studies have been conducted on the hole expansion workability of the high strength hot rolled steel sheet for automotive parts.

Toughness

For example, Patent Literature 1 proposes a hot rolled steel sheet with sheet thickness: 4.0 mm or more and 12 mm or less, having a composition containing, on a percent by mass basis, C: 0.04% to 0.12%, Si: 0.5% to 1.2%, Mn: 1.0% to 1.8%, P: 0.03% or less, S: 0.0030% or less, Al: 0.005% to 0.20%, N: 0.005% or less, Ti: 0.03% to 0.13%, and the balance being Fe and incidental impurities and a microstructure in which the area fraction of bainite phase is more than 95% and the average grain size of the bainite phase is 3 μm or less, wherein a difference between the Vickers hardness at the position at 50 μm from the surface layer and the Vickers hardness at the position one-quarter of the sheet thickness is specified to be 50 or less, and a difference between the Vickers hardness at the position one-quarter of the sheet thickness and the Vickers hardness at the position at one-half of the sheet thickness is specified to be 40 or less. It is mentioned that according to the technology proposed in Patent Literature 1, a high strength hot rolled steel sheet exhibiting excellent toughness and having tensile stress: 780 MPa or more is obtained by specifying the principal phase to be fine bainite and reducing the hardness distribution in the sheet thickness direction.

Patent Literature 2 proposes a method for manufacturing a steel sheet, including the steps of heating a steel material satisfying, on a percent by mass basis, C: 0.05% to 0.18%, Si: 0.10% to 0.60%, Mn: 0.90% to 2.0%, P: 0.025% or less (excluding 0%), S: 0.015% or less (excluding 0%), Al: 0.001% to 0.1%, and N: 0.002% to 0.01%, and the balance being Fe and incidental impurities, to 950° C. or higher and 1, 250° C. or lower, starting rolling, completing the rolling at 820° C. or higher, performing cooling to 600° C. to 700° C. at a cooling rate of 20° C./s or more, performing holding at that temperature range for 10 to 200 seconds or performing slow cooling and, thereafter, performing cooling to 300° C. or lower at a cooling rate of 5° C./s or more, wherein the metal microstructure is specified to be ferrite: 70% to 90%, martensite or a mixed phase of martensite and austenite: 3% to 15%, and the remainder: bainite (including the case of 0%) on an area fraction relative to the whole microstructure basis and, in addition, the average grain size of the above-described ferrite is specified to be 20 μm or less. It is mentioned that according to the technology proposed in Patent Literature 2, a high toughness steel sheet which has a tensile strength of 490 N/mm2 or more and which exhibits a low yield ratio, where the yield ratio is 70% or less, is obtained by specifying the metal microstructure to be a microstructure including ferrite having fine crystal grains, martensite or a mixed phase of martensite and austenite, and the like.

Patent Literature 3 proposes a method for manufacturing a thick high strength hot rolled steel sheet, including the steps of subjecting a steel material containing, on a percent by mass basis, C: 0.02% to 0.25%, Si: 1.0% or less, Mn: 0.3% to 2.3%, P: 0.03% or less, S: 0.03% or less, Al: 0.1% or less, Nb: 0.03% to 0.25%, and Ti: 0.001% to 0.10%, where (Ti+Nb/2)/C<4 is satisfied, to hot rolling, applying first cooling after finish rolling of the hot rolling is completed, where accelerated cooling is performed at an average cooling rate of hot-rolled sheet surface of 20° C./s or more and less than martensite formation critical cooling rate until the surface temperature reaches the Ar3 transformation temperature or lower and the Ms temperature or lower, applying second cooling, where quenching is performed until the sheet thickness center temperature reaches 350° C. or higher and lower than 600° C., performing coiling into the shape of a coil at a coiling temperature of 350° C. or higher and lower than 600° C. on a sheet thickness center temperature basis, and applying third cooling, where at least the position at one-quarter of the sheet thickness in the coil thickness direction to the position at three-quarters of the sheet thickness is held or retained at a temperature range of 350° C. to 600° C. for 30 minutes or more, sequentially. It is mentioned that according to the technology proposed in Patent Literature 3, a material for X65 grade or higher of high strength electric resistance welded steel pipe exhibiting excellent low-temperature toughness is obtained by specifying the microstructure of the hot rolled steel sheet to be a bainite phase or bainitic ferrite phase and, furthermore, adjusting the amount of grain boundary cementite to a specific value or less.

Hole Expansion Workability

For example, Patent Literature 4 describes a method for manufacturing a high strength hot rolled steel sheet, including the steps of heating a steel having a composition containing, on a percent by mass basis, C: 0.05% to 0.15%, Si: 0.2% to 1.2%, Mn: 1.0% to 2.0%, P: 0.04% or less, S: 0.005% or less, Ti: 0.05% to 0.15%, Al: 0.005% to 0.10%, N: 0.007% or less, and the balance being Fe and incidental impurities to 1,150° C. to 1,350° C., and preferably higher than 1,200° C. and 1,350° C. or lower, applying hot rolling which is completed at a finishing temperature of 850° C. to 950° C., and preferably higher than 900° C. and 950° C. or lower, applying cooling after the hot rolling is completed, where cooling to 530° C. is performed at an average cooling rate of 30° C./s or more, applying cooling to coiling temperature: 300° C. to 500° C. at an average cooling rate of 100° C./s or more, and performing coiling at that coiling temperature. It is mentioned that, according to this, the stretch flangeability and the fatigue resistance are considerably improved while high strength of TS: 780 MPa or more is maintained by allowing the microstructure to become composed of a bainite single phase having an average grain size of 5 μm or less, and preferably more than 3.0 μm and 5.0 μm or less and allowing 0.02% or more of solid solution Ti to remain. It is mentioned that the microstructure may be composed of 90% or more on an area fraction basis of bainite phase and a secondary phase other than the bainite phase, where the average grain size of the secondary phase is 3 μm or less, instead of the microstructure composed of the bainite single phase.

Patent Literature 5 describes a method for manufacturing a high strength hot rolled steel sheet, including the steps of subjecting a slab containing, on a percent by mass basis, C: 0.01% to 0.08%, Si: 0.30% to 1.50%, Mn: 0.50% to 2.50%, P: 0.03% or less, S: 0.005% or less, one or two of Ti: 0.01% to 0.20% and Nb: 0.01% to 0.04%, and the balance being Fe and incidental impurities to hot rolling, where the finish rolling temperature is specified to be the Ara transformation temperature to 950° C., performing cooling to 650° C. to 800° C. at a cooling rate of 20° C./s or more, performing air cooling for 2 to 15 s, performing further cooling to 350° C. to 600° C. at a cooling rate of 20° C./s or more, and performing coiling. It is mentioned that, according to this, a high strength hot rolled steel sheet having a ferrite bainite two-phase microstructure in which the proportion of ferrite having a grain size of 2 μm or more is 80% or more, having TS: 690 MPa or more, and exhibiting excellent hole expansion property and ductility is obtained. Also, it is mentioned that 0.0005% to 0.01% of one or two of Ca and REM may be contained.

Patent Literature 6 describes a high strength steel sheet exhibiting excellent hole expansion property and ductility. The high strength steel sheet described in Patent Literature 6 is a steel sheet containing, on a percent by mass basis, C: 0.01% to 0.20%, Si: 1.50% or less, Al: 1.5% or less, Mn: 0.5% to 3.5%, P: 0.2% or less, S: 0.0005% to 0.009%, N: 0.009% or less, Mg: 0.0006% to 0.01%, 0: 0.005% or less, one or two of Ti: 0.01% to 0.20% and Nb: 0.01% to 0.10%, and the balance being Fe and incidental impurities, wherein all three formulae below


[Mg %]≧([O %]/16×0.8)×24  (1)


[S %]≦([Mg %]/24−[O %]/16×0.8+0.00012)×32  (2)


[S %]≦0.0075/[Mn %]  (3)

are satisfied and the microstructure includes a bainite phase as a primary phase. It is mentioned that, according to this, a steel sheet having TS: 980 MPa or more and exhibiting excellent hole expansion property and ductility is produced. It is mentioned that according to the technology proposed in Patent Literature 3, the addition balance between O, Mg, Mn, and S is adjusted to some conditions, (Nb,Ti)N is allowed to become fine and uniform by utilizing composite precipitation of MgO and MgS, fine, uniform voids are generated in a cross-section of a punched hole, stress concentration during hole expansion working is mitigated and, thereby, the hole expansion property is improved.

PATENT LITERATURE

PTL 1: Japanese Unexamined Patent Application Publication No. 2012-062557

PTL 2: Japanese Unexamined Patent Application Publication No. 2007-056294

PTL 3: Japanese Unexamined Patent Application Publication No. 2010-174343

PTL 4: Japanese Unexamined Patent Application Publication No. 2012-12701

PTL 5: Japanese Unexamined Patent Application Publication No. 2002-180190

PTL 6: Japanese Unexamined Patent Application Publication No. 2005-120437

SUMMARY OF THE INVENTION Toughness

In the technology proposed in Patent Literature 1, the high strength hot rolled steel sheet having tensile strength: 980 MPa or more is obtained. However, the control of the bainite microstructure is insufficient and, thereby, there is a problem that excellent low-temperature toughness cannot be obtained stably.

Also, in the technology proposed in Patent Literature 2, the metal microstructure of the steel is specified to be the structure including a ferrite phase as a primary phase, although in the case where the tensile strength is in the 980 MPa class, the toughness of the ferrite phase may be degraded significantly.

Also, in the technology proposed in Patent Literature 3, an improvement of the low-temperature toughness by controlling the amount of grain boundary cementite is intended, although the hot rolled steel sheet strength is insufficient and, as shown in the example thereof, tensile strength: about 800 MPa is the maximum. In this regard, in the case where a high strength hot rolled steel sheet having tensile strength: 980 MPa or more is obtained on the basis of the technology proposed in Patent Literature 3, it is necessary that the C content be increased. However, the control of the grain boundary cementite becomes difficult as the C content increases, so that excellent toughness cannot be obtained stably in some cases.

The present invention solves the above-described problems included in the technologies of the related art advantageously, and it is an object to provide a high strength hot rolled steel sheet having high strength of tensile strength: 980 MPa or more, further exhibiting good toughness and, in one example, having a sheet thickness of 4 mm or more and 15 mm or less and a method for manufacturing the same.

Hole Expansion Property

In the technology described in Patent Literature 4, the aimed strength is tensile strength TS: 780 MPa or more, and when the C content is increased, high strength of tensile strength TS: 980 MPa or more can be obtained. However, if the C content is increased to further enhance the strength, control of the amount of precipitation of Ti carbides becomes difficult, and there is a problem that 0.02% or more of solid solution Ti required for improving hole expansion property cannot be left easily stably.

In the technology described in Patent Literature 5, the steel sheet microstructure is specified to be the mixed microstructure of ferrite in which the proportion of ferrite having a grain size of 2 μm or more is 80% or more+bainite. Therefore, there are problems that the resulting steel sheet strength is about 976 MPa at the maximum, further higher strength of tensile strength TS: 980 MPa or more cannot be achieved easily, and even if the high strength of tensile strength TS: 980 MPa or more is obtained, the toughness of the ferrite phase is degraded significantly and excellent hole expansion property cannot be obtained.

It is mentioned that in the technology described in Patent Literature 6, (Nb,Ti)N is allowed to become fine and uniform, in a cross-section of a punched hole, fine, uniform voids are generated, stress concentration during hole expansion working is mitigated and, thereby, the hole expansion property (hole expansion workability) is improved. However, there are problems that the distances between grains of (Nb,Ti)N are reduced by allowing (Nb,Ti)N to become fine and uniform, voids generated during local deformation are connected easily, and local elongation may be reduced.

The present invention solves such problems included in the technologies of the related art, and it is an object to provide a high strength hot rolled steel sheet exhibiting excellent hole expansion workability while the high strength of tensile strength: 980 MPa or more has and a method for manufacturing the same. In this regard, the high strength hot rolled steel sheet aimed in the present invention may be a steel sheet having a sheet thickness of 2 to 4 mm.

Toughness

In order to achieve the object, the present inventors conducted intensive research to improve the toughness of a hot rolled steel sheet while the high strength of tensile strength TS: 980 MPa or more had. Specifically, the bainite phase was noted, where it is known that the bainite phase has good strength-toughness balance in general, and various factors affecting the strength and the toughness of the hot rolled steel sheet, in which the primary phase of the microstructure was bainite, were studied. As a result, it was found that allowing laths of the bainite phase to become fine was very effective in enhancing strength and improving toughness of the hot rolled steel sheet. Then, further studies were conducted. As a result, it was found that the toughness was improved considerably while the high strength of tensile strength TS: 980 MPa or more was maintained by adding predetermined amounts of Ti and V, specifying the primary phase to be preferably more than 85% on an area fraction basis of bainite phase, specifying the lath interval of the bainite phase to be 400 nm or less in average, and specifying the length of long axis of the lath to be 5.0 μm or less in average.

The present invention has been completed on the basis of the above-described findings and additional studies. That is, an exemplary configuration of an embodiment of the present invention is as described below.

[1] A high strength hot rolled steel sheet having a tensile strength TS of 980 MPa or more and excellent toughness, comprising a composition and a microstructure,

the composition containing, on a percent by mass basis, C: 0.05% or more and 0.18% or less, Si: 1.0% or less, Mn: 1.0% or more and 3.5% or less, P: 0.04% or less, S: 0.006% or less, Al: 0.10% or less, N: 0.008% or less, Ti: 0.05% or more and 0.20% or less, V: more than 0.1% and 0.3% or less, and the balance being Fe and incidental impurities, and

the microstructure comprising a primary phase and a secondary phase,

the primary phase being a bainite phase having an area fraction of more than 85%,

the secondary phase being at least one of ferrite phase, martensite phase and retained austenite phase, the secondary phase having an area fraction of 0% or more and less than 15% in total,

the bainite phase having an average lath interval of laths of 400 nm or less, and the laths having an average long axis length of 5.0 μm or less.

[2] The high strength hot rolled steel sheet having excellent toughness, according to the item [1], wherein the composition further contains, on a percent by mass basis, at least one selected from Nb: 0.005% or more and 0.4% or less, B: 0.0002% or more and 0.0020% or less, Cu: 0.005% or more and 0.2% or less, Ni: 0.005% or more and 0.2% or less, Cr: 0.005% or more and 0.4% or less, and Mo: 0.005% or more and 0.4% or less.
[3] The high strength hot rolled steel sheet having excellent toughness, according to the item [1] or item [2], wherein the composition further contains, on a percent by mass basis, at least one selected from Ca: 0.0002% or more and 0.01% or less and REM: 0.0002% or more and 0.01% or less.
[4] A method for manufacturing a high strength hot rolled steel sheet having excellent toughness, including:

heating a steel having the composition according to any one of the items [1] to [3] to 1,200° C. or higher,

applying hot rolling having rough rolling and finish rolling in which the accumulated rolling reduction is 50% or more in a temperature range of 1,000° C. or lower and the finishing temperature is 820° C. or higher and 930° C. or lower,

starting cooling within 4.0 s after the hot rolling,

performing cooling at an average cooling rate of 20° C./s or more, and

performing coiling at a coiling temperature of 300° C. or higher and 450° C. or lower.

Hole Expansion Workability

In order to achieve the object, the present inventors conducted intensive research on various factors affecting the hole expansion workability while the high strength of tensile strength TS: 980 MPa or more has. As a result, it was found that when the primary phase in the microstructure was specified to be the bainite phase and high strength of tensile strength TS: 980 MPa or more had, cementite functioned as a starting point of void formation during hole expansion working or local deformation, and as the amount of cementite increased, voids were connected to each other easily, the local ductility was degraded, and the hole expansion workability was degraded. Also, it was found that as the grain size of cementite increased, coarse voids were formed in the punched surface by punching, which was a pretreatment of hole expansion working, and the hole expansion property was degraded.

Under these circumstances, the present inventors conducted further research and found that in order to improve the hole expansion property and, furthermore, the local ductility while the high strength of tensile strength TS: 980 MPa or more had, adjustment of the balance between the contents of C, Si, Ti, and V, further adjustment of cementite to 0.8% or less on a percent by mass basis and the average grain size of cementite to 150 nm or less by optimizing the production condition, and an increase in distance between cementite grains were important.

The present invention has been completed on the basis of the above-described findings and additional studies. That is, the gist of an exemplary embodiment of the present invention is as described below.

[5] A high strength hot rolled steel sheet having a tensile strength TS of 980 MPa or more and an excellent hole expansion property, comprising a composition and a microstructure,

the composition containing, on a percent by mass basis, C: more than 0.1% and 0.2% or less, Si: 1.0% or less, Mn: 1.5% to 2.5%, P: 0.05% or less, S: 0.005% or less, Al: 0.10% or less, N: 0.007% or less, Ti: 0.07% to 0.2%, V: more than 0.1% and 0.3% or less, and the balance being Fe and incidental impurities,

the microstructure comprising a primary phase and the remainder other than the primary phase,

the primary phase being a bainite phase having an area fraction of 90% or more,

the remainder being at least one selected from martensite phase, austenite phase and ferrite phase, and having an area fraction 10% or less, and

cementite dispersed in the microstructure having a mass percent of 0.8% or less and an average grain size of 150 nm or less. [6] The high strength hot rolled steel sheet according to the item [5], wherein the composition further contains, on a percent by mass basis, at least one selected from Nb: 0.005% to 0.1%, B: 0.0002% to 0.002%, Cu: 0.005% to 0.3%, Ni: 0.005% to 0.3%, Cr: 0.005% to 0.3%, and Mo: 0.005% to 0.3%.

[7] The high strength hot rolled steel sheet according to the item [5] or item [6], wherein the composition further contains, on a percent by mass basis, at least one selected from Ca: 0.0003% to 0.01% and REM: 0.0003% to 0.01%.
[8] A method for manufacturing a high strength hot rolled steel sheet having excellent hole expansion property, including: heating a steel material, applying hot rolling having rough rolling and finish rolling, applying cooling having two stages of first stage cooling and second stage cooling, and performing coiling to produce a hot rolled steel sheet,

wherein

the steel material is specified to be a steel material having a composition containing, on a percent by mass basis, C: more than 0.1% and 0.2% or less, Si: 1.0% or less, Mn: 1.5% to 2.5%, P: 0.05% or less, S: 0.005% or less, Al: 0.10% or less, N: 0.007% or less, Ti: 0.07% to 0.2%, V: more than 0.1% and 0.3% or less, and the balance being Fe and incidental impurities,

the heating is a treatment to heat the steel material to 1,200° C. or higher,

the finish rolling is rolling with finishing temperature: 850° C. to 950° C.,

the first stage cooling is cooling in which cooling is started within 1.5 s of completion of the above-described finish rolling and cooling to a first stage cooling stop temperature of 500° C. to 600° C. is performed at an average cooling rate of 20° C./s to 80° C./s,

the second stage cooling is cooling in which cooling to a second stage cooling stop temperature of 330° C. to 470° C. is performed at an average cooling rate of 90° C./s or more within 3 s of completion of the above-described first stage cooling, and

after completion of the second stage cooling, coiling is performed, where the coiling temperature is the second stage cooling stop temperature.

[9] The method for manufacturing a high strength hot rolled steel sheet, according to the item [8], wherein the composition further contains, on a percent by mass basis, at least one selected from Nb: 0.005% to 0.1%, B: 0.0002% to 0.002%, Cu: 0.005% to 0.3%, Ni: 0.005% to 0.3%, Cr: 0.005% to 0.3%, and Mo: 0.005% to 0.3%.
[10] The method for manufacturing a high strength hot rolled steel sheet, according to the item [8] or item [9], wherein the composition further contains, on a percent by mass basis, at least one selected from Ca: 0.0003% to 0.01% and REM: 0.0003% to 0.01%.

Toughness

According to an aspect of the present invention, a high strength hot rolled steel sheet having a tensile strength of 980 MPa or more and exhibiting excellent toughness is obtained. Therefore, the car body weight can be reduced while the safety of the automobile is ensured and an environmental load can be reduced by applying the present invention to structural parts and frameworks of automobiles, frames of trucks, and the like. In the case where a welded steel pipe produced from the hot rolled steel sheet according to the present invention serving as a material instead of the UOE pipe produced from a steel plate serving as a material is applied to a transport pipe, the productivity is improved and the cost can be further reduced.

Also, the present invention can stably produce a hot rolled steel sheet exhibiting improved toughness while high strength of tensile strength: 980 MPa or more has and, therefore, is very useful for the industry.

Hole Expansion Workability

According to an aspect of the present invention, a hot rolled steel sheet exhibiting considerably improved hole expansion workability can be produced while high strength of tensile strength: 980 MPa or more has, so that an industrially remarkable effect is exerted. Also, effects that the car body weight can be reduced while the safety of the automobile is ensured and an environmental load can be reduced are exerted by applying the hot rolled steel sheet according to an embodiment of the present invention to materials for chassis parts, structural parts and frameworks of automobiles, frames of trucks, and the like.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION First Embodiment Toughness

A first embodiment will be specifically described below.

To begin with, reasons for the limitation of the chemical composition of the hot rolled steel sheet according to an aspect of the present invention will be described. Hereafter the term “%” representing the chemical composition refers to “percent by mass” unless otherwise specified.

C: 0.05% or more and 0.18% or less

C enhances the strength of the steel and facilitates formation of bainite. Therefore, in an embodiment of the present invention, it is useful that the C content be 0.05% or more. On the other hand, if the C content is more than 0.18%, formation control of bainite becomes difficult, formation of hard martensite increases, and the toughness of the hot rolled steel sheet is degraded. Consequently, the C content may be specified to be 0.05% or more and 0.18% or less, preferably 0.08% or more and 0.17% or less, and more preferably more than 0.10% and 0.16% or less. In this regard, in the case where the amount of Mn is 2.5% or more and 3.5% or less, the amount of C is preferably 0.06% or more and 0.15% or less.

Si: 1.0% or less

Si is an element which suppresses coarse oxides and cementite to impair the toughness and which contributes to solute strengthening. If the content is more than 1.0%, the surface quality of the hot rolled steel sheet can be degraded significantly and degradation in the chemical conversion treatability and the corrosion resistance is caused. Therefore, the Si content may be specified to be 1.0% or less, and preferably 0.4% or more and 0.8% or less.

Mn: 1.0% or more and 3.5% or less

Mn is an element which contributes to enhancement of strength of the steel through solid solution and which facilitates formation of bainite through improvement of the hardenability. In order to obtain such effects, it is beneficial that the Mn content be 1.0% or more. On the other hand, if the Mn content is more than 3.5%, center segregation becomes considerable, and the toughness of the hot rolled steel sheet is degraded. Therefore, the Mn content may be specified to be 1.0% or more and 3.5% or less. In this regard, 1.5% or more and 3.0% or less is preferable and 1.8% or more and 2.5% or less is more preferable.

P: 0.04% or less

P is an element which contributes to enhancement of strength of the steel through solid solution but is an element which segregates at grain boundaries, in particular prior-austenite grain boundaries, to cause degradation in low-temperature toughness and workability. Consequently, it is preferable that the P content be minimized, although the content up to 0.04% can be allowable. Therefore, the P content may be specified to be 0.04% or less. However, when the P content is excessively reduced, an effect corresponding to an increase in the smelting cost is not obtained, so that the P content is specified to be preferably 0.003% or more and 0.03% or less, and more preferably 0.005% or more and 0.02% or less.

S: 0.006% or less

S forms coarse sulfides by bonding to Ti and Mn and degrades the workability of the hot rolled steel sheet. Consequently, it is preferable that the S content be minimized, although the content up to 0.006% can be allowable. Therefore, the S content may be specified to be 0.006% or less. However, when the S content is excessively reduced, an effect corresponding to an increase in the smelting cost is not obtained, so that the S content is specified to be preferably 0.0003% or more and 0.004% or less, and more preferably 0.0005% or more and 0.002% or less.

Al: 0.10% or less

Al is an element which functions as a deoxidizing agent and which is effective in improving cleanliness of the steel. On the other hand, excessive addition of Al causes increases in oxide inclusions, degrades the toughness of the hot rolled steel sheet and, in addition, causes an occurrence of flaw. Therefore, the Al content may be specified to be 0.10% or less, preferably 0.005% or more and 0.08% or less, and further preferably 0.01% or more and 0.05% or less.

N: 0.008% or less

Ni precipitates as nitrides by bonding to nitride-forming elements and contributes to making crystal grains fine. However, N bonds to Ti at a high temperature to form coarse nitrides easily and degrades the toughness of the hot rolled steel sheet. Consequently, the N content may be specified to be 0.008% or less, preferably 0.001% or more and 0.006% or less, and more preferably 0.002% or more and 0.005% or less.

Ti: 0.05% or more and 0.20% or less

Ti is a beneficial element in an embodiment of the present invention. Ti contributes to enhancement of strength of the steel through formation of carbonitrides to make crystal grains fine and through precipitation strengthening. Also, Ti forms many fine (Ti,V)C clusters at low temperatures of 300° C. or higher and 450° C. or lower, reduces the amount of cementite in the steel, and improve the toughness of the hot rolled steel sheet. In order to exert such effects, it is advantageous that the Ti content be 0.05% or more. On the other hand, if the Ti content is excessive and is more than 0.20%, the above-described effects are saturated, an increase in coarse precipitates is caused, and degradation in the toughness of the hot rolled steel sheet is caused. Therefore, the Ti content may be limited to within the range of 0.05% or more and 0.20% or less, and preferably 0.08% or more and 0.15% or less.

V: more than 0.1% and 0.3% or less

V is a beneficial element in an embodiment of the present invention. V contributes to enhancement of strength of the steel through formation of carbonitrides to make crystal grains fine and through precipitation strengthening. Also, V improves the hardenability and contributes to formation and making fine of bainite phase. In addition, V forms many fine (Ti,V)C clusters at low temperatures of 300° C. or higher and 450° C. or lower, reduces the amount of cementite in the steel, and improves the toughness of the hot rolled steel sheet. In order to exert such effects, it is advantageous that the V content be more than 0.1%. On the other hand, if the V content is excessive and is more than 0.3%, the above-described effects are saturated, so that the cost increases. Therefore, the V content may be limited to within the range of more than 0.1% and 0.3% or less, and preferably 0.15% or more and 0.25% or less.

The basic components of the hot rolled steel sheet according to an aspect of the present invention are as described above. The hot rolled steel sheet according to embodiments of the present invention may further contain, as necessary, at least one selected from Nb: 0.005% or more and 0.4% or less, B: 0.0002% or more and 0.0020% or less, Cu: 0.005% or more and 0.2% or less, Ni: 0.005% or more and 0.2% or less, Cr: 0.005% or more and 0.4% or less, and Mo: 0.005% or more and 0.4% or less for the purpose of, for example, improvement of toughness and enhancement of strength.

Nb: 0.005% or more and 0.4% or less

Nb is an element which contributes to enhancement of strength of the steel through formation of carbonitrides. In order to exert such an effect, it is preferable that the Nb content be 0.005% or more. On the other hand, if the Nb content is more than 0.4%, deformation resistance increases, so that a rolling force of hot rolling increases in production of the hot rolled steel sheet, a load to a rolling mill becomes too large, and rolling operation in itself may become difficult. Meanwhile, if the Nb content is more than 0.4%, coarse precipitates are formed and the toughness of the hot rolled steel sheet tends to be degraded. Therefore, the Nb content is preferably specified to be 0.005% or more and 0.4% or less. In this regard, 0.01% or more and 0.3% or less is more preferable and 0.02% or more and 0.2% or less is further preferable.

B: 0.0002% or more and 0.0020% or less

B is an element which segregates at austenite grain boundaries and which suppresses formation and growth of ferrite. Also, B is an element which improves the hardenability and which contributes to formation and making fine of bainite phase. In order to exert these effects, it is preferable that the B content be 0.0002% or more. However, if the B content is more than 0.0020%, formation of martensite phase is facilitated, so that the toughness of the hot rolled steel sheet may be degraded significantly. Therefore, in the case where B is contained, the content thereof is specified to be preferably 0.0002% or more and 0.0020% or less. In this regard, 0.0004% or more and 0.0012% or less is more preferable.

Cu: 0.005% or more and 0.2% or less

Cu is an element which contributes to enhancement of strength of the steel through solid solution. Also, Cu is an element which has a function of improving hardenability, which lowers, in particular, the bainite transformation temperature, and which contributes to making bainite phase fine. In order to obtain these effects, it is preferable that the Cu content be 0.005% or more, although if the content thereof is more than 0.2%, degradation in the surface quality of the hot rolled steel sheet is caused. Therefore, the Cu content is specified to be preferably 0.005% or more and 0.2% or less. In this regard, 0.01% or more and 0.15% or less is more preferable.

Ni: 0.005% or more and 0.2% or less

Ni is an element which contributes to enhancement of strength of the steel through solid solution. Also, Ni has a function of improving hardenability and facilitates formation of bainite phase. In order to obtain these effects, it is preferable that the Ni content be 0.005% or more. However, if the Ni content is more than 0.2%, a martensite phase is generated easily, and the toughness of the hot rolled steel sheet may be degraded significantly. Therefore, the Ni content is specified to be preferably 0.005% or more and 0.2% or less, and more preferably 0.01% or more and 0.15% or less.

Cr: 0.005% or more and 0.4% or less

Cr forms carbides and contributes to enhancement of strength of the hot rolled steel sheet. In order to exert this effect, it is preferable that the Cr content be 0.005% or more. On the other hand, if the Cr content is excessive and is more than 0.4%, it is feared that the corrosion resistance of the hot rolled steel sheet is degraded. Therefore, the Cr content is specified to be preferably 0.005% or more and 0.4% or less, and more preferably 0.01% or more and 0.2% or less.

Mo: 0.005% or more and 0.4% or less

Mo facilitates formation of bainite phase through improvement of the hardenability and contributes to improvement of the toughness and enhancement of strength of the hot rolled steel sheet. In order to obtain such effects, it is preferable that the Mo content be 0.005% or more. However, if the Mo content is more than 0.4%, a martensite phase is generated easily, and the toughness of the hot rolled steel sheet may be degraded. Therefore, the Mo content is specified to be preferably 0.005% or more and 0.4% or less, and more preferably 0.01% or more and 0.2% or less.

Meanwhile, the hot rolled steel sheet according to the present invention may contain, as necessary, one or two selected from Ca: 0.0002% or more and 0.01% or less and REM: 0.0002% or more and 0.01% or less.

Ca: 0.0002% or more and 0.01% or less

Ca is effective in controlling the shape of sulfide inclusions and improving bending workability and the toughness of the hot rolled steel sheet. In order to exert these effects, it is preferable that the Ca content be 0.0002% or more. However, if the Ca content is more than 0.01%, surface defects of the hot rolled steel sheet may be caused. Therefore, the Ca content is specified to be preferably 0.0002% or more and 0.01% or less. In this regard, 0.0004% or more and 0.005% or less is more preferable.

REM: 0.0002% or more and 0.01% or less

As with Ca, REM controls the shape of sulfide inclusions and improves adverse influences of sulfide inclusions on the bending workability and the toughness of the hot rolled steel sheet. In order to exert these effects, it is preferable that the REM content be 0.0002% or more. However, if the REM content is excessive and is more than 0.01%, the cleanliness of the steel tends to be degraded and the toughness of the hot rolled steel sheet tends to be degraded. Therefore, in the case where REM is contained, the content thereof is specified to be preferably 0.0002% or more and 0.01% or less. In this regard, 0.0004% or more and 0.005% or less is more preferable.

In an embodiment of the present invention, the remainder other than those described above is composed of Fe and incidental impurities. Examples of incidental impurities include Sb, Sn, and Zn. As for contents of them, Sb: 0.01% or less, Sn: 0.1% or less, and Zn: 0.01% or less are allowable.

Next, reasons for the limitation of the microstructure of the hot rolled steel sheet according to an aspect of the present invention will be described.

The hot rolled steel sheet according to an embodiment the present invention has a microstructure in which a primary phase is more than 85% on an area fraction basis of bainite phase, a secondary phase is at least one of ferrite phase, martensite phase, and retained austenite phase, 0% or more and less than 15% in total on an area fraction basis of secondary phase is contained, the average lath interval of laths of the above-described bainite phase may be 400 nm or less, and the average long axis length of the above-described laths may be 5.0 μm or less.

Fraction of bainite phase: more than 85% on an area fraction basis

The primary phase of the hot rolled steel sheet according to an embodiment of the present invention is a bainite phase having excellent strength-toughness balance. If the fraction of the bainite phase is 85% or less on an area fraction basis, a hot rolled steel sheet provided with predetermined strength and toughness may not be obtained. Therefore, the fraction of the bainite phase may be specified to be more than 85% on an area fraction basis, preferably 87% or more, and more preferably 90% or more. It is still more preferable that the fraction of the bainite phase be 100% on an area fraction basis and the microstructure be a bainite single phase microstructure.

Fraction of at least one of ferrite phase, martensite phase, and retained austenite phase (secondary phase): 0% or more and less than 15% in total on an area fraction basis

The hot rolled steel sheet according to an embodiment of the present invention may include a secondary phase, which is composed of at least one of ferrite phase, martensite phase, and retained austenite phase, as a microstructure other than the bainite phase serving as the primary phase. The microstructure is specified to be preferably a bainite single phase microstructure to impart predetermined strength and toughness to the hot rolled steel sheet. However, even in the case where at least one of ferrite phase, martensite phase, and retained austenite phase is included as the secondary phase, the total fraction of them of less than 15% on an area fraction basis is allowable. Therefore, the fraction of the above-described secondary phase in total is specified to be 0% or more and less than 15% on an area fraction basis, preferably 13% or less, and more preferably 11% or less.

Average lath interval of laths of bainite phase: 400 nm or less

Average long axis length of laths of bainite phase: 5.0 μm or less

It is very beneficial for enhancement of strength and enhancement of toughness of the hot rolled steel sheet to make laths of bainite phase fine. The present inventors found that the sizes of laths of bainite phase, specifically, the lath interval and the long axis length of the lath, were factors which influenced greatly the strength and the toughness of the hot rolled steel sheet. Consequently, in aspects of the present invention, predetermined strength and toughness are added to the hot rolled steel sheet by specifying the lath interval and the long axis length of the lath of bainite phase.

In the case where the average lath interval of laths of the bainite phase is more than 400 nm or the average long axis length of laths of the bainite phase is more than 5.0 μm, a hot rolled steel sheet exhibiting predetermined strength and toughness according to an embodiment of the present invention in combination may not be obtained. Therefore, the average lath interval of laths of the bainite phase may be specified to be 400 nm or less, and preferably 350 nm or less. Also, the average long axis length of laths of the bainite phase is specified to be 5.0 μm or less, and preferably 4.0 μm or less. In this regard, lower limits of the average lath interval of laths of the bainite and the average long axis length of laths of the bainite phase are not particularly specified. The lath interval and the long axis length are determined on the basis of the bainite transformation temperature and, therefore, usually the average lath interval of laths of the bainite phase is 100 nm or more and the average long axis length of laths of the bainite phase is 1.0 μm or more.

A high strength hot rolled steel sheet having a tensile strength of 980 MPa or more and having toughness required of a material for structural parts of automobiles and a material for steel pipes, e.g., line pipes, is obtained by specifying the composition and the microstructure, as described above. In this regard, the sheet thickness of the hot rolled steel sheet according to the present invention is not specifically limited, although the sheet thickness is specified to be preferably about 4 mm or more and 15 mm or less.

Next, a preferable method for manufacturing the hot rolled steel sheet according to an aspect of the present invention will be described.

Embodiments of the present invention may be characterized by heating a steel having the above-described composition to 1,200° C. or higher, applying hot rolling composed of rough rolling and finish rolling in which the accumulated rolling reduction is 50% or more in a temperature range of 1,000° C. or lower and the finishing temperature is 820° C. or higher and 930° C. or lower, starting cooling within 4.0 s of the hot rolling, performing cooling at an average cooling rate of 20° C./s or more, and performing coiling at a coiling temperature of 300° C. or higher and 450° C. or lower.

The method for manufacturing a steel is not necessarily particularly limited, and any common method can be applied, wherein a molten steel having the above-described composition is refined in a converter or the like, and a steel, e.g., a slab, is produced by a casting method, e.g., a continuous casting method. In this regard, an ingot-making and blooming method may be used.

Meanwhile, in one embodiment of the present invention, electro-magnetic stirrer (EMS), intentional bulging soft reduction casting (IBSR), and the like can be applied to reduce component segregation of the steel during continuous casting. Equiaxial crystals are formed in the sheet thickness center portion by applying an electro-magnetic stirrer treatment, so that segregation can be reduced. Also, in the case where the intentional bulging soft reduction casting is applied, segregation in the sheet thickness center portion can be reduced by preventing flowing of the molten steel in an unsolidified portion of the continuous casting slab. The toughness described below can be brought to a more excellent level by applying at least one of these segregation reduction treatments.

Heating temperature of steel: 1,200° C. or higher

In steel material, e.g., a slab, most of carbonitride-forming elements, e.g., Ti, are present as coarse carbonitrides. The presence of these coarse nonuniform precipitates causes degradation in various characteristics (for example, strength, toughness, and hole expansion workability) of the hot rolled steel sheet. Consequently, the steel material before hot rolling is heated to allow coarse precipitates to form solid solutions. In order to allow these coarse precipitates to form solid solutions sufficiently, it is advantageous that the heating temperature of the steel be 1,200° C. or higher. However, if the heating temperature of the steel is too high, an occurrence of slab flaw and reduction in yield due to scale-off are caused. Therefore, the heating temperature of the steel is specified to be preferably 1,350° C. or lower, and more preferably 1,220° C. or higher and 1,300° C. or lower.

In this regard, the steel material may be heated to the heating temperature of 1,200° C. or higher and is held for a predetermined time. If the holding time is more than 4,800 seconds, the amount of generation of scale increases and, as a result, scale biting and the like occurs easily in the following hot rolling step, and the surface quality of the hot rolled steel sheet tends to be degraded. Therefore, the holding time of the steel material in the temperature range of 1,200° C. or higher is specified to be preferably 4,800 seconds or less, and more preferably 4,000 seconds or less.

Following the heating of the steel material, the steel material may be subjected to hot rolling having rough rolling and finish rolling. The condition of the rough rolling is not specifically limited insofar as predetermined sheet bar dimensions are ensured. Following the rough rolling, the finish rolling is applied. In this regard, preferably, descaling is performed before the finish rolling or between stands during rolling. In the finish rolling, the accumulated rolling reduction may be specified to be 50% or more in a temperature range of 1,000° C. or lower and the finishing temperature may be specified to be 820° C. or higher and 930° C. or lower.

Accumulated rolling reduction in temperature range of 1,000° C. or lower: 50% or more

In order to make laths of the bainite phase fine, it is advantageous that the rolling reduction in a relatively low temperature range be increased and crystal grains after rolling be allowed to become crystal grains elongated in the rolling direction (crystal grains having a high elongation rate). If the accumulated rolling reduction at 1,000° C. or lower is less than 50%, it becomes difficult to make bainite having a predetermined lath structure (e.g., average lath interval: 400 nm or less, average long axis length: 5.0 μm or less), and the toughness of the hot rolled steel sheet is degraded. Therefore, the accumulated rolling reduction at 1,000° C. or lower may be specified to be 50% or more, and preferably 60% or more. However, if the accumulated rolling reduction in a temperature range of 1,000° C. or lower is excessively high, crystal grains are excessively elongated in the rolling direction and ferrite is generated easily, so that it may also be difficult to make bainite having a predetermined lath structure. Consequently, the accumulated rolling reduction in a temperature range of 1,000° C. or lower is specified to be preferably 80% or less.

Finishing temperature: 820° C. or higher and 930° C. or lower

If the finishing temperature of the finishing rolling is lower than 820° C., rolling is performed at a temperature of two-phase region of ferrite+austenite, so that a deformation microstructure remains after rolling and the toughness of the hot rolled steel sheet is degraded. On the other hand, if the finishing temperature is higher than 930° C., austenite grains grow, and a bainite phase of the hot rolled steel sheet obtained after cooling is coarsened. As a result, it becomes difficult to make a predetermined microstructure, and the toughness of the hot rolled steel sheet is degraded. Therefore, the finishing temperature may be specified to be 820° C. or higher and 930° C. or lower, and preferably 840° C. or higher and 920° C. or lower. Here, the finishing temperature refers to the surface temperature of a sheet.

Start of forced cooling: within 4.0 s of completion of finish rolling

Forced cooling may be started within 4.0 s of, preferably just after, completion of the finish rolling, cooling may be stopped at the coiling temperature, and coiling into the shape of a coil is performed. If the time from completion of the finish rolling to start of the forced cooling is more than 4.0 s and is long, austenite grains become coarse, and a bainite phase is coarsened. Also, austenite grains become coarse, so that the hardenability of the steel sheet increases and a martensite phase is generated easily. In the case where the bainite phase is coarsened and the martensite phase is generated easily, predetermined excellent toughness may not be obtained. Therefore, the forced cooling start time is limited to within 4.0 s of completion of the finish rolling.

Average cooling rate: 20° C./s or more

If the average cooling rate from the finishing temperature to the coiling temperature is less than 20° C./s, a bainite phase having a predetermined area fraction may not be obtained. Therefore, the above-described average cooling rate may be specified to be 20° C./s or more, and preferably 30° C./s or more. The upper limit of the average cooling rate is not particularly specified. However, if the average cooling rate is too large, the surface temperature becomes too low, and martensite is generated on the steel sheet surface easily. Therefore, the average cooling rate is specified to be preferably 60° C./s or less. In this regard, the above-described average cooling rate is specified to be an average cooling rate of the steel sheet surface.

Coiling temperature: 300° C. or higher and 450° C. or lower

If the coiling temperature is lower than 300° C., hard martensite phase and retained austenite phase are formed in the microstructure of the inside of the steel sheet. As a result, the hot rolled steel sheet may not be made a predetermined microstructure and predetermined toughness may not be obtained. On the other hand, if the coiling temperature is more than 450° C., ferrite and pearlite increase in the microstructure of the inside of the steel sheet. As a result, the lath interval of the bainite phase increases and, thereby, the toughness of the hot rolled steel sheet is degraded significantly. For the above-described reasons, the coiling temperature is specified to be within the range of 300° C. or higher and 450° C. or lower, and preferably 330° C. or higher and 430° C. or lower.

In this regard, after the coiling, the hot rolled steel sheet may be subjected to temper rolling following the common method or be subjected to pickling to remove scale formed on the surface. Alternatively, a galvanization process, e.g., hot dip galvanizing or electrogalvanizing, and a chemical conversion treatment may further be applied.

Example 1

A molten steel having the composition shown in Table 1 was refined in a converter, and a slab (steel) was produced by a continuous casting method. In the continuous casting, those other than Hot rolled steel sheet No. 1′ of Steel A1 in Tables 1 to 3 described below were subjected to electro-magnetic stirrer (EMS) for the purpose of segregation reduction treatment of the components. Subsequently, these steel materials were heated under the conditions shown in Table 2, and were subjected to hot rolling having rough rolling and finish rolling under the conditions shown in Table 2. After the finish rolling was completed, cooling was performed under the conditions shown in Table 2, and coiling was performed at coiling temperatures shown in Table 2, so that hot rolled steel sheets having sheet thicknesses shown in Table 2 were produced.

Test pieces were taken from the resulting hot rolled steel sheets, and microstructure observation, a tensile test, and a Charpy impact test were performed. The microstructure observation method and various testing methods were as described below.

(i) Microstructure Observation

Fraction of Microstructure

A test piece for a scanning electron microscope (SEM) was taken from the hot rolled steel sheet, a sheet thickness cross-section parallel to the rolling direction was polished and, thereafter, the microstructure was allowed to appear with a corrosive liquid (3% nital solution). Photographs were taken in three fields of view of each of the position at one-quarter of the sheet thickness and the position at one-half of the sheet thickness (center position of the sheet thickness) with a scanning electron microscope (SEM) at the magnification of 3,000 times, and the area fraction of each phase was quantified on the basis of an image treatment.

Lath Interval of Laths of Bainite Phase

A test piece having size: 10 mm×15 mm was taken from the hot rolled steel sheet, thin film samples for transmission electron microscope (TEM) observation of the position at one-quarter of the sheet thickness and the position at one-half of the sheet thickness (center position of the sheet thickness) were produced, and photographs were taken in ten fields of view of each position with TEM at the magnification of 30,000 times. Five straight lines at intervals of 10 mm were drawn at right angles to long axes of at least three laths which were shown in each photograph having a size of 120 mm×80 mm and which were successively arranged side by side. The length of each line segment between the intersection points of the straight line and the lath boundary was measured and the average value of the resulting lengths of the segments was specified to be the average lath interval.

Long Axis Length of Lath of Bainite Phase

A test piece for a scanning electron microscope (SEM) was taken from the hot rolled steel sheet, a sheet thickness cross-section parallel to the rolling direction was polished and, thereafter, the microstructure was allowed to appear with a corrosive liquid (3% nital solution). Photographs were taken in five fields of view of each of the position at one-quarter of the sheet thickness and the position at one-half of the sheet thickness (center position of the sheet thickness) with a scanning electron microscope (SEM) at the magnification of 10,000 times. The lengths of long axes of at least 10 laths which were shown in each photograph, where at least three laths were successively arranged side by side, were measured and the average value of the resulting lath long axis lengths was specified to be the average lath long axis length.

(ii) Tensile Test

JIS No. 5 test pieces (GL: 50 mm) were taken from the hot rolled steel sheet in such a way that the tensile direction and the rolling direction form a right angle. A tensile test was performed in conformity with JIS Z 2241 (2011) and the yield strength (yield point) YP, the tensile strength (TS), and the total elongation El were determined.

(iii) Charpy Impact Test

A subsize test piece (V-notch) having a thickness of 5 mm was taken from the hot rolled steel sheet in such a way that the longitudinal direction of the test piece and the rolling direction form a right angle. A Charpy impact test was performed in conformity with JIS Z 2242, the Charpy impact value (vE−50) at a temperature of −50° C. was measured, and the toughness was evaluated. Here, the hot rolled steel sheet having a sheet thickness of more than 5 mm was subjected to double-side polishing to produce a test piece having a sheet thickness of 5 mm. As for the hot rolled steel sheet having a sheet thickness of 5 mm or less, a test piece having the original sheet thickness was produced. Then, the test pieces were subjected to the charpy impact test. In the case where the measured vE−50 value was 40 J or more, the toughness was evaluated as good.

The obtained results are shown in Table 3 and Table 4.

TABLE 1 Chemical composition (percent by mass) Remainder: Fe and incidental impurities Steel C Si Mn P S Al N Ti V Others Remarks A1 0.06 0.3 3.3 0.017 0.0017 0.073 0.0038 0.15 0.20 — Invention steel B1 0.15 0.7 2.0 0.037 0.0032 0.036 0.0026 0.12 0.20 — Invention steel C1 0.21 0.8 2.2 0.026 0.0006 0.042 0.0033 0.10 0.15 — Comparative steel D1 0.14 1.3 2.0 0.005 0.0009 0.043 0.0042 0.11 0.30 — Comparative steel E1 0.15 0.2 2.3 0.026 0.0012 0.022 0.0031 0.12 0.15 — Invention steel F1 0.14 0.5 3.8 0.018 0.0006 0.038 0.0039 0.09 0.25 — Comparative steel G1 0.04 0.7 2.0 0.021 0.0028 0.031 0.0025 0.03 0.20 — Comparative steel H1 0.04 0.9 1.6 0.022 0.0015 0.028 0.0037 0.23 0.11 — Comparative steel I1 0.13 0.6 1.6 0.016 0.0010 0.010 0.0028 0.17 0.05 — Comparative steel J1 0.15 0.8 1.8 0.025 0.0010 0.047 0.0029 0.06 0.25 — Invention steel K1 0.15 0.3 3.0 0.027 0.0008 0.067 0.0074 0.08 0.20 — Invention steel L1 0.12 0.8 1.3 0.027 0.0021 0.019 0.0039 0.18 0.11 — Invention steel M1 0.18 0.7 1.7 0.016 0.0009 0.055 0.0047 0.09 0.20 — Invention steel N1 0.12 0.4 1.7 0.004 0.0009 0.037 0.0055 0.08 0.15 Nb: 0.02 Invention steel P1 0.14 0.3 1.8 0.030 0.0008 0.037 0.0070 0.12 0.20 Ni: 0.1, Cr: 0.1 Invention steel Q1 0.16 0.7 2.1 0.020 0.0011 0.047 0.0041 0.15 0.15 Mo: 0.15 Invention steel R1 0.15 0.5 1.9 0.035 0.0055 0.005 0.0034 0.11 0.22 B: 0.0005 Invention steel S1 0.16 0.9 2.4 0.029 0.0016 0.093 0.0033 0.11 0.30 Ca: 0.005 Invention steel T1 0.15 0.7 2.3 0.017 0.0045 0.031 0.0039 0.11 0.15 REM: 0.005 Invention steel U1 0.13 0.6 2.2 0.022 0.0035 0.027 0.0037 0.13 0.18 Cu: 0.1 Invention steel

TABLE 2 Hot rolling condition Finish rolling accumulated Average Hot rolled Slab heating rolling reduction Finishing Cooling cooling Coiling Sheet steel sheet temperature at 1000° temperature start time rate temperature thickness No. Steel (° C.) Cor lower (%) (° C.) (s)* (° C./s) (° C.) (mm) Remarks  1 A1 1220 80 920 1 40 360 4 Invention example   1′ 1220 80 910 1 40 360 4 Invention example  2 1220 80 910 1 40 470 4 Comparative example  3 1220 80 800 1 40 430 4 Comparative example  4 B1 1240 75 910 1.5 35 410 6 Invention example  5 1220 75 850 1.5 35 380 6 Invention example  6 C1 1220 55 900 3 25 390 12 Comparative example  7 D1 1240 75 880 1.5 35 350 6 Comparative example  8 E1 1220 55 950 2.5 25 370 10 Comparative example  9 1220 55 920 2.5 25 350 10 Invention example 10 F1 1220 55 840 3 25 380 12 Comparative example 11 G1 1220 60 870 2 30 320 8 Comparative example 12 H1 1240 50 910 3.5 20 430 14 Comparative example 13 I1 1220 75 880 1.5 35 380 6 Comparative example 14 J1 1220 45 910 3.5 20 400 14 Comparative example 15 1220 50 880 3.5 20 310 14 Invention example 16 K1 1240 75 900 1.5 10 370 6 Comparative example 17 1220 75 920 1.5 30 350 6 Invention example 18 L1 1280 80 890 1 35 380 4 Invention example 19 1170 80 850 1 40 430 4 Comparative example 20 M1 1260 60 900 2 30 280 8 Comparative example 21 1220 60 900 2 30 350 8 Invention example 22 N1 1220 55 920 3 25 330 12 Invention example 23 P1 1200 60 840 2 30 410 8 Invention example 24 Q1 1280 80 910 1 40 380 4 Invention example 25 R1 1220 80 880 1 35 350 4 Invention example 26 S1 1220 75 840 1.5 35 360 6 Invention example 27 T1 1220 75 910 1.5 35 390 6 Invention example 28 U1 1220 75 870 1.5 45 370 6 Invention example *time from completion of finish rolling to start of forced cooling

TABLE 3 Microstructure of hot rolled steel sheet** F + M + γarea B average lath interval B area fraction (%) fraction (%) (nm) B lath average long axis Hot rolled ¼ of ¼ of ¼ of length (μm) steel sheet sheet ½ of sheet sheet ½ of sheet sheet ½ of sheet ¼ of sheet ½ of sheet No. Steel thickness thickness thickness thickness thickness thickness thickness thickness Remarks  1 A1 88 90 12 10 260 290 2.9 3.5 Invention example   1′ 89 91 11  9 270 300 3.0 3.7 Invention example  2 83 84 17 16 290 320 2.8 3.4 Comparative example  3 82 83 18 17 280 310 2.2 2.5 Comparative example  4 B1 86 88 14 12 340 370 3.2 3.9 Invention example  5 87 89 13 11 330 350 2.8 3.1 Invention example  6 C1 87 90 13 10 380 400 4.6 4.9 Comparative example  7 D1 88 91 12  9 350 370 3.4 3.8 Comparative example  8 E1 88 90 12 10 330 360 5.2 5.8 Comparative example  9 88 90 12 10 320 350 4.5 4.9 Invention example 10 F1 87 91 13  9 360 380 4.2 4.6 Comparative example 11 G1 89 92 11  8 350 380 5.3 6.1 Comparative example 12 H1 86 88 14 12 520 560 4.2 4.7 Comparative example 13 I1 87 91 13  9 370 390 3.9 4.8 Comparative example 14 J1 87 90 13 10 420 440 5.7 6.5 Comparative example 15 90 92 10  8 380 390 4.6 4.9 Invention example 16 K1 81 84 19 16 410 440 3.3 3.8 Comparative example 17 88 92 12  8 280 330 3.5 3.9 Invention example 18 L1 87 88 13 12 370 380 2.8 3.5 Invention example 19 86 88 14 12 370 390 2.6 3.1 Comparative example 20 M1 82 85 18 15 310 350 4.1 4.8 Comparative example 21 88 90 12 10 340 370 4.1 4.7 Invention example 22 N1 89 91 11  9 370 390 3.4 4.1 Invention example 23 P1 86 89 14 11 310 350 3.4 3.9 Invention example 24 Q1 87 88 13 12 300 340 3.0 3.5 Invention example 25 R1 88 90 12 10 290 330 2.8 3.2 Invention example 26 S1 88 91 12  9 290 320 2.5 2.9 Invention example 27 T1 87 90 13 10 340 380 3.4 3.6 Invention example 28 U1 91 93  9  7 320 350 2.9 3.3 Invention example **B: bainite phase, F: ferrite phase, M: martensite phase, γ: retained austenite phase

TABLE 4 Mechanical characteristics of hot Hot rolled steel sheet rolled Yield Tensile Total steel stress strength elon- sheet YP TS gation vE−50 No. Steel (Mpa) (Mpa) El (%) (J) Remarks  1 A1 961 1117 12.8 67 Invention example   1′ 965 1119 11.8 62 Invention example  2 809  982 12.9 28 Comparative example  3 820 1012 11.7 31 Comparative example  4 B1 829  983 15.0 46 Invention example  5 877 1028 14.8 53 Invention example  6 C1 842  990 17.3 31 Comparative example  7 D1 999 1157 13.5 36 Comparative example  8 E1 846  988 17.1 28 Comparative example  9 879 1018 16.9 47 Invention example 10 F1 915 1072 16.0 27 Comparative example 11 G1 847  970 17.6 29 Comparative example 12 H1 894 1069 15.7 15 Comparative example 13 I1 862 1011 15.0 30 Comparative example 14 J1 786  928 18.9 19 Comparative example 15 932 1063 17.8 47 Invention example 16 K1 804  993 13.2 18 Comparative example 17 883 1023 15.3 68 Invention example 18 L1 899 1053 13.3 45 Invention example 19 819  978 13.6 34 Comparative example 20 M1 920 1122 12.5 23 Comparative example 21 878 1017 16.2 48 Invention example 22 N1 883 1014 17.8 60 Invention example 23 P1 836  992 15.7 43 Invention example 24 Q1 942 1104 12.6 52 Invention example 25 R1 942 1091 13.3 72 Invention example 26 S1 967 1124 13.7 69 Invention example 27 T1 844  993 15.2 44 Invention example 28 U1 923 1078 14.2 50 Invention example

The hot rolled steel sheets of Invention examples are hot rolled steel sheets having predetermined strength (e.g., TS: 980 MPa or more) and excellent toughness (e.g., vE−50 value: 40 J or more) in combination. Also, the hot rolled steel sheets of Invention examples have predetermined strength and excellent toughness at each of the position at ¼ of sheet thickness and the position at ½ of sheet thickness (sheet thickness center position) and, therefore, are hot rolled steel sheets having good characteristics in the entire region in the sheet thickness direction. On the other hand, the hot rolled steel sheets of Comparative examples out of the preferable scope of the present invention are unable to obtained predetermined strength or are unable to obtained sufficient toughness.

Second Embodiment Hole Expansion Workability

To begin with, reasons for the limitation of the composition of the hot rolled steel sheet according to an embodiment of the present invention will be described. In this regard, the term “%” representing the content of each component element refers to “percent by mass” unless otherwise specified.

C: more than 0.1% and 0.2% or less

C is a beneficial element in an embodiment of the present invention having a function of facilitating formation of bainite and enhancing strength of the steel. In order to obtain such effects, it is advantageous that the C content be more than 0.1%. On the other hand, C bonds to Fe to form cementite, so that if the C content is excessive, the number of cementite grains is increased, the distances between the cementite grains serving as starting points of voids are reduced, the local ductility is degraded, and the hole expansion workability is degraded. Also, if the C content is excessive and, e.g., is more than 0.2%, the weldability is degraded. Consequently, C may be limited to within the range of more than 0.1% and 0.2% or less. In this regard, 0.12% to 0.17% is preferable.

Si: 1.0% or less

Si is an element which contributes to enhancement of strength of the steel through solid solution and which has a function of suppressing generation of coarse cementite and, therefore, is a beneficial element in an embodiment of the present invention. In particular, Si increases the intervals between cementite grains serving as starting points of voids through the function of suppressing generation of coarse cementite and, thereby, contributes to improvement of the local ductility and the hole expansion workability. In order to obtain such effects, the content is desirably 0.1% or more. On the other hand, if the content is more than 1.0%, the surface quality of the steel sheet is degraded significantly, and degradation in the chemical conversion treatability and the corrosion resistance may be caused. Therefore, Si may be limited to 1.0% or less. In this regard, 0.5% to 0.9% is preferable.

Mn: 1.5% to 2.5%

Mn is an element which contributes to enhancement of strength of the steel through solid solution and, in addition, which facilitates formation of a bainite phase through improvement of the hardenability. In order to obtain such effects, it is advantageous that the Mn content be 1.5% or more. On the other hand, if the Mn content is more than 2.5%, center segregation becomes significant, appearances of punched surface of the steel sheet are degraded, and the hole expansion workability may be degraded. Consequently, the amount of Mn may be specified to be within the range of 1.5% to 2.5%. In this regard, the range of 1.7% to 2.2% is preferable.

P: 0.05% or less

P contributes to enhancement of strength of the steel through solid solution but segregates at grain boundaries, in particular prior-austenite grain boundaries, to cause degradation in low-temperature toughness and workability. Consequently, it is preferable that P be minimized, although the content up to 0.05% can be allowable. Therefore, P may be specified to be 0.05% or less. In this regard, 0.03% or less is preferable, and 0.02% or less is further preferable.

S: 0.005% or less

S forms coarse sulfides by bonding to Ti and Mn and degrades the workability. Consequently, it is preferable that S be minimized, although the content up to 0.005% can be allowable. Therefore, S may be limited to 0.005% or less. In this regard, 0.003% or less is preferable, and 0.001% or less is further preferable.

Al: 0.10% or less

Al is an element which functions as a deoxidizing agent and which is effective in improving cleanliness of the steel. In order to obtain such effects, the content is desirably 0.005% or more. On the other hand, if the content is excessive and is, e.g., more than 0.10%, increases in oxide inclusions are caused, an occurrence of flaw is caused and, in addition, the workability of the steel sheet is degraded. Therefore, Al may be limited to 0.10% or less. In this regard, 0.01% to 0.05% is preferable.

N: 0.007% or less

N precipitates as nitrides by bonding to nitride-forming elements and contributes to making crystal grains fine. However, N bonds to Ti at a high temperature to form coarse nitrides easily and serves as a starting point of a void during hole expansion working easily. Consequently, N is preferably minimized in an embodiment of the present invention, although up to 0.007% can be allowable. Therefore, N may be limited to 0.007% or less. In this regard, 0.006% or less is preferable, and 0.005% or less is further preferable.

Ti: 0.07% to 0.2%

Ti contributes to enhancement of strength of the steel through formation of carbonitrides to make crystal grains fine and through precipitation strengthening. Also, Ti forms many fine (Ti,V)C clusters at a temperature range of about 300° C. to 500° C. (coiling temperature), has a function of reducing the amount of cementite in the steel, and is a beneficial element in and embodiment of the present invention. In order to exert such effects, it is advantageous that the content be 0.07% or more. On the other hand, if the content is excessive and is more than 0.2%, the above-described effects are saturated, increases in coarse precipitates are caused, and degradation in the hole expansion workability may be caused. Also, Ti facilitates formation of a ferrite phase, so that a predetermined microstructure cannot be obtained and the hole expansion workability is degraded. Therefore, Ti may be limited to within the range of 0.07% to 0.2%. In this regard, 0.1% to 0.15% is preferable.

V: more than 0.1% and 0.3% or less

V is an element which contributes to enhancement of strength of the steel through formation of carbonitrides to make crystal grains fine and through precipitation strengthening and which also contributes to formation and making fine of bainite phase through an improvement of the hardenability. In addition, V forms many fine (Ti,V)C clusters in a temperature range of about 300° C. to 500° C. (coiling temperature), has a function of reducing the amount of cementite in the steel, and is a beneficial element in an embodiment of the present invention. In order to exert such effects, it is advantageous that the content be more than 0.1%. On the other hand, if the content is excessive and is more than 0.3%, the ductility may be degraded and, in addition, an increase in the cost is caused. Therefore, V may be limited to within the range of more than 0.1% and 0.3% or less. In this regard, 0.13% to 0.27% is preferable and 0.15% to 0.25% is further preferable.

The above-described components are the basic components. In the present invention, besides the basic composition, as necessary, at least one selected from Nb: 0.005% to 0.1%, B: 0.0002% to 0.002%, Cu: 0.005% to 0.3%, Ni: 0.005% to 0.3%, Cr: 0.005% to 0.3%, and Mo: 0.005% to 0.3% and/or one or two selected from Ca: 0.0003% to 0.01% and REM: 0.0003% to 0.01% may be further contained as selective elements.

At least one selected from Nb: 0.005% to 0.1%, B: 0.0002% to 0.002%, Cu: 0.005% to 0.3%, Ni: 0.005% to 0.3%, Cr: 0.005% to 0.3%, and Mo: 0.005% to 0.3%

Each of Nb, B, Cu, Ni, Cr, and Mo is an element which contributes to enhancement of strength of the steel and at least one may be selected and contained, as necessary.

Nb is an element which contributes to enhancement of strength of the steel through formation of carbonitrides. In order to exert such an effect, it is preferable that the content be 0.005% or more. On the other hand, if the content is more than 0.1%, deformation resistance increases, a rolling force of hot rolling increases, a load to a rolling mill becomes too large, rolling operation in itself becomes difficult and, in addition, coarse precipitates are formed, so that degradation in the workability is caused. Consequently, in the case where Nb is contained, Nb may be limited to within the range of preferably 0.005% to 0.1%. In this regard, 0.01% to 0.05% is more preferable and 0.02% to 0.04% is further preferable.

B is an element having functions of segregating at austenite grain boundaries, suppressing formation and growth of ferrite, improving hardenability, contributing to formation and making fine of bainite phase, and enhancing strength of the steel. In order to exert such effects, it is preferable that the content be 0.0002% or more. However, if the content is more than 0.002%, the workability is degraded significantly. Therefore, in the case where B is contained, B may be limited to within the range of preferably 0.0002% to 0.002%. In this regard, 0.0005% to 0.0015% is more preferable.

Cu is an element having functions of enhancing strength of the steel through solid solution and improving hardenability. In particular, Cu lowers the bainite transformation temperature and contributes to making bainite phase fine. In order to obtain such effects, it is preferable that the content be 0.005% or more, although if the content is more than 0.3%, degradation in the surface quality is caused. Therefore, in the case where Cu is contained, Cu may be limited to within the range of preferably 0.005% to 0.3%. In this regard, 0.01% to 0.2% is more preferable.

Ni is an element having functions of enhancing strength of the steel through solid solution, improving hardenability, and facilitating formation of bainite phase. In order to obtain such effects, it is preferable that the content be 0.005% or more. However, if the content is more than 0.3%, a martensite phase is generated easily, and the hole expansion workability is degraded significantly. Therefore, in the case where Ni is contained, Ni may be limited to within the range of preferably 0.005% to 0.3%. In this regard, 0.01% to 0.2% is more preferable.

Cr is an element which forms carbides and contributes to enhancement of strength of the steel. In order to exert such effects, it is preferable that the content be 0.005% or more. On the other hand, if the content is excessive and is more than 0.3%, the corrosion resistance of the steel is degraded. Therefore, in the case where Cr is contained, Cr is limited to within the range of preferably 0.005% to 0.3%. In this regard, 0.01% to 0.2% is more preferable.

Mo is an element having functions of improving hardenability, facilitating formation of bainite phase, and enhancing strength of the steel. In order to obtain such effects, it is preferable that the content be 0.005% or more. However, if the content is more than 0.3%, a martensite phase may be generated easily, and the hole expansion workability is degraded significantly. Therefore, in the case where Mo is contained, Mo may be limited to within the range of preferably 0.005% to 0.3%. In this regard, 0.01% to 0.2% is more preferable.

One or two selected from Ca: 0.0003% to 0.01% and REM: 0.0003% to 0.01%

Each of Ca and REM is an element which contributes to improvement of the hole expansion workability through shape control of inclusions and one or two may be selected and contained, as necessary.

Ca is an element which controls the shape of inclusions and which contributes to improvement of the hole expansion workability effectively. In order to exert such effects, it is advantageous that the content be 0.0003% or more. On the other hand, if the content is excessive and is more than 0.01%, the amount of inclusions increases and many surface defects are caused. Therefore, in the case where Ca is contained, Ca is limited to within the range of preferably 0.0003% to 0.01%.

As with Ca, REM is an element which controls the shape of sulfide inclusions to improve adverse influences of sulfide inclusions on the hole expansion workability and, thereby, contributes to improvement of the hole expansion workability. In order to exert such effects, it is advantageous that the content be 0.0003% or more. On the other hand, if the content is excessive and is more than 0.01%, the amount of inclusions increases, the cleanliness of the steel is degraded, and the hole expansion workability may be degraded. Therefore, in the case where REM is contained, REM may be limited to within the range of preferably 0.0003% to 0.01%.

The balance other than those described above is composed of Fe and incidental impurities. In this regard, as for the incidental impurities, O (oxygen): 0.005% or less, W: 0.1% or less, Ta: 0.1% or less, Co: 0.1% or less, Sb: 0.1% or less, Sn: 0.1% or less, Zr: 0.1% or less, and the like can be allowable.

Next, reasons for the limitation of the microstructure of the hot rolled steel sheet according to an aspect of the present invention will be described.

In the hot rolled steel sheet according to an embodiment of the present invention, the primary phase is specified to be a bainite phase. Here, the term “primary phase” refers to a phase having an area fraction of 90% or more. If a phase other than the bainite phase is specified to be the primary phase, predetermined high strength and good hole expansion workability cannot be obtained stably. Consequently, the primary phase may be specified to be bainite phase having an area fraction of 90% or more. In this regard, 92% or more is preferable, and 95% or more is more preferable.

The remainder other than the bainite phase serving as the primary phase may be at least one selected from martensite phase, austenite phase (retained austenite phase), and ferrite phase. The phases of the reminder other than the primary phase are specified to be 10% or less in total (including 0%) on an area fraction basis. If the phases of the reminder other than the bainite phase are more than 10%, predetermined high strength and good hole expansion workability may not be obtained stably. In particular, if the martensite phase increases, predetermined good hole expansion workability cannot be obtained stably.

The hot rolled steel sheet according to an embodiment of the present invention has the above-described microstructure, where the microstructure shows that cementite is dispersed in the microstructure. Cementite is present while being dispersed mainly in the bainite phase, although may be present in the phases other than bainite or at the phase boundaries. In the hot rolled steel sheet according to an embodiment of the present invention, cementite dispersed in the microstructure is specified to be 0.8% or less on a percent by mass basis and the average grain size is specified to be 150 nm or less.

In the case where a large amount of cementite is dispersed in the microstructure, where the proportion is more than 0.8% on a percent by mass basis, the number of dispersed cementite grains increases, voids started from cementite are connected easily during working, the local ductility is degraded, and the hole expansion workability may be degraded. Consequently, cementite may be limited to 0.8% or less on a percent by mass basis. In this regard, 0.6% or less is preferable, and 0.5% or less is more preferable.

Also, in the case where cementite is coarsened and the average grain size is more than 150 nm, coarse voids started from cementite are generated easily during working, and the hole expansion workability may be degraded. Consequently, the average grain size of cementite may be limited to 150 nm or less. In this regard, 130 nm or less is preferable, and 110 nm or less is further preferable.

Next, a preferable method for manufacturing the hot rolled steel sheet according to an aspect of the present invention will be described.

In an aspect of the present invention, a hot rolled steel sheet is produced through the steps of heating a steel, applying hot rolling having rough rolling and finish rolling, performing cooling composed of two stages of first stage cooling and second stage cooling, and performing coiling.

The method for manufacturing a steel serving as a starting material is not necessarily particularly limited, and any common manufacturing method can be applied, wherein a molten steel having the above-described composition is refined by a common refining method, e.g., a converter, and a steel, e.g., a slab, is produced by a common casting method, e.g., a continuous casting method. In this regard, an ingot-making and blooming method may be employed without problem.

Meanwhile, in an embodiment of the present invention, electro-magnetic stirrer (EMS), intentional bulging soft reduction casting (IBSR), and the like can be applied to reduce component segregation of the steel during continuous casting. Equiaxial crystals are formed in the sheet thickness center portion by applying an electro-magnetic stirrer treatment, so that segregation can be reduced. Also, in the case where the intentional bulging soft reduction casting is applied, segregation in the sheet thickness center portion can be reduced by preventing flowing of the molten steel in an unsolidified portion of the continuous casting slab. The elongation and the hole expansion workability in tensile characteristics described below can be brought to a more excellent level by applying at least one of these segregation reduction treatments.

Initially, the resulting steel may be heated to heating temperature: 1,200° C. or higher.

Heating temperature: 1,200° C. or higher

Carbonitride-forming elements, e.g., Ti, are contained in the steel employed in an embodiment of the present invention. Most of these carbonitride-forming elements are present as coarse carbonitrides (precipitates). In this regard, the presence of coarse carbonitride-forming elements, e.g., Ti, which remain coarse precipitates, causes reduction in the amount of fine precipitates, which contribute to solute strengthening. Consequently, the steel sheet strength is reduced. In order to allow these coarse precipitates to form solid solutions before hot rolling, the heating temperature may be limited to 1,200° C. or higher. In this regard, 1,220° C. to 1,350° C. is preferable.

Subsequently, the heated steel may be subjected to hot rolling composed of rough rolling and finish rolling.

The condition of the rough rolling is not specifically limited insofar as predetermined sheet bar dimensions are ensured. Following the rough rolling, the finish rolling with finishing temperature: 850° C. to 950° C. may be applied. In this regard, as a matter of course, descaling is performed before the finish rolling or between finish rolling stands during rolling.

Finishing temperature: 850° C. to 950° C.

If the finishing temperature is lower than 850° C., finish rolling is rolling in two-phase region of ferrite+austenite, so that a deformation microstructure remains after rolling and the hole expansion workability may be degraded. On the other hand, if the finishing temperature is high and is higher than 950° C., austenite grains grow, and a bainite phase of the hot rolled sheet obtained after cooling is coarsened. Consequently, the hole expansion workability may be degraded. Therefore, the finishing temperature may be limited to within the range of 850° C. to 950° C. In this regard, 870° C. to 930° C. is preferable. Here, the term “finishing temperature” refers to the surface temperature.

After the finish rolling is completed, cooling composed of two stages of first stage cooling and second stage cooling may be applied.

In the first stage cooling, cooling may be started within 1.5 s of, preferably just after, completion of the finish rolling, and cooling to a first stage cooling stop temperature of 500° C. to 600° C. may be performed at an average cooling rate of 20° C./s to 80° C./s.

If the time until cooling of the first stage cooling is started is long and is more than 1.5 s, austenite grains become coarse and a bainite phase may be coarsened. Also, if austenite grains become coarse, the hardenability of the steel sheet increases and a martensite phase is generated easily, so that predetermined excellent hole expansion workability cannot be obtained. Therefore, the cooling start time of the first stage cooling may be limited to within 1.5 s of completion of the finish rolling.

Meanwhile, if the average cooling rate of the first stage cooling is less than 20° C./s and, therefore, cooling becomes slow, formation of ferrite or coarse bainite is facilitated, and predetermined high strength or hole expansion workability may not be obtained. On the other hand, if quenching is performed at more than 80° C./s, martensite is generated easily to become hard, and the hole expansion workability may be degraded. Consequently, the average cooling rate of the first stage cooling may be limited to within the range of 20° C./s to 80° C./s. In this regard, 25° C./s to 60° C./s is preferable.

Meanwhile, if the first stage cooling stop temperature is lower than 500° C., a transition boiling region is reached, variations in steel sheet temperature increase, the microstructure becomes heterogeneous, and predetermined excellent hole expansion workability may not be obtained. On the other hand, if the first stage cooling stop temperature is a high temperature higher than 600° C., ferrite transformation is facilitated, and predetermined high strength may not be obtained. Consequently, the first stage cooling stop temperature may be limited to 500° C. to 600° C. In this regard, 520° C. to 580° C. is preferable.

The second stage cooling may be started just after or within 3 s of, preferably just after, completion of the first stage cooling, and cooling to a second stage cooling stop temperature of 330° C. to 470° C. may be performed at an average cooling rate of 90° C./s or more.

If the time until cooling of the second stage cooling is started is long and is more than 3 s, ferrite transformation is started and predetermined high strength may not be obtained. Therefore, the cooling start time of the second stage cooling may be limited to within 3 s of completion of the first stage cooling.

Meanwhile, if the average cooling rate of the second stage cooling is less than 90° C./s, generated bainite is coarsened, and predetermined hole expansion workability may not be obtained. Consequently, the average cooling rate of the second stage cooling may be limited to 90° C./s or more. In this regard, the upper limit of the average cooling rate of the second stage cooling is not specifically limited, although the upper limit may be about 250° C./s in association with the sheet thickness of a sheet to be cooled and the capability of cooling equipment. In this regard, 100° C./s to 200° C./s is preferable.

Meanwhile, if the second stage cooling stop temperature is lower than 330° C., hard martensite phase and retained austenite phase are formed in the steel sheet microstructure, a predetermined microstructure may not be obtained, and the hole expansion workability may be degraded. On the other hand, if the second stage cooling stop temperature is a high temperature higher than 470° C., a ferrite phase and a martensite phase increase in the steel sheet microstructure, predetermined microstructure cannot be obtained, and the hole expansion workability may be degraded significantly. Consequently, the second stage cooling stop temperature may be limited to 330° C. to 470° C. In this regard, 350° C. to 450° C. is preferable.

After cooling to the second stage cooling stop temperature is performed, hot rolled steel sheet (steel strip in coil) may be produced by performing coiling into the shape of a coil, where a coiling temperature is specified to be the second stage cooling stop temperature.

In this regard, the above-described temperature refers to a steel sheet surface temperature.

In this regard, after the coiling, the hot rolled steel sheet may further be subjected to temper rolling following the common method. Also, the resulting hot rolled steel sheet may be subjected to pickling to remove scale formed on the surface. Alternatively, after the pickling, a galvanization process, e.g., hot dip galvanizing or electrogalvanizing, and a chemical conversion treatment may further be applied.

Example 2

A molten steel having the composition shown in Table 5 was refined in a converter, and a slab (steel) was produced by a continuous casting method. In the continuous casting, those other than Hot rolled steel sheet No. 1′ of Steel A2 in Tables 5 to 7B described later were subjected to electro-magnetic stirrer (EMS) for the purpose of segregation reduction treatment of the components. Subsequently, these steels were heated under the conditions shown in Tables 6A and 6B, and were subjected to hot rolling composed of rough rolling and finish rolling under the conditions shown in Tables 6A and 6B. After the finish rolling was completed, cooling was performed under the conditions shown in Tables 6A and 6B, and coiling was performed at coiling temperatures shown in Tables 6A and 6B, so that hot rolled steel sheets having sheet thicknesses shown in Tables 6A and 6B were produced. Cooling of some hot rolled steel sheets were specified to be single stage cooling.

Test pieces were taken from the resulting hot rolled steel sheets, and microstructure observation, a tensile test, and a hole expanding test were performed. The testing methods were as described below.

(1) Microstructure Observation

A test piece for a microstructure observation was taken from the resulting hot rolled steel sheet, a sheet thickness cross-section parallel to the rolling direction was polished, and the microstructure was allowed to appear with a corrosive liquid (3% nital solution). The microstructure of the position at one-quarter of the sheet thickness was observed with a scanning electron microscope (SEM), and photographs of the microstructure were taken in three fields of view (magnification: 3,000 times). The microstructure fraction (area fraction) of each phase was calculated on the basis of identification of the microstructure and image analysis.

A test piece (size: 10 mm×15 mm) for replica was taken from the position at one-quarter of the sheet thickness of the resulting hot rolled steel sheet, a replica film was produced by a two-stage replica method, and cementite was taken. The resulting cementite was observed with a transmission electron microscope (TEM), and photographs were taken in five fields of view (magnification: 50,000 times). The grain size of each cementite was determined and the average grain size of cementite of the steel sheet concerned was determined by averaging. In this regard, in the case of cementite having an aspect ratio, the average value of the long axis length and the short axis length was specified to be the grain size of the cementite concerned.

A test piece (size: t×50×100 mm) for electrolytic residue extraction was taken from the resulting hot rolled steel sheet. The total thickness of the test piece was subjected to constant-current electrolysis in a 10 vol % AA electrolyte (10 vol % acetylacetone-1 mass % tetramethylammonium chloride methanol) at current density: 20 mA/cm2. The resulting electrolyte was filtrated and the electrolytic residue remaining on the filter paper was analyzed with an inductively-coupled plasma spectrophotometric analyzer to measure the amount of Fe in the electrolytic residue. It was assumed that quantified Fe was entirely Fe3C, and the amount of precipitated cementite was calculated on the basis of the following formula.


Fe3C (percent by mass)=(1.0716×[quantified Fe(g)])/[electrolyzed weight (g)]×100

In this regard, the atomic weight of Fe was specified to be 55.85 (g/mol) and the atomic weight of C was specified to be 12.01 (g/mol). Meanwhile, the electrolyzed weight was determined by cleaning the test piece for electrolysis after the electrolysis, measuring the weight, and subtracting the resulting weight from the test piece weight before electrolysis.

(2) Tensile Test

JIS No. 5 test pieces (GL: 50 mm) were taken from the resulting hot rolled steel sheet in such a way that the tensile direction and the rolling direction forma right angle. A tensile test was performed in conformity with JIS Z 2241 and the yield strength (yield point) YP, the tensile strength TS, and the elongation El were determined.

(3) Hole Expanding Test

A test piece (size: t×100×100 mm) for hole expanding test was taken from the resulting hot rolled steel sheet. In conformity with The Japan Iron and Steel Federation Standards JFST 1001, a punched hole was punched in the center of the test piece with a 10 mmφ punch, where clearance: 12.5% of sheet thickness. Thereafter, a 60° cone punch was inserted into the punched hole along the punching direction in such a way as to be pushed upward, and a hole diameter d mm at the point in time when a crack penetrated the sheet thickness was determined, and the hole expanding ratio k (%) defined by the following formula


λ(%)={(d−10)/10}×100

was calculated.

Also, a test piece (size: t×100×100 mm) for hole expanding test was taken from the resulting hot rolled steel sheet. A punched hole was punched in the center of the test piece with a 10 mmφ punch, where clearance: 25.0% of sheet thickness. Thereafter, a 60° cone punch was inserted into the punched hole along the punching direction in such a way as to be pushed upward, and a hole diameter d mm at the point in time when a crack penetrated the sheet thickness was determined, and the hole expanding ratio λ(%) was calculated by the above-described formula. In this regard, the clearance refers to the proportion (%) relative to the sheet thickness.

Then, the case where λ obtained by the hole expanding test performed with respect to the punched hole punched with a clearance of 12.5% was 60% or more and λ obtained by the hole expanding test performed with respect to the punched hole punched with a clearance of 25.0% was 40% or more was evaluated as good hole expansion workability.

The obtained results are shown in Tables 7A and 7B.

TABLE 5 Steel Chemical component (percent by mass) No. C Si Mn P S Al N Ti V Nb, B, Cu, Ni, Cr, Mo REM, Ca Remarks A2 0.11 0.5 2.4 0.014 0.0018 0.055 0.0031 0.15 0.24 — — Adaptation example B2 0.15 0.7 2.0 0.019 0.0014 0.087 0.0055 0.12 0.20 — — Adaptation example C2 0.18 0.8 2.2 0.019 0.0008 0.025 0.0062 0.10 0.15 — — Adaptation example D2 0.19 0.9 2.4 0.023 0.0010 0.079 0.0057 0.11 0.30 — — Adaptation example E2 0.15 0.2 2.3 0.015 0.0008 0.019 0.0065 0.15 0.25 — — Adaptation example F2 0.14 0.5 1.9 0.014 0.0039 0.037 0.0027 0.09 0.25 — — Adaptation example G2 0.12 0.7 2.0 0.008 0.0026 0.034 0.0056 0.15 0.20 — — Adaptation example H2 0.11 0.9 1.6 0.013 0.0015 0.075 0.0039 0.18 0.11 — — Adaptation example I2 0.13 0.6 1.6 0.013 0.0004 0.013 0.0022 0.17 0.15 — — Adaptation example J2 0.14 0.6 1.8 0.014 0.0021 0.043 0.0049 0.14 0.17 — — Adaptation example K2 0.13 0.5 1.9 0.011 0.0013 0.030 0.0043 0.08 0.20 Nb: 0.02 — Adaptation example L2 0.16 0.8 2.3 0.045 0.0011 0.031 0.0028 0.09 0.11 B: 0.0005 — Adaptation example M2 0.11 0.7 2.4 0.012 0.0009 0.045 0.0032 0.12 0.15 Ni: 0.2, Cr: 0.2 — Adaptation example N2 0.12 0.4 1.7 0.008 0.0020 0.024 0.0048 0.08 0.15 Nb: 0.01, Mo: 0.2 — Adaptation example O2 0.11 0.1 2.2 0.007 0.0014 0.021 0.0010 0.10 0.12 B: 0.0007 — Adaptation example P2 0.14 0.3 1.8 0.018 0.0008 0.055 0.0053 0.12 0.20 — Ca: 0.005 Adaptation example Q2 0.16 0.7 2.1 0.004 0.0028 0.035 0.0018 0.15 0.15 — REM: 0.005 Adaptation example R2 0.15 0.5 1.9 0.025 0.0023 0.041 0.0029 0.11 0.22 Cu: 0.1 — Adaptation example S2 0.19 0.9 2.4 0.015 0.0027 0.031 0.0063 0.11 0.30 — — Adaptation example T2 0.15 0.7 2.3 0.015 0.0019 0.052 0.0049 0.11 0.15 B: 0.001 — Adaptation example U2 0.05 1.3 2.4 0.019 0.0021 0.041 0.0033 0.16 — Nb: 04 — Comparative example V2 0.22 0.5 2.8 0.018 0.0014 0.029 0.0046 0.10 0.05 Nb: 0.02 — Comparative example W2 0.15 0.1 2.5 0.010 0.0090 0.035 0.0072 0.07 0.05 Cr: 0.2/Mo: 0.2 — Comparative example X2 0.12 0.3 2.1 0.037 0.0011 0.058 0.0052 0.09 0.11 — — Adaptation example Y2 0.05 0.5 1.9 0.016 0.0031 0.017 0.0041 0.15 — — — Comparative example Z2 0.14 0.7 1.2 0.016 0.0020 0.028 0.0047 0.09 0.30 — — Comparative example AA2 0.11 0.8 1.7 0.012 0.0016 0.020 0.0038 0.25 0.15 — — Comparative example AB2 0.14 0.7 2.3 0.016 0.0014 0.034 0.0035 0.05 0.20 — — Comparative example

TABLE 6A Production condition First stage cooling Second stage cooling Steel Heating Finishing Cooling Average Cooling stop Cooling Average Cooling stop sheet Steel temperature temperature start time* cooling rate temperature start cooling rate temperature No. No. (° C.) (° C.) (s) (° C./s) (° C.) time** (s) (° C./s) (° C.)  1 A2 1240 920 0.5 25 530 1.0 110 370   1′ A2 1240 920 0.6 25 520 1.0 110 370  2 A2 1230 910 0.5 30 560 2.5 150 420  3 A2 1210 900 1.5 35 540 3.0 100 310  4 B2 1230 910 0.5 45 560 3.0 100 410  5 B2 1240 920 0.0 25 540 1.5 160 370  6 B2 1240 890 1.0 30 560 2.5 100 440  7 C2 1220 910 0.5 25 570 2.0 140 360  8 C2 1260 910 0.0 20 560 0.0  95 430  9 D2 1250 930 0.5 35 550 3.0 100 370 10 D2 1240 910 1.0 30 570 2.5  95 440 11 E2 1230 900 1.5 45 540 2.5 120 380 12 E2 1250 920 0.0 25 550 1.5 110 430 13 F2 1270 910 0.5 25 540 1.5 110 490 14 F2 1260 890 1.0 25 540 1.5 100 380 15 G2 1220 890 1.0 25 540 1.5 130 380 16 G2 1250 930 0.5 30 540 2.0 100 430 17 H2 1270 910 0.0 25 570 2.0 120 350 18 H2 1250 920 0.5 30 570 2.5 100 380 19 H2 1230 900 1.0 10 560 0.0  95 420 20 I2 1210 920 1.0 30 540 2.0  95 370 21 I2 1210 910 0.5 25 530 1.0  10 440 22 J2 1270 920 0.5 30 540 2.0  90 380 23 J2 1260 910 1.0 30 560 2.5 120 440 24 K2 1250 920 0.5 25 550 1.5 120 360 25 K2 1250 900 0.0 25 550 1.5 100 410 26 L2 1220 900 0.5 25 540 1.5 110 360 Production condition Steel sheet Coiling temperature No. (° C.) Sheet thickness (mm) Remarks  1 370 2.9 Invention example   1′ 370 2.9 Invention example  2 420 2.0 Invention example  3 310 3.6 Comparative example  4 410 3.6 Invention example  5 370 2.0 Invention example  6 440 3.6 Invention example  7 360 2.3 Invention example  8 430 4.0 Invention example  9 370 3.6 Invention example 10 440 4.0 Invention example 11 380 2.6 Invention example 12 430 2.9 Invention example 13 490 2.9 Comparative example 14 380 3.6 Invention example 15 380 2.3 Invention example 16 430 2.9 Invention example 17 350 2.6 Invention example 18 380 3.6 Invention example 19 420 4.0 Comparative example 20 370 4.0 Invention example 21 440 4.0 Comparative example 22 380 4.0 Invention example 23 440 2.6 Invention example 24 360 2.6 Invention example 25 410 3.6 Invention example 26 360 2.9 Invention example *time from finish rolling completion **time from first stage cooling completion

TABLE 6B Production condition First stage cooling Second stage cooling Heating Finishing Cooling Average Cooling stop Cooling Average Cooling stop Steel Steel temperature temperature start time* cooling rate temperature start cooling rate temperature sheet No. No. (° C.) (° C.) (s) (° C./s) (° C.) time** (s) (° C./s) (° C.) 27 L2 1210 910 1.0 30 550 2.0  95 410 28 M2 1270 890 0.5 50 550 2.0  95 380 29 M2 1210 900 1.0 30 560 2.5 150 420 30 M2 1220 920 0.5 25 470 0.5 100 360 31 N2 1210 890 0.5 25 540 1.5 110 350 32 N2 1250 970 0.5 25 560 1.0 130 440 33 O2 1270 890 0.5 25 540 1.5 120 360 34 O2 1270 910 1.0 30 550 2.0 100 370 35 P2 1240 910 0.5 25 560 1.5 120 360 36 P2 1240 830 0.5 25 530 2.0 100 420 37 Q2 1220 910 1.0 30 540 2.0 100 380 38 Q2 1210 920 0.5 25 530 0.5 110 360 39 R2 1220 900 1.0 30 560 2.5 100 360 40 R2 1150 910 0.5 25 560 1.5  95 380 41 R2 1220 870 1.0 30 550 2.0 110 440 42 S2 1220 910 1.0 25 560 1.5  95 400 43 S2 1250 920 0.5 30 540 2.0 100 420 44 T2 1220 920 0.0 25 540 1.5 120 360 45 T2 1250 910 1.0 30 550 2.0 110 420 46 U2 1220 910 1.5 40 — —  40 480 47 V2 1240 890 1.0 30 560 2.5 120 440 48 W2 1200 900 0.5 25 — —  25 400 49 X2 1260 920 0.5 25 530 1.0 110 300 50 Y2 1250 880 1.0 30 540 2.5 100 420 51 Z2 1250 910 0.5 25 540 1.5 100 420 52 AA2 1220 920 0.5 25 540 1.0 110 400 53 AB2 1250 910 1.0 30 540 2.0 110 420 54 C2 1250 920 3.0 30 550 1.5 100 400 55 I2 1250 890 0.5 30 540 5.0 100 380 Production condition Steel Coiling temperature sheet No. (° C.) Sheet thickness (mm) Remarks 27 410 3.9 Invention example 28 380 3.9 Invention example 29 420 2.0 Invention example 30 360 3.6 Comparative example 31 350 3.2 Invention example 32 440 2.3 Comparative example 33 360 2.6 Invention example 34 370 3.6 Invention example 35 360 2.6 Invention example 36 420 3.6 Comparative example 37 380 3.6 Invention example 38 360 3.2 Invention example 39 360 3.6 Invention example 40 380 4.0 Comparative example 41 440 2.9 Invention example 42 400 4.0 Invention example 43 420 2.9 Invention example 44 360 2.6 Invention example 45 420 2.9 Invention example 46 480 3.6 Comparative example 47 440 2.6 Comparative example 48 400 4.0 Comparative example 49 300 3.2 Comparative example 50 420 3.6 Comparative example 51 420 3.6 Comparative example 52 400 3.2 Comparative example 53 420 2.9 Comparative example 54 400 3.2 Comparative example 55 380 3.2 Comparative example *time from finish rolling completion **time from first stage cooling completion

TABLE 7A Microstructure Steel Bainite Cementite Amount of Tensile characteristics sheet Steel phase area average grain cementite (percent Yield strength Tensile strength Elongation EI No. No. Type* fraction (%) size (nm) by mass) YP (MPa) TS (MPa) (%)  1 A2 B + M 94 101 0.51 954 1094 14.3   1′ A2 B + M 95 104 0.52 956 1097 13.4  2 A2 B + M 92 123 0.68 911 1069 16.6  3 A2 B + M 88  74 0.27 922 1124 12.6  4 B2 B + M 93 125 0.67 871 1018 16.9  5 B2 B 100  107 0.51 913 1038 14.2  6 B2 B + M 93 138 0.74 863 1003 16.7  7 C2 B + M 95 110 0.49 891 1017 14.9  8 C2 B + M 92 141 0.76 832  982 18.2  9 D2 B + M 94  92 0.44 966 1108 14.2 10 D2 B + M 91 123 0.72 905 1073 17.4 11 E2 B + M 94  89 0.53 957 1103 14.6 12 E2 B + M 92 111 0.73 914 1078 16.9 13 F2 B + M + F 88 173 0.99 779  927 19.8 14 F2 B + M 94 104 0.55 852  982 16.2 15 G2 B + M 95 102 0.47 950 1084 14.1 16 G2 B + M 92 134 0.75 890 1049 17.3 17 H2 B + M 95 111 0.46 966 1097 13.5 18 H2 B + M 94 125 0.58 939 1082 14.9 19 H2 B + M + F 85 143 0.74 892 1062 18.6 20 12 B + M 94 114 0.55 925 1061 14.7 21 12 B + F + P 79 142 0.89 872 1026 19.5 22 J2 B + M 94 116 0.58 894 1030 15.5 23 J2 B + M 91 143 0.69 844 1000 18.3 24 K2 B + M 95 102 0.49 877 1001 15.2 25 K2 B + M 93 125 0.69 841  983 17.5 26 L2 B + M 95 116 0.51 909 1038 14.6 Hole expansion workability Hole expanding ratio λ (%) Steel sheet No. Clearance: 12.5% Clearance: 25% Remarks  1 75 59 Invention example   1′ 65 48 Invention example  2 63 43 Invention example  3 45 28 Comparative example  4 62 46 Invention example  5 77 64 Invention example  6 64 42 Invention example  7 73 60 Invention example  8 65 44 Invention example  9 78 64 Invention example 10 63 41 Invention example 11 71 57 Invention example 12 64 48 Invention example 13 61 33 Comparative example 14 67 53 Invention example 15 67 53 Invention example 16 61 45 Invention example 17 71 58 Invention example 18 66 52 Invention example 19 48 32 Comparative example 20 72 58 Invention example 21 51 29 Comparative example 22 71 57 Invention example 23 63 47 Invention example 24 80 66 Invention example 25 64 45 Invention example 26 67 54 Invention example *B: bainite, F: ferrite, P: pearlite, M: martensite, θ: cementite, γ: retained austenite

TABLE 7B Microstructure Steel Bainite phase Cementite Amount of Tensile characteristics sheet Steel area fraction average grain cementite Yield strength Tensile strength Elongation EI No. No. Type* (%) size (nm) (percent by mass) YP (MPa) TS (MPa) (%) 27 L2 B + M + F 91 132 0.72 842  998 18.3 28 M2 B + M 94 119 0.58 997 1148 14.1 29 M2 B + M 95 137 0.71 993 1128 13.2 30 M2 B + M + F 87 110 0.50 984 1158 17.0 31 N2 B + M 95 105 0.49 879  999 14.7 32 N2 B + M 95 167 0.91 840  954 18.3 33 O2 B + M 95 117 0.58 865  987 15.3 34 O2 B + M 94 122 0.62 856  982 15.8 35 P2 B 100  102 0.51 896 1004 15.2 36 P2 B + M + F 82 129 0.75 845  982 17.3 37 Q2 B + M 94 119 0.58 924 1064 15.1 38 Q2 B + M 95 110 0.50 941 1074 14.2 39 R2 B + M 95  99 0.48 912 1041 14.6 40 R2 B + M 94 108 0.56 832  961 16.5 41 R2 B + M + F + γ 90 135 0.76 845 1001 18.3 42 S2 B + M 95  98 0.41 1082 1230 12.2 43 S2 B + M 95 107 0.53 1052 1195 12.5 44 T2 B + M 95 110 0.22 1063 1208 12.4 45 T2 B + M 95 137 0.58 1045 1188 12.5 46 U2 B + M + F 95 186 0.99 898 1020 14.3 47 V2 B + M 91 161 0.89 757  920 18.5 48 W2 B + M + F 85 143 0.71 815  980 19.0 49 X2 B + M 88  90 0.32 830 1024 13.6 50 Y2 B + M + F 92 159 0.93 699  910 18.2 51 Z2 B + M + F 80 114 0.55 843  980 20.1 52 AA2 B + F 87 128 0.46 853  992 12.3 53 AB2 B + M + F 92 134 0.82 820  952 17.2 54 C2 B + M 85 182 0.85 932 1048 12.9 55 I2 B + M + F 87 135 0.42 821  962 17.5 Hole expansion workability Hole expanding ratio λ (%) Steel sheet No. Clearance: 12.5% Clearance: 25% Remarks 27 62 44 Invention example 28 67 43 Invention example 29 65 42 Invention example 30 46 31 Comparative example 31 70 57 Invention example 32 55 29 Comparative example 33 65 52 Invention example 34 69 55 Invention example 35 76 63 Invention example 36 39 29 Comparative example 37 66 52 Invention example 38 74 61 Invention example 39 72 59 Invention example 40 42 33 Comparative example 41 63 42 Invention example 42 69 56 Invention example 43 62 56 Invention example 44 64 52 Invention example 45 63 43 Invention example 46 74 35 Comparative example 47 60 34 Comparative example 48 62 36 Comparative example 49 48 27 Comparative example 50 67 33 Comparative example 51 51 29 Comparative example 52 43 27 Comparative example 53 52 35 Comparative example 54 39 28 Comparative example 55 42 31 Comparative example *B: bainite, F: ferrite, P: pearlite, M: martensite, θ: cementite, γ: retained austenite

All Invention examples are high strength hot rolled steel sheets having high strength of tensile strength: 980 MPa or more and excellent hole expansion workability. On the other hand, Comparative examples out of the preferred scope of the present invention are unable to obtain predetermined tensile strength or exhibit degraded hole expansion workability.

Claims

1. A high strength hot rolled steel sheet having a tensile strength TS of 980 MPa or more, comprising a composition and a microstructure,

the composition containing, on a percent by mass basis,
C: 0.05% or more and 0.18% or less, Si: 1.0% or less,
Mn: 1.0% or more and 3.5% or less, P: 0.04% or less,
S: 0.006% or less, Al: 0.10% or less,
N: 0.008% or less, Ti: 0.05% or more and 0.20% or less,
V: more than 0.1% and 0.3% or less, and
the balance being Fe and incidental impurities, and
the microstructure comprising a primary phase and a secondary phase,
the primary phase being a bainite phase having an area fraction of more than 85%,
the secondary phase being at least one of ferrite phase, martensite phase, and retained austenite phase, the secondary phase having an area fraction of 0% or more and less than 15% in total,
the bainite phase having an average lath interval of laths of 400 nm or less, and the laths having an average long axis length of 5.0 μm or less.

2. The high strength hot rolled steel sheet according to claim 1, wherein the composition further contains, on a percent by mass basis, at least one selected from Nb: 0.005% or more and 0.4% or less, B: 0.0002% or more and 0.0020% or less, Cu: 0.005% or more and 0.2% or less, Ni: 0.005% or more and 0.2% or less, Cr: 0.005% or more and 0.4% or less, and Mo: 0.005% or more and 0.4% or less.

3. The high strength hot rolled steel sheet according to claim 1, wherein the composition further contains, on a percent by mass basis, at least one selected from Ca: 0.0002% or more and 0.01% or less and REM: 0.0002% or more and 0.01% or less.

4. A method for manufacturing a high strength hot rolled steel sheet, comprising:

heating a steel material having the composition according claim 1 to 1,200° C. or higher,
applying hot rolling having rough rolling and finish rolling, the finish rolling having an accumulated rolling reduction of 50% or more in a temperature range of 1,000° C. or lower and a finishing temperature of 820° C. or higher and 930° C. or lower,
starting cooling within 4.0 s after the hot rolling,
performing cooling at an average cooling rate of 20° C./s or more, and
performing coiling at a coiling temperature of 300° C. or higher and 450° C. or lower.

5. A high strength hot rolled steel sheet having a tensile strength TS of 980 MPa or more, comprising a composition and a microstructure,

the composition containing, on a percent by mass basis,
C: more than 0.1% and 0.2% or less, Si: 1.0% or less,
Mn: 1.5% to 2.5%, P: 0.05% or less,
S: 0.005% or less, Al: 0.10% or less,
N: 0.007% or less, Ti: 0.07% to 0.2%,
V: more than 0.1% and 0.3% or less, and
the balance being Fe and incidental impurities, the microstructure comprising a primary phase and the remainder other than the primary phase,
the primary phase being a bainite phase having an area fraction of 90% or more,
the remainder being at least one selected from martensite phase, austenite phase and ferrite phase and having an area fraction of 10% or less, and
cementite dispersed in the microstructure having a mass percent of 0.8% or less and an average grain size of 150 nm or less.

6. The high strength hot rolled steel sheet according to claim 5, wherein the composition further contains, on a percent by mass basis, at least one selected from Nb: 0.005% to 0.1%, B: 0.0002% to 0.002%, Cu: 0.005% to 0.3%, Ni: 0.005% to 0.3%, Cr: 0.005% to 0.3%, and Mo: 0.005% to 0.3%.

7. The high strength hot rolled steel sheet according to claim 5, wherein the composition further contains, on a percent by mass basis, at least one selected from Ca: 0.0003% to 0.01% and REM: 0.0003% to 0.01%.

8. A method for manufacturing a high strength hot rolled steel sheet comprising: heating a steel material, applying hot rolling having rough rolling and finish rolling, applying cooling having two stages of first stage cooling and second stage cooling, and performing coiling to produce a hot rolled steel sheet,

wherein
the steel material is specified to be a steel material having a composition containing, on a percent by mass basis,
C: more than 0.1% and 0.2% or less, Si: 1.0% or less,
Mn: 1.5% to 2.5%, P: 0.05% or less,
S: 0.005% or less, Al: 0.10% or less,
N: 0.007% or less, Ti: 0.07% to 0.2%,
V: more than 0.1% and 0.3% or less, and
the balance being Fe and incidental impurities,
the heating is a treatment to heat the steel material to 1,200° C. or higher,
the finish rolling is rolling with finishing temperature: 850° C. to 950° C.,
the first stage cooling is cooling in which cooling is started within 1.5 s of completion of the finish rolling and cooling to a first stage cooling stop temperature of 500° C. to 600° C. is performed at an average cooling rate of 20° C./s to 80° C./s,
the second stage cooling is cooling in which cooling to a second stage cooling stop temperature of 330° C. to 470° C. is performed at an average cooling rate of 90° C./s or more within 3 s of completion of the first stage cooling, and
after completion of the second stage cooling, coiling is performed, where the coiling temperature is the second stage cooling stop temperature.

9. The method for manufacturing a high strength hot rolled steel sheet, according to claim 8, wherein the composition further contains, on a percent by mass basis, at least one selected from Nb: 0.005% to 0.1%, B: 0.0002% to 0.002%, Cu: 0.005% to 0.3%, Ni: 0.005% to 0.3%, Cr: 0.005% to 0.3%, and Mo: 0.005% to 0.3%.

10. The method for manufacturing a high strength hot rolled steel sheet, according to claim 8, wherein the composition further contains, on a percent by mass basis, at least one selected from Ca: 0.0003% to 0.01% and REM: 0.0003% to 0.01%.

Patent History
Publication number: 20160076124
Type: Application
Filed: Mar 17, 2014
Publication Date: Mar 17, 2016
Applicant: JFE STEEL CORPORATION (Chiyoda-ku, Tokyo)
Inventors: Kazuhiko Yamazaki (Tokyo), Katsumi Nakajima (Tokyo), Chikara Kami (Tokyo)
Application Number: 14/784,455
Classifications
International Classification: C22C 38/58 (20060101); C22C 38/50 (20060101); C22C 38/46 (20060101); C22C 38/06 (20060101); C22C 38/02 (20060101); C22C 38/00 (20060101); C22C 38/16 (20060101); C22C 38/14 (20060101); C22C 38/12 (20060101); C22C 38/04 (20060101); C21D 6/00 (20060101); C21D 9/46 (20060101); C21D 7/13 (20060101); C21D 1/84 (20060101); B21B 1/22 (20060101);