NI-CONTAINING STEEL PLATE

An object of the present invention is to provide an Ni-containing steel plate which is low in cost and has excellent low-temperature toughness. In view of the object, the Ni-containing steel plate of the present invention has a chemical composition containing by mass % C: 0.01% to 0.15%, Si: 0.02% to 0.20%, Mn: 0.45% to 2.00%, P: 0.020% or less, 5: 0.005% or less, Al: 0.005% to 0.100% Ni: 5.0 to 8.0%, and the balance being Fe and incidental impurities, and has a microstructure containing less than 1.7% by volume fraction of retained austenite when cooled to liquid nitrogen temperature, and having an average grain size of crystal grains surrounded by high-angle grain boundaries with an orientation difference of 15° or more of 5 μm or less by equivalent circle diameter.

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Description
TECHNICAL FIELD

The present invention relates to an Ni-containing steel plate with excellent low-temperature toughness, in particular to a steel plate which is suitable for use as members such as storage tanks for liquefied natural gas.

BACKGROUND ART

Conventionally, for members such as overland storage tanks for liquefied natural gas (hereinafter, referred to as LNG) high Ni-containing steel plates which are excellent in mechanical properties at low temperatures have been commonly used. In particular, steel plates composed of high Ni-containing steel which contains Ni by 9 mass % (hereinafter, referred to as 9% Ni steel) have been commonly used, and they have, actually been applied in many cases.

Regarding 9% Ni steel, considerations on various properties such as mechanical properties and weldability have been made. For example, Steel and Iron by Furukimi Osamu, Suzuki Shigeharu, Nakano Yashifumi, 69(1982)5, S492 (NPL 1) discloses that low-temperature toughness is improved by reducing the amount of impurity elements such as P and S. Further, Handbook of Metal, 4th revised edition, edited by The Japan Institute of Metals and Materials, Maruzen, p 801 (NPL 2) discloses that low-temperature toughness is improved by stabilizing retained austenite. However, since Ni is an expensive metal, it is desired to reduce Ni content.

Techniques for obtaining steel plates which can be made to have an Ni content smaller than that of 9% Ni steel and has good low temperature toughness are disclosed in for example, WO2007/034576 (PTL 1), WO2007/080645 (PTL 2), JP2011-214099A (PTL 3). PTL 1 discloses that mechanical properties of a steel plate can be improved by predetermining the chemical composition of the steel plate, defining the amount, aspect ratio, and average equivalent circular diameter of austenite contained in the steel plate, and manufacturing the steel plate with a method to satisfy such definitions. Further, PTL 2 discloses that toughness of the heat-affected zone of a steel plate can be improved when the steel plate has a predetermined chemical composition and the Fe content obtained by an extraction residue method after a heat-cycle simulation test is more than a predetermined value. Further, PTL 3 discloses that a brittle crack-arrest property of steel can be improved when the steel has a predetermined chemical composition, with certain textures developed.

CITATION LIST Patent Literature

PTL 1: WO2007/034576

PTL 2: WO2007/080645

PTL 3: JP2011 -214099A

Non-Patent Literature

NPL 1: Steel and iron by Furukimi Osamu, Suzuki Shigeharu, Nakano Yoshifumi, 69(1982)5, S492

NPL 2: Handbook of Metal, 4th revised edition, edited by The Japan Institute of Metals and Materials, Maruzen, p 800-802

SUMMARY OF INVENTION Technical Problem

However, the techniques disclosed in PTL 1, 2 and 3 do not include definitions regarding the amount of austenite at around −165° C. where the LNG tanks are actually used, and consideration regarding low-temperature toughness when the techniques are applied to actual structures were not made. Further, there were no specific disclosures regarding the manufacturing method of the steel plates.

The present invention has been developed in view of such situation, and an object thereof is to provide an Ni-containing steel plate which is low in cost and has excellent low-temperature toughness.

Solution to Problem

The inventors of the present invention, as a result of intense investigation for providing an Ni-containing steel plate with excellent low-temperature toughness, discovered, that by containing C, Si, Mn, P, S, Al, and Ni as essential elements of a steel, and setting the amount of retained austenite contained in the steel after performing sub-zero treatment were cooling is performed until reaching liquid nitrogen temperature to be less than 1.7%, and setting the average grain size of crystal grains surrounded by high-angle grain boundaries with an orientation difference of 15° or more to 5 μm or less by equivalent circle diameter, excellent low-temperature toughness can be achieved even when the Ni content is reduced compared to conventional 9% Ni steel.

If the Ni content in steel is reduced to be smaller than that of 9% Ni Steel, even if retained austenite is stable at room temperature, it will be unstable at −165° C. where LNG tanks are used. Further, it is considered that toughness decreases when retained austenite exists at −165° C., because the retained austenite is transformed into martensite phase due to deformation induced transformation, at the tip of a crack formed in the steel material when the LNG tank fractures. Under the situation, by reducing the amount of retained austenite remaining after sub-zero treatment corresponding to −165° C. where LNG tanks are used, and forming a fine microstructure as described above, it is assumed that low-temperature toughness can be improved even if the Ni content in steel is reduced to be smaller than that of conventional 9% Ni steel.

The present invention is based on the above discoveries and it provides the following (1) to (4).

(1) An Ni-containing steel plate having, a chemical composition containing by mass % C: 0.01% to 0.15%, Si: 0.02% to 0.20%, Mn: 0.45% to 2.00%, P: 0.020% or less, S: 0.005% or less, Al: 0.005% to 0.100%, Ni: 5.0% to 8.0%, and the balance being Fe and incidental impurities, wherein

the steel plate has a microstructure containing less than 1.7% by volume fraction of retained austenite when cooled to liquid nitrogen temperature, and having an average grain size of crystal grains surrounded by high-angle grain boundaries with an orientation difference of 15° or more of 5 μm or less by equivalent circle diameter.

(2) The Ni-containing steel plate according to aspect (1), wherein the chemical composition further contains by mass % at least one element selected from Cr: 1.00% or less and Mo: 1.000% or less.

(3) The Ni-containing steel plate according to aspect (1) or (2), wherein the chemical composition further contains by mass % at least one element selected from Cu: 1.00% or less, V: 0.100% or less, Nb: 0.100% or less, Ti: 0.100% or less, and B: 0.0030% or less.

(4) The Ni-containing steel plate according to any one of aspects (1) to (3), wherein the chemical composition further contains by mass % at least one element selected from Ca: 0.0050% or less and REM: 0.0050% or less.

Advantageous Effect of Invention

According to the present invention, an Ni-containing steel plate containing less Ni content compared to 9% Ni steel but having low-temperature toughness equivalent to that of 9% Ni steel can be easily manufactured, and an industrially remarkable effect is provided.

DESCRIPTION OF EMBODIMENTS

Hereinafter, the Ni-containing steel plate according to the present invention will be explained in detail and separately based on chemical composition, microstructure, and manufacturing method.

Unless otherwise specified, the indication of “%” regarding composition shall stand for “mass %”.

(1) Chemical Composition

First, the chemical composition will be described.

C: 0.01% to 0.15%

C is an important element for solid solution strengthening of steel. If C content is less than 0.01%, sufficient strength cannot be obtained. On the other hand, adding C in an amount exceeding 0.15% would cause deterioration of weldability and workability. Therefore, C content is set to be in the range of 0.01% to 0.15%. Preferably, the range is from 0.03% to 0.10%.

Si: 0.02% to 0.20%

Si is an effective element as a deoxidizer in molten steel and an effective element for solid solution strengthening. If Si content is less than 0.02%, deoxidizing effect cannot be sufficiently obtained. On the other hand, adding Si in an amount exceeding 0.20% would cause problems such as reduction in ductility and toughness, and an increase of inclusions. Therefore, Si content is set to be in the range of 0.02% to 0.20% and preferably in the range of 0.03% to 0.10%.

Mn: 0.45% to 2.00%

Mn is an effective element from the viewpoint of ensuring quench hardenability and enhancing strength. If Mn content is less than 0.45%, the effect thereof cannot be sufficiently obtained. On the other hand, adding Mn in an amount exceeding 2.00% would cause deterioration of weldability. Therefore, Mn content is set to be in the range of 0.45% to 2.00%, and preferably in the range of 0.55% to 1.00%.

P: 0.020% or less

Although high P content in steel leads to deterioration of low temperature toughness, the content thereof of 0.020% or less would be acceptable. Therefore, the upper limit of P content is set to be 0.020%.

S: 0.005% or less

High S content in steel causes precipitation as MnS, and this, as an inclusion, becomes the fracture generation origin of high tensile strength steel and leads to deterioration of toughness. However, if the content thereof is 0.005% or less, it would cause no problem. Therefore, the upper limit of S content is set to be 0.005%.

Al: 0.005% to 0.100%

Al is an effective element as a deoxidizer in molten steel and an effective element for improving low-temperature toughness. If Al content is less than 0.005%, these effects cannot be sufficiently obtained. On the other hand, if the content thereof exceeds 0.100%, weldability will decrease. Therefore, Al content is set to be in the range of 0.005% to 0.100%, and preferably in the range of 0.020% to 0.050%.

Ni: 5.0 to 8.0%

Ni is an important element for the present invention, and it is an element that enhances quench hardenability and improves toughness of ferrite matrix. If Ni content is less than 5.0%, these effects cannot be sufficiently exhibited. On the other hand, if the content thereof exceeds 8.0%, costs will increase. Therefore, Ni content is set to be in a range of 5.0% to 8.0%. In addition, from the viewpoint of further reducing costs, it is desirable for Ni content to be in the range of 5.0% to 7.5%.

In addition to the above basic chemical compositions, it is possible to contain at least one element selected from Cr and Mo, as a first group of selected components, if necessary, in the following ranges.

Cr: 1.00% or less

Cr enhances quench hardenability and provides an effect of improving low-temperature toughness by refining martensite phase. However, if the content thereof exceeds 1.00%, it would cause deterioration of weldability and an increase in manufacturing costs. Therefore, when containing Cr, the content thereof is set to be in the range of 1.00% or less. In order to effectively exhibit the above effect, it is preferable for the Cr content to be 0.05% or more, and more preferably in the range of 0.10% to 0.75%.

Mo: 1.000% or less

Mo enhances quench hardenability and provides an effect of improving low-temperature toughness by refining martensite phase. However, if the content thereof exceeds 1.000%, it would cause deterioration of weldability and an increase in manufacturing costs. Therefore, when containing Mo, the content thereof is set to be in the range of 1.000% or less. In order to effectively exhibit the above effects, it is preferable for the content thereof to be 0.005% or more, and more preferably in the range of 0.010% to 0.500%.

Further, in the present invention, it is possible to contain at least one element selected from Cu, V, Nb, Ti, and B as a second group of selected components, if necessary, in the following ranges.

Cu: 1.00% or less

Cu is an element that enhances quench hardenability. However, if the content thereof exceeds 1.00%, it would cause reduction of hot workability and an increase in costs. Therefore, when containing Cu, the content thereof is set to be in the range of 1.00% or less. In order to effectively exhibit the above effect, it is preferable for the content thereof to be 0.05% or more.

V: 0.100% or less

V is an element that precipitates as carbonitride, has an effect of refining microstructures, and is useful for improving toughness. However, if the content thereof exceeds 0.100% it would cause deterioration of weldability. Therefore, when containing V, the content thereof is set to be in the range of 0.100% or less. In order to effectively exhibit the above effects, it is preferable for the content thereof to be 0.005% or more.

Nb: 0.100% or less

Nb is an element that precipitates as carbonitride, has an effect of refining microstructures, and is useful for improving toughness. However, if the content thereof exceeds 0.100%, it would cause deterioration of weldability. Therefore, when containing Nb, the content thereof is set to he in the range of 0.100% or less. In order to effectively exhibit the above effects, it is preferable for the content thereof to be 0.005% or more.

Ti: 0.100% or less

Ti has an effect of improving toughness by fixing solute N, which is harmful to toughness, as TiN. However, if the content thereof exceeds 0.100%, it would cause precipitation of a coarse carbonitride, and deteriorate toughness. Therefore, when containing Ti, the content thereof is set to be in the range of 0.100% or less. In order to effectively exhibit the above effect, it is preferable for the content thereof to be 0.005% or more, and more preferably in the range of 0.010% to 0.050%.

B: 0.0030% or less

B is an element that enhances quench hardenability when added to steel by a small amount. However, if the content thereof exceeds 0.0030%, it would cause deterioration of toughness. Therefore, when containing B, the content thereof is set to be in the range of 0.0030% or less. In order to effectively exhibit the above effect, it is preferable for the content thereof to be 0.0003% or more.

Further, in the present invention, it is possible to contain at least one element selected from Ca and REM as a third group of selected components, if necessary, in the following ranges.

Ca: 0.0050% or less

Ca is an element that fixes S and inhibits generation of MnS which becomes the cause of reduction in toughness. However, if the content thereof exceeds 0.0050%, it would cause an increase in the amount of inclusions existing in steel and lead to deterioration of toughness rather than providing the above effect. Therefore, when containing Ca, the content thereof is set to be in the range of 0.0050% or less. In order to effectively exhibit the above effect, it is preferable for the content thereof to be 0.0005% or more.

REM: 0.0050% or less

REM (Rare Earth Metal) is an element that fixes S and inhibits generation of MnS which becomes the cause of reduction in toughness. However, if the content thereof exceeds 0.0050%, it would cause an increase in the amount of inclusions existing in steel and lead to deterioration of toughness rather than providing the above effect. Therefore, when containing REM, the content thereof is set to be in the range of 0.0050% or less. In order to effectively exhibit the above effect, it is preferable for the content thereof to be 0.0005% or more.

The balance other than the components described above includes Fe and incidental impurities.

(2) Microstructure

Next, the microstructure will be described.

The Ni-containing steel plate of the present invention has the above chemical composition, and also has a microstructure containing less than 1.7% of retained austenite when cooled to liquid nitrogen temperature, and having an average grain size of crystal grains surrounded by high-angle grain boundaries with an orientation difference of 15° or more of 5 μm or less by equivalent circle diameter.

Since the steel plate of the present invention is used mainly in storage tanks for LNG, the microstructure at −165° C. where LNG tanks are used is important. Therefore, the microstructure after sub-zero treatment where the steel plate is held at liquid nitrogen temperature, is defined. If the amount of retained austenite remaining after sub-zero treatment is 1.7% or more by volume fraction, sufficient low-temperature toughness cannot be obtained. Some reports have been made that retained austenite improves low temperature toughness. However, for the Ni-containing steel plate of the present invention, retained austenite has a harmful effect on toughness. It is considered that this is due to the fact that, since in Ni-containing steel plate of the present invention, the Ni content is smaller than the Ni content in conventional 9% Ni steel, even if retained austenite exists at −165° C., it is unstable, and if the steel structure undergoes plastic deformation at the tip of a crack, the retained austenite transforms into martensite by plasticity-induced martensite phase transformation. Therefore, the amount of retained austenite when the steel plate is cooled to liquid nitrogen temperature is set to be less than 1.7% by volume fraction. This amount is preferably 1.0% or less, and more preferably 0.5% or less.

Further, if the average grain size of crystal grains surrounded by high-angle grain boundaries with an orientation difference of 15° or more exceeds 5 μm by equivalent circle diameter, sufficient low-temperature toughness cannot be obtained. Therefore, the average grain size of crystal grains surrounded by high-angle grain boundaries with an orientation difference of 15° or more is set to be 5 μm or less by equivalent circle diameter, and preferably 3 μm or less by equivalent circle diameter.

(3) Manufacturing Condition

Next, a preferable manufacturing condition for manufacturing the steel plate of the present invention having the above described chemical composition and the above microstructure will be described. The following manufacturing condition is merely an example of a condition for manufacturing the Ni-containing steel plate of the present invention, and as long as the Ni-containing steel plate of the present invention can be obtained, manufacturing condition for the present invention is not limited to the following manufacturing condition.

In the present invention, it is preferable to heat a slab or a steel billet having the above described chemical composition at a temperature range of 900° C. to 1100° C. for 10 hours or less, and then to subject it to hot rolling at a temperature range of 870° C. or lower so that the cumulative rolling reduction ratio is 40% or more and 70% or less and the finisher delivery temperature is between 700° C. and 820° C., and then to subject the obtained hot rolled steel plate to direct quenching treatment where quenching is immediately performed until reaching a temperature of 200° C. or lower at a cooling rate of 5° C./s or more, and then to heat the steel plate to a temperature range of 500° C. to 650° C. at a heating rate of 0.05° C./s to 1.0° C./s, and then to subject the steel plate to tempering by holding the temperature at the same temperature range for 10 minutes or more and 60 minutes or less.

Heating Temperature: 900° C. to 1100° C., Heating duration: 10 hours or less

In a case where the heating temperature is lower than 900° C., coarse AlN which precipitates during the stage of casting of the steel slab does not dissolve, and toughness decreases. Further, the following rolling conditions cannot be substantially satisfied. If the heating temperature exceeds 1100° C, austenite becomes coarse grains and toughness will decrease. If the heating duration exceeds 10 hours, austenite grains become coarse and toughness decreases. Therefore, the heating temperature is set to be between 900° C. and 1100° C., and the heating duration is 10 hours or less.

Rolling Reduction Ratio: Cumulative Rolling Reduction Ratio of 40% or more and 70% or less at 870° C. or lower

If the cumulative rolling reduction ratio in the non-recrystallized region of austenite at 870° C. or lower is less than 40%, refinement of martensite phase will not be sufficient, and toughness decreases. On the other hand, in a case where the cumulative rolling reduction ratio exceeds 70%, it is difficult to substantially perform rolling at the following finisher delivery temperature. Therefore, the rolling reduction ratio is set to be 40% or more and 70% or less at 870° C. or lower.

Finisher delivery temperature: 700° C. to 820° C.

If the finisher delivery temperature is lower than 700° C., it results in α-γ dual phase rolling so that bainite phase forms, and therefore a desired strength cannot be satisfied. On the other hand, if the finisher delivery temperature exceeds 820° C., it becomes substantially difficult to perform sufficient rolling reduction in the non-recrystallized region of austenite, a fine microstructure cannot be obtained, and toughness decreases. Therefore, the finisher delivery temperature is set to be in the range of 700° C. to 820° C.

Cooling (Direct Quenching): Start immediately after rolling

Cooling (direct quenching) is started immediately after rolling is finished. If cooling is not immediately started, bainite phase will generate, and. therefore a desired strength cannot be satisfied. Therefore, cooling is started immediately after rolling is finished. Here, “immediately” refers to a point in time within 120 seconds after the completion of rolling.

Cooling Rate; 5° C./s or more

In a case where the cooling rate is less than 5° C./s, transformation to martensite phase will not occur, and a desirable strength and toughness cannot be obtained. Therefore, the cooling rate is set to be 5° C./s or more. Preferably, the cooling rate is 10° C./s or more.

Cooling Stop Temperature: 200° C. or lower

In a case where the cooling stop temperature exceeds 200° C., transformation to martensite phase will not Occur uniformly in the steel plate, and a desirable strength and toughness cannot be obtained. Therefore, the cooling stop temperature is set to be 200° C. or lower.

Tempering Heating Rate: 0.05° C./s to 1.0° C./s

In a case where the tempering heating rate is less than 0.05° C./s, the precipitated carbide would become coarse, and toughness will decrease. On the other hand, in order to perform rapid short time beating where the tempering heating rate exceeds 1.0° C./s, induction heating facilities and the like will be required, and costs will increase. Therefore, the tempering heating rate is set to be in the range of 0.05° C./s to 1.0° C./s.

Tempering temperature: 500° C. to 650° C.

In a case where the tempering temperature is lower than 500° C., toughness improving effect caused by precipitation of fine carbides such as cementite cannot, be sufficiently obtained. On the other hand, in a case were the tempering temperature exceeds 650° C., coarse carbide precipitates, and toughness decreases. Therefore, the tempering temperature is set to be in the range of 500° C. to 650° C.

Tempering Holding Time: 10 minutes or more and 60 minutes or less

In a case where the tempering holding time is less than 10 minutes, toughness improving effect caused by precipitation of fine carbides such as cementite cannot be sufficiently obtained. On the other hand, in a case where the tempering holding time exceeds 60 minutes, toughness will decrease due to reasons such as precipitation of a coarse carbide. Further, manufacturing costs will increase. Therefore, the tempering holding time is set to be 10 minutes or more and 60 minutes or less. Cooling, after tempering may be performed by either water cooling or air cooling. However, if the cooling rate is too fast, the temperature difference between the surface and the inside of the steel plate becomes large and causes formation of strains inside the steel plate and low temperature toughness decreases. Therefore, the cooling rate is preferably 5° C./s or less.

In the aforementioned manufacturing condition, after direct quenching, dual phase heat treatment where the steel plate is heated to a temperature range from 650° C. to 800° C. at a heating rate of 0.1° C./s to 1.5° C./s, held at the same temperature range for 10 minutes or more and 60 minutes or less, and then subjected to quenching until reaching a temperature of 200° C. or lower at a cooling rate of 5° C./s or more, may be performed.

Dual Phase Heat Treatment Heating Rate: 0.1° C./s to 1.5° C./s

By performing dual phase heat treatment, part of the microstructure transforms into austenite, and as crystal grains become fine, tempering proceeds and thereby improves toughness. However, in a case where the dual phase heat treatment heating rate is less than 0.1° C./s, austenite grains become coarse and toughness decreases. Further, since the microstructure generated after cooling also becomes coarse, toughness decreases. On the other hand, in a case where the heating rate exceeds 1.5° C./s, induction heating facilities and the like are required, and costs increase. Therefore, the dual phase heat treatment heating rate is set to be in the range of 0.1° C./s to 1.5° C./s.

Dual Phase Heat Treatment Temperature: 650° C. to 800° C.

In a case where the dual phase heat treatment temperature is lower than 650° C., sufficient austenite reverse transformation does not occur, and refining effect of the microstructure cannot be obtained, and therefore a toughness improving effect cannot be obtained. Further, since the amount of austenite reverse transformation is small, C easily concentrates in austenite, and retained austenite increases. On the other hand, if the dual phase heat treatment temperature exceeds 800° C., reverse transformation austenite becomes coarse and toughness decreases. Further, since the microstructure after cooling becomes coarse, toughness decreases. Further, manufacturing costs increase. Therefore, the dual phase heat treatment temperature is set to be in the range of 650° C. to 800° C. In a case where the dual phase heat treatment temperature is high, the amount of reverse transformation austenite increases and the amount of concentration of C in reverse transformation austenite decreases compared to a case where the dual phase heat treatment temperature is low, and therefore the amount of martensite transformation caused h cooling after dual phase heat treatment increases, and the amount of retained austenite decreases. Therefore, the dual phase heat treatment temperature is preferably in the range of 720° C. to 780° C.

Dual Phase Heat Treatment Holding Time: 10 minutes or more and 60 minutes or less

If the dual phase heat treatment holding time is less than 10 minutes, sufficient austenite reverse transformation does not occur and toughness improving effect caused by refinement of the microstructure cannot be sufficiently obtained. On the other hand, in a case where the dual phase heat treatment holding time exceeds 60 minutes, austenite grains become coarse and toughness decreases. Further, since the microstructure generated after cooling also becomes coarse, toughness decreases. Since C concentrates in austenite, retained austenite increases. Manufacturing costs increase as well. Therefore, the dual phase heat treatment holding time is set to be 10 minutes or more and 60 minutes or less.

Cooling Rate after Dual Phase Heat Treatment: 5° C./s or more

In a case where the cooling rate is less than 5° C./s, transformation from austenite to martensite phase will not occur, and a desirable strength and toughness cannot be obtained. Further, if the cooling rate is slow, the amount of solute C in ferrite decreases as the temperature is lowered, and therefore C moves to austenite from the ferrite surrounding the reverse transformed austenite, and C concentrates in the austenite and the austenite tends to remain as retained austenite. Therefore, the cooling rate is set to be 5° C./s or more. Preferably, the cooling rate is 10° C./s or more.

Cooling Stop Temperature after Dual Phase Heat Treatment: 200° C. or lower

In a case where the cooling stop temperature exceeds 200° C., transformation to martensite phase will not occur uniformly in the steel plate, and a desirable strength and toughness cannot be obtained. Further, C concentrates in austenite and tends to remain as retained austenite. Therefore, the cooling stop temperature is set to be 200° C. or lower.

After performing the dual phase heat treatment and cooling, until reaching 200° C. or lower, tempering is conducted in the manner previously described. That is, the steel is heated to a temperature range of 500° C. to 650° C. at a heating rate of 0.05° C./s to 1.0° C./s, and then subjected to tempering by holding the temperature at the same temperature range for 10 minutes or more and 60 minutes or less.

EXAMPLES

Next, Examples of the present invention will be described.

Molten steels with the chemical compositions shown in table I were obtained by steelmaking in a vacuum melting, furnace and made into small-sized steel ingots (150 kg). These steels were heated in the conditions shown in table 2, subjected to hot rolling until reaching a plate thickness of 7 mm to 50 mm, and then subjected to quenching just after the rolling. Some of the steel plates were then subjected to tempering treatment. Regarding the rest of the steel plates, after quenching, they were subjected to dual phase heat treatment and then to tempering treatment. The obtained steel plates were each subjected to a tensile test, a Charpy impact test, a measurement of austenite volume fraction, and a measurement of grain size of crystal grains surrounded by high-angle grain boundaries with an orientation difference of 15° or more, in the manner described below.

TABLE 1 Steel Chemical Composition (mass %) No. C Si Mn P S Al Ni Cr Mo Cu V Nb Ti B Ca REM Remarks A 0.06 0.06 1.21 0.005 0.0011 0.035 5.7 Inventive Example B 0.07 0.09 0.95 0.010 0.0009 0.033 7.2 Inventive Example C 0.05 0.04 0.67 0.003 0.0012 0.029 7.8 0.12 Inventive Example D 0.09 0.03 1.06 0.009 0.0010 0.028 6.9 0.12 0.043 0.0023 Inventive Example E 0.03 0.05 0.88 0.004 0.0012 0.033 7.4 0.72 Inventive Example F 0.02 0.06 1.36 0.008 0.0011 0.036 7.6 0.03 Inventive Example G 0.05 0.08 0.63 0.006 0.0008 0.024 6.8 0.41 0.014 Inventive Example H 0.04 0.07 0.97 0.011 0.0008 0.031 7.3 0.23 0.015 0.0012 0.0018 Inventive Example I 0.06 0.05 1.02 0.005 0.0009 0.030 4.9 Comparative Example The underlined values are outside the scope of the invention.

Tensile Test

From each steel plate, at a position of a half the plate thickness, and in the roiling direction, a tensile test specimen having a parallel portion length of 30 mm, GL of 24 mm, a parallel portion diameter of 6φ was collected and subjected to a tensile test at room temperature. From the obtained stress-strain curve, tensile strength (TS) and yield strength (YS) were calculated. TS of 690 MPa or more and YS of 590 MPa or more are each considered as excellent TS and YS.

[Charpy Impact Test]

From each steel plate, at a position of a half the plate thickness, and in a direction orthogonal to the rolling direction, V-notch test specimens were collected in accordance with JIS Z2202 (1998) standard, and subjected to a Charpy impact test with 3 specimens per each temperature for each steel plate in accordance with JIS Z2242 (1998) standard, and absorbed energy at −196° C. was measured to evaluate base material toughness. Steel plates with an average value of absorbed energy (vE.196) of 3 specimens of 150 J or more are considered as having excellent base material toughness.

[Austenite Volume Fraction]

Samples collected from each steel plate at a position of a half the plate thickness in a direction orthogonal to the rolling direction were subjected to sub-zero treatment for 10 minutes in liquid nitrogen, and then the austenite volume fraction was measured by X-ray diffraction.

[Measurement of Grain Size of Crystal Grains]

Samples collected from each steel plate at a position of a half the plate thickness in a direction orthogonal to the rolling direction were polished and mirror finished, and subjected to EBSP analysis. Among the obtained data, a high-angle grain boundary where the orientation difference between two crystal grains contacting the grain boundary is 15° or more was selected and the average grain size by equivalent circle diameter of the region surrounded by the high-angle grain boundary was obtained.

The obtained results are shown in Table 2.

As shown in table 2, it has been confirmed that the inventive examples have excellent low-temperature toughness whereas the comparative examples outside the scope of the present invention have reduced low-temperature toughness.

TABLE 2 Temp. Dual Dual Cooling Rolling Start of Cool- Phase Heat Dual Phase Heat Rate Plate Heat- Reduc- Finisher of Starting Cool- ing Treatment Phase Heat Treatment after Dual Steel Thick- ing tion Delivery Cool- Cool- ing Stop Heating Treament Holding Phase Heat Plate Steel ness Temp. Ratio* Temp. ing** ing Rate Temp. Rate Temp. Time Treatment No. No. (mm) (° C.) (%) (° C.) (s) (° C.) (° C./s) (° C.) (° C./s) (° C.) (min) (° C./s) 1 A 25 1050 50 780 34 760 25 150 0.34 750 20 22 2 A 25 1050 60 750 35 730 25 150 0.37 640 20 22 3 A 25 1050 55 730 38 710 25 150 4 B 25 1050 0 900 30 880 25 150 0.33 740 30 22 5 B 25 1050 55 800 34 780 25 150 0.33 740 20 22 6 B 25 1050 55 800 34 780 25 150 0.33 630 30 22 7 B 25 1050 55 800 34 780 25 150 0.33 740 120 22 8 B 25 1050 55 800 34 780 25 150 0.33 740 30 2 9 B 25 1050 55 800 34 780 25 150 0.33 700 30 20 10 B 25 1000 60 750 36 730 25 150 0.31 730 20 22 11 B 25 1000 55 740 37 720 25 150 0.29 720 15 22 12 B 25 1000 50 800 33 780 25 100 13 B 25 1050 60 740 37 720 25 150 14 B 25 1000 55 910 39 870 25 100 15 B 7 1000 65 800 18 780 40 100 0.64 740 10 37 16 B 50 1050 50 780 60 760 12 150 0.19 720 20  9 17 C 25 1050 60 780 34 760 25 150 0.31 730 20 22 18 C 25 1050 55 800 33 780 25 150 19 C 25 1050 55 810 32 790 25 150 20 C 50 1050 60 800 32 780 12 100 0.20 720 20  9 21 D 25 1050 65 750 36 730 25 150 0.35 780 50 22 22 E 25 1200 50 800 33 780 25 100 0.34 750 20 22 23 E 25  950 55 790 33 770 25 150 0.33 740 20 22 24 F 25 1050 60 770 35 750 25 150 0.31 730 30 22 25 F 25 1050 30 800 33 780 25 150 0.19 720 20 22 26 G 25 1050 65 730 37 710 25 100 0.18 730 20 22 27 H 25 1050 55 750 36 730 25 100 0.29 700 20 22 28 H 25 1050 60 780 34 760 25 150 29 I 25 1050 60 800 33 780 25 150 0.34 760 20 22 Cooling Ave. Grain Stop Temp. Size by after Dual Tempering Tempering Cooling Austenite Equivalent Steel Phase Heat Heating Tempering Holding Rate after Volume Circle Plate Treatment Rate Temp. Time Tempering TS YS vE-196 Fraction Diameter No. (° C.) (° C.) (° C.) (min) (° C./s) (MPa) (MPa) (J) (%) (μm) Remarks 1 100 0.18 570 20 0.4 698 642 225 0.2 2.3 Inventive Example 2 150 0.19 580 20 0.4 711 382 121 2.0 5.6 Comparative Example 3 0.17 560 15 0.4 701 650 156 0.3 3.9 Inventive Example 4  75 0.18 570 15 0.4 722 699 120 0.2 5.8 Comparative Example 5 125 0.19 580 20 0.4 740 715 235 0.2 1.9 Inventive Example 6 125 0.19 580 20 0.4 810 785 56 2.6 3.6 Comparative Example 7 125 0.19 580 20 0.4 822 765 98 1.8 5.1 Comparative Example 8 100 0.19 580 20 0.4 752 722 110 1.8 3.2 Comparative Example 9 350 0.19 580 20 0.4 720 735 103 1.9 2.5 Comparative Example 10 100 0.19 580 25 0.4 731 685 220 0.3 1.4 Inventive Example 11 150 0.19 580 20 0.4 705 615 245 0.3 1.1 Inventive Example 12 0.18 570 40 0.4 715 682 160 0.1 4.3 Inventive Example 13 0.20 590 15 0.4 735 719 152 0.1 4.0 Inventive Example 14 100 0.19 580 20 0.4 730 705 116 0.3 7.3 Comparative Example 15 150 0.53 590 20 1.2 745 719 225 0.2 1.3 Inventive Example 16 100 0.13 600 15 0.2 721 674 167 1.4 2.5 Inventive Example 17 100 0.19 580 20 0.4 749 713 234 0.2 1.4 Inventive Example 18 0.2 590 30 0.4 720 687 174 0.3 4.1 Inventive Example 19 0.14 520 20 0.4 736 701 151 0.1 4.6 Inventive Example 20 100 0.11 560 15 0.2 732 699 173 1.1 2.0 Inventive Example 21 150 0.19 580 20 0.4 726 694 219 0.1 2.0 Inventive Example 22  75 0.17 560 15 0.4 713 689 88 0.5 6.7 Comparative Example 23 100 0.18 570 20 0.4 720 692 239 0.2 1.2 Inventive Example 24 100 0.2 590 20 0.4 721 675 215 0.1 1.5 Inventive Example 25 100 0.19 580 30 0.4 712 653 102 0.3 5.7 Comparative Example 26 150 0.13 600 25 0.4 716 665 258 0.2 1.0 Inventive Example 27 150 0.19 580 20 0.4 721 653 182 1.2 1.4 Inventive Example 28 0.23 630 20 0.4 702 643 176 0.2 4.1 Inventive Example 29 100 0.16 540 20 0.4 675 621 76 0.1 1.9 Comparative Example The underlined values are outside the scope of the invention. *Cumulative rolling reduction ratio at 870° C. or lower **Time from when finishing rolling is completed to when cooling is started

Claims

1. An Ni-containing steel plate having a chemical composition containing by mass % C: 0.01% to 0.15%, Si: 0.02% to 0.20%, Mn: 0.45% to 2.00%, P: 0.020% or less, S: 0.005% or less, Al: 0.005% to 0.100%, Ni: 5.0% to 8.0%, and the balance being Fe and incidental impurities, wherein

the steel plate has a microstructure containing less than 1.7% by volume fraction of retained austenite when cooled to liquid nitrogen temperature, and having an average grain size of crystal grains surrounded by high-angle grain boundaries with an orientation difference of 15° or more of 5 μm or less by equivalent circle diameter.

2. The Ni-containing steel plate according to claim 1, wherein the chemical composition further contains by mass % at least one element selected from Cr: 1.00% or less and Mo: 1.000% or less.

3. The Ni-containing steel plate according to claim 1, wherein the chemical composition further contains by mass % at least one element selected from Cu: 1.00% or less, V: 0.100% or less, Nb: 0.100% or less, Ti: 0.100% or less, and B: 0.0030% or less.

4. The Ni-containing steel plate according to claim 1, wherein the chemical composition further contains by mass % at least one element selected from Ca: 0.0050% or less and REM: 0.0050% or less.

5. The Ni-containing steel plate according to claim 2, wherein the chemical composition further contains by mass % at least one element selected from Cu: 1.00% or less, V: 0.100% or less, Nb: 0.100% or less, Ti: 0.100% or less, and B: 0.0030% or less.

6. The Ni-containing steel plate according to claim 2, wherein the chemical composition further contains by mass % at least one element selected from Ca: 0.0050% or less and REM: 0.0050% or less.

7. The Ni-containing steel plate according to claim 3, wherein the chemical composition further contains by mass % at least one element selected from Ca: 0.0050% or less and REM: 0.0050% or less.

8. The Ni-containing steel plate according to claim 5, wherein the chemical composition further contains by mass % at least one element selected from Ca: 0.0050% or less and REM: 0.0050% or less.

Patent History
Publication number: 20150147222
Type: Application
Filed: Jul 18, 2013
Publication Date: May 28, 2015
Inventors: Shinichi Miura (Kurashiki-shi), Yukio Shimbo (Kawasaki-shi), Nobuyuki Ishikawa (Fukuyama-shi)
Application Number: 14/406,405
Classifications
Current U.S. Class: Rare Earth Metal Containing (420/83); Lead, Bismuth, Selenium, Tellurium Or Calcium Containing (420/84); Nickel Containing (420/92); Nickel Containing (420/112); Nickel Containing, But 10 Percent Or Less (420/119)
International Classification: C22C 38/46 (20060101); C22C 38/14 (20060101); C22C 38/12 (20060101); C22C 38/00 (20060101); C22C 38/06 (20060101); C22C 38/04 (20060101); C22C 38/02 (20060101); C22C 38/16 (20060101); C22C 38/08 (20060101);