JP7156500B2 - Steel plate and its manufacturing method - Google Patents

Description

TECHNICAL FIELD The present invention relates to a steel sheet particularly excellent in stress corrosion cracking resistance suitable for structural steel used in cryogenic environments such as liquefied gas storage tanks, and a method for producing the same.

When a hot-rolled steel sheet is used for a structure for a liquefied gas storage tank, the use environment is extremely low, so not only the strength of the steel sheet but also toughness at extremely low temperatures is required. For example, when a hot-rolled steel plate is used in a liquefied natural gas storage tank, it is necessary to ensure excellent toughness at −164° C. or below, which is the boiling point of liquefied natural gas. If the low-temperature toughness of the steel material is poor, the safety of the cryogenic storage tank structure may not be maintained. Conventionally, 7% Ni steel sheets and 9% Ni steel sheets have been used to meet this requirement.

For example, Patent Literatures 1, 2 and 3 propose low-temperature steel sheets having performance equal to or higher than that of 9% Ni steel sheets with a Ni content lower than 9%.

WO2007/034576 WO2007/080646 JP 2011-241419 A

However, although the Ni-containing steel materials described in Patent Documents 1, 2 and 3 are excellent in low-temperature toughness, they do not mention stress corrosion cracking caused by hydrogen, and there is still room for investigation. For example, in the case of LNG tanks for ships, the use environment contains sulfides and chlorides, so there is a high possibility that hydrogen-induced stress corrosion cracking will occur. It is also required to have stress corrosion cracking resistance.

The present invention has been made in view of such problems, and an object of the present invention is to provide a steel sheet that is suitable for use in a low-temperature environment and has excellent resistance to stress corrosion cracking.

In order to solve the above problems, the present inventors conducted extensive research on the chemical composition and structure of steel sheets, and obtained the following findings.
(1) By adding Co, when corrosion progresses on the steel sheet surface, Co concentrates on the steel sheet surface, hydrogen penetration into the steel can be reduced, and crack propagation due to hydrogen embrittlement can be reduced.

(2) By setting the cooling rate to 1°C/s or more during cooling after hot rolling or cooling after heat treatment (quenching or two-phase region quenching), the structure up to 1 mm below the surface of the steel sheet is A fine structure is obtained in which the diameter of crystal grains surrounded by large-angle grain boundaries with a difference of 15° or more is 5 μm or less. This microstructure disperses the hydrogen trap sites, thereby reducing crack growth due to hydrogen embrittlement.

(3) By setting the maximum equivalent circle diameter of retained austenite in the surface layer of the steel sheet to 1 μm or less, the local concentration of hydrogen traps in the retained austenite can be dispersed, and crack propagation due to hydrogen embrittlement can be reduced.

The present invention was made by further studying the above findings, and the gist thereof is as follows.
1. in % by mass,
C: 0.01% or more and 0.15% or less,
Si: 0.01% or more and 1.00% or less,
Mn: 0.10% or more and 3.00% or less,
Al: 0.002% or more and 0.100% or less,
Ni: 5.0% or more and 10.0% or less,
N: 0.0010% or more and 0.0080% or less,
Co: more than 0% and 1.50% or less,
P: 0.030% or less and S: 0.0050% or less, with the balance being Fe and unavoidable impurities,
In the structure from the surface of the steel sheet to a position 1 mm deep, the average equivalent circle diameter of crystal grains surrounded by large-angle grain boundaries with a misorientation of 15° or more is 5 μm or less, and the maximum equivalent circle diameter of retained austenite grains is 1 μm. Steel plate which is below.

2. The component composition is further mass %,
Nb: 0.001% or more and 0.030% or less,
V: 0.01% or more and 0.10% or less,
Ti: 0.003% or more and 0.050% or less,
B: 0.0003% or more and 0.0100% or less,
Cu: 0.01% or more and 1.00% or less,
Cr: 0.01% or more and 1.50% or less,
Sn: 0.01% or more and 0.50% or less,
Sb: 0.01% or more and 0.50% or less,
2. The steel sheet according to 1 above, containing one or more selected from Mo: 0.03% or more and 1.00% or less and W: 0.05% or more and 2.00% or less.

3. The component composition is further mass %,
Ca: 0.0005% or more and 0.0050% or less,
Zr: 0.0005% or more and 0.0050% or less,
3. The steel sheet according to 1 or 2 above, containing one or more selected from Mg: 0.0005% or more and 0.0050% or less and REM: 0.0010% or more and 0.0100% or less.

4. In the method for manufacturing a steel sheet, the steel material having the chemical composition according to any one of 1 to 3 is heated, hot-rolled, and then cooled, wherein the temperature range in the cooling treatment is 600 ° C. or lower and 200 ° C. or higher. A method for manufacturing a steel sheet, wherein the average cooling rate at is 1 ° C / s or more.

5. 3. A steel sheet manufacturing method comprising heating a steel material having the chemical composition according to any one of 1 to 3, hot rolling, further heat-treating, and then cooling, wherein A method for producing a steel sheet, wherein the average cooling rate in a temperature range of °C or higher is 1 °C/s or higher.

ADVANTAGE OF THE INVENTION According to this invention, the steel plate which has high durability with respect to the stress corrosion cracking by hydrogen can be provided. By applying this steel plate to a steel structure used in a low-temperature environment, such as a liquefied gas storage tank, the safety of the steel structure can be improved, resulting in significant industrial effects.

Embodiments of the present invention will be described below. In addition, this invention is not limited to the following embodiment.
[Component composition]
First, the chemical composition of the steel sheet of the present invention and the reasons for its limitation will be described. In the present invention, in order to ensure excellent corrosion resistance, the chemical composition of the steel sheet is specified as follows. In addition, % display in a component composition shall mean the mass % unless otherwise indicated.

C: 0.01% or more and 0.15% or less C is effective in increasing the strength, and in order to obtain this effect, it is necessary to contain C in an amount of 0.01% or more. On the other hand, when the content exceeds 0.15%, the low temperature toughness is lowered. Therefore, C should be 0.01% or more and 0.15% or less. Preferably, it is 0.03% or more. Preferably, it is 0.10% or less.

Si: 0.01% or more and 1.00% or less Si acts as a deoxidizing agent and is not only necessary for steelmaking, but also dissolves in steel and has the effect of increasing the strength of the steel sheet by solid solution strengthening. . In order to obtain this effect, it is necessary to contain Si at 0.01% or more. On the other hand, when the content exceeds 1.00%, the low temperature toughness deteriorates. Therefore, Si should be 0.01% or more and 1.00% or less. Preferably, it is 0.03% or more. Preferably, it is 0.5% or less.

Mn: 0.10% or more and 3.00% or less Mn is an element effective in increasing the hardenability of steel and increasing the strength of the steel sheet. In order to obtain the effect, Mn needs to be contained in an amount of 0.01% or more. On the other hand, when the content exceeds 3.00%, corrosion cracking resistance is lowered. Therefore, Mn should be in the range of 0.10% or more and 3.00% or less. Preferably, it is 0.20% or more. It is preferably 2.00% or less, more preferably 1.00% or less.

Al: 0.002% or more and 0.100% or less Al acts as a deoxidizing agent and is most commonly used in the molten steel deoxidizing process for steel sheets. In addition, it has the effect of fixing solute N in the steel to form AlN and suppressing deterioration of toughness due to the reduction of solute N. On the other hand, if the content exceeds 0.100%, the toughness deteriorates, so the content is made 0.100% or less. Preferably, it is 0.010% or more. Preferably, it is 0.070% or less. More preferably, it is 0.020% or more. More preferably, it is 0.060% or less.

Ni: 5.0% or more and 10.0% or less Ni is an element extremely effective in improving the low temperature toughness of the steel sheet. On the other hand, since it is an expensive element, the steel sheet cost rises as the content increases. Therefore, in the present invention, the Ni content is set to 10.0% or less. However, if the Ni content is less than 5.0%, the strength of the steel sheet is lowered, and as a result of not being able to obtain stable retained austenite at low temperatures, the low-temperature toughness and strength of the steel sheet are lowered. Therefore, the Ni content is made 5.0% or more and 10.0% or less. Preferably, it is 9.5% or less. Preferably, it is 6.0% or more.

N: 0.0010% or more and 0.0080% or less N forms precipitates in steel. and a decrease in the toughness of the weld heat-affected zone. However, N is also an element that contributes to grain refinement of the base metal by forming AlN, and such an effect can be obtained by setting the N content to 0.0010% or more. Therefore, the N content should be 0.0010% or more and 0.0080% or less. It is preferably 0.0020% or more. More preferably, it is 0.0060% or less.

Co: more than 0% and 1.50% or less Co is an important element that concentrates in the surface layer of the steel sheet in a corrosive environment and contributes to suppressing corrosion cracking by reducing the entry of hydrogen. Therefore, the content must exceed 0%. Preferably, the Co content is 0.05% or more, more preferably 0.1% or more. However, even if the content exceeds 1.50%, the effect is saturated, and Co is an expensive element, so the maximum addition amount is set to 1.50%.

P: 0.030% or less When the P content exceeds 0.030%, the corrosion cracking resistance is lowered. Therefore, it is desirable to set the upper limit to 0.030% and reduce it as much as possible. Therefore, P is set to 0.030% or less. Since the smaller the P content, the better the properties, the P content is preferably 0.025% or less, more preferably 0.020% or less. Of course, the P content may be 0%, but since removal of P requires a high cost, it is preferably 0.002% or more from the viewpoint of cost.

S: 0.0050% or less S forms MnS in the steel and significantly deteriorates the low temperature toughness. It is preferably 0.0020% or less. The S content may of course be 0%, but it is preferable to set the S content to 0.0005% or more from the viewpoint of cost, because high cost is required for removing S.

The basic component composition includes the above elements and the balance is Fe and unavoidable impurities.
In the present invention, for the purpose of further improving strength and low-temperature toughness, in addition to the essential elements described above, the following elements can be contained as necessary.

Nb: 0.001% or more and 0.030% or less Nb is an element effective in improving the strength of the steel sheet. In order to obtain such effects, it is preferable to add 0.001% or more of Nb. On the other hand, if the content exceeds 0.030%, coarse carbonitrides may precipitate and deteriorate the toughness of the base material. Therefore, when Nb is contained, the content should be 0.001% or more and 0.030% or less. It is preferably 0.005% or more, more preferably 0.007% or more. It is preferably 0.025% or less, more preferably 0.022% or less.

V: 0.01% or more and 0.10% or less V is an element effective in improving the strength of the steel sheet. In order to obtain such effects, it is preferable to add V in an amount of 0.01% or more. On the other hand, when the content exceeds 0.10%, coarse carbonitrides precipitate and may become starting points of fracture. In addition, precipitates may become coarse and deteriorate the toughness of the base material. Therefore, when V is contained, it should be 0.01% or more and 0.10% or less. It is preferably 0.02% or more, more preferably 0.03% or more. It is preferably 0.09% or less, more preferably 0.08% or less.

Ti: 0.003% to 0.050% Ti is an element that precipitates as a nitride or carbonitride and is effective in improving the strength of the steel sheet. In order to obtain such effects, it is preferable to add Ti in an amount of 0.003% or more. On the other hand, when the content exceeds 0.050%, the precipitates become coarse, which may deteriorate the toughness of the base metal. In addition, coarse carbonitrides may precipitate and become starting points of fracture. Therefore, when Ti is contained, it is made 0.003% or more and 0.050% or less. It is preferably 0.005% or more, more preferably 0.007% or more. It is preferably 0.035% or less, more preferably 0.032% or less.

B: 0.0003% or more and 0.0100% or less B is an element effective in improving the strength of the steel sheet. In order to obtain such effects, it is preferable to add B in an amount of 0.0003% or more. On the other hand, when the content exceeds 0.0100%, coarse B precipitates may be formed and the toughness may be lowered. Therefore, B should be in the range of 0.0003% or more and 0.0100% or less. Preferably, it is 0.0030% or less.

Cu: 0.01% or more and 1.00% or less Cu is an element effective in increasing the strength of the steel sheet by improving the hardenability, but when the content exceeds 1.00%, the low-temperature toughness of the steel sheet decreases. There is a risk of Therefore, when Cu is contained, the content is preferably 1.00% or less. On the other hand, if it is less than 0.01%, the effect of increasing the strength cannot be obtained. More preferably, it is 0.10% or more and 0.30% or less.

Cr: 0.01% or more and 1.50% or less Cr is an element that contributes to improving the low-temperature toughness and corrosion resistance of high-Mn steel. For that purpose, it is preferable to set the amount of Cr to 0.01% or more. On the other hand, Cr may precipitate in the form of nitrides, carbides, carbonitrides, etc. during rolling. 1.50% is preferable. More preferably, it is 1.00% or less.

Mo: 0.03% or more and 1.00% or less Mo is an element effective in suppressing the temper embrittlement susceptibility of the steel sheet, and is also an element that increases the strength of the steel sheet without impairing the low temperature toughness. In order to obtain such effects, it is preferable to set the Mo content to 0.03% or more. On the other hand, when Mo exceeds 1.00%, there is a possibility that the low-temperature toughness is lowered. Therefore, when Mo is contained, the content is made 0.03% or more and 1.00% or less. More preferably, it is more than 0.05% and 0.30% or less.

Sn: 0.01% to 0.50% Sb: 0.01% to 0.50% W: 0.05% to 2.00% Sn, Sb and W are elements effective in improving corrosion resistance. be. These effects are exhibited when Sn and Sb are 0.01% or more and W is 0.05% or more. However, if any element is included in a large amount, the weldability and toughness may be deteriorated, which may be disadvantageous from the viewpoint of cost. Therefore, the Sn content is in the range of 0.01% to 0.50%, the Sb content is in the range of 0.01% to 0.50%, and the W content is in the range of 0.05% to 2.00%. do. Preferably, the Sn content is 0.02% or more and 0.25% or less, the Sb content is 0.02% or more and 0.25% or less, and the W content is 0.10% or more and 1.00% or less.

Furthermore, in the present invention, the following elements can be contained as necessary.
Ca: 0.0005% to 0.0050%, Zr: 0.0005% to 0.0050%, Mg: 0.0005% to 0.0050% and REM: 0.0010% to 0.0100% One or more of the following Ca, Zr, Mg and REM are elements useful for controlling the morphology of inclusions such as MnS and can be added as necessary. Here, the morphology control of inclusions refers to turning expanded sulfide-based inclusions into granular inclusions. Through the morphology control of this inclusion, toughness and sulfide stress corrosion cracking resistance are improved. In order to obtain such effects, it is preferable to contain Ca, Zr and Mg at 0.0005% or more and REM at 0.0010% or more. On the other hand, if any element is included in a large amount, the amount of non-metallic inclusions increases, and the low temperature toughness may rather deteriorate. Therefore, when Ca, Zr, and Mg are contained, each content is 0.0005% or more and 0.0050% or less, and when REM is contained, the content is 0.0010% or more and 0.0100% or less. More preferably, the Ca content is 0.0010% or more and 0.0040% or less, the Zr content is 0.0010% or more and 0.0040% or less, the Mg content is 0.0010% or more and 0.0040% or less, and the REM content is 0 0.0020% or more and 0.0100% or less.

[Surface structure]
Next, the structure from the surface of the steel sheet to a position at a depth of 1 mm (hereinafter also referred to as a surface layer structure) has an average equivalent circle diameter of 5 μm or less of crystal grains surrounded by large-angle grain boundaries with a misorientation of 15° or more. It is also important that the maximum equivalent circle diameter of the retained austenite grains is 1 μm or less.
First, it is necessary to set the average equivalent circle diameter of crystal grains surrounded by large-angle grain boundaries with a misorientation of 15° or more to 5 μm or less. This is because the amount of crystal grain boundaries with a misorientation of 15° or more, which serve as hydrogen trap sites, increases and disperses, so crack growth due to hydrogen embrittlement can be reduced. Furthermore, the average equivalent circle diameter of the crystal grains is preferably 4 μm or less, more preferably 3 μm or less.

The identification of crystal grains surrounded by large-angle grain boundaries with a misorientation of 15° or more and the identification of the average equivalent circle diameter of the crystal grains can be performed by the measurement method in the examples described later.

In order to make the average circle equivalent diameter of the crystal grains surrounded by the large-angle grain boundaries with a misorientation of 15° or more to be 5 μm or less, after the hot rolling, or if the heat treatment is performed after the hot rolling, the heat treatment is performed at a predetermined temperature. Cooling treatment is performed with an average cooling rate of 1°C/s or more in the region.

Furthermore, in the surface layer structure, the maximum equivalent circle diameter of retained austenite grains must be 1 μm or less. This is because by setting the maximum equivalent circle diameter to 1 μm or less, hydrogen traps in retained austenite are dispersed and local concentration of hydrogen traps is avoided, thereby reducing crack propagation due to hydrogen embrittlement. The amount of retained austenite in the surface layer structure is preferably 15% or less, more preferably 10% or less in terms of area ratio.

The structure of the steel sheet is preferably martensite and/or bainite. At that time, the area ratio of martensite and/or bainite is preferably 80% or more.

Next, conditions for manufacturing the steel sheet of the present invention will be described. That is, it can be produced by heating a steel material having the chemical composition described above, followed by hot rolling and cooling, or by further heat-treating and cooling after hot rolling. At that time, when cooling after hot rolling, or when heat treatment is performed after hot rolling, cooling after the heat treatment, the average cooling rate in a predetermined temperature range is set to 1 ° C./s or more. necessary to obtain The manufacturing conditions will be described below in the order of steps. In the following description, the temperature (°C) means the temperature at the central portion of the sheet thickness.

First, the reheating temperature of the steel material in hot rolling is preferably 1000° C. or higher and 1300° C. or lower.
[Reheating temperature of steel material: 1000°C or higher and 1300°C or lower]
The reason why the steel material is heated to 1000° C. or higher is to dissolve precipitates in the structure and homogenize the crystal grain size, etc. The heating temperature is preferably 1000° C. or higher and 1300° C. or lower. . That is, if the heating temperature is less than 900° C., the precipitates may not be sufficiently solid-dissolved, so there is a possibility that the desired characteristics cannot be obtained. On the other hand, if the temperature exceeds 1300° C., the crystal grain size may be coarsened to deteriorate the quality of the material, and excessive energy may be required for manufacturing, resulting in a decrease in productivity. It is more preferably in the range of 1050° C. or higher and 1250° C. or lower, further preferably in the range of 1100° C. or higher and 1250° C. or lower.

[Cooling after hot rolling]
The surface layer structure of the steel sheet is preferably a martensite and/or bainite structure, and in order to increase the large-angle grain boundaries contained in the structure and ensure excellent stress corrosion cracking resistance, a cooling treatment is performed after hot rolling. is applied, and the average cooling rate in the temperature range of 600° C. or lower and 200° C. or higher in the surface layer structure is set to 1° C./s or higher. That is, when the cooling rate in this cooling treatment is less than 1 ° C / s, the surface layer structure becomes an upper bainite structure, the large angle grain boundaries contained in the structure are reduced, the structure is not sufficiently refined, and the stress corrosion cracking resistance is reduced. I can't get it. There is no particular need to limit the upper limit of the average cooling rate.
In addition, when the heat treatment described later is performed after hot rolling, the cooling rate after this hot rolling need not be 1° C./s or more.

[Heat treatment after hot rolling]
The following heat treatment may be performed without cooling after hot rolling. As described above, the surface layer structure of the steel sheet is preferably a martensite and/or bainite structure, and in order to increase the large angle grain boundaries contained in the structure and to ensure excellent stress corrosion cracking resistance, hot When heat treatment is performed after rolling, it is preferable to harden (primary hardening) by heating to Ac ₃ point or more and 900° C. or less after hot rolling. That is, when the heating temperature is less than the Ac ₃ point or exceeds 900° C., the equivalent circle diameter of the large-angle grain boundary becomes coarse, and the desired properties may not be obtained.

As described above, when the above-described heat treatment is performed after hot rolling, it is necessary to control the cooling rate after the heat treatment. That is, the average cooling rate in the surface layer structure in the temperature range of 600° C. to 200° C. is set to 1° C./s or more.

Furthermore, as a heat treatment after hot rolling, instead of the above-described quenching ( _primary quenching), or after cooling after primary quenching _, heat treatment (secondary quenching) may be applied. By performing this secondary hardening, the low temperature toughness of the base material can be improved.
As described above, when the heat treatment (secondary quenching) is performed, it is necessary to control the cooling rate after the heat treatment. That is, the average cooling rate in the surface layer structure in the temperature range of 600° C. to 200° C. is set to 1° C./s or more.

In order to obtain properties such as high strength and excellent low-temperature toughness, it is effective to make the retained austenite grains in the surface layer structure fine grains with a diameter of 1 μm or less. For that purpose, it is preferable to temper by heating to a temperature of 500° C. or more and 650° C. or less after the final cooling. That is, if the tempering temperature is less than 500°C, it may become difficult to ensure low-temperature toughness. On the other hand, if the tempering temperature exceeds 650° C., coarse retained austenite may be formed, and desired properties may not be obtained.

After melting the steels A to W shown in Table 1 and making them into slabs, steel plates (Samples No. 1 to 26) with a thickness of 30 to 50 mm were produced under the production conditions shown in Table 2, and each sample was produced. It was subjected to the following Charpy impact test and stress corrosion cracking test. In addition, the interval of large-angle grain boundaries and the grain size of retained austenite in the surface layer structure of each sample were investigated.

A large-angle grain boundary is defined as a grain boundary with a grain boundary misorientation of 15° or more, and was identified using EBSD. Then, the crystal grain size was measured in an arbitrary range of 500×500 μm at a depth of 1 mm from the surface of the steel sheet, and the average value of the equivalent circle diameters of the crystal grains surrounded by the large-angle grain boundaries was obtained. In addition, those having a range of less than 0.1 μm surrounded by large-angle grain boundaries were excluded from the calculation.

In addition, the maximum equivalent circle diameter of retained austenite was determined from the crystal structure of retained austenite grains present in the same EBSD measurement region, and was the maximum equivalent circle diameter of the crystal grains recognized as austenite.

[Charpy impact test (low temperature toughness)]
For each sample, a V-notch test piece specified in JIS Z2242 was prepared, and a Charpy impact test was carried out according to JIS Z2242 at a test temperature of -196°C to measure absorbed energy. Three test pieces were tested for each sample, and the average value of 34 J or more was considered acceptable.

[Stress corrosion cracking test (stress corrosion cracking resistance)]
A DCB (Double-Cantilever-Beam) test was performed in accordance with NACE TM0177-96 2003 edition. The test environment was NACE TM0177 sol. A (initial pH 2.7)×100% H ₂ S gas saturation (0.1 MPa), and the immersion time was 336 hours. After immersion, KISSC was derived from wedge load and crack length. Three test pieces were tested for each sample, and the average value of 25 MPa√m or more was considered acceptable.
Table 2 shows the results obtained as described above.

Sample no. It was confirmed that Nos. 1 to 14, 23, and 26 ensure low-temperature toughness and have excellent resistance to stress corrosion cracking. On the other hand, the comparative examples (Sample Nos. 15 to 22, 24 and 25) outside the scope of the present invention had an absorbed energy lower than 34 J or a DCB test result of less than 25 MPa√m%, and the above target performance was not achieved. I wasn't satisfied.

Claims (5)

in % by mass,
C: 0.01% or more and 0.15% or less,
Si: 0.01% or more and 1.00% or less,
Mn: 0.10% or more and 3.00% or less,
Al: 0.002% or more and 0.100% or less,
Ni: 5.0% or more and 10.0% or less,
N: 0.0010% or more and 0.0080% or less,
Co: 0.05% or more and 1.50% or less,
P: 0.030% or less and S: 0.0050% or less, with the balance being Fe and unavoidable impurities,
In the structure from the surface of the steel sheet to a position 1 mm deep, the average equivalent circle diameter of crystal grains surrounded by large-angle grain boundaries with a misorientation of 15° or more is 5 μm or less, and the maximum equivalent circle diameter of retained austenite grains is 1 μm. Steel plate which is below. The component composition is further mass %,
Nb: 0.001% or more and 0.030% or less,
V: 0.01% or more and 0.10% or less,
Ti: 0.003% or more and 0.050% or less,
B: 0.0003% or more and 0.0100% or less,
Cu: 0.01% or more and 1.00% or less,
Cr: 0.01% or more and 1.50% or less,
Sn: 0.01% or more and 0.50% or less,
Sb: 0.01% or more and 0.50% or less,
The steel sheet according to claim 1, containing one or more selected from Mo: 0.03% or more and 1.00% or less and W: 0.05% or more and 2.00% or less. The component composition is further mass %,
Ca: 0.0005% or more and 0.0050% or less,
Zr: 0.0005% or more and 0.0050% or less,
The steel sheet according to claim 1 or 2, containing one or more selected from Mg: 0.0005% or more and 0.0050% or less and REM: 0.0010% or more and 0.0100% or less. 4. A method for manufacturing a steel sheet, wherein the steel material having the chemical composition according to any one of claims 1 to 3 is heated, hot-rolled, and then cooled, wherein an average temperature of 600 ° C. or less and 200 ° C. or more in the cooling process When the cooling rate is 1°C/s or more, the structure from the surface of the steel sheet to a depth of 1 mm has an average equivalent circle diameter of 5 μm or less of crystal grains surrounded by large-angle grain boundaries with a misorientation of 15° or more. A method for producing a steel sheet in which the maximum equivalent circle diameter of retained austenite grains is 1 µm or less . 4. A method for manufacturing a steel sheet, wherein the steel material having the chemical composition according to any one of claims 1 to 3 is heated, hot-rolled, further heat-treated, and then cooled, wherein the cooling treatment is performed at 600 ° C. or less. When the average cooling rate at 200°C or higher is 1°C/s or higher, the structure from the surface of the steel sheet to a depth of 1 mm corresponds to an average circle of crystal grains surrounded by large-angle grain boundaries with a misorientation of 15° or higher. A method for producing a steel sheet having a diameter of 5 μm or less and a maximum circle equivalent diameter of retained austenite grains of 1 μm or less .