JPWO2021117382A1 - Steel plate and its manufacturing method - Google Patents

Abstract

Provided is a steel sheet having excellent stress corrosion cracking resistance, which is suitable for use in a low temperature environment. 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 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: A large-angle grain boundary containing 0.0050% or less, having a residual Fe and a component composition which is an unavoidable impurity, and having a structure from the surface of the steel plate to a depth of 1 mm with an orientation difference of 15 ° or more. The average circle-equivalent diameter of the crystal grains surrounded by is 5 μm or less, and the maximum circle-equivalent diameter of the residual austenite grains is 1 μm or less.

Description

The present invention relates to a steel sheet having excellent stress corrosion cracking resistance, which is suitable for structural steel used in an extremely low temperature environment, such as a tank for a liquefied gas storage tank, 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 usage environment is extremely low, so that not only the strength of the steel sheet but also the toughness at an extremely low temperature is required. For example, when a hot-rolled steel sheet is used in a storage tank for liquefied natural gas, it is necessary to secure excellent toughness at -164 ° C. or lower, which is the boiling point of liquefied natural gas. If the low temperature toughness of the steel material is inferior, there is a risk that the safety of the structure for the cryogenic storage tank cannot be maintained. Therefore, there is a strong demand for improving the low temperature toughness of the applied steel sheet. Conventionally, a 7% Ni steel sheet or a 9% Ni steel sheet has been used in response to this requirement.

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

International Publication No. 2007/034576 International Publication No. 2007/080646 Japanese Unexamined Patent Publication No. 2011-241419

However, although the Ni-containing steel materials described in Patent Documents 1, 2 and 3 are excellent in low-temperature toughness, stress corrosion cracking caused by hydrogen is not mentioned, and there is still room for study. That is, for example, in the case of a marine LNG tank, since sulfide and chloride are contained in the usage environment, there is a high possibility that stress corrosion cracking due to hydrogen will occur, so that it is durable against stress corrosion cracking. It is also required to have stress corrosion cracking resistance.

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

In order to solve the above problems, the present inventors have conducted intensive studies on the composition and structure of the steel sheet, and obtained the following findings.
(1) By adding Co, when corrosion progresses on the surface of the steel sheet, Co is concentrated on the surface of the steel sheet, hydrogen intrusion into the steel can be reduced, and crack growth due to hydrogen embrittlement can be reduced.

(2) By setting the cooling rate to 1 ° C./s or higher in 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 oriented. It becomes a microstructure in which the diameter of the crystal grains surrounded by the large-angle grain boundaries with a difference of 15 ° or more is 5 μm or less. Then, this microstructure disperses the trap sites of hydrogen, so that crack growth due to hydrogen embrittlement can be reduced.

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

The present invention has been made by further studying the above findings, and the gist thereof is as follows.
1. 1. 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 less than 1.50%,
It contains P: 0.030% or less and S: 0.0050% or less, and has a component composition that is a balance Fe and an unavoidable impurity.
In the structure from the surface of the steel sheet to the position of 1 mm in depth, the average circle equivalent diameter of the crystal grains surrounded by the large angle grain boundaries with an orientation difference of 15 ° or more is 5 μm or less, and the maximum circle equivalent diameter of the residual austenite grains is 1 μm. The following steel plate.

2. 2. The composition of the components is further increased by 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 1 above, which contains 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. 3. The composition of the components is further increased by 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 1 or 2 above, which contains 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 producing a steel sheet in which a steel material having the component composition according to any one of 1 to 3 is heated, hot-rolled, and then cooled, the temperature range of 600 ° C. or lower and 200 ° C. or higher in the cooling treatment is performed. A method for manufacturing steel sheets with an average cooling rate of 1 ° C / s or higher.

5. In a method for producing a steel sheet in which a steel material having the component composition according to any one of 1 to 3 is heated, hot-rolled, and then heat-treated and then cooled, the temperature is 600 ° C. or lower in the cooling treatment. A method for manufacturing a steel sheet in which the average cooling rate in a temperature range of ° C or higher is 1 ° C / s or higher.

According to the present invention, it is possible to provide a steel sheet having high durability against stress corrosion cracking due to hydrogen. By applying this steel sheet to a steel structure used in a low temperature environment such as a tank for a liquefied gas storage tank, the safety of the steel structure can be improved, which brings about a remarkable industrial effect.

Hereinafter, embodiments of the present invention will be described. The present invention is not limited to the following embodiments.
[Ingredient composition]
First, the component composition of the steel sheet of the present invention and the reason for its limitation will be described. In the present invention, in order to ensure excellent corrosion resistance, the composition of the steel sheet is defined as follows. In addition, the% display in the component composition shall mean mass% unless otherwise specified.

C: 0.01% or more and 0.15% or less C is effective for increasing the strength, and in order to obtain the effect, it is necessary to contain C at 0.01% or more. On the other hand, if it is contained in excess of 0.15%, the low temperature toughness is lowered. Therefore, C is set to 0.01% or more and 0.15% or less. It is preferably 0.03% or more. It is preferably 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 has the effect of dissolving in steel and strengthening the solid solution to increase the strength of the steel sheet. .. In order to obtain this effect, it is necessary to contain Si at 0.01% or more. On the other hand, if it is contained in excess of 1.00%, the low temperature toughness deteriorates. Therefore, Si is set to 0.01% or more and 1.00% or less. It is preferably 0.03% or more. It is preferably 0.5% or less.

Mn: 0.10% or more and 3.00% or less Mn is an element that enhances the hardenability of steel and is effective in increasing the strength of steel sheets. In order to obtain the effect, Mn needs to be contained in an amount of 0.01% or more. On the other hand, if it is contained in excess of 3.00%, the corrosion cracking resistance is lowered. Therefore, Mn is set in the range of 0.10% or more and 3.00% or less. It is preferably 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 of steel sheets. Further, it has the effect of fixing the solid solution N in the steel to form AlN and suppressing the deterioration of toughness due to the reduction of the solid solution N. On the other hand, if it is contained in excess of 0.100%, the toughness is deteriorated, so the content is set to 0.100% or less. It is preferably 0.010% or more. It is preferably 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 extremely effective element for improving the low temperature toughness of steel sheets. On the other hand, since it is an expensive element, the cost of the steel sheet rises as its content increases. Therefore, in the present invention, the Ni content is set to 10.0% or less. However, when the Ni content is less than 5.0%, the strength of the steel sheet is lowered, and stable retained austenite cannot be obtained at a low temperature. As a result, the low temperature toughness and strength of the steel sheet are lowered. Therefore, the Ni content is set to 5.0% or more and 10.0% or less. It is preferably 9.5% or less. Preferably, it is 6.0% or more.

N: 0.0010% or more and 0.0080% or less N forms a precipitate in steel, and when the content exceeds 0.0080%, when the steel plate is welded to form a welded structure, it is a base material. And it causes a decrease in toughness of the weld heat affected zone. However, N is also an element that contributes to the 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 is 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 less than 1.50% Co is an important element that concentrates on the surface layer of steel sheets in a corrosive environment and contributes to suppressing corrosion cracking by reducing the intrusion of hydrogen. Therefore, it is necessary to contain more than 0%. The amount of Co is preferably 0.05% or more, more preferably 0.1% or more. However, even if it is contained in excess of 1.50%, the effect is saturated and Co is an expensive element, so the maximum addition amount is 1.50%.

P: 0.030% or less If P is contained in excess of 0.030%, the corrosion cracking resistance is lowered. Therefore, it is desirable to set 0.030% as the upper limit and reduce it as much as possible. Therefore, P is 0.030% or less. The lower the content of P, the better the characteristics. Therefore, the amount of P is preferably 0.025% or less, more preferably 0.020% or less. It goes without saying that the content of P may be 0%, but since it requires a high cost to remove P, it is preferably 0.002% or more from the viewpoint of cost.

S: 0.0050% or less S forms MnS in steel and significantly deteriorates low temperature toughness. Therefore, it is desirable to limit S to 0.0050% and reduce it as much as possible. It is preferably 0.0020% or less. It goes without saying that the content of S may be 0%, but since it requires a high cost to remove S, it is preferably 0.0005% or more from the viewpoint of cost.

Basically, it contains each of the above elements, and the balance is Fe and unavoidable impurities.
In the present invention, for the purpose of further improving the strength and low temperature toughness, the following elements can be contained, if necessary, in addition to the above essential elements.

Nb: 0.001% or more and 0.030% or less Nb is an element effective for improving the strength of the steel sheet. In order to obtain such an effect, it is preferable to add Nb in an amount of 0.001% or more. On the other hand, if it is contained in an amount of more than 0.030%, coarse carbonitride may be precipitated and the toughness of the base metal may be deteriorated. Therefore, when Nb is contained, it 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 for improving the strength of the steel sheet. In order to obtain such an effect, it is preferable to add V at 0.01% or more. On the other hand, if the content exceeds 0.10%, coarse carbonitride may precipitate and become a starting point of fracture. In addition, the precipitate may become coarse and the toughness of the base metal may be deteriorated. Therefore, when V is contained, it is 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% or more and 0.050% or less Ti is an element that precipitates as a nitride or carbonitride and is effective for improving the strength of steel sheets. In order to obtain such an effect, it is preferable to add Ti at 0.003% or more. On the other hand, if it is contained in excess of 0.050%, the precipitate may be coarsened and the toughness of the base metal may be deteriorated. In addition, coarse carbonitride may precipitate and become a starting point of fracture. Therefore, when Ti is contained, it is set to 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 for improving the strength of the steel sheet. In order to obtain such an effect, it is preferable to add B at 0.0003% or more. On the other hand, if it is contained in excess of 0.0100%, coarse B precipitates may be formed and the toughness may be lowered. Therefore, B is in the range of 0.0003% or more and 0.0100% or less. It is preferably 0.0030% or less.

Cu: 0.01% or more and 1.00% or less Cu is an element effective for increasing the strength of a steel sheet by improving 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 thereof 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. Therefore, when it is added, it is preferably 0.01% or more. 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 the improvement of low temperature toughness and corrosion resistance of high Mn steel. For that purpose, it is preferable that the amount of Cr is 0.01% or more. On the other hand, Cr may precipitate in the form of nitrides, carbides, carbonitrides, etc. during rolling, and the formation of such precipitates causes corrosion and fracture to reduce low temperature toughness. It is preferably 1.50%. More preferably, it is 1.00% or less.

Mo: 0.03% or more and 1.00% or less Mo is an element effective for suppressing the temper embrittlement sensitivity of a steel sheet, and is also an element for increasing the strength of a steel sheet without impairing low temperature toughness. In order to obtain such an effect, the Mo content is preferably 0.03% or more. On the other hand, if Mo exceeds 1.00%, the low temperature toughness may decrease. Therefore, when Mo is contained, the content thereof is set to 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% or more and 0.50% or less Sb: 0.01% or more and 0.50% or less W: 0.05% or more and 2.00% or less Sn, Sb and W are elements effective for 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 a large amount of any of the elements is contained, the weldability and toughness are deteriorated, which may be disadvantageous from the viewpoint of cost. Therefore, the Sn amount is in the range of 0.01% or more and 0.50% or less, the Sb amount is in the range of 0.01% or more and 0.50% or less, and the W amount is in the range of 0.05% or more and 2.00% or less. do. Preferably, the Sn amount is 0.02% or more and 0.25% or less, the Sb amount is 0.02% or more and 0.25% or less, and the W amount is 0.10% or more and 1.00% or less.

Further, in the present invention, the following elements can be contained, if necessary.
Ca: 0.0005% or more and 0.0050% or less, Zr: 0.0005% or more and 0.0050% or less, Mg: 0.0005% or more and 0.0050% or less and REM: 0.0010% or more and 0.0100% One or more of the following Ca, Zr, Mg and REM are elements useful for morphological control of inclusions such as MnS and can be added as needed. Here, the morphological control of inclusions means that the expanded sulfide-based inclusions are made into granular inclusions. Toughness and sulfide stress corrosion cracking resistance are improved through morphological control of the inclusions. In order to obtain such an effect, it is preferable that Ca, Zr and Mg are contained in an amount of 0.0005% or more and REM is contained in an amount of 0.0010% or more. On the other hand, if a large amount of any of the elements is contained, the amount of non-metal inclusions may increase and the low temperature toughness may decrease. Therefore, when Ca, Zr, and Mg are contained, the content is 0.0005% or more and 0.0050% or less, respectively, and when REM is contained, the content is 0.0010% or more and 0.0100% or less. More preferably, the Ca amount is 0.0010% or more and 0.0040% or less, the Zr amount is 0.0010% or more and 0.0040% or less, the Mg amount is 0.0010% or more and 0.0040% or less, and the REM amount is 0. It shall be 0020% or more and 0.0100% or less.

[Surface structure]
Next, in the structure from the surface of the steel sheet to the position where the depth is 1 mm (hereinafter, also referred to as the surface layer structure), the average circle equivalent diameter of the crystal grains surrounded by the large angle grain boundaries having an orientation difference of 15 ° or more is 5 μm or less. Moreover, it is important that the diameter corresponding to the maximum circle of the retained austenite grains is 1 μm or less.
First, it is necessary to set the average circle equivalent diameter of the crystal grains surrounded by the large-angle grain boundaries having an orientation difference of 15 ° or more to 5 μm or less. This is because the amount of grain boundaries having an orientation difference of 15 ° or more, which is a trap site for hydrogen, increases and is dispersed, so that crack growth due to hydrogen embrittlement can be reduced. Further, the average circle equivalent diameter of the crystal grains is preferably 4 μm or less, more preferably 3 μm or less.

The crystal grains surrounded by the large angle grain boundaries having an orientation difference of 15 ° or more and the average circle equivalent diameter of the crystal grains can be specified by the measurement method in the examples described later.

In order to reduce the average circle equivalent diameter of the crystal grains surrounded by the large angle grain boundaries with an orientation difference of 15 ° or more to 5 μm or less, the temperature is determined after hot rolling or, if heat treatment is performed after hot rolling, after the heat treatment. Perform cooling treatment with an average cooling rate of 1 ° C./s or higher in the region.

Further, in the surface layer structure, the maximum circle equivalent diameter of the retained austenite grains needs to be 1 μm or less. This is because by setting the diameter corresponding to the maximum circle to 1 μm or less, the hydrogen traps on the retained austenite are dispersed and the local concentration of the hydrogen traps is avoided, and as a result, the crack growth due to hydrogen embrittlement can be reduced. The amount of retained austenite in the surface layer structure is preferably 15% or less in terms of area ratio, and more preferably 10% or less.

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, the 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 above-mentioned composition and cooling it by hot rolling, or by further heat-treating it after hot rolling and cooling it. At that time, in the cooling after hot rolling, or in the case of cooling after the heat treatment, the average cooling rate in a predetermined temperature range should be 1 ° C./s or more to obtain the above-mentioned surface structure. Needed to get. Hereinafter, the manufacturing conditions will be described in order of process. In the following description, the temperature (° C.) means the temperature at the center of the plate 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 the precipitates in the structure to make the crystal grain size uniform, and 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 precipitate may not be sufficiently solid-solved, so that the desired characteristics may not be obtained. On the other hand, if it is heated above 1300 ° C., the material may be deteriorated due to the coarsening of the crystal grain size, and excessive energy may be required for production to reduce the productivity. More preferably, it is in the range of 1050 ° C or higher and 1250 ° C or lower, and further preferably 1100 ° C or higher and 1250 ° C or lower.

[Cooling after hot rolling]
In order to make the surface structure of the steel plate preferably a martensite and / or bainite structure and to increase the large grain boundaries contained in the structure to ensure excellent stress corrosion cracking resistance, a cooling treatment is performed after hot rolling. The average cooling rate of the surface layer structure in the temperature range of 600 ° C. or lower and 200 ° C. or higher 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 finely divided, and stress corrosion cracking resistance is deteriorated. I can't get it. The upper limit of the average cooling rate does not need to be particularly limited.
When the heat treatment described later is performed after hot rolling, it is not necessary to set the cooling speed after hot rolling to 1 ° C./s or higher.

[Heat treatment after hot rolling]
After hot rolling, the following heat treatment may be performed without cooling. As described above, in order to make the surface structure of the steel plate preferably a martensite and / or bainite structure and to increase the large grain boundaries contained in the structure to ensure excellent stress corrosion cracking resistance, it is hot. When heat treatment is performed after rolling, it is preferable to heat and quench (primary quenching) at _{3 points or more and 900 ° C. or less after hot rolling.} That is, if the heating temperature is _{less than 3} points of Ac or exceeds 900 ° C., the equivalent circle diameter of the large angle grain boundaries becomes coarse, and there is a possibility that the desired characteristics cannot be obtained.

As described above, when the above heat treatment is performed after hot rolling, it is necessary to control the speed of cooling after the heat treatment. That is, 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.

Further, as a heat treatment after hot rolling, a heat treatment (secondary) in which the above-mentioned quenching (primary quenching) is performed or the primary quenching is performed to cool the material, and then the material is heated to a temperature _{equal to or higher than the Ac 1} transformation point and lower than the Ac _{3 transformation point.} (Quenching) may be applied. By performing this secondary quenching, the low temperature toughness of the base metal can be improved.
As described above, when the above-mentioned heat treatment (secondary quenching) is performed, it is necessary to control the speed of cooling after the heat treatment. That is, 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.

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

After the steels A to W shown in Table 1 are melted and made into slabs, steel plates (Sample Nos. 1 to 26) having a thickness of 30 to 50 mm are manufactured according to the manufacturing conditions shown in Table 2, and each sample is prepared. It was subjected to the following Charpy impact test and stress corrosion cracking test. In addition, for each sample, the spacing between large grain boundaries and the residual austenite grain size in the surface structure were investigated.

Large-angle grain boundaries were defined as grain boundaries with grain boundary orientation differences of 15 ° or more, which were identified using EBSD. 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. The range surrounded by the large-angle grain boundaries of less than 0.1 μm was excluded from the calculation.

Further, the maximum circle-equivalent diameter of the retained austenite was set to the circle-equivalent diameter of the largest of the crystal grains recognized as austenite by specifying the retained austenite grains existing in the same EBSD measurement region from the crystal structure.

[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 in accordance with JIS Z2242 at a test temperature of -196 ° C., and the absorbed energy was measured. The test was carried out with three test pieces for each sample, and the case where the average value thereof was 34J or more was regarded as acceptable.

[Stress corrosion cracking test (stress corrosion cracking resistance)]
A DCB (Double-Cantilever-Beam) test was conducted in accordance with NACE TM0177-96 2003 version. Test environment, NACE TM0177 sol.A (initial pH2.7) × 100% H ₂ S gas saturation (0.1 MPa) immersion time was 336 hours. After the immersion was completed, KISSC was derived from the Wedge load and crack length. The test was carried out with three test pieces for each sample, and the case where the average value thereof was 25 MPa√m or more was regarded as acceptable.
The results obtained as described above are shown in Table 2.

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

Claims (5)

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 less than 1.50%,
It contains P: 0.030% or less and S: 0.0050% or less, and has a component composition that is a balance Fe and an unavoidable impurity.
In the structure from the surface of the steel sheet to the position of 1 mm in depth, the average circle equivalent diameter of the crystal grains surrounded by the large angle grain boundaries with an orientation difference of 15 ° or more is 5 μm or less, and the maximum circle equivalent diameter of the residual austenite grains is 1 μm. The following steel plate. The composition of the components is further increased by 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, which contains 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 composition of the components is further increased by 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, which contains 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. In a method for producing a steel sheet in which a steel material having the component composition according to any one of claims 1 to 3 is heated, hot-rolled, and then cooled, the average of 600 ° C. or lower and 200 ° C. or higher in the cooling treatment. A method for manufacturing steel sheets with a cooling rate of 1 ° C / s or higher. In a method for producing a steel sheet in which a steel material having the component composition according to any one of claims 1 to 3 is heated, hot-rolled, and then heat-treated and then cooled, the temperature is 600 ° C. or lower in the cooling treatment. A method for manufacturing a steel sheet, in which an average cooling rate of 200 ° C. or higher is 1 ° C./s or higher.