JPWO2020036090A1 - Steel plate and method of manufacturing the same - Google Patents

Abstract

Provided is a high Mn steel excellent in corrosion resistance, particularly in a salt corrosion environment. C: 0.20% to 0.70%, Si: 0.05% to 1.00%, Mn: 15.0% to 35.0%, P: 0.030% or less, S:0 0.0200% or less, Al: 0.010% or more and 0.100% or less, Cr: 0.5% or more and 8.0% or less and N: 0.0010% or more and 0.0300% or less, and the balance Fe and It has a composition of inevitable impurities, and 60% or more of the Cr is solid solution Cr.

Description

The present invention relates to a steel sheet suitable for structural steel used in a cryogenic environment such as a tank for a liquefied gas storage tank, and particularly excellent in corrosion resistance in a salt water corrosive environment, and a manufacturing method thereof.

Attempts have been made to use hot-rolled steel sheets for structures such as liquefied gas storage tanks. Since such a structure has an extremely low usage environment, the hot rolled steel sheet applied to the structure is required to have not only high strength but also excellent toughness at extremely low temperatures. 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 a boiling point of liquefied natural gas, -164°C or lower. If the low temperature toughness of the steel material is inferior, there is a risk that the safety as a structure for a cryogenic storage tank cannot be maintained, and therefore, there is a strong demand for improving the low temperature toughness of the applied steel material.

To meet this requirement, conventionally, an austenitic stainless steel having an austenitic structure that does not exhibit brittleness at extremely low temperatures, a 9% Ni steel, or a 5000 series aluminum alloy has been used. However, since these metal materials have high alloy costs and manufacturing costs, there is a demand for steel sheets that are inexpensive and have excellent cryogenic toughness. Therefore, as a new steel sheet replacing the conventional cryogenic steel, a high Mn steel having austenite structure by adding a large amount of Mn, which is a relatively inexpensive austenite stabilizing element, is applied as a structural steel sheet in a cryogenic environment. Is being considered.

However, when a steel sheet having an austenite structure is placed in a corrosive environment, there is a problem that stress corrosion cracking easily occurs when austenite grain boundaries are corroded by corrosion and a tensile stress is applied. At the manufacturing stage of structures such as liquefied gas storage tanks, the base metal of the steel plate may be exposed on the surface, and if the steel surface comes into contact with water vapor or water or oil containing corrosive substances such as salt, it will corrode the steel. Occurs. At this time, in the corrosion reaction on the surface of the steel sheet, iron produces oxides (rust) by the anode reaction, while hydrogen is generated by the cathode reaction of moisture, and hydrogen embrittlement is caused by the intrusion of hydrogen into the steel. The change occurs. If residual stress in bending or welding at the time of manufacture or load stress in the working environment acts on it, stress corrosion cracking may occur and the structure may be destroyed. The high Mn steel that has been conventionally studied may be inferior in corrosion resistance to 9% Ni steel and ordinary low alloy steel as well as austenitic stainless steel. Therefore, from the viewpoint of safety, it is important that the steel material used for the structure has not only high strength and toughness at cryogenic temperatures but also excellent corrosion resistance.

For example, in Patent Document 1, by adding Mn in an amount of 15 to 35%, Cu in an amount of 5% or less, and C and Cr in appropriate amounts, the machinability and the Charpy impact property of the heat-affected heat affected zone at -196°C. The steel material which improved is disclosed.

In Patent Document 2, C: 0.25 to 0.75%, Si: 0.05 to 1.0%, Mn: more than 20% and 35% or less, Ni: 0.1% or more and 7.0. %, Cr: 0.1% or more and less than 8.0% is added to improve the low temperature toughness, and a high Mn steel material is disclosed.

Furthermore, in patent document 3, 0.001 to 0.80% of C and 15 to 35% of Mn are contained, and by adding elements such as Cr, Ti, Si, Al, Mg, Ca, and REM, A high Mn steel material is disclosed which has improved cryogenic toughness of the material and welds.

Japanese Patent Publication No. 2015-508452 JP, 2016-84529, A JP, 2016-196703, A

However, the steel materials described in Patent Documents 1, 2 and 3 still have room for improvement in corrosion resistance when the austenitic steel materials described above are placed in a salt corrosive environment from the viewpoint of manufacturing cost for achieving strength and low temperature toughness. There is.

The present invention has been made in view of the above problems, and an object of the present invention is to provide a high Mn steel having excellent corrosion resistance, particularly corrosion resistance in a salt corrosion environment. Here, "excellent in corrosion resistance" is a test in conformity with the Slow Strain Rate Test Method of NACE Standard TM0111-2011 standard, and is immersed in artificial seawater (chloride ion concentration 18000 ppm) at a temperature of 23° C. and strain rate. It means that the breaking stress is 600 MPa or more when a constant velocity tensile test is performed at 4×10 ⁻⁷ inch/s.

In order to achieve the above-mentioned object, the inventors of the present invention have made extensive studies on various factors that determine the composition and manufacturing conditions of high Mn steel, and have obtained the following findings.
a. By adding Cr to the high Mn steel as a base and controlling the addition amount and the solid solution amount appropriately, it is possible to delay the initial corrosion reaction on the surface of the steel sheet in the salt water corrosion environment. Thereby, the amount of hydrogen penetrating into the steel can be reduced, and the above-mentioned stress corrosion cracking of the austenitic steel is suppressed.

b. Further, in order to effectively suppress the destruction of austenite from the crystal grain boundaries, it is effective to take measures to increase the crystal grain boundary strength. In particular, P is an element that easily segregates with Mn in the solidification process of the steel slab, and reduces the grain boundary strength at the portion intersecting with such a segregated portion. Therefore, it is necessary to reduce the impurity element such as P. On the other hand, B is an element that enhances the strength of the austenite grain boundary, and by adding B in addition to the reduction of the impurity element such as P, it becomes possible to further effectively suppress the grain boundary breakdown.

The present invention has been made by further studying the above findings, and the summary thereof is as follows.
1. In mass %,
C: 0.20% or more and 0.70% or less,
Si: 0.05% or more and 1.00% or less,
Mn: 15.0% or more and 35.0% or less,
P: 0.030% or less,
S: 0.0200% or less,
Al: 0.010% or more and 0.100% or less,
Cr: 0.5% or more and 8.0% or less,
N: 0.0010% or more and 0.0300% or less and B: 0.0003% or more and 0.0100% or less, with the balance Fe and unavoidable impurity component composition, and 60% or more of the Cr is solid. Steel plate that is molten Cr.

2. Further, the composition of the components is% by mass,
Nb: 0.003% or more and 0.030% or less,
V: 0.01% or more and 0.10% or less and Ti: 0.003% or more and 0.040% or less, and a steel sheet having excellent corrosion resistance as described in 1 above, which contains one or more.

3. Further, the composition of the components is% by mass,
Cu: 0.01% or more and 0.50% or less,
Ni: 0.01% or more and 0.50% or less,
Sn: 0.01% or more and 0.30% or less,
Sb: 0.01% or more and 0.30% or less,
The steel sheet according to 1 or 2 above, which contains one or more selected from Mo: 0.01% to 2.0% and W: 0.01% to 2.0%.

4. Further, the composition of the components is% by mass,
Ca: 0.0005% or more and 0.0050% or less,
The steel sheet according to the above 1, 2, or 3, containing one or more selected from Mg: 0.0005% to 0.0100% and REM: 0.0010% to 0.0200%.

5. After heating the steel material having the component composition according to any one of 1 to 4 above 1000° C. to 1300° C., hot rolling is performed at a finishing temperature of 750° C. or more and a rolled material temperature of 950° C. or less. A method for producing a steel sheet, which is carried out at a temperature of 600°C or higher for 30 minutes or less, and then cooled at an average cooling rate of 3°C/s or higher in a temperature range of 700°C or lower and 600°C or higher.

According to the present invention, it is possible to provide a steel sheet having excellent corrosion resistance, particularly corrosion resistance in a salt corrosion environment. Therefore, by using the steel sheet of the present invention for a steel structure used in a cryogenic environment, such as a tank for a liquefied gas storage tank, the safety and life of the steel structure are greatly improved, resulting in a remarkable industrial scale. Will bring the effect of. Further, the steel sheet of the present invention is cheaper than existing materials, and thus has an advantage of being economically advantageous.

Hereinafter, the steel sheet of the present invention will be described in detail. The present invention is not limited to the embodiments below.
[Ingredient composition]
First, the component composition of the steel sheet of the present invention and the reasons for limitation thereof 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, "%" showing a component composition shall mean the "mass %" unless there is particular notice.

C: 0.20% or more and 0.70% or less C is effective for increasing strength and is an inexpensive austenite stabilizing element, which is an important element for obtaining austenite. In order to obtain the effect, C needs to be contained at 0.20% or more. On the other hand, when the content exceeds 0.70%, excessive precipitation of Cr carbides and Nb, V, and Ti-based carbides is promoted, and these precipitates serve as the starting points of corrosion and lower the low temperature toughness. Therefore, C is set to 0.20% or more and 0.70% or less. Preferably, it is 0.25% or more and 0.60% or less.

Si: 0.05% or more and 1.00% or less Si acts as a deoxidizer and is not only necessary for steelmaking, but also has the effect of forming a solid solution with steel to strengthen the steel sheet by solid solution strengthening. .. In order to obtain such effects, Si needs to be contained in an amount of 0.05% or more. On the other hand, if the content exceeds 1.00%, the weldability and surface properties may deteriorate and the stress corrosion cracking resistance may decrease. Therefore, Si is set to 0.05% or more and 1.00% or less. Preferably, it is 0.07% or more and 0.50% or less.

Mn: 15.0% to 35.0% Mn is a relatively inexpensive austenite stabilizing element. In the present invention, it is an important element for achieving both strength and toughness at extremely low temperatures. In order to obtain the effect, Mn needs to be contained by 15.0% or more. On the other hand, if the content exceeds 35.0%, the effect of improving the toughness at cryogenic temperatures is saturated and the alloy cost is increased. In addition, weldability and cuttability are deteriorated. Further, it induces segregation of Mn and promotes the occurrence of stress corrosion cracking. Therefore, Mn is set to 15.0% or more and 35.0% or less. Preferably, the range is 18.0% or more and 28.0% or less.

P: 0.030% or less When P is contained in excess of 0.030%, it segregates at the grain boundaries, lowers the grain boundary strength, and becomes a starting point of stress corrosion cracking. For this reason, it is desirable to set 0.030% as the upper limit and reduce it as much as possible. The lower the content of P, the better the characteristics. Therefore, the content of P is preferably 0.024% or less, and more preferably 0.020% or less. On the other hand, if P is less than 0.001%, steelmaking requires a great deal of cost and the economic efficiency is impaired, so 0.001% or more is allowed from the economical viewpoint.

S: 0.0200% or less S degrades the low temperature toughness and ductility of the base material, so 0.0200% is the upper limit, and it is desirable to reduce as much as possible. Therefore, S is 0.0200% or less, preferably 0.0180% or less. On the other hand, if the content is less than 0.0001%, a large amount of cost is required for steel making and the economical efficiency is impaired. Therefore, from the economical viewpoint, the content of 0.0001% or more is allowed.

Al: 0.010% or more and 0.100% or less Al acts as a deoxidizing agent and is most commonly used in the molten steel deoxidizing process. Further, by fixing the solid solution N in the steel to form AlN, it has an effect of suppressing coarsening of crystal grains. Further, it has an effect of suppressing deterioration of toughness due to reduction of solute N. In order to obtain such an effect, Al needs to be contained by 0.01% or more. On the other hand, if the content exceeds 0.100%, coarse nitrides may be formed, which may cause corrosion or fracture, and may reduce the stress corrosion cracking resistance. Further, during welding, it diffuses into the weld metal portion and deteriorates the toughness of the weld metal. Therefore, Al is 0.100% or less. Preferably, it is 0.020% or more and 0.070% or less.

Cr: 0.5% or more and 8.0% or less and 60% or more of Cr is solid solution Cr
Cr, when contained in an appropriate amount, has the effect of delaying the initial corrosion reaction on the steel sheet surface in a saltwater corrosive environment. This effect is an important element that reduces the amount of hydrogen penetration into the steel sheet and improves the stress corrosion cracking resistance. In order to obtain such an effect, it is necessary to contain 0.5% or more. On the other hand, when the content of Cr exceeds 8.0%, the above-described effect obtained is saturated and the economy is rather impaired. Therefore, the Cr content is 0.5% or more and 8.0% or less. It is preferably 1.0% or more.

Here, of the added Cr, the solid solution content contributes to the improvement of the stress corrosion cracking resistance, but the precipitation content may hinder the improvement of the stress corrosion cracking resistance. It is important that at least 60% is solid solution Cr. That is, if the solid solution Cr is 60% or more of the content Cr, the above effects can be enjoyed, and the improvement of the stress corrosion cracking resistance due to the addition of Cr can be realized. The solid solution Cr content is preferably 70% or more of the Cr content, and more preferably 100%.

The solute Cr is a state in which solute atoms exist in the atomic state without forming precipitates and the like. Specifically, the amount of solid solution Cr was extracted by an electrolytic extraction method using a 10% AA (10% acetylacetone-1% tetramethylammonium chloride-methanol) solution obtained by collecting a test piece for electrolytic extraction from a steel plate. For the precipitate, it can be determined by measuring the amount of Cr in the precipitate by ICP emission spectrometry and subtracting it from the total Cr in the test piece.

N: 0.0010% or more and 0.0300% or less N is an austenite stabilizing element and is an element effective for improving cryogenic toughness. Further, it combines with Nb, V and Ti and finely precipitates as a nitride or a carbonitride to have an effect of suppressing stress corrosion cracking as a diffusible hydrogen trap site. To obtain such an effect, N needs to be contained in an amount of 0.0010% or more. On the other hand, if the content exceeds 0.0300%, not only the excessive nitride or carbonitride formation is promoted, the amount of solid solution element is decreased to lower the corrosion resistance, but also the toughness is decreased. Therefore, N is set to 0.0010% or more and 0.0300% or less. Preferably it is 0.0020% or more and 0.0150% or less.

B: 0.0003% or more and 0.0100% or less B is an element that enhances the strength of austenite grain boundaries, and is an element that suppresses cracks at the grain boundaries and is effective in improving stress corrosion cracking resistance. To obtain such an effect, B needs to be contained in an amount of 0.0003% or more. It is preferably 0.0005% or more, more preferably more than 0.0007% and more than 0.0010%. On the other hand, when the content exceeds 0.0100%, this effect is saturated. Therefore, B is limited to the range of 0.0100% or less. Preferably, it is 0.0070% or less.

In the present invention, for the purpose of further improving the corrosion resistance, Nb: 0.003% or more and 0.030% or less, V: 0.01% or more and 0.10, if necessary, in addition to the above essential elements. % Or less and Ti: 0.003% or more and 0.040% or less can be contained.

Nb: 0.003% or more and 0.030% or less Nb is an element that has the effect of suppressing stress corrosion cracking because it precipitates as carbonitrides and the precipitated carbonitrides function as trap sites for diffusible hydrogen. .. In order to obtain such effects, Nb is preferably contained in an amount of 0.003% or more. On the other hand, if the content is more than 0.030%, coarse carbonitrides may be precipitated and may become the starting point of fracture. In addition, the precipitate may become coarse and deteriorate the toughness of the base material. Therefore, when Nb is contained, the content is preferably 0.003% or more and 0.030% or less. More preferably, it is 0.005% or more and 0.025% or less, and further 0.007% or more and 0.022% or less.

V: 0.01% or more and 0.10% or less V is an element that has the effect of suppressing stress corrosion cracking because it precipitates as carbonitrides and the generated carbonitrides function as trap sites for diffusible hydrogen. .. In order to obtain such effects, V is preferably contained at 0.01% or more. On the other hand, if the content exceeds 0.10%, coarse carbonitrides may be precipitated, which may be the starting point of fracture. In addition, the precipitate may become coarse and deteriorate the toughness of the base material. Therefore, when V is contained, it is preferably 0.01% or more and 0.10% or less. More preferably, it is 0.02% or more and 0.09% or less, and further 0.03% or more and 0.08% or less.

Ti: 0.003% or more and 0.040% or less Ti precipitates as a nitride or carbonitride, and the generated nitride or carbonitride functions as a trap site for diffusible hydrogen, which suppresses stress corrosion cracking. It is an element that has an effect. In order to obtain such effects, it is preferable that the content of Ti is 0.003% or more. On the other hand, if the content exceeds 0.040%, the precipitate may be coarsened and the toughness of the base material may be deteriorated. In addition, coarse carbonitrides may be deposited and may become the starting point of fracture. Therefore, when Ti is contained, the content is preferably 0.003% or more and 0.040% or less. More preferably, it is 0.005% or more and 0.035% or less, and further 0.007% or more and 0.032% or less.

Furthermore, in the present invention, for the purpose of further improving the corrosion resistance, if necessary,
Cu: 0.01% or more and 0.50% or less, Ni: 0.01% or more and 0.50% or less, Sn: 0.01% or more and 0.30% or less, Sb: 0.01% or more and 0.30% Hereinafter, Mo: 0.01% or more and 2.0% or less, W: 0.01% or more and 2.0% or less, or one or more kinds can be contained.

That is, Cu, Ni, Sn, Sb, Mo, and W are elements that improve the corrosion resistance of high-Mn steel in a salt water corrosion environment by adding together with Cr. Here, Cu, Sn and Sb have an effect of suppressing the hydrogen generation reaction which is a cathode reaction by increasing the hydrogen overvoltage of the steel material. Ni forms a precipitate film on the surface of the steel material and physically suppresses permeation of corrosive anions such as Cl ⁻ into the base iron. Further, Cu, Ni, Sn, Sb, Mo and W are released as metal ions from the surface of the steel material during corrosion, and the corrosion product is densified, so that Cu, Ni, Sn, Sb, Mo Inhibits permeation of corrosive anions. Each Mo and W, of leaving as Mo ₄ ^2-and WO ₄ ^2-, by being adsorbed in corrosion products or steel sheet surface, and imparts a cationic permselective, penetration into the base steel corrosive anions Electrically suppress.

The above effects are manifested in high Mn steel when coexisting with Cr, and are manifested when 0.01% or more of each element is added. However, if any of these elements is contained in a large amount, weldability and toughness are deteriorated, which is also disadvantageous from the viewpoint of cost.
Therefore, the Cu content is 0.01% or more and 0.50% or less, the Ni content is 0.01% or more and 0.50% or less, and the Sn content is 0.01% or more and 0.30% or less, The amount of Sb is 0.01% or more and 0.30% or less, the amount of Mo is 0.01% or more and 2.0% or less, and the W amount is 0.01% or more and 2.0% or less. Is preferred.
More preferably, the Cu content is 0.02% or more and 0.40% or less, the Ni content is 0.02% or more and 0.40% or less, the Sn content is 0.02% or more and 0.25% or less, and the Sb content is 0. 0.02% or more and 0.25% or less, Mo amount is 0.02% or more and 0.40% or less, and W amount is 0.02% or more and 0.40% or less.

Similarly, in the present invention, for the purpose of further improving the corrosion resistance, if necessary,
One or more of Ca: 0.0005% or more and 0.0050% or less, Mg: 0.0005% or more and 0.0100% or less, and REM: 0.0010% or more and 0.0200% or less may be contained. it can.
That is, Ca, Mg, and REM are elements useful for controlling the morphology of inclusions, and can be contained if necessary. Here, the morphology control of inclusions means that expanded sulfide-based inclusions are made into granular inclusions. The ductility, toughness, and sulfide stress corrosion cracking resistance can be improved by controlling the morphology of the inclusions. In order to obtain such effects, it is preferable that Ca and Mg are contained at 0.0005% or more and REM is contained at 0.0010% or more. On the other hand, if any of these elements is contained in a large amount, the amount of non-metallic inclusions may increase, and the ductility, toughness, and sulfide stress corrosion cracking resistance may deteriorate. It may also be economically disadvantageous.

Therefore, 0.0005% or more and 0.0050% or less when Ca is contained, 0.0005% or more and 0.0100% or less when Mg is contained, and 0.0010% when REM is contained. It is preferably 0.0200% or more. More preferably, the Ca 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 0.0020% or more and 0.0150% or less.

Next, the manufacturing conditions of the present invention will be described. The temperature of the material to be rolled in the hot rolling step and the cooling rate in the subsequent cooling step mean the temperature and the cooling rate measured on the surface of the rolled material. That is, after heating a steel material having the above-described composition to 1000°C or higher and 1300°C or lower, hot rolling is performed at a rolling reduction ratio of 3 or more and 30 or less and a finishing temperature of 750°C or more, and a rolled material temperature of 950°C. A steel sheet is manufactured by applying a residence time at 600°C or higher for 30 minutes or less, and then cooling at an average cooling rate of 3°C/s or higher in a temperature range of 700°C or lower and 600°C or higher.

[Steel material heating temperature: 1000°C or more and 1300°C or less]
The reason why the steel material is heated to 1000° C. or higher is to dissolve the carbonitride in the structure as a solid solution to make the crystal grain size uniform. That is, when the heating temperature is less than 1000° C., the desired characteristics cannot be obtained because the carbonitride does not form a solid solution sufficiently. Further, if the heating temperature is higher than 1300°C, in addition to the deterioration of the material due to the coarsening of the crystal grain size, excessive energy is required and the productivity is lowered. Therefore, the upper limit of the heating temperature is set to 1300°C. It is preferably 1050°C or higher and 1250°C or lower, and more preferably 1070°C or higher and 1250°C or lower. In addition to the continuously cast slab, the steel material is preferably a steel material such as a slab or billet by a commonly known method such as an ingot making method. Needless to say, processing such as ladle refining and vacuum degassing may be added to the molten steel.

[Finishing temperature of hot rolling: 750°C or higher]
When the finishing temperature of hot rolling is less than 750°C, the amount of precipitated carbide during the rolling significantly increases, and the amount of solid solution Cr is secured even if the residence time at 600°C or more and 900°C or less is less than 30 minutes as described later. In some cases, it may not be possible to reduce corrosion resistance. Further, when rolling at a temperature lower than 750° C., the deformation resistance becomes large and an excessive load is applied to the manufacturing equipment. Therefore, the rolling finishing temperature is 750° C. or higher.

[Average cooling rate below 700°C and above 600°C: 3°C/s or above]
For the cooling after hot rolling, if the average cooling rate at 700°C or lower and 600°C or higher is less than 3°C/s, a large amount of precipitates such as Cr carbides are generated. Therefore, the average cooling rate should be 3°C/s or higher. limit. The range is preferably 10° C./s or more and 150° C./s or less.

[Residence time in temperature range of 950°C or lower and 600°C or higher: 30 minutes or less]
In hot rolling, when the material to be rolled stays in the temperature range of 950°C or lower and 600°C or higher for more than 30 minutes, a large amount of carbonitrides and carbides precipitate during rolling, and the required amount of solid solution Cr is In order to reduce the corrosion resistance and the cryogenic toughness, the residence time in the temperature range of 950°C or higher and 600°C or higher is restricted to 30 minutes or less. The range is preferably 5 minutes or more and 25 minutes or less.

Here, in order to set the residence time in the temperature range of 950° C. or lower and 600° C. or higher to 30 minutes or shorter, the length of the material to be rolled is set to 5000 mm or less, and the reduction ratio in hot rolling from the material to be rolled is 30 or less. Is preferred. That is, if the length of the material to be rolled is set to 5000 mm or less and the reduction ratio is set to 30 or less, the rolling time is shortened, and as a result, the residence time in the range of 950° C. or less and 600° C. or more can be 30 minutes or less. ..

As described above, the upper limit of the rolling reduction in hot rolling is preferably 30 or less. On the other hand, if the reduction ratio in the hot rolling is less than 3, the effect of promoting recrystallization and reducing the grain size is reduced, and as a result, coarse austenite grains remain, and that portion is preferentially oxidized, which results in corrosion resistance. There is a risk of deterioration. Therefore, it is preferable that the reduction ratio in hot rolling is 3 or more.
Here, the reduction ratio is defined by (plate thickness of rolling material to be subjected to hot rolling)/(plate thickness of steel sheet after hot rolling).

No. shown in Table 1 Sample Nos. 1 to 57 having a plate thickness of 6 mm or more and 50 mm or less under the manufacturing conditions shown in Table 2 after being melted to form a slab. Steel sheets 1 to 65 were manufactured. Then, the obtained steel sheet was subjected to the following corrosion resistance test. Table 2 also shows the results of measuring the amount of solid solution Cr by the above-mentioned electrolytic extraction method.

The corrosion resistance test was carried out in accordance with NACE Standard TM0111-2011 standard Slow Strain Rate Test Method (hereinafter, SSRT test). That is, as the shape of the test piece, a Type A round bar notched test piece was used, the test piece was immersed in artificial seawater (chloride ion concentration 18000 ppm) at a temperature of 23° C., and a constant velocity tensile test was performed at a strain rate of 4×10 ⁻⁷ inch/s. Was carried out. Here, the breaking stress of 600 MPa or more is excellent in stress corrosion cracking resistance.
Table 2 shows the results obtained as described above.

It was confirmed that the steel sheets according to the present invention (Sample Nos. 1 to 42) had a corrosion resistance of 600 MPa or more as a breaking stress in the SSRT test. On the other hand, in the comparative examples (Sample Nos. 43 to 65) out of the range of the present invention, the stress corrosion cracking resistance does not satisfy the above target performance.

Claims (5)

In mass %,
C: 0.20% or more and 0.70% or less,
Si: 0.05% or more and 1.00% or less,
Mn: 15.0% or more and 35.0% or less,
P: 0.030% or less,
S: 0.0200% or less,
Al: 0.010% or more and 0.100% or less,
Cr: 0.5% or more and 8.0% or less,
N: 0.0010% or more and 0.0300% or less and B: 0.0003% or more and 0.0100% or less, with the balance Fe and unavoidable impurity component composition, and 60% or more of the Cr is solid. Steel plate that is molten Cr. Further, the composition of the components is% by mass,
Nb: 0.003% or more and 0.030% or less,
The steel sheet according to claim 1, containing one or more selected from V: 0.01% to 0.10% and Ti: 0.003% to 0.040%. Further, the composition of the components is% by mass,
Cu: 0.01% or more and 0.50% or less,
Ni: 0.01% or more and 0.50% or less,
Sn: 0.01% or more and 0.30% or less,
Sb: 0.01% or more and 0.30% or less,
The steel sheet according to claim 1 or 2, containing one or more selected from Mo: 0.01% to 2.0% and W: 0.01% to 2.0%. Further, the composition of the components is% by mass,
Ca: 0.0005% or more and 0.0050% or less,
The steel sheet according to claim 1, 2 or 3, containing one or more selected from Mg: 0.0005% to 0.0100% and REM: 0.0010% to 0.0200%. After heating the steel material having the component composition according to any one of claims 1 to 4 to 1000°C or more and 1300°C or less, hot rolling is performed at a finishing temperature of 750°C or more and a material to be rolled temperature of 950°C. A method for producing a steel sheet, which is performed at a temperature of 600°C or higher for 30 minutes or less, and then cooled at an average cooling rate of 3°C/s or higher in a temperature range of 700°C or lower and 600°C or higher.