JP6904438B2 - Steel plate and its manufacturing method - Google Patents

Description

The present invention relates to a steel sheet suitable for structural steel used in an extremely low temperature environment such as a tank for a liquefied gas storage tank, and particularly excellent in corrosion resistance in a saltwater corrosion environment, and a method for producing the same.

Attempts have been made to use hot-rolled steel sheets for structures such as tanks for liquefied gas storage tanks. Since the environment in which such a structure is used is extremely low, 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 -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 material.

In response to this requirement, conventionally, austenitic stainless steel having an austenitic structure that does not show brittleness at extremely low temperatures, 9% Ni steel, or a 5000 series aluminum alloy has been used. However, since these metal materials have high alloying 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 to replace the conventional ultra-low temperature steel, a high Mn steel having an 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 an extremely low temperature environment. Is being considered.

However, when a steel plate having an austenite structure is placed in a corrosive environment, there is a problem that stress corrosion cracking is likely to occur when the austenite grain boundaries are eroded by corrosion and tensile stress is applied. At the manufacturing stage of structures for liquefied gas storage tanks, the base iron of the steel sheet may be exposed on the surface, and if the surface of the steel material comes into contact with water vapor containing corrosive substances such as salt, moisture, oil, etc., it will corrode the steel material. 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 water, and hydrogen embrittlement is caused by the invasion of hydrogen into the steel. Corrosion occurs. If residual stress during bending or welding during manufacturing or load stress in the usage environment acts on it, stress corrosion cracking may occur and the structure may be destroyed. The high Mn steels conventionally studied may be inferior in corrosion resistance to not only austenitic stainless steels but also 9% Ni steels and ordinary low alloy steels. Therefore, from the viewpoint of safety, it is important that the steel material used for the structure has high strength and toughness at extremely low temperatures, as well as excellent corrosion resistance.

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

Further, 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 low temperature toughness, and a high Mn steel material is disclosed.

Further, Patent Document 3 contains 0.001 to 0.80% of C and 15 to 35% of Mn, and by adding elements such as Cr, Ti, Si, Al, Mg, Ca and REM, the mother High Mn steel materials with improved ultra-low temperature toughness of materials and welds are disclosed.

Special Table 2015-508452 Japanese Unexamined Patent Publication No. 2016-84529 Japanese Unexamined Patent Publication No. 2016-196703

However, the steel materials described in Patent Documents 1, 2 and 3 still have room for improvement in terms of manufacturing cost for achieving strength and low temperature toughness, and in terms of corrosion resistance when the above-mentioned austenite steel material is placed in a salt-corroded environment. 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-corroded environment. Here, "excellent in corrosion resistance" is a test based on the Slow Straight Rate Test Method based on NACE Standard TM0111-2011, and is immersed in artificial seawater (chloride ion concentration 18000 ppm) at a temperature of 23 ° C. and has a 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 problems, the present inventors have diligently investigated various factors that determine the composition and production conditions of high Mn steel, and have obtained the following findings.
a. By adding Cr to the high Mn steel as a base and appropriately controlling the addition amount and the solid solution amount, the initial corrosion reaction on the steel sheet surface in the salt water corrosion environment can be delayed. As a result, the amount of hydrogen that penetrates into the steel can be reduced, and the stress corrosion cracking of the austenite steel described above is suppressed.

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

The present invention has been made by further studying the above findings, and the gist thereof is as follows.
1. 1. By mass%
C: 0.20% 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: 0.003% or more and 0.0100% or less, and has a component composition of the balance Fe and unavoidable impurities, and 60% or more of the Cr is solid. A steel plate that is molten Cr.

2. The composition of the components is further increased by mass%.
Nb: 0.003% or more and 0.030% or less,
The steel sheet having excellent corrosion resistance according to 1 above, which contains one or more selected from V: 0.01% or more and 0.10% or less and Ti: 0.003% or more and 0.040% or less.

3. 3. The composition of the components is further increased 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% or more and 2.0% or less and W: 0.01% or more and 2.0% or less.

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

5. After heating the steel material having the component composition according to any one of 1 to 4 to 1000 ° C. or higher and 1300 ° C. or lower, hot rolling is performed at a finishing temperature of 750 ° C. or higher and a material temperature to be rolled: 950 ° C. or lower. A method for producing a steel sheet in which a stay time at 600 ° C. or higher is set to 30 minutes or less, and then an average cooling rate in a temperature range of 700 ° C. or lower and 600 ° C. or higher is performed: 3 ° C./s 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-corroded environment. Therefore, by using the steel sheet of the present invention for a steel structure used in an extremely low temperature environment, for example, a tank for a liquefied gas storage tank, the safety and life of the steel structure are greatly improved, and as a result, the safety and life of the steel structure are significantly improved. Will bring about the effect of. Further, since the steel sheet of the present invention is cheaper than the existing materials, it also has an advantage of being excellent in economy.

Hereinafter, the steel sheet of the present invention will be described in detail. 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 component composition of the steel sheet is defined as follows. In addition, "%" representing a component composition shall mean "mass%" unless otherwise specified.

C: 0.20% or more and 0.70% or less C is an inexpensive austenite stabilizing element that is effective for increasing the strength and is an important element for obtaining austenite. In order to obtain the effect, C needs to be contained in an amount of 0.20% or more. On the other hand, if it is contained in an amount of more than 0.70%, excessive precipitation of Cr carbides and Nb, V, Ti-based carbides is promoted, and these precipitates become the starting point 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 deoxidizing material and is not only necessary for steelmaking, but also has the effect of dissolving in steel and increasing the strength of the steel sheet by solid solution strengthening. .. In order to obtain such an effect, Si needs to be contained in an amount of 0.05% or more. On the other hand, if it is contained in excess of 1.00%, the weldability and surface properties may be deteriorated and the stress corrosion cracking resistance may be lowered. 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% or more and 35.0% or less 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 in an amount of 15.0% or more. On the other hand, when it is contained in excess of 35.0%, the effect of improving toughness at extremely low temperatures is saturated, leading to an increase in alloy cost. In addition, weldability and cutability deteriorate. 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, it is in the range of 18.0% or more and 28.0% or less.

P: 0.030% or less If P is contained in excess of 0.030%, it segregates at the grain boundaries, lowers the grain boundary strength, and becomes the starting point for stress corrosion cracking. Therefore, it is desirable to limit the amount to 0.030% as much as possible. The lower the content of P, the better the characteristics. Therefore, 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%, a large cost is required for steelmaking and economic efficiency is impaired. Therefore, from the viewpoint of economic efficiency, a content of 0.001% or more is allowed.

S: 0.0200% or less Since S deteriorates the low temperature toughness and ductility of the base metal, it is desirable to limit it to 0.0200% and reduce it as much as possible. Therefore, S is 0.0200% or less, preferably 0.0180% or less. On the other hand, if it is less than 0.0001%, a large cost is required for steelmaking and economic efficiency is impaired. Therefore, from the viewpoint of economic efficiency, a content of 0.0001% or more is allowed.

Al: 0.010% or more and 0.100% or less Al acts as an antacid 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 the coarsening of crystal grains. Further, it has an effect of suppressing deterioration of toughness due to reduction of solid solution N. In order to obtain such an effect, Al needs to be contained in an amount of 0.01% or more. On the other hand, if it is contained in excess of 0.100%, coarse nitrides may be formed, which may become a starting point of corrosion or fracture, and the stress corrosion cracking resistance may be lowered. In addition, it diffuses into the weld metal portion during welding and deteriorates the toughness of the weld metal. Therefore, Al is set to 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 has the effect of delaying the initial corrosion reaction on the surface of the steel sheet in a saltwater corrosive environment when contained in an appropriate amount. This effect is an important element that reduces the amount of hydrogen that penetrates into the steel sheet and improves stress corrosion cracking resistance. In order to obtain such an effect, a content of 0.5% or more is required. On the other hand, if Cr exceeds 8.0%, the above-mentioned effect obtained will be saturated, and the economic efficiency will be impaired. Therefore, the amount of Cr is set to 0.5% or more and 8.0% or less. Preferably, it is 1.0% or more.

Here, the solid solution of the added Cr contributes to the improvement of the stress corrosion cracking resistance, but the precipitated component may adversely affect the improvement of the stress corrosion cracking resistance. It is essential that at least 60% is solid solution Cr. That is, when the solid solution Cr is 60% or more of the content Cr content, the above-mentioned effect can be enjoyed, and the stress corrosion cracking resistance can be improved by adding Cr. The solid solution Cr is preferably 70% or more, more preferably 100% of the Cr content.

The solid solution Cr is a state in which solute atoms exist in the atomic state without forming precipitates or the like. Specifically, the amount of solid-dissolved Cr was extracted by an electrolytic extraction method using a test piece for electrolytic extraction from a steel plate and using a 10% AA (10% acetylacetone-1% tetramethylammonium chloride-methanol) solution. The precipitate 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 has an effect of suppressing stress corrosion cracking as a trap site of diffusible hydrogen by binding to Nb, V and Ti and finely precipitating as a nitride or carbonitride. In order to obtain such an effect, N needs to be contained in an amount of 0.0010% or more. On the other hand, if it is contained in excess of 0.0300%, the formation of excess nitride or carbonitride is promoted, the amount of solid solution elements is lowered, the corrosion resistance is lowered, and the toughness is also lowered. Therefore, N is set to 0.0010% or more and 0.0300% or less. It is preferably 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, suppresses cracking at grain boundaries, and is an effective element for improving stress corrosion cracking resistance. In order 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, if it is contained in excess of 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, in addition to the above essential elements, Nb: 0.003% or more and 0.030% or less, V: 0.01% or more and 0.10, if necessary. % 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 having an effect of suppressing stress corrosion cracking because it is precipitated as a carbonitride and the precipitated carbonitride functions as a trap site for diffusible hydrogen. .. In order to obtain such an effect, Nb is preferably contained in an amount of 0.003% or more. On the other hand, if it is contained in excess of 0.030%, coarse carbonitride may be precipitated 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 Nb is contained, it 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 preferably 0.007% or more and 0.022% or less.

V: 0.01% or more and 0.10% or less V is an element having an effect of suppressing stress corrosion cracking because it is precipitated as carbonitride and the generated carbonitride functions as a trap site for diffusible hydrogen. .. In order to obtain such an effect, V is preferably contained in an amount of 0.01% or more. On the other hand, if it is contained in excess of 0.10%, coarse carbonitride may be precipitated and may 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 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 preferably 0.03% or more and 0.08% or less.

Ti: 0.003% or more and 0.040% or less Ti is precipitated as a nitride or carbonitride, and the generated nitride or carbonitride functions as a trap site for diffusible hydrogen, thus suppressing stress corrosion cracking. It is an element that has an effect. In order to obtain such an effect, Ti is preferably contained in an amount of 0.003% or more. On the other hand, if it is contained in excess of 0.040%, the precipitate may become coarse and the toughness of the base metal may be deteriorated. In addition, coarse carbonitride may precipitate and serve as a starting point for fracture. Therefore, when Ti is contained, it 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 preferably 0.007% or more and 0.032% or less.

Further, 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 can be contained in one or more types.

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 compound addition with Cr. Here, Cu, Sn and Sb have the 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 the permeation of corrosive anions such as ^{Cl − into the ground 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 products are densified to reach the steel interface (the interface between the rust layer and the base iron). Suppresses the 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 Is electrically suppressed.

The above effects become apparent in high Mn steel when coexisting with Cr, and are exhibited when 0.01% or more of each element is added. However, if a large amount of any of the elements is contained, the weldability and toughness are deteriorated, which is disadvantageous from the viewpoint of cost.
Therefore, the Cu amount is in the range of 0.01% or more and 0.50% or less, the Ni amount is in the range of 0.01% or more and 0.50% or less, and the Sn amount is in the range of 0.01% or more and 0.30% or less. The Sb amount shall be in the range of 0.01% or more and 0.30% or less, the Mo amount shall be in the range of 0.01% or more and 2.0% or less, and the W amount shall be in the range of 0.01% or more and 2.0% or less. Is preferable.
More preferably, the amount of Cu is 0.02% or more and 0.40% or less, the amount of Ni is 0.02% or more and 0.40% or less, the amount of Sn is 0.02% or more and 0.25% or less, and the amount of Sb is 0. The amount of Mo is 0.02% or more and 0.40% or less, and the amount of W 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,
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 contain one or more types. it can.
That is, Ca, Mg and REM are elements useful for controlling the morphology of inclusions and can be contained as needed. Here, the morphological control of inclusions means that the expanded sulfide-based inclusions are made into granular inclusions. Through morphological control of this inclusion, ductility, toughness and sulfide stress corrosion cracking resistance can be improved. In order to obtain such an effect, it is preferable that Ca 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 ductility, toughness, and sulfide stress corrosion cracking resistance may decrease. It may also be economically disadvantageous.

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

Next, the production conditions of the present invention will be described. The temperature of the material to be rolled in the hot rolling process and the cooling rate in the subsequent cooling process mean the temperature and the cooling rate measured on the surface of the rolled material. That is, after heating the steel material having the above-mentioned 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 lower and a finishing temperature of 750 ° C. or higher and a material temperature to be rolled: 950 ° C. The steel sheet is manufactured by subjecting the staying time at 600 ° C. or higher to 30 minutes or less, and then cooling at an average cooling rate of 700 ° C. or lower and 600 ° C. or higher: 3 ° C./s or higher.

[Heating 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 carbonitride in the structure to make the crystal grain size uniform. That is, when the heating temperature is less than 1000 ° C., the carbonitride is not sufficiently solid-solved, so that the desired characteristics cannot be obtained. Further, when heated above 1300 ° C., in addition to material deterioration due to coarsening of the crystal particle size, excessive energy is required and productivity is lowered, so the upper limit of the heating temperature is set to 1300 ° C. It is preferably in the range of 1050 ° C. or higher and 1250 ° C. or lower, and more preferably 1070 ° C. or higher and 1250 ° C. or lower. As the steel material, in addition to the continuously cast slab, it is preferable to use a steel material such as a slab or a billet by a commonly known method such as an ingot forming method. Needless to say, treatments such as ladle refining and vacuum degassing may be added to the molten steel.

[Finishing temperature for hot rolling: 750 ° C or higher]
When the finishing temperature of hot rolling is less than 750 ° C, the amount of carbides precipitated during the rolling increases remarkably, and the amount of solid solution Cr is secured even if the residence time at 600 ° C or higher and 900 ° C or lower is less than 30 minutes as described later. It may not be possible and the corrosion resistance will decrease. 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 finish temperature is set to 750 ° C. or higher.

[Average cooling rate at 700 ° C or lower and 600 ° C or higher: 3 ° C / s or higher]
For 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 will be generated, so the average cooling rate should be 3 ° C / s or higher. limit. It is preferably in the range of 10 ° C./s or more and 150 ° C./s or less.

[Stay time in the temperature range of 950 ° C or lower and 600 ° C or higher: 30 minutes or less]
In hot rolling, if the time for the material to be rolled to stay in the temperature range of 950 ° C or lower and 600 ° C or higher exceeds 30 minutes, a large amount of carbonitride and carbide will precipitate during rolling, and the required amount of solid-melt Cr will be required. The residence time in the temperature range of 950 ° C. or lower and 600 ° C. or higher is restricted to 30 minutes or less in order to reduce the corrosion resistance and cause the deterioration of the extremely low temperature toughness. The range is preferably 5 minutes or more and 25 minutes or less.

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

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

No. shown in Table 1. After melting the steels 1 to 57 into a slab, the sample No. having a plate thickness of 6 mm or more and 50 mm or less according to the production conditions shown in Table 2. Steel sheets 1 to 65 were manufactured. Next, 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 the Slow Straight Rate Test Method (hereinafter referred to as SSRT test) of the NACE Standard TM0111-2011 standard. That is, the shape of the test piece is a type A round bar notched test piece, immersed in artificial seawater (chloride ion concentration 18000 ppm) at a temperature of 23 ° C., and a constant velocity tensile test ^{at a strain rate of 4 × 10-7 inch / s.} Was carried out. Here, a breaking stress of 600 MPa or more is assumed to be excellent in stress corrosion cracking resistance.
The results obtained as described above are shown in Table 2.

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

Claims (5)

By 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: 0.003% or more and 0.0100% or less, and has a component composition of the balance Fe and unavoidable impurities, and 60% or more of the Cr is solid. A steel plate that is molten Cr. The composition of the components is further increased by mass%.
Nb: 0.003% or more and 0.030% or less,
The steel sheet according to claim 1, which contains one or more selected from V: 0.01% or more and 0.10% or less and Ti: 0.003% or more and 0.040% or less. The composition of the components is further increased by mass%.
Cu: 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, which contains one or more selected from Mo: 0.01% or more and 2.0% or less and W: 0.01% or more and 2.0% or less. The composition of the components is further increased by mass%.
Ca: 0.0005% or more and 0.0050% or less,
The steel sheet according to claim 1, 2 or 3, which contains one or more selected from Mg: 0.0005% or more and 0.0100% or less and REM: 0.0010% or more and 0.0200% or less. A method for producing a steel sheet according to any one of claims 1 to 4, wherein a steel material having the component composition according to any one of claims 1 to 4 is heated to 1000 ° C. or higher and 1300 ° C. or lower, and then heated. Inter-rolling is performed at a finishing temperature of 750 ° C. or higher, a material temperature to be rolled: 950 ° C. or lower and a residence time of 600 ° C. or higher for 30 minutes or less, and then an average cooling rate in a temperature range of 700 ° C. or lower and 600 ° C. or higher: 3 A method for manufacturing a steel sheet that cools at ° C / s or higher.