TW202012652A - Steel sheet and method for manufacturing same - Google Patents

Abstract

Provided is steel that has a high Mn content and excellent in corrosion resistance, particularly corrosion resistance in a corrosive salt environment. The steel has a composition comprising 0.20 to 0.70% of C, 0.05 to 1.00% of Si, 15.0 to 35.0% of Mn, 0.030% or less of P, 0.0200% or less of S, 0.010 to 0.100% of Al, 0.5 to 8.0% of Cr, and 0.0010 to 0.0300% of N, with the remainder comprising Fe and unavoidable impurities, and 60% or more of the Cr being solid Cr.

Description

Steel plate and its manufacturing method

The present invention relates to a steel plate that is preferably used for structural steel that can be used in an extremely low-temperature environment for tanks for liquefied gas storage tanks, especially in a salt water corrosion environment, and a method for manufacturing the same.

Attempt to use hot-rolled steel sheets in structures such as tanks for liquefied gas storage tanks. Since the use environment of the structure becomes extremely low temperature, the hot-rolled steel sheet used in the structure requires not only high strength but also excellent toughness at extremely low temperature. For example, when a hot-rolled steel sheet is used in a storage tank of liquefied natural gas, it is necessary to ensure excellent toughness at the boiling point of the liquefied natural gas, that is, below -164°C. If the low-temperature toughness of the steel material is poor, there is a risk that the safety as a structure for extremely low-temperature storage tanks cannot be maintained. Therefore, it is strongly required to improve the low-temperature toughness of the applied steel material.

With regard to the requirements, previously used Vostian iron-based stainless steel or 9% Ni steel or 5000-series aluminum alloy having a Vostian iron structure that does not exhibit brittleness at extremely low temperatures has been used previously. However, these metal materials have a high alloy cost or high manufacturing cost, so there is a need for a steel plate that is inexpensive and excellent in extremely low temperature toughness. Therefore, as a new steel plate that replaces the previous ultra-low temperature steel, it has been studied to use a high-Mn steel that becomes a Vostian iron structure by adding a large amount of Mn as a relatively inexpensive Vostian iron stabilizing element as an ultra-low temperature environment. Steel plate for the structure.

However, when a steel plate having a Vostian iron structure is placed in a corrosive environment, the Vostian iron grain boundaries are eroded due to corrosion, and there is a problem that stress corrosion cracking easily occurs when tensile stress is applied. In the manufacturing stage of structures for liquefied gas storage tanks, the steel base of the steel plate may sometimes be exposed. If the surface of the steel material is in contact with water vapor, water or oil containing corrosive substances such as salts, the steel material may corrode. At this time, in the corrosion reaction on the surface of the steel plate, iron generates oxides (rust) by the anode reaction, and on the other hand, hydrogen is generated by the cathode reaction of moisture, and hydrogen intrudes into the steel to cause hydrogen embrittlement. If residual stresses such as bending or welding during production or load stresses in the use environment work, stress corrosion cracking occurs, and there is a risk of damage to the structure. In the previously studied high Mn steel, compared with 9%Ni steel or the usual low alloy steel, the Vostian iron-based stainless steel also has poor corrosion resistance. Therefore, from the viewpoint of safety, it is of course important that the steel used in the structure has high strength and toughness at extremely low temperatures, and it is also important to have excellent corrosion resistance.

For example, Patent Document 1 discloses a steel material that improves the machinability and heat-affected heat-affected zone at -196°C by adding 15% to 35% of Mn, 5% or less of Cu, and appropriate amounts of C and Cr. Charpy impact characteristics.

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

Furthermore, Patent Document 3 discloses a high-Mn steel material containing 0.001% to 0.80% of C, 15% to 35% of Mn, and adding Cr, Ti, Si, Al, Mg, Ca, REM and other elements to improve The extremely low temperature toughness of the base metal and the welded part. [Prior Art Literature] [Patent Literature]

Patent Document 1: Japanese Patent Special Publication No. 2015-508452 Patent Document 2: Japanese Patent Laid-Open No. 2016-84529 Patent Document 3: Japanese Patent Laid-Open No. 2016-196703

[Problems to be solved by the invention] However, regarding the steel materials described in Patent Document 1, Patent Document 2, and Patent Document 3, in view of achieving the manufacturing cost of strength and low-temperature toughness, and corrosion resistance when the austenitic steel material is placed in a salt corrosion environment There is still room for improvement in terms of sex.

The present invention was made in view of the above problems, and an object thereof is to provide a high Mn steel having excellent corrosion resistance, particularly corrosion resistance in a salt corrosion environment. Here, the "excellent corrosion resistance" refers to the test of the Slow Strain Rate Test Method according to the National Association of Corrosion Engineers (NACE) standard TM0111-2011 standard, at the temperature When immersed in artificial seawater (chloride ion concentration 18000 ppm) at 23°C and subjected to a constant velocity tensile test at a strain rate of 4×10 ^-7 inch/s, the breaking stress was 600 MPa or more. [Means to solve the problem]

In order to achieve the above-mentioned problems, the inventors of the present invention have conducted research on various factors that determine the composition and production conditions of high Mn steel, and as a result, obtained the following insights. a. Based on high Mn steel, Cr is added thereto, and the amount of addition and the amount of solid solution are appropriately controlled, thereby delaying the initial corrosion reaction of the steel plate surface in a salt water corrosion environment. As a result, the amount of hydrogen intruding into the steel can be reduced, and stress corrosion cracking of the austenitic steel can be suppressed.

b. Furthermore, in order to effectively suppress the destruction of Vostian iron from the grain boundary, it is effective to increase the strength of the grain boundary. In particular, P is an element that tends to segregate together with Mn during the solidification of the steel sheet, and reduces the strength of the grain boundary at the part intersecting the segregation. Therefore, it is necessary to reduce impurity elements such as P. On the other hand, B is an element that increases the strength of the Vostian iron grain boundary. In addition to the reduction of impurity elements such as P, the addition of B can further effectively suppress grain boundary destruction.

The present invention is made by further researching the above findings, and its gist is as follows. 1. A steel plate containing 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, The remaining part has a composition of Fe and inevitable impurities, and 60% or more of the Cr is solid solution Cr.

2. The steel sheet excellent in corrosion resistance as described in 1 above, wherein the component composition further contains selected from 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 One or more than two.

3. The steel sheet according to 1 or 2 above, wherein the component composition further contains, in mass %, selected from 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, Mo: 0.01% or more and 2.0% or less, and W: 0.01% or more and 2.0% or less One or more than two.

4. The steel sheet according to 1, 2, or 3, wherein the composition of the component further contains 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 One or more than two.

5. A method of manufacturing a steel plate, heating a steel material having a composition as described in any one of 1 to 4 to 1000° C. or more and 1300° C. or less, and thereafter to a final temperature of 750° C. or more, Rolled material temperature: 950°C or lower and 600°C or higher residence time is set to 30 minutes or less and hot rolling is performed, followed by an average cooling rate in the temperature range of 700°C or lower and 600°C or higher: cooling at 3°C/s or higher. [Effect of invention]

According to the present invention, it is possible to provide a steel sheet excellent in corrosion resistance, especially in a salt corrosion environment. Therefore, by using the steel sheet of the present invention in a steel structure that can be used in an extremely low temperature environment such as a tank for a liquefied gas storage tank, the safety or life span of the steel structure is significantly improved, and as a result, industrial special Effect. In addition, the steel sheet of the present invention is cheaper than existing materials, and therefore has the advantage of being excellent in economy.

Hereinafter, the steel sheet of the present invention will be described in detail. Furthermore, the present invention is not limited to the following embodiments. [Composition] First, the composition of the steel sheet of the present invention and the reasons for its limitation will be described. In the present invention, in order to ensure excellent corrosion resistance, the composition of the steel sheet is specified as follows. In addition, "%" indicating the composition of components means "mass %" unless otherwise specified.

C: 0.20% or more and 0.70% or less C is effective for increasing strength, and is therefore a cheap stabilizing element of Vostian iron and an important element for obtaining Vostian iron. In order to obtain the effect, C must be contained in 0.20% or more. On the other hand, if the content exceeds 0.70%, excessive precipitation of Cr carbides and Nb, V, and Ti-based carbides is promoted, and these precipitates become a starting point of occurrence of corrosion, and low-temperature toughness decreases. 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 functions as a deoxidizing material, which is not only necessary for steel making, but also has the effect of solid solution in steel and strengthening the steel plate by solid solution strengthening. In order to obtain the effect, Si must be contained at 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 made 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 cheap stabilizing element of Vostian iron. In the present invention, it is an important element for coexisting strength and extremely low temperature toughness. In order to obtain the effect, Mn must be contained at 15.0% or more. On the other hand, when the content exceeds 35.0%, the effect of improving the extremely low-temperature toughness is saturated, and the alloy cost increases. In addition, weldability and cuttability deteriorate. Furthermore, it causes 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. It is preferably within a range of 18.0% or more and 28.0% or less.

P: 0.030% or less If P is contained in an amount exceeding 0.030%, segregation occurs at the grain boundary, and the grain boundary strength decreases to become a starting point of stress corrosion cracking. Therefore, 0.030% is made the upper limit, and it is desirable to reduce it as much as possible. The lower the content of P, the more the characteristics are improved. Therefore, it is preferably 0.024% or less, and more preferably 0.020% or less. On the other hand, in order to set P to less than 0.001%, steelmaking requires a large cost and the economy is impaired. Therefore, from the viewpoint of economy, the content is allowed to be 0.001% or more.

S: 0.0200% or less S deteriorates the low-temperature toughness or ductility of the base material, so 0.0200% is made the upper limit, and it is desirable to reduce it as much as possible. Therefore, S is set to 0.0200% or less, preferably 0.0180% or less. On the other hand, in order to make it less than 0.0001%, steel-making requires a large cost and the economy is impaired, so from the viewpoint of economy, 0.0001% or more is allowed to be contained.

Al: 0.010% or more and 0.100% or less Al functions as a deoxidizer and is most commonly used in the molten steel deoxidation process. In addition, the formation of AlN by fixing solid solution N in steel has an effect of suppressing coarsening of crystal grains. Furthermore, it has the effect of suppressing the deterioration of toughness caused by the decrease of solute N. In order to obtain the effect, Al must be contained in 0.01% or more. On the other hand, if the content exceeds 0.100%, coarse nitrides may be formed to become a starting point of corrosion or destruction, and stress corrosion cracking resistance may decrease. In addition, it diffuses in the weld metal portion during welding, deteriorating 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 an effect of delaying the initial corrosion reaction of the steel plate surface in a salt water corrosion environment by adding an appropriate amount. It is an important element that reduces the amount of hydrogen intrusion into the steel sheet and improves the stress corrosion cracking resistance by the above effect. In order to obtain the effect, it must be contained at 0.5% or more. On the other hand, if Cr exceeds 8.0%, the effect obtained is saturated, and on the contrary, the economy is impaired. Therefore, the amount of Cr is set to 0.5% or more and 8.0% or less. It is preferably 1.0% or more.

Here, the added solid solution component of Cr contributes to the improvement of stress corrosion cracking resistance, but since the precipitated component may hinder the improvement of stress corrosion cracking resistance, it is important that at least 60% of the Cr is solid Dissolve Cr. That is, if the solid solution Cr is 60% or more of the Cr content, the above effect can be enjoyed, and the stress corrosion cracking resistance caused by the addition of Cr can be improved. The solid solution Cr is preferably 70% or more of the Cr content, and more preferably 100%.

In addition, solid solution Cr is a state in which solute atoms exist in an atomic state without forming precipitates or the like. Specifically, the amount of solute Cr can be obtained by taking a test piece for electrolytic extraction from a steel plate, and by using 10% AA (10% acetone-1% tetramethylammonium chloride-methanol ) The precipitates extracted by the electrolytic extraction method of the solution are measured by inductively coupled plasma (ICP) luminescence analysis method, and the amount of Cr in the precipitates is measured and subtracted from all Cr in the test piece.

N: 0.0010% or more and 0.0300% or less N is an element for stabilizing Vostian iron and is an element effective for improving the extremely low temperature toughness. In addition, it combines with Nb, V, and Ti, and precipitates finely as a nitride or carbonitride, and as a trap site for diffusible hydrogen, it has an effect of suppressing stress corrosion cracking. In order to obtain the effect, N must be 0.0010% or more. On the other hand, if the content exceeds 0.0300%, the production of excessive nitrides or carbonitrides is promoted, the amount of solid solution elements decreases, and not only the corrosion resistance but also the toughness decreases. 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 the Vostian iron grain boundary, and is an element that is effective in suppressing cracks in the grain boundary and improving the resistance to stress corrosion cracking. In order to obtain the effect, B must contain 0.0003% or more. It is preferably 0.0005% or more, and more preferably more than 0.0007% and more than 0.0010%. On the other hand, if the content exceeds 0.0100%, the effect is saturated. Therefore, B is limited to the range of 0.0100% or less. It is preferably 0.0070% or less.

In the present invention, in order to further improve 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% or less, and Ti: 0.003% or more and 0.040% or less may be included as necessary .

Nb: 0.003% or more and 0.030% or less Nb precipitates as a carbonitride, and the precipitated carbonitride functions as a trap site for diffusible hydrogen, and therefore is an element having an effect of suppressing stress corrosion cracking. In order to obtain the above effect, Nb is preferably contained at 0.003% or more. On the other hand, if the content exceeds 0.030%, coarse carbonitrides may precipitate and become a starting point of destruction. In addition, the precipitates may become coarse and deteriorate the toughness of the base material. 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 it is 0.007% or more and 0.022% or less.

V: 0.01% or more and 0.10% or less V precipitates as a carbonitride, and the generated carbonitride functions as a trap site for diffusible hydrogen, so it is an element having an effect of suppressing stress corrosion cracking. In order to obtain the above effect, V is preferably contained at 0.01% or more. On the other hand, if the content exceeds 0.10%, coarse carbonitrides may precipitate and become a starting point of destruction. In addition, the precipitates 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 it is 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, and therefore is an element having an effect of suppressing stress corrosion cracking. In order to obtain the above effect, Ti is preferably contained at 0.003% or more. On the other hand, if the content exceeds 0.040%, the precipitates may coarsen and the toughness of the base material may deteriorate. In addition, coarse carbonitrides may precipitate and become the starting point of destruction. 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 it is 0.007% or more and 0.032% or less.

Furthermore, in the present invention, in order to further improve the corrosion resistance, if necessary Contains 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, Mo: 0.01% or more and 2.0% or less , W: one or more than 0.01% and less than 2.0%.

That is, Cu, Ni, Sn, Sb, Mo, and W are elements added in combination with Cr to improve the corrosion resistance of Mn steel in a salt water corrosion environment. Here, Cu, Sn, and Sb have the effect of suppressing the hydrogen generation reaction as a cathode reaction by increasing the hydrogen overvoltage of the steel material. Ni forms a precipitation film on the surface of the steel, which physically inhibits the penetration of corrosive anions such as Cl ^- through the steel base. In addition, Cu, Ni, Sn, Sb, Mo, and W dissociate from the steel surface as metal ions during corrosion. By densifying the corrosion products, the penetration of corrosive anions to the steel interface (the interface between the rust layer and the steel substrate) is suppressed. . Mo and W are free as Mo ₄ ^2- and WO ₄ ^2- , respectively, and are adsorbed on the corrosion product or the surface of the steel plate to impart selective permeability to cations, thereby electrically suppressing the penetration of corrosive anions to the steel substrate.

The above effect is manifested in the case of coexistence with Cr in high Mn steel, and appears when 0.01% or more of each element is added. However, if a large amount of arbitrary elements are also contained, weldability or toughness deteriorates, which is also disadvantageous from the viewpoint of cost. Therefore, it is preferable to set the range of the amount of Cu to be 0.01% or more and 0.50% or less, the amount of Ni to be 0.01% or more and 0.50% or less, the amount of Sn to be 0.01% or more and 0.30% or less, and the amount of Sb to 0.01 % Or more and 0.30% or less, Mo amount is 0.01% or more and 2.0% or less, and W amount is 0.01% or more and 2.0% or less. 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, the amount of Sb is 0.02% or more and 0.25% or less, and the amount of Mo 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, in order to further improve the corrosion resistance, as needed Contains 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. That is, Ca, Mg, and REM are elements that are useful for morphology control of inclusions, and may be included as necessary. Here, the morphology control of the inclusions means that the extended sulfide-based inclusions are made into granular inclusions. The shape control of the inclusions improves ductility, toughness, and resistance to sulfide stress corrosion cracking. In order to obtain the above effect, Ca and Mg are preferably contained at 0.0005% or more, and REM is preferably contained at 0.0010% or more. On the other hand, if a large amount of arbitrary elements are also contained, the amount of non-metallic inclusions may increase, but instead the ductility, toughness, and resistance to sulfide stress corrosion cracking may decrease. In addition, it is sometimes economically disadvantageous.

Therefore, when Ca is contained, it is preferably set to 0.0005% or more and 0.0050% or less, and when Mg is contained, it is preferably set to 0.0005% or more and 0.0100% or less, and when REM is contained, it is Preferably, it is 0.0010% or more and 0.0200% or less. More preferably, the amount of Ca is 0.0010% or more and 0.0040% or less, the amount of Mg is 0.0010% or more and 0.0040% or less, and the amount of REM is 0.0020% or more and 0.0150% or less.

Next, the manufacturing conditions of the present invention will be described. In addition, the temperature of the material to be rolled in the hot rolling step and the cooling rate in the subsequent cooling step refer to 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 more and 1300° C. or less, then the reduction ratio: 3 or more and 30 or less and the final temperature: 750° C. or more, the material to be rolled temperature: 950 The residence time at or below 600° C. is set to 30 minutes and hot rolling is performed, followed by cooling at an average cooling rate in the temperature range of 700° C. or lower and 600° C. or higher: 3° C./s or higher, thereby manufacturing a steel sheet.

[Heating temperature of steel materials: 1000°C or higher and 1300°C or lower] The reason why the steel material is heated to 1000° C. or higher is that the carbonitride in the structure is solid-dissolved, and the crystal grain size and the like are uniformized. That is, when the heating temperature is less than 1000° C., the carbonitride will not be sufficiently solid-dissolved, and thus the desired characteristics cannot be obtained. In addition, when heating exceeds 1300°C, in addition to deterioration of the material due to coarsening of the crystal grain size, excessive energy is required and productivity decreases, so 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 in a range of 1070°C or higher and 1250°C or lower. In addition, as the steel material, in addition to the continuous casting of the slab, it is preferable to produce the steel material such as a slab or a slab by a commonly known method such as a block method. In addition, of course, barrel refining or vacuum degassing can also be added to the molten steel.

[Final temperature of hot rolling: above 750℃] When the final temperature of hot rolling is less than 750°C, the amount of carbide precipitation during the rolling is significantly increased. As will be described later, even if the residence time of 600°C or more and 900°C or less is set to less than 30 In minutes, sometimes the amount of solid solution Cr cannot be confirmed, and the corrosion resistance decreases. In addition, when rolling is performed at less than 750°C, the deformation resistance becomes large, which causes an excessive load on the manufacturing equipment. Therefore, the finishing rolling temperature is set to 750°C or higher.

[Average cooling rate below 700°C and above 600°C: 3°C/s or above] Regarding cooling after hot rolling, when the average cooling rate of 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, so the average cooling rate is limited to 3°C/s the above. It is preferably in the range of 10°C/s or more and 150°C/s or less.

[Residence time in temperature range below 950°C and above 600°C: less than 30 minutes] If the rolled material stays in the temperature range below 950°C and above 600°C during hot rolling for more than 30 minutes, a large amount of carbonitrides or carbides are precipitated from the rolling, and the amount of required solid solution Cr is reduced, causing Since the corrosion resistance and the extremely low temperature toughness decrease, the residence time in the temperature range of 950°C or lower and 600°C or higher is limited to 30 minutes or less. It is preferably in the range of 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 less and 600° C. or more to 30 minutes or less, it is preferable to set the length of the rolled material to 5000 mm or less and to hot-roll from the rolled material The reduction ratio is set to 30 or less. That is, if the length of the material to be rolled is 5000 mm or less and the reduction ratio is 30 or less, the rolling time becomes shorter, and as a result, the residence time in the range of 950°C or less and 600°C or more can be set to 30 Less than minutes.

As described above, the upper limit of the hot rolling reduction ratio is preferably 30 or less. On the other hand, if the reduction ratio of hot rolling is less than 3, the effect of promoting recrystallization and achieving granulation becomes less, and as a result, coarse Vostian iron particles remain, and part of them are preferentially oxidized, thereby having corrosion resistance The risk of deterioration. Therefore, it is preferable to set the reduction ratio of hot rolling to 3 or more. Here, the reduction ratio is defined as (the thickness of the rolled material for hot rolling)/(the thickness of the steel sheet after hot rolling). [Example]

After the steels No. 1 to No. 57 shown in Table 1 were melted to make slabs, a sample No. with a plate thickness of 6 mm or more and 50 mm or less was produced under the manufacturing conditions shown in Table 2. 1～No.65 steel plate. Next, the obtained steel plate was subjected to the following corrosion resistance test. In addition, the results of measuring the amount of solid solution Cr by the electrolytic extraction method are also shown in Table 2.

The corrosion resistance test was carried out in accordance with the slow strain rate test method (hereinafter, SSRT test) of the NACE standard TM0111-2011. That is, for the shape of the test piece, a test piece with a Type A round bar cutout was used, immersed in artificial seawater (chloride ion concentration 18000 ppm) at a temperature of 23° C., at a strain rate of 4×10 ^-7 inch/s Carry out the constant velocity tensile test. Here, a fracture stress of 600 MPa or more is considered to be excellent in resistance to stress corrosion cracking. Table 2 shows the results obtained by the above.

下劃線表示本發明範圍外。[Table 1] [Table 1]

The underline indicates outside the scope of the present invention.

下劃線表示本發明範圍外。 *1）表示700℃～600℃的平均冷卻速度。 ※無法獲得沃斯田鐵組織，因此省略測定。[Table 2] [Table 2]

The underline indicates outside the scope of the present invention. *1) Indicates the average cooling rate from 700°C to 600°C. ※Unable to obtain Vostian iron organization, so the measurement is omitted.

According to the steel sheets of the present invention (Sample No. 1 to Sample No. 42), it was confirmed that the corrosion resistance satisfies 600 MPa or more as measured by the fracture stress of the SSRT test. On the other hand, in Comparative Examples (Sample No. 43 to Sample No. 65) that deviate from the scope of the present invention, the stress corrosion cracking resistance cannot satisfy the target performance.

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Claims (5)

A steel plate, the composition of which contains 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, The remaining part contains Fe and inevitable impurities, and more than 60% of the Cr is solid solution Cr. The steel sheet as described in item 1 of the patent application range, wherein the component composition further contains one or two or more selected from the following components in 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. The steel sheet as described in item 1 or item 2 of the patent application scope, wherein the composition of the component is further contained in one or two or more types selected from the following components in 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, Mo: 0.01% or more and 2.0% or less, and W: 0.01% or more and 2.0% or less. The steel plate as described in item 1, item 2 or item 3 of the patent application scope, wherein the composition of the component is further calculated as mass% and further contains one or more than two selected from the following components: 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. A method for manufacturing a steel plate, heating a steel material having the composition as described in any one of the patent application items 1 to 4 to 1000°C or more and 1300°C or less, and thereafter at a final temperature: 750°C Above, and the temperature of the material to be rolled: 950°C or lower and 600°C or higher, hot rolling is performed with the residence time set to 30 minutes or shorter, and then the average cooling rate is performed in the temperature range of 700°C or lower and 600°C or higher: 3 Cooling above ℃/s.