Austenitic stainless steel plate, hydrogen gas transmission pipe, valve, joint, and instrument

Abstract

Provided is an austenitic stainless steel sheet having a predetermined chemical composition, in which the difference between GSNo(1 / 4)ave, which is the average grain size number at 1 / 4 of sheet thickness, and GSNo(1 / 2)ave, which is the average grain size number at 1 / 2 of sheet thickness, satisfies [0<GSNo(1 / 4)ave-GSNo(1 / 2)ave], and among the grain size number at 1 / 4 of sheet thickness and the grain size number at 1 / 2 of sheet thickness, GSNomin, which is the minimum grain size number, is 4.5 or more, the tensile strength is 550 MPa or more, and the sheet thickness is 4.5 mm or more.

Description

NAME OF INVENTION: AUSTENITIC STAINLESS STEEL PLATE, HYDROGEN GAS TRANSMISSION PIPE, VALVE, JOINT, AND INSTRUMENT TECHNICAL FIELD

[0001] The present invention relates to an austenitic stainless steel plate, a hydrogen gas transmission pipe, a valve, a joint, and an instrument. BACKGROUND ART

[0002] In recent years, the hydrogen power generation is drawing attention for its ability to reduce emission of CO2 during a stage of power generation. In the hydrogen power generation, hydrogen is used as a fuel to generate electricity. An example of hydrogen power generation facilities is a facility described in Patent Document 1, for example. Various parts and members such as pipelines are installed in such a hydrogen power generation facility for a need of transporting a large amount of hydrogen. Those parts and members used in such a hydrogen power generation facility are required to have hydrogen embrittlement resistance. LIST OF PRIOR ART DOCUMENTS PATENT DOCUMENT

[0003] Patent Document 1: JP2021-141058A SUMMARY OF INVENTION TECHNICAL PROBLEM

[0004] Some of the parts and members used in hydrogen power generation facilities are produced from what is called a thick steel plate that has a large plate thickness. Here, to shape it into desired parts and members, it is necessary to introduce a certain amount of strain to the thick steel plate. However, a problem with a thick starting material such as a thick steel plate, into which a certain amount of strain is introduced, is that hydrogen embrittlement is likely to occur when used in a hydrogen environment. In addition, some of the parts and members used in hydrogen power generation facilities may require high strength. Accordingly, there is a demand for an austenitic stainless steel plate that has both good hydrogen embrittlement resistance and high strength.

[0005] An objective of the present invention is to solve the above-described problem and to provide an austenitic stainless steel plate that has a plate thickness of 4.5 mm or more, is excellent in hydrogen embrittlement resistance, and has high strength. SOLUTION TO PROBLEM

[0006] The gist of the present invention, which has been made to solve the abovedescribed problem, is an austenitic stainless steel plate as described below.

[0007] (1) An austenitic stainless steel plate that has a chemical composition containing, in mass%: C: 0.10% or less, Si: 1.0% or less, Mn: 8.0 to 10.0%, P: 0.050% or less, S: 0.0050% or less, Cr: 14.0 to 18.0%, Mo: 1.0% or less, Ni: 6.0 to 9.0%, Cu: 1.5% or less, Co: 0.01 to 1.0%, N: 0.25% or less, Al: 0 to 0.10%, Nb: 0 to 0.10%, Ti: 0 to 0.10%, B: 0 to 0.0050%, V: 0 to 0.50%, W: 0 to 0.50%, Ca: 0 to 0.010%, Mg: 0 to 0.010%, Zr: 0 to 0.50%, Ga: 0 to 0.05%, Hf: 0 to 0.10%, REM: 0 to 0.10%, and the balance: Fe and impurities, wherein M-value calculated by Formula (i) below is -90 to -20, the difference between GSNo(1 / 4)ave and GSNo(1 / 2)ave satisfies Formula (ii) below, where GSNo(1 / 4)ave is an average value of grain size number values of a 1 / 4 platethickness portion and GSNo(1 / 2)ave is an average value of grain size number values of a 1 / 2 plate-thickness portion, GSNomin is 4.5 or more, where GSNomin is the smallest grain size number value among the grain size number values of the 1 / 4 plate-thickness portion and the grain size number values of the 1 / 2 plate-thickness portion, a tensile strength is 550 MPa or more, and a plate thickness is 4.5 mm or more: M-value = 551 - 462(C + N) - 9.2Si - 8.1Mn - 13.7Cr - 29(Ni + Cu) - 18.2Mo ... (i) 0 < GSNo(1 / 4)ave - GSNo(1 / 2)ave ... (ii) where each element symbol in the Formula (i) represents a content (mass%) of each element contained in the austenitic stainless steel plate and is zero when the element is not contained.

[0008] (2) The austenitic stainless steel plate according to the above (1), wherein the chemical composition contains one or more elements selected from, in mass%: Al: 0.01 to 0.10%, Nb: 0.01 to 0.10%, Ti: 0.01 to 0.10%, B: 0.0002 to 0.0050%, V: 0.05 to 0.50%, W: 0.05 to 0.50%, Ca: 0.0002 to 0.010%, Mg: 0.0002 to 0.010%, Zr: 0.01 to 0.50%, Ga: 0.001 to 0.05%, Hf: 0.01 to 0.10%, and REM: 0.01 to 0.10%.

[0009] (3) The austenitic stainless steel plate according to the above (1) or (2), wherein in the 1 / 4 plate-thickness portion, an area fraction of a region that satisfies Formula (iii) below is 90% or more and an area fraction of a region that satisfies Formula (iv) below is 90% or more: Nis > 0.8 ... (iii) Mns > 0.8 ... (iv) where in the Formula (iii), Nis represents a degree of segregation of Ni, and in the Formula (iv), Mns represents a degree of segregation of Mn.

[0010] (4) The austenitic stainless steel plate according to any one of the above (1) to (3), wherein the austenitic stainless steel plate is used in a hydrogen gas environment.

[0011] (5) A hydrogen gas transmission pipe, including the austenitic stainless steel plate according to any one of the above (1) to (3).

[0012] (6) A valve, a joint, or an instrument for use with hydrogen gas, including the austenitic stainless steel plate according to any one of the above (1) to (3). ADVANTAGEOUS EFFECTS OF INVENTION

[0013] According to the present invention, it is possible to obtain an austenitic stainless steel plate that has a plate thickness of 4.5 mm or more, is excellent in hydrogen embrittlement resistance, and has high strength. DESCRIPTION OF EMBODIMENTS

[0014] The inventors have studied hydrogen embrittlement resistance of the thick steel plate and obtained the following findings.

[0015] (a) When shaped into desired parts and members, the thick steel plate is processed based on plastic deformation such as by tensile processing and bending. Such plastic deformation applies pre-strain to the thick steel plate. In general, as the plate thickness increases, the stress required for plastic deformation increases. On the other hand, the strain is not necessarily applied uniformly in all positions in the steel plate. For example, the amount of strain applied to a near-surface area of the steel plate and the amount of strain applied to a near-center area in the plate thickness are greatly different.

[0016] (b) One reason that can be given is the difference in grain size between a nearsurface area and an area near the center in the plate thickness. As a result of study conducted by the inventors, it has been revealed that, particularly in a high-Mn austenitic stainless steel and in the case of a thick steel plate that is more than 10 mm in the plate thickness, grains are finest in the outermost surface, whereas grains in a 1 / 4 platethickness portion, which is the near-surface area, tend to be slightly coarse due to columnar microstructures grown and coarsened during solidification. Furthermore, grains in a 1 / 2 plate-thickness portion, which is the near-center area in the plate thickness, tend to be finer than those in the 1 / 4 plate-thickness portion due to formation of equiaxial microstructures during solidification. That is, a difference is likely to occur in grain size between the 1 / 4 plate-thickness portion and the 1 / 2 plate-thickness portion, and grains in the 1 / 4 plate-thickness portion are likely to be coarser than grains in the 1 / 2 platethickness portion. As a result, greater amount of strain is likely to be accumulated in a region in which grains have a large diameter, so that hydrogen embrittlement is likely to occur in the region in which grains have a large diameter. The higher the strength of the steel plate, the more remarkable this tendency is.

[0017] (c) Based on the above, to increase hydrogen embrittlement resistance of the thick steel plate, it is desirable to make the difference in grain size in the steel plate smaller. However, rather than simply making the difference in grain size smaller, it is preferable that grains in the 1 / 4 plate-thickness portion are finer than those in the 1 / 2 plate-thickness portion from the viewpoint of hydrogen embrittlement resistance. To achieve such a metallographic structure, it is necessary to control a rolled shape ratio into a predetermined range during production. Furthermore, from the viewpoint of hydrogen embrittlement resistance, it is preferable to reduce coarse grains in the entire steel plate.

[0018] An embodiment of the present invention has been made based on the abovedescribed findings. The requirements of the austenitic stainless steel plate of the embodiment will now be described in detail.

[0019] 1. Chemical composition The reason for limitation for each element is described below. Note that in the description below, "%" for a content refers to "mass%".

[0020] C: 0.10% or less C (carbon) is an element that is effective for stabilization of the austenite phase and also contributes to improvement of hydrogen embrittlement resistance. However, excessively contained C promotes grain boundary precipitation of Cr carbide, and what is worse, is likely to lead to degradation of hydrogen embrittlement resistance. Accordingly, the content of C is 0.10% or less. The content of C is preferably 0.080% or less, more preferably 0.070% or less, and further preferably 0.060% or less. On the other hand, to obtain the above-described effects, the content of C is preferably 0.010% or more, more preferably 0.020% or more, further preferably 0.030% or more, and further preferably 0.040% or more.

[0021] Si: 1.0% or less Si (silicon) is an element that is effective for deoxidation and also contributes to improvement of hydrogen embrittlement resistance. However, excessively contained Si promotes generation of an intermetallic compound such as a g phase and causes degradation of hot workability and toughness. Accordingly, the content of Si is 1.0% or less. The content of Si is preferably 0.90% or less, more preferably 0.80% or less, further preferably 0.70% or less, and further preferably 0.60% or less. On the other hand, to obtain the above-described effects, the content of Si is preferably 0.10% or more, more preferably 0.20% or more, further preferably 0.30% or more, and further preferably 0.40% or more.

[0022] Mn: 8.0 to 10.0% Mn (manganese) is an element that is effective for stabilization of the austenite phase and contributes to improvement of hydrogen embrittlement resistance. Furthermore, Mn produces an effect of improving strength. Accordingly, the content of Mn is 8.0% or more. The content of Mn is preferably 8.5% or more. However, excessively contained Mn promotes generation of an s phase that is highly sensitive to hydrogen embrittlement, and what is worse, causes degradation of hydrogen embrittlement resistance. Accordingly, the content of Mn is 10.0% or less. The content of Mn is preferably 9.5% or less.

[0023] P: 0.050% or less P (phosphorus) is an element contained in the steel as an impunity and causes segregation, which leads to degradation of hydrogen embrittlement resistance. Accordingly, the content of P is 0.050% or less. The content of P is preferably 0.040% or less and more preferably 0.030% or less. On the other hand, excessively reducing P leads to an increase in production costs, and therefore, the content of P is preferably 0.001% or more and more preferably 0.010% or more.

[0024] S: 0.0050% or less S (sulfur) is an element contained in the steel as an impunity and causes formation of MnS, which leads to degradation of hydrogen embrittlement resistance. Accordingly, the content of S is 0.0050% or less. The content of S is preferably 0.0040% or less and more preferably 0.0030% or less. However, excessively reducing S leads to an increase in production costs. Accordingly, the content of S is preferably 0.0001% or more and more preferably 0.0002% or more.

[0025] Cr: 14.0 to 18.0% Cr (chromium) is an element that is contained in stainless steel at a certain amount and produces an effect of improving corrosion resistance. Accordingly, the content of Cr is 14.0% or more. The content of Cr is preferably 14.5% or more, more preferably 15.0% or more, and further preferably 15.5% or more. However, Cr is a ferrite-forming element. Accordingly, excessively contained Cr makes the austenite phase unstable and causes a large amount of carbide to precipitate, which leads to degradation of hydrogen embrittlement resistance. Accordingly, the content of Cr is 18.0% or less. The content of Cr is preferably 17.5% or less, more preferably 17.0% or less, and more preferably 16.5% or less.

[0026] Mo: 1.0% or less Mo (molybdenum) produces an effect of improving strength. However, excessively contained Mo promotes generation of a 5 ferrite phase, which leads to degradation of hydrogen embrittlement resistance. Accordingly, the content of Mo is 1.0% or less. The content of Mo is preferably 0.85% or less, more preferably 0.70% or less, further preferably 0.50% or less, and further preferably 0.30% or less. On the other hand, excessively reducing Mo leads to constraints in molten raw materials, and production costs increase. Accordingly, the content of Mo is preferably 0.01% or more, more preferably 0.05% or more, and further preferably 0.10% or more.

[0027] Ni: 6.0 to 9.0% Ni (nickel) is an element necessary to secure hydrogen embrittlement resistance and strength, along with Mn. Accordingly, the content of Ni is 6.0% or more. The content of Ni is preferably 6.3% or more, more preferably 6.7% or more, and further preferably 7.0% or more. However, excessively contained Ni is likely not only to lead to an increase in production costs but also to cause segregation, and what is worse, likely to lead to degradation of hydrogen embrittlement resistance. In addition, excessively contained Ni causes solid solution softening, leading to degradation of strength. Accordingly, the content of Ni is 9.0% or less. The content of Ni is preferably 8.5% or less, more preferably 8.0% or less, and further preferably 7.6% or less.

[0028] Cu: 1.5% or less Cu (copper) is an element introduced from raw materials such as scrap and is an element that is effective for stabilization of the austenite phase. On the other hand, Cu is a low melting point element and segregates at grain boundaries, so that a starting point of fracture is likely to occur. Accordingly, the content of Cu is 1.5% or less. The content of Cu is preferably 1.0% or less, more preferably 0.70% or less, and further preferably 0.50% or less. However, excessively reducing Cu leads to constraints in molten raw materials, and production costs increase. Accordingly, the content of Cu is preferably 0.01% or more, preferably 0.05% or more, and further preferably 0.10% or more.

[0029] Co: 0.01 to 1.0% Co (cobalt) is an important element and produces effects of improving corrosion resistance, stabilizing the austenite phase, and in particular in the case of a large plate thickness, improving hydrogen embrittlement resistance. Accordingly, the content of Co is 0.01% or more. The content of Co is preferably 0.05% or more, more preferably 0.10% or more, and further preferably 0.15% or more. However, excessively contained Co causes degradation of toughness and workability. Accordingly, the content of Co is 1.0% or less. The content of Co is preferably 0.70% or less and further preferably 0.50% or less.

[0030] N: 0.25% or less N (nitrogen) is an element that is effective for improving hydrogen embrittlement resistance similarly to Mn and Ni. However, excessively contained N may cause inner defects such as blowholes while being melted to occur, and what is worse, is likely to lead to degradation of hydrogen embrittlement resistance. Accordingly, the content of N is 0.25% or less. The content of N is preferably 0.20% or less and more preferably 0.18% or less. On the other hand, to obtain the above-described effects, the content of N is preferably 0.010% or more, more preferably 0.020% or more, further preferably 0.050% or more, and further preferably 0.10% or more.

[0031] In addition to the above-described elements, one or more elements selected from Al, Nb, Ti, B, V, W, Ca, Mg, Zr, Ga, Hf, and REM may further be contained to the extent indicated below. The reason for limitation for each element will be described.

[0032] Al: 0 to 0.10% Al (aluminum), which is an effective deoxidation element, suppresses grain boundary segregation of low melting point elements, producing an effect of enhancing grain boundaries. Accordingly, Al may be contained as necessary. However, Al is a ferrite-forming element. Accordingly, excessively contained Al makes the austenite phase unstable. Accordingly, the content of Al is 0.10% or less. The content of Al is preferably 0.07% or less and more preferably 0.05% or less. On the other hand, to obtain the above-described effects, the content of Al is preferably 0.01% or more and more preferably 0.02% or more.

[0033] Nb: 0 to 0.10% Nb (niobium) forms carbonitride and refines grains, producing an effect of enhancing grain boundaries. Accordingly, Nb may be contained as necessary. However, excessively contained Nb causes degradation of producibility during hot rolling and workability. Accordingly, the content of Nb is 0.10% or less. The content of Nb is preferably 0.08% or less and more preferably 0.07% or less. On the other hand, to obtain the above-described effects, the content of Nb is preferably 0.01% or more and more preferably 0.02% or more.

[0034] Ti: 0 to 0.10% Ti (titanium) forms carbonitride and refines grains, producing an effect of enhancing grain boundaries. Accordingly, Ti may be contained as necessary. However, excessively contained Ti causes degradation of producibility during hot rolling. Furthermore, a large amount of inclusions are formed and impact resistance may degrade. Accordingly, the content of Ti is 0.10% or less. The content of Ti is preferably 0.07% or less and more preferably 0.05% or less. On the other hand, to obtain the abovedescribed effects, the content of Ti is preferably 0.01% or more and more preferably 0.02% or more.

[0035] B: 0 to 0.0050% B (boron) enhances grain boundaries and produces effects of improving strength and improving impact resistance. Accordingly, B may be contained as necessary. However, excessively contained B only causes the effect to be saturated, and what is worse, may cause degradation of impact resistance. Accordingly, the content of B is 0.0050% or less. The content of B is preferably 0.0040% or less and more preferably 0.0030% or less. On the other hand, to obtain the above-described effects, the content of B is preferably 0.0002% or more, more preferably 0.0005% or more, and further preferably 0.0010% or more.

[0036] V: 0 to 0.50% V (vanadium) precipitates in the steel as a solid solution or carbonitride and produces an effect of improving strength. Accordingly, V may be contained as necessary. However, excessively contained V causes excessive formation of carbonitride, leading to degradation of producibility during hot rolling. Accordingly, the content of V is 0.50% or less. The content of V is preferably 0.40% or less, more preferably 0.30% or less, and further preferably 0.20% or less. On the other hand, to obtain the above-described effects, the content of V is preferably 0.05% or more and more preferably 0.10% or more.

[0037] W: 0 to 0.50% W (tungsten) produces effects of improving strength and corrosion resistance. Accordingly, W may be contained as necessary. However, excessively contained W leads to an increase in production costs. Accordingly, the content of W is 0.50% or less. The content of W is preferably 0.40% or less and more preferably 0.30% or less. On the other hand, to obtain the above-described effects, the content of W is preferably 0.05% or more and more preferably 0.10% or more.

[0038] Ca: 0 to 0.010% Ca (calcium) is an element that is effective for improving deoxidation and hot workability. Accordingly, Ca may be contained as necessary. However, excessively contained Ca is likely to cause segregation, and the segregation is likely to be a starting point of fracture. As a result, impact resistance may degrade. Accordingly, the content of Ca is 0.010% or less. The content of Ca is preferably 0.0050% or less and more preferably 0.0030% or less. On the other hand, to obtain the above-described effects, the content of Ca is preferably 0.0002% or more, more preferably 0.0005% or more, and further preferably 0.0010% or more.

[0039] Mg: 0 to 0.010% Mg (magnesium) is an element that is effective for improving deoxidation and hot workability. Accordingly, Mg may be contained as necessary. However, excessively contained Mg causes formation of a large amount of inclusions, which is likely to be a starting point of fracture, and as a result, the impact resistance may degrade. Accordingly, the content of Mg is 0.010% or less. The content of Mg is preferably 0.0050% or less, more preferably 0.0030% or less, and further preferably 0.0010% or less. On the other hand, to obtain the above-described effects, the content of Mg is preferably 0.0001% or more and more preferably 0.0002% or more.

[0040] Zr: 0 to 0.50% Zr (zirconium) produces a deoxidation effect. An effect of improving corrosion resistance is also produced. Accordingly, Zr may be contained as necessary. However, excessively contained Zr causes degradation of toughness and workability. Accordingly, the content of Zr is 0.50% or less. The content of Zr is preferably 0.30% or less, more preferably 0.10% or less, and further preferably 0.05% or less. On the other hand, to obtain the above-described effects, the content of Zr is preferably 0.01% or more and more preferably 0.02% or more.

[0041] Ga: 0 to 0.05% Ga (gallium) produces an effect of improving hot workability. Accordingly, Ga may be contained as necessary. However, excessively contained Ga causes degradation of producibility. Accordingly, the content of Ga is 0.05% or less. The content of Ga is preferably 0.04% or less and more preferably 0.02% or less. On the other hand, to obtain the above-described effects, the content of Ga is preferably 0.001% or more and more preferably 0.005% or more.

[0042] Hf: 0 to 0.10% Hf (hafnium) produces effects of improving strength and improving hydrogen embrittlement resistance. Accordingly, Hf may be contained as necessary. However, excessively contained Hf causes degradation of workability. Accordingly, the content of Hf is 0.10% or less. The content of Hf is preferably 0.07% or less, more preferably 0.05% or less, and further preferably 0.03% or less. On the other hand, to obtain the above-described effects, the content of Hf is preferably 0.001% or more, more preferably 0.005% or more, and further preferably 0.01% or more.

[0043] REM: 0 to 0.10% REM (rare earth metal) produces an effect of improving hot workability. An effect of improving corrosion resistance is also produced. Accordingly, REM may be contained as necessary. However, excessively contained REM not only causes the effect to be saturated but also causes degradation of hot workability. Accordingly, the content of REM is 0.10% or less. The content of REM is preferably 0.07% or less, more preferably 0.05% or less, and further preferably 0.03% or less. On the other hand, to obtain the above-described effects, the content of REM is preferably 0.001% or more, more preferably 0.005% or more, and further preferably 0.01% or more.

[0044] REM refers to a total of 17 elements of Sc, Y, and lanthanoid, and the content of REM refers to a total content of the elements. the form of misch metal.

[0045] In the chemical composition of the embodiment, the balance is Fe and impurities. Industrially, REM is often contained in austenitic stainless steel plate of the Here, "impurities" refer to components that are introduced due to various factors in raw materials such as ore and scrap and production processes when the austenitic stainless steel plate is industrially produced and that are acceptable to the extent that they do not adversely affect the embodiment.

[0046] M-value M-value calculated by Formula (i) below is an indicator that indicates the stability of the austenite phase in the austenitic stainless steel plate. In the austenitic stainless steel plate of the embodiment, the M-value is -90 to -20 for improving hydrogen embrittlement resistance and strength. M-value = 551 - 462(C + N) - 9.2Si - 8.1Mn - 13.7Cr - 29(Ni + Cu) - 18.2Mo ... (i) where each element symbol in the Formula (i) represents a content (mass%) of each element contained in the austenitic stainless steel plate and is zero when the element is not contained.

[0047] When the M-value is less than -90, alloy costs increase. Accordingly, the M-value is -90 or more. The M-value is preferably -80 or more and more preferably -70 or more.

[0048] On the other hand, when the M-value is more than -20, the stability of the austenite phase is lowered, transformation into an a' phase is likely to occur, and hydrogen embrittlement resistance degrades. Furthermore, a desired strength is less likely to be obtained. Accordingly, the M-value is -20 or less. The M-value is preferably -25 or less and more preferably -35 or less.

[0049] 2. Grain size number value 2-1. Difference between grain size numbers in near-surface area and grain size numbers in center portion The austenitic stainless steel plate of the embodiment requires that the difference between grain size numbers in the near-surface area and grain size numbers in the center portion is made smaller, and grains in the 1 / 4 plate-thickness portion are finer than those in the 1 / 2 plate-thickness portion to increase hydrogen embrittlement resistance.

[0050] Specifically, the difference between GSNo(1 / 4)ave, which is the average value of the grain size number values of the 1 / 4 plate-thickness portion, and GSNo(1 / 2)ave, which is the average value of the grain size number values of the 1 / 2 plate-thickness portion, (also referred to as "grain size number difference" or "the right side value of Formula (ii)") satisfies Formula (ii) below. 0 < GSNo(1 / 4)ave - GSNo(1 / 2)ave ... (ii)

[0051] From the viewpoint of hydrogen embrittlement resistance, the grain size number difference is preferably 0.1 or more and more preferably 0.3 or more. On the other hand, when the grain size number difference is more than 1.5, the difference in grain size number is too large in the steel plate, and what is worse, hydrogen embrittlement resistance is likely to degrade. Accordingly, the grain size number difference is preferably 1.5 or less and more preferably 1.0 or less.

[0052] Here, when the plate thickness is defined as t, the 1 / 4 plate-thickness portion refers to a position at 1 / 4t in the plate-thickness direction from the surface. Likewise, when the plate thickness is defined as t, the 1 / 2 plate-thickness portion refers to a position at 1 / 2t in the plate-thickness direction from the surface.

[0053] The average value of grain size number values of the 1 / 4 plate-thickness portion GSNo(1 / 4)ave and the average value of grain size number values of the 1 / 2 plate-thickness portion GSNo(1 / 2)ave are measured according to the procedure described below. Specifically, with a plane that is parallel to the plate-thickness direction and the longitudinal direction in the near-center area of the steel plate (hereinafter, referred to as an "L-section") being defined as an observation surface, crystal grain boundaries are revealed through a nitric acid electrolysis or aqua regia immersion, and thereafter, observation is performed with an optical microscope. The size of observation surface is the plate thickness x 30 mm (length in the longitudinal direction) and the magnification for observation is 100x. Then, for the 1 / 4 plate-thickness portion of the observation surface, measurements are taken in a total of 5 visual fields: the center in the longitudinal direction and 6-mm positions and 12-mm positions for opposite directions, respectively, grain size numbers are calculated linearly through a cutting method, and the average value thereof is taken as GSNo(1 / 4)ave described above. Likewise, for the 1 / 2 plate-thickness portion, measurements are taken in a total of 5 visual fields: the center in the longitudinal direction and 6-mm positions and 12-mm positions for opposite directions, respectively, grain size numbers are calculated linearly through a cutting method, and the average value thereof is taken as GSNo(1 / 2)ave described above. Other conditions for observation and measurement may be in compliance with JIS G 0551: 2020.

[0054] 2-2. Smallest grain size number The austenitic stainless steel plate of the embodiment suppresses formation of coarse grains to suppress penetration of hydrogen and improve hydrogen embrittlement resistance. Specifically, in observed visual fields, the smallest, that is, the coarsest, grain size number value GSNomin of a visual field is 4.5 or more among grain size number values of the 1 / 4 plate-thickness portion and grain size number values of the 1 / 2 platethickness portion. GSNomin is preferably 5.0 or more and more preferably 5.5 or more. Although not particularly limited, generally the upper limit of GSNomin is preferably 8.0 in consideration of production costs, constraints during production, and the like.

[0055] GSNomin can be obtained by determining the smallest grain size number value among 5 visual fields of the 1 / 4 plate-thickness portion and 5 visual fields of the 1 / 2 platethickness portion in observations performed for measuring GSNo(1 / 4)ave and GSNo(1 / 2)ave as described above.

[0056] 3. Tensile strength The tensile strength of the austenitic stainless steel plate of the embodiment at a normal temperature is 550 MPa or more in order to obtain a desired strength. The tensile strength is preferably 600 MPa or more and more preferably 650 MPa or more. Although not particularly limited, the upper limit of the tensile strength is generally 900 MPa. The tensile strength is preferably less than 800 MPa, more preferably 780 MPa or less, and further preferably 750 MPa or less. Note that the tensile strength at a normal temperature can be measured by conducting a tensile test at a room temperature (24°C). The conditions for the tensile test may be in accordance with common conditions.

[0057] 4. Plate thickness The austenitic stainless steel plate of the embodiment is directed to a thick steel plate that has a large plate thickness and is intended to be used in hydrogen power generation facilities. Accordingly, the plate thickness of the steel plate is 4.5 mm or more. The plate thickness is preferably 10 mm or more, more preferably 20 mm or more, and further preferably 30 mm or more. Although not particularly limited, the upper limit of the plate thickness is generally 100 mm.

[0058] 5. Segregated state It is preferable to control a segregated state in order to obtain good hydrogen embrittlement resistance. Segregation refers to phenomena of the concentration of dissolved elements being made non-uniform during solidification and is remarkable in the case of a thick steel plate. After completion as a steel plate, the segregated state, which partially remains in the metallographic structure, will have adverse effects on hydrogen embrittlement resistance later. In particular, in the near-surface 1 / 4 platethickness portion, the effect of segregation is remarkable, and pre-strain is locally accumulated in a segregated region, so that hydrogen embrittlement resistance is likely to degrade. Furthermore, any segregation is likely to cause transformation from the austenite phase into an a' phase, which is also one cause of likelihood of degradation of hydrogen embrittlement resistance.

[0059] While segregation is categorized into a positive segregation in which the element distribution is higher than the average concentration and a negative segregation in which the element distribution is lower than the average concentration, the austenitic stainless steel plate of the embodiment focuses mainly on the negative segregation from the viewpoint of hydrogen embrittlement resistance.

[0060] Based on the above, in the 1 / 4 plate-thickness portion, the area fraction of a region that satisfies Formula (iii) below is preferably 90% or more and the area fraction of a region that satisfies Formula (iv) below is preferably 90% or more: Nis > 0.8 ... (iii) Mns > 0.8 ... (iv) where in the Formula (iii), Nis represents the degree of segregation of Ni (nickel), and in the Formula (iv), Mns represents the degree of segregation of Mn (manganese).

[0061] In the chemical composition of the steel plate of the embodiment, elements that segregate and affect hydrogen embrittlement resistance are Ni and Mn. Accordingly, the degree of segregation of Ni and Mn is to be controlled. The degree of segregation refers to the concentration of a segregated portion to the average concentration of a predetermined element: the degree of segregation of Ni can be calculated by (local Ni concentration) / (average content of Ni of total thickness of steel plate), the degree of segregation Mn can be calculated by (local Mn concentration) / (average content of Mn of total thickness of steel plate), and the closer to one the degree of segregation is, the lower the segregation becomes.

[0062] When Nis is more than 0.8, that is, the degree of segregation of Ni is more than 0.8, and Formula (iii) is satisfied, no local concentration gradient occurs, and the negative segregation is reduced. Accordingly, from the viewpoint of degradation of hydrogen embrittlement resistance due to the negative segregation, in the 1 / 4 plate-thickness portion, the area fraction of a region that satisfies Formula (iii) is preferably 90% or more. The area fraction of the region that satisfies Formula (iii) is more preferably 95% or more.

[0063] Likewise, when Mns is more than 0.8, that is, the degree of segregation of Mn is more than 0.8, and Formula (iv) is satisfied, no local concentration gradient occurs, and the negative segregation is reduced. Accordingly, from the viewpoint of degradation of hydrogen embrittlement resistance due to the negative segregation, in the 1 / 4 platethickness portion, the area fraction of a region that satisfies Formula (iv) is preferably 90% or more. Accordingly, the area fraction of the region that satisfies Formula (iv) is preferably 90% or more and more preferably 95% or more.

[0064] Then, it is preferable that the area fraction of the region that satisfies Formula (iii) is 90% or more and the area fraction of the region that satisfies Formula (iv) is 90% or more.

[0065] Note that Nis and Mns in the 1 / 4 plate-thickness portion, as well as the area fraction of the region that satisfies Formula (iii) and the area fraction of the region that satisfies Formula (iv) may be measured according to the procedure described below. In a plane that is parallel to the plate-thickness direction and the plate-width direction of the steel plate (C-section), the 1 / 4 plate-thickness portion is taken as the center of the visual field, and an area of 2-mm square above and below the center is determined to be an observation visual field. Subsequently, area analysis is performed on the observation visual field by using EPMA under conditions of beam diameter: 6 jam, accelerating voltage: 15 kV, and irradiation current: 1.17 x 10-9 A. Through the area analysis by using EPMA, Ni concentration and Mn concentration are mapped and a region that satisfies Formula (iii) and a region that satisfies Formula (iv) are calculated in area fraction. Note that while it is preferable that the observations are performed near the center in the plate width of the steel plate where segregation is likely to occur, it is contemplated that whatever satisfies the above-described range falls within the scope of the invention for the embodiment regardless of the position in the plate-width direction.

[0066] 6. Application The austenitic stainless steel plate of the embodiment is suitable for use in a hydrogen gas environment, and in particular for parts and members in hydrogen power generation facilities. That is, it is preferably used for a hydrogen gas transmission pipe used in the facilities, and valves, joints, and instruments for use with hydrogen gas. The hydrogen gas is preferably compressed gas or liquefied gas, and the pressure of hydrogen gas is preferably in the range of 0.1 to 20 MPa, for example.

[0067] 7. Production method The austenitic stainless steel plate of the embodiment can stably be produced by, for example, a production method described below.

[0068] 7-1. Hot rolling process 7-1-1. Preliminary rolling The stainless steel that has a chemical composition described above is melted to produce a cast piece such as a slab. Next, the cast piece is heated to a predetermined temperature for hot rolling (hot rolling process). In the hot rolling process, the cast piece is first heated at a temperature of 1200oC or less and subjected to a preliminary rolling in which 3 or more passes are taken place, in each pass of which a rolled shape ratio mj is 0.5 or more.

[0069] In the production method of the austenitic stainless steel plate of the embodiment, the heating temperature for the preliminary rolling is 1200°C or less. The heating temperature for the preliminary rolling is preferably 1100°C or less. This is to suppress recrystallization and grain growth and cause strain to be accumulated in the 1 / 4 platethickness portion. Such accumulation of strain produces an effect of promoting recrystallization while being heated in heat treatment in the next process or finishing rolling to make grains in the 1 / 4 plate-thickness portion finer.

[0070] By performing the preliminary rolling in 3 or more passes, in each pass of which a rolled shape ratio mj is 0.5 or more, at the above-described heating temperature, it is possible to form grains that satisfy Formula (ii) and exhibit GSNomin of 4.5 or more. The rolled shape ratio mj is calculated by Formula (a) below.

[0071] [Math 1] — ^+1) £ / + ^ / +1 In the above formula, symbols are defined as described below: R: roll radius (mm) tj: entry-side plate thickness (mm) tj+1: exit-side plate thickness (mm) mt =

[0072]

[0073] In the hot rolling process, there is a plurality of rolling mills installed, each of which has a pair of rolling rolls. The steel plate passes through the plurality of rolling mills so that rolling elongation is achieved. Here, the action of the steel plate passing through and being rolled between rolls of the pair in one rolling mill is referred to as a "pass". Accordingly, R denotes a roll radius of the rolling roll, tj denotes a plate thickness before rolling at the time when the steel plate enters the rolling mill between passes, and tj+1 denotes a plate thickness after passing through the rolling mill. Note that since the rolled shape ratio mj is affected by the roll diameter, the rolling reduction, and the like of the rolling mill, it is sufficient to control them into appropriate ranges so that the rolled shape ratio mj is 0.5 or more. Other conditions for the preliminary rolling may be in accordance with common procedures as necessary.

[0074] 7-1-2. Heat treatment After the preliminary rolling, a heat treatment may be performed as necessary. The reason is that heat treatment makes it easy to allow the area fractions of the regions that satisfy Formulas (iii) and (iv) to be 90% or more in the 1 / 4 plate-thickness portion. The heat treatment temperature is preferably 1200 to 1300°C, and the heat treatment time is preferably 60 minutes or more.

[0075] 7-1-3. Finishing rolling In the hot rolling process, finishing rolling is performed after the preliminary rolling or after heat treatment when the heat treatment is performed as necessary. In the finishing rolling, 5 or more passes are preferably taken place in terms of the number of passes, in each of which the rolled shape ratio mj is 0.8 or more. The reason is that the grain size number difference falls to 1.0 or less, so that better hydrogen embrittlement resistance can be obtained. Note that the rolled shape ratio mj may be calculated as in the preliminary rolling. Furthermore, to obtain a microstructure that has a desired grain size number, the heating temperature for the steel plate in the finishing rolling is preferably in the range of 1150 to 1250°C. Other conditions for the finishing rolling may be in accordance with common procedures as necessary. After the finishing rolling, cooling in accordance with common procedures is carried out to attain the austenitic stainless steel plate.

[0076] 7-2. Others After the hot rolling process, annealing and pickling may be performed as necessary. Annealing conditions may be in accordance with common procedures: for example, the annealing temperature is preferably 1050 to 1150°C and the annealing time is preferably 5 to 15 minutes. Thereafter, cooling, and pickling as necessary, may be carried out to finally attain the austenitic stainless steel plate. The conditions for pickling may also be in accordance with common procedures.

[0077] Hereinunder, the austenitic stainless steel plate according to the present invention will specifically be described with reference to examples, whereas the embodiment is not limited to the examples. EXAMPLE

[0078] Stainless steels that each have a chemical composition indicated in Table 1 were melted to produce slabs that are 200 mm in thickness. The obtained slab was subsequently to preliminary rolling. In the preliminary rolling, the slab was heated at temperature listed in Table 2. The number of passes were counted for a pass in which the rolled shape ratio mj was 0.5 or more in the preliminary rolling. Thereafter, in some of the examples, heat treatment was performed under conditions listed in Table 2. For those examples in which heat treatment was performed as necessary after the preliminary rolling, the heat treatment was followed by heating to 1150 to 1200°C and finishing rolling. The number of passes were likewise counted for a pass in which the rolled shape ratio mj was 0.8 or more in the finishing rolling. Thereafter, annealing was performed in the temperature range of 1050 to 1100°C for 5 minutes, followed by cooling and pickling to attain the austenitic stainless steel plate.

[0079] [Table 1] Table 1 Steel Chemical composition (mass%, balance: Fe and impurities) M-value C Si Mn P S Cr Mo Ni Cu Co N Al Nb Ti B V w Ca Mg Zr Ga Hf REM A 0.055 0.45 9.1 0.013 0.0001 15.3 0.01 7.4 0.01 0.15 0.18 0.04 - 0.05 0.0003 0.15 - 0.0015 0.0003 - 0.02 - - -60 B 0.060 0.55 9.9 0.046 0.0002 15.7 0.03 6.1 1.4 0.45 0.055 - - - - - - - - - - - - -21 C 0.095 0.95 8.2 0.021 0.0035 17.8 0.95 6.5 0.20 0.05 0.012 - - - - - - - - - - - - -29 D 0.012 0.60 9.3 0.025 0.0014 14.1 0.25 8.9 0.35 0.35 0.19 - 0.05 - - - 0.30 - - - - - - -89 E 0.045 0.58 9.4 0.005 0.0006 15.5 0.10 6.2 0.10 0.20 0.24 - - - - - - - - 0.03 - 0.01 0.02 -59 F 0.038 0.45 8.9 0.013 0.0001 16.5 0.10 7.7 0.01 0.15 0.16 0.03 - - 0.0022 0.15 - - - - - - - -66 G 0.065 0.51 9.2 0.013 0.0001 17.2 0.10 6.5 0.01 0.15 0.17 0.04 - - - 0.10 - - - - - - - -63 H 0.082 0.45 8.4 0.013 0.0001 16.2 0.10 7.1 0.01 0.15 0.17 0.05 0.08 - - 0.08 - - - - - - - -67 I 0.11 0.58 9.2 0.021 0.0018 16.2 0.20 6.7 0.20 0.20 0.13 - - - - - - - - - - - - -65 J 0.045 0.55 10.5 0.022 0.0026 15.8 0.33 6.5 0.15 0.13 0.15 - - - - - - - - - - - - -45 K 0.056 0.61 9.2 0.051 0.0016 15.2 0.25 6.2 0.13 0.17 0.16 - - - - - - - - - - - - -25 L 0.053 0.59 9.1 0.022 0.0051 15.1 0.15 6.4 0.14 0.18 0.17 - - - - - - - - - - - - -30 M 0.065 0.62 8.8 0.021 0.0013 18.2 0.21 6.2 0.16 0.14 0.16 - - - - - - - - - - - - -68 N 0.045 0.48 8.1 0.018 0.0016 14.8 0.20 9.2 0.05 0.23 0.055 - - - - - - - - - - - - -40 O 0.062 0.53 9.1 0.021 0.0012 15.2 0.23 5.9 0.23 0.25 0.18 - - - - - - - - - - - - -30 P 0.057 0.52 9.3 0.018 0.0013 15.5 0.23 7.0 0.10 0.25 0.090 - - - - - - - - - - - - -19 Q 0.058 0.62 9.3 0.019 0.0013 15.6 0.11 6.0 0.22 <0.01 0.15 - - - - - - - - - - - - -22 R 0.035 0.58 8.8 0.023 0.0012 15.9 0.15 6.0 0.20 0.10 0.26 - - - - - - - - - - - - -62 Underline means that the chemical composition falls outside that of the austenitic stainless steel plate of the present embodiment. M-value=551-462(C+N)-9.2Si-8. lMn-13.7Cr-29(Ni+Cu)-18.2Mo ■■■(!)

[0080] On the obtained austenitic stainless steel plates, measurements were taken for GSNo(1 / 4)ave, GSNo(1 / 2)ave, GSNomin, and area fractions for regions that satisfy Formula (iii) and regions that satisfy Formula (iv) in the 1 / 4 plate-thickness portion according to the procedure described below. In addition, a slow strain rate tensile test (SSRT) was conducted for property evaluation according to the procedure described below.

[0081] (GSNo(1 / 4)ave, GSNo(1 / 2)ave, GSNomin) An L-section of the steel plate is taken as an observation surface. The size of the observation surface was the plate thickness x 30 mm (length in the longitudinal direction), and the magnification for observation was 100x. Then, for the 1 / 4 platethickness portion of the observation surface, measurements were taken in 5 visual fields, grain size numbers were calculated linearly through a cutting method, and the average value thereof was taken as GSNo(1 / 4)ave. Likewise, for the 1 / 2 plate-thickness portion, measurements were taken in 5 visual fields, grain size numbers were calculated linearly through a cutting method, and the average value thereof was taken as GSNo(1 / 2)ave. Other observation conditions were in accordance with JIS G 0551: 2020.

[0082] Furthermore, in observations for measuring GSNo(1 / 4)ave and GSNo(1 / 2)ave described above, the grain size number value of a visual field that exhibited the smallest grain size number value was taken as GSNomin. That is, the smallest grain size number value among 5 visual fields of the 1 / 4 plate-thickness portion and 5 visual fields of the 1 / 2 plate-thickness portion was taken as GSNomin.

[0083] (Area fractions of regions that satisfy Formulas (iii) and (iv) in 1 / 4 platethickness portion) In a C-section of the steel plate, the 1 / 4 plate-thickness portion was taken as the center of the visual field, and an area of 2-mm square above and below the center was taken as the observation visual field. Subsequently, area analysis by using EPMA was performed on the observation visual field under conditions of beam diameter: 6 Lim, accelerating voltage: 15 kV, and irradiation current: 1.17 x 10-9 A. Through the area analysis by using EPMA, Ni and Mn concentrations were mapped and a region that satisfies Formula (iii) and a region that satisfies Formula (iv) were calculated in area fraction. In the table, area fractions of regions that satisfied Formula (iii) in the 1 / 4 plate-thickness portion are listed under "area fraction of region in which Nis > 0.8", and area fractions of regions that satisfied Formula (iv) in the 1 / 4 plate-thickness portion are listed under "area fraction of region in which Mns > 0.8".

[0084] (SSRT) A SSRT was conducted to evaluate hydrogen embrittlement resistance. Test specimens each including a parallel portion that was a round bar of ¢3 mm x 20 mm length and a grip portion of ¢8 mm and the total length of 80 mm were collected from a plane that was at the plate thickness 1 / 4 position and parallel to the rolling surface such that the longitudinal direction of the test specimens coincided with the longitudinal direction of the steel plate. The test specimen was pulled until the displacement of the parallel portion reached 6 mm in the atmosphere, applying pre-strain of 30% elongation. Thereafter, the test was conducted by applying tensile stress in H2 gas at 20 MPa until rupture occurred. Note that the tensile speed was 0.036 mm / min and the strain speed was 3 x 10-5 / s.   Tensile strength and rupture elongation in the atmosphere were separately measured for each test specimen to calculate RTS (relative tensile strength) and REL (relative rupture elongation) described later. While round bar test specimens described above were used in the embodiment, when the plate thickness is less than 8 mm, test specimens each including, for example, a parallel portion that has a plate shape of the plate thickness x 4 mm width x 20 mm length and a grip portion of 25 mm width x 27 mm length, the total length of 94 mm (R10 mm from the parallel portion to the grip portion) may be used. Furthermore, when it is not possible to collect test specimens described above, tests may be conducted by using test specimens that have collectable sizes.

[0085] The obtained test results were evaluated in terms of RTS (relative tensile strength) and REL (relative rupture elongation). RTS and REL are calculated by the formulas below. RTS = "tensile strength as tested with tensile stress being applied in H2 at 20 MPa after applying pre-strain in the atmosphere" divided by "tensile strength in the atmosphere" ... (b) REL = "rupture elongation as tested with tensile stress being applied in H2 at 20 MPa after applying pre-strain in the atmosphere" divided by "rupture elongation in the atmosphere" ... (c)

[0086] When RTS was less than 1.00, hydrogen embrittlement resistance was determined to be poor, and C was entered. On the other hand, when RTS was 1.00 or more, hydrogen embrittlement resistance was determined to be good, and B was entered. Furthermore, when RTS was 1.00 or more and REL was 1.00 or more, hydrogen embrittlement resistance was determined to be excellent, and A was entered. The results are collectively indicated in Table 2 below.

[0087] [Table 2] to CD Table 2 Test No. Steel Production condition Material property evaluation Property evaluation Remarks Preliminary rolling Heat treatment Finishing rolling Grain size number Area fraction of region of Nis>0.8 (%) Area fraction of region of Mns0.8 (%) Tensile strength (MPa) Plate thickness (mm) SSRT Hydrogen embrittlement resistance Heating temperature (°C) Number of pass in which mj is 0.5 or more Number of pass in which mj is 0.8 or more Right side value of Formula (ii) GSNomln RTS REL 1 A 1090 4 1250°C, 6h None 1.0 5.5 90 90 680 40 1.00 0.95 B Inventive example 2 1090 4 1250°C, 6h 5 0.5 6.5 90 90 710 1.00 1.05 A Inventive example 3 B 1180 None Not performed None -1.5 4.5 85 85 580 50 0.90 0.80 C Comparative example 4 1100 4 Not performed None 1.0 5.5 85 85 600 1.00 0.90 B Inventive example 5 1100 4 1250°C, 6h 6 0.5 6.5 95 95 630 1.05 1.05 A Inventive example 6 C 1050 4 Not performed 6 0.5 6.0 95 85 560 40 1.00 0.95 B Inventive example 7 D 1100 3 Not performed None 1.0 5.0 80 80 690 20 1.00 0.85 B Inventive example 8 1250°C, 6h None 0.5 6.0 90 90 700 1.00 0.95 B Inventive example 9 6 0.3 6.5 95 95 720 1.05 1.05 A Inventive example 10 E 1180 None Not performed None -1.0 5.0 85 85 680 30 0.95 0.90 C Comparative example 11 1100 5 Not performed None 1.0 6.0 85 85 700 1.00 0.93 B Inventive example 12 1250°C, 6h 6 0.5 7.0 95 95 750 1.05 1.05 A Inventive example 13 F 1090 4 1250°C, 6h None 1.0 5.0 90 90 650 40 1.00 0.95 B Inventive example 14 1090 4 1250°C, 6h 5 0.5 6.0 95 95 670 40 1.00 1.05 A Inventive example 15 G 1100 4 1250°C, 6h 5 0.3 6.5 95 95 730 40 1.00 1.05 A Inventive example 16 H 1100 4 1250°C, 6h None 0.5 7.0 90 90 780 20 1.00 0.95 B Inventive example 17 1100 4 1250°C, 6h 6 0.3 7.5 95 95 820 20 1.00 1.05 A Inventive example 18 I 1100 4 Not performed None 0.5 5.5 90 90 670 30 0.90 0.80 C Comparative example 19 J 1100 5 Not performed None 0.5 5.0 85 85 650 30 0.90 0.80 C Comparative example 20 K 1100 5 Not performed None 0.5 5.0 85 85 670 30 0.90 0.85 c Comparative example 21 L 1100 5 Not performed None 0.5 5.0 85 85 660 30 0.90 0.80 c Comparative example 22 M 1100 4 Not performed None 0.5 5.5 90 90 680 30 0.95 0.80 c Comparative example 23 N 1100 5 Not performed None 1.0 4.5 90 90 540 30 0.90 0.90 c Comparative example 24 O 1100 5 Not performed None 1.0 4.5 85 85 660 30 0.90 0.90 c Comparative example 25 P 1100 5 Not performed None 1.0 4.5 85 85 650 30 0.90 0.90 c Comparative example 26 2 1100 5 Not performed None 1.0 5.0 85 85 670 30 0.95 0.90 c Comparative example 27 R 1100 4 Not performed None 0.5 5.5 90 90 680 30 0.85 0.80 c Comparative example Double underline means that the production condition falls outside the preferable condition. Underline means that the checmical composition falls outside the requirement of the present embodiment or the target property is not satisfied. 0<GSNo( l / 4)ave - GSNo( 1 / 2)^ ■ ■ ■ (ii)

[0088] Nos. 1, 2, 4, 5 to 9, and 11 to 17, which satisfied the requirements of the embodiment, exhibited good hydrogen embrittlement resistance. On the other hand, Nos. 3, 10, and 18 to 27, which did not satisfy the requirements of the embodiment, resulted in poor in hydrogen embrittlement resistance.

Claims

1. An austenitic stainless steel plate that has a chemical composition comprising, in mass%:C: 0.10% or less,Si: 1.0% or less,Mn: 8.0 to 10.0%,P: 0.050% or less,S: 0.0050% or less,Cr: 14.0 to 18.0%,Mo: 1.0% or less,Ni: 6.0 to 9.0%,Cu: 1.5% or less,Co: 0.01 to 1.0%,N: 0.25% or less,Al: 0 to 0.10%,Nb: 0 to 0.10%,Ti: 0 to 0.10%,B: 0 to 0.0050%,V: 0 to 0.50%,W: 0 to 0.50%,Ca: 0 to 0.010%,Mg: 0 to 0.010%,Zr: 0 to 0.50%,Ga: 0 to 0.05%,Hf: 0 to 0.10%,REM: 0 to 0.10%, andthe balance: Fe and impurities, whereinM-value calculated by Formula (i) below is -90 to -20,the difference between GSNo(1 / 4)ave and GSNo(1 / 2)ave satisfies Formula (ii)below, where GSNo(1 / 4)ave is an average value of grain size number values of a 1 / 4 platethickness portion and GSNo(1 / 2)ave is an average value of grain size number values of a 1 / 2 plate-thickness portion,GSNomin is 4.5 or more, where GSNomin is the smallest grain size number value among the grain size number values of the 1 / 4 plate-thickness portion and the grain size number values of the 1 / 2 plate-thickness portion,a tensile strength is 550 MPa or more, anda plate thickness is 4.5 mm or more:M-value = 551 - 462(C + N) - 9.2Si - 8.1Mn - 13.7Cr - 29(Ni + Cu) - 18.2Mo ... (i)0 < GSNo(1 / 4)ave - GSNo(1 / 2)ave ... (ii)where each element symbol in the Formula (i) represents a content (mass%) of each element contained in the austenitic stainless steel plate and is zero when the element is not contained.

2. The austenitic stainless steel plate according to claim 1, wherein the chemical composition contains one or more elements selected from, in mass%:Al: 0.01 to 0.10%,Nb: 0.01 to 0.10%,Ti: 0.01 to 0.10%,B: 0.0002 to 0.0050%,V: 0.05 to 0.50%,W: 0.05 to 0.50%,Ca: 0.0002 to 0.010%,Mg: 0.0002 to 0.010%,Zr: 0.01 to 0.50%,Ga: 0.001 to 0.05%,Hf: 0.01 to 0.10%, andREM: 0.01 to 0.10%.

3. The austenitic stainless steel plate according to claim 1, wherein in the 1 / 4 plate-thickness portion, an area fraction of a region that satisfies Formula (iii) below is 90% or more and an area fraction of a region that satisfies Formula (iv) below is 90% or more:Nis > 0.8 ... (iii)Mns > 0.8 ... (iv)where in the Formula (iii), Nis represents a degree of segregation of Ni, and in the Formula (iv), Mns represents a degree of segregation of Mn.

4. The austenitic stainless steel plate according to claim 2, wherein in the 1 / 4 plate-thickness portion, an area fraction of a region that satisfies Formula (iii) below is 90% or more and an area fraction of a region that satisfies Formula (iv) below is 90% or more:Nis > 0.8 ... (iii)Mns > 0.8 ... (iv)where in the Formula (iii), Nis represents a degree of segregation of Ni, and in the Formula (iv), Mns represents a degree of segregation of Mn.

5. The austenitic stainless steel plate according to any one of claims 1 to 4, wherein the austenitic stainless steel plate is used in a hydrogen gas environment.

6. A hydrogen gas transmission pipe, comprising the austenitic stainless steel plate according to any one of claims 1 to 4.

7. A valve, a joint, or an instrument for use with hydrogen gas, comprising the austenitic stainless steel plate according to any one of claims 1 to 4.

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