Austenitic stainless steel sheet, hydrogen gas delivery pipe, valve, joint, and meter

Abstract

The austenitic stainless steel plate has a prescribed chemical composition, and the difference between the average grain size number GSNO (1 / 4) ave in a 1 / 4 plate thickness portion and the average grain size number GSNO (1 / 2) ave in a 1 / 2 plate thickness portion satisfies [0lt; gSNO (1 / 4) ave-GSNO (1 / 2) ave], the smallest grain size number GSNO < min > among the grain size number of the 1 / 4 part of the sheet thickness and the grain size number of the 1 / 2 part of the sheet thickness is 4.5 or more, the tensile strength is 550 MPa or more, and the sheet thickness is 4.5 mm or more.

Description

Technical Field

[0001] This invention relates to austenitic stainless steel plates, hydrogen transmission piping, valves, fittings, and instruments. Background Technology

[0002] In recent years, hydrogen power generation, which can reduce CO2 emissions during the power generation phase, has attracted attention. Hydrogen power generation uses hydrogen as fuel for power generation. One example of a hydrogen power generation device is the device described in Patent Document 1. Such a hydrogen power generation device requires the transportation of large quantities of hydrogen and the installation of various components and parts, such as pipelines. The components and parts used in such a hydrogen power generation device are required to be resistant to hydrogen embrittlement.

[0003] Existing technical documents

[0004] Patent documents

[0005] Patent Document 1: Japanese Patent Application Publication No. 2021-141058 Summary of the Invention

[0006] The problem the invention aims to solve

[0007] Among the components and parts used in hydrogen power generation equipment, there are those made from thick steel plates, also known as heavy-duty steel plates. To machine these components into the desired shape, a certain amount of strain needs to be introduced into the thick steel plate. However, when thick steel plates are used in a hydrogen environment after introducing a certain amount of strain into the thick blank, hydrogen embrittlement can easily occur. Furthermore, high strength is sometimes required for the components and parts used in hydrogen power generation equipment. Therefore, austenitic stainless steel plates that possess both good resistance to hydrogen embrittlement and high strength are required.

[0008] The purpose of this invention is to solve the above-mentioned problems and provide austenitic stainless steel plates with a thickness of 4.5 mm or more, excellent resistance to hydrogen embrittlement, and high strength.

[0009] Solution for solving the problem

[0010] This invention was made to solve the above-mentioned problems, and its main purpose is austenitic stainless steel plates, etc.

[0011] (1) An austenitic stainless steel sheet, the chemical composition of which is given by weight%.

[0012] C: Below 0.10%

[0013] Si: below 1.0%

[0014] Mn: 8.0~10.0%,

[0015] P: below 0.050%

[0016] S: Below 0.0050%

[0017] Cr: 14.0~18.0%

[0018] Mo: 1.0% or less,

[0019] Ni: 6.0~9.0%,

[0020] Cu: below 1.5%

[0021] Co: 0.01~1.0%,

[0022] N: below 0.25%

[0023] Al: 0~0.10%

[0024] Nb: 0~0.10%,

[0025] Ti: 0~0.10%,

[0026] B: 0~0.0050%

[0027] V: 0~0.50%,

[0028] W: 0~0.50%,

[0029] Ca: 0~0.010%,

[0030] Mg: 0~0.010%,

[0031] Zr: 0~0.50%,

[0032] Ga: 0~0.05%,

[0033] Hf: 0~0.10%,

[0034] REM: 0~0.10%,

[0035] Balance: Fe and impurities,

[0036] The value of M calculated by the following formula (i) is -90 to -20.

[0037] The average grain size designation GSNo(1 / 4) for the 1 / 4 section of the plate thickness. ave The average grain size number of the 1 / 2 portion of the plate thickness, GSNo(1 / 2). ave The difference satisfies the following equation (ii),

[0038] The smallest grain size number (GSNo) among the grain size numbers of the 1 / 4 thickness portion and the 1 / 2 thickness portion of the plate mentioned above. min A score of 4.5 or higher

[0039] Tensile strength above 550MPa

[0040] The plate thickness is 4.5mm or more.

[0041] M value = 551 - 462(C+N) - 9.2Si - 8.1Mn - 13.7Cr - 29(Ni+Cu) - 18.2Mo···(i)

[0042] 0 <GSNo(1 / 4) ave -GSNo(1 / 2) ave ···(ii)

[0043] In the above formula (i), the element symbols represent the content (mass %) of each element contained in the austenitic stainless steel plate, and are sometimes marked as zero if they do not contain any element.

[0044] (2) According to the austenitic stainless steel plate described in (1) above, wherein,

[0045] The above chemical composition, by mass%, contains ingredients selected from...

[0046] Al: 0.01~0.10%

[0047] Nb: 0.01~0.10%

[0048] Ti: 0.01~0.10%,

[0049] B: 0.0002~0.0050%

[0050] V: 0.05~0.50%,

[0051] W: 0.05~0.50%,

[0052] Ca: 0.0002~0.010%

[0053] Mg: 0.0002~0.010%

[0054] Zr: 0.01~0.50%,

[0055] Ga: 0.001~0.05%

[0056] Hf: 0.01~0.10%, and

[0057] REM: 0.01% to 0.10% or more of the following.

[0058] (3) The austenitic stainless steel sheet according to (1) or (2) above,

[0059] Among them, in the portion of 1 / 4 of the plate thickness, the area ratio of the region satisfying the following formula (iii) is 90% or more, and the area ratio of the region satisfying the following formula (iv) is 90% or more.

[0060] Nis>0.8…(iii)

[0061] Mns>0.8…(iv)

[0062] In equation (iii) above, Nis represents the segregation degree of Ni, and in equation (iv) above, Mns represents the segregation degree of Mn.

[0063] (4) The austenitic stainless steel sheet according to any one of (1) to (3) above, which is used in a hydrogen environment.

[0064] (5) A hydrogen transmission piping system using any one of (1) to (3) above-mentioned austenitic stainless steel sheet.

[0065] (6) A valve, fitting or instrument for hydrogen use, which uses austenitic stainless steel sheet as described in any one of (1) to (3) above.

[0066] The effects of the invention

[0067] According to the present invention, austenitic stainless steel plates with a thickness of 4.5 mm or more, excellent resistance to hydrogen embrittlement, and high strength can be obtained. Detailed Implementation

[0068] The inventors have studied the hydrogen embrittlement resistance of thick steel plates and obtained the following insights.

[0069] (a) When forming thick steel plates into desired parts and components, processes based on plastic deformation, such as stretching and bending, are performed. This plastic deformation imparts a pre-strain to the thick steel plate. Generally, the stress required for plastic deformation increases with the plate thickness. Furthermore, strain is not uniformly imparted across the entire steel plate. For example, the strain imparted near the surface of the steel plate differs significantly from the strain imparted near the center of the plate.

[0070] (b) One reason for this is the difference in grain diameter between the surface area and the center of the plate thickness. Our research shows that, particularly in high-Mn austenitic stainless steels, in the case of thick steel plates exceeding 10 mm, the outermost surface has the finest grains, but the grains in the quarter-thickness portion near the surface tend to grow and coarsen during solidification, becoming coarse-grained. Furthermore, the grains in the half-thickness portion near the center of the plate thickness tend to be finer-grained compared to the quarter-thickness portion due to the formation of equiaxed grains during solidification. In other words, a difference in grain diameter easily occurs between the quarter-thickness and half-thickness portions, with the grains in the quarter-thickness portion being more prone to coarse-grained growth. As a result, a large amount of strain tends to accumulate in areas with larger grain diameters, and hydrogen embrittlement is more likely to occur in these areas. The higher the strength of the steel plate, the more pronounced this tendency becomes.

[0071] (c) Based on the above, in order to improve the hydrogen embrittlement resistance of thick steel plates, it is desirable to reduce the difference in grain diameter within the steel plate. However, from the viewpoint of hydrogen embrittlement resistance, it is preferable not to simply reduce the difference in grain diameter, but rather that the grains in the 1 / 4 portion of the plate thickness are finer than those in the 1 / 2 portion. To form such a metallographic structure, the rolling aspect ratio needs to be controlled within a specified range during manufacturing. Furthermore, from the viewpoint of hydrogen embrittlement resistance, it is preferable to reduce coarse grains throughout the steel plate.

[0072] One embodiment of the present invention is based on the above-described understanding. Hereinafter, the components of the austenitic stainless steel sheet of this embodiment will be described in detail.

[0073] 1. Chemical composition

[0074] The reasons for the limitations of each element are as follows. It should be noted that in the following explanation, the "%" for content refers to "mass %".

[0075] C: Below 0.10%

[0076] Carbon (C) is an effective element for stabilizing the austenitic phase and also contributes to improved resistance to hydrogen embrittlement. However, excessive C content promotes grain boundary precipitation of Cr-based carbides, which can actually reduce resistance to hydrogen embrittlement. Therefore, the C content is 0.10% or less. Preferably, the C content is 0.080% or less, more preferably 0.070% or less, and even more preferably 0.060% or less. On the other hand, to achieve the above-mentioned effects, the C content is preferably 0.010% or more, more preferably 0.020% or more, even more preferably 0.030% or more, and even more preferably 0.040% or more.

[0077] Si: below 1.0%

[0078] Silicon (Si) is an effective deoxidizing element and also contributes to improved resistance to hydrogen embrittlement. However, excessive Si content promotes the formation of intermetallic compounds such as the σ phase, reducing hot workability and toughness. Therefore, the Si content is 1.0% or less. Preferably, the Si content is 0.90% or less, more preferably 0.80% or less, even more preferably 0.70% or less, and even more preferably 0.60% or less. On the other hand, to obtain the above-mentioned effects, the Si content is preferably 0.10% or more, more preferably 0.20% or more, even more preferably 0.30% or more, and even more preferably 0.40% or more.

[0079] Mn: 8.0~10.0%

[0080] Manganese (Mn) is an effective element for stabilizing the austenitic phase, contributing to improved resistance to hydrogen embrittlement. It also increases strength. Therefore, the Mn content is 8.0% or more, preferably 8.5% or more. However, excessive Mn content promotes the formation of the highly hydrogen-embrittled ε phase, thus reducing resistance to hydrogen embrittlement. Therefore, the Mn content is 10.0% or less, preferably 9.5% or less.

[0081] P: below 0.050%

[0082] Phosphorus (P) is an element contained in steel as an impurity and can segregate, reducing its resistance to hydrogen embrittlement. Therefore, the P content is 0.050% or less. Preferably, the P content is 0.040% or less, more preferably 0.030% or less. On the other hand, excessively reducing P leads to increased manufacturing costs; therefore, the P content is preferably 0.001% or more, more preferably 0.010% or more.

[0083] S: Below 0.0050%

[0084] Sulfur (S) is an element contained in steel as an impurity, which forms MnS and reduces resistance to hydrogen embrittlement. Therefore, the S content is 0.0050% or less. The S content is preferably 0.0040% or less, more preferably 0.0030% or less. However, excessively reducing S increases manufacturing costs. Therefore, the S content is preferably 0.0001% or more, more preferably 0.0002% or more.

[0085] Cr: 14.0~18.0%

[0086] Chromium (Cr) is an element present in stainless steel in a certain amount, which improves its corrosion resistance. Therefore, the Cr content is 14.0% or more. Preferably, the Cr content is 14.5% or more, more preferably 15.0% or more, and even more preferably 15.5% or more. However, Cr is a ferrite-forming element. Therefore, when Cr is present in excess, the austenite phase becomes unstable, and a large amount of carbides precipitate, reducing resistance to hydrogen embrittlement. Therefore, the Cr content is 18.0% or less. Preferably, the Cr content is 17.5% or less, more preferably 17.0% or less, and even more preferably 16.5% or less.

[0087] Mo: 1.0% or less

[0088] Mo (molybdenum) has the effect of improving strength. However, excessive Mo content promotes the formation of the δ-ferrite phase, reducing resistance to hydrogen embrittlement. Therefore, the Mo content is 1.0% or less. Preferably, the Mo content is 0.85% or less, more preferably 0.70% or less, even more preferably 0.50% or less, and even more preferably 0.30% or less. On the other hand, excessively reducing Mo content leads to constraints on smelting raw materials and increases manufacturing costs. Therefore, the Mo content is preferably 0.01% or more, more preferably 0.05% or more, and even more preferably 0.10% or more.

[0089] Ni: 6.0~9.0%

[0090] Ni (nickel) is an element used in conjunction with Mn to ensure resistance to hydrogen embrittlement and strength. Therefore, the Ni content is 6.0% or more. Preferably, the Ni content is 6.3% or more, more preferably 6.7% or more, and even more preferably 7.0% or more. However, excessive Ni content not only increases manufacturing costs but also easily leads to segregation, which in turn reduces resistance to hydrogen embrittlement. Furthermore, excessive Ni content can sometimes cause solid solution softening and reduce strength. Therefore, the Ni content is 9.0% or less. Preferably, the Ni content is 8.5% or less, more preferably 8.0% or less, and even more preferably 7.6% or less.

[0091] Cu: below 1.5%

[0092] Cu (copper) is an element mixed in from raw materials such as waste, and it is effective in stabilizing the austenitic phase. On the other hand, Cu is a low-melting-point element, which tends to segregate at grain boundaries and easily becomes the starting point for fracture. Therefore, the Cu content is 1.5% or less. The Cu content is preferably 1.0% or less, more preferably 0.70% or less, and even more preferably 0.50% or less. However, excessively reducing the Cu content can lead to constraints on smelting raw materials and increased manufacturing costs. Therefore, the Cu content is preferably 0.01% or more, more preferably 0.05% or more, and even more preferably 0.10% or more.

[0093] Co: 0.01~1.0%

[0094] Cobalt (Co) is an important element that improves corrosion resistance, stabilizes the austenitic phase, and, particularly, enhances resistance to hydrogen embrittlement in thicker plates. Therefore, the Co content is 0.01% or more. Preferably, the Co content is 0.05% or more, more preferably 0.10% or more, and even more preferably 0.15% or more. However, excessive Co content reduces toughness and processability. Therefore, the Co content is 1.0% or less. Preferably, the Co content is 0.70% or less, and even more preferably 0.50% or less.

[0095] N: below 0.25%

[0096] Like Mn and Ni, nitrogen (N) is an effective element for improving resistance to hydrogen embrittlement. However, excessive N content can sometimes lead to internal defects such as porosity during smelting, which can actually reduce resistance to hydrogen embrittlement. Therefore, the N content is 0.25% or less. Preferably, the N content is 0.20% or less, more preferably 0.18% or less. On the other hand, to achieve the above-mentioned effects, the N content is preferably 0.010% or more, more preferably 0.020% or more, even more preferably 0.050% or more, and even more preferably 0.10% or more.

[0097] In addition to the elements mentioned above, the composition may further contain one or more elements selected from Al, Nb, Ti, B, V, W, Ca, Mg, Zr, Ga, Hf, and REM within the range shown below. The rationale for these limitations will be explained.

[0098] Al: 0~0.10%

[0099] In addition to being an effective deoxidizing element, Al (aluminum) also strengthens grain boundaries by suppressing the segregation of low-melting-point elements at grain boundaries. Therefore, it can be included as needed. However, Al is a ferrite-forming element. Therefore, excessive Al content can destabilize the austenite phase. Therefore, the Al content is 0.10% or less. The Al content is preferably 0.07% or less, more preferably 0.05% or less. On the other hand, to obtain the above-mentioned effects, the Al content is preferably 0.01% or more, more preferably 0.02% or more.

[0100] Nb: 0~0.10%

[0101] Niobium (Nb) forms carbonitrides, which refines the grain size and strengthens grain boundaries. Therefore, it can be included as needed. However, excessive Nb content reduces manufacturability and processability during hot rolling. Therefore, the Nb content is 0.10% or less. The Nb content is preferably 0.08% or less, more preferably 0.07% or less. On the other hand, to obtain the above-mentioned effects, the Nb content is preferably 0.01% or more, more preferably 0.02% or more.

[0102] Ti: 0~0.10%

[0103] Titanium (Ti) forms carbonitrides, which refines the grain size and strengthens grain boundaries. Therefore, it can be included as needed. However, excessive Ti content reduces manufacturability during hot rolling. Additionally, it can sometimes lead to the formation of numerous inclusions, reducing impact resistance. Therefore, the Ti content is 0.10% or less. Preferably, the Ti content is 0.07% or less, more preferably 0.05% or less. On the other hand, to achieve the above-mentioned effects, the Ti content is preferably 0.01% or more, more preferably 0.02% or more.

[0104] B: 0~0.0050%

[0105] Boron (B) has the effect of strengthening grain boundaries, increasing strength, and improving impact resistance. Therefore, it can be included as needed. However, even if B is included in excess, not only will its effect be saturated, but the impact resistance may sometimes decrease. Therefore, the B content is 0.0050% or less. The B content is preferably 0.0040% or less, more preferably 0.0030% or less. On the other hand, in order to obtain the above-mentioned effects, the B content is preferably 0.0002% or more, more preferably 0.0005% or more, and even more preferably 0.0010% or more.

[0106] V: 0~0.50%

[0107] Vanadium (V) can be dissolved in steel or precipitated as carbonitrides, thus improving its strength. Therefore, it can be included as needed. However, excessive V content can lead to excessive carbonitride formation, reducing manufacturability during hot rolling. Therefore, the V content is 0.50% or less. Preferably, the V content is 0.40% or less, more preferably 0.30% or less, and even more preferably 0.20% or less. On the other hand, to obtain the above-mentioned effects, the V content is preferably 0.05% or more, more preferably 0.10% or more.

[0108] W: 0~0.50%

[0109] Tungsten (W) has the effect of improving strength and corrosion resistance. Therefore, it can be included as needed. However, excessive W content increases manufacturing costs. Therefore, the W content is 0.50% or less. The W content is preferably 0.40% or less, more preferably 0.30% or less. On the other hand, in order to obtain the above-mentioned effects, the W content is preferably 0.05% or more, more preferably 0.10% or more.

[0110] Ca: 0~0.010%

[0111] Ca (calcium) is an effective element for improving deoxidation and heat workability. Therefore, it can be included as needed. However, excessive Ca content can easily lead to segregation, which can become the starting point for fracture. As a result, impact resistance may sometimes decrease. Therefore, the Ca content is 0.010% or less. The Ca content is preferably 0.0050% or less, more preferably 0.0030% or less. On the other hand, in order to obtain the above-mentioned effects, the Ca content is preferably 0.0002% or more, more preferably 0.0005% or more, and even more preferably 0.0010% or more.

[0112] Mg: 0~0.010%

[0113] Magnesium (Mg) is an effective element for improving deoxidation and hot workability. Therefore, it can be included as needed. However, when Mg is present in excess, inclusions will form in large quantities, easily becoming the starting point for fracture, which may result in a decrease in impact resistance. Therefore, the Mg content is 0.010% or less. The Mg content is preferably 0.0050% or less, more preferably 0.0030% or less, and even more preferably 0.0010% or less. On the other hand, in order to obtain the above-mentioned effects, the Mg content is preferably 0.0001% or more, more preferably 0.0002% or more.

[0114] Zr: 0~0.50%

[0115] Zirconium (Zr) has a deoxidizing effect. Additionally, it improves corrosion resistance. Therefore, it can be included as needed. However, excessive Zr content reduces toughness and processability. Therefore, the Zr content is 0.50% or less. Preferably, the Zr content is 0.30% or less, more preferably 0.10% or less, and even more preferably 0.05% or less. On the other hand, to obtain the above-mentioned effects, the Zr content is preferably 0.01% or more, more preferably 0.02% or more.

[0116] Ga: 0~0.05%

[0117] Gallium (Ga) improves thermal workability. Therefore, it can be included as needed. However, excessive Ga content reduces manufacturability. Therefore, the Ga content is 0.05% or less. The Ga content is preferably 0.04% or less, more preferably 0.02% or less. On the other hand, to obtain the above-mentioned effect, the Ga content is preferably 0.001% or more, more preferably 0.005% or more.

[0118] Hf: 0~0.10%

[0119] Hafnium (Hf) has the effect of improving strength and resistance to hydrogen embrittlement. Therefore, it can be included as needed. However, excessive Hf content reduces processability. Therefore, the Hf content is 0.10% or less. The Hf content is preferably 0.07% or less, more preferably 0.05% or less, and even more preferably 0.03% or less. On the other hand, in order to obtain the above-mentioned effects, the Hf content is preferably 0.001% or more, more preferably 0.005% or more, and even more preferably 0.01% or more.

[0120] REM: 0~0.10%

[0121] REM (rare earth elements) improves hot workability. It also improves corrosion resistance. Therefore, it can be included as needed. However, excessive REM content not only saturates its effect but also reduces hot workability. Therefore, the REM content is 0.10% or less. The REM content is preferably 0.07% or less, more preferably 0.05% or less, and even more preferably 0.03% or less. On the other hand, to obtain the above-mentioned effects, the REM content is preferably 0.001% or more, more preferably 0.005% or more, and even more preferably 0.01% or more.

[0122] REM refers to a total of 17 elements, including Sc, Y, and the lanthanides. The REM content mentioned above refers to the total content of these elements. Industrially, REM is mostly added in the form of mixed rare earth metals.

[0123] In the chemical composition of the austenitic stainless steel sheet of this embodiment, the balance is Fe and impurities. Here, "impurities" refers to components that may be mixed in during the industrial manufacturing of austenitic stainless steel sheets due to various reasons such as raw materials (ore, waste, etc.) and manufacturing processes, and are permissible within a range that does not adversely affect this embodiment.

[0124] M value

[0125] The M value calculated by the following formula (i) is an indicator of the stability of the austenitic phase in the austenitic stainless steel sheet. In the austenitic stainless steel sheet of this embodiment, the M value is -90 to -20 in order to improve resistance to hydrogen embrittlement and strength.

[0126] M value = 551 - 462(C+N) - 9.2Si - 8.1Mn - 13.7Cr - 29(Ni+Cu) - 18.2Mo…(i)

[0127] In the above formula (i), the element symbols represent the content (mass %) of each element contained in the austenitic stainless steel plate, and are sometimes marked as zero if they do not contain any element.

[0128] When the M value is less than -90, the cost of the alloy increases. Therefore, the M value is -90 or higher. Preferably, the M value is -80 or higher, and more preferably -70 or higher.

[0129] On the other hand, when the M value exceeds -20, the stability of the austenitic phase is low, making it prone to phase transformation to the α′ phase, thus reducing resistance to hydrogen embrittlement. Furthermore, it is difficult to obtain the desired strength. Therefore, the M value is -20 or less. The M value is preferably -25 or less, more preferably -35 or less.

[0130] 2. Grain size number

[0131] 2-1. The difference between the grain size near the surface and the grain size at the center.

[0132] In order to improve the resistance to hydrogen embrittlement, the austenitic stainless steel sheet of this embodiment needs to reduce the difference between the grain size near the surface and the grain size at the center, with the grain size in the 1 / 4 section of the sheet being finer than that in the 1 / 2 section.

[0133] Specifically, the average grain size designation GSNo(1 / 4) for the 1 / 4 section of the plate thickness. ave The average grain size number of the 1 / 2 portion of the plate thickness, GSNo(1 / 2). ave The difference (hereinafter also referred to as "grain size difference" or "the value on the right side of equation (ii)") satisfies the following equation (ii).

[0134] 0 <GSNo(1 / 4) ave -GSNo(1 / 2) ave …(ii)

[0135] From the viewpoint of resistance to hydrogen embrittlement, the aforementioned grain size difference is preferably 0.1 or more, and more preferably 0.3 or more. On the other hand, when the aforementioned grain size difference exceeds 1.5, the difference in grain size within the steel plate becomes too large, which can easily reduce the resistance to hydrogen embrittlement. Therefore, the aforementioned grain size difference is preferably 1.5 or less, and more preferably 1.0 or less.

[0136] Here, the 1 / 4 portion of the plate thickness refers to the position 1 / 4t away from the surface in the plate thickness direction when the plate thickness is set to t. Similarly, the 1 / 2 portion of the plate thickness refers to the position 1 / 2t away from the surface in the plate thickness direction when the plate thickness is set to t.

[0137] The average grain size designation GSNo(1 / 4) for the 1 / 4 section of the plate thickness. ave The average grain size designation GSNo(1 / 2) for the 1 / 2 portion of the plate thickness. ave The determination was performed according to the following steps. Specifically, a surface near the center of the steel plate, parallel to both the thickness direction and the long side direction (hereinafter referred to as the "L-section"), was used as the observation surface. After the grain boundaries were revealed by nitric acid electrolysis or aqua regia immersion, observation was conducted using an optical microscope. The size of the observation surface was plate thickness × 30 mm (length of the long side), and the magnification was 100x. Then, for the 1 / 4 thickness portion of the above observation surface, a total of five fields of view were measured at positions 6 mm and 12 mm along the long side, at the center, and in both directions. The grain size was calculated using the straight-line intercept method, and the average value was taken as the aforementioned GSNo(1 / 4). ave Similarly, for the 1 / 2 thickness portion, a total of 5 fields of view were measured at the center along the long side, at positions of 6 mm and 12 mm in both directions. The grain size was calculated using the straight line intercept method, and the average value was taken as the above GSNo(1 / 2). ave Other observation and measurement conditions shall comply with JIS G 0551:2020.

[0138] 2-2. Minimum grain size

[0139] To suppress hydrogen infiltration and improve resistance to hydrogen embrittlement, the austenitic stainless steel sheet of this embodiment suppresses the formation of coarse grains. Specifically, in the observed field of view, the smallest (i.e., the coarsest) grain size number (GSNo) in the field of view between the grain size numbers of the 1 / 4 thickness portion and the 1 / 2 thickness portion is selected. min A score of 4.5 or higher. The above GSNo. min Preferably, it is 5.0 or higher, and more preferably 5.5 or higher. It should be noted that the above GSNo... min There is no specific upper limit for the value, but considering manufacturing costs and constraints during manufacturing, 8.0 is usually preferred.

[0140] It should be noted that GSNo min The following results were obtained in order to determine the above GSNo(1 / 4) ave And GSNo(1 / 2) ave In the observations conducted, the smallest grain size number was determined in five fields of view in the 1 / 4 section and five fields of view in the 1 / 2 section.

[0141] 3. Tensile strength

[0142] To achieve the desired strength, the austenitic stainless steel sheet of this embodiment has a room temperature tensile strength of 550 MPa or higher. Preferably, the tensile strength is 600 MPa or higher, more preferably 650 MPa or higher. It should be noted that the upper limit of the tensile strength is not particularly limited, and is typically 900 MPa. Preferably, it is less than 800 MPa, more preferably 780 MPa or less, and even more preferably 750 MPa or less.

[0143] It should be noted that tensile strength at room temperature can be determined by conducting a tensile test at room temperature (24℃). The conditions for the tensile test can be set to general conditions.

[0144] 4.Plate thickness

[0145] The austenitic stainless steel sheet in this embodiment is intended to be a thick steel sheet for use in hydrogen power generation equipment. Therefore, the sheet thickness is 4.5 mm or more. Preferably, the sheet thickness is 10 mm or more, more preferably 20 mm or more, and even more preferably 30 mm or more. It should be noted that the upper limit of the sheet thickness is not particularly limited, and is typically 100 mm.

[0146] 5. Segregation state

[0147] To achieve good resistance to hydrogen embrittlement, it is preferable to control the state of segregation. Segregation is a phenomenon where the concentration of solute elements becomes uneven during solidification, and it tends to occur significantly in thick steel plates. Moreover, this segregation state can remain locally in the metallographic structure after the steel plate is manufactured, thus adversely affecting the resistance to hydrogen embrittlement. In particular, the effect of segregation is significant in the 1 / 4 of the plate thickness near the surface. In the segregated area, pre-strain accumulates locally, resulting in a decrease in resistance to hydrogen embrittlement. In addition, the occurrence of segregation facilitates the phase transformation from austenite to the α' phase, which is also a reason for the reduced resistance to hydrogen embrittlement.

[0148] It should be noted that segregation can be positive segregation, where the elemental distribution is higher than the average concentration, or negative segregation, where the elemental distribution is lower than the average concentration. However, in the austenitic stainless steel sheet of this embodiment, from the viewpoint of hydrogen embrittlement resistance, the focus is mainly on negative segregation.

[0149] Based on the above, it is preferable that the area ratio of the region satisfying the following formula (iii) is 90% or more at the 1 / 4 portion of the plate thickness, and the area ratio of the region satisfying the following formula (iv) is 90% or more.

[0150] Nis>0.8···(iii)

[0151] Mns>0.8···(iv)

[0152] In equation (iii) above, Nis represents the segregation degree of Ni (nickel), and in equation (iv) above, Mns represents the segregation degree of Mn (manganese).

[0153] In the chemical composition of the steel plate of this embodiment, Ni and Mn are elements that affect hydrogen embrittlement resistance due to segregation. Therefore, the segregation degree of Ni and Mn is controlled. It should be noted that the segregation degree refers to the concentration of the segregated portion relative to the average concentration of a specified element. The segregation degree of Ni can be calculated as (local Ni concentration) / (average Ni content of the total thickness of the steel plate), and the segregation degree of Mn can be calculated as (local Mn concentration) / (average Mn content of the total thickness of the steel plate). The closer the segregation degree is to 1, the less segregation occurs.

[0154] When Nis exceeds 0.8, meaning the segregation degree of Ni exceeds 0.8 and equation (iii) is satisfied, no local concentration gradient is generated, and negative segregation is reduced. Therefore, from the viewpoint of reduced resistance to hydrogen embrittlement caused by negative segregation, the area fraction of the region satisfying equation (iii) at 1 / 4 of the plate thickness is preferably 90% or more. The area fraction of the region satisfying equation (iii) is more preferably 95% or more.

[0155] Similarly, when Mns exceeds 0.8, i.e., the segregation degree of Mn exceeds 0.8 and equation (iv) is satisfied, no local concentration gradient is generated, and negative segregation is reduced. Therefore, from the viewpoint of reduced resistance to hydrogen embrittlement caused by negative segregation, the area fraction of the region satisfying equation (iv) at 1 / 4 of the plate thickness is preferably 90% or more. Therefore, the area fraction of the region satisfying equation (iv) is preferably 90% or more, and more preferably 95% or more.

[0156] Furthermore, it is preferable that the area satisfying equation (iii) is 90% or more, and the area satisfying equation (iv) is 90% or more.

[0157] It should be noted that the Nis and Mns values ​​at the 1 / 4 portion of the plate thickness, as well as the area ratios of the regions satisfying equation (iii) and (iv), can be determined according to the following steps. In a plane (section C) parallel to the thickness and width directions of the steel plate, the 1 / 4 portion of the plate thickness is taken as the center of the field of view, and a 2mm square area above and below this center is defined as the observation field. Then, for this observation field, an electron beam diameter of 6μm, an accelerating voltage of 15kV, and an irradiation current of 1.17×10⁻⁶ are applied. -9Under condition A, surface analysis using EPMA is performed. By using surface analysis with EPMA, Ni and Mn concentrations are mapped, and the regions satisfying equation (iii) and equation (iv) are calculated using area ratio. It should be noted that observation can be performed near the center of the steel plate width where segregation is prone to occur, but regardless of the location in the plate width direction, as long as the above-mentioned ranges are satisfied, it falls within the scope of the invention of this embodiment.

[0158] 6. Uses

[0159] The austenitic stainless steel sheet of this embodiment is suitable for use in hydrogen environments, particularly in components and parts of hydrogen power generation equipment. Specifically, it is preferably used in hydrogen transmission piping, valves, fittings, and instruments used in such equipment. It should be noted that the hydrogen is preferably a compressed or liquefied gas, and the hydrogen pressure is preferably in the range of 0.1 to 20 MPa.

[0160] 7. Manufacturing method

[0161] The austenitic stainless steel sheet of this embodiment can be stably manufactured, for example, by the following manufacturing method.

[0162] 7-1. Hot rolling process

[0163] 7-1-1. Preparatory rolling

[0164] Stainless steel with the above-mentioned chemical composition is smelted to produce steel billets such as slabs. Next, the steel billets are heated to a specified temperature and hot-rolled (hot rolling process). In this hot rolling process, the steel billets are first heated to a temperature below 1200°C, and then rolled to a shape ratio of m. j For passes with a thickness of 0.5 or higher, perform at least three pre-rolling passes.

[0165] In the manufacturing method of austenitic stainless steel sheet according to this embodiment, the pre-rolling heating temperature is 1200°C or below. Preferably, the pre-rolling heating temperature is 1100°C or below. This is to suppress recrystallization and grain growth by accumulating strain in the quarter-thickness portion of the sheet. Through this strain accumulation, recrystallization is promoted during the heat treatment or finishing rolling process in the next step, resulting in a finer grain structure in the quarter-thickness portion of the sheet.

[0166] By rolling at the above heating temperature to a shape ratio m j The pre-rolling process involves three or more passes to achieve a value of 0.5 or higher, which is sufficient to form a product that satisfies formula (ii) and GSNo. min The grain size is 4.5 or higher. It should be noted that the rolling shape is greater than m. j It can be calculated from the following formula (a).

[0167]

[0168] The symbols in the above formula are defined as follows.

[0169] R: Roller radius (mm)

[0170] t j Thickness of the inlet side panel (mm)

[0171] t j+1 Export side plate thickness (mm)

[0172] In the hot rolling process, multiple rolling mills, each with a pair of rolls, are arranged continuously. Steel sheets are rolled by passing between these mills. Here, the process of a steel sheet passing between the roll pairs of a single rolling mill and being rolled is called a pass. Therefore, R above is the roll radius, and t... j The thickness of the plate entering the rolling mill before rolling in one pass, t j+1 This refers to the plate thickness after rolling. It should be noted that the rolled shape is greater than m. j The rolling mill's roll diameter and reduction ratio are among the factors that affect the rolling profile; therefore, controlling these within appropriate ranges ensures that the rolled shape is within the acceptable range. j A value of 0.5 or higher is acceptable. It should be noted that other pre-rolling conditions can also follow conventional methods.

[0173] 7-1-2. Heat Treatment

[0174] After pre-rolling, heat treatment can be performed as needed. This is because, during heat treatment, the area satisfying equations (iii) and (iv) is likely to reach over 90% at 1 / 4 of the plate thickness. The preferred heat treatment temperature is 1200~1300℃, and the preferred heat treatment time is 60 minutes or more.

[0175] 7-1-3. Finish rolling

[0176] In the hot rolling process, after pre-rolling or, if necessary, heat treatment is performed followed by finish rolling. In finish rolling, it is preferable to roll a shape ratio of m... j A minimum of 5 passes are required to achieve a grain size difference of 0.8 or higher. This is because a grain size difference of less than 1.0 results in better resistance to hydrogen embrittlement. It should be noted that the rolling shape is more than m... j The calculations can be performed in the same manner as during pre-rolling. Furthermore, to obtain the desired grain size microstructure, the heating temperature of the steel plate during finish rolling is preferably in the range of 1150~1250℃. It should be noted that other finish rolling conditions can also be appropriately followed using conventional methods. After finish rolling, the plate is cooled using conventional methods to produce austenitic stainless steel sheets.

[0177] 7-2. Other

[0178] After the hot rolling process, annealing and pickling can be performed as needed. Annealing conditions can follow conventional methods; for example, the preferred annealing temperature is 1050~1150℃, and the preferred annealing time is 5~15 minutes. Then, it can be cooled to become an austenitic stainless steel sheet, or pickling can be performed as needed. Pickling conditions can also follow conventional methods appropriately.

[0179] The austenitic stainless steel sheet of the present invention will be described in more detail below through examples, but the embodiments are not limited to these examples.

[0180] Example

[0181] Smelting stainless steel with the chemical composition shown in Table 1, and manufacturing 200 mm thick slabs. The resulting slabs are then subjected to pre-rolling. During pre-rolling, heating is performed at the temperatures recorded in Table 2. It should be noted that the rolling shape ratio in the pre-rolling is m. j The number of passes is 0.5 or higher. Then, in some examples, heat treatment is performed under the conditions described in Table 2. For examples where heat treatment is required after pre-rolling, the temperature is raised to 1150~1200℃ after heat treatment, followed by finish rolling. It should be noted that, similarly, the rolling shape ratio m in count finish rolling is... j The number of passes is 0.8 or higher. Then, after annealing at a temperature range of 1050~1100℃ for 5 minutes, it is cooled, pickled, etc., to produce austenitic stainless steel sheet.

[0182] [Table 1]

[0183]

[0184] For the obtained austenitic stainless steel sheet, the GSNo (1 / 4) is determined according to the following steps. ave GSNo(1 / 2) ave GSNo min The area ratios of the regions satisfying equation (iii) and equation (iv) at 1 / 4 of the plate thickness. Additionally, for property evaluation, low strain rate tensile tests were performed following these steps.

[0185] (GSNo(1 / 4) ave GSNo(1 / 2) ave GSNo min )

[0186] The L-section of the steel plate was used as the observation surface. The size of the observation surface was plate thickness × 30 mm (length along the long side), and the magnification was 100x. Then, five fields of view were measured on the 1 / 4 thickness portion of the observation surface, and the grain size was calculated using the straight line intercept method. The average value was taken as the GSNo(1 / 4) mentioned above.ave Similarly, five fields of view were measured for the 1 / 2 thickness portion of the plate, and the grain size was calculated using the straight line intercept method. The average value of these values ​​was taken as the aforementioned GSNo(1 / 2). ave Other observation conditions were performed in accordance with JIS G 0551:2020.

[0187] In addition, in order to determine the above-mentioned GSNo(1 / 4) ave And GSNo(1 / 2) ave In the observations conducted, the grain size number of the field of view with the smallest grain size number was used as the GSNo. min That is, the smallest grain size among the five fields of view in the 1 / 4 thickness section and the five fields of view in the 1 / 2 thickness section is designated as GSNo. min .

[0188] (Area ratio of the region satisfying equations (iii) and (iv) at 1 / 4 of the plate thickness)

[0189] In section C of the steel plate, the portion representing 1 / 4 of the plate thickness is taken as the center of the field of view, and a 2mm square area above and below this center is defined as the observation field. Then, for this observation field, an electron beam diameter of 6μm, an accelerating voltage of 15kV, and an irradiation current of 1.17×10⁻⁶ are used. -9 Under condition A, surface analysis using EPMA was performed. Using EPMA surface analysis, Ni and Mn concentrations were mapped, and the regions satisfying equation (iii) and equation (iv) were calculated as area ratios. It should be noted that the area ratio of the region satisfying equation (iii) at the 1 / 4 portion of the plate thickness is recorded in the table as the area ratio of the region where Nis > 0.8, and the area ratio of the region satisfying equation (iv) at the 1 / 4 portion of the plate thickness is recorded as the area ratio of the region where Mns > 0.8.

[0190] (Low strain rate tensile test)

[0191] To evaluate resistance to hydrogen embrittlement, low strain rate tensile tests were conducted. A test piece with a parallel section of 3mm × 20mm length and a clamping section of 8mm, and a total length of 80mm, was collected from a quarter-thickness section parallel to the rolling surface, with its long side aligned with the long side of the steel plate. The test piece was stretched in atmosphere until the displacement of the parallel section reached 6mm, and a pre-strain of 30% elongation was applied. Then, tensile stress was applied in 20MPa H2 gas until fracture. It should be noted that the tensile speed was set to 0.036mm / min and the strain rate was set to 3 × 10⁻⁶ mm / min. -5 / s. It should be noted that for each test piece, in order to calculate the RTS (relative tensile strength) and REL (relative elongation at break) described later, the tensile strength and elongation at break in the atmosphere were also measured separately. It should be noted that in this embodiment, the above-described round bar test piece was used, but when the plate thickness is less than 8 mm, for example, a plate-shaped test piece with a parallel portion of plate thickness × width 4 mm × length 20 mm and a clamping portion of width 25 mm × length 27 mm, and a total length of 94 mm (R10 mm from the parallel portion to the clamping portion), can be used. Furthermore, if the above-described test piece cannot be collected, a test piece of a collectable size can be used for the test.

[0192] The experimental results were evaluated using RTS (relative tensile strength) and REL (relative elongation at break). It should be noted that RTS and REL are calculated using the following formulas.

[0193] RTS = Tensile strength under tensile stress at 20 MPaH2 after pre-straining in the atmosphere ÷ Tensile strength in the atmosphere…(b)

[0194] REL = Elongation at break after being pre-strained in the atmosphere and subjected to tensile stress at 20 MPa H2 ÷ Elongation at break in the atmosphere…(c)

[0195] When the RTS is less than 1.00, it is judged as poor resistance to hydrogen embrittlement and recorded as C. On the other hand, when the RTS is 1.00 or higher, it is judged as good resistance to hydrogen embrittlement and recorded as B. Furthermore, when both the RTS and REL are 1.00 or higher, it is judged as further good resistance to hydrogen embrittlement and recorded as A. The results are summarized in Table 2 below.

[0196] [Table 2]

[0197]

[0198] Requirements No. 1, 2, 4, 5-9, and 11-17 that meet the requirements of this embodiment exhibit good resistance to hydrogen embrittlement. On the other hand, requirements No. 3, 10, and 18-27 that do not meet the requirements of this embodiment exhibit poor resistance to hydrogen embrittlement.

Claims

1. An austenitic stainless steel sheet having a chemical composition, in mass%, of 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, 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, Nb: 0 to 0.10%, Ti: 0 to 0.10%, Ca: 0 to 0.010%, Mg: 0 to 0.010%, Ga: 0 to 0.05%, Hf: 0 to 0.10%, REM: 0 to 0.10%, balance: Fe and impurities, a value M calculated from the following formula (i) is -90 to -20, a tensile strength is 550 MPa or more, and a sheet 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) each element symbol in the formula (i) represents a content of each element contained in the austenitic stainless steel sheet, in mass%, and is zero when not contained.

2. The austenitic stainless steel sheet according to claim 1, wherein the chemical composition contains one or more selected from the group consisting of, in mass%, Nb: 0.01 to 0.10%, Ti: 0.01 to 0.10%, Ca: 0.0002 to 0.010%, Mg: 0.0002 to 0.010%, Ga: 0.001 to 0.05%, Hf: 0.01 to 0.10%, and REM: 0.01 to 0.10%.

3. The austenitic stainless steel sheet according to claim 1, wherein an area ratio of a region satisfying the following formula (iii) is 90% or more, and an area ratio of a region satisfying the following formula (iv) is 90% or more at a sheet thickness 1 / 4 portion. Nis > 0.8...(iii) Mns > 0.8...(iv) wherein, in the formula (iii), Nis represents a segregation degree of Ni, and in the formula (iv), Mns represents a segregation degree of Mn.

4. The austenitic stainless steel sheet according to claim 2, wherein an area ratio of a region satisfying the following formula (iii) is 90% or more, and an area ratio of a region satisfying the following formula (iv) is 90% or more at a sheet thickness 1 / 4 portion. Nis > 0.8...(iii) Mns > 0.8...(iv) wherein, in the formula (iii), Nis represents a segregation degree of Ni, and in the formula (iv), Mns represents a segregation degree of Mn.

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

6. A hydrogen gas delivery pipe using the austenitic stainless steel sheet according to any one of claims 1 to 4. Cr：14.0~18.0%、 7. A valve, joint or meter for hydrogen use, which uses the austenitic stainless steel sheet according to any one of claims 1 to 4. ​ ​ ​ ​ Al：0~0.10%、 ​ ​ B：0~0.0050%、 V：0~0.50%、 W：0~0.50%、 ​ ​ Zr：0~0.50%、 ​ ​ ​ ​ ​ GSNo(1 / 4) - GSNo(1 / 2) = 0.5 - 1.5 = -1 ave GSNo(1 / 4) - GSNo(1 / 2) = 0.5 - 1.5 = -1 ave the difference between the average grain size number of the 1 / 4 portion of the plate thickness and the average grain size number of the 1 / 2 portion of the plate thickness satisfies the following equation (ii), the minimum grain size number GSNo among the grain size number of the 1 / 4 portion of the plate thickness and the grain size number of the 1 / 2 portion of the plate thickness min is 4.5 or more, ​ ​ ​ 0 < GSNo (1 / 4) ave - GSNo (1 / 2) ave (ii) wherein ​ ​ ​ Al：0.01~0.10%、 ​ ​ B：0.0002~0.0050%、 V：0.05~0.50%、 W：0.05~0.50%、 ​ ​ Zr：0.01~0.50%、 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

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