Austenitic stainless steel sheet, hydrogen gas transport piping, 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

Austenitic stainless steel plates, hydrogen gas transport piping, valves, fittings, and instruments

[0001] The present invention relates to austenitic stainless steel plates, hydrogen gas transport piping, valves, fittings, and meters.

[0002] In recent years, CO ２ Hydrogen power generation, which has the potential to reduce CO2 emissions, has been attracting attention. Hydrogen power generation is a form of power generation that uses hydrogen as fuel. An example of a hydrogen power generation facility is the facility described in Patent Document 1. Such hydrogen power generation facilities require the transportation of large amounts of hydrogen, and various parts and components, such as pipelines, are installed. The parts and components used in such hydrogen power generation facilities are required to be resistant to hydrogen embrittlement.

[0003] Japanese Patent Application Laid-Open No. 2021-141058

[0004] Some parts and members used in hydrogen power generation facilities are manufactured from thick steel plates. To process the parts and members into the desired shapes, it is necessary to introduce a certain amount of strain into the steel plates. However, when a certain amount of strain is introduced into a thick material, such as a thick steel plate, and the material is used in a hydrogen environment, hydrogen embrittlement becomes more likely to occur. Furthermore, some parts and members used in hydrogen power generation facilities require high strength. Therefore, there is a demand for austenitic stainless steel plates that combine good hydrogen embrittlement resistance with high strength.

[0005] The present invention has an object to solve the above problems and to provide an austenitic stainless steel sheet having a thickness of 4.5 mm or more, excellent hydrogen embrittlement resistance, and high strength.

[0006] The present invention has been made to solve the above-mentioned problems, and the gist of the present invention is the following austenitic stainless steel sheet, etc.

[0007] (1) Chemical composition, in mass%, is: 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%, balance: Fe and impurities, the M value calculated by the following formula (i) is -90 to -20, and GSNo (1 / 4) is the average value of the grain size number at 1 / 4 of the plate thickness. ａｖｅ and GSNo (1 / 2), which is the average value of the grain size number at 1 / 2 of the plate thickness. ａｖｅ The difference between the GSNo. and the GSNo. satisfies the following formula (ii), and the GSNo. is the smallest of the GSNo. and the GSNo. of the 1 / 4 thickness portion and the 1 / 2 thickness portion. ｍｉｎ An austenitic stainless steel sheet having a tensile strength of 550 MPa or more, a thickness of 4.5 mm or more, and an M value of 551-462(C+N)-9.2Si-8.1Mn-13.7Cr-29(Ni+Cu)-18.2Mo (i), where 0<GSNo(1 / 4). ａｖｅ -GSNo (1 / 2) ａｖｅ ...(ii) However, each element symbol in the above formula (i) represents the content (mass%) of each element contained in the austenitic stainless steel sheet, and if the element is not contained, it is set to zero.

[0008] (2) The austenitic stainless steel sheet according to (1) above, wherein the chemical composition contains, in mass %, one or more elements selected from 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) An austenitic stainless steel sheet according to (1) or (2) above, in which, at a quarter-thickness portion, the area ratio of a region satisfying the following formula (iii) is 90% or more, and the area ratio of a region satisfying the following formula (iv) is 90% or more: Nis>0.8 (iii) Mns>0.8 (iv) where, in formula (iii), Nis represents the degree of segregation of Ni, and in formula (iv), Mns represents the degree of segregation of Mn.

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

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

[0012] (6) A valve, a joint, or a meter for hydrogen gas applications, which uses the austenitic stainless steel sheet according to any one of (1) to (3) above.

[0013] According to the present invention, it is possible to obtain an austenitic stainless steel sheet having a thickness of 4.5 mm or more, excellent resistance to hydrogen embrittlement, and high strength.

[0014] The present inventors have investigated the hydrogen embrittlement resistance of thick steel plates and have obtained the following findings.

[0015] (a) When steel plates are formed into desired parts and components, they undergo plastic deformation processes such as tensioning and bending. Such plastic deformation imparts pre-strain to the steel plates. Generally, the stress required for plastic deformation increases with increasing plate thickness. However, strain is not uniformly imparted to all positions on the steel plate. For example, the amount of strain imparted near the surface of the steel plate is significantly different from the amount of strain imparted near the center of the plate thickness.

[0016] (b) One of the reasons for this is the difference in grain size between the surface and the center of the plate thickness. As a result of research conducted by the present inventors, it was found that, particularly in high-Mn austenitic stainless steels, in thick steel plates exceeding 10 mm in thickness, the outermost surface is the finest grained, but the grains in the 1 / 4 portion of the plate thickness near the surface tend to become somewhat coarse due to the growth and coarsening of columnar crystal structures during solidification. Furthermore, the grains in the 1 / 2 portion of the plate thickness near the center of the plate thickness tend to become finer than those in the 1 / 4 portion due to the formation of an equiaxed crystal structure during solidification. In other words, a difference in grain size is likely to occur between the 1 / 4 portion and the 1 / 2 portion of the plate thickness, and the grains in the 1 / 4 portion are more likely to be coarser than the grains in the 1 / 2 portion of the plate thickness. As a result, a large amount of strain is likely to accumulate in the region with larger grain size, making the region with larger grain size more susceptible to hydrogen embrittlement. This tendency becomes more pronounced as the strength of the steel plate increases.

[0017] (c) In light of the above, in order to improve the hydrogen embrittlement resistance of thick steel plates, it is desirable to reduce the difference in crystal grain size within the steel plate. However, rather than simply reducing the difference in crystal grain size, it is preferable from the viewpoint of hydrogen embrittlement resistance that the crystal grains in the 1 / 4 portion of the plate thickness be finer than those in the 1 / 2 portion. To achieve such a metal structure, it is necessary to control the rolling shape ratio within a predetermined range during manufacturing. Furthermore, from the viewpoint of hydrogen embrittlement resistance, it is preferable to reduce coarse grains throughout the steel plate.

[0018] An embodiment of the present invention has been made based on the above findings. Hereinafter, each requirement of the austenitic stainless steel sheet of this embodiment will be described in detail.

[0019] 1. Chemical Composition The reasons for limiting the content of each element are as follows: In the following description, "%" in the content means "% by mass."

[0020] C: 0.10% or less Carbon (C) is an element effective in stabilizing the austenite phase and also contributes to improving hydrogen embrittlement resistance. However, excessive C content promotes grain boundary precipitation of Cr-based carbides, which in turn tends to reduce hydrogen embrittlement resistance. For this reason, the C content is 0.10% or less. The C content is preferably 0.080% or less, more preferably 0.070% or less, and even more preferably 0.060% or less. On the other hand, in order to obtain the above 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.

[0021] Si: 1.0% or less Si (silicon) is an effective element for deoxidation and also contributes to improving hydrogen embrittlement resistance. However, excessive Si content promotes the formation of intermetallic compounds such as σ phase, reducing hot workability and toughness. For this reason, the Si content is 1.0% or less. The Si content is preferably 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, in order to obtain the above 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.

[0022] Mn: 8.0 to 10.0% Mn (manganese) is an element effective in stabilizing the austenite phase and contributes to improving hydrogen embrittlement resistance. It also has the effect of improving strength. For this reason, the Mn content is 8.0% or more. The Mn content is preferably 8.5% or more. However, excessive Mn content promotes the formation of the ε phase, which is highly susceptible to hydrogen embrittlement, and actually reduces hydrogen embrittlement resistance. For this reason, the Mn content is 10.0% or less. The Mn content is preferably 9.5% or less.

[0023] P: 0.050% or less P (phosphorus) is an element contained in steel as an impurity, which causes segregation and reduces hydrogen embrittlement resistance. Therefore, the P content is 0.050% or less. The P content is preferably 0.040% or less, and more preferably 0.030% or less. On the other hand, excessive reduction of P leads to an increase in manufacturing costs, so the P content 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 steel as an impurity, and forms MnS, which reduces hydrogen embrittlement resistance. Therefore, the S content is 0.0050% or less. The S content is preferably 0.0040% or less, and more preferably 0.0030% or less. However, excessive reduction of S increases manufacturing costs. Therefore, the S content 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 contained in a certain amount in stainless steel and has the effect of improving corrosion resistance. Therefore, the Cr content is 14.0% or more. The Cr content is preferably 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, if Cr is contained in excess, the austenite phase becomes unstable and a large amount of carbides precipitates, reducing hydrogen embrittlement resistance. Therefore, the Cr content is 18.0% or less. The Cr content is preferably 17.5% or less, more preferably 17.0% or less, and even more preferably 16.5% or less.

[0026] Mo: 1.0% or less Mo (molybdenum) has the effect of improving strength. However, excessive Mo content promotes the formation of δ-ferrite phase, reducing hydrogen embrittlement resistance. Therefore, the Mo content is 1.0% or less. The Mo content is preferably 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, excessive reduction of Mo leads to restrictions on the melting raw materials and increases production 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.

[0027] Ni: 6.0 to 9.0% Ni (nickel), together with Mn, is an element necessary for ensuring hydrogen embrittlement resistance and strength. Therefore, the Ni content is 6.0% or more. The Ni content is preferably 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 production costs but also makes segregation more likely to occur, which in turn makes hydrogen embrittlement resistance more likely to decrease. In addition, excessive Ni content may cause solid solution softening and reduce strength. Therefore, the Ni content is 9.0% or less. The Ni content is preferably 8.5% or less, more preferably 8.0% or less, and even more preferably 7.6% or less.

[0028] Cu: 1.5% or less Cu (copper) is an element that is mixed in from raw materials such as scrap and is effective in stabilizing the austenite phase. On the other hand, Cu is a low-melting-point element that segregates at grain boundaries, making fracture initiation sites more likely to occur. For this reason, 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, excessive reduction of Cu leads to restrictions on the melting raw materials and increases production costs. For this reason, the Cu content is preferably 0.01% or more, preferably 0.05% or more, and even more preferably 0.10% or more.

[0029] Co: 0.01 to 1.0% Co (cobalt) is an important element that improves corrosion resistance, stabilizes the austenite phase, and, particularly when the plate thickness is large, has the effect of improving hydrogen embrittlement resistance. Therefore, the Co content is 0.01% or more. The Co content is preferably 0.05% or more, more preferably 0.10% or more, and even more preferably 0.15% or more. However, if Co is contained in excess, toughness and workability decrease. Therefore, the Co content is 1.0% or less. The Co content is preferably 0.70% or less, and even more preferably 0.50% or less.

[0030] N: 0.25% or less Like Mn and Ni, N (nitrogen) is an element effective in improving hydrogen embrittlement resistance. However, excessive N content may cause internal defects such as blowholes during melting, which may actually reduce hydrogen embrittlement resistance. For this reason, the N content is 0.25% or less. The N content is preferably 0.20% or less, and more preferably 0.18% or less. On the other hand, in order to obtain the above 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.

[0031] In addition to the above elements, one or more elements selected from Al, Nb, Ti, B, V, W, Ca, Mg, Zr, Ga, Hf, and REM may be contained within the ranges shown below. The reasons for limiting the amount of each element will be explained below.

[0032] Al: 0 to 0.10% Al (aluminum) is an effective deoxidizing element, and also has the effect of suppressing grain boundary segregation of low-melting point elements and strengthening the grain boundaries. Therefore, it may be contained as needed. However, Al is a ferrite-forming element. Therefore, if Al is contained in excess, the austenite phase becomes unstable. For this reason, the Al content is 0.10% or less. The Al content is preferably 0.07% or less, and more preferably 0.05% or less. On the other hand, in order to obtain the above effect, the Al content is preferably 0.01% or more, and more preferably 0.02% or more.

[0033] Nb: 0 to 0.10% Nb (niobium) forms carbonitrides, refines crystal grains, and strengthens grain boundaries. Therefore, it may be added as needed. However, excessive Nb content reduces manufacturability and workability during hot rolling. Therefore, the Nb content is 0.10% or less. The Nb content is preferably 0.08% or less, and more preferably 0.07% or less. On the other hand, in order to obtain the above effects, the Nb content is preferably 0.01% or more, and more preferably 0.02% or more.

[0034] Ti: 0 to 0.10% Ti (titanium) has the effect of forming carbonitrides, refining crystal grains, and strengthening grain boundaries. Therefore, it may be added as needed. However, excessive Ti content reduces manufacturability during hot rolling. Furthermore, a large amount of inclusions may be formed, resulting in reduced impact resistance. For this reason, the Ti content is 0.10% or less. The Ti content is preferably 0.07% or less, and more preferably 0.05% or less. On the other hand, to obtain the above effects, the Ti content is preferably 0.01% or more, and more preferably 0.02% or more.

[0035] B: 0 to 0.0050% B (boron) has the effect of strengthening grain boundaries, improving strength, and improving impact resistance. Therefore, it may be added as needed. However, if B is added in excess, not only will the effect saturate, but impact resistance may actually decrease. Therefore, the B content is 0.0050% or less. The B content is preferably 0.0040% or less, and more preferably 0.0030% or less. On the other hand, in order to obtain the above effects, the B content is preferably 0.0002% or more, more preferably 0.0005% or more, and even more preferably 0.0010% or more.

[0036] V: 0 to 0.50% V (vanadium) precipitates in steel as a solid solution or carbonitride, and has the effect of improving strength. Therefore, it may be added as needed. However, if excessive V is added, excessive carbonitrides are formed, reducing manufacturability during hot rolling. For this reason, the V content is 0.50% or less. The V content is preferably 0.40% or less, more preferably 0.30% or less, and even more preferably 0.20% or less. On the other hand, in order to obtain the above effect, the V content is preferably 0.05% or more, and more preferably 0.10% or more.

[0037] W: 0 to 0.50% W (tungsten) has the effect of improving strength and corrosion resistance. Therefore, it may be contained as needed. However, excessive W content increases manufacturing costs. For this reason, the W content is 0.50% or less. The W content is preferably 0.40% or less, and more preferably 0.30% or less. On the other hand, in order to obtain the above effects, the W content is preferably 0.05% or more, and more preferably 0.10% or more.

[0038] Ca: 0 to 0.010% Ca (calcium) is an element effective in deoxidizing and improving hot workability. Therefore, it may be added as needed. However, excessive Ca content makes segregation more likely to occur, and the segregation is more likely to become the starting point for fracture. As a result, impact resistance may decrease. For this reason, the Ca content is 0.010% or less. The Ca content is preferably 0.0050% or less, and more preferably 0.0030% or less. On the other hand, to obtain the above effects, the Ca content is preferably 0.0002% or more, more preferably 0.0005% or more, and even more preferably 0.0010% or more.

[0039] Mg: 0 to 0.010% Mg (magnesium) is an element effective in deoxidizing and improving hot workability. Therefore, it may be added as needed. However, excessive Mg content can result in the formation of a large amount of inclusions, which can easily become the starting point of fracture, resulting in a decrease in impact resistance. For this reason, 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, to obtain the above effects, the Mg content is preferably 0.0001% or more, and more preferably 0.0002% or more.

[0040] Zr: 0 to 0.50% Zr (zirconium) has a deoxidizing effect. It also has the effect of improving corrosion resistance. Therefore, it may be contained as needed. However, if Zr is contained in excess, toughness and workability decrease. For this reason, the Zr content is 0.50% or less. The Zr content is preferably 0.30% or less, more preferably 0.10% or less, and even more preferably 0.05% or less. On the other hand, in order to obtain the above effects, the Zr content is preferably 0.01% or more, and more preferably 0.02% or more.

[0041] Ga: 0 to 0.05% Ga (gallium) has the effect of improving hot workability. Therefore, it may be contained as necessary. However, if Ga is contained in excess, manufacturability decreases. Therefore, the Ga content is 0.05% or less. The Ga content is preferably 0.04% or less, and more preferably 0.02% or less. On the other hand, in order to obtain the above effect, the Ga content is preferably 0.001% or more, and more preferably 0.005% or more.

[0042] Hf: 0 to 0.10% Hf (hafnium) has the effect of improving strength and hydrogen embrittlement resistance. Therefore, it may be contained as needed. However, if Hf is contained in excess, workability decreases. 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 effects, the Hf content is preferably 0.001% or more, more preferably 0.005% or more, and even more preferably 0.01% or more.

[0043] REM: 0 to 0.10% REM (rare earth elements) have the effect of improving hot workability. They also have the effect of improving corrosion resistance. Therefore, they may be added as needed. However, if REM is added in excess, not only will the effect saturate, but hot workability will also decrease. For this reason, 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, in order to obtain the above effects, the REM content is preferably 0.001% or more, more preferably 0.005% or more, and even more preferably 0.01% or more.

[0044] REM refers to a total of 17 elements, including Sc, Y, and lanthanides, and the REM content refers to the total content of these elements. Industrially, REM is often added in the form of misch metal.

[0045] In the chemical composition of the austenitic stainless steel sheet of this embodiment, the balance is Fe and impurities. Here, "impurities" refer to components that are mixed in during the industrial production of austenitic stainless steel sheet due to various factors in raw materials such as ore and scrap, and in the manufacturing process, and are acceptable within a range that does not adversely affect this embodiment.

[0046] M Value The M value calculated by the following formula (i) is an index showing the stability of the austenitic phase in an austenitic stainless steel sheet. In the austenitic stainless steel sheet of this embodiment, the M value is -90 to -20 in order to improve hydrogen embrittlement resistance and strength. M Value = 551 - 462 (C + N) - 9.2 Si - 8.1 Mn - 13.7 Cr - 29 (Ni + Cu) - 18.2 Mo ... (i) Note that each element symbol in the above formula (i) represents the content (mass%) of each element contained in the austenitic stainless steel sheet, and if an element is not contained, it is set to zero.

[0047] If the M value is less than −90, the alloy cost increases. Therefore, 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, if the M value exceeds -20, the stability of the austenite phase is low, transformation to the α' phase is likely to occur, and hydrogen embrittlement resistance is reduced. Also, it becomes difficult to obtain the desired strength. For this reason, 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 2-1. Difference between grain size near the surface and grain size at the center In order to enhance hydrogen embrittlement resistance, the austenitic stainless steel sheet of this embodiment needs to have a small difference between the grain size near the surface and the grain size at the center, with the grains at ¼ of the sheet thickness being finer than those at ½.

[0050] Specifically, GSNo (1 / 4), which is the average value of the grain size number at 1 / 4 of the plate thickness, ａｖｅ and GSNo (1 / 2), which is the average value of the grain size number at 1 / 2 of the plate thickness. ａｖｅThe difference between the grain size (hereinafter simply referred to as the "crystal grain size difference" or the "right-hand side value of equation (ii)") satisfies the following equation (ii): 0<GSNo(1 / 4) ａｖｅ -GSNo (1 / 2) ａｖｅ ... (ii)

[0051] From the viewpoint of hydrogen embrittlement resistance, the grain size difference is preferably 0.1 or more, more preferably 0.3 or more. On the other hand, if the grain size difference exceeds 1.5, the difference in grain size within the steel sheet becomes too large, and the hydrogen embrittlement resistance is likely to decrease. Therefore, the grain size difference is preferably 1.5 or less, more preferably 1.0 or less.

[0052] Here, the quarter thickness portion refers to a position 1 / 4t from the surface in the thickness direction, where t is the thickness of the plate. Similarly, the half thickness portion refers to a position 1 / 2t from the surface in the thickness direction, where t is the thickness of the plate.

[0053] Average value of grain size number at 1 / 4 of plate thickness GS No (1 / 4) ａｖｅ and the average value of the grain size number at 1 / 2 of the plate thickness GS No (1 / 2) ａｖｅ is measured by the following procedure. Specifically, a surface parallel to the thickness direction and the longitudinal direction (hereinafter referred to as "L cross section") near the center of the steel sheet is used as the observation surface, and after the grain boundaries are revealed by nitric acid electrolysis or immersion in aqua regia, the surface is observed using an optical microscope. The size of the observation surface is the sheet thickness x 30 mm (longitudinal length), and the magnification during observation is 100x. Then, for a quarter of the sheet thickness of the observation surface, measurements are made in a total of five fields, at the center in the longitudinal direction and at positions 6 mm and 12 mm in both directions, and the grain size is calculated by a linear intercept method, and the average value is used as the GSNo (1 / 4). ａｖｅ Similarly, for the 1 / 2 thickness portion, measurements are taken in a total of five fields of view, at the center in the longitudinal direction and at positions 6 mm and 12 mm in both directions, and the grain size is calculated by the linear intercept method, and the average value is designated as the GS No. (1 / 2). ａｖｅ Other observation and measurement conditions may follow JIS G 0551:2020.

[0054] 2-2. Minimum Grain Size The austenitic stainless steel sheet of this embodiment suppresses the formation of coarse grains in order to suppress hydrogen penetration and improve hydrogen embrittlement resistance. Specifically, the minimum grain size number GSNo of the field of view with the coarsest grains among the grain size numbers at 1 / 4 of the plate thickness and the grain size numbers at 1 / 2 of the plate thickness in the observed field of view is used. ｍｉｎ is 4.5 or more. ｍｉｎ is preferably 5.0 or more, and more preferably 5.5 or more. ｍｉｎ Although there is no particular limitation on the upper limit of , taking into consideration production costs and restrictions during production, it is usually preferable that the upper limit be 8.0.

[0055] In addition, GSNo. ｍｉｎ is the above-mentioned GSNo (1 / 4) ａｖｅ and GSNo (1 / 2) ａｖｅ In the observation carried out to measure the grain size, the smallest grain size number is found among five visual fields at 1 / 4 of the plate thickness and five visual fields at 1 / 2 of the plate thickness.

[0056] 3. Tensile Strength The tensile strength of the austenitic stainless steel sheet of this embodiment at room temperature is 550 MPa or more to obtain the desired strength. The tensile strength is preferably 600 MPa or more, and more preferably 650 MPa or more. The upper limit of the tensile strength is not particularly limited, but is usually 900 MPa. It is preferably less than 800 MPa, more preferably 780 MPa or less, and even more preferably 750 MPa or less. The tensile strength at room temperature can be measured by conducting a tensile test at room temperature (24°C). The tensile test conditions may be general conditions.

[0057] 4. Plate Thickness The austenitic stainless steel plate of this embodiment is intended for thick steel plates intended for use in hydrogen power generation facilities. Therefore, 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 even more preferably 30 mm or more. The upper limit of the plate thickness is not particularly limited, but is usually 100 mm.

[0058] 5. Segregation State In order to obtain good hydrogen embrittlement resistance, it is preferable to control the segregation state. Segregation is a phenomenon in which the concentration of solute elements becomes non-uniform during solidification, and it is particularly likely to occur in thick steel plates. This segregation state may remain in part in the metal structure even after the steel plate is manufactured, adversely affecting hydrogen embrittlement resistance. The effect of segregation is particularly pronounced in the quarter-thickness portion near the surface, where pre-strain accumulates locally, resulting in a decrease in hydrogen embrittlement resistance. Furthermore, the occurrence of segregation also increases the likelihood of transformation from the austenite phase to the α' phase, which is another factor that can lead to a decrease in hydrogen embrittlement resistance.

[0059] Segregation can be classified into positive segregation, in which elements are distributed at higher than average concentrations, and negative segregation, in which elements are distributed at lower than average concentrations. In the austenitic stainless steel sheet of this embodiment, attention is mainly focused on negative segregation from the viewpoint of hydrogen embrittlement resistance.

[0060] In light of the above, it is preferable that, in the quarter thickness portion, 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: Nis>0.8 (iii) Mns>0.8 (iv) In the above formula (iii), Nis represents the segregation degree of Ni (nickel), and in the above formula (iv), Mns represents the segregation degree of Mn (manganese).

[0061] In the chemical composition of the steel sheet of this embodiment, elements that segregate and affect hydrogen embrittlement resistance include Ni and Mn. Therefore, the segregation degrees of Ni and Mn are controlled. The segregation degree refers to the concentration of a segregated portion relative to the average concentration of a given element. The Ni segregation degree can be calculated by (local Ni concentration) / (average Ni content throughout the entire thickness of the steel sheet), and the Mn segregation degree can be calculated by (local Mn concentration) / (average Mn content throughout the entire thickness of the steel sheet). The closer the segregation degree is to 1, the less segregation there is.

[0062] When Nis is greater than 0.8, i.e., when the degree of Ni segregation is greater than 0.8 and formula (iii) is satisfied, no local concentration gradient occurs and negative segregation is reduced. Therefore, from the viewpoint of reducing hydrogen embrittlement resistance due to negative segregation, the area ratio of the region satisfying formula (iii) in the quarter thickness portion is preferably 90% or more. The area ratio of the region satisfying formula (iii) is more preferably 95% or more.

[0063] Similarly, when Mns is greater than 0.8, i.e., when the degree of Mn segregation is greater than 0.8 and formula (iv) is satisfied, no local concentration gradient occurs and negative segregation is reduced. Therefore, from the viewpoint of reducing hydrogen embrittlement resistance due to negative segregation, it is preferable that the area ratio of the region satisfying formula (iv) is 90% or more in the quarter thickness portion. Therefore, it is preferable that the area ratio of the region satisfying formula (iv) is 90% or more, and more preferably 95% or more.

[0064] It is preferable that the area satisfying the formula (iii) is 90% or more, and the area satisfying the formula (iv) is 90% or more.

[0065] The Nis and Mns at the quarter thickness portion, as well as the area ratio of the region satisfying formula (iii) and the area ratio of the region satisfying formula (iv) may be measured by the following procedure. In a plane (C cross section) parallel to the thickness direction and width direction of the steel plate, the quarter thickness portion is set as the center of the field of view, and an observation field is set to a 2 mm square above and below the center. Next, a beam diameter of 6 μm, an acceleration voltage of 15 kV, and an irradiation current of 1.17 × 10 －９ Area analysis is performed using EPMA under the conditions of A. By the area analysis using EPMA, the Ni concentration and Mn concentration are mapped, and the area satisfying formula (iii) and the area satisfying formula (iv) are calculated in terms of area ratio. Note that the observation is preferably performed near the center of the steel sheet width where segregation is likely to occur, but regardless of the position in the sheet width direction, as long as the above ranges are satisfied, it is within the scope of the invention of this embodiment.

[0066] 6. Applications The austenitic stainless steel sheet of this embodiment is suitable for use in a hydrogen gas environment, particularly for parts and members of hydrogen power generation facilities. That is, it is preferably used for hydrogen gas transport piping used in the above-mentioned facilities, and valves, fittings, and meters for hydrogen gas applications. The hydrogen gas is preferably compressed or liquefied, and the pressure of the hydrogen gas is preferably in the range of 0.1 to 20 MPa, for example.

[0067] 7. Manufacturing Method The austenitic stainless steel sheet of this embodiment can be stably manufactured, for example, by the following manufacturing method.

[0068] 7-1. Hot Rolling Process 7-1-1. Pre-Rolling Stainless steel having the above chemical composition is melted and steel billets such as slabs are produced. Next, the steel billets are heated to a predetermined temperature and hot rolled (hot rolling process). In this hot rolling process, the steel billets are first heated to a temperature of 1200°C or less to obtain the rolling shape ratio m ｊ Preliminary rolling is performed by performing three or more passes in which the value of β is 0.5 or more.

[0069] In the manufacturing method of an austenitic stainless steel sheet according to this embodiment, the heating temperature in the preliminary rolling is 1200°C or lower. The heating temperature in the preliminary rolling is preferably 1100°C or lower. This is to suppress recrystallization and grain growth and accumulate strain in the quarter thickness portion. Such accumulation of strain promotes recrystallization in the subsequent heat treatment or finish rolling heating process, resulting in the effect of refinement of the grains in the quarter thickness portion.

[0070] At the above heating temperature, preliminary rolling is performed at a rolling shape ratio m ｊ By performing three or more passes where the value is 0.5 or more, the formula (ii) is satisfied and GSNo ｍｉｎ However, it is possible to form crystal grains having a rolling shape ratio m of 4.5 or more. ｊ is calculated by the following formula (a).

[0071]

[0072] In the above formula, each symbol is defined as follows: R: roll radius (mm) t ｊ : Entrance side plate thickness (mm) tｊ＋１ :Outside plate thickness (mm)

[0073] In the hot rolling process, a plurality of rolling mills each having a pair of rolling rolls are installed in series. The steel sheet is rolled by passing between these rolling mills. Here, the time when the steel sheet passes between the pair of rolls of one rolling mill is called a pass. Therefore, the above R is the radius of the rolling roll, and t ｊ is the thickness of the plate entering the rolling mill before rolling during one pass, and t ｊ＋１ is the thickness of the plate after passing through the rolling mill. ｊ Since it is affected by the roll diameter and reduction rate of the rolling mill, these must be controlled within an appropriate range to achieve the rolling shape ratio m ｊ The other conditions for the preliminary rolling may be in accordance with conventional methods.

[0074] 7-1-2. Heat Treatment After preliminary rolling, heat treatment may be performed as necessary. This is because heat treatment makes it easier for the area satisfying formulas (iii) and (iv) to be 90% or more in the quarter thickness portion. The heat treatment temperature is preferably 1200 to 1300°C, and the heat treatment time is preferably 60 minutes or longer.

[0075] 7-1-3. Finish rolling In the hot rolling process, after preliminary rolling or, if necessary, after heat treatment, finish rolling is performed. In the finish rolling, the shape factor m ｊ It is preferable to perform five or more passes so that the difference in grain size becomes 1.0 or less and better hydrogen embrittlement resistance can be obtained. ｊ can be calculated in the same way as in the preliminary rolling. In order to obtain a structure with the desired crystal grain size, the heating temperature of the steel sheet in the finish rolling is preferably in the range of 1150 to 1250°C. Other conditions for the finish rolling may be in accordance with conventional methods as appropriate. After the finish rolling, the steel sheet is cooled in accordance with conventional methods to obtain an austenitic stainless steel sheet.

[0076] 7-2. Other After the hot rolling process, annealing and pickling may be performed as needed. The annealing conditions may be in accordance with conventional methods; for example, the annealing temperature is preferably 1050 to 1150°C, and the annealing time is preferably 5 to 15 minutes. Thereafter, the sheet is cooled to finally obtain an austenitic stainless steel sheet, and pickling may be performed as needed. The pickling conditions may also be in accordance with conventional methods, as appropriate.

[0077] EXAMPLES The austenitic stainless steel sheet according to the present invention will be described in more detail below with reference to examples, but the present embodiment is not limited to these examples.

[0078] Stainless steels having the chemical compositions shown in Table 1 were melted and slabs of 200 mm in thickness were produced. The obtained slabs were pre-rolled. In the pre-rolling, they were heated at the temperatures shown in Table 2. The shape factor m ｊ The number of passes where the rolling shape ratio m was 0.5 or more was counted. Then, in some examples, heat treatment was performed under the conditions shown in Table 2. In examples where heat treatment was performed as necessary after preliminary rolling, the samples were heated to 1150 to 1200°C after the heat treatment and then subjected to finish rolling. ｊ The number of passes at which the tensile strength was 0.8 or more was counted. Thereafter, the sheet was annealed for 5 minutes in a temperature range of 1050 to 1100°C, and then cooled, pickled, etc. to obtain an austenitic stainless steel sheet.

[0079]

[0080] The obtained austenitic stainless steel sheet was subjected to the following procedure to obtain GS No. (1 / 4) ａｖｅ , GSNo (1 / 2) ａｖｅ , GSNo ｍｉｎ The area ratios of the region satisfying formula (iii) and the region satisfying formula (iv) at the quarter thickness portion of the sheet were measured. In addition, to evaluate the properties, a slow strain rate tensile test was performed using the following procedure.

[0081] (GSNo.(1 / 4) ａｖｅ , GSNo (1 / 2) ａｖｅ , GSNo ｍｉｎThe L-section of the steel plate was used as the observation surface. The size of the observation surface was the plate thickness x 30 mm (length in the longitudinal direction), and the magnification during observation was 100 times. Then, 5 fields of view were measured at 1 / 4 of the plate thickness of the observation surface, and the grain size was calculated by a linear intercept method, and the average value was used as the GS No. (1 / 4). ａｖｅ Similarly, for the 1 / 2 thickness portion, five visual fields were measured, the grain size was calculated by the linear intercept method, and the average value was designated as the GS No. (1 / 2). ａｖｅ Other observation conditions were in accordance with JIS G 0551:2020.

[0082] Also, the above-mentioned GSNo (1 / 4) ａｖｅ and GSNo (1 / 2) ａｖｅ The grain size number of the field of view where the grain size number was smallest in the observation conducted to measure the grain size was designated as GSNo. ｍｉｎ That is, the smallest grain size number among the five visual fields at the quarter thickness portion and the five visual fields at the half thickness portion was designated as GS No. ｍｉｎ It was decided.

[0083] (Area ratio of region satisfying formulas (iii) and (iv) at quarter thickness portion) In the C cross section of the steel plate, the quarter thickness portion was set as the center of the field of view, and an observation field was set to 2 mm square above and below the center. Subsequently, a beam diameter of 6 μm, an acceleration voltage of 15 kV, and an irradiation current of 1.17×10 were applied to the observation field. －９ Area analysis was performed using EPMA under the conditions of A. By the area analysis using EPMA, the Ni and Mn concentrations were mapped, and the area satisfying formula (iii) and the area satisfying formula (iv) were calculated as area ratios. In the table, the area ratio of the area satisfying formula (iii) at 1 / 4 of the plate thickness is described as the area ratio of the area where Nis > 0.8, and the area ratio of the area satisfying formula (iv) at 1 / 4 of the plate thickness is described as the area ratio of the area where Mns > 0.8.

[0084] (Slow Strain Rate Tensile Test) A slow strain rate tensile test was performed to evaluate hydrogen embrittlement resistance. A test piece was taken from a surface parallel to the rolling surface at the 1 / 4 position of the plate thickness, using a round bar with a parallel portion of Φ3 mm x length of 20 mm, a gripping portion of Φ8 mm, and a total length of 80 mm, so that the longitudinal direction coincided with the longitudinal direction of the steel plate. This test piece was pulled in air until the displacement of the parallel portion was 6 mm, and a pre-strain of 30% elongation was applied. Thereafter, a test piece was pulled in air using a 20 MPaH ２ The test was carried out by applying a tensile stress in gas until fracture. The tensile speed was 0.036 mm / min and the strain rate was 3×10 －５ / s. For each test piece, the tensile strength and elongation at break were also measured separately in air to calculate the RTS (relative tensile strength) and REL (relative elongation at break) described later. In this example, the round bar test piece was used. However, if the plate thickness is less than 8 mm, a test piece having a plate-like parallel portion measuring, for example, the plate thickness x width 4 mm x length 20 mm, a gripping portion having a width of 25 mm x length 27 mm, and a total length of 94 mm (R10 mm from the parallel portion to the gripping portion) may be used. If the above test piece cannot be obtained, the test may be performed using a test piece of a size that can be obtained.

[0085] The test results were evaluated in terms of RTS (relative tensile strength) and REL (relative elongation at break). RTS and REL are calculated using the following formula: RTS = Relative tensile strength after applying pre-strain in air and then straining to 20 MPaH ２ Tensile strength when tested with tensile stress applied in air ÷ Tensile strength in air (b) REL = Tensile strength when tested with tensile stress applied in air and then 20 MPaH ２ Breaking elongation when tested with tensile stress applied in air ÷ Breaking elongation in air (c)

[0086] When the RTS was less than 1.00, the hydrogen embrittlement resistance was judged to be poor and was marked C. On the other hand, when the RTS was 1.00 or more, the hydrogen embrittlement resistance was judged to be good and was marked B. Furthermore, when the RTS was 1.00 or more and the REL was 1.00 or more, the hydrogen embrittlement resistance was judged to be even better and was marked A. The results are summarized below in Table 2.

[0087]

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

Claims

1. Chemical composition, by mass%, is: C: 0.10% or less, Si: 1.0% or less, Mn: 8.0-10.0%, P: 0.050% or less, S: 0.0050% or less, Cr: 14.0-18.0%, Mo: 1.0% or less, Ni: 6.0-9.0%, Cu: 1.5% or less, Co: 0.01-1.0%, N: 0.25% or less, Al: 0-0.10%, Nb: 0-0.10%, Ti: 0-0.10%, B: 0-0.0050%, V: 0-0.50%, W: 0-0.50%, Ca: 0-0.010%, Mg: 0-0.010%, Zr: 0-0.50%, Ga: 0-0.05%, Hf: 0-0.10%, REM: 0-0.10%, balance: Fe and impurities, the M value calculated by the following formula (i) is -90 to -20, and GSNo (1 / 4) is the average value of the grain size number at the 1 / 4 part of the sheet thickness. ａｖｅ and GSNo (1 / 2), which is the average grain size number of the 1 / 2 part of the plate thickness. ａｖｅ The difference between the grain size number at the 1 / 4 plate thickness portion and the grain size number at the 1 / 2 plate thickness portion satisfies the following formula (ii), and ｍｉｎ is 4.5 or more, the tensile strength is 550 MPa or more, and the 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) However, each element symbol in the above formula (i) represents the content (mass%) of each element contained in the austenitic stainless steel sheet, and if the element is not contained, it is set to zero.

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

3. The austenitic stainless steel sheet according to claim 1, in which, at the 1 / 4 plate thickness portion, 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: Nis>0.8... (iii) Mns>0.8... (iv) In the above formula (iii), Nis represents the degree of segregation of Ni, and in the above formula (iv), Mns represents the degree of segregation of Mn.

4. The austenitic stainless steel sheet according to claim 2, wherein, at the 1 / 4 plate thickness portion, 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: Nis>0.8... (iii) Mns>0.8... (iv) In the above formula (iii), Nis represents the degree of segregation of Ni, and in the above formula (iv), Mns represents the degree of segregation 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 transport pipe using the austenitic stainless steel sheet according to any one of claims 1 to 4.

7. A valve, joint or meter for hydrogen gas use, which uses the austenitic stainless steel sheet according to any one of claims 1 to 4.