Austenitic stainless steel plate

Abstract

To provide an austenitic stainless steel plate that has excellent impact resistance and can prevent fractures even under the accumulation of strain in cryogenic conditions, such as those required for liquid hydrogen storage.SOLUTION: An austenitic stainless steel plate for liquid hydrogen storage tanks has a chemical composition comprising, in mass%, C: 0.1% or less, Si: 1.0% or less, Mn: 8.0-10.0%, P: 0.050% or less, S: 0.005% 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, an optional element, and the balance being Fe and impurities, with the M value of -90 or more to -20 or less, and the plate thickness of 4.5 mm or more.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to an austenitic stainless steel sheet. [Background technology]

[0002] In recent years, hydrogen gas has been attracting attention as a new energy source to replace fossil fuels. Hydrogen gas is a clean energy source that does not emit CO2. However, hydrogen gas can cause hydrogen embrittlement, which weakens materials. Therefore, Patent Document 1 discloses an austenitic stainless steel with improved resistance to hydrogen gas embrittlement. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2015-196842

[0004] In order to actually use hydrogen gas as an energy source, it is necessary to store large quantities of liquefied hydrogen compressed at cryogenic temperatures. For this reason, the construction of large storage tanks capable of accommodating liquefied hydrogen (hereinafter simply referred to as "liquefied hydrogen storage tanks") is being planned. Here, thick austenitic stainless steel plates, which are a material with excellent resistance to hydrogen gas embrittlement, are preferred for use as liquefied hydrogen storage tanks.

[0005] On the other hand, such austenitic stainless steel sheets are required not only to be resistant to hydrogen gas embrittlement but also to be fracture-resistant when used in architectural structures. For example, they are required to be able to suppress fracture even when strain accumulates due to an earthquake or other event. In particular, fracture is thought to be more likely to occur and propagate under extremely low temperatures, such as those used to store liquefied hydrogen. Therefore, impact resistance that can suppress fracture even in such harsh environments is required.

[0006] However, the austenitic stainless steel disclosed in Patent Document 1 does not consider the impact resistance characteristics described above, and therefore there is room for further improvement in the impact resistance characteristics. Summary of the Invention [Problem to be solved by the invention]

[0007] In view of the above, an object of the present invention is to solve the above problems and to provide an austenitic stainless steel sheet having excellent impact resistance that can suppress fracture even when strain accumulates at extremely low temperatures such as those used to store liquefied hydrogen. [Means for solving the problem]

[0008] 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.

[0009] (1) Chemical composition, in mass%, C: 0.10% or less, Si: 1.0% or less, Mn: 8.0 to 10.0%, P: 0.050% or less, S: 0.0050% or less, Cr: 14.0~18.0%, Mo: 1.0% or less Ni: 6.0-9.0% 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: 0~0.0050%, V: 0~0.5%, W: 0-0.5%, 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: 0~0.10%, The balance is Fe and impurities. The M value calculated by the following formula (i) is -90 or more and -20 or less, Austenitic stainless steel plate for liquefied hydrogen storage tanks, with a plate thickness of 4.5 mm or more. M value=551-462(C+N)-9.2Si-8.1Mn-13.7Cr-29(Ni+Cu)-18.2Mo...(i) However, each element symbol in the above formula (i) represents the content (mass%) of each element contained in the steel, and if the element is not contained, it is set to zero.

[0010] (2) The chemical composition is in mass%: Al: 0.01 to 0.10%, Nb: 0.01 to 0.10%, Ti: 0.01 to 0.10% B: 0.0002~0.0050%, V: 0.05~0.5%, W: 0.05 to 0.5%, 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: 0.01~0.10%, The austenitic stainless steel sheet for a liquefied hydrogen storage tank according to (1) above, containing one or more selected from the following:

[0011] (3) An austenitic stainless steel sheet for a liquefied hydrogen storage tank according to (1) above, in which, in the center of the sheet thickness, the area ratio of the region satisfying the following formula (ii) is 90% or more, and the area ratio of the region satisfying the following formula (iii) is 90% or more. Nis>0.75 (ii) Mns>0.75 (iii) In the above formulas (ii) and (iii), Nis represents the degree of segregation of Ni, and Mns represents the degree of segregation of Mn.

[0012] (4) An austenitic stainless steel sheet for a liquefied hydrogen storage tank according to (2) above, in which, in the center of the sheet thickness, the area ratio of the region satisfying the following formula (ii) is 90% or more, and the area ratio of the region satisfying the following formula (iii) is 90% or more. Nis>0.75 (ii) Mns>0.75 (iii) In the above formulas (ii) and (iii), Nis represents the degree of segregation of Ni, and Mns represents the degree of segregation of Mn.

[0013] (5) An austenitic stainless steel sheet for a liquefied hydrogen storage tank according to any one of (1) to (4) above, wherein, in the center of the sheet thickness, the width of a region satisfying the following formula (iv) is 0.03 mm or less, and the width of a region satisfying the following formula (v) is 0.03 mm or less. Nis<0.75 (iv) Mns<0.75 (v) In the above formulas (iv) and (v), Nis represents the degree of segregation of Ni, and Mns represents the degree of segregation of Mn. [Effects of the Invention]

[0014] According to the present invention, it is possible to obtain an austenitic stainless steel sheet having excellent impact resistance that can suppress fracture even when strain accumulates at extremely low temperatures such as those used to store liquefied hydrogen. DETAILED DESCRIPTION OF THE INVENTION

[0015] The present inventors have investigated the impact resistance characteristics in the environment in which liquefied hydrogen is stored, and have obtained the following findings (a) to (c).

[0016] (a) Large liquefied hydrogen storage tanks have a capacity of tens of thousands m 3These are large-scale building structures. For this reason, it is necessary to use materials that will not break even if an earthquake or other event occurs and strain accumulates. In particular, at the extremely low temperatures at which liquefied hydrogen is stored, specifically at temperatures below -235°C, it is thought that strain accumulation makes it more susceptible to breakage.

[0017] (b) From the viewpoint of hydrogen embrittlement resistance, it is desirable to use austenitic stainless steel plates with a specified chemical composition for liquefied hydrogen storage tanks. It is known that when strain occurs in austenitic stainless steels at the above-mentioned extremely low temperatures, the austenite phase (γ phase) transforms into the α' phase. The α' phase is more brittle and weaker than the γ phase, and therefore has the potential to become the starting point for fracture. Therefore, in order to suppress the initiation and propagation of fracture and improve impact resistance, it is desirable to suppress the transformation to the α' phase.

[0018] To suppress transformation to the α' phase, the chemical composition is set within a specified range. Specifically, the Mn content is increased and Co is an essential element. Furthermore, the M value, which is calculated from the content of each element and is an index of the stability of the γ phase, is set within the range of -90 to -20.

[0019] (c) Furthermore, transformation to the α' phase is likely to occur in areas where the concentration of added elements is uneven, i.e., where segregation occurs. Segregation often occurs when molten steel is melted and solidified, but it usually gradually disappears after processing and heat treatment. However, in the case of thick plates, the degree of processing is smaller than that of thin plates, and the number of heat treatment steps is fewer, so segregation is more likely to remain. Therefore, in order to eliminate segregation, it is preferable to first perform preliminary hot rolling, crush the segregated parts into small pieces, and then perform hot rolling again to eliminate the segregation. In this case, it is preferable to use a higher heating temperature in preliminary hot rolling to eliminate the segregation of Mn. It is also preferable to perform heat treatment after preliminary hot rolling.

[0020] An embodiment of the present invention has been made based on the above findings. Each requirement of this embodiment will be described in detail below.

[0021] 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."

[0022] C: 0.10% or less C is an element effective in stabilizing the austenite phase and also contributes to improving hydrogen gas embrittlement resistance. However, excessive C content promotes the precipitation of Cr-based carbides at grain boundaries, making them more likely to form fracture initiation points. As a result, impact resistance deteriorates. For this reason, the C content is set to 0.10% or less. The C content is preferably set to 0.08% or less, and more preferably set to 0.06% or less. On the other hand, to obtain the above effects, the C content is preferably set to 0.01% or more.

[0023] Si: 1.0% or less Si is an effective element for deoxidation and also contributes to improving hydrogen gas embrittlement resistance. However, excessive Si content promotes the formation of intermetallic compounds such as the σ phase, which makes it easier for fracture initiation points to form. As a result, impact resistance deteriorates. For this reason, the Si content is set to 1.0% or less. The Si content is more preferably set to 0.7% or less. On the other hand, to obtain the above effects, the Si content is preferably set to 0.3% or more.

[0024] Mn: 8.0 to 10.0% Mn is an element effective in stabilizing the austenite phase and contributes to improving hydrogen gas embrittlement resistance. It also has the effect of improving strength. For this reason, the Mn content is set to 8.0% or more. The Mn content is preferably set to 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 gas embrittlement resistance. Furthermore, excessive precipitation of MnS reduces impact resistance. For this reason, the Mn content is set to 10.0% or less. The Mn content is preferably set to 9.5% or less.

[0025] P:0.050% or less P is an element contained in steel as an impurity and is prone to segregation at grain boundaries. This can form fracture origins and reduce impact resistance. For this reason, the P content is set to 0.050% or less. The P content is preferably set to 0.040% or less, and more preferably set to 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 set to 0.010% or more.

[0026] S: 0.0050% or less S is an element contained in steel as an impurity, and can form MnS, which can form fracture origins and reduce impact resistance. Therefore, the S content is set to 0.0050% or less. The S content is preferably set to 0.0040% or less, and more preferably set to 0.0030% or less. However, excessive reduction of the S content increases manufacturing costs. Therefore, the S content is preferably set to 0.0002% or more.

[0027] Cr: 14.0 to 18.0% Cr is an element contained in a certain amount in stainless steel and has the effect of improving corrosion resistance. Therefore, the Cr content is set to 14.0% or more. However, Cr is a ferrite-forming element. Therefore, excessive Cr content destabilizes the austenite phase and reduces hydrogen gas embrittlement resistance. It also reduces impact resistance. Therefore, the Cr content is set to 18.0% or less. The Cr content is preferably set to 17.5% or less, and more preferably to 16.5% or less.

[0028] Mo: 1.0% or less Mo has the effect of improving strength. However, excessive content of Mo promotes the formation of the δ-ferrite phase and reduces hydrogen gas embrittlement resistance. Therefore, the Mo content is set to 1.0% or less. The Mo content is preferably set to 0.7% or less, and more preferably set to 0.5% or less. On the other hand, excessive reduction of the Mo content leads to restrictions on the melting raw materials and increases production costs. Therefore, the Mo content is preferably set to 0.05% or more.

[0029] Ni: 6.0 to 9.0% Ni, together with Mn, is an element necessary for ensuring hydrogen gas embrittlement resistance and impact resistance. Therefore, the Ni content is set to 6.0% or more. However, excessive Ni content increases manufacturing costs and makes segregation more likely to occur. Therefore, the Ni content is set to 9.0% or less. The Ni content is preferably set to 8.5% or less, and more preferably set to 7.6% or less.

[0030] Cu:1.5% or less Cu 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 them prone to forming fracture initiation points. For this reason, the Cu content is set to 1.5% or less. The Cu content is preferably set to 1.0% or less. However, excessive reduction of the Cu content leads to restrictions on the melting raw materials and increases manufacturing costs. For this reason, the Cu content is preferably set to 0.05% or more.

[0031] Co: 0.01 to 1.0% Co is an important element in the steel sheet of this embodiment and has the effect of improving impact resistance. It also has the effect of improving corrosion resistance and stabilizing the austenite phase. For this reason, the Co content is set to 0.01% or more. The Co content is preferably set to 0.05% or more, and more preferably set to 0.1% or more. However, if Co is contained in excess, toughness and workability will decrease. For this reason, the Co content is set to 1.0% or less. The Co content is preferably set to 0.7% or less.

[0032] N: 0.25% or less Like Mn and Ni, N is an element effective in improving hydrogen gas embrittlement resistance. However, excessive N content can cause internal defects such as blowholes during melting, making fracture initiation points more likely to occur. As a result, impact resistance is reduced. For this reason, the N content is set to 0.25% or less. The N content is preferably set to 0.20% or less, and more preferably set to 0.18% or less. On the other hand, to obtain the above effects, the N content is preferably set to 0.02% or more.

[0033] 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 each element will be explained below.

[0034] Al: 0 to 0.10% In addition to being an effective deoxidizing element, Al has the effect of suppressing the grain boundary segregation of low-melting point elements and strengthening the grain boundaries. As a result, it also has the effect of improving impact resistance. Therefore, it may be added as needed. However, since Al is a ferrite-forming element, excessive Al content destabilizes the austenite phase and reduces hydrogen gas embrittlement resistance. For this reason, the Al content is set to 0.10% or less. The Al content is preferably set to 0.05% or less. On the other hand, to obtain the above effects, the Al content is preferably set to 0.01% or more.

[0035] Nb: 0 to 0.10% Nb forms carbonitrides, refines crystal grains, and strengthens grain boundaries. As a result, it has the effect of improving impact resistance. Therefore, it may be added as needed. However, excessive Nb content reduces manufacturability and workability during hot rolling. Furthermore, a large amount of inclusions may be formed, which may reduce impact resistance. For this reason, the Nb content is set to 0.10% or less. The Nb content is preferably set to 0.07% or less. On the other hand, to obtain the above effects, the Nb content is preferably set to 0.01% or more.

[0036] Ti: 0 to 0.10% Ti forms carbonitrides, refines crystal grains, and strengthens grain boundaries. As a result, impact resistance is improved. Therefore, Ti 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 set to 0.10% or less. The Ti content is preferably set to 0.07% or less. On the other hand, to obtain the above effects, the Ti content is preferably set to 0.01% or more.

[0037] B: 0 to 0.0050% B 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 set to 0.0050% or less. The B content is preferably set to 0.0030% or less. On the other hand, in order to obtain the above effects, the B content is preferably set to 0.0002% or more.

[0038] V: 0 to 0.5% V 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 an excessive amount of V is added, excessive carbonitrides are formed, which reduces manufacturability during hot rolling. Also, impact resistance may be reduced. Therefore, the V content is preferably 0.5% or less. The V content is preferably 0.3% or less. On the other hand, to obtain the above effects, the V content is preferably 0.05% or more.

[0039] W: 0 to 0.5% W has the effect of improving strength and corrosion resistance. Therefore, it may be contained as needed. However, excessive W content increases manufacturing costs. Therefore, the W content is set to 0.5% or less. The W content is preferably set to 0.3% or less. On the other hand, in order to obtain the above effects, the W content is preferably set to 0.05% or more.

[0040] Ca: 0 to 0.010% Ca has the effect of suppressing the grain boundary segregation of low-melting point elements and strengthening the grain boundaries. As a result, impact resistance properties are improved. Therefore, it may be added as needed. However, if Ca is added in excess, segregation is more likely to occur and become the starting point for fracture. As a result, impact resistance properties may be reduced. For this reason, the Ca content is set to 0.010% or less. The Ca content is preferably set to 0.005% or less. On the other hand, to obtain the above effects, the Ca content is preferably set to 0.0002% or more.

[0041] Mg: 0 to 0.010% Mg has the effect of suppressing the grain boundary segregation of low-melting-point elements and strengthening the grain boundaries. As a result, impact resistance properties are improved. Therefore, Mg may be added as needed. However, excessive Mg content may result in the formation of a large amount of inclusions, which may easily become the starting point of fracture, resulting in a decrease in impact resistance properties. For this reason, the Mg content is set to 0.010% or less. The Mg content is preferably set to 0.005% or less. On the other hand, to obtain the above effects, the Mg content is preferably set to 0.0002% or more.

[0042] Zr: 0 to 0.50% Zr has a deoxidizing effect and also has the effect of improving corrosion resistance. Therefore, it may be contained as needed. However, excessive Zr content reduces toughness and workability. Furthermore, a large amount of inclusions may be formed, which may easily become the starting point of fracture, resulting in reduced impact resistance. For this reason, the Zr content is set to 0.50% or less. The Zr content is preferably set to 0.30% or less. On the other hand, to obtain the above effects, the Zr content is preferably set to 0.01% or more.

[0043] Ga: 0 to 0.05% Ga has the effect of improving hot workability. Therefore, it may be contained as necessary. However, excessive Ga content reduces manufacturability. Therefore, the Ga content is set to 0.05% or less. The Ga content is preferably set to 0.02% or less. On the other hand, in order to obtain the above effect, the Ga content is preferably set to 0.001% or more.

[0044] Hf: 0 to 0.10% Hf has the effect of improving strength and hydrogen embrittlement resistance. Therefore, it may be contained as needed. However, excessive Hf content reduces workability. Therefore, the Hf content is set to 0.10% or less. The Hf content is preferably set to 0.07% or less. On the other hand, in order to obtain the above effects, the Hf content is preferably set to 0.01% or more.

[0045] REM: 0 to 0.10% REM has the effect of improving hot workability. It also has the effect of improving corrosion resistance. Therefore, it may be added as needed. However, if REM is added in excess, not only will the effect saturate, but the hot workability will also decrease. Therefore, the REM content is set to 0.10% or less. The REM content is preferably set to 0.07% or less. On the other hand, to obtain the above effects, the REM content is preferably set to 0.01% or more.

[0046] REM refers to a total of 17 elements, including Sc, Y, and lanthanides, and the REM content above refers to the total content of these elements. In industry, REM is often added in the form of misch metal.

[0047] In the chemical composition 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.

[0048] M-value The M value calculated by the following formula (i) is an index showing the stability of the γ phase in an austenitic stainless steel sheet. In the austenitic stainless steel sheet of this embodiment, the M value is set to be −90 or more and −20 or less in order to improve the impact resistance.

[0049] M value=551-462(C+N)-9.2Si-8.1Mn-13.7Cr-29(Ni+Cu)-18.2Mo...(i) However, each element symbol in the above formula (i) represents the content (mass%) of each element contained in the steel, and if the element is not contained, it is set to zero.

[0050] If the M value is less than -90, a large amount of additive elements will be required, increasing the alloy cost. Therefore, the M value is set to -90 or more. The M value is preferably set to -80 or more, and more preferably set to -70 or more.

[0051] On the other hand, if the M value exceeds -20, the stability of the γ phase is low, transformation to the α' phase is likely to occur, and impact resistance properties are reduced. Therefore, the M value is set to -20 or less. The M value is preferably set to -25 or less, and more preferably set to -35 or less.

[0052] 2. Plate thickness The thickness of the austenitic stainless steel plate in this embodiment is 4.5 mm or more. This is because the strength required for a liquefied hydrogen storage tank can be ensured by setting the plate thickness within this range. The plate thickness is preferably 10 mm or more, and more preferably 20 mm or more. When used in a larger liquefied hydrogen storage tank, the plate thickness is preferably 30 mm or more. The upper limit of the plate thickness is not particularly limited, but is usually 100 mm.

[0053] 3.Segregation state In this embodiment, the segregation state is controlled to obtain good hydrogen embrittlement resistance and impact resistance. Segregation refers to a phenomenon in which the concentration of solute elements becomes non-uniform during solidification. This segregation state remains in part in the metal structure even after the steel sheet is manufactured, adversely affecting the hydrogen embrittlement resistance and impact resistance. 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, we focus primarily on negative segregation from the perspective of impact resistance.

[0054] 3-1. Area ratio of segregation Specifically, in the austenitic stainless steel sheet of this embodiment, it is preferable that, in the center portion of the sheet thickness, the area ratio of the region satisfying the following formula (ii) is 90% or more, and the area ratio of the region satisfying the following formula (iii) is 90% or more. Nis>0.75 (ii) Mns>0.75 (iii) In the above formulas (ii) and (iii), Nis represents the degree of segregation of Ni, and Mns represents the degree of segregation of Mn.

[0055] In the chemical composition of this embodiment, elements that segregate and affect impact 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 dividing the local Ni concentration by the average Ni content across the entire thickness of the steel sheet, and the Mn segregation degree can be calculated by dividing the local Mn concentration by the average Mn content across the entire thickness of the steel sheet. The closer the segregation degree is to 1, the less segregation there is.

[0056] When Nis is greater than 0.75 and the above formula (ii) is satisfied, no local concentration gradient occurs, and negative segregation is reduced. Similarly, when Mns is greater than 0.75 and the above formula (iii) is satisfied, no local concentration gradient occurs, and negative segregation is reduced. Therefore, in order to reduce negative segregation, it is preferable that the area ratio of the region satisfying the above formula (ii) is 90% or more, and the area ratio of the region satisfying the above formula (iii) is 90% or more, in the center of the sheet thickness, where segregation is most likely to occur. If the area ratio of the region satisfying the formula (ii) is less than 90% or the area ratio of the region satisfying the formula (iii) is less than 90%, many regions will have segregation, and transformation to the α' phase will be more likely to occur. As a result, it becomes difficult to sufficiently improve impact resistance at cryogenic temperatures. For this reason, it is preferable that the area ratio of the region satisfying the formula (ii) is 90% or more, and the area ratio of the region satisfying the formula (iii) is 90% or more. Furthermore, it is more preferable that the area ratio of the region satisfying the formula (ii) is 95% or more, and the area ratio of the region satisfying the formula (iii) is 95% or more.

[0057] The Nis and Mns in the center of the sheet thickness, as well as the area ratio of the region satisfying formula (ii) and the area ratio of the region satisfying formula (iii) can be measured by the following procedure. In a plane perpendicular to the rolled surface (cross section C), the center of the sheet thickness 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 condition A. By the area analysis using EPMA, the Ni and Mn concentrations are mapped, and the area ratio of the region satisfying formula (ii) and the area ratio of the region satisfying formula (iii) are calculated. 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.

[0058] 3-2. Width of negative segregation zone Furthermore, even if the area ratio of the region satisfying formula (ii) and the area ratio of the region satisfying formula (iii) are 90% or more and the area ratio of the region where negative segregation occurs is small, if there is even one part where negative segregation is large, transformation to the α' phase is likely to occur from that part. As a result, it becomes difficult to sufficiently improve the impact resistance.

[0059] Therefore, it is preferable that the width of the region satisfying the following formula (iv) is 0.03 mm or less, and the width of the region satisfying the following formula (v) is 0.03 mm or less, in the center of the plate thickness. Nis<0.75 (iv) Mns<0.75 (v) In the above formulas (iv) and (v), Nis represents the degree of segregation of Ni, and Mns represents the degree of segregation of Mn.

[0060] If the width of the region satisfying formula (iv) (hereinafter also referred to as the "Ni-negative segregation zone") in the thickness center exceeds 0.03 mm, or if the width of the region satisfying formula (v) (hereinafter also referred to as the "Mn-negative segregation zone") exceeds 0.03 mm, it becomes difficult to sufficiently improve impact resistance. Therefore, the widths of both the Ni-negative segregation zone and the Mn-negative segregation zone are preferably 0.03 mm or less, and more preferably 0.02 mm or less. The smaller the Ni-negative segregation zone and the Mn-negative segregation zone, the better.

[0061] The widths of the Ni-negative segregation band and the Mn-negative segregation band at the center of the sheet thickness may be measured by EPMA, as in the case of the segregation degree. For the EPMA observation, as in the case of the segregation degree, the C-section is used as the observation surface, the center of the sheet thickness of this surface is the center of the field of view, and the observation field is a 2 mm square above and below. The widths of each negative segregation band in the sheet thickness direction are measured in this observation field. It is preferable to perform the observation near the center of the sheet width of the steel sheet where segregation is likely to occur, but regardless of the position in the sheet width direction, as long as the above range is satisfied, it is within the scope of the present embodiment. Other measurement conditions are the same as those for the measurement of the segregation degree.

[0062] 4.Applications The austenitic stainless steel sheet of this embodiment is preferably used for liquefied hydrogen storage tanks because it has excellent resistance to hydrogen gas embrittlement and impact resistance at cryogenic temperatures. The austenitic stainless steel sheet of this embodiment has a high Mn content and excellent strength, allowing the thickness of the material for structures to be reduced, resulting in thinner walls.

[0063] 5. Manufacturing method A preferred method for producing the austenitic stainless steel sheet of this embodiment will now be described. The austenitic stainless steel sheet of this embodiment can be stably produced, for example, by the following production method.

[0064] Stainless steel having the above chemical composition is melted and produced into billets such as slabs. Next, the billets are heated to a predetermined temperature and hot rolled (hot rolling process). The heating temperature in hot rolling is preferably in the range of 1100 to 1250°C, and the rolling reduction is preferably 40% or more. This is because by setting the heating temperature and rolling reduction in the above ranges during hot rolling, it becomes easier to control the plate thickness to the desired value.

[0065] Here, it is preferable to perform preliminary hot rolling before the above-mentioned hot rolling. In preliminary hot rolling, it is preferable to heat the steel at a temperature of 1100 to 1250°C and roll it at a reduction rate in the range of 20 to 60%. By performing preliminary hot rolling under the above-mentioned conditions, it is possible to introduce strain into the steel billet and break down segregation finely while promoting the diffusion of Mn. As a result, it is easier to eliminate segregation in the subsequent heat treatment and hot rolling.

[0066] Since the steel of this embodiment has a high Mn content, if the heating temperature in preliminary hot rolling is less than 1100°C, Mn segregation is not sufficiently promoted. As a result, transformation to the α' phase is likely to occur, making it difficult to sufficiently improve impact resistance. For this reason, the heating temperature in preliminary hot rolling is preferably 1100°C or higher, more preferably 1150°C or higher. On the other hand, if the heating temperature in preliminary hot rolling exceeds 1250°C, subsequent coarsening of crystal grains may occur, leading to a deterioration in properties. For this reason, the heating temperature in preliminary hot rolling is preferably 1250°C or lower, more preferably 1230°C or lower. The lower limit of the heating temperature in preliminary hot rolling is not particularly limited, but is preferably, for example, 900°C or higher.

[0067] Furthermore, if the reduction rate in the preliminary hot rolling is less than 20%, it is difficult to introduce sufficient strain. As a result, it becomes difficult to eliminate segregation, and it becomes difficult to sufficiently improve impact resistance. For this reason, the reduction rate in the preliminary hot rolling is preferably 20% or more, and more preferably 40% or more. On the other hand, if the reduction rate in the preliminary hot rolling exceeds 60%, not only does the strain introduction effect saturate, but subsequent grain growth is promoted, resulting in coarsening of the crystal grains. As a result, the strength and hydrogen gas embrittlement resistance tend to decrease. For this reason, the reduction rate in the preliminary hot rolling is preferably 60% or less, and more preferably 55% or less.

[0068] In addition, it is preferable to perform heat treatment after preliminary hot rolling. The heat treatment conditions are preferably in the range of 1200 to 1300°C and for 60 minutes or more. If the heat treatment temperature is less than 1200°C, even if the segregated portions are made finer by preliminary hot rolling, it will be difficult to sufficiently eliminate the segregation thereafter. For this reason, the heat treatment temperature is preferably 1200°C or higher. On the other hand, if the heat treatment temperature exceeds 1200°C, the crystal grains will become coarse, which will in turn reduce the strength and hydrogen gas embrittlement resistance. Furthermore, abnormal oxidation of the steel sheet may occur, which may lead to increased production costs due to the addition of a grinding process, etc. For this reason, the heat treatment temperature is preferably 1300°C or lower.

[0069] The heat treatment time in the above heat treatment is preferably 60 minutes or more, because this allows segregation to be sufficiently eliminated. The upper limit of the heat treatment time is not particularly limited, but is usually 60 to 300 minutes from the viewpoint of manufacturability. Various processes, such as annealing, may be selected as necessary, and finally an appropriate cooling method may be selected to produce an austenitic stainless steel sheet.

[0070] 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. [Example]

[0071] Stainless steel having the chemical composition shown in Table 1 was melted and produced into a slab. The slab was then heated to a temperature range of 1100 to 1250°C and hot rolled. In some examples, prior to hot rolling, preliminary hot rolling was performed at a heating temperature of 1100 to 1250°C with a rolling reduction in the range of 20 to 60%. In some examples, after preliminary hot rolling, heat treatment was performed for 60 minutes at a heat treatment temperature of 1200 to 1300°C. After hot rolling, the slab was annealed for 5 minutes at 1050°C and then pickled to obtain an austenitic stainless steel sheet. The implementation status of each step is as shown in Table 2.

[0072] [Table 1]

[0073] (ii) Area ratio of the region that satisfies the formula (iii) and (ii) Area ratio of the region that satisfies the formula (iii). The area ratio of the region satisfying formula (ii) and the area ratio of the region satisfying formula (iii) were measured for the obtained austenitic stainless steel sheets by the following procedure. In a plane perpendicular to the rolled surface (C cross section), the center of the sheet thickness was set as the center of the field of view, and the observation field was 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 -9Area analysis was performed using EPMA under condition A. Through area analysis using EPMA, the Ni and Mn concentrations were mapped, and the area ratio of the region satisfying formula (ii) and the area ratio of the region satisfying formula (iii) were calculated. In the table, the area ratio of the region satisfying formula (ii) is referred to as the Ni segregation area ratio, and the area ratio of the region satisfying formula (iii) is referred to as the Mn segregation area ratio.

[0074] Width of negative segregation zone The width of the negative segregation band was measured for the obtained austenitic stainless steel sheet. As in the case of the degree of segregation, the C cross section was used as the observation surface, the center of the sheet thickness of this surface was used as the center of the field of view, and a 2 mm square area above and below was used as the observation field, and the width of the negative segregation band was measured. Note that other conditions were the same as those for the measurement of the degree of segregation.

[0075] Impact resistance evaluation The impact resistance properties at cryogenic temperatures were measured using the following procedure. Specifically, Charpy test specimens with a V-notch subsize (55 mm long, 10 mm wide, and 5 mm thick) were prepared. The Charpy test specimens were prepared in the L direction from the center of the plate. A 5% tensile prestrain was applied to the obtained Charpy test specimens at -253°C. A thermocouple was attached to the Charpy test specimen, which was then fixed to a jig and inserted into a cylindrical urethane cooling capsule. The specimen's temperature was measured while liquid helium was flowing through the cooling capsule. After reaching -253°C and holding for 10 seconds, the Charpy test was performed by impacting the cooling capsule from the C direction. The absorbed energy was adjusted by subtracting the cooling capsule (3.53 J). Note that all other conditions were in accordance with JIS Z 2242:2018.

[0076] The fracture surface of the test piece was visually observed, and if cracks were found, the fracture crack item was marked with "x". Furthermore, when the fracture surface was observed with an SEM at 500x magnification, if microcracks were found on the fracture surface, the fracture crack item was marked with "△". Furthermore, when the fracture surface was observed with an SEM, if no cracks were found, the fracture crack item was marked with "○".

[0077] Based on the above results, the Charpy impact value is 100J / cm 2 If the crack on the fracture surface is less than 100 J / cm, the item for the impact property judgment is marked as ×. Also, even if the item for crack on the fracture surface is marked as ×, the Charpy impact value is 100 J / cm. 2 If the crack on the fracture surface was rated as △ or ○, and the Charpy impact value was 100 J / cm or more, the impact property was judged as △. 2 More than 140J / cm 2 If the crack on the fracture surface was rated as ○ and the Charpy impact value was 140 J / cm or less, the impact properties were judged as ○. 2 In the above cases, the impact properties were judged as excellent. The results are summarized in Table 2 below.

[0078] [Table 2]

[0079] Test Nos. 1 to 11 satisfied the requirements of this embodiment and therefore had good impact resistance properties at cryogenic temperatures, whereas Test Nos. 12 to 21 did not satisfy the requirements of this embodiment and therefore had poor impact resistance properties at cryogenic temperatures. [Industrial Applicability]

[0080] The austenitic stainless steel sheet of this embodiment has excellent hydrogen embrittlement resistance and impact resistance, and is therefore suitable for use in steel sheets with a capacity of 10,000 m 3 This is suitable for the above-mentioned large land-based liquefied hydrogen storage tanks.

Claims

1. The 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: 14.0-18.0%, 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: 0-0.10%, Nb: 0 to 0.10%, Ti: 0 to 0.10%, B: 0 to 0.0050%, V: 0 to 0.5%, W: 0 to 0.5%, Ca: 0-0.010%, Mg: 0 to 0.010%, Zr: 0 to 0.50%, Ga: 0-0.05%, Hf: 0-0.10%, REM: 0-0.10%, The balance is Fe and impurities. The M value calculated by the following formula (i) is −90 or more and −20 or less, An austenitic stainless steel plate for a liquefied hydrogen storage tank, having a plate thickness of 4.5 mm or more. M value=551-462(C+N)-9.2Si-8.1Mn-13.7Cr-29(Ni+Cu)-18.2Mo...(i) However, each element symbol in the above formula (i) represents the content (mass %) of each element contained in the steel, and if the element is not contained, it is set to zero.

2. The chemical composition is, in mass %, Al: 0.01-0.10%, Nb: 0.01 to 0.10%, Ti: 0.01 to 0.10%, B: 0.0002 to 0.0050%, V: 0.05-0.5%, W: 0.05-0.5%, Ca: 0.0002-0.010%, Mg: 0.0002 to 0.010%, Zr: 0.01-0.50%, Ga: 0.001-0.05%, Hf: 0.01 to 0.10%, and REM: 0.01-0.10%, 2. The austenitic stainless steel sheet for a liquefied hydrogen storage tank according to claim 1, comprising one or more selected from the following:

3. 2. An austenitic stainless steel sheet for a liquefied hydrogen storage tank according to claim 1, wherein, in the center portion of the sheet thickness, an area ratio of a region satisfying the following formula (ii) is 90% or more, and an area ratio of a region satisfying the following formula (iii) is 90% or more: Nis>0.75... (ii) Mns>0.75... (iii) In the above formulas (ii) and (iii), Nis represents the degree of segregation of Ni, and Mns represents the degree of segregation of Mn.

4. 3. An austenitic stainless steel sheet for a liquefied hydrogen storage tank according to claim 2, wherein, in the center portion of the sheet thickness, an area ratio of a region satisfying the following formula (ii) is 90% or more, and an area ratio of a region satisfying the following formula (iii) is 90% or more: Nis>0.75... (ii) Mns>0.75... (iii) In the above formulas (ii) and (iii), Nis represents the degree of segregation of Ni, and Mns represents the degree of segregation of Mn.

5. 5. An austenitic stainless steel sheet for a liquefied hydrogen storage tank according to claim 1, wherein, in the center portion of the sheet thickness, a width of a region satisfying the following formula (iv) is 0.03 mm or less, and a width of a region satisfying the following formula (v) is 0.03 mm or less. Nis<0.75... (iv) Mns<0.75... (v) In the above formulas (iv) and (v), Nis represents the degree of segregation of Ni, and Mns represents the degree of segregation of Mn.

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