Austenitic stainless steel material

Abstract

Provided is an austenitic stainless steel material having high strength and excellent hydrogen embrittlement resistance under a high-pressure hydrogen gas environment. An austenitic stainless steel material according to the present disclosure has a chemical composition comprising, in mass %, C: 0.100% or less, Si: 1.000% or less, Mn: 8.00-20.00%, P: 0.050% or less, S: 0.0050% or less, Cr: 18.00-30.00%, Ni: 10.00-25.00%, N: more than 0.350 and less than 0.700%, V: 0.010-0.200%, Nb: 0.010-0.300%, Al: 0.200% or less, O: 0.0100% or less, with the remainder being composed of Fe and impurities. Furthermore, in said composition, Fn1, as defined by formula (1) in the specification, is 18.0 or more when the Ni content is less than 17.00% and is 22.0 or more when the Ni content is 17.00% or more, and in the steel material, the number density of a (Cr, Nb) composite precipitate having an equivalent circle diameter of 50 to 500 nm is 1.00 piece / μm2 or more.

Description

Austenitic stainless steel material

[0001] The present disclosure relates to austenitic stainless steel materials.

[0002] Recently, active research has been conducted into the practical application of transportation equipment, such as fuel cell vehicles, which use hydrogen as an energy source, and hydrogen stations that supply hydrogen to such transportation equipment. These hydrogen stations and transportation equipment are equipped with tanks that store high-pressure hydrogen and piping for high-pressure hydrogen. Hydrogen embrittlement is a problem for high-pressure hydrogen tanks and piping. Hydrogen embrittlement is a phenomenon in which hydrogen penetration into steel significantly impairs its ductility and toughness. In other words, austenitic stainless steel materials used for high-pressure hydrogen tanks and piping must have excellent resistance to hydrogen embrittlement in a high-pressure hydrogen gas environment.

[0003] Furthermore, the austenitic stainless steel materials used in tanks and piping for high-pressure hydrogen are required to have high strength in order to withstand high-pressure hydrogen.

[0004] Japanese Patent Application Laid-Open No. 2018-135592 (Patent Document 1) and Japanese Patent Application Laid-Open No. 2021-139007 (Patent Document 2) propose techniques for improving the hydrogen embrittlement resistance and strength of austenitic stainless steel materials.

[0005] The austenitic stainless steel material disclosed in Patent Document 1 is an austenitic stainless steel for high-pressure hydrogen, which contains, by mass%, C: 0.40 to 1.00%, Si: 1.00% or less, Mn: 2.00% or less, P: 0.040% or less, S: 0.030% or less, Ni: 8.00 to 14.00%, Cr: 16.00 to 21.00%, N: 0.09% or less, the balance being Fe and impurity elements, and further satisfies formula 1 (54.8C + 3.7Ni + 2.5Mn - 1.6Cr - 0.9Si + 266N - 39.6 > 0), is used as is after solution heat treatment, and has an area ratio of 23% or more of Cr carbides in the steel. Patent Document 1 discloses that this austenitic stainless steel material is inexpensive in terms of its components, and can achieve excellent yield strength and hardness as is after solution heat treatment without relying on cold working to improve strength, and also has excellent hydrogen embrittlement resistance at low temperatures.

[0006] The austenitic stainless steel material disclosed in Patent Document 2 contains, in mass %, C: 0.100% or less, Si: 1.00% or less, Mn: 1.50 to 6.00%, P: 0.050% or less, S: 0.030% or less, Ni: 4.0 to 12.0%, Cr: 17.0 to 19.0%, N: 0.12 to 0.30%, Nb: 0.01 to 0.20%, V: 0.01 to 0.10%, Mo: 0 to 0.10%, Cu: 0. The austenitic stainless steel material is characterized in that it contains 0.5% or less of Ni, the balance being Fe and impurities, and satisfies formulas (1) to (3), and in that in a cross section perpendicular to the longitudinal direction of the austenitic stainless steel material, the ratio A0 / A1 of the austenite area fraction A0 (%) at the center of the cross section to the austenite area fraction A1 (%) at a depth of 5 mm from the surface of the austenitic stainless steel material is 0.990 to 1.010. Patent Document 2 discloses that this austenitic stainless steel material is capable of achieving both high strength and excellent hydrogen embrittlement resistance, and has stable yield strength. -7.1+2.7Ni+0.49Cr+2.0Mo-2.0Si+0.75Mn-5.7C-24N≧10.00 (1) Ni+0.72Cr+0.88Mo+1.11Mn-0.27Si+12.93C+0.53Cu+7.55N≧25.00 (2) C+N≧0.22 (3)

[0007] JP 2018-135592 A JP 2021-139007 A

[0008] The austenitic stainless steel materials disclosed in the above-mentioned Patent Documents 1 and 2 achieve both high strength and improved hydrogen embrittlement resistance. However, an austenitic stainless steel material having high strength and excellent hydrogen embrittlement resistance in a high-pressure hydrogen gas environment may be obtained by means other than those disclosed in Patent Documents 1 and 2.

[0009] An object of the present disclosure is to provide an austenitic stainless steel material having high strength and excellent resistance to hydrogen embrittlement in a high-pressure hydrogen gas environment.

[0010] The austenitic stainless steel material according to the present disclosure has a chemical composition, in mass%, of C: 0.100% or less, Si: 1.000% or less, Mn: 8.00 to 20.00%, P: 0.050% or less, S: 0.0050% or less, Cr: 18.00 to 30.00%, Ni: 10.00 to 25.00%, N: more than 0.350 and less than 0.700%, V: 0.010 to 0.200%, Nb: 0.010 to 0.300%, Al: 0.200% or less, O: 0.0100% or less, Mo: 0 to 1.00%, W: 0 to 2.00%, Ti: 0 to 0.100%, Zr: 0 to 0.100%, Hf: 0 to 0.100%, Ta: 0 to 0.200%, Cu: 0 to 1.00%, Sn: 0 to 0.05%, Co: 0 to 2.00%, B: 0 to 0.020%, Mg: 0 to 0.0050%, Ca: 0 to 0.0050%, rare earth elements: 0 to 0.5000%, and the balance being Fe and impurities, wherein the chemical composition further satisfies Fn1 defined by formula (1) of 18.0 or more when the Ni content is less than 17.00%, and Fn1 defined by formula (1) of 22.0 or more when the Ni content is 17.00% or more, and wherein the number density of (Cr, Nb) composite precipitates having an equivalent circle diameter of 50 to 500 nm in the austenitic stainless steel material is 1.00 precipitates / μm ２ That is all. Fn1=Ni+0.02×Cr+0.52×Mn−0.48×Mo (1) Here, for each element symbol in formula (1), the content of the corresponding element in the chemical composition is substituted in mass %.

[0011] The austenitic stainless steel material according to the present disclosure has high strength and excellent resistance to hydrogen embrittlement in a high-pressure hydrogen gas environment.

[0012] The present inventors first investigated, from the standpoint of chemical composition, how to obtain high strength and excellent hydrogen embrittlement resistance for austenitic stainless steel materials intended for use in high-pressure hydrogen gas environments.

[0013] It has been known that increasing the nitrogen (N) content of austenitic stainless steel increases its strength. However, simply increasing the N content of austenitic stainless steel may not always result in a sufficient increase in the N content. Therefore, the present inventors conducted detailed studies to identify element contents that can increase the N content. As a result of their detailed studies, it was discovered that the N content in a steel can be increased by adjusting the balance of the chromium (Cr), nickel (Ni), manganese (Mn), and molybdenum (Mo) contents. Specifically, the present inventors discovered that by adjusting the Cr content to 18.00 to 30.00%, Ni content to 10.00 to 25.00%, Mn content to 8.00 to 20.00%, and Mo content to 0 to 1.00%, the N content can be stably increased to more than 0.350% but less than 0.700%.

[0014] That is, in mass%, C: 0.100% or less, Si: 1.000% or less, Mn: 8.00 to 20.00%, P: 0.050% or less, S: 0.0050% or less, Cr: 18.00 to 30.00%, Ni: 10.00 to 25.00%, N: more than 0.350 but less than 0.700%, V: 0.010 to 0.200%, Nb: 0.010 to 0.300%, Al: 0.200% or less, O: 0.0100% or less, Mo: 0 to 1.00%, W: 0 to 2.00%, Ti: 0 to 0.100% , Zr: 0 to 0.100%, Hf: 0 to 0.100%, Ta: 0 to 0.200%, Cu: 0 to 1.00%, Sn: 0 to 0.05%, Co: 0 to 2.00%, B: 0 to 0.020%, Mg: 0 to 0.0050%, Ca: 0 to 0.0050%, rare earth elements: 0 to 0.5000%, and the balance being Fe and impurities, there is a possibility that an austenitic stainless steel material having a chemical composition consisting of:

[0015] On the other hand, with the above-mentioned chemical composition, it has sometimes been impossible to achieve both high strength and excellent hydrogen embrittlement resistance in a high-pressure hydrogen gas environment. Specifically, the inventors have conducted detailed studies on factors that reduce the hydrogen embrittlement resistance in a high-pressure hydrogen gas environment in austenitic stainless steel materials having the above-mentioned chemical composition. As a result, it has become clear that the high N content in austenitic stainless steel materials having the above-mentioned chemical composition tends to reduce stacking fault energy.

[0016] When stacking fault energy is low, austenite is easily destabilized. As a result, the sensitivity of austenite to hydrogen increases, which may reduce hydrogen embrittlement resistance in a high-pressure hydrogen gas environment. Therefore, the present inventors considered stabilizing austenite in an austenitic stainless steel material with low stacking fault energy by satisfying the above-mentioned chemical composition containing a high N content. If austenite is stabilized, it may be possible to achieve both high strength and excellent hydrogen embrittlement resistance in a high-pressure hydrogen gas environment, even if stacking fault energy is reduced by satisfying the above-mentioned chemical composition containing a high N content.

[0017] Therefore, the present inventors focused on the stability of austenite in the microstructure of an austenitic stainless steel material having the above-described chemical composition and conducted detailed studies on methods for improving hydrogen embrittlement resistance in a high-pressure hydrogen gas environment while maintaining strength. As a result of detailed studies by the present inventors, it was revealed that in an austenitic stainless steel material having the above-described chemical composition, if Fn1, defined by the following formula (1), satisfies a predetermined value depending on the Ni content, hydrogen embrittlement resistance in a high-pressure hydrogen gas environment can be improved, provided that the other components of this embodiment are satisfied: Fn1 = Ni + 0.02 × Cr + 0.52 × Mn - 0.48 × Mo (1) Here, each element symbol in formula (1) is substituted with the content of the corresponding element in the chemical composition in mass %. When an element is not contained, "0" is substituted for the corresponding element symbol.

[0018] Fn1, as defined by formula (1), is an index of austenite stability in an austenitic stainless steel material having the above-described chemical composition. As a result of detailed studies by the present inventors, it has become clear that, assuming the above-described chemical composition, if the Ni content is 10.00 to less than 17.00%, and if Fn1 is 18.0 or more, and if the Ni content is 17.00 to 25.00%, and if Fn1 is 22.0 or more, hydrogen embrittlement resistance in a high-pressure hydrogen gas environment can be improved while maintaining strength. Therefore, the austenitic stainless steel material according to this embodiment satisfies the above-described chemical composition and further has Fn1 of 18.0 or more, or 22.0 or more, depending on the Ni content.

[0019] The present inventors further investigated a method for increasing the strength of austenitic stainless steel materials having the above-mentioned chemical composition containing Fn1 while maintaining hydrogen embrittlement resistance in a high-pressure hydrogen gas environment, and as a result, obtained the following findings.

[0020] The present inventors have investigated ways to increase the strength of steel materials by forming fine precipitates in the steel material. If fine precipitates can be dispersed in the steel material, the strength of the steel material can be increased. However, there is a concern that the hydrogen embrittlement resistance of the steel material may be reduced depending on the type of precipitates. Therefore, the present inventors have conducted a detailed investigation into fine precipitates that are less likely to reduce the hydrogen embrittlement resistance of the steel material, among the precipitates in the austenitic stainless steel material having the above-mentioned chemical composition including Fn1.

[0021] As a result of investigations by the present inventors, it has become clear that in an austenitic stainless steel material having the above-mentioned chemical composition containing Fn1, the strength of the steel material can be increased by precipitating a large number of (Cr, Nb) composite precipitates having an equivalent circle diameter of 50 to 500 nm. On the other hand, simply increasing the amount of (Cr, Nb) composite precipitates having an equivalent circle diameter of 50 to 500 nm in an attempt to increase the strength of the steel material has resulted in the formation of coarse precipitates and inclusions, which in turn coarsen the crystal grains due to abnormal grain growth, resulting in a decrease in the hydrogen embrittlement resistance of the steel material.

[0022] As a result of further detailed studies by the present inventors based on the above findings, it was found that in an austenitic stainless steel material having the above-mentioned chemical composition containing Fn1, the number density of (Cr, Nb) composite precipitates having a circle equivalent diameter of 50 to 500 nm is 1.00 particles / μm ２ Therefore, the austenitic stainless steel material according to this embodiment has the above-mentioned chemical composition containing Fn1, and further has a number density of (Cr, Nb) composite precipitates having a circle equivalent diameter of 50 to 500 nm in the steel material of 1.00 particles / μm ２ As a result, the austenitic stainless steel material according to this embodiment can achieve both high strength and excellent hydrogen embrittlement resistance in a high-pressure hydrogen gas environment.

[0023] The austenitic stainless steel material according to this embodiment, which was completed based on the above findings, has the following features.

[0024] [1] An austenitic stainless steel material having a chemical composition, in mass%, of C: 0.100% or less, Si: 1.000% or less, Mn: 8.00 to 20.00%, P: 0.050% or less, S: 0.0050% or less, Cr: 18.00 to 30.00%, Ni: 10.00 to 25.00%, N: more than 0.350 and less than 0.700%, V: 0.010 to 0.200%, Nb: 0.010 to 0.300%, Al: 0.200% or less, O: 0.0100% or less, Mo: 0 to 1.00%, W: 0 to 2.00%, Ti: 0 to 0.100%, Zr: 0 to 0.100%, Hf: 0 to 0.100%, Ta: 0 to 0.200%, Cu: 0 to 1.00%, Sn: 0 to 0.05%, Co: 0 to 2.00%, B: 0 to 0.020%, Mg: 0 to 0.0050%, Ca: 0 to 0.0050%, rare earth elements: 0 to 0.5000%, and the balance being Fe and impurities, wherein the chemical composition further satisfies Fn1 defined by formula (1) of 18.0 or more when the Ni content is less than 17.00%, and Fn1 defined by formula (1) of 22.0 or more when the Ni content is 17.00% or more, and wherein the number density of (Cr, Nb) composite precipitates having an equivalent circle diameter of 50 to 500 nm in the austenitic stainless steel material is 1.00 precipitates / μm ２ An austenitic stainless steel material having the above formula: Fn1=Ni+0.02×Cr+0.52×Mn−0.48×Mo (1) where each element symbol in formula (1) is substituted with the content of the corresponding element in the chemical composition in mass %.

[0025] [2] The austenitic stainless steel material according to [1], wherein the chemical composition is Mo: 0.01 to 1.00%, W: 0.01 to 2.00%, Ti: 0.001 to 0.100%, Zr: 0.001 to 0.100%, Hf: 0.001 to 0.100%, Ta: 0.001 to 0.200%, Cu: 0.01 to 1.00%, Sn: 0.01 to 0.05%, Co: 0.01 to 2.00%, B: 0.001 to 0.020%, Mg: 0.0001 to 0.0050%, Ca: 0.0001 to 0.0050%, and An austenitic stainless steel material containing one or more elements selected from the group consisting of: rare earth elements: 0.0001 to 0.5000%.

[0026] The austenitic stainless steel material according to this embodiment will be described in detail below. Unless otherwise specified, "%" regarding elements means mass %. In the following description, the austenitic stainless steel material will also be simply referred to as "steel material."

[0027] [Chemical Composition] The chemical composition of the austenitic stainless steel material according to this embodiment contains the following elements.

[0028] C: 0.100% or less Carbon (C) is unavoidably contained. In other words, the lower limit of the C content is greater than 0%. C forms carbides to increase the strength of the steel. C also dissolves in the steel to increase its strength. However, if the C content is too high, C forms excessive carbides at grain boundaries, reducing the hydrogen embrittlement resistance of the steel. Therefore, the C content is 0.100% or less. A preferred upper limit of the C content is 0.090%, more preferably 0.080%, even more preferably 0.070%, even more preferably 0.060%, and even more preferably 0.050%. To more effectively obtain the above effects, a preferred lower limit of the C content is 0.001%, even more preferably 0.002%, and even more preferably 0.003%.

[0029] Si: 1.000% or less Silicon (Si) is inevitably contained. That is, the lower limit of the Si content is greater than 0%. Si deoxidizes steel. However, if the Si content is too high, Si forms an excessive amount of intermetallic compounds, which reduces the hot workability and hydrogen embrittlement resistance of the steel. Therefore, the Si content is 1.000% or less. A preferred upper limit of the Si content is 0.900%, more preferably 0.800%, even more preferably 0.700%, and even more preferably 0.600%. On the other hand, excessive reduction of the Si content increases production costs. Therefore, considering normal industrial production, a preferred lower limit of the Si content is 0.001%, more preferably 0.005%, and even more preferably 0.010%.

[0030] Mn: 8.00 to 20.00% Manganese (Mn) increases the amount of dissolved N and increases the N content in steel. As a result, the strength of the steel increases. Mn also stabilizes austenite, improving the hydrogen embrittlement resistance of the steel. If the Mn content is too low, the above effects cannot be fully achieved. On the other hand, if the Mn content is too high, the hot workability of the steel decreases. Therefore, the Mn content is 8.00 to 20.00%. The preferred lower limit of the Mn content is 8.01%, more preferably 8.10%, even more preferably 8.30%, and even more preferably 8.50%. The preferred upper limit of the Mn content is 19.50%, even more preferably 19.00%, even more preferably 18.00%, even more preferably 17.00%, and even more preferably 16.00%.

[0031] P: 0.050% or less Phosphorus (P) is an unavoidable impurity. In other words, the lower limit of the P content is more than 0%. P reduces the hot workability and hydrogen embrittlement resistance of steel. Therefore, the P content is 0.050% or less. A preferred upper limit of the P content is 0.040%, more preferably 0.030%, and even more preferably 0.025%. The P content is preferably as low as possible. However, excessive reduction of the P content increases manufacturing costs. Therefore, considering normal industrial production, a preferred lower limit of the P content is 0.001%, and even more preferably 0.002%.

[0032] S: 0.0050% or less Sulfur (S) is an unavoidable impurity. In other words, the lower limit of the S content is more than 0%. S reduces the hot workability and hydrogen embrittlement resistance of steel. Therefore, the S content is 0.0050% or less. A preferred upper limit of the S content is 0.0040%, more preferably 0.0030%, and even more preferably 0.0020%. The S content is preferably as low as possible. However, excessive reduction of the S content increases manufacturing costs. Therefore, considering normal industrial production, a preferred lower limit of the S content is 0.0001%, more preferably 0.0002%, and even more preferably 0.0003%.

[0033] Cr: 18.00 to 30.00% Chromium (Cr) increases the amount of dissolved N and increases the N content in the steel material. As a result, it increases the strength of the steel material. If the Cr content is too low, the above effect cannot be sufficiently obtained. On the other hand, if the Cr content is too high, Cr nitrides are formed, and the hydrogen embrittlement resistance of the steel material decreases. Therefore, the Cr content is 18.00 to 30.00%. The lower limit of the Cr content is preferably 18.50%, more preferably 19.00%, and even more preferably 19.50%. The upper limit of the Cr content is preferably 29.50%, more preferably 29.10%, and even more preferably 28.70%.

[0034] Ni: 10.00 to 25.00% Nickel (Ni) stabilizes austenite and improves the hydrogen embrittlement resistance of steel. If the Ni content is too low, the above effect cannot be sufficiently obtained. On the other hand, if the Ni content is too high, the N content in the steel is not sufficiently increased, and the strength of the steel decreases. Therefore, the Ni content is 10.00 to 25.00%. A preferred lower limit of the Ni content is 10.05%, more preferably 10.10%, and even more preferably 10.30%. A preferred upper limit of the Ni content is 24.50%, more preferably 24.00%, even more preferably 23.00%, even more preferably 22.00%, and even more preferably 21.00%.

[0035] As described above, in the austenitic stainless steel material according to this embodiment, the value of Fn1 defined by formula (1) is adjusted depending on the Ni content. Specifically, when the Ni content is 10.00 to less than 17.00%, Fn1 is set to 18.0 or more. When the Ni content is 10.00 to less than 17.00%, the upper limit of the Ni content is preferably 16.99%, more preferably 16.90%, even more preferably 16.80%, even more preferably 16.50%, and even more preferably 16.30%. Furthermore, when the Ni content is 17.00 to 25.00%, Fn1 is set to 22.0 or more. When the Ni content is 17.00 to 25.00%, the lower limit of the Ni content is preferably 17.05%, even more preferably 17.10%, and even more preferably 17.30%.

[0036] N: More than 0.350% and less than 0.700% Nitrogen (N) dissolves in steel to increase the strength of the steel. If the N content is too low, the above effect cannot be sufficiently obtained. On the other hand, if the N content is too high, the hot workability of the steel decreases. Therefore, the N content is more than 0.350% and less than 0.700%. The lower limit of the N content is preferably 0.351%, more preferably 0.360%, and even more preferably 0.370%. The upper limit of the N content is preferably 0.699%, more preferably 0.690%, even more preferably 0.680%, and even more preferably 0.670%.

[0037] V: 0.010 to 0.200% Vanadium (V) forms precipitates to increase the strength of steel. If the V content is too low, the above effect cannot be sufficiently obtained. On the other hand, if the V content is too high, coarse precipitates are formed, and the hydrogen embrittlement resistance of the steel decreases. Therefore, the V content is 0.010 to 0.200%. The lower limit of the V content is preferably 0.015%, more preferably 0.020%, and even more preferably 0.025%. The upper limit of the V content is preferably 0.180%, more preferably 0.150%, and even more preferably 0.120%.

[0038] Nb: 0.010 to 0.300% Niobium (Nb) forms (Cr, Nb) composite precipitates in steel, thereby increasing the strength of the steel. If the Nb content is too low, the above effect cannot be sufficiently obtained. On the other hand, if the Nb content is too high, coarse precipitates are formed, reducing the hydrogen embrittlement resistance of the steel. Therefore, the Nb content is 0.010 to 0.300%. The lower limit of the Nb content is preferably 0.015%, more preferably 0.020%, and even more preferably 0.025%. The upper limit of the Nb content is preferably 0.250%, more preferably 0.200%, and even more preferably 0.150%.

[0039] Al: 0.200% or less Aluminum (Al) is inevitably contained. In other words, the lower limit of the Al content is greater than 0%. Al deoxidizes steel. Even if even a small amount of Al is contained, the above effect can be obtained to some extent. However, if the Al content is too high, oxides and intermetallic compounds are formed in the steel. In this case, the hydrogen embrittlement resistance of the steel decreases. Therefore, the Al content is 0.200% or less. A preferred upper limit of the Al content is 0.190%, more preferably 0.180%, and even more preferably 0.170%. A preferred lower limit of the Al content to more effectively obtain the above effect is 0.001%, and even more preferably 0.002%. In this specification, the Al content refers to the content of sol. Al (acid-soluble Al).

[0040] O: 0.0100% or less Oxygen (O) is an unavoidably contained impurity. In other words, the lower limit of the O content is more than 0%. O reduces the hot workability of steel. Therefore, the O content is 0.0100% or less. The O content is preferably as low as possible. However, excessive reduction of the O content increases manufacturing costs. Therefore, considering normal industrial production, the preferred lower limit of the O content is 0.0001%, and more preferably 0.0002%. The upper limit of the O content is preferably 0.0090%, more preferably 0.0080%, and even more preferably 0.0060%.

[0041] The balance of the chemical composition of the austenitic stainless steel material according to this embodiment is composed of Fe and impurities. Here, the term "impurities" refers to substances that are mixed in from raw materials such as ore or scrap or the manufacturing environment during industrial production of the austenitic stainless steel material, but are not intentionally contained and are allowed to a range that does not adversely affect the austenitic stainless steel material according to this embodiment.

[0042] [Regarding optional elements] The chemical composition of the austenitic stainless steel material according to this embodiment further contains: Mo: 0-1.00%, W: 0-2.00%, Ti: 0-0.100%, Zr: 0-0.100%, Hf: 0-0.100%, Ta: 0-0.200%, Cu: 0-1.00%, Sn: 0-0.05%, Co: 0-2.00%, B: 0-0.020%, Mg: 0-0.0050%, Ca: 0-0.0050%, and rare earth elements: 0-0.5000%. All of these elements are optional elements and do not necessarily need to be contained. In other words, the chemical composition of the austenitic stainless steel material according to this embodiment is such that, in place of a portion of Fe, Mo: 0.01 to 1.00%, W: 0.01 to 2.00%, Ti: 0.001 to 0.100%, Zr: 0.001 to 0.100%, Hf: 0.001 to 0.100%, Ta: 0.001 to 0.200%, Cu: 0.01 to 1.00%, Sn: 0.01 to 0.05%, Co: 0.01 to 2.00%, B: 0.001 to 0.020%, Mg: 0.0001 to 0.0050%, Ca: 0.0001 to 0.0050%, and Rare earth elements: 0.0001 to 0.5000%. These optional elements will be explained below.

[0043] [Regarding Mo and W] The chemical composition of the austenitic stainless steel material according to this embodiment may contain, in place of a portion of Fe, one or more elements selected from the group consisting of Mo and W. Both of these elements increase the strength of the steel material.

[0044] Mo: 0 to 1.00% Molybdenum (Mo) is an optional element and does not necessarily need to be contained. In other words, the Mo content may be 0%. When contained, Mo dissolves in steel to increase the strength of the steel. Even if even a small amount of Mo is contained, the above effect can be obtained to some extent. However, if the Mo content is too high, the strength of the steel becomes too high and the toughness of the steel decreases. Therefore, the Mo content is 0 to 1.00%, and when contained, the Mo content is 1.00% or less, that is, more than 0 to 1.00%. The preferred lower limit of the Mo content is 0.01%, and more preferably 0.02%. The preferred upper limit of the Mo content is 0.95%, more preferably 0.90%, even more preferably 0.80%, and even more preferably 0.70%.

[0045] W: 0 to 2.00% Tungsten (W) is an optional element and does not necessarily need to be included. In other words, the W content may be 0%. When included, W dissolves in steel to increase the strength of the steel. Even if even a small amount of W is included, the above effects can be achieved to some extent. However, if the W content is too high, the strength of the steel becomes too high and the toughness of the steel decreases. Therefore, the W content is 0 to 2.00%, and when included, it is 2.00% or less, that is, more than 0 to 2.00%. The preferred lower limit of the W content is 0.01%, more preferably 0.05%, and even more preferably 0.10%. The preferred upper limit of the W content is 1.90%, more preferably 1.80%, even more preferably 1.70%, even more preferably 1.60%, and even more preferably 1.50%.

[0046] [Regarding Ti, Zr, Hf, and Ta] The chemical composition of the austenitic stainless steel material according to this embodiment may contain one or more elements selected from the group consisting of Ti, Zr, Hf, and Ta in place of a portion of Fe. All of these elements increase the strength of the steel material.

[0047] Ti: 0 to 0.100% Titanium (Ti) is an optional element and does not necessarily need to be contained. In other words, the Ti content may be 0%. When contained, Ti forms nitrides and refines crystal grains through a pinning effect. As a result, Ti enhances the hydrogen embrittlement resistance of steel. Even if even a small amount of Ti is contained, the above effect can be achieved to some extent. However, if the Ti content is too high, excessive nitrides are formed, reducing the toughness of the steel. Therefore, the Ti content is 0 to 0.100%, and when contained, it is 0.100% or less, i.e., more than 0 to 0.100%. The preferred lower limit of the Ti content is 0.001%, more preferably 0.005%, and even more preferably 0.010%. The preferred upper limit of the Ti content is 0.090%, more preferably 0.080%, even more preferably 0.075%, and even more preferably 0.060%.

[0048] Zr: 0 to 0.100% Zirconium (Zr) is an optional element and does not necessarily need to be contained. In other words, the Zr content may be 0%. When contained, Zr forms nitrides and refines crystal grains through a pinning effect. As a result, Zr enhances the hydrogen embrittlement resistance of steel. Even if even a small amount of Zr is contained, the above effect can be achieved to some extent. However, if the Zr content is too high, excessive nitrides are formed, reducing the toughness of the steel. Therefore, the Zr content is 0 to 0.100%, and when contained, it is 0.100% or less, that is, more than 0 to 0.100%. The preferred lower limit of the Zr content is 0.001%, more preferably 0.005%, and even more preferably 0.010%. The preferred upper limit of the Zr content is 0.090%, more preferably 0.080%, even more preferably 0.070%, and even more preferably 0.060%.

[0049] Hf: 0 to 0.100% Hafnium (Hf) is an optional element and does not necessarily need to be contained. In other words, the Hf content may be 0%. When contained, Hf forms nitrides and refines crystal grains through a pinning effect. As a result, Hf enhances the hydrogen embrittlement resistance of steel. Even if even a small amount of Hf is contained, the above effect can be achieved to some extent. However, if the Hf content is too high, excessive nitrides are formed, reducing the toughness of the steel. Therefore, the Hf content is 0 to 0.100%, and when contained, it is 0.100% or less, that is, more than 0 to 0.100%. The preferred lower limit of the Hf content is 0.001%, more preferably 0.003%, and even more preferably 0.005%. The preferred upper limit of the Hf content is 0.095%, more preferably 0.090%, and even more preferably 0.080%.

[0050] Ta: 0 to 0.200% Tantalum (Ta) is an optional element and does not necessarily need to be contained. In other words, the Ta content may be 0%. When contained, Ta forms nitrides and refines crystal grains through a pinning effect. As a result, Ta enhances the hydrogen embrittlement resistance of steel. Even if even a small amount of Ta is contained, the above effect can be achieved to some extent. However, if the Ta content is too high, excessive nitrides are formed, reducing the toughness of the steel. Therefore, the Ta content is 0 to 0.200%, and when contained, it is 0.200% or less, i.e., more than 0 to 0.200%. The preferred lower limit of the Ta content is 0.001%, more preferably 0.003%, and even more preferably 0.005%. The preferred upper limit of the Ta content is 0.190%, more preferably 0.180%, even more preferably 0.170%, even more preferably 0.160%, and even more preferably 0.150%.

[0051] [Regarding Cu, Sn, and Co] The chemical composition of the austenitic stainless steel material according to this embodiment may contain, in place of a portion of Fe, one or more elements selected from the group consisting of Cu, Sn, and Co. All of these elements stabilize the austenite in the steel material and improve the hydrogen embrittlement resistance of the steel material.

[0052] Cu: 0 to 1.00% Copper (Cu) is an optional element and does not necessarily need to be contained. In other words, the Cu content may be 0%. When contained, Cu stabilizes austenite and improves the hydrogen embrittlement resistance of the steel. Even if even a small amount of Cu is contained, the above effect can be obtained to some extent. However, if the Cu content is too high, the hot workability of the steel deteriorates. Therefore, the Cu content is 0 to 1.00%, and when contained, the Cu content is 1.00% or less, that is, more than 0 to 1.00%. The preferred lower limit of the Cu content is 0.01%, more preferably 0.05%, and even more preferably 0.10%. The preferred upper limit of the Cu content is 0.90%, more preferably 0.85%, and even more preferably 0.80%.

[0053] Sn: 0 to 0.05% Tin (Sn) is an optional element and does not necessarily need to be contained. In other words, the Sn content may be 0%. When contained, Sn stabilizes austenite and improves the hydrogen embrittlement resistance of the steel. Even if even a small amount of Sn is contained, the above effect can be obtained to some extent. However, if the Sn content is too high, the manufacturing cost of the steel increases. Therefore, the Sn content is 0 to 0.05%, and when contained, the Sn content is 0.05% or less, that is, more than 0 to 0.05%. The preferred lower limit of the Sn content is 0.01%, and more preferably 0.02%. The preferred upper limit of the Sn content is 0.04%, and more preferably 0.03%.

[0054] Cobalt (Co) is an optional element and does not necessarily need to be contained. In other words, the Co content may be 0%. When contained, Co stabilizes austenite and improves the hydrogen embrittlement resistance of the steel. Even if even a small amount of Co is contained, the above effect can be obtained to some extent. However, if the Co content is too high, the manufacturing cost of the steel increases. Therefore, the Co content is 0 to 2.00%, and when contained, the Co content is 2.00% or less, that is, more than 0 to 2.00%. The preferred lower limit of the Co content is 0.01%, more preferably 0.10%, even more preferably 0.50%, and even more preferably 1.00%. The preferred upper limit of the Co content is 1.90%, even more preferably 1.80%, and even more preferably 1.70%.

[0055] [Regarding B, Mg, Ca, and rare earth elements] The chemical composition of the austenitic stainless steel material according to this embodiment may contain one or more elements selected from the group consisting of B, Mg, Ca, and rare earth elements in place of a portion of Fe. All of these elements improve the hot workability of the steel material.

[0056] B: 0 to 0.020% Boron (B) is an optional element and does not necessarily need to be contained. In other words, the B content may be 0%. When contained, B neutralizes S in the steel by fixing it as sulfides, improving the hot workability of the steel. Even if even a small amount of B is contained, the above effect can be achieved to some extent. However, if the B content is too high, numerous nitrides are formed, which actually reduces the hot workability of the steel. Therefore, the B content is 0 to 0.020%, and when contained, it is 0.020% or less, that is, more than 0 to 0.020%. The preferred lower limit of the B content is 0.001%, more preferably 0.002%, and even more preferably 0.003%. The preferred upper limit of the B content is 0.018%, more preferably 0.015%, even more preferably 0.010%, and even more preferably 0.008%.

[0057] Mg: 0 to 0.0050% Magnesium (Mg) is an optional element and does not necessarily need to be contained. In other words, the Mg content may be 0%. When contained, Mg neutralizes S in the steel by fixing it as sulfides, improving the hot workability of the steel. Even if even a small amount of Mg is contained, the above effect can be achieved to some extent. However, if the Mg content is too high, coarse oxides are formed, which actually reduces the hot workability of the steel. Therefore, the Mg content is 0 to 0.0050%, and when contained, it is 0.0050% or less, that is, more than 0 to 0.0050%. The preferred lower limit of the Mg content is 0.0001%. The preferred upper limit of the Mg content is 0.0045%, more preferably 0.0040%, even more preferably 0.0030%, even more preferably 0.0020%, and even more preferably 0.0010%.

[0058] Ca: 0 to 0.0050% Calcium (Ca) is an optional element and does not necessarily need to be contained. In other words, the Ca content may be 0%. When contained, Ca neutralizes S in the steel by fixing it as sulfides, improving the hot workability of the steel. Even if even a small amount of Ca is contained, the above effect can be achieved to some extent. However, if the Ca content is too high, coarse oxides are formed, which actually reduces the hot workability of the steel. Therefore, the Ca content is 0 to 0.0050%, and when contained, it is 0.0050% or less, that is, more than 0 to 0.0050%. The preferred lower limit of the Ca content is 0.0001%. The preferred upper limit of the Ca content is 0.0045%, more preferably 0.0040%, even more preferably 0.0030%, even more preferably 0.0020%, and even more preferably 0.0010%.

[0059] Rare Earth Elements: 0 to 0.5000% Rare earth elements (REM) are optional elements and do not necessarily need to be included. In other words, the REM content may be 0%. When included, REM neutralize the S in the steel by fixing it as sulfides, improving the hot workability of the steel. Even if even a small amount of REM is included, the above effect can be achieved to some extent. However, if the REM content is too high, coarse oxides are formed, which actually reduces the hot workability of the steel. Therefore, the REM content is 0 to 0.5000%, and when included, it is 0.5000% or less, that is, more than 0 to 0.5000%. The preferred lower limit of the REM content is 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%. The upper limit of the REM content is preferably 0.4900%, more preferably 0.4800%, even more preferably 0.4500%, even more preferably 0.4200%, and still more preferably 0.4000%.

[0060] In this specification, REM refers to one or more elements selected from the group consisting of scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanoids lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. In addition, the REM content in this specification refers to the total content of these elements.

[0061] [Formula (1)] The austenitic stainless steel material according to this embodiment has the above-described chemical composition, and when the Ni content is 10.00 to less than 17.00%, Fn1 defined by formula (1) is 18.0 or more, and when the Ni content is 17.00 to 25.00%, Fn1 defined by formula (1) is 22.0 or more. Fn1 = Ni + 0.02 × Cr + 0.52 × Mn - 0.48 × Mo (1) Here, the content of the corresponding element in the chemical composition is substituted for each element symbol in formula (1) in mass %. Note that when an element is not contained, "0" is substituted for the corresponding element symbol.

[0062] In addition, when the chemical composition of the austenitic stainless steel material does not contain the optional element Mo (i.e., when the Mo content is 0%), formula (1) can be transformed into the following formula (1A). In this case, in the austenitic stainless steel material according to this embodiment, when the Ni content is 10.00 to less than 17.00%, Fn1A defined by formula (1A) is 18.0 or more, and when the Ni content is 17.00 to 25.00%, Fn1A defined by formula (1A) is 22.0 or more. Fn1A = Ni + 0.02 × Cr + 0.52 × Mn (1A) Here, the content of the corresponding element in the chemical composition is substituted for each element symbol in formula (1A) in mass %.

[0063] Fn1 (= Ni + 0.02 × Cr + 0.52 × Mn - 0.48 × Mo) and Fn1A (= Ni + 0.02 × Cr + 0.52 × Mn) are indicators of austenite stability in austenitic stainless steel materials having the above-mentioned chemical composition. When the Ni content is 10.00 to less than 17.00%, increasing Fn1 to 18.0 or more can improve hydrogen embrittlement resistance in a high-pressure hydrogen gas environment while maintaining strength. Similarly, when the Ni content is 17.00 to 25.00%, increasing Fn1 to 22.0 or more can improve hydrogen embrittlement resistance in a high-pressure hydrogen gas environment while maintaining strength.

[0064] Therefore, in the austenitic stainless steel material according to this embodiment, while satisfying the above-mentioned chemical composition, when the Ni content is 10.00 to less than 17.00%, Fn1 is set to 18.0 or more, and when the Ni content is 17.00 to 25.00%, Fn1 is set to 22.0 or more. When the Ni content is 10.00 to less than 17.00%, the preferred lower limit of Fn1 is 18.1, more preferably 18.2, and even more preferably 18.3. When the Ni content is 17.00 to 25.00%, the preferred lower limit of Fn1 is 22.1, more preferably 22.2, and even more preferably 22.3. Although there are no particular limitations on the upper limit of Fn1, in the case of an austenitic stainless steel material having the above-mentioned chemical composition, the upper limit of Fn1 is substantially 36.0. The upper limit of Fn1 may be 35.0, 33.0, 30.0, or 28.0.

[0065] [(Cr, Nb) composite precipitates] The austenitic stainless steel material according to this embodiment has the above-described chemical composition containing Fn1, and further, in the austenitic stainless steel material, the number density of (Cr, Nb) composite precipitates having a circle equivalent diameter of 50 to 500 nm is 1.00 particles / μm ２ As a result, the austenitic stainless steel material according to this embodiment can achieve both high strength and excellent hydrogen embrittlement resistance in a high-pressure hydrogen gas environment. Hereinafter, in this specification, (Cr, Nb) composite precipitates having an equivalent circle diameter of 50 to 500 nm are also referred to as "specific precipitates."

[0066] The specific precipitates are finely dispersed in the austenitic stainless steel material, thereby increasing the strength of the steel material. The specific precipitates further refine the crystal grains, increasing the strength of the steel material, and also increasing the hydrogen embrittlement resistance of the steel material. In an austenitic stainless steel material having the above-mentioned chemical composition containing Fn1, if the number density of the specific precipitates is too low, the amount of the specific precipitates may be insufficient, and the strength may not be sufficiently increased. On the other hand, even if the amount of the specific precipitates is large, if the number density of the precipitates is too low, the coarse specific precipitates will dominate, and the hydrogen embrittlement resistance of the steel material will decrease. Therefore, in an austenitic stainless steel material having the above-mentioned chemical composition containing Fn1, the number density of the specific precipitates is 1.00 particles / μm２ If the above conditions are met, an austenitic stainless steel material having both high strength and excellent hydrogen embrittlement resistance can be obtained.

[0067] Therefore, the austenitic stainless steel material according to this embodiment has the above-mentioned chemical composition containing Fn1, and further, in the austenitic stainless steel material, the number density of the specific precipitates is 1.00 particles / μm ２ As a result, the austenitic stainless steel material according to this embodiment can achieve both high strength and excellent hydrogen embrittlement resistance.

[0068] In this embodiment, the preferred lower limit of the number density of the specific precipitates is 1.05 particles / μm ２ and more preferably 1.10 pieces / μm ２ and more preferably 1.20 pieces / μm ２ On the other hand, the upper limit of the number density of the specific precipitates is not particularly limited, but for example, it is 5.00 particles / μm ２ The upper limit of the number density of the specific precipitates is 4.50 particles / μm ２ 4.00 pieces / μm ２ 3.50 pieces / μm ２ may be.

[0069] In this embodiment, the number density of the specific precipitates is 1.00 particles / μm ２ However, in the austenitic stainless steel material having the above-mentioned chemical composition containing Fn1, the number density of the specific precipitates is 1.00 particles / μm ２ In the above cases, the amount of the specific precipitate is substantially 0.010 mass% or more. A more preferable lower limit of the amount of the specific precipitate is 0.015 mass%, even more preferably 0.020 mass%, and even more preferably 0.025 mass%. The upper limit of the amount of the specific precipitate is not particularly limited, and may be, for example, 0.300 mass%, 0.250 mass%, or 0.200 mass%.

[0070] In the austenitic stainless steel material having the above-mentioned chemical composition including Fn1, most of the specific precipitates are nitrides. That is, in the austenitic stainless steel material according to this embodiment, the number density of (Cr, Nb) composite nitrides having a circle equivalent diameter of 50 to 500 nm is 1.00 particles / μm ２ That's all. Here, (Cr, Nb) composite nitrides with an equivalent circle diameter of 50 to 500 nm precipitate in a temperature range of about 1060°C. (Cr, Nb) composite nitrides with an equivalent circle diameter of 50 to 500 nm are also relatively stable precipitates even at high temperatures. Therefore, the specific precipitates precipitated in the steel refine the austenite grain size through a pinning effect. As a result, it is believed that the strength can be further increased and hydrogen embrittlement resistance can be improved.

[0071] In this embodiment, the number density of specific precipitates in an austenitic stainless steel material is determined by the following method. Specifically, a micro-specimen for preparing an extraction replica is prepared from the austenitic stainless steel material. The size of the micro-specimen is, for example, 10 mm x 10 mm. After mirror-polishing the surface of the micro-specimen, the micro-specimen is immersed in a 3% nital etching solution for 10 minutes to etch the surface. The etched surface is covered with a carbon vapor-deposited film. The micro-specimen with its surface covered with the vapor-deposited film is immersed in a 5% nital etching solution for 20 minutes. The vapor-deposited film is peeled off from the immersed micro-specimen. The vapor-deposited film peeled off from the micro-specimen is washed with ethanol, then scooped up with a sheet mesh, and dried. In this embodiment, a Cu sheet mesh is used.

[0072] This vapor-deposited film (replica film) is observed with a transmission electron microscope (TEM). Specifically, an arbitrary position is identified from the vapor-deposited film, and the observation is performed at an acceleration voltage of 200 kV. The size of the observation field is, for example, 4.0 μm × 4.0 μm. In each observation field, particles with an equivalent circle diameter of 50 to 500 nm or less are identified. The particles can be identified from the contrast. In this specification, the term "particle" is not limited to circular (spherical) particles, but may also refer to small pieces having an angular shape or elongated oval pieces. The equivalent circle diameter of the precipitate can be determined by image analysis of the image observed in TEM observation.

[0073] The identified particles having an equivalent circle diameter of 50 to 500 nm or less are subjected to point analysis by energy dispersive X-ray spectrometry (EDS). The content of elements contained in each particle is determined by the EDS point analysis. In the EDS point analysis, the acceleration voltage is 200 kV. The target elements for point analysis are Fe, Nb, Cr, Si, Mn, V, Mo, Cu, and Ti. As a result of the elemental analysis by EDS performed under the above conditions, particles having a total content of Cr and Nb of 20 mass% or more are defined as "specific precipitates."

[0074] The number density (number / μm) of the specific precipitates was calculated based on the total number of specific precipitates identified in any three visual fields and the total area of ​​the visual fields. ２ In this embodiment, the number density of the specific precipitates is determined by rounding the obtained value to two decimal places.

[0075] In this embodiment, the amount of specific precipitates in the austenitic stainless steel material is determined by the following method. Specifically, a test piece is prepared from the austenitic stainless steel material. Note that the test piece is not particularly limited as long as it can provide an observation surface as described below.

[0076] After polishing the observation surface of the test piece to a mirror finish, the test piece is immersed in an electrolyte (10% acetylacetone-1% tetramethylammonium chloride-methanol solution) and electrolysis is performed. The orientation of the observation surface is not particularly limited. Here, the difference in mass between the test piece before electrolysis and the test piece after electrolysis is defined as the amount of electrolysis. The electrolyte after electrolysis is passed through a 0.2 μm filter to capture residue. The mass of the captured residue is measured. Furthermore, the obtained residue is acid-decomposed and ICP (inductively coupled plasma) optical emission spectrometry is performed. The mass of Cr and Nb in the residue is quantified from the Cr and Nb concentrations in the residue and the mass of the residue. The mass of Cr and Nb in the residue is divided by the amount of electrolysis to define the concentration (mass%) of Cr and Nb elements. In this embodiment, the obtained concentration (mass%) of Cr and Nb elements is defined as the precipitation amount (mass%) of specific precipitates in the austenitic stainless steel material. In the austenitic stainless steel material according to this embodiment, the residue captured by the above-described method has an equivalent circle diameter of 50 to 500 nm.

[0077] [Tensile strength] The austenitic stainless steel material according to this embodiment has the above-mentioned chemical composition containing Fn1, and further has (Cr, Nb) composite precipitates with a circle equivalent diameter of 50 to 500 nm in the steel material at a number density of 1.00 particles / μm ２ As a result, the austenitic stainless steel material according to this embodiment can achieve both high strength and excellent hydrogen embrittlement resistance in a high-pressure hydrogen gas environment. In this embodiment, the term "steel material has high strength" specifically means that the steel material has a tensile strength of 950 MPa or more.

[0078] The lower limit of the tensile strength is preferably 955 MPa, more preferably 960 MPa, even more preferably 970 MPa, and even more preferably 980 MPa. The upper limit of the tensile strength is not particularly limited, but is, for example, 1300 MPa. The upper limit of the tensile strength may be 1250 MPa, 1200 MPa, 1180 MPa, less than 1150 MPa, or 1149 MPa.

[0079] In this embodiment, the tensile strength of the austenitic stainless steel material is determined by the following method. Specifically, a round bar tensile test specimen is prepared from the austenitic stainless steel material. Here, if the austenitic stainless steel material is a steel pipe, the round bar tensile test specimen is prepared from the center position of the wall thickness. In this case, the longitudinal direction of the round bar tensile test specimen is parallel to the axial direction of the steel pipe. If the austenitic stainless steel material is a steel bar, the round bar tensile test specimen is prepared from the R / 2 position. In this case, the longitudinal direction of the round bar tensile test specimen is parallel to the axial direction of the steel bar. If the austenitic stainless steel material is a steel plate, the round bar tensile test specimen is prepared from the center position of the plate width and the center position of the plate thickness. In this case, the longitudinal direction of the round bar tensile test specimen is parallel to the rolling direction of the steel plate. The size of the round bar tensile test specimen is, for example, 8 mm in parallel part diameter.

[0080] The prepared round bar tensile test specimen is subjected to a tensile test at room temperature in air in accordance with JIS Z 2241:2011 to determine the tensile strength (MPa). In this embodiment, the tensile strength (MPa) is determined by rounding the obtained value to one decimal place.

[0081] [Hydrogen embrittlement resistance] The austenitic stainless steel material according to this embodiment has the above-mentioned chemical composition containing Fn1, and further has (Cr, Nb) composite precipitates with a circle equivalent diameter of 50 to 500 nm in the steel material at a number density of 1.00 particles / μm ２ As a result, the austenitic stainless steel material according to this embodiment can achieve both high strength and excellent hydrogen embrittlement resistance in a high-pressure hydrogen gas environment. Here, in this embodiment, the hydrogen embrittlement resistance of the steel material is evaluated according to the Ni content.

[0082] [When the Ni content is 10.00 to less than 17.00%] Specifically, a round bar tensile test specimen is prepared from the austenitic stainless steel material. Here, when the austenitic stainless steel material is a steel pipe, the round bar tensile test specimen is prepared from the center position of the wall thickness. In this case, the longitudinal direction of the round bar tensile test specimen is parallel to the axial direction of the steel pipe. When the austenitic stainless steel material is a steel bar, the round bar tensile test specimen is prepared from the R / 2 position. In this case, the longitudinal direction of the round bar tensile test specimen is parallel to the axial direction of the steel bar. When the austenitic stainless steel material is a steel plate, the round bar tensile test specimen is prepared from the center position of the plate width and the center position of the plate thickness. In this case, the longitudinal direction of the round bar tensile test specimen is parallel to the rolling direction of the steel plate. The size of the round bar tensile test specimen is, for example, 3 mm in parallel part diameter.

[0083] The prepared round bar tensile test specimens are subjected to a slow strain rate test (SSRT). Specifically, the specimens are subjected to a 3×10 tensile strain rate test at room temperature in the air. －６ A tensile test is carried out at a strain rate of 3 × 10 / s to determine the breaking elongation L0 (unit: %) in air. Furthermore, using another round bar tensile test piece, a strain rate of 3 × 10 / s is carried out in 45 MPa hydrogen gas at room temperature. －６ A tensile test is carried out at 1000 kJ / sec to determine the breaking elongation L1 (unit: %) in 45 MPa hydrogen gas. The ratio of the breaking elongation L1 (%) in high-pressure hydrogen gas to the breaking elongation L0 (%) in air is defined as the relative breaking elongation L. In other words, the relative breaking elongation L is defined as L = L1 / L0.

[0084] In this embodiment, when the Ni content is 10.00 to less than 17.00%, if the relative fracture elongation L obtained under the above conditions is 0.80 or more, the steel material is evaluated as having excellent hydrogen embrittlement resistance in a high-pressure hydrogen gas environment. In other words, when the Ni content is 10.00 to less than 17.00%, the austenitic stainless steel material according to this embodiment has a relative fracture elongation L obtained under the above conditions of 0.80 or more.

[0085] [When the Ni content is 17.00 to 25.00%] When the Ni content is 17.00 to 25.00%, similarly to when the Ni content is 10.00 to less than 17.00%, round bar tensile test specimens are prepared from the austenitic stainless steel material. A slow strain rate tensile test (SSRT) is performed on the prepared round bar tensile test specimens. At this time, the test is performed under the same conditions as when the Ni content is 10.00 to less than 17.00%, except that the hydrogen gas pressure is set to 85 MPa. That is, at room temperature in the atmosphere, a 3×10 －６ A tensile test is carried out at a strain rate of 3 × 10 / s to determine the breaking elongation L0 (unit: %) in air. Furthermore, using another round bar tensile test piece, a strain rate of 3 × 10 / s is carried out in 85 MPa hydrogen gas at room temperature. －６ A tensile test is carried out at 1000 kJ / sec to determine the breaking elongation L1 (unit: %) in 85 MPa hydrogen gas. The ratio of the breaking elongation L1 (%) in high-pressure hydrogen gas to the breaking elongation L0 (%) in air is defined as the relative breaking elongation L. In other words, the relative breaking elongation L is defined as L = L1 / L0.

[0086] In this embodiment, when the Ni content is 17.00 to 25.00%, if the relative fracture elongation L obtained under the above conditions is 0.80 or more, the steel material is evaluated as having excellent hydrogen embrittlement resistance in a high-pressure hydrogen gas environment. In other words, when the Ni content is 17.00 to 25.00%, the austenitic stainless steel material according to this embodiment has a relative fracture elongation L obtained under the above conditions of 0.80 or more.

[0087] [Austenite grain size] The austenitic stainless steel material according to this embodiment has the above-described chemical composition containing Fn1, and further contains (Cr, Nb) composite precipitates (specific precipitates) with a circle equivalent diameter of 50 to 500 nm in the steel material at a number density of 1.00 particles / μm ２ Preferably, the austenitic stainless steel material according to this embodiment has an austenite grain size (austenite grain size) of 20.0 μm or less.

[0088] As described above, the specific precipitates contained in the austenitic stainless steel material according to this embodiment refine the austenite grains by the pinning effect. ２ In the austenitic stainless steel material according to this embodiment containing the above, the austenite grains are refined, resulting in an austenite grain size of 20.0 μm or less.

[0089] The upper limit of the austenite grain size is more preferably 19.8 μm, even more preferably 19.5 μm, and even more preferably 19.0 μm. In this embodiment, a smaller austenite grain size is preferable. The lower limit of the austenite grain size is, for example, 4.0 μm.

[0090] In this embodiment, the crystal grain size of austenite grains in an austenitic stainless steel material is determined by the following method. Specifically, a test piece for microstructure observation is prepared from the austenitic stainless steel material. Here, when the austenitic stainless steel material is a steel pipe, a test piece having an observation surface including the pipe axial direction and the wall thickness direction is prepared. When the austenitic stainless steel material is a steel bar, a test piece having an observation surface including the axial direction and the cross-sectional radial direction is prepared. When the austenitic stainless steel material is a steel plate, a test piece having an observation surface including the rolling direction and the plate thickness direction is prepared. Note that the size of the test piece is not particularly limited as long as it is possible to obtain the observation surface described below.

[0091] The observation surface of the prepared test piece is polished to a mirror finish, and then electrolytic corrosion is performed using 10% oxalic acid-90% distilled water to reveal the austenite grain boundaries. The observation surface is observed using an optical microscope. The magnification for the microscope observation can be set appropriately depending on the grain size. Specifically, in the microscope observation, the magnification is set so that 50 or more austenite grains are included in the field of view, for example. Through the optical microscope observation, the grain size (μm) of the austenite grains is determined based on the average intercept length measurement method in accordance with JIS G 0551 (2020). In this embodiment, the grain size (μm) of the austenite grains is determined by rounding the obtained value to one decimal place.

[0092] [Shape of Austenitic Stainless Steel Material] The shape of the austenitic stainless steel material of this embodiment is not particularly limited. The austenitic stainless steel material of this embodiment may be a steel pipe, a steel plate, or a steel bar.

[0093] [Applications of Austenitic Stainless Steel Material] The austenitic stainless steel material according to this embodiment can be widely used in applications requiring high strength and hydrogen embrittlement resistance. Examples of such applications include steel materials for fuel tanks of transportation equipment that uses hydrogen as energy, and steel materials for piping connecting fuel tanks to combustion chambers. However, the austenitic stainless steel material according to this embodiment is not limited to applications in transportation equipment that uses high-pressure hydrogen gas as energy, or in hydrogen stations that supply hydrogen gas to transportation equipment.

[0094] [Manufacturing Method] A method for manufacturing an austenitic stainless steel material according to this embodiment will be described below. The method for manufacturing an austenitic stainless steel material described below is one example of a method for manufacturing an austenitic stainless steel material according to this embodiment. In other words, an austenitic stainless steel material having the above-described configuration may be manufactured by a manufacturing method other than the manufacturing method described below. However, the manufacturing method described below is a preferred example of a method for manufacturing an austenitic stainless steel material according to this embodiment.

[0095] An example of a method for manufacturing an austenitic stainless steel material according to this embodiment includes a step of preparing a raw material (preparation step), a step of hot working the prepared raw material to manufacture an intermediate steel material (hot working step), a step of cooling the manufactured intermediate steel material (cooling step), and a step of performing a specific precipitate precipitation treatment on the cooled intermediate steel material (specific precipitate precipitation treatment step). Each step will be described in detail below.

[0096] [Preparation Step] In the preparation step, the following components are added in mass %: C: 0.100% or less, Si: 1.000% or less, Mn: 8.00 to 20.00%, P: 0.050% or less, S: 0.0050% or less, Cr: 18.00 to 30.00%, Ni: 10.00 to less than 17.00%, N: more than 0.350 and less than 0.700%, V: 0.010 to 0.200%, Nb: 0.010 to 0.300%, Al: 0.200% or less, O: 0.0100% or less, Mo: 0 to 1.00%, W: 0 to 2.00%, Ti: 0-0.100%, Zr: 0-0.100%, Hf: 0-0.100%, Ta: 0-0.200%, Cu: 0-1.00%, Sn: 0-0.05%, Co: 0-2.00%, B: 0-0.020%, Mg: 0-0.0050%, Ca: 0-0.0050%, rare earth elements: 0-0.5000%, and the balance being Fe and impurities, and Fn1 (= Ni + 0.02 × Cr + 0.52 × Mn - 0.48 × Mo) ) is 18.0 or more, or a material that satisfies the following criteria by mass: C: 0.100% or less, Si: 1.000% or less, Mn: 8.00 to 20.00%, P: 0.050% or less, S: 0.0050% or less, Cr: 18.00 to 30.00%, Ni: 17.00 to 25.00%, N: more than 0.350 but less than 0.700%, V: 0.010 to 0.200%, Nb: 0.010 to 0.300%, Al: 0.200% or less, O: 0.0100% or less, Mo: 0 to 1 A material having a chemical composition of 0.00%, W: 0-2.00%, Ti: 0-0.100%, Zr: 0-0.100%, Hf: 0-0.100%, Ta: 0-0.200%, Cu: 0-1.00%, Sn: 0-0.05%, Co: 0-2.00%, B: 0-0.020%, Mg: 0-0.0050%, Ca: 0-0.0050%, rare earth elements: 0-0.5000%, and the balance being Fe and impurities, and having an Fn1 of 22.0 or greater is prepared. When manufacturing the material, for example, it can be manufactured by the following method.

[0097] Molten steel having the above-described chemical composition including Fn1 is produced by a known method. The produced molten steel is used to produce a cast material by a known casting method. For example, an ingot is produced by an ingot casting method. A cast piece (slab, bloom, billet, etc.) may be produced by a continuous casting method. The ingot may be subjected to hot working such as blooming or hot forging to produce a slab, bloom, or billet. A material is produced by the above steps.

[0098] [Hot Working Step] In the hot working step, the prepared material is hot worked to produce an intermediate steel material. The hot working is not particularly limited and may be, for example, hot forging, hot rolling, hot extrusion, etc. Hot forging is, for example, stretch forging. Hot rolling is, for example, performing multiple passes using a reverse rolling mill or a tandem rolling mill. Hot extrusion is, for example, hot extrusion using the Eugène-Séjournet method. The intermediate steel material is produced by the above manufacturing steps. The heating temperature before hot working is, for example, 1250°C.

[0099] Preferably, the temperature range from the start to the end of hot working is 1100 to 1250°C, and the time is 4 minutes or less. Here, "from the start to the end of hot working" means the time from when the material is extracted from the heating furnace to when the final hot working is completed. In other words, in the hot working process according to this embodiment, it is preferable that the time from when the material is extracted from the heating furnace to when the final hot working is completed is 4 minutes or less, and that the temperature of the material is 1100 to 1250°C from when the material is extracted from the heating furnace to when the final hot working is completed.

[0100] If the time from the start to the end of hot working is too long and / or the temperature range from the start to the end of hot working is too low, (Cr, Nb) complex nitrides precipitate and coarsen in the intermediate steel material during hot working. In this case, the amount of solute Nb in the steel material decreases, and the amount of fine (Cr, Nb) complex nitrides precipitated in the specific precipitate precipitation treatment process described below decreases. In this case, further, in the specific precipitate precipitation treatment process described below, the (Cr, Nb) complex nitrides precipitated during hot working further coarsen. In these cases, the number density of the specific precipitates decreases in the manufactured austenitic stainless steel material. Therefore, in the hot working process according to this embodiment, it is preferable that the temperature range from the start to the end of hot working be 1100 to 1250°C and the time be 4 minutes or less.

[0101] [Cooling Step] In the cooling step, the produced intermediate steel is cooled. The cooling method is not particularly limited and may be a well-known method. Preferably, the intermediate steel after hot working is rapidly cooled (water-cooled) using a water-cooling device. If the cooling rate is too slow, (Cr, Nb) complex nitrides may precipitate in the intermediate steel during cooling. In this case, in the specific precipitate precipitation treatment step described below, the (Cr, Nb) complex nitrides will coarsen and the number density of the specific precipitates will decrease in the produced austenitic stainless steel. Therefore, the cooling rate of the intermediate steel in the cooling step is preferably 15°C / min or more. The cooling stop temperature is not particularly limited, but is, for example, room temperature.

[0102] [Specific precipitate precipitation treatment step] In the specific precipitate precipitation treatment step, the cooled intermediate steel material is subjected to a specific precipitate precipitation treatment. Specifically, the intermediate steel material is held at 1000 to 1230°C for 15 to 90 minutes. Thereafter, the intermediate steel material is quenched. The quenching method is not particularly limited, and examples thereof include water quenching and oil quenching.

[0103] If the holding temperature in the specific precipitate precipitation treatment step is too low, the specific precipitates do not precipitate sufficiently, and a large amount of Cr nitrides is formed. As a result, hydrogen embrittlement resistance is reduced. On the other hand, if the holding temperature is too high, the specific precipitates dissolve in the steel material, and the amount of specific precipitates precipitated in the steel material is reduced. As a result, the desired strength is not obtained. Furthermore, if the holding time in the specific precipitate precipitation treatment step is too short, the specific precipitates do not precipitate sufficiently, and a large amount of Cr nitrides is formed. As a result, hydrogen embrittlement resistance is reduced. On the other hand, if the holding time is too long, austenite grains become coarse, and the desired strength is not obtained. Therefore, in the specific precipitate precipitation treatment step of this embodiment, it is preferable to hold at 1000 to 1230°C for 15 to 90 minutes.

[0104] The austenitic stainless steel material according to this embodiment can be manufactured by the above-described manufacturing method. Note that the above-described manufacturing method is one example, and other manufacturing methods may also be used. The effects of the austenitic stainless steel material according to this embodiment will be explained more specifically below using examples. Note that the conditions in the following examples are one example of conditions adopted to confirm the feasibility and effects of the austenitic stainless steel material according to this embodiment. Therefore, the austenitic stainless steel material according to this embodiment is not limited to this one example of conditions.

[0105] In Example 1, the effects of the austenitic stainless steel material according to this embodiment were confirmed when the Ni content was 10.00 to less than 17.00%. Specifically, molten steels having the chemical compositions shown in Tables 1-1 and 1-2 were melted in a high-frequency melting furnace to produce ingots. Table 2 also shows Fn1 calculated from the chemical composition of each test number and formula (1).

[0106]

[0107]

[0108]

[0109] The ingots produced for each test number were soaked at 1230°C for 1 hour and then hot forged to produce 70 mm thick blocks. The blocks for each test number were heated to 1230°C and then hot rolled to produce 9 mm thick plates (intermediate steel). In hot rolling, if the temperature range from extraction from the heating furnace to the end of final hot rolling was 1100-1250°C, then "A" is indicated in the "Temperature Range" column of the "Hot Working" section in Table 2. If the temperature range from extraction from the heating furnace to the end of final hot rolling was 950-1250°C, then "B" is indicated in the "Temperature Range" column of the "Hot Working" section in Table 2. Furthermore, if the time from extraction from the heating furnace to the end of final hot rolling was 3 minutes or less, then "A" is indicated in the "Processing Time" column of the "Hot Working" section in Table 2. If the time from removal from the heating furnace to the end of the final hot rolling is 6 to 7 minutes, the "Processing time" column in the "Hot processing" column in Table 2 is shown as "B."

[0110] The intermediate steel material of each test number was cooled to room temperature at a cooling rate of 15°C / min or more. Thereafter, a specific precipitate precipitation treatment was carried out on the intermediate steel material of each test number. The holding temperature (°C) and holding time (min) of the specific precipitate precipitation treatment carried out on each test number are shown in Table 2. Steel plates of each test number were manufactured by the above process. Note that cracks occurred in the steel plate of test number 1-57 during forging or hot working. Therefore, the specific precipitate precipitation treatment was not carried out on this steel plate.

[0111] [Evaluation Tests] The steel plates with each test number were subjected to the following tests: tensile test, specific precipitate amount measurement test, austenite grain size measurement test, and hydrogen embrittlement resistance evaluation test. Note that the steel plate with test number 1-57 developed cracks during forging or hot working. Therefore, the above evaluation tests were not performed on this steel plate.

[0112] [Tensile Test] A tensile test was conducted on the steel plate of each test number using the method described above. Specifically, a round bar tensile test specimen with a parallel section diameter of 8 mm was prepared from the center of the plate thickness of the steel plate of each test number. The axial direction of the round bar tensile test specimen was parallel to the rolling direction of the steel plate. A tensile test was conducted on the round bar tensile test specimen of each test number at room temperature in air in accordance with JIS Z 2241:2011. The tensile strength (MPa) of each test number obtained by the tensile test is shown in the "TS (MPa)" column of Table 2. The yield stress (MPa) of each test number obtained by the tensile test is also shown in the "YS (MPa)" column of Table 2.

[0113] [Specific Precipitate Amount Measurement Test] A specific precipitate amount measurement test was performed on the steel plates of each test number using the method described above. Specifically, test specimens were prepared from the center of the thickness of the steel plates of each test number using the method described above. The observation surface of the test specimen was perpendicular to the rolling direction of the steel plate. Electrolysis was performed on the test specimens of each test number using the method described above to capture residue. At this time, the difference in mass between the test specimen before and after electrolysis was defined as the amount of electrolysis. The obtained residue was acid-decomposed and subjected to ICP optical emission analysis to obtain the Cr and Nb concentrations in the residue. The masses of Cr and Nb in the residue were quantified from the Cr and Nb concentrations in the residue and the mass of the residue, and divided by the amount of electrolysis to obtain the concentrations (mass%) of Cr and Nb elements. The obtained concentrations of Cr and Nb elements were defined as the amount of precipitation of the specific precipitate (mass%). The amount of precipitation of the specific precipitate (mass%) for each test number is shown in Table 2.

[0114] [Austenite Grain Size Measurement Test] An austenite grain size measurement test was performed on the steel plates of each test number using the method described above. Specifically, test specimens for microstructure observation were prepared from the center of the plate thickness of the steel plates of each test number using the method described above. For the test specimens of each test number, austenite grain boundaries were revealed using the method described above, and the observation surface was observed using an optical microscope. The grain size (μm) of the austenite grains was determined using the optical microscope observation based on the average intercept length measurement method in accordance with JIS G 0551 (2020). The obtained austenite grain size (μm) for each test number is shown in the "γ grain size (μm)" column of Table 2.

[0115] [Hydrogen Embrittlement Resistance Evaluation Test] A hydrogen embrittlement resistance evaluation test was conducted for each steel plate according to the method described above. Specifically, a round bar tensile test specimen with a parallel section diameter of 3 mm was prepared from the center of the thickness of each steel plate. The axial direction of the round bar tensile test specimen was parallel to the rolling direction of the steel plate. A slow strain rate tensile test (SSRT) was conducted for each round bar tensile test specimen at room temperature in air to determine the fracture elongation L0 (%) in air. Another round bar tensile test specimen for each test number was conducted for each test specimen at room temperature in 45 MPa hydrogen gas to determine the fracture elongation L1 (%) in high-pressure hydrogen gas. In the SSRT, the strain rate was set to 3 × 10 －６ / sec. Furthermore, the relative breaking elongation L (L = L1 / L0) was determined for each test number. The relative breaking elongation L obtained for each test number is shown in Table 2.

[0116] [Evaluation Results] With reference to Tables 1-1, 1-2, and 2, the steel sheets of test numbers 1-1 to 1-53 had the above-described chemical compositions, Fn1 was 18.0 or more, and the manufacturing method also satisfied the above-described preferable conditions. These steel sheets had a number density of specific precipitates of 1.00 particles / μm ２ As a result, these steel sheets had a tensile strength of 950 MPa or more and had high strength. As a result, these steel sheets also had a relative fracture elongation L of 0.80 or more and had excellent hydrogen embrittlement resistance in a high-pressure hydrogen gas environment. Note that the amount of specific precipitates in all of these steel sheets was 0.010 mass% or more.

[0117] On the other hand, the steel plate of test number 1-54 had an excessively high Cr content and an excessively low N content, resulting in a relative fracture elongation L of less than 0.80, and the steel plate did not have excellent hydrogen embrittlement resistance in a high-pressure hydrogen gas environment.

[0118] The steel plate of test number 1-55 had an excessively low Cr content, resulting in a tensile strength of less than 950 MPa, and thus not having high strength.

[0119] The steel plate of test number 1-56 had an excessively low Ni content, resulting in a relative fracture elongation L of less than 0.80, and thus not having excellent hydrogen embrittlement resistance in a high-pressure hydrogen gas environment.

[0120] The steel plate of test number 1-58 had an excessively low Mn content, resulting in a relative fracture elongation L of less than 0.80, and thus not having excellent hydrogen embrittlement resistance in a high-pressure hydrogen gas environment.

[0121] The steel plate of test number 1-59 had an excessively low N content, resulting in a tensile strength of less than 950 MPa, and thus did not have high strength.

[0122] The steel plate of test number 1-60 had an excessively high Nb content, resulting in a relative fracture elongation L of less than 0.80, and thus not having excellent hydrogen embrittlement resistance in a high-pressure hydrogen gas environment.

[0123] The steel plate of test number 1-61 had a too low Nb content, and the number density of specific precipitates was 1.00 particles / μm ２ As a result, the tensile strength of this steel plate was less than 950 MPa, and it did not have high strength.

[0124] The steel plate of test number 1-62 had an Fn1 of less than 18.0, and as a result, the relative fracture elongation L of this steel plate was less than 0.80, and the steel plate did not have excellent hydrogen embrittlement resistance in a high-pressure hydrogen gas environment.

[0125] In the steel sheet of test number 1-63, the holding temperature in the specific precipitate precipitation treatment was too high, and the number density of the specific precipitates was 1.00 particles / μm ２ As a result, the tensile strength of this steel plate was less than 950 MPa, and it did not have high strength.

[0126] The steel plate of test number 1-64 was hot worked for too long, and the number density of specific precipitates was 1.00 pieces / μm ２ As a result, the relative fracture elongation L of this steel sheet was less than 0.80, and the steel sheet did not have excellent hydrogen embrittlement resistance in a high-pressure hydrogen gas environment.

[0127] In Example 2, the effects of the austenitic stainless steel material according to this embodiment were confirmed when the Ni content was 17.00 to 25.00%. Specifically, molten steels having the chemical compositions shown in Tables 3-1 and 3-2 were melted in a high-frequency melting furnace to produce ingots. Table 4 also shows the Fn1 calculated from the chemical composition of each test number and formula (1).

[0128]

[0129]

[0130]

[0131] The ingots produced for each test number were soaked at 1230°C for 1 hour and then hot forged to produce 70 mm thick blocks. The blocks for each test number were heated to 1230°C and then hot rolled to produce 9 mm thick plates (intermediate steel). In hot rolling, if the temperature range from extraction from the heating furnace to the end of final hot rolling was 1100-1250°C, then "A" is indicated in the "Temperature Range" column of the "Hot Working" section in Table 4. If the temperature range from extraction from the heating furnace to the end of final hot rolling was 950-1250°C, then "B" is indicated in the "Temperature Range" column of the "Hot Working" section in Table 4. Furthermore, if the time from extraction from the heating furnace to the end of final hot rolling was 3 minutes or less, then "A" is indicated in the "Processing Time" column of the "Hot Working" section in Table 4. If the time from extraction from the heating furnace to the end of the final hot rolling is 6 to 7 minutes, the "Processing time" column in the "Hot processing" column in Table 4 is indicated as "B."

[0132] The intermediate steel material of each test number was cooled to room temperature at a cooling rate of 15°C / min or more. Thereafter, a specific precipitate precipitation treatment was carried out on the intermediate steel material of each test number. The holding temperature (°C) and holding time (min) of the specific precipitate precipitation treatment carried out on each test number are shown in Table 4. Steel plates of each test number were manufactured by the above process. Note that cracks occurred in the steel plate of test number 2-59 during forging or hot working. Therefore, the specific precipitate precipitation treatment was not carried out on this steel plate.

[0133] [Evaluation Tests] The steel plates of each test number were subjected to the following tests: tensile test, specific precipitate amount measurement test, austenite grain size measurement test, and hydrogen embrittlement resistance evaluation test. Note that the steel plate of test number 2-59 developed cracks during forging or hot working. Therefore, the above evaluation tests were not performed on this steel plate.

[0134] [Tensile Test] A tensile test was performed on the steel plate of each test number using the method described above. Specifically, a round bar tensile test specimen with a parallel section diameter of 8 mm was prepared from the center of the plate thickness of the steel plate of each test number. The axial direction of the round bar tensile test specimen was parallel to the rolling direction of the steel plate. A tensile test was performed on the round bar tensile test specimen of each test number at room temperature in air in accordance with JIS Z 2241:2011. The tensile strength (MPa) of each test number obtained by the tensile test is shown in the "TS (MPa)" column of Table 4. The yield stress (MPa) of each test number obtained by the tensile test is also shown in the "YS (MPa)" column of Table 4.

[0135] [Specific Precipitate Amount Measurement Test] A specific precipitate amount measurement test was performed on the steel plates of each test number using the method described above. Specifically, test specimens were prepared from the center of the thickness of the steel plates of each test number using the method described above. The observation surface of the test specimen was perpendicular to the rolling direction of the steel plate. Electrolysis was performed on the test specimens of each test number using the method described above to capture residue. The difference in mass between the test specimen before and after electrolysis was defined as the amount of electrolysis. The resulting residue was acid-decomposed and subjected to ICP optical emission analysis to obtain the Cr and Nb concentrations in the residue. The masses of Cr and Nb in the residue were quantified from the Cr and Nb concentrations in the residue and the mass of the residue, and divided by the amount of electrolysis to obtain the concentrations (mass%) of Cr and Nb elements. The obtained concentrations of Cr and Nb elements were defined as the amount of precipitation of the specific precipitate (mass%). The amount of precipitation of the specific precipitate (mass%) for each test number is shown in Table 4.

[0136] [Austenite Grain Size Measurement Test] An austenite grain size measurement test was performed on the steel plates of each test number using the method described above. Specifically, test specimens for microstructure observation were prepared from the center of the plate thickness of the steel plates of each test number using the method described above. For the test specimens of each test number, austenite grain boundaries were revealed using the method described above, and the observation surface was observed using an optical microscope. The grain size (μm) of the austenite grains was determined using the optical microscope observation based on the average intercept length measurement method in accordance with JIS G 0551 (2020). The obtained austenite grain size (μm) for each test number is shown in the "γ grain size (μm)" column of Table 4.

[0137] [Hydrogen Embrittlement Resistance Evaluation Test] A hydrogen embrittlement resistance evaluation test was conducted for each steel plate according to the method described above. Specifically, a round bar tensile test specimen with a parallel section diameter of 3 mm was prepared from the center of the thickness of each steel plate. The axial direction of the round bar tensile test specimen was parallel to the rolling direction of the steel plate. A slow strain rate tensile test (SSRT) was conducted for each round bar tensile test specimen at room temperature in air to determine the fracture elongation L0 (%) in air. Another round bar tensile test specimen for each test number was conducted for each test specimen in 85 MPa hydrogen gas at room temperature to determine the fracture elongation L1 (%) in high-pressure hydrogen gas. In the SSRT, the strain rate was set to 3 × 10 －６ / sec. Furthermore, the relative breaking elongation L (L = L1 / L0) was determined for each test number. The relative breaking elongation L obtained for each test number is shown in Table 4.

[0138] [Evaluation Results] With reference to Tables 3-1, 3-2, and 4, the steel sheets of test numbers 2-1 to 2-55 had the above-described chemical compositions, Fn1 was 22.0 or more, and the manufacturing method also satisfied the above-described preferable conditions. These steel sheets had a number density of specific precipitates of 1.00 particles / μm ２ As a result, these steel sheets had a tensile strength of 950 MPa or more and had high strength. As a result, these steel sheets also had a relative fracture elongation L of 0.80 or more and had excellent hydrogen embrittlement resistance in a high-pressure hydrogen gas environment. Note that the amount of specific precipitates in all of these steel sheets was 0.010 mass% or more.

[0139] On the other hand, the steel plate of test number 2-56 had an excessively high Cr content, resulting in a relative fracture elongation L of less than 0.80, and thus not having excellent hydrogen embrittlement resistance in a high-pressure hydrogen gas environment.

[0140] The steel plate of test number 2-57 had an excessively low Cr content, resulting in a tensile strength of less than 950 MPa, and thus not having high strength.

[0141] The steel plate of test number 2-58 had too high a Ni content and too low a N content, resulting in a tensile strength of less than 950 MPa, and thus did not have high strength.

[0142] The steel plate of test number 2-60 had an excessively low Mn content, resulting in a relative fracture elongation L of less than 0.80, and thus not having excellent hydrogen embrittlement resistance in a high-pressure hydrogen gas environment.

[0143] The steel plate of test number 2-61 had an excessively low N content, resulting in a tensile strength of less than 950 MPa, and thus did not have high strength.

[0144] The steel plate of test number 2-62 had an excessively high Nb content, resulting in a relative fracture elongation L of less than 0.80, and thus not having excellent hydrogen embrittlement resistance in a high-pressure hydrogen gas environment.

[0145] The steel plate of test number 2-63 had a too low Nb content, and the number density of specific precipitates was 1.00 particles / μm ２ As a result, the tensile strength of this steel plate was less than 950 MPa, and it did not have high strength.

[0146] The steel plate of test number 2-64 had an Fn1 of less than 22.0, and as a result, the relative fracture elongation L of this steel plate was less than 0.80, and the steel plate did not have excellent hydrogen embrittlement resistance in a high-pressure hydrogen gas environment.

[0147] In the steel sheet of test number 2-65, the holding temperature in the specific precipitate precipitation treatment was too high, and the number density of the specific precipitates was 1.00 particles / μm ２ As a result, the tensile strength of this steel plate was less than 950 MPa, and it did not have high strength.

[0148] The steel plate of test number 2-66 was hot worked for too long, and the number density of specific precipitates was 1.00 pieces / μm ２ As a result, the relative fracture elongation L of this steel sheet was less than 0.80, and the steel sheet did not have excellent hydrogen embrittlement resistance in a high-pressure hydrogen gas environment.

[0149] The embodiments of the present disclosure have been described above. However, the above-described embodiments are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiments, and can be implemented by appropriately modifying the above-described embodiments within the scope of the present disclosure.

Claims

1. An austenitic stainless steel material having a chemical composition, in mass%, of C: 0.100% or less, Si: 1.000% or less, Mn: 8.00-20.00%, P: 0.050% or less, S: 0.0050% or less, Cr: 18.00-30.00%, Ni: 10.00-25.00%, N: over 0.350 and less than 0.700%, V: 0.010-0.200%, Nb: 0.010-0.300%, Al: 0.200% or less, O: 0.0100% or less, Mo: 0-1.00%, W: 0-2.00%, Ti: 0-0.100%, Zr: 0-0.100%, the chemical composition further comprises Hf: 0-0.100%, Ta: 0-0.200%, Cu: 0-1.00%, Sn: 0-0.05%, Co: 0-2.00%, B: 0-0.020%, Mg: 0-0.0050%, Ca: 0-0.0050%, rare earth elements: 0-0.5000%, and the balance being Fe and impurities; when the Ni content is less than 17.00%, Fn1 defined in formula (1) satisfies 18.0 or more, and when the Ni content is 17.00% or more, Fn1 defined in formula (1) satisfies 22.0 or more, and in the austenitic stainless steel material, the number density of (Cr, Nb) composite precipitates having an equivalent circle diameter of 50-500 nm is 1.00 precipitates / μm ２ The above austenitic stainless steel material: Fn1=Ni+0.02×Cr+0.52×Mn−0.48×Mo (1) where each element symbol in formula (1) is substituted with the content of the corresponding element in the chemical composition in mass%.

2. An austenitic stainless steel material according to claim 1, wherein the chemical composition is: Mo: 0.01-1.00%, W: 0.01-2.00%, Ti: 0.001-0.100%, Zr: 0.001-0.100%, Hf: 0.001-0.100%, Ta: 0.001-0.200%, Cu: 0.01-1.00%, Sn: 0.01-0.05%, Co: 0.01-2.00%, B: 0.001-0.020%, Mg: 0.0001-0.0050%, Ca: 0.0001-0.0050%, and An austenitic stainless steel material containing one or more elements selected from the group consisting of: rare earth elements: 0.0001 to 0.5000%.

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