Austenitic stainless steel material

Abstract

An austenitic stainless steel material that has high strength and has excellent hydrogen embrittlement resistance in a high-pressure hydrogen gas environment is provided. An austenitic stainless steel material of the present disclosure has a chemical composition consisting of, 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 to less than 0.700%, V: 0.010 to 0.200%, Nb: 0.010 to 0.300%, Al: 0.200% or less, and O: 0.0100% or less, with the balance being Fe and impurities, in which Fn1 defined by Formula (1) in the description is 18.0 or more when the content of Ni is less than 17.00% and is 22.0 or more when the content of Ni is 17.00% or more. In the steel material, a number density of (Cr, Nb) composite precipitates having an equivalent circular diameter of 50 to 500 nm is 1.00 / µm2 or more.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to an austenitic stainless steel material.BACKGROUND ART

[0002] Recently, research into the practical application of transportation equipment that utilizes hydrogen as energy, as typified by fuel cell vehicles, and hydrogen stations that supply hydrogen to such types of transportation equipment is being actively conducted. These hydrogen stations and transportation equipment are equipped with tanks for storing high-pressure hydrogen, and pipes for high-pressure hydrogen. Hydrogen embrittlement is an issue in tanks and pipes for high-pressure hydrogen. The term "hydrogen embrittlement" refers to a phenomenon whereby the ductility and toughness of a steel material are significantly reduced due to hydrogen penetrating into the steel material. That is, austenitic stainless steel materials used in tanks and pipes for high-pressure hydrogen are required to have excellent hydrogen embrittlement resistance in a high-pressure hydrogen gas environment.

[0003] Austenitic stainless steel materials used in tanks and pipes for high-pressure hydrogen are also required to have high strength in order to withstand hydrogen at high pressure.

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

[0005] Patent Literature 1 discloses an austenitic stainless steel material that is an austenitic stainless steel for high-pressure hydrogen which contains, in 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%, and N: 0.09% or less, with the balance being Fe and impurity elements, and which also satisfies Formula 1 (54.8C+3.7Ni+2.5Mn-1.6Cr-0.9Si+266N-39.6 > 0), wherein when the steel is used as it is after a solid solution heat treatment, Cr carbides are present in an amount equivalent to an area fraction of 23% or more in the steel. It is described in Patent Literature 1 that this austenitic stainless steel material can be produced using low-cost components, and excellent yield strength and hardness can be obtained in the state as it is after a solid solution heat treatment, without depending on increasing the strength by cold rolling, and that this austenitic stainless steel material is also excellent in hydrogen embrittlement resistance at low temperatures.

[0006] Patent Literature 2 discloses an austenitic stainless steel material which 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%, and Cu: 0 to 0.5%, with the balance being Fe and impurities, and which satisfies Formula (1) to Formula (3), wherein, in a cross section perpendicular to the longitudinal direction of the austenitic stainless steel material, a ratio A0 / A1 of an austenite area fraction A0 (%) at the center position of the cross section to an austenite area fraction A1 (%) at a position at a depth of 5 mm from the surface of the austenitic stainless steel material is 0.990 to 1.010. It is described in Patent Literature 2 that in this austenitic stainless steel material it is possible to achieve both high strength and excellent hydrogen embrittlement resistance, and the yield strength is stable. − 7.1 + 2.7 Ni + 0.49 Cr + 2.0 Mo − 2.0 Si + 0.75 Mn − 5.7 C − 24 N ≥ 10.00 Ni + 0.72 Cr + 0.88 Mo + 1.11 Mn − 0.27 Si + 12.93 C + 0.53 Cu + 7.55 N ≥ 25.00 C + N ≥ 0.22CITATION LISTPATENT LITERATURE

[0007] Patent Literature 1: Japanese Patent Application Publication No. 2018-135592 Patent Literature 2: Japanese Patent Application Publication No. 2021-139007 SUMMARY OF INVENTIONTECHNICAL PROBLEM

[0008] In the austenitic stainless steel materials disclosed in Patent Literatures 1 and 2 described above, high strength and an improvement in hydrogen embrittlement resistance are both achieved. However, an austenitic stainless steel material that has high strength and also has excellent hydrogen embrittlement resistance in a high-pressure hydrogen gas environment may also be obtained by different means from the means disclosed in Patent Literatures 1 and 2.

[0009] An objective of the present disclosure is to provide an austenitic stainless steel material that has high strength and also has excellent hydrogen embrittlement resistance in a high-pressure hydrogen gas environment.SOLUTION TO PROBLEM

[0010] An austenitic stainless steel material according to the present disclosure has a chemical composition consisting of, 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 to 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%, and rare earth metal: 0 to 0.5000%, with the balance being Fe and impurities, wherein: in the chemical composition, in addition, in a case where a content of Ni is less than 17.00%, Fn1 defined by Formula (1) is 18.0 or more, and in a case where a content of Ni is 17.00% or more, Fn1 defined by Formula (1) is 22.0 or more; and in the austenitic stainless steel material, a number density of (Cr, Nb) composite precipitates having an equivalent circular diameter of 50 to 500 nm is 1.00 / µm 2< or more; Fn 1 = Ni + 0.02 × Cr + 0.52 × Mn − 0.48 × Mo where, a content of a corresponding element in the chemical composition is substituted in percent by mass for each symbol of an element in Formula (1). ADVANTAGEOUS EFFECTS OF INVENTION

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

[0012] First, the present inventors investigated methods for obtaining high strength and excellent hydrogen embrittlement resistance in an austenitic stainless steel material for which use in a high-pressure hydrogen gas environment is envisioned, from the viewpoint of the chemical composition.

[0013] It is already known that the strength of an austenitic stainless steel material can be increased by increasing the content of nitrogen (N) in the austenitic stainless steel material. On the other hand, even when it is attempted to simply increase the content of N in an austenitic stainless steel material, there are some cases where the content of N cannot be sufficiently increased. Therefore, the present inventors carried out detailed studies regarding the content of elements that can increase the content of N. As a result of such detailed studies carried out by the present inventors, it has been revealed that the content of N in a steel material can be increased by adjusting the balance between the contents of chromium (Cr), nickel (Ni), manganese (Mn), and molybdenum (Mo). Specifically, the present inventors have found that by adjusting the contents of these elements to Cr: 18.00 to 30.00%, Ni: 10.00 to 25.00%, Mn: 8.00 to 20.00%, and Mo: 0 to 1.00%, the content of N can be stably increased to within the range of more than 0.350 to less than 0.700%.

[0014] That is, if an austenitic stainless steel material has a chemical composition consisting of, 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 to 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%, and rare earth metal: 0 to 0.5000%, with the balance being Fe and impurities, there is a possibility that high strength and excellent hydrogen embrittlement resistance in a high-pressure hydrogen gas environment can both be achieved.

[0015] On the other hand, there were some cases where it was not possible to achieve both high strength and excellent hydrogen embrittlement resistance in a high-pressure hydrogen gas environment in a chemical composition having the chemical composition described above. Specifically, with respect to an austenitic stainless steel material having the chemical composition described above, the present inventors conducted detailed studies regarding factors that reduce the hydrogen embrittlement resistance in a high-pressure hydrogen gas environment. As a result, it has been revealed that in an austenitic stainless steel material having the chemical composition described above, because the content of N is high, the stacking fault energy tends to be low.

[0016] In a case where the stacking fault energy is low, austenite is easily destabilized. As a result, there is a possibility that the sensitivity of austenite to hydrogen will increase and the hydrogen embrittlement resistance in a high-pressure hydrogen gas environment will decrease. Therefore, the present inventors have considered stabilizing the austenite in an austenitic stainless steel material in which the stacking fault energy is low due to satisfying the aforementioned chemical composition containing a high content of N. If the austenite is stabilized, there is a possibility that both high strength and excellent hydrogen embrittlement resistance in a high-pressure hydrogen gas environment can be achieved even if the stacking fault energy is low due to satisfying the aforementioned chemical composition containing a high content of N.

[0017] The present inventors therefore focused their attention on the stability of austenite in the microstructure of an austenitic stainless steel material having the chemical composition described above, and carried out detailed investigations regarding techniques for increasing the hydrogen embrittlement resistance in a high-pressure hydrogen gas environment while maintaining strength. As a result of the detailed studies by the present inventors, it has been revealed that in an austenitic stainless steel material having the chemical composition described above, if Fn1 defined by the following Formula (1) satisfies a predetermined value according to the content of Ni, on the condition that the other requirements of the present embodiment are satisfied, the hydrogen embrittlement resistance in a high-pressure hydrogen gas environment will be increased: Fn 1 = Ni + 0.02 × Cr + 0.52 × Mn − 0.48 × Mo where, the content of a corresponding element in the chemical composition is substituted in percent by mass for each symbol of an element in Formula (1). Note that, if an element is not contained, "0" is substituted for the corresponding symbol of an element.

[0018] Fn1 defined by Formula (1) is an index of the stability of austenite in an austenitic stainless steel material having the chemical composition described above. As a result of detailed studies by the present inventors, it has been revealed that, on the precondition that an austenitic stainless steel material has the chemical composition described above, if Fn1 is 18.0 or more in a case where the content of Ni is 10.00 to less than 17.00%, or if Fn1 is 22.0 or more in a case where the content of Ni is 17.00 to 25.00%, the hydrogen embrittlement resistance in a high-pressure hydrogen gas environment will be increased while maintaining the strength. Therefore, in an austenitic stainless steel material according to the present embodiment, in addition to satisfying the chemical composition described above, Fn1 is made 18.0 or more or 22.0 or more according to the content of Ni.

[0019] In addition, with respect to an austenitic stainless steel material having the chemical composition described above including Fn1, the present inventors investigated techniques for increasing strength while maintaining hydrogen embrittlement resistance in a high-pressure hydrogen gas environment. As a result, the present inventors obtained the following finding.

[0020] The present inventors investigated increasing the strength of a steel material by forming fine precipitates in the steel material. If fine precipitates are dispersed in a steel material, the strength of the steel material can be increased. On the other hand, depending on the type of precipitates, there is a concern that the hydrogen embrittlement resistance of the steel material will decrease. Therefore, the present inventors conducted detailed studies with respect to, among the precipitates in an austenitic stainless steel material having the chemical composition described above including Fn1, fine precipitates which are less likely to reduce the hydrogen embrittlement resistance of the steel material.

[0021] As a result of investigations conducted by the present inventors, it has been revealed that in an austenitic stainless steel material having the chemical composition described above including Fn1, if a large number of (Cr, Nb) composite precipitates having an equivalent circular diameter of 50 to 500 nm are caused to precipitate, the strength of the steel material will be increased. On the other hand, as a result of simply increasing the precipitated amount of (Cr, Nb) composite precipitates having an equivalent circular diameter of 50 to 500 nm in an attempt to increase the strength of the steel material, there were some cases in which coarse precipitates or inclusions were formed and grains became coarse due to abnormal grain growth, and consequently the hydrogen embrittlement resistance of the steel material decreased.

[0022] As a result of further detailed studies conducted by the present inventors based on the above findings, it has been revealed that in an austenitic stainless steel material having the chemical composition described above including Fn1, if the number density of (Cr, Nb) composite precipitates having an equivalent circular diameter of 50 to 500 nm is 1.00 / µm 2< or more, the strength can be increased while maintaining the hydrogen embrittlement resistance of the steel material. Therefore, the austenitic stainless steel material according to the present embodiment has the chemical composition described above including Fn1, and in addition, the number density of (Cr, Nb) composite precipitates having an equivalent circular diameter of 50 to 500 nm in the steel material is 1.00 / µm 2< or more. As a result, the austenitic stainless steel material according to the present embodiment can achieve both high strength and excellent hydrogen embrittlement resistance in a high-pressure hydrogen gas environment.

[0023] The gist of the austenitic stainless steel material according to the present embodiment, which was completed based on the above findings, is as follows. [1] An austenitic stainless steel material, having a chemical composition consisting of, 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 to 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%, and rare earth metal: 0 to 0.5000%, with the balance being Fe and impurities, wherein: in the chemical composition, in addition, in a case where a content of Ni is less than 17.00%, Fn1 defined by Formula (1) is 18.0 or more, and in a case where a content of Ni is 17.00% or more, Fn1 defined by Formula (1) is 22.0 or more; and in the austenitic stainless steel material, a number density of (Cr, Nb) composite precipitates having an equivalent circular diameter of 50 to 500 nm is 1.00 / µm 2< or more; Fn 1 = Ni + 0.02 × Cr + 0.52 × Mn − 0.48 × Mo where, a content of a corresponding element in the chemical composition is substituted in percent by mass for each symbol of an element in Formula (1). [2] The austenitic stainless steel material according to [1], wherein the chemical composition contains one or more elements selected from a group consisting of: 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 metal: 0.0001 to 0.5000%.

[0024] Hereunder, the austenitic stainless steel material according to the present embodiment is described in detail. The symbol "%" in relation to an element means "mass percent" unless otherwise stated. Further, in the following description, the austenitic stainless steel material is also referred to simply as "steel material".[Chemical composition]

[0025] The chemical composition of the austenitic stainless steel material according to the present embodiment contains the following elements.C: 0.100% or less

[0026] Carbon (C) is unavoidably contained. That is, the lower limit of the content of C is more than 0%. C forms carbides and increases the strength of the steel material. C also dissolves in the steel material and increases the strength of the steel material. However, if the content of C is too high, C will excessively form carbides at grain boundaries and will thereby reduce the hydrogen embrittlement resistance of the steel material. Therefore, the content of C is 0.100% or less. A preferable upper limit of the content of C is 0.090%, more preferably is 0.080%, further preferably is 0.070%, further preferably is 0.060%, and further preferably is 0.050%. A preferable lower limit of the content of C for more effectively obtaining the aforementioned advantageous effects is 0.001%, more preferably is 0.002%, and further preferably is 0.003%.Si: 1.000% or less

[0027] Silicon (Si) is unavoidably contained. That is, the lower limit of the content of Si is more than 0%. Si deoxidizes the steel. However, if the content of Si is too high, Si will excessively form intermetallic compounds and will thereby reduce the hot workability and hydrogen embrittlement resistance of the steel material. Therefore, the content of Si is 1.000% or less. A preferable upper limit of the content of Si is 0.900%, more preferably is 0.800%, further preferably is 0.700%, and further preferably is 0.600%. On the other hand, excessively reducing the content of Si will increase the production cost. Therefore, taking into consideration normal industrial production, a preferable lower limit of the content of Si is 0.001%, more preferably is 0.005%, and further preferably is 0.010%.Mn: 8.00 to 20.00%

[0028] Manganese (Mn) increases the dissolved amount of N, thereby increasing the content of N in the steel material. As a result, Mn increases the strength of the steel material. In addition, Mn stabilizes austenite and thereby increases the hydrogen embrittlement resistance of the steel material. If the content of Mn is too low, the aforementioned advantageous effects will not be sufficiently obtained. On the other hand, if the content of Mn is too high, hot workability of the steel material will decrease. Therefore, the content of Mn is 8.00 to 20.00%. A preferable lower limit of the content of Mn is 8.01%, more preferably is 8.10%, further preferably is 8.30%, and further preferably is 8.50%. A preferable upper limit of the content of Mn is 19.50%, more preferably is 19.00%, further preferably is 18.00%, further preferably is 17.00%, and further preferably is 16.00%.P: 0.050% or less

[0029] Phosphorus (P) is an impurity that is unavoidably contained. That is, the lower limit of the content of P is more than 0%. P reduces the hot workability and hydrogen embrittlement resistance of the steel material. Therefore, the content of P is 0.050% or less. A preferable upper limit of the content of P is 0.040%, more preferably is 0.030%, and further preferably is 0.025%. The content of P is preferably as low as possible. However, excessively reducing the content of P will increase the production cost. Therefore, taking into consideration normal industrial production, a preferable lower limit of the content of P is 0.001%, and more preferably is 0.002%.S: 0.0050% or less

[0030] Sulfur (S) is an impurity that is unavoidably contained. That is, the lower limit of the content of S is more than 0%. S reduces the hot workability and hydrogen embrittlement resistance of the steel material. Therefore, the content of S is 0.0050% or less. A preferable upper limit of the content of S is 0.0040%, more preferably is 0.0030%, and further preferably is 0.0020%. The content of S is preferably as low as possible. However, excessively reducing the content of S will increase the production cost. Therefore, taking into consideration normal industrial production, a preferable lower limit of the content of S is 0.0001%, more preferably is 0.0002%, and further preferably is 0.0003%.Cr: 18.00 to 30.00%

[0031] Chromium (Cr) increases the dissolved amount of N, thereby increasing the content of N in the steel material. As a result, Cr increases the strength of the steel material. If the content of Cr is too low, the aforementioned advantageous effect will not be sufficiently obtained. On the other hand, if the content of Cr is too high, Cr nitrides will be formed and the hydrogen embrittlement resistance of the steel material will decrease. Therefore, the content of Cr is 18.00 to 30.00%. The lower limit of the content of Cr is preferably 18.50%, more preferably is 19.00%, and further preferably is 19.50%. The upper limit of the content of Cr is preferably 29.50%, more preferably is 29.10%, and further preferably is 28.70%.Ni: 10.00 to 25.00%

[0032] Nickel (Ni) stabilizes austenite and thereby increases the hydrogen embrittlement resistance of the steel material. If the content of Ni is too low, the aforementioned advantageous effect will not be sufficiently obtained. On the other hand, if the content of Ni is too high, the content of N in the steel material will not be sufficiently increased and consequently the strength of the steel material will decrease. Therefore, the content of Ni is 10.00 to 25.00%. A preferable lower limit of the content of Ni is 10.05%, more preferably is 10.10%, and further preferably is 10.30%. A preferable upper limit of the content of Ni is 24.50%, more preferably is 24.00%, further preferably is 23.00%, further preferably is 22.00%, and further preferably is 21.00%.

[0033] Note that, as described above, in the austenitic stainless steel material according to the present embodiment, the value of Fn1 defined by Formula (1) is adjusted according to the content of Ni. Specifically, when the content of Ni is 10.00 to less than 17.00%, Fn1 is to be made 18.0 or more. When the content of Ni is 10.00 to less than 17.00%, a preferable upper limit of the content of Ni is 16.99%, more preferably is 16.90%, further preferably is 16.80%, further preferably is 16.50%, and further preferably is 16.30%. When the content of Ni is 17.00 to 25.00%, Fn1 is to be made 22.0 or more. When the content of Ni is 17.00 to 25.00%, a preferable lower limit of the content of Ni is 17.05%, more preferably is 17.10%, and further preferably is 17.30%.N: more than 0.350 to less than 0.700%

[0034] Nitrogen (N) dissolves in the steel material and thereby increases the strength of the steel material. If the content of N is too low, the aforementioned advantageous effect will not be sufficiently obtained. On the other hand, if the content of N is too high, hot workability of the steel material will decrease. Therefore, the content of N is more than 0.350 to less than 0.700%. The lower limit of the content of N is preferably 0.351%, more preferably is 0.360%, and further preferably is 0.370%. The upper limit of the content of N is preferably 0.699%, more preferably is 0.690%, further preferably is 0.680%, and further preferably is 0.670%.V: 0.010 to 0.200%

[0035] Vanadium (V) forms precipitates and thereby increases the strength of the steel material. If the content of V is too low, the aforementioned advantageous effect will not be sufficiently obtained. On the other hand, if the content of V is too high, coarse precipitates will be formed and the hydrogen embrittlement resistance of the steel material will decrease. Therefore, the content of V is 0.010 to 0.200%. The lower limit of the content of V is preferably 0.015%, more preferably is 0.020%, and further preferably is 0.025%. The upper limit of the content of V is preferably 0.180%, more preferably is 0.150%, and further preferably is 0.120%.Nb: 0.010 to 0.300%

[0036] Niobium (Nb) forms (Cr, Nb) composite precipitates in the steel material and thereby increases the strength of the steel material. If the content of Nb is too low, the aforementioned advantageous effect will not be sufficiently obtained. On the other hand, if the content of Nb is too high, coarse precipitates will be formed and the hydrogen embrittlement resistance of the steel material will decrease. Therefore, the content of Nb is 0.010 to 0.300%. The lower limit of the content of Nb is preferably 0.015%, more preferably is 0.020%, and further preferably is 0.025%. The upper limit of the content of Nb is preferably 0.250%, more preferably is 0.200%, and further preferably is 0.150%.Al: 0.200% or less

[0037] Aluminum (Al) is unavoidably contained. That is, the lower limit of the content of Al is more than 0%. Al deoxidizes the steel. If even a small amount of Al is contained, the aforementioned advantageous effect will be obtained to a certain extent. However, if the content of Al is too high, oxides and intermetallic compounds will form in the steel material. In such case, the hydrogen embrittlement resistance of the steel material will decrease. Therefore, the content of Al is 0.200% or less. A preferable upper limit of the content of Al is 0.190%, more preferably is 0.180%, and further preferably is 0.170%. A preferable lower limit of the content of Al for more effectively obtaining the aforementioned advantageous effect is 0.001%, and more preferably is 0.002%. In the present description, the term "content of Al" means the content of sol. Al (acid-soluble Al).O: 0.0100% or less

[0038] Oxygen (O) is an impurity that is unavoidably contained. That is, the lower limit of the content of O is more than 0%. O reduces hot workability of the steel material. Therefore, the content of O is 0.0100% or less. The content of O is preferably as low as possible. However, excessively reducing the content of O will increase the production cost. Therefore, taking into consideration normal industrial production, a preferable lower limit of the content of O is 0.0001%, and more preferably is 0.0002%. The upper limit of the content of O is preferably 0.0090%, more preferably is 0.0080%, and further preferably is 0.0060%.

[0039] The balance of the chemical composition of the austenitic stainless steel material according to the present embodiment is Fe and impurities. Here, the term "impurities" means substances which are mixed in from ore and scrap used as the raw material or from the production environment or the like when industrially producing the austenitic stainless steel material, and which are not intentionally contained but are permitted within a range that does not adversely affect the austenitic stainless steel material according to the present embodiment.[Regarding optional element]

[0040] The chemical composition of the austenitic stainless steel material according to the present embodiment further contains: 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%, and rare earth metal: 0 to 0.5000%. Each of these elements is an optional element, and does not have to be contained.

[0041] In other words, the chemical composition of the austenitic stainless steel material according to the present embodiment may contain, in lieu of a part of Fe, one or more elements selected from the group consisting of: 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 metal: 0.0001 to 0.5000%.

[0042] Each of these optional elements is described hereunder.[Regarding Mo and W]

[0043] The chemical composition of the austenitic stainless steel material according to the present embodiment may contain one or more elements selected from the group consisting of Mo and W in lieu of a part of Fe. Each of these elements increases the strength of the steel material.Mo: 0 to 1.00%

[0044] Molybdenum (Mo) is an optional element, and does not have to be contained. That is, the content of Mo may be 0%. When contained, Mo dissolves in the steel and increases the strength of the steel material. If the steel material includes even a small content of Mo, the aforementioned advantageous effect will be obtained to a certain extent. However, if the content of Mo is too high, the strength of the steel material will be too high and the toughness of the steel material will decrease. Therefore, the content of Mo is 0 to 1.00%, and when contained, the content of Mo is 1.00% or less, in other words, is more than 0 to 1.00%. A preferable lower limit of the content of Mo is 0.01%, and more preferably is 0.02%. A preferable upper limit of the content of Mo is 0.95%, more preferably is 0.90%, further preferably is 0.80%, and further preferably is 0.70%.W: 0 to 2.00%

[0045] Tungsten (W) is an optional element, and does not have to be contained. That is, the content of W may be 0%. When contained, W dissolves in the steel and increases the strength of the steel material. If even a small amount of W is contained, the aforementioned advantageous effect will be obtained to a certain extent. However, if the content of W is too high, the strength of the steel material will be too high and the toughness of the steel material will decrease. Therefore, the content of W is 0 to 2.00%, and when contained, the content of W is 2.00% or less, in other words, is more than 0 to 2.00%. A preferable lower limit of the content of W is 0.01%, more preferably is 0.05%, and further preferably is 0.10%. A preferable upper limit of the content of W is 1.90%, more preferably is 1.80%, further preferably is 1.70%, further preferably is 1.60%, and further preferably is 1.50%.[Regarding Ti, Zr, Hf, and Ta]

[0046] The chemical composition of the austenitic stainless steel material according to the present embodiment may contain one or more elements selected from the group consisting of Ti, Zr, Hf, and Ta in lieu of a part of Fe. Each of these elements increases the strength of the steel material.Ti: 0 to 0.100%

[0047] Titanium (Ti) is an optional element, and does not have to be contained. That is, the content of Ti may be 0%. When contained, Ti forms nitrides and refines the grains by a pinning effect. As a result, Ti increases the hydrogen embrittlement resistance of the steel material. If even a small amount of Ti is contained, the aforementioned advantageous effect will be obtained to a certain extent. However, if the content of Ti is too high, nitrides will be excessively formed and the toughness of the steel material will decrease. Therefore, the content of Ti is 0 to 0.100%, and when contained, the content of Ti is 0.100% or less, in other words, is more than 0 to 0.100%. A preferable lower limit of the content of Ti is 0.001%, more preferably is 0.005%, and further preferably is 0.010%. A preferable upper limit of the content of Ti is 0.090%, more preferably is 0.080%, further preferably is 0.075%, and further preferably is 0.060%.Zr: 0 to 0.100%

[0048] Zirconium (Zr) is an optional element, and does not have to be contained. That is, the content of Zr may be 0%. When contained, Zr forms nitrides and refines the grains by a pinning effect. As a result, Zr increases the hydrogen embrittlement resistance of the steel material. If even a small amount of Zr is contained, the aforementioned advantageous effect will be obtained to a certain extent. However, if the content of Zr is too high, nitrides will be excessively formed and the toughness of the steel material will decrease. Therefore, the content of Zr is 0 to 0.100%, and when contained, the content of Zr is 0.100% or less, in other words, is more than 0 to 0.100%. A preferable lower limit of the content of Zr is 0.001%, more preferably is 0.005%, and further preferably is 0.010%. A preferable upper limit of the content of Zr is 0.090%, more preferably is 0.080%, further preferably is 0.070%, and further preferably is 0.060%.Hf: 0 to 0.100%

[0049] Hafnium (Hf) is an optional element, and does not have to be contained. That is, the content of Hf may be 0%. When contained, Hf forms nitrides and refines the grains by a pinning effect. As a result, Hf increases the hydrogen embrittlement resistance of the steel material. If even a small amount of Hf is contained, the aforementioned advantageous effect will be obtained to a certain extent. However, if the content of Hf is too high, nitrides will be excessively formed and the toughness of the steel material will decrease. Therefore, the content of Hf is 0 to 0.100%, and when contained, the content of Hf is 0.100% or less, in other words, is more than 0 to 0.100%. A preferable lower limit of the content of Hf is 0.001%, more preferably is 0.003%, and further preferably is 0.005%. A preferable upper limit of the content of Hf is 0.095%, more preferably is 0.090%, and further preferably is 0.080%.Ta: 0 to 0.200%

[0050] Tantalum (Ta) is an optional element, and does not have to be contained. That is, the content of Ta may be 0%. When contained, Ta forms nitrides and refines the grains by a pinning effect. As a result, Ta increases the hydrogen embrittlement resistance of the steel material. If even a small amount of Ta is contained, the aforementioned advantageous effect will be obtained to a certain extent. However, if the content of Ta is too high, nitrides will be excessively formed and the toughness of the steel material will decrease. Therefore, the content of Ta is 0 to 0.200%, and when contained, the content of Ta is 0.200% or less, in other words, is more than 0 to 0.200%. A preferable lower limit of the content of Ta is 0.001%, more preferably is 0.003%, and further preferably is 0.005%. A preferable upper limit of the content of Ta is 0.190%, more preferably is 0.180%, further preferably is 0.170%, further preferably is 0.160%, and further preferably is 0.150%.[Regarding Cu, Sn, and Co]

[0051] The chemical composition of the austenitic stainless steel material according to the present embodiment may contain one or more elements selected from the group consisting of Cu, Sn, and Co in lieu of a part of Fe. Each of these elements stabilizes the austenite of the steel material and thereby increases the hydrogen embrittlement resistance of the steel material.Cu: 0 to 1.00%

[0052] Copper (Cu) is an optional element, and does not have to be contained. That is, the content of Cu may be 0%. When contained, Cu stabilizes austenite and thereby increases the hydrogen embrittlement resistance of the steel material. If the steel material includes even a small content of Cu, the aforementioned advantageous effect will be obtained to a certain extent. However, if the content of Cu is too high, hot workability of the steel material will decrease. Therefore, the content of Cu is 0 to 1.00%, and when contained, the content of Cu is 1.00% or less, in other words, is more than 0 to 1.00%. A preferable lower limit of the content of Cu is 0.01%, more preferably is 0.05%, and further preferably is 0.10%. A preferable upper limit of the content of Cu is 0.90%, more preferably is 0.85%, and further preferably is 0.80%.Sn: 0 to 0.05%

[0053] Tin (Sn) is an optional element, and does not have to be contained. That is, the content of Sn may be 0%. When contained, Sn stabilizes austenite and thereby increases the hydrogen embrittlement resistance of the steel material. If the steel material includes even a small content of Sn, the aforementioned advantageous effect will be obtained to a certain extent. However, if the content of Sn is too high, the production cost of the steel material will increase. Therefore, the content of Sn is 0 to 0.05%, and when contained, the content of Sn is 0.05% or less, in other words, is more than 0 to 0.05%. A preferable lower limit of the content of Sn is 0.01%, and more preferably is 0.02%. A preferable upper limit of the content of Sn is 0.04%, and more preferably is 0.03%.Co: 0 to 2.00%

[0054] Cobalt (Co) is an optional element, and does not have to be contained. That is, the content of Co may be 0%. When contained, Co stabilizes austenite and thereby increases the hydrogen embrittlement resistance of the steel material. If the steel material includes even a small content of Co, the aforementioned advantageous effect will be obtained to a certain extent. However, if the content of Co is too high, the production cost of the steel material will increase. Therefore, the content of Co is 0 to 2.00%, and when contained, the content of Co is 2.00% or less, in other words, is more than 0 to 2.00%. A preferable lower limit of the content of Co is 0.01%, more preferably is 0.10%, further preferably is 0.50%, and further preferably is 1.00%. A preferable upper limit of the content of Co is 1.90%, more preferably is 1.80%, and further preferably is 1.70%.[Regarding B, Mg, Ca, and rare earth metal]

[0055] The chemical composition of the austenitic stainless steel material according to the present embodiment may contain one or more elements selected from the group consisting of B, Mg, Ca, and rare earth metal in lieu of a part of Fe. Each of these elements increases hot workability of the steel material.B: 0 to 0.020%

[0056] Boron (B) is an optional element, and does not have to be contained. That is, the content of B may be 0%. When contained, B fixes S in the steel as a sulfide to render S harmless, and thereby increases hot workability of the steel material. If even a small amount of B is contained, the aforementioned advantageous effect will be obtained to a certain extent. However, if the content of B is too high, a large amount of nitrides will be formed and hot workability of the steel material will, on the contrary, decrease. Therefore, the content of B is 0 to 0.020%, and when contained, the content of B is 0.020% or less, in other words, is more than 0 to 0.020%. A preferable lower limit of the content of B is 0.001%, more preferably is 0.002%, and further preferably is 0.003%. A preferable upper limit of the content of B is 0.018%, more preferably is 0.015%, further preferably is 0.010%, and further preferably is 0.008%.Mg: 0 to 0.0050%

[0057] Magnesium (Mg) is an optional element, and does not have to be contained. That is, the content of Mg may be 0%. When contained, Mg fixes S in the steel as a sulfide to render S harmless, and thereby increases hot workability of the steel material. If even a small amount of Mg is contained, the aforementioned advantageous effect will be obtained to a certain extent. However, if the content of Mg is too high, coarse oxides will be formed and hot workability of the steel material will, on the contrary, decrease. Therefore, the content of Mg is 0 to 0.0050%, and when contained, the content of Mg is 0.0050% or less, in other words, is more than 0 to 0.0050%. A preferable lower limit of the content of Mg is 0.0001%. A preferable upper limit of the content of Mg is 0.0045%, more preferably is 0.0040%, further preferably is 0.0030%, further preferably is 0.0020%, and further preferably is 0.0010%.Ca: 0 to 0.0050%

[0058] Calcium (Ca) is an optional element, and does not have to be contained. That is, the content of Ca may be 0%. When contained, Ca fixes S in the steel as a sulfide to render S harmless, and thereby increases hot workability of the steel material. If even a small amount of Ca is contained, the aforementioned advantageous effect will be obtained to a certain extent. However, if the content of Ca is too high, coarse oxides will be formed and hot workability of the steel material will, on the contrary, decrease. Therefore, the content of Ca is 0 to 0.0050%, and when contained, the content of Ca is 0.0050% or less, in other words, is more than 0 to 0.0050%. A preferable lower limit of the content of Ca is 0.0001%. A preferable upper limit of the content of Ca is 0.0045%, more preferably is 0.0040%, further preferably is 0.0030%, further preferably is 0.0020%, and further preferably is 0.0010%.Rare earth metal: 0 to 0.5000%

[0059] Rare earth metal (REM) is an optional element, and does not have to be contained. That is, the content of REM may be 0%. When contained, REM fixes S in the steel as a sulfide to render S harmless, and thereby increases hot workability of the steel material. If even a small amount of REM is contained, the aforementioned advantageous effect will be obtained to a certain extent. However, if the content of REM is too high, coarse oxides will be formed and hot workability of the steel material will, on the contrary, decrease. Therefore, the content of REM is 0 to 0.5000%, and when contained, the content of REM is 0.5000% or less, in other words, is more than 0 to 0.5000%. A preferable lower limit of the content of REM is 0.0001%, more preferably is 0.0005%, and further preferably is 0.0010%. A preferable upper limit of the content of REM is 0.4900%, more preferably is 0.4800%, further preferably is 0.4500%, further preferably is 0.4200%, and further preferably is 0.4000%.

[0060] Note that, in the present description the term "REM" means one or more elements selected from the group consisting of scandium (Sc) which is the element with atomic number 21, yttrium (Y) which is the element with atomic number 39, and the elements from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71 that are lanthanoids. Further, in the present description, the term "content of REM" means the total content of these elements.[Formula (1)]

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

[0062] Note that, in a case where Mo that is an optional element is not contained in the chemical composition of the austenitic stainless steel material (that is, when the content of Mo is 0%), Formula (1) can be transformed into the following Formula (1A). In this case, in the austenitic stainless steel material according to the present embodiment, when the content of Ni is 10.00 to less than 17.00%, FnlA defined by Formula (1A) is made 18.0 or more, and when the content of Ni is 17.00 to 25.00%, FnlA defined by Formula (1A) is made 22.0 or more: Fn 1 A = Ni + 0.02 × Cr + 0.52 × Mn where, the content of a corresponding element in the chemical composition is substituted in percent by mass for each symbol of an element in Formula (1A).

[0063] Fn1 (= Ni+0.02×Cr+0.52×Mn-0.48×Mo) and FnlA (= Ni+0.02×Cr+0.52×Mn) are indexes of the stability of austenite in an austenitic stainless steel material having the chemical composition described above. In a case where the content of Ni is 10.00 to less than 17.00%, if Fn1 is increased to 18.0 or more, the hydrogen embrittlement resistance in a high-pressure hydrogen gas environment will be increased while maintaining the strength. Similarly, in a case where the content of Ni is 17.00 to 25.00%, if Fn1 is increased to 22.0 or more, the hydrogen embrittlement resistance in a high-pressure hydrogen gas environment will be increased while maintaining the strength.

[0064] Therefore, in the austenitic stainless steel material according to the present embodiment, in addition to satisfying the chemical composition described above, when the content of Ni is 10.00 to less than 17.00%, Fn1 is made 18.0 or more, and when the content of Ni is 17.00 to 25.00%, Fn1 is made 22.0 or more. In a case where the content of Ni is 10.00 to less than 17.00%, a preferable lower limit of Fn1 is 18.1, more preferably is 18.2, and further preferably is 18.3. In a case where the content of Ni is 17.00 to 25.00%, a preferable lower limit of Fn1 is 22.1, more preferably is 22.2, and further preferably is 22.3. Although the upper limit of Fn1 is not particularly limited, in the case of an austenitic stainless steel material having the chemical composition described above, the practical upper limit of Fn1 is 36.0. The upper limit of Fn1 may be 35.0, may be 33.0, may be 30.0, or may be 28.0.[(Cr, Nb) Composite Precipitates]

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

[0066] The specific precipitates disperse finely in the austenitic stainless steel material and increase the strength of the steel material. The specific precipitates also refine the grains and thereby increase the strength of the steel material and also increase the hydrogen embrittlement resistance of the steel material. In an austenitic stainless steel material having the chemical composition described above including Fn1, if the number density of the specific precipitates is too low, the amount of specific precipitates will be insufficient and the strength may not be increased sufficiently. On the other hand, even in a case where the amount of specific precipitates is large, if the number density of precipitates is too low, coarse specific precipitates will predominate, and the hydrogen embrittlement resistance of the steel material will decrease. Therefore, in an austenitic stainless steel material having the chemical composition described above including Fn1, if the number density of the specific precipitates is 1.00 / µm 2< or more, an austenitic stainless steel material in which both high strength and excellent hydrogen embrittlement resistance are achieved can be obtained.

[0067] Therefore, the austenitic stainless steel material according to the present embodiment has the chemical composition described above including Fn1, and furthermore, the number density of the specific precipitates in the austenitic stainless steel material is made 1.00 / µm 2< or more. As a result, the austenitic stainless steel material according to the present embodiment can achieve both high strength and excellent hydrogen embrittlement resistance.

[0068] In the present embodiment, a preferable lower limit of the number density of the specific precipitates is 1.05 / µm 2< , more preferably is 1.10 / µm 2< , and further preferably is 1.20 / µm 2< . On the other hand, although the upper limit of the number density of the specific precipitates is not particularly limited, for example, the upper limit is 5.00 / µm 2< . The upper limit of the number density of the specific precipitates may be 4.50 / µm 2< , may be 4.00 / µm 2< , or may be 3.50 / µm 2< .

[0069] In the present embodiment, it suffices that the number density of the specific precipitates is 1.00 / µm 2< or more, and the precipitated amount of the specific precipitates is not particularly limited. However, in an austenitic stainless steel material having the chemical composition described above including Fn1, in a case where the number density of the specific precipitates is 1.00 / µm 2< or more, the precipitated amount of the specific precipitates will, in practice, be 0.010% by mass or more. A more preferable lower limit of the precipitated amount of specific precipitates is 0.015% by mass, further preferably is 0.020% by mass, and further preferably is 0.025% by mass. The upper limit of the precipitated amount of specific precipitates is not particularly limited, and for example may be 0.300% by mass, may be 0.250% by mass, or may be 0.200% by mass.

[0070] Note that, in an austenitic stainless steel material having the chemical composition described above including Fn1, almost all of the specific precipitates are nitrides. That is, in the austenitic stainless steel material according to the present embodiment, the number density of (Cr, Nb) composite nitrides having an equivalent circular diameter of 50 to 500 nm is 1.00 / µm 2< or more. Here, (Cr, Nb) composite nitrides having an equivalent circular diameter of 50 to 500 nm precipitate in a temperature range of about 1060°C. In addition, (Cr, Nb) composite nitrides having an equivalent circular diameter of 50 to 500 nm are relatively stable precipitates even at high temperatures. Therefore, the specific precipitates thus precipitated in the steel material refine the austenite grain size by a pinning effect. It is believed that, as a result, the strength is further increased and the hydrogen embrittlement resistance is increased.

[0071] In the present embodiment, the number density of the specific precipitates in an austenitic stainless steel material is determined by the following method. Specifically, a micro test specimen for creating an extraction replica is prepared from the austenitic stainless steel material. The size of the micro test specimen is, for example, 10 mm × 10 mm. The surface of the micro test specimen is mirror-polished, and thereafter the micro test specimen is immersed for 10 minutes in a 3% nital etching reagent to etch the surface. The etched surface is then covered with a carbon deposited film. The micro test specimen whose surface is covered with the deposited film is immersed for 20 minutes in a 5% nital etching reagent. The deposited film is thereby peeled off from the immersed micro test specimen. The deposited film thus peeled off from the micro test specimen is cleaned with ethanol, and thereafter is scooped up with a sheet mesh and dried. Note that, in the present embodiment, a copper (Cu) sheet mesh is used.

[0072] The deposited film (replica film) is observed using a transmission electron microscope (TEM). Specifically, arbitrary locations are specified on the deposited film, and the specified locations are observed using an acceleration voltage of 200 kV. Note that, the size of the observation field is, for example, 4.0 µm × 4.0 µm. In each observation field, particles having an equivalent circular diameter of 50 to 500 nm or less are identified. Note that, it is possible to identify the particles based on the contrast. Note that, in the present description the term "particles" is not limited to circular (spherical) particles, and may also refer to small pieces that have an angular shape, or small pieces that have an elongated elliptical shape. Further, the equivalent circular diameter of precipitates can be determined by performing image analysis of an observation image in TEM observation.

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

[0074] The number density (pieces / µm 2< ) of the specific precipitates is calculated based on the total number of specific precipitates identified in the arbitrary three visual fields and the total area of the observation fields. Note that, in the present embodiment, a value obtained by rounding off to the second decimal place of the obtained numerical value is defined as the number density of the specific precipitates.

[0075] In the present embodiment, in addition, the precipitated amount of the specific precipitates in an austenitic stainless steel material is determined by the following method. Specifically, a test specimen is prepared from the austenitic stainless steel material. Note that, the test specimen is not particularly limited as long as an observation surface that is described later can be obtained.

[0076] After polishing the observation surface of the test specimen to obtain a mirror surface, the test specimen is immersed in an electrolyte (10% acetylacetone-1% tetramethylammonium chloride-methanol solution) and subjected to electrolysis. Note that, the orientation of the observation surface is not particularly limited. Here, the difference between the mass of the test specimen before electrolysis and the mass of the test specimen after electrolysis is defined as the electrolysis amount. The electrolyte after performing the electrolysis is passed through a 0.2 µm filter to capture residue. The mass of the captured residue is measured. In addition, the obtained residue is subjected to acid decomposition, and ICP (inductively coupled plasma) emission spectrometry is performed. The mass which Cr and Nb occupy in the residue is determined based on the concentration of Cr and Nb in the residue and the mass of the residue. The mass which Cr and Nb occupy in the residue is divided by the electrolysis amount, and the resulting value is defined as the concentration (mass%) of the Cr and Nb elements. In the present embodiment, the determined concentration (mass%) of the Cr and Nb elements is defined as the precipitated amount (mass%) of the specific precipitates in the austenitic stainless steel material. Note that, in the austenitic stainless steel material according to the present embodiment, the equivalent circular diameter of residue captured by the method described above will be 50 to 500 nm.[Tensile strength]

[0077] The austenitic stainless steel material according to the present embodiment has the chemical composition described above including Fn1, and furthermore the steel material contains (Cr, Nb) composite precipitates having an equivalent circular diameter of 50 to 500 nm which are present at a number density of 1.00 / µm 2< or more. As a result, the austenitic stainless steel material according to the present embodiment can achieve both high strength and excellent hydrogen embrittlement resistance in a high-pressure hydrogen gas environment. In the present embodiment, the phrase "steel material has high strength" means, specifically, that the tensile strength of the steel material is 950 MPa or more.

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

[0079] In the present 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, a 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 to be parallel to the pipe axis direction of the steel pipe. If the austenitic stainless steel material is a steel bar, the round bar tensile test specimen is prepared from an R / 2 position. In this case, the longitudinal direction of the round bar tensile test specimen is to be 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 a position which is the center position of the width and is the center position of the thickness. In this case, the longitudinal direction of the round bar tensile test specimen is to be parallel to the rolling elongation direction of the steel plate. Regarding the size of the round bar tensile test specimen, for example, the parallel portion diameter is to be 8 mm.

[0080] The prepared round bar tensile test specimen is subjected to a tensile test in accordance with JIS Z 2241: 2011 in air at normal temperature, and the tensile strength (MPa) is determined. Note that, in the present embodiment, a value obtained by rounding off decimals of the obtained numerical value is defined as the tensile strength (MPa).[Hydrogen embrittlement resistance]

[0081] The austenitic stainless steel material according to the present embodiment has the chemical composition described above including Fn1, and furthermore the steel material contains (Cr, Nb) composite precipitates having an equivalent circular diameter of 50 to 500 nm that are present at a number density of 1.00 / µm 2< or more. As a result, the austenitic stainless steel material according to the present embodiment can achieve both high strength and excellent hydrogen embrittlement resistance in a high-pressure hydrogen gas environment. Here, in the present embodiment, the hydrogen embrittlement resistance of the steel material is evaluated according to the content of Ni.[Case where content of Ni is 10.00 to less than 17.00%]

[0082] 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, a 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 to be parallel to the pipe axis direction of the steel pipe. If the austenitic stainless steel material is a steel bar, the round bar tensile test specimen is prepared from an R / 2 position. In this case, the longitudinal direction of the round bar tensile test specimen is to be 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 a position which is the center position of the width and is the center position of the thickness. In this case, the longitudinal direction of the round bar tensile test specimen is to be parallel to the rolling elongation direction of the steel plate. Regarding the size of the round bar tensile test specimen, for example, the parallel portion diameter is to be 3 mm.

[0083] The prepared round bar tensile test specimen is subjected to a slow strain rate test (SSRT). Specifically, a tensile test is performed at normal temperature in air at a strain rate of 3× 10 -6< / sec, and a breaking elongation L0 (unit is %) in air is determined. In addition, using another round bar tensile test specimen, a tensile test is performed at normal temperature in hydrogen gas at 45 MPa at a strain rate of 3×10 -6< / sec, and a breaking elongation L1 (unit is %) in hydrogen gas at 45 MPa is determined. The ratio of the determined breaking elongation L1 (%) in the high-pressure hydrogen gas to the determined breaking elongation L0 (%) in air is defined as a relative breaking elongation L. That is, the relative breaking elongation L is defined by L = L1 / L0.

[0084] In the present embodiment, in a case where the content of Ni is 10.00 to less than 17.00%, if the relative breaking elongation L determined under the conditions described above is 0.80 or more, it is evaluated that the steel material has excellent hydrogen embrittlement resistance in a high-pressure hydrogen gas environment. That is, in the austenitic stainless steel material according to the present embodiment, when the content of Ni is 10.00 to less than 17.00%, the relative breaking elongation L determined under the conditions described above is 0.80 or more.[Case where content of Ni is 17.00 to 25.00%]

[0085] In a case where the content of Ni is 17.00 to 25.00% also, a round bar tensile test specimen is prepared from the austenitic stainless steel material in a similar manner to a case where the content of Ni is 10.00 to less than 17.00%. The prepared round bar tensile test specimen is subjected to a slow strain rate test (SSRT). At such time, the test is performed under the same conditions as the case where the content of Ni is 10.00 to less than 17.00%, except that the pressure of the hydrogen gas is set to 85 MPa. That is, a tensile test is performed at normal temperature in air at a strain rate of 3×10 -6< / sec, and a breaking elongation L0 (unit is %) in air is determined. In addition, using another round bar tensile test specimen, a tensile test is performed at normal temperature in hydrogen gas at 85 MPa at a strain rate of 3×10 -6< / sec, and a breaking elongation L1 (unit is %) in hydrogen gas at 85 MPa is determined. The ratio of the determined breaking elongation L1 (%) in the high-pressure hydrogen gas to the determined breaking elongation L0 (%) in air is defined as a relative breaking elongation L. That is, the relative breaking elongation L is defined by L = L1 / L0.

[0086] In the present embodiment, in a case where the content of Ni is 17.00 to 25.00%, if the relative breaking elongation L determined under the conditions described above is 0.80 or more, it is evaluated that the steel material has excellent hydrogen embrittlement resistance in a high-pressure hydrogen gas environment. That is, in the austenitic stainless steel material according to the present embodiment, when the content of Ni is 17.00 to 25.00%, the relative breaking elongation L determined under the conditions described above is 0.80 or more.[Austenite grain size]

[0087] The austenitic stainless steel material according to the present embodiment has the chemical composition described above including Fn1, and furthermore the steel material contains (Cr, Nb) composite precipitates having an equivalent circular diameter of 50 to 500 nm (specific precipitates) which are present at a number density of 1.00 / µm 2< or more. Preferably, the grain size of austenite grains (austenite grain size) in the austenitic stainless steel material according to the present embodiment is 20.0 µm or less.

[0088] As mentioned above, the specific precipitates contained in the austenitic stainless steel material according to the present embodiment refine the austenite grains by a pinning effect. That is, in the austenitic stainless steel material according to the present embodiment containing the specific precipitates that are present at a number density of 1.00 / µm 2< or more, as a result of the austenite grains being refined, the austenite grain size is 20.0 µm or less.

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

[0090] In the present embodiment, the grain size of the austenite grains of the austenitic stainless steel material is determined by the following method. Specifically, a test specimen for microstructure observation is prepared from the austenitic stainless steel material. Here, if the austenitic stainless steel material is a steel pipe, a test specimen having an observation surface that includes the pipe axis direction and the wall thickness direction is prepared. If the austenitic stainless steel material is a steel bar, a test specimen having an observation surface that includes the axial direction and the cross-sectional radial direction is prepared. If the austenitic stainless steel material is a steel plate, a test specimen having an observation surface that includes the rolling elongation direction and the thickness direction is prepared. Note that, the size of the test specimen is not particularly limited as long as an observation surface that is described later can be obtained.

[0091] The observation surface of the prepared test specimen is polished to obtain a mirror surface, and thereafter electrolytic etching is performed using 10% oxalic acid-90% distilled water to reveal austenite grain boundaries. Observation of the observation surface is then performed using an optical microscope. Note that, the magnification in the microscopic observation can be set as appropriate according to the grain size. Specifically, in the microscopic observation, for example, the magnification is set so that 50 or more austenite grains are included in the visual field. By means of the optical microscope observation, the grain size (µm) of austenite grains is determined based on a method for measuring the mean intercept length in accordance with JIS G 0551 (2020). Note that, in the present embodiment, a value obtained by rounding off the obtained numerical value to the first decimal place is defined as the grain size (µm) of austenite grains.[Shape of austenitic stainless steel material]

[0092] The shape of the austenitic stainless steel material of the present embodiment is not particularly limited. The austenitic stainless steel material of the present embodiment may be a steel pipe, may be a steel plate, or may be a steel bar.[Applications of austenitic stainless steel material]

[0093] The austenitic stainless steel material according to the present embodiment can be widely applied to applications which require high strength and hydrogen embrittlement resistance. Such kinds of applications include, for example, steel materials for fuel tanks of transportation equipment which utilizes hydrogen as energy, and steel materials for pipes which connect a fuel tank to a combustion chamber. Note that, the austenitic stainless steel material according to the present embodiment is not limited to applications for transportation equipment which utilizes high-pressure hydrogen gas as energy, or hydrogen stations which supply hydrogen gas to transportation equipment.[Production method]

[0094] Hereunder, a method for producing the austenitic stainless steel material according to the present embodiment is described. The method for producing an austenitic stainless steel material that is described hereunder is one example of a method for producing the austenitic stainless steel material according to the present embodiment. That is, an austenitic stainless steel material composed as described above may also be produced by another production method that is different from the production method described hereunder. However, the production method described hereunder is a preferable example of a method for producing the austenitic stainless steel material according to the present embodiment.

[0095] One example of a method for producing the austenitic stainless steel material according to the present embodiment includes: a process of preparing a starting material (preparation process), a process of subjecting the prepared starting material to hot working to produce an intermediate steel material (hot working process), a process of cooling the produced intermediate steel material (cooling process), and a process of subjecting the cooled intermediate steel material to a specific precipitates precipitation treatment (specific precipitates precipitation treatment process). Each process is described in detail hereunder.[Preparation process]

[0096] In the preparation process, a starting material having a chemical composition consisting of, 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 to 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%, and rare earth metal: 0 to 0.5000%, with the balance being Fe and impurities, and in which Fn1 (= Ni+0.02×Cr+0.52×Mn-0.48×Mo) is 18.0 or more, or a starting material having a chemical composition consisting of, 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: 17.00 to 25.00%, N: more than 0.350 to 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%, and rare earth metal: 0 to 0.5000%, with the balance being Fe and impurities, and in which Fn1 is 22.0 or more is prepared. In the case of producing the starting material, for example, the starting material can be produced by the following method.

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

[0098] In the hot working process, the prepared starting material is subjected to hot working to produce an intermediate steel material. The hot working is not particularly limited, and for example the hot working is hot forging, hot rolling, hot extrusion, or the like. The hot forging is, for example, extend forging. As the hot rolling, for example, multi-pass rolling is performed using a reverse rolling mill or a tandem rolling mill. The hot extrusion is, for example, hot extrusion by the Ugine-Sejournet process. An intermediate steel material is produced by the above production process. The heating temperature before the hot working is, for example, 1250°C.

[0099] Preferably, from the start until the end of hot working, the temperature range is 1100 to 1250°C and the time period is four minutes or less. Here, the phrase "from the start until the end of hot working" means the time period from when the starting material is removed from the heating furnace until the final hot working ends. That is, in the hot working process according to the present embodiment, preferably the time period from when the starting material is removed from the heating furnace until the final hot working ends is four minutes or less, and the temperature of the starting material during the time period from when the starting material is removed from the heating furnace until the final hot working ends is 1100 to 1250°C.

[0100] If the time period from the start until the end of hot working is too long, and / or the temperature range from the start until the end of hot working is too low, (Cr, Nb) composite nitrides will precipitate and coarsen in the intermediate steel material during hot working. In such case, the amount of dissolved Nb in the steel material will decrease, and the precipitated amount of fine (Cr, Nb) composite nitrides in a specific precipitates precipitation treatment process to be described later will decrease. In this case, in addition, in the specific precipitates precipitation treatment process to be described later, (Cr, Nb) composite nitrides that precipitated during hot working will coarsen. In these cases, the number density of the specific precipitates in the produced austenitic stainless steel material will decrease. Therefore, in the hot working process according to the present embodiment, preferably the temperature range from the start until the end of hot working is 1100 to 1250°C, and the time period from the start until the end of hot working is four minutes or less.[Cooling process]

[0101] In the cooling process, the produced intermediate steel material is cooled. The cooling method is not particularly limited, and it suffices to use a well-known. Preferably the intermediate steel material after hot working is rapidly cooled (water-cooled) using a water cooling apparatus. If the cooling rate is too slow, (Cr, Nb) composite nitrides may precipitate in the intermediate steel material during cooling. In such case, in the specific precipitates precipitation treatment process to be described later, (Cr, Nb) composite nitrides will coarsen, and the number density of the specific precipitates in the produced austenitic stainless steel material will also decrease. Therefore, preferably the cooling rate of the intermediate steel material in the cooling process is set to 15°C / min or more. Further, although not particularly limited, the cooling stop temperature is, for example, room temperature.[Specific precipitates precipitation treatment process]

[0102] In the specific precipitates precipitation treatment process, the cooled intermediate steel material is subjected to a treatment for precipitating the specific precipitates. Specifically, the aforementioned intermediate steel material is held in the range of 1000 to 1230°C for 15 to 90 minutes. Thereafter, the intermediate steel material is rapidly cooled. Although not particularly limited, the method of rapid cooling is, for example, water cooling or oil cooling.

[0103] If the holding temperature in the specific precipitates precipitation treatment process is too low, the specific precipitates will not sufficiently precipitate, and a large amount of Cr nitrides will form. As a result, the hydrogen embrittlement resistance will decrease. On the other hand, if the holding temperature is too high, the specific precipitates will dissolve in the steel material, and the precipitated amount of the specific precipitates in the steel material will decrease. As a result, the desired strength will not be obtained. Further, if the holding time in the specific precipitates precipitation treatment process is too short, the specific precipitates will not sufficiently precipitate, and a large amount of Cr nitrides will form. As a result, the hydrogen embrittlement resistance will decrease. On the other hand, if the holding time is too long, austenite grains will coarsen and the desired strength will not be obtained. Therefore, in the specific precipitates precipitation treatment process of the present embodiment, preferably the intermediate steel material is held in the range of 1000 to 1230°C for 15 to 90 minutes.

[0104] The austenitic stainless steel material according to the present embodiment can be produced by the production method described above. Note that, the production method described above is one example, and the austenitic stainless steel material according to the present embodiment may also be produced by another production method. Hereunder, the advantageous effects of the austenitic stainless steel material according to the present embodiment are described more specifically by way of examples. Note that, the conditions adopted in the following examples are one example of conditions employed for confirming the feasibility and advantageous effects of the austenitic stainless steel material according to the present embodiment. Accordingly, the austenitic stainless steel material according to the present embodiment is not limited to this one example of conditions.EXAMPLE 1

[0105] In Example 1, the advantageous effects of the austenitic stainless steel material according to the present embodiment in a case where the content of Ni was 10.00 to less than 17.00% were confirmed. Specifically, molten steels having the chemical compositions shown in Table 1-1 and Table 1-2 were melted in a high frequency melting furnace to produce ingots. Further, for each test number, Fn1 determined based on the chemical composition and Formula (1) is shown in Table 2.[Table 1-1]

[0106] TABLE 1-1Test NumberChemical Composition (unit is mass%; balance is Fe and impurities)CSiMnPSCrNiNVNbAlOMo1-10.0100.3638.620.0090.000924.4316.950.4290.0650.0350.0480.0024-1-20.0160.07211.620.0040.000324.8712.060.6490.0250.0110.0550.0048-1-30.0020.1078.740.0190.000624.0715.430.4370.0400.0470.0500.0030-1-40.0130.28612.390.0160.000724.7412.920.6530.0700.0940.0070.0031-1-50.0060.07310.270.0170.000225.4315.990.5670.0200.0220.0400.0037-1-60.0180.39612.220.0030.000522.9714.710.5070.0910.0750.0800.0046-1-70.0380.3879.850.0070.000326.4414.880.6310.0780.0910.0900.0048-1-80.0330.48113.480.0150.000922.0612.820.5300.0670.0690.0990.0008-1-90.0310.48511.110.0100.000925.0615.720.5790.0840.0310.0080.0030-1-100.0260.00310.430.0110.000524.2313.630.5380.0590.0720.0090.0032-1-110.0120.14813.540.0070.000222.4913.160.5540.0240.0690.0860.0004-1-120.0390.38810.860.0190.000624.0913.550.5460.0460.0870.0030.0006-1-130.0120.42510.340.0030.000624.6912.790.5770.0880.0580.0310.0020-1-140.0010.0049.310.0130.000626.9614.160.6570.0490.0570.0660.0030-1-150.0360.32314.300.0030.000323.4412.990.6400.0260.0470.0460.0032-1-160.0290.23610.710.0010.000622.1712.240.4470.0810.0190.0350.0005-1-170.0240.05113.850.0090.000124.7314.130.6850.0470.0910.0650.0043-1-180.0290.4049.860.0020.000423.9412.780.5140.0950.0460.0840.00330.021-190.0070.3948.360.0060.000328.6216.470.6850.0170.0230.0460.0010-1-200.0100.4998.130.0080.000624.2113.970.4490.0240.0760.0170.0048-1-210.0250.2899.870.0180.000326.2815.760.6070.0180.0110.0920.0047-1-220.0020.0828.120.0050.000727.1016.040.5890.0180.0240.0170.0003-1-230.0370.02114.040.0180.000322.7612.520.5990.0110.0450.0840.0023-1-240.0080.33412.930.0060.000423.1413.790.5600.0130.0460.0180.0037-1-250.0400.22114.890.0100.000522.4212.090.6160.0980.0210.0510.0038-1-260.0090.1699.160.0030.000726.2113.860.6090.0640.0600.0850.0017-1-270.0160.26011.980.0170.000723.0715.270.4960.0800.0520.0760.0041-1-280.0380.15313.420.0080.000624.4913.210.6700.0650.0970.0620.0034-1-290.0190.0788.280.0060.000525.7116.810.4970.0110.0520.0200.0037-1-300.0360.13410.210.0050.000523.4313.310.4880.0570.0180.0490.0046-1-310.0250.08910.570.0060.000826.0312.260.6770.0350.0340.0690.0013-1-320.0030.49212.720.0130.001023.4012.270.5940.0470.0670.0570.0010-1-330.0160.15411.220.0110.000625.1913.620.6260.0650.0990.0920.0021-1-340.0370.32510.970.0110.000425.0315.720.5720.0200.0370.0680.0033-1-350.0190.3209.670.0190.000522.0212.610.3940.0520.0960.0390.0008-1-360.0190.1768.040.0140.000127.6115.750.6230.0480.0950.0700.00140.071-370.0270.2728.310.0030.001026.9113.800.6230.0700.0740.0340.0008-1-380.0110.23911.040.0030.001024.5812.030.6090.0440.0710.0180.0007-1-390.0330.19511.790.0120.000624.4912.630.6200.0980.0170.0780.0031-1-400.0400.39111.170.0180.000224.9613.480.6130.0440.0280.0210.0017-1-410.0090.1069.120.0170.000126.7115.330.6140.0780.0220.0380.0017-1-420.0180.32111.130.0080.000823.3913.130.5200.0260.0740.0440.00130.041-430.0220.1629.140.0030.000223.3712.880.4540.0810.0500.0830.0019-1-440.0110.1238.100.0030.000925.9113.440.5610.0250.0340.0130.0025-1-450.0370.36412.550.0130.001022.7514.170.5150.0700.0800.0130.00430.041-460.0360.21910.820.0160.000525.0913.870.6020.0590.0220.0700.0038-1-470.0980.08218.190.0150.000123.0516.130.3510.0580.0290.0550.0025-1-480.0120.93612.730.0120.000419.0112.750.4430.1900.0700.0240.0023-1-490.0220.07410.280.0200.000925.2615.650.5610.0560.2800.1900.0039-1-500.0130.3168.320.0110.000827.9416.440.6420.0670.0130.0950.00140.871-510.0370.28810.870.0020.000723.2512.030.5230.0950.0510.0400.0041-1-520.0110.33410.710.0010.000826.3013.600.6770.0170.0740.0680.0011-1-530.0050.2489.300.0080.000424.7615.720.4940.0650.0820.0120.0041-1-540.0330.1909.510.0070.000630.7616.330.3020.1520.1700.0810.0097-1-550.0830.25717.460.0240.000817.0812.560.3730.0700.2480.0880.0045-1-560.0640.41116.540.0160.000418.569.570.4820.1110.1050.0910.0006-1-570.0810.47021.380.0400.001018.6810.930.6400.1340.2230.1910.0044-1-580.0540.0687.190.0160.000327.0115.090.5660.0890.1810.1100.0092-1-590.0510.5698.200.0250.000822.1014.800.3100.0420.0620.0570.0097-1-600.0320.85914.570.0250.000120.6311.660.5030.0640.3120.0590.0046-1-610.0270.16511.290.0070.000325.7213.050.6710.086-0.0510.0008-1-620.0950.5378.350.0500.000827.1911.840.6760.1670.1710.1090.0043-1-630.0130.1598.600.0110.000825.9314.970.5530.0680.0940.0600.0015-1-640.0010.3848.800.0180.000225.9315.030.5590.0740.0690.0630.0036- [Table 1-2]

[0107] TABLE 1-2Test NumberChemical Composition (unit is mass%; balance is Fe and impurities)WTiZrHfTaCuSnCoBMgCaREM1-1--------0.005---1-2--------0.001---1-3--------0.003---1-4--------0.004---1-5--------0.002---1-6--------0.004---1-7--------0.004---1-8--------0.001---1-9--------0.003---1-10--------0.004---1-11--------0.005---1-12--------0.003---1-13------------1-14--------0.001---1-15--------0.004---1-16--------0.003---1-17------------1-18--------0.001---1-190.39-------0.004---1-20-0.010------0.005---1-21--0.094-----0.003---1-22---0.056----0.001---1-23----0.004---0.003---1-24-----0.49--0.004---1-25------0.03-0.002---1-26-------0.470.002---1-27--------0.0040.0001--1-28--------0.003-0.0001-1-29--------0.005--La:0.00431-30--------0.002--Ce:0.00601-31--------0.003--Y:0.00341-32--------0.004--Sm:0.00711-33--------0.005--Pr:0.00171-34--------0.005--Nd:0.00961-350.350.019----------1-36-----0.81--0.003---1-370.34-------0.0020.0001--1-38--0.016---0.00-0.002---1-39---0.010------0.0002-1-40-------1.960.003--La:0.00991-410.33---0.0010.63------1-42-0.006---------Ce:0.00391-430.30-----0.02-0.002--Y:0.00271-44--0.090----0.640.001--Sm:0.00341-45---0.015-0.76--0.002--Pr:0.00041-46------------1-47------------1-48------------1-49------------1-50-0.089----------1-511.98--0.089--------1-52----0.192----0.0047-Ce:0.14531-53--------0.018-0.0043La:0.49531-54--------0.012---1-55--------0.000---1-56--------0.010---1-57--------0.016---1-58--------0.018---1-59--------0.016---1-60--------0.012---1-61--------0.003---1-62--------0.018---1-63--------0.002---1-64--------0.004--- [Table 2]

[0108] TABLE 2Test NumberFn1Hot WorkingSpecific Precipitates Precipitation TreatmentTS (MPa)YS (MPa)Precipitated Amount of Specific Precipitates (mass%)Specific Precipitates Number Density ( / µm 2< )γ Grain Size (µm)Hydrogen Embrittlement ResistanceTemperature RangeWorking TimeHolding Temperature (°C)Holding Time (mins)Relative Breaking Elongation L (L1 / L0)1-121.9AA1060309835880.1672.228.30.821-218.6AA10606010565950.0541.5319.80.991-320.5AA1060159846090.1812.387.20.971-419.9AA11004511236870.2642.7615.00.891-521.8AA10603010856470.1181.9811.60.891-621.5AA11003010046010.1932.4612.30.831-720.5AA11406010846620.2232.5517.30.991-820.3AA1060459715890.1682.2412.90.831-922.0AA10603010486660.1242.0610.20.931-1019.5AA11004510445920.2592.7215.90.851-1120.7AA11006010815930.1942.4118.81.001-1219.7AA1140609585670.2262.6018.10.951-1318.7AA10606010055610.2162.5019.60.811-1419.5AA10606011256390.1792.3318.00.891-1520.9AA10606011046200.2042.4319.00.821-1618.3AA1060309856310.0851.766.50.941-1721.8AA11806011296940.2492.7312.70.861-1818.4AA11006010505790.1362.0816.00.951-1921.4AA11006010987160.0911.7614.70.901-2018.7AA1100309916160.1892.367.30.811-2121.4AA10604510276480.1412.1114.40.861-2220.8AA10603011467810.1752.327.40.881-2320.3AA11004510766600.1462.1113.00.871-2421.0AA10604510396360.1201.9513.40.861-2520.3AA11004511006490.0791.7314.00.941-2619.1AA11003011147440.2252.538.40.911-2722.0AA1100609925320.2222.5318.01.001-2820.7AA11404511366640.2922.8714.20.891-2921.6AA11003010876790.2132.527.81.001-3019.1AA10604510005400.1211.9814.30.881-3118.3AA10603011468180.1352.037.50.961-3219.4AA11009010616290.2242.6116.40.811-3320.0AA11406010986240.2662.7219.30.831-3421.9AA1100609735660.1902.3819.00.981-3518.1AA1100309566000.2722.797.20.941-3620.4AA11406010776280.3113.0016.10.901-3718.7AA11003011417740.2552.747.00.921-3818.3AA11003011407490.2102.487.80.811-3919.3AA10604511026530.0631.5313.00.971-4019.8AA10603011367290.1542.2110.50.901-4120.6AA10604510946860.0761.6912.30.981-4219.4AA11006010045470.2402.6916.20.941-4318.1AA1100459725400.2022.5113.90.861-4418.2AA11004510226400.1942.4513.80.891-4521.1AA1100459795840.2482.7014.50.851-4620.0AA10603011126900.0731.6310.10.941-4726.0AA11003010115850.0471.4312.10.991-4819.7AA11003010306440.0811.7312.80.821-4921.5AA11403011006210.2622.7216.70.841-5020.9AA11003010536570.0401.3813.80.871-5118.1AA11003010876810.1041.919.60.941-5219.7AA11003010666880.1402.1412.80.901-5321.1AA11003010055850.1272.0711.30.931-5421.9AA11804512148760.4263.4411.20.671-5522.0AA1100309054950.5623.9310.30.981-5618.5AA1140459575450.2552.7215.00.581-5722.4AA--------1-5819.4AA1180609795730.4493.4918.30.711-5919.5AA1060308564520.1462.109.40.971-6019.6AA1140309835560.6984.355.20.701-6119.4AA102030902512--31.20.871-6216.7AA11806010776660.4303.4114.70.791-6320.0AA1250458914980.0010.4446.80.921-6420.1BB10104510326810.0790.7112.80.66

[0109] The produced ingot of each test number was held at 1230°C for one hour, and thereafter subjected to hot forging to produce a block with a thickness of 70 mm. The block of each test number was heated to 1230°C, and thereafter subjected to hot rolling to produce a plate material (intermediate steel material) with a thickness of 9 mm. In the hot rolling, in a case where the temperature range from when the block was removed from the heating furnace until the final hot rolling ended was 1100 to 1250°C, "A" is shown in the column "Temperature Range" of the column "Hot Working" in Table 2. In a case where the temperature range from when the block was removed from the heating furnace until the final hot rolling ended was 950 to 1250°C, "B" is shown in the column "Temperature Range" of the column "Hot Working" in Table 2. In addition, in the hot rolling, in a case where the time period from when the block was removed from the heating furnace until the final hot rolling ended was three minutes or less, "A" is shown in the column "Working Time" of the column "Hot Working" in Table 2. In a case where the time period from when the block was removed from the heating furnace until the final hot rolling ended was six to seven minutes, "B" is shown in the column "Working Time" of the column "Hot Working" in Table 2.

[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, the intermediate steel material of each test number was subjected to a specific precipitates precipitation treatment. The holding temperature (°C) and holding time (mins) in the specific precipitates precipitation treatment performed for each test number are shown in Table 2. A steel plate of each test number was produced by the above process. Note that, in the steel plate of Test No. 1-57, a crack occurred during forging or hot working. Therefore, this steel plate was not subjected to a specific precipitates precipitation treatment.[Evaluation tests]

[0111] The steel plate of each test number was subjected to a tensile test, a test to measure the precipitated amount of specific precipitates, an austenite grain size measurement test, and a hydrogen embrittlement resistance evaluation test as described hereunder. Note that, in the steel plate of Test No. 1-57, a crack occurred during forging or hot working. Therefore, this steel plate was not subjected to the aforementioned evaluation tests.[Tensile test]

[0112] The steel plate of each test number was subjected to a tensile test by the method described above. Specifically, a round bar tensile test specimen in which the parallel portion diameter was 8 mm was prepared from a central portion of the thickness of the steel plate of each test number. The axial direction of the round bar tensile test specimen was parallel to the rolling elongation direction of the steel plate. The round bar tensile test specimen of each test number was subjected to a tensile test at normal temperature in air in accordance with JIS Z 2241: 2011. The tensile strength (MPa) of each test number determined by the tensile test is shown in the column "TS (MPa)" in Table 2. In addition, the yield stress (MPa) of each test number determined by the tensile test is shown in the column "YS (MPa)" in Table 2.[Test to measure precipitated amount of specific precipitates]

[0113] The steel plate of each test number was subjected to a test to measure the precipitated amount of specific precipitates by the method described above. Specifically, a test specimen was prepared by the method described above from a central portion of the thickness of the steel plate of each test number. The observation surface of the test specimen was a surface that was perpendicular to the rolling elongation direction of the steel plate. The test specimen of each test number was subjected to electrolysis by the method described above, and residue was captured. At such time, the difference in mass between the test specimen before the electrolysis and the test specimen after the electrolysis was defined as the electrolysis amount. The obtained residue was subjected to acid decomposition, and ICP emission spectrometry was performed to determine the concentration of Cr and Nb in the residue. The mass which Cr and Nb occupied in the residue was determined based on the concentration of Cr and Nb in the residue and the mass of the residue, and the determined mass of Cr and Nb was divided by the electrolysis amount to obtain the concentration (mass%) of the Cr and Nb elements. The obtained concentration of the Cr and Nb elements was defined as the precipitated amount (mass%) of the specific precipitates. The precipitated amount (mass%) of the specific precipitates of each test number is shown in Table 2.[Austenite grain size measurement test]

[0114] The steel plate of each test number was subjected to an austenite grain size measurement test by the method described above. Specifically, a test specimen for microstructure observation was prepared from a central portion of the thickness of the steel plate of each test number by the method described above. For the test specimen of each test number, austenite grain boundaries were revealed by the method described above, and observation of the observation surface was performed using an optical microscope. By means of the optical microscope observation, the grain size (µm) of austenite grains was determined based on a method for measuring the mean intercept length in accordance with JIS G 0551 (2020). The determined austenite grain size (µm) of each test number is shown in the column "γ Grain Size (µm)" in Table 2.[Hydrogen embrittlement resistance evaluation test]

[0115] The steel plate of each test number was subjected to a hydrogen embrittlement resistance evaluation test by the method described above. Specifically, round bar tensile test specimens having a parallel portion diameter of 3 mm were prepared from a central portion of the thickness of the steel plate of each test number. The axial direction of each round bar tensile test specimen was parallel to the rolling elongation direction of the steel plate. One round bar tensile test specimen of each test number was subjected to an SSRT (slow strain rate test) at normal temperature in air to determine a breaking elongation L0 (%) in air. Another round bar tensile test specimen of each test number was subjected to an SSRT at normal temperature in hydrogen gas at 45 MPa to determine a breaking elongation L1 (%) in high-pressure hydrogen gas. In each SSRT, the strain rate was set to 3×10 -6< / sec. In addition, a relative breaking elongation L (L = L1 / L0) was determined for each test number. The determined relative breaking elongation L of each test number is shown in Table 2.[Evaluation results]

[0116] Referring to Table 1-1, Table 1-2, and Table 2, the steel plates of Test Nos. 1-1 to 1-53 had the chemical composition described above, and in these steel plates the value of Fn1 was 18.0 or more, and the production method also satisfied the preferred conditions described above. In these steel plates, the number density of the specific precipitates was 1.00 / µm 2< or more. As a result, in these steel plates the tensile strength was 950 MPa or more, and thus these steel plates had high strength. Furthermore, as a result, in these steel plates the relative breaking elongation L was 0.80 or more, and thus these steel plates had excellent hydrogen embrittlement resistance in a high-pressure hydrogen gas environment. Note that, in each of these steel plates the precipitated amount of specific precipitates was 0.010% by mass or more.

[0117] On the other hand, in the steel plate of Test No. 1-54, the content of Cr was too high and the content of N was too low. As a result, in this steel plate the relative breaking elongation L was less than 0.80, and thus this steel plate did not have excellent hydrogen embrittlement resistance in a high-pressure hydrogen gas environment.

[0118] In the steel plate of Test No. 1-55, the content of Cr was too low. As a result, in this steel plate the tensile strength was less than 950 MPa, and thus this steel plate did not have high strength.

[0119] In the steel plate of Test No. 1-56, the content of Ni was too low. As a result, in this steel plate the relative breaking elongation L was less than 0.80, and thus this steel plate did not have excellent hydrogen embrittlement resistance in a high-pressure hydrogen gas environment.

[0120] In the steel plate of Test No. 1-58, the content of Mn was too low. As a result, in this steel plate the relative breaking elongation L was less than 0.80, and thus this steel plate did not have excellent hydrogen embrittlement resistance in a high-pressure hydrogen gas environment.

[0121] In the steel plate of Test No. 1-59, the content of N was too low. As a result, in this steel plate the tensile strength was less than 950 MPa, and thus this steel plate did not have high strength.

[0122] In the steel plate of Test No. 1-60, the content of Nb was too high. As a result, in this steel plate the relative breaking elongation L was less than 0.80, and thus this steel plate did not have excellent hydrogen embrittlement resistance in a high-pressure hydrogen gas environment.

[0123] In the steel plate of Test No. 1-61, the content of Nb was too low and the number density of the specific precipitates was less than 1.00 / µm 2< . As a result, in this steel plate the tensile strength was less than 950 MPa, and thus this steel plate did not have high strength.

[0124] In the steel plate of Test No. 1-62, Fn1 was less than 18.0. As a result, in this steel plate the relative breaking elongation L was less than 0.80, and thus this steel plate did not have excellent hydrogen embrittlement resistance in a high-pressure hydrogen gas environment.

[0125] In the steel plate of Test No. 1-63, the holding temperature in the specific precipitates precipitation treatment was too high, and hence the number density of the specific precipitates was less than 1.00 / µm 2< . As a result, in this steel plate the tensile strength was less than 950 MPa, and thus this steel plate did not have high strength.

[0126] In the steel plate of Test No. 1-64, the time period of hot working was too long, and hence the number density of the specific precipitates was less than 1.00 / µm 2< . As a result, in this steel plate the relative breaking elongation L was less than 0.80, and thus this steel plate did not have excellent hydrogen embrittlement resistance in a high-pressure hydrogen gas environment.EXAMPLE 2

[0127] In Example 2, the advantageous effects of the austenitic stainless steel material according to the present embodiment in a case where the content of Ni was 17.00 to 25.00% were confirmed. Specifically, molten steels having the chemical compositions shown in Table 3-1 and Table 3-2 were melted in a high frequency melting furnace to produce ingots. Further, for each test number, Fn1 determined based on the chemical composition and Formula (1) is shown in Table 4.[Table 3-1]

[0128] TABLE 3-1Test NumberChemical Composition (unit is mass%; balance is Fe and impurities)CSiMnPSCrNiNVNbAlOMo2-10.0160.0288.910.0190.001028.3617.080.6760.0130.0200.0400.0030-2-20.0080.1739.390.0010.000425.6918.540.5050.0990.0560.0850.0033-2-30.0170.16212.150.0060.000422.2317.050.4200.0880.0490.0260.0005-2-40.0040.22410.230.0120.000526.5017.050.6120.0460.0900.0500.0004-2-50.0380.33911.460.0140.000523.6117.550.4720.0870.1000.0780.0035-2-60.0140.4709.590.0020.000826.4517.390.5790.0580.0570.0540.0008-2-70.0180.1718.490.0190.000727.8717.360.6280.0790.0350.0860.0020-2-80.0060.44810.890.0060.000225.4517.640.5620.0220.0380.0030.0046-2-90.0310.4969.190.0170.000927.8318.280.6330.0180.0980.0810.0006-2-100.0250.46611.620.0060.000622.5117.310.4120.0550.0810.0620.0012-2-110.0070.2359.770.0040.000927.7017.210.6660.0100.0530.0480.0028-2-120.0230.1778.990.0030.000525.9717.760.5210.0180.0230.0760.0019-2-130.0150.0818.960.0120.000826.5317.370.5630.0460.0430.0900.0045-2-140.0140.41111.070.0180.000326.0517.120.6130.0920.0110.0930.0026-2-150.0220.2928.790.0130.000828.8318.420.6800.0600.0790.0360.0008-2-160.0210.1489.550.0050.001024.4517.420.4550.0670.0300.0480.0009-2-170.0190.3119.310.0100.000728.4718.620.6720.0580.0150.0750.0044-2-180.0190.1048.140.0030.000825.2318.560.4320.0610.0490.0060.0003-2-190.0070.0188.650.0030.000828.7317.720.6790.0110.0630.0990.00250.022-200.0170.36810.390.0030.000624.8217.140.5150.0850.0430.0510.0003-2-210.0040.15110.950.0170.000223.9517.050.4820.0320.0620.0510.0025-2-220.0100.3008.530.0130.000426.1417.450.5220.0200.0790.0630.0007-2-230.0110.36210.810.0140.000927.4817.660.6820.0310.0300.0950.0045-2-240.0400.1438.210.0200.000326.6119.000.5110.0540.0190.0750.0020-2-250.0070.2959.580.0110.000323.7617.810.4070.0190.0890.0100.0031-2-260.0340.0509.410.0020.000127.4117.210.6350.0410.0450.0440.0045-2-270.0370.3859.380.0180.000824.9418.480.4610.0710.0810.0790.0010-2-280.0330.15311.270.0030.000824.3417.200.5150.0900.0350.0150.0030-2-290.0160.08410.240.0170.000326.2517.590.5880.0460.0930.0890.0047-2-300.0290.3139.960.0100.000227.8317.120.6810.0980.0930.0990.0038-2-310.0170.4808.830.0110.000325.7818.810.4870.0120.0440.0550.0049-2-320.0290.4629.740.0120.000825.7417.920.5340.0700.0880.0630.0033-2-330.0380.4119.200.0130.000625.9418.630.5130.0450.0590.0890.0007-2-340.0380.35710.970.0030.000725.8517.150.5960.0360.0660.0870.0019-2-350.0350.25811.670.0120.000623.0917.470.4490.0500.0160.0240.0037-2-360.0180.3528.820.0090.000326.2218.770.5150.0740.0710.0350.0024-2-370.0020.1278.300.0080.000327.3518.590.5660.0460.0250.0790.00110.042-380.0050.0658.900.0120.000926.9017.620.5790.0870.0680.0320.0012-2-390.0310.42511.200.0100.000224.0317.470.4870.0750.0310.0910.0030-2-400.0390.1449.720.0020.000526.3417.760.5700.0440.0130.0860.0030-2-410.0020.06310.110.0060.000626.8717.460.6240.0160.0650.0750.0005-2-420.0360.1618.520.0030.000427.8918.710.6070.0300.0670.0930.0029-2-430.0070.4128.120.0110.000529.0218.850.6590.0220.0640.0450.00430.102-440.0280.4628.240.0180.000627.9818.040.6130.0330.0140.0310.0027-2-450.0370.22210.150.0110.000727.3117.960.6430.0520.0130.0500.0049-2-460.0080.3138.010.0120.000525.8017.910.4760.0460.0740.0130.00500.032-470.0150.2048.560.0180.000727.6517.400.6340.1290.0350.0650.0020-2-480.0930.65619.330.0170.001224.8319.280.6000.0180.0780.0810.0006-2-490.0210.96615.620.0060.000818.5117.290.3920.1850.0810.0280.0012-2-500.0090.13510.560.0040.001127.5024.210.3560.0100.0540.0420.0012-2-510.0210.1798.910.0030.000525.7217.700.5260.0190.2930.0210.0019-2-520.0130.0758.930.0120.000826.8518.600.5710.0420.0430.0620.00320.922-530.0130.1128.260.0120.003025.2117.200.5110.0720.0740.0300.0024-2-540.0370.1559.890.0020.000526.8117.600.5500.0980.0150.0860.0030-2-550.0350.1058.120.0030.000525.8917.870.5230.0780.0190.0850.0022-2-560.0390.4178.490.0270.000531.3823.430.3520.0630.0550.1360.0012-2-570.0190.42519.160.0350.000916.5318.070.3800.1010.2210.0930.0018-2-580.0410.1688.480.0300.000925.9926.510.1200.1880.2620.0360.0011-2-590.0040.86220.660.0250.000818.6224.620.3780.1920.0290.0460.0042-2-600.0500.3236.410.0340.000529.0119.360.6300.0380.2240.0200.0081-2-610.0280.25911.190.0220.000120.5518.150.2870.0910.2320.0820.0065-2-620.0120.86414.010.0190.000422.3817.260.5480.0190.3120.0870.0096-2-630.0360.44511.550.0020.000425.2017.250.5770.093-0.0280.0008-2-640.0990.8608.520.0420.000321.1317.010.4870.1380.1000.1310.0087-2-650.0360.3328.290.0150.000326.1018.810.4880.0440.0450.0510.0037-2-660.0250.0519.850.0070.000526.6317.510.5990.0240.0690.0400.0021- [Table 3-2]

[0129] TABLE 3-2Test NumberChemical Composition (unit is mass%; balance is Fe and impurities)WTiZrHfTaCuSnCoBMgCaREM2-1--------0.001---2-2--------0.005---2-3--------0.000---2-4--------0.005---2-5--------0.003---2-6--------0.002---2-7--------0.004---2-8--------0.005---2-9--------0.001---2-10--------0.002---2-11--------0.001---2-12--------0.003---2-13--------0.003---2-14--------0.004---2-15--------0.001---2-16--------0.003---2-17--------0.001---2-18--------0.004---2-19--------0.004---2-200.31-------0.003---2-21-0.052------0.004---2-22--0.011-----0.004---2-23---0.008----0.005---2-24----0.006---0.001---2-25-----0.67--0.001---2-26------0.03-0.002---2-27-------1.51----2-28--------0.0030.0001--2-29--------0.002-0.0002-2-30--------0.001--La:0.00742-31--------0.005--Ce:0.00992-32--------0.004--Y:0.00852-33--------0.001--Sm:0.00132-34--------0.004--Pr:0.00482-35--------0.002--Nd:0.00132-360.310.044------0.004---2-37-----0.54--0.004---2-380.34-------0.0050.0001--2-39--0.028---0.03-0.004---2-40---0.051----0.002-0.0001-2-41-------1.800.005--La:0.00712-420.32---0.0080.31--0.001---2-43-0.073------0.002--Ce:0.00842-440.32-----0.02-0.003--Y:0.00052-45--0.010----1.890.004--Sm:0.00682-46---0.094-0.77--0.004--Pr:0.00522-47------------2-48------------2-49------------2-50------------2-51------------2-52---------0.0045-Ce:0.17942-531.87-0.089---------2-54--------0.018-0.0042La:0.18012-55----0.186-------2-56--------0.016---2-57--------0.006---2-58--------0.001---2-59--------0.010---2-60--------0.006---2-61--------0.011---2-62--------0.017---2-63--------0.005---2-64--------0.001---2-65--------0.005---2-66--------0.001--- [Table 4]

[0130] TABLE 4Test NumberFn1Hot WorkingSpecific Precipitates Precipitation TreatmentTS (MPa)YS (MPa)Precipitated Amount of Specific Precipitates (mass%)Specific Precipitates Number Density ( / µm2)γ Grain Size (µm)Hydrogen Embrittlement ResistanceTemperature RangeWorking TimeHolding Temperature (°C)Holding Time (mins)Relative Breaking Elongation L (L1 / L0)2-122.3AA11003010946780.0701.6114.00.952-223.9AA10603010646270.0601.5711.60.902-323.8AA1020309726150.0631.596.70.832-422.9AA11401511267410.0981.869.50.882-524.0AA11801510456050.1552.1910.60.992-622.9AA11003011157190.0681.579.50.862-722.3AA10603010566070.0701.6317.70.852-823.8AA10203011337500.1041.856.30.852-923.6AA11404510946990.1562.2511.10.832-1023.8AA1100609855360.0981.8514.00.812-1122.8AA11009010776610.1372.0719.90.952-1223.0AA10603010376140.0901.819.80.992-1322.6AA10604510736460.0821.7411.70.942-1423.4AA10603010786290.0841.7218.00.912-1523.6AA11003011287550.0921.8112.30.902-1622.9AA10203010206030.0891.787.70.892-1724.0AA10606011337290.0891.8013.80.832-1823.3AA1060309515880.0931.829.10.992-1922.8AA11004511296890.0501.4814.70.802-2023.0AA1100309725670.1392.1114.10.952-2123.2AA1100309915480.1001.8913.60.982-2222.4AA10603010667160.0781.676.80.892-2323.8AA11003010526470.0621.6016.50.832-2423.8AA1060459835720.1322.0215.60.842-2523.3AA10603010085980.0751.708.40.862-2622.7AA10603011066820.1362.0415.60.892-2723.9AA10203010506900.0821.746.50.912-2823.5AA10203010626800.0791.716.70.982-2923.4AA10603010246300.1242.0113.20.972-3022.9AA11803010766800.0711.6014.30.852-3123.9AA1060609855490.0651.5617.50.982-3223.5AA11003010195800.0781.6617.90.912-3323.9AA11003010456210.0581.519.10.852-3423.4AA11006010075780.0761.7017.90.852-3524.0AA1060309855610.1272.0513.90.872-3623.9AA11003010325910.1121.8915.30.952-3723.4AA10603011297440.0391.338.00.982-3822.8AA11003010686750.0761.739.10.992-3923.8AA10603010676690.0531.457.00.942-4023.3AA10603011316620.0901.7611.30.942-4123.3AA11003011407320.0731.639.60.932-4223.7AA11009010656320.1011.8317.40.882-4323.6AA11003010426190.1302.0919.40.932-4422.9AA10606011306620.0881.7813.20.922-4523.8AA10603011006600.1232.0113.10.982-4622.6AA11003010396470.1382.087.30.912-4722.4AA11003010766120.0951.8416.90.872-4829.8AA11403010916670.1782.3310.90.812-4925.8AA1100309755160.1282.0512.00.832-5030.3AA11003010666590.1462.1716.90.892-5122.8AA12003010376140.2302.627.20.892-5223.3AA11003010906850.0711.6212.00.872-5322.0AA11003010295810.1412.0716.00.942-5423.3AA10603010816510.0731.6511.30.872-5522.6AA10603010846490.0711.6111.60.862-5628.5AA10603012037660.0521.4716.20.622-5728.4AA1060308694740.1532.2214.10.902-5831.4AA1180308924950.1181.9413.70.912-5935.7AA--------2-6023.3AA11806010786080.1362.0717.90.652-6124.4AA1180308444500.1342.128.00.812-6225.0AA11403010156260.1552.175.20.722-6323.8AA102030860450--22.10.922-6421.9AA1100459555030.1482.1919.60.782-6523.6AA1250459225120.0010.1925.80.852-6623.2BB10104510586660.0750.708.40.45

[0131] The produced ingot of each test number was held at 1230°C for one hour, and thereafter subjected to hot forging to produce a block with a thickness of 70 mm. The block of each test number was heated to 1230°C, and thereafter subjected to hot rolling to produce a plate material (intermediate steel material) with a thickness of 9 mm. In the hot rolling, in a case where the temperature range from when the block was removed from the heating furnace until the final hot rolling ended was 1100 to 1250°C, "A" is shown in the column "Temperature Range" of the column "Hot Working" in Table 4. In a case where the temperature range from when the block was removed from the heating furnace until the final hot rolling ended was 950 to 1250°C, "B" is shown in the column "Temperature Range" of the column "Hot Working" in Table 4. In addition, in the hot rolling, in a case where the time period from when the block was removed from the heating furnace until the final hot rolling ended was three minutes or less, "A" is shown in the column "Working Time" of the column "Hot Working" in Table 4. In a case where the time period from when the block was removed from the heating furnace until the final hot rolling ended was six to seven minutes, "B" is shown in the column "Working Time" of the column "Hot Working" in Table 4.

[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, the intermediate steel material of each test number was subjected to a specific precipitates precipitation treatment. The holding temperature (°C) and holding time (mins) in the specific precipitates precipitation treatment performed for each test number are shown in Table 4. A steel plate of each test number was produced by the above process. Note that, in the steel plate of Test No. 2-59, a crack occurred during forging or hot working. Therefore, this steel plate was not subjected to a specific precipitates precipitation treatment.[Evaluation tests]

[0133] The steel plate of each test number was subjected to a tensile test, a test to measure the precipitated amount of specific precipitates, an austenite grain size measurement test, and a hydrogen embrittlement resistance evaluation test as described hereunder. Note that, in the steel plate of Test No. 2-59, a crack occurred during forging or hot working. Therefore, this steel plate was not subjected to the aforementioned evaluation tests.[Tensile test]

[0134] The steel plate of each test number was subjected to a tensile test by the method described above. Specifically, a round bar tensile test specimen in which the parallel portion diameter was 8 mm was prepared from a central portion of the thickness of the steel plate of each test number. The axial direction of the round bar tensile test specimen was parallel to the rolling elongation direction of the steel plate. The round bar tensile test specimen of each test number was subjected to a tensile test at normal temperature in air in accordance with JIS Z 2241: 2011. The tensile strength (MPa) of each test number determined by the tensile test is shown in the column "TS (MPa)" in Table 4. In addition, the yield stress (MPa) of each test number determined by the tensile test is shown in the column "YS (MPa)" in Table 4.[Test to measure precipitated amount of specific precipitates]

[0135] The steel plate of each test number was subjected to a test to measure the precipitated amount of specific precipitates by the method described above. Specifically, a test specimen was prepared by the method described above from a central portion of the thickness of the steel plate of each test number. The observation surface of the test specimen was a surface that was perpendicular to the rolling elongation direction of the steel plate. The test specimen of each test number was subjected to electrolysis by the method described above, and residue was captured. At such time, the difference in mass between the test specimen before the electrolysis and the test specimen after the electrolysis was defined as the electrolysis amount. The obtained residue was subjected to acid decomposition, and ICP emission spectrometry was performed to determine the concentration of Cr and Nb in the residue. The mass which Cr and Nb occupied in the residue was determined based on the concentration of Cr and Nb in the residue and the mass of the residue, and the determined mass of Cr and Nb was divided by the electrolysis amount to obtain the concentration (mass%) of the Cr and Nb elements. The obtained concentration of the Cr and Nb elements was defined as the precipitated amount (mass%) of the specific precipitates. The precipitated amount (mass%) of the specific precipitates of each test number is shown in Table 4.[Austenite grain size measurement test]

[0136] The steel plate of each test number was subjected to an austenite grain size measurement test by the method described above. Specifically, a test specimen for microstructure observation was prepared from a central portion of the thickness of the steel plate of each test number by the method described above. For the test specimen of each test number, austenite grain boundaries were revealed by the method described above, and observation of the observation surface was performed using an optical microscope. By means of the optical microscope observation, the grain size (µm) of austenite grains was determined based on a method for measuring the mean intercept length in accordance with JIS G 0551 (2020). The determined austenite grain size (µm) of each test number is shown in the column "γ Grain Size (µm)" in Table 4.[Hydrogen embrittlement resistance evaluation test]

[0137] The steel plate of each test number was subjected to a hydrogen embrittlement resistance evaluation test by the method described above. Specifically, round bar tensile test specimens having a parallel portion diameter of 3 mm were prepared from a central portion of the thickness of the steel plate of each test number. The axial direction of each round bar tensile test specimen was parallel to the rolling elongation direction of the steel plate. One round bar tensile test specimen of each test number was subjected to an SSRT (slow strain rate test) at normal temperature in air to determine a breaking elongation L0 (%) in air. Another round bar tensile test specimen of each test number was subjected to an SSRT at normal temperature in hydrogen gas at 85 MPa to determine a breaking elongation L1 (%) in high-pressure hydrogen gas. In each SSRT, the strain rate was set to 3×10 -6< / sec. In addition, a relative breaking elongation L (L = L1 / L0) was determined for each test number. The determined relative breaking elongation L of each test number is shown in Table 4.[Evaluation results]

[0138] Referring to Table 3-1, Table 3-2, and Table 4, the steel plates of Test Nos. 2-1 to 2-55 had the chemical composition described above, and in these steel plates the value of Fn1 was 22.0 or more, and the production method also satisfied the preferred conditions described above. In these steel plates, the number density of the specific precipitates was 1.00 / µm 2< or more. As a result, in these steel plates the tensile strength was 950 MPa or more, and thus these steel plates had high strength. Furthermore, as a result, in these steel plates the relative breaking elongation L was 0.80 or more, and thus these steel plates had excellent hydrogen embrittlement resistance in a high-pressure hydrogen gas environment. Note that, in each of these steel plates the precipitated amount of specific precipitates was 0.010% by mass or more.

[0139] On the other hand, in the steel plate of Test No. 2-56, the content of Cr was too high. As a result, in this steel plate the relative breaking elongation L was less than 0.80, and thus this steel plate did not have excellent hydrogen embrittlement resistance in a high-pressure hydrogen gas environment.

[0140] In the steel plate of Test No. 2-57, the content of Cr was too low. As a result, in this steel plate the tensile strength was less than 950 MPa, and thus this steel plate did not have high strength.

[0141] In the steel plate of Test No. 2-58, the content of Ni was too high and the content of N was too low. As a result, in this steel plate the tensile strength was less than 950 MPa, and thus this steel plate did not have high strength.

[0142] In the steel plate of Test No. 2-60, the content of Mn was too low. As a result, in this steel plate the relative breaking elongation L was less than 0.80, and thus this steel plate did not have excellent hydrogen embrittlement resistance in a high-pressure hydrogen gas environment.

[0143] In the steel plate of Test No. 2-61, the content of N was too low. As a result, in this steel plate the tensile strength was less than 950 MPa, and thus this steel plate did not have high strength.

[0144] In the steel plate of Test No. 2-62, the content of Nb was too high. As a result, in this steel plate the relative breaking elongation L was less than 0.80, and thus this steel plate did not have excellent hydrogen embrittlement resistance in a high-pressure hydrogen gas environment.

[0145] In the steel plate of Test No. 2-63, the content of Nb was too low, and hence the number density of the specific precipitates was less than 1.00 / µm 2< . As a result, in this steel plate the tensile strength was less than 950 MPa, and thus this steel plate did not have high strength.

[0146] In the steel plate of Test No. 2-64, Fn1 was less than 22.0. As a result, in this steel plate the relative breaking elongation L was less than 0.80, and thus this steel plate did not have excellent hydrogen embrittlement resistance in a high-pressure hydrogen gas environment.

[0147] In the steel plate of Test No. 2-65, the holding temperature in the specific precipitates precipitation treatment was too high, and hence the number density of the specific precipitates was less than 1.00 / µm 2< . As a result, in this steel plate the tensile strength was less than 950 MPa, and thus this steel plate did not have high strength.

[0148] In the steel plate of Test No. 2-66, the time period of hot working was too long, and hence the number density of the specific precipitates was less than 1.00 / µm 2< . As a result, in this steel plate the relative breaking elongation L was less than 0.80, and thus this steel plate did not have excellent hydrogen embrittlement resistance in a high-pressure hydrogen gas environment.

[0149] An embodiment of the present disclosure has been described above. However, the embodiment described above is merely an example for carrying out the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiment, and can be implemented by appropriately modifying the above-described embodiment within a range that does not depart from the gist thereof.

Claims

1. An austenitic stainless steel material having a chemical composition consisting of, 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 to 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%, and rare earth metal: 0 to 0.5000%, with the balance being Fe and impurities, wherein: in the chemical composition, in addition, in a case where a content of Ni is less than 17.00%, Fnl defined by Formula (1) is 18.0 or more, and in a case where a content of Ni is 17.00% or more, Fnl defined by Formula (1) is 22.0 or more; and in the austenitic stainless steel material, a number density of (Cr, Nb) composite precipitates having an equivalent circular diameter of 50 to 500 nm is 1.00 / µm2 or more; Fn 1 = Ni + 0.02 × Cr + 0.52 × Mn − 0.48 × Mo where, a content of a corresponding element in the chemical composition is substituted in percent by mass for each symbol of an element in Formula (1).

2. The austenitic stainless steel material according to claim 1, wherein the chemical composition contains one or more elements selected from a group consisting of: 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 metal: 0.0001 to 0.5000%.

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