Austenitic stainless steel material

Abstract

Provided is an austenitic stainless steel material that has high strength and excellent fatigue crack propagation resistance in a hydrogen environment. The chemical composition of the austenitic stainless steel material of the present disclosure, as defined in the description, satisfies formula (1); the elemental content and grain size number (GSN) in μm satisfy formula (2); and the tensile strength (TS), which is 730 MPa or more, the number density of inclusions (NDI) having an equivalent circle diameter of 2.0 μm or more per 4.0 mm2, and the number density of carbonitrides (NDC) having an equivalent circle diameter of 1.0 μm or more per mm2 satisfy formula (3).　(1) Nb + V ≤ 0.55; (2) 10FnA – 0.2FnB + 4.5GSN ≥ 39.0, where FnA and FnB in formula (2) are defined by formulas (A) and (B);　(A) FnA = 2N + C; (B) FnB = -7.1 + 2.8Ni + 0.49Cr + 2Mo – 2Si + 0.75Mn – 5.7C – 24N; (3) 0.5NDI + NDC + 0.02TS ≤ 30.0.

Description

Austenitic stainless steel material

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

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

[0003] Furthermore, in recent years, there has been a demand for further compactness and weight reduction in hydrogen stations and transportation equipment, and therefore austenitic stainless steel materials used in high-pressure hydrogen tanks and piping are required to have not only excellent hydrogen embrittlement resistance but also high strength.

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

[0005] The austenitic stainless steel material disclosed in Patent Document 1 is an austenitic stainless steel for high-pressure hydrogen, containing, by mass%, 0.40 to 1.00% C, 1.00% or less Si, 2.00% or less Mn, 0.040% or less P, 0.030% or less S, 8.00 to 14.00% Ni, 16.00 to 21.00% Cr, and 0.09% or less N, with the balance being Fe and impurity elements, and further satisfying formula 1 (54.8C + 3.7Ni + 2.5Mn - 1.6Cr - 0.9Si + 266N - 39.6 > 0). This austenitic stainless steel material is used as is after solution heat treatment, and 23% or more of chromium carbides are present in the steel. Patent Document 1 states that, as a result, this austenitic stainless steel material can significantly increase the strength of the solution heat treatment state.

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

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

[0008] Incidentally, austenitic stainless steel materials used in hydrogen stations and transportation equipment may have minute defects formed on the steel surface. Furthermore, such austenitic stainless steel materials may be subjected to repeated stress amplitudes due to pressure fluctuations of high-pressure hydrogen gas. When stress is applied to steel materials with minute defects formed on the surface, the minute defects may become the initiation points of fatigue cracks, which may then propagate. In particular, when the thickness of a steel material is reduced due to high strength, there is a concern that the effects of fatigue crack propagation may become apparent. Therefore, it is preferable that austenitic stainless steel materials used in hydrogen stations and transportation equipment are resistant to fatigue crack propagation even when minute defects are formed.

[0009] Furthermore, it is believed that fatigue cracks in steel materials are more likely to propagate in a hydrogen environment than in an air environment. Therefore, austenitic stainless steel materials used in hydrogen stations and transportation equipment preferably have excellent fatigue crack growth resistance in a hydrogen environment in addition to high strength. In this specification, "having excellent fatigue crack growth resistance in a hydrogen environment" means that fatigue cracks are less likely to propagate even in a hydrogen environment.

[0010] The austenitic stainless steel materials disclosed in the above-mentioned Patent Documents 1 and 2 combine excellent hydrogen embrittlement resistance with high strength. However, Patent Documents 1 and 2 do not consider at all the fatigue crack growth resistance of the steel materials in a hydrogen environment.

[0011] An object of the present disclosure is to provide an austenitic stainless steel material having high strength and excellent fatigue crack growth resistance in a hydrogen environment.

[0012] The austenitic stainless steel material according to the present disclosure has a chemical composition, in mass %, of C: 0.005 to 0.150%, Si: 0.10 to 1.00%, Mn: 0.50 to 3.00%, P: 0.050% or less, S: 0.0100% or less, Cr: 17.0 to 25.0%, Ni: 6.5 to 12.0%, N: 0.15 to 0.35%, Al: 0.050% or less, Ca: 0.0005 to 0.0050%, O: 0.030% or less, Mo: 0 to 1.00%, Ti: 0 to 0.300%, Cu: 0 to 0.30%, Co: 0 to 0.30%, B: 0 to 0.010%, Mg: 0 to 0.100%, rare earth element: 0 to 0.0050%, one or more elements selected from the group consisting of Nb: 0.01 to 0.50%, and V: 0.01 to 0.50%, and the balance being Fe and impurities, satisfying formula (1), the element contents in the chemical composition and the grain size number GSNμm satisfy formula (2), the tensile strength TS is 730 MPa or more, and in the austenitic stainless steel material, the number density of inclusions having an equivalent circle diameter of 2.0μm or more is NDI pieces / 4.0mm ２The number density of carbonitrides having a circle equivalent diameter of 1.0 μm or more is defined as NDC pieces / mm ２ When the above definitions are used, the tensile strength TS, the number density NDI of the inclusions having an equivalent circle diameter of 2.0 μm or more, and the number density NDC of the carbonitrides having an equivalent circle diameter of 1.0 μm or more satisfy the following formula (3): Nb+V≦0.55 (1) 10FnA−0.2FnB+4.5GSN≧39.0 (2) where FnA and FnB in formula (2) are defined by formulas (A) and (B). FnA = 2N + C (A) FnB = -7.1 + 2.8Ni + 0.49Cr + 2Mo - 2Si + 0.75Mn - 5.7C - 24N (B) 0.5NDI + NDC + 0.02TS ≦ 30.0 (3) Here, for each element symbol in formulas (1), (A) and (B), the content of the corresponding element in the chemical composition is substituted in the unit of mass%. If the corresponding element is not contained, "0" is substituted for the corresponding element symbol. The grain size number is substituted for GSN in formula (2). NDI in formula (3) represents the number density of inclusions having a circle equivalent diameter of 2.0 μm or more in the unit: pieces / 4.0 mm ２ In the formula (3), NDC is the number density of carbonitrides having a circle equivalent diameter of 1.0 μm or more, expressed in units of pieces / mm ２ The tensile strength is substituted into TS in the formula (3) in the unit of MPa.

[0013] The austenitic stainless steel material according to the present disclosure has high strength and excellent fatigue crack growth resistance in a hydrogen environment.

[0014] FIG. 1 is a diagram showing the relationship between Fn3 (=0.5NDI+NDC+0.02TS) and the FCGA ratio, which is an index of fatigue crack growth resistance in a hydrogen environment, in this example.

[0015] The present inventors have investigated and studied austenitic stainless steel materials that have high strength and excellent fatigue crack growth resistance in a hydrogen environment, and as a result, have obtained the following findings.

[0016] The present inventors first investigated austenitic stainless steel materials having high strength and excellent fatigue crack propagation resistance in a hydrogen environment from the viewpoint of chemical composition, and found that the chemical composition was, in mass %, C: 0.005 to 0.150%, Si: 0.10 to 1.00%, Mn: 0.50 to 3.00%, P: 0.050% or less, S: 0.0100% or less, Cr: 17.0 to 25.0%, Ni: 6.5 to 12.0%, N: 0.15 to 0.35%, Al: 0.050% or less, Ca: 0.0005 to 0.0050%, O: 0.030% or less, Mo: 0 to 1.00%, Ti: 0 to 0.300%, Cu: 0. The inventors considered that an austenitic stainless steel material containing one or more elements selected from the group consisting of: 0.01-0.50% Nb, 0.01-0.50% Co, 0-0.30%, 0-0.010%, 0.010% B, 0-0.100%, rare earth elements, 0-0.0050%, 0.01-0.50% V, and the balance being Fe and impurities, may be able to achieve both high strength and excellent fatigue crack propagation resistance in a hydrogen environment.

[0017] Next, the inventors investigated the strength of austenitic stainless steel materials having the above-mentioned chemical composition. Specifically, the inventors investigated various methods for increasing the tensile strength (TS) to 730 MPa or more, which is considered to be high strength. As a result, the inventors obtained the following findings.

[0018] As a result of detailed studies by the inventors, it has become clear that in an austenitic stainless steel material having the above-described chemical composition, the tensile strength TS can be stably increased to 730 MPa or more by satisfying the following formula (2): 10FnA - 0.2FnB + 4.5GSN ≥ 39.0 (2) Here, FnA and FnB in formula (2) are defined by formulas (A) and (B). FnA = 2N + C (A) FnB = -7.1 + 2.8Ni + 0.49Cr + 2Mo - 2Si + 0.75Mn - 5.7C - 24N (B) Here, the element symbols in formulas (A) and (B) are substituted with the content of the corresponding element in the chemical composition in units of mass %. If the corresponding element is not contained, "0" is substituted for the corresponding element symbol. The grain size number is substituted for GSN in formula (2).

[0019] Fn2 is defined as 10FnA - 0.2FnB + 4.5GSN. FnA (= 2N + C) in Fn2 is an index of the solid solution strengthening mechanism due to nitrogen (N) and carbon (C). The higher FnA, the greater the effect of the solid solution strengthening mechanism. Furthermore, FnB (= -7.1 + 2.8Ni + 0.49Cr + 2Mo - 2Si + 0.75Mn - 5.7C - 24N) in Fn2 is an index of stacking fault energy. The lower FnB, the lower the stacking fault energy. Here, the lower the stacking fault energy, the more likely stacking faults are to occur. Furthermore, stacking faults increase the work hardening rate of steel and increase its strength. Therefore, the lower FnB, the greater the effect of the strengthening mechanism due to reduced stacking fault energy. Furthermore, GSN in Fn2 is the grain size number. The larger the GSN, the finer the crystal grains, and the greater the effect of the strengthening mechanism due to grain refinement.

[0020] In other words, Fn2 is an index showing the balance between the solid solution strengthening mechanism, the strengthening mechanism due to reduced stacking fault energy, and the strengthening mechanism due to grain refinement in an austenitic stainless steel material having the above-mentioned chemical composition. In an austenitic stainless steel material having the above-mentioned chemical composition, if Fn2 is 39.0 or more, the tensile strength TS can be stably increased to 730 MPa or more, provided that the other configurations of this embodiment are satisfied. Therefore, in the austenitic stainless steel material according to this embodiment, Fn2 is set to 39.0 or more.

[0021] The present inventors further investigated various methods for improving the fatigue crack growth resistance in a hydrogen environment for austenitic stainless steel materials having the above-mentioned chemical composition and an Fn2 of 39.0 or more, and as a result, the present inventors obtained the following findings.

[0022] As a result of detailed studies by the present inventors, it has become clear that in an austenitic stainless steel material having the above-mentioned chemical composition, Fn2 of 39.0 or more, and a tensile strength TS of 730 MPa or more, inclusions (oxides, sulfides, nitrides, etc.) in the steel material affect the fatigue crack growth resistance in a hydrogen environment. Here, for coarse inclusions in an austenitic stainless steel material having the above-mentioned chemical composition, the interface with the matrix can become a preferential path for fatigue crack growth. Therefore, if a large number of coarse inclusions are present in the steel material, there is a concern that the fatigue crack growth resistance of the steel material will be reduced.

[0023] Further investigations by the present inventors have revealed that in austenitic stainless steel materials with the above-mentioned chemical composition, not only coarse inclusions but also coarse carbonitrides affect fatigue crack growth resistance in a hydrogen environment. Similar to coarse inclusions, the interface between coarse carbonitrides and the matrix can serve as a fatigue crack growth path. Furthermore, the effect of carbonitrides on fatigue crack growth resistance may be more pronounced in a hydrogen environment.

[0024] Furthermore, the strength of the austenitic stainless steel material also affects the fatigue crack growth resistance in a hydrogen environment. Specifically, the higher the strength, the more likely the fatigue crack growth resistance is to deteriorate. On the other hand, as described above, the austenitic stainless steel material according to this embodiment has the above-described chemical composition, and by setting Fn2 to 39.0 or more, the tensile strength TS is increased to 730 MPa or more. As a result, the austenitic stainless steel material according to this embodiment is more likely to deteriorate its fatigue crack growth resistance in a hydrogen environment.

[0025] As a result of detailed studies by the present inventors based on the above findings, it has become clear that an austenitic stainless steel material having the above-mentioned chemical composition, Fn2 of 39.0 or more, and tensile strength TS of 730 MPa or more can stably improve fatigue crack propagation resistance in a hydrogen environment by satisfying the following formula (3): 0.5NDI+NDC+0.02TS≦30.0 (3) where NDI in formula (3) represents the number density of inclusions with a circle equivalent diameter of 2.0 μm or more, unit: pieces / 4.0 mm ２In the formula (3), NDC is the number density of carbonitrides having a circle equivalent diameter of 1.0 μm or more, expressed in units of pieces / mm ２ In formula (3), TS is substituted with tensile strength in units of MPa.

[0026] Fn3 is defined as 0.5NDI + NDC + 0.02TS. In this specification, inclusions with a circle equivalent diameter of 2.0 μm or more are also referred to as "coarse inclusions." In other words, NDI in Fn3 is the number density of coarse inclusions (numbers / 4.0 mm ２ ) In this specification, carbonitrides having a circle equivalent diameter of 1.0 μm or more are also referred to as "coarse carbonitrides." In other words, NDC in Fn3 is the number density (number / mm ２ ) means

[0027] Fn3 is an index of fatigue crack growth resistance in a hydrogen environment for an austenitic stainless steel material having the above-mentioned chemical composition, Fn2 of 39.0 or more, and tensile strength TS of 730 MPa or more. Below, the relationship between Fn3 and fatigue crack growth resistance in a hydrogen environment for an austenitic stainless steel material having the above-mentioned chemical composition, Fn2 of 39.0 or more, and tensile strength TS of 730 MPa or more will be specifically explained with reference to the drawings.

[0028] Fig. 1 is a diagram showing the relationship between Fn3 (= 0.5NDI + NDC + 0.02TS) and the FCGA ratio, which is an index of fatigue crack growth resistance in a hydrogen environment, in this example. Fig. 1 was created using Fn3 determined by a method described later and the FCGA ratio determined by a method described later for steel materials having the above-mentioned chemical composition, Fn2 of 39.0 or more, and tensile strength TS of 730 MPa or more, among the examples described later.

[0029] 1 , in a steel material having the above-described chemical composition, Fn2 of 39.0 or more, and tensile strength TS of 730 MPa or more, if Fn3 is 30.0 or less, the FCGA ratio is 7.5 or less, and the steel material exhibits excellent fatigue crack growth resistance in a hydrogen environment. Therefore, in this embodiment, the steel material has the above-described chemical composition, Fn2 of 39.0 or more, tensile strength TS of 730 MPa or more, and Fn3 is 30.0 or less. As a result, the steel material according to this embodiment can achieve both high strength and excellent fatigue crack growth resistance in a hydrogen environment.

[0030] Furthermore, the inventors have investigated various methods for setting Fn3 to 30.0 or less in an austenitic stainless steel material having the above-mentioned chemical composition, Fn2 of 39.0 or more, and tensile strength TS of 730 MPa or more. As a result, the inventors have found that, provided that the other conditions of this embodiment are met, Fn3 can be stably set to 30.0 or less if the above-mentioned chemical composition further satisfies the following formula (1): Nb + V ≦ 0.55 (1) Here, each element symbol in formula (1) is substituted with the content of the corresponding element in the chemical composition in units of mass %. If the corresponding element is not contained, "0" is substituted for the corresponding element symbol.

[0031] Fn1 is defined as Nb + V. Fn1 refers to the total amount of alloying elements that are likely to form coarse carbonitrides in a steel material having the above-mentioned chemical composition. If Fn1 is 0.55 or less, the number density of coarse carbonitrides decreases, and Fn3 can be stably set to 30.0 or less. Therefore, in the austenitic stainless steel material according to this embodiment, Fn1 is set to 0.55 or less.

[0032] In short, the austenitic stainless steel material according to this embodiment has the above-mentioned chemical composition, Fn1 is 0.55 or less, Fn2 is 39.0 or more, tensile strength TS is 730 MPa or more, and Fn3 is 30.0 or less. As a result, the austenitic stainless steel material according to this embodiment can achieve both high strength and excellent fatigue crack propagation resistance in a hydrogen environment.

[0033] The austenitic stainless steel material of this embodiment, which has been completed based on the above technical concept, has the following configuration.

[0034] [1] An austenitic stainless steel material having a chemical composition, in mass%, of C: 0.005 to 0.150%, Si: 0.10 to 1.00%, Mn: 0.50 to 3.00%, P: 0.050% or less, S: 0.0100% or less, Cr: 17.0 to 25.0%, Ni: 6.5 to 12.0%, N: 0.15 to 0.35%, Al: 0.050% or less, Ca: 0.0005 to 0.0050%, O: 0.030% or less, Mo: 0 to 1.00%, Ti: 0 to 0.300%, Cu: 0 to 0.30%, Co: 0 to 0.30%, B: 0 to 0.010%, Mg: 0 to 0.100%, rare earth element: 0 to 0.0050%, one or more elements selected from the group consisting of Nb: 0.01 to 0.50%, and V: 0.01 to 0.50%, and the balance being Fe and impurities, satisfying formula (1), the element contents in the chemical composition and the grain size number GSNμm satisfy formula (2), the tensile strength TS is 730 MPa or more, and in the austenitic stainless steel material, the number density of inclusions having an equivalent circle diameter of 2.0μm or more is NDI pieces / 4.0mm ２ The number density of carbonitrides having a circle equivalent diameter of 1.0 μm or more is defined as NDC pieces / mm ２An austenitic stainless steel material, wherein the tensile strength TS, the number density NDI of the inclusions having an equivalent circle diameter of 2.0 μm or more, and the number density NDC of the carbonitrides having an equivalent circle diameter of 1.0 μm or more satisfy the following formula (3): Nb + V≦0.55 (1) 10FnA − 0.2FnB + 4.5GSN≧39.0 (2) where FnA and FnB in formula (2) are defined by formulas (A) and (B). FnA = 2N + C (A) FnB = -7.1 + 2.8Ni + 0.49Cr + 2Mo - 2Si + 0.75Mn - 5.7C - 24N (B) 0.5NDI + NDC + 0.02TS ≦ 30.0 (3) Here, for each element symbol in formulas (1), (A) and (B), the content of the corresponding element in the chemical composition is substituted in the unit of mass%. If the corresponding element is not contained, "0" is substituted for the corresponding element symbol. The grain size number is substituted for GSN in formula (2). NDI in formula (3) represents the number density of inclusions having a circle equivalent diameter of 2.0 μm or more in the unit: pieces / 4.0 mm ２ In the formula (3), NDC is the number density of carbonitrides having a circle equivalent diameter of 1.0 μm or more, expressed in units of pieces / mm ２ The tensile strength is substituted into TS in the formula (3) in the unit of MPa.

[0035] [2] The austenitic stainless steel material according to [1], wherein the chemical composition contains, in mass%, one or more elements selected from the group consisting of Mo: 0.01 to 1.00%, Ti: 0.001 to 0.300%, Cu: 0.01 to 0.30%, Co: 0.01 to 0.30%, B: 0.001 to 0.010%, Mg: 0.001 to 0.100%, and rare earth elements: 0.0001 to 0.0050%.

[0036] The austenitic stainless steel material of this embodiment will be described in detail below. "%" for elements means mass % unless otherwise specified.

[0037] [Features of the Austenitic Stainless Steel Material of the Present Embodiment] The austenitic stainless steel material of the present embodiment satisfies the following features 1 to 5. (Feature 1) The chemical composition, in mass%, is: C: 0.005 to 0.150%, Si: 0.10 to 1.00%, Mn: 0.50 to 3.00%, P: 0.050% or less, S: 0.0100% or less, Cr: 17.0 to 25.0%, Ni: 6.5 to 12.0%, N: 0.15 to 0.35%, Al: 0.050% or less, Ca: 0.0005 to 0.0050%, O: 0.03 0% or less, Mo: 0-1.00%, Ti: 0-0.300%, Cu: 0-0.30%, Co: 0-0.30%, B: 0-0.010%, Mg: 0-0.100%, rare earth elements: 0-0.0050%, Nb: 0.01-0.50%, and V: 0.01-0.50%, with the balance consisting of Fe and impurities. (Feature 2) Satisfies formula (1). Nb + V ≦ 0.55 (1) Here, each element symbol in formula (1) is substituted with the content of the corresponding element in the chemical composition in units of mass %. If the corresponding element is not contained, "0" is substituted for the corresponding element symbol. (Feature 3) The element content in the chemical composition and the grain size number GSN μm satisfy formula (2). 10FnA - 0.2FnB + 4.5GSN ≥ 39.0 (2) Here, FnA and FnB in formula (2) are defined by formulas (A) and (B). FnA = 2N + C (A) FnB = -7.1 + 2.8Ni + 0.49Cr + 2Mo - 2Si + 0.75Mn - 5.7C - 24N (B) Here, each element symbol in formulas (A) and (B) is substituted with the content of the corresponding element in the chemical composition in units of mass %. If the corresponding element is not contained, "0" is substituted for the corresponding element symbol. GSN in formula (2) is substituted with the grain size number. (Feature 4) The tensile strength TS is 730 MPa or more. (Feature 5) In austenitic stainless steel materials, the number density of inclusions with a circle equivalent diameter of 2.0 μm or more is NDI pieces / 4.0 mm ２ The number density of carbonitrides having a circle equivalent diameter of 1.0 μm or more is defined as NDC pieces / mm ２When the formula (3) is defined as follows, the tensile strength TS, the number density NDI of inclusions having an equivalent circle diameter of 2.0 μm or more, and the number density NDC of carbonitrides having an equivalent circle diameter of 1.0 μm or more satisfy the formula (3): 0.5NDI+NDC+0.02TS≦30.0 (3) Here, NDI in formula (3) represents the number density of inclusions having an equivalent circle diameter of 2.0 μm or more, in units of pieces / 4.0 mm ２ In the formula (3), NDC is the number density of carbonitrides having a circle equivalent diameter of 1.0 μm or more, expressed in units of pieces / mm ２ In formula (3), TS is substituted with tensile strength in units of MPa.

[0038] [(Feature 1) Chemical Composition] The chemical composition of the austenitic stainless steel material of this embodiment contains the following elements.

[0039] C: 0.005 to 0.150% Carbon (C) forms carbides to increase the strength of austenitic stainless steel materials. C also increases the strength of steel materials through solid solution strengthening. If the C content is too low, the above effect cannot be fully achieved. On the other hand, if the C content is too high, excessive carbides are formed at the grain boundaries, reducing the toughness of the steel material. Therefore, the C content is 0.005 to 0.150%. The preferred lower limit of the C content is 0.006%, more preferably 0.008%, and even more preferably 0.010%. The preferred upper limit of the C content is 0.140%, more preferably 0.130%, even more preferably 0.120%, and even more preferably 0.100%.

[0040] Si: 0.10 to 1.00% Silicon (Si) deoxidizes steel. If the Si content is too low, the above effect cannot be sufficiently obtained. On the other hand, if the Si content is too high, intermetallic compounds are formed in excess, reducing the hot workability and toughness of the steel. Therefore, the Si content is 0.10 to 1.00%. A preferred lower limit of the Si content is 0.11%, more preferably 0.15%, and even more preferably 0.20%. A preferred upper limit of the Si content is 0.95%, more preferably 0.90%, and even more preferably 0.85%.

[0041] Mn: 0.50 to 3.00% Manganese (Mn) stabilizes austenite and suppresses the formation of deformation-induced martensite. Mn also increases the amount of dissolved N, thereby increasing the strength of the steel material through solid solution strengthening of N. If the Mn content is too low, the above effects cannot be sufficiently obtained. On the other hand, if the Mn content is too high, the ductility and hot workability of the steel material decrease. Therefore, the Mn content is 0.50 to 3.00%. The preferred lower limit of the Mn content is 0.52%, more preferably 0.55%, even more preferably 0.60%, and even more preferably 0.70%. The preferred upper limit of the Mn content is 2.95%, even more preferably 2.90%, even more preferably 2.80%, and even more preferably 2.70%.

[0042] P: 0.050% or less Phosphorus (P) is an unavoidable impurity. In other words, the lower limit of the P content is more than 0%. P reduces the hot workability and toughness of steel. Therefore, the P content is 0.050% or less. The lower the P content, the more preferable it is. However, excessive reduction of the P content increases manufacturing costs. Therefore, taking into account normal industrial production, the preferred lower limit of the P content is 0.001%, and more preferably 0.002%. The preferred upper limit of the P content is 0.045%, more preferably 0.040%, and even more preferably 0.035%.

[0043] S: 0.0100% or less Sulfur (S) is an unavoidable impurity. In other words, the lower limit of the S content is greater than 0%. S reduces the hot workability and toughness of steel. Therefore, the S content is 0.0100% or less. The lower the S content, the more preferable it is. However, excessive reduction of the S content increases manufacturing costs. Therefore, considering normal industrial production, the preferred lower limit of the S content is 0.0001%, and more preferably 0.0002%. The preferred upper limit of the S content is 0.0080%, more preferably 0.0060%, even more preferably 0.0050%, even more preferably 0.0030%, even more preferably 0.0028%, even more preferably 0.0025%, and even more preferably 0.0020%.

[0044] Cr: 17.0 to 25.0% Chromium (Cr) improves the corrosion resistance of steel. Cr also increases the amount of dissolved N, increasing the strength of the steel through solid solution strengthening of N. If the Cr content is too low, the above effects cannot be fully achieved. On the other hand, if the Cr content is too high, excessive Cr carbides are formed, reducing the ductility and toughness of the steel. Therefore, the Cr content is 17.0 to 25.0%. The preferred lower limit of the Cr content is 17.3%, more preferably 17.5%, and even more preferably 18.0%. The preferred upper limit of the Cr content is 24.5%, more preferably 24.0%, and even more preferably 23.5%.

[0045] Ni: 6.5 to 12.0% Nickel (Ni) stabilizes austenite and suppresses the formation of deformation-induced martensite. If the Ni content is too low, the above effect cannot be sufficiently obtained. On the other hand, if the Ni content is too high, the hot workability of the steel material decreases. Therefore, the Ni content is 6.5 to 12.0%. A preferred lower limit of the Ni content is 6.8%, and more preferably 7.0%. A preferred upper limit of the Ni content is 11.8%, more preferably 11.5%, even more preferably 10.5%, and even more preferably 10.0%.

[0046] N: 0.15 to 0.35% Nitrogen (N) stabilizes austenite. N also increases the strength of steel materials through solid solution strengthening. If the N content is too low, the above effects cannot be sufficiently obtained. On the other hand, if the N content is too high, the toughness and workability of the steel materials decrease. Therefore, the N content is 0.15 to 0.35%. The preferred lower limit of the N content is 0.16%, more preferably 0.18%, and even more preferably 0.20%. The preferred upper limit of the N content is 0.34%, more preferably 0.30%, and even more preferably 0.28%.

[0047] Al: 0.050% or less Aluminum (Al) is inevitably contained. In other words, the lower limit of the Al content is greater than 0%. Al deoxidizes steel. On the other hand, if the Al content is too high, many coarse inclusions are formed in the steel, reducing the fatigue crack propagation resistance of the steel. Therefore, the Al content is 0.050% or less. A preferred lower limit of the Al content is 0.001%, more preferably 0.002%, and even more preferably 0.003%. A preferred upper limit of the Al content is 0.045%, more preferably 0.040%, and even more preferably 0.035%. In this specification, the Al content refers to the content of sol. Al (acid-soluble Al).

[0048] Ca: 0.0005 to 0.0050% Calcium (Ca) fixes S as sulfides, improving the hot workability of steel. If the Ca content is too low, the above effect cannot be fully achieved. On the other hand, if the Ca content is too high, many coarse inclusions are formed in the steel, reducing the fatigue crack propagation resistance of the steel. Therefore, the Ca content is 0.0005 to 0.0050%. The preferred lower limit of the Ca content is 0.0006%, more preferably 0.0008%, even more preferably 0.0010%, and even more preferably 0.0015%. The preferred upper limit of the Ca content is 0.0045%, even more preferably 0.0040%, and even more preferably 0.0035%.

[0049] O: 0.030% or less Oxygen (O) is an unavoidably contained impurity. In other words, the lower limit of the O content is more than 0%. If the O content is too high, many coarse inclusions are formed in the steel material, and the fatigue crack propagation resistance of the steel material is reduced. Therefore, the O content is 0.030% or less. The O content is preferably as low as possible. However, excessive reduction of the O content increases manufacturing costs. Therefore, considering normal industrial production, the preferred lower limit of the O content is 0.001%, more preferably 0.002%. The preferred upper limit of the O content is 0.025%, more preferably 0.020%, and even more preferably 0.015%.

[0050] One or more elements selected from the group consisting of Nb: 0.01 to 0.50% and V: 0.01 to 0.50%. Niobium (Nb) and vanadium (V) both form carbides, nitrides, or carbonitrides (hereinafter referred to as "carbonitrides, etc.") to improve the fatigue crack propagation resistance of steel. Nb and V further refine crystal grains through the pinning effect of carbonitrides, etc., thereby increasing the strength of the steel. If at least one of Nb and V is contained in an amount of 0.01% or more, the above effects can be achieved to some extent. On the other hand, if the Nb content and / or V content is too high, the toughness and hot workability of the steel will decrease. Therefore, the chemical composition of the steel of this embodiment contains one or more elements selected from the group consisting of an Nb content of 0.50% or less and a V content of 0.50% or less. The preferred lower limit of the Nb content is 0.02%, more preferably 0.03%, and even more preferably 0.05%. The upper limit of the Nb content is preferably 0.48%, more preferably 0.45%, and even more preferably 0.40%. The lower limit of the V content is preferably 0.02%, more preferably 0.03%, and even more preferably 0.05%. The upper limit of the V content is preferably 0.48%, more preferably 0.45%, and even more preferably 0.40%.

[0051] The balance of the chemical composition of the austenitic stainless steel material according to this embodiment is composed of Fe and impurities, which refer to substances that are mixed in from raw materials such as ore or scrap or the manufacturing environment during industrial production of the austenitic stainless steel material and are acceptable within a range that does not adversely affect the austenitic stainless steel material according to this embodiment.

[0052] [Regarding optional elements] The chemical composition of the austenitic stainless steel material according to this embodiment may further contain, in place of a portion of Fe, one or more elements selected from the group consisting of Mo: 0-1.00%, Ti: 0-0.300%, Cu: 0-0.30%, Co: 0-0.30%, B: 0-0.010%, Mg: 0-0.100%, and rare earth elements: 0-0.0050%. All of these elements are optional elements. These optional elements will be described below.

[0053] [Regarding the first group (Mo, Ti, Cu, Co, and B)] The chemical composition of the austenitic stainless steel material according to this embodiment may contain, in place of a portion of Fe, one or more elements selected from the group consisting of Mo, Ti, Cu, Co, and B. All of these elements are optional elements, and increase the strength of the steel material.

[0054] Mo: 0 to 1.00% Molybdenum (Mo) is an optional element and does not necessarily need to be contained. In other words, the Mo content may be 0%. When contained, Mo increases the strength of the steel material. Even if even a small amount of Mo is contained, the above effect can be obtained to some extent. On the other hand, if the Mo content is too high, intermetallic compounds tend to precipitate. As a result, the ductility and toughness of the steel material decrease. Therefore, the Mo content is 0 to 1.00%. The preferred lower limit of the Mo content is more than 0%, more preferably 0.01%, and even more preferably 0.05%. The preferred upper limit of the Mo content is 0.95%, and even more preferably 0.90%.

[0055] Ti: 0 to 0.300% Titanium (Ti) is an optional element and does not necessarily need to be contained. In other words, the Ti content may be 0%. When contained, Ti forms carbonitrides and the like, which refine the crystal grains through a pinning effect. As a result, the strength of the steel material is increased. Even if even a small amount of Ti is contained, the above effect can be obtained to some extent. On the other hand, if the Ti content is too high, coarse carbonitrides and the like are formed, and the fatigue crack propagation resistance of the steel material is reduced. Therefore, the Ti content is 0 to 0.300%. The preferred lower limit of the Ti content is more than 0%, more preferably 0.001%, and even more preferably 0.002%. The preferred upper limit of the Ti content is 0.290%, more preferably 0.280%, and even more preferably 0.250%.

[0056] Cu: 0 to 0.30% Copper (Cu) is an optional element and does not need to be contained. In other words, the Cu content may be 0%. When contained, Cu stabilizes austenite. Cu also increases the strength of the steel material through solid solution strengthening. Even if even a small amount of Cu is contained, the above effects can be obtained to some extent. On the other hand, if the Cu content is too high, the hot workability of the steel material decreases. Therefore, the Cu content is 0 to 0.30%. The preferred lower limit of the Cu content is more than 0%, more preferably 0.01%, and even more preferably 0.02%. The preferred upper limit of the Cu content is 0.29%, more preferably 0.25%, and even more preferably 0.20%.

[0057] Co: 0 to 0.30% Cobalt (Co) is an optional element and does not necessarily need to be contained. In other words, the Co content may be 0%. When contained, Co stabilizes austenite. Co also increases the strength of the steel material through solid solution strengthening. Even if even a small amount of Co is contained, the above effects can be obtained to some extent. On the other hand, if the Co content is too high, the hot workability of the steel material decreases. Therefore, the Co content is 0 to 0.30%. The preferred lower limit of the Co content is more than 0%, more preferably 0.01%, even more preferably 0.02%, and even more preferably 0.05%. The preferred upper limit of the Co content is 0.28%, even more preferably 0.25%, and even more preferably 0.20%.

[0058] B: 0 to 0.010% Boron (B) is an optional element and does not need to be contained. In other words, the B content may be 0%. When contained, B increases the strength of the steel material. Even if even a small amount of B is contained, the above effect can be obtained to some extent. On the other hand, if the B content is too high, the hot workability of the steel material decreases. Therefore, the B content is 0 to 0.010%. The preferred lower limit of the B content is more than 0%, more preferably 0.001%, and even more preferably 0.003%. The preferred upper limit of the B content is 0.009%, more preferably 0.008%, and even more preferably 0.007%.

[0059] [Regarding Group 2 (Mg and Rare Earth Elements (REM)] The chemical composition of the austenitic stainless steel material according to this embodiment may contain one or more elements selected from the group consisting of Mg and rare earth elements (REM) in place of a portion of Fe. All of these elements are optional elements, and improve the hot workability of the steel material.

[0060] Mg: 0 to 0.100% Magnesium (Mg) is an optional element and does not necessarily need to be contained. In other words, the Mg content may be 0%. When contained, Mg fixes S as sulfides, improving the hot workability of the steel. Even if even a small amount of Mg is contained, the above effects can be obtained to some extent. On the other hand, if the Mg content is too high, the toughness and hot workability of the steel will decrease. Therefore, the Mg content is 0 to 0.100%. The preferred lower limit of Mg is more than 0%, more preferably 0.001%, even more preferably 0.005%, and even more preferably 0.010%. The preferred upper limit of the Mg content is 0.098%, even more preferably 0.095%, even more preferably 0.090%, and even more preferably 0.080%.

[0061] Rare earth elements (REM): 0 to 0.0050% Rare earth elements (REM) are optional elements and do not necessarily need to be contained. In other words, the REM content may be 0%. When contained, REM modifies the morphology of sulfides and improves the hot workability of steel. Even if even a small amount of REM is contained, the above effects can be obtained to some extent. On the other hand, if the REM content is too high, the toughness and hot workability of the steel will decrease. Therefore, the REM content is 0 to 0.0050%. The preferred lower limit of the REM content is more than 0%, more preferably 0.0001%, even more preferably 0.0002%, and even more preferably 0.0005%. The preferred upper limit of the REM content is 0.0045%, even more preferably 0.0040%, and even more preferably 0.0030%.

[0062] In this specification, REM refers to one or more elements selected from the group consisting of scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanoids lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. In addition, in this specification, the REM content refers to the total content of these elements.

[0063] [(Feature 2) Formula (1)] The austenitic stainless steel material according to this embodiment satisfies formula (1) on the assumption that it has the chemical composition described in feature 1. Nb + V ≦ 0.55 (1) Here, the content of the corresponding element in the chemical composition is substituted for each element symbol in formula (1) in units of mass %. When the corresponding element is not contained, "0" is substituted for the corresponding element symbol.

[0064] Fn1 (= Nb + V) refers to the total amount of alloying elements that easily form coarse carbonitrides (carbonitrides with an equivalent circle diameter of 1.0 μm or more) in a steel material having the above-mentioned chemical composition. Therefore, if Fn1 is too high, the number density of coarse carbonitrides becomes too high, and Fn3 becomes too high. As a result, fatigue crack propagation resistance in a hydrogen environment deteriorates. Therefore, in the austenitic stainless steel material according to this embodiment, Fn1 is set to 0.55 or less, assuming that the above-mentioned chemical composition is present. As a result, Fn3 can be stably set to 30.0 or less, assuming that the other configurations of this embodiment are satisfied.

[0065] The upper limit of Fn1 is preferably 0.52, more preferably 0.50, even more preferably 0.48, and even more preferably 0.45. The lower limit of Fn1 is not particularly limited, but is substantially 0.01. The lower limit of Fn1 is preferably 0.05, more preferably 0.10, and even more preferably 0.15.

[0066] [(Feature 3) Formula (2)] Assuming that the austenitic stainless steel material according to this embodiment has the chemical composition described in Feature 1, the element contents (mass%) in the chemical composition and the grain size number GSN (μm) satisfy formula (2). 10FnA-0.2FnB+4.5GSN≧39.0 (2) Here, FnA and FnB in formula (2) are defined by formulas (A) and (B). FnA=2N+C (A) FnB=-7.1+2.8Ni+0.49Cr+2Mo-2Si+0.75Mn-5.7C-24N (B) Here, the grain size number is substituted for GSN in formula (2). For each element symbol in formulas (A) and (B), the content of the corresponding element in the chemical composition is substituted in mass%. If the element in question is not contained, the corresponding element symbol is substituted with "0."

[0067] Fn2 (= 10FnA - 0.2FnB + 4.5GSN) is an index showing the balance between the solid solution strengthening mechanism, the strengthening mechanism due to reduced stacking fault energy, and the strengthening mechanism due to grain refinement in an austenitic stainless steel material having the above-mentioned chemical composition. If Fn2 is 39.0 or more, the tensile strength TS can be stably increased to 730 MPa or more, provided that the other configurations of this embodiment are satisfied. Therefore, in the austenitic stainless steel material according to this embodiment, Fn2 is set to 39.0 or more.

[0068] The lower limit of Fn2 is preferably 39.5, and more preferably 40.0. The upper limit of Fn2 is not particularly limited, but may be, for example, 55.0. The upper limit of Fn2 may be 52.0, 50.0, or 48.0.

[0069] FnA (= 2N + C) in Fn2 is an index of the solid solution strengthening mechanism. The higher FnA, the greater the effect of the solid solution strengthening mechanism. Furthermore, FnB (= -7.1 + 2.8Ni + 0.49Cr + 2Mo - 2Si + 0.75Mn - 5.7C - 24N) in Fn2 is an index of stacking fault energy. The lower FnB, the greater the effect of the strengthening mechanism due to reduced stacking fault energy. Furthermore, GSN in Fn2 means the grain size number of austenite grains in the microstructure. The larger the GSN, the finer the crystal grains, and the greater the effect of the strengthening mechanism due to grain refinement.

[0070] In the austenitic stainless steel material of this embodiment, Fn2 is required to be 39.0 or more, and FnA, FnB, and GSN are not particularly limited. FnA may be, for example, 0.31 to 0.85. The lower limit of FnA may be 0.35, 0.37, or 0.40. The upper limit of FnA may be 0.80, 0.75, or 0.70.

[0071] Furthermore, FnB may be, for example, 5.0 to 45.0. The lower limit of FnB may be 10.0, 12.0, or 14.0. The upper limit of FnB may be 40.0, 35.0, or 32.0. Furthermore, GSN may be, for example, 5.0 to 12.0. The lower limit of GSN may be 6.0, 7.0, 8.0, or 8.5. The upper limit of GSN may be 11.5, 11.0, or 10.5.

[0072] In the austenitic stainless steel material according to this embodiment, the grain size number GSN can be determined by the following method. Specifically, a test piece for microstructure observation is prepared from the austenitic stainless steel material according to this embodiment. When the steel material is a steel plate, the test piece is prepared from the center of the plate thickness. At this time, the plane including the rolling direction and the plate thickness direction of the steel plate is taken as the observation surface. When the steel material is a steel pipe, the test piece is prepared from the center of the wall thickness. At this time, the plane including the pipe axial direction and the wall thickness direction of the steel pipe is taken as the observation surface. When the steel material is a steel bar, the test piece is prepared from the R / 2 position on the cut surface. Here, the R / 2 position means the center position of the radius R of the cut surface. At this time, the plane including the axial and radial directions of the steel bar is taken as the observation surface. The size of the test piece is not particularly limited as long as it is possible to obtain the observation surface described below.

[0073] The observation surface of the prepared test specimen is polished to a mirror finish, and then etched using aqua regia (a solution of hydrochloric acid and nitric acid mixed at a ratio of 3:1) to reveal the austenite grain boundaries. Three fields of view are arbitrarily selected from the observation surface, and observed using an optical microscope to generate photographic images. The magnification for microscopic observation can be appropriately set depending on the grain size. More specifically, the size of each field of view may be 1000 μm × 1000 μm.

[0074] In each field of view, image analysis is performed on the obtained photographic image, and the grain size number is measured in accordance with ASTM E 112 (2021). The arithmetic average value of the grain size numbers in the three fields of view is defined as the grain size number GSN. The grain size number is obtained by rounding the obtained value to one decimal place.

[0075] [(Feature 4) Tensile Strength] The austenitic stainless steel material according to this embodiment has a tensile strength of 730 MPa or more. The upper limit of the tensile strength is not particularly limited, but is, for example, 1000 MPa. The lower limit of the tensile strength is preferably 740 MPa, and more preferably 750 MPa. The upper limit of the tensile strength is preferably 950 MPa, and more preferably 900 MPa.

[0076] In the austenitic stainless steel material according to this embodiment, the tensile strength can be determined by the following method. Specifically, a round bar test piece for a tensile test is prepared from the austenitic stainless steel material according to this embodiment. When the steel material is a steel plate, the round bar test piece is prepared from the center of the plate thickness. At this time, the axial direction of the round bar test piece is parallel to the rolling direction of the steel plate. When the steel material is a steel pipe, the round bar test piece is prepared from the center of the wall thickness. At this time, the axial direction of the round bar test piece is parallel to the axial direction of the steel pipe. When the steel material is a steel bar, the round bar test piece is prepared from the R / 2 position on the cut surface. At this time, the axial direction of the round bar test piece is parallel to the axial direction of the steel bar.

[0077] The round bar test piece used is JIS No. 14A as specified in JIS Z 2241:2011, with the diameter of the parallel part being 8.0 mm. A tensile test conforming to JIS Z 2241:2011 is performed on the round bar test piece at room temperature in the air to determine the tensile strength (MPa). The tensile strength (MPa) is calculated by rounding the obtained value to one decimal place.

[0078] [(Feature 5) Formula (3)] The austenitic stainless steel material according to this embodiment has a number density NDI (number / 4.0 mm) of inclusions (coarse inclusions) having an equivalent circle diameter of 2.0 μm or more. ２ ) and the number density NDC (pieces / mm ２ ) and the tensile strength TS satisfy the formula (3): 0.5NDI+NDC+0.02TS≦30.0 (3) where NDI in formula (3) represents the number density of coarse inclusions in units of pieces / 4.0 mm ２ In the formula (3), NDC is the number density of coarse carbonitrides in units of pieces / mm ２ In formula (3), TS is substituted with tensile strength in units of MPa.

[0079] Fn3 (= 0.5NDI + NDC + 0.02TS) is an index of fatigue crack growth resistance in a hydrogen environment. If Fn3 is 30.0 or less, the fatigue crack growth resistance of the steel material in a hydrogen environment is significantly improved, provided that the other characteristics of this embodiment are satisfied. Therefore, in the austenitic stainless steel material of this embodiment, Fn3 is set to 30.0 or less.

[0080] In the austenitic stainless steel material of this embodiment, it is sufficient that the tensile strength TS is 730 MPa or more and the Fn3 is 30.0 or less, and the number density NDI of the coarse inclusions and the number density NDC of the coarse carbonitrides are not particularly limited. The upper limit of the number density NDI of the coarse inclusions is, for example, 30.0 pieces / 4.0 mm ２ 25.0 pieces / 4.0 mm ２ 20.0 pieces / 4.0 mm ２ In this embodiment, the lower limit of the number density of the coarse inclusions is not particularly limited, and may be 0.0 pieces / 4.0 mm ２ However, in the austenitic stainless steel material having the above-mentioned chemical composition, the number density of the coarse inclusions is 2.0 pieces / 4.0 mm ２ Furthermore, the lower limit of the number density of coarse inclusions is 3.0 pieces / 4.0 mm ２ 3.5 pieces / 4.0 mm ２ may be.

[0081] Preferably, the upper limit of the number density NDI of coarse inclusions is 15.0 pieces / 4.0 mm ２ The number density NDI of coarse inclusions is 15.0 pieces / 4.0 mm ２ In this embodiment, a more preferable upper limit of the number density NDI of coarse inclusions is 14.5 pieces / 4.0 mm. ２ and more preferably 14.0 pieces / 4.0 mm ２ is.

[0082] The upper limit of the number density NDC of coarse carbonitrides is, for example, 40.0 pieces / mm ２ 30.0 pieces / mm ２ 20.0 pieces / mm２ 15.0 pieces / mm ２ In this embodiment, the lower limit of the number density of the coarse carbonitrides is not particularly limited, and may be 0.0 particles / mm ２ However, in the austenitic stainless steel material having the above-mentioned chemical composition, the number density of coarse carbonitrides is 0.4 particles / mm ２ Furthermore, the lower limit of the number density of coarse carbonitrides is 1.0 pieces / mm ２ 1.5 pieces / mm ２ may be.

[0083] Preferably, the upper limit of the number density NDC of coarse carbonitrides is 10.0 pieces / mm ２ The number density NDC of coarse carbonitrides is 10.0 pieces / mm ２ In this embodiment, a more preferable upper limit of the number density NDC of coarse carbonitrides is 9.5 pieces / mm ２ and more preferably 9.0 pieces / mm ２ is.

[0084] In the austenitic stainless steel material according to this embodiment, the number density NDI of coarse inclusions and the coarse carbonitrides NDC can be determined by the following method. First, a method for determining the number density NDI of coarse inclusions will be described. Specifically, a test piece for counting coarse inclusions is prepared from the austenitic stainless steel material according to this embodiment. When the steel material is a steel plate, the test piece is prepared from the center of the plate thickness. At this time, the plane including the rolling direction and the plate thickness direction of the steel plate is used as the observation surface. When the steel material is a steel pipe, the test piece is prepared from the center of the wall thickness. At this time, the plane including the pipe axial direction and the wall thickness direction of the steel pipe is used as the observation surface. When the steel material is a steel bar, the test piece is prepared from the R / 2 position on the cut surface. At this time, the plane including the axial direction and the radial direction of the steel bar is used as the observation surface.

[0085] The observation surface of the prepared test piece is polished with wet paper, and then buffed with colloidal silica. Three fields of view are selected from the observation surface and observed with an optical microscope to generate photographic images. At this time, the observation field area is 4.0 mm ２The observation magnification is set so that the area is 2.0 mm x 2.0 mm. At this time, it is preferable to provide an arbitrary mark in the observation field area in order to perform element concentration analysis (EDS analysis) on the observation field area, which will be described later. For example, a micro-Vickers indentation may be provided at the edge of the observation field area.

[0086] Using the generated photographic image, particles are identified based on contrast. Identifying particles based on contrast is naturally possible for those skilled in the art. Furthermore, the circle-equivalent diameter of the identified particles is determined. There are no particular limitations on the method for determining the circle-equivalent diameter, and any well-known method may be used. For example, the circle-equivalent diameter may be determined by image analysis.

[0087] Among the particles identified by the above method, particles having a circle-equivalent diameter of 2.0 μm or more (coarse particles) are identified. Element concentration analysis (EDS analysis) is performed on the identified coarse particles. In the EDS analysis, quantification is performed at an acceleration voltage of 20 kV. Based on the EDS analysis results of each particle, if the detected amounts of Cr, Nb, and V are all below the impurity level, the coarse particle is identified as a coarse inclusion. In other words, in this embodiment, an inclusion having a circle-equivalent diameter of 2.0 μm or more refers to a particle having a circle-equivalent diameter of 2.0 μm or more, and for which the detected amounts of Cr, Nb, and V are all below the impurity level as a result of EDS analysis.

[0088] The identified coarse inclusions are counted in each visual field. The arithmetic mean value of the number of coarse inclusions in the three visual fields is taken as the number density (NDI) of the coarse inclusions (numbers / 4.0 mm). ２ The number density NDI of coarse inclusions is determined by rounding the obtained value to one decimal place. As the EDS analyzer, an EDS Element for EDAX FlexSEM manufactured by Ametec Co., Ltd. can be used.

[0089] Next, a method for determining the number density NDC of coarse carbonitrides will be described. Specifically, a test piece for counting coarse carbonitrides is prepared from the austenitic stainless steel material according to this embodiment. When the steel material is a steel plate, the test piece is prepared from the center of the plate thickness. At this time, the plane including the rolling direction and plate thickness direction of the steel plate is used as the observation plane. When the steel material is a steel pipe, the test piece is prepared from the center of the wall thickness. At this time, the plane including the pipe axial direction and wall thickness direction of the steel pipe is used as the observation plane. When the steel material is a steel bar, the test piece is prepared from the R / 2 position on the cut surface. At this time, the plane including the axial direction and radial direction of the steel bar is used as the observation plane.

[0090] The observation surface of the prepared test piece is polished with wet paper, and then buffed with colloidal silica. Three fields of view are selected from the observation surface and observed with an optical microscope to generate photographic images. At this time, the observation field area is 1.0 mm ２ The observation magnification is set so that the area is 1.0 mm x 1.0 mm. At this time, any mark may be provided in the observation field area in order to perform element concentration analysis (EDS analysis) on the observation field area, which will be described later. For example, a micro-Vickers indentation may be provided at the edge of the observation field area.

[0091] Using the generated photographic image, particles are identified based on contrast. Identifying particles based on contrast is naturally possible for those skilled in the art. Furthermore, the circle-equivalent diameter of the identified particles is determined. There are no particular limitations on the method for determining the circle-equivalent diameter, and any well-known method may be used. For example, the circle-equivalent diameter may be determined by image analysis.

[0092] Among the particles identified by the above method, particles having an equivalent circle diameter of 1.0 μm or more (coarse particles) are identified. EDS analysis is performed on the identified coarse particles. In the EDS analysis, quantification is performed at an acceleration voltage of 20 kV. If at least one element of Cr, Nb, and V is detected above the impurity level based on the EDS analysis results of each particle, the coarse particle is identified as a coarse carbonitride. In other words, in this embodiment, carbonitrides having an equivalent circle diameter of 1.0 μm or more refer to particles having an equivalent circle diameter of 1.0 μm or more, and in which at least one element of Cr, Nb, and V is detected above the impurity level as a result of EDS analysis.

[0093] The identified coarse carbonitrides are counted in each visual field. The arithmetic mean value of the number of coarse carbonitrides in the three visual fields is taken as the number density NDC (number / mm ２ The number density NDC of the coarse carbonitrides is determined by rounding off the obtained value to one decimal place. As the EDS analyzer, an EDS Element for EDAX FlexSEM manufactured by Ametec Co., Ltd. can be used.

[0094] [Fatigue crack growth resistance] The austenitic stainless steel material according to this embodiment has the above-mentioned chemical composition, Fn1 is 0.55 or less, Fn2 is 39.0 or more, has a tensile strength TS of 730 MPa or more, and Fn3 is 30.0 or less. As a result, the austenitic stainless steel material according to this embodiment can achieve both high strength and excellent fatigue crack growth resistance in a hydrogen environment. Specifically, in this embodiment, the fatigue crack growth resistance of the steel material in a hydrogen environment is evaluated by the following method.

[0095] First, a test piece for a fatigue crack growth rate measurement test is prepared from the steel material according to this embodiment. A CT (Compact Tension) test piece specified in ASTM E 647 (2015) is used as the test piece for the crack growth test. When the steel material is a steel plate, a CT test piece is prepared from the center of the plate thickness. When the steel material is a steel pipe, a CT test piece is prepared from the center of the wall thickness. When the steel material is a steel bar, a test piece is prepared from the R / 2 position on the cut surface.

[0096] Using the prepared CT test specimen, a fatigue crack growth test conforming to ASTM E 647 (2015) is conducted at room temperature in air to determine the fatigue crack growth rate da / dN1 (m / cycle) in the stress intensity factor range ΔK (MPa√m) in which the Paris law holds. The fatigue crack growth test is conducted at a stress ratio of 0.1 and a frequency of 1 Hz. Furthermore, a new CT test specimen is conducted at room temperature in a 90 MPa hydrogen gas environment in accordance with ASTM E 647 (2015) to determine the fatigue crack growth rate da / dN2 (m / cycle) in the stress intensity factor range ΔK (MPa√m) in which the Paris law holds. The fatigue crack growth test is conducted at a stress ratio of 0.1 and a frequency of 1 Hz. Furthermore, the hydrogen-induced fatigue crack growth rate acceleration (FCGA ratio) is defined by the following formula (I): (FCGA ratio) = (da / dN2) / (da / dN1) (I)

[0097] If the obtained FCGA ratio is 7.5 or less, the steel is judged to have excellent fatigue crack growth resistance in a hydrogen environment. In this embodiment, the FCGA ratio of the steel is calculated by rounding the obtained value to one decimal place.

[0098] [Shape of Austenitic Stainless Steel Material] The shape of the austenitic stainless steel material of this embodiment is not particularly limited. The austenitic stainless steel material of this embodiment may be a steel plate, a steel pipe, or a steel bar. Here, the steel pipe may be a seamless steel pipe or a welded steel pipe.

[0099] [Applications of Austenitic Stainless Steel Material] The austenitic stainless steel material of this embodiment is widely applicable to applications requiring high strength and fatigue crack growth resistance. It is particularly suitable for applications requiring high strength and fatigue crack growth resistance in a hydrogen environment. Such applications include, for example, steel materials for fuel tanks of transportation equipment that uses hydrogen as energy, and steel materials for piping connecting fuel tanks to combustion chambers. Note that the austenitic stainless steel material of this embodiment is not limited to applications for transportation equipment that uses high-pressure hydrogen gas as energy, or hydrogen stations that supply hydrogen gas to transportation equipment. As described above, the austenitic stainless steel material of this embodiment is widely applicable to applications requiring high strength and / or fatigue crack growth resistance.

[0100] [Manufacturing Method] A method for manufacturing an austenitic stainless steel material according to this embodiment will be described below. The method for manufacturing an austenitic stainless steel material described below is one example of a method for manufacturing an austenitic stainless steel material according to this embodiment. Therefore, an austenitic stainless steel material having the above-described configuration may be manufactured by a manufacturing method other than the manufacturing method described below. However, the manufacturing method described below is a preferred example of a method for manufacturing an austenitic stainless steel material according to this embodiment.

[0101] An example of the method for manufacturing the austenitic stainless steel material of this embodiment includes the following steps: (Step 1) Preparation step (Step 2) First heat treatment step (Step 3) Hot working step (Step 4) Cold working step (Step 5) Second heat treatment step Each step will be described in detail below.

[0102] [(Step 1) Preparation Step] In the preparation step, a material that satisfies Features 1 and 2 is prepared. When producing the material, for example, it is produced by the following method.

[0103] Molten steel having a chemical composition that satisfies Features 1 and 2 is produced by a known method. The produced molten steel is used to produce a cast material by a known casting method. For example, an ingot is produced by an ingot casting method. A cast piece (slab, bloom, billet, etc.) may be produced by a continuous casting method. The ingot may be subjected to hot working such as blooming or hot forging to produce a slab, bloom, or billet. A material is produced by the above-mentioned steps.

[0104] [(Step 2) First Heat Treatment Step] In the first heat treatment step, the prepared material is heat-treated. Specifically, the material is held at a heat treatment temperature T1 (°C) for a holding time t1 (minutes). Here, the first heat treatment step satisfies the following conditions: (Condition 1) The heat treatment temperature T1 is set to 1150 to 1300°C, and the holding time t1 is set to 10 to 180 minutes.

[0105] [Regarding Condition 1] If the heat treatment temperature T1 is too low and / or the holding time t1 is too short, coarse carbonitrides in the material may not be sufficiently solid-dissolved. In this case, the number density NDC of the coarse carbonitrides in the produced austenitic stainless steel material will be too high, resulting in a large Fn3. On the other hand, if the heat treatment temperature T1 is too high and / or the holding time t1 is too long, the crystal grains may become extremely coarse. In this case, the grain size number GSN in the produced austenitic stainless steel material will be too large, resulting in a small Fn2. Therefore, it is preferable that the heat treatment temperature T1 be 1150 to 1300°C and the holding time t1 be 10 to 180 minutes.

[0106] In this embodiment, the first heat treatment step may be followed by a hot working step, which will be described later. That is, in a preferred manufacturing method according to this embodiment, the hot working step may be performed after the first heat treatment step without cooling the material. Furthermore, in a preferred manufacturing method according to this embodiment, the material may be cooled after the first heat treatment step, and then reheated before the hot working step is performed.

[0107] [(Step 3) Hot Working Step] In the hot working step, hot working is performed on the material heat-treated in the first heat treatment step. Hot working includes, for example, hot forging, hot rolling, hot extrusion, etc. Hot forging includes, for example, stretch forging. Hot rolling includes, for example, multiple pass rolling using a reverse rolling mill or a tandem rolling mill. Hot extrusion includes, for example, hot extrusion using the Eugène-Séjournet method. Note that multiple types of hot working may be performed. Specifically, other hot working may be performed after hot forging. An intermediate steel material is manufactured by the above manufacturing steps.

[0108] In the hot working step, the temperature of the material before the hot working is, for example, 950 to 1100°C. The area reduction rate in the hot working is, for example, 50% or more. When multiple types of hot working are performed, for example, the heating temperature before each hot working is 950 to 1100°C, and the cumulative area reduction rate of the multiple types of hot working is 50% or more.

[0109] Preferably, the finishing temperature in the hot working step is 900°C or higher. Here, the finishing temperature means the surface temperature of the intermediate steel immediately after the final hot working is performed. For example, if the hot working is hot rolling, it means the surface temperature of the intermediate steel at the exit side of the rolling stand where the final reduction is performed. For example, if the hot working is hot extrusion, it means the surface temperature of the intermediate steel at the exit side of the die. For example, if the hot working is hot forging, it means the surface temperature of the intermediate steel immediately after the final reduction is applied. The finishing temperature can be measured, for example, with a thermometer such as a radiation thermometer or a thermograph.

[0110] The hot working step satisfies the following conditions: (Condition 2) The average cooling rate CR of the intermediate steel material from the finishing temperature to the quenching start temperature is set to 0.5 to 1.2°C / sec.

[0111] [Regarding Condition 2] The average cooling rate CR of the intermediate steel from the finishing temperature to the quenching start temperature affects the amount of carbonitrides produced. If the average cooling rate CR is too slow, coarse NbV precipitates (Nb precipitates, V precipitates, and composite precipitates containing Nb and V) may form in the intermediate steel after hot working, or coarse Cr carbides may form at grain boundaries. In this case, these precipitates coarsen in the second heat treatment step described below. As a result, the number density NDC of the coarse carbonitrides in the produced austenitic stainless steel material becomes too high, resulting in a large Fn3. On the other hand, if the average cooling rate CR is too fast, the amount of fine carbonitrides produced may be insufficient. In this case, in the second heat treatment step described below, there will be a shortage of fine carbonitrides that act to refine the crystal grains by pinning the grain boundaries. As a result, the crystal grains in the produced austenitic stainless steel material may coarsen, resulting in a small Fn2. Therefore, the preferred average cooling rate CR of the intermediate steel from the finishing temperature to the quenching start temperature is 0.5 to 1.2°C / sec.

[0112] Here, the quenching start temperature is preferably 800 to 700°C. If the quenching start temperature is too high, the amount of fine carbonitrides that act to refine the crystal grain size will be insufficient. On the other hand, if the quenching start temperature is too low, coarse NbV precipitates may be formed in the intermediate steel material after hot working, or coarse Cr carbides may be formed at the grain boundaries. In these cases, the number density NDC of the coarse carbonitrides in the manufactured austenitic stainless steel material will be too high, and Fn3 will be large. Therefore, the quenching start temperature is preferably 800 to 700°C. The quenching in the hot working process is water cooling.

[0113] [(Step 4) Cold Working Step] In the cold working step, cold working is performed on the intermediate steel material after the hot working step. In this embodiment, the cold working is not particularly limited. Specifically, the cold working may be cold drawing or cold rolling. By performing the cold working step, strain is imparted to the intermediate steel material before the second heat treatment step. As a result, the strength of the steel material can be increased.

[0114] Preferably, the cold working ratio RD (%) in the cold working step is 20 to 60%. If the cold working ratio RD is 20% or more, the tensile strength of the manufactured steel material can be stably set to 730 MPa or more. Note that, although manufacturing is possible even if the cold working ratio RD exceeds 60%, a high cold working ratio RD increases the risk of edge cracking. Therefore, it is preferable to set the cold working ratio RD to 20 to 60%.

[0115] [(Step 5) Second Heat Treatment Step] In the second heat treatment step, heat treatment is performed on the intermediate steel material having the above-mentioned chemical composition. Specifically, the intermediate steel material is held at a heat treatment temperature T2 (°C) for a holding time t2 (minutes). After the holding time has elapsed, the intermediate steel material is quenched. The quenching is, for example, water cooling or oil cooling. Here, the second heat treatment step satisfies the following condition: (Condition 3) The heat treatment temperature T2 is 950 to 1150°C, and the holding time t2 is 5 to 30 minutes.

[0116] [Regarding Condition 3] If the heat treatment temperature T2 is too low and / or the holding time t2 is too short, the strain caused by cold working may not be eliminated, resulting in reduced workability. On the other hand, if the heat treatment temperature T2 is too high, the crystal grains may become coarse and Fn2 may become small. In this case, the manufactured austenitic stainless steel material will not achieve a tensile strength of 730 MPa or more. Furthermore, if the holding time t2 is too long, the above effect will saturate. Therefore, it is preferable that the heat treatment temperature T2 be 950 to 1150°C and the holding time t2 be 5 to 30 minutes.

[0117] By the above manufacturing process, an austenitic stainless steel material satisfying Features 1 to 5 can be manufactured.

[0118] The effects of the austenitic stainless steel material according to this embodiment will be described in more detail below using examples. Note that the conditions in the following examples are one example of conditions adopted to confirm the feasibility and effects of the austenitic stainless steel material according to this embodiment. Therefore, the austenitic stainless steel material according to this embodiment is not limited to this one example of conditions.

[0119] Molten steels having the chemical compositions shown in Tables 1-1, 1-2, and 1-3 were melted in a high-frequency melting furnace to produce ingots. Table 1-3 also shows the chemical composition of each steel code and Fn1 (= Nb + V) calculated from formula (1).

[0120]

[0121]

[0122]

[0123] The first heat treatment step shown in Table 2 was performed on the manufactured ingots. Specifically, the heat treatment temperature T1 (°C) listed in Table 2 was maintained for the holding time t1 (minutes). Then, hot forging and hot rolling were performed on the ingots of each test number to produce steel plates (intermediate steel materials) with a width of 200 mm and a thickness of 20 mm. Note that for all test numbers, the ingot temperatures before hot forging and hot rolling were 950 to 1100°C, respectively. Furthermore, for all test numbers, the cumulative reduction in area during hot working (hot forging and hot rolling) was 50%. Furthermore, for all test numbers, the finishing temperature of the hot working was 900°C or higher. The average cooling rate CR (°C / sec) from the finishing temperature to the quenching start temperature was as shown in Table 2. Note that for all test numbers, the quenching start temperature (°C) was 800 to 700°C. In addition, in all test numbers, quenching was performed by water cooling.

[0124]

[0125] A cold working process was carried out on the cooled intermediate steel material. The cold working ratio RD (%) of the cold working carried out on the intermediate steel material of each test number was as shown in Table 2. A second heat treatment process was carried out on the intermediate steel material of each test number that had been cold worked. The heat treatment temperature T2 (°C) in the second heat treatment process was as shown in Table 2. The holding time t2 (minutes) was 15 minutes for all test numbers. The intermediate steel material was water-cooled immediately after being extracted from the heat treatment furnace. Austenitic stainless steel material (steel plate) was produced by the above manufacturing process. The chemical composition of the steel code of each test number and FnA (= 2N + C) calculated from formula (A) are shown in Table 3. Furthermore, Table 3 shows the chemical composition of the steel code of each test number and FnB (=-7.1+2.8Ni+0.49Cr+2Mo-2Si+0.75Mn-5.7C-24N) calculated from formula (B).

[0126]

[0127] [Evaluation Tests] The following tests were carried out on the austenitic stainless steel materials with each test number: (Test 1) Grain size measurement test (Test 2) Coarse inclusion number density measurement test (Test 3) Coarse carbonitride number density measurement test (Test 4) Tensile test (Test 5) Fatigue crack propagation test Tests 1 to 5 are explained below.

[0128] [(Test 1) Grain Size Measurement Test] The grain size number of the austenitic stainless steel material for each test number was determined according to the method described above. The obtained grain size numbers GSN are shown in the "GSN" column of Table 3. Table 3 also shows FnA, FnB, GSN, and Fn2 (= 10FnA - 0.2FnB + 4.5GSN) determined from formula (2) for each test number.

[0129] [(Test 2) Coarse Inclusion Density Measurement Test] The coarse inclusion density of the austenitic stainless steel material of each test number was determined in accordance with the method described above. The area of ​​the observation field region was 4.0 mm ２The observation magnification was adjusted so that the number density NDI (number of particles / 4.0 mm) of the obtained coarse inclusions was 0.01. Furthermore, micro-Vickers indentations were made at the four corners of the observation field, and EDS analysis of the coarse particles in the same area as the observation field was carried out. Furthermore, for the EDS analysis, an EDS Element for EDAX FlexSEM manufactured by Ametec Co., Ltd. was used. ２ ) in Table 3, "NDI (pieces / 4.0 mm ２ ) column.

[0130] [(Test 3) Coarse Carbonitride Number Density Measurement Test] The number density of coarse carbonitrides in the austenitic stainless steel material of each test number was determined in accordance with the method described above. The area of ​​the observation field region was 1.0 mm ２ The observation magnification was adjusted so that the observation field area was 100%. EDS analysis of the coarse particles in the same area as the observation field area was carried out. For the EDS analysis, an EDS Element for EDAX FlexSEM manufactured by Ametec Co., Ltd. was used. The number density NDC (number / mm ２ ) in Table 3. ２ ) column.

[0131] [(Test 4) Tensile Test] The tensile strength of the austenitic stainless steel material of each test number was determined according to the method described above. The obtained tensile strength (MPa) is shown in the "TS (MPa)" column of Table 3.

[0132] [(Test 5) Fatigue crack growth test] The fatigue crack growth resistance of the austenitic stainless steel material of each test number in a hydrogen environment was evaluated in accordance with the method described above. Specifically, the hydrogen-induced fatigue crack growth acceleration (FCGA ratio) of the austenitic stainless steel material of each test number was determined in accordance with the method described above. The obtained FCGA ratios are shown in Table 3.

[0133] [Test Results] With reference to Tables 1-1, 1-2, 1-3, 2, and 3, the steel plates of Test Nos. 1 to 8 satisfied Features 1 to 5. As a result, these steel plates had a tensile strength TS of 730 MPa or more, and high strength was obtained. Furthermore, these steel plates had an FCGA ratio of 7.5 or less, and excellent fatigue crack propagation resistance in a hydrogen environment was obtained.

[0134] On the other hand, the steel plate of test number 9 had too low Fn2, and as a result, the tensile strength TS of this steel plate was less than 730 MPa, and high strength was not obtained.

[0135] The steel plate of test number 10 contained neither V nor Nb. Furthermore, this steel plate had too low Fn2. As a result, the tensile strength TS of this steel plate was less than 730 MPa, and high strength was not obtained.

[0136] The steel plate of test number 11 had too high Fn1 and too high Fn3, resulting in an FCGA ratio exceeding 7.5, and thus was unable to obtain excellent fatigue crack growth resistance in a hydrogen environment.

[0137] The steel plate of test number 12 had an excessively high Al content and an excessively high Fn3, resulting in an FCGA ratio exceeding 7.5, and thus was unable to obtain excellent fatigue crack growth resistance in a hydrogen environment.

[0138] The steel plate of test number 13 had an excessively high Ca content and an excessively high Fn3, resulting in an FCGA ratio exceeding 7.5, and thus was unable to obtain excellent fatigue crack growth resistance in a hydrogen environment.

[0139] The heat treatment temperature T1 in the first heat treatment step of the steel plate of test number 14 was too low. As a result, the Fn3 of this steel plate was too high. As a result, the FCGA ratio of this steel plate exceeded 7.5, and excellent fatigue crack growth resistance in a hydrogen environment was not obtained.

[0140] For the steel plate of test number 15, the heat treatment temperature T1 in the first heat treatment step was too low and the holding time t1 was too short. As a result, the Fn3 of this steel plate was too high. As a result, the FCGA ratio of this steel plate exceeded 7.5, and excellent fatigue crack growth resistance in a hydrogen environment was not obtained.

[0141] The cooling rate CR in the hot working process of the steel plate of test number 16 was too slow. As a result, the Fn3 of this steel plate was too high. As a result, the FCGA ratio of this steel plate exceeded 7.5, and excellent fatigue crack growth resistance in a hydrogen environment was not obtained.

[0142] The embodiments of the present disclosure have been described above. However, the above-described embodiments are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiments, and can be implemented by appropriately modifying the above-described embodiments within the scope of the present disclosure.

Claims

1. An austenitic stainless steel material having a chemical composition, in mass%, of C: 0.005 to 0.150%, Si: 0.10 to 1.00%, Mn: 0.50 to 3.00%, P: 0.050% or less, S: 0.0100% or less, Cr: 17.0 to 25.0%, Ni: 6.5 to 12.0%, N: 0.15 to 0.35%, Al: 0.050% or less, Ca: 0.0005 to 0.0050%, O: 0.030% or less, Mo: 0 to 1.00%, Ti: 0 to 0.300%, Cu: 0 to 0.30%, Co: 0 to 0.30%, B: 0 to 0.010%, Mg: 0 to 0.100%, rare earth element: 0 to 0.0050%, one or more elements selected from the group consisting of Nb: 0.01 to 0.50%, and V: 0.01 to 0.50%, and the balance being Fe and impurities, satisfying formula (1), the element contents in the chemical composition and the grain size number GSNμm satisfy formula (2), the tensile strength TS is 730 MPa or more, and in the austenitic stainless steel material, the number density of inclusions having an equivalent circle diameter of 2.0μm or more is NDI pieces / 4.0mm ２ The number density of carbonitrides having a circle equivalent diameter of 1.0 μm or more is defined as NDC pieces / mm ２ An austenitic stainless steel material, wherein the tensile strength TS, the number density NDI of the inclusions having an equivalent circle diameter of 2.0 μm or more, and the number density NDC of the carbonitrides having an equivalent circle diameter of 1.0 μm or more satisfy the following formula (3): Nb + V≦0.55 (1) 10FnA − 0.2FnB + 4.5GSN≧39.0 (2) where FnA and FnB in formula (2) are defined by formulas (A) and (B). FnA = 2N + C (A) FnB = -7.1 + 2.8Ni + 0.49Cr + 2Mo - 2Si + 0.75Mn - 5.7C - 24N (B) 0.5NDI + NDC + 0.02TS ≦ 30.0 (3) Here, for each element symbol in formulas (1), (A) and (B), the content of the corresponding element in the chemical composition is substituted in the unit of mass%. If the corresponding element is not contained, "0" is substituted for the corresponding element symbol. The grain size number is substituted for GSN in formula (2). NDI in formula (3) represents the number density of inclusions having a circle equivalent diameter of 2.0 μm or more in the unit: pieces / 4.0 mm ２ In the formula (3), NDC is the number density of carbonitrides having a circle equivalent diameter of 1.0 μm or more, expressed in units of pieces / mm ２ The tensile strength is substituted into TS in the formula (3) in the unit of MPa.

2. An austenitic stainless steel material according to claim 1, wherein the chemical composition contains, in mass%, one or more elements selected from the group consisting of Mo: 0.01 to 1.00%, Ti: 0.001 to 0.300%, Cu: 0.01 to 0.30%, Co: 0.01 to 0.30%, B: 0.001 to 0.010%, Mg: 0.001 to 0.100%, and rare earth elements: 0.0001 to 0.0050%.

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