Austenitic stainless steel and hydrogen resistant member

Abstract

The present invention relates to an austenitic stainless steel comprising C ≤ 0.10 mass%, Si ≤ 0.50 mass%, 3.0 mass% ≤ Mn ≤ 8.0 mass%, P ≤ 0.30 mass%, S ≤ 0.30 mass%, 7.0 mass% ≤ Ni ≤ 12.0 mass%, 18.0 mass% ≤ Cr ≤ 28.0 mass%, 1.0 mass% ≤ Mo ≤ 3.0 mass%, 0.03 mass% ≤ V ≤ 0.50 mass%, 0.0003 mass% ≤ B ≤ 0.0300 mass%, 0.0001 mass% ≤ Ca ≤ 0.0300 mass%, 0.35 mass% ≤ N ≤ 0.80 mass%, and 0.001 mass% ≤ Co ≤ 1.00 mass%, and Optionally, it consists of W ≤ 2.0 mass%, Zr ≤ 0.20 mass%, and Ta ≤ 0.50 mass%, with the remainder being Fe and unavoidable impurities, and has a number density of coarse alloy carbonitrides of 3 × 10⁵ pieces / mm² or less.

Description

Technology Field

[0001] The present invention relates to an austenitic stainless steel and a hydrogen-resistant member, and in particular, to an austenitic stainless steel having excellent strength and hydrogen embrittlement resistance, and a hydrogen-resistant member using said austenitic stainless steel. Background Technology

[0002] Recently, fuel cell vehicles using hydrogen as fuel and hydrogen stations supplying hydrogen to these vehicles have been developed. Since various devices used in fuel cell vehicles, hydrogen stations, etc. (hereinafter collectively referred to as "high-pressure hydrogen gas devices") are used in high-pressure hydrogen gas environments, the materials used in these devices are required to possess excellent hydrogen embrittlement resistance. Stainless steel (in particular, austenitic stainless steel with increased Ni equivalent) possesses excellent hydrogen embrittlement resistance and is suitable for this type of application.

[0003] Among austenitic stainless steels, SUS316L is known for its excellent resistance to hydrogen embrittlement. Currently, SUS316L is approved as a stainless steel with excellent hydrogen embrittlement resistance in accordance with the specifications for automotive compressed hydrogen containers stipulated in Japan's High Pressure Gas Safety Act. However, because SUS316L has low strength, when it is used as a structural component in high-pressure hydrogen gas devices, the component must be designed to be thick. Consequently, there is a problem in that an increase in the size and weight of the device cannot be avoided. To reduce the weight of fuel cell vehicles, make hydrogen stations compact, and achieve high-pressure operation at hydrogen stations, it is desirable for the stainless steel used for these applications to have high strength.

[0004] To solve these problems, various proposals have been made in the past.

[0005] For example, in Patent Document 1,

[0006] (a) containing predetermined amounts of C, Si, Mn, Cr, Ni, V, N and Al, with the remainder being Fe and impurities,

[0007] (b) A stainless steel for high-pressure hydrogen gas satisfying 2.5Cr + 3.4Mn < 300N is disclosed.

[0008] According to the description in that same patent document,

[0009] (A) Solid-solution strengthening by N is most effective for improving the strength of austenitic stainless steel, and as the amount of N added increases, strength improves but ductility and toughness decrease, and

[0010] (B) When the composition is adjusted to satisfy 2.5Cr+3.4Mn < 300N, the tensile strength is improved and the elongation is also improved.

[0011] In Patent Document 2,

[0012] (a) containing at least one of C, Si, Mn, Cr, Ni, Al, N and V and Nb in predetermined amounts, and the remainder being Fe and impurities,

[0013] (b) having a tensile strength of 800 MPa or more, and

[0014] (c) having a crystal particle size number of 8 or more, and

[0015] (d) Content of alloy carbonitrides having a maximum diameter of 50 nm to 1,000 nm is 0.4 pieces / μm 2 An austenitic stainless steel for high-pressure hydrogen gas, as described above, is disclosed.

[0016] According to the description in that same patent document,

[0017] (A) When nitrogen is used as a solute element, the strength of stainless steel can be improved, but the stacking fault energy is reduced, and thus the durability against hydrogen environment embrittlement is reduced, and

[0018] (B) When V and / or Nb are added to steel, fine alloy carbides are precipitated during solution heat treatment, and crystal grains are refined by the pinning effect, and

[0019] (C) When the grain size is refined, the resistance of high-nitrogen steel to hydrogen environment embrittlement can be improved.

[0020] In Patent Document 3,

[0021] (a) containing predetermined amounts of C, Si, Mn, P, S, Ni, Cr, Mo, N, Nb and V, with the remainder being Fe and impurities,

[0022] (b) Satisfying 15 ≤ 12.6C + 1.05Mn + Ni + 15N, and

[0023] (c) having a crystal particle size number of less than 8.0, and

[0024] (d) having a tensile strength of 690 MPa or more

[0025] Austenitic stainless steel is disclosed.

[0026] According to the description in that same patent document,

[0027] (A) If the crystal grain size number is 8.0 or less, excellent machinability can be obtained, and

[0028] (B) C, N, Mn, and Ni are all austenite-stabilizing elements, and optimizing the content of these elements stabilizes the austenite and improves hydrogen embrittlement resistance even when the grain size is coarse.

[0029] (C) When 1.0% or more of Mo is added, high tensile strength can be obtained even when the crystal grain number is less than 8.0.

[0030] In addition, in Patent Document 4,

[0031] (a) containing predetermined amounts of C, Si, Mn, P, S, Ni, Cr, N, Mo, V and Nb, with the remainder being Fe and impurities,

[0032] (b) having a crystal particle size number of 6.0 or higher, and

[0033] (c) having a tensile strength of 800 MPa or more,

[0034] (d) The difference between the maximum and minimum values ​​of tensile strength is 50 MPa or less, and

[0035] (e) The number of alloy carbonitrides with an equivalent circle diameter exceeding 1,000 nm is 10 / mm 2 That is all,

[0036] (f) The difference between the maximum and minimum values ​​of the determination particle size number is 1.5 or less

[0037] An austenitic stainless steel material is disclosed.

[0038] According to the description in that same patent document,

[0039] (A) When the crystal grain number is 6.0 or higher and the difference between the maximum and minimum values ​​of the crystal grain number (△GS) is 1.5 or lower, the difference between the maximum and minimum values ​​of the tensile strength is 50 MPa or lower, and

[0040] (B) When the difference between the initial temperature and the final temperature during hot working is 100℃ or less, △GS can be controlled to 1.5 or less, and

[0041] (C) 10 alloy carbonitrides / mm² with a crystal grain size number of 6.0 or greater and exceeding 1,000 nm 2In the above case, a tensile strength of 800 MPa or more can be obtained.

[0042] When materials with excellent hydrogen embrittlement resistance are used as structural members of high-pressure hydrogen gas devices, they are often subjected to treatments such as machining, cold working, and welding. Therefore, this type of material is required to possess not only excellent strength and hydrogen embrittlement resistance, but also excellent processability, such as machinability, cold workability, and weldability.

[0043] Additionally, to reduce the manufacturing and maintenance costs of the high-pressure hydrogen gas device, the materials used in the device may be used in a solution-treated (solution heat-treated) or welded state, and it is also desirable that they have a low content of expensive elements such as Ni.

[0044] In this regard, the stainless steel for high-pressure hydrogen gas described in Patent Document 1 has a strength of 700 MPa or more after solution treatment. However, the stainless steel described in the same patent document cannot achieve excellent workability because it has a high Mn content.

[0045] The austenitic stainless steel for high-pressure hydrogen gas described in Patent Document 2 achieves grain refinement and high strength by performing solution heat treatment, cold working, and secondary heat treatment. However, cold working and secondary heat treatment result in increased manufacturing costs.

[0046] In addition, the austenitic stainless steel described in Patent Document 3 is intended to be used in a hot-worked state. Therefore, the stainless steel described in the same patent document has an excessive amount of carbonitrides in the steel and low workability.

[0047] Similarly, in Patent Document 4, a relatively large amount of relatively coarse alloy carbonitrides are precipitated by performing heat treatment at a low temperature (from 930°C to less than 1000°C) after hot working of an austenitic stainless steel. Therefore, the stainless steel described in Patent Document 4 is also thought to have low workability. Prior art literature

[0048] WO2004 / 083477AWO2012 / 132992AWO2015 / 159554AWO2017 / 175739A The problem to be solved

[0049] The problem that the present invention aims to solve is to provide an austenitic stainless steel having excellent hydrogen embrittlement resistance, high strength, and excellent processability.

[0050] Additionally, another problem that the present invention aims to solve is to provide a hydrogen-resistant member using such austenitic stainless steel. means of solving the problem

[0051] The gist of the present invention for solving the above problem is as follows.

[0052] [1] As an austenitic stainless steel,

[0053] C ≤ 0.10 mass%,

[0054] Si ≤ 0.50 mass%,

[0055] 3.0 mass% ≤ Mn ≤ 8.0 mass%,

[0056] P ≤ 0.30 mass%,

[0057] S ≤ 0.30 mass%,

[0058] 7.0 mass% ≤ Ni ≤ 12.0 mass%,

[0059] 18.0 mass% ≤ Cr ≤ 28.0 mass%,

[0060] 1.0 mass% ≤ Mo ≤ 3.0 mass%,

[0061] 0.03 mass% ≤ V ≤ 0.50 mass%,

[0062] 0.0003 mass% ≤ B ≤ 0.0300 mass%,

[0063] 0.0001 mass% ≤ Ca ≤ 0.0300 mass%,

[0064] 0.35 mass% ≤ N ≤ 0.80 mass%, and

[0065] 0.001 mass% ≤ Co ≤ 1.00 mass%,

[0066] Optionally,

[0067] W ≤ 2.0 mass%,

[0068] Zr ≤ 0.20 mass%, and

[0069] Ta ≤ 0.50 mass%

[0070] It consists of,

[0071] The remainder is Fe and unavoidable impurities, and

[0072] 3 × 10 5 Pieces / mm 2 Austenitic stainless steel having a number density of coarse alloy carbonitrides below the following level.

[0073] Here, "coarse alloy carbonitride" refers to an alloy carbonitride having a circular equivalent diameter greater than 1,000 nm.

[0074] [2] In [1],

[0075] 0.3 mass% ≤ W ≤ 2.0 mass%,

[0076] 0.01 mass% ≤ Zr ≤ 0.20 mass%, and

[0077] 0.10 mass% < Ta ≤ 0.50 mass%

[0078] Austenitic stainless steel satisfying at least one additionally selected from the group consisting of

[0079] [3] Austenitic stainless steel having a tensile strength of 690 MPa or more as measured at 25°C in [1] or [2].

[0080] [4] An austenitic stainless steel having an austenite grain size number of less than 8.0 in any one of [1] to [3].

[0081] [5] An austenitic stainless steel having a reduction of area of ​​30% or more measured at 25°C in any one of [1] to [4].

[0082] [6] A hydrogen-resistant member comprising an austenitic stainless steel according to any one of [1] to [5].

[0083] [7] [6] In the case of austenitic stainless steel, the hydrogen-resistant member includes a portion that is in a solution-treated state.

[0084] [8] [6] or [7], a hydrogen-resistant member comprising a butt-welded portion, wherein the butt-welded portion has a welded tensile strength of 690 MPa or more as measured at 25°C.

[0085] Nitrogen (N) acts as an austenite-stabilizing element and a solid-solution strengthening element, but it is also an element that lowers stacking fault energy. Meanwhile, for example, silicon (Si) is an element that reduces grain boundary strength, while carbon (B) is an element that improves grain boundary strength. Therefore, when a relatively large amount of nitrogen is added to the steel and the composition is simultaneously optimized (specifically, the content of elements that reduce grain boundary strength is limited, and an appropriate amount of elements that improve grain boundary strength is added), an austenitic stainless steel with excellent hydrogen embrittlement resistance and high strength can be obtained. Additionally, the content of nickel (Ni) in the steel can be reduced, thereby lowering raw material costs.

[0086] Furthermore, the austenitic stainless steel according to the present invention exhibits high strength even in a solution-treated or welded state, for example. In addition, because the Mn content is relatively low and the water density of coarse alloy carbides is also low, the austenitic stainless steel has excellent machinability. Moreover, if manufacturing conditions are optimized, the grain size is appropriately coarsened, further improving machinability. Brief explanation of the drawing

[0087] Figure 1 shows the relationship between the Co content and the relative area reduction ratio at -60℃. Specific details for implementing the invention

[0088] Embodiments of the present invention are described in detail below.

[0089] [1. Austenitic Stainless Steel]

[0090] [1.1. Main Constituent Elements]

[0091] The austenitic stainless steel according to the present invention contains the following elements, with the remainder being Fe and unavoidable impurities. The types of added elements, the content range of the elements, and the reasons for limiting the content range are as follows.

[0092] (1) C ≤ 0.10 mass%:

[0093] In the present invention, C is an impurity. If the C content is excessive, a large amount of carbides precipitates, causing deterioration of toughness, ductility, and corrosion resistance. Therefore, the C content needs to be 0.10 mass% or less. The C content is preferably less than 0.05 mass%, and more preferably 0.03 mass% or less.

[0094] In the present invention, the lower the C content, the better. However, if the C content is reduced to an extreme, the manufacturing cost increases. Considering the manufacturing cost, the C content is preferably 0.0005 mass% or more. The C content is more preferably 0.001 mass% or more.

[0095] (2) Si ≤ 0.50 mass%:

[0096] In the present invention, Si is an impurity. Si combines with Ni and Cr to form intermetallic compounds. Si further promotes the growth of intermetallic compounds, such as the sigma phase. These intermetallic compounds reduce the hot workability of steel. In addition, if the Si content is excessive, the grain boundary strength decreases and the hydrogen embrittlement resistance decreases. Therefore, the Si content needs to be 0.50 mass% or less. The Si content is preferably 0.20 mass% or less, and more preferably 0.09 mass% or less.

[0097] In the present invention, the lower the Si content, the better. However, if the Si content is reduced to an extreme, the manufacturing cost increases. Considering the manufacturing cost, the Si content is preferably 0.001 mass% or more. The Si content is more preferably 0.01 mass% or more.

[0098] (3) 3.0 ≤ Mn ≤ 8.0 Mass%:

[0099] Mn stabilizes austenite and prevents the formation of martensite with high hydrogen embrittlement susceptibility. In addition, Mn improves the solubility of N in molten steel, thereby contributing to the improvement of strength.

[0100] To obtain this effect, the Mn content needs to be 3.0 mass% or more. The Mn content is preferably 5.1 mass% or more, and more preferably 5.5 mass% or more.

[0101] Meanwhile, if the Mn content is excessive, the stacking fault energy and grain boundary strength decrease, and the hydrogen embrittlement resistance decreases. In addition, if the Mn content is excessive, the toughness, ductility, and hot workability of the steel also decrease. Therefore, the Mn content needs to be 8.0 mass% or less. The Mn content is preferably 6.9 mass% or less, and more preferably 6.5 mass% or less.

[0102] (4) P ≤ 0.30 mass%:

[0103] In the present invention, P is an impurity. If the P content is excessive, the hot workability, toughness, and ductility of the steel are reduced. In addition, if the P content is excessive, the concern regarding solidification cracking during welding increases. Therefore, the P content needs to be 0.30 mass% or less. The P content is preferably less than 0.10 mass%, and more preferably 0.03 mass% or less.

[0104] In the present invention, the lower the P content, the better. However, if the P content is reduced to an extreme, the manufacturing cost increases. Considering the manufacturing cost, the P content is preferably 0.0005 mass% or more. The P content is more preferably 0.001 mass% or more.

[0105] (5) S ≤ 0.30 mass%:

[0106] In the present invention, S is an impurity. If the S content is excessive, the toughness, ductility, and hot workability of the steel are reduced. In addition, if the S content is excessive, the concern regarding cracking during welding increases. Therefore, the S content needs to be 0.30 mass% or less. The S content is preferably less than 0.10 mass%, and more preferably 0.09 mass% or less.

[0107] In the present invention, the lower the S content, the better. However, if the S content is reduced to an extreme, the manufacturing cost increases. Considering the manufacturing cost, the S content is preferably 0.0005 mass% or more. The S content is more preferably 0.001 mass% or more.

[0108] (6) 7.0 ≤ Ni ≤ 12.0 Mass%:

[0109] Ni stabilizes austenite and increases stacking fault energy, thereby improving hydrogen embrittlement resistance. To obtain this effect, the Ni content needs to be 7.0 mass% or more. The Ni content is preferably 9.0 mass% or more, and more preferably 9.5 mass% or more.

[0110] On the other hand, if the Ni content is excessive, raw material costs increase. In addition, if the Ni content is excessive, the solubility of N in the molten steel decreases, and strength deteriorates. Therefore, the Ni content needs to be 12.0 mass% or less. The Ni content is preferably 10.5 mass% or less, and more preferably 9.9 mass% or less.

[0111] (7) 18.0 ≤ Cr ≤ 28.0 Mass%:

[0112] Cr improves the corrosion resistance of steel. In addition, Cr improves the solubility of N in molten steel, thereby contributing to the improvement of strength. To obtain these effects, the Cr content needs to be 18.0 mass% or more. The Cr content is preferably 20.0 mass% or more, and more preferably 22.0 mass% or more.

[0113] Meanwhile, if the Cr content is excessive, intermetallic compounds or carbonitrides are prone to excessive precipitation, and the toughness, ductility, and corrosion resistance of the steel are reduced. Therefore, the Cr content needs to be 28.0 mass% or less. The Cr content is preferably 26.0 mass% or less, and more preferably 25.0 mass% or less.

[0114] (8) 1.0 ≤ Mo ≤ 3.0 Mass%:

[0115] Mo contributes to the improvement of strength by solid solution strengthening of austenite or forming carbonitrides. In addition, Mo improves the corrosion resistance of steel. To obtain these effects, the Mo content needs to be 1.0 mass% or more. The Mo content is preferably 1.5 mass% or more, and more preferably 1.8 mass% or more.

[0116] On the other hand, if the Mo content is excessive, intermetallic compounds or carbonitrides are prone to excessive precipitation, and the toughness and ductility of the steel decrease. In addition, if the Mo content is excessive, raw material costs also increase. Therefore, the Mo content needs to be 3.0 mass% or less. The Mo content is preferably 2.5 mass% or less, and more preferably 2.2 mass% or less.

[0117] (9) 0.03 ≤ V ≤ 0.50 mass%:

[0118] V improves the strength of steel by forming hard alloy carbonitrides. To obtain this effect, the V content needs to be 0.03 mass% or more. The V content is preferably 0.05 mass% or more, and more preferably 0.08 mass% or more.

[0119] Meanwhile, if the V content is excessive, alloy carbonitrides precipitate excessively, and the toughness and ductility of the steel decrease. Therefore, the V content needs to be 0.50 mass% or less. The V content is preferably 0.30 mass% or less, and more preferably 0.20 mass% or less.

[0120] (10) 0.0003 ≤ B ≤ 0.0300 mass%:

[0121] B segregates at grain boundaries to increase grain boundary-fixing force, thereby improving the strength of the steel. In addition, B prevents the embrittlement of the steel in a hydrogen environment and improves hydrogen embrittlement resistance. Furthermore, B improves the hot workability of the steel. To obtain these effects, the B content needs to be 0.0003 mass% or more. The B content is preferably 0.0005 mass% or more, and more preferably 0.0010 mass% or more.

[0122] Meanwhile, if the B content is excessive, the susceptibility of the molten steel to solidification cracking improves when welding is performed without using filler metal. Therefore, the B content needs to be 0.0300 mass% or less. The B content is preferably 0.0100 mass% or less, and more preferably 0.0050 mass% or less.

[0123] (11) 0.0001 ≤ Ca ≤ 0.0300 mass%:

[0124] Ca improves the hot workability of steel. To obtain this effect, the Ca content needs to be 0.0001 mass% or more. The Ca content is preferably 0.0003 mass% or more, and more preferably 0.0005 mass% or more.

[0125] On the other hand, if the Ca content is excessive, Ca and O combine to lower the cleanliness of the steel. As a result, hot workability is actually reduced, and toughness and ductility are also reduced. Therefore, the Ca content needs to be 0.0300 mass% or less. The Ca content is preferably 0.0150 mass% or less, and more preferably 0.0100 mass% or less.

[0126] (12) 0.35 ≤ N ≤ 0.80 Mass%:

[0127] N stabilizes austenite and improves hydrogen embrittlement resistance. In addition, N improves the strength of steel through solid solution strengthening and the formation of nitrides. In addition, N improves the corrosion resistance of steel. To obtain these effects, the N content needs to be 0.35 mass% or more.

[0128] The content of N is preferably 0.40 mass% or more, and more preferably 0.46 mass% or more.

[0129] On the other hand, if the N content is excessive, coarse nitrides are formed, and the toughness and ductility of the steel are reduced. In addition, if the N content is excessive, the hot workability of the steel is reduced, or there is a possibility that blowholes (defects) may form during welding. Furthermore, if N is excessive, the stacking fault energy is lowered, and hydrogen embrittlement resistance is reduced. Therefore, the N content needs to be 0.80 mass% or less. The N content is preferably 0.60 mass% or less, and more preferably 0.53 mass% or less.

[0130] It should be noted that while hydrogen embrittlement resistance may be reduced in high-nitrogen steel due to the deterioration of grain boundary strength, in the present invention, the deterioration of hydrogen embrittlement resistance can be prevented because the grain boundary strength is improved by optimizing the Si content, B content, etc.

[0131] (13) 0.001 ≤ Co ≤ 1.00 mass%:

[0132] An example of the effect of Co is,

[0133] (a) Effect of improving hydrogen embrittlement resistance by stabilizing the austenite structure;

[0134] (b) Effect of improving hydrogen embrittlement resistance by increasing stacking fault energy;

[0135] (c) Effect of improving strength by solid solution reinforcement;

[0136] (d) Effect of improving personality; and

[0137] (e) Includes the effect of suppressing the decrease in hydrogen embrittlement resistance after welding. To obtain this effect, the Co content needs to be 0.001 mass% or more. The Co content is preferably 0.07 mass% or more, more preferably 0.10 mass% or more, and even more preferably 0.15 mass% or more.

[0138] Meanwhile, if the Co content is excessive, costs increase. Therefore, the Co content is preferably 1.00 mass% or less.

[0139] [1.2. Sub-components]

[0140] The austenitic stainless steel according to the present invention may additionally contain one or more of the following elements in addition to the main constituent elements mentioned above. The types of added elements, the range of their content, and the reasons for limiting the range of their content are as follows.

[0141] (1) W ≤ 2.0 mass%:

[0142] W has the effect of improving corrosion resistance and the effect of improving strength through solid solution or the formation of carbonitrides. Accordingly, the austenitic stainless steel according to the present invention may further contain W. To obtain these effects, the content of W is preferably 0.3 mass% or more. The content of W is more preferably 0.8 mass% or more.

[0143] Meanwhile, if the W content is excessive, raw material costs increase. Therefore, the W content is preferably 2.0 mass% or less. The W content is more preferably 1.5 mass% or less.

[0144] (2) Zr ≤ 0.20 mass%:

[0145] Zr has the effect of forming crystallized oxides. Since crystallized oxides serve as a starting point for the formation of inclusions such as MnS and carbonitrides, the size of the inclusions can be reduced and toughness and ductility can be improved. Accordingly, the austenitic stainless steel according to the present invention may contain Zr in addition to or instead of W. To obtain this effect, the content of Zr is preferably 0.01 mass% or more. The content of Zr is more preferably 0.05 mass% or more.

[0146] On the other hand, if the Zr content is excessive, coarse oxides may be formed and toughness and ductility may be reduced. Therefore, the Zr content is preferably 0.20 mass% or less. The Zr content is more preferably 0.15 mass% or less.

[0147] (3) Ta ≤ 0.50 mass%:

[0148] Ta has the effect of improving strength through solid solution or through the formation of carbonitrides. Accordingly, the austenitic stainless steel according to the present invention may contain more Ta. The austenitic stainless steel may contain only Ta, or may additionally contain W and / or Zr in addition to Ta. To obtain this effect, the content of Ta is preferably greater than 0.10 mass%.

[0149] Meanwhile, if the Ta content is excessive, raw material costs increase. Therefore, the Ta content is preferably 0.50 mass% or less.

[0150] [1.3. Inevitable Impurities]

[0151] Unavoidable impurities refer to elements incorporated from the environment, such as ores or scrap used as raw materials for steel, or from the manufacturing process. Specific examples of unavoidable impurities include the following elements in addition to the previously mentioned C, Si, P, and S.

[0152] (1) Cu ≤ 0.5 mass%:

[0153] In the present invention, Cu is an impurity. If the content of Cu is excessive, the concern regarding solidification cracking during welding increases. Therefore, the content of Cu is preferably 0.5 mass% or less. The content of Cu is more preferably 0.4 mass% or less.

[0154] In the present invention, the lower the Cu content, the better. However, if the Cu content is reduced to an extreme, the manufacturing cost increases. Considering the manufacturing cost, the Cu content is preferably 0.005 mass% or more. The Cu content is more preferably 0.010 mass% or more.

[0155] (2) Al ≤ 0.10 mass%:

[0156] In the present invention, Al is an impurity. Similar to Si, Al has the effect of deoxidizing steel. However, if the Al content is excessive, excessive nitrides are formed, and the toughness and ductility of the steel are reduced.

[0157] In addition, if the Al content is excessive, the penetration depth during welding becomes shallow. Therefore, the Al content is preferably 0.10 mass% or less. The Al content is more preferably 0.05 mass% or less.

[0158] In the present invention, the lower the Al content, the better. However, if the Al content is reduced to an extreme, the manufacturing cost increases. Considering the manufacturing cost, the Al content is preferably 0.0005 mass% or more. The Al content is more preferably 0.001 mass% or more.

[0159] (3) O ≤ 0.050 mass%:

[0160] In the present invention, O is an impurity. O reduces the hot workability of the base material during manufacturing. In addition, O reduces the cleanliness of the steel and reduces the toughness and ductility of the steel. Therefore, the content of O is preferably 0.050 mass% or less. The content of O is more preferably 0.030 mass% or less, and even more preferably 0.010 mass% or less.

[0161] In the present invention, the lower the O content, the better. However, if the O content is reduced to an extreme, the manufacturing cost increases. Considering the manufacturing cost, the O content is preferably 0.0005 mass% or more. The O content is more preferably 0.001 mass% or more, and even more preferably 0.002 mass% or more.

[0162] (4) Other impurities:

[0163] The austenitic stainless steel according to the present invention may additionally contain the following components in the following amounts in addition to the components mentioned above. In such cases, these components are treated as unavoidable impurities in the present invention.

[0164] Nb < 0.05 mass%, Ta ≤ 0.10 mass%, Ti ≤ 0.10 mass%, Sn ≤ 0.03 mass%, Zn ≤ 0.03 mass%, Pd ≤ 0.03 mass%, Cd ≤ 0.03 mass%, Ag ≤ 0.03 mass%, Ga ≤ 0.03 mass%, As ≤ 0.03 mass%, La ≤ 0.03 mass%, Ce ≤ 0.03 mass%, Y ≤ 0.03 mass%, Sm ≤ 0.03 mass%, Se ≤ 0.03 mass%, Te ≤ 0.03 mass%, Bi ≤ 0.03 mass%, Pb ≤ 0.03 mass%, Mg ≤ 0.03 mass%, Sb ≤ 0.03 mass%, and REM ≤ 0.03 mass%.

[0165] [1.4. Characteristics]

[0166] [1.4.1. Number Density of Coarse Alloy Carbonitrides]

[0167] "Coarse alloy carbonitride" refers to an alloy carbonitride having a circular equivalent diameter exceeding 1,000 nm.

[0168] "Alloy carbonitride" refers to a material containing C or N or both among precipitates and inclusions containing carbonitride-forming elements such as Cr, V, Mo, W, Ta, Nb, or Ti as main components. Examples of alloy carbonitrides include the following:

[0169] (a) Cr2N;

[0170] (b) Z-phase (i.e., Cr(Nb,V)(C,N)); and

[0171] (c) MX-type carbonitride (M: Cr, V, Mo, Ta, Nb, Ti, etc., X: C, N).

[0172] "Number density of coarse alloy carbonitrides" is the number of coarse alloy carbonitrides per unit area (pieces / mm²) 2 It means ). Specifically, number density is determined by the following method.

[0173] That is, a sample of austenitic stainless steel material is taken to include the center of a cross-section perpendicular to the rolling or forging elongation direction. The upper observation area of ​​the sample is mirror-polished. Then, using a scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectrometer (EDS), any 10 fields of view (200 μm × 200 μm) in the observation area are observed to identify alloy carbonitrides from precipitates and inclusions in each field of view.

[0174] The circle equivalent diameter of the alloy carbonitride identified in each field of view is obtained through image analysis. The circle equivalent diameter refers to the diameter (nm) when the area of ​​the alloy carbonitride in the field of view is converted into a circle. The number of alloy carbonitrides (coarse alloy carbonitrides) having a circle equivalent diameter greater than 1,000 nm is counted. In the present invention, the average number of coarse alloy carbonitrides obtained in 10 fields of view is defined as the "number density of coarse alloy carbonitrides (pieces / mm²)".

[0175] When coarse alloy carbonitrides are excessively precipitated in the matrix phase, cuttability deteriorates due to tool wear caused by the coarse alloy carbonitrides. On the other hand, if the composition of the austenitic stainless steel is optimized and hot working and / or solution treatment is performed under appropriate conditions, the number density of coarse alloy carbonitrides can be reduced. To obtain excellent machinability, the number density of coarse alloy carbonitrides is preferably 3 × 10⁻⁶. 5 Pieces / mm 2 It is less than or equal to. The water density is more preferably 1 × 10⁻⁶ 4 Pieces / mm 2 Less than or equal to, and more preferably 1 × 10 3 Pieces / mm 2 It is as follows.

[0176] Meanwhile, since coarse alloy carbonitrides also contribute to tensile strength, if one wishes to obtain austenitic stainless steel with higher strength, the number density is preferably 1 piece / mm 2 Ideally, and more preferably 10 pieces / mm 2 That is all.

[0177] [1.4.2. Tensile Strength]

[0178] "Tensile strength" refers to the tensile strength obtained by performing a tensile test using a No. 14A test specimen having a parallel section diameter of 6 mm in accordance with JIS Z2241 (2011).

[0179] The austenitic stainless steel according to the present invention achieves a tensile strength of 690 MPa or more measured at 25°C by optimizing hot working conditions and / or solution treatment conditions. If the components are further optimized, the tensile strength may be 750 MPa or more, or 800 MPa or more.

[0180] [1.4.3. Determined Particle Size Number]

[0181] "Determined particle size number" refers to a value measured in accordance with JIS G0551 (2005). Specifically, the determined particle size number is determined by the following method.

[0182] That is, a test specimen for microscopic observation is taken from austenitic stainless steel. Using the taken specimen, the microscopic test method for grain size specified in JIS G0551 (2005) is performed to evaluate the grain size number.

[0183] More specifically, the surface of the test specimen is etched using a well-known etchant (such as glycerezia, Kalling's reagent, or Marbles' reagent) to expose grain boundaries on the surface. The grain size number for each of the 10 fields of view on the etched surface is obtained. The area of ​​each field of view is approximately 40 mm² 2 am.

[0184] The crystal grain size number of each field is evaluated by comparison with the reference chart of crystal grain size defined in 7.1.2 of JIS G0551 (2005). The average crystal grain size number of the field is defined as the crystal grain size number of the austenitic stainless steel according to the present invention.

[0185] In the austenitic stainless steel according to the present invention, since the composition is optimized, the hot working conditions and / or solution treatment conditions are optimized so that the grain size number of the austenite grains can be less than 8.0. When the grain size number is less than 8.0, the grain size increases appropriately and the cutting resistance is lowered. In addition, chips can be easily separated from the working material and the cutting tool during cutting, and chip handling is improved. That is, when the grain size number is less than 8.0, the machinability of the steel is improved. The grain size number is preferably 7.0 or less.

[0186] On the other hand, if the grain size number is too small, the grain size increases excessively, which may lower the tensile strength of the steel. Therefore, the grain size number is preferably 2.0 or higher. The grain size number is more preferably 3.0 or higher.

[0187] [1.4.4. Area Reduction Rate]

[0188] "Area reduction rate" is the ratio of the original cross-sectional area (S0) of the tensile test specimen before the test and the cross-sectional area (S) of the tensile test specimen after the test to the original cross-sectional area (S0) when performing a tensile test using a No. 14A test specimen having a parallel section diameter of 6 mm according to JIS Z2241 (2011). u The ratio of the difference between ) (=(S0-S u It refers to )×100 / S0).

[0189] The austenitic stainless steel according to the present invention can have an area reduction rate of 30% or more measured at 25°C by optimizing hot working conditions and / or solution treatment conditions. If the components are further optimized, the area reduction rate can be 40% or more, or 50% or more.

[0190] Here, in the present invention, "solution treatment" refers to a process of heating a steel material at 800°C to 1200°C for at least 1 minute and cooling the steel material at a cooling rate of water cooling, oil cooling, or air cooling, or an equivalent cooling rate.

[0191] [1.4.5. Hydrogen Embrittlement Resistance]

[0192] The characteristics of hydrogen embrittlement resistance can be evaluated based on the magnitude of the relative area reduction ratio.

[0193] Here, "relative area reduction ratio" refers to the value represented by the following equation (1). The larger the value of the relative area reduction ratio represented by equation (1), the better the hydrogen embrittlement resistance.

[0194] Relative area reduction ratio = A / B (1)

[0195] Here,

[0196] A is the area reduction rate of a round bar tensile test specimen when a low strain test is performed under conditions of a test temperature of normal temperature and a hydrogen gas test atmosphere at 87.5 MPa, and

[0197] B is the area reduction rate of a round bar tensile test specimen when a low strain test is performed under conditions of a test temperature of room temperature and a helium gas test atmosphere at 87.5 MPa.

[0198] For each measurement of A and B, a round bar tensile test specimen with a parallel section diameter of 4 mm was used, and the strain was 7×10 -5 It should be noted that it was / s

[0199] Furthermore, the relative area reduction ratio, which is an indicator of hydrogen embrittlement resistance, is known to be worse in low-temperature low-strain tests than in room-temperature low-strain tests. The austenitic stainless steel according to the present invention exhibits an excellent relative area reduction ratio even in low-temperature low-strain tests.

[0200] The relative area reduction ratio in the low-temperature low-strain test refers to the value represented by the following equation (2). The larger the value of the relative area reduction ratio represented by equation (2), the better the hydrogen embrittlement resistance.

[0201] Relative area reduction ratio = C / D (2)

[0202] Here,

[0203] C is the area reduction rate of a round bar tensile test specimen when a low strain test is performed under conditions of a test temperature of -60℃ and a hydrogen gas test atmosphere at 87.5 MPa, and

[0204] D is the reduction in fracture area of ​​a round bar tensile test specimen when a low strain test is performed under conditions of a test temperature of -60℃ and a helium gas test atmosphere at 87.5 MPa.

[0205] It should be noted that for each measurement of C and D of the material before welding, round bar tensile test specimens with a parallel section diameter of 4 mm were used. Meanwhile, for each measurement of C and D of the material after welding, plate-shaped tensile test specimens (parallel section width: 5 mm, parallel section thickness: 1.5 mm) were used. The strain was 7 × 10⁻⁶ regardless of the specimen shape and test temperature. -5 / s was.

[0206] The austenitic stainless steel according to the present invention has excellent hydrogen embrittlement resistance as its composition is optimized. In the austenitic stainless steel according to the present invention, when the composition and structure are optimized, the relative area reduction ratio may be 0.8 or higher. When the composition and / or structure are further optimized, the relative area reduction ratio may be 0.9 or higher.

[0207] [1.5. Purpose]

[0208] Since the austenitic stainless steel according to the present invention has excellent hydrogen embrittlement resistance, the austenitic stainless steel,

[0209] (a) as an austenitic stainless steel for high-pressure hydrogen gas, or

[0210] (b) It can be used as an austenitic stainless steel for liquid hydrogen environments.

[0211] In particular, since the austenitic stainless steel according to the present invention has excellent cryogenic toughness in addition to hydrogen embrittlement resistance, the austenitic stainless steel can be used as a material for, for example, the following components:

[0212] (a) A component for a liquid hydrogen pump pressurized hydrogen station; and

[0213] (b) Components used in a liquid hydrogen environment, such as liquid hydrogen valves and pump components.

[0214] [2. Method for Manufacturing Austenitic Stainless Steel]

[0215] The austenitic stainless steel according to the present invention is,

[0216] (a) Prepare an ingot by melting and casting raw materials mixed to obtain a predetermined composition, and

[0217] (b) perform primary hot working on the obtained ingot, and

[0218] (c) Perform a second hot working on the material obtained from the first hot working, and

[0219] (c) If necessary, perform cold working on the material after secondary hot working, and

[0220] (d) If necessary, perform solution treatment on the material after secondary hot working or cold working, and

[0221] (e) If necessary, this can be achieved by performing post-treatment on the material after secondary hot working, after cold working, or after solution treatment.

[0222] [2.1. Molten and Casting Processes]

[0223] First, raw materials mixed to have a predetermined composition are melted and cast. The methods and conditions of melting and casting are not particularly limited, and the optimal method and conditions can be selected according to the purpose. For example, electric furnaces, argon oxygen decarburization (AOD) furnaces, vacuum oxygen decarburization (VOD) furnaces, etc., can be used to produce molten steel.

[0224] It should be noted that if necessary, the obtained ingot may undergo homogenization heat treatment to remove segregation.

[0225] [2.2. 1st Hot Working Process]

[0226] Next, primary hot working is performed on the obtained ingot. Primary hot working is performed to break down the coarse cast structure and refine the structure, and simultaneously convert the ingot into steel materials such as slabs, blooms, and billets. The primary hot working method is not particularly limited, and the optimal method can be selected depending on the purpose. Examples of primary hot working methods include hot forging and hot rolling.

[0227] It should be noted that steel materials such as slabs, blooms, and billets can be produced directly from molten steel by a continuous casting method. In this case, the primary hot working process can be omitted.

[0228] [2.3. Second Hot Working Process]

[0229] Next, a second hot working process is performed on the material obtained from the first hot working process. The second hot working is performed to finish the material obtained from the first hot working process into a final product shape (e.g., steel plate, steel bar, wire rod, and steel pipe) or a shape close thereto. The method of the second hot working is not particularly limited, and the optimal method can be selected depending on the purpose. Examples of the second hot working method include hot rolling, hot extrusion, and hot piercing rolling.

[0230] The conditions for the secondary hot working are not particularly limited, and optimal conditions can be selected depending on the purpose. Additionally, the secondary hot working may be performed multiple times depending on the purpose. If the heating temperature of the steel prior to the secondary hot working is too low, the grain size may become excessively fine and machinability may be reduced. Therefore, the heating temperature is preferably 900°C or higher.

[0231] Meanwhile, if the heating temperature is too high, there is a risk of localized melting. Therefore, the heating temperature is preferably 1300℃ or lower.

[0232] When performing secondary hot working multiple times, the number density of grains and coarse alloy carbonitrides can be optimized by optimizing the temperature of the steel at the completion of the final secondary hot working. If the temperature of the steel is too low, the grains may become excessively fine and machinability may decrease. Therefore, the temperature of the steel is preferably 800°C or higher.

[0233] Meanwhile, if the temperature of the steel is too high, there is a risk of localized melting. Therefore, the temperature of the steel is preferably 1200℃ or lower.

[0234] [2.4. Cold Working Process]

[0235] Next, if necessary, cold working may be performed on the material after the second hot working. The cold working method is not particularly limited, and the optimal method can be selected depending on the purpose. For example, when cold working the material to form a steel pipe, it is preferable to use the cold drawing method. Alternatively, when processing the material to form a steel plate, it is preferable to use the cold rolling method.

[0236] [2.5. Solution Treatment Process]

[0237] Next, if necessary, a solution treatment can be performed on the material that has undergone secondary hot working or cold working. The solution treatment may be performed only once or multiple times.

[0238] The solution treatment temperature affects the properties of the material. If solution treatment is not performed or the solution treatment temperature is too low, the number density of coarse alloy carbonitrides may increase excessively and the area reduction rate may decrease. In addition, the grain size may become excessively refined and machinability may decrease. Therefore, the solution treatment temperature is preferably 800°C or higher. The solution treatment temperature is more preferably 1,000°C or higher.

[0239] Meanwhile, if the solution treatment temperature is too high, there is a risk of localized melting. Therefore, the solution treatment temperature is preferably 1,200℃ or lower.

[0240] The optimal holding time at the solution treatment temperature can be selected depending on the purpose. Generally, the longer the holding time at the solution treatment temperature, the lower the water density of the coarse alloy carbonitride.

[0241] Meanwhile, if the holding time is extended beyond what is necessary, the grains become excessively coarsened. The optimal holding time depends on the solution treatment temperature, but is generally 1 minute to 3 hours. After the holding time is finished, the material is cooled by water cooling, oil cooling, or air cooling, or at an equivalent cooling rate.

[0242] [2.6. Post-processing]

[0243] If necessary, additional post-treatment may be performed on materials that have undergone secondary hot working, cold working, or solution treatment. Examples of post-treatment include cutting, welding, and cold working. The resulting components are used for various purposes.

[0244] [3. Hydrogen-resistant component]

[0245] The hydrogen-resistant member according to the present invention contains austenitic stainless steel according to the present invention.

[0246] [3.1. Materials]

[0247] The austenitic stainless steel according to the present invention has excellent hydrogen embrittlement resistance because it has a predetermined composition. Other details regarding the composition of the austenitic stainless steel are as described above, so a description is omitted.

[0248] The austenitic stainless steel constituting the hydrogen-resistant member may be in any of the following states: a hot-worked state, a cold-worked state, a solution-treated state, or a state after necessary post-treatment following solution treatment. In order to reduce the water density of coarse alloy carbonitrides and lower manufacturing costs, the austenitic stainless steel constituting the hydrogen-resistant member preferably includes a portion in a solution-treated state.

[0249] Here, "including the part that is in a state of being processed for employment" means,

[0250] (a) where the entire austenitic stainless steel constituting the hydrogen-resistant member is in a solution-treated state, or

[0251] (b) Refers to a case where a portion of the austenitic stainless steel constituting the hydrogen-resistant member undergoes necessary post-treatment (e.g., machining and welding), while another portion remains in a solution-treated state.

[0252] [3.2. Shape]

[0253] The shape of the hydrogen-resistant member is not particularly limited, and the optimal shape can be selected according to the purpose. Examples of shapes of the hydrogen-resistant member include tubes, rods, wires, and plates.

[0254] Additionally, the hydrogen-resistant member may be a member comprising a weld obtained by welding members each having a predetermined shape. The type of seam (i.e., weld joint) of the weld is not particularly limited, and an optimal weld joint can be selected according to the purpose. Examples of weld joints include butt joints, T-joints, corner joints, lap joints, and edge joints.

[0255] If the hydrogen-resistant member includes a weld, the welding method is not particularly limited, and the optimal method can be selected according to the purpose. The welding method may be a welding method using filler material or a welding method without filler material. Examples of filler materials include YS316L, YS309LMo, YS308L, YS308H, YS308N2, and YS308LN.

[0256] Examples of welding methods using filler material include TIG welding, plasma welding, laser welding, MIG welding, MAG welding, and shielded metal arc welding.

[0257] Examples of welding methods that do not use filler material include TIG welding, plasma welding, and laser welding.

[0258] [3.3. Tensile Strength of Welded Joints]

[0259] When a hydrogen-resistant member includes a weldment, it is desirable that the member undergoes solution treatment before welding and has a tensile strength of 690 MPa or more measured at 25°C. The tensile strength measured at 25°C is more preferably 750 MPa or more, and even more preferably 800 MPa or more. When welding is performed using a high-strength member, a high-strength hydrogen-resistant member can be obtained.

[0260] In addition, by optimizing the composition and structure of austenitic stainless steel and optimizing the welding method and welding conditions, a hydrogen-resistant member with high strength even in the welded state can be obtained.

[0261] For example, when butt welding is performed using a TIG welding method with or without filler material at a heat input of 0.20 kJ / mm to 0.60 kJ / mm, a hydrogen-resistant member including a butt weld can be obtained. In this case, when the composition and structure of the austenitic stainless steel are optimized, a hydrogen-resistant member including a butt weld can be obtained, and the tensile strength of the butt weld in the welded state measured at 25°C is 690 MPa or higher. When the composition and structure of the austenitic stainless steel are further optimized, the tensile strength of the weld in the welded state measured at 25°C may be 750 MPa or higher, or 800 MPa or higher.

[0262] Here, "tensile strength of butt weld" refers to the tensile strength when a tensile test is performed using a No. 1A test specimen having a parallel section width of 12 mm and a plate thickness of 1.5 mm according to JIS Z3121 (2013).

[0263] [4. Function]

[0264] N is an austenite-stabilizing and solid solution-strengthening element, but it is also an element that lowers stacking fault energy. On the other hand, for example, Si is an element that reduces grain boundary strength, while B is an element that improves grain boundary strength. Therefore, if a relatively large amount of N is added to the steel and the composition is simultaneously optimized (specifically, by limiting the content of elements that reduce grain boundary strength and adding an appropriate amount of elements that improve grain boundary strength), an austenitic stainless steel with excellent hydrogen embrittlement resistance and high strength can be obtained. In addition, the content of Ni in the steel can be reduced, thereby lowering raw material costs.

[0265] Furthermore, the austenitic stainless steel according to the present invention exhibits high strength even in a solution-treated or welded state, for example. In addition, because the Mn content is relatively low and the number density of coarse alloy carbides is low, the austenitic stainless steel has excellent machinability. Moreover, if manufacturing conditions are optimized, the grain size is appropriately coarsened, and machinability is further improved.

[0266] [Example]

[0267] (Examples 1 to 10, Comparative Examples 1 to 9)

[0268] [1. Preparation of Samples]

[0269] In a vacuum induction furnace, 50 kg of steel having the composition shown in Table 1 was melted and cast into an ingot. Then, hot forging, hot rolling, solution treatment, and machining were performed on the ingot to produce a steel bar with a diameter of 30 mm. In Table 1, the steels of Examples 5 to 7 have the same composition as the steel of Example 4, and the steel of Comparative Example 8 has the same composition as the steel of Comparative Example 7. It should be noted that solution treatment was not performed in Example 7 and Comparative Example 7. Additionally, in Comparative Example 8, the solution treatment temperature was 700°C. Except for Example 7 and Comparative Examples 7 and 8, the solution treatment temperature was 900°C to 1,100°C.

[0270] In addition, two steel plates were prepared individually and butt-welded using the TIG welding method without using filler material at a heat input of 0.20 kJ / mm to 0.60 kJ / mm.

[0271] [Table 1]

[0272]

[0273] [Table 1 (continued)]

[0274]

[0275] [2. Test Method]

[0276] [2.1. Measurement of Crystal Particle Size]

[0277] Each steel bar was cut parallel to the rolling direction. A sample for crystal grain size measurement was taken using the cross-section near the center axis of the steel bar as the observation surface. Well-known electrolytic polishing was performed on the observation surface of each sample. After electrolytic polishing, the crystal grain size number was determined for the observation surface based on the method mentioned above.

[0278] [2.2. Measurement of Number Density of Coarse Alloy Carbonitrides]

[0279] The water density of the coarse alloy carbonitride was measured using the method mentioned above.

[0280] [2.3. Evaluation of Area Reduction Rate and Tensile Strength]

[0281] A round bar tensile test specimen was taken from the center of each steel bar. The parallel section of the round bar tensile test specimen was parallel to the rolling direction of the steel bar. The diameter of the parallel section was 6 mm. The tensile strength TS (MPa) was obtained by performing a tensile test on the round bar tensile test specimen at room temperature (25℃) in the atmosphere.

[0282] In addition, tensile tests were performed on each butt welded member. Specifically, a plate-shaped tensile test specimen having a weld at the center of the parallel section was prepared from the butt welded member. The width of the parallel section was 12 mm, and the thickness of the parallel section was 1.5 mm. The tensile strength TS (MPa) of the butt weld was obtained by performing a tensile test on the plate-shaped tensile test specimen at room temperature.

[0283] In a tensile test using a round bar tensile test specimen or a plate tensile test specimen, if the measured tensile strength TS (MPa) is 690 MPa or higher, which is the required strength of the base material, it was determined as "A (high strength)", and if the measured tensile strength TS (MPa) is less than 690 MPa, it was determined as "B".

[0284] In addition, the area reduction rate was calculated based on the fracture surface area of ​​the round bar tensile test specimen after the tensile test. If the area reduction rate was 30% or more, it was determined as "A (high area reduction rate)," and if the area reduction rate was less than 30%, it was determined as "B."

[0285] [2.4. Evaluation of Hydrogen Embrittlement Resistance]

[0286] [2.4.1. No Welding]

[0287] Low-strain tests were performed to evaluate hydrogen compatibility. The test temperature was room temperature or -60°C, and the test atmosphere was helium or hydrogen gas at 87.5 MPa. Round bar tensile specimens with a parallel section diameter of 4 mm were used as test specimens. The strain was 7 × 10⁻⁶ -5 / s was

[0288] After the low strain test, the area reduction rate in hydrogen gas and the area reduction rate in helium gas were individually calculated based on the fracture surface area of ​​the round bar tensile test specimen. In addition, based on these fracture area reduction rates, the relative area reduction ratio at room temperature (=A / B) and the relative area reduction ratio at -60℃ (=C / D) were calculated. In either case, a relative area reduction ratio of 0.9 or higher was judged as "S (particularly excellent hydrogen embrittlement resistance)," a relative area reduction ratio of 0.8 or higher but less than 0.9 was judged as "A (excellent hydrogen embrittlement resistance)," and a relative area reduction ratio of less than 0.8 was judged as "B."

[0289] It should be noted that a high-pressure hydrogen gas environment at -60℃ is the environment in which the area reduction rate is most significantly lower in austenitic stainless steel.

[0290] [2.4.2. Welding]

[0291] The low strain test at -60°C and the measurement of the relative area reduction ratio (=C / D) at -60°C were performed in the same manner as in "2.4.1 No Welding", except that a plate-type tensile test specimen having a weld at the center of the parallel section (width of the parallel section: 5 mm and thickness of the parallel section: 1.5 mm) was used as the test specimen.

[0292] [2.5. Relative Wear Evaluation]

[0293] A rod-shaped specimen was taken from the center of each steel bar. The parallel portion of the rod-shaped specimen was parallel to the rolling direction of the steel bar. The rod-shaped specimen has a circular cross-section and a diameter of 8 mm.

[0294] Peeling processing was performed on the rod-shaped test specimen for 5 minutes. A non-coated cemented carbide tool corresponding to JIS standard P20 was used as the tool for the peeling process.

[0295] The cutting speed was 100 m / min, the feed rate was 0.2 mm / rev, and the cut was 1.0 mm. No lubricant was used during the stripping process. The stripping process was performed under the above conditions, and the flank wear W1 (mm) of the cemented carbide tool was measured after the test.

[0296] In addition, a rod-shaped test specimen (hereinafter referred to as the "reference test specimen") having a chemical composition corresponding to SUS316 of the JIS standard was prepared. The shape of the reference test specimen was identical to that of the rod-shaped test specimen. Using the reference test specimen, a stripping treatment was performed under the same conditions as above, and the flank wear amount W0 (mm) of the cemented carbide tool was measured after the test.

[0297] Based on the measurement results, the relative wear ratio defined by the following equation (3) was calculated. When the relative wear ratio was 0.40 or higher, it was judged as "A (excellent machinability)", and when the relative wear ratio was less than 0.40, it was judged as "B".

[0298] Relative wear ratio = W0 / W1(3)

[0299] [3. Results]

[0300] The results are shown in Table 2. The figure shows the relationship between the Co content and the relative area reduction ratio (RRA) at -60°C. From Table 2 and the figure, the following can be confirmed.

[0301] (1) In Comparative Examples 1 and 2, the tensile strength was low. This is thought to be because the N content was low.

[0302] (2) In Comparative Example 3, the hydrogen embrittlement resistance was low. This is thought to be because the Si content was excessive.

[0303] (3) In Comparative Example 4, the hydrogen embrittlement resistance was low. This is thought to be because the B content was low.

[0304] (4) In Comparative Example 5, the number density of the coarse alloy carbonitride was high and the machinability was poor. This is thought to be because the excessive content of Nb increased the number density of the coarse alloy carbonitride and excessively promoted grain refinement, and also because the wear of the tool was accelerated by the coarse alloy carbonitride.

[0305] (5) In Comparative Example 6, the number density of the coarse alloy carbonitride was high and the machinability was poor. This is thought to be because the excessive content of Ti increased the number density of the coarse alloy carbonitride and excessively promoted grain refinement, and also because the wear of the tool was accelerated by the coarse alloy carbonitride.

[0306] (6) In Comparative Example 7, the number density of the coarse alloy carbonitride was high and the crystal grain size number was large. Therefore, it is thought that the machinability was poor. This is thought to be because the temperature at the time of completion of the final secondary hot working was low.

[0307] (8) In Comparative Example 8, the number density of the coarse alloy carbonitride was high and the crystal grain size number was large. Therefore, it is thought that the machinability was poor. This is thought to be because the solution treatment temperature was low.

[0308] (9) In each of Examples 1 to 10, the crystal grain size number was less than 8 and the hydrogen embrittlement resistance was excellent. In addition, the tensile strength was 690 MPa or higher and the area reduction rate was 30% or higher. In addition, the relative wear ratio was 0.40 or higher in all of Examples 1 to 10.

[0309] (10) Compared to Examples 5 to 7, Examples 1 to 4 had a lower water density of coarse alloy carbonitrides. This is thought to be because the hot working conditions and / or solution treatment conditions were more optimized.

[0310] (11) In Example 8, the tensile strength was improved compared to Examples 1 to 7 because W was included.

[0311] (12) In Example 9, the area reduction rate was improved compared to Examples 1 to 7 because Zr was included. This is thought to be because the inclusions such as MnS were minimal.

[0312] (13) In Example 10, the tensile strength was improved compared to Examples 1 to 7 because Ta was included.

[0313] (14) In the case of no welding, the relative area reduction ratio (RRA) at -60°C exceeded 0.9 even when the Co content was less than 0.001 mass%. On the other hand, in the case of welding, the relative area reduction ratio (RRA) at -60°C was less than 0.8 when the Co content was less than 0.001 mass% (see Comparative Example 9).

[0314] (15) In the case of no welding, the relative area reduction ratio (RRA) at -60°C gradually increased as the Co content increased. On the other hand, in the case of welding, the relative area reduction ratio (RRA) at -60°C rapidly increased as the Co content increased. In particular, when the Co content was 0.15 mass% or more, the relative area reduction ratio (RRA) at -60°C in the case of welding was quite equivalent to that in the case of no welding (see drawing).

[0315] [Table 2]

[0316]

[0317] [Table 2 (continued)]

[0318]

[0319] Although embodiments of the present invention have been described in detail above, the present invention is not limited to the above embodiments, and various modifications are possible within the scope of the present invention without departing from the gist of the present invention. This application is based on Japanese Patent Application No. 2023-165246 filed on September 27, 2023 and Japanese Patent Application No. 2024-118404 filed on July 24, 2024, the contents of which are incorporated by reference into this specification. Industrial applicability

[0320] The austenitic stainless steel according to the present invention can be used as a structural member for a high-pressure hydrogen gas device.

Claims

Claim 1 As an austenitic stainless steel, C ≤ 0.10 mass%, Si ≤ 0.50 mass%, 3.0 mass% ≤ Mn ≤ 8.0 mass%, P ≤ 0.30 mass%, S ≤ 0.30 mass%, 7.0 mass% ≤ Ni ≤ 12.0 mass%, 18.0 mass% ≤ Cr ≤ 28.0 mass%, 1.0 mass% ≤ Mo ≤ 3.0 mass%, 0.03 mass% ≤ V ≤ 0.50 mass%, 0.0003 mass% ≤ B ≤ 0.0300 mass%, 0.0001 mass% ≤ Ca ≤ 0.0300 mass%, 0.35 mass% ≤ N ≤ 0.80 mass%, and 0.001 mass% ≤ Co ≤ 1.00 mass%, and optionally, W ≤ 2.0 mass%, Zr It consists of ≤ 0.20 mass%, and Ta ≤ 0.50 mass%, with the remainder being Fe and unavoidable impurities, wherein among the unavoidable impurities, the content of Cu is 0.5 mass% or less, the content of Al is 0.10 mass% or less, and the content of O is 0.050 mass% or less, the crystal grain size number of the austenite grains is less than 8.0, and 3 × 10⁻⁶ 5 Pieces / mm 2 An austenitic stainless steel having a number density of coarse alloy carbonitrides less than or equal to the following, wherein "coarse alloy carbonitrides" refers to alloy carbonitrides having a circular equivalent diameter greater than 1,000 nm. Claim 2 An austenitic stainless steel according to claim 1, further satisfying at least one selected from the group consisting of 0.3 mass% ≤ W ≤ 2.0 mass%, 0.01 mass% ≤ Zr ≤ 0.20 mass%, and 0.10 mass% < Ta ≤ 0.50 mass%. Claim 3 Austenitic stainless steel according to claim 1 or 2, having a tensile strength of 690 MPa or more measured at 25℃. Claim 4 delete Claim 5 Austenitic stainless steel according to claim 1 or 2, wherein the area reduction rate measured at 25℃ is 30% or more. Claim 6 A hydrogen-resistant member comprising an austenitic stainless steel according to claim 1 or 2. Claim 7 In claim 6, the above-mentioned austenitic stainless steel comprises a portion in a solution-treated state, a hydrogen-resistant member. Claim 8 A hydrogen-resistant member comprising, wherein, in claim 6, a butt weld, the butt weld has a welded tensile strength of 690 MPa or more as measured at 25°C, and wherein, the "tensile strength of the butt weld" refers to the tensile strength when a tensile test is performed using a No. 1A test specimen having a parallel section width of 12 mm and a plate thickness of 1.5 mm in accordance with JIS Z3121 (2013), and the weld is formed at the center of the parallel section of the test specimen.

Images