Austenitic stainless steel and hydrogen-resistant member

Abstract

To provide an austenitic stainless steel having superior resistance to hydrogen embrittlement, high strength, and processability, and a hydrogen-resistant member made using the same.SOLUTION: An austenitic stainless steel comprises: C≤0.10 mass%, Si≤0.09 mass%, 5.1≤Mn≤7.0 mass%, P≤0.100 mass%, S≤0.100 mass%, Cu≤0.50 mass%, 9.5≤Ni≤14.0 mass%, 18.0≤Cr≤26.0 mass%, 0.5≤Mo≤5.0 mass%, 0.03≤V≤0.50 mass%, 0.03≤Nb≤0.50 mass%, 0.30≤N≤0.60 mass%, and 0.0003≤B≤0.0300 mass%, with the balance being Fe and unavoidable impurities. The austenite crystal grains in the austenitic stainless steel have a grain size number of less than 7.9.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to austenitic stainless steel and a hydrogen-resistant member, and more specifically, to austenitic stainless steel excellent in strength and hydrogen embrittlement resistance, and a hydrogen-resistant member using the same.

Background Art

[0002] In recent years, the development of fuel cell vehicles powered by hydrogen and hydrogen stations for supplying hydrogen to fuel cell vehicles has been underway. Various devices used in fuel cell vehicles, hydrogen stations, etc. (hereinafter, these may be collectively referred to as "equipment for high-pressure hydrogen gas"). Since they are used in a high-pressure hydrogen gas environment, excellent hydrogen embrittlement resistance is required for the materials used in these devices. Stainless steel (especially austenitic stainless steel with a high Ni equivalent) is excellent in hydrogen embrittlement resistance

[0003] and is suitable for this type of application. Among austenitic stainless steels, SUS316L is known as a material excellent in hydrogen embrittlement resistance. Currently, SUS316L is recognized as a stainless steel excellent in hydrogen embrittlement resistance in the automotive compressed hydrogen container standard specified by the High-Pressure Gas Safety Act. However, since SUS316L has low strength, when SUS316L is used as a structural member of equipment for high-pressure hydrogen gas, it is necessary to design it with a thick wall. As a result, there is a problem that the equipment cannot avoid being enlarged and made heavier. In order to achieve weight reduction of fuel cell vehicles, compactification of hydrogen stations, and high-pressure operation at hydrogen stations, it is preferable that the strength of the stainless steel used for these

[0004] Therefore, in order to solve this problem, various proposals have been made conventionally. For example, Patent Document 1 discloses (a) a steel and a coating on the surface of the steel, (b) the steel contains, by mass%, C: 0.100% or less, Si: 3.00% or less, Mn: 0.01 ~5.00%, P: 0.100% or less, S: 0.0050% or less, Ni: 7.00%~4 0.00%, Cr: 17.00%~28.00%, V: 0.010%~5.000%, with the balance being Fe and inevitable impurities, (c) the coating is a film in which the peak value of the V concentration in the depth direction from the outermost surface when the total amount of cation elements in the coating is 100% is 5.00 atomic% or more of austenitic stainless steel. It also discloses

[0005] In the same document, (A) By electrolyzing austenitic stainless steel added with V in a neutral solution after pickling, V can be concentrated in the surface coating, and (B) Even if a Cr-deficient layer exists on the surface of austenitic stainless steel, if V is concentrated in the surface coating, a decrease in corrosion resistance can be suppressed. This is described.

[0006] Patent Document 2 discloses (a) by mass%, C: 0.10% or less, Si: 1.0% or less, Mn: 3~8%, P: 0. 05% or less, S: 0.03% or less, Ni: 10~20%, Cr: 15%~30%, N: 0 .20~0.70%, Mo: 0~5.0%, V: 0~0.5%, and Nb: 0~0.5 %, with the balance being Fe and inevitable impurities, (b) the crystal grain size number is 6.0 or more, (c) the tensile strength is 800 Mpa or more, (d) The difference between the maximum and minimum values of the tensile strength is 50 MPa or less, (e) The number of alloy carbonitrides having a circle equivalent diameter exceeding 1000 nm in the steel is 10 pieces / mm 2 or less is disclosed an austenitic stainless steel.

[0007] In the same document, (A) When the crystal grain size number is 6.0 or more and the difference (ΔG S) between the maximum and minimum values of the crystal grain size number is 1.5 or less, the difference between the maximum and minimum values of the tensile strength becomes 50 Mpa or less, (B) When the difference between the initial temperature and the final temperature during hot working is 100 °C or less, ΔGS can be suppressed to 1.5 or less, and (C) When the crystal grain size number is 6.0 or more and the number of alloy carbonitrides exceeding 1000 nm is 1 0 pieces / mm 2 or less, a tensile strength of 800 MPa or more can be obtained is described.

[0008] In Patent Document 3, (a) By mass%, C: 0.005 to 0.07%, Si: 0.1 to 1.2%, Mn: 3.2 to 6.5%, Ni: 9 to 14%, total of at least one of Cu and Co: 0.005% or more and less than 3%, Cr: 19 to 24%, Mo: 1 to 4%, Nb: 0.05 to 0.4%, N: 0 .15 to 0.50%, Al: 0.05% or less, P: 0.03% or less, S: 0.002% or more and less than 0.02%, O: 0.02% or less, V: 0 to 0.5%, Ti: 0 to 0.5%, B: 0 to 0.01% %, Ca: 0 to 0.05%, Mg: 0 to 0.05%, REM: 0 to 0.5%, and the balance consists of Fe and inevitable impurities, (b) The amount of Nb analyzed as electrolytic extraction residue is 0.01 to 0.3% by mass disclosed an austenitic stainless steel.

[0009] In the same document, (A) The strength and ductility of austenitic stainless steel are related to the amount of Nb analyzed as electrolytic extraction residue, and (B) When the amount of Nb analyzed as electrolytic extraction residue exceeds 0.3% by mass, the ductility decreases, and (C) In order to suppress liquation cracking during welding, it is necessary to make the amount of Nb analyzed as electrolytic extraction residue 0 .01% by mass or more. This is described.

[0010] In Patent Document 4, (a) By mass, C: 0.10% or less, Si: 1.0% or less, Mn: 3% or more and less than 7%, Cr: 15 - 30%, Ni: 10% or more and less than 17%, Al: 0.1% or less, N: 0.10 - 0.50%, and at least one of V: 0.01 - 1.0% and Nb: 0.01 - 0.50% is contained, and the balance is composed of Fe and inevitable impurities, (b) P in the impurities is 0.050% or less and S is 0.050% or less, (c) The tensile strength is 800 MPa or more, the crystal grain size is No. 8 or more, (d) Austenitic stainless steel having alloy carbonitrides with a maximum diameter of 50 - 1000 nm and containing 0.4 pieces / μm or more in cross-sectional observation is disclosed. 2 is included and has

[0011] In the same document, (A) When nitrogen is utilized as a solid solution element, high strength of stainless steel is possible, but due to the reduction of stacking fault energy, the durability against hydrogen environment embrittlement decreases, and (B) When V and / or Nb is added to the steel, fine alloy carbides precipitate during solution heat treatment, and the crystal grains are refined by the pinning effect, and When the crystal grains are refined, the resistance of high-nitrogen steel to hydrogen environment embrittlement can be increased. This is described.

[0012] When a material excellent in hydrogen embrittlement resistance is used as a structural member of a high-pressure hydrogen gas equipment, the material is often subjected to machining such as cutting, cold working, and welding. Therefore, for this type of material, it is required not only to be excellent in strength and hydrogen embrittlement resistance but also to be excellent in workability such as machinability, cold workability, weldability, etc.

[0013] The austenitic stainless steels disclosed in Patent Documents 1 to 4 are high-strength, but further higher strength is desired in the future. For increasing the strength of austenitic stainless steel, a large amount of N addition is effective. However, austenitic stainless steel containing a large amount of N may generate nitrogen bubbles during casting or may have a problem of deterioration in hot workability, which may lead to a decrease in mass productivity and quality . Also, in order to add a large amount of N, it is necessary to adjust various composition balances such as Mn and Cr and increase the solid solution amount of N. Furthermore, with the increase in strength, the concern about hydrogen embrittlement may increase.

Prior Art Documents

Patent Documents

[0014]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Summary of the Invention

Problems to be Solved by the Invention

[0015] The problems to be solved by the present invention are to provide an austenitic stainless steel having excellent hydrogen embrittlement resistance and high strength, and further having excellent workability. Another problem to be solved by the present invention is to provide a hydrogen-resistant member using such an austenitic stainless steel.

Means for Solving the Problems

[0016] The gist of the present invention for solving the above problems is as follows.

[0017] [1] An austenitic stainless steel having the following composition. (1) The austenitic stainless steel C ≦ 0.10 mass%, Si ≦ 0.09 mass%, 5.1 ≦ Mn ≦ 7.0 mass%, P ≦ 0.100 mass%, S ≦ 0.100 mass%, Cu ≦ 0.50 mass%, 9.5 ≦ Ni ≦ 14.0 mass%, 18.0 ≦ Cr ≦ 26.0 mass%, 0.5 ≦ Mo ≦ 5.0 mass%, 0.03 ≦ V ≦ 0.50 mass%, 0.03 ≦ Nb ≦ 0.50 mass%, 0.30 ≦ N ≦ 0.60 mass%, 0.0003 ≦ B ≦ 0.0300 mass% and the balance consists of Fe and unavoidable impurities. (2) The austenitic stainless steel has a crystal grain size number of the austenite crystal grains less than 7.9

[0018] [2] The austenitic stainless steel described in [1] having a tensile strength of 690 MPa or more measured at 25°C. .

[0019] [3] The austenitic stainless steel described in [1] or [2] having an elongation at break of 30% or more measured at 25°C. .

[0020] [4] A hydrogen resistance member made of the austenitic stainless steel described in any one of [1] to [3]. .

[0021] [5] The austenitic stainless steel contains a portion in the solutionized state as described in [4]. Hydrogen resistance member.

[0022] [6] Having a butt welded portion, and the tensile strength of the butt welded portion as welded, measured at 25°C, is 690 MPa or more. The hydrogen resistance member described in [4] or [5].

Advantages of the Invention

[0023] N is an austenite stabilizing element and a solid solution strengthening element. The austenitic stainless steel according to the present invention has a high strength because of a large amount of N. Also, since it is not necessary to add a large amount of Ni to stabilize the austenite phase, the raw material cost can be reduced. Since the amount of Mn is increased to increase the amount of N in the austenitic stainless steel according to the present invention, the amount of N dissolved in the matrix phase is large. Also, since the amount of B is increased, the hot workability is improved. Therefore, the austenitic stainless steel according to the present invention is excellent in manufacturability despite containing a large amount of N. Since the amount of Mn is increased to increase the amount of N in the austenitic stainless steel according to the present invention, the amount of N dissolved in the matrix phase is large. Also, since the amount of B is increased, the hot workability is improved. Therefore, the austenitic stainless steel according to the present invention is excellent in manufacturability despite containing a large amount of N. ​

[0024] Austenitic stainless steels with a large amount of N are generally prone to hydrogen embrittlement. In particular, when the amount of Si becomes excessive, embrittlement is promoted at grain boundaries and twin boundaries, which are factors contributing to hydrogen embrittlement, and becomes less likely. The austenitic stainless steel according to the present invention increases the amount of N and at the same time reduces the amount of S and increases the amount of B that increases the grain boundary strength, so embrittlement at grain boundaries and twin boundaries is suppressed. Furthermore, high-N steels generally have poor machinability because of their high strength. In contrast, the austenitic stainless steel according to the present invention has a small crystal grain size number (large crystal grains), so it has high strength and excellent machinability.

Mode for Carrying Out the Invention

[0025] Hereinafter, an embodiment of the present invention will be described in detail. [1. Austenitic Stainless Steel] [1.1. Main Constituent Elements] The austenitic stainless steel according to the present invention contains the following elements, and the balance consists of Fe and inevitable impurities. The types of additive elements, their component ranges, and the reasons for their limitation are as follows.

[0026] (1) C ≦ 0.10 mass%: In the present invention, C is an impurity. When the amount of C becomes excessive, a large amount of carbides precipitate, and the toughness ductility and corrosion resistance decrease. Therefore, the amount of C needs to be 0.10 mass% or less . The amount of C is preferably 0.06 mass% or less, and more preferably 0.03 mass% or less. In the present invention, the smaller the amount of C, the better. However, an extreme reduction in the amount of C increases the manufacturing cost This causes an increase. Considering the manufacturing cost, the amount of C is preferably 0.0005 mass% or more, more preferably 0.001 mass% or more.

[0027] (2) Si ≤ 0.09 mass%: In the present invention, Si is an impurity. Si combines with Ni and Cr to form an intermetallic compound. Si further promotes the growth of intermetallic compounds such as sigma phase. These intermetallic compounds reduce the hot workability of steel. Furthermore, when the amount of Si becomes excessive, the grain boundary strength decreases, and the hydrogen embrittlement resistance characteristics deteriorate. Therefore, the amount of Si should be 0.09 mass% or less. The amount of Si is preferably 0.07 mass% or less, more preferably 0.05 mass% or less. In the present invention, the less the amount of Si, the better. However, an extreme reduction in the amount of Si causes an increase in manufacturing costs. Considering the manufacturing cost, the amount of Si is preferably 0.001 mass% or more, more preferably 0.01 mass% or more.

[0028] (3) 5.1 ≤ Mn ≤ 7.0 mass%: Mn stabilizes austenite and suppresses the formation of martensite with high hydrogen embrittlement susceptibility. In addition, Mn increases the solubility of N in the molten metal and contributes to the improvement of strength. To obtain such effects, the amount of Mn needs to be 5.1 mass% or more. The amount of Mn is preferably 5.2 mass% or more, more preferably 5.3 mass% or more. On the other hand, when the amount of Mn becomes excessive, the stacking fault energy and grain boundary strength decrease, and the hydrogen embrittlement resistance characteristics deteriorate. Furthermore, when the amount of Mn Therefore, the Mn content needs to be 7.0 mass% or less. The Mn content is preferably 6.0 mass% or less, and more preferably 5.8 mass% or less.

[0029] (4) P ≤ 0.100 mass%: In the present invention, P is an impurity. When the amount of P becomes excessive, the hot workability and ductility of the steel decrease. Also, when the amount of P becomes excessive, the concern about solidification cracking during welding increases. Therefore, the amount of P needs to be 0.100 mass% or less. The amount of P is preferably 0.050 mass% or less, and more preferably 0.030 mass% or less. In the present invention, the less the amount of P, the better. However, an extreme reduction in the amount of P causes an increase in the manufacturing cost. Considering the manufacturing cost, the amount of P is preferably 0.0005 mass% or more and more preferably 0.001 mass% or more.

[0030] (5) S ≤ 0.100 mass%: In the present invention, S is an impurity. When the amount of S becomes excessive, the hot workability and ductility of the steel decrease. Also, when the amount of S becomes excessive, the concern about solidification cracking during welding increases. Therefore, the amount of S needs to be 0.100 mass% or less. The amount of S is preferably 0.050 mass% or less, and more preferably 0.030 mass% or less. In the present invention, the less the amount of S, the better. However, an extreme reduction in the amount of S causes an increase in the manufacturing cost. Considering the manufacturing cost, the amount of S is preferably 0.0005 mass% or more and more preferably 0.001 mass% or more.

[0031] (6) Cu ≤ 0.50 mass%: In the present invention, Cu is an impurity. If the amount of Cu is excessive, the occurrence of solidification cracking during welding may occur. Therefore, the Cu content must be 0.50 mass% or less. , preferably 0.40 mass% or less. In the present invention, the smaller the Cu content, the better. However, an extreme reduction in the Cu content increases the manufacturing cost. Considering the manufacturing cost, the Cu content is preferably 0.005 mass% or more. It is preferable that the content of C is 0.010 mass % or more, and more preferable that the content of C is 0.010 mass % or more.

[0032] (7) 9.5≦Ni≦14.0mass%: Ni stabilizes austenite and increases stacking fault energy. To obtain this effect, the Ni content is 9.5ma The Ni content is preferably 10.5 mass% or more. It is more preferable that the content is 11.5 mass%. On the other hand, if the amount of Ni is excessive, the raw material cost increases. The solubility of N in the molten metal decreases, and the strength decreases. The Ni content is preferably 12.5 mass% or less. , and more preferably 11.9 mass% or less.

[0033] (8) 18.0≦Cr≦26.0mass%: Cr enhances the corrosion resistance of steel. It also increases the solubility of N in molten metal, improving strength. To obtain this effect, the Cr content must be 18.0 mass% or more. The Cr content is preferably 20.0 mass% or more, and more preferably 20.5 mass% or more. It is more preferable that the content is at least 5 mass %. On the one hand, when the Cr content becomes excessive, intermetallic compounds and carbonitrides tend to precipitate excessively, and the ductility and corrosion resistance of the steel decrease. Therefore, the Cr content needs to be 26.0 mass% or less . The Cr content is preferably 25.0 mass% or less, and more preferably 23.5 mass% or less.

[0034] (9) 0.5 ≦ Mo ≦ 5.0 mass%: Mo contributes to the improvement of strength by solid-solution strengthening austenite or forming carbonitrides. Also, Mo enhances the corrosion resistance of the steel. To obtain such effects , the Mo content needs to be 0.5 mass% or more. The Mo content is preferably 1.0 m ass% or more, and more preferably 1.5 mass% or more. On the other hand, when the Mo content becomes excessive, intermetallic compounds and carbonitrides tend to precipitate excessively, and the ductility of the steel decreases. Also, when the Mo content becomes excessive, the raw material cost also increases. Therefore, the M o content needs to be 5.0 mass% or less. The Mo content is preferably 4.0 mas s% or less, and more preferably 3.0 mass% or less.

[0035] (10) 0.03 ≦ V ≦ 0.50 mass%: V forms hard alloy carbonitrides and improves the strength of the steel. To obtain such effects , the V content needs to be 0.03 mass% or more. The V content is preferably 0. 05 mass% or more, and more preferably 0.10 mass% or more. On the other hand, when the V content becomes excessive, alloy carbonitrides precipitate excessively, and the ductility of the steel decreases. Therefore , the V content needs to be 0.50 mass% or less. The V content is preferably 0.4 ​It is 0 mass% or less, and more preferably 0.30 mass% or less.

[0036] (11) 0.03 ≦ Nb ≦ 0.50 mass%: Nb forms hard alloy carbonitrides and improves the strength of steel. To obtain such an effect the amount of Nb needs to be 0.03 mass% or more. The amount of Nb is preferably 0.05 mass% or more, and more preferably 0.10 mass% or more. On the other hand, when the amount of Nb becomes excessive, excessive alloy carbonitrides precipitate and the ductility of the steel decreases. Therefore, the amount of Nb needs to be 0.50 mass% or less. The amount of Nb is preferably 0.40 mass% or less, and more preferably 0.30 mass% or less.

[0037] (12) 0.30 ≦ N ≦ 0.60 mass%: N stabilizes austenite and improves hydrogen embrittlement resistance characteristics. Also, N increases the strength of steel by solid solution strengthening and nitride formation. Also, N improves the corrosion resistance of steel. Furthermore, since the chip breakability is improved by solid solution and nitride formation, the machinability is improved. To obtain such an effect, the amount of N needs to be 0.30 mass%. The amount of N is preferably 0.36 mass% or more, and more preferably 0.37 mass% or more.

[0038] On the other hand, when the amount of N becomes excessive, coarse nitrides are generated and the ductility of the steel decreases. Also, when the amount of N becomes excessive, the hot workability of the steel decreases, or blowholes (defects) are likely to be generated during welding. Furthermore, when the amount of N becomes excessive, the stacking fault energy decreases and the hydrogen embrittlement resistance characteristics decrease. Therefore, the amount of N needs to be 0.60 mass% or less. N The amount is preferably 0.55 mass% or less, more preferably 0.40 mass% or less. is more preferable. In high nitrogen steel, the hydrogen embrittlement resistance may deteriorate due to a decrease in grain boundary strength. However, in the present invention, the grain boundary strength is improved by optimizing the amounts of Si, B, etc., so that a decrease in the hydrogen embrittlement resistance can be suppressed.

[0039] (13) 0.0003 ≦ B ≦ 0.0300 mass%: B segregates at grain boundaries to increase grain boundary cohesion and improve the strength of the steel. Also, B suppresses embrittlement of the steel under a hydrogen environment and improves the hydrogen embrittlement resistance. Furthermore, B improves the hot workability of the steel. To obtain such effects, the amount of B needs to be 0.0003 mass% or more. The amount of B is preferably 0.0005 mass% or more, more preferably 0.001 0 mass% or more. On the other hand, when the amount of B becomes excessive, the susceptibility to solidification cracking of the molten metal increases when welding without using a filler metal. Therefore, the amount of B needs to be 0.0300 mass% or less. The amount of B is preferably 0.0100 mass% or less, more preferably 0.0050 mass% or less. is more preferable.

[0040] [1.2. Inseparable impurities] Inseparable impurities mean elements that are mixed in from ores, scraps used as raw materials for steel, or the environment of the manufacturing process, etc. Inseparable impurities specifically include, in addition to C, Si, P, S, and Cu described above, the following. Si, P, S, and Cu, the following are also included.

[0041] Ta ≦ 0.1 mass%, Ti ≦ 0.1 mass%, Al ≦ 0.1 mass%, W ≤ 0.3 mass%, O ≤ 0.05 mass%, Zr ≤ 0.05 mass%, Sn ≤ 0.03 mass%, Zn ≤ 0.03 mass%, Co ≤ 0.3 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%, Ca ≤ 0.03 mass%, REM ≤ 0.03 mass%.

[0042] [1.3. Characteristics] [1.3.1 Crystal Grain Size Number] The "crystal grain size number" refers to the value measured in accordance with JIS G0551 (2005). Specifically, the crystal grain size number is determined by the following method.

[0043] That is, a test piece for microscopic observation is taken from austenitic stainless steel. Using the collected test piece, the microscopic test method for crystal grain size specified in JIS G0551 (2005) is carried out to evaluate the crystal grain size number. More specifically, the surface of the test piece is corroded using a well-known corrosion solution (such as glyceredia, curling reagent, or Marble reagent, etc.) to reveal the crystal grain boundaries on the surface. On the corroded surface, in 10 fields of view, the crystal grain size number of each field of view is obtained. The area of 2 each field of view is approximately 40 mm². By comparing with the crystal grain size standard diagram specified in 7.1.2 of JIS G0551 (2005), the crystal grain size number in each field of view is evaluated. The average of the crystal grain size numbers of This is defined as the grain size number of the austenitic stainless steel.

[0044] The austenitic stainless steel according to the present invention has optimized components, so it is suitable for hot working. By optimizing the processing conditions and / or solution treatment conditions, the austenite grain size can be reduced. The grain size number is less than 7.9. If the grain size number is less than 7.9, the grains are properly aligned. The cutting resistance is reduced. This makes it easier to peel off from the cutting surface, improving chip disposal. The grain size number is preferably 7.5 or less, and 7.0 or less is preferable. It is more preferable that it is below. On the other hand, if the grain size number is too low, the grains become too large and the tensile strength of the steel decreases. Therefore, the grain size number is preferably 2.0 or more, and more preferably 3.0 or more. It is more preferable to do so.

[0045] 1.3.2 Tensile strength "Tensile strength" refers to the strength measured at 140°C with a diameter of 6 mm at the parallel section in accordance with JIS Z2241:2011. This refers to the tensile strength obtained by conducting a tensile test using an A test piece. The austenitic stainless steel according to the present invention is characterized in that it is hot worked and / or solution treated. By optimizing the processing conditions, the tensile strength measured at 25°C is 690 MPa or more. By further optimizing the components, the tensile strength can be increased to 750 MPa or more, or 800 MPa or more.

[0046] [1.3.3. Breaking drawing] "Fracture reduction" refers to a reduction in diameter of 14 mm at the parallel part in accordance with JIS Z2241:2011. The original cross-sectional area (S O ) of the tensile test specimen before the test when a tensile test is performed using Specimen A is defined as the ratio of the difference between the original cross-sectional area (S O ) and the cross-sectional area (S U ) of the tensile test specimen after the test to the original cross-sectional area (=(S O -S U )×100 / S U ).

[0047] The austenitic stainless steel according to the present invention has a fracture reduction of 30% or more measured at 25°C by optimizing the hot working conditions and / or the solution treatment conditions . By further optimizing the composition, the fracture reduction becomes 40% or more, or 50% or more . Here, in the present invention, "solution treatment" means a treatment in which a steel material is heated at 800°C to 1200°C for 1 minute or more and then cooled at a cooling rate of water cooling, oil cooling, air cooling or a cooling rate equivalent thereto .

[0048] [1.3.4. Hydrogen embrittlement resistance characteristics] The quality of the hydrogen embrittlement resistance characteristics can be evaluated by the magnitude of the relative reduction . Here, "relative reduction" means a value represented by the following formula (1). The relative reduction represented by formula (1) indicates that the hydrogen embrittlement resistance characteristics are better as the value increases .

[0049] Relative reduction = A / B …(1) However A is the fracture reduction of the round bar tensile test specimen when a low strain rate test is performed under the conditions of test temperature: room temperature, test atmosphere: hydrogen gas at 87.5 MPa . B is the fracture reduction of the round bar tensile test specimen when a low strain rate test is performed under the conditions of test temperature: room temperature, test atmosphere: helium gas at 87.5 MPa . In addition, for the measurement of A and B, round bar tensile test specimens with a parallel part diameter of 4 mm are used for both, and the strain rate is 7×10 -5 / s was used.

[0050] In addition, the relative reduction, which is an index of hydrogen embrittlement resistance, is inferior in the low strain rate test at low temperature compared to the low strain rate test at room temperature. It is known that the austenitic stainless steel according to the present invention shows excellent relative reduction even in the low strain rate test at low temperature. The relative reduction in the low strain rate test at low temperature refers to the value represented by the following formula (2). The relative reduction represented by formula (2) indicates that the better the hydrogen embrittlement resistance, the larger the value. represents.

[0051] Relative reduction = C / D …(2) C is the fracture reduction of the round bar tensile test piece when the low strain rate test is carried out under the conditions of test temperature: -60°C, test atmosphere: hydrogen gas at 87.5 MPa, D is the fracture reduction of the round bar tensile test piece when the low strain rate test is carried out under the conditions of test temperature: -60°C, test atmosphere: helium gas at 87.5 MPa. For the measurement of C and D, round bar tensile test pieces with a parallel part diameter of 4 mm are used, and the strain rate is 7×10 -5 / s was used.

[0052] Since the components of the austenitic stainless steel according to the present invention are optimized, it has excellent hydrogen embrittlement resistance. In the austenitic stainless steel according to the present invention, when the components and structure are optimized, the relative reduction is 0.8 or more. When the components and / or structure are further optimized, the relative reduction is 0.9 or more.

[0053] [1.4. Applications] Since the austenitic stainless steel according to the present invention has excellent hydrogen embrittlement resistance, (a) As austenitic stainless steel for high-pressure hydrogen gas, or (b) As austenitic stainless steel for liquid hydrogen environment it can be used.

[0054] In particular, the austenitic stainless steel according to the present invention, in addition to hydrogen embrittlement resistance, has excellent toughness at extremely low temperatures. Therefore, for example, it can be used as a material for components in liquid hydrogen environment such as (a) Components for liquid hydrogen pump-boosted hydrogen stations, (b) Valves for liquid hydrogen and components for pumps and the like.

[0055] [2. Manufacturing method of austenitic stainless steel] The austenitic stainless steel according to the present invention is (a) Raw materials compounded to have a predetermined composition are melted and cast, (b) Primary hot working is performed on the obtained ingot, (c) Secondary hot working is performed on the material obtained by the primary hot working, (c) If necessary, cold working is performed on the material after the secondary hot working (d) If necessary, solution treatment is performed on the material that has been secondary hot worked or cold worked (e) If necessary, post-processing is performed on the material that has been secondary hot worked, cold worked, or solution treated (e) If necessary, post-processing is performed on the material that has been secondary hot worked, cold worked, or solution treated and obtained in this way.

[0056] [2.1. Melting and casting process] First, raw materials compounded to have a predetermined composition are melted and cast. The method and conditions of melting and casting are not particularly limited, and the optimal method and conditions can be selected according to the purpose. For the production of molten steel, for example, an electric furnace, AOD (Argon Oxygen D and the like can be used. ​An ecarburization furnace, a VOD (Vacuum Oxygen Decarburization) furnace, etc. can be used. In addition, homogenization heat treatment for removing segregation may be performed on the obtained ingot as needed. Note that, if necessary, homogenization heat treatment for removing segregation may be performed on the obtained ingot. It is also acceptable.

[0057] [2.2. Primary hot working process] Next, primary hot working is performed on the obtained ingot. The primary hot working breaks up the coarse casting structure, refines the structure, and at the same time, is carried out to convert the ingot into steel materials such as slabs, blooms, billets, etc. The primary hot working method is not particularly limited, and the most suitable method can be selected according to the purpose. Examples of the primary hot working method include hot forging and hot rolling. For example, hot forging, hot rolling, etc. are available. In addition, steel materials such as slabs, blooms, billets, etc. may be directly manufactured from the produced molten steel by continuous casting. In this case, the primary hot working process can be omitted.

[0058] [2.3. Secondary hot working process] Next, secondary hot working is performed on the material obtained in the primary hot working process. The secondary hot working is carried out to finish the material obtained in the primary hot working process into the final product shape (for example, steel plates, bars, wire rods, steel pipes, etc.) or a shape close to it. The secondary hot working method is not particularly limited, and the most suitable method can be selected according to the purpose. Examples of the secondary hot working method include hot rolling, hot extrusion, hot piercing rolling, etc. For example, hot rolling, hot extrusion, hot piercing rolling, etc. are available.

[0059] The conditions of the secondary hot working are not particularly limited, and the most suitable conditions can be selected according to the purpose. In addition, the secondary hot working may be performed multiple times according to the purpose. If the heating temperature of the steel material before processing becomes too low, the crystal grains will be overly refined, and the machinability may decrease. Therefore, the heating temperature is preferably 900 °C or higher. On the other hand, if the heating temperature becomes too high, there is a concern of local melting. Therefore, the heating temperature is preferably 1300 °C or lower.

[0060] Also, when the secondary hot working is performed multiple times, optimizing the temperature of the steel material at the completion of the last secondary hot working can optimize the number density of crystal grains and coarse alloy carbonitrides. If the steel material temperature becomes too low, the crystal grains will be overly refined, and the machinability may decrease. Therefore, the steel material temperature is preferably 850 °C or higher. On the other hand, if the steel material temperature becomes too high, there is a concern of local melting. Therefore, the steel material temperature is preferably 1200 °C or lower.

[0061] [2.4. Cold working process] Next, if necessary, cold working may be performed on the material after the secondary hot working. The cold working method is not particularly limited, and an optimal method can be selected according to the purpose. For example, when cold working the material into a steel pipe, it is preferable to use the cold drawing method. Or when processing the material into a steel plate, it is preferable to use the cold rolling method.

[0062] [2.5. Solution treatment process] Next, if necessary, solution treatment may be performed on the material that has been secondary hot worked or cold worked. The solution treatment may be carried out only once, or may be carried out multiple times.

[0063] The solution treatment temperature affects the properties of the material. If the solution treatment is not performed, or ​​, when the solution treatment temperature is too low, the number density of coarse alloy carbonitrides becomes excessively high, and the fracture elongation may decrease. Also, the crystal grains may become excessively fine, and the machinability may decrease. Therefore, the solution treatment temperature is preferably 900 °C or higher. The solution treatment temperature is more preferably 1040 °C or higher. On the other hand, when the solution treatment temperature becomes too high, there is a concern of local melting. Therefore, the solution treatment temperature is preferably 1200 °C or lower. On the other hand, when the solution treatment temperature becomes too high, there is a concern of local melting. Therefore, the solution treatment temperature is preferably 1200 °C or lower.

[0064] The holding time at the solution treatment temperature can be selected as an optimal time according to the purpose. Generally, the longer the holding time of the solution treatment, the smaller the number density of coarse alloy carbonitrides. On the other hand, if the holding time is made longer than necessary, the crystal grains become excessively coarsened. The optimal holding time varies depending on the solution treatment temperature, but is usually 1 minute to 3 hours. After the holding time has ended, the material is cooled by water cooling, oil cooling, air cooling, or a cooling rate equivalent thereto. The optimal holding time varies depending on the solution treatment temperature, but is usually 1 minute to 3 hours. After the holding time has ended, the material is cooled by water cooling, oil cooling, air cooling, or a cooling rate equivalent thereto.

[0065] [2.6. Post-processing step] After performing the solution treatment on the material, secondary hot working, cold working, or further post-processing may be performed as necessary. Examples of post-processing include cutting, welding, cold working, etc. The member thus obtained is used for various applications. After performing the solution treatment on the material, secondary hot working, cold working, or further post-processing may be performed as necessary. Examples of post-processing include cutting, welding, cold working, etc.

[0066] [3. Hydrogen-resistant member] The hydrogen-resistant member according to the present invention is made of the austenitic stainless steel according to the present invention.

[0067] [3.1. Material] Since the austenitic stainless steel according to the present invention has a predetermined composition, it has excellent hydrogen embrittlement resistance characteristics. Regarding other points related to the composition of the austenitic stainless steel, ​​ , as described above, the explanation will be omitted.

[0068] The austenitic stainless steels that make up the hydrogen-resistant components are in the hot-worked state, cold-worked state, In the as-processed state, as-solution-treated state, or after solution-treatment with necessary post-processing The number density of the coarse alloy carbonitrides is reduced, In order to reduce manufacturing costs, the austenitic stainless steel that constitutes the hydrogen-resistant member is used. The loess steel preferably contains a portion that is in the as-solution-treated state.

[0069] Here, "including a portion that is in the state of being solution-treated" means (a) The entire austenitic stainless steel constituting the hydrogen-resistant component is in the state of being solution-treated. or (b) The part of the austenitic stainless steel that constitutes the hydrogen-resistant component is required to undergo post-processing. (e.g. cutting, welding, etc.) but other parts are in the state of being solution-annealed. Being in a good mood, This refers to.

[0070] [3.2. Shape] The shape of the hydrogen-resistant member is not particularly limited, and the most suitable shape can be selected according to the purpose. The hydrogen resistant member may be in the form of a tube, a rod, a wire, a plate, or the like. The hydrogen-resistant member is a member having a welded portion formed by welding members having a predetermined shape together. The type of the welded joint (i.e., the welded joint) is not particularly limited. Instead, it is possible to select the most suitable weld joint according to the purpose. For example, butt joints, T-joints, corner joints, lap joints, and edge joints.

[0071] When the hydrogen-resistant member includes a welded part, 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 a filler metal, or a welding method without using a filler metal. Examples of the filler metal include YS31 6L, YS309LMo, YS308L, YS308H, YS308N2, YS308L N, etc. Examples of the welding method using a filler metal include TIG welding method, plasma welding method, laser welding method, MIG welding method, MAG welding method, covered arc welding method, etc. Examples of the welding method without using a filler metal include TIG welding method, plasma welding method, laser welding method, etc.

[0072] [3.3. Tensile Strength of Welded Part] When the hydrogen-resistant member includes a welded part, the member before welding is solution-treated, and preferably has a tensile strength of 690 MPa or more measured at 25°C. The tensile strength 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.

[0073] In addition, by optimizing the composition and structure of austenitic stainless steel, as well as the welding method and welding conditions, a hydrogen-resistant member having high strength can be obtained even in the as-welded state. For example, when butt welding is performed using the TIG welding method, with or without using a filler metal, and the heat input is 0.20 - 0.60 kJ / mm, a hydrogen-resistant member including a butt weld joint can be obtained. At this time, by optimizing the composition and structure of austenitic stainless steel, a hydrogen-resistant member having a butt weld joint and having a tensile strength of the butt weld joint in the as-welded state measured at 25°C can be obtained. A hydrogen-resistant member with a strength of 690 MPa or more can be obtained. Austenitic stainless steel When the composition and structure of are further optimized, the tensile strength of the welded part is 750 MPa or more, or

[0074] 800 MPa or more. Here, the "tensile strength of the butt-welded part" refers to the tensile strength when a tensile test is performed using a No. 1A test piece with a parallel part width of 12 mm and a plate thickness of 1.5 mm in accordance with JIS Z3121:2013.

[0075] [4. Action] N is an austenite stabilizing element and also a solid-solution strengthening element. The austenitic stainless steel according to the present invention has a high strength because of a large amount of N. In addition, since it is not necessary to add a large amount of Ni to stabilize the austenite phase, the raw material cost can be reduced. In the austenitic stainless steel according to the present invention, since the amount of Mn is increased to increase the amount of N, the amount of N dissolved in the matrix phase is large. In addition, since the amount of B is also increased, the hot workability is improved. Therefore, the austenitic stainless steel according to the present invention is excellent in manufacturability despite containing a large amount of N.

[0076] Austenitic stainless steel with a large amount of N is generally prone to hydrogen embrittlement. Especially when the amount of Si becomes excessive, embrittlement is promoted at grain boundaries and twin boundaries, which are factors of hydrogen embrittlement. In the austenitic stainless steel according to the present invention, while increasing the amount of N, the amount of Si is reduced and the amount of B that increases the grain boundary strength is increased, so embrittlement at grain boundaries and twin boundaries is suppressed. Furthermore, high-N steel generally has poor machinability because of its high strength. In contrast, the present invention The austenitic stainless steel according to has a small crystal grain size number (large crystal grains),

Example

[0077] (Examples 1 to 5, Comparative Examples 1 to 5) [1. Preparation of Samples] 50 kg of steel with the composition shown in Table 1 was melted in a vacuum induction furnace and ingoted. Then, hot forging, hot rolling, solution treatment, and machining were performed on the ingot to produce round steel bars with a diameter of 30 mm. In Table 1, for Comparative Example 4, the solution treatment temperature was 850 °C. Otherwise, the solution treatment temperature was 980 °C to 1150 °C. Also, two steel plates were separately prepared, and butt welding was carried out by the TIG welding method without using a filler metal, with the heat input: 0.2

[0078]

Table 1

[0079] [2. Test Methods] [2.1. Crystal Grain Size Measurement] Each round steel bar was cut parallel to the rolling direction. Among the cut surfaces, a sample for crystal grain size measurement was taken with the surface near the central axis of the round steel bar as the observation surface. For the observation surface of each sample, known electrolytic polishing was carried out. Based on the above-mentioned method, the crystal grain size number was determined for the observation surface after electrolytic polishing.

[0080] [2.2. Evaluation of Drawing at Fracture and Tensile Strength] Round bar tensile test specimens were taken from the center of each round steel bar. The parallel part of the round bar tensile test specimen was parallel to the rolling direction of the round steel bar. The diameter of the parallel part was 6 mm. Using the round bar tensile test specimens 、Tensile tests were carried out at room temperature (25 °C) in the atmosphere to determine the tensile strength TS (MPa).

[0081] Tensile tests were also carried out on each member subjected to butt welding. Specifically, a plate-shaped tensile test piece having a welded portion at the center of the parallel portion was fabricated from the butt-welded member . A tensile test was carried out on the plate-shaped tensile test piece at room temperature to determine the tensile strength T S (MPa) of the butt-welded portion.

[0082] In the tensile test using either the round bar tensile test piece or the plate-shaped tensile test piece, when the measured tensile strength TS (MPa) was 690 MPa or more, which is the required strength of the base material, it was rated as "○ ( high strength))", and when it was less than 690 MPa, it was rated as "×". In addition, the reduction of area at fracture was calculated from the area of the fracture surface after the tensile test of the round bar tensile test piece. When the reduction of area at fracture was 30% or more, it was rated as "○ (large reduction of area at fracture)", and when it was less than 30% , it was rated as "×".

[0083] [2.3. Evaluation of hydrogen embrittlement characteristics] For the evaluation of hydrogen compatibility, a low strain rate test was carried out. The test temperature was room temperature or -60 °C , and the test atmosphere was in helium gas or hydrogen gas at 87.5 MPa. A round bar tensile test piece with a parallel portion diameter of 4 mm was used for the test piece. The strain rate was 7×10 / s. -5 / s. From the area of the fracture surface after the low strain rate test of the round bar tensile test piece, the reduction of area at fracture in hydrogen gas and the reduction of area at fracture in helium gas were calculated respectively. Furthermore, using these, the relative reduction of area at fracture (= A / B) at room temperature and the relative reduction of area at fracture (= C / D) at -60 °C were calculated. In any case , when the relative reduction of area at fracture was 0.9 or more, it was rated as "◎ (very excellent hydrogen embrittlement resistance characteristics)", and the phase in which the relative reduction of area at fracture was less than 0.9 was rated as "○ (excellent hydrogen embrittlement resistance characteristics)", and when it was less than 0.6, it was rated as "× (poor hydrogen embrittlement resistance characteristics)". Those with an equivalent diameter ratio of 0.8 or more and less than 0.9 are rated as "○ (excellent in hydrogen embrittlement resistance)", and those less than 0.80 are rated as "×". Note that the high-pressure hydrogen gas environment at -60°C is the environment in which the reduction of the reduction of area is most remarkable in austenitic stainless steel.

[0084] [2.4. Relative Wear Amount Evaluation] Bar-shaped test pieces were taken from the center of each bar steel. The parallel part of the bar-shaped test piece was parallel to the rolling direction of the bar steel. The cross-section of the bar-shaped test piece was circular, and the diameter was 8 mm.

[0085] Peeling processing was performed on the bar-shaped test piece for 5 minutes. The tool for peeling processing was not coated, and a cemented carbide tool equivalent to JIS standard P20 was used. The cutting speed was 100 m / min, the feed was 0.2 mm / rev, and the depth of cut was 1.0 mm. Lubricating oil was not used during the peeling process. Peeling processing was performed under the above conditions, and the flank wear amount W1 (mm) of the cemented carbide tool after the test was measured.

[0086] Furthermore, a bar-shaped test piece having a chemical composition equivalent to JIS standard SUS316 (hereinafter referred to as this "reference test piece") was prepared. The shape of the reference test piece was the same as that of the bar-shaped test piece. Peeling processing was performed on the reference test piece under the same conditions as above, and the flank wear amount W0 (mm) of the cemented carbide tool after the test was measured. Based on the measurement results, the relative wear amount ratio defined by the following formula (3) was obtained. Those with a relative wear amount ratio of 0.40 or more are rated as "○ (excellent in machinability)", and those less than 0.40 are rated as "×". Relative wear amount ratio = W0 / W1…(3)

[0087] ​​​​​​​​ [3. Results] The results are shown in Table 2. From Table 2, the following can be understood. (1) In Comparative Example 1, the tensile strength is low. This is considered to be because the amount of Mn is small and the amount of dissolved N is small , and the amount of N is small. Also, because the amount of N is small, the machinability is poor. (2) In Comparative Example 2, the hydrogen embrittlement resistance property is low. This is considered to be because the amount of B is small. (3) In Comparative Example 3, the hydrogen embrittlement resistance property is low. This is considered to be because the amount of Si is excessive .

[0088] (4) In Comparative Example 4, the crystal grain size number is large. For this reason, it is considered that the machinability is poor. This is considered to be because the temperature at the completion of the final secondary hot working is low. (5) In Comparative Example 5, the amount of N is small. This is because the amount of Ni is large and the amount of dissolved N is small . Also, because the amount of N is small, the tensile strength is low and the machinability is poor. Note that because the amount of Ni is large , Comparative Example 5 is also disadvantageous in terms of cost.

[0089] (6) In all of Examples 1 to 5, the crystal grain size number was less than 7.9, and the hydrogen embrittlement resistance property was excellent. Also, the tensile strength was 690 MPa or more in all cases, and the reduction of area at fracture was 30% or more . Furthermore, the relative wear amount ratio was 0.40 or more in all cases. (7) Examples 1 to 4 have higher hydrogen embrittlement resistance properties than Example 5. This is considered to be the result of the fact that in Examples 1 to 4 compared with Example 5 , the amount of B is relatively large and the grain boundary strength is more strengthened .

[0090]

Table 2

[0091] As described above, the embodiments of the present invention have been described in detail. However, the present invention is not limited to the above embodiments, and various modifications can be made without departing from the gist of the present invention.

Industrial Applicability

[0092] The austenitic stainless steel according to the present invention can be used as a structural member used for high-pressure hydrogen gas equipment.

Claims

1. An austenitic stainless steel having the following composition. (1) The austenitic stainless steel has C ≤ 0.10 mass%, Si ≤ 0.09 mass%, 5.1 ≤ Mn ≤ 7.0 mass%, P ≤ 0.100 mass%, S ≤ 0.100 mass%, Cu ≤ 0.50 mass%, 9.5 ≤ Ni ≤ 14.0 mass%, 18.0 ≤ Cr ≤ 26.0 mass%, 0.5 ≤ Mo ≤ 5.0 mass%, 0.03 ≤ V ≤ 0.50 mass%, 0.03 ≤ Nb ≤ 0.50 mass%, 0.30 ≤ N ≤ 0.60 mass%, 0.0003 ≤ B ≤ 0.0300 mass% and the balance consists of Fe and inevitable impurities. (2) The austenitic stainless steel has a crystal grain size number of the austenite crystal grains less than 7.

9.

2. The austenitic stainless steel according to claim 1, having a tensile strength of 690 MPa or more measured at 25°C.

3. The austenitic stainless steel according to claim 1, having a fracture elongation of 30% or more measured at 25°C.

4. A hydrogen resistance member made of the austenitic stainless steel according to claim 1.

5. The hydrogen resistance member according to claim 4, wherein the austenitic stainless steel includes a portion in the solutionized state.

6. It has a butt welded portion, and the tensile strength of the butt welded portion as welded, measured at 25°C The hydrogen resistance member according to claim 4, wherein the tensile strength is 690 MPa or more.

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