Austenitic stainless steel and hydrogen-resistant components

Abstract

To provide an austenitic stainless steel having excellent hydrogen embrittlement resistance and high strength, as well as excellent workability, and a hydrogen resistant member comprising the austenitic stainless steel.SOLUTION: An austenitic stainless steel comprises C≤0.10 mass%, Si≤0.50 mass%, 3.0≤Mn≤8.0 mass%, P≤0.30 mass%, S≤0.30 mass%, 7.0≤Ni≤12.0 mass%, 18.0≤Cr≤28.0 mass%, 1.0≤Mo≤3.0 mass%, 0.03≤V≤0.50 mass%, 0.0003≤B≤0.0300 mass%, 0.0001≤Ca≤0.0300 mass%, and 0.35≤N≤0.80 mass%, with the balance being Fe and inevitable impurities. The austenitic stainless steel has a number density of coarse alloy carbonitrides of 3×105 / mm2 or less. A hydrogen resistant member comprises the inventive austenitic stainless steel.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to austenitic stainless steel and hydrogen-resistant components, and more particularly to austenitic stainless steel with excellent strength and hydrogen embrittlement resistance, and hydrogen-resistant components using the same. [Background technology]

[0002] In recent years, development of fuel cell vehicles that use hydrogen as fuel, and hydrogen stations that supply hydrogen to fuel cell vehicles, has been progressing. Various equipment used in fuel cell vehicles and hydrogen stations (hereinafter collectively referred to as "high-pressure hydrogen gas equipment") are used in a high-pressure hydrogen gas environment, so the materials used in this equipment are required to have excellent resistance to hydrogen embrittlement. Stainless steel (especially austenitic stainless steel with a high Ni equivalent) has excellent resistance to hydrogen embrittlement and is suitable for this type of application.

[0003] Among austenitic stainless steels, SUS316L is known as a material with excellent resistance to hydrogen embrittlement. Currently, SUS316L is certified as a stainless steel with excellent resistance to hydrogen embrittlement in the standards for compressed hydrogen containers for automobiles stipulated in the High Pressure Gas Safety Act. However, because SUS316L has low strength, it is necessary to design it to be thick when used as a structural component for high-pressure hydrogen gas equipment. As a result, there is a problem in that the equipment becomes larger and heavier, which is unavoidable. In order to lighten fuel cell vehicles, make hydrogen stations more compact, and enable high-pressure operation at hydrogen stations, it is preferable that the strength of the stainless steel used in these applications is higher.

[0004] Therefore, various proposals have been made to solve this problem. For example, Patent Document 1 contains: (a) Containing a predetermined amount of C, Si, Mn, Cr, Ni, V, N, and Al, with the remainder being Fe and impurities, (b) Satisfying 2.5Cr + 3.4Mn < 300N Stainless steel for high-pressure hydrogen gas is disclosed.

[0005] The document states: (A) Solid solution strengthening with nitrogen is the most effective method for increasing the strength of austenitic stainless steel. Strength improves with increasing nitrogen content, but ductility and toughness decrease. (B) When the composition is adjusted to satisfy 2.5Cr + 3.4Mn < 300N, the tensile strength increases, and the elongation also increases. It is stated.

[0006] Patent Document 2 contains: (a) Containing a predetermined amount of C, Si, Mn, Cr, Ni, Al, N, and at least one of V and Nb, the remainder being Fe and impurities, (b) Tensile strength of 800 MPa or more, (c) The grain size number is 8 or higher, (d) The content of alloy carbonitrides with a maximum diameter of 50-1000 nm is 0.4 particles / μm 2 That's all. Austenitic stainless steel for high-pressure hydrogen gas is disclosed.

[0007] The document states: (A) While using nitrogen as a solid solution element can increase the strength of stainless steel, it lowers the stacking fault energy, thus reducing its resistance to hydrogen environmental embrittlement. (B) When V and / or Nb are added to the steel, fine alloy carbides precipitate during solution heat treatment, and the grain size is refined due to the pinning effect, and (C) Refining the crystal grains can increase the resistance of high-nitrogen steel to hydrogen environment embrittlement. It is stated.

[0008] Patent Document 3 contains: (a) Containing a predetermined amount of C, Si, Mn, P, S, Ni, Cr, Mo, N, Nb, and V, with the remainder being Fe and impurities, (b) It satisfies 15 ≦ 12.6C + 1.05Mn + Ni + 15N, (c) The crystal grain size number is less than 8.0, (d) The tensile strength is 690 MPa or more An austenitic stainless steel is disclosed.

[0009] In the same document, (A) When the crystal grain size number is 8.0 or less, excellent machinability can be obtained. (B) C, N, Mn, and Ni are all austenite stabilizing elements. When the contents of these elements are optimized, the austenite is stabilized, and even if the crystal grains are coarse, the hydrogen embrittlement resistance is increased. And, (C) When adding 1.0% or more of Mo, even if the crystal grain size number is less than 8.0, a high tensile strength can be obtained. is described.

[0010] Furthermore, in Patent Document 4, (a) It contains a predetermined amount of C, Si, Mn, P, S, Ni, Cr, N, Mo, V, and Nb, and the balance consists of Fe and 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 value and the minimum value of the tensile strength is 50 MPa or less, (e) The number of alloy carbonitrides with a circle equivalent diameter exceeding 1000 nm is 10 pieces / mm 2 or more, (f) The difference between the maximum value and the minimum value of the crystal grain size number is 1.5 or less. An austenitic stainless steel material is disclosed.

[0011] In the same document, (A) When the crystal grain size number is 6.0 or more and the difference (ΔGS) between the maximum value and the minimum value of the crystal grain size number is 1.5 or less, the difference between the maximum value and the minimum value 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) Set the crystal grain size number to 6.0 or more, and make the number of alloy carbonitrides exceeding 1000 nm 10 per mm 2 or more, and a tensile strength of 800 MPa or more can be obtained is described.

[0012] When a material with excellent hydrogen embrittlement resistance is used as a structural member of a high-pressure hydrogen gas device, the material is often subjected to processing such as cutting, cold working, and welding. Therefore, this type of material is required not only to have excellent strength and hydrogen embrittlement resistance but also to have excellent workability such as cutting workability, cold workability, and weldability. In addition, in order to reduce the manufacturing cost and maintenance cost of high-pressure hydrogen gas equipment, the materials used for these equipment can be used in the as-solution-treated (solution heat-treated) state or as-welded state, and preferably have a low content of expensive elements such as Ni.

[0013] Regarding these points, 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, since the stainless steel described in the same document has a high Mn content, excellent workability may not be obtained in some cases. 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 cause an increase in manufacturing cost.

[0014] Furthermore, the austenitic stainless steel described in Patent Document 3 is assumed to be used in the as-hot-worked state. Therefore, in the stainless steel described in the same document, the carbonitrides in the steel are excessive and the workability is low. Similarly, the austenitic stainless steel described in Patent Document 4 relatively precipitates a relatively large amount of relatively coarse alloy carbonitrides by performing heat treatment after hot working at a low temperature (930 °C to less than 1000 °C). Therefore, the stainless steel described in Patent Document 4 is also considered to have low workability. [Prior art documents] [Patent Documents]

[0015] [Patent Document 1] International Publication No. 2004 / 083477 [Patent Document 2] International Publication No. 2012 / 132992 [Patent Document 3] International Publication No. 2015 / 159554 [Patent Document 4] International Publication No. 2017 / 175739 [Overview of the project] [Problems that the invention aims to solve]

[0016] The problem that this invention aims to solve is to provide an austenitic stainless steel that has excellent hydrogen embrittlement resistance and high strength, and furthermore, excellent workability. Another problem that the present invention aims to solve is to provide a hydrogen-resistant component using such austenitic stainless steel. [Means for solving the problem]

[0017] The gist of the present invention, which solves the above problems, is as follows.

[0018] [1] Austenitic stainless steel having the following configuration: (1) The austenitic stainless steel is C ≤ 0.10 mass%, Si ≤ 0.50 mass%, 3.0 ≤ Mn ≤ 8.0 mass%, P ≤ 0.30 mass%, S ≤ 0.30 mass%, 7.0 ≤ Ni ≤ 12.0 mass%, 18.0 ≤ Cr ≤ 28.0 mass%, 1.0 ≤ Mo ≤ 3.0 mass%, 0.03 ≤ V ≤ 0.50 mass%, 0.0003 ≤ B ≤ 0.0300 mass%, 0.0001 ≤ Ca ≤ 0.0300 mass%, and, 0.35 ≤ N ≤ 0.80 mass% It contains [a certain component], with the remainder consisting of Fe and unavoidable impurities. (2) The austenitic stainless steel has a number density of 3 × 10¹⁶ coarse alloy carbonitrides. 5 pieces / mm 2 The following applies: However, "coarse alloy carbonitrides" refers to alloy carbonitrides with an equivalent circular diameter exceeding 1000 nm.

[0019] [2] W ≤ 2.0 mass%, and / or, Zr ≤ 0.20 mass% Austenitic stainless steel as described in [1], further including:

[0020] [3] Austenitic stainless steel as described in [1] or [2], wherein the tensile strength measured at 25°C is 690 MPa or higher.

[0021] [4] Austenitic stainless steel as described in any of [1] to [3], wherein the grain size number of the austenite grains is less than 8.0.

[0022] [5] Austenitic stainless steel as described in any of [1] to [4], wherein the reduction of area at fracture measured at 25°C is 30% or greater.

[0023] [6] A hydrogen-resistant component made of austenitic stainless steel as described in any of [1] to [5].

[0024] [7] The hydrogen-resistant member according to [6], wherein the austenitic stainless steel includes portions that are in the as-solution treated state.

[0025] [8] A hydrogen-resistant member according to [6] or [7], having a butt weld, wherein the as-welded tensile strength of the butt weld, measured at 25°C, is 690 MPa or more. [Effects of the Invention]

[0026] Ni is an austenite-stabilizing and solid-solution strengthening element, but it also reduces stacking fault energy. On the other hand, for example, Si is an element that reduces grain boundary strength, and B is an element that increases grain boundary strength. Therefore, by adding a relatively large amount of Ni to the steel while simultaneously optimizing the composition (specifically, limiting the amount of elements that reduce grain boundary strength and adding an appropriate amount of elements that increase grain boundary strength), an austenitic stainless steel with excellent hydrogen embrittlement resistance and high strength can be obtained. In addition, since the amount of Ni contained in the steel can be reduced, raw material costs can be reduced.

[0027] Furthermore, the austenitic stainless steel according to the present invention exhibits high strength even in its as-solution treated or as-welded state. In addition, it has excellent workability due to its relatively low Mn content and low number density of coarse alloy carbides. Moreover, optimizing the manufacturing conditions results in moderately coarse crystal grains, further improving workability. [Modes for carrying out the invention]

[0028] One embodiment of the present invention will be described in detail below. [1. Austenitic stainless steel] [1.1. Main constituent elements] 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, their component ranges, and the reasons for their limitations are as follows.

[0029] (1) C ≤ 0.10 mass %: In this invention, C is an impurity. If the amount of C is excessive, a large amount of carbides will precipitate, reducing toughness and corrosion resistance. Therefore, the amount of C needs to be 0.10 mass% or less. Preferably, the amount of C is less than 0.05 mass%, and more preferably 0.03 mass% or less. In this invention, a lower C content is preferable. However, an extreme reduction in the C content leads to increased manufacturing costs. Considering manufacturing costs, a C content of 0.0005 mass% or more is preferable. A C content of 0.001 mass% or more is even more preferable.

[0030] (2) Si ≤ 0.50 mass %: In this 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 the steel. Furthermore, if the amount of Si is excessive, the grain boundary strength decreases and the hydrogen embrittlement resistance decreases. Therefore, the amount of Si needs to be 0.50 mass% or less. Preferably, the amount of Si is 0.20 mass% or less, and more preferably 0.09 mass% or less. In this invention, a lower Si content is preferable. However, an extreme reduction in Si content leads to increased manufacturing costs. Considering manufacturing costs, a Si content of 0.001 mass% or more is preferable. More preferably, the Si content is 0.01 mass% or more.

[0031] (3) 3.0 ≤ Mn ≤ 8.0 mass%: Mn stabilizes austenite and suppresses the formation of martensite, which is highly susceptible to hydrogen embrittlement. Furthermore, Mn increases the solubility of nitrogen in the molten metal, contributing to improved strength. To achieve these effects, the Mn content must be 3.0 mass% or higher. Preferably, the Mn content is 5.1 mass% or higher, and more preferably, 5.5 mass% or higher. On the other hand, if the amount of Mn is excessive, the stacking fault energy and grain boundary strength decrease, and the hydrogen embrittlement resistance decreases. Furthermore, if the amount of Mn is excessive, the toughness and hot workability of the steel also decrease. Therefore, the amount of Mn needs to be 8.0 mass% or less. Preferably, the amount of Mn is 6.9 mass% or less, and more preferably 6.5 mass% or less.

[0032] (4) P ≤ 0.30 mass %: In this invention, P is an impurity. If the amount of P is excessive, the hot workability and toughness of the steel will decrease. Furthermore, if the amount of P is excessive, the risk of solidification cracking during welding will increase. Therefore, the amount of P needs to be 0.30 mass% or less. Preferably, the amount of P is less than 0.10 mass%, and more preferably 0.03 mass% or less. In this invention, a lower amount of phosphorus (P) is preferable. However, an extreme reduction in the amount of P leads to increased manufacturing costs. Considering manufacturing costs, a P amount of 0.0005 mass% or more is preferable. A P amount of 0.001 mass% or more is even more preferable.

[0033] (5) S ≤ 0.30 mass %: In this invention, sulfur (S) is an impurity. If the amount of sulfur is excessive, the toughness and hot workability of the steel will decrease. Furthermore, if the amount of sulfur is excessive, the risk of cracking during welding will increase. Therefore, the amount of sulfur needs to be 0.30 mass% or less. Preferably, the amount of sulfur is less than 0.10 mass%, and more preferably 0.09 mass% or less. In this invention, a lower sulfur content is preferable. However, an extreme reduction in the sulfur content leads to increased manufacturing costs. Considering manufacturing costs, a sulfur content of 0.0005 mass% or more is preferable. More preferably, the sulfur content is 0.001 mass% or more.

[0034] (6) 7.0 ≤ Ni ≤ 12.0 mass%: Ni improves hydrogen embrittlement resistance by stabilizing austenite and increasing stacking fault energy. To obtain these effects, the Ni content must be 7.0 mass% or more. Preferably, the Ni content is 9.0 mass% or more, and more preferably 9.5 mass% or more. On the other hand, if the amount of Ni is excessive, the raw material cost increases. Also, if the amount of Ni is excessive, the solubility of N in the molten metal decreases, and the strength decreases. Therefore, the amount of Ni needs to be 12.0 mass% or less. Preferably, the amount of Ni is 10.5 mass% or less, and more preferably 9.9 mass% or less.

[0035] (7) 18.0 ≤ Cr ≤ 28.0 mass%: Cr enhances the corrosion resistance of steel. Furthermore, Cr increases the solubility of nitrogen in the molten metal, contributing to improved strength. To achieve these effects, the Cr content must be 18.0 mass% or higher. Preferably, the Cr content is 20.0 mass% or higher, and more preferably, 22.0 mass% or higher. On the other hand, if the amount of Cr is excessive, intermetallic compounds and carbonitrides tend to precipitate excessively, reducing the toughness and corrosion resistance of the steel. Therefore, the amount of Cr needs to be 28.0 mass% or less. Preferably, the amount of Cr is 26.0 mass% or less, and more preferably 25.0 mass% or less.

[0036] (8) 1.0 ≤ Mo ≤ 3.0 mass %: Mo contributes to improved strength by solid solution strengthening of austenite or by forming carbonitrides. Mo also enhances the corrosion resistance of steel. To obtain these effects, the Mo content must be 1.0 mass% or higher. Preferably, the Mo content is 1.5 mass% or higher, and more preferably, 1.8 mass% or higher. On the other hand, if the amount of Mo is excessive, intermetallic compounds and carbonitrides tend to precipitate excessively, reducing the toughness and ductility of the steel. Furthermore, if the amount of Mo is excessive, the raw material cost also increases. Therefore, the amount of Mo needs to be 3.0 mass% or less. Preferably, the amount of Mo is 2.5 mass% or less, and more preferably, 2.2 mass% or less.

[0037] (9) 0.03 ≤ V ≤ 0.50 mass%: V forms hard alloy carbonitrides, improving the strength of the steel. To obtain this effect, the amount of V must be 0.03 mass% or more. Preferably, the amount of V is 0.05 mass% or more, and more preferably 0.08 mass% or more. On the other hand, if the amount of V is excessive, alloy carbonitrides precipitate excessively, reducing the toughness and ductility of the steel. Therefore, the amount of V needs to be 0.50 mass% or less. Preferably, the amount of V is 0.30 mass% or less, and more preferably 0.20 mass% or less.

[0038] (10) 0.0003 ≤ B ≤ 0.0300 mass%: B segregates at grain boundaries, increasing the grain boundary bonding force and improving the strength of the steel. Furthermore, B suppresses the embrittlement of steel in a hydrogen environment, improving its hydrogen embrittlement resistance. Additionally, B improves the hot workability of the steel. To obtain these effects, the amount of B must be 0.0003 mass% or more. Preferably, the amount of B is 0.0005 mass% or more, and more preferably, 0.0010 mass% or more. On the other hand, if the amount of B is excessive, the molten metal becomes more susceptible to solidification cracking when welding without using filler material. Therefore, the amount of B needs to be 0.0300 mass% or less. Preferably, the amount of B is 0.0100 mass% or less, and more preferably, 0.0050 mass% or less.

[0039] (11)0.0001≦Ca≦0.0300mass%: Ca enhances the hot workability of steel. To obtain this effect, the amount of Ca must be 0.0001 mass% or more. Preferably, the amount of Ca is 0.0003 mass% or more, and more preferably 0.0005 mass% or more. On the other hand, if the amount of Ca is excessive, Ca and O will combine, reducing the cleanliness of the steel. As a result, hot workability will decrease, and toughness and ductility will also decrease. Therefore, the amount of Ca needs to be 0.0300 mass% or less. Preferably, the amount of Ca is 0.0150 mass% or less, and more preferably, 0.0100 mass% or less.

[0040] (12) 0.35 ≤ N ≤ 0.80 mass%: Nitrogen (N) stabilizes austenite and improves hydrogen embrittlement resistance. Furthermore, N increases the strength of steel through solid solution strengthening and nitride formation. In addition, N improves the corrosion resistance of steel. To obtain these effects, the N content must be 0.35 mass% or higher. Preferably, the N content is 0.40 mass% or higher, and more preferably 0.46 mass% or higher.

[0041] On the other hand, if the amount of nitrogen is excessive, coarse nitrides are formed, reducing the toughness and ductility of the steel. Also, if the amount of nitrogen is excessive, the hot workability of the steel decreases, or blowholes (defects) are more likely to form during welding. Furthermore, if the amount of nitrogen is excessive, the stacking fault energy decreases, and the resistance to hydrogen embrittlement decreases. Therefore, the amount of nitrogen needs to be 0.80 mass% or less. Preferably, the amount of nitrogen is 0.60 mass% or less, and more preferably 0.53 mass% or less. In high-nitrogen steel, the hydrogen embrittlement resistance may decrease due to a reduction in grain boundary strength. However, in this invention, the grain boundary strength is improved by optimizing the Si content, B content, etc., thereby suppressing the decrease in hydrogen embrittlement resistance.

[0042] [1.2. Sub-constituent elements] The austenitic stainless steel according to the present invention may further contain one or more of the following elements in addition to the main constituent elements described above. The types of additive elements, their component ranges, and the reasons for their limitations are as follows.

[0043] (1) W ≤ 2.0 mass %: Water (W) has the effect of increasing corrosion resistance and increasing strength through the formation of solid solutions or carbonitrides. Therefore, austenitic stainless steel may contain more W. To obtain these effects, the amount of W is preferably 0.3 mass% or more. More preferably, the amount of W is 0.8 mass% or more. On the other hand, if the amount of W is excessive, the raw material cost increases. Therefore, the amount of W is preferably 2.0 mass% or less. More preferably, the amount of W is 1.5 mass% or less.

[0044] (2) Zr ≤ 0.20 mass%: Zr has the effect of forming crystallized carbides. Since crystallized carbides serve as a starting point for the formation of inclusions such as MnS and carbonitrides, the size of the inclusions can be reduced, improving toughness and ductility. For this reason, austenitic stainless steel may further contain Zr in place of or in addition to W. To obtain such effects, the amount of Zr is preferably 0.01 mass% or more. More preferably, the amount of Zr is 0.05 mass% or more. On the other hand, if the amount of Zr is excessive, it may form coarse oxides and reduce toughness and ductility. Therefore, the amount of Zr is preferably 0.20 mass% or less. More preferably, the amount of Zr is 0.15 mass% or less.

[0045] [1.3. Inevitable Impurities] Inevitable impurities refer to elements that are introduced from the ore or scrap used as raw materials for steel, or from the environment during the manufacturing process. Specifically, in addition to the C, Si, P, and S mentioned above, unavoidable impurities include the following:

[0046] (1) Cu ≤ 0.5 mass %: In this invention, Cu is an impurity. If the amount of Cu is excessive, the risk of solidification cracking during welding increases. Therefore, the amount of Cu is preferably 0.5 mass% or less. More preferably, the amount of Cu is 0.4 mass% or less. In this invention, a lower amount of Cu is preferable. However, an extreme reduction in the amount of Cu leads to increased manufacturing costs. Considering manufacturing costs, a Cu amount of 0.005 mass% or more is preferable. A Cu amount of 0.010 mass% or more is even more preferable.

[0047] (2) Al ≤ 0.10 mass %: In this invention, Al is an impurity. Like Si, Al has a deoxidizing effect on steel. However, if the amount of Al is excessive, excess nitrides are formed, reducing the toughness and ductility of the steel. Also, if the amount of Al is excessive, the penetration depth during welding becomes shallower. Therefore, the amount of Al is preferably 0.10 mass% or less. More preferably, the amount of Al is 0.05 mass% or less. In this invention, a lower amount of Al is preferable. However, an extreme reduction in the amount of Al leads to increased manufacturing costs. Considering manufacturing costs, an Al amount of 0.0005 mass% or more is preferable. More preferably, an Al amount of 0.001 mass% or more is preferable.

[0048] (3)O≦0.050mass% or less: In this invention, O is an impurity. O reduces the hot workability of the base material during manufacturing. O also reduces the cleanliness of the steel and reduces its toughness and ductility. Therefore, the amount of O is preferably 0.050 mass% or less. More preferably, the amount of O is 0.030 mass% or less, and even more preferably, 0.010 mass% or less. In this invention, the lower the oxygen content, the better. However, an extreme reduction in the oxygen content leads to increased manufacturing costs. Considering manufacturing costs, an oxygen content of 0.0005 mass% or more is preferable. More preferably, an oxygen content of 0.001 mass% or more is preferable, and even more preferably, 0.002 mass% or more is preferable.

[0049] [1.4. Characteristics] [1.4.1. Number density of coarse alloy carbonitrides] "Coarse alloy carbonitrides" refers to alloy carbonitrides with an equivalent circle diameter exceeding 1000 nm. "Number density of coarse alloy carbonitrides" refers to the number of coarse alloy carbonitrides per unit area (pieces / mm 2 ). Specifically, the number density is determined by the following method.

[0050] That is, a sample including the central part of a cross-section perpendicular to the rolling or forging elongation direction of the austenitic stainless steel material is collected. The observation area of the sample is mirror-polished. Then, in any 10 fields of view (200 μm × 200 μm) within the observation area, alloy carbonitrides are identified from the precipitates and inclusions in each field of view using a scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectrometer (EDS). The alloy carbonitrides in the present invention are defined as those containing C or N, or both, among the precipitates and inclusions. The equivalent circle diameter of the alloy carbonitrides identified in each field of view is determined by image analysis. The equivalent circle diameter means the diameter (nm) when the area of the alloy carbonitrides in the field of view is converted into a circle. The number of alloy carbonitrides with an equivalent circle diameter exceeding 1000 nm (coarse alloy carbonitrides) is measured. The average value of the number of coarse alloy carbonitrides obtained in each of the 10 fields of view is defined as the "number density of coarse alloy carbonitrides (pieces / mm 2 )" in the present invention.

[0051] When excessive coarse alloy carbonitrides precipitate in the matrix phase, the workability (machinability) deteriorates due to abrasive wear of the tool by the coarse alloy carbonitrides. On the other hand, by optimizing the composition of the austenitic stainless steel and performing hot working and / or solution treatment under appropriate conditions, the number density of the coarse alloy carbonitrides can be reduced. In order to obtain excellent workability, the number density of the coarse alloy carbonitrides is preferably 3×10 5 pieces / mm 2 or less. More preferably, the number density is 1×10 4 pieces / mm 2 or less, and even more preferably 1×10 3 pieces / mm 2 or less. On the other hand, even coarse alloy carbonitrides contribute to tensile strength, so if you want to obtain a higher-strength austenitic stainless steel, the number density should be 1 particle / mm². 2 The above is preferable, and more preferably 10 pieces / mm 2 That's all.

[0052] [1.4.2. Tensile Strength] "Tensile strength" refers to the tensile strength obtained by conducting a tensile test using a No. 14A test specimen with a parallel section diameter of 6 mm, in accordance with JIS Z2241:2011. The austenitic stainless steel according to the present invention achieves a tensile strength of 690 MPa or higher at 25°C by optimizing the hot working conditions and / or solution treatment conditions. Further optimization of the composition can result in a tensile strength of 750 MPa or higher, or even 800 MPa or higher.

[0053] [1.4.3. Grain Size Number] The "grain size number" refers to the value measured in accordance with JIS G0551 (2005). Specifically, the grain size number is determined by the following method.

[0054] Specifically, test specimens for microscopic observation are taken from austenitic stainless steel. Using the taken test specimens, the microscopic examination method for grain size specified in JIS G0551 (2005) is performed to evaluate the grain size number. More specifically, the surface of the test specimen is etched using a well-known etching solution (such as glyceresia, curling reagent, or marble reagent) to reveal the grain boundaries on the surface. The grain size number of each of the 10 fields of view on the etched surface is determined. The area of ​​each field of view is approximately 40 mm². 2 The grain size number in each field of view is evaluated by comparison with the standard grain size diagram specified in JIS G0551 (2005) 7.1.2. The average of the grain size numbers in each field of view is defined as the grain size number of the austenitic stainless steel according to the present invention.

[0055] The austenitic stainless steel according to the present invention has an optimized composition, so by optimizing the hot working conditions and / or solution treatment conditions, the grain size number of the austenite crystal grains becomes less than 8.0. When the grain size number is less than 8.0, the crystal grains become appropriately large, and the cutting resistance is reduced. In addition, the chips during cutting are easily separated from the workpiece and cutting tool, improving chip management. In other words, 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. On the other hand, if the grain size number becomes too low, the grains may become excessively large, which can reduce the tensile strength of the steel. Therefore, a grain size number of 2.0 or higher is preferable. A grain size number of 3.0 or higher is even more preferable.

[0056] [1.4.4. Breaking Axis] "Fracture reduction" refers to the difference between 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, when a tensile test is performed using a No. 14A test specimen with a parallel section diameter of 6 mm in accordance with JIS Z2241:2011. u The ratio of the difference between (=(S0-S)) u This refers to () × 100 / S0).

[0057] The austenitic stainless steel according to the present invention achieves a fracture reduction of 30% or more, measured at 25°C, by optimizing the hot working conditions and / or solution treatment conditions. Further optimization of the composition can result in a fracture reduction of 40% or more, or even 50% or more. In this invention, "solution treatment" refers to a process in which steel material is heated at 800°C to 1200°C for 1 minute or more, and then cooled by water cooling, oil cooling, air cooling, or a cooling rate equivalent thereto.

[0058] [1.4.5. Hydrogen Embrittlement Resistance] The quality of hydrogen embrittlement resistance can be evaluated by the relative aperture size. Here, "relative aperture" refers to the value expressed by the following equation (1). The larger the relative aperture expressed by equation (1), the better the hydrogen embrittlement resistance. Relative aperture = A / B …(1) however, A is the reduction of fracture of a round bar tensile test specimen when a low strain rate test was performed under the conditions of test temperature: room temperature, test atmosphere: hydrogen gas at 87.5 MPa, B is the reduction of fracture of a round bar tensile test specimen when a low strain rate test was performed under the conditions of test temperature: room temperature, test atmosphere: helium gas at 87.5 MPa. For measurements A and B, a round bar tensile test specimen with a parallel section diameter of 4 mm was used, and the strain rate was 7 × 10⁻⁶. -5 I used / s.

[0059] Furthermore, it is known that the relative reduction of area, an indicator of hydrogen embrittlement resistance, is inferior in low-strain-rate tests at low temperatures compared to low-strain-rate tests at room temperature. The austenitic stainless steel according to the present invention exhibits excellent relative reduction of area even in low-strain-rate tests at low temperatures. In low-temperature, low-strain rate tests, the relative aperture is the value expressed by the following equation (2). The larger the relative aperture expressed by equation (2), the better the hydrogen embrittlement resistance. Relative aperture = C / D …(2) C is the reduction of fracture of a round bar tensile test specimen when a low strain rate test was performed under the conditions of a test temperature of -60°C and a test atmosphere of hydrogen gas at 87.5 MPa. D is the reduction of fracture of a round bar tensile test specimen when a low strain rate test was performed under the conditions of a test temperature of -60°C and a test atmosphere of helium gas at 87.5 MPa. For measurements C and D, a round bar tensile test specimen with a parallel section diameter of 4 mm was used, and the strain rate was 7 × 10⁻⁶. -5 I used / s.

[0060] The austenitic stainless steel according to the present invention has an optimized composition and therefore exhibits excellent hydrogen embrittlement resistance. In the austenitic stainless steel according to the present invention, when the composition and microstructure are optimized, the relative reduction of area becomes 0.8 or higher. When the composition and / or microstructure are further optimized, the relative reduction of area becomes 0.9 or higher.

[0061] [1.5. Purpose] The austenitic stainless steel according to the present invention has excellent resistance to hydrogen embrittlement, (a) As an austenitic stainless steel for use with high-pressure hydrogen gas, or (b) As an austenitic stainless steel for liquid hydrogen environments It can be used.

[0062] In particular, the austenitic stainless steel according to the present invention has excellent toughness at cryogenic temperatures in addition to resistance to hydrogen embrittlement, for example, (a) Components for liquid hydrogen pump booster type hydrogen station, (b) Valves and pump components for liquid hydrogen It can be used as a material for components used in liquid hydrogen environments.

[0063] [2. Method for manufacturing austenitic stainless steel] The austenitic stainless steel according to the present invention is (a) The raw materials, which have been blended to have a predetermined composition, are melted and cast, (b) The obtained ingot is subjected to primary hot working, (c) The material obtained from the primary hot working is subjected to secondary hot working. (c) If necessary, perform cold working on the material after secondary hot working. (d) If necessary, the material that has undergone secondary hot working or cold working shall be subjected to solution treatment. (e) If necessary, perform secondary hot working, cold working, or post-processing on solution-treated materials. It is obtained by doing so.

[0064] [2.1. Melting and Casting Process] First, raw materials formulated to achieve a predetermined composition are melted and cast. The method and conditions of melting and casting are not particularly limited, and the most suitable method and conditions can be selected according to the purpose. For the production of molten steel, for example, electric furnaces, AOD (Argon Oxygen Decarburization) furnaces, VOD (Vacuum Oxygen Decarburization) furnaces, etc., can be used. Furthermore, the resulting ingot may be subjected to homogenization heat treatment to remove segregation, if necessary.

[0065] [2.2. Primary hot working process] Next, the resulting ingot is subjected to primary hot working. Primary hot working is performed to break down the coarse casting structure and refine the structure, while simultaneously transforming the ingot into steel material such as slabs, blooms, or billets. The primary hot working method is not particularly limited, and the most suitable method can be selected depending on the purpose. Examples of primary hot working methods include hot forging and hot rolling. Alternatively, steel materials such as slabs, blooms, and billets may be directly manufactured from the molten steel produced by continuous casting. In this case, the primary hot working process can be omitted.

[0066] [2.3. Secondary hot working process] Next, the material obtained in the primary hot working process is subjected to secondary hot working. Secondary hot working is performed to finish the material obtained in the primary hot working process into the final product shape (e.g., steel plate, steel bar, wire rod, steel pipe, 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 secondary hot working methods include hot rolling, hot extrusion, and hot drilling rolling.

[0067] The conditions for secondary hot working are not particularly limited, and the optimal conditions can be selected according to the purpose. Furthermore, secondary hot working may be performed multiple times depending on the purpose. The heating temperature of the steel material before secondary hot working is preferably between 900°C and 1300°C. Furthermore, when secondary hot working is performed multiple times, the steel temperature at the completion of the final secondary hot working is preferably between 800°C and 1200°C. This is to optimize the crystal grain size and the number density of coarse alloy carbonitrides.

[0068] [2.4. Cold working process] Next, if necessary, cold working may be performed on the material after secondary hot working. The cold working method is not particularly limited, and the most suitable 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. Alternatively, when processing the material into a steel plate, it is preferable to use the cold rolling method.

[0069] [2.5. Solution Treatment Process] Next, if necessary, the material that has undergone secondary hot working or cold working may be subjected to solution treatment. The solution treatment may be performed only once or multiple times.

[0070] The solution treatment temperature affects the properties of the material. If solution treatment is not performed, or if the solution treatment temperature is too low, the number density of coarse alloy carbonitrides may become excessively high, leading to a decrease in reduction at fracture. In addition, the crystal grains may become excessively fine, resulting in a decrease in machinability. Therefore, a solution treatment temperature of 800°C or higher is preferable. A solution treatment temperature of 1000°C or higher is even more preferable. On the other hand, if the solution treatment temperature becomes too high, there is a concern that localized melting may occur. Therefore, the solution treatment temperature is preferably 1200°C or lower.

[0071] The holding time at the solution treatment temperature can be selected to be optimal depending on the purpose. Generally, the longer the holding time during solution treatment, the lower the number density of coarse alloy carbonitrides. On the other hand, if the holding time is extended unnecessarily, the crystal grains will become excessively coarse. The optimal holding time varies depending on the solution treatment temperature, but is usually between 1 minute and 3 hours. After the holding time is complete, the material is cooled by water cooling, oil cooling, air cooling, or a cooling rate equivalent thereto.

[0072] [2.6. Post-processing process] After secondary hot working, cold working, or solution treatment, the material may be subjected to further post-processing as needed. Examples of post-processing include cutting, welding, and cold working. The resulting components can be used for various applications.

[0073] [3. Hydrogen-resistant materials] The hydrogen-resistant member according to the present invention is made of austenitic stainless steel according to the present invention.

[0074] [3.1. Materials] The austenitic stainless steel according to the present invention has a predetermined composition and therefore exhibits excellent resistance to hydrogen embrittlement. Further details regarding the composition of the austenitic stainless steel are as described above and will not be explained here.

[0075] The austenitic stainless steel constituting the hydrogen-resistant component may be in any of the following states: as hot-worked, as cold-worked, as solution-treated, or after necessary post-processing following solution treatment. In order to reduce the number density of coarse alloy carbonitrides and lower manufacturing costs, it is preferable that the austenitic stainless steel constituting the hydrogen-resistant component includes portions in the as-solution-treated state.

[0076] Here, "including the portion that remains in the solution-treated state" means: (a) The entire austenitic stainless steel constituting the hydrogen-resistant component is in the as-solution treated state, or (b) A portion of the austenitic stainless steel constituting the hydrogen-resistant component has undergone necessary post-processing (e.g., cutting, welding, etc.), but the other portion remains in the solution-treated state. It refers to.

[0077] [3.2. Shape] The shape of hydrogen-resistant components is not particularly limited, and the optimal shape can be selected according to the purpose. Examples of hydrogen-resistant component shapes include pipes, rods, wires, and plates. Furthermore, hydrogen-resistant members may also be members having welded joints formed by welding together members having a predetermined shape. The type of welded joint (i.e., welded joint) is not particularly limited, and the most suitable welded joint can be selected according to the purpose. Examples of welded joints include butt joints, T-joints, corner joints, lap joints, and edge joints.

[0078] When hydrogen-resistant components include welded joints, the welding method is not particularly limited, and the most suitable method can be selected according to the purpose. The welding method may be one that uses filler material, or one that does not. Examples of filler materials include YS316L, YS309LMo, YS308L, YS308H, YS308N2, and YS308LN. Examples of welding methods that use filler materials include TIG welding, plasma welding, laser welding, MIG welding, MAG welding, and shielded metal arc welding. Examples of welding methods that do not use filler material include TIG welding, plasma welding, and laser welding.

[0079] [3.3. Tensile strength of welded joints] When the hydrogen-resistant component includes a welded joint, it is preferable that the component before welding is solution-treated and has a tensile strength of 690 MPa or higher as measured at 25°C. More preferably, the tensile strength is 750 MPa or higher, and even more preferably, 800 MPa or higher. By welding using a high-strength component, a high-strength hydrogen-resistant component can be obtained.

[0080] Furthermore, by optimizing the composition and microstructure of austenitic stainless steel, as well as the welding method and welding conditions, hydrogen-resistant members with 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 filler material, with a heat input of 0.20 to 0.60 kJ / mm, a hydrogen-resistant member including the butt weld can be obtained. In this case, by optimizing the composition and microstructure of the austenitic stainless steel, a hydrogen-resistant member with a butt weld and a tensile strength of 690 MPa or more at the as-welded butt weld, measured at 25°C, can be obtained. Further optimization of the composition and microstructure of the austenitic stainless steel can result in a tensile strength of 750 MPa or more, or even 800 MPa or more, at the weld.

[0081] Here, "tensile strength of butt weld" refers to the tensile strength obtained when a tensile test is performed using a No. 1A test specimen with a parallel section width of 12 mm and a plate thickness of 1.5 mm, in accordance with JIS Z3121:2013.

[0082] [4. Effect] Ni is an austenite-stabilizing and solid-solution strengthening element, but it also reduces stacking fault energy. On the other hand, for example, Si is an element that reduces grain boundary strength, and B is an element that increases grain boundary strength. Therefore, by adding a relatively large amount of Ni to the steel while simultaneously optimizing the composition (specifically, limiting the amount of elements that reduce grain boundary strength and adding an appropriate amount of elements that increase grain boundary strength), an austenitic stainless steel with excellent hydrogen embrittlement resistance and high strength can be obtained. In addition, since the amount of Ni contained in the steel can be reduced, raw material costs can be reduced.

[0083] Furthermore, the austenitic stainless steel according to the present invention exhibits high strength even in its as-solution treated or as-welded state. In addition, it has excellent workability due to its relatively low Mn content and low number density of coarse alloy carbides. Moreover, optimizing the manufacturing conditions results in moderately coarse crystal grains, further improving workability. [Examples]

[0084] (Examples 1-9, Comparative Examples 1-8) [1. Sample Preparation] In a vacuum induction furnace, 50 kg of steel with the composition shown in Table 1 was melted and ingots were formed. Subsequently, the ingots were subjected to hot forging, hot rolling, solution treatment, and machining to produce steel bars 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. Note that solution treatment was not performed on Examples 7 and Comparative Example 7. In addition, the solution treatment temperature for Comparative Example 8 was 700°C. For the other examples, the solution treatment temperature was 900°C to 1100°C. In addition, two steel plates were prepared separately, and butt welding was performed using the TIG welding method without using filler material, with a heat input of 0.20 to 0.60 kJ / mm.

[0085] [Table 1]

[0086] [2. Test Method] [2.1. Grain Size Measurement] Each steel bar was cut parallel to the rolling direction. A sample for grain size measurement was taken from the cut surface, with the surface near the central axis of the steel bar used as the observation surface. The observation surface of each sample was subjected to the well-known electropolishing method. The grain size number was determined for the observation surface after electropolishing based on the method described above.

[0087] [2.2. Measurement of the number density of coarse alloy carbonitrides] The number density of coarse alloy carbonitrides was measured using the method described above.

[0088] [2.3. Evaluation of reduction of area at break and tensile strength] Tensile specimens were taken from the center of each steel bar. The parallel section of the tensile specimen was parallel to the rolling direction of the steel bar. The diameter of the parallel section was 6 mm. Tensile tests were performed on the tensile specimens at room temperature (25°C) in air to determine the tensile strength TS (MPa).

[0089] Furthermore, tensile tests were conducted on each component that underwent butt welding. Specifically, plate-shaped tensile test specimens were prepared from the butt-welded components, with the welded joint in the center of the parallel section. Tensile tests were performed on the plate-shaped tensile test specimens at room temperature, and the tensile strength TS (MPa) of the butt weld was determined.

[0090] In tensile tests using either round bar tensile test specimens or plate-shaped tensile test specimens, a measurement of tensile strength TS (MPa) of 690 MPa or higher, which is the required strength of the base material, was marked as "○ (high strength)," and a measurement of less than 690 MPa was marked as "×." Furthermore, the reduction in area at fracture was calculated from the area of ​​the fracture surface after the tensile test of the round bar tensile test specimen. A reduction in area at fracture of 30% or more was marked as "○ (large reduction in area at fracture)", and a reduction in area less than 30% was marked as "×".

[0091] [2.4. Evaluation of Hydrogen Embrittlement Characteristics] To evaluate hydrogen compatibility, a low strain rate test was conducted. The test temperature was room temperature or -60°C, and the test atmosphere was helium gas or hydrogen gas at 87.5 MPa. A round bar tensile test specimen with a parallel section diameter of 4 mm was used. The strain rate was 7 × 10⁻⁶. -5 I used / s. The reduction in fracture area in hydrogen gas and helium gas was calculated from the fracture surface area after low-strain rate testing of round bar tensile test specimens. Furthermore, the relative reduction in fracture area at room temperature (=A / B) and at -60°C (=C / D) were calculated using these values. In all cases, specimens with a relative reduction in fracture area of ​​0.8 or higher were marked "○ (excellent resistance to hydrogen embrittlement)", and those with a relative reduction in fracture area less than 0.80 were marked "×". Furthermore, a high-pressure hydrogen gas environment at -60°C is the environment in which the reduction of reduction at fracture is most significantly reduced in austenitic stainless steel.

[0092] [2.5. Relative wear evaluation] A rod-shaped test specimen was taken from the center of each steel bar. The parallel section of the rod-shaped test specimen was parallel to the rolling direction of the steel bar. The cross-section of the rod-shaped test specimen was circular, with a diameter of 8 mm.

[0093] A peeling process was performed on a rod-shaped test specimen for 5 minutes. The tool used for peeling was an uncoated carbide tool equivalent to JIS standard P20. The cutting speed was 100 m / min, the feed rate was 0.2 mm / rev, and the depth of cut was 1.0 mm. No lubricant was used during the peeling process. After performing the peeling process under the above conditions, the flank wear amount W1 (mm) of the carbide tool was measured.

[0094] Furthermore, a rod-shaped test specimen (hereinafter referred to as the "reference specimen") having a chemical composition equivalent to JIS standard SUS316 was prepared. The shape of the reference specimen was the same as that of the rod-shaped specimen. Using the reference specimen, peeling was performed under the same conditions as above, and the flank wear amount W0 (mm) of the carbide tool after the test was measured. Based on the measurement results, the relative wear ratio, defined by the following equation (3), was determined. A relative wear ratio of 0.40 or higher was marked as "○ (excellent machinability)," and a ratio less than 0.40 was marked as "×." Relative wear ratio = W0 / W1…(3)

[0095] [3. Results] The results are shown in Table 2. From Table 2, the following can be seen: (1) Comparative Examples 1 and 2 have low tensile strength. This is thought to be due to the low amount of nitrogen. (2) Comparative Example 3 has poor resistance to hydrogen embrittlement. This is thought to be due to an excessive amount of Si. (3) Comparative Example 4 has poor hydrogen embrittlement resistance. This is thought to be due to the low amount of B.

[0096] (4) Comparative Example 5 has a high number density of coarse alloy carbonitrides and poor machinability. This is thought to be because the excess Nb content leads to a high number density of coarse alloy carbonitrides, excessively promoting grain refinement and abrasive wear of the tool due to coarse alloy carbides. (5) Comparative Example 6 has a high number density of coarse alloy carbonitrides and poor machinability. This is thought to be because the excess Ti content leads to a high number density of coarse alloy carbonitrides, excessively promoting grain refinement and abrasive wear of the tool due to the coarse alloy carbonitrides.

[0097] (6) Comparative Example 7 has a high number density of coarse alloy carbonitrides and a large grain size number. Therefore, it is thought to have poor machinability. This is thought to be due to the low temperature at the completion of the final secondary hot working. (8) Comparative Example 8 has a high number density of coarse alloy carbonitrides and a large grain size number. For this reason, it is thought to have poor machinability. This is thought to be due to the low solution treatment temperature. (9) Examples 1 to 9 all had a grain size number of less than 8 and exhibited excellent resistance to hydrogen embrittlement. Furthermore, the tensile strength of all was 690 MPa or higher, and the reduction at fracture was 30% or higher. In addition, the relative wear ratio of all was 0.40 or higher.

[0098] (10) Compared to Examples 5-7, Examples 1-4 showed a lower number density of coarse alloy carbides. This is thought to be due to the optimization of the hot working conditions and / or solution treatment conditions. (11) Because Example 8 contains W, the tensile strength was improved compared to Examples 1 to 8. (12) Because Example 9 contains Zr, the number density of carbonitrides is increased compared to Examples 1-8, which improves the reduction of area at fracture.

[0099] [Table 2]

[0100] Although embodiments of the present invention have been described in detail above, the present invention is not limited in any way to the above embodiments, and various modifications are possible without departing from the spirit of the present invention. [Industrial applicability]

[0101] The austenitic stainless steel according to the present invention can be used as a structural component in equipment for high-pressure hydrogen gas.

Claims

1. Austenitic stainless steel having the following configuration: (1) The austenitic stainless steel is C≦0.10mass%, Si≦0.50mass%, 3.0≦Mn≦8.0mass%, P≦0.30mass%, S≦0.30mass%, 7.0≦Ni≦12.0mass%, 18.0≦Cr≦28.0mass%, 1.0≦Mo≦3.0mass%, 0.03≦V≦0.50mass%, 0.0003≦B≦0.0300mass%, 0.0001 ≤ Ca ≤ 0.0300 mass%, and, 0.35≦N≦0.80mass% It contains [a certain component], with the remainder consisting of Fe and unavoidable impurities. (2) The austenitic stainless steel has a number density of 3 × 10¹⁶ coarse alloy carbonitrides. 5 pieces / mm 2 The following applies: However, "coarse alloy carbonitrides" refers to alloy carbonitrides with an equivalent circular diameter exceeding 1000 nm.

2. W ≤ 2.0 mass%, and / or, Zr≦0.20mass% The austenitic stainless steel according to claim 1, further comprising:

3. The austenitic stainless steel according to claim 1 or 2, wherein the tensile strength measured at 25°C is 690 MPa or more.

4. The austenitic stainless steel according to claim 1 or 2, wherein the grain size number of the austenite grains is less than 8.

0.

5. The austenitic stainless steel according to claim 1 or 2, wherein the reduction of area at fracture measured at 25°C is 30% or more.

6. A hydrogen-resistant member made of austenitic stainless steel according to claim 1 or 2.

7. The hydrogen-resistant member according to claim 6, wherein the austenitic stainless steel includes portions that are in the as-solution treated state.

8. The hydrogen-resistant member according to claim 6, having a butt weld, wherein the as-welded tensile strength of the butt weld, measured at 25°C, is 690 MPa or more.

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