Austenitic stainless steel material, its manufacturing method, and hydrogen equipment

Abstract

This austenitic stainless steel material has a chemical composition comprising, by mass%, 0.20% or less of C, 2.0% or less of Si, 6.0-20.0% of Mn, 0.060% or less of P, 0.0080% or less of S, 10.0-18.0% of Cr, 4.0-12.0% of Ni, 0.01-0.30% of N, 4.0% or less of Cu, 3.0% or less of Mo, optional elements, and a balance being Fe and impurities, a value of A is 30.0-60.0, and, in the metal structure of a surface layer, the high-angle grain boundary ratio Gs satisfies [0.1<Gs<0.6].

Description

Technical Field

[0001] The present disclosure relates to austenitic stainless steel materials, a method for manufacturing the same, and hydrogen-related equipment.

Background Art

[0002] In recent years, hydrogen energy has attracted attention as a clean energy that does not emit greenhouse gases such as carbon dioxide. In order to utilize hydrogen energy, the establishment of hydrogen-related technologies such as hydrogen production, storage, and transportation is required.

[0003] On the other hand, there are various problems in the establishment of hydrogen-related technologies. One of them is the problem of hydrogen embrittlement. Hydrogen energy uses hydrogen gas as a fuel source. Therefore, for example, when a metal material is used in related devices such as hydrogen production devices and storage devices, a so-called hydrogen embrittlement problem occurs in which the material becomes brittle due to hydrogen gas.

[0004] From the viewpoints of manufacturing cost, strength, corrosion resistance, etc., austenitic stainless steel is one of the metal materials used in the above-mentioned related devices. Therefore, in order to suppress hydrogen embrittlement, austenitic stainless steel with improved hydrogen embrittlement resistance has been developed.

[0005] For example, Patent Documents 1 and 2 disclose high-Mn austenitic stainless steels having excellent hydrogen embrittlement resistance characteristics at low temperatures and economic efficiency. The austenitic stainless steels disclosed in Patent Documents 1 and 2 improve economic efficiency and hydrogen embrittlement resistance characteristics by adjusting the chemical composition to a predetermined amount.

[0006] In addition, members or parts of the above-mentioned hydrogen-related devices may be cold-worked and used with increased strength due to work hardening. Among austenitic stainless steels, there are cases where the hydrogen embrittlement resistance decreases after cold working. Therefore, Patent Document 3 discloses a stainless steel wire for hydrogen springs having good hydrogen resistance even when subjected to a certain amount of cold working.

[0007] In the steel wire disclosed in Patent Document 3, by adjusting the Cr content to 18% or more and the N content to 0.3% or more, the hydrogen resistance is enhanced, and good hydrogen resistance is achieved even when cold working is performed. Further, Patent Document 4 discloses an austenitic stainless steel for high-pressure hydrogen that achieves high strength and high hardness in the solution-treated state without cold working. In the austenitic stainless steel disclosed in Patent Document 4, by containing 0.40 to 1.00% of C and having 23% or more of Cr carbides present by area ratio, it has high strength even in the solution-treated state and also improves the hydrogen embrittlement resistance characteristics.

Prior Art Documents

Patent Documents

[0008]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Summary of the Invention

Problems to be Solved by the Invention

[0009] However, in the austenitic stainless steels disclosed in Patent Documents 1 and 2 described above, nothing is mentioned about the hydrogen embrittlement resistance after cold working. Therefore, there is still room for improvement in this property. Further, in the austenitic stainless steels disclosed in Patent Documents 3 and 4, compared with general austenitic stainless steels, the Cr content, N content, and C content are high, and there is still room for improvement from the viewpoints of economy and manufacturability.

[0010] Therefore, in the austenitic stainless steel materials used as members, parts, etc. of hydrogen-related devices, there is a problem that it is difficult to achieve both hydrogen embrittlement resistance and economic efficiency.

[0011] The present disclosure aims to solve the above problems and provide an austenitic stainless steel material that achieves both hydrogen embrittlement resistance and economic efficiency.

Means for Solving the Problems

[0012] The present disclosure has been made to solve the above problems and has the following austenitic stainless steel material as its gist.

[0013] (1) The chemical composition is, in mass%, C: 0.20% or less, Si: 2.0% or less, Mn: 6.0 to 20.0%, P: 0.060% or less, S: 0.0080% or less, Cr: 10.0 to 18.0%, Ni: 4.0 to 12.0%, N: 0.01 to 0.30%, Cu: 4.0% or less, Mo: 3.0% or less, Al: 0 to 0.20%, Ca: 0 to 0.01%, B: 0 to 0.01%, Mg: 0 to 0.01%, Nb: 0 to 1.0%, Ti: 0 to 1.0%, V: 0 to 1.0%, W: 0 to 2.0%, Zr: 0 to 1.0%, Co: 0 to 2.0%, Ga: 0 to 0.10%, Hf: 0 to 0.10%, REM: 0 to 0.10%, The balance is Fe and impurities, and the A value calculated by the following formula (i) is 30.0 to 60.0, An austenitic stainless steel material in which the ratio Gs of large-angle grain boundaries in the surface metal structure satisfies the following formula (ii). A value = 3.2Mn + 0.7Cr + 6.2Ni + 38.7N + 4.8Cu + 9.3Mo - 53 ···(i) 0.1 < Gs < 0.6 ···(ii) However, each element symbol in the above formula (i) represents the content (mass%) of each element contained in the steel. If not contained, it is taken as zero. Each symbol in the above formula is defined as follows. Gs: Ratio of large-angle grain boundaries

[0014] (2) The Vickers hardness is 250 - 500 HV1, In the crystal structure, in terms of area ratio, 97% or more is the fcc structure, The austenitic stainless steel material according to the above (1).

[0015] (3) The chemical composition is, in mass%, Al: 0.01 - 0.20%, Ca: 0.001 - 0.01%, B: 0.0002 - 0.01%, Mg: 0.0002 - 0.01%, Nb: 0.01 - 1.0%, Ti: 0.01 - 1.0%, V: 0.01 - 1.0%, W: 0.01 - 2.0%, Zr: 0.01 - 1.0%, Co: 0.01 - 2.0%, Ga: 0.01 - 0.10%, Hf: 0.01 - 0.10%, and REM: 0.01 - 0.10%, The austenitic stainless steel material according to the above (1), containing one or more selected from the above.

[0016] (4) The chemical composition is, in mass%, Al: 0.01 - 0.20%, Ca: 0.001 - 0.01%, B: 0.0002 - 0.01%, Mg: 0.0002 to 0.01%, Nb: 0.01 to 1.0%, Ti: 0.01 to 1.0%, V: 0.01 to 1.0%, W: 0.01 to 2.0%, Zr: 0.01 to 1.0%, Co: 0.01 to 2.0%, Ga: 0.01 to 0.10%, Hf: 0.01 to 0.10%, and REM: 0.01 to 0.10%, The austenitic stainless steel material according to (2) above, containing one or more selected from the above.

[0017] (5) The austenitic stainless steel material according to (1) above, used in a high-pressure hydrogen gas environment.

[0018] (6) The austenitic stainless steel material according to (2) above, used in a high-pressure hydrogen gas environment.

[0019] (7) The austenitic stainless steel material according to (3) above, used in a high-pressure hydrogen gas environment.

[0020] (8) The austenitic stainless steel material according to (4) above, used in a high-pressure hydrogen gas environment.

[0021] (9) A manufacturing method for manufacturing the austenitic stainless steel material according to any one of (1) to (8) above, including a step of performing solution treatment, a step of performing sub-zero treatment, and a step of performing cold working. A manufacturing method for austenitic stainless steel material.

[0022] (10) A hydrogen equipment including the austenitic stainless steel material according to any one of (1) to (8) above.

[0023] (11) The hydrogen equipment according to (10) above, wherein the hydrogen equipment is a tank body, a tank base, a liner, a pipe, a valve, or a heat exchanger.

Advantages of the Invention

[0024] According to the present disclosure, an austenitic stainless steel material that achieves both hydrogen embrittlement resistance and economic efficiency can be obtained.

Embodiments for Carrying Out the Invention

[0025] The inventors of the present invention conducted studies to obtain an austenitic stainless steel material that achieves both strength and hydrogen embrittlement resistance and economic efficiency, and obtained the following findings (a) to (d).

[0026] (a) In order to achieve both strength and elongation and hydrogen embrittlement resistance in a high-pressure hydrogen gas environment, it is effective to make the metal structure of the steel material have an fcc structure and control the ratio of large-angle grain boundaries. Making the fcc structure, more specifically, an austenite single-phase metal structure, is because martensite with a bcc structure is likely to cause hydrogen embrittlement when it is formed.

[0027] In addition, the above-mentioned large-angle grain boundaries are those with an orientation difference of 15 to 60° from the inside of the crystal grains. In the case of ordinary austenitic stainless steel materials, mainly, the large-angle grain boundaries become twin boundaries. And in the plastic deformation due to cold working during steel material production, by controlling the proportion of deformation twins, that is, large-angle grain boundaries, in the metal structure, high strength due to work hardening can be achieved. In addition, the hydrogen embrittlement resistance can also be improved. That is, even if plastic deformation occurs in the use environment, twin deformation starting from the large-angle grain boundaries occurs, and the large-angle grain boundary structure can be maintained, so the hydrogen embrittlement resistance is improved.

[0028] (b) Therefore, in order to cause plastic deformation by twinning, it is effective to control the A value, which is an index of the mode of plastic deformation in steel. By controlling the A value, the generation of large-angle grain boundaries can be promoted. Also, thereby, the deformation mechanism in plastic deformation mainly becomes twinning deformation. On the other hand, when elements such as Ni, N, Cu, and Mo are contained excessively, the generation of large-angle grain boundaries may be suppressed. Therefore, within the range of the chemical composition of the steel, it is effective to increase the content of Mn, which is excellent in economy, to control the A value.

[0029] (c) As a result, under the use environment, twinning deformation starting from large-angle grain boundaries occurs, the large-angle grain boundary structure is maintained, and the hydrogen embrittlement resistance is also maintained. Also, from the viewpoint of high strength, the Vickers hardness is preferably 250 HV1 or more. Also, in order to suppress the decrease in elongation in high-pressure hydrogen gas, that is, to ensure hydrogen embrittlement resistance, it is preferably 500 HV1 or less.

[0030] (d) In order to obtain the above-described metal structure, solution treatment and subsequent cold working are required in the steel manufacturing process. During cold working, in order to efficiently introduce the above-described large-angle grain boundaries into the crystal grains and exhibit work hardening, it is desirable to perform cold working after sub-zero treatment. This is because the sub-zero treatment has the effect of suppressing the cross-slip of dislocations and promoting deformation twins during plastic deformation by cold working.

[0031] The present disclosure has been made based on the above findings. Hereinafter, the requirements of one embodiment of the present disclosure will be described in detail.

[0032] 1. Chemical Composition The reasons for limiting each element are as follows. In the following description, "%" regarding the content means "mass%".

[0033] C: 0.20% or less C is an element effective for stabilizing the austenite phase and improves hydrogen embrittlement resistance. However, if C is contained in excess, it promotes the grain boundary precipitation of Cr-based carbides and reduces the weather resistance and hydrogen embrittlement resistance after cold working. Therefore, the C content should be 0.20% or less. The C content is preferably 0.15% or less, and more preferably 0.10% or less. On the other hand, in order to obtain the above effects, the C content is preferably 0.01% or more.

[0034] Si: 2.0% or less Si is an element effective for deoxidation and contributes to the improvement of hydrogen embrittlement resistance and the increase in strength by solid solution strengthening. However, if Si is contained in excess, it promotes the formation of intermetallic compounds such as the σ phase and reduces the cold workability. Therefore, the Si content should be 2.0% or less. The Si content is preferably 1.5% or less, more preferably 1.0% or less, and even more preferably 0.8% or less. On the other hand, in order to obtain the above effects, the Si content is preferably 0.1% or more.

[0035] Mn: 6.0 - 20.0% Mn is an element that stabilizes the austenite phase, promotes cold working and the subsequent occurrence of deformation twins, and contributes to the improvement of strength and hydrogen embrittlement resistance. In addition, it indirectly contributes to the reduction of expensive Ni by increasing the solid solubility limit of N. Therefore, the Mn content should be 6.0% or more. The Mn content is preferably 7.0% or more, and more preferably 8.0% or more. However, if Mn is contained in excess, it promotes the formation of the ε phase with high hydrogen embrittlement susceptibility and conversely reduces the hydrogen embrittlement resistance. Therefore, the Mn content should be 20.0% or less. The Mn content is preferably 17.0% or less, more preferably 15.0% or less, even more preferably 13.0% or less, and still more preferably 10.0% or less.

[0036] P: 0.060% or less P is an element contained in steel as an impurity. However, it may concentrate in the final stage of solidification, lower the melting point of the steel, and promote solidification cracking (hot cracking). Also, P may reduce the strength. Further, when the P content is high and the cold rolling reduction rate in finish rolling described later is high, the hydrogen embrittlement resistance may be reduced. Therefore, the P content should be 0.060% or less. From the viewpoint of improving the hot cracking resistance, the P content is preferably 0.030% or less, and more preferably 0.025% or less. On the other hand, if P is excessively reduced, it will lead to an increase in manufacturing cost. Therefore, the P content is preferably 0.005% or more.

[0037] S: 0.0080% or less S is an element contained in steel as an impurity. Similar to P, it may promote solidification cracking (hot cracking). Therefore, the S content should be 0.0080% or less. The S content is preferably 0.0030% or less, and more preferably 0.0020% or less. However, if S is excessively reduced, the manufacturing cost will increase. Therefore, the S content is preferably 0.0001% or more.

[0038] Cr: 10.0 - 18.0% Cr is an element necessary for improving the corrosion resistance in stainless steel. Therefore, the Cr content should be 10.0% or more. The Cr content is preferably 13.0% or more, and more preferably 14.0% or more. However, Cr is a ferrite-forming element. Therefore, if Cr is contained in excess, it will destabilize the austenite phase, reduce the hydrogen embrittlement resistance, and also reduce the cold workability. Therefore, the Cr content should be 18.0% or less. The Cr content is preferably 17.0% or less, and more preferably 16.0% or less.

[0039] Ni: 4.0 - 12.0% Like Mn, Ni stabilizes the austenite phase, promotes cold working and the subsequent formation of deformation twins, increases strength, and improves hydrogen embrittlement resistance. Therefore, the Ni content should be 4.0% or more. The Ni content is preferably more than 6.0%, more preferably 6.2% or more, and even more preferably 6.5% or more. However, if Ni is contained in excess, it suppresses deformation twins after cold working, leading to a decrease in strength and hydrogen embrittlement resistance. Therefore, the Ni content should be 12.0% or less. The Ni content is preferably 10.0% or less, more preferably 9.0% or less.

[0040] N: 0.01 - 0.30% Like Mn and Ni, N stabilizes the austenite phase, promotes cold working and the subsequent formation of deformation twins, increases strength, and improves hydrogen embrittlement resistance. Therefore, the N content should be 0.01% or more. When the effect of N is actively utilized, the N content is preferably 0.10% or more. However, if N is contained in excess, internal defects such as blowholes may occur, reducing hot workability, cold workability, and hydrogen embrittlement resistance. Therefore, the N content should be 0.30% or less. The N content is preferably 0.25% or less.

[0041] Cu: 4.0% or less Cu is an element mixed from raw materials such as scrap, stabilizes the austenite phase, and improves hydrogen embrittlement resistance. On the other hand, if Cu is contained in excess, in addition to manufacturability, it suppresses the formation of deformation twins after cold working, reducing strength and hydrogen embrittlement resistance. Therefore, the Cu content should be 4.0% or less. The Cu content is preferably 3.0% or less, more preferably less than 2.3%, even more preferably 1.5% or less. Also, the Cu content is even more preferably less than 1.0%, and most preferably 0.9% or less. However, if Cu is excessively reduced, it causes restrictions on the melting raw materials and increases the manufacturing cost. Therefore, the Cu content is preferably 0.01% or more.

[0042] Mo: Below 3.0% Mo is incorporated from raw materials such as scrap and is an element effective for hydrogen embrittlement resistance. On the other hand, if Mo is contained in excess, it promotes the formation of the δ-ferrite phase and reduces manufacturability. Also, it suppresses the formation of deformation twins after cold working and reduces strength and hydrogen embrittlement resistance. Therefore, the Mo content is set to 3.0% or less. The Mo content is preferably 2.0% or less, and more preferably 1.0% or less. However, if Mo is contained in excess, it causes restrictions on the melting raw materials and increases the manufacturing cost. Therefore, the Mo content is preferably 0.01% or more.

[0043] In addition to the above elements, one or more selected from Al, Ca, B, Mg, Nb, Ti, V, W, Zr, Co, Ga, Hf, and REM may be contained within the ranges shown below. The reasons for limiting each element will be explained.

[0044] Al: 0 to 0.20% In addition to being an effective deoxidizing element, Al has the effect of suppressing the grain boundary segregation of low melting point elements and strengthening the grain boundaries. As a result, manufacturability and the like are improved, and cold workability is enhanced. Therefore, it may be contained as needed. However, if Al is contained in excess, AlN is precipitated, which instead reduces manufacturability. Also, it reduces the solubility of N and the hydrogen embrittlement resistance decreases. Therefore, the Al content is 0.20% or less. The Al content is preferably 0.10% or less, and preferably 0.08% or less. On the other hand, in order to obtain the above effects, the Al content is preferably 0.01% or more.

[0045] Ca: 0 to 0.01% Ca has the effect of suppressing the grain boundary segregation of low melting point elements and strengthening the grain boundaries. As a result, manufacturability and the like are improved, and cold workability is improved. Therefore, it may be contained as necessary. However, if Ca is contained in excess, the formation of inclusions will instead reduce the manufacturability and corrosion resistance. Therefore, the Ca content should be 0.01% or less. The Ca content is preferably 0.005% or less. On the other hand, in order to obtain the above effect, the Ca content is preferably 0.001% or more.

[0046] B: 0 to 0.01% B strengthens the grain boundaries, improves the strength, and improves the hot workability and cold workability. Therefore, it may be contained as necessary. However, if B is contained in excess, the grain boundary precipitation of boron compounds (BN, BC, Cr2B) will be promoted, reducing the workability and corrosion resistance. Therefore, the B content should be 0.01% or less. The B content is preferably 0.005% or less. On the other hand, in order to obtain the above effect, the B content is preferably 0.0002% or more.

[0047] Mg: 0 to 0.01% Mg is an element having a deoxidizing effect and has the effect of improving the manufacturability. Therefore, it may be contained as necessary. However, if Mg is contained in excess, the manufacturability will decrease in refining and the like, and the manufacturing cost will increase. Therefore, the Mg content should be 0.01% or less. The Mg content is preferably 0.005% or less. On the other hand, in order to obtain the above effect, the Mg content is preferably 0.0002% or more, and more preferably 0.0005% or more.

[0048] Nb: 0 to 1.0% Nb forms carbonitrides, has the effect of refining crystal grains and strengthening grain boundaries. As a result, it contributes to the improvement of strength. Therefore, it may be contained as necessary. However, if Nb is contained in excess, the solid-solution N concentration decreases, leading to a decrease in hydrogen embrittlement resistance and weather resistance, as well as a decrease in hot workability and cold workability. Therefore, the Nb content should be 1.0% or less. The Nb content is preferably 0.50% or less. On the other hand, in order to obtain the above effects, the Nb content is preferably 0.01% or more.

[0049] Ti: 0 to 1.0% Ti forms carbonitrides, has the effect of refining crystal grains and strengthening grain boundaries. As a result, it has the effect of improving strength. Therefore, it may be contained as necessary. However, if Ti is contained in excess, the solid-solution N concentration decreases, leading to a decrease in hydrogen embrittlement resistance and weather resistance, as well as a decrease in hot workability and cold workability. Therefore, the Ti content should be 1.0% or less. The Ti content is preferably 0.50% or less. On the other hand, in order to obtain the above effects, the Ti content is preferably 0.01% or more.

[0050] V: 0 to 1.0% V dissolves in the steel or precipitates as carbonitrides, having the effect of improving strength. Therefore, it may be contained as necessary. However, if V is contained in excess, excessive carbonitrides are formed, leading to a decrease in hot workability and cold workability. Therefore, the V content should be 1.0% or less. The V content is preferably 0.50% or less. On the other hand, in order to obtain the above effects, the V content is preferably 0.01% or more.

[0051] W: 0 to 2.0% W has the effect of improving strength and weather resistance. Therefore, it may be contained as necessary. However, if W is contained in excess, manufacturability and raw material costs increase, so the W content should be 2.0% or less. The W content is preferably 1.0% or less. On the other hand, in order to obtain the above effects, the W content is preferably 0.01% or more.

[0052] Zr: 0 to 1.0% Zr has a deoxidizing effect and also has an effect of improving weather resistance. Therefore, it may be contained as necessary. However, if Zr is contained in excess, toughness and workability will decrease. Therefore, the Zr content should be 1.0% or less, preferably 0.50% or less. On the other hand, in order to obtain the above effects, the Zr content is preferably 0.01% or more.

[0053] Co: 0 to 2.0% Co has an effect of improving corrosion resistance and stabilizing the austenite phase. Therefore, it may be contained as necessary. However, if Co is contained in excess, the manufacturing cost will increase. Therefore, the Co content should be 2.0% or less, preferably 1.0% or less. On the other hand, in order to obtain the above effects, the Co content is preferably 0.01% or more.

[0054] Ga: 0 to 0.10% Ga has an effect of improving hot workability. Therefore, it may be contained as necessary. However, if Ga is contained in excess, manufacturability will decrease. Therefore, the Ga content should be 0.10% or less, preferably 0.05% or less. On the other hand, in order to obtain the above effects, the Ga content is preferably 0.01% or more.

[0055] Hf: 0 to 0.10% Hf has a deoxidizing effect and also has an effect of improving weldability. Therefore, it may be contained as necessary. However, if Hf is contained in excess, manufacturability such as refining will decrease. Therefore, the Hf content should be 0.10% or less, preferably 0.05% or less. On the other hand, in order to obtain the above effects, the Hf content is preferably 0.01% or more.

[0056] REM: 0 to 0.10% REM has a deoxidizing effect, an effect of improving productivity, and also an effect of improving corrosion resistance. Therefore, it may be contained as necessary. However, if REM is contained in excess, not only does its effect saturate, but rather the productivity in refining and the like decreases. Therefore, the REM content should be 0.10% or less. The REM content is preferably 0.05% or less. On the other hand, in order to obtain the above effects, the REM content is preferably 0.01% or more.

[0057] REM refers to a total of 17 elements including Sc, Y, and lanthanoids, and the above REM content means the total content of these elements. Industrially, REM is often added in the form of mischmetal.

[0058] In the chemical composition of this embodiment, the balance is Fe and impurities. Here, "impurities" means components that are mixed in due to raw materials such as ores and scraps and various factors in the manufacturing process when industrially manufacturing austenitic stainless steel materials, and are allowed within a range that does not adversely affect this embodiment.

[0059] A value In the chemical composition of the steel material of this embodiment, the A value, which is an index of the stability of the austenite phase and also an index of the plastic deformation mechanism, is set to 30.0 to 60.0. The A value is calculated by the following formula (i).

[0060] A value = 3.2Mn + 0.7Cr + 6.2Ni + 38.7N + 4.8Cu + 9.3Mo - 53 ···(i) However, each element symbol in the above formula (i) represents the content (mass%) of each element contained in the steel, and is set to zero if not contained.

[0061] When the value of A is less than 30.0, the stability of the austenite phase is low, and work-induced martensite is formed. Therefore, not only the crystal structure is face-centered cubic (fcc), but also the crystal grains with body-centered cubic (bcc) increase, and the hydrogen embrittlement resistance decreases. For this reason, the value of A shall be 30.0 or more. From the viewpoint of hydrogen embrittlement resistance, the value of A is preferably 32.0 or more, and more preferably 35.0 or more.

[0062] However, when the value of A exceeds 60.0, the occurrence of deformation twins after cold working is suppressed, and in a high-pressure hydrogen gas environment, it causes a decrease in strength and elongation. As a result, the hydrogen embrittlement resistance decreases. In addition, the raw material cost increases, leading to a decrease in productivity. For this reason, the value of A shall be 60.0 or less. From the viewpoints of strength, economy, and productivity after cold working, the value of A is preferably 50.0 or less.

[0063] 2. Ratio of high-angle grain boundaries In the austenitic stainless steel material of the present embodiment, the ratio of high-angle grain boundaries that affect twin deformation among the metal structures is within the following range. Specifically, in the metal structure of the surface layer, the ratio Gs of high-angle grain boundaries satisfies the following formula (ii). Note that the high-angle grain boundaries in the present embodiment refer to grain boundaries whose orientation difference from the inside of the crystal grains is in the range of 15 to 60°.

[0064] 0.1 < Gs < 0.6 ···(ii) Each symbol in the above formula is defined as follows. Gs: Ratio of high-angle grain boundaries

[0065] When the above-mentioned Gs is 0.1 or less, the formation of deformation twins starting from large-angle grain boundaries cannot be promoted after cold working. As a result, in a high-pressure hydrogen gas environment, the strength and elongation decrease, and the hydrogen embrittlement resistance deteriorates. Therefore, the above Gs should be more than 0.1. From the viewpoints of high strength and hydrogen embrittlement resistance after cold working, the above Gs is preferably 0.20 or more, and more preferably 0.30 or more. On the other hand, when the above Gs is 0.6 or more, the appearance of deformation twins after cold working is rather suppressed. As a result, in a high-pressure hydrogen gas environment, the strength and elongation decrease. Also, after cold working, the generation of processing-induced martensite starting from large-angle grain boundaries is induced, which also leads to a decrease in hydrogen embrittlement resistance. Therefore, the above Gs should be less than 0.6. From the viewpoints of high strength and hydrogen embrittlement resistance after cold working, the above Gs is preferably 0.5 or less.

[0066] The above Gs can be measured by a crystal orientation analysis system using EBSP. Specifically, taking the total thickness in the direction perpendicular to the cold-worked surface (rolling surface) as t, select one plane parallel to the working surface (rolling surface) from the surface to t / 4, and finish the surface with mirror polishing. It is preferable to use this surface as the observation surface and perform EBSP measurement by glow discharge optical emission spectrometry (GDS). For example, measurement can be performed on a plane at t / 8 and parallel to the working surface (rolling surface).

[0067] For example, when measuring under the conditions of GDS on a plane at t / 8 and parallel to the working surface (rolling surface), etc., the pulse sputtering mode with an output of 20 W and a pressure of 600 Pa is adopted, and the sputtering depth is preferably 0.1 μm or less. By using GDS, the measurement accuracy of EBSP can be maintained even in cold-worked steel materials. The measurement of EBSP preferably captures a region of 45 μm × 130 μm. The observation magnification at that time is preferably 2000, and it is sufficient if Gs satisfies the range of this embodiment in at least one field of view. Note that Gs can be calculated by the area fraction method in which the orientation difference from within the crystal grains is set to 15 to 60° in the crystal grain boundary map display of the crystal orientation analysis system. It is desirable to adopt the numerical value of the capture field of view in which the CI value (reliability index) exceeds 0.2 on average.

[0068] 3. Vickers hardness The austenitic stainless steel material of this embodiment is work-hardened and its hardness increases. Therefore, the Vickers hardness is preferably 250 to 500 HV1. If the Vickers hardness is less than 250 HV1, it becomes difficult to obtain a desired strength of 800 MPa or more. For this reason, the Vickers hardness is preferably 250 HV1 or more. From the viewpoints of strength and hydrogen embrittlement resistance, the Vickers hardness is more preferably 300 HV1. On the other hand, when the Vickers hardness exceeds 500, the tensile strength tends to exceed 1700 MPa, and the elongation of the material itself also decreases significantly. As a result, it also becomes difficult to maintain the elongation in a high-pressure hydrogen gas environment.

[0069] Also, in cold working and subsequent plastic deformation, when strain-induced martensite (body-centered cubic: bcc) is generated and the ratio of the austenite phase (face-centered cubic: fcc) decreases, it leads to a decrease in hydrogen embrittlement resistance. For this reason, the Vickers hardness is preferably 500 HV1 or less. From the viewpoint of hydrogen embrittlement resistance, the Vickers hardness is more preferably 450 HV1 or less.

[0070] The Vickers hardness may be measured under the following conditions in accordance with JIS Z 2244:2020. Specifically, a resin-embedded sample of the cross-section may be prepared, and the load may be set to 1 kg (9.8 N) near t / 2 and measured with a Vickers hardness tester. Also, the holding time of the test force may be loaded for 20 seconds.

[0071] 4. Crystal structure In the austenitic stainless steel material of this embodiment, in order to suppress a decrease in hydrogen embrittlement resistance, it is desirable to suppress the generation of strain-induced martensite after processing. For this reason, in the crystal structure, preferably 97% or more is the fcc structure in terms of area ratio.

[0072] If the area ratio of the crystal grains with an fcc structure is less than 97%, a large amount of δ-ferrite with a bcc structure and work-induced martensite are generated, which is not desirable from the viewpoint of hydrogen embrittlement resistance. Therefore, it is preferable that 97% or more of the area ratio has an fcc structure. Regarding δ-ferrite, it may be formed unavoidably in part, but even in such a case, if the area ratio of the crystal grains with a bcc structure is less than 3%, the influence on hydrogen embrittlement resistance is small and thus acceptable.

[0073] Regarding the crystal structure, it may be measured by the following procedure. Specifically, X-ray diffraction may be performed on the measurement surface of the above-described EBSP, and the area ratios of the grains with an fcc structure and the grains with a bcc structure may be calculated. The conditions for X-ray diffraction may be such that CuKα rays are used, the applied voltage is 40 kV, and 2θ is in the range of 30° ≤ 2θ ≤ 90°.

[0074] 5. Use The austenitic stainless steel material of the present embodiment is preferably used for hydrogen equipment that requires weight reduction and compactness for the purpose of increasing strength in a high-pressure hydrogen environment. Examples of hydrogen equipment include the main body of a tank for storing hydrogen gas and liquid hydrogen, the base of the tank, a liner, a pipe serving as a flow path for hydrogen gas, a valve, and a heat exchanger.

[0075] 6. Manufacturing method A preferred manufacturing method of the austenitic stainless steel material of the present embodiment will be described. The austenitic stainless steel material of the present embodiment can obtain the above-described effects as long as it has the above-described configuration regardless of the manufacturing method. However, for example, it can be stably manufactured by the following manufacturing method.

[0076] In the following description, for the sake of simplicity, the shape of the steel material will be described as a steel plate, but the shape of the steel material is not particularly limited. For example, shapes such as plate-shaped, rod-shaped, or tubular can be considered. A processing method corresponding to each shape may be selected.

[0077] The steel adjusted to the above chemical composition is melted and cast by a conventional method to obtain a steel slab to be subjected to hot rolling. Subsequently, hot rolling is carried out by a conventional method. The conditions during hot rolling are not particularly limited, but usually, the heating temperature of the steel slab is 1050 to 1250 °C, and the rolling ratio is preferably in the range of 20 to 99%. After hot rolling, annealing and pickling may be carried out as necessary. The annealing temperature at this time is not particularly limited, but for example, it may be in the range of 1050 to 1100 °C.

[0078] Subsequently, cold rolling and annealing are carried out as necessary. The annealing immediately before the final finish cold rolling is referred to as solution treatment, and the solution treatment is also carried out in the same manner. Also, pickling is carried out as necessary. Cold rolling is carried out, for example, in the range of a rolling ratio of 20 to 90%, and the subsequent annealing is preferably carried out at 1000 to 1150 °C with an isothermal holding for 1 to 600 seconds. Cold rolling, annealing, and pickling may be repeated a plurality of times for the purpose of finally obtaining a steel material with the required thickness.

[0079] Solution treatment is carried out immediately before the final finish cold rolling for work hardening. In the solution treatment, it is preferable to carry out heat treatment under the conditions of isothermal holding at 1000 to 1150 °C for 1 to 600 seconds. When the solution treatment temperature is less than 1000 °C, recrystallization becomes insufficient, and sufficient high-angle grain boundaries due to twin deformation are not introduced during the final finish cold rolling, so it becomes difficult for Gs to satisfy the formula (ii). For this reason, the solution treatment temperature is preferably 1000 °C or higher. On the other hand, when the solution treatment temperature exceeds 1150 °C, the crystal grains coarsen (exceed 0.1 mm), and it becomes difficult for Gs to satisfy the formula (ii) after cold working in the same manner as above. For this reason, the solution treatment temperature is preferably 1150 °C or lower.

[0080] Note that by performing sub-zero treatment immediately after the solution treatment, large-angle grain boundaries can be efficiently introduced into the crystal grains during the subsequent final finishing cold rolling, and the value of Gs can be increased. Therefore, it is preferable to perform sub-zero treatment. Sub-zero treatment refers to quenching treatment in which the temperature is rapidly cooled from the solution treatment temperature to 0°C or lower. When rapidly cooling, it is preferably carried out by a liquid method or a gas method using dry ice, liquid nitrogen, or carbon dioxide gas as the refrigerant in accordance with HSZ of JIS standards.

[0081] Finally, perform final finishing cold rolling to obtain a work-hardened steel material. By performing final finishing cold rolling after the solution treatment or after the subsequent sub-zero treatment, the value of Gs can be increased and good strength can be obtained. Therefore, it is preferable to perform final finishing cold rolling in the range of a rolling ratio of 10 to 80% after the solution treatment or after the sub-zero treatment. This is because if the rolling ratio is less than 10%, sufficient hardness and strength cannot be obtained. Also, it is difficult to increase the value of Gs. Therefore, the final finishing cold rolling ratio is preferably 10% or more.

[0082] On the other hand, if the rolling ratio exceeds 80%, the strength increases too much and the hydrogen embrittlement resistance decreases. Also, it becomes difficult to obtain the desired value of Gs. Therefore, the final finishing cold rolling ratio is preferably 80% or less. In order to make the hardness in the range of 300 to 450 HV1, the final finishing cold rolling ratio is preferably in the range of 20 to 70%. Although the steel plate has been described by way of example, for example, in the case of a bar-shaped or tubular steel material, the area reduction ratio may be in the range of 10 to 80%. That is, it may be adjusted so that the cold working ratio is in the range of 10 to 80%. Also, by adjusting the conditions within the above range and other conditions, the microstructure may be controlled to be austenitic stainless steel.

[0083] Hereinafter, the austenitic stainless steel material of the present embodiment will be described more specifically by way of examples, but the present embodiment is not limited to these examples.

Examples

[0084] Slabs with the composition shown in Table 1 were melted, heated to 1200 °C, and then hot-rolled to obtain hot-rolled sheets with a thickness of 5.0 mm. Next, the hot-rolled sheets were annealed and pickled at 1050 - 1100 °C, and then cold-rolled to a thickness of 2 mm to obtain cold-rolled sheets. Thereafter, the cold-rolled sheets were solution-treated at 1050 - 1100 °C for 30 seconds and then pickled. For some steel sheets, sub-zero treatment was performed. The sub-zero treatment was carried out using dry ice and ethanol as cryogenic agents by the liquid method described above, and the sheets were rapidly cooled from the solution-treatment temperature to -10 °C. Thereafter, regardless of the presence or absence of sub-zero treatment, the obtained steel sheets were finally cold-rolled in the range of 10 - 80% to obtain test specimens (austenitic stainless steel sheets) with a thickness of 0.4 - 1.8 mm.

[0085] [Table 1]

[0086] For the obtained test specimens, the calculation of Gs, hardness, crystal structure were examined, and the hydrogen embrittlement resistance was evaluated according to the following procedures.

[0087] (Calculation of Gs) Next, for the test specimens, sample preparation was carried out on the plane parallel to the rolling surface at the position of t / 8 by the method described above, and Gs was determined by the crystal orientation analysis system of EBSP. The observation magnification of EBSP was 2000, and Gs was taken as the value obtained from the field with a CI value ≧ 0.2. The measurement of Gs was carried out under the conditions described above.

[0088] (Measurement of hardness) Also, for the test specimens, resin-embedded specimens of the cross-section were prepared, the load was set to 1 kg (9.8 N) near t / 2, and a hardness test was carried out in accordance with JIS Z 2244:2020. In the test, a Vickers hardness tester was used, and the holding time of the test force was 20 seconds.

[0089] (Identification of crystal structure) Regarding the crystal structure, X-ray diffraction was performed on the measurement surface of EBSP, and the area ratios of grains with an fcc structure and grains with a bcc structure were calculated. The conditions for X-ray diffraction were as follows: CuKα radiation was used, the applied voltage was 40 kV, and the range of 2θ was 30° ≤ 2θ ≤ 90°. In addition, in the results of Table 2, for those with more than 3% of the grain area of the bcc structure, it was described as "present".

[0090] (Evaluation of hydrogen embrittlement resistance) Regarding hydrogen embrittlement resistance, the measurement was carried out according to the following procedure. A tensile test specimen with a parallel part width of 4 mm and a length of 20 mm was taken. Subsequently, for the above tensile test specimen, a low strain rate tensile test (hereinafter referred to as the "SSRT test") was carried out at -40 °C, in 70 MPa hydrogen and in 0.1 MPa nitrogen with a strain rate of 10 -5 / s. The evaluation of the SSRT test was carried out by measuring the tensile fracture strength and the tensile fracture elongation. Specifically, the hydrogen embrittlement resistance was evaluated using the numerical values calculated by the following formula.

[0091] Hydrogen embrittlement evaluation value of tensile fracture strength = (Tensile fracture strength in 70 MPa hydrogen) / (Tensile fracture strength in 0.1 MPa nitrogen) × 100 (%) ···(a) Hydrogen embrittlement evaluation value of tensile fracture elongation = (Tensile fracture elongation in 70 MPa hydrogen) / (Tensile fracture elongation in 0.1 MPa nitrogen) × 100 (%) ···(b)

[0092] When the hydrogen embrittlement evaluation value of the tensile fracture strength calculated from the above formula is 95% or more and the hydrogen embrittlement evaluation value of the tensile fracture elongation is 85% or more, it is described as "good" as having good hydrogen embrittlement resistance. On the other hand, when the hydrogen embrittlement evaluation value does not meet the above numerical values, it is described as "poor" as having poor hydrogen embrittlement resistance. Furthermore, when the hydrogen embrittlement evaluation value of the tensile fracture strength is 95% or more and the hydrogen embrittlement evaluation value of the tensile fracture elongation is 100% or more, it is described as "excellent" as having particularly excellent hydrogen embrittlement resistance. The results are summarized in Table 2 below.

[0093]

Table 2

[0094] Specimens No. 1 to 12 that satisfy the chemical composition, A value, and Gs of this embodiment achieved the target hydrogen embrittlement resistance. In particular, No. 1 to 4, 7, 11, and 12 have the preferred hardness and crystal structure of this embodiment, and the hydrogen embrittlement resistance was rated as "excellent", which is particularly excellent. No. 1 to 4 with "excellent" hydrogen embrittlement resistance satisfied the chemical composition and A value within the preferred range of this embodiment. Although No. 7, 11, and 12 deviate from the preferred chemical composition and A value of this embodiment, the Gs was increased by sub-zero treatment, and the hydrogen embrittlement resistance was improved to "excellent". Note that No. 9 has a relatively high P content and deviates from the preferred chemical composition of this embodiment. However, the finish cold rolling rate is low, and the hydrogen embrittlement resistance maintained "excellent" while the hardness decreased. Also, No. 10, like No. 9, has a relatively high P content and a high finish cold rolling rate. Therefore, the hardness increased, but the hydrogen embrittlement resistance was rated as "good".

[0095] On the other hand, No. 13 to 18 did not satisfy the requirements of this embodiment in terms of chemical composition, and No. 19 and 20 did not satisfy the requirements of this embodiment in terms of the A value among the chemical compositions. For this reason, even with sub-zero treatment, the hydrogen embrittlement resistance decreased. Also, No. 21 and No. 22 did not satisfy the range of the preferred finish cold rolling rate, so the value of Gs increased or decreased excessively, and the hydrogen embrittlement resistance also decreased.

[0096] (Appendix) (1) The chemical composition is in mass%, C: 0.20% or less, Si: 2.0% or less, Mn: 6.0 - 20.0%, P: 0.060% or less, S: 0.0080% or less, Cr: 10.0 - 18.0%, Ni: 4.0 - 12.0%, N: 0.01 - 0.30%, Cu: 4.0% or less, Mo: 3.0% or less, Al: 0 - 0.20%, Ca: 0 - 0.01%, B: 0 - 0.01%, Mg: 0 to 0.01%, Nb: 0 to 1.0%, Ti: 0 to 1.0%, V: 0 to 1.0%, W: 0 to 2.0%, Zr: 0 to 1.0%, Co: 0 to 2.0%, Ga: 0 to 0.10%, Hf: 0 to 0.10%, REM: 0 to 0.10%, The balance: Fe and impurities, The A value calculated by the following formula (i) is 30.0 to 60.0, An austenitic stainless steel material in which the ratio Gs of large-angle grain boundaries satisfies the following formula (ii) in the metal structure of the surface layer. A value = 3.2Mn + 0.7Cr + 6.2Ni + 38.7N + 4.8Cu + 9.3Mo - 53 ··· (i) 0.1 < Gs < 0.6 ··· (ii) However, each element symbol in the above formula (i) represents the content (mass%) of each element contained in the steel, and is zero if not contained. Each symbol in the above formula is defined as follows. Gs: Ratio of large-angle grain boundaries

[0097] (2) The Vickers hardness is 250 to 500 HV1, In the crystal structure, 97% or more is the fcc structure by area ratio, The austenitic stainless steel material according to the above (1).

[0098] (3) The chemical composition is in mass%, Al: 0.01 to 0.20%, Ca: 0.001 to 0.01%, B: 0.0002 to 0.01%, Mg: 0.0002 to 0.01%, Nb: 0.01 to 1.0%, Ti: 0.01 to 1.0%, V: 0.01 to 1.0%, W: 0.01 to 2.0%, Zr: 0.01 to 1.0%, Co: 0.01 to 2.0%, Ga: 0.01 to 0.10%, Hf: 0.01 to 0.10%, and REM: 0.01 to 0.10%, The austenitic stainless steel material according to the above (1) or (2), containing one or more selected from the above.

[0099] (4) The austenitic stainless steel material according to any one of the above (1) to (3), used in a high-pressure hydrogen gas environment.

[0100] (5) A manufacturing method for manufacturing the austenitic stainless steel material according to any one of the above (1) to (4), A step of performing solution treatment, A step of performing sub-zero treatment, A step of performing cold working, and having a manufacturing method for an austenitic stainless steel material.

[0101] (6) A hydrogen equipment including the austenitic stainless steel material according to any one of the above (1) to (4).

[0102] (7) The hydrogen equipment according to the above (6), wherein the hydrogen equipment is a main body of a tank, a base of a tank, a liner, a pipe, a valve, or a heat exchanger.

Industrial Applicability

[0103] The austenitic stainless steel material of the present embodiment has both high strength and hydrogen embrittlement resistance, and is suitable as a steel plate, bar, or pipe used by performing cold working in a high-pressure hydrogen gas environment. In addition, it is desirable to apply it to hydrogen equipment that requires weight reduction and compactness due to high strength. The steel material of the present embodiment can contribute to high strength by cold working and the reduction of thickness and weight of hydrogen equipment and parts associated therewith.

Claims

1. The chemical composition is, by mass percentage, C: 0.20% or less, Si: 2.0% or less, Mn: 6.0 - 20.0%, P: 0.060% or less, S: 0.0080% or less, Cr: 10.0 - 18.0%, Ni: 4.0 - 12.0%, N: 0.01 - 0.30%, Cu: 4.0% or less, Mo: 3.0% or less, Al: 0 - 0.20%, Ca: 0 - 0.01%, B: 0 - 0.01%, Mg: 0 - 0.01%, Nb: 0 - 1.0%, Ti: 0 - 1.0%, V: 0 - 1.0%, W: 0 - 2.0%, Zr: 0 - 1.0%, Co: 0 - 2.0%, Ga: 0 - 0.10%, Hf: 0 - 0.10%, REM: 0 - 0.10%, The balance is Fe and impurities, The A value calculated by the following formula (i) is 30.0 - 60.0, An austenitic stainless steel material in which, in the metal structure of the surface layer, the proportion Gs of grain boundaries having an orientation difference of 15 - 60° with respect to the inside of the crystal grains satisfies the following formula (ii). A value = 3.2Mn + 0.7Cr + 6.2Ni + 38.7N + 4.8Cu + 9.3Mo - 53... (i) 0.1 < Gs < 0.6... (ii) However, each element symbol in the above formula (i) represents the content (mass percentage) of each element contained in the steel, and is taken as zero if not contained, and each symbol in the above formula is defined as follows. Gs: The proportion of grain boundaries having an orientation difference of 15 - 60° with respect to the inside of the crystal grains

2. The Vickers hardness is 250 - 500 HV1, In the crystal structure, in terms of area ratio, 97% or more is the fcc structure, The austenitic stainless steel material according to Claim 1.

3. The chemical composition is, by mass percentage, Al: 0.01 - 0.20%, Ca: 0.001 - 0.01%, B: 0.0002 - 0.01%, Mg: 0.0002 - 0.01%, Nb: 0.01 - 1.0%, Ti: 0.01 - 1.0%, V: 0.01 - 1.0%, W: 0.01 - 2.0%, Zr: 0.01 - 1.0%, Co: 0.01 - 2.0%, Ga: 0.01 - 0.10%, Hf: 0.01 - 0.10%, and REM: 0.01 - 0.10%, The austenitic stainless steel material according to Claim 1, containing one or more selected from the above.

4. The chemical composition is, by mass percentage, Al: 0.01 - 0.20%, Ca: 0.001 - 0.01%, B: 0.0002 - 0.01%, Mg: 0.0002 - 0.01%, Nb: 0.01 - 1.0%, Ti: 0.01 - 1.0%, V: 0.01 - 1.0%, W: 0.01 to 2.0%, Zr: 0.01 to 1.0%, Co: 0.01 to 2.0%, Ga: 0.01 to 0.10%, Hf: 0.01 to 0.10%, and REM: 0.01 to 0.10%, The austenitic stainless steel material according to claim 2, containing one or more selected from the above.

5. The austenitic stainless steel material according to claim 1, used in a high-pressure hydrogen gas environment.

6. The austenitic stainless steel material according to claim 2, used in a high-pressure hydrogen gas environment.

7. The austenitic stainless steel material according to claim 3, used in a high-pressure hydrogen gas environment.

8. The austenitic stainless steel material according to claim 4, used in a high-pressure hydrogen gas environment.

9. A hydrogen equipment including the austenitic stainless steel material according to any one of claims 1 to 8.

10. The hydrogen equipment according to claim 9, wherein the hydrogen equipment is a main body of a tank, a base of a tank, a liner, a pipe, a valve, or a heat exchanger.

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