Austenitic stainless steel material, method for manufacturing the same, and hydrogen-using apparatus

Abstract

An austenitic stainless steel material, wherein a chemical composition of the austenitic stainless steel material is 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, arbitrary elements, balance: Fe and impurities, A value is 30.0 to 60.0, and in a metal structure of a surface layer, a proportion of large-angle grain boundaries Gs satisfies [0.1 < Gs < 0.6].

Description

Technical Field

[0001] This invention relates to austenitic stainless steel materials, methods for manufacturing the same, and equipment for hydrogen production. Background Technology

[0002] In recent years, hydrogen energy has attracted attention as a clean energy source that does not emit greenhouse gases such as carbon dioxide. There is a demand to establish hydrogen-related technologies, such as hydrogen production, storage, and transportation, based on the effective utilization of hydrogen energy.

[0003] On the other hand, various problems exist 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, when metallic materials are used in related devices such as hydrogen production and storage facilities, the so-called hydrogen embrittlement problem occurs, where materials become embrittled due to hydrogen gas.

[0004] From the perspectives of manufacturing cost, strength, and corrosion resistance, austenitic stainless steel is one of the metallic materials used in the aforementioned devices. Therefore, in order to suppress hydrogen embrittlement, austenitic stainless steel with improved resistance to hydrogen embrittlement has been developed.

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

[0006] Furthermore, for components or parts of the aforementioned hydrogen-related devices, there are cases where cold working is performed to increase strength through work hardening. However, in austenitic stainless steels, there are also cases where hydrogen embrittlement resistance decreases after cold working. Therefore, Patent Document 3 discloses a hydrogen-resistant stainless steel wire for springs that exhibits good hydrogen resistance even after certain cold working.

[0007] In the case of 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, hydrogen resistance is improved, achieving good hydrogen resistance even after cold working. Furthermore, Patent Document 4 discloses an austenitic stainless steel for high-pressure hydrogen, which achieves high strength and high hardness in a solution-treated state without relying on cold working. The austenitic stainless steel disclosed in Patent Document 4 contains 0.40 to 1.00% C and has Cr carbides present at an area ratio of 23% or more, thereby exhibiting high strength even in a solution-treated state and improving hydrogen embrittlement resistance.

[0008] Existing technical documents

[0009] Patent documents

[0010] Patent Document 1: Japanese Patent Application Publication No. 2019-143227

[0011] Patent Document 2: Japanese Patent Application Publication No. 2019-143228

[0012] Patent Document 3: Japanese Patent Application Publication No. 2009-084597

[0013] Patent Document 4: Japanese Patent Application Publication No. 2018-135592 Summary of the Invention

[0014] The problem the invention aims to solve

[0015] However, neither of the austenitic stainless steels disclosed in Patent Documents 1 and 2 mentions resistance to hydrogen embrittlement after cold working. Therefore, there is room for improvement in this characteristic. Furthermore, the austenitic stainless steels disclosed in Patent Documents 3 and 4 have higher Cr, N, and C contents compared to conventional austenitic stainless steels, suggesting further room for improvement from an economic and manufacturability standpoint.

[0016] Therefore, in the use of austenitic stainless steel materials as components and parts in hydrogen-related devices, there are issues such as the difficulty in balancing hydrogen embrittlement resistance and economy.

[0017] The purpose of this invention is to solve the above-mentioned problems and provide austenitic stainless steel materials that combine resistance to hydrogen embrittlement and economy.

[0018] Solution for solving the problem

[0019] This invention was made to solve the above-mentioned problems, and its main purpose is the following austenitic stainless steel material.

[0020] (1) An austenitic stainless steel material, wherein the chemical composition of the austenitic stainless steel material, expressed in mass percent, is as follows:

[0021] C: Below 0.20%

[0022] Si: below 2.0%

[0023] Mn: 6.0~20.0%

[0024] P: below 0.060%

[0025] S: below 0.0080%

[0026] Cr: 10.0–18.0%

[0027] Ni: 4.0–12.0%

[0028] N: 0.01~0.30%

[0029] Cu: below 4.0%

[0030] Mo: 3.0% or less,

[0031] Al: 0-0.20%

[0032] Ca: 0–0.01%

[0033] B: 0-0.01%

[0034] Mg: 0–0.01%

[0035] Nb: 0–1.0%

[0036] Ti: 0-1.0%

[0037] V: 0~1.0%

[0038] W: 0–2.0%

[0039] Zr: 0~1.0%

[0040] Co: 0-2.0%

[0041] Ga: 0-0.10%

[0042] Hf: 0~0.10%

[0043] REM: 0–0.10%

[0044] Balance: Fe and impurities,

[0045] The value of A calculated by equation (i) below is 30.0 to 60.0.

[0046] In the surface metallic microstructure, the proportion of large-angle grain boundaries Gs satisfies the following equation (ii): A = 3.2Mn + 0.7Cr + 6.2Ni + 38.7N + 4.8Cu + 9.3Mo - 53···(i) 0.1 < Gs < 0.6···(ii)

[0047] In equation (i) above, each element symbol represents the content (mass%) of each element contained in the steel, and is written as zero if the element is not present. The symbols in the above equation are defined as follows.

[0048] Gs: The proportion of large-angle grain boundaries.

[0049] (2) According to the austenitic stainless steel material described in (1) above, wherein,

[0050] Its Vickers hardness is 250-500 HV1, and in its crystal structure, more than 97% of it is of the fcc structure in terms of area ratio.

[0051] (3) According to the austenitic stainless steel material described in (1) above, wherein,

[0052] The chemical composition, expressed as a percentage by mass, contains one or more elements selected from the following elements.

[0053] Al: 0.01–0.20%

[0054] Ca: 0.001–0.01%

[0055] B: 0.0002~0.01%

[0056] Mg: 0.0002~0.01%

[0057] Nb: 0.01~1.0%

[0058] Ti: 0.01~1.0%

[0059] V: 0.01~1.0%

[0060] W: 0.01~2.0%

[0061] Zr: 0.01~1.0%

[0062] Co: 0.01~2.0%

[0063] Ga: 0.01~0.10%

[0064] Hf: 0.01~0.10%

[0065] REM: 0.01–0.10%.

[0066] (4) According to the austenitic stainless steel material described in (2) above, wherein,

[0067] The chemical composition, expressed as a percentage by mass, contains one or more elements selected from the following elements.

[0068] Al: 0.01–0.20%

[0069] Ca: 0.001–0.01%

[0070] B: 0.0002~0.01%

[0071] Mg: 0.0002~0.01%

[0072] Nb: 0.01~1.0%

[0073] Ti: 0.01~1.0%

[0074] V: 0.01~1.0%

[0075] W: 0.01~2.0%

[0076] Zr: 0.01~1.0%

[0077] Co: 0.01~2.0%

[0078] Ga: 0.01~0.10%

[0079] Hf: 0.01~0.10%

[0080] REM: 0.01–0.10%.

[0081] (5) The austenitic stainless steel material according to (1) above, wherein the austenitic stainless steel material is used in a high-pressure hydrogen environment.

[0082] (6) The austenitic stainless steel material according to (2) above, wherein the austenitic stainless steel material is used in a high-pressure hydrogen environment.

[0083] (7) The austenitic stainless steel material according to (3) above, wherein the austenitic stainless steel material is used in a high-pressure hydrogen environment.

[0084] (8) The austenitic stainless steel material according to (4) above, wherein the austenitic stainless steel material is used in a high-pressure hydrogen environment.

[0085] (9) A method for manufacturing an austenitic stainless steel material, wherein the austenitic stainless steel material described in any one of (1) to (8) above is manufactured by the method, wherein,

[0086] The manufacturing method of this austenitic stainless steel material has the following characteristics:

[0087] The process of solution treatment;

[0088] The process of performing sub-zero processing; and

[0089] The process of cold processing.

[0090] (10) A hydrogen processing device, wherein the hydrogen processing device comprises any one of the austenitic stainless steel materials described in (1) to (8) above.

[0091] (11) The hydrogen equipment according to (10) above, wherein the hydrogen equipment is the main body of the tank, the pipe head of the tank, the inner liner, the piping, the valve or the heat exchanger.

[0092] The effects of the invention

[0093] According to the present invention, austenitic stainless steel materials that combine resistance to hydrogen embrittlement and economy can be obtained. Detailed Implementation

[0094] In order to obtain austenitic stainless steel materials that balance strength, resistance to hydrogen embrittlement, and economy, the inventors have conducted research and obtained the following insights (a) to (d).

[0095] (a) To balance strength, elongation, and resistance to hydrogen embrittlement under high-pressure hydrogen conditions, it is effective to have the steel's microstructure be fcc structure and control the proportion of large-angle grain boundaries. The reason for setting it to fcc structure, or more specifically, austenitic single-phase microstructure, is that hydrogen embrittlement is more likely to occur when martensite with a bcc structure is formed.

[0096] Furthermore, the aforementioned large-angle grain boundaries refer to grain boundaries with an orientation difference of 15–60° relative to the grains within the grain. In the case of typical austenitic stainless steel materials, large-angle grain boundaries primarily become twin boundaries. Moreover, during the plastic deformation caused by cold working in steel manufacturing, by controlling the proportion of deformation twins (i.e., large-angle grain boundaries) within the metal microstructure, high strength resulting from work hardening can be achieved. Additionally, it can improve resistance to hydrogen embrittlement. That is, even if plastic deformation occurs under service conditions, it will become twin deformation originating from large-angle grain boundaries, maintaining the large-angle grain boundary structure, thus improving resistance to hydrogen embrittlement.

[0097] (b) Therefore, controlling the A value, which is a morphological indicator of plastic deformation in steel, is effective in order to induce twinning in plastic deformation. By controlling the A value, the formation of large-angle grain boundaries can be promoted. Furthermore, the deformation mechanism in plastic deformation is mainly twinning deformation. On the other hand, if elements such as Ni, N, Cu, and Mo are excessively present, the formation of large-angle grain boundaries may be suppressed. Therefore, within the range of the chemical composition of steel, controlling the A value by increasing the content of Mn, which has excellent economic efficiency, is effective.

[0098] (c) As a result, under the operating environment, it becomes a twinning deformation starting from large-angle grain boundaries, which can maintain the large-angle grain boundary structure and also maintain resistance to hydrogen embrittlement. In addition, from the viewpoint of high strength, the Vickers hardness is preferably 250 HV1 or higher. Furthermore, in order to suppress the decrease in elongation in high-pressure hydrogen gas, that is, to ensure resistance to hydrogen embrittlement, the Vickers hardness is preferably set to 500 HV1 or lower.

[0099] (d) To form the aforementioned metallic structure, solution treatment and subsequent cold working are required in the steel manufacturing process. During cold working, in order to efficiently introduce the aforementioned large-angle grain boundaries into the grains and achieve work hardening, it is desirable to perform cold working after performing sub-zero treatment. This is because sub-zero treatment has the effect of suppressing dislocation cross-slip during plastic deformation caused by cold working and promoting deformation twinning.

[0100] This invention is based on the above insights. The elements of one embodiment of this invention will be described in detail below.

[0101] 1. Chemical composition

[0102] The reasons for the limitations of each element are as follows. Furthermore, in the following explanation, "%" regarding content means "mass %".

[0103] C: Below 0.20%

[0104] C is an effective element for stabilizing the austenitic phase, improving resistance to hydrogen embrittlement. However, excessive C content promotes grain boundary precipitation of Cr-based carbides, reducing weather resistance and resistance to hydrogen embrittlement after cold working. Therefore, the C content is set to 0.20% or less. The C content is preferably set to 0.15% or less, and more preferably 0.10% or less. On the other hand, to obtain the above-mentioned effects, the C content is preferably set to 0.01% or more.

[0105] Si: below 2.0%

[0106] Si is an effective deoxidizing element, contributing to improved resistance to hydrogen embrittlement and strength based on solid solution strengthening. However, excessive Si content promotes the formation of intermetallic compounds such as the σ phase, reducing cold workability. Therefore, the Si content is set to 2.0% or less. Preferably, the Si content is set to 1.5% or less, more preferably 1.0% or less, and even more preferably 0.8% or less. On the other hand, to obtain the above-mentioned effects, the Si content is preferably set to 0.1% or more.

[0107] Mn: 6.0~20.0%

[0108] Mn is an element that stabilizes the austenite phase, promotes the formation of deformation twins during cold working and subsequent processes, and contributes to improved strength and resistance to hydrogen embrittlement. Furthermore, by increasing the solid solution limit of N, it indirectly helps to save on expensive Ni. Therefore, the Mn content is set to 6.0% or more. The Mn content is preferably 7.0% or more, more preferably 8.0% or more. However, if Mn is present in excess, it will promote the formation of the ε phase, which is highly susceptible to hydrogen embrittlement, thus reducing resistance to hydrogen embrittlement. Therefore, the Mn content is set to 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 even more preferably 10.0% or less.

[0109] P: below 0.060%

[0110] Phosphorus (P) is an element present in steel as an impurity, but it can accumulate during the final solidification process, lowering the melting point of the steel and thus promoting solidification cracking (high-temperature cracking). Furthermore, P can reduce strength. Additionally, when the P content is high and the finishing cold rolling rate (described later) is high, resistance to hydrogen embrittlement can decrease. Therefore, the P content is set to 0.060% or less. From the perspective of improving resistance to high-temperature cracking, the P content is preferably set to 0.030% or less, more preferably 0.025% or less. On the other hand, excessively reducing P leads to an increase in manufacturing costs; therefore, the P content is preferably set to 0.005% or more.

[0111] S: below 0.0080%

[0112] Sulfur (S) is an element present in steel as an impurity, and like phosphorus (P), it contributes to solidification cracking (high-temperature cracking). Therefore, the S content is set to 0.0080% or less. The S content is preferably set to 0.0030% or less, and more preferably 0.0020% or less. However, excessively reducing the S content will increase manufacturing costs. Therefore, the S content is preferably set to 0.0001% or more.

[0113] Cr: 10.0–18.0%

[0114] Cr is an essential element for improving the corrosion resistance of stainless steel. Therefore, the Cr content is set at 10.0% or more. Preferably, the Cr content is 13.0% or more, and more preferably 14.0% or more. However, Cr is a ferrite-forming element. Therefore, excessive Cr content will destabilize the austenite phase, reducing resistance to hydrogen embrittlement. It will also reduce cold workability. Therefore, the Cr content is set at 18.0% or less. Preferably, the Cr content is 17.0% or less, and more preferably 16.0% or less.

[0115] Ni: 4.0–12.0%

[0116] Similar to Mn, Ni stabilizes the austenite phase, promotes the formation of deformation twins during cold working and subsequent processes, thereby achieving high strength and improved resistance to hydrogen embrittlement. Therefore, the Ni content is set to 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, excessive Ni content inhibits deformation twinning after cold working, leading to a decrease in strength and resistance to hydrogen embrittlement. Therefore, the Ni content is set to 12.0% or less. The Ni content is preferably 10.0% or less, more preferably 9.0% or less.

[0117] N: 0.01~0.30%

[0118] Similar to Mn and Ni, N stabilizes the austenite phase, promotes the formation of deformation twins during cold working and subsequent processes, thereby achieving high strength and improving resistance to hydrogen embrittlement. Therefore, the N content is set to 0.01% or more. To actively utilize the effects of N, the N content is preferably set to 0.10% or more. However, if N is present in excess, internal defects such as porosity may occur, reducing hot and cold workability and resistance to hydrogen embrittlement. Therefore, the N content is set to 0.30% or less. The N content is preferably set to 0.25% or less.

[0119] Cu: below 4.0%

[0120] Cu is an element mixed in from raw materials such as waste to stabilize the austenite phase and improve resistance to hydrogen embrittlement. On the other hand, if Cu is present in excess, in addition to affecting manufacturability, it will also inhibit the formation of deformation twins after cold working, thus reducing strength and resistance to hydrogen embrittlement. Therefore, the Cu content is set to 4.0% or less. The Cu content is preferably set to 3.0% or less, more preferably less than 2.3%, and even more preferably less than 1.5%. Furthermore, the Cu content is even more preferably less than 1.0%, and most preferably less than 0.9%. However, if Cu is reduced excessively, it will lead to the restriction of molten raw materials, increasing manufacturing costs. Therefore, the Cu content is preferably set to 0.01% or more.

[0121] Mo: 3.0% or less

[0122] Mo is an element that is incorporated into raw materials such as waste and is effective in improving resistance to hydrogen embrittlement. On the other hand, if Mo is present in excess, it will promote the formation of the δ-ferrite phase, thus reducing manufacturability. In addition, it will inhibit the formation of deformation twins after cold working, thus reducing strength and resistance to hydrogen embrittlement. Therefore, the Mo content is set to 3.0% or less. The Mo content is preferably set to 2.0% or less, and more preferably to 1.0% or less. However, if Mo is present in excess, it will lead to the restriction of molten raw materials, increasing manufacturing costs. Therefore, the Mo content is preferably set to 0.01% or more.

[0123] Based on the elements mentioned above, the range shown below may further include one or more elements selected from Al, Ca, B, Mg, Nb, Ti, V, W, Zr, Co, Ga, Hf, and REM. The rationale for these limitations will be explained for each element.

[0124] Al: 0–0.20%

[0125] Al is an effective deoxidizing element and has the effect of strengthening grain boundaries by suppressing the segregation of low-melting-point elements at grain boundaries. As a result, it improves manufacturability and cold workability. Therefore, Al can be included as needed. However, if Al is included in excess, AlN will precipitate, which will reduce manufacturability. In addition, it will reduce the solubility of N and reduce resistance to hydrogen embrittlement. Therefore, the Al content is set to 0.20% or less. The Al content is preferably set to 0.10% or less, and more preferably to 0.08% or less. On the other hand, in order to obtain the above-mentioned effects, the Al content is preferably set to 0.01% or more.

[0126] Ca: 0–0.01%

[0127] Ca has the effect of suppressing grain boundary segregation of low-melting-point elements and strengthening grain boundaries. As a result, it improves manufacturability and cold workability. Therefore, Ca can be included as needed. However, if Ca is included in excess, the formation of inclusions will reduce manufacturability and corrosion resistance. Therefore, the Ca content is set to 0.01% or less. The Ca content is preferably set to 0.005% or less. On the other hand, in order to obtain the above-mentioned effects, the Ca content is preferably set to 0.001% or more.

[0128] B: 0-0.01%

[0129] Boron (B) strengthens grain boundaries, increases strength, and improves both hot and cold workability. Therefore, B can be included as needed. However, excessive B content promotes the precipitation of boron compounds (BN, BC, Cr2B) at grain boundaries, reducing workability and corrosion resistance. Therefore, the B content is set to 0.01% or less. Preferably, the B content is 0.005% or less. On the other hand, to achieve the above effects, the B content is preferably 0.0002% or more.

[0130] Mg: 0–0.01%

[0131] Mg is an element with a deoxidizing effect, which improves manufacturability. Therefore, Mg can be included as needed. However, if Mg is present in excess, manufacturability will decrease during refining and other processes, increasing manufacturing costs. Therefore, the Mg content is set to 0.01% or less. The Mg content is preferably set to 0.005% or less. On the other hand, in order to obtain the above-mentioned effect, the Mg content is preferably set to 0.0002% or more, and more preferably 0.0005% or more.

[0132] Nb: 0–1.0%

[0133] Nitrogen (Nb) forms carbonitrides, which refines the grain size and strengthens grain boundaries. This contributes to increased strength. Therefore, Nb can be included as needed. However, excessive Nb content reduces the concentration of dissolved N, leading to decreased resistance to hydrogen embrittlement and weathering, as well as reduced hot and cold workability. Therefore, the Nb content is set to 1.0% or less. Preferably, the Nb content is 0.50% or less. On the other hand, to achieve the above effects, the Nb content is preferably 0.01% or more.

[0134] Ti: 0~1.0%

[0135] Ti forms carbonitrides, which refines the grain size and strengthens grain boundaries. This results in increased strength. Therefore, Ti can be included as needed. However, excessive Ti content reduces the concentration of dissolved nitrogen, decreasing resistance to hydrogen embrittlement and weathering, as well as hot and cold workability. Therefore, the Ti content is set to 1.0% or less. Preferably, the Ti content is 0.50% or less. On the other hand, to achieve the above effects, the Ti content is preferably 0.01% or more.

[0136] V: 0~1.0%

[0137] V, through solid solution in steel or precipitation as carbonitrides, has the effect of increasing strength. Therefore, V can be included as needed. However, if V is present in excess, excessive carbonitride formation will occur, reducing hot workability and cold workability. Therefore, the V content is set to 1.0% or less. The V content is preferably set to 0.50% or less. On the other hand, to obtain the above-mentioned effects, the V content is preferably set to 0.01% or more.

[0138] W: 0–2.0%

[0139] W has the effect of improving strength and weather resistance. Therefore, W can be included as needed. However, if W is included in excess, manufacturing and raw material costs will increase; therefore, the W content is set to 2.0% or less. The W content is preferably set to 1.0% or less. On the other hand, in order to obtain the above-mentioned effects, the W content is preferably set to 0.01% or more.

[0140] Zr: 0~1.0%

[0141] Zr has a deoxidizing effect. Additionally, it improves weather resistance. Therefore, Zr can be included as needed. However, excessive Zr content reduces toughness and processability. Therefore, the Zr content is set to 1.0% or less. Preferably, the Zr content is 0.50% or less. On the other hand, to obtain the above-mentioned effects, the Zr content is preferably 0.01% or more.

[0142] Co: 0-2.0%

[0143] Co has the effect of improving corrosion resistance and stabilizing the austenitic phase. Therefore, Co can be included as needed. However, if Co is included in excess, the manufacturing cost will increase. Therefore, the Co content is set to 2.0% or less. The Co content is preferably set to 1.0% or less. On the other hand, in order to obtain the above-mentioned effects, the Co content is preferably set to 0.01% or more.

[0144] Ga: 0~0.10%

[0145] Ga improves heat workability. Therefore, Ga can be included as needed. However, excessive Ga content reduces manufacturability. Therefore, the Ga content is set to 0.10% or less. The Ga content is preferably set to 0.05% or less. On the other hand, to obtain the above-mentioned effect, the Ga content is preferably set to 0.01% or more.

[0146] Hf: 0~0.10%

[0147] Hf has a deoxidizing effect, which improves weldability. Therefore, Hf can be included as needed. However, if Hf is present in excess, manufacturability will decrease in refining and other aspects. Therefore, the Hf content is set to 0.10% or less. The Hf content is preferably set to 0.05% or less. On the other hand, in order to obtain the above-mentioned effect, the Hf content is preferably set to 0.01% or more.

[0148] REM: 0–0.10%

[0149] REM has a deoxidizing effect, which improves manufacturability. It also improves corrosion resistance. Therefore, REM can be included as needed. However, if REM is present in excess, not only will its effect be saturated, but manufacturability will also decrease in refining and other processes. Therefore, the REM content is set to 0.10% or less. The REM content is preferably set to 0.05% or less. On the other hand, to obtain the above-mentioned effects, the REM content is preferably set to 0.01% or more.

[0150] REM refers to a total of 17 elements, including Sc, Y, and the lanthanides. The REM content mentioned above refers to the total content of these elements. In industry, REM is mostly added in the form of mixed rare earth elements.

[0151] In the chemical composition of this embodiment, the balance is Fe and impurities. Here, "impurities" refers to components that may be mixed in during the industrial manufacturing of austenitic stainless steel materials due to various reasons such as raw materials (ore, waste, etc.) and manufacturing processes, and are acceptable within the range that do not adversely affect this embodiment.

[0152] A value

[0153] In the chemical composition of the steel in this embodiment, the A value, which is an indicator of the stability of the austenite phase and also an indicator of the plastic deformation mechanism, is set to 30.0 to 60.0. Furthermore, the A value is calculated using the following formula (i).

[0154] A value=3.2Mn+0.7Cr+6.2Ni+38.7N+4.8Cu+9.3Mo-53···(i)

[0155] In equation (i) above, the symbols of each element represent the content (mass%) of each element contained in the steel, and are written as zero if the element is not contained.

[0156] When the austenite value is less than 30.0, the stability of the austenite phase is low, leading to the formation of processing-induced martensite. Therefore, not only does the number of face-centered cubic (fcc) grains increase, but the number of body-centered cubic (bcc) grains also increases, resulting in decreased resistance to hydrogen embrittlement. Therefore, the austenite value is set to 30.0 or higher. From the viewpoint of hydrogen embrittlement resistance, the austenite value is preferably 32.0 or higher, and more preferably 35.0 or higher.

[0157] However, when the A value exceeds 60.0, the formation of deformation twins after cold working is suppressed, leading to a decrease in strength and elongation under high-pressure hydrogen conditions. Consequently, resistance to hydrogen embrittlement decreases. Furthermore, it increases raw material costs and reduces manufacturability. Therefore, the A value is set below 60.0. From the viewpoints of strength, economy, and manufacturability after cold working, the A value is preferably set below 50.0.

[0158] 2. Proportion of large-angle grain boundaries

[0159] In the austenitic stainless steel material of this embodiment, the proportion of large-angle grain boundaries that also affect twinning deformation in the metal structure is set within the following range. Specifically, in the surface metal structure, the proportion Gs of large-angle grain boundaries satisfies the following formula (ii). Furthermore, in this embodiment, a large-angle grain boundary refers to a grain boundary that is located in the range of 15 to 60° relative to the orientation difference within the grain.

[0160] 0.1 < Gs < 0.6 ... (ii)

[0161] The symbols in the above formula are defined as follows.

[0162] Gs: Proportion of large-angle grain boundaries

[0163] If 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, strength and elongation decrease under high-pressure hydrogen conditions, and resistance to hydrogen embrittlement decreases. Therefore, Gs is set to be greater than 0.1. From the viewpoint of high strength and resistance to hydrogen embrittlement after cold working, Gs is preferably set to 0.20 or more, more preferably 0.30 or more. On the other hand, if Gs is 0.6 or more, the occurrence of deformation twins after cold working can be suppressed. As a result, strength and elongation decrease under high-pressure hydrogen conditions. In addition, after cold working, the formation of processing-induced martensite starting from large-angle grain boundaries is induced, which also leads to a decrease in resistance to hydrogen embrittlement. Therefore, Gs is set to be less than 0.6. From the viewpoint of high strength and resistance to hydrogen embrittlement after cold working, Gs is preferably set to 0.5 or less.

[0164] The aforementioned Gs can be measured using a crystal orientation analysis system employing EBSP. Specifically, the total thickness in the direction perpendicular to the cold-worked surface (rolled surface) is defined as t. A surface parallel to the work surface (rolled surface) is selected within a range from its surface up to a position t / 4, and this surface is mirror-polished. Preferably, this surface is used as the observation surface, and EBSP measurements are performed using glow discharge luminescence analysis (GDS). Alternatively, for example, measurements can be performed on a surface at position t / 8 that is parallel to the work surface (rolled surface).

[0165] For example, regarding the conditions for GDS, it is preferable to set the output to a pulsed sputtering mode of 20W and 600Pa pressure when measuring at position t / 8 and on a surface parallel to the processing surface (rolled surface), so that the sputtering depth is 0.1μm or less. By using GDS, the measurement accuracy of EBSP can be maintained even in cold-worked steel. The EBSP measurement preferably covers an area of ​​45μm × 130μm. It is preferable to set the observation magnification to 2000, and Gs should meet the range of this embodiment in at least one field of view. Furthermore, Gs can be calculated by using the area fraction method with the orientation difference relative to the grain within the grain set to 15 to 60° in the grain boundary diagram display of the crystal orientation analysis system. For Gs, it is desirable to use a value of the observation field with an average CI value (confidence index) exceeding 0.2.

[0166] 3. Vickers hardness

[0167] The austenitic stainless steel material in this embodiment undergoes work hardening, resulting in increased hardness. Therefore, a Vickers hardness of 250–500 HV1 is preferably set. If the Vickers hardness is less than 250 HV1, it is difficult to obtain the desired strength of 800 MPa or higher. Therefore, a Vickers hardness of 250 HV1 or higher is preferred. From the viewpoint of strength and hydrogen embrittlement, a Vickers hardness of 300 HV1 is more preferable. On the other hand, when the Vickers hardness exceeds 500, the tensile strength easily exceeds 1700 MPa, and the elongation of the material itself decreases significantly. As a result, it is difficult to maintain elongation under high-pressure hydrogen conditions.

[0168] Furthermore, when the proportion of austenite phase (face-centered cubic: fcc) decreases due to the formation of work-induced martensite (body-centered cubic: bcc) during cold working and subsequent plastic deformation, it leads to a decrease in resistance to hydrogen embrittlement. Therefore, a Vickers hardness of 500 HV1 or less is preferably set. From the viewpoint of resistance to hydrogen embrittlement, a Vickers hardness of 450 HV1 or less is more preferably set.

[0169] Furthermore, Vickers hardness can be measured according to JIS Z 2244:2020 under the following conditions. Specifically, a resin specimen with a cross-section is embedded, a load of 1 kg (9.8 N) is applied at approximately t / 2, and the measurement is performed using a Vickers hardness tester. Additionally, the load is applied for 20 seconds.

[0170] 4. Crystal Structure

[0171] In the austenitic stainless steel material of this embodiment, in order to suppress the decrease in resistance to hydrogen embrittlement, it is desirable to suppress the formation of processing-induced martensite after processing. Therefore, in the crystal structure, an fcc structure with an area ratio of 97% or more is preferred.

[0172] If the area fraction of grains with an fcc structure is less than 97%, a large amount of δ-ferrite with a bcc structure and process-induced martensite will be generated, which is undesirable from the viewpoint of hydrogen embrittlement resistance. Therefore, in terms of area fraction, an fcc structure of 97% or more is preferred. In addition, δ-ferrite may sometimes form partially and unavoidably, but even in such cases, if the area fraction of grains with a bcc structure is less than 3%, the impact on hydrogen embrittlement resistance is small and therefore acceptable.

[0173] For crystal structures, the following steps can be taken for measurement. Specifically, X-ray diffraction is performed on the measurement plane of the aforementioned EBSP, and the grain area ratios of the fcc structure and the bcc structure are calculated. The X-ray diffraction conditions can be set as follows: CuKα rays are used, the applied voltage is set to 40 kV, and 2θ is set within the range of 30° ≤ 2θ ≤ 90°.

[0174] 5. Applications

[0175] The austenitic stainless steel material of this embodiment is intended for use in hydrogen processing equipment that requires lightweight and compact design for high strength under high-pressure hydrogen environments. Examples of hydrogen processing equipment include, for instance, the main body of a tank for storing gaseous and liquid hydrogen, the tank's inlet, inner liner, piping that forms the hydrogen flow path, valves, and heat exchangers.

[0176] 6. Manufacturing method

[0177] This section describes a preferred manufacturing method for the austenitic stainless steel material of this embodiment. As long as the austenitic stainless steel material of this embodiment has the above-described structure, its effects can be obtained regardless of the manufacturing method. For example, the austenitic stainless steel material of this embodiment can be stably manufactured using the following manufacturing method.

[0178] In the following description, for simplicity, the shape of the steel is described as a steel plate, but the shape of the steel is not particularly limited. For example, shapes such as plate, rod, or tube are conceivable. Simply choose the processing method corresponding to each shape.

[0179] Steel with the above-mentioned chemical composition is smelted and cast using conventional methods to obtain steel sheets for hot rolling. These sheets are then hot rolled using conventional methods. The conditions for hot rolling are not particularly limited; generally, it is preferred that the heating temperature of the steel sheets be set to 1050–1250°C and the rolling rate to be in the range of 20–99%. After hot rolling, annealing and pickling may be performed as needed. The annealing temperature is not particularly limited in this case; for example, it can be set to the range of 1050–1100°C.

[0180] Next, cold rolling and annealing are performed as needed. Furthermore, the annealing preceding the final finishing cold rolling is called solution treatment, and this is also performed. Additionally, pickling is performed as needed. For example, it is preferable to perform cold rolling with a rolling rate in the range of 20-90%, followed by isothermal holding at 1000-1150°C for 1-600 seconds for subsequent annealing. To obtain steel of the desired final thickness, cold rolling, annealing, and pickling can be repeated multiple times.

[0181] Solution treatment is performed before the final cold rolling for work hardening. During solution treatment, it is preferably performed isothermally at 1000–1150°C for 1–600 seconds. When the solution treatment temperature is below 1000°C, recrystallization becomes insufficient, and sufficient large-angle grain boundaries resulting from twinning deformation cannot be introduced during the final cold rolling; therefore, Gs is unlikely to satisfy equation (ii). Therefore, the solution treatment temperature is preferably set to 1000°C or higher. On the other hand, when the solution treatment temperature exceeds 1150°C, the grains coarsen (exceeding 0.1 mm), and similarly, after cold working, Gs is unlikely to satisfy equation (ii). Therefore, the solution treatment temperature is preferably set to 1150°C or lower.

[0182] Furthermore, immediately after the solution treatment described above, a sub-zero treatment is performed. This allows for the effective and efficient introduction of large-angle grain boundaries into the grains during the subsequent final finishing cold rolling, thereby increasing the Gs value. Therefore, a sub-zero treatment is preferred. Sub-zero treatment refers to a quenching process where the temperature is rapidly reduced from the solution treatment temperature to below 0°C. During quenching, it is preferable to perform a liquid or gaseous method based on JIS standard HSZ, using dry ice, liquid nitrogen, or carbon dioxide gas as the coolant.

[0183] Finally, a final finishing cold rolling process is performed to form work-hardened steel. By performing final finishing cold rolling after solution treatment or a subsequent sub-zero treatment, the value of Gs is increased, resulting in good strength. Therefore, after solution treatment or sub-zero treatment, final finishing cold rolling is preferably performed within a rolling rate range of 10% to 80%. This is because if the rolling rate is less than 10%, sufficient hardness and strength cannot be obtained. Furthermore, it is difficult to increase the value of Gs. Therefore, the final finishing cold rolling rate is preferably set to 10% or more.

[0184] On the other hand, when the rolling rate exceeds 80%, the strength increases excessively, while the resistance to hydrogen embrittlement decreases. Furthermore, it becomes difficult to obtain the desired Gs value. Therefore, the final finishing cold rolling rate is preferably set to 80% or less. To achieve a hardness in the range of 300–450 HV1, the final finishing cold rolling rate is preferably set to a range of 20–70%. Furthermore, for steel plates, examples are given; for instance, if it is bar or tubular steel, it is sufficient to set the reduction of area to a range of 10–80%. That is, it is sufficient to adjust the cold working rate to a range of 10–80%. Additionally, it is sufficient to control the metal structure to be an austenitic stainless steel by adjusting the above-mentioned range and other conditions.

[0185] The following examples illustrate the austenitic stainless steel material of this embodiment in more detail, but this embodiment is not limited to these examples.

[0186] Example

[0187] The slabs with the composition shown in Table 1 were melted, heated to 1200°C, and then hot-rolled to produce a hot-rolled sheet with a thickness of 5.0 mm. Next, the hot-rolled sheet was annealed at 1050–1100°C and pickled, then cold-rolled to a thickness of 2 mm to form a cold-rolled sheet. The cold-rolled sheet was then solution-treated at 1050–1100°C for 30 seconds, followed by pickling. Furthermore, a portion of the steel sheets underwent a sub-zero treatment. In the sub-zero treatment, dry ice and ethanol were used as coolants in the liquid method described above to rapidly cool from the solution treatment temperature to -10°C. Afterwards, regardless of whether a sub-zero treatment was performed, the resulting steel sheets were subjected to a final finishing cold rolling within the range of 10–80% to form sample materials (austenitic stainless steel sheets) with a thickness of 0.4–1.8 mm.

[0188] [Table 1]

[0189]

[0190] For the obtained sample material, the calculation of Gs, hardness, crystal structure and hydrogen embrittlement resistance were checked by the following steps.

[0191] (Calculation of Gs)

[0192] Next, for the sample material, at position t / 8 and parallel to the rolling plane, the sample was adjusted using the method described above, and Gs was determined using the crystal orientation analysis system of the EBSP. The observation magnification of the EBSP was set to 2000, and Gs was set to a value obtained from a field of view with a CI value ≥ 0.2. Gs was measured under the above conditions.

[0193] (Measurement of hardness)

[0194] In addition, for the sample material, resin was embedded in the prepared cross-section of the sample, and a load of 1 kg (9.8 N) was set at approximately t / 2. Hardness tests were conducted based on JIS Z 2244:2020. A Vickers hardness tester was used in the test, and the holding time of the test force was set to 20 seconds.

[0195] (Identification of crystal structure)

[0196] For the crystal structures, X-ray diffraction was performed on the measurement plane of the EBSP to calculate the grain area ratio of the fcc structure and the bcc structure. The X-ray diffraction conditions were set as follows: CuKα rays were used, the applied voltage was set to 40 kV, and 2θ was set in the range of 30° ≤ 2θ ≤ 90°. Furthermore, in the results in Table 2, samples with a grain area ratio exceeding 3% for the bcc structure were recorded as "present".

[0197] (Evaluation of resistance to hydrogen embrittlement)

[0198] For hydrogen embrittlement resistance, measurements were performed using the following procedure. Tensile test specimens with a parallel section width of 4 mm and a length of 20 mm were collected. Next, these tensile test specimens were subjected to a strain rate of 10 at -40°C, 70 MPa hydrogen, and 0.1 MPa nitrogen. -5 A low strain rate tensile test (hereinafter referred to as the "SSRT test") was conducted. In the evaluation of the SSRT test, tensile strength at break and elongation at break were measured. Specifically, resistance to hydrogen embrittlement was evaluated using values ​​calculated using the following formula.

[0199] The evaluation value of hydrogen embrittlement resistance to tensile fracture strength = (tensile fracture strength in 70 MPa hydrogen) / (tensile fracture strength in 0.1 MPa nitrogen) × 100 (%) ... (a)

[0200] The hydrogen embrittlement resistance evaluation value of tensile elongation at break = (tensile elongation at break in 70 MPa hydrogen) / (tensile elongation at break in 0.1 MPa nitrogen) × 100 (%) ···(b)

[0201] A tensile fracture strength with a hydrogen embrittlement resistance rating of 95% or higher and an elongation at break with a hydrogen embrittlement resistance rating of 85% or higher, calculated using the above formula, are considered to have good hydrogen embrittlement resistance and are denoted as "Good". Conversely, a hydrogen embrittlement resistance rating that does not meet the above values ​​is considered to have poor hydrogen embrittlement resistance and is denoted as "Poor". Furthermore, a tensile fracture strength with a hydrogen embrittlement resistance rating of 95% or higher and an elongation at break with a hydrogen embrittlement resistance rating of 100% or higher are considered to have exceptionally excellent hydrogen embrittlement resistance and are denoted as "Excellent". The results are summarized in Table 2 below.

[0202] [Table 2]

[0203] Table 2

[0204]

[0205] * indicates a deviation from the requirements of this implementation method.

[0206] **Refers to deviations from the preferred scope of this embodiment.

[0207] Underlined: refers to manufacturing conditions or characteristics that deviate from the preferred embodiments of this invention.

[0208] Sample materials No. 1 to 12, which meet the chemical composition, A value, and Gs of this embodiment, achieved the target hydrogen embrittlement resistance. In particular, Nos. 1 to 4, 7, 11, and 12 have the preferred hardness and crystal structure of this embodiment, and their hydrogen embrittlement resistance is "excellent". Nos. 1 to 4, which have "excellent" hydrogen embrittlement resistance, meet the preferred range of chemical composition and A value of this embodiment. Although Nos. 7, 11, and 12 deviate from the preferred chemical composition and A value of this embodiment, their hydrogen embrittlement resistance is increased to "excellent" by improving Gs through sub-zero treatment. Furthermore, No. 9 has a high P content, and although it deviates from the preferred chemical composition of this embodiment, its low finishing cold rolling rate maintains "excellent" hydrogen embrittlement resistance, while its hardness is reduced. Similarly to No. 9, No. 10 has a relatively high P content and a relatively high finishing cold rolling rate, thus increasing its hardness, but its hydrogen embrittlement resistance is "good".

[0209] On the other hand, Nos. 13 to 18 do not meet the requirements of this embodiment in terms of chemical composition, and Nos. 19 and 20 do not meet the requirements of this embodiment in terms of chemical composition, especially in terms of the A value. Therefore, even with sub-zero treatment, the resistance to hydrogen embrittlement is reduced. In addition, Nos. 21 and 22 do not meet the preferred range of finishing cold rolling rate, so the value of Gs increases or decreases excessively, and the resistance to hydrogen embrittlement is also reduced.

[0210] (Postscript)

[0211] (1) An austenitic stainless steel material, wherein the chemical composition of the austenitic stainless steel material, expressed in mass percent, is as follows:

[0212] C: Below 0.20%

[0213] Si: below 2.0%

[0214] Mn: 6.0~20.0%

[0215] P: below 0.060%

[0216] S: below 0.0080%

[0217] Cr: 10.0–18.0%

[0218] Ni: 4.0–12.0%

[0219] N: 0.01~0.30%

[0220] Cu: below 4.0%

[0221] Mo: 3.0% or less,

[0222] Al: 0-0.20%

[0223] Ca: 0–0.01%

[0224] B: 0-0.01%

[0225] Mg: 0–0.01%

[0226] Nb: 0–1.0%

[0227] Ti: 0-1.0%

[0228] V: 0~1.0%

[0229] W: 0–2.0%

[0230] Zr: 0~1.0%

[0231] Co: 0-2.0%

[0232] Ga: 0-0.10%

[0233] Hf: 0~0.10%

[0234] REM: 0–0.10%

[0235] Balance: Fe and impurities,

[0236] The value of A calculated by equation (i) below is 30.0 to 60.0.

[0237] In the surface metallic microstructure, the proportion of large-angle grain boundaries, Gs, satisfies the following equation (ii).

[0238] A value=3.2Mn+0.7Cr+6.2Ni+38.7N+4.8Cu+9.3Mo-53···(i)

[0239] 0.1 < Gs < 0.6 ... (ii)

[0240] In equation (i) above, each element symbol represents the content (mass%) of each element contained in the steel, and is written as zero if the element is not present. The symbols in the above equation are defined as follows.

[0241] Gs: The proportion of large-angle grain boundaries.

[0242] (2) According to the austenitic stainless steel material described in (1) above, wherein,

[0243] Its Vickers hardness is 250-500 HV1, and in its crystal structure, more than 97% of it is of the fcc structure in terms of area ratio.

[0244] (3) The austenitic stainless steel material according to (1) or (2) above, wherein,

[0245] The chemical composition, expressed as a percentage by mass, contains one or more elements selected from the following elements.

[0246] Al: 0.01–0.20%

[0247] Ca: 0.001–0.01%

[0248] B: 0.0002~0.01%

[0249] Mg: 0.0002~0.01%

[0250] Nb: 0.01~1.0%

[0251] Ti: 0.01~1.0%

[0252] V: 0.01~1.0%

[0253] W: 0.01~2.0%

[0254] Zr: 0.01~1.0%

[0255] Co: 0.01~2.0%

[0256] Ga: 0.01~0.10%

[0257] Hf: 0.01~0.10%

[0258] REM: 0.01–0.10%.

[0259] (4) The austenitic stainless steel material according to any one of (1) to (3) above, wherein the austenitic stainless steel material is used in a high-pressure hydrogen environment.

[0260] (5) A method for manufacturing an austenitic stainless steel material, wherein the austenitic stainless steel material described in any one of (1) to (4) above is manufactured by the method, wherein,

[0261] The manufacturing method of this austenitic stainless steel material has the following characteristics:

[0262] The process of solution treatment;

[0263] The process of performing sub-zero processing; and

[0264] The process of cold processing.

[0265] (6) A hydrogen processing device, wherein the hydrogen processing device comprises any one of the austenitic stainless steel materials described in (1) to (4) above.

[0266] (7) The hydrogen equipment according to (6) above, wherein the hydrogen equipment is the main body of the tank, the pipe head of the tank, the inner liner, the piping, the valve or the heat exchanger.

[0267] Industrial availability

[0268] The austenitic stainless steel material of this embodiment possesses both high strength and resistance to hydrogen embrittlement, making it suitable for use as steel plates, bars, and tubes for cold working in high-pressure hydrogen environments. Furthermore, it is desirable for application in hydrogen-related equipment requiring high strength, lightweight design, and compactness. The steel of this embodiment contributes to high strength through cold working, and consequently, thinner walls and lighter weight for hydrogen-related equipment and components.

Claims

1. An austenitic stainless steel material, wherein, The chemical composition of this austenitic stainless steel material is expressed in % by mass. C: Below 0.20% Si: below 2.0% Mn: 6.0~20.0%, P: below 0.060% S: Below 0.0080% Cr：10.0~18.0%、 Ni: 4.0~12.0% N：0.01~0.30%、 Cu: below 4.0% 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%, Balance: Fe and impurities, The value of A calculated by equation (i) below is 30.0~60.

0. In the surface metallic microstructure, the proportion of large-angle grain boundaries, Gs, satisfies the following equation (ii). The term "large-angle grain boundary" refers to a grain boundary with an orientation difference of 15-60° relative to the grain. A value=3.2Mn+0.7Cr+6.2Ni+38.7N+4.8Cu+9.3Mo-53···(i) 0.1 < Gs < 0.6 ···(ii) In equation (i) above, each element symbol represents the content of each element contained in the steel, and is written as zero if the element is not present. The unit of the content of each element is mass%, and each symbol in the above equation is defined as follows. Gs: The proportion of large-angle grain boundaries; Gs is calculated using the area fraction method, where the orientation difference relative to the grain is set to 15–60° in the grain boundary diagram display of the crystal orientation analysis system.

2. The austenitic stainless steel material according to claim 1, wherein, The Vickers hardness is 250~500 HV1. In crystal structures, those with an area ratio of over 97% are considered fcc structures.

3. The austenitic stainless steel material according to claim 1, wherein, The chemical composition, expressed as a percentage by mass, contains one or more elements selected from the following elements. 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% REM: 0.01~0.10%.

4. The austenitic stainless steel material according to claim 2, wherein, The chemical composition, expressed as a percentage by mass, contains one or more elements selected from the following elements. 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% REM: 0.01~0.10%.

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

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

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

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

9. A method for manufacturing an austenitic stainless steel material, wherein the austenitic stainless steel material according to any one of claims 1 to 8 is manufactured by the method, wherein... The manufacturing method of this austenitic stainless steel material has the following characteristics: The process of solution treatment; The process of performing sub-zero processing; and The process of cold processing.

10. A hydrogen processing device, wherein, The hydrogen-using equipment comprises austenitic stainless steel material as described in any one of claims 1 to 8.

11. The hydrogen processing equipment according to claim 10, wherein, The hydrogen-using equipment includes the main body of the tank, the tank head, the inner liner, piping, valves, or heat exchangers.

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