Stainless steel material

Abstract

To provide a stainless steel material that has excellent overall corrosion resistance and pitting corrosion resistance in supercritical corrosion environments, and excellent low-temperature toughness in cryogenic environments. [Solution] The stainless steel material according to this disclosure has the chemical composition described in the specification, satisfies formula (1), has a yield strength of 758 MPa or less, and its microstructure consists of retained austenite of 5-30% by volume, ferrite of 0-5.0%, and the remainder being martensite, and the elemental content, yield strength, and volume fraction of the microstructure satisfy formula (2). 0.10 ≤ Cu + W ≤ 2.20 (1) 0.01YS×(Ni+12Co) / (Vγ+3.3Vα′)≧0.0036 (2) In equations (1) and (2), the elemental symbols are substituted with the content of the corresponding element in units of mass%, in equation (2), YS is substituted with the yield strength in units of MPa, Vγ is substituted with the volume fraction of retained austenite, and Vα′ is substituted with the volume fraction of martensite in units of %.

Description

Technical Field

[0001] The present disclosure relates to stainless steel materials.

Background Art

[0002] Currently, the increasing concentration of carbon dioxide (CO2) on the ground has become a global problem. Therefore, efforts have been made to suppress CO2 emissions. Among such efforts to suppress CO2 emissions, in particular, CCUS has been attracting attention.

[0003] CCUS is an abbreviation for Carbon dioxide Capture, Utilization and Storage. That is, CCUS includes three technologies: CO2 recovery, utilization, and storage. Among these, as a technology for storing CO2, a technology for recovering CO2 emitted from industrial facilities such as power plants and factories and injecting and storing CO2 into depleted oil wells has been attracting attention.

[0004] Stainless steel materials used in such CO2 storage technologies are required to have excellent corrosion resistance. So far, stainless steel materials having excellent corrosion resistance have been proposed in JP-A-2011-190521 (Patent Document 1) and JP-A-2012-149317 (Patent Document 2).

[0005] The stainless steel material described in Patent Document 1 is a martensitic stainless steel material, with the following composition in mass%, C: 0.003-0.03%, Si: 0.01-1.0%, Mn: 3.0-6.0%, P: 0.05% or less, S: 0.003% or less, Ni: 1.0-3.0%, Cr: 15.0-18.0%, Mo: 0-1.0%, Cu: 0-2.0%, Ti: 0-0.05%, N: 0.05% or less, Al: 0.001-0.1%, O: 0.005% or less, and C+N: 0.0 It is characterized by containing 60% or less of [material], with the remainder being Fe and impurities, and having a γmax (=420×C%+470×N%+23×Ni%+9×Cu%+7×Mn%-11.5×Cr%-11.5×Si%-52×Al%+189) of 80 or more, and a γpot (=700×C%+800×N%+10×(Mn%+Cu%)+20×Ni%-9.3×Si%-6.2×Cr%-9.3×Mo%-74.4×Ti%-37.2×Al%+63.2) of 60 to 90. Patent Document 1 discloses that this martensitic stainless steel material has excellent mechanical properties and corrosion resistance in the welded joint.

[0006] The stainless steel material described in Patent Document 2 is a high-strength martensitic stainless steel seamless pipe for oil wells, containing, by mass%, C: 0.01% or less, Si: 0.5% or less, Mn: 0.1-2.0%, P: 0.03% or less, S: 0.005% or less, Cr: greater than 15.5% to 17.5%, Ni: 2.5-5.5%, Mo: 1.8-3.5%, Cu: 0.3-3.5%, V: 0.20% or less, Al: 0.05% or less, and N: 0.06% or less, with the remainder being Fe and impurities, having a yield strength of 655-862 MPa and a yield ratio of 0.90 or more, and exhibiting excellent resistance to carbon dioxide corrosion and sulfide pitting corrosion. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2011-190521 [Patent Document 2] Japanese Patent Publication No. 2012-149317 [Overview of the project] [Problems that the invention aims to solve]

[0008] Incidentally, in the carbon dioxide (CO2) storage technology mentioned above, in order to inject CO2 into depleted oil wells, the CO2 injected into the steel pipe is compressed and pressurized to bring the CO2 to a supercritical state. On the other hand, CO2 recovered from industrial facilities such as power plants and factories may contain SOx and O2. Here, SOx is a general term for sulfur oxides represented by SO2. SOx dissolves in water to form acidic compounds (such as sulfuric acid and sulfurous acid), causing general corrosion and pitting corrosion on the surface of steel materials.

[0009] Therefore, supercritical CO2 containing SOx and O2 forms an extremely severe corrosive environment. In this specification, the corrosive environment formed by supercritical CO2 containing SOx and O2 is referred to as a "supercritical corrosion environment." In other words, steel materials used in a supercritical corrosion environment require even better overall corrosion resistance and pitting corrosion resistance than those used in conventional corrosion environments.

[0010] In recent years, steel materials are sometimes required to have toughness in extremely low-temperature environments when storing CO2. Specifically, when the pressure of the stored carbon dioxide gas changes, the temperature of the stored gas may decrease due to the Joule-Thomson effect. In this case, steel materials may be required to have toughness in extremely low-temperature environments of -80°C, far below normal temperatures.

[0011] Therefore, stainless steel materials intended for use in CO2 storage technologies in such cryogenic environments require not only high strength and excellent resistance to overall corrosion and pitting corrosion in supercritical corrosion environments, but also excellent low-temperature toughness in cryogenic environments below -80°C. On the other hand, the stainless steel materials disclosed in Patent Documents 1 and 2 are not intended for use in such supercritical corrosion environments or cryogenic environments.

[0012] An object of the present disclosure is to provide a stainless steel material having excellent general corrosion resistance and pitting corrosion resistance in a supercritical corrosion environment and excellent low-temperature toughness in an extremely low-temperature environment.

Means for Solving the Problems

[0013] The stainless steel material according to the present disclosure is by mass%, C: 0.050% or less, Si: 1.00% or less, Mn: 1.00% or less, P: 0.050% or less, S: 0.0050% or less, Cr: 13.50 to less than 16.50%, Ni: 4.50 to 9.00%, Mo: 1.00 to 5.00%, Cu: 0.01 to 2.00%, Co: 0.01 to 1.00%, N: 0.200% or less, sol.Al: 0.001 to 0.100%, O: 0.020% or less, W: 0 to 2.00%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, B: 0 to 0.0050%, Rare earth element: 0 to 0.30%, V: 0 to 0.50%, Ti: 0 to 0.30%, Nb: 0 to 0.30%, Zr: 0 to 0.30%, Ta: 0 to 0.300%, Sn: 0 to 0.0100%, As: 0 to 0.0100%, Zn: 0 to 0.0100%, Pb: 0 to 0.0100%, Sb: 0 to 0.0100%, and the balance consists of Fe and impurities, satisfies formula (1), the yield strength is 758 MPa or less, The microstructure consists of 5 to 30% retained austenite, 0 to 5.0% ferrite, and the balance martensite by volume ratio, The content of the element, the yield strength, and the volume ratio of the microstructure satisfy formula (2). 0.10 ≦ Cu + W ≦ 2.20 (1) 0.01YS × (Ni + 12Co) / (Vγ + 3.3Vα′) ≧ 0.0036 (2) Here, in formula (1) and (2), the element symbols are substituted with the content of the corresponding element in units of mass%. When the corresponding element is not contained, "0" is substituted for the element symbol. In YS in formula (2), the yield strength is substituted in units of MPa. In Vγ in formula (2), the volume ratio of retained austenite in the microstructure is substituted in units of %, and in Vα′ in formula (2), the volume ratio of martensite in the microstructure is substituted in units of %.

Effect of the Invention

[0014] The stainless steel material according to the present disclosure has excellent general corrosion resistance and pitting corrosion resistance in a supercritical corrosion environment, and excellent low-temperature toughness in an extremely low-temperature environment.

Brief Description of the Drawings

[0015] [Figure 1] FIG. 1 is a diagram showing the relationship between Fn2 (= 0.01YS × (Ni + 12Co) / (Vγ + 3.3Vα′)) and the absorbed energy (J) at -80°C, which is an index of low-temperature toughness in an extremely low-temperature environment, in this example.

Mode for Carrying Out the Invention

[0016] The inventors first investigated, from the perspective of chemical composition, stainless steel materials that possess excellent overall corrosion resistance and pitting corrosion resistance in supercritical corrosion environments, as well as excellent low-temperature toughness in cryogenic environments, with an eye toward application to CO2 storage technology. As a result, in mass%, C: 0.050% or less, Si: 1.00% or less, Mn: 1.00% or less, P: 0.050% or less, S: 0.0050% or less, Cr: 13.50~16.50% or less, Ni: 4.50~9.00%, Mo: 1.00~5.00%, Cu: 0.01~2.00%, Co: 0.01~1.00%, N: 0.200% or less, sol.Al: 0.001~0.100%, O: 0.020% or less, W: 0~2.00%, Ca: 0~0.0100%, Mg: 0~0.0100%, B: 0~0.0050%, The inventors of this invention believe that a stainless steel material consisting of rare earth elements (0-0.30%), V (0-0.50%), Ti (0-0.30%), Nb (0-0.30%), Zr (0-0.30%), Ta (0-0.30%), Sn (0-0.0100%), As (0-0.0100%), Zn (0-0.0100%), Pb (0-0.0100%), Sb (0-0.0100%), and the remainder being Fe and impurities, may possess excellent overall corrosion resistance and pitting corrosion resistance in supercritical corrosion environments, as well as excellent low-temperature toughness in cryogenic environments.

[0017] The inventors further considered that, assuming the chemical composition described above, the corrosion resistance could be improved while simultaneously increasing low-temperature toughness by satisfying the following formula (1). 0.10 ≤ Cu + W ≤ 2.20 (1) Here, the elemental symbols in equation (1) are substituted with the content of the corresponding element in units of mass%. If the corresponding element is not present, "0" is substituted for that elemental symbol.

[0018] Fn1 is defined as Cu + W. Fn1 is an indicator of the balance between corrosion resistance and low-temperature toughness. If Fn1 is too low, sufficient corrosion resistance cannot be obtained. On the other hand, if Fn1 is too high, corrosion resistance can be obtained, but sufficient low-temperature toughness cannot be obtained. Therefore, assuming that the stainless steel material according to this embodiment has the above chemical composition, Fn1 is set to 0.10 to 2.20. As a result, assuming that the other configurations of this embodiment are satisfied, a stainless steel material is obtained that has excellent overall corrosion resistance and pitting corrosion resistance in supercritical corrosion environments, and excellent low-temperature toughness in cryogenic environments.

[0019] Next, the inventors focused on the microstructure to investigate how to obtain a desired stainless steel material. Here, the microstructure of the stainless steel material having the above-mentioned chemical composition consists of retained austenite, ferrite, and the remainder being martensite. As a result of the inventors' investigations, it became clear that a microstructure consisting of retained austenite at a volume fraction of 5-30%, ferrite at 0-5.0%, and the remainder being martensite has the potential to provide excellent overall corrosion resistance and pitting corrosion resistance in supercritical corrosion environments, as well as excellent low-temperature toughness in cryogenic environments.

[0020] Here, if the yield strength of the stainless steel is too high, its low-temperature toughness tends to decrease. Therefore, in this embodiment, the yield strength is set to 758 MPa or less. In other words, for a stainless steel having the above-mentioned chemical composition and microstructure and satisfying Fn1 of 0.10 to 2.20, if the yield strength is 758 MPa or less, it is possible to stably achieve both excellent overall corrosion resistance and pitting corrosion resistance in supercritical corrosion environments and excellent low-temperature toughness in cryogenic environments, provided that the other configurations of this embodiment are also satisfied.

[0021] On the other hand, even stainless steel materials having the above-mentioned chemical composition and microstructure, satisfying Fn1 of 0.10 to 2.20, and having a yield strength of 758 MPa or less, sometimes failed to exhibit excellent low-temperature toughness in cryogenic environments. Therefore, the inventors investigated methods to improve the excellent low-temperature toughness in cryogenic environments for stainless steel materials having the above-mentioned chemical composition and microstructure, satisfying Fn1 of 0.10 to 2.20, and having a yield strength of 758 MPa or less. As a result, it became clear that for stainless steel materials having the above-mentioned chemical composition and microstructure, satisfying Fn1 of 0.10 to 2.20, and having a yield strength of 758 MPa or less, excellent low-temperature toughness can be obtained even in cryogenic environments if the element content, yield strength, and volume fraction of the microstructure satisfy the following equation (2). 0.01YS×(Ni+12Co) / (Vγ+3.3Vα′)≧0.0036 (2) Here, the elemental symbols in equation (2) are substituted with the content of the corresponding element in units of mass%. In equation (2), YS is substituted with the yield strength in units of MPa. In equation (2), Vγ is substituted with the volume fraction of retained austenite in the microstructure in units of %, and Vα′ in equation (2) is substituted with the volume fraction of martensite in the microstructure in units of %.

[0022] Fn2 is defined as Fn2 = 0.01YS × (Ni + 12Co) / (Vγ + 3.3Vα′). Fn2 is an index of low-temperature toughness in an extremely low-temperature environment for stainless steel materials having the above-described chemical composition and microstructure, and satisfying Fn1 between 0.10 and 2.20. The relationship between Fn2 and low-temperature toughness will be explained in detail using the drawings. Figure 1 shows the relationship between Fn2 and the absorbed energy (J) at -80°C, which is an index of low-temperature toughness in an extremely low-temperature environment, in this embodiment. Figure 1 was created using Fn2 and the absorbed energy (J) at -80°C for an embodiment among the embodiments described later, in which the configuration other than Fn2 satisfies the conditions of this embodiment.

[0023] Referring to Figure 1, it can be confirmed that if Fn2 is 0.0036 or higher, the absorbed energy at -80°C will be 100 J or higher, and excellent low-temperature toughness can be obtained even in an extremely low-temperature environment. Therefore, the stainless steel material according to this embodiment has the above-mentioned chemical composition, satisfies Fn1 of 0.10 to 2.20, has a microstructure consisting of 5 to 30% retained austenite by volume fraction, 0 to 5.0% ferrite, and the remainder being martensite, has a yield strength of 758 MPa or less, and furthermore, Fn2 is 0.0036 or higher. As a result, the stainless steel material according to this embodiment can achieve both excellent overall corrosion resistance and pitting corrosion resistance in a supercritical corrosion environment and excellent low-temperature toughness in an extremely low-temperature environment.

[0024] Based on the above findings, the gist of the stainless steel material according to this embodiment is as follows:

[0025] [1] In mass%, C: 0.050% or less, Si: 1.00% or less, Mn: 1.00% or less, P: 0.050% or less, S: 0.0050% or less, Cr: 13.50-16.50% Ni: 4.50~9.00%, Mo: 1.00~5.00%, Cu: 0.01~2.00%, Co: 0.01~1.00%, N: 0.200% or less, sol.Al: 0.001~0.100%, O: 0.020% or less, W: 0~2.00%, Ca: 0~0.0100%, Mg: 0~0.0100%, B: 0~0.0050%, Rare earth elements: 0~0.30%, V: 0~0.50%, Ti: 0~0.30%, Nb: 0~0.30%, Zr: 0~0.30%, Ta: 0~0.300%, Sn: 0~0.0100%, As: 0~0.0100%, Zn: 0~0.0100%, Pb: 0~0.0100%, Sb: 0~0.0100%, and, The remainder consists of Fe and impurities. Satisfying equation (1), The yield strength is 758 MPa or less. The microstructure consists of 5-30% retained austenite by volume, 0-5.0% ferrite, and the remainder being martensite. The content of the element, the yield strength, and the volume fraction of the microstructure satisfy formula (2). Stainless steel material. 0.10 ≤ Cu + W ≤ 2.20 (1) 0.01YS×(Ni+12Co) / (Vγ+3.3Vα′)≧0.0036 (2) Here, the elemental symbols in equations (1) and (2) are substituted with the content of the corresponding element in units of mass%. If the corresponding element is not present, "0" is substituted for that elemental symbol. In equation (2), YS is substituted with the yield strength in units of MPa. In equation (2), Vγ is substituted with the volume fraction of retained austenite in the microstructure in units of %, and Vα′ in equation (2) is substituted with the volume fraction of martensite in the microstructure in units of %,.

[0026] [2] [1] Stainless steel material as described above, W: 0.01~2.00%, Ca: 0.0001~0.0100%, Mg: 0.0001~0.0100%, B: 0.0001~0.0050%, Rare earth elements: 0.01~0.30%, V: 0.01~0.50%, Ti: 0.01~0.30%, Nb: 0.01~0.30%, Zr: 0.01~0.30%, Ta: 0.001~0.300%, Sn: 0.0001~0.0100%, As: 0.0001~0.0100%, Zn: 0.0001~0.0100%, Pb: 0.0001~0.0100%, and, Contains one or more elements selected from the group consisting of Sb: 0.0001 to 0.0100%. Stainless steel material.

[0027] The shape of the stainless steel material according to this embodiment is not particularly limited. The stainless steel material according to this embodiment may be a steel pipe, a round steel bar (solid material), or a steel plate. A round steel bar refers to a steel bar with a circular cross-section perpendicular to the axial direction. The steel pipe may be a seamless steel pipe or a welded steel pipe.

[0028] The stainless steel material according to this embodiment will be described in detail below. In the following description, stainless steel material will also be simply referred to as "steel material." Furthermore, in the following description, resistance to overall corrosion and resistance to pitting corrosion will be collectively referred to as "corrosion resistance."

[0029] [Chemical composition] The chemical composition of the stainless steel material according to this embodiment contains the following elements. Unless otherwise specified, the "%" for elements refers to mass percentage.

[0030] C: 0.050% or less Carbon (C) is inevitably present. That is, the lower limit of the C content is greater than 0%. C forms carbides, increasing corrosion susceptibility. Therefore, if the C content is too high, the corrosion resistance of the steel in a supercritical corrosion environment will decrease, even if the content of other elements is within the range of this embodiment. Accordingly, the C content is 0.050% or less. The preferred upper limit of the C content is 0.048%, more preferably 0.045%, still more preferably 0.040%, and still more preferably 0.035%. It is preferable to have as low a C content as possible. However, an extreme reduction in the C content significantly increases manufacturing costs. Therefore, considering industrial production, the preferred lower limit of the C content is 0.001%, and more preferably 0.003%.

[0031] Si: 1.00% or less Silicon (Si) is inevitably present. That is, the lower limit of the Si content is greater than 0%. Si deoxidizes steel. On the other hand, if the Si content is too high, the hot workability of the steel material will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the Si content is 1.00% or less. The preferred upper limit of the Si content is 0.80%, more preferably 0.60%, more preferably 0.55%, and still more preferably 0.50%. The preferred lower limit of the Si content to more effectively obtain the above effects is 0.05%, more preferably 0.10%, more preferably 0.15%, and still more preferably 0.20%.

[0032] Mn: 1.00% or less Manganese (Mn) is inevitably present. That is, the lower limit of the Mn content is greater than 0%. Mn deoxidizes and desulfurizes steel. Mn also improves the hot workability of the steel. On the other hand, if the Mn content is too high, even if the content of other elements is within the range of this embodiment, the volume fraction of retained austenite may become too high, which may reduce the strength of the steel. Therefore, the Mn content is 1.00% or less. The preferred upper limit of the Mn content is 0.80%, more preferably 0.60%, and still more preferably 0.50%. The preferred lower limit of the Mn content to more effectively obtain the above effects is 0.01%, more preferably 0.02%, more preferably 0.03%, more preferably 0.05%, and still more preferably 0.08%.

[0033] P:0.050% or less Phosphorus (P) is inevitably present. That is, the lower limit of the P content is greater than 0%. P segregates at grain boundaries. Therefore, if the P content is too high, the corrosion resistance of the steel in a supercritical corrosion environment will decrease, even if the content of other elements is within the range of this embodiment. Accordingly, the P content is 0.050% or less. The preferred upper limit of the P content is 0.040%, more preferably 0.030%, and even more preferably 0.025%. It is preferable to have as low a P content as possible. However, an extreme reduction in the P content will significantly increase manufacturing costs. Therefore, considering industrial production, the preferred lower limit of the P content is 0.001%, and even more preferably 0.003%.

[0034] S: 0.0050% or less Sulfur (S) is inevitably present. That is, the lower limit of the S content is greater than 0%. S segregates at grain boundaries. Therefore, if the S content is too high, the corrosion resistance of the steel in a supercritical corrosion environment will decrease, even if the content of other elements is within the range of this embodiment. Accordingly, the S content is 0.0050% or less. The preferred upper limit of the S content is 0.0040%, more preferably 0.0030%, and even more preferably 0.0020%. It is preferable to have as low an S content as possible. However, an extreme reduction in the S content will significantly increase manufacturing costs. Therefore, considering industrial production, the preferred lower limit of the S content is 0.0001%, more preferably 0.0003%, and even more preferably 0.0004%.

[0035] Cr: 13.50-16.50% Chromium (Cr) forms a passive film on the surface of steel, enhancing its corrosion resistance in supercritical corrosion environments. If the Cr content is too low, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Cr content is too high, even if the content of other elements is within the range of this embodiment, the low-temperature toughness of the steel in cryogenic environments will decrease. Therefore, the Cr content is 13.50 to less than 16.50%. The preferred lower limit of the Cr content is 13.60%, more preferably 13.80%, even more preferably greater than 14.00%, and still more preferably 14.10%. The preferred upper limit of the Cr content is 16.49%, more preferably 16.40%, and still more preferably 16.30%.

[0036] Ni: 4.50~9.00% Nickel (Ni) enhances the low-temperature toughness of steel materials in cryogenic environments. If the Ni content is too low, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Ni content is too high, even if the content of other elements is within the range of this embodiment, the volume fraction of retained austenite becomes too high, reducing the strength of the steel material. Therefore, the Ni content is 4.50 to 9.00%. The preferred lower limit of the Ni content is 4.53%, more preferably 4.55%, and still more preferably 4.60%. The preferred upper limit of the Ni content is 8.90%, more preferably 8.80%, and still more preferably 8.70%.

[0037] Mo: 1.00~5.00% Molybdenum (Mo) enhances the corrosion resistance of steel materials in supercritical corrosion environments. If the Mo content is too low, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Mo content is too high, even if the content of other elements is within the range of this embodiment, the low-temperature toughness of the steel material in cryogenic environments will decrease. Therefore, the Mo content is 1.00 to 5.00%. The preferred lower limit of the Mo content is 1.03%, more preferably 1.05%, still more preferably 1.10%, and still more preferably 1.20%. The preferred upper limit of the Mo content is 4.90%, more preferably 4.80%, and still more preferably 4.70%.

[0038] Cu: 0.01~2.00% Copper (Cu) enhances the corrosion resistance of steel materials in supercritical corrosion environments. If the Cu content is too low, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Cu content is too high, even if the content of other elements is within the range of this embodiment, the low-temperature toughness of the steel material in cryogenic environments will decrease. Therefore, the Cu content is 0.01 to 2.00%. The preferred lower limit of the Cu content is 0.02%, more preferably 0.03%, and even more preferably 0.05%. The preferred upper limit of the Cu content is 1.97%, more preferably 1.95%, and even more preferably 1.90%.

[0039] Co: 0.01~1.00% Cobalt (Co) forms a film on the surface of steel, enhancing its corrosion resistance in supercritical corrosion environments. If the Co content is too low, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Co content is too high, the manufacturing cost becomes extremely high, even if the content of other elements is within the range of this embodiment. Therefore, the Co content is 0.01 to 1.00%. The preferred lower limit of the Co content is 0.02%, more preferably 0.05%, and even more preferably 0.10%. The preferred upper limit of the Co content is 0.95%, more preferably 0.90%, and even more preferably 0.80%.

[0040] N: 0.200% or less Nitrogen (N) is inevitably present. That is, the lower limit of the N content is greater than 0%. N stabilizes the retained austenite structure. On the other hand, if the N content is too high, the toughness and hot workability of the steel will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the N content is 0.200% or less. The preferred upper limit of the N content is 0.160%, more preferably 0.120%, still more preferably 0.080%, and still more preferably 0.060%. The preferred lower limit of the N content to more effectively obtain the above effects is 0.001%, more preferably 0.003%, and still more preferably 0.004%.

[0041] sol.Al:0.001~0.100% Aluminum (Al) deoxidizes steel. If the Al content is too low, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Al content is too high, even if the content of other elements is within the range of this embodiment, coarse oxides will be formed, reducing the low-temperature toughness of the steel. Therefore, the Al content is 0.001 to 0.100%. The preferred lower limit of the Al content is 0.003%, more preferably 0.005%, and even more preferably 0.010%. The preferred upper limit of the Al content is 0.080%, more preferably 0.060%, and even more preferably 0.050%. In this specification, Al content refers to the content of sol.Al (acid-soluble Al).

[0042] O: 0.020% or less Oxygen (O) is inevitably present. That is, the lower limit of the O content is greater than 0%. O forms oxides. Therefore, if the O content is too high, the corrosion resistance of the steel in a supercritical corrosion environment will decrease, even if the content of other elements is within the range of this embodiment. Accordingly, the O content is 0.020% or less. The preferred upper limit of the O content is 0.018%, more preferably 0.015%, and even more preferably 0.010%. It is preferable to have as low an O content as possible. However, an extreme reduction in O content increases manufacturing costs. Therefore, considering industrial production, the preferred lower limit of the O content is 0.001%, and even more preferably 0.002%.

[0043] The remainder of the chemical composition of the stainless steel material according to this embodiment consists of Fe and impurities. Here, impurities in the chemical composition refer to substances that are mixed in from raw materials such as ore, scrap, or the manufacturing environment during the industrial production of stainless steel material, and are acceptable within a range that does not adversely affect the stainless steel material according to this embodiment.

[0044] [Optional element] The chemical composition of the stainless steel material according to this embodiment may further include W in place of some of the Fe.

[0045] W: 0~2.00% Tungsten (W) is an optional element and may not be included. That is, the W content may be 0%. When included, W enhances the corrosion resistance of the steel material in a supercritical corrosion environment. Even if only a small amount of W is included, the above effect can be obtained to some extent. However, if the W content is too high, even if the content of other elements is within the range of this embodiment, the volume fraction of ferrite may become too high, which may reduce the strength of the steel material. Therefore, the W content is 0 to 2.00%. The preferred lower limit of the W content is greater than 0%, more preferably 0.01%, even more preferably 0.02%, and even more preferably 0.03%. The preferred upper limit of the W content is 1.90%, more preferably 1.85%, and even more preferably 1.80%.

[0046] The chemical composition of the stainless steel material according to this embodiment may further contain, in place of some of the Fe, one or more elements selected from the group consisting of Ca, Mg, B, and rare earth elements. Any of these elements are arbitrary and enhance the hot workability of the steel material.

[0047] Ca: 0~0.0100% Calcium (Ca) is an optional element and may not be present. That is, the Ca content may be 0%. When present, Ca neutralizes sulfur in the steel by fixing it as sulfides, thereby improving the hot workability of the steel. Even a small amount of Ca can provide some of the above effect. However, if the Ca content is too high, even if the content of other elements is within the range of this embodiment, the oxides in the steel will coarseen, reducing the corrosion resistance of the steel in a supercritical corrosion environment. Therefore, the Ca content is 0 to 0.0100%. The preferred lower limit of the Ca content is greater than 0%, more preferably 0.0001%, more preferably 0.0005%, more preferably 0.0008%, and more preferably 0.0010%. The preferred upper limit of the Ca content is 0.0080%, more preferably 0.0060%, and more preferably 0.0045%.

[0048] Mg: 0~0.0100% Magnesium (Mg) is an optional element and may not be present. That is, the Mg content may be 0%. If present, Mg neutralizes sulfur in the steel by fixing it as sulfides, thereby improving the hot workability of the steel. Even a small amount of Mg can provide the above effect to some extent. However, if the Mg content is too high, even if the content of other elements is within the range of this embodiment, the oxides in the steel will coarseen, reducing the corrosion resistance of the steel in a supercritical corrosion environment. Therefore, the Mg content is 0 to 0.0100%. The preferred lower limit of the Mg content is greater than 0%, more preferably 0.0001%, and even more preferably 0.0002%. The preferred upper limit of the Mg content is 0.0080%, more preferably 0.0060%, even more preferably 0.0040%, and even more preferably 0.0020%.

[0049] B: 0~0.0050% Boron (B) is an optional element and may not be present. That is, the B content may be 0%. If present, B neutralizes S in the steel by fixing it as sulfide, thereby improving the hot workability of the steel. Even a small amount of B can provide the above effect to some extent. However, if the B content is too high, boron nitride (BN) will be formed, even if the content of other elements is within the range of this embodiment, reducing the toughness of the steel. Therefore, the B content is 0 to 0.0050%. The preferred lower limit of the B content is greater than 0%, more preferably 0.0001%, and even more preferably 0.0003%. The preferred upper limit of the B content is 0.0040%, more preferably 0.0030%, and even more preferably 0.0020%.

[0050] Rare earth elements: 0~0.30% Rare earth elements (REMs) are optional and may not be present. In other words, the REM content may be 0%. When present, REMs neutralize sulfur in the steel by fixing it as sulfides, thereby improving the hot workability of the steel. Even a small amount of REM can provide some of the above effect. However, if the REM content is too high, even if the content of other elements is within the range of this embodiment, the oxides in the steel will coarseen, reducing the corrosion resistance of the steel in a supercritical corrosion environment. Therefore, the REM content is 0 to 0.30%. The preferred lower limit of the REM content is greater than 0%, more preferably 0.01%, and even more preferably 0.02%. The preferred upper limit of the REM content is 0.25%, more preferably 0.20%, and even more preferably 0.18%.

[0051] In this specification, REM refers to one or more elements selected from the group consisting of scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and the lanthanides lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. In this specification, REM content refers to the total content of these elements.

[0052] The chemical composition of the stainless steel material according to this embodiment may further contain one or more elements selected from the group consisting of V, Ti, Nb, Zr, and Ta in place of a portion of Fe. Any of these elements are arbitrary and enhance the strength of the steel material.

[0053] V: 0~0.50% Vanadium (V) is an optional element and may not be present. That is, the V content may be 0%. When present, V forms carbonitrides, increasing the strength of the steel. Even a small amount of V can provide some of the above effect. However, if the V content is too high, even if the content of other elements is within the range of this embodiment, the strength of the steel will become too high and the low-temperature toughness of the steel will decrease. Therefore, the V content is 0 to 0.50%. The preferred lower limit of the V content is greater than 0%, more preferably 0.01%, even more preferably 0.03%, and even more preferably 0.04%. The preferred upper limit of the V content is 0.40%, more preferably 0.30%, and even more preferably 0.25%.

[0054] Ti: 0~0.30% Titanium (Ti) is an optional element and may not be included. In other words, the Ti content may be 0%. When included, Ti forms carbonitrides, increasing the strength of the steel. Even a small amount of Ti will provide some of the above effect. However, if the Ti content is too high, even if the content of other elements is within the range of this embodiment, the strength of the steel will become too high and the low-temperature toughness of the steel will decrease. Therefore, the Ti content is 0 to 0.30%. The preferred lower limit of the Ti content is greater than 0%, more preferably 0.01%, and even more preferably 0.02%. The preferred upper limit of the Ti content is 0.25%, more preferably 0.20%, and even more preferably 0.18%.

[0055] Nb: 0~0.30% Niobium (Nb) is an optional element and may not be present. In other words, the Nb content may be 0%. If present, Nb forms carbonitrides, increasing the strength of the steel. Even a small amount of Nb can provide some of the above effect. However, if the Nb content is too high, even if the content of other elements is within the range of this embodiment, the strength of the steel will become too high and the low-temperature toughness of the steel will decrease. Therefore, the Nb content is 0 to 0.30%. The preferred lower limit of the Nb content is greater than 0%, more preferably 0.01%, and even more preferably 0.02%. The preferred upper limit of the Nb content is 0.25%, more preferably 0.20%, and even more preferably 0.18%.

[0056] Zr: 0~0.30% Zirconium (Zr) is an optional element and may not be present. That is, the Zr content may be 0%. If present, Zr forms carbonitrides, increasing the strength of the steel. Even a small amount of Zr will provide some of the above effect. However, if the Zr content is too high, even if the content of other elements is within the range of this embodiment, the strength of the steel will become too high and the low-temperature toughness of the steel will decrease. Therefore, the Zr content is 0 to 0.30%. The preferred lower limit of the Zr content is greater than 0%, more preferably 0.01%, and even more preferably 0.02%. The preferred upper limit of the Zr content is 0.25%, more preferably 0.20%, and even more preferably 0.18%.

[0057] Ta: 0~0.300% Tantalum (Ta) is an optional element and may not be present. That is, the Ta content may be 0%. When present, Ta forms carbonitrides, increasing the strength of the steel. Even a small amount of Ta will provide some of the above effect. However, if the Ta content is too high, even if the content of other elements is within the range of this embodiment, the strength of the steel will become too high and the low-temperature toughness of the steel will decrease. Therefore, the Ta content is 0 to 0.300%. The preferred lower limit of the Ta content is greater than 0%, more preferably 0.001%, more preferably 0.002%, and still more preferably 0.003%. The preferred upper limit of the Ta content is 0.290%, more preferably 0.280%, more preferably 0.260%, and still more preferably 0.250%.

[0058] The chemical composition of the stainless steel material according to this embodiment may further contain one or more elements selected from the group consisting of Sn, As, Zn, Pb, and Sb in place of a portion of Fe. Any of these elements are arbitrary and enhance the corrosion resistance of the steel material in a supercritical corrosion environment.

[0059] Sn: 0~0.0100% Tin (Sn) is an optional element and may not be present. That is, the Sn content may be 0%. If present, Sn enhances the corrosion resistance of the steel material in a supercritical corrosion environment. Even a small amount of Sn will provide the above effect to some extent. However, if the Sn content is too high, the hot workability of the steel material will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the Sn content is 0 to 0.0100%. The preferred lower limit of the Sn content is greater than 0%, more preferably 0.0001%, more preferably 0.0002%, and more preferably 0.0003%. The preferred upper limit of the Sn content is 0.0080%, more preferably 0.0060%, and more preferably 0.0050%.

[0060] As: 0~0.0100% Arsenic (As) is an optional element and may not be included. That is, the As content may be 0%. When included, As enhances the corrosion resistance of steel materials in supercritical corrosion environments. Even a small amount of As can provide the above effect to some extent. However, if the As content is too high, the corrosion resistance of steel materials in supercritical corrosion environments may actually decrease, even if the content of other elements is within the range of this embodiment. Therefore, the As content is 0 to 0.0100%. The preferred lower limit of the As content is greater than 0%, more preferably 0.0001%, more preferably 0.0002%, and more preferably 0.0003%. The preferred upper limit of the As content is 0.0080%, more preferably 0.0060%, and more preferably 0.0040%.

[0061] Zn: 0~0.0100% Zinc (Zn) is an optional element and may not be included. That is, the Zn content may be 0%. If included, Zn enhances the corrosion resistance of steel materials in supercritical corrosion environments. Even a small amount of Zn can provide the above effect to some extent. However, if the Zn content is too high, the corrosion resistance of steel materials in supercritical corrosion environments may actually decrease, even if the content of other elements is within the range of this embodiment. Therefore, the Zn content is 0 to 0.0100%. The preferred lower limit of the Zn content is greater than 0%, more preferably 0.0001%, more preferably 0.0002%, and more preferably 0.0003%. The preferred upper limit of the Zn content is 0.0080%, more preferably 0.0060%, and more preferably 0.0050%.

[0062] Pb: 0~0.0100% Lead (Pb) is an optional element and may not be present. That is, the Pb content may be 0%. If present, Pb enhances the corrosion resistance of the steel material in a supercritical corrosion environment. Even a small amount of Pb will provide some degree of the above effect. However, if the Pb content is too high, the hot workability of the steel material will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the Pb content is 0 to 0.0100%. The preferred lower limit of the Pb content is greater than 0%, more preferably 0.0001%, more preferably 0.0002%, and more preferably 0.0003%. The preferred upper limit of the Pb content is 0.0080%, more preferably 0.0060%, more preferably 0.0050%, and more preferably 0.0040%.

[0063] Sb: 0~0.0100% Antimony (Sb) is an optional element and may not be included. That is, the Sb content may be 0%. If included, Sb enhances the corrosion resistance of steel materials in supercritical corrosion environments. Even a small amount of Sb can provide the above effect to some extent. However, if the Sb content is too high, the manufacturing cost will increase drastically, even if the content of other elements is within the range of this embodiment. Therefore, the Sb content is 0 to 0.0100%. The preferred lower limit of the Sb content is greater than 0%, more preferably 0.0001%, more preferably 0.0002%, and more preferably 0.0003%. The preferred upper limit of the Sb content is 0.0080%, more preferably 0.0060%, more preferably 0.0050%, and more preferably 0.0040%.

[0064] [Fn1] Assuming that the stainless steel material according to this embodiment has the above-described chemical composition, it satisfies the following formula (1). 0.10 ≤ Cu + W ≤ 2.20 (1) Here, the elemental symbols in equation (1) are substituted with the content of the corresponding element in units of mass%. If the corresponding element is not present, "0" is substituted for that elemental symbol.

[0065] Fn1 (=Cu+W) is an index of the balance between corrosion resistance and low-temperature toughness in stainless steel materials having the above-mentioned chemical composition. Increasing Fn1 to 0.10 or higher improves the stability of the passivation film and enhances the corrosion resistance of the steel material in supercritical corrosion environments. On the other hand, if Fn1 exceeds 2.20, corrosion resistance is obtained, but sufficient low-temperature toughness is not obtained. Therefore, assuming that the stainless steel material according to this embodiment has the above-mentioned chemical composition, Fn1 is set to 0.10 to 2.20.

[0066] The preferred lower limit of Fn1 is 0.11, more preferably 0.12, and even more preferably 0.13. The preferred upper limit of Fn1 is 2.15, more preferably 2.10, and even more preferably 2.05. Fn1 is obtained by rounding the obtained value to the third decimal place.

[0067] [Yield strength] The yield strength of the stainless steel material according to this embodiment is 758 MPa or less. Here, the lower limit of the yield strength of the stainless steel material according to this embodiment is not particularly limited, but for example it is 552 MPa. In other words, the yield strength of the stainless steel material according to this embodiment may be 552 to 758 MPa. The preferred upper limit of the yield strength is 750 MPa, more preferably 740 MPa, and even more preferably 730 MPa. The preferred lower limit of the yield strength is 555 MPa, and even more preferably 560 MPa.

[0068] In this embodiment, the yield strength of the stainless steel material is determined by the following method. Specifically, a tensile test is performed in accordance with ASTM E8 / E8M(2022). A test specimen is prepared from the steel material according to this embodiment. If the steel material is a steel plate, a tensile test specimen is prepared from the center of the plate thickness. In this case, the longitudinal direction of the tensile test specimen is parallel to the rolling direction of the steel plate. If the steel material is a steel pipe, a tensile test specimen or an arc-shaped test specimen is prepared from the center of the wall thickness. In this case, the longitudinal direction of the tensile test specimen or the arc-shaped test specimen is parallel to the axial direction of the steel pipe. If the steel material is a round steel bar, a tensile test specimen is prepared from the R / 2 position. In this specification, the R / 2 position of a round steel bar means the center position of radius R in a cross section perpendicular to the axial direction of the round steel bar. In this case, the longitudinal direction of the tensile test specimen is parallel to the axial direction of the round steel bar.

[0069] The tensile test specimen has, for example, a parallel section diameter of 8.9 mm and a gauge length of 35.6 mm. If a round bar specimen cannot be prepared from a steel pipe, an arc-shaped specimen is prepared. The dimensions of the arc-shaped specimen are, for example, the total thickness, a width of 25.4 mm, and a gauge length of 50.8 mm. Using the prepared tensile test specimen, a tensile test is performed at room temperature (24 ± 3 °C) in accordance with ASTM E8 / E8M (2022). The 0.2% offset proof stress (MPa) obtained from the tensile test is defined as the yield strength (MPa). In this embodiment, the yield strength (MPa) is obtained by rounding the obtained value to the first decimal place.

[0070] [Microorganisms] The microstructure of the stainless steel material according to this embodiment consists of retained austenite at a volume fraction of 5-30%, ferrite at 0-5.0%, and the remainder being martensite. In this specification, when the microstructure is described as "consisting of retained austenite, ferrite, and martensite," it means that the amount of phases other than retained austenite, ferrite, and martensite in the microstructure is negligibly small. For example, in the chemical composition of the stainless steel material according to this embodiment, the volume fraction of precipitates and inclusions is negligibly small compared to the volume fractions of retained austenite, ferrite, and martensite. In other words, the microstructure of the stainless steel material according to this embodiment may contain trace amounts of precipitates, inclusions, etc., in addition to retained austenite, ferrite, and martensite.

[0071] As described above, in the microstructure of the stainless steel material according to this embodiment, the volume fraction of retained austenite is 5 to 30%. If the volume fraction of retained austenite is too low, the low-temperature toughness of the steel material in an extremely low-temperature environment will decrease. On the other hand, if the volume fraction of retained austenite is too high, the strength of the steel material will decrease. Therefore, in the microstructure of the stainless steel material according to this embodiment, the volume fraction of retained austenite is 5 to 30%. The preferred lower limit of the volume fraction of retained austenite is 6%, more preferably 7%, and still more preferably 8%. The preferred upper limit of the volume fraction of retained austenite is 29%, more preferably 28%, and still more preferably 27%.

[0072] As described above, in the microstructure of the stainless steel material according to this embodiment, the volume fraction of ferrite is 0 to 5.0%. In other words, ferrite may not be present in the microstructure of this embodiment. On the other hand, if the volume fraction of ferrite is too high, the ferrite will coarseen, and the low-temperature toughness of the steel material in an extremely low-temperature environment will decrease. Therefore, in the microstructure of the stainless steel material according to this embodiment, the volume fraction of ferrite is 0 to 5.0%. The lower limit of the volume fraction of ferrite may be greater than 0.0%, 0.1%, or 0.2%. The preferred upper limit of the volume fraction of ferrite is 4.8%, more preferably 4.6%, and even more preferably 4.5%.

[0073] As described above, the stainless steel material according to this embodiment has a microstructure consisting of retained austenite at a volume fraction of 5-30%, ferrite at a volume fraction of 0-5.0%, and the remainder being martensite. In this specification, "martensite" includes not only fresh martensite but also tempered martensite. Furthermore, the volume fraction of martensite is not particularly limited, but is substantially between 65.0% and 95.0%. The preferred lower limit of the volume fraction of martensite is 66.0%, more preferably 66.5%, and even more preferably 67.0%. The preferred upper limit of the volume fraction of martensite is 94.5%, more preferably 94.0%, and even more preferably 93.5%.

[0074] In this embodiment, the volume fraction of each phase in the microstructure is determined by the following method. Specifically, the volume fraction (%) of retained austenite and the volume fraction (%) of ferrite in the microstructure of the steel material are determined by the following method. The volume fraction (%) of martensite is determined by subtracting the determined volume fractions of retained austenite and ferrite from 100%.

[0075] [Method for measuring the volume fraction of retained austenite] The volume fraction of retained austenite in the microstructure of steel is determined by X-ray diffraction. Specifically, a test specimen for measuring the volume fraction of retained austenite is prepared from the steel material according to this embodiment. If the steel material is a steel plate, the test specimen is taken from the center of the plate thickness. If the steel material is a steel pipe, the test specimen is taken from the center of the wall thickness. If the steel material is a round bar, the test specimen is taken from the R / 2 position. The size of the test specimen is not particularly limited. For example, the test specimen is 15 mm × 15 mm × 2 mm thick. If the steel material is a steel pipe, the thickness direction of the test specimen is the diameter direction of the pipe. If the steel material is a round bar, the thickness direction of the test specimen is the radial direction. If the steel material is a steel plate, the thickness direction of the test specimen is the plate thickness direction. Using the prepared specimens, the X-ray diffraction intensity of the (110), (200), and (211) planes of the α-phase (martensite), the (111), (200), and (220) planes of the γ-phase (retained austenite) are measured, and the integrated intensity of each plane is calculated.

[0076] In measuring the X-ray diffraction intensity, the target of the X-ray diffractometer is set to Co (CoKα rays), and the output is set to 30kV-100mA. The measurement angle (2θ) is set to 45-105°. After calculation, the volume fraction Vγ (%) of retained austenite is calculated for each combination (3×3=9 pairs) of each face of the α phase and each face of the γ phase using equation (I). The average value of the volume fraction Vγ of the 9 pairs of retained austenite is then defined as the volume fraction (%) of retained austenite. Vγ=100 / {1+(Iα×Rγ) / (Iγ×Rα)} (I) Here, Iα is the integrated intensity of the α phase. Rα is the crystallographic theoretical calculation value of the α phase. Iγ is the integrated intensity of the γ phase. Rγ is the crystallographic theoretical calculation value of the γ phase. The values ​​of Rα and Rγ for each plane can be those incorporated into the residual γ quantitative analysis system included with RIGAK Corporation's product name RINT-TTR. The volume fraction of retained austenite is obtained by rounding the obtained value to the first decimal place.

[0077] [Method for measuring the volume fraction of ferrite] The volume fraction of ferrite in the microstructure of steel is determined by the point calculation method. Specifically, a test specimen for measuring the volume fraction of ferrite is prepared from the steel material according to this embodiment. If the steel material is a steel plate, the test specimen is taken from the center of the plate thickness. In this case, the observation surface of the test specimen is parallel to the rolling direction of the steel plate. If the steel material is a steel pipe, the test specimen is taken from the center of the wall thickness. In this case, the observation surface of the test specimen is parallel to the axial direction of the steel pipe. If the steel material is a round bar, the test specimen is taken from the R / 2 position. In this case, the observation surface of the test specimen is parallel to the axial direction of the round bar. The size of the test specimen is not particularly limited. After mechanically polishing the observation surface, the microstructure is revealed by electrolytic etching of the observation surface. Electrolytic etching is performed using a mixture of electrolyte: aqua regia (a solution of hydrochloric acid and nitric acid mixed in a 3:1 ratio) and glycerin, with a current density of 1 A / cm². 2 The electrolysis will be performed for 1 minute.

[0078] The electrolytically etched observation surface is observed using an optical microscope for 30 fields of view. Each field of view is a rectangle measuring 250 μm × 250 μm. The magnification is 400x. In each field of view, a person skilled in the art can distinguish between ferrite and other phases (retained austenite and martensite) based on contrast. Therefore, ferrite in each field of view is identified based on contrast. The area percentage of the identified ferrite is determined by the point calculation method in accordance with JIS G 0555 (2020).

[0079] Specifically, for the field of view, 20 vertical lines are drawn at equal intervals from the top to the bottom of the field of view. That is, the field of view is divided into 21 regions in the left-right direction by these 20 vertical lines. Furthermore, 20 horizontal lines are drawn at equal intervals from the left edge to the right edge of the field of view. That is, the field of view is divided into 21 regions in the up-down direction by these 20 horizontal lines. At this time, the intersections of the vertical and horizontal lines are called grid points. In other words, 400 grid points are arranged at equal intervals in the field of view. In accordance with JIS G 0555 (2020), the grid points that overlap with ferrite in the field of view are counted. The number of grid points that overlap with ferrite obtained in 30 fields of view is divided by the total number of grid points (400 × 30 = 12000) and defined as the ferrite area ratio. In this embodiment, the ferrite area ratio obtained by the above method is taken as the ferrite volume ratio (%). The volume fraction of ferrite is calculated by rounding the obtained value to two decimal places.

[0080] Using the volume fraction (%) of retained austenite obtained by the X-ray diffraction method described above and the volume fraction (%) of ferrite obtained by the point calculation method described above, the volume fraction (%) of martensite in the microstructure of the steel material is calculated using the following formula. Volume fraction of martensite (%) = 100 - {Volume fraction of retained austenite (%) + Volume fraction of ferrite (%)}

[0081] [Fn2] Assuming that the stainless steel material according to this embodiment has the above-described chemical composition and microstructure, the element content, yield strength, and volume fraction of the microstructure satisfy the following equation (2). 0.01YS×(Ni+12Co) / (Vγ+3.3Vα′)≧0.0036 (2) Here, the elemental symbols in equation (2) are substituted with the content of the corresponding element in units of mass%. In equation (2), YS is substituted with the yield strength in units of MPa. In equation (2), Vγ is substituted with the volume fraction of retained austenite in the microstructure in units of %, and Vα′ in equation (2) is substituted with the volume fraction of martensite in the microstructure in units of %.

[0082] Fn2 (=0.01YS × (Ni + 12Co) / (Vγ + 3.3Vα′)) is an index of low-temperature toughness in an extremely low-temperature environment for stainless steel materials having the above-mentioned chemical composition and microstructure, and satisfying Fn1 between 0.10 and 2.20. Here, the higher the yield strength of Fn2, the more likely the low-temperature toughness of the steel material is to decrease. Also, Ni and Co in Fn2 enhance the low-temperature toughness of the steel material. Furthermore, among the microstructure, retained austenite enhances low-temperature toughness, while martensite tends to significantly decrease it. Therefore, by adjusting the balance between yield strength, the volume fraction Vγ of retained austenite, the volume fraction Vα′ of martensite, and the content of Ni and Co, elements that enhance low-temperature toughness, it may be possible to obtain excellent low-temperature toughness even in extremely low-temperature environments.

[0083] Therefore, the stainless steel material according to this embodiment has the above-mentioned chemical composition and microstructure, with Fn1 satisfying 0.10 to 2.20, and furthermore, Fn2 being 0.0036 or higher. As a result, the stainless steel material according to this embodiment can achieve both excellent overall corrosion resistance and pitting corrosion resistance in supercritical corrosion environments, and excellent low-temperature toughness in cryogenic environments.

[0084] The preferred lower limit of Fn2 is 0.0037, more preferably 0.0038, and even more preferably 0.0039. The upper limit of Fn2 is not particularly limited, but is substantially 0.0113. The preferred upper limit of Fn2 is 0.0100, more preferably 0.0098, and even more preferably 0.0095. Fn2 is obtained by rounding the obtained value to the fifth decimal place.

[0085] [Corrosion resistance] The stainless steel material according to this embodiment has the above-described chemical composition, satisfies Fn1 of 0.10 to 2.20, has a yield strength of 758 MPa or less, and its microstructure consists of 5 to 30% retained austenite by volume fraction, 0 to 5.0% ferrite, and the remainder being martensite, with Fn2 of 0.0036 or more. As a result, the stainless steel material according to this embodiment has excellent corrosion resistance (resistance to overall corrosion and resistance to pitting corrosion) even in a supercritical corrosion environment. In this embodiment, excellent resistance to overall corrosion and resistance to pitting corrosion in a supercritical corrosion environment is evaluated by the following method.

[0086] Specifically, a test specimen for corrosion testing is taken from the stainless steel material according to this embodiment. If the steel material is a steel plate, the test specimen is taken from the center of the plate thickness. If the steel material is a steel pipe, the test specimen is taken from the center of the wall thickness. If the steel material is a round bar, the test specimen is taken from the R / 2 position. The size of the test specimen is not particularly limited, but for example, it may be 30 mm in length, 20 mm in width, and 2 mm in thickness.

[0087] The test specimen is placed in an autoclave. A 5.0% by mass sodium chloride aqueous solution, adjusted to pH 2.5, is poured into the autoclave so that the test specimen is submerged. A mixed gas of SO2, O2, and CO2 is pressurized and sealed into the autoclave to saturate the test solution, creating the test bath. At this time, the total pressure of the mixed gas is 300 bar, the SO2 concentration in the mixed gas is 50 ppm, and the O2 concentration in the mixed gas is 50 ppm. After sealing the autoclave, the test bath is maintained at 100°C, and the test specimen is immersed in the test bath for 96 hours while stirring.

[0088] The mass, density, and surface area of ​​the test specimen after 96 hours are determined, and the corrosion rate (mm / year) of the test specimen is calculated. In this embodiment, the corrosion rate is calculated by rounding the obtained value to the fourth decimal place. Furthermore, the surface of the test specimen after 96 hours is observed with a 10x magnification loupe to check for the presence or absence of pitting corrosion. If pitting corrosion is suspected based on observation with the loupe, it is further observed with a 100x magnification optical microscope to confirm the presence or absence of pitting corrosion. In this embodiment, if the corrosion rate obtained as a result of the corrosion test under the above conditions is 0.100 mm / year or less, it is evaluated as having excellent overall corrosion resistance even in a supercritical corrosion environment. In this embodiment, if no pitting corrosion is confirmed as a result of the corrosion test under the above conditions, it is evaluated as having excellent pitting corrosion resistance even in a supercritical corrosion environment.

[0089] [Low temperature toughness] The stainless steel material according to this embodiment has the above-described chemical composition, satisfies an Fn1 of 0.10 to 2.20, has a yield strength of 758 MPa or less, and its microstructure consists of 5 to 30% retained austenite by volume fraction, 0 to 5.0% ferrite, and the remainder being martensite, with an Fn2 of 0.0036 or more. As a result, the stainless steel material according to this embodiment has excellent low-temperature toughness in cryogenic environments. In this embodiment, the excellent low-temperature toughness in cryogenic environments is evaluated by the following method.

[0090] Specifically, full-size or sub-size V-notch test specimens are prepared from the stainless steel material according to this embodiment in accordance with API 5CT (2019). Here, if the steel material is a steel plate, the rolling direction of the steel plate is defined as the "L direction" (Longitudinal), and the width direction of the steel plate is defined as the "T direction" (Transverse). If the steel material is a steel pipe, the diameter direction of the steel pipe is defined as the "C direction", the axial direction of the steel pipe is defined as the "L direction", and the direction perpendicular to the C direction and the L direction is defined as the "T direction". If the steel material is a round steel bar, the diameter direction of the cross-section of the round steel bar is defined as the "C direction", the axial direction of the round steel bar is defined as the "L direction", and the direction perpendicular to the C direction and the L direction is defined as the "T direction".

[0091] A Charpy impact test is performed on the prepared V-notch specimens in accordance with JIS Z 2242 (2018) to determine the absorbed energy (J) at -80°C. If a sub-sized V-notch specimen is used, the obtained absorbed energy is divided by the reduction factor described in API 5CT (2019) to convert it to the absorbed energy of a full-sized V-notch specimen. In this embodiment, the absorbed energy (J) at -80°C is obtained by rounding the obtained value to the first decimal place.

[0092] In this embodiment, if the absorbed energy at -80°C determined by the above conditions is 100 J or more, it is evaluated as having excellent low-temperature toughness in an extremely low-temperature environment. In this specification, the absorbed energy at -80°C is also simply referred to as "absorbed energy".

[0093] [Shape of stainless steel material] As described above, the shape of the stainless steel material according to this embodiment is not particularly limited. Preferably, the stainless steel material according to this embodiment is a seamless steel pipe. When the stainless steel material according to this embodiment is a seamless steel pipe, even if the wall thickness is 5 mm or more, it has excellent corrosion resistance (resistance to overall corrosion and pitting corrosion) in supercritical corrosion environments and excellent low-temperature toughness in cryogenic environments.

[0094] [Manufacturing method] An example of a method for manufacturing stainless steel material according to this embodiment having the above-described configuration will be explained. Note that the method for manufacturing stainless steel material according to this embodiment is not limited to the method described below. An example of the method for manufacturing stainless steel material according to this embodiment includes a step of preparing intermediate steel material (preparation step), a step of performing quenching on the intermediate steel material (quenching step), and a step of performing tempering (tempering step). Each step will be described in detail below.

[0095] [Preparation process] In the preparation step, an intermediate steel material having the above-mentioned chemical composition is prepared. As long as the intermediate steel material has the above-mentioned chemical composition, the method of manufacturing the intermediate steel material is not particularly limited. The intermediate steel material referred to here is a plate-shaped steel material when the final product is a steel plate or welded steel pipe, a raw pipe when the final product is a seamless steel pipe, and a steel material with a circular cross-section perpendicular to the axial direction when the final product is a round steel bar.

[0096] The preparation process may include a process for preparing the raw materials (raw material preparation process) and a process for manufacturing intermediate steel materials by hot working the raw materials (hot working process). The following details the case in which the raw material preparation process and the hot working process are included.

[0097] [Material preparation process] In the material preparation process, the material is manufactured using molten steel having the chemical composition described above. The method of manufacturing the material is not particularly limited and any well-known method may be used. Specifically, a slab (slab, bloom, or billet) may be manufactured using a continuous casting method with molten steel. An ingot may be manufactured using a block-making method with molten steel. If necessary, a billet may be manufactured by bloc rolling of the slab, bloom, or ingot. The material (slab, bloom, or billet) is manufactured through the above process.

[0098] [Hot working process] In the hot working process, the prepared material is hot-worked to produce intermediate steel material. As mentioned above, if the steel material is a seamless steel pipe, the intermediate steel material corresponds to the raw pipe. First, the billet is heated in a heating furnace. The heating temperature is not particularly limited, but for example, it is 1100 to 1300°C. Hot working is performed on the billet extracted from the heating furnace to produce the raw pipe (seamless steel pipe). The method of hot working is not particularly limited and any well-known method is acceptable.

[0099] For example, the Mannesmann process may be used as a hot working method to produce the raw pipe. In this case, a round billet is perforated and rolled using a perforating machine. When perforating and rolling, the perforation ratio is not particularly limited, but for example, it is 1.0 to 4.0. The perforated and rolled round billet is further hot-rolled using a mandrel mill, reducer, sizing mill, etc., to produce the raw pipe. The cumulative reduction in surface area during the hot working process is, for example, 20 to 70%. The raw pipe may also be produced from the billet using other hot working methods. For example, if the steel material is a short, thick-walled steel pipe such as a coupling, the raw pipe may be produced by forging using the Erhardt process or similar methods. The raw pipe is produced by the above process. The wall thickness of the raw pipe is not particularly limited, but for example, it is 9 to 60 mm.

[0100] If the steel material is round steel, first the material is heated in a heating furnace. The heating temperature is not particularly limited, but for example, it is 1100 to 1300°C. Hot working is performed on the material extracted from the heating furnace to produce intermediate steel material with a circular cross-section perpendicular to the axial direction. Hot working is, for example, bloc rolling using a bloc rolling mill, or hot rolling using a continuous rolling mill. A continuous rolling mill has alternating horizontal stands with a pair of perforated rolls arranged vertically and vertical stands with a pair of perforated rolls arranged horizontally. If the steel material is steel plate, first the material is heated in a heating furnace. The heating temperature is not particularly limited, but for example, it is 1100 to 1300°C. Hot rolling is performed on the material extracted from the heating furnace using a bloc rolling mill and a continuous rolling mill to produce intermediate steel material in the shape of a steel plate.

[0101] Intermediate steel produced by hot working may be air-cooled (as-rolled). Intermediate steel produced by hot working may be quenched directly after hot working without cooling to room temperature, or it may be quenched after reheating after hot working. When quenching is performed directly after hot working or after reheating, cooling may be stopped or slowed during quenching. In this case, the occurrence of quenching cracks in the raw pipe can be suppressed. When quenching is performed directly after hot working or after reheating, stress relief annealing (SR) may be performed after quenching but before the next heat treatment process. In this case, residual stress in the raw pipe is removed.

[0102] As described above, the preparation process involves preparing intermediate steel. The intermediate steel may be manufactured by the preferred process described above, or it may be intermediate steel manufactured by a third party, or intermediate steel manufactured at a factory or business other than the one where the quenching and tempering processes described later are carried out. The quenching process will be described in detail below.

[0103] [Heat treatment process] The intermediate steel material produced in the hot working process is subjected to quenching (quenching process). Quenching is carried out by a well-known method. Specifically, the intermediate steel material after the hot working process is placed in a heat treatment furnace, held at the quenching temperature, and then rapidly cooled (quenched). Here, the quenching temperature refers to the temperature (°C) of the heat treatment furnace used to heat the intermediate steel material in the quenching process. The holding time in the quenching process refers to the time (minutes) that the intermediate steel material is held at the heat treatment temperature.

[0104] In the quenching process according to this embodiment, if the quenching temperature is too low, the intermediate steel material may not be heated sufficiently, and the resulting stainless steel material may not have the microstructure described above. On the other hand, if the quenching temperature is too high, the volume fraction of ferrite may become too high, causing the ferrite to coarseen, and the low-temperature toughness of the steel material may not be obtained in an extremely low-temperature environment. Therefore, in this embodiment, the preferred quenching temperature is 850 to 1000°C. The holding time at the quenching temperature is not particularly limited, but for example, it is 5 to 80 minutes.

[0105] As described above, in this embodiment, the intermediate steel material is held at the quenching temperature and then rapidly cooled (quenched). The quenching method is, for example, water cooling. The quenching method is not particularly limited. If the intermediate steel material is a raw pipe, for example, the raw pipe may be rapidly cooled by immersion in a water tank or oil tank, or the raw pipe may be rapidly cooled by shower cooling or mist cooling, by pouring or spraying cooling water onto the outer and / or inner surface of the raw pipe.

[0106] [Tempering process] A tempering process is further performed on the intermediate steel material after the quenching process. In this specification, "tempering" refers to the process of tempering the intermediate steel material after quenching. c1 This means reheating to a temperature below 1.5°C and holding it there. Here, the tempering temperature corresponds to the furnace temperature when heating and holding the intermediate steel material after quenching. The tempering time refers to the time from when the temperature of the intermediate steel material reaches the predetermined tempering temperature until it is removed from the heat treatment furnace.

[0107] In the tempering process according to this embodiment, the volume fraction of retained austenite is increased by holding the material at a predetermined temperature, and the strength of the intermediate steel is further adjusted. Preferably, in the tempering process according to this embodiment, the Ni content (mass%), the tempering temperature T (°C), and the tempering time t (minutes) satisfy the following equation (A). (T+273)×(20+log(t / 60))+210Ni≧18300 (A) Here, in equation (A), "T" is substituted with the tempering temperature in °C, "t" is substituted with the tempering time in minutes, and "Ni" is substituted with the Ni content of the intermediate steel in mass percent. Also, "log" in equation (A) means a logarithm with base 10 (common logarithm).

[0108] FnA is defined as (T + 273) × (20 + log(t / 60)) + 210Ni. FnA is a tempering parameter depending on the Ni content. If FnA is 18300 or more, Fn2 tends to be 0.0036 or more. Therefore, in the tempering process according to this embodiment, it is preferable to set FnA to 18300 or more.

[0109] The tempering temperature T (°C) and tempering time t (minutes) are not particularly limited, as long as FnA is 18300 or higher. The tempering temperature T may be, for example, 540 to 670°C. The tempering time t may be, for example, 10 to 180 minutes.

[0110] Through the above steps, stainless steel material according to this embodiment can be manufactured. However, as stated above, the stainless steel material according to this embodiment is not limited to the manufacturing method described above. The stainless steel material according to this embodiment will be described in more detail below with reference to examples. [Examples]

[0111] Molten steel having the chemical compositions shown in Tables 1A, 1B, and 1C was melted using a 50 kg vacuum melting furnace, and steel ingots were produced by the ingot-making method. In Tables 1B and 1C, "-" indicates that the content of the corresponding element was at an impurity level. For example, the W content, rare earth element (REM) content, V content, Ti content, Nb content, Zr content of steel A, and the Cu content of steel BD, rounded to the third decimal place, mean that they were 0%. Similarly, the Ta content of steel A, rounded to the fourth decimal place, means that they were 0%. Likewise, the Ca content, Mg content, B content, Sn content, As content, Zn content, Pb content, and Sb content of steel A, rounded to the fifth decimal place, mean that they were 0%.

[0112] [Table 1A]

[0113] [Table 1B]

[0114] [Table 1C]

[0115] Furthermore, Table 2 shows the chemical compositions listed in Tables 1A and 1B, and Fn1 (=Cu+W) determined from the definition described above.

[0116] [Table 2]

[0117] For each test number except test numbers 4 and 20, the ingot was heated at 1200-1250°C for 2 hours, followed by hot working to produce intermediate steel material (raw tube) with a wall thickness of 25.4 mm and an outer diameter of 177.8 mm. For test numbers 4 and 20, the ingot was heated at 1200-1250°C for 2 hours, followed by hot working to produce intermediate steel material (steel plate) with a thickness of 45 mm and a width of 60 mm. For each intermediate steel material of each test number, a quenching process and a tempering process were performed. Specifically, the intermediate steel material of each test number was held at the quenching temperature (°C) listed in the quenching process column of Table 2 for only 15 minutes, and then rapidly cooled. After that, the intermediate steel material was heated and held at the tempering temperature (°C) listed in the tempering process column of Table 2 for the holding time (minutes).

[0118] [Evaluation Test] Through the above process, steel samples corresponding to each test number were obtained. Tensile tests, microstructural observation tests, corrosion tests, and Charpy impact tests were performed on each of the obtained steel samples corresponding to each test number.

[0119] [Tensile test] Tensile tests were conducted on the steel materials for each test number in accordance with ASTM E8 / E8M(2022). Specifically, round bar tensile test specimens with a parallel section diameter of 8.9 mm and a gauge length of 35.6 mm were prepared from the steel materials for each test number using the method described above. Tensile tests were conducted on the round bar tensile test specimens for each test number at room temperature (24±3℃) in air to determine the 0.2% offset proof strength (MPa). The determined 0.2% offset proof strength was defined as the yield strength (MPa). The yield strengths obtained for each test number are shown in the "YS(MPa)" column of Table 3.

[0120] [Table 3]

[0121] [Microtissue observation test] Microstructural observation tests were performed on the steel samples for each test number using the method described above. Specifically, the volume fraction (%) of retained austenite was determined by X-ray diffraction using the method described above. Furthermore, the volume fraction (%) of ferrite was determined by the point calculation method in accordance with JIS G 0555 (2020), using the method described above. From the obtained volume fractions of retained austenite and ferrite, the volume fraction (%) of martensite was determined. The volume fraction of retained austenite for each test number is shown in the "Retained γ (Volume %)" column of Table 3. The volume fraction of ferrite for each test number is shown in the "Ferrite (Volume %)" column of Table 3. The volume fraction of martensite for each test number is shown in the "Martensite (Volume %)" column of Table 3. Furthermore, Table 3 shows Fn2 (=0.01YS×(Ni+12Co) / (Vγ+3.3Vα′)) obtained from the chemical composition listed in Table 1A, the yield strength listed in Table 3, the volume fraction of each phase of the microstructure listed in Table 3, and the definition described above.

[0122] [Corrosion Test] Corrosion tests were conducted on the steel materials for each test number to evaluate their resistance to overall corrosion and pitting corrosion in a supercritical corrosion environment. Specifically, test specimens for corrosion testing were prepared using the method described above. Corrosion tests were conducted on the prepared test specimens under the conditions described above to determine the corrosion rate (mm / year). Furthermore, the presence or absence of pitting corrosion was checked on the test specimens after the corrosion test using the method described above. The obtained corrosion rates (mm / year) are shown in Table 3. Test numbers in which no pitting corrosion was confirmed are indicated as "None" in the "Pitting Corrosion" column of Table 3. Test numbers in which pitting corrosion was confirmed are indicated as "Present" in the "Pitting Corrosion" column of Table 3.

[0123] [Charpy impact test] A Charpy impact test was performed on each steel sample according to JIS Z 2242 (2018). Specifically, full-size V-notch specimens were prepared according to API 5CT (2019) using the method described above. A Charpy impact test was then performed on the prepared V-notch specimens according to JIS Z 2242 (2018) to determine the absorbed energy (J) at -80°C. The absorbed energy at -80°C obtained for each test sample is shown in the "vE(-80°C)(J)" column of Table 3.

[0124] [Evaluation Results] Referring to Tables 1A, 1B, 1C, 2, and 3, the steel materials for test numbers 1 to 40 had an appropriate chemical composition and an Fn1 of 0.10 to 2.20. These steel materials were further manufactured by the preferred manufacturing method described in the specification. As a result, these steel materials had a yield strength of 758 MPa or less. These steel materials further had a microstructure consisting of 5 to 30% retained austenite, 0 to 5.0% ferrite, and the remainder martensite by volume fraction. These steel materials also had an Fn2 of 0.0036 or higher. As a result, these steel materials exhibited a corrosion rate of 0.100 mm / year or less in corrosion tests and had excellent overall corrosion resistance even in supercritical corrosion environments. Furthermore, no pitting corrosion was observed in these steel materials in corrosion tests, and they also exhibited excellent pitting corrosion resistance even in supercritical corrosion environments. Furthermore, these steels exhibited excellent low-temperature toughness even in extremely low-temperature environments, with absorbed energy exceeding 100 J at -80°C in Charpy impact tests.

[0125] On the other hand, the steel material for test number 41 had too low a chromium content. As a result, this steel material exhibited a corrosion rate exceeding 0.100 mm / year in the corrosion test, and did not possess excellent overall corrosion resistance in a supercritical corrosion environment. Furthermore, pitting corrosion was observed in this steel material during the corrosion test, indicating that it did not possess excellent pitting corrosion resistance in a supercritical corrosion environment.

[0126] The steel material in test number 42 had too low a nickel content. As a result, in the Charpy impact test, this steel material absorbed less than 100 J of energy at -80°C, and did not possess good low-temperature toughness in extremely cold environments.

[0127] The steel material in test number 43 had too low a molybdenum (Mo) content. As a result, this steel material exhibited a corrosion rate exceeding 0.100 mm / year in the corrosion test, and did not possess excellent overall corrosion resistance in a supercritical corrosion environment. Furthermore, pitting corrosion was observed in this steel material during the corrosion test, indicating that it did not possess excellent pitting corrosion resistance in a supercritical corrosion environment.

[0128] The steel sample for test number 44 had too low a copper content. As a result, this steel sample exhibited a corrosion rate exceeding 0.100 mm / year in the corrosion test, and did not possess excellent overall corrosion resistance in a supercritical corrosion environment. Furthermore, pitting corrosion was observed in this steel sample during the corrosion test, indicating a lack of excellent pitting corrosion resistance in a supercritical corrosion environment.

[0129] The steel materials in test numbers 45-47 had excessively high Fn1 values. As a result, these steel materials exhibited absorbed energy of less than 100 J at -80°C in Charpy impact tests, and did not possess excellent low-temperature toughness in cryogenic environments.

[0130] The steel materials in test numbers 48-50 had too low an Fn1 value. As a result, these steel materials exhibited corrosion rates exceeding 0.100 mm / year in the corrosion tests and did not possess excellent overall corrosion resistance in supercritical corrosion environments. Furthermore, pitting corrosion was observed in these steel materials during the corrosion tests, indicating a lack of excellent pitting corrosion resistance in supercritical corrosion environments.

[0131] The steel materials in test numbers 51-55 had too low an Fn2 value. As a result, these steel materials had an absorbed energy of less than 100 J at -80°C in the Charpy impact test, and did not possess good low-temperature toughness in cryogenic environments.

[0132] The steel materials in test numbers 56 and 57 had excessively high yield strength. As a result, these steel materials absorbed less than 100 J of energy at -80°C in the Charpy impact test, and did not possess excellent low-temperature toughness in cryogenic environments.

[0133] The embodiments of this disclosure have been described above. However, the embodiments described above are merely examples for implementing this disclosure. Therefore, this disclosure is not limited to the embodiments described above, and the embodiments described above can be modified as appropriate without departing from the spirit of this disclosure.

Claims

1. In mass percent, C: 0.050% or less, Si: 1.00% or less, Mn: 1.00% or less, P: 0.050% or less, S: 0.0050% or less, Cr: 13.50-16.50% Ni: 4.50-9.00%, Mo: 1.00-5.00%, Cu: 0.01-2.00%, Co: 0.01 to 1.00%, N: 0.200% or less, Sol. Al: 0.001–0.100%, O: 0.020% or less, W: 0-2.00%, Ca: 0-0.0100%, Mg: 0 to 0.0100%, B: 0 to 0.0050%, Rare earth elements: 0 to 0.30%, V: 0-0.50%, Ti: 0 to 0.30%, Nb: 0 to 0.30%, Zr: 0 to 0.30%, Ta: 0-0.300%, Sn: 0 to 0.0100%, As: 0 to 0.0100%, Zn: 0 to 0.0100%, Pb: 0 to 0.0100%, Sb: 0 to 0.0100%, and, The remainder consists of Fe and impurities. Satisfying equation (1), The yield strength is 758 MPa or less. The microstructure consists of 5-30% retained austenite by volume, 0-5.0% ferrite, and the remainder being martensite. The content of the element, the yield strength, and the volume fraction of the microstructure satisfy formula (2). Stainless steel material. 0.10 ≤ Cu + W ≤ 2.20 (1) 0.01YS×(Ni+12Co) / (Vγ+3.3Vα′)≧0.0036 (2) Here, the elemental symbols in equations (1) and (2) are substituted with the content of the corresponding element in units of mass%. If the corresponding element is not present, "0" is substituted for its elemental symbol. In equation (2), YS is substituted with the yield strength in units of MPa. In equation (2), Vγ is substituted with the volume fraction of retained austenite in the microstructure in units of %, and Vα′ in equation (2) is substituted with the volume fraction of martensite in the microstructure in units of %,.

2. A stainless steel material according to claim 1, W: 0.01-2.00%, Ca: 0.0001-0.0100%, Mg: 0.0001 to 0.0100%, B: 0.0001 to 0.0050%, Rare earth elements: 0.01-0.30%, V: 0.01-0.50%, Ti: 0.01 to 0.30%, Nb: 0.01-0.30%, Zr: 0.01 to 0.30%, Ta: 0.001-0.300%, Sn: 0.0001 to 0.0100%, As: 0.0001 to 0.0100%, Zn: 0.0001-0.0100%, Pb: 0.0001 to 0.0100%, and, Contains one or more elements selected from the group consisting of Sb: 0.0001 to 0.0100%, Stainless steel material.

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