Stainless steel

Abstract

To provide a stainless steel having excellent general corrosion resistance and pitting-corrosion resistance in a supercritical corrosive environment and excellent low-temperature toughness in an ultra-low temperature environment.SOLUTION: The stainless steel has a chemical composition described in the specification, and satisfies an expression (1), and in the stainless steel, a yield strength is 758 MPa or less, a micro structure thereof includes, in a volume fraction, ferrite of 25-80%, retained austenite of 10-35% and a balance martensite, and a content of chemical elements and the volume fraction of the micro structure satisfy an expression (2): 0.10≤Cu+W≤1.85 (1); and 0.0350≤(Ni+1.5Co) / (Vγ+3.3 Vα') (2). A content of a corresponding chemical element is assigned by unit:mass% to a symbol of chemical element in the expressions (1) and (2). In the expression (2), a volume fraction of the retained austenite in the micro structure is assigned by unit:% to Vγ and a volume fraction of the martensite in the micro structure is assigned by unit:% to Vα'.SELECTED DRAWING: Figure 1

Description

Technical Field

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

Background Art

[0002] Currently, the increase in the 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 attracted 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 attracted 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 Japanese Patent Application Laid-Open No. 2011-190521 (Patent Document 1) and Japanese Patent Application Laid-Open No. 2012-149317 (Patent Document 2).

[0005] The stainless steel material described in Patent Document 1 is a martensitic stainless steel material, and in mass%, C: 0.003 to 0.03%, Si: 0.01 to 1.0%, Mn: 3.0 to 6.0%, P: 0.05% or less, S: 0.003% or less, Ni: 1.0 to 3.0%, Cr: 15.0 to 18.0%, Mo: 0 to 1.0%, Cu: 0 to 2.0%, Ti: 0 to 0.05%, N: 0.05% or less, Al: 0.001 to 0.1%, О: 0.005% or less, and C + N: 0.060% or less, with the balance being composed of Fe and impurities, and γmax(=420×C% + 470×N% + 23×Ni% + 9×Cu% + 7×Mn% - 11.5×Cr% - 11.5×Si% - 52×Al% + 189) is 80 or more, and 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) is 60 to 90. This martensitic stainless steel material is disclosed in Patent Document 1 as being excellent in the mechanical properties and corrosion resistance of the welded part.

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

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0008] By the way, in the above-mentioned carbon dioxide (CO2) storage technology, in order to inject CO2 into a depleted oil well, the CO2 injected into the steel pipe is compressed and pressurized to bring the CO2 into a supercritical state. On the other hand, the 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 corrosion environment. In this specification, the corrosion environment formed by supercritical CO2 containing SOx and O2 is referred to as a "supercritical corrosion environment". That is, for steel materials used in a supercritical corrosion environment, higher general corrosion resistance and pitting corrosion resistance than those in conventional corrosion environments are required.

[0010] In recent years, furthermore, toughness at extremely low temperatures may be required for steel materials when storing CO2. Specifically, when a pressure change occurs in the stored carbon dioxide gas, the temperature of the stored gas may decrease due to the Joule-Thomson effect. In this case, the steel material may be required to have toughness at an extremely low temperature of -80°C, which is far lower than the normal temperature.

[0011] Therefore, stainless steel materials assumed to be applied to such CO2 storage technology in an extremely low temperature environment are required to have high strength, excellent general corrosion resistance and pitting corrosion resistance in a supercritical corrosion environment, and excellent low-temperature toughness in an extremely low temperature environment of -80°C or lower. On the other hand, the stainless steel materials disclosed in Patent Documents 1 and 2 are not assumed to be used in such a supercritical corrosion environment and an extremely low temperature environment.

[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: 16.50 to 20.50%, Ni: 4.00 to 9.00%, Mo: 1.50 to 6.00%, Cu: 0.01 to 1.80%, Co: 0.01 to 1.00%, N: 0.200% or less, sol.Al: 0.001 to 0.100%, O: 0.0200% or less, W: 0 to 1.80%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, B: 0 to 0.0050%, Rare earth elements: 0 to 0.100%, V: 0 to 0.50%, Ti: 0 to 0.300%, Nb: 0 to 0.300%, Zr: 0 to 0.200%, 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 25 to 80% ferrite, 10 to 35% retained austenite, and the balance martensite by volume ratio. The content of the above elements and the volume ratio of the above microstructure satisfy formula (2). 0.10 ≦ Cu + W ≦ 1.85 (1) 0.0350 ≦ (Ni + 1.5Co) / (Vγ + 3.3Vα′) (2) Here, in formula (1) and (2), the content of the corresponding element is substituted for the element symbol in units of mass%. When the corresponding element is not contained, "0" is substituted for the element symbol. Also, 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 %.

Advantages of the Invention

[0014] The stainless steel material according to the present disclosure has excellent overall 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

Embodiments for Carrying Out the Invention

[0016] First, assuming application to CO2 storage technology, the inventors examined stainless steel materials 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 from the perspective of chemical composition. As a result, in terms of 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: 16.50 to 20.50%, Ni: 4.00 to 9.00%, Mo: 1.50 to 6.00%, Cu: 0.01 to 1.80%, Co: 0.01 to 1.00%, N: 0.200% or less, sol.Al: 0.001 to 0.100%, O: 0.0200% or less, W: 0 to 1.80%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, B: 0 to 0.0050%, rare earth elements: 0 to 0.100%, V: 0 to 0.50%, Ti: 0 to 0.300%, Nb: 0 to 0.300%, Zr: 0 to 0.200%, 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: Fe and impurities. The inventors considered that such a stainless steel material may have excellent general corrosion resistance and pitting corrosion resistance in a supercritical corrosion environment and excellent low-temperature toughness in an extremely low-temperature environment.

[0017] Furthermore, on the premise of having the above chemical composition, the inventors considered that by satisfying the following formula (1) with the chemical composition, the low-temperature toughness can be enhanced while enhancing the corrosion resistance. 0.10 ≦ Cu + W ≦ 1.85 (1) Here, in the element symbols in formula (1), the content of the corresponding element is substituted in units of mass%. When the corresponding element is not contained, "0" is substituted for the element symbol.

[0018] Define Fn1 = Cu + W. Fn1 is an index 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, although corrosion resistance can be obtained, sufficient low-temperature toughness cannot be obtained. Therefore, on the premise that the stainless steel material according to this embodiment has the above chemical composition, Fn1 is set to 0.10 to 1.85. As a result, on the premise of satisfying other configurations of this embodiment, 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 can be obtained.

[0019] Next, the inventors focused on the microstructure and considered obtaining a desired stainless steel material. Here, the microstructure of the stainless steel material having the above chemical composition consists of ferrite, retained austenite, and the balance being martensite. As a result of the study by the inventors, if the microstructure consists of ferrite with a volume fraction of 25 to 80%, retained austenite with a volume fraction of 10 to 35%, and the balance being martensite, it is revealed that there is a possibility of obtaining excellent general corrosion resistance and pitting corrosion resistance in a supercritical corrosion environment and excellent low-temperature toughness in an extremely low-temperature environment.

[0020] Here, if the yield strength of the stainless steel material is too high, the low-temperature toughness tends to decrease. Therefore, in this embodiment, the yield strength is set to 758 MPa or less. That is, in a stainless steel material having the above chemical composition and microstructure and satisfying Fn1 of 0.10 to 1.85, if the yield strength is 758 MPa or less, on the condition of satisfying other configurations of this embodiment, excellent general corrosion resistance and pitting corrosion resistance in a supercritical corrosion environment and excellent low-temperature toughness in an extremely low-temperature environment can be stably achieved simultaneously.

[0021] On the other hand, even for a stainless steel material having the above chemical composition and microstructure, with Fn1 satisfying 0.10 to 1.85 and a yield strength of 758 MPa or less, excellent low-temperature toughness in an extremely low-temperature environment may not be obtained. Therefore, the inventors of the present invention studied a method for enhancing excellent low-temperature toughness in an extremely low-temperature environment for a stainless steel material having the above chemical composition and microstructure, with Fn1 satisfying 0.10 to 1.85 and a yield strength of 758 MPa or less. As a result, it was clarified that for a stainless steel material having the above chemical composition and microstructure, with Fn1 satisfying 0.10 to 1.85 and a yield strength of 758 MPa or less, excellent low-temperature toughness can be obtained even in an extremely low-temperature environment if the content of elements and the volume fraction of the microstructure satisfy the following formula (2). 0.0350 ≦ (Ni + 1.5Co) / (Vγ + 3.3Vα′) (2) Here, in the element symbols in formula (2), the content of the corresponding element is substituted in units of mass%. Also, in Vγ in formula (2), the volume fraction of retained austenite in the microstructure is substituted in units of %, and in Vα′ in formula (2), the volume fraction of martensite in the microstructure is substituted in units of %.

[0022] Define Fn2 = (Ni + 1.5Co) / (Vγ + 3.3Vα′). Fn2 is an index of low-temperature toughness in an extremely low-temperature environment for a stainless steel material having the above chemical composition and microstructure, with Fn1 satisfying 0.10 to 1.85. The relationship between Fn2 and low-temperature toughness will be specifically described with reference to the drawings. FIG. 1 is a diagram showing 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 the present embodiment. FIG. 1 was created using Fn2 and the absorbed energy (J) at -80°C for an example among the examples described later, in which the configuration other than Fn2 satisfies the conditions of the present embodiment.

[0023] Referring to Fig. 1, when Fn2 is 0.0350 or more, the absorbed energy at -80°C becomes 100 J or more, and it can be confirmed that excellent low-temperature toughness can be obtained even in an extremely low-temperature environment. Therefore, the stainless steel material according to the present embodiment has the above chemical composition, Fn1 satisfies 0.10 to 1.85, has a microstructure composed of 25 to 80% ferrite, 10 to 35% retained austenite, and the balance martensite by volume ratio, has a yield strength of 758 MPa or less, and further, Fn2 is 0.0350 or more. As a result, the stainless steel material according to the present 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 the present embodiment completed is as follows.

[0025] [1] 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: 16.50 to 20.50%, Ni: 4.00 to 9.00%, Mo: 1.50 to 6.00%, Cu: 0.01 to 1.80%, Co: 0.01 to 1.00%, N: 0.200% or less, sol.Al: 0.001 to 0.100%, O: 0.0200% or less, W: 0 to 1.80%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, B: 0 to 0.0050%, Rare earth elements: 0 to 0.100%, V: 0 to 0.50%, Ti: 0 to 0.300%, Nb: 0 to 0.300%, Zr: 0 to 0.200%, 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), has a yield strength of 758 MPa or less, the microstructure consists of ferrite with a volume fraction of 25 to 80%, retained austenite with a volume fraction of 10 to 35%, and the balance being martensite, the content of the said elements and the volume fraction of the said microstructure satisfy formula (2), a stainless steel material. 0.10 ≦ Cu + W ≦ 1.85 (1) 0.0350 ≦ (Ni + 1.5Co) / (Vγ + 3.3Vα′) (2) Here, for the element symbols in formulas (1) and (2), the content of the corresponding element is substituted in units of mass%. When the corresponding element is not contained, "0" is substituted for that element symbol. Also, for Vγ in formula (2), the volume fraction of retained austenite in the microstructure is substituted in units of %, and for Vα′ in formula (2), the volume fraction of martensite in the microstructure is substituted in units of %.

[0026] [2] The stainless steel material according to [1], W: 0.01 to 1.80%, Ca: 0.0001 to 0.0100%, Mg: 0.0001 to 0.0100%, B: 0.0001 to 0.0050%, rare earth elements: 0.001 to 0.100%, V: 0.01 to 0.50%, Ti: 0.001 to 0.300%, Nb: 0.001 to 0.300%, Zr: 0.001 to 0.200%, Sn: 0.0001 to 0.0100%, As: 0.0001 to 0.0100%, Zn: 0.0001 to 0.0100%, Pb: 0.0001 to 0.0100%, and Sb: 0.0001 to 0.0100%, containing one or more elements selected from the group consisting of stainless steel material.

[0027] Note that 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, may be a round steel (solid material), or may be a steel plate. Note that round steel means a bar steel having a circular cross-section perpendicular to the axial direction. Also, the steel pipe may be a seamless steel pipe or a welded steel pipe.

[0028] Hereinafter, the stainless steel material according to this embodiment will be described in detail. In the following description, the stainless steel material is also simply referred to as "steel material". Also, in the following description, general corrosion resistance and pitting corrosion resistance are 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. "% " regarding the elements means mass % unless otherwise specified.

[0030] C: 0.050% or less Carbon (C) is inevitably contained. That is, the lower limit of the C content is more than 0%. C forms carbides and increases the corrosion susceptibility. Therefore, if the C content is too high, even if the other element contents are within the range of this embodiment, the corrosion resistance of the steel material in a supercritical corrosion environment will decrease. Therefore, the C content is 0.050% or less. The preferable upper limit of the C content is 0.045%, more preferably 0.040%, and even more preferably 0.035%. The lower the C content, the better. However, an extreme reduction in the C content significantly increases the manufacturing cost. Therefore, considering industrial production, the preferable lower limit of the C content is 0.001%, more preferably 0.003%.

[0031] Si: Below 1.00% Silicon (Si) is inevitably contained. That is, the lower limit of the Si content is over 0%. Si deoxidizes steel. On the other hand, if the Si content is too high, even if the contents of other elements are within the range of this embodiment, the hot workability of the steel material will decrease. Therefore, the Si content is 1.00% or less. The preferable upper limit of the Si content is 0.95%, more preferably 0.90%, and even more preferably 0.85%. The preferable lower limit of the Si content for obtaining the above effects more effectively is 0.05%, more preferably 0.10%, even more preferably 0.15%, and even more preferably 0.20%.

[0032] Mn: Below 1.00% Manganese (Mn) is inevitably contained. That is, the lower limit of the Mn content is over 0%. Mn deoxidizes steel and desulfurizes steel. Mn also improves the hot workability of the steel material. On the other hand, if the Mn content is too high, even if the contents of other elements are within the range of this embodiment, the volume fraction of retained austenite may become too high and the strength of the steel material may decrease. Therefore, the Mn content is 1.00% or less. The preferable upper limit of the Mn content is 0.80%, more preferably 0.60%, and even more preferably 0.50%. The preferable lower limit of the Mn content for obtaining the above effects more effectively is 0.01%, more preferably 0.02%, and even more preferably 0.03%.

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

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

[0035] Cr: 16.50 - 20.50% Chromium (Cr) forms a passive film on the surface of steel materials to enhance the corrosion resistance of the steel materials in a supercritical corrosion environment. If the Cr content is too low, even if the contents of other elements are within the scope of this embodiment, the above effects cannot be fully obtained. On the other hand, if the Cr content is too high, even if the contents of other elements are within the scope of this embodiment, the low-temperature toughness of the steel materials in an extremely low-temperature environment will decrease. Therefore, the Cr content is 16.50 - 20.50%. The preferable lower limit of the Cr content is 16.55%, more preferably 16.60%, and even more preferably 16.80%. The preferable upper limit of the Cr content is 20.45%, more preferably 20.40%, and even more preferably 20.30%.

[0036] Ni: 4.00 - 9.00% Nickel (Ni) enhances the low-temperature toughness of steel materials in an extremely low-temperature environment. If the Ni content is too low, even if the contents of other elements are within the scope of this embodiment, the above effects cannot be fully obtained. On the other hand, if the Ni content is too high, even if the contents of other elements are within the scope of this embodiment, the volume fraction of retained austenite becomes too high and the strength of the steel materials decreases. Therefore, the Ni content is 4.00 - 9.00%. The preferable lower limit of the Ni content is 4.05%, more preferably 4.10%, and even more preferably 4.20%. The preferable upper limit of the Ni content is 8.90%, more preferably 8.80%, and even more preferably 8.70%.

[0037] Mo: 1.50 - 6.00% Molybdenum (Mo) enhances the corrosion resistance of steel in supercritical corrosion environments. If the Mo content is too low, even if the contents of other elements are within the scope of this embodiment, the above effects cannot be fully obtained. On the other hand, if the Mo content is too high, even if the contents of other elements are within the scope of this embodiment, the low-temperature toughness of the steel in extremely low-temperature environments will decrease. Therefore, the Mo content is 1.50 - 6.00%. The preferable lower limit of the Mo content is 1.55%, more preferably 1.60%, still more preferably 1.70%, and even more preferably 1.80%. The preferable upper limit of the Mo content is 5.90%, more preferably 5.80%, still more preferably 5.60%.

[0038] Cu: 0.01 - 1.80% Copper (Cu) enhances the corrosion resistance of steel in supercritical corrosion environments. If the Cu content is too low, even if the contents of other elements are within the scope of this embodiment, the above effects cannot be fully obtained. On the other hand, if the Cu content is too high, even if the contents of other elements are within the scope of this embodiment, the low-temperature toughness of the steel in extremely low-temperature environments will decrease. Therefore, the Cu content is 0.01 - 1.80%. The preferable lower limit of the Cu content is 0.02%, more preferably 0.03%, still more preferably 0.05%. The preferable upper limit of the Cu content is 1.78%, more preferably 1.75%, still more preferably 1.70%.

[0039] Co: 0.01 - 1.00% Cobalt (Co) forms a film on the surface of the steel to enhance the corrosion resistance of the steel in supercritical corrosion environments. If the Co content is too low, even if the contents of other elements are within the scope of this embodiment, the above effects cannot be fully obtained. On the other hand, if the Co content is too high, even if the contents of other elements are within the scope of this embodiment, the manufacturing cost will increase extremely. Therefore, the Co content is 0.01 - 1.00%. The preferable lower limit of the Co content is 0.02%, more preferably 0.05%, still more preferably 0.10%. The preferable upper limit of the Co content is 0.80%, more preferably 0.60%, still more preferably 0.50%.

[0040] N: Below 0.200% Nitrogen (N) is inevitably contained. That is, the lower limit of the N content is over 0%. N stabilizes the retained austenite structure. On the other hand, if the N content is too high, even if the contents of other elements are within the scope of this embodiment, the toughness and hot workability of the steel material will decrease. Therefore, the N content is 0.200% or less. The preferable upper limit of the N content is 0.160%, more preferably 0.120%, still more preferably 0.080%, and even more preferably 0.060%. The preferable lower limit of the N content for obtaining the above effects more effectively is 0.001%, more preferably 0.003%, and still more preferably 0.004%.

[0041] sol.Al: 0.001 - 0.100% Aluminum (Al) deoxidizes the steel. If the Al content is too low, even if the contents of other elements are within the scope of this embodiment, the above effects cannot be sufficiently obtained. On the other hand, if the Al content is too high, even if the contents of other elements are within the scope of this embodiment, coarse oxides are generated and the low-temperature toughness of the steel material decreases. Therefore, the Al content is 0.001 - 0.100%. The preferable lower limit of the Al content is 0.003%, more preferably 0.005%, and still more preferably 0.010%. The preferable upper limit of the Al content is 0.080%, more preferably 0.060%, and still more preferably 0.050%. The Al content referred to in this specification means the content of sol.Al (acid-soluble Al).

[0042] O: 0.0200% or less Oxygen (O) is inevitably contained. That is, the lower limit of the O content is more than 0%. O forms oxides. Therefore, if the O content is too high, even if the contents of other elements are within the range of this embodiment, the corrosion resistance of the steel material in the supercritical corrosion environment will decrease. Thus, the O content is 0.0200% or less. The preferable upper limit of the O content is 0.0150%, more preferably 0.0100%, still more preferably 0.0060%, and even more preferably 0.0040%. It is preferable that the O content is as low as possible. However, an extreme reduction in the O content increases the manufacturing cost. Therefore, considering industrial production, the preferable lower limit of the O content is 0.0001%, more preferably 0.0003%.

[0043] The remainder of the chemical composition of the stainless steel material according to this embodiment consists of Fe and impurities. Here, the impurities in the chemical composition are those mixed in from ores, scraps, or manufacturing environments as raw materials when industrially manufacturing the stainless steel material, and are meant to be those allowed 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 contain W instead of a part of Fe.

[0045] W: 0 to 1.80% Tungsten (W) is an optional element and may not be contained. That is, the W content may be 0%. When contained, W enhances the corrosion resistance of the steel material in a supercritical corrosion environment. Even if a small amount of W is contained, the above effect can be obtained to a certain extent. However, if the W content is too high, even if the contents of other elements are within the scope of this embodiment, the volume fraction of ferrite may become too high and the strength of the steel material may decrease. Therefore, the W content is 0 to 1.80%. The preferable lower limit of the W content is more than 0%, more preferably 0.01%, still more preferably 0.02%, and still more preferably 0.03%. The preferable upper limit of the W content is 1.78%, more preferably 1.75%, and still more preferably 1.70%.

[0046] The chemical composition of the stainless steel material according to this embodiment may further contain at least one element selected from the group consisting of Ca, Mg, B, and rare earth elements in place of a part of Fe. All of these elements are optional elements and enhance the hot workability of the steel material.

[0047] Ca: 0 to 0.0100% Calcium (Ca) is an optional element and may not be contained. That is, the Ca content may be 0%. When contained, Ca immobilizes S in the steel material as sulfide to make it harmless and enhances the hot workability of the steel material. Even if a small amount of Ca is contained, the above effect can be obtained to a certain extent. However, if the Ca content is too high, even if the contents of other elements are within the scope of this embodiment, the oxides in the steel material become coarser and the corrosion resistance of the steel material in a supercritical corrosion environment decreases. Therefore, the Ca content is 0 to 0.0100%. The preferable lower limit of the Ca content is more than 0%, more preferably 0.0001%, still more preferably 0.0005%, still more preferably 0.0008%, and still more preferably 0.0010%. The preferable upper limit of the Ca content is 0.0080%, more preferably 0.0060%, and still more preferably 0.0040%.

[0048] Mg: 0 to 0.0100% Magnesium (Mg) is an optional element and may not be contained. That is, the Mg content may be 0%. When contained, Mg immobilizes S in the steel as sulfide to render it harmless and improves the hot workability of the steel. Even if a small amount of Mg is contained, the above effects can be obtained to a certain extent. However, if the Mg content is too high, even if the contents of other elements are within the scope of this embodiment, the oxides in the steel become coarser and the corrosion resistance of the steel in a supercritical corrosion environment decreases. Therefore, the Mg content is 0 to 0.0100%. The preferable lower limit of the Mg content is more than 0%, more preferably 0.0001%, and even more preferably 0.0002%. The preferable upper limit of the Mg content is 0.0080%, more preferably 0.0060%, and even more preferably 0.0040%.

[0049] B: 0 to 0.0050% Boron (B) is an optional element and may not be contained. That is, the B content may be 0%. When contained, B immobilizes S in the steel as sulfide to render it harmless and improves the hot workability of the steel. Even if a small amount of B is contained, the above effects can be obtained to a certain extent. However, if the B content is too high, even if the contents of other elements are within the scope of this embodiment, boron nitride (BN) is formed and the toughness of the steel decreases. Therefore, the B content is 0 to 0.0050%. The preferable lower limit of the B content is more than 0%, more preferably 0.0001%, and even more preferably 0.0003%. The preferable upper limit of the B content is 0.0040%, more preferably 0.0030%, and even more preferably 0.0020%.

[0050] Rare earth elements: 0 to 0.100% The rare earth element (REM) is an optional element and may not be contained. That is, the REM content may be 0%. When contained, REM immobilizes S in the steel material as sulfide to detoxify it and improves the hot workability of the steel material. The above effects can be obtained to some extent even if a small amount of REM is contained. However, if the REM content is too high, even if the contents of other elements are within the range of this embodiment, the oxides in the steel material become coarser and the corrosion resistance of the steel material in a supercritical corrosion environment decreases. Therefore, the REM content is 0 to 0.100%. The preferable lower limit of the REM content is more than 0%, more preferably 0.001%, and even more preferably 0.002%. The preferable upper limit of the REM content is 0.095%, more preferably 0.090%, and even more preferably 0.085%.

[0051] In addition, REM in this specification means one or more elements selected from the group consisting of scandium (Sc) with an atomic number of 21, yttrium (Y) with an atomic number of 39, and lanthanum (La) with an atomic number of 57 to lutetium (Lu) with an atomic number of 71, which are lanthanoids. Further, the REM content in this specification means 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, and Zr instead of a part of Fe. All of these elements are optional elements and increase the strength of the steel material.

[0053] V: 0 to 0.50% Vanadium (V) is an optional element and may not be contained. That is, the V content may be 0%. When contained, V forms carbonitrides and increases the strength of the steel. Even if a small amount of V is contained, the above effect can be obtained to a certain extent. However, if the V content is too high, even if the contents of other elements are within the scope 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 more 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%, even more preferably 0.20%.

[0054] Ti: 0 to 0.300% Titanium (Ti) is an optional element and may not be contained. That is, the Ti content may be 0%. When contained, Ti forms carbonitrides and increases the strength of the steel. Even if a small amount of Ti is contained, the above effect can be obtained to a certain extent. However, if the Ti content is too high, even if the contents of other elements are within the scope 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 0.300%. The preferred lower limit of the Ti content is more than 0%, more preferably 0.001%, even more preferably 0.002%, and even more preferably 0.003%. The preferred upper limit of the Ti content is 0.250%, more preferably 0.200%, even more preferably 0.150%, even more preferably 0.100%, and even more preferably 0.080%.

[0055] Nb: 0 to 0.300% Niobium (Nb) is an optional element and may not be contained. That is, the Nb content may be 0%. When contained, Nb forms carbonitrides and increases the strength of the steel material. Even a small amount of Nb contained can obtain the above effects to a certain extent. However, if the Nb content is too high, even if the contents of other elements are within the range of this embodiment, the strength of the steel material becomes too high and the low-temperature toughness of the steel material decreases. Therefore, the Nb content is 0 to 0.300%. The preferable lower limit of the Nb content is more than 0%, more preferably 0.001%, still more preferably 0.002%, and still more preferably 0.003%. The preferable upper limit of the Nb content is 0.280%, more preferably 0.240%, still more preferably 0.200%, and still more preferably 0.180%.

[0056] Zr: 0 to 0.200% Zirconium (Zr) is an optional element and may not be contained. That is, the Zr content may be 0%. When contained, Zr forms carbonitrides and increases the strength of the steel material. Even a small amount of Zr contained can obtain the above effects to a certain extent. However, if the Zr content is too high, even if the contents of other elements are within the range of this embodiment, the strength of the steel material becomes too high and the low-temperature toughness of the steel material decreases. Therefore, the Zr content is 0 to 0.200%. The preferable lower limit of the Zr content is more than 0%, more preferably 0.001%, still more preferably 0.002%, and still more preferably 0.003%. The preferable upper limit of the Zr content is 0.180%, more preferably 0.160%, still more preferably 0.150%, and still more preferably 0.140%.

[0057] 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 part of Fe. All of these elements are optional elements and enhance the corrosion resistance of the steel material in a supercritical corrosion environment.

[0058] Sn: 0 to 0.0100% Tin (Sn) is an optional element and may not be contained. That is, the Sn content may be 0%. When contained, Sn enhances the corrosion resistance of the steel in a supercritical corrosion environment. Even if a small amount of Sn is contained, the above effect can be obtained to a certain extent. However, if the Sn content is too high, the hot workability of the steel will decrease even if the contents of other elements are within the scope of this embodiment. Therefore, the Sn content is 0 to 0.0100%. The preferred lower limit of the Sn content is more than 0%, more preferably 0.0001%, still more preferably 0.0002%, and even more preferably 0.0003%. The preferred upper limit of the Sn content is 0.0080%, more preferably 0.0060%, still more preferably 0.0050%.

[0059] As: 0 to 0.0100% Arsenic (As) is an optional element and may not be contained. That is, the As content may be 0%. When contained, As enhances the corrosion resistance of the steel in a supercritical corrosion environment. Even if a small amount of As is contained, the above effect can be obtained to a certain extent. However, if the As content is too high, the corrosion resistance of the steel in a supercritical corrosion environment may conversely decrease even if the contents of other elements are within the scope of this embodiment. Therefore, the As content is 0 to 0.0100%. The preferred lower limit of the As content is more than 0%, more preferably 0.0001%, still more preferably 0.0002%, and even more preferably 0.0003%. The preferred upper limit of the As content is 0.0080%, more preferably 0.0060%, still more preferably 0.0040%, and even more preferably 0.0020%.

[0060] Zn: 0 to 0.0100% Zinc (Zn) is an optional element and may not be contained. That is, the Zn content may be 0%. When contained, Zn enhances the corrosion resistance of the steel material in a supercritical corrosion environment. Even if a small amount of Zn is contained, the above effect can be obtained to a certain extent. However, if the Zn content is too high, even if the contents of other elements are within the scope of this embodiment, the corrosion resistance of the steel material in a supercritical corrosion environment may conversely decrease. Therefore, the Zn content is 0 to 0.0100%. The preferable lower limit of the Zn content is more than 0%, more preferably 0.0001%, still more preferably 0.0002%, and still more preferably 0.0003%. The preferable upper limit of the Zn content is 0.0080%, more preferably 0.0060%, and still more preferably 0.0055%.

[0061] Pb: 0 to 0.0100% Lead (Pb) is an optional element and may not be contained. That is, the Pb content may be 0%. When contained, Pb enhances the corrosion resistance of the steel material in a supercritical corrosion environment. Even if a small amount of Pb is contained, the above effect can be obtained to a certain extent. However, if the Pb content is too high, even if the contents of other elements are within the scope of this embodiment, the hot workability of the steel material decreases. Therefore, the Pb content is 0 to 0.0100%. The preferable lower limit of the Pb content is more than 0%, more preferably 0.0001%, still more preferably 0.0002%, and still more preferably 0.0003%. The preferable upper limit of the Pb content is 0.0080%, more preferably 0.0060%, still more preferably 0.0050%, and still more preferably 0.0030%.

[0062] Sb: 0 to 0.0100% Antimony (Sb) is an optional element and may not be contained. That is, the Sb content may be 0%. When contained, Sb enhances the corrosion resistance of the steel material in a supercritical corrosion environment. Even if a small amount of Sb is contained, the above effect can be obtained to a certain extent. However, if the Sb content is too high, even if the contents of other elements are within the scope of this embodiment, the manufacturing cost will increase extremely. Therefore, the Sb content is 0 to 0.0100%. The preferable lower limit of the Sb content is more than 0%, more preferably 0.0001%, still more preferably 0.0002%, and still more preferably 0.0003%. The preferable upper limit of the Sb content is 0.0080%, more preferably 0.0060%, still more preferably 0.0040%, and still more preferably 0.0020%.

[0063] [Fn1] On the premise that the stainless steel material according to this embodiment has the above chemical composition, it satisfies the following formula (1). 0.10 ≦ Cu + W ≦ 1.85 (1) Here, in the element symbols in formula (1), the contents of the corresponding elements are substituted in units of mass%. When the corresponding element is not contained, "0" is substituted for the element symbol.

[0064] Fn1 (= Cu + W) is an index of the balance between corrosion resistance and low-temperature toughness in the stainless steel material having the above chemical composition. If Fn1 is increased to 0.10 or more, the stability of the passive film increases, and the corrosion resistance of the steel material in a supercritical corrosion environment increases. On the other hand, if Fn1 exceeds 1.85, although corrosion resistance can be obtained, sufficient low-temperature toughness cannot be obtained. Therefore, on the premise that the stainless steel material according to this embodiment has the above chemical composition, Fn1 is set to 0.10 to 1.85.

[0065] The preferable lower limit of Fn1 is 0.11, more preferably 0.12, and still more preferably 0.13. The preferable upper limit of Fn1 is 1.84, more preferably 1.83, and still more preferably 1.82. Note that Fn1 is obtained by rounding off the third decimal place of the obtained numerical value.

[0066] [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. That is, the yield strength of the stainless steel material according to this embodiment may be 552 to 758 MPa. The preferable upper limit of the yield strength is 750 MPa, more preferably 740 MPa, and even more preferably 730 MPa. The preferable lower limit of the yield strength is 560 MPa, more preferably 570 MPa.

[0067] In this embodiment, the yield strength of the stainless steel material is determined by the following method. Specifically, a tensile test is performed by a method conforming to ASTM E8 / E8M (2022). A test piece is prepared from the steel material according to this embodiment. When the steel material is a steel plate, a tensile test piece is prepared from the central part of the plate thickness. In this case, the longitudinal direction of the tensile test piece is parallel to the rolling direction of the steel plate. When the steel material is a steel pipe, a round bar test piece or an arc-shaped test piece is prepared as a tensile test piece from the central part of the wall thickness. In this case, the longitudinal direction of the round bar test piece or the arc-shaped test piece is parallel to the pipe axis direction of the steel pipe. When the steel material is a round steel, a tensile test piece is prepared from the R / 2 position. In this specification, the R / 2 position of the round steel means the central position of the radius R in a cross section perpendicular to the axial direction of the round steel. In this case, the longitudinal direction of the tensile test piece is parallel to the axial direction of the round steel.

[0068] The tensile test piece is, for example, a round bar test piece with a parallel part diameter of 8.9 mm and a gauge length of 35.6 mm. When a round bar test piece cannot be prepared from the steel pipe, an arc-shaped test piece is prepared. The size of the arc-shaped test piece is, for example, the thickness is the full wall thickness, the width is 25.4 mm, and the gauge length is 50.8 mm. Using the prepared tensile test piece, a tensile test is carried out at room temperature (24 ± 3°C) in accordance with ASTM E8 / E8M (2022). The 0.2% offset yield strength (MPa) obtained by the tensile test is defined as the yield strength (MPa). In this embodiment, the yield strength (MPa) is obtained by rounding off the first decimal place of the obtained numerical value.

[0069] [Microstructure] The microstructure of the stainless steel material according to this embodiment consists of ferrite with a volume ratio of 25 to 80%, retained austenite with a volume ratio of 10 to 35%, and the balance being martensite. In this specification, the microstructure being "consisting of ferrite, retained austenite, and martensite" means that the phases other than ferrite, retained austenite, and martensite in the microstructure are so few that they can be ignored. For example, in the chemical composition of the stainless steel material according to this embodiment, the volume ratio of precipitates and inclusions is negligibly small compared to the volume ratios of ferrite, retained austenite, and martensite. That is, the microstructure of the stainless steel material according to this embodiment may contain a small amount of precipitates, inclusions, etc. in addition to ferrite, retained austenite, and martensite.

[0070] As described above, in the microstructure of the stainless steel material according to this embodiment, the volume ratio of ferrite is 25 to 80%. If the volume ratio of ferrite is too low, the corrosion resistance of the steel material in a supercritical corrosion environment decreases. On the other hand, if the volume ratio of ferrite is too high, the ferrite coarsens and the low-temperature toughness of the steel material in an extremely low-temperature environment decreases. Therefore, in the microstructure of the stainless steel material according to this embodiment, the volume ratio of ferrite is 25 to 80%. The preferable lower limit of the volume ratio of ferrite is 26%, more preferably 27%, and even more preferably 28%. The preferable upper limit of the volume ratio of ferrite is 75%, more preferably 70%, and even more preferably 65%.

[0071] As described above, in the microstructure of the stainless steel material according to the present embodiment, the volume ratio of retained austenite is 10 to 35%. If the volume ratio of retained austenite is too low, the low-temperature toughness of the steel material in an extremely low-temperature environment decreases. On the other hand, if the volume ratio of retained austenite is too high, the strength of the steel material decreases. Therefore, in the microstructure of the stainless steel material according to the present embodiment, the volume ratio of retained austenite is 10 to 35%. The preferable lower limit of the volume ratio of retained austenite is 10%, more preferably 11%, and even more preferably 12%. The preferable upper limit of the volume ratio of retained austenite is 34%, more preferably 33%, and even more preferably 32%.

[0072] As described above, the stainless steel material according to the present embodiment has a microstructure composed of 25 to 80% ferrite, 10 to 35% retained austenite, and the balance being martensite by volume ratio. In this specification, "martensite" includes not only fresh martensite but also tempered martensite. Also, the volume ratio of martensite is not particularly limited, but is, for example, 10 to 50%. The preferable lower limit of the volume ratio of martensite is 11%, more preferably 12%, and even more preferably 13%. The preferable upper limit of the volume ratio of martensite is 49%, more preferably 48%, and even more preferably 47%.

[0073] In the present embodiment, the volume ratio of each phase of the microstructure is determined by the following method. Specifically, the volume ratio (%) of retained austenite and the volume ratio (%) of ferrite in the microstructure of the steel material are determined by the following method. The volume ratio of martensite (%) is obtained by subtracting the obtained volume ratio of retained austenite and the volume ratio of ferrite from 100%.

[0074] [Measurement method of volume ratio of retained austenite] The volume fraction of retained austenite in the microstructure of the steel material is determined by X-ray diffraction. Specifically, a test piece for measuring the volume fraction of retained austenite is produced from the steel material according to this embodiment. When the steel material is a steel plate, the test piece is sampled from the center of the plate thickness. When the steel material is a steel pipe, the test piece is sampled from the center of the wall thickness. When the steel material is a round steel, the test piece is sampled from the R / 2 position. The size of the test piece is not particularly limited. The test piece is, for example, 15 mm × 15 mm × 2 mm thick. When the steel material is a steel pipe, the thickness direction of the test piece is the pipe diameter direction. When the steel material is a round steel, the thickness direction of the test piece is the diameter direction. When the steel material is a steel plate, the thickness direction of the test piece is the plate thickness direction. Using the produced test piece, the X-ray diffraction intensities of the (110) plane of the α-phase (martensite), the (200) plane of the α-phase, the (211) plane of the α-phase, the (111) plane of the γ-phase (retained austenite), the (200) plane of the γ-phase, and the (220) plane of the γ-phase are measured, and the integrated intensity of each plane is calculated.

[0075] In the measurement of the X-ray diffraction intensity, the target of the X-ray diffractometer is set to Co (CoKα ray), and the output is set to 30 kV - 100 mA. The measurement angle (2θ) is set to 45 to 105°. After calculation, the volume fraction Vγ (%) of retained austenite is calculated using Equation (I) for each combination (3 × 3 = 9 combinations) of each plane of the α-phase and each plane of the γ-phase. Then, the average value of the volume fractions Vγ of the 9 combinations of retained austenite is 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 calculated value of the α-phase. Iγ is the integrated intensity of the γ-phase. Rγ is the crystallographic theoretical calculated value of the γ-phase. As the values of Rα and Rγ for each plane, the values incorporated in the retained γ quantitative analysis system attached to Rigaku Corporation's product name RINT-TTR can be used. Note that the volume fraction of retained austenite is obtained by rounding off the first decimal place of the obtained numerical value.

[0076] [Method for Measuring the Volume Fraction of Ferrite] The volume fraction of ferrite in the microstructure of the steel material is determined by the point counting method. Specifically, a test piece for measuring the volume fraction of ferrite is prepared from the steel material according to this embodiment. When the steel material is a steel plate, the test piece is sampled from the center of the plate thickness. In this case, the observation surface of the test piece is a surface parallel to the rolling direction of the steel plate. When the steel material is a steel pipe, the test piece is sampled from the center of the wall thickness. In this case, the observation surface of the test piece is a surface parallel to the pipe axis direction of the steel pipe. When the steel material is a round bar, the test piece is sampled from the R / 2 position. In this case, the observation surface of the test piece is a surface parallel to the axial direction of the round bar. Note that the size of the test piece is not particularly limited. After mechanically polishing the observation surface, the observation surface is electrolytically etched to reveal the structure. The electrolytic etching is carried out with an electrolytic solution: a mixed solution of aqua regia (a solution mixed at 3:1 of hydrochloric acid: nitric acid) and glycerin, a current density: 1 A / cm 2 , and an electrolytic time: 1 minute.

[0077] The electrolytically etched observation surface is observed in 30 fields of view using an optical microscope. The observation field of view is a rectangle of 250 μm × 250 μm. Note that the observation magnification is 400 times. In each observation field of view, ferrite and other phases (retained austenite and martensite) can be distinguished from each other by contrast by those skilled in the art. Therefore, ferrite in each observation field of view is specified based on the contrast. The area fraction of the specified ferrite is determined by the point counting method conforming to JIS G 0555(2020).

[0078] Specifically, for the observation field of view, 20 vertical lines are drawn at equal intervals from the upper end to the lower end of the observation field of view. That is, due to the 20 vertical lines, the observation field of view is divided into 21 regions in the left-right direction. Further, for the observation field of view, 20 horizontal lines are drawn at equal intervals from the left end to the right end of the observation field of view. That is, due to the 20 horizontal lines, the observation field of view is divided into 21 regions in the up-down direction. At this time, the intersection points of the vertical lines and the horizontal lines are called lattice points. That is, 400 lattice points are arranged at equal intervals in the observation field of view. In accordance with JIS G 0555(2020), the lattice points overlapping with ferrite are counted in the observation field of view. The number of lattice points overlapping with ferrite obtained in 30 fields of view is divided by the total number of lattice points (400×30 = 12000), and is 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 (%). Note that the ferrite volume ratio is obtained by rounding the first decimal place of the obtained numerical value.

[0079] Using the volume ratio (%) of retained austenite obtained by the above X-ray diffraction method and the volume ratio (%) of ferrite obtained by the above point counting method, the volume ratio (%) of martensite in the microstructure of the steel material is obtained by the following formula. Volume ratio of martensite (%) = 100 - {Volume ratio of retained austenite (%) + Volume ratio of ferrite (%)}

[0080] [Fn2] On the premise that the stainless steel material according to this embodiment has the above chemical composition and microstructure, the content of elements and the volume ratio of the microstructure satisfy the following formula (2). 0.0350 ≦ (Ni + 1.5Co) / (Vγ + 3.3Vα′) (2) Here, in the element symbols in formula (2), the content of the corresponding element is substituted in units of mass%. Also, 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 %.

[0081] Fn2 (= (Ni + 1.5Co) / (Vγ + 3.3Vα')) is an index of low-temperature toughness in a cryogenic environment for a stainless steel material having the above-described chemical composition and microstructure, where Fn1 satisfies 0.10 to 1.85. As described above, among Fn2, Ni and Co enhance the low-temperature toughness of the steel material. Further, among the microstructures, retained austenite enhances low-temperature toughness, while martensite is likely to significantly reduce low-temperature toughness. Therefore, depending on the volume fraction Vα' of martensite, by increasing the contents of elements Ni and Co that enhance low-temperature toughness, it may be possible to obtain excellent low-temperature toughness even in a cryogenic environment.

[0082] Therefore, the stainless steel material according to the present embodiment has the above-described chemical composition and microstructure, where Fn1 satisfies 0.10 to 1.85, and further, Fn2 is 0.0350 or more. As a result, the stainless steel material according to the present embodiment can achieve both excellent general corrosion resistance and pitting corrosion resistance in a supercritical corrosion environment and excellent low-temperature toughness in a cryogenic environment.

[0083] The preferable lower limit of Fn2 is 0.0360, more preferably 0.0365, and even more preferably 0.0370. The upper limit of Fn2 is not particularly limited, but is substantially 0.2442. The preferable upper limit of Fn2 is 0.2000, more preferably 0.1500, and even more preferably 0.1200. Note that Fn2 is obtained by rounding off the fifth decimal place of the obtained numerical value.

[0084] [Corrosion Resistance] The stainless steel material according to the present embodiment has the above-described chemical composition, where Fn1 satisfies 0.10 to 1.85, the yield strength is 758 MPa or less, the microstructure consists of ferrite with a volume fraction of 25 to 80%, retained austenite with a volume fraction of 10 to 35%, and the balance being martensite, and Fn2 is 0.0350 or more. As a result, the stainless steel material according to the present embodiment has excellent corrosion resistance (general corrosion resistance and pitting corrosion resistance) even in a supercritical corrosion environment. In the present embodiment, the excellent general corrosion resistance and pitting corrosion resistance in a supercritical corrosion environment are evaluated by the following method.

[0085] Specifically, test pieces for corrosion tests are taken from the stainless steel materials according to this embodiment. When the steel material is a steel plate, the test pieces are taken from the central part of the plate thickness. When the steel material is a steel pipe, the test pieces are taken from the central part of the wall thickness. When the steel material is a round steel, the test pieces are taken from the R / 2 position. The size of the test pieces is not particularly limited. For example, the length is 30 mm, the width is 20 mm, and the thickness is 2 mm.

[0086] The test pieces are sealed in an autoclave. An aqueous sodium chloride solution with a concentration of 5.0 mass% adjusted to a pH of 2.5 is injected into the autoclave so that the test pieces are immersed. A mixed gas of SO2, O2, and CO2 is pressure-sealed into the autoclave to saturate the test solution, forming a 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 pieces are immersed for 96 hours while stirring the test bath.

[0087] After 96 hours, the mass, density, and surface area of the test pieces are determined, and the corrosion rate (mm / year) of the test pieces is determined. In this embodiment, the corrosion rate is obtained by rounding the fourth decimal place of the obtained numerical value. Furthermore, the surface of the test pieces after 96 hours is observed with a magnifying glass with a magnification of 10 times to confirm the presence or absence of pitting corrosion. If pitting corrosion is suspected based on the observation with the magnifying glass, further observation is performed with an optical microscope with a magnification of 100 times to confirm the presence or absence of pitting corrosion. In this embodiment, as a result of the corrosion test under the above conditions, if the obtained corrosion rate is 0.100 mm / year or less, it is evaluated that the material has excellent general corrosion resistance even in a supercritical corrosion environment. In this embodiment, furthermore, as a result of the corrosion test under the above conditions, if pitting corrosion is not confirmed, it is evaluated that the material has excellent pitting corrosion resistance even in a supercritical corrosion environment.

[0088] [Low-temperature toughness] The stainless steel material according to this embodiment has the above-described chemical composition, where Fn1 satisfies 0.10 to 1.85, the yield strength is 758 MPa or less, the microstructure consists of ferrite with a volume fraction of 25 to 80%, retained austenite with a volume fraction of 10 to 35%, and the balance being martensite, and Fn2 is 0.0350 or more. As a result, the stainless steel material according to this embodiment has excellent low-temperature toughness in an extremely low-temperature environment. In this embodiment, the excellent low-temperature toughness in an extremely low-temperature environment is evaluated by the following method.

[0089] Specifically, from the stainless steel material according to this embodiment, in accordance with API 5CT (2019), full-size or sub-size V-notch test pieces are prepared. Here, when 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). When the steel material is a steel pipe, the diameter direction of the steel pipe is defined as the "C direction", the pipe axis 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". When the steel material is a round bar, the cross-sectional diameter direction of the round bar is defined as the "C direction", the axial direction of the round 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".

[0090] For the prepared V-notch test pieces, a Charpy impact test in accordance with JIS Z 2242 (2018) is carried out to obtain the absorbed energy (J) at -80°C. When using sub-size V-notch test pieces, the obtained absorbed energy is divided by the reduction factor described in API 5CT (2019) and converted to the absorbed energy of full-size V-notch test pieces. In this embodiment, the absorbed energy (J) at -80°C is obtained by rounding the first decimal place of the obtained numerical value.

[0091] In this embodiment, when the absorbed energy at -80°C obtained under 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 the "absorbed energy".

[0092] [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 (general corrosion resistance and pitting corrosion resistance) in a supercritical corrosion environment and excellent low-temperature toughness in an extremely low-temperature environment.

[0093] [Manufacturing Method] An example of the manufacturing method of the stainless steel material according to this embodiment having the above configuration will be described. Note that the manufacturing method of the stainless steel material according to this embodiment is not limited to the manufacturing method described below. An example of the manufacturing method of the stainless steel material of this embodiment includes a step of preparing an intermediate steel material (preparation step), a step of quenching the intermediate steel material (quenching step), and a step of tempering (tempering step). Hereinafter, each step will be described in detail.

[0094] [Preparation Step] In the preparation step, an intermediate steel material having the above chemical composition is prepared. The manufacturing method of the intermediate steel material is not particularly limited as long as the intermediate steel material has the above chemical composition. The intermediate steel material referred to here is a plate-shaped steel material when the final product is a steel plate or a welded steel pipe, a plain pipe when the final product is a seamless steel pipe, and a steel material having a circular cross-section perpendicular to the axial direction when the final product is a round steel.

[0095] The preparation step may include a step of preparing a raw material (raw material preparation step) and a step of hot-working the raw material to manufacture an intermediate steel material (hot-working step). Hereinafter, the case including the raw material preparation step and the hot-working step will be described in detail.

[0096] [Raw Material Preparation Step] In the material preparation process, materials are manufactured using molten steel having the above-described chemical composition. The method for manufacturing the materials is not particularly limited and may be a well-known method. Specifically, a slab (slab, bloom, or billet) may be manufactured by continuous casting using molten steel. An ingot may be manufactured by an ingot-making method using molten steel. If necessary, a slab, bloom, or ingot may be chunk-rolled to produce a billet. Materials (slab, bloom, or billet) are manufactured through the above steps.

[0097] [Hot working process] In the hot working process, the prepared materials are hot-worked to produce intermediate steel materials. As described above, when the steel material is a seamless steel pipe, the intermediate steel material corresponds to a plain pipe. First, the billet is heated in a heating furnace. The heating temperature is not particularly limited, but for example, it is 1100 - 1300 °C. Hot working is performed on the billet extracted from the heating furnace to produce a plain pipe (seamless steel pipe). The method of hot working is not particularly limited and may be a well-known method.

[0098] For example, the Mannesmann method may be performed as hot working to produce a plain pipe. In this case, a round billet is pierced and rolled by a piercing mill. When piercing and rolling, the piercing ratio is not particularly limited, but for example, it is 1.0 - 4.0. The pierced and rolled round billet is further hot-rolled by a mandrel mill, a reducer, a sizing mill, etc. to form a plain pipe. The cumulative reduction ratio in the hot working process is, for example, 20 - 70%. Other hot working methods may be performed to produce a plain pipe from the billet. For example, when the steel material is a short and thick-walled steel pipe such as a coupling, a plain pipe may be produced by forging such as the Erhardt method. A plain pipe is manufactured through the above steps. The wall thickness of the plain pipe is not particularly limited, but for example, it is 9 - 60 mm.

[0099] When the steel material is round steel, first, the material is heated in a heating furnace. The heating temperature is not particularly limited, for example, it is 1100 - 1300°C. Hot working is performed on the material extracted from the heating furnace to produce an intermediate steel material with a circular cross-section perpendicular to the axial direction. The hot working is, for example, block rolling by a block rolling mill or hot rolling by a continuous rolling mill. The continuous rolling mill has a horizontal stand with a pair of grooved rolls arranged side by side in the vertical direction and a vertical stand with a pair of grooved rolls arranged side by side in the horizontal direction, which are alternately arranged. When the steel material is a steel plate, first, the material is heated in a heating furnace. The heating temperature is not particularly limited, for example, it is 1100 - 1300°C. Hot rolling is performed on the material extracted from the heating furnace using a block rolling mill and a continuous rolling mill to produce an intermediate steel material in the shape of a steel plate.

[0100] The intermediate steel material produced by hot working may be air-cooled (As-Rolled). The intermediate steel material produced by hot working may be directly quenched after hot working without cooling to room temperature, or may be reheated (reheated) after hot working and then quenched. When directly quenching after hot working or quenching after reheating, cooling may be stopped or slow cooling may be performed during quenching. In this case, the occurrence of burning cracks in the plain tube can be suppressed. When directly quenching after hot working or quenching after reheating, further, stress relief annealing (SR) may be performed after quenching and before the heat treatment in the next process. In this case, the residual stress of the plain tube is removed.

[0101] As described above, in the preparation process, an intermediate steel material is prepared. The intermediate steel material may be produced by the above-described preferred process, or an intermediate steel material produced by a third party, or an intermediate steel material produced at another factory or other business office other than the factory where the subsequent quenching process and tempering process are performed may be prepared. Hereinafter, the quenching process will be described in detail.

[0102] [Quenching Process] Quenching is performed on the intermediate steel material manufactured in the hot working process (quenching process). The quenching is carried out by a well-known method. Specifically, the intermediate steel material after the hot working process is charged into a heat treatment furnace, held at the quenching temperature, and then rapidly cooled (quenched). Here, the quenching temperature means the temperature (°C) of the heat treatment furnace for heating the intermediate steel material in the quenching process. The holding time in the quenching process means the time (minutes) during which the intermediate steel material is held at the heat treatment temperature.

[0103] In the quenching process according to this embodiment, if the quenching temperature is too low, the heating of the intermediate steel material is insufficient, and in the manufactured stainless steel material, the above-mentioned microstructure may not be obtained. On the other hand, if the quenching temperature is too high, the volume fraction of ferrite becomes too high, the ferrite coarsens, and the low-temperature toughness of the steel material may not be obtained in an extremely low-temperature environment. Therefore, in this embodiment, the preferable quenching temperature is 850 to 1100 °C. The holding time at the quenching temperature is not particularly limited, but for example, it is 5 to 80 minutes.

[0104] As described above, in this embodiment, after the intermediate steel material is held at the quenching temperature, it is rapidly cooled (quenched). The quenching method is, for example, water cooling. The quenching method is not particularly limited. When the intermediate steel material is a plain tube, for example, the plain tube may be rapidly cooled by immersing it in a water tank or an oil tank, or water may be poured or sprayed onto the outer surface and / or inner surface of the plain tube by shower cooling or mist cooling to rapidly cool the plain tube.

[0105] [Tempering process] A tempering process is further performed on the intermediate steel material after the quenching process. In this specification, "tempering" means reheating and holding the intermediate steel material after quenching at a temperature below the A c1 point. Here, the tempering temperature corresponds to the temperature of the furnace when the intermediate steel material after quenching is heated and held. The tempering time means the time from when the temperature of the intermediate steel material reaches a predetermined tempering temperature until it is extracted from the heat treatment furnace.

[0106] In the tempering process according to this embodiment, by holding at a predetermined temperature, the volume fraction of retained austenite is increased, and further the strength of the intermediate steel material is adjusted. Therefore, 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 formula (A). (T + 273)×(20 + log(t / 60)) + 210Ni ≥ 18300 (A) Here, in the formula (A), the tempering temperature is substituted for "T" in the unit of °C, the tempering time is substituted for "t" in the unit of minutes, and the Ni content of the intermediate steel material is substituted for "Ni" in the unit of mass %. Also, "log" in the formula (A) means the logarithm with base 10 (common logarithm).

[0107] Define FnA = (T + 273)×(20 + log(t / 60)) + 210Ni. FnA is a tempering parameter according to the Ni content. If FnA is 18300 or more, Fn2 is likely to be 0.0350 or more. Therefore, in the tempering process according to this embodiment, it is preferable that FnA is 18300 or more.

[0108] Note that the tempering temperature T (°C) and the tempering time t (minutes) only need to satisfy that FnA is 18300 or more, and are not particularly limited. The tempering temperature T may be, for example, 540 to 670 °C. The tempering time t may be, for example, 10 to 180 minutes.

[0109] By the above process, the stainless steel material according to this embodiment can be manufactured. As described above, the stainless steel material according to this embodiment is not limited to the above manufacturing method. Hereinafter, the stainless steel material according to this embodiment will be described more specifically by examples.

Examples

[0110] The molten steel having the chemical compositions shown in Table 1A, Table 1B, and Table 1C was melted using a 50 kg vacuum melting furnace, and steel ingots were produced by the ingot casting method. In Table 1B and Table 1C, "-" means that the content of the corresponding element was at the impurity level. For example, the W content and V content of Steel A, rounded to the third decimal place, were 0%, meaning that they were less than 0.005%. Similarly, the REM content, Ti content, Nb content, and Zr content of Steel A, rounded to the fourth decimal place, were 0%, meaning that they were less than 0.0005%. Similarly, 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, were 0%, meaning that they were less than 0.00005%.

[0111]

Table 1A

[0112]

Table 1B

[0113]

Table 1C

[0114] Furthermore, the chemical compositions described in Table 1A and Table 1B and Fn1 (= Cu + W) obtained from the above definitions are shown in Table 2.

[0115]

Table 2

[0116] For each test number except test numbers 4 and 20, the ingot was heated at 1200 - 1250 °C for 2 hours and then hot - worked to produce an intermediate steel material (plain pipe) 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 and then hot - worked to produce an intermediate steel material (steel plate) with a thickness of 45 mm and a width of 60 mm. For the intermediate steel materials of each test number, a quenching process and a tempering process were carried out. Specifically, the intermediate steel material of each test number was held at 900 °C for 15 minutes and then rapidly cooled. Then, the intermediate steel material was heated and held at the tempering temperature (°C) described in the tempering column of Table 2 for the holding time (minutes).

[0117] [Evaluation Test] Through the above - mentioned processes, steel materials of each test number were obtained. For the obtained steel materials of each test number, a tensile test, a microstructure observation test, a corrosion test, and a Charpy impact test were carried out.

[0118] [Tensile Test] For the steel materials of each test number, a tensile test was carried out in accordance with ASTM E8 / E8M (2022). Specifically, by the above - mentioned method, round - bar tensile test specimens with a parallel - part diameter of 8.9 mm and a gauge length of 35.6 mm were prepared from the steel materials of each test number. Using the round - bar tensile test specimens of each test number, a tensile test was carried out at room temperature (24 ± 3 °C) in the air, and the 0.2% offset yield strength (MPa) was obtained. The obtained 0.2% offset yield strength was defined as the yield strength (MPa). The yield strengths of each test number obtained were shown in the "YS (MPa)" column of Table 3.

[0119]

Table 3

[0120] [Microstructure Observation Test] For the steel materials of each test number, a microstructure observation test was carried out by the above-mentioned method. Specifically, the volume fraction (%) of retained austenite was determined by the X-ray diffraction method carried out by the above-mentioned method. Furthermore, the volume fraction (%) of ferrite was determined by the point counting method compliant with JIS G 0555(2020) carried out by the above-mentioned method. From the obtained volume fraction of retained austenite and the volume fraction of ferrite, the volume fraction (%) of martensite was determined. The obtained volume fraction of ferrite for each test number is shown in the "Ferrite (volume %)" column of Table 3. The obtained volume fraction of retained austenite for each test number is shown in the "Retained γ (volume %)" column of Table 3. The obtained volume fraction of martensite for each test number is shown in the "Martensite (volume %)" column of Table 3. Furthermore, Fn2 (= (Ni + 1.5Co) / (Vγ + 3.3Vα')) obtained from the chemical composition described in Table 1A, the volume fractions of each phase of the microstructure described in Table 3, and the above-mentioned definition is shown in Table 3.

[0121] [Corrosion test] For the steel materials of each test number, a corrosion test was carried out to evaluate the general corrosion resistance and pitting corrosion resistance in a supercritical corrosion environment. Specifically, test pieces for the corrosion test were prepared by the above-mentioned method. For the prepared test pieces, a corrosion test was carried out under the above-mentioned conditions to obtain the corrosion rate (mm / year). Furthermore, for the test pieces after the corrosion test, the presence or absence of pitting corrosion was confirmed by the above-mentioned method. The obtained corrosion rate (mm / year) is shown in Table 3. Also, for the test numbers where pitting corrosion was not confirmed, "E (Excellent)" is shown in the "Pitting corrosion" column of Table 3. Furthermore, for the test numbers where pitting corrosion was confirmed, "NA (Not Acceptable)" is shown in the "Pitting corrosion" column of Table 3.

[0122] [Charpy impact test] For the steel materials with each test number, a Charpy impact test was carried out in accordance with JIS Z 2242 (2018). Specifically, in the above-mentioned method, a full-size V-notch test piece was fabricated in accordance with API 5CT (2019). For the fabricated V-notch test piece, a Charpy impact test was carried out in accordance with JIS Z 2242 (2018) to obtain the absorbed energy (J) at -80°C. The absorbed energy at -80°C for each test number obtained was shown in the column of "vE(-80°C)(J)" in Table 3.

[0123] [Evaluation Results] Referring to Table 1A, Table 1B, Table 1C, Table 2, and Table 3, the steel materials with test numbers 1 to 33 had appropriate chemical compositions and satisfied Fn1 of 0.10 to 1.85. These steel materials were further manufactured by the preferred manufacturing method described in the specification. As a result, the yield strength of these steel materials was 758 MPa or less. These steel materials further had a microstructure consisting of 25 to 80% ferrite, 10 to 35% retained austenite, and the balance martensite by volume ratio. Fn2 of these steel materials was 0.0350 or more. As a result, these steel materials had a corrosion rate of 0.100 mm / year or less in the corrosion test and had excellent general corrosion resistance even in a supercritical corrosion environment. Pitting corrosion was not confirmed in the corrosion test for these steel materials, and they had excellent pitting corrosion resistance even in a supercritical corrosion environment. These steel materials further had an absorbed energy at -80°C of 100 J or more in the Charpy impact test and had excellent low-temperature toughness even in an extremely low-temperature environment.

[0124] On the other hand, the steel material with test number 34 had too low a Cr content. As a result, the corrosion rate of this steel material exceeded 0.100 mm / year in the corrosion test and did not have excellent general corrosion resistance in a supercritical corrosion environment. Pitting corrosion was further confirmed in the corrosion test for this steel material, and it did not have excellent pitting corrosion resistance in a supercritical corrosion environment.

[0125] The steel material of Test No. 35 had too low Ni content. As a result, in the Charpy impact test, the absorbed energy at -80 °C of this steel material was less than 100 J, and it did not have excellent low-temperature toughness in the cryogenic environment.

[0126] The steel material of Test No. 36 had too low Cu content. As a result, in the corrosion test, the corrosion rate of this steel material exceeded 0.100 mm / year, and it did not have excellent general corrosion resistance in the supercritical corrosion environment. Moreover, pitting corrosion was confirmed in the corrosion test of this steel material, and it did not have excellent pitting corrosion resistance in the supercritical corrosion environment.

[0127] The steel material of Test No. 37 had too high Cu content and furthermore too high Fn1. As a result, in the Charpy impact test, the absorbed energy at -80 °C of this steel material was less than 100 J, and it did not have excellent low-temperature toughness in the cryogenic environment.

[0128] The steel materials of Test Nos. 38 and 39 had too high Fn1. As a result, in the Charpy impact test, the absorbed energy at -80 °C of these steel materials was less than 100 J, and they did not have excellent low-temperature toughness in the cryogenic environment.

[0129] The steel materials of Test Nos. 40 to 42 had too low Fn1. As a result, in the corrosion test, the corrosion rate of these steel materials exceeded 0.100 mm / year, and they did not have excellent general corrosion resistance in the supercritical corrosion environment. Moreover, pitting corrosion was confirmed in the corrosion test of these steel materials, and they did not have excellent pitting corrosion resistance in the supercritical corrosion environment.

[0130] The steel materials of Test Nos. 43 to 47 had too low Fn2. As a result, in the Charpy impact test, the absorbed energy at -80 °C of these steel materials was less than 100 J, and they did not have excellent low-temperature toughness in the cryogenic environment.

[0131] The embodiments of the present disclosure have been described above. However, the above-described embodiments are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiments, and the above-described embodiments can be appropriately modified and implemented without departing from the spirit thereof.

Claims

1. By mass percentage, 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: 16.50 - 20.50%, Ni: 4.00 - 9.00%, Mo: 1.50 - 6.00%, Cu: 0.01 - 1.80%, Co: 0.01 - 1.00%, N: 0.200% or less, sol. Al: 0.001 - 0.100%, O: 0.0200% or less, W: 0 - 1.80%, Ca: 0 - 0.0100%, Mg: 0 - 0.0100%, B: 0 - 0.0050%, Rare earth elements: 0 - 0.100%, V: 0 - 0.50%, Ti: 0 - 0.300%, Nb: 0 - 0.300%, Zr: 0 - 0.200%, Sn: 0 - 0.0100%, As: 0 - 0.0100%, Zn: 0 - 0.0100%, Pb: 0 - 0.0100%, Sb: 0 - 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 25 - 80% ferrite, 10 - 35% retained austenite, and the balance martensite by volume fraction, The content of the said elements and the volume fraction of the said microstructure satisfy formula (2), Stainless steel material. 0.10 ≦ Cu + W ≦ 1.85 (1) 0.0350 ≦ (Ni + 1.5Co) / (Vγ + 3.3Vα′) (2) Here, for the element symbols in formulas (1) and (2), the content of the corresponding element is substituted in units of mass%. When the corresponding element is not contained, "0" is substituted for the element symbol. Also, in formula (2), Vγ is substituted with the volume fraction of retained austenite in the microstructure in units of %, and Vα′ in formula (2) is substituted with the volume fraction of martensite in the microstructure in units of %.

2. The stainless steel material according to Claim 1, wherein W: 0.01 - 1.80%, Ca: 0.0001 - 0.0100%, Mg: 0.0001 - 0.0100%, B: 0.0001 - 0.0050%, Rare earth elements: 0.001 - 0.100%, V: 0.01 - 0.50%, Ti: 0.001 - 0.300%, Nb: 0.001 - 0.300%, Zr: 0.001 - 0.200%, Sn: 0.0001 - 0.0100%, As: 0.0001 - 0.0100%, Zn: 0.0001 - 0.0100%, Pb: 0.0001 - 0.0100%, and, A stainless steel material containing at least one element selected from the group consisting of Sb: 0.0001 to 0.0100%. Stainless steel material.

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