Stainless steel material

Abstract

The present invention provides a stainless steel material which has high strength, excellent overall corrosion resistance and stress corrosion cracking resistance in a supercritical corrosive environment, and excellent low temperature toughness in an extremely low temperature environment. A stainless steel material according to the present disclosure has the chemical composition set forth in the description, and has a microstructure in which Fn1 defined by formula (1) is 150.0 or more, Fn2 defined by formula (2) is 24.0 or more, and which is composed of 30.0-85.0% by volume of a ferrite phase and 0.1-40.0% by volume of a retained austenite phase, with the balance being made up of a martensite phase. The yield strength is 552-758 MPa, and the amount of precipitated V in the stainless steel material is 0.005-0.130 mass%.　(1): Fn1 = 576.5 - 2660.7 × C - 7.8 × Cr - 11.3 × Mo - 20.9 × Ni - 10.6 × Cu (2): Fn2 = 2 × Cr + 2 × Mo + 19 × V + 28 × N - 887 × C

Description

Stainless steel material

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

[0002] Currently, carbon dioxide (CO ２ The rise in CO ２ Efforts to reduce CO emissions are underway. ２ Among efforts to curb CO2 emissions, CCUS has been attracting particular attention.

[0003] CCUS is an abbreviation for Carbon dioxide Capture, Utilization and Storage. ２ It includes three technologies: CO capture, utilization, and storage. ２ CO2 emitted from industrial facilities such as power plants and factories is stored as a technology. ２ and CO2 ２ Technology to inject and store gas has been attracting attention.

[0004] Such CO ２ Stainless steel materials used in storage technology are required to have a yield strength of, for example, 80 ksi or more (552 MPa or more). To date, stainless steel materials having a yield strength of 80 ksi or more have been proposed in Japanese Patent Laid-Open No. 2011-190521 (Patent Document 1) and Japanese Patent 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 contains, 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%, O: 0.005% or less, and C+N: 0.0 The martensitic stainless steel material has a yield strength of 400 to 800 MPa, and is characterized in that it contains 60% or less of C, the balance being Fe and impurities, and has 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. This martensitic stainless steel material has a yield strength of 400 to 800 MPa.

[0006] The stainless steel material described in Patent Document 2 is a high-strength martensitic stainless steel seamless steel pipe for oil wells, which contains, by 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: over 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 Fe and impurities, and has a yield strength of 655 to 862 MPa and a yield ratio of 0.90 or more, and is excellent in carbon dioxide corrosion resistance and sulfide pitting corrosion resistance.

[0007] JP 2011-190521 A JP 2012-149317 A

[0008] By the way, the above-mentioned carbon dioxide (CO ２ ) storage technology involves storing CO in depleted oil wells. ２ In order to inject CO into the steel pipe ２ is compressed and pressurized to produce CO ２ On the other hand, CO recovered from industrial facilities such as power plants and factories is ２ SOx, NOx and O２ Here, SOx is SO ２ NOx is a general term for sulfur oxides, such as NO ２ SOx and NOx dissolve in water to form acidic compounds (sulfuric acid, sulfurous acid, nitric acid, nitrous acid, etc.), which cause general corrosion and cracks on the steel surface.

[0009] Therefore, SOx, NOx and O ２ Supercritical CO containing ２ In this specification, SOx, NOx and O ２ Supercritical CO containing ２ The corrosive environment formed by the above phenomenon is called a "supercritical corrosive environment." That is, steel materials used in supercritical corrosive environments are required to have better general corrosion resistance and stress corrosion cracking resistance than those in conventional corrosive environments.

[0010] In recent years, CO ２ When storing CO, the steel material may be required to have toughness in an extremely low temperature environment. ２ When a pressure change of 1000 MPa or more occurs, the temperature of the stored gas may drop due to the Joule-Thomson effect. In this case, the steel may be required to have toughness in an extremely low temperature environment of -70°C, which is far below normal temperatures.

[0011] Therefore, CO in such a cryogenic environment ２ Stainless steel materials that are expected to be applied to storage technologies for the above-mentioned gases are required to have high strength, excellent general corrosion resistance and stress corrosion cracking resistance in supercritical corrosion environments, as well as excellent low-temperature toughness in cryogenic environments of −70° C. or below. On the other hand, the stainless steel materials disclosed in Patent Documents 1 and 2 are not expected to be used in such supercritical corrosion environments or cryogenic environments.

[0012] An object of the present disclosure is to provide a stainless steel material having high strength, excellent general corrosion resistance and stress corrosion cracking resistance in supercritical corrosion environments, and excellent low-temperature toughness in cryogenic environments.

[0013] The stainless steel material according to the present disclosure contains, in mass %, C: 0.030% or less, Si: 1.00% or less, Mn: 0.30% or less, P: 0.030% or less, S: 0.0050% or less, Cr: more than 18.0% to 21.5%, Ni: more than 5.00% to 8.00%, Mo: more than 0.60% to 3.50%, Al: 0.005 to 0.050%, V: 0.01 to 0.30%, N: 0.0030 to 0.1000%, Ti: 0.100% or less, Cu: 0.01 to less than 1.00%, O: 0.020% or less, W: 0 to 2.00%, Nb: 0 to 0.150%, Co: 0 to 0.80%, The stainless steel material comprises B: 0 to 0.0050%, Ca: 0 to 0.0050%, Mg: 0 to 0.0100%, rare earth elements: 0 to 0.0100%, and the balance: Fe and impurities, wherein Fn1 defined by formula (1) is 150.0 or more, and Fn2 defined by formula (2) is 24.0 or more, and has a microstructure consisting of, by volume, 30.0 to 85.0% ferrite phase, 0.1 to 40.0% retained austenite phase, and the balance being martensite phase, and has a yield strength of 552 to 758 MPa, and in the stainless steel material, the amount of precipitated V is 0.005 to 0.130 mass%. Fn1 = 576.5 - 2660.7 x C - 7.8 x Cr - 11.3 x Mo - 20.9 x Ni - 10.6 x Cu (1) Fn2 = 2 x Cr + 2 x Mo + 19 x V + 28 x N - 887 x C (2) Here, the element symbols in formulas (1) and (2) are substituted with the contents of the corresponding elements in mass%.

[0014] The stainless steel material according to the present disclosure has high strength, excellent resistance to general corrosion and stress corrosion cracking in supercritical corrosive environments, and excellent low-temperature toughness in cryogenic environments.

[0015] First, the inventors ２ Assuming application to CO storage technology, the inventors have investigated obtaining a stainless steel material having a yield strength of 552 to 758 MPa. ２The inventors have investigated and studied methods for obtaining a yield strength of 552 to 758 MPa, excellent general corrosion resistance and stress corrosion cracking resistance in supercritical corrosion environments, and excellent low-temperature toughness in cryogenic environments for stainless steel materials that are expected to be applied to storage technology for nuclear fuel. As a result, the inventors have obtained the following findings.

[0016] The present inventors first focused on the chemical composition to investigate how to obtain a desired stainless steel material, and as a result, found that the following components were present: C: 0.030% or less, Si: 1.00% or less, Mn: 0.30% or less, P: 0.030% or less, S: 0.0050% or less, Cr: more than 18.0% to 21.5%, Ni: more than 5.00% to 8.00%, Mo: more than 0.60% to 3.50%, Al: 0.005 to 0.050%, V: 0.01 to 0.30%, N: 0.0030 to 0.1000%, Ti: 0.100% or less, Cu: 0.01 to less than 1.00%, O: 0.020% or less, W: 0. The inventors considered that a stainless steel material consisting of Cr: 0.0 ...

[0017] Next, the inventors of the present invention focused on the microstructure and investigated how to obtain a desired stainless steel material. Here, a stainless steel material having the above-mentioned chemical composition has a microstructure consisting of a ferrite phase, a retained austenite phase, and the remainder being a martensite phase. The ferrite phase improves the corrosion resistance (general corrosion resistance and stress corrosion cracking resistance) of the steel material in a supercritical corrosion environment. On the other hand, if there is too much ferrite phase, the strength of the stainless steel material decreases. The retained austenite phase improves the low-temperature toughness in an extremely low-temperature environment. On the other hand, if there is too much retained austenite phase, the strength of the stainless steel material decreases. The martensite phase improves the strength of the stainless steel material. On the other hand, if there is too much martensite phase, the toughness decreases.

[0018] As a result of investigations by the present inventors based on these findings, it has become clear that a microstructure consisting of 30.0 to 85.0% by volume of ferrite phase, 0.1 to 40.0% by volume of retained austenite phase, and the remainder being martensite phase, may be able to achieve a yield strength of 552 to 758 MPa, excellent general corrosion resistance and stress corrosion cracking resistance in supercritical corrosion environments, and excellent low-temperature toughness in cryogenic environments.

[0019] Furthermore, the inventors have found through their studies that in stainless steel materials having the above-described chemical composition, if Fn1 defined by formula (1) is 150.0 or more, the volume fraction of the retained austenite phase in the microstructure can be stably set to 0.1 to 40.0%. Fn1=576.5-2660.7×C-7.8×Cr-11.3×Mo-20.9×Ni-10.6×Cu (1) Here, the element symbols in formula (1) are substituted with the contents of the corresponding elements in mass%.

[0020] Fn1 is an index of the volume fraction of the retained austenite phase in a stainless steel material having the above-described chemical composition. If Fn1 is 150.0 or more, the volume fraction of the retained austenite phase in the microstructure can be stably kept to 40.0% or less. As a result, the yield strength can be made 552 MPa or more. Therefore, in the stainless steel material according to this embodiment, Fn1 is set to 150.0 or more, assuming that the stainless steel material has the above-described chemical composition.

[0021] As a result of further investigations by the present inventors, it has become clear that in a stainless steel material having the above-mentioned chemical composition and microstructure and an Fn1 of 150.0 or more, if Fn2 defined by formula (2) is 24.0 or more, the corrosion resistance of the steel material in a supercritical corrosion environment can be improved. Fn2=2×Cr+2×Mo+19×V+28×N−887×C (2) Here, the element symbols in formula (2) are substituted with the contents of the corresponding elements in mass%.

[0022] Fn2 is an index of corrosion resistance of a stainless steel material having the above-mentioned chemical composition in a supercritical corrosion environment. If Fn2 is 24.0 or more, the corrosion resistance of the steel material in a supercritical corrosion environment can be improved. Therefore, in the stainless steel material according to this embodiment, Fn2 is set to 24.0 or more, assuming that the stainless steel material has the above-mentioned chemical composition and Fn1 is 150.0 or more.

[0023] On the other hand, even in stainless steel materials having the above-mentioned chemical composition and microstructure and satisfying an Fn1 of 150.0 or more, there are cases in which a yield strength of 552 to 758 MPa, excellent general corrosion resistance and stress corrosion cracking resistance in a supercritical corrosion environment, and excellent low-temperature toughness in an extremely low-temperature environment cannot be obtained. Therefore, the present inventors have focused on precipitates in the steel material and investigated a method for obtaining a desired stainless steel material.

[0024] The above-described chemical composition contains Cr: more than 18.0 to 21.5% and Mo: more than 0.60 to 3.50%. Cr and Mo dissolve in the steel material to enhance the corrosion resistance of the steel material. On the other hand, Cr and Mo easily bond with carbon (C) in the steel material to form carbides. When Cr or Mo forms carbides, the amount of Cr or Mo in solid solution decreases, which may result in an insufficient increase in the corrosion resistance of the steel material. Therefore, the inventors focused on vanadium (V) as an element that forms carbides and fixes C. The formation of V carbides may fix C in the steel material, thereby ensuring the amount of Cr and Mo in solid solution. In this case, excellent general corrosion resistance and stress corrosion cracking resistance in supercritical corrosion environments may be obtained.

[0025] As a result of detailed studies by the present inventors based on the above findings, it has been revealed that in a stainless steel material having the above-mentioned chemical composition and microstructure, in which Fn1 is 150.0 or more and Fn2 is 24.0 or more, if the amount of precipitated V is 0.005 to 0.130 mass%, a yield strength of 552 to 758 MPa, excellent general corrosion resistance and stress corrosion cracking resistance in a supercritical corrosion environment, and excellent low-temperature toughness in an extremely low-temperature environment can be obtained.

[0026] The details of why a stainless steel material having the above-mentioned chemical composition containing Fn1 and Fn2 and microstructure can achieve a yield strength of 552 to 758 MPa, excellent general corrosion resistance and stress corrosion cracking resistance in a supercritical corrosion environment, and excellent low-temperature toughness in a cryogenic environment when the amount of precipitated V is 0.005 to 0.130 mass% are not clear. However, the present inventors speculate as follows.

[0027] As described above, if V can fix C, the amount of soluble Cr and Mo increases, improving the corrosion resistance of the steel. In other words, increasing the amount of precipitated V is thought to improve the general corrosion resistance and stress corrosion cracking resistance of the steel in a supercritical corrosion environment. Meanwhile, V precipitates, including V carbides, increase the strength of the steel. In other words, if the amount of precipitated V is too high, the strength of the steel becomes too high, and excellent low-temperature toughness in a cryogenic environment cannot be obtained. Therefore, the inventors speculate that a precipitated V amount of 0.005 to 0.130 mass% will achieve the desired yield strength, excellent general corrosion resistance and stress corrosion cracking resistance in a supercritical corrosion environment, and excellent low-temperature toughness in a cryogenic environment.

[0028] It is possible that a stainless steel material having the above-mentioned chemical composition containing Fn1 and Fn2 and a microstructure with a precipitated V content of 0.005 to 0.130 mass% may be able to obtain a yield strength of 552 to 758 MPa, excellent general corrosion resistance and stress corrosion cracking resistance in supercritical corrosion environments, and excellent low-temperature toughness in cryogenic environments by a mechanism other than the above. However, it is proven by the examples described below that a stainless steel material having the above-mentioned chemical composition containing Fn1 and Fn2 and a microstructure with a precipitated V content of 0.005 to 0.130 mass% may be able to obtain a yield strength of 552 to 758 MPa, excellent general corrosion resistance and stress corrosion cracking resistance in supercritical corrosion environments, and excellent low-temperature toughness in cryogenic environments by a precipitated V content of 0.005 to 0.130 mass%.

[0029] The stainless steel material according to this embodiment, which was completed based on the above findings, has the following features.

[0030] [1] A stainless steel material comprising, in mass%, C: 0.030% or less, Si: 1.00% or less, Mn: 0.30% or less, P: 0.030% or less, S: 0.0050% or less, Cr: over 18.0 to 21.5%, Ni: over 5.00 to 8.00%, Mo: over 0.60 to 3.50%, Al: 0.005 to 0.050%, V: 0.01 to 0.30%, N: 0.0030 to 0.1000%, Ti: 0.100% or less, Cu: 0.01 to less than 1.00%, O: 0.020% or less, W: 0 to 2.00%, Nb: 0 to 0.150%, Co: 0 to 0.80%, A stainless steel material comprising B: 0 to 0.0050%, Ca: 0 to 0.0050%, Mg: 0 to 0.0100%, rare earth elements: 0 to 0.0100%, and the balance: Fe and impurities, wherein Fn1 defined by formula (1) is 150.0 or more, and Fn2 defined by formula (2) is 24.0 or more, and wherein the stainless steel material has a microstructure consisting of, by volume, 30.0 to 85.0% ferrite phase, 0.1 to 40.0% retained austenite phase, and the balance being martensite phase, and wherein the stainless steel material has a yield strength of 552 to 758 MPa, and wherein the amount of precipitated V in the stainless steel material is 0.005 to 0.130% by mass. Fn1 = 576.5 - 2660.7 x C - 7.8 x Cr - 11.3 x Mo - 20.9 x Ni - 10.6 x Cu (1) Fn2 = 2 x Cr + 2 x Mo + 19 x V + 28 x N - 887 x C (2) Here, the element symbols in formulas (1) and (2) are substituted with the contents of the corresponding elements in mass%.

[0031] [2] The stainless steel material according to [1], containing one or more elements selected from the group consisting of W: 0.01 to 2.00%, Nb: 0.001 to 0.150%, Co: 0.01 to 0.80%, B: 0.0001 to 0.0050%, Ca: 0.0001 to 0.0050%, Mg: 0.0001 to 0.0100%, and rare earth elements: 0.0001 to 0.0100%.

[0032] 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 bar (solid material), or a steel plate. The round bar refers to a steel bar having a circular cross section perpendicular to the axial direction. The steel pipe may be a seamless steel pipe or a welded steel pipe.

[0033] The stainless steel material according to this embodiment will be described in detail below. In the following description, the stainless steel material will also be simply referred to as a "steel material." In the following description, general corrosion resistance and stress corrosion cracking resistance will also be collectively referred to as "corrosion resistance."

[0034] [Chemical Composition] The chemical composition of the stainless steel material according to this embodiment contains the following elements: "%" relating to elements means mass % unless otherwise specified.

[0035] C: 0.030% or less Carbon (C) is unavoidably contained. That is, the lower limit of the C content is greater than 0%. C combines with Cr and Mo to form carbides. Therefore, if the C content is too high, even if the contents of other elements are within the ranges of this embodiment, the amount of soluble Cr and Mo decreases, resulting in a decrease in the corrosion resistance of the steel in a supercritical corrosion environment. Therefore, the C content is 0.030% or less. A preferred upper limit of the C content is 0.025%, more preferably 0.020%, and even more preferably 0.010%. The C content should be as low as possible. However, an extreme reduction in the C content significantly increases production costs. Therefore, considering industrial production, a preferred lower limit of the C content is 0.001%, more preferably 0.003%.

[0036] Si: 1.00% or less Silicon (Si) is unavoidably contained. That is, the lower limit of the Si content is more than 0%. Si deoxidizes steel. On the other hand, if the Si content is too high, the hot workability of the steel material will deteriorate even if the contents of other elements are within the ranges of this embodiment. Therefore, the Si content is 1.00% or less. A preferred upper limit of the Si content is 0.95%, more preferably 0.90%. To more effectively obtain the above effects, a preferred lower limit of the Si content is 0.10%, more preferably 0.15%, even more preferably 0.20%, and even more preferably 0.25%.

[0037] Mn: 0.30% or less Manganese (Mn) is unavoidably contained. 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 steel. On the other hand, if the Mn content is too high, even if the contents of other elements are within the ranges of this embodiment, the volume fraction of the retained austenite phase may become too high, resulting in a decrease in the strength of the steel. Therefore, the Mn content is 0.30% or less. A preferred upper limit of the Mn content is 0.25%, more preferably 0.20%, and even more preferably 0.15%. To more effectively obtain the above effects, a preferred lower limit of the Mn content is 0.01%, more preferably 0.02%, and even more preferably 0.03%.

[0038] P: 0.030% or less Phosphorus (P) is unavoidably contained. 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 contents of other elements are within the ranges of this embodiment. Therefore, the P content is 0.030% or less. A preferred upper limit of the P content is 0.025%, more preferably 0.020%, and even more preferably 0.015%. The P content should be as low as possible. However, an extreme reduction in the P content significantly increases manufacturing costs. Therefore, considering industrial production, a preferred lower limit of the P content is 0.001%, more preferably 0.003%.

[0039] S: 0.0050% or less Sulfur (S) is unavoidably contained. 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 contents of other elements are within the ranges of this embodiment. Therefore, the S content is 0.0050% or less. A preferred upper limit of the S content is 0.0040%, more preferably 0.0030%. The S content should be as low as possible. However, an extreme reduction in the S content significantly increases manufacturing costs. Therefore, considering industrial production, a preferred lower limit of the S content is 0.0001%, more preferably 0.0003%, and even more preferably 0.0004%.

[0040] Cr: More than 18.0% to 21.5% Chromium (Cr) forms a passive film as an oxide on the surface of a steel material, thereby enhancing the corrosion resistance of the steel material in a supercritical corrosion environment. Cr also dissolves in the steel material, enhancing the corrosion resistance of the steel material in a supercritical corrosion environment. If the Cr content is too low, the above effects cannot be sufficiently achieved even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Cr content is too high, the low-temperature toughness of the steel material in a cryogenic environment decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the Cr content is more than 18.0% to 21.5%. The preferred lower limit of the Cr content is 18.1%, more preferably 18.2%, and even more preferably 18.3%. The preferred upper limit of the Cr content is 21.4%, more preferably 21.2%, and even more preferably 21.0%.

[0041] Ni: More than 5.00% to 8.00% Nickel (Ni) enhances the corrosion resistance of steel materials in supercritical corrosion environments. If the Ni content is too low, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Ni content is too high, the volume fraction of the retained austenite phase becomes too high, resulting in a decrease in the strength of the steel material, even if the contents of other elements are within the ranges of this embodiment. Therefore, the Ni content is more than 5.00% to 8.00%. The preferred lower limit of the Ni content is 5.01%, more preferably 5.05%, even more preferably 5.10%, and even more preferably 5.15%. The preferred upper limit of the Ni content is 7.95%, even more preferably 7.90%, and even more preferably 7.85%.

[0042] Mo: More than 0.60% to 3.50% Molybdenum (Mo) dissolves in steel to enhance the corrosion resistance of the steel in a supercritical corrosion environment. If the Mo content is too low, the above effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Mo content is too high, the low-temperature toughness of the steel in a cryogenic environment decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the Mo content is more than 0.60% to 3.50%. A preferred lower limit of the Mo content is 0.65%, more preferably 1.00%, and even more preferably 1.50%. A preferred upper limit of the Mo content is 3.48%, more preferably 3.45%, and even more preferably 3.40%.

[0043] Al: 0.005 to 0.050% Aluminum (Al) deoxidizes steel. If the Al content is too low, the above effect cannot be sufficiently achieved even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Al content is too high, coarse oxides are formed, reducing the low-temperature toughness of the steel material even if the contents of other elements are within the ranges of this embodiment. Therefore, the Al content is 0.005 to 0.050%. A preferred lower limit of the Al content is 0.008%, more preferably 0.010%, and even more preferably 0.015%. A preferred upper limit of the Al content is 0.045%, more preferably 0.040%, and even more preferably 0.035%. The Al content referred to in this specification refers to the content of sol. Al (acid-soluble Al).

[0044] V: 0.01 to 0.30% Vanadium (V) fixes C in the steel material as carbides. As a result, the formation of Cr carbides and Mo carbides is suppressed, and the corrosion resistance of the steel material in supercritical corrosion environments is improved. If the V content is too low, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the V content is too high, the low-temperature toughness of the steel material decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the V content is 0.01 to 0.30%. The preferred lower limit of the V content is 0.02%, more preferably 0.03%, and even more preferably 0.04%. The preferred upper limit of the V content is 0.28%, and even more preferably 0.25%.

[0045] N: 0.0030 to 0.1000% Nitrogen (N) enhances the corrosion resistance of steel in supercritical corrosion environments. If the N content is too low, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the N content is too high, the toughness and hot workability of the steel will decrease even if the contents of other elements are within the ranges of this embodiment. Therefore, the N content is 0.0030 to 0.1000%. A preferred lower limit of the N content is 0.0035%, more preferably 0.0040%, and even more preferably 0.0045%. A preferred upper limit of the N content is 0.0990%, and even more preferably 0.0980%.

[0046] Ti: 0.100% or less Titanium (Ti) is an unavoidable impurity. That is, the lower limit of the Ti content is greater than 0%. Ti may be further added. When contained, Ti forms carbides to increase the strength of the steel material. Even if even a small amount of Ti is contained, the above effect can be obtained to some extent. On the other hand, if the Ti content is too high, even if the contents of other elements are within the ranges of this embodiment, the strength of the steel material becomes too high and the low-temperature toughness of the steel material decreases. Therefore, the Ti content is 0.100% or less. The preferred upper limit of the Ti content is 0.095%, more preferably 0.090%, even more preferably 0.085%, and even more preferably 0.080%. The lower limit of the Ti content may be 0.001%, 0.002%, or 0.003%.

[0047] Cu: 0.01 to less than 1.00% Copper (Cu) enhances the corrosion resistance of steel in supercritical corrosion environments. If the Cu content is too low, the above effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Cu content is too high, the hot workability of the steel will decrease even if the contents of other elements are within the ranges of this embodiment. Therefore, the Cu content is 0.01 to less than 1.00%. A preferred lower limit of the Cu content is 0.02%, more preferably 0.03%, and even more preferably 0.05%. A preferred upper limit of the Cu content is 0.99%, more preferably 0.97%, even more preferably 0.95%, and even more preferably 0.90%.

[0048] O: 0.020% or less Oxygen (O) is unavoidably contained. In other words, 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 contents of other elements are within the ranges of this embodiment. Therefore, the O content is 0.020% or less. A preferred upper limit of the O content is 0.018%, more preferably 0.016%, and even more preferably 0.014%. The O content is preferably as low as possible. However, an extreme reduction in the O content increases manufacturing costs. Therefore, considering industrial production, a preferred lower limit of the O content is 0.001%, more preferably 0.002%, and even more preferably 0.003%.

[0049] The balance of the chemical composition of the stainless steel material according to this embodiment is composed of Fe and impurities. Here, the impurities in the chemical composition refer to substances that are mixed in from raw materials such as ore and scrap, or the manufacturing environment, during industrial production of the stainless steel material, and are acceptable within a range that does not adversely affect the stainless steel material according to this embodiment.

[0050] [Optional Elements] The chemical composition of the stainless steel material according to this embodiment may further contain W instead of a portion of Fe.

[0051] W: 0 to 2.00% Tungsten (W) is an optional element and does not necessarily need to be contained. That is, the W content may be 0%. When contained, W, like Mo, enhances the corrosion resistance of steel in supercritical corrosion environments. Even if even a small amount of W is contained, the above effect can be obtained to some extent. However, if the W content is too high, even if the contents of other elements are within the ranges of this embodiment, the volume fraction of the ferrite phase may become too high, resulting in a decrease in the strength of the steel. Therefore, the W content is 0 to 2.00%. The preferred lower limit of the W content is more 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 1.95%, even more preferably 1.90%, and even more preferably 1.85%.

[0052] The chemical composition of the stainless steel material according to this embodiment may further contain Nb, Co, and B instead of part of Fe. These elements are optional elements and may be contained as impurities. These elements may also be added.

[0053] Nb: 0 to 0.150% Niobium (Nb) is an optional element and does not necessarily need to be contained. That is, the Nb content may be 0%. When contained, Nb is contained as an impurity. Nb may also be added. When contained, Nb forms carbides to increase the strength of the steel material. Even if even a small amount of Nb is contained, the above effect can be obtained to some extent. However, if the Nb content is too high, the toughness of the steel material will decrease even if the contents of other elements are within the ranges of this embodiment. Therefore, the Nb content is 0 to 0.150%. The preferred upper limit of the Nb content is 0.145%, more preferably 0.140%, and even more preferably 0.135%. The lower limit of the Nb content may be greater than 0%, may be 0.001%, or may be 0.003%.

[0054] Cobalt (Co) is an optional element and does not necessarily need to be contained. That is, the Co content may be 0%. When contained, Co is contained as an impurity. Co may also be added. When contained, Co dissolves in austenite to enhance the corrosion resistance of the steel. Even if even a small amount of Co is contained, the above effects can be obtained to some extent. However, if the Co content is too high, the manufacturing cost will increase dramatically even if the contents of other elements are within the ranges of this embodiment. Therefore, the Co content is 0 to 0.80%. The preferred upper limit of the Co content is 0.78%, more preferably 0.76%, and even more preferably 0.72%. The lower limit of the Co content may be greater than 0%, may be 0.01%, or may be 0.03%.

[0055] B: 0 to 0.0050% Boron (B) is an optional element and does not necessarily need to be contained. That is, the B content may be 0%. When contained, B is contained as an impurity. B may also be added. When contained, B segregates at grain boundaries to improve the hot workability of the steel. Even if even a small amount of B is contained, the above effect can be obtained to some extent. On the other hand, if the B content is too high, boron nitride (BN) is formed, even if the contents of other elements are within the ranges of this embodiment, and the toughness of the steel decreases. Therefore, the B content is 0 to 0.0050%. The preferred upper limit of the B content is 0.0045%, more preferably 0.0040%, and even more preferably 0.0035%. The lower limit of the B content may be more than 0%, may be 0.0001%, or may be 0.0003%.

[0056] 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 Ca, Mg, and rare earth elements in place of a portion of Fe. All of these elements are optional elements and improve the hot workability of the steel material.

[0057] Ca: 0 to 0.0050% Calcium (Ca) is an optional element and does not necessarily need to be contained. That is, the Ca content may be 0%. When contained, Ca neutralizes S in the steel by fixing it as sulfides, thereby improving the hot workability of the steel. Even if even a small amount of Ca is contained, the above effects can be achieved to some extent. However, if the Ca content is too high, even if the contents of other elements are within the ranges of this embodiment, oxides in the steel will coarsen, reducing the corrosion resistance of the steel in supercritical corrosion environments. Therefore, the Ca content is 0 to 0.0050%. The preferred lower limit of the Ca content is more than 0%, more preferably 0.0001%, even more preferably 0.0005%, even more preferably 0.0008%, and even more preferably 0.0010%. The preferred upper limit of the Ca content is 0.0048%, even more preferably 0.0045%, and even more preferably 0.0040%.

[0058] Mg: 0 to 0.0100% Magnesium (Mg) is an optional element and does not necessarily need to be contained. That is, the Mg content may be 0%. When contained, Mg neutralizes S in the steel by fixing it as sulfides, thereby improving the hot workability of the steel. Even if even a small amount of Mg is contained, the above effects can be achieved to some extent. However, if the Mg content is too high, even if the contents of other elements are within the ranges of this embodiment, oxides in the steel will coarsen, reducing the corrosion resistance of the steel in supercritical corrosion environments. 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 0.0095%, more preferably 0.0090%, and even more preferably 0.0080%.

[0059] Rare Earth Elements: 0 to 0.0100% Rare earth elements (REM) are optional elements and may not be present. That is, the REM content may be 0%. When present, REM neutralizes S in the steel by fixing it as sulfides, thereby improving the hot workability of the steel. Even if even a small amount of REM is present, the above effects can be achieved to some extent. However, if the REM content is too high, even if the contents of other elements are within the ranges of this embodiment, oxides in the steel will coarsen, reducing the corrosion resistance of the steel in supercritical corrosion environments. Therefore, the REM content is 0 to 0.0100%. The preferred lower limit of the REM content is greater than 0%, more preferably 0.0001%, and even more preferably 0.0002%. The preferred upper limit of the REM content is 0.0095%, more preferably 0.0090%, and even more preferably 0.0080%.

[0060] 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 lanthanoids lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. In addition, in this specification, the REM content refers to the total content of these elements.

[0061] [Fn1] The stainless steel material according to this embodiment has the above-described chemical composition, and Fn1 defined by the following formula (1) is 150.0 or more: Fn1=576.5-2660.7×C-7.8×Cr-11.3×Mo-20.9×Ni-10.6×Cu (1) Here, the element symbols in formula (1) are substituted with the contents of the corresponding elements in mass%.

[0062] Fn1 is an index of the volume fraction of the retained austenite phase in a stainless steel material having the above-described chemical composition. If Fn1 is 150.0 or more, the volume fraction of the retained austenite phase in the microstructure can be stably kept to 40.0% or less. As a result, the yield strength can be made 552 MPa or more. Therefore, in the stainless steel material according to this embodiment, Fn1 is set to 150.0 or more, assuming that the stainless steel material has the above-described chemical composition.

[0063] The preferred lower limit of Fn1 is 151.0, more preferably 152.0, and even more preferably 153.0. The upper limit of Fn1 is not particularly limited. However, when the above-mentioned chemical composition is used, the upper limit of Fn1 is substantially 324.7. The upper limit of Fn1 may be 320.0, 310.0, or 300.0. Fn1 is calculated by rounding the obtained value to one decimal place.

[0064] [Fn2] The stainless steel material according to this embodiment has the above-described chemical composition, with Fn1 being 150.0 or more, and Fn2 defined by the following formula (2) being 24.0 or more: Fn2=2×Cr+2×Mo+19×V+28×N−887×C (2) Here, the element symbols in formula (2) are substituted with the contents of the corresponding elements in mass%.

[0065] Fn2 is an index of corrosion resistance of a stainless steel material having the above-mentioned chemical composition in a supercritical corrosion environment. If Fn2 is 24.0 or more, the corrosion resistance of the steel material in a supercritical corrosion environment can be improved. Therefore, in the stainless steel material according to this embodiment, Fn2 is set to 24.0 or more, assuming that the stainless steel material has the above-mentioned chemical composition and Fn1 is 150.0 or more.

[0066] The preferred lower limit of Fn2 is 24.5, more preferably 25.0, even more preferably 25.5, and even more preferably 26.0. The upper limit of Fn2 is not particularly limited. However, when the above-mentioned chemical composition is used, the upper limit of Fn2 is substantially 58.5. The upper limit of Fn2 may be 55.0, 50.0, or 48.0. Note that Fn2 is calculated by rounding the obtained value to one decimal place.

[0067] [Microstructure] The stainless steel material according to this embodiment has the above-described chemical composition, Fn1 is 150.0 or more, Fn2 is 24.0 or more, and further has a microstructure consisting of, by volume fraction, 30.0 to 85.0% ferrite phase, 0.1 to 40.0% retained austenite phase, and the remainder martensite phase.

[0068] In this specification, the microstructure "consisting of a ferrite phase, a retained austenite phase, and a martensite phase" means that the amount of phases other than the ferrite phase, the retained austenite phase, and the martensite phase 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 the ferrite phase, the retained austenite phase, and the martensite phase. In other words, the microstructure of the stainless steel material according to this embodiment may contain minute amounts of precipitates, inclusions, etc. in addition to the ferrite phase, the retained austenite phase, and the martensite phase.

[0069] As described above, in the microstructure of the stainless steel material according to this embodiment, the volume fraction of the ferrite phase is 30.0 to 85.0%. If the volume fraction of the ferrite phase is too low, the corrosion resistance of the steel material in a supercritical corrosion environment decreases. On the other hand, if the volume fraction of the ferrite phase is too high, the strength of the steel material decreases. Therefore, in the microstructure of the stainless steel material according to this embodiment, the volume fraction of the ferrite phase is 30.0 to 85.0%. A preferred lower limit of the volume fraction of the ferrite phase is 31.0%, more preferably 32.0%, and even more preferably 33.0%. A preferred upper limit of the volume fraction of the ferrite phase is 84.0%, more preferably 83.0%, and even more preferably 82.0%.

[0070] As described above, in the microstructure of the stainless steel material according to this embodiment, the volume fraction of the retained austenite phase is 0.1 to 40.0%. If the volume fraction of the retained austenite phase 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 fraction of the retained austenite phase is too high, the strength of the steel material decreases. Therefore, in the microstructure of the stainless steel material according to this embodiment, the volume fraction of the retained austenite phase is 0.1 to 40.0%. A preferred lower limit of the volume fraction of the retained austenite phase is 0.2%, more preferably 0.3%, and even more preferably 0.5%. A preferred upper limit of the volume fraction of the retained austenite phase is 39.5%, more preferably 39.0%, even more preferably 38.5%, and even more preferably 38.0%.

[0071] As described above, the stainless steel material according to this embodiment has a microstructure consisting of, by volume, 30.0 to 85.0% ferrite phase, 0.1 to 40.0% retained austenite phase, and the remainder martensite phase. The volume fraction of the martensite phase is not particularly limited, but is, for example, 5.0 to 50.0%. The preferred lower limit of the volume fraction of the martensite phase is 5.5%, more preferably 6.0%, and even more preferably 6.5%. The preferred upper limit of the volume fraction of the martensite phase is 49.0%, more preferably 48.5%, and even more preferably 48.0%.

[0072] In this embodiment, the volume fraction of each phase in the microstructure is determined by the following method. Specifically, the volume fraction (%) of the retained austenite phase and the volume fraction (%) of the ferrite phase in the microstructure of the steel material are determined by the following method. The determined volume fractions of the retained austenite phase and the ferrite phase are subtracted from 100% to determine the volume fraction (%) of the martensite phase.

[0073] [Method for measuring the volume fraction of the retained austenite phase] The volume fraction of the retained austenite phase in the microstructure of a steel material is determined by X-ray diffraction. Specifically, a test piece for measuring the volume fraction of the retained austenite phase is prepared from the steel material according to this embodiment. When the steel material is a steel plate, the test piece is taken from the center of the plate thickness. When the steel material is a steel pipe, the test piece is taken from the center of the wall thickness. When the steel material is a round bar, the test piece is taken from the R / 2 position. In this specification, the R / 2 position of the round bar means the center position of the radius R in a cross section perpendicular to the axial direction of the round bar. The size of the test piece is not particularly limited. For example, the test piece is 15 mm x 15 mm x 2 mm thick. When the steel material is a steel plate, the thickness direction of the test piece is the plate thickness direction. When the steel material is a steel pipe, the thickness direction of the test piece is the pipe radial direction. When the steel material is a round bar, the thickness direction of the test piece is the radial direction. Using the prepared test specimen, the X-ray diffraction intensity of each of the (110) plane of the α phase (ferrite phase and martensite phase), the (200) plane of the α phase, the (211) plane of the α phase, the (111) plane of the γ phase (retained austenite phase), the (200) plane of the γ phase, and the (220) plane of the γ phase is measured, and the integrated intensity of each plane is calculated.

[0074] In measuring the X-ray diffraction intensity, the target of the X-ray diffractometer is Co (CoKα radiation), and the output is 30 kV-100 mA. The measurement angle (2θ) is 45 to 105°. After calculation, the volume fraction Vγ (%) of the retained austenite phase is calculated using formula (I) for each combination of each α phase face and each γ phase face (3 × 3 = 9 pairs). The average value of the nine pairs of the volume fraction Vγ (%) of the retained austenite phase is then defined as the volume fraction (%) of the retained austenite phase. Vγ = 100 / {1 + (Iα × Rγ) / (Iγ × Rα)} (I) where Iα is the integrated intensity of the α phase. Rα is the theoretically calculated value of the crystallographically calculated value of the α phase. Iγ is the integrated intensity of the γ phase. Rγ is the theoretically calculated value of the crystallographically calculated value of the γ phase. The values ​​of Rα and Rγ on each surface can be those incorporated into the retained austenite quantitative analysis system attached to the RINT-TTR product manufactured by Rigaku Corporation. The volume fraction of the retained austenite phase is calculated by rounding the obtained value to one decimal place.

[0075] [Method for measuring the volume fraction of ferrite phase] The volume fraction of the ferrite phase in the microstructure of a steel material is determined by the point counting method. Specifically, a test piece for measuring the volume fraction of the ferrite phase is prepared from the steel material according to this embodiment. When the steel material is a steel plate, the test piece is taken from the center of the plate thickness. Note that the size of the test piece is not particularly limited. When the steel material is a steel pipe, the test piece is taken from the center of the wall thickness. When the steel material is a round bar, the test piece is taken from the R / 2 position. Furthermore, when the steel material is a steel plate, 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 observation surface of the test piece is a surface parallel to the axial direction of the steel pipe. When the steel material is a round bar, the observation surface of the test piece is a surface parallel to the axial direction of the round bar. After mechanically polishing the observation surface, the observation surface is electrolytically etched to reveal the structure. Electrolytic etching was performed using a mixture of aqua regia (a solution of hydrochloric acid and nitric acid mixed at a ratio of 3:1) and glycerin, with a current density of 1 A / cm ２ The electrolysis time is 1 minute.

[0076] The electrolytically etched observation surface is observed using an optical microscope in 30 fields of view. The observation fields are rectangular, measuring 250 μm × 250 μm. The observation magnification is 400x. In each observation field, a person skilled in the art can distinguish the ferrite phase from other phases (retained austenite phase and martensite phase) based on the contrast. Therefore, the ferrite phase in each observation field is identified based on the contrast. The area ratio of the identified ferrite phase is calculated using the point counting method in accordance with JIS G 0555 (2020).

[0077] Specifically, 20 equally spaced vertical lines are drawn from the top to the bottom of the observation field. The 20 vertical lines divide the observation field into 21 regions in the left-right direction. Furthermore, 20 equally spaced horizontal lines are drawn from the left to the right of the observation field. The 20 horizontal lines divide the observation field into 21 regions in the up-down direction. The intersections of the vertical and horizontal lines are called lattice points. In other words, 400 lattice points are arranged at equal intervals in the observation field. In accordance with JIS G 0555 (2020), the number of lattice points overlapping with the ferrite phase in the observation field is counted. The number of lattice points overlapping with the ferrite phase obtained in 30 fields is divided by the total number of lattice points (400 x 30 = 12,000) to define the ferrite phase area ratio. In this embodiment, the area ratio of the ferrite phase obtained by the above method is referred to as the volume ratio (%) of the ferrite phase. The volume fraction of the ferrite phase is determined by rounding the obtained value to one decimal place.

[0078] Using the volume fraction (%) of the retained austenite phase obtained by the X-ray diffraction method and the volume fraction (%) of the ferrite phase obtained by the point calculation method, the volume fraction (%) of the martensite phase in the microstructure of the steel material is calculated by the following formula: Volume fraction (%) of martensite phase = 100.0 - {Volume fraction (%) of retained austenite phase + Volume fraction (%) of ferrite phase}

[0079] [Yield Strength] The stainless steel material according to this embodiment has the above-described chemical composition, Fn1 is 150.0 or more, Fn2 is 24.0 or more, and has a microstructure consisting of 30.0 to 85.0% by volume of ferrite phase, 0.1 to 40.0% by volume of retained austenite phase, and the balance being martensite phase, and further has a yield strength of 552 to 758 MPa. The preferred lower limit of the yield strength is 555 MPa, more preferably 558 MPa, and even more preferably 560 MPa. The preferred upper limit of the yield strength is 755 MPa, and even more preferably 750 MPa.

[0080] In this embodiment, the yield strength of the stainless steel material is determined by the following method. Specifically, a tensile test is performed according to ASTM E8 / E8M (2022). Test specimens are prepared from the steel material according to this embodiment. When 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. When the steel material is a steel pipe, a round bar test specimen or a circular arc test specimen is prepared as the tensile test specimen from the center of the wall thickness. In this case, the longitudinal direction of the round bar test specimen or the circular arc test specimen is parallel to the axial direction of the steel pipe. When the steel material is a round bar, a tensile test specimen is prepared from the R / 2 position. In this case, the longitudinal direction of the tensile test specimen is parallel to the axial direction of the round bar.

[0081] The tensile test specimen is, for example, a round bar test specimen with a parallel portion diameter of 8.9 mm and a gauge length of 35.6 mm. If a round bar test specimen cannot be prepared from a steel pipe, a circular arc test specimen is prepared. The size of the circular arc test specimen is, for example, the full wall 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 by the tensile test is defined as the yield strength (MPa). The yield strength (MPa) is calculated by rounding the obtained value to one decimal place.

[0082] [Precipitated V Amount] The stainless steel material according to this embodiment has the above-described chemical composition, Fn1 is 150.0 or more, Fn2 is 24.0 or more, has the above-described microstructure, has a yield strength of 552 to 758 MPa, and further has a precipitated V amount of 0.005 to 0.130 mass% in the stainless steel material.

[0083] In steel materials having the above-described chemical composition, V precipitates may include V nitrides, V carbonitrides, and the like in addition to V carbides. However, in steel materials having the above-described chemical composition, most of the V precipitates are V carbides. Therefore, by increasing the amount of precipitated V, the amount of C dissolved in the stainless steel material can be reduced. As a result, the general corrosion resistance and stress corrosion cracking resistance of the steel material in a supercritical corrosion environment are improved.

[0084] On the other hand, V precipitates, including V carbides, increase the strength of the steel material and reduce its low-temperature toughness. Therefore, if the amount of precipitated V is excessively increased, the strength of the steel material may become too high, and the yield strength may exceed 758 MPa. In this case, excellent low-temperature toughness in a cryogenic environment cannot be obtained. Therefore, in the stainless steel material according to this embodiment, the amount of precipitated V is set to 0.005 to 0.130 mass%, assuming that the material has the above-described chemical composition, Fn1 is 150.0 or more, Fn2 is 24.0 or more, and the material has the above-described microstructure.

[0085] In the stainless steel material according to this embodiment, the lower limit of the amount of precipitated V is preferably 0.006 mass%, more preferably 0.007 mass%, and even more preferably 0.008 mass%, and the upper limit of the amount of precipitated V is preferably 0.128 mass%, more preferably 0.126 mass%, and even more preferably 0.124 mass%.

[0086] In this embodiment, the amount of precipitated V in a stainless steel material is determined by the following method. Specifically, a cylindrical test piece having a diameter of 4 mm and a length of 50 mm is prepared from the stainless steel material according to this embodiment. When the steel material is a steel plate, the cylindrical test piece is prepared from the center of the plate thickness. In this case, the longitudinal direction of the cylindrical test piece is parallel to the rolling direction of the steel plate. When the steel material is a steel pipe, the cylindrical test piece is prepared from the center of the wall thickness. In this case, the longitudinal direction of the cylindrical test piece is parallel to the axial direction of the steel pipe. When the steel material is a round bar, the cylindrical test piece is prepared from the R / 2 position. In this case, the longitudinal direction of the cylindrical test piece is parallel to the axial direction of the round bar.

[0087] The cylindrical test piece is immersed in an electrolyte (10% acetylacetone-1% tetramethylammonium chloride-methanol solution) to carry out preliminary electrolysis. The preliminary electrolysis is carried out at a temperature of 15 to 30°C and a current of 1000 mA to a depth of approximately 100 μm from the surface of the steel material. The cylindrical test piece after preliminary electrolysis is immersed in an alcohol solution and subjected to ultrasonic cleaning to remove surface deposits. The mass M of the cylindrical test piece after ultrasonic cleaning is ０ (g) is measured.

[0088] After the mass measurement, the cylindrical test piece is immersed in an electrolyte (10% acetylacetone-1% tetramethylammonium chloride-methanol solution) and subjected to constant current electrolysis. The electrolyte used in the constant current electrolysis is a new electrolyte, not the electrolyte used in the preliminary electrolysis. The constant current electrolysis is performed at a temperature of 15 to 30°C and a current density of 20 mA / cm. ２ The mass of the cylindrical test piece after constant current electrolysis is M １ Furthermore, the cylindrical test piece after constant current electrolysis is further immersed in an alcohol solution and subjected to ultrasonic cleaning to remove any surface deposits.

[0089] The electrolyte used in the constant-current electrolysis and the alcohol solution used in the subsequent ultrasonic cleaning are passed through a 0.2 μm filter to capture the residue. The captured residue is decomposed with acid and subjected to ICP (inductively coupled plasma) emission spectrometry to quantify the amount of V in the residue. The amount of V in the resulting residue was determined by the mass difference (=M ０ -M １) to determine the V content (mass%) in the residue, which is defined as the amount of precipitated V (mass%). In this embodiment, the amount of precipitated V (mass%) is determined by rounding the obtained value to the fourth decimal place.

[0090] [Corrosion Resistance] The stainless steel material according to this embodiment has the above-described chemical composition, Fn1 of 150.0 or more, Fn2 of 24.0 or more, the above-described microstructure, a yield strength of 552 to 758 MPa, and further, the amount of precipitated V in the stainless steel material is 0.005 to 0.130 mass%. As a result, the stainless steel material according to this embodiment has excellent corrosion resistance (general corrosion resistance and stress corrosion cracking resistance) even in a supercritical corrosion environment. In this embodiment, excellent general corrosion resistance and stress corrosion cracking resistance in a supercritical corrosion environment are evaluated by the following method.

[0091] Specifically, test pieces for corrosion testing are prepared from the stainless steel material according to this embodiment. When the steel material is a steel plate, the test piece is prepared from the center of the plate thickness. In this case, the longitudinal direction of the test piece is parallel to the rolling direction of the steel plate. When the steel material is a steel pipe, the test piece is prepared from the center of the wall thickness. In this case, the longitudinal direction of the test piece is parallel to the axial direction of the steel pipe. When the steel material is a round bar, the test piece is prepared from the R / 2 position. In this case, the longitudinal direction of the test piece is parallel to the axial direction of the round bar. The size of the test piece is, for example, 75 mm in length, 10 mm in width, and 2 mm in thickness.

[0092] In accordance with NACE TM0316 (2016), a stress equivalent to 100% of the actual yield stress is applied to the test specimen by four-point bending. The stressed test specimen is sealed in an autoclave together with the test jig. A 5.0 mass% sodium chloride aqueous solution is poured into the autoclave so that the test specimen is immersed. SO 4 is poured into the autoclave. ２ and O ２ and NO ２ and CO ２ The mixed gas is pressurized and sealed in to saturate the test solution to form a test bath. At this time, the total pressure of the mixed gas is 300 bar, and the SO ２ The concentration was 30 ppm, and the O ２ The concentration was 30 ppm, and the NO in the mixed gas ２The concentration was 30 ppm, and the remainder was CO ２ 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.

[0093] The mass, density, and surface area of ​​the test specimen before stress loading and the test specimen after 96 hours were determined, and the corrosion rate (mm / year) of the test specimen was calculated. In this embodiment, the corrosion rate was calculated by rounding the obtained value to the fourth decimal place. Furthermore, the surface of the test specimen after 96 hours was observed with a magnifying glass at 10x magnification to confirm the presence or absence of cracks. If cracks are suspected based on the observation with the magnifying glass, the area where cracks are suspected is further cut out in the longitudinal direction of the test specimen, and the cross section is observed with an optical microscope at 100x magnification to confirm the presence or absence of cracks. 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, the test specimen is evaluated as having excellent general corrosion resistance even in a supercritical corrosion environment. Furthermore, in this embodiment, if no cracks are observed as a result of the corrosion test under the above conditions, the test specimen is evaluated as having excellent stress corrosion cracking resistance even in a supercritical corrosion environment.

[0094] [Low temperature toughness] The stainless steel material according to this embodiment has the above-described chemical composition, Fn1 is 150.0 or more, Fn2 is 24.0 or more, the above-described microstructure, a yield strength of 552 to 758 MPa, and further, the amount of precipitated V in the stainless steel material is 0.005 to 0.130 mass%. As a result, the stainless steel material according to this embodiment has excellent low temperature toughness in a cryogenic environment. In this embodiment, the excellent low temperature toughness in a cryogenic environment is evaluated by the following method.

[0095] Specifically, a T-direction full-size or sub-size V-notch test piece is prepared from the stainless steel material according to this embodiment in accordance with API 5CRA (2019). Here, when the steel material is a steel plate, the plate width direction of the steel plate is defined as the "T direction". When the steel material is a steel pipe, the pipe diameter direction of the steel pipe is defined as the "C direction", the pipe axial direction of the steel pipe is defined as the "L direction", and the direction perpendicular to the C direction and 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 L direction is defined as the "T direction".

[0096] The prepared V-notch test specimen is subjected to a Charpy impact test in accordance with JIS Z 2242 (2018) to determine the absorbed energy (J) at -70°C. When a sub-size V-notch test specimen is used, the obtained absorbed energy is divided by the reduction factor described in API 5CRA (2019) to convert it to the absorbed energy of a full-size V-notch test specimen. In this embodiment, the absorbed energy (J) at -70°C is determined by rounding the obtained value to one decimal place.

[0097] In this embodiment, when the absorbed energy at −70° C. obtained under the above conditions is 60 J or more, the steel is evaluated as having excellent low-temperature toughness in a cryogenic environment. In this specification, the absorbed energy at −70° C. is also simply referred to as “absorbed energy.”

[0098] [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 stress corrosion cracking resistance) in a supercritical corrosion environment and excellent low-temperature toughness in an extremely low-temperature environment.

[0099] [Manufacturing Method] An example of a method for manufacturing a stainless steel material according to this embodiment having the above-described configuration will be described. Note that the method for manufacturing a stainless steel material according to this embodiment is not limited to the manufacturing method described below. This example of a method for manufacturing a stainless steel material according to this embodiment includes a step of preparing an intermediate steel material (preparation step), a step of quenching the intermediate steel material (quenching step), a step of precipitating V carbide (V carbide precipitation step), and a step of tempering (tempering step). Each step will be described in detail below.

[0100] [Preparation step] 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, there is no particular limitation on the method for manufacturing the intermediate steel material. 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 mother pipe when the final product is a seamless steel pipe, or a steel material having a circular cross section perpendicular to the axial direction when the final product is a round steel bar.

[0101] The preparation step may include a step of preparing a material (material preparation step) and a step of hot working the material to produce an intermediate steel material (hot working step). Below, the case where the material preparation step and the hot working step are included will be described in detail.

[0102] [Material Preparation Step] In the material preparation step, a material is produced using molten steel having the above-described chemical composition. The method for producing the material is not particularly limited and may be a well-known method. Specifically, a cast piece (slab, bloom, or billet) may be produced using the molten steel by a continuous casting method. An ingot may be produced using the molten steel by an ingot casting method. If necessary, the slab, bloom, or ingot may be subjected to blooming to produce a billet. The material (slab, bloom, or billet) is produced through the above steps.

[0103] [Hot working process] In the hot working process, a prepared raw material is hot worked to produce an intermediate steel material. As described above, when the steel material is a seamless steel pipe, the intermediate steel material corresponds to a mother pipe. First, a billet is heated in a heating furnace. The heating temperature is not particularly limited, but is, for example, 1100 to 1300°C. The billet extracted from the heating furnace is hot worked to produce a mother pipe (seamless steel pipe). The hot working method is not particularly limited, and a well-known method may be used.

[0104] For example, a mother pipe may be produced by performing the Mannesmann process as hot working. In this case, a round billet is pierced and rolled using a piercing mill. When piercing and rolling is performed, the piercing ratio is not particularly limited, but is, for example, 1.0 to 4.0. The piercing-rolled round billet is further hot-rolled using a mandrel mill, a reducer, a sizing mill, or the like to produce a mother pipe. The cumulative area reduction rate in the hot working process is, for example, 20 to 70%. A mother pipe may also be produced from the billet by performing other hot working methods. For example, when the steel material is a short, thick-walled steel pipe such as a coupling, the mother pipe may be produced by forging using the Erhardt process or the like. A mother pipe is produced through the above steps. The wall thickness of the mother pipe is not particularly limited, but is, for example, 9 to 60 mm.

[0105] When the steel material is round steel, the material is first heated in a heating furnace. The heating temperature is not particularly limited, but is, for example, 1100 to 1300°C. The material extracted from the heating furnace is hot worked to produce an intermediate steel material having a circular cross section perpendicular to the axial direction. The hot working is, for example, blooming using a blooming mill or hot rolling using a continuous rolling mill. The continuous rolling mill has horizontal stands each having a pair of grooved rolls arranged side by side in the vertical direction, and vertical stands each having a pair of grooved rolls arranged side by side in the horizontal direction, arranged alternately. When the steel material is a steel plate, the material is first heated in a heating furnace. The heating temperature is not particularly limited, but is, for example, 1100 to 1300°C. The material extracted from the heating furnace is hot rolled using a blooming mill and a continuous rolling mill to produce an intermediate steel material in the shape of a steel plate.

[0106] An intermediate steel material produced by hot working may be air-cooled (as-rolled). An intermediate steel material produced by hot working may be quenched directly after hot working without being cooled to room temperature, or may be quenched after reheating (reheating) after hot working. When quenching is performed directly after hot working or after reheating, cooling may be stopped or slow cooling may be performed during quenching. In this case, the occurrence of quench cracks in the mother pipe can be suppressed. When quenching is performed directly after hot working or after reheating, stress relief annealing (SR) may be performed after quenching and before the next heat treatment step. In this case, residual stress in the mother pipe is removed.

[0107] As described above, in the preparation step, an intermediate steel material is prepared. The intermediate steel material may be manufactured by the above-mentioned preferred process, or may be manufactured by a third party, or may be manufactured in a factory or business establishment other than the factory where the quenching and tempering steps described below are performed. The heat treatment step will be described in detail below.

[0108] [Quenching process] The intermediate steel material produced in the hot working process is quenched (quenching process). Quenching is performed by a well-known method. Specifically, the intermediate steel material after the hot working process is loaded into a heat treatment furnace, held at a 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) the intermediate steel material is held at the quenching temperature.

[0109] 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 above-described microstructure. On the other hand, if the quenching temperature is too high, V carbides may form a solid solution, and the amount of precipitated V in the resulting stainless steel material may not be sufficient. Therefore, in this embodiment, the preferred quenching temperature is 850 to 1050°C. The holding time at the quenching temperature is not particularly limited, but is, for example, 5 to 80 minutes.

[0110] 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. When the intermediate steel material is a mother pipe, the mother pipe may be rapidly cooled, for example, by immersing it in a water bath or an oil bath, or the mother pipe may be rapidly cooled by pouring or spraying cooling water onto the outer surface and / or inner surface of the mother pipe using shower cooling or mist cooling.

[0111] [V Carbide Precipitation Step] The quenched intermediate steel is subjected to heat treatment for V carbide precipitation (V carbide precipitation step). Specifically, in the V carbide precipitation step, the intermediate steel is maintained at 490 to 510°C for 8 to 20 minutes. In an intermediate steel having the above-described chemical composition, V carbide is likely to precipitate at 490 to 510°C. Here, the maintenance time in the V carbide precipitation step refers to the time (minutes) for which the intermediate steel is maintained at 490 to 510°C. Note that in the V carbide precipitation step, the intermediate steel may be maintained at a constant temperature, or the temperature may be changed within the range of 490 to 510°C. In other words, in the V carbide precipitation step according to this embodiment, it is sufficient that the temperature of the intermediate steel is maintained within the range of 490 to 510°C for 8 to 20 minutes.

[0112] In the V carbide precipitation step according to this embodiment, if the maintenance time is too short, the precipitation of V carbide will be insufficient, and the amount of V precipitated in the produced stainless steel material may not be sufficient. On the other hand, if the maintenance time is too long, V carbide will precipitate excessively, and the amount of V precipitated in the produced stainless steel material may be too high. Therefore, in this embodiment, the maintenance time is preferably 8 to 20 minutes.

[0113] [Tempering step] The intermediate steel material after the V carbide precipitation step is further subjected to a tempering step. In this specification, "tempering" refers to the process of tempering the intermediate steel material after quenching. ｃ１ The tempering temperature corresponds to the furnace temperature at which the intermediate steel material is heated and held 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 extracted from the heat treatment furnace.

[0114] In the tempering process according to this embodiment, the volume fraction of the retained austenite phase is increased by maintaining the steel at a predetermined temperature, and the strength of the intermediate steel is also adjusted. Therefore, if the tempering temperature is too low, the above-mentioned microstructure may not be obtained in the manufactured stainless steel material. In this case, the yield strength may also become too high. On the other hand, if the tempering temperature is too high, the strength may decrease, and the yield strength may become too low. Therefore, in this embodiment, the preferred tempering temperature is 530 to 650°C.

[0115] Furthermore, if the holding time in the tempering process according to this embodiment is too short, the above-mentioned microstructure may not be obtained in the manufactured stainless steel material. In this case, the yield strength may become too high. On the other hand, if the holding time is too long, Cu precipitates in the steel material may increase, causing the yield strength to become too high. Therefore, in this embodiment, the preferred holding time is 20 to 70 minutes.

[0116] Preferably, the V carbide precipitation step and the tempering step are carried out consecutively. That is, it is preferable that after the intermediate steel material is maintained at 490 to 510°C in the V carbide precipitation step, the intermediate steel material is heated to the tempering temperature (°C) without being cooled, and held at that temperature.

[0117] The stainless steel material according to this embodiment can be manufactured by the above steps. As mentioned 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.

[0118] Molten steel having the chemical composition shown in Tables 1A and 1B was melted using a 50 kg vacuum melting furnace, and steel ingots were produced by an ingot casting method. Note that "-" in Tables 1A and 1B indicates that the content of the corresponding element was at the impurity level. For example, the W content and Co content of Test No. 1 were rounded to two decimal places to mean 0%. Similarly, the Nb content of Test No. 1 was rounded to three decimal places to mean 0%. Similarly, the B content, Ca content, Mg content, and REM content of Test No. 1 were rounded to five decimal places to mean 0%. Similarly, the V content of Test No. 25 was rounded to two decimal places to mean 0%. Similarly, the Cu content of Test No. 32 was rounded to two decimal places to mean 0%.

[0119]

[0120]

[0121] Furthermore, the chemical compositions listed in Tables 1A and 1B and Fn1 (= 576.5 - 2660.7 × C - 7.8 × Cr - 11.3 × Mo - 20.9 × Ni - 10.6 × Cu) and Fn2 (= 2 × Cr + 2 × Mo + 19 × V + 28 × N - 887 × C) calculated from the above definitions are shown in Table 2.

[0122]

[0123] The ingots of each test number were heated at 1250°C for 2 hours, and then hot-worked to produce intermediate steel materials (steel plates) with a thickness of 45 mm and a width of 60 mm. The intermediate steel materials of each test number were subjected to a quenching process, a V-carbide precipitation process, and a tempering process. Specifically, the intermediate steel materials of each test number were held at the quenching temperature (°C) for the holding time (minutes) listed in the quenching process column of Table 2, and then rapidly cooled. The intermediate steel materials of each test number were then held at 490 to 510°C for the holding time (minutes) listed in Table 2, and then heated and held at the tempering temperature (°C) for the holding time (minutes) listed in the tempering process column of Table 2.

[0124] [Evaluation Tests] Steel sheets of each test number were obtained through the above steps. The obtained steel sheets of each test number were subjected to a microstructure observation test, a tensile test, a V precipitate amount measurement test, a corrosion test, and a Charpy impact test.

[0125] [Microstructure Observation Test] A microstructure observation test was performed on the steel sheets of each test number using the method described above. Specifically, the volume fraction (%) of the retained austenite phase was determined using the X-ray diffraction method performed using the method described above. Furthermore, the volume fraction (%) of the ferrite phase was determined using the point counting method in accordance with JIS G 0555 (2020) performed using the method described above. The volume fraction (%) of the martensite phase was calculated from the volume fractions of the retained austenite phase and the ferrite phase obtained. The volume fraction of the ferrite phase obtained for each test number is shown in the "Ferrite (volume %)" column of Table 3. The volume fraction of the retained austenite phase obtained for each test number is shown in the "Residual γ (volume %)" column of Table 3. The volume fraction of the martensite phase obtained for each test number is shown in the "Martensite (volume %)" column of Table 3.

[0126]

[0127] [Tensile Test] A tensile test was performed on the steel plate of each test number in accordance with ASTM E8 / E8M (2022). Specifically, a round bar tensile test specimen with a parallel diameter of 8.9 mm and a gauge length of 35.6 mm was prepared from the center of the plate thickness of the steel plate of each test number. The longitudinal direction of the round bar tensile test specimen was parallel to the rolling direction of the steel plate. Using the round bar tensile test specimen of each test number, a tensile test was performed at room temperature (24±3°C) in air to determine the 0.2% offset yield strength (MPa). The determined 0.2% offset yield strength was defined as the yield strength (MPa). The obtained yield strength for each test number is shown in the "YS (MPa)" column of Table 3.

[0128] [Measurement test for precipitated V amount] The precipitated V amount was determined for the steel plate of each test number by the method described above. Specifically, a cylindrical test piece having a diameter of 4 mm and a length of 50 mm was prepared from the center of the plate thickness of each test number by the method described above, and preliminary electrolysis was performed by the method described above. Thereafter, ultrasonic cleaning was performed by the method described above for the cylindrical test piece of each test number, and the mass M ０Furthermore, constant current electrolysis was performed on the cylindrical test piece of each test number by the method described above, and the mass M １ Furthermore, the cylindrical test piece of each test number was subjected to ultrasonic cleaning by the method described above.

[0129] The electrolyte used in the constant-current electrolysis and the alcohol solution used in the subsequent ultrasonic cleaning were passed through a 0.2 μm filter to capture the residue. The captured residue was decomposed with acid and subjected to ICP emission spectrometry to quantify the amount of V in the residue. Furthermore, the amount of V in the resulting residue was calculated by calculating the mass difference (=M ０ -M １ The amount of precipitated V (mass %) was calculated by dividing the amount of precipitated V by the test number. The amount of precipitated V obtained for each test number is shown in the "Amount of precipitated V (mass %)" column of Table 3.

[0130] [Corrosion Test] A corrosion test was performed on the steel plate with each test number to evaluate its general corrosion resistance and stress corrosion cracking resistance in a supercritical corrosion environment. Specifically, test specimens for the corrosion test were prepared using the method described above. A corrosion test was performed on the prepared test specimens under the conditions described above to determine the corrosion rate (mm / year). Furthermore, the test specimens after the corrosion test were checked for the presence or absence of cracks using the method described above. The obtained corrosion rates (mm / year) are shown in Table 3. Furthermore, test numbers in which no cracks were observed are indicated as "EX (Excellent)" in the "Stress Corrosion Cracking Resistance" column of Table 3. Furthermore, test numbers in which cracks were observed are indicated as "NA (Not Acceptable)" in the "Stress Corrosion Cracking Resistance" column of Table 3.

[0131] [Charpy Impact Test] A Charpy impact test was performed on the steel plate of each test number in accordance with JIS Z 2242 (2018). Specifically, a full-size V-notch test piece in the T direction was prepared from the center of the plate thickness of the steel plate of each test number in accordance with API 5CRA (2019). A Charpy impact test in accordance with JIS Z 2242 (2018) was performed on the prepared V-notch test piece to determine the absorbed energy (J) at -70 ° C. The absorbed energy at -70 ° C. for each test number obtained is shown in the "vE (-70 ° C) (J)" column in Table 3.

[0132] [Evaluation Results] Referring to Tables 1A, 1B, 2, and 3, the steel plates with test numbers 1 to 21 had appropriate chemical compositions, with Fn1 of 150.0 or more and Fn2 of 24.0 or more. These steel plates were also manufactured by the preferred manufacturing method described in the specification. As a result, these steel plates had a microstructure consisting of 30.0 to 85.0% by volume of ferrite phase, 0.1 to 40.0% by volume of retained austenite phase, and the remainder being martensite phase. These steel plates also had a yield strength of 552 to 758 MPa. These steel plates also had a precipitated V content of 0.005 to 0.130 mass%. As a result, in a corrosion test, these steel plates exhibited a corrosion rate of 0.100 mm / year or less, demonstrating excellent general corrosion resistance even in a supercritical corrosion environment. Furthermore, no cracking was observed in the corrosion test, demonstrating excellent stress corrosion cracking resistance even in a supercritical corrosion environment. Furthermore, in a Charpy impact test, these steel plates exhibited an absorbed energy of 60 J or more at −70° C., demonstrating excellent low-temperature toughness even in an extremely low-temperature environment.

[0133] On the other hand, the steel plate of test number 22 had an Fn1 of less than 150.0. As a result, this steel plate had an excessive amount of retained austenite phase and a yield strength of less than 552 MPa. In other words, the desired yield strength was not obtained.

[0134] The steel plate of test number 23 had an Fn2 of less than 24.0. As a result, the corrosion rate of this steel plate exceeded 0.100 mm / year in the corrosion test, and this steel plate did not have excellent general corrosion resistance in a supercritical corrosion environment. Furthermore, cracks were observed in this steel plate in the corrosion test, and this steel plate did not have excellent stress corrosion cracking resistance in a supercritical corrosion environment.

[0135] The steel plate of test number 24 had an Fn2 of less than 24.0. As a result, cracks were confirmed in the corrosion test for this steel plate, and it was found that this steel plate did not have excellent stress corrosion cracking resistance in a supercritical corrosion environment.

[0136] The steel plate of test number 25 had an excessively low V content. As a result, the amount of precipitated V in this steel plate was less than 0.005 mass%. As a result, the corrosion rate of this steel plate exceeded 0.100 mm / year in the corrosion test, and this steel plate did not have excellent general corrosion resistance in a supercritical corrosion environment. Furthermore, cracks were observed in this steel plate in the corrosion test, and this steel plate did not have excellent stress corrosion cracking resistance in a supercritical corrosion environment.

[0137] The steel plate of test number 26 had an excessively high C content. As a result, the corrosion rate of this steel plate exceeded 0.100 mm / year in the corrosion test, and this steel plate did not have excellent general corrosion resistance in a supercritical corrosion environment. Furthermore, cracks were observed in this steel plate in the corrosion test, and this steel plate did not have excellent stress corrosion cracking resistance in a supercritical corrosion environment.

[0138] The steel plate of test number 27 had an excessively high Cr content, and as a result, the absorbed energy of this steel plate at −70° C. in the Charpy impact test was less than 60 J, and it did not have excellent low-temperature toughness in an extremely low-temperature environment.

[0139] The steel plate of test number 28 had an excessively low Cr content. As a result, the corrosion rate of this steel plate exceeded 0.100 mm / year in the corrosion test, and this steel plate did not have excellent general corrosion resistance in a supercritical corrosion environment. Furthermore, cracks were observed in this steel plate in the corrosion test, and this steel plate did not have excellent stress corrosion cracking resistance in a supercritical corrosion environment.

[0140] The steel plate of test number 29 had an excessively high Ni content. As a result, the steel plate had an excessively large amount of retained austenite phase, resulting in a yield strength of less than 552 MPa. In other words, the desired yield strength was not obtained.

[0141] The steel plate of test number 30 had an excessively high Mo content, and as a result, the absorbed energy of this steel plate at −70° C. in the Charpy impact test was less than 60 J, and it did not have excellent low-temperature toughness in an extremely low-temperature environment.

[0142] The steel plate of test number 31 had an excessively low Mo content. As a result, the corrosion rate of this steel plate exceeded 0.100 mm / year in the corrosion test, and this steel plate did not have excellent general corrosion resistance in a supercritical corrosion environment. Furthermore, cracks were observed in this steel plate in the corrosion test, and this steel plate did not have excellent stress corrosion cracking resistance in a supercritical corrosion environment.

[0143] The steel plate of test number 32 had an excessively low Cu content. As a result, the corrosion rate of this steel plate exceeded 0.100 mm / year in the corrosion test, and this steel plate did not have excellent general corrosion resistance in a supercritical corrosion environment. Furthermore, cracks were observed in this steel plate in the corrosion test, and this steel plate did not have excellent stress corrosion cracking resistance in a supercritical corrosion environment.

[0144] The steel plate of test number 33 was subjected to the V carbide precipitation process for a long period of time. As a result, the amount of precipitated V in this steel plate exceeded 0.130 mass%. As a result, the yield strength of this steel plate exceeded 758 MPa. In other words, the desired yield strength was not obtained. Furthermore, in a Charpy impact test, the absorbed energy of this steel plate at −70°C was less than 60 J, indicating that it did not have excellent low-temperature toughness in a cryogenic environment.

[0145] For the steel plates of test numbers 34 and 35, the maintenance time for the V carbide precipitation step was too short. As a result, the amount of precipitated V in these steel plates was less than 0.005 mass%. As a result, the corrosion rate of these steel plates exceeded 0.100 mm / year in the corrosion test, and they did not have excellent general corrosion resistance in a supercritical corrosion environment. Furthermore, cracks were confirmed in these steel plates in the corrosion test, and they did not have excellent stress corrosion cracking resistance in a supercritical corrosion environment.

[0146] The tempering temperatures of the steel plates of test numbers 36 and 37 were too low. As a result, the retained austenite phase in these steel plates was too small. As a result, the absorbed energy of these steel plates at −70°C in the Charpy impact test was less than 60 J, and they did not have excellent low-temperature toughness in a cryogenic environment.

[0147] The steel plates of test numbers 38 and 39 were held for too long a time in the tempering process, resulting in a yield strength of more than 758 MPa, meaning that the desired yield strength was not achieved.

[0148] 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 can be implemented by appropriately modifying the above-described embodiments within the scope of the present disclosure.

Claims

1. A stainless steel material, by mass%, C: 0.030% or less, Si: 1.00% or less, Mn: 0.30% or less, P: 0.030% or less, S: 0.0050% or less, Cr: more than 18.0 to 21.5%, Ni: more than 5.00 to 8.00%, Mo: more than 0.60 to 3.50%, Al: 0.005 to 0.050%, V: 0.01 to 0.30%, N: 0.0030 to 0.1000%, Ti: 0.100% or less, Cu: 0.01 to less than 1.00%, O: 0.020% or less, W: 0 to 2.00%, Nb: 0 to 0.150%, Co: 0 to 0.80%, B: 0 to 0.0050%, Ca: 0 to 0.0050%, Mg: 0 to 0.0100%, rare earth elements: 0 to 0.0100%, and the balance consisting of Fe and impurities, where Fn1 defined by formula (1) is 150.0 or more, Fn2 defined by formula (2) is 24.0 or more, having a microstructure consisting of a ferrite phase with a volume fraction of 30.0 to 85.0%, a retained austenite phase of 0.1 to 40.0%, and the balance being a martensite phase, with a yield strength of 552 to 758 MPa, and in the said stainless steel material, the precipitated V amount is 0.005 to 0.130 mass%, a stainless steel material. Fn1 = 576.5 - 2660.7×C - 7.8×Cr - 11.3×Mo - 20.9×Ni - 10.6×Cu (1) Fn2 = 2×Cr + 2×Mo + 19×V + 28×N - 887×C (2) Here, for the element symbols in formulas (1) and (2), the contents of the corresponding elements are substituted in mass%.

2. The stainless steel material according to claim 1, containing one or more elements selected from the group consisting of W: 0.01 to 2.00%, Nb: 0.001 to 0.150%, Co: 0.01 to 0.80%, B: 0.0001 to 0.0050%, Ca: 0.0001 to 0.0050%, Mg: 0.0001 to 0.0100%, and rare earth elements: 0.0001 to 0.0100%.

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