Duplex stainless steel material

Abstract

Provided is a duplex stainless steel material excelling in resistance to general corrosion and in resistance to stress corrosion cracking, even in a supercritical corrosive environment. The duplex stainless steel material according to the present disclosure has the chemical composition described in the description and has a microstructure comprising 35-65% by volume of ferrite, with the remainder being austenite. In the duplex stainless steel material according to the present disclosure, a dislocation density ρ(α) in the ferrite and a dislocation density ρ(γ) in the austenite satisfy the following relational expression (1).　(1): 0.3 < ρ(γ) / ρ(α) < 4.0 In relational expression (1), the dislocation density in the austenite is substituted with m-2 for ρ(γ), and the dislocation density in the ferrite is substituted with m-2 for ρ(α).

Description

Duplex stainless steel material

[0001] The present disclosure relates to steel products, and more particularly to duplex stainless steel products.

[0002] Currently, the rise in carbon dioxide (CO2) concentrations on land has become a global problem. As a result, efforts to curb CO2 emissions are underway. Among these efforts to curb CO2 emissions, CCUS has attracted particular attention.

[0003] CCUS is an abbreviation for Carbon Dioxide Capture, Utilization and Storage. In other words, CCUS includes three technologies: CO2 capture, utilization, and storage. Of these, a CO2 storage technology that has been attracting attention is one that captures CO2 emitted from industrial facilities such as power plants and factories and injects it into depleted oil wells for storage.

[0004] The steel materials used in this CO2 storage technology must have high strength because CO2 is injected into depleted oil wells. Furthermore, CO2 is a corrosive substance that corrodes steel. Therefore, the steel materials used in CO2 storage technology must have excellent corrosion resistance in a corrosive environment containing a large amount of CO2.

[0005] Duplex stainless steel materials having a two-phase structure of ferrite and austenite are known as steel materials with excellent corrosion resistance in corrosive environments. Duplex stainless steel materials are disclosed, for example, in Japanese Patent Laid-Open Publication Nos. 5-132741 (Patent Document 1) and 9-195003 (Patent Document 2).

[0006] The duplex stainless steel material disclosed in Patent Document 1 has a chemical composition, in mass%, of C: 0.03% or less, Si: 1.0% or less, Mn: 1.5% or less, P: 0.040% or less, S: 0.008% or less, sol. Al: 0.040% or less, Ni: 5.0 to 9.0%, Cr: 23.0 to 27.0%, Mo: 2.0 to 4.0%, W: more than 1.5 to 5.0%, N: 0.24 to 0.32%, with the balance being Fe and unavoidable impurities, and PREW (= Cr + 3.3 (Mo + 0.5W) + 16N) is 40 or more.

[0007] The duplex stainless steel material disclosed in Patent Document 2 contains, by mass%, C: 0.12% or less, Si: 1% or less, Mn: 2% or less, Ni: 3 to 12%, Cr: 20 to 35%, Mo: 0.5 to 10%, W: over 3 to 8%, Co: 0.01 to 2%, Cu: 0.1 to 5%, N: 0.05 to 0.5%, and the balance being Fe and unavoidable impurities.

[0008] In the duplex stainless steel materials disclosed in Patent Documents 1 and 2, the corrosion resistance is improved by adjusting the chemical composition.

[0009] JP-A-5-132741 JP-A-9-195003

[0010] In the carbon dioxide storage technology described above, in order to inject CO2 into depleted oil wells, CO2 is compressed and pressurized into steel pipes to bring the CO2 to a supercritical state. Meanwhile, CO2 recovered from industrial facilities such as power plants and factories contains SOx, NOx, and O2. Here, SOx is a general term for sulfur oxides, such as SO2. NOx is a general term for nitrogen oxides, such as NO2. SOx and NOx dissolve in water to form acidic compounds (sulfuric acid, sulfurous acid, nitric acid, nitrous acid, etc.), which cause general corrosion and cracking of steel surfaces. Furthermore, the presence of H2S in the formation water in the storage well promotes corrosion and cracking of steel. Therefore, supercritical CO2 containing SOx, NOx, O2, and H2S creates an extremely corrosive environment. In this specification, the corrosive environment created by supercritical CO2 containing SOx, NOx, O2, and H2S is referred to as a "supercritical corrosive environment."

[0011] In other words, steel materials used in supercritical corrosion environments are required to have better general corrosion resistance and stress corrosion cracking resistance than those in conventional corrosive environments. On the other hand, the duplex stainless steel materials disclosed in Patent Documents 1 and 2 are not intended for use in such supercritical corrosion environments.

[0012] An object of the present disclosure is to provide a duplex stainless steel material that has excellent general corrosion resistance and stress corrosion cracking resistance even in a supercritical corrosion environment.

[0013] The duplex stainless steel material according to the present disclosure has, in mass %, C: 0.050% or less, Si: 1.00% or less, Mn: 0.40 to 3.00%, P: 0.050% or less, S: 0.0050% or less, Cr: 20.00 to 30.00%, Cu: 0.05 to less than 1.50%, Ni: 2.00 to 10.00%, Mo: 0.80 to 5.00%, W: 0.01 to 3.00%, N: 0.001 to 0.350%, Co: 0.10 to 1.00%, Sn: 0.001 to 0.050%, Al: 0.050% or less, V: 0.01 to 0.50%, Ti: 0.001 to 0.500%, The steel sheet has an average particle size of 1000 nm and a microstructure consisting of 35 to 65% by volume of ferrite and the remainder being austenite, and the dislocation density ρ(α) in the ferrite and the dislocation density ρ(γ) in the austenite satisfy the following formula (1): 0.3<ρ(γ) / ρ(α)<4.0 (1) Here, ρ(γ) in the formula (1) represents the dislocation density in the austenite. -2 ρ(α) is the dislocation density in the ferrite -2 is assigned.

[0014] The duplex stainless steel material according to the present disclosure has excellent general corrosion resistance and stress corrosion cracking resistance even in supercritical corrosive environments.

[0015] The present inventors have investigated duplex stainless steel materials that have excellent general corrosion resistance and stress corrosion cracking resistance in a supercritical corrosive environment formed by supercritical CO2 containing SOx, NOx, O2, and H2S.

[0016] The present inventors first investigated, from the viewpoint of chemical composition, steel materials having excellent general corrosion resistance and excellent stress corrosion cracking resistance in supercritical corrosion environments. As a result, the inventors have found that the composition is, in mass %, C: 0.050% or less, Si: 1.00% or less, Mn: 0.40 to 3.00%, P: 0.050% or less, S: 0.0050% or less, Cr: 20.00 to 30.00%, Cu: 0.05 to less than 1.50%, Ni: 2.00 to 10.00%, Mo: 0.80 to 5.00%, W: 0.01 to 3.00%, N: 0.001 to 0.350%, Co: 0.10 to 1.00%, Sn: 0.001 to 0.050%, Al: 0.050% or less, V: 0.01 to 0.50%, Ti: 0.001 to 0.500%, C It was considered that a duplex stainless steel material consisting of a: 0.0001 to 0.0100%, B: 0.0001 to 0.0050%, O: 0.010% or less, Mg: 0 to 0.010%, rare earth elements: 0 to 0.010%, Zr: 0 to 0.010%, Nb: 0 to 0.500%, Ta: 0 to 0.100%, As: 0 to 0.050%, Zn: 0 to 0.010%, Pb: 0 to 0.010%, Sb: 0 to 0.010%, and the balance being Fe and impurities, may have excellent general corrosion resistance and excellent stress corrosion cracking resistance even in a supercritical corrosion environment.

[0017] On the other hand, even duplex stainless steel materials having the above-mentioned chemical composition sometimes do not have sufficient stress corrosion cracking resistance in supercritical corrosion environments. Therefore, the present inventors further investigated means for improving stress corrosion cracking resistance in supercritical corrosion environments.

[0018] Here, the duplex stainless steel material having the above-mentioned chemical composition has a microstructure consisting of 35 to 65% by volume of ferrite and the remainder being austenite. In this specification, "consisting of ferrite and austenite" means that the amount of phases other than ferrite and austenite in the microstructure is negligibly small.

[0019] The present inventors further conducted detailed studies on methods for improving the stress corrosion cracking resistance of duplex stainless steel materials having the above-described chemical composition and microstructure in supercritical corrosion environments. Specifically, the present inventors focused on dislocations in duplex stainless steel materials. Regions of high dislocation density in steel materials are believed to be prone to initiation sites for corrosion-induced cracks. Furthermore, when stress is applied, cracks may propagate from the regions of high dislocation density. In other words, if there are regions of locally high dislocation density in duplex stainless steel materials, there is a concern that the stress corrosion cracking resistance of the duplex stainless steel materials may be reduced. The present inventors believed that, particularly in supercritical corrosion environments, the localization of dislocations may easily cause a localized reduction in stress corrosion cracking resistance.

[0020] As a result of further detailed studies by the present inventors taking the above findings into consideration, it has been found that a duplex stainless steel material having the above-mentioned chemical composition and a microstructure consisting of 35 to 65% by volume of ferrite and the remainder being austenite has excellent stress corrosion cracking resistance even in a supercritical corrosion environment if the dislocation density ρ(α) in ferrite and the dislocation density ρ(γ) in austenite satisfy the following formula (1): 0.3<ρ(γ) / ρ(α)<4.0 (1) where ρ(γ) in formula (1) represents the dislocation density in austenite m -2 ρ(α) is the dislocation density in ferrite, m -2 is assigned.

[0021] The reason why a duplex stainless steel material having the above-described chemical composition and a microstructure consisting of 35 to 65% by volume of ferrite and the remainder being austenite, has excellent stress corrosion cracking resistance even in a supercritical corrosion environment when the dislocation density ρ(α) in ferrite and the dislocation density ρ(γ) in austenite satisfy the above-described formula (1), is not clear in detail. However, the present inventors speculate as follows.

[0022] In a duplex stainless steel material having the above-described chemical composition, increasing the dislocation density of the duplex stainless steel material by work hardening or the like can result in localized dislocations. In this case, the dislocation density increases locally, which can easily result in a local decrease in corrosion resistance. On the other hand, if the ratio of the dislocation density ρ(α) in ferrite to the dislocation density ρ(γ) in austenite is controlled within a certain range, the localization of dislocation density in the duplex stainless steel material may be alleviated. The inventors speculate that this will result in the alleviation of the localized increase in dislocation density, thereby improving the stress corrosion cracking resistance of the duplex stainless steel material even in a supercritical corrosion environment.

[0023] It is possible that a duplex stainless steel material having the above-described chemical composition and microstructure may have excellent corrosion resistance even in a supercritical corrosion environment due to a mechanism other than the above, in which the dislocation density ρ(α) in ferrite and the dislocation density ρ(γ) in austenite satisfy the above-described formula (1). However, it has been proven in the examples described below that a duplex stainless steel material having the above-described chemical composition and microstructure may have excellent stress corrosion cracking resistance even in a supercritical corrosion environment due to the dislocation density ρ(α) in ferrite and the dislocation density ρ(γ) in austenite satisfying the above-described formula (1).

[0024] The duplex stainless steel material according to this embodiment, which was completed based on the above findings, has the following features.

[0025] [1] In mass%, C: 0.050% or less, Si: 1.00% or less, Mn: 0.40 to 3.00%, P: 0.050% or less, S: 0.0050% or less, Cr: 20.00 to 30.00%, Cu: 0.05 to less than 1.50%, Ni: 2.00 to 10.00%, Mo: 0.80 to 5.00%, W: 0.01 to 3.00%, N: 0.001 to 0.350%, Co: 0.10 to 1.00%, Sn: 0.001 to 0.050%, Al: 0.050% or less, V: 0.01 to 0.50%, Ti: 0.001 to 0.500%, Ca: 0.0001 to 0.0100%, B: 0.0001 to 0.0050%, O: 0.010% or less, Mg: 0 to 0.010%, rare earth elements: 0 to 0.010%, Zr: 0 to 0.010%, Nb: 0 to 0.500%, Ta: 0 to 0.100%, As: 0 to 0.050%, Zn: 0 to 0.010%, Pb: 0 to 0.010%, Sb: 0 to 0.010%, and the balance being Fe and impurities; the microstructure is composed of 35 to 65% by volume of ferrite and the balance being austenite; and the dislocation density ρ(α) in the ferrite and the dislocation density ρ(γ) in the austenite satisfy the following formula (1): Duplex stainless steel material. 0.3<ρ(γ) / ρ(α)<4.0 (1) Here, ρ(γ) in the formula (1) represents the dislocation density in the austenite. -2 ρ(α) is the dislocation density in the ferrite -2 is substituted.

[0026] [2] The duplex stainless steel material according to [1], containing one or more elements selected from the group consisting of Mg: 0.001 to 0.010%, rare earth elements: 0.001 to 0.010%, Zr: 0.001 to 0.010%, Nb: 0.001 to 0.500%, Ta: 0.001 to 0.100%, As: 0.001 to 0.050%, Zn: 0.001 to 0.010%, Pb: 0.001 to 0.010%, and Sb: 0.001 to 0.010%.

[0027] The shape of the duplex stainless steel material according to this embodiment is not particularly limited. The duplex 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.

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

[0029] [Chemical Composition] The chemical composition of the duplex stainless steel material according to this embodiment contains the following elements: "%" relating to elements means mass % unless otherwise specified.

[0030] C: 0.050% or less Carbon (C) is unavoidably contained. That is, the lower limit of the C content is greater than 0%. C forms Cr carbides at grain boundaries, increasing the corrosion susceptibility at the grain boundaries. Therefore, if the C 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 C content is 0.050% or less. A preferred upper limit of the C content is 0.048%, more preferably 0.045%. The C content is preferably 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 0.005%.

[0031] Si: 1.00% or less Silicon (Si) is inevitably 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 toughness of the steel material decreases 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%, and more preferably 0.90%. To more effectively obtain the above effects, a preferred lower limit of the Si content is 0.05%, more preferably 0.10%, even more preferably 0.20%, and even more preferably 0.25%.

[0032] Mn: 0.40 to 3.00% Manganese (Mn) deoxidizes and desulfurizes steel. Mn also improves the hot workability of steel. If the Mn content is too low, the above effects cannot be fully achieved even if the contents of other elements are within the ranges of this embodiment. On the other hand, Mn combines with sulfur (S) to form Mn sulfides. Therefore, if the Mn content is too high, even if the contents of other elements are within the ranges of this embodiment, many coarse Mn sulfides are formed, reducing the corrosion resistance of the steel in supercritical corrosion environments. Therefore, the Mn content is 0.40 to 3.00%. The preferred lower limit of the Mn content is 0.45%, more preferably 0.50%, even more preferably 0.60%, and even more preferably 0.70%. The preferred upper limit of the Mn content is 2.95%, more preferably 2.90%, and even more preferably 2.80%.

[0033] P: 0.050% 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 0.050% or less. A preferred upper limit of the P content is 0.045%, more preferably 0.040%, and even more preferably 0.030%. 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%.

[0034] 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%.

[0035] Cr: 20.00 to 30.00% Chromium (Cr) forms a passive film as an oxide on the surface of a steel material, thereby improving the corrosion resistance of the steel material in a supercritical corrosion environment. If the Cr 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 Cr 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 Cr content is 20.00 to 30.00%. The preferred lower limit of the Cr content is 20.05%, more preferably 20.10%, and even more preferably 20.20%. The preferred upper limit of the Cr content is 29.95%, more preferably 29.90%, and even more preferably 29.80%.

[0036] Furthermore, the duplex stainless steel material according to this embodiment exhibits even higher corrosion resistance if the Cr content is greater than 24.00% but less than 30.00%. That is, the Cr content may be 20.00% to 24.00%, or greater than 24.00% to 30.00%. When the Cr content is 20.00% to 24.00%, a more preferable upper limit of the Cr content is 23.95%, even more preferably 23.90%, and even more preferably 23.80%. When the Cr content is greater than 24.00% but less than 30.00%, a more preferable lower limit of the Cr content is 24.01%, even more preferably 24.05%, and even more preferably 24.10%.

[0037] Cu: 0.05 to less than 1.50% Copper (Cu) enhances the corrosion resistance of steel in supercritical corrosion environments. If the Cu content is too low, the above effect cannot be fully achieved, 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 0.05 to less than 1.50%. A preferred lower limit of the Cu content is 0.06%, more preferably 0.07%, even more preferably 0.10%, even more preferably 0.15%, even more preferably 0.20%, and even more preferably 0.25%. A preferred upper limit of the Cu content is 1.49%, even more preferably less than 1.45%, even more preferably 1.44%, and even more preferably 1.40%.

[0038] Ni: 2.00 to 10.00% Nickel (Ni) stabilizes the austenite structure of the steel material. Ni also enhances the corrosion resistance of the steel material 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 austenite volume fraction may become too high, even if the contents of other elements are within the ranges of this embodiment, and the strength of the steel material may decrease. Therefore, the Ni content is 2.00 to 10.00%. The preferred lower limit of the Ni content is 2.50%, more preferably 3.00%, even more preferably 3.50%, and even more preferably 4.00%. The preferred upper limit of the Ni content is 9.80%, even more preferably 9.60%, and even more preferably 9.20%.

[0039] Mo: 0.80 to 5.00% Molybdenum (Mo) enhances the corrosion resistance of steel in supercritical corrosion environments. 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 hot workability of the steel deteriorates even if the contents of other elements are within the ranges of this embodiment. Therefore, the Mo content is 0.80 to 5.00%. A preferred lower limit of the Mo content is 0.85%, more preferably 0.90%, and even more preferably 1.20%. A preferred upper limit of the Mo content is 4.95%, more preferably 4.90%, even more preferably 4.80%, and even more preferably 4.60%.

[0040] W: 0.01 to 3.00% Tungsten (W) enhances the corrosion resistance of steel in supercritical corrosion environments. If the W content is too low, the above effect cannot be fully achieved, even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the W content is too high, the strength of the steel will be too high, and the toughness of the steel will decrease, even if the contents of other elements are within the ranges of this embodiment. Therefore, the W content is 0.01 to 3.00%. The preferred lower limit of the W content is 0.02%, more preferably 0.03%, even more preferably 0.04%, even more preferably 0.06%, even more preferably 0.10%, and even more preferably 0.15%. The preferred upper limit of the W content is 2.70%, even more preferably 2.50%, even more preferably 2.00%, and even more preferably 1.50%.

[0041] N: 0.001 to 0.350% Nitrogen (N) stabilizes the austenitic structure of steel. N also 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 0.001 to 0.350%. A preferred lower limit of the N content is 0.003%, more preferably 0.005%, and even more preferably 0.008%. A preferred upper limit of the N content is 0.345%, and even more preferably 0.340%.

[0042] Co: 0.10 to 1.00% Cobalt (Co) forms a coating on the surface of a steel material, improving the corrosion resistance of the steel material in a supercritical corrosion environment. If the Co 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 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 0.10 to 1.00%. The preferred lower limit of the Co content is 0.11%, more preferably 0.13%, and even more preferably 0.15%. The preferred upper limit of the Co content is 0.95%, more preferably 0.90%, and even more preferably 0.85%.

[0043] Sn: 0.001 to 0.050% Tin (Sn) enhances the corrosion resistance of steel in supercritical corrosion environments. If the Sn 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 Sn content is too high, the hot workability of the steel decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the Sn content is 0.001 to 0.050%. A preferred lower limit of the Sn content is 0.002%, more preferably 0.003%, and even more preferably 0.005%. A preferred upper limit of the Sn content is 0.045%, and even more preferably 0.040%.

[0044] Al: 0.050% or less Aluminum (Al) is inevitably contained. That is, the lower limit of the Al content is more than 0%. Al deoxidizes steel. On the other hand, if the Al content is too high, coarse oxide-based inclusions are formed, reducing the toughness of the steel material, even if the contents of other elements are within the ranges of this embodiment. Therefore, the Al content is 0.050% or less. A preferred upper limit of the Al content is 0.045%, more preferably 0.040%. To more effectively obtain the above effects, a preferred lower limit of the Al content is 0.001%, more preferably 0.005%, and even more preferably 0.010%. Note that the Al content referred to in this specification means the content of "acid-soluble Al," that is, sol. Al.

[0045] V: 0.01 to 0.50% Vanadium (V) increases the strength of steel. If the V 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 V content is too high, the strength of the steel becomes too high, and the toughness of the steel decreases, even if the contents of other elements are within the ranges of this embodiment. Therefore, the V content is 0.01 to 0.50%. A preferred lower limit of the V content is 0.02%, more preferably 0.03%, and even more preferably 0.05%. A preferred upper limit of the V content is 0.45%, and even more preferably 0.40%.

[0046] Ti: 0.001 to 0.500% Titanium (Ti) forms carbonitrides and increases the strength of steel. If the Ti 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 Ti content is too high, the strength of the steel becomes too high, and the toughness of the steel decreases, even if the contents of other elements are within the ranges of this embodiment. Therefore, the Ti content is 0.001 to 0.500%. A preferred lower limit of the Ti content is 0.003%, more preferably 0.005%, even more preferably 0.007%, and even more preferably 0.010%. A preferred upper limit of the Ti content is 0.350%, more preferably 0.250%, even more preferably 0.200%, even more preferably 0.180%, even more preferably 0.160%, and even more preferably 0.150%.

[0047] Ca: 0.0001 to 0.0100% Calcium (Ca) neutralizes S in the steel material by fixing it as sulfides, thereby improving the hot workability of the steel material. If the Ca 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 Ca content is too high, even if the contents of other elements are within the ranges of this embodiment, the oxides in the steel material become coarse, reducing the corrosion resistance of the steel material in supercritical corrosion environments. Therefore, the Ca content is 0.0001 to 0.0100%. The preferred lower limit of the Ca content is 0.0003%, more preferably 0.0005%, even more preferably 0.0006%, even more preferably 0.0008%, and even more preferably 0.0010%. The upper limit of the Ca content is preferably 0.0080%, more preferably 0.0060%, further preferably 0.0055%, and still further preferably 0.0050%.

[0048] B: 0.0001 to 0.0050% Boron (B) suppresses the segregation of S to grain boundaries in steel and improves the hot workability of the steel. If the B content is too low, the above effect cannot be fully achieved even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the B content is too high, boron nitride (BN) is formed, reducing the toughness of the steel, even if the contents of other elements are within the ranges of this embodiment. Therefore, the B content is 0.0001 to 0.0050%. The preferred lower limit of the B content is 0.0005%, more preferably 0.0010%, and even more preferably 0.0015%. The preferred upper limit of the B content is 0.0045%, more preferably 0.0040%, and even more preferably 0.0035%.

[0049] O: 0.010% 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 0.010% or less. A preferred upper limit of the O content is 0.009%, more preferably 0.008%, and even more preferably 0.006%. 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%.

[0050] The balance of the chemical composition of the duplex 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 or scrap or the manufacturing environment during industrial production of the duplex stainless steel material, and are acceptable within a range that does not adversely affect the duplex stainless steel material according to this embodiment.

[0051] [Optional Elements] The chemical composition of the duplex stainless steel material according to this embodiment may further contain one or more elements selected from the group consisting of 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.

[0052] Mg: 0 to 0.010% 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 sulfide, thereby improving the hot workability of the steel. Even if even a small amount of Mg is contained, the above effects can be obtained 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 0.010%. The preferred lower limit of the Mg content is more than 0%, more preferably 0.001%, and even more preferably 0.002%. The preferred upper limit of the Mg content is 0.009%, more preferably 0.008%, even more preferably 0.006%, and even more preferably 0.005%.

[0053] Rare Earth Elements: 0 to 0.010% Rare earth elements (REM) are optional elements and do not necessarily need to be contained. That is, the REM content may be 0%. When contained, REM neutralizes S in the steel by fixing it as sulfides, improving the hot workability of the steel. Even if even a small amount of REM is contained, 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 0.010%. The preferred lower limit of the REM content is more than 0%, more preferably 0.001%, and even more preferably 0.002%. The preferred upper limit of the REM content is 0.009%, and even more preferably 0.008%.

[0054] 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.

[0055] The chemical composition of the duplex stainless steel material according to this embodiment may further contain one or more elements selected from the group consisting of Zr, Nb, and Ta in place of a portion of Fe. All of these elements are optional elements and increase the strength of the steel material.

[0056] Zr: 0 to 0.010% Zirconium (Zr) is an optional element and does not necessarily need to be contained. That is, the Zr content may be 0%. When contained, Zr forms carbonitrides and increases the strength of the steel material. Even if even a small amount of Zr is contained, the above effect can be obtained to some extent. However, if the Zr content is too high, even if the contents of other elements are within the ranges of this embodiment, the strength of the steel material will be too high and the toughness of the steel material will decrease. Therefore, the Zr content is 0 to 0.010%. The preferred lower limit of the Zr content is more than 0%, more preferably 0.001%, even more preferably 0.002%, and even more preferably 0.003%. The preferred upper limit of the Zr content is 0.009%, even more preferably 0.008%.

[0057] Nb: 0 to 0.500% 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 forms carbonitrides and increases 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, even if the contents of other elements are within the ranges of this embodiment, the strength of the steel material will be too high and the toughness of the steel material will decrease. Therefore, the Nb content is 0 to 0.500%. The preferred lower limit of the Nb content is more than 0%, more preferably 0.001%, even more preferably 0.005%, even more preferably 0.010%, and even more preferably 0.050%. The preferred upper limit of the Nb content is 0.400%, even more preferably 0.300%, even more preferably 0.200%, even more preferably 0.175%, even more preferably 0.160%, and even more preferably 0.150%.

[0058] Ta: 0 to 0.100% Tantalum (Ta) is an optional element and does not necessarily need to be contained. That is, the Ta content may be 0%. When contained, Ta forms carbonitrides and increases the strength of the steel material. Even if even a small amount of Ta is contained, the above effect can be obtained to some extent. However, if the Ta content is too high, even if the contents of other elements are within the ranges of this embodiment, the strength of the steel material will be too high and the toughness of the steel material will decrease. Therefore, the Ta content is 0 to 0.100%. The preferred lower limit of the Ta content is more than 0%, more preferably 0.001%, even more preferably 0.002%, even more preferably 0.003%, and even more preferably 0.005%. The preferred upper limit of the Ta content is 0.095%, even more preferably 0.090%, and even more preferably 0.080%.

[0059] The chemical composition of the duplex stainless steel material according to this embodiment may further contain one or more elements selected from the group consisting of As, Zn, Pb, and Sb in place of a portion of Fe. All of these elements are optional elements, and enhance the corrosion resistance of the steel material in supercritical corrosion environments.

[0060] As: 0 to 0.050% Arsenic (As) is an optional element and does not necessarily need to be contained. That is, the As content may be 0%. When contained, As enhances the corrosion resistance of steel in supercritical corrosion environments. Even if even a small amount of As is contained, the above effect can be obtained to some extent. However, if the As content is too high, even if the contents of other elements are within the ranges of this embodiment, the corrosion resistance of the steel in supercritical corrosion environments may actually decrease. Therefore, the As content is 0 to 0.050%. The preferred lower limit of the As content is more than 0%, more preferably 0.001%, and even more preferably 0.002%. The preferred upper limit of the As content is 0.045%, more preferably 0.040%, and even more preferably 0.035%.

[0061] Zn: 0 to 0.010% Zinc (Zn) is an optional element and does not necessarily need to be contained. That is, the Zn content may be 0%. When contained, Zn enhances the corrosion resistance of the steel material in a supercritical corrosion environment. Even if even a small amount of Zn is contained, the above effect can be obtained to some extent. However, if the Zn content is too high, the corrosion resistance of the steel material in a supercritical corrosion environment may actually decrease even if the contents of other elements are within the ranges of this embodiment. Therefore, the Zn content is 0 to 0.010%. The preferred lower limit of the Zn content is more than 0%, more preferably 0.001%, even more preferably 0.002%, and even more preferably 0.003%. The preferred upper limit of the Zn content is 0.009%, even more preferably 0.008%, and even more preferably 0.007%.

[0062] Pb: 0 to 0.010% Lead (Pb) is an optional element and may not be contained. That is, the Pb content may be 0%. When contained, Pb enhances the corrosion resistance of steel in supercritical corrosion environments. Even if even a small amount of Pb is contained, the above effect can be obtained to some extent. However, if the Pb content is too high, numerous defects will occur on the surface of the steel after hot working, even if the contents of other elements are within the ranges of this embodiment. Therefore, the Pb content is 0 to 0.010%. The preferred lower limit of the Pb content is more than 0%, more preferably 0.001%, even more preferably 0.002%, and even more preferably 0.003%. The preferred upper limit of the Pb content is 0.009%, even more preferably 0.008%, and even more preferably 0.007%.

[0063] Sb: 0 to 0.010% Antimony (Sb) is an optional element and does not necessarily need to be contained. That is, the Sb content may be 0%. When contained, Sb enhances the corrosion resistance of steel in supercritical corrosion environments. Even if even a small amount of Sb is contained, the above effect can be obtained to some extent. However, if the Sb content is too high, even if the contents of other elements are within the ranges of this embodiment, the manufacturing cost will increase dramatically. Therefore, the Sb content is 0 to 0.010%. The preferred lower limit of the Sb content is more than 0%, more preferably 0.001%, even more preferably 0.002%, and even more preferably 0.003%. The preferred upper limit of the Sb content is 0.009%, even more preferably 0.008%.

[0064] Preferably, the duplex stainless steel material according to this embodiment has the above-mentioned chemical composition and further has a Cr content of more than 24.00 to 30.00%, which further enhances the corrosion resistance of the duplex stainless steel material according to this embodiment. More specifically, the chemical composition of the duplex stainless steel material according to this embodiment is, in mass %, C: 0.050% or less, Si: 1.00% or less, Mn: 0.40 to 3.00%, P: 0.050% or less, S: 0.0050% or less, Cr: more than 24.00 to 30.00%, Cu: 0.05 to less than 1.50%, Ni: 2.00 to 10.00%, Mo: 0.80 to 5.00%, W: 0.01 to 3.00%, N: 0.001 to 0.350%, Co: 0.10 to 1.00%, Sn: 0.001 to 0.050%, Al: 0.050% or less, V: 0.0 0.010%, Zn: 0-0.010%, Pb: 0-0.010%, Sb: 0-0.010%, and the balance is Fe and impurities, the corrosion resistance is further improved, provided that the other components of this embodiment are satisfied.

[0065] In other words, the duplex stainless steel material according to this embodiment contains, in mass%, C: 0.050% or less, Si: 1.00% or less, Mn: 0.40 to 3.00%, P: 0.050% or less, S: 0.0050% or less, Cr: 20.00 to 24.00%, Cu: 0.05 to less than 1.50%, Ni: 2.00 to 10.00%, Mo: 0.80 to 5.00%, W: 0.01 to 3.00%, N: 0.001 to 0.350%, Co: 0.10 to 1.00%, Sn: 0.001 to 0.050%, and Al: 0.050% or more. or less, V: 0.01 to 0.50%, Ti: 0.001 to 0.500%, Ca: 0.0001 to 0.0100%, B: 0.0001 to 0.0050%, O: 0.010% or less, Mg: 0 to 0.010%, rare earth elements: 0 to 0.010%, Zr: 0 to 0.010%, Nb: 0 to 0.500%, Ta: 0 to 0.100%, As: 0 to 0.050%, Zn: 0 to 0.010%, Pb: 0 to 0.010%, Sb: 0 to 0.010%, and the balance being Fe and impurities.

[0066] [Microstructure] The duplex stainless steel material according to this embodiment has the above-described chemical composition, a microstructure consisting of 35 to 65% by volume of ferrite and the remainder being austenite, and a dislocation density ratio (ρ(γ) / ρ(α)) described below of more than 0.3 and less than 4.0. As a result, the duplex 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.

[0067] In this specification, the microstructure "consisting of ferrite and austenite" means that the amount of phases other than ferrite and austenite in the microstructure is negligibly small. For example, in the microstructure of the duplex stainless steel material according to this embodiment, the volume fraction of precipitates and inclusions is negligibly small compared to the volume fraction of ferrite and austenite. In other words, the microstructure of the duplex stainless steel material according to this embodiment may contain minute amounts of precipitates, inclusions, etc. in addition to ferrite and austenite.

[0068] As described above, the microstructure of the duplex stainless steel material according to this embodiment has a ferrite volume fraction of 35 to 65%. If the ferrite volume fraction is too low, the corrosion resistance of the steel material may decrease. On the other hand, if the ferrite volume fraction is too high, the toughness and hot workability of the steel material may decrease. Therefore, in the microstructure of the duplex stainless steel material according to this embodiment, the ferrite volume fraction is 35 to 65%. A preferred lower limit of the ferrite volume fraction is 36%, and more preferably 37%. A preferred upper limit of the ferrite volume fraction is 64%, and more preferably 63%.

[0069] In this embodiment, the volume fraction of ferrite in the duplex stainless steel material can be determined by a method in accordance with ASTM E562 (2019). A test piece for microstructure observation is prepared from the duplex stainless steel material according to this embodiment. When the steel material is a steel plate, a test piece having an observation surface measuring 5 mm in the rolling direction and 5 mm in the plate width direction is prepared from the center of the plate thickness. When the steel material is a steel pipe, a test piece having an observation surface measuring 5 mm in the pipe axial direction and 5 mm in the pipe circumferential direction is prepared from the center of the wall thickness. In this specification, the circumferential direction of the steel pipe refers to a direction perpendicular to the pipe axial direction and the pipe radial direction. When the steel material is a round steel bar, a test piece having an observation surface measuring 5 mm in the axial direction and 5 mm in the circumferential direction is prepared from the R / 2 position. In this specification, the R / 2 position of the round steel bar refers to the center position of the radius R in a cross section perpendicular to the axial direction of the round steel bar. In addition, in this specification, the circumferential direction of the round steel bar refers to a direction perpendicular to the axial and radial directions. The size of the test piece is not particularly limited as long as the above observation surface can be obtained.

[0070] The observation surface of the prepared test piece is mirror-polished. The mirror-polished observation surface is electrolytically etched in a 7% potassium hydroxide etching solution to reveal the structure. The observation surface with the revealed structure is observed using an optical microscope in 10 fields of view. The area of ​​each field of view is, for example, 1.00 mm. 2(Magnification: 100x). Ferrite is identified from the contrast in each field of view. The area ratio of the identified ferrite is measured using a point counting method in accordance with ASTM E562 (2019). In this embodiment, the arithmetic average value of the obtained ferrite area ratios in 10 fields of view is defined as the ferrite volume ratio (%). In this embodiment, the ferrite volume ratio (%) is calculated by rounding the obtained value to one decimal place.

[0071] [Dislocation Density Ratio] The duplex stainless steel material according to this embodiment has the above-described chemical composition and microstructure, and the dislocation density ρ(α) in ferrite and the dislocation density ρ(γ) in austenite satisfy the following formula (1): 0.3<ρ(γ) / ρ(α)<4.0 (1) where ρ(γ) in formula (1) represents the dislocation density in austenite m -2 ρ(α) is the dislocation density in ferrite, m -2 is assigned.

[0072] Fn1 is defined as ρ(γ) / ρ(α). Fn1 refers to the distribution ratio (dislocation density ratio) of the dislocation density in austenite to the dislocation density in ferrite in a duplex stainless steel material having the above-mentioned chemical composition and microstructure. The larger the dislocation density ratio Fn1, the more dislocations are localized in austenite. The smaller the dislocation density ratio Fn1, the more dislocations are localized in ferrite. In other words, if the dislocation density ratio Fn1 is too high, the dislocation density in austenite increases locally, significantly reducing the stress corrosion cracking resistance of the steel material in a supercritical corrosion environment. On the other hand, if the dislocation density ratio Fn1 is too low, the dislocation density in ferrite increases locally, reducing the stress corrosion cracking resistance of the steel material in a supercritical corrosion environment. Therefore, in the duplex stainless steel material according to this embodiment, the dislocation density ratio Fn1 is greater than 0.3 and less than 4.0. The preferred lower limit of the dislocation density ratio Fn1 is 0.4, and more preferably 0.5. The upper limit of the dislocation density ratio Fn1 is preferably 3.9, and more preferably 3.8.

[0073] In this embodiment, the dislocation density ratio Fn1 can be determined by the following method. A thin film sample for dislocation density measurement is prepared from the duplex stainless steel material according to this embodiment. Specifically, a test piece is cut out from the duplex stainless steel material. Then, a thin film sample is prepared from the cut out test piece by electrolytic polishing using the Twin Jet method. When the steel material is a steel plate, a thin film sample having an observation surface perpendicular to the rolling direction is prepared from a test piece cut out from the center of the plate thickness. When the steel material is a steel pipe, a thin film sample having an observation surface perpendicular to the pipe axial direction is prepared from a test piece cut out from the center of the wall thickness. When the steel material is a round bar, a thin film sample having an observation surface perpendicular to the axial direction is prepared from a test piece cut out from the R / 2 position. The sizes of the test piece and the thin film sample are not particularly limited as long as the observation field described below can be obtained.

[0074] Ferrite and austenite are identified in the observation surface of the obtained thin film sample. Ferrite and austenite in the observation surface can be identified by identifying the crystal structure by electron beam diffraction. The identified field of view is subjected to structural observation using a transmission electron microscope (hereinafter also referred to as "TEM"). The area of ​​the observation field is not particularly limited, and may be an area obtained at a magnification that makes it easy to observe dislocations. The area of ​​the observation field is, for example, 100 nm x 100 nm to 800 nm x 800 nm. Furthermore, the volume (m) of each observation field is calculated from the area and thickness of the observation field. 3 The thickness of the observation field is determined from the total integrated intensity of the electron energy loss intensity spectrum (EELS) and the integrated intensity of the zero-loss spectrum for the thin film sample.

[0075] The structure observation of the observation field is performed at an acceleration voltage of 300 kV under diffraction conditions suitable for dislocation observation. The diffraction conditions suitable for dislocation observation refer to conditions under which two-beam approximation is possible, in which a transmitted wave and one diffracted wave are excited. Specifically, for austenite, these refer to conditions under which a reciprocal lattice vector g = 40-2 is excited, and for ferrite, these refer to conditions under which a reciprocal lattice vector g = 200 or 30-1 is excited. In this embodiment, the thin film sample is tilted to achieve diffraction conditions suitable for dislocation observation, and the observation region of the thin film sample is observed under bright-field observation. Note that, instead of bright-field observation, dislocations may be observed using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Observation using HAADF-STEM allows dislocations to be observed more easily than bright-field observation.

[0076] Furthermore, the observation field is photographed by performing an appropriate time exposure. Dislocations are identified from the contrast of the generated photographic image, and the length of the dislocations is measured. The length of the dislocations can be measured by a known method. For example, the length of the dislocations identified based on the contrast may be obtained by image analysis. The total length (m) of the dislocations in the obtained ferrite in the five fields of view and the total volume (m) of the ferrite in the five fields of view are calculated. 3 ) and the dislocation density ρ(α)(m -2 Similarly, the sum (m) of the dislocation lengths in the obtained austenite in the five visual fields and the total volume (m 3 ) and the dislocation density ρ(γ)(m -2 ) is required.

[0077] The dislocation density ρ(α) (m -2 ) versus the dislocation density in austenite ρ(γ) (m -2 ) is calculated. In this embodiment, the dislocation density ratio Fn1 is calculated by rounding the calculated value to one decimal place.

[0078] In this embodiment, the dislocation density ρ(α) (m -2 ) and dislocation density ρ(γ)(m -2 ) is not particularly limited as long as the dislocation density ratio Fn1 is greater than 0.3 and less than 4.0. In the duplex stainless steel material according to this embodiment, the dislocation density ρ(α)(m -2 ) is, for example, 1.0 × 10 13 ~8.0 x 10 15 (m -2 In the duplex stainless steel material according to this embodiment, the dislocation density ρ(γ)(m -2 ) is, for example, 1.0 × 10 13 ~8.0 x 10 15 (m -2 ) The dislocation density in ferrite ρ(α) (m -2 ) is 1.0 × 10 13 ~8.0 x 10 15 (m -2 ) and the dislocation density in austenite ρ(γ)(m -2 ) is 1.0 × 10 13 ~8.0 x 10 15 (m -2 ) a duplex stainless steel material having excellent corrosion resistance in a supercritical corrosion environment can be obtained, provided that the other configurations of this embodiment are satisfied.

[0079] [Corrosion Resistance] The duplex stainless steel material according to this embodiment has the chemical composition and microstructure described above, and has a dislocation density ratio Fn1 that is greater than 0.3 and less than 4.0. As a result, the duplex stainless steel material according to this embodiment has excellent corrosion resistance (general corrosion resistance and stress corrosion cracking resistance) even in supercritical corrosion environments. In this embodiment, excellent general corrosion resistance and stress corrosion cracking resistance in supercritical corrosion environments are evaluated by the following method.

[0080] As described above, the corrosion resistance of the duplex stainless steel material according to this embodiment is further improved when the Cr content is greater than 24.00% to 30.00%. Therefore, in this embodiment, the excellent general corrosion resistance and stress corrosion cracking resistance in a supercritical corrosion environment are evaluated according to the Cr content.

[0081] [When Cr: 20.00 to 24.00%] When the Cr content in the chemical composition is 20.00 to 24.00%, the excellent general corrosion resistance and stress corrosion cracking resistance of the steel material in a supercritical corrosion environment are evaluated by the following method. Specifically, a test piece for a corrosion test is prepared from the duplex stainless steel material according to this embodiment. When the steel material is a steel plate, the test piece is prepared from the center position 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 position 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, 30 mm in length, 20 mm in width, and 2 mm in thickness.

[0082] In accordance with ASTM G39-99 (2011), 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 then sealed in an autoclave along with the test jig. A 5.0 mass% sodium chloride aqueous solution is poured into the autoclave so that the test specimen is immersed. A mixed gas of SO2, O2, NO2, H2S, and CO2 is pressurized and sealed into the autoclave to saturate the test solution, forming a test bath. The total pressure of the mixed gas is 130 bar, the SO2 concentration in the mixed gas is 0.01 vol%, the O2 concentration in the mixed gas is 0.05 vol%, the NO2 gas concentration in the mixed gas is 0.01 vol%, and the H2S gas concentration is 0.01 vol%. After sealing the autoclave, the test bath is maintained at 90°C, and the test specimen is immersed for 720 hours while the test bath is stirred.

[0083] The mass, density, and surface area of ​​the test specimen after 720 hours are determined, and the corrosion rate (mm / year) of the test specimen is calculated. In this embodiment, the corrosion rate is calculated by rounding the obtained value to the nearest four decimal places. Furthermore, the surface of the test specimen after 720 hours is observed with a 10x magnification loupe to confirm the presence or absence of cracks. If the occurrence of cracks is suspected based on the observation with the loupe, the presence or absence of cracks is further confirmed by observation with an optical microscope at 100x magnification. In this embodiment, when the Cr content is 20.00 to 24.00%, 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, when the Cr content is 20.00 to 24.00%, 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.

[0084] [When Cr: More than 24.00 to 30.00%] When the Cr content in the chemical composition is more than 24.00 to 30.00%, the excellent general corrosion resistance and stress corrosion cracking resistance of the steel in a supercritical corrosion environment are evaluated using the following method. Specifically, a test specimen for the corrosion test is prepared in the same manner as when the Cr content is 20.00 to 24.00%. Furthermore, using the prepared test specimen, a corrosion test is conducted in the same manner as when the Cr content is 20.00 to 24.00%. At this time, the mixed gas sealed in the autoclave is set to a total pressure of 130 bar, an SO2 concentration of 0.02 vol%, an O2 concentration of 0.05 vol%, an NO2 gas concentration of 0.02 vol%, and an H2S gas concentration of 0.02 vol%. The corrosion test is conducted under the same other conditions as when the Cr content is 20.00 to 24.00% described above.

[0085] The mass, density, and surface area of ​​the test specimen after 720 hours are determined, and the corrosion rate (mm / year) of the test specimen is calculated. In this embodiment, the corrosion rate is calculated by rounding the obtained value to the nearest four decimal places. Furthermore, the surface of the test specimen after 720 hours is observed with a 10x magnification loupe to confirm the presence or absence of cracks. If the occurrence of cracks is suspected based on the observation with the loupe, the presence or absence of cracks is further confirmed by observation with an optical microscope at 100x magnification. In this embodiment, when the Cr content is greater than 24.00 to 30.00%, 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, when the Cr content is greater than 24.00 to 30.00%, 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.

[0086] [Yield strength] The yield strength of the duplex stainless steel material according to this embodiment is not particularly limited. The yield strength of the duplex stainless steel material according to this embodiment is, for example, 552 to 1000 MPa. The lower limit of the yield strength of the duplex stainless steel material according to this embodiment may be 565 MPa, 586 MPa, 621 MPa, or 655 MPa. The upper limit of the yield strength of the duplex stainless steel material according to this embodiment may be 965 MPa, less than 965 MPa, or 931 MPa.

[0087] The yield strength of the duplex stainless steel material according to this embodiment can be determined by the following method. Specifically, a tensile test is performed according to ASTM E8 / E8M (2022). A test specimen is 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 circular arc-shaped test specimen having a thickness equal to the wall thickness of the steel pipe, a width of 25.4 mm, and a gauge length of 50.8 mm is prepared. In this case, the longitudinal direction of the circular arc-shaped 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.

[0088] When preparing a tensile test piece, the size of the tensile test piece is, for example, 6 mm in parallel diameter and 24 mm in gauge length. A tensile test is performed using the test piece at room temperature (25°C) in the atmosphere. In this embodiment, the 0.2% offset proof stress obtained from the tensile test is defined as the yield strength (MPa). In this embodiment, the yield strength (MPa) is determined by rounding the obtained value to one decimal place.

[0089] [Shape of duplex stainless steel material] As described above, the shape of the duplex stainless steel material according to this embodiment is not particularly limited. Preferably, the duplex stainless steel material according to this embodiment is a seamless steel pipe. When the duplex stainless steel material according to this embodiment is a seamless steel pipe, it has excellent corrosion resistance (general corrosion resistance and stress corrosion cracking resistance) in a supercritical corrosion environment, even if the wall thickness is 5 mm or more.

[0090] [Manufacturing Method] An example of a method for manufacturing a duplex stainless steel material according to this embodiment having the above-described configuration will be described. Note that the method for manufacturing a duplex stainless steel material according to this embodiment is not limited to the manufacturing method described below. The example of a method for manufacturing a duplex stainless steel material according to this embodiment includes a material preparation step, a hot working step, a first cold working step, a solution treatment step, and a second cold working step. Each manufacturing step will be described in detail below.

[0091] [Material Preparation Step] In the material preparation step according to this embodiment, a material having the above-described chemical composition is prepared. The material may be prepared by manufacturing or by purchasing from a third party. In other words, the method for preparing the material is not particularly limited.

[0092] When manufacturing a raw material, for example, it is manufactured by the following method. Molten steel having the above-mentioned chemical composition is manufactured. A cast piece (slab, bloom, or billet) is manufactured using the molten steel by a continuous casting method. A steel ingot (ingot) may be manufactured using the molten steel by an ingot-making method. If necessary, the slab, bloom, or ingot may be subjected to blooming to manufacture a billet. The raw material is manufactured by the above-mentioned steps.

[0093] [Hot Working Step] In the hot working step according to this embodiment, the raw material prepared in the raw material preparation step is hot worked to produce an intermediate steel material. In this specification, the intermediate steel material refers to a plate-shaped steel material if the final product is a steel plate, a mother pipe if the final product is a steel pipe, a bar-shaped steel material having a circular cross section perpendicular to the axial direction if the final product is a round bar, or a wire-shaped steel material if the final product is a wire rod. The hot working may be hot forging, hot extrusion, or hot rolling. The hot working method is not particularly limited and may be a well-known method.

[0094] When the intermediate steel material is a mother pipe (seamless steel pipe), for example, the hot working step may involve the Eugène-Séjournet method or the Erhardt push bench method (i.e., hot extrusion), or piercing-rolling by the Mannesmann method (i.e., hot rolling). The hot working may be carried out only once or multiple times. For example, the above-mentioned piercing-rolling may be carried out on the raw material, and then the above-mentioned hot extrusion may be carried out. For example, the above-mentioned piercing-rolling may be carried out on the raw material, and then elongation-rolling may be carried out. That is, in the hot working step, hot working is carried out by a well-known method to produce an intermediate steel material having a desired shape.

[0095] [First cold working step] In the first cold working step according to this embodiment, cold working is performed on the intermediate steel material that has been subjected to the hot working step. The cold working may be cold rolling or cold drawing. That is, in the first cold working step, well-known cold working may be performed under well-known conditions. For example, the temperature of the intermediate steel material during cold working may be from room temperature to less than 150°C.

[0096] Here, the area reduction rate Rd1 (%) of the intermediate steel material in the first cold working step is defined as follows. The area reduction rate Rd1 (%) in the first cold working step is not particularly limited, but is, for example, 2 to 30%. Rd1 (%) = {1 - (cross-sectional area perpendicular to the working direction of the intermediate steel material after the first cold working step / cross-sectional area perpendicular to the working direction of the intermediate steel material before the first cold working step)} × 100

[0097] [Solution Treatment Step] In the solution treatment step according to this embodiment, the intermediate steel material that has been subjected to the first cold working step is subjected to solution treatment. The method of solution treatment is not particularly limited, and a well-known method may be used. For example, the intermediate steel material is charged into a heat treatment furnace, held at a desired temperature, and then rapidly cooled. In this case, the temperature at which the solution treatment is performed (heat treatment temperature) refers to the temperature (°C) of the heat treatment furnace for carrying out the solution treatment. The time for which the intermediate steel material is held at the solution treatment temperature (holding time) refers to the time (minutes) for which the intermediate steel material is held at the heat treatment temperature.

[0098] Preferably, the heat treatment temperature in the solution treatment step of this embodiment is 950 to 1150°C. If the heat treatment temperature is too low, the ferrite volume fraction of the duplex stainless steel material after solution treatment will be less than 35%, which may result in a decrease in the corrosion resistance of the manufactured duplex stainless steel material. On the other hand, if the heat treatment temperature is too high, the ferrite volume fraction of the duplex stainless steel material after solution treatment will exceed 65%, which may result in a decrease in the corrosion resistance of the steel material.

[0099] Therefore, when an intermediate steel material is charged into a heat treatment furnace, held at a desired temperature, and then rapidly cooled to perform solution treatment, the solution treatment temperature is preferably 950 to 1150°C. A more preferable lower limit of the solution treatment temperature is 960°C, and even more preferably 970°C. A more preferable upper limit of the solution treatment temperature is 1140°C, and even more preferably 1120°C.

[0100] When the intermediate steel material is charged into a heat treatment furnace, held at a desired temperature, and then rapidly cooled to perform solution treatment, the solution treatment time is not particularly limited and may be performed under well-known conditions. The solution treatment time is, for example, 5 to 180 minutes. The rapid cooling method is, for example, water cooling.

[0101] [Second Cold Working Step] In the second cold working step according to this embodiment, cold working is performed on the intermediate steel material that has been subjected to the solution treatment step. The cold working may be cold rolling or cold drawing. That is, in the second cold working step, well-known cold working may be performed under well-known conditions, as in the first cold working step. For example, the temperature of the intermediate steel material during cold working may be from room temperature to less than 150°C.

[0102] Here, the cross-sectional area reduction rate Rd2 (%) of the intermediate steel material in the second cold working step is defined as follows: Rd2 (%) = {1 - (cross-sectional area perpendicular to the working direction of the intermediate steel material after the second cold working step / cross-sectional area perpendicular to the working direction of the intermediate steel material before the second cold working step)} × 100

[0103] The area reduction rate Rd2 (%) in the second cold working step has a significant effect on the strength of the duplex stainless steel material produced. In other words, adjusting the area reduction rate Rd2 allows the strength of the produced duplex stainless steel material to be adjusted. However, if the area reduction rate Rd2 is too large, the dislocation density of austenite increases, and the dislocation density ratio Fn1 may become 4.0 or more. Therefore, in this embodiment, the area reduction rate Rd2 is set to 20% or less. Note that, in the second cold working step according to this embodiment, the lower limit of the area reduction rate Rd2 is not particularly limited and may be, for example, 1% or 3%.

[0104] Thus, in the preferred manufacturing method of the duplex stainless steel material according to this embodiment, a material preparation step, a hot working step, a first cold working step, a solution treatment step, and a second cold working step are performed. Here, the ratio Fn1 (=ρ(γ) / ρ(α)) of the dislocation density ρ(γ) in austenite to the dislocation density ρ(α) in ferrite is strongly influenced by the cold working step among the steps of the preferred manufacturing method described above, and its value changes. That is, in the preferred manufacturing method described above, the value of the dislocation density ratio Fn1 changes depending on the balance between the first cold working step and the second cold working step.

[0105] Therefore, in a preferred manufacturing method according to this embodiment, the area reduction rate Rd1 (%) in the first cold working step and the area reduction rate Rd2 (%) in the second cold working step satisfy the following formula (A). As a result, a duplex stainless steel material having the above-described chemical composition and microstructure and a dislocation density ratio Fn1 of greater than 0.3 and less than 4.0 can be consistently manufactured: Rd1 / Rd2>(Ni+20N+10Sn+4Co+0.5Mn+0.5Cu) / (Cr+3Mo+2Si) (A) where Rd1 in formula (A) represents the area reduction rate in the first cold working step (%), Rd2 represents the area reduction rate in the second cold working step (%), and the element symbols represent the contents of the corresponding elements in mass%.

[0106] Here, cold working before solution treatment promotes recrystallization during solution treatment, making it easier to reduce the variation in grain size. That is, the area reduction rate Rd1 (%) in the first cold working step affects the variation in grain size after solution treatment. If the variation in grain size after solution treatment is small, dislocations are more likely to be uniformly distributed between ferrite and austenite by cold working in the second cold working step. In this case, the dislocation density ratio Fn1 is likely to be small.

[0107] On the other hand, as described above, if the area reduction rate Rd2 (%) in the second cold working step is too large, the dislocation density of austenite tends to increase, and the dislocation density ratio Fn1 tends to become large. Therefore, in a preferred manufacturing method according to this embodiment, Rd1 relative to Rd2 is specified. That is, by increasing Rd1 to a certain level or more in accordance with Rd2, the grain size of the intermediate steel material in the second cold working step can be adjusted in advance. That is, it is possible to suppress a local increase in the dislocation density ρ(γ) in austenite in the second cold working step. As a result, the dislocation density ratio Fn1 can be reduced.

[0108] Furthermore, FnA is defined as (Ni + 20N + 10Sn + 4Co + 0.5Mn + 0.5Cu) / (Cr + 3Mo + 2Si). FnA is an index showing the degree of grain uniformity in the microstructure of a duplex stainless steel material having the above-mentioned chemical composition. The larger FnA, the greater the tendency for grain variation to increase. Therefore, even when FnA is large, increasing Rd1 in accordance with Rd2 will increase the effect of grain uniformity.

[0109] Therefore, in a preferred manufacturing method according to this embodiment, the ratio of Rd1 to Rd2 is set to be greater than FnA. In this case, it is possible to suppress a local increase in the dislocation density ρ(γ) in the austenite during the second cold working step. As a result, it is possible to reduce the dislocation density ratio Fn1. Thus, according to the preferred manufacturing method according to this embodiment, it is possible to stably manufacture a duplex stainless steel material having the above-described chemical composition and microstructure, and further satisfying the dislocation density ratio Fn1 of more than 0.3 but less than 4.0.

[0110] [Other Steps] The manufacturing method according to this embodiment may include manufacturing steps other than those described above. For example, the duplex stainless steel material according to this embodiment may be subjected to an aging heat treatment. Aging heat treatment means that the manufactured duplex stainless steel material is maintained at a desired temperature. In this case, the aging heat treatment may be performed by a well-known method, and is not particularly limited. For example, the duplex stainless steel material according to this embodiment may also be subjected to a pickling treatment. In this case, the pickling treatment may be performed by a well-known method, and is not particularly limited. Furthermore, other well-known post-treatments may be performed on the duplex stainless steel material that has been subjected to the second cold working step.

[0111] The duplex stainless steel material according to this embodiment can be manufactured by the above steps. Note that the above-described method for manufacturing the duplex stainless steel material is one example, and the duplex stainless steel material may be manufactured by other methods. The present invention will be described in more detail below with reference to examples.

[0112] In Example 1, a duplex stainless steel material having a chemical composition with a Cr content of 20.00 to 24.00% was investigated. Specifically, molten steel having the chemical composition shown in Tables 1A, 1B, and 1C 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 1C indicates that the content of the corresponding element was at the impurity level. For example, the Mg content, REM content, Zr content, Nb content, Ta content, As content, Zn content, Pb content, and Sb content of Test No. 1 were rounded to the nearest four decimal places and were 0%. Similarly, the W content of Test No. 36 was rounded to the nearest three decimal places and was 0%. Furthermore, Table 2 shows the chemical compositions shown in Tables 1A and 1B and FnA (=(Ni+20N+10Sn+4Co+0.5Mn+0.5Cu) / (Cr+3Mo+2Si)) calculated from the above definition.

[0113]

[0114]

[0115]

[0116]

[0117] Ingots of each steel type were hot worked to produce mother pipes (seamless steel pipes). The mother pipes of each test number that had been hot worked were subjected to first cold working at a cross-sectional area reduction rate Rd1 (%) shown in Table 2. Furthermore, the mother pipes of each test number were solution treated at the treatment temperature (°C) and holding time (minutes) shown in Table 2. Furthermore, the mother pipes of each test number that had been solution treated were subjected to second cold working at a cross-sectional area reduction rate Rd2 (%) shown in Table 2. The ratio of the cross-sectional area reduction rate Rd1 (%) of the first cold working to the cross-sectional area reduction rate Rd2 (%) of the second cold working for each test number is shown in the "Rd1 / Rd2" column in Table 2. Both the first cold working and the second cold working were performed by cold drawing.

[0118] [Evaluation Tests] Seamless steel pipes of each test number were obtained through the above steps. Tensile tests, microstructure observation tests, dislocation density ratio measurement tests, and corrosion tests were carried out on the obtained seamless steel pipes of each test number.

[0119] [Tensile Test] A tensile test was performed on the seamless steel pipe of each test number in accordance with ASTM E8 / E8M (2022) to determine the yield strength. Specifically, a circular arc-shaped test specimen for the tensile test was prepared from the seamless steel pipe of each test number. The circular arc-shaped test specimen had the same thickness as the wall thickness of the steel pipe, a width of 25.4 mm, and a gauge length of 50.8 mm. A tensile test was performed using the circular arc-shaped test specimen of each test number at room temperature (25°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 (Yield Strength) for each test number is shown in the "YS (MPa)" column of Table 3. Note that the seamless steel pipe of each test number all had a yield strength of 552 to 1000 MPa.

[0120]

[0121] [Microstructure Observation Test] Microstructure observation was performed on the seamless steel pipe of each test number to determine the volume fraction of ferrite. Specifically, a test piece for microstructure observation was prepared from the center of the wall thickness of the seamless steel pipe of each test number, having an observation surface of 5 mm in the pipe axis direction × 5 mm in the pipe circumferential direction. The observation surface of the test piece of each test number was polished to a mirror finish and subjected to electrolytic corrosion in a 7% potassium hydroxide etching solution. The observation surface, on which the structure was revealed by electrolytic corrosion, was observed in 10 fields of view using an optical microscope. The area of ​​each field of view was 1.00 mm 2 (magnification 100x).

[0122] In each visual field of each test number, the microstructure contained negligible amounts of phases other than ferrite and austenite. That is, the seamless steel pipe of each test number had a microstructure consisting of ferrite and austenite. In each visual field of each test number, ferrite and austenite were identified based on contrast. The area fraction of the identified ferrite was determined using the point counting method in accordance with ASTM E562 (2019). The arithmetic average of the area fractions of ferrite in 10 visual fields was taken as the ferrite volume fraction (%). The ferrite volume fraction (%) determined for each test number is shown in Table 3.

[0123] [Dislocation density ratio measurement test] A dislocation density ratio measurement test was carried out on the seamless steel pipe of each test number to determine the dislocation density ratio Fn1 (=ρ(γ) / ρ(α)). Specifically, a thin film sample was prepared from the seamless steel pipe of each test number by the above-mentioned method. Furthermore, the thin film sample of each test number was used to measure the dislocation density ρ(α) (m -2 ), and the dislocation density ρ(γ)(m -2 In this embodiment, dislocations were observed by bright field observation. For each test number, the dislocation density ρ(α) in the ferrite was 1.0 × 10 13 ~8.0 x 10 15 (m -2 ) and the dislocation density ρ(γ) in austenite is 1.0 × 10 13 ~8.0 x 10 15 (m -2 ) was obtained. -2 ) and ρ(γ)(m -2 The dislocation density ratio Fn1 (=ρ(γ) / ρ(α)) was calculated from the above. The calculated dislocation density ratio Fn1 is shown in the "Dislocation density ratio Fn1 (=ρ(γ) / ρ(α))" column in Table 3.

[0124] [Corrosion Test] A corrosion test was conducted on the steel material of 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 conducted on the prepared test specimens under the conditions described above to determine the corrosion rate (mm / year). Furthermore, the presence or absence of cracks was confirmed for the test specimens after the corrosion test using the method described above. In the corrosion test, the total pressure of the mixed gas was 130 bar, the SO2 concentration in the mixed gas was 0.01 vol%, the O2 concentration in the mixed gas was 0.05 vol%, the NO2 gas concentration in the mixed gas was 0.01 vol%, and the H2S gas concentration was 0.01 vol%. The obtained corrosion rates (mm / year) are shown in Table 3. In addition, test numbers in which no cracks were observed are marked with "E (Excellent)" in the "Crack" column of Table 3. Furthermore, for test numbers where cracks were confirmed, the "Cracks" column in Table 3 is marked with "NA (Not Acceptable)."

[0125] [Evaluation Results] Referring to Tables 1A, 1B, 1C, 2, and 3, the seamless steel pipes of test numbers 1 to 33 had appropriate chemical compositions. Furthermore, the manufacturing methods used for these seamless steel pipes were the preferred manufacturing methods described in the specification. As a result, these seamless steel pipes had a ferrite volume fraction of 35 to 65% and a dislocation density ratio Fn1 of more than 0.3 and less than 4.0. As a result, these seamless steel pipes were determined to have excellent corrosion resistance in corrosion resistance tests. That is, the seamless steel pipes of test numbers 1 to 33 had excellent general corrosion resistance and stress corrosion cracking resistance, even in a supercritical corrosion environment.

[0126] On the other hand, the seamless steel pipe of test number 34 had an excessively low Cr content. As a result, the corrosion rate of this seamless steel pipe exceeded 0.100 mm / year. In other words, this seamless steel pipe did not have excellent general corrosion resistance in a supercritical corrosion environment.

[0127] The seamless steel pipe of test number 35 had an excessively low Cu content. As a result, the corrosion rate of this seamless steel pipe exceeded 0.100 mm / year. In other words, this seamless steel pipe did not have excellent general corrosion resistance in a supercritical corrosion environment.

[0128] The seamless steel pipe of test number 36 had an excessively low W content. As a result, the corrosion rate of this seamless steel pipe exceeded 0.100 mm / year. In other words, this seamless steel pipe did not have excellent general corrosion resistance in a supercritical corrosion environment.

[0129] The seamless steel pipes of test numbers 37 and 38 had an excessively large reduction in area Rd2 in the second cold working step. As a result, the dislocation density ratio Fn1 of these seamless steel pipes was 4.0 or more. As a result, cracks were confirmed in these seamless steel pipes in the corrosion test. In other words, these seamless steel pipes did not have excellent stress corrosion cracking resistance in a supercritical corrosion environment.

[0130] In the seamless steel pipes of test numbers 39 and 40, the area reduction ratio Rd1 in the first cold working step, the area reduction ratio Rd2 in the second cold working step, and FnA did not satisfy formula (A). As a result, the dislocation density ratio Fn1 of these seamless steel pipes was 4.0 or more. As a result, cracks were confirmed in these seamless steel pipes in the corrosion test. In other words, these seamless steel pipes did not have excellent stress corrosion cracking resistance in a supercritical corrosion environment.

[0131] In Example 2, duplex stainless steel materials having a chemical composition with a Cr content of more than 24.00 to 30.00% were investigated. Specifically, molten steel having the chemical composition shown in Tables 4A, 4B, and 4C was melted using a 50 kg vacuum melting furnace, as in Example 1, and steel ingots were produced by an ingot-making method. Note that, as in Example 1, "-" in Tables 4A, 4B, and 4C indicates that the content of the corresponding element was at the impurity level. Specifically, the Sn content of Test No. 77 was 0%, rounded to the nearest four decimal places. Furthermore, the chemical compositions shown in Tables 4A and 4B and FnA (= (Ni + 20N + 10Sn + 4Co + 0.5Mn + 0.5Cu) / (Cr + 3Mo + 2Si)) calculated from the above definition are shown in Table 5.

[0132]

[0133]

[0134]

[0135]

[0136] The ingots of each steel type were hot worked to produce mother pipes (seamless steel pipes). The mother pipes of each test number that had been hot worked were subjected to first cold working at a cross-sectional area reduction rate Rd1 (%) shown in Table 5. Furthermore, the mother pipes of each test number were solution treated at the treatment temperature (°C) and holding time (minutes) shown in Table 5. Furthermore, the mother pipes of each test number that had been solution treated were subjected to second cold working at a cross-sectional area reduction rate Rd2 (%) shown in Table 5. The ratio of the cross-sectional area reduction rate Rd1 (%) of the first cold working to the cross-sectional area reduction rate Rd2 (%) of the second cold working for each test number is shown in the "Rd1 / Rd2" column in Table 5. Both the first cold working and the second cold working were performed by cold drawing.

[0137] [Evaluation Tests] Seamless steel pipes of each test number were obtained by the above steps. As in Example 1, a tensile test, a microstructure observation test, a dislocation density ratio measurement test, and a corrosion test were performed on the obtained seamless steel pipes of each test number.

[0138] [Tensile Test] A tensile test was performed on the seamless steel pipe of each test number in accordance with ASTM E8 / E8M (2022) to determine the yield strength. Specifically, a circular arc-shaped test specimen for the tensile test was prepared from the seamless steel pipe of each test number. The circular arc-shaped test specimen had the same thickness as the wall thickness of the steel pipe, a width of 25.4 mm, and a gauge length of 50.8 mm. A tensile test was performed using the circular arc-shaped test specimen of each test number at room temperature (25°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 (Yield Strength) for each test number is shown in the "YS (MPa)" column of Table 6. Note that the seamless steel pipe of each test number all had a yield strength of 552 to 1000 MPa.

[0139]

[0140] [Microstructure Observation Test] Microstructure observation was performed on the seamless steel pipe of each test number to determine the volume fraction of ferrite. Specifically, a test piece for microstructure observation was prepared from the center of the wall thickness of the seamless steel pipe of each test number, having an observation surface of 5 mm in the pipe axis direction × 5 mm in the pipe circumferential direction. The observation surface of the test piece of each test number was polished to a mirror finish and subjected to electrolytic corrosion in a 7% potassium hydroxide etching solution. The observation surface, on which the structure was revealed by electrolytic corrosion, was observed in 10 fields of view using an optical microscope. The area of ​​each field of view was 1.00 mm 2 (magnification 100x).

[0141] In each visual field of each test number, the microstructure contained negligible amounts of phases other than ferrite and austenite. That is, the seamless steel pipe of each test number had a microstructure consisting of ferrite and austenite. In each visual field of each test number, ferrite and austenite were identified based on contrast. The area fraction of the identified ferrite was determined using the point counting method in accordance with ASTM E562 (2019). The arithmetic average of the area fractions of ferrite in 10 visual fields was taken as the ferrite volume fraction (%). The ferrite volume fraction (%) determined for each test number is shown in Table 6.

[0142] [Dislocation density ratio measurement test] A dislocation density ratio measurement test was carried out on the seamless steel pipe of each test number to determine the dislocation density ratio Fn1 (=ρ(γ) / ρ(α)). Specifically, a thin film sample was prepared from the seamless steel pipe of each test number by the above-mentioned method. Furthermore, the thin film sample of each test number was used to measure the dislocation density ρ(α) (m -2 ), and the dislocation density ρ(γ)(m -2 In this embodiment, dislocations were observed by bright field observation. For each test number, the dislocation density ρ(α) in the ferrite was 1.0 × 10 13 ~8.0 x 10 15 (m -2 ) and the dislocation density ρ(γ) in austenite is 1.0 × 10 13 ~8.0 x 10 15 (m -2 ) was obtained. -2 ) and ρ(γ)(m -2The dislocation density ratio Fn1 (=ρ(γ) / ρ(α)) was calculated from the above. The calculated dislocation density ratio Fn1 is shown in the "Dislocation density ratio Fn1 (=ρ(γ) / ρ(α))" column in Table 6.

[0143] [Corrosion Test] A corrosion test was conducted on the steel material of 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 conducted on the prepared test specimens under the conditions described above to determine the corrosion rate (mm / year). Furthermore, the presence or absence of cracks was confirmed for the test specimens after the corrosion test using the method described above. In the corrosion test, the total pressure of the mixed gas was 130 bar, the SO2 concentration in the mixed gas was 0.02 vol%, the O2 concentration in the mixed gas was 0.05 vol%, the NO2 gas concentration in the mixed gas was 0.02 vol%, and the H2S gas concentration was 0.02 vol%. The obtained corrosion rates (mm / year) are shown in Table 6. In addition, test numbers in which no cracks were observed are marked with "E (Excellent)" in the "Crack" column of Table 6. Furthermore, for test numbers in which cracks were confirmed, the "Cracks" column in Table 6 is marked with "NA (Not Acceptable)."

[0144] [Evaluation Results] Referring to Tables 4A, 4B, 4C, 5, and 6, the seamless steel pipes of test numbers 41 to 73 had appropriate chemical compositions. Furthermore, the manufacturing methods used for these seamless steel pipes were the preferred manufacturing methods described in the specification. As a result, these seamless steel pipes had a ferrite volume fraction of 35 to 65% and a dislocation density ratio Fn1 of more than 0.3 and less than 4.0. As a result, these seamless steel pipes were determined to have excellent corrosion resistance in corrosion tests. That is, the seamless steel pipes of test numbers 41 to 73 had excellent general corrosion resistance and stress corrosion cracking resistance, even in supercritical corrosion environments.

[0145] On the other hand, the seamless steel pipe of test number 74 had an excessively low Cu content. As a result, the corrosion rate of this seamless steel pipe exceeded 0.100 mm / year. In other words, this seamless steel pipe did not have excellent general corrosion resistance in a supercritical corrosion environment.

[0146] The seamless steel pipe of test number 75 had an excessively low W content. As a result, the corrosion rate of this seamless steel pipe exceeded 0.100 mm / year. In other words, this seamless steel pipe did not have excellent general corrosion resistance in a supercritical corrosion environment.

[0147] The seamless steel pipe of test number 76 had an excessively low Co content. As a result, cracks were observed in the corrosion test. That is, the seamless steel pipe did not have excellent stress corrosion cracking resistance in a supercritical corrosion environment.

[0148] The seamless steel pipe of test number 77 had an excessively low Sn content. As a result, cracks were observed in this seamless steel pipe in the corrosion test. In other words, this seamless steel pipe did not have excellent stress corrosion cracking resistance in a supercritical corrosion environment.

[0149] The seamless steel pipes of test numbers 78 and 79 had an excessively large reduction in area Rd2 in the second cold working process. As a result, the dislocation density ratio Fn1 of these seamless steel pipes was 4.0 or more. As a result, cracks were confirmed in these seamless steel pipes in the corrosion test. In other words, these seamless steel pipes did not have excellent stress corrosion cracking resistance in a supercritical corrosion environment.

[0150] In the seamless steel pipes of test numbers 80 and 81, the area reduction ratio Rd1 in the first cold working step, the area reduction ratio Rd2 in the second cold working step, and FnA did not satisfy formula (A). As a result, the dislocation density ratio Fn1 of these seamless steel pipes was 4.0 or more. As a result, cracks were confirmed in these seamless steel pipes in the corrosion test. In other words, these seamless steel pipes did not have excellent stress corrosion cracking resistance in a supercritical corrosion environment.

[0151] 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. In mass%, C: 0.050% or less, Si: 1.00% or less, Mn: 0.40 to 3.00%, P: 0.050% or less, S: 0.0050% or less, Cr: 20.00 to 30.00%, Cu: 0.05 to less than 1.50%, Ni: 2.00 to 10.00%, Mo: 0.80 to 5.00%, W: 0.01 to 3.00%, N: 0.001 to 0.350%, Co: 0.10 to 1.00%, Sn: 0.001 to 0.050%, Al: 0.050% or less, V: 0.01 to 0.50%, Ti: 0.001 to 0.500%, Ca: 0.0001 to 0.0100%, B: 0.0001 to 0.0050%, O: 0.010% or less, Mg: 0 to 0.010%, rare earth elements: 0 to 0.010%, Zr: 0 to 0.010%, Nb: 0 to 0.500%, Ta: 0 to 0.100%, As: 0 to 0.050%, Zn: 0 to 0.010%, Pb: 0 to 0.010%, Sb: 0 to 0.010%, and the balance being Fe and impurities; the microstructure is composed of 35 to 65% by volume of ferrite and the balance being austenite; and the dislocation density ρ(α) in the ferrite and the dislocation density ρ(γ) in the austenite satisfy the following formula (1): Duplex stainless steel material. 0.3<ρ(γ) / ρ(α)<4.0 (1) Here, ρ(γ) in the formula (1) represents the dislocation density in the austenite. -2 ρ(α) is the dislocation density in the ferrite -2 is assigned.

2. The duplex stainless steel material according to claim 1, containing one or more elements selected from the group consisting of Mg: 0.001 to 0.010%, rare earth elements: 0.001 to 0.010%, Zr: 0.001 to 0.010%, Nb: 0.001 to 0.500%, Ta: 0.001 to 0.100%, As: 0.001 to 0.050%, Zn: 0.001 to 0.010%, Pb: 0.001 to 0.010%, and Sb: 0.001 to 0.010%.

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