Duplex stainless steel material

Abstract

Provided is a duplex stainless steel material having excellent general corrosion resistance and stress corrosion cracking resistance even in a supercritical corrosion environment. The duplex stainless steel material according to the present disclosure has the chemical composition described in the specification and has a microstructure composed of ferrite with a volume fraction of 35 to 65% and the balance being austenite. The duplex stainless steel material according to the present disclosure further satisfies the following formula (1) for the dislocation density ρ(α) in ferrite and the dislocation density ρ(γ) in austenite. 0.3 < ρ(γ) / ρ(α) < 4.0 (1) Here, the dislocation density in austenite is substituted for ρ(γ) in formula (1). -2 And the dislocation density in ferrite is substituted for ρ(α). -2 Is substituted.

Description

Technical Field

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

Background Art

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

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

[0004] For steel materials used in such CO2 storage technologies, high strength is required to inject CO2 into depleted oil wells. In addition, CO2 is a corrosive substance that corrodes steel materials. Therefore, for steel materials used in CO2 storage technologies, excellent corrosion resistance in a corrosive environment containing a large amount of CO2 is required.

[0005] As steel materials with excellent corrosion resistance in a corrosive environment, duplex stainless steel materials having a duplex structure of ferrite and austenite are known. Duplex stainless steel materials are disclosed, for example, in Japanese Patent Application Laid-Open No. 5-132741 (Patent Document 1) and Japanese Patent Application Laid-Open No. 9-195003 (Patent Document 2).

[0006] The duplex stainless steel disclosed in Patent Document 1 has a chemical composition by 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%, and the balance being Fe and inevitable impurities, and PREW (= Cr + 3.3(Mo + 0.5W) + 16N) is 40 or more.

[0007] The duplex stainless steel 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: more than 3 to 8%, Co: 0.01 to 2%, Cu: 0.1 to 5%, N: 0.05 to 0.5%, and the balance is Fe and inevitable impurities.

[0008] In the duplex stainless steels disclosed in Patent Document 1 and Patent Document 2, the corrosion resistance is enhanced by adjusting the chemical composition.

Prior Art Documents

Patent Documents

[0009]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0010] Incidentally, in the above-described carbon dioxide storage technology, in order to inject CO2 into depleted oil wells, the CO2 to be injected into the steel pipe is compressed and pressurized to bring the CO2 into a supercritical state. On the other hand, the CO2 recovered from industrial facilities such as power plants and factories contains SOx, NOx, and O2. Here, SOx is a general term for sulfur oxides represented by SO2. NOx is a general term for nitrogen oxides represented by NO2. SOx and NOx dissolve in water to form acidic compounds (such as sulfuric acid, sulfurous acid, nitric acid, and nitrous acid), causing general corrosion and cracking on the surface of steel materials. Also, when H2S is present in the formation water existing in the well to be stored, the corrosion and cracking of steel materials are promoted. Therefore, supercritical CO2 containing SOx, NOx, O2, and H2S forms an extremely severe corrosion environment. In this specification, the corrosion environment formed by supercritical CO2 containing SOx, NOx, O2, and H2S is referred to as a "supercritical corrosion environment".

[0011] That is, for steel materials used in a supercritical corrosion environment, higher general corrosion resistance and stress corrosion cracking resistance than those in conventional corrosion environments are required. On the other hand, the duplex stainless steel materials disclosed in Patent Documents 1 and 2 are not assumed to be used in such a supercritical corrosion environment.

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

Means for Solving the Problems

[0013] The duplex stainless steel material according to the present disclosure is by 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 - 5.00%, W: 0.01 - 3.00%, N: 0.001 - 0.350%, Co: 0.10 - 1.00%, Sn: 0.001 - 0.050%, Al: 0.050% or less, V: 0.01 - 0.50%, Ti: 0.001 - 0.500%, Ca: 0.0001 - 0.0100%, B: 0.0001 - 0.0050%, O: 0.010% or less, Mg: 0 - 0.010%, Rare earth elements: 0 - 0.010%, Zr: 0 - 0.010%, Nb: 0 - 0.500%, Ta: 0 - 0.100%, As: 0 - 0.050%, Zn: 0 - 0.010%, Pb: 0 - 0.010%, Sb: 0 - 0.010%, and the balance consists of Fe and impurities, the microstructure consists of 35 - 65% ferrite by volume fraction and the balance austenite, 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, in formula (1), ρ(γ) is the dislocation density in the austenite in m -2 and ρ(α) is the dislocation density in the ferrite in m -2 and are substituted.

Advantages of the Invention

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

Modes for Carrying Out the Invention

[0015] The inventors examined a duplex stainless steel material having excellent general corrosion resistance and stress corrosion cracking resistance in a supercritical corrosion environment formed by supercritical CO2 containing SOx, NOx, O2, and H2S.

[0016] First, the inventors examined a steel material having excellent general corrosion resistance and excellent stress corrosion cracking resistance in a supercritical corrosion environment from the perspective of chemical composition. As a result, the inventors found that, by 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: less than 0.05 to 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, a duplex stainless steel material may have excellent general corrosion resistance and excellent stress corrosion cracking resistance even in a supercritical corrosion environment.

[0017] On the other hand, even a duplex stainless steel material having the above chemical composition may not sufficiently obtain stress corrosion cracking resistance in a supercritical corrosion environment. Therefore, the inventors further examined means for enhancing stress corrosion cracking resistance in a supercritical corrosion environment.

[0018] Here, the duplex stainless steel material having the above-described chemical composition has a microstructure composed of ferrite with a volume fraction of 35 to 65% and the balance being austenite. In this specification, "consisting of ferrite and austenite" means that the phases other than ferrite and austenite in the microstructure are so few as to be negligible.

[0019] The inventors further examined in detail a method for enhancing the stress corrosion cracking resistance of the duplex stainless steel material having the above-described chemical composition and microstructure in a supercritical corrosion environment. Specifically, the inventors focused on the dislocations in the duplex stainless steel material. Here, it is considered that regions with a high dislocation density in the steel material are likely to be the starting points of cracks due to corrosion. Furthermore, when stress is applied, cracks may propagate from regions with a high dislocation density. That is, if there are regions in the duplex stainless steel material where the dislocation density is locally increased, there is a concern that the stress corrosion cracking resistance of the duplex stainless steel material may decrease. In particular, in a supercritical corrosion environment, the inventors considered that local reduction in stress corrosion cracking resistance due to dislocation localization may be likely to become apparent.

[0020] As a result of further detailed examination by the inventors considering the above findings, in the duplex stainless steel material having the above-described chemical composition and a microstructure composed of ferrite with a volume fraction of 35 to 65% and the balance being austenite, if the dislocation density ρ(α) in ferrite and the dislocation density ρ(γ) in austenite satisfy the following formula (1), it has been clarified that excellent stress corrosion cracking resistance can be obtained even in a supercritical corrosion environment. 0.3 < ρ(γ) / ρ(α) < 4.0 (1) Here, the dislocation density in austenite is substituted for ρ(γ) in formula (1). -2 And the dislocation density in ferrite is substituted for ρ(α). -2 in the formula.

[0021] In a duplex stainless steel having the above chemical composition and a microstructure composed of ferrite with a volume fraction of 35 to 65% and the balance being austenite, the reason why excellent stress corrosion cracking resistance is exhibited even in a supercritical corrosion environment when the dislocation density ρ(α) in the ferrite and the dislocation density ρ(γ) in the austenite satisfy the above formula (1) has not been clarified in detail. However, the present inventors have made the following speculation.

[0022] In a duplex stainless steel having the above chemical composition, when the dislocation density of the duplex stainless steel is increased by work hardening or the like, dislocations may locally enter. In this case, the dislocation density locally increases, and local deterioration of corrosion resistance is likely to occur. On the other hand, if the ratio of the dislocation density ρ(α) in the ferrite and the dislocation density ρ(γ) in the austenite is controlled within a certain range, localization of the dislocation density in the duplex stainless steel may be alleviated. As a result, the increase in the local dislocation density is alleviated, and the present inventors speculate that the stress corrosion cracking resistance of the duplex stainless steel may be enhanced even in a supercritical corrosion environment.

[0023] In addition, there may be a possibility that excellent corrosion resistance is exhibited even in a supercritical corrosion environment when the dislocation density ρ(α) in the ferrite and the dislocation density ρ(γ) in the austenite satisfy the above formula (1) in a duplex stainless steel having the above chemical composition and microstructure by a mechanism other than the above mechanism. However, it has been proven by the examples described later that excellent stress corrosion cracking resistance is exhibited even in a supercritical corrosion environment when the dislocation density ρ(α) in the ferrite and the dislocation density ρ(γ) in the austenite satisfy the above formula (1) in a duplex stainless steel having the above chemical composition and microstructure.

[0024] The gist of the duplex stainless steel according to the present embodiment completed based on the above findings is as follows.

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

[0026] [2] The duplex stainless steel material according to [1], Mg: 0.001 to 0.010%, rare earth element: 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%, and contains one or more elements selected from the group consisting of duplex stainless steel material.

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

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

[0030] C: 0.050% or less Carbon (C) is inevitably contained. That is, the lower limit of the C content is more than 0%. C forms Cr carbides at the grain boundaries, increasing the corrosion susceptibility at the grain boundaries. Therefore, if the C content is too high, even if the contents of other elements are within the range of this embodiment, the corrosion resistance of the steel material in the supercritical corrosion environment will decrease. Accordingly, the C content is 0.050% or less. The preferable upper limit of the C content is 0.048%, and more preferably 0.045%. It is preferable that the C content is as low as possible. However, an extreme reduction in the C content significantly increases the manufacturing cost. Therefore, considering industrial production, the preferable lower limit of the C content is 0.001%, and 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 the steel. On the other hand, if the Si content is too high, even if the contents of other elements are within the range of this embodiment, the toughness of the steel material will decrease. Therefore, the Si content is 1.00% or less. The preferable upper limit of the Si content is 0.95%, and more preferably 0.90%. The preferable lower limit of the Si content for more effectively obtaining the above effects is 0.05%, more preferably 0.10%, more preferably 0.20%, and more preferably 0.25%.

[0032] Mn: 0.40 - 3.00% Manganese (Mn) deoxidizes steel and desulfurizes steel. Mn further improves the hot workability of steel. If the Mn content is too low, even if the contents of other elements are within the range of this embodiment, the above effects cannot be sufficiently obtained. On the other hand, Mn combines with sulfur (S) to form Mn sulfide. Therefore, if the Mn content is too high, even if the contents of other elements are within the range of this embodiment, a large number of coarse Mn sulfides are formed, and the corrosion resistance of the steel in a supercritical corrosion environment decreases. Therefore, the Mn content is 0.40 to 3.00%. The preferable lower limit of the Mn content is 0.45%, more preferably 0.50%, still more preferably 0.60%, and even more preferably 0.70%. The preferable upper limit of the Mn content is 2.95%, more preferably 2.90%, still more preferably 2.80%.

[0033] P: 0.050% or less Phosphorus (P) is inevitably contained. That is, the lower limit of the P content is more than 0%. P segregates at grain boundaries. Therefore, if the P content is too high, even if the contents of other elements are within the range of this embodiment, the corrosion resistance of the steel in a supercritical corrosion environment decreases. Therefore, the P content is 0.050% or less. The preferable upper limit of the P content is 0.045%, more preferably 0.040%, still more preferably 0.030%. It is preferable that the P content is as low as possible. However, an extreme reduction in the P content significantly increases the manufacturing cost. Therefore, considering industrial production, the preferable lower limit of the P content is 0.001%, more preferably 0.003%.

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

[0035] Cr: 20.00 - 30.00% Chromium (Cr) forms a passive film on the surface of the steel material as an oxide, enhancing the corrosion resistance of the steel material in a supercritical corrosion environment. If the Cr content is too low, even if the contents of other elements are within the scope of this embodiment, the above effects cannot be sufficiently obtained. On the other hand, if the Cr content is too high, even if the contents of other elements are within the scope of this embodiment, the hot workability of the steel material decreases. Therefore, the Cr content is 20.00 - 30.00%. The preferable lower limit of the Cr content is 20.05%, more preferably 20.10%, and even more preferably 20.20%. The preferable upper limit of the Cr content is 29.95%, more preferably 29.90%, and even more preferably 29.80%.

[0036] In addition, for the duplex stainless steel material according to this embodiment, if the Cr content is over 24.00 - 30.00%, the corrosion resistance is further enhanced. That is, the Cr content may be 20.00 - 24.00% or over 24.00 - 30.00%. When the Cr content is 20.00 - 24.00%, the more preferable upper limit of the Cr content is 23.95%, more preferably 23.90%, and even more preferably 23.80%. When the Cr content is over 24.00 - 30.00%, the more preferable lower limit of the Cr content is 24.01%, more preferably 24.05%, and even more preferably 24.10%.

[0037] Cu: less than 0.05 to 1.50% Copper (Cu) enhances the corrosion resistance of steel materials in a supercritical corrosion environment. If the Cu content is too low, even if the contents of other elements are within the scope of this embodiment, the above effects cannot be fully obtained. On the other hand, if the Cu content is too high, even if the contents of other elements are within the scope of this embodiment, the hot workability of the steel material will deteriorate. Therefore, the Cu content is less than 0.05 to 1.50%. The preferable lower limit of the Cu content is 0.06%, more preferably 0.07%, still more preferably 0.10%, still more preferably 0.15%, still more preferably 0.20%, still more preferably 0.25%. The preferable upper limit of the Cu content is 1.49%, more preferably less than 1.45%, still more preferably 1.44%, still more preferably 1.40%.

[0038] Ni: 2.00 to 10.00% Nickel (Ni) stabilizes the austenite structure of steel materials. Ni further enhances the corrosion resistance of steel materials in a supercritical corrosion environment. If the Ni content is too low, even if the contents of other elements are within the scope of this embodiment, the above effects cannot be fully obtained. On the other hand, if the Ni content is too high, even if the contents of other elements are within the scope of this embodiment, the volume fraction of austenite may become too high and the strength of the steel material may decrease. Therefore, the Ni content is 2.00 to 10.00%. The preferable lower limit of the Ni content is 2.50%, more preferably 3.00%, still more preferably 3.50%, still more preferably 4.00%. The preferable upper limit of the Ni content is 9.80%, more preferably 9.60%, still more preferably 9.20%.

[0039] Mo: 0.80 to 5.00% Molybdenum (Mo) enhances the corrosion resistance of steel materials in supercritical corrosion environments. If the Mo content is too low, even if the contents of other elements are within the scope of this embodiment, the above effect cannot be sufficiently obtained. On the other hand, if the Mo content is too high, even if the contents of other elements are within the scope of this embodiment, the hot workability of the steel material deteriorates. Therefore, the Mo content is 0.80 to 5.00%. The preferred lower limit of the Mo content is 0.85%, more preferably 0.90%, still more preferably 1.20%. The preferred upper limit of the Mo content is 4.95%, more preferably 4.90%, still 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 materials in supercritical corrosion environments. If the W content is too low, even if the contents of other elements are within the scope of this embodiment, the above effect cannot be sufficiently obtained. On the other hand, if the W content is too high, even if the contents of other elements are within the scope of this embodiment, the strength of the steel material becomes too high and the toughness of the steel material deteriorates. 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%, still more preferably 0.04%, even more preferably 0.06%, still more preferably 0.10%, and even more preferably 0.15%. The preferred upper limit of the W content is 2.70%, more preferably 2.50%, still more preferably 2.00%, and even more preferably 1.50%.

[0041] N: 0.001 to 0.350% Nitrogen (N) stabilizes the austenite structure of steel. N also enhances the corrosion resistance of steel in a supercritical corrosion environment. If the N content is too low, even if the contents of other elements are within the scope of this embodiment, the above effects cannot be fully obtained. On the other hand, if the N content is too high, even if the contents of other elements are within the scope of this embodiment, the toughness and hot workability of the steel decrease. Therefore, the N content is 0.001 - 0.350%. The preferred lower limit of the N content is 0.003%, more preferably 0.005%, and even more preferably 0.008%. The preferred upper limit of the N content is 0.345%, more preferably 0.340%.

[0042] Co: 0.10 - 1.00% Cobalt (Co) forms a film on the surface of the steel to enhance the corrosion resistance of the steel in a supercritical corrosion environment. If the Co content is too low, even if the contents of other elements are within the scope of this embodiment, the above effects cannot be fully obtained. On the other hand, if the Co content is too high, even if the contents of other elements are within the scope of this embodiment, the manufacturing cost increases extremely. Therefore, the Co content is 0.10 - 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 - 0.050% Tin (Sn) enhances the corrosion resistance of steel in a supercritical corrosion environment. If the Sn content is too low, even if the contents of other elements are within the scope of this embodiment, the above effects cannot be fully obtained. On the other hand, if the Sn content is too high, even if the contents of other elements are within the scope of this embodiment, the hot workability of the steel decreases. Therefore, the Sn content is 0.001 - 0.050%. The preferred lower limit of the Sn content is 0.002%, more preferably 0.003%, and even more preferably 0.005%. The preferred upper limit of the Sn content is 0.045%, more preferably 0.040%.

[0044] Al: Below 0.050% 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, even if the contents of other elements are within the scope of this embodiment, coarse oxide inclusions are generated and the toughness of the steel material decreases. Therefore, the Al content is 0.050% or less. The preferable upper limit of the Al content is 0.045%, and more preferably 0.040%. The preferable lower limit of the Al content for obtaining the above effect more effectively 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 - 0.50% Vanadium (V) increases the strength of the steel material. If the V content is too low, even if the contents of other elements are within the scope of this embodiment, the above effect cannot be obtained sufficiently. On the other hand, if the V content is too high, even if the contents of other elements are within the scope of this embodiment, the strength of the steel material becomes too high and the toughness of the steel material decreases. Therefore, the V content is 0.01 - 0.50%. The preferable lower limit of the V content is 0.02%, more preferably 0.03%, and even more preferably 0.05%. The preferable upper limit of the V content is 0.45%, and more preferably 0.40%.

[0046] Ti: 0.001 - 0.500% Titanium (Ti) forms carbonitrides and increases the strength of steel materials. If the Ti content is too low, even if the contents of other elements are within the scope of this embodiment, the above effects cannot be fully obtained. On the other hand, if the Ti content is too high, even if the contents of other elements are within the scope of this embodiment, the strength of the steel material becomes too high and the toughness of the steel material decreases. Therefore, the Ti content is 0.001 to 0.500%. The preferable lower limit of the Ti content is 0.003%, more preferably 0.005%, still more preferably 0.007%, and even more preferably 0.010%. The preferable upper limit of the Ti content is 0.350%, more preferably 0.250%, still more preferably 0.200%, even more preferably 0.180%, still more preferably 0.160%, and even more preferably 0.150%.

[0047] Ca: 0.0001 to 0.0100% Calcium (Ca) detoxifies S in the steel material by fixing it as sulfide and improves the hot workability of the steel material. If the Ca content is too low, even if the contents of other elements are within the scope of this embodiment, the above effects cannot be fully obtained. On the other hand, if the Ca content is too high, even if the contents of other elements are within the scope of this embodiment, the oxides in the steel material coarsen and the corrosion resistance of the steel material in a supercritical corrosion environment decreases. Therefore, the Ca content is 0.0001 to 0.0100%. The preferable lower limit of the Ca content is 0.0003%, more preferably 0.0005%, still more preferably 0.0006%, even more preferably 0.0008%, and even more preferably 0.0010%. The preferable upper limit of the Ca content is 0.0080%, more preferably 0.0060%, still more preferably 0.0055%, and even more preferably 0.0050%.

[0048] B: 0.0001 to 0.0050% Boron (B) suppresses the segregation of S to the grain boundaries in the steel material, enhancing the hot workability of the steel material. If the B content is too low, even if the contents of other elements are within the scope of this embodiment, the above effects cannot be fully obtained. On the other hand, if the B content is too high, even if the contents of other elements are within the scope of this embodiment, boron nitride (BN) is generated, reducing the toughness of the steel material. Therefore, the B content is 0.0001 - 0.0050%. The preferable lower limit of the B content is 0.0005%, more preferably 0.0010%, and even more preferably 0.0015%. The preferable 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 inevitably contained. That is, the lower limit of the O content is more than 0%. O forms oxides. Therefore, if the O content is too high, even if the contents of other elements are within the scope of this embodiment, the corrosion resistance of the steel material in a supercritical corrosion environment decreases. Therefore, the O content is 0.010% or less. The preferable upper limit of the O content is 0.009%, more preferably 0.008%, and even more preferably 0.006%. It is preferable that the O content is as low as possible. However, an extreme reduction in the O content increases the manufacturing cost. Therefore, considering industrial production, the preferable lower limit of the O content is 0.001%, 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 consists of Fe and impurities. Here, the impurities in the chemical composition refer to those mixed in from ores, scraps, or manufacturing environments as raw materials when industrially manufacturing the duplex stainless steel material, and are those allowed within a range that does not adversely affect the duplex stainless steel material according to this embodiment.

[0051] [Optional element] 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 part of Fe. These elements are all optional elements, which improve the hot workability of the steel material.

[0052] Mg: 0 - 0.010% Magnesium (Mg) is an optional element and may not be contained. That is, the Mg content may be 0%. When contained, Mg immobilizes S in the steel material as sulfide to render it harmless and improves the hot workability of the steel material. Even if a little 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 scope of this embodiment, the oxides in the steel material become coarse, and the corrosion resistance of the steel material in a supercritical corrosion environment decreases. Therefore, the Mg content is 0 - 0.010%. The preferable lower limit of the Mg content is more than 0%, more preferably 0.001%, and even more preferably 0.002%. The preferable upper limit of the Mg content is 0.009%, more preferably 0.008%, and even more preferably 0.006%, and even more preferably 0.005%.

[0053] Rare earth elements: 0 - 0.010% The rare earth elements (REM) are optional elements and may not be contained. That is, the REM content may be 0%. When contained, REM immobilizes S in the steel material as sulfide to render it harmless and improves the hot workability of the steel material. Even if a little REM is contained, the above effects can be obtained to some extent. However, if the REM content is too high, even if the contents of other elements are within the scope of this embodiment, the oxides in the steel material become coarse, and the corrosion resistance of the steel material in a supercritical corrosion environment decreases. Therefore, the REM content is 0 - 0.010%. The preferable lower limit of the REM content is more than 0%, more preferably 0.001%, and even more preferably 0.002%. The preferable upper limit of the REM content is 0.009%, more preferably 0.008%.

[0054] In addition, REM in this specification means one or more elements selected from the group consisting of scandium (Sc) with an atomic number of 21, yttrium (Y) with an atomic number of 39, and lanthanum (La) with an atomic number of 57 to lutetium (Lu) with an atomic number of 71, which are lanthanoids. Further, the REM content in this specification means the total content of these elements.

[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 part 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 may not be contained. That is, the Zr content may be 0%. When contained, Zr forms carbonitrides and increases the strength of the steel material. Even if a little Zr is contained, the above effects can be obtained to some extent. However, if the Zr content is too high, even if the contents of other elements are within the range of this embodiment, the strength of the steel material becomes too high and the toughness of the steel material decreases. Therefore, the Zr content is 0 to 0.010%. The preferable lower limit of the Zr content is more than 0%, more preferably 0.001%, still more preferably 0.002%, and still more preferably 0.003%. The preferable upper limit of the Zr content is 0.009%, more preferably 0.008%.

[0057] Nb: 0 to 0.500% Niobium (Nb) is an optional element and may not be contained. That is, the Nb content may be 0%. When contained, Nb forms a carbonitride and increases the strength of the steel material. Even if a small amount of Nb is contained, the above effects can be obtained to a certain extent. However, if the Nb content is too high, even if the contents of other elements are within the scope of this embodiment, the strength of the steel material becomes too high and the toughness of the steel material decreases. Therefore, the Nb content is 0 to 0.500%. The preferable lower limit of the Nb content is more than 0%, more preferably 0.001%, still more preferably 0.005%, still more preferably 0.010%, still more preferably 0.050%. The preferable upper limit of the Nb content is 0.400%, more preferably 0.300%, still more preferably 0.200%, still more preferably 0.175%, still more preferably 0.160%, still more preferably 0.150%.

[0058] Ta: 0 to 0.100% Tantalum (Ta) is an optional element and may not be contained. That is, the Ta content may be 0%. When contained, Ta forms a carbonitride and increases the strength of the steel material. Even if a small amount of Ta is contained, the above effects can be obtained to a certain extent. However, if the Ta content is too high, even if the contents of other elements are within the scope of this embodiment, the strength of the steel material becomes too high and the toughness of the steel material decreases. Therefore, the Ta content is 0 to 0.100%. The preferable lower limit of the Ta content is more than 0%, more preferably 0.001%, still more preferably 0.002%, still more preferably 0.003%, still more preferably 0.005%. The preferable upper limit of the Ta content is 0.095%, more preferably 0.090%, still 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 part of Fe. All of these elements are optional elements and enhance the corrosion resistance of the steel material in a supercritical corrosion environment.

[0060] As: 0 to 0.050% Arsenic (As) is an optional element and may not be contained. That is, the As content may be 0%. When contained, As enhances the corrosion resistance of the steel material in a supercritical corrosion environment. Even if a small amount of As is contained, the above effect can be obtained to a certain extent. However, if the As content is too high, the corrosion resistance of the steel material in a supercritical corrosion environment may instead decrease even if the contents of other elements are within the scope of this embodiment. 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 may not be contained. That is, the Zn content may be 0%. When contained, Zn enhances the corrosion resistance of the steel material in a supercritical corrosion environment. Even if a small amount of Zn is contained, the above effect can be obtained to a certain extent. However, if the Zn content is too high, the corrosion resistance of the steel material in a supercritical corrosion environment may instead decrease even if the contents of other elements are within the scope 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%, 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 the steel material in a supercritical corrosion environment. Even if a small amount of Pb is contained, the above effect can be obtained to a certain extent. However, if the Pb content is too high, a large number of flaws will occur on the surface of the steel material after hot working even if the contents of other elements are within the scope of this embodiment. Therefore, the Pb content is 0 to 0.010%. The preferable lower limit of the Pb content is more than 0%, more preferably 0.001%, still more preferably 0.002%, and still more preferably 0.003%. The preferable upper limit of the Pb content is 0.009%, more preferably 0.008%, and still more preferably 0.007%.

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

[0064] Preferably, the duplex stainless steel material according to the present embodiment has the above-described chemical composition and further has a Cr content of more than 24.00% to 30.00% Cr. In this case, the duplex stainless steel material according to the present embodiment has further enhanced corrosion resistance. More specifically, the duplex stainless steel material according to the present embodiment has a chemical composition in mass% of 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: 0.01 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 composed of Fe and impurities. On the premise of satisfying other configurations of the present embodiment, the corrosion resistance is further enhanced.

[0065] In other words, the duplex stainless steel material according to the present embodiment has, by 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: less than 0.05 to 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 may have a chemical composition consisting of Fe and impurities.

[0066] [Microstructure] The duplex stainless steel material according to the present embodiment has the above-described chemical composition, has a microstructure consisting of 35 to 65% ferrite and the balance austenite by volume ratio, and has a dislocation density ratio (ρ(γ) / ρ(α)) described later of more than 0.3 and less than 4.0. As a result, the duplex stainless steel material according to the present 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 being "composed of ferrite and austenite" means that the phases other than ferrite and austenite in the microstructure are so few as to be negligible. For example, in the microstructure of the duplex stainless steel material according to the present embodiment, the volume ratios of precipitates and inclusions are negligibly small compared to the volume ratios of ferrite and austenite. That is, the microstructure of the duplex stainless steel material according to the present embodiment may contain a minute amount of precipitates, inclusions, etc. in addition to ferrite and austenite.

[0068] As described above, in the microstructure of the duplex stainless steel material according to the present embodiment, the volume ratio of ferrite is 35 to 65%. If the volume ratio of ferrite is too low, the corrosion resistance of the steel material may decrease. On the other hand, if the volume ratio of ferrite 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 the present embodiment, the volume ratio of ferrite is 35 to 65%. The preferable lower limit of the volume ratio of ferrite is 36%, more preferably 37%. The preferable upper limit of the volume ratio of ferrite is 64%, more preferably 63%.

[0069] In the present embodiment, the volume ratio of ferrite in the duplex stainless steel material can be determined by a method conforming to ASTM E562 (2019). A test piece for microstructure observation is prepared from the duplex stainless steel material according to the present embodiment. When the steel material is a steel plate, a test piece having an observation surface with a rolling direction of 5 mm and a plate width direction of 5 mm is prepared from the center of the plate thickness. When the steel material is a steel pipe, a test piece having an observation surface with a pipe axis direction of 5 mm and a pipe circumferential direction of 5 mm is prepared from the center of the wall thickness. In this specification, the pipe circumferential direction of the steel pipe means a direction perpendicular to the pipe axis direction and the pipe diameter direction. When the steel material is a round bar, a test piece having an observation surface with an axial direction of 5 mm and a circumferential direction of 5 mm is prepared from the R / 2 position. In this specification, the R / 2 position of the round bar means the central position of the radius R in a cross section perpendicular to the axial direction of the round bar. Also, in this specification, the circumferential direction of the round bar means a direction perpendicular to the axial direction and the radial direction. Note that once the above observation surface is obtained, the size of the test piece is not particularly limited.

[0070] The observation surface of the prepared test piece is mirror-polished. The mirror-polished observation surface is electrolytically corroded in a 7% potassium hydroxide corrosion solution to reveal the structure. The observation surface on which the structure is revealed is observed in 10 fields of view using an optical microscope. The area of each field of view is, for example, 1.00 mm 2(Magnification: 100 times). In each field of view, ferrite is identified from the contrast. The area ratio of the identified ferrite is measured by a point counting method conforming to ASTM E562 (2019). In this embodiment, the arithmetic mean value of the area ratios of the obtained ferrite in 10 fields of view is defined as the volume ratio (%) of ferrite. In this embodiment, the volume ratio (%) of ferrite is obtained by rounding off the first decimal place of the obtained numerical value.

[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) Here, in formula (1), the dislocation density in austenite is substituted for ρ(γ), and -2 the dislocation density in ferrite is substituted for ρ(α). -2

[0072] Define Fn1 = ρ(γ) / ρ(α). Fn1 means the distribution ratio (dislocation density ratio) of the dislocation density in austenite to the dislocation density in ferrite in the duplex stainless steel material having the above-described chemical composition and microstructure. The larger the dislocation density ratio Fn1, the more the dislocations are localized in austenite. The smaller the dislocation density ratio Fn1, the more the dislocations are localized in ferrite. That is, if the dislocation density ratio Fn1 is too high, the dislocation density in austenite increases locally, and the stress corrosion cracking resistance of the steel material in a supercritical corrosion environment is significantly reduced. On the other hand, if the dislocation density ratio Fn1 is too low, the dislocation density in ferrite increases locally, and the stress corrosion cracking resistance of the steel material in a supercritical corrosion environment is reduced. 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 preferable lower limit of the dislocation density ratio Fn1 is 0.4, and more preferably 0.5. The preferable upper limit of the dislocation density ratio Fn1 is 3.9, and more preferably 3.8.

[0073] ​In this embodiment, the dislocation density ratio Fn1 can be obtained by the following method. A thin film sample for measuring the dislocation density 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. Further, 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 plane perpendicular to the rolling direction is prepared from a test piece cut out from the central part of the plate thickness. When the steel material is a steel pipe, a thin film sample having an observation plane perpendicular to the pipe axis direction is prepared from a test piece cut out from the central part of the wall thickness. When the steel material is a round steel, a thin film sample having an observation plane perpendicular to the axial direction is prepared from a test piece cut out from the R / 2 position. Also, the sizes of the test piece and the thin film sample are not particularly limited as long as the observation field of view described later can be obtained.

[0074] On the observation plane of the obtained thin film sample, ferrite and austenite are specified. Ferrite and austenite in the observation plane can be specified by identifying the crystal structure by electron beam diffraction. For the specified field of view, microstructure observation is carried out using a transmission electron microscope (hereinafter also referred to as "TEM"). The area of the observation field of view is not particularly limited and may be an area obtained at a magnification at which dislocations are easily observed. The area of the observation field of view is, for example, 100 nm × 100 nm to 800 nm × 800 nm. Further, from the area of the observation field of view and the thickness of the observation field of view, the volume (m 3 ) of each observation field of view is obtained. The thickness of the observation field of view is obtained from the total integrated intensity of the electron energy loss intensity spectrum (EELS) for the thin film sample and the integrated intensity of the zero-loss spectrum.

[0075] The observation of the tissue in the observation field is carried out with an acceleration voltage of 300 kV under diffraction conditions suitable for dislocation observation. The diffraction conditions suitable for dislocation observation mean the conditions under which two-wave approximation is possible with one transmitted wave and one diffracted wave excited. Specifically, for austenite, it means the condition under which the reciprocal lattice vector g = 40 - 2 is excited, and for ferrite, it means the condition under which the reciprocal lattice vector g = 200 or 30 - 1 is excited. In this embodiment, the thin film sample is tilted so as to be under diffraction conditions suitable for dislocation observation, and the observation region of the thin film sample is observed in bright field. Note that instead of bright field observation, dislocations may be observed by high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM). In the observation by HAADF-STEM, dislocations can be observed more simply than in bright field observation.

[0076] Furthermore, by performing appropriate time exposure, the observation field is photographed. For the generated photographic image, dislocations are identified from the contrast and the length of the dislocations is measured. Note that the length of the dislocations can be measured by a well-known method. For example, the length of the dislocations identified based on the contrast may be obtained by image analysis. Based on the total length (m) of the dislocations in the obtained ferrite in 5 fields of view and the total volume (m 3 ) of the ferrite in 5 fields of view, the dislocation density ρ(α) (m -2 ) in the ferrite is obtained. Similarly, based on the total length (m) of the dislocations in the obtained austenite in 5 fields of view and the total volume (m 3 ) of the austenite in 5 fields of view, the dislocation density ρ(γ) (m -2 ) in the austenite is obtained.

[0077] The ratio Fn1 (= ρ(γ) / ρ(α)) of the dislocation density ρ(γ) (m -2 ) in austenite to the dislocation density ρ(α) (m -2 ) in ferrite obtained by the above method is obtained. In this embodiment, the dislocation density ratio Fn1 is obtained by rounding off the second decimal place of the obtained numerical value.

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

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

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

[0081] [When [Cr: 20.00 to 24.00%]] In the chemical composition, when the Cr content 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 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 central 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 central position of the wall thickness. In this case, the longitudinal direction of the test piece is parallel to the pipe axis direction of the steel pipe. When the steel material is a round steel, the test piece is prepared from the R / 2 position. In this case, the longitudinal direction of the test piece is parallel to the axis direction of the round steel. 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 corresponding to 100% of the actual yield stress is applied to the test piece by four-point bending. The test piece to which the stress is applied is enclosed in an autoclave together with the test jig. 5.0 mass% aqueous sodium chloride solution is injected into the autoclave so that the test piece is immersed. A mixed gas of SO2, O2, NO2, H2S, and CO2 is pressure-sealed into the autoclave to saturate the test solution to form a test bath. At this time, the total pressure of the mixed gas is 130 bar, the SO2 concentration in the mixed gas is 0.01% by volume, the O2 concentration in the mixed gas is 0.05% by volume, the NO2 gas concentration in the mixed gas is 0.01% by volume, and the H2S gas concentration is 0.01% by volume. After sealing the autoclave, the test bath is maintained at 90 °C, and the test piece is immersed for 720 hours while stirring the test bath.

[0083] After 720 hours have elapsed, determine the mass, density, and surface area of the test piece, and determine the corrosion rate (mm / year) of the test piece. In this embodiment, the corrosion rate is determined by rounding off the fourth decimal place of the obtained numerical value. Further, observe the surface of the test piece after 720 hours have elapsed with a magnifying glass having a magnification of 10 times to confirm the presence or absence of cracks. If the occurrence of cracks is suspected by observation with the magnifying glass, further observe with an optical microscope having a magnification of 100 times to confirm the presence or absence of cracks. In this embodiment, when the Cr content is 20.00 to 24.00%, as a result of the corrosion test under the above conditions, if the obtained corrosion rate is 0.100 mm / year or less, it is evaluated that it has excellent general corrosion resistance even in a supercritical corrosion environment. In this embodiment, further, when the Cr content is 20.00 to 24.00%, as a result of the corrosion test under the above conditions, if no cracks are confirmed, it is evaluated that it has excellent stress corrosion cracking resistance even in a supercritical corrosion environment.

[0084] [When [Cr: over 24.00 to 30.00%]] In the chemical composition, when the Cr content is over 24.00 to 30.00%, evaluate the excellent general corrosion resistance and stress corrosion cracking resistance of the steel material in a supercritical corrosion environment by the following method. Specifically, in the same manner as when the Cr content is 20.00 to 24.00%, prepare a test piece for the corrosion test. Further, using the prepared test piece, conduct a corrosion test in the same manner as when the Cr content is 20.00 to 24.00%. At this time, for the mixed gas pressure-sealed in the autoclave, the total pressure of the mixed gas is 130 bar, the SO2 concentration in the mixed gas is 0.02% by volume, the O2 concentration in the mixed gas is 0.05% by volume, the NO2 gas concentration in the mixed gas is 0.02% by volume, and the H2S gas concentration is 0.02% by volume. Conduct the corrosion test under the same conditions as when the Cr content is 20.00 to 24.00% described above.

[0085] After 720 hours, determine the mass, density, and surface area of the test piece, and determine the corrosion rate (mm / year) of the test piece. In this embodiment, the corrosion rate is determined by rounding off the fourth decimal place of the obtained numerical value. Further, observe the surface of the test piece after 720 hours with a magnifying glass having a magnification of 10 times to confirm the presence or absence of cracks. When the occurrence of cracks is suspected by observation with a magnifying glass, further observe with an optical microscope having a magnification of 100 times to confirm the presence or absence of cracks. In this embodiment, when the Cr content exceeds 24.00% to 30.00%, if the obtained corrosion rate is 0.100 mm / year or less as a result of the corrosion test under the above conditions, it is evaluated that it has excellent general corrosion resistance even in a supercritical corrosion environment. In this embodiment, further, when the Cr content exceeds 24.00% to 30.00%, if no cracks are confirmed as a result of the corrosion test under the above conditions, it is evaluated that it has 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 in accordance with the method specified in ASTM E8 / E8M (2022). A test piece is prepared from the steel material of this embodiment. When the steel material is a steel plate, a tensile test piece is prepared from the center of the plate thickness. In this case, the longitudinal direction of the tensile test piece is parallel to the rolling direction of the steel plate. When the steel material is a steel pipe, an arc-shaped test piece with 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 arc-shaped test piece is parallel to the pipe axis direction of the steel pipe. When the steel material is a round bar, a tensile test piece is prepared from the R / 2 position. In this case, the longitudinal direction of the tensile test piece is parallel to the axial direction of the round bar.

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

[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, even if the wall thickness is 5 mm or more, it has excellent corrosion resistance (general corrosion resistance and stress corrosion cracking resistance) in a supercritical corrosion environment.

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

[0091] [Material Preparation Process] In the material preparation process according to this embodiment, a material having the above-described chemical composition is prepared. The material may be prepared by manufacturing it or by purchasing it from a third party. That is, the method of preparing the material is not particularly limited.

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

[0093] [Hot Working Process] In the hot working process according to this embodiment, the material prepared in the above material preparation process is hot-worked to manufacture an intermediate steel material. In this specification, the intermediate steel material is a plate-shaped steel material when the final product is a steel plate, a seamless pipe when the final product is a steel pipe, a bar-shaped steel material with a circular cross-section perpendicular to the axial direction when the final product is a round steel, and a wire-shaped steel material when the final product is a wire rod. The hot working may be hot forging, hot extrusion, or hot rolling. The method of hot working is not particularly limited and may be a well-known method.

[0094] When the intermediate steel material is a seamless pipe, in the hot working process, for example, the Eugene Cédulne method or the Erhardt push bench method (i.e., hot extrusion) may be performed, or piercing rolling by the Mannesmann method (i.e., hot rolling) may be performed. Note that the hot working may be performed only once or multiple times. For example, after performing the above-described piercing rolling on the material, the above-described hot extrusion may be performed. For example, further, after performing the above-described piercing rolling on the material, stretch rolling may be performed. That is, in the hot working process, hot working is performed by a well-known method to manufacture an intermediate steel material having a desired shape.

[0095] [First Cold Working Process] In the first cold working process according to the present embodiment, cold working is performed on the intermediate steel material on which the hot working process has been performed. The cold working may be cold rolling or cold drawing. That is, in the first cold working process, 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 room temperature to less than 150°C.

[0096] Here, the cross-sectional reduction rate Rd1 (%) of the intermediate steel material in the first cold working process is defined as follows. Note that the cross-sectional reduction rate Rd1 (%) in the first cold working process 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 process / cross-sectional area perpendicular to the working direction of the intermediate steel material before the first cold working process)} × 100

[0097] [Solution treatment process] In the solution treatment process according to the present embodiment, solution treatment is performed on the intermediate steel material on which the first cold working process has been performed. The method of solution treatment is not particularly limited and may be a well-known method. 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 (heat treatment temperature) at which the solution treatment is performed means the temperature (°C) of the heat treatment furnace for performing the solution treatment. The holding time at the solution treatment temperature means the time (minutes) during which the intermediate steel material is held at the heat treatment temperature.

[0098] Preferably, the heat treatment temperature in the solution treatment process of the present 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 the solution treatment becomes less than 35%, and the corrosion resistance of the manufactured duplex stainless steel material may decrease. On the other hand, if the heat treatment temperature is too high, the volume fraction of ferrite in the duplex stainless steel material after the solution treatment exceeds 65%, and conversely, the corrosion resistance of the steel material may decrease.

[0099] Therefore, when charging the intermediate steel material into a heat treatment furnace, holding it at a desired temperature, and then rapidly cooling it to perform a 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 more preferably 970 °C. A more preferable upper limit of the solution treatment temperature is 1140 °C, and more preferably 1120 °C.

[0100] When charging the intermediate steel material into a heat treatment furnace, holding it at a desired temperature, and then rapidly cooling it to perform a solution treatment, the solution treatment time is not particularly limited and may be carried out 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 process] In the second cold working process according to this embodiment, cold working is performed on the intermediate steel material on which the above solution treatment process has been carried out. The cold working may be cold rolling or cold drawing. That is, in the second cold working process, similar to the first cold working process, well-known cold working may be carried out under well-known conditions. For example, the temperature of the intermediate steel material during cold working may be room temperature to less than 150 °C.

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

[0103] The cross-sectional reduction rate Rd2 (%) in the second cold working process has a great influence on the strength of the duplex stainless steel produced. That is, by adjusting the cross-sectional reduction rate Rd2, the strength of the produced duplex stainless steel can be adjusted. However, if the cross-sectional reduction rate Rd2 is too large, the dislocation density of austenite increases, and the dislocation density ratio Fn1 may be 4.0 or more. Therefore, in this embodiment, the cross-sectional reduction rate Rd2 is set to 20% or less. In addition, in the second cold working process according to this embodiment, the lower limit of the cross-sectional reduction rate Rd2 is not particularly limited, and for example, it may be 1% or 3%.

[0104] In this way, in the preferred manufacturing method of the duplex stainless steel according to this embodiment, a material preparation process, a hot working process, a first cold working process, a solution treatment process, and a second cold working process are carried out. 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 affected by cold working among the respective processes of the above-described preferred manufacturing method, and its value changes. That is, in the above-described preferred manufacturing method, the value of the dislocation density ratio Fn1 changes depending on the balance between the first cold working process and the second cold working process.

[0105] Therefore, in the preferred manufacturing method according to this embodiment, the cross-sectional reduction rate Rd1 (%) in the first cold working process and the cross-sectional reduction rate Rd2 (%) in the second cold working process satisfy the following formula (A). As a result, it is possible to stably produce a duplex stainless steel having the above-described chemical composition and microstructure and satisfying that the dislocation density ratio Fn1 is more than 0.3 and less than 4.0. Rd1 / Rd2 > (Ni + 20N + 10Sn + 4Co + 0.5Mn + 0.5Cu) / (Cr + 3Mo + 2Si) (A) Here, in formula (A), the cross-sectional reduction rate in the first cold working process is substituted for Rd1 in %, the cross-sectional reduction rate in the second cold working process is substituted for Rd2 in %, and the content of the corresponding element is substituted for the element symbol in mass %.

[0106] Here, by performing cold working before solution treatment, recrystallization is promoted during solution treatment, and the variation in the grain size of crystal grains is likely to be reduced. That is, the cross-sectional reduction rate Rd1 (%) in the first cold working step affects the variation in crystal grains after solution treatment. If the variation in the grain size of crystal grains after solution treatment is small, dislocations are 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 cross-sectional reduction rate Rd2 (%) in the second cold working step is too large, the dislocation density of austenite is likely to increase, and the dislocation density ratio Fn1 is likely to increase. Therefore, in the preferable manufacturing method according to the present embodiment, Rd1 with respect to Rd2 is defined. That is, by increasing Rd1 to a certain extent or more according to Rd2, the crystal grains of the intermediate steel material in the second cold working step can be pre-refined. That is, it is possible to suppress the 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, define FnA = (Ni + 20N + 10Sn + 4Co + 0.5Mn + 0.5Cu) / (Cr + 3Mo + 2Si). FnA is an index indicating the degree of grain refinement in the microstructure of a duplex stainless steel material having the above-described chemical composition. The larger FnA is, the larger the variation in crystal grains is likely to be. Therefore, even when FnA is large, if Rd1 is increased according to Rd2, the effect of grain refinement is enhanced.

[0109] Therefore, in the preferred manufacturing method according to the present embodiment, the ratio of Rd1 to Rd2 is made larger than FnA. In this case, 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. Thus, according to the preferred manufacturing method according to the present embodiment, a duplex stainless steel material having the above-described chemical composition and microstructure and further satisfying that the dislocation density ratio Fn1 is more than 0.3 and less than 4.0 can be stably manufactured.

[0110] [Other processes] The manufacturing method according to the present embodiment may include manufacturing processes other than the above. For example, age heat treatment may be performed on the duplex stainless steel material according to the present embodiment. Age heat treatment means holding the manufactured duplex stainless steel material at a desired temperature. In this case, the age heat treatment may be performed by a well-known method and is not particularly limited. For example, further, pickling treatment may be performed on the duplex stainless steel material according to the present embodiment. In this case, the pickling treatment may be performed by a well-known method and is not particularly limited. Further, other well-known post-treatments may be performed on the duplex stainless steel material on which the second cold working step has been performed.

[0111] Through the above processes, the duplex stainless steel material according to the present embodiment can be manufactured. Note that the above-described manufacturing method of the duplex stainless steel material is an example, and the duplex stainless steel material may be manufactured by other methods. Hereinafter, the present invention will be described in more detail with reference to examples.

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 compositions shown in Table 1A, Table 1B, and Table 1C was melted using a 50 kg vacuum melting furnace, and steel ingots were produced by the ingot casting method. Note that "-" in Table 1A and Table 1C means 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 mean that, after rounding to the fourth decimal place, they were 0%. Similarly, the W content of Test No. 36 means that, after rounding to the third decimal place, it was 0%. Further, the chemical compositions described in Table 1A and Table 1B and FnA (= (Ni + 20N + 10Sn + 4Co + 0.5Mn + 0.5Cu) / (Cr + 3Mo + 2Si)) obtained from the above definitions are shown in Table 2.

[0113]

Table 1A

[0114]

Table 1B

[0115]

Table 1C

[0116]

Table 2

[0117] Hot working was carried out on the ingots of each steel type to produce plain pipes (seamless steel pipes). For the plain pipes of each test number on which hot working was carried out, the first cold working was carried out at the cross-sectional reduction rate Rd1 (%) shown in Table 2. Furthermore, solution heat treatment was carried out on the plain pipes of each test number at the treatment temperature (°C) and holding time (minutes) shown in Table 2. Furthermore, for the plain pipes of each test number on which solution heat treatment was carried out, the second cold working was carried out at the cross-sectional reduction rate Rd2 (%) shown in Table 2. The ratio of the cross-sectional reduction rate Rd1 (%) of the first cold working to the cross-sectional reduction rate Rd2 (%) of the second cold working for each test number is shown in the "Rd1 / Rd2" column of Table 2. Note that both the first cold working and the second cold working were carried out by cold drawing.

[0118] [Evaluation Test] By the above steps, seamless steel pipes of each test number were obtained. 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] Tensile tests were carried out on the seamless steel pipes of each test number in accordance with ASTM E8 / E8M (2022) to determine the yield strength. Specifically, arc-shaped test pieces for tensile tests were fabricated from the seamless steel pipes of each test number. The arc-shaped test pieces 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. Using the arc-shaped test pieces of each test number, tensile tests were carried out at room temperature (25°C) in air to determine the 0.2% offset yield strength (MPa). The obtained 0.2% offset yield strength was defined as the yield strength (MPa). The yield strength obtained for each test number is shown in the "YS (MPa)" column of Table 3. Note that the seamless steel pipes of each test number all satisfied a yield strength of 552 to 1000 MPa.

[0120]

Table 3

[0121] [Microstructure Observation Test] For seamless pipes of each test number, microstructural observation was carried out to determine the volume fraction of ferrite. Specifically, from the central part of the wall thickness of the seamless pipes of each test number, test pieces for microstructural observation with an observation surface of 5 mm in the pipe axis direction × 5 mm in the pipe circumferential direction were prepared. The observation surfaces of the test pieces of each test number were polished to a mirror surface and electrolytically corroded in a 7% potassium hydroxide corrosion solution. The observation surfaces on which the microstructure was revealed by electrolytic corrosion were observed in 10 fields using an optical microscope. The area of each field was 1.00 mm 2 (magnification 100 times).

[0122] In each field of each test number, the phases other than ferrite and austenite in the microstructure were negligibly small. That is, the seamless pipes of each test number had a microstructure composed of ferrite and austenite. In each field of each test number, ferrite and austenite were specified based on the contrast. The area ratio of the specified ferrite was determined by the point counting method in accordance with ASTM E562 (2019). The arithmetic mean value of the area ratios of ferrite in 10 fields was taken as the ferrite volume fraction (%). The ferrite volume fractions (%) of each test number obtained are shown in Table 3.

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

[0124] [Corrosion test] For the steel materials of each test number, a corrosion test was carried out to evaluate the general corrosion resistance and stress corrosion cracking resistance in a supercritical corrosion environment. Specifically, test pieces for the corrosion test were prepared by the above method. For the prepared test pieces, a corrosion test was carried out under the above conditions to obtain the corrosion rate (mm / year). Further, for the test pieces after the corrosion test, the presence or absence of cracks was confirmed by the above method. In the corrosion test, the total pressure of the mixed gas was 130 bar, the SO2 concentration in the mixed gas was 0.01% by volume, the O2 concentration in the mixed gas was 0.05% by volume, the NO2 gas concentration in the mixed gas was 0.01% by volume, and the H2S gas concentration was 0.01% by volume. The obtained corrosion rate (mm / year) is shown in Table 3. Also, for the test numbers where no cracks were confirmed, "E (Excellent)" is shown in the "Crack" column of Table 3. Further, for the test numbers where cracks were confirmed, "NA (Not Acceptable)" is shown in the "Crack" column of Table 3.

[0125] [Evaluation results] Referring to Table 1A, Table 1B, Table 1C, Table 2, and Table 3, the seamless steel pipes of test numbers 1 to 33 had appropriate chemical compositions. Further, the manufacturing methods implemented 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 ratio of 35 to 65% and satisfied a dislocation density ratio Fn1 of more than 0.3 and less than 4.0. As a result, these seamless steel pipes were judged to have excellent corrosion resistance in the corrosion resistance test. 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 one hand, for the seamless steel pipe of Test No. 34, the Cr content was too low. As a result, the corrosion rate of this seamless steel pipe exceeded 0.100 mm / year. That is, this seamless steel pipe did not have excellent general corrosion resistance in the supercritical corrosion environment.

[0127] For the seamless steel pipe of Test No. 35, the Cu content was too low. As a result, the corrosion rate of this seamless steel pipe exceeded 0.100 mm / year. That is, this seamless steel pipe did not have excellent general corrosion resistance in the supercritical corrosion environment.

[0128] For the seamless steel pipe of Test No. 36, the W content was too low. As a result, the corrosion rate of this seamless steel pipe exceeded 0.100 mm / year. That is, this seamless steel pipe did not have excellent general corrosion resistance in the supercritical corrosion environment.

[0129] For the seamless steel pipes of Test Nos. 37 and 38, the cross-sectional reduction rate Rd2 in the second cold working process was too large. As a result, the dislocation density ratio Fn1 of these seamless steel pipes became 4.0 or more. As a result, cracks were confirmed in these seamless steel pipes in the corrosion test. That is, these seamless steel pipes did not have excellent stress corrosion cracking resistance in the supercritical corrosion environment.

[0130] For the seamless steel pipes of Test Nos. 39 and 40, the cross-sectional reduction rate Rd1 in the first cold working process, the cross-sectional reduction rate Rd2 in the second cold working process, and FnA did not satisfy Equation (A). As a result, the dislocation density ratio Fn1 of these seamless steel pipes became 4.0 or more. As a result, cracks were confirmed in these seamless steel pipes in the corrosion test. That is, these seamless steel pipes did not have excellent stress corrosion cracking resistance in the supercritical corrosion environment.

Examples

[0131] In Example 2, a duplex stainless steel material having a chemical composition with a Cr content exceeding 24.00% to 30.00% was investigated. Specifically, molten steel having the chemical compositions shown in Table 4A, Table 4B, and Table 4C was melted using a 50 kg vacuum melting furnace in the same manner as in Example 1, and an ingot was produced by the ingot casting method. Note that, similar to Example 1, "-" in Table 4A, Table 4B, and Table 4C means that the content of the corresponding element was at the impurity level. Specifically, it means that the Sn content of test number 77 was 0% after rounding the fourth decimal place. Furthermore, the chemical compositions described in Table 4A and Table 4B and FnA (= (Ni + 20N + 10Sn + 4Co + 0.5Mn + 0.5Cu) / (Cr + 3Mo + 2Si)) obtained from the above definition are shown in Table 5.

[0132]

Table 4A

[0133]

Table 4B

[0134]

Table 4C

[0135]

Table 5

[0136] For each ingot of each steel type, hot working was carried out to produce a seamless tube (seamless steel pipe). For each seamless tube of each test number on which hot working was carried out, the first cold working was carried out at the cross-sectional reduction rate Rd1 (%) described in Table 5. Further, for each seamless tube of each test number, solution treatment was carried out at the treatment temperature (°C) and holding time (minutes) described in Table 5. Further, for each seamless tube of each test number on which solution treatment was carried out, the second cold working was carried out at the cross-sectional reduction rate Rd2 (%) described in Table 5. The ratio of the cross-sectional reduction rate Rd1 (%) of the first cold working to the cross-sectional reduction rate Rd2 (%) of the second cold working for each test number is shown in the "Rd1 / Rd2" column of Table 5. Note that both the first cold working and the second cold working were carried out by cold drawing.

[0137] [Evaluation Test] Through the above steps, seamless steel pipes of each test number were obtained. For each seamless steel pipe of each test number obtained, in the same manner as in Example 1, a tensile test, a microstructure observation test, a dislocation density ratio measurement test, and a corrosion test were carried out.

[0138] [Tensile Test] For each seamless steel pipe of each test number, a tensile test was carried out in accordance with ASTM E8 / E8M (2022) to obtain the yield strength. Specifically, an arc-shaped test piece for the tensile test was prepared from each seamless steel pipe of each test number. The arc-shaped test piece 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. Using the arc-shaped test pieces of each test number, a tensile test was carried out at room temperature (25°C) in the air to obtain the 0.2% offset yield strength (MPa). The obtained 0.2% offset yield strength was defined as the yield strength (MPa). The yield strength of each test number obtained was shown in the "YS (MPa)" column of Table 6. Note that the yield strength of each seamless steel pipe of each test number satisfied 552 to 1000 MPa.

[0139]

Table 6

[0140] [Microstructure Observation Test] For seamless pipes of each test number, microstructural observation was carried out to determine the volume fraction of ferrite. Specifically, from the central part of the wall thickness of the seamless pipes of each test number, test pieces for microstructural observation with an observation surface of 5 mm in the pipe axis direction × 5 mm in the pipe circumferential direction were prepared. The observation surfaces of the test pieces of each test number were polished to a mirror finish and electrolytically etched in a 7% potassium hydroxide etching solution. The observation surfaces on which the microstructure was revealed by electrolytic etching were observed in 10 fields of view using an optical microscope. The area of each field of view was 1.00 mm 2 (magnification 100 times).

[0141] In each field of view of each test number, the phases other than ferrite and austenite in the microstructure were negligibly small. That is, the seamless pipes of each test number had a microstructure composed of ferrite and austenite. In each field of view of each test number, ferrite and austenite were specified based on the contrast. The area ratio of the specified ferrite was determined by the point counting method in accordance with ASTM E562 (2019). The arithmetic mean value of the area ratios of ferrite in 10 fields of view was taken as the ferrite volume fraction (%). The ferrite volume fractions (%) of each test number obtained are shown in Table 6.

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

[0143] [Corrosion test] For the steel materials of each test number, a corrosion test was carried out to evaluate the general corrosion resistance and stress corrosion cracking resistance in a supercritical corrosion environment. Specifically, test pieces for the corrosion test were prepared by the above method. For the prepared test pieces, a corrosion test was carried out under the above conditions to obtain the corrosion rate (mm / year). Further, for the test pieces after the corrosion test, the presence or absence of cracks was confirmed by the above method. In the corrosion test, the total pressure of the mixed gas was 130 bar, the SO2 concentration in the mixed gas was 0.02% by volume, the O2 concentration in the mixed gas was 0.05% by volume, the NO2 gas concentration in the mixed gas was 0.02% by volume, and the H2S gas concentration was 0.02% by volume. The obtained corrosion rate (mm / year) is shown in Table 6. Also, for the test numbers where no cracks were confirmed, "E (Excellent)" is shown in the "Crack" column of Table 6. Further, for the test numbers where cracks were confirmed, "NA (Not Acceptable)" is shown in the "Crack" column of Table 6.

[0144] [Evaluation results] Referring to Table 4A, Table 4B, Table 4C, Table 5, and Table 6, the seamless steel pipes of test numbers 41 to 73 had appropriate chemical compositions. Further, the manufacturing methods implemented 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 satisfied a dislocation density ratio Fn1 of more than 0.3 and less than 4.0. As a result, these seamless steel pipes were judged to have excellent corrosion resistance in the corrosion test. That is, the seamless steel pipes of test numbers 41 to 73 had excellent general corrosion resistance and stress corrosion cracking resistance even in a supercritical corrosion environment.

[0145] On the one hand, for the seamless steel pipe with test number 74, the Cu content was too low. As a result, the corrosion rate of this seamless steel pipe exceeded 0.100 mm / year. That is, this seamless steel pipe did not have excellent general corrosion resistance in the supercritical corrosion environment.

[0146] For the seamless steel pipe with test number 75, the W content was too low. As a result, the corrosion rate of this seamless steel pipe exceeded 0.100 mm / year. That is, this seamless steel pipe did not have excellent general corrosion resistance in the supercritical corrosion environment.

[0147] For the seamless steel pipe with test number 76, the Co content was too low. As a result, cracks were confirmed in this seamless steel pipe during the corrosion test. That is, this seamless steel pipe did not have excellent stress corrosion cracking resistance in the supercritical corrosion environment.

[0148] For the seamless steel pipe with test number 77, the Sn content was too low. As a result, cracks were confirmed in this seamless steel pipe during the corrosion test. That is, this seamless steel pipe did not have excellent stress corrosion cracking resistance in the supercritical corrosion environment.

[0149] For the seamless steel pipes with test numbers 78 and 79, the cross-sectional reduction rate Rd2 in the second cold working process was too large. As a result, the dislocation density ratio Fn1 of these seamless steel pipes became 4.0 or more. As a result, cracks were confirmed in these seamless steel pipes during the corrosion test. That is, these seamless steel pipes did not have excellent stress corrosion cracking resistance in the supercritical corrosion environment.

[0150] For the seamless steel pipes with test numbers 80 and 81, the cross-sectional reduction rate Rd1 in the first cold working process, the cross-sectional reduction rate Rd2 in the second cold working process, and FnA did not satisfy Equation (A). As a result, the dislocation density ratio Fn1 of these seamless steel pipes became 4.0 or more. As a result, cracks were confirmed in these seamless steel pipes during the corrosion test. That is, these seamless steel pipes did not have excellent stress corrosion cracking resistance in the 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 the above-described embodiments can be appropriately modified and implemented without departing from the spirit thereof.

Claims

1. In mass percent, C: 0.050% or less, Si: 1.00% or less, Mn: 0.40-3.00%, P: 0.050% or less, S: 0.0050% or less, Cr: 20.00-30.00%, Cu: 0.05 to less than 1.50% Ni: 2.00-10.00%, Mo: 0.80-5.00%, W: 0.01-3.00%, N: 0.001-0.350%, Co: 0.10-1.00%, Sn: 0.001 to 0.050%, Al: 0.050% or less, V: 0.01-0.50%, Ti: 0.001 to 0.500%, Ca: 0.0001-0.0100%, 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: 0-0.100%, As: 0 to 0.050%, Zn: 0 to 0.010%, Pb: 0 to 0.010%, Sb: 0 to 0.010%, and The balance is Fe and impurities, The microstructure is composed of 35 to 65% by volume of ferrite and the remainder of austenite, 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.

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

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