Duplex stainless steel material

Abstract

Provided is a duplex stainless steel material having excellent general corrosion resistance and pitting corrosion resistance even in a supercritical corrosive environment. The duplex stainless steel material according to the present disclosure has a chemical composition described in the description, results in Fn1 satisfying at least 40.0 as defined by formula (1), and has a microstructure including ferrite and austenite. In the duplex stainless steel material according to the present disclosure, the total number density of coarse Mn sulfides having an equivalent circle diameter of at least 1.0 µm and coarse Ca sulfides having an equivalent circle diameter of at least 2.0 µm is at most 0.50 pieces / mm2, and the total number density NDA pieces / mm2 of coarse Mn sulfides and coarse Ca sulfides in ferrite and the total number density NDG pieces / mm2 of coarse Mn sulfides and coarse Ca sulfides in austenite satisfy formula (2).　(1): Fn1=Cr+3.3(Mo+0.5W)+16N+2Ni+Cu+2Co+10Sn (2): NDA / NDG<0.50

Description

Duplex stainless steel material

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

[0002] Currently, the increase in the concentration of carbon dioxide (CO ２ ) in the atmosphere has become a global problem. Therefore, efforts have been made to suppress the emission of CO ２ . Among such efforts to suppress the emission of CO ２ , in particular, CCUS has attracted attention.

[0003] CCUS is an abbreviation of Carbon dioxide Capture, Utilization and Storage. That is, CCUS includes three technologies: recovery, utilization, and storage of CO ２ . Among these, as a technology for storing CO ２ , a technology for recovering CO ２ emitted from industrial facilities such as power plants and factories and injecting and storing CO ２ into depleted oil wells has attracted attention.

[0004] For the steel materials used in such CO ２ storage technology, high strength is required for injecting CO ２ into depleted oil wells. In addition, CO ２ is a corrosive substance that corrodes steel materials. Therefore, for the steel materials used in CO ２ storage technology, excellent corrosion resistance in a corrosive environment containing a large amount of CO ２ is required.

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

[0006] The duplex stainless steel material disclosed in Patent Document 1 has a chemical composition consisting of, by mass%, 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: greater than 1.5% to 5.0%, N: 0.24 to 0.32%, with the remainder being Fe and unavoidable impurities, and has a PREW (=Cr + 3.3(Mo + 0.5W) + 16N) of 40 or more.

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

[0008] The duplex stainless steel materials disclosed in Patent Documents 1 and 2 have improved corrosion resistance by adjusting their chemical composition.

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

[0010] By the way, in the carbon dioxide storage technology mentioned above, CO2 is released into depleted oil wells. ２ CO2 is injected into the steel pipe in order to inject CO2. ２ Compressing and increasing the pressure of CO ２ To bring it into a supercritical state. Meanwhile, CO2 recovered from industrial facilities such as power plants and factories. ２ It contains SOx and O ２ It includes SOx, where SOx is SO ２ SOx is a general term for sulfur oxides, represented by [specific example]. SOx dissolves in water to form acidic compounds (such as sulfuric acid and sulfurous acid), causing widespread corrosion on the surface of steel materials. Also, O ２ This causes pitting corrosion. Therefore, SOx and O ２ Supercritical CO2 containing ２ This creates an extremely harsh corrosive environment. In this specification, SOx and O ２ Supercritical CO2 containing ２ The corrosion environment formed by this process is called a "supercritical corrosion environment."

[0011] In other words, steel materials used in supercritical corrosion environments require even better overall corrosion resistance and pitting corrosion resistance than those used in conventional corrosion environments. The duplex stainless steel materials disclosed in Patent Documents 1 and 2 are not intended for use in such supercritical corrosion environments.

[0012] The purpose of this disclosure is to provide a duplex stainless steel material that has excellent overall corrosion resistance and pitting corrosion resistance, even in supercritical corrosion environments.

[0013] The duplex stainless steel material according to this disclosure has the following composition in mass%, C: 0.050% or less, Si: 1.00% or less, Mn: 0.40 to 3.00%, P: 0.050% or less, S: 0.0050% or less, Cr: 20.00 to 24.50%, Cu: 0.10 to 3.00%, Ni: 2.00 to 10.00%, Mo: 0.80 to 5.00%, W: 0.01 to 1.50%, N: 0.001 to 0.350%, Co: 0.10 to 1.00%, Sn: 0.001 to 0.050%, sol. Assuming that the content of each element is within the above range, the composition is as follows: Al: 0.050% or less, V: 0.01 to 0.50%, Ti: 0.001 to 0.050%, Ca: 0.0005 to 0.0100%, B: 0.0015 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 remainder is Fe and impurities, and that the content of each element is within the above range, When Fn1, as defined by formula (1), is 40.0 or greater, and in the duplex stainless steel material, particles with an equivalent circle diameter of 1.0 μm or more, a Mn content of 10% by mass or more, and a S content of 10% by mass or more are defined as coarse Mn sulfides, and particles with an equivalent circle diameter of 2.0 μm or more, a Ca content of 20% by mass or more, a S content of 10% by mass or more, and a Mn content of less than 10% by mass are defined as coarse Ca sulfides, then the total number density of the coarse Mn sulfides and the coarse Ca sulfides is 0.50 particles / mm２ The following is true: The microstructure of the duplex stainless steel consists of ferrite and austenite, and the total number density of the coarse Mn sulfide and coarse Ca sulfide in the ferrite is NDA particles / mm³. ２ Defined as follows, the total number density of the coarse Mn sulfide and the coarse Ca sulfide in the austenite is NDG particles / mm³. ２ When defined as above, NDA and NDG satisfy equation (2). Fn1 = Cr + 3.3(Mo + 0.5W) + 16N + 2Ni + Cu + 2Co + 10Sn (1) NDA / NDG < 0.50 (2) Here, the elemental symbols in equation (1) are substituted with the content of the corresponding element in mass percent.

[0014] The duplex stainless steel material according to this disclosure exhibits excellent overall corrosion resistance and pitting corrosion resistance, even in supercritical corrosion environments.

[0015] The inventors of this invention have identified SOx and O ２ Supercritical CO2 containing ２ We investigated duplex stainless steel materials that exhibit excellent resistance to overall corrosion and pitting corrosion in supercritical corrosion environments formed by [unspecified conditions].

[0016] The inventors first investigated steel materials with excellent overall corrosion resistance and excellent pitting corrosion resistance in supercritical corrosion environments from the perspective of chemical composition. As a result, in mass%, C: 0.050% or less, Si: 1.00% or less, Mn: 0.40 to 3.00%, P: 0.050% or less, S: 0.0050% or less, Cr: 20.00 to 24.50%, Cu: 0.10 to 3.00%, Ni: 2.00 to 10.00%, Mo: 0.80 to 5.00%, W: 0.01 to 1.50%, N: 0.001 to 0.350%, Co: 0.10 to 1.00%, Sn: 0.001 to 0.050%, sol. We considered that a steel material having a chemical composition of Al: 0.050% or less, V: 0.01 to 0.50%, Ti: 0.001 to 0.050%, Ca: 0.0005 to 0.0100%, B: 0.0015 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 remainder being Fe and impurities, could potentially provide excellent overall corrosion resistance and excellent pitting corrosion resistance in a supercritical corrosion environment.

[0017] On the other hand, even with duplex stainless steel materials having the above-mentioned chemical composition, sufficient resistance to overall corrosion could not be obtained in supercritical corrosion environments. Therefore, the inventors further investigated means to improve overall corrosion resistance in supercritical corrosion environments.

[0018] Specifically, the inventors focused on the effects of each element in the chemical composition. It has been known that among the elements in the aforementioned chemical composition, Cr, Mo, W, and N enhance overall corrosion resistance in normal corrosive environments containing chlorides such as seawater. Therefore, the inventors considered that these elements may also enhance overall corrosion resistance in supercritical corrosion environments. Furthermore, the inventors investigated elements other than Cr, Mo, W, and N that enhance overall corrosion resistance in supercritical corrosion environments. As a result, it became clear that in supercritical corrosion environments, not only Cr, Mo, W, and N, but also Ni, Cu, Co, and Sn enhance the overall corrosion resistance of duplex stainless steel materials.

[0019] Therefore, the inventors further investigated the relationship between the content of Cr, Mo, W, N, Ni, Cu, Co, and Sn in duplex stainless steel and the overall corrosion resistance in a supercritical corrosion environment. As a result, it was found that in duplex stainless steel having the above-mentioned chemical composition, if Fn1, as defined by formula (1), is 40.0 or higher, the overall corrosion resistance in a supercritical corrosion environment can be enhanced. Fn1 = Cr + 3.3(Mo + 0.5W) + 16N + 2Ni + Cu + 2Co + 10Sn (1) Here, the content of the corresponding element is substituted in mass percent for the element symbols in formula (1).

[0020] On the other hand, even with duplex stainless steel materials having the above-mentioned chemical composition and satisfying an Fn1 of 40.0 or higher, excellent pitting corrosion resistance could not be sufficiently obtained in supercritical corrosion environments. Therefore, the inventors further investigated means to improve pitting corrosion resistance in supercritical corrosion environments.

[0021] As mentioned above, supercritical corrosion environments include high-temperature, high-pressure supercritical CO2. ２ Not only gases, but also SOx gas and O ２ It also contains gases. SOx gas and O in supercritical corrosion environments. ２ The gas readily dissolves in water, making the environment more acidic. Therefore, in such a supercritical corrosion environment, if inclusions are present on the surface of duplex stainless steel, these inclusions are more likely to dissolve. When the inclusions dissolve, depressions are formed on the surface. Furthermore, depressions formed by the dissolution of coarse inclusions tend to become starting points for pitting corrosion in a supercritical corrosion environment.

[0022] Further investigations by the inventors revealed that in a supercritical corrosion environment, not all coarse inclusions (oxides, sulfides, nitrides, etc.) present on the surface of duplex stainless steel material become the starting point for pitting corrosion. Rather, coarse Mn sulfides and coarse Ca sulfides dissolve, forming depressions on the steel surface. In other words, if the number density of coarse Mn sulfides and coarse Ca sulfides can be reduced, it may be possible to improve the pitting corrosion resistance of duplex stainless steel material having the above-mentioned chemical composition.

[0023] Specifically, the inventors focused on Mn sulfides with an equivalent circular diameter of 1.0 μm or more, and Ca sulfides with an equivalent circular diameter of 2.0 μm or more. Hereinafter, in this specification, particles having a Mn content of 10% by mass or more and an S content of 10% by mass or more are also referred to as Mn sulfides. Similarly, in this specification, particles having a Ca content of 20% by mass or more, an S content of 10% by mass or more, and a Mn content of less than 10% by mass are also referred to as Ca sulfides. Furthermore, in this specification, Mn sulfides with an equivalent circular diameter of 1.0 μm or more are also referred to as "coarse Mn sulfides." Similarly, in this specification, Ca sulfides with an equivalent circular diameter of 2.0 μm or more are also referred to as "coarse Ca sulfides." In this specification, coarse Mn sulfides and coarse Ca sulfides are collectively referred to as "specific inclusions."

[0024] The inventors hypothesized that if the formation of large, coarse Mn sulfides could be suppressed, as well as the formation of large, coarse Ca sulfides, it might be possible to improve the pitting corrosion resistance of duplex stainless steel materials having the above-mentioned chemical composition. Specifically, the total number density of specific inclusions (coarse Mn sulfides and coarse Ca sulfides) is 0.50 particles / mm³. ２ We hypothesized that, under the following conditions, a duplex stainless steel material with the aforementioned chemical composition could achieve excellent pitting corrosion resistance.

[0025] On the other hand, if the chemical composition described above is satisfied, Fn1 is 40.0 or higher, and the total number density of specific inclusions is 0.50 particles / mm³, then it is a suitable material. ２ Even under the following conditions, excellent pitting corrosion resistance could not always be obtained. Therefore, the inventors conducted a detailed investigation into cases where excellent pitting corrosion resistance could not be obtained and considered means to improve pitting corrosion resistance. As a result, it became clear that when excellent pitting corrosion resistance could not be obtained, pitting corrosion was more likely to occur in the ferrite part of the microstructure of duplex stainless steel.

[0026] Here, the duplex stainless steel material having the above-described chemical composition has a microstructure consisting of ferrite and austenite. In this specification, "consisting of ferrite and austenite" means that the amount of phases other than ferrite and austenite is negligibly small. Furthermore, austenite has superior corrosion resistance to ferrite. Therefore, in the duplex stainless steel material having the above-described chemical composition, there is a concern that selective pitting corrosion may occur in the ferrite. Accordingly, the inventors considered that if specific inclusions in the duplex stainless steel material could be concentrated in the austenite, it might be possible to suppress the occurrence of selective pitting corrosion in the ferrite.

[0027] Based on the above findings, the inventors conducted a detailed study and found that the chemical composition satisfies the above requirements, Fn1 is 40.0 or higher, and the total number density of specific inclusions is 0.50 particles / mm³. ２ After reducing the following, the total number density NDA (numbers / mm³) of specific inclusions in ferrite is calculated. ２ ) and the total number density of specific inclusions in austenite NDG (items / mm³) ２ It was found that if the following conditions are met, excellent resistance to overall corrosion and pitting corrosion can be obtained even in a supercritical corrosion environment. NDA / NDG < 0.50 (2)

[0028] Fn2 is defined as NDA / NDG. Fn2 is an indicator of the extent to which specific inclusions (coarse Mn sulfides and coarse Ca sulfides) are present in the ferrite. If Fn2 is less than 0.50, the number density of specific inclusions in the ferrite can be sufficiently reduced. As a result, provided that the other configurations of this embodiment are met, the occurrence of selective pitting corrosion in the ferrite can be suppressed. On the other hand, if Fn2 is 0.50 or higher, the ferrite contains a large amount of specific inclusions, raising concerns about selective pitting corrosion.

[0029] Therefore, the duplex stainless steel material according to this embodiment has the above-described chemical composition, an Fn1 of 40.0 or higher, and a total number density of specific inclusions of 0.50 particles / mm³. ２The following conditions are met, and furthermore, Fn2 is set to less than 0.50. As a result, the duplex stainless steel material according to this embodiment has excellent overall corrosion resistance and pitting corrosion resistance even in supercritical corrosion environments.

[0030] Based on the above findings, the gist of the duplex stainless steel material according to this embodiment is as follows.

[0031] [1] Duplex stainless steel material, with 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: 20.00 to 24.50%, Cu: 0.10 to 3.00%, Ni: 2.00 to 10.00%, Mo: 0.80 to 5.00%, W: 0.01 to 1.50%, N: 0.001 to 0.350%, Co: 0.10 to 1.00%, Sn: 0.001 to 0.050%, sol. Assuming that the content of each element is within the above range, the composition is as follows: Al: 0.050% or less, V: 0.01 to 0.50%, Ti: 0.001 to 0.050%, Ca: 0.0005 to 0.0100%, B: 0.0015 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 remainder is Fe and impurities, and that the content of each element is within the above range, When Fn1, as defined by formula (1), is 40.0 or greater, and in the duplex stainless steel material, particles with an equivalent circle diameter of 1.0 μm or more, a Mn content of 10% by mass or more, and a S content of 10% by mass or more are defined as coarse Mn sulfides, and particles with an equivalent circle diameter of 2.0 μm or more, a Ca content of 20% by mass or more, a S content of 10% by mass or more, and a Mn content of less than 10% by mass are defined as coarse Ca sulfides, then the total number density of the coarse Mn sulfides and the coarse Ca sulfides is 0.50 particles / mm ２The following is true: The microstructure of the duplex stainless steel consists of ferrite and austenite, and the total number density of the coarse Mn sulfide and coarse Ca sulfide in the ferrite is NDA particles / mm³. ２ Defined as follows, the total number density of the coarse Mn sulfide and the coarse Ca sulfide in the austenite is NDG particles / mm³. ２ When defined as above, the NDA and NDG satisfy formula (2) for a duplex stainless steel material. Fn1 = Cr + 3.3(Mo + 0.5W) + 16N + 2Ni + Cu + 2Co + 10Sn (1) NDA / NDG < 0.50 (2) Here, the elemental symbols in formula (1) are substituted with the content of the corresponding element in mass percent.

[0032] [2] A duplex stainless steel material as described in [1], wherein it contains one or more elements selected from the group consisting of Mg: 0.001 to 0.010%, rare earth elements: 0.001 to 0.010%, Zr: 0.001 to 0.010%, Nb: 0.001 to 0.500%, Ta: 0.001 to 0.100%, As: 0.001 to 0.050%, Zn: 0.001 to 0.010%, Pb: 0.001 to 0.010%, and Sb: 0.001 to 0.010%.

[0033] [3] A duplex stainless steel material as described in [1] or [2], wherein the duplex stainless steel material is a steel pipe.

[0034] The shape of the duplex stainless steel material according to this embodiment is not particularly limited. The duplex stainless steel material according to this embodiment may be a steel pipe, a round steel bar (solid material), or a steel plate. A round steel bar refers to a steel bar with a circular cross-section perpendicular to the axial direction. A steel pipe may be a seamless steel pipe or a welded steel pipe.

[0035] The duplex stainless steel material according to this embodiment will be described in detail below.

[0036] [Chemical Composition] The chemical composition of the duplex stainless steel material according to this embodiment contains the following elements. Unless otherwise specified, "%" for elements means mass percent.

[0037] C: 0.050% or less. Carbon (C) is inevitably present. That is, the lower limit of the C content is greater than 0%. C forms Cr carbides at the grain boundaries, increasing corrosion susceptibility at the grain boundaries. Therefore, if the C content is too high, the corrosion resistance of the steel will decrease, even if the content of other elements is within the range of this embodiment. Accordingly, the C content is 0.050% or less. The preferred upper limit of the C content is 0.048%, and more preferably 0.045%. It is preferable to have as low a C content as possible. However, an extreme reduction in the C content significantly increases manufacturing costs. Therefore, considering industrial production, the preferred lower limit of the C content is 0.001%, more preferably 0.003%, and still more preferably 0.005%.

[0038] Si: 1.00% or less. Silicon (Si) is inevitably present. That is, the lower limit of the Si content is greater than 0%. Si deoxidizes steel. On the other hand, if the Si content is too high, the toughness of the steel material will decrease even if the content of other elements is within the range of this embodiment. Therefore, the Si content is 1.00% or less. The preferred upper limit of the Si content is 0.90%, more preferably 0.80%, and even more preferably 0.70%. The preferred lower limit of the Si content to more effectively obtain the above effects is 0.01%, more preferably 0.05%, even more preferably 0.10%, and even more preferably 0.20%.

[0039] Mn: 0.40-3.00% Manganese (Mn) deoxidizes and desulfurizes steel. Mn also improves the hot workability of the steel. If the Mn content is too low, the above effects cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, Mn combines with sulfur (S) to form Mn sulfides. Therefore, if the Mn content is too high, even if the content of other elements is within the range of this embodiment, a large number of coarse Mn sulfides will be formed, and the pitting corrosion resistance of the steel in a supercritical corrosion environment will decrease. Therefore, the Mn content is 0.40-3.00%. The preferred lower limit of the Mn content is 0.50%, and more preferably 0.60%. The preferred upper limit of the Mn content is 2.90%, and more preferably 2.70%.

[0040] P: 0.050% or less. Phosphorus (P) is inevitably present. That is, the lower limit of the P content is greater than 0%. P segregates at grain boundaries. Therefore, if the P content is too high, the corrosion resistance of the steel will decrease, even if the content of other elements is within the range of this embodiment. Accordingly, the P content is 0.050% or less. The preferred upper limit of the P content is 0.045%, more preferably 0.040%, and even more preferably 0.030%. It is preferable to have as low a P content as possible. However, an extreme reduction in the P content will significantly increase manufacturing costs. Therefore, considering industrial production, the preferred lower limit of the P content is 0.001%, and even more preferably 0.003%.

[0041] S: 0.0050% or less. Sulfur (S) is inevitably present. That is, the lower limit of the S content is greater than 0%. S segregates at grain boundaries. Therefore, if the S content is too high, the corrosion resistance of the steel will decrease, even if the content of other elements is within the range of this embodiment. Accordingly, the S content is 0.0050% or less. The preferred upper limit of the S content is 0.0040%, more preferably 0.0030%, and even more preferably 0.0020%. It is preferable to have as low an S content as possible. However, an extreme reduction in the S content will significantly increase manufacturing costs. Therefore, considering industrial production, the preferred lower limit of the S content is 0.0001%, more preferably 0.0003%, and even more preferably 0.0005%.

[0042] Cr: 20.00 to 24.50% Chromium (Cr), as an oxide, forms a passive film on the surface of the steel material, improving the overall corrosion resistance and pitting corrosion resistance of the steel material in a supercritical corrosion environment. If the Cr content is too low, the above effect cannot be fully obtained even if the content of other elements is within the range of this embodiment. On the other hand, if the Cr content is too high, the hot workability of the steel material will decrease even if the content of other elements is within the range of this embodiment. Therefore, the Cr content is 20.00 to 24.50%. The preferred lower limit of the Cr content is 20.20%, more preferably 20.50%, and still more preferably 21.00%. The preferred upper limit of the Cr content is 24.30%, more preferably 24.00%, and still more preferably 23.80%.

[0043] Cu: 0.10-3.00% Copper (Cu) enhances the overall corrosion resistance of steel materials in supercritical corrosion environments. If the Cu content is too low, the above effect cannot be fully obtained even if the content of other elements is within the range of this embodiment. On the other hand, if the Cu content is too high, the hot workability of the steel material decreases even if the content of other elements is within the range of this embodiment. Therefore, the Cu content is 0.10-3.00%. The preferred lower limit of the Cu content is 0.15%, more preferably 0.20%, and even more preferably 0.25%. The preferred upper limit of the Cu content is 2.90%, more preferably 2.75%, and even more preferably 2.50%.

[0044] Ni: 2.00 to 10.00% Nickel (Ni) stabilizes the austenite of the steel. Ni further enhances the overall corrosion resistance of the steel in supercritical corrosion environments. If the Ni content is too low, the above effects cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Ni content is too high, even if the content of other elements is within the range of this embodiment, the volume fraction of austenite becomes too high, and the yield strength of the steel decreases. Therefore, the Ni content is 2.00 to 10.00%. The preferred lower limit of the Ni content is 2.50%, more preferably 3.00%, still more preferably 3.50%, and still more preferably 4.00%. The preferred upper limit of the Ni content is 9.80%, more preferably 9.60%, still more preferably 9.30%, and still more preferably 9.00%.

[0045] Mo: 0.80-5.00% Molybdenum (Mo) enhances the overall corrosion resistance of steel materials in supercritical corrosion environments. If the Mo content is too low, the above effect cannot be fully obtained even if the content of other elements is within the range of this embodiment. On the other hand, if the Mo content is too high, the hot workability of the steel material will decrease even if the content of other elements is within the range of this embodiment. Therefore, the Mo content is 0.80-5.00%. The preferred lower limit of the Mo content is 0.85%, more preferably 1.00%, and even more preferably 1.50%. The preferred upper limit of the Mo content is 4.80%, more preferably 4.50%, and even more preferably 4.30%.

[0046] W: 0.01 to 1.50% Tungsten (W) enhances the overall corrosion resistance of steel materials in supercritical corrosion environments. If the W content is too low, the above effect cannot be fully obtained even if the content of other elements is within the range of this embodiment. On the other hand, if the W content is too high, the toughness of the steel material will decrease even if the content of other elements is within the range of this embodiment. Therefore, the W content is 0.01 to 1.50%. The preferred lower limit of the W content is 0.05%, more preferably 0.07%, more preferably 0.10%, and still more preferably 0.15%. The preferred upper limit of the W content is 1.45%, more preferably 1.35%, and still more preferably 1.25%.

[0047] N: 0.001 to 0.350% Nitrogen (N) stabilizes the austenite of the steel. N further enhances the overall corrosion resistance of the steel in supercritical corrosion environments. If the N content is too low, the above effects cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the N content is too high, the hot workability of the steel will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the N content is 0.001 to 0.350%. The preferred lower limit of the N content is 0.003%, more preferably 0.005%, and even more preferably 0.010%. The preferred upper limit of the N content is 0.345%, and even more preferably 0.340%.

[0048] Co: 0.10-1.00% Cobalt (Co) forms a film on the surface of the steel material, improving its resistance to overall corrosion and pitting corrosion in supercritical corrosion environments. If the Co content is too low, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Co content is too high, the manufacturing cost will increase drastically, even if the content of other elements is within the range of this embodiment. Therefore, the Co content is 0.10-1.00%. The preferred lower limit of the Co content is 0.11%, more preferably 0.12%, 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 0.80%.

[0049] Sn: 0.001 to 0.050% Tin (Sn) enhances the overall corrosion resistance and pitting corrosion resistance of steel materials in supercritical corrosion environments. If the Sn content is too low, the above effects cannot be fully obtained even if the content of other elements is within the range of this embodiment. On the other hand, if the Sn content is too high, the hot workability of the steel material will decrease even if the content of other elements is within the range of this embodiment. Therefore, the Sn content is 0.001 to 0.050%. The preferred lower limit of the Sn content is 0.002%, more preferably 0.003%, and still more preferably 0.005%. The preferred upper limit of the Sn content is 0.045%, more preferably 0.040%, and still more preferably 0.035%.

[0050] sol. Al: 0.050% or less. Aluminum (Al) is inevitably present. That is, the lower limit of the Al content is greater than 0%. Al deoxidizes steel. On the other hand, if the Al content is too high, even if the content of other elements is within the range of this embodiment, coarse oxide inclusions will be formed, reducing the toughness of the steel. Therefore, the Al content is 0.050% or less. The preferred lower limit of the Al content is 0.001%, more preferably 0.005%, and even more preferably 0.010%. The preferred upper limit of the Al content is 0.045%, and even more preferably 0.040%. In this specification, the Al content refers to the content of "acid-soluble Al," that is, sol. Al.

[0051] V: 0.01 to 0.50% Vanadium (V) increases the strength of the steel. If the V content is too low, the above effect cannot be fully obtained even if the content of other elements is within the range of this embodiment. On the other hand, if the V content is too high, even if the content of other elements is within the range of this embodiment, the strength of the steel becomes too high and the toughness of the steel decreases. Therefore, the V content is 0.01 to 0.50%. The preferred lower limit of the V content is 0.02%, more preferably 0.03%, more preferably 0.05%, and still more preferably 0.10%. The preferred upper limit of the V content is 0.45%, more preferably 0.40%, and still more preferably 0.30%.

[0052] Ti: 0.001 to 0.050% Titanium (Ti) forms carbonitrides, increasing the strength of the steel. If the Ti content is too low, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Ti content is too high, even if the content of other elements is within the range of this embodiment, the strength of the steel becomes too high and the toughness of the steel decreases. Therefore, the Ti content is 0.001 to 0.050%. The preferred lower limit of the Ti content is 0.002%, more preferably 0.003%, and still more preferably 0.005%. The preferred upper limit of the Ti content is 0.045%, more preferably 0.040%, more preferably 0.030%, more preferably 0.025%, and still more preferably 0.020%.

[0053] Ca: 0.0005 to 0.0100% Calcium (Ca) combines with sulfur in the steel to form Ca sulfides, suppressing the formation of coarse Mn sulfides. As a result, the number density of coarse Mn sulfides in the steel is reduced, improving the pitting corrosion resistance of the steel in a supercritical corrosion environment. If the Ca content is too low, the above effect cannot be fully obtained even if the content of other elements is within the range of this embodiment. On the other hand, if the Ca content is too high, even if the content of other elements is within the range of this embodiment, a large number of coarse Ca sulfides will be formed, reducing the pitting corrosion resistance of the steel in a supercritical corrosion environment. Therefore, the Ca content is 0.0005 to 0.0100%. The preferred lower limit of the Ca content is 0.0006%, more preferably 0.0007%, and even more preferably 0.0008%. The preferred upper limit for the Ca content is 0.0090%, more preferably 0.0080%, even more preferably 0.0060%, and even more preferably 0.0050%.

[0054] B: 0.0015 to 0.0050% Boron (B) suppresses segregation of S at grain boundaries in steel and improves the hot workability of the steel. If the B content is too low, the above effect cannot be fully obtained even if the content of other elements is within the range of this embodiment. On the other hand, if the B content is too high, boron nitride (BN) is formed even if the content of other elements is within the range of this embodiment, which reduces the toughness of the steel. Therefore, the B content is 0.0015 to 0.0050%. The preferred lower limit of the B content is 0.0018%, more preferably 0.0020%, and even more preferably 0.0025%. The preferred upper limit of the B content is 0.0045%, more preferably 0.0040%, and even more preferably 0.0035%.

[0055] O: 0.010% or less. Oxygen (O) is inevitably present. That is, the lower limit of the O content is greater than 0%. O forms oxides, which reduce the toughness of the steel. Therefore, if the O content is too high, the corrosion resistance of the steel will decrease, even if the content of other elements is within the range of this embodiment. Accordingly, the O content is 0.010% or less. The preferred upper limit of the O content is 0.009%, more preferably 0.008%, and even more preferably 0.007%. It is preferable to have as low an O content as possible. However, an extreme reduction in the O content will significantly increase manufacturing costs. Therefore, considering industrial production, the preferred lower limit of the O content is 0.001%, and even more preferably 0.003%.

[0056] The remainder of the chemical composition of the duplex stainless steel material according to this embodiment consists of Fe and impurities. Here, impurities in the chemical composition refer to substances that are mixed in from raw materials such as ore, scrap, or the manufacturing environment during the industrial production of duplex stainless steel material, and are acceptable within a range that does not adversely affect the duplex stainless steel material according to this embodiment.

[0057] [Optional Elements] The chemical composition of the duplex stainless steel material according to this embodiment may further contain, in place of a portion of Fe, one or more elements selected from the group consisting of Mg and rare earth elements. All of these elements are optional and enhance the hot workability of the steel material.

[0058] Mg: 0-0.010% Magnesium (Mg) is an optional element and may not be present. That is, the Mg content may be 0%. If present, Mg neutralizes sulfur in the steel by fixing it as sulfides, thereby improving the hot workability of the steel. Even a small amount of Mg will provide the above effect to some extent. However, if the Mg content is too high, even if the content of other elements is within the range of this embodiment, the oxides in the steel will coarseen, reducing the toughness of the steel. Therefore, the Mg content is 0-0.010%. The preferred lower limit of the Mg content is greater than 0%, more preferably 0.001%, more preferably 0.002%, and more preferably 0.003%. The preferred upper limit of the Mg content is 0.009%, and more preferably 0.008%.

[0059] Rare earth elements: 0 to 0.010% Rare earth elements (REM) are optional elements and do not need to be included. That is, the REM content may be 0%. If included, REM detoxifies S in the steel by fixing it as sulfides and improves the hot workability of the steel. Even a small amount of REM can provide the above effect to some extent. However, if the REM content is too high, even if the content of other elements is within the range of this embodiment, the oxides in the steel will coarseen, reducing the toughness of the steel. Therefore, the REM content is 0 to 0.010%. The preferred lower limit of the REM content is greater than 0%, more preferably 0.001%, more preferably 0.002%, and more preferably 0.003%. The preferred upper limit of the REM content is 0.009%, and more preferably 0.008%.

[0060] In this specification, REM refers to one or more elements selected from the group consisting of scandium (Sc), atomic number 21; yttrium (Y), atomic number 39; and lanthanides, lanthanum (La), atomic number 57 to lutetium (Lu), atomic number 71. In this specification, REM content refers to the total content of these elements.

[0061] The chemical composition of the duplex stainless steel material according to this embodiment may further contain one or more elements selected from the group consisting of Zr, Nb, and Ta in place of a portion of Fe. All of these elements are arbitrary and enhance the strength of the steel material.

[0062] Zr: 0 to 0.010% Zirconium (Zr) is an optional element and may not be included. That is, the Zr content may be 0%. If included, Zr forms carbonitrides, increasing the strength of the steel. Even a small amount of Zr will provide some of the above effect. However, if the Zr content is too high, even if the content of other elements is within the range of this embodiment, the strength of the steel will become too high and the toughness of the steel will decrease. Therefore, the Zr content is 0 to 0.010%. The preferred lower limit of the Zr content is greater than 0%, more preferably 0.001%, more preferably 0.002%, and more preferably 0.003%. The preferred upper limit of the Zr content is 0.009%, and more preferably 0.008%.

[0063] Nb: 0-0.500% Niobium (Nb) is an optional element and may not be present. That is, the Nb content may be 0%. If present, Nb forms carbonitrides, increasing the strength of the steel. Even a small amount of Nb will provide some of the above effect. However, if the Nb content is too high, even if the content of other elements is within the range of this embodiment, the strength of the steel will become too high and the toughness of the steel will decrease. Therefore, the Nb content is 0-0.500%. The preferred lower limit of the Nb content is greater than 0%, more preferably 0.001%, more preferably 0.005%, more preferably 0.010%, and more preferably 0.050%. The preferred upper limit of the Nb content is 0.400%, more preferably 0.300%, and more preferably 0.200%.

[0064] Ta: 0 to 0.100% Tantalum (Ta) is an optional element and may not be included. That is, the Ta content may be 0%. If included, Ta forms carbonitrides, increasing the strength of the steel. Even if only a small amount of Ta is included, the above effect can be obtained to some extent. However, if the Ta content is too high, even if the content of other elements is within the range of this embodiment, the strength of the steel will become too high and the toughness of the steel will decrease. Therefore, the Ta content is 0 to 0.100%. The preferred lower limit of the Ta content is greater than 0%, more preferably 0.001%, more preferably 0.002%, more preferably 0.003%, and more preferably 0.005%. The preferred upper limit of the Ta content is 0.095%, more preferably 0.090%, and more preferably 0.080%.

[0065] The chemical composition of the duplex stainless steel material according to this embodiment may further contain one or more elements selected from the group consisting of As, Zn, Pb, and Sb in place of a portion of Fe. All of these elements are arbitrary and enhance the corrosion resistance of the steel material.

[0066] As: 0-0.050% Arsenic (As) is an optional element and may not be present. That is, the As content may be 0%. If present, As enhances the overall corrosion resistance of steel materials in supercritical corrosion environments. Even a small amount of As can provide the above effect to some extent. However, if the As content is too high, even if the content of other elements is within the range of this embodiment, the overall corrosion resistance and pitting corrosion resistance of steel materials in supercritical corrosion environments may actually decrease. Therefore, the As content is 0-0.050%. The preferred lower limit of the As content is greater 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%.

[0067] Zn: 0-0.010% Zinc (Zn) is an optional element and may not be included. That is, the Zn content may be 0%. If included, Zn enhances the overall corrosion resistance of steel materials in supercritical corrosion environments. Even a small amount of Zn can provide the above effect to some extent. However, if the Zn content is too high, even if the content of other elements is within the range of this embodiment, the overall corrosion resistance and pitting corrosion resistance of steel materials in supercritical corrosion environments may actually decrease. Therefore, the Zn content is 0-0.010%. The preferred lower limit of the Zn content is greater than 0%, more preferably 0.001%, more preferably 0.002%, and more preferably 0.003%. The preferred upper limit of the Zn content is 0.009%, more preferably 0.008%, and more preferably 0.007%.

[0068] Pb: 0-0.010% Lead (Pb) is an optional element and may not be present. That is, the Pb content may be 0%. If present, Pb enhances the overall corrosion resistance of the steel material in a supercritical corrosion environment. Even a small amount of Pb can provide the above effect to some extent. However, if the Pb content is too high, numerous defects will occur on the surface of the steel material after hot working, even if the content of other elements is within the range of this embodiment. Therefore, the Pb content is 0-0.010%. The preferred lower limit of the Pb content is greater than 0%, more preferably 0.001%, more preferably 0.002%, and more preferably 0.003%. The preferred upper limit of the Pb content is 0.009%, more preferably 0.008%, and more preferably 0.007%.

[0069] Sb: 0-0.010% Antimony (Sb) is an optional element and may not be present. That is, the Sb content may be 0%. If present, Sb enhances the overall corrosion resistance of steel materials in supercritical corrosion environments. Even a small amount of Sb can provide the above effect to some extent. However, if the Sb content is too high, the manufacturing cost will increase drastically, even if the content of other elements is within the range of this embodiment. Therefore, the Sb content is 0-0.010%. The preferred lower limit of the Sb content is greater than 0%, more preferably 0.001%, more preferably 0.002%, and more preferably 0.003%. The preferred upper limit of the Sb content is 0.009%, and more preferably 0.008%.

[0070] [Fn1] The duplex stainless steel material according to this embodiment has an Fn1 of 40.0 or more, as defined by formula (1), assuming that the content of each element in the chemical composition is within the above range. Fn1 = Cr + 3.3(Mo + 0.5W) + 16N + 2Ni + Cu + 2Co + 10Sn (1) Here, the elemental symbols in formula (1) are substituted with the mass percentage content of the corresponding element.

[0071] Fn1 is an index of the overall corrosion resistance of duplex stainless steel materials in a supercritical corrosion environment. As mentioned above, supercritical CO ２ The gas contains SOx gas and O ２ In a supercritical corrosion environment containing gas, not only Cr, Mo, W, and N, but also Ni, Cu, Co, and Sn contribute to improved overall corrosion resistance among the elements of the above-mentioned chemical composition. In other words, Fn1 is a parameter formula set considering the contribution of each element to overall corrosion resistance in a supercritical corrosion environment for a duplex stainless steel material having the above-mentioned chemical composition. In a duplex stainless steel material having the above-mentioned chemical composition, if Fn1 is 40.0 or higher, overall corrosion resistance in a supercritical corrosion environment is significantly improved. Therefore, in this embodiment, Fn1 is 40.0 or higher.

[0072] The preferred lower limit of Fn1 is 40.5, more preferably 41.0, even more preferably 41.5, and even more preferably 42.0. The upper limit of Fn1 is not particularly limited, but is substantially 74.6. The upper limit of Fn1 may also be 70.0, 68.0, or 65.0. Fn1 is obtained by rounding the obtained value to the second decimal place.

[0073] [Total number density of specific inclusions] The duplex stainless steel material according to this embodiment has the above-described chemical composition, an Fn1 of 40.0 or more, and furthermore, a total number density of specific inclusions of 0.50 pieces / mm ２ The following applies. In this specification, "specific inclusions" is a general term for particles (coarse Mn sulfides) having an equivalent circular diameter of 1.0 μm or more, and having a Mn content of 10% by mass or more and an S content of 10% by mass or more among the specified elements identified by the method described below, and particles (coarse Ca sulfides) having an equivalent circular diameter of 2.0 μm or more, and having a Ca content of 20% by mass or more, an S content of 10% by mass or more and a Mn content of less than 10% by mass among the specified elements identified by the method described below.

[0074] In this specification, the unit area (1 mm) ２ The sum of the number densities of coarse Mn sulfides and coarse Ca sulfides per unit area is used to determine the total number density of specific inclusions (pieces / mm³). ２ ) is defined as follows. Hereafter, the total number density of specific inclusions will also be called the total number density of specific inclusions ND (Number Density).

[0075] As described above, in a supercritical corrosion environment, inclusions on the surface of steel materials dissolve easily, and specific inclusions (coarse Mn sulfides and coarse Ca sulfides) tend to become the starting point for pitting corrosion. Therefore, in this embodiment, the total number density ND of specific inclusions is set to 0.50 inclusions / mm ２ The following reductions are achieved. As a result, provided that the other components of this embodiment are met, the duplex stainless steel material exhibits excellent overall corrosion resistance and pitting corrosion resistance even in supercritical corrosion environments.

[0076] A preferred upper limit for the total number density ND of specific inclusions is 0.49 inclusions / mm³. ２ And more preferably 0.48 pieces / mm ２And more preferably 0.45 pieces / mm ２ The lower limit of the total number density ND of specific inclusions is not particularly limited, and is 0.00 particles / mm³. ２ This may also be the case. In the duplex stainless steel material according to this embodiment, the lower limit of the total number density ND of specific inclusions is, for example, 0.01 pieces / mm ２ It may be 0.05 pieces / mm ２ It may be 0.10 pieces / mm ２ It may be 0.15 pieces / mm ２ This is also acceptable. The method for determining the total number density ND of specific inclusions will be described later.

[0077] [Microstructure] The microstructure of the duplex stainless steel material according to this embodiment consists of ferrite and austenite. In this specification, "consisting of ferrite and austenite" means that the amount of phases other than ferrite and austenite is negligibly small. For example, the volume fraction of precipitates and inclusions in the microstructure of the duplex stainless steel material according to this embodiment is negligibly low compared to the volume fraction of ferrite and austenite. In other words, the microstructure of the duplex stainless steel material according to this embodiment may contain trace amounts of precipitates, inclusions, etc., in addition to ferrite and austenite.

[0078] In the microstructure of the duplex stainless steel material according to this embodiment, the volume fractions of ferrite and austenite are not particularly limited. However, in the microstructure of the duplex stainless steel material according to this embodiment having the above-described chemical composition, for example, the volume fraction of ferrite may be 35 to 65%. In this case, the volume fraction of austenite will be 35 to 65%. The method for determining the volume fractions of ferrite and austenite in the microstructure will be described later.

[0079] [Fn2] The duplex stainless steel material according to this embodiment has the above-described chemical composition, Fn1 is 40.0 or higher, and the total number density ND of specific inclusions is 0.50 pieces / mm ２ The following is the result: it has a microstructure consisting of ferrite and austenite, and furthermore, the total number density NDA (numbers / mm³) of specific inclusions in the ferrite. ２) and the total number density of specific inclusions in austenite NDG (items / mm³) ２ ) and satisfy the following equation (2): NDA / NDG < 0.50 (2)

[0080] Fn2 (=NDA / NDG) is an index indicating the extent to which specific inclusions (coarse Mn sulfides and coarse Ca sulfides) are present in the ferrite. Austenite has superior corrosion resistance to ferrite. Therefore, in duplex stainless steel materials with the above-mentioned chemical composition, there is a concern that selective pitting corrosion may occur in the ferrite.

[0081] On the other hand, if Fn2 is less than 0.50, the number density of specific inclusions in the ferrite can be sufficiently reduced. As a result, subject to the other conditions of this embodiment being met, the occurrence of selective pitting corrosion in the ferrite can be suppressed. Therefore, in this embodiment, Fn2 is reduced to less than 0.50. As a result, subject to the other conditions of this embodiment being met, the duplex stainless steel material has excellent overall corrosion resistance and pitting corrosion resistance even in supercritical corrosion environments.

[0082] As described above, in the duplex stainless steel material according to this embodiment, the volume fractions of ferrite and austenite in the microstructure are, for example, 35 to 65%. In this case, if Fn2 is less than 0.50, the number of specific inclusions in the ferrite will be less than the number of specific inclusions in the austenite. Therefore, the pitting corrosion resistance of ferrite can be stably improved.

[0083] A preferred upper limit for Fn2 is 0.49, more preferably 0.48, and even more preferably 0.47. The lower limit for Fn2 is not particularly limited and may be 0.00. In the duplex stainless steel material according to this embodiment, the lower limit for Fn2 may be 0.01, 0.05, 0.10, or 0.15.

[0084] In this embodiment, the total number density of specific inclusions is ND (number of inclusions / mm²). ２The volume fractions (%) of ferrite and austenite in the microstructure, and Fn2 can be determined by the following method. First, a test piece for microstructure observation is prepared from the duplex stainless steel material according to this embodiment, and the volume fractions (%) of ferrite and austenite are determined. Specifically, if the steel material is a steel plate, a test piece is prepared from the center of the plate thickness. At this time, the observation surface of the test piece includes the rolling direction and the plate thickness direction, but does not include a region of 1 mm in the plate thickness direction from the surface of the steel plate. If the steel material is a steel pipe, a test piece is prepared from the center of the wall thickness. At this time, the observation surface of the test piece includes the pipe axis direction and the pipe diameter direction (wall thickness direction), but does not include a region of 1 mm in the wall thickness direction from the inner and outer surfaces of the steel pipe. If the steel material is a round steel bar, a test piece is prepared from the R / 2 position in a cross section perpendicular to the axial direction of the round steel bar. In this specification, the R / 2 position of a round steel bar means the central position of radius R in a cross section perpendicular to the axial direction of the round steel bar. In this case, the observation surface of the test specimen shall include the axial and radial directions, but shall not include a region of 1 mm in the radial direction from the surface of the round steel.

[0085] The observation surface of the prepared test specimen is mirror-polished. The mirror-polished observation surface is electrolytically etched to reveal the microstructure. The electrolytic etching solution is a mixture of aqua regia (hydrochloric acid:nitric acid = 3:1) and glycerin, with a current density of 1 A / cm². ２ The electrolysis is performed for 1 minute. The observation surface from which the tissue has been exposed is observed in 10 fields using an optical microscope. The observation field is a 250 μm × 250 μm square. In each observation field, ferrite and austenite are identified from the contrast. The area percentage of the identified ferrite is measured using the point method in accordance with JIS G0555 (2020). In this embodiment, the arithmetic mean of the obtained ferrite area percentages in the 10 fields is defined as the ferrite volume percentage (%). The ferrite volume percentage (%) is obtained by rounding the obtained value to the first decimal place. The austenite volume percentage (%) is obtained by subtracting the obtained ferrite volume percentage (%) from 100.

[0086] Further, another test specimen for microstructural observation is prepared from the duplex stainless steel material according to this embodiment, and the total number density ND (numbers / mm³) of specific inclusions is determined. ２), and Fn2 are determined. Test specimens for microstructural observation are prepared using the method described above. The size of the test specimen is not particularly limited, but the area of ​​the observation surface is 200 mm². ２ It is preferable to use a size of 20 mm x 10 mm or larger. 200 mm from one test piece. ２ If the above observation surface cannot be secured, prepare multiple test pieces and measure 200 mm. ２ The observation surface described above may be obtained. After polishing the observation surface of the prepared test specimen to a mirror finish, measurements are taken. First, the observation surface is observed using a scanning electron microscope (SEM), and particles in the observation surface are identified from the contrast. Elemental concentration analysis (EDS analysis) is performed on each identified particle. In the EDS analysis, the acceleration voltage is set to 20 kV, and the target elements are quantified as N, O, Mg, Al, Si, P, S, Ca, Ti, Cr, Mn, Fe, Cu, Zr, and Nb. Based on the EDS analysis results of each particle, when the total content of N, O, Mg, Al, Si, P, S, Ca, Ti, Cr, Mn, Fe, Cu, Zr, and Nb is set to 100% by mass, particles with a Mn content of 10% by mass or more and an S content of 10% by mass or more are identified as "Mn sulfides". Similarly, based on the EDS analysis results of each particle, when the total content of N, O, Mg, Al, Si, P, S, Ca, Ti, Cr, Mn, Fe, Cu, Zr, and Nb is set to 100% by mass, particles with a Ca content of 20% by mass or more, an S content of 10% by mass or more, and a Mn content of less than 10% by mass are identified as "Ca sulfides".

[0087] Among the Mn sulfides in the observation surface, Mn sulfides with an equivalent circle diameter of 1.0 μm or more are designated as "coarse Mn sulfides." In other words, in this specification, "coarse Mn sulfides" means particles with an equivalent circle diameter of 1.0 μm or more, and when the total content of N, O, Mg, Al, Si, P, S, Ca, Ti, Cr, Mn, Fe, Cu, Zr, and Nb is defined as 100% by mass, the Mn content is 10% by mass or more and the S content is 10% by mass or more. Similarly, among the Ca sulfides in each observation surface, Ca sulfides with an equivalent circle diameter of 2.0 μm or more are designated as "coarse Ca sulfides." In other words, in this specification, "coarse Ca sulfide" means particles with an equivalent circular diameter of 2.0 μm or more, and where, when the total content of N, O, Mg, Al, Si, P, S, Ca, Ti, Cr, Mn, Fe, Cu, Zr, and Nb is defined as 100% by mass, the Ca content is 20% by mass or more, the S content is 10% by mass or more, and the Mn content is less than 10% by mass. Coarse Mn sulfide and coarse Ca sulfide identified on the observation surface by the above method are counted as "specific inclusions".

[0088] Furthermore, the microstructure (phase) containing each specific inclusion is identified by electrolytic etching of the same observation surface. Specifically, the microstructure is revealed by electrolytic etching of the observation surface. The electrolytic etching solution is a mixture of aqua regia (hydrochloric acid:nitric acid = 3:1) and glycerin, with a current density of 1 A / cm². ２ The electrolysis is performed for 1 minute. The observation surface from which the tissue has been revealed is observed with an optical microscope, and ferrite is identified from the contrast. For each specific inclusion on the observation surface, it is determined whether it is contained in ferrite or austenite. The number of specific inclusions contained in ferrite on the observation surface is counted. The number of specific inclusions contained in austenite on the observation surface is determined by subtracting the number of specific inclusions contained in ferrite from the total number of specific inclusions on the entire observation surface. For alignment purposes when observing the same observation surface, for example, an indentation may be formed on the edge of the observation surface beforehand. Furthermore, the position coordinates on the observation surface of the specific inclusions identified by the method described above may be obtained.

[0089] Similar observations are performed on three or more observation surfaces. Based on the total number of specific inclusions counted on all observation surfaces and the total area of ​​all observation surfaces, the total number density of specific inclusions ND (items / mm²) is calculated. ２ The total number density (NDA) of specific inclusions in ferrite is calculated based on the total number of specific inclusions contained in the ferrite counted on all observation surfaces and the total area of ​​the ferrite on all observation surfaces. ２ The total area of ​​ferrite on all observation surfaces is calculated by multiplying the total area of ​​all observation surfaces by the volume fraction (%) of ferrite obtained by the method described above. Furthermore, based on the total number of specific inclusions contained in the austenite counted on all observation surfaces and the total area of ​​austenite on all observation surfaces, the total number density NDG (items / mm³) of specific inclusions in the austenite is calculated. ２ ) is calculated. Note that the total area of ​​austenite on all observation surfaces is calculated by subtracting the total area of ​​ferrite on all observation surfaces from the total area of ​​all observation surfaces.

[0090] The total number density NDA (numbers / mm³) of specific inclusions in the obtained ferrite ２ ) and the total number density of specific inclusions in austenite NDG (items / mm³) ２ Using the above, Fn2 (= NDA / NDG) is calculated. Note that the total number density of specific inclusions ND (pieces / mm) is calculated. ２ ), total number density of specific inclusions in ferrite NDA (pieces / mm³) ２ ), total number density of specific inclusions in austenite NDG (inclusions / mm³) ２ ), and Fn2 are obtained by rounding the obtained numerical value to the third decimal place. Furthermore, observation of specific inclusions can be performed using a scanning electron microscope equipped with a compositional analysis function (SEM-EDS instrument). As an SEM-EDS instrument, for example, the automated analyzer manufactured by FEI (ASPEX), trade name: Metals Quality Analyzer, can be used.

[0091] [Resistance to overall corrosion and pitting corrosion] The duplex stainless steel material according to this embodiment has the above-described chemical composition, an Fn1 of 40.0 or higher, and a total number density ND of specific inclusions of 0.50 pieces / mm ２The following is true: it has a microstructure consisting of ferrite and austenite, and its Fn2 is less than 0.50. As a result, the duplex stainless steel material according to this embodiment has excellent overall corrosion resistance and pitting corrosion resistance even in a supercritical corrosion environment. In this embodiment, excellent overall corrosion resistance and pitting corrosion resistance in a supercritical corrosion environment are evaluated by the following method.

[0092] Specifically, a test specimen for corrosion testing is prepared from the duplex stainless steel material according to this embodiment. If the steel material is a steel pipe, the test specimen is prepared from the center of the wall thickness. In this case, the longitudinal direction of the test specimen is parallel to the axial direction of the steel pipe. If the steel material is a round bar, the test specimen is prepared from the R / 2 position. In this case, the longitudinal direction of the test specimen is parallel to the axial direction of the round bar. If the steel material is a steel plate, the test specimen is prepared from the center of the plate thickness. In this case, the longitudinal direction of the test specimen is parallel to the rolling direction of the steel plate. The test specimen is, for example, 30 mm in length, 20 mm in width, and 2 mm in thickness.

[0093] Place the test specimen in the autoclave and add 0.01 volume% SO ２ Gas and 0.05 volume% of O ２ A 5.0% by mass aqueous solution of sodium chloride saturated with gas is poured in so that the test specimen is immersed. CO2 is then added to the autoclave at a total pressure of 130 bar. ２ The corrosion test will begin after pressurizing and filling the autoclave with gas. The corrosion test will last for 96 hours, and the temperature inside the autoclave will be maintained at 100°C during the test.

[0094] The mass, density, and surface area of ​​the test specimen after 96 hours are determined, and the corrosion rate (mm / year) of the test specimen is calculated. In this embodiment, the corrosion rate is calculated by rounding the obtained value to the fourth decimal place. Furthermore, the surface of the test specimen after 96 hours is observed with a magnifying glass at 10x to check for the presence or absence of pitting corrosion. If the occurrence of pitting corrosion is suspected based on observation with the magnifying glass, the specimen is further observed with an optical microscope at 100x magnification to confirm the presence or absence of pitting corrosion. In this embodiment, if the corrosion rate obtained as a result of the corrosion test under the above conditions is 0.100 mm / year or less, it is evaluated as having excellent overall corrosion resistance even in a supercritical corrosion environment. In this embodiment, if no pitting corrosion is confirmed as a result of the corrosion test under the above conditions, it is evaluated as having excellent pitting corrosion resistance even in a supercritical corrosion environment.

[0095] [Yield Strength] The yield strength of the duplex stainless steel material according to this embodiment is not particularly limited. For example, the yield strength of the duplex stainless steel material according to this embodiment is 413 to 1035 MPa. In this embodiment, the lower limit of the yield strength of the duplex stainless steel material may be 427 MPa, 441 MPa, or 448 MPa. In this embodiment, the upper limit of the yield strength of the duplex stainless steel material may be 1020 MPa, 1007 MPa, or 1000 MPa.

[0096] 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 ASTM E8 / E8M (2022). A test specimen is prepared from the steel material according to this embodiment. If the steel material is a steel plate, a round bar-shaped tensile test specimen is prepared from the center of the plate thickness. In this case, the longitudinal direction of the tensile test specimen is parallel to the rolling direction of the steel plate. If the steel material is a steel pipe, a round bar-shaped tensile test specimen or an arc-shaped tensile test specimen is prepared from the center of the wall thickness. In this case, the longitudinal direction of the tensile test specimen is parallel to the axial direction of the steel pipe. If the steel material is a round steel bar, a round bar-shaped tensile test specimen is prepared from the R / 2 position. In this case, the longitudinal direction of the tensile test specimen is parallel to the axial direction of the round steel bar.

[0097] The dimensions of the round bar-shaped tensile test specimen are, for example, a parallel section diameter of 6.35 mm and a gauge length of 25.4 mm. The dimensions of the arc-shaped tensile test specimen are, for example, a total thickness of 25.4 mm in width and a gauge length of 50.8 mm. Using the prepared test specimens, a tensile test is performed at room temperature (25°C) in air, in accordance with the ASTM E8 / E8M (2022) method. In this embodiment, the 0.2% offset proof 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 obtained value to the first decimal place.

[0098] [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. The duplex stainless steel material according to this embodiment may be, for example, a steel pipe, a steel plate, a round steel bar, or a wire rod. Preferably, the duplex stainless steel material according to this embodiment is a seamless steel pipe. When the duplex stainless steel material according to this embodiment is a seamless steel pipe, it has excellent overall corrosion resistance and pitting corrosion resistance in a supercritical corrosion environment, even if the wall thickness is 5 mm or more.

[0099] [Manufacturing Method] An example of a manufacturing method for duplex stainless steel material according to this embodiment, having the above-described configuration, will be explained. Note that the manufacturing method for duplex stainless steel material according to this embodiment is not limited to the manufacturing method described below. The example of the manufacturing method for duplex stainless steel material according to this embodiment includes a steelmaking process, a hot working process, and a solution treatment process. Each manufacturing process will be described in detail below.

[0100] [Steelmaking Process] The steelmaking process according to this embodiment includes a process for producing molten steel (refining process) and a process for producing raw materials by casting using molten steel (raw material production process). Each process will be described below.

[0101] [Smelting Process] In the smelting process, first, molten steel containing Cr is placed in a ladle, and decarburization is performed on the molten steel in the ladle under atmospheric pressure. This process is called the rough decarburization smelting process. Slag is generated by the decarburization process in the rough decarburization smelting process. After the rough decarburization smelting process, slag generated by the decarburization process floats on the surface of the molten steel. In the rough decarburization smelting process, Cr in the molten steel oxidizes and Cr ２ O ３ It generates. Cr ２ O ３ It is absorbed into the slag. Therefore, add a deoxidizing agent to the ladle and remove the Cr from the slag. ２ O ３ The chromium is reduced, and Cr is recovered into the molten steel. This process is called the Cr reduction process. The crude decarburization refining process and the Cr reduction process are carried out, for example, by the electric furnace method, the converter method, or the AOD (Argon Oxygen Decarburization) method. After the Cr reduction process, the slag is removed from the molten steel. This process is called the slag removal process.

[0102] In the case of chromium-containing steel, the carbon activity is reduced by the chromium, thus suppressing the decarburization reaction. Therefore, a final decarburization treatment is performed on the molten steel after the slag removal process. This process is called the final decarburization refining process. In the final decarburization refining process, the decarburization treatment is carried out under reduced pressure. If the decarburization treatment is carried out under reduced pressure, the partial pressure of CO gas in the atmosphere (P) ＣＯ The carbon content (C) in the slag is reduced, and the oxidation of Cr in the molten steel is suppressed. Therefore, if the decarburization treatment is carried out under reduced pressure, the carbon concentration in the molten steel can be further reduced while suppressing the oxidation of Cr. After the final decarburization refining process, a deoxidizing agent is added to the molten steel to reduce the carbon content in the slag. ２ O ３ A Cr reduction treatment is performed again to reduce the chromium. This process is called the Cr reduction treatment process. The final decarburization refining process and the Cr reduction treatment process after the final decarburization refining process may be carried out by, for example, the VOD (Vacuum Oxygen Decarburization) method or the RH (Ruhrstahl-Heraeus) method.

[0103] After the Cr reduction process, the molten steel in the ladle undergoes final compositional adjustment and temperature adjustment before the material manufacturing process. This process is called the compositional adjustment process. The compositional adjustment process is carried out, for example, by LT (Ladle Treatment). In the latter half of the compositional adjustment process, Ca is added to the molten steel. Here, the time from the addition of Ca until the Ca is uniformly dispersed in the molten steel is defined as the "uniform mixing time τ". The uniform mixing time τ can be calculated by the following equation (A): τ = 800 × ε －０．４ (A) Here, ε is the stirring power density of molten steel at LT and is defined by equation (B). ε = 28.5(Q / W) × T × log(1 + H / 1.48) (B) Here, Q is the upward blowing gas flow rate (Nm ３ The formula is (min). W is the mass of molten steel (t). T is the temperature of the molten steel (K). H is the depth of the molten steel in the ladle (steel bath depth) (m).

[0104] In the component adjustment process, the molten steel temperature in the ladle is maintained at 1500 to 1700°C. Furthermore, Ca is added to the molten steel, and the holding time after the uniform mixing time τ has elapsed is defined as the "holding time t" (seconds). Preferably, in this embodiment, the holding time t after the uniform mixing time τ has elapsed is 60 seconds or more.

[0105] If the holding time t is too short, Ca may not be able to sufficiently modify the Mn sulfides in the molten steel. In this case, a large number of coarse Mn sulfides remain in the steel. As a result, the number density of specific inclusions in the manufactured duplex stainless steel becomes too high. Therefore, in the refining process according to this embodiment, it is preferable to set the holding time t after the uniform mixing time τ has elapsed to 60 seconds or more.

[0106] As described above, in the refining process of this embodiment, the holding time t after the uniform mixing time τ in the component adjustment process is set to 60 seconds or more. In the component adjustment process of this embodiment, there is no particular upper limit to the holding time t after the uniform mixing time τ has elapsed, but for example it is 3600 seconds.

[0107] [Material Manufacturing Process] The material is manufactured using the molten steel produced by the refining process described above. The material is either a slab or an ingot. Specifically, a slab is manufactured using molten steel by continuous casting. The slab may be a slab, a bloom, or a billet. Alternatively, an ingot may be made using molten steel by ingot forming. A billet may be manufactured by further processes such as bloc rolling on the slab or ingot.

[0108] In the duplex stainless steel material having the chemical composition described above, austenite is formed during the solidification of molten steel, and the temperature range in which the formed austenite grows is 1350 to 1100°C. Therefore, in this embodiment, the cooling rate in the 1350 to 1100°C range is controlled to cause specific inclusions to be unevenly distributed within the austenite. Specifically, if the cooling rate of the material in the 1350 to 1100°C range is slower than usual, austenite will preferentially nucleate and grow near the inclusions, making it easier for the austenite to incorporate the inclusions.

[0109] On the other hand, if the cooling rate of the material between 1350 and 1100°C is too fast, the material becomes supercooled, and the formation and growth of austenite tends to occur randomly. As a result, in the manufactured duplex stainless steel, the uneven distribution of specific inclusions in the austenite becomes less likely, and the Fn2 may exceed 0.50. Conversely, if the cooling rate of the material between 1350 and 1100°C is too slow, the austenite tends to coarseen. As a result, the desired mechanical properties may not be obtained in the manufactured duplex stainless steel.

[0110] Therefore, in the material manufacturing process according to this embodiment, it is preferable to set the cooling rate to 8.0 to 25.0°C / min when the material temperature is between 1350 and 1100°C. The method for controlling the cooling rate when the material temperature is between 1350 and 1100°C is not particularly limited and any well-known method may be used. For example, when manufacturing materials by continuous casting, the cooling rate can be controlled by adjusting the amount of cooling water (specific water content) used to cool the slab. For example, when manufacturing materials by ingot forming, the cooling rate can be controlled by the material of the mold and the water cooling of the mold.

[0111] In this case, measuring the internal temperature of the material is difficult in actual operation. Furthermore, in this embodiment, since the material is cooled in a steady state, the cooling rate based on the surface temperature of the material and the cooling rate inside the material are equivalent. More specifically, in the material of this embodiment, it was confirmed that the cooling rate when the internal temperature of the material is 1350 to 1100°C is equivalent to the cooling rate when the surface temperature of the material is 1200 to 950°C. Therefore, in the material manufacturing process of this embodiment, if the internal temperature of the material cannot be controlled, it is preferable to set the cooling rate when the surface temperature of the material is 1200 to 950°C to 8.0 to 25.0°C / min. The surface temperature of the material can be measured using a non-contact infrared radiation thermometer. The material is manufactured by the above process.

[0112] [Hot Working Process] In the hot working process according to this embodiment, the material prepared in the steelmaking process described above is hot-worked to produce intermediate steel material. In this specification, intermediate steel material refers to plate-shaped steel material when the final product is a steel plate, a raw pipe when the final product is a steel pipe, a steel material with a circular cross-section when the final product is a round steel bar, 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.

[0113] When the intermediate steel material is a raw pipe (seamless steel pipe), the hot working process may involve, for example, the Eugène Séjournet method or the Erhardt push bench method (i.e., hot extrusion), or the Mannesmann method of perforation rolling (i.e., hot rolling). The hot working may be performed only once or multiple times. For example, the above-mentioned perforation rolling may be performed on the material, followed by the above-mentioned hot extrusion. For example, after the above-mentioned perforation rolling may be performed on the material, stretch rolling may be performed. In other words, the raw pipe is manufactured by performing hot working in the hot working process using a well-known method. The heating temperature of the material during hot working is, for example, 1000 to 1280°C.

[0114] Furthermore, if the steel material is round steel or steel plate, the intermediate steel material may be manufactured as follows. If the steel material is round steel, first, the material is heated in a heating furnace. The heating temperature is not particularly limited, but for example, it is 1000 to 1280°C. Hot working is performed on the material extracted from the heating furnace to produce an intermediate steel material with a circular cross-section perpendicular to the axial direction. Hot working is, for example, bract rolling using a bract rolling mill, or hot rolling using a continuous rolling mill. A continuous rolling mill has alternating horizontal stands with a pair of perforated rolls arranged vertically and vertical stands with a pair of perforated rolls arranged horizontally.

[0115] When the steel material is a steel plate, the material is first heated in a heating furnace. The heating temperature is not particularly limited, but for example, it is 1000 to 1280°C. The material extracted from the heating furnace is then hot-rolled using a bloc rolling mill and a continuous rolling mill to produce intermediate steel material in the shape of a steel plate.

[0116] [Solution Treatment Process] In the solution treatment process, the intermediate steel material is subjected to solution treatment. The method of solution treatment is not particularly limited and any well-known method may be used. For example, the intermediate steel material is charged into a heat treatment furnace, held at a desired temperature, and then rapidly cooled. When the intermediate steel material is charged into a heat treatment furnace, held at a desired temperature, and then rapidly cooled to perform solution treatment, the solution temperature refers to the temperature (°C) of the heat treatment furnace used to perform the solution treatment. In this case, the solution time refers to the time during which the intermediate steel material is held at the solution temperature.

[0117] Preferably, the solution treatment temperature in the solution treatment process of this embodiment is set to 900 to 1200°C. If the solution treatment temperature is too low, precipitates (for example, σ phase, which is an intermetallic compound) may remain in the intermediate steel material after solution treatment. In this case, the corrosion resistance of the manufactured duplex stainless steel material will decrease.

[0118] When intermediate steel material is charged into a heat treatment furnace, held at a desired temperature, and then rapidly cooled to perform solution treatment, the solution treatment time is not particularly limited and can be carried out under well-known conditions. For example, the solution treatment time is 5 to 180 minutes. For example, the rapid cooling method is water cooling.

[0119] [Other Processes] If necessary, pickling may be performed on the duplex stainless steel material that has undergone solution treatment. In this case, the pickling may be carried out by any known method and is not particularly limited. Furthermore, cold working may be performed on the duplex stainless steel material that has undergone solution treatment. Cold working can increase the strength of the duplex stainless steel material. Even when cold working is performed, the material must have the above-mentioned chemical composition, an Fn1 of 40.0 or higher, and a total number density ND of specific inclusions of 0.50 particles / mm². ２ The following conditions apply to duplex stainless steel materials having a microstructure consisting of ferrite and austenite, and with an Fn2 of less than 0.50, which will exhibit excellent overall corrosion resistance and pitting corrosion resistance even in supercritical corrosion environments.

[0120] The duplex stainless steel material according to this embodiment can be manufactured through the above process. Note that the above-described method for manufacturing the duplex stainless steel material is merely an example, and the duplex stainless steel material according to this embodiment may be manufactured by other methods. The duplex stainless steel material according to this embodiment will be described in more detail below with reference to examples. Note that the examples described below are examples of conditions adopted to confirm the feasibility and effectiveness of the duplex stainless steel material according to this embodiment. In other words, the duplex stainless steel material according to this embodiment is not limited to the examples described below.

[0121] Duplex stainless steel materials (seamless steel pipes) having the chemical compositions shown in Tables 1-1 to 1-3 were manufactured by the following method. In Tables 1-1 to 1-3, "-" in the "Chemical Composition" column means that the content of the corresponding element is 0% when rounded to the least significant decimal place as described in the embodiment. Specifically, the Mg content, REM content, Zr content, Nb content, Ta content, As content, Zn content, Pb content, and Sb content of test number 1 were 0% when rounded to the fourth decimal place.

[0122]

[0123]

[0124]

[0125] The molten steel for each test number was manufactured as follows: Molten steel containing Cr was placed in a ladle, and the well-known crude decarburization refining process and Cr reduction treatment process were carried out by the AOD method. After the Cr reduction treatment process, a slag removal process was carried out to remove slag from the molten steel. Furthermore, the well-known finish decarburization refining process and Cr reduction treatment process were carried out by the VOD method.

[0126] After the Cr reduction treatment process using the VOD method, the molten steel in the ladle underwent final compositional adjustment and temperature adjustment before the material manufacturing process using LT. The molten steel temperature was 1500-1700°C in all cases. Furthermore, Ca was added to the molten steel. After adding Ca, the holding time t (seconds) after the uniform mixing time τ was adjusted as shown in Table 2. Molten steel for each test number was produced using the above process.

[0127]

[0128] Billets were manufactured using the molten steel produced by continuous casting. During billet production, the cooling rate at which the billet temperature was between 1350 and 1100°C was adjusted as shown in Table 2, "Cooling Rate at 1350-1100°C (°C / min)". The cooling rate was adjusted by changing the specific water content. Since it was difficult to measure the internal temperature of the billet, in this example, the cooling rate at which the billet surface temperature was between 1200 and 950°C was measured and considered to be the cooling rate at which the billet internal temperature was between 1350 and 1100°C. In addition, the surface temperature of the billet was measured using an infrared radiation thermometer.

[0129] After heating the manufactured billets to 1250°C, they were hot-rolled using the Mannesmann process to produce raw pipes (seamless steel pipes) with an outer diameter of 244.48 mm and a wall thickness of 13.84 mm. Furthermore, the raw pipes were subjected to solution treatment. The solution treatment temperature was 1080°C and the treatment time was 15 minutes. After the solution treatment time, the raw pipes were water-cooled. Cold working was then performed on the water-cooled seamless steel pipes of test numbers 21 to 27. In this example, cold drawing was performed as the cold working process. Through the above process, steel materials (seamless steel pipes) of each test number were manufactured.

[0130] [Evaluation Test] For each steel material with a test number produced, a microstructure observation test, a tensile test, and a corrosion test were conducted. Note that for the steel material with test number 41, since a flaw occurred during the manufacturing process, none of the evaluation tests were conducted.

[0131] [Microstructure Observation Test] For each steel material with a test number except test number 41, a microstructure observation test was conducted to identify the microstructure and determine the total number density ND (number / mm ２ ) and Fn2. Specifically, using the above method, a test piece for microstructure observation was prepared to identify the microstructure. Furthermore, using the above method, the total number density ND of the specific inclusions was determined. Additionally, using the above method, Fn2 was determined. When determining the total number density ND of the specific inclusions and Fn2, the area of the observation surface was 200 - 300 mm ２ and the observation test was conducted on three observation surfaces.

[0132] Each steel material with a test number except test number 41 had a microstructure composed of ferrite and austenite. Furthermore, the volume ratio of ferrite satisfied 35 - 65%. Also, the obtained total number density ND (number / mm ２ ) is shown in the "Total Number Density ND (number / mm ２ )" column in Table 2. Also, from the total number density NDA (number / mm ２ ) of the specific inclusions in the ferrite and the total number density NDG (number / mm ２ ) of the specific inclusions in the austenite for each steel material with the obtained test number, Fn2 (= NDA / NDG) was determined. The obtained Fn2 is shown in Table 2. Note that as described above, since test number 41 did not undergo the microstructure observation test, "-" (no evaluation) is shown in Table 2.

[0133] [Tensile Test] Tensile tests were conducted on the steel materials for each test number except test number 41, in accordance with ASTM E8 / E8M (2022), to determine the yield strength. Specifically, arc-shaped tensile test specimens were prepared for the tensile tests using the method described above. The arc-shaped tensile test specimens had the same thickness as the wall thickness of the seamless steel pipe, a width of 25.4 mm, and a gauge length of 50.8 mm. Tensile tests were conducted on the tensile test specimens for each test number at room temperature (25°C) in air to determine the 0.2% offset proof strength (MPa). The determined 0.2% offset proof strength was defined as the yield strength (MPa). The yield strengths obtained for each test number are shown in the "YS (MPa)" column of Table 2. Note that all steel materials for each test number except test number 41 met the yield strength requirement of 413 to 1035 MPa. As mentioned above, a tensile test was not performed for test number 41, so it is marked with "-" (no evaluation) in Table 2.

[0134] [Corrosion Test] Corrosion tests were conducted on the steel materials for each test number except test number 41 to evaluate their resistance to overall corrosion and pitting corrosion in a supercritical corrosion environment. Specifically, test specimens for corrosion testing were prepared using the method described above. Corrosion tests were conducted on the prepared test specimens under the conditions described above to determine the corrosion rate (mm / year). Furthermore, the presence or absence of pitting corrosion was checked on the test specimens after the corrosion test using the method described above. The obtained corrosion rates (mm / year) are shown in Table 2. Test numbers in which no pitting corrosion was confirmed are indicated with "E (Excellent)" in the "Pitting Corrosion" column of Table 2. Test numbers in which pitting corrosion was confirmed are indicated with "NA (Not Acceptable)" in the "Pitting Corrosion" column of Table 2. As mentioned above, no corrosion test was conducted on test number 41, so "-" (no evaluation) is shown in Table 2.

[0135] [Evaluation Results] Referring to Tables 1-1, 1-2, 1-3, and 2, the duplex stainless steel materials of test numbers 1 to 27 had an appropriate chemical composition, an Fn1 of 40.0 or higher, and the manufacturing method also met the conditions of the preferred manufacturing method described above. Furthermore, these steel materials had a microstructure consisting of ferrite and austenite, and the total number density ND of specific inclusions was 0.50 particles / mm³. ２The following is the case where Fn2 was less than 0.50. As a result, the corrosion rate of these steel materials was 0.100 mm / year or less, and pitting corrosion was not confirmed. That is, these steel materials had excellent general corrosion resistance and pitting corrosion resistance even in a supercritical corrosion environment.

[0136] On the other hand, the steel material of test number 28 had too low Cr content. As a result, the corrosion rate of this steel material exceeded 0.100 mm / year. That is, this steel material did not have excellent general corrosion resistance in a supercritical corrosion environment.

[0137] The steel material of test number 29 had too low Cu content. As a result, the corrosion rate of this steel material exceeded 0.100 mm / year. That is, this steel material did not have excellent general corrosion resistance in a supercritical corrosion environment.

[0138] The steel material of test number 30 had too low W content. As a result, the corrosion rate of this steel material exceeded 0.100 mm / year. That is, this steel material did not have excellent general corrosion resistance in a supercritical corrosion environment.

[0139] The steel material of test number 31 had too low Co content. As a result, pitting corrosion was confirmed in the corrosion test of this steel material. That is, this steel material did not have excellent pitting corrosion resistance in a supercritical corrosion environment.

[0140] The steel material of test number 32 had too low Sn content. As a result, pitting corrosion was confirmed in the corrosion test of this steel material. That is, this steel material did not have excellent pitting corrosion resistance in a supercritical corrosion environment.

[0141] The steel material of test number 33 had too high Al content. Furthermore, the total number density ND of specific inclusions in this steel material exceeded 0.50 pieces / mm ２ As a result, pitting corrosion was confirmed in the corrosion test of this steel material. That is, this steel material did not have excellent pitting corrosion resistance in a supercritical corrosion environment.

[0142] The steel material of test number 34 had too low Ca content. Furthermore, the total number density ND of specific inclusions in this steel material was 0.50 pieces / mm ２It exceeded the limit. As a result, pitting corrosion was confirmed in the corrosion test. In other words, this steel did not have good resistance to pitting corrosion in a supercritical corrosion environment.

[0143] The steel materials in test numbers 35-37 had too low an Fn1 value. As a result, these steel materials exhibited a corrosion rate exceeding 0.100 mm / year. In other words, these steel materials did not possess excellent overall corrosion resistance in a supercritical corrosion environment.

[0144] In the steel materials tested for test numbers 38-40, the holding time t after the uniform mixing time τ following the addition of Ca was too short. Furthermore, these steel materials had a total number density ND of specific inclusions of 0.50 particles / mm³. ２ This exceeded the limit. As a result, pitting corrosion was observed in these steel materials during corrosion tests. In other words, these steel materials did not possess excellent resistance to pitting corrosion in a supercritical corrosion environment.

[0145] As mentioned above, the steel material for test number 41 developed a defect during the manufacturing process.

[0146] The steel sample for test number 42 underwent an excessively rapid cooling rate between 1350 and 1100°C. Furthermore, this steel sample had an Fn2 value exceeding 0.50. As a result, pitting corrosion was observed in the corrosion test. In other words, this steel sample lacked excellent resistance to pitting corrosion in a supercritical corrosion environment.

[0147] The embodiments of this disclosure have been described above. However, the embodiments described above are merely examples for implementing this disclosure. Therefore, this disclosure is not limited to the embodiments described above, and the embodiments described above can be modified as appropriate without departing from the spirit of this disclosure.

Claims

1. Duplex stainless steel material, with 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: 20.00 to 24.50%, Cu: 0.10 to 3.00%, Ni: 2.00 to 10.00%, Mo: 0.80 to 5.00%, W: 0.01 to 1.50%, N: 0.001 to 0.350%, Co: 0.10 to 1.00%, Sn: 0.001 to 0.050%, sol. Assuming that the content of each element is within the above range, the composition is as follows: Al: 0.050% or less, V: 0.01 to 0.50%, Ti: 0.001 to 0.050%, Ca: 0.0005 to 0.0100%, B: 0.0015 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 remainder is Fe and impurities, and that the content of each element is within the above range, When Fn1, as defined by formula (1), is 40.0 or greater, and in the duplex stainless steel material, particles with an equivalent circle diameter of 1.0 μm or more, a Mn content of 10% by mass or more, and a S content of 10% by mass or more are defined as coarse Mn sulfides, and particles with an equivalent circle diameter of 2.0 μm or more, a Ca content of 20% by mass or more, a S content of 10% by mass or more, and a Mn content of less than 10% by mass are defined as coarse Ca sulfides, then the total number density of the coarse Mn sulfides and the coarse Ca sulfides is 0.50 particles / mm ２ The following is true: The microstructure of the duplex stainless steel consists of ferrite and austenite, and the total number density of coarse Mn sulfide and coarse Ca sulfide in the ferrite is NDA particles / mm³. ２ Defined as follows, the total number density of the coarse Mn sulfide and the coarse Ca sulfide in the austenite is NDG particles / mm³. ２ When defined as above, the NDA and NDG satisfy formula (2) for a duplex stainless steel material. Fn1 = Cr + 3.3(Mo + 0.5W) + 16N + 2Ni + Cu + 2Co + 10Sn (1) NDA / NDG < 0.50 (2) Here, the elemental symbols in formula (1) are substituted with the content of the corresponding element in mass percent.

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

3. A duplex stainless steel material according to claim 1 or claim 2, wherein the duplex stainless steel material is a steel pipe.

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