Stainless steel material

Abstract

Provided is a stainless steel material having excellent general corrosion resistance and pitting corrosion resistance even in a supercritical corrosive environment. A stainless steel material according to the present disclosure has a chemical composition described in the description, results in Fn1 being at least 32.0 as defined by formula (1), and has a microstructure including, in volume fraction, 35-70% of ferrite and 0-15% of residual austenite, the balance being tempered martensite. In the stainless steel material, 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 0.60 particles / mm2 or less. The total number density NDF particles / mm2 of coarse Mn sulfides and coarse Ca sulfides in ferrite and the total number density NDO particles / mm2 of coarse Mn sulfides and coarse Ca sulfides in phases other than ferrite satisfy formula (2).　(1): Fn1=Cr+3.3(Mo+0.5W)+16N+2Ni+Cu+2Co+10Sn (2): NDO / NDF<0.60

Description

Stainless steel material

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

[0002] In oil wells and gas wells (hereinafter, oil wells and gas wells are collectively referred to as "oil wells"), there are environments containing a large amount of corrosive substances. Corrosive substances include, for example, hydrogen sulfide (H ２ S) gas and carbonic acid (CO ２ ) gas and other corrosive gases. Here, in this specification, an environment containing hydrogen sulfide and carbon dioxide gas is referred to as a "sour environment". Steel materials for oil wells used in a sour environment are required to have sulfide stress cracking resistance (Sulfide Stress Cracking resistance: hereinafter referred to as SSC resistance).

[0003] So far, stainless steel materials having excellent SSC resistance have been proposed in Japanese Patent Application Laid-Open No. 2005-336599 (Patent Document 1) and Japanese Patent Application Laid-Open No. 2015-110822 (Patent Document 2).

[0004] The stainless steel material disclosed in Patent Document 1 is a high-strength stainless steel pipe for a line pipe, and in mass%, C: 0.001 to 0.015%, Si: 0.01 to 0.5%, Mn: 0.1 to 1.8%, P: 0.03% or less, S: 0.005% or less, Cr: 15 to 18%, Ni: less than 0.5 to 5.5%, Mo: 0.5 to 3.5%, V: 0.02 to 0.2%, N: 0.001 to 0.015%, O: 0.006% or less, and the balance is Fe and impurities, and satisfies the formulas (Cr + 0.65Ni + 0.6Mo + 0.55Cu - 20C ≧ 18.5), (Cr + Mo + 0.3Si - 43.5C - 0.4Mn - Ni - 0.3Cu - 9N ≧ 11.5), and (C + N ≦ 0.025). This stainless steel material is disclosed in Patent Document 1 to have a high strength with a yield strength of 413 MPa or more and excellent sulfide stress corrosion cracking resistance.

[0005] The stainless steel material disclosed in Patent Document 2 is a high-strength seamless steel pipe for oil wells, and in mass %, C: 0.05% or less, Si: 0.5% or less, Mn: 0.15 to 1.0%, P: 0.030% or less, S: 0.005% or less, Cr: 15.5 to 17.5%, Ni: 3.0 to 6.0%, Mo: 1.5 to 5.0%, Cu: 4.0% or less, W: 0.1 to 2.5%, N: 0.15% or less, and the balance is Fe and impurities, and satisfies the formulas (-5.9×(7.82 + 27C - 0.91Si + 0.21Mn - 0.9Cr + Ni - 1.1Mo + 0.2Cu + 11N) ≧ 13.0), (Cu + Mo + 0.5W ≧ 5.8), and (Cu + Mo + W + Cr + 2Ni ≦ 34.5). This stainless steel material is disclosed in Patent Document 2 as having a high strength with a yield strength of 758 MPa or more and excellent sulfide stress corrosion cracking resistance.

[0006] Japanese Patent Application Laid-Open No. 2005-336599 Japanese Patent Application Laid-Open No. 2015-110822

[0007] By the way, in recent years, the increase in the concentration of carbon dioxide (CO ２ ) on the ground 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 been attracting attention. CCUS is an abbreviation for Carbon dioxide Capture, Utilization and Storage. That is, CCUS includes three technologies: the recovery, utilization, and storage of CO ２ . Among these, as a technology for storing CO ２ , a technology that recovers CO ２ emitted from industrial facilities such as power plants and factories and injects and stores CO ２ into depleted oil wells has been attracting attention.

[0008] Here, in the above-described CO ２ storage technology, in order to inject CO ２ into a depleted oil well, the CO ２ injected into the steel pipe is compressed and pressurized to bring CO ２ into a supercritical state. On the other hand, the CO ２ recovered from industrial facilities such as power plants and factoriesIt 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."

[0009] 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 stainless steel materials disclosed in Patent Documents 1 and 2 are not intended for use in such supercritical corrosion environments.

[0010] The purpose of this disclosure is to provide stainless steel materials that have excellent overall corrosion resistance and pitting corrosion resistance, even in supercritical corrosion environments.

[0011] The stainless steel material according to this disclosure has a chemical composition in mass percent of: C: 0.050% or less, Si: 1.00% or less, Mn: 1.00% or less, P: 0.050% or less, S: 0.0050% or less, Cr: 13.50 to 20.50%, Ni: 3.50 to 9.00%, Mo: 1.50 to 6.00%, Cu: 0.01 to 3.00%, Co: 0.05 to 1.00%, Ca: 0.0005 to 0.0100%, Sn: 0.0003 to 0.0100%, N: 0.200% or less, sol. Al: 0.100% or less, O: 0.020% or less, W: 0 to 1.80%, Mg: 0 to 0.0100%, B: 0 to 0.0050%, Rare earth elements: 0 to 0.30%, V: 0 to 0.50%, Ti: 0 to 0.300%, Ta: 0 to 0.30%, Nb: 0 to 0.30%, Zr: 0 to 0.30%, Zn: 0 to 0.0100%, Pb: 0 to 0.0100%, Sb: 0 to 0.0100%, As: 0 to 0.0100%, and the remainder being Fe and impurities, and assuming that the content of each element is within the above range, Fn1 as defined by formula (1) is 32.0 or more, and in the stainless steel material, When particles with an equivalent 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 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, the total number density of the coarse Mn sulfides and the coarse Ca sulfides is 0.60 particles / mm². ２ The following is the case: The microstructure of the stainless steel material consists of 35-70% ferrite, 0-15% retained austenite, and the remainder being tempered martensite, and the total number density of the coarse Mn sulfide and coarse Ca sulfide in the ferrite of the microstructure is NDF particles / mm³. ２ Defined as follows, the total number density of the coarse Mn sulfide and the coarse Ca sulfide in the phase other than the ferrite in the microstructure is NDO particles / mm³. ２When defined as above, the NDF and NDO satisfy equation (2). Fn1 = Cr + 3.3(Mo + 0.5W) + 16N + 2Ni + Cu + 2Co + 10Sn (1) NDO / NDF < 0.60 (2) Here, the elemental symbols in equation (1) are substituted with the content of the corresponding element in mass percent. If the corresponding element is not contained, "0" is substituted for that elemental symbol.

[0012] The stainless steel material according to this disclosure has excellent overall corrosion resistance and pitting corrosion resistance, even in supercritical corrosion environments.

[0013] The inventors of this invention have identified SOx and O ２ Supercritical CO2 containing ２ We investigated stainless steel materials that possess excellent resistance to overall corrosion and pitting corrosion in supercritical corrosion environments formed by [unspecified conditions].

[0014] 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: 1.00% or less, P: 0.050% or less, S: 0.0050% or less, Cr: 13.50-20.50%, Ni: 3.50-9.00%, Mo: 1.50-6.00%, Cu: 0.01-3.00%, Co: 0.05-1.00%, Ca: 0.0005-0.0100%, Sn: 0.0003-0.0100%, N: 0.200% or less, sol. We considered that a steel material having a chemical composition of Al: 0.100% or less, O: 0.020% or less, W: 0 to 1.80%, Mg: 0 to 0.0100%, B: 0 to 0.0050%, rare earth elements: 0 to 0.30%, V: 0 to 0.50%, Ti: 0 to 0.300%, Ta: 0 to 0.30%, Nb: 0 to 0.30%, Zr: 0 to 0.30%, Zn: 0 to 0.0100%, Pb: 0 to 0.0100%, Sb: 0 to 0.0100%, As: 0 to 0.0100%, with the remainder being Fe and impurities, could potentially provide excellent overall corrosion resistance and excellent pitting corrosion resistance in a supercritical corrosion environment.

[0015] On the other hand, even stainless steel materials having the above-mentioned chemical composition sometimes did not exhibit sufficient overall corrosion resistance in supercritical corrosion environments. Therefore, the inventors further investigated means to improve overall corrosion resistance in supercritical corrosion environments.

[0016] 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 stainless steel materials.

[0017] Therefore, the inventors further investigated the relationship between the content of Cr, Mo, W, N, Ni, Cu, Co, and Sn in stainless steel materials and their resistance to overall corrosion in supercritical corrosion environments. As a result, it was found that in stainless steel materials having the above-mentioned chemical composition, if Fn1, as defined by formula (1), is 32.0 or higher, the resistance to overall corrosion in supercritical corrosion environments can be enhanced. Fn1 = Cr + 3.3(Mo + 0.5W) + 16N + 2Ni + Cu + 2Co + 10Sn (1) Here, the content of the corresponding element in mass percent is substituted for the element symbol in formula (1). If the corresponding element is not contained, "0" is substituted for that element symbol.

[0018] On the other hand, even stainless steel materials having the above-mentioned chemical composition and satisfying an Fn1 of 32.0 or higher sometimes failed to provide sufficient pitting corrosion resistance in supercritical corrosion environments. Therefore, the inventors further investigated means to improve pitting corrosion resistance in supercritical corrosion environments.

[0019] As mentioned above, supercritical corrosion environments include high-temperature, high-pressure supercritical CO2. ２ Not only gases, but also SOx gas and O ２It 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 stainless steel, these inclusions are more likely to dissolve. When 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.

[0020] 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 stainless steel material become the starting point for pitting corrosion. Rather, coarse Mn sulfides and coarse Ca sulfides dissolve, forming depressions on the surface of the steel material. 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 stainless steel material having the above-mentioned chemical composition.

[0021] 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."

[0022] 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 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 0.60 particles / mm³. ２ We hypothesized that, under the following conditions, stainless steel materials with the aforementioned chemical composition could achieve excellent pitting corrosion resistance.

[0023] On the other hand, if the chemical composition described above is satisfied, Fn1 is 32.0 or higher, and the total number density of specific inclusions is 0.60 particles / mm³, then the material satisfies the above-mentioned chemical composition, has a total number density of 32.0 or higher, and has a total number density of 0.60 particles / mm³. 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 tempered martensite of the stainless steel microstructure.

[0024] Here, the stainless steel material having the above-described chemical composition has a microstructure consisting of 35-70% ferrite, 0-15% retained austenite, and the remainder being tempered martensite, by volume fraction. In this specification, "consisting of ferrite, retained austenite, and tempered martensite" means that the amount of phases other than ferrite, retained austenite, and tempered martensite is negligibly small.

[0025] Here, retained austenite is easily dispersed finely within the microstructure. Therefore, retained austenite does not significantly affect the corrosion resistance of the steel. Furthermore, ferrite has superior corrosion resistance to tempered martensite compared to ferrite. For this reason, in stainless steel materials with the above-mentioned chemical composition, there is a concern that selective pitting corrosion may occur in the tempered martensite. Therefore, the inventors considered that if specific inclusions in the stainless steel material could be concentrated in the ferrite, it might be possible to suppress the occurrence of selective pitting corrosion in the tempered martensite.

[0026] Based on the above findings, the inventors conducted a detailed study and found that the chemical composition satisfies the above requirements, Fn1 is 32.0 or higher, and the total number density of specific inclusions is 0.60 particles / mm³. ２ After reducing the following, the total number density NDF (numbers / mm³) of specific inclusions in ferrite is calculated. ２ ) and the total number density NDO (numbers / mm³) of specific inclusions in the phase other than ferrite. ２It 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. NDO / NDF < 0.60 (2)

[0027] Fn2 is defined as NDO / NDF. As mentioned above, retained austenite is easily dispersed finely within the microstructure. Therefore, specific inclusions in phases other than ferrite essentially correspond to specific inclusions in tempered martensite. In other words, Fn2 is an indicator of the extent to which specific inclusions (coarse Mn sulfides and coarse Ca sulfides) are present in the tempered martensite. If Fn2 is less than 0.60, the number density of specific inclusions in the tempered martensite 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 tempered martensite can be suppressed. On the other hand, if Fn2 is 0.60 or higher, there is a concern that the tempered martensite contains many specific inclusions and selective pitting corrosion occurs.

[0028] Therefore, the stainless steel material according to this embodiment has the above-described chemical composition, an Fn1 of 32.0 or higher, and a total number density of specific inclusions of 0.60 particles / mm³. ２ The following conditions are met, and furthermore, Fn2 is set to less than 0.60. As a result, the stainless steel material according to this embodiment has excellent overall corrosion resistance and pitting corrosion resistance even in a supercritical corrosion environment.

[0029] Based on the above findings, the gist of the stainless steel material according to this embodiment is as follows:

[0030] [1] Stainless steel material having a chemical composition in mass% of: C: 0.050% or less, Si: 1.00% or less, Mn: 1.00% or less, P: 0.050% or less, S: 0.0050% or less, Cr: 13.50 to 20.50%, Ni: 3.50 to 9.00%, Mo: 1.50 to 6.00%, Cu: 0.01 to 3.00%, Co: 0.05 to 1.00%, Ca: 0.0005 to 0.0100%, Sn: 0.0003 to 0.0100%, N: 0.200% or less, sol. Al: 0.100% or less, O: 0.020% or less, W: 0 to 1.80%, Mg: 0 to 0.0100%, B: 0 to 0.0050%, Rare earth elements: 0 to 0.30%, V: 0 to 0.50%, Ti: 0 to 0.300%, Ta: 0 to 0.30%, Nb: 0 to 0.30%, Zr: 0 to 0.30%, Zn: 0 to 0.0100%, Pb: 0 to 0.0100%, Sb: 0 to 0.0100%, As: 0 to 0.0100%, and the remainder being Fe and impurities, and assuming that the content of each element is within the above range, Fn1 as defined by formula (1) is 32.0 or more, and in the stainless steel material, When particles with an equivalent 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 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, the total number density of the coarse Mn sulfides and the coarse Ca sulfides is 0.60 particles / mm². ２ The following is the case: The microstructure of the stainless steel material consists of 35-70% ferrite, 0-15% retained austenite, and the remainder being tempered martensite, and the total number density of the coarse Mn sulfide and coarse Ca sulfide in the ferrite of the microstructure is NDF particles / mm³. ２ Defined as follows, the total number density of the coarse Mn sulfide and the coarse Ca sulfide in the phase other than the ferrite in the microstructure is NDO particles / mm³. ２When defined as above, the stainless steel material such that the NDF and NDO satisfy formula (2): Fn1 = Cr + 3.3(Mo + 0.5W) + 16N + 2Ni + Cu + 2Co + 10Sn (1) NDO / NDF < 0.60 (2) Here, the elemental symbols in formula (1) are substituted with the content of the corresponding element in mass percent. If the corresponding element is not contained, "0" is substituted for that elemental symbol.

[0031] [2] The stainless steel material described in [1], wherein the chemical composition is: W: 0.01 to 1.80%, Mg: 0.0001 to 0.0100%, B: 0.0001 to 0.0050%, rare earth elements: 0.01 to 0.30%, V: 0.01 to 0.50%, Ti: 0.001 to 0.300%, Ta: 0.01 to 0.30%, Nb: 0.01 to 0.30%, Zr: 0.01 to 0.30%, Zn: 0.0001 to 0.0100%, Pb: 0.0001 to 0.0100%, Sb: 0.0001 to 0.0100%, and Stainless steel material containing one or more elements selected from the group consisting of As: 0.0001 to 0.0100%.

[0032] The shape of the stainless steel material according to this embodiment is not particularly limited. The stainless steel material according to this embodiment may be a steel pipe, a round 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. The steel pipe may be a seamless steel pipe or a welded steel pipe.

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

[0034] [Chemical Composition] The chemical composition of the stainless steel material according to this embodiment contains the following elements. Unless otherwise specified, "%" for elements means mass percent.

[0035] 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 carbides, increasing corrosion susceptibility. Therefore, if the C content is too high, even if the content of other elements is within the range of this embodiment, a large number of coarse carbides will be formed, reducing the corrosion resistance of the steel material in a supercritical corrosion environment. Therefore, the C content is 0.050% or less. The preferred upper limit of the C content is 0.045%, more preferably 0.040%, and even more preferably 0.030%. 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 even more preferably 0.005%.

[0036] 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 hot workability 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 0.95%, more preferably 0.90%, more preferably 0.70%, more preferably 0.60%, and more preferably 0.50%. The preferred lower limit of the Si content to more effectively obtain the above effects is 0.10%, more preferably 0.15%, more preferably 0.20%, and more preferably 0.25%.

[0037] Manganese (Mn) is inevitably present in amounts of 1.00% or less. In other words, the lower limit of the Mn content is greater than 0%. Mn increases the hardenability of steel and thus increases its strength. 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 corrosion resistance of the steel in a supercritical corrosion environment will decrease. For this reason, the Mn content is 1.00% or less. The preferred lower limit of the Mn content to effectively obtain the above effects is 0.01%, more preferably 0.03%, more preferably 0.05%, and still more preferably 0.07%. The preferred upper limit of the Mn content is 0.80%, more preferably 0.70%, more preferably 0.60%, and still more preferably 0.55%.

[0038] 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 in a supercritical corrosion environment will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the P content is 0.050% or less. The preferred upper limit of the P content is 0.040%, more preferably 0.030%, and even more preferably 0.025%. 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%, more preferably 0.003%, and even more preferably 0.005%.

[0039] 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 in a supercritical corrosion environment will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the S content is 0.0050% or less. The preferred upper limit of the S content is 0.0040%, more preferably 0.0030%, more preferably 0.0020%, and still more preferably 0.0015%. 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 still more preferably 0.0004%.

[0040] Cr: 13.50-20.50% Chromium (Cr) forms a passive film on the surface of the steel material, improving its corrosion resistance in supercritical corrosion environments. 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 13.50-20.50%. The preferred lower limit of the Cr content is 13.55%, more preferably 13.60%, and still more preferably 13.70%. The preferred upper limit of the Cr content is 20.40%, more preferably 20.20%, and still more preferably less than 20.00%.

[0041] Ni: 3.50-9.00% Nickel (Ni) enhances the corrosion resistance of steel materials in supercritical corrosion environments. If the Ni 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 Ni content is too high, even if the content of other elements is within the range of this embodiment, the volume fraction of retained austenite becomes too high, and the strength of the steel material decreases. Therefore, the Ni content is 3.50-9.00%. The preferred lower limit of the Ni content is 3.55%, more preferably 3.60%, still more preferably 3.80%, and still more preferably 4.00%. The preferred upper limit of the Ni content is 8.90%, more preferably 8.70%, and still more preferably 8.50%.

[0042] Mo: 1.50-6.00% Molybdenum (Mo) enhances the 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 low-temperature toughness of steel materials in cryogenic environments will decrease even if the content of other elements is within the range of this embodiment. Therefore, the Mo content is 1.50-6.00%. The preferred lower limit of the Mo content is 1.55%, more preferably 1.60%, and even more preferably 1.80%. The preferred upper limit of the Mo content is 5.90%, more preferably 5.70%, and even more preferably 5.50%.

[0043] Cu: 0.01 to 3.00% Copper (Cu) enhances the 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 0.01 to 3.00%. The preferred lower limit of the Cu content is 0.02%, more preferably 0.03%, and even more preferably 0.05%. The preferred upper limit of the Cu content is 2.90%, more preferably 2.70%, and even more preferably 2.50%.

[0044] Co: 0.05 to 1.00% Cobalt (Co) enhances the corrosion resistance of steel materials 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 low-temperature toughness of the steel material decreases even if the content of other elements is within the range of this embodiment. Therefore, the Co content is 0.05 to 1.00%. The preferred lower limit of the Co content is 0.10%, and more preferably 0.15%. The preferred upper limit of the Co content is 0.95%, more preferably 0.90%, and still more preferably 0.80%.

[0045] Ca: 0.0005 to 0.0100% Calcium (Ca) neutralizes sulfur in steel by fixing it as sulfides, thereby improving the corrosion resistance of steel in supercritical corrosion environments. 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, the oxides in the steel become coarser, and the corrosion resistance of the steel actually decreases. Therefore, the Ca content is 0.0005 to 0.0100%. The preferred lower limit of the Ca content is 0.0006%, more preferably 0.0008%, and even more preferably 0.0010%. The preferred upper limit of the Ca content is 0.0080%, more preferably 0.0060%, and even more preferably 0.0055%.

[0046] Sn: 0.0003 to 0.0100% Tin (Sn) enhances the corrosion resistance of steel materials in supercritical corrosion environments. If the Sn 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 Sn content is too high, the low-temperature toughness of the steel material decreases even if the content of other elements is within the range of this embodiment. Therefore, the Sn content is 0.0003 to 0.0100%. The preferred lower limit of the Sn content is 0.0005%, more preferably 0.0008%, and even more preferably 0.0010%. The preferred upper limit of the Sn content is 0.0090%, more preferably 0.0080%, and even more preferably 0.0070%.

[0047] N: 0.200% or less. Nitrogen (N) is inevitably present. That is, the lower limit of the N content is greater than 0%. N enhances the corrosion resistance of steel materials in supercritical corrosion environments. On the other hand, if the N content is too high, the toughness and hot workability of the steel material will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the N content is 0.200% or less. The preferred upper limit of the N content is 0.160%, more preferably 0.130%, more preferably 0.100%, more preferably 0.090%, and more preferably 0.080%. The preferred lower limit of the N content to more effectively obtain the above effects is 0.005%, more preferably 0.010%, and more preferably 0.015%.

[0048] sol. Al: 0.100% 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 oxides will be formed, and the corrosion resistance of the steel in a supercritical corrosion environment will decrease. Therefore, the Al content is 0.100% or less. The preferred upper limit of the Al content is 0.080%, more preferably 0.075%, more preferably 0.060%, and still more preferably 0.050%. The preferred lower limit of the Al content to more effectively obtain the above effects is 0.007%, and more preferably 0.010%. In this specification, Al content refers to the content of sol. Al (acid-soluble Al).

[0049] O: 0.020% or less. Oxygen (O) is inevitably present. That is, the lower limit of the O content is greater than 0%. O forms oxides. Therefore, if the O content is too high, the corrosion resistance of the steel will decrease, even if the content of other elements is within the range of this embodiment. Accordingly, the O content is 0.020% or less. The preferred upper limit of the O content is 0.018%, more preferably 0.016%, and even more preferably 0.014%. It is preferable to have as low an O content as possible. However, an extreme reduction in the O content increases manufacturing costs. Therefore, considering industrial production, the preferred lower limit of the O content is 0.001%, more preferably 0.002%, and even more preferably 0.003%.

[0050] The remainder of the chemical composition of the 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 stainless steel material, and are acceptable within a range that does not adversely affect the stainless steel material according to this embodiment.

[0051] [Optional elements] The chemical composition of the stainless steel material according to this embodiment may further contain W in place of a portion of Fe.

[0052] W: 0-1.80% Tungsten (W) is an optional element and may not be included. That is, the W content may be 0%. If included, W, like Mo, enhances the corrosion resistance of steel materials in supercritical corrosion environments. Even if only a small amount of W is included, the above effect can be obtained to some extent. However, if the W content is too high, even if the content of other elements is within the range of this embodiment, the volume fraction of ferrite may become too high, which may reduce the strength of the steel material. Therefore, the W content is 0-1.80%. The preferred lower limit of the W content is greater than 0%, more preferably 0.01%, more preferably 0.02%, and still more preferably 0.03%. The preferred upper limit of the W content is 1.76%, more preferably 1.60%, and still more preferably 1.50%.

[0053] The chemical composition of the 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, B, and rare earth elements. All of these elements are arbitrary and enhance the hot workability of the steel material.

[0054] Mg: 0 to 0.0100% Magnesium (Mg) is an optional element and may not be included. That is, the Mg content may be 0%. If included, Mg detoxifies S in the steel by fixing it as sulfides, thereby improving the hot workability of the steel. Even if only a small amount of Mg is included, the above effect can be obtained 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, and the corrosion resistance of the steel will decrease. Therefore, the Mg content is 0 to 0.0100%. The preferred lower limit of the Mg content is greater than 0%, more preferably 0.0001%, and even more preferably 0.0005%. The preferred upper limit of the Mg content is 0.0080%, more preferably 0.0060%, more preferably 0.0040%, more preferably 0.0030%, and even more preferably 0.0020%.

[0055] B: 0 to 0.0050% Boron (B) is an optional element and may not be present. That is, the B content may be 0%. If present, B detoxifies S in the steel by fixing it as sulfide, thereby improving the hot workability of the steel. Even if only a small amount of B is present, the above effect can be obtained to some extent. However, if the B content is too high, boron nitrides will be formed even if the content of other elements is within the range of this embodiment, and the corrosion resistance of the steel will decrease. Therefore, the B content is 0 to 0.0050%. The preferred lower limit of the B content is greater than 0%, more preferably 0.0001%, and even more preferably 0.0003%. The preferred upper limit of the B content is 0.0040%, more preferably 0.0030%, even more preferably 0.0025%, and even more preferably 0.0020%.

[0056] Rare earth elements: 0-0.30% 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, and the corrosion resistance of the steel will decrease. Therefore, the REM content is 0-0.30%. The preferred lower limit of the REM content is greater than 0%, more preferably 0.01%, and even more preferably 0.02%. The preferred upper limit of the REM content is 0.28%, more preferably 0.25%, and even more preferably 0.20%.

[0057] 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.

[0058] The chemical composition of the stainless steel material according to this embodiment may further contain one or more elements selected from the group consisting of V, Ti, Ta, Nb, and Zr in place of a portion of Fe. Any of these elements are arbitrary and increase the strength of the steel material.

[0059] V: 0-0.50% Vanadium (V) is an optional element and may not be included. That is, the V content may be 0%. If included, V forms carbonitrides, increasing the strength of the steel. Even a small amount of V can provide some of the above effect. However, 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 will become too high and the corrosion resistance of the steel will decrease. Therefore, the V content is 0-0.50%. The preferred lower limit of the V content is greater than 0%, more preferably 0.01%, and even more preferably 0.02%. The preferred upper limit of the V content is 0.49%, more preferably 0.45%, more preferably 0.40%, more preferably 0.30%, and even more preferably 0.20%.

[0060] Ti: 0 to 0.300% Titanium (Ti) is an optional element and may not be included. That is, the Ti content may be 0%. If included, Ti forms carbonitrides, increasing the strength of the steel. Even a small amount of Ti can provide some of the above effect. However, 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 will become too high and the corrosion resistance of the steel will decrease. Therefore, the Ti content is 0 to 0.300%. The preferred lower limit of the Ti content is greater than 0%, more preferably 0.001%, more preferably 0.002%, and still more preferably 0.003%. The preferred upper limit of the Ti content is 0.290%, more preferably 0.280%, more preferably 0.250%, and still more preferably 0.200%.

[0061] Ta: 0-0.30% 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 a small amount of Ta can provide some of the above effect. 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 corrosion resistance of the steel will decrease. Therefore, the Ta content is 0-0.30%. The preferred lower limit of the Ta content is greater than 0%, more preferably 0.01%, more preferably 0.02%, and still more preferably 0.05%. The preferred upper limit of the Ta content is 0.28%, and more preferably 0.25%.

[0062] Nb: 0-0.30% 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 can 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 corrosion resistance of the steel will decrease. Therefore, the Nb content is 0-0.30%. The preferred lower limit of the Nb content is greater than 0%, more preferably 0.01%, more preferably 0.02%, and more preferably 0.05%. The preferred upper limit of the Nb content is 0.28%, and more preferably 0.25%.

[0063] Zr: 0-0.30% 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 can 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 corrosion resistance of the steel will decrease. Therefore, the Zr content is 0-0.30%. The preferred lower limit of the Zr content is greater than 0%, more preferably 0.01%, more preferably 0.02%, and more preferably 0.05%. The preferred upper limit of the Zr content is 0.28%, and more preferably 0.25%.

[0064] The chemical composition of the stainless steel material according to this embodiment may further contain one or more elements selected from the group consisting of Zn, Pb, Sb, and As, in place of a portion of Fe. All of these elements are arbitrary and enhance the corrosion resistance of the steel material.

[0065] Zn: 0 to 0.0100% Zinc (Zn) is an optional element and may not be included. That is, the Zn content may be 0%. If included, Zn enhances the corrosion resistance of the steel. Even a small amount of Zn can provide the above effect to some extent. However, if the Zn content is too high, the corrosion resistance of the steel may actually decrease, even if the content of other elements is within the range of this embodiment. Therefore, the Zn content is 0 to 0.0100%. The preferred lower limit of the Zn content is greater than 0%, more preferably 0.0001%, more preferably 0.0002%, and more preferably 0.0003%. The preferred upper limit of the Zn content is 0.0080%, more preferably 0.0070%, more preferably 0.0060%, more preferably 0.0050%, and more preferably 0.0040%.

[0066] Pb: 0 to 0.0100% Lead (Pb) is an optional element and may not be present. That is, the Pb content may be 0%. If present, Pb enhances the corrosion resistance of the steel. Even a small amount of Pb will provide some degree of the above effect. However, if the Pb 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 Pb content is 0 to 0.0100%. The preferred lower limit of the Pb content is greater than 0%, more preferably 0.0001%, more preferably 0.0002%, and more preferably 0.0003%. The preferred upper limit of the Pb content is 0.0090%, more preferably 0.0080%, more preferably 0.0070%, and more preferably 0.0065%.

[0067] Sb: 0 to 0.0100% Antimony (Sb) is an optional element and may not be included. That is, the Sb content may be 0%. If included, Sb enhances the corrosion resistance of the steel. Even a small amount of Sb will provide some degree of the above effect. However, if the Sb 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. Therefore, the Sb content is 0 to 0.0100%. The preferred lower limit of the Sb content is greater than 0%, more preferably 0.0001%, more preferably 0.0002%, and more preferably 0.0003%. The preferred upper limit of the Sb content is 0.0090%, more preferably 0.0080%, and more preferably 0.0075%.

[0068] As: 0 to 0.0100% Arsenic (As) is an optional element and may not be present. That is, the As content may be 0%. If present, As enhances the corrosion resistance of the steel. Even a small amount of As can provide some of the above effect. However, if the As 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. Therefore, the As content is 0 to 0.0100%. The preferred lower limit of the As content is greater than 0%, more preferably 0.0001%, more preferably 0.0002%, and still more preferably 0.0003%. The preferred upper limit of the As content is 0.0080%, more preferably 0.0075%, more preferably 0.0060%, more preferably 0.0040%, and still more preferably 0.0020%.

[0069] [Fn1] The stainless steel material according to this embodiment has an Fn1 of 32.0 or more, as defined by formula (1), assuming that the content of each element in the chemical composition is within the range described above. Fn1 = Cr + 3.3(Mo + 0.5W) + 16N + 2Ni + Cu + 2Co + 10Sn (1) Here, the element symbol in formula (1) is substituted with the mass percentage content of the corresponding element. If the corresponding element is not contained, "0" is substituted for that element symbol.

[0070] Fn1 is an index of the overall corrosion resistance of 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 stainless steel materials having the above-mentioned chemical composition. In stainless steel materials having the above-mentioned chemical composition, if Fn1 is 32.0 or higher, overall corrosion resistance in a supercritical corrosion environment is significantly improved. Therefore, in this embodiment, Fn1 is 32.0 or higher.

[0071] The preferred lower limit of Fn1 is 32.5, more preferably 32.8, and even more preferably 33.0. The upper limit of Fn1 is not particularly limited, but is substantially 69.6. The upper limit of Fn1 may also be 60.0, 55.0, or 45.0. Fn1 is obtained by rounding the obtained value to the second decimal place.

[0072] [Total number density of specific inclusions] The stainless steel material according to this embodiment has the above-described chemical composition, an Fn1 of 32.0 or higher, and furthermore, a total number density of specific inclusions of 0.60 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.

[0073] 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).

[0074] 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 0.60 inclusions / mm ２ The following reductions are achieved. As a result, provided that the other components of this embodiment are met, the stainless steel material exhibits excellent overall corrosion resistance and pitting corrosion resistance even in supercritical corrosion environments.

[0075] A preferred upper limit for the total number density ND of specific inclusions is 0.59 inclusions / mm³. ２ And more preferably 0.58 pieces / mm ２And more preferably 0.55 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 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 This is also acceptable. The method for determining the total number density ND of specific inclusions will be described later.

[0076] [Microstructure] The stainless steel material according to this embodiment has the above-described chemical composition, an Fn1 of 32.0 or more, and a microstructure consisting of 35-70% ferrite, 0-15% retained austenite, and the remainder being tempered martensite, by volume fraction.

[0077] In this specification, when the microstructure is described as "consisting of ferrite, retained austenite, and tempered martensite," it means that the amount of phases other than ferrite, retained austenite, and tempered martensite in the microstructure is negligibly small. For example, in the microstructure of the stainless steel material according to this embodiment having the above-described chemical composition, the volume fraction of precipitates and inclusions is negligibly small compared to the volume fraction of ferrite, retained austenite, and tempered martensite. In other words, the microstructure of the stainless steel material according to this embodiment may contain trace amounts of precipitates, inclusions, etc., in addition to ferrite, retained austenite, and tempered martensite.

[0078] As described above, in the microstructure of the stainless steel material according to this embodiment, the volume fraction of ferrite is 35 to 70%. If the volume fraction of ferrite is too low, the corrosion resistance of the steel material in a supercritical corrosion environment will decrease. On the other hand, if the volume fraction of ferrite is too high, the strength of the steel material will decrease. Therefore, in the microstructure of the stainless steel material according to this embodiment, the volume fraction of ferrite is 35 to 70%. The preferred lower limit of the volume fraction of ferrite is 36%, more preferably 38%, and still more preferably 40%. The preferred upper limit of the volume fraction of ferrite is 69%, more preferably 67%, and still more preferably 65%.

[0079] As described above, in the microstructure of the stainless steel material according to this embodiment, the volume fraction of retained austenite is 0 to 15%. That is, retained austenite may not be present in the microstructure of the stainless steel material according to this embodiment. On the other hand, if the volume fraction of retained austenite is too high, the strength of the steel material will decrease. Therefore, in the microstructure of the stainless steel material according to this embodiment, the volume fraction of retained austenite is 0 to 15%. The preferred upper limit of the volume fraction of retained austenite is 14%, more preferably 12%, and even more preferably 10%. The lower limit of the volume fraction of retained austenite may be greater than 0%, 1%, or 2%.

[0080] As described above, the stainless steel material according to this embodiment has a microstructure consisting of 35 to 70% ferrite by volume, 0 to 15% retained austenite, and the remainder being tempered martensite. In the microstructure of the stainless steel material according to this embodiment, the volume percentage of tempered martensite is specifically 30 to 65%. The preferred lower limit of the volume percentage of tempered martensite is 31%, more preferably 33%, and still more preferably 35%. The preferred upper limit of the volume percentage of tempered martensite is 64%, more preferably 62%, and still more preferably 60%.

[0081] In this embodiment, the volume fraction of each phase in the microstructure is determined by the following method. Specifically, the volume fraction (%) of retained austenite and the volume fraction (%) of ferrite in the microstructure of the steel material are determined by the following method. The volume fraction (%) of tempered martensite is determined by subtracting the determined volume fractions of retained austenite and ferrite from 100%.

[0082] [Method for Measuring the Volume Fraction of Retained Austenite] The volume fraction of retained austenite in the microstructure of steel is determined by X-ray diffraction. Specifically, a test specimen for measuring the volume fraction of retained austenite is prepared from the steel material according to this embodiment. If the steel material is a steel plate, the test specimen is taken from the center of the plate thickness. If the steel material is a steel pipe, the test specimen is taken from the center of the wall thickness. If the steel material is a round bar, the test specimen is taken from the R / 2 position. In this specification, the R / 2 position of a round bar means the center position of radius R in a cross section perpendicular to the axial direction of the round bar. The size of the test specimen is not particularly limited. For example, the test specimen is 15 mm × 15 mm × 2 mm thick. If the steel material is a steel plate, the thickness direction of the test specimen is the plate thickness direction. If the steel material is a steel pipe, the thickness direction of the test specimen is the pipe diameter direction. If the steel material is a round bar, the thickness direction of the test specimen is the radial direction. Using the prepared test specimens, the X-ray diffraction intensity of the (110) plane of the α-phase (martensite), the (200) plane of the α-phase, the (211) plane of the α-phase, the (111) plane of the γ-phase (retained austenite), the (200) plane of the γ-phase, and the (220) plane of the γ-phase are measured, and the integrated intensity of each plane is calculated.

[0083] In measuring the X-ray diffraction intensity, the target of the X-ray diffractometer is set to Co (CoKα rays), and the output is set to 30kV-100mA. The measurement angle (2θ) is set to 45 to 105°. After calculation, the volume fraction Vγ (%) of retained austenite is calculated for each combination (3 × 3 = 9 sets) of each face of the α phase and each face of the γ phase using equation (I). The average value of the volume fraction Vγ (%) of the 9 sets of retained austenite is defined as the volume fraction (%) of retained austenite. Vγ = 100 / {1 + (Iα × Rγ) / (Iγ × Rα)} (I) Here, Iα is the integrated intensity of the α phase. Rα is the crystallographic theoretical calculation value of the α phase. Iγ is the integrated intensity of the γ phase. Rγ is the crystallographic theoretical calculation value of the γ phase. The Rα and Rγ values ​​for each surface can be obtained using the values ​​incorporated into the residual gamma quantitative analysis system included with the RINT-TTR product manufactured by Rigaku Corporation. The volume fraction of retained austenite is calculated by rounding the obtained value to the first decimal place.

[0084] [Method for Measuring the Volume Percentage of Ferrite] The volume percentage of ferrite in the microstructure of steel material is determined by the point calculation method. Specifically, a test specimen for measuring the volume percentage of ferrite is prepared from the steel material according to this embodiment. If the steel material is a steel plate, the test specimen is taken from the center of the plate thickness. The size of the test specimen is not particularly limited. If the steel material is a steel pipe, the test specimen is taken from the center of the wall thickness. If the steel material is a round bar, the test specimen is taken from the R / 2 position. Furthermore, if the steel material is a steel plate, the observation surface of the test specimen is parallel to the rolling direction of the steel plate. If the steel material is a steel pipe, the observation surface of the test specimen is parallel to the axial direction of the steel pipe. If the steel material is a round bar, the observation surface of the test specimen is parallel to the axial direction of the round bar. After mechanical polishing the observation surface, the observation surface is electrolytically etched to reveal the microstructure. Electrolytic etching uses a mixture of electrolyte: aqua regia (a solution of hydrochloric acid and nitric acid in a 3:1 ratio) and glycerin, with a current density of 1 A / cm². ２ The electrolysis will be performed for 1 minute.

[0085] The electrolytically etched observation surface is observed using an optical microscope for 30 fields of view. Each field of view is a 250 μm × 250 μm square. The magnification is 400x. In each field of view, a person skilled in the art can distinguish between ferrite and other phases (retained austenite and tempered martensite) based on contrast. Therefore, ferrite in each field of view is identified based on contrast. The area ratio of the identified ferrite is determined by the point calculation method in accordance with JIS G 0555 (2020).

[0086] Specifically, regarding the field of view, 20 vertical lines are drawn at equal intervals from the top to the bottom of the field of view. That is, the field of view is divided into 21 regions in the left-right direction by these 20 vertical lines. Furthermore, 20 horizontal lines are drawn at equal intervals from the left edge to the right edge of the field of view. That is, the field of view is divided into 21 regions in the up-down direction by these 20 horizontal lines. At this time, the intersections of the vertical and horizontal lines are called grid points. In other words, 400 grid points are arranged at equal intervals in the field of view. In accordance with JIS G 0555 (2020), the grid points that overlap with ferrite in the field of view are counted. The number of grid points that overlap with ferrite obtained in 30 fields of view is divided by the total number of grid points (400 × 30 = 12000) and defined as the area ratio of ferrite. In this embodiment, the area ratio of ferrite obtained by the above method is defined as the volume ratio (%) of ferrite. The volume fraction of ferrite is calculated by rounding the obtained value to the first decimal place.

[0087] Using the volume fraction (%) of retained austenite obtained by the X-ray diffraction method described above and the volume fraction (%) of ferrite obtained by the point calculation method described above, the volume fraction (%) of tempered martensite in the microstructure of the steel material is calculated using the following formula: Volume fraction (%) of tempered martensite = 100 - {Volume fraction (%) of retained austenite + Volume fraction (%) of ferrite}

[0088] [Fn2] The stainless steel material according to this embodiment has the above-described chemical composition, Fn1 is 32.0 or higher, and the total number density ND of specific inclusions is 0.60 pieces / mm ２The microstructure consists of 35-70% ferrite, 0-15% retained austenite, and the remainder being tempered martensite, and furthermore, the total number density of specific inclusions in the ferrite is NDF (inclusions / mm³). ２ ) and the total number density NDO (numbers / mm³) of specific inclusions in the phase other than ferrite. ２ ) and satisfy the following equation (2): NDO / NDF < 0.60 (2)

[0089] Fn2 (= NDO / NDF) is an index indicating the extent to which specific inclusions (coarse Mn sulfides and coarse Ca sulfides) are present in the tempered martensite. As mentioned above, retained austenite is easily dispersed finely within the microstructure. Therefore, specific inclusions in phases other than ferrite essentially correspond to specific inclusions in the tempered martensite. Furthermore, ferrite has superior corrosion resistance to tempered martensite compared to ferrite. For this reason, in stainless steel materials with the above chemical composition, there is a concern that selective pitting corrosion may occur in the tempered martensite.

[0090] On the other hand, if Fn2 is less than 0.60, the number density of specific inclusions in the tempered martensite 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 tempered martensite can be suppressed. Therefore, in this embodiment, Fn2 is reduced to less than 0.60. As a result, subject to the other conditions of this embodiment being met, the stainless steel material has excellent overall corrosion resistance and pitting corrosion resistance even in supercritical corrosion environments.

[0091] As described above, in this embodiment, the volume fraction of ferrite in the microstructure of the stainless steel material is 35 to 70%, and the volume fraction of tempered martensite is 30 to 65%. In this case, if Fn2 is less than 0.60, the number density of specific inclusions in the tempered martensite will be less than the number density of specific inclusions in the ferrite. Therefore, the pitting corrosion resistance of the tempered martensite can be stably improved.

[0092] A preferred upper limit for Fn2 is 0.59, more preferably 0.58, and even more preferably 0.57. The lower limit for Fn2 is not particularly limited and may be 0.00. In the stainless steel material according to this embodiment, the lower limit for Fn2 may be 0.01, 0.10, 0.20, or 0.30.

[0093] In this embodiment, the total number density of specific inclusions is ND (number of inclusions / mm²). ２ ), and Fn2 can be determined by the following method. Specifically, a test piece for microstructural observation is prepared from the stainless steel material according to this embodiment. More specifically, if the steel material is a steel plate, a test piece is prepared from the center of the plate thickness. In this case, the observation surface of the test piece includes the rolling direction and the plate thickness direction, and does not include an area 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. In this case, the observation surface of the test piece includes the pipe axis direction and the pipe diameter direction (wall thickness direction), and does not include an area 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 center 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 piece includes the axial and radial directions, and does not include an area of ​​1 mm in the radial direction from the surface of the round steel bar. The size of the test specimen is not particularly limited, but the observation surface area 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 above observations may also be obtained.

[0094] 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) to identify particles in the observation surface based on 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 for 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 mass%, particles with a Mn content of 10 mass% or more and an S content of 10 mass% or more are identified as "Mn sulfide". Similarly, based on the EDS analysis results of each particle, if 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, then 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 sulfide".

[0095] 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 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". Based on the total number of specific inclusions counted on the observation surface and the area of ​​the observation surface, the total number density of specific inclusions ND (particles / mm²) is calculated. ２ )

[0096] 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 where the tissue has been revealed is observed with an optical microscope, and ferrite is identified from the contrast. As mentioned above, ferrite and other phases (retained austenite and tempered martensite) can be identified from the contrast within the observation surface.

[0097] For each specific inclusion on the observation surface, it is determined whether it is contained in ferrite or in a phase other than ferrite (tempered martensite and retained austenite). As mentioned above, since retained austenite is finely dispersed, specific inclusions in phases other than ferrite can be considered as specific inclusions contained in tempered martensite. The specific inclusions contained in ferrite on the observation surface are counted. The number of specific inclusions contained in phases other than ferrite on the observation surface is obtained 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 measuring 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 may be obtained for the specific inclusions identified by the method described above.

[0098] 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 (NDF) of specific inclusions in the 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 non-ferrite phase counted on all observation surfaces and the total area of ​​the non-ferrite phase on all observation surfaces, the total number density NDO (numbers / mm³) of specific inclusions in the non-ferrite phase is calculated. ２ ) is calculated. Note that the total area of ​​the non-ferrite phase 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.

[0099] Using the total number density NDF of specific inclusions in the obtained ferrite and the total number density NDO of specific inclusions in the non-ferrite phase, Fn2 (= NDO / NDF) is calculated. Note that the total number density ND (inclusions / mm³) of specific inclusions is calculated as follows: ２), total number density NDF (numbers / mm³) of specific inclusions in ferrite ２ ), the total number density NDO (numbers / mm³) of specific inclusions in the phase other than ferrite. 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.

[0100] [Resistance to overall corrosion and pitting corrosion] The stainless steel material according to this embodiment has the above-described chemical composition, an Fn1 of 32.0 or higher, and a total number density ND of specific inclusions of 0.60 particles / mm ２ The following microstructure is found: it has a microstructure consisting of 35-70% ferrite by volume, 0-15% retained austenite, and the remainder being tempered martensite, with an Fn2 of less than 0.60. As a result, the 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.

[0101] Specifically, a test specimen for corrosion testing is prepared from the 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.

[0102] Place the test specimen in the autoclave and add 0.003 volume% SO ２ Gas and 0.01 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. ２The 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.

[0103] 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.

[0104] [Yield Strength] The yield strength of the stainless steel material according to this embodiment is not particularly limited. For example, the yield strength of the stainless steel material according to this embodiment is 552 to 965 MPa. In this embodiment, the lower limit of the yield strength of the stainless steel material may be 553 MPa, 556 MPa, or 565 MPa. In this embodiment, the upper limit of the yield strength of the stainless steel material may be 958 MPa, 951 MPa, or 945 MPa.

[0105] The yield strength of the 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.

[0106] 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.

[0107] [Shape of Stainless Steel Material] As described above, the shape of the stainless steel material according to this embodiment is not particularly limited. Preferably, the stainless steel material according to this embodiment is a seamless steel pipe. When the stainless steel material according to this embodiment is a seamless steel pipe, even if the wall thickness is 5 mm or more, it has excellent corrosion resistance in supercritical corrosion environments (resistance to overall corrosion and resistance to stress corrosion cracking), and excellent low-temperature toughness in cryogenic environments.

[0108] [Manufacturing Method] An example of a manufacturing method for stainless steel material according to this embodiment, having the above-described configuration, will be explained. Note that the manufacturing method for stainless steel material according to this embodiment is not limited to the manufacturing method described below. The example of the manufacturing method for stainless steel material according to this embodiment includes a steelmaking process, a hot working process, a quenching process, and a tempering process. Each manufacturing process will be described in detail below.

[0109] [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.

[0110] [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 ２O 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.

[0111] 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.

[0112] 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 the 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).

[0113] 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.

[0114] 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 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.

[0115] 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.

[0116] [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.

[0117] In stainless steel materials having the chemical composition described above, the temperature range in which ferrite is formed during the solidification of molten steel 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 ferrite. Specifically, if the cooling rate of the material in the 1350 to 1100°C range is slower than usual, ferrite will preferentially nucleate and grow from the vicinity of the inclusions. As a result, inclusions are more easily incorporated near the center of the ferrite grains. As the temperature of the steel decreases further, austenite is formed from the interface of the ferrite grains. Furthermore, the austenite thus formed is transformed into martensite in the quenching process described later. In other words, if the cooling rate of the material in the 1350 to 1100°C range is slower than usual, inclusions are more likely to be unevenly distributed within the ferrite in the manufactured steel material.

[0118] On the other hand, if the cooling rate of the material between 1350 and 1100°C is too fast, the material will become supercooled, and the formation and growth of ferrite will tend to occur randomly. As a result, the uneven distribution of specific inclusions in the ferrite may not occur in the manufactured stainless steel, and the Fn2 may exceed 0.60. Conversely, if the cooling rate of the material between 1350 and 1100°C is too slow, the ferrite will tend to coarseen. As a result, the desired mechanical properties may not be obtained in the manufactured stainless steel.

[0119] 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.

[0120] 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.

[0121] [Hot Working Process] In the hot working process, the manufactured material is hot-worked to produce intermediate steel material. If the steel material is a seamless steel pipe, the intermediate steel material corresponds to the raw pipe. First, the billet is heated in a heating furnace. The heating temperature is not particularly limited, but for example, it is 1100 to 1300°C. Hot working is performed on the billet extracted from the heating furnace to produce the raw pipe (seamless steel pipe). The method of hot working is not particularly limited and any well-known method may be used.

[0122] For example, the Mannesmann process may be used as a hot working method to produce the raw pipe. In this case, a round billet is perforated and rolled using a perforating machine. When perforating and rolling, the perforation ratio is not particularly limited, but for example, it is 1.0 to 4.0. The perforated and rolled round billet is further hot-rolled using a mandrel mill, reducer, sizing mill, etc., to produce the raw pipe. The cumulative reduction in surface area during the hot working process is, for example, 20 to 70%. The raw pipe may also be produced from the billet using other hot working methods. For example, if the steel material is a short, thick-walled steel pipe such as a coupling, the raw pipe may be produced by forging using the Erhardt process or similar methods. The raw pipe is produced by the above process. The wall thickness of the raw pipe is not particularly limited, but for example, it is 9 to 60 mm.

[0123] 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 1100 to 1300°C. Hot working is performed on the material extracted from the heating furnace to produce 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. If the steel material is steel plate, first the material is heated in a heating furnace. The heating temperature is not particularly limited, but for example it is 1100 to 1300°C. Hot rolling is performed on the material extracted from the heating furnace using a bract rolling mill and a continuous rolling mill to produce intermediate steel material in the shape of a steel plate.

[0124] Intermediate steel materials manufactured by hot working may be air-cooled (As-Rolled). Intermediate steel materials manufactured by hot working may be quenched directly after hot working without cooling to room temperature, or they may be reheated after hot working before quenching. When quenching is performed directly after hot working or after reheating, cooling may be stopped or slowed during quenching. In this case, the occurrence of quenching cracks in the raw pipe can be suppressed. When quenching is performed directly after hot working or after reheating, stress relief annealing (SR) may be performed after quenching but before the next heat treatment process. In this case, residual stress in the raw pipe is removed.

[0125] [Quenching Process] In the quenching process, the intermediate steel material produced in the hot working process is subjected to quenching. Quenching is carried out by a well-known method. Specifically, the intermediate steel material after the hot working process may be placed in a heat treatment furnace, held at the quenching temperature, and then rapidly cooled (quenched). In this case, the temperature (°C) of the heat treatment furnace used to heat the intermediate steel material in the quenching process is also called the quenching temperature.

[0126] If the quenching temperature is too low, the intermediate steel material may not be heated sufficiently, and the resulting stainless steel material may not have the microstructure described above. On the other hand, if the quenching temperature is too high, the volume fraction of ferrite may become too high, and the desired mechanical properties may not be obtained. Therefore, in this embodiment, the preferred quenching temperature is 850 to 1150°C. The holding time at the quenching temperature is not particularly limited, but for example, it is 5 to 80 minutes.

[0127] The method of rapid cooling in the quenching process is not particularly limited, but for example, water cooling is used. If the intermediate steel material is a raw tube, for example, the raw tube may be rapidly cooled by immersion in a water tank or oil tank, or the raw tube may be rapidly cooled by shower cooling or mist cooling, by pouring or spraying cooling water onto the outer and / or inner surface of the raw tube.

[0128] Furthermore, after the hot working process, the intermediate steel material may be quenched (direct quenching) immediately after hot working without cooling it to room temperature, or the raw tube may be placed in a supplemental heating furnace before its temperature drops after hot working, maintained at the quenching temperature, and then quenched.

[0129] [Tempering Process] After quenching, the intermediate steel material undergoes a tempering process. In the tempering process, the intermediate steel material is placed in a heat treatment furnace and held at the tempering temperature to adjust the yield strength of the steel material. At this time, the temperature (°C) of the heat treatment furnace used to heat the intermediate steel material in the tempering process is also called the tempering temperature.

[0130] If the tempering temperature is too low, the strength may become too high, and the corrosion resistance of the steel may decrease. On the other hand, if the tempering temperature is too high, the desired yield strength may not be obtained. Therefore, in this embodiment, the preferred tempering temperature is 550 to 700°C. The holding time at the tempering temperature is not particularly limited, but for example, it is 10 to 180 minutes. It is well known to those skilled in the art that the yield strength of the steel can be adjusted by appropriately adjusting the tempering temperature according to the chemical composition. Therefore, the tempering conditions are adjusted so that the yield strength of the steel reaches the desired value.

[0131] Through the above steps, stainless steel material according to this embodiment can be manufactured. However, as stated above, the stainless steel material according to this embodiment is not limited to the manufacturing method described above. The stainless steel material according to this embodiment will be described in more detail below with reference to examples.

[0132] Stainless steel materials (seamless steel pipes) having the chemical compositions shown in Tables 1A to 1C were manufactured by the following method. In Tables 1A to 1C, a "-" 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 Cu content in test number 33, the W content in test number 2, and the REM, V, Ta, Nb, and Zr content in test number 1 were 0% when rounded to the third decimal place. Similarly, the Ti content in test number 1 was 0% when rounded to the fourth decimal place. Similarly, the Sn content in test number 35, and the Mg, B, Zn, Pb, Sb, and As content in test number 1 were 0% when rounded to the fifth decimal place.

[0133]

[0134]

[0135]

[0136] 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.

[0137] 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.

[0138]

[0139] 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.

[0140] After heating the manufactured billets to 1250°C, they were hot-rolled using the Mannesmann process to produce raw tubes (seamless steel tubes) with an outer diameter of 244.48 mm and a wall thickness of 13.84 mm. Furthermore, each raw tube with a test number was subjected to quenching by holding it at 910°C for 15 minutes and then rapidly cooling it. Subsequently, each raw tube with a test number was subjected to tempering by holding it at 580-680°C for 30-60 minutes. Through the above manufacturing process, seamless steel tubes with each test number were produced.

[0141] [Evaluation Tests] Microstructural observation tests, specific inclusion observation tests, tensile tests, and corrosion tests were performed on each steel sample with the corresponding test number. However, none of the evaluation tests were performed on steel sample number 44 because a defect occurred during the manufacturing process.

[0142] [Microstructure Observation Test] For each steel sample with test number 44, a microstructure observation test was conducted using the method described above to determine the volume fraction of the microstructure. Specifically, the volume fraction (%) of retained austenite was determined by X-ray diffraction using the method described above. Furthermore, the volume fraction (%) of ferrite was determined by the point calculation method in accordance with JIS G 0555 (2020), using the method described above. From the obtained volume fractions of retained austenite and ferrite, the volume fraction (%) of tempered martensite was determined. For each steel sample with test number 44, the microstructure consisted of 35-70% ferrite, 0-15% retained austenite, and the remainder being tempered martensite, in terms of volume fraction. The volume fraction of tempered martensite was between 30-65%.

[0143] [Specific Inclusion Observation Test] For each steel material with test number excluding test number 44, a specific inclusion observation test was performed using the method described above, and the total number density of specific inclusions ND (items / mm²) was determined. ２ Specifically, a specimen for microstructural observation was prepared using the method described above, and the total number density ND of specific inclusions was determined. Furthermore, Fn2 was determined using the method described above. The observation surface area was 200-300 mm². ２ Observation tests were conducted on three observation surfaces. As mentioned above, since no specific inclusion observation test was performed for test number 44, it is indicated as "-" (no evaluation) in Table 2.

[0144] [Tensile Test] For each steel material with test number 44, tensile tests were conducted in accordance with ASTM E8 / E8M (2022) using the method described above to determine the yield strength. Specifically, arc-shaped tensile test specimens were prepared for the tensile test 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 using 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 strength (YS (MPa)) obtained for each test number is shown in the "YS (MPa)" column of Table 2. As mentioned above, a tensile test was not conducted for test number 44, so "-" (no evaluation) is shown in Table 2.

[0145] [Corrosion Test] For each steel material with test number 44, excluding test number 44, corrosion tests were conducted using the method described above to evaluate the 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. For test numbers where pitting corrosion was not confirmed, "E (Excellent)" is indicated in the "Pitting Corrosion" column of Table 2. For test numbers where pitting corrosion was confirmed, "NA (Not Acceptable)" is indicated in the "Pitting Corrosion" column of Table 2. As mentioned above, no corrosion test was conducted on test number 44, so "-" (no evaluation) is shown in Table 2.

[0146] [Evaluation Results] Referring to Tables 1A, 1B, 1C, and 2, the stainless steel materials for test numbers 1 to 29 had an appropriate chemical composition, an Fn1 of 32.0 or higher, and the manufacturing method also met the conditions of the preferred manufacturing method described above. Furthermore, these steel materials had a total number density ND of specific inclusions of 0.60 pieces / mm³. ２The following conditions were observed, with Fn2 being less than 0.60. As a result, these steel materials exhibited a corrosion rate of 0.100 mm / year or less, and no pitting corrosion was observed. In other words, these steel materials possessed excellent resistance to overall corrosion and pitting corrosion, even in supercritical corrosion environments.

[0147] On the other hand, the steel material in test number 30 had too low a chromium content. As a result, the corrosion rate of this steel material exceeded 0.100 mm / year. In other words, this steel material did not have good overall corrosion resistance in a supercritical corrosion environment.

[0148] The steel material in test number 31 had too low a Ni content. As a result, the corrosion rate of this steel material exceeded 0.100 mm / year. In other words, this steel material did not have good overall corrosion resistance in a supercritical corrosion environment.

[0149] The steel material used in test number 32 had too low a molybdenum (Mo) content. As a result, pitting corrosion was observed in this steel material during the corrosion test. In other words, this steel material did not possess excellent resistance to pitting corrosion in a supercritical corrosion environment.

[0150] The steel material in test number 33 had too low a copper content. As a result, the corrosion rate of this steel material exceeded 0.100 mm / year. In other words, this steel material did not possess excellent overall corrosion resistance in a supercritical corrosion environment.

[0151] The steel material in test number 34 had too low a Co content. As a result, pitting corrosion was observed in this steel material during the corrosion test. In other words, this steel material did not possess excellent resistance to pitting corrosion in a supercritical corrosion environment.

[0152] The steel sample for test number 35 had too low a Sn content. As a result, pitting corrosion was observed in this steel sample during the corrosion test. In other words, this steel sample did not possess excellent resistance to pitting corrosion in a supercritical corrosion environment.

[0153] The steel sample in test number 36 had an excessively high aluminum content. As a result, pitting corrosion was observed in this steel sample during the corrosion test. In other words, this steel sample did not possess excellent resistance to pitting corrosion in a supercritical corrosion environment.

[0154] The steel sample for test number 37 had too low a calcium content. As a result, pitting corrosion was observed in this steel sample during the corrosion test. In other words, this steel sample did not possess excellent resistance to pitting corrosion in a supercritical corrosion environment.

[0155] The steel materials in test numbers 38-40 had too low an Fn1 value. As a result, these steel materials exhibited corrosion rates exceeding 0.100 mm / year. In other words, these steel materials did not possess excellent overall corrosion resistance in a supercritical corrosion environment.

[0156] In the steel materials tested for test numbers 41-43, 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 0.60 particles / mm³. ２ The threshold was exceeded. 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.

[0157] As mentioned above, the steel material for test number 44 developed a defect during the manufacturing process.

[0158] The steel sample for test number 45 underwent an excessively rapid cooling rate between 1350 and 1100°C. Furthermore, this steel sample had an Fn2 value exceeding 0.60. 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.

[0159] 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. Stainless steel material, with a chemical composition in mass% of: C: 0.050% or less, Si: 1.00% or less, Mn: 1.00% or less, P: 0.050% or less, S: 0.0050% or less, Cr: 13.50 to 20.50%, Ni: 3.50 to 9.00%, Mo: 1.50 to 6.00%, Cu: 0.01 to 3.00%, Co: 0.05 to 1.00%, Ca: 0.0005 to 0.0100%, Sn: 0.0003 to 0.0100%, N: 0.200% or less, sol. Al: 0.100% or less, O: 0.020% or less, W: 0 to 1.80%, Mg: 0 to 0.0100%, B: 0 to 0.0050%, Rare earth elements: 0 to 0.30%, V: 0 to 0.50%, Ti: 0 to 0.300%, Ta: 0 to 0.30%, Nb: 0 to 0.30%, Zr: 0 to 0.30%, Zn: 0 to 0.0100%, Pb: 0 to 0.0100%, Sb: 0 to 0.0100%, As: 0 to 0.0100%, and the remainder being Fe and impurities, and assuming that the content of each element is within the above range, Fn1 as defined by formula (1) is 32.0 or more, and in the stainless steel material, When particles with an equivalent 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 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, the total number density of the coarse Mn sulfides and the coarse Ca sulfides is 0.60 particles / mm². ２ The following is the case: The microstructure of the stainless steel material consists of 35-70% ferrite, 0-15% retained austenite, and the remainder being tempered martensite, and the total number density of the coarse Mn sulfide and coarse Ca sulfide in the ferrite of the microstructure is NDF particles / mm³. ２ Defined as follows, the total number density of the coarse Mn sulfide and the coarse Ca sulfide in the phase other than the ferrite in the microstructure is NDO particles / mm³. ２ When defined as above, the stainless steel material such that the NDF and NDO satisfy formula (2): Fn1 = Cr + 3.3(Mo + 0.5W) + 16N + 2Ni + Cu + 2Co + 10Sn (1) NDO / NDF < 0.60 (2) Here, the elemental symbols in formula (1) are substituted with the content of the corresponding element in mass percent. If the corresponding element is not contained, "0" is substituted for that elemental symbol.

2. The stainless steel material according to claim 1, wherein the chemical composition is: W: 0.01 to 1.80%, Mg: 0.0001 to 0.0100%, B: 0.0001 to 0.0050%, rare earth elements: 0.01 to 0.30%, V: 0.01 to 0.50%, Ti: 0.001 to 0.300%, Ta: 0.01 to 0.30%, Nb: 0.01 to 0.30%, Zr: 0.01 to 0.30%, Zn: 0.0001 to 0.0100%, Pb: 0.0001 to 0.0100%, Sb: 0.0001 to 0.0100%, and Stainless steel material containing one or more elements selected from the group consisting of As: 0.0001 to 0.0100%.

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