Stainless steel material

Abstract

Provided is a stainless steel material which has high strength, excellent corrosion resistance in a high CO2 environment, and excellent low temperature toughness in an extremely low temperature environment. A stainless steel material according to the present disclosure has the chemical composition set forth in the description. With respect to the stainless steel material, Fn1 is 22.00 or more, Fn2 is 1.00 or more, the yield strength is not less than 552 MPa but less than 758 MPa, the microstructure is composed of more than 5.0% by volume but not more than 30.0% by volume of austenite and the balance made up of ferrite and martensite, and mixed structures of austenite and martensite having a length of 10 µm or more in the C direction of the stainless steel material have an average length in the C direction of 30 µm or less, and the area fraction of austenite having an equivalent circle diameter of 3.0 µm or more is 0.0050 or less.　(1): Fn1 = Cr + 3Mo + 0.5Ni - 5.0Mn, (2): Fn2 = 7 / (1 + exp(- 18 + Mn - Ni + Cr + 0.2Mo + Cu + 0.01YS))

Description

Stainless steel material

[0001] This disclosure relates to stainless steel materials.

[0002] Currently, carbon dioxide (CO2) on Earth ２ The rising concentration of CO2 is becoming a global problem. ２ Efforts have been made to reduce CO2 emissions. ２ In the efforts to reduce emissions, CCUS (Carbon Capture and Use) has been attracting particular attention.

[0003] CCUS is an abbreviation for Carbon Dioxide Capture, Utilization and Storage. In other words, CCUS is CO ２ This includes three technologies: recovery, utilization, and storage. Of these, CO ２ As a technology for storing CO2, CO2 is used to store CO2 emitted from industrial facilities such as power plants and factories. ２ The CO2 is recovered and added to the depleted oil wells. ２ A technology for injecting and storing substances is attracting attention.

[0004] This kind of CO ２ Stainless steel materials used in storage technology are required to have a yield strength of, for example, 80 ksi or higher (552 MPa or higher). Stainless steel materials having a yield strength of 80 ksi or higher have been proposed in Japanese Patent Application Publication No. 2011-190521 (Patent Document 1) and Japanese Patent Application Publication No. 2012-149317 (Patent Document 2).

[0005] The stainless steel material described in Patent Document 1 is a martensitic stainless steel material. In terms of mass%, C: 0.003 to 0.03%, Si: 0.01 to 1.0%, Mn: 3.0 to 6.0%, P: 0.05% or less, S: 0.003% or less, Ni: 1.0 to 3.0%, Cr: 15.0 to 18.0%, Mo: 0 to 1.0%, Cu: 0 to 2.0%, Ti: 0 to 0.05%, N: 0.05% or less, Al: 0.001 to 0.1%, О: 0.005% or less, and C + N: 0.060% or less are contained, and the balance consists of Fe and impurities. γmax (= 420 × C% + 470 × N% + 23 × Ni% + 9 × Cu% + 7 × Mn% - 11.5 × Cr% - 11.5 × Si% - 52 × Al% + 189) is 80 or more, and γpot (= 700 × C% + 800 × N% + 10 × (Mn% + Cu%) + 20 × Ni% - 9.3 × Si% - 6.2 × Cr% - 9.3 × Mo% - 74.4 × Ti% - 37.2 × Al% + 63.2) is 60 to 90. This martensitic stainless steel material has a yield strength of 400 to 800 MPa.

[0006] The stainless steel material described in Patent Document 2 is a high-strength martensitic stainless seamless steel pipe for oil wells. In terms of mass%, C: 0.01% or less, Si: 0.5% or less, Mn: 0.1 to 2.0%, P: 0.03% or less, S: 0.005% or less, Cr: more than 15.5 to 17.5%, Ni: 2.5 to 5.5%, Mo: 1.8 to 3.5%, Cu: 0.3 to 3.5%, V: 0.20% or less, Al: 0.05% or less, N: 0.06% or less are contained, and the balance consists of Fe and impurities. It has a yield strength of 655 to 862 MPa and a yield ratio of 0.90 or more, and is excellent in corrosion resistance to carbon dioxide gas and sulfide pitting corrosion.

[0007] Japanese Patent Application Laid-Open No. 2011-190521 Japanese Patent Application Laid-Open No. 2012-149317

[0008] As described above, in the carbon dioxide (CO ２ ) storage technology, CO ２ is injected into depleted oil wells. At this time, the CO ２ gas to be injected may be compressed and boosted to a high pressure of about 200 atm. At this time, furthermore, the CO ２In some cases, CO2 dissolves in the geological water, creating an acidic environment inside depleted oil wells. ２ The presence of acidic compounds as impurities can sometimes create an acidic environment inside depleted oil wells. Therefore, CO ２ The steel used in CO storage technology is not only high-strength, but also high-pressure CO ２ Corrosion resistance in gas-containing environments may be required. Hereinafter, in this specification, high-pressure CO2 at 200 atm ２ The environment containing gases is called "high CO2". ２ It is also called the "environment."

[0009] In recent years, CO ２ The steel used in CO2 storage technology sometimes requires toughness in extremely low-temperature environments. Specifically, the steel used in CO2 storage technology requires toughness in extremely low-temperature environments. ２ When pressure changes occur, the Joule-Thomson effect can cause the temperature of the stored gas to decrease. In this case, steel materials may be required to have toughness in extremely low-temperature environments, such as -70°C, far below normal temperatures.

[0010] Therefore, such CO ２ The stainless steel material intended for application in storage technology has high strength and high CO2 content. ２ In addition to excellent corrosion resistance in the environment, excellent low-temperature toughness in extremely low-temperature environments below -70°C is also required. On the other hand, the stainless steel materials disclosed in Patent Documents 1 and 2 are such high CO ２ It is not intended for use in general environments, nor for use in extremely low-temperature environments.

[0011] The purpose of this disclosure is to achieve high strength and high CO ２ The objective is to provide a stainless steel material that possesses excellent corrosion resistance in environmental conditions and excellent low-temperature toughness in cryogenic environments.

[0012] The stainless steel material according to this disclosure has a chemical composition in mass percent of: C: 0.040% or less, Si: 0.01 to 1.00%, Mn: 0.01 to 0.50%, P: 0.030% or less, S: 0.0050% or less, Cr: 16.0% to less than 18.0%, Ni: greater than 5.00% to 8.00%, Mo: greater than 0.60% to 3.50%, Al: 0.001% to 0.100%, O: 0.0200% or less, N: 0.0200% or less, V: 0 to 0.30%, Ti: 0 to 0.100%, Nb: 0 to 0.100%, Ta: 0 to 0.100%, Zr: 0 to 0.100%, Cu: 0 to less than 1.00%. W: 0-0.50%, Co: 0-0.80%, Ca: 0-0.0050%, Mg: 0-0.0050%, B: 0-0.0050%, Rare earth elements: 0-0.0100%, As: 0-0.100%, Sn: 0-0.100%, Sb: 0-0.100%, Zn: 0-0.100%, Pb: 0-0.100%, and the remainder: Fe and impurities, with Fn1 defined by formula (1) being 22.00 or higher, Fn2 defined by formula (2) being 1.00 or higher, and a yield strength of 552-758 MPa. The microstructure consists of austenite comprising more than 5.0% to 30.0% by volume, with the remainder being ferrite and martensite. Among the mixed structures of austenite and martensite, the mixed structures satisfying the C-direction length of the stainless steel material being 10 μm or more have an average C-direction length of 30 μm or less. Among the austenite, the area fraction of austenite with an equivalent circular diameter of 3.0 μm or more is 0.0050 or less. Fn1 = Cr + 3Mo + 0.5Ni - 5.0Mn (1) Fn2 = 7 / (1 + exp(-18 + Mn - Ni + Cr + 0.2Mo + Cu + 0.01YS)) (2) Here, the C-direction of the stainless steel material means the thickness direction if the stainless steel material is a stainless steel plate, the diameter direction if the stainless steel material is a stainless steel pipe, and the radial direction of the cross-section perpendicular to the axial direction if the stainless steel material is a stainless steel round bar. Furthermore, the elemental symbols in formulas (1) and (2) are substituted with the content of the corresponding element in units of mass%. If the corresponding element is not present, "0" is substituted for that elemental symbol.Furthermore, in equation (2), the yield strength in units of MPa is substituted for YS.

[0013] The stainless steel material disclosed herein offers high strength and high CO2. ２ It possesses excellent corrosion resistance in environmental conditions and excellent low-temperature toughness in extremely low-temperature environments.

[0014] Figure 1 shows the relationship between the area fraction of coarse γ (austenite with an equivalent circle diameter of 3.0 μm or more) and the absorbed energy vE (J) at -70°C, which is an indicator of low-temperature toughness in an extremely low-temperature environment, in this embodiment.

[0015] The inventors first considered CO ２ With an eye towards its application to storage technology, the inventors investigated obtaining a stainless steel material having a yield strength of less than 552 to 758 MPa. In other words, the inventors investigated obtaining a stainless steel material having a yield strength of less than 552 to 758 MPa. ２ In stainless steel materials intended for application in storage technology, a yield strength of 552 to less than 758 MPa and high CO2 ２ We investigated and examined methods for obtaining excellent corrosion resistance in environmental conditions and excellent low-temperature toughness in cryogenic environments. As a result, the inventors obtained the following findings.

[0016] Initially, the inventors focused on the chemical composition to obtain a desired stainless steel material. As a result, the chemical composition was found to be as follows (by mass%): C: 0.040% or less, Si: 0.01 to 1.00%, Mn: 0.01 to 0.50%, P: 0.030% or less, S: 0.0050% or less, Cr: 16.0% to less than 18.0%, Ni: greater than 5.00% to 8.00%, Mo: greater than 0.60% to 3.50%, Al: 0.001% to 0.100%, O: 0.0200% or less, N: 0.0200% or less, V: 0 to 0.30%, Ti: 0 to 0.100%, Nb: 0 to 0.100%, Ta: 0 to 0.100%, Z If the stainless steel material consists of r: 0-0.100%, Cu: 0-1.00%, W: 0-0.50%, Co: 0-0.80%, Ca: 0-0.0050%, Mg: 0-0.0050%, B: 0-0.0050%, rare earth elements: 0-0.0100%, As: 0-0.100%, Sn: 0-0.100%, Sb: 0-0.100%, Zn: 0-0.100%, Pb: 0-0.100%, and the remainder being Fe and impurities, then it will have a yield strength of 552-758 MPa and high CO2 content. ２ We believed that this material could potentially provide excellent corrosion resistance in environmental conditions and excellent low-temperature toughness in extremely low-temperature environments.

[0017] On the other hand, even if a stainless steel material has the above-mentioned chemical composition, if it has a yield strength of less than 552 to 758 MPa, high CO ２ There were cases where it was not possible to achieve both excellent corrosion resistance in the environment and excellent low-temperature toughness in extremely low-temperature environments. Therefore, the present inventors have developed a stainless steel material having the above-mentioned chemical composition that maintains yield strength while achieving high CO2 ２ Further investigations were conducted to explore methods for improving corrosion resistance in environmental conditions and low-temperature toughness in cryogenic environments.

[0018] As a result of further investigation by the present inventors, in stainless steel materials having the above-mentioned chemical composition, if Fn1, defined by the following formula (1), is 22.00 or higher, then even if the other configurations of this embodiment are satisfied, the yield strength is less than 552 to 758 MPa, and high CO ２It has been revealed that the corrosion resistance in the environment can be enhanced. Fn1 = Cr + 3Mo + 0.5Ni - 5.0Mn (1) Here, in the element symbols in formula (1), the content of the corresponding element is substituted in units of mass%.

[0019] Fn1 is an index of the corrosion resistance of the stainless steel material having the above chemical composition in a high CO ２ environment. When Fn1 is 22.00 or more, on the condition that other configurations of the present embodiment are satisfied, the corrosion resistance of the stainless steel material in a high CO ２ environment is enhanced. On the other hand, when Fn1 is less than 22.00, even if other configurations of the present embodiment are satisfied, the corrosion resistance of the stainless steel material in a high CO ２ environment cannot be sufficiently enhanced. Therefore, the stainless steel material according to the present embodiment sets Fn1 to 22.00 or more on the premise of satisfying the above chemical composition.

[0020] As a result of further studies by the present inventors, in the stainless steel material having the above chemical composition, if Fn2 defined by the following formula (2) is 1.00 or more, on the condition that other configurations of the present embodiment are satisfied, even if it has a yield strength of less than 552 to 758 MPa, it has been revealed that the low-temperature toughness in an extremely low-temperature environment can be enhanced. Fn2 = 7 / (1 + exp(−18 + Mn − Ni + Cr + 0.2Mo + Cu + 0.01YS)) (2) Here, in the element symbols in formula (2), the content of the corresponding element is substituted in units of mass%. When the corresponding element is not contained, "0" is substituted for the element symbol. Also, in YS in formula (2), the yield strength is substituted in units of MPa.

[0021] Fn2 is an index of the low-temperature toughness of the stainless steel material having the above chemical composition in an extremely low-temperature environment. When Fn2 is 1.00 or more, on the condition that other configurations of the present embodiment are satisfied, the low-temperature toughness of the stainless steel material in an extremely low-temperature environment is enhanced. On the other hand, when Fn2 is less than 1.00, even if other configurations of the present embodiment are satisfied, the low-temperature toughness of the stainless steel material in an extremely low-temperature environment cannot be sufficiently enhanced. Therefore, the stainless steel material according to the present embodiment sets Fn2 to 1.00 or more on the premise of satisfying the above chemical composition.

[0022] As a result of further studies by the present inventors, it has been clarified that in the stainless steel material having the above chemical composition, the microstructure of the stainless steel material has a significant influence on the low-temperature toughness in an extremely low-temperature environment. Therefore, the present inventors focused on the microstructure of a stainless steel material having the above chemical composition, satisfying Fn1 of 22.00 or more, Fn2 of 1.00 or more, and having a yield strength of less than 552 to 758 MPa, and tried to improve the low-temperature toughness in an extremely low-temperature environment. As a result, the following findings were obtained.

[0023] First, the microstructure of the stainless steel material having the above chemical composition consists of austenite, ferrite, and martensite. Furthermore, as a result of studies by the present inventors, it was considered that if the microstructure has austenite of more than 5.0 to 30.0% by volume fraction and the balance consists of ferrite and martensite, there is a possibility of improving the low-temperature toughness in an extremely low-temperature environment.

[0024] Here, in the microstructure of the stainless steel material having the above chemical composition, austenite is finely dispersed and present in martensite. Hereinafter, in this specification, martensite in which austenite is dispersed is also referred to as "mixed structure". The mixed structure has a martensite matrix and high hardness. Therefore, if a coarse mixed structure is formed, there is a concern that the toughness of the stainless steel material may decrease.

[0025] Therefore, the present inventors carefully studied the mixed structure that is likely to reduce the low-temperature toughness in an extremely low-temperature environment. As a result, it has been clarified that in an extremely low-temperature environment, the size in the C direction of the steel material with the mixed structure affects the low-temperature toughness. Here, the C direction of the stainless steel material means the plate thickness direction of the steel plate when the stainless steel material is a stainless steel plate, the pipe diameter direction of the steel pipe when the stainless steel material is a stainless steel pipe, and the radial direction of the cross section perpendicular to the axial direction of the round steel when the stainless steel material is a stainless steel round bar.

[0026] Next, among the mixed structures in the microstructure of stainless steel, a mixed structure satisfying a length of 10 μm or more in the C direction is defined as a "coarse mixed structure." As a result of the inventors' studies, it has become clear that the stainless steel material according to this embodiment has excellent low-temperature toughness even in an extremely low-temperature environment, provided that the average length of the coarse mixed structure in the C direction is 30 μm or less, and the other configurations of this embodiment are also satisfied.

[0027] Further investigations by the inventors revealed that in cryogenic environments, not only the size of the mixed structure but also the size of the austenite within the mixed structure significantly affects low-temperature toughness. In other words, simply reducing the size of the coarse mixed structure is not enough; if a large number of fine austenite particles are dispersed within the mixed structure after reducing the size of the coarse mixed structure, it may be possible to suppress the decrease in low-temperature toughness caused by the mixed structure, even in cryogenic environments.

[0028] Therefore, in the stainless steel material according to this embodiment, specifically, the area fraction of austenite with an equivalent circular diameter of 3.0 μm or more is set to 0.0050 or less. As a result, it has become clear that excellent low-temperature toughness can be obtained even in an extremely low-temperature environment, provided that the other configurations of this embodiment are satisfied. Hereinafter, in this specification, austenite will also be referred to as "γ". Furthermore, in this specification, austenite with an equivalent circular diameter of 3.0 μm or more will also be referred to as "coarse γ". The relationship between the area fraction of coarse γ and low-temperature toughness in an extremely low-temperature environment will be explained below with reference to the drawings.

[0029] Figure 1 shows the relationship between the area fraction of coarse γ (austenite with an equivalent circle diameter of 3.0 μm or more) and the absorbed energy vE (J) at -70°C, which is an indicator of low-temperature toughness in an extremely low-temperature environment, in this embodiment. Figure 1 was prepared using the area fraction of coarse γ obtained by the method described later and the absorbed energy vE (J) at -70°C obtained by the method described later for a stainless steel material that has the above-described chemical composition, satisfies Fn1 of 22.00 or more, satisfies Fn2 of 1.00 or more, has a yield strength of 552 to less than 758 MPa, has a microstructure consisting of more than 5.0 to 30.0% austenite by volume fraction and the remainder being ferrite and martensite, and further satisfies the average length in the C direction of the mixed structure that satisfies the length in the C direction of 10 μm or more and 30 μm or less, using the method described later.

[0030] Referring to Figure 1, in a stainless steel material having the above-described chemical composition, satisfying Fn1 of 22.00 or more, satisfying Fn2 of 1.00 or more, having the above-described yield strength, and having the above-described microstructure, if the area fraction of coarse γ is 0.0050 or less, the absorbed energy vE at -70°C will be 40 J or more, confirming that it has excellent low-temperature toughness even in an extremely low-temperature environment. On the other hand, if the area fraction of coarse γ exceeds 0.0050, the absorbed energy vE at -70°C will be less than 40 J, confirming that it does not have excellent low-temperature toughness in an extremely low-temperature environment.

[0031] Therefore, the stainless steel material according to this embodiment has the above-described chemical composition, satisfies Fn1 of 22.00 or more, satisfies Fn2 of 1.00 or more, has a yield strength of 552 to less than 758 MPa, has a microstructure consisting of more than 5.0 to 30.0% austenite by volume fraction and the remainder being ferrite and martensite, satisfies the average length in the C direction of the mixed structure having a C direction length of 10 μm or more and 30 μm or less, and further satisfies the area fraction of coarse γ of 0.0050 or less. As a result, the stainless steel material according to this embodiment has high strength and high CO ２ It possesses excellent corrosion resistance in environmental conditions and excellent low-temperature toughness in extremely low-temperature environments.

[0032] Based on the above findings, the gist of the stainless steel material according to this embodiment is as follows:

[0033] [1] Stainless steel material, having a chemical composition in mass% of: C: 0.040% or less, Si: 0.01 to 1.00%, Mn: 0.01 to 0.50%, P: 0.030% or less, S: 0.0050% or less, Cr: 16.0% to less than 18.0%, Ni: greater than 5.00% to 8.00%, Mo: greater than 0.60% to 3.50%, Al: 0.001% to 0.100%, O: 0.0200% or less, N: 0.0200% or less, V: 0 to 0.30%, Ti: 0 to 0.100%, Nb: 0 to 0.100%, Ta: 0 to 0.100%, Zr: 0 to 0.100%, Cu: 0 to less than 1.00%. W: 0-0.50%, Co: 0-0.80%, Ca: 0-0.0050%, Mg: 0-0.0050%, B: 0-0.0050%, Rare earth elements: 0-0.0100%, As: 0-0.100%, Sn: 0-0.100%, Sb: 0-0.100%, Zn: 0-0.100%, Pb: 0-0.100%, and the remainder: Fe and impurities, with Fn1 defined by formula (1) being 22.00 or higher, Fn2 defined by formula (2) being 1.00 or higher, and a yield strength of 552-758 MPa. A stainless steel material having a microstructure in which, by volume fraction, more than 5.0% to 30.0% austenite and the remainder being ferrite and martensite, wherein, among the mixed structures of austenite and martensite, the mixed structures satisfying the C-direction length of the stainless steel material to be 10 μm or more have an average C-direction length of 30 μm or less, and the area fraction of austenite with an equivalent circular diameter of 3.0 μm or more is 0.0050 or less. Fn1 = Cr + 3Mo + 0.5Ni - 5.0Mn (1) Fn2 = 7 / (1 + exp(-18 + Mn - Ni + Cr + 0.2Mo + Cu + 0.01YS)) (2) Here, the C-direction of the stainless steel material means the thickness direction if the stainless steel material is a stainless steel plate, the diameter direction if the stainless steel material is a stainless steel pipe, and the radial direction of the cross-section perpendicular to the axial direction if the stainless steel material is a stainless steel round bar. Furthermore, the elemental symbols in formulas (1) and (2) are substituted with the content of the corresponding element in units of mass percent.If the corresponding element is not present, "0" is substituted for its element symbol. Furthermore, the yield strength in units of MPa is substituted for YS in equation (2).

[0034] [2] Stainless steel material as described in [1], wherein the chemical composition is: V: 0.01 to 0.30%, Ti: 0.001 to 0.100%, Nb: 0.001 to 0.100%, Ta: 0.001 to 0.100%, Zr: 0.001 to 0.100%, Cu: 0.01 to less than 1.00%, W: 0.01 to less than 0.50%, Co: 0.01 to 0.80%, Ca: 0.0001 to 0.0050%, Mg: 0.0001 to 0.0050%, B: 0.0001 to 0.0050%, rare earth elements: 0.0001 to 0.0100%, As: 0.001 to 0.100%, A stainless steel material containing one or more elements selected from the group consisting of Sn: 0.001 to 0.100%, Sb: 0.001 to 0.100%, Zn: 0.001 to 0.100%, and Pb: 0.001 to 0.100%.

[0035] [3] A stainless steel material according to [1] or [2], wherein the stainless steel material is a stainless steel pipe, and the C direction is the radial direction of the stainless steel pipe.

[0036] 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.

[0037] 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."

[0038] [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.

[0039] C: 0.040% or less. Carbon (C) is inevitably present. That is, the lower limit of the C content is greater than 0%. C forms carbides, which increase the strength of the steel. Therefore, if the C 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 low-temperature toughness of the steel in an extremely low-temperature environment will decrease. Therefore, the C content is 0.040% or less. The preferred upper limit of the C content is 0.035%, and 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%, and more preferably 0.003%.

[0040] Si: 0.01 to 1.00% Silicon (Si) deoxidizes steel. If the Si 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 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 0.01 to 1.00%. The preferred lower limit of the Si content is 0.05%, and more preferably 0.10%. The preferred upper limit of the Si content is 0.80%, more preferably 0.60%, and still more preferably 0.50%.

[0041] Mn: 0.01-0.50% Manganese (Mn) deoxidizes and desulfurizes steel. If the Mn content is too low, the above effects cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Mn content is too high, even if the content of other elements is within the range of this embodiment, high CO2 ２ The corrosion resistance of steel materials in the environment may decrease. Therefore, the Mn content is 0.01 to 0.50%. The preferred lower limit of the Mn content is 0.02%, more preferably 0.03%, and even more preferably 0.05%. The preferred upper limit of the Mn content is 0.46%, more preferably 0.44%, and even more preferably 0.42%.

[0042] P: 0.030% 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, even if the content of other elements is within the range of this embodiment, the low-temperature toughness of the steel material in an extremely low-temperature environment will decrease. Therefore, the P content is 0.030% or less. The preferred upper limit of the P content is 0.025%, more preferably 0.020%, and even more preferably 0.015%. 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 0.002%, and even more preferably 0.004%.

[0043] 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, even if the content of other elements is within the range of this embodiment, the low-temperature toughness of the steel material in an extremely low-temperature environment will decrease. Therefore, the S content is 0.0050% or less. The preferred upper limit of the S content is 0.0040%, more preferably 0.0030%, and even more preferably 0.0020%. It is preferable to have as low an S content as possible. However, an extreme reduction in the S content will significantly increase manufacturing costs. Therefore, considering industrial production, the preferred lower limit of the S content is 0.0001%, and even more preferably 0.0002%.

[0044] Cr: 16.0-18.0% Chromium (Cr), as an oxide, forms a passive film on the surface of the steel material, resulting in high CO ２The corrosion resistance of steel materials in the environment is enhanced. 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 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 Cr content is 16.0 to less than 18.0%. The preferred lower limit of the Cr content is 16.1%, and more preferably 16.2%. The preferred upper limit of the Cr content is 17.9%, more preferably 17.8%, and still more preferably 17.7%.

[0045] Ni: Greater than 5.00% to 8.00% Nickel (Ni) enhances the low-temperature toughness of steel materials in cryogenic 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 austenite becomes too high, and the strength of the steel material decreases. Therefore, the Ni content is greater than 5.00% to 8.00%. The preferred lower limit of the Ni content is 5.01%, more preferably 5.05%, even more preferably 5.07%, and even more preferably 5.10%. The preferred upper limit of the Ni content is 7.70%, more preferably 7.60%, even more preferably 7.50%, even more preferably 7.30%, and even more preferably 7.10%.

[0046] Mo: Over 0.60% to 3.50% Molybdenum (Mo) is dissolved in the steel material, resulting in high CO ２ The corrosion resistance of steel materials in the environment is enhanced. 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 greater than 0.60% to 3.50%. The preferred lower limit of the Mo content is 0.61%, more preferably 0.80%, and even more preferably 1.00%. The preferred upper limit of the Mo content is 3.30%, more preferably 3.20%, and even more preferably 3.10%.

[0047] Al: 0.001 to 0.100% Aluminum (Al) deoxidizes steel. If the Al 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 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 low-temperature toughness of the steel will decrease. Therefore, the Al content is 0.001 to 0.100%. The preferred lower limit of the Al content is 0.002%, more preferably 0.003%, and still more preferably 0.005%. The preferred upper limit of the Al content is 0.080%, more preferably 0.060%, and still more preferably 0.050%. In this specification, Al content refers to the content of sol. Al (acid-soluble Al).

[0048] O: 0.0200% or less. Oxygen (O) is inevitably present. In other words, the lower limit of O content is greater than 0%. O forms oxides. Therefore, if the O content is too high, even if the content of other elements is within the range of this embodiment, high CO2 levels will result. ２ The corrosion resistance of steel materials in the environment decreases. Therefore, the oxygen content is 0.0200% or less. The preferred upper limit of the oxygen content is 0.0150%, more preferably 0.0120%, and even more preferably 0.0100%. It is preferable to have an oxygen content as low as possible. However, an extreme reduction in the oxygen content increases manufacturing costs. Therefore, considering industrial production, the preferred lower limit of the oxygen content is 0.0001%, more preferably 0.0003%, and even more preferably 0.0005%.

[0049] N: 0.0200% or less. Nitrogen (N) is inevitably present. In other words, the lower limit of the N content is greater than 0%. N forms fine nitrides, suppressing grain coarsening. As a result, it increases the yield strength of the steel. On the other hand, if the N content is too high, even if the content of other elements is within the range of this embodiment, coarse nitrides will be formed, reducing the corrosion resistance and low-temperature toughness of the steel. Therefore, the N content is 0.0200% or less. The preferred upper limit of the N content is 0.0150%, more preferably 0.0120%, and even more preferably 0.0100%. The preferred lower limit of the N content to more effectively obtain the above effects is 0.0005%, and even more preferably 0.0010%.

[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 one or more elements selected from the group consisting of V, Ti, Nb, Ta, and Zr in place of a portion of Fe. All of these elements are optional and enhance the strength of the steel material.

[0052] V: 0-0.30% Vanadium (V) is an optional element and may not be present. That is, the V content may be 0%. If present, 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 low-temperature toughness of the steel in an extremely low-temperature environment will decrease. Therefore, the V content is 0-0.30%. 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 0.25%, more preferably 0.20%, and even more preferably 0.15%.

[0053] Ti: 0 to 0.100% 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 low-temperature toughness of the steel in an extremely low-temperature environment will decrease. Therefore, the Ti content is 0 to 0.100%. 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 0.080%, more preferably 0.050%, more preferably 0.040%, and still more preferably 0.035%.

[0054] Nb: 0 to 0.100% 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 to 0.100%. The preferred lower limit of the Nb content is greater than 0%, more preferably 0.001%, more preferably 0.002%, and more preferably 0.003%. The preferred upper limit of the Nb content is 0.070%, more preferably 0.050%, and more preferably 0.040%.

[0055] Ta: 0 to 0.100% Tantalum (Ta) is an optional element and may not be included. That is, the Ta content may be 0%. If included, Ta forms carbonitrides, increasing the strength of the steel. Even 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 to 0.100%. The preferred lower limit of the Ta content is greater than 0%, more preferably 0.001%, more preferably 0.002%, and more preferably 0.003%. The preferred upper limit of the Ta content is 0.070%, more preferably 0.050%, and more preferably 0.040%.

[0056] Zr: 0 to 0.100% 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 to 0.100%. The preferred lower limit of the Zr content is greater than 0%, more preferably 0.001%, more preferably 0.002%, and more preferably 0.003%. The preferred upper limit of the Zr content is 0.070%, more preferably 0.050%, and more preferably 0.040%.

[0057] 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 Cu, W, and Co in place of a portion of Fe. These elements are all arbitrary elements and enhance the corrosion resistance of the steel material.

[0058] Cu: 0 to less than 1.00% Copper (Cu) is an optional element and may not be included. That is, the Cu content may be 0%. If included, Cu enhances the corrosion resistance of the steel. Even a small amount of Cu will provide the above effect to some extent. However, if the Cu 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 Cu content is 0 to less than 1.00%. The preferred lower limit of the Cu content is greater than 0%, more preferably 0.01%, and even more preferably 0.02%. The preferred upper limit of the Cu content is 0.99%, more preferably 0.80%, even more preferably 0.60%, even more preferably less than 0.50%, and even more preferably 0.49%.

[0059] W: 0 to less than 0.50% Tungsten (W) is an optional element and may not be included. That is, the W content may be 0%. If included, W enhances the corrosion resistance of the steel. 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, the strength of the steel may decrease even if the content of other elements is within the range of this embodiment. Therefore, the W content is 0 to less than 0.50%. 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 0.49%, more preferably 0.45%, more preferably 0.43%, and still more preferably 0.41%.

[0060] Co: 0-0.80% Cobalt (Co) is an optional element and may not be included. That is, the Co content may be 0%. If included, Co enhances the corrosion resistance of the steel. Even a small amount of Co will provide the above effect to some extent. However, if the Co content is too high, the low-temperature toughness of the steel will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the Co content is 0-0.80%. The preferred lower limit of the Co content is greater than 0%, more preferably 0.01%, more preferably 0.02%, and more preferably 0.03%. The preferred upper limit of the Co content is 0.75%, more preferably 0.70%, and more preferably 0.65%.

[0061] 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 Ca, Mg, B, and rare earth elements. All of these elements are arbitrary and enhance the hot workability of the steel material.

[0062] Ca: 0 to 0.0050% Calcium (Ca) is an optional element and may not be present. That is, the Ca content may be 0%. If present, Ca 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 Ca is present, the above effect can be obtained to some extent. However, 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 will coarseen, and the corrosion resistance of the steel will decrease. Therefore, the Ca content is 0 to 0.0050%. The preferred lower limit of the Ca content is greater than 0%, more preferably 0.0001%, more preferably 0.0003%, and still more preferably 0.0005%. The preferred upper limit of the Ca content is 0.0045%, more preferably 0.0040%, and still more preferably 0.0037%.

[0063] Mg: 0-0.0050% 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 0-0.0050%. The preferred lower limit of the Mg content is greater than 0%, more preferably 0.0001%, more preferably 0.0003%, and still more preferably 0.0005%. The preferred upper limit of the Mg content is 0.0045%, more preferably 0.0040%, and still more preferably 0.0037%.

[0064] B: 0 to 0.0050% Boron (B) is an optional element and may not be included. That is, the B content may be 0%. If included, B improves the hot workability of the steel material. Even if only a small amount of B is included, the above effect can be obtained to some extent. However, if the B content is too high, even if the content of other elements is within the range of this embodiment, boron nitride will be formed and the corrosion resistance of the steel material 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%, more preferably 0.0003%, and still more preferably 0.0005%. The preferred upper limit of the B content is 0.0045%, more preferably 0.0040%, and still more preferably 0.0037%.

[0065] Rare Earth Elements: 0 to 0.0100% 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, thereby improving the hot workability of the steel. Even a small amount of REM can provide the above effect to some extent. However, if the REM content is too high, even if the content of other elements is within the range of this embodiment, the oxides in the steel will coarseen, reducing the corrosion resistance of the steel. Therefore, the REM content is 0 to 0.0100%. The preferred lower limit of the REM content is greater than 0%, more preferably 0.0001%, more preferably 0.0003%, and more preferably 0.0005%. The preferred upper limit of the REM content is 0.0090%, more preferably 0.0070%, and more preferably 0.0050%.

[0066] 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 As, Sn, Sb, Zn, and Pb in place of a portion of Fe. All of these elements are arbitrary and enhance the corrosion resistance of the steel material.

[0067] As: 0 to 0.100% 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 low-temperature toughness 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 0.100%. The preferred lower limit of the As content is greater than 0%, more preferably 0.001%, more preferably 0.002%, and more preferably 0.003%. The preferred upper limit of the As content is 0.070%, more preferably 0.050%, and more preferably 0.040%.

[0068] Sn: 0 to 0.100% Tin (Sn) is an optional element and may not be included. That is, the Sn content may be 0%. If included, Sn enhances the corrosion resistance of the steel. Even a small amount of Sn will provide some degree of the above effect. However, if the Sn content is too high, the low-temperature toughness of the steel will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the Sn content is 0 to 0.100%. The preferred lower limit of the Sn content is greater than 0%, more preferably 0.001%, more preferably 0.002%, and more preferably 0.003%. The preferred upper limit of the Sn content is 0.070%, more preferably 0.050%, and more preferably 0.040%.

[0069] Sb: 0-0.100% Antimony (Sb) is an optional element and may not be present. That is, the Sb content may be 0%. If present, 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 low-temperature toughness of the steel will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the Sb content is 0-0.100%. The preferred lower limit of the Sb content is greater than 0%, more preferably 0.001%, more preferably 0.002%, and more preferably 0.003%. The preferred upper limit of the Sb content is 0.070%, more preferably 0.050%, and more preferably 0.040%.

[0070] Zn: 0 to 0.100% 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 will provide some degree of the above effect. However, if the Zn 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 Zn content is 0 to 0.100%. The preferred lower limit of the Zn content is greater than 0%, more preferably 0.001%, more preferably 0.002%, and more preferably 0.003%. The preferred upper limit of the Zn content is 0.070%, more preferably 0.050%, and more preferably 0.040%.

[0071] Pb: 0 to 0.100% 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 0.100%. The preferred lower limit of the Pb content is greater than 0%, more preferably 0.001%, more preferably 0.002%, and more preferably 0.003%. The preferred upper limit of the Pb content is 0.070%, more preferably 0.050%, and more preferably 0.040%.

[0072] [Fn1] Assuming that the content of each element in the chemical composition of the stainless steel material according to this embodiment is within the above range, Fn1 as defined by formula (1) is 22.00 or more. Fn1 = Cr + 3Mo + 0.5Ni - 5.0Mn (1) Here, the content of the corresponding element in unit: mass% is substituted for the element symbol in formula (1).

[0073] Fn1 is a stainless steel material having the above-mentioned chemical composition, high CO ２ This is an indicator of corrosion resistance in the environment. If Fn1 is 22.00 or higher, provided that the other configurations of this embodiment are met, high CO ２The corrosion resistance of stainless steel materials in the environment is improved. Therefore, the stainless steel material according to this embodiment has an Fn1 of 22.00 or higher, provided that it satisfies the above-mentioned chemical composition.

[0074] The preferred lower limit of Fn1 is 22.10, more preferably 22.50, and even more preferably 23.00. The upper limit of Fn1 is not particularly limited, but is substantially less than 32.45. The upper limit of Fn1 may be 32.00, 31.00, or 30.00. Fn1 is obtained by rounding the obtained value to the third decimal place.

[0075] [Yield Strength] The stainless steel material according to this embodiment has a yield strength of 552 to less than 758 MPa. The preferred lower limit of the yield strength is 555 MPa, more preferably 558 MPa, and even more preferably 560 MPa. The preferred upper limit of the yield strength is 757 MPa, more preferably 755 MPa, and even more preferably 750 MPa.

[0076] In this embodiment, the yield strength of the stainless steel material is 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 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 test specimen or an arc-shaped test specimen is prepared as a tensile test specimen from the center of the wall thickness. In this case, the longitudinal direction of the round bar test specimen or the arc-shaped test specimen is parallel to the axial direction of the steel pipe. If the steel material is a round steel bar, a tensile test specimen is prepared from the R / 2 position. 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 longitudinal direction of the tensile test specimen is parallel to the axial direction of the round steel bar.

[0077] The tensile test specimen is, for example, a round bar specimen with a parallel section diameter of 4 mm and a gauge length of 16 mm. If a round bar specimen cannot be prepared from a steel pipe, an arc-shaped specimen is prepared. The dimensions of the arc-shaped specimen are, for example, the total thickness, a width of 25.4 mm, and a gauge length of 50.8 mm. Using the prepared tensile test specimen, a tensile test is performed at room temperature (24 ± 3 °C) in accordance with ASTM E8 / E8M (2022). The 0.2% offset proof stress (MPa) obtained from the tensile test is defined as the yield strength (MPa). The yield strength (MPa) is calculated by rounding the obtained value to the first decimal place.

[0078] [Fn2] The stainless steel material according to this embodiment has an Fn2 of 1.00 or more, as defined by formula (2), assuming that the content of each element in the chemical composition is within the above range and the yield strength is less than 552 to 758 MPa. Fn2 = 7 / (1 + exp(-18 + Mn - Ni + Cr + 0.2Mo + Cu + 0.01YS)) (2) Here, the content of the corresponding element is substituted for the element symbol in formula (2) in units of mass%. If the corresponding element is not contained, "0" is substituted for that element symbol. Also, the yield strength is substituted for YS in formula (2) in units of MPa.

[0079] Fn2 is an index of the low-temperature toughness of a stainless steel material having the above-described chemical composition in an extremely low-temperature environment. When Fn2 is 1.00 or higher, the low-temperature toughness of the stainless steel material in an extremely low-temperature environment is increased, provided that the other configurations of this embodiment are also satisfied. On the other hand, when Fn2 is less than 1.00, the low-temperature toughness of the stainless steel material in an extremely low-temperature environment cannot be sufficiently increased, even if the other configurations of this embodiment are also satisfied. Therefore, the stainless steel material according to this embodiment has an Fn2 of 1.00 or higher, provided that the above-described chemical composition is satisfied.

[0080] The preferred lower limit of Fn2 is 1.01, more preferably 1.02, and even more preferably 1.03. The upper limit of Fn2 is not particularly limited, but is substantially 6.91. The upper limit of Fn2 may also be 6.50, 6.00, or 5.80. Fn2 is obtained by rounding the obtained value to the third decimal place.

[0081] [Microstructure] The stainless steel material according to this embodiment has the above-described chemical composition, with an Fn1 of 22.00 or more, a yield strength of 552 to less than 758 MPa, an Fn2 of 1.00 or more, and a microstructure consisting of more than 5.0% to 30.0% austenite by volume fraction, with the remainder being ferrite and martensite.

[0082] In this specification, when it is stated that the microstructure consists of "austenite and the remainder being ferrite and martensite," it means that the amount of phases other than austenite, ferrite, and 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 fractions of austenite, ferrite, and 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 austenite, ferrite, and martensite.

[0083] As described above, in the microstructure of the stainless steel material according to this embodiment, the volume fraction of austenite is greater than 5.0% to 30.0%. If the volume fraction of austenite is too low, the low-temperature toughness of the steel material in an extremely low-temperature environment will decrease. On the other hand, if the volume fraction of 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 austenite is greater than 5.0% to 30.0%. The preferred lower limit of the volume fraction of austenite is 5.1%, more preferably 6.0%, still more preferably 7.0%, still more preferably 8.0%, and still more preferably 9.0%. The preferred upper limit of the volume fraction of austenite is 29.7%, more preferably 29.5%, still more preferably 29.2%, and still more preferably 29.0%.

[0084] The remainder of the microstructure of the stainless steel material according to this embodiment consists of ferrite and martensite. In the stainless steel material according to this embodiment having the above-described chemical composition, the volume fraction of ferrite is, for example, 10 to 60%. Similarly, in the stainless steel material according to this embodiment having the above-described chemical composition, the volume fraction of martensite is, for example, 10 to less than 85%.

[0085] In this embodiment, the volume fraction of austenite in the microstructure is determined by the following method. Specifically, the volume fraction of austenite in the microstructure of the steel material is determined by X-ray diffraction. First, a test specimen for measuring the volume fraction of 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. 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 (ferrite and martensite), the (200) plane of the α phase, the (211) plane of the α phase, the (111) plane of the γ phase (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.

[0086] 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 45-105°. After calculation, the volume fraction Vγ (%) of austenite is calculated for each combination (3 x 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 austenite is defined as the volume fraction (%) of 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 γ quantitative analysis system included with the RINT-TTR product manufactured by Rigaku Corporation. The volume fraction of austenite is calculated by rounding the obtained value to two decimal places.

[0087] Furthermore, the volume fractions of ferrite and martensite in the microstructure are determined by the following method. Specifically, the volume fraction of ferrite in the microstructure of steel is determined by the point calculation method. First, a test piece for measuring the volume fraction of ferrite is prepared from the steel material according to this embodiment. If the steel material is a steel plate, the test piece is taken from the center of the plate thickness. At this time, the observation surface of the test piece is a plane parallel to the rolling direction of the steel plate. If the steel material is a steel pipe, the test piece is taken from the center of the wall thickness. At this time, the observation surface of the test piece is a plane parallel to the pipe axis direction of the steel pipe. If the steel material is a round bar, the test piece is taken from the R / 2 position. At this time, the observation surface of the test piece is a plane parallel to the axis direction of the round bar. The size of the test piece is not particularly limited. After mechanically 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.

[0088] 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 (a mixed structure in which austenite is dispersed in martensite) based on contrast. Therefore, ferrite in each field of view is identified based on contrast. The area fraction of the identified ferrite is determined by the point calculation method in accordance with ASTM E562 (2019).

[0089] 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 to the right 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 ASTM E562 (2019), 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 ferrite area ratio. In this embodiment, the ferrite area ratio obtained by the above method is taken as the ferrite volume ratio (%). The volume fraction of ferrite is calculated by rounding the obtained value to the first decimal place.

[0090] Using the volume fraction (%) of 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 martensite in the microstructure of the steel material is calculated using the following formula: Volume fraction (%) of martensite = 100 - {Volume fraction (%) of austenite + Volume fraction (%) of ferrite}

[0091] [Mixed Structure] As described above, in the microstructure of the stainless steel material according to this embodiment, austenite is finely dispersed within the martensite. In this specification, martensite in which austenite is dispersed is also referred to as a "mixed structure". In the stainless steel material according to this embodiment, the average length in the C direction of the mixed structure that satisfies the C direction length of the stainless steel material to be 10 μm or more is set to 30 μm or less.

[0092] Here, if the stainless steel material is a stainless steel plate, the C direction of the stainless steel material corresponds to the thickness direction of the stainless steel plate. If the stainless steel material is a stainless steel pipe, the C direction of the stainless steel material corresponds to the diameter direction (wall thickness direction) of the stainless steel pipe. If the stainless steel material is a stainless steel round bar, the C direction of the stainless steel material corresponds to the diameter direction of the cross-section perpendicular to the axial direction of the stainless steel round bar. In this embodiment, if the stainless steel material is a stainless steel plate, the thickness direction of the stainless steel plate is defined as the C direction, and the average length in the C direction of the mixed structure of austenite and martensite that satisfies the length in the C direction of 10 μm or more is 30 μm or less. Furthermore, in this embodiment, if the stainless steel material is a stainless steel pipe, the diameter direction of the stainless steel pipe is defined as the C direction, and the average length in the C direction of the mixed structure of austenite and martensite that satisfies the length in the C direction of 10 μm or more is 30 μm or less. In this embodiment, if the stainless steel material is a stainless steel round bar, the radial direction of the cross-section perpendicular to the axial direction of the stainless steel round bar is defined as the C direction, and the average length in the C direction of the mixed structure of austenite and martensite that satisfies the condition that the length in the C direction is 10 μm or more is 30 μm or less.

[0093] As described above, among the mixed structures in the microstructure of stainless steel, a mixed structure whose C-direction length is 10 μm or more is also called a "coarse mixed structure." In other words, in the stainless steel material according to this embodiment, the average C-direction length of the coarse mixed structure is 30 μm or less. As a result, assuming that the other components of this embodiment are met, the stainless steel material has excellent low-temperature toughness even in extremely low-temperature environments.

[0094] The preferred upper limit for the average length of the coarse mixed structure in the C direction may be 29 μm, more preferably 28 μm, and even more preferably 27 μm. The lower limit for the average length of the coarse mixed structure in the C direction is 0 μm. In other words, the stainless steel material according to this embodiment does not have to contain a coarse mixed structure. If the stainless steel material contains a coarse mixed structure, its average length in the C direction is 10 μm or more.

[0095] In this embodiment, the average length of the coarse mixed structure in the C direction is determined by the following method. First, a test specimen for observing the coarse mixed structure 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. At this time, the observation surface of the test specimen shall be a plane that includes the rolling direction (L direction) and the thickness direction (C direction) of the steel plate. If the steel material is a steel pipe, the test specimen is taken from the center of the wall thickness. At this time, the observation surface of the test specimen shall be a plane that includes the axial direction (L direction) and the radial direction (C direction) of the steel pipe. If the steel material is a round steel bar, the test specimen is taken from the R / 2 position. At this time, the observation surface of the test specimen shall be a plane that includes the axial direction (L direction) of the round steel bar and the radial direction (C direction) of the cross-section perpendicular to the axial direction. The size of the test specimen is not particularly limited, but it shall be a size that has an observation surface that can secure five or more observation fields of 200 μm × 200 μm.

[0096] The test specimen is embedded in resin, and the observation surface, which has been polished to a mirror finish, is immersed in a Virela etching solution (a mixture of ethanol, hydrochloric acid, and picric acid) for about 60 seconds to reveal the microstructure by etching. The etched observation surface is observed using an optical microscope for at least five fields of view. As described above, the observation field of view is a 200 μm × 200 μm square.

[0097] A line segment is drawn in the C direction at any position in the observation field. Preferably, the observation field is a square with the horizontal axis in the L direction and the vertical axis in the C direction, and a vertical line segment is drawn at the center of the observation field in the left-right direction. The intersection point between the phase boundary between the mixed tissue and the ferrite phase and the vertical line is determined, and the length of the mixed tissue in the C direction is determined. The ferrite phase and the mixed tissue can be identified from the contrast. As a result of determining the length of each mixed tissue in the C direction, mixed tissue with a length of 10 μm or more in the C direction is identified as "coarse mixed tissue". The length of the coarse mixed tissue in the C direction is determined for each of the five or more observation fields. The arithmetic mean of the obtained lengths in the C direction is defined as the average length of the coarse mixed tissue in the C direction. The average length of the coarse mixed tissue in the C direction is obtained by rounding the obtained value to the first decimal place. In this case, if no coarse mixed tissue is confirmed in five or more observation fields, the average length of the coarse mixed tissue in the C direction is set to 0 μm.

[0098] [Area fraction of coarse γ] In the stainless steel material according to this embodiment, the area fraction of austenite with an equivalent circle diameter of 3.0 μm or more in the microstructure is 0.0050 or less. As described above, in this specification, austenite is also referred to as "γ". In this specification, austenite with an equivalent circle diameter of 3.0 μm or more is further referred to as "coarse γ". In addition, in the stainless steel material according to this embodiment, austenite is finely dispersed in martensite to form a mixed structure.

[0099] In other words, in the stainless steel material according to this embodiment, γ is finely dispersed in the mixed structure. On the other hand, if the area fraction of coarse γ is too high, the γ dispersed in the mixed structure becomes coarser, and the amount of fine γ decreases. As a result, it is thought that the low-temperature toughness of the stainless steel material in an extremely low-temperature environment will decrease. Therefore, in the stainless steel material according to this embodiment, the area fraction of coarse γ in the microstructure is set to 0.0050 or less.

[0100] In this embodiment, the preferred upper limit of the area fraction of coarse γ is 0.0049, more preferably 0.0048, and even more preferably 0.0047. The lower limit of the area fraction of coarse γ is 0.0000. In other words, coarse γ may not be present in the microstructure of the stainless steel material according to this embodiment.

[0101] In this embodiment, the area fraction of coarse γ is determined by the following method. First, a test piece for measuring the area fraction of coarse γ is prepared from the steel material according to this embodiment. If the steel material is a steel plate, the test piece is taken from the center of the plate thickness. At this time, the observation surface of the test piece shall be a plane that includes the rolling direction (L direction) and the thickness direction (C direction) of the steel plate. If the steel material is a steel pipe, the test piece is taken from the center of the wall thickness. At this time, the observation surface of the test piece shall be a plane that includes the axial direction (L direction) and the radial direction (C direction) of the steel pipe. If the steel material is a round steel bar, the test piece is taken from the R / 2 position. At this time, the observation surface of the test piece shall be a plane that includes the axial direction (L direction) of the round steel bar and the radial direction (C direction) of the cross-section perpendicular to the axial direction. The size of the test piece is not particularly limited, but it shall be a size that has an observation surface that can secure four or more observation fields of 100 μm × 100 μm.

[0102] The specimen is embedded in resin, the observation surface is polished to a mirror finish, and then polished with colloidal silica. Electron backscatter diffraction (EBD) measurements are performed on the polished observation surface in 100 μm × 100 μm fields at 0.1 μm intervals. The accelerating voltage for EBSD measurements is set to 15 kV. From the obtained EBSD measurement results, the phase at the measurement point (whether or not it is austenite) is identified. From the identified results, the equivalent circle diameter of each austenite is determined, and austenite with an equivalent circle diameter of 3.0 μm or more is defined as "coarse γ". The area fraction of coarse γ in the four fields is calculated from the area of ​​coarse γ and the area of ​​austenite in each field. The area fraction of coarse γ is obtained by rounding the obtained value to the fifth decimal place.

[0103] [Corrosion Resistance] The stainless steel material according to this embodiment has the above-described chemical composition, satisfies Fn1 of 22.00 or more, satisfies Fn2 of 1.00 or more, has a yield strength of 552 to less than 758 MPa, has a microstructure consisting of more than 5.0 to 30.0% austenite by volume fraction and the remainder being ferrite and martensite, satisfies the average length in the C direction of the mixed structure having a length in the C direction of 10 μm or more, satisfies 30 μm or less, and satisfies the area fraction of coarse γ of 0.0050 or less. As a result, the stainless steel material according to this embodiment has high strength and high CO ２ It has excellent corrosion resistance in the environment and excellent low-temperature toughness in cryogenic environments. In this embodiment, high CO ２ Excellent corrosion resistance in the environment is evaluated by the following method.

[0104] 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 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. 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. The size of the test specimen is, for example, 30.0 mm in length, 20.0 mm in width, and 2.0 mm in thickness.

[0105] Pour a 20% by mass sodium chloride aqueous solution into the autoclave so that the test specimen is immersed. CO2 is added to the autoclave. ２ The gas is pressurized and sealed into the test solution to a total pressure of 200 bar, saturating it to create the test bath. After sealing the autoclave, the test bath is maintained at 100°C, and the test specimens are immersed in the test bath for 96 hours while stirring.

[0106] For the test specimen before stress loading and after 96 hours, the mass, volume, and surface area 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. 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, high CO ２It is evaluated as having excellent corrosion resistance even in environmental conditions.

[0107] [Low Temperature Toughness] The stainless steel material according to this embodiment has the above-described chemical composition, satisfies Fn1 of 22.00 or more, satisfies Fn2 of 1.00 or more, has a yield strength of 552 to less than 758 MPa, has a microstructure consisting of more than 5.0 to 30.0% austenite by volume fraction and the remainder being ferrite and martensite, satisfies the average length in the C direction of the mixed structure having a length in the C direction of 10 μm or more, satisfies 30 μm or less, and satisfies the area fraction of coarse γ of 0.0050 or less. As a result, the stainless steel material according to this embodiment has high strength and high CO ２ It possesses excellent corrosion resistance in the environment and excellent low-temperature toughness in cryogenic environments. In this embodiment, the excellent low-temperature toughness in cryogenic environments is evaluated by the following method.

[0108] Specifically, a full-size or sub-size V-notch test specimen in the T-direction is prepared from the stainless steel material according to this embodiment, in accordance with API 5CRA (2019). Here, if the steel material is a steel plate, the width direction of the steel plate is defined as the "T-direction". If the steel material is a steel pipe, the diameter direction of the steel pipe is defined as the "C-direction", the axial direction of the steel pipe is defined as the "L-direction", and the direction perpendicular to the C-direction and L-direction is defined as the "T-direction". If the steel material is a round steel bar, the diameter direction of the cross-sectional section of the round steel bar is defined as the "C-direction", the axial direction of the round steel bar is defined as the "L-direction", and the direction perpendicular to the C-direction and L-direction is defined as the "T-direction". Note that if a 1 / 2-size Charpy test specimen in the T-direction cannot be obtained due to the size of the steel material (plate thickness and width of steel plates, diameter and wall thickness of steel pipes, etc.), a full-size or sub-size Charpy test specimen in the L-direction is prepared.

[0109] A Charpy impact test is performed on the prepared V-notch specimen in accordance with JIS Z 2242 (2018) to determine the absorbed energy (J) at -70°C. If a sub-sized V-notch specimen is used, the obtained absorbed energy is divided by the reduction factor described in API 5CRA (2019) to convert it to the absorbed energy of a full-sized V-notch specimen. In this embodiment, the absorbed energy (J) at -70°C is obtained by rounding the obtained value to the first decimal place.

[0110] In this embodiment, if the absorbed energy at -70°C determined by the above conditions is 40 J or more, it is evaluated as having excellent low-temperature toughness in an extremely low-temperature environment. In this specification, the absorbed energy at -70°C is also simply referred to as "absorbed energy".

[0111] [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, high strength and high CO ２ It possesses excellent corrosion resistance in environmental conditions and excellent low-temperature toughness in extremely low-temperature environments.

[0112] [Manufacturing Method] An example of a method for manufacturing stainless steel material according to this embodiment having the above-described configuration will be explained. Note that the method for manufacturing stainless steel material according to this embodiment is not limited to the method described below. An example of the method for manufacturing stainless steel material according to this embodiment includes a step of preparing intermediate steel material (preparation step), a step of performing quenching on the intermediate steel material (quenching step), and a step of performing tempering (tempering step). Each step will be described in detail below.

[0113] [Preparation Process] In the preparation process, intermediate steel material having the above-mentioned chemical composition is prepared. As long as the intermediate steel material has the above-mentioned chemical composition, the method of manufacturing the intermediate steel material is not particularly limited. The intermediate steel material referred to here is plate-shaped steel material when the final product is a steel plate or welded steel pipe, raw pipe when the final product is a seamless steel pipe, and steel material with a circular cross-section perpendicular to the axial direction when the final product is a round steel bar.

[0114] The preparation process may include a process for preparing the raw materials (raw material preparation process) and a process for manufacturing intermediate steel materials by hot working the raw materials (hot working process). The following details the case in which the raw material preparation process and the hot working process are included.

[0115] [Material Preparation Process] In the material preparation process, the material is manufactured using molten steel having the chemical composition described above. The method of manufacturing the material is not particularly limited and any well-known method is acceptable. Specifically, cast slabs (slabs, blooms, or billets) may be manufactured using a continuous casting method with molten steel. Ingots may be manufactured using a block-making method with molten steel. If necessary, billets may be manufactured by bloc rolling of the slabs, blooms, or ingots. The material (slabs, blooms, or billets) is manufactured through the above process.

[0116] [Hot Working Process] In the hot working process, the prepared material is hot-worked to produce intermediate steel material. As mentioned above, 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. Specifically, the heating temperature T (°C) in the hot working process is, for example, 1100 to 1300°C. Hot working is performed on the billet extracted from the heating furnace to produce the raw pipe (seamless steel pipe).

[0117] In the hot working process according to this embodiment, the method of hot working is not particularly limited and any well-known method may be used, but it is preferable that the cross-sectional reduction rate RA (%) in the entire hot working process satisfies the following formula (A): RA ≥ 41 / (1 + exp(180 - 0.1(T + 273.15) + 0.5Mn + 5Ni - 2.8Cr - 3.6Mo)) + 9 (A) Here, the elemental symbols in formula (A) are substituted with the content of the corresponding element in units of mass%. Also, T in formula (A) is substituted with the heating temperature of the hot working process in units of °C.

[0118] FnA is defined as FnA = 41 / (1 + exp(180 - 0.1(T + 273.15) + 0.5Mn + 5Ni - 2.8Cr - 3.6Mo)) + 9. FnA represents the minimum value of the cross-sectional reduction rate RA (%) when a material having the above chemical composition is hot-worked to produce intermediate steel. If RA is greater than or equal to FnA, the average length of the coarse mixed structure in the C direction in the manufactured stainless steel can be stably kept below 30 μm. Therefore, in the hot-working process according to this embodiment, it is preferable to make the cross-sectional reduction rate RA (%) greater than or equal to FnA.

[0119] As mentioned above, the method of hot working in the hot working process according to this embodiment is not particularly limited. For example, the Mannesmann process may be used as the hot working method to produce the raw pipe. In this case, the 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.

[0120] For example, other hot working methods may be used to manufacture the raw pipe from the billet. For example, if the steel material is a short, thick-walled steel pipe such as a coupling, the raw pipe may be manufactured by forging such as the Erhardt process. The raw pipe is manufactured through the above process. The wall thickness of the raw pipe is not particularly limited, but is, for example, 9 to 60 mm.

[0121] For example, if the steel material is a round bar, the material is first heated in a heating furnace. The heating temperature T (°C) is, for example, 1100 to 1300°C, similar to the method for manufacturing the raw tube described above. Hot working is performed on the material extracted from the heating furnace to produce an intermediate steel material with a circular cross-section perpendicular to the axial direction. Hot working is, for example, bubbling rolling using a bubbling mill, or hot rolling using a continuous rolling mill. A continuous rolling mill has alternating horizontal stands with a pair of hole-shaped rolls arranged vertically and vertical stands with a pair of hole-shaped rolls arranged horizontally.

[0122] For example, if the steel material is a steel plate, the material is first heated in a heating furnace. The heating temperature T (°C) is, for example, 1100 to 1300°C, similar to the method for manufacturing the raw pipe described above. The material extracted from the heating furnace is subjected to hot rolling using a bract mill and a continuous rolling mill to produce an intermediate steel material in the shape of a steel plate. Even with these methods, it is preferable that the cross-sectional area reduction rate RA (%) in hot working be FnA or greater. The cross-sectional area reduction rate RA (%) is defined by the following formula: RA (%) = {1 - (cross-sectional area of ​​the intermediate steel material perpendicular to the processing direction after the hot working process / cross-sectional area of ​​the material perpendicular to the processing direction before the hot working process)} × 100

[0123] 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.

[0124] As described above, the preparation process involves preparing intermediate steel. The intermediate steel may be manufactured by the preferred process described above, or it may be intermediate steel manufactured by a third party, or intermediate steel manufactured at a factory or business other than the one where the quenching and tempering processes described later are carried out. The quenching process will be described in detail below.

[0125] [Quenching Process] Quenching is performed on the intermediate steel material manufactured in the hot working process (quenching process). Quenching is carried out by a well-known method. Specifically, the intermediate steel material after the hot working process is placed in a heat treatment furnace, held at the quenching temperature, and then rapidly cooled (quenched). Here, the quenching temperature refers to the temperature (°C) of the heat treatment furnace used to heat the intermediate steel material in the quenching process. The holding time in the quenching process refers to the time (minutes) that the intermediate steel material is held at the quenching temperature.

[0126] In the quenching process according to this embodiment, if the quenching temperature is too low, the intermediate steel material may not be heated sufficiently, and the resulting stainless steel material may not have the microstructure described above. On the other hand, if the quenching temperature is too high, the mechanical properties of the resulting stainless steel material may not be obtained. Therefore, in this embodiment, the preferred quenching temperature is 820 to 1100°C. The holding time at the quenching temperature is not particularly limited, but for example, it is 5 to 120 minutes.

[0127] As described above, in this embodiment, the intermediate steel material is held at the quenching temperature and then rapidly cooled (quenched). The quenching method is, for example, water cooling. At this time, it is preferable to set the cooling rate CRQ (°C / min) in the 300-80°C range to 6-2400°C / min. If the cooling rate CRQ in the 300-80°C range is too slow, the manufactured stainless steel material may not have the microstructure described above. On the other hand, if the cooling rate CRQ in the 300-80°C range is too fast, the elemental concentration into austenite in the microstructure may not be sufficient, and many austenite-forming elements may be left behind in the martensite. As a result, austenite is more likely to grow during the tempering process described later, and the area fraction of coarse γ in the manufactured stainless steel material may become too high. Therefore, in the quenching process according to this embodiment, it is preferable to set the cooling rate CRQ (°C / min) in the 300-80°C range to 6-2400°C / min.

[0128] Furthermore, the quenching method is not particularly limited as long as the cooling rate CRQ (°C / min) at 300-80°C is between 6 and 2400°C / min. If the intermediate steel material is a raw tube, for example, the raw tube may be rapidly cooled by immersion in a water or oil bath, or by shower cooling or mist cooling, by pouring or spraying cooling water onto the outer and / or inner surface of the raw tube.

[0129] [Tempering Process] A tempering process is further performed on the intermediate steel material after the quenching process. In this specification, "tempering" refers to the process of tempering the intermediate steel material after quenching. ｃ１This means reheating and holding the material at a temperature below 1.5°C. Here, the tempering temperature corresponds to the furnace temperature at which the intermediate steel material is heated and held after quenching. The holding time in the tempering process refers to the time from when the temperature of the intermediate steel material reaches the predetermined tempering temperature until it is removed from the heat treatment furnace.

[0130] In the tempering process according to this embodiment, it is preferable to set the heating rate HRT (°C / min) in the 100-510°C range to 8°C / min or higher. If the heating rate HRT in the 100-510°C range is too slow, austenite may grow in the microstructure. As a result, the area fraction of coarse γ in the manufactured stainless steel material may become too high. On the other hand, the upper limit of the heating rate HRT in the 100-510°C range is not particularly limited and may be, for example, 100°C / min.

[0131] In this embodiment, it is preferable that the cooling rate CRQ (°C / min) in the quenching process at 300 to 80°C, the heating rate HRT (°C / min) in the tempering process at 100 to 510°C, and FnB, defined by the following formula (B), are 160 or less. FnB = CRQ / HRT (B) Here, in formula (B), CRQ is substituted with the cooling rate in the quenching process at 300 to 80°C in units of °C / min, and HRT is substituted with the heating rate in the tempering process at 100 to 510°C in units of °C / min.

[0132] FnB is an indicator of austenite coarsening in the microstructure in the stainless steel manufacturing method according to this embodiment. If FnB is 160 or less, austenite coarsening can be stably suppressed. As a result, the area fraction of coarseness γ in the manufactured stainless steel can be stably reduced to 0.0050 or less. Therefore, in the stainless steel manufacturing method according to this embodiment, FnB is set to 160 or less, assuming that the cooling rate CRQ at 300 to 80°C in the quenching process is 6 to 2400°C / min and the heating rate HRT at 100 to 510°C in the tempering process is 8°C / min or more.

[0133] Furthermore, the yield strength of the manufactured stainless steel can be adjusted by adjusting the holding temperature during the tempering process. Therefore, in this embodiment, the preferred tempering temperature is 530 to 670°C. Moreover, if the holding time is too short during the tempering process according to this embodiment, the above-mentioned microstructure may not be obtained in the manufactured stainless steel. In this case, the yield strength may become too high. On the other hand, if the holding time is too long, the above effect will saturate. Therefore, in this embodiment, the preferred holding time is 20 to 200 minutes.

[0134] 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.

[0135] Molten steel having the chemical compositions shown in Tables 1A, 1B, and 1C was melted using a 50 kg vacuum melting furnace, and steel ingots were produced by the ingot-making method. In Tables 1B and 1C, "-" indicates that the content of the corresponding element was at an impurity level. For example, the V, Cu, W, and Co content in test number 1 was 0%, rounded to the third decimal place. Similarly, the Ti, Nb, Ta, Zr, As, Sn, Sb, Zn, and Pb content in test number 1 was 0%, rounded to the fourth decimal place. Similarly, the Ca, Mg, B, and REM content in test number 1 was 0%, rounded to the fifth decimal place.

[0136]

[0137]

[0138]

[0139] After heating the ingots for each test number at 1250°C for 2 hours, hot working was performed to produce intermediate steel materials (steel plates) with a thickness of 15 mm. At this time, the heating temperature T (°C), the cross-sectional reduction rate RA (%), and FnA (= 41 / (1 + exp(180 - 0.1(T + 273.15) + 0.5Mn + 5Ni - 2.8Cr - 3.6Mo)) + 9) obtained from formula (A) are shown in Table 2.

[0140]

[0141] Furthermore, a quenching process and a tempering process were performed on the intermediate steel material for each test number. Specifically, the intermediate steel material for each test number was held at the quenching temperature (°C) indicated in the quenching process column of Table 2 for the holding time (minutes), and then rapidly cooled. At this time, the cooling rate CRQ (°C / min) in the quenching process from 300 to 80°C was as shown in Table 2. The intermediate steel material for each test number was further held at the tempering temperature (°C) indicated in the tempering process column of Table 2 for the holding time (minutes). At this time, the heating rate HRT (°C / min) in the tempering process from 100 to 510°C was as shown in Table 2. Furthermore, for each test number, FnB (=CRQ / HRT) obtained from formula (B) is shown in Table 2.

[0142] [Evaluation Tests] Through the above process, steel plates of each test number were obtained. Tensile tests, microstructural observation tests, mixed structure observation tests, coarse gamma observation tests, corrosion tests, and Charpy impact tests were performed on each of the obtained steel plates of each test number.

[0143] [Tensile Test] Tensile tests were conducted on each steel plate according to ASTM E8 / E8M (2022). Specifically, round bar tensile test specimens with a parallel section diameter of 4 mm and a gauge length of 16 mm were prepared from the center of the thickness of each steel plate. The longitudinal direction of the round bar tensile test specimen was parallel to the rolling direction of the steel plate. Tensile tests were conducted using the round bar tensile test specimens for each test number at room temperature (24 ± 3°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). Table 3 shows the elemental content for each test number and Fn1 (= Cr + 3Mo + 0.5Ni - 5.0Mn) calculated from equation (1). Table 3 shows the elemental content, yield strength YS, and Fn2 (= 7 / (1 + exp(-18 + Mn-Ni + Cr + 0.2Mo + Cu + 0.01YS))) obtained from equation (2) for each test number. The yield strength obtained for each test number is shown in the "YS (MPa)" column of Table 3.

[0144]

[0145] [Microstructure Observation Test] Microstructure observation tests were performed on the steel plates of each test number using the method described above. Specifically, the volume fraction (%) of 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 ASTM E562 (2019), using the method described above. The obtained volume fractions of austenite for each test number are shown in the "γ (volume %)" column of Table 3. The volume fraction of ferrite ranged from 10 to 60%, and the remainder of the microstructure was martensite.

[0146] [Mixed Structure Observation Test] A mixed structure observation test was performed on the steel plates of each test number using the method described above. Specifically, the corroded observation surface was observed with an optical microscope using the method described above, and the average length in the C direction of the coarse mixed structure was determined. The average length in the C direction of the coarse mixed structure for each test number obtained is shown in Table 3.

[0147] [Coarse Gamma Observation Test] A coarse gamma observation test was conducted on the steel plates of each test number using the method described above. Specifically, the area fraction of coarse gamma was determined by EBSD measurement using the method described above. The area fractions of coarse gamma obtained for each test number are shown in Table 3.

[0148] [Corrosion Test] A corrosion test was conducted on the steel plates of each test number, and high CO2 levels were observed. ２ The corrosion resistance in the environment was evaluated. 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, and the corrosion rate (mm / year) was determined. For test numbers where the obtained corrosion rate (mm / year) was 0.100 mm / year or less, "EX (EXcellent)" is indicated in the "Corrosion Resistance" column of Table 3. Furthermore, for test numbers where the obtained corrosion rate (mm / year) exceeded 0.100 mm / year, "NA (Not Acceptable)" is indicated in the "Corrosion Resistance" column of Table 3.

[0149] [Charpy Impact Test] A Charpy impact test was performed on each steel plate according to JIS Z 2242 (2018). Specifically, a full-size V-notch specimen in the T-direction was prepared from the center of the thickness of each steel plate according to API 5CRA (2019). A Charpy impact test was performed on the prepared V-notch specimen according to JIS Z 2242 (2018) to determine the absorbed energy (J) at -70°C. The absorbed energy at -70°C obtained for each test number is shown in the "vE (-70°C) (J)" column of Table 3.

[0150] [Evaluation Results] Referring to Tables 1A, 1B, 1C, 2, and 3, the steel sheets for test numbers 1 to 29 had an appropriate chemical composition, a yield strength of 552 to less than 758 MPa, an Fn1 of 22.00 or higher, and an Fn2 of 1.00 or higher. These steel sheets were further manufactured by the preferred manufacturing method described in the specification. As a result, these steel sheets had a microstructure consisting of more than 5.0% to 30.0% austenite by volume fraction, with the remainder being ferrite and martensite. Furthermore, the average length of the coarse mixed structure in the C direction of these steel sheets was 30 μm or less. Furthermore, the area fraction of coarse γ in these steel sheets was 0.0050 or less. As a result, in the corrosion test, these steel sheets had a corrosion rate of 0.100 mm / year or less and high CO ２These steel plates exhibited excellent corrosion resistance even in extremely cold environments. As a result, in Charpy impact tests, these steel plates absorbed more than 40 J of energy at -70°C, demonstrating excellent low-temperature toughness even in cryogenic environments.

[0151] On the other hand, in the hot working process, the cross-sectional reduction rate of steel plates numbered 30 and 31 was less than FnA. As a result, the average length of the coarse mixed structure in the C direction of these steel plates exceeded 30 μm. Consequently, in the Charpy impact test, these steel plates absorbed less than 40 J at -70°C, indicating that they did not possess good low-temperature toughness in an extremely low-temperature environment.

[0152] In test number 32, the heating rate (HRT) during the tempering process between 100 and 510°C was less than 8°C / min, and the FnB (Functional Fiber Bound) exceeded 160. As a result, the area fraction of coarse γ (gamma) in this steel sheet exceeded 0.0050. Consequently, in the Charpy impact test, the absorbed energy at -70°C was less than 40 J, indicating that this steel sheet did not possess excellent low-temperature toughness in an extremely low-temperature environment.

[0153] The steel plate of test number 33 had a heating rate (HRT) of less than 8°C / min during the tempering process between 100°C and 510°C. As a result, the area fraction of coarse γ in this steel plate exceeded 0.0050. Consequently, in the Charpy impact test, this steel plate absorbed less than 40 J at -70°C, indicating that it did not possess good low-temperature toughness in an extremely low-temperature environment.

[0154] The steel plate of test number 34 had a cooling rate CRQ exceeding 2400°C / min during the quenching process at 300-80°C. As a result, the area fraction of coarse γ in this steel plate exceeded 0.0050. Consequently, in the Charpy impact test, this steel plate absorbed less than 40 J at -70°C, indicating that it did not possess good low-temperature toughness in an extremely low-temperature environment.

[0155] In test number 35, the cooling rate CRQ during the quenching process at 300-80°C exceeded 2400°C / min, and the FnB exceeded 160. As a result, the area fraction of coarse γ in this steel sheet exceeded 0.0050. Consequently, in the Charpy impact test, the absorbed energy at -70°C was less than 40 J, indicating that this steel sheet did not possess excellent low-temperature toughness in an extremely low-temperature environment.

[0156] Steel plates numbered 36 and 37 had an FnB value exceeding 160. As a result, the area fraction of coarse γ in these steel plates exceeded 0.0050. Consequently, in the Charpy impact test, these steel plates had an absorbed energy of less than 40 J at -70°C, indicating that they did not possess good low-temperature toughness in cryogenic environments.

[0157] The steel plate of test number 38 had an Fn1 of less than 22.00. As a result, in the corrosion test, this steel plate showed a corrosion rate exceeding 0.100 mm / year and high CO2 ２ It did not exhibit excellent corrosion resistance in that environment.

[0158] The steel plates for test numbers 39 and 40 had an Fn2 of less than 1.00. As a result, these steel plates absorbed less than 40 J of energy at -70°C in the Charpy impact test, indicating that they did not possess good low-temperature toughness in extremely low-temperature environments.

[0159] The steel sheet in test number 41 had too low a Ni content, resulting in an Fn2 of less than 1.00. As a result, the volume fraction of austenite in the microstructure of this steel sheet was 5.0% or less. Consequently, in the Charpy impact test, this steel sheet absorbed less than 40 J at -70°C, indicating that it did not possess good low-temperature toughness in an extremely low-temperature environment.

[0160] The steel plate in test number 42 had too high a Mn content and an Fn1 of less than 22.00. As a result, in the corrosion test, this steel plate showed a corrosion rate exceeding 0.100 mm / year and high CO2. ２ It did not exhibit excellent corrosion resistance in that environment.

[0161] The steel plate in test number 43 had too high a phosphorus (P) content. As a result, in the Charpy impact test, this steel plate absorbed less than 40 J of energy at -70°C, indicating that it did not possess good low-temperature toughness in extremely low-temperature environments.

[0162] 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 percent, C: 0.040% or less, Si: 0.01 to 1.00%, Mn: 0.01 to 0.50%, P: 0.030% or less, S: 0.0050% or less, Cr: 16.0% to less than 18.0%, Ni: greater than 5.00% to 8.00%, Mo: greater than 0.60% to 3.50%, Al: 0.001% to 0.100%, O: 0.0200% or less, N: 0.0200% or less, V: 0 to 0.30%, Ti: 0 to 0.100%, Nb: 0 to 0.100%, Ta: 0 to 0.100%, Zr: 0 to 0.100%, Cu: 0 to less than 1.00%. W: 0-0.50%, Co: 0-0.80%, Ca: 0-0.0050%, Mg: 0-0.0050%, B: 0-0.0050%, Rare earth elements: 0-0.0100%, As: 0-0.100%, Sn: 0-0.100%, Sb: 0-0.100%, Zn: 0-0.100%, Pb: 0-0.100%, and the remainder: Fe and impurities, with Fn1 defined by formula (1) being 22.00 or higher, Fn2 defined by formula (2) being 1.00 or higher, and a yield strength of 552-758 MPa. A stainless steel material having a microstructure in which, by volume fraction, more than 5.0% to 30.0% austenite and the remainder being ferrite and martensite, wherein, among the mixed structures of austenite and martensite, the mixed structures satisfying the C-direction length of the stainless steel material to be 10 μm or more have an average C-direction length of 30 μm or less, and the area fraction of austenite with an equivalent circular diameter of 3.0 μm or more is 0.0050 or less. Fn1 = Cr + 3Mo + 0.5Ni - 5.0Mn (1) Fn2 = 7 / (1 + exp(-18 + Mn - Ni + Cr + 0.2Mo + Cu + 0.01YS)) (2) Here, the C-direction of the stainless steel material means the thickness direction if the stainless steel material is a stainless steel plate, the diameter direction if the stainless steel material is a stainless steel pipe, and the radial direction of the cross-section perpendicular to the axial direction if the stainless steel material is a stainless steel round bar. Furthermore, the elemental symbols in formulas (1) and (2) are substituted with the content of the corresponding element in units of mass percent.If the corresponding element is not present, "0" is substituted for its element symbol. Furthermore, the yield strength in units of MPa is substituted for YS in equation (2).

2. The stainless steel material according to claim 1, wherein the chemical composition is: V: 0.01 to 0.30%, Ti: 0.001 to 0.100%, Nb: 0.001 to 0.100%, Ta: 0.001 to 0.100%, Zr: 0.001 to 0.100%, Cu: 0.01 to less than 1.00%, W: 0.01 to less than 0.50%, Co: 0.01 to 0.80%, Ca: 0.0001 to 0.0050%, Mg: 0.0001 to 0.0050%, B: 0.0001 to 0.0050%, rare earth elements: 0.0001 to 0.0100%, As: 0.001 to 0.100%, A stainless steel material containing one or more elements selected from the group consisting of Sn: 0.001 to 0.100%, Sb: 0.001 to 0.100%, Zn: 0.001 to 0.100%, and Pb: 0.001 to 0.100%.

3. A stainless steel material according to claim 1 or claim 2, wherein the stainless steel material is a stainless steel pipe, and the C direction is the radial direction of the stainless steel pipe.

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