Stainless Steel

Abstract

We provide stainless steel materials that have high strength of 110 ksi or more, excellent SSC resistance, and excellent low-temperature toughness in cryogenic environments. [Solution] The stainless steel material disclosed herein has the chemical composition described in the specification, a yield strength of 758 MPa or more, and a microstructure consisting of, by volume, 0-20% ferrite, 0-15% retained austenite, and the balance martensite, and the element contents and yield strength satisfy formula (1), and the element contents satisfy formula (2). 0.15≦(Sn+As+Sb) / {(Cu+Ni) / YS}≦1.00 (1) (Ni+2Co) / Sn≧900 (2) Here, the element symbols in formulas (1) and (2) are substituted with the content of the corresponding element in units of mass%. When the corresponding element is not contained, "0" is substituted for the element symbol. Furthermore, the yield strength in units of MPa is substituted for YS in formula (1).

Description

[Technical field]

[0001] The present disclosure relates to stainless steel materials. [Background technology]

[0002] Some oil wells and gas wells (hereinafter, oil wells and gas wells are collectively referred to as "oil wells") have environments that contain a large amount of corrosive substances. Examples of corrosive substances are corrosive gases such as hydrogen sulfide (H2S) gas and carbon dioxide (CO2) gas. In this specification, an environment that contains hydrogen sulfide and carbon dioxide gas is referred to as a "sour environment." Oil well steel materials used in sour environments are required to have sulfide stress cracking resistance (hereinafter, referred to as SSC resistance).

[0003] In recent years, the deepening of oil wells has led to a demand for higher strength oil well steel materials. Specifically, 80 ksi class (yield strength of 80 to less than 95 ksi, i.e., less than 552 to 655 MPa) and 95 ksi class (yield strength of 95 to less than 110 ksi, i.e., less than 655 to 758 MPa) oil well steel materials have been widely used. Recently, there has been a demand for oil well steel materials of 110 ksi or more (yield strength of 758 MPa or more). In other words, in recent years, there has been a demand for oil well steel materials that combine high strength of 110 ksi or more with excellent SSC resistance.

[0004] Stainless steel materials having high strength and excellent SSC resistance have been proposed so far in JP 2005-336599 A (Patent Document 1) and JP 2015-110822 A (Patent Document 2).

[0005] The stainless steel material disclosed in Patent Document 1 is a high-strength stainless steel pipe for line pipe, and contains, in mass%, C: 0.001 to 0.015%, Si: 0.01 to 0.5%, Mn: 0.1 to 1.8%, P: 0.03% or less, S: 0.005% or less, Cr: 15 to 18%, Ni: 0.5 to less than 5.5%, Mo: 0.5 to 3.5%, V: 0.0 The stainless steel material has a composition of Cr+0.2-0.2%, N: 0.001-0.015%, O: 0.006% or less, and the balance being Fe and impurities, and satisfies the formulas (Cr+0.65Ni+0.6Mo+0.55Cu-20C≧18.5), (Cr+Mo+0.3Si-43.5C-0.4Mn-Ni-0.3Cu-9N≧11.5), and (C+N≦0.025). Patent Document 1 discloses that this stainless steel material has a high strength of 413 MPa or more in yield strength and excellent resistance to sulfide stress corrosion cracking.

[0006] The stainless steel material disclosed in Patent Document 2 is a high-strength stainless steel seamless pipe for oil wells, which contains, by mass%, C: 0.05% or less, Si: 0.5% or less, Mn: 0.15 to 1.0%, P: 0.030% or less, S: 0.005% or less, Cr: 15.5 to 17.5%, Ni: 3.0 to 6.0%, Mo: 1.5 to 5.0%, Cu: 4.0% or less, W: 0.1 to 2.5%, N: 0.15% or less, and the balance being Fe and impurities, satisfying the formula (-5.9 x (7.82 + 27C-0.91Si + 0.21Mn-0.9Cr + Ni-1.1Mo + 0.2Cu + 11N) ≧ 13.0), the formula (Cu + Mo + 0.5W ≧ 5.8), and the formula (Cu + Mo + W + Cr + 2Ni ≦ 34.5). Patent Document 2 discloses that this stainless steel material has high strength with a yield strength of 758 MPa or more and excellent resistance to sulfide stress corrosion cracking. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] JP 2005-336599 A [Patent Document 2] JP 2015-110822 A Summary of the Invention [Problem to be solved by the invention]

[0008] In recent years, the rise in carbon dioxide (CO2) concentrations on land has become a global problem. As a result, efforts to curb CO2 emissions have been underway. Among these efforts to curb CO2 emissions, CCUS has been attracting particular attention. CCUS stands for Carbon dioxide Capture, Utilization and Storage. In other words, CCUS includes three technologies: CO2 capture, utilization, and storage. Of these, a technology that has been attracting attention as a CO2 storage technology is one that captures CO2 emitted from industrial facilities such as power plants and factories, and injects and stores the CO2 into depleted oil wells.

[0009] Here, when storing CO2, the steel material may be required to have toughness in a cryogenic environment. Specifically, when the pressure of the stored CO2 gas changes, the temperature of the stored gas may drop due to the Joule-Thomson effect. In this case, the steel material may be required to have toughness in a cryogenic environment of -80°C, which is far below normal temperatures. In other words, stainless steel materials intended for use in CCUS applications in addition to OCTG applications are required to have not only high strength and excellent SSC resistance, but also low-temperature toughness in a cryogenic environment of -80°C or below.

[0010] The above Patent Documents 1 and 2 propose techniques for increasing the yield strength of steel materials and improving SSC resistance. However, stainless steel materials having excellent SSC resistance while increasing the yield strength may be obtained by techniques other than those proposed in the above Patent Documents 1 and 2. Furthermore, Patent Documents 1 and 2 do not consider low-temperature toughness in an extremely low-temperature environment of -80°C or lower.

[0011] An object of the present disclosure is to provide a stainless steel material having high strength of 110 ksi or more, excellent SSC resistance, and excellent low-temperature toughness in an extremely low temperature environment. [Means for solving the problem]

[0012] The stainless steel material according to the present disclosure is In mass%, C: 0.050% or less, Si: 1.00% or less, Mn: 1.00% or less, P: 0.050% or less, S: 0.0050% or less, Cr: 13.50~16.50% or less, Mo: 0.50-5.00%, Ni: 1.00-7.00%, Cu: 0.01-3.00%, Co: 0.10-1.50%, Sn: 0.0005~0.0100%, Sol.Al: 0.005~0.050%, N: 0.150% or less, O: 0.0050% or less, W: 0-1.60%, As: 0~0.0100%, Sb: 0 to 0.0100%, Ca: 0 to 0.0050%, Mg: 0 to 0.0050%, B: 0~0.0050%, Rare earth elements: 0~0.100% V: 0~0.50%, Ti: 0 to 0.300%, Nb: 0 to 0.300%, Zr: 0 to 0.200%, Zn: 0 to 0.0100%, Pb: 0 to 0.0100%, and The balance is composed of Fe and impurities. The yield strength is 758 MPa or more, The microstructure is composed of 0 to 20% ferrite, 0 to 15% retained austenite, and the remainder martensite, by volume fraction. The contents of the elements and the yield strength satisfy formula (1), The contents of the elements satisfy formula (2). 0.15≦(Sn+As+Sb) / {(Cu+Ni) / YS}≦1.00 (1) (Ni+2Co) / Sn≧900 (2) Here, the element symbols in formulas (1) and (2) are substituted with the content of the corresponding element in units of mass%. When the corresponding element is not contained, "0" is substituted for the element symbol. Furthermore, the yield strength in units of MPa is substituted for YS in formula (1). Effect of the Invention

[0013] The stainless steel material according to the present disclosure has high strength of 110 ksi or more, excellent SSC resistance, and excellent low-temperature toughness in cryogenic environments. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] First, the present inventors have studied, from the viewpoint of chemical composition, stainless steel materials having high strength of 110 ksi or more, excellent SSC resistance, and excellent low-temperature toughness in cryogenic environments. Specifically, the present inventors have found that tin (Sn), arsenic (As), and antimony (Sb), which have not received attention up to now, have the potential to enhance SSC resistance. As a result of further detailed studies by the present inventors, it has become clear that Sn in particular significantly enhances SSC resistance, and that As and Sb may assist the effect of Sn in enhancing SSC resistance.

[0015] Therefore, the present inventors have conducted detailed studies on the Sn, As and Sb contents that can sufficiently improve the SSC resistance of stainless steel materials. As a result, the present inventors have found that the following contents are sufficient to sufficiently improve the SSC resistance of stainless steel materials: C: 0.050% or less, Si: 1.00% or less, Mn: 1.00% or less, P: 0.050% or less, S: 0.0050% or less, Cr: 13.50 to less than 16.50%, Mo: 0.50 to 5.00%, Ni: 1.00 to 7.00%, Cu: 0.01 to 3.00%, Co: 0.10 to 1.50%, Sn: 0.0005 to 0.0100%, sol.Al: 0.005 to 0.050%, N: 0.150% or less, O: 0.0050% or less, W: 0 to 1.60%, As: 0 to 0.01 It was considered that a stainless steel material consisting of the following elements: 0-0.00%, Sb: 0-0.0100%, Ca: 0-0.0050%, Mg: 0-0.0050%, B: 0-0.0050%, rare earth elements: 0-0.100%, V: 0-0.50%, Ti: 0-0.300%, Nb: 0-0.300%, Zr: 0-0.200%, Zn: 0-0.0100%, Pb: 0-0.0100%, and the balance: Fe and impurities, may have high strength of 110 ksi or more, excellent SSC resistance, and excellent low-temperature toughness in cryogenic environments.

[0016] Here, the microstructure of the stainless steel material having the above-mentioned chemical composition is composed of ferrite, retained austenite, and the balance is martensite. The present inventors have found that in the stainless steel material having the above-mentioned chemical composition, if the microstructure is composed of 0 to 20% ferrite, 0 to 15% retained austenite, and the balance is martensite, the high strength of 110 ksi or more, excellent SSC resistance, and excellent low-temperature toughness in an extremely low temperature environment can be stably improved. That is, in the stainless steel material according to this embodiment, the microstructure is composed of 0 to 20% ferrite, 0 to 15% retained austenite, and the balance is martensite, by volume. In this specification, "composed of ferrite, retained austenite, and martensite" means that the phases other than ferrite, retained austenite, and martensite are negligibly small.

[0017] The present inventors further conducted detailed studies on a method for improving SSC resistance while maintaining yield strength for a stainless steel material having the above-mentioned chemical composition and microstructure and having a yield strength of 758 MPa or more. As a result of detailed studies by the present inventors, it was revealed that in a stainless steel material having the above-mentioned chemical composition and microstructure and having a yield strength of 758 MPa or more, if the element contents and yield strength satisfy formula (1), the SSC resistance of the steel material can be significantly improved. 0.15≦(Sn+As+Sb) / {(Cu+Ni) / YS}≦1.00 (1) Here, the content of the corresponding element in units of mass% is substituted for the element symbol in formula (1). If the corresponding element is not contained, "0" is substituted for the element symbol. Furthermore, the yield strength in units of MPa is substituted for YS in formula (1).

[0018] Fn1 is defined as (Sn+As+Sb) / {(Cu+Ni) / YS}. As mentioned above, As and Sb assist the effect of Sn in increasing the SSC resistance of steel. Furthermore, by setting the ratio of the Sn, As and Sb contents to the Cu and Ni contents within a certain range, the SSC resistance of the steel is significantly increased. On the other hand, the higher the yield strength of the steel, the more likely the SSC resistance of the steel is to decrease. Therefore, the denominator of Fn1 is the ratio of the Cu and Ni contents to the yield strength. In this way, the ratio of the Sn, As and Sb contents to the Cu and Ni contents adjusted according to the yield strength is defined as Fn1. In other words, Fn1 is an index that increases the SSC resistance due to the synergistic effect of Sn, As and Sb, and Cu and Ni, adjusted according to the yield strength.

[0019] As a result of detailed studies by the present inventors based on the above findings, it has become clear that in a stainless steel material having the above-mentioned chemical composition and microstructure, if Fn1 is 0.15 to 1.00, it is possible to achieve both a yield strength of 758 MPa or more and excellent SSC resistance. In other words, the stainless steel material according to this embodiment has the above-mentioned chemical composition and a yield strength of 758 MPa or more, and the element contents and yield strength satisfy formula (1). As a result, the stainless steel material according to this embodiment can achieve both high strength and excellent SSC resistance.

[0020] On the other hand, in the case of a stainless steel material having the above-mentioned chemical composition, microstructure, yield strength of 758 MPa or more, and Fn1 satisfying 0.15 to 1.00, there are cases where low-temperature toughness in a cryogenic environment is not sufficient. Therefore, the present inventors have investigated a method for improving low-temperature toughness in a cryogenic environment while maintaining yield strength and SSC resistance. As a result, it has been found that in the case of a stainless steel material having the above-mentioned chemical composition, microstructure, yield strength of 758 MPa or more, and Fn1 satisfying 0.15 to 1.00, the low-temperature toughness in a cryogenic environment can be improved while maintaining strength and SSC resistance, if the content of elements satisfies formula (2). (Ni+2Co) / Sn≧900 (2) Here, the element symbols in formula (2) are substituted with the contents of the corresponding elements in units of mass %.

[0021] It is defined as Fn2=(Ni+2Co) / Sn. Fn2 is an index of low-temperature toughness in a cryogenic environment. As described above, Sn has the effect of significantly increasing the SSC resistance of steel. On the other hand, as a result of the study by the present inventors, it has become clear that Sn may decrease the low-temperature toughness of steel. In particular, in a cryogenic environment of -80°C, the effect of Sn on low-temperature toughness is likely to become apparent. Therefore, Ni and Co, which have the effect of increasing the low-temperature toughness of steel in a cryogenic environment, are adjusted according to the Sn content. As a result, it is possible to increase the low-temperature toughness in a cryogenic environment while maintaining the strength and SSC resistance of the stainless steel material.

[0022] Specifically, the stainless steel material according to this embodiment has the above-mentioned chemical composition, microstructure, and yield strength of 758 MPa or more, Fn1 satisfies 0.15 to 1.00, and Fn2 is set to be 900 or more. As a result, the stainless steel material according to this embodiment has a yield strength of 758 MPa or more (high strength of 110 ksi or more), excellent SSC resistance, and excellent low-temperature toughness in an extremely low temperature environment.

[0023] The stainless steel material according to this embodiment, which was completed based on the above findings, has the following features.

[0024] [1] A stainless steel material, In mass%, C: 0.050% or less, Si: 1.00% or less, Mn: 1.00% or less, P: 0.050% or less, S: 0.0050% or less, Cr: 13.50~16.50% or less, Mo: 0.50-5.00%, Ni: 1.00-7.00%, Cu: 0.01-3.00%, Co: 0.10-1.50%, Sn: 0.0005~0.0100%, Sol.Al: 0.005~0.050%, N: 0.150% or less, O: 0.0050% or less, W: 0-1.60%, As: 0~0.0100%, Sb: 0 to 0.0100%, Ca: 0 to 0.0050%, Mg: 0 to 0.0050%, B: 0~0.0050%, Rare earth elements: 0~0.100% V: 0~0.50%, Ti: 0 to 0.300%, Nb: 0 to 0.300%, Zr: 0 to 0.200%, Zn: 0 to 0.0100%, Pb: 0 to 0.0100%, and The balance is composed of Fe and impurities. The yield strength is 758 MPa or more, The microstructure is composed of 0 to 20% ferrite, 0 to 15% retained austenite, and the remainder martensite, by volume fraction. The contents of the elements and the yield strength satisfy formula (1), The content of the element satisfies formula (2), Stainless steel material. 0.15≦(Sn+As+Sb) / {(Cu+Ni) / YS}≦1.00 (1) (Ni+2Co) / Sn≧900 (2) Here, the element symbols in formulas (1) and (2) are substituted with the content of the corresponding element in units of mass%. When the corresponding element is not contained, "0" is substituted for the element symbol. Furthermore, the yield strength in units of MPa is substituted for YS in formula (1).

[0025] [2] The stainless steel material according to [1], W: 0.01 to 1.60%, As: 0.0001 to 0.0100%, Sb: 0.0001 to 0.0100%, Ca: 0.0001 to 0.0050%, Mg: 0.0001 to 0.0050%, B: 0.0001~0.0050%, Rare earth elements: 0.001~0.100% V: 0.01 to 0.50%, Ti: 0.001 to 0.300%, Nb: 0.001 to 0.300%, Zr: 0.001 to 0.200%, Zn: 0.0001 to 0.0100%, and Pb: 0.0001 to 0.0100%, containing one or more elements selected from the group consisting of Stainless steel material.

[0026] 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 bar (solid material), or a steel plate. The round bar means a steel bar having a circular cross section perpendicular to the axial direction. The steel pipe may be a seamless steel pipe or a welded steel pipe.

[0027] The stainless steel material according to this embodiment will be described in detail below. In the following description, the stainless steel material will also be simply referred to as "steel material."

[0028] [Chemical composition] The chemical composition of the stainless steel material according to this embodiment contains the following elements: "%" for elements means mass % unless otherwise specified.

[0029] C: 0.050% or less Carbon (C) is inevitably contained. That is, the lower limit of the C content is more than 0%. C forms carbides and increases corrosion sensitivity. Therefore, if the C content is too high, the SSC resistance of the steel material decreases even if the contents of other elements are within the range of this embodiment. If the C content is too high, the low-temperature toughness of the steel material may also decrease. Therefore, the C content is 0.050% or less. The preferred upper limit of the C content is 0.049%, more preferably 0.047%, and even more preferably 0.045%. The C content is preferably as low as possible. However, an extreme reduction in the C content increases the manufacturing cost. Therefore, in consideration of industrial production, the preferred lower limit of the C content is 0.001%, more preferably 0.003%, and even more preferably 0.005%.

[0030] Si: 1.00% or less Silicon (Si) is inevitably contained. That is, the lower limit of the Si content is more than 0%. Si deoxidizes steel. On the other hand, if the Si content is too high, the hot workability of the steel material decreases even if the contents of other elements are within the range of this embodiment. Therefore, the Si content is 1.00% or less. The preferred lower limit of the Si content for effectively obtaining the above effects is 0.01%, more preferably 0.05%, more preferably 0.10%, and even more preferably 0.15%. The preferred upper limit of the Si content is 0.80%, more preferably 0.60%, more preferably 0.50%, and even more preferably 0.45%.

[0031] Mn: 1.00% or less Manganese (Mn) is inevitably contained. That is, the lower limit of the Mn content is more than 0%. Mn improves the hardenability of the steel material and increases the strength of the steel material. On the other hand, Mn may segregate to grain boundaries together with impurity elements such as P and S. Therefore, if the Mn content is too high, the SSC resistance and low-temperature toughness of the steel material will decrease even if the contents of other elements are within the range of this embodiment. Therefore, the Mn content is 1.00% or less. The preferred lower limit of the Mn content for effectively obtaining the above effects is 0.01%, more preferably 0.03%, more preferably 0.05%, more preferably 0.10%, and more preferably 0.15%. The preferred upper limit of the Mn content is 0.80%, more preferably 0.60%, more preferably 0.50%, and more preferably 0.45%.

[0032] P:0.050% or less Phosphorus (P) is an impurity that is inevitably contained. That is, the lower limit of the P content is more than 0%. P segregates at grain boundaries and makes SSC more likely to occur. Therefore, if the P content is too high, even if the contents of other elements are within the range of this embodiment, the SSC resistance of the steel material is significantly reduced. Therefore, the P content is 0.050% or less. A preferable upper limit of the P content is 0.040%, more preferably 0.030%, and even more preferably 0.025%. It is preferable that the P content is as low as possible. However, an extreme reduction in the P content increases the manufacturing cost. Therefore, in consideration of industrial production, a preferable lower limit of the P content is 0.001%, more preferably 0.002%, and even more preferably 0.003%.

[0033] S: 0.0050% or less Sulfur (S) is an impurity that is inevitably contained. That is, the lower limit of the S content is more than 0%. S, like P, segregates at grain boundaries and makes SSC more likely to occur. Therefore, if the S content is too high, even if the contents of other elements are within the range of this embodiment, the SSC resistance of the steel material is significantly reduced. Therefore, the S content is 0.0050% or less. The preferred upper limit of the S content is 0.0040%, more preferably 0.0030%, more preferably 0.0025%, and even more preferably 0.0020%. The S content is preferably as low as possible. However, an extreme reduction in the S content increases the manufacturing cost. Therefore, in consideration of industrial production, the preferred lower limit of the S content is 0.0001%, more preferably 0.0002%, and even more preferably 0.0003%.

[0034] Cr: 13.50~16.50% or less Chromium (Cr) forms a passive film on the surface of a steel material to enhance the SSC resistance of the steel material. If the Cr content is too low, the above effect cannot be sufficiently obtained even if the contents of other elements are within the range of this embodiment. On the other hand, if the Cr content is too high, intermetallic compounds and Cr carbonitrides are likely to be formed in the steel material even if the contents of other elements are within the range of this embodiment. As a result, the SSC resistance and low-temperature toughness of the steel material are reduced. Therefore, the Cr content is 13.50 to less than 16.50%. The preferred lower limit of the Cr content is 13.80%, more preferably more than 14.00%, more preferably 14.05%, and more preferably 14.10%. The preferred upper limit of the Cr content is 16.49%, more preferably 16.45%, more preferably 16.40%, and more preferably 16.25%.

[0035] Mo: 0.50-5.00% Molybdenum (Mo) forms Mo sulfides to improve the SSC resistance of steel materials. Mo also dissolves in steel materials to improve the strength of the steel materials. If the Mo content is too low, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Mo content is too high, austenite is difficult to stabilize even if the contents of other elements are within the ranges of this embodiment. As a result, the volume fraction of ferrite becomes too high, and the SSC resistance and low-temperature toughness of the steel materials may decrease. Therefore, the Mo content is 0.50 to 5.00%. The preferred lower limit of the Mo content is 0.55%, more preferably 0.60%, and more preferably 0.65%. The preferred upper limit of the Mo content is 4.90%, more preferably 4.80%, and more preferably 4.70%.

[0036] Ni: 1.00-7.00% Nickel (Ni) enhances the SSC resistance of steel material by a synergistic effect with Sn, As and Sb. Ni further enhances the low-temperature toughness of steel material, which is reduced by Sn. If the Ni content is too low, the above effect cannot be sufficiently obtained even if the contents of other elements are within the range of this embodiment. On the other hand, if the Ni content is too high, the hydrogen diffusion coefficient in the steel material decreases, and the SSC resistance of the steel material may decrease even if the contents of other elements are within the range of this embodiment. Therefore, the Ni content is 1.00 to 7.00%. The preferred lower limit of the Ni content is 1.03%, more preferably 1.05%, and more preferably 1.10%. The preferred upper limit of the Ni content is 6.95%, and more preferably 6.90%.

[0037] Cu: 0.01-3.00% Copper (Cu) enhances the SSC resistance of steel material by a synergistic effect with Sn, As and Sb. If the Cu content is too low, the above effect cannot be sufficiently obtained even if the contents of other elements are within the range of this embodiment. On the other hand, if the Cu content is too high, the low-temperature toughness of the steel material decreases even if the contents of other elements are within the range of this embodiment. Therefore, the Cu content is 0.01 to 3.00%. The preferred lower limit of the Cu content is 0.01%, more preferably 0.02%, and even more preferably 0.03%. The preferred upper limit of the Cu content is 2.95%, more preferably 2.90%, and even more preferably 2.85%.

[0038] Co: 0.10-1.50% Cobalt (Co) increases the low-temperature toughness of the steel material, which is reduced by Sn. If the Co content is too low, the above effect cannot be sufficiently obtained even if the contents of other elements are within the range of this embodiment. On the other hand, if the Co content is too high, the low-temperature toughness of the steel material decreases even if the contents of other elements are within the range of this embodiment. Therefore, the Co content is 0.10 to 1.50%. The preferred lower limit of the Co content is 0.12%, and more preferably 0.15%. The preferred upper limit of the Co content is 1.40%, and more preferably 1.30%, and more preferably 1.25%.

[0039] Sn: 0.0005~0.0100% Tin (Sn) enhances the SSC resistance of steel materials. If the Sn content is too low, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Sn content is too high, the low-temperature toughness of the steel material decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the Sn content is 0.0005 to 0.0100%. The preferred lower limit of the Sn content is 0.0006%, more preferably 0.0008%, and even more preferably 0.0010%. The preferred upper limit of the Sn content is 0.0098%, more preferably 0.0095%, and even more preferably 0.0090%.

[0040] Sol.Al: 0.005~0.050% Aluminum (Al) deoxidizes steel. If the Al content is too low, the above effect cannot be sufficiently obtained even if the contents of other elements are within the range of this embodiment. On the other hand, if the Al content is too high, coarse oxides are generated, and the SSC resistance and low-temperature toughness of the steel material are reduced even if the contents of other elements are within the range of this embodiment. Therefore, the Al content is 0.005 to 0.050%. The preferred lower limit of the Al content is 0.007%, and more preferably 0.010%. The preferred upper limit of the Al content is 0.048%, and more preferably 0.045%. The Al content in this specification means the content of sol.Al (acid-soluble Al).

[0041] N: 0.150% or less Nitrogen (N) is inevitably contained. That is, the lower limit of the N content is more than 0%. N forms Ti nitrides with Ti and suppresses the coarsening of crystal grains. As a result, the yield strength of the steel material is increased. On the other hand, if the N content is too high, even if the contents of other elements are within the range of this embodiment, coarse nitrides are generated, and the SSC resistance and low-temperature toughness of the steel material are reduced. Therefore, the N content is 0.150% or less. The preferable lower limit of the N content for effectively obtaining the above effects is 0.001%, more preferably 0.003%, and more preferably 0.005%. The preferable upper limit of the N content is 0.148%, more preferably 0.145%, and more preferably 0.140%.

[0042] O: 0.0050% or less Oxygen (O) is an impurity that is inevitably contained. That is, the lower limit of the O content is more than 0%. O forms oxides and reduces the SSC resistance and low-temperature toughness of the steel material. Therefore, if the O content is too high, the SSC resistance and low-temperature toughness of the steel material will decrease even if the contents of other elements are within the range of this embodiment. Therefore, the O content is 0.0050% or less. A preferred upper limit of the O content is 0.0048%, more preferably 0.0045%, and even more preferably 0.0040%. The O content is preferably as low as possible. However, an extreme reduction in the O content increases the manufacturing cost. Therefore, in consideration of industrial production, a preferred lower limit of the O content is 0.0001%, more preferably 0.0003%, and even more preferably 0.0005%.

[0043] The balance of the chemical composition of the stainless steel material according to the present embodiment is composed of Fe and impurities. Here, the impurities in the chemical composition refer to substances that are mixed in from raw materials such as ore and scrap, or the manufacturing environment, during industrial production of the stainless steel material, and are permissible within a range that does not adversely affect the stainless steel material according to the present embodiment.

[0044] [Optional element] The chemical composition of the stainless steel material according to this embodiment may further contain W instead of a portion of Fe.

[0045] W: 0~1.60% Tungsten (W) is an optional element and may not be contained. That is, the W content may be 0%. When contained, W enhances the SSC resistance of the steel material. If even a small amount of W is contained, the above effect can be obtained to some extent. However, if the W content is too high, even if the contents of other elements are within the range of this embodiment, the volume fraction of ferrite becomes too high, and the SSC resistance and low-temperature toughness of the steel material may decrease. Therefore, the W content is 0 to 1.60%. The preferred lower limit of the W content is more than 0%, more preferably 0.01%, more preferably 0.02%, and more preferably 0.03%. The preferred upper limit of the W content is 1.58%, more preferably 1.55%, and more preferably 1.53%.

[0046] The chemical composition of the stainless steel material according to the present embodiment may further contain one or more elements selected from the group consisting of As and Sb in place of a portion of Fe. All of these elements assist the effect of Sn in enhancing the SSC resistance of the steel material.

[0047] As: 0 to 0.0100% Arsenic (As) is an optional element and may not be contained. That is, the As content may be 0%. When contained, As assists the effect of Sn in increasing the SSC resistance of the steel material. If even a small amount of As is contained, the above effect can be obtained to a certain extent. On the other hand, if the As content is too high, As segregates to grain boundaries even if the contents of other elements are within the range of this embodiment, and the SSC resistance of the steel material decreases. Therefore, the As content is 0 to 0.0100%. The preferred lower limit of the As content is more than 0%, more preferably 0.0001%, more preferably 0.0003%, and more preferably 0.0005%. The preferred upper limit of the As content is 0.0080%, more preferably 0.0060%, and more preferably 0.0050%.

[0048] Sb: 0 to 0.0100% Antimony (Sb) is an optional element and may not be contained. That is, the Sb content may be 0%. When contained, Sb assists the effect of Sn in increasing the SSC resistance of the steel material. If even a small amount of Sb is contained, the above effect can be obtained to a certain extent. On the other hand, if the Sb content is too high, even if the contents of other elements are within the range of this embodiment, Sb segregates at the grain boundary, and the SSC resistance of the steel material decreases. Therefore, the Sb content is 0 to 0.0100%. The preferred lower limit of the Sb content is 0.0001%, more preferably 0.0003%, and more preferably 0.0005%. The preferred upper limit of the Sb content is 0.0080%, more preferably 0.0060%, and more preferably 0.0050%.

[0049] The chemical composition of the stainless steel material according to the present embodiment may further contain one or more elements selected from the group consisting of Ca, Mg, B, and rare earth elements in place of a portion of Fe. All of these elements are optional elements, and improve the hot workability of the steel material.

[0050] Ca: 0 to 0.0050% Calcium (Ca) is an optional element and may not be contained. That is, the Ca content may be 0%. When contained, Ca fixes S in the steel material as sulfides to render it harmless, and improves the hot workability of the steel material. If even a small amount of Ca is contained, the above effect can be obtained to a certain extent. However, if the Ca content is too high, even if the contents of other elements are within the range of this embodiment, the oxides in the steel material become coarse, and the SSC resistance and low-temperature toughness of the steel material decrease. Therefore, the Ca content is 0 to 0.0050%. The preferred lower limit of the Ca content is more than 0%, more preferably 0.0001%, more preferably 0.0005%, more preferably 0.0008%, and more preferably 0.0010%. The preferred upper limit of the Ca content is 0.0048%, and more preferably 0.0045%.

[0051] Magnesium: 0 to 0.0050% Magnesium (Mg) is an optional element and may not be contained. That is, the Mg content may be 0%. When contained, Mg fixes S in the steel material as sulfides to render it harmless, and improves the hot workability of the steel material. If even a small amount of Mg is contained, the above effects can be obtained to a certain extent. However, if the Mg content is too high, even if the contents of other elements are within the range of this embodiment, the oxides in the steel material become coarse, and the SSC resistance and low-temperature toughness of the steel material decrease. Therefore, the Mg content is 0 to 0.0050%. The preferable lower limit of the Mg content is more than 0%, more preferably 0.0001%, and more preferably 0.0002%. The preferable upper limit of the Mg content is 0.0048%, and more preferably 0.0045%.

[0052] B: 0 to 0.0050% Boron (B) is an optional element and may not be contained. That is, the B content may be 0%. When contained, B fixes S in the steel material as sulfides to render it harmless, and improves the hot workability of the steel material. If even a small amount of B is contained, the above effect can be obtained to a certain extent. However, if the B content is too high, boron nitride (BN) is formed even if the contents of other elements are within the range of this embodiment, and the low-temperature toughness of the steel material decreases. Therefore, the B content is 0 to 0.0050%. The preferred lower limit of the B content is more than 0%, more preferably 0.0001%, and more preferably 0.0003%. The preferred upper limit of the B content is 0.0040%, more preferably 0.0030%, and more preferably 0.0025%.

[0053] Rare earth elements: 0~0.100% Rare earth elements (REM) are optional elements and may not be contained. That is, the REM content may be 0%. When contained, REM fixes S in the steel material as sulfides to render it harmless, and improves the hot workability of the steel material. The above effect can be obtained to some extent if even a small amount of REM is contained. However, if the REM content is too high, even if the contents of other elements are within the range of this embodiment, the oxides in the steel material become coarse, and the SSC resistance and low-temperature toughness of the steel material decrease. Therefore, the REM content is 0 to 0.100%. The preferred lower limit of the REM content is more than 0%, more preferably 0.001%, and more preferably 0.002%. The preferred upper limit of the REM content is 0.095%, more preferably 0.090%, and more preferably 0.085%.

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

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

[0056] V: 0~0.50% Vanadium (V) is an optional element and may not be contained. That is, the V content may be 0%. When contained, V forms carbonitrides and increases the strength of the steel material. The above effect can be obtained to some extent if even a small amount of V is contained. However, if the V content is too high, even if the contents of other elements are within the range of this embodiment, the strength of the steel material becomes too high and the low-temperature toughness of the steel material decreases. Therefore, the V content is 0 to 0.50%. The preferable lower limit of the V content is more than 0%, more preferably 0.01%, more preferably 0.03%, and more preferably 0.05%. The preferable upper limit of the V content is 0.48%, more preferably 0.45%, and more preferably 0.40%.

[0057] Ti: 0 to 0.300% Titanium (Ti) is an optional element and may not be contained. That is, the Ti content may be 0%. When contained, Ti forms carbonitrides and increases the strength of the steel material. The above effect can be obtained to some extent if even a small amount of Ti is contained. However, if the Ti content is too high, even if the contents of other elements are within the range of this embodiment, the strength of the steel material becomes too high and the low-temperature toughness of the steel material decreases. Therefore, the Ti content is 0 to 0.300%. The preferred lower limit of the Ti content is more than 0%, more preferably 0.001%, more preferably 0.002%, and more preferably 0.003%. The preferred upper limit of the Ti content is 0.250%, more preferably 0.200%, more preferably 0.150%, more preferably 0.100%, and more preferably 0.080%.

[0058] Nb: 0 to 0.300% Niobium (Nb) is an optional element and may not be contained. That is, the Nb content may be 0%. When contained, Nb forms carbonitrides and increases the strength of the steel material. If even a small amount of Nb is contained, the above effect can be obtained to some extent. However, if the Nb content is too high, even if the contents of other elements are within the range of this embodiment, the strength of the steel material becomes too high and the low-temperature toughness of the steel material decreases. Therefore, the Nb content is 0 to 0.300%. The preferred lower limit of the Nb content is more than 0%, more preferably 0.001%, more preferably 0.002%, more preferably 0.003%, more preferably 0.005%, and more preferably 0.010%. The preferred upper limit of the Nb content is 0.280%, more preferably 0.240%, more preferably 0.200%, and more preferably 0.180%.

[0059] Zr: 0 to 0.200% Zirconium (Zr) is an optional element and may not be contained. That is, the Zr content may be 0%. When contained, Zr forms carbonitrides and increases the strength of the steel material. The above effect can be obtained to some extent if even a small amount of Zr is contained. However, if the Zr content is too high, even if the contents of other elements are within the range of this embodiment, the strength of the steel material becomes too high and the low-temperature toughness of the steel material decreases. Therefore, the Zr content is 0 to 0.200%. The preferable lower limit of the Zr content is more than 0%, more preferably 0.001%, more preferably 0.005%, more preferably 0.010%, and more preferably 0.015%. The preferable upper limit of the Zr content is 0.180%, more preferably 0.150%, more preferably 0.100%, and more preferably 0.050%.

[0060] The chemical composition of the stainless steel material according to the present embodiment may further contain one or more elements selected from the group consisting of Zn and Pb instead of a part of Fe. All of these elements are optional elements, and enhance the SSC resistance of the steel material.

[0061] Zn: 0 to 0.0100% Zinc (Zn) is an optional element and may not be contained. That is, the Zn content may be 0%. When contained, Zn enhances the SSC resistance of the steel material. If even a small amount of Zn is contained, the above effect can be obtained to a certain extent. However, if the Zn content is too high, the SSC resistance of the steel material may be decreased even if the contents of other elements are within the range of this embodiment. Therefore, the Zn content is 0 to 0.0100%. The preferred lower limit of the Zn content is more than 0%, more preferably 0.0001%, more preferably 0.0002%, and more preferably 0.0003%. The preferred upper limit of the Zn content is 0.0090%, more preferably 0.0080%, more preferably 0.0060%, and more preferably 0.0055%.

[0062] Pb: 0~0.0100% Lead (Pb) is an optional element and may not be contained. That is, the Pb content may be 0%. When contained, Pb enhances the SSC resistance of the steel material. If even a small amount of Pb is contained, the above effect can be obtained to a certain extent. However, if the Pb content is too high, the hot workability of the steel material decreases even if the contents of other elements are within the range of this embodiment. Therefore, the Pb content is 0 to 0.0100%. The preferred lower limit of the Pb content is more than 0%, more preferably 0.0001%, more preferably 0.0002%, and more preferably 0.0003%. The preferred upper limit of the Pb content is 0.0080%, more preferably 0.0060%, more preferably 0.0050%, and more preferably 0.0040%.

[0063] [Yield strength] The yield strength YS of the stainless steel material according to this embodiment is 758 MPa or more (110 ksi or more). The upper limit of the yield strength YS of the stainless steel material according to this embodiment is not particularly limited, but is, for example, 1069 MPa (155 ksi). That is, the yield strength YS of the stainless steel material according to this embodiment may be 758 to 1069 MPa. The preferred lower limit of the yield strength YS is 760 MPa, more preferably 770 MPa, and even more preferably 780 MPa. The upper limit of the yield strength YS may be 1034 MPa or 1000 MPa.

[0064] In this embodiment, the yield strength of the stainless steel material is determined by the following method. Specifically, a tensile test is performed according to ASTM E8 / E8M (2022). A test piece is prepared from the steel material according to this embodiment. When the steel material is a steel plate, a tensile test piece is prepared from the center of the plate thickness. In this case, the longitudinal direction of the tensile test piece is parallel to the rolling direction of the steel plate. When the steel material is a steel pipe, a round bar test piece or a circular arc test piece is prepared as a tensile test piece from the center of the wall thickness. In this case, the longitudinal direction of the round bar test piece or the circular arc test piece is parallel to the axial direction of the steel pipe. When the steel material is a round steel, a tensile test piece is prepared from the R / 2 position. In this specification, the R / 2 position of the round steel means the central position of the radius R in a cross section perpendicular to the axial direction of the round steel. In this case, the longitudinal direction of the tensile test piece is parallel to the axial direction of the round steel.

[0065] The tensile test piece is, for example, a round bar test piece with a parallel part diameter of 8.9 mm and a gauge length of 35.6 mm. When a round bar test piece cannot be made from a steel pipe, a circular arc test piece is made. The size of the circular arc test piece is, for example, the total thickness, width of 25.4 mm, and gauge length of 50.8 mm. Using the made tensile test piece, a tensile test is performed at room temperature (24±3°C) in accordance with ASTM E8 / E8M(2022). The 0.2% offset yield strength (MPa) obtained by the tensile test is defined as the yield strength (MPa). In this embodiment, the yield strength (MPa) is obtained by rounding off the obtained numerical value to the nearest tenth.

[0066] [Microstructure] The microstructure of the stainless steel material according to the present embodiment is composed of 0-20% by volume of ferrite, 0-15% by volume of retained austenite, and the remainder being martensite. In this specification, the microstructure "composed of ferrite, retained austenite, and martensite" means that the phases other than ferrite, retained austenite, and martensite in the microstructure are negligibly small. For example, in the chemical composition of the stainless steel material according to the present embodiment, the volume fraction of precipitates and inclusions is negligibly small compared to the volume fractions of ferrite, retained austenite, and martensite. That is, the microstructure of the stainless steel material according to the present embodiment may contain minute amounts of precipitates, inclusions, etc. in addition to ferrite, retained austenite, and martensite.

[0067] As described above, in the microstructure of the stainless steel material according to this embodiment, the volume fraction of ferrite is 0 to 20%. In other words, ferrite does not have to be included in the microstructure. On the other hand, if the volume fraction of ferrite is too high, the strength of the steel material decreases. Therefore, in the microstructure of the stainless steel material according to this embodiment, the volume fraction of ferrite is 0 to 20%. A preferred upper limit of the volume fraction of ferrite is 19%, and more preferably 18%. The lower limit of the volume fraction of ferrite may be more than 0%, may be 1%, or may be 2%.

[0068] As described above, in the microstructure of the stainless steel material according to this embodiment, the volume fraction of retained austenite is 0 to 15%. In other words, retained austenite does not have to be included in the microstructure. On the other hand, if the volume fraction of retained austenite is too high, the strength of the steel material decreases. Therefore, in the microstructure of the stainless steel material according to this embodiment, the volume fraction of retained austenite is 0 to 15%. A preferred upper limit of the volume fraction of retained austenite is 14%, more preferably 13%, and even more preferably 12%. The lower limit of the volume fraction of retained austenite may be more than 0%, may be 1%, or may be 2%.

[0069] As described above, the stainless steel material according to the present embodiment has a microstructure consisting of 0 to 20% by volume of ferrite, 0 to 15% by volume of retained austenite, and the balance being martensite. In this specification, "martensite" includes not only fresh martensite but also tempered martensite. The volume fraction of martensite is not particularly limited, but is substantially 65 to 100%. The preferred lower limit of the volume fraction of martensite is 70%, more preferably 72%, and further preferably 75%. The preferred upper limit of the volume fraction of martensite is 100%. Therefore, the stainless steel material according to the present embodiment may have a microstructure consisting of martensite. In other words, the microstructure of the stainless steel material according to the present embodiment may be a martensite single phase.

[0070] In this embodiment, the volume fraction of each phase in the microstructure is determined by the following method. Specifically, the volume fraction (%) of retained austenite and the volume fraction (%) of ferrite in the microstructure of the steel material are determined by the following method. The determined volume fractions of retained austenite and ferrite are subtracted from 100% to determine the volume fraction (%) of martensite.

[0071] [Method for measuring volume fraction of retained austenite] The volume fraction of the retained austenite in the microstructure of the steel material is determined by X-ray diffraction. Specifically, a test piece for measuring the volume fraction of the retained austenite is prepared from the steel material according to this embodiment. When the steel material is a steel plate, the test piece is taken from the center of the plate thickness. When the steel material is a steel pipe, the test piece is taken from the center of the wall thickness. When the steel material is a round steel bar, the test piece is taken from the R / 2 position. The size of the test piece is not particularly limited. The test piece is, for example, 15 mm x 15 mm x 2 mm thick. When the steel material is a steel plate, the thickness direction of the test piece is the plate thickness direction. When the steel material is a steel pipe, the thickness direction of the test piece is the pipe diameter direction. When the steel material is a round steel bar, the thickness direction of the test piece is the diameter direction. Using the prepared test pieces, the X-ray diffraction intensities of the (110) plane of the α phase (martensite), the (200) plane of the α phase, the (211) plane of the α phase, the (111) plane of the γ phase (retained austenite), the (200) plane of the γ phase, and the (220) plane of the γ phase are measured, and the integrated intensity of each plane is calculated.

[0072] In the measurement of X-ray diffraction intensity, the target of the X-ray diffraction device is Co (CoKα radiation), and the output is 30 kV-100 mA. The measurement angle (2θ) is 45 to 105°. After the calculation, the volume fraction of the retained austenite Vγ (%) is calculated for each combination (3×3=9 pairs) of each face of the α phase and each face of the γ phase using formula (I). Then, the average value of the volume fraction of the retained austenite Vγ (%) of the 9 pairs is defined as the volume fraction of the retained austenite (%). Vγ=100 / {1+(Iα×Rγ) / (Iγ×Rα)} (I) Here, Iα is the integrated intensity of the α phase. Rα is the theoretically calculated crystallographic value of the α phase. Iγ is the integrated intensity of the γ phase. Rγ is the theoretically calculated crystallographic value of the γ phase. The values ​​of Rα and Rγ for each surface can be those incorporated in the residual γ quantitative analysis system attached to the RINT-TTR product manufactured by Rigaku Corporation. The volume fraction of retained austenite is calculated by rounding off the obtained value to one decimal place.

[0073] [Method of measuring ferrite volume fraction] The volume fraction of ferrite in the microstructure of the steel material is determined by the point counting method. Specifically, a test piece for measuring the volume fraction of ferrite is prepared from the steel material according to this embodiment. When the steel material is a steel plate, the test piece is taken from the center of the plate thickness. When the steel material is a steel pipe, the test piece is taken from the center of the wall thickness. When the steel material is a round steel bar, the test piece is taken from the R / 2 position. The size of the test piece is not particularly limited. When the steel material is a steel plate, the observation surface of the test piece is a surface parallel to the rolling direction of the steel plate. When the steel material is a steel pipe, the observation surface of the test piece is a surface parallel to the axial direction of the steel pipe. When the steel material is a round steel bar, the observation surface of the test piece is a surface parallel to the axial direction of the round steel bar. After mechanically polishing the observation surface, the observation surface is electrolytically etched to reveal the structure. Electrolytic etching was performed with a mixture of aqua regia (a solution of hydrochloric acid and nitric acid mixed in a ratio of 3:1) and glycerin, and a current density of 1A / cm. 2 The electrolysis time is 1 minute.

[0074] The electrolytically etched observation surface is observed in 30 fields of view using an optical microscope. The observation field is a rectangle of 250 μm × 250 μm. The observation magnification is 400 times. In each observation field, a person skilled in the art can distinguish between ferrite and other phases (residual austenite and martensite) from the contrast. Therefore, the ferrite in each observation field is identified based on the contrast. The area ratio of the identified ferrite is calculated using a point counting method in accordance with JIS G 0555 (2020).

[0075] Specifically, 20 vertical lines are drawn at equal intervals from the top to the bottom of the observation field. That is, the observation field is divided into 21 regions in the left-right direction by the 20 vertical lines. Furthermore, 20 horizontal lines are drawn at equal intervals from the left end to the right end of the observation field. That is, the observation field is divided into 21 regions in the up-down direction by the 20 horizontal lines. At this time, the intersections of the vertical and horizontal lines are called lattice points. That is, 400 lattice points are arranged at equal intervals in the observation field. In accordance with JIS G 0555 (2020), the lattice points that overlap with ferrite are counted in the observation field. The number of lattice points that overlap with ferrite obtained in 30 fields is divided by the total number of lattice points (400 × 30 = 12000) and defined as the ferrite area ratio. In this embodiment, the area ratio of ferrite obtained by the above method is defined as the volume ratio (%) of ferrite. The volume fraction of ferrite is determined by rounding off the obtained value to one decimal place.

[0076] Using the volume fraction (%) of retained austenite obtained by the above-mentioned X-ray diffraction method and the volume fraction (%) of ferrite obtained by the above-mentioned point calculation method, the volume fraction (%) of martensite in the microstructure of the steel material is calculated according to the following formula. Martensite volume fraction (%) = 100 - {Retained austenite volume fraction (%) + Ferrite volume fraction (%)}

[0077] [Formula (1)] The stainless steel material according to this embodiment has the above-mentioned chemical composition and yield strength of 758 MPa or more, and the element contents and yield strength YS satisfy formula (1). As a result, the stainless steel material according to this embodiment has a yield strength of 758 MPa or more, excellent SSC resistance, and excellent low-temperature toughness in an extremely low-temperature environment, provided that the other configurations of this embodiment are satisfied. 0.15≦(Sn+As+Sb) / {(Cu+Ni) / YS}≦1.00 (1) Here, the content of the corresponding element in units of mass% is substituted for the element symbol in formula (1). If the corresponding element is not contained, "0" is substituted for the element symbol. Furthermore, the yield strength in units of MPa is substituted for YS in formula (1).

[0078] Fn1 (=(Sn+As+Sb) / {(Cu+Ni) / YS}) is an index for enhancing SSC resistance by the synergistic effect of Sn, As, and Sb on the one hand, and Cu and Ni on the other, which is adjusted according to the yield strength. On the premise that the steel has the above-mentioned chemical composition and microstructure and the yield strength YS is 758 MPa or more, if Fn1 is 0.15 to 1.00, the steel has stable and excellent SSC resistance. Therefore, in this embodiment, Fn1 is set to 0.15 to 1.00. The preferable lower limit of Fn1 is 0.16, and more preferably 0.17. The preferable upper limit of Fn1 is 0.99, and more preferably 0.98. In this embodiment, Fn1 is calculated by rounding off the obtained numerical value to two decimal places.

[0079] [Formula (2)] The stainless steel material according to this embodiment has an element content within the above-mentioned range of chemical composition that satisfies formula (2). As a result, the stainless steel material according to this embodiment has a yield strength of 758 MPa or more, excellent SSC resistance, and excellent low-temperature toughness in an extremely low-temperature environment, provided that the other configurations of this embodiment are satisfied. (Ni+2Co) / Sn≧900 (2) Here, the element symbols in formula (2) are substituted with the contents of the corresponding elements in units of mass %.

[0080] Fn2 (= (Ni + 2Co) / Sn) is an index of low temperature toughness in a cryogenic environment. On the premise that the steel has the above-mentioned chemical composition and microstructure, has a yield strength YS of 758 MPa or more, and satisfies Fn1 of 0.15 to 1.00, if Fn2 is 900 or more, the steel has stable and excellent low temperature toughness even in a cryogenic environment. Therefore, in this embodiment, Fn2 is 900 or more. A preferable lower limit of Fn2 is 901, more preferably 905, and more preferably 910. The upper limit of Fn2 is not particularly limited, but is substantially 22000. The upper limit of Fn2 may be 20000, 19000, 17000, or 15000. In this embodiment, Fn2 is obtained by rounding off the first decimal place of the obtained numerical value.

[0081] [SSC resistance] The stainless steel material according to this embodiment has the above-mentioned chemical composition and microstructure, a yield strength of 758 MPa or more, Fn1 satisfies 0.15 to 1.00, and Fn2 satisfies 900 or more. As a result, the stainless steel material according to this embodiment has high strength, excellent SSC resistance, and excellent low-temperature toughness in an extremely low temperature environment. In this embodiment, excellent SSC resistance is defined in the following way.

[0082] Specifically, the SSC resistance test is performed by a method conforming to NACE TM0177-2016 Method A. A round bar test piece is prepared from the stainless steel material according to this embodiment. When the steel material is a steel plate, the round bar test piece is prepared from the center of the plate thickness. In this case, the axial direction of the round bar test piece is parallel to the rolling direction of the steel plate. When the steel material is a steel pipe, the round bar test piece is prepared from the center of the wall thickness. In this case, the axial direction of the round bar test piece is parallel to the axial direction of the steel pipe. When the steel material is a round steel bar, the round bar test piece is prepared from the R / 2 position. In this case, the axial direction of the round bar test piece is parallel to the axial direction of the round steel bar. The size of the round bar test piece is, for example, a diameter of 6.35 mm and a length of the parallel part of 25.4 mm.

[0083] The test solution is a 0.17% by mass sodium chloride aqueous solution adjusted to pH 2.7 with acetic acid. A stress equivalent to 90% of the actual yield stress is applied to the prepared round bar test piece. The test solution at 24°C is poured into a test vessel so that the round bar test piece to which the stress has been applied is immersed, forming a test bath. After the test bath is degassed, a mixed gas of 0.03 atm H2S gas and 0.97 atm CO2 gas is blown into the test bath to saturate the test bath. The test bath saturated with the mixed gas is held at 24°C for 720 hours. In the SSC resistance test carried out under the above conditions, the steel material according to this embodiment has no cracks after 720 hours. In this specification, "no cracks are confirmed" means that no cracks are confirmed when the test piece after the test is observed by the naked eye.

[0084] [Low temperature toughness] The stainless steel material according to this embodiment has the above-mentioned chemical composition and microstructure, a yield strength of 758 MPa or more, Fn1 satisfies 0.15 to 1.00, and Fn2 satisfies 900 or more. As a result, the stainless steel material according to this embodiment has high strength, excellent SSC resistance, and excellent low-temperature toughness in a cryogenic environment. In this embodiment, excellent low-temperature toughness in a cryogenic environment is defined in the following manner.

[0085] Specifically, a full-size or sub-size V-notch test piece is prepared from the stainless steel material according to this embodiment in accordance with API 5CT (2019). Here, when the steel material is a steel plate, the rolling direction of the steel plate is defined as the "L direction" (longitudinal), and the plate width direction of the steel plate is defined as the "T direction" (transverse). When the steel material is a steel pipe, the pipe diameter direction of the steel pipe is defined as the "C direction", the pipe axial direction of the steel pipe is defined as the "L direction", and the direction perpendicular to the C direction and the L direction is defined as the "T direction". When the steel material is a round bar, the cross-sectional diameter direction of the round bar is defined as the "C direction", the axial direction of the round bar is defined as the "L direction", and the direction perpendicular to the C direction and the L direction is defined as the "T direction".

[0086] The prepared V-notch test specimen is subjected to a Charpy impact test in accordance with JIS Z 2242 (2018) to obtain the absorbed energy (J) at -80°C. When a sub-size V-notch test specimen is used, the obtained absorbed energy is divided by the reduction factor described in API 5CT (2019) to convert it to the absorbed energy of a full-size V-notch test specimen. In this embodiment, the absorbed energy (J) at -80°C is obtained by rounding off the obtained value to the first decimal place.

[0087] In this embodiment, when the absorbed energy at -80°C obtained under the above conditions is 60 J or more, it is evaluated as having excellent low-temperature toughness in a cryogenic environment. Note that in this specification, the absorbed energy at -80°C is also simply referred to as "absorbed energy".

[0088] [Stainless steel material shape] As described above, the shape of the stainless steel material according to the present embodiment is not particularly limited. Preferably, the stainless steel material according to the present embodiment is a seamless steel pipe. When the stainless steel material according to the present embodiment is a seamless steel pipe, even if the wall thickness is 5 mm or more, it has high strength, excellent SSC resistance, and excellent low-temperature toughness in an extremely low temperature environment.

[0089] [Manufacturing method] An example of a method for manufacturing a stainless steel material according to this embodiment having the above-mentioned configuration will be described. Note that the method for manufacturing a stainless steel material according to this embodiment is not limited to the manufacturing method described below. The example of a method for manufacturing a stainless steel material according to this embodiment includes a step of preparing an intermediate steel material (preparation step), a step of quenching the intermediate steel material (quenching step), and a step of tempering (tempering step). Each step will be described in detail below.

[0090] [Preparation process] In the preparation step, an 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 manufacturing method of the intermediate steel material is not particularly limited. The intermediate steel material referred to here is a plate-shaped steel material when the final product is a steel plate or a welded steel pipe, a blank pipe when the final product is a seamless steel pipe, and a steel material having a circular cross section perpendicular to the axial direction when the final product is a round steel.

[0091] The preparation step may include a step of preparing a material (material preparation step) and a step of hot working the material to produce an intermediate steel material (hot working step). Hereinafter, the case including the material preparation step and the hot working step will be described in detail.

[0092] [Material preparation process] In the material preparation step, a material is manufactured using molten steel having the above-mentioned chemical composition. The method for manufacturing the material is not particularly limited and may be a well-known method. Specifically, a cast piece (slab, bloom, or billet) may be manufactured using the molten steel by a continuous casting method. An ingot may be manufactured using the molten steel by an ingot casting method. If necessary, the slab, bloom, or ingot may be rolled to manufacture a billet. The material (slab, bloom, or billet) is manufactured by the above steps.

[0093] [Hot processing process] In the hot working process, the prepared material is hot worked to produce an intermediate steel material. As described above, when the steel material is a seamless steel pipe, the intermediate steel material corresponds to a mother pipe. First, the billet is heated in a heating furnace. The heating temperature is not particularly limited, but is, for example, 1100 to 1300°C. The billet extracted from the heating furnace is hot worked to produce a mother pipe (seamless steel pipe). The method of hot working is not particularly limited, and may be a well-known method.

[0094] For example, the Mannesmann process may be carried out as the hot working to produce a mother pipe. In this case, a round billet is pierced and rolled by a piercing machine. When piercing and rolling is carried out, the piercing ratio is not particularly limited, but is, for example, 1.0 to 4.0. The pierced and rolled round billet is further hot rolled by a mandrel mill, a reducer, a sizing mill, or the like to produce a mother pipe. The cumulative reduction in area in the hot working process is, for example, 20 to 70%. A mother pipe may be produced from the billet by carrying out another hot working method. For example, when the steel material is a short, thick-walled steel pipe such as a coupling, the mother pipe may be produced by forging such as the Erhardt process. The mother pipe is produced by the above steps. The wall thickness of the mother pipe is not particularly limited, but is, for example, 9 to 60 mm.

[0095] When the steel material is a round bar, the material is first heated in a heating furnace. The heating temperature is not particularly limited, but is, for example, 1100 to 1300°C. The material extracted from the heating furnace is subjected to hot processing to produce an intermediate steel material having a circular cross section perpendicular to the axial direction. The hot processing is, for example, blooming rolling by a blooming mill or hot rolling by a continuous rolling mill. The continuous rolling mill alternately arranges horizontal stands having a pair of grooved rolls arranged side by side in the vertical direction and vertical stands having a pair of grooved rolls arranged side by side in the horizontal direction. When the steel material is a steel plate, the material is first heated in a heating furnace. The heating temperature is not particularly limited, but is, for example, 1100 to 1300°C. The material extracted from the heating furnace is subjected to hot rolling using a blooming mill and a continuous rolling mill to produce an intermediate steel material in the shape of a steel plate.

[0096] The intermediate steel produced by hot working may be air-cooled (as-rolled). The intermediate steel produced by hot working may be quenched directly after hot working without being cooled to room temperature, or may be quenched after reheating (reheating) after hot working. When quenching is performed directly after hot working or after reheating, cooling may be stopped or slow cooling may be performed during quenching. In this case, it is possible to suppress the occurrence of quenching cracks in the mother pipe. When quenching is performed directly after hot working or after reheating, stress relief annealing (SR) may be performed after quenching and before the heat treatment in the next process. In this case, residual stress in the mother pipe is removed.

[0097] As described above, in the preparation step, an intermediate steel material is prepared. The intermediate steel material may be manufactured by the above-mentioned preferred steps, or an intermediate steel material manufactured by a third party, or an intermediate steel material manufactured in a factory or business establishment other than the factory where the quenching step and tempering step described below are performed, may be prepared. The heat treatment step will be described in detail below.

[0098] [Quenching process] In the quenching process, the intermediate steel material produced in the hot working process is quenched. Quenching is performed by a well-known method. Specifically, the intermediate steel material after the hot working process may be loaded into a heat treatment furnace, held at the quenching temperature, and then rapidly cooled (quenched). In this case, the temperature (°C) of the heat treatment furnace for heating the intermediate steel material in the quenching process is also called the quenching temperature.

[0099] If the quenching temperature is too low, the intermediate steel material may not be heated sufficiently, and the above-mentioned microstructure may not be obtained in the manufactured stainless steel material. On the other hand, if the quenching temperature is too high, the volume fraction of ferrite may become too high, and excellent low-temperature toughness may not be obtained. Therefore, in this embodiment, the preferred quenching temperature is 850 to 1150°C. The holding time at the quenching temperature is not particularly limited, but is, for example, 5 to 80 minutes.

[0100] The quenching method in the quenching step is not particularly limited, but may be, for example, water cooling. When the intermediate steel material is a mother tube, the mother tube may be quenched, for example, by immersing it in a water tank or an oil tank, or the mother tube may be quenched by pouring or spraying cooling water onto the outer and / or inner surface of the mother tube by shower cooling or mist cooling.

[0101] After the hot working process, the intermediate steel may be quenched (direct quenched) immediately after the hot working without being cooled to room temperature, or the raw pipe after hot working may be loaded into a reheating furnace before the temperature of the raw pipe after hot working drops, and held at the quenching temperature, after which quenching may be performed.

[0102] [Tempering process] The intermediate steel material after quenching is further subjected to a tempering process. In the tempering process, the intermediate steel material is loaded into a heat treatment furnace and held at the tempering temperature to adjust the yield strength of the steel material. At this time, the temperature (℃) of the heat treatment furnace for heating the intermediate steel material in the tempering process is also called the tempering temperature.

[0103] If the tempering temperature is too low, the strength may become too high, and the SSC resistance and low-temperature toughness may decrease. On the other hand, if the tempering temperature is too high, the desired yield strength may not be obtained. Therefore, in this embodiment, the preferable tempering temperature is 550 to 700°C. The holding time at the tempering temperature is not particularly limited, but is, for example, 10 to 180 minutes. It is well known to those skilled in the art that the yield strength of the steel material can be adjusted by appropriately adjusting the tempering temperature according to the chemical composition. Therefore, the tempering conditions are adjusted so that the yield strength of the steel material is 758 MPa or more.

[0104] The stainless steel material according to the present embodiment can be manufactured by the above steps. As described above, the stainless steel material according to the present embodiment is not limited to the above manufacturing method. The stainless steel material according to the present embodiment will be described in more detail below with reference to examples. EXAMPLES

[0105] Molten steel having the chemical composition shown in Tables 1A, 1B, and 1C was melted using a 50 kg vacuum melting furnace, and steel ingots were produced by the ingot casting method. In Tables 1B and 1C, "-" means that the content of the corresponding element was at the impurity level. For example, the W content and V content of Test No. 1 are rounded to the nearest third decimal place and are 0%. Similarly, the REM content, Ti content, Nb content, and Zr content of Test No. 1 are rounded to the nearest fourth decimal place and are 0%. Similarly, the As content, Sb content, Ca content, Mg content, B content, Zn content, and Pb content of Test No. 1 are rounded to the nearest fifth decimal place and are 0%.

[0106] [Table 1A]

[0107] [Table 1B]

[0108] [Table 1C]

[0109] The ingots of each test number were heated at 1200-1250°C for 2 hours, and then hot worked to produce intermediate steel materials (blank pipes) with a wall thickness of 25.4 mm and an outer diameter of 177.8 mm. The intermediate steel materials of each test number were subjected to a quenching process and a tempering process. Specifically, the intermediate steel materials of each test number were held at 910°C for 15 minutes, and then quenched by rapid cooling. Then, the intermediate steel materials of each test number were tempered by holding them at 580-680°C for 30-60 minutes. Seamless steel pipes of each test number were produced by the above manufacturing process.

[0110] [Evaluation test] The seamless steel pipes having each test number obtained were subjected to a tensile test, a microstructure observation test, an SSC resistance test, and a Charpy impact test.

[0111] [Tensile test] A tensile test was conducted on the seamless steel pipe of each test number in accordance with ASTM E8 / E8M(2022). Specifically, a round bar tensile test specimen with a parallel part diameter of 8.9 mm and a gauge length of 35.6 mm was prepared from the center of the wall thickness of the seamless steel pipe of each test number. The longitudinal direction of the round bar tensile test specimen was parallel to the rolling direction of the steel plate. A tensile test was conducted at room temperature (24±3°C) in air using the round bar tensile test specimen of each test number to determine the 0.2% offset yield strength (MPa). The obtained 0.2% offset yield strength was defined as the yield strength YS (MPa). The obtained yield strength YS of each test number is shown in the "YS (MPa)" column of Table 2.

[0112] [Table 2]

[0113] From the chemical compositions shown in Tables 1A to 1C, the yield strengths YS shown in Table 2, and the above-mentioned definitions, Fn1 (=(Sn+As+Sb) / {(Cu+Ni) / YS}) was determined for the seamless steel pipe of each test number. The obtained Fn1 for each test number is shown in Table 2. Furthermore, from the chemical compositions shown in Tables 1A to 1C, and the above-mentioned definitions, Fn2 (=(Ni+2Co) / Sn) was determined for the seamless steel pipe of each test number. The obtained Fn2 for each test number is shown in Table 2.

[0114] [Microstructure observation test] A microstructure observation test was performed on the seamless steel pipes of each test number by the method described above. Specifically, the volume fraction (%) of retained austenite was determined by the X-ray diffraction method performed by the method described above. Furthermore, the volume fraction (%) of ferrite was determined by the point counting method in accordance with JIS G 0555 (2020) performed by the method described above. The volume fraction (%) of martensite was determined from the volume fractions of retained austenite and ferrite obtained. The volume fraction of ferrite obtained for each test number is shown in the "Ferrite (volume%)" column of Table 2. The volume fraction of retained austenite obtained for each test number is shown in the "Residual γ (volume%)" column of Table 2. The volume fraction of martensite obtained for each test number is shown in the "Martensite (volume%)" column of Table 2.

[0115] [SSC resistance test] The seamless steel pipes of each test number were subjected to the SSC resistance test by the above-mentioned method. Specifically, the SSC resistance test was performed on three round bar test pieces prepared by the above-mentioned method in accordance with NACE TM0177-2016 Method A. The axial direction of the test pieces was parallel to the pipe axial direction. A tensile stress equivalent to 90% of the actual yield stress was applied in the axial direction of the round bar test pieces of each test number. The test solution used was a 0.17 mass% sodium chloride aqueous solution adjusted to pH 2.7 with acetic acid.

[0116] The test solution at 24°C was poured into each of the three test vessels to form the test bath. The three round bar test specimens to which stress had been applied were immersed one by one in the test bath in a different test vessel. After degassing each test bath, a mixture of 0.03 atm H2S gas and 0.97 atm CO2 gas was blown into the test bath to saturate it. The test bath saturated with the mixed gas was held at 24°C for 720 hours. The round bar test specimens of each test number after 720 hours were observed for the occurrence of sulfide stress cracking (SSC). Specifically, the round bar test specimens after 720 hours were observed with the naked eye. As a result of the observation, those in which no cracks were found in all three test specimens were judged to be "E" (Excellent). On the other hand, those in which cracks were found in at least one test specimen were judged to be "NA" (Not Acceptable). The evaluation results for each test number are shown in the "SSC Resistance" column in Table 2.

[0117] [Charpy impact test] A Charpy impact test was conducted on the seamless steel pipes with each test number in accordance with JIS Z 2242 (2018). Specifically, full-size V-notch test specimens were prepared in accordance with API 5CT (2019) using the method described above. A Charpy impact test was conducted on the prepared V-notch test specimens in accordance with JIS Z 2242 (2018) to determine the absorbed energy (J) at -80°C. The absorbed energy at -80°C for each test number obtained is shown in the "vE(-80°C)(J)" column in Table 2.

[0118] [Evaluation Results] With reference to Tables 1A, 1B, 1C, and 2, the seamless steel pipes of test numbers 1 to 17 had appropriate chemical compositions, yield strengths of 758 MPa or more, and microstructures consisting of 0 to 20% ferrite, 0 to 15% retained austenite, and the balance martensite by volume fraction. These seamless steel pipes further had Fn1 of 0.15 to 1.00 and Fn2 of 900 or more. As a result, these seamless steel pipes were judged to have excellent SSC resistance in the SSC resistance test. These seamless steel pipes further had absorbed energy of 60 J or more at -80°C in the Charpy impact test, and were judged to have excellent low-temperature toughness even in an extremely low-temperature environment.

[0119] On the other hand, the Mo content in test number 18 was too low, and as a result, this seamless steel pipe was determined not to have excellent SSC resistance in the SSC resistance test.

[0120] Test No. 19 had too low Ni content and too low Fn2, and as a result, the absorbed energy of this seamless steel pipe at -80°C was less than 60 J, and it was determined that this seamless steel pipe did not have excellent low-temperature toughness in an extremely low temperature environment.

[0121] Test No. 20 had too low Ni content and too low Co content. Furthermore, Fn2 was too low. As a result, the volume fraction of ferrite in the microstructure of this seamless steel pipe exceeded 20%, and the yield strength was less than 758 MPa. As a result, the absorbed energy of this seamless steel pipe at -80°C was less than 60 J, and it was determined that this seamless steel pipe did not have excellent low-temperature toughness in an extremely low-temperature environment.

[0122] Test No. 21 had too high an Sn content and too low an Fn2, and as a result, the absorbed energy of this seamless steel pipe at -80°C was less than 60 J, and it was determined that this seamless steel pipe did not have excellent low-temperature toughness in an extremely low-temperature environment.

[0123] Test No. 22 had too low an Sn content and too low an Fn1, and as a result, this seamless steel pipe was judged not to have excellent SSC resistance in the SSC resistance test.

[0124] Test No. 23 had an excessively low Sn content, and as a result, this seamless steel pipe was determined not to have excellent SSC resistance in the SSC resistance test.

[0125] Test Nos. 24 to 26 had too high Fn1, and as a result, these seamless steel pipes were determined not to have excellent SSC resistance in the SSC resistance test.

[0126] Test Nos. 27 to 29 had too low Fn1, and as a result, these seamless steel pipes were judged not to have excellent SSC resistance in the SSC resistance test.

[0127] Test Nos. 30 to 32 had too low Fn2, and as a result, these seamless steel pipes had absorbed energy of less than 60 J at −80° C., and were determined not to have excellent low-temperature toughness in an extremely low-temperature environment.

[0128] The embodiments of the present disclosure have been described above. However, the above-described embodiments are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiments, and can be implemented by appropriately modifying the above-described embodiments without departing from the spirit of the present disclosure.

Claims

1. A stainless steel material, In mass percent, C: 0.050% or less, Si: 1.00% or less, Mn: 1.00% or less, P: 0.050% or less, S: 0.0050% or less, Cr: 13.50 to less than 16.50%; Mo: 0.50-5.00%, Ni: 1.00-7.00%, Cu: 0.01-3.00%, Co: 0.10 to 1.50%, Sn: 0.0005-0.0100%, sol. Al: 0.005 to 0.050%, N: 0.150% or less, O: 0.0050% or less, W: 0-1.60%, As: 0 to 0.0100%, Sb: 0 to 0.0100%, Ca: 0-0.0050%, Mg: 0 to 0.0050%, B: 0 to 0.0050%, Rare earth elements: 0-0.100% V: 0-0.50%, Ti: 0-0.300%, Nb: 0 to 0.300%, Zr: 0-0.200%, Zn: 0 to 0.0100%, Pb: 0 to 0.0100%, and The balance is composed of Fe and impurities. The yield strength is 758 MPa or more, The microstructure is composed of, by volume, 0 to 20% ferrite, 0 to 15% retained austenite, and the remainder martensite; The contents of the elements and the yield strength satisfy formula (1), The content of the element satisfies formula (2), Stainless steel material. 0.15≦(Sn+As+Sb) / {(Cu+Ni) / YS}≦1.00 (1) (Ni+2Co) / Sn≧900 (2) Here, the element symbols in formulas (1) and (2) are substituted with the content of the corresponding element in units of mass %. When the corresponding element is not contained, "0" is substituted for the element symbol. Furthermore, the yield strength in units of MPa is substituted for YS in formula (1).

2. The stainless steel material according to claim 1, W: 0.01-1.60%, As: 0.0001 to 0.0100%, Sb: 0.0001 to 0.0100%, Ca: 0.0001-0.0050%, Mg: 0.0001 to 0.0050%, B: 0.0001 to 0.0050%, Rare earth elements: 0.001-0.100% V: 0.01-0.50%, Ti: 0.001-0.300%, Nb: 0.001-0.300%, Zr: 0.001 to 0.200%, Zn: 0.0001 to 0.0100%, and Pb: 0.0001 to 0.0100%, containing one or more elements selected from the group consisting of Stainless steel material.

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