Stainless steel

Abstract

To provide a stainless steel having high intensity of 110 ksi or more, excellent SSC resistance and excellent low-temperature toughness in an ultra-low temperature environment.SOLUTION: The stainless steel has a chemical composition described in the specification, and in the stainless steel, a yield strength is 758 MPa or more, a micro structure thereof includes, in a volume fraction, ferrite of 35-70%, retained austenite of 0-15% and a balance martensite, and a content of chemical elements and the yield strength satisfy an expression (1) and the content of chemical elements satisfies an expression (2): 0.10≤(Sn+As+Sb) / {(Cu+Ni) / YS}≤1.00 (1); and (Ni+2Co) / Sn≥900 (2). A content of a corresponding chemical element is assigned by unit:mass% to a symbol of chemical element in the expressions (1) and (2). When a corresponding chemical element is not contained, "0" is assigned to the chemical element. The yield strength is assigned by unit:MPa to the YS in the expression (1).SELECTED DRAWING: None

Description

Technical Field

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

Background Art

[0002] In oil wells and gas wells (hereinafter, oil wells and gas wells are collectively referred to as "oil wells"), there are environments containing a large amount of corrosive substances. Corrosive substances are, for example, corrosive gases such as hydrogen sulfide (H2S) gas and carbonic acid (CO2) gas. Here, in this specification, an environment containing hydrogen sulfide and carbon dioxide gas is referred to as a "sour environment". Oil well steel materials used in a sour environment are required to have sulfide stress cracking resistance (Sulfide Stress Cracking resistance: hereinafter referred to as SSC resistance).

[0003] In recent years, due to the deepening of oil wells, higher strength of oil well steel materials has been required. Specifically, oil well steel materials of 80 ksi grade (yield strength is less than 80 to 95 ksi, that is, less than 552 to 655 MPa) and 95 ksi grade (yield strength is less than 95 to 110 ksi, that is, less than 655 to 758 MPa) have been widely used. Recently, furthermore, oil well steel materials with a yield strength of 110 ksi or more (758 MPa or more) have begun to be demanded. That is, in recent years, oil well steel materials that achieve both high strength of 110 ksi or more and excellent SSC resistance have been demanded.

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

[0005] The stainless steel material disclosed in Patent Document 1 is a high-strength stainless steel pipe for line pipes. By mass, C: 0.001 to 0.015%, Si: 0.01 to 0.5%, Mn: 0.1 to 1.8%, P: 0.03% or less, S: 0.005% or less, Cr: 15 to 18%, Ni: less than 0.5 to 5.5%, Mo: 0.5 to 3.5%, V: 0.02 to 0.2%, N: 0.001 to 0.015%, O: 0.006% or less, and the balance is Fe and impurities, and it satisfies the formulas (Cr + 0.65Ni + 0.6Mo + 0.55Cu - 20C ≥ 18.5), (Cr + Mo + 0.3Si - 43.5C - 0.4Mn - Ni - 0.3Cu - 9N ≥ 11.5), and (C + N ≤ 0.025). This stainless steel material is disclosed in Patent Document 1 as having a high strength with a yield strength of 413 MPa or more and excellent sulfide stress corrosion cracking resistance.

[0006] The stainless steel material disclosed in Patent Document 2 is a high-strength seamless stainless steel pipe for oil wells. 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 is Fe and impurities, and it satisfies the formulas (-5.9×(7.82 + 27C - 0.91Si + 0.21Mn - 0.9Cr + Ni - 1.1Mo + 0.2Cu + 11N) ≥ 13.0), (Cu + Mo + 0.5W ≥ 5.8), and (Cu + Mo + W + Cr + 2Ni ≤ 34.5). This stainless steel material is disclosed in Patent Document 2 as having a high strength with a yield strength of 758 MPa or more and excellent sulfide stress corrosion cracking resistance.

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0008] By the way, in recent years, the increase in the concentration of carbon dioxide (CO2) on the ground has become a global problem. Therefore, efforts have been made to suppress CO2 emissions. Among such efforts to suppress CO2 emissions, in particular, CCUS has been attracting attention. CCUS is an abbreviation for Carbon dioxide Capture, Utilization and Storage. That is, CCUS includes three technologies: carbon dioxide recovery, utilization, and storage. Among these, as a technology for storing CO2, a technology that recovers CO2 emitted from industrial facilities such as power plants and factories and injects and stores CO2 into depleted oil wells has been attracting attention.

[0009] Here, when storing CO2, toughness at extremely low temperatures may be required for steel materials. Specifically, when a pressure change occurs in the stored CO2 gas, the temperature of the stored gas may decrease due to the Joule-Thomson effect. In this case, steel materials may be required to have toughness in an extremely low temperature environment of -80°C, which is much lower than normal temperature. That is, in addition to oil well pipe applications, stainless steel materials assumed to be applied to CCUS applications are required not only to have high strength and excellent SSC resistance but also to have low-temperature toughness in an extremely low temperature environment of -80°C or lower.

[0010] Patent Documents 1 and 2 propose technologies 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 other technologies other than the technologies proposed in Patent Documents 1 and 2. In addition, Patent Documents 1 and 2 do not discuss 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 a 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 Problems

[0012] The stainless steel material according to the present disclosure is by 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: 16.50 - 19.50%, Mo: 1.50 - 6.00%, Ni: 3.50 - 9.00%, Cu: 0.03 - 3.00%, Co: 0.05 - 1.00%, Sn: 0.0005 - 0.0100%, sol.Al: 0.005 - 0.100%, N: 0.200% or less, O: 0.0200% or less, W: 0 - 1.80%, As: 0 - 0.0100%, Sb: 0 - 0.0100%, Ca: 0 - 0.0100%, Mg: 0 - 0.0100%, 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 consists of Fe and impurities, the yield strength is 758 MPa or more, the microstructure consists of ferrite with a volume fraction of 35 - 70%, retained austenite of 0 - 15%, and the balance being martensite, the content of the said elements and the said yield strength satisfy formula (1), the content of the said elements satisfies formula (2). 0.10 ≦ (Sn + As + Sb) / {(Cu + Ni) / YS} ≦ 1.00 (1) (Ni + 2Co) / Sn ≥ 900 (2) Here, in formulas (1) and (2), the element symbols are substituted with the contents of the corresponding elements in units of mass%. When the corresponding element is not contained, "0" is substituted for the element symbol. Also, in YS in formula (1), the yield strength is substituted in units of MPa.

Effect of the Invention

[0013] The stainless steel material according to the present disclosure has a high strength of 110 ksi or more, excellent SSC resistance, and excellent low-temperature toughness in an extremely low-temperature environment.

Mode for Carrying Out the Invention

[0014] First, the present inventors examined a stainless steel material having a high strength of 110 ksi or more, excellent SSC resistance, and excellent low-temperature toughness in an extremely low-temperature environment from the viewpoint of chemical composition. Specifically, the present inventors found that tin (Sn), arsenic (As), and antimony (Sb), which have not been focused on heretofore, may enhance SSC resistance. As a result of further detailed examination by the present inventors, in particular, Sn significantly enhances SSC resistance, and As and Sb may assist the effect of Sn in enhancing SSC resistance.

[0015] Therefore, the present inventors have carefully studied the Sn, As, and Sb contents that can sufficiently enhance the SSC resistance of stainless steel materials. As a result, the present inventors have found that, by 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: 16.50 - 19.50%, Mo: 1.50 - 6.00%, Ni: 3.50 - 9.00%, Cu: 0.03 - 3.00%, Co: 0.05 - 1.00%, Sn: 0.0005 - 0.0100%, sol.Al: 0.005 - 0.100%, N: 0.200% or less, O: 0.0200% or less, W: 0 - 1.80%, As: 0 - 0.0100%, Sb: 0 - 0.0100%, Ca: 0 - 0.0100%, Mg: 0 - 0.0100%, 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 being Fe and impurities, the stainless steel material may have a high strength of 110 ksi or more, excellent SSC resistance, and excellent low-temperature toughness in an extremely low-temperature environment.

[0016] Here, the microstructure of the stainless steel material having the above chemical composition consists of ferrite, retained austenite, and the balance being martensite. The present inventors have found that in the stainless steel material having the above chemical composition, if the microstructure consists of ferrite with a volume fraction of 35 - 70%, retained austenite with a volume fraction of 0 - 15%, and the balance being 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 enhanced. That is, in the stainless steel material according to the present embodiment, the microstructure consists of ferrite with a volume fraction of 35 - 70%, retained austenite with a volume fraction of 0 - 15%, and the balance being martensite. In this specification, "consisting of ferrite, retained austenite, and martensite" means that the phases other than ferrite, retained austenite, and martensite are so few as to be negligible.

[0017] The inventors further examined in detail a method for enhancing SSC resistance while maintaining the yield strength for stainless steel materials having the above-described chemical composition and microstructure and a yield strength of 758 MPa or more. As a result of the detailed examination by the inventors, it has been clarified that in stainless steel materials having the above-described chemical composition and microstructure and a yield strength of 758 MPa or more, if the content of elements and the yield strength satisfy the formula (1), the SSC resistance of the steel material can be significantly enhanced. 0.10 ≦ (Sn + As + Sb) / {(Cu + Ni) / YS} ≦ 1.00 (1) Here, in the element symbols in the formula (1), 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. Further, in YS in the formula (1), the yield strength is substituted in units of MPa.

[0018] Define Fn1 = (Sn + As + Sb) / {(Cu + Ni) / YS}. As described above, As and Sb assist the effect of Sn in enhancing the SSC resistance of the steel material. Further, by setting the ratio of the contents of Sn, As, and Sb to the contents of Cu and Ni within a certain range, the SSC resistance of the steel material is significantly increased. On the other hand, the higher the yield strength of the steel material, the more likely the SSC resistance of the steel material is to decrease. Therefore, the denominator of Fn1 is set to the ratio of the contents of Cu and Ni to the yield strength. In this way, the ratio of the contents of Sn, As, and Sb to the contents of Cu and Ni, adjusted according to the yield strength, is defined as Fn1. That is, Fn1 is an index for enhancing 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 the detailed examination by the inventors based on the above findings, it has been clarified that in the stainless steel material having the above-described chemical composition and microstructure, if Fn1 is 0.10 to 1.00, it is possible to achieve both a yield strength of 758 MPa or more and excellent SSC resistance. That is, the stainless steel material according to the present embodiment satisfies the formula (1) with respect to the content of elements and the yield strength on the premise of having the above-described chemical composition and a yield strength of 758 MPa or more. As a result, the stainless steel material according to the present 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 chemical composition, microstructure, and yield strength of 758 MPa or more, and satisfying Fn1 of 0.10 to 1.00, the low-temperature toughness in an extremely low-temperature environment may not be sufficiently obtained. Therefore, the present inventors examined a method for enhancing the low-temperature toughness in an extremely low-temperature environment while maintaining the yield strength and SSC resistance. As a result, it was found that in a stainless steel material having the above chemical composition, microstructure, and yield strength of 758 MPa or more, and satisfying Fn1 of 0.10 to 1.00, if the content of the element satisfies formula (2), the low-temperature toughness in an extremely low-temperature environment can be enhanced while maintaining the strength and SSC resistance. (Ni + 2Co) / Sn ≥ 900 (2) Here, in the element symbols in formula (2), the content of the corresponding element is substituted in units of mass%.

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

[0022] Specifically, in the stainless steel material according to the present embodiment, it has the above chemical composition, microstructure, and yield strength of 758 MPa or more, satisfies Fn1 of 0.10 to 1.00, and further sets Fn2 to 900 or more. As a result, the stainless steel material according to the present 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] Based on the above findings, the gist of the stainless steel material according to the present embodiment completed is as follows.

[0024] [1] A stainless steel material, by 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: 16.50 - 19.50%, Mo: 1.50 - 6.00%, Ni: 3.50 - 9.00%, Cu: 0.03 - 3.00%, Co: 0.05 - 1.00%, Sn: 0.0005 - 0.0100%, sol.Al: 0.005 - 0.100%, N: 0.200% or less, O: 0.0200% or less, W: 0 - 1.80%, As: 0 - 0.0100%, Sb: 0 - 0.0100%, Ca: 0 - 0.0100%, Mg: 0 - 0.0100%, 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 consists of Fe and impurities, the yield strength is 758 MPa or more, the microstructure consists of 35 - 70% ferrite, 0 - 15% retained austenite, and the balance martensite by volume fraction, the content of the elements and the yield strength satisfy formula (1), the content of the elements satisfies formula (2), Stainless steel material. 0.10 ≦ (Sn + As + Sb) / {(Cu + Ni) / YS} ≦ 1.00 (1) (Ni + 2Co) / Sn ≧ 900 (2) Here, for the element symbols in formulas (1) and (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, for YS in formula (1), the yield strength is substituted in units of MPa.

[0025] [2] The stainless steel material according to [1], W: 0.01 to 1.80%, As: 0.0001 to 0.0100%, Sb: 0.0001 to 0.0100%, Ca: 0.0001 to 0.0100%, Mg: 0.0001 to 0.0100%, B: 0.0001 to 0.0050%, Rare earth elements: 0.001 to 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] Note that 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, may be a round steel (solid material), or may be a steel plate. Note that round steel means a bar steel having a circular cross-section perpendicular to the axial direction. Also, the steel pipe may be a seamless steel pipe or a welded steel pipe.

[0027] Hereinafter, the stainless steel material according to this embodiment will be described in detail. In the following description, the stainless steel material is also 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. "% " regarding the 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 susceptibility. Therefore, if the C 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 decreases. 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 preferable upper limit of the C content is 0.049%, more preferably 0.047%, and even more preferably 0.045%. It is preferable that the C content is as low as possible. However, an extreme reduction in the C content increases the manufacturing cost. Therefore, considering industrial production, the preferable 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, even if the contents of other elements are within the range of this embodiment, the hot workability of the steel material decreases. Therefore, the Si content is 1.00% or less. The preferable lower limit of the Si content for effectively obtaining the above effects is 0.01%, more preferably 0.05%, even more preferably 0.10%, and even more preferably 0.15%. The preferable upper limit of the Si content is 0.80%, more preferably 0.60%, even 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 enhances the hardenability of the steel material and increases the strength of the steel material. On the other hand, Mn may segregate at the grain boundaries together with impurity elements such as P and S. Therefore, if the Mn content is too high, even if the contents of other elements are within the range of this embodiment, the SSC resistance and low-temperature toughness of the steel material will decrease. Therefore, the Mn content is 1.00% or less. The preferable lower limit of the Mn content for effectively obtaining the above effects is 0.01%, more preferably 0.03%, and even more preferably 0.05%. The preferable upper limit of the Mn content is 0.80%, more preferably 0.60%, even more preferably 0.50%, and even more preferably 0.45%.

[0032] P: 0.050% or less Phosphorus (P) is an inevitably contained impurity. That is, the lower limit of the P content is more than 0%. P segregates at the crystal grain boundaries and makes SSC 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 will significantly decrease. Therefore, the P content is 0.050% or less. The 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, considering industrial production, the 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 inevitable impurity. That is, the lower limit of the S content is more than 0%. Similar to P, S segregates at the crystal grain boundaries, making 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 will be significantly reduced. Thus, the S content is 0.0050% or less. The preferable upper limit of the S content is 0.0040%, more preferably 0.0030%, still more preferably 0.0025%, and even more preferably 0.0020%. It is preferable that the S content is as low as possible. However, an extreme reduction in the S content increases the manufacturing cost. Therefore, considering industrial production, the preferable lower limit of the S content is 0.0001%, more preferably 0.0002%, and still more preferably 0.0003%.

[0034] Cr: 16.50 - 19.50% Chromium (Cr) forms a passive film on the surface of the steel material, enhancing the SSC resistance of the steel material. If the Cr content is too low, even if the contents of other elements are within the range of this embodiment, the above effects cannot be fully obtained. On the other hand, if the Cr content is too high, even if the contents of other elements are within the range of this embodiment, it becomes difficult to stabilize austenite. As a result, the volume fraction of ferrite becomes too high, and the SSC resistance and low-temperature toughness of the steel material decrease. Therefore, the Cr content is 16.50 - 19.50%. The preferable lower limit of the Cr content is 16.55%, more preferably 16.60%, and still more preferably 16.65%. The preferable upper limit of the Cr content is 19.45%, more preferably 19.40%, and still more preferably 19.30%.

[0035] Mo: 1.50 - 6.00% Molybdenum (Mo) forms Mo sulfides to enhance the SSC resistance of steel. Mo further dissolves in the steel to increase the strength of the steel. If the Mo content is too low, even if the contents of other elements are within the scope of this embodiment, the above effects cannot be fully obtained. On the other hand, if the Mo content is too high, even if the contents of other elements are within the scope of this embodiment, it becomes difficult to stabilize austenite. As a result, the volume fraction of ferrite becomes too high, and the SSC resistance and low-temperature toughness of the steel may decrease. Therefore, the Mo content is 1.50 to 6.00%. The preferable lower limit of the Mo content is 1.51%, more preferably 1.55%, and even more preferably 1.60%. The preferable upper limit of the Mo content is 5.50%, more preferably 5.00%, even more preferably 4.50%, and even more preferably 4.00%.

[0036] Ni: 3.50 to 9.00% Nickel (Ni) enhances the SSC resistance of steel due to the synergistic effect with Sn, As, and Sb. Ni further increases the low-temperature toughness of the steel decreased by Sn. If the Ni content is too low, even if the contents of other elements are within the scope of this embodiment, the above effects cannot be fully obtained. On the other hand, if the Ni content is too high, even if the contents of other elements are within the scope of this embodiment, the hydrogen diffusion coefficient in the steel decreases, and the SSC resistance of the steel may decrease. Therefore, the Ni content is 3.50 to 9.00%. The preferable lower limit of the Ni content is 3.51%, more preferably 3.55%, and even more preferably 3.60%. The preferable upper limit of the Ni content is 8.50%, more preferably 8.00%, even more preferably 7.50%, and even more preferably 7.00%.

[0037] Cu: 0.03 to 3.00% Copper (Cu) enhances the SSC resistance of steel materials due to its synergistic effect with Sn, As, and Sb. If the Cu content is too low, the above effect cannot be fully obtained even if the contents of other elements are within the scope of this embodiment. On the other hand, if the Cu content is too high, the low-temperature toughness of the steel material will decrease even if the contents of other elements are within the scope of this embodiment. Therefore, the Cu content is 0.03 to 3.00%. The preferred lower limit of the Cu content is 0.04%, more preferably 0.05%, and even more preferably 0.10%. The preferred upper limit of the Cu content is 2.95%, more preferably 2.90%, and even more preferably 2.80%. means the content of dissolved Al).

[0038] Co: 0.05 to 1.00% Cobalt (Co) enhances the low-temperature toughness of steel materials reduced by Sn. If the Co content is too low, the above effect cannot be fully obtained even if the contents of other elements are within the scope of this embodiment. On the other hand, if the Co content is too high, the low-temperature toughness of the steel material will instead decrease even if the contents of other elements are within the scope of this embodiment. Therefore, the Co content is 0.05 to 1.00%. The preferred lower limit of the Co content is 0.06%, more preferably 0.08%, even more preferably 0.10%, and even more preferably 0.12%. The preferred upper limit of the Co content is 0.98%, more preferably 0.95%, and even more preferably 0.90%.

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

[0040] sol.Al: 0.005 to 0.100% Aluminum (Al) deoxidizes steel. If the Al content is too low, even if the contents of other elements are within the scope of this embodiment, the above effect cannot be sufficiently obtained. On the other hand, if the Al content is too high, even if the contents of other elements are within the scope of this embodiment, coarse oxides are generated, and the SSC resistance and low-temperature toughness of the steel decrease. Therefore, the Al content is 0.005 to 0.100%. The preferable lower limit of the Al content is 0.007%, more preferably 0.010%. The preferable upper limit of the Al content is 0.080%, more preferably 0.060%, even more preferably 0.050%, and even more preferably 0.045%. The Al content referred to in this specification means the content of sol.Al (acid-soluble Al).

[0041] N: 0.200% or less Nitrogen (N) is inevitably contained. That is, the lower limit of the N content is more than 0%. N forms Ti and Ti nitrides 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 0.200% or less. The preferable lower limit of the N content for effectively obtaining the above effects is 0.001%, more preferably 0.003%, still more preferably 0.005%. The preferable upper limit of the N content is 0.180%, more preferably 0.170%, still more preferably 0.150%, and even more preferably 0.145%.

[0042] O: 0.0200% or less Oxygen (O) is an impurity 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, even if the contents of other elements are within the range of this embodiment, the SSC resistance and low-temperature toughness of the steel material are reduced. Therefore, the O content is 0.0200% or less. The preferable upper limit of the O content is 0.0150%, more preferably 0.0100%, still more preferably 0.0050%, even more preferably 0.0030%, and even more preferably 0.0020%. It is preferable that the O content is as low as possible. However, an extreme reduction in the O content increases the manufacturing cost. Therefore, considering industrial production, the preferable lower limit of the O content is 0.0001%, more preferably 0.0003%, still more preferably 0.0005%.

[0043] The remainder of the chemical composition of the stainless steel material according to this embodiment consists of Fe and impurities. Here, the impurities in the chemical composition mean those mixed from ores, scraps, or manufacturing environments as raw materials when industrially manufacturing the stainless steel material, and are allowed within a range that does not adversely affect the stainless steel material according to this embodiment.

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

[0045] W: 0 to 1.80% 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. Even if 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 may become too high, and the SSC resistance and low-temperature toughness of the steel material may decrease. Therefore, the W content is 0 to 1.80%. The preferable lower limit of the W content is more than 0%, more preferably 0.01%, still more preferably 0.02%, and still more preferably 0.03%. The preferable upper limit of the W content is 1.78%, more preferably 1.75%, and still more preferably 1.70%.

[0046] 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 and Sb in place of a part 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 enhancing the SSC resistance of the steel material. Even if a small amount of As is contained, the above effect can be obtained to some extent. On the other hand, if the As content is too high, even if the contents of other elements are within the range of this embodiment, As segregates at the grain boundaries, and the SSC resistance of the steel material decreases. Therefore, the As content is 0 to 0.0100%. The preferable lower limit of the As content is more than 0%, more preferably 0.0001%, still more preferably 0.0003%, and still more preferably 0.0005%. The preferable upper limit of the As content is 0.0090%, more preferably 0.0080%.

[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 Sn in enhancing the SSC resistance of the steel material. Even if 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 boundaries, resulting in a decrease in the SSC resistance of the steel material. Therefore, the Sb content is 0 to 0.0100%. The preferable lower limit of the Sb content is 0.0001%, more preferably 0.0003%, and even more preferably 0.0005%. The preferable upper limit of the Sb content is 0.0090%, more preferably 0.0080%.

[0049] The chemical composition of the stainless steel material according to this embodiment may further contain at least one element selected from the group consisting of Ca, Mg, B, and rare earth elements in place of a part of Fe. All of these elements are optional elements and enhance the hot workability of the steel material.

[0050] Ca: 0 to 0.0100% Calcium (Ca) is an optional element and may not be contained. That is, the Ca content may be 0%. When contained, Ca immobilizes S in the steel material as sulfide to make it harmless and enhances the hot workability of the steel material. Even if 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 coarsen, resulting in a decrease in the SSC resistance and low-temperature toughness of the steel material. Therefore, the Ca content is 0 to 0.0100%. The preferable lower limit of the Ca content is more than 0%, more preferably 0.0001%, even more preferably 0.0005%, even more preferably 0.0008%, and even more preferably 0.0010%. The preferable upper limit of the Ca content is 0.0080%, more preferably 0.0060%, and even more preferably 0.0050%.

[0051] Mg: 0 to 0.0100% Magnesium (Mg) is an optional element and may not be contained. That is, the Mg content may be 0%. When contained, Mg immobilizes S in the steel as sulfide to render it harmless and improves the hot workability of the steel. Even if 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 scope of this embodiment, the oxides in the steel become coarsened, and the SSC resistance and low-temperature toughness of the steel decrease. Therefore, the Mg content is 0 to 0.0100%. The preferable lower limit of the Mg content is more than 0%, more preferably 0.0001%, and even more preferably 0.0002%. The preferable upper limit of the Mg content is 0.0080%, more preferably 0.0060%, and even more preferably 0.0050%.

[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 immobilizes S in the steel as sulfide to render it harmless and improves the hot workability of the steel. Even if a small amount of B is contained, the above effects can be obtained to a certain extent. However, if the B content is too high, even if the contents of other elements are within the scope of this embodiment, boron nitride (BN) is formed, and the low-temperature toughness of the steel decreases. Therefore, the B content is 0 to 0.0050%. The preferable lower limit of the B content is more than 0%, more preferably 0.0001%, and even more preferably 0.0003%. The preferable upper limit of the B content is 0.0040%, more preferably 0.0030%, and even more preferably 0.0025%.

[0053] Rare earth elements: 0 to 0.100% The rare earth element (REM) is an optional element and may not be contained. That is, the REM content may be 0%. When contained, REM immobilizes S in the steel material as sulfide to detoxify it and improves the hot workability of the steel material. Even if a small amount of REM is contained, the above effects can be obtained to some extent. However, if the REM content is too high, even if the contents of other elements are within the range of this embodiment, the oxides in the steel material will coarsen, and the SSC resistance and low-temperature toughness of the steel material will decrease. Therefore, the REM content is 0 to 0.100%. The preferable lower limit of the REM content is more than 0%, more preferably 0.001%, and even more preferably 0.002%. The preferable upper limit of the REM content is 0.095%, more preferably 0.090%, and even more preferably 0.085%.

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

[0055] 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, and Zr in place of a part of Fe. All of these elements are optional elements and increase the strength of the steel material.

[0056] V: 0 to 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 a carbonitride and increases the strength of the steel material. The above effects can be obtained to a certain extent even if 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%, still more preferably 0.03%, and still more preferably 0.05%. The preferable upper limit of the V content is 0.48%, more preferably 0.45%, and still 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 a carbonitride and increases the strength of the steel material. The above effects can be obtained to a certain extent even if 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 preferable lower limit of the Ti content is more than 0%, more preferably 0.001%, still more preferably 0.002%, and still more preferably 0.003%. The preferable upper limit of the Ti content is 0.250%, more preferably 0.200%, still more preferably 0.150%, still more preferably 0.100%, and still 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. Even if 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 scope 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 preferable lower limit of the Nb content is more than 0%, more preferably 0.001%, still more preferably 0.002%, still more preferably 0.003%, still more preferably 0.005%, and still more preferably 0.010%. The preferable upper limit of the Nb content is 0.280%, more preferably 0.240%, still more preferably 0.200%, and still 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. Even if a small amount of Zr is contained, the above effect can be obtained to some extent. However, if the Zr content is too high, even if the contents of other elements are within the scope of this embodiment, the strength of the steel material becomes too high and the 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%, still more preferably 0.005%, still more preferably 0.010%, and still more preferably 0.015%. The preferable upper limit of the Zr content is 0.180%, more preferably 0.160%, still more preferably 0.150%, and still more preferably 0.140%.

[0060] The chemical composition of the stainless steel material according to this embodiment may further contain one or more elements selected from the group consisting of Zn and Pb in place 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. Even if a little Zn is contained, the above effect can be obtained to some extent. However, if the Zn content is too high, the SSC resistance of the steel material may conversely decrease even when the contents of other elements are within the range of this embodiment. Therefore, the Zn content is 0 to 0.0100%. The preferable lower limit of the Zn content is more than 0%, more preferably 0.0001%, still more preferably 0.0002%, and still more preferably 0.0003%. The preferable upper limit of the Zn content is 0.0080%, more preferably 0.0060%, and still more preferably 0.0055%.

[0062] Pb: 0 to 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. Even if a little Pb is contained, the above effect can be obtained to some extent. However, if the Pb content is too high, the hot workability of the steel material decreases even when the contents of other elements are within the range of this embodiment. Therefore, the Pb content is 0 to 0.0100%. The preferable lower limit of the Pb content is more than 0%, more preferably 0.0001%, still more preferably 0.0002%, and still more preferably 0.0003%. The preferable upper limit of the Pb content is 0.0080%, more preferably 0.0060%, still more preferably 0.0050%, and still more preferably 0.0030%.

[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. For example, it is 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 preferable 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 in accordance with 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 central part 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 tensile test piece or an arc-shaped test piece is prepared from the central part of the wall thickness. In this case, the longitudinal direction of the tensile test piece or the arc-shaped test piece is parallel to the pipe axis direction of the steel pipe. When the steel material is a round bar, a tensile test piece is prepared from the R / 2 position. In this specification, the R / 2 position of the round bar means the central position of the radius R in a cross section perpendicular to the axial direction of the round bar. In this case, the longitudinal direction of the tensile test piece is parallel to the axial direction of the round bar.

[0065] The tensile test piece is, for example, 8.9 mm in diameter of the parallel part and 35.6 mm in gauge length. When a round bar test piece cannot be prepared from the steel pipe, an arc-shaped test piece is prepared. The size of the arc-shaped test piece is, for example, the full wall thickness in thickness, 25.4 mm in width, and 50.8 mm in gauge length. Using the prepared tensile test piece, a tensile test is carried out 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 first decimal place of the obtained numerical value.

[0066] [Microstructure] The microstructure of the stainless steel material according to this embodiment consists of ferrite with a volume fraction of 35 to 70%, retained austenite with a volume fraction of 0 to 15%, and the balance being martensite. In this specification, the microstructure being "composed of ferrite, retained austenite, and martensite" means that the phases other than ferrite, retained austenite, and martensite in the microstructure are so few that they can be ignored. For example, in the chemical composition of the stainless steel material according to this 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 this embodiment may contain a minute amount 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 35 to 70%. If the volume fraction of ferrite is too low, the SSC resistance of the steel material decreases. On the other hand, if the volume fraction of ferrite is too high, the ferrite coarsens and the low-temperature toughness of the steel material in an extremely low-temperature environment decreases. Therefore, in the microstructure of the stainless steel material according to this embodiment, the volume fraction of ferrite is 35 to 70%. The preferable lower limit of the volume fraction of ferrite is 36%, more preferably 37%, and even more preferably 38%. The preferable upper limit of the volume fraction of ferrite is 68%, more preferably 67%, and even more preferably 65%.

[0068] As described above, in the microstructure of the stainless steel material according to the present embodiment, the volume ratio of retained austenite is 0 to 15%. That is, retained austenite may not be included in the microstructure. If retained austenite is contained, the low-temperature toughness of the steel material is enhanced. On the other hand, if the volume ratio of retained austenite is too high, the strength of the steel material decreases. Therefore, in the microstructure of the stainless steel material according to the present embodiment, the volume ratio of retained austenite is 0 to 15%. The preferable upper limit of the volume ratio of retained austenite is 14%, more preferably 13%, and even more preferably 12%. The preferable lower limit of the volume ratio of retained austenite for obtaining the above effects more effectively is more than 0%, more preferably 1%, even more preferably 2%, and even more preferably 3%.

[0069] As described above, the stainless steel material according to the present embodiment has a microstructure composed of 35 to 70% ferrite, 0 to 15% retained austenite, and the balance martensite by volume ratio. In this specification, "martensite" includes not only fresh martensite but also tempered martensite. Also, the volume ratio of martensite is not particularly limited, but is substantially 15 to 65%. The preferable lower limit of the volume ratio of martensite is 16%, more preferably 17%, and even more preferably 18%. The preferable upper limit of the volume ratio of martensite is 62%, more preferably 60%, and even more preferably 55%.

[0070] In the present embodiment, the volume ratio of each phase of the microstructure is determined by the following method. Specifically, the volume ratio (%) of retained austenite and the volume ratio (%) of ferrite in the microstructure of the steel material are determined by the following method. The volume ratio of martensite (%) is obtained by subtracting the obtained volume ratio of retained austenite and the volume ratio of ferrite from 100%.

[0071] [Measurement method of volume ratio of retained austenite] The volume ratio of retained austenite in the microstructure of the steel material is determined by X-ray diffraction. Specifically, a test piece for measuring the volume ratio of 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 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 × 15 mm × 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 bar, the thickness direction of the test piece is the diameter direction. Using the prepared test piece, 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 the X-ray diffraction intensity, the target of the X-ray diffractometer is set to Co (CoKα ray), and the output is set to 30 kV - 100 mA. The measurement angle (2θ) is set to 45 - 105°. After calculation, the volume ratio Vγ (%) of retained austenite is calculated using Equation (I) for each combination (3 × 3 = 9 combinations) of each plane of the α-phase and each plane of the γ-phase. Then, the average value of the volume ratios Vγ of the 9 combinations of retained austenite is defined as the volume ratio (%) of retained austenite. Vγ = 100 / {1 + (Iα × Rγ) / (Iγ × Rα)} (I) Here, Iα is the integrated intensity of the α-phase. Rα is the crystallographic theoretical calculated value of the α-phase. Iγ is the integrated intensity of the γ-phase. Rγ is the crystallographic theoretical calculated value of the γ-phase. The values of Rα and Rγ for each plane can be the values incorporated in the retained γ quantitative analysis system attached to Rigaku Corporation's product named RINT-TTR. Note that the volume ratio of retained austenite is obtained by rounding the first decimal place of the obtained numerical value.

[0073] [Method for Measuring the Volume Ratio of Ferrite] 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 sampled from the center of the plate thickness. When the steel material is a steel pipe, the test piece is sampled from the center of the wall thickness. When the steel material is a round bar, the test piece is sampled from the R / 2 position. Note that the size of the test piece is not particularly limited. Also, 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 pipe axis direction of the steel pipe. When the steel material is a round bar, the observation surface of the test piece is a surface parallel to the axial direction of the round bar. After the observation surface is mechanically polished, the observation surface is electrolytically etched to reveal the microstructure. The electrolytic etching is carried out with an electrolytic solution: a mixed solution of aqua regia (a solution mixed at 3:1 of hydrochloric acid: nitric acid) and glycerin, a current density: 1 A / cm 2 , and an electrolytic time: 1 minute.

[0074] The electrolytically etched observation surface is observed in 30 fields of view using an optical microscope. The observation fields of view are rectangles of 250 μm × 250 μm. Note that the observation magnification is 400 times. In each observation field of view, ferrite and other phases (retained austenite and martensite) can be distinguished by those skilled in the art from the contrast. Therefore, the ferrite in each observation field of view is specified based on the contrast. The area fraction of the specified ferrite is determined by the point-counting method conforming to JIS G 0555 (2020).

[0075] Specifically, for the observation field of view, 20 vertical lines are drawn at equal intervals from the upper end to the lower end of the observation field of view. That is, due to the 20 vertical lines, the observation field of view is divided into 21 regions in the left-right direction. Further, for the observation field of view, 20 horizontal lines are drawn at equal intervals from the left end to the right end of the observation field of view. That is, due to the 20 horizontal lines, the observation field of view is divided into 21 regions in the up-down direction. At this time, the intersection points of the vertical lines and the horizontal lines are called lattice points. That is, 400 lattice points are arranged at equal intervals in the observation field of view. In accordance with JIS G 0555(2020), the lattice points overlapping with ferrite are counted in the observation field of view. The number of lattice points overlapping with ferrite obtained in 30 fields of view is divided by the total number of lattice points (400×30 = 12000), and defined as the ferrite area ratio. In this embodiment, the ferrite area ratio obtained by the above method is used as the ferrite volume ratio (%). Note that the ferrite volume ratio is obtained by rounding the first decimal place of the obtained numerical value.

[0076] Using the volume ratio (%) of retained austenite obtained by the above X-ray diffraction method and the volume ratio (%) of ferrite obtained by the above point counting method, the volume ratio (%) of martensite in the microstructure of the steel material is obtained by the following formula. Volume ratio of martensite (%) = 100 - {Volume ratio of retained austenite (%) + Volume ratio of ferrite (%)}

[0077] [Equation (1)] The stainless steel material according to this embodiment satisfies the following relationship between the content of elements and the yield strength YS within the range of the above chemical composition and a yield strength of 758 MPa or more. 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. 0.10 ≦ (Sn + As + Sb) / {(Cu + Ni) / YS} ≦ 1.00 (1) Here, for the element symbols in Equation (1), the content of the corresponding element is substituted in units of mass%. When the corresponding element is not contained, "0" is substituted for that element symbol. Also, for YS in Equation (1), the yield strength is substituted in units of MPa.

[0078] Fn1 = (Sn + As + Sb) / {(Cu + Ni) / YS} is an index for enhancing SSC resistance due to the synergistic effect of Sn, As, and Sb and Cu and Ni, which is adjusted according to the yield strength. On the premise that the steel has the above chemical composition and microstructure and the yield strength YS is 758 MPa or more, if Fn1 is 0.10 to 1.00, the steel has stable and excellent SSC resistance. Therefore, in this embodiment, Fn1 is set to 0.10 to 1.00. The preferable lower limit of Fn1 is 0.11, more preferably 0.12. The preferable upper limit of Fn1 is 0.99, more preferably 0.98. In this embodiment, Fn1 is obtained by rounding off the third decimal place of the obtained numerical value.

[0079] [Formula (2)] The stainless steel material according to this embodiment satisfies the content of elements within the range of the above chemical composition in 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 it satisfies other configurations of this embodiment. (Ni + 2Co) / Sn ≥ 900 (2) Here, the content of the corresponding element is substituted for the element symbol in Formula (2) in units of mass%.

[0080] Fn2 (= (Ni + 2Co) / Sn) is an index of low-temperature toughness in an extremely low-temperature environment. On the premise that the steel has the above chemical composition, microstructure, and a yield strength YS of 758 MPa or more, and Fn1 satisfies 0.10 to 1.00, if Fn2 is 900 or more, the steel has stable and excellent low-temperature toughness even in an extremely low-temperature environment. Therefore, in this embodiment, Fn2 is set to 900 or more. The preferable lower limit of Fn2 is 901, more preferably 905, and even 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 chemical composition, microstructure, and a yield strength of 758 MPa or more, Fn1 satisfies 0.10 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 by the following method.

[0082] Specifically, an SSC resistance test is carried out 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 central part of the plate thickness. In this case, the axial direction of the round bar test piece is set to be 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 central part of the wall thickness. In this case, the axial direction of the round bar test piece is set to be parallel to the pipe axis direction of the steel pipe. When the steel material is a round steel, 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 set to be parallel to the axial direction of the round steel. The size of the round bar test piece is, for example, a diameter of 6.35 mm and a parallel part length of 25.4 mm.

[0083] The test solution used is an aqueous solution of 5.0 mass% sodium chloride (NACE solution A) adjusted to pH 2.7 with acetic acid. A stress corresponding 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 the test container so that the round bar test piece to which stress has been applied is immersed, and a test bath is formed. After degassing the test bath, a mixed gas of 0.03 atm of H2S gas and 0.97 atm of 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. The steel material according to this embodiment does not show cracks after 720 hours in the SSC resistance test conducted under the above conditions. In this specification, "no cracks are confirmed" means that no cracks are confirmed when the test piece after the test is observed with the naked eye.

[0084] [Low-temperature toughness] The stainless steel material according to this embodiment has the above-described chemical composition, microstructure, and a yield strength of 758 MPa or more, satisfies 0.10 to 1.00 for Fn1, and satisfies 900 or more for Fn2. 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, the excellent low-temperature toughness in an extremely low-temperature environment is defined by the following method.

[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 axis 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 steel, the cross-sectional diameter direction of the round steel is defined as the "C direction", the axis direction of the round steel 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] For the fabricated V-notch test piece, a Charpy impact test compliant with JIS Z 2242 (2018) is carried out to obtain the absorbed energy (J) at -80°C. When using a sub-size V-notch test piece, the obtained absorbed energy is divided by the reduction factor described in API 5CT (2019) and converted to the absorbed energy of a full-size V-notch test piece. In this embodiment, the absorbed energy (J) at -80°C is obtained by rounding off the first decimal place of the obtained numerical value.

[0087] In this embodiment, when the absorbed energy at -80°C obtained under the above conditions is 100 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 -80°C is also simply referred to as "absorbed energy".

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

[0089] [Manufacturing method] An example of the manufacturing method of the stainless steel material according to this embodiment having the above configuration will be described. Note that the manufacturing method of the stainless steel material according to this embodiment is not limited to the manufacturing method described below. An example of the manufacturing method of the stainless steel material of 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). Hereinafter, each step will be described in detail.

[0090] [Preparation step] In the preparation step, an intermediate steel material having the above-described chemical composition is prepared. The manufacturing method of the intermediate steel material is not particularly limited as long as the intermediate steel material has the above chemical composition. Here, the intermediate steel material is a plate-shaped steel material when the final product is a steel plate or a welded steel pipe, a plain 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 raw material (raw material preparation step) and a step of hot-working the raw material to manufacture an intermediate steel material (hot-working step). Hereinafter, the case including the raw material preparation step and the hot-working step will be described in detail.

[0092] [Raw Material Preparation Step] In the raw material preparation step, a raw material is manufactured using molten steel having the above-described chemical composition. The manufacturing method of the raw material is not particularly limited and may be a well-known method. Specifically, a slab (slab, bloom, or billet) may be manufactured by continuous casting using molten steel. An ingot may be manufactured by the ingot-making method using molten steel. If necessary, a billet may be manufactured by block rolling a slab, bloom, or ingot. The raw material (slab, bloom, or billet) is manufactured by the above steps.

[0093] [Hot-Working Step] In the hot-working step, the prepared raw material is hot-worked to manufacture an intermediate steel material. As described above, when the steel material is a seamless steel pipe, the intermediate steel material corresponds to a plain pipe. First, the billet is heated in a heating furnace. The heating temperature is not particularly limited, for example, it is 1100 to 1300 °C. Hot-working is performed on the billet extracted from the heating furnace to manufacture a plain 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 a hot working process to manufacture a plain pipe. In this case, a round billet is pierced and rolled by a piercing mill. When piercing and rolling, the piercing ratio is not particularly limited, but for example, it is 1.0 to 4.0. The pierced and rolled round billet is further hot-rolled by a mandrel mill, a reducer, a sizing mill, etc. to form a plain pipe. The cumulative area reduction rate in the hot working process is, for example, 20 to 70%. Other hot working methods may be carried out to manufacture a plain pipe from a billet. For example, when the steel material is a short and thick steel pipe such as a coupling, a plain pipe may be manufactured by forging such as the Erhardt method. The plain pipe is manufactured by the above steps. The wall thickness of the plain pipe is not particularly limited, but for example, it is 9 to 60 mm.

[0095] When the steel material is round steel, first, the material is heated in a heating furnace. The heating temperature is not particularly limited, but for example, it is 1100 to 1300 °C. Hot working is carried out on the material extracted from the heating furnace to manufacture intermediate steel material having a circular cross-section perpendicular to the axial direction. The hot working is, for example, block rolling by a block rolling mill or hot rolling by a continuous rolling mill. The continuous rolling mill has a horizontal stand having a pair of grooved rolls arranged side by side in the vertical direction and a vertical stand having a pair of grooved rolls arranged side by side in the horizontal direction, which are alternately arranged. When the steel material is a steel plate, first, the material is heated in a heating furnace. The heating temperature is not particularly limited, but for example, it is 1100 to 1300 °C. Hot rolling is carried out on the material extracted from the heating furnace using a block rolling mill and a continuous rolling mill to manufacture intermediate steel material in the shape of a steel plate.

[0096] The intermediate steel material produced by hot working may be air-cooled (As-Rolled). The intermediate steel material produced by hot working may be quenched directly after hot working without cooling to room temperature, or may be quenched after post-heating (reheating) after hot working. When quenching is carried out directly after hot working or after post-heating, cooling may be stopped or slow cooling may be carried out during quenching. In this case, the occurrence of burning cracks in the plain pipe can be suppressed. When quenching is carried out directly after hot working or after post-heating, further, stress relief annealing (SR) may be carried out after quenching and before the heat treatment in the next process. In this case, the residual stress of the plain pipe is removed.

[0097] As described above, in the preparation process, an intermediate steel material is prepared. The intermediate steel material may be produced by the above-described preferred process, or an intermediate steel material produced by a third party, or an intermediate steel material produced at another factory or other business office other than the factory where the subsequent quenching process and tempering process are carried out may be prepared. Hereinafter, the heat treatment process will be described in detail.

[0098] [Quenching Process] In the quenching process, quenching is carried out on the intermediate steel material produced in the hot working process. Quenching is carried out by a well-known method. Specifically, the intermediate steel material after the hot working process may be charged 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 referred to as the quenching temperature.

[0099] If the quenching temperature is too low, the heating of the intermediate steel material may be insufficient, and in the produced stainless steel material, the above-described microstructure may not be obtained. 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 the present embodiment, the preferred quenching temperature is 850 to 1150°C. The holding time at the quenching temperature is not particularly limited, but for example, it is 5 to 80 minutes.

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

[0101] In addition, after the hot working process, quenching (direct quenching) may be carried out immediately after hot working without cooling the intermediate steel material to room temperature, or after charging it into a reheating furnace and maintaining it at the quenching temperature before the temperature of the plain pipe after hot working drops, quenching may be carried out.

[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 charged 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 (°C) of the heat treatment furnace for heating the intermediate steel material in the tempering process is also referred to as 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 500 to 700°C. The holding time at the tempering temperature is not particularly limited, but for example, it is 10 to 180 minutes. It is well known to those skilled in the art that the yield strength of the steel 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 becomes 758 MPa or more.

[0104] Through the above steps, the stainless steel material according to this embodiment can be manufactured. As described above, the stainless steel material according to this embodiment is not limited to the above manufacturing method. Hereinafter, the stainless steel material according to this embodiment will be described more specifically by way of examples.

Examples

[0105] The molten steel having the chemical compositions shown in Table 1A, Table 1B, and Table 1C was melted using a 50 kg vacuum melting furnace, and steel ingots were produced by the ingot casting method. In Table 1B and Table 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 mean that, after rounding off the third decimal place, they were 0%. Similarly, the REM content, Ti content, Nb content, and Zr content of Test No. 1 mean that, after rounding off the fourth decimal place, they were 0%. Similarly, the As content, Sb content, Ca content, Mg content, B content, Zn content, and Pb content of Test No. 1 mean that, after rounding off the fifth decimal place, they were 0%.

[0106]

Table 1A

[0107]

Table 1B

[0108]

Table 1C

[0109] After heating the ingots of each test number at 1200 to 1250 °C for 2 hours, hot working was carried out to produce intermediate steel materials (plain pipes) with a wall thickness of 25.4 mm and an outer diameter of 177.8 mm. For the intermediate steel materials of each test number, a quenching process and a tempering process were carried out. Specifically, after holding the intermediate steel materials of each test number at 910 °C for 15 minutes, quenching with rapid cooling was carried out. Then, tempering was carried out by holding the intermediate steel materials of each test number at 570 to 600 °C for 30 to 60 minutes. By the above manufacturing process, seamless steel pipes of each test number were manufactured.

[0110] [Evaluation Test] For the obtained seamless steel pipes of each test number, a tensile test, a microstructure observation test, an SSC resistance test, and a Charpy impact test were carried out.

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

[0112]

Table 2

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

[0114] [Microstructure observation test] For seamless steel pipes of each test number, a microstructure observation test was carried out by the above method. Specifically, the volume fraction (%) of retained austenite was determined by the X-ray diffraction method carried out by the above method. Furthermore, the volume fraction (%) of ferrite was determined by the point-counting method compliant with JIS G 0555(2020) carried out by the above method. From the obtained volume fraction of retained austenite and the volume fraction of ferrite, the volume fraction (%) of martensite was determined. The obtained volume fraction of ferrite for each test number is shown in the column of "Ferrite (volume %)" in Table 2. The obtained volume fraction of retained austenite for each test number is shown in the column of "Retained γ (volume %)" in Table 2. The obtained volume fraction of martensite for each test number is shown in the column of "Martensite (volume %)" in Table 2.

[0115] [SSC resistance test] For seamless steel pipes of each test number, an SSC resistance test was carried out by the above method. Specifically, for three round bar test pieces fabricated by the above method, an SSC resistance test was carried out by a method compliant with NACE TM0177-2016 Method A. Note that the axial direction of the test piece was parallel to the pipe axis direction. A tensile stress corresponding to 90% of the actual yield stress was applied in the axial direction of the round bar test piece of each test number. As the test solution, an aqueous solution of 5.0 mass% sodium chloride (NACE solution A) adjusted to pH 2.7 with acetic acid was used.

[0116] Three test containers were each filled with a test solution at 24°C to serve as test baths. Three round bar test pieces subjected to stress were immersed one by one in the test baths of different test containers. After degassing each test bath, a mixed gas of 0.03 atm of H2S gas and 0.97 atm of 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. For each round bar test piece of each test number after holding for 720 hours, the presence or absence of sulfide stress cracking (SSC) was observed. Specifically, the round bar test pieces after holding for 720 hours were observed with the naked eye. As a result of the observation, those in which no cracks were confirmed in all three test pieces were judged as "E" (Excellent). On the other hand, those in which cracks were confirmed in at least one test piece were judged as "NA" (Not Acceptable). The evaluation results of each test number are shown in the "SSC resistance" column of Table 2.

[0117] [Charpy impact test] For each seamless pipe of each test number, a Charpy impact test was carried out in accordance with JIS Z 2242 (2018). Specifically, in the above-mentioned method, a full-size V-notch test piece was prepared in accordance with API 5CT (2019). A Charpy impact test in accordance with JIS Z 2242 (2018) was carried out on the prepared V-notch test piece to obtain the absorbed energy (J) at -80°C. The absorbed energy at -80°C for each test number obtained was shown in the column of "vE(-80°C)(J)" in Table 2.

[0118] [Evaluation results] Referring to Table 1A, Table 1B, Table 1C, and Table 2, the seamless steel pipes with test numbers 1 to 25 had an appropriate chemical composition, a yield strength of 758 MPa or more, and a microstructure consisting of 35 to 70% ferrite, 0 to 15% retained austenite, and the balance being martensite by volume ratio. These seamless steel pipes further satisfied that Fn1 was 0.10 to 1.00 and Fn2 was 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 an absorbed energy of 100 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, for test numbers 26 and 27, the Ni content was too low. As a result, these seamless steel pipes were judged not to have excellent SSC resistance in the SSC resistance test. These seamless steel pipes further had an absorbed energy of less than 100 J at -80°C and were judged not to have excellent low-temperature toughness in an extremely low-temperature environment.

[0120] For test numbers 28 and 29, the Cu content was too low. As a result, these seamless steel pipes were judged not to have excellent SSC resistance in the SSC resistance test.

[0121] For test numbers 30 and 31, the Co content was too low. As a result, these seamless steel pipes had an absorbed energy of less than 100 J at -80°C and were judged not to have excellent low-temperature toughness in an extremely low-temperature environment.

[0122] For test numbers 32 and 33, the Sn content was too low. As a result, these seamless steel pipes were judged not to have excellent SSC resistance in the SSC resistance test.

[0123] For test numbers 34 to 36, Fn1 was too high. As a result, these seamless steel pipes were judged not to have excellent SSC resistance in the SSC resistance test.

[0124] For Test Nos. 37 to 39, Fn1 was too low. As a result, these seamless steel pipes were judged not to have excellent SSC resistance in the SSC resistance test.

[0125] For Test Nos. 40 to 42, Fn2 was too low. As a result, the absorbed energy of these seamless steel pipes at -80°C was less than 100 J, and they were judged not to have excellent low-temperature toughness in an extremely low-temperature environment.

[0126] The embodiments of the present disclosure have been described above. However, the above-described embodiments are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiments, and the above-described embodiments can be appropriately modified and implemented without departing from the spirit thereof.

Claims

1. A stainless steel material, by mass percentage, 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: 16.50 - 19.50%, Mo: 1.50 - 6.00%, Ni: 3.50 - 9.00%, Cu: 0.03 - 3.00%, Co: 0.05 - 1.00%, Sn: 0.0005 - 0.0100%, sol.Al: 0.005 - 0.100%, N: 0.200% or less, O: 0.0200% or less, W: 0 - 1.80%, As: 0 - 0.0100%, Sb: 0 - 0.0100%, Ca: 0 - 0.0100%, Mg: 0 - 0.0100%, 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 consists of Fe and impurities, the yield strength is 758 MPa or more, the microstructure consists of ferrite with a volume fraction of 35 - 70%, retained austenite of 0 - 15%, and the balance martensite, the content of the said elements and the said yield strength satisfy formula (1), the content of the said elements satisfies formula (2), a stainless steel material. 0.10 ≦ (Sn + As + Sb) / {(Cu + Ni) / YS} ≦ 1.00 (1) (Ni + 2Co) / Sn ≧ 900 (2) Here, in formula (1) and (2), the element symbols are substituted with the contents of the corresponding elements in units of mass%. When the corresponding element is not contained, "0" is substituted for the element symbol. Also, in YS in formula (1), the yield strength is substituted in units of MPa.

2. The stainless steel material according to Claim 1, W: 0.01 - 1.80%, As: 0.0001 - 0.0100%, Sb: 0.0001 - 0.0100%, Ca: 0.0001 - 0.0100%, Mg: 0.0001 - 0.0100%, B: 0.0001 - 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 - 0.200%, Zn: 0.0001 - 0.0100%, and Pb: 0.0001 - 0.0100%, containing one or more elements selected from the group consisting of, a stainless steel material.

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