Steel material

Abstract

Provided is a steel material having high strength and excellent SSC resistance in a high H2S environment. A steel material according to the present disclosure contains, in terms of mass%, 0.15% to less than 0.30% of C, 0.05-1.00% of Si, 0.05% to less than 0.30% of Mn, 0.020% or less of P, 0.0050% or less of S, 0.10-1.00% of Cr, 0.85-2.50% of Mo, 0.002-0.020% of Ti, 0.002-0.050% of Nb, 0.01-0.30% of V, 0.0001-0.0030% of Ca, 0.0005-0.0050% of B and 0.005-0.100% of Al, with the remainder comprising Fe and impurities, has a yield strength of 862 MPa to less than 965 MPa, and is such that the content of elements, the grain size number (GSN) of prior austenite grains and the content of P close to prior austenite grain boundaries (P_seg.) satisfy formula (1).　Formula (1): (Mn+P_seg.) / (2.5Mo+GSN)≤0.125.

Description

steel material

[0001] The present disclosure relates to steel products, and more particularly to steel products suitable for use in high H2S environments.

[0002] The deepening of oil wells and gas wells (hereinafter, oil wells and gas wells are collectively referred to simply as "oil wells") has led to a demand for higher strength oil well steel materials, typified by oil well steel pipes. Specifically, 80 ksi grade (yield strength of 80 to less than 95 ksi, i.e., 552 to less than 655 MPa) and 95 ksi grade (yield strength of 95 to less than 110 ksi, i.e., 655 to less than 758 MPa) oil well steel materials are widely used, and recently, there has been an increasing demand for 110 ksi grade (yield strength of 110 to less than 125 ksi, i.e., 758 to less than 862 MPa) and 125 ksi grade (yield strength of 125 to less than 140 ksi, i.e., 862 to less than 965 MPa) oil well steel pipes.

[0003] On the other hand, many deep wells are in sour environments containing corrosive hydrogen sulfide. In this specification, a sour environment refers to an acidic environment containing hydrogen sulfide. Note that a sour environment may also contain carbon dioxide. Oil well steel pipes used in such sour environments are required to have not only high strength but also sulfide stress cracking resistance (hereinafter referred to as SSC resistance).

[0004] Techniques for improving the strength and SSC resistance of steel materials are proposed in Japanese Patent Laid-Open No. 2006-28612 (Patent Document 1) and Japanese Patent Laid-Open No. 2017-166060 (Patent Document 2).

[0005] The steel material disclosed in Patent Document 1 is a steel for steel pipes, which contains, by mass%, 0.2 to 0.7% C, 0.01 to 0.8% Si, 0.1 to 1.5% Mn, 0.005% or less S, 0.03% or less P, 0.0005 to 0.1% Al, 0.005 to 0.05% Ti, 0.0004 to 0.005% Ca, 0.007% or less N, 0.1 to 1.5% Cr, 0.2 to 1.0% Mo, and the balance being Fe and impurities. This steel material further contains non-metallic inclusions containing Ca, Al, Ti, N, O, and S, with a (Ca%) / (Al%) ratio of 0.55 to 1.72 and a (Ca%) / (Ti%) ratio of 0.7 to 19. Patent Document 1 describes that this steel has a high yield strength exceeding 758 MPa and excellent SSC resistance.

[0006] The steel material disclosed in Patent Document 2 is a material for high-strength steel pipes for oil wells, and consists of, in mass%, C: 0.20 to 0.45%, Si: 0.05 to 0.40%, Mn: 0.3 to 0.9%, P: 0.015% or less, S: 0.005% or less, Al: 0.005 to 0.10%, N: 0.001 to 0.006%, Cr: 0.1 to 0.8%, Mo: 0.1 to 1.6%, V: 0.02 to 0.2%, Nb: 0.001 to 0.04%, B: 0.0003 to 0.0030%, O (oxygen): 0.0030% or less, and the balance being Fe and unavoidable impurities. Furthermore, this steel material has a Rockwell hardness HRC that satisfies the formula (15.6 × [%C] + 29.2 ≦ HRC < 60.5 × [%C] + 31.1). Patent Document 2 states that this steel material makes it possible to obtain steel pipes having a yield strength of 758 to less than 862 MPa and excellent SSC resistance.

[0007] JP 2006-28612 A JP 2017-166060 A

[0008] Recently, attention has been paid to wells in even more severe environments. For example, development has been carried out for wells in environments containing high-pressure hydrogen sulfide (HS) gas. Specifically, in sour environments where the partial pressure of HS gas is as high as 10 atm (hereinafter, a sour environment where the partial pressure of HS gas is 10 atm is referred to as a "high HS environment"), more severe SSC resistance is required than in sour environments containing normal-pressure HS gas. Therefore, there is a demand for steel materials that have high SSC resistance even in such high HS environments.

[0009] The above Patent Documents 1 and 2 propose steel materials having high strength and excellent SSC resistance. However, the above Patent Documents 1 and 2 do not consider SSC resistance in a high H2S environment at all.

[0010] An object of the present disclosure is to provide a steel material having high strength and excellent SSC resistance even in a high H2S environment.

[0011] The steel material according to the present disclosure contains, in mass %, C: 0.15 to less than 0.30%, Si: 0.05 to 1.00%, Mn: 0.05 to less than 0.30%, P: 0.020% or less, S: 0.0050% or less, Cr: 0.10 to 1.00%, Mo: 0.85 to 2.50%, Ti: 0.002 to 0.020%, Nb: 0.002 to 0.050%, V: 0.01 to 0.30%, Ca: 0.0001 to 0.0030%, B: 0.0005 to 0.0050%, Al: 0.005 to 0.100%, N: 0.0050% or less, O: 0.0020% or less, The alloy comprises Cu: 0 to 0.50%, Ni: 0 to 0.50%, W: 0 to 1.00%, Zr: 0 to 0.50%, Mg: 0 to 0.0100%, rare earth elements: 0 to 0.0100%, and the balance being Fe and impurities, and has a yield strength of 862 to less than 965 MPa, and the contents of the elements, the grain size number GSN of prior austenite grains in the microstructure, and the P content P_seg. in the vicinity of the prior austenite grain boundaries satisfy formula (1): (Mn+P_seg.) / (2.5Mo+GSN)≦0.125 (1) Here, the element symbols in formula (1) are substituted with the contents of the corresponding elements in units of mass %. Also, P_seg. in formula (1) satisfies formula (1): The P content in the vicinity of the prior austenite grain boundary is substituted into , and the grain size number of the prior austenite grain is substituted into GSN in formula (1).

[0012] The steel material according to the present disclosure has high strength and excellent SSC resistance even in a high H2S environment.

[0013] FIG. 1 is a diagram showing the relationship between Fn1 (=(Mn+P_seg.) / (2.5Mo+GSN)) in this example and the number of SSC occurrences (pieces), which is an index of SSC resistance.

[0014] First, the inventors considered obtaining a high-strength steel material having a yield strength of 125 ksi (862 to less than 965 MPa). That is, the inventors investigated and examined a method for obtaining a 125 ksi-class yield strength and excellent SSC resistance even in a high-HS environment in a steel material intended for use in oil wells. As a result, the inventors obtained the following findings.

[0015] Next, the inventors focused on the grain boundaries of prior austenite grains (hereinafter, prior austenite grains are also referred to as "prior γ grains") and considered improving the SSC resistance of steel materials. Here, there is a concern that if the prior γ grain boundaries of a steel material become embrittled and the cracking sensitivity of the prior γ grain boundaries increases, the SSC resistance of the steel material will decrease. On the other hand, if the embrittlement of the prior γ grain boundaries of a steel material can be suppressed and the prior γ grain boundaries can be strengthened, there is a possibility that the SSC resistance of the steel material can be improved.

[0016] Here, manganese (Mn) in the chemical composition tends to concentrate at prior γ grain boundaries, embrittling them. Mn also promotes the concentration of phosphorus (P) at prior γ grain boundaries, further embrittling them. On the other hand, if molybdenum (Mo) concentrates at prior γ grain boundaries, it can strengthen the prior γ grain boundaries. As a result of studies by the present inventors, it has become clear that reducing the Mn content of a steel material to less than 0.30% and increasing the Mo content to 0.85% or more may suppress the embrittlement of the prior γ grain boundaries of the steel material and strengthen the prior γ grain boundaries of the steel material.

[0017] That is, in mass%, C: 0.15 to less than 0.30%, Si: 0.05 to 1.00%, Mn: 0.05 to less than 0.30%, P: 0.020% or less, S: 0.0050% or less, Cr: 0.10 to 1.00%, Mo: 0.85 to 2.50%, Ti: 0.002 to 0.020%, Nb: 0.002 to 0.050%, V: 0.01 to 0.30%, Ca: 0.0001 to 0.0030%, B: 0.0005 to 0.0050 %, Al: 0.005 to 0.100%, N: 0.0050% or less, O: 0.0020% or less, Cu: 0 to 0.50%, Ni: 0 to 0.50%, W: 0 to 1.00%, Zr: 0 to 0.50%, Mg: 0 to 0.0100%, rare earth elements: 0 to 0.0100%, and the balance being Fe and impurities, the inventors considered that there is a possibility that a 125 ksi-class yield strength and excellent SSC resistance can both be achieved.

[0018] On the other hand, even with the above-mentioned chemical composition, excellent SSC resistance may not be obtained in a high-HS environment. Therefore, the present inventors further investigated in detail the factors that cause the above-mentioned steel material to have a lower SSC resistance in a high-HS environment. As a result, it was revealed that even in a steel material having the above-mentioned chemical composition, the effect of embrittlement of prior-γ grain boundaries becomes significant in a high-HS environment, and the SSC resistance of the steel material is likely to be reduced. Therefore, the present inventors conducted a detailed investigation into further strengthening of the prior-γ grain boundaries of a steel material having the above-mentioned chemical composition.

[0019] As a result of detailed studies by the inventors, it was found that a steel material having the above-described chemical composition can stably achieve both a 125 ksi-class yield strength and SSC resistance in a high-HS environment if the Mn content, Mo content, prior γ grain grain size number (GSN), and P content (P_seg.) near the prior γ grain boundaries satisfy the following formula (1): (Mn + P_seg.) / (2.5Mo + GSN) ≦ 0.125 (1) Here, the element symbols in formula (1) are substituted with the contents of the corresponding elements in units of mass %. Furthermore, the P content (mol.%) near the prior austenite grain boundaries is substituted for P_seg. in formula (1), and the grain size number of the prior austenite grains is substituted for GSN in formula (1).

[0020] Fn1 is defined as (Mn + P_seg.) / (2.5Mo + GSN). The relationship between Fn1 and SSC resistance in a high HS environment will be explained in more detail below with reference to the drawings. FIG. 1 is a diagram showing the relationship between Fn1 (= (Mn + P_seg.) / (2.5Mo + GSN)) in this example and the number of SSC occurrences (pieces), which is an index of SSC resistance. FIG. 1 was created using Fn1 determined by a method described later for steel materials having the above-mentioned chemical composition and a yield strength of 862 to less than 965 MPa in the examples described later, and the number of SSC occurrences (pieces), also determined by a method described later, in a 10 atm HS gas environment.

[0021] Referring to FIG. 1 , in a steel material having the above-described chemical composition and a yield strength of 862 to less than 965 MPa, when Fn1 is 0.125 or less, the number of SSC occurrences in a 10 atm HS gas environment is zero, and the steel material has excellent SSC resistance even in a high HS environment. On the other hand, in a steel material having the above-described chemical composition and a yield strength of 862 to less than 965 MPa, when Fn1 exceeds 0.125, the number of SSC occurrences in a 10 atm HS gas environment is one or more, and the steel material does not have excellent SSC resistance in a high HS environment. Therefore, the steel material according to this embodiment has the above-described chemical composition and a yield strength of 862 to less than 965 MPa, and Fn1 is set to 0.125 or less. As a result, the steel material according to this embodiment has high strength and excellent SSC resistance even in a high HS environment.

[0022] The details of why a steel material having the above-described chemical composition and a yield strength of 862 to less than 965 MPa has excellent SSC resistance even in a high-HS environment if Fn1 is 0.125 or less have not been clarified. However, the inventors speculate as follows. Here, the above-described Fn1 (= (Mn + P_seg.) / (2.5Mo + GSN)) is an index showing the degree of embrittlement of the prior-γ grain boundary. As described above, the higher the Mn content, the more Mn concentrates at the prior-γ grain boundary, making the prior-γ grain boundary more likely to become embrittled. Furthermore, the higher the P content P_seg. near the prior-γ grain boundary (hereinafter, the P content P_seg. near the prior-γ grain boundary will also be referred to as the "grain boundary P content P_seg."), the more likely the prior-γ grain boundary becomes embrittled.

[0023] On the other hand, the higher the Mo content, the more Mo concentrates at the prior γ grain boundaries, strengthening them. Furthermore, the larger the prior γ grain grain size number (GSN) is, the smaller the prior γ grain grain size becomes, which increases the area occupied by the prior γ grain boundaries in the steel material, potentially mitigating embrittlement of the prior γ grain boundaries. Therefore, the inventors speculate that, in the steel material according to this embodiment, by setting Fn1 to 0.125 or less, assuming the above-described chemical composition and a yield strength of 862 to less than 965 MPa, excellent SSC resistance can be stably obtained even in a high-HS environment. It is possible, however, that excellent SSC resistance can be stably obtained even in a high-HS environment through a mechanism different from that speculated by the inventors. However, the fact that excellent SSC resistance can be obtained even in a high-HS environment by setting Fn1 to 0.125 or less, assuming the above-described chemical composition and a yield strength of 862 to less than 965 MPa, is proven by the examples described below.

[0024] The gist of the steel material according to this embodiment, which was completed based on the above findings, is as follows.

[0025] [1] In mass%, C: 0.15 to less than 0.30%, Si: 0.05 to 1.00%, Mn: 0.05 to less than 0.30%, P: 0.020% or less, S: 0.0050% or less, Cr: 0.10 to 1.00%, Mo: 0.85 to 2.50%, Ti: 0.002 to 0.020%, Nb: 0.002 to 0.050%, V: 0.01 to 0.30%, Ca: 0.0001 to 0.0030%, B: 0.0005 to 0.0050%, Al: 0.005 to 0.100%, N: 0.0050% or less, O: 0.0020% or less, Cu: 0 to 0.50%, A steel material comprising Ni: 0 to 0.50%, W: 0 to 1.00%, Zr: 0 to 0.50%, Mg: 0 to 0.0100%, rare earth elements: 0 to 0.0100%, and the balance being Fe and impurities, having a yield strength of 862 to less than 965 MPa, and the contents of the elements, the grain size number GSN of prior austenite grains in the microstructure, and the P content P_seg. in the vicinity of the prior austenite grain boundaries satisfying formula (1): (Mn+P_seg.) / (2.5Mo+GSN)≦0.125 (1) Here, the element symbols in formula (1) are substituted with the contents of the corresponding elements in units of mass %. Furthermore, P_seg. in formula (1) represents the P content in the vicinity of the prior austenite grain boundaries in mol. %, and the grain size number of the prior austenite grains is substituted for GSN in formula (1).

[0026] [2] The steel material according to [1], containing one or more elements selected from the group consisting of Cu: 0.01 to 0.50%, Ni: 0.01 to 0.50%, W: 0.01 to 1.00%, Zr: 0.01 to 0.50%, Mg: 0.0001 to 0.0100%, and rare earth elements: 0.0001 to 0.0100%.

[0027] [3] The steel material according to [1] or [2], wherein the steel material is a seamless steel pipe.

[0028] The shape of the steel material according to this embodiment is not particularly limited. The steel material according to this embodiment may be a steel pipe, a round bar (solid material), or a steel plate. Note that round bar refers to 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.

[0029] The steel material according to this embodiment will be described in detail below. Unless otherwise specified, "%" regarding elements means mass %.

[0030] [Chemical Composition] The chemical composition of the steel material according to this embodiment contains the following elements.

[0031] C: 0.15 to less than 0.30% Carbon (C) improves the hardenability of steel and increases its strength. If the C 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 C content is too high, even if the contents of other elements are within the ranges of this embodiment, excessive carbides are formed in the steel, reducing the SSC resistance of the steel. Therefore, the C content is 0.15 to less than 0.30%. A preferred lower limit of the C content is 0.16%, more preferably 0.17%. A preferred upper limit of the C content is 0.29%, more preferably 0.28%, and even more preferably 0.27%.

[0032] Si: 0.05 to 1.00% Silicon (Si) deoxidizes steel. If the Si content is too low, the above effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Si content is too high, the SSC resistance of the steel material decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the Si content is 0.05 to 1.00%. A preferred lower limit of the Si content is 0.06%, more preferably 0.08%, and even more preferably 0.10%. A preferred upper limit of the Si content is 0.96%, more preferably 0.92%, and even more preferably 0.90%.

[0033] Mn: 0.05 to less than 0.30% Manganese (Mn) improves the hardenability of steel and increases its strength. If the Mn content is too low, the above effects cannot be fully achieved even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Mn content is too high, prior γ grain boundaries become embrittled, reducing the SSC resistance of the steel, even if the contents of other elements are within the ranges of this embodiment. If the Mn content is too high, it further promotes an increase in the grain boundary P content P_seg., reducing the SSC resistance of the steel. Therefore, the Mn content is 0.05 to less than 0.30%. The preferred lower limit of the Mn content is 0.06%, more preferably 0.08%, and even more preferably 0.10%. The preferred upper limit of the Mn content is 0.29%, more preferably 0.28%.

[0034] P: 0.020% or less Phosphorus (P) is an impurity. That is, the lower limit of the P content is greater than 0%. If the P content is too high, even if the contents of other elements are within the ranges of this embodiment, the grain boundary P content (P_seg.) becomes too high, and the SSC resistance of the steel material decreases. Therefore, the P content is 0.020% or less. A preferred upper limit of the P content is 0.018%, more preferably 0.015%. The P content is preferably as low as possible. However, an extreme reduction in the P content significantly increases manufacturing costs. Therefore, considering industrial production, a preferred lower limit of the P content is 0.001%, more preferably 0.002%, and even more preferably 0.003%.

[0035] S: 0.0050% or less Sulfur (S) is an impurity. That is, the lower limit of the S content is more than 0%. If the S content is too high, even if the contents of other elements are within the ranges of this embodiment, coarse sulfide-based inclusions are formed, and the SSC resistance of the steel material decreases. Therefore, the S content is 0.0050% or less. A preferred upper limit of the S content is 0.0045%, more preferably 0.0040%, and even more preferably 0.0035%. The S content is preferably as low as possible. However, an extreme reduction in the S content significantly increases manufacturing costs. Therefore, considering industrial production, a preferred lower limit of the S content is 0.0001%, more preferably 0.0002%, and even more preferably 0.0003%.

[0036] Cr: 0.10 to 1.00% Chromium (Cr) improves the hardenability of steel and increases its strength. If the Cr content is too low, the above effects cannot be sufficiently achieved even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Cr content is too high, the strength of the steel becomes too high, even if the contents of other elements are within the ranges of this embodiment, and the SSC resistance of the steel decreases. Therefore, the Cr content is 0.10 to 1.00%. A preferred lower limit of the Cr content is 0.15%, and more preferably 0.20%. A preferred upper limit of the Cr content is 0.95%, more preferably 0.90%, even more preferably 0.85%, and even more preferably 0.80%.

[0037] Mo: 0.85 to 2.50% Molybdenum (Mo) concentrates at prior γ grain boundaries and strengthens them. As a result, the SSC resistance of the steel material is improved. 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, even if the contents of other elements are within the ranges of this embodiment, excessive fine carbides are formed, resulting in an excessively high strength of the steel material and a decrease in the SSC resistance of the steel material. Therefore, the Mo content is 0.85 to 2.50%. A preferred lower limit of the Mo content is 0.87%, and more preferably 0.90%. A preferred upper limit of the Mo content is 2.20%, and more preferably 2.00%.

[0038] Ti: 0.002 to 0.020% Titanium (Ti) forms fine nitrides and refines the prior γ grains of the steel material through a pinning effect. As a result, the grain size number (GSN) of the prior γ grains increases, and the SSC resistance of the steel material improves. If the Ti content is too low, the above effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Ti content is too high, coarse nitrides are formed, and the SSC resistance of the steel material decreases, even if the contents of other elements are within the ranges of this embodiment. Therefore, the Ti content is 0.002 to 0.020%. The preferred lower limit of the Ti content is 0.003%, and more preferably 0.005%. The preferred upper limit of the Ti content is 0.018%, more preferably 0.016%, and even more preferably 0.014%.

[0039] Nb: 0.002 to 0.050% Niobium (Nb) forms carbides, nitrides, or carbonitrides (hereinafter referred to as "carbonitrides, etc.") and refines the prior γ grains of the steel material through a pinning effect. As a result, the grain size number GSN of the prior γ grains increases, and the SSC resistance of the steel material improves. If the Nb content is too low, the above effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Nb content is too high, even if the contents of other elements are within the ranges of this embodiment, excessive amounts of carbonitrides, etc. are formed, and the SSC resistance of the steel material decreases. Therefore, the Nb content is 0.002 to 0.050%. A preferred lower limit of the Nb content is 0.003%, and more preferably 0.005%. A preferred upper limit of the Nb content is 0.045%, and more preferably 0.040%.

[0040] V: 0.01 to 0.30% Vanadium (V) forms carbonitrides and the like to increase the strength of the steel material. If the V content is too low, the above effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the V content is too high, the strength of the steel material becomes too high, even if the contents of other elements are within the ranges of this embodiment, and the SSC resistance of the steel material decreases. Therefore, the V content is 0.01 to 0.30%. A preferred lower limit of the V content is 0.02%, more preferably 0.03%, and even more preferably 0.05%. A preferred upper limit of the V content is 0.28%, more preferably 0.25%, and even more preferably 0.22%.

[0041] Ca: 0.0001 to 0.0030% Calcium (Ca) forms sulfides and reduces Mn sulfides at prior γ grain boundaries. As a result, the SSC resistance of the steel material is improved. If the Ca content is too low, the above effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Ca content is too high, coarse oxides are generated, reducing the SSC resistance of the steel material even if the contents of other elements are within the ranges of this embodiment. Therefore, the Ca content is 0.0001 to 0.0030%. A preferred lower limit of the Ca content is 0.0003%, and more preferably 0.0005%. A preferred upper limit of the Ca content is 0.0028%, and more preferably 0.0025%.

[0042] B: 0.0005 to 0.0050% Boron (B) improves the hardenability of steel and increases its strength. Furthermore, B concentrates at prior γ grain boundaries, reducing the grain boundary P content (P_seg.). As a result, the SSC resistance of the steel is improved. If the B content is too low, the above effects cannot be fully achieved even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the B content is too high, coarse B nitrides are formed, reducing the SSC resistance of the steel, even if the contents of other elements are within the ranges of this embodiment. Therefore, the B content is 0.0005 to 0.0050%. The preferred lower limit of the B content is 0.0006%, more preferably 0.0008%, and even more preferably 0.0010%. The preferred upper limit of the B content is 0.0048%, and even more preferably 0.0045%.

[0043] Al: 0.005 to 0.100% Aluminum (Al) deoxidizes steel. If the Al content is too low, the above effect cannot be sufficiently achieved even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Al content is too high, coarse oxide-based inclusions are formed, reducing the SSC resistance of the steel material even if the contents of other elements are within the ranges of this embodiment. Therefore, the Al content is 0.005 to 0.100%. A preferred lower limit of the Al content is 0.005%, more preferably 0.010%, even more preferably 0.015%, and even more preferably 0.020%. A preferred upper limit of the Al content is 0.090%, more preferably 0.080%, even more preferably 0.070%, even more preferably 0.065%, and even more preferably 0.060%. As used herein, the "Al" content refers to the content of "acid-soluble Al," i.e., "sol. Al."

[0044] N: 0.0050% or less Nitrogen (N) is unavoidably contained. That is, the lower limit of the N content is greater than 0%. N forms nitrides and refines the prior γ grains of the steel material through a pinning effect. As a result, the grain size number GSN of the prior γ grains increases, and the SSC resistance of the steel material improves. On the other hand, if the N content is too high, even if the contents of other elements are within the ranges of this embodiment, coarse nitrides are formed, and the SSC resistance of the steel material decreases. Therefore, the N content is 0.0050% or less. A preferred upper limit of the N content is 0.0048%, more preferably 0.0045%, and even more preferably 0.0040%. To more effectively obtain the above effects, a preferred lower limit of the N content is 0.0003%, more preferably 0.0005%, and even more preferably 0.0010%.

[0045] O: 0.0020% or less Oxygen (O) is an unavoidable impurity. In other words, the lower limit of the O content is more than 0%. If the O content is too high, even if the contents of other elements are within the ranges of this embodiment, coarse oxides are formed, and the SSC resistance of the steel material decreases. Therefore, the O content is 0.0020% or less. A preferred upper limit of the O content is 0.0018%, more preferably 0.0015%. The O content is preferably as low as possible. However, an extreme reduction in the O content significantly increases manufacturing costs. Therefore, considering industrial production, a preferred lower limit of the O content is 0.0001%, more preferably 0.0005%, and even more preferably 0.0008%.

[0046] The balance of the chemical composition of the steel material according to this embodiment is composed of Fe and impurities. Here, the impurities refer to substances that are mixed in from raw materials such as ore and scrap, or the manufacturing environment, during industrial production of the steel material, and are acceptable within a range that does not adversely affect the steel material according to this embodiment.

[0047] [Optional Elements] The chemical composition of the steel material described above may further contain one or more elements selected from the group consisting of Cu and Ni in place of a portion of Fe. All of these elements are optional elements, and they improve the hardenability and strength of the steel material.

[0048] Cu: 0 to 0.50% Copper (Cu) is an optional element and does not necessarily need to be contained. That is, the Cu content may be 0%. When contained, Cu improves the hardenability of the steel material and increases its strength. Even if even a small amount of Cu is contained, the above effects can be obtained to some extent. However, if the Cu content is too high, even if the contents of other elements are within the ranges of this embodiment, the hardenability of the steel material will be too high and the SSC resistance of the steel material will decrease. Therefore, the Cu content is 0 to 0.50%. The preferred lower limit of the Cu content is more than 0%, more preferably 0.01%, and even more preferably 0.02%. The preferred upper limit of the Cu content is 0.40%, more preferably 0.30%, even more preferably 0.20%, and even more preferably 0.10%.

[0049] Ni: 0 to 0.50% Nickel (Ni) is an optional element and does not necessarily need to be contained. That is, the Ni content may be 0%. When contained, Ni improves the hardenability and strength of the steel. Ni also dissolves in the steel to improve the toughness of the steel. Even if even a small amount of Ni is contained, these effects can be obtained to some extent. However, if the Ni content is too high, localized corrosion is promoted and the SSC resistance of the steel decreases, even if the contents of other elements are within the ranges of this embodiment. Therefore, the Ni content is 0 to 0.50%. The preferred lower limit of the Ni content is more than 0%, more preferably 0.01%, even more preferably 0.02%, and even more preferably 0.04%. The preferred upper limit of the Ni content is 0.40%, even more preferably 0.30%, even more preferably 0.20%, and even more preferably 0.10%.

[0050] The chemical composition of the above-mentioned steel material may further contain W instead of a part of Fe.

[0051] W: 0 to 1.00% Tungsten (W) is an optional element and does not necessarily need to be contained. That is, the W content may be 0%. When W is contained, it forms a protective corrosion film in sour environments and suppresses hydrogen penetration into the steel material. As a result, the SSC resistance of the steel material is improved. Even if even a small amount of W is contained, the above effects can be achieved to some extent. However, if the W content is too high, even if the contents of other elements are within the ranges of this embodiment, coarse carbides will form in the steel material, reducing the low-temperature toughness and SSC resistance of the steel material. Therefore, the W content is 0 to 1.00%. The preferred lower limit of the W content is more than 0%, more preferably 0.01%, even more preferably 0.03%, even more preferably 0.05%, and even more preferably 0.10%. The preferred upper limit of the W content is 0.90%, even more preferably 0.80%, even more preferably 0.60%, and even more preferably 0.50%.

[0052] The chemical composition of the steel material described above may further contain one or more elements selected from the group consisting of Zr, Mg, and rare earth elements in place of a portion of Fe. All of these elements are optional elements, and they render S in the steel material harmless as sulfides, thereby improving the SSC resistance of the steel material.

[0053] Zr: 0 to 0.50% Zirconium (Zr) is an optional element and does not necessarily need to be contained. That is, the Zr content may be 0%. When contained, Zr neutralizes S in the steel material as sulfides, thereby improving the SSC resistance of the steel material. Even if even 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 ranges of this embodiment, oxides in the steel material will coarsen, reducing the SSC resistance of the steel material. Therefore, the Zr content is 0 to 0.50%. The preferred lower limit of the Zr content is more than 0%, more preferably 0.01%, and even more preferably 0.03%. The preferred upper limit of the Zr content is 0.40%, more preferably 0.30%, even more preferably 0.20%, and even more preferably 0.10%.

[0054] Mg: 0 to 0.0100% Magnesium (Mg) is an optional element and does not necessarily need to be contained. That is, the Mg content may be 0%. When contained, Mg renders S in the steel material harmless as sulfides, thereby improving the SSC resistance of the steel material. Even if even a small amount of Mg is contained, the above effect can be obtained to some extent. However, if the Mg content is too high, even if the contents of other elements are within the ranges of this embodiment, oxides in the steel material will coarsen, and the SSC resistance of the steel material will decrease. Therefore, the Mg content is 0 to 0.0100%. The preferred lower limit of the Mg content is more than 0%, more preferably 0.0001%, even more preferably 0.0005%, even more preferably 0.0010%, and even more preferably 0.0020%. The upper limit of the Mg content is preferably 0.0090%, more preferably 0.0080%, even more preferably 0.0070%, even more preferably 0.0060%, and still more preferably 0.0050%.

[0055] Rare earth elements (REM): 0 to 0.0100% Rare earth elements (REM) are optional elements and do not necessarily need to be contained. That is, the REM content may be 0%. When contained, REM neutralizes S in the steel material as sulfides, improving the SSC resistance of the steel material. Even if even a small amount of REM is contained, the above effect can be obtained to some extent. However, if the REM content is too high, even if the contents of other elements are within the ranges of this embodiment, oxides in the steel material will coarsen, reducing the SSC resistance of the steel material. Therefore, the REM content is 0 to 0.0100%. The preferred lower limit of the REM content is more than 0%, more preferably 0.0001%, even more preferably 0.0005%, even more preferably 0.0010%, and even more preferably 0.0020%. The upper limit of the REM content is preferably 0.0090%, more preferably 0.0080%, even more preferably 0.0070%, and still more preferably 0.0060%.

[0056] 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 lanthanoids 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.

[0057] [Yield Strength] The yield strength of the steel material according to this embodiment is 862 to less than 965 MPa (125 to less than 140 ksi). The yield strength in this specification means the stress at 0.7% elongation obtained in a tensile test in accordance with ASTM E8 / E8M (2022).

[0058] The yield strength of the steel material according to this embodiment is determined by the following method. Specifically, a tensile test specimen is prepared from the steel material according to this embodiment. When the steel material is a steel plate, the tensile test specimen is prepared from the center of the plate thickness. In this case, the longitudinal direction of the tensile test specimen is parallel to the rolling direction of the steel plate. When the steel material is a steel pipe, the tensile test specimen is prepared from the center of the wall thickness. In this case, the longitudinal direction of the tensile test specimen is parallel to the axial direction of the steel pipe. When the steel material is a round bar, the tensile test specimen 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 specimen is parallel to the axial direction of the round bar.

[0059] The tensile test specimen is, for example, a round bar test specimen with a parallel portion diameter of 8.9 mm and a gauge length of 35.6 mm. When the steel material is a steel pipe, a circular arc-shaped test specimen may be used as the tensile test specimen. In this case, the size of the circular arc-shaped test specimen is, for example, the full wall thickness, a width of 25.4 mm, and a gauge length of 50.8 mm. The longitudinal direction of the circular arc-shaped test specimen is parallel to the axial direction of the steel pipe. Using the prepared tensile test specimen, a tensile test is performed in air at room temperature (25°C) in accordance with ASTM E8 / E8M (2022). The stress at 0.7% elongation obtained by the tensile test is defined as the yield strength (MPa). In this embodiment, the yield strength (MPa) is determined by rounding the obtained value to one decimal place.

[0060] [Fn1] Assuming that the steel material according to this embodiment has the above-described chemical composition, the element contents, the grain size number GSN of the prior austenite grains in the microstructure, and the P content P_seg. near the prior austenite grain boundaries satisfy formula (1): (Mn + P_seg.) / (2.5Mo + GSN) ≦ 0.125 (1) Here, the element symbols in formula (1) are substituted with the contents of the corresponding elements in units of mass %. Furthermore, the P content near the prior austenite grain boundaries is substituted for P_seg. in formula (1), and the grain size number of the prior austenite grains is substituted for GSN in formula (1).

[0061] As described above, the P content P_seg. near the prior γ grain boundaries is also referred to as the "grain boundary P content P_seg." Furthermore, Fn1 (= (Mn + P_seg.) / (2.5Mo + GSN)) is an index showing the degree of embrittlement of the prior γ grain boundaries. The higher the Mn content, the more Mn concentrates at the prior γ grain boundaries, making the prior γ grain boundaries more likely to become embrittled. Furthermore, the higher the grain boundary P content P_seg., the more likely the prior γ grain boundaries become embrittled. On the other hand, the higher the Mo content, the more Mo concentrates at the prior γ grain boundaries, strengthening the prior γ grain boundaries. Furthermore, the larger the grain size number GSN of the prior γ grains, the smaller the grain size of the prior γ grains, increasing the area occupied by the prior γ grain boundaries in the steel material, making it easier to mitigate embrittlement of the prior γ grain boundaries.

[0062] That is, the steel material according to this embodiment does not simply refine prior γ grains or reduce the amount of grain boundary P, but adjusts the Mn content, which is an element that embrittles prior γ grain boundaries, the Mo content, which is an element that strengthens prior γ grain boundaries, the grain size number GSN of the prior γ grains, and the amount of grain boundary P P_seg. As a result, the steel material according to this embodiment can stably achieve both a 125 ksi-class yield strength and excellent SSC resistance even in a high HS environment.

[0063] The upper limit of Fn1 is preferably 0.124, and more preferably 0.122. The lower limit of Fn1 is not particularly limited. On the other hand, in a steel material having the above-mentioned chemical composition, the lower limit of Fn1 may be, for example, 0.060, 0.080, or 0.090.

[0064] The grain size number GSN of the prior γ grains is not particularly limited as long as it satisfies formula (1). On the other hand, in a steel material having the above-mentioned chemical composition and satisfying formula (1), the grain size number GSN of the prior γ grains is, for example, 5.0 to 12.0. A preferred lower limit of the grain size number GSN of the prior γ grains is 5.5, and more preferably 6.0. The upper limit of the grain size number GSN of the prior γ grains may be 11.5 or 11.0.

[0065] The P content near the prior γ grain boundaries (grain boundary P content) P_seg. is not particularly limited as long as it satisfies formula (1). On the other hand, in a steel material having the above-described chemical composition and satisfying formula (1), the grain boundary P content P_seg. is, for example, 1.7 mol.% or less. A preferred upper limit of the grain boundary P content P_seg. is 1.6 mol.%, and more preferably 1.5 mol.%. The lower limit of the grain boundary P content P_seg. may be 0.0 mol.%, 0.3 mol.%, 0.5 mol.%, or 1.0 mol.%.

[0066] In this embodiment, Fn1 can be determined by the following method. First, the grain size number GSN of the prior γ grains of the steel material according to this embodiment is determined. Specifically, a test piece for measuring the prior γ grain size D is prepared from the steel material according to this embodiment. When the steel material is a steel plate, a test piece is prepared from the center of the plate thickness, with the plane including the rolling direction and the plate thickness direction as the observation surface. When the steel material is a steel pipe, a test piece is prepared from the center of the wall thickness, with the plane including the pipe axial direction and the pipe radial direction as the observation surface. When the steel material is a round bar, a test piece is prepared with the R / 2 position in the center and the plane including the axial and radial directions as the observation surface.

[0067] The test piece is embedded in resin, and the observation surface is polished to a mirror finish. Then, it is immersed for about 60 seconds in a solution of saturated aqueous picric acid mixed with an appropriate amount of surfactant, and the prior γ grain boundaries are revealed by etching. The area of ​​the observation surface is not limited, but for example, 100 mm 2 (10 mm × 10 mm). The grain size number GSN is determined by optical microscope observation based on the cutting method in accordance with JIS G 0551 (2020). In this embodiment, the grain size number GSN of the prior γ grains is determined by rounding the obtained value to one decimal place.

[0068] Next, the grain boundary P content P_seg. of the steel material according to this embodiment is determined. Specifically, a notched round bar test specimen is prepared from the steel material according to this embodiment. The size of the notched round bar test specimen is, for example, 3 mm in diameter and 17 mm in axial length. Furthermore, a notch is formed in the center of the axial length of the round bar test specimen. The notch is, for example, a V-notch with a depth of 0.7 mm, and is formed around the entire circumference of the round bar. When the steel material is a steel plate, the test specimen is prepared from the center of the plate thickness. In this case, the axial direction of the test specimen is parallel to the rolling direction of the steel plate. When the steel material is a steel pipe, the test specimen is prepared from the center of the wall thickness. In this case, the axial direction of the test specimen is parallel to the axial direction of the steel pipe. When the steel material is a round bar, the test specimen is prepared from the R / 2 position. In this case, the axial direction of the test specimen is parallel to the axial direction of the round bar.

[0069] The prepared test piece was placed in a vacuum chamber attached to the Auger electron spectrometer, and the degree of vacuum in the chamber was adjusted to 10 -7 Pa or less. For example, a PHI680 manufactured by ULVAC-PHI, Inc. is used as the Auger electron spectrometer. A test piece cooled to -120°C or less in a chamber is subjected to impact fracture to obtain a prior γ grain boundary fracture surface. In this embodiment, a plurality of measurement points are identified on each of a plurality of prior γ grain boundary fracture surfaces. Specifically, for example, a total of 10 or more measurement points are identified on three or more prior γ grain boundary fracture surfaces. Furthermore, the measurement points identify regions of the prior γ grain boundary fracture surface other than precipitates. Whether or not a region is a precipitate can be determined from the contrast by a person skilled in the art.

[0070] The identified measurement points are subjected to elemental concentration analysis by Auger electron spectroscopy. In Auger electron spectroscopy, an acceleration voltage of 10 kV and a sample current of 10 nA are used, and the target elements are quantified as P, S, C, Cr, Fe, Mn, Si, and Mo. In the differential Auger spectrum, the Auger peaks of each element used for quantitative calculation are: P: 102-130 eV, S: 131-165 eV, C: 234-292 eV, Cr: 510-540 eV, Fe: 685-715 eV, Mn: 535-545 eV, Si: 1588-1640 eV, and Mo: 169-199 eV. Quantitative values ​​are obtained for the obtained Auger peaks of each element using the peak intensity and relative sensitivity coefficient. In this case, the peak intensity is determined as the difference between the maximum and minimum values ​​of the peak (so-called peak-to-peak intensity). Furthermore, the relative sensitivity coefficient can be a value provided by the device manufacturer.

[0071] For each measurement point, the P content (mol %) is defined as the P content (mol %) when the total content of P, S, C, Cr, Fe, Mn, Si, and Mo is taken as 100 mol %. The arithmetic average of the P contents (mol %) at all measurement points is defined as the grain boundary P content P_seg. (mol %). That is, in this embodiment, the P content P_seg. near the prior austenite grain boundary means the P content (mol %) when the total content of P, S, C, Cr, Fe, Mn, Si, and Mo is taken as 100 mol % as quantified by Auger electron spectroscopy on the prior austenite grain boundary fracture surface. Note that in this embodiment, the grain boundary P content P_seg. (mol %) is calculated by rounding the obtained value to one decimal place. In this embodiment, Fn1 (=(Mn+P_seg.) / (2.5Mo+GSN)) is calculated by rounding off the obtained value to the fourth decimal place.

[0072] [SSC Resistance] The steel material according to this embodiment has the above-described chemical composition and satisfies formula (1). As a result, the steel material according to this embodiment achieves both a 125 ksi-class yield strength and excellent SSC resistance in a high-HS environment. In this embodiment, the phrase "the steel material has excellent SSC resistance" is defined as follows.

[0073] Specifically, in this embodiment, the SSC resistance of the steel material is evaluated by an SSC resistance test carried out in a 0.1 atm H2S gas environment using a method conforming to NACE TM0177-2016 Method A, and a four-point bending test carried out in a 10 atm H2S gas environment using a method conforming to NACE TM0316 (2016).

[0074] First, we will explain the SSC resistance test performed in a 0.1 atm H2S gas environment using a method conforming to NACE TM0177-2016 Method A. Round bar test specimens are prepared from the steel material according to this embodiment. The size of the round bar test specimen is, for example, 6.35 mm in diameter and 25.4 mm in length at the parallel portion. When the steel material is a steel plate, the round bar test specimen is prepared from the center of the plate thickness. In this case, the axial direction of the round bar test specimen is parallel to the rolling direction of the steel plate. When the steel material is a steel pipe, the round bar test specimen is prepared from the center of the wall thickness. In this case, the axial direction of the round bar test specimen is parallel to the axial direction of the steel pipe. When the steel material is a round bar, the round bar test specimen is prepared from the R / 2 position. In this case, the axial direction of the round bar test specimen is parallel to the axial direction of the round bar.

[0075] A stress equivalent to 90% of the actual yield stress is applied to the prepared round bar test specimen. A mixed aqueous solution of 5.0 mass% sodium chloride and 0.4 mass% sodium acetate (NACE solution B), adjusted to pH 3.5 with acetic acid, is used as the test solution. The test solution at 24°C is poured into a test vessel so that the stressed round bar test specimen is immersed, forming a test bath. After degassing the test bath, a mixed gas of 0.1 atm H2S gas and 0.9 atm CO2 gas is blown into the test bath to saturate it. The test bath saturated with the mixed gas is maintained at 24°C for 720 hours.

[0076] Next, a four-point bending test performed in a 10 atm H2S gas environment according to a method in accordance with NACE TM0316 (2016) will be described. Test specimens for the four-point bending test are prepared from the steel material according to this embodiment. The size of the test specimen is, for example, 75 mm in length, 10 mm in width, and 2 mm in thickness. When the steel material is a steel plate, the test specimen is prepared from the center of the plate thickness. In this case, the longitudinal direction of the test specimen is parallel to the rolling direction of the steel plate. When the steel material is a steel pipe, the test specimen is prepared from the center of the wall thickness. In this case, the longitudinal direction of the test specimen is parallel to the axial direction of the steel pipe. When the steel material is a round bar, the test specimen is prepared from the R / 2 position. In this case, the longitudinal direction of the test specimen is parallel to the axial direction of the round bar.

[0077] In accordance with NACE TM0316 (2016), a stress equivalent to 90% of the actual yield stress is applied to the test specimen by four-point bending. The stressed test specimen is then sealed in an autoclave along with the test jig. The test solution is a mixed aqueous solution of 5.0 mass% sodium chloride and 0.4 mass% sodium acetate, adjusted to pH 4.0 with acetic acid. The test solution is poured into the autoclave so that the test specimen is immersed. H2S gas is pressurized and sealed into the autoclave to saturate the test solution, forming a test bath. The H2S gas pressure is 10 atm. After sealing the autoclave, the test bath is maintained at 24°C, and the test specimen is immersed for 720 hours while the test bath is stirred.

[0078] In this embodiment, if no cracks are found after 720 hours in the SSC resistance test and four-point bending test carried out under the above conditions, the steel material is judged to have excellent SSC resistance. In this specification, "no cracks are found" means that no cracks are found when the test piece is observed with the naked eye after the test.

[0079] [Microstructure] The microstructure of the steel material according to this embodiment is mainly composed of tempered martensite and tempered bainite. Specifically, the microstructure of the steel material according to this embodiment has a total volume fraction of tempered martensite and tempered bainite of 90% or more. The remainder of the microstructure is, for example, ferrite or pearlite. A steel material having the above-described chemical composition, a yield strength of 862 to less than 965 MPa, and satisfying formula (1) has excellent SSC resistance if it has a microstructure in which the total volume fraction of tempered martensite and tempered bainite is 90% or more. Therefore, in this embodiment, if a steel material having the above-described chemical composition, a yield strength of 862 to less than 965 MPa, and satisfying formula (1) has excellent SSC resistance, the microstructure of the steel material is determined to have a total volume fraction of tempered martensite and tempered bainite of 90% or more.

[0080] When the volume fractions of tempered martensite and tempered bainite are determined by observation, they can be determined by the following method. First, a test piece having a specific observation surface is prepared from the steel material according to this embodiment. When the steel material is a steel plate, a test piece is prepared from the center of the plate thickness, with the observation surface being a plane including the rolling direction and the plate thickness direction. When the steel material is a steel pipe, a test piece is prepared from the center of the wall thickness, with the observation surface being a plane including the pipe axial direction and the pipe radial direction. When the steel material is a round bar, a test piece is prepared with the R / 2 position in the center, with the observation surface being a plane including the axial and radial directions.

[0081] The observation surface of the test piece is polished to a mirror finish, and then immersed in a nital etching solution for about 10 seconds to reveal the structure by etching. The etched observation surface is observed using a SEM in 10 fields of view as secondary electron images. The field area is, for example, 0.01 mm. 2(Magnification: 1000x). In each field of view, tempered martensite and tempered bainite are identified from the contrast. The area fractions of the identified tempered martensite and tempered bainite are calculated. The method for calculating the area fractions is not particularly limited, and any known method may be used. For example, the area fractions of tempered martensite and tempered bainite can be calculated by image analysis. In this embodiment, the arithmetic mean values ​​of the area fractions of tempered martensite and tempered bainite calculated in all fields of view are defined as the volume fractions of tempered martensite and tempered bainite.

[0082] [Shape of Steel Material] As described above, the shape of the steel material according to this embodiment is not particularly limited. The steel material is, for example, a steel pipe, a steel plate, or a round bar. When the steel material is a steel pipe for oil wells, the preferred wall thickness is 9 to 60 mm. More preferably, the steel material according to this embodiment is a seamless steel pipe. When the steel material according to this embodiment is a seamless steel pipe, even a thick seamless steel pipe with a wall thickness of 15 mm or more can achieve both a 125 ksi-class yield strength and excellent SSC resistance in a high-HS environment.

[0083] [Manufacturing method] A method for manufacturing a steel material according to this embodiment will be described below. A method for manufacturing a seamless steel pipe will be described below as an example of a steel material according to this embodiment. The method for manufacturing a seamless steel pipe includes a step of preparing a mother pipe (preparation step) and a step of quenching and tempering the mother pipe to form a seamless steel pipe (quenching step and tempering step). Note that the manufacturing method according to this embodiment is not limited to the manufacturing method described below. Each step will be described in detail below.

[0084] [Preparation step] 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, there is no particular limitation on the method for manufacturing the intermediate steel material. The intermediate steel material referred to here is a plate-shaped steel material if the final product is a steel plate, a mother pipe if the final product is a steel pipe, or a steel bar having a circular cross section perpendicular to the axial direction if the final product is a round steel bar.

[0085] 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). Below, the case where the material preparation step and the hot working step are included will be described in detail.

[0086] [Material Preparation Step] In the material preparation step, a material is produced using molten steel having the above-described chemical composition. The method for producing the material is not particularly limited and may be a well-known method. Specifically, a cast piece (slab, bloom, or billet) may be produced using the molten steel by a continuous casting method. An ingot may be produced using the molten steel by an ingot casting method. If necessary, the slab, bloom, or ingot may be subjected to blooming to produce a billet. The material (slab, bloom, or billet) is produced through the above steps.

[0087] [Hot working process] In the hot working process, a prepared raw material is hot worked to produce an intermediate steel material. When the steel material is a seamless steel pipe, the intermediate steel material corresponds to a mother pipe. First, a 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 hot working method is not particularly limited, and a well-known method may be used.

[0088] The hot working method is not particularly limited, but when the intermediate steel material is a mother pipe, it is preferable to carry out the Mannesmann mandrel process as hot working to produce a mother pipe. In this case, a heated solid round billet is pierced and rolled using a piercing mill to produce a hollow round billet. When piercing and rolling is performed, the piercing ratio is not particularly limited, but is, for example, 1.0 to 4.0. The piercing-rolled round billet, i.e., the hollow round billet, is further hot rolled using a mandrel mill, a reducer, a sizing mill, or the like to produce a mother pipe.

[0089] Preferably, in the elongation rolling according to this embodiment, the temperature of the hollow round billet is set to 1100°C or less. More specifically, in this embodiment, the temperature of the hollow round billet at the entry side of the elongation rolling mill (e.g., a mandrel mill) is preferably set to 1100°C or less. If the temperature of the hollow round billet is 1100°C or less, strain is likely to be introduced into the hollow round billet by elongation rolling. In this case, prior γ grains are refined in the microstructure of the hollow round billet, and the concentration of P at the prior γ grain boundaries is likely to be alleviated. As a result, the grain boundary P content P_seg. can be reduced in the produced steel material. Note that if the temperature of the hollow round billet at the entry side of the elongation rolling mill is too low, the load on the elongation rolling mill and the sizing mill increases. Therefore, the lower limit of the temperature of the hollow round billet at the entry side of the elongation rolling mill is preferably set to 950°C or more.

[0090] Preferably, in the elongation rolling according to this embodiment, the reduction ratio in the first rolling pass of the elongation rolling is set to 24.5% or more. Here, in the elongation rolling according to this embodiment, the elongation rolling may be performed using a elongation mill consisting of multiple rolling stands. In this case, each rolling stand has multiple rolls arranged around a pass line through which the hollow round billet passes, and the rolls are arranged along the pass line. The number of rolls included in the rolling stand is not limited, and a two-roll type rolling stand or a three-roll type rolling stand may be used.

[0091] That is, "the first rolling in the elongation rolling" specifically means, when elongation is carried out using a two-roll rolling stand, rolling carried out by the first and second stands of the elongation mill arranged along the pass line of the hollow round billet. Similarly, when elongation is carried out using a three-roll rolling stand, it means rolling carried out by the first and second stands arranged along the pass line of the hollow round billet. In other words, when a elongation mill consisting of multiple rolling stands is used, it is preferable that the reduction ratio Rm (%) in the first and second stands of the elongation mill be 24.5% or more.

[0092] Hereinafter, in this specification, the "reduction ratio Rm (%) in the first and second stands of the elongating mill" will also be referred to as the "first-stage reduction ratio Rm (%) of elongating mill." More specifically, when a elongating mill consisting of a plurality of rolling stands is used, the first-stage reduction ratio Rm (%) of elongating mill is defined by the following formula (A): first-stage reduction ratio Rm (%) of elongating mill = 100 × {(wall thickness of hollow blank after piercing-rolling - wall thickness of hollow blank after the second stand of the elongating mill) / wall thickness of hollow blank after piercing-rolling} (A)

[0093] If the first-stage reduction ratio of elongation rolling Rm is 24.5% or more, strain is likely to be introduced into the hollow round billet during elongation rolling. In this case, prior γ grains are refined in the microstructure of the hollow round billet, and the concentration of P at the prior γ grain boundaries is likely to be alleviated. As a result, the grain boundary P content P_seg. can be reduced in the manufactured steel material. Therefore, in this embodiment, it is preferable to set the first-stage reduction ratio of elongation rolling to 24.5% or more. Note that the upper limit of the first-stage reduction ratio of elongation rolling is not particularly limited, but is, for example, 50.0%.

[0094] After elongation rolling, sizing rolling or reducing rolling may be performed. These hot rolling processes may be performed using known equipment and by known methods, and are not particularly limited. The mother pipe produced by hot working may be air-cooled (as-rolled), or may be quenched directly after hot working without being cooled to room temperature, or may be reheated after hot working and then quenched. The quenching step and tempering step will be described in detail below.

[0095] [Quenching Process] In the quenching process, the prepared intermediate steel (blank pipe) is quenched. In this specification, "quenching" refers to rapidly cooling the intermediate steel at or above the A3 point. If the quenching temperature is too high, the prior γ grains may become coarse, which may reduce the SSC resistance of the steel. Therefore, the quenching temperature is preferably 850 to 1000°C. In this specification, the quenching temperature corresponds to the surface temperature of the intermediate steel measured with a thermometer installed at the outlet of the final hot working equipment when quenching is performed directly after hot working. Furthermore, the quenching temperature corresponds to the temperature of the furnace where quenching or reheating is performed when quenching is performed after reheating or reheating after hot working.

[0096] The quenching method involves, for example, continuously cooling the intermediate steel material (mother pipe) from the quenching start temperature to continuously lower the surface temperature of the mother pipe. The method of continuous cooling is not particularly limited, and any well-known method may be used. Examples of continuous cooling methods include a method of immersing the mother pipe in a water bath for cooling, or a method of accelerating the cooling of the mother pipe by shower water cooling or mist cooling.

[0097] If the cooling rate during quenching is too slow, the microstructure may not be mainly composed of tempered martensite and tempered bainite. In this case, the mechanical properties (yield strength of 125 ksi class) specified in this embodiment cannot be obtained. Furthermore, in this case, the steel material does not have excellent SSC resistance. Therefore, as described above, in the steel manufacturing method according to this embodiment, the intermediate steel material is rapidly cooled during quenching. Specifically, in the quenching process, the average cooling rate in the range of the surface temperature of the intermediate steel material (blank pipe) during quenching from 800 to 500°C is defined as the cooling rate during quenching, CR. 800-500 More specifically, the cooling rate during quenching, CR 800-500 is determined from the temperature measured at the slowest cooling location within the cross section of the intermediate steel being quenched (for example, the center of the thickness of the intermediate steel when both surfaces are forcedly cooled).

[0098] Preferred cooling rate during quenching: CR 800-500 The cooling rate CR during quenching is preferably 300°C / min or more. 800-500The lower limit of the cooling rate during quenching, CR, is 450°C / min, and more preferably 600°C / min. 800-500 The upper limit of the rate is not particularly specified, but is, for example, 60,000° C. / min.

[0099] Preferably, the mother pipe is heated in the austenite region multiple times and then quenched. In this case, the austenite grains are refined, thereby improving the SSC resistance of the steel material. By performing quenching multiple times, heating in the austenite region may be repeated multiple times, or by performing normalizing and quenching, heating in the austenite region may be repeated multiple times. Furthermore, quenching and tempering, which will be described later, may be combined and performed multiple times. That is, quenching and tempering may be performed multiple times. In this case, the SSC resistance of the steel material is further improved. The tempering process will be described in detail below.

[0100] [Tempering step] In the tempering step, the intermediate steel material that has been subjected to the above-mentioned quenching is tempered. c1 The tempering temperature corresponds to the temperature of the heat treatment furnace when the intermediate steel material is heated and held at a temperature below the tempering point after quenching. The tempering time refers to the time from when the temperature of the intermediate steel material reaches the predetermined tempering temperature until it is extracted from the heat treatment furnace.

[0101] The tempering temperature is adjusted appropriately depending on the chemical composition of the steel and the yield strength to be obtained. That is, the tempering temperature is adjusted for an intermediate steel having the chemical composition of this embodiment to adjust the yield strength of the steel to 125 ksi class (862 to less than 965 MPa). In the tempering process of this embodiment, the preferred tempering temperature is 600 to 700°C. Furthermore, in the tempering process of this embodiment, the preferred tempering time is 10 to 240 minutes.

[0102] The steel material according to this embodiment can be manufactured by the above-described manufacturing method. However, as mentioned above, the above-described manufacturing method is only an example, and the steel material may be manufactured by other manufacturing methods. Hereinafter, the present invention will be described in more detail with reference to examples.

[0103] Molten steels having the chemical compositions shown in Tables 1A and 1B were produced. Note that "-" in Table 1B means that the content of each element was at the impurity level. Specifically, the Cu content, Ni content, W content, and Zr content of Test No. 1 were rounded to two decimal places to mean that they were 0%. The Mg content and REM content of Test No. 1 were rounded to five decimal places to mean that they were 0%.

[0104]

[0105]

[0106] Using the molten steel of each test number, solid round billets having the diameters listed in Table 2 were produced by continuous casting. The solid round billets of each test number were heated and subjected to piercing-rolling to produce hollow round billets. The hollow round billets after piercing-rolling were subjected to elongation rolling and sizing rolling to produce mother pipes of each test number. The outer diameter (mm) and wall thickness (mm) of the mother pipe of each test number are shown in Table 2. In this embodiment, elongation rolling was performed using a two-roll mandrel mill. For each test number, the surface temperature of the hollow round billet at the entry side of the elongation rolling mill (mandrel mill) is shown in the "Entry Temperature (°C)" column of Table 2. The temperature of the hollow round billet was measured using a non-contact thermometer. Furthermore, for each test number, the first-stage elongation rolling reduction rate Rm (%) (the reduction rate (%) in the first and second stands of the mandrel mill) is shown in the "Rm (%)" column of Table 2.

[0107]

[0108] The blank tubes of each test number were quenched by heating at the temperatures (°C) for the times (minutes) shown in Table 2, followed by rapid cooling. 800-500 ) all satisfied the range of 300 to 60,000°C / min. Furthermore, tempering was carried out by holding the samples at the temperatures (°C) for the times (min) shown in Table 2. Seamless steel pipes with each test number were produced by the above method.

[0109] [Evaluation Tests] The seamless steel pipes manufactured with each test number were subjected to the following tests: tensile test, prior γ grain observation test, grain boundary P amount measurement test, and SSC resistance test.

[0110] [Tensile Test] Round bar test specimens with a parallel section diameter of 8.9 mm and a gauge length of 35.6 mm were prepared from the center of the wall thickness of each seamless steel pipe using the method described above. Tensile tests were performed on the prepared round bar test specimens at room temperature (25°C) in air in accordance with ASTM E8 / E8M (2022). The stress at 0.7% elongation obtained in the tensile test was defined as the yield strength (MPa). Furthermore, the maximum stress during uniform elongation obtained in the tensile test was defined as the tensile strength (MPa). The obtained yield strengths (MPa) are shown in the "YS (MPa)" column of Table 3. The obtained tensile strengths (MPa) are shown in the "TS (MPa)" column of Table 3.

[0111]

[0112] [Prior γ Grain Observation Test] Test specimens were prepared from the center of the wall thickness of each seamless steel pipe by the method described above, with the observation surface being a plane including the pipe axial direction and the pipe diameter direction. The prepared test specimens were etched by the method described above to reveal the prior γ grain boundaries. 2 The grain size numbers (GSN) of the prior γ grains were determined for the observation surface (10 mm × 10 mm) by optical microscope observation based on the cutting method in accordance with JIS G 0551 (2020). The obtained grain size numbers (GSN) of the prior γ grains are shown in the "Prior γ grain size number (GSN)" column of Table 3.

[0113] [Grain Boundary P Content Measurement Test] Notched round bar test specimens with a diameter of 3 mm and an axial length of 17 mm were prepared from the central portion of the wall thickness of each seamless steel pipe using the method described above. For the notched round bar test specimens, a 0.7 mm deep V-notch-shaped notch was formed around the entire circumference of the round bar at the central portion of the axial length. The prepared notched round bar test specimens were impact fractured using the method described above to obtain a prior γ grain boundary fracture surface. Furthermore, 10 measurement points were identified using the method described above on the obtained prior γ grain boundary fracture surface. Element concentration analysis was performed using Auger electron spectroscopy using the method described above for the identified measurement points to obtain the P content (mol.%) at each measurement point. The arithmetic mean of the P content (mol.%) at the 10 measurement points was defined as the grain boundary P content P_seg. (mol.%). The obtained grain boundary P content P_seg. (mol.%) is shown in Table 3.

[0114] Furthermore, for each test number, Fn1 (=(Mn+P_seg.) / (2.5Mo+GSN)) was calculated from the Mn content (mass%), Mo content (mass%), prior γ grain grain size number GSN, and grain boundary P content P_seg. (mol.%). The obtained Fn1 is shown in Table 3.

[0115] [SSC Resistance Test] A SSC resistance test was performed on the seamless steel pipe of each test number in a 0.1 atm H2S gas environment using a method conforming to NACE TM0177-2016 Method A, and a four-point bending test was performed in a 10 atm H2S gas environment using a method conforming to NACE TM0316 (2016).

[0116] First, round bar test specimens with a parallel section diameter of 6.35 mm and a parallel section length of 25.4 mm were prepared from the center of the wall thickness of each seamless steel pipe using the method described above. A stress equivalent to 90% of the actual yield stress was applied to the prepared round bar test specimens in accordance with NACE TM0316 (2016). A test solution (a mixed aqueous solution of 5.0 mass% sodium chloride and 0.4 mass% sodium acetate) at 24°C was poured into a test vessel so that the stressed round bar test specimens were immersed, forming a test bath. After degassing the test bath, a mixed gas of 0.1 atm H2S gas and 0.9 atm CO2 gas was blown into the test bath to saturate it. The test bath saturated with the mixed gas was maintained at 24°C for 720 hours.

[0117] The test pieces after 720 hours of holding were visually observed. If no cracks were observed, they were judged to have excellent SSC resistance ("EX" (Excellent) in the "0.1 atm H2S" column in Table 3). On the other hand, if cracks were observed, they were judged to have poor SSC resistance ("NA" (Not Acceptable) in the "0.1 atm H2S" column in Table 3).

[0118] Next, three test specimens measuring 75 mm in length, 10 mm in width, and 2 mm in thickness were prepared from the central portion of the wall thickness of each seamless steel pipe using the method described above. Each prepared test specimen was subjected to four-point bending to a stress equivalent to 90% of the actual yield stress. The test solution (a mixed aqueous solution of 5.0 mass% sodium chloride and 0.4 mass% sodium acetate, adjusted to pH 4.0 with acetic acid) was poured into an autoclave so that the test specimens were immersed. H2S gas was pressurized and sealed into the autoclave at 10 atm to saturate the test solution, forming a test bath. After sealing the autoclave, the test bath was maintained at 24°C, and the test specimens were immersed for 720 hours while the test bath was stirred.

[0119] The test pieces after the holding were observed with the naked eye. The number of test pieces in which cracks were confirmed was counted among the three test pieces and defined as the "number of SSC occurrences (pieces)." The obtained number of SSC occurrences (pieces) in the 10 atm H2S gas environment is shown in the "10 atm H2S" column in Table 3.

[0120] [Evaluation Results] With reference to Tables 1A, 1B, 2 and 3, the seamless steel pipes of test numbers 1 to 18 had appropriate chemical compositions, yield strengths of 862 to less than 965 MPa, and Fn1 of 0.125 or less. As a result, the number of seamless steel pipes that experienced SSC in a 10 atm HS gas environment was zero, and these seamless steel pipes had excellent SSC resistance even in a high HS environment.

[0121] On the other hand, the seamless steel pipe of test number 19 had an excessively high Mn content and an excessively high Fn1, resulting in one or more pipes experiencing SSC in a 10 atm H2S gas environment, and the seamless steel pipe did not have excellent SSC resistance in a high H2S environment.

[0122] The seamless steel pipe of test number 20 had an excessively low Mo content and an excessively high Fn1, resulting in one or more SSC occurrences in this seamless steel pipe in a 10 atm H2S gas environment, and the seamless steel pipe did not have excellent SSC resistance in a high H2S environment.

[0123] The seamless steel pipes of test numbers 21 and 22 had too high a Mn content, too low a Mo content, and too high a Fn1, and as a result, these seamless steel pipes had one or more SSC occurrences in a 10 atm H2S gas environment, and did not have excellent SSC resistance in a high H2S environment.

[0124] In the seamless steel pipes of test numbers 23 to 27, the temperature of the hollow round billet in elongation rolling was too high, and Fn1 was too high. As a result, these seamless steel pipes had one or more SSC occurrences in a 10 atm HS gas environment, and did not have excellent SSC resistance in a high HS environment.

[0125] The seamless steel pipes of test numbers 28 to 31 had too small a first-stage reduction ratio Rm of elongation rolling and too high a Fn1, and as a result, these seamless steel pipes had one or more SSC occurrences in a 10 atm H2S gas environment, and did not have excellent SSC resistance in a high H2S environment.

[0126] The seamless steel pipes of test numbers 32 to 34 had too high Fn1. As a result, these seamless steel pipes had one or more SSC occurrences in a 10 atm H2S gas environment, and did not have excellent SSC resistance in a high H2S environment.

[0127] 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 within the scope of the present disclosure.

Claims

1. In mass%, C: 0.15 to less than 0.30%, Si: 0.05 to 1.00%, Mn: 0.05 to less than 0.30%, P: 0.020% or less, S: 0.0050% or less, Cr: 0.10 to 1.00%, Mo: 0.85 to 2.50%, Ti: 0.002 to 0.020%, Nb: 0.002 to 0.050%, V: 0.01 to 0.30%, Ca: 0.0001 to 0.0030%, B: 0.0005 to 0.0050%, Al: 0.005 to 0.100%, N: 0.0050% or less, O: 0.0020% or less, Cu: 0 to 0.50%, A steel material comprising Ni: 0 to 0.50%, W: 0 to 1.00%, Zr: 0 to 0.50%, Mg: 0 to 0.0100%, rare earth elements: 0 to 0.0100%, and the balance being Fe and impurities, having a yield strength of 862 to less than 965 MPa, and the contents of the elements, the grain size number GSN of prior austenite grains in the microstructure, and the P content P_seg. in the vicinity of the prior austenite grain boundaries satisfying formula (1): (Mn+P_seg.) / (2.5Mo+GSN)≦0.125 (1) Here, the element symbols in formula (1) are substituted with the contents of the corresponding elements in units of mass %. Furthermore, P_seg. in formula (1) represents the P content in the vicinity of the prior austenite grain boundaries in mol. %, and the grain size number of the prior austenite grains is substituted for GSN in formula (1).

2. The steel material according to claim 1, containing one or more elements selected from the group consisting of Cu: 0.01 to 0.50%, Ni: 0.01 to 0.50%, W: 0.01 to 1.00%, Zr: 0.01 to 0.50%, Mg: 0.0001 to 0.0100%, and rare earth elements: 0.0001 to 0.0100%.

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

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