steel

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

[Technical Field]

[0001] This disclosure relates to steel materials, and more specifically, to steel materials suitable for use in high H2S environments. [Background technology]

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

[0003] On the other hand, many deep wells are sour environments containing corrosive hydrogen sulfide. In this specification, a sour environment means an acidic environment containing hydrogen sulfide. Note that sour environments may also contain carbon dioxide. Steel pipes used for oil wells in such sour environments are required not only to be high in strength but also to be resistant to sulfide stress cracking (SSC resistance).

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

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

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

[0007] [Patent Document 1] Japanese Patent Publication No. 2006-28612 [Patent Document 2] Japanese Patent Publication No. 2017-166060 [Overview of the project] [Problems that the invention aims to solve]

[0008] Incidentally, in recent years, attention has also been focused on wells in even harsher environments. For example, development has been underway for wells in environments containing high-pressure hydrogen sulfide (H2S) gas. Specifically, in a sour environment where the partial pressure of H2S gas is as high as 10 atm (hereinafter, a sour environment with an H2S gas partial pressure of 10 atm will be referred to as a "high H2S environment"), a more stringent resistance to SSC (Steel-Sulfur Contamination) is required than in a sour environment containing H2S gas at normal pressure. Therefore, there is a growing demand for steel materials that possess high SSC resistance even in such high H2S environments.

[0009] Patent documents 1 and 2 above propose steel materials that have high strength and excellent resistance to SSC (steam-steam corrosion). However, patent documents 1 and 2 do not consider SSC resistance in high H2S environments at all.

[0010] The purpose of this disclosure is to provide a steel material that has high strength and excellent SSC resistance even in high H2S environments. [Means for solving the problem]

[0011] The steel materials disclosed herein are In mass percent, C: Less than 0.15-0.30% Si: 0.05~1.00%, Mn: 0.05-0.30% P: 0.020% or less, S: 0.0050% or less, Cr: 0.10~1.00%, Mo: 0.85~2.50%, Ti: 0.002~0.020%, Nb: 0.002~0.050%, V: 0.01~0.30%, Ca: 0.0001~0.0030%, B: 0.0005~0.0050%, Al: 0.005~0.100%, N: 0.0050% or less, O: less than 0.0020%, 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 consists of Fe and impurities, the yield strength is less than 862 to 965 MPa, the content of the above elements, the crystal 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, in the element symbols in formula (1), the content of the corresponding element is substituted in units of mass%. Also, in P_seg. in formula (1), the P content near the prior austenite grain boundaries is substituted in mol.%, and in GSN in formula (1), the crystal grain size number of the prior austenite grains is substituted.

Advantages of the Invention

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

Brief Description of the Drawings

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

Modes for Carrying Out the Invention

[0014] First, the inventors considered obtaining a steel material with a yield strength of 125 ksi class (less than 862 to 965 MPa) as a high-strength material. In other words, the inventors investigated and examined methods for obtaining a yield strength of 125 ksi class and excellent SSC resistance even in high H2S environments 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") to improve the SSC resistance of steel materials. Here, there is a concern that if the prior γ grain boundaries of the steel material become brittle and the crack susceptibility of the prior γ grain boundaries increases, the SSC resistance of the steel material will decrease. On the other hand, if the brittleness of the prior γ grain boundaries of the steel material can be suppressed and the prior γ grain boundaries can be strengthened, it may be possible to improve the SSC resistance of the steel material.

[0016] Here, manganese (Mn) in the chemical composition tends to concentrate at the prior γ grain boundaries and embrittle them. Mn further promotes the concentration of phosphorus (P) at the prior γ grain boundaries, further embrittlement of the prior γ grain boundaries. On the other hand, if molybdenum (Mo) concentrates at the prior γ grain boundaries, it can strengthen them. As a result of our investigations, it has become clear that reducing the Mn content of the steel 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 and strengthen them.

[0017] In other words, in mass percent, C: less than 0.15-0.30%, Si: 0.05-1.00%, Mn: less than 0.05-0.30%, P: 0.020% or less, S: 0.0050% or less, Cr: 0.10-1.00%, Mo: 0.85-2.50%, Ti: 0.002-0.020%, Nb: 0.002-0.050%, V: 0.01-0.30%, Ca: 0.0001-0.0030%, B: 0.0005-0.0050% The inventors believe that a steel material consisting of %, Al: 0.005~0.100%, N: 0.0050% or less, O: 0.0020% or less, Cu: 0~0.50%, Ni: 0~0.50%, W: 0~1.00%, Zr: 0~0.50%, Mg: 0~0.0100%, rare earth elements: 0~0.0100%, and the remainder being Fe and impurities, may be able to achieve both a yield strength of 125 ksi class and excellent SSC resistance.

[0018] On the other hand, even with the above-mentioned chemical composition, excellent SSC resistance could not be obtained in high-H2S environments. Therefore, the inventors further investigated the factors that reduce SSC resistance in high-H2S environments for the above-mentioned steel materials. As a result, it became clear that in high-H2S environments, even steel materials with the above-mentioned chemical composition are significantly affected by the embrittlement of prior γ grain boundaries, and the SSC resistance of the steel material tends to decrease. Therefore, the inventors further investigated strengthening of prior γ grain boundaries in steel materials with the above-mentioned chemical composition.

[0019] As a result of detailed studies by the inventors, it has become clear that in steel materials having the above-mentioned chemical composition, if the Mn content, Mo content, grain size number GSN of the prior γ grains, and P content P_seg. near the prior γ grain boundaries satisfy the following equation (1), then a yield strength of 125 ksi class and resistance to SSC in a high H2S environment can be stably achieved simultaneously. (Mn+P_seg.) / (2.5Mo+GSN)≦0.125 (1) Here, the elemental symbols in equation (1) are substituted with the content of the corresponding element in units of mass%. Also, P_seg. in equation (1) is substituted with the P content near the prior austenite grain boundary in mol%, and GSN in equation (1) is substituted with the grain size number of the prior austenite grain.

[0020] Fn1 is defined as Fn1 = (Mn + P_seg.) / (2.5Mo + GSN). Below, the relationship between Fn1 and SSC resistance in a high H2S environment will be explained in more detail using the figures. Figure 1 shows the relationship between Fn1 (= (Mn + P_seg.) / (2.5Mo + GSN)) and the number of SSCs generated, which is an indicator of SSC resistance, in this embodiment. Figure 1 was created using Fn1 obtained by the method described later and the number of SSCs generated in a 10 atm H2S gas environment, obtained by the method described later, for a steel material having the above chemical composition and a yield strength of 862 to less than 965 MPa, from the embodiment described later.

[0021] Referring to Figure 1, in steel materials having the above-mentioned chemical composition and a yield strength of less than 862 to 965 MPa, if Fn1 is 0.125 or less, the number of SSCs generated in a 10 atm H2S gas environment is 0, indicating excellent SSC resistance even in a high H2S environment. On the other hand, in steel materials having the above-mentioned chemical composition and a yield strength of less than 862 to 965 MPa, if Fn1 exceeds 0.125, the number of SSCs generated in a 10 atm H2S gas environment becomes 1 or more, indicating that excellent SSC resistance in a high H2S environment was not observed. Therefore, the steel material according to this embodiment is based on the premise that it has the above-mentioned chemical composition and a yield strength of less than 862 to 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 H2S environment.

[0022] Furthermore, the reason why steel materials having the above-mentioned chemical composition and a yield strength of less than 862-965 MPa exhibit excellent SSC resistance even in high H2S environments if Fn1 is 0.125 or less remains unclear. However, the inventors of this invention speculate as follows. Here, Fn1 (=(Mn+P_seg.) / (2.5Mo+GSN)) is an index indicating the degree of embrittlement of the prior γ grain boundary. As described above, the higher the Mn content, the more Mn is concentrated in the prior γ grain boundary, making the prior γ grain boundary more prone to embrittlement. Also, 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 is also called "grain boundary P content P_seg."), the more prone the prior γ grain boundary is to embrittlement.

[0023] On the other hand, the higher the Mo content, the more Mo is concentrated in the prior γ grain boundaries, thereby strengthening them. Furthermore, the larger the crystal grain size number GSN of the prior γ grains, the smaller the crystal grain size of the prior γ grains becomes, increasing the area occupied by the prior γ grain boundaries in the steel, which may make it easier to mitigate the embrittlement of the prior γ grain boundaries. Therefore, the inventors speculate that, assuming the steel material according to this embodiment has the above-mentioned chemical composition and a yield strength of 862 to less than 965 MPa, setting Fn1 to 0.125 or less may stably provide excellent SSC resistance even in a high H2S environment. It should be noted that it is possible that excellent SSC resistance is stably obtained even in a high H2S environment through a mechanism different from the inventors' speculation. However, assuming the above-mentioned chemical composition and a yield strength of 862 to less than 965 MPa, it has been proven by the examples described later that excellent SSC resistance can be obtained even in a high H2S environment by setting Fn1 to 0.125 or less.

[0024] Based on the above findings, the gist of the steel material according to this embodiment is as follows:

[0025] [1] In mass percent, C: Less than 0.15-0.30% Si: 0.05~1.00%, Mn: 0.05-0.30% P: 0.020% or less, S: 0.0050% or less, Cr: 0.10~1.00%, Mo: 0.85~2.50%, Ti: 0.002~0.020%, Nb: 0.002~0.050%, V: 0.01~0.30%, Ca: 0.0001~0.0030%, B: 0.0005~0.0050%, Al: 0.005~0.100%, N: 0.0050% or less, O: 0.0020% or less, Cu: 0~0.50%, Ni: 0~0.50%, W: 0~1.00%, Zr: 0~0.50%, Mg: 0~0.0100%, Rare earth elements: 0-0.0100%, and, The remainder consists of Fe and impurities. The yield strength is less than 862-965 MPa. The content of the aforementioned element, 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 equation (1). Steel material. (Mn+P_seg.) / (2.5Mo+GSN)≦0.125 (1) Here, the elemental symbols in equation (1) are substituted with the content of the corresponding element in units of mass%. Also, P_seg. in equation (1) is substituted with the P content near the prior austenite grain boundary in mol%, and GSN in equation (1) is substituted with the grain size number of the prior austenite grain.

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

[0027] [3] The steel material described in [1] or [2], The aforementioned 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 steel bar (solid material), or a steel plate. A round steel bar refers to a steel bar with a circular cross-section perpendicular to the axial direction. Furthermore, 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, the "%" in relation to elements refers to mass percentage.

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

[0031] C: Less than 0.15-0.30% Carbon (C) enhances the hardenability and strength of steel. If the C content is too low, the above effects cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the C content is too high, even if the content of other elements is within the range of this embodiment, excessive carbides will be formed in the steel, reducing the steel's resistance to carbon dioxide (SSC). Therefore, the C content is less than 0.15-0.30%. The preferred lower limit of the C content is 0.16%, and more preferably 0.17%. The preferred upper limit of the C content is 0.29%, more preferably 0.28%, and still more preferably 0.27%.

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

[0033] Mn: 0.05-0.30% Manganese (Mn) enhances the hardenability and strength of steel. If the Mn content is too low, the above effects cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Mn content is too high, even if the content of other elements is within the range of this embodiment, the prior γ grain boundaries become brittle, and the SSC resistance of the steel decreases. If the Mn content is too high, it further promotes an increase in the grain boundary P content P_seg., further decreasing the SSC resistance of the steel. Therefore, the Mn content is between 0.05% and less than 0.30%. The preferred lower limit of the Mn content is 0.06%, more preferably 0.08%, and still more preferably 0.10%. The preferred upper limit of the Mn content is 0.29%, and still 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 content of other elements is within the range of this embodiment, the grain boundary P content P_seg. will become too high, and the SSC resistance of the steel will decrease. Therefore, the P content is 0.020% or less. The preferred upper limit of the P content is 0.018%, and more preferably 0.015%. It is preferable to have as low a P content as possible. However, an extreme reduction in P content will significantly increase manufacturing costs. Therefore, considering industrial production, the preferred lower limit of the P content is 0.001%, more preferably 0.002%, and still 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 greater than 0%. If the S content is too high, even if the content of other elements is within the range of this embodiment, coarse sulfide inclusions will be formed, reducing the SSC resistance of the steel. Therefore, the S content is 0.0050% or less. The preferred upper limit of the S content is 0.0045%, more preferably 0.0040%, and even more preferably 0.0035%. It is preferable to have as low an S content as possible. However, an extreme reduction in the S content will significantly increase manufacturing costs. Therefore, considering industrial production, the preferred lower limit of the S content is 0.0001%, more preferably 0.0002%, and even more preferably 0.0003%.

[0036] Cr: 0.10~1.00% Chromium (Cr) enhances the hardenability and strength of steel. If the Cr content is too low, the above effects cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Cr content is too high, even if the content of other elements is within the range of this embodiment, the strength of the steel becomes too high, and the steel's resistance to SSC (Steel-Sealed Carbon) deteriorates. Therefore, the Cr content is 0.10 to 1.00%. The preferred lower limit of the Cr content is 0.15%, and more preferably 0.20%. The preferred upper limit of the Cr content is 0.95%, more preferably 0.90%, more preferably 0.85%, and still more preferably 0.80%.

[0037] Mo: 0.85~2.50% Molybdenum (Mo) concentrates at the prior γ grain boundaries, strengthening them. As a result, the steel's resistance to thermal corrosion (SSC) increases. If the Mo content is too low, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Mo content is too high, even if the content of other elements is within the range of this embodiment, fine carbides are excessively formed, increasing the strength of the steel too much and reducing its resistance to SSC. Therefore, the Mo content is 0.85 to 2.50%. The preferred lower limit of the Mo content is 0.87%, and more preferably 0.90%. The preferred upper limit of the Mo content is 2.20%, and more preferably 2.00%.

[0038] Ti: 0.002~0.020% Titanium (Ti) forms fine nitrides, which refine the prior γ grains of the steel material through a pinning effect. As a result, the crystal grain size number GSN of the prior γ grains increases, improving the steel material's resistance to scaling and corrosion (SSC). If the Ti content is too low, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Ti content is too high, even if the content of other elements is within the range of this embodiment, coarse nitrides will be formed, reducing the steel material's resistance to SSC. 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 still more preferably 0.014%.

[0039] Nb: 0.002~0.050% 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 content of other elements is within the range of this embodiment. On the other hand, if the Nb content is too high, even if the content of other elements is within the range of this embodiment, an excessive amount of carbonitrides, etc. will be generated, and the SSC resistance of the steel material will decrease. Therefore, the Nb content is 0.002 to 0.050%. The preferred lower limit of the Nb content is 0.003%, and more preferably 0.005%. The preferred upper limit of the Nb content is 0.045%, and more preferably 0.040%.

[0040] V: 0.01~0.30% Vanadium (V) forms carbonitrides and other materials, thereby increasing the strength of the steel. If the V content is too low, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the V content is too high, even if the content of other elements is within the range of this embodiment, the strength of the steel becomes too high, and the steel's resistance to SSC (Steel-Sealed Carbon) deteriorates. Therefore, the V content is 0.01 to 0.30%. The preferred lower limit of the V content is 0.02%, more preferably 0.03%, and even more preferably 0.05%. The preferred upper limit of the V content is 0.28%, more preferably 0.25%, and even more preferably 0.22%.

[0041] Ca: 0.0001~0.0030% Calcium (Ca) forms sulfides, reducing Mn sulfides at prior γ grain boundaries. As a result, the steel's resistance to SSC (Steel-Sulfur-Cleaning) is enhanced. If the Ca content is too low, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Ca content is too high, even if the content of other elements is within the range of this embodiment, coarse oxides are formed, reducing the steel's resistance to SSC. Therefore, the Ca content is 0.0001 to 0.0030%. The preferred lower limit of the Ca content is 0.0003%, and more preferably 0.0005%. The preferred upper limit of the Ca content is 0.0028%, and more preferably 0.0025%.

[0042] B: 0.0005~0.0050% Boron (B) enhances the hardenability of steel, thereby increasing its strength. Furthermore, B concentrates at the prior γ grain boundaries, reducing the amount of P at the grain boundaries (P_seg.). As a result, the steel's resistance to thermal corrosion (SSC) is improved. If the B content is too low, the above effects cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the B content is too high, even if the content of other elements is within the range of this embodiment, coarse B nitrides are formed, reducing the steel's resistance to SSC. 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: 0.005~0.100% Aluminum (Al) deoxidizes steel. If the Al content is too low, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Al content is too high, even if the content of other elements is within the range of this embodiment, coarse oxide inclusions will be formed, reducing the SSC resistance of the steel. Therefore, the Al content is 0.005 to 0.100%. The preferred lower limit of the Al content is 0.005%, more preferably 0.010%, more preferably 0.015%, and still more preferably 0.020%. The preferred upper limit of the Al content is 0.090%, more preferably 0.080%, more preferably 0.070%, more preferably 0.065%, and still more preferably 0.060%. As used herein, "Al" content refers to the content of "acid-soluble Al," that is, "sol.Al."

[0044] N: 0.0050% or less Nitrogen (N) is inevitably present. That is, the lower limit of the N content is greater than 0%. N forms nitrides, which refine the prior γ grains of the steel material through a pinning effect. As a result, the crystal 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 content of other elements is within the range of this embodiment, coarse nitrides will be formed, and the SSC resistance of the steel material will decrease. Therefore, the N content is 0.0050% or less. The preferred upper limit of the N content is 0.0048%, more preferably 0.0045%, and even more preferably 0.0040%. The preferred lower limit of the N content to more effectively obtain the above effects 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. Therefore, the lower limit of the O content is greater than 0%. If the O content is too high, even if the content of other elements is within the range of this embodiment, coarse oxides will form, reducing the SSC resistance of the steel material. Therefore, the O content is 0.0020% or less. The preferred upper limit of the O content is 0.0018%, and more preferably 0.0015%. It is preferable to have as low an O content as possible. However, an extreme reduction in O content significantly increases manufacturing costs. Therefore, considering industrial production, the preferred lower limit of the O content is 0.0001%, more preferably 0.0005%, and still more preferably 0.0008%.

[0046] The remainder of the chemical composition of the steel material according to this embodiment consists of Fe and impurities. Here, impurities refer to substances that are mixed in from the raw materials such as ore, scrap, or the manufacturing environment during the industrial production of steel material, and are acceptable as long as they do not adversely affect the steel material according to this embodiment.

[0047] [Optional element] 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 some of the Fe. Any of these elements are arbitrary and enhance the hardenability and strength of the steel material.

[0048] Cu: 0~0.50% Copper (Cu) is an optional element and may not be included. That is, the Cu content may be 0%. When included, Cu increases the hardenability and strength of the steel. Even a small amount of Cu will provide some of the above effects. However, if the Cu content is too high, even if the content of other elements is within the range of this embodiment, the hardenability of the steel will become too high and the steel's resistance to SSC will decrease. Therefore, the Cu content is 0 to 0.50%. The preferred lower limit of the Cu content is greater than 0%, more preferably 0.01%, and even more preferably 0.02%. The preferred upper limit of the Cu content is 0.40%, more preferably 0.30%, even more preferably 0.20%, and even more preferably 0.10%.

[0049] Ni: 0~0.50% Nickel (Ni) is an optional element and may not be present. That is, the Ni content may be 0%. When present, Ni increases the hardenability and strength of the steel. Furthermore, Ni dissolves in the steel, increasing its toughness. Even a small amount of Ni will provide these effects to some extent. However, if the Ni content is too high, localized corrosion will be accelerated, even if the content of other elements is within the range of this embodiment, and the steel's resistance to corrosion corrosion (SSC) will decrease. Therefore, the Ni content is 0 to 0.50%. The preferred lower limit of the Ni content is greater than 0%, more preferably 0.01%, more preferably 0.02%, and still more preferably 0.04%. The preferred upper limit of the Ni content is 0.40%, more preferably 0.30%, more preferably 0.20%, and still more preferably 0.10%.

[0050] The chemical composition of the steel material described above may also contain W in place of some of the Fe.

[0051] W: 0~1.00% Tungsten (W) is an optional element and may not be included. That is, the W content may be 0%. When included, W forms a protective corrosion film in a sour environment, suppressing hydrogen penetration into the steel. As a result, the steel's SSC resistance is increased. Even a small amount of W can provide some of the above effect. However, if the W content is too high, even if the content of other elements is within the range of this embodiment, coarse carbides will form in the steel, reducing the steel's low-temperature toughness and SSC resistance. Therefore, the W content is 0 to 1.00%. The preferred lower limit of the W content is greater than 0%, more preferably 0.01%, more preferably 0.03%, more preferably 0.05%, and more preferably 0.10%. The preferred upper limit of the W content is 0.90%, more preferably 0.80%, more preferably 0.60%, and more preferably 0.50%.

[0052] The chemical composition of the steel material described above may further contain, in place of some of the Fe, one or more elements selected from the group consisting of Zr, Mg, and rare earth elements. Any of these elements are arbitrary and neutralize the S in the steel material as sulfides, thereby improving the steel's resistance to sulfuric acid (SSC).

[0053] Zr: 0~0.50% Zirconium (Zr) is an optional element and may not be included. That is, the Zr content may be 0%. If included, Zr detoxifies the sulfur in the steel as sulfides, improving the steel's SSC resistance. Even a small amount of Zr will provide some degree of the above effect. However, if the Zr content is too high, even if the content of other elements is within the range of this embodiment, the oxides in the steel will coarseen, reducing the steel's SSC resistance. Therefore, the Zr content is 0 to 0.50%. The preferred lower limit of the Zr content is greater 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: 0~0.0100% Magnesium (Mg) is an optional element and may not be present. That is, the Mg content may be 0%. If present, Mg detoxifies the sulfur in the steel as sulfides, improving the steel's SSC resistance. Even a small amount of Mg can provide some of the above effect. However, if the Mg content is too high, even if the content of other elements is within the range of this embodiment, the oxides in the steel will coarseen, reducing the steel's SSC resistance. Therefore, the Mg content is 0 to 0.0100%. The preferred lower limit of the Mg content is greater than 0%, more preferably 0.0001%, more preferably 0.0005%, more preferably 0.0010%, and more preferably 0.0020%. The preferred upper limit of the Mg content is 0.0090%, more preferably 0.0080%, more preferably 0.0070%, more preferably 0.0060%, and more preferably 0.0050%.

[0055] Rare earth elements (REM): 0~0.0100% Rare earth elements (REMs) are optional and do not need to be included. That is, the REM content may be 0%. If included, REMs neutralize sulfur in the steel as sulfides, thereby improving the steel's resistance to sulfur dioxide (SSC). Even a small amount of REM can provide the above effect to some extent. However, if the REM content is too high, even if the content of other elements is within the range of this embodiment, the oxides in the steel will coarseen, reducing the steel's resistance to sulfur dioxide (SSC). Therefore, the REM content is 0 to 0.0100%. The preferred lower limit of the REM content is greater than 0%, more preferably 0.0001%, more preferably 0.0005%, more preferably 0.0010%, and more preferably 0.0020%. The preferred upper limit of the REM content is 0.0090%, more preferably 0.0080%, more preferably 0.0070%, and 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 lanthanides from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. In this specification, 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 less than 862 to 965 MPa (less than 125 to 140 ksi). In this specification, yield strength refers to 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. If 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. If 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. If the steel material is a round steel bar, the tensile test specimen is prepared from the R / 2 position. In this specification, the R / 2 position of a round steel bar means the center position of radius R in a cross section perpendicular to the axial direction of the round steel bar. In this case, the longitudinal direction of the tensile test specimen is parallel to the axial direction of the round steel bar.

[0059] The tensile test specimen is, for example, a round bar specimen with a parallel section diameter of 8.9 mm and a gauge length of 35.6 mm. If the steel material is a steel pipe, an arc-shaped specimen may be used as the tensile test specimen. In this case, the dimensions of the arc-shaped specimen are, for example, the total wall thickness, a width of 25.4 mm, and a gauge length of 50.8 mm. The longitudinal direction of the arc-shaped specimen is parallel to the axial direction of the steel pipe. Using the prepared tensile test specimen, a tensile test is performed at room temperature (25°C) in air, 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 obtained by rounding the obtained value to the first decimal place.

[0060] [Fn1] Assuming that the steel material according to this embodiment has the above-described chemical composition, the element content, 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 equation (1). (Mn+P_seg.) / (2.5Mo+GSN)≦0.125 (1) Here, the elemental symbols in equation (1) are substituted with the content of the corresponding element in units of mass%. Also, P_seg. in equation (1) is substituted with the P content near the prior austenite grain boundary in mol%, and GSN in equation (1) is substituted with the grain size number of the prior austenite grain.

[0061] As mentioned above, the P content P_seg. near the prior γ grain boundary is also called the "grain boundary P content P_seg.". Furthermore, Fn1 (=(Mn+P_seg.) / (2.5Mo+GSN)) is an index indicating the degree of embrittlement of the prior γ grain boundary. The higher the Mn content, the more Mn is concentrated in the prior γ grain boundary, making the prior γ grain boundary more prone to embrittlement. Also, the higher the grain boundary P content P_seg., the more prone the prior γ grain boundary is to embrittlement. On the other hand, the higher the Mo content, the more Mo is concentrated in the prior γ grain boundary, strengthening the prior γ grain boundary. In addition, the larger the crystal grain size number GSN of the prior γ grain, the smaller the crystal grain size of the prior γ grain, increasing the area occupied by the prior γ grain boundary in the steel material, and making it easier to mitigate the embrittlement of the prior γ grain boundary.

[0062] In other words, the steel material according to this embodiment does not simply refine the prior γ grains or reduce the amount of P at the grain boundaries, but rather adjusts the Mn content, which is an element that embrittles the prior γ grain boundaries, the Mo content, which is an element that strengthens the prior γ grain boundaries, the crystal grain size number GSN of the prior γ grains, and the amount of P at the grain boundaries P_seg. As a result, the steel material according to this embodiment can stably achieve both a yield strength of 125 ksi class and excellent SSC resistance, even in a high H2S environment.

[0063] A preferred upper limit for Fn1 is 0.124, and more preferably 0.122. The lower limit for Fn1 is not particularly limited. On the other hand, in steel materials having the above-mentioned chemical composition, the lower limit for 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 steel materials 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. The 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 (grain boundary P content) P_seg. near the prior γ grain boundary is not particularly limited, as long as it satisfies formula (1). On the other hand, in steel materials having the above chemical composition and satisfying formula (1), the grain boundary P content P_seg. is, for example, 1.7 mol.% or less. The 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 particle 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. If 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 that includes the rolling direction and the plate thickness direction. If 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 that includes the pipe axis direction and the pipe radial direction. If the steel material is a round steel bar, a test piece is prepared with the R / 2 position in the center, with the observation surface being a plane that includes the axial direction and the radial direction.

[0067] The test specimen is embedded in resin, the observation surface is polished to a mirror finish, and then immersed for about 60 seconds in a solution of saturated picric acid aqueous solution mixed with an appropriate amount of surfactant to reveal the prior γ grain boundaries by etching. The area of ​​the observation surface is not limited, but for example, 100 mm 2 The size is set to (10 mm × 10 mm). The grain size number GSN is determined by optical microscope observation based on the sectioning method in accordance with JIS G 0551 (2020). In this embodiment, the grain size number GSN of the prior γ grain is obtained by rounding the obtained value to the second 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, the notch is formed in the center of the axial length of the round bar test specimen. The notch is, for example, a V-notch shape with a depth of 0.7 mm and is formed around the entire circumference of the round bar. If the steel material is a steel plate, the test specimen is prepared from the center of the plate thickness. In this case, the axial direction of the test specimen is parallel to the rolling direction of the steel plate. If the steel material is a steel pipe, the test specimen is prepared from the center of the wall thickness. In this case, the axial direction of the test specimen is parallel to the pipe axis direction of the steel pipe. If the steel material is a round steel 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 steel bar.

[0069] The prepared test specimens are placed in a vacuum chamber attached to the Auger electron spectrometer, and the vacuum level inside the chamber is set to 10 -7 The pressure should be Pa or less. An Auger electron spectrometer, for example, the PHI680 manufactured by ULVAC-PHI, Inc., is used. A test specimen cooled to -120°C or below in a chamber is impact fractured to obtain a prior γ grain boundary fracture surface. In this embodiment, multiple measurement points are identified from each of the multiple prior γ grain boundary fracture surfaces. Specifically, for example, a total of 10 or more measurement points are identified from three or more prior γ grain boundary fracture surfaces. In this case, the measurement points identify areas of the prior γ grain boundary fracture surface other than precipitates. Whether or not it is a precipitate can be determined from the contrast by a person skilled in the art.

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

[0071] For each measurement point, the P content (mol.%) is defined as the P content (mol.%) at that measurement point, assuming the total content of P, S, C, Cr, Fe, Mn, Si, and Mo is 100 mol.%. The arithmetic mean of the P content (mol.%) at all measurement points is defined as the grain boundary P content P_seg. (mol.%). In other words, in this embodiment, the P content P_seg. near the prior austenite grain boundary means the P content (mol.%) at the fracture surface of the prior austenite grain boundary, quantified by Auger electron spectroscopy, assuming the total content of P, S, C, Cr, Fe, Mn, Si, and Mo is 100 mol.%. In this embodiment, the grain boundary P content P_seg. (mol.%) is obtained by rounding the obtained value to the second decimal place. In this embodiment, Fn1 (=(Mn+P_seg.) / (2.5Mo+GSN)) is obtained by rounding the resulting 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 yield strength of 125 ksi class and excellent SSC resistance in high H2S environments. In this embodiment, "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 a SSC resistance test conducted in accordance with NACE TM0177-2016 Method A under an H2S gas environment of 0.1 atm, and a four-point bending test conducted in accordance with NACE TM0316 (2016) under an H2S gas environment of 10 atm.

[0074] First, we will describe the SSC resistance test performed in accordance with NACE TM0177-2016 Method A under an H2S gas environment of 0.1 atm. A round bar test specimen is prepared from the steel material according to this embodiment. The size of the round bar test specimen is, for example, a parallel section diameter of 6.35 mm and a parallel section length of 25.4 mm. If 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. If 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 pipe axis direction of the steel pipe. If the steel material is a round steel 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 axis direction of the round steel 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% by mass sodium chloride and 0.4% by 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 container so that the stressed round bar test specimen is immersed in it, creating the 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 conducted in accordance with NACE TM0316 (2016) under an H2S gas environment at 10 atm will be described. A test specimen for the four-point bending test is 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. If the steel material is a steel plate, the test specimen is prepared from the center of the plate thickness. In this case, the longitudinal direction of the test specimen is parallel to the rolling direction of the steel plate. If the steel material is a steel pipe, the test specimen is prepared from the center of the wall thickness. In this case, the longitudinal direction of the test specimen is parallel to the axial direction of the steel pipe. If the steel material is a round steel 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 steel 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, along with the test fixture, is sealed in an autoclave. A mixed aqueous solution of 5.0% by mass sodium chloride and 0.4% by mass sodium acetate, adjusted to pH 4.0 with acetic acid, is used as the test solution. 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 and create the test bath. At this time, the H2S gas pressure is set to 10 atm. After sealing the autoclave, the test bath is maintained at 24°C, and the test specimen is immersed in the test bath for 720 hours while stirring the bath.

[0078] In this embodiment, if no cracks are observed after 720 hours in the SSC resistance test and the four-point bending test conducted under the above conditions, it is determined that "the steel material has excellent SSC resistance." In this specification, "no cracks observed" means that no cracks are observed when the test specimen is observed with the naked eye after the test.

[0079] [Microorganisms] The microstructure of the steel material according to this embodiment mainly consists 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-mentioned chemical composition and 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-mentioned chemical composition and a yield strength of 862 to less than 965 MPa, and satisfying formula (1), has excellent SSC resistance, it is determined that the microstructure of that steel material has a total volume fraction of tempered martensite and tempered bainite of 90% or more.

[0080] Furthermore, when determining the volume fraction of tempered martensite and tempered bainite by observation, the following method can be used. First, a test piece having a specific observation surface is prepared from the steel material according to this embodiment. If 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 surface that includes the rolling direction and the plate thickness direction. If 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 surface that includes the pipe axis direction and the pipe radial direction. If the steel material is a round steel bar, a test piece is prepared with the R / 2 position in the center, with the observation surface being a surface that includes the axial direction and the radial direction.

[0081] After polishing the observation surface of the test specimen to a mirror finish, it is immersed in Nital etching solution for about 10 seconds to reveal the microstructure by etching. The etched observation surface is then observed using a scanning electron microscope (SEM) to obtain 10 fields of view using secondary electron imaging. The field of view area is, for example, 0.01 mm². 2(Magnification 1000x). In each field of view, tempered martensite and tempered bainite are identified from the contrast. The area ratio of the identified tempered martensite and tempered bainite is determined. The method for determining the area ratio is not particularly limited and any well-known method may be used. For example, the area ratio of tempered martensite and tempered bainite can be determined by image analysis. In this embodiment, the arithmetic mean of the area ratios of tempered martensite and tempered bainite obtained in all fields of view is defined as the volume ratio 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 may be, for example, a steel pipe, a steel plate, or a round steel 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-walled seamless steel pipe with a wall thickness of 15 mm or more can achieve both a yield strength of 125 ksi class and excellent SSC resistance in a high H2S environment.

[0083] [Manufacturing method] A method for manufacturing steel materials according to this embodiment will be described below. As an example of steel materials according to this embodiment, a method for manufacturing seamless steel pipes will be described. The method for manufacturing seamless steel pipes comprises a step of preparing a raw pipe (preparation step) and a step of performing quenching and tempering on the raw pipe to produce seamless steel pipes (quenching and tempering step). Note that the manufacturing method according to this embodiment is not limited to the method described below. Each step will be described in detail below.

[0084] [Preparation process] In the preparation step, an intermediate steel material having the above-mentioned chemical composition is prepared. As long as the intermediate steel material has the above-mentioned chemical composition, the method of manufacturing the intermediate steel material is not particularly limited. The intermediate steel material referred to here is a plate-shaped steel material if the final product is a steel plate, a raw pipe if the final product is a steel pipe, and a steel bar with a circular cross-section perpendicular to the axial direction if the final product is a round steel bar.

[0085] The preparation process may include a process for preparing the raw materials (raw material preparation process) and a process for manufacturing intermediate steel materials by hot working the raw materials (hot working process). The following details the case in which the raw material preparation process and the hot working process are included.

[0086] [Material preparation process] In the material preparation process, the material is manufactured using molten steel having the chemical composition described above. The method of manufacturing the material is not particularly limited and any well-known method may be used. Specifically, a slab (slab, bloom, or billet) may be manufactured using a continuous casting method with molten steel. An ingot may be manufactured using a block-making method with molten steel. If necessary, a billet may be manufactured by bloc rolling of the slab, bloom, or ingot. The material (slab, bloom, or billet) is manufactured through the above process.

[0087] [Hot working process] In the hot working process, the prepared material is hot-worked to produce intermediate steel material. If the steel material is a seamless steel pipe, the intermediate steel material corresponds to the raw pipe. First, the billet is heated in a heating furnace. The heating temperature is not particularly limited, but for example, it is 1100 to 1300°C. Hot working is performed on the billet extracted from the heating furnace to produce the raw pipe (seamless steel pipe). The method of hot working is not particularly limited and any well-known method may be used.

[0088] The method of hot working is not particularly limited, but when the intermediate steel material is a raw tube, it is preferable to manufacture the raw tube by performing the Mannesmann mandrel process as the hot working method. In this case, a heated solid round billet is perforated and rolled using a perforating machine to manufacture a hollow round billet. When perforating and rolling, the perforation ratio is not particularly limited, but for example, it is 1.0 to 4.0. The perforated and rolled round billet, i.e., the hollow round billet, is further hot-rolled using a mandrel mill, reducer, sizing mill, etc., to manufacture a raw tube.

[0089] Preferably, in the stretch rolling process according to this embodiment, the temperature of the hollow round billet is set to 1100°C or lower. More specifically, in this embodiment, it is preferable to set the temperature of the hollow round billet at the entrance of the stretch rolling mill (for example, a mandrel mill) to 1100°C or lower. If the temperature of the hollow round billet is 1100°C or lower, strain is more easily introduced into the hollow round billet by stretch rolling. In this case, the prior γ grains in the microstructure of the hollow round billet are refined, and the concentration of P at the prior γ grain boundaries is easily mitigated. As a result, the amount of grain boundary P P_seg. can be reduced in the manufactured steel material. However, if the temperature of the hollow round billet at the entrance of the stretch rolling mill is too low, the load on the stretch rolling mill and the constant-diameter rolling mill will increase. Therefore, it is preferable to set the lower limit of the temperature of the hollow round billet at the entrance of the stretch rolling mill to 950°C or higher.

[0090] Preferably, in the stretch rolling process according to this embodiment, the degree of processing in the first roll of the stretch rolling process is 24.5% or more. Here, in the stretch rolling process according to this embodiment, stretch rolling may be carried out using a stretch rolling mill consisting of multiple rolling stands. In this case, each rolling stand is equipped with multiple rolling rolls arranged around a pass line through which the hollow round billet passes, and is arranged along the pass line. The number of rolling rolls equipped in a rolling stand is not limited, and a two-roll rolling stand may be used, or a three-roll rolling stand may be used.

[0091] In other words, "the first rolling of the stretch rolling process" specifically refers to the rolling performed by the first and second stands of the stretch rolling mill, which are arranged along the pass line of the hollow round billet, when stretch rolling is performed using a two-roll type rolling stand. Similarly, when stretch rolling is performed using a three-roll type rolling stand, it refers to the rolling performed by the first and second rolling stands, which are arranged along the pass line of the hollow round billet. In short, when using a stretch rolling mill consisting of multiple rolling stands, it is preferable that the degree of processing Rm(%) in the first and second stands of the stretch rolling mill be 24.5% or more.

[0092] Hereinafter, in this specification, "degree of processing Rm(%) at the first and second stands of the stretch-rolling mill" will also be referred to as "first-stage stretch-rolling degree of processing Rm(%)". More specifically, when a stretch-rolling mill consisting of multiple rolling stands is used, the first-stage stretch-rolling degree of processing Rm(%) is defined by the following formula (A). The degree of processing in the first stage of stretch rolling, Rm (%) = 100 × {(wall thickness of the hollow tube after perforation rolling - wall thickness of the hollow tube after the second stand of the stretch rolling mill) / wall thickness of the hollow tube after perforation rolling} (A)

[0093] If the initial degree of deformation Rm in stretch rolling is 24.5% or higher, strain is more easily introduced into the hollow round billet during stretch rolling. In this case, the 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 more easily mitigated. As a result, the amount of grain boundary P P_seg. can be reduced in the manufactured steel material. Therefore, in this embodiment, it is preferable to set the initial degree of deformation in stretch rolling to 24.5% or higher. The upper limit of the initial degree of deformation in stretch rolling is not particularly limited, but for example, it is 50.0%.

[0094] Furthermore, after stretch rolling, diameter rolling or drawing rolling may be performed. These hot rolling processes may be carried out using well-known equipment and methods, and are not particularly limited. In addition, the raw tubes produced by hot working may be air-cooled (as-rolled), or may be quenched directly after hot working without cooling to room temperature, or may be reheated after hot working before quenching. The quenching and tempering processes will be described in detail below.

[0095] [Heat treatment process] In the quenching process, the prepared intermediate steel material (raw tube) is subjected to quenching. In this specification, "quenching" means rapidly cooling intermediate steel material with an A3 point or higher. If the quenching temperature is too high, the crystal grains of the prior γ grains may become coarse, which may reduce the steel material's SSC resistance. Therefore, a quenching temperature of 850 to 1000°C is preferred. In this specification, the quenching temperature corresponds to the surface temperature of the intermediate steel material measured by a thermometer installed at the exit of the apparatus performing the final hot working when quenching is performed directly after hot working. Furthermore, the quenching temperature corresponds to the temperature of the furnace used for reheating or additional heating when quenching is performed after additional heating or reheating following hot working.

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

[0097] If the cooling rate during quenching is too slow, the resulting microstructure may not consist mainly 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 will not have good SSC resistance. Therefore, as described above, in the steel material manufacturing method according to this embodiment, the intermediate steel material is rapidly cooled during quenching. Specifically, in the quenching process, the average cooling rate during which the surface temperature of the intermediate steel material (raw tube) during quenching is in the range of 800 to 500°C is set to the quenching cooling rate CR. 800-500 This is defined as the cooling rate CR during quenching. More specifically, the cooling rate CR during quenching. 800-500 This is determined from the temperature measured at the slowest-cooling point within the cross-section of the intermediate steel being hardened (for example, the center of the intermediate steel thickness when both surfaces are forced-cooled).

[0098] Preferred cooling rate during quenching CR 800-500 The temperature is 300°C / min or higher. A more preferable quenching cooling rate is CR. 800-500The lower limit is 450 °C / min, more preferably 600 °C / min. Cooling rate CR during quenching 800-500 The upper limit is not particularly defined, but for example, it is 60000 °C / min.

[0099] Preferably, after heating the plain tube in the austenite region multiple times, quenching is performed. In this case, since the austenite grains are refined, the SSC resistance of the steel material is enhanced. By performing quenching multiple times, heating in the austenite region may be repeated multiple times, or heating in the austenite region may be repeated multiple times by performing normalizing and quenching. Also, quenching and tempering described later may be combined and performed multiple times. That is, multiple quenching and tempering operations may be performed. In this case, the SSC resistance of the steel material is further enhanced. Hereinafter, the tempering process will be described in detail.

[0100] [Tempering Process] In the tempering process, tempering is performed on the intermediate steel material on which the above-described quenching has been performed. In this specification, "tempering" means reheating the intermediate steel material after quenching to a temperature below the A c1 point and holding it. Here, the tempering temperature corresponds to the temperature of the heat treatment furnace when heating and holding the intermediate steel material after quenching. The tempering time means the time from when the temperature of the intermediate steel material reaches a predetermined tempering temperature until it is extracted from the heat treatment furnace.

[0101] The tempering temperature is appropriately adjusted according to the chemical composition of the steel material and the yield strength to be obtained. That is, for the intermediate steel material having the chemical composition of the present embodiment, the tempering temperature is adjusted so that the yield strength of the steel material is adjusted to the 125 ksi grade (less than 862 - 965 MPa). In the tempering process according to the present embodiment, the preferred tempering temperature is 600 - 700 °C. Also, in the tempering process of the present embodiment, the tempering time is preferably 10 - 240 minutes.

[0102] The steel material according to this embodiment can be manufactured by the manufacturing method described above. However, as stated above, the above manufacturing method is just one example, and the material may be manufactured by other manufacturing methods. The present invention will be described in more detail below with reference to examples. [Examples]

[0103] Molten steel having the chemical compositions shown in Tables 1A and 1B was produced. In Table 1B, "-" indicates that the content of each element is at the impurity level. Specifically, the Cu, Ni, W, and Zr content in test number 1 were 0% when rounded to the third decimal place. The Mg and REM content in test number 1 were 0% when rounded to the fifth decimal place.

[0104] [Table 1A]

[0105] [Table 1B]

[0106] Using the molten steel of each test number, solid round billets with the diameters listed in Table 2 were manufactured by continuous casting. Hollow round billets were manufactured by heating and perforating rolling of the solid round billets of each test number. Drawing rolling and constant diameter rolling were performed on the hollow round billets after perforating rolling to manufacture the raw tubes of each test number. The outer diameter (mm) and wall thickness (mm) of the raw tubes of each test number are shown in Table 2. In this embodiment, drawing 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 drawing mill (mandrel mill) is shown in the "Entry Side 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 of drawing rolling, Rm (%) (degree of processing (%) at the first and second stands of the mandrel mill), is shown in the "Rm (%)" column of Table 2.

[0107] [Table 2]

[0108] For each test tube, quenching was performed by heating at the temperature (°C) and time (minutes) listed in Table 2, followed by rapid cooling. Note that the cooling rate (CR) during quenching was also specified. 800-500 All of the samples met the requirements of 300-60000°C / min. Furthermore, tempering was performed by holding the samples at the temperatures (°C) listed in Table 2 for the specified time (minutes). Using the above method, seamless steel pipes for each test number were manufactured.

[0109] [Evaluation Test] For each test number of the manufactured seamless steel pipes, the following tests were performed: tensile tests, prior γ grain observation tests, grain boundary P content measurement tests, and SSC resistance tests.

[0110] [Tensile test] For each test number, round bar 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 the seamless steel pipe using the method described above. Tensile tests were performed on the prepared round bar specimens at room temperature (25°C) in air, in accordance with ASTM E8 / E8M(2022). The stress at 0.7% elongation obtained from the tensile test was defined as the yield strength (MPa). Furthermore, the maximum stress during uniform elongation obtained from 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] [Table 3]

[0112] [Old γ-granule observation test] For each test number, a specimen was prepared from the center of the wall thickness of the seamless steel pipe using the method described above, with the observation surface encompassing both the axial and radial directions of the pipe. The prepared specimen was then etched using the method described above to reveal the prior γ-grain boundaries. (100mm) 2For an observation surface measuring (10 mm × 10 mm), the grain size number GSN of the prior γ grains was determined by optical microscopy, based on the sectioning method compliant 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] For each test number, a notched round bar specimen with a diameter of 3 mm and an axial length of 17 mm was prepared from the center of the wall thickness of the seamless steel pipe using the method described above. In the notched round bar specimen, a V-notch shaped section with a depth of 0.7 mm was formed around the entire circumference of the round bar in the center of the axial length. The prepared notched round bar specimen was fractured by impact using the method described above to obtain the prior γ grain boundary fracture surface. Furthermore, 10 measurement points were identified from the obtained prior γ grain boundary fracture surface using the method described above. Elemental concentration analysis was performed on the identified measurement points by Auger electron spectroscopy using the method described above 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 amount P_seg. (mol.%). The obtained grain boundary P amounts P_seg. (mol.%) are 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%), the grain size number GSN of the prior γ grains, and the grain boundary P content P_seg. (mol.%). The obtained Fn1 values ​​are shown in Table 3.

[0115] [SSC resistance test] For each test number of seamless steel pipe, an SSC resistance test was performed in accordance with NACE TM0177-2016 Method A under an H2S gas environment of 0.1 atm, and a four-point bending test was performed in accordance with NACE TM0316 (2016) under an H2S gas environment of 10 atm.

[0116] First, round bar 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 test number's 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 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 the test container so that the stressed round bar specimens were immersed, creating 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 specimens were observed visually after 720 hours of holding. If no cracks were observed, it was determined that the specimen had excellent SSC resistance (indicated as "EX" (Excellent) in the "0.1atmH2S" column in Table 3). On the other hand, if cracks were observed, it was determined that the specimen did not have excellent SSC resistance (indicated as "NA" (Not Acceptable) in the "0.1atmH2S" 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 center of the wall thickness of each test number's seamless steel pipe using the method described above. A stress equivalent to 90% of the actual yield stress was applied to each of the prepared test specimens by four-point bending. The test solution (a mixed aqueous solution of 5.0% by mass sodium chloride and 0.4% by 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 at 10 atm was pressurized into the autoclave to saturate the test solution and create a test bath. After sealing the autoclave, the test bath was maintained at 24°C, and the test specimens were immersed in the test bath for 720 hours while stirring the bath.

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

[0120] [Evaluation Results] Referring to Tables 1A, 1B, 2, and 3, the seamless steel pipes for test numbers 1 to 18 had appropriate chemical composition, yield strengths of less than 862 to 965 MPa, and Fn1 values ​​of 0.125 or less. As a result, these seamless steel pipes produced zero SSCs in a 10 atm H2S gas environment, demonstrating excellent SSC resistance even in high H2S environments.

[0121] On the other hand, the seamless steel pipe of test number 19 had too high a Mn content and too high an Fn1 level. As a result, this seamless steel pipe produced one or more SSCs in a 10 atm H2S gas environment, and did not exhibit good SSC resistance in high H2S environments.

[0122] The seamless steel pipe in test number 20 had too low a Mo content and too high an Fn1. As a result, this seamless steel pipe produced one or more SSCs in a 10 atm H2S gas environment, and did not exhibit good SSC resistance in high H2S environments.

[0123] The seamless steel pipes in test numbers 21 and 22 had excessively high Mn content, excessively low Mo content, and excessively high Fn1. As a result, these seamless steel pipes produced one or more SSCs in a 10 atm H2S gas environment, and did not exhibit good SSC resistance in high H2S environments.

[0124] In test numbers 23-27, the temperature of the hollow round billet during stretch rolling was too high, resulting in an excessively high Fn1. As a result, these seamless steel pipes produced one or more SSCs in a 10 atm H2S gas environment, and did not exhibit good SSC resistance in high H2S environments.

[0125] The seamless steel pipes in test numbers 28-31 had an insufficient Rm (initial degree of deformation Rm) and an excessively high Fn1 (factory density). As a result, these seamless steel pipes produced one or more SSCs (steam-scaling corrosion cells) in a 10 atm H2S gas environment, indicating they did not exhibit good SSC resistance in high H2S environments.

[0126] The seamless steel pipes in test numbers 32-34 had excessively high Fn1 values. As a result, these seamless steel pipes produced one or more SSCs in a 10 atm H2S gas environment, indicating they did not exhibit good SSC resistance in high H2S environments.

[0127] The embodiments of this disclosure have been described above. However, the embodiments described above are merely examples for implementing this disclosure. Therefore, this disclosure is not limited to the embodiments described above, and the embodiments described above can be modified as appropriate without departing from the spirit of this disclosure.

Claims

1. In mass percent, C: 0.15-0.30% Si: 0.05-1.00%, Mn: 0.05% to less than 0.30% P: 0.020% or less, S: 0.0050% or less, Cr: 0.10-1.00%, Mo: 0.85-2.50%, Ti: 0.002 to 0.020%, Nb: 0.002 to 0.050%, V: 0.01-0.30%, Ca: 0.0001-0.0030%, B: 0.0005-0.0050%, Al: 0.005-0.100%, N: 0.0050% or less, O: 0.0020% or less, Cu: 0 to 0.50%, Ni: 0 to 0.50%, W: 0-1.00%, Zr: 0 to 0.50%, Mg: 0 to 0.0100%, Rare earth elements: 0-0.0100%, and, The remainder consists of Fe and impurities. The yield strength is between 862 and less than 965 MPa. In the microstructure, the total volume percentage of tempered martensite and tempered bainite is 90% or more. The content of the aforementioned element, 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 equation (1), Steel material. (Mn+P_seg.) / (2.5Mo+GSN)≦0.125 (1) Here, the elemental symbols in equation (1) are substituted with the content of the corresponding element in units of mass%. Also, P_seg. in equation (1) is substituted with the P content near the prior austenite grain boundary in mol. %, and GSN in equation (1) is substituted with the grain size number of the prior austenite grain.

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

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

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