Steel material

Abstract

Provided is a steel material having high strength and excellent pitting resistance in a high H2S environment. A steel material according to the present disclosure contains, in terms of mass%, 0.20-0.35% of C, 0.10-1.50% of Si, 0.05-0.55% of Mn, 0.050% or less of P, 0.0100% or less of S, 0.20-1.00% of Cr, 0.20-1.50% of Mo, 0.003-0.030% of Ti, 0.010-0.100% of Al, 0.0100% or less of N and 0.0050% or less of O, with the remainder comprising Fe and impurities, has a yield strength of 758 MPa to less than 965 MPa, and is such that in a microstructure, the standard deviation of the grain size number of prior austenite grains is 0.80 or less.

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] As oil wells and gas wells (hereinafter, oil wells and gas wells will be collectively referred to simply as "oil wells") become deeper, there is a demand for higher strength oil well steel materials, such as 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 oil well steel materials of 110 ksi or more (yield strength of 758 MPa or more).

[0003] Furthermore, in oil wells, the environment may contain corrosive hydrogen sulfide (HS) gas. Therefore, steel materials intended for use as oil well steel materials are required to have not only high strength but also excellent corrosion resistance. Furthermore, oil well steel materials may be subjected to stress during use. For this reason, sulfide stress cracking resistance (hereinafter referred to as SSC resistance) has been used as an indicator of the excellent corrosion resistance of oil well steel materials.

[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 an environment containing HS gas at a pressure of 15 atm (hereinafter, an environment containing HS gas at 15 atm is referred to as a "high HS environment"), the corrosion behavior differs from that in an environment containing HS gas at normal pressure. Therefore, there is a demand for steel materials that have high corrosion resistance even in such high HS environments.

[0009] As described above, SSC resistance has been used as an indicator of excellent corrosion resistance in oil well steel materials. Specifically, Patent Documents 1 and 2 disclose techniques for improving the SSC resistance of steel materials. On the other hand, in high-HS environments, pitting corrosion and crevice corrosion may be the initiation point for stress corrosion cracking. Therefore, oil well steel materials intended for use in high-HS environments are also required to have resistance to pitting corrosion and / or crevice corrosion (hereinafter referred to as "pitting corrosion resistance"). However, little research has been conducted on the pitting corrosion resistance of oil well steel materials in high-HS environments.

[0010] An object of the present disclosure is to provide a steel product having high strength and excellent pitting corrosion resistance in high H2S environments.

[0011] The steel material according to the present disclosure contains, in mass %, C: 0.20 to 0.35%, Si: 0.10 to 1.50%, Mn: 0.05 to 0.55%, P: 0.050% or less, S: 0.0100% or less, Cr: 0.20 to 1.00%, Mo: 0.20 to 1.50%, Ti: 0.003 to 0.030%, Al: 0.010 to 0.100%, N: 0.0100% or less, O: 0.0050% or less, Sb: 0 to 0.50%, Cu: 0 to 0.50%, Ni: 0 to 0.50%, Co: 0 to 0.50%, Zr: 0 to 0.0040%, Nb: 0 to 0.150%, The steel sheet has a yield strength of 758 to less than 965 MPa, and a microstructure in which the standard deviation of the grain size number of prior austenite grains is 0.80 or less.

[0012] The steel material according to the present disclosure has high strength and excellent pitting corrosion resistance in high H2S environments.

[0013] FIG. 1 is a diagram showing the relationship between the standard deviation σ of the grain size number of prior austenite grains and the number of pits (number of pits), which is an index of pitting corrosion resistance, in this example.

[0014] First, the inventors considered obtaining a high-strength steel material having a yield strength of 110 to less than 140 ksi (758 to less than 965 MPa). Next, the inventors investigated, from the standpoint of chemical composition, steel materials having a yield strength of 110 to less than 140 ksi and excellent pitting corrosion resistance in a high-HS environment. As a result, the following contents were found in mass %: C: 0.20 to 0.35%, Si: 0.10 to 1.50%, Mn: 0.05 to 0.55%, P: 0.050% or less, S: 0.0100% or less, Cr: 0.20 to 1.00%, Mo: 0.20 to 1.50%, Ti: 0.003 to 0.030%, Al: 0.010 to 0.100%, N: 0.0100% or less, O: 0.0050% or less, Sb: 0 to 0.50%, Cu: 0 to 0.50%, Ni: 0 to The present inventors considered that a steel material consisting of the following elements: Cr: 0.50%, Co: 0-0.50%, Zr: 0-0.0040%, Nb: 0-0.150%, V: 0-0.500%, B: 0-0.0030%, Ca: 0-0.0040%, Mg: 0-0.0040%, rare earth elements: 0-0.0040%, and the balance being Fe and impurities, may be able to achieve both a yield strength of 110 to less than 140 ksi and excellent pitting corrosion resistance in a high-HS environment.

[0015] Furthermore, the present inventors have focused on the microstructure of steel materials and conducted detailed studies on methods for improving pitting corrosion resistance in high-HS environments. As a result of detailed studies by the present inventors, it has become clear that in steel materials having the above-mentioned chemical composition and a yield strength of 110 to less than 140 ksi, the standard deviation σ of the grain size number of prior austenite grains in the microstructure affects the pitting corrosion resistance of the steel materials in high-HS environments. This point will be specifically explained using drawings.

[0016] Fig. 1 is a diagram showing the relationship between the standard deviation σ of the grain size numbers of prior austenite grains and the number of pits (number of pits), which is an index of pitting resistance, in this example. Note that in this specification, prior austenite grains are also referred to as "prior γ grains." Fig. 1 was created using the value of the standard deviation σ of the grain size numbers of prior γ grains and the number of pits (number of pits), which is an index of pitting resistance, for examples described later in which the configurations other than the standard deviation σ of the grain size numbers of prior γ grains satisfy the conditions of this embodiment.

[0017] 1 , in a steel material having the above-described chemical composition and a yield strength of 110 to less than 140 ksi, if the standard deviation σ of the prior γ grain size number is 0.80 or less, the number of pits, which is an index of pitting corrosion resistance, is 10 or less, and it can be confirmed that the steel material has excellent pitting corrosion resistance. Therefore, the steel material according to this embodiment has the above-described chemical composition, has a yield strength of 110 to less than 140 ksi, and further has the standard deviation σ of the prior γ grain size number is 0.80 or less. As a result, the steel material according to this embodiment has high strength and excellent pitting corrosion resistance even in a high-HS environment.

[0018] The reason why a steel material having the above-mentioned chemical composition and a yield strength of 110 to less than 140 ksi has excellent pitting corrosion resistance even in a high-HS environment when the standard deviation σ of the prior γ grain size number is 0.80 or less is not clear in detail. However, the present inventors speculate as follows.

[0019] First, a high-HS environment has a higher hydrogen sulfide concentration than an atmospheric-pressure HS environment, making it easier for a corrosion film to form on the surface of a steel material. Furthermore, the corrosion film formed on the surface of a steel material has the effect of protecting the surface of the steel material. Therefore, in a high-HS environment, the use of a corrosion film may effectively improve the pitting corrosion resistance of the steel material. Here, the grain boundaries of a steel material have a mismatched atomic arrangement compared to the grain interior, making the corrosion rate more likely to be high. Therefore, if a corrosion film can be formed quickly and uniformly on the surface of a steel material, the pitting corrosion resistance in a high-HS environment may be improved. On the other hand, if prior γ grain boundaries, which are grain boundaries at the time of heating to the austenite region, are uniformly distributed in the steel material, the supply of metal ions for film formation may be uniform. In other words, if prior γ grain boundaries are uniformly distributed in the steel material, a uniform corrosion film can be formed quickly, potentially improving the pitting corrosion resistance of the steel material.

[0020] Here, when the standard deviation σ of the grain size number of the prior γ grains is large, regions where coarse prior γ grains (coarse grains) are unevenly distributed and regions where fine prior γ grains (fine grains) are unevenly distributed are generated in the steel material. In this case, the supply of metal ions for film formation becomes uneven, and the corrosion film may not be formed quickly and uniformly. As a result, there is a concern that a local decrease in pitting corrosion resistance may become apparent, particularly in regions where coarse grains are unevenly distributed. On the other hand, when the standard deviation σ of the grain size number of the prior γ grains is 0.80 or less, the corrosion film may be formed quickly and uniformly, and the occurrence of pitting corrosion on the steel surface may be suppressed.

[0021] The inventors speculate that, based on the above mechanism, if a steel material having the above-mentioned chemical composition and yield strength has a standard deviation σ of the prior γ grain size number of 0.80 or less, then the pitting corrosion resistance in a high-HS environment will be enhanced. Note that it is possible that excellent pitting corrosion resistance in a high-HS environment may be achieved by a mechanism different from that speculated by the inventors. However, the fact that a steel material having the above-mentioned chemical composition and yield strength has excellent pitting corrosion resistance even in a high-HS environment if the standard deviation σ of the prior γ grain size number of 0.80 or less is demonstrated by the examples described below.

[0022] The gist of the steel material according to this embodiment, which was completed based on the above findings, is as follows.

[0023] [1] In mass%, C: 0.20 to 0.35%, Si: 0.10 to 1.50%, Mn: 0.05 to 0.55%, P: 0.050% or less, S: 0.0100% or less, Cr: 0.20 to 1.00%, Mo: 0.20 to 1.50%, Ti: 0.003 to 0.030%, Al: 0.010 to 0.100%, N: 0.0100% or less, O: 0.0050% or less, Sb: 0 to 0.50%, Cu: 0 to 0.50%, Ni: 0 to 0.50%, Co: 0 to 0.50%, Zr: 0 to 0.0040%, Nb: 0 to 0.150%, A steel material comprising: V: 0 to 0.500%, B: 0 to 0.0030%, Ca: 0 to 0.0040%, Mg: 0 to 0.0040%, rare earth elements: 0 to 0.0040%, and the balance being Fe and impurities; having a yield strength of 758 to less than 965 MPa; and having a microstructure in which the standard deviation of the grain size number of prior austenite grains is 0.80 or less.

[0024] [2] The steel material according to [1], containing one or more elements selected from the group consisting of Sb: 0.01 to 0.50%, Cu: 0.01 to 0.50%, Ni: 0.01 to 0.50%, Co: 0.01 to 0.50%, Zr: 0.0001 to 0.0040%, Nb: 0.001 to 0.150%, V: 0.001 to 0.500%, B: 0.0001 to 0.0030%, Ca: 0.0001 to 0.0040%, Mg: 0.0001 to 0.0040%, and rare earth elements: 0.0001 to 0.0040%.

[0025] [3] The steel material according to [1] or [2], containing Si: more than 0.50 to 1.50%.

[0026] [4] The steel material according to any one of [1] to [3], wherein the steel material is a seamless steel pipe.

[0027] 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. The 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.

[0028] The steel material according to this embodiment will be described in detail below. In the following description, "corrosion resistance" refers to resistance to general corrosion, including pitting corrosion resistance and SSC resistance. "%" related to elements refers to mass % unless otherwise specified.

[0029] [Chemical Composition] The chemical composition of the steel material according to this embodiment contains the following elements.

[0030] C: 0.20 to 0.35% Carbon (C) improves the hardenability of steel and increases its strength. If the C 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 C content is too high, even if the contents of other elements are within the ranges of this embodiment, the amount of carbides will be too large, and the corrosion resistance of the steel will decrease. Therefore, the C content is 0.20 to 0.35%. The preferred lower limit of the C content is 0.21%, more preferably 0.22%, and even more preferably 0.23%. The preferred upper limit of the C content is 0.34%, more preferably 0.33%, and even more preferably 0.32%.

[0031] Si: 0.10 to 1.50% Silicon (Si) deoxidizes steel. Si also promotes the formation of a corrosion film on the surface of steel. As a result, the pitting corrosion resistance of steel in a high-HS environment is improved. If the Si 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 Si content is too high, the pitting corrosion resistance of the steel decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the Si content is 0.10 to 1.50%. A preferred lower limit of the Si content is 0.12%, more preferably 0.14%, and even more preferably 0.18%. A preferred upper limit of the Si content is 1.48%, more preferably 1.46%, and even more preferably 1.40%.

[0032] Preferably, the lower limit of the Si content is more than 0.50%. That is, the Si content is preferably more than 0.50% to 1.50%. A steel material having a Si content of more than 0.50% to 1.50% while satisfying the contents of other elements of this embodiment has high strength and, in a high-HS environment, has excellent pitting corrosion resistance as well as excellent general corrosion resistance. Therefore, the Si content is preferably more than 0.50% to 1.50%. A more preferable lower limit of the Si content is 0.51%, even more preferably 0.53%, even more preferably 0.55%, and even more preferably 0.60%.

[0033] Mn: 0.05 to 0.55% Manganese (Mn) deoxidizes steel. Mn also improves the hardenability and strength of steel. 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, excessive Mn sulfides may be formed. Mn sulfides act as the starting point for pitting corrosion. Therefore, even if the contents of other elements are within the ranges of this embodiment, excessive Mn sulfides may be formed, reducing the pitting corrosion resistance of the steel in a high-HS environment. Therefore, the Mn content is 0.05 to 0.55%. The preferred lower limit of the Mn content is 0.06%, and more preferably 0.08%. The preferred upper limit of the Mn content is 0.50%, more preferably 0.45%, and even more preferably 0.40%.

[0034] P: 0.050% 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, P segregates at grain boundaries, reducing the corrosion resistance of the steel in a high-HS environment. Therefore, the P content is 0.050% or less. A preferred upper limit of the P content is 0.048%, more preferably 0.045%, and even more preferably 0.040%. The lower the P content, the better. 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: 0.0100% 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 contents of other elements are within the ranges of this embodiment, coarse sulfides are formed, reducing the pitting corrosion resistance of the steel in a high-HS environment. Therefore, the S content is 0.0100% or less. A preferred upper limit of the S content is 0.0095%, more preferably 0.0080%, even more preferably 0.0060%, and even more preferably 0.0040%. The S content should be 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 0.0003%, and even more preferably 0.0005%.

[0036] Cr: 0.20 to 1.00% Chromium (Cr) promotes the formation of a corrosion film on the surface of a steel material. As a result, the pitting corrosion resistance of the steel material in a high-HS environment is improved. If the Cr content is too low, the above effect cannot be fully 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, coarse precipitates are formed, reducing the pitting corrosion resistance of the steel material, even if the contents of other elements are within the ranges of this embodiment. Therefore, the Cr content is 0.20 to 1.00%. The preferred lower limit of the Cr content is 0.21%, more preferably 0.23%, even more preferably 0.25%, and even more preferably 0.30%. The preferred upper limit of the Cr content is 0.98%, more preferably 0.96%, and even more preferably 0.95%.

[0037] Mo: 0.20 to 1.50% Molybdenum (Mo) contributes to stabilizing the corrosion film on the steel surface. As a result, the pitting corrosion resistance of the steel in a high-HS environment 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, the above effects saturate. Therefore, the Mo content is 0.20 to 1.50%. The preferred lower limit of the Mo content is 0.21%, more preferably 0.24%, and even more preferably 0.30%. The preferred upper limit of the Mo content is 1.49%, more preferably 1.45%, even more preferably 1.40%, and even more preferably 1.30%.

[0038] Ti: 0.003 to 0.030% Titanium (Ti) bonds with N to form nitrides, which refine the grains of the steel material through a pinning effect. As a result, the pitting corrosion resistance of the steel material is improved. 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, even if the contents of other elements are within the ranges of this embodiment, the Ti nitrides become coarse, and the pitting corrosion resistance of the steel material is reduced. Therefore, the Ti content is 0.003 to 0.030%. 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 0.028%, and more preferably 0.025%.

[0039] Al: 0.010 to 0.100% Aluminum (Al) deoxidizes steel. If the Al content is too low, the above effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Al content is too high, coarse oxide-based inclusions are formed, reducing the pitting corrosion resistance of the steel material, even if the contents of other elements are within the ranges of this embodiment. Therefore, the Al content is 0.010 to 0.100%. A preferred lower limit of the Al content is 0.015%, more preferably 0.020%, and even more preferably 0.025%. A preferred upper limit of the Al content is 0.080%, more preferably 0.070%, and even more preferably 0.060%. As used herein, the "Al" content refers to the content of "acid-soluble Al," that is, "sol. Al."

[0040] N: 0.0100% or less Nitrogen (N) is unavoidably contained. That is, the lower limit of the N content is greater than 0%. N combines with Ti to form nitrides, which refine the grains of the steel material through a pinning effect. As a result, the pitting corrosion resistance of the steel material is improved. 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 pitting corrosion resistance of the steel material is actually reduced. Therefore, the N content is 0.0100% or less. A preferred upper limit of the N content is 0.0080%, more preferably 0.0060%, and even more preferably 0.0050%. To more effectively obtain the above effects, a preferred lower limit of the N content is 0.0005%, more preferably 0.0010%, even more preferably 0.0020%, and even more preferably 0.0025%.

[0041] O: 0.0050% or less Oxygen (O) is an impurity. That is, the lower limit of the O content is greater 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, reducing the pitting corrosion resistance of the steel material. Therefore, the O content is 0.0050% or less. A preferred upper limit of the O content is 0.0040%, more preferably 0.0030%, and even more preferably 0.0020%. 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 0.0003%, and even more preferably 0.0005%.

[0042] 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.

[0043] [Optional Elements] The chemical composition of the above-mentioned steel material may further contain one or more elements selected from the group consisting of Sb, Cu, Ni, Co, and Zr in place of a portion of Fe. Any of these elements suppresses hydrogen penetration into the steel material and improves the SSC resistance of the steel material in a high H2S environment.

[0044] Sb: 0 to 0.50% Antimony (Sb) is an optional element and does not necessarily need to be contained. That is, the Sb content may be 0%. When contained, Sb suppresses hydrogen penetration into steel in a high-HS environment. As a result, the SSC resistance of the steel in a high-HS environment is improved. Even if even a small amount of Sb is contained, the above effect can be obtained to some extent. However, if the Sb content is too high, the hot workability of the steel will deteriorate even if the contents of other elements are within the ranges of this embodiment. Therefore, the Sb content is 0 to 0.50%. The preferred lower limit of the Sb content is more than 0%, more preferably 0.01%, even more preferably 0.02%, even more preferably 0.03%, and even more preferably 0.05%. The preferred upper limit of the Sb content is 0.45%, even more preferably 0.40%.

[0045] 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 suppresses hydrogen penetration into steel in a high-HS environment. As a result, the SSC resistance of the steel in a high-HS environment is improved. Even if even a small amount of Cu is contained, the above effect can be achieved 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 will be too high and the toughness of the steel will be reduced. 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%, even more preferably 0.02%, and even more preferably 0.05%. The preferred upper limit of the Cu content is 0.48%, even more preferably 0.45%, and even more preferably 0.40%.

[0046] Ni: 0 to 0.50% Nickel (Ni) is an optional element and may not be contained. That is, the Ni content may be 0%. When contained, Ni forms a corrosion film in a high-HS environment, suppressing hydrogen penetration into the steel material. As a result, the SSC resistance of the steel material in a high-HS environment is improved. Even if even a small amount of Ni is contained, the above effect can be achieved to some extent. However, if the Ni content is too high, localized corrosion will be promoted and the corrosion resistance of the steel material will decrease, 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 0.05%. The preferred upper limit of the Ni content is 0.48%, even more preferably 0.45%, and even more preferably 0.40%.

[0047] Cobalt (Co) is an optional element and does not necessarily need to be contained. That is, the Co content may be 0%. When contained, Co forms a corrosion film in a high-HS environment, suppressing hydrogen penetration into the steel material. As a result, the SSC resistance of the steel material in a high-HS environment is improved. Even if even a small amount of Co is contained, the above effects can be achieved to some extent. However, if the Co content is too high, even if the contents of other elements are within the ranges of this embodiment, the hardenability of the steel material decreases, resulting in a decrease in the strength of the steel material. Therefore, the Co content is 0 to 0.50%. The preferred lower limit of the Co content is more than 0%, more preferably 0.01%, even more preferably 0.02%, even more preferably 0.03%, and even more preferably 0.05%. The preferred upper limit of the Co content is 0.48%, even more preferably 0.45%, and even more preferably 0.40%.

[0048] Zr: 0 to 0.0040% 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 stabilizes corrosion films in high-HS environments and suppresses hydrogen penetration into the steel material. As a result, the SSC resistance of the steel material in high-HS environments is improved. Even if even a small amount of Zr is contained, the above effects can be achieved 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 corrosion resistance of the steel material. Therefore, the Zr content is 0 to 0.0040%. The preferred lower limit of the Zr content is greater than 0%, more preferably 0.0001%, even more preferably 0.0003%, even more preferably 0.0006%, even more preferably 0.0010%, and even more preferably 0.0015%. The upper limit of the Zr content is preferably 0.0038%, more preferably 0.0035%, and even more preferably 0.0030%.

[0049] The chemical composition of the steel material described above may further contain, in place of a portion of Fe, one or more elements selected from the group consisting of Nb, V, and B. All of these elements increase the strength of the steel material.

[0050] Nb: 0 to 0.150% Niobium (Nb) is an optional element and may not be contained. That is, the Nb content may be 0%. When contained, Nb bonds with C or N to form carbides, nitrides, or carbonitrides (hereinafter also referred to as "carbonitrides, etc."), and refines the grains of the steel material through a pinning effect. As a result, the strength of the steel material is increased. In this case, the corrosion resistance of the steel material is also improved. Even if even a small amount of Nb is contained, the above effects can be obtained to some extent. However, if the Nb content is too high, even if the contents of other elements are within the ranges of this embodiment, excessive carbonitrides, etc. are formed, and the corrosion resistance of the steel material is reduced. Therefore, the Nb content is 0 to 0.150%. The preferred lower limit of the Nb content is more than 0%, more preferably 0.001%, even more preferably 0.003%, even more preferably 0.005%, and even more preferably 0.010%. The upper limit of the Nb content is preferably 0.100%, more preferably 0.080%, and even more preferably 0.060%.

[0051] V: 0 to 0.500% Vanadium (V) is an optional element and does not necessarily need to be contained. That is, the V content may be 0%. When contained, V forms carbonitrides and the like, and refines the grains of the steel material through a pinning effect. As a result, the strength of the steel material is increased. In this case, the corrosion resistance of the steel material is also improved. Even if even a small amount of V is contained, the above effect can be obtained to some extent. However, if the V content is too high, even if the contents of other elements are within the ranges of this embodiment, excessive carbonitrides and the like will be formed, reducing the corrosion resistance of the steel material. Therefore, the V content is 0 to 0.500%. The preferred lower limit of the V content is more than 0%, more preferably 0.001%, even more preferably 0.005%, even more preferably 0.010%, even more preferably 0.030%, and even more preferably 0.050%. The preferred upper limit of the V content is 0.400%, even more preferably 0.350%, and even more preferably 0.300%.

[0052] B: 0 to 0.0030% Boron (B) is an optional element and does not necessarily need to be contained. That is, the B content may be 0%. When contained, B dissolves in the steel to improve the hardenability and strength of the steel. Even if even a small amount of B is contained, the above effects can be obtained to some extent. However, if the B content is too high, even if the contents of other elements are within the ranges of this embodiment, coarse nitrides are formed, reducing the corrosion resistance of the steel. Therefore, the B content is 0 to 0.0030%. The preferred lower limit of the B content is more than 0%, more preferably 0.0001%, even more preferably 0.0005%, and even more preferably 0.0010%. The preferred upper limit of the B content is 0.0029%, even more preferably 0.0025%.

[0053] The chemical composition of the steel material described above may further contain, in place of a portion of Fe, one or more elements selected from the group consisting of Ca, Mg, and rare earth elements. All of these elements are optional elements, and render S in the steel material harmless as sulfides. As a result, all of these elements improve the corrosion resistance of the steel material.

[0054] Ca: 0 to 0.0040% Calcium (Ca) is an optional element and does not necessarily need to be contained. That is, the Ca content may be 0%. When contained, Ca neutralizes S in the steel material as sulfides, thereby improving the corrosion resistance of the steel material. Even if even a small amount of Ca is contained, the above effect can be achieved to some extent. However, if the Ca 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, thereby reducing the corrosion resistance of the steel material. Therefore, the Ca content is 0 to 0.0040%. The preferred lower limit of the Ca content is more than 0%, more preferably 0.0001%, even more preferably 0.0003%, even more preferably 0.0006%, and even more preferably 0.0010%. The preferred upper limit of the Ca content is 0.0035%, even more preferably 0.0030%.

[0055] Mg: 0 to 0.0040% 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 neutralizes S in the steel material by converting it into sulfides, thereby improving the corrosion resistance of the steel material. Even if even a small amount of Mg is contained, the above effect can be achieved 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, thereby reducing the corrosion resistance of the steel material. Therefore, the Mg content is 0 to 0.0040%. The preferred lower limit of the Mg content is greater than 0%, more preferably 0.0001%, even more preferably 0.0003%, even more preferably 0.0006%, and even more preferably 0.0010%. The preferred upper limit of the Mg content is 0.0035%, even more preferably 0.0030%.

[0056] Rare Earth Elements: 0 to 0.0040% 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 corrosion resistance of the steel material. Even if even a small amount of REM is contained, the above effects can be achieved to some extent, even if the contents of other elements are within the ranges of this embodiment. However, if the REM content is too high, oxides in the steel material will coarsen, thereby reducing the corrosion resistance of the steel material, even if the contents of other elements are within the ranges of this embodiment. Therefore, the REM content is 0 to 0.0040%. The preferred lower limit of the REM content is greater than 0%, more preferably 0.0001%, even more preferably 0.0003%, even more preferably 0.0006%, and even more preferably 0.0010%. The preferred upper limit of the REM content is 0.0035%, even more preferably 0.0030%.

[0057] 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.

[0058] As described above, the Si content is preferably more than 0.50% and less than 1.50%. That is, the steel material according to this embodiment contains, in mass %, C: 0.20 to 0.35%, Si: more than 0.50 to 1.50%, Mn: 0.05 to 0.55%, P: 0.050% or less, S: 0.0100% or less, Cr: 0.20 to 1.00%, Mo: 0.20 to 1.50%, Ti: 0.003 to 0.030%, Al: 0.010 to 0.100%, N: 0.0100% or less, O: 0.0050% or less, Sb: 0 to 0.50%, Cu: 0 to 0.50%, Ni: A steel material containing 0 to 0.50%, Co: 0 to 0.50%, Zr: 0 to 0.0040%, Nb: 0 to 0.150%, V: 0 to 0.500%, B: 0 to 0.0030%, Ca: 0 to 0.0040%, Mg: 0 to 0.0040%, rare earth elements: 0 to 0.0040%, and the balance being Fe and impurities, has a yield strength of 110 to less than 140 ksi, excellent pitting corrosion resistance in a high-HS environment, and excellent general corrosion resistance in a high-HS environment.

[0059] [Yield Strength] The yield strength of the steel material according to this embodiment is 758 to less than 965 MPa (110 to less than 140 ksi). The yield strength in this specification means the 0.2% offset proof stress obtained in a tensile test in accordance with ASTM E8 / E8M (2022).

[0060] 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 or the arc-shaped 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.

[0061] The tensile test specimen is, for example, a round bar test specimen with a parallel portion diameter of 6.0 mm and a gauge length of 30.0 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 dimensions of the circular arc-shaped test specimen are, for example, the full wall thickness, a width of 25.4 mm, and a gauge length of 50.8 mm. Using the prepared tensile test specimen, a tensile test is performed in air at room temperature (25°C) in accordance with ASTM E8 / E8M (2022). The 0.2% offset yield strength obtained by the tensile test is defined as the yield strength (MPa). In this embodiment, the yield strength (MPa) is calculated by rounding the obtained value to one decimal place.

[0062] [Standard deviation σ of grain size number] The steel material according to this embodiment has the above-described chemical composition, a yield strength of 758 to less than 965 MPa, and further, the standard deviation σ of the grain size number of prior austenite grains (prior γ grains) is 0.80 or less. As a result, the steel material according to this embodiment has excellent pitting corrosion resistance in a high-HS environment, even though it has a yield strength of 758 to less than 965 MPa.

[0063] Here, when the standard deviation σ of the grain size numbers of prior γ grains is large, regions where coarse prior γ grains (coarse grains) are unevenly distributed and regions where fine prior γ grains (fine grains) are unevenly distributed occur in the steel material. As a result, in a high-HS environment, there is a possibility that a local decrease in pitting corrosion resistance is likely to become apparent in the regions where coarse grains are unevenly distributed. Therefore, in the steel material according to this embodiment, the standard deviation σ of the grain size numbers of prior γ grains is set to 0.80 or less.

[0064] In this embodiment, the upper limit of the standard deviation σ of the grain size numbers of the prior γ grains is preferably 0.79, more preferably 0.78, and even more preferably 0.77. In the steel material according to this embodiment, the standard deviation σ of the grain size numbers of the prior γ grains is preferably smaller. In other words, the lower limit of the standard deviation σ of the grain size numbers of the prior austenite grains may be 0.00, 0.05, 0.10, or 0.15.

[0065] In the steel material according to this embodiment, the grain size number of the prior γ grains is not particularly limited as long as the standard deviation σ is 0.80 or less. In this embodiment, the grain size number of the prior γ grains is, for example, 0.0 or more. The lower limit of the grain size number of the prior γ grains may be 0.5. Preferably, the grain size number of the prior γ grains is 5.0 or more. In this case, the corrosion resistance of the steel material can be more stably obtained. A more preferable lower limit of the grain size number of the prior γ grains is 6.0, even more preferably 7.0, even more preferably 7.5, and even more preferably 8.0. The upper limit of the grain size number of the prior γ grains may be 13.0, 12.0, 11.0, or 10.0.

[0066] In the steel material according to this embodiment, the standard deviation σ of the grain size number of prior γ grains is determined by the following method. Specifically, test pieces for microstructure observation are prepared from the steel material according to this embodiment. When the steel material is a steel plate, the test piece is prepared from the center of the plate thickness. When the steel material is a steel pipe, the test piece is prepared from the center of the wall thickness. When the steel material is a round bar, the test piece is prepared from the R / 2 position. The size of the test piece is not particularly limited as long as it can provide an observation surface, which will be described later.

[0067] The observation surface of the prepared test piece is polished to a mirror finish, and then immersed for approximately 60 seconds in a solution of picric acid saturated aqueous solution mixed with an appropriate amount of surfactant to reveal the prior γ grain boundaries by etching. Ten fields of view are selected from the observation surface and observed using an optical microscope to generate photographic images. The magnification for microscopic observation can be appropriately set depending on the crystal grain size. Specifically, the magnification is set so that, for example, 50 or more crystal grains are included in the field of view.

[0068] In each field of view, image analysis is performed on the obtained photographic image, and the grain size number is measured in accordance with ASTM E112 (2021). That is, one grain size number is obtained for each observation field. The standard deviation of the obtained 10 grain size numbers is calculated and defined as the standard deviation σ of the grain size numbers of the prior austenite grains. The standard deviation σ of the grain size numbers of the prior austenite grains is calculated by rounding the obtained value to two decimal places.

[0069] [Pitting corrosion resistance] The steel material according to this embodiment has the above-described chemical composition, a yield strength of 758 to less than 965 MPa, and a standard deviation σ of the grain size number of prior austenite grains (prior γ grains) of 0.80 or less. As a result, the steel material according to this embodiment has excellent pitting corrosion resistance in a high-HS environment, even though it has a yield strength of 758 to less than 965 MPa. In this embodiment, having excellent pitting corrosion resistance in a high-HS environment is defined as follows.

[0070] A pitting corrosion resistance test is carried out on the steel material according to this embodiment. Specifically, test pieces for a four-point bending test are prepared from the steel material according to this embodiment. When the steel material is a steel plate, the test piece is prepared from the center of the plate thickness. When the steel material is a steel pipe, the test piece is prepared from the center of the wall thickness. When the steel material is a round bar, the test piece is prepared from the R / 2 position. The size of the test piece is, for example, 30 mm in length, 30 mm in width, and 3 mm in thickness.

[0071] The test specimens are sealed in an autoclave. A 5.0 mass% sodium chloride aqueous solution is poured into the autoclave. The test specimens are not immersed in the test solution (test bath) but are kept in the gas phase region of the autoclave. 15 atm of H2S gas is pressurized and sealed into the autoclave to saturate the test solution, forming a test bath. After sealing the autoclave, the test bath is kept at 24°C, and the test specimens are kept in the test bath for 720 hours while stirring.

[0072] After 720 hours, the surface of a 30 mm long and 30 mm wide test piece is observed to check for the presence or absence of pitting corrosion. Pitting corrosion is defined as corrosion with a depth of 50 μm or more and an equivalent circle diameter of 40 μm or more. Specifically, the surface of the test piece is observed with a magnifying glass at a magnification of 10 times to check for the presence or absence of pitting corrosion. If pitting corrosion is confirmed, the number of pits is counted. In this embodiment, if the number of pits is 10 or less as a result of the pitting corrosion resistance test under the above conditions, the test piece is evaluated as having excellent pitting corrosion resistance even in a high-HS environment.

[0073] [General Corrosion Resistance] Preferably, the steel material according to this embodiment has the chemical composition described above, a Si content of more than 0.50% and less than 1.50%, a yield strength of 758 to less than 965 MPa, and a standard deviation σ of the grain size number of prior austenite grains (prior γ grains) of 0.80 or less. As a result, even if the steel material according to this embodiment has a yield strength of 758 to less than 965 MPa, it has excellent pitting corrosion resistance in a high-HS environment and also has excellent general corrosion resistance in a high-HS environment. In this embodiment, having excellent general corrosion resistance in a high-HS environment is defined as follows.

[0074] A corrosion test is performed on the steel material according to this embodiment. Specifically, a test specimen is prepared in the same manner as in the pitting corrosion resistance test described above. The test specimen is sealed in an autoclave. A 5.0 mass % sodium chloride aqueous solution is poured into the autoclave. The test specimen is then immersed in the test solution (test bath). After sealing the autoclave under the same conditions, the test bath is maintained at 24°C, and the test specimen is maintained for 720 hours while stirring the test bath. The difference between the mass of the test specimen before the test and the mass of the test specimen after 720 hours of immersion, from which corrosion scale has been removed, is calculated. The corrosion rate (mm / year) of the test specimen is calculated by dividing the resulting mass difference by the surface area, density, and immersion time of the test specimen. In this embodiment, if the corrosion rate obtained as a result of the corrosion test under the above conditions is 0.40 mm / year or less, the test specimen is evaluated as having excellent general corrosion resistance even in a high-HS environment.

[0075] [Microstructure] 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. If the microstructure of a steel material having the above-described chemical composition contains a total volume fraction of tempered martensite and tempered bainite of 90% or more, the microstructure will have a yield strength of 758 to less than 965 MPa (110 to less than 140 ksi) and excellent pitting corrosion resistance in a high-HS environment, provided that the other configurations of this embodiment are satisfied. That is, in this embodiment, if a steel material has a yield strength of 758 to less than 965 MPa (110 to less than 140 ksi) and excellent pitting corrosion resistance in a high-HS environment, the microstructure is determined to have a total volume fraction of tempered martensite and tempered bainite of 90% or more.

[0076] 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 an 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 and the observation surface being a plane including the axial and radial directions.

[0077] 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 in 10 fields of view as secondary electron images using a scanning electron microscope (SEM). 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.

[0078] [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-walled seamless steel pipe with a wall thickness of 15 mm or more can achieve both a yield strength of 110 to less than 140 ksi and excellent pitting corrosion resistance in a high-HS environment.

[0079] [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.

[0080] [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 blank 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.

[0081] 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.

[0082] [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.

[0083] [Hot working process] In the hot working process, the prepared 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 billet extracted from the heating furnace is hot worked to produce a mother pipe (seamless steel pipe).

[0084] In this embodiment, the conditions for heating the billet are preferably as follows: Heating temperature T: 1150 to 1300°C Holding time t: 30 to 500 minutes Furthermore, the heating temperature T (°C) and the holding time t (minutes) satisfy the following formula (A): 30000≦(273+T)×(20+Log(t / 60))≦32000 (A)

[0085] Here, the heating temperature T of the billet refers to the temperature (°C) of the heat treatment furnace when the billet is heated. The holding time t when the billet is heated refers to the time (minutes) for which the billet is held at the heating temperature T. If the heating temperature T is too high, the austenite grains may become coarse. On the other hand, if the heating temperature T is too low, the billet may not be heated enough, which may place too much strain on the hot working equipment. Furthermore, if the holding time t when heating the billet is too short, the billet may not be heated enough, which may place too much strain on the hot working equipment. On the other hand, if the holding time t is too long, the heating effect will saturate.

[0086] In this embodiment, it is preferable to further satisfy the above formula (A). Here, LMP is defined as (273 + T) × (20 + Log(t / 60)). If the LMP is too low, the billet may not be heated sufficiently, resulting in temperature unevenness in the material during hot working. In this case, the variation in prior γ grains increases, and the standard deviation σ of the grain size numbers of the prior γ grains increases. On the other hand, if the LMP is too high, Ostwald ripening of pinning particles, typified by Ti nitrides, may be promoted, resulting in unevenness in the size and distribution of the pinning particles. In this case, the variation in grain size of the prior γ grains increases, and the variation in the grain size numbers of the prior γ grains increases. As a result, the standard deviation σ of the grain size numbers of the prior γ grains increases. Therefore, in the hot working process according to this embodiment, it is preferable to set the LMP when heating the billet to 30,000 to 32,000.

[0087] The raw material extracted from the heating furnace is hot worked to produce an intermediate steel material. Although the hot working method is not particularly limited, when the intermediate steel material is a mother pipe, it is preferable to carry out the Mannesmann process to produce the mother pipe. In this case, the round billet is pierced and rolled using a piercing mill. 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 is further hot rolled using a mandrel mill, a reducer, a sizing mill, or the like to produce a mother pipe.

[0088] When a mother pipe is manufactured using the Mannesmann process, the time from piercing-rolling to hot rolling (elongation) using a mandrel mill is preferably 30 to 180 seconds. In this specification, the time from piercing-rolling to the start of elongation is also referred to as the "piercing-elongation holding time." By providing a certain amount of piercing-elongation holding time, austenite grains tend to grow during holding, resulting in a uniform austenite grain size. Therefore, the standard deviation σ of the grain size number of prior γ grains can be reduced in the manufactured steel. On the other hand, if the piercing-elongation holding time is too long, the temperature of the intermediate steel may decrease, making it impossible to obtain hot workability. In this case, the austenite grains may grow too much, increasing the standard deviation σ of the grain size number of prior γ grains. Therefore, in the hot working process according to this embodiment, the piercing-elongation holding time is preferably 30 to 180 seconds.

[0089] The intermediate steel material 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 process will be described in detail below.

[0090] [Quenching Process] In the quenching process, the prepared intermediate steel material is quenched. In this specification, "quenching" means rapidly cooling the intermediate steel material at the A3 point or above. The preferred quenching temperature is 850 to 1000°C. If the quenching temperature is too high, prior γ grains may become coarse, making it impossible to obtain corrosion resistance. Therefore, the quenching temperature is preferably 850 to 1000°C.

[0091] In this specification, the quenching temperature corresponds to the surface temperature of the intermediate steel material measured by a thermometer installed on the outlet side of the equipment that performs the final hot working when quenching is performed directly after hot working. Furthermore, the quenching temperature corresponds to the temperature of the furnace that performs the quenching or reheating when quenching is performed after reheating or reheating after hot working.

[0092] 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.

[0093] If the cooling rate during quenching is too slow, the microstructure may not be mainly composed of martensite and bainite, and in this case, the mechanical properties specified in this embodiment (yield strength of 110 to less than 140 ksi) may not be obtained.

[0094] Therefore, as described above, in the steel manufacturing method according to this embodiment, the intermediate steel is rapidly cooled during quenching. Specifically, in the quenching step, the average cooling rate in the range of the surface temperature of the intermediate steel (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).

[0095] Preferred cooling rate during quenching: CR 800-500 The cooling rate CR during quenching is preferably 300°C / min or more. 800-500 The 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.

[0096] Preferably, the mother pipe is heated in the austenite region multiple times and then quenched. In this case, the austenite grains before quenching are refined, thereby improving the pitting corrosion 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 pitting corrosion resistance of the steel material is further improved. The tempering process will be described in detail below.

[0097] [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 furnace temperature at which the intermediate steel material is heated and held 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.

[0098] 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 758 to less than 965 MPa. In the tempering process of this embodiment, the preferred tempering temperature is 660 to 740°C. Furthermore, in the tempering process of this embodiment, the preferred tempering time is 20 to 180 minutes.

[0099] The steel material according to this embodiment can be manufactured by the above manufacturing method. In the above manufacturing method, a method for manufacturing a seamless steel pipe has been described as an example. However, the steel material according to this embodiment may be a steel plate or have other shapes. Furthermore, as mentioned above, the above manufacturing method is 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. Note that the conditions in the following examples are one example of conditions adopted to confirm the feasibility and effects of the steel material according to this embodiment. Therefore, the steel material according to this embodiment is not limited to this one example of conditions.

[0100] 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 Sb content, Cu content, Ni content, and Co content of Test No. 1 were rounded to two decimal places to mean that they were 0%. The Nb content and V content of Test No. 1 were rounded to four decimal places to mean that they were 0%. Furthermore, the Zr content, B content, Ca content, Mg content, and REM content of Test No. 1 were rounded to five decimal places to mean that they were 0%.

[0101]

[0102]

[0103] Using the molten steel of each test number, a round billet was produced by continuous casting. The round billet of each test number was heated in a heating furnace and hot-worked. The heating temperature T (°C), holding time t (minutes) in the heating before hot-working, and LMP (= (273 + T) × (20 + Log (t / 60))) calculated from the above formula (A) were as shown in Table 2.

[0104]

[0105] Piercing-rolling and elongation-rolling were carried out on the heated round billet. The time from the end of piercing-rolling to the start of elongation-rolling (piercing-elongation holding time) is shown in Table 2. In Table 2, "A (Appropriate)" in the piercing-elongation holding time column means that the piercing-elongation holding time was 30 to 180 seconds. In Table 2, "S (Short)" in the piercing-elongation holding time column means that the piercing-elongation holding time was less than 30 seconds. In Table 2, "L (Long)" in the piercing-elongation holding time column means that the piercing-elongation holding time exceeded 180 seconds.

[0106] The produced mother pipes were quenched and tempered. Specifically, the mother pipes of each test number were quenched by holding them at the temperature (°C) shown in Table 2 for the time (minutes) shown in Table 2, followed by water cooling. The quenched mother pipes of each test number were further tempered by holding them at the temperature (°C) shown in Table 2 for the time (minutes) shown in Table 2. Through the above production process, seamless steel pipes of each test number were obtained.

[0107] [Evaluation Tests] The seamless steel pipes having the respective test numbers after tempering were subjected to the following grain size number measurement test, tensile test, pitting corrosion resistance test, and general corrosion resistance test.

[0108] [Grain size number measurement test] A grain size number measurement test was performed on the seamless steel pipe of each test number, and the standard deviation σ of the grain size number of the prior γ grains was determined. Specifically, the test pieces prepared by the above-mentioned method were subjected to microscopic observation by the above-mentioned method. Image analysis was performed on the photographic images obtained by microscopic observation, and the grain size numbers were measured in accordance with ASTM E112 (2021). The grain size numbers obtained in 10 fields of view for each test number are shown in Table 3A. The average value and standard deviation σ of the grain size numbers obtained from the 10 obtained grain size numbers are shown in Table 3A.

[0109]

[0110] [Tensile Test] A tensile test was conducted on each seamless steel pipe according to ASTM E8 / E8M (2022). Specifically, a round bar test piece with a parallel section diameter of 6.0 mm and a gauge length of 30.0 mm was prepared as a tensile test piece from the center of the wall thickness of each seamless steel pipe according to ASTM E8 / E8M (2022). The longitudinal direction of the tensile test piece was parallel to the axial direction of the steel pipe. Using the prepared tensile test piece, a tensile test was conducted in air at room temperature (25°C) according to ASTM E8 / E8M (2022). The 0.2% offset proof stress obtained by the tensile test was defined as the yield strength (MPa). The obtained yield strength (MPa) for each seamless steel pipe according to ASTM E8 / E8M (2022) is shown in Table 3B as "YS (MPa)."

[0111]

[0112] [Pitting corrosion resistance test] A pitting corrosion resistance test was conducted on the seamless steel pipes of each test number to evaluate pitting corrosion resistance. Specifically, the test specimens prepared by the above-mentioned method were kept in an autoclave under the above-mentioned conditions for 720 hours. After 720 hours, the presence or absence of pitting corrosion was confirmed on the surfaces of the test specimens by the above-mentioned method. The number of pits was counted for the test specimens of each test number to obtain the number of pits. The obtained number of pits is shown in Table 3B.

[0113] [General Corrosion Resistance Test] A general corrosion resistance test was conducted on the seamless steel pipe of each test number to evaluate general corrosion resistance. Specifically, the test specimens prepared by the above-mentioned method were kept in an autoclave under the above-mentioned conditions for 720 hours. After 720 hours, the corrosion rate of the surface of the test specimen was determined by the above-mentioned method. The obtained corrosion rate (mm / year) for the test specimen of each test number is shown in Table 3B.

[0114] [Evaluation Results] With reference to Tables 1A, 1B, 2, 3A, and 3B, the seamless steel pipes of test numbers 1 to 25 had appropriate chemical compositions, yield strengths of 758 to less than 965 MPa, and standard deviations of the prior γ grain grain size numbers of 0.80 or less. As a result, these seamless steel pipes had 10 or fewer pits, and were judged to have excellent pitting corrosion resistance even in a high-HS environment.

[0115] The seamless steel pipes of test numbers 1, 3 to 5, 7, 8, 10, 13 to 16, 18, and 20 to 25 further satisfied the Si content requirement of more than 0.50% to 1.50%. As a result, it was determined that these seamless steel pipes had a corrosion rate of 0.40 mm / year or less and had excellent general corrosion resistance even in a high-HS environment.

[0116] On the other hand, the seamless steel pipe of test number 26 had an excessively low Si content, and as a result, the number of pitting corrosion in this seamless steel pipe exceeded 10, and it was determined that this seamless steel pipe did not have excellent pitting corrosion resistance in a high H2S environment.

[0117] The seamless steel pipe of test number 27 had an excessively low Cr content, resulting in more than 10 pits, and was therefore judged not to have excellent pitting corrosion resistance in a high H2S environment.

[0118] The seamless steel pipe of test number 28 had an excessively low Mo content, resulting in more than 10 pits, and was therefore judged not to have excellent pitting corrosion resistance in a high H2S environment.

[0119] The seamless steel pipe of test number 29 had a holding time of too short a time after piercing and drawing. As a result, the standard deviation of the prior γ grain size number of this seamless steel pipe exceeded 0.80. As a result, the number of pits in this seamless steel pipe exceeded 10, and it was determined that this seamless steel pipe did not have excellent pitting corrosion resistance in a high H2S environment.

[0120] The seamless steel pipe of test number 30 had a piercing-drawing holding time that was too long. As a result, the standard deviation of the prior γ grain size number of this seamless steel pipe exceeded 0.80. As a result, the number of pits in this seamless steel pipe exceeded 10, and it was determined that this seamless steel pipe did not have excellent pitting corrosion resistance in a high-HS environment.

[0121] The seamless steel pipe of test number 31 had an LMP of more than 32,000 when heated before hot working. As a result, the standard deviation of the prior γ grain size number of this seamless steel pipe exceeded 0.80. As a result, the number of pits in this seamless steel pipe exceeded 10, and it was determined that this seamless steel pipe did not have excellent pitting corrosion resistance in a high H2S environment.

[0122] The seamless steel pipe of test number 32 had an LMP of less than 30,000 when heated before hot working. As a result, the standard deviation of the prior γ grain size number of this seamless steel pipe exceeded 0.80. As a result, the number of pits in this seamless steel pipe exceeded 10, and it was determined that this seamless steel pipe did not have excellent pitting corrosion resistance in a high H2S environment.

[0123] 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: 0.20 to 0.35%, Si: 0.10 to 1.50%, Mn: 0.05 to 0.55%, P: 0.050% or less, S: 0.0100% or less, Cr: 0.20 to 1.00%, Mo: 0.20 to 1.50%, Ti: 0.003 to 0.030%, Al: 0.010 to 0.100%, N: 0.0100% or less, O: 0.0050% or less, Sb: 0 to 0.50%, Cu: 0 to 0.50%, Ni: 0 to 0.50%, Co: 0 to 0.50%, Zr: 0 to 0.0040%, Nb: 0 to 0.150%, A steel material comprising: V: 0 to 0.500%, B: 0 to 0.0030%, Ca: 0 to 0.0040%, Mg: 0 to 0.0040%, rare earth elements: 0 to 0.0040%, and the balance being Fe and impurities; having a yield strength of 758 to less than 965 MPa; and having a microstructure in which the standard deviation of the grain size number of prior austenite grains is 0.80 or less.

2. The steel material according to claim 1, containing one or more elements selected from the group consisting of Sb: 0.01 to 0.50%, Cu: 0.01 to 0.50%, Ni: 0.01 to 0.50%, Co: 0.01 to 0.50%, Zr: 0.0001 to 0.0040%, Nb: 0.001 to 0.150%, V: 0.001 to 0.500%, B: 0.0001 to 0.0030%, Ca: 0.0001 to 0.0040%, Mg: 0.0001 to 0.0040%, and rare earth elements: 0.0001 to 0.0040%.

3. The steel material according to claim 1 or 2, containing Si: more than 0.50 to 1.50%.

4. A steel material according to any one of claims 1 to 3, wherein the steel material is a seamless steel pipe.

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