steel

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

Technical Field

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

Background Art

[0002] With the deepening of wellbores in oil wells and gas wells (hereinafter, oil wells and gas wells are collectively referred to simply as "oil wells"), higher strength of steel materials for oil wells, typified by steel pipes for oil wells, is required. Specifically, steel materials for oil wells of 80 ksi grade (yield strength is 80 to less than 95 ksi, that is, 552 to less than 655 MPa) and 95 ksi grade (yield strength is 95 to less than 110 ksi, that is, 655 to less than 758 MPa) are widely used, and recently, steel materials for oil wells with a yield strength of 110 ksi or more (758 MPa or more) have begun to be required.

[0003] In oil wells, furthermore, corrosive hydrogen sulfide (H2S) gas may be contained in the environment. Therefore, steel materials assumed to be used as steel materials for oil wells are required not only to have high strength but also excellent corrosion resistance. In addition, stress may be applied to the steel materials for oil wells during use. Therefore, sulfide stress cracking resistance (Sulfide Stress Cracking resistance: hereinafter referred to as SSC resistance) has been used as an index of excellent corrosion resistance of steel materials for oil wells.

[0004] Techniques for increasing the strength and SSC resistance of steel materials have been proposed in JP-A-2006-28612 (Patent Document 1) and JP-A-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 Initiative] [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 environments containing H2S gas at high pressures of 15 atm (hereinafter, environments containing H2S gas at 15 atm will be referred to as "high H2S environments"), the corrosion pattern differs from that in environments containing H2S gas at normal pressure. Therefore, there is a growing demand for steel materials that have high corrosion resistance even in such high H2S environments.

[0009] As mentioned above, SSC resistance has been used as an indicator of superior corrosion resistance in steel materials for oil wells. Specifically, Patent Documents 1 and 2 disclose technologies for improving the SSC resistance of steel materials. On the other hand, in high H2S environments, pitting corrosion and crevice corrosion can be the starting point for stress corrosion cracking. Therefore, steel materials for oil wells intended for use in high H2S environments are also required to have corrosion resistance to pitting corrosion and / or crevice corrosion (hereinafter referred to as "pitting corrosion resistance"). However, there has been little study on the pitting corrosion resistance of steel materials for oil wells in high H2S environments.

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

[0011] The steel materials disclosed herein are In mass percent, C: 0.20~0.35%, Si: 0.10~1.50%, Mn: 0.05~0.55%, P: 0.050% or less, S: 0.0100% or less, Cr: 0.20~1.00%, Mo: 0.20~1.50%, Ti: 0.003~0.030%, Al: 0.010~0.100%, N: less than 0.0100%, O: less than 0.0050%, 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%, 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 consists of Fe and impurities, the yield strength is less than 758 to 965 MPa, In the microstructure, the standard deviation of the crystal grain size number of the prior austenite grains is 0.80 or less.

Advantages of the Invention

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

Brief Description of the Drawings

[0013] [Figure 1] Figure 1 is a diagram showing the relationship between the standard deviation σ of the crystal grain size number of the prior austenite grains and the number of pitting corrosion (pieces), which is an index of pitting corrosion resistance, in the present embodiment.

Modes for Carrying Out the Invention

[0014] First, the inventors considered obtaining a steel material with a yield strength of less than 110-140 ksi (less than 758-965 MPa) as high strength. Next, the inventors investigated a steel material with a yield strength of less than 110-140 ksi and excellent pitting corrosion resistance in a high H2S environment from the perspective of chemical composition. As a result, in mass%, C: 0.20~0.35%, Si: 0.10~1.50%, Mn: 0.05~0.55%, P: 0.050% or less, S: 0.0100% or less, Cr: 0.20~1.00%, Mo: 0.20~1.50%, Ti: 0.003~0.030%, Al: 0.010~0.100%, N: 0.0100% or less, O: 0.0050% or less, Sb: 0~0.50%, Cu: 0~0.50%, Ni: 0~ We considered that a steel material consisting of 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 remainder being Fe and impurities could potentially achieve both a yield strength of less than 110-140 ksi and excellent pitting corrosion resistance in high H2S environments.

[0015] Furthermore, the inventors focused on the microstructure of steel materials and conducted detailed studies on methods to improve pitting corrosion resistance in high H2S environments. As a result of these detailed studies, it became clear that for steel materials having the above-mentioned chemical composition and a yield strength of less than 110-140 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 H2S environments. This point will be explained in detail with reference to the drawings.

[0016] Figure 1 shows the relationship between the standard deviation σ of the grain size number of prior austenite grains and the number of pitted grains, which is an indicator of pitting resistance, in this embodiment. In this specification, prior austenite grains are also referred to as "prior γ grains." Figure 1 was created using the value of the standard deviation σ of the grain size number of prior γ grains and the number of pitted grains, which is an indicator of pitting resistance, for an embodiment among the embodiments described later, in which all components except the standard deviation σ of the grain size number of prior γ grains satisfy the conditions of this embodiment.

[0017] Referring to Figure 1, in a steel material having the above-described chemical composition and a yield strength of less than 110-140 ksi, if the standard deviation σ of the grain size number of the prior γ grains is 0.80 or less, the number of pitted corrosion sites, which is an indicator of pitting corrosion resistance, will be 10 or less, confirming that it has excellent pitting corrosion resistance. Therefore, the steel material according to this embodiment has the above-described chemical composition, a yield strength of less than 110-140 ksi, and furthermore, the standard deviation σ of the grain size number of the prior γ grains 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 H2S environment.

[0018] In steel materials having the above-mentioned chemical composition and a yield strength of less than 110-140 ksi, the reason why they exhibit excellent pitting corrosion resistance even in high H2S environments if the standard deviation σ of the grain size number of prior γ grains is 0.80 or less remains unclear. However, the inventors of this invention speculate as follows.

[0019] First, a high H2S environment has a higher hydrogen sulfide concentration compared to an atmospheric pressure H2S environment, making it easier for a corrosion film to form on the surface of steel. Furthermore, the corrosion film formed on the surface of steel has a protective effect. Therefore, in a high H2S environment, it may be possible to effectively improve the pitting corrosion resistance of steel by utilizing the corrosion film. Here, the grain boundaries of steel have a mismatch in atomic arrangement compared to within the grains, and the corrosion rate tends to be faster. Therefore, if a corrosion film can be formed rapidly and uniformly on the surface of steel, it may be possible to improve pitting corrosion resistance in a high H2S environment. On the other hand, if the prior γ grain boundaries, which are the grain boundaries at the time of heating to the austenite region, are uniformly distributed in the steel, the supply of metal ions for film formation may be uniform. In other words, if the prior γ grain boundaries are uniformly distributed in the steel, a uniform corrosion film can be formed rapidly, potentially improving the pitting corrosion resistance of steel.

[0020] Here, if the standard deviation σ of the grain size number of the prior γ grains is large, regions with uneven distribution of coarse prior γ grains (coarse grains) and regions with uneven distribution of fine prior γ grains (fine grains) will occur in the steel material. In this case, the supply of metal ions for film formation will be uneven, and the corrosion film may not be formed quickly and uniformly. As a result, there is a concern that a localized decrease in pitting corrosion resistance will become apparent, especially in regions where coarse grains are unevenly distributed. On the other hand, if 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] Based on the above mechanism, the inventors surmise that if the standard deviation σ of the grain size number of prior γ grains is 0.80 or less in a steel material having the above-mentioned chemical composition and yield strength, then its pitting corrosion resistance in a high H2S environment may be enhanced. It should be noted that there may be other mechanisms that result in excellent pitting corrosion resistance in a high H2S environment. However, the examples described below demonstrate that if the standard deviation σ of the grain size number of prior γ grains is 0.80 or less in a steel material having the above-mentioned chemical composition and yield strength, then it exhibits excellent pitting corrosion resistance even in a high H2S environment.

[0022] Based on the above findings, the gist of the steel material according to this embodiment is as follows:

[0023] [1] In mass percent, C: 0.20~0.35%, Si: 0.10~1.50%, Mn: 0.05~0.55%, P: 0.050% or less, S: 0.0100% or less, Cr: 0.20~1.00%, Mo: 0.20~1.50%, Ti: 0.003~0.030%, Al: 0.010~0.100%, N: 0.0100% or less, O: 0.0050% or less, Sb: 0~0.50%, Cu: 0~0.50%, Ni: 0~0.50%, 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 remainder consists of Fe and impurities. The yield strength is less than 758-965 MPa. In the microstructure, the standard deviation of the grain size number of prior austenite grains is 0.80 or less. Steel material.

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

[0025] [3] The steel material described in [1] or [2], Contains Si: over 0.50% to 1.50%. Steel material.

[0026] [4] Steel materials described in any one of items [1] to [3], The aforementioned steel material is a seamless steel pipe. Steel material.

[0027] The shape of the steel material in this embodiment is not particularly limited. The steel material in 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. A 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. Unless otherwise specified, "%" for elements refers to mass percentage.

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

[0030] C: 0.20~0.35% 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, there will be too many carbides, 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: 0.10~1.50% Silicon (Si) deoxidizes steel. Si further promotes the formation of a corrosion film on the surface of the steel material. As a result, the pitting corrosion resistance of the steel material in a high H2S environment is increased. 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 pitting corrosion resistance of the steel material decreases even if the content of other elements is within the range of this embodiment. Therefore, the Si content is 0.10 to 1.50%. The preferred lower limit of the Si content is 0.12%, more preferably 0.14%, and still more preferably 0.18%. The preferred upper limit of the Si content is 1.48%, more preferably 1.46%, and still more preferably 1.40%.

[0032] Preferably, the lower limit of the Si content is greater than 0.50%. That is, the Si content is preferably greater than 0.50% to 1.50%. Steel materials that satisfy the Si content of greater than 0.50% to 1.50% while also satisfying the other elemental content requirements of this embodiment have high strength and, in a high H2S environment, possess not only excellent pitting corrosion resistance but also excellent overall corrosion resistance. Therefore, it is preferable that the Si content be greater than 0.50% to 1.50%. A further preferred lower limit for the Si content is 0.51%, even more preferably 0.53%, even more preferably 0.55%, and even more preferably 0.60%.

[0033] Mn: 0.05~0.55% Manganese (Mn) deoxidizes steel. Mn also improves the hardenability and strength of the 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, excessive Mn sulfide may be generated. Here, Mn sulfide becomes the starting point for pitting corrosion. Therefore, even if the content of other elements is within the range of this embodiment, excessive Mn sulfide may be generated, and the pitting corrosion resistance of the steel in a high H2S environment may decrease. Accordingly, 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 still 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 content of other elements is within the range of this embodiment, P will segregate at the grain boundaries, reducing the corrosion resistance of the steel in a high H2S environment. Therefore, the P content is 0.050% or less. The preferred upper limit of the P content is 0.048%, more preferably 0.045%, and still more preferably 0.040%. It is preferable to have as low a P content as possible. However, an extreme reduction in the 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: 0.0100% 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 content of other elements is within the range of this embodiment, coarse sulfides will be formed, reducing the pitting corrosion resistance of the steel in a high H2S environment. Therefore, the S content is 0.0100% or less. The preferred upper limit of the S content is 0.0095%, more preferably 0.0080%, more preferably 0.0060%, and still more preferably 0.0040%. 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 0.0003%, and still more preferably 0.0005%.

[0036] Cr: 0.20~1.00% Chromium (Cr) promotes the formation of a corrosion film on the surface of steel. As a result, the pitting corrosion resistance of steel in high H2S environments is enhanced. If the Cr 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 Cr content is too high, even if the content of other elements is within the range of this embodiment, coarse precipitates will be formed, and the pitting corrosion resistance of steel will decrease. 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%, still more preferably 0.25%, and still more preferably 0.30%. The preferred upper limit of the Cr content is 0.98%, more preferably 0.96%, and still more preferably 0.95%.

[0037] Mo: 0.20~1.50% Molybdenum (Mo) contributes to stabilizing the corrosion film on the surface of steel materials. As a result, the pitting corrosion resistance of steel materials in high H2S environments is enhanced. 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, the above effect becomes saturated. 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: 0.003~0.030% Titanium (Ti) combines with nitrogen to form nitrides, and the pinning effect refines the crystal grains of the steel. As a result, the pitting corrosion resistance of the steel is improved. 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, the Ti nitrides become coarser, and the pitting corrosion resistance of the steel decreases. 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: 0.010~0.100% 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, and the pitting corrosion resistance of the steel will decrease. Therefore, the Al content is 0.010 to 0.100%. The preferred lower limit of the Al content is 0.015%, more preferably 0.020%, and even more preferably 0.025%. The preferred upper limit of the Al content is 0.080%, more preferably 0.070%, and even more preferably 0.060%. As used herein, "Al" content refers to "acid-soluble Al," that is, the content of "sol.Al."

[0040] N: 0.0100% or less Nitrogen (N) is inevitably present. That is, the lower limit of the N content is greater than 0%. N combines with Ti to form nitrides, and the pinning effect refines the crystal grains of the steel. As a result, the pitting corrosion resistance of the steel is increased. 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 pitting corrosion resistance of the steel will actually decrease. Therefore, the N content is 0.0100% or less. The preferred upper limit of the N content is 0.0080%, more preferably 0.0060%, and even more preferably 0.0050%. The preferred lower limit of the N content to more effectively obtain the above effect 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 content of other elements is within the range of this embodiment, coarse oxides will be formed, and the pitting corrosion resistance of the steel will decrease. Therefore, the O content is 0.0050% or less. The preferred upper limit of the O content is 0.0040%, more preferably 0.0030%, and even more preferably 0.0020%. It is preferable to have as low an O content as possible. However, an extreme reduction in the O content will significantly increase manufacturing costs. Therefore, considering industrial production, the preferred lower limit of the O content is 0.0001%, more preferably 0.0003%, and even more preferably 0.0005%.

[0042] 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.

[0043] [Optional element] The chemical composition of the steel described above may further contain, in place of some of the Fe, one or more elements selected from the group consisting of Sb, Cu, Ni, Co, and Zr. All of these elements suppress the penetration of hydrogen into the steel, thereby improving the steel's resistance to SSC in high H2S environments.

[0044] Sb: 0~0.50% Antimony (Sb) is an optional element and may not be present. That is, the Sb content may be 0%. When present, Sb suppresses the penetration of hydrogen into the steel material in a high H2S environment. As a result, the SSC resistance of the steel material in a high H2S environment is increased. Even if only a small amount of Sb is present, the above effect can be obtained to some extent. However, if the Sb content is too high, the hot workability of the steel material will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the Sb content is 0 to 0.50%. The preferred lower limit of the Sb content is greater than 0%, more preferably 0.01%, more preferably 0.02%, more preferably 0.03%, and more preferably 0.05%. The preferred upper limit of the Sb content is 0.45%, and more preferably 0.40%.

[0045] 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 suppresses the penetration of hydrogen into the steel material in a high H2S environment. As a result, the steel material's resistance to SSC in a high H2S environment is increased. Even if only a small amount of Cu is included, the above effect can be obtained to some extent. 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 material will become too high and the toughness of the steel material 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%, more preferably 0.02%, and still more preferably 0.05%. The preferred upper limit of the Cu content is 0.48%, more preferably 0.45%, and still more preferably 0.40%.

[0046] Ni: 0~0.50% Nickel (Ni) is an optional element and may not be present. That is, the Ni content may be 0%. If present, Ni forms a corrosion film in a high H2S environment, suppressing hydrogen penetration into the steel. As a result, the steel's resistance to SSC in a high H2S environment is increased. Even a small amount of Ni can provide the above effect 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, reducing the corrosion resistance of the steel. 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 0.05%. The preferred upper limit of the Ni content is 0.48%, more preferably 0.45%, and still more preferably 0.40%.

[0047] Co: 0~0.50% Cobalt (Co) is an optional element and may not be present. In other words, the Co content may be 0%. When present, Co forms a corrosion film in a high H2S environment, suppressing hydrogen penetration into the steel. As a result, the steel's resistance to SSC in a high H2S environment is enhanced. Even a small amount of Co can provide some of the above effect. However, if the Co content is too high, even if the content of other elements is within the range of this embodiment, the hardenability of the steel will decrease, and the strength of the steel will decline. Therefore, the Co content is 0 to 0.50%. The preferred lower limit of the Co content is greater than 0%, more preferably 0.01%, more preferably 0.02%, more preferably 0.03%, and more preferably 0.05%. The preferred upper limit of the Co content is 0.48%, more preferably 0.45%, and more preferably 0.40%.

[0048] Zr: 0~0.0040% Zirconium (Zr) is an optional element and may not be included. That is, the Zr content may be 0%. If included, Zr stabilizes the corrosion film in a high H2S environment and suppresses the penetration of hydrogen into the steel. As a result, the SSC resistance of the steel in a high H2S environment is increased. Even if only a small amount of Zr is included, the above effect can be obtained to some extent. 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, and the corrosion resistance of the steel will decrease. 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%, more preferably 0.0003%, more preferably 0.0006%, more preferably 0.0010%, and more preferably 0.0015%. The preferred upper limit for the Zr content is 0.0038%, more preferably 0.0035%, and even more preferably 0.0030%.

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

[0050] Nb: 0~0.150% Niobium (Nb) is an optional element and may not be present. In other words, the Nb content may be 0%. When present, Nb combines with C or N to form carbides, nitrides, or carbonitrides (hereinafter also referred to as "carbonitrides, etc."), and refines the crystal grains of the steel material through a pinning effect. As a result, the strength of the steel material increases. In this case, the corrosion resistance of the steel material also increases. Even if only a small amount of Nb is present, the above effects can be obtained to some extent. However, if the Nb content is too high, even if the content of other elements is within the range of this embodiment, carbonitrides, etc. will be formed in excess, and the corrosion resistance of the steel material will decrease. Therefore, the Nb content is 0 to 0.150%. The preferred lower limit of the Nb content is greater than 0%, more preferably 0.001%, more preferably 0.003%, more preferably 0.005%, and more preferably 0.010%. The preferred upper limit for the Nb content is 0.100%, more preferably 0.080%, and even more preferably 0.060%.

[0051] V: 0~0.500% Vanadium (V) is an optional element and may not be present. That is, the V content may be 0%. When present, V forms carbonitrides, etc., and refines the crystal grains of the steel material through a pinning effect. As a result, the strength of the steel material increases. In this case, the corrosion resistance of the steel material also increases. Even if only a small amount of V is present, the above effects can be obtained to some extent. However, if the V content is too high, even if the content of other elements is within the range of this embodiment, carbonitrides, etc. will be formed in excess, and the corrosion resistance of the steel material will decrease. Therefore, the V content is 0 to 0.500%. The preferred lower limit of the V content is greater than 0%, more preferably 0.001%, more preferably 0.005%, more preferably 0.010%, more preferably 0.030%, and more preferably 0.050%. The preferred upper limit of the V content is 0.400%, more preferably 0.350%, and more preferably 0.300%.

[0052] B: 0~0.0030% Boron (B) is an optional element and may not be present. That is, the B content may be 0%. If present, B dissolves in the steel, improving its hardenability and increasing its strength. Even a small amount of B will provide some of the above effects. However, if the B content is too high, even if the content of other elements is within the range of this embodiment, coarse nitrides will form, 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 greater than 0%, more preferably 0.0001%, more preferably 0.0005%, and more preferably 0.0010%. The preferred upper limit of the B content is 0.0029%, and more preferably 0.0025%.

[0053] 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 Ca, Mg, and rare earth elements. All of these elements are arbitrary and neutralize the S in the steel material as sulfides. As a result, all of these elements enhance the corrosion resistance of the steel material.

[0054] Ca: 0~0.0040% Calcium (Ca) is an optional element and may not be present. That is, the Ca content may be 0%. When present, Ca detoxifies S in the steel as sulfides, improving the corrosion resistance of the steel. Even a small amount of Ca can provide the above effect to some extent. However, if the Ca 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, and the corrosion resistance of the steel will actually decrease. Therefore, the Ca content is 0 to 0.0040%. The preferred lower limit of the Ca content is greater than 0%, more preferably 0.0001%, more preferably 0.0003%, more preferably 0.0006%, and more preferably 0.0010%. The preferred upper limit of the Ca content is 0.0035%, and more preferably 0.0030%.

[0055] Mg: 0~0.0040% Magnesium (Mg) is an optional element and may not be present. That is, the Mg content may be 0%. When present, Mg neutralizes sulfur in the steel as sulfides, thereby improving the corrosion resistance of the steel. Even a small amount of Mg can provide the above effect to some extent. 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, and the corrosion resistance of the steel will actually decrease. 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%, more preferably 0.0003%, more preferably 0.0006%, and more preferably 0.0010%. The preferred upper limit of the Mg content is 0.0035%, and more preferably 0.0030%.

[0056] Rare earth elements: 0~0.0040% Rare earth elements (REMs) are optional elements 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 corrosion resistance of the steel. Even if only a small amount of REM is included, the above effect can be obtained to some extent even if the content of other elements is within the range of this embodiment. 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, and the corrosion resistance of the steel will actually decrease. 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%, more preferably 0.0003%, more preferably 0.0006%, and more preferably 0.0010%. The preferred upper limit of the REM content is 0.0035%, and 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 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.

[0058] As mentioned above, the Si content is preferably greater than 0.50% to 1.50%. In other words, the steel material according to this embodiment has the following composition in mass%, C: 0.20-0.35%, Si: greater than 0.50% to 1.50%, Mn: 0.05-0.55%, P: 0.050% or less, S: 0.0100% or less, Cr: 0.20-1.00%, Mo: 0.20-1.50%, Ti: 0.003-0.030%, Al: 0.010-0.100%, N: 0.0100% or less, O: 0.0050% or less, Sb: 0-0.50%, Cu: 0-0.50%, Ni: If the steel consists of 0-0.50% of the following elements: 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 remainder being Fe and impurities, it will have a yield strength of less than 110-140 ksi, excellent pitting corrosion resistance in high H2S environments, and excellent overall corrosion resistance in high H2S environments.

[0059] [Yield strength] The yield strength of the steel material according to this embodiment is less than 758 to 965 MPa (less than 110 to 140 ksi). In this specification, yield strength refers to the 0.2% offset proof strength 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. 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 or the arc-shaped 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.

[0061] The tensile test specimen is, for example, a round bar specimen with a parallel section diameter of 6.0 mm and a gauge length of 30.0 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. 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 0.2% offset proof strength obtained from 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.

[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 less than 758 to 965 MPa, and furthermore, the standard deviation σ of the grain size number of the prior austenite grains (prior γ grains) is 0.80 or less. As a result, even though the steel material according to this embodiment has a yield strength of less than 758 to 965 MPa, it exhibits excellent pitting corrosion resistance in high H2S environments.

[0063] Here, if the standard deviation σ of the grain size number of the prior γ grains is large, regions with uneven distribution of coarse prior γ grains (coarse grains) and regions with uneven distribution of fine prior γ grains (fine grains) will occur in the steel. As a result, in a high H2S environment, a localized decrease in pitting corrosion resistance may become apparent in the regions where coarse grains are unevenly distributed. Therefore, in the steel according to this embodiment, the standard deviation σ of the grain size number of the prior γ grains is set to 0.80 or less.

[0064] In this embodiment, the preferred upper limit of the standard deviation σ of the grain size number of the prior γ grains is 0.79, more preferably 0.78, and even more preferably 0.77. In the steel material according to this embodiment, a smaller standard deviation σ of the grain size number of the prior γ grains is preferable. That is, the lower limit of the standard deviation σ of the grain size number 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 higher. 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 higher. In this case, the corrosion resistance of the steel material can be obtained more stably. A further preferred lower limit for 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 for 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 the prior γ grains is determined by the following method. Specifically, a test specimen for microstructural observation is prepared from the steel material according to this embodiment. If the steel material is a steel plate, the test specimen is prepared from the center of the plate thickness. If the steel material is a steel pipe, the test specimen is prepared from the center of the wall thickness. If the steel material is a round bar, the test specimen is prepared from the R / 2 position. The size of the test specimen is not particularly limited, as long as an observation surface, as described later, can be obtained.

[0067] After polishing the observation surface of the prepared test specimen to a mirror finish, it is immersed for about 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 arbitrary fields of view are selected from the observation surface and observed with an optical microscope to generate photographic images. The magnification for microscopic observation can be appropriately set according to the crystal grain size. Specifically, for example, the magnification is set so that 50 or more crystal grains are included in the field of view.

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

[0069] [Pitting corrosion resistance] The steel material according to this embodiment has the above-described chemical composition, a yield strength of less than 758 to 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 H2S environment, even though it has a yield strength of less than 758 to 965 MPa. In this embodiment, having excellent pitting corrosion resistance in a high H2S environment is defined as follows.

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

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

[0072] After 720 hours, the surface of a 30 mm long and 30 mm wide specimen is observed to check for the presence or absence of pitting corrosion. Pitting corrosion is identified as having a depth of 50 μm or more and an equivalent circle diameter of 40 μm or more. Specifically, the surface of the aforementioned specimen is observed with a magnifying glass with a magnification of 10x 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, it is evaluated as having excellent pitting corrosion resistance even in a high H2S environment.

[0073] [Completely corrosion resistant] Preferably, the steel material according to this embodiment has the above-described chemical composition, a Si content of more than 0.50% to 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 with a yield strength of less than 758 to 965 MPa, the steel material according to this embodiment has excellent pitting corrosion resistance in high H2S environments, and furthermore, excellent overall corrosion resistance in high H2S environments. In this embodiment, having excellent overall corrosion resistance in high H2S environments 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 the pitting corrosion resistance test described above. The test specimen is sealed in an autoclave. A 5.0% by mass sodium chloride aqueous solution is poured into the autoclave. The test specimen is 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 kept in the test bath 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 and removal of corrosion scale is determined. The corrosion rate (mm / year) of the test specimen is determined by dividing the obtained 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, it is evaluated that the material has excellent overall corrosion resistance even in a high H2S environment.

[0075] [Microorganisms] 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 the steel material having the above chemical composition contains a total volume fraction of tempered martensite and tempered bainite of 90% or more, then, provided that the other components of this embodiment are satisfied, it will achieve both a yield strength of 758 to less than 965 MPa (less than 110 to less than 140 ksi) and excellent pitting corrosion resistance in a high H2S environment. In other words, in this embodiment, if the steel material has a yield strength of 758 to less than 965 MPa (less than 110 to less than 140 ksi) and excellent pitting corrosion resistance in a high H2S environment, then it is determined that the microstructure has a total volume fraction of tempered martensite and tempered bainite of 90% or more.

[0076] 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 an 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 with the observation surface being a plane that includes the rolling direction and the thickness direction, starting from the center of the plate thickness. If the steel material is a steel pipe, a test piece is prepared with the observation surface being a plane that includes the pipe axis direction and the pipe radial direction, starting from the center of the wall thickness. If the steel material is a round steel bar, a test piece is prepared with the R / 2 position in the center and the observation surface being a plane that includes the axial direction and the radial direction.

[0077] 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 as secondary electron images. 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.

[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 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 if it is a thick-walled seamless steel pipe with a wall thickness of 15 mm or more, it is possible to achieve both a yield strength of less than 110 to 140 ksi and excellent pitting corrosion resistance in a high H2S environment.

[0079] [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.

[0080] [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.

[0081] 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.

[0082] [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.

[0083] [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. Hot working is performed on the billet extracted from the heating furnace to produce the raw pipe (seamless steel pipe).

[0084] In this embodiment, it is preferable to heat the billet under the following conditions. Heating temperature T: 1150~1300℃ Holding time: 30~500 minutes Furthermore, the heating temperature T (°C) and the holding time t (minutes) satisfy the following equation (A). 30000≦(273+T)×(20+Log(t / 60))≦32000 (A)

[0085] Here, the billet heating temperature T refers to the temperature (°C) of the heat treatment furnace used to heat the billet. The holding time t in billet heating refers to the time (minutes) during 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 sufficiently, and the hot working equipment may be overloaded. Similarly, if the holding time t in billet heating is too short, the billet may not be heated sufficiently, and the hot working equipment may be overloaded. On the other hand, if the holding time t is too long, the effect of heating will saturate.

[0086] In this embodiment, it is also preferable to satisfy the above equation (A). Here, LMP is defined as (273 + T) × (20 + Log(t / 60)). If LMP is too low, the billet may not be heated sufficiently, and temperature unevenness may occur 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 number of the prior γ grains increases. On the other hand, if LMP is too high, Ostwald growth of pinned particles, such as Ti nitrides, is promoted, and unevenness may occur in the size and distribution of pinned particles. In this case, the variation in particle size of the prior γ grains increases, and the variation in the grain size number of the prior γ grains increases. As a result, the standard deviation σ of the grain size number 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 30000 to 32000.

[0087] Intermediate steel materials are manufactured by hot working on the material extracted from the heating furnace. The method of hot working is not particularly limited, but if the intermediate steel material is a raw tube, it is preferable to manufacture the raw tube by performing the Mannesmann process. In this case, a round billet is perforated and rolled using a perforating machine. 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 is further hot-rolled using a mandrel mill, reducer, sizing mill, etc., to make a raw tube.

[0088] When manufacturing a raw tube using the Mannesmann process, it is preferable that the time between drilling and rolling and the start of hot rolling (stretching) using a mandrel mill be 30 to 180 seconds. In this specification, the time between drilling and rolling and the start of stretching is also referred to as the "drilling-stretching holding time." Having a certain amount of drilling-stretching holding time allows austenite grains to grow during holding, and the size of the austenite grains tends to become more uniform. Therefore, the standard deviation σ of the grain size number of the prior γ grains can be reduced in the manufactured steel material. On the other hand, if the drilling-stretching holding time is too long, the temperature of the intermediate steel material may drop, and hot workability may not be obtained. In this case, the austenite grains may grow too much, and the standard deviation σ of the grain size number of the prior γ grains may actually increase. Therefore, in the hot working process according to this embodiment, it is preferable to set the drilling-stretching holding time to 30 to 180 seconds.

[0089] Intermediate steel materials manufactured by hot working may be air-cooled (as-rolled), or may be directly quenched after hot working without cooling to room temperature, or may be reheated after hot working before quenching. The quenching process will be described in detail below.

[0090] [Heat treatment process] In the quenching process, the prepared intermediate steel material is subjected to quenching. In this specification, "quenching" means rapidly cooling the intermediate steel material with an A3 point or higher. The preferred quenching temperature is 850 to 1000°C. If the quenching temperature is too high, the prior γ grains may coarseen, and corrosion resistance may not be obtained. Therefore, a quenching temperature of 850 to 1000°C is preferred.

[0091] 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 additional heating or reheating when quenching is performed after additional heating or reheating following hot working.

[0092] 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.

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

[0094] Therefore, as described above, in the steel manufacturing method according to this embodiment, the intermediate steel material is rapidly cooled during quenching. Specifically, in the quenching process, the average cooling rate in the range of 800 to 500°C for the surface temperature of the intermediate steel material (raw tube) during quenching is defined as 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).

[0095] Preferred cooling rate during quenching CR800-500 The temperature is 300°C / min or higher. A more preferable quenching cooling rate is CR. 800-500 The lower limit is 450°C / min, and more preferably 600°C / min. Cooling rate CR during quenching 800-500 There is no specific upper limit, but for example, it is 60,000°C / min.

[0096] Preferably, the raw tube is heated multiple times in the austenite region before quenching. In this case, the austenite grains before quenching are refined, thus increasing the pitting corrosion resistance of the steel. Heating in the austenite region may be repeated multiple times by performing quenching multiple times, or heating in the austenite region may be repeated multiple times by performing normalizing and quenching. Alternatively, quenching and tempering, which will be described later, may be combined and performed multiple times. That is, multiple quenching and tempering processes may be performed. In this case, the pitting corrosion resistance of the steel is further increased. The tempering process will be described in detail below.

[0097] [Tempering process] In the tempering process, the intermediate steel material that has undergone the above-described quenching is tempered. In this specification, "tempering" refers to the process of tempering the intermediate steel material after quenching. c1 This means reheating to a temperature below 1.5°C and holding it there. Here, the tempering temperature corresponds to the furnace temperature when heating and holding the intermediate steel material 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 removed from the heat treatment furnace.

[0098] The tempering temperature is adjusted as appropriate according to the chemical composition of the steel and the yield strength to be obtained. In other words, for an intermediate steel having the chemical composition of this embodiment, the tempering temperature is adjusted to adjust the yield strength of the steel to less than 758 to 965 MPa. In the tempering process according to this embodiment, the preferred tempering temperature is 660 to 740°C. In addition, in the tempering process according to this embodiment, the preferred tempering time is 20 to 180 minutes.

[0099] The steel material according to this embodiment can be manufactured by the manufacturing method described above. In the above-described manufacturing method, a method for manufacturing a seamless steel pipe was explained as an example. However, the steel material according to this embodiment may be a steel plate or other shape. Furthermore, 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. The conditions in the following examples are just one example of conditions adopted to confirm the feasibility and effect 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. [Examples]

[0100] 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 Sb, Cu, Ni, and Co content in test number 1 were 0%, rounded to the third decimal place. The Nb and V content in test number 1 were 0%, rounded to the fourth decimal place. Furthermore, the Zr, B, Ca, Mg, and REM content in test number 1 were 0%, rounded to the fifth decimal place.

[0101] [Table 1A]

[0102] [Table 1B]

[0103] Round billets were manufactured using the molten steel of each test number by continuous casting. Hot working was performed on each round billet of each test number by heating it in a heating furnace. The heating temperature T (°C), holding time t (minutes) during the heating before hot working, and LMP (=(273+T)×(20+Log(t / 60))) calculated from the above formula (A) are as shown in Table 2.

[0104] [Table 2]

[0105] Perforation rolling and stretch rolling were performed on heated round billets. The time from the end of perforation rolling to the start of stretch rolling (perforation-stretch holding time) is shown in Table 2. In Table 2, "A (Appropriate)" in the perforation-stretch holding time column means that the perforation-stretch holding time was between 30 and 180 seconds. In Table 2, "S (Short)" in the perforation-stretch holding time column means that the perforation-stretch holding time was less than 30 seconds. In Table 2, "L (Long)" in the perforation-stretch holding time column means that the perforation-stretch holding time exceeded 180 seconds.

[0106] The manufactured raw tubes were subjected to quenching and tempering. Specifically, each raw tube with a test number was quenched by holding it at the temperature (°C) specified in Table 2 for the time (minutes) specified in Table 2, followed by water cooling. The quenched raw tubes with each test number were then tempered by holding them at the temperature (°C) specified in Table 2 for the time (minutes) specified in Table 2. Through the above manufacturing process, seamless steel tubes with each test number were obtained.

[0107] [Evaluation Test] The seamless steel pipes of each test number after tempering, as described above, were subjected to the following tests: grain size number measurement test, tensile test, pitting corrosion resistance test, and overall corrosion resistance test.

[0108] [Grain size number measurement test] For each test number of seamless steel pipe, a grain size number measurement test was performed to determine the standard deviation σ of the grain size number of the prior γ grains. Specifically, the test specimens prepared by the method described above were observed under a microscope using the method described above. Image analysis was performed on the photographic images obtained from the microscope observation, and the grain size number was measured in accordance with ASTM E112 (2021). For each test number, the grain size numbers obtained from 10 fields of view 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 also shown in Table 3A.

[0109] [Table 3A]

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

[0111] [Table 3B]

[0112] [Pitting resistance test] A pitting corrosion resistance test was conducted on the seamless steel pipes for each test number to evaluate their resistance to pitting corrosion. Specifically, the test specimens prepared using the method described above were kept in an autoclave for 720 hours under the conditions described above. After 720 hours, the presence or absence of pitting corrosion was checked on the surface of the test specimens using the method described above. The number of pitting corrosion lesions was counted for each test specimen to obtain the number of pitting lesions. The obtained number of pitting corrosion lesions is shown in Table 3B.

[0113] [Overall corrosion resistance test] For each test number of seamless steel pipe, a total corrosion resistance test was conducted to evaluate its overall corrosion resistance. Specifically, the test specimens prepared using the method described above were kept in an autoclave for 720 hours under the conditions described above. After 720 hours, the corrosion rate was determined on the surface of the test specimens using the method described above. The obtained corrosion rates (mm / year) for each test specimen are shown in Table 3B.

[0114] [Evaluation Results] Referring to Tables 1A, 1B, 2, 3A, and 3B, the seamless steel pipes for test numbers 1 to 25 had an appropriate chemical composition, a yield strength of less than 758 to 965 MPa, and a standard deviation of the grain size number of the prior γ grains of 0.80 or less. As a result, these seamless steel pipes had 10 or fewer pitting lesions and were judged to have excellent pitting corrosion resistance even in a high H2S environment.

[0115] The seamless steel pipes for test numbers 1, 3-5, 7, 8, 10, 13-16, 18, and 20-25 also met the requirement of having a Si content of over 0.50% to 1.50%. As a result, these seamless steel pipes were judged to have excellent overall corrosion resistance even in high H2S environments, with a corrosion rate of 0.40 mm / year or less.

[0116] On the other hand, the seamless steel pipe in test number 26 had too low a Si content. As a result, this seamless steel pipe had more than 10 pitting corrosion sites, and was judged to lack excellent pitting corrosion resistance in a high H2S environment.

[0117] The seamless steel pipe in test number 27 had too low a chromium content. As a result, this seamless steel pipe had more than 10 pitting lesions and was judged to lack excellent pitting corrosion resistance in a high H2S environment.

[0118] The seamless steel pipe in test number 28 had too low a molybdenum (Mo) content. As a result, this seamless steel pipe had more than 10 pitting lesions and was judged to lack excellent pitting corrosion resistance in a high H2S environment.

[0119] The seamless steel pipe in test number 29 had an insufficient drilling-stretching holding time. As a result, the standard deviation of the grain size number of the prior γ grains in this seamless steel pipe exceeded 0.80. Consequently, this seamless steel pipe had more than 10 pitting lesions and was judged to lack excellent pitting corrosion resistance in a high H2S environment.

[0120] The seamless steel pipe in test number 30 had an excessively long puncture-stretch holding time. As a result, the standard deviation of the grain size number of the prior γ grains in this seamless steel pipe exceeded 0.80. Consequently, this seamless steel pipe had more than 10 pitting lesions and was judged to lack excellent pitting corrosion resistance in a high H2S environment.

[0121] In test number 31, the seamless steel pipe showed an LMP (Low Mass Factor) exceeding 32,000 during heating before hot working. As a result, the standard deviation of the grain size number of the prior γ grains in this seamless steel pipe exceeded 0.80. Consequently, this seamless steel pipe was found to have more than 10 pitting corrosion sites, and was judged to lack excellent pitting corrosion resistance in a high H2S environment.

[0122] In test number 32, the seamless steel pipe had an LMP of less than 30,000 during heating before hot working. As a result, the standard deviation of the grain size number of the prior γ grains in this seamless steel pipe exceeded 0.80. Consequently, this seamless steel pipe had more than 10 pitting lesions and was judged to lack excellent pitting corrosion resistance in a high H2S environment.

[0123] 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: 0.20-0.35%, Si: 0.10 to 1.50%, Mn: 0.05-0.55%, P: 0.050% or less, S: 0.0100% or less, Cr: 0.20-1.00%, Mo: 0.20-1.50%, Ti: 0.003 to 0.030%, Al: 0.010-0.100%, 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%, V: 0 to 0.500%, B: 0 to 0.0030%, Ca: 0-0.0040%, Mg: 0 to 0.0040%, Rare earth elements: 0-0.0040%, and The remainder consists of Fe and impurities. The yield strength is between 758 and less than 965 MPa. In microorganisms, The total volume percentage of tempered martensite and tempered bainite is 90% or more. The average grain size number of the old austenite grains is 8.0 or higher, and the standard deviation is 0.80 or lower. Steel material.

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

3. The steel material according to claim 1, Si: Contains more than 0.50% to 1.50% Steel material.

4. The steel material according to claim 2, Si: Contains more than 0.50% to 1.50% Steel material.

5. A steel material according to any one of claims 1 to 4, The aforementioned steel material is a seamless steel pipe. Steel material.

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