steel material

Abstract

Provided is a steel material having high strength and excellent pitting corrosion resistance in a high H2S environment. The steel material of the present disclosure has, 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, and the balance being Fe and impurities, and has a yield strength of 758 MPa or more and less than 965 MPa, and in the microstructure, the standard deviation of the grain size number of the prior austenite grains is 0.80 or less.

Description

Technical Field

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

[0002] Due to the increasing depth of oil and gas wells (hereinafter, oil and gas wells are simply referred to as "oil wells"), there is a demand for high-strength steel for oil wells, represented by oil well steel pipes. Specifically, 80ksi grade (yield strength of 80ksi or higher and less than 95ksi, i.e., 552MPa or higher and less than 655MPa) and 95ksi grade (yield strength of 95ksi or higher and less than 110ksi, i.e., 655MPa or higher and less than 758MPa) oil well steels have been widely used, and recently, there has been a further demand for oil well steels with a yield strength of 110ksi or higher (yield strength of 758MPa or higher).

[0003] Oil wells sometimes contain corrosive hydrogen sulfide (H2S) gas in the environment. Therefore, steel intended for use in oil wells requires not only high strength but also excellent corrosion resistance. Furthermore, oil well steel in use is sometimes subjected to stress. Therefore, resistance to sulfide stress cracking (hereinafter referred to as SSC resistance) is used as an indicator of excellent corrosion resistance for oil well steel.

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

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

[0006] The steel disclosed in Patent Document 2 is a billet for high-strength oil well steel pipes, which, by mass%, comprises: C: 0.20~0.45%, Si: 0.05~0.40%, Mn: 0.3~0.9%, P: less than 0.015%, S: less than 0.005%, 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): less than 0.0030%, with the balance being Fe and unavoidable impurities. The Rockwell hardness HRC of this steel also satisfies the formula (15.6×[%C]+29.2≤HRC<60.5×[%C]+31.1). Patent document 2 describes that, based on this steel, it is possible to obtain a steel pipe with a yield strength of 758 MPa or more and less than 862 MPa and excellent resistance to SSC.

[0007] Existing technical documents

[0008] Patent documents

[0009] Patent Document 1: Japanese Patent Application Publication No. 2006-28612

[0010] Patent Document 2: Japanese Patent Application Publication No. 2017-166060 Summary of the Invention

[0011] The problem the invention aims to solve

[0012] Therefore, in recent years, wells operating in more severe environments have also attracted attention. For example, wells have been developed for environments containing high-pressure hydrogen sulfide (H2S) gas. Specifically, the corrosion pattern in environments containing 15 atm of high-pressure H2S gas (hereinafter referred to as "high H2S environments") differs from that in environments containing H2S gas at normal pressure. Therefore, there is a need for steels that exhibit high corrosion resistance even in such high H2S environments.

[0013] As mentioned above, resistance to stress corrosion cracking (SSC) has been used as an indicator of excellent corrosion resistance in oil well steels to date. Specifically, patent documents 1 and 2 disclose techniques for improving the SSC resistance of steels. On the other hand, in high-H2S environments, pitting and crevice corrosion can also become the starting point for stress corrosion cracking. Therefore, for oil well steels intended for use in high-H2S environments, resistance to pitting and / or crevice corrosion (hereinafter referred to as "pitting resistance") is also required. However, there has been almost no research on the pitting resistance of oil well steels in high-H2S environments.

[0014] The purpose of this disclosure is to provide steel with high strength and excellent pitting resistance in high H2S environments.

[0015] Solution for solving the problem

[0016] The steel disclosed herein

[0017] In terms of mass %

[0018] C: 0.20~0.35%

[0019] Si: 0.10~1.50%

[0020] Mn: 0.05~0.55%,

[0021] P: below 0.050%

[0022] S: Below 0.0100%

[0023] Cr: 0.20~1.00%

[0024] Mo: 0.20~1.50%,

[0025] Ti: 0.003~0.030%

[0026] Al: 0.010~0.100%

[0027] N: below 0.0100%

[0028] O: Below 0.0050%

[0029] Sb: 0~0.50%,

[0030] Cu: 0~0.50%,

[0031] Ni: 0~0.50%

[0032] Co: 0~0.50%,

[0033] Zr: 0~0.0040%

[0034] Nb: 0~0.150%,

[0035] V: 0~0.500%

[0036] B: 0~0.0030%

[0037] Ca: 0~0.0040%

[0038] Mg: 0~0.0040%,

[0039] Rare earth elements: 0~0.0040%, and

[0040] The balance consists of Fe and impurities.

[0041] The yield strength of the steel is above 758 MPa and less than 965 MPa.

[0042] In the microstructure, the standard deviation of the grain size number of the original austenite grains is below 0.80.

[0043] The effects of the invention

[0044] The steel disclosed herein possesses high strength and excellent resistance to pitting corrosion in high H2S environments. Attached Figure Description

[0045] Figure 1 This is a graph showing the relationship between the standard deviation σ of the grain size number of the original austenite grains in this embodiment and the number of pits, which is an indicator of pitting resistance. Detailed Implementation

[0046] First, the inventors considered obtaining steel with a yield strength of 110 ksi or higher and less than 140 ksi (758 MPa or higher and less than 965 MPa), which is considered as high strength. Next, the inventors conducted research from the viewpoint of chemical composition on steel with a yield strength of 110 ksi or higher and less than 140 ksi and excellent pitting corrosion resistance in high H2S environments. The results indicate that for steel with the following composition (by mass%): C: 0.20~0.35%, Si: 0.10~1.50%, Mn: 0.05~0.55%, P: below 0.050%, S: below 0.0100%, Cr: 0.20~1.00%, Mo: 0.20~1.50%, Ti: 0.003~0.030%, Al: 0.010~0.100%, N: below 0.0100%, O: below 0.0050%, Sb: 0~0.50%, the following conditions are considered acceptable. With 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 balance being Fe and impurities, it is possible to achieve a yield strength of above 110 ksi and below 140 ksi, as well as excellent pitting corrosion resistance in high H2S environments.

[0047] Furthermore, the inventors focused on the microstructure of steel and conducted a detailed study on methods to improve pitting corrosion resistance in high H2S environments. The results of their detailed research indicate that, in steels with the aforementioned chemical composition and a yield strength of 110 ksi or higher and less than 140 ksi, the standard deviation σ of the grain size index of the original austenite grains in the microstructure affects the pitting corrosion resistance of the steel in high H2S environments. This is explained in detail using the accompanying drawings.

[0048] Figure 1 This is a graph showing the relationship between the standard deviation σ of the grain size number of the original austenite grains in this embodiment and the number of pits, which is an indicator of pitting corrosion resistance. It should be noted that in this specification, the original austenite grains are also referred to as "original γ grains". Figure 1 This is an embodiment described later in which the configuration other than the standard deviation σ of the original γ grain size number satisfies the conditions of this embodiment. It is made using the value of the standard deviation σ of the original γ grain size number and the number of pits (pieces) as an indicator of pitting resistance.

[0049] Reference Figure 1 In steels having the aforementioned chemical composition and a yield strength of 110 ksi or higher and less than 140 ksi, if the standard deviation σ of the original γ grain size number is 0.80 or lower, the number of pits, an indicator of pitting corrosion resistance, is 10 or lower, confirming excellent pitting corrosion resistance. Therefore, the steel of this embodiment has the aforementioned chemical composition, a yield strength of 110 ksi or higher and less than 140 ksi, and further, a standard deviation σ of the original γ grain size number is 0.80 or lower. As a result, the steel of this embodiment exhibits high strength and excellent pitting corrosion resistance even in high H2S environments.

[0050] In steels having the above-mentioned chemical composition and a yield strength of 110 ksi or more and less than 140 ksi, the reasons why they exhibit excellent pitting corrosion resistance even in high H2S environments are not yet clear. However, the inventors speculate as follows.

[0051] First, compared to atmospheric pressure H2S environments, high H2S environments have higher hydrogen sulfide concentrations, making it easier for a corrosion film to form on the surface of steel. This corrosion film then protects the steel surface. Therefore, in high H2S environments, utilizing this corrosion film can potentially improve the steel's resistance to pitting corrosion. Here, the atomic arrangement at the grain boundaries of the steel becomes mismatched compared to within the grains, leading to a faster corrosion rate. Therefore, if a corrosion film can be formed rapidly and uniformly on the steel surface, it is possible to improve pitting corrosion resistance in high H2S environments. On the other hand, if the original γ-grain boundaries (i.e., those at the point of heating to the austenite region) are uniformly distributed in the steel, the supply of metal ions for film formation can become more uniform. That is, if the original γ-grain boundaries are uniformly distributed in the steel, a uniform corrosion film can be formed rapidly, potentially improving the steel's resistance to pitting corrosion.

[0052] When the standard deviation σ of the original γ grain size number is large, regions of uneven coarse γ grains (coarse grains) and uneven fine γ grains (fine grains) are formed in the steel. In this case, the supply of metal ions for coating formation becomes uneven, and the corrosion coating may not form rapidly and uniformly. As a result, especially in regions with uneven coarse grains, the local reduction in pitting corrosion resistance may become significant. On the other hand, if the standard deviation σ of the original γ grain size number is 0.80 or less, the corrosion coating forms rapidly and uniformly, potentially suppressing pitting corrosion on the steel surface.

[0053] Based on the above mechanism, the inventors speculated that if the standard deviation σ of the original γ grain size number is less than or equal to 0.80 in steels with the above chemical composition and yield strength, the pitting corrosion resistance in high H2S environments might be improved. It should be noted that excellent pitting corrosion resistance in high H2S environments could also be achieved through mechanisms different from the inventors' speculation. However, in steels with the above chemical composition and yield strength, if the standard deviation σ of the original γ grain size number is less than or equal to 0.80, the examples described later demonstrate excellent pitting corrosion resistance even in high H2S environments.

[0054] The main points of the steel used in this embodiment, based on the above insights, are as follows. [1]

[0056] A type of steel, which is expressed as a percentage by mass.

[0057] C: 0.20~0.35%

[0058] Si: 0.10~1.50%

[0059] Mn: 0.05~0.55%,

[0060] P: below 0.050%

[0061] S: Below 0.0100%

[0062] Cr: 0.20~1.00%

[0063] Mo: 0.20~1.50%,

[0064] Ti: 0.003~0.030%

[0065] Al: 0.010~0.100%

[0066] N: below 0.0100%

[0067] O: Below 0.0050%

[0068] Sb: 0~0.50%,

[0069] Cu: 0~0.50%,

[0070] Ni: 0~0.50%

[0071] Co: 0~0.50%,

[0072] Zr: 0~0.0040%

[0073] Nb: 0~0.150%,

[0074] V: 0~0.500%

[0075] B: 0~0.0030%

[0076] Ca: 0~0.0040%

[0077] Mg: 0~0.0040%,

[0078] Rare earth elements: 0~0.0040%, and

[0079] The balance consists of Fe and impurities.

[0080] The yield strength of the steel is above 758 MPa and less than 965 MPa.

[0081] In the microstructure, the standard deviation of the grain size number of the original austenite grains is below 0.80. [2]

[0083] According to the steel described in [1], it contains selected free...

[0084] Sb: 0.01~0.50%

[0085] Cu: 0.01~0.50%,

[0086] Ni: 0.01~0.50%

[0087] Co: 0.01~0.50%,

[0088] Zr: 0.0001~0.0040%

[0089] Nb: 0.001~0.150%,

[0090] V: 0.001~0.500%

[0091] B: 0.0001~0.0030%

[0092] Ca: 0.0001~0.0040%

[0093] Mg: 0.0001~0.0040%, and

[0094] Rare earth elements: one or more elements in a group consisting of 0.0001 to 0.0040%. [3]

[0096] According to the steel described in [1] or [2], it contains

[0097] Si: More than 0.50% and less than 1.50%. [4]

[0099] According to any one of [1] to [3], the steel, wherein,

[0100] The steel is a seamless steel pipe.

[0101] It should be noted that the shape of the steel in this embodiment is not particularly limited. The steel in this embodiment can be a steel pipe, a round bar (solid material), or a steel plate. It should be noted that round bar refers to a bar with a circular cross-section perpendicular to the axial direction. In addition, the steel pipe can be a seamless steel pipe or a welded steel pipe.

[0102] The steel used in this embodiment will now be described in detail. It should be noted that "corrosion resistance" as used below refers to a broad range of resistance to general corrosion, including resistance to pitting corrosion and SSC corrosion. Unless otherwise specified, "%" related to elements refers to mass%.

[0103] [Chemical Composition]

[0104] The steel in this embodiment contains the following elements in its chemical composition.

[0105] C: 0.20~0.35%

[0106] Carbon (C) improves the hardenability and strength of steel. If the C content is too low, the above-mentioned effects cannot be fully achieved even if the contents of other elements are within the range of this embodiment. On the other hand, if the C content is too high, carbides become excessive even if the contents of other elements are within the range of this embodiment, and the corrosion resistance of the steel decreases. Therefore, the C content is 0.20~0.35%. 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%.

[0107] Si: 0.10~1.50%

[0108] Silicon (Si) deoxidizes steel. Si further promotes the formation of a corrosion coating on the steel surface. As a result, the pitting corrosion resistance of steel in high H2S environments is improved. If the Si content is too low, the above-mentioned effects cannot be fully achieved even if the contents of other elements are within the range 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 range of this embodiment. Therefore, the Si content is 0.10~1.50%. The preferred lower limit of the Si content is 0.12%, more preferably 0.14%, and even more preferably 0.18%. The preferred upper limit of the Si content is 1.48%, more preferably 1.46%, and even more preferably 1.40%.

[0109] The lower limit of the Si content is preferably greater than 0.50%. That is, the Si content is preferably greater than 0.50% and less than 1.50%. Based on satisfying the other element contents of this embodiment, steel with a Si content of greater than 0.50% and less than 1.50% exhibits high strength and, in high H2S environments, excellent resistance to both pitting and general corrosion. Therefore, the Si content is preferably set to greater than 0.50% and less than 1.50%. A further preferred lower limit for the Si content is 0.51%, more preferably 0.53%, more preferably 0.55%, and more preferably 0.60%.

[0110] Mn: 0.05~0.55%

[0111] Manganese (Mn) deoxidizes steel. Mn further 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 range of this embodiment. On the other hand, if the Mn content is too high, excessive Mn sulfides may sometimes be formed. Here, Mn sulfides become the starting point for pitting corrosion. Therefore, even if the contents of other elements are within the range of this embodiment, excessive Mn sulfides may be formed, and the pitting corrosion resistance of steel in a high H2S environment may decrease. Therefore, the Mn content is 0.05~0.55%. The preferred lower limit of the Mn content is 0.06%, more preferably 0.08%. The preferred upper limit of the Mn content is 0.50%, more preferably 0.45%, and more preferably 0.40%.

[0112] P: below 0.050%

[0113] Phosphorus (P) is an impurity. That is, the lower limit of P content exceeds 0%. If the P content is too high, even if the contents of other elements are 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 P content is 0.048%, more preferably 0.045%, and even more preferably 0.040%. The P content is preferably as low as possible. However, extreme reductions in P content would significantly increase manufacturing costs. Therefore, considering industrial production, the preferred lower limit of P content is 0.001%, more preferably 0.002%, and even more preferably 0.003%.

[0114] S: Below 0.0100%

[0115] Sulfur (S) is an impurity. That is, the lower limit of S content exceeds 0%. If the S content is too high, even if the contents of other elements are within the range of this embodiment, coarse sulfides will be formed, reducing the pitting corrosion resistance of steel in a high H2S environment. Therefore, the S content is 0.0100% or less. The preferred upper limit of S content is 0.0095%, more preferably 0.0080%, more preferably 0.0060%, and more preferably 0.0040%. The S content is preferably as low as possible. However, an extreme reduction in S content would significantly increase manufacturing costs. Therefore, considering industrial production, the preferred lower limit of S content is 0.0001%, more preferably 0.0003%, and more preferably 0.0005%.

[0116] Cr: 0.20~1.00%

[0117] Chromium (Cr) promotes the formation of a corrosion coating on the surface of steel. As a result, the pitting corrosion resistance of steel in high H2S environments is improved. If the Cr content is too low, the above-mentioned effect cannot be fully obtained even if the contents of other elements are within the range of this embodiment. On the other hand, if the Cr content is too high, coarse precipitates will form even if the contents of other elements are within the range of this embodiment, and the pitting corrosion resistance of the steel will decrease. Therefore, the Cr content is 0.20~1.00%. The preferred lower limit of the Cr content is 0.21%, more preferably 0.23%, more preferably 0.25%, and more preferably 0.30%. The preferred upper limit of the Cr content is 0.98%, more preferably 0.96%, and more preferably 0.95%.

[0118] Mo: 0.20~1.50%

[0119] Molybdenum (Mo) contributes to the stabilization of the corrosion coating on the steel surface. As a result, the pitting corrosion resistance of steel in high H2S environments is improved. If the Mo content is too low, the above-mentioned effects cannot be fully obtained even if the contents of other elements are within the range of this embodiment. On the other hand, if the Mo content is too high, the above-mentioned effects are saturated. Therefore, the Mo content is 0.20~1.50%. The preferred lower limit of the Mo content is 0.21%, more preferably 0.24%, and more preferably 0.30%. The preferred upper limit of the Mo content is 1.49%, more preferably 1.45%, more preferably 1.40%, and more preferably 1.30%.

[0120] Ti: 0.003~0.030%

[0121] Titanium (Ti) combines with nitrogen to form nitrides, which refine the grain size of the steel through a pinning effect. As a result, the steel's resistance to pitting corrosion is improved. If the Ti content is too low, the above-mentioned effect cannot be fully achieved even if the contents of other elements are within the range of this embodiment. On the other hand, if the Ti content is too high, the Ti nitrides will coarsen even if the contents of other elements are within the range of this embodiment, and the steel's resistance to pitting corrosion will decrease. Therefore, the Ti content is 0.003% to 0.030%. The preferred lower limit of the Ti content is 0.003%, more preferably 0.005%. The preferred upper limit of the Ti content is 0.028%, more preferably 0.025%.

[0122] Al: 0.010~0.100%

[0123] Aluminum (Al) deoxidizes steel. If the Al content is too low, the aforementioned effect cannot be fully achieved even if the contents of other elements are within the range of this embodiment. On the other hand, if the Al content is too high, coarse oxide inclusions are formed even if the contents of other elements are within the range of this embodiment, reducing the pitting corrosion resistance of the steel. 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%. The "Al" content mentioned in this specification refers to the content of "acid-soluble Al," i.e., "sol.Al."

[0124] N: below 0.0100%

[0125] Nitrogen (N) is unavoidably present. That is, the lower limit of the N content exceeds 0%. N combines with Ti to form nitrides, which, through a pinning effect, refine the grain size of the steel. As a result, the steel's resistance to pitting corrosion is improved. On the other hand, if the N content is too high, even if the contents of other elements are within the range of this embodiment, coarse nitrides will form, and the steel's resistance to pitting corrosion 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 for more effectively obtaining the above-mentioned effect is 0.0005%, more preferably 0.0010%, more preferably 0.0020%, and even more preferably 0.0025%.

[0126] O: Below 0.0050%

[0127] Oxygen (O) is an impurity. That is, the lower limit of O content exceeds 0%. If the O content is too high, even if the contents of other elements are within the range of this embodiment, coarse oxides will form, reducing the pitting corrosion resistance of the steel. Therefore, the O content is 0.0050% or less. The preferred upper limit of 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 O content would significantly increase manufacturing costs. Therefore, considering industrial production, the preferred lower limit of O content is 0.0001%, more preferably 0.0003%, and even more preferably 0.0005%.

[0128] The steel in this embodiment has a chemical composition balance of Fe and impurities. Here, impurities refer to substances that are permissible during the industrial manufacturing of steel, which may be introduced from the ore, waste, or manufacturing environment used as raw materials, and are within the range that do not adversely affect the steel of this embodiment.

[0129] [Any element]

[0130] The chemical composition of the aforementioned steel may also contain one or more elements selected from the group consisting of Sb, Cu, Ni, Co, and Zr to replace a portion of the Fe. These elements all inhibit the intrusion of hydrogen into the steel and improve the SSC resistance of the steel in high H2S environments.

[0131] Sb: 0~0.50%

[0132] Antimony (Sb) can be any element, or it can be absent. That is, the Sb content can be 0%. When present, Sb inhibits the intrusion of hydrogen into the steel in a high H2S environment. As a result, the SSC resistance of the steel in a high H2S environment is improved. The above-mentioned effect is achieved to some extent as long as a small amount of Sb is present. However, if the Sb content is too high, the hot workability of the steel decreases even if the contents of other elements are within the range 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%, more preferably 0.02%, more preferably 0.03%, and more preferably 0.05%. The preferred upper limit of the Sb content is 0.45%, more preferably 0.40%.

[0133] Cu: 0~0.50%

[0134] Copper (Cu) can be any element, or it can be absent. That is, the Cu content can be 0%. When present, Cu inhibits the intrusion of hydrogen into the steel in a high H2S environment. As a result, the SSC resistance of the steel in a high H2S environment is improved. The above-mentioned effect is achieved to some extent with only a small amount of Cu. However, if the Cu content is too high, the hardenability of the steel will become too high, and the toughness of the steel will decrease, even if the contents of other elements are within the range of this embodiment. 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%, more preferably 0.02%, and more preferably 0.05%. The preferred upper limit of the Cu content is 0.48%, more preferably 0.45%, and more preferably 0.40%.

[0135] Ni: 0~0.50%

[0136] Nickel (Ni) is an arbitrary element and may be absent. That is, the Ni content can be 0%. When present, Ni forms a corrosion film in a high H2S environment, inhibiting the intrusion of hydrogen into the steel. As a result, the SSC resistance of the steel in a high H2S environment is improved. The above-mentioned effect is achieved to some extent as long as a small amount of Ni is present. However, if the Ni content is too high, even if the contents of other elements are within the range of this embodiment, it will promote localized corrosion and reduce the corrosion resistance of the steel. 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%, more preferably 0.02%, and more preferably 0.05%. The preferred upper limit of the Ni content is 0.48%, more preferably 0.45%, and more preferably 0.40%.

[0137] Co: 0~0.50%

[0138] Cobalt (Co) can be any element, or it can be absent. That is, the Co content can be 0%. When present, Co forms a corrosion coating in a high H2S environment, inhibiting the intrusion of hydrogen into the steel. As a result, the SSC resistance of the steel in a high H2S environment is improved. The above-mentioned effect is achieved to some extent as long as a small amount of Co is present. However, if the Co content is too high, the hardenability of the steel will decrease, and the strength of the steel will decrease, even if the contents of other elements are within the range of this embodiment. 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%, 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%.

[0139] Zr: 0~0.0040%

[0140] Zirconium (Zr) can be any element or may be absent. That is, the Zr content can be 0%. When present, Zr stabilizes the corrosion coating in a high H2S environment, inhibiting hydrogen intrusion into the steel. As a result, the SSC resistance of the steel in a high H2S environment is improved. The above-mentioned effect is achieved to some extent even with only a small amount of Zr. However, if the Zr content is too high, even if the contents of other elements are within the range of this embodiment, the oxides in the steel will become coarser, 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 more 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 of the Zr content is 0.0038%, more preferably 0.0035%, and more preferably 0.0030%.

[0141] The chemical composition of the aforementioned steel may also contain one or more elements selected from the group consisting of Nb, V, and B to replace a portion of the Fe. These elements all improve the strength of the steel.

[0142] Nb: 0~0.150%

[0143] Niobium (Nb) can be any element, or it may be absent. That is, the Nb content can be 0%. When present, Nb combines with C or N to form carbides, nitrides, or carbonitrides (hereinafter also referred to as "carbonitrides, etc."), which refine the grain size of the steel through a pinning effect. As a result, the strength of the steel increases. In this case, the corrosion resistance of the steel is further improved. The above-mentioned effects are achieved to some extent as long as a small amount of Nb is present. However, if the Nb content is too high, even if the contents of other elements are within the range of this embodiment, excessive carbonitrides, etc., will be formed, and the corrosion resistance of the steel will decrease. 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%, more preferably 0.003%, more preferably 0.005%, and more preferably 0.010%. The preferred upper limit of the Nb content is 0.100%, more preferably 0.080%, and more preferably 0.060%.

[0144] V: 0~0.500%

[0145] Vanadium (V) can be any element, or it can be absent. That is, the V content can be 0%. When present, V forms carbonitrides, etc., which refine the grain size of the steel through a pinning effect. As a result, the strength of the steel increases. In this case, the corrosion resistance of the steel is further improved. The above-mentioned effects are achieved to some extent even with only a small amount of V. However, if the V content is too high, carbonitrides, etc., will form excessively even if the contents of other elements are within the range of this embodiment, and the corrosion resistance of the steel will decrease. 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%, 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%.

[0146] B: 0~0.0030%

[0147] Boron (B) can be any element, or it can be absent. That is, the B content can be 0%. When present, B dissolves in steel, improving the hardenability and strength of the steel. Even a small amount of B is sufficient to achieve these effects to some extent. However, if the B content is too high, even if the contents of other elements are 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 over 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%, more preferably 0.0025%.

[0148] The chemical composition of the aforementioned steel may also contain one or more elements selected from the group consisting of Ca, Mg, and rare earth elements to replace a portion of the Fe. These elements are arbitrary and neutralize the S in the steel in the form of sulfides. As a result, these elements all improve the corrosion resistance of the steel.

[0149] Ca: 0~0.0040%

[0150] Calcium (Ca) can be any element, or it can be absent. That is, the Ca content can be 0%. When present, Ca neutralizes sulfur (S) in the steel as sulfides, improving the steel's corrosion resistance. Even a small amount of Ca achieves this effect to some extent. However, if the Ca content is too high, even if the contents of other elements are within the range of this embodiment, the oxides in the steel will become coarser, and the steel's corrosion resistance will actually decrease. Therefore, the Ca content is 0 to 0.0040%. The preferred lower limit of the Ca content is over 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%, more preferably 0.0030%.

[0151] Mg: 0~0.0040%

[0152] Magnesium (Mg) can be any element, or it can be absent. That is, the Mg content can be 0%. When present, Mg neutralizes sulfur (S) in the steel as sulfides, improving the steel's corrosion resistance. Even a small amount of Mg achieves this effect to some extent. However, if the Mg content is too high, even if the contents of other elements are within the range of this embodiment, the oxides in the steel will become coarser, and the steel's corrosion resistance will actually decrease. Therefore, the Mg content is 0 to 0.0040%. The preferred lower limit of the Mg content is over 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%, more preferably 0.0030%.

[0153] Rare earth elements: 0~0.0040%

[0154] Rare earth elements (REMs) can be any element, or they can be absent. That is, the REM content can be 0%. When present, REMs neutralize sulfur in the steel in the form of sulfides, improving the steel's corrosion resistance. Even with only a small amount of REM, the above-mentioned effects are achieved to some extent, even if the contents of other elements are within the range of this embodiment. However, if the REM content is too high, even if the contents of other elements are within the range of this embodiment, the oxides in the steel will become coarser, 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 more 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%, more preferably 0.0030%.

[0155] It should be noted that REM in this specification refers to one or more elements selected from the group consisting of scandium (Sc) atomic number 21, yttrium (Y) atomic number 39, and lanthanum (La) atomic number 57 to lutetium (Lu) atomic number 71, which are lanthanide elements. Furthermore, REM content in this specification refers to the total content of these elements.

[0156] As described above, the Si content is preferably greater than 0.50% and less than 1.50%. That is, the steel in this embodiment is the following steel with the following content (by mass%): C: 0.20~0.35%, Si: greater than 0.50% and less than 1.50%, Mn: 0.05~0.55%, P: less than 0.050%, S: less than 0.0100%, 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: 0~0.50%, Cu With the following composition: 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 balance being Fe and impurities, it possesses a yield strength of over 110 ksi and less than 140 ksi, excellent pitting corrosion resistance in high H2S environments, and excellent overall corrosion resistance in high H2S environments.

[0157] [Yield Strength]

[0158] The steel used in this embodiment has a yield strength of 758 MPa or more and less than 965 MPa (110 ksi or more and less than 140 ksi). The yield strength referred to in this specification is the 0.2% conditional yield strength obtained in a tensile test according to ASTM E8 / E8M (2022).

[0159] The yield strength of the steel in this embodiment is determined by the following method. Specifically, tensile test pieces are made from the steel of this embodiment. When the steel is a steel plate, a tensile test piece is made from the center of the plate thickness. In this case, the long side direction of the tensile test piece is parallel to the rolling direction of the steel plate. When the steel is a steel pipe, a tensile test piece is made from the center of the wall thickness. In this case, the long side direction of the tensile test piece or the arc-shaped test piece is parallel to the pipe axis direction. When the steel is a round bar, a tensile test piece is made from the R / 2 position. In this specification, the R / 2 position of the round bar refers to the center position of radius R in a section perpendicular to the axial direction of the round bar. In this case, the long side direction of the tensile test piece is parallel to the axial direction of the round bar.

[0160] 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. When the steel is a steel pipe, an arc-shaped specimen can be used as the tensile test specimen. In this case, the size of the arc-shaped specimen is, for example, the thickness equal to the total wall thickness, the width of 25.4 mm, and the gauge length of 50.8 mm. Using the prepared tensile test specimen, a tensile test according to ASTM E8 / E8M (2022) is performed at room temperature (25°C) in the atmosphere. The 0.2% conditional yield strength obtained through the tensile test is defined as the yield strength (MPa). It should be noted that in this embodiment, the yield strength (MPa) is calculated by rounding the first decimal place of the obtained value.

[0161] [Standard deviation σ for grain size designation]

[0162] The steel of this embodiment has the above-described chemical composition, a yield strength of 758 MPa or more and less than 965 MPa, and furthermore, a standard deviation σ of the grain size number of the original austenite grains (original γ grains) of 0.80 or less. As a result, the steel of this embodiment, even with a yield strength of 758 MPa or more and less than 965 MPa, exhibits excellent pitting corrosion resistance in high H2S environments.

[0163] Here, when the standard deviation σ of the original γ grain size number is large, regions with both coarse and fine original γ grains (coarse grains) are formed in the steel. As a result, in high H2S environments, the reduction in local pitting corrosion resistance may become more pronounced in regions with coarse grain inhomogeneity. Therefore, for the steel of this embodiment, the standard deviation σ of the original γ grain size number is 0.80 or less.

[0164] In this embodiment, the preferred upper limit of the standard deviation σ of the original γ grain grain size number is 0.79, more preferably 0.78, and even more preferably 0.77. In the steel of this embodiment, it is preferable that the standard deviation σ of the original γ grain grain size number is small. That is, the lower limit of the standard deviation σ of the original austenite grain size number can be 0.00, 0.05, 0.10, or 0.15.

[0165] It should be noted that in the steel of this embodiment, there is no particular limitation as long as the standard deviation σ of the original γ grain size number is 0.80 or less. In this embodiment, the original γ grain size number is, for example, 0.0 or more. The lower limit of the original γ grain size number can be 0.5. Preferably, the original γ grain size number is 5.0 or more. In this case, the corrosion resistance of the steel can be obtained more stably. A further preferred lower limit of the original γ grain size number is 6.0, further preferred is 7.0, further preferred is 7.5, and further preferred is 8.0. The upper limit of the original γ grain size number can be 13.0, 12.0, 11.0, or 10.0.

[0166] In the steel of this embodiment, the standard deviation σ of the original γ grain grain size number is determined by the following method. Specifically, test pieces for microstructural observation are made from the steel of this embodiment. When the steel is a steel plate, the test piece is made from the center of the plate thickness. When the steel is a steel pipe, the test piece is made from the center of the wall thickness. When the steel is a round bar, the test piece is made from the R / 2 position. It should be noted that the size of the test piece is not particularly limited, as long as the observation surface described later can be obtained.

[0167] After grinding the observation surface of the prepared test specimen to a mirror finish, it was immersed in a solution of a suitable amount of surfactant mixed with a saturated aqueous solution of picric acid for about 60 seconds, revealing the original γ grain boundaries through etching. Ten arbitrary fields of view were determined from the observation surface, and observations were performed using an optical microscope to generate photographic images. It should be noted that the magnification during microscopic observation can be appropriately set according to the grain diameter. Specifically, in microscopic observation, the magnification can be set, for example, to include more than 50 grains within the field of view.

[0168] In each field of view, image analysis was performed on the obtained photographic images, and grain size numbers were determined according to ASTM E112 (2021). That is, one grain size number was obtained for each field of view. The standard deviation of the ten obtained grain size numbers was calculated and defined as the standard deviation σ of the original austenite grain size number. It should be noted that the standard deviation σ of the original austenite grain size number is obtained by rounding the third decimal place of the obtained value.

[0169] [Resistance to pitting corrosion]

[0170] The steel of this embodiment has the above-described chemical composition, a yield strength of 758 MPa or more and less than 965 MPa, and a standard deviation σ of the grain size number of the original austenite grains (original γ grains) of 0.80 or less. As a result, the steel of this embodiment, even with a yield strength of 758 MPa or more and less than 965 MPa, exhibits excellent pitting corrosion resistance in high H2S environments. In this embodiment, excellent pitting corrosion resistance in high H2S environments is defined as follows.

[0171] A pitting corrosion resistance test was performed on the steel of this embodiment. Specifically, a test piece for a four-point bending test was made from the steel of this embodiment. When the steel is a sheet, the test piece was made from the center of the sheet thickness. When the steel is a pipe, the test piece was made from the center of the wall thickness. When the steel is a round bar, the test piece was made from the R / 2 position. The size of the test piece was, for example, 30 mm in length, 30 mm in width, and 3 mm in thickness.

[0172] Seal the test piece in an autoclave. Inject a 5.0% (w / w) sodium chloride aqueous solution into the autoclave. Note that the test piece is not immersed in the test solution (test bath) but remains in the gas phase region of the autoclave. Pressurize the autoclave by introducing 15 atm of H₂S gas to saturate the test solution, thus creating the test bath. After sealing the autoclave, maintain the test bath at 24°C and keep the test piece stirred for 720 hours.

[0173] The surface of the test piece, measuring 30 mm in length and 30 mm in width, was observed after 720 hours to confirm the presence or absence of pitting corrosion. Pitting corrosion specifically refers to pitting 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 was observed using a 10x magnifying glass to confirm the presence or absence of pitting corrosion. If pitting corrosion was confirmed, the number of pits was counted. In this embodiment, the results of the pitting corrosion resistance test under the above conditions are as follows: when the number of pits is 10 or less, it is evaluated as having excellent pitting corrosion resistance even in high H2S environments.

[0174] [Resistant to overall corrosion]

[0175] The preferred steel of this embodiment has the above-described chemical composition, with a Si content exceeding 0.50% and below 1.50%, a yield strength of 758 MPa or more and less than 965 MPa, and a standard deviation σ of the grain size number of the original austenite grains (original γ grains) of 0.80 or less. As a result, the steel of this embodiment, even with a yield strength of 758 MPa or more and less than 965 MPa, exhibits excellent resistance to pitting corrosion in high H2S environments, and consequently, excellent resistance to general corrosion in high H2S environments. In this embodiment, excellent resistance to general corrosion in high H2S environments is defined as follows.

[0176] A corrosion test was conducted on the steel used in this embodiment. Specifically, test pieces were prepared in the same manner as the pitting corrosion resistance test described above. The test pieces were sealed in an autoclave. A 5.0% (w / w) sodium chloride aqueous solution was injected into the autoclave. It should be noted that the test pieces were immersed in the test solution (test bath). Under the same conditions, after sealing the autoclave, the test bath was maintained at 24°C, and the test pieces were kept for 720 hours while stirring the test bath. The difference between the mass of the test piece before the test and the mass of the test piece after immersion for 720 hours to remove the oxide scale caused by corrosion was calculated. The corrosion rate (mm / year) of the test piece was calculated by dividing the obtained mass difference by the surface area, density, and immersion time of the test piece. In this embodiment, the corrosion test results under the above conditions are as follows: when the obtained corrosion rate is 0.40 mm / year or less, it is evaluated as having excellent resistance to general corrosion even in a high H2S environment.

[0177] [Microstructure]

[0178] For the microstructure of the steel in this embodiment, the total volume fraction of tempered martensite and tempered bainite is 90% or more. The balance of the microstructure is, for example, ferrite or pearlite. If the microstructure of the steel having the above chemical composition contains a total volume fraction of tempered martensite and tempered bainite of 90% or more, then, provided that other configurations of this embodiment are satisfied, a yield strength of 758 MPa or more and less than 965 MPa (110 ksi or more and less than 140 ksi) and excellent pitting corrosion resistance in a high H2S environment are also considered. That is, in this embodiment, if the steel has a yield strength of 758 MPa or more and less than 965 MPa (110 ksi or more and less than 140 ksi) and excellent pitting corrosion resistance in a high H2S environment, then it is determined that the total volume fraction of tempered martensite and tempered bainite in the microstructure is 90% or more.

[0179] It should be noted that the volume fractions of tempered martensite and tempered bainite can be obtained by observation using the following method. First, a test piece with an observation surface is prepared from the steel of this embodiment. When the steel is a sheet, a test piece with an observation surface including both the rolling direction and the thickness direction is prepared from the center of the sheet thickness. When the steel is a pipe, a test piece with an observation surface including both the pipe axis direction and the pipe diameter direction is prepared from the center of the wall thickness. When the steel is a round bar, a test piece with an observation surface including both the R / 2 position at the center and both the axial and radial directions is prepared.

[0180] After grinding the observation surface of the test piece to a mirror finish, it was immersed in a nitric acid-ethanol etching solution for approximately 10 seconds to perform etching-based tissue visualization. Using a scanning electron microscope (SEM), 10 fields of view were observed on the etched observation surface in secondary electron images. The field of view area was, for example, 0.01 mm². 2 (Magnification 1000x). Tempered martensite and tempered bainite are determined by contrast in each field of view. The area ratios of the determined tempered martensite and tempered bainite are calculated. The method for calculating the area ratios is not particularly limited and can be a known method. For example, the area ratios of tempered martensite and tempered bainite can be calculated through image analysis. In this embodiment, the arithmetic mean of the area ratios of tempered martensite and tempered bainite calculated in all fields of view is defined as the volume ratio of tempered martensite and tempered bainite.

[0181] [Shape of steel]

[0182] As described above, the shape of the steel used in this embodiment is not particularly limited. The steel can be, for example, steel pipe, steel plate, or round steel. When the steel is an oil well pipe, a wall thickness of 9 to 60 mm is preferred. More preferably, the steel used in this embodiment is a seamless steel pipe. When the steel used in 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 a yield strength of 110 ksi or more but less than 140 ksi, and excellent pitting corrosion resistance in high H2S environments.

[0183] [Manufacturing Method]

[0184] The manufacturing method of the steel according to this embodiment will be described. Hereinafter, as an example of the steel according to this embodiment, the manufacturing method of a seamless steel pipe will be described. The manufacturing method of a seamless steel pipe includes: a process of preparing a tube blank (preparation process); and a process of quenching and tempering the tube blank to produce a seamless steel pipe (quenching process and tempering process). It should be noted that the manufacturing method of this embodiment is not limited to the manufacturing method described below. Each process will be described in detail below.

[0185] [Preparation Process]

[0186] In the preparation process, intermediate steel with the above-mentioned chemical composition is prepared. As long as the intermediate steel has the above-mentioned chemical composition, the manufacturing method of the intermediate steel is not particularly limited. The intermediate steel referred to here is plate-shaped steel when the final product is steel plate, tube blank when the final product is steel pipe, and bar steel with a circular cross-section perpendicular to the axial direction when the final product is round steel.

[0187] The preparation process may include: the process of preparing the billet (bill preparation process); and the process of hot working the billet to manufacture intermediate steel (hot working process). The following details the case including the billet preparation process and the hot working process.

[0188] [Burnt material preparation process]

[0189] In the billet preparation process, billets are manufactured using molten steel with the aforementioned chemical composition. The method of billet manufacturing is not particularly limited and can be any known method. Specifically, molten steel can be used to manufacture cast billets (slabs, large billets, or small billets) through continuous casting. Molten steel can also be used to manufacture ingots through ingot casting. Small billets can also be manufactured by initial rolling slabs, large billets, or ingots as needed. Billets (slabs, large billets, or small billets) are manufactured through the above processes.

[0190] [Heat treatment process]

[0191] In the hot working process, the prepared billet is hot-worked to produce intermediate steel. In the case of seamless steel pipe, the intermediate steel is equivalent to a tube blank. First, a small square billet is heated in a furnace. The small square billet, removed from the furnace, is then hot-worked to produce a tube blank (seamless steel pipe).

[0192] In this embodiment, the conditions for heating the small square billet are preferably as described below.

[0193] Heating temperature T: 1150~1300℃

[0194] Holding time t: 30~500 minutes

[0195] Furthermore, the heating temperature T (°C) and the holding time t (minutes) satisfy the following equation (A).

[0196] 30000≤(273+T)×(20+Log(t / 60))≤32000 (A)

[0197] Here, the heating temperature T of the small square billet refers to the temperature (°C) of the heat treatment furnace when heating the small square billet. The holding time t during the heating of the small square billet refers to the time (minutes) spent holding it at the heating temperature T. If the heating temperature T is too high, the austenite grains may sometimes coarsen. On the other hand, if the heating temperature T is too low, the small square billet is underheated, sometimes placing an excessive load on the heat processing equipment. In addition, if the holding time t during the heating of the small square billet is too short, the small square billet is underheated, sometimes placing an excessive load on the heat processing equipment. On the other hand, even if the holding time t is too long, the heating effect will be saturated.

[0198] In this embodiment, it is further preferable to satisfy the above formula (A). Here, LMP is defined as LMP = (273 + T) × (20 + Log(t / 60)). If LMP is too low, the small billet will be underheated, and temperature inhomogeneity may sometimes occur in the billet during hot working. In this case, the deviation of the original γ grains increases, and the standard deviation σ of the grain size number of the original γ grains increases. On the other hand, if LMP is too high, Ostwald ripening of pinned particles, represented by Ti nitrides, is promoted, and inhomogeneity may sometimes occur in the size and distribution of the pinned particles. In this case, the deviation of the grain size in the original γ grains increases, and the deviation of the grain size number of the original γ grains increases. As a result, the standard deviation σ of the grain size number of the original γ grains increases. Therefore, in the hot working process of this embodiment, it is preferable to set the LMP when heating the small billet to 30000~32000.

[0199] The billet drawn from the heating furnace is hot-worked to produce intermediate steel. The hot-working method is not particularly limited; however, when the intermediate steel is a tube billet, the Mannesmann process is preferred. In this case, the round steel billet is pierced and rolled using a piercing mill. The piercing ratio is not particularly limited, for example, it is 1.0 to 4.0. The pierced and rolled round steel billet is then further hot-rolled using a mandrel-type seamless tube mill, a reducing mill, a sizing mill, etc., to produce a tube billet.

[0200] When manufacturing tube blanks using the Mannesmann process, it is preferable to set the time from piercing rolling to hot rolling (stretch rolling) using a mandrel-type seamless tube mill to 30 to 180 seconds. In this specification, the time from piercing rolling to the start of stretch rolling is also referred to as the "piercing-stretch holding time." With a certain piercing-stretch holding time, austenite grains grow during holding, tending towards uniformity in austenite grain size. Therefore, in the manufactured steel, the standard deviation σ of the original γ grain grain size can be reduced. On the other hand, if the piercing-stretch holding time is too long, the temperature of the intermediate steel decreases, sometimes resulting in poor hot workability. In this case, austenite grains further overgrow, and sometimes the standard deviation σ of the original γ grain grain size increases instead. Therefore, in the hot working process of this embodiment, it is preferable to set the piercing-stretch holding time to 30 to 180 seconds.

[0201] Intermediate steel manufactured through hot working can be air-cooled (As-Rolled), or it can be quenched directly after hot working without cooling to room temperature, or it can be quenched after reheating (reheating) following hot working. The quenching process is described in detail below.

[0202] [Quenching process]

[0203] In the quenching process, the prepared intermediate steel is quenched. In this specification, "quenching" refers to the rapid cooling of intermediate steel with an A3 point or higher. The preferred quenching temperature is 850~1000℃. If the quenching temperature is too high, the original γ grains will coarsen, and sometimes corrosion resistance may not be obtained. Therefore, the preferred quenching temperature is 850~1000℃.

[0204] In this specification, quenching temperature refers to the surface temperature of the intermediate steel, measured by a thermometer installed on the outlet side of the apparatus used for the final hot working, when quenching is performed directly after hot working. Furthermore, when quenching is performed after reheating or reheating following hot working, the quenching temperature is equivalent to the temperature of the furnace used for reheating or reheating.

[0205] Quenching methods include, for example, continuously cooling the intermediate steel (tube blank) from the quenching start temperature, causing the surface temperature of the tube blank to decrease continuously. There are no particular limitations on the continuous cooling method; any known method can be used. Examples of continuous cooling methods include immersing the tube blank in a water bath for cooling, or accelerating the cooling of the tube blank through water spraying or mist cooling.

[0206] If the cooling rate during quenching is too slow, the microstructure may not become a martensitic and bainitic matrix. In this case, the mechanical properties specified in this embodiment (yield strength of 110 ksi or more and less than 140 ksi) cannot be obtained.

[0207] Therefore, as described above, in the steel manufacturing method of this embodiment, the intermediate steel is rapidly cooled during quenching. Specifically, in the quenching process, the average cooling rate within the range of 800~500°C of the surface temperature of the intermediate steel (tube blank) during quenching is defined as the quenching cooling rate CR. 800-500 More specifically, the cooling rate CR during quenching 800-500 The temperature is determined based on the part of the intermediate steel that is cooled most slowly within its cross-section (e.g., the center of the intermediate steel thickness in the case of forced cooling of both surfaces).

[0208] Preferred cooling rate CR during quenching 800-500 The quenching rate is 300℃ / minute or higher. A more preferred cooling rate during quenching is CR. 800-500 The lower limit is 450℃ / min, more preferably 600℃ / min. The cooling rate CR during quenching... 800-500 There is no specific upper limit, such as 60,000℃ / minute.

[0209] Preferably, the billet is heated multiple times in the austenitic region before quenching. In this case, the austenitic grains before quenching are refined, thus improving the steel's resistance to pitting corrosion. This heating of the austenitic region can be repeated through multiple quenching processes, or through normalizing and quenching. Alternatively, quenching and tempering (described later) can be combined and performed multiple times. That is, multiple quenching and tempering processes can also be performed. In this case, the steel's resistance to pitting corrosion is further improved. The tempering process will be described in detail below.

[0210] [Tempering process]

[0211] In the tempering process, the intermediate steel that has undergone the above-mentioned quenching is tempered. In this specification, "tempering" refers to tempering the quenched intermediate steel at a temperature less than A. c1 The intermediate steel is then reheated and held at a specified temperature. Here, the tempering temperature is equivalent to the furnace temperature at which the quenched intermediate steel is heated and held. The tempering time refers to the time from when the intermediate steel reaches the specified tempering temperature until it is removed from the heat treatment furnace.

[0212] The tempering temperature is adjusted appropriately based on the chemical composition of the steel and the desired yield strength. Specifically, for intermediate steel with the chemical composition of this embodiment, the tempering temperature is adjusted to achieve a yield strength of 758 MPa or higher and less than 965 MPa. In the tempering process of this embodiment, a tempering temperature of 660–740°C is preferred. Furthermore, in the tempering process of this embodiment, the tempering time is preferably set to 20–180 minutes.

[0213] The steel of this embodiment can be manufactured using the above manufacturing method. In the above manufacturing method, a method for manufacturing a seamless steel pipe was described as an example. However, the steel of this embodiment can also be a steel plate or other shapes. Furthermore, as described above, the above manufacturing method is an example, and it can also be manufactured using other manufacturing methods. Hereinafter, the present invention will be further described in detail through examples. It should be noted that the conditions in the following examples are examples used to confirm the feasibility and effectiveness of the steel of this embodiment. Therefore, the steel of this embodiment is not limited to these specific conditions.

[0214] Example

[0215] Molten steel with the chemical compositions shown in Tables 1A and 1B is produced. It should be noted that the "-" in Table 1B indicates that the content of each element is at the impurity level. Specifically, the Sb, Cu, Ni, and Co contents in Test No. 1 are rounded to 0% to the third decimal place. The Nb and V contents in Test No. 1 are rounded to 0% to the fourth decimal place. Furthermore, the Zr, B, Ca, Mg, and REM contents in Test No. 1 are rounded to 0% to the fifth decimal place.

[0216] [Table 1A]

[0217]

[0218] [Table 1B]

[0219]

[0220] Round steel billets were manufactured using molten steel from each test number via continuous casting. The round steel billets from each test number were then heated in a furnace for hot working. The heating temperature T (°C), holding time t (minutes), and LMP (=(273+T)×(20+Log(t / 60))) calculated from the above formula (A) are shown in Table 2.

[0221] [Table 2]

[0222]

[0223] The heated round steel billet is subjected to piercing rolling and stretching rolling. The time from the end of piercing rolling to the start of stretching rolling (piercing-stretching holding time) is shown in Table 2. It should be noted that in Table 2, "A (Appropriate)" in the piercing-stretching holding time column refers to a piercing-stretching holding time of 30-180 seconds. "S (Short)" in the piercing-stretching holding time column refers to a piercing-stretching holding time of less than 30 seconds. "L (Long)" in the piercing-stretching holding time column refers to a piercing-stretching holding time exceeding 180 seconds.

[0224] The manufactured tube blanks are then quenched and tempered. Specifically, for each test number of the tube blank, water quenching is performed after holding it at the temperature (°C) and the corresponding time (minutes) recorded in Table 2. The quenched tube blanks for each test number are then further tempered at the temperature (°C) and the corresponding time (minutes) recorded in Table 2. Through these manufacturing processes, seamless steel tubes for each test number are obtained.

[0225] [Evaluation Test]

[0226] The following tests were performed on the seamless steel pipes after tempering, including grain size determination, tensile testing, pitting corrosion resistance testing, and general corrosion resistance testing.

[0227] [Grain Size Numbering Test]

[0228] Grain size determination tests were conducted on seamless steel pipes for each test number to determine the standard deviation σ of the original γ grain grains. Specifically, for test pieces prepared using the above method, microscopic observation was performed using the same method. Image analysis was performed on the microscopic images obtained, and the grain size was determined according to 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 grain size numbers are shown in Table 3A.

[0229] [Table 3A]

[0230]

[0231] [Tension Test]

[0232] For each test number of seamless steel pipe, tensile tests were performed 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 from the center of the wall thickness of the seamless steel pipe for each test number. The long side of the tensile test piece was parallel to the pipe axis. Using the prepared tensile test piece, tensile tests were performed at room temperature (25°C) in the atmosphere according to ASTM E8 / E8M (2022). The 0.2% conditional yield strength obtained through the tensile test was defined as the yield strength (MPa). For each test number of seamless steel pipe, the obtained yield strength (MPa) is shown as "YS (MPa)" in Table 3B.

[0233] [Table 3B]

[0234]

[0235] [Pitting resistance test]

[0236] Pitting resistance tests were conducted on seamless steel pipes for each test number to evaluate their pitting resistance. Specifically, test pieces prepared using the above method were kept in an autoclave for 720 hours under the above conditions. After 720 hours, the surface of the test pieces was checked for pitting using the above method. For each test piece with a specific test number, the number of pits was counted to obtain the total number of pits. The obtained number of pits is shown in Table 3B.

[0237] [Full Corrosion Resistance Test]

[0238] A comprehensive corrosion resistance test was conducted on the seamless steel pipes for each test number to evaluate their resistance to comprehensive corrosion. Specifically, test pieces prepared using the above method were kept in an autoclave for 720 hours under the above conditions. The corrosion rate of the test pieces after 720 hours was determined using the above method. The obtained corrosion rates (mm / year) for the test pieces for each test number are shown in Table 3B.

[0239] [Evaluation Results]

[0240] Referring to Tables 1A, 1B, 2, 3A, and 3B, the seamless steel pipes tested from 1 to 25 had appropriate chemical compositions, yield strengths above 758 MPa and below 965 MPa, and standard deviations of the original γ-grain grain size numbering below 0.80. As a result, these seamless steel pipes exhibited fewer than 10 pitting corrosions, indicating excellent pitting corrosion resistance even in high H2S environments.

[0241] The Si content of the seamless steel pipes tested (numbers 1, 3-5, 7, 8, 10, 13-16, 18, and 20-25) further met the requirement of being greater than 0.50% and less than 1.50%. As a result, the corrosion rate of these seamless steel pipes was less than 0.40 mm / year, indicating excellent resistance to overall corrosion even in high H2S environments.

[0242] On the other hand, the Si content of the seamless steel pipe in test number 26 was too low. As a result, the number of pitting corrosions on this seamless steel pipe exceeded 10, indicating that it did not have excellent pitting corrosion resistance in a high H2S environment.

[0243] The seamless steel pipe in test number 27 had an excessively low Cr content. As a result, the seamless steel pipe had more than 10 pitting corrosions, indicating that it did not have excellent pitting corrosion resistance in a high H2S environment.

[0244] The seamless steel pipe in test number 28 had an excessively low Mo content. As a result, the seamless steel pipe had more than 10 pitting corrosions, indicating that it did not have excellent pitting corrosion resistance in a high H2S environment.

[0245] The piercing-tensile holding time of the seamless steel pipe in test number 29 was too short. As a result, the standard deviation of the original γ grain size of the seamless steel pipe exceeded 0.80. Consequently, the number of pitting corrosions on the seamless steel pipe exceeded 10, indicating that it did not have excellent pitting corrosion resistance in a high H2S environment.

[0246] The piercing-tensile holding time of the seamless steel pipe in test number 30 was too long. As a result, the standard deviation of the original γ grain size of the seamless steel pipe exceeded 0.80. Consequently, the number of pitting corrosions on the seamless steel pipe exceeded 10, indicating that it did not have excellent pitting corrosion resistance in a high H2S environment.

[0247] In test 31, the seamless steel pipe underwent heating before hot working, with an LMP exceeding 32000. As a result, the standard deviation of the original γ grain size of this seamless steel pipe exceeded 0.80. Consequently, the number of pitting corrosions on this seamless steel pipe exceeded 10, indicating that it does not possess excellent pitting corrosion resistance in a high H2S environment.

[0248] In test 32, the seamless steel pipe had an LMP value less than 30,000 during heating before hot working. As a result, the standard deviation of the original γ grain size of this seamless steel pipe exceeded 0.80. Consequently, the number of pitting corrosions on this seamless steel pipe exceeded 10, indicating that it did not possess excellent pitting corrosion resistance in a high H2S environment.

[0249] The embodiments of this disclosure have been described above. However, the above embodiments are merely illustrative examples for implementing this disclosure. Therefore, this disclosure is not limited to the above embodiments, and appropriate modifications can be made to the above embodiments without departing from its spirit.

Claims

1. A type of steel, which, in terms of mass%, is C：0.20~0.35%、 Si: 0.10~1.50% Mn: 0.05~0.55%, P: below 0.050% S: Below 0.0100% Cr：0.20~1.00%、 Mo: 0.20~1.50%, Ti: 0.003~0.030% Al：0.010~0.100%、 N: below 0.0100% O: Below 0.0050% 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 balance consists of Fe and impurities. The yield strength of the steel is above 758 MPa and less than 965 MPa. In the microstructure, the standard deviation of the grain size number of the original austenite grains is below 0.

80.

2. The steel according to claim 1, wherein it contains selected... 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 Rare earth elements: one or more elements in a group consisting of 0.0001 to 0.0040%.

3. The steel according to claim 1 or 2, wherein it contains Si: More than 0.50% and less than 1.50%.

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

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