Steel material

Abstract

Provided is a steel material having both high strength of 150 ksi grade and excellent SSC resistance. This steel material contains, in mass%, 0.15-0.45% of C, 0.05-1.00% of Si, 0.05-1.00% of Mn, 0.030% or less of P, 0.0050% or less of S, 0.005-0.100% of Al, 0.30-1.50% of Cr, 0.40-2.00% of Mo, 0.002-0.020% of Ti, 0.002-0.100% of Nb, 0.05-0.30% of V, 0.0005-0.0040% of B, 0.0100% or less of N, 0.0040% or less of O, and the balance being Fe and impurities, has a yield strength of more than 1034 MPa and 1172 MPa or less, and in the steel material, the number density of Al oxides having a major diameter of 5.0 [mu] m or more is less than 30 / 200 mm2, and the number density of Si oxides having a major diameter of 5.0 [

Description

Technical Field

[0001] This disclosure pertains to steel. Background Technology

[0002] Due to the increasing depth of oil and gas wells (hereinafter, oil and gas wells are collectively 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 / or 95ksi grade (yield strength of 95ksi or higher and less than 110ksi, i.e., 655MPa or higher and less than 758MPa) oil well steel is widely used. Recently, there has been a further demand for 110ksi grade (yield strength of 110ksi or higher and less than 125ksi, i.e., 758MPa or higher and less than 862MPa) oil well steel.

[0003] Oil wells sometimes contain corrosive gases such as hydrogen sulfide (H2S) and / or carbon dioxide (CO2). Therefore, steel intended for use in oil wells requires not only high strength but also excellent corrosion resistance. Furthermore, oil well steel is subjected to stress during use. Therefore, resistance to sulfide stress cracking (SSC resistance) is used as an indicator of excellent corrosion resistance in oil well steel.

[0004] Techniques for improving the strength and SSC resistance of steel have been proposed in Japanese Patent Application Publication No. 2006-28612 (Patent Document 1), International Publication No. 2008 / 123422 (Patent Document 2), and Japanese Patent Application Publication No. 2017-166060 (Patent Document 3).

[0005] The steel disclosed in Patent Document 1 is a steel for steel pipes, with the following composition by mass%: 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, where the ratio of (Ca%) to (Al%) is 0.55~1.72 and the ratio of (Ca%) to (Ti%) 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] Patent Document 2 discloses a low-alloy steel containing, by mass percent: C: 0.10~0.20%, Si: 0.05~1.0%, Mn: 0.05~1.5%, Cr: 1.0~2.0%, Mo: 0.05~2.0%, Al: less than 0.10%, and Ti: 0.002~0.05%, with Ceq (=C+(Mn / 6)+(Cr+Mo+V) / 5) being 0.65 or more, and the balance being Fe and impurities. Among the impurities, P: less than 0.025%, S: less than 0.010%, N: less than 0.007%, and B: less than 0.0003%. In this steel, Mn with a particle size of 1 μm or more... 23 The C6 precipitate was 0.1 particles / mm. 2 The following is described in Patent Document 2: This steel has a yield strength of 654~793MPa and excellent resistance to SSC even under high-pressure hydrogen sulfide environment.

[0007] The steel disclosed in Patent Document 3 is a billet for high-strength oil well steel pipes, with the following composition by mass%: 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). According to this steel, Patent Document 3 describes a steel pipe that can be obtained with a yield strength of 758 MPa or more and less than 862 MPa and excellent resistance to SSC.

[0008] Existing technical documents

[0009] Patent documents

[0010] Patent Document 1: Japanese Patent Application Publication No. 2006-28612

[0011] Patent Document 2: International Publication No. 2008 / 123422

[0012] Patent Document 3: Japanese Patent Application Publication No. 2017-166060 Summary of the Invention

[0013] The problem the invention aims to solve

[0014] According to the technology disclosed in Patent Documents 1 to 3, oil well steel with excellent SSC resistance can be obtained. However, oil well steel with excellent SSC resistance can also be obtained using technologies other than those disclosed in Patent Documents 1 to 3.

[0015] Furthermore, in recent years, with the increasing severity of the oil well environment, higher yield strength is required for steel used in oil wells. Specifically, oil well steel with a yield strength of 150 ksi (yield strength exceeding 150 ksi but below 170 ksi, i.e., exceeding 1034 MPa but below 1172 MPa) is required. On the other hand, none of the aforementioned patent documents 1-3 have studied the combination of high yield strength (150 ksi) and excellent resistance to SSC (Self-Strength Curve Strain).

[0016] The purpose of this disclosure is to provide steel with high strength and excellent resistance to SSC, which is of grade 150 ksi (above 1034 MPa and below 1172 MPa).

[0017] Solution for solving the problem

[0018] The steel disclosed herein is expressed as a percentage by mass.

[0019] C: 0.15~0.45%

[0020] Si: 0.05~1.00%

[0021] Mn: 0.05~1.00%,

[0022] P: below 0.030%

[0023] S: Below 0.0050%

[0024] Al: 0.005~0.100%

[0025] Cr: 0.30~1.50%

[0026] Mo: 0.40~2.00%

[0027] Ti: 0.002~0.020%

[0028] Nb: 0.002~0.100%

[0029] V: 0.05~0.30%,

[0030] B: 0.0005~0.0040%

[0031] N: below 0.0100%

[0032] O: Below 0.0040%

[0033] Cu: 0~0.50%,

[0034] Ni: 0~0.50%,

[0035] W: 0~0.50%,

[0036] Ca: 0~0.0100%

[0037] Mg: 0~0.0100%

[0038] Zr: 0~0.0100%

[0039] Rare earth elements: 0~0.0100%, and

[0040] The balance consists of Fe and impurities.

[0041] Yield strength exceeding 1034 MPa but below 1172 MPa

[0042] Among the aforementioned steels,

[0043] The number density of Al oxides with a major diameter of 5.0 μm or more, an Al content of 20% or more by mass, and an O content of 10% or more is less than 30 per 200 mm. 2 ,

[0044] The number density of Si oxides with an Al content of less than 20%, a Si content of more than 20%, an O content of more than 10%, and a major diameter of more than 5.0 μm is less than or equal to 5 per 200 mm. 2 .

[0045] The effects of the invention

[0046] The steel disclosed herein can achieve both high strength (above 1034 MPa and below 1172 MPa) and excellent resistance to SSC. Attached Figure Description

[0047] Figure 1 This represents the number density (numbers / 200mm) of coarse Si oxides (Si oxides with a major diameter of 5.0 μm or more) in this embodiment. 2 A graph showing the relationship between the number of SSC roots (roots) and SSC resistance as an indicator of SSC resistance. Detailed Implementation

[0048] First, the inventors focused on chemical composition and studied how to obtain a steel that balances yield strength of 150 kSi and excellent resistance to SSC (sulfate-strength precipitates). As a result, the inventors determined that, in mass percent, the following composition is suitable: C: 0.15~0.45%, Si: 0.05~1.00%, Mn: 0.05~1.00%, P: ≤0.030%, S: ≤0.0050%, Al: 0.005~0.100%, Cr: 0.30~1.50%, Mo: 0.40~2.00%, Ti: 0.002~0.020%, Nb: 0.002~0.100%, V: 0.05~0.3%. Steel with the following composition: 0%, B: 0.0005~0.0040%, N: less than 0.0100%, O: less than 0.0040%, Cu: 0~0.50%, Ni: 0~0.50%, W: 0~0.50%, Ca: 0~0.0100%, Mg: 0~0.0100%, Zr: 0~0.0100%, rare earth elements: 0~0.0100%, and the balance being Fe and impurities, can potentially achieve a yield strength of 150ksi and excellent resistance to SSC.

[0049] Next, the inventors investigated various methods to improve SSC resistance for steels with the aforementioned chemical composition and a yield strength of 150 ksi. Specifically, the inventors believed that if coarse oxide inclusions could be reduced, it would be possible to improve SSC resistance while maintaining the yield strength. Here, in steels with the aforementioned chemical composition, Al oxides, primarily Al2O3, tend to coarsen. Therefore, the inventors first focused on coarse Al oxides.

[0050] The inventors' research results clearly show that, in steel with the above-mentioned chemical composition and a yield strength of 150 ksi, if the number density of Al oxides with a major diameter of 5.0 μm or more is less than 30 per 200 mm... 2 This may improve SSC resistance. In this specification, particles with an Al content of 20% or more and an O content of 10% or more by mass are also referred to as "Al oxides". In this specification, Al oxides with a major diameter of 5.0 μm or more are also referred to as "coarse Al oxides".

[0051] Here, Al oxides are hard oxides that easily reduce the corrosion resistance of steel. Especially when the yield strength is increased to 150 ksi, the effect of coarse Al oxides becomes more pronounced, and SSC resistance is significantly reduced. Therefore, in the steel of this embodiment with the above chemical composition and a yield strength of 150 ksi, the number density of coarse Al oxides is set to less than 30 per 200 mm. 2 .

[0052] On the other hand, even with the above chemical composition, the number density of coarse Al oxides is less than 30 per 200 mm. 2 Even with a yield strength of 150 ksi, steels sometimes cannot consistently achieve excellent SSC resistance. Therefore, the inventors have developed a method for producing steels with the aforementioned chemical composition, a yield strength of 150 ksi, and a coarse Al oxide density of less than 30 particles / 200 mm. 2 Various methods for stably obtaining excellent SSC resistance were investigated for steel. The detailed research conducted by the inventors clarified that, with the aforementioned chemical composition and a yield strength of 150 ksi, the number density of coarse Al oxides is less than 30 per 200 mm². 2 In steel, if not only coarse Al oxides but also coarse Si oxides can be reduced, it is possible to stably obtain excellent SSC resistance.

[0053] In this specification, particles with an Al content of less than 20%, a Si content of 20% or more, and an O content of 10% or more (by mass%) are also referred to as "Si oxides". In this specification, Si oxides with a major diameter of 5.0 μm or more are also referred to as "coarse Si oxides". Hereinafter, coarse Al oxides with the above chemical composition, a yield strength of 150 kSi, and a particle density of less than 30 particles / 200 mm are considered to be coarse. 2 For steel, the relationship between coarse Si oxide and SSC resistance is specifically illustrated using the attached diagram.

[0054] Figure 1 This represents the number density (numbers / 200mm) of coarse Si oxides (Si oxides with a major diameter of 5.0 μm or more) in this embodiment. 2 A graph showing the relationship between the number of SSC roots (roots) and SSC resistance as an indicator of SSC resistance. Figure 1 For the embodiments described later, the chemical composition must meet the above requirements, the yield strength must be at least 150 kSi, and the number density of coarse Al oxides must be less than 30 per 200 mm. 2 The steel is determined using the method described later, which yields the number density (numbers / 200mm) of coarse Si oxide particles. 2 It is made by generating the number of roots (roots) from the SSC obtained by the method described later.

[0055] Reference Figure 1 With the above-mentioned chemical composition, a yield strength of 150 ksi, and a coarse Al oxide density of less than 30 per 200 mm, 2 In steel, if the density of coarse Si oxide particles is less than or equal to 5 per 200 mm 2Therefore, the number of SSCs produced becomes 0, demonstrating excellent SSC resistance. Thus, in this embodiment, with the aforementioned chemical composition and a yield strength of 150 ksi, the number density of coarse Al oxides is set to less than 30 per 200 mm. 2 Therefore, the number density of coarse Si oxide particles is set to be less than or equal to 5 per 200 mm. 2 As a result, the steel of this embodiment can achieve both a yield strength of 150 ksi and excellent resistance to SSC.

[0056] The detailed reasons for improving the SSC resistance of steel by reducing the number density of coarse Si oxides are not yet clear. However, the inventors speculate as follows: When manufacturing steel with the above chemical composition, deoxidation using aluminum (Al) is mainly carried out in the steelmaking process. Therefore, in steel with the above chemical composition, research is conducted on Al oxides, represented by Al2O3, without focusing on the small amount of Si oxides. However, when the yield strength is increased to the 150 ksi level, not only coarse Al oxides but also small amounts of coarse Si oxides may easily make the reduction in SSC resistance more prominent. Therefore, the inventors speculate that by reducing the number density of coarse Al oxides to less than 30 per 200 mm, the SSC resistance can be improved. 2 It also ensures that the number density of coarse Si oxide particles is less than or equal to 5 per 200 mm. 2 Even with a yield strength of 150 ksi, can excellent SSC resistance be consistently obtained?

[0057] It should be noted that, based on a mechanism different from that proposed by the inventors, the SSC resistance of the steel may also be improved. However, with the above-mentioned chemical composition, a yield strength of 150 ksi, and a coarse Al oxide density of less than 30 per 200 mm, the SSC resistance of the steel may still be improved. 2 In steel, the number density of coarse Si oxide particles is set to less than or equal to 5 per 200 mm. 2 Excellent SSC resistance can be obtained, as demonstrated by the examples described later.

[0058] The main points of the steel used in this embodiment, based on the above insights, are as follows. [1]

[0060] A type of steel, which is expressed as a percentage by mass.

[0061] C: 0.15~0.45%

[0062] Si: 0.05~1.00%

[0063] Mn: 0.05~1.00%,

[0064] P: below 0.030%

[0065] S: Below 0.0050%

[0066] Al: 0.005~0.100%

[0067] Cr: 0.30~1.50%

[0068] Mo: 0.40~2.00%

[0069] Ti: 0.002~0.020%

[0070] Nb: 0.002~0.100%

[0071] V: 0.05~0.30%,

[0072] B: 0.0005~0.0040%

[0073] N: below 0.0100%

[0074] O: Below 0.0040%

[0075] Cu: 0~0.50%,

[0076] Ni: 0~0.50%,

[0077] W: 0~0.50%,

[0078] Ca: 0~0.0100%

[0079] Mg: 0~0.0100%

[0080] Zr: 0~0.0100%

[0081] Rare earth elements: 0~0.0100%, and

[0082] The balance consists of Fe and impurities.

[0083] Yield strength exceeding 1034 MPa but below 1172 MPa

[0084] Among the aforementioned steels,

[0085] The number density of Al oxides with a major diameter of 5.0 μm or more, an Al content of 20% or more by mass, and an O content of 10% or more is less than 30 per 200 mm. 2 ,

[0086] The number density of Si oxides with an Al content of less than 20%, a Si content of more than 20%, an O content of more than 10%, and a major diameter of more than 5.0 μm is less than or equal to 5 per 200 mm. 2 . [2]

[0088] According to the steel described in [1], it contains selected free...

[0089] Cu: 0.01~0.50%,

[0090] Ni: 0.01~0.50%

[0091] W: 0.01~0.50%,

[0092] Ca: 0.0001~0.0100%

[0093] Mg: 0.0001~0.0100%

[0094] Zr: 0.0001~0.0100%, and

[0095] Rare earth elements: one or more elements in a group consisting of 0.0001 to 0.0100%. [3]

[0097] According to the steel described in [1] or [2], wherein,

[0098] The steel mentioned above is seamless steel pipe.

[0099] The shape of the steel used in this embodiment is not particularly limited. The steel can be a steel pipe, a round bar (solid), 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. Furthermore, the steel pipe can be a seamless steel pipe or a welded steel pipe.

[0100] The steel used in this embodiment will be described in detail below. Unless otherwise specified, "%" related to elements refers to mass%.

[0101] [Chemical Composition]

[0102] The steel in this embodiment contains the following elements in its chemical composition.

[0103] C: 0.15~0.45%

[0104] Carbon (C) improves the hardenability and strength of steel. C also promotes the spheroidization of carbides during tempering in the manufacturing process, improving the steel's resistance to saturated steel (SSC). 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, even if the contents of other elements are within the range of this embodiment, there will be too many carbides, and the steel's resistance to SSC will decrease. Therefore, the C content is 0.15% to 0.45%. The preferred lower limit of the C content is 0.18%, more preferably 0.20%, more preferably 0.22%, and more preferably 0.23%. The preferred upper limit of the C content is 0.40%, more preferably 0.38%, more preferably 0.35%, and more preferably 0.30%.

[0105] Si: 0.05~1.00%

[0106] Silicon (Si) deoxidizes steel. If the Si 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 Si content is too high, large amounts of coarse Si oxides may form even if the contents of other elements are within the range of this embodiment, reducing the steel's resistance to SSC. Therefore, the Si content is 0.05 to 1.00%. The preferred lower limit of the Si content is 0.10%, more preferably 0.15%, and more preferably 0.20%. The preferred upper limit of the Si content is 0.85%, more preferably 0.75%, more preferably 0.60%, more preferably 0.50%, and more preferably 0.40%.

[0107] Mn: 0.05~1.00%

[0108] Manganese (Mn) deoxidizes steel. Mn also improves the hardenability of steel. If the Mn 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 Mn content is too high, coarse sulfide inclusions will form even if the contents of other elements are within the range of this embodiment, reducing the steel's resistance to SSC. Therefore, the Mn content is 0.05~1.00%. The preferred lower limit of the Mn content is 0.06%, more preferably 0.08%, and more preferably 0.10%. The preferred upper limit of the Mn content is 0.90%, more preferably 0.80%, more preferably 0.70%, more preferably 0.60%, and more preferably 0.50%.

[0109] P: below 0.030%

[0110] 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 still segregate at the grain boundaries, reducing the SSC resistance of the steel. Therefore, the P content is 0.030% or less. The preferred upper limit of P content is 0.025%, more preferably 0.020%, more preferably 0.015%, and more preferably 0.010%. The P content is preferably as low as possible. However, an extreme reduction 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 more preferably 0.003%.

[0111] S: Below 0.0050%

[0112] 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, S will segregate at the grain boundaries, reducing the SSC resistance of the steel. Therefore, the S content is 0.0050% or less. The preferred upper limit of S content is 0.0040%, more preferably 0.0032%, more preferably 0.0030%, more preferably 0.0020%, and more preferably 0.0015%. 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 0.0002%, and more preferably 0.0003%.

[0113] Al: 0.005~0.100%

[0114] Aluminum (Al) deoxidizes steel. If the Al content is too low, even if the contents of other elements are within the range of this embodiment, the above-mentioned effect cannot be fully obtained, and the SSC resistance of the steel decreases. On the other hand, if the Al content is too high, even if the contents of other elements are within the range of this embodiment, a large amount of coarse Al oxides will be formed, and the SSC resistance of the steel decreases. Therefore, the Al content is 0.005~0.100%. The preferred lower limit of the Al content is 0.010%, more preferably 0.015%, and more preferably 0.020%. The preferred upper limit of the Al content is 0.080%, more preferably 0.060%, more preferably 0.040%, and more preferably 0.035%. The "Al" content mentioned in this specification refers to the content of "acid-soluble Al", that is, "sol.Al".

[0115] Cr: 0.30~1.50%

[0116] Chromium (Cr) improves the hardenability of steel. Cr also increases the tempering softening resistance of steel, enabling high-temperature tempering. As a result, the steel's resistance to sintering corrosion cracking (SSC) is improved. If the Cr 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 Cr content is too high, the SSC resistance of the steel will decrease even if the contents of other elements are within the range of this embodiment. Therefore, the Cr content is 0.30~1.50%. The preferred lower limit of the Cr content is 0.35%, more preferably 0.40%, and more preferably 0.50%. The preferred upper limit of the Cr content is 1.40%, more preferably 1.30%, more preferably 1.20%, more preferably 1.10%, and more preferably 1.05%.

[0117] Mo: 0.40~2.00%

[0118] Molybdenum (Mo) improves the hardenability of steel. Mo also increases the tempering softening resistance of steel, enabling high-temperature tempering. As a result, the steel's resistance to sintering corrosion cracking (SSC) 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 0.40~2.00%. The preferred lower limit of the Mo content is 0.45%, more preferably 0.50%, more preferably 0.51%, more preferably 0.55%, and more preferably 0.60%. The preferred upper limit of the Mo content is 1.80%, more preferably 1.60%, more preferably 1.40%, and more preferably 1.30%.

[0119] Ti: 0.002~0.020%

[0120] Titanium (Ti) combines with nitrogen to form nitrides, which refine the grain size of the steel through a pinning effect. As a result, the strength of the steel is improved. If the Ti 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 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 SSC resistance of the steel will decrease. Therefore, the Ti content is 0.002~0.020%. The preferred lower limit of the Ti content is 0.003%, more preferably 0.004%. The preferred upper limit of the Ti content is 0.018%, more preferably 0.015%, more preferably 0.010%, and more preferably 0.008%.

[0121] Nb: 0.002~0.100%

[0122] Niobium (Nb) combines with C and / or N to form carbides, nitrides, or carbonitrides (hereinafter referred to as "carbonitrides, etc."). Carbonitrides, etc., refine the grain size of steel through a pinning effect, thereby improving the steel's resistance to sintering stress (SSC). Nb also forms fine carbides during tempering, increasing the steel's resistance to tempering softening and thus its strength. If the Nb 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 Nb content is too high, carbonitrides, etc., will be formed excessively even if the contents of other elements are within the range of this embodiment, reducing the steel's resistance to SSC. Therefore, the Nb content is 0.002 to 0.100%. The preferred lower limit of the Nb content is 0.005%, more preferably 0.010%, more preferably 0.015%, and more preferably 0.020%. The preferred upper limit of the Nb content is 0.080%, more preferably 0.060%, and more preferably 0.040%.

[0123] V: 0.05~0.30%

[0124] Vanadium (V) forms carbonitrides and the like. Carbonitrides and the like refine the grain size of the steel through a pinning effect, improving the steel's resistance to sintering stress (SSC). V also forms fine carbides during tempering, increasing the steel's resistance to tempering softening and thus its strength. If the V content is too low, even if the contents of other elements are within the range of this embodiment, the above-mentioned effects cannot be fully obtained. On the other hand, if the V content is too high, even if the contents of other elements are within the range of this embodiment, excessive carbonitrides and the like will form, reducing the steel's resistance to SSC. Therefore, the V content is 0.05 to 0.30%. The preferred lower limit of the V content is 0.06%, more preferably 0.07%, and even more preferably 0.08%. The preferred upper limit of the V content is 0.25%, more preferably 0.20%, and even more preferably 0.15%.

[0125] B: 0.0005~0.0040%

[0126] Boron (B) dissolves in steel, improving its hardenability and strength. If the B content is too low, even with other elements within the range specified in this embodiment, the aforementioned effects cannot be fully achieved. Conversely, if the B content is too high, even with other elements within the range specified in this embodiment, coarse nitrides will form, reducing the steel's resistance to SSC (Superficial Staining Concentration). Therefore, the B content is 0.0005~0.0040%. The preferred lower limit for the B content is 0.0006%, more preferably 0.0008%. The preferred upper limit for the B content is 0.0035%, more preferably 0.0030%, more preferably 0.0025%, and more preferably 0.0020%.

[0127] N: below 0.0100%

[0128] Nitrogen (N) is unavoidably present. That is, the lower limit of 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 strength of the steel is improved. However, 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 SSC resistance of the steel will decrease. Therefore, the N content is 0.0100% or less. The preferred upper limit of N content is 0.0080%, more preferably 0.0060%, more preferably 0.0050%, and more preferably 0.0040%. The preferred lower limit of N content for more effectively obtaining the above-mentioned effect is 0.0005%, more preferably 0.0010%, more preferably 0.0015%, and more preferably 0.0020%.

[0129] O: Below 0.0040%

[0130] 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 SSC resistance of the steel. Therefore, the O content is 0.0040% or less. The preferred upper limit of O content is 0.0035%, more preferably 0.0033%, more preferably 0.0030%, more preferably 0.0025%, and 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 0.0002%, and more preferably 0.0003%.

[0131] 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.

[0132] [Any element]

[0133] The chemical composition of the aforementioned steel may also contain one or more elements selected from the group consisting of Cu and Ni to replace a portion of the Fe. These elements are arbitrary and improve the hardenability of the steel.

[0134] Cu: 0~0.50%

[0135] Copper (Cu) can be any element, or it can be absent. That is, the Cu content can be 0%. When present, Cu improves the hardenability and strength of the steel. Even a small amount of Cu is sufficient to achieve these effects to some extent. However, if the Cu content is too high, the SSC resistance 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%, and more preferably 0.02%. The preferred upper limit of the Cu content is 0.35%, more preferably 0.25%, more preferably 0.15%, more preferably 0.10%, and more preferably 0.05%.

[0136] Ni: 0~0.50%

[0137] Nickel (Ni) is an arbitrary element and may be absent. That is, the Ni content can be 0%. When present, Ni improves the hardenability and strength of the steel. Ni also dissolves in steel, improving its resistance to SSC (Self-Staining Corrosion). These effects are achieved to some extent even with a small amount of Ni. On the other hand, 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 SSC 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%, and more preferably 0.02%. The preferred upper limit of the Ni content is 0.30%, more preferably 0.20%, more preferably 0.10%, and more preferably 0.05%.

[0138] The chemical composition of the steel mentioned above may also contain W to replace a portion of Fe.

[0139] W: 0~0.50%

[0140] Tungsten (W) can be any element, or it can be absent. That is, the W content can be 0%. When present, W forms a protective corrosion coating in an acidic environment, inhibiting the intrusion of hydrogen into the steel. This improves the SSC resistance of the steel. Even a small amount of W is sufficient to achieve the above-mentioned effect. However, if the W content is too high, even if the contents of other elements are within the range of this embodiment, coarse carbides will form in the steel, reducing the SSC resistance. Therefore, the W content is 0 to 0.50%. The preferred lower limit of the W content is more than 0%, more preferably 0.01%, more preferably 0.03%, and more preferably 0.05%. The preferred upper limit of the W content is less than 0.50%, and more preferably 0.48%.

[0141] The chemical composition of the aforementioned steel may also contain one or more elements selected from the group consisting of Ca, Mg, Zr, 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 improve the steel's resistance to SSC.

[0142] Ca: 0~0.0100%

[0143] 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 resistance to sulfur dioxide (SSC). Even a small amount of Ca is sufficient to achieve the above-mentioned effect. 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, reducing the steel's resistance to SSC. Therefore, the Ca content is 0 to 0.0100%. The preferred lower limit of the Ca content is over 0%, more preferably 0.0001%, more preferably 0.0003%, and more preferably 0.0006%. The preferred upper limit of the Ca content is 0.0040%, more preferably 0.0025%, and more preferably 0.0020%.

[0144] Mg: 0~0.0100%

[0145] 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 resistance to SSC. Even a small amount of Mg is sufficient to achieve the above-mentioned effect. 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, reducing the steel's resistance to SSC. Therefore, the Mg content is 0 to 0.0100%. The preferred lower limit of the Mg content is over 0%, more preferably 0.0001%, more preferably 0.0003%, and more preferably 0.0006%. The preferred upper limit of the Mg content is 0.0040%, more preferably 0.0025%, and more preferably 0.0020%.

[0146] Zr: 0~0.0100%

[0147] Zirconium (Zr) can be any element or may be absent. That is, the Zr content can be 0%. When present, Zr neutralizes sulfur in the steel in the form of sulfides, improving the steel's resistance to sulfur oxides (SSCs). Even a small amount of Zr is sufficient to achieve the above-mentioned effect. 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, reducing the steel's SSC resistance. Therefore, the Zr content is 0 to 0.0100%. The preferred lower limit of the Zr content is over 0%, more preferably 0.0001%, more preferably 0.0003%, and more preferably 0.0006%. The preferred upper limit of the Zr content is 0.0040%, more preferably 0.0025%, and more preferably 0.0020%.

[0148] Rare earth elements (REM): 0~0.0100%

[0149] 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 (S) in the steel in the form of sulfides, improving the steel's resistance to sulfur dioxide (SSC). REMs also combine with phosphorus (P) in the steel to suppress P segregation at grain boundaries. Therefore, the reduction in the steel's resistance to SSC caused by P segregation can be suppressed. Even a small amount of REMs is sufficient to achieve the above-mentioned effects. 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, reducing the steel's resistance to SSC. Therefore, the REM content is 0 to 0.0100%. The preferred lower limit of the REM content is more than 0%, more preferably 0.0001%, more preferably 0.0003%, and more preferably 0.0006%. The preferred upper limit of the REM content is 0.0040%, more preferably 0.0025%, and more preferably 0.0020%.

[0150] 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.

[0151] [Yield Strength]

[0152] The steel of this embodiment has a yield strength exceeding 1034 MPa and below 1172 MPa (exceeding 150 ksi and below 170 ksi). The yield strength referred to in this specification is the stress (0.65% yield strength) obtained at 0.65% elongation in a tensile test at room temperature (25°C) according to ASTM E8 / E8M (2021). The steel of this embodiment has the above-described chemical composition and, by satisfying the number density of coarse Al oxides and the number density of coarse Si oxides described later, exhibits excellent resistance to SSC even with a yield strength exceeding 1034 MPa and below 1172 MPa.

[0153] The yield strength of the steel in this embodiment is determined by the following method. First, a round bar test piece is made from the steel of this embodiment. When the steel is a steel plate, a round bar test piece is made from the center of the plate thickness. In this case, the axial direction of the round bar test piece is set parallel to the rolling direction of the steel plate. When the steel is a steel pipe, a round bar test piece is made from the center of the wall thickness. In this case, the axial direction of the round bar test piece is set parallel to the pipe axis direction. When the steel is a round bar, a round bar test piece is made at the R / 2 position. In this specification, the R / 2 position refers to the center position of radius R in the cross-section perpendicular to the axial direction of the round bar. In this case, the axial direction of the round bar test piece is set parallel to the axial direction of the round bar. The size of the round bar test piece is, for example, 8.9 mm in diameter at the parallel portion and 35.6 mm in gauge length. Using the prepared round bar test piece, a tensile test was conducted at room temperature (25°C) in atmospheric conditions according to the method of ASTM E8 / E8M (2021). The stress at 0.65% elongation (0.65% yield strength) is defined as the yield strength (MPa). It should be noted that in this embodiment, the yield strength (MPa) is obtained by rounding the first decimal place of the obtained value.

[0154] [Number density of coarse Al oxides]

[0155] The steel of this embodiment has the above-described chemical composition and a yield strength exceeding 1034 MPa and below 1172 MPa, and furthermore, the number density of coarse Al oxides is less than 30 per 200 mm. 2 As described above, in this specification, particles with an Al content of 20% or more and an O content of 10% or more by mass are also referred to as "Al oxides". As described above, in this specification, Al oxides with a major diameter of 5.0 μm or more are also referred to as "coarse Al oxides". That is, coarse Al oxides refer to particles with a major diameter of 5.0 μm or more and an Al content of 20% or more and an O content of 10% or more by mass.

[0156] As described above, in the manufacture of steel with the aforementioned chemical composition, deoxidation using aluminum (Al) is primarily performed during the steelmaking process. Therefore, a large amount of Al oxides tends to form in steel with the aforementioned chemical composition. Furthermore, Al oxides are hard oxides, which easily reduce the corrosion resistance of the steel. Particularly when the yield strength is high (around 150 ksi), the effect of coarse Al oxides becomes more pronounced, and SSC resistance is significantly reduced. Therefore, in the steel of this embodiment, which has the aforementioned chemical composition and a yield strength exceeding 1034 MPa and below 1172 MPa, the number density of coarse Al oxides is set to less than 30 per 200 mm². 2 .

[0157] In this embodiment, the preferred upper limit for the number density of coarse Al oxides is 28 per 200 mm. 2 Further preferred is 25 pieces / 200mm 2 Further preferred is 22 per 200mm 2 In this embodiment, the lower limit of the number density of coarse Al oxides is not particularly limited, and can also be 0 per 200 mm. 2 The lower limit for the number density of coarse Al oxides can be, for example, 5 per 200 mm. 2 It can be 7 pieces / 200mm 2 It can also be 9 per 200mm 2 The method for determining the number density of coarse Al oxides will be described later.

[0158] [Number density of coarse Si oxides]

[0159] The steel of this embodiment has the above-described chemical composition and a yield strength exceeding 1034 MPa and below 1172 MPa, and the number density of coarse Al oxides is less than 30 per 200 mm. 2 Furthermore, the number density of coarse Si oxide particles is less than or equal to 5 per 200 mm. 2 As described above, in this specification, particles with an Al content of less than 20%, a Si content of 20% or more, and an O content of 10% or more by mass are also referred to as "Si oxides". As described above, in this specification, Si oxides with a major diameter of 5.0 μm or more are also referred to as "coarse Si oxides". That is, coarse Si oxides refer to particles with an Al content of less than 20% (by mass), a Si content of 20% or more, an O content of 10% or more, and a major diameter of 5.0 μm or more.

[0160] As mentioned above, Si oxides have not received much attention to date due to their small quantity. However, even with a high yield strength of 150 kSi, not only coarse Al oxides but also small quantities of coarse Si oxides can easily lead to a significant decrease in SSC resistance. Therefore, by not only reducing the number density of coarse Al oxides to less than 30 per 200 mm², 2 It also ensures that the number density of coarse Si oxide particles is less than or equal to 5 per 200 mm. 2 Even when the yield strength is increased to the 150 ksi level, it is possible to consistently obtain excellent resistance to SSC. Therefore, the steel of this embodiment has the above-mentioned chemical composition and a yield strength exceeding 1034 MPa and below 1172 MPa, and the number density of coarse Al oxides in the steel is set to be less than 30 per 200 mm. 2 Therefore, the number density of coarse Si oxide particles is set to be less than or equal to 5 per 200 mm. 2 .

[0161] In this embodiment, the preferred upper limit for the number density of coarse Si oxide particles is 4 per 200 mm. 2 Further preferred are 3 per 200mm 2 In this embodiment, the lower limit of the number density of coarse Si oxides is not particularly limited, and can also be 0 per 200 mm. 2 The lower limit for the number density of coarse Si oxide particles can also be, for example, 1 particle / 200 mm. 2 .

[0162] In this embodiment, the number density of coarse Al oxides and the number density of coarse Si oxides in the steel can be determined by the following method. First, a test piece is made from the steel of this embodiment, with the surface including the rolling direction and the reduction direction as the observation surface. Specifically, when the steel is a steel plate, a test piece is made from the center of the plate thickness, with the surface including the rolling direction and the plate thickness direction as the observation surface. When the steel is a steel pipe, a test piece is made from the center of the wall thickness, with the surface including the pipe axis direction and the pipe diameter direction as the observation surface. When the steel is a round bar, a test piece is made with the surface including the R / 2 position in the center and including both axial and radial directions as the observation surface.

[0163] The observation surface of the prepared test piece is ground to a mirror finish before measurement. There is no limit to the area of ​​the observation surface; for example, it can be set to 300 mm². 2(20mm × 15mm). The number of Si oxide particles with a major diameter greater than 5.0μm was determined within the observation surface. Specifically, the particles in the observation surface were first identified based on contrast. Elemental concentration analysis (EDS) was performed on each identified particle. In the EDS analysis, the accelerating voltage was set to 20kV, and the target elements were N, O, Mg, Al, Si, P, S, Ca, Ti, Cr, Mn, Fe, Cu, Zr, and Nb for quantification. Based on the EDS analysis results of each particle, if the Al content was greater than 20% and the O content was greater than 10% by mass, the particle was identified as an "Al oxide". Based on the EDS analysis results of each particle, if the Al content was less than 20%, the Si content was greater than 20%, and the O content was greater than 10% by mass, the particle was further identified as a "Si oxide".

[0164] Among the Al oxides identified in the observation plane, Al oxides with a major diameter of 5.0 μm or more (coarse Al oxides) are selected, and the total number of coarse Al oxides is calculated. Furthermore, among the Si oxides identified in the observation plane, Si oxides with a major diameter of 5.0 μm or more (coarse Si oxides) are selected, and the total number of coarse Si oxides is calculated. It should be noted that the major diameters of the Al oxides and Si oxides can be determined using known methods. Additionally, in this specification, the major diameter of the Al oxides and Si oxides refers to the largest line segment (μm) connecting any two points on the outer periphery of the Al oxide and Si oxide in the observation plane.

[0165] Based on the total number of coarse Al oxides and the total area of ​​the observation surface, the number density of coarse Al oxides (numbers / 200mm²) was calculated. 2 Furthermore, based on the total number of coarse Si oxide particles and the total area of ​​the observation surface, the number density of coarse Si oxide particles (particles / 200mm²) was calculated. 2 It should be noted that, in this embodiment, the number density of coarse Al oxide particles (particles / 200 mm²) is... 2 ) and the number density of coarse Si oxides (numbers / 200mm²) 2 All obtained values ​​are rounded to the nearest decimal place. Furthermore, the number density of coarse Al oxides and coarse Si oxides can be determined using a scanning electron microscope equipped with compositional analysis capabilities (SEM-EDS apparatus). For example, an automated analysis apparatus manufactured by FEI (ASPEX), under the trade name Metals Quality Analyzer, can be used for SEM-EDS.

[0166] [SSC Resistance]

[0167] The SSC resistance of the steel according to this embodiment can be evaluated by an SSC resistance test performed according to NACE TM0177-2016 Method A. Specifically, it can be evaluated by the following method.

[0168] A mixed aqueous solution of 5.0% by mass sodium chloride and 0.4% by mass sodium acetate (NACEsolution D), adjusted to pH 5.0 with hydrochloric acid, was used as the test solution. Round bar test pieces were prepared from the steel of this embodiment. When the steel is a steel plate, the round bar test piece was prepared from the center of the plate thickness. In this case, the axial direction of the round bar test piece was set parallel to the rolling direction of the steel plate. When the steel is a steel pipe, the round bar test piece was prepared from the center of the wall thickness. In this case, the axial direction of the round bar test piece was set parallel to the pipe axis direction. When the steel is a round bar, the round bar test piece was prepared from the R / 2 position. In this case, the axial direction of the round bar test piece was set parallel to the axial direction of the round bar. The size of the round bar test piece was, for example, a diameter of 6.35 mm and a parallel portion length of 25.4 mm.

[0169] A stress equivalent to 90% of the actual yield stress was applied to the prepared round bar test piece. A test bath was prepared by immersing the stressed round bar test piece in a test container filled with a test solution at 24°C. After degassing the test bath, a mixture of 0.01 atm H₂S gas and 0.99 atm CO₂ gas was blown into the test bath to saturate it. The gas-saturated test bath was maintained at 24°C for 720 hours. In the SSC resistance test performed under the above conditions, no cracks were detected in the steel of this embodiment after 720 hours. It should be noted that, in this specification, "no cracks detected" means that no cracks were detected by visual inspection of the test piece after the test.

[0170] [Microstructure]

[0171] For the microstructure of the steel in this embodiment, the sum of the volume fractions 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 sum of 90% or more of tempered martensite and tempered bainite, then, provided that other configurations of this embodiment are satisfied, a yield strength of 150 ksi and excellent resistance to SSC can be achieved. That is, in this embodiment, if the steel achieves both a yield strength of 150 ksi and excellent resistance to SSC, it is determined that the sum of the volume fractions of tempered martensite and tempered bainite in the microstructure is 90% or more.

[0172] 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.

[0173] 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. Ten fields of view were then observed using a scanning electron microscope (SEM) to obtain secondary electron images of the etched observation surface. 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.

[0174] [Manufacturing Method]

[0175] 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 billet (steelmaking process); a process of hot-working the billet to produce a tube blank (hot working 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.

[0176] [Steelmaking process]

[0177] In the steelmaking process, firstly, molten iron produced using known methods is refined in a converter (primary refining). The molten steel after primary refining undergoes secondary refining. In secondary refining, alloying elements are added to adjust the composition, producing molten steel that meets the aforementioned chemical composition.

[0178] Secondary refining may involve, for example, performing RH (Ruhrstahl-Hausen) vacuum degassing. Following this, the alloy composition is finalized. Composite refining can also be performed during secondary refining. In this case, prior to RH vacuum degassing, refining processes using, for example, LF (Ladle Furnace) or VAD (Vacuum Arc Degassing) are performed.

[0179] Billets are manufactured using molten steel that has undergone secondary refining. Specifically, billets (slabs, large billets, or small billets) are manufactured using molten steel that has undergone secondary refining via continuous casting. In continuous casting, molten steel is first poured from the ladle into the tundish. At this time, filler sand is usually inserted into the nozzle to seal it. Therefore, sometimes the filler sand is mixed with the molten steel as it is poured from the ladle into the tundish. In addition, when manufacturing billets with the above-mentioned chemical composition, Si oxide is sometimes used as filler sand. In this case, it is possible to introduce Si oxide into the manufactured billet.

[0180] Therefore, in this embodiment, to prevent Si oxide sealed in the ladle nozzle from being introduced into the tundish, the molten steel is separated from the Si oxide. The method for separating the Si oxide is not particularly limited; for example, the following method can be used. A metal plate with an incline is positioned below the ladle nozzle and above the opening of the tundish. When the ladle nozzle is opened, first, the Si oxide is discharged from the nozzle, followed by the molten steel. Here, the Si oxide is lighter than the molten steel. Therefore, the Si oxide discharged from the nozzle is guided out of the tundish opening along the incline of the metal plate. The incline of the metal plate can be set, for example, by arranging a metal plate machined into a cone shape without a bottom surface with its apex directly below the ladle nozzle, or by other methods. Furthermore, a single metal plate can be used, or multiple metal plates can be stacked. Moreover, the thickness of the metal plate is not particularly limited, for example, it is about 1 to 10 mm.

[0181] After the Si oxide is discharged from the nozzle, the molten steel is discharged. At this time, the molten steel discharged from the nozzle, together with the metal plate, is introduced into the tundish through the opening. That is, in this embodiment, part or all of the metal plate may be introduced into the tundish and mixed into the molten steel. Therefore, the metal plate in this embodiment is preferably a metal plate composed of alloying elements contained in the molten steel. As a metal plate composed of alloying elements contained in the molten steel, an aluminum plate can be used, for example. It should be noted that in this specification, an aluminum plate refers to a metal plate composed of aluminum and the balance being impurities.

[0182] Preferably, the metal plate is removed from below the nozzle after the Si oxide has been discharged from the nozzle and before the molten steel has been discharged. This prevents the Si oxide adhering to the metal plate from mixing into the molten steel. It should be noted that the method for removing the metal plate from below the nozzle is not particularly limited; for example, a hole may be formed in a portion of the metal plate, and a rod with a hook at its tip may be used for removal. In this case, the metal plate can be removed by hooking the hook at the tip of the rod into the hole in the metal plate and pulling the rod. Using the above method, the Si oxide can be separated from the molten steel, allowing the molten steel to be introduced into the tundish. It should be noted that the method for separating Si oxide from the molten steel is not limited to the methods described above.

[0183] Next, the prepared molten steel is cast to produce a billet. The casting method is not particularly limited; for example, continuous casting is acceptable. When producing the billet using continuous casting, the following method is preferred.

[0184] The casting speed in a continuous casting machine is preferably set to 1.0 to 3.0 m / min. If the casting speed is too slow, Al oxide agglomerates may sometimes form in the billet. In this case, the manufactured steel contains a large amount of coarse Al oxides, reducing the steel's resistance to SSC (sulfate-carbon dioxide). On the other hand, if the casting speed is too fast, the Al oxides cannot float to the surface of the molten steel, sometimes leaving a large amount of Al oxides in the billet. In this case, the manufactured steel also contains a large amount of coarse Al oxides, reducing the steel's resistance to SSC. Therefore, the casting speed in a continuous casting machine is preferably set to 1.0 to 3.0 m / min.

[0185] When manufacturing billets using continuous casting, it is even more preferable to electromagnetically stir the molten steel within the mold. Specifically, by setting the electromagnetic stirring within the mold to a current value of 330-450A, it is less likely to form Al oxide agglomerates in the billet. If the current value in the electromagnetic stirring within the mold is too low, the molten steel is not sufficiently stirred, and sometimes Al oxide agglomerates form in the billet. In this case, the manufactured steel contains a large amount of coarse Al oxides, reducing the steel's resistance to SSC (Supersonic Staining). On the other hand, if the current value in the electromagnetic stirring within the mold is too high, the manufacturing equipment may sometimes be overloaded. Therefore, in this embodiment, it is preferable to set the electromagnetic stirring within the mold to a current value of 330-450A. Using the above method, molten steel is cast to produce billets.

[0186] [Heat treatment process]

[0187] In the hot working process, the prepared billet is hot-worked to produce intermediate steel. When the steel is a seamless steel pipe, the intermediate steel is equivalent to a pipe blank. First, a small square billet is heated in a furnace. The heating temperature is not particularly limited, for example, 1100~1300℃. The small square billet, removed from the furnace, is then hot-worked to produce a pipe blank (seamless steel pipe). The hot working method is not particularly limited and can be a known method.

[0188] For example, the Mannesmann process can also be used to manufacture tube blanks as a hot working process. In this case, the round billet is pierced and rolled using a piercing mill. The piercing ratio is not particularly limited in the case of piercing rolling, and is, for example, 1.0 to 4.0. The pierced and rolled round billet is then further hot-rolled using a mandrel-type seamless tube mill, a reducing mill, a sizing mill, etc., to produce a tube blank. The cumulative reduction of section during the hot working process is, for example, 20% to 70%.

[0189] Other hot working methods can also be used to manufacture tube blanks from small square billets. For example, in the case of thick-walled steel with short dimensions, such as pipe fittings, tube blanks can also be manufactured by forging, such as the Arachne punching method. The tube blanks are manufactured through the above processes. The wall thickness of the tube blanks is not particularly limited, for example, 9~60mm.

[0190] When the steel is round bar, the billet is first heated in a furnace. The heating temperature is not particularly limited, but can be, for example, 1100~1300°C. The billet removed from the furnace is then hot-worked to produce intermediate steel with a circular cross-section perpendicular to the axial direction. Hot-working can be performed, for example, by primary rolling on a primary rolling mill or by hot rolling on a continuous rolling mill. In a continuous rolling mill, horizontal stands with a pair of roll passes arranged vertically and vertical stands with a pair of roll passes arranged horizontally are arranged alternately.

[0191] When the steel product is a steel plate, the billet is first heated in a furnace. The heating temperature is not particularly limited, but can be, for example, 1100~1300℃. The billet, taken from the furnace, is then hot-rolled using a primary rolling mill and a continuous rolling mill to produce intermediate steel in the shape of a steel plate.

[0192] Tube blanks manufactured by hot working can be air-cooled (As-Rolled). Tube blanks manufactured by hot working can also be directly quenched after hot working without cooling to room temperature, or they can be quenched after being reheated (reheated) after hot working.

[0193] When quenching is performed directly after hot working or after reheating, cooling can be stopped or slowed during quenching. This helps prevent quenching cracks in the billet. Furthermore, when quenching is performed directly after hot working or after reheating, stress-relief annealing (SR) can be performed after quenching and before the next heat treatment process. In this case, residual stress in the billet is removed.

[0194] As described above, in the hot working process, the prepared billet is hot-worked to produce intermediate steel. The quenching process will be described in detail below.

[0195] [Quenching process]

[0196] In the quenching process, the prepared intermediate steel (tube blank) 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 800~1000℃. If the quenching temperature is too high, the original γ grains become coarser, and sometimes the steel's SSC resistance will decrease. Therefore, the preferred quenching temperature is 800~1000℃.

[0197] In this specification, when direct quenching is performed after hot working, the quenching temperature is equivalent to the surface temperature of the intermediate steel measured by a thermometer installed on the exit side of the apparatus for performing the final 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 where the reheating or reheating is performed.

[0198] Quenching methods include, for example, continuously cooling the intermediate steel (tube blank) from the quenching start temperature, thereby continuously reducing the surface temperature of the tube blank. There are no particular limitations on the continuous cooling method; any known method may be used. Examples of continuous cooling methods include immersing the tube blank in a water bath for cooling and / or accelerating the cooling of the tube blank through spray cooling or mist cooling.

[0199] If the cooling rate during quenching is too slow, the microstructure will not become primarily martensite and bainite, and the mechanical properties specified in this embodiment (yield strength exceeding 1034 MPa and below 1172 MPa) cannot be obtained. Consequently, excellent resistance to SSC cannot be achieved in this case.

[0200] 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 800-500The temperature is determined based on the temperature measured at the part of the intermediate steel that is cooled most slowly within the cross-section of the quenched intermediate steel (e.g., the center of the intermediate steel thickness in the case of forced cooling of both surfaces).

[0201] 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.

[0202] 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 sintering stress (SSC). Multiple quenchings can be performed to repeatedly heat the billet in the austenitic region, or normalizing and quenching can be performed to repeatedly heat the billet in the austenitic region. Alternatively, quenching and tempering (described later) can be combined and performed multiple times. That is, multiple quenching and tempering can also be performed. In this case, the steel's resistance to SSC is further improved. The tempering process will be described in detail below.

[0203] [Tempering process]

[0204] In the tempering process, the tube blank 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 the 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 duration the intermediate steel is held at the tempering temperature.

[0205] The tempering temperature is appropriately adjusted according to the chemical composition of the seamless steel pipe and the desired yield strength. Specifically, for a billet with the chemical composition of this embodiment, the tempering temperature is adjusted to adjust the yield strength of the seamless steel pipe to be greater than 1034 MPa and less than 1172 MPa. It should be noted that those skilled in the art can, of course, adjust the tempering temperature to adjust the yield strength of the seamless steel pipe to be greater than 1034 MPa and less than 1172 MPa. Specifically, in the tempering process of this embodiment, the preferred tempering temperature is 640~660°C.

[0206] If the tempering time is too short, it may be impossible to obtain a microstructure consisting mainly of tempered martensite and tempered bainite. On the other hand, if the tempering time is too long, the above-mentioned effects will saturate. Therefore, in the tempering process of this embodiment, the tempering time is preferably set to 10 to 90 minutes. A more preferred lower limit for the tempering time is 15 minutes. A more preferred upper limit for the tempering time is 80 minutes.

[0207] The steel of this embodiment can be manufactured using the above manufacturing method. It should be noted that the manufacturing method described above is an example of manufacturing a seamless steel pipe. However, the steel of this embodiment can also be a steel plate and / or other shapes. The manufacturing method for steel plates and / or other shapes also includes, for example, a preparation process, a quenching process, and a tempering process, similar to the manufacturing method described above. Furthermore, the above manufacturing method is an example, and other manufacturing methods can also be used to manufacture the steel.

[0208] The present invention will be further described in detail below through examples.

[0209] Example

[0210] Molten steel with the chemical compositions shown in Tables 1-1 and 1-2 is produced. It should be noted that the "-" in Table 1-2 indicates that the content of each element is at the impurity level. Specifically, the Cu, Ni, and W contents of steel A are rounded to 0% to the third decimal place. Furthermore, the Ca, Mg, Zr, and rare earth element (REM) contents of steel A are rounded to 0% to the fifth decimal place.

[0211] [Table 1-1]

[0212]

[0213] [Table 1-2]

[0214]

[0215] Using the aforementioned molten steel, round billets are manufactured using a continuous casting method. In the continuous casting method, when molten steel is introduced from the ladle into the tundish, a metal plate shaped like a cone without a bottom surface is positioned above the opening of the tundish with its apex directly below the nozzle of the ladle. Table 2 shows whether a metal plate of the aforementioned shape is positioned above the opening of the tundish. Specifically, if a metal plate of the aforementioned shape is positioned above the opening of the tundish, it is indicated as "A" in the "Metal Plate" column of Table 2. If a metal plate of the aforementioned shape is not positioned above the opening of the tundish, it is indicated as "B" in the "Metal Plate" column of Table 2. It should be noted that the metal plate of the aforementioned shape positioned above the opening of the tundish is an aluminum plate. Specifically, three aluminum plates with a thickness of 2 mm are overlapped. Furthermore, when the metal plate is positioned, after the Si oxide is discharged from the nozzle and before the molten steel is discharged, the metal plate is removed from below the nozzle using a rod with a hook formed at the tip. Then, round billets are cast from the molten steel at the casting speed described in Table 2. It should be noted that, at this time, electromagnetic stirring is performed inside the mold using the current values ​​recorded in Table 2.

[0216] [Table 2]

[0217]

[0218] After holding the prepared round billets of each test number at 1250°C for 1 hour, hot rolling was performed using the Mannesmann-mandrel method to produce tube blanks (seamless steel tubes) of each test number. Then, the resulting tube blanks of each test number were quenched. Specifically, the tube blanks of each test number were held at the temperature (°C) recorded in the "Quenching Process" column of Table 2 for the corresponding time (minutes) before undergoing water-spray quenching. Furthermore, for test number 3, after the above quenching, it was held at 900°C for 10 minutes before further quenching by water-spray quenching. It should be noted that, in each test number, the cooling rate CR during quenching... 800-500 All are within the range of 480~30000℃ / min. Here, the temperature (℃) of the quenching process is the temperature (℃) of the heat treatment furnace for heating the tube blank. Furthermore, the time (minutes) of the quenching process is the time (minutes) during which the tube blank is held at the quenching temperature.

[0219] The tube blanks for each test number were then tempered. Specifically, each tube blank for each test number was tempered at the temperature (°C) recorded in the "Tempering Process" column of Table 2 for the corresponding time (minutes). Here, the tempering temperature (°C) recorded in Table 2 refers to the temperature (°C) of the tempering furnace used to heat the tube blanks. Furthermore, the tempering time (minutes) recorded in Table 2 refers to the time (minutes) the tube blanks were held at the tempering temperature. Through the above manufacturing process, seamless steel tubes for each test number were obtained.

[0220] [Evaluation Test]

[0221] For the seamless steel pipes of each test number after the above tempering, the following tensile tests, coarse Al oxide and coarse Si oxide number density determination tests, and SSC resistance tests were performed.

[0222] [Tension Test]

[0223] Tensile tests were performed on seamless steel pipes of each test number to determine the yield strength. The tensile tests were conducted according to ASTM E8 / E8M (2021). Round bar test pieces with a parallel section diameter of 8.9 mm and a gauge length of 35.6 mm were prepared from the central portion of the wall thickness of the seamless steel pipes of each test number. The axial direction of the round bar test pieces was parallel to the pipe axis. Using the prepared round bar test pieces, tensile tests were performed at room temperature (25°C) in atmospheric conditions to obtain the yield strength (MPa) of the seamless steel pipes of each test number. It should be noted that in this embodiment, the stress at 0.65% elongation (0.65% yield strength) obtained in the tensile test is defined as the yield strength. The obtained yield strength (MPa) is shown as "YS (MPa)" in Table 3.

[0224] [Table 3]

[0225]

[0226] [Number Density Measurement Experiment of Coarse Al Oxide and Coarse Si Oxide]

[0227] For seamless steel pipes of each test number, the number density of coarse Al oxide and coarse Si oxide was determined to obtain the number density of Al oxide (coarse Al oxide) and Si oxide (coarse Si oxide) with a length diameter of 5.0 μm or more. Test pieces prepared from the central portion of the wall thickness of the seamless steel pipes of each test number were used to determine the number density of coarse Al oxide and coarse Si oxide using the above method. The obtained number density of coarse Al oxide (numbers / 200 mm) was then calculated. 2 Table 3 shows the "coarse Al oxides (number / 200mm)". 2 The column indicates the number density of coarse Si oxide particles (particles / 200 mm²). 2 Table 3 shows the "coarse Si oxides (number / 200mm)". 2 )"column.

[0228] [SSC Resistance Test]

[0229] For each test number of seamless steel pipe, SSC resistance was evaluated by conducting a SSC resistance test according to NACE TM0177-2016 Method A. Specifically, round bar test pieces with a diameter of 6.35 mm and a parallel section length of 25.4 mm were prepared from the central portion of the wall thickness of each test number of seamless steel pipe. Three of the prepared test pieces were subjected to the SSC resistance test. It should be noted that the axial direction of the test piece was parallel to the pipe axis.

[0230] Tensile stress was applied axially to the round bar test pieces of each test number. The applied stress was then adjusted to 90% of the actual yield stress of each steel plate. The test solution used was a mixed aqueous solution of 5.0% by mass sodium chloride and 0.4% by mass sodium acetate adjusted to pH 5.0 with hydrochloric acid (NACE solution D). The test solution at 24°C was injected into three test containers as test baths. The three stressed round bar test pieces were immersed one by one in the test baths of different test containers. After degassing each test bath, a mixture of 0.01 atm H₂S gas and 0.99 atm CO₂ gas was blown into the test bath to saturate it. The gas-saturated test baths were maintained at 24°C for 720 hours.

[0231] For the round bar test pieces of each test number after holding for 720 hours, observe whether sulfide stress cracks (SSCs) have formed. Specifically, observe the round bar test pieces after holding for 720 hours with the naked eye. For each test number, the number of test pieces with SSCs out of the three round bar test pieces is shown in the "Number of test pieces with SSCs (pieces)" column of Table 3.

[0232] [Evaluation Results]

[0233] Referring to Tables 1-1, 1-2, 2, and 3, the seamless steel pipes tested 1-12 had appropriate chemical compositions and their manufacturing methods met the aforementioned preferred conditions. As a result, these seamless steel pipes exhibited yield strengths exceeding 1034 MPa and below 1172 MPa, and a coarse Al oxide density of less than 30 particles / 200 mm². 2 Furthermore, the number density of coarse Si oxide particles is less than or equal to 5 per 200 mm. 2 As a result, these seamless steel pipes exhibited zero SSC formation in the SSC resistance test. That is, the seamless steel pipes tested (numbers 1-12) possessed a yield strength exceeding 1034 MPa and below 1172 MPa, along with excellent SSC resistance. It should be noted that the sum of the volume fractions of tempered martensite and tempered bainite in the microstructure of these seamless steel pipes was determined to be over 90%.

[0234] On the other hand, the seamless steel pipes tested in experiments 13 and 14 were cast at excessively high speeds during the steelmaking process. As a result, the number density of coarse Al oxide particles in these seamless steel pipes was greater than or equal to 30 particles / 200 mm. 2 As a result, these seamless steel pipes produced more than one SSC in the SSC resistance test, and did not have excellent SSC resistance.

[0235] Seamless steel pipes tested (numbers 15-17) were produced without the use of sheet metal in the steelmaking process. As a result, the coarse Si oxide number density in these seamless steel pipes exceeded 5 per 200 mm. 2 As a result, these seamless steel pipes produced more than one SSC in the SSC resistance test, and did not have excellent SSC resistance.

[0236] The seamless steel pipe in test number 18 had an excessively high oxygen content. As a result, this seamless steel pipe produced more than one SSC in the SSC resistance test and did not exhibit excellent SSC resistance.

[0237] The Mo content of the seamless steel pipe in test number 19 was too low. As a result, the seamless steel pipe produced more than one SSC in the SSC resistance test and did not have excellent SSC resistance.

[0238] The seamless steel pipe in test number 20 had an excessively high sulfur content. As a result, this seamless steel pipe produced more than one SSC in the SSC resistance test and did not exhibit excellent SSC resistance.

[0239] 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 steel material, which consists of, in mass % C：0.15~0.45%、 Si: 0.05 to 1.00%, Mn: 0.05 to 1.00%, P: 0.030% or less, S: 0.0050% or less, Al：0.005~0.100%、 Cr：0.30~1.50%、 Mo: 0.40 to 2.00%, Ti: 0.002 to 0.020%, Nb: 0.002 to 0.100%, V：0.05~0.30%、 B：0.0005~0.0040%、 N: 0.0100% or less, O: 0.0040% or less, Cu: 0 to 0.50%, Ni: 0 to 0.50%, W：0~0.50%、 Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Zr：0~0.0100%、 a rare earth element: 0 to 0.0100%, and the balance being Fe and impurities, a yield strength exceeding 1034 MPa and being 1172 MPa or less, in the steel material, The number density of Al oxide having a length of 5.0 μm or more and an Al content of 20% or more and an O content of 10% or more by mass% is less than 30 pieces / 200 mm 2 , The number density of Si oxides having an Al content of less than 20% by mass, a Si content of 20% or more, an O content of 10% or more, and a length diameter of 5.0 μm or more is 5 or less per 200 mm 2 .

2. The steel material according to claim 1, which contains one or more elements selected from the group consisting of Cu: 0.01 to 0.50%, Ni: 0.01 to 0.50%, W：0.01~0.50%、 Ca: 0.0001 to 0.0100%, Mg: 0.0001 to 0.0100%, Zr: 0.0001 to 0.0100%, and a rare earth element: 0.0001 to 0.0100%.

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

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