Steel material

Abstract

Provided is a steel material having both high strength and excellent hydrogen embrittlement resistance. A steel material according to the present disclosure comprises, in mass%, 0.15 to 0.45% of C, 0.05 to 1.00% of Si, 0.05 to 1.00% of Mn, 0.030% or less of P, 0.0050% or less of S, 0.005 to 0.100% of Al, 0.30 to 1.50% of Cr, 0.40 to 2.00% of Mo, 0.002 to 0.020% of Ti, 0.002 to 0.100% of Nb, 0.05 to 0.30% of V, 0.0005 to 0.0040% of B, 0.0100% or less of N, 0.0040% or less of O, and the balance of Fe and impurities, and has a yield strength of 965 MPa or more. In the steel material, the number density of Si oxides having a major axis of 5.0 [mu] m or more, an Si content of 20% or more in mass%, and an O content of 10% or more is 5 / 200 mm2 or less.

Description

Technical Field

[0001] This disclosure pertains to steel. Background Technology

[0002] Oil wells and / or gas wells (hereinafter collectively referred to as "oil wells") contain environments containing large amounts of corrosive substances. These corrosive substances include, for example, corrosive gases such as hydrogen sulfide. In this specification, environments containing hydrogen sulfide are referred to as "acidic environments." The temperature of an acidic environment also depends on the depth of the well, ranging from ambient temperature to approximately 200°C.

[0003] Steels used in such acidic environments include, for example, oil well steel used as oil well pipes and / or pipeline steel used as line pipes. In recent years, due to the increasing depth of oil wells, there has been a demand for higher strength oil well steel and similar products.

[0004] On the other hand, when steel is used in acidic environments, the steel surface comes into contact with corrosive substances, resulting in an electrochemical reaction that produces hydrogen on the steel surface. Due to this hydrogen, the steel is prone to hydrogen embrittlement cracking, exemplified by sulfide stress cracking (SSC). Therefore, steel used in acidic environments requires high strength as well as excellent resistance to hydrogen embrittlement.

[0005] Japanese Patent Application Publication No. 2011-246798 (Patent Document 1) and Japanese Patent Application Publication No. 2015-38247 (Patent Document 2) disclose technologies for improving the resistance to hydrogen embrittlement of steel used in acidic environments.

[0006] In Patent Document 1, a specified amount of dissolved Mo is ensured in the oil well pipe made of low-alloy steel to refine the original austenite grains and disperse M2C-type precipitates. This improves resistance to SSC (Segregation of Sludge Strains). Patent Document 1 further improves resistance to hydrogen embrittlement by forming Mo segregation regions at the original austenite grain boundaries.

[0007] In Patent Document 2, hydrogen embrittlement resistance is improved in oil well steel pipes made of low alloy steel by suppressing Mo segregation regions as much as possible.

[0008] Furthermore, the development of fuel cell vehicles powered by hydrogen and the practical application of hydrogen stations supplying hydrogen to these vehicles are currently underway. High-pressure hydrogen is stored in high-pressure hydrogen accumulators installed at hydrogen stations. Additionally, the development of fuel cell vehicles equipped with high-pressure hydrogen cylinders is also progressing. The steel used in high-pressure hydrogen containers, such as high-pressure hydrogen accumulators and / or high-pressure hydrogen cylinders, requires high strength and excellent resistance to hydrogen embrittlement.

[0009] Japanese Patent Application Publication No. 2009-74122 (Patent Document 3) discloses a technique for improving the hydrogen embrittlement resistance of steel used in high-pressure hydrogen containers. In Patent Document 3, by increasing the V and Mo content in a low-alloy steel compared to the past, the morphology of carbides at the original austenite grain boundaries is improved, thereby enhancing the hydrogen embrittlement resistance.

[0010] Existing technical documents

[0011] Patent documents

[0012] Patent Document 1: Japanese Patent Application Publication No. 2011-246798

[0013] Patent Document 2: Japanese Patent Application Publication No. 2015-38247

[0014] Patent Document 3: Japanese Patent Application Publication No. 2009-74122 Summary of the Invention

[0015] The problem the invention aims to solve

[0016] According to the technology disclosed in Patent Documents 1 to 3, the hydrogen embrittlement resistance of steel intended for use in acidic environments and / or for high-pressure hydrogen containers can be improved. However, steel with high strength and excellent hydrogen embrittlement resistance can also be obtained by means other than those described in Patent Documents 1 to 3.

[0017] The purpose of this disclosure is to provide a steel that combines high strength with excellent resistance to hydrogen embrittlement.

[0018] Solution for solving the problem

[0019] The steel disclosed herein is expressed as a percentage by mass.

[0020] C: 0.15~0.45%

[0021] Si: 0.05~1.00%

[0022] Mn: 0.05~1.00%,

[0023] P: below 0.030%

[0024] S: Below 0.0050%

[0025] Al: 0.005~0.100%

[0026] Cr: 0.30~1.50%

[0027] Mo: 0.40~2.00%

[0028] Ti: 0.002~0.020%

[0029] Nb: 0.002~0.100%

[0030] V: 0.05~0.30%,

[0031] B: 0.0005~0.0040%

[0032] N: below 0.0100%

[0033] O: Below 0.0040%

[0034] Cu: 0~0.50%,

[0035] Ni: 0~0.50%

[0036] W: 0~0.50%,

[0037] Ca: 0~0.0100%

[0038] Mg: 0~0.0100%

[0039] Zr: 0~0.0100%

[0040] Rare earth elements: 0~0.0100%, and

[0041] The balance consists of Fe and impurities.

[0042] Yield strength above 965 MPa

[0043] Among the aforementioned steels,

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

[0045] The effects of the invention

[0046] The steel disclosed herein combines high strength with excellent resistance to hydrogen embrittlement. 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 The graph shows the relationship between the relative fracture stress (=BS1 / BS0), which is an indicator of hydrogen embrittlement resistance. Detailed Implementation

[0048] The inventors first envisioned its use in acidic environments and / or in high-pressure hydrogen containers, and studied the development of a high-strength steel with a yield strength of 965 MPa or higher (140 kSi or higher). Furthermore, focusing on chemical composition, the inventors investigated a steel that balances a yield strength of 965 MPa or higher with excellent resistance to hydrogen embrittlement. As a result, the inventors determined that, by 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: 0.05–0.30%. Steels containing the following components and the balance being Fe and impurities may possess a yield strength of over 965 MPa and excellent resistance to hydrogen embrittlement: 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%.

[0049] On the other hand, even steels with the aforementioned chemical composition, while possessing a yield strength of 965 MPa or higher, sometimes fail to exhibit excellent resistance to hydrogen embrittlement under high-pressure hydrogen conditions. Therefore, the inventors conducted a detailed study on the main reasons for the reduced hydrogen embrittlement resistance of steels with the aforementioned chemical composition and a yield strength of 965 MPa or higher. The results clarified that steels with the aforementioned chemical composition may contain coarse Si oxides. If the steel contains coarse Si oxides, its resistance to hydrogen embrittlement may be reduced.

[0050] In this specification, Si oxides with a major diameter of 5.0 μm or more and a Si content of 20% or more and an O content of 10% or more by mass are also referred to as "coarse Si oxides". The inventors' detailed research has clarified that setting the number density of coarse Si oxides to less than or equal to 5 per 200 mm is effective. 2 It achieves both a yield strength exceeding 965 MPa and excellent resistance to hydrogen embrittlement. This is illustrated in the accompanying diagram.

[0051] 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. 2The graph shows the relationship between the relative fracture stress (=BS1 / BS0), which is an indicator of hydrogen embrittlement resistance. Figure 1 The steel having the above-described chemical composition and yield strength of 965 MPa or more in the embodiments described later is manufactured using the number density of coarse Si oxides and the relative fracture stress determined by the method described later.

[0052] Reference Figure 1 In steels with the above chemical composition and a yield strength of 965 MPa or higher, if the density of coarse Si oxide particles is less than or equal to 5 per 200 mm², 2 When the relative fracture stress is 0.85 or higher, excellent resistance to hydrogen embrittlement can be confirmed. Therefore, the steel of this embodiment has the above-mentioned chemical composition and yield strength of 965 MPa or higher, and furthermore, the number density of coarse Si oxides 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 over 965 MPa and excellent resistance to hydrogen embrittlement.

[0053] Regarding the method of reducing the number density of coarse Si oxide particles to less than or equal to 5 per 200 mm² 2 The reasons for improving the hydrogen embrittlement resistance of steel are not yet clear in detail. 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, although Al oxides, represented by Al2O3, have been studied, Si oxides have not been considered. However, Si oxides, especially coarse Si oxides with a length diameter of 5.0 μm or more, are more likely to reduce the hydrogen embrittlement resistance of steel than Al oxides. Therefore, the inventors speculate that by reducing the number density of coarse Si oxides to less than or equal to 5 per 200 mm... 2 Whether it can improve the hydrogen embrittlement resistance of steel.

[0054] It should be noted that, based on a mechanism different from the inventors' hypothesis, the hydrogen embrittlement resistance of the steel may also be improved. The steel possesses the aforementioned chemical composition, with a coarse Si oxide number density of less than or equal to 5 per 200 mm². 2 As a result, a yield strength of over 965 MPa and excellent resistance to hydrogen embrittlement are achieved, as demonstrated in the examples described later. Therefore, the steel of this embodiment possesses the aforementioned chemical composition and a yield strength of over 965 MPa, and the number density of coarse Si oxides in the steel is less than or equal to 5 per 200 mm². 2 As a result, the steel of this embodiment can achieve both a yield strength of over 965 MPa and excellent resistance to hydrogen embrittlement.

[0055] The main points of the steel used in this embodiment, based on the above insights, are as follows. [1]

[0057] A type of steel, which is expressed as a percentage by mass.

[0058] C: 0.15~0.45%

[0059] Si: 0.05~1.00%

[0060] Mn: 0.05~1.00%,

[0061] P: below 0.030%

[0062] S: Below 0.0050%

[0063] Al: 0.005~0.100%

[0064] Cr: 0.30~1.50%

[0065] Mo: 0.40~2.00%

[0066] Ti: 0.002~0.020%

[0067] Nb: 0.002~0.100%

[0068] V: 0.05~0.30%,

[0069] B: 0.0005~0.0040%

[0070] N: below 0.0100%

[0071] O: Below 0.0040%

[0072] Cu: 0~0.50%,

[0073] Ni: 0~0.50%

[0074] W: 0~0.50%,

[0075] Ca: 0~0.0100%

[0076] Mg: 0~0.0100%

[0077] Zr: 0~0.0100%

[0078] Rare earth elements: 0~0.0100%, and

[0079] The balance consists of Fe and impurities.

[0080] Yield strength above 965 MPa

[0081] Among the aforementioned steels,

[0082] The number density of Si oxides with a major diameter of 5.0 μm or more, a Si content of 20% or more by mass%, and an O content of 10% or more is less than or equal to 5 per 200 mm. 2 . [2]

[0084] According to the steel described in [1], it contains selected free...

[0085] Cu: 0.01~0.50%,

[0086] Ni: 0.01~0.50%

[0087] W: 0.01~0.50%,

[0088] Ca: 0.0001~0.0100%

[0089] Mg: 0.0001~0.0100%

[0090] Zr: 0.0001~0.0100%, and

[0091] Rare earth elements: one or more elements in a group consisting of 0.0001 to 0.0100%. [3]

[0093] According to the steel described in [1] or [2], wherein,

[0094] The steel mentioned above is seamless steel pipe.

[0095] 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.

[0096] The steel used in this embodiment can be optionally oil well steel pipe, pipeline steel pipe, or high-pressure hydrogen container steel pipe. Here, in this specification, "oil well steel pipe" refers to steel pipe used as an oil well pipe. Oil well pipe is a general term for casing, pipes, and drill pipes used for oil or gas well excavation, crude oil or natural gas extraction, etc.

[0097] In addition, in this specification, "pipeline steel pipe" refers to steel pipe used for pipeline applications, constituting a pipeline for transporting production fluids (crude oil or natural gas) collected from oil or gas wells. Examples of pipelines include oil discharge pipelines that transport production fluids from oil or gas wells, collection pipelines that collect and transport production fluids transported by oil discharge pipelines to primary processing facilities, trunk lines that transport production fluids that have undergone primary processing such as dehydration to markets, and distribution pipelines that transport production fluids to consumers.

[0098] In this specification, "steel pipe for high-pressure hydrogen containers" refers to steel pipes used in high-pressure hydrogen containers that store high-pressure hydrogen gas, standardized according to ISO 11439, ANSI / NGV, the High Pressure Gas Safety Law, and the Container Safety Code. Examples of high-pressure hydrogen containers include high-pressure hydrogen accumulators installed in hydrogen stations and high-pressure hydrogen cylinders mounted in fuel cell vehicles.

[0099] The steel used in this embodiment will be described in detail below. Unless otherwise specified, "%" related to elements refers to mass%.

[0100] [Chemical Composition]

[0101] The steel in this embodiment contains the following elements in its chemical composition.

[0102] C: 0.15~0.45%

[0103] 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 hydrogen embrittlement (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, reducing the steel's resistance to hydrogen embrittlement. Therefore, the C content is 0.15~0.45%. 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%, and more preferably 0.35%.

[0104] Si: 0.05~1.00%

[0105] 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 hydrogen embrittlement. 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%.

[0106] Mn: 0.05~1.00%

[0107] 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 hydrogen embrittlement resistance of the steel. 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%, more preferably 0.50%, and more preferably 0.40%.

[0108] P: below 0.030%

[0109] 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 hydrogen embrittlement 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%.

[0110] S: Below 0.0050%

[0111] 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 hydrogen embrittlement 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 0.0031%, 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%.

[0112] Al: 0.005~0.100%

[0113] 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 hydrogen embrittlement 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, coarse Al oxides will be formed, and the hydrogen embrittlement 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".

[0114] Cr: 0.30~1.50%

[0115] 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 hydrogen embrittlement is improved. If the Cr 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 Cr content is too high, the hydrogen embrittlement resistance of the steel decreases 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%.

[0116] Mo: 0.40~2.00%

[0117] 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 hydrogen embrittlement is improved. If the Mo 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 Mo content is too high, coarse carbides will form even if the contents of other elements are within the range of this embodiment, reducing the steel's resistance to hydrogen embrittlement. Therefore, the Mo content is 0.40~2.00%. The preferred lower limit of the Mo content is 0.45%, more preferably 0.49%, more preferably 0.50%, 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%.

[0118] Ti: 0.002~0.020%

[0119] Titanium (Ti) combines with nitrogen to form nitrides, which, through a pinning effect, refine the grain size of the steel. As a result, the steel's resistance to hydrogen embrittlement is improved. If the Ti content is too low, even if the contents of other elements are within the range of this embodiment, the aforementioned effect cannot be fully achieved. On the other hand, if the Ti content is too high, even if the contents of other elements are within the range of this embodiment, the Ti nitrides will become coarse, and the steel's resistance to hydrogen embrittlement 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%.

[0120] Nb: 0.002~0.100%

[0121] 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 hydrogen embrittlement. Nb also forms fine carbides during tempering, increasing the steel's resistance to temper 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 hydrogen embrittlement. 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 for Nb content is 0.080%, more preferably 0.060%, and even more preferably 0.040%.

[0122] V: 0.05~0.30%

[0123] 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 hydrogen embrittlement. V also forms fine carbides during tempering, increasing the steel's resistance to temper 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 aforementioned effects cannot be fully achieved. 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 hydrogen embrittlement. 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%.

[0124] B: 0.0005~0.0040%

[0125] Boron (B) dissolves in steel, improving its hardenability and strength. B also inhibits grain boundary segregation of phosphorus (P), enhancing the steel's resistance to hydrogen embrittlement. If the B content is too low, the aforementioned effects cannot be fully achieved even with the contents of other elements within the range specified in this embodiment. Conversely, if the B content is too high, coarse nitrides will form even with the contents of other elements within the range specified in this embodiment, reducing the steel's resistance to hydrogen embrittlement. Therefore, the B content is 0.0005 to 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%.

[0126] N: below 0.0100%

[0127] 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, reducing the steel's resistance to hydrogen embrittlement. 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 0.0045%. 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%.

[0128] O: Below 0.0040%

[0129] 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 hydrogen embrittlement 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%.

[0130] 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.

[0131] [Any element]

[0132] 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.

[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 improves the steel's resistance to hydrogen embrittlement. Even a small amount of Cu is sufficient to achieve the aforementioned effect. However, if the Cu content is too high, the hot workability 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%.

[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 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%.

[0137] The chemical composition of the steel mentioned above may also contain W to replace a portion of Fe.

[0138] W: 0~0.50%

[0139] 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. As a result, the steel's resistance to hydrogen embrittlement is improved. 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 steel's resistance to hydrogen embrittlement. 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%, more preferably 0.48%.

[0140] 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 hydrogen embrittlement.

[0141] Ca: 0~0.0100%

[0142] 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 hydrogen embrittlement. 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 hydrogen embrittlement. 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%.

[0143] Mg: 0~0.0100%

[0144] 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 hydrogen embrittlement. 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 hydrogen embrittlement. 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%.

[0145] Zr: 0~0.0100%

[0146] Zirconium (Zr) can be any element, or it can 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 hydrogen embrittlement. 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 resistance to hydrogen embrittlement. 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%.

[0147] Rare earth elements (REM): 0~0.0100%

[0148] 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 hydrogen embrittlement. 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 hydrogen embrittlement 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, and the steel's resistance to hydrogen embrittlement will decrease. 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%.

[0149] 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.

[0150] [Yield Strength]

[0151] The steel of this embodiment has a yield strength of 965 MPa or higher (140 ksi or higher). The yield strength referred to in this specification is the stress (0.65% yield strength) obtained in a tensile test at room temperature (25°C) according to ASTM E8 / E8M (2021) with 0.65% elongation. The steel of this embodiment has the above-described chemical composition and, by satisfying the number density of coarse Si oxides described later, exhibits excellent resistance to SSC even with a yield strength of 965 MPa or higher. It should be noted that in this embodiment, the upper limit of the yield strength is not particularly limited, for example, it is 1172 MPa. In this embodiment, the preferred lower limit of the yield strength is 986 MPa, more preferably 1000 MPa, more preferably 1030 MPa, more preferably exceeding 1034 MPa, more preferably 1035 MPa, and more preferably 1040 MPa.

[0152] 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.

[0153] [Number density of coarse Si oxides]

[0154] The steel of this embodiment has the above-described chemical composition and a yield strength of 965 MPa or more. Furthermore, the number density of coarse Si oxides in the steel is less than or equal to 5 per 200 mm². 2 As described above, in this specification, particles with 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 a major diameter of 5.0 μm or more and a Si content of 20% or more and an O content of 10% or more by mass.

[0155] As mentioned above, Si oxides have not received much attention to date due to their small quantity. However, even with a small number of coarse Si oxides, the reduction in hydrogen embrittlement resistance can easily become apparent when the yield strength is high (above 965 MPa). Therefore, by setting the number density of coarse Si oxides to less than or equal to 5 per 200 mm², [further research is needed]. 2 Even when the yield strength is increased to 965 MPa or higher, excellent resistance to hydrogen embrittlement can still be stably obtained. Therefore, the steel of this embodiment has the above-mentioned chemical composition and a yield strength of 965 MPa or higher, and furthermore, the number density of coarse Si oxides in the steel is set to be less than or equal to 5 per 200 mm². 2 .

[0156] 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 .

[0157] In this embodiment, 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 central portion of the plate width and the portion of the plate thickness t / 4, with the surface including the rolling direction and the plate thickness direction as the observation surface. Here, the portion of the plate thickness t / 4 refers to the position at a depth t / 4 from the surface of the steel plate when the plate thickness is set to t. When the steel is a steel pipe, a test piece is made from the central portion 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 round steel, a test piece is made with the surface including the R / 2 position in the center and including both the axial and radial directions as the observation surface.

[0158] 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 Si content (by mass%) was greater than 20% and the O content was greater than 10%, the particle was identified as a "Si oxide".

[0159] 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 determined, and the total number of coarse Si oxides is calculated. It should be noted that the major diameter of the Si oxides can be determined using known methods. Furthermore, in this specification, the major diameter of a Si oxide refers to the largest line segment (μm) connecting any two points on the outer periphery of the Si oxide in the observation plane.

[0160] 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 Si oxide particles (particles / 200mm²) is... 2 The obtained value is rounded to the nearest decimal place. Furthermore, the number density of 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.

[0161] [Resistance to hydrogen embrittlement]

[0162] The steel of this embodiment has the above-mentioned chemical composition and a yield strength of 965 MPa or more, and the number density of coarse Si oxides in the steel is less than or equal to 5 per 200 mm². 2 As a result, the steel of this embodiment combines high strength with excellent resistance to hydrogen embrittlement. In this embodiment, the excellent resistance to hydrogen embrittlement can be evaluated using the following methods.

[0163] Test pieces for evaluating hydrogen embrittlement resistance are made from the steel of this embodiment. The test piece is a round bar test piece with an annular notch. For example, the outer diameter of the parallel portion of the test piece is 4.0 mm, the length of the parallel portion is 25 mm, and an annular notch is formed at the center of the long side of the parallel portion. In this case, regarding the notch shape, the notch depth is 0.3 mm, the notch angle is 60°, and the radius of curvature at the bottom of the notch is 0.125 mm. When the steel is a steel plate, a round bar test piece is made from the center of the plate width and the portion of the plate thickness t / 4. 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 of the steel pipe. When the steel is a round bar, a round bar test piece is made from the R / 2 position. In this case, the axial direction of the round bar test piece is set parallel to the axial direction of the round bar.

[0164] The prepared circular rod test piece with a ring-shaped notch was charged with hydrogen using the cathode hydrogen charging method. Specifically, a room-temperature cathode hydrogen charging solution was prepared. The cathode hydrogen charging solution was an aqueous solution containing 5% sodium chloride solution at room temperature, 30 g / L NH4SCN, and an acetate buffer solution. The pH was adjusted to 3.5 before the experiment using the acetate buffer solution.

[0165] With the annularly notched round bar test piece immersed in the cathode hydrogen-filled solution, the potential is set to -1.5V and the filling time is set to 24 hours to fill the annularly notched round bar test piece with hydrogen. At this time, it is preferable to form a zinc-plated coating on the surface of the hydrogen-filled annularly notched round bar test piece to prevent hydrogen from leaking to the outside.

[0166] For hydrogen-filled round bar specimens with ring-shaped notches, tensile tests were performed using a low strain rate testing machine (SSRT) at room temperature (25°C) in atmospheric conditions. The strain rate was set to 4.2 × 10⁻⁶. -6 The fracture stress BS1 (MPa) is calculated per second. For a round bar test piece with annular notches that has not been hydrogen-filled, a tensile test is performed under the same conditions, and the fracture stress BS0 (MPa) is calculated. It should be noted that in this embodiment, the fracture stress (MPa) is calculated by rounding the first decimal place. Furthermore, the ratio of the obtained fracture stress BS1 (MPa) to the fracture stress BS0 (MPa) is defined as the relative fracture stress (=BS1 / BS0). In this embodiment, if the relative fracture stress is 0.85 or higher, it is judged to have excellent hydrogen embrittlement resistance. It should be noted that in this embodiment, the relative fracture stress is calculated by rounding the third decimal place.

[0167] [Microstructure]

[0168] 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 965 MPa or more and excellent resistance to hydrogen embrittlement can be achieved. That is, in this embodiment, if the steel achieves both a yield strength of 965 MPa or more and excellent resistance to hydrogen embrittlement (SSC), then it is determined that the sum of the volume fractions of tempered martensite and tempered bainite in the microstructure is 90% or more.

[0169] It should be noted that the volume fractions of tempered martensite and tempered bainite, determined by observation, can be obtained 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 width and a portion of the sheet thickness t / 4. 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 axial and radial directions, encompassing a central R / 2 position, is prepared.

[0170] 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.

[0171] [Manufacturing Method]

[0172] 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.

[0173] [Steelmaking process]

[0174] 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.

[0175] 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.

[0176] 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.

[0177] 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.

[0178] 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.

[0179] 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.

[0180] The molten steel is cast using the methods described above to produce billets. The preferred billet is a small square billet with a circular cross-section (round billet). The method for producing the billet is not particularly limited. For example, molten steel can be cast into round billets using continuous casting. Alternatively, molten steel can be cast to produce small square billets with a rectangular cross-section, or large square billets. In these cases, initial rolling is preferred to produce round billets from small or large square billets with a rectangular cross-section.

[0181] [Heat treatment process]

[0182] 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.

[0183] 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%.

[0184] 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.

[0185] 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.

[0186] 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.

[0187] 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.

[0188] 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.

[0189] 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.

[0190] [Quenching process]

[0191] 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℃.

[0192] 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.

[0193] 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.

[0194] If the cooling rate during quenching is too slow, the microstructure will not form a martensitic and bainitic matrix, and the mechanical properties specified in this embodiment (yield strength of 965 MPa or higher) cannot be obtained. Consequently, excellent resistance to SSC cannot be achieved in this case.

[0195] 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 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).

[0196] 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.

[0197] 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.

[0198] [Tempering process]

[0199] 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.

[0200] The tempering temperature is adjusted appropriately based on 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 achieve a yield strength of 965 MPa or higher for the seamless steel pipe. It should be noted that those skilled in the art can, of course, adjust the tempering temperature to achieve a yield strength of 965 MPa or higher for the seamless steel pipe. Specifically, in the tempering process of this embodiment, a preferred tempering temperature is 640~680°C.

[0201] 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.

[0202] 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.

[0203] The present invention will be further described in detail below through examples.

[0204] Example

[0205] 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.

[0206] [Table 1-1]

[0207]

[0208] [Table 1-2]

[0209]

[0210] 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 used overlapping. 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 front end.

[0211] [Table 2]

[0212]

[0213] 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 5, 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.

[0214] 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.

[0215] [Evaluation Test]

[0216] For the seamless steel pipes of each test number after tempering, the following tensile tests, coarse Si oxide number density determination tests, and hydrogen embrittlement resistance evaluation tests were performed.

[0217] [Tension Test]

[0218] 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.

[0219] [Table 3]

[0220]

[0221] [Number Density Measurement Experiment of Coarse Si Oxides]

[0222] For seamless steel pipes of each test number, a number density test was conducted to determine the number density of coarse Si oxides (coarse Si oxides) in diameters greater than 5.0 μm. 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 Si oxides using the method described above. The obtained number density of coarse Si oxides (numbers / 200 mm) was then calculated. 2 Table 3 shows the "coarse Si oxides (number / 200mm)". 2 )"column.

[0223] [Evaluation Test for Hydrogen Embrittlement Resistance]

[0224] Two circular bar test pieces with annular notches were prepared from the central portion of the wall thickness of the seamless steel pipe for each test number. The outer diameter of the parallel portion of each test piece was 4.0 mm, and the length of the parallel portion was 25 mm. An annular notch was formed at the center of the long side of the parallel portion. For the notch shape, the depth was 0.3 mm, the notch angle was 60°, and the radius of curvature at the bottom of the notch was 0.125 mm. It should be noted that the circular bar test pieces were prepared with the axial direction of the bar parallel to the rolling direction (pipe axis direction) of the seamless steel pipe.

[0225] Hydrogen charging was performed on one of two circular rod test pieces with ring-shaped cuts using the cathode hydrogen charging method. Specifically, a room-temperature cathode hydrogen charging solution was prepared. The cathode hydrogen charging solution was an aqueous solution containing 5% sodium chloride (by mass) at room temperature, 30 g / L NH4SCN, and an acetate buffer solution. The pH was adjusted to 3.5 before the experiment using the acetate buffer solution.

[0226] With the annularly notched round bar test piece immersed in a cathodic hydrogen-filled solution, the potential was set to -1.5V and the filling time to 24 hours, and the round bar test piece with the annular notch was hydrogen-filled. That is, an acidic environment was simulated by hydrogen filling. A zinc-plated coating was then formed on the surface of the hydrogen-filled round bar test piece under the same conditions in each test number, ensuring that hydrogen inside the round bar test piece would not leak to the outside. It should be noted that the other round bar test piece with the annular notch was not hydrogen-filled.

[0227] For round bar test specimens with a galvanized coating and annular notches, a low strain rate testing machine (SSRT) was used at room temperature and atmospheric pressure at 4.2 × 10⁻⁶. -6 Tensile tests were conducted at a strain rate of / second to determine the fracture stress BS1 (MPa) in the hydrogen environment.

[0228] Furthermore, for the uncharged, annularly cut cylindrical test specimens of each test number, a low strain rate testing machine (SSRT) was used at room temperature and in atmospheric conditions at a strain rate of 4.2 × 10⁻⁶. -6 A tensile test was conducted at a strain rate of / second to determine the fracture stress BS0 (MPa) in the atmosphere.

[0229] The obtained atmospheric fracture stress BS0 (MPa) is shown in the "Atmospheric BS0 (MPa)" column of the "Notched Tensile Test Results" section in Table 3. The obtained fracture stress BS1 (MPa) in the hydrogen environment is shown in the "Hydrogen Environment BS1 (MPa)" column of the "Notched Tensile Test Results" section in Table 3. The relative fracture stress (BS1 / BS0) calculated from the atmospheric fracture stress BS0 (MPa) and the hydrogen environment fracture stress BS1 (MPa) is shown in Table 3.

[0230] [Evaluation Results]

[0231] Referring to Tables 1-1, 1-2, 2, and 3, the seamless steel pipes tested 1-17 had appropriate chemical compositions and their manufacturing methods met the aforementioned preferred conditions. As a result, these seamless steel pipes exhibited yield strengths of 965 MPa or higher, and furthermore, a coarse Si oxide number density of less than or equal to 5 per 200 mm². 2As a result, these seamless steel pipes exhibited a relative fracture stress of over 0.85 in the hydrogen embrittlement resistance evaluation test. That is, the seamless steel pipes tested (numbers 1-17) combined a yield strength of over 965 MPa with excellent hydrogen embrittlement resistance. It should be noted that the total volume fraction of tempered martensite and tempered bainite in the microstructure of these seamless steel pipes was determined to be over 90%.

[0232] Seamless steel pipes tested (numbers 18-22) were produced without the use of sheet metal in the steelmaking process. As a result, the number density of coarse Si oxide particles in these seamless steel pipes exceeded 5 per 200 mm. 2 As a result, in the hydrogen embrittlement resistance evaluation test, the relative fracture stress of these seamless steel pipes was less than 0.85, indicating that they did not have excellent hydrogen embrittlement resistance.

[0233] The seamless steel pipe in test number 23 had an excessively high oxygen content. As a result, in the hydrogen embrittlement resistance evaluation test, the relative fracture stress of this seamless steel pipe was less than 0.85, indicating that it did not exhibit excellent hydrogen embrittlement resistance.

[0234] The Mo content of the seamless steel pipe in test number 24 was too low. As a result, the relative fracture stress of this seamless steel pipe was less than 0.85 in the hydrogen embrittlement resistance evaluation test, and it did not have excellent hydrogen embrittlement resistance.

[0235] The seamless steel pipe in test number 25 had an excessively high sulfur content. As a result, in the hydrogen embrittlement resistance evaluation test, the relative fracture stress of this seamless steel pipe was less than 0.85, indicating that it did not exhibit excellent hydrogen embrittlement resistance.

[0236] 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 of 965 MPa or more, in the steel material, The number density of Si oxides having a length of 5.0 μm or more and a Si content of 20% or more and an O content of 10% or more by mass 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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