Steel material

Abstract

The present invention provides a steel material which achieves both high strength and excellent hydrogen embrittlement resistance. A steel material according to the present disclosure contains, 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, and 0.0040% or less of O, with the balance being made up of Fe and impurities, and has a yield strength of 965 MPa or more. In the steel material, the number density of Si oxide grains having an Si content of 20% by mass or more, an O content of 10% by mass or more, and a length of 5.0 μm or more is 5 per 200 mm2 or less.

Description

steel material

[0001] The present disclosure relates to steel products.

[0002] Some oil wells and gas wells (hereinafter, oil wells and gas wells are collectively referred to as "oil wells") contain environments that contain a large amount of corrosive substances. Examples of corrosive substances include corrosive gases such as hydrogen sulfide. In this specification, an environment containing hydrogen sulfide is referred to as a "sour environment." The temperature of a sour environment ranges from room temperature to approximately 200°C, depending on the depth of the well.

[0003] Steel materials used in such sour environments include, for example, oil well steel materials used as oil country tubular goods, line pipe steel materials used as line pipes, etc. In recent years, as oil wells have become deeper, there has been a demand for higher strength oil well steel materials, etc.

[0004] On the other hand, when a steel material is used in a sour environment, the surface of the steel material comes into contact with a corrosive substance, causing an electrochemical reaction and generating hydrogen on the surface of the steel material. This hydrogen makes the steel material prone to hydrogen embrittlement cracking, typified by sulfide stress corrosion cracking (SSC). Therefore, steel materials used in sour environments are required to have not only high strength but also excellent hydrogen embrittlement resistance.

[0005] Techniques for improving hydrogen embrittlement resistance in steel materials used in sour environments are disclosed in Japanese Patent Laid-Open No. 2011-246798 (Patent Document 1) and Japanese Patent Laid-Open No. 2015-38247 (Patent Document 2).

[0006] In Patent Document 1, a predetermined amount of solute Mo is secured in an oil well steel pipe made of low alloy steel, prior austenite grains are refined, and M2C-type precipitates are dispersed, thereby improving SSC resistance. Furthermore, Patent Document 1 also forms Mo segregation regions at prior austenite grain boundaries, further improving hydrogen embrittlement resistance.

[0007] In Patent Document 2, in an oil well steel pipe made of low alloy steel, the Mo segregation region is suppressed as much as possible, thereby improving hydrogen embrittlement resistance.

[0008] More recently, progress has been made in the development of fuel cell vehicles that run on hydrogen fuel, and in the practical application of hydrogen stations that supply hydrogen to fuel cell vehicles. High-pressure hydrogen gas is stored in high-pressure hydrogen pressure vessels installed at hydrogen stations. Development of fuel cell vehicles equipped with high-pressure hydrogen cylinders is also underway. The steel materials used in such high-pressure hydrogen containers, such as high-pressure hydrogen pressure vessels and high-pressure hydrogen cylinders, are required to have high strength as well as excellent hydrogen embrittlement resistance.

[0009] A technology for improving hydrogen embrittlement resistance in steel materials used in high-pressure hydrogen containers is proposed in JP 2009-74122 A (Patent Document 3). In Patent Document 3, in a steel material made of low alloy steel, the V content and Mo content are increased compared to conventional steel materials, thereby improving the morphology of carbides at prior austenite grain boundaries and improving hydrogen embrittlement resistance.

[0010] JP 2011-246798 A JP 2015-38247 A JP 2009-74122 A

[0011] The techniques disclosed in the above Patent Documents 1 to 3 can improve the hydrogen embrittlement resistance of steel materials intended for use in sour environments or in high-pressure hydrogen containers. However, steel materials having high strength and excellent hydrogen embrittlement resistance may be obtained by means other than those described in the above Patent Documents 1 to 3.

[0012] An object of the present disclosure is to provide a steel material that combines high strength with excellent hydrogen embrittlement resistance.

[0013] The steel material according to the present disclosure contains, in mass %, C: 0.15 to 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 to 0.100%, Cr: 0.30 to 1.50%, Mo: 0.40 to 2.00%, Ti: 0.002 to 0.020%, Nb: 0.002 to 0.100%, V: 0.05 to 0.30%, B: 0.0005 to 0.0040%, N: 0.0100% or less, O: 0.0040% or less, Cu: 0 to 0.50%, Ni: 0 to 0.50%, W: 0 to 0.50%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, rare earth elements: 0 to 0.0100%, and the balance being Fe and impurities, and the steel material has a yield strength of 965 MPa or more, and in the steel material, the Si content is 20% or more, the O content is 10% or more, and the number density of Si oxides having a major axis of 5.0 μm or more is 5 / 200 mm 2 The following is the result.

[0014] The steel material according to the present disclosure combines high strength with excellent resistance to hydrogen embrittlement.

[0015] FIG. 1 shows the number density (particles / 200 mm ) of coarse Si oxide particles (Si oxide particles with a major axis of 5.0 μm or more) in this example. 2 ) and the relative fracture stress (=BS1 / BS0), which is an index of hydrogen embrittlement resistance.

[0016] The present inventors first investigated the possibility of obtaining a high-strength steel material having a yield strength of 965 MPa or more (140 ksi or more), assuming use in sour environments and for high-pressure hydrogen containers. Furthermore, the present inventors focused on the chemical composition and investigated the possibility of obtaining a steel material that has both a yield strength of 965 MPa or more and excellent hydrogen embrittlement resistance. As a result, the inventors have found that the composition is, in mass %, C: 0.15 to 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 to 0.100%, Cr: 0.30 to 1.50%, Mo: 0.40 to 2.00%, Ti: 0.002 to 0.020%, Nb: 0.002 to 0.100%, V: 0.05 to 0.30%, B: 0.0005 to 0.0005%. The present inventors considered that a steel material consisting of the following elements may be possible: 0.040%, N: 0.0100% or less, O: 0.0040% or less, 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, which may achieve both a yield strength of 965 MPa or more and excellent hydrogen embrittlement resistance.

[0017] On the other hand, even if a steel material has the above-mentioned chemical composition, if it has a yield strength of 965 MPa or more, it may not be possible to obtain excellent hydrogen embrittlement resistance in a high-pressure hydrogen gas environment. Therefore, the inventors of the present invention conducted a detailed study on the factors that cause deterioration in hydrogen embrittlement resistance of steel materials having the above-mentioned chemical composition and a yield strength of 965 MPa or more. As a result, it became clear that there is a concern that coarse Si oxides may be contained in the steel material with the above-mentioned chemical composition. If coarse Si oxides are contained in the steel material, the hydrogen embrittlement resistance of the steel material may be deteriorated.

[0018] In this specification, Si oxides having a Si content of 20% or more, an O content of 10% or more, and a major axis of 5.0 μm or more, in mass %, are also referred to as "coarse Si oxides." As a result of detailed studies by the present inventors, it was found that the number density of coarse Si oxides is 5 particles / 200 mm 2It has been found that by satisfying the following conditions, it is possible to achieve both a yield strength of 965 MPa or more and excellent hydrogen embrittlement resistance. This point will be specifically explained with reference to the drawings.

[0019] FIG. 1 shows the number density (particles / 200 mm ) of coarse Si oxide particles (Si oxide particles with a major axis of 5.0 μm or more) in this example. 2 1 is a graph showing the relationship between the number density of coarse Si oxides determined by a method to be described later and the relative fracture stress (=BS1 / BS0), which is an index of hydrogen embrittlement resistance. Fig. 1 was created using the number density of coarse Si oxides determined by a method to be described later for steel materials having the above-mentioned chemical composition and a yield strength of 965 MPa or more, among the examples to be described later.

[0020] Referring to FIG. 1, in the steel material having the above-mentioned chemical composition and a yield strength of 965 MPa or more, the number density of coarse Si oxides is 5 particles / 200 mm 2 Therefore, it can be confirmed that the steel material according to this embodiment has the above-mentioned chemical composition and a yield strength of 965 MPa or more, and further has a number density of coarse Si oxides of 5 particles / 200 mm 2 As a result, the steel material according to this embodiment can achieve both a yield strength of 965 MPa or more and excellent hydrogen embrittlement resistance.

[0021] The density of coarse Si oxide particles is 5 particles / 200 mm. 2 The details of why the hydrogen embrittlement resistance of steel is improved by the following are not clear. However, the inventors speculate as follows: When manufacturing steel having the above-mentioned chemical composition, deoxidation is mainly carried out with aluminum (Al) in the steelmaking process. Therefore, although Al oxides, typified by Al2O3, have been studied for steel having the above-mentioned chemical composition, Si oxides have not been given much attention. However, Si oxides present in small numbers, particularly coarse Si oxides with a major axis of 5.0 μm or more, may be more likely to reduce the hydrogen embrittlement resistance of steel than Al oxides. Therefore, it is desirable to set the number density of coarse Si oxides to 5 / 200 mm 2The present inventors speculate that by reducing the hydrogen embrittlement content to or below, the hydrogen embrittlement resistance of the steel material can be improved.

[0022] It is possible that the hydrogen embrittlement resistance of the steel material is enhanced by a mechanism different from that speculated by the inventors. 2 As a result, it is possible to achieve both a yield strength of 965 MPa or more and excellent hydrogen embrittlement resistance, as will be demonstrated by the examples described later. Therefore, the steel material according to 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 material is 5 particles / 200 mm 2 As a result, the steel material according to this embodiment can achieve both a yield strength of 965 MPa or more and excellent hydrogen embrittlement resistance.

[0023] The gist of the steel material according to this embodiment, which was completed based on the above findings, is as follows.

[0024] [1] A steel material comprising, in mass%, C: 0.15 to 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 to 0.100%, Cr: 0.30 to 1.50%, Mo: 0.40 to 2.00%, Ti: 0.002 to 0.020%, Nb: 0.002 to 0.100%, V: 0.05 to 0.30%, B: 0.0005 to 0.0040%, N: 0.0100% or less, O: 0.0040% or less, Cu: 0 to 0.50%, Ni: 0 to 0.50%, W: 0 to 0.50%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, rare earth elements: 0 to 0.0100%, and the balance being Fe and impurities, and the steel material has a yield strength of 965 MPa or more, and in the steel material, the Si content is 20% or more, the O content is 10% or more, and the number density of Si oxides having a major axis of 5.0 μm or more is 5 / 200 mm 2 The following are steel materials.

[0025] [2] The steel material according to [1], containing 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 to 0.50%, Ca: 0.0001 to 0.0100%, Mg: 0.0001 to 0.0100%, Zr: 0.0001 to 0.0100%, and rare earth elements: 0.0001 to 0.0100%.

[0026] [3] The steel material according to [1] or [2], wherein the steel material is a seamless steel pipe.

[0027] The shape of the steel material according to this embodiment is not particularly limited. The steel material according to this embodiment may be a steel pipe, a round bar (solid material), or a steel plate. Note that round bar refers to a steel bar having a circular cross section perpendicular to the axial direction. The steel pipe may be a seamless steel pipe or a welded steel pipe.

[0028] The steel material according to this embodiment may be any of steel pipes for oil wells, steel pipes for line pipes, and steel pipes for high-pressure hydrogen containers. Here, in this specification, "steel pipes for oil wells" means steel pipes used as oil well tubular goods. Oil well tubular goods is a general term for casings, tubing, and drill pipes used in drilling oil or gas wells, extracting crude oil or natural gas, etc.

[0029] In this specification, "steel pipe for line pipe" means steel pipe for use as a line pipe constituting a pipeline for transporting produced fluids (crude oil or natural gas) extracted from oil or gas wells. Examples of pipelines include flow lines that transport produced fluids from oil or gas wells, gathering lines that collect the produced fluids transported along the flow lines and transport them to primary treatment facilities, trunk lines that transport produced fluids that have undergone primary treatment such as dehydration to the vicinity of markets, and distribution lines that transport them to consumers.

[0030] In this specification, "steel pipe for high-pressure hydrogen containers" refers to steel pipes that are standardized by ISO 11439, ANSI / NGV, the High-Pressure Gas Safety Act, Illustrative Standards of Container Safety Regulations, etc. and are used for high-pressure hydrogen containers that store high-pressure hydrogen gas. Examples of high-pressure hydrogen containers include high-pressure hydrogen accumulators installed at hydrogen stations and high-pressure hydrogen cylinders installed in fuel cell vehicles.

[0031] The steel material according to this embodiment will be described in detail below. Unless otherwise specified, "%" regarding elements means mass %.

[0032] [Chemical Composition] The chemical composition of the steel material according to this embodiment contains the following elements.

[0033] C: 0.15 to 0.45% Carbon (C) improves the hardenability and strength of steel. Furthermore, C promotes the spheroidization of carbides during tempering during the manufacturing process, thereby improving the SSC resistance of the steel. If the C content is too low, the above effects cannot be fully achieved, even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the C content is too high, even if the contents of other elements are within the ranges of this embodiment, the amount of carbides will be too high, and the hydrogen embrittlement resistance of the steel will be reduced. 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%, even more preferably 0.22%, and even more preferably 0.23%. The preferred upper limit of the C content is 0.40%, even more preferably 0.38%, and even more preferably 0.35%.

[0034] Si: 0.05 to 1.00% Silicon (Si) deoxidizes steel. If the Si content is too low, the above effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Si content is too high, a large number of coarse Si oxides are formed, which may reduce the hydrogen embrittlement resistance of the steel material, even if the contents of other elements are within the ranges of this embodiment. Therefore, the Si content is 0.05 to 1.00%. A preferred lower limit of the Si content is 0.10%, more preferably 0.15%, and even more preferably 0.20%. A preferred upper limit of the Si content is 0.85%, more preferably 0.75%, even more preferably 0.60%, even more preferably 0.50%, and even more preferably 0.40%.

[0035] Mn: 0.05 to 1.00% Manganese (Mn) deoxidizes steel. Mn also improves the hardenability of steel. If the Mn content is too low, the above effects cannot be sufficiently obtained, even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Mn content is too high, coarse sulfide-based inclusions are formed, even if the contents of other elements are within the ranges of this embodiment, and the hydrogen embrittlement resistance of the steel is reduced. Therefore, the Mn content is 0.05 to 1.00%. A preferred lower limit of the Mn content is 0.06%, more preferably 0.08%, and even more preferably 0.10%. A preferred upper limit of the Mn content is 0.90%, more preferably 0.80%, even more preferably 0.70%, even more preferably 0.60%, even more preferably 0.50%, and even more preferably 0.40%.

[0036] P: 0.030% or less Phosphorus (P) is an impurity. That is, the lower limit of the P content is greater than 0%. If the P content is too high, even if the contents of other elements are within the ranges of this embodiment, P segregates at grain boundaries, degrading the hydrogen embrittlement resistance of the steel material. Therefore, the P content is 0.030% or less. A preferred upper limit of the P content is 0.025%, more preferably 0.020%, even more preferably 0.015%, and even more preferably 0.010%. The P content is preferably as low as possible. However, an extreme reduction in the P content significantly increases manufacturing costs. Therefore, considering industrial production, a preferred lower limit of the P content is 0.001%, more preferably 0.002%, and even more preferably 0.003%.

[0037] S: 0.0050% or less Sulfur (S) is an impurity. That is, the lower limit of the S content is greater than 0%. If the S content is too high, even if the contents of other elements are within the ranges of this embodiment, S segregates at grain boundaries, reducing the hydrogen embrittlement resistance of the steel material. Therefore, the S content is 0.0050% or less. A preferred upper limit of the S content is 0.0040%, more preferably 0.0031%, even more preferably 0.0030%, even more preferably 0.0020%, and even more preferably 0.0015%. The S content should be as low as possible. However, an extreme reduction in the S content significantly increases manufacturing costs. Therefore, considering industrial production, a preferred lower limit of the S content is 0.0001%, more preferably 0.0002%, and even more preferably 0.0003%.

[0038] Al: 0.005 to 0.100% Aluminum (Al) deoxidizes steel. If the Al content is too low, the above effect is not sufficiently obtained, even if the contents of other elements are within the ranges of this embodiment, and the hydrogen embrittlement resistance of the steel material is reduced. On the other hand, if the Al content is too high, coarse Al oxides are formed, even if the contents of other elements are within the ranges of this embodiment, and the hydrogen embrittlement resistance of the steel material is reduced. Therefore, the Al content is 0.005 to 0.100%. A preferred lower limit of the Al content is 0.010%, more preferably 0.015%, and even more preferably 0.020%. A preferred upper limit of the Al content is 0.080%, more preferably 0.060%, even more preferably 0.040%, and even more preferably 0.035%. As used herein, the "Al" content refers to the content of "acid-soluble Al," that is, "sol. Al."

[0039] Cr: 0.30 to 1.50% Chromium (Cr) improves the hardenability of steel. Cr also increases the temper softening resistance of steel, enabling high-temperature tempering. As a result, the hydrogen embrittlement resistance of the steel is improved. If the Cr content is too low, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Cr content is too high, the hydrogen embrittlement resistance of the steel is reduced even if the contents of other elements are within the ranges of this embodiment. Therefore, the Cr content is 0.30 to 1.50%. A preferred lower limit of the Cr content is 0.35%, more preferably 0.40%, and even more preferably 0.50%. A preferred upper limit of the Cr content is 1.40%, more preferably 1.30%, even more preferably 1.20%, even more preferably 1.10%, and even more preferably 1.05%.

[0040] Mo: 0.40 to 2.00% Molybdenum (Mo) improves the hardenability of steel. Mo also increases the temper softening resistance of steel, enabling high-temperature tempering. As a result, the hydrogen embrittlement resistance of the steel is improved. If the Mo content is too low, the above effects cannot be fully achieved, even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Mo content is too high, coarse carbides are formed, even if the contents of other elements are within the ranges of this embodiment, and the hydrogen embrittlement resistance of the steel is reduced. Therefore, the Mo content is 0.40 to 2.00%. The preferred lower limit of the Mo content is 0.45%, more preferably 0.49%, even more preferably 0.50%, even more preferably 0.55%, and even more preferably 0.60%. The preferred upper limit of the Mo content is 1.80%, even more preferably 1.60%, even more preferably 1.40%, and even more preferably 1.30%.

[0041] Ti: 0.002 to 0.020% Titanium (Ti) combines with N to form nitrides, which refine the grains of the steel material through a pinning effect. As a result, the hydrogen embrittlement resistance of the steel material is improved. If the Ti content is too low, the above effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Ti content is too high, even if the contents of other elements are within the ranges of this embodiment, the Ti nitrides become coarse, and the hydrogen embrittlement resistance of the steel material is reduced. Therefore, the Ti content is 0.002 to 0.020%. The preferred lower limit of the Ti content is 0.003%, and more preferably 0.004%. The preferred upper limit of the Ti content is 0.018%, more preferably 0.015%, even more preferably 0.010%, and even more preferably 0.008%.

[0042] Nb: 0.002 to 0.100% Niobium (Nb) combines with C and / or N to form carbides, nitrides, or carbonitrides (hereinafter referred to as "carbonitrides, etc."). Carbonitrides, etc., refine the grains of steel through a pinning effect, improving the hydrogen embrittlement resistance of the steel. Nb also forms fine carbides during tempering, improving the temper softening resistance of the steel and increasing its strength. If the Nb content is too low, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Nb content is too high, excessive carbonitrides, etc. are formed, even if the contents of other elements are within the ranges of this embodiment, and the hydrogen embrittlement resistance of the steel is reduced. Therefore, the Nb content is 0.002 to 0.100%. A preferred lower limit of the Nb content is 0.005%, more preferably 0.010%, even more preferably 0.015%, and still more preferably 0.020%. The upper limit of the Nb content is preferably 0.080%, more preferably 0.060%, and even more preferably 0.040%.

[0043] V: 0.05 to 0.30% Vanadium (V) forms carbonitrides and the like. Carbonitrides and the like refine the grains of steel through a pinning effect, improving the hydrogen embrittlement resistance of the steel. V also forms fine carbides during tempering, increasing the temper softening resistance of the steel and increasing its strength. If the V content is too low, the above effects cannot be fully achieved even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the V content is too high, excessive carbonitrides and the like are formed, even if the contents of other elements are within the ranges of this embodiment, and the hydrogen embrittlement resistance of the steel is reduced. Therefore, the V content is 0.05 to 0.30%. A preferred lower limit of the V content is 0.06%, more preferably 0.07%, and even more preferably 0.08%. A preferred upper limit of the V content is 0.25%, more preferably 0.20%, and even more preferably 0.15%.

[0044] B: 0.0005 to 0.0040% Boron (B) dissolves in steel to improve the hardenability and strength of the steel. B also suppresses grain boundary segregation of P, improving the hydrogen embrittlement resistance of the steel. If the B content is too low, the above effects cannot be fully achieved, even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the B content is too high, coarse nitrides are formed, even if the contents of other elements are within the ranges of this embodiment, and the hydrogen embrittlement resistance of the steel is reduced. Therefore, the B content is 0.0005 to 0.0040%. The preferred lower limit of the B content is 0.0006%, and more preferably 0.0008%. The preferred upper limit of the B content is 0.0035%, more preferably 0.0030%, even more preferably 0.0025%, and even more preferably 0.0020%.

[0045] N: 0.0100% or less Nitrogen (N) is unavoidably contained. That is, the lower limit of the N content is greater than 0%. N combines with Ti to form nitrides, which refine the grains of the steel material through a pinning effect. As a result, the strength of the steel material is increased. However, if the N content is too high, even if the contents of other elements are within the ranges of this embodiment, coarse nitrides are formed, and the hydrogen embrittlement resistance of the steel material is reduced. Therefore, the N content is 0.0100% or less. The preferred upper limit of the N content is 0.0080%, more preferably 0.0060%, even more preferably 0.0050%, and even more preferably 0.0045%. To more effectively obtain the above effects, the preferred lower limit of the N content is 0.0005%, more preferably 0.0010%, even more preferably 0.0015%, and even more preferably 0.0020%.

[0046] O: 0.0040% or less Oxygen (O) is an impurity. That is, the lower limit of the O content is greater than 0%. If the O content is too high, even if the contents of other elements are within the ranges of this embodiment, coarse oxides are formed, and the hydrogen embrittlement resistance of the steel material is reduced. Therefore, the O content is 0.0040% or less. A preferred upper limit of the O content is 0.0035%, more preferably 0.0033%, even more preferably 0.0030%, even more preferably 0.0025%, and even more preferably 0.0020%. The O content is preferably as low as possible. However, an extreme reduction in the O content significantly increases manufacturing costs. Therefore, considering industrial production, a preferred lower limit of the O content is 0.0001%, more preferably 0.0002%, and even more preferably 0.0003%.

[0047] The balance of the chemical composition of the steel material according to this embodiment is composed of Fe and impurities. Here, the impurities refer to substances that are mixed in from raw materials such as ore and scrap, or the manufacturing environment, during industrial production of the steel material, and are acceptable within a range that does not adversely affect the steel material according to this embodiment.

[0048] [Optional Elements] The chemical composition of the steel material described above may further contain one or more elements selected from the group consisting of Cu and Ni in place of a portion of Fe. All of these elements are optional elements and improve the hardenability of the steel material.

[0049] Cu: 0 to 0.50% Copper (Cu) is an optional element and does not necessarily need to be contained. That is, the Cu content may be 0%. When contained, Cu improves the hydrogen embrittlement resistance of the steel material. Even if even a small amount of Cu is contained, the above effect can be obtained to some extent. However, if the Cu content is too high, the hot workability of the steel material will deteriorate even if the contents of other elements are within the ranges 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 even more preferably 0.02%. The preferred upper limit of the Cu content is 0.35%, more preferably 0.25%, even more preferably 0.15%, even more preferably 0.10%, and even more preferably 0.05%.

[0050] Ni: 0 to 0.50% Nickel (Ni) is an optional element and does not necessarily need to be contained. That is, the Ni content may be 0%. When contained, Ni improves the hardenability and strength of the steel material. Ni also dissolves in the steel to improve the SSC resistance of the steel material. Even if even a small amount of Ni is contained, these effects can be obtained to some extent. However, if the Ni content is too high, localized corrosion is promoted and the SSC resistance of the steel material decreases, even if the contents of other elements are within the ranges of this embodiment. Therefore, the Ni content is 0 to 0.50%. The preferred lower limit of the Ni content is more than 0%, more preferably 0.01%, and even more preferably 0.02%. The preferred upper limit of the Ni content is 0.30%, more preferably 0.20%, even more preferably 0.10%, and even more preferably 0.05%.

[0051] The chemical composition of the above-mentioned steel material may further contain W instead of a part of Fe.

[0052] W: 0 to 0.50% Tungsten (W) is an optional element and does not necessarily need to be contained. That is, the W content may be 0%. When W is contained, it forms a protective corrosion film in sour environments, suppressing hydrogen penetration into the steel material. As a result, the hydrogen embrittlement resistance of the steel material is improved. Even if even a small amount of W is contained, the above effect can be obtained to some extent. However, if the W content is too high, even if the contents of other elements are within the ranges of this embodiment, coarse carbides will form in the steel material, reducing the hydrogen embrittlement resistance of the steel material. 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%, even more preferably 0.03%, and even more preferably 0.05%. The preferred upper limit of the W content is less than 0.50%, even more preferably 0.48%.

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

[0054] Ca: 0 to 0.0100% Calcium (Ca) is an optional element and does not necessarily need to be contained. That is, the Ca content may be 0%. When contained, Ca neutralizes S in the steel material as sulfides, improving the hydrogen embrittlement resistance of the steel material. Even if even a small amount of Ca is contained, the above effect can be obtained to some extent. However, if the Ca content is too high, even if the contents of other elements are within the ranges of this embodiment, oxides in the steel material will coarsen, and the hydrogen embrittlement resistance of the steel material will deteriorate. Therefore, the Ca content is 0 to 0.0100%. The preferred lower limit of the Ca content is more than 0%, more preferably 0.0001%, even more preferably 0.0003%, and even more preferably 0.0006%. The preferred upper limit of the Ca content is 0.0040%, even more preferably 0.0025%, and even more preferably 0.0020%.

[0055] Mg: 0 to 0.0100% Magnesium (Mg) is an optional element and does not necessarily need to be contained. That is, the Mg content may be 0%. When contained, Mg neutralizes S in the steel material as sulfides, improving the hydrogen embrittlement resistance of the steel material. Even if even a small amount of Mg is contained, the above effect can be obtained to some extent. However, if the Mg content is too high, even if the contents of other elements are within the ranges of this embodiment, oxides in the steel material will coarsen, reducing the hydrogen embrittlement resistance of the steel material. Therefore, the Mg content is 0 to 0.0100%. The preferred lower limit of the Mg content is more than 0%, more preferably 0.0001%, even more preferably 0.0003%, and even more preferably 0.0006%. The preferred upper limit of the Mg content is 0.0040%, even more preferably 0.0025%, and even more preferably 0.0020%.

[0056] Zr: 0 to 0.0100% Zirconium (Zr) is an optional element and does not necessarily need to be contained. That is, the Zr content may be 0%. When contained, Zr neutralizes S in the steel material as sulfides, improving the hydrogen embrittlement resistance of the steel material. Even if even a small amount of Zr is contained, the above effect can be obtained to some extent. However, if the Zr content is too high, even if the contents of other elements are within the ranges of this embodiment, oxides in the steel material will coarsen, reducing the hydrogen embrittlement resistance of the steel material. Therefore, the Zr content is 0 to 0.0100%. The preferred lower limit of the Zr content is more than 0%, more preferably 0.0001%, even more preferably 0.0003%, and even more preferably 0.0006%. The preferred upper limit of the Zr content is 0.0040%, even more preferably 0.0025%, and even more preferably 0.0020%.

[0057] Rare earth elements (REM): 0 to 0.0100% Rare earth elements (REM) are optional elements and may not be included. That is, the REM content may be 0%. When included, REM neutralizes S in the steel as sulfides, improving the hydrogen embrittlement resistance of the steel. REM also combines with P in the steel to suppress P segregation at grain boundaries. Therefore, deterioration of the hydrogen embrittlement resistance of the steel due to P segregation is suppressed. Even if even a small amount of REM is included, the above effects can be achieved to some extent. However, if the REM content is too high, oxides in the steel will coarsen, even if the contents of other elements are within the ranges of this embodiment, and the hydrogen embrittlement resistance of the steel will be reduced. Therefore, the REM content is 0 to 0.0100%. The preferred lower limit of the REM content is greater than 0%, more preferably 0.0001%, even more preferably 0.0003%, and even more preferably 0.0006%. The upper limit of the REM content is preferably 0.0040%, more preferably 0.0025%, and even more preferably 0.0020%.

[0058] In this specification, REM refers to one or more elements selected from the group consisting of scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanoids lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. In addition, in this specification, the REM content refers to the total content of these elements.

[0059] [Yield Strength] The yield strength of the steel material according to this embodiment is 965 MPa or more (140 ksi or more). The yield strength referred to in this specification means the stress at 0.65% elongation (0.65% proof stress) obtained in a tensile test at room temperature (25°C) in accordance with ASTM E8 / E8M (2021). The steel material according to this embodiment has the above-mentioned chemical composition and satisfies the number density of coarse Si oxides described below, thereby having excellent SSC resistance even when the yield strength is 965 MPa or more. In this embodiment, the upper limit of the yield strength is not particularly limited, but is, for example, 1172 MPa. In this embodiment, the preferred lower limit of the yield strength is 986 MPa, more preferably 1000 MPa, even more preferably 1030 MPa, even more preferably greater than 1034 MPa, even more preferably 1035 MPa, and even more preferably 1040 MPa.

[0060] The yield strength of the steel material according to this embodiment is determined by the following method. First, a round bar test specimen is prepared from the steel material according to this embodiment. When the steel material is a steel plate, the round bar test specimen is prepared from the center of the plate thickness. In this case, the axial direction of the round bar test specimen is parallel to the rolling direction of the steel plate. When the steel material is a steel pipe, the round bar test specimen is prepared from the center of the wall thickness. In this case, the axial direction of the round bar test specimen is parallel to the axial direction of the steel pipe. When the steel material is a round bar, the round bar test specimen is prepared from the R / 2 position. In this specification, the R / 2 position refers to the center position of the radius R in a cross section perpendicular to the axial direction of the round bar. In this case, the axial direction of the round bar test specimen is parallel to the axial direction of the round bar. The size of the round bar test specimen is, for example, 8.9 mm in parallel part diameter and 35.6 mm in gauge length. Using the prepared round bar test specimen, a tensile test is performed at room temperature (25°C) in the air according to a method in accordance with ASTM E8 / E8M (2021), and the obtained stress at 0.65% elongation (0.65% proof stress) is defined as the yield strength (MPa). In this embodiment, the yield strength (MPa) is calculated by rounding the obtained value to one decimal place.

[0061] [Number Density of Coarse Si Oxides] The steel material according to this embodiment has the above-described chemical composition and a yield strength of 965 MPa or more, and further has a number density of coarse Si oxides in the steel material of 5 particles / 200 mm 2As described above, in this specification, particles having a Si content of 20% or more and an O content of 10% or more, in mass %, are also referred to as "Si oxides." As described above, in this specification, Si oxides having a major axis of 5.0 μm or more are also referred to as "coarse Si oxides." In other words, coarse Si oxides refer to particles having a Si content of 20% or more, an O content of 10% or more, and a major axis of 5.0 μm or more, in mass %.

[0062] As mentioned above, silicon oxides have not received much attention due to their small number. However, when a steel has a high yield strength of 965 MPa or more, even a small number of coarse silicon oxides may easily cause a decrease in hydrogen embrittlement resistance. Therefore, the number density of coarse silicon oxides is set to 5 / 200 mm. 2 By making the steel material have the above-mentioned chemical composition and a yield strength of 965 MPa or more, it is possible to stably obtain excellent hydrogen embrittlement resistance even if the yield strength is increased to 965 MPa or more. 2 The following applies.

[0063] In this embodiment, the preferred upper limit of the number density of the coarse Si oxides is 4 pieces / 200 mm 2 and more preferably 3 pieces / 200 mm 2 In this embodiment, the lower limit of the number density of the coarse Si oxides is not particularly limited, and is 0 pieces / 200 mm 2 The lower limit of the number density of the coarse Si oxides may be, for example, 1 piece / 200 mm 2 may be.

[0064] In this embodiment, the number density of coarse Si oxides in a steel material can be determined by the following method. First, a test piece is prepared from the steel material according to this embodiment, with the observation surface being a plane including the rolling direction and the reduction direction. Specifically, when the steel material is a steel plate, a test piece is prepared from the plate width center and the plate thickness t / 4 portion, with the observation surface being a plane including the rolling direction and the plate thickness direction. Here, the plate thickness t / 4 portion means a depth position of t / 4 from the surface of the steel plate, where t is the thickness of the steel plate. When the steel material is a steel pipe, a test piece is prepared from the wall thickness center, with the observation surface being a plane including the pipe axial direction and the pipe radial direction. When the steel material is a round bar, a test piece is prepared, with the observation surface being a plane including the R / 2 position in the center and including the axial and radial directions.

[0065] The observation surface of the prepared test piece is polished to a mirror finish before measurement. The area of ​​the observation surface is not limited, but for example, 300 mm 2 The sample is measured in a size of 20 mm x 15 mm. The number of Si oxide particles with a major axis of 5.0 μm or more is determined on the observation surface. Specifically, particles on the observation surface are first identified based on contrast. Element concentration analysis (EDS analysis) is then performed on each identified particle. In the EDS analysis, an acceleration voltage is set to 20 kV, and the target elements are quantified as N, O, Mg, Al, Si, P, S, Ca, Ti, Cr, Mn, Fe, Cu, Zr, and Nb. Based on the EDS analysis results for each particle, if the Si content is 20% or more and the O content is 10% or more by mass, the particle is identified as a "Si oxide."

[0066] Among the Si oxides identified on the observation surface, Si oxides with a major axis of 5.0 μm or more (coarse Si oxides) are identified, and the total number of coarse Si oxides is calculated. The major axis of Si oxides can be calculated by a well-known method. In this specification, the major axis of Si oxides refers to the longest line segment (μm) among line segments connecting any two points on the periphery of the Si oxide on the observation surface.

[0067] Based on the total number of coarse Si oxides and the total area of ​​the observation surface, the number density of the coarse Si oxides (pieces / 200 mm 2 In this embodiment, the number density (particles / 200 mm) of coarse Si oxide particles is calculated. 2) is obtained by rounding off the obtained value to one decimal place. The number density of coarse Si oxides can be measured using a scanning electron microscope equipped with a composition analysis function (SEM-EDS device). As the SEM-EDS device, for example, an automatic analyzer manufactured by FEI (ASPEX) under the trade name of Metals Quality Analyzer can be used.

[0068] [Hydrogen embrittlement resistance] The steel material according to this embodiment has the above-described chemical composition and a yield strength of 965 MPa or more, and the number density of coarse Si oxides in the steel material is 5 pieces / 200 mm 2 As a result, the steel material according to this embodiment achieves both high strength and excellent hydrogen embrittlement resistance. In this embodiment, excellent hydrogen embrittlement resistance can be evaluated by the following method.

[0069] Test specimens for evaluating hydrogen embrittlement resistance are prepared from the steel material according to this embodiment. The test specimen is a round bar test specimen with an annular notch. For example, the outer diameter of the parallel portion of the test specimen is 4.0 mm, the length of the parallel portion is 25 mm, and an annular notch is formed at the longitudinal center position of the parallel portion. In this case, the notch shape is 0.3 mm deep, the notch angle is 60°, and the radius of curvature of the notch bottom is 0.125 mm. When the steel material is a steel plate, a round bar test specimen is prepared from the center of the plate width and the plate thickness t / 4 portion. In this case, the axial direction of the round bar test specimen is parallel to the rolling direction of the steel plate. When the steel material is a steel pipe, a round bar test specimen is prepared from the center of the wall thickness. In this case, the axial direction of the round bar test specimen is parallel to the pipe axis direction of the steel pipe. When the steel material is a round steel, a round bar test specimen is prepared from the R / 2 position. In this case, the axial direction of the round bar test piece is parallel to the axial direction of the round steel.

[0070] The prepared circularly notched round bar test specimens were charged with hydrogen by cathodic hydrogen charging. Specifically, a room-temperature cathodic hydrogen charging solution was prepared. The cathodic hydrogen charging solution was an aqueous solution containing 5 mass% sodium chloride solution, 30 g / L NH4SCN, and an acetate buffer solution at room temperature. The pH of the solution was adjusted to 3.5 using the acetate buffer solution before testing.

[0071] The annular notched round bar test specimen was immersed in the cathodic hydrogen charging solution and charged with hydrogen at a potential of −1.5 V for 24 hours. At this time, a zinc plating film is preferably formed on the surface of the hydrogen-charged annular notched round bar test specimen to prevent hydrogen from leaking out of the annular notched round bar test specimen.

[0072] A tensile test was carried out on the hydrogen-charged circularly notched round bar specimen in air at room temperature (25°C) using a slow strain rate testing machine (SSRT). -6 The fracture stress BS1 (MPa) is calculated as the fracture stress BS0 (MPa) / second. A tensile test is performed under the same conditions on an annular notched round bar test piece that is not charged with hydrogen, and the fracture stress BS0 (MPa) is calculated. In this embodiment, the fracture stress (MPa) is calculated by rounding the obtained value to the nearest tenth. 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 more, it is determined that the specimen has excellent hydrogen embrittlement resistance. In this embodiment, the relative fracture stress is calculated by rounding the obtained value to the nearest tenth.

[0073] [Microstructure] The microstructure of the steel material according to this embodiment has a total volume fraction of tempered martensite and tempered bainite of 90% or more. The remainder of the microstructure is, for example, ferrite or pearlite. If the microstructure of a steel material having the above-described chemical composition contains a total volume fraction of tempered martensite and tempered bainite of 90% or more, it can achieve both a yield strength of 965 MPa or more and excellent hydrogen embrittlement resistance, provided that the other configurations of this embodiment are satisfied. That is, in this embodiment, if a steel material achieves both a yield strength of 965 MPa or more and excellent SSC resistance, it is determined that the microstructure has a total volume fraction of tempered martensite and tempered bainite of 90% or more.

[0074] When the volume fractions of tempered martensite and tempered bainite are determined by observation, they can be determined by the following method. First, a test piece having an observation surface is prepared from the steel material according to this embodiment. When the steel material is a steel plate, a test piece is prepared from the center of the plate width and the plate thickness t / 4 part, with the plane including the rolling direction and the plate thickness direction as the observation surface. When the steel material is a steel pipe, a test piece is prepared from the center of the wall thickness, with the plane including the pipe axial direction and the pipe radial direction as the observation surface. When the steel material is a round bar, a test piece is prepared with the R / 2 position in the center and with the plane including the axial and radial directions as the observation surface.

[0075] The observation surface of the test piece is polished to a mirror finish, and then immersed in a nital etching solution for about 10 seconds to reveal the structure by etching. The etched observation surface is observed in 10 fields of view as secondary electron images using a scanning electron microscope (SEM). The field area is, for example, 0.01 mm. 2 (Magnification: 1000x). In each field of view, tempered martensite and tempered bainite are identified from the contrast. The area fractions of the identified tempered martensite and tempered bainite are calculated. The method for calculating the area fractions is not particularly limited, and any known method may be used. For example, the area fractions of tempered martensite and tempered bainite can be calculated by image analysis. In this embodiment, the arithmetic mean values ​​of the area fractions of tempered martensite and tempered bainite calculated in all fields of view are defined as the volume fractions of tempered martensite and tempered bainite.

[0076] [Manufacturing method] A method for manufacturing a steel material according to this embodiment will be described. Below, a method for manufacturing a seamless steel pipe will be described as an example of a steel material according to this embodiment. The manufacturing method for a seamless steel pipe includes a step of preparing a material (steelmaking step), a step of hot working the material to manufacture a mother pipe (hot working step), and a step of quenching and tempering the mother pipe to produce a seamless steel pipe (quenching step and tempering step). Note that the manufacturing method according to this embodiment is not limited to the manufacturing method described below. Each step will be described in detail below.

[0077] [Steelmaking Process] In the steelmaking process, first, molten pig iron produced by a known method is refined in a converter (primary refining). The molten steel produced by the primary refining is then subjected to secondary refining. In the secondary refining, alloy elements are added to adjust the composition, thereby producing molten steel that satisfies the above-mentioned chemical composition.

[0078] In the secondary refining, for example, a Ruhrstahl-Hausen (RH) vacuum degassing treatment is carried out. After that, the alloy composition is finally adjusted. In the secondary refining, a combined refining may be carried out. In this case, a refining treatment using a Ladle Furnace (LF) or a Vacuum Arc Degassing (VAD) is carried out before the RH vacuum degassing treatment.

[0079] A material is produced using molten steel that has undergone secondary refining. Specifically, a cast piece (slab, bloom, or billet) is produced by continuous casting using the molten steel that has undergone secondary refining. In the continuous casting process, molten steel is first poured from a ladle into a tundish. At this time, packing sand is usually enclosed in the nozzle of the ladle to seal the nozzle. Therefore, packing sand may be mixed in with the molten steel from the ladle to the tundish. Furthermore, when producing a material having the above-mentioned chemical composition, silicon oxide may be used as packing sand. In this case, there is a concern that silicon oxide may be introduced into the produced material.

[0080] Therefore, in this embodiment, the molten steel and the Si oxide are separated to prevent the Si oxide sealed in the ladle nozzle from being introduced into the tundish. The method for separating the Si oxide is not particularly limited, but the following method can be used, for example: A sloped metal plate is placed below the ladle nozzle and above the opening of the tundish. When the ladle nozzle is opened, the Si oxide is discharged first 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 along the slope of the metal plate to the outside of the opening of the tundish. The slope of the metal plate may be achieved, for example, by placing a metal plate machined into a bottomless cone shape with its apex positioned directly below the ladle nozzle, or by other methods. Furthermore, a single metal plate may be used, or multiple metal plates may be stacked. Furthermore, the thickness of the metal plate is not particularly limited, but may be, for example, about 1 to 10 mm.

[0081] After the Si oxides are discharged from the nozzle, the molten steel is discharged. At this time, the molten steel discharged from the nozzle is introduced into the tundish together with the metal plate through the opening. That is, in this embodiment, a part or all of the metal plate may be introduced into the tundish and mixed with the molten steel. Therefore, the metal plate in this embodiment is preferably a metal plate made of alloy elements contained in the molten steel. As a metal plate made of alloy elements contained in the molten steel, for example, an aluminum plate can be used. Note that in this specification, the aluminum plate means a metal plate made of aluminum and the remainder made of impurities.

[0082] Preferably, the metal plate is removed from below the nozzle after the Si oxides have been discharged from the nozzle but before the molten steel is discharged. In this case, it is possible to prevent Si oxides adhering to the metal plate from being mixed into the molten steel. The method for removing the metal plate from below the nozzle is not particularly limited. For example, a hole may be formed in a part of the metal plate and the metal plate may be removed using a rod with a hook at its tip. In this case, the hook at the tip of the rod can be hooked into the hole in the metal plate and the rod can be pulled to remove the metal plate. By the above method, it is possible to separate the Si oxides from the molten steel and introduce the molten steel into the tundish. The method for separating the Si oxides from the molten steel is not limited to the above method.

[0083] Molten steel is cast by the above method to produce a material. The material is preferably a billet with a circular cross section (round billet). The method for producing the material is not particularly limited. For example, molten steel may be cast into a round billet by continuous casting. Alternatively, molten steel may be cast to produce a billet with a rectangular cross section, or a bloom. In these cases, it is preferable to perform blooming to produce a round billet from the billet with a rectangular cross section or the bloom.

[0084] [Hot working process] In the hot working process, a prepared raw material is hot worked to produce an intermediate steel material. When the steel material is a seamless steel pipe, the intermediate steel material corresponds to a mother pipe. First, a billet is heated in a heating furnace. The heating temperature is not particularly limited, but is, for example, 1100 to 1300°C. The billet extracted from the heating furnace is hot worked to produce a mother pipe (seamless steel pipe). The hot working method is not particularly limited, and a well-known method may be used.

[0085] For example, a mother pipe may be produced by carrying out the Mannesmann process as the hot working. In this case, a round billet is pierced and rolled using a piercing mill. When piercing and rolling is performed, the piercing ratio is not particularly limited, but is, for example, 1.0 to 4.0. The piercing-rolled round billet is further hot-rolled using a mandrel mill, a reducer, a sizing mill, or the like to produce a mother pipe. The cumulative area reduction rate in the hot working process is, for example, 20 to 70%.

[0086] Other hot working methods may be used to produce a mother pipe from the billet. For example, in the case of a short, thick-walled steel material such as a coupling, the mother pipe may be produced by forging using the Erhardt method or the like. A mother pipe is produced through the above steps. The thickness of the mother pipe is not particularly limited, but is, for example, 9 to 60 mm.

[0087] When the steel material is round steel, the material is first heated in a heating furnace. The heating temperature is not particularly limited, but is, for example, 1100 to 1300°C. The material extracted from the heating furnace is subjected to hot working to produce an intermediate steel material having a circular cross section perpendicular to the axial direction. The hot working is, for example, blooming using a blooming mill or hot rolling using a continuous rolling mill. The continuous rolling mill has horizontal stands each having a pair of grooved rolls arranged side by side in the vertical direction, and vertical stands each having a pair of grooved rolls arranged side by side in the horizontal direction, arranged alternately.

[0088] When the steel material is a steel plate, the material is first heated in a heating furnace. The heating temperature is not particularly limited, but is, for example, 1100 to 1300°C. The material extracted from the heating furnace is hot-rolled using a blooming mill and a continuous rolling mill to produce an intermediate steel material in the shape of a steel plate.

[0089] The mother pipe produced by hot working may be air-cooled (as-rolled). The mother pipe produced by hot working may be quenched directly after the hot working without being cooled to room temperature, or may be quenched after being reheated after the hot working.

[0090] When quenching is performed directly after hot working or after reheating, cooling may be stopped or slow cooling may be performed during quenching. In this case, the occurrence of quench cracks in the mother pipe can be suppressed. When quenching is performed directly after hot working or after reheating, stress relief annealing (SR) may be performed after quenching and before the next heat treatment. In this case, residual stress in the mother pipe is removed.

[0091] As described above, in the hot working step, the prepared material is hot worked to produce an intermediate steel material. The quenching step will be described in detail below.

[0092] [Quenching Process] In the quenching process, the prepared intermediate steel material (blank pipe) is quenched. In this specification, "quenching" means rapidly cooling the intermediate steel material at or above the A3 point. The preferred quenching temperature is 800 to 1000°C. If the quenching temperature is too high, the prior γ grains may become coarse, which may reduce the SSC resistance of the steel material. Therefore, the quenching temperature is preferably 800 to 1000°C.

[0093] In this specification, the quenching temperature corresponds to the surface temperature of the intermediate steel material measured by a thermometer installed on the outlet side of the equipment that performs the final hot working when quenching is performed directly after hot working. Furthermore, the quenching temperature corresponds to the temperature of the furnace that performs the quenching or reheating when quenching is performed after reheating or reheating after hot working.

[0094] The quenching method involves, for example, continuously cooling the intermediate steel material (mother pipe) from the quenching start temperature to continuously lower the surface temperature of the mother pipe. The method of continuous cooling is not particularly limited, and any well-known method may be used. Examples of continuous cooling methods include a method of immersing the mother pipe in a water bath for cooling, or a method of accelerating the cooling of the mother pipe by shower water cooling or mist cooling.

[0095] If the cooling rate during quenching is too slow, the microstructure will not be mainly composed of martensite and bainite, and the mechanical properties specified in this embodiment (yield strength of 965 MPa or more) will not be obtained. In this case, excellent SSC resistance will also not be obtained.

[0096] Therefore, as described above, in the steel manufacturing method according to this embodiment, the intermediate steel is rapidly cooled during quenching. Specifically, in the quenching step, the average cooling rate in the range of the surface temperature of the intermediate steel (blank pipe) during quenching from 800 to 500°C is defined as the cooling rate during quenching CR 800-500 More specifically, the cooling rate during quenching, CR 800-500 is determined from the temperature measured at the slowest cooling location within the cross section of the intermediate steel being quenched (for example, the center of the thickness of the intermediate steel when both surfaces are forcedly cooled).

[0097] Preferred cooling rate during quenching: CR 800-500The cooling rate CR during quenching is preferably 300°C / min or more. 800-500 The lower limit of the cooling rate during quenching, CR, is 450°C / min, and more preferably 600°C / min. 800-500 The upper limit of the rate is not particularly specified, but is, for example, 60,000° C. / min.

[0098] Preferably, the mother pipe is heated in the austenite region multiple times and then quenched. In this case, the austenite grains before quenching are refined, thereby improving the SSC resistance of the steel material. By performing quenching multiple times, heating in the austenite region may be repeated multiple times, or by performing normalizing and quenching, heating in the austenite region may be repeated multiple times. Furthermore, quenching and tempering, which will be described later, may be combined and performed multiple times. That is, quenching and tempering may be performed multiple times. In this case, the SSC resistance of the steel material is further improved. The tempering process will be described in detail below.

[0099] [Tempering Step] In the tempering step, the above-mentioned quenched mother pipe is tempered. In this specification, "tempering" refers to the process of tempering the intermediate steel material after quenching. c1 The tempering temperature corresponds to the furnace temperature when the intermediate steel material is heated and held at the tempering temperature after quenching. The tempering time refers to the time the intermediate steel material is held at the tempering temperature.

[0100] The tempering temperature is adjusted appropriately depending on the chemical composition of the seamless steel pipe and the yield strength to be obtained. That is, for a mother pipe having the chemical composition of this embodiment, the tempering temperature is adjusted to adjust the yield strength of the seamless steel pipe to 965 MPa or more. Note that it is naturally possible for a person skilled in the art to adjust the yield strength of the seamless steel pipe to 965 MPa or more by adjusting the tempering temperature. Specifically, in the tempering process according to this embodiment, the preferred tempering temperature is 640 to 680°C.

[0101] If the tempering time is too short, a microstructure mainly composed of tempered martensite and tempered bainite may not be obtained. On the other hand, if the tempering time is too long, the above effects will saturate. Therefore, in the tempering step of this embodiment, the tempering time is preferably 10 to 90 minutes. A more preferable lower limit of the tempering time is 15 minutes. A more preferable upper limit of the tempering time is 80 minutes.

[0102] The steel material according to this embodiment can be manufactured by the above manufacturing method. In the above manufacturing method, a method for manufacturing a seamless steel pipe has been described as an example. However, the steel material according to this embodiment may be a steel plate or other shape. Similar to the above manufacturing method, a manufacturing method for a steel plate or other shape also includes, for example, a preparation step, a quenching step, and a tempering step. Furthermore, the above manufacturing method is an example, and the steel material may be manufactured by other manufacturing methods.

[0103] The present invention will be explained in more detail below with reference to examples.

[0104] Molten steels having the chemical compositions shown in Tables 1-1 and 1-2 were produced. Note that "-" in Table 1-2 means that the content of each element was at the impurity level. Specifically, the Cu content, Ni content, and W content of Steel A were rounded to two decimal places to mean that they were 0%. Furthermore, the Ca content, Mg content, Zr content, and rare earth element (REM) content of Steel A were rounded to five decimal places to mean that they were 0%.

[0105]

[0106]

[0107] The molten steel was used to produce a round billet by continuous casting. In the continuous casting process, when molten steel was introduced from the ladle into the tundish, a metal plate processed into a conical shape without a base was placed above the opening of the tundish, with its apex positioned directly below the nozzle of the ladle. Table 2 shows whether or not a metal plate with the above shape was placed above the opening of the tundish. Specifically, when a metal plate with the above shape was placed above the opening of the tundish, "A" is indicated in the "Metal Plate" column of Table 2. When a metal plate with the above shape was not placed above the opening of the tundish, "B" is indicated in the "Metal Plate" column of Table 2. The metal plate with the above shape placed above the opening of the tundish was an aluminum plate. Specifically, three 2 mm thick aluminum plates were used in a stack. When a metal plate was placed, it was removed from below the nozzle using a rod with a hook at its tip after silicon oxide was discharged from the nozzle and before the molten steel was discharged.

[0108]

[0109] The produced round billet of each test number was held at 1250°C for 1 hour, and then hot-rolled by the Mannesmann-mandrel method to produce a mother pipe (seamless steel pipe) of each test number. Furthermore, the obtained mother pipe of each test number was quenched. Specifically, the mother pipe of each test number was held at the temperature (°C) for the time (minutes) shown in the "Quenching step" column of Table 2, and then quenched by shower water cooling. Furthermore, for test number 5, after the above-mentioned quenching, it was held at 900°C for 10 minutes, and then further quenched by shower water cooling. Note that for each test number, the cooling rate during quenching, CR 800-500 The quenching rates were all within the range of 480 to 30,000°C / min. Here, the temperature (°C) in the quenching process was the temperature (°C) of the heat treatment furnace in which the mother pipe was heated. Furthermore, the quenching process time (minutes) was the time (minutes) during which the mother pipe was held at the quenching temperature.

[0110] The obtained mother pipes of each test number were tempered. Specifically, the mother pipes of each test number were tempered by holding them at the temperature (°C) for the time (minutes) shown in the "Tempering step" column of Table 2. Here, the tempering temperature (°C) shown in Table 2 is the temperature (°C) of the tempering furnace in which the mother pipes were heated. Furthermore, the tempering time (minutes) shown in Table 2 is the time (minutes) for which the mother pipes were held at the tempering temperature. Through the above manufacturing process, seamless steel pipes of each test number were obtained.

[0111] [Evaluation Tests] The seamless steel pipes having the respective test numbers after tempering were subjected to a tensile test, a coarse Si oxide number density measurement test, and a hydrogen embrittlement resistance evaluation test, all of which will be described below.

[0112] [Tensile Test] A tensile test was conducted on each seamless steel pipe to determine its yield strength. The tensile test was conducted in accordance with ASTM E8 / E8M (2021). Round bar test specimens with a parallel section diameter of 8.9 mm and a gauge length of 35.6 mm were prepared from the center of the wall thickness of each seamless steel pipe. The axial direction of the round bar test specimen was parallel to the axial direction of the seamless steel pipe. Using the prepared round bar test specimens, a tensile test was conducted at room temperature (25°C) in air to obtain the yield strength (MPa) of each seamless steel pipe. In this example, the stress at 0.65% elongation (0.65% proof stress) obtained in the tensile test was defined as the yield strength. The obtained yield strength (MPa) is shown in Table 3 as "YS (MPa)."

[0113]

[0114] [Measurement test for number density of coarse Si oxides] A measurement test for number density of coarse Si oxides was carried out for the seamless steel pipe of each test number, and the number density of Si oxides (coarse Si oxides) having a major axis of 5.0 μm or more was determined. The number density of coarse Si oxides was determined by the above-mentioned method using test pieces prepared from the central part of the wall thickness of the seamless steel pipe of each test number. The number density of the coarse Si oxides (pieces / 200 mm 2 ) was compared with the "coarse Si oxides (pieces / 200 mm 2 ) column.

[0115] [Hydrogen embrittlement resistance evaluation test] Two round bar test specimens with an annular notch were prepared from the central portion of the wall thickness of each seamless steel pipe. The outer diameter of the parallel portion of each test specimen was 4.0 mm, the length of the parallel portion was 25 mm, and an annular notch was formed at the longitudinal center of the parallel portion. The notch shape had a notch depth of 0.3 mm, a notch angle of 60°, and a curvature radius of the notch bottom of 0.125 mm. The round bar test specimens were prepared so that the axial direction of the round bar test specimens was parallel to the rolling direction (pipe axial direction) of the seamless steel pipe.

[0116] One of the two annularly notched round bar specimens was charged with hydrogen by the cathodic hydrogen charging method. Specifically, a room-temperature cathodic hydrogen charging solution was prepared. The cathodic hydrogen charging solution was an aqueous solution containing 5 mass% sodium chloride solution, 30 g / L NH4SCN, and an acetate buffer solution at room temperature. The pH of the solution was adjusted to 3.5 using the acetate buffer solution before the test.

[0117] The annular notched round bar test specimens were immersed in a cathodic hydrogen charging solution and charged with hydrogen at a potential of -1.5 V for 24 hours. In other words, charging with hydrogen simulated a sour environment. A zinc plating film was formed on the surface of the hydrogen-charged annular notched round bar test specimen under the same conditions for each test number, preventing hydrogen leakage from the annular notched round bar test specimen. Hydrogen was not charged to the other annular notched round bar test specimen.

[0118] A round bar specimen with a circular notch on which a zinc plating film was formed was subjected to a 4.2 x 10 strain rate test at room temperature in air using a slow strain rate testing machine (SSRT). -6 A tensile test was carried out at a strain rate of 1 / sec, and the breaking stress BS1 (MPa) in a hydrogen environment was determined.

[0119] Furthermore, for each test number, a circularly notched round bar specimen that was not charged with hydrogen was subjected to a 4.2 × 10 strain rate test at room temperature in air using a slow strain rate testing machine (SSRT). -6 A tensile test was carried out at a strain rate of 1 / sec, and the breaking stress BS0 (MPa) in air was determined.

[0120] The obtained breaking stress BS0 (MPa) in air is shown in the "BS0 (MPa) in air" column under "Notched tensile test results" in Table 3. The obtained breaking stress BS1 (MPa) in a hydrogen environment is shown in the "BS1 (MPa) in hydrogen environment" column under "Notched tensile test results" in Table 3. Table 3 shows the relative breaking stress (BS1 / BS0) calculated from the breaking stress BS0 (MPa) in air and the breaking stress BS1 (MPa) in a hydrogen environment.

[0121] [Evaluation Results] With reference to Tables 1-1, 1-2, 2, and 3, the seamless steel pipes of test numbers 1 to 17 had appropriate chemical compositions, and the manufacturing methods also satisfied the above-mentioned preferable conditions. As a result, these seamless steel pipes had a yield strength of 965 MPa or more, and furthermore, the number density of coarse Si oxides was 5 / 200 mm 2 or less. As a result, these seamless steel pipes had a relative fracture stress of 0.85 or more in the hydrogen embrittlement resistance evaluation test. That is, the seamless steel pipes of test numbers 1 to 17 had both a yield strength of 965 MPa or more and excellent hydrogen embrittlement resistance. It was determined that the total volume fraction of tempered martensite and tempered bainite in the microstructure of these seamless steel pipes was 90% or more.

[0122] For seamless steel pipes of test numbers 18 to 22, no metal plate was used in the steelmaking process. As a result, for these seamless steel pipes, the number density of coarse Si oxides was 5 pieces / 200 mm 2 As a result, in the hydrogen embrittlement resistance evaluation test, these seamless steel pipes had a relative fracture stress of less than 0.85, and did not have excellent hydrogen embrittlement resistance.

[0123] The seamless steel pipe of test number 23 had an excessively high O content. As a result, in the hydrogen embrittlement resistance evaluation test, this seamless steel pipe had a relative fracture stress of less than 0.85, and did not have excellent hydrogen embrittlement resistance.

[0124] The seamless steel pipe of test number 24 had an excessively low Mo content, and as a result, in the hydrogen embrittlement resistance evaluation test, the seamless steel pipe had a relative fracture stress of less than 0.85, and did not have excellent hydrogen embrittlement resistance.

[0125] The seamless steel pipe of test number 25 had an excessively high S content. As a result, in the hydrogen embrittlement resistance evaluation test, this seamless steel pipe had a relative fracture stress of less than 0.85, and did not have excellent hydrogen embrittlement resistance.

[0126] The embodiments of the present disclosure have been described above. However, the above-described embodiments are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiments, and can be implemented by appropriately modifying the above-described embodiments within the scope of the present disclosure.

Claims

1. A steel material having, in mass%, the following components: C: 0.15-0.45%, Si: 0.05-1.00%, Mn: 0.05-1.00%, P: 0.030% or less, S: 0.0050% or less, Al: 0.005-0.100%, Cr: 0.30-1.50%, Mo: 0.40-2.00%, Ti: 0.002-0.020%, Nb: 0.002-0.100%, V: 0.05-0.30%, B: 0.0005-0.0040%, N: 0.0100% or less, O: 0.0040% or less, Cu: 0-0.50%, Ni: 0-0.50%, The steel material is made up of 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, and has a yield strength of 965 MPa or more, and in the steel material, the Si content is, by mass%, 20% or more, the O content is 10% or more, and the number density of Si oxides having a major axis of 5.0 μm or more is 5 / 200 mm 2 The following are steel materials.

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

3. A steel material according to claim 1 or 2, which is a seamless steel pipe.

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