Steel material

Abstract

Provided is a steel material having high strength and excellent hydrogen embrittlement resistance even if the grain diameter of the prior austenite grains exceeds 5.0 [mu] m. A steel material according to the present disclosure has a chemical composition comprising, in mass%, 0.20% to 0.45% of C, 0.05% to 1.50% of Si, 0.01% to 1.00% of Mn, 0.030% or less of P, 0.0100% or less of S, 0.40% to 1.10% of Cr, 0.40% to 1.30% of Mo, 0.01% to 0.30% of V, 0.005% to 0.100% of Nb, 0.001% to 0.030% of Ti, 0.005% to 0.100% of Al, 0.0005% to 0.0050% of B, 0.0100% or less of N, 0.0050% or less of O, and the balance of Fe and impurities, the steel material having a prior austenite crystal grain diameter GS of more than 5.0 [mu] m and 30.0 [mu] m or less, a yield strength sigma of 862 to 965 MPa, a yield point

Description

Technical Field

[0001] This disclosure relates to steel, and more specifically, to steel used in acidic environments and steel used in high-pressure hydrogen containers. Background Technology

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

[0003] Steels used in such acidic environments include, for example, oil well steel used as oil well tubing and pipeline steel used as line pipes. In recent years, with 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, causing an electrochemical reaction that generates hydrogen on the steel surface. This hydrogen readily leads to hydrogen embrittlement cracking, exemplified by sulfide stress corrosion cracking (SSC). Therefore, steel used in acidic environments requires not only high strength but also 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 techniques for improving the resistance to hydrogen embrittlement in steels 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 (Sedimentation-Sensitive Contamination). Furthermore, Patent Document 1 further enhances resistance to hydrogen embrittlement by forming Mo segregation regions within the original austenite grain boundaries.

[0007] In Patent Document 2, the resistance to hydrogen embrittlement is improved in oil well steel pipes made of low alloy steel by suppressing the Mo segregation region 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. High-pressure hydrogen containers, such as high-pressure hydrogen accumulators and cylinders, require steel with high strength and excellent resistance to hydrogen embrittlement.

[0009] Japanese Patent Application Publication No. 2009-74122 (Patent Document 3) discloses a technique for improving hydrogen embrittlement resistance in 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 previous methods, the morphology of carbides at the original austenite grain boundaries is improved, thereby enhancing 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

[0015] Patent Document 4: Japanese Patent Application Publication No. 2017-210645 Summary of the Invention

[0016] The problem the invention aims to solve

[0017] According to the technology disclosed in Patent Documents 1 to 3, the hydrogen embrittlement resistance of steel intended for use in acidic environments and in high-pressure hydrogen containers can be improved. However, steel with high strength and excellent hydrogen embrittlement resistance can also be obtained through other solutions besides those described in Patent Documents 1 to 3.

[0018] However, to date, techniques have been proposed to improve the hydrogen embrittlement resistance of steel by refining the original austenite grains. For example, specifically, Japanese Patent Application Publication No. 2017-210645 (Patent Document 4) discloses that "the original austenite grains are fine grains with a grain size of 6 μm or less, thereby reducing hydrogen embrittlement fracture starting from the original austenite grain boundaries" (paragraph

[0012] of Patent Document 4). Thus, techniques for improving the hydrogen embrittlement resistance of steel by refining the original austenite grains have been studied.

[0019] On the other hand, for steels intended for use in acidic environments and high-pressure hydrogen containers, excessive refinement of the original austenite grains is undesirable when considering industrial production. Specifically, for steels intended for use in acidic environments and high-pressure hydrogen containers, manufacturing costs increase drastically when the original austenite grain diameter is 5.0 μm or less. Therefore, for steels intended for use in acidic environments and high-pressure hydrogen containers, it is preferable to achieve both high strength and excellent resistance to hydrogen embrittlement even when the original austenite grain diameter exceeds 5.0 μm.

[0020] The purpose of this disclosure is to provide steels that possess high strength and excellent resistance to hydrogen embrittlement even when the original austenite grain diameter exceeds 5.0 μm.

[0021] Solution for solving the problem

[0022] The steel disclosed herein has a chemical composition in mass percent as follows:

[0023] C: 0.20%~0.45%

[0024] Si: 0.05~1.50%

[0025] Mn: 0.01~1.00%

[0026] P: below 0.030%

[0027] S: below 0.0100%

[0028] Cr: 0.40–1.10%

[0029] Mo: 0.40–1.30%

[0030] V: 0.01~0.30%

[0031] Nb: 0.005~0.100%

[0032] Ti: 0.001~0.030%

[0033] Al: 0.005~0.100%

[0034] B: 0.0005~0.0050%

[0035] N: below 0.0100%

[0036] O: Below 0.0050%

[0037] W: 0–2.00%

[0038] Co: 0-0.20%

[0039] Mg: 0~0.0100%

[0040] Ca: 0~0.0100%

[0041] Rare earth elements: 0~0.0100%

[0042] Cu: 0–0.40%

[0043] Ni: 0-0.20%

[0044] Sn: 0–0.10%, and

[0045] Balance: Fe and impurities,

[0046] The original austenite grain diameter GS of the steel is greater than 5.0 μm and less than 30.0 μm.

[0047] The yield strength σ is 862–965 MPa.

[0048] The yield point decreases by Δσ by more than 40 MPa.

[0049] The yield point elongation Δε is above 1.5%.

[0050] The effects of the invention

[0051] The steel disclosed herein exhibits high strength and excellent resistance to hydrogen embrittlement even when the original austenite grain diameter exceeds 5.0 μm. Attached Figure Description

[0052] Figure 1 This is a graph showing the stress-strain curve of the steel in this embodiment.

[0053] Figure 2 This is a graph showing the stress-strain curves of steels with the same chemical composition, yield strength, and original γ grain diameter as those in this embodiment, but which do not have excellent resistance to hydrogen embrittlement.

[0054] Figure 3 It is Figure 1 The image is obtained by magnifying a portion of it.

[0055] Figure 4 It is Figure 1 The image is obtained by magnifying a portion of it. Detailed Implementation

[0056] The inventors first envisioned its use in acidic environments and in high-pressure hydrogen containers, and investigated methods to obtain steel with a yield strength of 862–965 MPa (125–140 ksi, hereinafter also referred to as "125 ksi grade"), which is considered to be high strength. Specifically, for steel intended for use in acidic environments and in high-pressure hydrogen containers, the inventors investigated and researched methods to improve hydrogen embrittlement resistance even when the original austenite grain diameter GS (hereinafter, the original austenite grain diameter GS is also referred to as "original γ grain diameter GS") is greater than 5.0 μm. As a result, the inventors obtained the following insights.

[0057] The inventors first focused on the chemical composition. The results showed that, in terms of mass percent, the composition was: C: 0.20%–0.45%, Si: 0.05–1.50%, Mn: 0.01–1.00%, P: less than 0.030%, S: less than 0.0100%, Cr: 0.40–1.10%, Mo: 0.40–1.30%, V: 0.01–0.30%, Nb: 0.005–0.100%, Ti: 0.001–0.030%, Al: 0.005–0.100%, B: 0.0005–0.0050%. Steels with the following chemical composition can achieve a yield strength of 125 ksi and excellent resistance to hydrogen embrittlement, even if the original γ grain diameter GS exceeds 5.0 μm.

[0058] Therefore, the inventors manufactured various steels having the aforementioned chemical composition, yield strength of 125 ksi, and original gamma grain diameter GS exceeding 5.0 μm, and evaluated their resistance to hydrogen embrittlement. The detailed results of the inventors' research clearly show that for steels having the aforementioned chemical composition, yield strength of 125 ksi, and original gamma grain diameter GS exceeding 5.0 μm, the resistance to hydrogen embrittlement is significantly improved when the stress-strain curve has a characteristic shape. This will be specifically explained using the accompanying drawings.

[0059] Figure 1 This is a graph showing the stress-strain curve of the steel in this embodiment. Figure 2 This is a graph showing the stress-strain curves of steels with chemical composition, yield strength, and original γ grain diameter GS that are the same as those in this embodiment but do not have excellent resistance to hydrogen embrittlement. Figure 1 and Figure 2 All results were obtained through tensile tests, which will be described later. Figure 1 and Figure 2 The steels shown all possess the aforementioned chemical composition and yield strength of 125 ksi, with a primary γ grain diameter (GS) exceeding 5.0 μm. On the other hand, Figure 1 The steel shown has excellent resistance to hydrogen embrittlement. Figure 2 The steel shown does not have excellent resistance to hydrogen embrittlement.

[0060] Reference Figure 1 , Figure 1 The stress-strain curve shown can be divided into three regions. (Refer to...) Figure 1 First, it can be confirmed that the stress monotonically increases from 0 MPa as the strain increases from 0% (elastic deformation region). It can also be confirmed that if the strain increases further after the stress reaches its maximum value, the stress decreases sharply, and even with further increases in strain, the stress becomes approximately constant (Luders deformation region). Finally, it can be confirmed that when the strain increases further, the stress increases again, and then decreases (plastic deformation region). On the other hand, referring to... Figure 2 , Figure 2 The stress-strain curve shown can be divided into two regions. (Refer to...) Figure 2 First, we can identify the region where the stress monotonically increases from 0 MPa as the strain increases from 0% (elastic deformation region). Then, we can identify the region where the stress decreases after a further increase in strain (plastic deformation region).

[0061] That is, if comparing Figure 1 and Figure 2 ,but Figure 1 The stress-strain curves shown indicate a region between the elastic and plastic deformation regions where stress decreases sharply, and even with increased strain, the stress remains approximately constant (Luders deformation region). Here, the magnitude of the stress decrease that occurs when stress drops sharply from the elastic deformation region is called the yield point drop Δσ (MPa). Furthermore, the magnitude of the strain in the region where stress remains approximately constant even with increased strain is called the yield point elongation Δε (%). These are illustrated in more detail with the accompanying figures.

[0062] Figure 3 and Figure 4 It is Figure 1 The image is obtained by magnifying a portion of it. Figure 3 It is Figure 1 The image is an enlarged representation of the region with strain of 0–2.0% and stress of 850–1050 MPa. (Refer to...) Figure 3Let P0 be the point in the stress-strain curve where the stress reaches its maximum value in the elastic deformation region, and let σ0 (MPa) be the stress at point P0. (Refer to...) Figure 3 Furthermore, the intersection of the stress-strain curve and the straight line L is set as P, and the stress at point P is set as σ (MPa). The straight line L is obtained by shifting it parallel to the elastic deformation region in the stress-strain curve by 0.2% in the positive direction of strain. That is, the stress σ at point P is equivalent to the so-called 0.2% conditional yield strength. In this specification, the difference between the stress σ0 (MPa) at point P0 and the stress σ (MPa) at point P is defined as the yield point decrease Δσ (MPa). It should be noted that in this specification, the yield strength (MPa) is defined as the stress σ (MPa) at point P.

[0063] Figure 4 It is Figure 1 The image is a magnified representation of the region with strain ranging from 0% to 4.0%. (Refer to...) Figure 4 Let Q be the point where the plastic deformation region begins in the stress-strain curve. Here, the strain (%) at point Q is defined as the yield elongation Δε (%). It should be noted that point Q is the inflection point of the stress-strain curve, which can be determined by those skilled in the art.

[0064] The inventors have conducted a more detailed study on the yield point decrease Δσ (MPa) and yield point elongation Δε (%) as defined above. Further detailed research by the inventors indicates that for steels with the aforementioned chemical composition, a yield strength σ of 125 ksi, and a primary γ grain diameter GS exceeding 5.0 μm, excellent resistance to hydrogen embrittlement can be obtained if the yield point decrease Δσ is 40 MPa or more and the yield point elongation Δε is 1.5% or more.

[0065] In steels with the aforementioned chemical composition, a yield strength σ of 125 ksi, and a primary γ grain diameter GS exceeding 5.0 μm, the reasons why excellent resistance to hydrogen embrittlement can be obtained if the yield point decrease Δσ is 40 MPa or more and the yield elongation Δε is 1.5% or more are not yet clear. However, the inventors speculate as follows: The stress limit for hydrogen embrittlement fracture in steel may be closely related to plastic deformation. While the details are unclear, the more difficult it is for plastic deformation to occur in the steel, the higher the stress limit for hydrogen embrittlement fracture may be.

[0066] Here, refer to Figure 1 In steels exhibiting a yield point decrease of Δσ, elastic deformation is maintained even under stresses exceeding the yield strength σ. That is, the greater the yield point decrease Δσ, the less likely plastic deformation occurs, and the higher the stress limit for hydrogen embrittlement fracture may be. In other words, a greater yield point decrease Δσ may result in improved resistance to hydrogen embrittlement.

[0067] Reference Figure 1 Furthermore, the larger the elongation at yield point Δε, the less likely plastic deformation will occur even with increased strain applied to the steel. Therefore, a larger elongation at yield point Δε may increase the stress limit for hydrogen embrittlement fracture. In other words, a larger elongation at yield point Δε may improve resistance to hydrogen embrittlement.

[0068] Based on the above mechanism, the inventors hypothesize that by reducing the yield point Δσ of steel with the above-described chemical composition, a yield strength σ of 125 ksi, and an original γ grain diameter GS exceeding 5.0 μm to 40 MPa or more, and the yield elongation Δε to 1.5% or more, the hydrogen embrittlement resistance can be improved. It should be noted that the hydrogen embrittlement resistance of the steel could also be improved based on a mechanism different from the above. However, the examples described later demonstrate that if the steel has the above-described chemical composition, a yield point reduction Δσ of 40 MPa or more, and a yield elongation Δε of 1.5% or more, then even if the original γ grain diameter GS is 5.0 μm or more, a high yield strength σ of 125 ksi and excellent hydrogen embrittlement resistance can be obtained.

[0069] The steel used in this embodiment, based on the above insights, has the following composition. [1]

[0071] A type of steel, the chemical composition of which is expressed in mass percent as follows:

[0072] C: 0.20%~0.45%

[0073] Si: 0.05~1.50%

[0074] Mn: 0.01~1.00%

[0075] P: below 0.030%

[0076] S: below 0.0100%

[0077] Cr: 0.40–1.10%

[0078] Mo: 0.40–1.30%

[0079] V: 0.01~0.30%

[0080] Nb: 0.005~0.100%

[0081] Ti: 0.001~0.030%

[0082] Al: 0.005~0.100%

[0083] B: 0.0005~0.0050%

[0084] N: below 0.0100%

[0085] O: Below 0.0050%

[0086] W: 0–2.00%

[0087] Co: 0-0.20%

[0088] Mg: 0~0.0100%

[0089] Ca: 0~0.0100%

[0090] Rare earth elements: 0~0.0100%

[0091] Cu: 0–0.40%

[0092] Ni: 0-0.20%

[0093] Sn: 0–0.10%, and

[0094] Balance: Fe and impurities,

[0095] The aforementioned steel has a pre-austenite grain diameter GS exceeding 5.0 μm but below 30.0 μm, and a yield strength σ of 862–965 MPa.

[0096] The yield point decreases by Δσ by more than 40 MPa.

[0097] The yield point elongation Δε is above 1.5%. [2]

[0099] According to the steel described in [1], wherein,

[0100] The aforementioned chemical composition contains selected free radicals.

[0101] W: 0.01~2.00%

[0102] Co: 0.01~0.20%

[0103] Mg: 0.0001~0.0100%

[0104] Ca: 0.0001~0.0100%

[0105] Rare earth elements: 0.0001~0.0100%

[0106] Cu: 0.01–0.40%

[0107] Ni: 0.01–0.20%, and

[0108] Sn: One or more elements in a group consisting of 0.01% to 0.10%. [3]

[0110] According to the steel described in [1], wherein,

[0111] The aforementioned grain diameter GS of the original austenite grain, the aforementioned yield strength σ, and the aforementioned yield point elongation Δε satisfy the following equation (1).

[0112] σ×Δε / GS 2 ≥12 (1)

[0113] Here, the yield strength of the steel is substituted into σ in Equation (1) in MPa, the elongation at the yield point of the steel is substituted into Δε in Equation (1) in %, and the grain diameter of the original austenite grains of the steel is substituted into GS in Equation (1) in μm. [4]

[0115] According to the steel described in [2], wherein,

[0116] The aforementioned grain diameter GS of the original austenite grain, the aforementioned yield strength σ, and the aforementioned yield point elongation Δε satisfy the following equation (1).

[0117] σ×Δε / GS 2 ≥12 (1)

[0118] Here, the yield strength of the steel is substituted into σ in Equation (1) in MPa, the elongation at the yield point of the steel is substituted into Δε in Equation (1) in %, and the grain diameter of the original austenite grains of the steel is substituted into GS in Equation (1) in μm. [5]

[0120] According to any one of [1] to [4], the steel, wherein,

[0121] The aforementioned steel is any one of oil well steel pipes, pipeline steel pipes, and high-pressure hydrogen container steel pipes. [6]

[0123] According to any one of [1] to [4], the steel, wherein,

[0124] The aforementioned steel is any one of seamless steel pipes for oil wells, seamless steel pipes for pipelines, and seamless steel pipes for high-pressure hydrogen containers. [7]

[0126] According to the steel described in [5], wherein,

[0127] The aforementioned high-pressure hydrogen container steel pipe is either a high-pressure hydrogen accumulator steel pipe or a high-pressure hydrogen cylinder steel pipe. [8]

[0129] According to the steel described in [6], wherein,

[0130] The aforementioned seamless steel pipe for high-pressure hydrogen containers is either a seamless steel pipe for high-pressure hydrogen accumulators or a seamless steel pipe for high-pressure hydrogen cylinders.

[0131] In this specification, "steel pipe for oil wells" refers to steel pipes used as oil well casings. Oil well casings are a general term for casings, pipelines, and drill pipes used for the excavation of oil or gas wells, and the extraction of crude oil or natural gas. "Seamless steel pipe for oil wells" means that the steel pipes used for oil wells are seamless steel pipes.

[0132] In this specification, "line pipe" refers to steel pipe used for pipeline applications, constituting pipelines for transporting production fluids (crude oil or natural gas) collected from oil or gas wells. Examples of such pipelines include fluid pipelines transporting production fluids from oil or gas wells, collection pipelines that collect and transport production fluids transported by fluid 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 fluids to consumers. "Seamless steel pipe for pipeline applications" means that the line pipe is a seamless steel pipe.

[0133] In this specification, "steel pipe for high-pressure hydrogen containers" refers to the steel pipe used in high-pressure hydrogen containers that store high-pressure hydrogen, as standardized by ISO 11439, ANSI / NGV, the High Pressure Gas Safety Law, and the Example Standards for Container Safety Regulations. Examples of high-pressure hydrogen containers include high-pressure hydrogen accumulators installed in hydrogen stations and high-pressure hydrogen cylinders installed in fuel cell vehicles. "Seamless steel pipe for high-pressure hydrogen containers" means that the steel pipe for high-pressure hydrogen containers is a seamless steel pipe.

[0134] The steel used in this embodiment will be described in detail below. Unless otherwise specified, "%" for elements refers to mass percentage.

[0135] [Chemical Composition]

[0136] The steel in this embodiment contains the following elements in its chemical composition.

[0137] C: 0.20%~0.45%

[0138] Carbon (C) improves hardenability, making the microstructure of steel primarily composed of tempered martensite and tempered bainite. As a result, the steel's resistance to hydrogen embrittlement is improved. C also forms carbides or carbonitrides, increasing the steel's strength. If the C content is too low, even with the contents of other elements within the range of this embodiment, the aforementioned effects cannot be fully achieved. On the other hand, if the C content is too high, even with the contents of other elements within the range of this embodiment, there will be excessive carbides in the steel. In this case, the steel's strength becomes excessively high, and its resistance to hydrogen embrittlement decreases. Therefore, the C content is 0.20% to 0.45%. The preferred lower limit of the C content is 0.21%, more preferably 0.22%, and even more preferably 0.24%. The preferred upper limit of the C content is 0.40%, more preferably 0.38%, and even more preferably 0.36%.

[0139] Si: 0.05–1.50%

[0140] Silicon (Si) deoxidizes steel, reducing inclusions in the steel. As a result, the steel's resistance to hydrogen embrittlement is improved. If the Si content is too low, the above-mentioned effect cannot be fully obtained even if the contents of other elements are within the range of this embodiment. On the other hand, if the Si content is too high, the steel's resistance to hydrogen embrittlement decreases even if the contents of other elements are within the range of this embodiment. Therefore, the Si content is 0.05% to 1.50%. The preferred lower limit of the Si content is 0.06%, more preferably 0.08%, further preferably 0.10%, and even more preferably 0.13%. The preferred upper limit of the Si content is 1.45%, more preferably 1.42%, further preferably 1.40%, further preferably 1.38%, and even more preferably 1.35%.

[0141] Mn: 0.01~1.00%

[0142] Manganese (Mn) deoxidizes steel. Mn also improves hardenability and increases the strength of the steel. If the Mn content is too low, even if the contents of other elements are within the range of this embodiment, the above-mentioned effects cannot be fully obtained. On the other hand, if the Mn content is too high, even if the contents of other elements are within the range of this embodiment, coarse sulfide inclusions will be formed, reducing the hydrogen embrittlement resistance of the steel. Therefore, the Mn content is 0.01 to 1.00%. The preferred lower limit of the Mn content is 0.02%, more preferably 0.04%, and even more preferably 0.10%. The preferred upper limit of the Mn content is 0.97%, more preferably 0.95%, even more preferably 0.90%, and even more preferably 0.80%.

[0143] P: below 0.030%

[0144] Phosphorus (P) is an unavoidable 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 steel's resistance to hydrogen embrittlement. Therefore, the P content is 0.030% or less. The P content is preferably as low as possible. However, extreme reductions in P content would significantly increase manufacturing costs. Therefore, considering industrial production, the preferred lower limit of P content is 0.001%, more preferably 0.002%, and even more preferably 0.003%. The preferred upper limit of P content is 0.025%, more preferably 0.023%, even more preferably 0.021%, and even more preferably 0.020%.

[0145] S: below 0.0100%

[0146] Sulfur (S) is an unavoidable 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 steel's resistance to hydrogen embrittlement. Therefore, the S content is 0.0100% or less. The S content is preferably as low as possible. However, extreme reductions 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%, further preferably 0.0003%, and even more preferably 0.0005%. The preferred upper limit of S content is 0.0090%, more preferably 0.0080%, and even more preferably 0.0070%.

[0147] Cr: 0.40–1.10%

[0148] Chromium (Cr) improves the hardenability of steel and enhances its resistance to hydrogen embrittlement. Cr also improves the steel's resistance to temper softening, enabling high-temperature tempering. As a result, the steel's resistance to hydrogen embrittlement is improved. If the Cr content is too low, even if the contents of other elements are within the range of this embodiment, the above-mentioned effects cannot be fully obtained. On the other hand, if the Cr content is too high, even if the contents of other elements are within the range of this embodiment, coarse carbides will be formed, reducing the steel's resistance to hydrogen embrittlement. Therefore, the Cr content is 0.40 to 1.10%. The preferred lower limit of the Cr content is 0.43%, more preferably 0.44%, further preferably 0.46%, and even more preferably 0.50%. The preferred upper limit of the Cr content is 1.08%, more preferably 1.05%, further preferably 0.99%, and even more preferably 0.95%.

[0149] Mo: 0.40–1.30%

[0150] Molybdenum (Mo) improves the hardenability of steel. Mo also improves the steel's resistance to temper softening, enabling high-temperature tempering. As a result, the steel's resistance to hydrogen embrittlement is improved. Mo also segregates at the original austenite grain boundaries, further enhancing the steel's resistance to hydrogen embrittlement. If the Mo content is too low, even with the contents of other elements within the range of this embodiment, the aforementioned effects cannot be fully achieved. On the other hand, if the Mo content is too high, even with the contents of other elements within the range of this embodiment, coarse carbides will form, reducing the steel's resistance to hydrogen embrittlement. Therefore, the Mo content is 0.40% to 1.30%. The preferred lower limit of the Mo content is 0.43%, more preferably 0.44%, further preferably 0.48%, and even more preferably 0.55%. The preferred upper limit of the Mo content is 1.25%, more preferably 1.24%, further preferably 1.20%, and even more preferably 1.17%.

[0151] V: 0.01~0.30%

[0152] Vanadium (V) forms carbides, nitrides, or carbonitrides (hereinafter referred to as "carbonitrides, etc."), increasing the strength of steel. If the V content is too low, the aforementioned 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 V content is too high, carbonitrides, etc., will be excessively formed even if the contents of other elements are within the range of this embodiment. As a result, the strength of the steel becomes excessively high, and the resistance to hydrogen embrittlement of the steel decreases. Therefore, the V content is 0.01 to 0.30%. The preferred lower limit of the V content is 0.02%, more preferably 0.03%, further preferably 0.04%, further preferably 0.06%, and further preferably 0.08%. The preferred upper limit of the V content is 0.28%, more preferably 0.24%, and further preferably 0.22%.

[0153] Nb: 0.005~0.100%

[0154] Niobium (Nb) forms carbonitrides and the like. During the austenitizing heat treatment before quenching, the pinning effect of austenite grain boundaries refines the grains of the steel, increasing the yield point and decreasing the yield strength by Δσ. As a result, the steel's resistance to hydrogen embrittlement is improved. Nb also forms fine carbides during tempering, improving the steel's resistance to temper softening and increasing its strength. If the Nb content is too low, even if the contents of other elements are within the range of this embodiment, the above-mentioned effects cannot be fully obtained. On the other hand, if the Nb content is too high, even if the contents of other elements are within the range of this embodiment, excessive amounts of carbonitrides and the like are formed, and the steel's resistance to hydrogen embrittlement is reduced. Therefore, the Nb content is 0.005 to 0.100%. The preferred lower limit of the Nb content is 0.006%, more preferably 0.007%, further preferably 0.010%, and even more preferably 0.015%. The preferred upper limit for Nb content is 0.097%, more preferably 0.095%, even more preferably 0.090%, and even more preferably 0.085%.

[0155] Ti: 0.001~0.030%

[0156] Titanium (Ti) forms fine precipitates such as Ti nitrides. During the austenitizing heat treatment before quenching, the austenite grains are refined through a pinning effect, improving the hydrogen embrittlement resistance of the steel. If the Ti content is too low, the above-mentioned effect cannot be fully obtained even if the contents of other elements are within the range of this embodiment. On the other hand, if the Ti content is too high, coarse Ti nitrides will be formed even if the contents of other elements are within the range of this embodiment. Coarse Ti nitrides become the initiation point of cracks. As a result, the hydrogen embrittlement resistance of the steel is reduced. Therefore, the Ti content is 0.001 to 0.030%. The preferred lower limit of the Ti content is 0.002%, more preferably 0.003%, and even more preferably 0.005%. The preferred upper limit of the Ti content is 0.029%, more preferably 0.028%, even more preferably 0.027%, and even more preferably 0.025%.

[0157] Al: 0.005~0.100%

[0158] Aluminum (Al) deoxidizes steel. Al also combines with nitrogen to form Al nitrides, which refine the grains through a pinning effect, improving the steel's resistance to hydrogen embrittlement. If the Al content is too low, even if the contents of other elements are within the range of this embodiment, the above-mentioned effects cannot be fully obtained. On the other hand, if the Al content is too high, even if the contents of other elements are within the range of this embodiment, coarse oxides will be formed, reducing the steel's resistance to hydrogen embrittlement. Therefore, the Al content is 0.005% to 0.100%. The preferred lower limit of the Al content is 0.006%, more preferably 0.008%, and even more preferably 0.012%. The preferred upper limit of the Al content is 0.095%, more preferably 0.090%, and even more preferably 0.085%. It should be noted that the "Al" content mentioned in this specification refers to the content of "acid-soluble Al," i.e., "sol.Al."

[0159] B: 0.0005~0.0050%

[0160] Boron (B) improves the hardenability and strength of steel. B also suppresses grain boundary segregation of phosphorus (P), improving the steel's resistance to hydrogen embrittlement. If the B content is too low, the aforementioned 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 B content is too high, coarse B nitrides will form even if the contents of other elements are within the range of this embodiment. These coarse B nitrides become the initiation point for cracks. As a result, the steel's resistance to hydrogen embrittlement decreases. Therefore, the B content is 0.0005% to 0.0050%. The preferred lower limit of the B content is 0.0006%, more preferably 0.0008%, and even more preferably 0.0010%. The preferred upper limit of the B content is 0.0045%, more preferably 0.0040%, even more preferably 0.0035%, and even more preferably 0.0030%.

[0161] N: below 0.0100%

[0162] Nitrogen (N) is unavoidably present. That is, the lower limit of the N content exceeds 0%. N combines with Ti to form nitrides, which, during the austenitizing heat treatment before quenching, refine the austenite grains of the steel through a pinning effect, thereby improving the strength of the steel. On the other hand, if the N content is too high, even if the contents of other elements are within the range of this embodiment, coarse nitrides will form, reducing the steel's resistance to hydrogen embrittlement. Therefore, the N content is 0.0100% or less. The preferred lower limit of the N content for more effectively obtaining the above-mentioned effect is 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%. The preferred upper limit of the N content is 0.0096%, more preferably 0.0090%, even more preferably 0.0080%, and even more preferably 0.0070%.

[0163] O: below 0.0050%

[0164] Oxygen (O) is an unavoidable 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 steel's resistance to hydrogen embrittlement. Therefore, the O content is 0.0050% or less. The O content is preferably as low as possible. However, extreme reductions in O content significantly increase manufacturing costs. Therefore, considering industrial production, the preferred lower limit of O content is 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%. The preferred upper limit of O content is 0.0045%, more preferably 0.0040%, and even more preferably 0.0035%.

[0165] The chemical composition of the steel in this embodiment consists of Fe and impurities. Here, impurities in the chemical composition refer to impurities that are permissible during the industrial manufacturing of steel, arising from raw materials such as ore, waste, or the manufacturing environment, and are not intentionally present, provided they do not adversely affect the steel of this embodiment.

[0166] [Any element]

[0167] The chemical composition of the steel in this embodiment may also contain one or more elements selected from the group consisting of W and Co to replace a portion of the Fe. These elements are arbitrary and may not be present. When present, W and Co improve the steel's resistance to hydrogen embrittlement.

[0168] W: 0~2.00%

[0169] Tungsten (W) is an arbitrary element and may be absent. That is, the W content can be 0%. When present, W forms a corrosion coating on the surface of the steel in an acidic environment. As a result, hydrogen intrusion into the steel is suppressed, and the steel's resistance to hydrogen embrittlement is improved. The above-mentioned effects can be achieved to some extent with only a small amount of W. However, if the W 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 W content is 0 to 2.00%. The preferred lower limit of the W content is 0.01%, more preferably 0.05%, and even more preferably 0.08%. The preferred upper limit of the W content is 1.50%, more preferably 1.20%, and even more preferably 1.00%.

[0170] Co: 0-0.20%

[0171] Cobalt (Co) can be any element, or it can be absent. That is, the Co content can be 0%. When present, Co improves the hydrogen embrittlement resistance of steel. Co also dissolves in steel, improving its hardenability and strength. Even a small amount of Co is sufficient to achieve these effects to some extent. However, if the Co content is too high, its effect becomes saturated. Therefore, the Co content is 0–0.20%. The preferred lower limit for the Co content is 0.01%, more preferably 0.02%, and even more preferably 0.04%. The preferred upper limit for the Co content is 0.18%, more preferably 0.15%, even more preferably 0.12%, and even more preferably 0.10%.

[0172] The chemical composition of the steel in this embodiment may also contain one or more elements selected from the group consisting of Mg, Ca, and rare earth elements (REM) to replace a portion of Fe. These elements are arbitrary and may not be present. When present, Mg, Ca, and rare earth elements (REM) improve the steel's resistance to hydrogen embrittlement.

[0173] Mg: 0~0.0100%

[0174] Magnesium (Mg) is an arbitrary element and may be absent. That is, the Mg content can be 0%. When present, Mg combines with sulfur (S) in the steel, precipitating as fine Mg sulfides. Consequently, Mg reduces Mn sulfides. The hydrogen embrittlement resistance of the steel is improved through the combined effect of these two factors. The above-mentioned effects can be achieved to some extent with only a small amount of Mg. 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 coarsen, and the hydrogen embrittlement resistance of the steel will decrease. Therefore, the Mg content is 0 to 0.0100%. The preferred lower limit of the Mg content is 0.0001%, more preferably 0.0003%, further preferably 0.0006%, and even more preferably 0.0010%. The preferred upper limit of the Mg content is 0.0090%, more preferably 0.0080%, further preferably 0.0070%, further preferably 0.0060%, further preferably 0.0050%, further preferably 0.0040%, and even more preferably 0.0030%.

[0175] Ca: 0~0.0100%

[0176] Calcium (Ca) is an arbitrary element and may be absent. That is, the Ca content can be 0%. When present, Ca combines with S in the steel to precipitate as fine Ca sulfides. Consequently, Ca reduces Mn sulfides. The hydrogen embrittlement resistance of the steel is improved through the combined effect of these two factors. The above-mentioned effects can be achieved to some extent with only a small amount of Ca. 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 coarsen, and the hydrogen embrittlement resistance of the steel will decrease. Therefore, the Ca content is 0 to 0.0100%. The preferred lower limit of the Ca content is 0.0001%, more preferably 0.0003%, further preferably 0.0005%, and even more preferably 0.0007%. The preferred upper limit of the Ca content is 0.0080%, more preferably 0.0060%, further preferably 0.0050%, and even more preferably 0.0040%.

[0177] Rare earth elements (REM): 0~0.0100%

[0178] Rare earth elements (REMs) can be any element, or they can be absent. That is, the REM content can be 0%. When present, REMs combine with sulfur (S) in the steel, precipitating as fine REM sulfides. Consequently, REMs reduce Mn sulfides. The hydrogen embrittlement resistance of the steel is improved through the combined effect of these two factors. The above-mentioned effects can be achieved to some extent with only a small amount of REMs. 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 coarsen, and the hydrogen embrittlement resistance of the steel will decrease. Therefore, the REM content is 0 to 0.0100%. The preferred lower limit of the REM content is 0.0001%, more preferably 0.0003%, further preferably 0.0005%, and even more preferably 0.0010%. The preferred upper limit of the REM content is 0.0090%, more preferably 0.0080%, further preferably 0.0070%, further preferably 0.0060%, further preferably 0.0050%, and even more preferably 0.0040%.

[0179] In this specification, REM 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, the REM content in this specification refers to the total content of these elements.

[0180] The chemical composition of the steel in this embodiment may also contain one or more elements selected from the group consisting of Cu, Ni, and Sn to replace a portion of the Fe. These elements are arbitrary and may not be present. When present, Cu, Ni, and Sn all improve the steel's resistance to hydrogen embrittlement.

[0181] Cu: 0–0.40%

[0182] Copper (Cu) is an arbitrary element and may be absent. That is, the Cu content can be 0%. When present, Cu improves the steel's resistance to hydrogen embrittlement. Cu also dissolves in the steel, improving its hardenability and strength. Even a small amount of Cu can achieve these effects to some extent. However, if the Cu content is too high, the hot workability of the steel decreases, even if the contents of other elements are within the range of this embodiment. Therefore, the Cu content is 0 to 0.40%. The preferred lower limit of the Cu content is 0.01%, more preferably 0.02%, further preferably 0.03%, further preferably 0.05%, and further preferably 0.07%. The preferred upper limit of the Cu content is 0.38%, more preferably 0.36%, and further preferably 0.34%.

[0183] Ni: 0–0.20%

[0184] Nickel (Ni) is an arbitrary element and may be absent. That is, the Ni content can be 0%. When present, Ni improves the steel's resistance to hydrogen embrittlement. Ni also dissolves in the steel, improving its hardenability and strength. Even a small amount of Ni can achieve these effects to some extent. However, if the Ni content is too high, the manufacturing cost will increase drastically, even if the contents of other elements are within the range of this embodiment. Therefore, the Ni content is 0 to 0.20%. The preferred lower limit of the Ni content is 0.01%, more preferably 0.02%, further preferably 0.03%, and even more preferably 0.05%. The preferred upper limit of the Ni content is 0.19%, more preferably 0.18%, and even more preferably 0.17%.

[0185] Sn: 0–0.10%

[0186] Tin (Sn) is an arbitrary element and may be absent. That is, the Sn content can be 0%. When present, Sn improves the resistance of steel to hydrogen embrittlement. Even a small amount of Sn can achieve the above-mentioned effects to some extent. However, if the Sn content is too high, the hot workability of the steel decreases even if the contents of other elements are within the range of this embodiment. Therefore, the Sn content is 0 to 0.10%. The preferred lower limit of the Sn content is 0.01%, more preferably 0.02%, further preferably 0.03%, and even more preferably 0.04%. The preferred upper limit of the Sn content is 0.09%, more preferably 0.08%, and even more preferably 0.07%.

[0187] [Grain diameter of the original austenite grain (GS)]

[0188] In the steel of this embodiment, the original austenite grain diameter GS is greater than 5.0 μm and less than 30.0 μm. In this specification, the original austenite grain diameter GS (original γ grain diameter GS) refers to the original austenite grain diameter determined according to the average intercept method specified in JIS G0551:2020.

[0189] Hydrogen embrittlement is prone to occur when hydrogen accumulates at grain boundaries. In steels with the above chemical composition, if the original austenite grains are fine, the area of ​​the original austenite grain boundaries increases. In this case, even if the amount of hydrogen absorbed and stored in the steel is the same, the amount of hydrogen stored per unit area of ​​the original austenite grain boundaries decreases. Therefore, if the original austenite grains are fine, the hydrogen embrittlement resistance of the steel can be improved. Specifically, if the original γ grain diameter GS of the steel with the above chemical composition is 30.0 μm or less, then even if the original γ grain diameter GS exceeds 5.0 μm, it is possible to achieve both a high yield strength of 125 ksi and excellent hydrogen embrittlement resistance, provided that other configurations of this embodiment are satisfied.

[0190] It should be noted that the original γ grain diameter GS is affected by the chemical composition of the steel and the manufacturing method, and its value varies. Specifically, as mentioned above, during the austenitizing heat treatment before quenching, Ti and / or Nb form fine precipitates such as Ti nitrides and / or Nb carbonitrides, which refine the grains through the pinning effect of austenite grain boundaries. Furthermore, as described later, if the quenching temperature in the quenching process is too high, the grains coarsen, and the original γ grain diameter GS of the manufactured steel sometimes becomes larger. Thus, those skilled in the art can, to some extent, control the original γ grain diameter GS by adjusting the chemical composition of the steel and the manufacturing method.

[0191] In the steel of this embodiment, the preferred upper limit of the original γ grain diameter GS is 28.0 μm, more preferably 26.0 μm, even more preferably 24.0 μm, and even more preferably 20.0 μm. In the steel of this embodiment, a small original γ grain diameter GS is preferred. However, as described above, for steels intended for use in acidic environments and in high-pressure hydrogen containers, manufacturing costs increase drastically when the original γ grain diameter GS is 5.0 μm or less. Therefore, for the steel of this embodiment, the lower limit of the original γ grain diameter GS is exceeded by 5.0 μm. Whether the original γ grain diameter GS is 5.5 μm or more, or 6.0 μm or more, the steel of this embodiment can sometimes achieve both a high yield strength of 125 ksi and excellent resistance to hydrogen embrittlement.

[0192] [Method for determining the grain diameter (GS) of the original austenite grains]

[0193] In this embodiment, the original γ grain diameter GS of the steel can be determined by the following method. The original austenite grain diameter GS is determined according to the average intercept method specified in JIS G0551:2020.

[0194] First, test pieces with observation surfaces are collected from the steel. When the steel is a steel plate, a test piece with an observation surface including the plate thickness t / 4 position (the area to be observed) and parallel to the rolling direction is collected from the center of the plate width. Here, the plate thickness t / 4 position refers to the position at a depth of 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 with an observation surface including the central portion of the wall thickness (the area to be observed) and parallel to the pipe axis is collected. When the steel is round bar, a test piece with an observation surface including the R / 2 position (the area to be observed) and parallel to the rolling direction is collected. It should be noted that in this specification, round bar refers to a bar steel bar with a circular cross-section perpendicular to the axial direction. Furthermore, the R / 2 position refers to the center position of radius R in the cross-section of the round bar perpendicular to the axial direction. The size of the test piece is not particularly limited. For example, the size of the test piece is set to 10 mm in length in the rolling direction × 5 mm in width direction × 10 mm in thickness direction. The surface containing both the rolling direction and the thickness direction (the test piece size is 10 mm × 10 mm in the above case) is used as the observation surface.

[0195] The observation surface of the test piece was mirror-polished. After mirror polishing, the observation surface was immersed in picrine etching solution for about 10 seconds to expose the grain boundaries of the original austenite grains through etching. Using a scanning electron microscope (SEM), any 10 fields of view of the etched observation surface were observed in secondary electron images, generating photographic images. The area of ​​each field of view was, for example, 500 μm × 500 μm (magnification 200x).

[0196] Using the generated photographic images, the grain size number was evaluated according to the mean intercept method specified in JIS G0551:2020. Based on the evaluated grain size number, the grain diameter of the original austenite grain in each field of view was calculated. The arithmetic mean of the grain diameters of the original austenite grains calculated in the 10 fields of view was defined as the original austenite grain diameter GS (original γ grain diameter GS) (μm).

[0197] [Yield strength σ of steel]

[0198] In the steel of this embodiment, the yield strength σ is 862–965 MPa. In this specification, the yield strength σ refers to the 0.2% conditional yield strength obtained by a tensile test performed according to the method of JIS Z2241:2011. By satisfying other configurations of this embodiment, the steel of this embodiment exhibits excellent resistance to hydrogen embrittlement, even with a primary γ grain diameter GS exceeding 5.0 μm and a yield strength σ of 862–965 MPa.

[0199] In this embodiment, the preferred lower limit for the yield strength σ is 865 MPa. In this embodiment, the preferred upper limit for the yield strength σ is less than 965 MPa, more preferably 960 MPa, and even more preferably 955 MPa. It should be noted that the method for determining the yield strength σ will be described later.

[0200] [Yield point decrease of steel Δσ]

[0201] In the steel of this embodiment, the yield point drop Δσ is 40 MPa or more. In this specification, the yield point drop Δσ refers to the difference between the maximum stress in the elastic deformation region (upper yield point) obtained by a tensile test performed according to the method of JIS Z2241:2011 and 0.2% of the conditional yield strength. That is, in this embodiment, the yield point drop Δσ is defined as... Figure 3 The stress σ0 (MPa) at point P0 shown is... Figure 3 The difference in stress σ (MPa) at point P is shown (Δσ = σ0 - σ).

[0202] It should be noted that, referring to Figure 2 ,exist Figure 2 The stress-strain curve shown does not exhibit a region of sharp stress decrease. Therefore, in the case of such a stress-strain curve, it is impossible to define a point equivalent to point P0, and thus, the yield point drop Δσ cannot be defined. On the other hand, for the curve shown as... Figure 1 For the steel shown in the stress-strain curve, even at stresses higher than the yield strength σ, elastic deformation is maintained. That is, the greater the yield point decrease Δσ, the less likely plastic deformation will occur, and the stress limit for hydrogen embrittlement fracture may be increased. In other words, the greater the yield point decrease Δσ, the higher the resistance to hydrogen embrittlement may be.

[0203] Here, for steels having the above-mentioned chemical composition, a primary γ-grain diameter GS exceeding 5.0 μm and less than 30.0 μm, and a yield strength σ of 862–965 MPa, if the yield point decrease Δσ is 40 MPa or more, excellent hydrogen embrittlement resistance can be stably obtained, provided that other configurations of this embodiment are satisfied. Therefore, in the steel of this embodiment, the yield point decrease Δσ is 40 MPa or more.

[0204] In this embodiment, the preferred lower limit of the yield point decrease Δσ is 44 MPa, more preferably 48 MPa, further preferably 55 MPa, and even more preferably 60 MPa. In this embodiment, the upper limit of the yield point decrease Δσ is not particularly limited. For example, the upper limit of the yield point decrease Δσ for the steel in this embodiment could be 200 MPa, 150 MPa, 130 MPa, or 120 MPa. It should be noted that the method for measuring the yield point decrease Δσ will be described later.

[0205] [Elongation at yield point of steel Δε]

[0206] In the steel of this embodiment, the yield point elongation Δε is 1.5% or more. In this specification, the yield point elongation Δε refers to the magnitude of the strain at the beginning of the plastic deformation region obtained by a tensile test performed according to the method of JIS Z2241:2011. That is, in this embodiment, the yield point elongation Δε is defined as... Figure 4 The strain (%) at point Q is shown.

[0207] It should be noted that, referring to Figure 2 ,exist Figure 2 In the stress-strain curve shown, there is no region where the stress remains constant even as the strain increases. Therefore, in the case of such a stress-strain curve, it is impossible to define a point equivalent to point Q, and it is impossible to define the yield elongation Δε. On the other hand, for the curve shown as... Figure 1 For the steel shown in the stress-strain curve, the larger the yield elongation Δε, the less likely plastic deformation will occur even with an increase in the amount of strain introduced into the steel. Therefore, the larger the yield elongation Δε, the higher the stress limit for hydrogen embrittlement fracture may be. In other words, the larger the yield elongation Δε, the higher the resistance to hydrogen embrittlement may be.

[0208] Here, for steels having the above-mentioned chemical composition, a primary γ-grain diameter GS exceeding 5.0 μm and less than 30.0 μm, a yield strength σ of 862–965 MPa, and a yield point drop Δσ of 40 MPa or more, excellent resistance to hydrogen embrittlement can be stably obtained if the yield point elongation Δε is 1.5% or more. Therefore, in the steel of this embodiment, the yield point elongation Δε is 1.5% or more.

[0209] In this embodiment, the preferred lower limit of the yield point elongation Δε is 1.6%, more preferably 1.8%, further preferably 2.0%, and even more preferably 2.2%. In this embodiment, the upper limit of the yield point elongation Δε is not particularly limited. For example, the upper limit of the yield point elongation Δε of the steel in this embodiment could be 4.0%, 3.8%, 3.6%, or 3.5%. It should be noted that the method for determining the yield point drop Δσ will be described later.

[0210] It should be noted that, as Figure 2 As shown, even for steels with the same chemical composition, original γ grain diameter GS, and yield strength σ as in this embodiment, it may be impossible to define the yield point decrease Δσ and yield point elongation Δε. Here, in steels with the aforementioned chemical composition, the yield point decrease Δσ and / or yield point elongation Δε are determined based on the microstructure of the steel (phases, precipitates, and inclusions), and / or the state of the precipitates (crystal structure, size, and volume fraction), and / or the state of dislocations in the steel (dislocation density, dislocation arrangement, and the ratio of edge dislocations to screw dislocations, etc.), and their equilibrium. Therefore, for the steel of this embodiment, by appropriately controlling the microstructure and / or the state of the precipitates, the state of the dislocations in the steel, and their equilibrium, it is considered that the yield point decrease Δσ is 40 MPa or more, and the yield point elongation Δε is 1.5% or more.

[0211] [Methods for determining yield strength σ, yield point depression Δσ, and yield point elongation Δε]

[0212] The yield strength σ, yield point depression Δσ, and yield point elongation Δε of the steel in this embodiment can be determined by the following method. A tensile test is performed according to the method in JIS Z2241:2011. Round bar test pieces are prepared from the steel of this embodiment. When the steel is a steel plate, round bar test pieces are prepared from the center of the plate width and the center of the plate thickness. When the steel is a steel pipe, round bar test pieces are prepared from the center of the wall thickness. When the steel is a round bar, round bar test pieces are prepared from the R / 2 position. The dimensions of the round bar test piece are, for example, a parallel portion diameter of 6.0 mm and a parallel portion length of 40 mm. It should be noted that the round bar test piece is prepared with its axial direction parallel to the rolling direction of the steel.

[0213] Tensile tests were conducted using the prepared round bar test specimens at room temperature (25°C) in atmospheric conditions. The elastic deformation region and the plastic deformation region were determined based on the stress-strain curve obtained from the tensile test. It is, of course, easy for those skilled in the art to determine the elastic and plastic deformation regions. In the stress-strain curve, the point where the stress in the elastic deformation region shows the maximum stress is defined as P0, and the stress at point P0 is defined as σ0 (MPa). Furthermore, the intersection point of the stress-strain curve with the straight line L, which is parallel to the elastic deformation region and shifted 0.2% parallel to the positive direction of strain, is defined as P, and the stress at point P is defined as σ (MPa). It should be noted that in this specification, "strain" refers to the so-called nominal strain.

[0214] The stress σ at point P is defined as the yield strength σ (MPa). It should be noted that the yield strength σ (MPa) is obtained by rounding the first decimal place of the obtained value. As mentioned above, the yield strength σ is equivalent to 0.2% of the conditional yield strength. The difference between the stress σ0 at point P0 and the stress σ at point P is defined as the yield point drop Δσ (MPa). It should be noted that the yield point drop Δσ (MPa) is obtained by rounding the first decimal place of the obtained value. Furthermore, the point where the plastic deformation region begins in the stress-strain curve is defined as Q. The strain (%) at point Q is defined as the yield point elongation Δε (%). It should be noted that the yield point elongation Δε (%) is obtained by rounding the second decimal place of the obtained value.

[0215] [Equation (1)]

[0216] The steel in this embodiment preferably has a primary γ grain diameter GS, yield strength σ, and yield point elongation Δε that satisfy the following formula (1). If the steel in this embodiment satisfies formula (1), it has superior resistance to hydrogen embrittlement.

[0217] σ×Δε / GS 2 ≥12 (1)

[0218] Here, the yield strength of the steel is substituted into σ in Equation (1) in MPa, the elongation at the yield point of the steel is substituted into Δε in Equation (1) in %, and the grain diameter of the original austenite grains of the steel is substituted into GS in Equation (1) in μm.

[0219] Defined as Fn1=σ×Δε / GS 2Fn1 is an indicator of hydrogen embrittlement resistance. Provided that other configurations of this embodiment are satisfied, if Fn1 is 12 or higher, the steel exhibits superior hydrogen embrittlement resistance. Therefore, the steel of this embodiment preferably has the above-mentioned chemical composition, with an original γ-grain diameter GS exceeding 5.0 μm and below 30.0 μm, a yield strength σ of 862–965 MPa, a yield point decrease Δσ of 40 MPa or higher, a yield point elongation Δε of 1.5% or higher, and furthermore, an Fn1 of 12 or higher.

[0220] The lower limit of Fn1 is further preferred to be greater than 12, and more preferably 13. The upper limit of Fn1 is not particularly limited, for example, it can be 148, 135, 130, or 125. It should be noted that Fn1 is obtained by rounding the first decimal place of the obtained value.

[0221] [Resistance to hydrogen embrittlement]

[0222] The steel of this embodiment has the above-described chemical composition, with an original γ grain diameter GS exceeding 5.0 μm and less than 30.0 μm, a yield strength σ of 862–965 MPa, a yield point depression Δσ of 40 MPa or more, and a yield elongation Δε of 1.5% or more. As a result, the steel of this embodiment exhibits high strength and excellent resistance to hydrogen embrittlement even when the original γ grain diameter GS exceeds 5.0 μm. In this embodiment, the excellent resistance to hydrogen embrittlement can be evaluated using the following method.

[0223] Test pieces for evaluating hydrogen embrittlement resistance are made from the steel described in this embodiment. The test piece is a round bar test piece with an annular notch. For example, the 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 parallel portion along its length. 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, the round bar test piece is made from the center of the plate width and at a position equal to t / 4 of the plate thickness. When the steel is a steel pipe, the round bar test piece is made from the center of the wall thickness. When the steel is a round bar, the round bar test piece is made from a position equal to R / 2.

[0224] The prepared circular rod test piece with a ring-shaped notch was hydrogen-charged 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, 30 g / L NH4SCN, and an acetate buffer solution at room temperature. The pH was adjusted to 3.5 before the experiment using the acetate buffer solution.

[0225] With the annularly notched cylindrical test piece immersed in a cathode hydrogen-charging solution, the potential is set to -1.5V and the hydrogen charging time is set to 24 hours to char the cylindrical test piece with the annular notch. Preferably, a zinc-plated coating is formed on the surface of the hydrogen-charged cylindrical test piece with the annular notch to prevent hydrogen from leaking to the outside.

[0226] For hydrogen-filled round bar specimens with ring-shaped notches, tensile tests were performed at room temperature (25°C) in the atmosphere using a slow strain rate testing machine (SSRT). The strain rate was set to 4.2 × 10⁻⁶. -6 The fracture stress BS1 is calculated per second. It should be noted that the fracture stress BS1 is obtained by rounding the first decimal place of the obtained value. In this embodiment, if the fracture stress BS1 obtained under the above conditions is 950 MPa or higher, it is considered to have excellent hydrogen embrittlement resistance. In this embodiment, if the fracture stress BS1 obtained under the above conditions is further 970 MPa or higher, it is considered to have even better hydrogen embrittlement resistance.

[0227] [Microstructure]

[0228] In the microstructure of the steel of this embodiment, the total area fraction of tempered martensite and tempered bainite is 90% or more. The balance of the microstructure is, for example, ferrite and / or pearlite. In this embodiment, when the steel has the above-described chemical composition, the original γ grain diameter GS is greater than 5.0 μm and less than 30.0 μm, the yield strength σ is 862 to 965 MPa, the yield point depression Δσ is 40 MPa or more, and the yield point elongation Δε is 1.5% or more, it can be determined that the total area fraction of tempered martensite and tempered bainite in the steel is 90% or more.

[0229] [Method for determining the total area ratio of tempered martensite and tempered bainite]

[0230] The total area ratio of tempered martensite and tempered bainite in the microstructure of the steel in this embodiment can also be determined by the following method. A test piece with an observation surface is prepared from the steel. When the steel is a steel plate, a test piece with an observation surface including the plate thickness t / 4 position (the area to be observed) and parallel to the rolling direction is collected from the center of the plate width. When the steel is a steel tube, a test piece with an observation surface including the wall thickness (the area to be observed) and parallel to the tube axis is prepared. When the steel is a round bar, a test piece with an observation surface including the R / 2 position (the area to be observed) and parallel to the rolling direction is collected. The size of the test piece is not particularly limited. For example, the size of the test piece is 10 mm in length in the rolling direction × 5 mm in width direction × 10 mm in thickness direction. When the steel is a steel plate, the thickness direction corresponds to the plate thickness direction, and the width direction corresponds to the plate width direction. When the steel is a steel tube, the rolling direction corresponds to the tube axis direction, the thickness direction corresponds to the wall thickness direction, and the width direction corresponds to the direction perpendicular to both the tube axis direction and the wall thickness direction (circumferential direction). When the steel is round bar, the rolling direction is equivalent to the axial direction, the thickness direction is equivalent to the radial direction, and the width direction is equivalent to the direction perpendicular to both the rolling direction and the radial direction (circumferential direction). The surface containing the rolling direction and the thickness direction (the test piece size is 10mm × 10mm in the above case) is used as the observation surface.

[0231] After grinding the observation surface of the test piece to a mirror finish, it was immersed in a nitric acid-alcohol etching solution for approximately 10 seconds to expose the tissue through etching. Using a scanning electron microscope (SEM), ten random fields of view within the observation area of ​​the etched observation surface were observed in secondary electron images. For steel plates, the observation area was at the t / 4 position of the plate thickness. For steel pipes, the observation area was at the central part of the wall thickness. For round steel bars, the observation area was at the R / 2 position. The area of ​​each of the ten fields of view within the observation area was, for example, 400 μm. 2 (Multiplier 5000x).

[0232] In each field of view, tempered martensite and tempered bainite were identified. In each field of view, tempered martensite and tempered bainite can be distinguished from other microstructures (ferrite, pearlite, etc.) based on their morphology. Specifically, a microstructure with a layered structure can be identified as pearlite. A microstructure containing lath-like and / or lenticular structures can be identified as tempered martensite and tempered bainite. A microstructure without a lower microstructure within the grain can be identified as ferrite.

[0233] The total area ratio of tempered martensite and tempered bainite is determined. There is no particular limitation on the method for determining the total area ratio; any known method can be used. For example, the total area ratio of tempered martensite and tempered bainite can be determined through image analysis. In this embodiment, the arithmetic mean of the total area ratios of tempered martensite and tempered bainite obtained from all fields of view (10 fields of view) is defined as the total area ratio (%) of tempered martensite and tempered bainite.

[0234] [Steel shapes and uses]

[0235] The shape of the steel used in this embodiment is not particularly limited. The steel used in this embodiment can be a steel pipe, a steel plate, or a round steel bar.

[0236] Preferably, the steel used in this embodiment is any one of oil well steel pipe, pipeline steel pipe, and high-pressure hydrogen container steel pipe. Oil well steel pipe refers to steel pipe used for oil well applications. Examples of oil well pipes include casing, pipes, and drill pipes used for drilling oil or gas wells, collecting crude oil or natural gas, etc. Pipeline steel pipe refers to steel pipe used for pipeline applications, constituting pipelines for transporting production fluids (crude oil or natural gas) collected from oil or gas wells. Examples of pipelines include fluid pipelines transporting production fluids from oil or gas wells, collection pipelines collecting and transporting production fluids transported by fluid pipelines to primary processing facilities, trunk lines transporting production fluids that have undergone primary processing such as dehydration to markets, and distribution pipelines transporting to consumers. High-pressure hydrogen container steel pipe refers to steel pipe used in high-pressure hydrogen containers that store high-pressure hydrogen gas, standardized by ISO 11439, ANSI / NGV, the High-Pressure Gas Safety Law, and the Example Standards for Container Safety Regulations. The steel used in this embodiment can be a steel pipe for a high-pressure hydrogen container, or any of the steel pipes for a high-pressure hydrogen accumulator or a steel pipe for a high-pressure hydrogen cylinder.

[0237] More preferably, the steel used in this embodiment is any one of seamless steel pipes for oil wells, seamless steel pipes for pipelines, and seamless steel pipes for high-pressure hydrogen containers. Seamless steel pipes for oil wells refer to steel pipes used for oil wells that are seamless steel pipes. Seamless steel pipes for pipelines refer to steel pipes used for pipelines that are seamless steel pipes. Seamless steel pipes for high-pressure hydrogen containers refer to steel pipes used for high-pressure hydrogen containers that are seamless steel pipes. The steel used in this embodiment can be seamless steel pipes for high-pressure hydrogen containers, or seamless steel pipes for high-pressure hydrogen accumulators, or seamless steel pipes for high-pressure hydrogen cylinders.

[0238] [Manufacturing Method]

[0239] Hereinafter, an example of a method for manufacturing the steel according to this embodiment will be described. It should be noted that the manufacturing method described below is an example, and the manufacturing method of the steel according to this embodiment is not limited to this. That is, as long as the steel of this embodiment having the above-described structure can be manufactured, the manufacturing method described below is not limited. However, the manufacturing method described below is a suitable manufacturing method for manufacturing the steel of this embodiment.

[0240] An example of the steel manufacturing method of this embodiment includes the following steps.

[0241] (Process 1) Blank preparation process

[0242] (Process 2) Hot working process

[0243] (Step 3) Quenching and Tempering

[0244] (Process 4) Low-temperature heat treatment process

[0245] The manufacturing method satisfies the following conditions in the low-temperature heat treatment process of step 4.

[0246] (Condition 1) Keep at 150-250℃ for more than 10 minutes.

[0247] The following is a description of each process.

[0248] [(Process 1) Blank Preparation Process]

[0249] In the billet preparation process, firstly, molten steel with the aforementioned chemical composition is produced using a known refining method. The produced molten steel is then used to manufacture billets via continuous casting. Here, a billet refers to a slab, large billet, or small billet. Alternatively, the molten steel can be used to manufacture steel ingots via ingot casting instead of billets. Depending on the requirements, slabs, large billets, or steel ingots can be hot-rolled to produce small billets. Through the above manufacturing processes, billets (slabs, large billets, or small billets) are produced.

[0250] [(Process 2) Hot working process]

[0251] In the hot working process, the prepared billet is hot-worked to produce intermediate steel. When the final product is a steel pipe, 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 a tube blank (seamless steel pipe). For example, the Mannesmann process is used for hot working to produce the tube blank. In this case, a small square billet is pierced using a piercing mill. The pierced small square billet is then stretched using a mandrel-type seamless tube mill. Furthermore, as needed, the stretched small square billet is sizing rolled using a reducing mill or a sizing mill. Through these processes, the tube blank is manufactured.

[0252] Tube blanks can also be manufactured from small square billets using hot working methods other than the Mannesmann process. For example, in the case of thick-walled steel with short dimensions, such as pipe fittings, tube blanks can be manufactured by forging, such as the Arachne punching process, or by hot extrusion.

[0253] When the final product is a steel plate, for example, one or more rolling mills containing a pair of rolls are used to hot roll the billet (slab) to produce intermediate steel (steel). The heating temperature before hot rolling is not particularly limited, for example, 1100 to 1300°C.

[0254] When the final product is round steel, for example, intermediate steel (round steel) is manufactured by sizing a billet (large square billet) using a sizing mill and / or hot rolling using a continuous rolling mill. That is, round steel can be produced by sizing the billet, by hot rolling the billet using a continuous rolling mill without sizing, or by sizing the billet using both a sizing mill and hot rolling using a continuous rolling mill. Multiple rolling mills in a continuous rolling mill are arranged in a row, each mill including a pair of rolls. When sizing is performed, the pre-sizing heating temperature is not particularly limited, for example, 1100–1300°C. When hot rolling is performed using a continuous rolling mill, the pre-sizing heating temperature is not particularly limited, for example, 1100–1300°C.

[0255] (Process 3) Quenching and tempering process

[0256] In the quenching and tempering process, the intermediate steel after the hot working process is subjected to quenching and tempering processes.

[0257] [Quenching process]

[0258] In the quenching process, the intermediate steel produced in the hot working process is quenched. Quenching is carried out using known methods. Specifically, the hot-worked intermediate steel is placed in a heat treatment furnace and held at the quenching temperature. The quenching temperature is A. C3 Above the phase transformation point, for example, 900–1000°C. After maintaining the intermediate steel at the quenching temperature, it is rapidly cooled (quenched).

[0259] If the quenching temperature is too high, the grains coarsen, and the original γ grain diameter GS sometimes becomes excessively large in the manufactured steel. On the other hand, if the quenching temperature is too low, coarse carbides sometimes remain in the manufactured steel. In this case, a yield strength σ of 862 MPa or higher cannot be obtained. Therefore, in the quenching process of this embodiment, the quenching temperature is A. C3 Above the phase transition point, specifically, preferably 900–1000℃.

[0260] There is no particular limitation on the holding time at the quenching temperature, for example, it is 10 to 60 minutes. The quenching method is, for example, water cooling. There are no particular restrictions on the quenching method. When the steel is a steel pipe, the billet can be rapidly cooled by immersing it in a water bath or oil bath, or by injecting cooling water into the outer and / or inner surface of the billet, or by spraying cooling water from a nozzle.

[0261] It should be noted that after hot working, quenching can be performed immediately after hot working without cooling the steel to room temperature (direct quenching), or the steel can be placed in a reheating furnace and kept at the quenching temperature before quenching.

[0262] [Tempering process]

[0263] In the tempering process, the steel after the quenching process is tempered. During the tempering process, the yield strength σ of the steel is adjusted to 862–965 MPa. It should be noted that those skilled in the art can adjust the yield strength σ of the steel to 862–965 MPa by adjusting the conditions of the tempering process.

[0264] As long as the yield strength σ of the manufactured steel is 862–965 MPa, there are no particular limitations on the tempering temperature and holding time. Specifically, the tempering temperature is, for example, 650℃ to A. C1 Phase transformation point. The holding time at the tempering temperature is not particularly limited, for example, it is 10 to 180 minutes. The yield strength of the steel with the aforementioned chemical composition can be adjusted by appropriately adjusting the tempering temperature according to its chemical composition. Here, tempering temperature refers to the furnace temperature (°C) in the heat treatment furnace, and the holding time at the tempering temperature refers to the time spent in the furnace (from the time it is loaded into the heat treatment furnace until it is removed).

[0265] It should be noted that the quenching and tempering processes can be performed once or multiple times. For example, the quenching and tempering processes can be performed again after the initial quenching and tempering processes. In cases where multiple quenching and tempering processes have been performed, the original γ grain diameter GS in the manufactured steel may sometimes become smaller.

[0266] (Process 4) Low-temperature heat treatment process

[0267] In the low-temperature heat treatment process, for steel that has undergone quenching and tempering, low-temperature heat treatment is performed under the conditions described in condition 1 below.

[0268] [Regarding condition 1]

[0269] (Condition 1) Keep at 150-250℃ for more than 10 minutes.

[0270] In this embodiment, the steel after the tempering process is held at 150–250°C for at least 10 minutes. Here, the heat treatment temperature of the low-temperature heat treatment process refers to the furnace temperature (°C) in the heat treatment furnace, and the heat treatment time refers to the time spent in the furnace (from the time it is loaded into the heat treatment furnace until it is removed).

[0271] By heat treatment at 150–250°C, dislocations may be stabilized. If dislocations are stabilized, they become less susceptible to plastic deformation. As a result, the yield point decreases by Δσ to 40 MPa or more, and the yield elongation Δε becomes 1.5% or more. Therefore, in the low-temperature heat treatment process of this embodiment, heat treatment is performed at 150–250°C for 10 minutes or more.

[0272] If the heat treatment temperature is too low, the aforementioned effects cannot be fully achieved. That is, dislocations are not sufficiently stabilized, and in the manufactured steel, the yield point decrease Δσ sometimes cannot be defined, or the yield point decrease Δσ becomes below 40 MPa. In this case, the yield point elongation Δε sometimes cannot be defined, or the yield point elongation Δε becomes less than 1.5%. On the other hand, even if the heat treatment temperature is too high, dislocation stabilization is sometimes insufficient. Therefore, in the manufactured steel, the yield point decrease Δσ sometimes cannot be defined, or the yield point decrease Δσ becomes less than 40 MPa. In this case, the yield point elongation Δε sometimes cannot be defined, or the yield point elongation Δε becomes less than 1.5%.

[0273] If the heat treatment time is too short, the aforementioned effects cannot be fully achieved. That is, dislocations are not sufficiently stabilized, and in the manufactured steel, the yield point drop Δσ may sometimes be undefined, or Δσ may fall below 40 MPa. In this case, the yield point elongation Δε may also sometimes be undefined, or Δε may fall below 1.5%. On the other hand, even if the heat treatment time is too long, the aforementioned effects become saturated. Therefore, there is no particular upper limit to the heat treatment time, for example, 60 minutes.

[0274] By performing the above manufacturing processes, the steel of this embodiment can be manufactured. Hereinafter, the effects of the steel of this embodiment will be further explained in detail with reference to embodiments. The various conditions in the embodiments described below are examples of conditions adopted to confirm the feasibility and effects of the steel of this embodiment. Therefore, the steel of this embodiment is not limited to the one example of conditions described in the embodiments.

[0275] Example

[0276] Manufacture steel (steel plate) with the chemical composition shown in Tables 1-1 and 1-2. It should be noted that "-" in Tables 1-1 and 1-2 indicates that the content of the corresponding element is below the impurity level.

[0277] [Table 1-1]

[0278] Table 1-1

[0279]

[0280] [Table 1-2]

[0281] Table 1-2

[0282]

[0283] The steel samples for each specimen symbol were manufactured using the following method. Steel ingots with the chemical compositions listed in Tables 1-1 and 1-2 were manufactured by casting. The steel ingots were then hot-forged to produce 50 mm thick blocks.

[0284] The stock material undergoes a hot working process. Specifically, the stock material is heated to 1250°C. The heated stock material is then hot-rolled to produce steel (plate) with a thickness of 15mm. The produced steel is then allowed to cool naturally to room temperature.

[0285] For steel that has cooled naturally to room temperature, quenching and tempering processes are performed. Specifically, for the steel, after holding at the temperature (°C) for only a time (minutes) in the "Quenching Process" column of Table 2, water quenching is performed. The quenched steel is then tempered. It is held at the temperature (°C) for only a time (minutes) in the "Tempering Process" column of Table 2. Afterwards, the tempered steel undergoes a low-temperature heat treatment process. It is held at the temperature (°C) for only a time (minutes) in the "Low-Temperature Heat Treatment Process" column of Table 2. It should be noted that the low-temperature heat treatment process is not performed for steel with the sample symbol ZR. Through the above manufacturing processes, steel (steel plates) with each sample symbol are manufactured.

[0286] [Table 2]

[0287] Table 2

[0288]

[0289] [Evaluation Test]

[0290] For the manufactured steel, tests were conducted to determine the original γ grain diameter, perform tensile tests, and evaluate its resistance to hydrogen embrittlement.

[0291] [Original γ-grain diameter determination experiment]

[0292] From the center of the steel plate width of each specimen symbol, a test piece is prepared with the observation surface covering the plate thickness t / 4 position of the observation area and including both the rolling direction and the plate thickness direction. The dimensions of the test piece are 10 mm in the rolling direction, 5 mm in the plate width direction, and 10 mm in the plate thickness direction. The 10 mm length in the rolling direction × 10 mm length in the plate thickness direction is used as the observation surface. For the collected test pieces, the original γ grain diameter GS (μm) is calculated according to the above method. At this time, the field of view area is set to 500 μm × 500 μm (magnification 200x). The obtained original γ grain diameter GS (μm) is shown in the "Original γ Grain Diameter GS (μm)" column of Table 3.

[0293] [Table 3]

[0294] Table 3

[0295]

[0296] [Tension Test]

[0297] Round bar test pieces were collected from the center of the plate thickness of the steel for each specimen symbol. The diameter of the parallel portion of the round bar test piece was 6.0 mm, and the length of the parallel portion was 40 mm. The axis of the round bar test piece was parallel to the rolling direction of the steel. Using the round bar test pieces, the yield strength σ (MPa), yield point drop Δσ (MPa), and yield point elongation Δε (%) were determined according to the above method. The maximum stress in the uniform elongation during the tensile test was defined as the tensile strength (MPa). The obtained yield strength σ is shown in the "Yield Strength σ (MPa)" column of Table 3. The obtained tensile strength is shown in the "Tensile Strength (MPa)" column of Table 3. The obtained yield point drop Δσ is shown in the "Yield Point Drop Δσ (MPa)" column of Table 3. The obtained yield point elongation Δε is shown in the "Yield Point Elongation Δε (%)" column of Table 3. It should be noted that if the yield point drop Δσ and yield point elongation Δε cannot be defined, "-" is shown in Table 3. Furthermore, based on the obtained original γ grain diameter GS (μm), yield strength σ (MPa), yield point elongation Δε (%), and the above definition, Fn1 (=σ×Δε / GS) is obtained. 2 The obtained Fn1 is shown in the "Fn1" column of Table 3.

[0298] [Evaluation Test for Hydrogen Embrittlement Resistance]

[0299] Two circular bar test pieces with annular notches were prepared from the center of the steel plate width and at a position t / 4 of the plate thickness for each specimen symbol. The 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 length of the parallel portion. The notch 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.

[0300] The cathode hydrogen charging method was used to charge one of two circular rod test pieces with ring-shaped cuts with hydrogen. Specifically, a room-temperature cathode hydrogen charging solution was prepared. The cathode hydrogen charging solution was an aqueous solution containing 5% sodium chloride, 30 g / L NH4SCN, and an acetate buffer solution at room temperature. The pH was adjusted to 3.5 before the experiment using the acetate buffer solution.

[0301] With the circular test piece with annular notches immersed in a cathodic hydrogen-charging solution, the potential was set to -1.5V and the hydrogen charging time was set to 24 hours to perform hydrogen charging on the circular test piece with annular notches. That is, an acidic environment was simulated by hydrogen charging. A zinc-plated coating was formed on the surface of the hydrogen-charged circular test piece with annular notches under the same conditions for each sample symbol, ensuring that hydrogen inside the circular test piece with annular notches would not leak to the outside. It should be noted that the other circular test piece with annular notches was not subjected to hydrogen charging.

[0302] For round bar test pieces with a galvanized coating and a ring-shaped notch, a slow strain rate testing machine (SSRT) was used at room temperature and in atmospheric conditions at a speed of 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.

[0303] Furthermore, for the un-hydrogenated circular bar test pieces with annular cuts of each specimen symbol, a slow strain rate testing machine (SSRT) was used at room temperature and in the atmosphere 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.

[0304] The obtained fracturing stress BS0 (MPa) in the atmosphere is shown in the "Atmosphere (MPa)" column of the "Results of Notched Tensile Tests" section in Table 3. The obtained fracturing stress BS1 (MPa) in the hydrogen environment is shown in the "Hydrogen Environment (MPa)" column of the "Results of Notched Tensile Tests" section in Table 3.

[0305] [Evaluation Results]

[0306] Referring to Tables 1-1, 1-2, 2, and 3, for samples A to X, the following conditions are met: the original γ grain diameter GS is greater than 5.0 μm but less than 30.0 μm; the yield strength σ is 862–965 MPa; the yield point drop Δσ is greater than 40 MPa; and the yield elongation Δε is greater than 1.5%. As a result, the fracture stress BS1 in a hydrogen environment is greater than 950 MPa, exhibiting excellent resistance to hydrogen embrittlement. That is, these steels possess high strength and excellent resistance to hydrogen embrittlement even when the original γ grain diameter GS exceeds 5.0 μm. It should be noted that the total area ratio of tempered martensite and tempered bainite in the microstructure of samples A to X is greater than 90%.

[0307] The sample symbols A to U further specify that Fn1 is 12 or higher. As a result, the fracture stress BS1 in the hydrogen environment becomes 970 MPa or higher, exhibiting superior resistance to hydrogen embrittlement.

[0308] On the other hand, the carbon content of sample ZA was too low. As a result, the fracture stress BS1 of this steel in a hydrogen environment became less than 950 MPa, and it did not have excellent resistance to hydrogen embrittlement.

[0309] The carbon content in sample ZB was too high. As a result, the yield strength σ of this steel exceeded 965 MPa. Consequently, the fracture stress BS1 of this steel in a hydrogen environment became less than 950 MPa, and it did not exhibit excellent resistance to hydrogen embrittlement.

[0310] The Si content of the sample with the symbol ZC was too low. As a result, the fracture stress BS1 of this steel in a hydrogen environment became less than 950 MPa, and it did not have excellent resistance to hydrogen embrittlement.

[0311] The Si content of the sample with the symbol ZD was too high. As a result, the fracture stress BS1 of this steel in a hydrogen environment became lower than 950 MPa, and it did not have excellent resistance to hydrogen embrittlement.

[0312] The sample with the designation ZE had an excessively high Mn content. As a result, the fracture stress BS1 of this steel in a hydrogen environment became lower than 950 MPa, and it did not exhibit excellent resistance to hydrogen embrittlement.

[0313] The Cr content in the sample symbol ZF was too low. As a result, the fracture stress BS1 of this steel in a hydrogen environment became less than 950 MPa, and it did not have excellent resistance to hydrogen embrittlement.

[0314] The Cr content in the sample with the symbol ZG was too high. As a result, the fracture stress BS1 of this steel in a hydrogen environment became lower than 950 MPa, and it did not have excellent resistance to hydrogen embrittlement.

[0315] The Mo content in the sample with the symbol ZH was too low. As a result, the fracture stress BS1 of this steel in a hydrogen environment became less than 950 MPa, and it did not have excellent resistance to hydrogen embrittlement.

[0316] The sample with the symbol ZI had an excessively high Mo content. As a result, the fracture stress BS1 of this steel in a hydrogen environment became less than 950 MPa, and it did not exhibit excellent resistance to hydrogen embrittlement.

[0317] The V content of the sample symbol ZJ is too low. As a result, the yield strength σ of the steel becomes less than 862 MPa. Consequently, the fracture stress BS1 of the steel in a hydrogen environment becomes less than 950 MPa, and it does not have excellent resistance to hydrogen embrittlement.

[0318] The V content in sample ZK was too high. As a result, the yield strength σ of the steel exceeded 965 MPa. Consequently, the fracture stress BS1 of the steel in a hydrogen environment became less than 950 MPa, and it did not exhibit excellent resistance to hydrogen embrittlement.

[0319] The Nb content of the sample symbol ZL is too low. As a result, the yield point drop Δσ cannot be defined for this steel. Consequently, the fracture stress BS1 of this steel in a hydrogen environment becomes less than 950 MPa, and it does not exhibit excellent resistance to hydrogen embrittlement.

[0320] The Nb content of the sample with the designation ZM was too high. As a result, the fracture stress BS1 of this steel in a hydrogen environment became lower than 950 MPa, and it did not have excellent resistance to hydrogen embrittlement.

[0321] The sample with the ZN symbol had an excessively high Ti content. As a result, the fracture stress BS1 of the steel in a hydrogen environment became less than 950 MPa, and it did not exhibit excellent resistance to hydrogen embrittlement.

[0322] The boron content in the sample symbol ZO was too low. As a result, the fracture stress BS1 of this steel in a hydrogen environment became less than 950 MPa, and it did not have excellent resistance to hydrogen embrittlement.

[0323] The B content in sample ZP was too high. As a result, the fracture stress BS1 of this steel in a hydrogen environment became lower than 950 MPa, and it did not have excellent resistance to hydrogen embrittlement.

[0324] The nitrogen content in sample ZQ was too high. As a result, the fracture stress BS1 of this steel in a hydrogen environment became less than 950 MPa, and it did not have excellent resistance to hydrogen embrittlement.

[0325] The specimen, designated ZR, did not undergo a low-temperature heat treatment process. As a result, the yield point drop Δσ and yield elongation Δε of this steel cannot be defined. Consequently, the fracture stress BS1 of this steel in a hydrogen environment becomes below 950 MPa, and it does not exhibit excellent resistance to hydrogen embrittlement.

[0326] The heat treatment temperature of the low-temperature heat treatment process for sample ZS was too low. As a result, the yield point drop Δσ and yield elongation Δε of this steel could not be defined. Consequently, the fracture stress BS1 of this steel in a hydrogen environment became below 950 MPa, and it did not exhibit excellent resistance to hydrogen embrittlement.

[0327] The heat treatment temperature of the low-temperature heat treatment process for sample ZT was too high. As a result, the yield point decrease Δσ could not be defined for this steel. Consequently, the yield elongation Δε of this steel became less than 1.5%. Consequently, the fracture stress BS1 of this steel in a hydrogen environment became less than 950 MPa, and it did not exhibit excellent resistance to hydrogen embrittlement.

[0328] The heat treatment time for the low-temperature heat treatment process of sample ZU was too short. As a result, the yield point of the steel decreased by Δσ to less than 40 MPa. Consequently, the fracture stress BS1 of the steel in a hydrogen environment became less than 950 MPa, and it did not exhibit excellent resistance to hydrogen embrittlement.

[0329] The embodiments of this disclosure have been described above. However, the above embodiments are merely examples for implementing this disclosure. Therefore, this disclosure is not limited to the above embodiments, and can be implemented by appropriately modifying the above embodiments without departing from its spirit.

Claims

1. A type of steel, the chemical composition of which, by mass%, is C: 0.20%–0.45%. Si: 0.05~1.50% Mn: 0.01~1.00% P: below 0.030% S: below 0.0100% Cr：0.40～1.10％、 Mo: 0.40–1.30% V：0.01～0.30％、 Nb: 0.005~0.100% Ti: 0.001~0.030% Al：0.005～0.100％、 B：0.0005～0.0050％、 N: below 0.0100% O: below 0.0050% W：0～2.00％、 Co: 0-0.20% Mg: 0~0.0100% Ca: 0~0.0100% Rare earth elements: 0~0.0100% Cu: 0–0.40% Ni: 0-0.20% Sn: 0–0.10%, and Balance: Fe and impurities, The original austenite grain diameter GS of the steel is greater than 5.0 μm and less than 30.0 μm, and the yield strength σ is 862~965MPa. The yield point decreases by Δσ by more than 40 MPa. The yield point elongation Δε is above 1.5%.

2. The steel according to claim 1, wherein, The chemical composition contains selected... W：0.01～2.00％、 Co: 0.01~0.20% Mg: 0.0001~0.0100% Ca: 0.0001~0.0100% Rare earth elements: 0.0001~0.0100% Cu: 0.01–0.40% Ni: 0.01–0.20%, and Sn: One or more elements in a group consisting of 0.01% to 0.10%.

3. The steel according to claim 1, wherein, The grain diameter GS of the original austenite grain, the yield strength σ, and the elongation at the yield point Δε satisfy the following equation (1). σ×Δε / GS 2 ≥12 (1) Here, the yield strength of the steel is substituted into σ in Equation (1) in MPa, the elongation at the yield point of the steel is substituted into Δε in Equation (1) in %, and the grain diameter of the original austenite grains of the steel is substituted into GS in Equation (1) in μm.

4. The steel according to claim 2, wherein, The grain diameter GS of the original austenite grain, the yield strength σ, and the elongation at the yield point Δε satisfy the following equation (1). σ×Δε / GS 2 ≥12 (1) Here, the yield strength of the steel is substituted into σ in Equation (1) in MPa, the elongation at the yield point of the steel is substituted into Δε in Equation (1) in %, and the grain diameter of the original austenite grains of the steel is substituted into GS in Equation (1) in μm.

5. The steel according to any one of claims 1 to 4, wherein, The steel is any one of oil well steel pipes, pipeline steel pipes, and high-pressure hydrogen container steel pipes.

6. The steel according to any one of claims 1 to 4, wherein, The steel is any one of seamless steel pipes for oil wells, seamless steel pipes for pipelines, and seamless steel pipes for high-pressure hydrogen containers.

7. The steel according to claim 5, wherein, The steel pipe for the high-pressure hydrogen container is either the steel pipe for the high-pressure hydrogen accumulator or the steel pipe for the high-pressure hydrogen cylinder.

8. The steel according to claim 6, wherein, The seamless steel pipe for high-pressure hydrogen containers is either a seamless steel pipe for high-pressure hydrogen accumulators or a seamless steel pipe for high-pressure hydrogen cylinders.

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