Steel material

Abstract

Provided is a steel material which has high strength and excellent hydrogen embrittlement resistance. A steel material according to the present disclosure has the chemical composition set forth in the description, and has a tensile strength of 900-1,100 MPa. When the total content of Mo, V, Fe, Mn, Cr, Nb, and Ti in a precipitate in the steel material is taken as 100 mol% and a precipitate in which the total of the Mo content and the V content in the precipitate is 60 mol% or more is defined as a specific precipitate, the Mo content [Mo] (mol%) in the specific precipitate and the V content [V] (mol%) in the specific precipitate satisfy formula (1).　(1): 2.0 ≤ [Mo] / [V] ≤ 3.3 The Mo content in the specific precipitate is substituted in the unit of mol% into [Mo] in the formula (1), and the V content in the specific precipitate is substituted in the unit of mol% into [V] in the formula (1).

Description

steel material

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

[0002] Some oil wells and gas wells (hereinafter collectively referred to as "oil wells") contain environments with a high concentration of corrosive substances. These corrosive substances include, for example, corrosive gases such as hydrogen sulfide. In this specification, an environment containing hydrogen sulfide is referred to as a "sour environment." The temperature of a sour environment varies depending on the depth of the well, but is typically between room temperature and approximately 200°C.

[0003] Examples of steel materials used in such sour environments include steel materials for oil wells, used for oil well tubular construction, and steel materials for line pipes. In recent years, with the increasing depth of oil wells, there has been a demand for higher strength in steel materials for oil wells and other applications.

[0004] On the other hand, when steel is used in a sour environment, the steel surface comes into contact with corrosive substances, causing an electrochemical reaction that generates hydrogen on the steel surface. This hydrogen makes the steel susceptible to hydrogen embrittlement cracking, typified by sulfide stress cracking (SSC). Therefore, steel used in a sour environment requires not only high strength but also excellent resistance to hydrogen embrittlement.

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

[0006] Patent Document 1 describes how, in an oil well steel pipe made of low-alloy steel, a predetermined amount of solid-solution Mo is secured, prior austenite grains are refined, and M2C type precipitates are dispersed. This improves resistance to SSC (Steel-Steel-Crossing) corrosion. Patent Document 1 further improves resistance to hydrogen embrittlement by forming Mo segregation regions at prior austenite grain boundaries.

[0007] Patent Document 2 describes how hydrogen embrittlement resistance is enhanced in oil well steel pipes made of low-alloy steel by forming Mo segregation regions.

[0008] More recently, the development of fuel cell vehicles that run on hydrogen as fuel, and the practical application of hydrogen stations that supply hydrogen to these vehicles, are progressing. High-pressure hydrogen gas is stored in high-pressure hydrogen accumulators installed at hydrogen stations. In addition, the development of vehicles equipped with high-pressure hydrogen cylinders as fuel cell vehicles is also progressing. The steel materials used for high-pressure hydrogen containers such as these high-pressure hydrogen accumulators and high-pressure hydrogen cylinders require not only high strength but also excellent resistance to hydrogen embrittlement.

[0009] A technology to improve hydrogen embrittlement resistance in steel materials used in high-pressure hydrogen containers is proposed in Japanese Patent Publication No. 2009-74122 (Patent Document 3). In Patent Document 3, in a steel material made of low-alloy steel, the morphology of carbides at prior austenite grain boundaries is improved by increasing the V content and Mo content compared to conventional materials, thereby improving hydrogen embrittlement resistance.

[0010] Japanese Patent Publication No. 2011-246798, Japanese Patent Publication No. 2015-38247, Japanese Patent Publication No. 2009-74122

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

[0012] The purpose of this disclosure is to provide a steel material having high strength and excellent resistance to hydrogen embrittlement.

[0013] The steel material according to this disclosure has a chemical composition in mass percent of: C: 0.15-0.40%, Si: 0.10-0.50%, Mn: 0.05-0.60%, P: 0.050% or less, S: 0.0100% or less, Cr: 0.50-1.50%, Mo: 0.80-1.20%, Ti: 0.002-0.010%, V: 0.08-0.15%, Nb: 0.010-0.050%, Al: 0.001-0.100%, B: 0.0001-0.0050%, N: 0.0100% or less, O: 0.0100% or less, Mg: 0-0.0050%, Ca: 0-0.0050%. The steel material consists of rare earth elements: 0-0.0050%, Cu: 0-0.50%, Ni: 0-0.50%, Co: 0-0.50%, W: 0-0.50%, Zr: 0-0.0100%, and the remainder being Fe and impurities, with a tensile strength of 900-1100 MPa. In the steel material, when the total content of Mo, V, Fe, Mn, Cr, Nb, and Ti in the precipitate is set to 100 mol%, precipitates satisfying the sum of the Mo content and V content in the precipitate being 60 mol% or more are defined as specific precipitates. The Mo content [Mo] (mol%) in the specific precipitate and the V content [V] (mol%) in the specific precipitate satisfy formula (1). 2.0 ≤ [Mo] / [V] ≤ 3.3 (1) Here, [Mo] in equation (1) is substituted with the Mo content in the specified precipitate in units of mol%, and [V] in equation (1) is substituted with the V content in the specified precipitate in units of mol%.

[0014] The steel material according to this disclosure has high strength and excellent resistance to hydrogen embrittlement.

[0015] Figure 1 shows the relationship between Fn1 (= [Mo] / [V]) and relative elongation, an indicator of hydrogen embrittlement resistance, when the Mn content is greater than 0.20% to 0.60%. Figure 2 shows the relationship between Fn1 (= [Mo] / [V]) and relative elongation, an indicator of hydrogen embrittlement resistance, when the Mn content is between 0.05% and 0.20%.

[0016] The inventors first considered obtaining a steel material with a tensile strength of 900 to 1100 MPa, assuming use in sour environments and in high-pressure hydrogen containers. In other words, the inventors investigated and examined methods to improve hydrogen embrittlement resistance even when the tensile strength of a steel material intended for use in sour environments and in high-pressure hydrogen containers is 900 to 1100 MPa. As a result, the inventors obtained the following findings.

[0017] Next, the inventors focused on the chemical composition and investigated a steel material that achieves both a tensile strength of 900 to 1100 MPa and excellent hydrogen embrittlement resistance. Specifically, the inventors considered the following composition in mass%, C: 0.15 to 0.40%, Si: 0.10 to 0.50%, Mn: 0.05 to 0.60%, P: 0.050% or less, S: 0.0100% or less, Cr: 0.50 to 1.50%, Mo: 0.80 to 1.20%, Ti: 0.002 to 0.010%, V: 0.08 to 0.15%, Nb: 0.010 to 0.050%, Al: 0.001 to 0.100%, B: 0.0001 to 0.0050%, N: 0.01 We believe that a steel material having a chemical composition consisting of 0% or less of O, 0.0100% or less of Mg, 0 to 0.0050%, Ca, 0 to 0.0050%, rare earth elements, 0 to 0.0050%, Cu, 0 to 0.50%, Ni, 0 to 0.50%, Co, 0 to 0.50%, W, 0 to 0.50%, Zr, 0 to 0.0100%, and the remainder being Fe and impurities, could potentially achieve both a tensile strength of 900 to 1100 MPa and excellent resistance to hydrogen embrittlement.

[0018] On the other hand, even with steel materials having the above-mentioned chemical composition, excellent hydrogen embrittlement resistance could not be obtained when the tensile strength was 900 to 1100 MPa. Therefore, the inventors of the present invention investigated in detail a method to improve the hydrogen embrittlement resistance of steel materials having the above-mentioned chemical composition while maintaining the tensile strength. As a result, the following findings were obtained.

[0019] Here, steel materials having the above-mentioned chemical composition and a tensile strength of 900 to 1100 MPa have a large number of precipitates dispersed within the steel material. Therefore, the inventors focused on the precipitates in the steel material and investigated various methods to improve the hydrogen embrittlement resistance of the steel material while maintaining its tensile strength. As a result of the inventors' investigations, among the precipitates in steel materials having the above-mentioned chemical composition, precipitates mainly composed of metal elements V and Mo are particularly easily dispersed into a fine atmosphere. Therefore, precipitates mainly composed of metal elements V and Mo may have a high ability to trap hydrogen that has penetrated into the steel material.

[0020] Therefore, the inventors defined a specific precipitate as one in which, when the total content of Mo, V, Fe, Mn, Cr, Nb, and Ti in the precipitate is 100 mol%, the sum of the Mo content and V content in the precipitate is 60 mol% or more, and investigated in detail the relationship between the specific precipitate and the hydrogen embrittlement resistance of the steel material. As a result, it became clear that the ratio of the Mo content and V content in the specific precipitate affects the hydrogen embrittlement resistance of the steel material.

[0021] Based on the above findings, detailed studies by the present inventors revealed that for steel materials having the above-mentioned chemical composition, if the Mo content [Mo] (mol%) in the specific precipitate and the V content [V] (mol%) in the specific precipitate satisfy formula (1), the hydrogen embrittlement resistance of the steel material can be improved while maintaining tensile strength. 2.0 ≤ [Mo] / [V] ≤ 3.3 (1) Here, [Mo] in formula (1) is substituted with the Mo content in the specific precipitate in units of mol%, and [V] in formula (1) is substituted with the V content in the specific precipitate in units of mol%.

[0022] Fn1 is defined as [Mo] / [V]. Fn1 represents the ratio of Mo content to V content in a specific precipitate (a precipitate mainly composed of V and Mo metal elements). If Fn1 is too low, the V ratio in the specific precipitate becomes too high, and the hydrogen embrittlement resistance of the steel cannot be sufficiently improved. On the other hand, if Fn1 is too high, the Mo ratio in the specific precipitate becomes too high, and the hydrogen embrittlement resistance of the steel cannot be sufficiently improved. This point will be explained in detail using the diagram.

[0023] Figure 1 shows the relationship between Fn1 (= [Mo] / [V]) and relative elongation, an indicator of hydrogen embrittlement resistance, when the Mn content is greater than 0.20% to 0.60%. Figure 1 was created using Fn1 and relative elongation obtained by the method described later, for an example from the examples described later in which the chemical composition excluding Mo and V satisfies the range of this embodiment, the Mn content is greater than 0.20% to 0.60%, and the tensile strength is 900 to 1100 MPa. Referring to Figure 1, it can be confirmed that for steel materials with a chemical composition excluding Mo and V that satisfies the range of this embodiment and a tensile strength of 900 to 1100 MPa, if Fn1 is 2.0 to 3.3, the relative elongation is 0.80 or higher, indicating excellent hydrogen embrittlement resistance.

[0024] Furthermore, Figure 2 shows the relationship between Fn1 (= [Mo] / [V]) and relative elongation, which is an indicator of hydrogen embrittlement resistance, when the Mn content is 0.05 to 0.20%. Figure 2 was created using Fn1 and relative elongation obtained by the method described later, similar to Figure 1. Referring to Figure 2, it can be confirmed that for steel materials with a chemical composition excluding Mo and V that satisfies the range of this embodiment, a Mn content of 0.05 to 0.20%, and a tensile strength of 900 to 1100 MPa, if Fn1 is 2.0 to 3.3, the relative elongation will be 0.80 or higher, and even 0.90 or higher, indicating excellent hydrogen embrittlement resistance.

[0025] In other words, referring to Figures 1 and 2, it can be confirmed that for steel materials with a chemical composition excluding Mo and V that satisfies the range of this embodiment and a tensile strength of 900 to 1100 MPa, if Fn1 is 2.0 to 3.3, the relative elongation will be 0.80 or more, indicating excellent hydrogen embrittlement resistance. The steel material according to this embodiment has the above-mentioned chemical composition, a tensile strength of 900 to 1100 MPa, and furthermore, Fn1 is set to 2.0 to 3.3. As a result, the steel material according to this embodiment can achieve both a tensile strength of 900 to 1100 MPa and excellent hydrogen embrittlement resistance.

[0026] Furthermore, the detailed reason why setting Fn1 to 2.0 to 3.3 improves the hydrogen embrittlement resistance of the steel while maintaining tensile strength remains unclear. However, the inventors speculate as follows: In the steel having the above chemical composition, the specific precipitates are almost entirely carbides. Also, V and Mo have different atomic radii, with Mo having a larger atomic radius. Therefore, it is presumed that carbides composed of V and Mo contain metal elements with different atomic radii, causing strain in the crystal lattice. In other words, if the ratio of V to Mo in the carbides changes, the magnitude of the strain generated in the crystal lattice may also change.

[0027] Here, the strain generated in the crystal lattice of the carbide can potentially act as a hydrogen trapping site. Therefore, by adjusting the ratio of V to Mo in the carbide, the strain generated in the crystal lattice can be increased, potentially improving the hydrogen trapping ability of the carbide. As a result, the inventors speculate that the hydrogen embrittlement resistance of the steel material may be improved.

[0028] It is possible that the hydrogen embrittlement resistance of the steel material is enhanced while maintaining its tensile strength for reasons different from those speculated by the inventors above. However, it has been proven by the examples described later that steel materials having the above-mentioned chemical composition and satisfying Fn1 of 2.0 to 3.3 can achieve both a tensile strength of 900 to 1100 MPa and excellent hydrogen embrittlement resistance.

[0029] Based on the above findings, the gist of the steel material according to this embodiment is as follows:

[0030] [1] Steel material having a chemical composition in mass percent of: C: 0.15-0.40%, Si: 0.10-0.50%, Mn: 0.05-0.60%, P: 0.050% or less, S: 0.0100% or less, Cr: 0.50-1.50%, Mo: 0.80-1.20%, Ti: 0.002-0.010%, V: 0.08-0.15%, Nb: 0.010-0.050%, Al: 0.001-0.100%, B: 0.0001-0.0050%, N: 0.0100% or less, O: 0.0100% or less, Mg: 0-0.0050% A steel material comprising Ca: 0-0.0050%, rare earth elements: 0-0.0050%, Cu: 0-0.50%, Ni: 0-0.50%, Co: 0-0.50%, W: 0-0.50%, Zr: 0-0.0100%, and the remainder being Fe and impurities, having a tensile strength of 900-1100 MPa, wherein, in the steel material, when the total content of Mo, V, Fe, Mn, Cr, Nb, and Ti in the precipitates is set to 100 mol%, precipitates satisfying the sum of the Mo content and V content in the precipitates being 60 mol% or more are defined as specific precipitates, and the Mo content [Mo] (mol%) in the specific precipitates and the V content [V] (mol%) in the specific precipitates satisfy formula (1). 2.0 ≤ [Mo] / [V] ≤ 3.3 (1) Here, [Mo] in equation (1) is substituted with the Mo content in the specified precipitate in units of mol%, and [V] in equation (1) is substituted with the V content in the specified precipitate in units of mol%.

[0031] [2] A steel material as described in [1], wherein the chemical composition contains one or more elements selected from the group consisting of: Mg: 0.0001 to 0.0050%, Ca: 0.0001 to 0.0050%, rare earth elements: 0.0001 to 0.0050%, Cu: 0.01 to 0.50%, Ni: 0.01 to 0.50%, Co: 0.01 to 0.50%, W: 0.01 to 0.50%, and Zr: 0.0001 to 0.0100%.

[0032] [3] A steel material as described in [1] or [2], wherein the steel material is any of the following: a steel pipe for oil wells, a steel pipe for line pipes, and a steel pipe for high-pressure hydrogen containers.

[0033] [4] A steel material as described in [3], wherein the steel pipe for high-pressure hydrogen container is either a steel pipe for high-pressure hydrogen accumulator or a steel pipe for high-pressure hydrogen cylinder.

[0034] The shape of the steel material in this embodiment is not particularly limited. The steel material in this embodiment may be a steel pipe, a round steel bar (solid material), or a steel plate. A round steel bar refers to a steel bar with a circular cross-section perpendicular to the axial direction. A steel pipe may be a seamless steel pipe or a welded steel pipe.

[0035] In this specification, "oil well steel pipe" means a steel pipe used as an oil well pipe. An oil well pipe is a general term for casings, tubing, and drill pipes used in drilling oil or gas wells, extracting crude oil or natural gas, etc. "Seamless oil well steel pipe" means that the oil well steel pipe is a seamless steel pipe.

[0036] In this specification, "steel pipe for line pipes" means a steel pipe used for line pipe applications that constitute a pipeline for transporting production fluids (crude oil or natural gas) extracted from an oil well or gas well. Pipelines include, for example, flow lines that transport production fluids from oil wells or gas wells, gathering lines that collect the production fluids transported by the flow lines and transport them to a primary processing facility, trunk lines that transport the production fluids that have undergone primary processing such as dewatering to the vicinity of the market, and distribution lines that transport them to consumers. "Seamless steel pipe for line pipes" means that the steel pipe for line pipes is a seamless steel pipe.

[0037] In this specification, the "steel pipe for high-pressure hydrogen containers" is standardized by ISO 11439, ANSI / NGV, the High-Pressure Gas Safety Act, the Container Safety Regulations exemplary standards, etc., and means a steel pipe used for high-pressure hydrogen containers in which high-pressure hydrogen gas is stored. High-pressure hydrogen containers are, for example, high-pressure hydrogen accumulators installed at hydrogen stations and high-pressure hydrogen cylinders mounted on fuel cell vehicles. The "seamless steel pipe for high-pressure hydrogen containers" means that the steel pipe for high-pressure hydrogen containers is a seamless steel pipe.

[0038] Hereinafter, the steel material according to this embodiment will be described in detail. "%" regarding elements means mass% unless otherwise specified.

[0039] [Chemical Composition] The chemical composition of the steel material according to this embodiment contains the following elements.

[0040] C: 0.15 to 0.40% Carbon (C) enhances hardenability and makes the microstructure of the steel material mainly tempered martensite and tempered bainite. As a result, the hydrogen embrittlement resistance characteristics of the steel material are enhanced. C further forms carbides or carbonitrides and increases the strength of the steel material. If the C content is too low, even if the contents of other elements are within the range of this embodiment, the above effects cannot be sufficiently obtained. On the other hand, if the C content is too high, even if the contents of other elements are within the range of this embodiment, the carbides in the steel material become excessively numerous, and the hydrogen embrittlement resistance characteristics of the steel material deteriorate. Therefore, the C content is 0.15 to 0.40%. The preferable lower limit of the C content is 0.16%, more preferably 0.18%. The preferable upper limit of the C content is 0.39%, more preferably 0.37%, and even more preferably 0.36%.

[0041] Si: 0.10 to 0.50% Silicon (Si) deoxidizes steel and reduces inclusions in the steel material. As a result, the hydrogen embrittlement resistance characteristics of the steel material are enhanced. If the Si content is too low, even if the contents of other elements are within the scope of this embodiment, the above effects cannot be sufficiently obtained. On the other hand, if the Si content is too high, even if the contents of other elements are within the scope of this embodiment, the hydrogen embrittlement resistance characteristics of the steel material deteriorate. Therefore, the Si content is 0.10 to 0.50%. The preferable lower limit of the Si content is 0.11%, more preferably 0.13%, still more preferably 0.14%, and still more preferably 0.15%. The preferable upper limit of the Si content is 0.49%, more preferably 0.48%, and still more preferably 0.47%.

[0042] Mn: 0.05 to 0.60% Manganese (Mn) deoxidizes steel. Mn further enhances hardenability and the hydrogen embrittlement resistance characteristics of the steel material. If the Mn content is too low, even if the contents of other elements are within the scope of this embodiment, the above effects cannot be sufficiently obtained. On the other hand, if the Mn content is too high, even if the contents of other elements are within the scope of this embodiment, a large number of Mn sulfides are formed and the hydrogen embrittlement resistance characteristics of the steel material deteriorate. Therefore, the Mn content is 0.05 to 0.60%. The preferable lower limit of the Mn content is 0.06%, more preferably 0.08%. The preferable upper limit of the Mn content is 0.58%, more preferably 0.55%.

[0043] Preferably, the Mn content is 0.05 to 0.20%. In this case, the steel material has more excellent hydrogen embrittlement resistance characteristics. That is, in the chemical composition of the steel material according to this embodiment, the Mn content may be 0.05 to 0.20% or may be more than 0.20 to 0.60%. When the Mn content is 0.05 to 0.20%, the preferable upper limit of the Mn content is 0.18%, more preferably 0.16%. When the Mn content is more than 0.20 to 0.60%, the preferable lower limit of the Mn content is 0.22%, more preferably 0.25%.

[0044] P: 0.050% or less. Phosphorus (P) is an unavoidable impurity. In other words, the lower limit of the P content is greater than 0%. If the P content is too high, even if the content of other elements is within the range of this embodiment, P will segregate at the grain boundaries, reducing the hydrogen embrittlement resistance of the steel. Therefore, the P content is 0.050% or less. It is preferable to have as low a P content as possible. However, an extreme reduction in the P content significantly increases manufacturing costs. Therefore, considering industrial production, the preferred lower limit of the P content is 0.001%, more preferably 0.002%, even more preferably 0.003%, even more preferably 0.004%, and even more preferably 0.005%. The preferred upper limit for the P content is 0.048%, more preferably 0.047%, even more preferably 0.045%, even more preferably 0.044%, even more preferably 0.040%, even more preferably 0.030%, even more preferably 0.025%, and even more preferably 0.020%.

[0045] S: 0.0100% or less. Sulfur (S) is an unavoidable impurity. In other words, the lower limit of the S content is greater than 0%. If the S content is too high, even if the content of other elements is within the range of this embodiment, S will segregate at the grain boundaries, reducing the hydrogen embrittlement resistance of the steel. Therefore, the S content is 0.0100% or less. It is preferable to have as low an S content as possible. However, an extreme reduction in the S content significantly increases manufacturing costs. Therefore, considering industrial production, the preferred lower limit of the S content is 0.0001%, more preferably 0.0003%, even more preferably 0.0005%, and even more preferably 0.0008%. The preferred upper limit of the S content is 0.0090%, more preferably 0.0086%, even more preferably 0.0080%, even more preferably 0.0060%, and even more preferably 0.0050%.

[0046] Cr: 0.50-1.50% Chromium (Cr) enhances the hardenability of steel and improves its resistance to hydrogen embrittlement. Furthermore, Cr increases the tempering softening resistance of steel, enabling high-temperature tempering. As a result, the resistance to hydrogen embrittlement of steel is improved. If the Cr content is too low, the above effects cannot be fully obtained even if the content of other elements is within the range of this embodiment. On the other hand, if the Cr content is too high, even if the content of other elements is within the range of this embodiment, coarse carbides will be formed, and the resistance to hydrogen embrittlement of steel will decrease. Therefore, the Cr content is 0.50-1.50%. The preferred lower limit of the Cr content is 0.51%, more preferably 0.53%, and even more preferably 0.55%. The preferred upper limit of the Cr content is 1.48%, more preferably 1.45%, even more preferably 1.42%, and even more preferably 1.40%.

[0047] Mo: 0.80-1.20% Molybdenum (Mo), together with V, forms specific precipitates that enhance the hydrogen embrittlement resistance of the steel. Mo further enhances the hardenability of the steel and improves its hydrogen embrittlement resistance. If the Mo content is too low, the above effects cannot be fully obtained even if the content of other elements is within the range of this embodiment. On the other hand, if the Mo content is too high, even if the content of other elements is within the range of this embodiment, the Mo ratio in the specific precipitates becomes too high, and the hydrogen embrittlement resistance of the steel decreases. Therefore, the Mo content is 0.80-1.20%. The preferred lower limit of the Mo content is 0.82%, more preferably 0.85%. The preferred upper limit of the Mo content is 1.19%, more preferably 1.18%, and even more preferably 1.15%.

[0048] Ti: 0.002 to 0.010% Titanium (Ti) forms fine precipitates such as Ti nitrides, and by the pinning effect, it refines the prior γ grains, thereby improving the hydrogen embrittlement resistance of the steel. If the Ti content is too low, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of this embodiment. On the other hand, if the Ti content is too high, coarse Ti nitrides will be generated even if the content of other elements is within the range of this embodiment. Coarse Ti nitrides become the starting point for cracks. As a result, the hydrogen embrittlement resistance of the steel decreases. Therefore, the Ti content is 0.002 to 0.010%. The preferred lower limit of the Ti content is 0.003%, and more preferably 0.004%. The preferred upper limit of the Ti content is 0.009%, and more preferably 0.008%.

[0049] V: 0.08-0.15% Vanadium (V), together with Mo, forms specific precipitates that enhance the hydrogen embrittlement resistance of the steel. If the V content is too low, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the V content is too high, even if the content of other elements is within the range of this embodiment, the V ratio in the specific precipitates becomes too high, and the hydrogen embrittlement resistance of the steel decreases. Therefore, the V content is 0.08-0.15%. The preferred lower limit of the V content is 0.09%, and more preferably 0.10%. The preferred upper limit of the V content is 0.14%, and more preferably 0.13%.

[0050] Nb: 0.010 to 0.050% Niobium (Nb) forms carbides, nitrides, or carbonitrides (hereinafter referred to as "carbonitrides, etc.") and refines the prior γ grains through a pinning effect, thereby improving the hydrogen embrittlement resistance of the steel. Furthermore, Nb forms fine carbides during tempering, increasing the tempering softening resistance of the steel and improving the strength of the steel. If the Nb content is too low, the above effects cannot be fully obtained even if the content of other elements is within the range of this embodiment. On the other hand, if the Nb content is too high, even if the content of other elements is within the range of this embodiment, an excessive amount of carbonitrides, etc. will be generated, and the hydrogen embrittlement resistance of the steel will actually decrease. Therefore, the Nb content is 0.010 to 0.050%. The preferred lower limit of the Nb content is 0.011%, more preferably 0.013%, and even more preferably 0.015%. The preferred upper limit for the Nb content is 0.048%, more preferably 0.047%, even more preferably 0.046%, and even more preferably 0.045%.

[0051] Al: 0.001 to 0.100% Aluminum (Al) deoxidizes steel. Al further combines with N to form Al nitrides, which refine the crystal grains through a pinning effect, thereby improving the hydrogen embrittlement resistance of the steel. If the Al content is too low, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Al content is too high, even if the content of other elements is within the range of this embodiment, coarse oxides will be formed, and the hydrogen embrittlement resistance of the steel will decrease. Therefore, the Al content is 0.001 to 0.100%. The preferred lower limit of the Al content is 0.003%, more preferably 0.005%, and even more preferably 0.008%. The preferred upper limit of the Al content is 0.098%, more preferably 0.095%, and even more preferably 0.090%. In this specification, "Al" content refers to "acid-soluble Al," that is, the content of "sol. Al."

[0052] B: 0.0001 to 0.0050% Boron (B) enhances the hardenability of steel and improves its resistance to hydrogen embrittlement. If the B content is too low, the above effect cannot be fully obtained even if the content of other elements is within the range of this embodiment. On the other hand, if the B content is too high, even if the content of other elements is within the range of this embodiment, coarse B nitrides may be formed, which can become the starting point for cracks. As a result, the resistance to hydrogen embrittlement of steel decreases. Therefore, the B content is 0.0001 to 0.0050%. The preferred lower limit of the B content is 0.0002%, more preferably 0.0003%, and even more preferably 0.0005%. The preferred upper limit of the B content is 0.0049%, more preferably 0.0048%, even more preferably 0.0047%, and even more preferably 0.0045%.

[0053] N: 0.0100% or less. Nitrogen (N) is inevitably present. In other words, the lower limit of the N content is greater than 0%. N combines with Ti to form nitrides, which refine the prior γ grains through a pinning effect, thereby improving the hydrogen embrittlement resistance of the steel. On the other hand, if the N content is too high, even if the content of other elements is within the range of this embodiment, coarse nitrides will be formed, and the hydrogen embrittlement resistance of the steel will decrease. Therefore, the N content is 0.0100% or less. A preferred lower limit for the N content to more effectively obtain the above effect is 0.0001%, more preferably 0.0003%, and even more preferably 0.0005%. A preferred upper limit for the N content is 0.0096%, more preferably 0.0095%, and even more preferably 0.0092%.

[0054] O: 0.0100% or less. Oxygen (O) is an unavoidable impurity. In other words, the lower limit of the O content is greater than 0%. If the O content is too high, even if the content of other elements is within the range of this embodiment, coarse oxides will be formed, and the hydrogen embrittlement resistance of the steel will decrease. Therefore, the O content is 0.0100% or less. It is preferable that the O content be as low as possible. However, an extreme reduction in the O content will significantly increase manufacturing costs. Therefore, considering industrial production, the preferred lower limit of the O content is 0.0001%, more preferably 0.0003%, and even more preferably 0.0005%. The preferred upper limit of the O content is 0.0098%, more preferably 0.0095%, and even more preferably 0.0090%.

[0055] The remainder of the chemical composition of the steel material according to this embodiment consists of Fe and impurities. Here, impurities in the chemical composition refer to substances that are mixed in from raw materials such as ore, scrap, or the manufacturing environment during the industrial production of steel material, and are not intentionally included, but are acceptable within a range that does not adversely affect the steel material according to this embodiment.

[0056] [Optional Elements] The chemical composition of the steel material according to this embodiment may further contain one or more elements selected from the group consisting of Mg, Ca, and rare earth elements (REM) in place of a portion of Fe. Any of these elements are optional and may not be included. If included, Mg, Ca, and rare earth elements (REM) enhance the hydrogen embrittlement resistance of the steel material.

[0057] Mg: 0 to 0.0050% Magnesium (Mg) is an optional element and may not be included. In other words, the Mg content may be 0%. If included, Mg forms sulfides with S in the steel, improving the hydrogen embrittlement resistance of the steel. Even if only a small amount of Mg is included, the above effect can be obtained to some extent. However, if the Mg content is too high, even if the content of other elements is within the range of this embodiment, the oxides in the steel will coarseen, and the hydrogen embrittlement resistance of the steel will decrease. Therefore, the Mg content is 0 to 0.0050%. The preferred lower limit of the Mg content is 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%. The preferred upper limit of the Mg content is 0.0048%, and more preferably 0.0045%.

[0058] Ca: 0 to 0.0050% Calcium (Ca) is an optional element and may not be present. In other words, the Ca content may be 0%. If present, Ca forms sulfides with S in the steel, improving the hydrogen embrittlement resistance of the steel. Even a small amount of Ca can provide some of the above effect. However, if the Ca content is too high, even if the content of other elements is within the range of this embodiment, the oxides in the steel will coarseen, reducing the hydrogen embrittlement resistance of the steel. Therefore, the Ca content is 0 to 0.0050%. The preferred lower limit of the Ca content is 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%. The preferred upper limit of the Ca content is 0.0048%, and more preferably 0.0045%.

[0059] Rare earth elements (REM): 0 to 0.0050% Rare earth elements (REM) are optional elements and may not be included. In other words, the REM content may be 0%. If included, REM forms sulfides with S in the steel, improving the hydrogen embrittlement resistance of the steel. Even a small amount of REM can provide the above effect to some extent. However, if the REM content is too high, even if the content of other elements is within the range of this embodiment, the oxides in the steel will coarseen, reducing the hydrogen embrittlement resistance of the steel. Therefore, the REM content is 0 to 0.0050%. The preferred lower limit of the REM content is 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%. The preferred upper limit of the REM content is 0.0048%, and more preferably 0.0045%.

[0060] In this specification, REM means one or more elements selected from the group consisting of scandium (Sc), atomic number 21; yttrium (Y), atomic number 39; and lanthanides, lanthanum (La), atomic number 57 to lutetium (Lu), atomic number 71. In this specification, REM content means the total content of these elements.

[0061] The chemical composition of the steel material according to this embodiment may further contain one or more elements selected from the group consisting of Cu, Ni, and Co in place of a portion of Fe. Any of these elements are optional and may not be included. If included, Cu, Ni, and Co all enhance the hardenability of the steel material and improve its resistance to hydrogen embrittlement.

[0062] Cu: 0-0.50% Copper (Cu) is an optional element and may not be included. In other words, the Cu content may be 0%. If included, Cu improves the hardenability of the steel and enhances its resistance to hydrogen embrittlement. Even a small amount of Cu will provide some of the above effects. However, if the Cu content is too high, the hot workability of the steel will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the Cu content is 0-0.50%. The preferred lower limit of the Cu content is 0.01%, more preferably 0.05%. The preferred upper limit of the Cu content is 0.48%, more preferably 0.45%, even more preferably 0.40%, and even more preferably 0.36%.

[0063] Ni: 0-0.50% Nickel (Ni) is an optional element and may not be included. In other words, the Ni content may be 0%. If included, Ni improves the hardenability of the steel and enhances its resistance to hydrogen embrittlement. Even a small amount of Ni will provide some of the above effects. However, if the Ni content is too high, the manufacturing cost will increase drastically, even if the content of other elements is within the range of this embodiment. Therefore, the Ni content is 0-0.50%. The preferred lower limit of the Ni content is 0.01%, more preferably 0.05%. The preferred upper limit of the Ni content is 0.48%, more preferably 0.45%, even more preferably 0.40%, and even more preferably 0.30%.

[0064] Co: 0-0.50% Cobalt (Co) is an optional element and may not be included. In other words, the Co content may be 0%. If included, Co enhances the hardenability of the steel and improves its resistance to hydrogen embrittlement. Even a small amount of Co will provide some of the above effects. However, if the Co content is too high, the effect will saturate. Therefore, the Co content is 0-0.50%. The preferred lower limit of the Co content is 0.01%, more preferably 0.03%. The preferred upper limit of the Co content is 0.48%, more preferably 0.45%, even more preferably 0.40%, and even more preferably 0.35%.

[0065] The chemical composition of the steel material according to this embodiment may further contain one or more elements selected from the group consisting of W and Zr in place of a portion of Fe. These elements are all optional and may not be included. If included, W and Zr form fine carbides, thereby increasing the strength of the steel material.

[0066] W: 0-0.50% Tungsten (W) is an optional element and may not be included. That is, the W content may be 0%. If included, W forms fine carbides, increasing the strength of the steel. Even if only a small amount of W is included, the above effect can be obtained to some extent. However, if the W content is too high, even if the content of other elements is within the range of this embodiment, coarse carbides will be formed, and the SSC resistance of the steel will decrease. Therefore, the W content is 0-0.50%. The preferred lower limit of the W content is 0.01%, more preferably 0.02%, and even more preferably 0.05%. The preferred upper limit of the W content is 0.45%, more preferably 0.40%, even more preferably 0.30%, and even more preferably 0.25%.

[0067] Zr: 0 to 0.0100% Zirconium (Zr) is an optional element and may not be included. That is, the Zr content may be 0%. If included, Zr forms fine carbides, increasing the strength of the steel. Even if only a small amount of Zr is included, the above effect can be obtained to some extent. However, if the Zr content is too high, even if the content of other elements is within the range of this embodiment, coarse oxides will be formed, and the SSC resistance of the steel will decrease. Therefore, the Zr content is 0 to 0.0100%. The preferred lower limit of the Zr content is 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%. The preferred upper limit of the Zr content is 0.0095%, more preferably 0.0090%, and even more preferably 0.0085%.

[0068] [Tensile Strength] In the steel material according to this embodiment, the tensile strength is 900 to 1100 MPa. In this specification, tensile strength means the maximum stress in uniform elongation obtained by a tensile test performed in accordance with JIS Z2241:2011. Provided that the other components of this embodiment are met, the steel material according to this embodiment has excellent hydrogen embrittlement resistance even if the tensile strength is 900 to 1100 MPa.

[0069] In this embodiment, the preferred lower limit of the tensile strength is 910 MPa, more preferably 924 MPa, and even more preferably 931 MPa. In this embodiment, the preferred upper limit of the tensile strength is 1096 MPa, more preferably 1089 MPa, and even more preferably 1082 MPa. In this embodiment, the yield strength of the steel is not particularly limited. The yield strength of the steel may be, for example, 825 to 1050 MPa.

[0070] The tensile strength and yield strength of the steel material according to this embodiment can be determined by the following method. Specifically, a tensile test specimen is prepared from the steel material according to this embodiment in accordance with the High Pressure Gas Safety Association standard KHKS 0220 (2020) "Standards for Ultra-High Pressure Gas Equipment". More specifically, the tensile test specimen is a round bar tensile test specimen, and it is prepared so that the axis of the round bar tensile test specimen coincides with a specific position on the steel material. For example, the diameter of the parallel section of the round bar tensile test specimen is 8.0 mm and the length of the parallel section is 56 mm.

[0071] More specifically, if the steel material is a steel plate, the position at plate thickness t / 4 is designated as the specific position. Here, the position at plate thickness t / 4 means the position at a depth of t / 4 from the surface of the steel plate, where t is the thickness of the steel plate. Also, the axial direction of the round bar tensile test specimen is parallel to the width direction of the steel plate. If the steel material is a steel pipe, the position at wall thickness t / 4 on the outer surface side of the steel pipe is designated as the specific position. Here, the position at wall thickness t / 4 on the outer surface side of the steel pipe means the position at a depth of t / 4 from the outer surface of the steel pipe, where t is the thickness of the steel pipe. Also, the axial direction of the round bar tensile test specimen is perpendicular to the pipe axis direction and pipe diameter direction of the steel pipe. If a round bar tensile test specimen cannot be made from a steel pipe, an arc-shaped test specimen is made. The size of the arc-shaped test specimen is, for example, the thickness is the total wall thickness, the width is 25.4 mm, and the gauge length is 50.8 mm. If the steel material is a round steel bar, the R / 2 position is designated as the specific position. Here, the R / 2 position of the round steel bar refers to the midpoint of radius R in a cross-section perpendicular to the axial direction of the round steel bar. Furthermore, the axial direction of the round bar tensile test specimen is perpendicular to the axial and radial directions of the round steel bar.

[0072] Tensile tests are performed on the prepared tensile specimens in accordance with JIS Z2241:2011 at room temperature (25°C) in air. The maximum stress (MPa) in uniform elongation obtained from the tensile test is defined as the tensile strength. The 0.2% offset proof stress (MPa) obtained from the tensile test is defined as the yield strength. Both the tensile strength (MPa) and the yield strength (MPa) are obtained by rounding the obtained values ​​to the first decimal place.

[0073] [Specific Precipitates] In the steel material according to this embodiment, when the total content of Mo, V, Fe, Mn, Cr, Nb, and Ti in the precipitates is 100 mol%, precipitates that satisfy the condition that the sum of the Mo content and V content in the precipitates is 60 mol% or more are defined as "specific precipitates." As described above, specific precipitates are almost entirely carbides. Furthermore, both Mo and V are contained in specific precipitates. Therefore, specific precipitates essentially mean composite carbides of V and Mo.

[0074] Here, the composite carbide of V and Mo may contain metal elements other than V and Mo (such as Fe and Cr). Furthermore, the Mo content [Mo] (mol%) and the V content [V] (mol%) in the specific precipitate are not particularly limited, as long as their sum is 60 mol% or more. However, since both Mo and V are contained in the specific precipitate, the Mo content [Mo] in the specific precipitate is 1 to 99 mol%, and the V content [V] in the specific precipitate is 1 to 99 mol%.

[0075] The steel material according to this embodiment further satisfies equation (1) in terms of the Mo content [Mo] (mol%) in the specific precipitate and the V content [V] (mol%) in the specific precipitate. 2.0 ≤ [Mo] / [V] ≤ 3.3 (1) Here, in equation (1), [Mo] is substituted with the Mo content in the specific precipitate in units of mol%, and [V] in equation (1) is substituted with the V content in the specific precipitate in units of mol%.

[0076] As described above, Fn1 (= [Mo] / [V]) represents the ratio of Mo content to V content in a specific precipitate. If Fn1 is too low, the V ratio in the specific precipitate becomes too high, and the hydrogen embrittlement resistance of the steel cannot be sufficiently improved. On the other hand, if Fn1 is too high, the Mo ratio in the specific precipitate becomes too high, and the hydrogen embrittlement resistance of the steel cannot be sufficiently improved. Therefore, assuming that the steel according to this embodiment has the above-described chemical composition, Fn1 is set to 2.0 to 3.3.

[0077] The preferred lower limit of Fn1 is 2.1, more preferably 2.2, and still more preferably 2.3. The preferred upper limit of Fn1 is 3.2, more preferably 3.1, and still more preferably 3.0.

[0078] In this embodiment, the Mo content [Mo] (mol%), the V content [V] (mol%), and Fn1 in the specific precipitate can be determined by the following method. First, a micro-test piece for making an extraction replica is prepared from the steel material according to this embodiment. Specifically, if the steel material is a steel plate, a micro-test piece is prepared from the position t / 4 of the plate thickness. If the steel material is a steel pipe, a micro-test piece is prepared from the position t / 4 of the wall thickness on the inner surface side of the steel pipe. Here, the position t / 4 of the wall thickness on the inner surface side of the steel pipe means a depth of t / 4 from the inner surface of the steel pipe, where t is the wall thickness of the steel pipe. If the steel material is a round steel bar, a micro-test piece is prepared from the R / 2 position.

[0079] The size of the micro-test specimen is, for example, 10 mm x 10 mm. After mirror polishing the surface of the micro-test specimen, it is immersed in an electrolyte solution (10% acetylacetone - 1% tetramethylammonium chloride - methanol solution) and constant potential electrolytic etching is performed. In constant potential electrolytic etching, the potential is set to -100 mV (vs SCE) and the charge is 5 C / cm². 2 It will be implemented as follows.

[0080] A surface subjected to constant potential electrolytic etching is covered with a carbon vapor-deposited film. A micro specimen covered with the vapor-deposited film is immersed in a 5% Nital etching solution for 20 minutes. The vapor-deposited film is peeled off the immersed micro specimen. The vapor-deposited film peeled off the micro specimen is washed with ethanol, then scooped up with a sheet mesh and dried. In this embodiment, a Cu sheet mesh is used.

[0081] This vapor-deposited film (replica film) is observed using a transmission electron microscope (TEM). For the observation of specific precipitates, an arbitrary position is selected from the vapor-deposited film, and the observation is performed at a magnification of 100,000x and an acceleration voltage of 200kV. Preferably, the total area of ​​the observation field is 8 μm. 2 The above is the conclusion. Specifically, for example, the size of the observation field will be 0.4 μm × 0.4 μm, and the number of observation fields will be 50.

[0082] In each observation field, precipitates are identified. These precipitates can be identified by their contrast. Point analysis is performed on the identified precipitates using energy-dispersive X-ray spectroscopy (EDS). The elemental content in each precipitate is determined by the EDS point analysis. In the EDS point analysis, the acceleration voltage is set to 200 kV, and the target elements are quantified as Mo, V, Fe, Mn, Cr, Nb, and Ti. Based on the EDS analysis results for each precipitate, when the total content of Mo, V, Fe, Mn, Cr, Nb, and Ti is set to 100 mol%, precipitates where the sum of Mo content and V content is 60 mol% or more are identified as specific precipitates.

[0083] For each specific precipitate identified in 50 fields of view, the Mo content (mol%) is determined. The arithmetic mean of the Mo content (mol%) in all obtained specific precipitates is defined as the Mo content [Mo] (mol%) in the specific precipitate. Similarly, for each specific precipitate identified in 50 fields of view, the V content (mol%) is determined. The arithmetic mean of the V content (mol%) in all obtained specific precipitates is defined as the V content [V] (mol%) in the specific precipitate. Furthermore, Fn1 (= [Mo] / [V]) is calculated from the Mo content [Mo] (mol%) and the V content [V] (mol%) in the specific precipitate. Note that both the Mo content [Mo] (mol%) and the V content [V] (mol%) in the specific precipitate are obtained by rounding the obtained values ​​to the first decimal place. Furthermore, Fn1 is obtained by rounding the second decimal place of the resulting value.

[0084] [Hydrogen Embrittlement Resistance] The steel material according to this embodiment has the above-described chemical composition, a tensile strength of 900 to 1100 MPa, and an Fn1 of 2.0 to 3.3. As a result, the steel material according to this embodiment can achieve both a tensile strength of 900 to 1100 MPa and excellent hydrogen embrittlement resistance. In this embodiment, having excellent hydrogen embrittlement resistance is defined as follows.

[0085] Specifically, tensile test specimens are prepared from the steel material according to this embodiment using a method compliant with the High Pressure Gas Safety Association standard KHKS 0220 (2020) "Standards for Ultra-High Pressure Gas Equipment". More specifically, the tensile test specimens are made as round bar tensile test specimens, and are prepared so that the axis of the round bar tensile test specimen coincides with a specific position on the steel material. For example, the diameter of the parallel section of the round bar tensile test specimen is 4.0 mm and the length of the parallel section is 25 mm.

[0086] More specifically, if the steel material is a steel plate, the position at plate thickness t / 4 is designated as the specific position. Also, the axial direction of the round bar tensile test specimen is parallel to the width direction of the steel plate. If the steel material is a steel pipe, the position at wall thickness t / 4 on the inner surface side of the steel pipe is designated as the specific position. Also, the axial direction of the round bar tensile test specimen is perpendicular to the axial direction and diameter direction of the steel pipe. If the steel material is a round steel bar, the position R / 2 is designated as the specific position. Also, the axial direction of the round bar tensile test specimen is perpendicular to the axial direction and diameter direction of the round steel bar.

[0087] The prepared round bar tensile test specimens are charged with hydrogen using the cathode hydrogen charging method. Specifically, a cathode hydrogen charging solution is prepared at room temperature. The cathode hydrogen charging solution is an aqueous solution containing a 5% by mass sodium chloride aqueous solution, 30 g / L NH4SCN, and acetate buffer at room temperature, and the pH before the test is adjusted to pH 3.5 with the acetate buffer. With the round bar tensile test specimens immersed in the cathode hydrogen charging solution, the potential is set to -1.5 V and the charging time to 24 hours to charge the round bar tensile test specimens with hydrogen. At this time, preferably, a zinc plating film is formed on the surface of the hydrogen-charged round bar tensile test specimens to prevent hydrogen from leaking out of the round bar tensile test specimens.

[0088] Tensile tests are performed on both hydrogen-charged and uncharged round bar tensile test specimens using a low strain rate testing machine (SSRT) at room temperature (25°C) in air. The strain rate is set to 4.2 × 10⁻⁶. -6The test is performed in seconds. The elongation at fracture, L1 (%), is determined from a hydrogen-charged round bar tensile test specimen. The elongation at maximum load, L0 (%), is determined from a round bar tensile test specimen that is not hydrogen-charged. Note that the elongation at fracture, L1 (%), in the hydrogen environment and the elongation at maximum load, L0 (%), in the atmospheric environment are obtained by rounding the obtained values ​​to the first decimal place.

[0089] Using the obtained fracture elongation L1 (%) in a hydrogen environment and the elongation L0 (%) under maximum load in an atmospheric environment, the relative elongation is calculated using the following formula. If the calculated relative elongation is 0.80 or higher, it is judged to have excellent hydrogen embrittlement resistance. The relative elongation is calculated by rounding the obtained value to the third decimal place. Also, if the relative elongation exceeds 1.00, it is judged to be "1.00". Relative elongation = L1 / L0

[0090] As described above, the steel material according to this embodiment has even better hydrogen embrittlement resistance if the Mn content in its chemical composition is 0.05 to 0.20%. Here, in this specification, even better hydrogen embrittlement resistance means that the relative elongation obtained under the above conditions is 0.90 or higher.

[0091] [Microstructure] In the microstructure of the steel material according to this embodiment, the total area ratio of tempered martensite and tempered bainite is 90% or more. The remainder of the microstructure is, for example, ferrite and / or pearlite. In this embodiment, if the steel material has the above-described chemical composition, a tensile strength of 900 to 1100 MPa, an Fn1 of 2.0 to 3.3, and excellent hydrogen embrittlement resistance, it can be determined that the total area ratio of tempered martensite and tempered bainite is 90% or more.

[0092] [Method for Measuring the Total Area Ratio of Tempered Martensite and Tempered Bainite] The total area ratio of tempered martensite and tempered bainite in the microstructure of the steel material of this embodiment can also be determined by the following method. A test piece having an observation surface is prepared from the steel material. If the steel material is a steel plate, a test piece is prepared that includes the plate thickness t / 4 position, which is the area to be observed, from the center of the plate width, and has an observation surface parallel to the rolling direction. If the steel material is a steel pipe, a test piece is prepared that includes the wall thickness center position, which is the area to be observed, and has an observation surface parallel to the pipe axis direction. If the steel material is a round bar, a test piece is prepared that includes the R / 2 position, which is the area to be observed, and has an observation surface parallel to the rolling direction. 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. If the steel material 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 material is a steel pipe, the rolling direction corresponds to the axial direction of the pipe, the thickness direction corresponds to the wall thickness direction, and the width direction corresponds to the direction perpendicular to the axial direction and wall thickness direction (circumferential direction). When the steel material is a round bar, the rolling direction corresponds to the axial direction, the thickness direction corresponds to the radial direction, and the width direction corresponds to the direction perpendicular to the rolling direction and radial direction (circumferential direction). The surface including the rolling direction and the thickness direction (a 10 mm x 10 mm surface in the case of the test piece size described above) is used as the observation surface.

[0093] After polishing the observation surface of the test specimen to a mirror finish, it is immersed in Nital etching solution for about 10 seconds to reveal the microstructure by etching. Ten arbitrary fields of view within the observation area of ​​the etched observation surface are observed as secondary electron images using a scanning electron microscope (SEM). If the steel material is a steel plate, the observation area is at the position t / 4 of the plate thickness. If the steel material is a steel pipe, the observation area is at the center of the wall thickness. If the steel material is a round bar, the observation area is at the R / 2 position. The area of ​​each of the ten fields of view within the observation area is, for example, 400 μm. 2 (Magnification 5000 times).

[0094] In each field of view, tempered martensite and tempered bainite are identified. In each field of view, tempered martensite and tempered bainite can be distinguished from other tissues (ferrite, pearlite, etc.) based on their morphology. Specifically, tissues with lamellar structure can be identified as pearlite. Tissues containing lath or lenses can be identified as tempered martensite and tempered bainite. Tissues without substructure within the grains can be identified as ferrite.

[0095] The total area ratio of the identified tempered martensite and tempered bainite is determined. The method for determining the total area ratio is not particularly limited and may be a well-known method. For example, the total area ratio of tempered martensite and tempered bainite can be determined by image analysis. In this embodiment, the arithmetic mean of the total area ratios of tempered martensite and tempered bainite obtained in all fields (10 fields) is defined as the total area ratio of tempered martensite and tempered bainite (%).

[0096] [Shape and Use of Steel Material] The shape of the steel material according to this embodiment is not particularly limited. The steel material according to this embodiment may be a steel pipe, a steel plate, or a steel bar (round steel).

[0097] Preferably, the steel material in this embodiment is one of the following: oil well steel pipes, line pipe steel pipes, and high-pressure hydrogen container steel pipes. Oil well steel pipes refer to steel pipes used for oil well tubing applications. Oil well tubing includes, for example, casings, tubing, and drill pipes used in drilling oil or gas wells and extracting crude oil or natural gas. Line pipe steel pipes refer to steel pipes used for line pipe applications that constitute pipelines for transporting production fluids (crude oil or natural gas) extracted from oil or gas wells. Pipelines include, for example, flow lines that transport production fluids from oil or gas wells, gathering lines that collect the production fluids transported by the flow lines and transport them to primary processing facilities, trunk lines that transport the production fluids that have undergone primary processing such as dewatering to the vicinity of the market, and distribution lines that transport them to consumers. Steel pipes for high-pressure hydrogen containers are standardized by ISO 11439, ANSI / NGV, the High-Pressure Gas Safety Act, the Container Safety Regulations Exemplary Standards, etc., and refer to steel pipes used in high-pressure hydrogen containers that store high-pressure hydrogen gas. The steel material in this embodiment may be steel pipes for high-pressure hydrogen containers, or it may be steel pipes for high-pressure hydrogen accumulators or steel pipes for high-pressure hydrogen cylinders.

[0098] More preferably, the steel material of this embodiment is one of the following: seamless steel pipes for oil wells, seamless steel pipes for line pipes, and seamless steel pipes for high-pressure hydrogen containers. Seamless steel pipes for oil wells mean that the steel pipes for oil wells are seamless steel pipes. Seamless steel pipes for line pipes mean that the steel pipes for line pipes are seamless steel pipes. Seamless steel pipes for high-pressure hydrogen containers mean that the steel pipes for high-pressure hydrogen containers are seamless steel pipes. The steel material of this embodiment may be seamless steel pipes for high-pressure hydrogen containers, seamless steel pipes for high-pressure hydrogen accumulators, or seamless steel pipes for high-pressure hydrogen cylinders.

[0099] [Manufacturing Method] An example of a manufacturing method for steel materials according to this embodiment is described below. Note that the manufacturing method described below is just one example, and the manufacturing method for steel materials according to this embodiment is not limited to this example. In other words, as long as steel materials having the above-described configuration can be manufactured according to this embodiment, the method is not limited to the manufacturing method described below. However, the manufacturing method described below is a preferred manufacturing method for manufacturing steel materials according to this embodiment.

[0100] An example of a steel manufacturing method according to this embodiment includes a material manufacturing process, a hot working process, and a heat treatment process. Each process will be described below.

[0101] [Material Manufacturing Process] In the material manufacturing process, the material is manufactured using molten steel having the chemical composition described above. Specifically, in the material manufacturing process according to this embodiment, a slab or ingot is manufactured using molten steel. It is preferable to manufacture the material from the obtained slab or ingot by hot forging.

[0102] When manufacturing slabs from molten steel, the slabs are produced by continuous casting. On the other hand, when manufacturing steel ingots from molten steel, the steel ingots are produced by the ingot-making method. Either method may be used in the material manufacturing process according to this embodiment. Furthermore, the continuous casting method and the ingot-making method may be any well-known method. Hereinafter, in this specification, slabs or steel ingots will be collectively referred to as "slabs, etc."

[0103] The material is manufactured by hot forging the obtained slab or the like. Specifically, first, the slab or the like is heated in a heating furnace. At this time, the heating temperature of the slab or the like is not particularly limited, but for example, it is 1100 to 1300°C. Next, hot forging is performed on the heated slab or the like. The reduction ratio in hot forging is not particularly limited, but for example, it is 10 to 50%.

[0104] Preferably, the cast slab after hot forging is held at 500 to 750°C for 30 minutes or more during the cooling process. Here, 500 to 750°C is the temperature range for the precipitation of composite carbides of V and Mo. Therefore, by holding the cast slab after hot forging at 500 to 750°C for 30 minutes or more, a large number of composite carbides of V and Mo precipitate in the cast slab. In the hot working process described later, most of these composite carbides of V and Mo dissolve into the steel, forming regions in the steel where V, Mo, and C are concentrated. As a result, in the tempering process described later, specific precipitates satisfying Fn1 of 2.0 to 3.3 precipitate. Therefore, in the material manufacturing process according to this embodiment, it is preferable to hold the cast slab after hot forging at 500 to 750°C for 30 minutes or more during the cooling process. Furthermore, it is preferable to allow cast slabs, etc., that have been held at 500 to 750°C for 30 minutes or more to be air-cooled to room temperature.

[0105] In the preferred manufacturing method described above, the material was manufactured by hot forging. However, the material manufacturing process according to this embodiment is not limited to hot forging; for example, the material may be manufactured by bract rolling. Even in this case, it is preferable to hold the material at 500 to 750°C for 30 minutes or more after bract rolling. The material (slab, bloom, or billet) is manufactured by the above process. The hot working process will be described below.

[0106] [Hot Working Process] In the hot working process, the prepared material is hot-worked to produce intermediate steel material. As mentioned above, when the steel material is a seamless steel pipe, the intermediate steel material corresponds to the raw pipe. First, the billet is heated in a heating furnace. The heating temperature is not particularly limited, but for example, it is 1100 to 1300°C. Hot working is performed on the billet extracted from the heating furnace to produce the raw pipe (seamless steel pipe). The method of hot working is not particularly limited and any well-known method is acceptable.

[0107] For example, the Mannesmann process may be used as a hot working method to produce the raw pipe. In this case, a round billet is perforated and rolled using a perforating machine. When perforating and rolling, the perforation ratio is not particularly limited, but for example, it is 1.0 to 4.0. The perforated and rolled round billet is further hot-rolled using a mandrel mill, reducer, sizing mill, etc., to produce the raw pipe. The raw pipe may also be produced from the billet by other hot working methods. For example, if the steel material is a short, thick-walled steel pipe such as a coupling, the raw pipe may be produced by forging using the Erhardt process or similar methods. The raw pipe is produced by the above process. The wall thickness of the raw pipe is not particularly limited, but for example, it is 9 to 60 mm.

[0108] If the steel material is round steel, first the material is heated in a heating furnace. The heating temperature is not particularly limited, but for example it is 1100 to 1300°C. Hot working is performed on the material extracted from the heating furnace to produce intermediate steel material with a circular cross-section perpendicular to the axial direction. Hot working is, for example, bract rolling using a bract rolling mill or hot rolling using a continuous rolling mill. A continuous rolling mill has alternating horizontal stands with a pair of perforated rolls arranged vertically and vertical stands with a pair of perforated rolls arranged horizontally. If the steel material is steel plate, first the material is heated in a heating furnace. The heating temperature is not particularly limited, but for example it is 1100 to 1300°C. Hot rolling is performed on the material extracted from the heating furnace using a bract rolling mill and a continuous rolling mill to produce intermediate steel material in the shape of a steel plate.

[0109] Intermediate steel produced by hot working may be air-cooled (As-Rolled). Intermediate steel produced by hot working may be quenched directly after hot working without cooling to room temperature, or it may be quenched after reheating (additional heating) after hot working. When quenching is performed directly after hot working or after reheating, cooling may be stopped or slowed during quenching. In this case, the occurrence of quenching cracks in the raw pipe can be suppressed. When quenching is performed directly after hot working or after reheating, stress relief annealing (SR) may be performed after quenching but before the next heat treatment process. In this case, residual stress in the raw pipe is removed. Intermediate steel is produced by the above process. The heat treatment process will be described below.

[0110] [Heat Treatment Process] The heat treatment process according to this embodiment includes a quenching process and a tempering process. Each process will be described below.

[0111] [Quenching Process] In the quenching process, the intermediate steel material is subjected to quenching. Quenching is carried out by a well-known method. In this specification, "quenching" means rapidly cooling the intermediate steel material above the A3 transformation point. Here, the holding temperature in quenching corresponds to the temperature of the heat treatment furnace when the intermediate steel material is heated and held. The holding time in quenching means the time from when the temperature of the intermediate steel material reaches a predetermined holding temperature until it is removed from the heat treatment furnace.

[0112] In the quenching process according to this embodiment, the holding temperature is preferably set to FnA (°C) to 1000°C, as defined by the following formula (A): FnA = 890 + 400 × V (A) Here, "V" in formula (A) is substituted with the V content in units of mass%.

[0113] FnA is an index indicating the temperature at which the composite carbide of V and Mo solid-solves in the intermediate steel material having the above-described chemical composition. If the holding temperature of the intermediate steel material is FnA (°C) or higher, most of the V and Mo composite carbide remaining in the intermediate steel material can be solid-solved in the intermediate steel material. On the other hand, if the holding temperature during quenching exceeds 1000°C, the prior γ grains may coarseen, and excellent hydrogen embrittlement resistance may not be obtained. Therefore, it is preferable to set the holding temperature during quenching to FnA (°C) to 1000°C. The quenching time is not particularly limited, but for example, it is 10 to 60 minutes.

[0114] The quenching method involves continuously cooling the intermediate steel material from the quenching start temperature, thereby continuously lowering the surface temperature of the intermediate steel material. The method of continuous cooling is not particularly limited and any well-known method may be used. Examples of continuous cooling methods include immersing the intermediate steel material in a water bath for cooling, or accelerating the cooling of the intermediate steel material by shower water cooling or mist cooling. The tempering process will be described below.

[0115] [Tempering Process] In the tempering process, the intermediate steel material that has undergone quenching is tempered. In this specification, "tempering" refers to the process of tempering the intermediate steel material after quenching. c1 This means reheating and holding the material at a temperature below its transformation point. Here, the tempering holding temperature corresponds to the temperature of the heat treatment furnace when the intermediate steel material is heated and held after quenching. The tempering holding time refers to the time from when the temperature of the intermediate steel material reaches the predetermined holding temperature until it is removed from the heat treatment furnace.

[0116] In the tempering process according to this embodiment, the heating rate to the tempering holding temperature is preferably 5 to 20°C / min. Specifically, the average heating rate when the temperature of the intermediate steel material during tempering is in the range of 100 to 600°C is 5 to 20°C / min. If the heating rate is too fast, composite carbides of V and Mo may not precipitate sufficiently in the intermediate steel material. As a result, Fn1 may not meet the desired range, and the hydrogen embrittlement resistance of the manufactured steel material may not be sufficiently improved. On the other hand, if the heating rate is too slow, the above effect will saturate. Therefore, it is preferable to set the average heating rate when the temperature of the intermediate steel material during tempering is in the range of 100 to 600°C to 5 to 20°C / min.

[0117] Here, the tempering temperature is adjusted as appropriate according to the chemical composition of the steel material and the tensile strength to be obtained. In other words, for an intermediate steel material having the chemical composition of this embodiment, the tempering temperature is adjusted to adjust the tensile strength of the steel material to 900 to 1100 MPa. In the tempering process according to this embodiment, the preferred tempering temperature is 660°C to A c1 This is the transformation point. Furthermore, in the tempering process of this embodiment, the tempering time is preferably 5 to 240 minutes.

[0118] By carrying out the above manufacturing process, the steel material according to this embodiment can be manufactured. However, as stated above, the above manufacturing method is just one example, and it may be manufactured by other manufacturing methods. The effects of the steel material according to this embodiment will be explained in more detail below with reference to examples. However, the various conditions in the examples described below are just one example of conditions adopted to confirm the feasibility and effects of the steel material according to this embodiment. Therefore, the steel material according to this embodiment is not limited to the one example of conditions described in the examples.

[0119] In Example 1, we investigated steel materials having a chemical composition with a Mn content of more than 0.20% to 0.60%. Specifically, we produced molten steel having the chemical compositions shown in Tables 1A and 1B. In Table 1B, "-" indicates that the content of each element is at the impurity level. Specifically, in Test No. 1, the Cu, Ni, Co, and W content was 0%, rounded to the third decimal place. In Test No. 1, the Mg, Ca, REM, and Zr content was 0%, rounded to the fifth decimal place.

[0120]

[0121]

[0122] The steel materials for each test number were manufactured using the following method: Ingots were produced from molten steel having the chemical compositions listed in Tables 1A and 1B. Hot forging was performed on the obtained ingots for each test number to produce block materials. After hot forging, the block materials were held at the holding temperature (°C) and holding time (minutes) shown in the "Post-Forging Cooling Process" column of Table 2, and then cooled to room temperature.

[0123]

[0124] Hot working was performed on the block material for each test number. Specifically, the block material was heated to 1250°C. After heating, the block material was hot-rolled to produce 30 mm thick intermediate steel material (steel plate). Furthermore, for the intermediate steel material for each test number, quenching was performed by holding it at the holding temperature (°C) and holding time (minutes) shown in the "Quenching Process" column of Table 2, followed by water cooling. For the intermediate steel material for each test number after quenching, tempering was performed by raising the temperature in the range of 100 to 600°C at the heating rate (°C / min) shown in the "Tempering Process" column of Table 2, and holding it at the holding temperature (°C) and holding time (minutes). Through the above manufacturing process, steel material (steel plate) for each test number was manufactured.

[0125] [Evaluation Tests] Tensile tests, specific precipitate evaluation tests, and SSRT tests were performed on each steel plate with the corresponding test number that was manufactured.

[0126] [Tensile Test] Tensile tests were conducted on the steel plates of each test number using the method described above to determine the tensile strength and yield strength. Specifically, round bar tensile test specimens were prepared from the steel plates of each test number using a method compliant with KHKS 0220 (2020). The round bar tensile test specimens had a parallel section diameter of 8.0 mm and a parallel section length of 56 mm. The plate thickness t / 4 position was designated as a specific position, and the axial direction of the round bar tensile test specimen was parallel to the plate width direction of the steel material. Using the prepared round bar tensile test specimens, tensile tests were conducted at room temperature (25°C) in air in accordance with JIS Z2241:2011, and the maximum stress in the uniform elongation obtained was defined as the tensile strength (MPa). The 0.2% offset proof stress (MPa) obtained in a similar tensile test was defined as the yield strength. The obtained tensile strength (MPa) and yield strength (MPa) for the steel material of each test number are shown in Table 3.

[0127]

[0128] [Specific Precipitate Evaluation Test] For each steel plate with a test number, the Mo content [Mo] (mol%) and V content [V] (mol%) in the specific precipitate were determined using the method described above. Specifically, micro specimens for creating extraction replicas were prepared from the t / 4 position of the steel plate thickness for each test number. After mirror polishing the surface of the specimens, constant potential electrolytic etching was performed under the conditions described above. Furthermore, replica films were prepared using the method described above, and TEM observation was performed under the conditions described above. At this time, the size of each observation field was set to 0.4 μm × 0.4 μm (100,000x magnification), and the number of fields was set to 50.

[0129] In each observation field, precipitates were identified based on contrast, and elemental concentration analysis was performed using EDS. The conditions for EDS analysis were as described above. By EDS analysis, the Mo content (mol%) and V content (mol%) were quantified, assuming the total content of Mo, V, Fe, Mn, Cr, Nb, and Ti was 100 mol%. Precipitates whose sum of Mo content (mol%) and V content (mol%) was 60 mol% or more were defined as specific precipitates. Furthermore, the arithmetic mean of the Mo content (mol%) of all specific precipitates identified from all observation fields was defined as the Mo content [Mo] (mol%) of the specific precipitate. Similarly, the arithmetic mean of the V content (mol%) of all specific precipitates identified from all observation fields was defined as the V content [V] (mol%) of the specific precipitate. Fn1 (= [Mo] / [V]) was determined from the Mo content [Mo] and V content [V] of the obtained specific precipitates. The obtained Fn1 (= [Mo] / [V]) for each steel material of each test number is shown in Table 3.

[0130] [SSRT Test] For each steel plate with a test number, a tensile test was performed using a low strain rate tester (SSRT) in the manner described above to evaluate its hydrogen embrittlement resistance. Specifically, a round bar tensile test specimen with a parallel section diameter of 4.0 mm and a parallel section length of 25 mm was prepared from the t / 4 position of the steel plate thickness for each test number. Hydrogen was charged to one of the two round bar tensile test specimens in the manner described above. The cathode hydrogen charging solution was an aqueous solution containing a 5% by mass sodium chloride aqueous solution at room temperature, 30 g / L NH4SCN, and acetate buffer. The pH before the test was adjusted to pH 3.5 with the acetate buffer.

[0131] A round bar tensile test specimen was immersed in a cathode hydrogen charging solution, and hydrogen was charged to the specimen at a potential of -1.5V for 24 hours. A zinc plating film was formed on the surface of the hydrogen-charged round bar tensile test specimen to prevent hydrogen from leaking out. The conditions for forming the zinc plating film were the same for all test specimens. One of the round bar tensile test specimens was not charged with hydrogen.

[0132] A tensile test specimen of a zinc-plated round bar was subjected to a low strain rate test (SSRT) at room temperature (25°C) in air, yielding a result of 4.2 × 10⁻⁶. -6 Tensile tests were conducted at a strain rate of 10¹⁶ / second to determine the elongation at break L1 (%) in a hydrogen environment. Furthermore, for round bar tensile test specimens that were not charged with hydrogen, a low strain rate tester (SSRT) was used at room temperature (25°C) in air to determine 4.2 × 10¹⁶. -6 Tensile tests were conducted at a strain rate of 1 / second to determine the elongation L0 (%) under maximum load in an atmospheric environment.

[0133] The relative elongation (= L1 / L0) was calculated using the fracture elongation L1 (%) obtained in the hydrogen environment and the elongation L0 (%) at maximum load in the atmospheric environment. If the relative elongation exceeded 1.00, it was determined that the relative elongation was "1.00".

[0134] [Evaluation Results] Referring to Tables 1A, 1B, 2, and 3, the steel plates of test numbers 1 to 11 had the above-described chemical composition, a tensile strength of 900 to 1100 MPa, and an Fn1 of 2.0 to 3.3. As a result, these steel plates had a relative elongation of 0.80 or higher and exhibited excellent hydrogen embrittlement resistance. In other words, these steel materials possessed high strength and excellent hydrogen embrittlement resistance. Furthermore, it was determined that the total area ratio of tempered martensite and tempered bainite in the microstructure of the steel materials of test numbers 1 to 11 was 90% or higher.

[0135] On the other hand, the steel sheet of test number 12 had too low a Mo content and too low an Fn1. As a result, this steel sheet had a relative elongation of less than 0.80 and did not possess excellent hydrogen embrittlement resistance.

[0136] The steel sheet in test number 13 had too high a Mo content and too high an Fn1 level. As a result, this steel sheet had a relative elongation of less than 0.80 and did not possess excellent hydrogen embrittlement resistance.

[0137] The steel sheet in test number 14 had too high a V content and too low an Fn1. As a result, this steel sheet had a relative elongation of less than 0.80 and did not possess excellent hydrogen embrittlement resistance.

[0138] The steel plates of test numbers 15 and 16 were held at too high a temperature during the cooling process after forging. As a result, these steel plates had too low an Fn1 value. Consequently, these steel plates had a relative elongation of less than 0.80 and did not possess excellent hydrogen embrittlement resistance.

[0139] The steel plates of test numbers 17 and 18 had an insufficient holding time during the cooling process after forging. As a result, these steel plates had too low an Fn1 value. Consequently, these steel plates had a relative elongation of less than 0.80 and did not possess excellent hydrogen embrittlement resistance.

[0140] The steel plates in test numbers 19-21 were held at too low a temperature during the quenching process. As a result, these steel plates had too low an Fn1 value. Consequently, these steel plates had a relative elongation of less than 0.80 and did not possess excellent hydrogen embrittlement resistance.

[0141] In test number 22, the heating rate was too fast during the tempering process. As a result, the Fn1 value of this steel sheet was too low. Consequently, the relative elongation of this steel sheet was less than 0.80, and it did not possess excellent hydrogen embrittlement resistance.

[0142] In Example 2, we investigated steel materials having a chemical composition with a Mn content of 0.05 to 0.20%. Specifically, we produced molten steel having the chemical compositions shown in Tables 4A and 4B. In Table 4B, "-" indicates that the content of each element is at the impurity level. Specifically, in test number 23, the Cu, Ni, Co, and W content was 0% when rounded to the third decimal place. In test number 23, the Mg, Ca, REM, and Zr content was 0% when rounded to the fifth decimal place.

[0143]

[0144]

[0145] Similar to Example 1, the steel materials for each test number were manufactured by the following method. Ingots were produced from molten steel having the chemical compositions listed in Tables 4A and 4B. Hot forging was performed on the obtained ingots for each test number to produce block materials. After hot forging, the block materials were held at the holding temperature (°C) and holding time (minutes) shown in the "Post-Forging Cooling Process" column of Table 5, and then cooled to room temperature.

[0146]

[0147] Similar to Example 1, hot working was performed on the block material for each test number. Specifically, the block material was heated to 1250°C. After heating, hot rolling was performed on the block material to produce intermediate steel material (steel plate) with a thickness of 30 mm. Furthermore, for the intermediate steel material for each test number, quenching was performed by holding it at the holding temperature (°C) and holding time (minutes) shown in the "Quenching Process" column of Table 5, followed by water cooling. For the intermediate steel material for each test number after quenching, tempering was performed by raising the temperature in the range of 100 to 600°C at the heating rate (°C / min) shown in the "Tempering Process" column of Table 5, and holding it at the holding temperature (°C) and holding time (minutes). Through the above manufacturing process, steel material (steel plate) for each test number was manufactured.

[0148] [Evaluation Tests] In the same manner as in Example 1, tensile tests, specific precipitate evaluation tests, and SSRT tests were performed on each steel plate with the corresponding test number that was manufactured.

[0149] [Tensile Test] Similar to Example 1, tensile tests were performed on the steel plates of each test number using the method described above to determine the tensile strength and yield strength. Similar to Example 1, round bar tensile test specimens were prepared from the steel plates of each test number. Similar to Example 1, tensile tests were performed, and the maximum stress in the uniform elongation obtained was defined as the tensile strength (MPa). The 0.2% offset proof stress (MPa) obtained in the same tensile test was defined as the yield strength. The obtained tensile strength (MPa) and yield strength (MPa) for the steel materials of each test number are shown in Table 6.

[0150]

[0151] [Specific Precipitate Evaluation Test] Similar to Example 1, the Mo content [Mo] (mol%) and V content [V] (mol%) in the specific precipitates were determined for each steel plate with the test number using the method described above. Similar to Example 1, replica films were prepared and TEM observation was performed under the conditions described above. Furthermore, similar to Example 1, the Mo content (mol%) and V content (mol%) were quantified by EDS analysis, assuming the total content of Mo, V, Fe, Mn, Cr, Nb, and Ti was 100 mol%. Precipitates whose sum of Mo content (mol%) and V content (mol%) was 60 mol% or more were defined as specific precipitates. Furthermore, the arithmetic mean of the Mo content (mol%) in all specific precipitates identified from all observation fields was defined as the Mo content [Mo] (mol%) in the specific precipitates. Similarly, the arithmetic mean of the V content (mol%) in all specific precipitates identified from all observation fields was defined as the V content [V] (mol%) in the specific precipitate. From the obtained Mo content [Mo] and V content [V] in the specific precipitate, Fn1 (= [Mo] / [V]) was calculated. The obtained Fn1 (= [Mo] / [V]) for each steel material of each test number is shown in Table 6.

[0152] [SSRT Test] Similar to Example 1, for the steel plates of each test number, a tensile test was carried out using a low strain rate testing machine (SSRT) by the above method to evaluate the hydrogen embrittlement resistance characteristics. Similar to Example 1, round bar tensile test pieces were prepared, and one of the two round bar tensile test pieces was charged with hydrogen. Similar to Example 1, for the round bar tensile test piece with a zinc plating film formed, using a low strain rate testing machine (SSRT), at room temperature (25 °C) in the atmosphere, a tensile test was carried out at a strain rate of 4.2×10 -6 / s, and the elongation at fracture L1 (%) in a hydrogen environment was determined. Further, for the round bar tensile test piece not charged with hydrogen, using a low strain rate testing machine (SSRT), at room temperature (25 °C) in the atmosphere, a tensile test was carried out at a strain rate of 4.2×10 -6 / s, and the elongation L0 (%) at the maximum load in an air environment was determined.

[0153] Using the obtained elongation at fracture L1 (%) in a hydrogen environment and the elongation L0 (%) at the maximum load in an air environment, the relative elongation (=L1 / L0) was determined. When the relative elongation exceeded 1.00, the relative elongation was judged to be "1.00".

[0154] [Evaluation Results] Referring to Table 4A, Table 4B, Table 5 and Table 6, the steel plates of test numbers 23 to 33 had the above chemical compositions, the tensile strength was 900 to 1100 MPa, and Fn1 satisfied 2.0 to 3.3. As a result, these steel plates had a relative elongation of 0.90 or more and had more excellent hydrogen embrittlement resistance characteristics. That is, these steel materials had high strength and excellent hydrogen embrittlement resistance characteristics. In addition, it was judged that the steel materials of test numbers 23 to 33 had a total area ratio of tempered martensite and tempered bainite of 90% or more in the microstructure.

[0155] On the other hand, the steel plate of test number 34 had too low Mo content and too low Fn1. As a result, this steel plate had a relative elongation of less than 0.80 and did not have excellent hydrogen embrittlement resistance characteristics.

[0156] The steel plate of test number 35 had too high Mo content and too high Fn1. As a result, this steel plate had a relative elongation of less than 0.80 and did not have excellent hydrogen embrittlement resistance characteristics.

[0157] The steel sheet in test number 36 had too high a V content and too low an Fn1. As a result, this steel sheet had a relative elongation of less than 0.80 and did not possess excellent hydrogen embrittlement resistance.

[0158] The steel plates of test numbers 37 and 38 were held at too high a temperature during the cooling process after forging. As a result, these steel plates had too low an Fn1 value. Consequently, these steel plates had a relative elongation of less than 0.80 and did not possess excellent hydrogen embrittlement resistance.

[0159] The steel plates of test numbers 39 and 40 had an insufficient holding time during the cooling process after forging. As a result, these steel plates had too low an Fn1 value. Consequently, these steel plates had a relative elongation of less than 0.80 and did not possess excellent hydrogen embrittlement resistance.

[0160] The steel plates in test numbers 41-43 were held at too low a temperature during the quenching process. As a result, these steel plates had too low an Fn1 value. Consequently, these steel plates had a relative elongation of less than 0.80 and did not possess excellent hydrogen embrittlement resistance.

[0161] In the tempering process of steel plate No. 44, the heating rate was too fast. As a result, the Fn1 value of this steel plate was too low. Consequently, the relative elongation of this steel plate was less than 0.80, and it did not possess excellent hydrogen embrittlement resistance.

[0162] The embodiments of this disclosure have been described above. However, the embodiments described above are merely examples for implementing this disclosure. Therefore, this disclosure is not limited to the embodiments described above, and the embodiments described above can be modified as appropriate without departing from the spirit of this disclosure.

Claims

1. Steel material, with a chemical composition in mass percent, C: 0.15-0.40%, Si: 0.10-0.50%, Mn: 0.05-0.60%, P: 0.050% or less, S: 0.0100% or less, Cr: 0.50-1.50%, Mo: 0.80-1.20%, Ti: 0.002-0.010%, V: 0.08-0.15%, Nb: 0.010-0.050%, Al: 0.001-0.100%, B: 0.0001-0.0050%, N: 0.0100% or less, O: 0.0100% or less, Mg: 0-0.0050% A steel material comprising Ca: 0-0.0050%, rare earth elements: 0-0.0050%, Cu: 0-0.50%, Ni: 0-0.50%, Co: 0-0.50%, W: 0-0.50%, Zr: 0-0.0100%, and the remainder being Fe and impurities, having a tensile strength of 900-1100 MPa, wherein, in the steel material, when the total content of Mo, V, Fe, Mn, Cr, Nb, and Ti in the precipitates is set to 100 mol%, precipitates satisfying the sum of the Mo content and V content in the precipitates being 60 mol% or more are defined as specific precipitates, and the Mo content [Mo] (mol%) in the specific precipitates and the V content [V] (mol%) in the specific precipitates satisfy formula (1). 2.0 ≤ [Mo] / [V] ≤ 3.3 (1) Here, [Mo] in equation (1) is substituted with the Mo content in the specified precipitate in units of mol%, and [V] in equation (1) is substituted with the V content in the specified precipitate in units of mol%.

2. A steel material according to claim 1, wherein the chemical composition contains one or more elements selected from the group consisting of: Mg: 0.0001 to 0.0050%, Ca: 0.0001 to 0.0050%, rare earth elements: 0.0001 to 0.0050%, Cu: 0.01 to 0.50%, Ni: 0.01 to 0.50%, Co: 0.01 to 0.50%, W: 0.01 to 0.50%, and Zr: 0.0001 to 0.0100%.

3. A steel material according to claim 1 or claim 2, wherein the steel material is any of the following: a steel pipe for oil wells, a steel pipe for line pipes, and a steel pipe for high-pressure hydrogen containers.

4. A steel material according to claim 3, wherein the steel pipe for high-pressure hydrogen container is either a steel pipe for a high-pressure hydrogen accumulator or a steel pipe for a high-pressure hydrogen cylinder.

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