Steel material

Abstract

Provided is a steel material having high strength and excellent hydrogen embrittlement resistance. A steel material according to the present disclosure has the chemical composition described in the description, satisfies formula (1), and has a tensile strength of 900-1100 MPa. In the steel material, the Mn sulfide number density NT satisfies 0.300 / mm2 or less, and the number density NA / mm2 of Mn sulfides having a major axis of 7 μm or more and the number density NB / mm2 of Mn sulfides having a major axis of 1-3 μm satisfy formula (2).　(1): 0.8Cr + Mn + αMo > 2.50 (2): NA / (NA + NB) ≤ 0.40 Here, each element symbol in formula (1) is substituted with the content of the corresponding element in units of mass%. In formula (1), α = 2 when B_eff defined in the description is 0.0005 or more, and α = 1 when B_eff is less than 0.0005.

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, such as those used for oil well tubular construction, and steel materials for line pipes, such as those used 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 M ２ C-type precipitates are dispersed. This enhances resistance to SSC (Stein-Stein-Crystal) formation. Patent Document 1 further enhances resistance to hydrogen embrittlement by forming Mo segregation regions at the 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 the present disclosure has a chemical composition in mass %, C: 0.15 to 0.40%, Si: 0.10 to 0.50%, Mn: 0.05 to 0.20%, P: 0.050% or less, S: 0.0100% or less, Cr: 0.50 to 1.50%, Mo: 0.80 to 2.00%, Ti: 0.002 to 0.010%, V: 0.08 to 0.30%, Nb: 0.010 to 0.050%, Al: 0.001 to 0.100%, B: 0.0001 to 0.0050%, N: 0.0100% or less, O: 0.0100% or less, 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 balance: consisting of Fe and impurities, satisfies formula (1), has a tensile strength of 900 to 1100 MPa, and in the steel material, the number density N of Mn sulfides Ｔ is 0.300 pieces / mm ２ or less, and among the Mn sulfides, the number density N of Mn sulfides having a major axis of 7 μm or more Ａ pieces / mm ２ and the number density N of Mn sulfides having a major axis of 1 to 3 μm Ｂ pieces / mm ２ satisfy formula (2). 0.8Cr + Mn + αMo > 2.50 (1) N Ａ / (N Ａ + N Ｂ ) ≦ 0.40 (2) Here, in the element symbols in formula (1), the content of the corresponding element is substituted in units of mass %. α in formula (1) is 2 when B_eff defined by the following formulas (3) and (4) is 0.0005 or more, and α = 1 when the B_eff is less than 0.0005. When N - 0.293Ti ≧ 0: B_eff = B - 0.772(N - 0.293Ti) (3) When N - 0.293Ti < 0: B_eff = B (4) Here, in the element symbols in formulas (3) and (4), the content of the corresponding element is substituted in units of mass %.

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

[0015] Figure 1 shows Fn2(=N Ａ / (N Ａ +N Ｂ )) and the lower limit stress intensity factor K, which is an indicator of hydrogen embrittlement resistance. １Ｈ Value (MPa·m) １／２ This is a diagram showing the relationship with ).

[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] The inventors focused on chemical composition and investigated steel materials that achieve both a tensile strength of 900 to 1100 MPa and excellent resistance to hydrogen embrittlement. In this case, Mn sulfides may form in the steel material. On the other hand, Mn sulfides are easily stretched and become coarse during the steel manufacturing process. Therefore, the inventors considered that if the formation of Mn sulfides could be suppressed, the resistance to hydrogen embrittlement of the steel material could be improved. Specifically, the inventors found that reducing the Mn content to 0.05 to 0.20% could suppress the formation of Mn sulfides and potentially improve the resistance to hydrogen embrittlement of the steel material.

[0018] In other words, the inventors have determined that the composition of the material is as follows, in mass%, C: 0.15-0.40%, Si: 0.10-0.50%, Mn: 0.05-0.20%, P: 0.050% or less, S: 0.0100% or less, Cr: 0.50-1.50%, Mo: 0.80-2.00%, Ti: 0.002-0.010%, V: 0.08-0.30%, Nb: 0.010-0.050%, Al: 0.001-0.100%, B: 0.0001-0.0050%, 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.

[0019] As a result of our investigations, we found that by reducing the Mn content to 0.05-0.20%, the number density of Mn sulfides in the steel material having the above chemical composition was reduced. Ｔ 0.300 pieces / mm ２ It can be reduced to the following extent. As a result, steel materials having the above-mentioned chemical composition may be able to achieve both a tensile strength of 900 to 1100 MPa and excellent resistance to hydrogen embrittlement. In this specification, "Mn sulfide" means particles identified by the method described later, in which the sum of Mn content and S content by mass is 70% or more.

[0020] On the other hand, in steel materials having the above-mentioned chemical composition, there is a concern that the hardenability of the steel material will decrease due to the reduction of the Mn content to 0.05 to 0.20%. If the hardenability of the steel material decreases, the hydrogen embrittlement resistance of the steel material will decrease. Therefore, the inventors investigated a method to stably improve the hardenability of steel materials having the above-mentioned chemical composition. As a result, it became clear that in steel materials having the above-mentioned chemical composition, if the chemical composition further satisfies the following formula (1), then, provided that the other configurations of this embodiment are also satisfied, it is possible to achieve both a tensile strength of 900 to 1100 MPa and excellent hydrogen embrittlement resistance. 0.8Cr + Mn + αMo > 2.50 (1) Here, the elemental symbols in formula (1) are substituted with the content of the corresponding element in units of mass%. In equation (1), α = 2 if B_eff, defined in equations (3) and (4) below, is 0.0005 or greater, and α = 1 if B_eff is less than 0.0005. When N - 0.293Ti ≥ 0: B_eff = B - 0.772(N - 0.293Ti) (3) When N - 0.293Ti < 0: B_eff = B (4) Here, the elemental symbols in equations (3) and (4) are substituted with the content of the corresponding element in units of mass%.

[0021] Here, Fn1 is defined as 0.8Cr + Mn + αMo. The value of α in Fn1 is determined according to the value of B_eff, defined in equations (3) and (4) above. B_eff corresponds to the amount of B dissolved in the steel. In other words, Fn1 is an indicator of the hardenability of the steel having the above chemical composition. If Fn1 is too low, the hardenability of the steel cannot be sufficiently increased, and the hydrogen embrittlement resistance of the steel decreases. On the other hand, if Fn1 exceeds 2.50, the hardenability of the steel is sufficiently increased, and the hydrogen embrittlement resistance of the steel can be stably improved. Therefore, assuming the steel according to this embodiment has the above chemical composition, Fn1 is set to be greater than 2.50.

[0022] On the other hand, having the above chemical composition, with Fn1 exceeding 2.50 and the number density of Mn sulfide N Ｔ 0.300 pieces / mm ２Even with steel materials that meet the following conditions, excellent hydrogen embrittlement resistance could not be obtained when they had a tensile strength of 900 to 1100 MPa. Therefore, the present inventors have developed a steel material having the above-mentioned chemical composition, with Fn1 exceeding 2.50, and a number density of Mn sulfide N Ｔ 0.300 pieces / mm ２ For steel materials that meet the following criteria, various methods were investigated to improve hydrogen embrittlement resistance while maintaining tensile strength.

[0023] As a result of detailed investigations by the present inventors, the chemical composition described above is found to have an Fn1 value greater than 2.50 and a number density of Mn sulfide N Ｔ 0.300 pieces / mm ２ In steel materials that satisfy the following conditions, it has become clear that the distribution of Mn sulfide particle size affects the hydrogen embrittlement resistance of the steel. In other words, simply the number density N of Mn sulfides Ｔ The inventors of this invention believe that by not only reducing the amount of Mn sulfide but also controlling its size, it may be possible to improve hydrogen embrittlement resistance while maintaining tensile strength.

[0024] Therefore, the inventors focused on the distribution of the equivalent circle diameter of Mn sulfides and investigated various methods to improve hydrogen embrittlement resistance while maintaining strength. As a result, the inventors found a chemical composition having the above-mentioned composition, with Fn1 exceeding 2.50, and a number density of Mn sulfides N Ｔ 0.300 pieces / mm ２ In steel materials that satisfy the following conditions, the number density of Mn sulfides with a major axis of 7 μm or larger is N Ａ pieces / mm ２ And the number density N of Mn sulfides with a major axis of 1 to 3 μm Ｂ pieces / mm ２ It has become clear that if the following equation (2) is satisfied, excellent hydrogen embrittlement resistance can be obtained even if the tensile strength is 900 to 1100 MPa. Ａ / (N Ａ +N Ｂ ) ≤ 0.40 (2)

[0025] In this specification, Mn sulfides with a major axis of 7 μm or more are also referred to as "coarse Mn sulfides." Furthermore, Mn sulfides with a major axis of 1 to 3 μm are also referred to as "fine Mn sulfides." In addition, Fn2 = NＡ / (N Ａ +N Ｂ ) is defined as follows. Fn2 represents the ratio of coarse Mn sulfides to the total of coarse Mn sulfides and fine Mn sulfides. The relationship between Fn2 and hydrogen embrittlement resistance will be explained in detail below using diagrams.

[0026] Figure 1 shows Fn2(=N Ａ / (N Ａ +N Ｂ )) and the lower limit stress intensity factor K, which is an indicator of hydrogen embrittlement resistance. １Ｈ Value (MPa·m) １／２ This figure shows the relationship with ). Figure 1 shows an example described later that has the above chemical composition, Fn1 exceeds 2.50, and the number density of Mn sulfide N Ｔ 0.300 pieces / mm ２ For an example that satisfies the following conditions and has a tensile strength of 900 to 1100 MPa, the Fn2 obtained by the method described later and the lower limit stress intensity factor K obtained by the method described later are used. １Ｈ Value (MPa·m) １／２ It was created using ) and .

[0027] Referring to Figure 1, the chemical composition described above is obtained, Fn1 is greater than 2.50, and the number density of Mn sulfide is N Ｔ 0.300 pieces / mm ２ For steel materials that satisfy the following conditions and have a tensile strength of 900 to 1100 MPa, if Fn2 is 0.40 or less, the lower limit stress intensity factor K １Ｈ The value is 30 MPa·m １／２ The above confirms that it possesses excellent hydrogen embrittlement resistance. Therefore, the steel material according to this embodiment has the above-mentioned chemical composition, Fn1 exceeds 2.50, and the number density of Mn sulfides is N Ｔ 0.300 pieces / mm ２ The following conditions are met, the steel material has a tensile strength of 900 to 1100 MPa, and furthermore, Fn2 is 0.40 or less. 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.

[0028] Based on the above findings, the gist of the steel material according to this embodiment is as follows:

[0029] [1] Steel material having a chemical composition in mass percent of: C: 0.15-0.40%, Si: 0.10-0.50%, Mn: 0.05-0.20%, P: 0.050% or less, S: 0.0100% or less, Cr: 0.50-1.50%, Mo: 0.80-2.00%, Ti: 0.002-0.010%, V: 0.08-0.30%, 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% The composition consists of 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, satisfying formula (1), having a tensile strength of 900-1100 MPa, and in the steel material, the number density of Mn sulfide is N Ｔ 0.300 pieces / mm ２ The following conditions must be met, and the number density of Mn sulfides with a major axis of 7 μm or larger is N Ａ pieces / mm ２ And the number density N of Mn sulfides with a major axis of 1 to 3 μm Ｂ pieces / mm ２ The following steel material satisfies equation (2): 0.8Cr + Mn + αMo > 2.50 (1) N Ａ / (N Ａ +N Ｂ ) ≤ 0.40 (2) Here, the content of the corresponding element is substituted for the element symbol in formula (1) in units of mass%. In formula (1), α = 2 when B_eff, defined in the following formulas (3) and (4), is 0.0005 or more, and α = 1 when B_eff is less than 0.0005. When N - 0.293Ti ≥ 0: B_eff = B - 0.772 (N - 0.293Ti) (3) When N - 0.293Ti < 0: B_eff = B (4) Here, the content of the corresponding element is substituted for the element symbol in formulas (3) and (4) in units of mass%.

[0030] [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%.

[0031] [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.

[0032] [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.

[0033] 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.

[0034] 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.

[0035] 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.

[0036] In this specification, "steel pipe for high-pressure hydrogen containers" refers to steel pipes used in high-pressure hydrogen containers that store high-pressure hydrogen gas, as standardized by ISO 11439, ANSI / NGV, the High-Pressure Gas Safety Act, the Container Safety Regulations Exemplary Standards, etc. 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. "Seamless steel pipe for high-pressure hydrogen containers" means that the steel pipe for high-pressure hydrogen containers is a seamless steel pipe.

[0037] The steel material according to this embodiment will be described in detail below. Unless otherwise specified, the "%" for elements refers to mass percent.

[0038] [Chemical Composition] The chemical composition of the steel material according to this embodiment contains the following elements.

[0039] C: 0.15-0.40% Carbon (C) enhances hardenability and makes the microstructure of the steel primarily composed of tempered martensite and tempered bainite. As a result, the hydrogen embrittlement resistance of the steel is improved. C further forms carbides or carbonitrides, increasing the strength of the steel. If the C 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 C content is too high, even if the content of other elements is within the range of this embodiment, there will be an excessive amount of carbides in the steel, and the hydrogen embrittlement resistance of the steel will decrease. Therefore, the C content is 0.15-0.40%. The preferred lower limit of the C content is 0.16%, and more preferably 0.18%. The preferred upper limit of the C content is 0.39%, and more preferably 0.36%.

[0040] Si: 0.10-0.50% Silicon (Si) deoxidizes the steel and reduces inclusions in the steel material. As a result, the hydrogen embrittlement resistance of the steel material is improved. If the Si 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 Si content is too high, the hydrogen embrittlement resistance of the steel material will decrease even if the content of other elements is within the range of this embodiment. Therefore, the Si content is 0.10-0.50%. The preferred lower limit of the Si content is 0.11%, more preferably 0.13%, and even more preferably 0.15%. The preferred upper limit of the Si content is 0.49%, and more preferably 0.48%.

[0041] Mn: 0.05-0.20% Manganese (Mn) deoxidizes steel. Mn further enhances hardenability and improves the hydrogen embrittlement resistance of the steel. If the Mn 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 Mn content is too high, even if the content of other elements is within the range of this embodiment, a large number of Mn sulfides will be formed, and the hydrogen embrittlement resistance of the steel will decrease. Therefore, the Mn content is 0.05-0.20%. The preferred lower limit of the Mn content is 0.06%, and more preferably 0.08%. The preferred upper limit of the Mn content is 0.18%, and more preferably 0.16%.

[0042] 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 of the P content is 0.048%, more preferably 0.046%, even more preferably 0.045%, and even more preferably 0.044%.

[0043] 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 0.0098%, more preferably 0.0095%, and even more preferably 0.0092%.

[0044] 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%, and even more preferably 1.40%.

[0045] Mo: 0.80-2.00% Molybdenum (Mo) enhances the hardenability of steel and improves its resistance to hydrogen embrittlement. Mo further increases the tempering softening resistance of steel, enabling high-temperature tempering. As a result, the resistance to hydrogen embrittlement of steel is improved. Mo further strengthens prior γ grain boundaries, improving the resistance to hydrogen embrittlement of steel. 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, coarse carbides are formed, and the resistance to hydrogen embrittlement of steel decreases. Therefore, the Mo content is 0.80-2.00%. The preferred lower limit of the Mo content is 0.83%, more preferably 0.86%, and even more preferably 0.88%. The preferred upper limit of the Mo content is 1.96%, more preferably 1.90%, and even more preferably 1.88%.

[0046] 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%.

[0047] V: 0.08-0.30% Vanadium (V) forms carbides, nitrides, or carbonitrides (hereinafter referred to as "carbonitrides, etc.") and enhances the hydrogen embrittlement resistance of 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, carbonitrides, etc. will be excessively generated, and the hydrogen embrittlement resistance of steel will decrease. Therefore, the V content is 0.08-0.30%. 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 0.29%, and more preferably 0.28%.

[0048] Nb: 0.010 to 0.050% Niobium (Nb) forms carbonitrides and other materials, and by the pinning effect, it refines the prior γ grains, 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 and other materials 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 of the Nb content is 0.047%, and more preferably 0.045%.

[0049] 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."

[0050] 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 0.0047%, and even more preferably 0.0045%.

[0051] 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%.

[0052] 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%.

[0053] 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.

[0054] [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.

[0055] Mg: 0-0.0050% Magnesium (Mg) is an optional element and may not be present. In other words, the Mg content may be 0%. If present, Mg combines with S in the steel and precipitates as fine Mg sulfides. In addition, Mg reduces Mn sulfides. The combined effects of these two factors enhance the hydrogen embrittlement resistance of the steel. Even a small amount of Mg can provide some of the above effects. 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, reducing the hydrogen embrittlement resistance of the steel. Therefore, the Mg content is 0-0.0050%. 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%, more preferably 0.0045%, and even more preferably 0.0030%.

[0056] 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 combines with S in the steel and precipitates as fine Ca sulfides. In addition, Ca reduces Mn sulfides. The combined effects of these two factors enhance the hydrogen embrittlement resistance of the steel. Even a small amount of Ca can provide some of the above effects. 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%.

[0057] 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 combines with S in the steel and precipitates as fine REM sulfides. In addition, REM reduces Mn sulfides. The combined effects of these two factors enhance the hydrogen embrittlement resistance of the steel. Even a small amount of REM can provide some of the above effects. 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 for the REM content is 0.0040%, more preferably 0.0030%, and even more preferably 0.0025%.

[0058] 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.

[0059] 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.

[0060] 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 0.40%, more preferably 0.30%, and even more preferably 0.25%.

[0061] 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 0.45%, more preferably 0.40%, and even more preferably 0.30%.

[0062] 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 0.45%, more preferably 0.40%, and even more preferably 0.38%.

[0063] 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.

[0064] W: 0-0.50% Tungsten (W) is an optional element and may not be present. That is, the W content may be 0%. When present, W forms fine carbides, increasing the strength of the steel. Even a small amount of W can provide the above effect 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, reducing the steel's resistance to SSC (Steel-Sealing Carbon). 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%, and more preferably 0.40%.

[0065] 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 0.0080%, more preferably 0.0060%, even more preferably 0.0040%, and even more preferably 0.0020%.

[0066] [Formula (1)] Assuming that the steel material according to this embodiment has the above-described chemical composition, it satisfies the following formula (1): 0.8Cr + Mn + αMo > 2.50 (1) Here, the content of the corresponding element is substituted for the element symbol in formula (1) in units of mass%. In formula (1), α = 2 when B_eff, defined in the following formulas (3) and (4), is 0.0005 or more, and α = 1 when B_eff is less than 0.0005. When N - 0.293Ti ≥ 0: B_eff = B - 0.772 (N - 0.293Ti) (3) When N - 0.293Ti < 0: B_eff = B (4) Here, the content of the corresponding element is substituted for the element symbol in formulas (3) and (4) in units of mass%.

[0067] Fn1 (= 0.8Cr + Mn + αMo) is an index of the hardenability of steel having the above-mentioned chemical composition. If Fn1 is too low, the hardenability of the steel cannot be sufficiently increased, and the hydrogen embrittlement resistance of the steel decreases. On the other hand, if Fn1 exceeds 2.50, the hardenability of the steel is sufficiently increased, and the hydrogen embrittlement resistance of the steel can be stably increased. Therefore, assuming that the steel according to this embodiment has the above-mentioned chemical composition, Fn1 is set to be greater than 2.50.

[0068] The preferred lower limit of Fn1 is 2.51, more preferably 2.55, and even more preferably 2.60. The upper limit of Fn1 is not particularly limited, but for example, it is 5.00. The upper limit of Fn1 may also be 4.90 or 4.80. Fn1 is obtained by rounding the third decimal place of the obtained value.

[0069] As described above, α in Fn1 is determined based on B_eff. Here, B_eff corresponds to the amount (mass%) of B estimated to be dissolved in the steel. B combines with N to form B nitride. On the other hand, N combines with Ti to form Ti nitride. Therefore, by fixing N with Ti, the formation of B nitride is suppressed, and the amount of dissolved B increases. The amount of dissolved B estimated in this way is defined as B_eff (mass%) by equations (3) and (4). In this embodiment, B_eff (mass%) is obtained by rounding the obtained value to the fifth decimal place.

[0070] Furthermore, if B_eff, as defined in equation (3) or (4), is 0.0005 or greater, then α = 2 is substituted into Fn1. On the other hand, if B_eff, as defined in equation (3) or (4), is less than 0.0005, then α = 1 is substituted into Fn1. Here, the numerical range of B_eff is not particularly limited, but is practically between -0.0072 and 0.0050.

[0071] [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.

[0072] 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.

[0073] 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 56.0 mm.

[0074] 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. 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 of the steel pipe is designated as the specific position. Here, the position at wall thickness t / 4 on the outer surface 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. The axial direction of the round bar tensile test specimen is perpendicular to the axial direction and 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. Furthermore, if the steel material is a round steel bar, the R / 2 position is designated as the specific position. In this specification, the R / 2 position of a round steel bar means 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.

[0075] 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.

[0076] [Mn sulfide] The steel material according to this embodiment has the above chemical composition, Fn1 exceeds 2.50, has a tensile strength of 900 to 1100 MPa, and furthermore, the number density of Mn sulfide is N Ｔ 0.300 pieces / mm ２ The following conditions must be met, and the number density of Mn sulfides with a major axis of 7 μm or larger is N Ａ pieces / mm ２ And the number density N of Mn sulfides with a major axis of 1 to 3 μm Ｂ pieces / mm ２ The equation (2) is satisfied. N Ａ / (N Ａ +N Ｂ ) ≤ 0.40 (2)

[0077] As described above, in this specification, "Mn sulfide" means particles identified by the method described later, in which the sum of Mn content and S content is 70% or more by mass. Furthermore, in this embodiment, the major axis of the Mn sulfide is not particularly limited, but is substantially 1 to 20 μm. That is, in this embodiment, the number density N of Mn sulfides with a major axis of 1 to 20 μm Ｔ 0.300 pieces / mm ２ The following is the number density N of Mn sulfides (coarse Mn sulfides) with a major axis of 7 to 20 μm. Ａ (pcs / mm ２ ) and the number density N of Mn sulfides (fine Mn sulfides) with a major axis of 1 to 3 μm Ｂ (pcs / mm ２ ) and satisfy equation (2).

[0078] Here, Fn2(=N Ａ / (N Ａ +N Ｂ )) represents the ratio of coarse Mn sulfides to the total of coarse Mn sulfides and fine Mn sulfides. The chemical composition described above, Fn1 exceeding 2.50, having a tensile strength of 900 to 1100 MPa, and the number density of Mn sulfides N Ｔ 0.300 pieces / mm ２ For steel materials that satisfy the following conditions, if Fn2 exceeds 0.40, excellent hydrogen embrittlement resistance cannot be obtained. On the other hand, for the above-mentioned steel materials, if Fn2 is 0.40 or less, the lower limit stress intensity factor K １Ｈ The value remains stable at 30 MPa·m １／２ As described above, it possesses excellent resistance to hydrogen embrittlement. Therefore, the steel material according to this embodiment has an Fn2 of 0.40 or less.

[0079] Furthermore, in the steel material according to this embodiment, the Mn content is reduced to 0.05 to 0.20%, resulting in a Mn sulfide number density N Ｔ 0.300 pieces / mm ２Reduce as follows. Here, usually, the number of Mn sulfides with a smaller major axis is more likely to be larger than that of Mn sulfides with a larger major axis. On the other hand, the steel material according to this embodiment reduces the Mn content to reduce the number density of Mn sulfides. In this case, the number of Mn sulfides with a smaller major axis is likely to decrease, and as a result, the number ratio of Mn sulfides with a larger major axis relatively increases. That is, the steel material according to this embodiment reduces the Mn content to reduce the number density N of Mn sulfides Ｔ to 0.300 pieces / mm ２ or less. As a result, it is considered that the influence on the hydrogen embrittlement resistance characteristics due to coarse Mn sulfides is more likely to be manifested.

[0080] Therefore, in the steel material according to this embodiment, the ratio (Fn2) of the number density N Ａ (pieces / mm ２ ) of coarse Mn sulfides to the total of the number density N Ａ (pieces / mm ２ ) of coarse Mn sulfides and the number density N Ｂ (pieces / mm ２ ) of fine Mn sulfides is reduced to 0.40 or less. 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 characteristics on the condition that other configurations of this embodiment are satisfied.

[0081] In this embodiment, the preferable upper limit of Fn2 is 0.39, more preferably 0.38, and still more preferably 0.37. In this embodiment, the lower limit of Fn2 is not particularly limited and may be 0.00. However, for a steel material having the above chemical composition, Fn1 exceeding 2.50, a tensile strength of 900 to 1100 MPa, and the number density N of Mn sulfides Ｔ being 0.300 pieces / mm ２ or less, the lower limit of Fn2 is, for example, 0.05. The lower limit of Fn2 may be 0.06 or 0.08.

[0082] In this embodiment, the number density N Ｔ (pieces / mm ２ ) of Mn sulfides, the number density N Ａ (pieces / mm ２ ) of coarse Mn sulfides, and the number density N of fine Mn sulfidesＢ (pcs / mm ２ ) and Fn2 are determined by the following method. Specifically, a test piece is prepared from the steel material according to this embodiment, with the observation surface being a surface that includes the rolling direction and the reduction direction. If the steel material is a steel plate, a test piece is prepared from the center of the plate thickness, with the observation surface being a surface that includes the rolling direction and the plate thickness direction. If the steel material is a steel pipe, a test piece is prepared from the center of the wall thickness, with the observation surface being a surface that includes the pipe axis direction and the wall thickness direction. If the steel material is a round steel bar, a test piece is prepared with the R / 2 position in the center, with the observation surface being a surface that includes the axial direction and the radial direction in the cross-section. The size of the test piece is not particularly limited, but for example, a test piece with an observation surface of 22 mm × 22 mm is used.

[0083] The observation surface of the prepared test specimen is polished to a mirror finish. From the polished observation surface, 200 mm ２ Observe the above areas. For example, set the area of ​​each field of view to 25 mm². ２ The microscope is set to 5 mm x 5 mm, with a magnification of 1000x, and eight fields of view are observed. On the observation surface, particles with a major axis of 1 μm or larger are identified based on contrast. The major axis of each particle can be determined by image analysis. Elemental concentration analysis (EDS analysis) is performed on the identified particles with a major axis of 1 μm or larger. In the EDS analysis, the acceleration voltage is set to 20 kV, and the target elements are quantified as N, O, Na, Mg, Al, Si, P, S, Cl, K, Ca, Ti, V, Mn, Cu, Zr, and Nb.

[0084] Based on the EDS analysis results of each particle, particles with mass percentages of 0-5% Na, 0-5% K, and 0-5% Cl are defined as specific particles. Furthermore, among the specific particles, those with a total Mn and S content of 70% or more are identified as Mn sulfides. In other words, in the steel material according to this embodiment, Mn sulfides are defined as particles with a major axis of 1 μm or more that satisfy the mass percentages of 70% or more total Mn and S, 0-5% Na, 0-5% K, and 0-5% Cl based on the EDS analysis results. There is no particular upper limit to the major axis of Mn sulfides, but for example, it is 20 μm.

[0085] In each field of view, the Mn sulfides identified by the above method are counted. Furthermore, using the major axis of each particle identified by the above method, the coarse Mn sulfides in each field of view are counted. Similarly, using the major axis of each particle identified by the above method, the fine Mn sulfides in each field of view are counted. Based on the total number of Mn sulfides identified in all fields of view and the total area of ​​all fields of view, the number density N of Mn sulfides is calculated. Ｔ (pcs / mm ２ The number density N of coarse Mn sulfides is calculated based on the total number of coarse Mn sulfides identified in all fields of view and the total area of ​​all fields of view. Ａ (pcs / mm ２ The number density N of the fine Mn sulfides is determined based on the total number of fine Mn sulfides identified in all fields of view and the total area of ​​all fields of view. Ｂ (pcs / mm ２ ) is determined. Also, the number density N of the obtained coarse Mn sulfide is calculated. Ａ (pcs / mm ２ ) and the number density N of fine Mn sulfides Ｂ (pcs / mm ２ From equation (2) and equation (2), we can derive Fn² (= N Ａ / (N Ａ +N Ｂ ))

[0086] In this embodiment, the number density of Mn sulfide is N Ｔ (pcs / mm ２ ) and the number density N of coarse Mn sulfides Ａ (pcs / mm ２ ) and the number density N of fine Mn sulfides Ｂ (pcs / mm ２ ) are both obtained by rounding the fourth decimal place of the obtained value. In this embodiment, Fn2 is obtained by rounding the third decimal place of the obtained value. Note that the number density of Mn sulfide is N Ｔ (pcs / mm ２ ) and the number density N of coarse Mn sulfides Ａ (pcs / mm ２ ) and the number density N of fine Mn sulfides Ｂ (pcs / mm ２This can be determined using a scanning electron microscope (SEM) equipped with a compositional analysis function (SEM-EDS instrument). For example, the Metals Quality Analyzer, an automated inclusion analyzer manufactured by FEI (ASPEX), can be used as an SEM-EDS instrument.

[0087] [Hydrogen Embrittlement Resistance] The steel material according to this embodiment has the above-described chemical composition, with Fn1 exceeding 2.50 and a number density of Mn sulfide N Ｔ 0.300 pieces / mm ２ The following conditions are met, the material has a tensile strength of 900 to 1100 MPa, and Fn2 is 0.40 or less. 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.

[0088] In this embodiment, the hydrogen embrittlement resistance was determined by a rising load test conducted in accordance with the High Pressure Gas Safety Association standard KHKS 0220 (2020) "Standards for Ultra-High Pressure Gas Equipment", using the lower limit stress intensity factor K. １Ｈ Value (MPa·m) １／２ The evaluation is based on the following. Specifically, rising load test specimens are prepared from the steel material according to this embodiment in accordance with KHKS 0220 (2020). More specifically, if the steel material is a steel plate, the rising load test specimen is prepared so that the surface of the test specimen is 2 mm from the surface of the steel plate in the thickness direction. In this case, the axial direction of the load application of the test specimen is also parallel to the width direction of the steel plate. If the steel material is a steel pipe, the rising load test specimen is prepared so that the surface of the test specimen is 2 mm from the inner surface of the steel pipe in the thickness direction. In this case, the axial direction of the load application of the test specimen is also perpendicular to the axial direction and diameter direction of the pipe. If the steel material is a round steel bar, the rising load test specimen is prepared so that the surface of the test specimen is 2 mm from the surface of the round steel bar in the diameter direction of the cross-section. In this case, the axial direction of the load application of the test specimen is also perpendicular to the axial direction and diameter direction of the round steel bar. The fatigue pre-crack in the rising load test specimen is introduced with the aim of reaching 3 mm.

[0089] The rising load test will be conducted at room temperature in both an atmospheric environment and a hydrogen gas environment at 90 MPa. The test speed will be 3 × 10⁻¹⁰ load line opening displacement speed. －４ The load is set to mm / sec, and the opening displacement is measured with a clip gauge. The lower limit stress intensity factor K is calculated using the load at the initiation point of hydrogen-induced crack propagation and the fatigue pre-crack length obtained from fracture surface measurements. １Ｈ Value (MPa·m) １／２ The lower limit stress intensity factor K is determined. The starting point of hydrogen-induced crack propagation is determined by superimposing the load-opening displacement curves of the atmospheric environment and the hydrogen gas environment. In this embodiment, the lower limit stress intensity factor K is determined. １Ｈ Value (MPa·m) １／２ The result is obtained by rounding the obtained number to the first decimal place.

[0090] In this embodiment, the lower limit stress intensity factor K obtained under the above conditions １Ｈ The value is 30 MPa·m １／２ If the above conditions are met, it is judged to possess excellent resistance to hydrogen embrittlement.

[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, the chemical composition is as described above, Fn1 is greater than 2.50, and the number density of Mn sulfide is N Ｔ 0.300 pieces / mm ２ If a steel material satisfies the following conditions, has a tensile strength of 900 to 1100 MPa, has an Fn2 of 0.40 or less, and possesses excellent hydrogen embrittlement resistance, then 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.

[0103] The obtained slab or ingot is subjected to hot forging to produce the material. In this material production process according to this embodiment, it is preferable to perform hot forging in two stages: a first hot forging and a second hot forging. Specifically, first, the slab or ingot is heated in a heating furnace. At this time, the heating temperature of the slab or ingot is preferably 1180 to 1250°C. If the heating temperature is too high, the Mn sulfide in the slab or ingot will coarseen, and the Fn2 in the manufactured steel may become too high. On the other hand, if the heating temperature is too low, the load on the equipment used for hot forging will increase. Therefore, in this material production process according to this embodiment, it is preferable to set the heating temperature in the first hot forging to 1180 to 1250°C.

[0104] A first hot forging is performed on the heated slab or ingot. The maximum reduction ratio R1 (%) in the first hot forging is preferably 15 to 45%. Here, the maximum reduction ratio R1 (%) in the first hot forging is defined by the following formula (A): Maximum reduction ratio R1 = 100 × {1 - (cross-sectional area of ​​the surface perpendicular to the direction E1 of the material after the first hot forging / cross-sectional area of ​​the surface perpendicular to the direction E1 of the material before the first hot forging)} (A) Here, "direction E1 of the material" in formula (A) means the direction in which the material is stretched the most by the first hot forging. In other words, the maximum reduction ratio R1 means the reduction ratio obtained by focusing on the surface in which the cross-sectional area of ​​the material is reduced the most before and after the first hot forging.

[0105] If the maximum reduction ratio R1 of the first hot forging is 15% or more, the Mn sulfides in the slab or ingot will be fragmented. As a result, the major axis of the Mn sulfides in the slab or ingot will be shortened, and the amount of Fn2 in the manufactured steel can be reduced. On the other hand, if the maximum reduction ratio R1 of the first hot forging is too high, the load on the equipment performing the hot forging will increase. Therefore, in the material manufacturing process according to this embodiment, it is preferable to set the maximum reduction ratio R1 of the first hot forging to 15 to 45%.

[0106] A second hot forging is performed on a slab or ingot that has undergone a first hot forging. Specifically, the slab or ingot that has undergone the first hot forging is heated again in a heating furnace. Here, it is preferable that the heating temperature of the slab or ingot in the second hot forging is also 1180 to 1250°C. As mentioned above, if the heating temperature is too high, the Mn sulfides in the slab or ingot will coarseen, and the Fn2 may become too high in the manufactured steel. On the other hand, if the heating temperature in the second hot forging is 1180 to 1250°C, the Mn sulfides that were stretched by the first hot forging may be broken up into multiple fine Mn sulfides. As a result, the Fn2 can be stably reduced in the manufactured steel. The inventors speculate that this is because the fine Mn sulfides after breaking have a lower interfacial energy compared to the stretched Mn sulfides. Therefore, in the material manufacturing process according to this embodiment, it is preferable that the heating temperature for the second hot forging is also 1180 to 1250°C.

[0107] A second hot forging is performed on the heated cast slab or steel ingot. The maximum reduction ratio R2 (%) in the second hot forging is preferably 15 to 45%. Here, the maximum reduction ratio R2 (%) in the second hot forging is defined by the following formula (B): Maximum reduction ratio R2 = 100 × {1 - (cross-sectional area of ​​the surface perpendicular to the direction E2 of the material after the second hot forging / cross-sectional area of ​​the surface perpendicular to the direction E2 of the material before the second hot forging)} (B) Here, "direction E2 of the material" in formula (B) means the direction in which the material is stretched the most by the second hot forging. In other words, the maximum reduction ratio R2 means the reduction ratio obtained by focusing on the surface in which the cross-sectional area of ​​the material is reduced the most before and after the second hot forging.

[0108] If the maximum reduction ratio R2 of the second hot forging is 15% or more, the Mn sulfides in the slab or ingot will be fragmented. As a result, the major axis of the Mn sulfides in the slab or ingot will be shortened, and the amount of Fn2 in the manufactured steel can be reduced. On the other hand, if the maximum reduction ratio R2 of the second hot forging is too high, the load on the equipment performing the hot forging will increase. Therefore, in the material manufacturing process according to this embodiment, it is preferable to set the maximum reduction ratio R2 of the second hot forging to 15 to 45%.

[0109] In this embodiment, it is preferable that the maximum reduction ratio R (%) in the entire hot forging process be 50% or more. Here, the maximum reduction ratio R in the entire hot forging process is defined by the following formula (C): Maximum reduction ratio R = 100 × {1 - (cross-sectional area of ​​the surface perpendicular to the direction E of the material after the second hot forging / cross-sectional area of ​​the surface perpendicular to the direction E of the material before the first hot forging process)} (C) Here, "direction E of the material" in formula (C) means the direction in which the material is stretched the most by the two hot forging processes. That is, the maximum reduction ratio R means the reduction ratio obtained by focusing on the surface in which the cross-sectional area of ​​the material is reduced the most before and after the two hot forging processes.

[0110] If the maximum reduction ratio R for the entire hot forging process is 50% or more, the Mn sulfides in the slab or ingot are sufficiently fragmented. As a result, the major axis of the Mn sulfides in the slab or ingot is shortened, and the amount of Fn2 in the manufactured steel can be reduced. Therefore, in the material manufacturing process according to this embodiment, it is preferable to set the maximum reduction ratio R for the entire hot forging process to 50% or more.

[0111] 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 that heating at 1180 to 1250°C is performed between the first and second bract rolling processes. The material (slab, bloom, or billet) is manufactured by the above process. The hot working process will now be described.

[0112] [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.

[0113] 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.

[0114] 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.

[0115] 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.

[0116] [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.

[0117] [Quenching Process] In the quenching process, the intermediate steel material is subjected to quenching. The quenching is carried out by a well-known method. In this specification, "quenching" refers to A ３ This means rapidly cooling intermediate steel materials that are above their 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 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.

[0118] The preferred holding temperature is A Ｃ３ The transformation point is ~1000°C. If the holding temperature is too high, the prior γ grains may coarseen, and excellent hydrogen embrittlement resistance may not be obtained. On the other hand, if the holding temperature is too low, the desired microstructure may not be obtained, and the desired mechanical properties may not be obtained. Therefore, the holding temperature in quenching is A Ｃ３ The temperature is preferably between the transformation point and 1000°C. The holding time is not particularly limited, but for example, it is 10 to 60 minutes.

[0119] 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.

[0120] [Tempering Process] In the tempering process, the intermediate steel material that has undergone the above-described 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 holding temperature in tempering corresponds to the temperature of the heat treatment furnace when the intermediate steel material is heated and held after quenching. The holding time in tempering 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.

[0121] The holding temperature during tempering is adjusted appropriately according to the chemical composition of the steel and the tensile strength to be obtained. In other words, for an intermediate steel having the chemical composition of this embodiment, the holding temperature is adjusted to adjust the tensile strength of the steel to 900 to 1100 MPa. In the tempering process according to this embodiment, the preferred holding temperature is 660°C to A ｃ１ It is below the transformation point. Furthermore, in the tempering process of this embodiment, the holding time is preferably 5 to 240 minutes.

[0122] 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.

[0123] Molten steel having the chemical compositions shown in Tables 1A and 1B was produced. In Table 1B, "-" indicates that the content of each element is at the impurity level. Specifically, the Cu, Ni, Co, and W content in test number 1 was 0%, rounded to the third decimal place. The Mg, Ca, REM, and Zr content in test number 1 was 0%, rounded to the fifth decimal place.

[0124]

[0125]

[0126] 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 under the conditions listed in Table 2. Specifically, after heating the ingot at the heating temperature listed in Table 2, the first hot forging was performed with the maximum reduction ratio R1 listed in Table 2. Furthermore, after reheating at the heating temperature listed in Table 2, the second hot forging was performed with the maximum reduction ratio R2 listed in Table 2. At this time, the maximum reduction ratio R for the entire two hot forgings was as shown in Table 2. Block materials were manufactured under the above manufacturing conditions.

[0127]

[0128] 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). The intermediate steel material produced by the above method was allowed to cool to room temperature. Furthermore, quenching was performed on the intermediate steel material for each test number by holding it at the holding temperature (°C) and holding time (minutes) shown in the "Quenching" column of Table 2, followed by water cooling. After quenching, tempering was performed on the intermediate steel material for each test number by holding it at the holding temperature (°C) and holding time (minutes) shown in the "Tempering" column of Table 2. The steel material (steel plate) for each test number was produced by the above manufacturing process. For the steel plates of each test number produced, the elemental content and B_eff calculated from formula (3) or (4) are shown in Table 3. Furthermore, Table 3 shows the elemental content for each test number obtained, B_eff, and Fn1 (= 0.8Cr + Mn + αMo) calculated from equation (1).

[0129]

[0130] [Evaluation Tests] Tensile tests, Mn sulfide number density tests, and rising load tests were performed on each steel plate with the specified test number.

[0131] [Tensile Test] 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. 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 56.0 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 performed 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. For each test number of steel material, the obtained tensile strength is denoted as "TS (MPa)" and the yield strength (MPa) is denoted as "YS (MPa)," as shown in Table 3.

[0132] [Number Density Measurement Test of Mn Sulfides] For each steel plate with the specified test number, the number density of Mn sulfides N was measured using the method described above. Ｔ (pcs / mm ２ ), number density N of coarse Mn sulfides Ａ (pcs / mm ２ ), and the number density N of fine Mn sulfides Ｂ (pcs / mm ２ The following was determined. Specifically, test specimens were prepared from the steel plates of each test number, with the observation surface being a surface that included both the rolling direction and the thickness direction (reduction direction). Eight 5 mm x 5 mm observation fields were identified on the mirror-polished observation surface at a magnification of 1000x. Particles with a major axis of 1 μm or larger were identified from the contrast, and elemental concentration analysis was performed by EDS. The conditions for the EDS analysis were as described above.

[0133] Based on EDS analysis, particles satisfying the following mass percentages were defined as specific particles: Na 0-5%, K 0-5%, and Cl 0-5%. Among the specific particles, those satisfying the following mass percentages of Mn and S totaling 70% or more and having a major axis of 1 μm or more were identified as Mn sulfides. Among the specific particles, those satisfying the following mass percentages of Mn and S totaling 70% or more and having a major axis of 7-20 μm were identified as coarse Mn sulfides. Among the specific particles, those satisfying the following mass percentages of Mn and S totaling 70% or more and having a major axis of 1-3 μm were identified as fine Mn sulfides.

[0134] The number of identified Mn sulfides is counted and divided by the total area of ​​the observation field to determine the number density N of the Mn sulfides. Ｔ (pcs / mm ２ Similarly, the number density N of the identified coarse Mn sulfides was calculated by counting them and dividing by the total area of ​​the observation field. Ａ (pcs / mm ２ Similarly, the number density N of the identified fine Mn sulfides was calculated by counting them and dividing by the total area of ​​the observation field. Ｂ (pcs / mm ２ ) was determined. Furthermore, the number density N of coarse Mn sulfide was calculated. Ａ (pcs / mm ２ ) and the number density N of fine Mn sulfides Ｂ (pcs / mm ２From equation (2) and equation (2), we can derive Fn² (= N Ａ / (N Ａ +N Ｂ The number density N of the obtained Mn sulfide was calculated. Ｔ (pcs / mm ２ ) and the number density N of coarse Mn sulfides Ａ (pcs / mm ２ ) and the number density N of fine Mn sulfides Ｂ (pcs / mm ２ Table 3 shows the values ​​of ) and Fn2.

[0135] [Rising Load Test] Rising load tests were performed on the steel plates of each test number using the method described above to evaluate their hydrogen embrittlement resistance. Specifically, rising load test specimens were prepared from the steel plates of each test number in accordance with KHKS 0220 (2020). Specifically, the surface of the test specimen was prepared so that it was located 2 mm from the surface of the steel plate in the thickness direction. In this case, the load application axis direction of the test specimen was also made parallel to the width direction of the steel plate. A 3 mm fatigue precrack was introduced into the prepared rising load test specimens.

[0136] For each test numbered rising load test specimen, a rising load test was performed at room temperature in both an atmospheric environment and a hydrogen gas environment at 90 MPa, in accordance with KHKS 0220 (2020). The test speed was 3 × 10⁻¹⁰ load line opening displacement speed. －４ The load was set to mm / sec, and the opening displacement was measured with a clip gauge. The lower limit stress intensity factor K was calculated using the load at the initiation point of hydrogen-induced crack propagation and the fatigue pre-crack length obtained from fracture surface measurements. １Ｈ Value (MPa·m) １／２ The lower limit stress intensity factor K was calculated. １Ｈ Value (MPa·m) １／２ Table 3 shows the results.

[0137] [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, with Fn1 exceeding 2.50, a tensile strength TS of 900 to 1100 MPa, and a number density of Mn sulfide N Ｔ 0.300 pieces / mm ２The following conditions were met and Fn2 was 0.40 or less. As a result, these steel plates had a lower limit stress intensity factor K １Ｈ The value is 30 MPa·m １／２ The above results indicate that the steels possessed excellent resistance to hydrogen embrittlement. In other words, these steels had high strength and excellent resistance to hydrogen embrittlement. Furthermore, it was determined that the total area ratio of tempered martensite and tempered bainite in the microstructure of steels numbered 1 to 11 was 90% or more.

[0138] On the other hand, the steel plate of test number 12 had an Fn1 of 2.50 or less. As a result, this steel plate had a lower limit stress intensity factor K １Ｈ The value is 30 MPa·m １／２ The result was less than [a certain value], and it did not possess excellent hydrogen embrittlement resistance.

[0139] The steel plate in test number 13 had too low a carbon content, and its Fn1 was less than 2.50. As a result, this steel plate had a lower limit stress intensity factor K １Ｈ The value is 30 MPa·m １／２ The result was less than [a certain value], and it did not possess excellent hydrogen embrittlement resistance.

[0140] The steel plates in test numbers 14 and 15 had too high a Mn content. As a result, these steel plates had a high number density of Mn sulfides. Ｔ 0.300 pieces / mm ２ It exceeded this. As a result, these steel plates have a lower limit stress intensity factor K １Ｈ The value is 30 MPa·m １／２ The result was less than [a certain value], and it did not possess excellent hydrogen embrittlement resistance.

[0141] The steel plates of test numbers 16 and 17 had too small a maximum reduction ratio R throughout the hot forging process. As a result, these steel plates had an Fn2 of over 0.40. Consequently, these steel plates had a lower limit stress intensity factor K １Ｈ The value is 30 MPa·m １／２ The result was less than [a certain value], and it did not possess excellent hydrogen embrittlement resistance.

[0142] The steel plates in test numbers 18 and 19 were heated to too high a temperature during hot forging, and a second hot forging was not performed. As a result, the Fn2 of these steel plates exceeded 0.40. Consequently, the lower limit stress intensity factor K of these steel plates１Ｈ The value is 30 MPa·m １／２ The result was less than [a certain value], and it did not possess excellent hydrogen embrittlement resistance.

[0143] The steel plates in test numbers 20 and 21 were heated to too high a temperature during hot forging. As a result, the Fn2 of these steel plates exceeded 0.40. Consequently, the lower limit stress intensity factor K of these steel plates was １Ｈ The value is 30 MPa·m １／２ The result was less than [a certain value], and it did not possess excellent hydrogen embrittlement resistance.

[0144] 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. A steel material, the chemical composition of which, in mass %, is as follows: C: 0.15 to 0.40%, Si: 0.10 to 0.50%, Mn: 0.05 to 0.20%, P: 0.050% or less, S: 0.0100% or less, Cr: 0.50 to 1.50%, Mo: 0.80 to 2.00%, Ti: 0.002 to 0.010%, V: 0.08 to 0.30%, Nb: 0.010 to 0.050%, Al: 0.001 to 0.100%, B: 0.0001 to 0.0050%, N: 0.0100% or less, O: 0.0100% or less, 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 balance: consisting of Fe and impurities, satisfying formula (1), having a tensile strength of 900 to 1100 MPa, and in the steel material, the number density N of Mn sulfides Ｂ , Ａ , Ａ , Ｂ , ２ is 0.300 pieces / mm ２ or less, and among the Mn sulfides, the number density N of Mn sulfides having a major axis of 7 μm or more Ａ pieces / mm ２ and the number density N of Mn sulfides having a major axis of 1 to 3 μm Ｂ pieces / mm ２ satisfy formula (2). A steel material. 0.8Cr + Mn + αMo > 2.50 (1) N Ａ / (N Ａ + N Ｂ ) ≤ 0.40 (2) Here, in the element symbols in formula (1), the contents of the corresponding elements are substituted in units of mass %. α in formula (1) is 2 when B_eff defined by the following formulas (3) and (4) is 0.0005 or more, and α is 1 when the B_eff is less than 0.0005. When N - 0.293Ti ≥ 0: B_eff = B - 0.772(N - 0.293Ti) (3) When N - 0.293Ti < 0: B_eff = B (4) Here, in the element symbols in formulas (3) and (4), the contents of the corresponding elements are substituted in units of mass %.

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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