Steel material

Abstract

Provided is a steel material which has high strength and excellent hydrogen embrittlement resistance. A steel material according to the present disclosure has a chemical composition comprising, in terms of mass%, 0.25-0.30% of C, 0.10-0.50% of Si, 0.05-0.60% of Mn, not more than 0.050% of P, not more than 0.0100% of S, 0.001-0.100% of Al, 0.50-1.50% of Cr, 0.80-2.00% of Mo, 0.002-0.010% of Ti, 0.08-0.30% of V, 0.010-0.050% of Nb, 0.0001-0.0050% of B, not more than 0.0100% of N, and not more than 0.0100% of O, with the remainder being Fe and impurities. The steel material has a tensile strength of 900-1,100 MPa, a prior austenite grain size of not more than 20.0 μm, a dislocation density of not more than 7.0×1014m-2, and a Q value, obtained from an X-ray diffraction line profile, of not less than 2.35.

Description

steel material

[0001] The present disclosure relates to steel products, and more particularly to steel products used in sour environments and steel products utilized in high-pressure hydrogen vessels.

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

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

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

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

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

[0007] In Patent Document 2, in an oil well steel pipe made of low alloy steel, a Mo segregated region is formed to improve hydrogen embrittlement resistance.

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

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

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

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

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

[0013] The steel material according to the present disclosure has a chemical composition, in mass%, of C: 0.25 to 0.30%, Si: 0.10 to 0.50%, Mn: 0.05 to 0.60%, P: 0.050% or less, S: 0.0100% or less, Al: 0.001 to 0.100%, 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%, 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%, W: 0 to 0.50%, Co: 0 to 0.50%, Cu: 0 to 0.50%, Ni: 0 to 0.50%, Sn: 0 to 0.0100%, and the balance being Fe and impurities, the tensile strength is 900 to 1100 MPa, the grain size of the prior austenite grains is 20.0 μm or less, and the dislocation density is 7.0 × 10 14 m -2 or less, and the Q value obtained by analyzing the line profile of X-ray diffraction of the steel material by the modified Williamson-Hall / Warren-Averbach method is 2.35 or more.

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

[0015] FIG. 1 is a diagram showing the relationship between the Q value and the relative fracture stress ratio, which is an index of hydrogen embrittlement resistance, in this example.

[0016] The present inventors first investigated the possibility of obtaining a high-strength steel material having a tensile strength of 900 to 1100 MPa, assuming use in sour environments and for high-pressure hydrogen containers. That is, the present inventors investigated and examined methods for improving the hydrogen embrittlement resistance of steel materials intended for use in sour environments and for high-pressure hydrogen containers, even if the tensile strength is 900 to 1100 MPa. As a result, the present inventors obtained the following findings.

[0017] Next, the inventors focused on the chemical composition and investigated steel materials that combine high strength with hydrogen embrittlement resistance. As a result, the following was found to be the composition, in mass%, of steel: C: 0.25-0.30%, Si: 0.10-0.50%, Mn: 0.05-0.60%, P: 0.050% or less, S: 0.0100% or less, Al: 0.001-0.100%, 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%, B: 0.0001-0.0050%, N: 0.0100% It was thought that a steel material having a chemical composition consisting of O: 0.0100% or less, Mg: 0 to 0.0050%, Ca: 0 to 0.0050%, rare earth elements: 0 to 0.0050%, W: 0 to 0.50%, Co: 0 to 0.50%, Cu: 0 to 0.50%, Ni: 0 to 0.50%, Sn: 0 to 0.0100%, and the balance being Fe and impurities, could potentially achieve a tensile strength of 900 to 1100 MPa and excellent hydrogen embrittlement resistance.

[0018] Furthermore, the inventors have focused on the microstructure of the steel material and studied steel materials that combine high strength with hydrogen embrittlement resistance. Here, hydrogen embrittlement is likely to occur when hydrogen accumulates at grain boundaries. In a steel material having the above-mentioned chemical composition, if the prior austenite grains (hereinafter, the prior austenite grains are also referred to as "prior γ grains") are fine, the area of ​​the prior γ grain boundaries increases. In this case, even if the amount of hydrogen absorbed in the steel material is the same, the amount of hydrogen accumulated per unit area of ​​the prior γ grain boundaries decreases. Therefore, if the prior γ grains are fine, the hydrogen embrittlement resistance of the steel material can be improved.

[0019] Therefore, the present inventors considered refining the prior γ grains in a steel material to improve the hydrogen embrittlement resistance of the steel material while maintaining a tensile strength of 900 to 1100 MPa. As a result, it was found that if the grain size of the prior γ grains in the steel material is 20.0 μm or less, excellent hydrogen embrittlement resistance can be improved while maintaining a tensile strength of 900 to 1100 MPa, provided that the other configurations of this embodiment are satisfied. Hereinafter, in this specification, the grain size of prior austenite grains will also be referred to as the "prior γ grain size."

[0020] Furthermore, the inventors have focused on dislocations and investigated ways to improve the hydrogen embrittlement resistance of steel materials having the above-mentioned chemical composition. Here, increasing the dislocation density in a steel material increases the strength of the steel material. However, dislocations have the potential to absorb hydrogen. Therefore, if the dislocation density in a steel material increases, the amount of hydrogen absorbed by the steel material also increases, raising concerns that the hydrogen embrittlement resistance of the steel material may be reduced.

[0021] Therefore, the present inventors have considered reducing the dislocation density in the steel material to improve the hydrogen embrittlement resistance of the steel material while maintaining a tensile strength of 900 to 1100 MPa. As a result, the dislocation density of the steel material was reduced to 7.0 × 10 14 m -2 It has been found that if the content of Cr is reduced to the following range, the hydrogen embrittlement resistance of the steel material can be improved while maintaining a tensile strength of 900 to 1100 MPa, provided that the other configurations of this embodiment are satisfied.

[0022] On the other hand, if the alloy has the above-mentioned chemical composition, the prior γ grain size satisfies 20.0 μm or less, and the dislocation density is 7.0×10 14 m -2 Even if a steel material satisfies the following requirements, a steel material having a tensile strength of 900 to 1100 MPa may not be able to obtain excellent hydrogen embrittlement resistance. Therefore, the present inventors have conducted further detailed studies on methods for improving hydrogen embrittlement resistance.

[0023] Specifically, the inventors have focused on dislocation components and investigated methods for improving the hydrogen embrittlement resistance of steel materials. As a result of detailed investigations by the inventors, it has become clear that the higher the Q value obtained by analyzing an X-ray diffraction (XRD) line profile using the modified Williamson-Hall / Warren-Averbach method, the more significantly the hydrogen embrittlement resistance of the steel material improves. In this specification, the XRD line profile refers to the intensity curve of diffracted X-rays generated by XRD.

[0024] The Q value obtained by analyzing the XRD line profile is an index of the dislocation component. Here, the dislocation components include edge dislocations and screw dislocations. An edge dislocation is a defect in which an atomic plane is removed from the dislocation line, and the dislocation line and the Burgers vector are perpendicular. A screw dislocation is a defect in which an atomic plane is shifted around the dislocation line, and the dislocation line and the Burgers vector are parallel. Furthermore, the higher the Q value, the greater the proportion of screw dislocations in dislocations.

[0025] As a result of detailed investigations by the present inventors based on the above findings, it was found that the alloy having the above chemical composition, a tensile strength of 900 to 1100 MPa, a prior γ grain size of 20.0 μm or less, and a dislocation density of 7.0 × 10 14 m -2 It has been revealed that the hydrogen embrittlement resistance of the following steel materials can be significantly improved if the Q value is 2.35 or more. The relationship between the Q value and the hydrogen embrittlement resistance will be specifically explained below with reference to the drawings.

[0026] Fig. 1 is a diagram showing the relationship between the Q value and the relative fracture stress ratio, which is an index of hydrogen embrittlement resistance, in this example. Fig. 1 shows the relationship between the Q value and the relative fracture stress ratio, which is an index of hydrogen embrittlement resistance, in the examples described later. The examples have the above-mentioned chemical composition, the tensile strength is 900 to 1100 MPa, the prior γ grain size is 20.0 μm or less, and the dislocation density is 7.0 × 10 14 m -2 The following examples were used to prepare the steel sheets: The tensile strength, prior γ grain size, dislocation density, and Q value were determined by the methods described below.

[0027] Referring to FIG. 1, the alloy has the above-mentioned chemical composition, a tensile strength of 900 to 1100 MPa, a prior γ grain size of 20.0 μm or less, and a dislocation density of 7.0×10 14 m -2 In the following steel material, if the Q value is 2.35 or more, the relative fracture stress ratio is 0.80 or more, and it can be confirmed that the steel material exhibits stable and excellent hydrogen embrittlement resistance. Therefore, the steel material according to this embodiment has the above-mentioned chemical composition, a tensile strength of 900 to 1100 MPa, a prior γ grain size of 20.0 μm or less, and a dislocation density of 7.0 × 10 14 m -2or less, and furthermore, the Q value is set to 2.35 or more. As a result, the steel material according to this embodiment can achieve both high strength and excellent hydrogen embrittlement resistance.

[0028] The reason why the hydrogen embrittlement resistance of a steel material is improved by satisfying a Q value of 2.35 or more is not clear in detail. However, when the steel material has the above-mentioned chemical composition, the prior γ grain size is 20.0 μm or less, and the dislocation density is 7.0 × 10 14 m -2 It is proven by the examples described below that the hydrogen embrittlement resistance of the following steel materials is improved when the Q value is 2.35 or more.

[0029] The gist of the steel material according to this embodiment, which was completed based on the above findings, is as follows.

[0030] [1] A steel material having a chemical composition, in mass%, of C: 0.25 to 0.30%, Si: 0.10 to 0.50%, Mn: 0.05 to 0.60%, P: 0.050% or less, S: 0.0100% or less, Al: 0.001 to 0.100%, 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%, 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%, W: 0 to 0.50%, Co: 0 to 0.50%, Cu: 0 to 0.50%, Ni: 0 to 0.50%, Sn: 0 to 0.0100%, and the balance being Fe and impurities, the tensile strength is 900 to 1100 MPa, the grain size of the prior austenite grains is 20.0 μm or less, and the dislocation density is 7.0 × 10 14 m -2 or less, and a Q value obtained by analyzing an X-ray diffraction line profile of the steel material by a modified Williamson-Hall / Warren-Averbach method is 2.35 or more.

[0031] [2] The steel material according to [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%, W: 0.01 to 0.50%, Co: 0.01 to 0.50%, Cu: 0.01 to 0.50%, Ni: 0.01 to 0.50%, and Sn: 0.0001 to 0.0100%.

[0032] [3] The steel material according to [1] or [2], wherein the steel material is any one of a steel pipe for oil wells, a steel pipe for line pipes, and a steel pipe for high-pressure hydrogen containers.

[0033] [4] The steel material according to [3], wherein the steel pipe for a 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.

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

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

[0036] In this specification, "steel pipe for line pipe" means steel pipe for use as a line pipe constituting a pipeline for transporting produced fluids (crude oil or natural gas) extracted from oil or gas wells. Examples of pipelines include flow lines that transport produced fluids from oil or gas wells, gathering lines that collect the produced fluids transported along the flow lines and transport them to primary treatment facilities, trunk lines that transport produced fluids that have undergone primary treatment such as dehydration to the vicinity of markets, and distribution lines that transport them to consumers. "Seamless steel pipe for line pipe" means that the steel pipe for line pipe is a seamless steel pipe.

[0037] In this specification, "steel pipe for high-pressure hydrogen containers" means steel pipe that is standardized by ISO 11439, ANSI / NGV, the High-Pressure Gas Safety Act, Illustrative Standards of Container Safety Regulations, etc. and is used for high-pressure hydrogen containers that store high-pressure hydrogen gas. Examples of high-pressure hydrogen containers are high-pressure hydrogen accumulators installed at hydrogen stations and high-pressure hydrogen cylinders installed in fuel cell vehicles. "Seamless steel pipe for high-pressure hydrogen containers" means that the steel pipe for high-pressure hydrogen containers is a seamless steel pipe.

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

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

[0040] C: 0.25 to 0.30% Carbon (C) improves hardenability and causes the microstructure of the steel material to be mainly tempered martensite and tempered bainite. As a result, the hydrogen embrittlement resistance of the steel material is improved. C also forms carbides or carbonitrides, increasing the strength of the steel material. If the C content is too low, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the C content is too high, even if the contents of other elements are within the ranges of this embodiment, the amount of carbides in the steel material will be excessively large, and the hydrogen embrittlement resistance of the steel material will be reduced. Therefore, the C content is 0.25 to 0.30%. A preferable lower limit of the C content is 0.26%. A preferable upper limit of the C content is 0.29%.

[0041] Si: 0.10 to 0.50% Silicon (Si) deoxidizes steel and reduces inclusions in the steel. As a result, the hydrogen embrittlement resistance of the steel is improved. If the Si content is too low, the above effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Si content is too high, the hydrogen embrittlement resistance of the steel is reduced even if the contents of other elements are within the ranges of this embodiment. Therefore, the Si content is 0.10 to 0.50%. The preferred lower limit of the Si content is 0.12%, more preferably 0.13%, and even more preferably 0.15%. The preferred upper limit of the Si content is 0.49%, more preferably 0.48%.

[0042] Mn: 0.05 to 0.60% Manganese (Mn) deoxidizes steel. Mn also improves hardenability and enhances the hydrogen embrittlement resistance of the steel. If the Mn content is too low, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Mn content is too high, coarse sulfide-based inclusions are formed, reducing the hydrogen embrittlement resistance of the steel, even if the contents of other elements are within the ranges of this embodiment. Therefore, the Mn content is 0.05 to 0.60%. A preferred lower limit of the Mn content is 0.06%, more preferably 0.08%. A preferred upper limit of the Mn content is 0.58%, more preferably 0.56%.

[0043] 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 contents of other elements are within the ranges of this embodiment, P segregates at grain boundaries, reducing the hydrogen embrittlement resistance of the steel. Therefore, the P content is 0.050% or less. The lower the P content, the better. 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%, and even more preferably 0.003%. The preferred upper limit of the P content is 0.049%, more preferably 0.047%, and even more preferably 0.045%.

[0044] 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 contents of other elements are within the ranges of this embodiment, S segregates at grain boundaries, reducing the hydrogen embrittlement resistance of the steel. Therefore, the S content is 0.0100% or less. The lower the S content, the better. 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 0.0006%. The preferred upper limit of the S content is 0.0098%, more preferably 0.0095%, and even more preferably 0.0090%.

[0045] Al: 0.001 to 0.100% Aluminum (Al) deoxidizes steel. Furthermore, Al bonds with N to form Al nitrides, which refine the crystal grains through a pinning effect and improve the hydrogen embrittlement resistance of the steel. If the Al content is too low, the above effect cannot be fully achieved, even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Al content is too high, coarse oxides are formed, reducing the hydrogen embrittlement resistance of the steel, even if the contents of other elements are within the ranges of this embodiment. 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 0.010%. The preferred upper limit of the Al content is 0.095%, more preferably 0.090%, and even more preferably 0.085%. Note that the "Al" content in this specification refers to the content of "acid-soluble Al," i.e., "sol. Al."

[0046] Cr: 0.50 to 1.50% Chromium (Cr) improves the hardenability of steel and enhances its hydrogen embrittlement resistance. Cr also increases the temper softening resistance of steel, enabling high-temperature tempering. As a result, the hydrogen embrittlement resistance of steel is improved. If the Cr content is too low, the above effects cannot be fully achieved, even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Cr content is too high, coarse carbides are formed, even if the contents of other elements are within the ranges of this embodiment, and the hydrogen embrittlement resistance of the steel is reduced. Therefore, the Cr content is 0.50 to 1.50%. The preferred lower limit of the Cr content is 0.51%, more preferably 0.52%, even more preferably 0.54%, 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 1.35%.

[0047] Mo: 0.80 to 2.00% Molybdenum (Mo) improves the hardenability of steel. Mo also increases the temper softening resistance of steel, enabling high-temperature tempering. As a result, the hydrogen embrittlement resistance of steel is improved. Mo also strengthens prior γ grain boundaries, improving the hydrogen embrittlement resistance of steel. If the Mo content is too low, the above effects cannot be fully achieved even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Mo content is too high, coarse carbides are formed, reducing the hydrogen embrittlement resistance of the steel, even if the contents of other elements are within the ranges of this embodiment. Therefore, the Mo content is 0.80 to 2.00%. The preferred lower limit of the Mo content is 0.83%, more preferably 0.86%, and even more preferably 0.90%. The preferred upper limit of the Mo content is 1.96%, more preferably 1.90%, and even more preferably 1.86%.

[0048] Ti: 0.002 to 0.010% Titanium (Ti) forms fine precipitates such as Ti nitrides, which refine prior γ grains through a pinning effect, thereby improving the hydrogen embrittlement resistance of the steel. If the Ti content is too low, the above effect cannot be fully achieved even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Ti content is too high, coarse Ti nitrides are formed even if the contents of other elements are within the ranges of this embodiment. The coarse Ti nitrides become crack initiation sites. As a result, the hydrogen embrittlement resistance of the steel is reduced. Therefore, the Ti content is 0.002 to 0.010%. The preferred lower limit of the Ti content is 0.003%, more preferably 0.004%. The preferred upper limit of the Ti content is 0.009%, more preferably 0.008%.

[0049] V: 0.08 to 0.30% Vanadium (V) forms carbides, nitrides, or carbonitrides (hereinafter referred to as "carbonitrides, etc.") and improves the hydrogen embrittlement resistance of the steel material. If the V content is too low, the above effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the V content is too high, even if the contents of other elements are within the ranges of this embodiment, excessive carbonitrides, etc. are formed, and the hydrogen embrittlement resistance of the steel material deteriorates. Therefore, the V content is 0.08 to 0.30%. A preferred lower limit of the V content is 0.09%, more preferably 0.10%. A preferred upper limit of the V content is 0.28%, more preferably 0.26%, and even more preferably 0.25%.

[0050] Nb: 0.010 to 0.050% Niobium (Nb) forms carbonitrides and the like, and refines prior γ grains through a pinning effect, thereby improving the hydrogen embrittlement resistance of the steel. Nb also forms fine carbides during tempering, improving the temper softening resistance of the steel and increasing its strength. If the Nb content is too low, the above effects cannot be fully achieved, even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Nb content is too high, even if the contents of other elements are within the ranges of this embodiment, excessive carbonitrides and the like are formed, which actually reduces the hydrogen embrittlement resistance of the steel. 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%, more preferably 0.045%.

[0051] B: 0.0001 to 0.0050% Boron (B) improves the hardenability of steel and increases its strength. Furthermore, B suppresses grain boundary segregation of P, thereby improving the hydrogen embrittlement resistance of the steel. If the B content is too low, the above effects cannot be fully achieved, even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the B content is too high, coarse B nitrides are formed, even if the contents of other elements are within the ranges of this embodiment. These coarse B nitrides become crack initiation sites. As a result, the hydrogen embrittlement resistance of the steel is reduced. Therefore, the B content is 0.0001 to 0.0050%. The preferred lower limit of the B content is 0.0003%, more preferably 0.0005%, and even more preferably 0.0008%. The preferred upper limit of the B content is 0.0048%, more preferably 0.0046%, and even more preferably 0.0045%.

[0052] N: 0.0100% or less Nitrogen (N) is unavoidably contained. In other words, the lower limit of the N content is greater than 0%. N combines with Ti to form nitrides, which refine prior γ grains through a pinning effect and improve the hydrogen embrittlement resistance of the steel. On the other hand, if the N content is too high, coarse nitrides are formed, even if the contents of other elements are within the ranges of this embodiment, and the hydrogen embrittlement resistance of the steel is reduced. Therefore, the N content is 0.0100% or less. To more effectively obtain the above effects, the lower limit of the N content is preferably 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%. The upper limit of the N content is preferably 0.0096%, more preferably 0.0095%, and even more preferably 0.0090%.

[0053] 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 contents of other elements are within the ranges of this embodiment, coarse oxides are formed, and the hydrogen embrittlement resistance of the steel material is reduced. Therefore, the O content is 0.0100% or less. The O content is preferably as low as possible. However, an extreme reduction in the O content significantly increases manufacturing costs. Therefore, considering industrial production, the preferred lower limit of the O content is 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%. The preferred upper limit of the O content is 0.0096%, more preferably 0.0090%, and even more preferably 0.0080%.

[0054] The balance of the chemical composition of the steel material according to the present embodiment is composed of Fe and impurities. Here, the impurities in the chemical composition refer to substances that are mixed in from raw materials such as ore or scrap or the manufacturing environment during industrial production of the steel material, but are not intentionally contained and are allowed within a range that does not adversely affect the steel material according to the present embodiment.

[0055] [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. All of these elements are optional elements and may not be contained. When contained, Mg, Ca, and rare earth elements (REM) improve the hydrogen embrittlement resistance of the steel material.

[0056] Mg: 0 to 0.0050% Magnesium (Mg) is an optional element and does not necessarily need to be contained. In other words, the Mg content may be 0%. When contained, Mg combines with S in the steel and precipitates as fine Mg sulfides. Concomitantly, Mg reduces Mn sulfides. Both of these effects improve the hydrogen embrittlement resistance of the steel. Even if even a small amount of Mg is contained, the above effect can be achieved to some extent. However, if the Mg content is too high, even if the contents of other elements are within the ranges of this embodiment, oxides in the steel will coarsen, reducing the hydrogen embrittlement resistance of the steel. Therefore, the Mg content is 0 to 0.0050%. The preferred lower limit of the Mg content is 0.0001%, more preferably 0.0003%, even more preferably 0.0006%, and even more preferably 0.0010%. The preferred upper limit of the Mg content is 0.0045%, more preferably 0.0040%.

[0057] Ca: 0 to 0.0050% Calcium (Ca) is an optional element and does not necessarily need to be contained. In other words, the Ca content may be 0%. When contained, Ca bonds with S in the steel and precipitates as fine Ca sulfides. Concomitantly, Ca reduces Mn sulfides. Both of these effects improve the hydrogen embrittlement resistance of the steel. Even if even a small amount of Ca is contained, the above effect can be achieved to some extent. However, if the Ca content is too high, even if the contents of other elements are within the ranges of this embodiment, the oxides in the steel will coarsen, 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 0.0003%, and even more preferably 0.0005%. The preferred upper limit of the Ca content is 0.0045%, more preferably 0.0040%.

[0058] Rare earth elements (REM): 0 to 0.0050% Rare earth elements (REM) are optional elements and do not necessarily need to be contained. In other words, the REM content may be 0%. When contained, REM combines with S in the steel and precipitates as fine REM sulfides. Concomitantly, REM reduces Mn sulfides. Both of these effects improve the hydrogen embrittlement resistance of the steel. Even if even a small amount of REM is contained, the above effect can be achieved to some extent. However, if the REM content is too high, even if the contents of other elements are within the ranges of this embodiment, the oxides in the steel will coarsen, 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 0.0003%, even more preferably 0.0005%, and even more preferably 0.0010%. The upper limit of the REM content is preferably 0.0045%, and more preferably 0.0040%.

[0059] In this specification, REM refers to one or more elements selected from the group consisting of scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and the lanthanoids lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. In addition, in this specification, the REM content refers to the total content of these elements.

[0060] 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 Co, instead of a portion of Fe. These elements are optional elements and may not be contained. When contained, W and Co improve the hydrogen embrittlement resistance of the steel material.

[0061] W: 0 to 0.50% Tungsten (W) is an optional element and does not necessarily need to be contained. That is, the W content may be 0%. When W is contained, W forms a corrosion film on the surface of the steel in a sour environment. As a result, hydrogen penetration into the steel is suppressed, and the hydrogen embrittlement resistance of the steel is improved. Even if even a small amount of W is contained, the above effect can be obtained to some extent. However, if the W content is too high, even if the contents of other elements are within the ranges of this embodiment, coarse carbides are formed, and the hydrogen embrittlement resistance of the steel is reduced. Therefore, the W content is 0 to 0.50%. The preferred lower limit of the W content is 0.01%, more preferably 0.03%. The preferred upper limit of the W content is 0.48%, more preferably 0.45%.

[0062] Co: 0 to 0.50% Cobalt (Co) is an optional element and does not necessarily need to be contained. In other words, the Co content may be 0%. When contained, Co improves the hydrogen embrittlement resistance of the steel. Co also dissolves in the steel to improve the hardenability and strength of the steel. Even if even a small amount of Co is contained, the above effects can be obtained to some extent. However, if the Co content is too high, the effect saturates. Therefore, the Co content is 0 to 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%.

[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 Cu, Ni, and Sn, instead of a portion of Fe. These elements are optional elements and may not be contained. When contained, Cu, Ni, and Sn all improve the hydrogen embrittlement resistance of the steel material.

[0064] Cu: 0 to 0.50% Copper (Cu) is an optional element and does not necessarily need to be contained. In other words, the Cu content may be 0%. When contained, Cu improves the hydrogen embrittlement resistance of the steel material. Cu also dissolves in the steel material to improve the hardenability and strength of the steel material. Even if even a small amount of Cu is contained, the above effects can be obtained to some extent. However, if the Cu content is too high, the hot workability of the steel material will deteriorate even if the contents of other elements are within the ranges of this embodiment. Therefore, the Cu content is 0 to 0.50%. The preferred lower limit of the Cu content is 0.01%, more preferably 0.03%. The preferred upper limit of the Cu content is 0.48%, more preferably 0.45%.

[0065] Ni: 0 to 0.50% Nickel (Ni) is an optional element and does not necessarily need to be contained. In other words, the Ni content may be 0%. When contained, Ni improves the hydrogen embrittlement resistance of the steel. Ni also dissolves in the steel to improve the hardenability and strength of the steel. Even if even a small amount of Ni is contained, the above effects can be obtained to some extent. However, if the Ni content is too high, the manufacturing cost will increase dramatically even if the contents of other elements are within the ranges of this embodiment. Therefore, the Ni content is 0 to 0.50%. The preferred lower limit of the Ni content is 0.01%, more preferably 0.03%. The preferred upper limit of the Ni content is 0.49%, more preferably 0.48%, and even more preferably 0.45%.

[0066] Sn: 0 to 0.0100% Tin (Sn) is an optional element and does not necessarily need to be contained. In other words, the Sn content may be 0%. When contained, Sn improves the hydrogen embrittlement resistance of the steel material. Even if even a small amount of Sn is contained, the above effect can be obtained to some extent. However, if the Sn content is too high, the hot workability of the steel material will deteriorate even if the contents of other elements are within the ranges of this embodiment. Therefore, the Sn content is 0 to 0.0100%. The preferred lower limit of the Sn content is 0.0001%, more preferably 0.0005%, even more preferably 0.0010%, and even more preferably 0.0015%. The preferred upper limit of the Sn content is 0.0098%, more preferably 0.0095%.

[0067] [Tensile Strength of Steel Material] The steel material according to this embodiment has a tensile strength of 900 to 1100 MPa. In this specification, tensile strength means the maximum stress during uniform elongation obtained by a tensile test carried out according to a method in accordance with JIS Z2241:2011. The steel material according to this embodiment has excellent hydrogen embrittlement resistance even when the tensile strength is 900 to 1100 MPa, provided that the other configurations of this embodiment are satisfied.

[0068] 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. Note that in this embodiment, the yield strength of the steel material is not particularly limited. The yield strength of the steel material may be, for example, 758 to 1000 MPa.

[0069] 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 is performed according to a method in accordance with JIS Z2241:2011. A test specimen is prepared from the steel material according to this embodiment. When the steel material is a steel plate, a tensile test specimen is prepared from the center of the plate thickness. In this case, the longitudinal direction of the tensile test specimen is parallel to the rolling direction of the steel plate. When the steel material is a steel pipe, a tensile test specimen is prepared from the center of the wall thickness. In this case, the longitudinal direction of the tensile test specimen is parallel to the axial direction of the steel pipe. When the steel material is a round bar, a tensile test specimen is prepared from the R / 2 position. In this specification, the R / 2 position of the round bar means the center position of the radius R in a cross section perpendicular to the axial direction of the round bar. In this case, the longitudinal direction of the tensile test specimen is parallel to the axial direction of the round bar.

[0070] The tensile test specimen is, for example, a round bar test specimen with a parallel portion diameter of 6.0 mm and a parallel portion length of 40 mm. If a round bar test specimen cannot be prepared from a steel pipe, a circular arc test specimen is prepared. The size of the circular arc test specimen is, for example, the total wall thickness, a width of 25.4 mm, and a gauge length of 50.8 mm. Using the prepared tensile test specimen, a tensile test is performed in air at room temperature (25°C) in accordance with JIS Z2241:2011. The maximum stress (MPa) during uniform elongation obtained by the tensile test is defined as the tensile strength. The 0.2% offset proof stress (MPa) obtained by the tensile test is defined as the yield strength. Both the tensile strength (MPa) and the yield strength (MPa) are calculated by rounding the obtained values ​​to one decimal place.

[0071] [Grain size of prior austenite grains] In the steel material according to this embodiment, the grain size of the prior austenite grains (prior γ grains) is 20.0 μm or less. In this specification, the grain size of the prior austenite grains (prior γ grain size) means the grain size of the prior austenite grains determined in accordance with the mean intercept method specified in JIS G0551:2020.

[0072] As described above, hydrogen embrittlement is likely to occur when hydrogen accumulates at grain boundaries. In a steel material having the above-described chemical composition, if the prior γ grains are fine, the area of ​​the prior γ grain boundaries increases. In this case, even if the amount of hydrogen absorbed in the steel material is the same, the amount of hydrogen accumulated per unit area of ​​the prior γ grain boundaries decreases. Therefore, if the prior γ grains are fine, the hydrogen embrittlement resistance of the steel material can be improved. Specifically, if the prior γ grain size of a steel material having the above-described chemical composition is 20.0 μm or less, both a tensile strength of 900 to 1100 MPa and excellent hydrogen embrittlement resistance can be achieved, provided that the other configurations of this embodiment are satisfied.

[0073] In the steel material according to this embodiment, the upper limit of the prior γ grain size is preferably 19.0 μm, more preferably 18.5 μm, and even more preferably 18.0 μm. In the steel material according to this embodiment, the smaller the prior γ grain size, the better. Note that in steel materials intended for use in sour environments or for high-pressure hydrogen containers, the lower limit of the prior γ grain size may be, for example, 2.0 μm, 3.0 μm, 4.0 μm, 5.0 μm, or more than 5.0 μm.

[0074] The prior γ grain size of the steel material according to this embodiment can be determined by the following method. Specifically, it is determined in accordance with the measurement method of the mean intercept method specified in JIS G0551:2020. A test specimen having an observation surface is prepared from the steel material according to this embodiment. When the steel material is a steel plate, a test specimen is prepared having an observation surface that includes the plate thickness t / 4 position, which is the observation target region, from the plate width center, and that includes the rolling direction and the plate thickness direction. Note that in this specification, the plate thickness t / 4 position means the t / 4 depth position from the surface of the steel plate, where t is the plate thickness of the steel plate. When the steel material is a steel pipe, a test specimen is prepared having an observation surface that includes the wall thickness center, which is the observation target region, and that includes the pipe axial direction and the pipe radial direction (wall thickness direction). When the steel material is a round steel, a test specimen is prepared having an observation surface that includes the R / 2 position, which is the observation target region, and that includes the axial direction and the cross-sectional radial direction. The size of the test specimen is, for example, 10 mm in length in the rolling direction, 5 mm in width direction, and 10 mm in thickness direction. The surface including the rolling direction and the thickness direction (the surface of 10 mm x 10 mm in the case of the test piece size described above) is used as the observation surface.

[0075] The observation surface of the test piece is mirror-polished. After mirror-polishing, the observation surface is immersed in a picral etching solution for about 10 seconds to expose the grain boundaries of the prior austenite grains by etching. Ten random fields of view in the observation target region of the etched observation surface are observed using a scanning electron microscope (SEM) as secondary electron images to generate photographic images. The area of ​​each field of view is, for example, 500 μm × 500 μm (magnification 200 times).

[0076] The generated photographic images are used to evaluate the grain size number in accordance with the measurement method of the mean intercept method specified in JIS G0551:2020. The grain size of the prior austenite grains in each visual field is determined from the evaluated grain size number. The arithmetic mean value of the grain size of the prior austenite grains determined in 10 visual fields is defined as the grain size of the prior austenite grains (prior γ grain size) (μm). The prior γ grain size (μm) is determined by rounding the obtained value to one decimal place.

[0077] [Dislocation Density] The steel material according to this embodiment has a dislocation density of 7.0 × 10 14 m -2 In this specification, the dislocation density is the dislocation density (m -2 As described above, in this specification, the XRD line profile means the intensity curve of diffracted X-rays generated by XRD.

[0078] As described above, increasing the dislocation density in a steel material increases the strength of the steel material. However, dislocations have the potential to absorb hydrogen. In other words, if the dislocation density in a steel material increases, the amount of hydrogen absorbed by the steel material also increases, raising concerns that the hydrogen embrittlement resistance of the steel material may deteriorate. Therefore, if the dislocation density is reduced to a level that maintains strength, the hydrogen embrittlement resistance of the steel material can be improved while maintaining high strength. Specifically, when the dislocation density of a steel material having the above-described chemical composition is 7.0 × 10 14 m -2 If the above conditions are met, both a tensile strength of 900 to 1100 MPa and excellent hydrogen embrittlement resistance can be achieved, provided that the other configurations of this embodiment are met.

[0079] That is, in the steel material according to this embodiment, if the dislocation density is too high, excellent hydrogen embrittlement resistance may not be obtained. On the other hand, if the dislocation density is too low, a tensile strength of 900 to 1100 MPa may not be obtained. Therefore, in the steel material according to this embodiment, the dislocation density is 7.0 × 10 14 m -2 The following applies.

[0080] In the steel material according to this embodiment, the preferred upper limit of the dislocation density is 6.9 × 10 14 m -2 and more preferably 6.8 × 10 14 m -2 and more preferably 6.6 × 10 14 m -2 The lower limit of the dislocation density is, for example, 0.3 × 10 14 m -2 In the steel material according to this embodiment, the preferred lower limit of the dislocation density is 0.5 × 10 14 m -2 and more preferably 0.7 × 10 14 m -2 and more preferably 0.8 × 10 14 m -2 is.

[0081] In this embodiment, the dislocation density can be determined by the following method. Specifically, a test piece for XRD is prepared from the steel material according to this embodiment. When the steel material is a steel plate, the test piece is prepared from the center of the plate thickness. When the steel material is a steel pipe, the test piece is prepared from the center of the wall thickness. When the steel material is a round bar, the test piece is prepared from the R / 2 position. The size of the test piece is, for example, 20 mm wide x 20 mm long x 2 mm thick. The thickness direction of the test piece is the thickness direction of the steel material (T direction: plate thickness direction, pipe diameter direction or diameter direction). In this case, the observation surface of the test piece is a surface with a width of 20 mm x length of 20 mm.

[0082] The observation surface of the test piece is mirror-polished, and then electrolytically polished using 10% by volume of perchloric acid (acetic acid solvent) to remove surface distortion. An XRD line profile is obtained for the observation surface after electrolytic polishing using an X-ray diffractometer. In XRD, the half-width ΔK is measured using a CoKα ray source, a tube voltage of 30 kV, and a tube current of 100 mA. Furthermore, to measure the half-width derived from the X-ray diffractometer, LaB6 (lanthanum hexaboride) powder is used. From the obtained line profile, the half-width ΔK of the peaks of the (110), (211), and (220) planes of the body-centered cubic structure (iron) is determined.

[0083] The non-uniform strain ε of the test piece is calculated from the half-width ΔK calculated by the above-mentioned method and the Williamson-Hall equation (Equation (A)). Here, in formula (A), θ is the diffraction angle, λ is the wavelength of the X-ray, and D is the crystallite diameter.

[0084] Furthermore, the dislocation density ρ (m -2 ) can be obtained. -2 ) is a unit of 10 14 m -2 The resulting number is expressed in exponential notation, and the mantissa is rounded off to one decimal place to obtain the value. Here, in formula (B), b is the Burgers vector (b=0.248 (nm)) of the body-centered cubic structure (iron).

[0085] [Q value] The steel material according to this embodiment has a Q value of 2.35 or more obtained by analyzing an X-ray diffraction (XRD) line profile by the modified Williamson-Hall / Warren-Averbach method. Hereinafter, in this specification, the Q value obtained by analyzing an XRD line profile by the modified Williamson-Hall / Warren-Averbach method will also be simply referred to as the "Q value."

[0086] As described above, the Q value obtained by analyzing the XRD line profile is an index of dislocation components. Dislocation components include edge dislocations and screw dislocations. Furthermore, when the Q value is 2.35 or more, the proportion of screw dislocations among the dislocation components is 0.75 or more. In other words, in the steel material according to this embodiment, screw dislocations account for 75% or more of the dislocation components.

[0087] In this embodiment, the preferred lower limit of the Q value is 2.36, and more preferably 2.37. While the upper limit of the Q value is not particularly limited in the steel material according to this embodiment, the upper limit of the Q value is substantially 2.67 in the steel material having the above-described chemical composition. In this case, the proportion of screw dislocations among the dislocation components is 1.00. Furthermore, in this embodiment, the upper limit of the Q value may be 2.60, 2.55, 2.50, or 2.47.

[0088] Here, the peak position, height, shape, etc. of the XRD line profile change due to various factors. Specifically, for example, if the dislocation density increases, the width of the line profile widens. Therefore, the characteristics of steel materials, such as dislocation density, have been quantified by analyzing the line profile. Specifically, the Williamson-Hall method and the Warren-Averbach method have been used to analyze XRD line profiles. In recent years, these analysis methods have been further improved, and a modified Williamson-Hall / Warren-Averbach method, which takes anisotropy into account and introduces a contrast factor, has been used.

[0089] Specifically, the Q value in this specification is defined by the following formula (C). Here, C in formula (C) hkl is the average contrast factor of the (hkl) plane. C h00 is the average contrast factor (C h00 = 0.256). Q is the Q value. H hkl is a crystal orientation parameter of the (hkl) plane and is defined by the following formula (D): In formula (D), h, k, and l are Miller indices corresponding to the (hkl) plane.

[0090] Furthermore, for steel materials having the above-mentioned chemical composition, when the modified Williamson-Hall / Warren-Averbach method is used, the (110), (200), (211), (220), (310), and (222) planes of the XRD line profile will be discussed. Hereinafter, the (110), (200), (211), (220), (310), and (222) planes will also be collectively referred to as (hkl) planes.

[0091] In this embodiment, the Q value can be determined by the following method. An XRD line profile is obtained as described in the above method for determining dislocation density. From the obtained line profile, the half-width ΔK of the peak of the (hkl) plane is calculated. hkl and the scattering vector K of the peak of the (hkl) plane hkl and asks for.

[0092] The half-width ΔK of the peak of the obtained (hkl) plane hkl and the scattering vector K of the peak of the (hkl) plane hkl and the average contrast factor C of the (hkl) plane hkl satisfies the following formula (E). Here, α is a parameter that depends on the crystallite size, and Φ is a parameter that represents the degree of processing.

[0093] Furthermore, by transforming the above formulas (C) and (E), the following formula (F) is obtained.

[0094] As described above, the left side of the formula (F) is the crystal orientation parameter H of the (hkl) plane. hkl Therefore, a regression analysis using the least squares method was performed to determine whether the left side of equation (F) is H hkl The α that shows the best linearity with respect to the square of σ is calculated. Furthermore, the Q value can be calculated from the slope and intercept of the line obtained by regression analysis. The Q value is calculated by rounding the obtained value to two decimal places. The Q value calculated by the above method is unitless.

[0095] [Hydrogen embrittlement resistance] The steel material according to this embodiment has the above-mentioned chemical composition, a tensile strength of 900 to 1100 MPa, a prior γ grain size of 20.0 μm or less, and a dislocation density of 7.0 × 10 14 m -2 or less, and the Q value is 2.35 or more. As a result, the steel material according to this embodiment achieves both a tensile strength of 900 to 1100 MPa and excellent hydrogen embrittlement resistance. In this embodiment, excellent hydrogen embrittlement resistance can be evaluated by the following method.

[0096] Test specimens for evaluating hydrogen embrittlement resistance are prepared from the steel material according to this embodiment. The test specimen is a round bar test specimen with an annular notch. For example, the outer diameter of the parallel portion of the test specimen is 4.0 mm, the length of the parallel portion is 25 mm, and an annular notch is formed at the longitudinal center position of the parallel portion. In this case, the notch shape is 0.3 mm deep, the notch angle is 60°, and the radius of curvature of the notch bottom is 0.125 mm. When the steel material is a steel plate, a round bar test specimen is prepared from the center of the plate width and the plate thickness t / 4 position. In this case, the longitudinal direction of the round bar test specimen with annular notch is parallel to the rolling direction of the steel plate. When the steel material is a steel pipe, a round bar test specimen is prepared from the center of the wall thickness. In this case, the longitudinal direction of the round bar test specimen with annular notch is parallel to the axial direction of the steel pipe. When the steel material is a round steel, a round bar test specimen is prepared from the R / 2 position. In this case, the longitudinal direction of the circularly notched round bar test piece is parallel to the axial direction of the round steel.

[0097] The prepared circularly notched round bar test specimens were charged with hydrogen by cathodic hydrogen charging. Specifically, a room-temperature cathodic hydrogen charging solution was prepared. The cathodic hydrogen charging solution was an aqueous solution containing 5 mass% sodium chloride solution, 30 g / L NH4SCN, and an acetate buffer solution at room temperature. The pH of the solution was adjusted to 3.5 using the acetate buffer solution before testing.

[0098] The annular notched round bar test specimen was immersed in the cathodic hydrogen charging solution and charged with hydrogen at a potential of −1.5 V for 24 hours. At this time, a zinc plating film is preferably formed on the surface of the hydrogen-charged annular notched round bar test specimen to prevent hydrogen from leaking out of the annular notched round bar test specimen.

[0099] The hydrogen-charged annular notched round bar test specimens and the non-hydrogen-charged annular notched round bar test specimens were subjected to tensile tests in air at room temperature (25°C) using a slow strain rate testing machine (SSRT). -6 / sec. The breaking stress BS1 is determined from the hydrogen-charged annular notched round bar test piece. The breaking stress BS0 is determined from the hydrogen-uncharged annular notched round bar test piece. The breaking stresses BS0 and BS1 are calculated by rounding the obtained values ​​to one decimal place.

[0100] The relative fracture stress ratio is calculated using the obtained fracture stresses BS0 and BS1 according to the following formula. If the calculated relative fracture stress ratio is 0.80 or more, it is determined that the material has excellent hydrogen embrittlement resistance. The relative fracture stress ratio is calculated by rounding the obtained value to two decimal places. Relative fracture stress ratio = BS1 / BS0

[0101] [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 steel material has the above-mentioned chemical composition, a tensile strength of 900 to 1100 MPa, a prior γ grain size of 20.0 μm or less, and a dislocation density of 7.0 × 10 14 m -2 If the Q value is 2.35 or more, the steel material can be determined to have a total area ratio of tempered martensite and tempered bainite of 90% or more.

[0102] [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. When the steel material is a steel plate, a test piece is taken from the center of the plate width, including the plate thickness t / 4 position, which is the observation target region, and having an observation surface parallel to the rolling direction. When the steel material is a steel pipe, a test piece is prepared that includes the wall thickness center, which is the observation target region, and has an observation surface parallel to the pipe axial direction. When the steel material is a round bar, a test piece is taken that includes the R / 2 position, which is the observation target region, and has an observation surface parallel to the rolling direction. The size of the test piece is not particularly limited. The size of the test piece is, for example, 10 mm in the length direction in the rolling direction, 5 mm in the width direction, and 10 mm in the thickness direction. When 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 pipe axial direction, the thickness direction corresponds to the wall thickness direction, and the width direction corresponds to the direction perpendicular to the pipe axial direction and the wall thickness direction (circumferential direction). When the steel material is a round steel, 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 the radial direction (circumferential direction). The surface including the rolling direction and the thickness direction (the surface of 10 mm × 10 mm when the test piece size is as described above) is used as the observation surface.

[0103] After polishing the observation surface of the test piece to a mirror finish, it is immersed in a nital etching solution for about 10 seconds to reveal the structure by etching. Ten arbitrary fields within the observation area of ​​the etched observation surface are observed as secondary electron images using a scanning electron microscope (SEM). When the steel material is a steel plate, the observation area is at the plate thickness t / 4 position. When the steel material is a steel pipe, the observation area is at the center of the wall thickness. When the steel material is a round bar, the observation area is at the R / 2 position. The area of ​​each of the ten fields within the observation area is, for example, 400 μm 2 (Magnification: 5000x).

[0104] In each field, tempered martensite and tempered bainite are identified. In each field, tempered martensite and tempered bainite can be distinguished from other structures (ferrite, pearlite, etc.) based on their morphology. Specifically, a structure with a lamellar structure can be identified as pearlite. A structure containing laths and lenses can be identified as tempered martensite and tempered bainite. A structure without a substructure within the grains can be identified as ferrite.

[0105] The total area ratio of the specified tempered martensite and tempered bainite is calculated. The method for calculating the total area ratio is not particularly limited, and any known method may be used. For example, the total area ratio of the tempered martensite and tempered bainite can be calculated by image analysis. In this embodiment, the arithmetic mean value of the total area ratios of the tempered martensite and tempered bainite calculated in all visual fields (10 visual fields) is defined as the total area ratio (%) of the tempered martensite and tempered bainite.

[0106] [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).

[0107] Preferably, the steel material of this embodiment is any one of steel pipes for oil wells, steel pipes for line pipes, and steel pipes for high-pressure hydrogen containers. Steel pipes for oil wells refer to steel pipes for use as oil well tubular goods. Oil well tubular goods include, for example, casings, tubing, drill pipes, etc. used for drilling oil or gas wells and extracting crude oil or natural gas. Steel pipes for line pipes refer to steel pipes for use as line pipes that constitute pipelines that transport produced fluids (crude oil or natural gas) extracted from oil or gas wells. Examples of pipelines include flow lines that transport produced fluids from oil or gas wells, gathering lines that collect the produced fluids transported along the flow lines and transport them to primary treatment facilities, trunk lines that transport produced fluids that have undergone primary treatment such as dehydration to the vicinity of markets, and distribution lines that transport them to consumers. Steel pipe for high-pressure hydrogen containers is standardized by ISO 11439, ANSI / NGV, the High-Pressure Gas Safety Act, Illustrative Standards of Container Safety Regulations, etc., and refers to steel pipe used for high-pressure hydrogen containers in which high-pressure hydrogen gas is stored. The steel material of this embodiment may be a steel pipe for high-pressure hydrogen containers, or may be a steel pipe for high-pressure hydrogen accumulators, or a steel pipe for high-pressure hydrogen cylinders.

[0108] More preferably, the steel material of this embodiment is any one of a seamless steel pipe for oil wells, a seamless steel pipe for line pipes, and a seamless steel pipe for high-pressure hydrogen containers. A seamless steel pipe for oil wells means that the steel pipe for oil wells is a seamless steel pipe. A seamless steel pipe for line pipes means that the steel pipe for line pipes is a seamless steel pipe. A seamless steel pipe for high-pressure hydrogen containers means that the steel pipe for high-pressure hydrogen containers is a seamless steel pipe. The steel material of this embodiment may be a seamless steel pipe for high-pressure hydrogen containers, or may be any one of a seamless steel pipe for high-pressure hydrogen accumulators and a seamless steel pipe for high-pressure hydrogen cylinders.

[0109] [Manufacturing method] An example of a method for manufacturing a steel material according to this embodiment will be described below. Note that the manufacturing method described below is just one example, and the manufacturing method for a steel material according to this embodiment is not limited to this. In other words, as long as the steel material according to this embodiment having the above-described configuration can be manufactured, the manufacturing method is not limited to the manufacturing method described below. However, the manufacturing method described below is a suitable manufacturing method for manufacturing a steel material according to this embodiment.

[0110] An example of the method for manufacturing a steel material according to this embodiment includes a material preparation step, a hot working step, and a heat treatment step. Each step will be described below.

[0111] [Material Preparation Step] In the material preparation step, a material is produced using molten steel having the above-described chemical composition. The method for producing the material is not particularly limited and may be a well-known method. Specifically, a cast piece (slab, bloom, or billet) may be produced using the molten steel by a continuous casting method. An ingot may be produced using the molten steel by an ingot-making method. If necessary, the slab, bloom, or ingot may be subjected to blooming to produce a billet. The material (slab, bloom, or billet) is produced through the above steps. The hot working step will be described below.

[0112] [Hot working process] In the hot working process, a prepared raw material is hot worked to produce an intermediate steel material. As described above, when the steel material is a seamless steel pipe, the intermediate steel material corresponds to a mother pipe. First, a billet is heated in a heating furnace. The heating temperature is not particularly limited, but is, for example, 1100 to 1300°C. The billet extracted from the heating furnace is hot worked to produce a mother pipe (seamless steel pipe). The hot working method is not particularly limited, and a well-known method may be used.

[0113] For example, a mother pipe may be produced by carrying out the Mannesmann process as hot working. In this case, a round billet is pierced and rolled using a piercing mill. When piercing and rolling is performed, the piercing ratio is not particularly limited, but is, for example, 1.0 to 4.0. The piercing-rolled round billet is further hot-rolled using a mandrel mill, a reducer, a sizing mill, or the like to produce a mother pipe. A mother pipe may also be produced from the billet by carrying out other hot working methods. For example, when the steel material is a short, thick-walled steel pipe such as a coupling, the mother pipe may be produced by forging using the Erhardt process or the like. A mother pipe is produced through the above steps. The wall thickness of the mother pipe is not particularly limited, but is, for example, 9 to 60 mm.

[0114] When the steel material is round steel, the material is first heated in a heating furnace. The heating temperature is not particularly limited, but is, for example, 1100 to 1300°C. The material extracted from the heating furnace is hot worked to produce an intermediate steel material having a circular cross section perpendicular to the axial direction. The hot working is, for example, blooming using a blooming mill or hot rolling using a continuous rolling mill. The continuous rolling mill has horizontal stands each having a pair of grooved rolls arranged side by side in the vertical direction, and vertical stands each having a pair of grooved rolls arranged side by side in the horizontal direction, arranged alternately. When the steel material is a steel plate, the material is first heated in a heating furnace. The heating temperature is not particularly limited, but is, for example, 1100 to 1300°C. The material extracted from the heating furnace is hot rolled using a blooming mill and a continuous rolling mill to produce an intermediate steel material in the shape of a steel plate.

[0115] In this case, the area reduction ratio R of the intermediate steel material in the hot working is preferably 60% or more. The area reduction ratio is defined by the following formula (I): Area reduction ratio R (%) = 100 × cross-sectional area perpendicular to the working direction of the intermediate steel material after hot working / cross-sectional area perpendicular to the working direction of the material before hot working (I)

[0116] If the area reduction rate R in the hot working step is too small, the austenite grains may not be sufficiently refined, and the prior γ grain size of the produced steel may become coarse. Therefore, in the hot working step according to this embodiment, the area reduction rate R is set to 60% or more. As a result, provided that the other manufacturing conditions of this embodiment are satisfied, the prior γ grain size of the produced steel can be stably set to 20.0 μm or less. Note that in this embodiment, the upper limit of the area reduction rate R in the hot working step is not particularly limited, but may be, for example, 85%.

[0117] The intermediate steel produced by hot working may be air-cooled (as-rolled). The intermediate steel produced by hot working may be quenched directly after the hot working without being cooled to room temperature, or may be quenched after reheating (reheating) after the hot working. When quenching is performed directly after the hot working or after reheating, cooling may be stopped or slow cooling may be performed during quenching. In this case, the occurrence of quench cracks in the mother pipe can be suppressed. When quenching is performed directly after the hot working or after reheating, stress relief annealing (SR) may be performed after quenching and before the next heat treatment step. In this case, residual stress in the mother pipe is removed. The intermediate steel is produced by the above steps. The heat treatment steps are described below.

[0118] [Heat Treatment Step] The heat treatment step according to this embodiment preferably includes a dislocation adjustment step, a quenching step, and a tempering step. Each step will be described below.

[0119] [Dislocation Adjustment Step] In the dislocation adjustment step according to this embodiment, the intermediate steel material produced in the hot working step is subjected to a heat treatment for adjusting dislocations, thereby adjusting the dislocation components of edge dislocations and screw dislocations. By performing the dislocation adjustment step, the proportion of screw dislocations in the total dislocations can be stably set to 0.75 or more, and the Q value of the produced steel material can be stably set to 2.35 or more.

[0120] Specifically, in the dislocation adjustment step, the intermediate steel is held at 980 to 1080°C for 30 to 90 minutes. If the holding temperature in the dislocation adjustment step is too low, the proportion of screw dislocations may not be increased sufficiently, and the Q value of the manufactured steel may not be able to be 2.35 or higher. In this case, excellent hydrogen embrittlement resistance may not be obtained. On the other hand, if the holding temperature in the dislocation adjustment step is too high, prior γ grains may become coarse, and excellent hydrogen embrittlement resistance may not be obtained. Therefore, in the dislocation adjustment step according to this embodiment, the holding temperature of the intermediate steel is preferably 980 to 1080°C.

[0121] If the holding time in the dislocation adjustment step is too short, the proportion of screw dislocations may not be increased sufficiently, and the Q value of the manufactured steel may not be able to be 2.35 or more. In this case, excellent hydrogen embrittlement resistance may not be obtained. On the other hand, if the holding time in the dislocation adjustment step is too long, the above effect will saturate. Therefore, in the dislocation adjustment step according to this embodiment, the holding time of the intermediate steel is preferably 30 to 90 minutes.

[0122] In this specification, the holding temperature in the dislocation adjustment step corresponds to the temperature of the heat treatment furnace when the intermediate steel material is heated and held therein, and the holding time refers to the time from when the temperature of the intermediate steel material reaches a predetermined holding temperature to when the intermediate steel material is extracted from the heat treatment furnace.

[0123] Furthermore, after hot working, the dislocation adjustment step may be carried out immediately after the hot working without cooling the intermediate steel to room temperature, or the dislocation adjustment step may be carried out after the intermediate steel is once cooled to room temperature. In either case, the dislocation adjustment step can be carried out by charging the intermediate steel into a heating furnace at a desired temperature. The quenching step will now be described.

[0124] [Quenching Process] In the quenching process, the intermediate steel material that has been subjected to the dislocation adjustment process is quenched. Quenching is performed by a well-known method. In this specification, "quenching" means rapidly cooling the intermediate steel material at or above the A3 transformation point. Here, the quenching temperature corresponds to the temperature of the heat treatment furnace when the intermediate steel material is heated and held. The quenching time refers to the time from when the temperature of the intermediate steel material reaches a predetermined quenching temperature to when it is extracted from the heat treatment furnace.

[0125] The preferred quenching temperature is A C3 The transformation point is 960°C or higher. If the quenching temperature is too high, the prior γ grains will become coarse and excellent hydrogen embrittlement resistance may not be obtained. On the other hand, if the quenching temperature is too low, the dislocation density will not be reduced sufficiently and excellent hydrogen embrittlement resistance may not be obtained. Therefore, the quenching temperature is set to A C3 The temperature is preferably from the transformation point to 960° C. The quenching time is not particularly limited, but is, for example, 10 to 60 minutes.

[0126] The quenching method is, for example, to continuously cool 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 a method of cooling the intermediate steel material by immersing it in a water tank, or a method of accelerating the cooling of the intermediate steel material by shower water cooling or mist cooling.

[0127] It is preferable that the dislocation adjustment step and the quenching step are carried out consecutively. For example, the dislocation adjustment step and the quenching step may be carried out using the same heat treatment furnace by changing the temperature inside the heat treatment furnace. Furthermore, for example, the dislocation adjustment step and the quenching step may be carried out using two heat treatment furnaces with different temperatures. The tempering step will be described below.

[0128] [Tempering step] In the tempering step, the intermediate steel material that has been subjected to the above-mentioned quenching is tempered. c1 It means reheating at a temperature below the transformation point and holding it there. Here, the tempering temperature corresponds to the temperature of the heat treatment furnace when the intermediate steel material is heated and held there after quenching. The tempering time refers to the time from when the temperature of the intermediate steel material reaches the predetermined tempering temperature until it is extracted from the heat treatment furnace.

[0129] The tempering temperature is adjusted appropriately depending on the chemical composition of the steel material and the tensile strength to be obtained. That is, the tempering temperature is adjusted for the intermediate steel material having the chemical composition of this embodiment to adjust the tensile strength of the steel material to 900 to 1100 MPa. In the tempering process according to this embodiment, the preferred tempering temperature is 670°C to 1100 MPa. c1 In the tempering step of this embodiment, the tempering time is preferably 5 to 240 minutes.

[0130] The steel material according to this embodiment can be manufactured by carrying out the above manufacturing steps. However, as mentioned above, the above manufacturing method is one example, and the steel material may be manufactured by other manufacturing methods. The effects of the steel material according to this embodiment will be explained more specifically below using examples. However, the various conditions in the examples described below are 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.

[0131] Molten steels having the chemical compositions shown in Tables 1A and 1B were produced. Note that "-" in Tables 1A and 1B indicates that the content of each element was at the impurity level. Specifically, the Nb content of Test No. 35 was rounded to four decimal places to mean 0%. The W, Co, Cu, and Ni contents of Test No. 1 were rounded to two decimal places to mean 0%. The Mg, Ca, REM, and Sn contents of Test No. 1 and the B content of Test No. 37 were rounded to five decimal places to mean 0%.

[0132]

[0133]

[0134] The steel materials of each test number were produced by the following method: Ingots were produced from molten steel having the chemical compositions shown in Tables 1A and 1B. The obtained ingots of each test number were subjected to hot forging to produce blocks of 50 mm in thickness.

[0135] The blocks of each test number were subjected to hot working. Specifically, the blocks were heated to 1250°C. The heated blocks were hot rolled to produce intermediate steel materials (steel plates) with a thickness of 15 mm. The area reduction rates (%) of the intermediate steel materials of each test number in the hot working were as shown in Table 2. The intermediate steel materials produced by the above method were allowed to cool to room temperature.

[0136]

[0137] The steel material of each test number that had been naturally cooled to room temperature was subjected to a heat treatment. Specifically, a dislocation adjustment process was carried out on the intermediate steel material of each test number, in which the material was held at the holding temperature (°C) for the holding time (minutes) shown in the "Dislocation Adjustment" column in Table 2. Furthermore, the intermediate steel material of each test number that had undergone the dislocation adjustment process was quenched by holding the material at the quenching temperature (°C) for the quenching time (minutes) shown in the "Quenching" column in Table 2, followed by water cooling. After quenching, the intermediate steel material of each test number was tempered by holding the material at the tempering temperature (°C) for the tempering time (minutes) shown in the "Tempering" column in Table 2. Steel materials (steel plates) of each test number were produced by the above manufacturing process.

[0138] [Evaluation Tests] The produced steel materials with each test number were subjected to a tensile test, a prior γ grain size measurement test, a dislocation density and Q value measurement test, and a hydrogen embrittlement resistance evaluation test.

[0139] [Tensile Test] A tensile test was performed on the steel material of each test number to determine the tensile strength and yield strength. Specifically, a round bar test specimen was prepared from the center of the plate thickness of the steel material of each test number. The round bar test specimen had a parallel section diameter of 6.0 mm and a parallel section length of 40 mm. The axial direction of the round bar test specimen was parallel to the rolling direction of the steel material. Using the prepared tensile test specimen, a tensile test was performed in air at room temperature (25°C) in accordance with JIS Z2241:2011, and the maximum stress during uniform elongation obtained was defined as the tensile strength (MPa). The 0.2% offset proof stress (MPa) obtained in the same tensile test was defined as the yield strength. For the steel material of each test number, the obtained tensile strength was defined as "TS (MPa)" and the yield strength (MPa) was defined as "YS (MPa)". Table 3 shows the tensile strength and yield strength (MPa) obtained for each steel material.

[0140]

[0141] [Prior γ Grain Size Measurement Test] A prior γ grain size measurement test was performed on the steel material of each test number to determine the prior γ grain size. Specifically, a test specimen was prepared from the width center of the steel material of each test number, including the thickness t / 4 position, which was the observation target region, and including the rolling direction and the thickness direction as the observation surface. The size of the test specimen was 10 mm in the rolling direction, 5 mm in the width direction, and 10 mm in the thickness direction. The observation surface was 10 mm in the rolling direction × 10 mm in the thickness direction. The prior γ grain size (μm) of the prepared test specimen was determined according to the above-mentioned method. At this time, the field area was 500 μm × 500 μm (magnification 200 times). The obtained prior γ grain size (μm) is shown in the "Prior γ grain size (μm)" column in Table 3.

[0142] [Dislocation density and Q value measurement test] Dislocation density and Q value measurement tests were performed on the steel material of each test number to determine the dislocation density and Q value. Specifically, using the method described above, a test piece measuring 20 mm wide x 20 mm long x 2 mm thick was prepared from the center of the plate thickness of the steel material of each test number. At this time, the thickness direction of the test piece was prepared so as to be parallel to the plate thickness direction of the steel material. After mirror polishing the 20 mm wide x 20 mm long surface (observation surface) of the test piece, electrolytic polishing was performed using 10% by volume of perchloric acid (acetic acid solvent). For the observation surface after electrolytic polishing, an XRD line profile was obtained using the method described above. From the obtained XRD line profile, the dislocation density (m -2 ) was obtained. From the obtained XRD line profile, the Q value was further obtained by the above-mentioned method. -2 ) in Table 3. 14 / m 2 The Q values ​​obtained are shown in Table 3.

[0143] [Hydrogen Embrittlement Resistance Evaluation Test] A hydrogen embrittlement resistance evaluation test was conducted on the steel material of each test number to evaluate its hydrogen embrittlement resistance. Specifically, two round bar test specimens with an annular notch were prepared from the center of the plate width and the plate thickness t / 4 position of the steel material of each test number. The outer diameter of the parallel portion of each test specimen was 4.0 mm, the length of the parallel portion was 25 mm, and an annular notch was formed at the center position in the longitudinal direction of the parallel portion. The notch shape had a notch depth of 0.3 mm, a notch angle of 60°, and a curvature radius of the notch bottom of 0.125 mm.

[0144] One of the two annularly notched round bar specimens was charged with hydrogen by the cathodic hydrogen charging method. Specifically, a room-temperature cathodic hydrogen charging solution was prepared. The cathodic hydrogen charging solution was an aqueous solution containing 5 mass% sodium chloride solution, 30 g / L NH4SCN, and an acetate buffer solution at room temperature. The pH of the solution was adjusted to 3.5 using the acetate buffer solution before the test.

[0145] The annular-notched round bar test specimens were immersed in a cathodic hydrogen charging solution and charged with hydrogen at a potential of -1.5 V for 24 hours. In other words, charging with hydrogen simulated a sour environment. A zinc plating coating was formed on the surface of the hydrogen-charged annular-notched round bar test specimens under the same conditions for each sample code, preventing hydrogen leakage from the annular-notched round bar test specimens. The other annular-notched round bar test specimen was not charged with hydrogen.

[0146] A round bar specimen with a circular notch on which a zinc plating film was formed was subjected to a 4.2 x 10 strain rate test at room temperature in air using a slow strain rate testing machine (SSRT). -6 A tensile test was carried out at a strain rate of 1 / sec, and the breaking stress BS1 (MPa) in a hydrogen environment was determined.

[0147] Furthermore, for the circularly notched round bar specimens of each test number that were not charged with hydrogen, a slow strain rate testing machine (SSRT) was used to test the specimens at room temperature in air at a strain rate of 4.2 × 10 -6 A tensile test was carried out at a strain rate of 1 / sec, and the breaking stress BS0 (MPa) in air was determined.

[0148] The obtained rupture stress BS0 (MPa) in the air is shown in the "BS0 (MPa)" column of the "Hydrogen embrittlement resistance" column in Table 3. The obtained rupture stress BS1 (MPa) in the hydrogen environment is shown in the "BS1 (MPa)" column of the "Hydrogen embrittlement resistance" column in Table 3. Furthermore, the ratio of the rupture stress BS1 in the hydrogen environment to the obtained rupture stress BS0 in the air (= BS1 / BS0) is shown in the "Relative rupture stress ratio" column of the "Hydrogen embrittlement resistance" column.

[0149] [Test Results] Referring to Tables 1A, 1B, 2 and 3, the steel materials of test numbers 1 to 20 have the above-mentioned chemical compositions, tensile strengths TS of 900 to 1100 MPa, prior γ grain sizes of 20.0 μm or less, and dislocation densities of 7.0 × 10 14 m -2 or less, and the Q value was 2.35 or more. As a result, these steel materials had a relative fracture stress ratio of 0.80 or more, and excellent hydrogen embrittlement resistance. In other words, these steel materials had high strength and excellent hydrogen embrittlement resistance. It was determined that the steel materials of test numbers 1 to 20 had a total area ratio of tempered martensite and tempered bainite of 90% or more in the microstructure.

[0150] On the other hand, the steel material of test number 21 had an excessively high C content, resulting in a relative fracture stress ratio of less than 0.80, and the steel material did not have excellent hydrogen embrittlement resistance.

[0151] The steel material of test number 22 had an excessively low C content, resulting in a relative fracture stress ratio of less than 0.80, and thus did not have excellent hydrogen embrittlement resistance.

[0152] The steel material of test number 23 had an excessively high Si content, resulting in a relative fracture stress ratio of less than 0.80, and thus did not have excellent hydrogen embrittlement resistance.

[0153] The steel material of test number 24 had an excessively low Si content, resulting in a relative fracture stress ratio of less than 0.80, and thus did not have excellent hydrogen embrittlement resistance.

[0154] The steel material of test number 25 had an excessively high Mn content, resulting in a relative fracture stress ratio of less than 0.80, and thus did not have excellent hydrogen embrittlement resistance.

[0155] The steel material of test number 26 had an excessively low Mn content, resulting in a relative fracture stress ratio of less than 0.80, and thus did not have excellent hydrogen embrittlement resistance.

[0156] The steel material of test number 27 had an excessively high Cr content, resulting in a relative fracture stress ratio of less than 0.80, and thus did not have excellent hydrogen embrittlement resistance.

[0157] The Cr content of the steel material of test number 28 was too low, resulting in a relative fracture stress ratio of less than 0.80, and the steel material did not have excellent hydrogen embrittlement resistance.

[0158] The steel material of test number 29 had an excessively high Mo content, resulting in a relative fracture stress ratio of less than 0.80, and thus did not have excellent hydrogen embrittlement resistance.

[0159] The steel material of test number 30 had an excessively low Mo content, resulting in a relative fracture stress ratio of less than 0.80, and thus did not have excellent hydrogen embrittlement resistance.

[0160] The steel material of test number 31 had an excessively high Ti content, resulting in a relative fracture stress ratio of less than 0.80, and thus did not have excellent hydrogen embrittlement resistance.

[0161] The steel material of test number 32 had an excessively high V content, resulting in a relative fracture stress ratio of less than 0.80, and the steel material did not have excellent hydrogen embrittlement resistance.

[0162] The steel material of test number 33 had an excessively low V content, resulting in a relative fracture stress ratio of less than 0.80, and thus did not have excellent hydrogen embrittlement resistance.

[0163] The steel material of test number 34 had an excessively high Nb content, resulting in a relative fracture stress ratio of less than 0.80, and thus did not have excellent hydrogen embrittlement resistance.

[0164] The steel material of test number 35 had an excessively low Nb content. As a result, the prior γ grain size of this steel material exceeded 20.0 μm. As a result, the relative fracture stress ratio of this steel material was less than 0.80, and this steel material did not have excellent hydrogen embrittlement resistance.

[0165] The steel material of test number 36 had an excessively high B content, resulting in a relative fracture stress ratio of less than 0.80, and the steel material did not have excellent hydrogen embrittlement resistance.

[0166] The steel material of test number 37 had an excessively low B content, resulting in a relative fracture stress ratio of less than 0.80, and the steel material did not have excellent hydrogen embrittlement resistance.

[0167] The steel material of test number 38 had an excessively high N content, resulting in a relative fracture stress ratio of less than 0.80, and the steel material did not have excellent hydrogen embrittlement resistance.

[0168] The tempering temperature of the steel material of test number 39 was too low. As a result, the dislocation density of this steel material was 7.0 × 10 14 m -2 As a result, the relative fracture stress ratio of this steel was less than 0.80, and it did not have excellent hydrogen embrittlement resistance.

[0169] The dislocation adjustment process was not performed on the steel material of test number 40. As a result, the dislocation density of this steel material was 7.0 × 10 14 m -2 and the Q value was less than 2.35. As a result, the relative fracture stress ratio of this steel was less than 0.80, and it did not have excellent hydrogen embrittlement resistance.

[0170] The holding temperature in the dislocation adjustment step for the steel material of test number 41 was too low. As a result, the Q value of this steel material was less than 2.35. As a result, the relative fracture stress ratio of this steel material was less than 0.80, and this steel material did not have excellent hydrogen embrittlement resistance.

[0171] The holding time in the dislocation adjustment step for the steel material of test number 42 was too short. As a result, the Q value of this steel material was less than 2.35. As a result, the relative fracture stress ratio of this steel material was less than 0.80, and this steel material did not have excellent hydrogen embrittlement resistance.

[0172] The holding temperature in the dislocation adjustment process for the steel material of test number 43 was too high. As a result, the prior γ grain size of this steel material exceeded 20.0 μm. As a result, the relative fracture stress ratio of this steel material was less than 0.80, and the steel material did not have excellent hydrogen embrittlement resistance.

[0173] The steel material of test number 44 had an area reduction rate R in the hot working process that was too low. As a result, the prior γ grain size of this steel material exceeded 20.0 μm. As a result, the relative fracture stress ratio of this steel material was less than 0.80, and it did not have excellent hydrogen embrittlement resistance.

[0174] The steel material of test number 45 was not quenched. As a result, the dislocation density of this steel material was 7.0 × 10 14 m -2 As a result, the relative fracture stress ratio of this steel was less than 0.80, and it did not have excellent hydrogen embrittlement resistance.

[0175] The embodiments of the present disclosure have been described above. However, the above-described embodiments are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiments, and can be implemented by appropriately modifying the above-described embodiments within the scope of the present disclosure.

Claims

1. A steel material having a chemical composition, in mass%, of C: 0.25 to 0.30%, Si: 0.10 to 0.50%, Mn: 0.05 to 0.60%, P: 0.050% or less, S: 0.0100% or less, Al: 0.001 to 0.100%, 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%, 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%, W: 0 to 0.50%, Co: 0 to 0.50%, Cu: 0 to 0.50%, Ni: 0 to 0.50%, Sn: 0 to 0.0100%, and the balance being Fe and impurities, the tensile strength is 900 to 1100 MPa, the grain size of the prior austenite grains is 20.0 μm or less, and the dislocation density is 7.0 × 10 14 m -2 or less, and a Q value obtained by analyzing an X-ray diffraction line profile of the steel material by a modified Williamson-Hall / Warren-Averbach method is 2.35 or more.

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%, W: 0.01 to 0.50%, Co: 0.01 to 0.50%, Cu: 0.01 to 0.50%, Ni: 0.01 to 0.50%, and Sn: 0.0001 to 0.0100%.

3. A steel material according to claim 1 or 2, wherein the steel material is any one of 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 a 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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