Steel materials

Abstract

Provide a steel material having high strength and excellent hydrogen embrittlement resistance characteristics. The steel material according to the present disclosure has a chemical composition in mass %, 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, and the balance consisting of Fe and impurities, a tensile strength of 900 to 1100 MPa, an original austenite 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 obtained from the line profile of X-ray diffraction is 2.35 or more.

Description

Technical Field

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

Background Art

[0002] In oil wells and gas wells (hereinafter, oil wells and gas wells are collectively referred to as "oil wells"), there are environments containing a large amount of corrosive substances. The corrosive substances are, for example, corrosive gases such as hydrogen sulfide. In this specification, an environment containing hydrogen sulfide is referred to as a "sour environment". The temperature of the sour environment depends on the depth of the well, but is about room temperature to 200°C.

[0003] Examples of steel materials used in such sour environments include steel materials for oil wells applied as oil well pipes and steel materials for line pipes applied as line pipes. In recent years, with the deepening of oil wells, higher strength of steel materials for oil wells and the like has been demanded.

[0004] On the other hand, when a steel material is used in a sour environment, the surface of the steel material comes into contact with a corrosive substance, an electrochemical reaction occurs, and hydrogen is generated on the surface of the steel material. Due to this hydrogen, hydrogen embrittlement cracks typified by sulfide stress cracking (SSC) are likely to occur in the steel material. Therefore, for steel materials used in sour environments, excellent hydrogen embrittlement resistance characteristics are also required together with high strength.

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

[0006] In Patent Document 1, in an oil well steel pipe made of low alloy steel, a predetermined amount of dissolved Mo is ensured, the prior austenite grains are refined, and M2C type precipitates are dispersed. Thereby, the SSC resistance is enhanced. In Patent Document 1, further, by forming a Mo segregation region at the prior austenite grain boundary, the hydrogen embrittlement resistance characteristics are further enhanced.

[0007] In Patent Document 2, in the steel pipe for oil wells made of low alloy steel, by forming a Mo segregation region, the hydrogen embrittlement resistance property is enhanced.

[0008] More recently, the development of fuel cell vehicles that run on hydrogen as fuel and the practical application of hydrogen stations that supply hydrogen to fuel cell vehicles have been promoted. High-pressure hydrogen gas is stored in the high-pressure hydrogen accumulators installed in hydrogen stations. Also, as fuel cell vehicles, the development of vehicles equipped with high-pressure hydrogen cylinders is also underway. For the steel materials used in high-pressure hydrogen containers typified by such high-pressure hydrogen accumulators and high-pressure hydrogen cylinders, excellent hydrogen embrittlement resistance properties are required together with high strength.

[0009] A technique for enhancing the hydrogen embrittlement resistance property in the steel material used for high-pressure hydrogen containers has been proposed in Japanese Unexamined Patent Application Publication No. 2009-74122 (Patent Document 3). In Patent Document 3, in the steel material made of low alloy steel, by increasing the V content and the Mo content more than before, the morphology of the carbides at the prior austenite grain boundaries is improved, and the hydrogen embrittlement resistance property is enhanced.

Prior Art Documents

Patent Documents

[0010]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0011] According to the technologies disclosed in the above Patent Documents 1 to 3, it is possible to enhance the hydrogen embrittlement resistance characteristics of steel materials assumed to be used in sour environments or for high-pressure hydrogen containers. However, even if steel materials having high strength and excellent hydrogen embrittlement resistance characteristics are obtained by means other than those described in the above Patent Documents 1 to 3, it may be acceptable.

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

Means for Solving the Problems

[0013] The steel material according to the present disclosure is The chemical composition is, in mass %, 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 consists of Fe and impurities, the tensile strength is 900 to 1100 MPa, The crystal grain size of the old austenite grains is 20.0 μm or less, and the dislocation density is 7.0×10 14 m -2 or less, and for the steel material, the Q value obtained by analyzing the line profile of X-ray diffraction by the modified Williamson-Hall / Warren-Averbach method is 2.35 or more.

Advantages of the Invention

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

Brief Description of the Drawings

[0015]

Figure 1

Modes for Carrying Out the Invention

[0016] First, the inventors considered obtaining a steel material having a tensile strength of 900 to 1100 MPa as high strength, assuming use in a sour environment or application to a high-pressure hydrogen container. That is, the inventors investigated and studied methods for enhancing the hydrogen embrittlement resistance characteristics even when the tensile strength is 900 to 1100 MPa for steel materials assumed to be used in a sour environment or applied to a high-pressure hydrogen container. As a result, the inventors obtained the following findings.

[0017] Next, the inventors focused on the chemical composition and studied steel materials that achieve both high strength and hydrogen embrittlement resistance. As a result, in terms of mass percentage, if the steel material has a chemical composition of 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% or less, O: 0.0100% or less, Mg: 0 - 0.0050%, Ca: 0 - 0.0050%, rare earth elements: 0 - 0.0050%, W: 0 - 0.50%, Co: 0 - 0.50%, Cu: 0 - 0.50%, Ni: 0 - 0.50%, Sn: 0 - 0.0100%, and the balance being Fe and impurities, it is considered possible to obtain a tensile strength of 900 - 1100 MPa and excellent hydrogen embrittlement resistance characteristics.

[0018] Furthermore, the inventors focused on the microstructure in the steel material and studied steel materials that achieve both high strength and hydrogen embrittlement resistance. Here, hydrogen embrittlement is likely to occur when hydrogen accumulates at grain boundaries. In a steel material having the above 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 occluded 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 characteristics of the steel material can be enhanced.

[0019] Therefore, the inventors considered refining the prior γ grains in the steel material to enhance the hydrogen embrittlement resistance characteristics of the steel material while maintaining a tensile strength of 900 - 1100 MPa. As a result, it has been clarified that if the crystal grain size of the prior γ grains in the steel material is 20.0 μm or less, excellent hydrogen embrittlement resistance characteristics can be enhanced while maintaining a tensile strength of 900 - 1100 MPa on the condition that the other configurations of the present embodiment are satisfied. Hereinafter, in this specification, the crystal grain size of the prior austenite grains is also referred to as the "prior γ grain size".

[0020] Furthermore, the present inventors focused on dislocations and considered ways to enhance the hydrogen embrittlement resistance of steel materials having the above-described chemical composition. Here, increasing the dislocation density in the steel material increases the strength of the steel material. However, dislocations may absorb hydrogen. Therefore, if the dislocation density in the steel material increases, there is a concern that the amount of hydrogen absorbed by the steel material will also increase, resulting in a decrease in the hydrogen embrittlement resistance of the steel material.

[0021] Therefore, the present inventors considered reducing the dislocation density in the steel material and enhancing the hydrogen embrittlement resistance of the steel material while maintaining a tensile strength of 900 to 1100 MPa. As a result, if the dislocation density of the steel material is reduced to 7.0×10 14 m -2 or less, it has been found that the hydrogen embrittlement resistance of the steel material can be enhanced while maintaining a tensile strength of 900 to 1100 MPa on the condition that the other configurations of the present embodiment are satisfied.

[0022] On the other hand, even in the case of a steel material having the above-described chemical composition, satisfying an old γ grain size of 20.0 μm or less and a dislocation density of 7.0×10 14 m -2 or less, there were cases where excellent hydrogen embrittlement resistance could not be obtained in a steel material having a tensile strength of 900 to 1100 MPa. Therefore, the present inventors further examined in detail methods for enhancing the hydrogen embrittlement resistance.

[0023] Specifically, the present inventors focused on the components of dislocations and examined methods for enhancing the hydrogen embrittlement resistance of steel materials. As a result of detailed examination by the present inventors, it has been clarified that the higher the Q value obtained by analyzing the line profile of X-ray diffraction (XRD) by the modified Williamson-Hall / Warren-Averbach method, the more significantly the hydrogen embrittlement resistance of the steel material increases. In this specification, the XRD line profile means 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 component includes edge dislocations and screw dislocations. An edge dislocation is a defect where an atomic plane is missing from the dislocation line, and the dislocation line and the Burgers vector are perpendicular. A screw dislocation is a defect where the atomic plane is displaced around the dislocation line, and the dislocation line and the Burgers vector are parallel. Furthermore, the higher the Q value, the higher the proportion of screw dislocations in the dislocations.

[0025] As a result of the detailed study by the inventors based on the above findings, for steel materials having the above chemical composition, a tensile strength of 900 to 1100 MPa, an old γ grain size of 20.0 μm or less, and a dislocation density of 7.0×10 14 m -2 It has been clarified that if the Q value is 2.35 or more in the following steel materials, the hydrogen embrittlement resistance characteristics can be significantly improved. Hereinafter, the relationship between the Q value and the hydrogen embrittlement resistance characteristics will be specifically described 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 the hydrogen embrittlement resistance characteristics, in this example. FIG. 1 was created using the examples described below. The tensile strength, the old γ grain size, the dislocation density, and the Q value were determined by the methods described below. 14 m -2 For steel materials having the above chemical composition, a tensile strength of 900 to 1100 MPa, an old γ grain size of 20.0 μm or less, and a dislocation density of 7.0×10

[0027] Referring to FIG. 1, for steel materials having the above chemical composition, a tensile strength of 900 to 1100 MPa, an old γ grain size of 20.0 μm or less, and a dislocation density of 7.0×10 14 m -2 It can be confirmed that if the Q value is 2.35 or more in the following steel materials, the relative fracture stress ratio will be 0.80 or more, showing stable and excellent hydrogen embrittlement resistance characteristics. Therefore, the steel material according to this embodiment has the above chemical composition, a tensile strength of 900 to 1100 MPa, an old γ grain size of 20.0 μm or less, and a dislocation density of 7.0×10 14 m -2The following is the case, and further, 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 characteristics.

[0028] The details of the reason why the hydrogen embrittlement resistance characteristics of the steel material are enhanced by satisfying that the Q value is 2.35 or more are not clear. However, based on the above chemical composition, the prior γ grain size is 20.0 μm or less, and the dislocation density is 7.0×10 14 m -2 or less. It has been proven by the examples described later that the hydrogen embrittlement resistance characteristics of the steel material are enhanced by satisfying that the Q value is 2.35 or more.

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

[0030] [1] A steel material, whose chemical composition is in mass%, 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 consists of Fe and impurities, the tensile strength is 900 to 1100 MPa, the crystal grain size of prior austenite grains is 20.0 μm or less, the dislocation density is 7.0×10 14 m -2 or less, for the steel material, the Q value obtained by analyzing the line profile of X-ray diffraction by the modified Williamson-Hall / Warren-Averbach method is 2.35 or more, Steel material.

[0031] [2] The steel material according to [1], wherein the chemical composition is Mg: 0.0001 to 0.0050%, Ca: 0.0001 to 0.0050%, rare earth element: 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%, contains one or more elements selected from the group consisting of Steel material.

[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, Steel material.

[0033] [4] The steel material according to [3], wherein the steel pipe for high-pressure hydrogen containers is any one of a steel pipe for high-pressure hydrogen accumulators and a steel pipe for high-pressure hydrogen cylinders, Steel material.

[0034] Note that 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 steel (solid material), or a steel plate. Note that the round steel means a bar steel having a circular cross section perpendicular to the axial direction. Also, the steel pipe may be a seamless steel pipe or a welded steel pipe.

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

[0036] In this specification, the "steel pipe for line pipe" means a steel pipe for line pipe use that constitutes a pipeline for transporting production fluid (crude oil or natural gas) collected from an oil well or a gas well. The pipeline is, for example, a flow line for transporting production fluid from an oil well or a gas well, a gathering line for gathering the production fluid transported by the flow line and transporting it to a primary treatment facility, a trunk line for transporting the production fluid that has undergone primary treatment such as dehydration to the vicinity of the market, and a distribution line for transporting it to consumers. The "seamless steel pipe for line pipe" means that the line pipe steel pipe is a seamless steel pipe.

[0037] In this specification, the "steel pipe for high-pressure hydrogen container" is standardized by ISO11439, ANSI / NGV, High-Pressure Gas Safety Act, Container Safety Regulations Example Standards, etc., and means a steel pipe used for a high-pressure hydrogen container in which high-pressure hydrogen gas is stored. The high-pressure hydrogen container is, for example, a high-pressure hydrogen accumulator installed at a hydrogen station or a high-pressure hydrogen cylinder mounted on a fuel cell vehicle. The "seamless steel pipe for high-pressure hydrogen container" means that the high-pressure hydrogen container steel pipe is a seamless steel pipe.

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

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

[0040] C: 0.25 - 0.30% Carbon (C) enhances hardenability and makes the microstructure of the steel material mainly tempered martensite and tempered bainite. As a result, the hydrogen embrittlement resistance characteristics of the steel material are enhanced. C further forms carbides or carbonitrides to increase the strength of the steel material. If the C content is too low, even if the contents of other elements are within the scope of this embodiment, the above effects cannot be fully obtained. On the other hand, if the C content is too high, even if the contents of other elements are within the scope of this embodiment, the carbides in the steel material become excessively numerous, and the hydrogen embrittlement resistance characteristics of the steel material deteriorate. Therefore, the C content is 0.25 - 0.30%. The preferable lower limit of the C content is 0.26%. The preferable upper limit of the C content is 0.29%.

[0041] Si: 0.10 - 0.50% Silicon (Si) deoxidizes the steel and reduces inclusions in the steel material. As a result, the hydrogen embrittlement resistance characteristics of the steel material are enhanced. If the Si content is too low, even if the contents of other elements are within the scope of this embodiment, the above effects cannot be fully obtained. On the other hand, if the Si content is too high, even if the contents of other elements are within the scope of this embodiment, the hydrogen embrittlement resistance characteristics of the steel material deteriorate. Therefore, the Si content is 0.10 - 0.50%. The preferable lower limit of the Si content is 0.12%, more preferably 0.13%, and still more preferably 0.15%. The preferable upper limit of the Si content is 0.49%, more preferably 0.48%.

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

[0043] P: 0.050% or less Phosphorus (P) is an impurity inevitably contained. That is, the lower limit of the P content is more than 0%. If the P content is too high, even if the contents of other elements are within the scope of this embodiment, P segregates at the grain boundaries, and the hydrogen embrittlement resistance characteristics of the steel decrease. 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 the manufacturing cost. Therefore, considering industrial production, the preferable lower limit of the P content is 0.001%, more preferably 0.002%, and even more preferably 0.003%. The preferable 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 inevitably contained impurity. That is, the lower limit of the S content is more than 0%. If the S content is too high, even if the contents of other elements are within the scope of this embodiment, S segregates at the grain boundaries, and the hydrogen embrittlement resistance of the steel material deteriorates. Therefore, the S content is 0.0100% or less. It is preferable that the S content is as low as possible. However, an extreme reduction in the S content significantly increases the manufacturing cost. Therefore, considering industrial production, the preferable lower limit of the S content is 0.0001%, more preferably 0.0003%, still more preferably 0.0005%, and even more preferably 0.0006%. The preferable upper limit of the S content is 0.0098%, more preferably 0.0095%, and still more preferably 0.0090%.

[0045] Al: 0.001 to 0.100% Aluminum (Al) deoxidizes the steel. Al further combines with N to form Al nitride, and due to the pinning effect, the crystal grains are refined, enhancing the hydrogen embrittlement resistance of the steel material. If the Al content is too low, even if the contents of other elements are within the scope of this embodiment, the above effects cannot be fully obtained. On the other hand, if the Al content is too high, even if the contents of other elements are within the scope of this embodiment, coarse oxides are generated, and the hydrogen embrittlement resistance of the steel material deteriorates. Therefore, the Al content is 0.001 to 0.100%. The preferable lower limit of the Al content is 0.003%, more preferably 0.005%, and still more preferably 0.010%. The preferable upper limit of the Al content is 0.095%, more preferably 0.090%, and still more preferably 0.085%. Note that the "Al" content referred to in this specification means the content of "acid-soluble Al", that is, "sol.Al".

[0046] Cr: 0.50 to 1.50% Chromium (Cr) enhances the hardenability of the steel material and improves the hydrogen embrittlement resistance of the steel material. Cr further increases the tempering softening resistance of the steel material and enables high-temperature tempering. As a result, the hydrogen embrittlement resistance of the steel material is enhanced. If the Cr content is too low, even if the contents of other elements are within the scope of this embodiment, the above effects cannot be fully obtained. On the other hand, if the Cr content is too high, even if the contents of other elements are within the scope of this embodiment, coarse carbides are formed and the hydrogen embrittlement resistance of the steel material decreases. Therefore, the Cr content is 0.50 to 1.50%. The preferable lower limit of the Cr content is 0.51%, more preferably 0.52%, still more preferably 0.54%, and even more preferably 0.55%. The preferable upper limit of the Cr content is 1.48%, more preferably 1.45%, still more preferably 1.35%.

[0047] Mo: 0.80 to 2.00% Molybdenum (Mo) enhances the hardenability of the steel material. Mo further increases the tempering softening resistance of the steel material and enables high-temperature tempering. As a result, the hydrogen embrittlement resistance of the steel material is enhanced. Mo further strengthens the prior γ grain boundaries and improves the hydrogen embrittlement resistance of the steel material. If the Mo content is too low, even if the contents of other elements are within the scope of this embodiment, the above effects cannot be fully obtained. On the other hand, if the Mo content is too high, even if the contents of other elements are within the scope of this embodiment, coarse carbides are formed and the hydrogen embrittlement resistance of the steel material decreases. Therefore, the Mo content is 0.80 to 2.00%. The preferable lower limit of the Mo content is 0.83%, more preferably 0.86%, still more preferably 0.90%. The preferable upper limit of the Mo content is 1.96%, more preferably 1.90%, still more preferably 1.86%.

[0048] Ti: 0.002 to 0.010% Titanium (Ti) forms fine precipitates such as Ti nitride, and refines the prior γ grains by the pinning effect, thereby enhancing the hydrogen embrittlement resistance characteristics of the steel material. If the Ti content is too low, even if the contents of other elements are within the scope of this embodiment, the above effects cannot be sufficiently obtained. On the other hand, if the Ti content is too high, even if the contents of other elements are within the scope of this embodiment, coarse Ti nitride is generated. Coarse Ti nitride becomes a crack initiation point. As a result, the hydrogen embrittlement resistance characteristics of the steel material deteriorate. Therefore, the Ti content is 0.002 to 0.010%. The preferable lower limit of the Ti content is 0.003%, more preferably 0.004%. The preferable 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 enhances the hydrogen embrittlement resistance characteristics of the steel material. If the V content is too low, even if the contents of other elements are within the scope of this embodiment, the above effects cannot be sufficiently obtained. On the other hand, if the V content is too high, even if the contents of other elements are within the scope of this embodiment, carbonitrides, etc. are excessively generated, and the hydrogen embrittlement resistance characteristics of the steel material deteriorate. Therefore, the V content is 0.08 to 0.30%. The preferable lower limit of the V content is 0.09%, more preferably 0.10%. The preferable upper limit of the V content is 0.28%, more preferably 0.26%, and still more preferably 0.25%.

[0050] Nb: 0.010 to 0.050% Niobium (Nb) forms carbides and nitrides, etc., and refines the original γ grains by the pinning effect, thereby enhancing the hydrogen embrittlement resistance of the steel material. Nb further forms fine carbides during tempering, enhancing the tempering softening resistance of the steel material and increasing the strength of the steel material. If the Nb content is too low, even if the contents of other elements are within the scope of this embodiment, the above effects cannot be fully obtained. On the other hand, if the Nb content is too high, even if the contents of other elements are within the scope of this embodiment, excessive amounts of carbides and nitrides, etc., are generated, and the hydrogen embrittlement resistance of the steel material rather decreases. Therefore, the Nb content is 0.010 to 0.050%. The preferable lower limit of the Nb content is 0.011%, more preferably 0.013%, and even more preferably 0.015%. The preferable upper limit of the Nb content is 0.047%, more preferably 0.045%.

[0051] B: 0.0001 to 0.0050% Boron (B) enhances the hardenability of the steel material and increases the strength of the steel material. B further suppresses the grain boundary segregation of P and enhances the hydrogen embrittlement resistance of the steel material. If the B content is too low, even if the contents of other elements are within the scope of this embodiment, the above effects cannot be fully obtained. On the other hand, if the B content is too high, even if the contents of other elements are within the scope of this embodiment, coarse B nitrides are generated. Coarse B nitrides become the starting points of cracks. As a result, the hydrogen embrittlement resistance of the steel material decreases. Therefore, the B content is 0.0001 to 0.0050%. The preferable lower limit of the B content is 0.0003%, more preferably 0.0005%, and even more preferably 0.0008%. The preferable 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 inevitably contained. That is, the lower limit of the N content is more than 0%. N combines with Ti to form nitrides, and due to the pinning effect, the old γ grains are refined, enhancing the hydrogen embrittlement resistance of the steel. On the other hand, if the N content is too high, even if the contents of other elements are within the scope of this embodiment, coarse nitrides are formed, and the hydrogen embrittlement resistance of the steel decreases. Therefore, the N content is 0.0100% or less. The preferable lower limit of the N content for obtaining the above effects more effectively is 0.0001%, more preferably 0.0005%, and still more preferably 0.0010%. The preferable upper limit of the N content is 0.0096%, more preferably 0.0095%, and still more preferably 0.0090%.

[0053] O: 0.0100% or less Oxygen (O) is an inevitably contained impurity. That is, the lower limit of the O content is more than 0%. If the O content is too high, even if the contents of other elements are within the scope of this embodiment, coarse oxides are formed, and the hydrogen embrittlement resistance of the steel decreases. Therefore, the O content is 0.0100% or less. It is preferable that the O content is as low as possible. However, an extreme reduction in the O content significantly increases the manufacturing cost. Therefore, considering industrial production, the preferable lower limit of the O content is 0.0001%, more preferably 0.0005%, and still more preferably 0.0010%. The preferable upper limit of the O content is 0.0096%, more preferably 0.0090%, and still more preferably 0.0080%.

[0054] The remainder of the chemical composition of the steel according to this embodiment consists of Fe and impurities. Here, the impurities in the chemical composition are those mixed from ores, scraps, or manufacturing environments as raw materials during the industrial production of the steel, and are not intentionally contained, meaning those allowed within a range that does not adversely affect the steel according to this embodiment.

[0055] [Optional element] 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) instead of a part of Fe. All of these elements are optional elements and may not be contained. When contained, Mg, Ca, and rare earth elements (REM) enhance the hydrogen embrittlement resistance characteristics of the steel material.

[0056] Mg: 0 to 0.0050% Magnesium (Mg) is an optional element and may not be contained. That is, the Mg content may be 0%. When contained, Mg combines with S in the steel material and precipitates as fine Mg sulfide. Along with this, Mg reduces Mn sulfide. Due to both of these effects, the hydrogen embrittlement resistance characteristics of the steel material are enhanced. Even if a little Mg is contained, the above effects can be obtained to some extent. However, if the Mg content is too high, even if the contents of other elements are within the range of this embodiment, the oxides in the steel material coarsen and the hydrogen embrittlement resistance characteristics of the steel material deteriorate. Therefore, the Mg content is 0 to 0.0050%. The preferable lower limit of the Mg content is 0.0001%, more preferably 0.0003%, still more preferably 0.0006%, and even more preferably 0.0010%. The preferable 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 may not be contained. That is, the Ca content may be 0%. When contained, Ca combines with S in the steel and precipitates as fine Ca sulfide. Along with this, Ca reduces Mn sulfide. Due to both of these effects, the hydrogen embrittlement resistance characteristics of the steel are enhanced. Even if a small amount of Ca is contained, the above effects can be obtained to a certain extent. However, if the Ca content is too high, even if the contents of other elements are within the range of this embodiment, the oxides in the steel coarsen and the hydrogen embrittlement resistance characteristics of the steel deteriorate. Therefore, the Ca content is 0 to 0.0050%. The preferable lower limit of the Ca content is 0.0001%, more preferably 0.0003%, and even more preferably 0.0005%. The preferable 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 may not be contained. That is, the REM content may be 0%. When contained, REM combines with S in the steel and precipitates as fine REM sulfide. Along with this, REM reduces Mn sulfide. Due to both of these effects, the hydrogen embrittlement resistance characteristics of the steel are enhanced. Even if a small amount of REM is contained, the above effects can be obtained to a certain extent. However, if the REM content is too high, even if the contents of other elements are within the range of this embodiment, the oxides in the steel coarsen and the hydrogen embrittlement resistance characteristics of the steel deteriorate. Therefore, the REM content is 0 to 0.0050%. The preferable 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 preferable upper limit of the REM content is 0.0045%, more preferably 0.0040%.

[0059] In this specification, REM means one or more elements selected from the group consisting of scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71 which are lanthanoids. Further, the REM content in this specification means 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 in place of a part of Fe. All of these elements are optional elements and may not be contained. When contained, W and Co enhance the hydrogen embrittlement resistance characteristics of the steel material.

[0061] W: 0 to 0.50% Tungsten (W) is an optional element and may not be contained. That is, the W content may be 0%. When contained, W forms a corrosion film on the surface of the steel material in a sour environment. As a result, the ingress of hydrogen into the steel material is suppressed and the hydrogen embrittlement resistance characteristics of the steel material are enhanced. The above effects can be obtained to some extent if W is contained even slightly. However, if the W content is too high, even if the contents of other elements are within the range of this embodiment, coarse carbides are generated and the hydrogen embrittlement resistance characteristics of the steel material deteriorate. Therefore, the W content is 0 to 0.50%. The preferable lower limit of the W content is 0.01%, more preferably 0.03%. The preferable 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 may not be contained. That is, the Co content may be 0%. When contained, Co enhances the hydrogen embrittlement resistance characteristics of the steel material. Co further dissolves in the steel material to enhance the hardenability of the steel material and increase the strength of the steel material. Even if a small amount of Co is contained, the above effects can be obtained to a certain 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 in place of a part of Fe. All of these elements are optional elements and may not be contained. When contained, Cu, Ni, and Sn all enhance the hydrogen embrittlement resistance characteristics of the steel material.

[0064] Cu: 0 to 0.50% Copper (Cu) is an optional element and may not be contained. That is, the Cu content may be 0%. When contained, Cu enhances the hydrogen embrittlement resistance characteristics of the steel material. Cu further dissolves in the steel material to enhance the hardenability of the steel material and increase the strength of the steel material. Even if a small amount of Cu is contained, the above effects can be obtained to a certain extent. However, if the Cu content is too high, the hot workability of the steel material deteriorates even if the contents of other elements are within the range 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 may not be contained. That is, the Ni content may be 0%. When contained, Ni enhances the hydrogen embrittlement resistance characteristics of the steel material. Ni further dissolves in the steel material to enhance the hardenability of the steel material and increase the strength of the steel material. Even if a small amount of Ni is contained, the above effects can be obtained to a certain extent. However, if the Ni content is too high, even if the contents of other elements are within the scope of this embodiment, the manufacturing cost will increase extremely. Therefore, the Ni content is 0 to 0.50%. The preferable lower limit of the Ni content is 0.01%, more preferably 0.03%. The preferable 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 may not be contained. That is, the Sn content may be 0%. When contained, Sn enhances the hydrogen embrittlement resistance characteristics of the steel material. Even if a small amount of Sn is contained, the above effects can be obtained to a certain extent. However, if the Sn content is too high, even if the contents of other elements are within the scope of this embodiment, the hot workability of the steel material will deteriorate. Therefore, the Sn content is 0 to 0.0100%. The preferable 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 preferable upper limit of the Sn content is 0.0098%, more preferably 0.0095%.

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

[0068] In this embodiment, the preferable lower limit of the tensile strength is 910 MPa, more preferably 924 MPa, and even more preferably 931 MPa. In this embodiment, the preferable 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 in accordance with the method specified in JIS Z2241:2011. A test piece is prepared from the steel material according to this embodiment. When the steel material is a steel plate, a tensile test piece is prepared from the center of the plate thickness. In this case, the longitudinal direction of the tensile test piece is parallel to the rolling direction of the steel plate. When the steel material is a steel pipe, a tensile test piece is prepared from the center of the wall thickness. In this case, the longitudinal direction of the tensile test piece is parallel to the pipe axis direction of the steel pipe. When the steel material is a round bar, a tensile test piece is prepared from the R / 2 position. In this specification, the R / 2 position of the round bar means the central 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 piece is parallel to the axial direction of the round bar.

[0070] The tensile test piece is, for example, a round bar test piece with a diameter of 6.0 mm in the parallel part and a parallel part length of 40 mm. When a round bar test piece cannot be prepared from the steel pipe, an arc-shaped test piece is prepared. The size of the arc-shaped test piece is, for example, the full wall thickness for the thickness, a width of 25.4 mm, and a gauge length of 50.8 mm. Using the prepared tensile test piece, a tensile test is carried out at room temperature (25°C) in the air in accordance with JIS Z2241:2011. The maximum stress (MPa) during the uniform elongation obtained by the tensile test is defined as the tensile strength. Also, the 0.2% offset yield strength (MPa) obtained by the tensile test is defined as the yield strength. Note that both the tensile strength (MPa) and the yield strength (MPa) are obtained by rounding off the first decimal place of the obtained numerical value.

[0071] [Crystal grain size of the old austenite grains] In the steel material according to the present embodiment, the crystal grain size of the prior austenite grains (prior γ grains) is 20.0 μm or less. In this specification, the crystal grain size of the prior austenite grains (prior γ grain size) means the crystal grain size of the prior austenite grains determined in accordance with the measurement method of the average section method specified in JIS G0551:2020.

[0072] As described above, hydrogen embrittlement is likely to occur when hydrogen accumulates at grain boundaries. In the steel material having the above 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 occluded 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 characteristics of the steel material can be enhanced. Specifically, if the prior γ grain size of the steel material having the above chemical composition is 20.0 μm or less, on the premise that the other configurations of the present embodiment are satisfied, a tensile strength of 900 to 1100 MPa and excellent hydrogen embrittlement resistance characteristics can be achieved simultaneously.

[0073] In the steel material according to the present embodiment, the preferable upper limit of the prior γ grain size is 19.0 μm, more preferably 18.5 μm, and still more preferably 18.0 μm. In the steel material according to the present embodiment, it is preferable that the prior γ grain size is smaller. In the case of steel materials assumed to be used 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 section method specified in JIS G0551:2020. A test piece having an observation surface is prepared from the steel material according to this embodiment. When the steel material is a steel plate, a test piece having an observation surface including the position of the plate thickness t / 4 which is the observation target area from the center of the plate width and including the rolling direction and the plate thickness direction is prepared. In this specification, the position of the plate thickness t / 4 means the position at a depth of t / 4 from the surface of the steel plate when the plate thickness of the steel plate is t. When the steel material is a steel pipe, a test piece having an observation surface including the center of the wall thickness which is the observation target area and including the pipe axis direction and the pipe diameter direction (wall thickness direction) is prepared. When the steel material is a round steel, a test piece having an observation surface including the position of R / 2 which is the observation target area and including the axial direction and the cross-sectional diameter direction is prepared. The size of the test piece is, for example, 10 mm in length in the rolling direction × 5 mm in the width direction × 10 mm in the thickness 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.

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

[0076] Using the generated photographic image, the grain size number is evaluated in accordance with the measurement method of the mean section method specified in JIS G0551:2020. From the evaluated grain size number, the crystal grain size of the prior austenite grains in each field of view is determined. The arithmetic mean value of the crystal grain sizes of the prior austenite grains determined in 10 fields of view is defined as the crystal grain size (prior γ grain size) (μm) of the prior austenite grains. The prior γ grain size (μm) is obtained by rounding off the second decimal place of the obtained numerical value.

[0077] [Dislocation density] The steel material according to this embodiment has a dislocation density of 7.0×1014 m -2 is as follows. In this specification, the dislocation density means the dislocation density (m -2 ) obtained by analyzing the line profile of X-ray diffraction (XRD) by the Williamson-Hall method. 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 the steel material increases the strength of the steel material. However, dislocations may absorb hydrogen. That is, if the dislocation density in the steel material increases, the amount of hydrogen absorbed by the steel material also increases, and there is a concern that the hydrogen embrittlement resistance characteristics of the steel material may deteriorate. Therefore, if the dislocation density is reduced to such an extent that the strength can be maintained, the hydrogen embrittlement resistance characteristics of the steel material can be improved while maintaining high strength. Specifically, if the dislocation density of the steel material having the above chemical composition is 7.0×10 14 m -2 or less, on the premise that the other configurations of this embodiment are satisfied, a tensile strength of 900 to 1100 MPa and excellent hydrogen embrittlement resistance characteristics can be achieved simultaneously.

[0079] That is, in the steel material according to this embodiment, if the dislocation density is too high, excellent hydrogen embrittlement resistance characteristics 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 or less.

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

[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 central part of the plate thickness. When the steel material is a steel pipe, the test piece is prepared from the central part of the wall thickness. When the steel material is a round steel, the test piece is prepared from the R / 2 position. The size of the test piece is, for example, 20 mm in width × 20 mm in length × 2 mm in thickness. 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 of 20 mm in width × 20 mm in length.

[0082] The observation surface of the test piece is mirror-polished, and further, electrolytic polishing is performed using 10 vol% perchloric acid (acetic acid solvent) to remove the surface layer strain. For the observation surface after electrolytic polishing, an X-ray diffraction apparatus is used to obtain an XRD line profile. In XRD, the line source is CoKα ray, the tube voltage is 30 kV, the tube current is 100 mA, and the half-value width ΔK is measured. Further, in order to measure the half-value width derived from the X-ray diffraction apparatus, a powder of LaB6 (lanthanum hexaboride) is used. Among the obtained line profiles, the half-value widths ΔK of the peaks of the (110), (211), and (220) planes of the body-centered cubic structure (iron) are determined.

[0083] The non-uniform strain ε of the test piece is determined from the half-value width ΔK obtained by the above method and the Williamson-Hall equation (Equation (A)). [Equation] Here, in Equation (A), θ: diffraction angle, λ: wavelength of X-ray, D: crystallite size, are meant.

[0084] Furthermore, using the obtained non-uniform strain ε and Equation (B), the dislocation density ρ (m -2 ) can be determined. Note that the dislocation density (m-2 ) takes the unit as 10 14 m -2 and rounds the second decimal place of the numerical value of the mantissa obtained by expressing the obtained numerical value in exponential notation.

Number

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

[0086] As described above, the Q value obtained by analyzing the line profile of XRD is an index of the dislocation component. In addition, there are edge dislocations and screw dislocations in the dislocation component. Further, when the Q value is 2.35 or more, among the dislocation components, the proportion occupied by the screw dislocation is 0.75 or more. That is, in the steel material according to the present embodiment, among the dislocation components, the screw dislocation occupies 75% or more.

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

[0088] Here, the XRD line profile changes in its peak position, height, shape, etc. due to various factors. Specifically, for example, if the dislocation density increases, the width of the line profile broadens. Therefore, by analyzing the line profile, the quantification of the properties of the steel material represented by the dislocation density has been carried out. Specifically, the Williamson-Hall method and the Warren-Averbach method have been used as analysis methods for the XRD line profile. In recent years, these analysis methods have been further improved, and a modified Williamson-Hall / Warren-Averbach method that takes anisotropy into account and introduces a contrast factor has been used.

[0089] Specifically, in this specification, the Q value is defined by the following formula (C).

Equation

Equation

[0090] Also, in the steel material having the above chemical composition, when using the modified Williamson-Hall / Warren-Averbach method, among the XRD line profiles, the (110), (200), (211), (220), (310), and (222) planes are discussed. Hereinafter, the (110), (200), (211), (220), (310), and (222) planes are collectively referred to as the (hkl) plane.

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

[0092] The obtained half-value width ΔK of the peak of the (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 satisfy the following formula (E).

Equation

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

Equation

[0094] As described above, the left side of formula (F) has a linear dependence on the square of the crystal orientation parameter H of the (hkl) plane hkl . Therefore, a regression analysis by the least squares method is performed to obtain α that shows the best linearity with respect to the square of H hkl . Furthermore, the Q value can be obtained from the slope and intercept of the straight line obtained by the regression analysis. The Q value is obtained by rounding the third decimal place of the obtained numerical value. Also, the Q value obtained by the above method is unitless.

[0095] [Hydrogen embrittlement resistance properties] The steel material according to this embodiment has the above-mentioned chemical composition, a tensile strength of 900 to 1100 MPa, an old γ grain size of 20.0 μm or less, and a dislocation density of 7.0×10 14 m -2The following is the case where 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 characteristics. In this embodiment, the excellent hydrogen embrittlement resistance characteristics can be evaluated by the following method.

[0096] A test piece for evaluating hydrogen embrittlement resistance characteristics is produced from the steel material according to this embodiment. The test piece is a round bar test piece with an annular notch. The test piece has, for example, an outer diameter of 4.0 mm in the parallel part, a length of 25 mm in the parallel part, and an annular notch is formed at the central position in the longitudinal direction of the parallel part. At this time, regarding the notch shape, the depth of the notch is 0.3 mm, the notch angle is 60°, and the radius of curvature at the bottom of the notch is 0.125 mm. When the steel material is a steel plate, a round bar test piece is produced from the central part of the plate width and at the position of t / 4 of the plate thickness. In this case, the longitudinal direction of the round bar test piece with an annular notch is parallel to the rolling direction of the steel plate. When the steel material is a steel pipe, a round bar test piece is produced from the central part of the wall thickness. In this case, the longitudinal direction of the round bar test piece with an annular notch is parallel to the pipe axis direction of the steel pipe. When the steel material is a round steel, a round bar test piece is produced from the R / 2 position. In this case, the longitudinal direction of the round bar test piece with an annular notch is parallel to the axial direction of the round steel.

[0097] Hydrogen is charged into the produced round bar test piece with an annular notch by the cathodic hydrogen charging method. Specifically, a cathodic hydrogen charging solution at room temperature is prepared. The cathodic hydrogen charging solution is an aqueous solution containing a 5 mass% sodium chloride aqueous solution at room temperature, 30 g / L of NH4SCN, and an acetic acid buffer solution, and the pH before the test is adjusted to pH 3.5 with the acetic acid buffer solution.

[0098] With the round bar test piece with an annular notch immersed in the cathodic hydrogen charging solution, hydrogen is charged into the round bar test piece with an annular notch with a potential of -1.5 V and a charging time of 24 hours. At this time, preferably, a zinc plating film is formed on the surface of the round bar test piece with an annular notch charged with hydrogen so that the hydrogen in the round bar test piece with an annular notch does not leak to the outside.

[0099] For each of a hydrogen-charged round bar test piece with an annular notch and a hydrogen-uncharged round bar test piece with an annular notch, a tensile test is carried out at room temperature (25 °C) in the atmosphere using a low strain rate testing machine (SSRT). At this time, the strain rate is set to 4.2×10 -6 / s. The fracture stress BS1 is determined from the hydrogen-charged round bar test piece with an annular notch. The fracture stress BS0 is determined from the hydrogen-uncharged round bar test piece with an annular notch. Note that the fracture stress BS0 and the fracture stress BS1 are determined by rounding off the first decimal place of the obtained numerical values.

[0100] Using the obtained fracture stress BS0 and fracture stress BS1, the relative fracture stress ratio is determined by the following formula. When the obtained relative fracture stress ratio is 0.80 or more, it is determined that the material has excellent hydrogen embrittlement resistance characteristics. Note that the relative fracture stress ratio is determined by rounding off the third decimal place of the obtained numerical value. Relative fracture stress ratio = BS1 / BS0

[0101] [Microstructure] In the microstructure of the steel material according to the present 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 the present embodiment, when the steel material has the above chemical composition, the tensile strength is 900 to 1100 MPa, the prior γ grain size is 20.0 μm or less, the dislocation density is 7.0×10 14 m -2 or less, and the Q value is 2.35 or more, it can be determined that the total area ratio of tempered martensite and tempered bainite of the steel material is 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 having an observation surface parallel to the rolling direction and including the plate thickness t / 4 position, which is the observation target area, is sampled from the center of the plate width. When the steel material is a steel pipe, a test piece having an observation surface parallel to the pipe axis direction and including the center of the wall thickness, which is the observation target area, is prepared. When the steel material is a round bar, a test piece having an observation surface parallel to the rolling direction and including the R / 2 position, which is the observation target area, is sampled. The size of the test piece is not particularly limited. For example, the size of the test piece is 10 mm in the length direction of rolling × 5 mm in the width direction × 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 axis direction, the thickness direction corresponds to the wall thickness direction, and the width direction corresponds to the direction (circumferential direction) perpendicular to the pipe axis direction and the wall thickness direction. When the steel material is a round bar, the rolling direction corresponds to the axial direction, the thickness direction corresponds to the radial direction, and the width direction corresponds to the direction (circumferential direction) perpendicular to the rolling direction and the radial 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. Any 10 fields of view within the observation target area of the etched observation surface are observed using a scanning electron microscope (SEM) in the secondary electron image. When the steel material is a steel plate, the observation target area is the plate thickness t / 4 position. When the steel material is a steel pipe, the observation target area is the center of the wall thickness. When the steel material is a round bar, the observation target area is the R / 2 position. The area of each of the 10 fields of view within the observation target area is, for example, 400 μm 2 (magnification 5000 times).

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

[0105] Determine the total area ratio of the identified tempered martensite and tempered bainite. The method for determining the total area ratio is not particularly limited and may be a well-known method. For example, the total area ratio of tempered martensite and tempered bainite can be determined by image analysis. In this embodiment, the arithmetic mean value of the total area ratios of tempered martensite and tempered bainite obtained in all fields of view (10 fields of view) is defined as the total area ratio (%) of 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 of this embodiment may be a steel pipe, a steel plate, or a round steel (bar steel).

[0107] Preferably, the steel material of the present embodiment 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. The steel pipe for oil wells means a steel pipe for oil well applications. Oil well pipes are, for example, casings, tubing, drill pipes, etc. used for drilling oil wells or gas wells, and extracting crude oil or natural gas. The steel pipe for line pipes means a steel pipe for line pipe applications that constitutes a pipeline for transporting production fluids (crude oil or natural gas) collected from oil wells or gas wells. Pipelines are, for example, flow lines for transporting production fluids from oil wells or gas wells, gathering lines for gathering the production fluids transported by flow lines and transporting them to a primary treatment facility, trunk lines for transporting the production fluids that have undergone primary treatment such as dehydration to the vicinity of the market, and distribution lines for transporting them to consumers. The steel pipe for high-pressure hydrogen containers is standardized according to ISO11439, ANSI / NGV, the High-Pressure Gas Safety Act, the Container Safety Regulations Exemplary Standards, etc., and means a steel pipe used for high-pressure hydrogen containers in which high-pressure hydrogen gas is stored. The steel material of the present embodiment may be a steel pipe for high-pressure hydrogen containers, or may be any one of a steel pipe for high-pressure hydrogen accumulators and a steel pipe for high-pressure hydrogen cylinders.

[0108] More preferably, the steel material of the present 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. The seamless steel pipe for oil wells means that the steel pipe for oil wells is a seamless steel pipe. The seamless steel pipe for line pipes means that the steel pipe for line pipes is a seamless steel pipe. The 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 the present 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] Hereinafter, an example of the manufacturing method of the steel material according to this embodiment will be described. Note that the manufacturing method described below is an example, and the manufacturing method of the steel material according to this embodiment is not limited thereto. That is, as long as the steel material according to this embodiment having the above-described configuration can be manufactured, it is not limited to the manufacturing method described below. However, the manufacturing method described below is a suitable manufacturing method for manufacturing the steel material according to this embodiment.

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

[0111] [Raw Material Preparation Step] In the raw material preparation step, a raw material is manufactured using the molten steel having the above-described chemical composition. The manufacturing method of the raw material is not particularly limited, and a well-known method may be used. Specifically, a slab (slab, bloom, or billet) may be manufactured by continuous casting using molten steel. An ingot may be manufactured by an ingot-making method using molten steel. If necessary, a slab, bloom, or ingot may be block-rolled to manufacture a billet. The raw material (slab, bloom, or billet) is manufactured through the above steps. Hereinafter, the hot working step will be described.

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

[0113] For example, the Mannesmann method may be carried out as hot working to manufacture a plain tube. In this case, a round billet is pierced and rolled by a piercing mill. When piercing and rolling, the piercing ratio is not particularly limited, but for example, it is 1.0 to 4.0. The pierced and rolled round billet is further hot-rolled by a mandrel mill, a reducer, a sizing mill, etc. to obtain a plain tube. Other hot working methods may be carried out to manufacture a plain tube from a billet. For example, when the steel material is a short and thick steel pipe like a coupling, a plain tube may be manufactured by forging such as the Erhardt method. The plain tube is manufactured by the above steps. The wall thickness of the plain tube is not particularly limited, but for example, it is 9 to 60 mm.

[0114] When the steel material is round steel, first, the material is heated in a heating furnace. The heating temperature is not particularly limited, but for example, it is 1100 to 1300 °C. Hot working is carried out on the material extracted from the heating furnace to manufacture intermediate steel material with a circular cross-section perpendicular to the axial direction. The hot working is, for example, block rolling by a block rolling mill or hot rolling by a continuous rolling mill. The continuous rolling mill has a horizontal stand having a pair of grooved rolls arranged side by side in the vertical direction and a vertical stand having a pair of grooved rolls arranged side by side in the horizontal direction, which are alternately arranged. When the steel material is a steel plate, first, the material is heated in a heating furnace. The heating temperature is not particularly limited, but for example, it is 1100 to 1300 °C. Hot rolling is carried out on the material extracted from the heating furnace using a block rolling mill and a continuous rolling mill to manufacture intermediate steel material in the shape of a steel plate.

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

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

[0117] The intermediate steel material produced by hot working may be air-cooled (As-Rolled). The intermediate steel material produced by hot working may be directly quenched after hot working without being cooled to room temperature, or may be quenched after preheating (reheating) after hot working. When directly quenching after hot working or quenching after preheating, cooling may be stopped or slow cooling may be carried out during quenching. In this case, the occurrence of burning cracks in the plain tube can be suppressed. Further, when directly quenching after hot working or quenching after preheating, stress relief annealing (SR) may be carried out after quenching and before the heat treatment of the next process. In this case, the residual stress of the plain tube is removed. The intermediate steel material is manufactured by the above steps. Hereinafter, the heat treatment process will be described.

[0118] [Heat Treatment Process] In the heat treatment process according to this embodiment, preferably, it includes a dislocation adjustment process, a quenching process, and a tempering process. Hereinafter, each process will be described.

[0119] [Dislocation Adjustment Process] In the dislocation adjustment process according to this embodiment, heat treatment for adjusting dislocations is performed on the intermediate steel material produced in the hot working process to adjust the dislocation components between edge dislocations and screw dislocations. By performing the dislocation adjustment process, the ratio of screw dislocations in the dislocations can be stably set to 0.75 or more, and the Q value in the produced steel material can be stably set to 2.35 or more.

[0120] Specifically, in the dislocation adjustment process, the intermediate steel material is held at 980 to 1080 °C for 30 to 90 minutes. If the holding temperature in the dislocation adjustment process is too low, the ratio of helical dislocations cannot be increased sufficiently, and the Q value of the produced steel material may not be 2.35 or more. In this case, excellent hydrogen embrittlement resistance characteristics cannot be obtained. On the other hand, if the holding temperature in the dislocation adjustment process is too high, the old γ grains may coarsen, and excellent hydrogen embrittlement resistance characteristics may not be obtained. Therefore, in the dislocation adjustment process according to this embodiment, it is preferable that the holding temperature of the intermediate steel material is 980 to 1080 °C.

[0121] If the holding time in the dislocation adjustment process is too short, the ratio of helical dislocations cannot be increased sufficiently, and the Q value of the produced steel material may not be 2.35 or more. In this case, excellent hydrogen embrittlement resistance characteristics cannot be obtained. On the other hand, even if the holding time in the dislocation adjustment process is too long, the above effects saturate. Therefore, in the dislocation adjustment process according to this embodiment, it is preferable that the holding time of the intermediate steel material is 30 to 90 minutes.

[0122] In addition, in this specification, the holding temperature in the dislocation adjustment process corresponds to the temperature of the heat treatment furnace when heating and holding the intermediate steel material. The holding time means the time from when the temperature of the intermediate steel material reaches the predetermined holding temperature until it is extracted from the heat treatment furnace.

[0123] Also, after hot working, the dislocation adjustment process may be carried out immediately after hot working without cooling the intermediate steel material to room temperature, or the dislocation adjustment process may be carried out after once cooling the intermediate steel material to room temperature. In any case, the intermediate steel material can be charged into a heating furnace at a desired temperature and the dislocation adjustment process can be carried out. Hereinafter, the quenching process will be described.

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

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

[0126] The quenching method is, for example, continuously cooling the intermediate steel material from the quenching start temperature and continuously lowering the surface temperature of the intermediate steel material. The method of continuous cooling treatment is not particularly limited and may be a well-known method. The method of continuous cooling treatment is, for example, a method of immersing the intermediate steel material in a water tank for cooling or a method of accelerating the cooling of the intermediate steel material by shower water cooling or mist cooling.

[0127] It should be noted that the dislocation adjustment process and the quenching process are preferably carried out continuously. For example, using the same heat treatment furnace and changing the temperature in the heat treatment furnace, the dislocation adjustment process and the quenching process may be carried out. For example, furthermore, using two heat treatment furnaces with different temperatures, the dislocation adjustment process and the quenching process may be carried out. Hereinafter, the tempering process will be described.

[0128] [Tempering Process] In the tempering process, tempering is performed on the intermediate steel material on which the above-mentioned quenching has been carried out. In this specification, "tempering" means tempering the intermediate steel material after quenching at A c1It means reheating and holding at a temperature below the transformation point. Here, the tempering temperature corresponds to the temperature of the heat treatment furnace when reheating and holding the intermediate steel material after quenching. The tempering time means the time from when the temperature of the intermediate steel material reaches a predetermined tempering temperature until it is extracted from the heat treatment furnace.

[0129] The tempering temperature is appropriately adjusted according to the chemical composition of the steel material and the tensile strength to be obtained. That is, for the intermediate steel material having the chemical composition of this embodiment, the tempering temperature is adjusted so that the tensile strength of the steel material is adjusted to 900 - 1100 MPa. In the tempering process according to this embodiment, the preferable tempering temperature is 670°C - A c1 is the transformation point. Also, in the tempering process of this embodiment, the tempering time is preferably 5 - 240 minutes.

[0130] By implementing the above manufacturing process, the steel material according to this embodiment can be manufactured. However, as described above, the above manufacturing method is an example, and it may be manufactured by other manufacturing methods. Hereinafter, the effects of the steel material according to this embodiment will be described more specifically with reference to examples. However, the various conditions in the examples described below are one example of the 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 the conditions described in the examples.

Example

[0131] Molten steel having the chemical compositions shown in Table 1A and Table 1B was produced. Note that “-” in Table 1A and Table 1B means that the content of each element is at the impurity level. Specifically, the Nb content of test number 35 means that it was 0% after rounding the fourth decimal place. The W, Co, Cu, and Ni contents of test number 1 mean that they were 0% after rounding the third decimal place. The Mg, Ca, REM, and Sn contents of test number 1 and the B content of test number 37 mean that they were 0% after rounding the fifth decimal place.

[0132]

Table 1A

[0133] [Table 1B]

[0134] The steel materials of each test number were manufactured by the following method. Ingots were produced from molten steel having the chemical compositions described in Table 1A and Table 1B. For each ingot of each test number obtained, hot forging was carried out to produce a block material with a thickness of 50 mm.

[0135] Hot working was carried out on the block materials of each test number. Specifically, the block materials were heated to 1250 °C. Hot rolling was carried out on the heated block materials to produce intermediate steel materials (steel plates) with a thickness of 15 mm. At this time, for each intermediate steel material of each test number, the area reduction rate (%) in hot working was as shown in Table 2. The intermediate steel materials manufactured by the above method were allowed to cool to room temperature.

[0136] [Table 2]

[0137] Heat treatment was carried out on the steel materials of each test number cooled to room temperature. Specifically, for each intermediate steel material of each test number, a dislocation adjustment process of holding at the holding temperature (°C) in the "Dislocation Adjustment" column shown in Table 2 for the holding time (minutes) was carried out. Further, for each intermediate steel material of each test number on which the dislocation adjustment process was carried out, quenching was carried out by holding at the quenching temperature (°C) in the "Quenching" column shown in Table 2 for the quenching time (minutes) and then water cooling. For each intermediate steel material of each test number after quenching, tempering was carried out by holding at the tempering temperature (°C) in the "Tempering" column shown in Table 2 for the tempering time (minutes). By the above manufacturing process, the steel materials (steel plates) of each test number were manufactured.

[0138] [Evaluation Test] Tensile tests, measurement tests of the old γ grain size, measurement tests of the dislocation density and Q value, and evaluation tests of the hydrogen embrittlement resistance characteristics were carried out on the manufactured steel materials of each test number.

[0139] [Tensile test] Tensile tests were carried out on the steel materials with each test number to determine the tensile strength and yield strength. Specifically, round bar test pieces were prepared from the central part of the plate thickness of the steel materials with each test number. The round bar test pieces were round bar test pieces with a diameter of 6.0 mm in the parallel part and a parallel part length of 40 mm. The axial direction of the round bar test pieces was parallel to the rolling direction of the steel materials. Using the prepared tensile test pieces, a tensile test was carried out at room temperature (25°C) in the atmosphere in accordance with JIS Z2241:2011, and the maximum stress during the obtained uniform elongation was defined as the tensile strength (MPa). The 0.2% offset yield strength (MPa) obtained from the same tensile test was defined as the yield strength. For the steel materials with each test number, the obtained tensile strength was designated as "TS (MPa)" and the yield strength (MPa) was designated as "YS (MPa)", and they are shown in Table 3.

[0140] [Table 3]

[0141] [Old γ grain size measurement test] Old γ grain size measurement tests were carried out on the steel materials with each test number to determine the old γ grain size. Specifically, test pieces were prepared from the central part of the plate width of the steel materials with each test number, with the observation surface being the surface including the position of plate thickness t / 4 in the observation target area and including the rolling direction and the plate thickness direction. The size of the test pieces was 10 mm in length in the rolling direction, 5 mm in length in the plate width direction, and 10 mm in length in the plate thickness direction. The area of 10 mm in length in the rolling direction × 10 mm in length in the plate thickness direction was taken as the observation surface. For the prepared test pieces, the old γ grain size (μm) was determined in accordance with the above-mentioned method. At this time, the field of view area was 500 μm × 500 μm (magnification 200 times). The obtained old γ grain size (μm) is shown in the column of "Old γ grain size (μm)" in Table 3.

[0142] [Dislocation density and Q value measurement test] For the steel materials with each test number, a dislocation density and Q-value measurement test was carried out to obtain the dislocation density and Q-value. Specifically, by the above method, a test piece with a width of 20 mm × a length of 20 mm × a thickness of 2 mm was prepared from the central part of the plate thickness of the steel material with each test number. At this time, the thickness direction of the test piece was prepared to be parallel to the plate thickness direction of the steel material. After mirror polishing the surface (observation surface) of the test piece with a width of 20 mm × a length of 20 mm, electrolytic polishing was carried out using 10 vol% perchloric acid (acetic acid solvent). For the observation surface after electrolytic polishing, an XRD line profile was obtained by the above method. From the obtained XRD line profile, the dislocation density (m -2 ) was obtained. Further, from the obtained XRD line profile, the Q-value was obtained by the above method. The obtained dislocation density (m -2 ) was shown in the column of "Dislocation density (10 14 / m 2 )" in Table 3. The obtained Q-value was shown in Table 3.

[0143] [Hydrogen embrittlement resistance property evaluation test] For the steel materials with each test number, a hydrogen embrittlement resistance property evaluation test was carried out to evaluate the hydrogen embrittlement resistance property. Specifically, two round bar test pieces with an annular notch were prepared from the central part of the plate width and at the position of t / 4 of the plate thickness of the steel material with each test number. The outer diameter of the parallel part of each test piece was 4.0 mm, the length of the parallel part was 25 mm, and an annular notch was formed at the central position in the longitudinal direction of the parallel part. In the notch shape, the depth of the notch was 0.3 mm, the notch angle was 60°, and the curvature radius of the notch bottom was 0.125 mm.

[0144] By the cathode hydrogen charging method, hydrogen was charged to one of the two round bar test pieces with an annular notch. Specifically, a cathode hydrogen charging solution at room temperature was prepared. The cathode hydrogen charging solution was an aqueous solution containing a 5 mass% aqueous sodium chloride solution at room temperature, 30 g / L of NH4SCN, and an acetic acid buffer solution, and the pH before the test was adjusted to pH 3.5 with the acetic acid buffer solution.

[0145] With a circular-notch round bar test piece immersed in a cathode hydrogen charging solution, the potential was set at -1.5 V and the charging time at 24 hours to charge the circular-notch round bar test piece with hydrogen. That is, by charging hydrogen, a sour environment was simulated. A zinc plating film was formed on the surface of the hydrogen-charged circular-notch round bar test piece under the same conditions for each sample code, and the hydrogen inside the circular-notch round bar test piece was prevented from leaking to the outside. For another circular-notch round bar test piece, hydrogen was not charged.

[0146] For the circular-notch round bar test piece with a zinc plating film formed thereon, a tensile test was carried out at a strain rate of 4.2×10 -6 / s at room temperature in the atmosphere using a low strain rate testing machine (SSRT), and the breaking stress BS1 (MPa) in the hydrogen environment was determined.

[0147] Furthermore, for the circular-notch round bar test piece without hydrogen charging for each test number, a tensile test was carried out at a strain rate of 4.2×10 -6 / s at room temperature in the atmosphere using a low strain rate testing machine (SSRT), and the breaking stress BS0 (MPa) in the atmosphere was determined.

[0148] The obtained breaking stress BS0 (MPa) in the atmosphere is shown in the "BS0 (MPa)" column in the "Hydrogen Embrittlement Resistance Characteristics" column in Table 3. The obtained breaking stress BS1 (MPa) in the hydrogen environment is shown in the "BS1 (MPa)" column in the "Hydrogen Embrittlement Resistance Characteristics" column in Table 3. Furthermore, the ratio of the breaking stress BS1 in the hydrogen environment to the breaking stress BS0 in the atmosphere (= BS1 / BS0) is shown in the "Relative Breaking Stress Ratio" column in the "Hydrogen Embrittlement Resistance Characteristics" column.

[0149] [Test Results] Referring to Table 1A, Table 1B, Table 2, and Table 3, the steel materials of Test Nos. 1 to 20 have the above chemical compositions, the tensile strength TS 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 -2The following was the case, 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 had excellent hydrogen embrittlement resistance characteristics. That is, these steel materials had high strength and excellent hydrogen embrittlement resistance characteristics. Note that the steel materials with test numbers 1 to 20 were judged to have 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 with test number 21 had too high a C content. As a result, this steel material had a relative fracture stress ratio of less than 0.80 and did not have excellent hydrogen embrittlement resistance characteristics.

[0151] The steel material with test number 22 had too low a C content. As a result, this steel material had a relative fracture stress ratio of less than 0.80 and did not have excellent hydrogen embrittlement resistance characteristics.

[0152] The steel material with test number 23 had too high an Si content. As a result, this steel material had a relative fracture stress ratio of less than 0.80 and did not have excellent hydrogen embrittlement resistance characteristics.

[0153] The steel material with test number 24 had too low an Si content. As a result, this steel material had a relative fracture stress ratio of less than 0.80 and did not have excellent hydrogen embrittlement resistance characteristics.

[0154] The steel material with test number 25 had too high an Mn content. As a result, this steel material had a relative fracture stress ratio of less than 0.80 and did not have excellent hydrogen embrittlement resistance characteristics.

[0155] The steel material with test number 26 had too low an Mn content. As a result, this steel material had a relative fracture stress ratio of less than 0.80 and did not have excellent hydrogen embrittlement resistance characteristics.

[0156] The steel material with test number 27 had too high a Cr content. As a result, this steel material had a relative fracture stress ratio of less than 0.80 and did not have excellent hydrogen embrittlement resistance characteristics.

[0157] The steel material of Test No. 28 had too low a Cr content. 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 characteristics.

[0158] The steel material of Test No. 29 had too high a Mo content. 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 characteristics.

[0159] The steel material of Test No. 30 had too low a Mo content. 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 characteristics.

[0160] The steel material of Test No. 31 had too high a Ti content. 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 characteristics.

[0161] The steel material of Test No. 32 had too high a V content. 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 characteristics.

[0162] The steel material of Test No. 33 had too low a V content. 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 characteristics.

[0163] The steel material of Test No. 34 had too high a Nb content. 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 characteristics.

[0164] The steel material of Test No. 35 had too low a 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 it did not have excellent hydrogen embrittlement resistance characteristics.

[0165] The steel material of Test No. 36 had too high a B content. 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 characteristics.

[0166] For the steel material with test number 37, the B content was too low. 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 characteristics.

[0167] For the steel material with test number 38, the N content was too high. 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 characteristics.

[0168] For the steel material with test number 39, the tempering temperature was too low. As a result, the dislocation density of this steel material exceeded 7.0×10 14 m -2 . 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 characteristics.

[0169] For the steel material with test number 40, the dislocation adjustment process was not carried out. As a result, the dislocation density of this steel material exceeded 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 material was less than 0.80, and it did not have excellent hydrogen embrittlement resistance characteristics.

[0170] For the steel material with test number 41, the holding temperature of the dislocation adjustment process 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 it did not have excellent hydrogen embrittlement resistance characteristics.

[0171] For the steel material with test number 42, the holding time of the dislocation adjustment process 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 it did not have excellent hydrogen embrittlement resistance characteristics.

[0172] For the steel material with test number 43, the holding temperature of the dislocation adjustment process was too high. As a result, the prior γ grain size of this steel material exceeded 20.0μm. 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 characteristics.

[0173] For the steel material of Test No. 44, the reduction ratio R in the hot working process 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 characteristics.

[0174] For the steel material of Test No. 45, quenching was not performed. As a result, the dislocation density of this steel material exceeded 7.0×10 14 m -2 . 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 characteristics.

[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 the above-described embodiments can be appropriately modified and implemented without departing from the spirit thereof.

Claims

1. A steel material, whose chemical composition is in mass %, 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% or less, O: 0.0100% or less, Mg: 0 - 0.0050%, Ca: 0 - 0.0050%, Rare earth elements: 0 - 0.0050%, W: 0 - 0.50%, Co: 0 - 0.50%, Cu: 0 - 0.50%, Ni: 0 - 0.50%, Sn: 0 - 0.0100%, and, the balance consists of Fe and impurities, whose tensile strength is 900 - 1100 MPa, the crystal grain size of prior austenite grains is 20.0 μm or less, The dislocation density is 7.0×10 14 m -2 or less, for the said steel material, the Q value obtained by analyzing the line profile of X-ray diffraction by the modified Williamson-Hall / Warren-Averbach method is 2.35 or more, Steel material.

2. The steel material according to Claim 1, wherein the chemical composition Mg: 0.0001 - 0.0050%, Ca: 0.0001 - 0.0050%, Rare earth elements: 0.0001 - 0.0050%, W: 0.01 - 0.50%, Co: 0.01 - 0.50%, Cu: 0.01 - 0.50%, Ni: 0.01 - 0.50%, and, Sn: 0.0001 - 0.0100%, contains one or more elements selected from the group consisting of, Steel material.

3. The steel material according to Claim 1 or Claim 2, wherein the said steel material is any one of steel pipes for oil wells, steel pipes for line pipes, and steel pipes for high-pressure hydrogen containers, Steel material.

4. The steel material according to Claim 3, wherein the said steel pipe for high-pressure hydrogen containers is any one of steel pipes for high-pressure hydrogen accumulators and steel pipes for high-pressure hydrogen cylinders, Steel material.