JP7564498B1 - Steel - Google Patents

Abstract

The present disclosure provides a steel material having high strength and excellent hydrogen embrittlement resistance even when the crystal grain size of the prior austenite grains exceeds 5.0 μm. The steel material according to the present disclosure has a chemical composition, in mass %, of C: 0.20% to 0.45%, Si: 0.05 to 1.50%, Mn: 0.01 to 1.00%, P: 0.030% or less, S: 0.0100% or less, Cr: 0.40 to 1.10%, Mo: 0.40 to 1.30%, V: 0.01 to 0.30%, Nb: 0.005 to 0.100%, Ti: 0.001 to 0.030%, Al: 0.10 to 0.20%, and Mn: 0.05 to 0.10%. : 0.005 to 0.100%, B: 0.0005 to 0.0050%, N: 0.0100% or less, O: 0.0050% or less, and the balance being Fe and impurities, the crystal grain size GS of the prior austenite grains is more than 5.0 to 30.0 μm, the yield strength σ is 862 to 965 MPa, the yield point drop Δσ is 40 MPa or more, and the yield point elongation Δε is 1.5% or more.

Description

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

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

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

On the other hand, when steel is used in a sour environment, the surface of the steel comes into contact with a corrosive substance, causing an electrochemical reaction and generating hydrogen on the surface of the steel. This hydrogen makes the steel susceptible to hydrogen embrittlement cracking, typified by sulfide stress corrosion cracking (SSC). Therefore, steel used in a sour environment is required to have not only high strength but also excellent hydrogen embrittlement resistance.

Technology for improving hydrogen embrittlement resistance in steel materials used in sour environments is disclosed in JP 2011-246798 A (Patent Document 1) and JP 2015-38247 A (Patent Document 2).

In Patent Document 1, a predetermined amount of dissolved Mo is secured, prior austenite grains are refined, and _M2C type precipitates are dispersed in an oil well steel pipe made of low alloy steel, thereby improving SSC resistance. Patent Document 1 further improves hydrogen embrittlement resistance by forming Mo segregation regions at prior austenite grain boundaries.

In Patent Document 2, in an oil well steel pipe made of low alloy steel, the hydrogen embrittlement resistance is improved by minimizing Mo segregation regions.

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

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

JP 2011-246798 A JP 2015-38247 A JP 2009-74122 A JP 2017-210645 A

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

Incidentally, techniques have been proposed to improve the hydrogen embrittlement resistance of steel materials by refining prior austenite grains in the steel material. For example, Japanese Patent Laid-Open Publication No. 2017-210645 (Patent Document 4) specifically discloses that "by making prior austenite grains fine with a grain size of 6 μm or less, it is possible to reduce hydrogen embrittlement fracture originating from prior austenite grain boundaries" (paragraph [0012] of Patent Document 4). In this way, techniques have been considered to improve the hydrogen embrittlement resistance of steel materials by refining prior austenite grains.

On the other hand, for steel materials intended for use in sour environments or for high-pressure hydrogen containers, excessive refinement of the prior austenite grains is undesirable in consideration of industrial production. Specifically, for steel materials intended for use in sour environments or for high-pressure hydrogen containers, if the crystal grain size of the prior austenite grains is 5.0 μm or less, the manufacturing costs will be extremely high. Therefore, for steel materials intended for use in sour environments or for high-pressure hydrogen containers, it is preferable to achieve both high strength and excellent hydrogen embrittlement resistance, even if the crystal grain size of the prior austenite grains exceeds 5.0 μm.

The objective of the present disclosure is to provide a steel material having high strength and excellent resistance to hydrogen embrittlement even when the crystal grain size of the prior austenite grains exceeds 5.0 μm.

The steel material according to the present disclosure is
The chemical composition, in mass%, is
C: 0.20% to 0.45%,
Si: 0.05-1.50%,
Mn: 0.01-1.00%,
P: 0.030% or less,
S: 0.0100% or less,
Cr: 0.40-1.10%,
Mo: 0.40-1.30%,
V: 0.01-0.30%,
Nb: 0.005-0.100%,
Ti: 0.001 to 0.030%,
Al: 0.005-0.100%,
B: 0.0005-0.0050%,
N: 0.0100% or less,
O: 0.0050% or less,
W: 0-2.00%,
Co: 0 to 0.20%,
Mg: 0 to 0.0100%,
Ca: 0-0.0100%,
Rare earth elements: 0 to 0.0100%,
Cu: 0 to 0.40%,
Ni: 0 to 0.20%,
Sn: 0 to 0.10%; and
The balance is Fe and impurities,
The grain size GS of the prior austenite grains is more than 5.0 to 30.0 μm,
The yield strength σ is 862 to 965 MPa,
The yield point drop Δσ is 40 MPa or more,
The yield point elongation Δε is 1.5% or more.

The steel material disclosed herein has high strength and excellent resistance to hydrogen embrittlement even when the crystal grain size of the prior austenite grains exceeds 5.0 μm.

FIG. 1 is a diagram showing a stress-strain curve of the steel material according to the present embodiment. FIG. 2 is a diagram showing the stress-strain curve of a steel material that has the same chemical composition, yield strength, and prior γ grain size as the present embodiment but does not have excellent hydrogen embrittlement resistance. FIG. 3 is an enlarged view of a portion of FIG. FIG. 4 is an enlarged view of a portion of FIG.

The inventors first investigated obtaining a high-strength steel material with a yield strength of 862 to 965 MPa (125 to 140 ksi, hereinafter also referred to as "125 ksi class"), assuming use in sour environments and for high-pressure hydrogen containers. In other words, the inventors investigated and examined a method for improving hydrogen embrittlement resistance for steel materials assumed to be used in sour environments and for high-pressure hydrogen containers, even when the yield strength is 125 ksi class and the crystal grain size GS of the prior austenite grains (hereinafter the crystal grain size GS of the prior austenite grains is also referred to as "prior gamma grain size GS") is greater than 5.0 μm. As a result, the inventors obtained the following findings.

The inventors first focused on the chemical composition. As a result, the following components were found to be present in mass percent: C: 0.20%-0.45%, Si: 0.05-1.50%, Mn: 0.01-1.00%, P: 0.030% or less, S: 0.0100% or less, Cr: 0.40-1.10%, Mo: 0.40-1.30%, V: 0.01-0.30%, Nb: 0.005-0.100%, Ti: 0.001-0.030%, Al: 0.005-0.100%, B: 0.0005-0.0050%, N: 0.0100% or less, O: 0. It was considered that if a steel material had a chemical composition of 0.0050% or less, W: 0-2.00%, Co: 0-0.20%, Mg: 0-0.0100%, Ca: 0-0.0100%, rare earth elements: 0-0.0100%, Cu: 0-0.40%, Ni: 0-0.20%, Sn: 0-0.10%, and the balance being Fe and impurities, it would be possible to obtain a 125 ksi-class yield strength and excellent hydrogen embrittlement resistance even if the prior gamma grain size GS exceeds 5.0 μm.

Therefore, the inventors manufactured various steel materials having the above-mentioned chemical composition, 125 ksi-class yield strength, and prior gamma grain size GS of more than 5.0 μm, and evaluated their hydrogen embrittlement resistance. As a result of detailed investigation by the inventors, it was revealed that in steel materials having the above-mentioned chemical composition, 125 ksi-class yield strength, and prior gamma grain size GS of more than 5.0 μm, when the stress-strain curve has a characteristic shape, the hydrogen embrittlement resistance is significantly improved. This point will be specifically explained using the drawings.

Figure 1 is a diagram showing the stress-strain curve of the steel material according to this embodiment. Figure 2 is a diagram showing the stress-strain curve of a steel material that has the same chemical composition, yield strength, and prior gamma grain size GS as this embodiment but does not have excellent hydrogen embrittlement resistance. Both Figures 1 and 2 were obtained by the tensile test described below. Both of the steel materials shown in Figures 1 and 2 had the above-mentioned chemical composition and a yield strength of 125 ksi class, and a prior gamma grain size GS of more than 5.0 μm. On the other hand, the steel material shown in Figure 1 had excellent hydrogen embrittlement resistance, while the steel material shown in Figure 2 did not have excellent hydrogen embrittlement resistance.

With reference to FIG. 1, the stress-strain curve shown in FIG. 1 can be divided into three regions. With reference to FIG. 1, first, a region (elastic deformation region) in which the stress increases monotonically from 0 MPa as the strain increases from 0% can be confirmed. With further increase in strain after the stress reaches its maximum value, a region (Luders deformation region) in which the stress drops sharply and the stress remains almost constant even if the strain increases further can be confirmed. With further increase in strain thereafter, a region (plastic deformation region) in which the stress increases again and then drops can be confirmed. On the other hand, with reference to FIG. 2, the stress-strain curve shown in FIG. 2 can be divided into two regions. With reference to FIG. 2, first, a region (elastic deformation region) in which the stress increases monotonically from 0 MPa as the strain increases from 0% can be confirmed. With further increase in strain thereafter, a region (plastic deformation region) in which the stress increases and then drops can be confirmed.

In other words, comparing Figures 1 and 2, the stress-strain curve shown in Figure 1 has a region (Luders deformation region) between the elastic deformation region and the plastic deformation region where the stress suddenly drops and remains almost constant even as the strain increases. Here, the magnitude of the drop in stress when the stress suddenly drops from the elastic deformation region is called the yield point drop Δσ (MPa). Furthermore, the magnitude of strain in the region where the stress remains almost constant even as the strain increases is called the yield point elongation Δε (%). These will be explained in more detail using the drawings.

Figures 3 and 4 are enlarged views of a portion of Figure 1. Figure 3 is an enlarged view of the region of Figure 1 where the strain is 0 to 2.0% and the stress is 850 to 1050 MPa. Referring to Figure 3, the point where the stress in the elastic deformation region of the stress-strain curve is the maximum value is defined as P0, and the stress at point P0 is defined as σ0 (MPa). Furthermore, referring to Figure 3, the intersection point of the stress-strain curve and the straight line L is defined as P, and the stress at point P is defined as σ (MPa). The straight line L is parallel to the elastic deformation region of the stress-strain curve and is a straight line that is translated 0.2% in the positive direction of the strain. In other words, the stress σ at point P corresponds to the so-called 0.2% offset yield strength. In this specification, the difference between the stress σ0 (MPa) at point P0 and the stress σ (MPa) at point P is defined as the yield point drop Δσ (MPa). In this specification, the yield strength (MPa) is defined as the stress σ (MPa) at point P.

Figure 4 is an enlarged view of the 0-4.0% strain region of Figure 1. Referring to Figure 4, the point on the stress-strain curve where the plastic deformation region begins is designated as Q. Here, the strain (%) at point Q is defined as the yield point elongation Δε (%). Point Q is the inflection point of the stress-strain curve, and can be identified by a person skilled in the art.

The present inventors further studied the yield point drop Δσ (MPa) and yield point elongation Δε (%) defined above. As a result of further detailed study by the present inventors, it was revealed that in a steel material having the above-mentioned chemical composition and 125 ksi-class yield strength σ and a prior γ grain size GS of more than 5.0 μm, if the yield point drop Δσ is 40 MPa or more and the yield point elongation Δε is 1.5% or more, excellent hydrogen embrittlement resistance can be obtained.

Details of why excellent hydrogen embrittlement resistance can be obtained in steel having the above-mentioned chemical composition, 125 ksi-class yield strength σ, and prior γ grain size GS exceeding 5.0 μm, if the yield point drop Δσ is 40 MPa or more and the yield point elongation Δε is 1.5% or more, are not clear. However, the inventors speculate as follows. The stress limit at which hydrogen embrittlement fracture occurs in steel may be closely related to plastic deformation. Although details are not clear, the more difficult it is for the steel to undergo plastic deformation, the higher the stress limit at which hydrogen embrittlement fracture occurs.

Now, referring to Figure 1, in steel materials for which a yield point drop Δσ has been confirmed, elastic deformation is maintained even at stresses higher than the yield strength σ. In other words, the larger the yield point drop Δσ, the less likely plastic deformation will occur, and the higher the stress limit at which hydrogen embrittlement fracture occurs. In other words, the larger the yield point drop Δσ, the higher the hydrogen embrittlement resistance.

Referring further to Figure 1, the larger the yield point elongation Δε, the less likely plastic deformation will occur even if the amount of strain introduced into the steel increases. Therefore, the larger the yield point elongation Δε, the higher the potential stress limit for hydrogen embrittlement fracture. In other words, the larger the yield point elongation Δε, the higher the potential for hydrogen embrittlement resistance.

The inventors speculate that the hydrogen embrittlement resistance of a steel material having the above-mentioned chemical composition, 125 ksi-grade yield strength σ, and a prior γ grain size GS of more than 5.0 μm is improved by setting the yield point drop Δσ to 40 MPa or more and the yield point elongation Δε to 1.5% or more. It is possible that the hydrogen embrittlement resistance of the steel material is improved by a mechanism different from the above mechanism. However, it has been proven by the examples described below that a steel material having the above-mentioned chemical composition, a yield point drop Δσ of 40 MPa or more, and a yield point elongation Δε of 1.5% or more can obtain a high 125 ksi-grade yield strength σ and excellent hydrogen embrittlement resistance even if the prior γ grain size GS is 5.0 μm or more.

The steel material of this embodiment, completed based on the above findings, has the following configuration.

[1]
A steel material,
The chemical composition, in mass%, is
C: 0.20% to 0.45%,
Si: 0.05-1.50%,
Mn: 0.01-1.00%,
P: 0.030% or less,
S: 0.0100% or less,
Cr: 0.40-1.10%,
Mo: 0.40-1.30%,
V: 0.01-0.30%,
Nb: 0.005-0.100%,
Ti: 0.001 to 0.030%,
Al: 0.005-0.100%,
B: 0.0005-0.0050%,
N: 0.0100% or less,
O: 0.0050% or less,
W: 0-2.00%,
Co: 0 to 0.20%,
Mg: 0 to 0.0100%,
Ca: 0-0.0100%,
Rare earth elements: 0 to 0.0100%,
Cu: 0 to 0.40%,
Ni: 0 to 0.20%,
Sn: 0 to 0.10%; and
The balance is Fe and impurities,
The grain size GS of the prior austenite grains is more than 5.0 to 30.0 μm,
The yield strength σ is 862 to 965 MPa,
The yield point drop Δσ is 40 MPa or more,
The yield point elongation Δε is 1.5% or more.
Steel.

[2]
The steel material according to [1],
The chemical composition is
W: 0.01-2.00%,
Co: 0.01-0.20%,
Mg: 0.0001 to 0.0100%,
Ca: 0.0001-0.0100%,
Rare earth elements: 0.0001 to 0.0100%,
Cu: 0.01-0.40%,
Ni: 0.01 to 0.20%, and
Sn: 0.01 to 0.10%,
Contains one or more elements selected from the group consisting of
Steel.

[3]
The steel material according to [1],
The grain size GS of the prior austenite grains, the yield strength σ, and the yield point elongation Δε satisfy the following formula (1):
Steel.
σ×Δε/GS ² ≧12 (1)
Here, the yield strength of the steel material is substituted in MPa in σ in formula (1), the yield point elongation of the steel material is substituted in %, and the grain size of prior austenite grains of the steel material is substituted in μm in GS in formula (1).

[4]
The steel material according to [2],
The grain size GS of the prior austenite grains, the yield strength σ, and the yield point elongation Δε satisfy the following formula (1):
Steel.
σ×Δε/GS ² ≧12 (1)
Here, the yield strength of the steel material is substituted in MPa in σ in formula (1), the yield point elongation of the steel material is substituted in %, and the grain size of prior austenite grains of the steel material is substituted in μm in GS in formula (1).

[5]
The steel material according to any one of [1] to [4],
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.

[6]
The steel material according to any one of [1] to [4],
The steel material 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.
Steel.

[7]
The steel material according to [5],
The steel pipe for a high-pressure hydrogen container is either a steel pipe for a high-pressure hydrogen storage tank or a steel pipe for a high-pressure hydrogen cylinder.
Steel.

[8]
The steel material according to [6],
The seamless steel pipe for a high-pressure hydrogen container is either a seamless steel pipe for a high-pressure hydrogen storage tank or a seamless steel pipe for a high-pressure hydrogen cylinder.
Steel.

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

In this specification, "steel pipe for line pipe" means steel pipe for line pipe use constituting a pipeline for transporting produced fluid (crude oil or natural gas) extracted from an oil well or gas well. Examples of pipelines include flow lines that transport produced fluid from oil wells or gas wells, gathering lines that collect the produced fluid transported by the flow lines and transport it to a primary treatment facility, trunk lines that transport produced fluid that has undergone primary treatment such as dehydration to the vicinity of the market, and distribution lines that transport it to consumers. "Seamless steel pipe for line pipe" means that the steel pipe for line pipe is a seamless steel pipe.

In this specification, "steel pipe for high-pressure hydrogen containers" refers to steel pipe that is standardized by ISO11439, ANSI/NGV, the High Pressure Gas Safety Act, Illustrative Standards for Container Safety Regulations, etc. and is used for high-pressure hydrogen containers in which high-pressure hydrogen gas is stored. Examples of high-pressure hydrogen containers are high-pressure hydrogen accumulators installed at hydrogen stations and high-pressure hydrogen cylinders mounted on fuel cell vehicles. "Seamless steel pipe for high-pressure hydrogen containers" means that the steel pipe for high-pressure hydrogen containers is a seamless steel pipe.

The steel material according to this embodiment will be described in detail below. "%" for elements means mass % unless otherwise specified.

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

C: 0.20% to 0.45%
Carbon (C) improves hardenability and makes the microstructure of the steel mainly composed of tempered martensite and tempered bainite. As a result, the hydrogen embrittlement resistance of the steel is improved. C also forms carbides or carbonitrides. If the C content is too low, the above effects cannot be sufficiently obtained even if the contents of other elements are within the range of this embodiment. For example, even if the contents of other elements are within the ranges of this embodiment, the amount of carbides in the steel material will be excessively large. In this case, the strength of the steel material will be too high, and the hydrogen embrittlement resistance of the steel material will be reduced. Therefore, the C content is 0.20% to 0.45%. The lower limit of the C content is preferably 0.21%, more preferably 0.22%, and further preferably 0.24%. The upper limit of the C content is preferably 0.40%, more preferably 0.38%, and further preferably 0.36%.

Si: 0.05-1.50%
Silicon (Si) deoxidizes steel and reduces inclusions in the steel, which increases the hydrogen embrittlement resistance of the steel. If the Si content is too low, the other element contents will be On the other hand, if the Si content is too high, the hydrogen resistance of the steel material may be poor even if the contents of other elements are within the ranges of the present embodiment. The embrittlement resistance is deteriorated. Therefore, the Si content is 0.05 to 1.50%. The lower limit of the Si content is preferably 0.06%, more preferably 0.08%, and further preferably The upper limit of the Si content is preferably 1.45%, more preferably 1.42%, and even more preferably 1.40%. %, more preferably 1.38%, and even more preferably 1.35%.

Mn: 0.01-1.00%
Manganese (Mn) deoxidizes the steel. Mn also improves the hardenability and strength of the steel. If the Mn content is too low, the other element contents may be within the range of this embodiment. On the other hand, if the Mn content is too high, even if the contents of other elements are within the ranges of this embodiment, coarse sulfide-based inclusions are generated, The hydrogen embrittlement resistance of the steel material is deteriorated. Therefore, the Mn content is 0.01 to 1.00%. The lower limit of the Mn content is preferably 0.02%, and more preferably 0.04%. The upper limit of the Mn content is preferably 0.97%, more preferably 0.95%, even more preferably 0.90%, and still more preferably 0. . 80%.

P: 0.030% or less Phosphorus (P) is an unavoidably contained impurity. 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 range of this embodiment, P will segregate at the grain boundaries, and the hydrogen embrittlement resistance of the steel material will decrease. Therefore, the P content is 0.030% or less. It is preferable that the P content is as low as possible. However, an extreme reduction in the P content significantly increases the manufacturing cost. Therefore, in consideration of industrial production, the preferred lower limit of the P content is 0.001%, more preferably 0.002%, and even more preferably 0.003%. The preferred upper limit of the P content is 0.025%, more preferably 0.023%, even more preferably 0.021%, and even more preferably 0.020%.

S: 0.0100% or less Sulfur (S) is an impurity that is inevitably contained. 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 range of this embodiment, S will segregate at the grain boundaries, and the hydrogen embrittlement resistance of the steel material will decrease. 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, in consideration of industrial production, the preferred lower limit of the S content is 0.0001%, more preferably 0.0002%, even more preferably 0.0003%, and even more preferably 0.0005%. The preferred upper limit of the S content is 0.0090%, more preferably 0.0080%, and even more preferably 0.0070%.

Cr:0.40~1.10%
Chromium (Cr) enhances the hardenability of steel and improves its hydrogen embrittlement resistance. Cr also enhances the tempering softening resistance of steel, making high-temperature tempering possible. As a result, the hydrogen embrittlement resistance of steel is improved. If the Cr content is too low, the above effects cannot be sufficiently obtained even if the contents of other elements are within the range of this embodiment. Even if the content of Cr is within the range of this embodiment, coarse carbides are generated, and the hydrogen embrittlement resistance of the steel material is reduced. Therefore, the Cr content is set to 0.40 to 1.10%. The lower limit of the Cr content is preferably 0.43%, more preferably 0.44%, further preferably 0.46%, and further preferably 0.50%. The upper limit is preferably 1.08%, more preferably 1.05%, further preferably 0.99%, and further preferably 0.95%.

Mo: 0.40~1.30%
Molybdenum (Mo) improves the hardenability of steel. Mo also improves the temper softening resistance of steel, making high-temperature tempering possible. As a result, the hydrogen embrittlement resistance of steel is improved. Mo also improves the hardenability of steel. It segregates at the austenite grain boundaries and improves the hydrogen embrittlement resistance of the steel material. If the Mo content is too low, the above effect cannot be sufficiently obtained even if the contents of other elements are within the range of this embodiment. On the other hand, if the Mo content is too high, even if the contents of other elements are within the ranges of this embodiment, coarse carbides are generated, and the hydrogen embrittlement resistance of the steel material is reduced. The Mo content is 0.40 to 1.30%. The lower limit of the Mo content is preferably 0.43%, more preferably 0.44%, and even more preferably 0.48%. The upper limit of the Mo content is preferably 1.25%, more preferably 1.24%, further preferably 1.20%, and further preferably 1.17%. It is.

V:0.01~0.30%
Vanadium (V) forms carbides, nitrides, or carbonitrides (hereinafter referred to as "carbonitrides, etc.") and increases the strength of steel. If the V content is too low, the contents of other elements will be too high. Even if the V content is within the range of the embodiment, the above-mentioned 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 range of the embodiment, carbonitrides, etc. As a result, the strength of the steel becomes too high, and the hydrogen embrittlement resistance of the steel deteriorates. Therefore, the V content is 0.01 to 0.30%. The lower limit is 0.02%, more preferably 0.03%, further preferably 0.04%, further preferably 0.06%, and further preferably 0.08%. The upper limit of the content is preferably 0.28%, more preferably 0.24%, and further preferably 0.22%.

Nb: 0.005-0.100%
Niobium (Nb) forms carbonitrides and the like, and during austenitizing heat treatment before quenching, it refines the crystal grains of the steel material by the pinning effect of the austenite grain boundaries, thereby increasing the yield point drop Δσ of the steel material. Nb also forms fine carbides during tempering to increase the temper softening resistance of the steel, thereby increasing the strength of the steel. If the Nb content is too low, the steel will not be able to withstand the effects of other elements. On the other hand, if the Nb content is too high, the carbon nanotube may not be able to obtain the above-mentioned effects even if the contents of the other elements are within the ranges of the present embodiment. If excessive amounts of nitrides are formed, the hydrogen embrittlement resistance of the steel material will be reduced. Therefore, the Nb content is 0.005 to 0.100%. The preferred lower limit of the Nb content is 0.006 %, more preferably 0.007%, even more preferably 0.010%, and even more preferably 0.015%. The upper limit of the Nb content is preferably 0.097%, more preferably 0.095%, further preferably 0.090%, and further preferably 0.085%.

Ti: 0.001-0.030%
Titanium (Ti) forms fine precipitates such as Ti nitrides, and during the austenitizing heat treatment before quenching, it refines the austenite grains through a pinning effect, thereby improving the hydrogen embrittlement resistance of the steel. If the Ti content is too low, the above-mentioned effects cannot be sufficiently obtained even if the contents of the other elements are within the ranges of this embodiment. Even if the Ti content is within the range of the morphology, coarse Ti nitrides are generated. The coarse Ti nitrides become the starting points of cracks. As a result, the hydrogen embrittlement resistance of the steel material is reduced. Therefore, the Ti content is The Ti content is preferably 0.001 to 0.030%. The lower limit of the Ti content is preferably 0.002%, more preferably 0.003%, and further preferably 0.005%. The upper limit is 0.029%, more preferably 0.028%, further preferably 0.027%, and further preferably 0.025%.

Al: 0.005-0.100%
Aluminum (Al) deoxidizes steel. Furthermore, Al combines with N to form Al nitrides, which refines the crystal grains through a pinning effect and improves the hydrogen embrittlement resistance of steel. If the content of Al is too low, the above-mentioned effect cannot be sufficiently obtained even if the contents of other elements are within the range of this embodiment. Even if the Al content is within the range of the form, coarse oxides are generated, and the hydrogen embrittlement resistance of the steel material is deteriorated. Therefore, the Al content is 0.005 to 0.100%. The lower limit of the Al content is preferably 0.006%, more preferably 0.008%, and further preferably 0.012%. The upper limit of the Al content is preferably 0.095%, and more preferably 0.090%. %, and more preferably 0.085%. In this specification, the "Al" content means the content of "acid-soluble Al", that is, "sol. Al".

B: 0.0005-0.0050%
Boron (B) improves the hardenability of steel and increases its strength. B also suppresses the grain boundary segregation of P and improves the hydrogen embrittlement resistance of steel. If the B content is too low, If the B content is too high, the above effect cannot be sufficiently obtained even if the other element contents are within the range of this embodiment. Even if the B content is 0.0005 to 0.5%, coarse B nitrides are formed. The coarse B nitrides become the starting points of cracks. As a result, the hydrogen embrittlement resistance of the steel material is reduced. Therefore, the B content should be 0.0005 to 0.5%. The preferred lower limit of the B content is 0.0006%, more preferably 0.0008%, and further preferably 0.0010%. The preferred upper limit of the B content is 0. The content of the manganese oxide is preferably 0.0045%, more preferably 0.0040%, even more preferably 0.0035%, and even more preferably 0.0030%.

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 during austenitizing heat treatment before quenching, the austenite grains of the steel are refined by the pinning effect, thereby increasing the strength of the steel. On the other hand, if the N content is too high, even if the contents of other elements are within the range of this embodiment, coarse nitrides are formed, and the hydrogen embrittlement resistance of the steel is reduced. Therefore, the N content is 0.0100% or less. The preferred lower limit of the N content to more effectively obtain the above effect is 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%. The preferred upper limit of the N content is 0.0096%, more preferably 0.0090%, even more preferably 0.0080%, and even more preferably 0.0070%.

O: 0.0050% or less Oxygen (O) is an impurity that is inevitably contained. 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 range of this embodiment, coarse oxides are formed, and the hydrogen embrittlement resistance of the steel material is reduced. Therefore, the O content is 0.0050% 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, in consideration of industrial production, the preferred lower limit of the O content is 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%. The preferred upper limit of the O content is 0.0045%, more preferably 0.0040%, and even more preferably 0.0035%.

The remainder of the chemical composition of the steel material according to this embodiment is composed of Fe and impurities. Here, impurities in the chemical composition refer to substances that are mixed in from raw materials such as ore, scrap, or the manufacturing environment when the steel material is industrially manufactured, and are not intentionally contained, but are permissible within a range that does not adversely affect the steel material according to this embodiment.

[Optional element]
The chemical composition of the steel material according to the present embodiment may further contain, instead of a part of Fe, one or more elements selected from the group consisting of W and Co. All of these elements are optional elements and may not be contained. When contained, W and Co enhance the hydrogen embrittlement resistance of the steel material.

W: 0 to 2.00%
Tungsten (W) is an optional element and may not be contained. That is, the W content may be 0%. When W is contained, W forms a corrosion film on the surface of the steel material in a sour environment. As a result, hydrogen penetration into the steel material is suppressed, and the hydrogen embrittlement resistance of the steel material is improved. If even a small amount of W is contained, the above effect can be obtained to a certain extent. 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 of the steel material is reduced. Therefore, the W content is 0 to 2.00%. The preferred lower limit of the W content is 0.01%, more preferably 0.05%, and even more preferably 0.08%. The preferred upper limit of the W content is 1.50%, more preferably 1.20%, and even more preferably 1.00%.

Co: 0-0.20%
Cobalt (Co) is an optional element and may not be contained. In other words, the Co content may be 0%. When contained, Co enhances the hydrogen embrittlement resistance of the steel material. Furthermore, Co dissolves in the steel material to improve the hardenability of the steel material and increase the strength of the steel material. If even 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 effects are not obtained. Therefore, the Co content is 0 to 0.20%. The lower limit of the Co content is preferably 0.01%, more preferably 0.02%, and further preferably 0.04%. The upper limit of the Co content is preferably 0.18%, more preferably 0.15%, further preferably 0.12%, and further preferably 0.10%.

The chemical composition of the steel material according to this embodiment may further contain, in place of a portion of Fe, one or more elements selected from the group consisting of Mg, Ca, and rare earth elements (REM). All of these elements are optional elements and do not have to be contained. When contained, Mg, Ca, and rare earth elements (REM) enhance the hydrogen embrittlement resistance of the steel material.

Mg: 0-0.0100%
Magnesium (Mg) is an optional element and may not be contained. In other words, the Mg content may be 0%. If magnesium is contained, it combines with S in the steel material to form fine Mg sulfides. As a result, Mg reduces Mn sulfides. Both of these effects improve the hydrogen embrittlement resistance of steel. Even if only a small amount of Mg is included, 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 ranges of this embodiment, the oxides in the steel material will become coarse, and the hydrogen embrittlement resistance of the steel material will deteriorate. The Mg content is 0 to 0.0100%. The lower limit of the Mg content is preferably 0.0001%, more preferably 0.0003%, further preferably 0.0006%, and further preferably is 0.0010%. The upper limit of the Mg content is preferably 0.0090%, more preferably 0.0080%, further preferably 0.0070%, further preferably 0.0060%, and further preferably 0.0050%. %, more preferably 0.0040%, and even more preferably 0.0030%.

Ca: 0~0.0100%
Calcium (Ca) is an optional element and may not be contained. In other words, the Ca content may be 0%. When calcium is contained, calcium bonds with sulfur in the steel material to form fine calcium sulfides. Ca precipitates as manganese sulfides. In addition, Ca reduces manganese sulfides. Both of these effects improve the hydrogen embrittlement resistance of the steel. Even if the content of Ca is even a small amount, 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 ranges of this embodiment, the oxides in the steel material will become coarse, and the hydrogen embrittlement resistance of the steel material will deteriorate. The Ca content is 0 to 0.0100%. The lower limit of the Ca content is preferably 0.0001%, more preferably 0.0003%, further preferably 0.0005%, and further preferably It is 0.0007%. The upper limit of the Ca content is preferably 0.0080%, more preferably 0.0060%, further preferably 0.0050%, and further preferably 0.0040%.

Rare earth elements (REM): 0 to 0.0100%
Rare earth elements (REM) are optional elements and may not be contained. In other words, the REM content may be 0%. When contained, REM combines with S in the steel material to form fine REM sulfides. In addition, REM reduces Mn sulfides. Both of these effects improve the hydrogen embrittlement resistance of steel. Even if only a small amount of REM is included, the above effects can be achieved to some extent. However, if the REM content is too high, even if the contents of other elements are within the ranges of this embodiment, the oxides in the steel material will become coarse, and the hydrogen embrittlement resistance of the steel material will deteriorate. The REM content is 0 to 0.0100%. The lower limit of the REM content is preferably 0.0001%, more preferably 0.0003%, even more preferably 0.0005%, and even more preferably is 0.0010%. The upper limit of the REM content is preferably 0.0090%, more preferably 0.0080%, further preferably 0.0070%, further preferably 0.0060%, and further preferably 0.0050%. %, and more preferably 0.0040%.

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

The chemical composition of the steel material according to this embodiment may further contain, in place of a portion of Fe, one or more elements selected from the group consisting of Cu, Ni, and Sn. All of these elements are optional elements and do not have to be contained. When contained, Cu, Ni, and Sn all increase the hydrogen embrittlement resistance of the steel material.

Cu: 0-0.40%
Copper (Cu) is an optional element and may not be contained. In other words, the Cu content may be 0%. When contained, Cu enhances the hydrogen embrittlement resistance of the steel material. Cu Furthermore, Cu dissolves in the steel material to improve the hardenability of the steel material and increase its strength. Even if even 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, other Even if the element content is within the range of this embodiment, the hot workability of the steel material is reduced. Therefore, the Cu content is 0 to 0.40%. The preferable lower limit of the Cu content is 0.01 %, more preferably 0.02%, further preferably 0.03%, further preferably 0.05%, and further preferably 0.07%. is 0.38%, more preferably 0.36%, and further preferably 0.34%.

Ni: 0-0.20%
Nickel (Ni) is an optional element and may not be contained. In other words, the Ni content may be 0%. When contained, Ni enhances the hydrogen embrittlement resistance of the steel material. Ni Ni also dissolves in the steel material to improve the hardenability and strength of the steel material. Even if even 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, other Even if the element content is within the range of this embodiment, the manufacturing cost will be extremely high. Therefore, the Ni content is 0 to 0.20%. The preferable lower limit of the Ni content is 0.01%. The upper limit of the Ni content is preferably 0.19%, more preferably 0.02%, further preferably 0.03%, and further preferably 0.05%. 18%, and more preferably 0.17%.

Sn: 0-0.10%
Tin (Sn) is an optional element and may not be contained. In other words, the Sn content may be 0%. When contained, Sn enhances the hydrogen embrittlement resistance of the steel material. Sn However, if the Sn content is too high, the hot workability of the steel material is deteriorated even if the contents of other elements are within the range of this embodiment. Therefore, the Sn content is 0 to 0.10%. The lower limit of the Sn content is preferably 0.01%, more preferably 0.02%, and further preferably 0.03%. The upper limit of the Sn content is preferably 0.09%, more preferably 0.08%, and further preferably 0.07%.

[Grain size GS of prior austenite grains]
In the steel material according to the present embodiment, the grain size GS of the prior austenite grains is more than 5.0 to 30.0 μm. In this specification, the grain size GS of the prior austenite grains (prior γ grain size GS) means the grain size of the prior austenite grains determined in accordance with the measurement method of the average intercept method specified in JIS G0551:2020.

Hydrogen embrittlement is likely to occur when hydrogen accumulates at grain boundaries. In a steel material having the above-mentioned chemical composition, if the prior austenite grains are fine, the area of the prior austenite grain boundaries increases. In this case, even if the amount of hydrogen absorbed in the steel material is the same, the amount of hydrogen accumulated per unit area of the prior austenite grain boundaries decreases. Therefore, if the prior austenite grains are fine, the hydrogen embrittlement resistance of the steel material can be improved. Specifically, if the prior γ grain size GS of the steel material having the above-mentioned chemical composition is 30.0 μm or less, it is possible to achieve both a high yield strength of 125 ksi class and excellent hydrogen embrittlement resistance even if the prior γ grain size GS is more than 5.0 μm, provided that the other configurations of this embodiment are satisfied.

The value of the prior gamma grain size GS varies depending on the chemical composition of the steel and the manufacturing method. Specifically, as described above, Ti and Nb form fine precipitates such as Ti nitrides and Nb carbonitrides during the austenitizing heat treatment before quenching, and refine the crystal grains by the pinning effect of the austenite grain boundaries. Also, as described later, if the quenching temperature in the quenching process is too high, the crystal grains may become coarse, and the prior gamma grain size GS of the manufactured steel may become large. In this way, it is possible for a person skilled in the art to control the prior gamma grain size GS to some extent by adjusting the chemical composition of the steel and the manufacturing method.

In the steel material according to the present embodiment, the preferred upper limit of the prior γ grain size GS is 28.0 μm, more preferably 26.0 μm, even more preferably 24.0 μm, and even more preferably 20.0 μm. In the steel material according to the present embodiment, the prior γ grain size GS is preferably small. However, as described above, in the steel material intended for use in sour environments or for use in high-pressure hydrogen containers, if the prior γ grain size GS is 5.0 μm or less, the manufacturing cost increases extremely. Therefore, in the steel material according to the present embodiment, the lower limit of the prior γ grain size GS is set to more than 5.0 μm. Furthermore, in the steel material according to the present embodiment, even if the prior γ grain size GS is 5.5 μm or more or 6.0 μm or more, there are cases in which the high yield strength of 125 ksi class and excellent hydrogen embrittlement resistance can be achieved at the same time.

[Method for measuring grain size GS of prior austenite grains]
In this embodiment, the prior γ grain size GS of the steel material can be determined by the following method. The grain size GS of the prior austenite grains is determined in accordance with the measurement method of the mean intercept method specified in JIS G0551:2020.

First, a test piece having an observation surface is taken from the steel material. When the steel material is a steel plate, a test piece having an observation surface parallel to the rolling direction is taken from the center of the plate width, including the plate thickness t/4 position, which is the observation target area. Here, the plate thickness t/4 position means a t/4 depth position from the surface of the steel plate, where the plate thickness of the steel plate is t. When the steel material is a steel pipe, a test piece having an observation surface parallel to the pipe axis direction is taken, including the wall thickness center, which is the observation target area. When the steel material is a round steel, a test piece having an observation surface parallel to the rolling direction is taken, including the R/2 position, which is the observation target area. In this specification, round steel means a steel bar having a circular cross section perpendicular to the axial direction. In addition, the R/2 position means the center position of the radius R in the cross section perpendicular to the axial direction of the round steel. The size of the test piece is not particularly limited. For example, the size of the test piece is 10 mm in the rolling direction length x 5 mm in the width direction x 10 mm in the thickness direction. The surface including the rolling direction and the thickness direction (the surface of 10 mm×10 mm in the case of the test piece size described above) is used as the observation surface.

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

Using the generated photographic image, the grain size number is evaluated in accordance with the measurement method of the average intercept method specified in JIS G0551:2020. The grain size of the prior austenite grains in each field of view is determined from the evaluated grain size number. The arithmetic average value of the grain size of the prior austenite grains determined in 10 fields of view is defined as the grain size GS of the prior austenite grains (prior γ grain size GS) (μm).

[Yield strength of steel σ]
In the steel material according to this embodiment, the yield strength σ is 862 to 965 MPa. In this specification, the yield strength σ means the 0.2% offset proof stress obtained by a tensile test performed according to a method in accordance with JIS Z2241:2011. The steel material according to this embodiment satisfies the other configurations of this embodiment, and therefore has excellent hydrogen embrittlement resistance even if the prior γ grain size GS is more than 5.0 μm and the yield strength σ is 862 to 965 MPa.

In this embodiment, the preferred lower limit of the yield strength σ is 865 MPa. In this embodiment, the preferred upper limit of the yield strength σ is less than 965 MPa, more preferably 960 MPa, and even more preferably 955 MPa. The method for measuring the yield strength σ will be described later.

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

With reference to Figure 2, the stress-strain curve shown in Figure 2 does not have an area where the stress drops suddenly. Therefore, in the case of such a stress-strain curve, it is not possible to define a point equivalent to point P0, and the yield point drop Δσ cannot be defined. On the other hand, in a steel material showing a stress-strain curve such as that shown in Figure 1, elastic deformation is maintained even at stresses higher than the yield strength σ. In other words, the larger the yield point drop Δσ, the less likely plastic deformation will occur, and the higher the stress limit at which hydrogen embrittlement fracture occurs. In other words, the larger the yield point drop Δσ, the higher the potential for hydrogen embrittlement resistance.

Here, in a steel material having the above-mentioned chemical composition, a prior γ grain size GS of more than 5.0 to 30.0 μm, and a yield strength σ of 862 to 965 MPa, if the yield point drop Δσ is 40 MPa or more, excellent hydrogen embrittlement resistance can be stably obtained, provided that the other configurations of this embodiment are satisfied. Therefore, in the steel material according to this embodiment, the yield point drop Δσ is set to 40 MPa or more.

In this embodiment, the preferred lower limit of the yield point drop Δσ is 44 MPa, more preferably 48 MPa, even more preferably 55 MPa, and even more preferably 60 MPa. In this embodiment, the upper limit of the yield point drop Δσ is not particularly limited. The upper limit of the yield point drop Δσ of the steel material according to this embodiment may be, for example, 200 MPa, 150 MPa, 130 MPa, or 120 MPa. The method for measuring the yield point drop Δσ will be described later.

[Yield point elongation of steel Δε]
In the steel material according to this embodiment, the yield point elongation Δε is 1.5% or more. In this specification, the yield point elongation Δε means the magnitude of strain at which the plastic deformation region starts, obtained by a tensile test performed according to a method in accordance with JIS Z2241:2011. That is, in this embodiment, the yield point elongation Δε is defined as the strain (%) at point Q shown in FIG.

With reference to Figure 2, the stress-strain curve shown in Figure 2 does not have a region where the stress remains constant even as the strain increases. Therefore, in the case of such a stress-strain curve, it is not possible to define a point equivalent to point Q, and the yield point elongation Δε cannot be defined. On the other hand, in a steel material showing a stress-strain curve such as that shown in Figure 1, the larger the yield point elongation Δε, the less likely plastic deformation will occur even if the amount of strain introduced into the steel material increases. Therefore, the larger the yield point elongation Δε, the higher the potential for the occurrence of hydrogen embrittlement fracture stress limit. In other words, the larger the yield point elongation Δε, the higher the potential for hydrogen embrittlement resistance.

Here, in a steel material having the above-mentioned chemical composition, a prior γ grain size GS of more than 5.0 to 30.0 μm, a yield strength σ of 862 to 965 MPa, and a yield point drop Δσ of 40 MPa or more, if the yield point elongation Δε is 1.5% or more, excellent hydrogen embrittlement resistance can be stably obtained. Therefore, in the steel material according to this embodiment, the yield point elongation Δε is set to 1.5% or more.

In this embodiment, the preferred lower limit of the yield point elongation Δε is 1.6%, more preferably 1.8%, even more preferably 2.0%, and even more preferably 2.2%. In this embodiment, the upper limit of the yield point elongation Δε is not particularly limited. The upper limit of the yield point elongation Δε of the steel material according to this embodiment may be, for example, 4.0%, 3.8%, 3.6%, or 3.5%. The method for measuring the yield point drop Δσ will be described later.

As shown in FIG. 2, even if a steel material has a chemical composition, prior γ grain size GS, and yield strength σ that overlap with this embodiment, there may be cases where the yield point drop Δσ and the yield point elongation Δε cannot be defined. Here, in a steel material having the above-mentioned chemical composition, the yield point drop Δσ and the yield point elongation Δε are determined by the microstructure (phases, precipitates, and inclusions) of the steel material, and/or the state of the precipitates (crystal structure, size, and volume fraction), and/or the state of dislocations in the steel material (dislocation density, dislocation arrangement, and the ratio of edge dislocations to screw dislocations, etc.), and the balance between these. Therefore, it is considered that the steel material according to this embodiment has a yield point drop Δσ of 40 MPa or more and a yield point elongation Δε of 1.5% or more as a result of appropriately controlling the microstructure of the steel material, and/or the state of the precipitates, the state of the dislocations in the steel material, and the balance between these.

[Method of measuring yield strength σ, yield point drop Δσ, and yield point elongation Δε]
The yield strength σ, yield point drop Δσ, and yield point elongation Δε of the steel material according to this embodiment can be obtained by the following method. A tensile test is performed according to the method of JIS Z2241:2011. A round bar test piece is prepared from the steel material according to this embodiment. When the steel material is a steel plate, the round bar test piece is prepared from the center of the plate width and the center of the plate thickness. When the steel material is a steel pipe, the round bar test piece is prepared from the center of the wall thickness. When the steel material is a round steel, the round bar test piece is prepared from the R/2 position. The size of the round bar test piece is, for example, a parallel part diameter of 6.0 mm and a parallel part length of 40 mm. The round bar test piece is prepared so that the axial direction of the round bar test piece is parallel to the rolling direction of the steel material.

A tensile test is carried out at room temperature (25°C) in air using the prepared round bar test piece. The elastic deformation region and the plastic deformation region are identified from the stress-strain curve obtained by the tensile test. The elastic deformation region and the plastic deformation region can be identified by a person skilled in the art. The point on the stress-strain curve where the stress shows the maximum value in the elastic deformation region is defined as P0, and the stress at point P0 is defined as σ0 (MPa). Furthermore, the intersection point between the stress-strain curve and a straight line L that is parallel to the elastic deformation region and is translated 0.2% in the positive direction of the strain is defined as P, and the stress at point P is defined as σ (MPa). In this specification, "strain" means the so-called nominal strain.

The stress σ at the obtained point P is defined as the yield strength σ (MPa). The yield strength σ (MPa) is obtained by rounding off the obtained numerical value to the nearest tenth. As described above, the yield strength σ corresponds to the 0.2% offset yield strength. The difference between the stress σ0 at the obtained point P0 and the stress σ at point P is defined as the yield point drop Δσ (MPa). The yield point drop Δσ (MPa) is obtained by rounding off the obtained numerical value to the nearest tenth. Furthermore, the point on the stress-strain curve where the plastic deformation region begins is defined as Q. The strain (%) at the obtained point Q is defined as the yield point elongation Δε (%). The yield point elongation Δε (%) is obtained by rounding off the obtained numerical value to the nearest tenth.

[Formula (1)]
In the steel material according to the present embodiment, the prior γ grain size GS, the yield strength σ, and the yield point elongation Δε preferably satisfy the following formula (1): If the steel material according to the present embodiment satisfies formula (1), it has even better hydrogen embrittlement resistance.
σ×Δε/GS ² ≧12 (1)
Here, the yield strength of the steel material is substituted in the unit of MPa for σ in formula (1), the yield point elongation of the steel material is substituted in the unit of % for Δε in formula (1), and the crystal grain size of prior austenite grains of the steel material is substituted in the unit of μm for GS in formula (1).

It is defined as Fn1=σ×Δε/ ^GS2 . Fn1 is an index of hydrogen embrittlement resistance. On the premise that the other configurations of this embodiment are satisfied, if Fn1 is 12 or more, the steel material has even better hydrogen embrittlement resistance. Therefore, it is preferable that the steel material according to this embodiment has the above-mentioned chemical composition, the prior γ grain size GS is more than 5.0 to 30.0 μm, the yield strength σ is 862 to 965 MPa, the yield point drop Δσ is 40 MPa or more, the yield point elongation Δε is 1.5% or more, and further, Fn1 is 12 or more.

A more preferred lower limit for Fn1 is greater than 12, and even more preferably 13. The upper limit for Fn1 is not particularly limited, but may be, for example, 148, 135, 130, or 125. Fn1 is calculated by rounding off the obtained numerical value to one decimal place.

[Hydrogen embrittlement resistance]
The steel material according to this embodiment has the above-mentioned chemical composition, a prior γ grain size GS of more than 5.0 to 30.0 μm, a yield strength σ of 862 to 965 MPa, a yield point drop Δσ of 40 MPa or more, and a yield point elongation Δε of 1.5% or more. As a result, the steel material according to this embodiment has high strength and excellent hydrogen embrittlement resistance even if the prior γ grain size GS is more than 5.0 μm. In this embodiment, the excellent hydrogen embrittlement resistance can be evaluated by the following method.

A test piece for evaluating hydrogen embrittlement resistance is prepared from the steel material according to this embodiment. The test piece is a round bar test piece with an annular notch. For example, the parallel part of the test piece has a diameter of 4.0 mm and a length of 25 mm, and an annular notch is formed at the longitudinal center position of the parallel part. At this time, the notch shape has a notch depth of 0.3 mm, a notch angle of 60°, and a curvature radius of the notch bottom of 0.125 mm. When the steel material is a steel plate, a round bar test piece is prepared from the center of the plate width and the plate thickness t/4 position. When the steel material is a steel pipe, a round bar test piece is prepared from the center of the wall thickness. When the steel material is a round steel, a round bar test piece is prepared from the R/2 position.

Hydrogen was charged to the prepared circular notched round bar test specimen by cathodic hydrogen charging. Specifically, a cathodic hydrogen charging solution at room temperature was prepared. The cathodic hydrogen charging solution was an aqueous solution containing 5 mass% sodium chloride solution at room temperature, 30 g/L NH ₄ SCN, and acetate buffer, and the pH before the test was adjusted to pH 3.5 using the acetate buffer.

With the annular notched round bar test specimen immersed in the cathodic hydrogen charging solution, hydrogen is charged into the annular notched round bar test specimen at a potential of -1.5 V for a charging time of 24 hours. At this time, a zinc plating film is preferably formed on the surface of the annular notched round bar test specimen that has been charged with hydrogen, to prevent hydrogen from leaking out from within the annular notched round bar test specimen.

A tensile test is performed on the hydrogen-charged annular notched round bar test piece in air at room temperature (25°C) using a slow strain rate testing machine (SSRT). At this time, the strain rate is set to 4.2 x ^10-6 /sec to determine the breaking stress BS1. The breaking stress BS1 is determined by rounding the obtained value to the nearest tenth. In this embodiment, if the breaking stress BS1 obtained under the above conditions is 950 MPa or more, it is determined that the specimen has excellent hydrogen embrittlement resistance. Furthermore, in this embodiment, if the breaking stress BS1 obtained under the above conditions is 970 MPa or more, it is determined that the specimen has even more excellent hydrogen embrittlement resistance.

[Microstructure]
In the microstructure of the steel material according to this embodiment, the total area ratio of tempered martensite and tempered bainite is 90% or more. The remainder of the microstructure is, for example, ferrite and/or pearlite. In this embodiment, when the steel material has the above-mentioned chemical composition, the prior γ grain size GS is more than 5.0 to 30.0 μm, the yield strength σ is 862 to 965 MPa, the yield point drop Δσ is 40 MPa or more, and the yield point elongation Δε is 1.5% or more, the steel material can be determined to have a total area ratio of tempered martensite and tempered bainite of 90% or more.

[Method for measuring 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 obtained 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, including the plate thickness t/4 position, which is the observation target region, is taken from the plate width center. When the steel material is a steel pipe, a test piece having an observation surface parallel to the tube axis direction, including the wall thickness center, which is the observation target region, is prepared. When the steel material is a round steel, a test piece having an observation surface parallel to the rolling direction, including the R/2 position, which is the observation target region. The size of the test piece is not particularly limited. The size of the test piece is, for example, 10 mm in the length in the rolling direction x 5 mm in the width direction x 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 perpendicular to the pipe axis direction and the wall thickness direction (circumferential direction). When the steel material is a round steel, the rolling direction corresponds to the pipe axis direction, the thickness direction corresponds to the radial direction, and the width direction corresponds to the direction perpendicular to the rolling direction and the radial direction (circumferential direction). The surface including the rolling direction and the thickness direction (the surface of 10 mm x 10 mm in the case of the test piece size described above) is the observation surface.

After the observation surface of the test piece is polished 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 area of the etched observation surface are observed as secondary electron images using a scanning electron microscope (SEM). When the steel material is a steel plate, the observation area is at the plate thickness t/4 position. When the steel material is a steel pipe, the observation area is at the center of the wall thickness. When the steel material is a round bar, the observation area is at the R/2 position. The area of each of the 10 fields of view within the observation area is, for example, 400 μm ² (magnification 5000 times).

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

The total area ratio of the identified tempered martensite and tempered bainite is determined. The method for determining the total area ratio is not particularly limited, and any well-known method may be used. For example, the total area ratio of tempered martensite and tempered bainite can be determined by image analysis. In this embodiment, the arithmetic average value of the total area ratio of tempered martensite and tempered bainite determined in all fields of view (10 fields of view) is defined as the total area ratio (%) of tempered martensite and tempered bainite.

[Shapes and uses of steel materials]
The shape of the steel material according to the present embodiment is not particularly limited. The steel material according to the present embodiment may be a steel pipe, a steel plate, or a round bar.

Preferably, the steel material of this embodiment is any one of steel pipes for oil wells, steel pipes for line pipes, and steel pipes for high-pressure hydrogen containers. Steel pipes for oil wells refer to steel pipes for use as oil well pipes. Oil well pipes are, for example, casings, tubing, drill pipes, etc. used for drilling oil wells or gas wells, extracting crude oil or natural gas, etc. Steel pipes for line pipes refer to steel pipes for use as line pipes that constitute pipelines for transporting production fluids (crude oil or natural gas) extracted from oil wells or gas wells. Pipelines include, for example, flow lines that transport production fluids from oil wells or gas wells, gathering lines that collect the production fluids transported by the flow lines and transport them to primary treatment facilities, trunk lines that transport production fluids that have been subjected to primary treatment such as dehydration to the vicinity of the market, and distribution lines that transport them to consumers. The steel pipe for high-pressure hydrogen containers is standardized by ISO11439, ANSI/NGV, the High Pressure Gas Safety Act, the Container Safety Regulations Exemplary Standards, etc., and refers to a steel pipe used for high-pressure hydrogen containers in which high-pressure hydrogen gas is stored. The steel material of this embodiment may be a steel pipe for high-pressure hydrogen containers, a steel pipe for high-pressure hydrogen accumulators, or a steel pipe for high-pressure hydrogen cylinders.

More preferably, the steel material of this embodiment is any one of seamless steel pipes for oil wells, seamless steel pipes for line pipes, and seamless steel pipes for high-pressure hydrogen containers. Seamless steel pipes for oil wells means that the steel pipes for oil wells are seamless steel pipes. Seamless steel pipes for line pipes means that the steel pipes for line pipes are seamless steel pipes. Seamless steel pipes for high-pressure hydrogen containers means that the steel pipes for high-pressure hydrogen containers are seamless steel pipes. The steel material of this embodiment may be seamless steel pipes for high-pressure hydrogen containers, or may be seamless steel pipes for high-pressure hydrogen storage tanks or seamless steel pipes for high-pressure hydrogen cylinders.

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

An example of a method for manufacturing a steel material according to this embodiment includes the following steps.
(Step 1) Material preparation step (Step 2) Hot working step (Step 3) Quenching and tempering step (Step 4) Low-temperature heat treatment step In this manufacturing method, the following conditions are satisfied in step 4, the low-temperature heat treatment step.
(Condition 1) Hold at 150 to 250° C. for 10 minutes or more.
Each step will be described below.

[(Process 1) Material preparation process]
In the material preparation process, first, molten steel having the above-mentioned chemical composition is produced by a known refining method. The produced molten steel is then used to produce a cast piece by a continuous casting method. Here, the cast piece is called a slab. Instead of the cast piece, an ingot may be produced by an ingot casting method using the above molten steel. If necessary, the slab, bloom or ingot may be hot-rolled to produce a billet. Through the above-described manufacturing process, a material (slab, bloom, or billet) is manufactured.

[(Step 2) Hot working step]
In the hot working process, the prepared material is hot worked to produce an intermediate steel material. When the final product is a steel pipe, the material is first heated in a heating furnace. The heating temperature is not particularly limited, but is, for example, 1100 to 1300°C. The material extracted from the heating furnace is hot worked to produce a mother pipe (seamless steel pipe). For example, the Mannesmann process is carried out as the hot working to produce a mother pipe. In this case, the billet is pierced and rolled using a piercing machine. The billet after piercing and rolling is subjected to elongation rolling using a mandrel mill. Furthermore, as necessary, the billet after elongation rolling is subjected to sizing rolling using a reducer or a sizing mill. Through the above steps, a mother pipe is produced.

A blank pipe may be manufactured from the billet by a hot working method other than the Mannesmann process. For example, in the case of a short, thick-walled steel material such as a coupling, the blank pipe may be manufactured by forging such as the Erhardt process, or by hot extrusion.

When the final product is a steel plate, for example, the raw material (slab) is hot-rolled using one or more rolling mills including a pair of roll groups to produce an intermediate steel material (steel material). The heating temperature before hot rolling is not particularly limited, but is, for example, 1100 to 1300°C.

When the final product is round steel, for example, the raw material (bloom) is subjected to blooming using a blooming mill and/or hot rolling using a continuous rolling mill to produce intermediate steel (round steel). In other words, the raw material may be bloomed to produce round steel, or the raw material may not be bloomed but hot rolled using a continuous rolling mill to produce round steel, or the raw material may be bloomed using a blooming mill and hot rolled using a continuous rolling mill to produce round steel. A continuous rolling mill has multiple rolling stands lined up in a row, and each stand includes a pair of rolling rolls. When blooming is performed, the heating temperature before blooming is not particularly limited, but is, for example, 1100 to 1300°C. When hot rolling is performed using a continuous rolling mill, the heating temperature before hot rolling is not particularly limited, but is, for example, 1100 to 1300°C.

[(Step 3) Quenching and tempering step]
In the quenching and tempering process, the intermediate steel material after the hot working process is subjected to the quenching and tempering processes.

[Quenching process]
In the quenching process, the intermediate steel material produced in the hot working process is quenched. Quenching is performed by a well-known method. Specifically, the intermediate steel material after hot working is loaded into a heat treatment furnace and held at the quenching temperature. The quenching temperature is equal to or higher than the A _C3 transformation point, for example, 900 to 1000°C. After holding the intermediate steel material at the quenching temperature, it is rapidly cooled (quenched).

If the quenching temperature is too high, the crystal grains may become coarse, and the prior γ grain size GS may become too large in the manufactured steel material. On the other hand, if the quenching temperature is too low, coarse carbides may remain in the manufactured steel material. In this case, a yield strength σ of 862 MPa or more cannot be obtained. Therefore, in the quenching process according to this embodiment, the quenching temperature is equal to or higher than the A _C3 transformation point, and specifically, it is preferable that the quenching temperature is 900 to 1000°C.

The holding time at the quenching temperature is not particularly limited, but is, for example, 10 to 60 minutes. The quenching method is, for example, water cooling. The quenching method is not particularly limited. When the steel material is a steel pipe, the blank pipe may be quenched by immersing it in a water tank or an oil tank, or the blank pipe may be quenched by pouring cooling water onto the outer and/or inner surface of the blank pipe or by spraying cooling water from a nozzle.

After hot working, the steel may be quenched (direct quenching) immediately after hot working without being cooled to room temperature, or the steel may be loaded into a reheating furnace before the temperature of the hot worked steel drops and held at the quenching temperature before quenching is performed.

[Tempering process]
In the tempering process, the steel material after the quenching process is tempered. In the tempering process, the yield strength σ of the steel material is adjusted to 862 to 965 MPa. Note that a person skilled in the art can adjust the yield strength σ of the steel material to 862 to 965 MPa by adjusting the conditions of the tempering process.

The tempering temperature and holding time are not particularly limited as long as the yield strength σ of the produced steel material is 862 to 965 MPa. Specifically, the tempering temperature is, for example, 650°C to the A _C1 transformation point. The holding time at the tempering temperature is not particularly limited, but is, for example, 10 to 180 minutes. The yield strength of the steel material having the above-mentioned chemical composition is adjusted by appropriately adjusting the tempering temperature according to the chemical composition. Here, the tempering temperature means the furnace temperature (°C) in the heat treatment furnace, and the holding time at the tempering temperature means the in-furnace time (the time from charging into the heat treatment furnace to being extracted).

The quenching and tempering processes may be performed once each, or multiple times. For example, after performing the quenching and tempering processes, the quenching and tempering processes may be performed again. When the quenching and tempering processes are performed multiple times, the prior gamma grain size GS may become smaller in the manufactured steel material.

[(Step 4) Low-temperature heat treatment step]
In the low-temperature heat treatment step, the steel material after the quenching and tempering steps is subjected to a low-temperature heat treatment under the conditions described in Condition 1 below.

[Regarding Condition 1]
(Condition 1) Hold at 150 to 250° C. for 10 minutes or more.
In this embodiment, the steel material after the tempering process is held at 150 to 250° C. for 10 minutes or more. Here, the heat treatment temperature in the low-temperature heat treatment process means the furnace temperature (° C.) in the heat treatment furnace, and the heat treatment time means the time spent in the furnace (the time from when the steel material is charged into the heat treatment furnace until when the steel material is extracted).

Heat treatment at 150 to 250°C may stabilize dislocations. If dislocations are stabilized, plastic deformation is less likely to occur. As a result, the yield point drop Δσ becomes 40 MPa or more, and the yield point elongation Δε becomes 1.5% or more. Therefore, in the low-temperature heat treatment process according to this embodiment, heat treatment is performed at 150 to 250°C for 10 minutes or more.

If the heat treatment temperature is too low, the above effect cannot be sufficiently obtained. In other words, the dislocations are not sufficiently stabilized, and the yield point drop Δσ cannot be defined in the manufactured steel, or the yield point drop Δσ may be less than 40 MPa. In this case, the yield point elongation Δε may not be defined, or the yield point elongation Δε may be less than 1.5%. On the other hand, if the heat treatment temperature is too high, the dislocations may not be sufficiently stabilized. Therefore, in the manufactured steel, the yield point drop Δσ may not be defined, or the yield point drop Δσ may be less than 40 MPa. In this case, the yield point elongation Δε may not be defined, or the yield point elongation Δε may be less than 1.5%.

If the heat treatment time is too short, the above effects are not fully achieved. In other words, the dislocations are not sufficiently stabilized, and the yield point drop Δσ in the manufactured steel may not be defined, or the yield point drop Δσ may be less than 40 MPa. In this case, the yield point elongation Δε may not be defined, or the yield point elongation Δε may be less than 1.5%. On the other hand, if the heat treatment time is too long, the above effects will saturate. Therefore, the upper limit of the heat treatment time is not particularly limited, but is, for example, 60 minutes.

By carrying out the above manufacturing process, the steel material according to this embodiment can be manufactured. Below, the effects of the steel material according to this embodiment will be explained in more detail using examples. The various conditions in the examples described below are one example of conditions adopted to confirm the feasibility and effects of the steel material according to this embodiment. Therefore, the steel material according to this embodiment is not limited to the one example of conditions described in the examples.

Steel materials (steel plates) were manufactured having the chemical compositions shown in Tables 1-1 and 1-2. Note that "-" in Tables 1-1 and 1-2 indicates that the content of the corresponding element is below the impurity level.

The steel materials with each sample code were manufactured by the following method. Ingots having the chemical compositions shown in Tables 1-1 and 1-2 were manufactured by casting. The ingots were hot forged to produce blocks of material 50 mm thick.

The block material was subjected to a hot working process. Specifically, the block material was heated to 1250°C. After heating, the block material was hot rolled to produce a steel material (steel plate) having a thickness of 15 mm. The produced steel material was allowed to cool to room temperature.

The steel material that had been allowed to cool to room temperature was subjected to quenching and tempering processes. Specifically, the steel material was quenched by holding it at the temperature (°C) in the "Quenching process" column shown in Table 2 for the time (minutes), and then water-cooling. The steel material after quenching was tempered. It was held at the temperature (°C) in the "Tempering process" column shown in Table 2 for the time (minutes). Then, the tempered steel material was subjected to a low-temperature heat treatment process. It was held at the temperature (°C) in the "Low-temperature heat treatment process" column shown in Table 2 for the time (minutes). Note that the low-temperature heat treatment process was not performed on the steel material with sample code ZR. Steel materials (steel plates) with each sample code were manufactured using the above manufacturing process.

[Evaluation test]
The manufactured steel was subjected to a prior-γ grain size measurement test, a tensile test, and a hydrogen embrittlement resistance evaluation test.

[Old gamma particle size measurement test]
A test piece was prepared from the center of the sheet width of each sample steel material, including the sheet thickness t/4 position, which is the observation target region, and including the rolling direction and the sheet thickness direction as the observation surface. The size of the test piece was 10 mm in the rolling direction, 5 mm in the sheet width direction, and 10 mm in the sheet thickness direction. The observation surface was 10 mm in the rolling direction x 10 mm in the sheet thickness direction. The prior γ grain size GS (μm) of the collected test piece was determined according to the above-mentioned method. At this time, the field area was 500 μm x 500 μm (magnification 200 times). The obtained prior γ grain size GS (μm) is shown in the "prior γ grain size GS (μm)" column in Table 3.

[Tensile test]
A round bar test piece was taken from the center of the plate thickness of the steel material of each sample code. The diameter of the parallel part of the round bar test piece was 6.0 mm, and the length of the parallel part was 40 mm. The axial direction of the round bar test piece was parallel to the rolling direction of the steel material. Using the round bar test piece, the yield strength σ (MPa), yield point drop Δσ (MPa), and yield point elongation Δε (%) were obtained according to the above-mentioned method. The maximum stress during uniform elongation in the tensile test was defined as the tensile strength (MPa). The obtained yield strength σ is shown in the "Yield strength σ (MPa)" column in Table 3. The obtained tensile strength is shown in the "Tensile strength (MPa)" column in Table 3. The obtained yield point drop Δσ is shown in the "Yield point drop Δσ (MPa)" column in Table 3. The obtained yield point elongation Δε is shown in the "Yield point elongation Δε (%)" column in Table 3. In addition, when the yield point drop Δσ and the yield point elongation Δε cannot be defined, they are indicated as "-" in Table 3. Furthermore, Fn1 (=σ×Δε/GS ² ) was obtained from the obtained prior γ grain size GS (μm), yield strength σ (MPa), and yield point elongation Δε (%), as well as the above definitions. The obtained Fn1 is shown in the "Fn1" column in Table 3.

[Hydrogen embrittlement resistance evaluation test]
Two round bar test pieces with an annular notch were prepared from the center of the plate width and the plate thickness t/4 position of each sample steel. The 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 center position in the longitudinal direction of the parallel part. The notch shape had a notch depth of 0.3 mm, a notch angle of 60°, and a curvature radius of the notch bottom of 0.125 mm.

Hydrogen was charged to one of the two annular notched round bar specimens by the cathodic hydrogen charging method. Specifically, a cathodic hydrogen charging solution was prepared at room temperature. The cathodic hydrogen charging solution was an aqueous solution containing 5 mass% sodium chloride solution at room temperature, 30 g/L NH ₄ SCN, and an acetate buffer solution, and the pH of the solution before the test was adjusted to pH 3.5 using the acetate buffer solution.

The annular notched round bar test specimens were immersed in the cathodic hydrogen charging solution and charged with hydrogen at a potential of -1.5 V for a charging time of 24 hours. In other words, a sour environment was simulated by charging with hydrogen. A zinc plating film was formed under the same conditions for each sample code on the surface of the hydrogen-charged annular notched round bar test specimens to prevent hydrogen from leaking out from within the annular notched round bar test specimens. Hydrogen was not charged to the other annular notched round bar test specimen.

A tensile test was performed on the annular notched round bar specimens having the zinc plating coating at room temperature in air at a strain rate of 4.2 × 10 ^-6 /sec using a slow strain rate testing machine (SSRT), and the breaking stress BS1 (MPa) in a hydrogen environment was determined.

Furthermore, a tensile test was carried out on annularly notched round bar test pieces of each sample code that had not been charged with hydrogen at room temperature in air at a strain rate of 4.2×10 ⁻⁶ /sec using a slow strain rate testing machine (SSRT), and the breaking stress in air, BS0 (MPa), was determined.

The obtained breaking stress BS0 (MPa) in air is shown in the "Air (MPa)" column of the "Notched tensile test results" column in Table 3. The obtained breaking stress BS1 (MPa) in a hydrogen environment is shown in the "Hydrogen environment (MPa)" column of the "Notched tensile test results" column in Table 3.

[Evaluation Results]
With reference to Tables 1-1, 1-2, 2, and 3, the samples A to X had the above-mentioned chemical composition, the prior γ grain size GS was greater than 5.0 to 30.0 μm, the yield strength σ was 862 to 965 MPa, the yield point drop Δσ was 40 MPa or more, and the yield point elongation Δε was 1.5% or more. As a result, the fracture stress BS1 in a hydrogen environment was 950 MPa or more, and the steel had excellent hydrogen embrittlement resistance. In other words, even if the prior γ grain size GS was greater than 5.0 μm, these steel materials had high strength and excellent hydrogen embrittlement resistance. It was determined that the total area ratio of tempered martensite and tempered bainite in the microstructure of the samples A to X was 90% or more.

Furthermore, samples A to U had Fn1 of 12 or more. As a result, the fracture stress BS1 in a hydrogen environment was 970 MPa or more, and the samples had even better resistance to hydrogen embrittlement.

On the other hand, sample ZA had too low a C content. As a result, the rupture stress BS1 of this steel in a hydrogen environment was less than 950 MPa, and it did not have excellent hydrogen embrittlement resistance.

Sample ZB had too high a C content. As a result, the yield strength σ of this steel exceeded 965 MPa. As a result, the fracture stress BS1 in a hydrogen environment was less than 950 MPa, and this steel did not have excellent hydrogen embrittlement resistance.

Sample ZC had too low a Si content. As a result, the steel had a fracture stress BS1 in a hydrogen environment of less than 950 MPa and did not have excellent hydrogen embrittlement resistance.

Sample ZD had too high a Si content. As a result, the steel had a fracture stress BS1 in a hydrogen environment of less than 950 MPa and did not have excellent hydrogen embrittlement resistance.

Sample code ZE had too high a Mn content. As a result, the steel had a fracture stress BS1 in a hydrogen environment of less than 950 MPa and did not have excellent hydrogen embrittlement resistance.

Sample code ZF had too low a Cr content. As a result, the steel had a fracture stress BS1 in a hydrogen environment of less than 950 MPa and did not have excellent hydrogen embrittlement resistance.

Sample ZG had too high a Cr content. As a result, the steel had a fracture stress BS1 in a hydrogen environment of less than 950 MPa and did not have excellent hydrogen embrittlement resistance.

Sample ZH had too low a Mo content. As a result, the steel had a fracture stress BS1 in a hydrogen environment of less than 950 MPa and did not have excellent hydrogen embrittlement resistance.

Sample ZI had too high a Mo content. As a result, the steel had a fracture stress BS1 in a hydrogen environment of less than 950 MPa and did not have excellent hydrogen embrittlement resistance.

Sample ZJ had too low a V content. As a result, the yield strength σ of this steel was less than 862 MPa. As a result, the fracture stress BS1 in a hydrogen environment was less than 950 MPa, and this steel did not have excellent hydrogen embrittlement resistance.

Sample ZK had too high a V content. As a result, the yield strength σ of this steel exceeded 965 MPa. As a result, the fracture stress BS1 in a hydrogen environment was less than 950 MPa, and this steel did not have excellent hydrogen embrittlement resistance.

The Nb content of sample ZL was too low. As a result, the yield drop Δσ of this steel could not be defined. As a result, the fracture stress BS1 in a hydrogen environment was less than 950 MPa, and this steel did not have excellent hydrogen embrittlement resistance.

Sample ZM had too high a Nb content. As a result, the steel had a fracture stress BS1 in a hydrogen environment of less than 950 MPa and did not have excellent hydrogen embrittlement resistance.

Sample ZN had too high a Ti content. As a result, the steel had a fracture stress BS1 in a hydrogen environment of less than 950 MPa and did not have excellent hydrogen embrittlement resistance.

Sample code ZO had too low a B content. As a result, the steel had a fracture stress BS1 in a hydrogen environment of less than 950 MPa and did not have excellent hydrogen embrittlement resistance.

Sample ZP had too high a B content. As a result, the steel had a fracture stress BS1 in a hydrogen environment of less than 950 MPa and did not have excellent hydrogen embrittlement resistance.

Sample ZQ had too high a N content. As a result, the steel had a fracture stress BS1 in a hydrogen environment of less than 950 MPa and did not have excellent hydrogen embrittlement resistance.

Sample ZR did not undergo a low-temperature heat treatment process. As a result, the yield point drop Δσ and yield point elongation Δε could not be defined for this steel. As a result, the rupture stress BS1 in a hydrogen environment was less than 950 MPa, and this steel did not have excellent hydrogen embrittlement resistance.

For sample ZS, the heat treatment temperature in the low-temperature heat treatment process was too low. As a result, the yield point drop Δσ and yield point elongation Δε could not be defined for this steel. As a result, the fracture stress BS1 in a hydrogen environment was less than 950 MPa, and this steel did not have excellent hydrogen embrittlement resistance.

For sample code ZT, the heat treatment temperature in the low-temperature heat treatment process was too high. As a result, the yield point drop Δσ of this steel could not be defined. As a result, the yield point elongation Δε of this steel was less than 1.5%. As a result, the fracture stress BS1 in a hydrogen environment was less than 950 MPa, and this steel did not have excellent hydrogen embrittlement resistance.

For sample ZU, the heat treatment time in the low-temperature heat treatment step was too short. As a result, the yield point drop Δσ of this steel was less than 40 MPa. As a result, the fracture stress BS1 in a hydrogen environment was less than 950 MPa, and the steel did not have excellent hydrogen embrittlement resistance.

The above describes the embodiments of the present disclosure. However, the above-described embodiments are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiments, and can be implemented by modifying the above-described embodiments as appropriate within the scope of the spirit of the present disclosure.

Claims (8)

A steel material,
The chemical composition, in mass%, is
C: 0.20% to 0.45%,
Si: 0.05-1.50%,
Mn: 0.01-1.00%,
P: 0.030% or less,
S: 0.0100% or less,
Cr: 0.40-1.10%,
Mo: 0.40-1.30%,
V: 0.01-0.30%,
Nb: 0.005-0.100%,
Ti: 0.001 to 0.030%,
Al: 0.005-0.100%,
B: 0.0005-0.0050%,
N: 0.0100% or less,
O: 0.0050% or less,
W: 0-2.00%,
Co: 0 to 0.20%,
Mg: 0 to 0.0100%,
Ca: 0-0.0100%,
Rare earth elements: 0 to 0.0100%,
Cu: 0 to 0.40%,
Ni: 0 to 0.20%,
Sn: 0 to 0.10%; and
The balance is Fe and impurities,
In the microstructure of the steel material, the total area ratio of tempered martensite and tempered bainite is 90% or more,
The grain size GS of the prior austenite grains is more than 5.0 to 30.0 μm,
The yield strength σ is 862 to 965 MPa,
The yield point drop Δσ is 40 MPa or more,
The yield point elongation Δε is 1.5% or more.
Steel. The steel material according to claim 1,
The chemical composition is
W: 0.01-2.00%,
Co: 0.01-0.20%,
Mg: 0.0001 to 0.0100%,
Ca: 0.0001-0.0100%,
Rare earth elements: 0.0001 to 0.0100%,
Cu: 0.01-0.40%,
Ni: 0.01 to 0.20%, and
Sn: 0.01 to 0.10%,
Contains one or more elements selected from the group consisting of
Steel. The steel material according to claim 1,
The grain size GS of the prior austenite grains, the yield strength σ, and the yield point elongation Δε satisfy the following formula (1):
Steel.
σ×Δε/GS ² ≧12 (1)
Here, the yield strength of the steel material is substituted in the unit of MPa for σ in formula (1), the yield point elongation of the steel material is substituted in the unit of % for Δε in formula (1), and the crystal grain size of prior austenite grains of the steel material is substituted in the unit of μm for GS in formula (1). The steel material according to claim 2,
The grain size GS of the prior austenite grains, the yield strength σ, and the yield point elongation Δε satisfy the following formula (1):
Steel.
σ×Δε/GS ² ≧12 (1)
Here, the yield strength of the steel material is substituted in MPa in σ in formula (1), the yield point elongation of the steel material is substituted in %, and the grain size of prior austenite grains of the steel material is substituted in μm in GS in formula (1). The steel material according to any one of claims 1 to 4,
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. The steel material according to any one of claims 1 to 4,
The steel material 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.
Steel. The steel material according to claim 5,
The steel pipe for a high-pressure hydrogen container is either a steel pipe for a high-pressure hydrogen storage tank or a steel pipe for a high-pressure hydrogen cylinder.
Steel. The steel material according to claim 6,
The seamless steel pipe for a high-pressure hydrogen container is either a seamless steel pipe for a high-pressure hydrogen storage tank or a seamless steel pipe for a high-pressure hydrogen cylinder.
Steel.

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