EP4692397A1

EP4692397A1 - Steel material

Info

Publication number: EP4692397A1
Application number: EP24784704.9A
Authority: EP
Inventors: Yusaku TOMIO
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 2023-04-06
Filing date: 2024-03-15
Publication date: 2026-02-11
Also published as: CN120958162A; JP7564498B1; JPWO2024209910A1; MX2025011587A

Abstract

A steel material that has high strength and excellent hydrogen embrittlement resistance even when the grain size of prior-austenite grains is more than 5.0 µm is provided. In a steel material according to the present disclosure, the chemical composition consists of, in mass%, 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: 0.005 to 0.100%, B: 0.0005 to 0.0050%, N: 0.0100% or less, and O: 0.0050% or less, with the balance being Fe and impurities. In the steel material, a grain size GS of prior-austenite grains is more than 5.0 to 30.0 µm, a yield strength σ is 862 to 965 MPa, a yield point drop Δσ is 40 MPa or more, and a yield point elongation Δε is 1.5% or more.

Description

TECHNICAL FIELD

The present disclosure relates to a steel material, and more particularly relates to a steel material to be used in a sour environment and a steel material to be utilized for high-pressure hydrogen containers.

BACKGROUND ART

The environments of oil wells and gas wells (hereinafter, oil wells and gas wells are collectively referred to as "oil wells") include environments which contain large amounts of corrosive substances. Examples of such corrosive substances include a corrosive gas such as hydrogen sulfide. In the present description, an environment containing hydrogen sulfide is referred to as a "sour environment". The temperature of a sour environment ranges from normal temperature to about 200°C, although the temperature also depends on the depth of the well.
Examples of steel materials used in such kind of sour environments include steel materials for oil wells which are applied as oil country tubular goods, and steel materials for line pipes which are applied as line pipes. In recent years, oil wells are being made deeper, and consequently there is a demand to increase the strength of steel materials for oil wells and the like.
On the other hand, when a steel material is used in a sour environment, the surface of the steel material comes into contact with corrosive substances, which causes an electrochemical reaction that generates hydrogen on the steel material surface. The hydrogen on the steel material surface is liable to cause hydrogen embrittlement cracking, which is typified by sulfide stress corrosion cracking (SSC), in the steel material. Therefore, a steel material to be used in a sour environment also needs to have excellent hydrogen embrittlement resistance, in addition to high strength.
Techniques for increasing hydrogen embrittlement resistance in a steel material to be used in a sour environment are disclosed in Japanese Patent Application Publication No. 2011-246798 (Patent Literature 1) and Japanese Patent Application Publication No. 2015-38247 (Patent Literature 2).
Patent Literature 1 discloses an oil-well steel pipe composed of a low alloy steel in which a prescribed amount of dissolved Mo is secured, prior-austenite grains are refined, and M₂C-type precipitates are dispersed. By this means, SSC resistance is improved. Furthermore, in Patent Literature 1, hydrogen embrittlement resistance is further improved by forming Mo segregation regions at prior-austenite grain boundaries.
Patent Literature 2 discloses an oil-well steel pipe composed of a low alloy steel in which hydrogen embrittlement resistance is improved by suppressing the occurrence of Mo segregation regions as much as possible.
Furthermore, recently the development of fuel cell vehicles that are vehicles which run on hydrogen as fuel, as well as research into the practical use of hydrogen stations where fuel cell vehicles are supplied with hydrogen have been progressing. High-pressure hydrogen gas is stored in high-pressure hydrogen storage vessels in hydrogen service installed at a hydrogen station. Further, as fuel cell vehicles, the development of automotive vehicles equipped with high-pressure hydrogen cylinders has been also progressing. A steel material to be utilized for such kind of high-pressure hydrogen containers as high-pressure hydrogen storage vessels in hydrogen service and high-pressure hydrogen cylinders also needs to have excellent hydrogen embrittlement resistance as well as high strength.
A technique for improving hydrogen embrittlement resistance in a steel material for use in high-pressure hydrogen containers is proposed in Japanese Patent Application Publication No. 2009-74122 (Patent Literature 3). Patent Literature 3 discloses a steel material composed of a low alloy steel in which, by making the content of V and the content of Mo higher than in conventional steel materials, the morphology of carbides at prior-austenite grain boundaries is improved to thereby improve the hydrogen embrittlement resistance.

CITATION LIST

PATENT LITERATURE

Patent Literature 1: Japanese Patent Application Publication No. 2011-246798
Patent Literature 2: Japanese Patent Application Publication No. 2015-38247
Patent Literature 3: Japanese Patent Application Publication No. 2009-74122
Patent Literature 4: Japanese Patent Application Publication No. 2017-210645

SUMMARY OF INVENTION

TECHNICAL PROBLEM

According to the techniques disclosed in the aforementioned Patent Literatures 1 to 3, the hydrogen embrittlement resistance of a steel material for which use in a sour environment or use for high-pressure hydrogen containers is assumed can be improved. However, a steel material having high strength and excellent hydrogen embrittlement resistance may also be obtained by means other than the means disclosed in the aforementioned Patent Literatures 1 to 3.
In this connection, a technique that makes the prior-austenite grains in a steel material fine to thereby improve the hydrogen embrittlement resistance of the steel material has already been proposed. For example, specifically, Japanese Patent Application Publication No. 2017-210645 (Patent Literature 4) discloses that "by the prior-austenite grains being fine grains with a grain size of 6 µm or less, hydrogen embrittlement fractures for which prior-austenite grain boundaries serve as starting points can be reduced" (paragraph [0012] of Patent Literature 4). Thus, techniques for making the prior-austenite grains in a steel material fine to improve the hydrogen embrittlement resistance of the steel material have been investigated.
On the other hand, with regard to steel materials for which use in a sour environment or use for high-pressure hydrogen containers is assumed, when taking industrial production into consideration, it is not preferable to excessively refine prior-austenite grains. Specifically, in steel materials for which use in a sour environment or use for high-pressure hydrogen containers is assumed, in a case where the grain size of the prior-austenite grains is 5.0 µm or less, the production cost will increase extremely. Therefore, in a steel material for which use in a sour environment or use for high-pressure hydrogen containers is assumed, it is preferable that both high strength and excellent hydrogen embrittlement resistance can be achieved even when the grain size of the prior-austenite grains is more than 5.0 µm.
An objective of the present disclosure is to provide a steel material which has high strength and excellent hydrogen embrittlement resistance even when the grain size of the prior-austenite grains is more than 5.0 µm.

SOLUTION TO PROBLEM

A steel material according to the present disclosure has a chemical composition consisting of, in mass%,

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: 0.005 to 0.100%,
B: 0.0005 to 0.0050%,
N: 0.0100% or less,
O: 0.0050% or less,
W: 0 to 2.00%,
Co: 0 to 0.20%,
Mg: 0 to 0.0100%,
Ca: 0 to 0.0100%,
rare earth metal: 0 to 0.0100%,
Cu: 0 to 0.40%,
Ni: 0 to 0.20%, and
Sn: 0 to 0.10%,
with the balance being Fe and impurities,
wherein:
- a grain size GS of prior-austenite grains is more than 5.0 to 30.0 µm,
- a yield strength σ is 862 to 965 MPa,
- a yield point drop Δσ is 40 MPa or more, and
- a yield point elongation Δε is 1.5% or more.

ADVANTAGEOUS EFFECTS OF INVENTION

The steel material according to the present disclosure has high strength and excellent hydrogen embrittlement resistance even when the grain size of prior-austenite grains is more than 5.0 µm.

BRIEF DESCRIPTION OF DRAWINGS

[FIG. 1] FIG. 1 is a view illustrating a stress-strain curve of a steel material according to the present embodiment.
[FIG. 2] FIG. 2 is a view illustrating a stress-strain curve of a steel material in which the chemical composition, yield strength, and prior-y grain size duplicate those of the present embodiment, but which does not have excellent hydrogen embrittlement resistance.
[FIG. 3] FIG. 3 is a view in which one part of FIG. 1 is enlarged.
[FIG. 4] FIG. 4 is a view in which one part of FIG. 1 is enlarged.

DESCRIPTION OF EMBODIMENTS

The present inventors initially conducted studies with a view to obtaining a steel material having a yield strength of 862 to 965 MPa (125 to 140 ksi, hereunder, also referred to as "125 ksi grade") as a high strength, for which use in a sour environment or use for high-pressure hydrogen containers is assumed. That is, with respect to a steel material for which use in a sour environment or use for high-pressure hydrogen containers is assumed, the present inventors conducted studies and investigations regarding means for improving the hydrogen embrittlement resistance even when the yield strength is 125 ksi grade and a grain size GS of prior-austenite grains (hereunder, the grain size GS of prior-austenite grains is also referred to as "prior-γ grain size GS") is larger than 5.0 µm. As a result, the present inventors obtained the following findings.
The present inventors firstly conducted studies that focused on the chemical composition. As a result, the present inventors considered that if a steel material has a chemical composition consisting of, in mass%, 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: 0.005 to 0.100%, B: 0.0005 to 0.0050%, N: 0.0100% or less, O: 0.0050% or less, W: 0 to 2.00%, Co: 0 to 0.20%, Mg: 0 to 0.0100%, Ca: 0 to 0.0100%, rare earth metal: 0 to 0.0100%, Cu: 0 to 0.40%, Ni: 0 to 0.20%, and Sn: 0 to 0.10%, with the balance being Fe and impurities, there is a possibility that a yield strength of 125 ksi grade and excellent hydrogen embrittlement resistance can be obtained even if the prior-y grain size GS is more than 5.0 µm.
Therefore, the present inventors produced various steel materials having the aforementioned chemical composition, a yield strength of 125 ksi grade, and a prior-y grain size GS of more than 5.0 µm, and evaluated the hydrogen embrittlement resistance of the steel materials. As a result of detailed studies conducted by the present inventors, it was revealed that in a steel material having the aforementioned chemical composition, a yield strength of 125 ksi grade, and a prior-y grain size GS of more than 5.0 µm, in a case where the stress-strain curve has a characteristic shape, the hydrogen embrittlement resistance markedly improves. This point will now be described specifically using the accompanying drawings.
FIG. 1 is a view illustrating a stress-strain curve of a steel material according to the present embodiment. FIG. 2 is a view illustrating a stress-strain curve of a steel material in which the chemical composition, yield strength, and prior-y grain size GS duplicate those of the present embodiment, but which does not have excellent hydrogen embrittlement resistance. FIG. 1 and FIG. 2 were each obtained by performing a tensile test to be described later. Each of the steel materials illustrated in FIG. 1 and FIG. 2 had the chemical composition described above and a yield strength of 125 ksi grade, and in each steel material the prior-y grain size GS was more than 5.0 µm. On the other hand, although the steel material illustrated in FIG. 1 had excellent hydrogen embrittlement resistance, the steel material illustrated in FIG. 2 did not have excellent hydrogen embrittlement resistance.
Referring to FIG. 1, the stress-strain curve illustrated in FIG. 1 can be divided into three regions. Referring to FIG. 1, first, a region where the stress monotonically increases from 0 MPa as the strain increases from 0% can be confirmed (elastic deformation region). When the strain increases further after the stress becomes a maximum value, a region where the stress rapidly decreases and then, even when the strain increases further, the stress is approximately constant can be confirmed (Luders deformation region). When the strain further increases thereafter, a region where the stress increases once again, and thereafter the stress decreases can be confirmed (plastic deformation region). On the other hand, referring to FIG. 2, the stress-strain curve illustrated in FIG. 2 can be divided into two regions. Referring to FIG. 2, first, a region where the stress monotonically increases from 0 MPa as the strain increases from 0% can be confirmed (elastic deformation region). Thereafter, when the strain further increases, a region where the stress increases and thereafter the stress decreases can be confirmed (plastic deformation region).
That is, comparing FIG. 1 and FIG. 2, in the stress-strain curve illustrated in FIG. 1, between the elastic deformation region and the plastic deformation region there is a region (Luders deformation region) where the stress rapidly decreases and then, even when the strain increases, the stress is approximately constant. Here, the size of a decrease in the stress when the stress rapidly decreases from the elastic deformation region is referred to as "yield point drop Δσ (MPa)". Further, the amount of the strain in the region where the stress is approximately constant even when the strain increases is referred to as "yield point elongation Δε (%)". These will now be described more specifically using the accompanying drawings.
FIG. 3 and FIG. 4 are each a view in which one part of FIG. 1 is enlarged. FIG. 3 is a view in which, in the stress-strain curve illustrated in FIG. 1, a region where the strain is 0 to 2.0% and the stress is 850 to 1050 MPa is enlarged. Referring to FIG. 3, in the stress-strain curve, a point where the stress exhibits a maximum value in the elastic deformation region is defined as "P0", and the stress at the point P0 is defined as "a0" (MPa). Referring further to FIG. 3, a point of intersection between the stress-strain curve and a straight line L is defined as "P", and the stress at the point P is defined as "σ" (MPa). The straight line L is parallel to the stress-strain curve in the elastic deformation region, and is a straight line parallely translated by 0.2% in the positive direction of the strain. That is, the stress σ at the point P corresponds to so-called "0.2% offset proof stress". In the present description, the difference between the stress σ0 (MPa) at the point P0 and the stress σ (MPa) at the point P is defined as "yield point drop Δσ (MPa)". Note that, in the present description, the yield strength (MPa) is defined as the stress σ (MPa) at the point P.
FIG. 4 is a view in which, in the stress-strain curve illustrated in FIG. 1, a region where the strain is 0 to 4.0% is enlarged. Referring to FIG. 4, in the stress-strain curve, the point where the plastic deformation region begins is defined as "Q". Here, the strain (%) at the point Q is defined as "yield point elongation Δε (%)". Note that, the point Q is a point of inflection in the stress-strain curve, and is a point which a person skilled in the art is fully capable of identifying.
The present inventors conducted further detailed studies regarding the yield point drop Δσ (MPa) and the yield point elongation Δε (%) defined as described above. As a result of such further detailed studies conducted by the present inventors, it was revealed that in a steel material having the aforementioned chemical composition and a yield strength σ of 125 ksi grade and in which the prior-y grain size GS is more than 5.0 µm, when 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 are obtained.
The reason why, in a steel material having the aforementioned chemical composition and a yield strength σ of 125 ksi grade and in which the prior-y grain size GS is more than 5.0 µm, excellent hydrogen embrittlement resistance are obtained when the yield point drop Δσ is 40 MPa or more and the yield point elongation Δε is 1.5% or more has not been clarified in detail. However, the present inventors surmise that the reason is as follows. There is a possibility that the critical stress for hydrogen embrittlement fractures in a steel material is closely related to plastic deformation. Although the details have not been clarified, there is a possibility that the more difficult it is for plastic deformation of a steel material to occur, the higher the critical stress for hydrogen embrittlement fractures becomes.
Here, referring to FIG. 1, in a steel material in which a yield point drop Δσ is confirmed, elastic deformation is maintained even at a higher stress than the yield strength σ. That is, there is a possibility that the greater the yield point drop Δσ is, the more difficult it is for plastic deformation to occur, and the higher that the critical stress for hydrogen embrittlement fractures becomes. In other words, there is a possibility that the larger the yield point drop Δσ is, the more that the hydrogen embrittlement resistance improves.
Furthermore, referring to FIG. 1, as the yield point elongation Δε increases, the more difficult it is for plastic deformation to occur even when the amount of strain introduced into the steel material increases. Therefore, there is a possibility that as the yield point elongation Δε becomes larger, the critical stress for hydrogen embrittlement fractures increases. In other words, there is a possibility that the larger the yield point elongation Δε is, the more the hydrogen embrittlement resistance improves.
The present inventors surmise that in a steel material having the aforementioned chemical composition and a yield strength σ of 125 ksi grade and in which the prior-y grain size GS is more than 5.0 µm, by making the yield point drop Δσ of the steel material 40 MPa or more and making the yield point elongation Δε 1.5% or more, the hydrogen embrittlement resistance is improved because of the mechanism described above. Note that, there is also a possibility that the hydrogen embrittlement resistance of the aforementioned steel material is improved by a mechanism that is different to the mechanism described above. However, the fact that when a steel material has the aforementioned chemical composition, a yield point drop Δσ of 40 MPa or more, and a yield point elongation Δε of 1.5% or more, a high yield strength σ of 125 ksi grade and excellent hydrogen embrittlement resistance is obtained even when the prior-y grain size GS is 5.0 µm or more has been proven by examples which are described later.
The steel material according to the present embodiment, which has been completed based on the findings described above, is as follows.

[1] A steel material having a chemical composition consisting of, in mass%,
- 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: 0.005 to 0.100%,
- B: 0.0005 to 0.0050%,
- N: 0.0100% or less,
- O: 0.0050% or less,
- W: 0 to 2.00%,
- Co: 0 to 0.20%,
- Mg: 0 to 0.0100%,
- Ca: 0 to 0.0100%,
- rare earth metal: 0 to 0.0100%,
- Cu: 0 to 0.40%,
- Ni: 0 to 0.20%, and
- Sn: 0 to 0.10%,
- with the balance being Fe and impurities,
- wherein:
  - a grain size GS of prior-austenite grains is more than 5.0 to 30.0 µm,
  - a yield strength σ is 862 to 965 MPa,
  - a yield point drop Δσ is 40 MPa or more, and
  - a yield point elongation Δε is 1.5% or more.
[2] The steel material according to [1], wherein the chemical composition contains one or more elements selected from a group consisting of:
- W: 0.01 to 2.00%,
- Co: 0.01 to 0.20%,
- Mg: 0.0001 to 0.0100%,
- Ca: 0.0001 to 0.0100%,
- rare earth metal: 0.0001 to 0.0100%,
- Cu: 0.01 to 0.40%,
- Ni: 0.01 to 0.20%, and
- Sn: 0.01 to 0.10%.
[3] The steel material according to [1], wherein:
- the grain size GS of prior-austenite grains, the yield strength σ, and the yield point elongation Δε satisfy the following Formula (1): $σ \times Δε / {GS}^{2} \geq 12$
- where, the yield strength of the steel material in units of MPa is substituted for σ in Formula (1), the yield point elongation of the steel material in units of % is substituted for Δε in Formula (1), and the grain size of prior-austenite grains of the steel material in units of µm is substituted for GS in Formula (1).
[4] The steel material according to [2], wherein:
- the grain size GS of prior-austenite grains, the yield strength σ, and the yield point elongation Δε satisfy the following Formula (1): $σ \times Δε / {GS}^{2} \geq 12$
- where, the yield strength of the steel material in units of MPa is substituted for σ in Formula (1), the yield point elongation of the steel material in units of % is substituted for Δε in Formula (1), and the grain size of prior-austenite grains of the steel material in units of µm is substituted for GS in Formula (1).
[5] The steel material according to any one of [1] to [4], wherein:
the steel material is any one of an oil-well steel pipe, a steel pipe for line pipes, and a steel pipe for high-pressure hydrogen containers.
[6] The steel material according to any one of [1] to [4], wherein:
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.
[7] The steel material according to [5], wherein:
the steel pipe for high-pressure hydrogen containers is any one of a steel pipe for high-pressure hydrogen storage vessels in hydrogen service or a steel pipe for high-pressure hydrogen cylinders.
[8] The steel material according to [6], wherein:
the seamless steel pipe for high-pressure hydrogen containers is any one of a seamless steel pipe for high-pressure hydrogen storage vessels in hydrogen service or a seamless steel pipe for high-pressure hydrogen cylinders.

In the present description the term "oil-well steel pipe" means a steel pipe that is used for oil country tubular goods. The term "oil country tubular goods" is a generic term for casing pipes, tubing pipes, and drilling pipes which are used for drilling an oil well or a gas well, extracting crude oil or natural gas, and the like. The term "seamless steel pipe for oil wells" means an oil-well steel pipe that is a seamless steel pipe.
In the present description, the term "steel pipe for line pipes" means a steel pipe that is used for line pipes which constitute a pipeline that transmits production fluid (crude oil or natural gas) extracted from an oil well or a gas well. Examples of a pipeline include a flow line which transmits production fluid from an oil well or a gas well, a gathering line which gathers production fluid transmitted by a flow line and transmits the production fluid to a primary treatment facility, a trunk line that transmits production fluid subjected to a primary treatment such as dehydration to the outskirts of a market, and a distribution line that transmits oil or gas or the like to consumers. The term "seamless steel pipe for line pipes" means that a steel pipe for line pipes is a seamless steel pipe.
In the present description, the term "steel pipe for high-pressure hydrogen containers" means a steel pipe that is used for high-pressure hydrogen containers in which high-pressure hydrogen gas is stored and which are standardized in ISO 11439, ANSI/NGV, and the Container Safety Rules-Exemplified Standard of the High Pressure Gas Safety Act and the like. High-pressure hydrogen containers are, for example, high-pressure hydrogen storage vessels in hydrogen service installed at a hydrogen station or high-pressure hydrogen cylinders with which fuel cell vehicles are equipped. The term "seamless steel pipe for high-pressure hydrogen containers" means that a steel pipe for high-pressure hydrogen containers is a seamless steel pipe.
Hereunder, the steel material according to the present embodiment is described in detail. The symbol "%" in relation to an element means mass percent unless otherwise stated.

[Chemical composition]

The chemical composition of the steel material according to the present embodiment contains the following elements.

C: 0.20% to 0.45%

Carbon (C) increases hardenability, and makes the microstructure of the steel material a microstructure that is mainly composed of tempered martensite and tempered bainite. As a result, the hydrogen embrittlement resistance of the steel material is improved. C also forms carbides or carbo-nitrides, and thereby increases the strength of the steel material. If the content of C is too low, the aforementioned advantageous effects will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment. On the other hand, if the content of C is too high, even if the contents of other elements are within the range of the present embodiment, the amount of carbides in the steel material will be excessively large. In such case, strength of the steel material will be too high, and the hydrogen embrittlement resistance of the steel material will decrease. Therefore, the content of C is to be 0.20% to 0.45%. A preferable lower limit of the content of C is 0.21%, more preferably is 0.22%, and further preferably is 0.24%. A preferable upper limit of the content of C is 0.40%, more preferably is 0.38%, and further preferably is 0.36%.

Si: 0.05 to 1.50%

Silicon (Si) deoxidizes the steel and reduces inclusions in the steel material. As a result, the hydrogen embrittlement resistance of the steel material is improved. If the content of Si is too low, the aforementioned advantageous effect will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment. On the other hand, if the content of Si is too high, the hydrogen embrittlement resistance of the steel material will decrease even if the contents of other elements are within the range of the present embodiment. Therefore, the content of Si is to be 0.05 to 1.50%. A preferable lower limit of the content of Si is 0.06%, more preferably is 0.08%, further preferably is 0.10%, and further preferably is 0.13%. A preferable upper limit of the content of Si is 1.45%, more preferably is 1.42%, further preferably is 1.40%, further preferably is 1.38%, and further preferably is 1.35%.

Mn: 0.01 to 1.00%

Manganese (Mn) deoxidizes the steel. Mn also increases hardenability of the steel material and increases strength of the steel material. If the content of Mn is too low the aforementioned advantageous effects will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment. On the other hand, if the content of Mn is too high, even if the contents of other elements are within the range of the present embodiment, coarse sulfide-based inclusions will form and the hydrogen embrittlement resistance of the steel material will decrease. Therefore, the content of Mn is to be 0.01 to 1.00%. A preferable lower limit of the content of Mn is 0.02%, more preferably is 0.04%, and further preferably is 0.10%. A preferable upper limit of the content of Mn is 0.97%, more preferably is 0.95%, further preferably is 0.90%, and further preferably is 0.80%.

P: 0.030% or less

Phosphorus (P) is an impurity which is unavoidably contained. That is, the lower limit of the content of P is more than 0%. If the content of P is too high, even if the contents of other elements are within the range of the present embodiment, P will segregate to grain boundaries and the hydrogen embrittlement resistance of the steel material will decrease. Therefore, the content of P is to be 0.030% or less. The content of P is preferably as low as possible. However, extremely reducing the content of P will greatly increase the production cost. Therefore, when taking industrial production into consideration, a preferable lower limit of the content of P is 0.001%, more preferably is 0.002%, and further preferably is 0.003%. A preferable upper limit of the content of P is 0.025%, more preferably is 0.023%, further preferably is 0.021%, and further preferably is 0.020%.

S: 0.0100% or less

Sulfur (S) is an impurity which is unavoidably contained. That is, the lower limit of the content of S is more than 0%. If the content of S is too high, even if the contents of other elements are within the range of the present embodiment, S will segregate to grain boundaries and the hydrogen embrittlement resistance of the steel material will decrease. Therefore, the content of S is to be 0.0100% or less. The content of S is preferably as low as possible. However, extremely reducing the content of S will greatly increase the production cost. Therefore, when taking industrial production into consideration, a preferable lower limit of the content of S is 0.0001%, more preferably is 0.0002%, further preferably is 0.0003%, and further preferably is 0.0005%. A preferable upper limit of the content of S is 0.0090%, more preferably is 0.0080%, and further preferably is 0.0070%.

Cr: 0.40 to 1.10%

Chromium (Cr) increases hardenability of the steel material and improves the hydrogen embrittlement resistance of the steel material. Cr also increases the temper softening resistance of the steel material and thereby enables high-temperature tempering. As a result, the hydrogen embrittlement resistance of the steel material is improved. If the content of Cr is too low, the aforementioned advantageous effects will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment. On the other hand, if the content of Cr is too high, even if the contents of other elements are within the range of the present embodiment, coarse carbides will form and the hydrogen embrittlement resistance of the steel material will decrease. Therefore, the content of Cr is to be 0.40 to 1.10%. A preferable lower limit of the content of Cr is 0.43%, more preferably is 0.44%, further preferably is 0.46%, and further preferably is 0.50%. A preferable upper limit of the content of Cr is 1.08%, more preferably is 1.05%, further preferably is 0.99%, and further preferably is 0.95%.

Mo: 0.40 to 1.30%

Molybdenum (Mo) increases hardenability of the steel material. Mo also increases the temper softening resistance of the steel material and thereby enables high-temperature tempering. As a result, the hydrogen embrittlement resistance of the steel material is improved. In addition, Mo segregates to prior-austenite grain boundaries and improves the hydrogen embrittlement resistance of the steel material. If the content of Mo is too low, the aforementioned advantageous effects will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment. On the other hand, if the content of Mo is too high, even if the contents of other elements are within the range of the present embodiment, coarse carbides will form and the hydrogen embrittlement resistance of the steel material will decrease. Therefore, the content of Mo is to be 0.40 to 1.30%. A preferable lower limit of the content of Mo is 0.43%, more preferably is 0.44%, further preferably is 0.48%, and further preferably is 0.55%. A preferable upper limit of the content of Mo is 1.25%, more preferably is 1.24%, further preferably is 1.20%, and further preferably is 1.17%.

V: 0.01 to 0.30%

Vanadium (V) forms carbides, nitrides or carbo-nitrides (hereinafter, referred to as "carbo-nitrides and the like"), and thereby increases strength of the steel material. If the content of V is too low, the aforementioned advantageous effect will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment. On the other hand, if the content of V is too high, carbo-nitrides and the like will excessively form even if the contents of other elements are within the range of the present embodiment. As a result, strength of the steel material will be too high, and the hydrogen embrittlement resistance of the steel material will decrease. Therefore, the content of V is to be 0.01 to 0.30%. A preferable lower limit of the content of V is 0.02%, more preferably is 0.03%, further preferably is 0.04%, further preferably is 0.06%, and further preferably is 0.08%. A preferable upper limit of the content of V is 0.28%, more preferably is 0.24%, and further preferably is 0.22%.

Nb: 0.005 to 0.100%

Niobium (Nb) forms carbo-nitrides and the like, which refine the grains of the steel material by the pinning effect at austenite grain boundaries when performing an austenitizing heat treatment before quenching, thereby increasing the yield point drop Δσ of the steel material. As a result, the hydrogen embrittlement resistance of the steel material is improved. Nb also forms fine carbides during tempering and thereby increases the temper softening resistance of the steel material and increases strength of the steel material. If the content of Nb is too low, the aforementioned advantageous effects will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment. On the other hand, if the content of Nb is too high, even if the contents of other elements are within the range of the present embodiment, carbo-nitrides and the like will excessively form and the hydrogen embrittlement resistance of the steel material will, on the contrary, decrease. Therefore, the content of Nb is to be 0.005 to 0.100%. A preferable lower limit of the content of Nb is 0.006%, more preferably is 0.007%, further preferably is 0.010%, and further preferably is 0.015%. A preferable upper limit of the content of Nb is 0.097%, more preferably is 0.095%, further preferably is 0.090%, and further preferably is 0.085%.

Ti: 0.001 to 0.030%

Titanium (Ti) form fine precipitates such as Ti nitrides, which refine the austenite grains by the pinning effect when performing an austenitizing heat treatment before quenching, and thereby improve the hydrogen embrittlement resistance of the steel material. If the content of Ti is too low, the aforementioned advantageous effect will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment. On the other hand, if the content of Ti is too high, coarse Ti nitrides will form even if the contents of other elements are within the range of the present embodiment. The coarse Ti nitrides will act as starting points for cracks. As a result, the hydrogen embrittlement resistance of the steel material will decrease. Therefore, the content of Ti is to be 0.001 to 0.030%. A preferable lower limit of the content of Ti is 0.002%, more preferably is 0.003%, and further preferably is 0.005%. A preferable upper limit of the content of Ti is 0.029%, more preferably is 0.028%, further preferably is 0.027%, and further preferably is 0.025%.

Al: 0.005 to 0.100%

Aluminum (Al) deoxidizes the steel. Al also combines with N to form Al nitrides, which refine the grains by the pinning effect and thereby improve the hydrogen embrittlement resistance of the steel material. If the content of Al is too low the aforementioned advantageous effects will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment. On the other hand, if the content of Al is too high, even if the contents of other elements are within the range of the present embodiment, coarse oxides will form and the hydrogen embrittlement resistance of the steel material will decrease. Therefore, the content of Al is to be 0.005 to 0.100%. A preferable lower limit of the content of Al is 0.006%, more preferably is 0.008%, and further preferably is 0.012%. A preferable upper limit of the content of Al is 0.095%, more preferably is 0.090%, and further preferably is 0.085%. Note that, as used in the present description, the term content of "Al" means the content of "acid-soluble Al", that is, "sol. Al".

B: 0.0005 to 0.0050%

Boron (B) increases hardenability of the steel material and increases strength of the steel material. B also suppresses grain-boundary segregation of P and thereby improves the hydrogen embrittlement resistance of the steel material. If the content of B is too low, the aforementioned advantageous effects will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment. On the other hand, if the content of B is too high, coarse B nitrides will form even if the contents of other elements are within the range of the present embodiment. The coarse B nitrides will act as starting points for cracks. As a result, the hydrogen embrittlement resistance of the steel material will decrease. Therefore, the content of B is to be 0.0005 to 0.0050%. A preferable lower limit of the content of B is 0.0006%, more preferably is 0.0008%, and further preferably is 0.0010%. A preferable upper limit of the content of B is 0.0045%, more preferably is 0.0040%, further preferably is 0.0035%, and further preferably is 0.0030%.

N: 0.0100% or less

Nitrogen (N) is unavoidably contained. That is, the lower limit of the content of N is more than 0%. N combines with Ti to form nitrides, which refine the austenite grains of the steel material by the pinning effect when performing an austenitizing heat treatment before quenching, thereby increasing strength of the steel material. On the other hand, if the content of N is too high, even if the contents of other elements are within the range of the present embodiment, coarse nitrides will be formed and the hydrogen embrittlement resistance of the steel material will decrease. Therefore, the content of N is to be 0.0100% or less. A preferable lower limit of the content of N for more effectively obtaining the aforementioned advantageous effect is 0.0001%, more preferably is 0.0005%, and further preferably is 0.0010%. A preferable upper limit of the content of N is 0.0096%, more preferably is 0.0090%, further preferably is 0.0080%, and further preferably is 0.0070%.

O: 0.0050% or less

Oxygen (O) is an impurity which is unavoidably contained. That is, the lower limit of the content of O is more than 0%. If the content of O is too high, even if the contents of other elements are within the range of the present embodiment, coarse oxides will be formed and the hydrogen embrittlement resistance of the steel material will decrease. Therefore, the content of O is to be 0.0050% or less. The content of O is preferably as low as possible. However, extremely reducing the content of O will greatly increase the production cost. Therefore, when taking industrial production into consideration, a preferable lower limit of the content of O is 0.0001%, more preferably is 0.0005%, and further preferably is 0.0010%. A preferable upper limit of the content of O is 0.0045%, more preferably is 0.0040%, and further preferably is 0.0035%.
The balance of the chemical composition of the steel material according to the present embodiment is Fe and impurities. Here, the term "impurities" in the chemical composition means substances which are mixed in from ore and scrap used as the raw material or from the production environment or the like when industrially producing the steel material, and which are not intentionally contained but are permitted within a range that does not adversely affect the steel material according to the present embodiment.

[Optional elements]

The chemical composition of the steel material according to the present embodiment may further contain one or more elements selected from the group consisting of W and Co in lieu of a part of Fe. Each of these elements is an optional element, and does not have to be contained. When contained, W and Co each improve the hydrogen embrittlement resistance of the steel material.

W: 0 to 2.00%

Tungsten (W) is an optional element, and does not have to be contained. That is, the content of W may be 0%. When contained, in a sour environment, W forms a corrosion coating on the surface of the steel material. As a result, the penetration of hydrogen 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 aforementioned advantageous effect will be obtained to a certain extent. However, if the content of W is too high, even if the contents of other elements are within the range of the present embodiment, coarse carbides will form and the hydrogen embrittlement resistance of the steel material will decrease. Therefore, the content of W is to be 0 to 2.00%. A preferable lower limit of the content of W is 0.01%, more preferably is 0.05%, and further preferably is 0.08%. A preferable upper limit of the content of W is 1.50%, more preferably is 1.20%, and further preferably is 1.00%.

Co: 0 to 0.20%

Cobalt (Co) is an optional element, and does not have to be contained. That is, the content of Co may be 0%. When contained, Co improves the hydrogen embrittlement resistance of the steel material. Co also dissolves in the steel material and thereby increases hardenability of the steel material and increases strength of the steel material. If even a small amount of Co is contained, the aforementioned advantageous effects will be obtained to a certain extent. However, if the content of Co is too high, the advantageous effects of Co will be saturated. Therefore, the content of Co is to be 0 to 0.20%. A preferable lower limit of the content of Co is 0.01%, more preferably is 0.02%, and further preferably is 0.04%. A preferable upper limit of the content of Co is 0.18%, more preferably is 0.15%, further preferably is 0.12%, and further preferably is 0.10%.
The chemical composition of the steel material according to the present embodiment may further contain one or more elements selected from the group consisting of Mg, Ca, and rare earth metal (REM) in lieu of a part of Fe. Each of these elements is an optional element, and does not have to be contained. When contained, Mg, Ca, and rare earth metal (REM) each improve the hydrogen embrittlement resistance of the steel material.

Mg: 0 to 0.0100%

Magnesium (Mg) is an optional element, and does not have to be contained. That is, the content of Mg may be 0%. When contained, Mg combines with S in the steel material, and precipitates as fine Mg sulfides. Incidental with this, Mg causes Mn sulfides to decrease. By these two effects, Mg improves the hydrogen embrittlement resistance of the steel material. If even a small amount of Mg is contained, the aforementioned advantageous effect will be obtained to a certain extent. However, if the content of Mg is too high, even if the contents of other elements are within the range of the present embodiment, oxides in the steel material will coarsen and the hydrogen embrittlement resistance of the steel material will decrease. Therefore, the content of Mg is to be 0 to 0.0100%. A preferable lower limit of the content of Mg is 0.0001%, more preferably is 0.0003%, further preferably is 0.0006%, and further preferably is 0.0010%. A preferable upper limit of the content of Mg is 0.0090%, more preferably is 0.0080%, further preferably is 0.0070%, further preferably is 0.0060%, further preferably is 0.0050%, further preferably is 0.0040%, and further preferably is 0.0030%.

Ca: 0 to 0.0100%

Calcium (Ca) is an optional element, and does not have to be contained. That is, the content of Ca may be 0%. When contained, Ca combines with S in the steel material, and precipitates as fine Ca sulfides. Incidental with this, Ca causes Mn sulfides to decrease. By these two effects, Ca improves the hydrogen embrittlement resistance of the steel material. If even a small amount of Ca is contained, the aforementioned advantageous effect will be obtained to a certain extent. However, if the content of Ca is too high, even if the contents of other elements are within the range of the present embodiment, oxides in the steel material will coarsen and the hydrogen embrittlement resistance of the steel material will decrease. Therefore, the content of Ca is to be 0 to 0.0100%. A preferable lower limit of the content of Ca is 0.0001%, more preferably is 0.0003%, further preferably is 0.0005%, and further preferably is 0.0007%. A preferable upper limit of the content of Ca is 0.0080%, more preferably is 0.0060%, further preferably is 0.0050%, and further preferably is 0.0040%.

Rare earth metal (REM): 0 to 0.0100%

Rare earth metal (REM) is an optional element, and does not have to be contained. That is, the content of REM may be 0%. When contained, REM combines with S in the steel material, and precipitates as fine REM sulfides. Incidental with this, REM causes Mn sulfides to decrease. By these two effects, REM improves the hydrogen embrittlement resistance of the steel material. If even a small amount of REM is contained, the aforementioned advantageous effect will be obtained to a certain extent. However, if the content of REM is too high, even if the contents of other elements are within the range of the present embodiment, oxides in the steel material will coarsen and the hydrogen embrittlement resistance of the steel material will decrease. Therefore, the content of REM is to be 0 to 0.0100%. A preferable lower limit of the content of REM is 0.0001%, more preferably is 0.0003%, further preferably is 0.0005%, and further preferably is 0.0010%. A preferable upper limit of the content of REM is 0.0090%, more preferably is 0.0080%, further preferably is 0.0070%, further preferably is 0.0060%, further preferably is 0.0050%, and further preferably is 0.0040%.
In the present description the term "REM" means one or more types of element selected from the group consisting of scandium (Sc) which is the element with atomic number 21, yttrium (Y) which is the element with atomic number 39, and the elements from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71 that are lanthanoids. Further, in the present description, the term "content of REM" refers to the total content of these elements.
The chemical composition of the steel material according to the present embodiment may further contain one or more elements selected from the group consisting of Cu, Ni, and Sn in lieu of a part of Fe. Each of these elements is an optional element, and does not have to be contained. When contained, Cu, Ni, and Sn each improve the hydrogen embrittlement resistance of the steel material.

Cu: 0 to 0.40%

Copper (Cu) is an optional element, and does not have to be contained. That is, the content of Cu may be 0%. When contained, Cu improves the hydrogen embrittlement resistance of the steel material. Cu also dissolves in the steel material and thereby increases hardenability of the steel material and increases strength of the steel material. If even a small amount of Cu is contained, the aforementioned advantageous effects will be obtained to a certain extent. However, if the content of Cu is too high, hot workability of the steel material will decrease even if the contents of other elements are within the range of the present embodiment. Therefore, the content of Cu is to be 0 to 0.40%. A preferable lower limit of the content of Cu is 0.01%, more preferably is 0.02%, further preferably is 0.03%, further preferably is 0.05%, and further preferably is 0.07%. A preferable upper limit of the content of Cu is 0.38%, more preferably is 0.36%, and further preferably is 0.34%.

Ni: 0 to 0.20%

Nickel (Ni) is an optional element, and does not have to be contained. That is, the content of Ni may be 0%. When contained, Ni improves the hydrogen embrittlement resistance of the steel material. Ni also dissolves in the steel material and thereby increases hardenability of the steel material and increases strength of the steel material. If even a small amount of Ni is contained, the aforementioned advantageous effects will be obtained to a certain extent. However, if the content of Ni is too high, the production cost will increase extremely even if the contents of other elements are within the range of the present embodiment. Therefore, the content of Ni is to be 0 to 0.20%. A preferable lower limit of the content of Ni is 0.01%, more preferably is 0.02%, further preferably is 0.03%, and further preferably is 0.05%. A preferable upper limit of the content of Ni is 0.19%, more preferably is 0.18%, and further preferably is 0.17%.

Sn: 0 to 0.10%

Tin (Sn) is an optional element, and does not have to be contained. That is, the content of Sn may be 0%. When contained, Sn improves the hydrogen embrittlement resistance of the steel material. If even a small amount of Sn is contained, the aforementioned advantageous effect will be obtained to a certain extent. However, if the content of Sn is too high, hot workability of the steel material will decrease even if the contents of other elements are within the range of the present embodiment. Therefore, the content of Sn is to be 0 to 0.10%. A preferable lower limit of the content of Sn is 0.01%, more preferably is 0.02%, further preferably is 0.03%, and further preferably is 0.04%. A preferable upper limit of the content of Sn is 0.09%, more preferably is 0.08%, and further preferably is 0.07%.

[Grain size GS of prior-austenite grains]

In the steel material according to the present embodiment, a grain size GS of prior-austenite grains is more than 5.0 to 30.0 µm. In the present description, the term "grain size GS of prior-austenite grains (prior-y grain size GS)" means the grain size of prior-austenite grains that is determined in accordance with the measurement method of the mean intercept method that is specified in JIS G 0551: 2020.
Hydrogen embrittlement is liable to occur in a case where hydrogen has accumulated at grain boundaries. In a steel material having the chemical composition described above, if the prior-austenite grains are fine, the area of the prior-austenite grain boundaries will increase. In such a case, even though the amount of hydrogen that is absorbed in the material is the same, the amount of hydrogen accumulated per unit area of the prior-austenite grain boundaries will decrease. Therefore, if the prior-austenite grains are fine, the hydrogen embrittlement resistance of the steel material can be improved. Specifically, if the prior-y grain size GS of a steel material having the chemical composition described above is 30.0 µm or less, on the precondition that the other requirements of the present embodiment are satisfied, even when the prior-y grain size GS is more than 5.0 µm, a high yield strength of 125 ksi grade and excellent hydrogen embrittlement resistance can both be achieved.
Note that, the prior-y grain size GS is influenced by the chemical composition of the steel material and the production method, and the value of the prior-y grain size GS varies as a result. Specifically, as described above, Ti or Nb form fine precipitates such as Ti nitrides or Nb carbo-nitrides and the like during an austenitizing heat treatment before quenching, which refine the grains by the pinning effect at austenite grain boundaries. Further, as will be described later, if the quenching temperature in a quenching process is too high, in some cases the grains will coarsen and the prior-y grain size GS of the produced steel material will be large. Controlling the prior-y grain size GS to a certain extent in this way by adjusting the chemical composition of the steel material and the production method is something that a person skilled in the art is fully capable of doing.
In the steel material according to the present embodiment, a preferable upper limit of the prior-y grain size GS is 28.0 µm, more preferably is 26.0 µm, further preferably is 24.0 µm, and further preferably is 20.0 µm. In the steel material according to the present embodiment, the smaller that the prior-y grain size GS is, the more preferable it is. However, as mentioned above, in a steel material for which use in a sour environment or use for high-pressure hydrogen containers is assumed, in a case where the prior-y grain size GS is made 5.0 µm or less, the production cost increases extremely. Therefore, in the steel material according to the present embodiment, the lower limit of the prior-y grain size GS is defined as more than 5.0 µm. Furthermore, even if the prior-y grain size GS of the steel material according to the present embodiment is 5.5 µm or more, or even 6.0 µm or more, a yield strength of 125 ksi grade and excellent hydrogen embrittlement resistance can also both be achieved in some cases.

[Method for measuring grain size GS of prior-austenite grains]

In the present embodiment, the prior-y grain size GS of the steel material can be determined by the following method. The grain size GS of prior-austenite grains is determined in accordance with the measurement method of the mean intercept method specified in JIS G 0551: 2020.
First, a test specimen having an observation surface is taken from the steel material. If the steel material is a steel plate, a test specimen that has an observation surface that is parallel to the rolling elongation direction and that includes a thickness t/4 position which is the observation target region is taken from a center portion of the width. Here, the term "thickness t/4 position" means, in a case where the thickness of the steel plate is defined as "t", a position which is at a depth of t/4 from the surface of the steel plate. If the steel material is a steel pipe, a test specimen that has an observation surface that is parallel to the pipe axis direction and that includes a center portion of the wall thickness which is the observation target region is taken. If the steel material is a round steel bar, a test specimen that has an observation surface that is parallel to the rolling elongation direction and that includes an R/2 position which is the observation target region is taken. Note that, in the present description, the term "round steel bar" refers to a steel bar in which a cross section in a direction perpendicular to an axial direction is a circular shape. In addition, the term "R/2 position" means the center position of a radius R in a cross section perpendicular to the axial direction of the round steel bar. The size of the test specimen is not particularly limited. For example, the test specimen is made a size that is 10 mm in length in the rolling elongation direction × 5 mm in the width direction × 10 mm in the thickness direction. A surface that includes the rolling elongation direction and the thickness direction (surface of 10 mm × 10 mm in a case where the test specimen has the size described above) is adopted as the observation surface.
The observation surface of the test specimen is mirror polished. After the mirror polishing, the observation surface is immersed for about 10 seconds in a picral etching reagent to reveal the grain boundaries of prior-austenite grains by etching. An arbitrary 10 visual fields of the observation target region of the etched observation surface are observed by means of a secondary electron image obtained using a scanning electron microscope (SEM), and photographic images are generated. An area of each visual field is set to, for example, 500 µm × 500 µm (magnification of 200×).
The generated photographic images are used to evaluate the grain size number in accordance with the measurement method of the mean intercept method specified in JIS G 0551: 2020. The grain size of the prior-austenite grains in each visual field is determined based on the evaluated grain size number. The arithmetic average value of the grain sizes of the prior-austenite grains determined in the 10 visual fields is defined as the grain size GS of the prior-austenite grains (prior-y grain size GS) (µm).

[Yield strength σ of steel material]

In the steel material according to the present embodiment, a yield strength σ is 862 to 965 MPa. In the present description, the term "yield strength σ" means 0.2% offset proof stress that is obtained by a tensile test carried out by a method in accordance with JIS Z 2241: 2011. By satisfying the other requirements of the present embodiment, the steel material according to the present embodiment has excellent hydrogen embrittlement resistance even when the prior-y grain size GS is more than 5.0 µm and the yield strength σ is 862 to 965 MPa.
In the present embodiment, a preferable lower limit of the yield strength σ is 865 MPa. In the present embodiment, a preferable upper limit of the yield strength σ is less than 965 MPa, more preferably is 960 MPa, and further preferably is 955 MPa. Note that, a method for measuring the yield strength σ is described later.

[Yield point drop Δσ of steel material]

In the steel material according to the present embodiment, a yield point drop Δσ is 40 MPa or more. In the present description, the term "yield point drop Δσ" means the difference between the 0.2% offset proof stress and the maximum value (upper yield point) of stress in an elastic deformation region which is obtained by a tensile test carried out by a method in accordance with JIS Z 2241: 2011. That is, in the present embodiment, the yield point drop Δσ is defined as the difference between the stress σ0 (MPa) at the point P0 illustrated in FIG. 3 and the stress σ (MPa) at the point P illustrated in FIG. 3 (Δσ = σ0 - σ).
Note that, referring to FIG. 2, in the stress-strain curve illustrated in FIG. 2, a region in which the stress rapidly decreases does not exist. Therefore, in the case of this kind of stress-strain curve, a point corresponding to the point P0 cannot be defined, and hence the yield point drop Δσ cannot be defined. On the other hand, in a steel material that exhibits a stress-strain curve like the curve illustrated in FIG. 1, elastic deformation is maintained even at a higher stress than the yield strength σ. That is, there is a possibility that the greater the yield point drop Δσ is, the more difficult it is for plastic deformation to occur, and the higher that the critical stress for hydrogen embrittlement fractures becomes. In other words, there is a possibility that the larger the yield point drop Δσ is, the more that the hydrogen embrittlement resistance improves.
Here, in a steel material which has the chemical composition described above and in which the prior-y grain size GS is more than 5.0 to 30.0 µm and the yield strength σ is 862 to 965 MPa, if the yield point drop Δσ is 40 MPa or more, on the precondition that the other requirements of the present embodiment are satisfied, excellent hydrogen embrittlement resistance will be stably obtained. Therefore, in the steel material according to the present embodiment, the yield point drop Δσ is made 40 MPa or more.
In the present embodiment, a preferable lower limit of the yield point drop Δσ is 44 MPa, more preferably is 48 MPa, further preferably is 55 MPa, and further preferably is 60 MPa. In the present 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 the present embodiment, for example, may be 200 MPa, may be 150 MPa, may be 130 MPa, or may be 120 MPa. Note that, a method for measuring the yield point drop Δσ is described later.

[Yield point elongation Δε of steel material]

In the steel material according to the present embodiment, a yield point elongation Δε is 1.5% or more. In the present description, the term "yield point elongation Δε" means the amount of the strain at which the plastic deformation region starts, that is obtained by a tensile test carried out by a method in accordance with JIS Z 2241: 2011. That is, in the present embodiment, the yield point elongation Δε is defined as the strain (%) at the point Q illustrated in FIG. 4.
Note that, referring to FIG. 2, a region where the stress is constant even when the strain increases does not exist in the stress-strain curve illustrated in FIG. 2. Therefore, in the case of this kind of stress-strain curve, a point corresponding to the point Q cannot be defined, and hence the yield point elongation Δε cannot be defined. On the other hand, in a steel material that exhibits a stress-strain curve like the curve illustrated in FIG. 1, the larger that the yield point elongation Δε is, the more difficult it is for plastic deformation to occur even when the amount of strain introduced into the steel material increases. Therefore, there is a possibility that as the yield point elongation Δε becomes larger, the critical stress for hydrogen embrittlement fractures increases. In other words, there is a possibility that the larger the yield point elongation Δε is, the more the hydrogen embrittlement resistance improves.
Here, in a steel material which has the chemical composition described above, and in which the prior-y grain size GS is more than 5.0 to 30.0 µm, the yield strength σ is 862 to 965 MPa, and the yield point drop Δσ is 40 MPa or more, if the yield point elongation Δε is 1.5% or more, excellent hydrogen embrittlement resistance will be stably obtained. Therefore, in the steel material according to the present embodiment, the yield point elongation Δε is made 1.5% or more.
In the present embodiment, a preferable lower limit of the yield point elongation Δε is 1.6%, more preferably is 1.8%, further preferably is 2.0%, and further preferably is 2.2%. In the present 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 the present embodiment, for example, may be 4.0%, may be 3.8%, may be 3.6%, or may be 3.5%. Note that, a method for measuring the yield point drop Δσ is described later.
Note that, as illustrated in FIG. 2, even in the case of a steel material in which the chemical composition, the prior-y grain size GS, and the yield strength σ duplicate those of the present embodiment, a case can also occur in which the yield point drop Δσ and the yield point elongation Δε cannot be defined. Here, in a steel material having the chemical composition described above, the yield point drop Δσ and the yield point elongation Δε are each determined by the microstructure (phases, precipitates, and inclusions) of the steel material, and/or the state (crystal structure, size, and volume ratio) of precipitates, and/or the state of dislocations (dislocation density, arrangement of dislocations, and proportion of edge dislocations and screw dislocations and the like) in the steel material, and, in addition, the balance between these factors. Therefore, it is considered that the steel material according to the present embodiment is a steel material in which the yield point drop Δσ becomes 40 MPa or more and the yield point elongation Δε becomes 1.5% or more as a result of the microstructure of the steel material and/or the state of precipitates in the steel material, the state of dislocations in the steel material, and, in addition, the balance between these factors being appropriately controlled.

[Method for 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 the present embodiment can be determined by the following method. A tensile test is carried out by a method in accordance with JIS Z 2241: 2011. A round bar specimen is prepared from the steel material according to the present embodiment. If the steel material is a steel plate, the round bar specimen is prepared from a location which is at the center portion of the width and is at the center portion of the thickness. If the steel material is a steel pipe, the round bar specimen is prepared from the center portion of the wall thickness. If the steel material is a round steel bar, the round bar specimen is prepared from an R/2 position. Regarding the size of the round bar specimen, for example, the round bar specimen has a parallel portion diameter of 6.0 mm and a parallel portion length of 40 mm. Note that, the round bar specimen is to be prepared in a manner so that the axial direction of the round bar specimen is parallel with the rolling elongation direction of the steel material.
A tensile test is carried out in air at normal temperature (25°C) using the prepared round bar specimen. An elastic deformation region and a plastic deformation region are identified from a stress-strain curve obtained by the tensile test. A person skilled in the art is fully capable of identifying the elastic deformation region and the plastic deformation region. In the stress-strain curve, a point at which the stress exhibits a maximum value in the elastic deformation region is defined as "P0", and the stress at the point P0 is defined as "σ0 (MPa)". In addition, a point of intersection between the stress-strain curve and a straight line L which is parallel to the stress-strain curve in the elastic deformation region and parallelly translated by 0.2% in the positive direction of the strain is defined as "P", and the stress at the point P is defined as "σ (MPa)". Note that, in the present description, the term "strain" means so-called "nominal strain".
The stress σ at the obtained point P is defined as the yield strength σ (MPa). Note that, a value obtained by rounding off decimals of the obtained numerical value is adopted as the yield strength σ (MPa). As described above, the yield strength σ corresponds to the 0.2% offset proof stress. A difference between the stress σ0 at the obtained point P0 and the stress σ at the point P is defined as the yield point drop Δσ (MPa). Note that, a value obtained by rounding off decimals of the obtained numerical value is adopted as the yield point drop Δσ (MPa). In addition, in the stress-strain curve, a point where the plastic deformation region starts is defined as "Q". The strain (%) at the obtained point Q is defined as the yield point elongation Δε (%). Note that, a value obtained by rounding off to the first decimal place of the obtained numerical value is adopted as the yield point elongation Δε (%).

[Formula (1)]

In the steel material according to the present embodiment, preferably, the prior-y grain size GS, the yield strength σ, and the yield point elongation Δε satisfy the following Formula (1). When the steel material according to the present embodiment satisfies Formula (1), the steel material has even more excellent hydrogen embrittlement resistance. $σ \times Δε / {GS}^{2} \geq 12$
Where, the yield strength of the steel material in units of MPa is substituted for σ in Formula (1), the yield point elongation of the steel material in units of % is substituted for Δε in Formula (1), and the grain size of prior-austenite grains of the steel material in units of µm is substituted for GS in Formula (1).
Let Fn1 be defined as Fn1 = σ×Δε/GS². Fn1 is an index of hydrogen embrittlement resistance. On the precondition that the other requirements of the present embodiment are satisfied, if Fn1 is 12 or more, the steel material will have even more excellent hydrogen embrittlement resistance. Therefore, the steel material according to the present embodiment has the chemical composition described above, the prior-y 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, and in addition, preferably Fn1 is 12 or more.
A more preferable lower limit of Fn1 is more than 12, and further preferably is 13. The upper limit of Fn1 is not particularly limited, and for example may be 148, may be 135, may be 130, or may be 125. Note that, a value obtained by rounding off decimals of the obtained numerical value is adopted as Fn1.

[Hydrogen embrittlement resistance]

The steel material according to the present embodiment has the chemical composition described above, and in the steel material the prior-y 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. As a result, the steel material according to the present embodiment has high strength and excellent hydrogen embrittlement resistance even when the prior-y grain size GS is more than 5.0 µm. In the present embodiment, whether or not a steel material has excellent hydrogen embrittlement resistance can be evaluated by the following method.
A test specimen for evaluating hydrogen embrittlement resistance is prepared from the steel material according to the present embodiment. A round bar specimen with an annular notch is adopted as the test specimen. In the test specimen, for example, the diameter of a parallel portion is 4.0 mm, the length of the parallel portion is 25 mm, and an annular notch is formed at a central position in the longitudinal direction of the parallel portion. At such time, with regard to the notch shape, the notch depth is 0.3 mm, the notch angle is 60°, and the radius of curvature of the notch root is 0.125 mm. If the steel material is a steel plate, a round bar specimen is prepared from a location which is at the center portion of the width and is at a thickness t/4 position. If the steel material is a steel pipe, the round bar specimen is prepared from a center portion of the wall thickness. If the steel material is a round steel bar, the round bar specimen is prepared from an R/2 position.
The prepared round bar specimen with an annular notch is charged with hydrogen by a cathodic hydrogen charging method. Specifically, a cathodic hydrogen charging solution at normal temperature is prepared. An aqueous solution containing 5% by mass of a sodium chloride aqueous solution, 30 g/L of NH₄SCN, and an acetate buffer solution at normal temperature is adopted as the cathodic hydrogen charging solution, and the pH before testing is adjusted to pH 3.5 with the acetate buffer solution.
In a state in which the round bar specimen with an annular notch is immersed in the cathodic hydrogen charging solution, the round bar specimen with an annular notch is subjected to charging with hydrogen for a charging time of 24 hours at a potential of -1.5 V. At such time, it is preferable to form a zinc plating film on the surface of the round bar specimen with an annular notch that has been charged with hydrogen, to prevent the hydrogen in the round bar specimen with an annular notch leaking to outside.
The round bar specimen with an annular notch charged with hydrogen is subjected to a tensile test at normal temperature (25°C) in air using a slow strain rate tester (SSRT). At such time, a breaking stress BS1 at a strain rate of 4.2 × 10^-6/sec is determined. Note that, a value obtained by rounding off decimals of the obtained numerical value is adopted as the breaking stress BS1. In the present embodiment, if the breaking stress BS1 obtained under the aforementioned conditions is 950 MPa or more, it is determined that the steel material has excellent hydrogen embrittlement resistance. Further, in the present embodiment, if the breaking stress BS1 obtained under the aforementioned conditions is 970 MPa or more, it is determined that the steel material has even more excellent hydrogen embrittlement resistance.

[Microstructure]

In the microstructure of the steel material according to the present embodiment, the total area fraction of tempered martensite and tempered bainite is 90% or more. The balance of the microstructure is for example, ferrite and/or pearlite. In the present embodiment, in a case where a steel material has the chemical composition described above, and the prior-y 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, it can be determined that, for the relevant steel material, the total area fraction of tempered martensite and tempered bainite is 90% or more.

[Method for measuring total area fraction of tempered martensite and tempered bainite]

The total area fraction of tempered martensite and tempered bainite in the microstructure of the steel material of the present embodiment can also be determined by the following method. A test specimen having an observation surface is prepared from the steel material. If the steel material is a steel plate, a test specimen that has an observation surface that is parallel to the rolling elongation direction and that includes a thickness t/4 position that is an observation target region is prepared from a center portion of the width. If the steel material is a steel pipe, a test specimen that has an observation surface that is parallel to the pipe axis direction and that includes a center portion of the wall thickness which is the observation target region is prepared. If the steel material is a round steel bar, a test specimen that has an observation surface that is parallel to the rolling elongation direction and that includes an R/2 position which is the observation target region is prepared. The size of the test specimen is not particularly limited. The size of the test specimen is, for example, 10 mm in length in the rolling elongation direction × 5 mm in the width direction × 10 mm in the thickness direction. If the steel material is a steel plate, the thickness direction corresponds to the plate thickness direction, and the width direction corresponds to the plate width direction. If the steel material is a steel pipe, the rolling elongation direction corresponds to the pipe axis direction, the thickness direction corresponds to the wall thickness direction, and the width direction corresponds to the direction (circumferential direction) which is perpendicular to the pipe axis direction and the wall thickness direction. If the steel material is a round steel bar, the rolling elongation direction corresponds to the axial direction, the thickness direction corresponds to the radial direction, and the width direction corresponds to the direction (circumferential direction) which is perpendicular to the rolling elongation direction and the radial direction. A surface that includes the rolling elongation direction and the thickness direction (a surface of 10 mm × 10 mm in a case where the size of the test specimen is as described above) is adopted as the observation surface.
After polishing the observation surface of the test specimen to obtain a mirror surface, the test specimen is immersed for about 10 seconds in a nital etching reagent to reveal the microstructure by etching. An arbitrary 10 visual fields within the observation target region of the etched observation surface are observed by means of a secondary electron image obtained using a scanning electron microscope (SEM). If the steel material is a steel plate, the observation target region is a thickness t/4 position. If the steel material is a steel pipe, the observation target region is a center portion of the wall thickness. If the steel material is a round steel bar, the observation target region is an R/2 position. Each of the 10 visual field areas within the observation target region is, for example, 400 µm² (magnification of 5000×).
In each visual field, tempered martensite and tempered bainite are identified. In each visual field, it is possible to distinguish tempered martensite and tempered bainite from the other structures (ferrite, pearlite and the like) based on the morphology. Specifically, a structure that has a lamellar structure can be identified as pearlite. Structures including a lath-shaped structure or a lens-shaped structure can be identified as tempered martensite and tempered bainite. A structure without a substructure in the grain can be identified as ferrite.
The total area fraction of the identified tempered martensite and tempered bainite is determined. The method for determining the total area fraction is not particularly limited, and a well-known method can be used. For example, the total area fraction of tempered martensite and tempered bainite can be determined by image analysis. In the present embodiment, the arithmetic average value of the total area fractions of tempered martensite and tempered bainite determined in all of the visual fields (10 visual fields) is defined as the total area fraction (%) of tempered martensite and tempered bainite.

[Shape and uses of steel material]

The shape of the steel material according to the present embodiment is not particularly limited. The steel material of the present embodiment may be a steel pipe, may be a steel plate, or may be a round steel bar.
Preferably, the steel material of the present embodiment is any one of an oil-well steel pipe, a steel pipe for line pipes, and a steel pipe for high-pressure hydrogen containers. The term "oil-well steel pipe" means a steel pipe that is used for oil country tubular goods. Oil country tubular goods are, for example, casing pipes, tubing pipes, and drilling pipes which are used for drilling an oil well or a gas well, extracting crude oil or natural gas, and the like. The term "steel pipe for line pipes" means a steel pipe that is used for line pipes which constitute a pipeline that transmits production fluid (crude oil or natural gas) extracted from an oil well or a gas well. Examples of a pipeline include a flow line which transmits production fluid from an oil well or a gas well, a gathering line which gathers production fluid transmitted by a flow line and transmits the production fluid to a primary treatment facility, a trunk line that transmits production fluid subjected to a primary treatment such as dehydration to the outskirts of a market, and a distribution line that transmits oil or gas or the like to consumers. The term "steel pipe for high-pressure hydrogen containers" means a steel pipe that is used for high-pressure hydrogen containers in which high-pressure hydrogen gas is stored and which are standardized in ISO 11439, ANSI/NGV, and the Container Safety Rules-Exemplified Standard of the High Pressure Gas Safety Act and the like. The steel material of the present embodiment may be a steel pipe for high-pressure hydrogen containers and may be any one of a steel pipe for high-pressure hydrogen storage vessels in hydrogen service or a steel pipe for high-pressure hydrogen cylinders.
More preferably, the steel material of the present embodiment is any one of a seamless steel pipe for oil wells, a seamless steel pipe for line pipes, and a seamless steel pipe for high-pressure hydrogen containers. The term "seamless steel pipe for oil wells" means that an oil-well steel pipe is a seamless steel pipe. The term "seamless steel pipe for line pipes" means that a steel pipe for line pipes is a seamless steel pipe. The term "seamless steel pipe for high-pressure hydrogen containers" means that a steel pipe for high-pressure hydrogen containers is a seamless steel pipe. The steel material of the present embodiment may be a seamless steel pipe for high-pressure hydrogen containers and may be any one of a seamless steel pipe for high-pressure hydrogen storage vessels in hydrogen service or a seamless steel pipe for high-pressure hydrogen cylinders.

[Production method]

One example of a method for producing the steel material according to the present embodiment will now be described. Note that, the production method described hereunder is one example, and a method for producing the steel material according to the present embodiment is not limited to the following production method. That is, as long as the steel material according to the present embodiment that is composed as described above can be produced, a method for producing the steel material is not limited to the production method described hereunder. However, the production method described hereunder is a suitable method for producing the steel material according to the present embodiment.
One example of a method for producing the steel material according to the present embodiment includes the following steps.

(Step 1) Starting material preparation process
(Step 2) Hot working process
(Step 3) Quenching and tempering processes
(Step 4) Low-temperature heat treatment process

The present production method satisfies the following condition in the low-temperature heat treatment process of step 4.
(Condition 1) Hold at 150 to 250°C for 10 minutes or more.
Each process is described hereunder.

[(Step 1) Starting material preparation process]

In the starting material preparation process, first, molten steel having the chemical composition described above is produced by a well-known refining method. The produced molten steel is used to produce a cast piece by a continuous casting process. Here, the cast piece is a slab, a bloom, or a billet. Instead of the cast piece, an ingot may be produced by an ingot-making process using the aforementioned molten steel. As needed, the slab, bloom, or ingot may be subjected to hot rolling to produce a billet. The starting material (slab, bloom, or billet) is produced by the above production process.

[(Step 2) Hot working process]

In the hot working process, the prepared starting material is subjected to hot working to produce an intermediate steel material. If the end product is a steel pipe, first, the starting material is heated in a heating furnace. Although not particularly limited, the heating temperature is, for example, 1100 to 1300°C. After being extracted from the heating furnace, the starting material is subjected to hot working to produce a hollow shell (seamless steel pipe). For example, the Mannesmann process is performed as the hot working to produce a hollow shell. In this case, a billet is subjected to piercing-rolling using a piercing machine. The billet after the piercing-rolling is subjected to tube drawing using a mandrel mill. In addition, as needed, the billet after the tube drawing is subjected to diameter adjusting rolling using a reducer or a sizing mill. A hollow shell is produced by the above process.
A hollow shell may also be produced from the billet by a hot working method other than the Mannesmann process. For example, in a case where the steel material is a heavy-wall steel material of a short length such as a coupling, a hollow shell may be produced by forging by the Ehrhardt process or the like, or a hollow shell may be produced by a hot-extrusion process.
If the end product is a steel plate, for example, the starting material (slab) is subjected to hot rolling using one or a plurality of rolling mills including pairs of rolls to thereby produce an intermediate steel material (steel material). Although not particularly limited, the heating temperature prior to hot rolling is, for example, 1100 to 1300°C.
If the end product is a round steel bar, for example, the starting material (bloom) is subjected to blooming using a blooming mill and/or hot rolling using a continuous mill to produce an intermediate steel material (round steel bar). That is, the starting material may be subjected to blooming and thereby made into a round steel bar, or the starting material may be subjected to hot rolling using a continuous mill and thereby made into a round steel bar without performing blooming, or the starting material may be subjected to blooming using a blooming mill and subjected to hot rolling using a continuous mill and thereby made into a round steel bar. A continuous mill includes a plurality of roll stands arranged in a single row, with each stand including a pair of rolling rolls. In the case of performing blooming, although not particularly limited, the heating temperature prior to the blooming is, for example, 1100 to 1300°C. In the case of performing hot rolling using a continuous mill, although not particularly limited, the heating temperature prior to the hot rolling is, for example, 1100 to 1300°C.

[(Step 3) Quenching and tempering processes]

In the quenching and tempering processes, a quenching process and a tempering process are performed on the intermediate steel material after the hot working process.

[Quenching process]

In the quenching process, the intermediate steel material produced in the hot working process is subjected to quenching. The quenching is carried out by a well-known method. Specifically, the intermediate steel material after the hot working process is loaded into a heat treatment furnace and held at a quenching temperature. The quenching temperature is equal to or higher than the A_C3 transformation point, and for example is 900 to 1000°C. After being held at the quenching temperature, the intermediate steel material is rapidly cooled (quenched).
If the quenching temperature is too high, the grains will coarsen and in some cases the prior-y grain size GS in the produced steel material will become too large. On the other hand, if the quenching temperature is too low, in some cases coarse carbides will remain in the produced steel material. In such a case, a yield strength σ of 862 MPa or more will not be obtained. Therefore, in the quenching process according to the present embodiment, the quenching temperature is to be equal to or higher than the A_C3 transformation point, and specifically the quenching temperature is preferably 900 to 1000°C.
Although not particularly limited, the holding time at the quenching temperature is, for example, 10 to 60 minutes. The quenching method is, for example, water cooling. The quenching method is not particularly limited. In a case where the steel material is a steel pipe, the hollow shell may be rapidly cooled by immersing the hollow shell in a water bath or an oil bath, or the hollow shell may be rapidly cooled by pouring cooling water or jetting cooling water from a nozzle onto the outer surface and/or inner surface of the hollow shell.
Note that, after hot working, quenching (direct quenching) may be performed immediately after the hot working without cooling the steel material to normal temperature, or quenching may be performed after the steel material was held at the quenching temperature after being loaded into a holding furnace before the temperature of the steel material decreased after the hot working.

[Tempering process]

In the tempering process, the steel material after the quenching process is subjected to tempering. In the tempering process, the yield strength σ of the steel material is adjusted to 862 to 965 MPa. Note that, adjusting the yield strength σ of the steel material to 862 to 965 MPa by adjusting the conditions of the tempering process is something which can be practiced by a person skilled in the art.
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, within the range of 650°C to the A_C1 transformation point. Although not particularly limited, the holding time at the tempering temperature is, for example, within the range of 10 to 180 minutes. The yield strength of a steel material having the chemical composition described above is adjusted by appropriately adjusting the tempering temperature according to the chemical composition. Here, the term "tempering temperature" means the furnace temperature (°C) in the heat treatment furnace, and the term "holding time at the tempering temperature" means the in-furnace time (the time from when the steel material is loaded into the heat treatment furnace until being extracted therefrom).
Note that the quenching process and the tempering process may be performed one time each, or may be performed multiple times. For example, after performing the quenching process and the tempering process, the quenching process and the tempering process may be performed again. When the quenching process and the tempering process are performed multiple times, in some cases the prior-y grain size GS in the produced steel material may decrease.

[(Step 4) Low-temperature heat treatment process]

In the low-temperature heat treatment process, the steel material after the quenching and tempering processes is subjected to a low-temperature heat treatment under the condition described in Condition 1 hereunder.

[Regarding Condition 1]

(Condition 1) Hold at 150 to 250°C for 10 minutes or more.
In the present 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) of the heat treatment furnace, and the heat treatment time means the in-furnace time (the time from when the steel material is loaded into the heat treatment furnace until being extracted therefrom).
There is a possibility that dislocations will be stabilized by performing a heat treatment at 150 to 250°C. If dislocations are stabilized, it will be difficult for plastic deformation to occur. As a result, the yield point drop Δσ will be 40 MPa or more, and the yield point elongation Δε will be 1.5% or more. Therefore, in the low-temperature heat treatment process according to the present embodiment, a heat treatment at 150 to 250°C for 10 minutes or more is performed.
If the heat treatment temperature is too low, the aforementioned advantageous effects will not be sufficiently obtained. That is, dislocations will not be sufficiently stabilized, and there may be cases where, in the produced steel material, the yield point drop Δσ cannot be defined or the yield point drop Δσ is less than 40 MPa. In such a case, furthermore, it may not be possible to define the yield point elongation Δε, or the yield point elongation Δε will be less than 1.5%. On the other hand, if the heat treatment temperature is too high, sufficient stabilization of dislocations will not be obtained in some cases. Consequently, there may be cases where, in the produced steel material, the yield point drop Δσ cannot be defined or the yield point drop Δσ is less than 40 MPa. In such a case, furthermore, it may not be possible to define the yield point elongation Δε, or the yield point elongation Δε will be less than 1.5%.
If the heat treatment time is too short, the aforementioned advantageous effects will not be sufficiently obtained. That is, dislocations will not be sufficiently stabilized, and there may be cases where, in the produced steel material, the yield point drop Δσ cannot be defined or the yield point drop Δσ is less than 40 MPa. In such a case, furthermore, it may not be possible to define the yield point elongation Δε or the yield point elongation Δε will be less than 1.5%. On the other hand, if the heat treatment time is too long, the aforementioned effects will be saturated. Therefore, although not particularly limited, the upper limit of the heat treatment time is, for example, 60 minutes.
The steel material according to the present embodiment can be produced by performing the above production process. Hereunder, the advantageous effects of the steel material according to the present embodiment are described more specifically by way of examples. The various conditions adopted in the examples described hereunder are one example of conditions adopted for confirming the feasibility and advantageous effects of the steel material according to the present embodiment. Accordingly, the steel material according to the present embodiment is not limited to the one example of conditions described in the examples.

EXAMPLES

Steel materials (steel plates) having the chemical compositions shown in Table 1-1 and Table 1-2 were produced. Note that, the symbol "-" in Table 1-1 and Table 1-2 indicates that the content of the corresponding element was at the level of an impurity or less.

[Table 1-1]

TABLE 1-1

Sample Symbol	Chemical Composition (unit is mass%; balance is Fe and impurities)
Sample Symbol	C	Si	Mn	P	S	Cr	Mo	V	Nb	Ti	Al	B
A	0.32	0.79	0.15	0.009	0.0074	0.46	0.86	0.08	0.015	0.011	0.030	0.0019
B	0.35	0.91	0.09	0.010	0.0079	0.98	0.86	0.18	0.068	0.016	0.022	0.0029
C	0.33	1.09	0.38	0.006	0.0017	0.98	0.48	0.19	0.085	0.017	0.082	0.0025
D	0.35	0.53	0.04	0.012	0.0038	0.73	1.12	0.15	0.032	0.024	0.018	0.0008
E	0.23	1.01	0.89	0.017	0.0003	0.91	1.00	0.06	0.055	0.027	0.040	0.0029
F	0.30	1.03	0.81	0.006	0.0039	0.62	0.67	0.23	0.078	0.004	0.032	0.0028
G	0.22	1.03	0.41	0.004	0.0077	0.51	0.53	0.03	0.071	0.002	0.012	0.0028
H	0.35	0.89	0.55	0.009	0.0082	0.63	0.95	0.19	0.089	0.013	0.062	0.0023
I	0.29	1.13	0.84	0.002	0.0032	0.64	1.00	0.11	0.066	0.014	0.064	0.0027
J	0.30	1.05	0.22	0.014	0.0002	0.51	0.65	0.09	0.010	0.026	0.033	0.0013
K	0.33	0.42	0.30	0.015	0.0080	0.63	0.77	0.23	0.057	0.021	0.063	0.0006
L	0.29	1.17	0.49	0.020	0.0011	0.87	1.11	0.06	0.067	0.003	0.054	0.0021
M	0.29	0.15	0.39	0.004	0.0029	0.58	0.82	0.10	0.036	0.010	0.084	0.0023
N	0.31	0.41	0.28	0.002	0.0007	0.51	0.74	0.22	0.073	0.010	0.053	0.0025
O	0.25	0.68	0.08	0.020	0.0030	0.73	1.07	0.13	0.088	0.018	0.089	0.0009
P	0.33	0.74	0.17	0.010	0.0024	0.74	0.48	0.17	0.074	0.022	0.057	0.0008
Q	0.32	0.79	0.80	0.010	0.0046	0.86	0.77	0.24	0.080	0.023	0.070	0.0008
R	0.25	0.54	0.58	0.011	0.0010	0.68	0.88	0.08	0.047	0.011	0.054	0.0026
S	0.32	0.94	0.16	0.011	0.0086	0.77	0.68	0.18	0.073	0.014	0.036	0.0019
T	0.29	1.26	0.45	0.013	0.0079	0.58	0.87	0.10	0.076	0.021	0.036	0.0029
U	0.31	0.75	0.13	0.015	0.0012	0.60	0.96	0.12	0.063	0.019	0.086	0.0025
V	0.24	0.62	0.37	0.009	0.0058	0.97	0.66	0.21	0.037	0.010	0.030	0.0028
W	0.24	0.54	0.88	0.017	0.0007	0.89	0.62	0.23	0.054	0.027	0.087	0.0022
X	0.23	0.77	0.42	0.011	0.0040	0.69	0.80	0.15	0.062	0.016	0.057	0.0020
ZA	0.18	1.00	0.52	0.009	0.0046	0.88	0.53	0.15	0.081	0.001	0.082	0.0029
ZB	0.47	0.90	0.10	0.008	0.0080	0.90	1.10	0.21	0.044	0.027	0.028	0.0021
ZC	0.31	0.03	0.43	0.021	0.0038	0.51	0.94	0.23	0.046	0.012	0.012	0.0016
ZD	0.22	1.60	0.50	0.005	0.0056	0.58	0.94	0.09	0.011	0.015	0.039	0.0025
ZE	0.26	0.68	1.06	0.009	0.0022	0.78	1.09	0.08	0.038	0.020	0.053	0.0028
ZF	0.24	0.62	0.37	0.009	0.0058	0.38	0.66	0.21	0.037	0.010	0.030	0.0028
ZG	0.29	0.09	0.49	0.002	0.0046	1.20	1.17	0.14	0.019	0.013	0.016	0.0024
ZH	0.31	0.73	0.59	0.017	0.0031	0.70	0.36	0.07	0.045	0.004	0.040	0.0028
ZI	0.28	1.08	0.15	0.007	0.0078	0.84	1.37	0.24	0.052	0.005	0.038	0.0022
ZJ	0.29	0.85	0.64	0.005	0.0071	0.83	0.86	-	0.045	0.009	0.032	0.0028
ZK	0.32	0.08	0.51	0.002	0.0020	0.53	0.53	0.36	0.061	0.021	0.084	0.0013
ZL	0.33	0.27	0.85	0.000	0.0022	0.81	1.04	0.19	0.003	0.015	0.047	0.0024
ZM	0.34	0.61	0.46	0.002	0.0025	0.67	0.91	0.23	0.106	0.018	0.010	0.0023
ZN	0.33	0.68	0.62	0.017	0.0057	0.89	0.63	0.17	0.054	0.033	0.023	0.0024
ZO	0.24	0.54	0.88	0.017	0.0007	0.89	0.62	0.23	0.054	0.027	0.087	0.0001
ZP	0.29	1.21	0.43	0.013	0.0003	0.91	0.44	0.21	0.089	0.022	0.066	0.0062
ZQ	0.24	0.08	0.30	0.016	0.0043	0.91	0.86	0.15	0.062	0.011	0.055	0.0012
ZR	0.31	0.62	0.64	0.002	0.0073	0.87	0.50	0.21	0.012	0.015	0.028	0.0028
ZS	0.26	0.26	0.24	0.016	0.0005	0.49	0.53	0.21	0.083	0.020	0.008	0.0015
ZT	0.26	0.73	0.09	0.009	0.0018	0.97	0.49	0.19	0.088	0.010	0.062	0.0008
ZU	0.31	0.75	0.13	0.015	0.0012	0.60	0.96	0.18	0.015	0.019	0.086	0.0025

[Table 1-2]

TABLE 1-2

Sample Symbol	Chemical Composition (unit is mass%; balance is Fe and impurities)
Sample Symbol	N	O	W	Co	Mg	Ca	REM	Cu	Ni	Sn
A	0.0044	0.0035	-	-	-	-	-	-	-	-
B	0.0069	0.0023	-	-	-	-	-	-	-	-
C	0.0050	0.0010	-	-	-	-	-	-	-	-
D	0.0065	0.0037	-	-	-	-	-	-	-	-
E	0.0012	0.0020	-	-	-	-	-	-	-	-
F	0.0034	0.0014	-	-	-	-	-	-	-	-
G	0.0016	0.0018	-	-	-	-	-	-	-	-
H	0.0047	0.0020	-	-	-	-	-	-	-	-
I	0.0021	0.0036	1.50	-	-	-	-	-	-	-
J	0.0015	0.0020	-	0.15	-	-	-	-	-	-
K	0.0041	0.0014	0.80	0.06	-	-	-	-	-	-
L	0.0011	0.0040	-	-	0.0028	-	-	-	-	-
M	0.0047	0.0010	-	-	-	0.0032	-	-	-	-
N	0.0047	0.0044	-	-	-	-	0.0081	-	-	-
O	0.0029	0.0016	-	-	-	0.0016	0.0067	-	-	-
P	0.0019	0.0017	-	-	-	-	-	0.30	-	-
Q	0.0026	0.0025	-	-	-	-	-	-	0.15	-
R	0.0014	0.0018	-	-	-	-	-	-	-	0.08
S	0.0035	0.0018	-	-	-	-	-	0.25	0.12	-
T	0.0051	0.0023	-	-	-	-	-	0.28	-	0.06
U	0.0034	0.0031	-	-	-	-	-	-	-	-
V	0.0052	0.0017	-	-	-	-	-	-	-	-
W	0.0038	0.0024	-	-	-	-	-	-	-	-
X	0.0032	0.0023	-	-	-	-	-	-	-	-
ZA	0.0021	0.0021	-	-	-	-	-	-	-	-
ZB	0.0053	0.0014	-	-	-	-	-	-	-	-
ZC	0.0021	0.0032	-	-	-	-	-	-	-	-
ZD	0.0022	0.0024	-	-	-	-	-	-	-	-
ZE	0.0015	0.0014	-	-	-	-	-	-	-	-
ZF	0.0052	0.0017	-	-	-	-	-	-	-	-
ZG	0.0058	0.0022	-	-	-	-	-	-	-	-
ZH	0.0036	0.0011	-	-	-	-	-	-	-	-
ZI	0.0013	0.0019	-	-	-	-	-	-	-	-
ZJ	0.0025	0.0037	-	-	-	-	-	-	-	-
ZK	0.0032	0.0032	-	-	-	-	-	-	-	-
ZL	0.0017	0.0025	-	-	-	-	-	-	-	-
ZM	0.0049	0.0014	-	-	-	-	-	-	-	-
ZN	0.0018	0.0018	-	-	-	-	-	-	-	-
ZO	0.0038	0.0024	-	-	-	-	-	-	-	-
ZP	0.0073	0.0043	-	-	-	-	-	-	-	-
ZQ	0.0124	0.0040	-	-	-	-	-	-	-	-
ZR	0.0018	0.0016	-	-	-	-	-	-	-	-
ZS	0.0022	0.0036	-	-	-	-	-	-	-	-
ZT	0.0030	0.0013	-	-	-	-	-	-	-	-
ZU	0.0034	0.0031	-	-	-	-	-	-	-	-

The steel materials denoted by the respective sample symbols were produced by the following method. Ingots having the chemical compositions described in Table 1-1 and Table 1-2 were produced by a casting process. Each ingot was subjected to hot forging to produce a block material having a thickness of 50 mm.
Each block material was subjected to a hot working process. Specifically, the block material was heated to 1250°C. After being heated, the block material was subjected to hot rolling to produce a steel material (steel plate) having a thickness of 15 mm. The produced steel material was allowed to cool to normal temperature.
After being allowed to cool to normal temperature, the steel material was subjected to quenching and tempering processes. Specifically, after the steel material was held at a temperature (°C) for a time (min) which are each described in the column "Quenching Process" shown in Table 2, the steel material was subjected to quenching by water cooling. The steel material after quenching was subjected to tempering. The steel material was held at a temperature (°C) for a time (min) which are each described in the column "Tempering Process" shown in Table 2. Thereafter, the steel material after tempering was subjected to a low-temperature heat treatment process. The steel material was held at a temperature (°C) for a time (min) which are each described in the column "Low-temperature Heat Treatment Process" shown in Table 2. Note that, the low-temperature heat treatment process was not performed on the steel materials denoted by sample symbols ZR. The steel materials (steel plates) denoted by the respective sample symbols were produced by the above production process.

[Table 2]

TABLE 2

Sample Symbol	Quenching Process		Tempering Process		Low-temperature Heat Treatment Process
Sample Symbol	Temperature (°C)	Time (mins)	Temperature (°C)	Time (mins)	Temperature (°C)	Time (mins)
A	900	15	690	30	200	30
B	920	15	705	30	200	30
C	900	15	705	30	220	30
D	920	15	700	30	240	30
E	920	15	690	30	180	30
F	900	15	705	30	180	15
G	900	15	690	30	180	15
H	920	15	705	30	200	15
I	920	15	695	30	220	30
J	900	15	690	30	180	30
K	900	15	705	30	180	30
L	920	15	690	30	180	30
M	900	15	695	30	180	30
N	900	15	705	30	160	30
O	920	15	700	30	240	30
P	900	15	705	30	220	30
Q	920	15	705	30	240	30
R	920	15	690	30	240	30
S	900	15	705	30	220	30
T	900	15	695	30	220	30
U	920	15	705	30	160	30
V	920	15	705	30	240	30
W	920	15	705	30	220	30
X	900	15	705	30	180	15
ZA	900	15	690	30	180	30
ZB	940	15	705	30	200	30
ZC	900	15	705	30	200	30
ZD	920	15	690	30	220	30
ZE	920	15	690	30	220	30
ZF	920	15	705	30	180	30
ZG	940	15	700	30	200	30
ZH	900	15	690	30	160	30
ZI	920	15	705	30	240	30
ZJ	920	15	695	30	160	30
ZK	900	15	695	30	180	30
ZL	920	15	705	30	160	30
ZM	920	15	705	30	160	30
ZN	920	15	705	30	200	30
ZO	920	15	705	30	220	30
ZP	900	15	705	30	160	30
ZQ	920	15	700	30	240	30
ZR	940	15	705	30	-	-
ZS	900	15	705	30	100	30
ZT	920	15	705	30	300	30
ZU	920	15	705	30	200	5

[Evaluation tests]

The produced steel materials were subjected to a prior-y grain size measurement test, a tensile test, and a hydrogen embrittlement resistance evaluation test.

[Prior-y grain size measurement test]

A test specimen in which a face that included a thickness t/4 position as the observation target region and included the rolling elongation direction and the thickness direction was adopted as an observation surface was prepared from a center portion of the width of each of the steel materials denoted by the respective sample symbols. The size of the test specimen was 10 mm in length in the rolling elongation direction, 5 mm in length in the plate width direction, and 10 mm in length in the thickness direction. A region of 10 mm in length in the rolling elongation direction × 10 mm in length in the thickness direction was adopted as the observation surface. The prior-y grain size GS (µm) of the obtained test specimen was determined in accordance with the method described above. At such time, the visual field area was set to 500 µm × 500 µm (magnification of 200×). The determined prior-y grain size GS (µm) is shown in the column "Prior-y Grain Size GS (µm)" in Table 3.

[Table 3]

TABLE 3

Sample Symbol	Prior-y Grain Size GS (µm)	Yield Strength σ (MPa)	Tensile Strength (MPa)	Yield Point Drop Δσ (MPa)	Yield Point Elongation Δε (%)	Fn1	Notched Tensile Test Result
Sample Symbol	Prior-y Grain Size GS (µm)	Yield Strength σ (MPa)	Tensile Strength (MPa)	Yield Point Drop Δσ (MPa)	Yield Point Elongation Δε (%)	Fn1	In Air (MPa)	In Hydrogen Environment (MPa)
A	10.8	884	968	64	1.6	12	1205	1044
B	7.8	887	963	80	2.6	38	1200	1056
C	9.7	915	980	64	3.1	30	1218	972
D	11.2	928	990	58	2.3	17	1228	1094
E	8.7	959	997	73	2.5	32	1236	1080
F	9.5	942	1005	66	3.1	32	1244	1011
G	5.2	881	950	102	3.0	98	1186	1003
H	6.7	951	1011	90	2.8	59	1251	1079
I	8.5	939	1004	74	2.5	32	1243	1081
J	12.6	920	980	44	2.2	13	1218	995
K	6.0	941	1003	96	2.9	76	1242	1047
L	7.8	940	965	80	2.6	40	1202	1106
M	7.2	910	978	85	2.7	47	1216	1051
N	5.5	929	1010	100	3.0	92	1250	1043
O	7.1	943	1005	86	2.7	51	1244	1101
P	5.4	923	978	101	3.0	95	1216	992
Q	7.6	916	965	82	2.6	41	1202	1039
R	9.2	948	1030	68	2.5	28	1271	1054
S	5.2	957	992	102	3.0	106	1231	1033
T	9.8	883	967	63	3.1	29	1204	1050
U	15.2	862	961	58	3.1	12	1197	984
V	16.8	868	970	48	3.0	9	1207	963
W	16.5	872	958	42	3.2	10	1195	951
X	11.2	881	967	40	1.6	11	1208	964
ZA	5.4	886	954	101	3.0	91	1190	802
ZB	12.8	985	1037	42	2.3	14	1278	868
ZC	6.5	864	940	91	2.8	57	1176	862
ZD	12.6	914	968	44	2.7	16	1205	842
ZE	11.1	952	997	60	2.3	18	1236	871
ZF	10.8	925	956	64	2.3	18	1193	803
ZG	9.2	907	957	68	2.6	28	1194	890
ZH	6.6	864	935	90	2.8	56	1170	770
ZI	9.4	899	948	67	2.4	24	1184	921
ZJ	9.8	834	896	63	2.4	21	1129	838
ZK	5.5	994	1043	100	2.9	95	1285	801
ZL	20.2	878	952	-	2.5	5	1188	840
ZM	9.9	883	960	62	2.4	22	1197	846
ZN	9.0	925	954	70	2.5	29	1190	804
ZO	9.3	870	947	68	2.5	25	1183	802
ZP	5.4	890	953	108	3.1	95	1189	791
ZQ	8.3	887	939	76	2.6	33	1175	843
ZR	13.9	872	946	-	-	-	1182	769
ZS	5.2	883	945	-	-	-	1181	802
ZT	5.3	927	991	-	1.0	33	1230	795
ZU	13.4	899	986	37	2.6	13	1224	923

[Tensile test]

A round bar specimen was taken from a center portion of the thickness of the steel material denoted by each sample symbol. The diameter of a parallel portion of the round bar specimen was 6.0 mm, and the length of the parallel portion was 40 mm. The axial direction of the round bar specimen was parallel to the rolling elongation direction of the steel material. The yield strength σ (MPa), the yield point drop Δσ (MPa), and the yield point elongation Δε (%) were determined in accordance with the method described above using the round bar specimen. 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 column "Yield Strength σ (MPa)" in Table 3. The obtained tensile strength is shown in the column "Tensile Strength (MPa)" in Table 3. The obtained yield point drop Δσ is shown in the column "Yield Point Drop Δσ (MPa)" in Table 3. The obtained yield point elongation Δε is shown in the column "Yield Point Elongation Δε (%)" in Table 3. Note that, a case where the yield point drop Δσ or the yield point elongation Δε could not be defined is indicated by the symbol "-" in Table 3. In addition, Fn1 (= σ×Δε/GS²) was determined based on the obtained prior-y grain size GS (µm), yield strength σ (MPa), and yield point elongation Δε (%), and the definition described above. The obtained value of Fn1 is shown in the column "Fn1" in Table 3.

[Hydrogen embrittlement resistance evaluation test]

Two round bar specimens with an annular notch were prepared from a location which was at the center portion of the width and at a thickness t/4 position of each of the steel materials denoted by the respective sample symbols. The diameter of the parallel portion of each test specimen was 4.0 mm, the length of the parallel portion was 25 mm, and an annular notch was formed at the center position in the longitudinal direction of the parallel portion. With regard to the notch shape, the notch depth was 0.3 mm, the notch angle was 60°, and the radius of curvature of the notch root was 0.125 mm.
One of the two round bar specimens with an annular notch was charged with hydrogen by a cathodic hydrogen charging method. Specifically, a cathodic hydrogen charging solution at normal temperature was prepared. An aqueous solution containing 5% by mass of a sodium chloride aqueous solution, 30 g/L of NH₄SCN, and an acetate buffer solution at normal temperature was adopted as the cathodic hydrogen charging solution, and the pH before testing was adjusted to pH 3.5 with the acetate buffer solution.
In a state in which the round bar specimen with an annular notch was immersed in the cathodic hydrogen charging solution, the round bar specimen with an annular notch was subjected to hydrogen charging at a potential of -1.5 V for a charging time of 24 hrs. That is, a sour environment was simulated by charging with hydrogen. Under the same conditions for each sample symbol, a zinc plating film was formed on the surface of the round bar specimen with an annular notch charged with hydrogen, to prevent the hydrogen in the round bar specimen with an annular notch leaking to outside. Note that, the other one of the two round bar specimens with an annular notch was not charged with hydrogen.
The round bar specimen with an annular notch on which the zinc plating film was formed was subjected to a tensile test in air at normal temperature at a strain rate of 4.2×10^-6/sec using a slow strain rate tester (SSRT), and a breaking stress BS1 (MPa) in a hydrogen environment was determined.
In addition, the round bar specimen with an annular notch without being charged with hydrogen of each sample symbol was subjected to a tensile test in air at normal temperature at a strain rate of 4.2×10^-6/sec using a slow strain rate tester (SSRT), and a breaking stress BS0 (MPa) in air was determined.
The obtained breaking stress BS0 (MPa) in air is shown in the column "In Air (MPa)" in the column "Notched Tensile Test Result" in Table 3. The obtained breaking stress BS1 (MPa) in the hydrogen environment is shown in the column "In Hydrogen Environment (MPa)" in the column "Notched Tensile Test Result" in Table 3.

[Evaluation Results]

Referring to Table 1-1, Table 1-2, Table 2, and Table 3, the steel materials denoted by sample symbols A to X had the chemical composition described above, and in these steel materials the prior-y grain size GS satisfied the condition of being in the range of more 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 breaking stress BS1 in the hydrogen environment was 950 MPa or more, and thus these steel materials had excellent hydrogen embrittlement resistance. That is, even though the prior-y grain size GS was more than 5.0 µm, these steel materials had high strength and excellent hydrogen embrittlement resistance. Note that, for the steel materials denoted by sample symbols A to X, it was determined that the total area fraction of tempered martensite and tempered bainite in the microstructure was 90% or more.
In addition, for the steel materials denoted by sample symbols A to U, Fn1 was 12 or more. As a result, the breaking stress BS1 in the hydrogen environment was 970 MPa or more, and thus these steel materials had even more excellent hydrogen embrittlement resistance.
On the other hand, in the steel material denoted by sample symbol ZA, the content of C was too low. As a result, for this steel material the breaking stress BS1 in the hydrogen environment was less than 950 MPa, and the steel material did not have excellent hydrogen embrittlement resistance.
In the steel material denoted by sample symbol ZB, the content of C was too high. As a result, in this steel material the yield strength σ was more than 965 MPa. Consequently, in addition, for this steel material the breaking stress BS1 in the hydrogen environment was less than 950 MPa, and the steel material did not have excellent hydrogen embrittlement resistance.
In the steel material denoted by sample symbol ZC, the content of Si was too low. As a result, for this steel material the breaking stress BS1 in the hydrogen environment was less than 950 MPa, and the steel material did not have excellent hydrogen embrittlement resistance.
In the steel material denoted by sample symbol ZD, the content of Si was too high. As a result, for this steel material the breaking stress BS1 in the hydrogen environment was less than 950 MPa, and the steel material did not have excellent hydrogen embrittlement resistance.
In the steel material denoted by sample symbol ZE, the content of Mn was too high. As a result, for this steel material the breaking stress BS1 in the hydrogen environment was less than 950 MPa, and the steel material did not have excellent hydrogen embrittlement resistance.
In the steel material denoted by sample symbol ZF, the content of Cr was too low. As a result, for this steel material the breaking stress BS1 in the hydrogen environment was less than 950 MPa, and the steel material did not have excellent hydrogen embrittlement resistance.
In the steel material denoted by sample symbol ZG, the content of Cr was too high. As a result, for this steel material the breaking stress BS1 in the hydrogen environment was less than 950 MPa, and the steel material did not have excellent hydrogen embrittlement resistance.
In the steel material denoted by sample symbol ZH, the content of Mo was too low. As a result, for this steel material the breaking stress BS1 in the hydrogen environment was less than 950 MPa, and the steel material did not have excellent hydrogen embrittlement resistance.
In the steel material denoted by sample symbol ZI, the content of Mo was too high. As a result, for this steel material the breaking stress BS1 in the hydrogen environment was less than 950 MPa, and the steel material did not have excellent hydrogen embrittlement resistance.
In the steel material denoted by sample symbol ZJ, the content of V was too low. As a result, in this steel material the yield strength σ was less than 862 MPa. Consequently, in addition, for this steel material the breaking stress BS1 in the hydrogen environment was less than 950 MPa, and the steel material did not have excellent hydrogen embrittlement resistance.
In the steel material denoted by sample symbol ZK, the content of V was too high. As a result, in this steel material the yield strength σ was more than 965 MPa. Consequently, in addition, for this steel material the breaking stress BS1 in the hydrogen environment was less than 950 MPa, and the steel material did not have excellent hydrogen embrittlement resistance.
In the steel material denoted by sample symbol ZL, the content of Nb was too low. As a result, the yield point drop Δσ could not be defined for this steel material. In addition, as a result, for this steel material the breaking stress BS1 in the hydrogen environment was less than 950 MPa, and the steel material did not have excellent hydrogen embrittlement resistance.
In the steel material denoted by sample symbol ZM, the content of Nb was too high. As a result, for this steel material the breaking stress BS1 in the hydrogen environment was less than 950 MPa, and the steel material did not have excellent hydrogen embrittlement resistance.
In the steel material denoted by sample symbol ZN, the content of Ti was too high. As a result, for this steel material the breaking stress BS1 in the hydrogen environment was less than 950 MPa, and the steel material did not have excellent hydrogen embrittlement resistance.
In the steel material denoted by sample symbol ZO, the content of B was too low. As a result, for this steel material the breaking stress BS1 in the hydrogen environment was less than 950 MPa, and the steel material did not have excellent hydrogen embrittlement resistance.
In the steel material denoted by sample symbol ZP, the content of B was too high. As a result, for this steel material the breaking stress BS1 in the hydrogen environment was less than 950 MPa, and the steel material did not have excellent hydrogen embrittlement resistance.
In the steel material denoted by sample symbol ZQ, the content of N was too high. As a result, for this steel material the breaking stress BS1 in the hydrogen environment was less than 950 MPa, and the steel material did not have excellent hydrogen embrittlement resistance.
The steel material denoted by sample symbol ZR was not subjected to a low-temperature heat treatment process. As a result, the yield point drop Δσ and the yield point elongation Δε could not be defined for this steel material. In addition, as a result, for this steel material the breaking stress BS1 in the hydrogen environment was less than 950 MPa, and the steel material did not have excellent hydrogen embrittlement resistance.
For the steel material denoted by sample symbol ZS, the heat treatment temperature in the low-temperature heat treatment process was too low. As a result, the yield point drop Δσ and the yield point elongation Δε could not be defined for this steel material. In addition, as a result, for this steel material the breaking stress BS1 in the hydrogen environment was less than 950 MPa, and the steel material did not have excellent hydrogen embrittlement resistance.
For the steel material denoted by sample symbol ZT, the heat treatment temperature in the low-temperature heat treatment process was too high. As a result, the yield point drop Δσ could not be defined for this steel material. Furthermore, as a result, in this steel material the yield point elongation Δε was less than 1.5%. In addition, as a result, for this steel material the breaking stress BS1 in the hydrogen environment was less than 950 MPa, and the steel material did not have excellent hydrogen embrittlement resistance.
For the steel material denoted by sample symbol ZU, the heat treatment time in the low-temperature heat treatment process was too short. As a result, in this steel material the yield point drop Δσ was less than 40 MPa. In addition, as a result, for this steel material the breaking stress BS1 in the hydrogen environment was less than 950 MPa, and the steel material did not have excellent hydrogen embrittlement resistance.
An embodiment of the present disclosure has been described above. However, the embodiment described above is merely an example for carrying out the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiment, and can be implemented by appropriately modifying the above-described embodiment within a range not departing from the gist thereof.

Claims

A steel material having a chemical composition consisting of, in mass%,
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: 0.005 to 0.100%,

B: 0.0005 to 0.0050%,

N: 0.0100% or less,

O: 0.0050% or less,

W: 0 to 2.00%,

Co: 0 to 0.20%,

Mg: 0 to 0.0100%,

Ca: 0 to 0.0100%,

rare earth metal: 0 to 0.0100%,

Cu: 0 to 0.40%,

Ni: 0 to 0.20%, and

Sn: 0 to 0.10%,

with the balance being Fe and impurities,

wherein:
a grain size GS of prior-austenite grains is more than 5.0 to 30.0 µm,

a yield strength σ is 862 to 965 MPa,

a yield point drop Δσ is 40 MPa or more, and

a yield point elongation Δε is 1.5% or more.
The steel material according to claim 1, wherein the chemical composition contains one or more elements selected from the group consisting of:
W: 0.01 to 2.00%,

Co: 0.01 to 0.20%,

Mg: 0.0001 to 0.0100%,

Ca: 0.0001 to 0.0100%,

rare earth metal: 0.0001 to 0.0100%,

Cu: 0.01 to 0.40%,

Ni: 0.01 to 0.20%, and

Sn: 0.01 to 0.10%.
The steel material according to claim 1, wherein:
the grain size GS of prior-austenite grains, the yield strength σ, and the yield point elongation Δε satisfy the following Formula (1): $σ \times Δε / {GS}^{2} \geq 12$

where, the yield strength of the steel material in units of MPa is substituted for σ in Formula (1), the yield point elongation of the steel material in units of % is substituted for Δε in Formula (1), and the grain size of prior-austenite grains of the steel material in units of µm is substituted for GS in Formula (1).
The steel material according to claim 2, wherein:
the grain size GS of prior-austenite grains, the yield strength σ, and the yield point elongation Δε satisfy the following Formula (1): $σ \times Δε / {GS}^{2} \geq 12$

where, the yield strength of the steel material in units of MPa is substituted for σ in Formula (1), the yield point elongation of the steel material in units of % is substituted for Δε in Formula (1), and the grain size of prior-austenite grains of the steel material in units of µm is substituted for GS in Formula (1).
The steel material according to any one of claim 1 to claim 4, wherein:
the steel material is any one of an oil-well steel pipe, a steel pipe for line pipes, and a steel pipe for high-pressure hydrogen containers.
The steel material according to any one of claim 1 to claim 4, wherein:
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.
The steel material according to claim 5, wherein:
the steel pipe for high-pressure hydrogen containers is any one of a steel pipe for high-pressure hydrogen storage vessels in hydrogen service or a steel pipe for high-pressure hydrogen cylinders.
The steel material according to claim 6, wherein:
the seamless steel pipe for high-pressure hydrogen containers is any one of a seamless steel pipe for high-pressure hydrogen storage vessels in hydrogen service or a seamless steel pipe for high-pressure hydrogen cylinders.