EP4512921A1

EP4512921A1 - Steel material

Info

Publication number: EP4512921A1
Application number: EP23791942.8A
Authority: EP
Inventors: Hiroyuki Fuji; Shinji Yoshida; Yuji Arai; Keiichi Kondo
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 2022-04-22
Filing date: 2023-04-21
Publication date: 2025-02-26
Also published as: MX2024012876A; JP7364993B1; EP4512921A4; JPWO2023204294A1

Abstract

A steel material that has high strength and excellent SSC resistance is provided. A steel material according to the present disclosure consists of, in mass%, C: 0.20 to 0.35%, Si: 0.60 to 1.30%, Mn: 0.05 to 0.25%, P: 0.050% or less, S: 0.0100% or less, Al: 0.010 to 0.100%, N: 0.0100% or less, Cr: 0.20 to 1.00%, Mo: 0.10 to 1.00%, and Ti: 0.003 to 0.030%, with the balance being Fe and impurities. In a case where the yield strength is 110 ksi grade, EE defined by Formula (1) is 2.75 or more and FN defined by Formula (2) is 0.185 or more, and in a case where the yield strength is 125 ksi grade, EE is 3.00 or more and FN is 0.200 or more.

EE = -0.25C + 2Si - 5.8Mn + 2.1Cr + Mo + 4.1Zr + 2.6Sb + 0.3Cu + 0.4Ni + 1.5Co

FN = {EE/(D}^{0 .9})

(D: average equivalent circular diameter (µm) of prior-austenite grains)

Description

TECHNICAL FIELD

The present disclosure relates to a steel material, and more particularly relates to a steel material that is suitable for use in a sour environment.

BACKGROUND ART

Due to the deepening of oil wells and gas wells (hereunder, oil wells and gas wells are collectively referred to as simply "oil wells"), there is a demand to increase the strength of steel materials for use in oil wells that are typified by oil-well steel pipes. Specifically, oil-well steel materials of 80 ksi grade (yield strength is 80 to less than 95 ksi, that is, 552 to less than 655 MPa) and 95 ksi grade (yield strength is 95 to less than 110 ksi, that is, 655 to less than 758 MPa) are being widely utilized. Furthermore, recently, there is an emerging need for oil-well steel materials of 110 ksi grade (yield strength is 758 MPa to less than 862 MPa) and 125 ksi grade (yield strength is 862 to less than 965 MPa).
In addition, most deep wells are sour environments containing corrosive hydrogen sulfide. In the present description, the term "sour environment" means an acidified environment containing hydrogen sulfide. Note that, in some cases a sour environment may also contain carbon dioxide. Steel materials to be used in such sour environments are required to have not only high strength, but also to have sulfide stress cracking resistance (hereunder, referred to as "SSC resistance").
Technology for increasing the SSC resistance of steel materials that are typified by oil-well steel pipes is proposed in Japanese Patent Application Publication No. 2000-297344 (Patent Literature 1) and International Application Publication No. WO2008/123422 (Patent Literature 2).
Patent Literature 1 discloses a steel material containing, in mass%, C: 0.15 to 0.3%, Cr: 0.2 to 1.5%, Mo: 0.1 to 1%, V: 0.05 to 0.3%, and Nb: 0.003 to 0.1%. In this steel material, the total amount of precipitating carbides is within the range of 1.5 to 4% by mass, the proportion that MC-type carbides occupy among the total amount of carbides is within the range of 5 to 45% by mass, and when the wall thickness of the product is defined as t (mm), the proportion of M₂₃C₆-type carbides is (200/t) or less in percent by mass. In this steel material, SSC resistance is increased by suppressing the proportion of M₂₃C₆-type carbides.
Patent Literature 2 discloses a steel material that contains, in mass%, C: 0.10 to 0.20%, Si: 0.05 to 1.0%, Mn: 0.05 to 1.5%, Cr: 1.0 to 2.0%, Mo: 0.05 to 2.0%, Al: 0.10% or less, and Ti: 0.002 to 0.05%, with Ceq (= C + (Mn/6) + (Cr + Mo + V)/5) being 0.65 or more, and the balance being Fe and impurities, in which, among the impurities, P is 0.025% or less, S is 0.010% or less, N is 0.007% or less, and B is less than 0.0003%. In this steel material, the number density of M₂₃C₆-type precipitates having a grain size of 1 µm or more is 0.1 pieces/mm² or less. In this steel material also, similarly to Patent Literature 1, SSC resistance is increased by suppressing the proportion of M₂₃C₆-type carbides.

CITATION LIST

PATENT LITERATURE

Patent Literature 1: Japanese Patent Application Publication No. 2000-297344
Patent Literature 2: International Application Publication No. WO2008/123422

SUMMARY OF INVENTION

TECHNICAL PROBLEM

As described above, recently, as the severity of oil well environments increases, there is an increasing demand for steel materials capable of achieving both a high strength of 110 ksi grade or more (758 MPa or more) and excellent SSC resistance. In the aforementioned Patent Literatures 1 and 2, it is attempted to achieve both high strength and SSC resistance by suppressing precipitates. However, a steel material that can achieve both high strength and SSC resistance may also be obtained by means other than the means disclosed in Patent Literatures 1 and 2.
An objective of the present disclosure is to provide a steel material that has both high strength and excellent SSC resistance in a sour environment.

SOLUTION TO PROBLEM

A steel material according to the present disclosure is as follows.
A steel material having a chemical composition consisting of, in mass%,

C: 0.20 to 0.35%,
Si: 0.60 to 1.30%,
Mn: 0.05 to 0.25%,
P: 0.050% or less,
S: 0.0100% or less,
Al: 0.010 to 0.100%,
N: 0.0100% or less,
Cr: 0.20 to 1.00%,
Mo: 0.10 to 1.00%,
Ti: 0.003 to 0.030%,
O: 0.0050% or less,
Zr: 0 to 0.0040%,
Sb: 0 to 0.50%,
Cu: 0 to 0.50%,
Ni: 0 to 0.50%,
Co: 0 to 0.50%,
Ca: 0 to 0.0040%,
Mg: 0 to 0.0040%,
rare earth metal: 0 to 0.0040%,
Nb: 0 to 0.150%,
V: 0 to 0.500%, and
B: 0 to 0.0030%,
with the balance being Fe and impurities,
wherein:
- a yield strength is 758 to less than 965 MPa,
- in a case where the yield strength is 758 to less than 862 MPa, EE defined by Formula (1) is 2.75 or more, and FN defined by Formula (2) is 0.185 or more, and
- in a case where the yield strength is 862 to less than 965 MPa, the EE is 3.00 or more and the FN is 0.200 or more; EE = -0.25C + 2Si - 5.8Mn + 2.1Cr + Mo + 4.1Zr + 2.6Sb + 0.3Cu + 0.4Ni + 1.5Co $FN = EE / (D^{0.9})$
- where, a content in percent by mass of a corresponding element is substituted for each symbol of an element in Formula (1); and an average equivalent circular diameter in units of µm of prior-austenite grains in the steel material is substituted for D in Formula (2).

ADVANTAGEOUS EFFECT OF INVENTION

The steel material according to the present disclosure has high strength and has excellent SSC resistance in a sour environment.

BRIEF DESCRIPTION OF DRAWINGS

[FIG. 1] FIG. 1 is a schematic diagram for describing a process in which hydrogen penetrates into the interior of a steel material from the surface of the steel material.
[FIG. 2A] FIG. 2A is a view illustrating the relation between FN and a fracture toughness value Kissc (MPa√m) obtained in a DCB test performed on steel materials having a chemical composition satisfying Feature 1 and having a yield strength of 110 ksi grade (758 to less than 862 MPa) and in which EE defined by Formula (1) was 2.75 or more.
[FIG. 2B] FIG. 2B is a view illustrating the relation between FN and a fracture toughness value Kissc (MPa√m) obtained in a DCB test performed on steel materials having a chemical composition satisfying Feature 1 and having a yield strength of 125 ksi grade (862 to less than 965 MPa) and in which EE defined by Formula (1) was 3.00 or more.
[FIG. 3A] FIG. 3A is a side view of a DCB test specimen defined in NACE TM0177-2016 Method D.
[FIG. 3B] FIG. 3B is a perspective view of a wedge that is driven into the DCB test specimen illustrated in FIG. 3A.

DESCRIPTION OF EMBODIMENT

The present inventors carried out investigations and studies pertaining to a steel material that has high strength and has excellent SSC resistance in a sour environment. As a result, the present inventors obtained the following findings.
First, the present inventors conducted studies from the viewpoint of the chemical composition with regard to a steel material that has high strength and has excellent SSC resistance in a sour environment. As a result, the present inventors considered that if the chemical composition of a steel material satisfies the following Feature 1, there is a possibility that a high strength of 110 ksi grade (758 to less than 862 MPa) to 125 ksi grade (862 to less than 965 MPa), and excellent SSC resistance in a sour environment will be obtained.

(Feature 1)

The chemical composition consists of, in mass%, C: 0.20 to 0.35%, Si: 0.60 to 1.30%, Mn: 0.05 to 0.25%, P: 0.050% or less, S: 0.0100% or less, Al: 0.010 to 0.100%, N: 0.0100% or less, Cr: 0.20 to 1.00%, Mo: 0.10 to 1.00%, Ti: 0.003 to 0.030%, O: 0.0050% or less, Zr: 0 to 0.0040%, Sb: 0 to 0.50%, Cu: 0 to 0.50%, Ni: 0 to 0.50%, Co: 0 to 0.50%, Ca: 0 to 0.0040%, Mg: 0 to 0.0040%, rare earth metal: 0 to 0.0040%, Nb: 0 to 0.150%, V: 0 to 0.500%, and B: 0 to 0.0030%, with the balance being Fe and impurities.
Therefore, the present inventors conducted studies regarding means for further increasing the SSC resistance in a steel material having a chemical composition satisfying Feature 1.
It is considered that SSC occurs through the following process. FIG. 1 is a schematic diagram for describing a process in which hydrogen penetrates into the interior of a steel material from the surface of the steel material. Referring to FIG. 1, initially, when a steel material 1 corrodes in a sour environment, the surface of the steel material 1 enters an electrochemically active state. Then, Fe in the steel material 1 becomes Fe²⁺ and dissolves in the environment. At such time, electrons e^- are released to the outside of the steel material 1 (A: occurrence of corrosion). Hydrogen ions H⁺ that are present in the environment receive electrons e^- released from the steel material 1 and are reduced, and are then adsorbed on the surface of the steel material 1 as adsorbed hydrogen atoms H_ad (B: adsorption reaction of hydrogen ions). Due to the above mechanism, a plurality of adsorbed hydrogen atoms H_ad are present on the surface of the steel material 1. Almost all of these adsorbed hydrogen atoms H_ad combine together to form hydrogen gas Hz, and are released from the surface of the steel material 1 into the environment. However, among the plurality of adsorbed hydrogen atoms H_ad present on the surface of the steel material 1, a very small proportion of adsorbed hydrogen atoms H_ad penetrate into the interior of the steel material 1 from the surface of the steel material 1 and become absorbed hydrogen atoms H_ab (C: penetration of hydrogen). Then, SSC occurs and propagates in the steel material due to the absorbed hydrogen atoms H_ab. That is, the absorbed hydrogen atoms H_ab promote not only the occurrence of SSC, but also propagation of the SSC.
Considering the mechanism by which SSC occurs that is described above, the present inventors considered that in order to suppress the occurrence and propagation of SSC and obtain excellent SSC resistance, it is effective to suppress the penetration of hydrogen from the steel material surface. Therefore, the present inventors conducted studies regarding means for suppressing the penetration of hydrogen from the steel material surface.
First, the present inventors carried out investigations and studies regarding elements that affect the penetration of hydrogen from the surface of steel materials. As a result, the present inventors obtained the following finding regarding steel materials that have a chemical composition which satisfies Feature 1.
As mentioned above, in a case where the steel material surface has entered an electrochemically active state (A in FIG. 1), adsorbed hydrogen atoms H_ad attach to the steel material surface (B in FIG. 1). The adsorbed hydrogen atoms H_ad then become absorbed hydrogen atoms H_ab which penetrate into the interior of the steel material (C in FIG. 1). Therefore, in order to suppress the penetration of hydrogen from the steel material surface, it is effective to suppress electrochemical activity on the steel material surface.
Si, Cr, Mo, Zr, Sb, Cu, Ni, and Co suppress electrochemical activity on a steel material surface in a sour environment. As a result, these elements suppress penetration of hydrogen from the steel material surface. On the other hand, C and Mn promote electrochemical activity on a steel material surface in a sour environment. As a result, these elements promote the penetration of hydrogen from the steel material surface.
Based on the above finding, the present inventors considered that if the content of these hydrogen penetration suppressing elements (Si, Cr, Mo, Zr, Sb, Cu, Ni, and Co) and the content of these hydrogen penetration promoting elements (C and Mn) are appropriately adjusted, the penetration of hydrogen from the steel material surface can be suppressed by electrochemical action. Therefore, the present inventors studied the relation between hydrogen penetration suppressing elements, hydrogen penetration promoting elements, and SSC resistance in steel materials having a chemical composition satisfying Feature 1. As a result, the present inventors considered that by increasing electrochemical elements EE defined by the following Formula (1), excellent SSC resistance in a sour environment will be obtained in a steel material that has high strength: EE = -0.25C + 2Si - 5.8Mn + 2.1Cr + Mo + 4.1Zr + 2.6Sb + 0.3Cu + 0.4Ni + 1.5Co where, a content in percent by mass of a corresponding element is substituted for each symbol of an element in Formula (1).
In general, there is a tendency for the SSC resistance of a steel material to decrease as the strength of the steel material increases. Therefore, the present inventors considered that it is necessary to suppress hydrogen penetration to a greater extent in a case where the strength of a steel material is high. Therefore, the present inventors conducted studies regarding EE in a case where the yield strength of a steel material is 110 ksi grade (758 to less than 862 MPa), and EE in a case where the yield strength of a steel material is 125 ksi grade (862 to less than 965 MPa). As a result, it was revealed that if EE is made 2.75 or more in a case where the yield strength is 110 ksi grade, and EE is made 3.00 or more in a case where the yield strength is 125 ksi grade, excellent SSC resistance can be obtained even though the steel material has high strength.
However, in steel materials whose chemical composition satisfied Feature 1, when only the aforementioned electrochemical elements were adjusted, in some cases a high strength of 110 ksi grade to 125 ksi grade and excellent SSC resistance were not sufficiently obtained. Therefore, the present inventors conducted further studies.
Here, the present inventors considered that the occurrence and propagation of SSC in a steel material whose chemical composition satisfies Feature 1 are influenced not only by the aforementioned electrochemical elements, but also by physical elements that depends on the microstructure. Therefore, the present inventors also conducted studies regarding means for increasing the SSC resistance of a steel material from the viewpoint of physical elements, and not just from the viewpoint of electrochemical elements. As a result, the present inventors discovered that the average equivalent circular diameter (µm) of prior-austenite grains in a steel material acts synergistically with the aforementioned electrochemical elements and markedly influences the SSC resistance of the steel material.
Based on the above findings, the present inventors also investigated the relation among the aforementioned electrochemical elements (hydrogen penetration suppressing elements and hydrogen penetration promoting elements), the physical element (average equivalent circular diameter of prior-austenite grains), and SSC resistance. As a result, the present inventors discovered that by adjusting FN defined by the following Formula (2) according to the strength while also adjusting EE so as to be within the aforementioned range, excellent SSC resistance is obtained even when the steel material has a high strength of 110 ksi grade to 125 ksi grade: $FN = EE / (D^{0.9})$
where, the average equivalent circular diameter in units of µm of prior-austenite grains in the steel material is substituted for D in Formula (2).
FN is an index which indicates the degree to which the electrochemical elements (hydrogen penetration suppressing elements and hydrogen penetration promoting elements) and the physical element (average equivalent circular diameter of prior-austenite grains) influence SSC resistance. In a case where a steel material has a chemical composition that satisfies Feature 1 and has a yield strength of 110 ksi grade, EE is to be made 2.75 or more, and FN is to be made 0.185 or more. Further, in a case where a steel material has a chemical composition that satisfies Feature 1 and has a yield strength of 125 ksi grade, EE is to be made 3.00 or more, and FN is to be made 0.200 or more. In such a case, excellent SSC resistance is obtained even when the steel material has a high strength of 110 ksi grade to 125 ksi grade. This point is described hereunder.
FIG. 2A is a view illustrating the relation between FN and a fracture toughness value Kissc (MPa^m) obtained in a DCB test performed on steel materials having a chemical composition satisfying Feature 1 and having a yield strength of 110 ksi grade (758 to less than 862 MPa) and in which EE was 2.75 or more. FIG. 2A was prepared based on data obtained in Example 1 to be described later.
Referring to FIG. 2A, when FN is 0.185 or more, the fracture toughness value Kissc is a high value of 25.0 MPa√m or more, and excellent SSC resistance is obtained. However, when FN is less than 0.185, the fracture toughness value Kissc markedly decreases to less than 25.0 MPa√m. Accordingly, by making FN 0.185 or more, excellent SSC resistance is obtained in a steel material of 110 ksi grade (758 to less than 862 MPa).
FIG. 2B is a view illustrating the relation between FN and the fracture toughness value Kissc (MPa√m) obtained in a DCB test performed on steel materials having a chemical composition satisfying Feature 1 and having a yield strength of 125 ksi grade (862 to less than 965 MPa) and in which EE was 3.00 or more. FIG. 2B was prepared based on data obtained in Example 2 to be described later.
Referring to FIG. 2B, when FN is 0.200 or more, the fracture toughness value Kissc is a high value of 24.0 MPa√m or more, and excellent SSC resistance is obtained. However, when FN is less than 0.200, the fracture toughness value Kissc markedly decreases to less than 24.0 MPa√m. Accordingly, by making FN 0.200 or more, excellent SSC resistance is obtained in a steel material of 125 ksi grade (862 to less than 965 MPa).
The relation between the electrochemical elements and the physical element and the SSC resistance described above is a relation that is surmised by the present inventors, and there is also a possibility that excellent SSC resistance is obtained by a mechanism that is different from the mechanism described above. However, the fact that in a steel material whose chemical composition satisfies Feature 1, excellent SSC resistance is obtained even when the steel material has a high strength of 110 ksi grade (758 to less than 862 MPa) to 125 ksi grade (862 to less than 965 MPa) if EE is 2.75 or more and FN is 0.185 or more in a case where the yield strength is 110 ksi grade, and if EE is 3.00 or more and FN is 0.200 or more in a case where the yield strength is 125 ksi grade has been proven by examples that are described later.
A 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 0.35%,
- Si: 0.60 to 1.30%,
- Mn: 0.05 to 0.25%,
- P: 0.050% or less,
- S: 0.0100% or less,
- Al: 0.010 to 0.100%,
- N: 0.0100% or less,
- Cr: 0.20 to 1.00%,
- Mo: 0.10 to 1.00%,
- Ti: 0.003 to 0.030%,
- O: 0.0050% or less,
- Zr: 0 to 0.0040%,
- Sb: 0 to 0.50%,
- Cu: 0 to 0.50%,
- Ni: 0 to 0.50%,
- Co: 0 to 0.50%,
- Ca: 0 to 0.0040%,
- Mg: 0 to 0.0040%,
- rare earth metal: 0 to 0.0040%,
- Nb: 0 to 0.150%,
- V: 0 to 0.500%, and,
- B: 0 to 0.0030%,
- with the balance being Fe and impurities,
- wherein:
  - a yield strength is 758 to less than 965 MPa,
  - in a case where the yield strength is 758 to less than 862 MPa, EE defined by Formula (1) is 2.75 or more, and FN defined by Formula (2) is 0.185 or more, and
  - in a case where the yield strength is 862 to less than 965 MPa, the EE is 3.00 or more and the FN is 0.200 or more; EE = -0.25C + 2Si - 5.8Mn + 2.1Cr + Mo + 4.1Zr + 2.6Sb + 0.3Cu + 0.4Ni + 1.5Co $FN = EE / (D^{0.9})$
  - where, a content in percent by mass of a corresponding element is substituted for each symbol of an element in Formula (1); and an average equivalent circular diameter in units of µm of prior-austenite grains in the steel material is substituted for D in Formula (2).
[2] The steel material according to [1], wherein the chemical composition contains one or more kinds of element selected from a group consisting of:
- Zr: 0.0001 to 0.0040%,
- Sb: 0.01 to 0.50%,
- Cu: 0.01 to 0.50%,
- Ni: 0.01 to 0.50%,
- Co: 0.01 to 0.50%,
- Ca: 0.0001 to 0.0040%,
- Mg: 0.0001 to 0.0040%,
- rare earth metal: 0.0001 to 0.0040%,
- Nb: 0.001 to 0.150%,
- V: 0.001 to 0.500%, and
- B: 0.0001 to 0.0030%.
[3] The steel material according to [1] or [2], wherein:
the steel material is an oil-well steel pipe.

Hereunder, the steel material of the present embodiment is described in detail. Note that, the symbol "%" in relation to elements means "mass percent" unless specifically stated otherwise.

[Features of steel material of present embodiment]

The steel material of the present embodiment satisfies the following Feature 1 to Feature 3.

(Feature 1)

The chemical composition consists of, in mass%, C: 0.20 to 0.35%, Si: 0.60 to 1.30%, Mn: 0.05 to 0.25%, P: 0.050% or less, S: 0.0100% or less, Al: 0.010 to 0.100%, N: 0.0100% or less, Cr: 0.20 to 1.00%, Mo: 0.10 to 1.00%, Ti: 0.003 to 0.030%, O: 0.0050% or less, Zr: 0 to 0.0040%, Sb: 0 to 0.50%, Cu: 0 to 0.50%, Ni: 0 to 0.50%, Co: 0 to 0.50%, Ca: 0 to 0.0040%, Mg: 0 to 0.0040%, rare earth metal: 0 to 0.0040%, Nb: 0 to 0.150%, V: 0 to 0.500%, and B: 0 to 0.0030%, with the balance being Fe and impurities.

(Feature 2)

The yield strength is 758 to less than 965 MPa.

(Feature 3)

In a case where the yield strength is 758 to less than 862 MPa, EE defined by Formula (1) is 2.75 or more and FN defined by Formula (2) is 0.185 or more,
and in a case where the yield strength is 862 to less than 965 MPa, EE is 3.00 or more and FN is 0.200 or more; EE = -0.25C + 2Si - 5.8Mn + 2.1Cr + Mo + 4.1Zr + 2.6Sb + 0.3Cu + 0.4Ni + 1.5Co $FN = EE / (D^{0.9})$
where, the content in percent by mass of a corresponding element is substituted for each symbol of an element in Formula (1). The average equivalent circular diameter in units of µm of prior-austenite grains in the steel material is substituted for D in Formula (2).
Hereunder, Feature 1 to Feature 3 are described.

[(Feature 1) Regarding chemical composition]

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

C: 0.20 to 0.35%

Carbon (C) increases the strength of the steel material by increasing hardenability of the steel material and forming carbides. C also promotes spheroidization of carbides during tempering in the production process, and thereby increases the SSC resistance of the steel material. If the content of C is less than 0.20%, the aforementioned advantageous effects will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment.
If the content of C is more than 0.35%, coarse carbides will excessively form. Furthermore, C is a hydrogen penetration promoting element. Therefore, even if the contents of other elements are within the range of the present embodiment, the SSC resistance of the steel material will decrease.
Therefore, the content of C is 0.20 to 0.35%.
A preferable lower limit of the content of C is 0.22%, more preferably is 0.23%, further preferably is 0.24%, and further preferably is 0.25%.
A preferable upper limit of the content of C is 0.32%, more preferably is 0.30%, further preferably is 0.28%, and further preferably is 0.27%.

Si: 0.60 to 1.30%

Silicon (Si) is a hydrogen penetration suppressing element, and suppresses penetration of hydrogen from the steel material surface. As a result, the SSC resistance of the steel material increases. If the content of Si is less than 0.60%, 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 more than 1.30%, prior-austenite grains will coarsen even if the contents of other elements are within the range of the present embodiment. In such a case, although the amount of hydrogen penetration from the steel material surface will be suppressed by the electrochemical action of Si, the SSC resistance of the steel material will decrease due to a physical action.
Therefore, the content of Si is 0.60 to 1.30%.
A preferable lower limit of the content of Si is 0.62%, more preferably is 0.65%, further preferably is 0.70%, further preferably is 0.72%, further preferably is 0.75%, and further preferably is 0.80%.
A preferable upper limit of the content of Si is 1.28%, more preferably is 1.25%, and further preferably is 1.20%.

Mn: 0.05 to 0.25%

Manganese (Mn) deoxidizes the steel. Mn also increases hardenability of the steel material. If the content of Mn is less than 0.05%, 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, Mn is a hydrogen penetration promoting element. If the content of Mn is more than 0.25%, Mn sulfides will excessively form. The Mn sulfides will act as starting points for pitting. Therefore, if Mn sulfides excessively form, the corrosion rate will increase and penetration of hydrogen into the steel material will be promoted. As a result, even if the contents of other elements are within the range of the present embodiment, the SSC resistance of the steel material will decrease.
Therefore, the content of Mn is 0.05 to 0.25%.
A preferable lower limit of the content of Mn is 0.06%, more preferably is 0.07%, further preferably is 0.08%, and further preferably is 0.10%.
A preferable upper limit of the content of Mn is 0.24%, more preferably is 0.23%, further preferably is 0.22%, further preferably is 0.20%, and further preferably is 0.18%.

P: 0.050% or less

Phosphorus (P) is an impurity. That is, the content of P is more than 0%. If the content of P is more than 0.050%, even if the contents of other elements are within the range of the present embodiment, P will segregate to grain boundaries and the SSC resistance of the steel material will decrease.
Therefore, the content of P is 0.050% or less.
The content of P is preferably as low as possible. However, excessively reducing the content of P will significantly increase the production cost. Therefore, when taking industrial production into consideration, a preferable lower limit of the content of P is 0.001%, and more preferably is 0.003%.
A preferable upper limit of the content of P is 0.030%, more preferably is 0.025%, further preferably is 0.020%, and further preferably is 0.015%.

S: 0.0100% or less

Sulfur (S) is an impurity. That is, the content of S is more than 0%. If the content of S is more than 0.0100%, even if the contents of other elements are within the range of the present embodiment, S will segregate to grain boundaries and the SSC resistance of the steel material will decrease.
Therefore, the content of S is 0.0100% or less.
The content of S is preferably as low as possible. However, excessively reducing the content of S will significantly 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%, and further preferably is 0.0003%.
A preferable upper limit of the content of S is 0.0070%, more preferably is 0.0050%, further preferably is 0.0030%, further preferably is 0.0025%, further preferably is 0.0020%, and further preferably is 0.0015%.

Al: 0.010 to 0.100%

Aluminum (Al) deoxidizes the steel. If the content of Al is less than 0.010%, 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 Al is more than 0.100%, even if the contents of other elements are within the range of the present embodiment, coarse oxide-based inclusions will form. Consequently, the SSC resistance of the steel material will decrease.
Therefore, the content of Al is 0.010 to 0.100%.
A preferable lower limit of the content of Al is 0.012%, more preferably is 0.015%, further preferably is 0.020%, and further preferably is 0.025%.
A preferable upper limit of the content of Al is 0.080%, more preferably is 0.070%, and further preferably is 0.060%.
In the present description, the content of "Al" means the content of "acid-soluble Al", that is, "sol. Al".

N: 0.0100% or less

Nitrogen (N) is unavoidably contained. That is, a lower limit of the content of N is more than 0%. N combines with Ti to form nitrides, and refines the grains of the steel material by a pinning effect. As a result, the strength of the steel material increases.
However, if the content of N is more than 0.0100%, coarse nitrides will form even if the contents of other elements are within the range of the present embodiment. As a result, the SSC resistance of the steel material will decrease.
Therefore, the content of N is 0.0100% or less.
A preferable lower limit of the content of N is 0.0001%, more preferably is 0.0005%, further preferably is 0.0010%, further preferably is 0.0015%, and further preferably is 0.0020%.
A preferable upper limit of the content of N is 0.0070%, more preferably is 0.0060%, further preferably is 0.0050%, further preferably is 0.0045%, and further preferably is 0.0040%.

Cr: 0.20 to 1.00%

Chromium (Cr) increases hardenability of the steel material. Cr also functions as a hydrogen penetration suppressing element. Specifically, in a sour environment, Cr stabilizes a corrosion product film that is formed on the steel material surface and thereby suppresses penetration of hydrogen into the steel material. As a result, the SSC resistance of the steel material increases. If the content of Cr is less than 0.20%, 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 Cr is more than 1.00%, the hardness of the steel material will become excessively high even if the contents of other elements are within the range of the present embodiment. Consequently, the SSC resistance of the steel material will decrease.

Therefore, the content of Cr is 0.20 to 1.00%.

A preferable lower limit of the content of Cr is 0.25%, more preferably is 0.30%, further preferably is 0.35%, further preferably is 0.40%, further preferably is 0.45%, further preferably is 0.50%, further preferably is 0.55%, and further preferably is 0.60%.
A preferable upper limit of the content of Cr is 0.98%, more preferably is 0.95%, and further preferably is 0.90%.

Mo: 0.10 to 1.00%

Molybdenum (Mo) functions as a hydrogen penetration suppressing element. Specifically, in a sour environment, Mo stabilizes a corrosion product film that is formed on the steel material surface and thereby suppresses penetration of hydrogen into the steel material. As a result, the SSC resistance of the steel material increases. Mo also increases hardenability of the steel material. In addition, Mo increases the temper softening resistance of the steel material and thereby enables high-temperature tempering. As a result, the SSC resistance of the steel material increases. If the content of Mo is less than 0.10%, 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 more than 1.00%, the aforementioned advantageous effects will be saturated.
Therefore, the content of Mo is 0.10 to 1.00%.
A preferable lower limit of the content of Mo is 0.20%, more preferably is 0.25%, further preferably is 0.30%, and further preferably is 0.35%.
A preferable upper limit of the content of Mo is 0.95%, more preferably is 0.90%, further preferably is 0.85%, further preferably is 0.80%, and further preferably is 0.70%.

Ti: 0.003 to 0.030%

Titanium (Ti) combines with N to form nitrides, and refines the grains of the steel material by a pinning effect. As a result, the strength of the steel material increases. If the content of Ti is less than 0.003%, 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 more than 0.030%, coarse Ti nitrides will form even if the contents of other elements are within the range of the present embodiment. As a result, the SSC resistance of the steel material will decrease.
Therefore, the content of Ti is 0.003 to 0.030%.
A preferable lower limit of the content of Ti is 0.004%, and more preferably is 0.005%.
A preferable upper limit of the content of Ti is 0.028%, more preferably is 0.025%, further preferably is 0.022%, and further preferably is 0.020%.

O: 0.0050% or less

Oxygen (O) is an impurity. That is, the lower limit of the content of O is more than 0%. If the content of O is more than 0.0050%, even if the contents of other elements are within the range of the present embodiment, coarse oxides will form. As a result, the low-temperature toughness and SSC resistance of the steel material will decrease.
Therefore, the content of O is 0.0050% or less.
The content of O is preferably as low as possible. However, excessively reducing the content of O will significantly 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 0.0002%, and further preferably is 0.0003%.
A preferable upper limit of the content of O is 0.0040%, more preferably is 0.0030%, further preferably is 0.0025%, and further preferably is 0.0020%.
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:

Zr: 0 to 0.0040%,
Sb: 0 to 0.50%,
Cu: 0 to 0.50%,
Ni: 0 to 0.50%,
Co: 0 to 0.50%,
Ca: 0 to 0.0040%,
Mg: 0 to 0.0040%,
rare earth metal: 0 to 0.0040%,
Nb: 0 to 0.150%,
V: 0 to 0.500%, and
B: 0 to 0.0030%.

These optional elements are described hereunder.

[First group: Regarding Zr, Sb, Cu, Ni, Co, Ca, Mg, and rare earth metal (REM)]

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 Zr, Sb, Cu, Ni, Co, Ca, Mg, and rare earth metal (REM) in lieu of a part of Fe. Each of these elements is an optional element, and increases the SSC resistance of the steel material.

Zr: 0 to 0.0040%

Zirconium (Zr) is an optional element, and does not have to be contained. That is, the content of Zr may be 0%.
When Zr is contained, that is, when the content of Zr is more than 0%, Zr functions as a hydrogen penetration suppressing element. Specifically, in a sour environment, Zr stabilizes a corrosion product film formed on the steel material surface and thereby suppresses penetration of hydrogen into the steel material. As a result, the SSC resistance of the steel material increases. If even a small amount of Zr is contained, the aforementioned advantageous effect will be obtained to a certain extent.
However, if the content of Zr is more than 0.0040%, coarse oxides will form even if the contents of other elements are within the range of the present embodiment. As a result, the SSC resistance of the steel material will decrease.
Therefore, the content of Zr is 0 to 0.0040%.
A preferable lower limit of the content of Zr is 0.0001%, more preferably is 0.0003%, further preferably is 0.0006%, further preferably is 0.0010%, and further preferably is 0.0015%.
A preferable upper limit of the content of Zr is 0.0038%, more preferably is 0.0035%, and further preferably is 0.0032%.

Sb: 0 to 0.50%

Antimony (Sb) is an optional element, and does not have to be contained. That is, the content of Sb may be 0%.
When Sb is contained, that is, when the content of Sb is more than 0%, Sb functions as a hydrogen penetration suppressing element. Specifically, under a sour environment, Sb suppresses penetration of hydrogen into the steel material. As a result, the SSC resistance of the steel material increases. If even a small amount of Sb is contained, the aforementioned advantageous effect will be obtained to a certain extent.
However, if the content of Sb is more than 0.50%, 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 Sb is 0 to 0.50%.
A preferable lower limit of the content of Sb is 0.01%, more preferably is 0.03%, further preferably is 0.05%, and further preferably is 0.08%.
A preferable upper limit of the content of Sb is 0.40%, more preferably is 0.35%, further preferably is 0.30%, and further preferably is 0.25%.

Cu: 0 to 0.50%

Copper (Cu) is an optional element, and does not have to be contained. That is, the content of Cu may be 0%.
When Cu is contained, that is, when the content of Cu is more than 0%, Cu functions as a hydrogen penetration suppressing element. Specifically, under a sour environment, Cu concentrates at the interface between the corrosion product film and the base metal. As a result, surface activity of the base metal is suppressed, and penetration of hydrogen into the steel material is suppressed. Consequently, the SSC resistance of the steel material increases. Cu also dissolves in the steel material and increases hardenability of the steel material and increases the 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 more than 0.50%, Cu will precipitate in the steel material. The precipitated Cu will tend to trap hydrogen. Therefore, even if the contents of other elements are within the range of the present embodiment, the SSC resistance of the steel material will decrease.
Therefore, the content of Cu is 0 to 0.50%.
A preferable lower limit of the content of Cu is 0.01%, more preferably is 0.02%, and further preferably is 0.05%.
A preferable upper limit of the content of Cu is 0.40%, more preferably is 0.38%, further preferably is 0.35%, and further preferably is 0.30%.

Ni: 0 to 0.50%

Nickel (Ni) is an optional element, and does not have to be contained. That is, the content of Ni may be 0%.
When Ni is contained, that is, when the content of Ni is more than 0%, Ni functions as a hydrogen penetration suppressing element. Specifically, under a sour environment, Ni concentrates at the interface between the corrosion product film and the base metal. As a result, surface activity of the base metal is suppressed, and penetration of hydrogen into the steel material is suppressed. Consequently, the SSC resistance of the steel material increases. If even a small amount of Ni is contained, the aforementioned advantageous effect will be obtained to a certain extent.
However, if the content of Ni is more than 0.50%, even if the contents of other elements are within the range of the present embodiment, it will become easy for local corrosion to proceed, and a hydrogen embrittlement resistance characteristic of the steel material will decrease.
Therefore, the content of Ni is 0 to 0.50%.
A preferable lower limit of the content of Ni is 0.01%, more preferably is 0.05%, and further preferably is 0.07%.
A preferable upper limit of the content of Ni is 0.45%, more preferably is 0.40%, further preferably is 0.35%, and further preferably is 0.32%.

Co: 0 to 0.50%

Cobalt (Co) is an optional element, and does not have to be contained. That is, the content of Co may be 0%.
When Co is contained, that is, when the content of Co is more than 0%, Co functions as a hydrogen penetration suppressing element. Specifically, under a sour environment, Co concentrates at the interface between the corrosion product film and the base metal. As a result, surface activity of the base metal is suppressed, and penetration of hydrogen into the steel material is suppressed. Consequently, the SSC resistance of the steel material increases. If even a small amount of Co is contained, the aforementioned advantageous effect will be obtained to a certain extent.
However, if the content of Co is more than 0.50%, even if the contents of other elements are within the range of the present embodiment, hardenability of the steel material will decrease and the strength of the steel material will decrease.

Therefore, the content of Co is 0 to 0.50%.

A preferable lower limit of the content of Co is 0.01%, more preferably is 0.02%, further preferably is 0.03%, further preferably is 0.05%, and further preferably is 0.08%.
A preferable upper limit of the content of Co is 0.40%, more preferably is 0.30%, further preferably is 0.20%, and further preferably is 0.15%.

Ca: 0 to 0.0040%

Calcium (Ca) is an optional element, and does not have to be contained. That is, the content of Ca may be 0%.
When Ca is contained, that is, when the content of Ca is more than 0%, Ca renders S in the steel material harmless by forming sulfides, and thereby increases the SSC 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 more than 0.0040%, coarse oxides will form even if the contents of other elements are within the range of the present embodiment. As a result, the SSC resistance of the steel material will decrease.
Therefore, the content of Ca is 0 to 0.0040%.
A preferable lower limit of the content of Ca is 0.0001%, and more preferably is 0.0003%.
A preferable upper limit of the content of Ca is 0.0030%, more preferably is 0.0020%, further preferably is 0.0015%, and further preferably is 0.0012%.

Mg: 0 to 0.0040%

Magnesium (Mg) is an optional element, and does not have to be contained. That is, the content of Mg may be 0%.
When Mg is contained, that is, when the content of Mg is more than 0%, Mg renders S in the steel material harmless by forming sulfides, and thereby increases the SSC 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 more than 0.0040%, coarse oxides will form even if the contents of other elements are within the range of the present embodiment. As a result, the SSC resistance of the steel material will decrease.
Therefore, the content of Mg is 0 to 0.0040%.
A preferable lower limit of the content of Mg is 0.0001%, and more preferably is 0.0003%.
A preferable upper limit of the content of Mg is 0.0030%, more preferably is 0.0025%, further preferably is 0.0020%, and further preferably is 0.0015%.

Rare earth metal: 0 to 0.0040%

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 REM is contained, that is, when the content of REM is more than 0%, REM renders S in the steel material harmless by forming sulfides, and thereby increases the SSC resistance of the steel material. REM also combines with P in the steel material and thereby suppresses segregation of P to grain boundaries. Therefore, a decrease in the SSC resistance of the steel material that is attributable to segregation of P is suppressed. If even a small amount of REM is contained, the aforementioned advantageous effects will be obtained to a certain extent.
However, if the content of REM is more than 0.0040%, coarse oxides will form even if the contents of other elements are within the range of the present embodiment. As a result, the SSC resistance of the steel material will decrease.
Therefore, the content of REM is 0 to 0.0040%.
A preferable lower limit of the content of REM is 0.0001%, more preferably is 0.0003%, and further preferably is 0.0005%.
A preferable upper limit of the content of REM is 0.0035%, more preferably is 0.0030%, and further preferably is 0.0025%.
Note that, 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" means the total content of these elements.

[Second group: Regarding Nb, V, and B]

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 Nb, V, and B in lieu of a part of Fe. Each of these elements is an optional element, and increases the strength of the steel material.

Nb: 0 to 0.150%

Niobium (Nb) is an optional element, and does not have to be contained. That is, the content of Nb may be 0%.
When Nb is contained, Nb combines with C and/or N to form Nb carbo-nitrides and the like. These Nb carbo-nitrides and the like refine the grains of the steel material by a pinning effect. As a result, the strength of the steel material increases. If even a small amount of Nb is contained, the aforementioned advantageous effect will be obtained to a certain extent.
However, if the content of Nb is more than 0.150%, coarse Nb carbo-nitrides and the like will be formed even if the contents of other elements are within the range of the present embodiment. As a result, the SSC resistance of the steel material will decrease.
Therefore, the content of Nb is 0 to 0.150%.
A preferable lower limit of the content of Nb is 0.001%, more preferably is 0.003%, further preferably is 0.005%, further preferably is 0.008%, and further preferably is 0.012%.
A preferable upper limit of the content of Nb is 0.100%, more preferably is 0.050%, further preferably is 0.030%, further preferably is 0.025%, and further preferably is 0.020%.

V: 0 to 0.500%

Vanadium (V) is an optional element, and does not have to be contained. That is, the content of V may be 0%.
When V is contained, V combines with C and/or N to form V carbo-nitrides and the like. These V carbo-nitrides and the like refine the grains of the steel material by a pinning effect. As a result, the strength of the steel material increases. If even a small amount of V is contained, the aforementioned advantageous effect will be obtained to a certain extent.
However, if the content of V is more than 0.500%, the toughness 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 V is 0 to 0.500%.
A preferable lower limit of the content of V is 0.001%, more preferably is 0.005%, and further preferably is 0.010%.
A preferable upper limit of the content of V is 0.300%, more preferably is 0.250%, further preferably is 0.200%, further preferably is 0.150%, further preferably is 0.120%, and further preferably is 0.100%.

B: 0 to 0.0030%

Boron (B) is an optional element, and does not have to be contained. That is, the content of B may be 0%.
When B is contained, B dissolves in the steel material and increases hardenability of the steel material and increases the strength of the steel material. If even a small amount of B is contained, the aforementioned advantageous effect will be obtained to a certain extent.
However, if the content of B is more than 0.0030%, coarse B nitrides will form even if the contents of other elements are within the range of the present embodiment. As a result, the SSC resistance of the steel material will decrease.
Therefore, the content of B is 0 to 0.0030%.
A preferable lower limit of the content of B is 0.0001%, more preferably is 0.0005%, and further preferably is 0.0008%.
A preferable upper limit of the content of B is 0.0028%, more preferably is 0.0025%, and further preferably is 0.0023%.

[(Feature 2) Regarding yield strength]

The yield strength of the steel material according to the present embodiment is 758 to less than 965 MPa (110 ksi grade to 125 ksi grade). In a case where the steel material of the present embodiment satisfies Feature 1 and Feature 3, the steel material has excellent SSC resistance even though the yield strength is a high strength of 758 to less than 965 MPa.

[Method for measuring yield strength]

The yield strength is measured by the following method. A tensile test is carried out in accordance with ASTM E8/E8M (2021). Specifically, a tensile test specimen is taken from the steel material. The size of the tensile test specimen is not particularly limited. For example, a round bar tensile test specimen in which the diameter of the parallel portion is 6.0 mm and the gage length is 30.0 mm is used as the tensile test specimen.
If the steel material is a steel pipe, the tensile test specimen is to be taken from a central portion of the wall thickness. In this case, the longitudinal direction of the tensile test specimen is to be made parallel to the axial direction of the steel pipe. If the steel material is a steel plate, the tensile test specimen is to be taken from a central portion of the thickness. In this case, the longitudinal direction of the tensile test specimen is to be made parallel to the rolling direction of the steel plate. If the steel material is a round steel bar, the tensile test specimen is to be taken from an R/2 portion. In the present description, the term "round steel bar" means a steel bar in which a cross section perpendicular to the axial direction is a circular shape. The term "R/2 portion" means the central portion of a radius R in a cross section perpendicular to the axial direction (rolling direction) of the round steel bar. The longitudinal direction of the tensile test specimen is to be made parallel to the axial direction of the round steel bar.
A tensile test is carried out in air at normal temperature (24 ±3°C) using the taken tensile test specimen. The obtained 0.2% offset yield stress (MPa) is defined as the yield strength (MPa).

[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 microstructure of a steel material which has a chemical composition satisfying Feature 1 and which has a yield strength satisfying Feature 2, the total area fraction of tempered martensite and tempered bainite will be 90% or more. Accordingly, if the chemical composition of a steel material satisfies Feature 1 and the yield strength of the steel material satisfies Feature 2, it can be regarded that the total area fraction of tempered martensite and tempered bainite in the microstructure of the relevant steel material is 90% or more.

[Microstructure observation method]

The total area fraction of tempered martensite and tempered bainite can be determined by the following method. First, a test specimen is taken from the steel material.
If the steel material is a steel pipe, a test specimen having an observation surface with dimensions of 10 mm in the pipe axis direction and 10 mm in the pipe radius direction is to be taken from a central portion of the wall thickness. In a case where the steel material is a steel pipe with a wall thickness of less than 10 mm, a test specimen having an observation surface with dimensions of 10 mm in the pipe axis direction and the wall thickness of the steel pipe in the pipe radius direction is to be taken.
If the steel material is a steel plate, a test specimen having an observation surface with dimensions of 10 mm in the rolling direction and 10 mm in the thickness direction is to be taken from a central portion of the thickness. In a case where the steel material is a steel plate with a thickness of less than 10 mm, a test specimen having an observation surface with dimensions of 10 mm in the rolling direction and the thickness of the steel plate in the thickness direction is to be taken.
If the steel material is a round steel bar, a test specimen is to be taken from a cross section parallel to the axial direction (rolling direction) of the round steel bar. Specifically, a test specimen having an observation surface with dimensions of 10 mm in the axial direction and 10 mm in the radial direction of the aforementioned cross section and which includes the R/2 portion at the center thereof is to be taken. In a case where the diameter of the cross section is less than 10 mm, a test specimen having an observation surface with dimensions of 10 mm in the axial direction and the diameter of the round steel bar in the radial direction of the cross section and which includes the R/2 portion is to be taken.
The observation surface of the test specimen is polished to obtain a mirror surface. The polished observation surface is then immersed for about 10 seconds in a nital etching reagent to perform etching of the observation surface. The etched observation surface is observed in 10 visual fields by means of a secondary electron image using a scanning electron microscope (SEM). The visual field area is, for example, 10,000 µm² (magnification of × 1000).
In each visual field, tempered martensite and tempered bainite are identified based on the contrast. In each visual field, tempered martensite and tempered bainite can be distinguished 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.

[(Feature 3) Regarding electrochemical elements EE and FN]

In the steel material of the present embodiment, in addition, in a case where the yield strength is 758 to less than 862 MPa (110 ksi grade), EE defined by Formula (1) is 2.75 or more and FN defined by Formula (2) is 0.185 or more, and in a case where the yield strength is 862 to less than 965 MPa (125 ksi grade), EE is 3.00 or more and FN is 0.200 or more; EE = -0.25C + 2Si - 5.8Mn + 2.1Cr + Mo + 4.1Zr + 2.6Sb + 0.3Cu + 0.4Ni + 1.5Co $FN = EE / (D^{0.9})$
where, the content in percent by mass of a corresponding element is substituted for each symbol of an element in Formula (1). The average equivalent circular diameter in units of µm of prior-austenite grains in the steel material is substituted for D in Formula (2).

[Regarding EE defined by Formula (1)]

In EE, the elements Si, Cr, Mo, Zr, Sb, Cu, Ni, and Co are elements that suppress penetration of hydrogen from the steel material surface (hydrogen penetration suppressing elements). In EE, the elements C and Mn are elements that promote penetration of hydrogen from the steel material surface (hydrogen penetration promoting elements). EE is an index of an electrochemical hydrogen penetration suppression effect in the steel material.
In a case where the yield strength of the steel material is 110 ksi grade (758 to less than 862 MPa), if EE is 2.75 or more, hydrogen penetration suppressing elements are sufficiently more than hydrogen penetration promoting elements. Therefore, penetration of hydrogen from the steel material surface is suppressed electrochemically. As a result, on the precondition that Feature 1 and Feature 2 are satisfied and also that FN is 0.185 or more, excellent SSC resistance is obtained.
A preferable lower limit of EE in a case where the yield strength is 110 ksi grade is 2.78, more preferably is 2.80, further preferably is 2.85, further preferably is 2.90, further preferably is 2.95, and further preferably is 3.00.
Although the upper limit of EE is not particularly limited, a preferable upper limit of EE is 6.60, more preferably is 6.00, further preferably is 5.80, and further preferably is 5.50.
In a case where the yield strength of the steel material is 125 ksi grade (862 to less than 965 MPa), because the strength is higher than 110 ksi grade, it is necessary to suppress hydrogen penetration to a greater extent in order to increase the SSC resistance. If EE is 3.00 or more, in a steel material with a yield strength of 125 ksi grade, the penetration of hydrogen from the steel material surface will be electrochemically suppressed. As a result, on the precondition that Feature 1 and Feature 2 are satisfied and also that FN is 0.200 or more, excellent SSC resistance is obtained.
A preferable lower limit of EE in a case where the yield strength is 125 ksi grade is 3.10, more preferably is 3.15, further preferably is 3.20, further preferably is 3.25, further preferably is 3.30, and further preferably is 3.35.
Although the upper limit of EE is not particularly limited, a preferable upper limit of EE is 6.60, more preferably is 6.00, further preferably is 5.80, and further preferably is 5.50.

[Regarding FN defined by Formula (2)]

As mentioned above, in a steel material whose chemical composition satisfies Feature 1, a synergistic effect between electrochemical elements EE defined by Formula (1) and a physical element is effective for suppressing penetration of hydrogen from the steel material surface.
In a case where the yield strength of a steel material whose chemical composition satisfies Feature 1 is 110 ksi grade (758 to less than 862 MPa), if EE is 2.75 or more and FN is 0.185 or more, as illustrated in FIG. 2A, the SSC resistance markedly increases.
A preferable lower limit of FN in a case where the yield strength is 110 ksi grade is 0.187, more preferably is 0.190, further preferably is 0.192, and further preferably is 0.195. Although the upper limit of FN is not particularly limited, a preferable upper limit of FN is 0.580, more preferably is 0.550, further preferably is 0.500, and further preferably is 0.450.
Further, in a case where the yield strength of a steel material whose chemical composition satisfies Feature 1 is 125 ksi grade (862 to less than 965 MPa), if EE is 3.00 or more and FN is 0.200 or more, as illustrated in FIG. 2B, the SSC resistance markedly increases.
A preferable lower limit of FN in a case where the yield strength is 125 ksi grade is 0.205, more preferably is 0.210, further preferably is 0.215, and further preferably is 0.220. Although the upper limit of FN is not particularly limited, a preferable upper limit of FN is 0.580, more preferably is 0.550, further preferably is 0.500, and further preferably is 0.450.

[Method for determining average equivalent circular diameter D of prior-austenite grains]

The average equivalent circular diameter D (µm) of prior-austenite grains of the steel material according to the present embodiment is determined by the following method. First, a test specimen is taken from the steel material.
If the steel material is a steel pipe, a test specimen having an observation surface with dimensions of 10 mm in the pipe axis direction and 10 mm in the pipe radius direction is to be taken from a central portion of the wall thickness. In a case where the steel material is a steel pipe with a wall thickness of less than 10 mm, a test specimen having an observation surface with dimensions of 10 mm in the pipe axis direction and the wall thickness of the steel pipe in the pipe radius direction is to be taken.
If the steel material is a steel plate, a test specimen having an observation surface with dimensions of 10 mm in the rolling direction and 10 mm in the thickness direction is to be taken from a central portion of the thickness. In a case where the steel material is a steel plate with a thickness of less than 10 mm, a test specimen having an observation surface with dimensions of 10 mm in the rolling direction and the thickness of the steel plate in the thickness direction is to be taken.
If the steel material is a round steel bar, a test specimen is to be taken from a cross section parallel to the axial direction (rolling direction) of the round steel bar. Specifically, a test specimen having an observation surface with dimensions of 10 mm in the axial direction and 10 mm in the radial direction of the aforementioned cross section and which includes the R/2 portion at the center thereof is to be taken. In a case where the diameter of the cross section is less than 10 mm, a test specimen having an observation surface with dimensions of 10 mm in the axial direction and the diameter of the round steel bar in the radial direction of the cross section and which includes the R/2 portion is to be taken.
The test specimen is embedded in resin, and the observation surface is polished to obtain a mirror surface. The test specimen after polishing is immersed for about 60 seconds in an aqueous solution saturated with picric acid. By this means, the observation surface is etched and prior-austenite grain boundaries are revealed on the observation surface. Ten visual fields on the etched observation surface are observed at a magnification of ×420 using an optical microscope. The visual field area of each visual field is set to a rectangle of 450 µm × 450 µm. The grain size No. of each prior-austenite grain in each visual field is determined by an intercept method in accordance with JIS G 0551 (2020). At such time, the number of grid points, which are points of intersection between grid lines, is set to 16. The arithmetic average value of the grain size Nos. of the prior-austenite grains determined in the 10 visual fields is determined. The average area of the prior-austenite grains is calculated based on the arithmetic average value of the grain size Nos. of the prior-austenite grains. The equivalent circular diameter is calculated based on the calculated average area of the prior-austenite grains. Here, the term "equivalent circular diameter" refers to the diameter of a circle having the same area as the average area of the prior-austenite grains. The calculated equivalent circular diameter is defined as the average equivalent circular diameter D (µm) of the prior-austenite grains. The average equivalent circular diameter D is to be made an integer obtained by rounding off decimals of the calculated value.
Note that, a preferable upper limit of the average equivalent circular diameter of the prior-austenite grains is 40 µm, more preferably is 35 µm, further preferably is 30 µm, and further preferably is 25 µm.
A preferable lower limit of the average equivalent circular diameter of the prior-austenite grains is 10 µm, more preferably is 15 µm, and further preferably is 17 µm.

[Shape of steel material]

The shape of the steel material according to the present embodiment is not particularly limited. The steel material is, for example, a steel pipe, a steel plate, or a round steel bar.
Preferably, the steel material of the present embodiment is an oil-well steel pipe. The oil-well steel pipe is, for example, a casing pipe, a tubing pipe, a drilling pipe or the like which are used for drilling an oil well or a gas well, extracting crude oil or natural gas, and the like. In a case where the steel material is an oil-well steel pipe, the wall thickness is, for example, 9 to 60 mm.

[Regarding advantageous effect of steel material of present embodiment]

The steel material of the present embodiment satisfies the aforementioned Feature 1 to Feature 3. Therefore, even though the steel material of the present embodiment has a high strength of 110 ksi grade (758 to less than 862 MPa) to 125 ksi grade (862 to less than 965 MPa), excellent SSC resistance is obtained.

[SSC resistance evaluation method]

The SSC resistance is evaluated by a normal-temperature SSC resistance evaluation test and a low-temperature SSC resistance evaluation test conducted in accordance with NACE TM0177-2016 Method A, and a DCB test conducted in accordance with NACE TM0177-2016 Method D.

[Normal-temperature SSC resistance evaluation test]

In the normal-temperature SSC resistance evaluation test, a mixed aqueous solution containing 5.0% by mass of sodium chloride and 0.5% by mass of acetic acid (NACE solution A) is employed as the test solution. A round bar specimen is taken from the steel material.
If the steel material is a steel pipe, the round bar specimen is to be taken from a central portion of the wall thickness. In this case, the longitudinal direction of the round bar specimen is to be made parallel to the axial direction of the steel pipe. If the steel material is a steel plate, the round bar specimen is to be taken from a central portion of the thickness. In this case, the longitudinal direction of the round bar specimen is to be made parallel to the rolling direction of the steel plate. If the steel material is a round steel bar, the round bar specimen is to be taken from an R/2 portion. In this case, the longitudinal direction of the round bar specimen is to be made parallel to the axial direction of the round steel bar. Regarding the size of the round bar specimen, for example, the round bar specimen has a diameter of 6.35 mm and a parallel portion length of 25.4 mm. Three round bar specimens are taken from the steel material.
A stress equivalent to 90% of the actual yield stress is applied to each round bar specimen. The test solution at 24°C is poured into a test vessel so that the round bar specimen to which the stress has been applied is immersed therein, and this is adopted as the test bath. After degassing the test bath, H₂S gas is blown into the test bath and is caused to saturate in the test bath. Specifically, H₂S gas at 1 atm is blown into the test bath. The test bath into which the H₂S gas has been blown is held at 24°C for 720 hours.

[Low-temperature SSC resistance evaluation test]

In the low-temperature SSC resistance evaluation test, similarly to the normal-temperature SSC resistance evaluation test, NACE solution A is adopted as the test solution. Similarly to the normal-temperature SSC resistance evaluation test, three round bar specimens are taken from the steel material. Regarding the size of the round bar specimen, for example, the round bar specimen has a diameter of 6.35 mm and a parallel portion length of 25.4 mm. Note that, the longitudinal direction of the round bar specimen is to be made the same direction as in the case of the normal-temperature SSC resistance evaluation test.
A stress equivalent to 85% of the actual yield stress is applied to each round bar specimen. The test solution at 4°C is poured into a test vessel so that the round bar specimen to which the stress has been applied is immersed therein, and this is adopted as the test bath. After degassing the test bath, H₂S gas is blown into the test bath and is caused to saturate in the test bath. Specifically, H₂S gas at 1 atm is blown into the test bath. The test bath into which the H₂S gas has been blown is held at 4°C for 720 hours.
In the normal-temperature SSC resistance evaluation test and the low-temperature SSC resistance evaluation test, after being held for 720 hours, each round bar specimen is observed to confirm whether or not sulfide stress cracking (SSC) occurred. Specifically, after being held for 720 hours, each round bar specimen is observed with the naked eye and using a projector with a magnification of ×10.
With respect to the steel material of the present embodiment, as the result of the observation, cracking is not confirmed in any of the three round bar specimens in the normal-temperature SSC resistance evaluation test, and cracking is also not confirmed in any of the three round bar specimens in the low-temperature SSC resistance evaluation test.

[DCB test]

The DCB test is carried out by the following method. An aqueous solution containing 5.0% by mass of sodium chloride is adopted as a test solution. A DCB test specimen illustrated in FIG. 3A is taken from the steel material.
If the steel material is a steel pipe, the DCB test specimen is to be taken from a central portion of the wall thickness. In this case, the longitudinal direction of the DCB test specimen is to be made parallel to the axial direction of the steel pipe. If the steel material is a steel plate, the DCB test specimen is to be taken from a central portion of the thickness. In this case, the longitudinal direction of the DCB test specimen is to be made parallel to the rolling direction of the steel plate. If the steel material is a round steel bar, the DCB test specimen is to be taken from an R/2 portion. In this case, the longitudinal direction of the DCB test specimen is to be made parallel to the axial direction of the round steel bar. In addition, a wedge illustrated in FIG. 3B is taken from the steel material. A thickness t of the wedge is to be 3.10 (mm).
Referring to FIG. 3A, the aforementioned wedge is driven in between the arms of the DCB test specimen. The DCB test specimen into which the wedge has been driven is then enclosed in a test vessel. Thereafter, a test solution is poured into the test vessel so as to leave a vapor phase portion, and this is adopted as a test bath. The amount of the test bath is to be 1 L per test specimen. Next, N₂ gas is blown into the test bath for three hours to degas the test bath until the dissolved oxygen in the test bath becomes 20 ppb or less.
H₂S gas is blown into the degassed test bath to make the test bath a corrosive environment. Specifically, H₂S gas at 5 atm (0.5 MPa) is blown into the test bath. The pH of the test bath is adjusted so as to be within the range of 3.5 to 4.0 throughout the immersion period. The inside of the test vessel is held at 24 ±3°C for 14 days (336 hours) while stirring the test bath. After being held for 14 days, the DCB test specimen is taken out from the test vessel.
A pin is inserted into a hole formed in the tip of the arms of the DCB test specimen that is taken out from the test vessel, a notch portion is opened with a tensile testing machine, and a wedge releasing stress P is measured. In addition, the notch in the DCB test specimen is released in liquid nitrogen, and a crack propagation length "a" of the DCB test specimen during immersion in the test bath is measured. The crack propagation length "a" is measured visually using a vernier calipers. A fracture toughness value Kissc (MPa^m) is determined using the following formula based on the measured wedge releasing stress P and the crack propagation length "a". $K_{1SSC} = \frac{Pa (2 \sqrt{3} + 2.38 \frac{h}{a}) {(\frac{B}{Bn})}^{\frac{1}{\sqrt{3}}}}{{Bh}^{\frac{3}{2}}}$
In the above formula, h (mm) represents the height of each arm of the DCB test specimen. B (mm) represents the thickness of the DCB test specimen. Bn (mm) represents the web thickness of the DCB test specimen. These are defined in NACE TM0177-2016 Method D.
With respect to the steel material of the present embodiment, in a case where the steel material has a yield strength of 110 ksi grade (758 to less than 862 MPa), if the fracture toughness value Kissc determined in the DCB test is 25.0 MPa^m or more, it is determined that excellent fracture toughness was obtained in the DCB test. In a case where the steel material has a yield strength of 125 ksi grade (862 to less than 965 MPa), if the fracture toughness value Kissc determined in the DCB test is 24.0 MPa√m or more, it is determined that excellent fracture toughness was obtained in the DCB test.
The above can be summarized as follows.
The phrase "excellent SSC resistance is obtained" in a case where the steel material has a yield strength of 110 ksi grade (758 to less than 862 MPa) means that a crack is not confirmed in the normal-temperature SSC resistance evaluation test at 24°C and in the low-temperature SSC resistance evaluation test at 4°C which are conducted in accordance with NACE TM0177-2016 Method A, and also that a fracture toughness value Kissc obtained in the DCB test conducted in accordance with NACE TM0177-2016 Method D is 25.0 MPa^m or more.
The phrase "excellent SSC resistance is obtained" in a case where the steel material has a yield strength of 125 ksi grade (862 to less than 965 MPa) means that a crack is not confirmed in the normal-temperature SSC resistance evaluation test at 24°C and in the low-temperature SSC resistance evaluation test at 4°C which are conducted in accordance with NACE TM0177-2016 Method A, and also that a fracture toughness value Kissc obtained in the DCB test conducted in accordance with NACE TM0177-2016 Method D is 24.0 MPa^m or more.

[Production method]

A method for producing a seamless steel pipe will now be described as an example of a method for producing the steel material according to the present embodiment. The method for producing the steel material described hereunder is one example for producing the steel material of the present embodiment. Accordingly, a steel material composed as described above may also be produced by a production method other than the production method described hereinafter. However, the production method described hereunder is a preferred example of the method for producing the steel material of the present embodiment.
One example of the method for producing a seamless steel pipe of the present embodiment includes the following processes.

(Process 1) Starting material preparation process
(Process 2) Hot working process
(Process 3) Quenching process
(Process 4) Tempering process

The principal production condition in the above production method is as follows.
(Condition 1) In process 4, if the yield strength of the steel material to be produced is 110 ksi grade (758 to less than 862 MPa), FA defined by Formula (A) is to be 2500 or less. Further, in process 4, if the yield strength of the steel material to be produced is 125 ksi grade (862 to less than 965 MPa), FA defined by Formula (A) is to be 2400 or less; $FA = T \times (- 3.0 C + 4.7 Si - 4.34 Mn - 2.4 Cr + 2.2 Mo - 2.2 Cu - 3.2 Ni) \times {(t / 60)}^{0.5}$
where, the content in percent by mass of a corresponding element in the steel material is substituted for each symbol of an element in Formula (A). The tempering temperature (°C) is substituted for T. A holding time t (min) at the tempering temperature T is substituted for t.

Hereunder, each process is described.

[(Process 1) Starting material preparation process]

In the starting material preparation process, a starting material is produced using a molten steel having a chemical composition satisfying Feature 1. The method for producing the starting material is not particularly limited, and it suffices to use a well-known method. Specifically, a cast piece (a slab, a bloom, or a billet) may be produced by a continuous casting process using the molten steel. An ingot may also be produced by an ingot-making process using the molten steel. A starting material (a slab, a bloom, or a billet) is produced by the above process.

[(Process 2) Hot working process]

In the hot working process, the prepared starting material is subjected to hot working to produce an intermediate steel material. The method of hot working for producing the intermediate steel material is not particularly limited. The hot working may be hot forging, may be hot extrusion, or may be hot rolling.
If the steel material is a steel pipe, the hot working process is, for example, as follows. When using a bloom or an ingot as the starting material, first the starting material is subjected to blooming using a blooming mill to produce a billet. Although not particularly limited, the heating temperature before blooming is, for example, 1100 to 1350°C.
Piercing-rolling according to the Mannesmann process is performed using a billet produced by blooming or using a billet produced by the continuous casting process in the starting material preparation process. In this case, first, the billet is heated in a heating furnace. Although not particularly limited, the heating temperature is, for example, 1100 to 1350°C. After being taken out from the heating furnace, the billet is subjected to piercing-rolling to produce an intermediate steel material (hollow shell). When performing the piercing-rolling, although not particularly limited, the piercing ratio is, for example, 1.0 to 4.0. The billet after piercing-rolling is subjected to elongating using a mandrel mill. In addition, as necessary, the billet after elongating is subjected to diameter adjusting rolling using a stretch reducing mill or a sizing mill. An intermediate steel material (hollow shell) is produced by the above processes. Although not particularly limited, the cumulative reduction of area in the hot working process is, for example, 20 to 70%.
In a case where the steel material is a steel pipe, instead of piercing-rolling according to the Mannesmann process, the billet may be subjected to the Ugine-Sejournet process or the Ehrhardt push bench process (that is, hot extrusion) to produce an intermediate steel material (hollow shell).
If the steel material is a steel plate, the hot working process is, for example, as follows. A slab is subjected to rough rolling using a reverse rolling mill to produce a sheet bar. Although not particularly limited, the heating temperature before rough rolling is, for example, 1100 to 1350°C. In addition, the sheet bar is subjected to finish rolling using a tandem rolling mill to produce an intermediate steel material (steel plate).
If the steel material is a round steel bar, the hot working process is, for example, as follows. When using a bloom or an ingot as the starting material, first, the starting material is subjected to blooming using a blooming mill to produce a billet. Although not particularly limited, the heating temperature before blooming is, for example, 1100 to 1350°C.
The billet produced by blooming or the billet produced by a continuous casting process in the starting material preparation process is heated. Although not particularly limited, the heating temperature is, for example, 1100 to 1350°C. The heated billet is subjected to finish rolling using a continuous mill to produce an intermediate steel material (round steel bar). In a continuous mill, a horizontal stand having a pair of grooved rolls arranged one on the other in the vertical direction, and a vertical stand having a pair of grooved rolls arranged side by side in the horizontal direction are arranged in an alternating manner.
The intermediate steel material produced by the above hot working may be air-cooled. The intermediate steel material produced by the hot working may also be subjected to direct quenching after the hot working without being cooled to normal temperature, or may be subjected to quenching after undergoing supplementary heating (reheating) after the hot working. In a case where direct quenching is performed after hot working, or quenching is performed after performing supplementary heating after hot working, stress relief annealing (SR treatment) may be performed in order to eliminate residual stress before the quenching process that is the next process.

[(Process 3) Quenching process]

In the quenching process, the intermediate steel material produced by the hot working process is subjected to quenching. The quenching is performed by a well-known method. Specifically, the intermediate steel material after the hot working process is charged into a heat treatment furnace and held at a quenching temperature. The quenching temperature is to be equal to or higher than the A_c3 transformation point. However, if the quenching temperature is too high, the prior-austenite grains may become coarse. Therefore, the quenching temperature is, for example, 800 to 950°C. After the intermediate steel material has been held at the quenching temperature, the intermediate steel material is rapidly cooled (quenched). 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 or oil cooling. The quenching method is not particularly limited. For example, the intermediate steel material may be rapidly cooled by immersing the intermediate steel material in a water bath or an oil bath. If the intermediate steel material is a steel pipe, the steel pipe may be rapidly cooled by pouring or jetting cooling water onto the outer surface and/or the inner surface of the steel pipe by shower cooling or mist cooling.
If the intermediate steel material is a hollow shell (seamless steel pipe), quenching (direct quenching) may be performed immediately after the hot working, without cooling the hollow shell to normal temperature after the hot working process. Further, quenching may be performed after the hollow shell has been held at the quenching temperature after being charged into a holding furnace before the temperature of the hollow shell decreased after the hot working.
In the present description, in a case where direct quenching is performed after hot working, the quenching temperature corresponds to the surface temperature of the intermediate steel material that is measured by a thermometer placed on the exit side of the apparatus that performs the final hot working. Further, in a case where quenching is performed after supplementary heating or reheating is performed after hot working, the quenching temperature corresponds to the temperature of the furnace that performs the supplementary heating or the reheating.

[(Process 4) Tempering process]

In the tempering process, the intermediate steel material after quenching is further subjected to tempering. In the tempering process, the yield strength of the steel material can be adjusted by appropriately adjusting the tempering temperature according to the chemical composition. Specifically, the tempering conditions are adjusted so that the yield strength of the steel material becomes 110 ksi grade (758 to less than 862 MPa) to 125 ksi grade (862 to less than 965 MPa).
In the tempering process, a tempering temperature T is to be set to 660 to 740°C, and a holding time t at the tempering temperature T is to be set to 20 to 180 minutes. In the tempering process, in addition, FA defined by Formula (A) is adjusted according to the strength. Specifically, if the yield strength of the steel material to be produced is 110 ksi grade, FA is to be 2500 or less. If the yield strength of the steel material to be produced is 125 ksi grade, FA is to be 2400 or less. $FA = T \times (- 3.0 C + 4.7 Si - 4.4 Mn - 2.4 Cr + 2.2 Mo - 2.2 Cu - 3.2 Ni) \times {(t / 60)}^{0.5}$
where, the content in percent by mass of a corresponding element in the steel material is substituted for each symbol of an element in Formula (A). The tempering temperature (°C) is substituted for T. The holding time t (min) at the tempering temperature T is substituted for t.
As mentioned above, in the steel material of the present embodiment, because of a synergetic effect between electrochemical elements (EE defined by Formula (1)) and a physical element (average equivalent circular diameter of prior-austenite grains), even though the steel material has a high strength of 758 to less than 965 MPa, excellent SSC resistance is obtained. FA is an index for appropriately adjusting electrochemical elements that constitutes the numerator of Formula (2), the physical element that constitutes the denominator of Formula (2), and the strength of the steel material.
In a case where the strength of the steel material to be produced is 110 ksi grade, if FA is 2500 or less, on the precondition that the chemical composition satisfies Feature 1, the relation among the electrochemical elements, the physical element, and the strength can be appropriately adjusted. Therefore, a steel material satisfying Feature 1 to Feature 3 can be produced.
Similarly, in a case where the strength of the steel material to be produced is 125 ksi grade, if FA is 2400 or less, on the precondition that the chemical composition satisfies Feature 1, the relation among the electrochemical elements, the physical element, and the strength can be appropriately adjusted. Therefore, a steel material satisfying Feature 1 to Feature 3 can be produced.
The steel material of the present embodiment can be produced by the processes described above. Note that, the production method described above is a description of one example of the method for producing the steel material according to the present embodiment. The steel material according to the present embodiment may also be produced by a production method other than the production method described above. Even in such a case, as long as the steel material satisfies Feature 1 to Feature 3, a high strength of 110 ksi grade (758 to less than 862 MPa) to 125 ksi grade (862 to less than 965 MPa) and excellent SSC resistance will be obtained.

EXAMPLE 1

Hereunder, the advantageous effect of the steel material of the present embodiment is described more specifically by way of examples. The conditions adopted in the following examples are one example of conditions adopted for confirming the feasibility and advantageous effect of the steel material of the present embodiment. Accordingly, the steel material of the present embodiment is not limited to this one example of conditions.
In Example 1, the SSC resistance of steel materials having a yield strength of 110 ksi grade (758 to less than 862 MPa) was investigated. Specifically, steel materials (seamless steel pipes) having the chemical compositions shown in Table 1-1 and Table 1-2 were produced.

[Table 1-1]

TABLE 1-1

Test number	Chemical composition (unit is mass%; balance is Fe and impurities)
Test number	C	Si	Mn	P	S	Al	N	Cr	Mo	Ti	O
1	0.26	0.63	0.08	0.020	0.0009	0.013	0.0085	0.96	0.97	0.007	0.0046
2	0.22	1.28	0.08	0.018	0.0006	0.086	0.0036	0.99	0.14	0.007	0.0028
3	0.27	0.71	0.13	0.003	0.0007	0.060	0.0047	0.95	0.51	0.007	0.0010
4	0.27	0.86	0.13	0.010	0.0004	0.031	0.0032	0.75	0.93	0.004	0.0017
5	0.26	1.19	0.12	0.008	0.0007	0.027	0.0036	0.89	0.50	0.005	0.0012
6	0.28	1.01	0.11	0.011	0.0009	0.035	0.0031	0.69	0.52	0.008	0.0019
7	0.27	1.10	0.13	0.008	0.0011	0.042	0.0050	0.81	0.31	0.011	0.0013
8	0.27	0.74	0.13	0.003	0.0007	0.060	0.0047	1.00	0.51	0.007	0.0010
9	0.34	0.94	0.23	0.012	0.0010	0.038	0.0038	0.95	0.89	0.005	0.0022
10	0.21	1.06	0.11	0.012	0.0010	0.041	0.0041	0.94	0.80	0.006	0.0019
11	0.26	1.14	0.16	0.047	0.0010	0.039	0.0036	0.94	0.79	0.006	0.0020
12	0.29	1.05	0.20	0.012	0.0093	0.038	0.0039	0.97	0.92	0.007	0.0016
13	0.27	0.80	0.10	0.006	0.0006	0.044	0.0039	0.75	0.32	0.006	0.0012
14	0.26	0.72	0.15	0.012	0.0005	0.034	0.0034	0.78	0.71	0.005	0.0016
15	0.25	1.27	0.16	0.002	0.0005	0.033	0.0027	1.00	0.86	0.006	0.0025
16	0.27	1.25	0.13	0.008	0.0008	0.037	0.0041	0.97	0.29	0.004	0.0017
17	0.27	1.24	0.19	0.010	0.0008	0.039	0.0042	0.98	0.69	0.014	0.0016
18	0.26	1.11	0.11	0.005	0.0012	0.032	0.0041	0.96	0.41	0.006	0.0016
19	0.29	1.22	0.14	0.010	0.0016	0.031	0.0050	0.95	0.36	0.021	0.0018
20	0.28	0.97	0.13	0.007	0.0008	0.029	0.0032	0.99	0.50	0.009	0.0009
21	0.26	1.11	0.23	0.008	0.0006	0.036	0.0047	0.98	0.68	0.009	0.0012
22	0.25	1.18	0.12	0.011	0.0007	0.038	0.0042	0.95	0.66	0.006	0.0008
23	0.25	1.00	0.13	0.012	0.0006	0.040	0.0037	0.93	0.53	0.003	0.0018
24	0.25	1.25	0.12	0.006	0.0009	0.035	0.0035	1.00	0.45	0.010	0.0017
25	0.30	1.12	0.11	0.009	0.0005	0.036	0.0031	0.98	0.47	0.017	0.0016
26	0.26	1.03	0.14	0.006	0.0006	0.032	0.0037	0.97	0.84	0.012	0.0016
27	0.26	1.21	0.12	0.010	0.0009	0.035	0.0039	0.95	0.91	0.026	0.0016
28	0.27	1.21	0.10	0.020	0.0006	0.035	0.0040	0.31	0.85	0.007	0.0018
29	0.22	0.85	0.08	0.018	0.0006	0.023	0.0022	0.34	0.64	0.006	0.0035
30	0.31	0.88	0.24	0.019	0.0010	0.044	0.0041	0.96	0.41	0.013	0.0016
31	0.25	0.63	0.13	0.010	0.0006	0.038	0.0041	0.31	0.56	0.007	0.0014
32	0.30	0.60	0.24	0.005	0.0005	0.031	0.0033	0.23	0.39	0.010	0.0015
33	0.34	0.99	0.25	0.016	0.0013	0.047	0.0041	0.57	0.97	0.005	0.0016
34	0.25	1.27	0.13	0.012	0.0006	0.040	0.0037	0.45	0.53	0.016	0.0011
35	0.26	1.20	0.11	0.013	0.0007	0.036	0.0040	0.48	0.72	0.008	0.0013
36	0.28	1.28	0.23	0.021	0.0014	0.038	0.0042	0.58	0.88	0.006	0.0016
37	0.28	1.74	0.24	0.007	0.0005	0.033	0.0035	0.86	0.77	0.004	0.0013
38	0.27	1.34	0.22	0.011	0.0009	0.037	0.0039	0.89	0.54	0.006	0.0010
39	0.23	0.26	0.07	0.011	0.0008	0.041	0.0039	0.70	0.55	0.007	0.0014
40	0.22	0.81	0.51	0.012	0.0010	0.037	0.0044	0.89	0.94	0.005	0.0019

[Table 1-2]

TABLE 1-2

Test number	Chemical composition (unit is mass%; balance is Fe and impurities)
Test number	Zr	Sb	Cu	Ni	Co	Ca	Mg	REM	Nb	V	B
1
2		0.05	0.23	0.21
3
4
5
6
7
8
9
10
11
12
13
14
15
16	0.0029
17		0.11
18			0.30
19				0.24
20					0.12
21						0.0010
22							0.0013
23								0.0022
24									0.016
25										0.099
26											0.0021
27	0.0032	0.21	0.28	0.31	0.12
28	0.0011		0.11	0.13
29			0.34	0.32
30
31
32
33
34
35			0.22	0.21
36				0.20
37
38
39
40

A blank space in Table 1-2 means that the content of the corresponding element was at the level of an impurity. Note that, EE of each test number is shown in the column "EE" in Table 2.

[Table 2]

TABLE 2

Test number	EE	Quenching condition	Tempering conditions		FA	YS (MPa)	D (µm)	FN	SSC resistance		Kissc (MPa√m)
Test number	EE	Quenching temperature (°C)	Tempering temperature T (°C)	Holding time t (min)	FA	YS (MPa)	D (µm)	FN	24°C	4°C	Kissc (MPa√m)
1	3.72	920	705	30	827	840	17	0.290	E	E	27.1
2	4.54	950	690	30	858	813	32	0.201	E	E	27.8
3	3.10	920	710	60	566	801	17	0.242	E	E	28.5
4	3.40	950	710	60	2063	808	23	0.202	E	E	28.6
5	3.99	950	720	60	2339	799	25	0.220	E	E	26.8
6	3.28	920	705	60	2052	824	21	0.212	E	E	28.2
7	3.39	900	695	60	1756	809	20	0.229	E	E	30.9
8	3.27	900	710	60	581	801	17	0.255	E	E	28.5
9	3.35	900	720	60	1486	823	20	0.226	E	E	26.7
10	4.20	920	710	60	2394	818	23	0.250	E	E	30.0
11	4.05	950	710	60	2383	824	24	0.232	E	E	29.8
12	3.82	920	710	60	2046	820	18	0.284	E	E	33.3
13	2.85	900	710	60	1004	807	18	0.211	E	E	33.6
14	2.85	920	710	60	1160	816	20	0.192	E	E	29.3
15	4.51	990	700	30	1983	825	43	0.153	B	B	23.0
16	4.02	920	705	60	1976	815	23	0.239	E	E	28.0
17	4.34	900	715	60	2394	826	21	0.281	E	E	28.4
18	4.03	950	705	60	1333	819	23	0.240	E	E	27.1
19	4.01	920	725	60	1444	838	22	0.248	E	E	27.3
20	3.88	920	715	60	1338	812	21	0.250	E	E	31.7
21	3.56	900	720	60	1850	817	21	0.230	E	E	32.3
22	4.26	890	720	60	2477	809	22	0.264	E	E	32.5
23	3.67	920	700	60	1618	800	21	0.237	E	E	30.9
24	4.29	920	720	60	2295	793	20	0.290	E	E	26.9
25	4.06	900	725	60	1857	794	18	0.301	E	E	34.5
26	4.06	950	715	60	2120	816	24	0.232	E	E	27.0
27	5.51	880	720	100	2317	841	17	0.430	E	E	34.5
28	3.36	920	695	30	2411	775	23	0.200	E	E	26.0
29	2.77	850	690	30	880	815	19	0.195	E	E	25.5
30	2.72	920	700	100	676	813	19	0.192	B	B	23.9
31	1.65	900	680	100	1867	806	18	0.123	B	B	21.8
32	0.61	850	730	100	1103	778	18	0.045	B	B	22.8
33	2.61	920	700	100	2981	819	24	0.150	B	B	23.0
34	3.20	950	695	100	4247	815	27	0.165	B	B	20.3
35	3.58	950	690	100	3253	784	27	0.184	B	B	23.5
36	3.33	920	690	100	3624	791	26	0.178	B	B	22.3
37	4.59	920	720	30	3010	812	30	0.215	B	B	19.8
38	3.75	950	710	30	1793	811	29	0.181	B	B	21.3
39	2.08	900	665	30	-116	804	16	0.171	B	B	20.3
40	1.42	900	710	30	419	819	17	0.111	B	B	20.4

Blooms were produced by a continuous casting process using molten steels. Thereafter, each bloom was subjected to blooming to produce a round billet with a diameter of 310 mm. The heating temperature before blooming was 1100 to 1350°C.
The round billets produced by the blooming were subjected to hot working. Specifically, the round billets were charged into a heating furnace, and heated at 1100 to 1350°C. After being taken out from the heating furnace, the round billets were subjected to hot rolling (hot working) by the Mannesmann process to thereby produce hollow shells (seamless steel pipes) of each test number. At such time, the piercing ratio was within the range of 1.0 to 4.0, and the cumulative reduction of area in the hot working was within the range of 20 to 70%.
Each hollow shell after the hot working was subjected to quenching. The quenching temperature (°C) in the quenching is shown in the column "Quenching temperature (°C)" of the "Quenching condition" column in Table 2. The holding time at the quenching temperature was set to 15 minutes. The hollow shell after quenching was subjected to tempering. The tempering temperature T (°C) in the tempering is shown in the column "Tempering temperature T (°C)" of the "Tempering conditions" column in Table 2. The holding time t (min) at the tempering temperature T is shown in the column "Holding time t (min)" of the "Tempering conditions" column in Table 2. Further, FAin the tempering is shown in the column "FA" in Table 2. Steel materials (seamless steel pipes) were produced by the above production process.

[Evaluation Tests]

The steel material (seamless steel pipe) of each test number was subjected to the following evaluation tests.

(Test 1) Microstructure observation test
(Test 2) Test to measure average equivalent circular diameter D of prior-austenite grains
(Test 3) Yield strength evaluation test
(Test 4) Normal-temperature SSC resistance evaluation test and low-temperature SSC resistance evaluation test
(Test 5) DCB test

Each test is described hereunder.

[(Test 1) Microstructure observation test]

The total area fraction (%) of tempered martensite and tempered bainite of the steel material of each test number was determined by the following method. A test specimen having an observation surface with dimensions of 10 mm in the pipe axis direction and 10 mm in the pipe radius direction was taken from a central portion of the wall thickness of the steel material (seamless steel pipe) of each test number. Note that, in a case where the steel material was a steel pipe whose wall thickness was less than 10 mm, a test specimen having an observation surface with dimensions of 10 mm in the pipe axis direction and the wall thickness of the steel pipe in the pipe radius direction was taken.
Using the taken test specimen, the total area fraction (%) of tempered martensite and tempered bainite was determined by the method described above in the section [Microstructure observation method]. As a result, it was determined that the total area fraction of tempered martensite and tempered bainite in each test number was 90% or more.

[(Test 2) Test to measure average equivalent circular diameter D of prior-austenite grains]

The average equivalent circular diameter D (µm) of prior-austenite grains of the steel material of each test number was determined by the following method. A test specimen having an observation surface with dimensions of 10 mm in the pipe axis direction and 10 mm in the pipe radius direction was taken from a central portion of the wall thickness of the steel material (seamless steel pipe) of each test number. Note that, in a case where the steel material was a steel pipe whose wall thickness was less than 10 mm, a test specimen having an observation surface with dimensions of 10 mm in the pipe axis direction and the wall thickness of the steel pipe in the pipe radius direction was taken.
Using the taken test specimen, the average equivalent circular diameter D (µm) of prior-austenite grains was determined by the method described above in the section [Method for determining average equivalent circular diameter D of prior-austenite grains]. The determined average equivalent circular diameter D of prior-austenite grains is shown in the column "D (µm)" in Table 2.

[(Test 3) Yield strength evaluation test]

The yield strength (MPa) of the steel material of each test number was determined by the following method. A round bar tensile test specimen was taken from a central portion of the wall thickness of the steel material (seamless steel pipe) of each test number. Regarding the size of the round bar tensile test specimen, the diameter of the parallel portion was 6.0 mm and the gage length was 30.0 mm. The longitudinal direction of the round bar tensile test specimen was parallel to the pipe axis direction of the steel material (seamless steel pipe).
The taken round bar tensile test specimen was used to determine the yield strength (MPa) by the method described above in the section [Method for measuring yield strength]. The determined yield strength is shown in the column "YS (MPa)" in Table 2.

[(Test 4) Normal-temperature SSC resistance evaluation test and low-temperature SSC resistance evaluation test]

The steel material of each test number was subjected to an SSC resistance evaluation test in accordance with NACE TM0177-2016 Method A by the following method.
The SSC resistance at 24°C was evaluated by the method described above in the section [Normal-temperature SSC resistance evaluation test]. Note that, round bar specimens were taken from the central portion of the wall thickness of the steel material (seamless steel pipe) of each test number. Regarding the size of the round bar specimen, the diameter was 6.35 mm and the length of the parallel portion was 25.4 mm. The longitudinal direction of the round bar specimen was parallel to the pipe axis direction of the steel material (seamless steel pipe). H₂S gas at 1 atm was blown into the test bath.
In addition, the SSC resistance at 4°C was evaluated by the method described above in the section [Low-temperature SSC resistance evaluation test]. Note that, round bar specimens were taken from the central portion of the wall thickness of the steel material (seamless steel pipe) of each test number. Regarding the size of the round bar specimen, the diameter was 6.35 mm and the length of the parallel portion was 25.4 mm. The longitudinal direction of the round bar specimen was parallel to the pipe axis direction of the steel material (seamless steel pipe). H₂S gas at 1 atm was blown into the test bath.
If the result of the normal-temperature SSC resistance evaluation test was that a crack was not confirmed in any of the three round bar specimens, it was determined that excellent normal-temperature SSC resistance was obtained (indicated by "E (Excellent)" in the column "24°C" of the column "SSC resistance" in Table 2). On the other hand, if a crack was confirmed in one or more of the three round bar specimens, it was determined that excellent normal-temperature SSC resistance was not obtained (indicated by "B (Bad)" in the column "24°C" of the column "SSC resistance" in Table 2).
Further, if the result of the low-temperature SSC resistance evaluation test was that a crack was not confirmed in any of the three round bar specimens, it was determined that excellent low-temperature SSC resistance was obtained (indicated by "E (Excellent)" in the column "4°C" of the column "SSC resistance" in Table 2). On the other hand, if a crack was confirmed in one or more of the three round bar specimens, it was determined that excellent low-temperature SSC resistance was not obtained (indicated by "B (Bad)" in the column "4°C" of the column "SSC resistance" in Table 2).

[(Test 5) DCB test]

The fracture toughness value Kissc (MPa√m) of the steel material of each test number was determined by the method described above in the section [DCB test]. The obtained fracture toughness value Kissc (MPa√m) is shown in the column "Kissc (MPa√m)" in Table 2.
Note that, a DCB test specimen illustrated in FIG. 3A was taken from the central portion of the wall thickness of the steel material (seamless steel pipe) of each test number. The longitudinal direction of the DCB test specimen was parallel to the pipe axis direction of the steel material (seamless steel pipe). In addition, a wedge illustrated in FIG. 3B was taken from the steel material. A thickness t of the wedge was 3.10 mm. H₂S gas at 5 atm (0.5 MPa) was blown into the test bath.

[Test results]

Referring to Table 1-1, Table 1-2, and Table 2, in each of Test Nos. 1 to 14 and 16 to 29, the chemical composition of the steel material (seamless steel pipe) was appropriate, and the production method was also appropriate. Therefore, the steel materials of these test numbers satisfied Feature 1 to Feature 3. As a result, excellent normal-temperature SSC resistance and excellent low-temperature SSC resistance were obtained, and the fracture toughness value Kissc was 25.0 MPa√m or more. Even though the steel materials of these test numbers had a high strength of 110 ksi grade (758 to less than 862 MPa), excellent SSC resistance was obtained.
On the other hand, in Test No. 15, although the chemical composition was appropriate, the quenching temperature was too high. Therefore, FN was too low. As a result, excellent normal-temperature SSC resistance and excellent low-temperature SSC resistance were not obtained, and the fracture toughness value Kissc was less than 25.0 MPa√m.
In Test No. 30, although the chemical composition was appropriate, EE was too low. Further, in Test Nos. 31 and 32, although the chemical composition was appropriate, EE was too low and FN was also too low. In Test No. 33, although the chemical composition was appropriate, EE was too low and FA was too high. As a result, FN was too low. Therefore, in these test numbers, excellent normal-temperature SSC resistance and excellent low-temperature SSC resistance were not obtained, and the fracture toughness value Kissc was less than 25.0 MPa√m.
In Test Nos. 34 to 36, although the chemical composition was appropriate, FA that was a production condition was too high. Therefore, FN was too low. Consequently, excellent normal-temperature SSC resistance and excellent low-temperature SSC resistance were not obtained, and the fracture toughness value Kissc was less than 25.0 MPa√m.
In Test Nos. 37 and 38, the content of Si was too high. Consequently, excellent normal-temperature SSC resistance and excellent low-temperature SSC resistance were not obtained, and the fracture toughness value Kissc was less than 25.0 MPa√m.
In Test No. 39, the content of Si was too low. In addition, because the content of Si was too low, EE and FN were too low. Consequently, excellent normal-temperature SSC resistance and excellent low-temperature SSC resistance were not obtained, and the fracture toughness value Kissc was less than 25.0 MPa√m.
In Test No. 40, the content of Mn was too high. In addition, because the content of Mn was too high, EE and FN were too low. Consequently, excellent normal-temperature SSC resistance and excellent low-temperature SSC resistance were not obtained, and the fracture toughness value Kissc was less than 25.0 MPa√m.

EXAMPLE 2

In Example 2, the SSC resistance of steel materials having a yield strength of 125 ksi grade (862 to less than 965 MPa) was investigated.
Steel materials (seamless steel pipes) having the chemical compositions shown in Table 1-1 and Table 1-2 were produced. Specifically, blooms were produced by a continuous casting process using molten steels. Thereafter, each bloom was subjected to blooming to produce a round billet with a diameter of 310 mm. The heating temperature before blooming was 1100 to 1350°C.
The round billets produced by the blooming were subjected to hot working. Specifically, the round billets were charged into a heating furnace, and heated at 1100 to 1350°C. After being taken out from the heating furnace, the round billets were subjected to hot rolling (hot working) by the Mannesmann process to thereby produce hollow shells (seamless steel pipes) of each test number. At such time, the piercing ratio was within the range of 1.0 to 4.0, and the cumulative reduction of area in the hot working was within the range of 20 to 70%.
Each hollow shell after the hot working was subjected to quenching. The quenching temperature (°C) in the quenching is shown in the column "Quenching temperature (°C)" of the "Quenching condition" column in Table 3. The holding time at the quenching temperature was set to 15 minutes. The hollow shell after quenching was subjected to tempering. The tempering temperature T (°C) in the tempering is shown in the column "Tempering temperature T (°C)" of the "Tempering conditions" column in Table 3. The holding time t (min) at the tempering temperature T is shown in the column "Holding time t (min)" of the "Tempering conditions" column in Table 3. FA in the tempering is shown in the column "FA" in Table 3. Steel materials (seamless steel pipes) were produced by the above production process. Note that, EE of each test number is shown in the column "EE" in Table 3.

[Table 3]

TABLE 3

Test number	EE	Quenching condition	Tempering conditions		FA	YS (MPa)	D (µm)	FN	SSC resistance		Kissc (MPaVm)
Test number	EE	Quenching temperature (°C)	Tempering temperature T (°C)	Holding time t (min)	FA	YS (MPa)	D (µm)	FN	24°C	4°C	Kissc (MPaVm)
1	3.72	920	695	30	815	952	17	0.290	E	E	26.2
2	4.54	950	680	30	845	940	32	0.201	E	E	25.5
3	3.10	920	680	60	542	905	17	0.242	E	E	26.4
4	3.40	950	695	60	2020	910	23	0.202	E	E	27.7
5	3.99	950	690	60	2242	889	25	0.220	E	E	26.3
6	3.28	920	690	60	2009	910	21	0.212	E	E	26.0
7	3.39	900	690	60	1743	869	20	0.229	E	E	28.6
8	3.27	900	680	60	556	905	17	0.255	E	E	26.4
9	3.35	900	695	60	1434	879	20	0.226	E	E	24.2
10	4.20	920	695	60	2344	889	23	0.250	E	E	28.0
11	4.05	950	685	60	2299	900	24	0.232	E	E	27.3
12	3.82	920	700	60	2017	894	18	0.284	E	E	30.0
13	2.85	900	690	60	976	871	18	0.211	B	B	22.5
14	2.85	920	695	60	1136	917	20	0.192	B	B	22.2
15	4.51	990	685	30	1941	958	43	0.153	B	B	21.1
16	4.02	920	685	60	1920	883	23	0.239	E	E	25.5
17	4.34	900	695	60	2327	869	21	0.281	E	E	26.9
18	4.03	950	690	60	1305	873	23	0.240	E	E	25.9
19	4.01	920	680	60	1355	883	22	0.248	E	E	26.4
20	3.88	920	690	60	1291	902	21	0.250	E	E	28.8
21	3.56	900	700	60	1798	866	21	0.230	E	E	29.7
22	4.26	890	695	60	2391	862	22	0.264	E	E	30.2
23	3.67	920	690	60	1595	903	21	0.237	E	E	28.6
24	4.29	920	690	60	2199	879	20	0.290	E	E	24.4
25	406	900	690	60	1768	891	18	0.301	E	E	30.9
26	406	950	705	60	2090	864	24	0.232	E	E	25.0
27	5.51	880	690	100	2221	887	17	0.430	E	E	31.7
28	3.36	920	680	30	2358	890	23	0.200	E	E	24.2
29	2.77	850	675	30	861	932	19	0.195	B	B	21.8
30	2.72	920	685	100	661	882	19	0.192	B	B	22.7
31	1.65	900	660	100	1812	920	18	0.123	B	B	20.9
32	0.61	850	670	100	1012	896	18	0.045	B	B	20.6
33	2.61	920	680	100	2896	877	24	0.150	B	B	20.1
34	3.20	950	680	100	4155	920	27	0.165	B	B	19.8
35	3.58	950	680	100	3206	928	27	0.184	B	B	21.1
36	3.33	920	685	100	3597	907	26	0.178	B	B	21.0
37	4.59	920	695	30	2905	885	30	0.215	B	B	19.0
38	3.75	950	690	30	1743	880	29	0.181	B	B	20.0
39	208	900	695	30	-121	877	16	0.171	B	B	19.5
40	1.42	900	690	30	407	929	17	0.111	B	B	19.2

[Evaluation Tests]

Similarly to Example 1, the steel material (seamless steel pipe) of each test number was subjected to the following evaluation tests.

[(Test 1) Microstructure observation test]

Using the same method as the method used for the microstructure observation test of Example 1, the total area fraction (%) of tempered martensite and tempered bainite of the steel material of each test number was determined. As a result, it was determined that the total area fraction of tempered martensite and tempered bainite in each test number was 90% or more.

Using the same method as the method used for the test to measure the average equivalent circular diameter D of prior-austenite grains of Example 1, the average equivalent circular diameter D (µm) of prior-austenite grains of the steel material of each test number was determined. The obtained average equivalent circular diameter D of prior-austenite grains is shown in the column "D (µm)" in Table 3.

[(Test 3) Yield strength evaluation test]

Using the same method as the method used for the yield strength evaluation test of Example 1, the yield strength (MPa) of the steel material of each test number was determined. The obtained yield strength is shown in the column "YS (MPa)" in Table 3.

The steel material of each test number was subjected to an SSC resistance evaluation test in accordance with NACE TM0177-2016 Method A by the following method.
The SSC resistance at 24°C was evaluated by the method described above in the section [Normal-temperature SSC resistance evaluation test]. Note that, round bar specimens were taken from the central portion of the wall thickness of the steel material (seamless steel pipe) of each test number. Regarding the size of the round bar specimen, the diameter was 6.35 mm and the length of the parallel portion was 25.4 mm. The longitudinal direction of the round bar specimen was parallel to the pipe axis direction of the steel material (seamless steel pipe). H₂S gas at 1 atm was blown into the test bath.
In addition, the SSC resistance at 4°C was evaluated by the method described above in the section [Low-temperature SSC resistance evaluation test]. Note that, round bar specimens were taken from the central portion of the wall thickness of the steel material (seamless steel pipe) of each test number. Regarding the size of the round bar specimen, the diameter was 6.35 mm and the length of the parallel portion was 25.4 mm. The longitudinal direction of the round bar specimen was parallel to the pipe axis direction of the steel material (seamless steel pipe). H₂S gas at 1 atm was blown into the test bath.
If the result of the normal-temperature SSC resistance evaluation test was that a crack was not confirmed in any of the three round bar specimens, it was determined that excellent normal-temperature SSC resistance was obtained (indicated by "E (Excellent)" in the column "24°C" of the column "SSC resistance" in Table 3). On the other hand, if a crack was confirmed in one or more of the three round bar specimens, it was determined that excellent normal-temperature SSC resistance was not obtained (indicated by "B (Bad)" in the column "24°C" of the column "SSC resistance" in Table 3).
Further, if the result of the low-temperature SSC resistance evaluation test was that a crack was not confirmed in any of the three round bar specimens, it was determined that excellent low-temperature SSC resistance was obtained (indicated by "E (Excellent)" in the column "4°C" of the column "SSC resistance" in Table 3). On the other hand, if a crack was confirmed in one or more of the three round bar specimens, it was determined that excellent low-temperature SSC resistance was not obtained (indicated by "B (Bad)" in the column "4°C" of the column "SSC resistance" in Table 3).

[(Test 5) DCB test]

The fracture toughness value Kissc (MPa^m) of the steel material of each test number was determined by the method described above in the section [DCB test]. The obtained fracture toughness value Kissc (MPa√m) is shown in the column "Kissc (MPa√m)" in Table 3.
Note that, a DCB test specimen illustrated in FIG. 3A was taken from the central portion of the wall thickness of the steel material (seamless steel pipe) of each test number. The longitudinal direction of the DCB test specimen was parallel to the pipe axis direction of the steel material (seamless steel pipe). In addition, a wedge illustrated in FIG. 3B was taken from the steel material. A thickness t of the wedge was 3.10 mm. H₂S gas at 5 atm (0.5 MPa) was blown into the test bath.

[Test results]

Referring to Table 1-1, Table 1-2, and Table 3, in each of Test Nos. 1 to 12 and 16 to 28, the chemical composition of the steel material (seamless steel pipe) was appropriate, and the production method was also appropriate. Therefore, the steel materials of these test numbers satisfied Feature 1 to Feature 3. As a result, excellent normal-temperature SSC resistance and excellent low-temperature SSC resistance were obtained, and the fracture toughness value Kissc was 24.0 MPa√m or more. Even though the steel materials of these test numbers had a high strength of 125 ksi grade (862 to less than 965 MPa), excellent SSC resistance was obtained.
On the other hand, in Test No. 13, although the chemical composition was appropriate, EE was too low. Further, in Test Nos. 14 and 29 to 32, although the chemical composition was appropriate, EE was too low and FN was also too low. In Test No. 33, although the chemical composition was appropriate, EE was too low and FA was too high. As a result, FN was too low. Therefore, in these test numbers, excellent normal-temperature SSC resistance and excellent low-temperature SSC resistance were not obtained, and the fracture toughness value Kissc was less than 24.0 MPa√m.
In Test No. 15, although the chemical composition was appropriate, the quenching temperature was too high. Therefore, FN was too low. Consequently, excellent normal-temperature SSC resistance and excellent low-temperature SSC resistance were not obtained, and the fracture toughness value Kissc was less than 24.0 MPa^m.
In Test Nos. 34 to 36, although the chemical composition was appropriate, FA that was a production condition was too high. Therefore, FN was too low. Consequently, excellent normal-temperature SSC resistance and excellent low-temperature SSC resistance were not obtained, and the fracture toughness value Kissc was less than 24.0 MPa√m.
In Test Nos. 37 and 38, the content of Si was too high. Consequently, excellent normal-temperature SSC resistance and excellent low-temperature SSC resistance were not obtained, and the fracture toughness value Kissc was less than 24.0 MPa^m.
In Test No. 39, the content of Si was too low. In addition, because the content of Si was too low, EE and FN were too low. Consequently, excellent normal-temperature SSC resistance and excellent low-temperature SSC resistance were not obtained, and the fracture toughness value Kissc was less than 24.0 MPa√m.
In Test No. 40, the content of Mn was too high. In addition, because the content of Mn was too high, EE and FN were too low. Consequently, excellent normal-temperature SSC resistance and excellent low-temperature SSC resistance were not obtained, and the fracture toughness value Kissc was less than 24.0 MPa√m.
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 that does not depart from the gist of the present disclosure.

Claims

A steel material having a chemical composition consisting of, in mass%,
C: 0.20 to 0.35%,

Si: 0.60 to 1.30%,

Mn: 0.05 to 0.25%,

P: 0.050% or less,

S: 0.0100% or less,

Al: 0.010 to 0.100%,

N: 0.0100% or less,

Cr: 0.20 to 1.00%,

Mo: 0.10 to 1.00%,

Ti: 0.003 to 0.030%,

O: 0.0050% or less,

Zr: 0 to 0.0040%,

Sb: 0 to 0.50%,

Cu: 0 to 0.50%,

Ni: 0 to 0.50%,

Co: 0 to 0.50%,

Ca: 0 to 0.0040%,

Mg: 0 to 0.0040%,

rare earth metal: 0 to 0.0040%,

Nb: 0 to 0.150%,

V: 0 to 0.500%, and

B: 0 to 0.0030%,

with the balance being Fe and impurities,

wherein:
a yield strength is 758 to less than 965 MPa,

in a case where the yield strength is 758 to less than 862 MPa, EE defined by Formula (1) is 2.75 or more, and FN defined by Formula (2) is 0.185 or more, and

in a case where the yield strength is 862 to less than 965 MPa, the EE is 3.00 or more and the FN is 0.200 or more; EE = -0.25C + 2Si - 5.8Mn + 2.1Cr + Mo + 4.1Zr + 2.6Sb + 0.3Cu + 0.4Ni + 1.5Co $FN = EE / (D^{0.9})$

where, a content in percent by mass of a corresponding element is substituted for each symbol of an element in Formula (1); and an average equivalent circular diameter in units of µm of prior-austenite grains in the steel material is substituted for D in Formula (2).
The steel material according to claim 1, wherein the chemical composition contains one or more kinds of element selected from a group consisting of:
Zr: 0.0001 to 0.0040%,

Sb: 0.01 to 0.50%,

Cu: 0.01 to 0.50%,

Ni: 0.01 to 0.50%,

Co: 0.01 to 0.50%,

Ca: 0.0001 to 0.0040%,

Mg: 0.0001 to 0.0040%,

rare earth metal: 0.0001 to 0.0040%,

Nb: 0.001 to 0.150%,

V: 0.001 to 0.500%, and

B: 0.0001 to 0.0030%.
The steel material according to claim 1 or claim 2, wherein:
the steel material is an oil-well steel pipe.