EP4481062A1

EP4481062A1 - Steel material suitable for use in sour environments

Info

Publication number: EP4481062A1
Application number: EP23756422.4A
Authority: EP
Inventors: Hiroki KAMITANI; Yuji Arai; Jin KAWASAKI; Keiichi Kondo
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 2022-02-17
Filing date: 2023-02-16
Publication date: 2024-12-25
Also published as: WO2023157897A1; JPWO2023157897A1; MX2024009724A; JP7406177B1; EP4481062A4; US20250066891A1

Abstract

A steel material that has high strength and has excellent SSC resistance in a sour environment is provided. A steel material according to the present disclosure consists of, in mass%, C: 0.15 to 0.45%, Si: 0.05 to 1.00%, Mn: 0.05 to 0.30%, P: 0.030% or less, S: 0.0050% or less, Al: 0.005 to 0.100%, Cr: 0.30 to 1.10%, Mo: 0.40 to 2.00%, Ti: 0.002 to 0.020%, Nb: 0.002 to 0.100%, B: 0.0005 to 0.0040%, N: 0.0100% or less, and O: less than 0.0040%, with the balance being Fe and impurities, and has a yield strength of 862 MPa or more. In the steel material, a number density of Si oxides in which, in mass%, a content of Si is 20% or more and a content of O is 10% or more, and which have a major axis of 5.0 µm or more is 5 /100 mm² or less, and in a case where the yield strength is 931 MPa or more, the number density of the Si oxides is 5 /200 mm² or less.

Description

TECHNICAL FIELD

The present disclosure relates to a steel material, and more particularly relates to a steel material 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 enhance the strength of steel materials for oil wells typified by oil-well steel pipes. Specifically, oil-well steel pipes 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 in widespread use, and recently requests are also starting to be made for oil-well steel pipes of 110 ksi grade or more (yield strength is 758 MPa or more).
Furthermore, most deep wells are sour environments that contain 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. Oil-well steel pipes for use in such sour environments are required to have not only high strength, but to also have sulfide stress cracking resistance (hereunder, referred to as "SSC resistance"). Thus, steel materials having high strength and excellent SSC resistance have begun to be demanded.
Techniques for increasing the SSC resistance of steel materials as typified by oil-well steel pipes are proposed in Japanese Patent Application Publication No. 2000-297344 (Patent Literature 1), Japanese Patent Application Publication No. 2001-271134 (Patent Literature 2), and International Application Publication No. WO2008/123422 (Patent Literature 3).
Patent Literature 1 discloses a steel for oil wells 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 for oil wells, 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 taken as t (mm), the proportion of M₂₃C₆-type carbides is (200/t) or less in percent by mass. It is described in Patent Literature 1 that this steel for oil wells is excellent in SSC resistance.
Patent Literature 2 discloses a low-alloy steel material that consists of, in mass%, C: 0.2 to 0.35%, Si: 0.05 to 0.5%, Mn: 0.1 to 1%, P: 0.025% or less, S: 0.01% or less, Cr: 0.1 to 1.2%, Mo: 0.1 to 1%, B: 0.0001 to 0.005%, Al: 0.005 to 0.1%, N: 0.01% or less, V: 0.05 to 0.5%, Ni: 0.1% or less, W: 1.0% or less, and O: 0.01% or less, with the balance being Fe and impurities, and satisfies the formula (0.03≤Mo×V≤0.3) and the formula (0.5×Mo-V+GS/10≥1) and has a yield strength of 1060 MPa or more. Note that, "GS" in the formula represents the ASTM grain size number of prior-austenite grains. It is described in Patent Literature 2 that this low-alloy steel material is excellent in SSC resistance.
Patent Literature 3 discloses a low-alloy steel consisting of, 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. Among the impurities, the low-alloy steel contains P: 0.025% or less, S: 0.010% or less, N: 0.007% or less, and B: less than 0.0003%. In the low-alloy steel, the number density of M₂₃C₆-type precipitates having a grain size of 1 µm or more is 0.1 /mm² or less. It is described in Patent Literature 3 that in this low-alloy steel, the SSC resistance is enhanced.

CITATION LIST

PATENT LITERATURE

Patent Literature 1: Japanese Patent Application Publication No. 2000-297344
Patent Literature 2: Japanese Patent Application Publication No. 2001-271134
Patent Literature 3: International Application Publication No. WO2008/123422

SUMMARY OF INVENTION

TECHNICAL PROBLEM

As described above, in recent years, accompanying the increasing severity of oil well environments, there is a demand for steel materials that have excellent SSC resistance. Specifically, there is a demand for steel materials that achieve both high strength and excellent SSC resistance. Therefore, a steel material (for example, an oil-well steel material) that achieves both high strength and excellent SSC resistance may be obtained by a technique other than the techniques disclosed in the aforementioned Patent Literatures 1 to 3.
An objective of the present disclosure is to provide a steel material that has high strength and has excellent SSC resistance in a sour environment.

SOLUTION TO PROBLEM

A steel material according to the present disclosure consists of, in mass%,

C: 0.15 to 0.45%,
Si: 0.05 to 1.00%,
Mn: 0.05 to 0.30%,
P: 0.030% or less,
S: 0.0050% or less,
Al: 0.005 to 0.100%,
Cr: 0.30 to 1.10%,
Mo: 0.40 to 2.00%,
Ti: 0.002 to 0.020%,
Nb: 0.002 to 0.100%,
B: 0.0005 to 0.0040%,
N: 0.0100% or less,
O: less than 0.0040%,
V: 0 to 0.30%,
Cu: 0 to 0.50%,
Ni: 0 to 0.50%,
W: 0 to 1.50%,
Ca: 0 to 0.0100%,
Mg: 0 to 0.0100%,
Zr: 0 to 0.0100%, and
rare earth metal: 0 to 0.0100%,
with the balance being Fe and impurities,
wherein:
- a yield strength is 862 MPa or more, and
- in the steel material,
- a number density of Si oxides in which, in mass%, a content of Si is 20% or more and a content of O is 10% or more, and which have a major axis of 5.0 µm or more is 5 /100 mm² or less, and
- in a case where the yield strength is 931 MPa or more, a number density of the Si oxides is 5 /200 mm² or less.

ADVANTAGEOUS EFFECTS 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 view illustrating the relation between the number density (/100 mm²) of coarse Si oxides having a major axis of 5.0 µm or more and the number of specimens in which SSC occurred (specimens) in an SSC resistance test with respect to steel materials having a yield strength of 862 to less than 931 MPa among the present examples.
[FIG. 2] FIG. 2 is a view illustrating the relation between the number density (/200 mm²) of coarse Si oxides having a major axis of 5.0 µm or more and the number of specimens in which SSC occurred (specimens) in an SSC resistance test with respect to steel materials having a yield strength of 931 MPa or more among the present examples.

DESCRIPTION OF EMBODIMENTS

First, the present inventors conducted studies regarding obtaining a steel material having a yield strength of 125 ksi (862 MPa) or more as high strength. That is, the present inventors conducted investigations and studies regarding a method for obtaining a yield strength of 125 ksi or more and excellent SSC resistance in a sour environment in a steel material for which use in a sour environment is assumed. As a result, the present inventors obtained the following findings.
First, focusing on the chemical composition, the present inventors conducted studies regarding obtaining a steel material having a yield strength of 125 ksi or more and excellent SSC resistance in a sour environment. As a result, the present inventors considered that by decreasing the content of manganese (Mn) to 0.30% or less, there is a possibility of increasing the SSC resistance of a steel material while maintaining the strength of the steel material. Mn combines with sulfur (S) in a steel material to form Mn sulfides. Mn sulfides are easily elongated by rolling, and tend to become inclusions that have a long major axis. Furthermore, Mn sulfides that have a long major axis are liable to become starting points of fractures in a sour environment. Therefore, there is a possibility that, by reducing the content of Mn to 0.30% or less, formation of Mn sulfides will be suppressed and the SSC resistance of the steel material will be increased.
That is, the present inventors considered that if a steel material consists of, in mass%, C: 0.15 to 0.45%, Si: 0.05 to 1.00%, Mn: 0.05 to 0.30%, P: 0.030% or less, S: 0.0050% or less, Al: 0.005 to 0.100%, Cr: 0.30 to 1.10%, Mo: 0.40 to 2.00%, Ti: 0.002 to 0.020%, Nb: 0.002 to 0.100%, B: 0.0005 to 0.0040%, N: 0.0100% or less, O: less than 0.0040%, V: 0 to 0.30%, Cu: 0 to 0.50%, Ni: 0 to 0.50%, W: 0 to 1.50%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, and rare earth metal: 0 to 0.0100%, with the balance being Fe and impurities, there is a possibility that the steel material will have a yield strength of 125 ksi or more and, furthermore, that excellent SSC resistance will be obtained in a sour environment.
On the other hand, even in the case of steel materials having the chemical composition described above, when the steel materials had a yield strength of 125 ksi or more, in some cases excellent SSC resistance was not obtained in a sour environment. Therefore, the present inventors conducted detailed studies regarding the cause of such a decrease in SSC resistance with respect to steel materials having the chemical composition described above and a yield strength of 125 ksi or more. As a result, it was revealed that, in the case of a steel material having the chemical composition described above, there is a concern that coarse Si oxides will be included in the steel material. If coarse Si oxides are included in the steel material, there is a possibility that the SSC resistance of the steel material will decrease.
In the present description, Si oxides in which, in mass%, the content of Si is 20% or more and the content of O is 10% or more, and which have a major axis of 5.0 µm or more are also referred to as "coarse Si oxides". The present inventors also conducted detailed studies regarding the cause of a decrease in SSC resistance for each yield strength with respect to steel materials having the chemical composition described above. As a result, the present inventors obtained the following findings.
Specifically, it was revealed that in a steel material having the chemical composition described above, in a case where the yield strength is less than 135 ksi (less than 931 MPa), by making the number density of coarse Si oxides 5 /100 mm² or less, a high yield strength of 125 ksi or more and excellent SSC resistance can both be achieved. This point will be described specifically using the drawings.
FIG. 1 is a view illustrating the relation between the number density (/100 mm²) of coarse Si oxides (Si oxides having a major axis of 5.0 µm or more) and the number of specimens in which SSC occurred (specimens) in an SSC resistance test with respect to steel materials having a yield strength of 125 to less than 135 ksi among the present examples. FIG. 1 was prepared using a number density of coarse Si oxides determined by a method to be described later, and the number of specimens in which SSC occurred (specimens) as the result of an SSC resistance test conducted by a method to be described later with respect to steel materials which, among examples to be described later, had the chemical composition described above and had a yield strength of 125 to less than 135 ksi.
Referring to FIG. 1, it can be confirmed that in steel materials having the chemical composition described above and a yield strength of 125 to less than 135 ksi, when the number density of coarse Si oxides was 5 /100 mm² or less, SSC did not occur in an SSC resistance test and excellent SSC resistance was obtained. Therefore, in the present embodiment, a steel material has the chemical composition described above, has a yield strength of 125 to less than 135 ksi, and the number density of coarse Si oxides in the steel material is made 5 /100 mm² or less. As a result, a yield strength of 125 ksi or more and excellent SSC resistance can both be achieved.
On the other hand, in the case of steel materials whose yield strength was 135 ksi or more (931 MPa or more) among the steel materials having a yield strength of 125 ksi or more, even when the number density of coarse Si oxides was 5 /100 mm² or less, in some cases excellent SSC resistance was not obtained in a sour environment. As the result of further detailed studies conducted by the present inventors it was revealed that in the steel materials having a yield strength of 135 ksi or more, a high yield strength and excellent SSC resistance can both be achieved by further reducing the number density of coarse Si oxides to make the number density of coarse Si oxides 5 /200 mm² or less. This point will be described specifically using the drawings.
FIG. 2 is a view illustrating the relation between the number density (/200 mm²) of coarse Si oxides (Si oxides having a major axis of 5.0 µm or more) and the number of specimens in which SSC occurred (specimens) in an SSC resistance test with respect to steel materials having a yield strength of 135 ksi or more among the present examples. FIG. 2 was prepared using a number density of coarse Si oxides determined by a method to be described later, and the number of specimens in which SSC occurred (specimens) as the result of an SSC resistance test conducted by a method to be described later with respect to steel materials which, among examples to be described later, had the chemical composition described above and had a yield strength of 135 ksi or more.
Referring to FIG. 2, it can be confirmed that in steel materials having the chemical composition described above and a yield strength of 135 ksi or more, when the number density of coarse Si oxides was 5 /200 mm² or less, SSC did not occur in an SSC resistance test and excellent SSC resistance was obtained. Therefore, in the present embodiment, a steel material has the chemical composition described above, has a yield strength of 135 ksi or more, and the number density of coarse Si oxides in the steel material is made 5 /200 mm² or less. As a result, a yield strength of 135 ksi or more and excellent SSC resistance can both be achieved.
The reason why the SSC resistance of a steel material is increased by lowering the number density of coarse Si oxides has not been clarified in detail. However, the present inventors surmise that the reason is as follows. When producing a steel material having the chemical composition described above, during the steelmaking process, deoxidation is performed mainly by aluminum (Al). Therefore, with regard to steel materials which have the chemical composition described above, although consideration has been given to Al oxides that are typified by Al₂O₃, attention has not been focused on Si oxides. However, there is a possibility that Si oxides, which are small in number, and in particular coarse Si oxides whose major axis is 5.0 µm or more, are more likely to decrease the SSC resistance of the steel material than Al oxides. Therefore, the present inventors surmise that by reducing the number density of coarse Si oxides to 5 /100 mm² or less, and furthermore to 5 /200 mm² or less, the SSC resistance of the steel material can be increased.
Note that, there is also a possibility that the SSC resistance of the steel material is increased by a mechanism which is different from the mechanism surmised by the present inventors. The fact that in a steel material having the chemical composition described above, as a result of the number density of coarse Si oxides being 5 /100 mm² or less, both a yield strength of 125 to less than 135 ksi and excellent SSC resistance can be achieved has been demonstrated by examples that are described later. In addition, the fact that in a steel material having the chemical composition described above, as a result of the number density of coarse Si oxides being 5 /200 mm² or less, both a yield strength of 135 ksi or more and excellent SSC resistance can be achieved has been demonstrated by examples that are described later. Therefore, a steel material according to the present embodiment has the chemical composition described above, has a yield strength of 862 MPa or more, and the number density of coarse Si oxides in the steel material is 5 /100 mm² or less, and in a case where the yield strength is 931 MPa or more, the number density of coarse Si oxides in the steel material is 5 /200 mm² or less. As a result, the steel material according to the present embodiment can achieve both a yield strength of 125 ksi or more and excellent SSC resistance.
The gist of the steel material according to the present embodiment, which has been completed based on the findings described above, is as follows.

[1] A steel material consisting of, in mass%,
- C: 0.15 to 0.45%,
- Si: 0.05 to 1.00%,
- Mn: 0.05 to 0.30%,
- P: 0.030% or less,
- S: 0.0050% or less,
- Al: 0.005 to 0.100%,
- Cr: 0.30 to 1.10%,
- Mo: 0.40 to 2.00%,
- Ti: 0.002 to 0.020%,
- Nb: 0.002 to 0.100%,
- B: 0.0005 to 0.0040%,
- N: 0.0100% or less,
- O: less than 0.0040%,
- V: 0 to 0.30%,
- Cu: 0 to 0.50%,
- Ni: 0 to 0.50%,
- W: 0 to 1.50%,
- Ca: 0 to 0.0100%,
- Mg: 0 to 0.0100%,
- Zr: 0 to 0.0100%, and
- rare earth metal: 0 to 0.0100%,
- with the balance being Fe and impurities,
- wherein:
  - a yield strength is 862 MPa or more, and
  - in the steel material,
  - a number density of Si oxides in which, in mass%, a content of Si is 20% or more and a content of O is 10% or more, and which have a major axis of 5.0 µm or more is 5 /100 mm² or less, and
  - in a case where the yield strength is 931 MPa or more, a number density of the Si oxides is 5 /200 mm² or less.
[2] The steel material according to [1], containing one or more elements selected from a group consisting of:
- V: 0.01 to 0.30%,
- Cu: 0.01 to 0.50%,
- Ni: 0.01 to 0.50%,
- W: 0.01 to 1.50%,
- Ca: 0.0001 to 0.0100%,
- Mg: 0.0001 to 0.0100%,
- Zr: 0.0001 to 0.0100%, and
- rare earth metal: 0.0001 to 0.0100%.
[3] The steel material according to [1] or [2], wherein:
the steel material is an oil-well steel pipe.

In the present description, the oil-well steel pipe may be a steel pipe used for oil country tubular goods. The oil-well steel pipe may be a seamless steel pipe, or may be a welded steel pipe. The oil country tubular goods are, for example, steel pipes that are used for casing pipes or tubing pipes.
Preferably, the oil-well steel pipe according to the present embodiment is a seamless steel pipe. If the oil-well steel pipe according to the present embodiment is a seamless steel pipe, even if the wall thickness thereof is 15 mm or more, the oil-well steel pipe has a yield strength of 125 ksi or more and has excellent SSC resistance in a sour environment.
The shape of the steel material according to the present embodiment is not particularly limited. That is, the steel material according to the present embodiment may be a steel pipe, may be a round steel bar (solid material), or may be a steel plate. Note that, the term "round steel bar" refers to a steel bar in which a cross section in a direction perpendicular to the axial direction is a circular shape. Further, the steel pipe may be a seamless steel pipe or may be a welded steel pipe.
Hereunder, the steel material according to the present embodiment is described in detail. The symbol "%" in relation to an element means mass percent unless otherwise stated.

[Chemical composition]

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

C: 0.15 to 0.45%

Carbon (C) increases hardenability of the steel material and increases strength of the steel material. C also promotes spheroidization of carbides during tempering in the production process, and thereby increases the SSC resistance of the steel material. If carbides are dispersed, strength of the steel material increases further. If the content of C is too low, the aforementioned advantageous effects will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment. On the other hand, if the content of C is too high, even if the contents of other elements are within the range of the present embodiment, there will be too many carbides formed and toughness of the steel material will decrease. In addition, if the content of C is too high, in some cases quench cracking will easily occur during quenching in the production process. Therefore, the content of C is to be 0.15 to 0.45%. A preferable lower limit of the content of C is 0.18%, more preferably is 0.20%, further preferably is 0.22%, and further preferably is 0.25%. A preferable upper limit of the content of C is 0.40%, more preferably is 0.38%, and further preferably is 0.35%.

Si: 0.05 to 1.00%

Silicon (Si) deoxidizes the steel. If the content of Si is too low, the aforementioned advantageous effect will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment. On the other hand, if the content of Si is too high, 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 Si is to be 0.05 to 1.00%. A preferable lower limit of the content of Si is 0.10%, more preferably is 0.15%, and further preferably is 0.20%. A preferable upper limit of the content of Si is 0.85%, more preferably is 0.75%, further preferably is 0.60%, further preferably is 0.50%, and further preferably is 0.40%.

Mn: 0.05 to 0.30%

Manganese (Mn) deoxidizes the steel. Mn also increases hardenability of the steel material. If the content of Mn is too low, the aforementioned advantageous effects will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment. On the other hand, if the content of Mn is too high, even if the contents of other elements are within the range of the present embodiment, coarse sulfide-based inclusions will form and the SSC resistance of the steel material will decrease. Therefore, the content of Mn is to be 0.05 to 0.30%. A preferable lower limit of the content of Mn is 0.06%, more preferably is 0.08%, and further preferably is 0.10%. A preferable upper limit of the content of Mn is 0.28%, more preferably is 0.25%, and further preferably is 0.20%.

P: 0.030% or less

Phosphorus (P) is an impurity. That is, the lower limit of the content of P is more than 0%. If the content of P is too high, even if the contents of other elements are within the range of the present embodiment, P will segregate to grain boundaries and the SSC resistance of the steel material will decrease. Therefore, the content of P is to be 0.030% or less. A preferable upper limit of the content of P is 0.025%, more preferably is 0.020%, and further preferably is 0.015%. The content of P is preferably as low as possible. However, extremely reducing the content of P will greatly increase the production cost. Therefore, when taking industrial production into consideration, a preferable lower limit of the content of P is 0.001%, more preferably is 0.002%, and further preferably is 0.003%.

S: 0.0050% or less

Sulfur (S) is an impurity. That is, the lower limit of the content of S is more than 0%. If the content of S is too high, even if the contents of other elements are within the range of the present embodiment, S will segregate to grain boundaries and the SSC resistance of the steel material will decrease. Therefore, the content of S is to be 0.0050% or less. A preferable upper limit of the content of S is 0.0040%, more preferably is 0.0030%, and further preferably is 0.0020%. The content of S is preferably as low as possible. However, extremely reducing the content of S will greatly increase the production cost. Therefore, when taking industrial production into consideration, a preferable lower limit of the content of S is 0.0001%, more preferably is 0.0002%, and further preferably is 0.0003%.

Al: 0.005 to 0.100%

Aluminum (Al) deoxidizes the steel. If the content of Al is too low, even if the contents of other elements are within the range of the present embodiment, the aforementioned advantageous effect will not be sufficiently obtained and the SSC resistance of the steel material will decrease. On the other hand, if the content of Al is too high, even if the contents of other elements are within the range of the present embodiment, coarse oxide-based inclusions will form and the SSC resistance of the steel material will decrease. Therefore, the content of Al is to be 0.005 to 0.100%. A preferable lower limit of the content of Al is 0.010%, more preferably is 0.015%, and further preferably is 0.020%. A preferable upper limit of the content of Al is 0.080%, more preferably is 0.060%, and further preferably is 0.040%. As used in the present description, the term content of "Al" means the content of "acid-soluble Al", that is, "sol. Al".

Cr: 0.30 to 1.10%

Chromium (Cr) increases hardenability of the steel material. Cr also increases 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 Cr is too low, the aforementioned advantageous effects will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment. On the other hand, if the content of Cr is too high, the SSC resistance of the steel material will decrease even if the contents of other elements are within the range of the present embodiment. Therefore, the content of Cr is to be 0.30 to 1.10%. A preferable lower limit of the content of Cr is 0.35%, more preferably is 0.40%, and further preferably is 0.50%. A preferable upper limit of the content of Cr is 1.00%, more preferably is 0.90%, and further preferably is 0.80%.

Mo: 0.40 to 2.00%

Molybdenum (Mo) increases hardenability of the steel material. Mo also increases 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 too low, the aforementioned advantageous effects will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment. On the other hand, if the content of Mo is too high, the aforementioned advantageous effects will be saturated. Therefore, the content of Mo is to be 0.40 to 2.00%. A preferable lower limit of the content of Mo is 0.45%, more preferably is 0.50%, and further preferably is 0.60%. A preferable upper limit of the content of Mo is 1.80%, more preferably is 1.60%, and further preferably is 1.40%.

Ti: 0.002 to 0.020%

Titanium (Ti) combines with N to form nitrides, and refines the grains of the steel material by the pinning effect. As a result, strength of the steel material increases. If the content of Ti is too low, the aforementioned advantageous effect will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment. On the other hand, if the content of Ti is too high, even if the contents of other elements are within the range of the present embodiment, Ti nitrides will coarsen and the SSC resistance of the steel material will decrease. Therefore, the content of Ti is to be 0.002 to 0.020%. A preferable lower limit of the content of Ti is 0.003%, and more preferably is 0.004%. A preferable upper limit of the content of Ti is 0.018%, further preferably is 0.015%, and further preferably is 0.010%.

Nb: 0.002 to 0.100%

Niobium (Nb) combines with C and/or N to form carbides, nitrides, or carbo-nitrides (hereunder, also referred to as "carbo-nitrides and the like"). The carbo-nitrides and the like refine the grains of the steel material by the pinning effect, and thereby increase low-temperature toughness and the SSC resistance of the steel material. Nb also forms fine carbides during tempering and thereby increases temper softening resistance of the steel material and increases strength of the steel material. If the content of Nb is too low, the aforementioned advantageous effects will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment. On the other hand, if the content of Nb is too high, even if the contents of other elements are within the range of the present embodiment, carbo-nitrides and the like will excessively form and the SSC resistance of the steel material will decrease. Therefore, the content of Nb is to be 0.002 to 0.100%. A preferable lower limit of the content of Nb is 0.005%, more preferably is 0.010%, further preferably is 0.015%, and further preferably is 0.020%. A preferable upper limit of the content of Nb is 0.080%, more preferably is 0.060%, and further preferably is 0.040%.

B: 0.0005 to 0.0040%

Boron (B) dissolves in the steel and thereby increases hardenability of the steel material and increases strength of the steel material. If the content of B is too low, the aforementioned advantageous effect will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment. On the other hand, if the content of B is too high, even if the contents of other elements are within the range of the present embodiment, coarse nitrides will form and the SSC resistance of the steel material will decrease. Therefore, the content of B is to be 0.0005 to 0.0040%. A preferable lower limit of the content of B is 0.0006%, and more preferably is 0.0008%. A preferable upper limit of the content of B is 0.0035%, more preferably is 0.0030%, further preferably is 0.0025%, and further preferably is 0.0020%.

N: 0.0100% or less

Nitrogen (N) is unavoidably contained. That is, the lower limit of the content of N is more than 0%. N combines with Ti to form nitrides, thereby refining the grains of the steel material by the pinning effect. As a result, strength of the steel material increases. However, if the content of N is too high, even if the contents of other elements are within the range of the present embodiment, coarse nitrides will be formed and the SSC resistance of the steel material will decrease. Therefore, the content of N is to be 0.0100% or less. A preferable upper limit of the content of N is 0.0080%, more preferably is 0.0060%, and further preferably is 0.0040%. A preferable lower limit of the content of N for more effectively obtaining the aforementioned advantageous effect is 0.0005%, more preferably is 0.0010%, further preferably is 0.0015%, and further preferably is 0.0020%.

O: less than 0.0040%

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 too high, even if the contents of other elements are within the range of the present embodiment, coarse oxides will form and the SSC resistance of the steel material will decrease. Therefore, the content of O is to be less than 0.0040%. A preferable upper limit of the content of O is 0.0035%, more preferably is 0.0030%, further preferably is 0.0025%, and further preferably is 0.0020%. The content of O is preferably as low as possible. However, extremely reducing the content of O will greatly increase the production cost. Therefore, when taking industrial production into consideration, a preferable lower limit of the content of O is 0.0001%, more preferably is 0.0002%, and further preferably is 0.0003%.
The balance of the chemical composition of the steel material according to the present embodiment is Fe and impurities. Here, the term "impurities" refers to substances which, when industrially producing the steel material, are mixed in from ore or scrap that is used as the raw material or from the production environment or the like, and which are allowed 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 described above may further contain V in lieu of a part of Fe.

V: 0 to 0.30%

Vanadium (V) is an optional element, and does not have to be contained. That is, the content of V may be 0%. When contained, V forms carbo-nitrides and the like. The carbo-nitrides and the like refine the grains of the steel material by the pinning effect, and thereby increase the SSC resistance of the steel material. V also forms fine carbides during tempering and thereby increases temper softening resistance of the steel material and increases strength of the steel material. If even a small amount of V is contained, the aforementioned advantageous effects will be obtained to a certain extent. However, if the content of V is too high, even if the contents of other elements are within the range of the present embodiment, carbo-nitrides and the like will excessively form and the SSC resistance of the steel material will decrease. Therefore, the content of V is to be 0 to 0.30%. A preferable lower limit of the content of V is more than 0%, more preferably is 0.01%, further preferably is 0.02%, further preferably is 0.05%, and further preferably is 0.07%. A preferable upper limit of the content of V is 0.25%, more preferably is 0.20%, and further preferably is 0.15%.
The chemical composition of the steel material described above may further contain one or more elements selected from the group consisting of Cu and Ni in lieu of a part of Fe. Each of these elements is an optional element, and increases hardenability of the steel material.

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 contained, Cu increases hardenability of the steel material and increases strength of the steel material. If even a small amount of Cu is contained, the aforementioned advantageous effect will be obtained to a certain extent. However, if the content of Cu is too high, even if the contents of other elements are within the range of the present embodiment, hardenability of the steel material will be too high and the SSC resistance of the steel material will decrease. Therefore, the content of Cu is to be 0 to 0.50%. A preferable lower limit of the content of Cu is more than 0%, more preferably is 0.01%, further preferably is 0.02%, and further preferably is 0.05%. A preferable upper limit of the content of Cu is 0.35%, more preferably is 0.25%, further preferably is 0.15%, and further preferably is 0.10%.

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 contained, Ni increases hardenability of the steel material and increases strength of the steel material. Ni also dissolves in the steel and increases low-temperature toughness of the steel material. If even a small amount of Ni is contained, these advantageous effects will be obtained to a certain extent. However, if the content of Ni is too high, even if the contents of other elements are within the range of the present embodiment, local corrosion will be promoted and the SSC resistance of the steel material will decrease. Therefore, the content of Ni is to be 0 to 0.50%. A preferable lower limit of the content of Ni is more than 0%, more preferably is 0.01%, and further preferably is 0.02%. A preferable upper limit of the content of Ni is 0.30%, more preferably is 0.20%, and further preferably is 0.10%.
The chemical composition of the steel material described above may further contain W in lieu of a part of Fe.

W: 0 to 1.50%

Tungsten (W) is an optional element, and does not have to be contained. That is, the content of W may be 0%. When contained, in sour environments W forms a protective corrosion coating and suppresses penetration of hydrogen into the steel material. As a result, the SSC resistance of the steel material is increased. If even a small amount of W is contained, the aforementioned advantageous effect will be obtained to a certain extent. However, if the content of W is too high, even if the contents of other elements are within the range of the present embodiment, coarse carbides will form in the steel material, and low-temperature toughness and SSC resistance of the steel material will decrease. Therefore, the content of W is to be 0 to 1.50%. A preferable lower limit of the content of W is more than 0%, more preferably is 0.01%, further preferably is 0.03%, and further preferably is 0.05%. A preferable upper limit of the content of W is 1.30%, and more preferably is 1.10%.
The chemical composition of the steel material described above may further contain one or more elements selected from the group consisting of Ca, Mg, Zr and rare earth metal in lieu of a part of Fe. Each of these elements is an optional element, and each element renders S in the steel material harmless by forming sulfides. As a result, these elements increase the SSC resistance of the steel material.

Ca: 0 to 0.0100%

Calcium (Ca) is an optional element, and does not have to be contained. That is, the content of Ca may be 0%. When contained, Ca 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 too high, even if the contents of other elements are within the range of the present embodiment, oxides in the steel material will coarsen and the SSC resistance of the steel material will decrease. Therefore, the content of Ca is to be 0 to 0.0100%. A preferable lower limit of the content of Ca is more than 0%, more preferably is 0.0001%, further preferably is 0.0003%, and further preferably is 0.0006%. A preferable upper limit of the content of Ca is 0.0040%, more preferably is 0.0025%, and further preferably is 0.0020%.

Mg: 0 to 0.0100%

Magnesium (Mg) is an optional element, and does not have to be contained. That is, the content of Mg may be 0%. When contained, Mg 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 too high, even if the contents of other elements are within the range of the present embodiment, oxides in the steel material will coarsen and the SSC resistance of the steel material will decrease. Therefore, the content of Mg is to be 0 to 0.0100%. A preferable lower limit of the content of Mg is more than 0%, more preferably is 0.0001%, further preferably is 0.0003%, and further preferably is 0.0006%. A preferable upper limit of the content of Mg is 0.0040%, more preferably is 0.0025%, and further preferably is 0.0020%.

Zr: 0 to 0.0100%

Zirconium (Zr) is an optional element, and does not have to be contained. That is, the content of Zr may be 0%. When contained, Zr 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 Zr is contained, the aforementioned advantageous effect will be obtained to a certain extent. However, if the content of Zr is too high, even if the contents of other elements are within the range of the present embodiment, oxides in the steel material will coarsen and the SSC resistance of the steel material will decrease. Therefore, the content of Zr is to be 0 to 0.0100%. A preferable lower limit of the content of Zr is more than 0%, more preferably is 0.0001%, further preferably is 0.0003%, and further preferably is 0.0006%. A preferable upper limit of the content of Zr is 0.0040%, more preferably is 0.0025%, and further preferably is 0.0020%.

Rare earth metal (REM): 0 to 0.0100%

Rare earth metal (REM) is an optional element, and does not have to be contained. That is, the content of REM may be 0%. When contained, REM 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 the grain boundaries. Therefore, a decrease in the SSC resistance of the steel material 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 even if the contents of other elements are within the range of the present embodiment. However, if the content of REM is too high, even if the contents of other elements are within the range of the present embodiment, oxides in the steel material will coarsen and the SSC resistance of the steel material will decrease. Therefore, the content of REM is to be 0 to 0.0100%. A preferable lower limit of the content of REM is more than 0%, more preferably is 0.0001%, further preferably is 0.0003%, and further preferably is 0.0006%. A preferable upper limit of the content of REM is 0.0040%, more preferably is 0.0025%, and further preferably is 0.0020%.
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. In the present description, the term "content of REM" refers to the total content of these elements.

[Yield strength]

The yield strength of the steel material according to the present embodiment is 862 MPa or more. As used in the present description, the term "yield strength" means the stress at a time of 0.65% elongation (0.65% proof stress) obtained in a tensile test. By having the chemical composition described above and satisfying a requirement regarding a number density of coarse Si oxides to be described later, the steel material according to the present embodiment has excellent SSC resistance in a sour environment even when the yield strength of the steel material is 862 MPa or more. Note that, in the present embodiment, although not particularly limited, the upper limit of the yield strength of the steel material is, for example, 1069 MPa (155 ksi), and preferably is 1034 MPa (150 ksi).
The yield strength of the steel material according to the present embodiment can be determined by the following method. Specifically, a tensile test is carried out by a method in accordance with ASTM E8/E8M (2021). First, a round bar specimen is prepared from the steel material according to the present embodiment. If the steel material is a steel plate, the round bar specimen is prepared from the center portion of the thickness. In this case, the axial direction of the round bar specimen is to be made a direction that is parallel to the rolling elongation direction of the steel plate. If the steel material is a steel pipe, the round bar specimen is prepared from the center portion of the wall thickness. In this case, the axial direction of the round bar specimen is to be made a direction that is parallel to the axial direction of the steel pipe. If the steel material is a round steel bar, the round bar specimen is prepared from an R/2 position. In the present description, the term "R/2 position" means the center position of a radius R in a cross section perpendicular to the axial direction of the round steel bar. In this case, the axial direction of the round bar specimen is to be made a direction that is 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 parallel portion diameter of 8.9 mm and a gage length of 35.6 mm. A tensile test is carried out in the atmosphere at normal temperature (25°C) using the round bar specimen, and the obtained stress at a time of 0.65% elongation (0.65% proof stress) is defined as the yield strength (MPa). Note that, a value obtained by rounding off decimals of the obtained numerical value is adopted as the yield strength (MPa) in the present embodiment.

[Number density of coarse Si oxides]

As described above, in the present description, Si oxides in which, in mass%, the content of Si is 20% or more and the content of O is 10% or more, and which have a major axis of 5.0 µm or more are also referred to as "coarse Si oxides". The steel material according to the present embodiment has the chemical composition described above and the yield strength described above, and in addition, in the steel material, the number density of Si oxides in which, in mass%, the content of Si is 20% or more and the content of O is 10% or more, and which have a major axis of 5.0 µm or more (coarse Si oxides) is 5 /100 mm² or less. Furthermore, in a case where the yield strength of the steel material according to the present embodiment is 931 MPa or more, the number density of the coarse Si oxides in the steel material is 5 /200 mm² or less.
That is, in the steel material according to the present embodiment, in a case where the yield strength is 862 to less than 931 MPa, the number density of coarse Si oxides is 5 /100 mm² or less (that is, 10/200 mm² or less), and in a case where the yield strength is 931 MPa or more, the number density of coarse Si oxides is 5 /200 mm² or less. As a result, the steel material according to the present embodiment can achieve both a yield strength of 125 ksi or more and excellent SSC resistance. In the steel material according to the present embodiment, in a case where the yield strength is 862 to less than 931 MPa, a preferable upper limit of the number density of coarse Si oxides is 4 /100 mm², and more preferably is 3 /100 mm². In the steel material according to the present embodiment, in a case where the yield strength is 931 MPa or more, a preferable upper limit of the number density of coarse Si oxides is 4 /200 mm², and more preferably is 3 /200 mm². Note that, in the steel material according to the present embodiment, the lower limit of the number density of coarse Si oxides is not particularly limited, and may be 0 /100 mm², that is, 0 /200 mm².
In the present embodiment, the number density of coarse Si oxides in the steel material can be determined by the following method. First, a test specimen in which a face including the rolling elongation direction and the rolling reduction direction is adopted as an observation surface is prepared from the steel material according to the present embodiment. Specifically, if the steel material is a steel plate, a test specimen in which a face including the rolling elongation direction and the thickness direction is adopted as the observation surface is prepared from a center portion of the thickness. If the steel material is a steel pipe, a test specimen in which a face including the pipe axis direction and the pipe radius direction is adopted as the observation surface is prepared from a center portion of the wall thickness. If the steel material is a round steel bar, a test specimen which includes an R/2 position at the center thereof and in which a face including the axial direction and the radial direction is adopted as the observation surface is prepared.
After polishing the observation surface of the prepared test specimen to obtain a mirror surface, measurement is performed. Although the area of the observation surface is not limited, for example, the area is set to a size of 300 mm² (20 mm × 15 mm). On the observation surface, the number of Si oxides having a major axis of 5.0 µm or more is determined. Specifically, first, particles at the observation surface are identified based on contrast. Each of the identified particles is subjected to an element concentration analysis (EDS analysis). The EDS analysis is conducted with an accelerating voltage of 20 kV for quantification of N, O, Mg, Al, Si, P, S, Ca, Ti, Cr, Mn, Fe, Cu, Zr, and Nb as elements to be analyzed. Based on the EDS analysis result for each particle, particles in which, in mass%, the content of Si is 20% or more, and the content of O is 10% or more are identified as "Si oxides".
Among the Si oxides identified at the observation surface, Si oxides having a major axis of 5.0 µm or more (coarse Si oxides) are identified, and the total number of the coarse Si oxides is determined. Note that, the major axis of the Si oxides can be determined by a well-known method. Further, in the present description, the term "major axis" of the Si oxides means, at the observation surface, the longest line segment (µm) among line segments linking an arbitrary two points on the outer circumference of the each of the Si oxides. The number density of coarse Si oxides (/100 mm² or /200 mm²) is determined based on the total number of the coarse Si oxides and the gross area of the observation surface. Note that, in the present embodiment, a number obtained by rounding off decimals of the obtained numerical value is adopted as the number density of coarse Si oxides (/100 mm² or /200 mm²). Further, measurement of the number density of coarse Si oxides can be performed using an apparatus in which a scanning electron microscope is provided with a composition analysis function (SEM-EDS apparatus). For example, an automatic analyzer having the trade name "Metals Quality Analyzer" manufactured by FEI (ASPEX) Company can be used as the SEM-EDS apparatus.

[SSC resistance]

The SSC resistance of the steel material according to the present embodiment can be evaluated by an SSC resistance test conducted by a method carried out in accordance with NACE TM0177-2016 Method A. Specifically, the SSC resistance can be evaluated by the following method.
A mixed aqueous solution containing 5.0% by mass of sodium chloride and 0.4% by mass of sodium acetate that is adjusted to pH 3.5 with acetic acid (NACE solution B) is employed as the test solution. A round bar specimen is prepared from the steel material according to the present embodiment. If the steel material is a steel plate, the round bar specimen is prepared from the center portion of the thickness. In this case, the axial direction of the round bar specimen is to be made a direction that is parallel to the rolling elongation direction of the steel plate. If the steel material is a steel pipe, the round bar specimen is prepared from the center portion of the wall thickness. In this case, the axial direction of the round bar specimen is to be made a direction that is parallel to the axial direction of the steel pipe. If the steel material is a round steel bar, the round bar specimen is prepared from an R/2 position. In this case, the axial direction of the round bar specimen is to be made a direction that is 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 the length of a parallel portion is 25.4 mm. Note that, the axial direction of the round bar specimen is parallel to the rolling elongation direction of the steel material.

[Case where yield strength is less than 931 MPa]

In a case where the yield strength of the steel material is less than 931 MPa, a stress equivalent to 90% of the actual yield stress is applied to the prepared round bar specimen. The test solution at 24°C is poured into a test vessel in a manner so that the round bar specimen to which the stress has been applied is immersed therein, and this is adopted as a test bath. After degassing the test bath, a gaseous mixture of H₂S gas at 0.1 atm and CO₂ gas at 0.9 atm is blown into the test bath to saturate the test bath. The test bath saturated with the gaseous mixture is held at 24°C for 1440 hours. In a case where the steel material according to the present embodiment has a yield strength of less than 931 MPa, in an SSC resistance test conducted under the above conditions, cracking is not confirmed after 1440 hours elapses.

[Case where yield strength is 931 MPa or more]

In a case where the yield strength of the steel material is 931 MPa or more, a stress equivalent to 90% of the actual yield stress is applied to the prepared round bar specimen. The test solution at 24°C is poured into a test vessel in a manner so that the round bar specimen to which the stress has been applied is immersed therein, and this is adopted as a test bath. After degassing the test bath, a gaseous mixture of H₂S gas at 0.01 atm and CO₂ gas at 0.99 atm is blown into the test bath to saturate the test bath. The test bath saturated with the gaseous mixture is held at 24°C for 1440 hours. In a case where the steel material according to the present embodiment has a yield strength of 931 MPa or more, in an SSC resistance test conducted under the above conditions, cracking is not confirmed after 1440 hours elapses.

[Microstructure]

In the microstructure of the steel material according to the present embodiment, the total of the volume ratios of tempered martensite and tempered bainite is 90% or more. The balance of the microstructure is, for example, ferrite or pearlite. If the total of the volume ratios of tempered martensite and tempered bainite contained in the microstructure of a steel material having the chemical composition described above is 90% or more, on the condition that the other requirements of the present embodiment are satisfied, the steel material will exhibit excellent SSC resistance in a sour environment. That is, in the present embodiment, if the steel material has excellent SSC resistance, it is determined that the total of the volume ratios of tempered martensite and tempered bainite contained in the microstructure is 90% or more.
Note that, in the case of determining the volume ratios of tempered martensite and tempered bainite by observation, the volume ratios can be determined by the following method. First, a test specimen having an observation surface is prepared from the steel material according to the present embodiment. If the steel material is a steel plate, a test specimen in which a face including the rolling elongation direction and the thickness direction is adopted as the observation surface is prepared from a center portion of the thickness. If the steel material is a steel pipe, a test specimen in which a face including the pipe axis direction and the pipe radius direction is adopted as the observation surface is prepared from a center portion of the wall thickness. If the steel material is a round steel bar, a test specimen which includes an R/2 position at the center thereof, and in which a face including the axial direction and the radial direction is adopted as the observation surface is prepared.
After polishing the observation surface of the test specimen to obtain a mirror surface, the test specimen is immersed for about 10 seconds in a nital etching reagent to reveal the microstructure by etching. The etched observation surface is observed by means of a secondary electron image obtained using a scanning electron microscope (SEM), and the observation is performed in 10 visual fields. The area of each visual field is, for example, 0.01 mm² (magnification of 1000×). In each visual field, tempered martensite and tempered bainite are identified based on contrast. The area fractions of the identified tempered martensite and tempered bainite are determined. The method for determining the area fractions is not particularly limited, and a well-known method can be used. For example, the area fractions of tempered martensite and tempered bainite can be determined by image analysis. In the present embodiment, an arithmetic average value of the area fractions of tempered martensite and tempered bainite determined in all of the visual fields is defined as the volume ratio of tempered martensite and tempered bainite.

[Shape of steel material]

As described above, 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. In a case where the steel material is an oil-well steel pipe, a preferable wall thickness is 9 to 60 mm. More preferably, the steel material according to the present embodiment is a seamless steel pipe. In a case where the steel material according to the present embodiment is a seamless steel pipe, even when the steel material is a heavy-wall seamless steel pipe with a wall thickness of 15 mm or more, the steel material has a yield strength of 125 ksi or more and has excellent SSC resistance in a sour environment.

[Production method]

A method for producing the steel material according to the present embodiment will now be described. Hereunder, a method for producing a seamless steel pipe as one example of the steel material according to the present embodiment is described. The method for producing a seamless steel pipe includes a process of preparing a starting material (steelmaking process), a process of subjecting the starting material to hot working to produce a hollow shell (hot working process), and a process of subjecting the hollow shell to quenching and tempering to make a seamless steel pipe (quenching process and tempering process). Note that, a production method according to the present embodiment is not limited to the production method described below. Each process is described in detail hereunder.

[Steelmaking process]

In the steelmaking process, firstly, molten iron produced by a well-known method is subjected to refining using a converter (primary refining). The molten steel that has undergone the primary refining is then subjected to secondary refining. In the secondary refining, alloy elements that were subjected to composition adjustment are added to the molten steel to thereby produce a molten steel that satisfies the chemical composition described above.
In the secondary refining, for example, an RH (Ruhrstahl-Hausen) vacuum degassing treatment is performed. Thereafter, final adjustment of the alloy elements is performed. In the secondary refining, composite refining may be performed. In such case, prior to the RH vacuum degassing treatment, for example, a refining treatment that uses an LF (ladle furnace) or VAD (vacuum arc degassing) is performed.
A starting material is produced using the molten steel that underwent the secondary refining. Specifically, a cast piece (a slab, a bloom, or a billet) is produced by a continuous casting process using the molten steel subjected to the secondary refining. In the continuous casting process, first, molten steel is poured from a ladle into a tundish. At such time, in order to seal the nozzle of the ladle, usually the nozzle is filled with sand. Therefore, in some cases, the sand for filling may get mixed in together with the molten steel poured from the ladle to the tundish. Further, when producing a starting material having the chemical composition described above, in some cases Si oxides may be used as the sand for filling. In such a case, there is a concern that Si oxides will be introduced into the produced starting material.
Therefore, in the present embodiment, in order to prevent Si oxides which are filled in the nozzle of the ladle being introduced into the tundish, the molten steel and the Si oxides are separated. Although the method for separating Si oxides from the molten steel is not particularly limited, for example the following method can be used. An inclined metal plate is placed at a position that is below the nozzle of the ladle and is above the opening of the tundish. When the nozzle of the ladle is opened, first, Si oxides are discharged from the nozzle, and next molten steel is discharged. Here, the Si oxides are light in comparison to the molten steel. Therefore, the Si oxides discharged from the nozzle are guided to outside of the opening of the tundish along the inclination of the metal plate. The inclination of the metal plate may be provided, for example, by arranging a metal plate machined into a conical shape without a bottom surface in a manner so that the apex of the conically shape metal plate is directly below the nozzle of the ladle, or may be provided by the other methods. Further, one metal plate may be used, or a plurality of metal plates may be stacked on each other and used. In addition, although not particularly limited, the thickness of the metal plate is, for example, about 1 to 10 mm.
After the Si oxides have been discharged from the nozzle, the molten steel is discharged. At such time, the molten steel that is discharged from the nozzle is introduced into the tundish through the opening together with the metal plate. That is, in the present embodiment, a part or all of the metal plate may be introduced into the tundish and mixed into the molten steel. Therefore, the metal plate in the present embodiment is preferably a metal plate composed of an alloying element contained in the molten steel. For example, an aluminum plate can be used as a metal plate composed of an alloying element contained in the molten steel. Note that, in the present description, the term "aluminum plate" means a metal plate which is formed of aluminum and the balance of impurities.
Preferably, after the Si oxides have been discharged from the nozzle, the metal plate is removed from below the nozzle before discharging the molten steel. In this case, Si oxides that adhered to the metal plate can be prevented from becoming mixed in with the molten steel. As a result, in the produced steel material, the number density of coarse Si oxides can be reduced to 5 /200 mm² or less in some cases. Therefore, in the present embodiment, it is preferable to remove the metal plate from below the nozzle at a timing that is after the Si oxides have been discharged from the nozzle and is before the molten steel is discharged.
A method for removing the metal plate from below the nozzle is not particularly limited, and for example a hole may be formed in advance in one part of the metal plate, and the metal plate may be removed using a rod that has a hook formed at the front end thereof. In this case, the metal plate can be removed by hooking the hook that is formed at the front end of the rod into the hole in the metal plate and then pulling the rod. By using the method described above, Si oxides can be separated from the molten steel, and the molten steel can be introduced into the tundish. Note that, a method for separating the Si oxides from the molten steel is not limited to the method described above.
The molten steel is cast and a starting material is produced by the above method. Preferably, the starting material is a billet having a circular cross section (a round billet). A method for producing the starting material is not particularly limited. For example, the molten steel may be cast into a round billet by a continuous casting process. Alternatively, the molten steel may be cast to produce a billet having a rectangular cross section, or to produce a bloom. In these cases, it is preferable to perform blooming to produce a round billet from the billet having a rectangular cross section or the bloom.

[Hot working process]

In the hot working process, the prepared starting material is subjected to hot working to produce an intermediate steel material. If the steel material is a seamless steel pipe, the intermediate steel material corresponds to a hollow shell. First, a billet is heated in a heating furnace. Although not particularly limited, the heating temperature is, for example, 1100 to 1300°C. After the billet is extracted from the heating furnace, the billet is subjected to hot working to produce a hollow shell (seamless steel pipe). The method of hot working is not particularly limited, and it suffices to use a well-known method.
For example, the Mannesmann process may be performed as hot working to produce a hollow shell. In this case, a round billet is subjected to piercing-rolling using a piercing machine. When performing piercing-rolling, although not particularly limited, for example, the piercing ratio is 1.0 to 4.0. The round billet subjected to piercing-rolling is further subjected to hot rolling with a mandrel mill, a reducer, a sizing mill or the like to produce a hollow shell. The cumulative reduction of area in the hot working process is, for example, 20 to 70%.
A hollow shell may be produced from the billet by the other hot working methods. For example, in a case where the steel material is a heavy-wall steel material of a short length such as a coupling, a hollow shell may be produced by forging by the Ehrhardt process or the like. A hollow shell is produced by the above process. Although not particularly limited, the wall thickness of the hollow shell is, for example, 9 to 60 mm.
The hollow shell produced by hot working may be air-cooled (as-rolled). The hollow shell produced by hot working may 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 of performing direct quenching after the hot working, or performing quenching after supplementary heating, cooling may be stopped midway through the quenching process or slow cooling may be performed. In this case, the occurrence of quench cracking in the hollow shell can be suppressed. In addition, in the case of performing direct quenching after hot working, or performing quenching after supplementary heating, stress relief annealing (SR) may be performed at a time that is after quenching and before the heat treatment of the next process. In this case, residual stress of the hollow shell is eliminated.
If the steel material is a round steel bar, first, the starting material is heated in a heating furnace. Although not particularly limited, the heating temperature is, for example, 1100 to 1300°C. The starting material extracted from the heating furnace is subjected to hot working to produce an intermediate steel material in which a cross section perpendicular to the axial direction is a circular shape. The hot working is, for example, blooming that is performed using a blooming mill or hot rolling that is performed using a continuous mill. 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 alternately arranged.
If the steel material is a steel plate, first, the starting material is heated in a heating furnace. Although not particularly limited, the heating temperature is, for example, 1100 to 1300°C. The starting material extracted from the heating furnace is subjected to hot rolling using a blooming mill and a continuous mill to produce an intermediate steel material having a steel plate shape.
As described above, in the hot working process, the prepared starting material is subjected to hot working to produce an intermediate steel material. Hereunder, the quenching process is described in detail.

[Quenching process]

In the quenching process, the prepared intermediate steel material (hollow shell) is subjected to quenching. As used in the present description, the term "quenching" means rapidly cooling the intermediate steel material which is at a temperature not lower than the A₃ point. A preferable quenching temperature is 800 to 1000°C. If the quenching temperature is too high, in some cases prior-γ grains will become coarse and the SSC resistance of the steel material will decrease. Therefore, a quenching temperature in the range of 800 to 1000°C is preferable.
In the present description, in a case where direct quenching is performed after hot working, the term "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 after hot working, the term "quenching temperature" corresponds to the temperature of the furnace that performs the supplementary heating or reheating.
The quenching method is a method that, for example, continuously cools the intermediate steel material (hollow shell) from the quenching starting temperature and continuously decreases the surface temperature of the hollow shell. The method of performing the continuous cooling treatment is not particularly limited, and a well-known method can be used. The method of performing the continuous cooling treatment is, for example, a method that cools the hollow shell by immersing the hollow shell in a water bath, or a method that cools the hollow shell in an accelerated manner by shower water cooling or mist cooling.
If the cooling rate during quenching is too slow, the microstructure will not become a microstructure that is principally composed of martensite and bainite, and the mechanical property defined in the present embodiment (yield strength of 125 ksi or more) will not be obtained. In such case, in addition, excellent low-temperature toughness and SSC resistance will not be obtained.
Therefore, as described above, in the method for producing the steel material according to the present embodiment, the intermediate steel material is rapidly cooled during quenching. Specifically, in the quenching process, the average cooling rate when the surface temperature of the intermediate steel material (hollow shell) is within the range of 800 to 500°C during quenching is defined as "cooling rate during quenching CR_800-500". More specifically, the cooling rate during quenching CR_800-500 is determined based on a temperature measured at a region that is most slowly cooled within a cross-section of the intermediate steel material that is being quenched (for example, in the case of forcedly cooling both surfaces, the cooling rate is measured at the center portion of the thickness of the intermediate steel material).
A preferable cooling rate during quenching CR_800-500 is 300°C/min or more. A more preferable lower limit of the cooling rate during quenching CR_800-500 is 450°C/min, and further preferably is 600°C/min. Although an upper limit of the cooling rate during quenching CR_800-500 is not particularly defined, the upper limit is, for example, 60000°C/min.
Preferably, quenching is performed after performing heating of the hollow shell in the austenite zone a plurality of times. In this case, the SSC resistance of the steel material increases because austenite grains are refined prior to quenching. Heating in the austenite zone may be repeated a plurality of times by performing quenching a plurality of times, or heating in the austenite zone may be repeated a plurality of times by performing normalizing and quenching. Further, quenching and tempering that is described later may be performed in combination a plurality of times. That is, both quenching and tempering may be performed a plurality of times. In such case, the SSC resistance of the steel material increases further. The tempering process is described in detail hereunder.

[Tempering process]

In the tempering process, the hollow shell on which the aforementioned quenching was performed is subjected to tempering. In the present description, the term "tempering" means reheating the intermediate steel material after quenching to a temperature that is less than the A_c1 point and holding the intermediate steel material at that temperature. Here, the term "tempering temperature" corresponds to the temperature of the furnace when the intermediate steel material after quenching is heated and held at the relevant temperature. The term "tempering time" means the period of time from when the temperature of the intermediate steel material reaches a predetermined tempering temperature until the steel material is extracted from the heat treatment furnace.
The tempering temperature is appropriately adjusted in accordance with the chemical composition of the seamless steel pipe and the yield strength to be obtained. That is, for a hollow shell having the chemical composition of the present embodiment, the tempering temperature is adjusted so as to adjust the yield strength of the seamless steel pipe to 862 MPa or more. Note that, a person skilled in the art is fully capable of adjusting the tempering temperature so as to adjust the yield strength of the seamless steel pipe to 862 MPa or more, and to 931 MPa or more. Specifically, in the tempering process according to the present embodiment, a preferable tempering temperature is 650 to 690°C. A more preferable lower limit of the tempering temperature is 655°C. A more preferable upper limit of the tempering temperature is 685°C.
If the tempering time is too short, in some cases a microstructure that is principally composed of tempered martensite and tempered bainite may not be obtained. On the other hand, if the tempering time is too long, the aforementioned advantageous effect will be saturated. Therefore, in the tempering process of the present embodiment, preferably the tempering time is set within a range of 10 to 90 minutes. A more preferable lower limit of the tempering time is 15 minutes. A more preferable upper limit of the tempering time is 80 minutes.
The steel material according to the present embodiment can be produced by the production method described above. Note that, in the foregoing description of the production method, a method for producing a seamless steel pipe has been described as one example. However, the steel material according to the present embodiment may also be a steel plate or the other shapes. A method for producing a steel plate or a steel material of the other shapes also include, for example, a preparation process, a quenching process, and a tempering process, similarly to the production method described above. Further, the production method described above is an example, and the steel material may also be produced by the other production methods.
Hereunder, the present invention is described more specifically by way of examples.

EXAMPLE 1

In Example 1, steel materials having a yield strength of 862 to less than 931 MPa were evaluated. Specifically, first, molten steels having the chemical compositions shown in Table 1-1 and Table 1-2 were produced. Note that, the symbol "-" in Table 1-2 means that the content of the relevant element was at the level of an impurity. Specifically, "-" means that the content of V, the content of Cu, the content of Ni, and the content of W of steel A were each 0% when rounded off to second decimal places. In addition, "-" means that the content of Ca, the content of Mg, the content of Zr, and the content of rare earth metal (REM) of steel A were each 0% when rounded off to fourth decimal places.

[Table 1-1]

TABLE 1-1

Steel	Chemical Composition (unit is mass%; balance is Fe and impurities)
Steel	C	Si	Mn	P	S	Al	Cr	Mo	Ti	Nb	B
A	0.26	0.29	0.13	0.006	0.0012	0.032	0.75	0.95	0.006	0.027	0.0013
B	0.27	0.28	0.12	0.007	0.0011	0.025	0.72	0.89	0.006	0.028	0.0011
C	0.25	0.25	0.10	0.005	0.0010	0.028	0.70	1.00	0.005	0.029	0.0012
D	0.27	0.28	0.29	0.006	0.0014	0.025	0.71	0.99	0.006	0.028	0.0011
E	0.26	0.27	0.18	0.006	0.0014	0.030	0.72	0.91	0.005	0.027	0.0013
F	0.25	0.28	0.20	0.006	0.0015	0.030	0.72	0.89	0.005	0.025	0.0013
G	0.27	0.27	0.19	0.005	0.0011	0.027	0.74	0.91	0.006	0.029	0.0013
H	0.26	0.26	0.15	0.005	0.0012	0.030	0.77	1.20	0.006	0.025	0.0013
I	0.32	0.30	0.12	0.006	0.0011	0.033	0.85	0.90	0.007	0.025	0.0011
J	0.28	0.29	0.09	0.007	0.0015	0.032	0.79	0.98	0.005	0.025	0.0013
K	0.26	0.25	0.10	0.006	0.0010	0.029	0.75	0.70	0.007	0.027	0.0013
L	0.28	0.29	0.09	0.005	0.0011	0.032	0.72	0.94	0.005	0.029	0.0011
M	0.23	0.28	0.81	0.012	0.0024	0.032	1.05	0.41	0.004	0.025	0.0013
N	0.27	0.25	0.22	0.006	0.0012	0.035	0.73	0.25	0.006	0.028	0.0011
O	0.27	0.28	0.12	0.006	0.0055	0.025	0.75	0.95	0.006	0.029	0.0013
P	0.26	0.27	0.13	0.035	0.0011	0.028	0.75	0.95	0.006	0.028	0.0013
Q	0.27	0.30	0.10	0.005	0.0012	0.025	0.84	0.88	0.004	0.030	0.0012

[Table 1-2]

TABLE 1-2

Steel	Chemical Composition (unit is mass%; balance is Fe and impurities)
Steel	N	O	V	Cu	Ni	W	Ca	Mg	Zr	REM
A	0.0033	0.0010	-	-	-	-	-	-	-	-
B	0.0045	0.0008	-	-	-	-	-	-	-	-
C	0.0042	0.0010	-	-	-	-	-	-	-	-
D	0.0031	0.0010	0.10	-	-	-	-	-	-	-
E	0.0039	0.0012	0.10	0.04	-	-	-	-	-	-
F	0.0040	0.0009	0.10	-	0.03	-	-	-	-	-
G	0.0038	0.0012	0.09	-	-	1.00	-	-	-	-
H	0.0035	0.0011	0.10	-	-	-	0.0012	-	-	-
I	0.0044	0.0015	-	-	-	-	-	0.0015	-	-
J	0.0039	0.0015	0.10	-	-	-	-	-	0.0013	-
K	0.0042	0.0012	0.10	-	-	-	-	-	-	0.0014
L	0.0040	0.0038	-	-	-	-	-	-	-	-
M	0.0040	0.0011	-	-	-	-	-	-	-	-
N	0.0045	0.0020	-	-	-	-	-	-	-	-
O	0.0045	0.0012	0.10	-	-	-	-	-	-	-
P	0.0034	0.0010	0.10	-	-	-	-	-	-	-
Q	0.0045	0.0045	-	-	-	-	-	-	-	-

Round billets were produced by a continuous casting process using the molten steels described above. In the continuous casting process, when introducing some of the molten steels into a tundish from a ladle, a metal plate which had been machined into a conical shape without a bottom surface was arranged above the opening of the tundish in a manner so that the apex of the conically shape metal plate was directly below the nozzle of the ladle. Whether or not a metal plate having the aforementioned shape was arranged above the opening of the tundish is indicated in Table 2. Specifically, a case where a metal plate having the aforementioned shape was arranged above the opening of the tundish is indicated by "A" in the column "Metal Plate" in Table 2. A case where a metal plate having the aforementioned shape was not arranged above the opening of the tundish is indicated by "B" in the column "Metal Plate" in Table 2. Note that aluminum plates were used as metal plates having the aforementioned shape arranged above the opening of the tundish. Specifically, three aluminum plates each having a thickness of 2 mm were stacked on top of each other and used. Further, in Test Nos. 3, 8 to 10, and 12, at a timing that was after Si oxides were discharged from the nozzle and was before the molten steel was discharged, the metal plates were removed from below the nozzle using a rod that had a hook formed at the front end thereof.

[Table 2]

TABLE 2

Test Number	Steel	Metal Plate	Quenching		Tempering		YS (MPa)	Coarse Si Oxides (/100 mm²)	Number of Specimens in which SSC Occurred (Specimens)
Test Number	Steel	Metal Plate	Quenching Temperature (°C)	Quenching Time (mins)	Tempering Temperature (°C)	Tempering Time (mins)	YS (MPa)	Coarse Si Oxides (/100 mm²)	Number of Specimens in which SSC Occurred (Specimens)
1	A	A	920	10	650	30	929	3	0
2	B	A	920	10	650	30	925	3	0
3	C	A	920	10	650	30	917	2	0
4	D	A	900	15	690	30	888	3	0
5	E	A	900	15	685	30	910	3	0
6	F	A	920	10	685	30	915	3	0
7	G	A	920	10	680	30	924	3	0
8	H	A	900	15	680	30	918	2	0
9	I	A	900	10	680	30	920	2	0
10	J	A	920	10	675	60	918	2	0
11	K	A	920	10	675	60	924	3	0
12	L	A	920	10	650	30	922	2	0
13	A	B	920	10	655	30	883	7	2
14	B	B	900	10	660	30	880	6	2
15	M	A	920	10	685	30	903	2	2
16	N	A	920	10	680	30	921	3	2
17	O	A	920	10	685	30	917	1	1
18	P	A	920	10	685	40	917	2	2
19	Q	A	920	10	650	30	930	4	3

The produced round billets of Test Nos. 1 to 19 were held at 1250°C for one hour, and thereafter were subjected to hot rolling by the Mannesmann-mandrel process to produce hollow shells (seamless steel pipes) of Test Nos. 1 to 19. In addition, the obtained hollow shells of Test Nos. 1 to 19 were subjected to quenching. Specifically, the hollow shells of Test Nos. 1 to 19 were held at quenching temperatures (°C) for quenching times (mins) which are each described in the column "Quenching" in Table 2, and thereafter were subjected to quenching by shower water cooling. Note that, the cooling rates during quenching CR_800-500 were within the range of 480 to 30000°C/min for Test Nos. 1 to 19. Here, the temperatures (°C) of the heat treatment furnace that heated the hollow shells were adopted as the quenching temperatures (°C) described in Table 2. Further, the times (mins) for which the hollow shells were held at the quenching temperatures were adopted as the quenching time (mins) described in Table 2.
The obtained hollow shells of Test Nos. 1 to 19 were subjected to tempering. Specifically, tempering of the hollow shells of Test Nos. 1 to 19 was carried out by holding the hollow shells at tempering temperatures (°C) for tempering times (mins) which are each described in the column "Tempering" in Table 2. Here, the temperatures (°C) of the tempering furnace that heated the hollow shells were adopted as the tempering temperature (°C) described in Table 2. Further, the times (mins) for which the hollow shells were held at the tempering temperature were adopted as the tempering time (mins) described in Table 2. Seamless steel pipes of Test Nos. 1 to 19 were obtained by the production process described above.

[Evaluation tests]

The seamless steel pipes of Test Nos. 1 to 19 after the tempering described above were subjected to a tensile test, a test to measure the number density of coarse Si oxides, and an SSC resistance test.

[Tensile test]

The seamless steel pipes of Test Nos. 1 to 19 were subjected to a tensile test, and the yield strength was determined. The tensile test was carried out in accordance with ASTM E8/E8M (2021). Round bar specimens having a parallel portion diameter of 8.9 mm and a gage length of 35.6 mm were prepared from the center portion of the wall thickness of the seamless steel pipes of Test Nos. 1 to 19. The axial direction of the round bar specimen was parallel to the axial direction of the seamless steel pipe. The tensile tests were carried out in the atmosphere at normal temperature (25°C) using the prepared round bar specimens, and the yield strength (MPa) of the seamless steel pipes of Test Nos. 1 to 19 were determined. Note that, in the present examples, the stress at a time of 0.65% elongation (0.65% proof stress) obtained in the tensile test was defined as the yield strength. For each of Test Nos. 1 to 19, the obtained yield strength (MPa) is shown in Table 2 as "YS (MPa)".

[Test to measure number density of coarse Si oxides]

Tests to measure the number density of coarse Si oxides were carried out on the seamless steel pipes of Test Nos. 1 to 19, and the number density of Si oxides having a major axis of 5.0 µm or more (coarse Si oxides) was determined. The number density of coarse Si oxides was determined by the method described above using test specimens prepared from the center portion of the wall thickness of the seamless steel pipes of Test Nos. 1 to 19. For each of Test Nos. 1 to 19, the obtained number density of coarse Si oxides (/100 mm²) is shown in the column "Coarse Si Oxides (/100 mm²)" in Table 2.

[SSC resistance test]

The seamless steel pipes of Test Nos. 1 to 19 were subjected to an SSC resistance test conducted by a method carried out in accordance with NACE TM0177-2016 Method A, and the SSC resistance was evaluated. Specifically, round bar specimens of 6.35 mm in diameter in which the length of a parallel portion was 25.4 mm were prepared from the center portion of the wall thickness of the seamless steel pipes of Test Nos. 1 to 19. The SSC resistance test was performed on three test specimens among the prepared test specimens. Note that the axial direction of each test specimen was parallel to the pipe axis direction.
Tensile stress was applied in the axial direction of the round bar specimens of Test Nos. 1 to 19. At this time, the applied stress was adjusted so as to be 90% of the actual yield stress of each seamless steel pipe. A mixed aqueous solution containing 5.0% by mass of sodium chloride and 0.4% by mass of sodium acetate that was adjusted to pH 3.5 with acetic acid (NACE solution B) was used as the test solution. The test solution at 24°C was poured into three test vessels, and these were adopted as test baths. Three round bar specimens to which the stress was applied were immersed individually in mutually different test vessels as the test baths. After each test bath was degassed, a gaseous mixture of H₂S gas at 0.1 atm and CO₂ gas at 0.9 atm was blown into each test bath to saturate the test baths. The test baths saturated with the gaseous mixture of H₂S gas at 0.1 atm and CO₂ gas at 0.9 atm were held at 24°C for 1440 hours.
After being held for 1440 hours, the round bar specimens of Test Nos. 1 to 19 were observed to determine whether or not sulfide stress cracking (SSC) had occurred. Specifically, after being held for 1440 hours, the round bar specimens were observed with the naked eye. For Test Nos. 1 to 19, the number of round bar specimens in which SSC had occurred among the three round bar specimens is shown in the column "Number of Specimens in which SSC Occurred (Specimens)" in Table 2.

[Test results]

Referring to Table 1-1, Table 1-2, and Table 2, the chemical compositions of the seamless steel pipes of Test Nos. 1 to 12 were appropriate, and the production methods of Test Nos. 1 to 12 also satisfied the preferable conditions described above. As a result, for each of these seamless steel pipes, the yield strength was 862 to less than 931 MPa, and the number density of coarse Si oxides was 5 /100 mm² or less. As a result, for each of these seamless steel pipes, SSC did not occur in the SSC resistance test. That is, the seamless steel pipes of Test Nos. 1 to 12 had a yield strength of 862 to less than 931 MPa and had excellent SSC resistance.
On the other hand, in the seamless steel pipes of Test Nos. 13 and 14, the number density of coarse Si oxides was more than 5 /100 mm². As a result, in the SSC resistance test, SSC occurred in test specimens. That is, the seamless steel pipes of Test Nos. 13 and 14 did not have excellent SSC resistance.
In the seamless steel pipe of Test No. 15, the content of Mn was too high. As a result, in the SSC resistance test, SSC occurred in test specimens. That is, the seamless steel pipe of Test No. 15 did not have excellent SSC resistance.
In the seamless steel pipe of Test No. 16, the content of Mo was too low. As a result, in the SSC resistance test, SSC occurred in test specimens. That is, the seamless steel pipe of Test No. 16 did not have excellent SSC resistance.
In the seamless steel pipe of Test No. 17, the content of S was too high. As a result, in the SSC resistance test, SSC occurred in a test specimen. That is, the seamless steel pipe of Test No. 17 did not have excellent SSC resistance.
In the seamless steel pipe of Test No. 18, the content of P was too high. As a result, in the SSC resistance test, SSC occurred in test specimens. That is, the seamless steel pipe of Test No. 18 did not have excellent SSC resistance.
In the seamless steel pipe of Test No. 19, the content of O was too high. As a result, in the SSC resistance test, SSC occurred in test specimens. That is, the seamless steel pipe of Test No. 19 did not have excellent SSC resistance.

EXAMPLE 2

In Example 2, steel materials having a yield strength of 931 MPa or more were evaluated. Specifically, first, molten steels having the chemical compositions shown in Table 3-1 and Table 3-2 were produced. Note that, the symbol "-" in Table 3-2 means that the content of the relevant element was at the level of an impurity. Specifically, "-" means that the content of Cu, the content of Ni, and the content of W of steel R were each 0% when rounded off to second decimal places. In addition, "-" means that the content of Ca, the content of Mg, the content of Zr, and the content of rare earth metal (REM) of steel R were each 0% when rounded off to fourth decimal places.

[Table 3-1]

TABLE 3-1

Steel	Chemical Composition (unit is mass%; balance is Fe and impurities)
Steel	C	Si	Mn	P	S	Al	Cr	Mo	Ti	Nb	B
R	0.29	0.25	0.14	0.006	0.0012	0.025	0.75	0.92	0.005	0.025	0.0013
S	0.22	0.29	0.21	0.008	0.0009	0.028	0.72	0.85	0.004	0.028	0.0012
T	0.26	0.29	0.12	0.007	0.0013	0.035	0.70	0.93	0.006	0.026	0.0013
U	0.29	0.31	0.11	0.006	0.0009	0.032	0.85	1.05	0.006	0.027	0.0011
V	0.25	0.80	0.06	0.007	0.0011	0.031	0.92	0.93	0.004	0.025	0.0011
W	0.22	0.25	0.09	0.007	0.0008	0.033	0.51	1.18	0.006	0.032	0.0012
X	0.24	0.32	0.09	0.008	0.0012	0.033	0.55	0.92	0.004	0.032	0.0011
Y	0.26	0.28	0.15	0.008	0.0011	0.035	1.04	0.82	0.006	0.025	0.0011
Z	0.23	0.31	0.12	0.009	0.0013	0.029	1.09	0.91	0.005	0.032	0.0012
AA	0.27	0.32	0.13	0.009	0.0012	0.024	0.85	0.88	0.005	0.025	0.0011
AB	0.28	0.32	0.22	0.008	0.0014	0.031	0.65	0.94	0.006	0.025	0.0011
AC	0.23	0.25	0.21	0.006	0.0013	0.035	0.79	0.99	0.006	0.031	0.0011
AD	0.26	0.24	0.15	0.006	0.0012	0.025	1.02	0.33	0.006	0.015	0.0013
AE	0.28	0.22	0.11	0.008	0.0055	0.025	0.52	0.93	0.006	0.025	0.0013

[Table 3-2]

TABLE 3-2

Steel	Chemical Composition (unit is mass%; balance is Fe and impurities)
Steel	N	O	V	Cu	Ni	W	Ca	Mg	Zr	REM
R	0.0033	0.0011	0.09	-	-	-	-	-	-
S	0.0035	0.0012	0.10	-	-	-	-	-	-	-
T	0.0030	0.0013	0.09	-	-	-	-	-	-	-
U	0.0033	0.0025	0.09	0.03	-	-	-	-	-	-
V	0.0032	0.0009	0.09	-	0.03	-	-	-	-	-
W	0.0042	0.0011	0.10	-	-	0.50	-	-	-	-
X	0.0039	0.0012	0.09	-	-	-	-	-	0.0011	-
Y	0.0032	0.0025	0.10	-	-	-	0.0012	-	-	-
Z	0.0041	0.0013	0.11	-	-	-	-	0.0015	-	-
AA	0.0042	0.0031	0.12	-	-	-	-	-	-	0.0012
AB	0.0045	0.0012	0.10	-	0.04	0.50	0.0012	-	-	-
AC	0.0031	0.0042	0.10	-	-	-	-	-	-	-
AD	0.0042	0.0014	0.10	-	-	-	-	-	-	-
AE	0.0045	0.0015	0.10	-	-	-	-	-	-	-

Round billets were produced by a continuous casting process using the molten steels described above. In the continuous casting process, when pouring some of the molten steels into a tundish from a ladle, a metal plate which had been machined into a conical shape without a bottom surface was arranged above the opening of the tundish in a manner so that the apex of the conically shape metal plate was directly below the nozzle of the ladle. Whether or not a metal plate having the aforementioned shape was arranged above the opening of the tundish is indicated in Table 4. Specifically, a case where a metal plate having the aforementioned shape was arranged above the opening of the tundish is indicated by "A" in the column "Metal Plate" in Table 4. A case where a metal plate having the aforementioned shape was not arranged above the opening of the tundish is indicated by "B" in the column "Metal Plate" in Table 4. Note that aluminum plates were used as the metal plates having the aforementioned shape arranged above the opening of the tundish. Specifically, three aluminum plates each having a thickness of 2 mm were stacked on top of each other and used. Further, in the examples where the metal plates were arranged, at a timing that was after Si oxides were discharged from the nozzle and was before the molten steel was discharged, the metal plates were removed from below the nozzle using a rod that had a hook formed at the front end thereof.

[Table 4]

TABLE 4

Test Number	Steel	Metal Plate	Quenching		Tempering		YS (MPa)	Coarse Si Oxides (/100 mm²)	Number of Specimens in which SSC Occurred (Specimens)
Test Number	Steel	Metal Plate	Quenching Temperature (°C)	Quenching Time (mins)	Tempering Temperature (°C)	Tempering Time (mins)	YS (MPa)	Coarse Si Oxides (/100 mm²)	Number of Specimens in which SSC Occurred (Specimens)
20	R	A	920	10	670	30	1016	5	0
21	R	A	920	10	665	30	1015	5	0
22	R	A	920	10	670	30	1005	5	0
23	S	A	920	10	670	30	982	3	0
24	T	A	920	10	680	30	931	2	0
25	U	A	920	10	670	30	1029	4	0
26	V	A	920	10	675	60	945	3	0
27	W	A	920	10	670	30	1004	4	0
28	X	A	900	10	675	30	993	4	0
29	Y	A	920	10	670	60	977	3	0
30	Z	A	900	10	675	30	1027	4	0
31	AA	A	900	10	675	30	1020	3	0
32	AB	A	920	10	670	30	1007	2	0
33	R	B	920	10	670	30	1031	9	2
34	U	B	920	10	675	30	1031	7	1
35	V	B	920	10	670	30	1027	8	1
36	W	B	920	10	670	30	1011	6	2
37	Y	B	920	10	670	30	1022	9	1
38	AC	A	920	10	670	30	991	2	2
39	AD	A	920	10	670	30	1023	4	2
40	AE	A	920	10	670	30	1005	2	2

The produced round billets of Test Nos. 20 to 40 were held at 1250°C for one hour, and thereafter were subjected to hot rolling by the Mannesmann-mandrel process to produce hollow shells (seamless steel pipes) of Test Nos. 20 to 40. In addition, the obtained hollow shells of Test Nos. 20 to 40 were subjected to quenching. Specifically, the hollow shells of Test Nos. 20 to 40 were held at quenching temperatures (°C) for quenching times (mins) which are each described in the column "Quenching" in Table 4, and thereafter were subjected to quenching by shower water cooling. Note that, in each of Test Nos. 20 to 40, the cooling rate during quenching CR_800-500 was within the range of 480 to 30000°C/min. Here, the temperatures (°C) of the heat treatment furnace that heated the hollow shells were adopted as the quenching temperatures (°C) described in Table 4. Further, the times (mins) for which the hollow shells were held at the quenching temperatures were adopted as the quenching times (mins) described in Table 4.
In addition, the hollow shell of Test No. 22 was subjected to a second quenching. Specifically, the hollow shell of Test No. 22 was held at 900°C in the heat treatment furnace for 10 minutes, and thereafter was subjected to quenching by shower water cooling. Note that, in the second quenching performed on the hollow shell of Test No. 22 also, the cooling rate during quenching CR_800-500 was within the range of 480 to 30000°C/min.
The obtained hollow shells of Test Nos. 20 to 40 were subjected to tempering. Specifically, tempering of the hollow shells of Test Nos. 20 to 40 was carried out by holding the hollow shells at the tempering temperatures (°C) for the tempering times (mins) which are each described in the column "Tempering" in Table 4. Here, the temperatures (°C) of the tempering furnace that heated the hollow shells were adopted as the tempering temperatures (°C) described in Table 4. Further, the times (mins) for which the hollow shells were held at the tempering temperatures were adopted as the tempering times (mins) described in Table 4. Seamless steel pipes of Test Nos. 20 to 40 were obtained by the production process described above.

[Evaluation tests]

The seamless steel pipes of Test Nos. 20 to 40 after the tempering described above were subjected to a tensile test, a test to measure the number density of coarse Si oxides, and an SSC resistance test which are described hereunder.

[Tensile test]

The seamless steel pipes of Test Nos. 20 to 40 were subjected to a tensile test, and the yield strength was determined. The tensile test was carried out in accordance with ASTM E8/E8M (2021). Round bar specimens having a parallel portion diameter of 8.9 mm and a gage length of 35.6 mm were prepared from the center portion of the wall thickness of the seamless steel pipes of Test Nos. 20 to 40. The axial direction of the round bar specimen was parallel to the axial direction of the seamless steel pipe. The tensile tests were carried out in the atmosphere at normal temperature (25°C) using the prepared round bar specimens, and the yield strength (MPa) of the seamless steel pipes of Test Nos. 20 to 40 were determined. Note that, in the present examples, the stress at a time of 0.65% elongation (0.65% proof stress) obtained in the tensile test was defined as the yield strength. For each of Test Nos. 20 to 40, the obtained yield strength (MPa) is shown in Table 4 as "YS (MPa)".

[Test to measure number density of coarse Si oxides]

Tests to measure the number density of coarse Si oxides were carried out on the seamless steel pipes of Test Nos. 20 to 40, and the number density of Si oxides having a major axis of 5.0 µm or more (coarse Si oxides) was determined. The number density of coarse Si oxides was determined by the method described above using test specimens prepared from the center portion of the wall thickness of the seamless steel pipes of Test Nos. 20 to 40. For each of Test Nos. 20 to 40, the obtained number density of coarse Si oxides (/200 mm²) is shown in the column "Coarse Si Oxides (/200 mm²)" in Table 4.

[SSC resistance test]

The seamless steel pipes of Test Nos. 20 to 40 were subjected to an SSC resistance test conducted by a method carried out in accordance with NACE TM0177-2016 Method A, and the SSC resistance was evaluated. Specifically, round bar specimens of 6.35 mm in diameter in which the length of a parallel portion was 25.4 mm were prepared from the center portion of the wall thickness of the seamless steel pipes of Test Nos. 20 to 40. The SSC resistance test was performed on three test specimens among the prepared test specimens. Note that the axial direction of each test specimen was parallel to the pipe axis direction.
Tensile stress was applied in the axial direction of the round bar specimens of Test Nos. 20 to 40. At this time, the applied stress was adjusted so as to be 90% of the actual yield stress of each steel plate. A mixed aqueous solution containing 5.0% by mass of sodium chloride and 0.4% by mass of sodium acetate that was adjusted to pH 3.5 with acetic acid (NACE solution B) was used as the test solution. The test solution at 24°C was poured into three test vessels, and these were adopted as test baths. Three round bar specimens to which the stress was applied were immersed individually in mutually different test vessels as the test baths. After each test bath was degassed, a gaseous mixture of H₂S gas at 0.01 atm and CO₂ gas at 0.99 atm was blown into each test bath to saturate the test baths. The test baths saturated with the gaseous mixture of H₂S gas at 0.01 atm and CO₂ gas at 0.99 atm were held at 24°C for 1440 hours.
After being held for 1440 hours, the round bar specimens of Test Nos. 20 to 40 were observed to determine whether or not sulfide stress cracking (SSC) had occurred. Specifically, after being held for 1440 hours, the round bar specimens were observed with the naked eye. For Test Nos. 20 to 40, the number of round bar specimens in which SSC had occurred among the three round bar specimens is shown in the column "Number of Specimens in which SSC Occurred (Specimens)" in Table 4.

[Test results]

Referring to Table 3-1, Table 3-2, and Table 4, the chemical compositions of the seamless steel pipes of Test Nos. 20 to 32 were appropriate, and the production methods of Test Nos. 20 to 32 also satisfied the preferable conditions described above. As a result, for each of these seamless steel pipes, the yield strength was 931 MPa or more, and the number density of coarse Si oxides was 5 /200 mm² or less. As a result, for each of these seamless steel pipes, SSC did not occur in the SSC resistance test. That is, the seamless steel pipes of Test Nos. 20 to 32 had a yield strength of 931 MPa or more and had excellent SSC resistance.
On the other hand, in the seamless steel pipes of Test Nos. 33 to 37, the number density of coarse Si oxides was more than 5 /200 mm². As a result, in the SSC resistance test, SSC occurred in test specimens. That is, the seamless steel pipes of Test Nos. 33 to 37 did not have excellent SSC resistance.
In the seamless steel pipe of Test No. 38, the content of O was too high. As a result, in the SSC resistance test, SSC occurred in test specimens. That is, the seamless steel pipe of Test No. 38 did not have excellent SSC resistance.
In the seamless steel pipe of Test No. 39, the content of Mo was too low. As a result, in the SSC resistance test, SSC occurred in test specimens. That is, the seamless steel pipe of Test No. 39 did not have excellent SSC resistance.
In the seamless steel pipe of Test No. 40, the content of S was too high. As a result, in the SSC resistance test, SSC occurred in test specimens. That is, the seamless steel pipe of Test No. 40 did not have excellent SSC resistance.
An embodiment of the present disclosure has been described above. However, the embodiment described above is merely an example for carrying out the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiment, and can be implemented by appropriately modifying the above-described embodiment within a range not departing from the spirit thereof.

Claims

A steel material consisting of, in mass%,
C: 0.15 to 0.45%,

Si: 0.05 to 1.00%,

Mn: 0.05 to 0.30%,

P: 0.030% or less,

S: 0.0050% or less,

Al: 0.005 to 0.100%,

Cr: 0.30 to 1.10%,

Mo: 0.40 to 2.00%,

Ti: 0.002 to 0.020%,

Nb: 0.002 to 0.100%,

B: 0.0005 to 0.0040%,

N: 0.0100% or less,

O: less than 0.0040%,

V: 0 to 0.30%,

Cu: 0 to 0.50%,

Ni: 0 to 0.50%,

W: 0 to 1.50%,

Ca: 0 to 0.0100%,

Mg: 0 to 0.0100%,

Zr: 0 to 0.0100%, and

rare earth metal: 0 to 0.0100%,

with the balance being Fe and impurities,

wherein:
a yield strength is 862 MPa or more, and

in the steel material,

a number density of Si oxides in which, in mass%, a content of Si is 20% or more and a content of O is 10% or more, and which have a major axis of 5.0 µm or more is 5 /100 mm² or less, and

in a case where the yield strength is 931 MPa or more, a number density of the Si oxides is 5 /200 mm² or less.
The steel material according to claim 1, containing one or more elements selected from a group consisting of:
V: 0.01 to 0.30%,

Cu: 0.01 to 0.50%,

Ni: 0.01 to 0.50%,

W: 0.01 to 1.50%,

Ca: 0.0001 to 0.0100%,

Mg: 0.0001 to 0.0100%,

Zr: 0.0001 to 0.0100%, and

rare earth metal: 0.0001 to 0.0100%.
The steel material according to claim 1 or claim 2, wherein:
the steel material is an oil-well steel pipe.