JP7406177B1 - Steel suitable for use in sour environments - Google Patents

Abstract

Provided is a steel material having high strength and excellent SSC resistance in a sour environment. The steel material according to the present disclosure has C: 0.15 to 0.45%, Si: 0.05 to 1.00%, Mn: 0.05 to 0.30%, and P: 0.030% or less in mass %. , S: 0.0050% or less, Al: 0.005-0.100%, Cr: 0.30-1.10%, Mo: 0.40-2.00%, Ti: 0.002-0. 020%, Nb: 0.002 to 0.100%, B: 0.0005 to 0.0040%, N: 0.0100% or less, O: less than 0.0040%, and the balance consists of Fe and impurities. , the yield strength is 862 MPa or more. In the steel material, the Si content is 20% or more, the O content is 10% or more, and the number density of Si oxides with a major axis of 5.0 μm or more is 5 pieces/100 mm ² or less in mass%. When the yield strength is 931 MPa or more, the number density of Si oxides is 5 pieces/200 mm ² or less.

Description

TECHNICAL FIELD The present disclosure relates to steel materials, and more particularly to steel materials suitable for use in sour environments.

BACKGROUND OF THE INVENTION As oil wells and gas wells (hereinafter, oil wells and gas wells are collectively referred to as "oil wells") become deeper, there is a demand for higher strength steel materials for oil wells, such as steel pipes for oil wells. Specifically, oil well steel pipes of 80 ksi class (yield strength of less than 80 to 95 ksi, that is, less than 552 to 655 MPa) and 95 ksi class (yield strength of less than 95 to 110 ksi, that is, less than 655 to 758 MPa) are widely used. Recently, steel pipes for oil wells with a strength of 110 ksi or more (yield strength of 758 MPa or more) have begun to be required.

Additionally, many deep wells are sour environments containing corrosive hydrogen sulfide. As used herein, a sour environment refers to an acidified environment containing hydrogen sulfide. Note that in a sour environment, carbon dioxide may be included. Steel pipes for oil wells used in such sour environments are required not only to have high strength but also to have sulfide stress cracking resistance (hereinafter referred to as SSC resistance). As described above, there is a growing demand for steel materials that have high strength and excellent SSC resistance.

Technologies for improving the SSC resistance of steel materials, such as steel pipes for oil wells, are disclosed in Japanese Patent Application Laid-open No. 2000-297344 (Patent Document 1), Japanese Patent Application Publication No. 2001-271134 (Patent Document 2), and International Publication No. 2008/2008/ This is proposed in No. 123422 (Patent Document 3).

The oil well steel disclosed in Patent Document 1 has, in mass %, C: 0.15 to 0.3%, Cr: 0.2 to 1.5%, Mo: 0.1 to 1%, and V: Contains 0.05 to 0.3% and Nb: 0.003 to 0.1%. In this oil well steel, the total amount of precipitated carbides is 1.5 to 4% by mass, the proportion of MC type carbides in the total amount of carbides is 5 to 45% by mass, and the proportion of M ₂₃ C ₆ type carbides is 5 to 45% by mass. The ratio is less than (200/t) mass% of the product, where the wall thickness is t (mm). Patent Document 1 describes that this steel for oil wells has excellent SSC resistance.

The low alloy steel material disclosed in Patent Document 2 has, 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 Contains ~0.1%, N: 0.01% or less, V: 0.05-0.5%, Ni: 0.1% or less, W: 1.0% or less, O: 0.01% or less The remainder consists of Fe and impurities, 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 means the ASTM grain size number of prior austenite grains. Patent Document 2 describes that this low alloy steel material has excellent SSC resistance.

The low alloy steel disclosed in Patent Document 3 has, in mass%, C: 0.10 to 0.20%, Si: 0.05 to 1.0%, Mn: 0.05 to 1.5%, Contains Cr: 1.0 to 2.0%, Mo: 0.05 to 2.0%, Al: 0.10% or less, and Ti: 0.002 to 0.05%, and Ceq ( =C+(Mn/6)+(Cr+Mo+V)/5) is 0.65 or more, the balance consists of Fe and impurities, and among the impurities, P: 0.025% or less, S: 0.010% or less, N : 0.007% or less, B: less than 0.0003%. In this low alloy steel, the number of M ₂₃ C ₆ type precipitates with a grain size of 1 μm or more is 0.1 pieces/mm ² or less. Patent Document 3 states that this low alloy steel has improved SSC resistance.

Japanese Patent Application Publication No. 2000-297344 Japanese Patent Application Publication No. 2001-271134 International Publication No. 2008/123422

As mentioned above, in recent years, as oil well environments have become more severe, steel materials with excellent SSC resistance have been required. Specifically, there is a growing demand for steel materials that have both high strength and excellent SSC resistance. Therefore, steel materials (for example, steel materials for oil wells) that have both high strength and excellent SSC resistance may be obtained by other technologies than those disclosed in Patent Documents 1 to 3 above.

An object of the present disclosure is to provide a steel material that has high strength and excellent SSC resistance in sour environments.

The steel material according to the present disclosure is
In mass%,
C: 0.15-0.45%,
Si: 0.05-1.00%,
Mn: 0.05-0.30%,
P: 0.030% or less,
S: 0.0050% or less,
Al: 0.005-0.100%,
Cr: 0.30-1.10%,
Mo: 0.40-2.00%,
Ti: 0.002 to 0.020%,
Nb: 0.002-0.100%,
B: 0.0005-0.0040%,
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: 0-1.50%,
Ca: 0-0.0100%,
Mg: 0 to 0.0100%,
Zr: 0 to 0.0100%,
Rare earth elements: 0 to 0.0100%, and
The remainder consists of Fe and impurities,
The yield strength is 862 MPa or more,
In the steel material,
In terms of mass %, the Si content is 20% or more, the O content is 10% or more, and the number density of Si oxides with a major axis of 5.0 μm or more is 5 pieces/100 mm ² or less,
When the yield strength is 931 MPa or more, the number density of the Si oxides is 5 pieces/200 mm ² or less.

Steel materials according to the present disclosure have high strength and excellent SSC resistance in sour environments.

Figure 1 shows the number density (pieces/100 mm ² ) of coarse Si oxides with a major axis of 5.0 μm or more in steel materials with a yield strength of 862 to less than 931 MPa in this example, and the SSC occurrence in the SSC resistance test. It is a figure showing the relationship with the number (pieces). Figure 2 shows the number density (pieces/200 mm 2 ) of coarse Si oxides with a major axis of 5.0 μm or more in the steel material with a yield strength of 931 MPa or more in this example, and the number of SSC occurrences (pieces/200 mm ² ) in the SSC resistance test. It is a figure showing a relationship with a book).

First, the present inventors considered obtaining a steel material having a yield strength of 125 ksi (862 MPa) or more as high strength. In other words, the present inventors investigated and studied methods for obtaining a yield strength of 125 ksi or more and excellent SSC resistance in a sour environment in a steel material intended for use in a sour environment. As a result, the present inventors obtained the following findings.

First, the present inventors focused on the chemical composition and studied to obtain a steel material having a yield strength of 125 ksi or more and excellent SSC resistance in a sour environment. As a result, it was thought that by reducing the manganese (Mn) content to 0.30% or less, it is possible to improve the SSC resistance of the steel material while maintaining the strength of the steel material. Mn combines with sulfur (S) in the steel material to form Mn sulfide. Mn sulfide is easily stretched by rolling and tends to form inclusions with a long major axis. Furthermore, Mn sulfide having a long major axis tends to become a starting point of destruction in a sour environment. Therefore, by reducing the Mn content to 0.30% or less, it is possible to suppress the formation of Mn sulfides and improve the SSC resistance of steel materials.

In other words, the present inventors, 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: 0-0.50%, Ni: 0-0.50%, W: 0-1.50%, Ca: 0-0.0100%, Mg: 0-0.0100%, Zr: 0 Steel materials consisting of ~0.0100%, rare earth elements: 0~0.0100%, and the balance Fe and impurities have a yield strength of 125 ksi or more, and also have excellent SSC resistance in sour environments. I thought there was a possibility that it would happen.

On the other hand, even if the steel material has the above-mentioned chemical composition, if it has a yield strength of 125 ksi or more, excellent SSC resistance may not be obtained in a sour environment. Therefore, the present inventors conducted a detailed study on the factors that reduce the SSC resistance of steel materials having the above-mentioned chemical composition and yield strength of 125 ksi or more. As a result, it has become clear that there is a concern that coarse Si oxides may be contained in the steel material having the above-mentioned chemical composition. If coarse Si oxides are included in the steel material, the SSC resistance of the steel material may be reduced.

In this specification, a Si oxide having a Si content of 20% or more, an O content of 10% or more, and a major axis of 5.0 μm or more in mass % is also referred to as a "coarse Si oxide." The present inventors further examined in detail the factors that cause the SSC resistance to decrease for each yield strength of steel materials having the above-mentioned chemical composition. As a result, the present inventors obtained the following knowledge.

Specifically, in steel materials having the above chemical composition, if the yield strength is less than 135 ksi (less than 931 MPa), by setting the number density of coarse Si oxides to 5 pieces/100 mm ² or less, a high yield strength of 125 ksi or more can be achieved. It has become clear that it is possible to achieve both this and excellent SSC resistance. This point will be specifically explained using the drawings.

Figure 1 shows the number density (pieces/100 mm ² ) of coarse Si oxides (Si oxides with a major axis of 5.0 μm or more) and SSC resistance in steel materials with a yield strength of less than 125 to 135 ksi in this example. FIG. 3 is a diagram showing the relationship with the number of SSCs (number of cells) generated in a sex test. FIG. 1 shows the number density of coarse Si oxides determined by the method described below for steel materials having the above-mentioned chemical composition and yield strength of less than 125 to 135 ksi, and the number density of coarse Si oxides obtained by the method described later. It was created using the results of the SSC resistance test and the number of pieces in which SSC occurred.

Referring to FIG. 1, for steel materials having the above-mentioned chemical composition and yield strength of less than 125 to 135 ksi, if the number density of coarse Si oxides is 5 pieces/100 mm ² or less, SSC It can be confirmed that excellent SSC resistance was obtained without occurrence of . Therefore, in this embodiment, it has the above-mentioned chemical composition, a yield strength of less than 125 to 135 ksi, and the number density of coarse Si oxides is 5 pieces/100 mm ² or less. As a result, it is possible to achieve both a yield strength of 125 ksi or more and excellent SSC resistance.

On the other hand, among steel materials with a yield strength of 125 ksi or more, steel materials with a yield strength of 135 ksi or more (931 MPa or more) have excellent SSC resistance in a sour environment even if the number density of coarse Si oxides is 5 pieces/100 mm2 ^or less. There were times when I couldn't get it. As a result of further detailed study by the present inventors, in steel materials with a yield strength of 135 ksi or more, by further reducing the number density of coarse Si oxides to 5 pieces/200 mm ² or less, high yield strength and excellent It has become clear that it is possible to achieve both SSC resistance and SSC resistance. This point will be specifically explained using the drawings.

Figure 2 shows the number density (pieces/200 mm ² ) of coarse Si oxides (Si oxides with a major axis of 5.0 μm or more) and the SSC resistance test in steel materials with a yield strength of 135 ksi or more in this example. It is a figure which shows the relationship with the number (number) of SSC occurrences in . Figure 2 shows the number density of coarse Si oxides determined by the method described later and the resistance value determined by the method described later for steel materials having the above-mentioned chemical composition and yield strength of 135 ksi or more among the examples described later. It was created using the results of the SSC test and the number of pieces in which SSC occurred.

Referring to FIG. 2, in steel materials having the above chemical composition and yield strength of 135 ksi or more, if the number density of coarse Si oxides is 5 pieces/200 mm ² or less, SSC will occur in the SSC resistance test. It can be confirmed that excellent SSC resistance was obtained without any problems. Therefore, in this embodiment, it has the above-mentioned chemical composition and a yield strength of 135 ksi or more, and the number density of coarse Si oxides is 5 pieces/200 mm ² or less. As a result, it is possible to achieve both a yield strength of 135 ksi or more and excellent SSC resistance.

The details of why the SSC resistance of steel materials is improved by reducing the number density of coarse Si oxides have not been clarified. However, the present inventors speculate as follows. When manufacturing a steel material having the above-mentioned chemical composition, deoxidation using aluminum (Al) is mainly performed in the steel manufacturing process. Therefore, for steel materials having the above-mentioned chemical composition, although Al oxides such as Al ₂ O ₃ have been studied, no attention has been paid to Si oxides. However, a small number of Si oxides, particularly coarse Si oxides with a major axis of 5.0 μm or more, may more easily deteriorate the SSC resistance of steel materials than Al oxides. Therefore, the present inventors believe that the SSC resistance of steel materials may be improved by lowering the number density of coarse Si oxides to 5 pieces/100 mm ² or less, or even 5 pieces/200 mm ² or less. is guessing.

Note that it is possible that the SSC resistance of the steel material is enhanced by a mechanism different from that inferred by the present inventors. As a result of having the above-mentioned chemical composition and having a number density of coarse Si oxides of 5 pieces/100 mm ² or less, it is possible to achieve both a yield strength of less than 125 to 135 ksi and excellent SSC resistance. Proven by example. Furthermore, as a result of having the above-mentioned chemical composition and having a number density of coarse Si oxides of 5 pieces/200 mm ² or less, it is possible to achieve both a yield strength of 135 ksi or more and excellent SSC resistance. Proven by example. Therefore, the steel material according to the present embodiment has the above-mentioned chemical composition and a yield strength of 862 MPa or more, the number density of coarse Si oxides in the steel material is 5 pieces/100 mm ² or less, and the yield strength is 931 MPa or more. In this case, the number density of coarse Si oxides in the steel material is 5 pieces/200 mm ² or less. As a result, the steel material according to this embodiment can have both a yield strength of 125 ksi or more and excellent SSC resistance.

The gist of the steel material according to this embodiment, which was completed based on the above knowledge, is as follows.

[1]
A steel material,
In mass%,
C: 0.15-0.45%,
Si: 0.05-1.00%,
Mn: 0.05-0.30%,
P: 0.030% or less,
S: 0.0050% or less,
Al: 0.005-0.100%,
Cr: 0.30-1.10%,
Mo: 0.40-2.00%,
Ti: 0.002 to 0.020%,
Nb: 0.002-0.100%,
B: 0.0005-0.0040%,
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: 0-1.50%,
Ca: 0-0.0100%,
Mg: 0 to 0.0100%,
Zr: 0 to 0.0100%,
Rare earth elements: 0 to 0.0100%, and
The remainder consists of Fe and impurities,
The yield strength is 862 MPa or more,
In the steel material,
In terms of mass %, the Si content is 20% or more, the O content is 10% or more, and the number density of Si oxides with a major axis of 5.0 μm or more is 5 pieces/100 mm ² or less,
When the yield strength is 931 MPa or more, the number density of the Si oxides is 5 pieces/200 mm ² or less,
Steel material.

[2]
The steel material according to [1],
V: 0.01-0.30%,
Cu: 0.01 to 0.50%,
Ni: 0.01-0.50%,
W: 0.01-1.50%,
Ca: 0.0001-0.0100%,
Mg: 0.0001-0.0100%,
Zr: 0.0001 to 0.0100%, and
Rare earth element: Contains one or more elements selected from the group consisting of 0.0001 to 0.0100%,
Steel material.

[3]
The steel material according to [1] or [2],
The steel material is a steel pipe for oil wells,
Steel material.

In this specification, the oil country steel pipe may be an oil country tubular product. The oil well steel pipe may be a seamless steel pipe or a welded steel pipe. Oil country tubular goods are, for example, steel pipes used for casing and tubing applications.

The oil well steel pipe according to this embodiment is preferably 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 is 15 mm or more, it has a yield strength of 125 ksi or more and excellent SSC resistance in a sour environment.

The shape of the steel material according to this embodiment is not particularly limited. That is, the steel material according to this embodiment may be a steel pipe, a round steel (solid material), or a steel plate. Note that the round steel means a steel bar whose cross section perpendicular to the axial direction is circular. Further, the steel pipe may be a seamless steel pipe or a welded steel pipe.

Hereinafter, the steel material according to this embodiment will be explained in detail. "%" with respect to elements means mass % unless otherwise specified.

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

C: 0.15-0.45%
Carbon (C) improves the hardenability of steel and increases its strength. Furthermore, C promotes the spheroidization of carbides during tempering during the manufacturing process and improves the SSC resistance of the steel material. If the carbides are dispersed, the strength of the steel material will further increase. If the C content is too low, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the C content is too high, even if the contents of other elements are within the ranges of this embodiment, carbides will be too large and the toughness of the steel material will decrease. Furthermore, if the C content is too high, quench cracking may be more likely to occur during quenching during the manufacturing process. Therefore, the C content is 0.15-0.45%. The preferable lower limit of the C content is 0.18%, more preferably 0.20%, still more preferably 0.22%, and still more preferably 0.25%. A preferable upper limit of the C content is 0.40%, more preferably 0.38%, and still more preferably 0.35%.

Si: 0.05-1.00%
Silicon (Si) deoxidizes steel. If the Si content is too low, even if the contents of other elements are within the ranges of this embodiment, the above effects cannot be sufficiently obtained. On the other hand, if the Si content is too high, the SSC resistance of the steel material will decrease even if the other element contents are within the ranges of this embodiment. Therefore, the Si content is 0.05-1.00%. The lower limit of the preferable Si content is 0.10%, more preferably 0.15%, and still more preferably 0.20%. The preferable upper limit of the Si content is 0.85%, more preferably 0.75%, even more preferably 0.60%, still more preferably 0.50%, and even more preferably 0.40%. %.

Mn: 0.05-0.30%
Manganese (Mn) deoxidizes steel. Mn further improves the hardenability of the steel material. If the Mn content is too low, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Mn content is too high, even if the contents of other elements are within the ranges of the present embodiment, coarse sulfide-based inclusions will be generated and the SSC resistance of the steel material will be reduced. Therefore, the Mn content is 0.05-0.30%. The lower limit of the Mn content is preferably 0.06%, more preferably 0.08%, and still more preferably 0.10%. A preferable upper limit of the Mn content is 0.28%, more preferably 0.25%, and still more preferably 0.20%.

P: 0.030% or less Phosphorus (P) is an impurity. That is, the lower limit of the P content is over 0%. If the P content is too high, even if the contents of other elements are within the ranges of this embodiment, P will segregate at grain boundaries and the SSC resistance of the steel material will decrease. Therefore, the P content is 0.030% or less. A preferable upper limit of the P content is 0.025%, more preferably 0.020%, and even more preferably 0.015%. It is preferable that the P content is as low as possible. However, extreme reduction in P content significantly increases manufacturing costs. Therefore, when considering industrial production, the preferable lower limit of the P content is 0.001%, more preferably 0.002%, and still more preferably 0.003%.

S: 0.0050% or less Sulfur (S) is an impurity. That is, the lower limit of the S content is more than 0%. If the S content is too high, even if the contents of other elements are within the ranges of this embodiment, S will segregate at grain boundaries and the SSC resistance of the steel material will decrease. Therefore, the S content is 0.0050% or less. A preferable upper limit of the S content is 0.0040%, more preferably 0.0030%, and still more preferably 0.0020%. It is preferable that the S content is as low as possible. However, extreme reduction in S content significantly increases manufacturing costs. Therefore, when considering industrial production, the preferable lower limit of the S content is 0.0001%, more preferably 0.0002%, and still more preferably 0.0003%.

Al: 0.005-0.100%
Aluminum (Al) deoxidizes steel. If the Al content is too low, even if the contents of other elements are within the ranges of this embodiment, the above effects will not be sufficiently obtained, and the SSC resistance of the steel material will decrease. On the other hand, if the Al content is too high, even if the contents of other elements are within the ranges of this embodiment, coarse oxide-based inclusions will be generated and the SSC resistance of the steel material will be reduced. Therefore, the Al content is 0.005-0.100%. The preferable lower limit of the Al content is 0.010%, more preferably 0.015%, and still more preferably 0.020%. A preferable upper limit of the Al content is 0.080%, more preferably 0.060%, and still more preferably 0.040%. The "Al" content as used herein means the content of "acid-soluble Al", that is, "sol.Al".

Cr:0.30~1.10%
Chromium (Cr) improves the hardenability of steel materials. Cr further increases the temper softening resistance of the steel material and enables high temperature tempering. As a result, the SSC resistance of the steel material increases. If the Cr content is too low, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Cr content is too high, the SSC resistance of the steel material will decrease even if the other element contents are within the ranges of this embodiment. Therefore, the Cr content is between 0.30 and 1.10%. The lower limit of the Cr content is preferably 0.35%, more preferably 0.40%, and still more preferably 0.50%. A preferable upper limit of the Cr content is 1.00%, more preferably 0.90%, and still more preferably 0.80%.

Mo: 0.40~2.00%
Molybdenum (Mo) improves the hardenability of steel materials. Mo further increases the temper softening resistance of the steel material and enables high temperature tempering. As a result, the SSC resistance of the steel material increases. If the Mo content is too low, even if the contents of other elements are within the ranges of this embodiment, the above effects cannot be sufficiently obtained. On the other hand, if the Mo content is too high, the above effects will be saturated. Therefore, the Mo content is 0.40-2.00%. The lower limit of the Mo content is preferably 0.45%, more preferably 0.50%, and even more preferably 0.60%. A preferable upper limit of the Mo content is 1.80%, more preferably 1.60%, and still more preferably 1.40%.

Ti: 0.002-0.020%
Titanium (Ti) combines with N to form a nitride, and the pinning effect refines the crystal grains of the steel material. As a result, the strength of the steel material increases. If the Ti content is too low, even if the contents of other elements are within the ranges of this embodiment, the above effects cannot be sufficiently obtained. On the other hand, if the Ti content is too high, even if the contents of other elements are within the ranges of this embodiment, Ti nitrides will become coarse and the SSC resistance of the steel material will decrease. Therefore, the Ti content is 0.002 to 0.020%. The lower limit of the Ti content is preferably 0.003%, more preferably 0.004%. A preferable upper limit of the Ti content is 0.018%, more preferably 0.015%, and still more preferably 0.010%.

Nb: 0.002-0.100%
Niobium (Nb) combines with C and/or N to form carbides, nitrides, or carbonitrides (hereinafter referred to as "carbonitrides, etc."). Carbonitrides and the like refine the crystal grains of the steel material due to the pinning effect, and improve the low temperature toughness and SSC resistance of the steel material. Furthermore, Nb forms fine carbides during tempering, increases the temper softening resistance of the steel material, and increases the strength of the steel material. If the Nb content is too low, even if the contents of other elements are within the ranges of this embodiment, the above effects cannot be sufficiently obtained. On the other hand, if the Nb content is too high, even if the content of other elements is within the range of this embodiment, carbonitrides and the like will be excessively generated, resulting in a decrease in the SSC resistance of the steel material. Therefore, the Nb content is 0.002-0.100%. The lower limit of the Nb content is preferably 0.005%, more preferably 0.010%, even more preferably 0.015%, and still more preferably 0.020%. A preferable upper limit of the Nb content is 0.080%, more preferably 0.060%, and still more preferably 0.040%.

B: 0.0005-0.0040%
Boron (B) is dissolved in steel to improve the hardenability of the steel and increase the strength of the steel. If the B content is too low, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the B content is too high, even if the contents of other elements are within the ranges of this embodiment, coarse nitrides will be generated and the SSC resistance of the steel material will decrease. Therefore, the B content is 0.0005 to 0.0040%. The lower limit of the B content is preferably 0.0006%, more preferably 0.0008%. The upper limit of the B content is preferably 0.0035%, more preferably 0.0030%, even more preferably 0.0025%, and even more preferably 0.0020%.

N: 0.0100% or less Nitrogen (N) is unavoidably contained. That is, the lower limit of the N content is over 0%. N combines with Ti to form nitrides and refines the crystal grains of the steel material due to the pinning effect. As a result, the strength of the steel material increases. However, if the N content is too high, coarse nitrides will be formed even if the contents of other elements are within the ranges of this embodiment, and the SSC resistance of the steel material will deteriorate. Therefore, the N content is 0.0100% or less. A preferable upper limit of the N content is 0.0080%, more preferably 0.0060%, and still more preferably 0.0040%. The preferable lower limit of the N content in order to obtain the above effects more effectively is 0.0005%, more preferably 0.0010%, still more preferably 0.0015%, and still more preferably 0.0020%. It is.

O: Less than 0.0040% Oxygen (O) is an impurity. That is, the lower limit of the O content is over 0%. If the O content is too high, even if the contents of other elements are within the ranges of this embodiment, coarse oxides will be formed and the SSC resistance of the steel material will decrease. Therefore, the O content is less than 0.0040%. A preferable upper limit of the O content is 0.0035%, more preferably 0.0030%, still more preferably 0.0025%, and still more preferably 0.0020%. It is preferable that the O content is as low as possible. However, extreme reduction in O content significantly increases manufacturing costs. Therefore, when considering industrial production, the preferable lower limit of the O content is 0.0001%, more preferably 0.0002%, and still more preferably 0.0003%.

The remainder of the chemical composition of the steel material according to this embodiment consists of Fe and impurities. Here, impurities are those that are mixed in from ores used as raw materials, scraps, or the manufacturing environment when steel products are industrially manufactured, and to the extent that they do not adversely affect the steel products according to this embodiment. means permissible.

[Optional element]
The chemical composition of the above-mentioned steel material may further contain V in place of a part of Fe.

V: 0-0.30%
Vanadium (V) is an optional element and may not be included. That is, the V content may be 0%. When contained, V forms carbonitrides and the like. Carbonitrides and the like make the crystal grains of steel materials finer due to their pinning effect, thereby improving the SSC resistance of steel materials. Furthermore, V forms fine carbides during tempering, increases the temper softening resistance of the steel material, and increases the strength of the steel material. If even a small amount of V is contained, the above effects can be obtained to some extent. However, if the V content is too high, even if the contents of other elements are within the ranges of this embodiment, carbonitrides and the like will be excessively produced, resulting in a decrease in the SSC resistance of the steel material. Therefore, the V content is 0-0.30%. The lower limit of the V content is preferably more than 0%, more preferably 0.01%, even more preferably 0.02%, even more preferably 0.05%, and even more preferably 0.07%. It is. A preferable upper limit of the V content is 0.25%, more preferably 0.20%, and still more preferably 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 place of a part of Fe. All of these elements are optional elements and improve the hardenability of the steel material.

Cu: 0-0.50%
Copper (Cu) is an optional element and may not be included. That is, the Cu content may be 0%. When contained, Cu improves the hardenability of the steel material and increases the strength of the steel material. If even a small amount of Cu is contained, the above effects can be obtained to some extent. However, if the Cu content is too high, even if the contents of other elements are within the ranges of this embodiment, the hardenability of the steel material will become too high and the SSC resistance of the steel material will decrease. Therefore, the Cu content is 0-0.50%. The preferable lower limit of the Cu content is more than 0%, more preferably 0.01%, even more preferably 0.02%, and still more preferably 0.05%. The upper limit of the Cu content is preferably 0.35%, more preferably 0.25%, even more preferably 0.15%, and still more preferably 0.10%.

Ni: 0-0.50%
Nickel (Ni) is an optional element and may not be included. That is, the Ni content may be 0%. When contained, Ni improves the hardenability of the steel material and increases the strength of the steel material. Ni further dissolves in steel and improves the low-temperature toughness of the steel material. If even a small amount of Ni is contained, these effects can be obtained to some extent. However, if the Ni content is too high, local corrosion will be promoted and the SSC resistance of the steel material will be reduced even if the other element contents are within the ranges of this embodiment. Therefore, the Ni content is 0 to 0.50%. The preferable lower limit of the Ni content is more than 0%, more preferably 0.01%, and still more preferably 0.02%. A preferable upper limit of the Ni content is 0.30%, more preferably 0.20%, and still more preferably 0.10%.

The chemical composition of the above-mentioned steel material may further contain W in place of a part of Fe.

W: 0-1.50%
Tungsten (W) is an optional element and may not be included. That is, the W content may be 0%. When included, W forms a protective corrosion film in a sour environment and inhibits hydrogen from penetrating into the steel material. This improves the SSC resistance of the steel material. If even a small amount of W is contained, the above effects can be obtained to some extent. However, if the W content is too high, even if the contents of other elements are within the ranges of this embodiment, coarse carbides will be generated in the steel material, resulting in a decrease in the low-temperature toughness and SSC resistance of the steel material. Therefore, the W content is 0 to 1.50%. The lower limit of the W content is preferably more than 0%, more preferably 0.01%, even more preferably 0.03%, and still more preferably 0.05%. The upper limit of the W content is preferably 1.30%, more preferably 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 elements in place of a part of Fe. All of these elements are optional elements, and render S in the steel material harmless as sulfide. As a result, these elements improve the SSC resistance of steel materials.

Ca: 0~0.0100%
Calcium (Ca) is an optional element and may not be included. That is, the Ca content may be 0%. When contained, Ca renders S in the steel material harmless as sulfide and improves the SSC resistance of the steel material. If even a small amount of Ca is contained, the above effects can be obtained to some extent. However, if the Ca content is too high, even if the contents of other elements are within the ranges of this embodiment, the oxides in the steel material will become coarse and the SSC resistance of the steel material will decrease. Therefore, the Ca content is 0 to 0.0100%. The preferable lower limit of the Ca content is more than 0%, more preferably 0.0001%, still more preferably 0.0003%, and still more preferably 0.0006%. A preferable upper limit of the Ca content is 0.0040%, more preferably 0.0025%, and still more preferably 0.0020%.

Mg: 0-0.0100%
Magnesium (Mg) is an optional element and may not be included. That is, the Mg content may be 0%. When contained, Mg renders S in the steel material harmless as sulfide and improves the SSC resistance of the steel material. If even a small amount of Mg is contained, the above effects can be obtained to some extent. However, if the Mg content is too high, even if the contents of other elements are within the ranges of this embodiment, the oxides in the steel material will become coarse and the SSC resistance of the steel material will decrease. Therefore, the Mg content is 0 to 0.0100%. The preferable lower limit of the Mg content is more than 0%, more preferably 0.0001%, still more preferably 0.0003%, and still more preferably 0.0006%. A preferable upper limit of the Mg content is 0.0040%, more preferably 0.0025%, and still more preferably 0.0020%.

Zr: 0~0.0100%
Zirconium (Zr) is an optional element and may not be included. That is, the Zr content may be 0%. When contained, Zr renders S in the steel material harmless as sulfide and improves the SSC resistance of the steel material. If even a small amount of Zr is contained, the above effects can be obtained to some extent. However, if the Zr content is too high, even if the contents of other elements are within the ranges of this embodiment, the oxides in the steel material will become coarse and the SSC resistance of the steel material will decrease. Therefore, the Zr content is 0 to 0.0100%. The preferable lower limit of the Zr content is more than 0%, more preferably 0.0001%, still more preferably 0.0003%, and still more preferably 0.0006%. A preferable upper limit of the Zr content is 0.0040%, more preferably 0.0025%, and still more preferably 0.0020%.

Rare earth elements (REM): 0 to 0.0100%
Rare earth elements (REM) are optional elements and may not be included. That is, the REM content may be 0%. When contained, REM renders S in the steel material harmless as sulfide and improves the SSC resistance of the steel material. REM further combines with P in the steel material to suppress segregation of P at grain boundaries. Therefore, a decrease in SSC resistance of the steel material due to P segregation is suppressed. If even a small amount of REM is contained, the above effects can be obtained to some extent even if the contents of other elements are within the range of this embodiment. However, if the REM content is too high, even if the contents of other elements are within the ranges of this embodiment, the oxides in the steel material will become coarse and the SSC resistance of the steel material will decrease. Therefore, the REM content is between 0 and 0.0100%. A preferable lower limit of the REM content is more than 0%, more preferably 0.0001%, still more preferably 0.0003%, and still more preferably 0.0006%. A preferable upper limit of the REM content is 0.0040%, more preferably 0.0025%, and still more preferably 0.0020%.

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

[Yield strength]
The yield strength of the steel material according to this embodiment is 862 MPa or more. The yield strength as used herein means the stress at 0.65% elongation (0.65% yield strength) obtained in a tensile test. The steel material according to this embodiment has the above-mentioned chemical composition and satisfies the number density of coarse Si oxides described below, so that it has excellent SSC resistance in a sour environment even if the yield strength is 862 MPa or more. In this embodiment, the upper limit of the yield strength of the steel material is not particularly limited, but is, for example, 1069 MPa (155 ksi), preferably 1034 MPa (150 ksi).

The yield strength of the steel material according to this embodiment can be determined by the following method. Specifically, a tensile test is performed in accordance with ASTM E8/E8M (2021). A round bar test piece is produced from the steel material according to this embodiment. If the steel material is a steel plate, prepare a round bar test piece from the center of the plate thickness. In this case, the axial direction of the round bar test piece is parallel to the rolling direction of the steel plate. If the steel material is a steel pipe, prepare a round bar test piece from the center of the wall thickness. In this case, the axial direction of the round bar test piece is parallel to the axial direction of the steel pipe. When the steel material is a round bar, a round bar test piece is prepared from the R/2 position. In this specification, the R/2 position means the center position of the radius R in a cross section perpendicular to the axial direction of the round steel. In this case, the axial direction of the round bar test piece is parallel to the axial direction of the round steel. The size of the round bar test piece is, for example, a parallel part diameter of 8.9 mm and a gauge length of 35.6 mm. Using a round bar test piece, perform a tensile test in the atmosphere at room temperature (25°C), and define the stress at 0.65% elongation (0.65% proof stress) as the yield strength (MPa). do. In this embodiment, the yield strength (MPa) is determined by rounding the obtained value to the first decimal place.

[Number density of coarse Si oxide]
As mentioned above, in this specification, a Si oxide having a Si content of 20% or more, an O content of 10% or more, and a major axis of 5.0 μm or more in terms of mass % is referred to as a "coarse Si oxide". ” is also called. The steel material according to the present embodiment has the above-mentioned chemical composition and the above-mentioned yield strength, and further has a Si content of 20% or more and an O content of 10% or more in mass%. The number density of Si oxides (coarse Si oxides) having a major axis of 5.0 μm or more is 5 pieces/100 mm ² or less. Further, in the steel material according to the present embodiment, when the yield strength is 931 MPa or more, the number density of coarse Si oxides is 5 pieces/200 mm ² or less.

That is, in the steel material according to the present embodiment, when the yield strength is less than 862 to 931 MPa, the number density of coarse Si oxides is 5 pieces/100 mm ² or less (that is, 10 pieces/200 mm ² or less), and the yield strength is When the pressure is 931 MPa or more, the number density of coarse Si oxides is 5 pieces/200 mm ² or less. As a result, the steel material according to this embodiment can have both a yield strength of 125 ksi or more and excellent SSC resistance. In the steel material according to the present embodiment, when the yield strength is less than 862 to 931 MPa, the preferable upper limit of the number density of coarse Si oxides is 4 pieces/100 mm ² , more preferably 3 pieces/100 mm ² . In the steel material according to the present embodiment, when the yield strength is 931 MPa or more, the preferable upper limit of the number density of coarse Si oxides is 4 pieces/200 mm ² , more preferably 3 pieces/200 mm ² . In addition, in the steel material according to this embodiment, the lower limit of the number density of coarse Si oxides is not particularly limited, and may be 0 pieces/100 mm ² , that is, 0 pieces/200 mm ² .

In this embodiment, the number density of coarse Si oxides in the steel material can be determined by the following method. First, a test piece whose observation surface is a surface including the rolling direction and the rolling direction is prepared from the steel material according to the present embodiment. Specifically, when the steel material is a steel plate, a test piece is prepared from the central part of the plate thickness, with the plane including the rolling direction and the plate thickness direction as the observation plane. When the steel material is a steel pipe, a test piece is prepared from the center of the wall thickness, with the surface including the pipe axis direction and the pipe diameter direction as the observation surface. When the steel material is a round steel, a test piece is prepared whose observation surface is a surface including the R/2 position in the center and including the axial direction and the radial direction.

After polishing the observation surface of the prepared test piece to a mirror surface, measurements are performed. Although the area of the observation surface is not limited, it is, for example, 300 mm ² (20 mm x 15 mm). In the observation plane, the number of Si oxides having a major axis of 5.0 μm or more is determined. Specifically, particles on the observation surface are first identified based on contrast. Element concentration analysis (EDS analysis) is performed on each identified particle. In the EDS analysis, the acceleration voltage is set to 20 kV, and target elements are quantified as N, O, Mg, Al, Si, P, S, Ca, Ti, Cr, Mn, Fe, Cu, Zr, and Nb. Based on the EDS analysis results of each particle, if the Si content is 20% or more and the O content is 10% or more in mass %, the particle is identified as "Si oxide".

Among the Si oxides identified on the observation surface, Si oxides with a major axis of 5.0 μm or more (coarse Si oxides) are identified, and the total number of coarse Si oxides is determined. Note that the long axis of the Si oxide can be determined by a well-known method. In this specification, the long axis of the Si oxide means the largest line segment (μm) among the line segments connecting any two points on the outer periphery of the Si oxide on the observation plane. The number density of coarse Si oxides (pieces/100 mm ² or pieces/200 mm ² ) is determined based on the total number of coarse Si oxides and the total area of the observation surface. In this embodiment, the number density of coarse Si oxides (numbers/100 mm ² or numbers/200 mm ² ) is determined by rounding the obtained value to the first decimal place. Further, the number density of coarse Si oxides can be measured using a scanning electron microscope equipped with a composition analysis function (SEM-EDS device). As the SEM-EDS device, for example, an automatic analyzer manufactured by FEI (ASPEX), trade name: Metals Quality Analyzer, can be used.

[SSC resistance]
The SSC resistance of the steel material according to the present embodiment can be evaluated by an SSC resistance test conducted in accordance with NACE TM0177-2016 Method A. Specifically, it can be evaluated using the following method.

A mixed aqueous solution of 5.0 mass% sodium chloride and 0.4 mass% sodium acetate (NACE solution B) adjusted to pH 3.5 with acetic acid is used as the test solution. A round bar test piece is produced from the steel material according to this embodiment. If the steel material is a steel plate, prepare a round bar test piece from the center of the plate thickness. In this case, the axial direction of the round bar test piece is parallel to the rolling direction of the steel plate. If the steel material is a steel pipe, prepare a round bar test piece from the center of the wall thickness. In this case, the axial direction of the round bar test piece is parallel to the axial direction of the steel pipe. When the steel material is a round bar, a round bar test piece is prepared from the R/2 position. In this case, the axial direction of the round bar test piece is parallel to the axial direction of the round steel. The size of the round bar test piece is, for example, 6.35 mm in diameter and 25.4 mm in length of the parallel part. Note that the axial direction of the round bar test piece was parallel to the rolling direction of the steel material.

[When yield strength is less than 931 MPa]
When 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 test piece. A test solution at 24° C. is poured into a test container so that the stressed round bar test piece is immersed therein to form a test bath. After the test bath is degassed, a mixed gas of 0.1 atm H ₂ S gas and 0.9 atm CO ₂ gas is blown into the test bath to saturate it. The test bath saturated with gas mixture is maintained at 24° C. for 1440 hours. When the steel material according to this embodiment has a yield strength of less than 931 MPa, no cracking is observed after 1440 hours in the SSC resistance test conducted under the above conditions.

[When yield strength is 931 MPa or more]
When 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 test piece. A test solution at 24° C. is poured into a test container so that the stressed round bar test piece is immersed therein to form a test bath. After the test bath is degassed, a mixed gas of 0.01 atm H ₂ S gas and 0.99 atm CO ₂ gas is blown into the test bath to saturate it. The test bath saturated with gas mixture is maintained at 24° C. for 1440 hours. In the steel material according to the present embodiment, when the yield strength is 931 MPa or more, no cracking is observed after 1440 hours in the SSC resistance test conducted under the above conditions.

[Microstructure]
In the microstructure of the steel material according to this embodiment, the total volume fraction of tempered martensite and tempered bainite is 90% or more. The remainder of the microstructure is, for example, ferrite or pearlite. If the microstructure of the steel material having the above-mentioned chemical composition contains a total volume fraction of tempered martensite and tempered bainite of 90% or more, it will be excellent in a sour environment, provided that the other configurations of this embodiment are satisfied. It shows excellent SSC resistance. That is, in this embodiment, if the steel material has excellent SSC resistance, it is determined that the microstructure has a total volume fraction of tempered martensite and tempered bainite of 90% or more.

In addition, when determining the volume fraction of tempered martensite and tempered bainite by observation, it can be determined by the following method. First, a test piece having an observation surface is produced from the steel material according to this embodiment. When the steel material is a steel plate, a test piece is prepared from the center of the plate thickness, with the plane including the rolling direction and the plate thickness direction as the observation plane. When the steel material is a steel pipe, a test piece is prepared from the center of the wall thickness, with the surface including the pipe axis direction and the pipe diameter direction as the observation surface. When the steel material is a round steel, a test piece is prepared whose observation surface is a surface including the R/2 position in the center and including the axial direction and the radial direction.

After polishing the observation surface of the test piece to a mirror surface, it is immersed in a nital corrosive solution for about 10 seconds to reveal the structure by etching. The etched observation surface is observed in 10 fields of view using a secondary electron image using a scanning electron microscope (SEM). The field of view area is, for example, 0.01 mm ² (1000x magnification). In each field of view, tempered martensite and tempered bainite are identified from the contrast. The area ratio of the specified tempered martensite and tempered bainite is determined. The method for determining the area ratio is not particularly limited, and any known method may be used. For example, the area ratio of tempered martensite and tempered bainite can be determined by image analysis. In the present embodiment, the arithmetic mean value of the area ratios of tempered martensite and tempered bainite determined in all visual fields is defined as the volume ratio of tempered martensite and tempered bainite.

[Shape of steel material]
As mentioned above, the shape of the steel material according to this embodiment is not particularly limited. Examples of the steel materials include steel pipes, steel plates, and round steel. When the steel material is a steel pipe for oil wells, the preferred wall thickness is 9 to 60 mm. More preferably, the steel material according to this embodiment is a seamless steel pipe. When the steel material according to this embodiment is a seamless steel pipe, even if it is a thick seamless steel pipe with a wall thickness of 15 mm or more, it has a yield strength of 125 ksi or more and excellent SSC resistance in a sour environment.

[Production method]
A method for manufacturing steel materials according to this embodiment will be described. Hereinafter, a method for manufacturing a seamless steel pipe will be described as an example of the steel material according to the present embodiment. The manufacturing method of seamless steel pipes consists of a process of preparing the material (steel manufacturing process), a process of hot working the material to produce the base pipe (hot working process), and quenching and tempering the base pipe. and a step of forming a seamless steel pipe (quenching step and tempering step). Note that the manufacturing method according to this embodiment is not limited to the manufacturing method described below. Each step will be explained in detail below.

[Steelmaking process]
In the steelmaking process, first, hot metal produced by a well-known method is refined in a converter (primary refining). Secondary refining is performed on the molten steel that has been primary refined. In the secondary refining, alloying elements are added for component adjustment to produce molten steel that satisfies the above-mentioned chemical composition.

The secondary refining is performed by, for example, RH (Ruhrstahl-Hausen) vacuum degassing treatment. After that, final adjustment of alloy components is performed. In the secondary refining, composite refining may be performed. In this case, a refining process using, for example, an LF (Ladle Furnace) or a VAD (Vacuum Arc Degassing) is performed before the RH vacuum degassing process.

Materials are manufactured using molten steel that has undergone secondary refining. Specifically, a slab (slab, bloom, or billet) is manufactured by a continuous casting method using molten steel that has been subjected to secondary refining. In the continuous casting method, molten steel is first poured from a ladle into a tundish. At this time, in order to seal the nozzle of the ladle, the nozzle is usually filled with packed sand. Therefore, packing sand may get mixed in with the molten steel from the ladle into the tundish. Further, when producing a material having the above-mentioned chemical composition, Si oxide may be used as packing sand. In this case, there is a concern that Si oxide may be introduced into the manufactured material.

Therefore, in this embodiment, in order to prevent the Si oxide sealed in the nozzle of the ladle from being introduced into the tundish, the molten steel and the Si oxide are separated. Although the method of separating Si oxide is not particularly limited, for example, the following method can be used. A beveled metal plate is placed below the nozzle of the ladle and above the opening of the tundish. When the nozzle of the ladle is opened, Si oxide is first discharged from the nozzle, followed by molten steel. Here, Si oxide is lighter than molten steel. Therefore, the Si oxide discharged from the nozzle is guided out of the opening of the tundish along the slope of the metal plate. The inclination of the metal plate may be provided, for example, by arranging a cone-shaped metal plate without a bottom surface so that the apex is directly below the nozzle of the ladle, or by other methods. Good too. Further, a single metal plate may be used, or a plurality of metal plates may be used in a stacked manner. Further, the thickness of the metal plate is not particularly limited, but is, for example, about 1 to 10 mm.

After the Si oxide is discharged from the nozzle, molten steel is discharged. At this time, the molten steel discharged from the nozzle is introduced into the tundish through the opening along with the metal plate. That is, in this embodiment, part or all of the metal plate may be introduced into the tundish and mixed into the molten steel. Therefore, the metal plate in this embodiment is preferably a metal plate made of an alloy element contained in molten steel. For example, an aluminum plate can be used as the metal plate made of an alloying element contained in the molten steel. Note that in this specification, an aluminum plate means a metal plate made of aluminum and the remainder impurities.

Preferably, after the Si oxide is discharged from the nozzle and before the molten steel is discharged, the metal plate is removed from below the nozzle. In this case, it is possible to prevent Si oxide adhering to the metal plate from being mixed into the molten steel. As a result, in the manufactured steel material, the number density of coarse Si oxides may be reduced to 5 pieces/200 mm ² or less. Therefore, in this embodiment, it is preferable to remove the metal plate from below the nozzle after the Si oxide is discharged from the nozzle but before the molten steel is discharged.

The method for removing the metal plate from below the nozzle is not particularly limited, but for example, a hole may be formed in a part of the metal plate and the metal plate may be removed using a rod having a hook at the tip. In this case, the metal plate can be removed by hooking the hook at the tip of the rod into the hole in the metal plate and pulling the rod. By the above method, Si oxide can be separated from molten steel and the molten steel can be introduced into the tundish. Note that the method for separating Si oxide from molten steel is not limited to the above method.

By the above method, molten steel is cast to produce a material. The material is preferably a billet with a circular cross section (round billet). The method of manufacturing the material is not particularly limited. For example, molten steel may be cast into round billets using a continuous casting method. Alternatively, a billet with a rectangular cross section may be produced by casting molten steel, or a bloom may be produced. In these cases, it is preferable to carry out blooming rolling to produce a round billet from a billet or bloom having a rectangular cross section.

[Hot processing process]
In the hot working step, the prepared material is hot worked to produce an intermediate steel material. When the steel material is a seamless steel pipe, the intermediate steel material corresponds to the base pipe. First, the billet is heated in a heating furnace. The heating temperature is not particularly limited, but is, for example, 1100 to 1300°C. Hot working is performed on the billet extracted from the heating furnace to produce a raw pipe (seamless steel pipe). The hot working method is not particularly limited, and may be any known method.

For example, the raw pipe may be manufactured by implementing the Mannesmann method as hot working. In this case, the round billet is pierced and rolled using a piercer. In the case of piercing rolling, the piercing ratio is not particularly limited, but is, for example, 1.0 to 4.0. The hole-rolled round billet is further hot-rolled using a mandrel mill, reducer, sizing mill, etc. to form a blank tube. The cumulative area reduction rate in the hot working step is, for example, 20 to 70%.

Other hot working methods may be used to produce blank tubes from billets. For example, in the case of a short thick-walled steel material such as a coupling, the raw tube may be manufactured by forging such as the Erhard method. A raw pipe is manufactured through the above steps. The wall thickness of the raw tube is not particularly limited, but is, for example, 9 to 60 mm.

The raw tube manufactured by hot working may be air-cooled (As-Rolled). The raw tube manufactured by hot working may be quenched directly after hot working without being cooled to room temperature, or may be quenched after reheating (reheating) after hot working. good.

When directly quenching after hot working or quenching after reheating, cooling may be stopped during quenching or slow cooling may be performed. In this case, it is possible to suppress the occurrence of quench cracks in the raw pipe. In the case where direct quenching is performed after hot working or quenching is performed after reheating, stress relief annealing (SR) may be performed after quenching and before the next step of heat treatment. In this case, residual stress in the raw pipe is removed.

If the steel material is round steel, first the material is heated in a heating furnace. The heating temperature is not particularly limited, but is, for example, 1100 to 1300°C. Hot working is performed on the material extracted from the heating furnace to produce an intermediate steel material having a circular cross section perpendicular to the axial direction. The hot working is, for example, blooming rolling using a blooming mill or hot rolling using a continuous rolling mill. A continuous rolling mill has a horizontal stand having a pair of grooved rolls arranged in parallel in the vertical direction and a vertical stand having a pair of grooved rolls arranged in parallel in the horizontal direction, which are arranged alternately.

When the steel material is a steel plate, the material is first heated in a heating furnace. The heating temperature is not particularly limited, but is, for example, 1100 to 1300°C. The raw material extracted from the heating furnace is hot-rolled using a blooming mill and a continuous rolling mill to produce an intermediate steel material in the shape of a steel plate.

As described above, in the hot working step, the prepared material is hot worked to produce an intermediate steel material. The hardening process will be explained in detail below.

[Quenching process]
In the quenching process, the prepared intermediate steel material (raw pipe) is quenched. In this specification, "quenching" means quenching an intermediate steel material having an _A3 point or higher. The preferred quenching temperature is 800-1000°C. If the quenching temperature is too high, the crystal grains of the prior γ grains may become coarse and the SSC resistance of the steel material may decrease. Therefore, the quenching temperature is preferably 800 to 1000°C.

In this specification, the quenching temperature refers to the surface temperature of the intermediate steel material measured with a thermometer installed on the exit side of the equipment that performs the final hot working when quenching is performed directly after hot working. Equivalent to. Furthermore, the quenching temperature corresponds to the temperature of the furnace in which the reheating or reheating is performed when quenching is performed after the hot working.

In the quenching method, for example, the intermediate steel material (raw tube) is continuously cooled from the quenching start temperature to continuously lower the surface temperature of the empty tube. The method of continuous cooling treatment is not particularly limited, and any known method may be used. Examples of the continuous cooling treatment include a method in which the raw tube is cooled by immersing it in a water tank, and a method in which the raw tube is acceleratedly cooled by shower water cooling or mist cooling.

If the cooling rate during quenching is too slow, a microstructure consisting mainly of martensite and bainite will not be obtained, and the mechanical properties defined in this embodiment (yield strength of 125 ksi or more) will not be obtained. Furthermore, in this case, excellent low temperature toughness and excellent SSC resistance cannot be obtained.

Therefore, as described above, in the method for manufacturing a 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 (raw pipe) during quenching is in the range of 800 to 500°C is defined as the cooling rate during quenching CR _800-500 . More specifically, the cooling rate CR _800-500 during quenching is determined by the part of the intermediate steel material to be quenched that is cooled slowest in its cross section (for example, in the case of forced cooling on both surfaces, the center of the thickness of the intermediate steel material). determined from the temperature measured at

A preferable cooling rate CR _800-500 during quenching is 300° C./min or more. The lower limit of the cooling rate CR _800-500 during quenching is more preferably 450°C/min, and even more preferably 600°C/min. The upper limit of the cooling rate CR _800-500 during quenching is not particularly specified, but is, for example, 60000° C./min.

Preferably, the raw pipe is heated in the austenite region multiple times and then quenched. In this case, since the austenite grains before quenching are refined, the SSC resistance of the steel material increases. By performing quenching multiple times, heating in the austenite region may be repeated multiple times, or by performing normalization and quenching, heating in the austenite region may be repeated multiple times. Further, quenching and tempering, which will be described later, may be combined and performed multiple times. That is, quenching and tempering may be performed multiple times. In this case, the SSC resistance of the steel material is further improved. The tempering process will be described in detail below.

[Tempering process]
In the tempering step, the raw tube that has been hardened as described above is tempered. In this specification, "tempering" means reheating and holding the intermediate steel material after quenching at a temperature below the A _c1 point. Here, the tempering temperature corresponds to the temperature of the furnace at which the intermediate steel material after quenching is heated and held. Tempering time means the time from when the temperature of the intermediate steel material reaches a predetermined tempering temperature until it is extracted from the heat treatment furnace.

The tempering temperature is appropriately adjusted depending on the chemical composition of the seamless steel pipe and the desired yield strength. That is, the yield strength of the seamless steel pipe is adjusted to 862 MPa or more by adjusting the tempering temperature for the raw pipe having the chemical composition of this embodiment. Note that it is naturally possible for those skilled in the art to adjust the yield strength of the seamless steel pipe to 862 MPa or more and 931 MPa or more by adjusting the tempering temperature. Specifically, in the tempering process according to this embodiment, the preferred 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, a microstructure consisting mainly of tempered martensite and tempered bainite may not be obtained. On the other hand, if the tempering time is too long, the above effect will be saturated. Therefore, in the tempering step of this embodiment, the tempering time is preferably 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 this embodiment can be manufactured by the above manufacturing method. In addition, in the above-mentioned manufacturing method, the manufacturing method of a seamless steel pipe was demonstrated as an example. However, the steel material according to this embodiment may be a steel plate or other shapes. Similar to the above-described manufacturing method, the manufacturing method for steel plates and other shapes includes, for example, a preparation process, a quenching process, and a tempering process. Furthermore, the above-mentioned manufacturing method is just an example, and other manufacturing methods may be used.

Hereinafter, the present invention will be explained in more detail with reference to Examples.

In Example 1, steel materials having a yield strength of 862 to less than 931 MPa were evaluated. Specifically, first, molten steel having the chemical composition shown in Tables 1-1 and 1-2 was produced. Note that "-" in Table 1-2 means that the content of each element is at an impurity level. Specifically, the V content, Cu content, Ni content, and W content of Steel A were rounded to the second decimal place, meaning that they were 0%. Further, the Ca content, Mg content, Zr content, and rare earth element (REM) content of Steel A were rounded to the fifth decimal place, meaning that they were 0%.

A round billet was manufactured using the above molten steel by a continuous casting method. In the continuous casting method, when introducing molten steel from the ladle to the tundish, a metal plate processed into a conical shape without a bottom is placed above the opening of the tundish so that the apex is directly below the nozzle of the ladle. Placed. Table 2 shows whether a metal plate of the above shape was placed above the opening of the tundish. Specifically, when a metal plate having the above shape is placed above the opening of the tundish, "A" is shown in the "Metal plate" column of Table 2. When the metal plate having the above shape is not placed above the opening of the tundish, "B" is shown in the "Metal plate" column of Table 2. Note that the metal plate having the above shape and arranged above the opening of the tundish was an aluminum plate. Specifically, three aluminum plates with a thickness of 2 mm were stacked and used. In test numbers 3, 8 to 10, and 12, after the Si oxide was discharged from the nozzle, but before the molten steel was discharged, a rod with a hook at the tip was used to remove the metal from below the nozzle. The board was removed.

The manufactured round billets of test numbers 1 to 19 were held at 1250 ° C. for 1 hour, and then hot rolled using the Mannesmann-mandrel method to produce raw pipes (seamless steel pipes) of test numbers 1 to 19. Furthermore, the obtained raw tubes with test numbers 1 to 19 were quenched. Specifically, the raw tubes of test numbers 1 to 19 were held at the quenching temperature (°C) listed in the "Quenching" column of Table 2 for the quenching time (minutes), and then quenched by shower water cooling. In addition, in test numbers 1 to 19, the cooling rate CR _800-500 during quenching was all within the range of 480 to 30,000°C/min. Here, the quenching temperature (°C) listed in Table 2 was the temperature (°C) of the heat treatment furnace in which the raw tube was heated. Furthermore, the quenching time (minutes) listed in Table 2 was the time (minutes) during which the raw tube was held at the quenching temperature.

The obtained blank tubes with test numbers 1 to 19 were tempered. Specifically, the raw tubes of Test Numbers 1 to 19 were tempered by holding them at the tempering temperature (°C) listed in the "Tempering" column of Table 2 for the tempering time (minutes). Here, the tempering temperature (°C) listed in Table 2 was the temperature (°C) of the tempering furnace in which the raw tube was heated. Further, the tempering time (minutes) listed in Table 2 was the time (minutes) during which the raw tube was maintained at the tempering temperature. Through the above manufacturing process, seamless steel pipes with test numbers 1 to 19 were obtained.

[Evaluation test]
The tensile test, coarse Si oxide number density measurement test, and SSC resistance test described below were conducted on the seamless steel pipes with test numbers 1 to 19 after tempering.

[Tensile test]
A tensile test was conducted on the seamless steel pipes with test numbers 1 to 19 to determine the yield strength. The tensile test was conducted in accordance with ASTM E8/E8M (2021). Round bar test pieces with a parallel part diameter of 8.9 mm and a gauge length of 35.6 mm were prepared from the thick center portions of the seamless steel pipes of test numbers 1 to 19. The axial direction of the round bar test piece was parallel to the pipe axial direction of the seamless steel pipe. Using the prepared round bar test pieces, a tensile test was conducted at room temperature (25° C.) in the atmosphere to obtain the yield strength (MPa) of the seamless steel pipes of test numbers 1 to 19. In this example, the stress at 0.65% elongation (0.65% yield strength) obtained in the tensile test was defined as the yield strength. For test numbers 1 to 19, the yield strength (MPa) obtained is shown in Table 2 as "YS (MPa)".

[Number density measurement test of coarse Si oxide]
A coarse Si oxide number density measurement test was conducted on the seamless steel pipes of test numbers 1 to 19 to determine the number density of Si oxides (coarse Si oxides) with a major diameter of 5.0 μm or more. The number density of coarse Si oxides was determined by the method described above using test pieces prepared from the thick center portions of seamless steel pipes with test numbers 1 to 19. For test numbers 1 to 19, the number density (pieces/100 mm ² ) of coarse Si oxides obtained is shown in the "Coarse Si oxides (pieces/100 mm ² )" column of Table 2.

[SSC resistance test]
SSC resistance tests were conducted on seamless steel pipes with test numbers 1 to 19 in accordance with NACE TM0177-2016 Method A to evaluate SSC resistance. Specifically, round bar test pieces with a diameter of 6.35 mm and a parallel portion length of 25.4 mm were prepared from the thick center portions of the seamless steel pipes of test numbers 1 to 19. An SSC resistance test was conducted on three of the prepared test pieces. Note that the axial direction of the test piece was parallel to the tube axis direction.

Tensile stress was applied in the axial direction of the round bar test pieces with test numbers 1 to 19. At this time, the applied stress was adjusted to be 90% of the actual yield stress of each steel plate. The test solution used was a mixed aqueous solution (NACE solution B) of 5.0 mass% sodium chloride and 0.4 mass% sodium acetate, which was adjusted to pH 3.5 with acetic acid. A test solution at 24° C. was injected into three test containers to form test baths. Three stress-applied round bar test pieces were immersed one by one in a test bath in a different test container. After each test bath was degassed, a mixed gas of 0.1 atm H ₂ S gas and 0.9 atm CO ₂ gas was blown into the test bath to saturate it. A test bath saturated with a mixed gas of 0.01 atm H ₂ S gas and 0.99 atm CO ₂ gas was maintained at 24° C. for 1440 hours.

After holding for 1440 hours, the round bar test pieces of test numbers 1 to 19 were observed for the occurrence of sulfide stress cracking (SSC). Specifically, the round bar test piece after being held for 1440 hours was observed with the naked eye. For test numbers 1 to 19, the number of specimens in which SSC occurred among the three round bar test pieces is shown in the "Number of specimens in which SSC occurred (pieces)" column of Table 2.

[Test results]
Referring to Tables 1-1, 1-2, and 2, the chemical compositions of the seamless steel pipes of test numbers 1 to 12 were appropriate, and the manufacturing methods also satisfied the above-mentioned preferable conditions. As a result, these seamless steel pipes had a yield strength of 862 to less than 931 MPa, and a number density of coarse Si oxides of 5 pieces/100 mm ² or less. As a result, SSC did not occur in these seamless steel pipes in the SSC resistance test. That is, the seamless steel pipes of test numbers 1 to 12 had yield strengths of 862 to less than 931 MPa and excellent SSC resistance.

On the other hand, in the seamless steel pipes of test numbers 13 and 14, the number density of coarse Si oxides exceeded 5 pieces/100 mm ² . As a result, SSC occurred in the test piece in the SSC resistance test. That is, the seamless steel pipes of test numbers 13 and 14 did not have excellent SSC resistance.

The seamless steel pipe of test number 15 had too high a Mn content. As a result, SSC occurred in the test piece in the SSC resistance test. That is, the seamless steel pipe of test number 15 did not have excellent SSC resistance.

The seamless steel pipe of test number 16 had too low Mo content. As a result, SSC occurred in the test piece in the SSC resistance test. That is, the seamless steel pipe of test number 16 did not have excellent SSC resistance.

The seamless steel pipe of test number 17 had too high a S content. As a result, SSC occurred in the test piece in the SSC resistance test. That is, the seamless steel pipe of test number 17 did not have excellent SSC resistance.

The seamless steel pipe of test number 18 had too high a P content. As a result, SSC occurred in the test piece in the SSC resistance test. That is, the seamless steel pipe of test number 18 did not have excellent SSC resistance.

The seamless steel pipe of test number 19 had too high an O content. As a result, SSC occurred in the test piece in the SSC resistance test. That is, the seamless steel pipe of Test No. 19 did not have excellent SSC resistance.

In Example 2, steel materials with a yield strength of 931 MPa or more were evaluated. Specifically, first, molten steel having the chemical composition shown in Table 3-1 and Table 3-2 was produced. Note that "-" in Table 3-2 means that the content of each element is at the impurity level. Specifically, the Cu content, Ni content, and W content of Steel R were rounded to the second decimal place, meaning that they were 0%. Further, the Ca content, Mg content, Zr content, and rare earth element (REM) content of Steel R were rounded to the fifth decimal place, meaning that they were 0%.

A round billet was manufactured using the above molten steel by a continuous casting method. In the continuous casting method, when introducing molten steel from the ladle to the tundish, a metal plate processed into a conical shape without a bottom is placed above the opening of the tundish so that the apex is directly below the nozzle of the ladle. Placed. Table 4 shows whether a metal plate having the above shape was placed above the opening of the tundish. Specifically, when a metal plate having the above shape is placed above the opening of the tundish, "A" is shown in the "Metal plate" column of Table 4. When the metal plate having the above shape is not placed above the opening of the tundish, "B" is shown in the "Metal plate" column of Table 4. Note that the metal plate having the above shape and arranged above the opening of the tundish was an aluminum plate. Specifically, three aluminum plates with a thickness of 2 mm were stacked and used. In addition, in the embodiment in which the metal plate of the above shape is arranged, after the Si oxide is discharged from the nozzle and before the molten steel is discharged, a rod with a hook formed at the tip is used to remove the metal plate from below the nozzle. was removed.

The manufactured round billets with test numbers 20 to 40 were held at 1250°C for 1 hour, and then hot rolled using the Mannesmann-mandrel method to produce base pipes (seamless steel pipes) with test numbers 20 to 40. Furthermore, the obtained raw tubes with test numbers 20 to 40 were quenched. Specifically, the raw tubes with test numbers 20 to 40 were held at the quenching temperature (°C) listed in the "Quenching" column of Table 4 for the quenching time (minutes), and then quenched by shower water cooling. In addition, in test numbers 20 to 40, the cooling rate CR _800-500 during quenching was within the range of 480 to 30,000°C/min. Here, the quenching temperature (°C) listed in Table 4 was the temperature (°C) of the heat treatment furnace in which the raw tube was heated. Furthermore, the quenching time (minutes) listed in Table 4 was the time (minutes) during which the raw tube was held at the quenching temperature.

Furthermore, the raw tube of test number 22 was quenched a second time. Specifically, the raw tube of test number 3 was held in a heat treatment furnace at 900° C. for 10 minutes, and then quenched by shower water cooling. In addition, in the second quenching performed on the raw tube of test number 22, the cooling rate CR _800-500 during quenching was within the range of 480 to 30,000°C/min.

The obtained blank tubes with test numbers 20 to 40 were tempered. Specifically, the raw tubes of test numbers 20 to 40 were tempered by holding them at the tempering temperature (°C) listed in the "Tempering" column of Table 4 for the tempering time (minutes). Here, the tempering temperature (°C) listed in Table 4 was the temperature (°C) of the tempering furnace in which the raw tube was heated. Furthermore, the tempering time (minutes) listed in Table 4 was the time (minutes) during which the raw tube was maintained at the tempering temperature. Through the above manufacturing process, seamless steel pipes with test numbers 20 to 40 were obtained.

[Evaluation test]
The tensile test, coarse Si oxide number density measurement test, and SSC resistance test described below were conducted on the seamless steel pipes with test numbers 20 to 40 after tempering.

[Tensile test]
A tensile test was conducted on the seamless steel pipes with test numbers 20 to 40 to determine the yield strength. The tensile test was conducted in accordance with ASTM E8/E8M (2021). A round bar test piece with a parallel part diameter of 8.9 mm and a gauge length of 35.6 mm was prepared from the thick center part of seamless steel pipes with test numbers 20 to 40. The axial direction of the round bar test piece was parallel to the pipe axial direction of the seamless steel pipe. Using the prepared round bar test pieces, a tensile test was conducted at room temperature (25° C.) in the atmosphere to obtain the yield strengths (MPa) of seamless steel pipes with test numbers 20 to 40. In this example, the stress at 0.65% elongation (0.65% yield strength) obtained in the tensile test was defined as the yield strength. For test numbers 20 to 40, the yield strength (MPa) obtained is shown in Table 4 as "YS (MPa)".

[Number density measurement test of coarse Si oxide]
A coarse Si oxide number density measurement test was performed on seamless steel pipes with test numbers 20 to 40 to determine the number density of Si oxides (coarse Si oxides) with a major diameter of 5.0 μm or more. The number density of coarse Si oxides was determined by the method described above using test pieces prepared from the center of the wall thickness of seamless steel pipes with test numbers 20 to 40. For test numbers 20 to 40, the number density (pieces/200 mm ² ) of coarse Si oxides obtained is shown in the "Coarse Si oxides (pieces/200 mm ² )" column of Table 4.

[SSC resistance test]
SSC resistance tests were conducted on seamless steel pipes with test numbers 20 to 40 in accordance with NACE TM0177-2016 Method A to evaluate SSC resistance. Specifically, round bar test pieces with a diameter of 6.35 mm and a parallel portion length of 25.4 mm were prepared from the thick center portions of seamless steel pipes with test numbers 20 to 40. An SSC resistance test was conducted on three of the prepared test pieces. Note that the axial direction of the test piece was parallel to the tube axis direction.

Tensile stress was applied in the axial direction of round bar test pieces with test numbers 20 to 40. At this time, the applied stress was adjusted to be 90% of the actual yield stress of each steel plate. The test solution used was a mixed aqueous solution (NACE solution B) of 5.0 mass% sodium chloride and 0.4 mass% sodium acetate, which was adjusted to pH 3.5 with acetic acid. A test solution at 24° C. was injected into three test containers to form test baths. Three stress-applied round bar test pieces were immersed one by one in a test bath in a different test container. After each test bath was degassed, a mixed gas of 0.01 atm H ₂ S gas and 0.99 atm CO ₂ gas was blown into the test bath to saturate it. A test bath saturated with a mixed gas of 0.01 atm H ₂ S gas and 0.99 atm CO ₂ gas was maintained at 24° C. for 1440 hours.

After holding for 1440 hours, the round bar test pieces with test numbers 20 to 40 were observed for the occurrence of sulfide stress cracking (SSC). Specifically, the round bar test piece after being held for 1440 hours was observed with the naked eye. For test numbers 20 to 40, the number of specimens in which SSC occurred among the three round bar test pieces is shown in the "Number of specimens in which SSC occurred (pieces)" column of 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 numbers 20 to 32 were appropriate, and the manufacturing methods also satisfied the above-mentioned preferable conditions. As a result, these seamless steel pipes had a yield strength of 931 MPa or more and a number density of coarse Si oxides of 5 pieces/200 mm ² or less. As a result, SSC did not occur in these seamless steel pipes in the SSC resistance test. That is, the seamless steel pipes with test numbers 20 to 32 had a yield strength of 931 MPa or more and excellent SSC resistance.

On the other hand, in the seamless steel pipes of test numbers 33 to 37, the number density of coarse Si oxides exceeded 5 pieces/200 mm ² . As a result, SSC occurred in the test piece in the SSC resistance test. That is, the seamless steel pipes of test numbers 33 to 37 did not have excellent SSC resistance.

The seamless steel pipe of test number 38 had too high an O content. As a result, SSC occurred in the test piece in the SSC resistance test. That is, the seamless steel pipe of test number 38 did not have excellent SSC resistance.

The seamless steel pipe of test number 39 had too low Mo content. As a result, SSC occurred in the test piece in the SSC resistance test. That is, the seamless steel pipe of test number 39 did not have excellent SSC resistance.

The seamless steel pipe of test number 40 had too high a S content. As a result, SSC occurred in the test piece in the SSC resistance test. That is, the seamless steel pipe of test number 40 did not have excellent SSC resistance.

The embodiments of the present disclosure have been described above. However, the embodiments described above are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the embodiments described above, and the embodiments described above can be modified and implemented as appropriate without departing from the spirit thereof.

Claims (3)

A steel material,
In mass%,
C: 0.15-0.45%,
Si: 0.05-1.00%,
Mn: 0.05-0.30%,
P: 0.030% or less,
S: 0.0050% or less,
Al: 0.005-0.100%,
Cr: 0.30-1.10%,
Mo: 0.40-2.00%,
Ti: 0.002 to 0.020%,
Nb: 0.002-0.100%,
B: 0.0005-0.0040%,
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: 0-1.50%,
Ca: 0-0.0100%,
Mg: 0 to 0.0100%,
Zr: 0 to 0.0100%,
Rare earth elements: 0 to 0.0100%, and
The remainder consists of Fe and impurities,
The yield strength is 862 MPa or more,
The microstructure has a total volume fraction of tempered martensite and tempered bainite of 90% or more,
In the steel material,
In terms of mass %, the Si content is 20% or more, the O content is 10% or more, and the number density of Si oxides with a major axis of 5.0 μm or more is 5 pieces/100 mm ² or less,
When the yield strength is 931 MPa or more, the number density of the Si oxides is 5 pieces/200 mm ² or less,
Steel material. The steel material according to claim 1,
V: 0.01-0.30%,
Cu: 0.01 to 0.50%,
Ni: 0.01-0.50%,
W: 0.01-1.50%,
Ca: 0.0001-0.0100%,
Mg: 0.0001-0.0100%,
Zr: 0.0001 to 0.0100%, and
Rare earth element: Contains one or more elements selected from the group consisting of 0.0001 to 0.0100%,
Steel material. The steel material according to claim 1 or claim 2,
The steel material is a steel pipe for oil wells,
Steel material.

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