WO2023204294A1

WO2023204294A1 - Steel material

Info

Publication number: WO2023204294A1
Application number: PCT/JP2023/015878
Authority: WO
Inventors: 浩行富士; 晋士吉田; 勇次荒井; 桂一近藤
Original assignee: 日本製鉄株式会社
Priority date: 2022-04-22
Filing date: 2023-04-21
Publication date: 2023-10-26

Abstract

Provided is a steel material that has high strength and excellent SSC resistance. The steel material according to the present disclosure contains, by mass%, 0.20%–0.35% C, 0.60%–1.30% Si, 0.05%–0.25% Mn, no more than 0.050% P, no more than 0.0100% S, 0.010%–0.100% Al, no more than 0.0100% N, 0.20%–1.00% Cr, 0.10%-1.00% Mo, and 0.003%–0.030% Ti, the remainder being Fe and impurities. If the steel material is 110 ksi-grade, EE of formula (1) is at least 2.75 and FN of formula (2) is at least 0.185. If the steel material is 125 ksi-grade, EE is at least 3.00 and FN is at least 0.200.　Formula (1): EE = -0.25C+2Si-5.8Mn+2.1Cr+Mo+4.1Zr+2.6Sb+0.3Cu+0.4Ni+1.5Co. Formula (2): FN = EE/(D0.9) (D: average equivalent circle diameter (μm) of prior austenite grains).

Description

steel material

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

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 well applications, such as steel pipes for oil wells. Specifically, oil well steel materials 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. has been done. Recently, oil well steel materials of 110 ksi class (yield strength of 758 MPa to less than 862 MPa) and 125 ksi class (yield strength of 862 to less than 965 MPa) have begun to be sought.

Furthermore, many deep wells are in sour environments containing corrosive hydrogen sulfide. As used herein, a sour environment refers to an acidified environment containing hydrogen sulfide. Note that the sour environment may also contain carbon dioxide. Steel materials used in such a sour environment are required not only to have high strength but also to have sulfide stress cracking resistance (hereinafter referred to as SSC resistance).

A technique for increasing the SSC resistance of steel materials, typified by steel pipes for oil wells, has been proposed in Japanese Patent Application Laid-Open No. 2000-297344 (Patent Document 1) and International Publication No. 2008/123422 (Patent Document 2).

The steel material disclosed in Patent Document 1 has C: 0.15 to 0.3%, Cr: 0.2 to 1.5%, Mo: 0.1 to 1%, V: 0. 05 to 0.3%, and Nb: 0.003 to 0.1%. In this steel material, the total amount of 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 1.5 to 4% by mass. When is t (mm), it is less than (200/t)% by mass. In this steel material, the SSC resistance is improved by suppressing the proportion of M ₂₃ C ₆ type carbide.

The steel material disclosed in Patent Document 2 has, in mass%, C: 0.10 to 0.20%, Si: 0.05 to 1.0%, Mn: 0.05 to 1.5%, and 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 steel material, 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. Similarly to Patent Document 1, this steel material also has improved SSC resistance by suppressing the proportion of M ₂₃ C ₆ type carbides.

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

As mentioned above, recently, as oil well environments have become harsher, there has been a growing demand for steel materials that can both have high strength of 110 ksi class or higher (758 MPa or higher) and excellent SSC resistance. In

Patent Documents

1 and 2, an attempt is made to achieve both high strength and SSC resistance by controlling precipitates. However, a steel material capable of achieving both high strength and excellent SSC resistance may be obtained by means other than

Patent Documents

1 and 2.

An objective 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 has the following configuration.

The chemical composition is in mass%,
C: 0.20-0.35%,
Si: 0.60-1.30%,
Mn: 0.05-0.25%,
P: 0.050% or less,
S: 0.0100% or less,
Al: 0.010-0.100%,
N: 0.0100% or less,
Cr: 0.20-1.00%,
Mo: 0.10-1.00%,
Ti: 0.003 to 0.030%,
O: 0.0050% or less,
Zr: 0 to 0.0040%,
Sb: 0 to 0.50%,
Cu: 0 to 0.50%,
Ni: 0 to 0.50%,
Co: 0 to 0.50%,
Ca: 0-0.0040%,
Mg: 0 to 0.0040%,
Rare earth elements: 0 to 0.0040%,
Nb: 0 to 0.150%,
V: 0 to 0.500%,
B: 0 to 0.0030%, and
The remainder consists of Fe and impurities,
The yield strength is less than 758 to 965 MPa,
When the yield strength is less than 758 to 862 MPa, EE defined by formula (1) is 2.75 or more, FN defined by formula (2) is 0.185 or more,
When the yield strength is less than 862 to 965 MPa, the EE is 3.00 or more, and the FN is 0.200 or more.
Steel material.
EE=-0.25C+2Si-5.8Mn+2.1Cr+Mo+4.1Zr+2.6Sb+0.3Cu+0.4Ni+1.5Co (1)
FN=EE/(D ^0.9 ) (2)
Here, each element symbol in formula (1) is substituted with the content in mass % of the corresponding element. D in equation (2) is substituted with the average circular equivalent diameter in μm of the prior austenite grains in the steel material.

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

FIG. 1 is a schematic diagram for explaining a process in which hydrogen penetrates into the interior of a steel material from the surface of the steel material. Fig. 2A shows the FN and , is a diagram showing the relationship between the fracture toughness value K _1SSC (MPa√m) obtained in the DCB test. Figure 2B shows the FN and , is a diagram showing the relationship between the fracture toughness value K _1SSC (MPa√m) obtained in the DCB test. FIG. 3A is a side view of a DCB test piece specified in NACE TM0177-2016 Method D. FIG. 3B is a perspective view of a wedge driven into the DCB specimen shown in FIG. 3A.

The present inventors investigated and studied steel materials that have high strength and excellent SSC resistance in sour environments. As a result, the present inventors obtained the following findings.

First, the present inventors investigated a steel material having high strength and excellent SSC resistance in a sour environment from the viewpoint of chemical composition. As a result, if the chemical composition of the steel satisfies the following characteristic 1, high strength of 110 ksi class (less than 758 to 862 MPa) to 125 ksi class (less than 862 to 965 MPa) and excellent SSC resistance in sour environments can be obtained. I thought there was a possibility that it would happen.
(Feature 1)
The chemical composition is in mass%, C: 0.20 to 0.35%, Si: 0.60 to 1.30%, Mn: 0.05 to 0.25%, P: 0.050% or less, S : 0.0100% or less, Al: 0.010 to 0.100%, N: 0.0100% or less, Cr: 0.20 to 1.00%, Mo: 0.10 to 1.00%, Ti: 0.003 to 0.030%, O: 0.0050% or less, Zr: 0 to 0.0040%, Sb: 0 to 0.50%, Cu: 0 to 0.50%, Ni: 0 to 0. 50%, Co: 0 to 0.50%, Ca: 0 to 0.0040%, Mg: 0 to 0.0040%, Rare earth elements: 0 to 0.0040%, Nb: 0 to 0.150%, V : 0 to 0.500%, B: 0 to 0.0030%, and the balance consists of Fe and impurities.

Therefore, the present inventors investigated means for further increasing the SSC resistance of steel materials whose chemical composition satisfies Feature 1.

SSC is thought to occur through the following process. FIG. 1 is a schematic diagram for explaining a process in which hydrogen penetrates into the interior of a steel material from the surface of the steel material. Referring to FIG. 1, first, when steel material 1 corrodes in a sour environment, the surface of steel material 1 becomes electrochemically active. Then, Fe in the steel material 1 becomes Fe ²⁺ and dissolves in the environment. At this time, electrons e ^- are released to the outside of the steel material 1 (A: occurrence of corrosion). Hydrogen ions H ⁺ present in the environment receive electrons e ⁻ emitted from the steel material 1, are reduced, and are adsorbed on the surface of the steel material 1 as adsorbed hydrogen atoms H _ad (B: adsorption reaction of hydrogen ions). Due to the above mechanism, a plurality of adsorbed hydrogen _atoms Had exist on the surface of the steel material 1. Most of these adsorbed _hydrogen atoms H _ad combine with each other to become hydrogen gas H ₂ and are released from the surface of the steel material 1 into the environment. However, among the plurality of adsorbed hydrogen atoms H _ad existing on the surface of the steel material 1, a small portion of the adsorbed hydrogen atoms H _ad enters into the interior of the steel material 1 from the surface of the steel material 1, and becomes the invading hydrogen atoms H _ab . (C: Invasion of hydrogen). Then, SSC occurs and propagates in the steel material due to the invading hydrogen atoms H _ab . In other words, the invading hydrogen atoms H _ab promote not only the generation but also the propagation of SSC.

Considering the above mechanism of SSC generation, the present inventors have determined that in order to suppress the generation and propagation of SSC and obtain excellent SSC resistance, it is necessary to suppress the penetration of hydrogen from the surface of the steel material. was considered to be effective. Therefore, the present inventors investigated means for suppressing hydrogen from penetrating from the surface of the steel material.

The inventors first investigated and studied elements that affect the penetration of hydrogen from the surface of steel materials. As a result of studies, the present inventors obtained the following findings regarding steel materials whose chemical composition satisfies Feature 1.

As described above, when the surface of the steel material becomes electrochemically active (A in FIG. 1), adsorbed hydrogen atoms H _ad adhere to the surface of the steel material (B in FIG. 1). Then, the adsorbed hydrogen atoms H _ad become interstitial hydrogen atoms H _ab and enter into the steel material (C in FIG. 1). Therefore, in order to suppress the intrusion of hydrogen from the surface of the steel material, it is effective to suppress the electrochemical activity on the surface of the steel material.

Si, Cr, Mo, Zr, Sb, Cu, Ni, and Co suppress the electrochemical activity of the steel surface in a sour environment. As a result, these elements suppress the intrusion of hydrogen from the surface of the steel material. On the other hand, C and Mn promote electrochemical activity on the surface of steel in a sour environment. As a result, these elements promote hydrogen penetration from the steel surface.

Based on the above findings, the present inventors determined the content of these hydrogen penetration inhibiting elements (Si, Cr, Mo, Zr, Sb, Cu, Ni, and Co) and the hydrogen penetration promoting elements (C, It was thought that if the content of Mn) was appropriately adjusted, hydrogen penetration from the steel surface could be suppressed by electrochemical action. Therefore, in steel materials whose chemical composition satisfies Characteristic 1, the relationship between hydrogen penetration inhibiting elements, hydrogen penetration promoting elements, and SSC resistance was investigated. As a result, it was thought that excellent SSC resistance in a sour environment could be obtained in a steel material having high strength by increasing the electrochemical elements (EE) defined by the following formula (1).
EE=-0.25C+2Si-5.8Mn+2.1Cr+Mo+4.1Zr+2.6Sb+0.3Cu+0.4Ni+1.5Co (1)
Here, each element symbol in formula (1) is substituted with the content in mass % of the corresponding element.

Generally, as the strength of steel increases, the SSC resistance of the steel tends to decrease. Therefore, the present inventors considered that it is necessary to further suppress hydrogen intrusion when the strength of the steel material is high. Therefore, the present inventors investigated EE when the yield strength of the steel material is 110 ksi class (less than 758 to 862 MPa) and EE when the yield strength of the steel material is 125 ksi class (less than 862 to 965 MPa). . As a result, if the EE is 2.75 or more when the yield strength is 110 ksi class, and the EE is 3.00 or more when the yield strength is 125 ksi class, excellent SSC resistance can be obtained even with high strength. It turned out that it was possible.

However, in steel materials whose chemical composition satisfies Feature 1, high strength of 110 ksi class to 125 ksi class and excellent SSC resistance may not be sufficiently obtained by simply adjusting the electrochemical elements described above. Therefore, the inventor conducted further study.

Here, the present inventors considered that the occurrence and propagation of SSC in a steel material whose chemical composition satisfies Feature 1 is affected not only by the above-mentioned electrochemical factors but also by physical factors due to the microstructure. Therefore, the present inventors further investigated means for increasing the SSC resistance of steel materials not only from the viewpoint of electrochemical factors but also from the viewpoint of physical factors. As a result, the present inventors found that the average equivalent circle diameter (μm) of prior austenite grains in steel material acts synergistically with the above-mentioned electrochemical factors and significantly influences the SSC resistance of steel material. found out.

Based on the above findings, the present inventors further investigated the above-mentioned electrochemical factors (hydrogen penetration inhibiting element and hydrogen penetration promoting element), physical factors (average equivalent circular diameter of prior austenite grains), and SSC resistance. We examined the relationship with gender. As a result, by adjusting the EE to the above range and further adjusting the FN defined by the following formula (2) according to the strength, even if the strength is high from 110ski to 125ksi, The present inventors have discovered that excellent SSC resistance can be obtained.
FN=EE/(D ^0.9 ) (2)
Here, the average circular equivalent diameter in μm of prior austenite grains in the steel material is substituted for D in equation (2).

FN is an index indicating the degree of influence of electrochemical factors (hydrogen penetration suppressing elements and hydrogen penetration promoting elements) and physical factors (average equivalent circular diameter of prior austenite grains) on SSC resistance. If the steel material has a chemical composition that satisfies Characteristic 1 and has a yield strength of 110 ksi class, the EE should be 2.75 or more and the FN should be 0.185 or more. Further, in the case of a steel material whose chemical composition satisfies Characteristic 1 and whose yield strength is 125 ksi class, the EE should be 3.00 or more and the FN should be 0.200 or more. In this case, excellent SSC resistance can be obtained even if the strength is as high as 110 to 125 ksi. This point will be explained below.

Figure 2A shows the FN and fracture obtained in the DCB test for a steel material whose chemical composition satisfies Feature 1, whose yield strength is 110 ksi class (758 to less than 862 MPa), and whose EE is 2.75 or more. It is a figure which shows the relationship with toughness value _K1SSC (MPa√m). FIG. 2A was created based on data obtained in Example 1, which will be described later.

Referring to FIG. 2A, when FN is 0.185 or more, the fracture toughness value _K1SSC is as high as 25.0 MPa√m or more, and excellent SSC resistance is obtained. However, if FN is less than 0.185, the fracture toughness value K _1SSC decreases significantly to less than 25.0 MPa√m. Therefore, by setting FN to 0.185 or more, excellent SSC resistance can be obtained in 110 ksi class (less than 758 to 862 MPa) steel materials.

Figure 2B shows the FN and fracture obtained in the DCB test for a steel material whose chemical composition satisfies Feature 1, whose yield strength is 125 ksi class (862 to less than 965 MPa), and whose EE is 3.00 or more. It is a figure which shows the relationship with toughness value _K1SSC (MPa√m). FIG. 2B was created based on data obtained in Example 2, which will be described later.

Referring to FIG. 2B, when FN is 0.200 or more, the fracture toughness value _K1SSC is as high as 24.0 MPa√m or more, and excellent SSC resistance is obtained. However, if FN is less than 0.200, the fracture toughness value K _1SSC decreases significantly to less than 24.0 MPa√m. Therefore, by setting FN to 0.200 or more, excellent SSC resistance can be obtained in 125 ksi class (less than 862 to 965 MPa) steel materials.

The relationship between the electrochemical and physical factors and SSC resistance described above is only an estimate, and there is a possibility that the excellent SSC resistance is obtained by a mechanism different from that described above. However, in a steel material whose chemical composition satisfies characteristic 1, if the yield strength is 110 ksi class (less than 758 to 862 MPa), the EE is 2.75 or more, the FN is 0.185 or more, and the yield strength is 125 ksi class. (less than 862 to 965 MPa), if EE is 3.00 or more and FN is 0.200 or more, excellent SSC resistance can be obtained even with high strength of 110 ksi class to 125 ksi class. This is also proven in the examples described below.

The steel material according to this embodiment, which was completed based on the above findings, has the following configuration.

[1]
The chemical composition is in mass%,
C: 0.20-0.35%,
Si: 0.60-1.30%,
Mn: 0.05-0.25%,
P: 0.050% or less,
S: 0.0100% or less,
Al: 0.010-0.100%,
N: 0.0100% or less,
Cr: 0.20-1.00%,
Mo: 0.10-1.00%,
Ti: 0.003 to 0.030%,
O: 0.0050% or less,
Zr: 0 to 0.0040%,
Sb: 0 to 0.50%,
Cu: 0 to 0.50%,
Ni: 0 to 0.50%,
Co: 0 to 0.50%,
Ca: 0-0.0040%,
Mg: 0 to 0.0040%,
Rare earth elements: 0 to 0.0040%,
Nb: 0 to 0.150%,
V: 0 to 0.500%,
B: 0 to 0.0030%, and
The remainder consists of Fe and impurities,
The yield strength is less than 758 to 965 MPa,
When the yield strength is less than 758 to 862 MPa, EE defined by formula (1) is 2.75 or more, FN defined by formula (2) is 0.185 or more,
When the yield strength is less than 862 to 965 MPa, the EE is 3.00 or more, and the FN is 0.200 or more.
Steel material.
EE=-0.25C+2Si-5.8Mn+2.1Cr+Mo+4.1Zr+2.6Sb+0.3Cu+0.4Ni+1.5Co (1)
FN=EE/(D ^0.9 ) (2)
Here, each element symbol in formula (1) is substituted with the content in mass % of the corresponding element. D in equation (2) is substituted with the average circular equivalent diameter in μm of the prior austenite grains in the steel material.

[2]
The steel material according to [1],
The chemical composition is
Zr: 0.0001 to 0.0040%,
Sb: 0.01 to 0.50%,
Cu: 0.01 to 0.50%,
Ni: 0.01-0.50%,
Co: 0.01 to 0.50%,
Ca: 0.0001-0.0040%,
Mg: 0.0001 to 0.0040%,
Rare earth elements: 0.0001-0.0040%,
Nb: 0.001 to 0.150%,
V: 0.001 to 0.500%, and
B: 0.0001 to 0.0030%,
Containing one or more selected from the group consisting of
Steel material.

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

Hereinafter, the steel material of this embodiment will be explained in detail. Note that "%" regarding elements means mass % unless otherwise specified.

[Characteristics of the steel material of this embodiment]
The steel material of this embodiment satisfies the following characteristics 1 to 3.
(Feature 1)
The chemical composition is in mass%, C: 0.20 to 0.35%, Si: 0.60 to 1.30%, Mn: 0.05 to 0.25%, P: 0.050% or less, S : 0.0100% or less, Al: 0.010 to 0.100%, N: 0.0100% or less, Cr: 0.20 to 1.00%, Mo: 0.10 to 1.00%, Ti: 0.003 to 0.030%, O: 0.0050% or less, Zr: 0 to 0.0040%, Sb: 0 to 0.50%, Cu: 0 to 0.50%, Ni: 0 to 0. 50%, Co: 0 to 0.50%, Ca: 0 to 0.0040%, Mg: 0 to 0.0040%, Rare earth elements: 0 to 0.0040%, Nb: 0 to 0.150%, V : 0 to 0.500%, B: 0 to 0.0030%, and the balance consists of Fe and impurities.
(Feature 2)
Yield strength is 758 to less than 965 MPa.
(Feature 3)
When the yield strength is less than 758 to 862 MPa, EE defined by formula (1) is 2.75 or more, FN defined by formula (2) is 0.185 or more,
When the yield strength is less than 862 to 965 MPa, the EE is 3.00 or more and the FN is 0.200 or more.
EE=-0.25C+2Si-5.8Mn+2.1Cr+Mo+4.1Zr+2.6Sb+0.3Cu+0.4Ni+1.5Co (1)
FN=EE/(D ^0.9 ) (2)
Here, each element symbol in formula (1) is substituted with the content in mass % of the corresponding element. D in equation (2) is substituted with the average circular equivalent diameter in μm of prior austenite grains in the steel material.
Features 1 to 3 will be explained below.

[(Feature 1) Regarding chemical composition]
The chemical composition of the steel material of this embodiment contains the following elements.

C: 0.20-0.35%
Carbon (C) increases the strength of steel by increasing the hardenability of the steel and forming carbides. Furthermore, C promotes the spheroidization of carbides during tempering during the manufacturing process and improves the SSC resistance of the steel material. If the C content is less than 0.20%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the range of this embodiment.
If the C content exceeds 0.35%, coarse carbides are excessively produced. Further, C is an element that promotes hydrogen penetration. Therefore, even if the contents of other elements are within the range of this embodiment, the SSC resistance of the steel material is reduced.
Therefore, the C content is 0.20-0.35%.
The preferable lower limit of the C content is 0.22%, more preferably 0.23%, still more preferably 0.24%, and still more preferably 0.25%.
A preferable upper limit of the C content is 0.32%, more preferably 0.30%, still more preferably 0.28%, and still more preferably 0.27%.

Si: 0.60-1.30%
Silicon (Si) is a hydrogen intrusion suppressing element and suppresses hydrogen intrusion from the surface of the steel material. This increases the SSC resistance of the steel material. If the Si content is less than 0.60%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the range of this embodiment.
On the other hand, if the Si content exceeds 1.30%, the prior austenite grains become coarse even if the contents of other elements are within the range of this embodiment. In this case, although the amount of hydrogen penetrating from the surface of the steel material is suppressed by the electrochemical action of Si, the SSC resistance of the steel material decreases due to the physical action.
Therefore, the Si content is 0.60-1.30%.
The lower limit of the Si content is preferably 0.62%, more preferably 0.65%, even more preferably 0.70%, even more preferably 0.72%, and even more preferably 0.75%. %, more preferably 0.80%.
A preferable upper limit of the Si content is 1.28%, more preferably 1.25%, and still more preferably 1.20%.

Mn: 0.05-0.25%
Manganese (Mn) deoxidizes steel. Mn further improves the hardenability of the steel material. If the Mn content is less than 0.05%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the range of this embodiment.
On the other hand, Mn is an element that promotes hydrogen penetration. If the Mn content exceeds 0.25%, excessive Mn sulfide will be produced. Mn sulfide becomes a starting point for pitting corrosion. Therefore, if Mn sulfide is produced excessively, the corrosion rate will increase and hydrogen penetration into the steel material will be promoted. As a result, even if the contents of other elements are within the range of this embodiment, the SSC resistance of the steel material decreases.
Therefore, the Mn content is 0.05-0.25%.
The preferable lower limit of the Mn content is 0.06%, more preferably 0.07%, even more preferably 0.08%, and still more preferably 0.10%.
A preferable upper limit of the Mn content is 0.24%, more preferably 0.23%, even more preferably 0.22%, still more preferably 0.20%, and even more preferably 0.18%. %.

P: 0.050% or less Phosphorus (P) is an impurity. That is, the P content is more than 0%. If the P content exceeds 0.050%, 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 0.050% or less.
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 0.003%.
The upper limit of the P content is preferably 0.030%, more preferably 0.025%, even more preferably 0.020%, and still more preferably 0.015%.

S: 0.0100% or less Sulfur (S) is an impurity. That is, the S content is more than 0%. If the S content exceeds 0.0100%, 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 0.0100% or less.
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%.
A preferable upper limit of the S content is 0.0070%, more preferably 0.0050%, even more preferably 0.0030%, still more preferably 0.0025%, and even more preferably 0.0020%. %, more preferably 0.0015%.

Al: 0.010-0.100%
Aluminum (Al) deoxidizes steel. If the Al content is less than 0.010%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the range of this embodiment.
On the other hand, if the Al content exceeds 0.100%, coarse oxide-based inclusions will be generated even if the contents of other elements are within the range of this embodiment. Therefore, the SSC resistance of the steel material decreases.
Therefore, the Al content is 0.010 to 0.100%.
The lower limit of the Al content is preferably 0.012%, more preferably 0.015%, even more preferably 0.020%, and still more preferably 0.025%.
A preferable upper limit of the Al content is 0.080%, more preferably 0.070%, and still more preferably 0.060%.
The "Al" content as used herein means the content of "acid-soluble Al", that is, "sol.Al".

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 exceeds 0.0100%, coarse nitrides will be formed even if the contents of other elements are within the range of this embodiment. As a result, the SSC resistance of the steel material decreases.
Therefore, the N content is 0.0100% or less.
The preferable lower limit of the N content is 0.0001%, more preferably 0.0005%, even more preferably 0.0010%, still more preferably 0.0015%, and even more preferably 0.0020%. %.
A preferable upper limit of the N content is 0.0070%, more preferably 0.0060%, even more preferably 0.0050%, still more preferably 0.0045%, and even more preferably 0.0040%. %.

Cr:0.20~1.00%
Chromium (Cr) improves the hardenability of steel materials. Cr further functions as a hydrogen intrusion suppressing element. Specifically, Cr stabilizes the corrosion product film formed on the surface of the steel material in a sour environment, thereby suppressing hydrogen from penetrating into the steel material. As a result, the SSC resistance of the steel material increases. If the Cr content is less than 0.20%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the range of this embodiment.
On the other hand, if the Cr content exceeds 1.00%, the hardness of the steel material will become excessively high even if the contents of other elements are within the range of this embodiment. Therefore, the SSC resistance of the steel material decreases.
Therefore, the Cr content is between 0.20 and 1.00%.
The preferable lower limit of the Cr content is 0.25%, more preferably 0.30%, even more preferably 0.35%, still more preferably 0.40%, even more preferably 0.45%. %, more preferably 0.50%, still more preferably 0.55%, still more preferably 0.60%.
The upper limit of the Cr content is preferably 0.98%, more preferably 0.95%, and still more preferably 0.90%.

Mo: 0.10~1.00%
Molybdenum (Mo) functions as a hydrogen penetration inhibiting element. Specifically, Mo stabilizes the corrosion product film formed on the surface of the steel material in a sour environment, thereby suppressing hydrogen from penetrating into the steel material. As a result, the SSC resistance of the steel material increases. Mo further improves the hardenability of the steel material. 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 less than 0.10%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the range of this embodiment.
On the other hand, if the Mo content exceeds 1.00%, the above effects will be saturated.
Therefore, the Mo content is 0.10-1.00%.
The lower limit of the Mo content is preferably 0.20%, more preferably 0.25%, even more preferably 0.30%, and still more preferably 0.35%.
The preferable upper limit of the Mo content is 0.95%, more preferably 0.90%, even more preferably 0.85%, even more preferably 0.80%, and even more preferably 0.70%. %.

Ti: 0.003~0.030%
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 less than 0.003%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the range of this embodiment.
On the other hand, if the Ti content exceeds 0.030%, coarse Ti nitrides will be produced even if the contents of other elements are within the range of this embodiment. As a result, the SSC resistance of the steel material decreases.
Therefore, the Ti content is 0.003 to 0.030%.
The lower limit of the Ti content is preferably 0.004%, more preferably 0.005%.
A preferable upper limit of the Ti content is 0.028%, more preferably 0.025%, still more preferably 0.022%, and still more preferably 0.020%.

O: 0.0050% or less Oxygen (O) is an impurity. That is, the lower limit of the O content is over 0%. If the O content exceeds 0.0050%, coarse oxides will be produced even if the contents of other elements are within the range of this embodiment. As a result, the low-temperature toughness and SSC resistance of the steel material decrease.
Therefore, the O content is 0.0050% or less.
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%.
A preferable upper limit of the O content is 0.0040%, more preferably 0.0030%, still more preferably 0.0025%, and still more preferably 0.0020%.

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

[Optional Elements]
The chemical composition of the steel material of this embodiment further includes:
Zr: 0 to 0.0040%,
Sb: 0 to 0.50%,
Cu: 0 to 0.50%,
Ni: 0 to 0.50%,
Co: 0 to 0.50%,
Ca: 0-0.0040%,
Mg: 0 to 0.0040%,
Rare earth elements: 0 to 0.0040%,
Nb: 0 to 0.150%,
V: 0 to 0.500%, and
B: 0 to 0.0030%,
It may contain one or more elements selected from the group consisting of.
These arbitrary elements will be explained below.

[Group 1: Regarding Zr, Sb, Cu, Ni, Co, Ca, Mg and rare earth elements (REM)]
The chemical composition of the steel material according to the present embodiment further includes one or more elements selected from the group consisting of Zr, Sb, Cu, Ni, Co, Ca, Mg, and rare earth elements (REM) in place of a part of Fe. You may. All of these elements are optional elements and improve the SSC resistance of the steel material.

Zr: 0 to 0.0040%
Zirconium (Zr) is an optional element and may not be included. That is, the Zr content may be 0%.
When Zr is contained, that is, when the Zr content is more than 0%, Zr functions as a hydrogen penetration inhibiting element. Specifically, Zr stabilizes the corrosion product film formed on the surface of the steel material in a sour environment and suppresses hydrogen from penetrating into the steel material. As a result, the SSC resistance of the steel material increases. If even a small amount of Zr is contained, the above effects can be obtained to some extent.
However, if the Zr content exceeds 0.0040%, coarse oxides will be produced even if the contents of other elements are within the range of this embodiment. As a result, the SSC resistance of the steel material decreases.
Therefore, the Zr content is 0 to 0.0040%.
The preferable lower limit of the Zr content is 0.0001%, more preferably 0.0003%, even more preferably 0.0006%, still more preferably 0.0010%, and even more preferably 0.0015%. %.
A preferable upper limit of the Zr content is 0.0038%, more preferably 0.0035%, and still more preferably 0.0032%.

Sb: 0-0.50%
Antimony (Sb) is an optional element and may not be included. That is, the Sb content may be 0%.
When Sb is contained, that is, when the Sb content is more than 0%, Sb functions as a hydrogen penetration suppressing element. Specifically, Sb suppresses hydrogen from entering the steel material in a sour environment. As a result, the SSC resistance of the steel material increases. If even a small amount of Sb is contained, the above effects can be obtained to some extent.
However, if the Sb content exceeds 0.50%, the hot workability of the steel material will decrease even if the contents of other elements are within the range of this embodiment.
Therefore, the Sb content is 0 to 0.50%.
The preferable lower limit of the Sb content is 0.01%, more preferably 0.03%, still more preferably 0.05%, and still more preferably 0.08%.
A preferable upper limit of the Sb content is 0.40%, more preferably 0.35%, still more preferably 0.30%, and still more preferably 0.25%.

Cu: 0-0.50%
Copper (Cu) is an optional element and may not be included. That is, the Cu content may be 0%.
When Cu is contained, that is, when the Cu content is more than 0%, Cu functions as a hydrogen penetration suppressing element. Specifically, Cu concentrates at the interface between the corrosion product film and the base material in a sour environment. This suppresses the surface activity of the base material and suppresses hydrogen from entering the steel material. As a result, the SSC resistance of the steel material increases. Furthermore, Cu is dissolved in the steel material to improve the hardenability of the steel material, thereby increasing 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 exceeds 0.50%, Cu will precipitate into the steel material. Precipitated Cu tends to trap hydrogen. Therefore, even if the contents of other elements are within the range of this embodiment, the SSC resistance of the steel material is reduced.
Therefore, the Cu content is 0-0.50%.
The preferable lower limit of the Cu content is 0.01%, more preferably 0.02%, and still more preferably 0.05%.
A preferable upper limit of the Cu content is 0.40%, more preferably 0.38%, still more preferably 0.35%, and still more preferably 0.30%.

Ni: 0-0.50%
Nickel (Ni) is an optional element and may not be included. That is, the Ni content may be 0%.
When Ni is contained, that is, when the Ni content is more than 0%, Ni functions as a hydrogen penetration suppressing element. Specifically, Ni concentrates at the interface between the corrosion product film and the base material in a sour environment. This suppresses the surface activity of the base material and suppresses hydrogen from entering the steel material. As a result, the SSC resistance of the steel material increases. If even a small amount of Ni is contained, the above effects can be obtained to some extent.
However, if the Ni content exceeds 0.50%, even if the contents of other elements are within the ranges of this embodiment, local corrosion tends to progress and the hydrogen embrittlement resistance of the steel material deteriorates.
Therefore, the Ni content is 0 to 0.50%.
The preferable lower limit of the Ni content is 0.01%, more preferably 0.05%, and still more preferably 0.07%.
A preferable upper limit of the Ni content is 0.45%, more preferably 0.40%, still more preferably 0.35%, and still more preferably 0.32%.

Co: 0-0.50%
Cobalt (Co) is an optional element and may not be included. That is, the Co content may be 0%.
When Co is contained, that is, when the Co content is more than 0%, Co functions as a hydrogen penetration suppressing element. Specifically, Co concentrates at the interface between the corrosion product film and the base material in a sour environment. This suppresses the surface activity of the base material and suppresses hydrogen from entering the steel material. As a result, the SSC resistance of the steel material increases. If even a small amount of Co is contained, the above effects can be obtained to some extent.
However, if the Co content exceeds 0.50%, even if the contents of other elements are within the ranges of this embodiment, the hardenability of the steel material decreases, and the strength of the steel material decreases.
Therefore, the Co content is 0-0.50%.
The preferable lower limit of the Co content is 0.01%, more preferably 0.02%, even more preferably 0.03%, still more preferably 0.05%, and even more preferably 0.08%. %.
A preferable upper limit of the Co content is 0.40%, more preferably 0.30%, still more preferably 0.20%, and still more preferably 0.15%.

Ca: 0-0.0040%
Calcium (Ca) is an optional element and may not be included. That is, the Ca content may be 0%.
When Ca is contained, that is, when the Ca content is more than 0%, 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 exceeds 0.0040%, coarse oxides will be produced even if the contents of other elements are within the range of this embodiment. As a result, the SSC resistance of the steel material decreases.
Therefore, the Ca content is 0 to 0.0040%.
The lower limit of the Ca content is preferably 0.0001%, more preferably 0.0003%.
A preferable upper limit of the Ca content is 0.0030%, more preferably 0.0020%, still more preferably 0.0015%, and still more preferably 0.0012%.

Mg: 0-0.0040%
Magnesium (Mg) is an optional element and may not be included. That is, the Mg content may be 0%.
When Mg is contained, that is, when the Mg content is more than 0%, 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 exceeds 0.0040%, coarse oxides will be produced even if the contents of other elements are within the range of this embodiment. As a result, the SSC resistance of the steel material decreases.
Therefore, the Mg content is 0 to 0.0040%.
The lower limit of the Mg content is preferably 0.0001%, more preferably 0.0003%.
The upper limit of the Mg content is preferably 0.0030%, more preferably 0.0025%, even more preferably 0.0020%, and still more preferably 0.0015%.

Rare earth elements: 0-0.0040%
Rare earth elements (REM) are optional elements and may not be included. That is, the REM content may be 0%.
When REM is contained, that is, when the REM content is more than 0%, 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 the 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.
However, if the REM content exceeds 0.0040%, coarse oxides will be produced even if the contents of other elements are within the range of this embodiment. As a result, the SSC resistance of the steel material decreases.
Therefore, the REM content is between 0 and 0.0040%.
The lower limit of the REM content is preferably 0.0001%, more preferably 0.0003%, and even more preferably 0.0005%.
A preferable upper limit of the REM content is 0.0035%, more preferably 0.0030%, and still more preferably 0.0025%.

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.

[Group 2: Regarding Nb, V and B]
The chemical composition of the steel material according to the present embodiment may further contain one or more elements selected from the group consisting of Nb, V, and B in place of a part of Fe. All of these elements are optional elements and increase the strength of the steel material.

Nb: 0-0.150%
Niobium (Nb) is an optional element and may not be included. That is, the Nb content may be 0%.
When Nb is contained, Nb combines with C and/or N to form Nb carbonitride or the like. These Nb carbonitrides etc. refine the crystal grains of the steel material due to the pinning effect. As a result, the strength of the steel material increases. If even a small amount of Nb is contained, the above effects can be obtained to some extent.
However, if the Nb content exceeds 0.150%, coarse Nb carbonitrides and the like will be generated even if the contents of other elements are within the range of this embodiment. As a result, the SSC resistance of the steel material decreases.
Therefore, the Nb content is between 0 and 0.150%.
The lower limit of the Nb content is preferably 0.001%, more preferably 0.003%, even more preferably 0.005%, even more preferably 0.008%, and still more preferably 0.012%. %.
A preferable upper limit of the Nb content is 0.100%, more preferably 0.050%, even more preferably 0.030%, still more preferably 0.025%, and even more preferably 0.020%. %.

V: 0-0.500%
Vanadium (V) is an optional element and may not be included. That is, the V content may be 0%.
When V is contained, V combines with C and/or N to form V carbonitride or the like. These V carbonitrides etc. refine the crystal grains of the steel material due to the pinning effect. As a result, the strength of the steel material increases. If even a small amount of V is contained, the above effects can be obtained to some extent.
However, if the V content exceeds 0.500%, the toughness of the steel material will decrease even if the contents of other elements are within the range of this embodiment.
Therefore, the V content is 0-0.500%.
The lower limit of the V content is preferably 0.001%, more preferably 0.005%, and still more preferably 0.010%.
A preferable upper limit of the V content is 0.300%, more preferably 0.250%, even more preferably 0.200%, still more preferably 0.150%, and even more preferably 0.120%. %, more preferably 0.100%.

B: 0-0.0030%
Boron (B) is an optional element and may not be included. That is, the B content may be 0%.
When B is contained, B forms a solid solution in the steel material, improves the hardenability of the steel material, and increases the strength of the steel material. If even a small amount of B is contained, the above effects can be obtained to some extent.
However, if the B content exceeds 0.0030%, coarse B nitrides will be produced even if the contents of other elements are within the range of this embodiment. As a result, the SSC resistance of the steel material decreases.
Therefore, the B content is 0 to 0.0030%.
The lower limit of the B content is preferably 0.0001%, more preferably 0.0005%, and even more preferably 0.0008%.
A preferable upper limit of the B content is 0.0028%, more preferably 0.0025%, and still more preferably 0.0023%.

[(Feature 2) Regarding yield strength]
The yield strength of the steel material according to this embodiment is 758 to less than 965 MPa (110 ksi class to 125 ksi class). The steel material of this embodiment has excellent SSC resistance even if it has a high yield strength of less than 758 to 965 MPa when characteristics 1 and 3 are satisfied.

[Method of measuring yield strength]
Yield strength is measured by the following method. A tensile test is performed in accordance with ASTM E8/E8M (2021). Specifically, a tensile test piece is taken from the steel material. The size of the tensile test piece is not particularly limited. The tensile test piece is, for example, a round bar tensile test piece with a parallel part diameter of 6.0 mm and a gage length of 30.0 mm.
If the steel material is a steel pipe, take a tensile test piece from the center of the wall thickness. In this case, the longitudinal direction of the tensile test piece is parallel to the axial direction of the steel pipe. If the steel material is a steel plate, take a tensile test piece from the center of the plate thickness. In this case, the longitudinal direction of the tensile test piece is parallel to the rolling direction of the steel plate. When the steel material is round steel, a tensile test piece is taken from section R/2. In this specification, round steel means a steel bar whose cross section perpendicular to the axial direction is circular. The R/2 section means the center of the radius R in a cross section perpendicular to the axial direction (rolling direction) of the round steel. The longitudinal direction of the tensile test piece shall be parallel to the axial direction of the round steel.
Using the collected tensile test piece, a tensile test is conducted in the atmosphere at room temperature (24±3°C). The obtained 0.2% offset proof stress (MPa) is defined as yield strength (MPa).

[Microstructure]
In the microstructure of the steel material according to this embodiment, the total area ratio of tempered martensite and tempered bainite is 90% or more. The remainder of the microstructure is, for example, ferrite and/or pearlite.

In the microstructure of a steel material whose chemical composition satisfies Feature 1 and whose yield strength satisfies Feature 2, the total area ratio of tempered martensite and tempered bainite is 90% or more. Therefore, if the chemical composition of a steel material satisfies feature 1 and the yield strength satisfies feature 2, the total area ratio of tempered martensite and tempered bainite can be considered to be 90% or more in the microstructure of the steel material. can.

[Microstructure observation method]
The total area ratio of tempered martensite and tempered bainite can be determined by the following method. First, a test piece is taken from the steel material.
When the steel material is a steel pipe, a test piece having an observation surface of 10 mm in the tube axis direction and 10 mm in the tube diameter direction from the center of the wall thickness is taken. If the steel material is a steel pipe with a wall thickness of less than 10 mm, a test piece having an observation surface of the wall thickness of the steel pipe 10 mm in the pipe axis direction and in the pipe radial direction is taken.
When the steel material is a steel plate, a test piece having an observation surface extending 10 mm in the rolling direction and 10 mm in the plate thickness direction from the center of the plate thickness is taken. If the steel material is a steel plate with a thickness of less than 10 mm, a test piece having an observation surface of 10 mm in the rolling direction and the thickness of the steel plate in the thickness direction is taken.
If the steel material is a round steel, a test piece is taken from a cross section perpendicular to the axial direction (rolling direction) of the round steel. Specifically, a test piece is taken that includes the R/2 part in the center and has an observation surface of 10 mm in the axial direction and 10 mm in the radial direction of the cross section. If the diameter of the cross section is less than 10 mm, take a test piece that includes the R/2 portion and has an observation surface that is 10 mm in the axial direction and has a diameter in the radial direction of the cross section.

Polish the observation surface of the test piece to a mirror surface. After polishing, the observation surface is immersed in a nital corrosive solution for about 10 seconds to be etched. 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, 10000 μm ² (1000x magnification).

In each field of view, tempered martensite and tempered bainite are identified from the contrast. In each field of view, tempered martensite and tempered bainite can be distinguished from other structures (ferrite, pearlite, etc.) based on their morphology. Specifically, the tissue having lamellae can be identified as pearlite. The structure containing laths and lenses can be identified as tempered martensite and tempered bainite. A structure without substructure within the grain can be identified as ferrite.

The total area ratio of the specified tempered martensite and tempered bainite is determined. The method for determining the total area ratio is not particularly limited, and any known method may be used. For example, the total area ratio of tempered martensite and tempered bainite can be determined by image analysis. In this embodiment, the arithmetic mean value of the total area ratio of tempered martensite and tempered bainite determined in all fields of view (10 fields of view) is defined as the total area ratio (%) of tempered martensite and tempered bainite.
[(Feature 3) Regarding electrochemical elements EE and FN]
Furthermore, in the steel material of this embodiment, when the yield strength is 758 to less than 862 MPa (110 ksi class), the EE defined by formula (1) is 2.75 or more, and the FN defined by formula (2) is When it is 0.185 or more and the yield strength is 862 to less than 965 MPa (125 ksi class), EE is 3.00 or more and FN is 0.200 or more.
EE=-0.25C+2Si-5.8Mn+2.1Cr+Mo+4.1Zr+2.6Sb+0.3Cu+0.4Ni+1.5Co (1)
FN=EE/(D ^0.9 ) (2)
Here, each element symbol in formula (1) is substituted with the content in mass % of the corresponding element. D in equation (2) is substituted with the average circular equivalent diameter in μm of prior austenite grains in the steel material.

[About EE defined by formula (1)]
In EE, Si, Cr, Mo, Zr, Sb, Cu, Ni, and Co are elements that suppress hydrogen penetration from the surface of the steel material (hydrogen penetration suppressing elements). In EE, C and Mn are elements that promote hydrogen penetration from the steel surface (hydrogen penetration promoting elements). EE is an index of electrochemical hydrogen penetration suppression effect in steel materials.

When the yield strength of the steel material is 110 ksi class (less than 758 to 862 MPa), if the EE is 2.75 or more, the hydrogen penetration inhibiting element is sufficiently larger than the hydrogen penetration promoting element. Therefore, hydrogen penetration from the steel surface is electrochemically suppressed. As a result, excellent SSC resistance can be obtained on the premise that Features 1 and 2 are satisfied and FN is 0.185 or more.

When the yield strength is 110 ksi class, the preferable lower limit of EE is 2.78, more preferably 2.80, even more preferably 2.85, still more preferably 2.90, even more preferably It is 2.95, more preferably 3.00.
The upper limit of EE is not particularly limited, but the preferable upper limit of EE is 6.60, more preferably 6.00, still more preferably 5.80, and still more preferably 5.50.

If the yield strength of the steel material is 125 ksi class (less than 862 to 965 MPa), the strength is higher than that of 110 ksi class, so in order to improve SSC resistance, it is necessary to further suppress hydrogen intrusion. When EE is 3.00 or more, hydrogen penetration from the surface of the steel material is electrochemically suppressed in a steel material having a yield strength of 125 ksi class. As a result, excellent SSC resistance can be obtained on the premise that Features 1 and 2 are satisfied and FN is 0.200 or more.

When the yield strength is 125 ksi class, the preferable lower limit of EE is 3.10, more preferably 3.15, still more preferably 3.20, still more preferably 3.25, even more preferably 3.30, more preferably 3.35.
The upper limit of EE is not particularly limited, but the preferable upper limit of EE is 6.60, more preferably 6.00, still more preferably 5.80, and still more preferably 5.50.

[About FN defined by formula (2)]
As mentioned above, in order to suppress the penetration of hydrogen from the steel material surface in a steel material whose chemical composition satisfies characteristic 1, a synergistic effect between the electrochemical element EE defined by formula (1) and the physical element is required. is valid.

If the yield strength of the steel material whose chemical composition satisfies Feature 1 is 110 ksi class (less than 758 to 862 MPa), if EE is 2.75 or more and FN is 0.185 or more, as shown in Figure 2A , SSC resistance increases significantly.

When the yield strength is 110 ksi class, the preferable lower limit of FN is 0.187, more preferably 0.190, still more preferably 0.192, and still more preferably 0.195. Although the upper limit of FN is not particularly limited, the preferable upper limit of FN is 0.580, more preferably 0.550, still more preferably 0.500, and even more preferably 0.450.

In addition, if the yield strength of the steel material whose chemical composition satisfies characteristic 1 is 125 ksi class (less than 862 to 965 MPa), if EE is 3.00 or more and FN is 0.200 or more, then As shown, the SSC resistance is significantly increased.

When the yield strength is 125 ksi class, the preferable lower limit of FN is 0.205, more preferably 0.210, still more preferably 0.215, and still more preferably 0.220. Although the upper limit of FN is not particularly limited, the preferable upper limit of FN is 0.580, more preferably 0.550, still more preferably 0.500, and still more preferably 0.450.

[How to determine the average circular equivalent diameter D of prior austenite grains]
The average equivalent circle diameter D (μm) of the prior austenite grains of the steel material according to this embodiment is determined by the following method. First, a test piece is taken from the steel material.

When the steel material is a steel pipe, a test piece having an observation surface of 10 mm in the tube axis direction and 10 mm in the tube diameter direction from the center of the wall thickness is taken. If the steel material is a steel pipe with a wall thickness of less than 10 mm, a test piece having an observation surface of the wall thickness of the steel pipe 10 mm in the pipe axis direction and in the pipe radial direction is taken.
When the steel material is a steel plate, a test piece having an observation surface extending 10 mm in the rolling direction and 10 mm in the plate thickness direction from the center of the plate thickness is taken. If the steel material is a steel plate with a thickness of less than 10 mm, a test piece having an observation surface of 10 mm in the rolling direction and the thickness of the steel plate in the thickness direction is taken.
If the steel material is a round steel, a test piece is taken from a cross section perpendicular to the axial direction (rolling direction) of the round steel. Specifically, a test piece is taken that includes the R/2 part in the center and has an observation surface of 10 mm in the axial direction and 10 mm in the radial direction of the cross section. If the diameter of the cross section is less than 10 mm, take a test piece that includes the R/2 portion and has an observation surface that is 10 mm in the axial direction and has a diameter in the radial direction of the cross section.

Embed the test piece in resin and polish the observation surface to a mirror surface. The polished test piece is immersed in a saturated aqueous solution of picric acid for about 60 seconds. As a result, the observation surface is etched, and prior austenite grain boundaries appear on the observation surface. The etched observation surface is observed using an optical microscope at 420 times magnification for 10 fields of view. The visual field area of each visual field is a rectangle of 450 μm×450 μm. In accordance with JIS G 0551 (2020), the grain size number of prior austenite grains in each field of view is determined by a cutting method. At this time, the number of grid points, which are the intersections of the grid lines, is set to 16. The arithmetic mean value of the grain size numbers of prior austenite grains determined in 10 fields of view is determined. The average area of the prior austenite grains is calculated based on the arithmetic mean value of the grain size numbers of the prior austenite grains. The equivalent circle diameter is calculated from the calculated average area of the prior austenite grains. Here, the equivalent circle diameter is the diameter of a circle having the same area as the average area of prior austenite grains. The calculated equivalent circle diameter is defined as the average equivalent circle diameter D (μm) of the prior austenite grains. The average equivalent circle diameter D is an integer obtained by rounding off the calculated value to the first decimal place.

The preferable upper limit of the average equivalent circle diameter of the prior austenite grains is 40 μm, more preferably 35 μm, still more preferably 30 μm, and even more preferably 25 μm.
The lower limit of the average equivalent circle diameter of the prior austenite grains is preferably 10 μm, more preferably 15 μm, and even more preferably 17 μm.

[Shape of steel material]
The shape of the steel material according to this embodiment is not particularly limited. The steel material is, for example, a steel pipe, a steel plate, or a round steel.

Preferably, the steel material of this embodiment is a steel pipe for oil wells. Steel pipes for oil wells are, for example, casings, tubing, drill pipes, etc. used for drilling oil or gas wells, extracting crude oil or natural gas, and the like. When the steel material is a steel pipe for oil wells, the wall thickness is, for example, 9 to 60 mm.

[About the effects of the steel material of this embodiment]
The steel material of this embodiment satisfies the characteristics 1 to 3 described above. Therefore, the steel material of this embodiment has excellent SSC resistance despite having a high strength of 110 ksi class (758 to less than 862 MPa) to 125 ksi class (862 to less than 965 MPa).

[SSC resistance evaluation method]
SSC resistance is evaluated by a room temperature SSC resistance evaluation test and a low temperature SSC resistance evaluation test based on NACE TM0177-2016 Method A, and a DCB test based on NACE TM0177-2016 Method D.

[Room temperature SSC resistance evaluation test]
In the room temperature SSC resistance evaluation test, a mixed aqueous solution (NACE solution A) of 5.0 mass% sodium chloride and 0.5 mass% acetic acid is used as the test solution. Collect a round bar test piece from the steel material.
If the steel material is a steel pipe, take a round bar test piece from the center of the wall thickness. In this case, the longitudinal direction of the round bar test piece is parallel to the axial direction of the steel pipe. If the steel material is a steel plate, take a round bar test piece from the center of the plate thickness. In this case, the longitudinal direction of the round bar test piece is parallel to the rolling direction of the steel plate. If the steel material is round steel, take a round bar test piece from section R/2. In this case, the longitudinal 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. Take three round bar test pieces from the steel material.

A stress equivalent to 90% of the actual yield stress is applied to the round bar test piece. A test solution at 24° C. is poured into a test container so that the round rod test piece to which stress is applied is immersed, and this is used as a test bath. After degassing the test bath, H ₂ S gas is bubbled into the test bath to saturate it. Specifically, 1 atm H ₂ S gas is blown into the test bath. The test bath, flushed with H ₂ S gas, is held at 24° C. for 720 hours.

[Low temperature SSC resistance evaluation test]
In the low temperature SSC resistance evaluation test, NACE solution A is used as the test solution as in the room temperature SSC resistance evaluation test. As in the normal temperature SSC resistance evaluation test, three round bar test pieces are taken from the steel material. 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 longitudinal direction of the round bar test piece is the same as in the room temperature SSC resistance evaluation test.

A stress equivalent to 85% of the actual yield stress is applied to the round bar test piece. A test solution at 4° C. is poured into a test container so that the round rod test piece to which stress is applied is immersed, and this is used as a test bath. After degassing the test bath, H ₂ S gas is bubbled into the test bath to saturate it. Specifically, 1 atm H ₂ S gas is blown into the test bath. The test bath, flushed with H ₂ S gas, is maintained at 4° C. for 720 hours.

In the room temperature SSC resistance evaluation test and the low temperature SSC resistance evaluation test, the presence or absence of sulfide stress cracking (SSC) in the round bar test piece is observed after holding for 720 hours. Specifically, the round bar test piece after being held for 720 hours is observed with the naked eye and using a projector with 10x magnification.
In the steel material of this embodiment, as a result of observation, no cracks were confirmed in all three round bar test pieces in the room temperature SSC resistance evaluation test, and all three round bar test pieces in the low temperature SSC resistance evaluation test. No cracks are observed.

[DCB test]
The DCB test is carried out in the following manner. A 5.0% by mass aqueous sodium chloride solution is used as the test solution. A DCB test piece shown in FIG. 3A is taken from the steel material.
When the steel material is a steel pipe, a DCB test piece is taken from the center of the wall thickness. In this case, the longitudinal direction of the DCB test piece is parallel to the axial direction of the steel pipe. When the steel material is a steel plate, a DCB test piece is taken from the center of the plate thickness. In this case, the longitudinal direction of the DCB test piece is parallel to the rolling direction of the steel plate. If the steel material is round steel, take a DCB test piece from section R/2. In this case, the longitudinal direction of the DCB test piece is parallel to the axial direction of the round steel. A wedge shown in FIG. 3B is further extracted from the steel material. The thickness t of the wedge is 3.10 (mm).

Referring to FIG. 3A, the wedge is driven between the arms of the DCB test piece. The DCB test piece with the wedge driven into it is sealed in a test container. Thereafter, the test solution is poured into the test container leaving the gas phase behind to form a test bath. The amount of test bath is 1 L per test piece. Subsequently, N ₂ gas is blown into the test bath for 3 hours to degas the test bath until the dissolved oxygen in the test bath becomes 20 ppb or less.

H ₂ S gas is blown into the degassed test bath to create a corrosive environment. Specifically, 5 atm (0.5 MPa) H ₂ S gas is blown into the test bath. The pH of the test bath is in the range 3.5-4.0 throughout the immersion. While stirring the test bath, the inside of the test container is maintained at 24±3° C. for 14 days (336 hours). The DCB test piece is taken out from the test container after holding.

A pin is inserted into a hole formed at the end of the arm of the taken out DCB test piece, the notch is opened using a tensile tester, and the wedge release stress P is measured. Furthermore, the notch of the DCB test piece is opened in liquid nitrogen, and the crack growth length a of the DCB test piece while immersed in the test bath is measured. The crack growth length a can be measured visually using a caliper. Based on the measured wedge release stress P and the crack growth length a, the fracture toughness value K _1SSC (MPa√m) is determined using the following equation.

In the above formula, h (mm) is the height of each arm of the DCB test piece. B (mm) is the thickness of the DCB test piece. Bn (mm) is the web thickness of the DCB specimen. These are defined in NACE TM0177-2016 Method D.

In the case of the steel material of this embodiment, in the case of 110 ksi class (less than 758 to 862 MPa), if the fracture toughness value _K1SSC determined by the DCB test is 25.0 MPa√m or more, excellent fracture toughness can be obtained in the DCB test. I judge that. In the case of 125 ksi class (less than 862 to 965 MPa), if the fracture toughness value K _1SSC determined by the DCB test is 24.0 MPa√m or more, it is judged that excellent fracture toughness was obtained in the DCB test.

The above can be summarized as follows.
"Excellent SSC resistance can be obtained" in the case of 110 ksi class (758 to less than 862 MPa) means that the evaluation test for room temperature SSC resistance at 24°C and low temperature resistance at 4°C in accordance with NACE TM0177-2016 Method A is used. This means that no cracks were confirmed in the SSC property evaluation test, and the fracture toughness value _K1SSC obtained in the DCB test in accordance with NACE TM0177-2016 Method D is 25.0 MPa√m or more.

In the case of 125 ksi class (less than 862 to 965 MPa), "excellent SSC resistance can be obtained" refers to the evaluation test of room temperature SSC resistance at 24°C and low temperature resistance at 4°C in accordance with NACE TM0177-2016 Method A. It means that no cracks were confirmed in the SSC property evaluation test, and the fracture toughness value _K1SSC obtained in the DCB test in accordance with NACE TM0177-2016 Method D is 24.0 MPa√m or more.

[Production method]
As an example of the method for manufacturing a steel material according to the present embodiment, a method for manufacturing a seamless steel pipe will be described. The method for manufacturing steel materials described below is an example for manufacturing the steel materials of this embodiment. Therefore, the steel material having the above-mentioned configuration may be manufactured by a manufacturing method other than the manufacturing method described below. However, the manufacturing method described below is a preferable example of the method for manufacturing the steel material of this embodiment.

An example of the method for manufacturing a seamless steel pipe of this embodiment includes the following steps.
(Process 1) Material preparation process (Process 2) Hot processing process (Process 3) Quenching process (Process 4) Tempering process

The main manufacturing conditions in the above manufacturing method are as follows.
(Condition 1) In step 4, if the yield strength of the steel material to be manufactured is 110 ksi class (758 to less than 862 MPa), FA defined by formula (A) is 2500 or less. Further, in step 4, if the yield strength of the steel material to be manufactured is 125 ksi class (less than 862 to 965 MPa), FA defined by formula (A) is set to 2400 or less.
FA=T×(-3.0C+4.7Si-4.4Mn-2.4Cr+2.2Mo-2.2Cu-3.2Ni)×(t/60) ^0.5 (A)
Here, each element symbol in formula (A) is substituted with the content in mass % of the corresponding element in the steel material. The tempering temperature (° C.) is substituted for T. The holding time t (minutes) at the tempering temperature T is substituted for t.

Each step will be explained below.

[(Process 1) Material preparation process]
In the material preparation step, a material is manufactured using molten steel whose chemical composition satisfies Feature 1. The method for producing the material is not particularly limited, and any known method may be used. Specifically, a slab (slab, bloom, or billet) may be manufactured by a continuous casting method using molten steel. An ingot may be manufactured by an ingot-forming method using molten steel. A material (slab, bloom, or billet) is manufactured through the above steps.

[(Step 2) Hot processing step]
In the hot working step, the prepared material is hot worked to produce an intermediate steel material. The hot working method for manufacturing the intermediate steel material is not particularly limited. The hot working may be hot forging, hot extrusion, or hot rolling.

When the steel material is a steel pipe, the hot working process is, for example, as follows. When using a bloom or an ingot as a raw material, first, the raw material is subjected to blooming rolling using a blooming mill to produce a billet. The heating temperature before blooming is not particularly limited, but is, for example, 1100 to 1350°C.

Using a billet manufactured by blooming rolling or a billet manufactured by a continuous casting method in the material preparation process, piercing rolling is performed using the Mannesmann method. In this case, the billet is first heated in a heating furnace. The heating temperature is not particularly limited, but is, for example, 1100 to 1350°C. The billet extracted from the heating furnace is subjected to piercing rolling to produce an intermediate steel material (base pipe). The perforation ratio in the perforation rolling is not particularly limited, but is, for example, 1.0 to 4.0. The billet after piercing and rolling is subjected to elongation rolling using a mandrel mill. Furthermore, if necessary, the billet after elongation rolling is subjected to sizing rolling using a reducer or a sizing mill. Through the above steps, an intermediate steel material (raw pipe) is manufactured. The cumulative area reduction rate in the hot working process is not particularly limited, but is, for example, 20 to 70%.

When the steel material is a steel pipe, instead of piercing and rolling using the Mannesmann method, the billet is subjected to the Eugene-Séjournet method or the Erhardt push bench method (i.e., hot extrusion) to produce an intermediate steel material (raw pipe). may be manufactured.

When the steel material is a steel plate, the hot working process is, for example, as follows. Rough rolling is performed on the slab using a reverse rolling mill to produce a rough bar. The heating temperature before rough rolling is not particularly limited, but is, for example, 1100 to 1350°C. Furthermore, finish rolling is performed on the rough bar using a tandem rolling mill to produce an intermediate steel material (steel plate).

When the steel material is round steel, the hot working process is, for example, as follows. When using a bloom or an ingot as a raw material, first, the raw material is subjected to blooming rolling using a blooming mill to produce a billet. The heating temperature before blooming is not particularly limited, but is, for example, 1100 to 1350°C.

A billet manufactured by blooming rolling or a billet manufactured by a continuous casting method in the material preparation process is heated. The heating temperature is not particularly limited, but is, for example, 1100 to 1350°C. Finish rolling is performed on the heated billet using a continuous rolling mill to produce an intermediate steel material (round steel). 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.

The intermediate steel material manufactured by the above hot working may be air cooled. Intermediate steel products manufactured by hot working may also be quenched directly after hot working without being cooled to room temperature, or quenching may be performed after reheating (reheating) after hot working. Good too. When quenching is performed directly after hot working, or when quenching is performed after reheating after hot working, stress relief annealing (SR treatment) is performed before the next quenching step in order to remove residual stress. May be implemented.

[(Step 3) Quenching step]
In the quenching process, the intermediate steel material produced in the hot working process is quenched. Hardening is carried out by a well-known method. Specifically, the intermediate steel material after the hot working step is charged into a heat treatment furnace and held at the quenching temperature. The quenching temperature shall be at least the _AC3 transformation point. However, if the quenching temperature is too high, the prior austenite grains may become coarse. Therefore, the quenching temperature is, for example, 800 to 950°C. After holding the intermediate steel material at the quenching temperature, it is rapidly cooled (quenched). The holding time at the quenching temperature is not particularly limited, but is, for example, 10 to 60 minutes.

The quenching method is, for example, water cooling or oil cooling. The quenching method is not particularly limited. For example, the intermediate steel material may be quenched by immersing it in a water tank or an oil tank. When the intermediate steel material is a steel pipe, the steel pipe may be rapidly cooled by shower cooling or mist cooling by pouring or spraying cooling water onto the outer surface and/or inner surface of the steel pipe.

When the intermediate steel material is a raw pipe (seamless steel pipe), quenching (direct quenching) may be performed immediately after hot working without cooling the raw pipe to room temperature after the hot working step. Furthermore, before the temperature of the mother tube after hot working decreases, it may be charged into a reheating furnace and maintained at the quenching temperature, and then quenched.

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.

[(Step 4) Tempering step]
In the tempering step, the intermediate steel material after quenching is further tempered. In the tempering step, the yield strength of the steel material can be adjusted by appropriately adjusting the tempering temperature depending on the chemical composition. Specifically, the tempering conditions are adjusted so that the yield strength of the steel material is 110 ksi class (758 to less than 862 MPa) to 125 ksi class (862 to less than 965 MPa).

In the tempering step, the tempering temperature T is 660 to 740°C, and the holding time t at the tempering temperature T is 20 to 180 minutes. In the tempering process, FA defined by formula (A) is further adjusted according to the strength. Specifically, when the yield strength of the manufactured steel material is 110 ksi class, FA is set to 2500 or less. When the yield strength of the manufactured steel material is 125 ksi class, FA is set to 2400 or less.
FA=T×(-3.0C+4.7Si-4.4Mn-2.4Cr+2.2Mo-2.2Cu-3.2Ni)×(t/60) ^0.5 (A)
Here, each element symbol in formula (A) is substituted with the content in mass % of the corresponding element in the steel material. The tempering temperature (° C.) is substituted for T. The holding time (minutes) at the tempering temperature T is substituted for t.

As mentioned above, the steel material of this embodiment has a pressure of 758 to 965 MPa due to the synergistic effect of the electrochemical element (EE defined by formula (1)) and the physical element (average equivalent circle diameter of prior austenite grains). Excellent SSC resistance can be obtained even with a high strength of less than FA is an index for appropriately adjusting the electrochemical elements that make up the numerator of formula (2), the physical elements that make up the denominator of formula (2), and the strength of the steel material.

If the strength of the manufactured steel is 110ksi class, and the FA is 2500 or less, the relationship between electrochemical elements, physical elements, and strength should be appropriately determined, assuming that the chemical composition satisfies characteristic 1. Can be adjusted. Therefore, steel materials satisfying Features 1 to 3 can be manufactured.

Similarly, if the strength of the manufactured steel is 125 ksi class, and the FA is 2400 or less, the relationship between electrochemical elements, physical elements, and strength is determined based on the premise that the chemical composition satisfies characteristic 1. can be adjusted appropriately. Therefore, steel materials satisfying Features 1 to 3 can be manufactured.

Through the above steps, the steel material of this embodiment can be manufactured. In addition, in the above-mentioned manufacturing method, an example of the manufacturing method of the steel material by this embodiment was demonstrated. The steel material according to this embodiment may also be manufactured by a manufacturing method other than the manufacturing method described above. Even in this case, if the steel material satisfies Features 1 to 3, high strength of 110 ksi class (758 to less than 862 MPa) to 125 ksi class (862 to less than 965 MPa) and excellent SSC resistance can be obtained.

The effects of the steel material of this embodiment will be explained in more detail with examples. The conditions in the following examples are examples of conditions adopted to confirm the feasibility and effects of the steel material of this embodiment. Therefore, the steel material of this embodiment is not limited to this one example condition.

In Example 1, the SSC resistance of a steel material having a yield strength of 110 ksi class (758 to less than 862 MPa) was investigated. Specifically, steel materials (seamless steel pipes) having chemical compositions shown in Tables 1-1 and 1-2 were manufactured.

Blank areas in Table 1-2 mean that the content of the corresponding element is at the impurity level. Note that the EE of each test number is shown in the "EE" column of Table 2.

A bloom was manufactured using a continuous casting method using molten steel. Thereafter, the bloom was subjected to blooming rolling to produce a round billet with a diameter of 310 mm. The heating temperature before blooming was 1100 to 1350°C.

Hot working was performed on a round billet manufactured by blooming. Specifically, the round billet was placed in a heating furnace and heated at 1100 to 1350°C. The round billet extracted from the heating furnace was hot rolled (hot worked) by the Mannesmann method to produce raw pipes (seamless steel pipes) of each test number. At this time, the perforation ratio was within the range of 1.0 to 4.0, and the cumulative area reduction rate during hot working was within the range of 20 to 70%.

Hardening was performed on the raw tube after hot working. The quenching temperature (°C) in quenching is shown in "Quenching temperature (°C)" in the "Quenching conditions" column of Table 2. The holding time at the quenching temperature was 15 minutes. After quenching, the raw tube was tempered. The tempering temperature T (°C) in tempering is shown in "Tempering temperature T (°C)" in the "Tempering conditions" column of Table 2. The holding time t (minutes) at the tempering temperature T is shown in "Holding time t (minutes)" in the "Tempering conditions" column of Table 2. The FA during tempering is shown in the "FA" column of Table 2. Through the above manufacturing process, a steel material (seamless steel pipe) was manufactured.

[Evaluation test]
The following evaluation tests were conducted on the steel materials (seamless steel pipes) of each test number.
(Test 1) Microstructure observation test (Test 2) Measurement test of average equivalent circle diameter D of prior austenite grains (Test 3) Yield strength evaluation test (Test 4) Room temperature SSC resistance evaluation test and low temperature SSC resistance evaluation test (Test 5) DCB Test Each test will be explained below.

[(Test 1) Microstructure observation test]
The total area ratio (%) of tempered martensite and tempered bainite of the steel material of each test number was determined by the following method. A test piece having an observation surface of 10 mm in the pipe axis direction and 10 mm in the pipe diameter direction was taken from the center of the wall thickness of the steel material (seamless steel pipe) of each test number. In addition, in the case where the steel material was a steel pipe with a wall thickness of less than 10 mm, a test piece having an observation surface of the wall thickness of the steel pipe 10 mm in the pipe axis direction and in the pipe radial direction was taken.

Using the sampled test piece, the total area ratio (%) of tempered martensite and tempered bainite was determined by the method described in the above-mentioned [Microstructure observation method]. As a result, in all test numbers, the total area ratio of tempered martensite and tempered bainite was 90% or more.

[(Test 2) Average circular equivalent diameter D measurement test of prior austenite grains]
The average equivalent circular diameter D (μm) of prior austenite grains of steel materials of each test number was determined by the following method. A test piece having an observation surface of 10 mm in the pipe axis direction and 10 mm in the pipe diameter direction was taken from the center of the wall thickness of the steel material (seamless steel pipe) of each test number. In addition, in the case where the steel material was a steel pipe with a wall thickness of less than 10 mm, a test piece having an observation surface of the wall thickness of the steel pipe 10 mm in the pipe axis direction and in the pipe radial direction was taken.

Using the sampled test piece, the average equivalent circle diameter D (μm) of the prior austenite grains was determined by the method described in [How to determine the average equivalent circle diameter D of the prior austenite grains] above. The average equivalent circular diameter D of the obtained prior austenite grains is shown in the "D (μm)" column in Table 2.

[(Test 3) Yield strength evaluation test]
The yield strength (MPa) of the steel material of each test number was determined by the following method. A round bar tensile test piece was taken from the center of the wall thickness of the steel material (seamless steel pipe) of each test number. The size of the round bar tensile test piece was 6.0 mm in parallel part diameter and 30.0 mm in gage distance. The longitudinal direction of the round bar tensile test piece was parallel to the pipe axis direction of the steel material (seamless steel pipe).

Using the collected round bar tensile test piece, the yield strength (MPa) was determined by the method described in the above-mentioned [Method for measuring yield strength]. The yield strength obtained is shown in the "YS (MPa)" column of Table 2.

[(Test 4) Room temperature SSC resistance evaluation test and low temperature SSC resistance evaluation test]
An SSC resistance evaluation test based on NACE TM0177-2016 Method A of the steel materials with each test number was conducted using the following method.

The SSC resistance at 24°C was evaluated by the method described in the above-mentioned [Room Temperature SSC Resistance Evaluation Test]. In addition, a round bar test piece was taken from the center of the wall thickness of the steel material (seamless steel pipe) of each test number. The size of the round bar test piece was 6.35 mm in diameter and 25.4 mm in length of the parallel part. The longitudinal direction of the round bar test piece was parallel to the pipe axis direction of the steel material (seamless steel pipe). The test bath was blown with 1 atm H ₂ S gas.

Furthermore, the SSC resistance at 4° C. was evaluated by the method described in [Low-temperature SSC resistance evaluation test]. In addition, a round bar test piece was taken from the center of the wall thickness of the steel material (seamless steel pipe) of each test number. The size of the round bar test piece was 6.35 mm in diameter and 25.4 mm in length of the parallel part. The longitudinal direction of the round bar test piece was parallel to the pipe axis direction of the steel material (seamless steel pipe). The test bath was blown with 1 atm H ₂ S gas.

As a result of the room temperature SSC resistance evaluation test, if no cracks were confirmed in all three round bar test pieces, it was judged that excellent room temperature SSC resistance was obtained (the "SSC resistance" column in Table 2 Displayed as "E" (Excellent) in the "24°C" column). On the other hand, if cracks were confirmed in one or more of the three round bar test pieces, it was determined that excellent room temperature SSC resistance was not obtained ("24℃ resistance" in the "SSC resistance" column in Table 2). ” column).

In addition, as a result of the low-temperature SSC resistance evaluation test, if no cracks were confirmed in all three round bar test pieces, it was judged that excellent low-temperature SSC resistance was obtained ("SSC resistance" in Table 2). (Displayed as "E" (Excellent) in the "4°C" column). On the other hand, if cracks were confirmed in one or more of the three round bar test pieces, it was determined that excellent low-temperature SSC resistance was not obtained ("4℃ resistance" in the "SSC resistance" column in Table 2). ” column).

[(Test 5) DCB test]
The fracture toughness value K _1SSC (MPa√m) of the steel material of each test number was determined by the method described in the above-mentioned [DCB test]. The obtained fracture toughness value K _1SSC (MPa√m) is shown in the “K _1SSC (MPa√m)” column in Table 2.

Note that the DCB test piece shown in FIG. 3A was taken from the center of the wall thickness of the steel material (seamless steel pipe) of each test number. The longitudinal direction of the DCB test piece was parallel to the pipe axis direction of the steel material (seamless steel pipe). Furthermore, a wedge shown in FIG. 3B was collected from the steel material. The thickness t of the wedge was 3.10 mm. 5 atm (0.5 MPa) of H ₂ S gas was blown into the test bath.

[Test results]
Referring to Tables 1-1, 1-2, and 2, the chemical compositions of the steel materials (seamless steel pipes) of test numbers 1 to 14 and 16 to 29 were appropriate, and the manufacturing methods were also appropriate. Therefore, the steel materials with these test numbers satisfied Features 1 to 3. As a result, excellent room temperature SSC resistance and excellent low temperature SSC resistance were obtained, and the fracture toughness value _K1SSC was 25.0 MPa√m or more. The steel materials with these test numbers had excellent SSC resistance despite having a high strength of 110 ksi class (758 to less than 862 MPa).

On the other hand, in test number 15, although the chemical composition was appropriate, the quenching temperature was too high. Therefore, FN was too low. As a result, excellent room temperature SSC resistance and excellent low temperature SSC resistance were not obtained, and the fracture toughness value _K1SSC was less than 25.0 MPa√m.

In test number 30, although the chemical composition was appropriate, the EE was too low. Further, in test numbers 31 and 32, although the chemical composition was appropriate, the EE was too low and the FN was also too low. In test number 33, although the chemical composition was appropriate, the EE was too low and the FA was too high. As a result, FN was too low. Therefore, with these test numbers, excellent room temperature SSC resistance and excellent low temperature SSC resistance were not obtained, and the fracture toughness value K _1SSC was less than 25.0 MPa√m.

In test numbers 34 to 36, although the chemical composition was appropriate, the FA, which was the manufacturing condition, was too high. Therefore, FN was too low. Therefore, excellent room temperature SSC resistance and excellent low temperature SSC resistance were not obtained, and the fracture toughness value _K1SSC was less than 25.0 MPa√m.

In test numbers 37 and 38, the Si content was too high. Therefore, excellent room temperature SSC resistance and excellent low temperature SSC resistance were not obtained, and the fracture toughness value _K1SSC was less than 25.0 MPa√m.

In test number 39, the Si content was too low. Furthermore, since the Si content was too low, EE and FN were too low. Therefore, excellent room temperature SSC resistance and excellent low temperature SSC resistance were not obtained, and the fracture toughness value _K1SSC was less than 25.0 MPa√m.

In test number 40, the Mn content was too high. Furthermore, the Mn content was too high, so the EE and FN were too low. Therefore, excellent room temperature SSC resistance and excellent low temperature SSC resistance were not obtained, and the fracture toughness value _K1SSC was less than 25.0 MPa√m.

In Example 2, the SSC resistance of steel materials having a yield strength of 125 ksi class (less than 862 to 965 MPa) was investigated.

Steel materials (seamless steel pipes) having the chemical compositions shown in Tables 1-1 and 1-2 were manufactured. Specifically, the bloom was manufactured by a continuous casting method using molten steel. Thereafter, the bloom was subjected to blooming rolling to produce a round billet with a diameter of 310 mm. The heating temperature before blooming was 1100 to 1350°C.

Hardening was performed on the raw tube after hot working. The quenching temperature (°C) in the quenching is shown in "Quenching temperature (°C)" in the "Quenching conditions" column of Table 3. The holding time at the quenching temperature was 15 minutes. After quenching, the raw tube was tempered. The tempering temperature T (°C) in tempering is shown in "Tempering temperature T (°C)" in the "Tempering conditions" column of Table 3. The holding time t (minutes) at the tempering temperature T is shown in “Holding time t (minutes)” in the “Tempering conditions” column of Table 3. The FA during tempering is shown in the "FA" column of Table 3. Through the above manufacturing process, a steel material (seamless steel pipe) was manufactured. Note that the EE of each test number is shown in the "EE" column of Table 3.

[Evaluation test]
As in Example 1, the following evaluation tests were conducted on the steel materials (seamless steel pipes) of each test number.
(Test 1) Microstructure observation test (Test 2) Measurement test of average equivalent circle diameter D of prior austenite grains (Test 3) Yield strength evaluation test (Test 4) Room temperature SSC resistance evaluation test and low temperature SSC resistance evaluation test (Test 5) DCB test

[(Test 1) Microstructure observation test]
By the same method as the microstructure observation test of Example 1, the total area ratio (%) of tempered martensite and tempered bainite of the steel material of each test number was determined. As a result, in all test numbers, the total area ratio of tempered martensite and tempered bainite was 90% or more.

[(Test 2) Average circular equivalent diameter D measurement test of prior austenite grains]
The average equivalent circle diameter D (μm) of the prior austenite grains of the steel material of each test number was determined by the same method as the measurement test for the average equivalent circle diameter D of the prior austenite grains in Example 1. The average equivalent circular diameter D of the obtained prior austenite grains is shown in the "D (μm)" column in Table 3.

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

As a result of the room temperature SSC resistance evaluation test, if no cracks were confirmed in all three round bar test pieces, it was judged that excellent room temperature SSC resistance was obtained (the "SSC resistance" column in Table 3 Displayed as "E" (Excellent) in the "24°C" column). On the other hand, if cracks were confirmed in one or more of the three round bar test pieces, it was determined that excellent room temperature SSC resistance was not obtained ("24℃ resistance" in the "SSC resistance" column in Table 3). ” column).

In addition, as a result of the low-temperature SSC resistance evaluation test, if no cracks were confirmed in all three round bar test pieces, it was judged that excellent low-temperature SSC resistance was obtained ("SSC resistance" in Table 3). (Displayed as "E" (Excellent) in the "4°C" column). On the other hand, if cracks were confirmed in one or more of the three round bar test pieces, it was determined that excellent low-temperature SSC resistance was not obtained ("4℃ resistance" in the "SSC resistance" column in Table 3). ” column).

[(Test 5) DCB test]
The fracture toughness value K _1SSC (MPa√m) of the steel material of each test number was determined by the method described in the above-mentioned [DCB test]. The obtained fracture toughness value K _1SSC (MPa√m) is shown in the “K _1SSC (MPa√m)” column in Table 3.

[Test results]
Referring to Tables 1-1, 1-2, and 3, the chemical compositions of the steel materials (seamless steel pipes) of test numbers 1 to 12 and 16 to 28 were appropriate, and the manufacturing methods were also appropriate. Therefore, the steel materials with these test numbers satisfied Features 1 to 3. As a result, excellent room temperature SSC resistance and excellent low temperature SSC resistance were obtained, and the fracture toughness value _K1SSC was 24.0 MPa√m or more. The steel materials with these test numbers had excellent SSC resistance despite having a high strength of 125 ksi class (less than 862 to 965 MPa).

On the other hand, in test number 13, although the chemical composition was appropriate, the EE was too low. Further, in test numbers 14 and 29 to 32, although the chemical composition was appropriate, the EE was too low and the FN was also too low. In test number 33, although the chemical composition was appropriate, the EE was too low and the FA was too high. As a result, FN was too low. Therefore, with these test numbers, excellent room temperature SSC resistance and excellent low temperature SSC resistance were not obtained, and the fracture toughness value K _1SSC was less than 24.0 MPa√m.

In test number 15, although the chemical composition was appropriate, the quenching temperature was too high. Therefore, FN was too low. Therefore, excellent room temperature SSC resistance and excellent low temperature SSC resistance were not obtained, and the fracture toughness value _K1SSC was less than 24.0 MPa√m.

In test numbers 34 to 36, although the chemical composition was appropriate, the FA, which was the manufacturing condition, was too high. Therefore, FN was too low. Therefore, excellent room temperature SSC resistance and excellent low temperature SSC resistance were not obtained, and the fracture toughness value _K1SSC was less than 24.0 MPa√m.

In test numbers 37 and 38, the Si content was too high. Therefore, excellent room temperature SSC resistance and excellent low temperature SSC resistance were not obtained, and the fracture toughness value _K1SSC was less than 24.0 MPa√m.

In test number 39, the Si content was too low. Furthermore, since the Si content was too low, EE and FN were too low. Therefore, excellent room temperature SSC resistance and excellent low temperature SSC resistance were not obtained, and the fracture toughness value _K1SSC was less than 24.0 MPa√m.

In test number 40, the Mn content was too high. Furthermore, the Mn content was too high, so the EE and FN were too low. Therefore, excellent room temperature SSC resistance and excellent low temperature SSC resistance were not obtained, and the fracture toughness value _K1SSC was less than 24.0 MPa√m.

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

The chemical composition is in mass%,
C: 0.20-0.35%,
Si: 0.60-1.30%,
Mn: 0.05-0.25%,
P: 0.050% or less,
S: 0.0100% or less,
Al: 0.010-0.100%,
N: 0.0100% or less,
Cr: 0.20-1.00%,
Mo: 0.10-1.00%,
Ti: 0.003 to 0.030%,
O: 0.0050% or less,
Zr: 0 to 0.0040%,
Sb: 0 to 0.50%,
Cu: 0 to 0.50%,
Ni: 0 to 0.50%,
Co: 0 to 0.50%,
Ca: 0-0.0040%,
Mg: 0 to 0.0040%,
Rare earth elements: 0 to 0.0040%,
Nb: 0 to 0.150%,
V: 0 to 0.500%,
B: 0 to 0.0030%, and
The remainder consists of Fe and impurities,
The yield strength is less than 758 to 965 MPa,
When the yield strength is less than 758 to 862 MPa, EE defined by formula (1) is 2.75 or more, FN defined by formula (2) is 0.185 or more,
When the yield strength is less than 862 to 965 MPa, the EE is 3.00 or more, and the FN is 0.200 or more.
Steel material.
EE=-0.25C+2Si-5.8Mn+2.1Cr+Mo+4.1Zr+2.6Sb+0.3Cu+0.4Ni+1.5Co (1)
FN=EE/(D 0.9 ) (2)
Here, each element symbol in formula (1) is substituted with the content in mass % of the corresponding element. D in equation (2) is substituted with the average circular equivalent diameter in μm of the prior austenite grains in the steel material.
The steel material according to claim 1,
The chemical composition is
Zr: 0.0001 to 0.0040%,
Sb: 0.01 to 0.50%,
Cu: 0.01 to 0.50%,
Ni: 0.01-0.50%,
Co: 0.01 to 0.50%,
Ca: 0.0001-0.0040%,
Mg: 0.0001 to 0.0040%,
Rare earth elements: 0.0001-0.0040%,
Nb: 0.001 to 0.150%,
V: 0.001 to 0.500%, and
B: 0.0001 to 0.0030%,
Containing one or more selected from the group consisting of
Steel material.
The steel material according to claim 1 or claim 2,
The steel material is a steel pipe for oil wells,
Steel material.