JP7417180B1 - steel material - Google Patents

Abstract

To provide steel materials that have both high strength and excellent fracture toughness. The steel material according to the present disclosure includes C: 0.10 to 0.45%, Si: 1.00% or less, Mn: 0.01 to 1.00%, Al: 0.001 to 0.100%, Cr: 0 .1 to 2.0%, Mo: 0.20 to 2.00%, Ca: 0.0005 to 0.0200%, and Mg: 0.0005 to 0.0200%. Contains at least one element of Ti: 0.001 to 0.300%, Nb: 0.001 to 0.300%, and V: 0.01 to 0.50%, and the remainder is composed of Fe and impurities and satisfies formulas (1) and (2) described in the specification. The yield strength is less than 758 to 862 MPa, the equivalent circle diameter is 100 nm or less, and when the total content of Mo, Nb, V, and Ti is defined as 100 mass%, the Mo content is 50 mass%. The number density of MX-type precipitates is 20/μm2 or more.

Description

BACKGROUND OF THE INVENTION This disclosure relates to steel materials, and more particularly to steel materials suitable for use in oil wells.

BACKGROUND OF THE INVENTION As oil wells and gas wells (hereinafter, oil wells and gas wells are collectively referred to as "oil wells") become deeper, there is a demand for higher strength steel materials for oil wells, such as steel pipes for oil wells. Specifically, oil well steel 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. Recently, there has been a demand for oil well steel materials of 110 ksi class (yield strength of less than 758 to 862 MPa).

Oil wells may also contain corrosive hydrogen sulfide gas (H ₂ S), carbon dioxide gas (CO ₂ ), and the like. Therefore, steel materials expected to be used as oil well steel materials are required not only to have high strength but also to have excellent corrosion resistance. Moreover, in steel materials for oil wells, stress is applied to the steel materials during use. Therefore, sulfide stress cracking resistance (hereinafter referred to as SSC resistance) has been used as an index of the excellent corrosion resistance of oil well steel materials.

Techniques for increasing the strength and SSC resistance of steel materials are disclosed in Japanese Patent Application Publication No. 2006-28612 (Patent Document 1), International Publication No. 2008/123422 (Patent Document 2), and Japanese Patent Application Publication No. 2017-166060 (Patent Document 1). This is proposed in document 3).

The steel material disclosed in Patent Document 1 is a steel for steel pipes, and in mass %, C: 0.2 to 0.7%, Si: 0.01 to 0.8%, Mn: 0.1 to 1 .5%, S: 0.005% or less, P: 0.03% or less, Al: 0.0005-0.1%, Ti: 0.005-0.05%, Ca: 0.0004-0. 005%, N: 0.007% or less, Cr: 0.1 to 1.5%, Mo: 0.2 to 1.0%, and the remainder consists of Fe and impurities. This steel material further has (Ca%)/(Al%) in the inclusions of nonmetallic inclusions containing Ca, Al, Ti, N, O, and S from 0.55 to 1.72, and (Ca% )/(Ti%) is 0.7 to 19. Patent Document 1 describes that this steel material has a high yield strength exceeding 758 MPa and excellent SSC resistance.

The steel material disclosed in Patent Document 2 is a low alloy steel, and in mass %, C: 0.10 to 0.20%, Si: 0.05 to 1.0%, Mn: 0.05 to 1 .5%, Cr: 1.0 to 2.0%, Mo: 0.05 to 2.0%, Al: 0.10% or less, and Ti: 0.002 to 0.05%, 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. N: 0.007% or less, B: less than 0.0003%. This steel material further contains M ₂₃ C ₆ type precipitates with a grain size of 1 μm or more at a rate of 0.1 pieces/mm ² or less. Patent Document 2 describes that this steel material has a yield strength of 654 to 793 MPa and has excellent SSC resistance even in a high-pressure hydrogen sulfide environment.

The steel material disclosed in Patent Document 3 is a material for high-strength steel pipes for oil wells, and in mass %, C: 0.20 to 0.45%, Si: 0.05 to 0.40%, Mn: 0 .3 to 0.9%, P: 0.015% or less, S: 0.005% or less, Al: 0.005 to 0.10%, N: 0.001 to 0.006%, Cr: 0. 1-0.8%, Mo: 0.1-1.6%, V: 0.02-0.2%, Nb: 0.001-0.04%, B: 0.0003-0.0030% , O (oxygen): 0.0030% or less, the remainder consisting of Fe and inevitable impurities. Furthermore, the Rockwell hardness HRC of this steel material satisfies the formula (15.6×[%C]+29.2≦HRC<60.5×[%C]+31.1). Patent Document 3 states that with this steel material, a steel pipe having a yield strength of less than 758 to 862 MPa and excellent SSC resistance can be obtained.

Japanese Patent Application Publication No. 2006-28612 International Publication No. 2008/123422 Japanese Patent Application Publication No. 2017-166060

By the way, in steel materials for oil wells, minute flaws may be formed on the surface of the steel materials during transportation or drilling. Furthermore, as mentioned above, stress is applied to the steel material for oil wells during use. Therefore, when stress is applied to a steel material with microscopic flaws formed on its surface, the microscopic flaws may become starting points for cracks, and the cracks may propagate. Therefore, steel materials for oil wells are required to have resistance to destruction even if minute flaws are formed.

In this specification, when minute flaws are formed in a steel material and stress is applied, the steel material is said to have excellent fracture toughness if it has high resistance to fracture. That is, the better the fracture toughness is, the less likely it is that fracture will occur even if stress is applied to a steel material with minute flaws formed thereon. On the other hand, generally speaking, the higher the yield strength of a steel material, the more easily its fracture toughness tends to decrease. Therefore, steel materials for oil wells are required to have both high yield strength and excellent fracture toughness. However, in the above-mentioned Patent Documents 1 to 3, the fracture toughness of the steel material is not studied.

An object of the present disclosure is to provide a steel material that has both high strength and excellent fracture toughness.

The steel material according to the present disclosure is
In mass%,
C: 0.10-0.45%,
Si: 1.00% or less,
Mn: 0.01-1.00%,
P: 0.050% or less,
S: 0.0050% or less,
Al: 0.001-0.100%,
Cr: 0.1-2.0%,
Mo: 0.20-2.00%,
N: 0.010% or less,
W: 0-0.50%,
Co: 0 to 0.50%,
Ni: 0 to 0.50%,
Rare earth elements: 0 to 0.020%,
Cu: 0 to 0.50%, and
B: Contains 0 to 0.0100%,
Ca: 0.0005 to 0.0200%, and
Contains one or more elements selected from the group consisting of Mg: 0.0005 to 0.0200%,
Ti: 0.001-0.300%,
Nb: 0.001 to 0.300%, and
V: Contains one or more elements selected from the group consisting of 0.01 to 0.50%,
The remainder consists of Fe and impurities,
satisfies formula (1) and formula (2),
The yield strength is less than 758 to 862 MPa,
In the steel material,
The equivalent circle diameter is 100 nm or less,
When the total content of Mo, Nb, V, and Ti is defined as 100% by mass, the number density of MX type precipitates having a Mo content of more than 50% by mass is 20 pieces/μm ² or more.
Mn×Sp≦12.0 (1)
0.05≦7×Ti+2×Nb+3×V≦0.80 (2)
Here, the Mn content is substituted in mass % for "Mn" in formula (1), and the S content is substituted in ppm for "Sp". The content of the corresponding element in mass % is substituted for the element symbol in formula (2). Note that when the corresponding element is not contained, "0" is substituted for the element symbol.

The steel material according to the present disclosure can have both high strength and excellent fracture toughness.

Figure 1 shows the Mo content of the Mo-based MX type precipitate (having an equivalent circle diameter of 100 nm or less and the total content of Mo, Nb, V, and Ti being defined as 100% by mass) in this example. FIG. 3 is a diagram showing the relationship between the number density (pieces/μm ² ) of MX-type precipitates (exceeding 50% by mass) and the CTOD value (mm), which is an index of fracture toughness. FIG. 2A is a schematic diagram of a single edge notched bend (SENB) test piece used in the CTOD test for evaluating the fracture toughness of steel materials in this embodiment. FIG. 2B is an enlarged view of region 10 of FIG. 2A.

First, the present inventors investigated increasing the strength and fracture toughness of steel materials intended for use in oil wells from the viewpoint of chemical composition. As a result, the present inventors found that, in mass %, C: 0.10 to 0.45%, Si: 1.00% or less, Mn: 0.01 to 1.00%, P: 0.050% or less. , S: 0.0050% or less, Al: 0.001 to 0.100%, Cr: 0.1 to 2.0%, Mo: 0.20 to 2.00%, N: 0.010% or less, W: 0 to 0.50%, Co: 0 to 0.50%, Ni: 0 to 0.50%, rare earth elements: 0 to 0.020%, Cu: 0 to 0.50%, and B: Contains 0 to 0.0100%, contains one or more elements selected from the group consisting of Ca: 0.0005 to 0.0200%, and Mg: 0.0005 to 0.0200%, Ti: 0 Contains one or more elements selected from the group consisting of .001 to 0.300%, Nb: 0.001 to 0.300%, and V: 0.01 to 0.50%, with the balance being Fe and impurities. We considered that it is possible to achieve both a high yield strength of less than 758 to 862 MPa (110 ksi class) and excellent fracture toughness if the steel material is made of:

The present inventors next focused on Mn sulfide in steel materials. Mn sulfide is easily stretched and coarsened by hot working. Moreover, when coarse Mn sulfides are formed in steel materials, the fracture toughness of the steel materials is significantly reduced. Therefore, the present inventors thought that if it was possible to prevent the formation of coarse Mn sulfides in a steel material, it would be possible to improve the fracture toughness while maintaining the yield strength of the steel material.

Therefore, the present inventors investigated various methods for reducing coarse Mn sulfides in steel materials having the above-mentioned chemical composition. As a result, it was revealed that in a steel material having the above-mentioned chemical composition, coarse Mn sulfides in the steel material can be reduced if the chemical composition satisfies the following formula (1).
Mn×Sp≦12.0 (1)
Here, the Mn content is substituted in mass % for "Mn" in formula (1), and the S content is substituted in ppm for "Sp".

Define Fn1=Mn×Sp. Fn1 is an index of Mn sulfide in the steel material. If Fn1 exceeds 12.0, many coarse Mn sulfides are formed in the steel material, and the fracture toughness of the steel material is reduced. Therefore, on the premise that the steel material according to this embodiment has the above-mentioned chemical composition, Fn1 is set to 12.0 or less. As a result, it is possible to achieve both a yield strength of less than 758 to 862 MPa and excellent fracture toughness, provided that the other configurations of this embodiment are satisfied.

On the other hand, even if a steel material has the above-mentioned chemical composition and satisfies formula (1), it may not be possible to stably achieve both a yield strength of less than 758 to 862 MPa and excellent fracture toughness. Therefore, the present inventors investigated various methods for stably increasing the fracture toughness while maintaining the yield strength of a steel material that has the above-mentioned chemical composition and satisfies formula (1).

Specifically, the present inventors focused on fine precipitates and studied ways to improve the fracture toughness of steel materials. As a result of detailed study by the present inventors, in a steel material having the above-mentioned chemical composition and satisfying formula (1), if Mo-enriched MX-type precipitates are finely dispersed, the yield strength can be improved. It has become clear that excellent fracture toughness can be stably increased while maintaining the same.

First, in the steel material having the above-mentioned chemical composition, the MX type precipitates are mainly those with an equivalent circle diameter of 100 nm or less, and those with an equivalent circle diameter exceeding 100 nm are negligibly small. Therefore, in this specification, when the equivalent circle diameter is 100 nm or less and the total content of Mo, Nb, V, and Ti is defined as 100 mass%, the MX type with a Mo content of more than 50 mass% The precipitate is also referred to as "Mo-based MX type precipitate."

FIG. 1 is a diagram showing the relationship between the number density (pieces/μm ² ) of Mo-based MX type precipitates and the CTOD value (mm), which is an index of fracture toughness, in this example. Figure 1 shows the number density of Mo-based MX type precipitates (pieces/μm ² ) and the CTOD value (mm) obtained by the CTOD test described below. Note that all of the steel materials shown in FIG. 1 had a yield strength of 758 to less than 862 MPa.

Referring to FIG. 1, in a steel material having the above-mentioned chemical composition, satisfying formula (1), and having a yield strength of less than 758 to 862 MPa, the number density of Mo-based MX type precipitates is 20 pieces/μm. If it is ² or more, it can be confirmed that the CTOD value is stable and 0.15 mm or more. Therefore, the steel material according to this embodiment has the above-mentioned chemical composition, satisfies formula (1), and has a number density of Mo-based MX type precipitates of 20 pieces/μm ² or more. As a result, even if the steel material according to the present embodiment has a yield strength of less than 758 to 862 MPa, it has excellent fracture toughness with a CTOD value of 0.15 mm or more.

In addition, when the equivalent circle diameter is 100 nm or less and the total content of Mo, Nb, V, and Ti is defined as 100% by mass, MX-type precipitates (Mo-based The details of the mechanism by which fracture toughness can be increased while maintaining the yield strength of the steel material by setting the number density of MX type precipitates to 20/μm ² or more are not clear. However, the present inventors speculate as follows. In the above chemical composition, most of the MX type precipitates with an equivalent circle diameter of 100 nm or less are carbides, and are mainly composed of MC type carbides. MC type carbides tend to be finely dispersed in steel materials. On the other hand, if the dispersed MC type carbide is too hard, it is difficult to improve the fracture toughness of the steel material. Therefore, if the MC type carbide is a Mo-based MC type carbide in which Mo is relatively concentrated, the hardness of the MC type carbide decreases. In other words, MC type carbide having an appropriate hardness can be finely dispersed in the steel material. As a result, fracture toughness can be increased while maintaining the strength of the steel material.

It is also possible that the fracture toughness of the steel material is increased while the yield strength of the steel material is maintained through a mechanism different from the above-mentioned speculation by the present inventors. However, a steel material that has the above chemical composition, satisfies formula (1), and has a number density of Mo-based MX type precipitates of 20 pieces/μm2 ^or more has excellent fracture toughness while maintaining yield strength. This is proven by the examples described later.

As a result of further detailed study by the present inventors based on the above findings, in addition to having the above-mentioned chemical composition and satisfying formula (1), the chemical composition also satisfies the following formula (2). It has become clear that the number density of Mo-based MX type precipitates can be stably increased to 20 pieces/μm ² or more.
0.05≦7×Ti+2×Nb+3×V≦0.80 (2)
Here, the content of the corresponding element in mass % is substituted for the element symbol in formula (2). Note that when the corresponding element is not contained, "0" is substituted for the element symbol.

It is defined as Fn2=7×Ti+2×Nb+3×V. Fn2 is an index regarding the precipitation state of carbides. Ti, Nb, and/or V form MX type precipitates. If Fn2 is too low, MX type precipitates themselves cannot be sufficiently formed. As a result, the number density of Mo-based MX type precipitates decreases. On the other hand, if Fn2 is too high, the Mo content in the MX type precipitates will decrease. As a result, the number density of Mo-based MX type precipitates decreases. Therefore, in the steel material according to this embodiment, Fn2 is set to 0.05 to 0.80 on the premise that it has the above-mentioned chemical composition and satisfies formula (1). As a result, the number density of Mo-based MX type precipitates can be stably increased to 20 pieces/μm ² or more.

As described above, the steel material according to the present embodiment has the above-mentioned chemical composition, satisfies formulas (1) and (2), has a yield strength of less than 758 to 862 MPa, and further has Mo-based MX type precipitation. The number density of objects is 20 pieces/μm ² or more. As a result, the steel material according to this embodiment can have both high strength and excellent fracture toughness.

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

[1]
A steel material,
In mass%,
C: 0.10-0.45%,
Si: 1.00% or less,
Mn: 0.01-1.00%,
P: 0.050% or less,
S: 0.0050% or less,
Al: 0.001-0.100%,
Cr: 0.1-2.0%,
Mo: 0.20-2.00%,
N: 0.010% or less,
W: 0-0.50%,
Co: 0 to 0.50%,
Ni: 0 to 0.50%,
Rare earth elements: 0 to 0.020%,
Cu: 0 to 0.50%, and
B: Contains 0 to 0.0100%,
Ca: 0.0005 to 0.0200%, and
Contains one or more elements selected from the group consisting of Mg: 0.0005 to 0.0200%,
Ti: 0.001-0.300%,
Nb: 0.001 to 0.300%, and
V: Contains one or more elements selected from the group consisting of 0.01 to 0.50%,
The remainder consists of Fe and impurities,
satisfies formula (1) and formula (2),
The yield strength is less than 758 to 862 MPa,
In the steel material,
The equivalent circle diameter is 100 nm or less,
When the total content of Mo, Nb, V, and Ti is defined as 100% by mass, the number density of MX type precipitates with a Mo content of more than 50% by mass is 20 pieces/μm ² or more,
Steel material.
Mn×Sp≦12.0 (1)
0.05≦7×Ti+2×Nb+3×V≦0.80 (2)
Here, the Mn content is substituted in mass % for "Mn" in formula (1), and the S content is substituted in ppm for "Sp". The content of the corresponding element in mass % is substituted for the element symbol in formula (2). Note that when the corresponding element is not contained, "0" is substituted for the element symbol.

[2]
The steel material according to [1],
W: 0.01-0.50%,
Co: 0.01 to 0.50%,
Ni: 0.01-0.50%,
Rare earth elements: 0.001-0.020%,
Cu: 0.01 to 0.50%, and
B: Contains one or more elements selected from the group consisting of 0.0001 to 0.0100%,
Steel material.

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

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

In this specification, the oil country steel pipe may be an oil country tubular product. Oil country tubular goods are, for example, steel pipes used for casing and tubing applications. The oil well steel pipe according to this embodiment is preferably a seamless steel pipe. If the steel pipe for oil wells according to this embodiment is a seamless steel pipe, even if the wall thickness is 15 mm or more, it is possible to achieve both a yield strength of 758 to less than 862 MPa (110 ksi class) and excellent fracture toughness.

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

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

C: 0.10-0.45%
Carbon (C) improves the hardenability of steel and increases its strength. If the C content is too low, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the C content is too high, quench cracking may easily occur during quenching during the manufacturing process even if the contents of other elements are within the ranges of this embodiment. Therefore, the C content is between 0.10 and 0.45%. The preferable lower limit of the C content is 0.12%, more preferably 0.15%, and still more preferably 0.20%. A preferable upper limit of the C content is 0.40%, more preferably 0.38%, and still more preferably 0.37%.

Si: 1.00% or less Silicon (Si) is unavoidably contained. That is, the lower limit of the Si content is over 0%. Si deoxidizes steel. On the other hand, if the Si content is too high, even if the content of other elements is within the range of this embodiment, the formation of carbides will be suppressed and the fracture toughness of the steel material will decrease. Therefore, the Si content is 1.00% or less. The preferable upper limit of the Si content is 0.90%, more preferably 0.80%, even more preferably 0.75%, still more preferably 0.60%, and even more preferably 0.50%. %. The lower limit of the Si content is preferably 0.05%, more preferably 0.10%, and still more preferably 0.15% to effectively obtain the above effects.

Mn: 0.01-1.00%
Manganese (Mn) deoxidizes steel. Mn further improves the hardenability of the steel material and increases the strength of the steel material. If the Mn content is too low, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Mn content is too high, coarse Mn sulfides will be formed even if the contents of other elements are within the ranges of this embodiment, and the fracture toughness of the steel material will decrease. Therefore, the Mn content is 0.01 to 1.00%. The lower limit of the Mn content is preferably 0.03%, more preferably 0.05%, and still more preferably 0.10%. The upper limit of the Mn content is preferably 0.90%, more preferably 0.85%, even more preferably 0.80%, and still more preferably 0.75%.

P: 0.050% or less Phosphorus (P) is an impurity. That is, the lower limit of the P content is over 0%. If the P content is too high, even if the contents of other elements are within the ranges of this embodiment, P will segregate at grain boundaries and the fracture toughness of the steel material will decrease. Therefore, the P content is 0.050% or less. A preferable upper limit of the P content is 0.040%, more preferably 0.030%, still more preferably 0.020%, and still more preferably 0.015%. It is preferable that the P content is as low as possible. However, extreme reduction in P content significantly increases manufacturing costs. Therefore, when considering industrial production, the preferable lower limit of the P content is 0.001%, more preferably 0.002%, and still more preferably 0.003%.

S: 0.0050% or less Sulfur (S) is an impurity. That is, the lower limit of the S content is more than 0%. If the S content is too high, coarse Mn sulfides will be formed even if the contents of other elements are within the ranges of this embodiment, and the fracture toughness of the steel material will decrease. Therefore, the S content is 0.0050% or less. A preferable upper limit of the S content is 0.0040%, more preferably 0.0030%, and still more preferably 0.0020%. It is preferable that the S content is as low as possible. However, extreme reduction in S content significantly increases manufacturing costs. Therefore, when considering industrial production, the preferable lower limit of the S content is 0.0001%, more preferably 0.0005%, and still more preferably 0.0010%.

Al: 0.001-0.100%
Aluminum (Al) deoxidizes steel. If the Al content is too low, even if the contents of other elements are within the ranges of this embodiment, the above effects will not be sufficiently obtained and the corrosion resistance of the steel material will decrease. On the other hand, if the Al content is too high, even if the contents of other elements are within the ranges of this embodiment, coarse oxide-based inclusions will be generated and the corrosion resistance of the steel material will be reduced. Therefore, the Al content is 0.001 to 0.100%. The preferable lower limit of the Al content is 0.005%, more preferably 0.010%, still more preferably 0.020%, and still more preferably 0.025%. A preferable upper limit of the Al content is 0.080%, more preferably 0.060%, and still more preferably 0.050%. The "Al" content as used herein means the content of "acid-soluble Al", that is, "sol.Al".

Cr: 0.1-2.0%
Chromium (Cr) improves the hardenability of steel materials. Cr further increases the temper softening resistance of the steel material and enables high temperature tempering. As a result, the fracture toughness of the steel material increases. If the Cr content is too low, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Cr content is too high, the corrosion resistance of the steel material will decrease even if the other element contents are within the ranges of this embodiment. Therefore, the Cr content is between 0.1 and 2.0%. The lower limit of the Cr content is preferably 0.2%, more preferably 0.4%. A preferable upper limit of the Cr content is 1.9%, more preferably 1.8%, still more preferably 1.5%, and still more preferably 1.0%.

Mo: 0.20~2.00%
Molybdenum (Mo) improves the hardenability of steel materials. Mo further forms Mo-based MX type precipitates to improve the fracture toughness of the steel material. If the Mo content is too low, even if the contents of other elements are within the ranges of this embodiment, the above effects cannot be sufficiently obtained. On the other hand, if the Mo content is too high, the above effects will be saturated. Therefore, the Mo content is between 0.20 and 2.00%. The lower limit of the Mo content is preferably 0.25%, more preferably 0.30%, and still more preferably 0.50%. The upper limit of the Mo content is preferably 1.90%, more preferably 1.80%, even more preferably 1.60%, and still more preferably 1.40%.

N: 0.010% or less Nitrogen (N) is unavoidably contained. That is, the lower limit of the N content is over 0%. N combines with Ti to form nitrides and refines the crystal grains of the steel material due to the pinning effect. As a result, the strength of the steel material increases. However, if the N content is too high, coarse nitrides will be formed even if the contents of other elements are within the ranges of this embodiment, and the fracture toughness of the steel material will decrease. Therefore, the N content is 0.010% or less. A preferable upper limit of the N content is 0.008%, more preferably 0.006%. The lower limit of the N content is preferably 0.001%, more preferably 0.002%, and still more preferably 0.003% in order to more effectively obtain the above effects.

The chemical composition of the steel material according to this embodiment contains one or more elements selected from the group consisting of Ca and Mg. That is, in the chemical composition of the steel material according to the present embodiment, the content of either Ca or Mg may be 0%. All of these elements improve the hot workability of steel materials.

Ca: 0.0005-0.0200%
Calcium (Ca) fixes S in steel materials as sulfide, rendering it harmless and improving the corrosion resistance of steel materials. However, if the Ca content is too high, even if the contents of other elements are within the ranges of this embodiment, the oxides in the steel material will become coarse and the fracture toughness of the steel material will decrease. Therefore, when contained, the Ca content is 0.0005 to 0.0200%. The preferable lower limit of the Ca content is more than 0.0006%, more preferably 0.0008%, and still more preferably 0.0010%. A preferable upper limit of the Ca content is 0.0150%, more preferably 0.0100%, still more preferably 0.0060%, and still more preferably 0.0040%.

Mg: 0.0005-0.0200%
Magnesium (Mg) fixes S in steel materials as sulfide, rendering it harmless and improving the corrosion resistance of steel materials. However, if the Mg content is too high, even if the contents of other elements are within the ranges of this embodiment, the oxides in the steel material will become coarse and the fracture toughness of the steel material will decrease. Therefore, when included, the Mg content is 0.0005 to 0.0200%. The lower limit of the Mg content is preferably more than 0.0006%, more preferably 0.0008%, and still more preferably 0.0010%. A preferable upper limit of the Mg content is 0.0150%, more preferably 0.0100%, still more preferably 0.0060%, and still more preferably 0.0040%.

The chemical composition of the steel material according to this embodiment contains one or more elements selected from the group consisting of Ti, Nb, and V. That is, the chemical composition of the steel material according to the present embodiment may have a content of 0% except for any one of Ti, Nb, and V. All of these elements form composite MX-type precipitates together with Mo and improve the fracture toughness of the steel material.

Ti: 0.001-0.300%
Titanium (Ti) forms Mo-based MX type precipitates together with Mo, and improves the fracture toughness of steel materials. The Mo-based MX type precipitates containing Ti further refine the crystal grains of the steel material due to the pinning effect and improve the fracture toughness of the steel material. However, if the Ti content is too high, even if the contents of other elements are within the range of this embodiment, the Ti content in the Mo-based MX-type precipitates will become too high, and the Ti content in the Mo-based MX-type precipitates will be too high. Mo content decreases. As a result, the fracture toughness of the steel material is rather reduced. Therefore, when included, the Ti content is 0.001 to 0.300%. The lower limit of the Ti content is preferably 0.002%, more preferably 0.003%, even more preferably 0.005%, and still more preferably 0.010%. A preferable upper limit of the Ti content is 0.250%, more preferably 0.150%, even more preferably 0.100%, still more preferably 0.080%, and still more preferably 0.060%. %.

Nb: 0.001-0.300%
Niobium (Nb) forms Mo-based MX type precipitates together with Mo, and improves the fracture toughness of steel materials. The Mo-based MX type precipitates containing Nb further refine the crystal grains of the steel material due to the pinning effect and improve the fracture toughness of the steel material. Nb further increases the temper softening resistance of the steel material and increases the strength of the steel material. However, if the Nb content is too high, even if the contents of other elements are within the range of this embodiment, the Nb content in the Mo-based MX-type precipitates will be too high, and the Nb content in the Mo-based MX-type precipitates will be too high. Mo content decreases. As a result, the fracture toughness of the steel material is rather reduced. Therefore, when included, the Nb content is 0.001 to 0.300%. The preferable lower limit of the Nb content is 0.003%, more preferably 0.005%, and still more preferably 0.010%. A preferable upper limit of the Nb content is 0.250%, more preferably 0.150%, still more preferably 0.100%, and still more preferably 0.080%.

V:0.01~0.50%
Vanadium (V) forms Mo-based MX type precipitates together with Mo, and improves the fracture toughness of steel materials. The Mo-based MX type precipitates containing V further refine the crystal grains of the steel material due to the pinning effect and improve the fracture toughness of the steel material. V further increases the temper softening resistance of the steel material and increases the strength of the steel material. However, if the V content is too high, even if the content of other elements is within the range of this embodiment, the V content in the Mo-based MX-type precipitate will be too high, and the V content in the Mo-based MX-type precipitate will be too high. Mo content decreases. As a result, the fracture toughness of the steel material is rather reduced. Therefore, when contained, the V content is 0.01 to 0.50%. The lower limit of the V content is preferably 0.01%, more preferably 0.02%, and even more preferably 0.05%. A preferable upper limit of the V content is 0.40%, more preferably 0.30%, and still more preferably 0.20%.

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

[Optional element]
The chemical composition of the steel material described above may further contain one or more elements selected from the group consisting of W, Co, Ni, and rare earth elements in place of a part of Fe. All of these elements increase the corrosion resistance of steel materials.

W: 0-0.50%
Tungsten (W) is an optional element and may not be included. That is, the W content may be 0%. When contained, W forms a protective corrosion film in a corrosive environment and suppresses hydrogen from penetrating into the steel material. As a result, the corrosion resistance of the steel material is improved. If even a small amount of W is contained, the above effects can be obtained to some extent. However, if the W content is too high, even if the contents of other elements are within the ranges of this embodiment, coarse carbides will be generated in the steel material, resulting in a decrease in the corrosion resistance of the steel material. Therefore, the W content is 0 to 0.50%. The lower limit of the W content is preferably more than 0%, more preferably 0.01%. The upper limit of the W content is preferably 0.40%, more preferably 0.30%, even more preferably 0.20%, and still more preferably 0.10%.

Co: 0-0.50%
Cobalt (Co) is an optional element and may not be included. That is, the Co content may be 0%. When contained, Co forms a protective corrosion film in a corrosive environment and suppresses hydrogen from penetrating into the steel material. As a result, the corrosion resistance of the steel material is improved. If even a small amount of Co is contained, the above effects can be obtained to some extent. However, if the Co content is too high, even if the contents of other elements are within the ranges of this embodiment, the hardenability of the steel material will be reduced and the strength of the steel material will be reduced. Therefore, the Co content is 0-0.50%. The preferable lower limit of the Co content is more than 0%, more preferably 0.01%, and still more preferably 0.02%. A preferable upper limit of the Co content is 0.40%, more preferably 0.30%, and still more preferably 0.20%.

Ni: 0-0.50%
Nickel (Ni) is an optional element and may not be included. That is, the Ni content may be 0%. When contained, Ni dissolves in solid solution in the steel and improves the corrosion resistance of the steel material. If even a small amount of Ni is contained, these effects can be obtained to some extent. However, if the Ni content is too high, local corrosion will be promoted and the corrosion resistance of the steel material will be reduced even if the other element contents are within the ranges of this embodiment. Therefore, the Ni content is 0 to 0.50%. The preferable lower limit of the Ni content is more than 0%, more preferably 0.01%, and still more preferably 0.02%. A preferable upper limit of the Ni content is 0.40%, more preferably 0.30%, still more preferably 0.20%, and still more preferably 0.15%.

Rare earth elements (REM): 0 to 0.020%
Rare earth elements (REM) are optional elements and may not be included. That is, the REM content may be 0%. When contained, REM fixes S in the steel material as sulfide, rendering it harmless and improving the corrosion resistance of the steel material. If even a small amount of REM is contained, the above effects can be obtained to some extent even if the contents of other elements are within the range of this embodiment. However, if the REM content is too high, even if the contents of other elements are within the ranges of this embodiment, the oxides in the steel material will become coarse and the corrosion resistance of the steel material will decrease. Therefore, the REM content is between 0 and 0.020%. A preferable lower limit of the REM content is more than 0%, more preferably 0.001%, still more preferably 0.003%, and still more preferably 0.005%. A preferable upper limit of the REM content is 0.018%, more preferably 0.015%.

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.

The chemical composition of the steel material described above may further contain one or more elements selected from the group consisting of Cu and B in place of a part of Fe. All of these elements improve the hardenability of the steel material and increase the strength of the steel material.

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

B: 0-0.0100%
Boron (B) is an optional element and may not be contained. That is, the B content may be 0%. When contained, B increases 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 is too high, even if the contents of other elements are within the ranges of this embodiment, coarse nitrides will be generated and the corrosion resistance of the steel material will deteriorate. Therefore, the B content is 0 to 0.0100%. The preferable lower limit of the B content is more than 0%, more preferably 0.0001%, still more preferably 0.0003%, and still more preferably 0.0005%. A preferable upper limit of the B content is 0.0080%, more preferably 0.0060%.

[Formula (1)]
The steel material according to this embodiment satisfies the following formula (1) on the premise that it has the above-mentioned chemical composition. As a result, the steel material according to the present embodiment can have both a yield strength of less than 758 to 862 MPa and excellent fracture toughness, provided that the other configurations of the present embodiment are satisfied.
Mn×Sp≦12.0 (1)
Here, the Mn content is substituted in mass % for "Mn" in formula (1), and the S content is substituted in ppm for "Sp".

Fn1 (=Mn×Sp) is an index of Mn sulfide in the steel material. If Fn1 exceeds 12.0, many coarse Mn sulfides are formed in the steel material, and the fracture toughness of the steel material is reduced. Therefore, on the premise that the steel material according to this embodiment has the above-mentioned chemical composition, Fn1 is set to 12.0 or less. As a result, it is possible to achieve both a yield strength of less than 758 to 862 MPa and excellent fracture toughness, provided that the other configurations of this embodiment are satisfied.

A more preferable upper limit of Fn1 is 11.5, still more preferably 11.0, and still more preferably 10.0. Note that the lower limit of Fn1 is not particularly limited, and may be, for example, 0.1. However, when considering industrial production, the preferable lower limit of Fn1 is 0.3, more preferably 0.5.

[Formula (2)]
The steel material according to this embodiment has the above-mentioned chemical composition and satisfies the following formula (2) on the premise that formula (1) is satisfied. As a result, the steel material according to the present embodiment can have both a yield strength of less than 758 to 862 MPa and excellent fracture toughness, provided that the other configurations of the present embodiment are satisfied.
0.05≦7×Ti+2×Nb+3×V≦0.80 (2)
Here, the content of the corresponding element in mass % is substituted for the element symbol in formula (2). Note that when the corresponding element is not contained, "0" is substituted for the element symbol.

Fn2 (=7×Ti+2×Nb+3×V) is an index regarding the precipitation state of carbides. Ti, Nb, and/or V form MX type precipitates. If Fn2 is too low, MX type precipitates themselves cannot be sufficiently formed. As a result, the number density of Mo-based MX type precipitates decreases. On the other hand, if Fn2 is too high, the Mo content in the MX type precipitates will decrease. As a result, the number density of Mo-based MX type precipitates decreases. Therefore, in the steel material according to this embodiment, Fn2 is set to 0.05 to 0.80 on the premise that it has the above-mentioned chemical composition and satisfies formula (1). As a result, the number density of Mo-based MX type precipitates can be stably increased to 20 pieces/μm ² or more.

The lower limit of Fn2 is preferably 0.08, more preferably 0.10, and still more preferably 0.15. A preferable upper limit of Fn2 is 0.75, more preferably 0.70, and still more preferably 0.60.

[Yield strength]
The steel material according to this embodiment has the above-mentioned chemical composition, satisfies formulas (1) and (2), and has a number density of Mo-based MX type precipitates of 20/μm ² or more. As a result, the steel material according to the present embodiment has excellent fracture toughness even if the yield strength is less than 758 to 862 MPa. In short, the yield strength of the steel material according to this embodiment is less than 758 to 862 MPa. In this specification, yield strength refers to 0.6% total elongation yield strength (MPa) obtained by a tensile test at room temperature (24±3°C) in accordance with ASTM E8/E8M (2021) described below. means.

Specifically, in this embodiment, the yield strength of the steel material is determined by the following method. First, a tensile test piece is produced from the steel material according to this embodiment. 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 mm and a gage length of 30 mm. If the steel material is a steel pipe, prepare 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. When the steel material is a round steel, a tensile test piece is prepared from the R/2 position. In this specification, the R/2 position of the round steel means the center position of the radius R in a cross section perpendicular to the axial direction of the round steel. In this case, the longitudinal direction of the tensile test piece is parallel to the axial direction of the round steel. If the steel material is a steel plate, prepare 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. Using the prepared tensile test piece, perform a tensile test at room temperature (24 ± 3 ° C.) in the atmosphere in accordance with ASTM E8/E8M (2021) to determine 0.6% total elongation yield strength (MPa). . The obtained 0.6% total elongation yield strength is defined as yield strength (MPa).

[Mo-based MX type precipitate]
The steel material according to this embodiment has the above-mentioned chemical composition, satisfies formulas (1) and (2), has an equivalent circle diameter of 100 nm or less, and contains Mo, Nb, V, and Ti. When the total content is defined as 100% by mass, the number density of MX type precipitates having an Mo content of more than 50% by mass is 20 pieces/μm ² or more. As a result, the steel material according to this embodiment has excellent fracture toughness even if the yield strength is less than 758 to 862 MPa. As mentioned above, in this specification, when the equivalent circle diameter is 100 nm or less and the total content of Mo, Nb, V, and Ti is defined as 100% by mass, the Mo content exceeds 50% by mass. The MX type precipitate is also referred to as "Mo-based MX type precipitate".

Here, most of the MX type precipitates with an equivalent circle diameter of 100 nm or less are MC type carbides. In addition, MC type carbides tend to be finely dispersed in steel materials. On the other hand, depending on the elements constituting the MC type carbide, the hardness of the MC type carbide may become too high. In this case, it is difficult to increase the fracture toughness while maintaining the strength of the steel material. Therefore, in this embodiment, an MC type carbide containing Nb and/or V and/or Ti is made into a composite carbide with Mo to form an MC type carbide enriched with Mo (Mo-based MX type precipitate). . In this embodiment, the number density of Mo-based MX type precipitates is further increased. As a result, precipitates of appropriate hardness are finely dispersed in the steel material, and the fracture toughness of the steel material can be increased while maintaining the yield strength of the steel material.

Therefore, in this embodiment, it has the above-mentioned chemical composition, satisfies formulas (1) and (2), and has a number density of Mo-based MX type precipitates of 20 pieces/μm ² or more. A preferable lower limit of the number density of Mo-based MX type precipitates is 21 pieces/μm ² , more preferably 23 pieces/μm ² , and even more preferably 25 pieces/μm ² . Although the upper limit of the number density of Mo-based MX type precipitates is not particularly limited, it is, for example, 200 pieces/μm ² .

In this embodiment, the number density of Mo-based MX type precipitates is determined by the following method. First, a micro test piece for producing an extraction replica is produced from the steel material according to this embodiment. If the steel material is a steel pipe, take a micro specimen from the center of the wall thickness. If the steel material is round steel, take a micro test piece from the R/2 position. If the steel material is a steel plate, take a micro test piece from the center of the plate thickness. The size of the micro test piece is, for example, 10 mm x 10 mm. After mirror polishing the surface of the micro test piece, the micro test piece is immersed in a 3% nital corrosive solution for 10 minutes to corrode the surface. The corroded surface is covered with a carbon-deposited film. A micro specimen whose surface is covered with a vapor-deposited film is immersed in a 5% nital corrosive solution for 20 minutes. The deposited film is peeled off from the immersed micro test piece. The deposited film peeled off from the micro test piece is washed with ethanol, then scooped out with a sheet mesh and dried. Note that in this embodiment, a sheet mesh made of Cu is used.

This deposited film (replica film) is observed with a transmission electron microscope (TEM). Specifically, an arbitrary position is specified from the deposited film and observed at an observation magnification of 100,000 times and an accelerating voltage of 200 kV. Note that the size of the observation field is, for example, 2.0 μm×3.0 μm. In each observation field, particles with an equivalent circle diameter of 100 nm or less are identified. Note that particles can be identified from the contrast. In this specification, the term "particles" is not limited to circular (spherical) particles, and may be small pieces having an angular shape or elongated elliptical pieces. Further, the equivalent circle diameter of the precipitate can be determined by image analysis of an observed image in TEM observation. In the present embodiment, the lower limit of the equivalent circle diameter of the identified particles having an equivalent circle diameter of 100 nm or less is 10 nm. That is, in this embodiment, particles having an equivalent circle diameter of 10 to 100 nm are specified.

Point analysis is performed on the identified particles using energy dispersive X-ray spectrometry (EDS). The content of elements contained in each particle is determined by EDS point analysis. In the EDS point analysis, the accelerating voltage is 200 kV. In addition, the target elements of point analysis are quantified as Mo, Nb, V, and Ti. Here, when the total content of Mo, Nb, V, and Ti is defined as 100% by mass, when the Mo content exceeds 70% by mass, M ₂ X type precipitates are likely to occur. Therefore, in this embodiment, when the total content of Mo, Nb, V, and Ti is defined as 100% by mass, a precipitate with a Mo content of 70% by mass or less is specified as an MX-type precipitate. Therefore, in this embodiment, when the total content of Mo, Nb, V, and Ti determined by EDS point analysis is defined as 100% by mass, particles with a Mo content of more than 50 to 70% by mass are , it is identified as a Mo-based MX type precipitate.

The number density (pieces/μm ² ) of Mo-based MX-type precipitates is determined based on the total number of Mo-based MX-type precipitates identified in each observation field and the total area of the observation field. In this embodiment, the number density of Mo-based MX type precipitates is determined by rounding the obtained value to the first decimal place.

[Fracture toughness]
The steel material according to this embodiment has the above-mentioned chemical composition, satisfies formulas (1) and (2), and has a number density of Mo-based MX type precipitates of 20/μm ² or more. As a result, the steel material according to this embodiment has excellent fracture toughness even if the yield strength is less than 758 to 862 MPa. In this embodiment, excellent fracture toughness means that the CTOD value is 0.15 mm or more, as determined by a CTOD test at room temperature (25°C) in accordance with ISO 12135 (2021) described below. do.

Specifically, in this embodiment, the fracture toughness of steel material is evaluated by the following method. First, a single edge notched bend (SENB) test piece shown in FIGS. 2A and 2B is produced from the steel material according to the present embodiment. FIG. 2A is a schematic diagram of a SENB test piece used in the CTOD test for evaluating the fracture toughness of steel materials in this embodiment. FIG. 2B is an enlarged view of region 10 of FIG. 2A. Referring to FIG. 2A, the SENB test piece has a thickness B of 10 mm, a width W of 20 mm, and a length L of 100 mm. If the steel material is a steel pipe, prepare a SENB test piece from the center of the wall thickness. In this case, the longitudinal direction of the SENB test piece is parallel to the axial direction of the steel pipe. If the steel material is round steel, prepare a SENB test piece from the R/2 position. In this case, the longitudinal direction of the SENB test piece is parallel to the axial direction of the round steel. When the steel material is a steel plate, a SENB test piece is prepared from the center of the plate thickness. In this case, the longitudinal direction of the SENB test piece is parallel to the rolling direction of the steel plate.

Referring to FIG. 2A, the SENB test piece has a notch formed in the width W direction at the center position in the length L direction. The notch in the SENB specimen is formed by machining. Further, referring to FIG. 2B, the notch of the SENB test piece has a width of 2 mm and a depth of 8 mm. A fatigue test to introduce a pre-crack is performed on the prepared SENB test piece. In this embodiment, the initial relative crack length a ₀ /W is set to 0.50. Specifically, the fatigue test is conducted at room temperature (25° C.) so that the fatigue pre-crack introduced at the tip of the notch is 2 mm.

A CTOD test is conducted at room temperature (25° C.) in accordance with ISO 12135 (2021) on the SENB test piece in which a fatigue pre-crack has been introduced. The CTOD value (mm) is determined based on ISO 12135 (2021) from the load at break in the load-opening amount curve obtained by the CTOD test and the amount of plastic component of the clip gauge opening displacement. Note that the same test is conducted three times and the minimum CTOD value (mm) is defined as the CTOD value (mm) of the steel material. In this embodiment, the CTOD value of the steel material is determined by rounding the obtained value to the second decimal place.

[Microstructure]
In the microstructure of the steel material according to this embodiment, the total volume fraction of tempered martensite and tempered bainite is 90% or more. The remainder of the microstructure is, for example, ferrite or pearlite. If the microstructure of the steel material having the above-mentioned chemical composition contains a total volume fraction of tempered martensite and tempered bainite of 90% or more, the yield strength is 110 ksi, provided that the other configurations of this embodiment are satisfied. (758 to less than 862 MPa). That is, in this embodiment, if the yield strength of the steel material is 110 ksi class, it is determined that the microstructure of the steel material has a total volume fraction of tempered martensite and tempered bainite of 90% or more.

In addition, when determining the volume fraction of tempered martensite and tempered bainite by observation, it can be determined by the following method. First, a test piece is made from steel material. When the steel material is a steel pipe, a test piece is prepared that has an observation surface of 10 mm in the pipe axis direction and 8 mm in the wall thickness (pipe diameter) direction from the center of the wall thickness. In addition, when the wall thickness of the steel pipe is less than 10 mm, a test piece is prepared that has an observation surface of the wall thickness of the steel pipe 10 mm in the pipe axis direction and in the pipe radial direction. When the steel material is a round steel, a test piece is prepared that includes the R/2 position in the center and has an observation surface of 10 mm in the axial direction and 8 mm in the radial direction of the cross section. In addition, when the diameter of the cross section of the round steel is less than 10 mm, a test piece is prepared that includes the R/2 position, has an observation surface of 10 mm in the axial direction, and has a diameter in the radial direction of the cross section. 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 thickness direction from the central position of the plate thickness is prepared. In addition, when the plate thickness of the steel plate is 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 plate thickness direction is prepared.

After polishing the observation surface of the test piece to a mirror surface, it is immersed in a nital corrosive solution for about 10 seconds to reveal the structure by etching. The etched observation surface is observed in 10 fields of view using a secondary electron image using a scanning electron microscope (SEM). The field of view area is, for example, 10000 μm ² (1000x magnification). In each field of view, tempered martensite and tempered bainite are identified from the contrast. The area ratio of the specified tempered martensite and tempered bainite is determined. The method for determining the area ratio is not particularly limited, and any known method may be used. For example, the area ratio of tempered martensite and tempered bainite can be determined by image analysis. In the present embodiment, the arithmetic mean value of the area ratios of tempered martensite and tempered bainite determined in all visual fields is defined as the volume ratio of tempered martensite and tempered bainite.

[Production method]
A method for manufacturing steel materials according to this embodiment will be explained. Hereinafter, a method for manufacturing a seamless steel pipe will be described as an example of the steel material according to the present embodiment. A method for manufacturing a seamless steel pipe includes a step of preparing a mother tube (preparation step) and a step of quenching and tempering the mother tube to form a seamless steel tube (quenching step and tempering step). Note that the manufacturing method according to this embodiment is not limited to the manufacturing method described below. Each step will be explained in detail below.

[Preparation process]
In the preparation step, an intermediate steel material having the above-mentioned chemical composition is prepared. As long as the intermediate steel material has the above chemical composition, the method for producing the intermediate steel material is not particularly limited. The intermediate steel material here is a plate-shaped steel material when the final product is a steel plate, and is a raw pipe when the final product is a steel pipe.

The preparation step may include a step of preparing a material (material preparation step) and a step of hot working the material to produce an intermediate steel material (hot working step). Hereinafter, a case including a material preparation process and a hot working process will be described in detail.

[Material preparation process]
In the material preparation step, a material is manufactured using molten steel having the above-mentioned chemical composition. 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. If necessary, the slab, bloom, or ingot may be bloomed and rolled to produce a billet. A material (slab, bloom, or billet) is manufactured through the above steps.

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

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

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

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

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

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

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

As described above, intermediate steel materials are prepared in the preparation process. The intermediate steel material may be manufactured by the above-mentioned preferred process, or may be manufactured by a third party, or at another factory or other business office other than the factory where the quenching process and tempering process described below are carried out. You may also prepare an intermediate steel material manufactured by The hardening process will be explained in detail below.

[Quenching process]
In the quenching process, the prepared intermediate steel material (raw pipe) is quenched. In this specification, "quenching" means quenching an intermediate steel material having an _A3 point or higher. Here, in this specification, the temperature of the intermediate steel material immediately before quenching when quenching is performed is also referred to as quenching temperature. Here, in the quenching process according to the present embodiment, after heating at a medium temperature, heating at a high temperature is performed, and then rapid cooling is performed. That is, the hardening process according to this embodiment includes a medium temperature heating process, a high temperature heating process, and a rapid cooling process. Each step will be explained in detail below.

[Medium temperature heating process]
In the medium temperature heating step, the prepared intermediate steel material (raw pipe) is heated from room temperature to a heating temperature and held. In this way, in the medium temperature heating step, fine Mo-based MX type precipitates are precipitated in the intermediate steel material. Specifically, the preferred heating temperature in the medium temperature heating step is 400 to less than 600°C. If the heating temperature is too low, the amount of Mo-based MX type precipitates deposited in the medium temperature heating step will decrease. As a result, the number density of Mo-based MX type precipitates in the manufactured steel material decreases. On the other hand, if the heating temperature is too high, the Mo-based MX-type precipitates will grow too much, the Mo-based MX-type precipitates will become coarse in the medium temperature heating process, and the number density of the Mo-based MX-type precipitates in the manufactured steel material will decrease. descend.

Therefore, in the medium temperature heating step according to the present embodiment, the preferred heating temperature is 400 to less than 600°C. A more preferable lower limit of the heating temperature in the medium temperature heating step is 410°C, still more preferably 420°C, and even more preferably 430°C. A more preferable upper limit of the heating temperature in the medium temperature heating step is 590°C, still more preferably 580°C, and even more preferably 570°C.

The preferred holding time in the medium temperature heating step is 20 to 120 minutes. If the holding time is too short, the amount of Mo-based MX type precipitates deposited in the medium temperature heating step will decrease. As a result, the number density of Mo-based MX type precipitates in the manufactured steel material decreases. On the other hand, if the holding time is too long, the Mo-based MX-type precipitates grow too much, and the Mo-based MX-type precipitates become coarse in the medium temperature heating step. As a result, the number density of Mo-based MX type precipitates in the manufactured steel material decreases.

Therefore, in the medium temperature heating step according to this embodiment, the preferred holding time is 20 to 120 minutes. A more preferable lower limit of the holding time in the medium temperature heating step is 25 minutes. A more preferable upper limit of the holding time in the medium temperature heating step is 100 minutes, and even more preferably 90 minutes.

[High temperature heating process]
In the high temperature heating step, the intermediate steel material (raw pipe) heated in the medium temperature heating step is heated from the heating temperature in the medium temperature heating step to the heating temperature in the high temperature heating step and held. In this way, in the high temperature heating step, the microstructure of the steel material is transformed into a single austenite phase. As a result, the intermediate steel material can be hardened through the subsequent rapid cooling step. Specifically, the preferred heating temperature in the high temperature heating step is 880 to 1000°C. If the heating temperature is too low, the microstructure of the intermediate steel material will not be sufficiently transformed, and the hardening effect will not be sufficiently obtained. As a result, the mechanical properties specified in this embodiment cannot be obtained in the manufactured steel material. On the other hand, if the heating temperature is too high, the austenite grains will become coarse. If the heating temperature is too high, many of the fine Mo-based MX type precipitates precipitated in the medium temperature heating step will further dissolve. As a result, the fracture toughness of the manufactured steel material decreases.

Therefore, in the high temperature heating step according to this embodiment, the preferred heating temperature is 880 to 1000°C. The lower limit of the heating temperature in the high temperature heating step is more preferably 890°C, and even more preferably 900°C. A more preferable upper limit of the heating temperature in the high temperature heating step is 990°C, and even more preferably 980°C.

The preferred holding time in the high temperature heating step is 10 to 90 minutes. If the holding time is too short, the microstructure of the intermediate steel material will not be sufficiently transformed, and the hardening effect will not be sufficiently obtained. As a result, the mechanical properties specified in this embodiment cannot be obtained in the manufactured steel material. On the other hand, if the holding time is too long, the above effects will be saturated.

Therefore, in the high temperature heating step according to the present embodiment, the preferred holding time is 10 to 90 minutes. A more preferable lower limit of the holding time in the high temperature heating step is 15 minutes. The upper limit of the holding time in the high temperature heating step is more preferably 80 minutes, and even more preferably 60 minutes.

[Quick cooling process]
In the quenching step, the intermediate steel material (raw pipe) heated in the high-temperature heating step is quenched. In the quenching process, the intermediate steel material (raw pipe) is continuously cooled, and the surface temperature of the raw pipe is continuously lowered. The method of continuous cooling treatment is not particularly limited, and any known method may be used. Examples of the continuous cooling treatment include a method of cooling the raw tube by immersing it in a water tank, and a method of accelerating cooling of the raw tube by shower water cooling or mist cooling.

If the cooling rate during quenching is too slow, the microstructure will not consist mainly of martensite and bainite, and the mechanical properties defined in this embodiment will not be obtained. Here, in the quenching process according to the present embodiment, the average cooling rate in a range where the surface temperature of the intermediate steel material (raw pipe) during quenching is in the range of 800 to 500°C is defined as the cooling rate during quenching CR _800-500 . Specifically, the cooling rate CR _800-500 during quenching is determined at the slowest cooling rate in the cross section of the intermediate steel material to be quenched (for example, in the case of forced cooling on both surfaces, the center of the thickness of the intermediate steel material). Determined from the measured temperature.

In the rapid cooling step according to the present embodiment, a preferable cooling rate CR _800-500 during quenching is 300° C./min or more. More preferably, the lower limit of the cooling rate CR _800-500 during quenching is 450°C/min, and even more preferably 600°C/min. The upper limit of the cooling rate CR _800-500 during quenching is not particularly specified, but is, for example, 60000° C./min.

Through the above steps, the quenching step according to this embodiment can be performed. In addition, in the quenching step according to the present embodiment, the intermediate steel material may be heated in the austenite region multiple times and then quenched. However, in this case, in the first quenching, a medium temperature heating step, a high temperature heating step, and a rapid cooling step are performed. That is, in the second and subsequent quenching, it is preferable not to perform the medium temperature heating step. If a medium-temperature heating step is also performed in the second and subsequent quenching, Mo-based MX type precipitates may become coarse in the second and subsequent medium-temperature heating steps. As a result, the number density of Mo-based MX type precipitates in the produced steel may decrease. Therefore, when hardening is performed two or more times, it is preferable that the second and subsequent hardenings include a high temperature heating step and a rapid cooling step. The tempering process will be explained in detail below.

[Tempering process]
In the tempering step, after the above-mentioned hardening is performed, tempering is performed. In this specification, "tempering" means reheating and holding the intermediate steel material after quenching to a temperature below the A _c1 point. The tempering temperature is appropriately adjusted depending on the chemical composition of the steel material and the desired yield strength. That is, the tempering temperature of the intermediate steel material (raw pipe) having the chemical composition of this embodiment is adjusted to adjust the yield strength of the steel material to, for example, 110 ksi class (less than 758 to 862 MPa). Here, the tempering temperature corresponds to the temperature of a heat treatment furnace when heating and holding the intermediate steel material after quenching. Tempering time means the time from when the temperature of the intermediate steel material reaches a predetermined tempering temperature until it is extracted from the heat treatment furnace.

The tempering temperature is appropriately adjusted depending on the chemical composition of the steel material and the desired yield strength. That is, for the intermediate steel material (raw pipe) having the chemical composition of this embodiment, the tempering temperature is adjusted to adjust the yield strength of the steel material to less than 758 to 862 MPa. In the tempering process according to this embodiment, the preferred tempering temperature is 690 to 740°C. A more preferable lower limit of the tempering temperature is 695°C. A more preferable upper limit of the tempering temperature is 735°C.

If the tempering time is too short, a microstructure consisting mainly of tempered martensite and tempered bainite may not be obtained. On the other hand, if the tempering time is too long, the above effect will be saturated. Therefore, in the tempering step of this embodiment, the tempering time is preferably 20 to 180 minutes. A more preferable lower limit of the tempering time is 30 minutes. A more preferable upper limit of the tempering time is 150 minutes, and even more preferably 120 minutes.

The steel material according to this embodiment can be manufactured by the above manufacturing method. In addition, in the above-mentioned manufacturing method, the manufacturing method of a steel pipe was demonstrated as an example. However, the steel material according to this embodiment may be a steel plate or other shapes. Similar to the above-described manufacturing method, the manufacturing method for steel plates and other shapes includes, for example, a preparation process, a quenching process, and a tempering process. Furthermore, the above-mentioned manufacturing method is just an example, and other manufacturing methods may be used.

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

180 kg of molten steel having the chemical composition shown in Tables 1-1 and 1-2 was produced. Note that "-" in Tables 1-1 and 1-2 means that the content of the corresponding element was at the impurity level. Specifically, this means that the V content, W content, Co content, Ni content, and Cu content of Steel R were rounded to the second decimal place and were 0%. Similarly, the Ti content, Nb content, and rare earth element (REM) content of Steel R were rounded to the fourth decimal place, meaning that they were 0%. Similarly, the Ca content of Steel C, the Mg content of Steel R, and the B content of Steel R are rounded to the fifth decimal place and mean that they were 0%. Furthermore, Table 2 shows Fn1 and Fn2 determined from the chemical composition shown in Table 1 and the above definition.

An ingot was manufactured using the above molten steel. The ingot was hot rolled to produce a steel plate with a thickness of 20 mm. After hot rolling, the steel plates of each test number were cooled to room temperature and then quenched and tempered. In the quenching process, a medium-temperature heating process and a high-temperature heating process were performed, followed by a rapid cooling process. Specifically, the steel plates of each test number were held at the heating temperature (° C.) listed in the “medium temperature heating step” column of Table 2 for the holding time (minutes). Further, the steel plates of each test number were held at the heating temperature (°C) listed in the "High Temperature Heating Step" column of Table 2 for 20 minutes, and then rapidly cooled with shower water. In each test number, the cooling rate CR _800-500 during quenching was within the range of 300 to 800°C/min. Moreover, the heating temperature (°C) listed in Table 2 was the temperature (°C) of the heat treatment furnace in which the steel plate was heated. Further, the holding time (minutes) listed in Table 2 was the time (minutes) during which the steel plate was held at the heating temperature.

The obtained steel plates of each test number were tempered. Specifically, the steel plates of each test number were tempered by holding them at the tempering temperature (° C.) listed in the “Tempering Step” column of Table 2 for the holding time (minutes). Here, the tempering temperature (°C) listed in Table 2 was the temperature (°C) of the tempering furnace in which the steel plate was heated. Furthermore, the holding time (minutes) listed in Table 2 was the time (minutes) during which the steel plate was held at the tempering temperature. Through the above manufacturing process, steel plates of each test number were obtained.

[Evaluation test]
A tensile test, a Mo-based MX precipitate number density measurement test, and a fracture toughness test, which will be described below, were conducted on the steel plates of each test number obtained.

[Tensile test]
A tensile test was performed on the steel plate of each test number to determine the yield strength and tensile strength. The tensile test was conducted in accordance with ASTM E8/E8M (2021). Specifically, a round bar tensile test piece with a parallel portion diameter of 6 mm and a gage length of 30 mm was prepared from the thickness center position of the steel plate of each test number. The longitudinal direction of the round bar tensile test piece was parallel to the rolling direction of the steel plate. Using the produced round bar tensile test pieces, a tensile test was conducted at room temperature (25° C.) in the atmosphere to obtain the yield strength (MPa) of the steel plate of each test number. In this example, the 0.6% total elongation yield strength obtained in the tensile test was defined as the yield strength. Further, the maximum stress during uniform elongation was defined as tensile strength (MPa). Table 3 shows the obtained yield strength as "YS (MPa)" and the obtained tensile strength as "TS (MPa)" for each test number.

[Number density measurement test of Mo-based MX type precipitates]
A test for measuring the number density of Mo-based MX-type precipitates was performed on the steel plates of each test number to determine the number density of Mo-based MX-type precipitates. Specifically, a micro test piece was prepared from the center position of the thickness of the steel plate of each test number. A replica film was prepared using the obtained micro test piece by the method described above, and the replica film was observed using a TEM. The conditions for TEM observation were as follows: observation magnification was 100,000 times, acceleration voltage was 200 kV, and observation field size was 2.0 μm×3.0 μm. In the observation field, particles with an equivalent circular diameter of 100 nm or less were identified using the method described above. Point analysis by EDS was performed on the identified particles having an equivalent circle diameter of 100 nm or less using the method described above. When the total content of Mo, Nb, V, and Ti determined by EDS point analysis is defined as 100% by mass, particles with a Mo content of more than 50 to 70% by mass are classified as Mo-based MX-type precipitates. It was defined as The number density (pieces/μm ² ) of Mo-based MX-type precipitates was determined based on the total number of Mo-based MX-type precipitates identified in each observation field and the total area of the observation field. Table 3 shows the number density (pieces/μm ² ) of the Mo-based MX type precipitates obtained for each test number.

[Fracture toughness test]
A fracture toughness test was performed on the steel plate of each test number, and the CTOD value was determined. Specifically, SENB test pieces shown in FIG. 2A were prepared from the center position of the thickness of the steel plates of each test number. The longitudinal direction of the SENB test piece was parallel to the rolling direction of the steel plate. The width W direction of the SENB test piece was parallel to the width direction of the steel plate. As shown in FIGS. 2A and 2B, a notch with a depth of 8 mm in the width W direction was formed in the SENB specimen by machining. A 2 mm fatigue pre-crack was introduced at the notch tip of the SENB specimen. At this time, the fatigue test was conducted at room temperature (24±3°C).

A CTOD test was conducted at room temperature (25° C.) in accordance with ISO 12135 (2021) on the SENB test piece in which a fatigue pre-crack was introduced. The CTOD value (mm) was determined based on ISO 12135 (2021) from the load at break in the load-opening amount curve obtained by the CTOD test and the amount of plastic component of the clip gauge opening displacement. In this example, the load application rate in the CTOD test was 20.94 kN/min, and the Young's modulus was 212,000 MPa. The same test was conducted three times, and the minimum CTOD value (mm) was defined as the CTOD value (mm) of the steel material. Table 3 shows the CTOD values (mm) obtained for each test number.

[Test results]
Table 3 shows the test results.

Referring to Table 1-1, Table 1-2, Table 2, and Table 3, the chemical compositions of the steel plates of test numbers 1 to 16 were appropriate, and the yield strength was 758 to less than 862 MPa (110 ksi class). Ta. Furthermore, these steel plates had Fn1 of 12.0 or less and Fn2 of 0.05 to 0.80. Furthermore, these steel plates had a number density of Mo-based MX type precipitates of 20 pieces/μm ² or more. As a result, these steel plates had a CTOD value of 0.15 mm or more and exhibited excellent fracture toughness.

On the other hand, for the steel plate of test number 17, the holding time in the medium temperature heating step was too short. As a result, this steel plate had a number density of Mo-based MX type precipitates of less than 20 pieces/μm ² . As a result, this steel plate had a CTOD value of less than 0.15 mm and did not exhibit excellent fracture toughness.

In the steel plate of test number 18, the heating temperature in the medium temperature heating step was too low. As a result, this steel plate had a number density of Mo-based MX type precipitates of less than 20 pieces/μm ² . As a result, this steel plate had a CTOD value of less than 0.15 mm and did not exhibit excellent fracture toughness.

For the steel plate of test number 19, the heating temperature in the medium temperature heating step was too high. As a result, this steel plate had a number density of Mo-based MX type precipitates of less than 20 pieces/μm ² . As a result, this steel plate had a CTOD value of less than 0.15 mm and did not exhibit excellent fracture toughness.

The steel plate of test number 20 had too high Fn2. As a result, this steel plate had a number density of Mo-based MX type precipitates of less than 20 pieces/μm ² . As a result, this steel plate had a CTOD value of less than 0.15 mm and did not exhibit excellent fracture toughness.

The steel plate of Test No. 21 contained neither Ti, Nb, nor V, and furthermore, Fn2 was too low. As a result, this steel plate had a number density of Mo-based MX type precipitates of less than 20 pieces/μm ² . As a result, this steel plate had a CTOD value of less than 0.15 mm and did not exhibit excellent fracture toughness.

The steel plate of test number 22 had too low Mo content. As a result, this steel plate had a number density of Mo-based MX type precipitates of less than 20 pieces/μm ² . As a result, this steel plate had a CTOD value of less than 0.15 mm and did not exhibit excellent fracture toughness.

The steel plate of test number 23 had too high Mn content and too high Fn1. As a result, this steel plate had a CTOD value of less than 0.15 mm and did not exhibit excellent fracture toughness.

The steel plate of test number 24 had too high Fn1. As a result, this steel plate had a CTOD value of less than 0.15 mm and did not exhibit excellent fracture toughness.

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

Claims (3)

A steel material,
In mass%,
C: 0.10-0.45%,
Si: 1.00% or less,
Mn: 0.01-1.00%,
P: 0.050% or less,
S: 0.0050% or less,
Al: 0.001-0.100%,
Cr: 0.1-2.0%,
Mo: 0.20-2.00%,
N: 0.010% or less,
W: 0-0.50%,
Co: 0 to 0.50%,
Ni: 0 to 0.50%,
Rare earth elements: 0 to 0.020%,
Cu: 0 to 0.50% ,
B: 0 to 0.0100% , and
Contains Ti: 0.001 to 0.300%,
Ca: 0.0005 to 0.0200%, and
Contains one or more elements selected from the group consisting of Mg: 0.0005 to 0.0200%,
Nb: 0.001 to 0.300%, and
V: Contains one or more elements selected from the group consisting of 0.01 to 0.50%,
The remainder consists of Fe and impurities,
satisfies formula (1) and formula (2),
The yield strength is less than 758 to 862 MPa,
In the steel material,
The equivalent circle diameter is 10 to 100 nm ,
When the total content of Mo, Nb, V, and Ti is defined as 100% by mass, the number density of particles with a Mo content of more than 50 to 70% by mass is 20 particles/μm ² or more,
Steel material.
Mn×Sp≦ 11.0 (1)
0.05≦7×Ti+2×Nb+3×V≦0.80 (2)
Here, the Mn content is substituted in mass % for "Mn" in formula (1), and the S content is substituted in ppm for "Sp". The content of the corresponding element in mass % is substituted for the element symbol in formula (2). Note that when the corresponding element is not contained, "0" is substituted for the element symbol. The steel material according to claim 1,
W: 0.01-0.50%,
Co: 0.01 to 0.50%,
Ni: 0.01-0.50%,
Rare earth elements: 0.001-0.020%,
Cu: 0.01 to 0.50%, and
B: Contains one or more elements selected from the group consisting of 0.0001 to 0.0100%,
Steel material. The steel material according to claim 1 or claim 2,
The steel material is a steel pipe for oil wells,
Steel material.

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