Steels suitable for use in sour environments

Abstract

The present disclosure provides a steel material having high yield strength and excellent fracture toughness in a low-temperature sour environment. The steel material according to the present disclosure has a chemical composition as described in the specification and has a yield strength σ YS The Mo content in the vicinity of the prior austenite grain boundary in the steel is 758 MPa or more. Mo The steel material according to the present disclosure further has a yield strength σ YS When the value of Fn1 defined by the formula (1) is less than 862 MPa, the yield strength σ YS is 862 MPa or more, Fn1 defined by formula (1) satisfies −520 or more. Fn1 = 83 × (γ Mo -10 -7 ×√ρ)-σ YS (1) Here, γ in equation (1) Mo is substituted with the Mo content in the vicinity of the prior austenite grain boundary in mass%, and ρ in equation (1) is the dislocation density m -2 and σ in Eq. (1) YS is substituted for the yield strength in MPa.

Description

[Technical field]

[0001] The present disclosure relates to steel products, and more particularly, to steel products suitable for use in sour environments. [Background technology]

[0002] As oil wells and gas wells (hereinafter, oil wells and gas wells are collectively referred to simply as "oil wells") become deeper, there is a demand for higher strength oil well steel materials, such as oil well steel pipes. Specifically, 80 ksi class (yield strength of 80 to less than 95 ksi, i.e., less than 552 to 655 MPa) and 95 ksi class (yield strength of 95 to less than 110 ksi, i.e., less than 655 to 758 MPa) oil well steel materials are widely used, and recently, there has been a demand for 110 ksi class (yield strength of 110 to less than 125 ksi, i.e., less than 758 to 862 MPa), 125 ksi class (yield strength of 125 to less than 140 ksi, i.e., less than 862 to 965 MPa), and 140 ksi or more (yield strength of 140 ksi or more, i.e., 965 MPa or more) oil well steel pipes.

[0003] On the other hand, many deep wells are in sour environments containing corrosive hydrogen sulfide. In this specification, a sour environment means an acidic environment containing hydrogen sulfide. Note that a sour environment may also contain carbon dioxide. Oil well steel pipes used in such sour environments are required to have not only high strength but also sulfide stress cracking resistance (hereinafter referred to as SSC resistance).

[0004] Techniques for improving the SSC resistance of steel materials, such as oil well steel pipes, have been proposed in JP 2000-297344 A (Patent Document 1), JP 2001-271134 A (Patent Document 2), and WO 2008 / 123422 (Patent Document 3).

[0005] The oil well steel disclosed in Patent Document 1 contains, by mass%, C: 0.15-0.3%, Cr: 0.2-1.5%, Mo: 0.1-1%, V: 0.05-0.3%, and Nb: 0.003-0.1%. This oil well steel contains precipitated carbides in a total amount of 1.5-4 mass%, MC carbides in the total amount of carbides in a ratio of 5-45 mass%, and M 23 The proportion of C6 carbides is (200 / t) mass% or less when the wall thickness of the product is t (mm). Patent Document 1 states that this oil well steel has excellent SSC resistance.

[0006] The low alloy steel material disclosed in Patent Document 2 contains, by mass%, C: 0.2-0.35%, Si: 0.05-0.5%, Mn: 0.1-1%, P: 0.025% or less, S: 0.01% or less, Cr: 0.1-1.2%, Mo: 0.1-1%, B: 0.0001-0.005%, Al: 0.005-0.1%, N: 0.01% or less, V: 0.05-0.5%, Ni: 0.1% or less, W: 1.0% or less, O: 0.01% or less, the balance being Fe and impurities, and satisfies the formula (0.03≦Mo×V≦0.3) and the formula (0.5×Mo-V+GS / 10≧1), and has a yield strength of 1060 MPa or more. In addition, GS in the formula means the ASTM grain size number of prior austenite grains. Patent Document 2 states that this low-alloy steel material has excellent SSC resistance.

[0007] The low alloy steel disclosed in Patent Document 3 contains, by mass%, C: 0.10-0.20%, Si: 0.05-1.0%, Mn: 0.05-1.5%, Cr: 1.0-2.0%, Mo: 0.05-2.0%, Al: 0.10% or less, and Ti: 0.002-0.05%, and Ceq (=C+(Mn / 6)+(Cr+Mo+V) / 5) is 0.65 or more, with the balance being Fe and impurities, of which P: 0.025% or less, S: 0.010% or less, N: 0.007% or less, and B: less than 0.0003%. This low alloy steel contains M having a grain size of 1 μm or more. 23 C6 type precipitates are 0.1 pieces / mm 2 The following is a description thereof. Patent Document 3 states that this low alloy steel has improved SSC resistance. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] JP 2000-297344 A [Patent Document 2] JP 2001-271134 A [Patent Document 3] International Publication No. 2008 / 123422 Summary of the Invention [Problem to be solved by the invention]

[0009] In recent years, attention has been focused on the ability to suppress the propagation of cracks formed in steel materials in sour environments, among other SSC resistance properties. When stress is applied to steel materials with microscopic flaws, there is a concern that the microscopic flaws may become the starting points of cracks, causing the cracks to propagate. On the other hand, if the propagation of cracks can be suppressed, it is believed that the steel materials will have excellent resistance to fracture (i.e., excellent fracture toughness). For this reason, there has been a demand for oil well steel materials that are high in strength and also have excellent fracture toughness in sour environments.

[0010] Furthermore, in recent years, development of deep wells below sea level has also become active. For example, in so-called deep-sea oil fields with a depth of 2000m or more, the water temperature is low. In addition, generally, the lower the environmental temperature, the more likely it is that fracture toughness will decrease. Therefore, there is an increasing demand for oil well steel materials, such as oil well steel pipes, that have high strength and excellent fracture toughness even in low-temperature sour environments.

[0011] The above Patent Documents 1 to 3 propose steel materials having excellent SSC resistance. However, the above Patent Documents 1 to 3 do not consider fracture toughness in a low-temperature sour environment.

[0012] An object of the present disclosure is to provide a steel material having high yield strength and excellent fracture toughness in low temperature sour environments. [Means for solving the problem]

[0013] The steel material according to the present disclosure is In mass%, C: more than 0.20~0.60%, Si: 0.05 to 2.00%, Mn: 0.02 to 0.60%, P: 0.025% or less, S: 0.0100% or less, Al: 0.005 to 0.100%, Cr: 0.20~1.50%, Mo: 0.35-1.50%, V: 0.01 to 0.60%, Ti: 0.002 to 0.050%, B: 0.0001~0.0050%, N: 0.0100% or less, O: 0.0100% or less, Nb: 0 to 0.030%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, Rare earth elements: 0~0.0100%, Co: 0-0.50%, W: 0 to 0.50%, Ni: 0 to 0.20%, Cu: 0 to 0.50%, and the balance being Fe and impurities, Yield strength σ YS is 758MPa or more, In the above steel material, the Mo content γ Mo is 5.0 mass% or more, The yield strength σ YS is less than 862 MPa, Fn1 defined by formula (1) satisfies -300 or more, The yield strength σ YS is 862 MPa or more, Fn1 defined by formula (1) is -520 or more; Steel material. Fn1 = 83 × (γMo -10 -7 ×√ρ)-σ YS (1) Here, γ in equation (1) Mo is substituted with the Mo content in the vicinity of the prior austenite grain boundary in mass%, and ρ in formula (1) is substituted with the dislocation density m -2 and σ in Eq. (1) YS The yield strength in MPa is substituted for . Effect of the Invention

[0014] The steel material according to the present disclosure has high yield strength and excellent fracture toughness in low temperature sour environments. [Brief description of the drawings]

[0015] [Figure 1] FIG. 1 is a diagram showing the relationship between Fn1 (=83×(γMo-10×√ρ)-σYS) and the fracture toughness value K1SSC value, which is an index of fracture toughness in a low-temperature sour environment, in an example having a yield strength of 110 ksi class (758 to less than 862 MPa) among the present embodiments. [Diagram 2] FIG. 2 is a diagram showing the relationship between Fn1 (=83×(γMo-10-7×√ρ)-σYS) and the fracture toughness value K1SSC value, which is an index of fracture toughness in a low-temperature sour environment, in an example of this embodiment having a yield strength of 125 ksi or more (862 MPa or more). [Diagram 3] FIG. 3 is a schematic diagram of the observation surface of a polished test piece for determining the grain boundary Mo content. [Figure 4] FIG. 4 is a schematic diagram of a measurement area 10 where the grain boundaries of prior austenite grains are identified. [Diagram 5] FIG. 5 is a schematic diagram showing how a thin film sample TP is prepared from the measurement region 10 shown in FIG. [Figure 6] FIG. 6 is a perspective view of the thin film sample TP shown in FIG. [Figure 7] FIG. 7 is a schematic diagram for explaining the relationship between the electron beam and the prior austenite grain boundary GB in the thin film sample TP during TEM observation and elemental analysis by EDS. [Figure 8] FIG. 8 is a schematic diagram showing the relationship between the irradiation direction of the electron beam EB during TEM observation and elemental analysis by EDS, the prior austenite grain boundary GB on the surface 10 of the thin film sample TP, and the element concentration profile. [Figure 9] FIG. 9 shows an example of the Mo content on a line segment SL that is centered on the prior austenite grain boundary GB and perpendicular to the prior austenite grain boundary GB. [Figure 10] FIG. 10 is a schematic diagram for explaining the formula (2). [Figure 11] FIG. 11 shows a side view and a cross-sectional view of a DCB test piece used in an evaluation test of the fracture toughness of a steel material in a low-temperature sour environment (DCB test) in this embodiment. [Figure 12] FIG. 12 is a perspective view of a wedge used in an evaluation test of the fracture toughness of a steel material in a low-temperature sour environment (DCB test) in this embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] The present inventors have investigated and examined a method for obtaining a steel material having high yield strength and excellent fracture toughness in a low-temperature sour environment, and have obtained the following findings.

[0017] Specifically, the inventors aimed to obtain a steel material having a high yield strength of 758 MPa or more (110 ksi or more). Therefore, the inventors first investigated, from the viewpoint of chemical composition, a steel material having a yield strength of 110 ksi or more and excellent fracture toughness in a low-temperature sour environment. As a result, the inventors found that, in mass%, C: over 0.20 to 0.60%, Si: 0.05 to 2.00%, Mn: 0.02 to 0.60%, P: 0.025% or less, S: 0.0100% or less, Al: 0.005 to 0.100%, Cr: 0.20 to 1.50%, Mo: 0.35 to 1.50%, V: 0.01 to 0.60%, Ti: 0.002 to 0.050%, B: 0.0001 to 0.0050%, N: 0.0100% or less, O: 0.0100% It was believed that a steel material having a yield strength of 110 ksi or more and excellent fracture toughness in a low-temperature sour environment could be obtained if the steel material consisted of the following composition: Nb: 0-0.030%, Ca: 0-0.0100%, Mg: 0-0.0100%, Zr: 0-0.0100%, rare earth elements: 0-0.0100%, Co: 0-0.50%, W: 0-0.50%, Ni: 0-0.20%, Cu: 0-0.50%, and the balance being Fe and impurities.

[0018] Next, the inventors of the present invention have investigated various methods for improving the fracture toughness in a low-temperature sour environment for a steel material having the above-mentioned chemical composition and a yield strength of 110 ksi or more. As a result, it has been revealed that in a steel material having the above-mentioned chemical composition and yield strength, a decrease in the toughness of the prior austenite grain boundaries in the steel material is likely to become apparent in a low-temperature sour environment, and cracks are likely to propagate along the prior austenite grain boundaries in the steel material. In other words, in a steel material having the above-mentioned chemical composition and yield strength, if the prior austenite grain boundaries can be strengthened, there is a possibility that excellent fracture toughness can be obtained. Hereinafter, in this specification, prior austenite grains are also referred to as "prior γ grains".

[0019] Therefore, the present inventors focused on molybdenum (Mo) as an element that contributes to strengthening the prior γ grain boundaries. If Mo is concentrated at the prior γ grain boundaries, the prior γ grain boundaries can be strengthened, and there is a possibility that the propagation of cracks can be suppressed even in a low-temperature sour environment. As a result, there is a possibility that excellent fracture toughness can be obtained in a low-temperature sour environment. As a result of detailed studies by the present inventors, in a steel material having the above-mentioned chemical composition and a yield strength of 110 ksi or more, the Mo content near the prior γ grain boundaries is Mo It was revealed that there is a possibility of obtaining excellent fracture toughness if the Mo content in the vicinity of the prior γ grain boundary is 5.0 mass% or more. Mo The grain boundary Mo content γ Mo "It is also called.

[0020] On the other hand, the above-mentioned chemical composition and yield strength are obtained by reducing the grain boundary Mo content γ Mo Even if the steel material has a dislocation density of 5.0 mass% or more, it may not be possible to obtain stable and excellent fracture toughness in a low-temperature sour environment. Therefore, the present inventors have further studied in detail the factors that reduce the fracture toughness of the above-mentioned steel material. As a result, in a steel material having the above-mentioned chemical composition and yield strength, the dislocation density in the steel material ρ and the yield strength of the steel material σ YS and the grain boundary Mo content γ Mo It was revealed that the fracture toughness in low-temperature sour environments is affected by the above.

[0021] Here, Fn1 = 83 × (γ Mo -10 -7 ×√ρ)-σ YS Fn1 is an index of fracture toughness in a low-temperature sour environment. Hereinafter, the yield strength σ YS When the yield strength is less than 862MPa (when the yield strength is less than 758 to 862MPa), YS The cases where the compressive strength is 862 MPa or more and 862 MPa or more will be specifically described with reference to the drawings.

[0022] FIG. 1 shows the Fn1(=83×(γ Mo -10 -7 ×√ρ)-σ YS) and the fracture toughness value K, which is an index of fracture toughness in a low-temperature sour environment. 1SSC FIG. 1 is a graph showing the relationship between the yield strength σ YS is 110ksi class, and the grain boundary Mo content γ Mo The yield strength σ YS and the grain boundary Mo content γ Mo , dislocation density ρ, and fracture toughness value K 1SSC The value was determined by the method described below.

[0023] Referring to FIG. 1, the above-mentioned chemical composition, 110 ksi-class yield strength, and 5.0 mass% or more of grain boundary Mo content γ Mo For steels having Fn1 of -300 or more, the fracture toughness value K 1SSC On the other hand, for the above steel, if Fn1 is less than -300, the fracture toughness value K 1SSC Therefore, the steel material according to the present embodiment has the above-mentioned chemical composition and a grain boundary Mo content γ of 5.0 mass% or more. Mo and the yield strength σ YS When Fn1 is 758 to less than 862 MPa, Fn1 is set to be −300 or more.

[0024] Furthermore, FIG. 2 shows that in the present embodiment, Fn1(=83×(γ Mo -10 -7 ×√ρ)-σ YS ) and the fracture toughness value K, which is an index of fracture toughness in a low-temperature sour environment. 1SSC FIG. 2 is a graph showing the relationship between the yield strength σ YS is 125 ksi or more, and the grain boundary Mo content γ Mo The yield strength σ YS and the grain boundary Mo content γ Mo , dislocation density ρ, and fracture toughness value K 1SSC The value was determined by the method described below.

[0025] Referring to FIG. 2, the above-mentioned chemical composition, the yield strength of 125 ksi or more, and the grain boundary Mo content γ of 5.0 mass% or more Mo For steels having this property, if Fn1 is -520 or more, the fracture toughness value K 1SSC It can be confirmed that the fracture toughness value K of the above steel material is 24.1 MPa√m or more, and that the fracture toughness is stable and excellent. On the other hand, if Fn1 is less than -520, the fracture toughness value K 1SSC Therefore, the steel material according to the present embodiment has the above-mentioned chemical composition and a grain boundary Mo content γ of 5.0 mass% or more. Mo and the yield strength σ YS is 862 MPa or more, Fn1 is set to -520 or more.

[0026] In addition, the above chemical composition and yield strength σ YS and the grain boundary Mo content γ Mo The reason why the fracture toughness in a low-temperature sour environment can be improved by adjusting Fn1 in a steel material having the above properties is not clear in detail. However, the inventors speculate as follows. As described above, Fn1 is related to the yield strength σ YS and the grain boundary Mo content γ Mo and the dislocation density ρ. Here, the dislocations are YS It is believed that dislocation density ρ increases the yield strength while absorbing hydrogen. Therefore, if the dislocation density ρ is too high, the hydrogen absorbed in the dislocations will be more likely to remain in the prior γ grain boundaries, and there is a concern that the prior γ grain boundaries will become more likely to become embrittled. In other words, if the dislocation density ρ can be reduced, the amount of hydrogen remaining in the prior γ grain boundaries can be reduced, and it is possible to strengthen the prior γ grain boundaries. However, as mentioned above, the dislocation density ρ has a large effect on the yield strength σ YS Therefore, rather than simply reducing the dislocation density ρ, it is important to improve the yield strength σ YS and the grain boundary Mo content γ Mo By controlling the relationship between the dislocation density ρ and the dislocation density ρ, it is possible to improve the fracture toughness in a low-temperature sour environment in a steel material having the above-mentioned chemical composition.

[0027] The inventors presume that the steel material according to this embodiment can obtain high strength and excellent fracture toughness in a low-temperature sour environment due to the above mechanism. YS and the grain boundary Mo content γ Mo In steels having the above chemical composition and yield strength σ, it may be possible to improve the fracture toughness in low-temperature sour environments by adjusting Fn1. YS and the grain boundary Mo content γ Mo In the steel material having the above properties, it is possible to increase the fracture toughness in a low-temperature sour environment by adjusting Fn1, as will be demonstrated in the examples described later.

[0028] The gist of the steel material according to this embodiment, which was completed based on the above findings, is as follows.

[0029] [1] A steel material, In mass%, C: more than 0.20~0.60%, Si: 0.05 to 2.00%, Mn: 0.02 to 0.60%, P: 0.025% or less, S: 0.0100% or less, Al: 0.005 to 0.100%, Cr: 0.20~1.50%, Mo: 0.35-1.50%, V: 0.01 to 0.60%, Ti: 0.002 to 0.050%, B: 0.0001~0.0050%, N: 0.0100% or less, O: 0.0100% or less, Nb: 0 to 0.030%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, Rare earth elements: 0~0.0100%, Co: 0-0.50%, W: 0 to 0.50%, Ni: 0 to 0.20%, Cu: 0 to 0.50%, and the balance being Fe and impurities, Yield strength σ YS is 758MPa or more, In the above steel material, the Mo content γ Mo is 5.0 mass% or more, The yield strength σ YS is less than 862 MPa, Fn1 defined by formula (1) satisfies -300 or more, The yield strength σ YS is 862 MPa or more, Fn1 defined by formula (1) is -520 or more; Steel material. Fn1 = 83 × (γ Mo -10 -7 ×√ρ)-σ YS (1) Here, γ in equation (1) Mo is substituted with the Mo content in the vicinity of the prior austenite grain boundary in mass%, and ρ in formula (1) is substituted with the dislocation density m -2 and σ in Eq. (1) YS The yield strength in MPa is substituted for .

[0030] [2] The steel material according to [1], Nb: 0.001 to 0.030%, Ca: 0.0001 to 0.0100%, Mg: 0.0001 to 0.0100%, Zr: 0.0001 to 0.0100%, Rare earth elements: 0.0001~0.0100%, Co: 0.01 to 0.50%, W: 0.01 to 0.50%, Ni: 0.01 to 0.20%, and Cu: 0.01 to 0.50%, containing one or more elements selected from the group consisting of Steel material.

[0031] [3] The steel material according to [1] or [2], The steel material is a steel pipe for oil wells.

[0032] 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 bar (solid material), or a steel plate. The round bar means a steel bar having a circular cross section perpendicular to the axial direction. The steel pipe may be a seamless steel pipe or a welded steel pipe.

[0033] The steel material according to this embodiment will be described in detail below.

[0034] [Chemical composition] The chemical composition of the steel material according to this embodiment contains the following elements: "%" for each element means mass % unless otherwise specified.

[0035] C: More than 0.20~0.60% Carbon (C) improves the hardenability of steel material and increases the strength of the steel material. C also promotes spheroidization of carbides during tempering in the manufacturing process and increases the SSC resistance of the steel material. Dispersion of carbides further increases the strength of the steel material. If the C content is too low, 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 C content is too high, even if the contents of other elements are within the range of this embodiment, the amount of carbides becomes too large, and the toughness of the steel material decreases. If the C content is too high, furthermore, quench cracks may easily occur during quenching in the manufacturing process. Therefore, the C content is more than 0.20 to 0.60%. The preferred lower limit of the C content is 0.22%, more preferably 0.24%, and more preferably 0.25%. The preferred upper limit of the C content is 0.55%, more preferably 0.50%, and more preferably 0.45%.

[0036] Silicon: 0.05 to 2.00% Silicon (Si) deoxidizes steel. If the Si content is too low, the above effect 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 is too high, the SSC resistance of the steel material decreases even if the contents of other elements are within the range of this embodiment. Therefore, the Si content is 0.05 to 2.00%. The preferred lower limit of the Si content is 0.15%, and more preferably 0.20%. The preferred upper limit of the Si content is 1.80%, and more preferably 1.60%, and more preferably 1.50%, and more preferably 1.40%.

[0037] Mn: 0.02 to 0.60% Manganese (Mn) deoxidizes steel. Mn also improves the hardenability of steel and increases its strength. If the Mn content is too low, 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 Mn content is too high, Mn segregates to the grain boundaries together with impurities such as P and S, even if the contents of other elements are within the range of this embodiment, and the fracture toughness of the steel in a low-temperature sour environment decreases. Therefore, the Mn content is 0.02 to 0.60%. The preferred lower limit of the Mn content is 0.03%, more preferably 0.04%, and more preferably 0.06%. The preferred upper limit of the Mn content is 0.55%, more preferably 0.50%, and more preferably 0.45%.

[0038] P:0.025% or less Phosphorus (P) is an impurity. That is, the lower limit of the P content is more than 0%. If the P content is too high, even if the contents of other elements are within the range of this embodiment, P will segregate at the grain boundaries, and the fracture toughness of the steel material in a low-temperature sour environment will decrease. Therefore, the P content is 0.025% or less. A preferred upper limit of the P content is 0.020%, more preferably 0.015%. The P content is preferably as low as possible. However, an extreme reduction in the P content significantly increases the manufacturing cost. Therefore, in consideration of industrial production, a preferred lower limit of the P content is 0.001%, more preferably 0.002%, and even more preferably 0.003%.

[0039] S: 0.0100% or less Sulfur (S) is an impurity. That is, the lower limit of the S content is more than 0%. If the S content is too high, even if the contents of other elements are within the range of this embodiment, S will segregate at the grain boundaries, and the SSC resistance of the steel material will decrease. Therefore, the S content is 0.0100% or less. A preferred upper limit of the S content is 0.0075%, more preferably 0.0050%, and even more preferably 0.0030%. The S content is preferably as low as possible. However, an extreme reduction in the S content significantly increases the manufacturing cost. Therefore, in consideration of industrial production, a preferred lower limit of the S content is 0.0001%, more preferably 0.0002%, and even more preferably 0.0003%.

[0040] Al: 0.005 to 0.100% Aluminum (Al) deoxidizes steel. If the Al content is too low, the above effect cannot be sufficiently obtained even if the contents of other elements are within the range of this embodiment. As a result, the SSC resistance of the steel material is reduced. On the other hand, if the Al content is too high, coarse oxide-based inclusions are generated, even if the contents of other elements are within the range of this embodiment, and the SSC resistance of the steel material is reduced. Therefore, the Al content is 0.005 to 0.100%. The preferred lower limit of the Al content is 0.015%, and more preferably 0.020%. The preferred upper limit of the Al content is 0.080%, and more preferably 0.060%. The "Al" content in this specification means the content of "acid-soluble Al", that is, "sol.Al".

[0041] Cr: 0.20~1.50% Chromium (Cr) improves the hardenability of steel and increases its strength. Cr also improves the temper softening resistance of steel and enables high-temperature tempering. As a result, the fracture toughness of steel in a low-temperature sour environment is increased. If the Cr content is too low, 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 is too high, the SSC resistance of the steel decreases even if the contents of other elements are within the range of this embodiment. Therefore, the Cr content is 0.20 to 1.50%. The preferred lower limit of the Cr content is 0.25%, more preferably 0.30%, more preferably 0.35%, and even more preferably 0.40%. The preferred upper limit of the Cr content is 1.40%, and more preferably 1.30%.

[0042] Mo: 0.35-1.50% Molybdenum (Mo) improves the hardenability of steel and increases its strength. Mo also increases the grain boundary Mo content γ Mo and improves the fracture toughness of the steel material in a low-temperature sour environment. If the Mo content is too low, 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 is too high, the above effects are saturated. Therefore, the Mo content is 0.35 to 1.50%. The preferred lower limit of the Mo content is 0.40%, more preferably 0.45%, and even more preferably 0.50%. The preferred upper limit of the Mo content is 1.40%, more preferably 1.30%, and even more preferably 1.25%.

[0043] V: 0.01 to 0.60% Vanadium (V) combines with C or N to form carbides, nitrides, or carbonitrides (hereinafter also referred to as "carbonitrides, etc."), and refines the grains of the steel material by the pinning effect. As a result, the fracture toughness of the steel material in a low-temperature sour environment is increased. V also forms fine carbides during tempering to increase the tempering softening resistance of the steel material and increase the strength of the steel material. If the V content is too low, 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 V content is too high, the toughness of the steel material decreases even if the contents of other elements are within the range of this embodiment. Therefore, the V content is 0.01 to 0.60%. The preferred lower limit of the V content is 0.02%, more preferably 0.04%, and more preferably 0.06%. The preferred upper limit of the V content is 0.40%, more preferably 0.30%, more preferably 0.25%, and more preferably 0.20%.

[0044] Ti: 0.002~0.050% Titanium (Ti) combines with N to form nitrides, and the pinning effect refines the crystal grains of the steel material. As a result, the fracture toughness of the steel material in a low-temperature sour environment is increased. If the Ti content is too low, 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 is too high, even if the contents of other elements are within the range of this embodiment, the Ti nitrides become coarse, and the SSC resistance of the steel material decreases. Therefore, the Ti content is 0.002 to 0.050%. The preferred lower limit of the Ti content is 0.003%, and more preferably 0.005%. The preferred upper limit of the Ti content is 0.030%, and more preferably 0.020%.

[0045] B: 0.0001 to 0.0050% Boron (B) dissolves in steel to improve the hardenability of the steel material and increase the strength of the steel material. If the B content is too low, 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 B content is too high, coarse nitrides are generated and the SSC resistance of the steel material decreases even if the contents of other elements are within the range of this embodiment. Therefore, the B content is 0.0001 to 0.0050%. The preferred lower limit of the B content is 0.0003%, and more preferably 0.0007%. The preferred upper limit of the B content is 0.0030%, and more preferably 0.0025%, and more preferably 0.0020%, and more preferably 0.0015%.

[0046] N: 0.0100% or less Nitrogen (N) is inevitably contained. That is, the lower limit of the N content is more than 0%. N combines with Ti to form nitrides, and the grains of the steel are refined by the pinning effect. As a result, the fracture toughness of the steel in a low-temperature sour environment is increased. However, if the N content is too high, even if the contents of other elements are within the range of this embodiment, coarse nitrides are formed, and the fracture toughness of the steel in a low-temperature sour environment is rather reduced. Therefore, the N content is 0.0100% or less. The preferred upper limit of the N content is 0.0060%, more preferably 0.0050%, and more preferably 0.0045%. The preferred lower limit of the N content to more effectively obtain the above effects is 0.0005%, more preferably 0.0010%, more preferably 0.0015%, and more preferably 0.0020%.

[0047] O: 0.0100% or less Oxygen (O) is an impurity. That is, the lower limit of the O content is more than 0%. If the O content is too high, even if the contents of other elements are within the range of this embodiment, coarse oxides are formed, and the low-temperature toughness and SSC resistance of the steel material are reduced. Therefore, the O content is 0.0100% or less. A preferred upper limit of the O content is 0.0050%, more preferably 0.0030%, and even more preferably 0.0020%. The O content is preferably as low as possible. However, an extreme reduction in the O content significantly increases the manufacturing cost. Therefore, in consideration of industrial production, a preferred lower limit of the O content is 0.0001%, more preferably 0.0002%, and even more preferably 0.0003%.

[0048] The balance of the chemical composition of the steel material according to the present embodiment is composed of Fe and impurities. Here, the term "impurities" refers to substances that are mixed in from raw materials such as ore and scrap, or from the manufacturing environment, during industrial production of the steel material, and that are permissible within a range that does not adversely affect the steel material according to the present embodiment.

[0049] [Optional element] The chemical composition of the above-mentioned steel material may further contain Nb instead of a portion of Fe.

[0050] Nb: 0 to 0.030% Niobium (Nb) is an optional element and may not be contained. That is, the Nb content may be 0%. When contained, Nb forms carbonitrides and the like, and refines the grains of the steel material by the pinning effect. As a result, the fracture toughness of the steel material in a low-temperature sour environment is increased. Nb further forms fine carbides during tempering to increase the tempering softening resistance of the steel material and increase the strength of the steel material. If even a small amount of Nb is contained, the above effects can be obtained to some extent. However, if the Nb content is too high, even if the contents of other elements are within the range of this embodiment, excessive carbonitrides and the like are generated, and the SSC resistance of the steel material decreases. Therefore, the Nb content is 0 to 0.030%. The preferable lower limit of the Nb content is more than 0%, more preferably 0.001%, more preferably 0.002%, more preferably 0.003%, more preferably 0.005%, and more preferably 0.007%. The upper limit of the Nb content is preferably 0.025%, and more preferably 0.020%.

[0051] The chemical composition of the steel material described above may further contain, instead of a part of Fe, one or more elements selected from the group consisting of Ca, Mg, Zr, and rare earth elements. All of these elements are optional elements, and render S in the steel material harmless as sulfides. As a result, these elements increase the SSC resistance of the steel material.

[0052] Ca: 0 to 0.0100% Calcium (Ca) is an optional element and may not be contained. That is, the Ca content may be 0%. When contained, Ca renders S in the steel material harmless as sulfides, and enhances the SSC resistance of the steel material. If even a small amount of Ca is contained, the above effect can be obtained to a certain degree. However, if the Ca content is too high, even if the contents of other elements are within the range of this embodiment, the oxides in the steel material become coarse, and the SSC resistance of the steel material is rather reduced. Therefore, the Ca content is 0 to 0.0100%. The preferred lower limit of the Ca content is more than 0%, more preferably 0.0001%, more preferably 0.0003%, more preferably 0.0006%, and more preferably 0.0010%. The preferred upper limit of the Ca content is 0.0040%, more preferably 0.0030%, more preferably 0.0025%, and more preferably 0.0020%.

[0053] Magnesium: 0 to 0.0100% Magnesium (Mg) is an optional element and may not be contained. That is, the Mg content may be 0%. When contained, Mg renders S in the steel material harmless as sulfides, and enhances the SSC resistance of the steel material. If even a small amount of Mg is contained, the above effect can be obtained to a certain degree. However, if the Mg content is too high, even if the contents of other elements are within the range of this embodiment, the oxides in the steel material become coarse, and the SSC resistance of the steel material decreases. Therefore, the Mg content is 0 to 0.0100%. The preferable lower limit of the Mg content is more than 0%, more preferably 0.0001%, more preferably 0.0003%, more preferably 0.0006%, and more preferably 0.0010%. The preferable upper limit of the Mg content is 0.0040%, more preferably 0.0030%, more preferably 0.0025%, and more preferably 0.0020%.

[0054] Zr: 0 to 0.0100% Zirconium (Zr) is an optional element and may not be contained. That is, the Zr content may be 0%. When contained, Zr renders S in the steel harmless as sulfides, and enhances the SSC resistance of the steel. If even a small amount of Zr is contained, the above effect can be obtained to some extent. However, if the Zr content is too high, even if the contents of other elements are within the range of this embodiment, the oxides in the steel are coarsened, and the SSC resistance of the steel is reduced. Therefore, the Zr content is 0 to 0.0100%. The preferable lower limit of the Zr content is more than 0%, more preferably 0.0001%, more preferably 0.0003%, more preferably 0.0006%, and more preferably 0.0010%. The preferable upper limit of the Zr content is 0.0040%, more preferably 0.0030%, more preferably 0.0025%, and more preferably 0.0020%.

[0055] Rare earth elements (REM): 0~0.0100% Rare earth elements (REM) are optional elements and may not be contained. That is, the REM content may be 0%. When contained, REM renders S in the steel harmless as sulfides, and enhances the SSC resistance of the steel. REM further binds to P in the steel and suppresses the segregation of P at the grain boundaries. Therefore, the deterioration of the SSC resistance of the steel caused by the segregation of P is suppressed. If even a small amount of REM is contained, the above effect 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, the oxides in the steel become coarse, and the SSC resistance of the steel decreases even if the contents of other elements are within the range of this embodiment. Therefore, the REM content is 0 to 0.0100%. The preferable lower limit of the REM content is more than 0%, more preferably 0.0001%, more preferably 0.0003%, and even more preferably 0.0006%. The upper limit of the REM content is preferably 0.0040%, more preferably 0.0030%, and even more preferably 0.0025%.

[0056] In this specification, REM refers to one or more elements selected from the group consisting of scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and the lanthanides lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. In addition, in this specification, the REM content refers to the total content of these elements.

[0057] The chemical composition of the above-mentioned steel material may further contain, instead of a part of Fe, one or more elements selected from the group consisting of Co and W. All of these elements are optional elements, and they form a protective corrosion film in a sour environment and suppress the penetration of hydrogen into the steel material. As a result, these elements increase the SSC resistance of the steel material.

[0058] Co: 0 to 0.50% Cobalt (Co) is an optional element and may not be contained. That is, the Co content may be 0%. When Co is contained, Co forms a protective corrosion film in a sour environment and suppresses the penetration of hydrogen into the steel material. As a result, the SSC resistance of the steel material is improved. If even a small amount of Co is contained, the above effect can be obtained to a certain extent. However, if the Co content is too high, even if the contents of other elements are within the range of this embodiment, the hardenability of the steel material decreases and the strength of the steel material decreases. Therefore, the Co content is 0 to 0.50%. The preferred lower limit of the Co content is more than 0%, more preferably 0.01%, more preferably 0.02%, more preferably 0.03%, and more preferably 0.05%. The preferred upper limit of the Co content is 0.45%, and more preferably 0.40%.

[0059] W: 0~0.50% Tungsten (W) is an optional element and may not be contained. That is, the W content may be 0%. When W is contained, W forms a protective corrosion film in a sour environment and suppresses the penetration of hydrogen into the steel material. As a result, the SSC resistance of the steel material is improved. If even a small amount of W is contained, the above effect can be obtained to a certain extent. However, if the W content is too high, even if the contents of other elements are within the range of this embodiment, coarse carbides are generated in the steel material, and the low-temperature toughness and SSC resistance of the steel material are reduced. Therefore, the W content is 0 to 0.50%. The preferred lower limit of the W content is more than 0%, more preferably 0.01%, more preferably 0.02%, more preferably 0.03%, and more preferably 0.05%. The preferred upper limit of the W content is 0.45%, and more preferably 0.40%.

[0060] The chemical composition of the steel material described above may further contain one or more elements selected from the group consisting of Ni and Cu in place of a portion of Fe. All of these elements are optional elements and improve the hardenability of the steel material.

[0061] Ni: 0 to 0.20% Nickel (Ni) is an optional element and may not be contained. That is, the Ni content may be 0%. When contained, Ni improves the hardenability of the steel material and increases the strength of the steel material. Ni also dissolves in the steel material to improve the low-temperature toughness of the steel material. Even if even a small amount of Ni is contained, these effects can be obtained to a certain extent. However, if the Ni content is too high, even if the contents of other elements are within the range of this embodiment, local corrosion is promoted and the SSC resistance of the steel material decreases. Therefore, the Ni content is 0 to 0.20%. The preferred lower limit of the Ni content is more than 0%, more preferably 0.01%, and more preferably 0.02%. The preferred upper limit of the Ni content is 0.15%, more preferably 0.10%, more preferably 0.09%, more preferably 0.08%, and more preferably 0.06%.

[0062] Cu: 0-0.50% Copper (Cu) is an optional element and may not be contained. That is, the Cu content may be 0%. When contained, Cu enhances the hardenability of the steel material and enhances 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 range of this embodiment, the hardenability of the steel material becomes too high, and the SSC resistance of the steel material decreases. Therefore, the Cu content is 0 to 0.50%. The preferred lower limit of the Cu content is more than 0%, more preferably 0.01%, more preferably 0.02%, and more preferably 0.05%. The preferred upper limit of the Cu content is 0.35%, and more preferably 0.25%.

[0063] [Yield strength σ YS ] The yield strength σ of the steel material according to this embodiment YS is 758 MPa or more (110 ksi or more). The yield strength in this specification means the 0.2% offset proof stress obtained in a tensile test in accordance with ASTM E8 / E8M (2021). The upper limit of the yield strength of the steel material according to this embodiment is not particularly limited. However, it has been proven by the examples described later that the steel material according to this embodiment has excellent fracture toughness in a low-temperature sour environment at least in the range of yield strength of 758 to 965 MPa. Therefore, the yield strength of the steel material according to this embodiment includes at least 758 to 965 MPa (110 to 140 ksi). That is, the yield strength of the steel material according to this embodiment includes at least 758 to less than 862 MPa (110 ksi class) and 862 to 965 MPa (125 ksi class).

[0064] The yield strength of the steel material according to this embodiment can be determined by the following method. Specifically, a tensile test is performed by a method conforming to ASTM E8 / E8M (2021). A round bar test piece is prepared from the steel material according to this embodiment. When the steel material is a steel plate, a round bar test piece is prepared from the center of the plate thickness. In this case, the axial direction of the round bar test piece is parallel to the rolling direction of the steel plate. When the steel material is a steel pipe, a round bar test piece is prepared from the center of the wall thickness. In this case, the axial direction of the round bar test piece is parallel to the axial direction of the steel pipe. When the steel material is a round steel, a round bar test piece is prepared from the R / 2 position. In this case, the axial direction of the round bar test piece is parallel to the axial direction of the round steel. In this specification, the R / 2 position means the central position of the radius R in a cross section perpendicular to the axial direction of the round steel. The size of the round bar test piece is, for example, 4 mm in parallel part diameter and 16 mm in gauge length. A tensile test is carried out in air at room temperature (25°C) using a round bar test piece, and the obtained 0.2% offset proof stress is defined as the yield strength (MPa). In this embodiment, the yield strength (MPa) is calculated by rounding off the obtained value to the first decimal place.

[0065] [Grain boundary Mo content γ Mo ] The grain boundary Mo content γ of the steel material according to this embodiment Mo As described above, in this specification, the grain boundary Mo content γ Mo means the Mo content in the vicinity of the prior austenite grain boundary. In this embodiment, the Mo content γ Mo (Grain boundary Mo content γ Mo ) is defined as follows: Of the steel materials according to this embodiment, One end point of a 10 nm line segment SL perpendicular to the grain boundary of a prior austenite grain is defined as E1, and the other end point is defined as E2. At each measurement point arranged at a pitch of 0.2 nm from the end point E1 to the end point E2 of the line segment SL, the Mo content (mass%) at each measurement point on the line segment SL is determined by elemental analysis using energy dispersive X-ray spectroscopy; In the line segment SL, The measurement point on the line segment SL where the Mo content is maximum is defined as the Mo peak measurement point, The region having a width of 2.0 nm centered on the Mo peak measurement point is defined as the Mo-enriched region. A region between the end point E1 and the Mo-enriched region is defined as a first Mo intragranular region, A region between the end point E2 and the Mo-enriched region is defined as a second Mo intragranular region, The arithmetic average value of the Mo content at all the measurement points in the first Mo intragranular region is defined as γ Mo1 (% by mass), The arithmetic average value of the Mo content at all the measurement points in the second Mo intragranular region is defined as γ Mo2 (mass%), Grain boundary Mo content γ Mo is defined by the following equation (2). Gamma Mo = {(sum of Mo content at all the measurement points on the line segment SL × 0.2) - γ Mo1 × (the distance between the end point E1 and the Mo peak measurement point)−γ Mo2 × (the distance between the end point E2 and the Mo peak measurement point)} / 0.8+(γ Mo1 +γ Mo2 ) / twenty two)

[0066] More specifically, in this embodiment, the grain boundary Mo content γ Mo can be obtained by the following method. First, the grain boundary Mo content γ Mo A test piece for measurement is prepared. Specifically, a test piece is prepared in which the surface perpendicular to the C direction of the steel material is the observation surface. In other words, the observation surface of the test piece includes the L direction and the T direction. Here, when the steel material is a steel plate, the rolling direction is defined as the L direction, the plate thickness direction as the T direction, and the plate width direction as the C direction. When the steel material is a steel pipe, the pipe axial direction is defined as the L direction, the pipe radial direction (wall thickness direction) as the T direction, and the direction perpendicular to the L direction and the T direction as the C direction. When the steel material is a round bar, the axial direction is defined as the L direction, the radial direction (R direction) as the T direction, and the direction perpendicular to the L direction and the T direction as the C direction.

[0067] In addition, when preparing a test piece, a test piece including a specific position of the steel material in the center is prepared. At this time, the specific position of the steel material is the grain boundary Mo content γ Mo It means a position where it is possible to stably measure. Specifically, when the steel material is a steel plate, the plate thickness t / 4 position is defined as the specific position. In this specification, the plate thickness t / 4 position means the central position between the plate thickness central part and the steel plate surface in the plate thickness direction of the steel plate. When the steel material is a steel pipe, the wall thickness central part is defined as the specific position. When the steel material is a round bar, the R / 2 position is defined as the specific position. The size of the test piece is not particularly limited, and may be, for example, a test piece of 10 mm in the L direction × 5 mm in the C direction × 8 mm in the T direction.

[0068] The observation surface of the test piece is mechanically polished, and then chemically polished with a colloidal silica suspension. The polished observation surface is observed in an area including the specific position described above with an optical microscope. Specifically, FIG. 3 is a schematic diagram of the observation surface of the polished test piece for determining the grain boundary Mo amount. The L direction and T direction in FIG. 3 are defined as described above. Referring to FIG. 3, the observation target area 100 of the observation surface of the polished test piece is observed with an optical microscope at 200 times magnification to obtain an optical microscope image. Furthermore, as described above, the observation target area 100 includes the specific position described above. In the observation target area 100, an arbitrary rectangular measurement area 10 of 100 μm×100 μm is selected. At this time, the measurement area 10 is selected so as to include the specific position described above. Marks 20 are applied to multiple corners of the measurement area 10 so that the position of the selected measurement area 10 can be specified. The mark 20 may be, for example, an indentation made by a micro Vickers hardness tester.

[0069] Electron Backscatter Diffraction (EBSD) analysis is performed on the selected measurement area 10 to obtain crystal orientation information of the martensite phase. In the EBSD analysis, the step size is set to 0.1 μm. Based on the obtained crystal orientation information and the Kurdjumov-Sachs relationship, the grain boundaries (prior γ grain boundaries) of the prior γ grains (prior austenite grains) are identified.

[0070] Fig. 4 is a schematic diagram of a measurement region 10 in which a prior γ grain boundary is identified. With reference to Fig. 4, a prior γ grain boundary GB in which the crystal orientation difference between adjacent prior γ grains is 18° or more is selected from among the prior γ grain boundaries identified in the measurement region 10. Then, a thin film sample TP perpendicular to the selected prior γ grain boundary GB is prepared.

[0071] A method for preparing the thin film sample TP will be described in more detail. Fig. 5 is a schematic diagram showing how the thin film sample TP is prepared from the measurement region 10 shown in Fig. 4. As shown in Fig. 5, a plate-shaped thin film sample TP having an upper surface perpendicular to the selected prior γ grain boundary GB is collected from the measurement region 10 by focused ion beam (FIB) processing. As the FIB processing device, for example, a product name SMI3050SE manufactured by Hitachi High-Tech Science Corporation can be used.

[0072] The collected thin film sample TP is shown in FIG. 6. FIG. 6 is a perspective view of the thin film sample TP shown in FIG. 5. In the thin film sample TP, surface 10 corresponds to a part of the measurement region 10. Surface 30 is a surface perpendicular to surface 10. The prior γ grain boundary GB selected on surface 10 (measurement region 10) is naturally also observed on surface 30. The film thickness T10 of the thin film sample TP is 100 nm.

[0073] The prepared thin film sample TP is observed using a transmission electron microscope (TEM). In the TEM observation, a TEM equipped with an elemental analyzer (EDS analyzer) using energy dispersive X-ray spectrometry (EDS analyzer) is used. For example, a transmission electron microscope with a spherical aberration corrector manufactured by JEOL Ltd. (product name: NEOARM) can be used as the TEM. In addition, an EDS analyzer manufactured by JEOL Ltd. (product name: JED-2300T) can be used as the EDS analyzer attached to the TEM.

[0074] FIG. 7 is a schematic diagram for explaining the relationship between the electron beam and the prior γ grain boundary GB in the thin film sample TP during TEM observation and elemental analysis by EDS. As shown in FIG. 7, in TEM observation, the thin film sample TP is first positioned so that the electron beam EB is incident on the surface 30 of the thin film sample TP. Then, the thin film sample TP is tilted so that the prior γ grain boundary GB on the surface 10 is parallel to the incident direction of the electron beam EB. Then, as viewed from the incident direction of the electron beam EB, a line segment SL with a total length of 10 nm is identified that is centered on the prior γ grain boundary GB on the surface 30 and perpendicular to the prior γ grain boundary GB. Here, one end point of the line segment SL is defined as end point E1, and the other end point is defined as end point E2 as viewed from the incident direction of the electron beam EB. The distance between end point E1 and end point E2 is 10 nm.

[0075] Next, elemental analysis is performed by EDS at each measurement point arranged at a pitch of 0.2 nm from the end point E1 to the end point E2 of the line segment SL. The Mo content (mass%) at each measurement point on the line segment SL is determined.

[0076] FIG. 8 is a schematic diagram showing the relationship between the irradiation direction of the electron beam EB during TEM observation and elemental analysis by EDS, the prior γ grain boundary GB on the surface 10 of the thin film sample TP, and the element concentration profile. As shown in FIG. 8(A), when the electron beam EB is not parallel to the prior γ grain boundary GB on the surface 10 of the thin film sample TP, the peak of the element concentration profile PR on the line segment SL obtained by elemental analysis by EDS becomes broad. On the other hand, as shown in FIG. 8(B), when the electron beam EB is parallel to the prior γ grain boundary GB on the surface 10 of the thin film sample TP, the peak of the element concentration profile PR on the line segment SL becomes sharp. Therefore, when elemental analysis by EDS is performed, the thin film sample TP is tilted so that the electron beam EB is parallel to the prior γ grain boundary GB on the surface 10 of the thin film sample TP.

[0077] After determining the Mo content (mass%) at each measurement point on the line segment SL by elemental analysis using EDS, the grain boundary Mo content γ Mo Fig. 9 shows an example of the Mo content on a line segment SL that is centered on the prior γ grain boundary GB and perpendicular to the prior γ grain boundary GB. With reference to Fig. 9, the following definitions are given. The measurement point on line segment SL where the Mo content is maximum is defined as Mo peak measurement point P. The 2.0 nm wide region centered on the Mo peak measurement point P is defined as the Mo-enriched region GB0. The region between the end point E1 of the line segment SL and the Mo-enriched region GB0 is defined as the first Mo grain intragranular region BM1. The region between the end point E2 of the line segment SL and the Mo-enriched region GB0 is defined as the second Mo intragranular region BM2. The arithmetic mean value of the Mo content at all measurement points in the first Mo grain region BM1 was calculated as γ Mo1 (% by mass). The arithmetic mean value of the Mo content at all measurement points in the second Mo intragranular region BM2 was calculated as γ Mo2 (% by mass).

[0078] Referring to FIG. 9, the γ Mo1 (mass%) and γ in the second Mo grain region BM2 Mo2 (mass%) is regarded as the Mo content in the grains of adjacent prior γ grains across the prior γ grain boundary GB. Mo1 (mass%) and γ Mo2 In some cases, the average Mo content (γ Mo1 , γ Mo2 ) is required.

[0079] Mo content at each measurement point on the line segment SL, γ Mo1 , γ Mo2 Using the above, the grain boundary Mo content γ Mo is defined as equation (2). Gamma Mo = {(sum of Mo content at all measurement points on line segment SL × 0.2) - γ Mo1 × (distance between end point E1 and Mo peak measurement point P)-γ Mo2 × (distance between end point E2 and Mo peak measurement point P)} / 0.8+(γ Mo1 +γ Mo2 ) / twenty two)

[0080] Here, equation (2) is divided as follows: A = (sum of Mo content at all measurement points on line segment SL × 0.2) B=γ Mo1 × (distance between end point E1 and Mo peak measurement point P) C=γ Mo2 × (distance between end point E2 and Mo peak measurement point P) D = (γ Mo1 +γ Mo2 ) / 2

[0081] A corresponds to the total area of ​​the Mo content distribution in FIG. 9, that is, the total amount of Mo content on the line segment SL. The "0.2" in A means 0.2 nm, which is the pitch of the measurement points. B corresponds to the total amount of Mo content in the region BM10 between the end point E1 and the Mo peak measurement point P on the line segment SL. The average Mo content in the region BM10 is γ Mo1 Region BM10 corresponds to an ideal intragranular region of a prior γ grain when the Mo peak measurement point P is assumed to be an ideal prior γ grain boundary.

[0082] C corresponds to the total amount of Mo content in the region BM20 between the end point E2 and the Mo peak measurement point P of the line segment SL. Mo2 (% by mass) is used. Region BM20 corresponds to the ideal intragranular region of a prior γ grain when Mo peak measurement point P is assumed to be an ideal prior γ grain boundary. FIG. 10 is a schematic diagram for explaining formula (2). As shown in FIG. 10, (ABC) in formula (2) is a value obtained by subtracting the total Mo content in ideal intragranular regions BM10 and BM20 adjacent to each other across the ideal prior γ grain boundary GB from the total Mo content on line segment SL. That is, (ABC) in formula (2) corresponds to the difference ΔMo in Mo content in the Mo-enriched region GB0 in FIG. 10.

[0083] As described above, ideally, the prior γ grain boundary GB perpendicular to the line segment SL does not exist as a wide range (line) on the line segment SL like the Mo-enriched region GB0, but exists in an extremely narrow range centered on the Mo peak measurement point P. Therefore, in this specification, the 0.8 nm-wide region on the line segment SL centered on the Mo peak measurement point P is assumed to be the prior γ grain boundary GB region.

[0084] As described above, it is assumed that on the line segment SL, the ideal prior γ grain boundary GB is located in a 0.8 nm wide region centered on the Mo peak measurement point P, and that ideal intragranular regions BM10 and BM20 exist in other regions as described above. In this case, it is assumed that the ideal distribution of Mo content on the line segment SL is such that the Mo content is maximum at the ideal prior γ grain boundary GB, and that the Mo content is constant at the average Mo content in the ideal intragranular regions BM10 and BM20. The above-mentioned D corresponds to the average Mo content in the ideal intragranular regions.

[0085] Under the above assumptions, the value obtained by dividing ΔMo (=ABC) by 0.8 plus the average value D of the Mo content in the ideal intragranular regions BM10 and BM20, i.e., formula (2), is considered to be the Mo content (mass%) in the vicinity of the prior γ grain boundary GB. The grain boundary Mo content γ, defined by formula (2) obtained by the above method, Mo is an index of the Mo content in the vicinity of the prior γ grain boundary GB. In this embodiment, the grain boundary Mo content (mass%) is calculated by rounding off the obtained numerical value to one decimal place.

[0086] In the steel material according to the present embodiment, the grain boundary Mo content defined as above is 5.0 mass% or more. If the grain boundary Mo content is too low, the prior γ grain boundaries cannot be sufficiently strengthened, and the above-mentioned chemical composition and yield strength σ YS In a steel material having the above structure, the fracture toughness in a low-temperature sour environment cannot be sufficiently increased. On the other hand, if the grain boundary Mo content is 5.0 mass% or more, the fracture toughness in a low-temperature sour environment can be increased, provided that the other configurations of this embodiment are satisfied.

[0087] Therefore, in this embodiment, the above-mentioned chemical composition and the yield strength σ of 758 MPa or more are used. YS On the premise that the grain boundary Mo content is 5.0 mass% or more. A preferable lower limit of the grain boundary Mo content is 5.5 mass%, more preferably 6.0 mass%, more preferably 6.5 mass%, more preferably 7.0 mass%, more preferably 7.3 mass%, more preferably 7.5 mass%, and more preferably 7.7 mass%. The upper limit of the grain boundary Mo content is not particularly limited, but may be 30.0 mass%, 25.0 mass%, or 20.0 mass%.

[0088] [Fn1] The steel material according to the present embodiment has a yield strength σ YS When the value of Fn1 defined by the formula (1) is less than 862 MPa, the yield strength σ YS is 862 MPa or more, Fn1 defined by formula (1) satisfies −520 or more. Fn1 = 83 × (γ Mo -10 -7 ×√ρ)-σ YS (1) Here, γ in equation (1) Mo is substituted with the Mo content in the vicinity of the prior austenite grain boundary in mass%, and ρ in equation (1) is the dislocation density m -2 and σ in Eq. (1) YS is substituted for the yield strength in MPa.

[0089] Fn1(=83×(γ Mo -10 -7 ×√ρ)-σ YS ) is an index of fracture toughness in a low-temperature sour environment. The above chemical composition, yield strength σYS, and grain boundary Mo content γ Mo For steels with and, the yield strength σ YS When Fn1 is less than 862MPa, if Fn1 is -300 or more, it has excellent fracture toughness even in a low-temperature sour environment. Mo In addition, the yield strength σ YSWhen Fn1 is 862 MPa or more and Fn2 is -520 or more, the material has excellent fracture toughness even in a low-temperature sour environment.

[0090] Yield strength σ YS When the yield strength σ is less than 862 MPa, the lower limit of Fn1 is preferably −290, more preferably −280, and still more preferably −270. YS When the yield strength σ is less than 862 MPa, the upper limit of Fn1 is not particularly limited, but is, for example, 1606. YS When the yield strength σ is 862 MPa or more, the lower limit of Fn1 is preferably −515, more preferably −510, even more preferably −505, and even more preferably −500. YS When the compressive strength is 862 MPa or more, the upper limit of Fn1 is not particularly limited, but is 1408, for example.

[0091] In addition, the dislocation density ρ in Fn1 is not particularly limited as long as Fn1 satisfies a predetermined value. YS When the strain rate is less than 862 MPa, the dislocation density ρ is, for example, 2.3 to 7.0 × 10 14 m -2 In this case, the preferred upper limit of the dislocation density ρ is 6.9×10 14 m -2 and more preferably 6.8×10 14 m -2 In this case, the preferred lower limit of the dislocation density ρ is 2.5×10 14 m -2 More preferably, it is greater than 2.7×10 14 m -2 More preferably, it is greater than 2.8×10 14 m -2 and more preferably 3.0×10 14 m -2 In the steel material having the above chemical composition, the yield strength σ YS When the strain rate is 862 MPa or more, the dislocation density ρ is, for example, 7.0 to 12.0 × 10 14 m -2 In this case, the preferred upper limit of the dislocation density ρ is 11.9×1014 m -2 and more preferably 11.5×10 14 m -2 and more preferably 11.2×10 14 m -2 and more preferably 11.0×10 14 m -2 It is.

[0092] The dislocation density ρ of the steel material according to this embodiment can be obtained by the old Williamson-Hall method. Specifically, a test piece for measuring the dislocation density is prepared from the steel material according to this embodiment. When the steel material is a steel plate, the test piece is prepared from the center of the plate thickness. When the steel material is a steel pipe, the test piece is prepared from the center of the wall thickness. When the steel material is a round steel, the test piece is prepared from the R / 2 position. The size of the test piece is, for example, 20 mm wide x 20 mm long x 2 mm thick. The thickness direction of the test piece is the thickness direction of the steel material (T direction: plate thickness direction, pipe diameter direction or diameter direction). In this case, the observation surface of the test piece is a surface of 20 mm wide x 20 mm long. The observation surface of the test piece is mirror polished, and further electrolytic polishing is performed using 10% by volume of perchloric acid (acetic acid solvent) to remove distortion of the surface layer. For the surface observed after electrolytic polishing, the half-width ΔK of the peaks for the (110), (211), and (220) planes of the body-centered cubic structure (iron) is obtained by X-ray diffraction (XRD).

[0093] In XRD, the half-width ΔK is measured with a CoKα ray source, a tube voltage of 30 kV, and a tube current of 100 mA. Furthermore, LaB6 (lanthanum hexaboride) powder is used to measure the half-width derived from the X-ray diffraction device.

[0094] The non-uniform strain ε of the test piece is calculated from the half-width ΔK calculated by the above-mentioned method and the Williamson-Hall equation (Equation (3)). ΔK×cosθ / λ=0.9 / D+2ε×sinθ / λ (3) In the formula (3), θ is the diffraction angle, λ is the wavelength of the X-ray, and D is the crystallite diameter.

[0095] Furthermore, the dislocation density ρ(m -2 ) can be obtained. ρ=14.4×ε 2 / b 2 (4) Here, in formula (4), b is the Burgers vector of the body-centered cubic structure (iron) (b=0.248 (nm)).

[0096] [Fracture toughness in low-temperature sour environments] The steel material according to the present embodiment has the above-mentioned chemical composition and a yield strength σ YS The grain boundary Mo content γ Mo is 5.0 mass% or more, and the yield strength σ YS When Fn1 is less than 862MPa, Fn1 is -300 or more, and the yield strength σ YS When Fn1 is 862 MPa or more, Fn2 satisfies -520 or more. As a result, the steel material according to this embodiment has a yield strength of 110 ksi or more and excellent fracture toughness in a low-temperature sour environment. In this embodiment, the excellent fracture toughness in a low-temperature sour environment is evaluated by a DCB test in accordance with NACE TM0177-2016 Method D. Specifically, in this embodiment, having excellent fracture toughness in a low-temperature sour environment is defined as follows.

[0097] A DCB test piece shown in FIG. 11 and a wedge shown in FIG. 12 are prepared from the steel material according to this embodiment. When the steel material is a steel plate, the DCB test piece and the wedge are prepared from the center of the plate thickness. When the steel material is a steel pipe, the DCB test piece and the wedge are prepared from the center of the wall thickness. When the steel material is a round bar, the DCB test piece and the wedge are prepared from the R / 2 position. The longitudinal direction of the DCB test piece (left-right direction in the figure) is parallel to the L direction of the steel material (rolling direction, tube axial direction or axial direction). The thickness t1 of the wedge is 3.10 mm.

[0098] Referring to FIG. 11, the prepared wedge is driven between the arms of the DCB test piece. The DCB test piece with the wedge driven in is sealed in a test vessel. The test solution is poured into the test vessel leaving the gas phase to form a test bath. The test solution is a mixed aqueous solution of 5.0 mass% sodium chloride and 0.4 mass% sodium acetate, adjusted to pH 3.5 with acetic acid (NACE solution B). The amount of the test bath is 1 L per test piece. Next, N2 gas is blown into the test bath for 3 hours to degas it until the dissolved oxygen in the test bath is 20 ppb or less.

[0099] A mixture of 1% H2S gas and 99% CO2 gas is blown into the degassed test bath at a total pressure of 1 atm to create a corrosive environment in the test bath. The test bath is stirred while the test vessel is kept at 4°C for 14 days (336 hours). After this time, the DCB test piece is removed from the test vessel.

[0100] A pin is inserted into the hole formed at the end of the arm of the removed DCB test piece, and the notch is opened with a tensile testing machine to measure the wedge release stress P. Furthermore, the notch of the DCB test piece is released in liquid nitrogen, and the crack propagation length a of the DCB test piece while immersed in the test bath is measured. The crack propagation length a can be measured visually using a caliper. Based on the measured wedge release stress P and crack propagation length a, the fracture toughness value K is calculated using equation (5). 1SSC The value (MPa√m) is calculated. In formula (5), h (mm) is the height of each arm of the DCB test specimen, B (mm) is the thickness of the DCB test specimen, and Bn (mm) is the web thickness of the DCB test specimen. These are specified in NACE TM0177-2016 Method D.

[0101]

number

[0102] In this embodiment, the yield strength σ YS When the temperature is less than 862MPa, the K obtained as a result of the DCB test performed under the above conditions 1SSCIf the value is 29.5 MPa√m or more, it is determined that the steel has excellent fracture toughness in a low-temperature sour environment. YS When the temperature is 862 MPa or more, the K obtained as a result of the DCB test performed under the above conditions 1SSC If the value is 24.1 MPa√m or more, it is determined that the steel has excellent fracture toughness in a low-temperature sour environment.

[0103] [Microstructure] The microstructure of the steel material according to this embodiment has a total volume fraction of tempered martensite and tempered bainite of 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 758 MPa (110 ksi) or more, and the steel material exhibits excellent fracture toughness in a low-temperature sour environment, provided that other configurations of this embodiment are satisfied. That is, in this embodiment, if the steel material has a yield strength of 758 MPa (110 ksi) or more and excellent fracture toughness in a low-temperature sour environment, the microstructure is determined to have a total volume fraction of tempered martensite and tempered bainite of 90% or more.

[0104] When the volume fractions of tempered martensite and tempered bainite are determined by observation, they can be determined by the following method. First, a test piece having an observation surface is prepared from the steel material according to this embodiment. When the steel material is a steel plate, a test piece is prepared from the center of the plate thickness, with a surface including the rolling direction and the plate thickness direction as the observation surface. When the steel material is a steel pipe, a test piece is prepared from the center of the plate thickness, with a surface including the pipe axial direction and the pipe radial direction as the observation surface. When the steel material is a round bar, a test piece is prepared that includes the R / 2 position in the center and has a surface including the axial direction and the radial direction as the observation surface.

[0105] The observation surface of the test piece is polished to a mirror finish, and then immersed in a nital etching solution for about 10 seconds to reveal the structure by etching. The etched observation surface is observed in 10 fields of view using a scanning electron microscope (SEM) in secondary electron images. The field area is, for example, 0.01 mm 2 (magnification 1000x). In each field of view, tempered martensite and tempered bainite are identified from the contrast. The area ratios of the identified tempered martensite and tempered bainite are determined. The method for determining the area ratios is not particularly limited, and any known method may be used. For example, the area ratios of tempered martensite and tempered bainite can be determined by image analysis. In this embodiment, the arithmetic average value of the area ratios of tempered martensite and tempered bainite determined in all fields of view is defined as the volume ratios of tempered martensite and tempered bainite.

[0106] [Steel shape] As described above, 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, and a round bar. When the steel material is a steel pipe for oil wells, the preferred wall thickness is 9 to 60 mm. More preferably, the steel material according to this embodiment is a seamless steel pipe. When the steel material according to this embodiment is a seamless steel pipe, even a thick seamless steel pipe having a wall thickness of 15 mm or more has a yield strength of 110 ksi or more and excellent fracture toughness in a low-temperature sour environment.

[0107] [Manufacturing method] A method for manufacturing a steel material according to this embodiment will be described below. A method for manufacturing a seamless steel pipe will be described below as an example of the steel material according to this embodiment. The method for manufacturing a seamless steel pipe includes a step of preparing a mother pipe (preparation step), and a step of quenching and tempering the mother pipe to produce a seamless steel pipe (quenching step and tempering step). Note that the manufacturing method according to this embodiment is not limited to the manufacturing method described below. Each step will be described in detail below.

[0108] [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-mentioned chemical composition, the manufacturing method of the intermediate steel material is not particularly limited. The intermediate steel material referred to here is a plate-shaped steel material when the final product is a steel plate, a blank pipe when the final product is a steel pipe, and a steel material having a circular cross section perpendicular to the axial direction when the final product is a round steel.

[0109] 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, the case including the material preparation step and the hot working step will be described in detail.

[0110] [Material preparation process] In the material preparation step, a material is manufactured using molten steel having the above-mentioned chemical composition. The method for manufacturing the material is not particularly limited and may be a well-known method. Specifically, a cast piece (slab, bloom, or billet) may be manufactured using the molten steel by a continuous casting method. An ingot may be manufactured using the molten steel by an ingot casting method. If necessary, the slab, bloom, or ingot may be rolled to manufacture a billet. The material (slab, bloom, or billet) is manufactured by the above steps.

[0111] [Hot processing process] In the hot working process, 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 a mother 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. The billet extracted from the heating furnace is hot worked to produce a mother pipe (seamless steel pipe). The method of hot working is not particularly limited, and may be a well-known method.

[0112] For example, the Mannesmann process may be carried out as the hot working to produce a mother pipe. In this case, a round billet is pierced and rolled using a piercing machine. When piercing and rolling is performed, the piercing ratio is not particularly limited, but is, for example, 1.0 to 4.0. The pierced and rolled round billet is further hot rolled using a mandrel mill, a reducer, a sizing mill, or the like to produce a mother pipe. The cumulative reduction in area in the hot working process is, for example, 20 to 70%.

[0113] Other hot working methods may be used to produce a mother pipe from the billet. For example, in the case of a short, thick-walled steel material such as a coupling, the mother pipe may be produced by forging using the Erhardt method or the like. The mother pipe is produced by the above steps. The thickness of the mother pipe is not particularly limited, but is, for example, 9 to 60 mm.

[0114] When the steel material is round steel, 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 material extracted from the heating furnace is subjected to hot processing to produce an intermediate steel material having a circular cross section perpendicular to the axial direction. The hot processing is, for example, blooming rolling by a blooming mill, or hot rolling by a continuous rolling mill. In the continuous rolling mill, a horizontal stand having a pair of grooved rolls arranged side by side in the vertical direction and a vertical stand having a pair of grooved rolls arranged side by side in the horizontal direction are alternately arranged.

[0115] 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 material removed from the heating furnace is subjected to hot rolling using a blooming mill and a continuous rolling mill to produce an intermediate steel material in the shape of a steel plate.

[0116] The mother pipe produced by hot working may be air-cooled (as-rolled). The mother pipe produced 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. When quenching is performed directly after hot working or after reheating, cooling may be stopped or slow cooling may be performed during quenching. In this case, it is possible to suppress the occurrence of quench cracks in the mother pipe. When quenching is performed directly after hot working or after reheating, stress relief annealing (SR) may be performed after quenching and before the heat treatment in the next step. In this case, residual stress in the mother pipe is removed.

[0117] As described above, in the preparation step, an intermediate steel material is prepared. The intermediate steel material may be manufactured by the above-mentioned preferred step, or an intermediate steel material manufactured by a third party, or an intermediate steel material manufactured in a factory or business establishment other than the factory where the quenching step and tempering step described below are performed, may be prepared. The quenching step will be described in detail below.

[0118] [Quenching process] In the quenching process, quenching is performed on the prepared intermediate steel material (blank pipe). In this specification, "quenching" means rapidly cooling the intermediate steel material at the A3 point or higher. The quenching temperature is preferably 800 to 1000°C. If the quenching temperature is too high, the prior γ grains may become coarse, and the SSC resistance of the steel material may decrease. Therefore, the quenching temperature is preferably 800 to 1000°C.

[0119] In this specification, the quenching temperature corresponds to the surface temperature of the intermediate steel material measured by a thermometer installed at the outlet side of the device performing the final hot working when quenching is performed directly after hot working. Furthermore, the quenching temperature corresponds to the temperature of the furnace where quenching or reheating is performed when quenching is performed after reheating or reheating after hot working.

[0120] The quenching method is, for example, to continuously cool the intermediate steel material (mother pipe) from the quenching start temperature to continuously lower the surface temperature of the mother pipe. The method of continuous cooling is not particularly limited and may be a well-known method. For example, the method of continuous cooling is a method of immersing the mother pipe in a water tank for cooling, or a method of accelerating cooling the mother pipe by shower water cooling or mist cooling.

[0121] If the cooling rate during quenching is too slow, the microstructure may not be mainly composed of tempered martensite and tempered bainite. In this case, the mechanical properties (yield strength of 125 ksi or more) specified in this embodiment are not obtained. Furthermore, excellent low-temperature toughness and excellent SSC resistance are not obtained.

[0122] Therefore, as described above, in the steel manufacturing method according to the present embodiment, the intermediate steel is quenched during quenching. Specifically, in the quenching step, the average cooling rate in the range of the surface temperature of the intermediate steel (blank pipe) during quenching from 800 to 500°C is defined as the quenching cooling rate CR. 800-500 More specifically, the cooling rate during quenching, CR 800-500 is determined from the temperature measured at the slowest cooling location in the cross section of the intermediate steel being quenched (e.g., the center of the intermediate steel thickness when both surfaces are forced cooled).

[0123] Preferred cooling rate during quenching: CR 800-500 The cooling rate during quenching is preferably 300°C / min or more. 800-500 The lower limit of the cooling rate during quenching is 450° C. / min, and more preferably 600° C. / min. 800-500 The upper limit is not particularly specified, but is, for example, 60,000° C. / min.

[0124] Preferably, the mother tube is heated in the austenite region multiple times and then quenched. In this case, the austenite grains before quenching are refined, and the SSC resistance of the steel material is improved. By performing quenching multiple times, heating in the austenite region may be repeated multiple times, or by performing normalizing and quenching, heating in the austenite region may be repeated multiple times. Furthermore, quenching and tempering, which will be described later, may be combined and performed multiple times. That is, quenching and tempering may be performed multiple times. In this case, the SSC resistance of the steel material is further improved. The tempering process will be described in detail below.

[0125] [Tempering process] In the tempering process, the above-mentioned quenching is performed, and then tempering is performed. In this specification, "tempering" refers to the process of tempering the intermediate steel material after quenching to A. c1 The term "holding temperature" refers to the temperature of the furnace when the intermediate steel material is heated and held after quenching. The term "holding time" refers to the time from when the temperature of the intermediate steel material reaches the predetermined holding temperature to when the intermediate steel material is extracted from the heat treatment furnace.

[0126] When manufacturing steel materials for oil well applications, the holding temperature has been set to 630 to 730°C in order to improve SSC resistance and the like. On the other hand, as a result of detailed studies by the present inventors, it has become clear that Mo can be selectively enriched in the prior γ grain boundaries by performing tempering at 450 to 600°C, which is lower than the temperature usually used. Therefore, in the tempering process according to the present embodiment, tempering is performed at 450 to 600°C. On the other hand, if tempering is simply performed at 450 to 600°C, the dislocation density ρ may become too high, and Fn1 may become too low.

[0127] Therefore, in the tempering process according to the present embodiment, tempering is performed at 450 to 600°C for a certain holding time or more, and the diffusion distance during the entire tempering is increased. Specifically, in the present embodiment, the diffusion distance √Dt during tempering is defined by the following formula (6).

[0128]

number

[0129] Furthermore, s in equation (6) 2 is defined by the following equation (7).

[0130]

number

[0131] More specifically, in this embodiment, the diffusion distance √Dt L In this embodiment, the diffusion distance √Dt L is 1.3 nm or more, and the diffusion length of the entire tempering √Dt To It is preferable that the diffusion distance √Dt is 60.0 nm or more. L and √Dt To As long as the above range is satisfied, the tempering process is not particularly limited. For example, a so-called two-stage tempering may be performed in which tempering at a high temperature is performed and then tempering at 450 to 600°C is performed. For example, only tempering at 450 to 600°C may be performed.

[0132] Hereinafter, each step will be described for the case where tempering at 450 to 600° C. (hereinafter also referred to as "medium temperature tempering") is carried out after tempering at high temperature (high temperature tempering step).

[0133] [High temperature tempering process] When the high-temperature tempering process is carried out, the quenched intermediate steel material (blank pipe) is heated from room temperature to a holding temperature, and then held at the holding temperature for a holding time in the high-temperature tempering process. In the high-temperature tempering process, the holding temperature is preferably 640 to 740°C. A more preferable lower limit of the holding temperature in the high-temperature tempering process is 645°C, and more preferably 650°C. A more preferable upper limit of the holding temperature in the high-temperature tempering process is 730°C. In the high-temperature tempering process according to this embodiment, the holding time is preferably 5 to 120 minutes. A more preferable upper limit of the holding time in the high-temperature tempering process is 100 minutes. A more preferable lower limit of the holding time in the high-temperature tempering process is 10 minutes, and more preferably 20 minutes.

[0134] In this embodiment, when the high-temperature tempering process is performed, the diffusion distance in the high-temperature tempering process is √Dt H Define it as √Dt H can be calculated using the holding temperature (°C) and holding time (seconds) in the high-temperature tempering process and the above formulas (6) and (7). H is not particularly limited.

[0135] [Medium temperature tempering process] In the intermediate temperature tempering process, the intermediate steel material (blank pipe) is held at 450 to 600°C. In the intermediate temperature tempering process, Mo is concentrated at the prior γ grain boundaries by holding at 450 to 600°C. In this case, the holding time is preferably 100 minutes or more. If the holding time in the intermediate temperature tempering process is too short, the grain boundary Mo amount may not be sufficiently increased in the manufactured steel material. The upper limit of the holding time is not particularly limited, but may be, for example, 8333 hours.

[0136] In the intermediate temperature tempering process according to the present embodiment, the diffusion distance √Dt L It is preferable to set the diffusion length √Dt to 1.3 nm or more. L If the diffusion length √Dt is too low, the grain boundary Mo content may not be increased sufficiently in the produced steel material. LThe preferred lower limit of the diffusion length √Dt is 1.3 nm or more. L The upper limit is not particularly limited, but is, for example, 250.0 nm.

[0137] When tempering is performed in two stages, the diffusion distance throughout the tempering process is √Dt To is the diffusion distance √Dt in the high-temperature tempering process H and the diffusion distance √Dt in the intermediate temperature tempering process L Specifically, it can be calculated using the following formula (8).

[0138]

number

[0139] As described above, the tempering process according to the present embodiment may be performed only with the intermediate temperature tempering process. In this case, the diffusion distance √Dt To is the diffusion distance √Dt during the intermediate temperature tempering process L The tempering process according to the present embodiment may be performed in three or more stages. Even in this case, the diffusion length √Dt L is 1.3 nm or more, and the diffusion distance of the entire tempering √Dt To It is preferable to set the thickness to 60.0 nm or more.

[0140] In the tempering process, the holding temperature and holding time are appropriately adjusted to obtain a steel material having a yield strength of 758 MPa or more. It is fully possible for a person skilled in the art to obtain a yield strength of 758 MPa or more by performing tempering on an intermediate steel material (blank pipe) having the chemical composition of this embodiment with appropriately adjusted holding temperature and holding time.

[0141] According to the above manufacturing method, the steel material according to the present embodiment can be manufactured. In the above manufacturing method, a method for manufacturing a seamless steel pipe has been described as an example. However, the steel material according to the present embodiment may be a steel plate or other shape. Similarly to the above manufacturing method, the manufacturing method for a steel plate or other shape also includes, for example, a preparation step, a quenching step, and a tempering step. However, the above manufacturing method is only an example, and the steel material may be manufactured by other manufacturing methods. Hereinafter, the present disclosure will be described more specifically with reference to examples. EXAMPLES

[0142] In Example 1, steel materials having a yield strength of less than 862 MPa were investigated. Specifically, 180 kg of molten steel having the chemical composition shown in Table 1-1 and Table 1-2 was produced. In Table 1-2, "-" means that the content of each element is at the impurity level. Specifically, the Co content, W content, Ni content, and Cu content of Test No. 1-1 are rounded off to the second decimal place and are 0%. The Nb content of Test No. 1-1 is rounded off to the fourth decimal place and are 0%. Furthermore, the Ca content, Mg content, Zr content, and REM content of Test No. 1-1 are rounded off to the fifth decimal place and are 0%.

[0143] [Table 1-1]

[0144] [Table 1-2]

[0145] Ingots were produced using the molten steel of test numbers 1-1 to 1-26. The produced ingots were hot rolled to produce steel plates with a plate thickness of 15 mm. The steel plates of test numbers 1-1 to 1-26 after hot rolling were allowed to cool to room temperature (25°C). The steel plates of test numbers 1-1 to 1-26 after cooling were held at the quenching temperature (920°C) for 20 minutes, and then quenched by immersing the steel plates in a water tank. At this time, the cooling rate during quenching (CR800-500 ) was 600℃ / min. The quenching temperature and the cooling rate CR during quenching were measured using a sheathed K thermocouple inserted in the center of the thickness of the steel plate. 800-500 was measured.

[0146] After quenching, the steel plates of test numbers 1-1 to 1-26 were tempered. In this example, the tempering at high temperature is also called the first tempering, and the tempering at medium temperature is also called the second tempering. The holding temperature (°C), holding time (min), and diffusion distance (√Dt H (nm) or √Dt L (nm)) are shown in Table 2. Furthermore, the diffusion distance √Dt H , √Dt L , and the diffusion distance throughout the tempering process, √Dt, calculated from equation (8) To The thickness (nm) of the specimen is shown in Table 2. In Table 2, "-" in the "First tempering" column means that the first tempering was not performed. Similarly, in Table 2, "-" in the "Second tempering" column means that the second tempering was not performed.

[0147] [Table 2]

[0148] Here, the tempering holding temperature in this example was the temperature of the heat treatment furnace in which tempering was performed, and further, the tempering holding time in this example was the time from when the temperature of the steel plate of each test number reached a predetermined tempering temperature until it was extracted from the heat treatment furnace.

[0149] [Evaluation test] The tempered steel plates of test numbers 1-1 to 1-26 were subjected to a tensile test, a dislocation density measurement test, a grain boundary Mo amount measurement test, and a DCB test, which will be described below.

[0150] [Tensile test] Tensile tests were conducted on the steel plates of test numbers 1-1 to 1-26 by the method described above. Specifically, round bar tensile test pieces with a parallel part diameter of 4 mm and a gauge length of 16 mm were prepared from the center of the plate thickness of the steel plates of test numbers 1-1 to 1-26. The axial direction of the round bar tensile test pieces was parallel to the rolling direction of the steel plates. Using the round bar test pieces of test numbers 1-1 to 1-26, tensile tests were conducted in accordance with ASTM E8 / E8M (2021) at room temperature (25°C) in air to obtain the yield strength (MPa) of the steel plates of test numbers 1-1 to 1-26. The obtained yield strengths were referred to as "yield strength σ YS The values ​​are shown in Table 3 as "(MPa)".

[0151] [Table 3]

[0152] [Dislocation density measurement test] A dislocation density measurement test was carried out for the steel plates of test numbers 1-1 to 1-26 by the method described above. Specifically, test pieces of 20 mm width × 20 mm length × 2 mm thickness were prepared from the center of the plate thickness of the steel plates of test numbers 1-1 to 1-26. The half-widths ΔK of the peaks of the (110), (211), and (220) planes were obtained by the X-ray diffraction method by the method described above. Using the obtained half-widths ΔK, the dislocation densities ρ(m -2 The obtained dislocation density was defined as the dislocation density ρ(10 14 m -2 )" in Table 3.

[0153] [Grain boundary Mo content measurement test] For the steel plates of test numbers 1-1 to 1-26, a grain boundary Mo content measurement test was carried out by the above-mentioned method. Specifically, test pieces were prepared that included the thickness t / 4 position of the steel plates of test numbers 1-1 to 1-26 in the center and had a length of 10 mm in the rolling direction, a width of 5 mm, and a thickness of 8 mm. The grain boundary Mo content (mass%) was obtained by the above-mentioned method using the prepared test pieces. The obtained grain boundary Mo content was referred to as "grain boundary Mo content γ Mo The yield strengths obtained are shown in Table 3 as "(mass%)". YS (MPa) and dislocation density ρ(m -2 ) and grain boundary Mo content γMo Fn1 was obtained using (mass%) and formula (1). The obtained Fn1 is shown in Table 3.

[0154] [DCB Test] The DCB test was carried out on the steel plates of test numbers 1-1 to 1-26 by the above-mentioned method. Specifically, the above-mentioned DCB test pieces and wedges were prepared from the center of the plate thickness of the steel plates of test numbers 1-1 to 1-26. Using the prepared test pieces and wedges, a DCB test was carried out in accordance with NACE TM0177-2016 Method D under the above-mentioned conditions. The fracture toughness values ​​K obtained by the DCB test carried out by the above-mentioned method were 1SSC The value is "K 1SSC The values ​​are shown in Table 3 as "(MPa√m)".

[0155] [Evaluation Results] With reference to Tables 1-1, 1-2, 2, and 3, the chemical compositions of the steel plates of test numbers 1-1 to 1-14 were appropriate, and the yield strength was 758 MPa or more (110 ksi or more). Mo was 5.0 mass% or more, and Fn1 was -300 or more. 1SSC The values ​​were 29.5 MPa or more. In other words, these steel plates had a yield strength of 110 ksi or more and excellent fracture toughness in a low-temperature sour environment.

[0156] On the other hand, the steel plates of test numbers 1-15 and 1-16 were not subjected to the intermediate temperature tempering process. As a result, the Fn1 of these steel plates was less than -300. 1SSC The values ​​were less than 29.5 MPa. In other words, these steel plates did not have excellent fracture toughness in a low-temperature sour environment.

[0157] The steel plate of test number 1-17 had a too high holding temperature and too short holding time in the intermediate temperature tempering process. As a result, the Fn1 of this steel plate was less than -300. 1SSC The value was less than 29.5 MPa. In other words, this steel plate did not have excellent fracture toughness in a low-temperature sour environment.

[0158] The steel plates of test numbers 1-18 and 1-19 had too short a holding time in the intermediate temperature tempering process. As a result, the Fn1 of these steel plates was less than -300. 1SSC The values ​​were less than 29.5 MPa. In other words, these steel plates did not have excellent fracture toughness in a low-temperature sour environment.

[0159] The steel plate of test number 1-20 had a diffusion distance √Dt L As a result, the Fn1 of this steel plate was less than -300. 1SSC The value was less than 29.5 MPa. In other words, this steel plate did not have excellent fracture toughness in a low-temperature sour environment.

[0160] The steel plate of test number 1-21 has a diffusion distance √Dt L As a result, the Fn1 of this steel plate was less than -300. 1SSC The value was less than 29.5 MPa. In other words, this steel plate did not have excellent fracture toughness in a low-temperature sour environment.

[0161] The steel plate of test number 1-22 had too low a Cr content. 1SSC The value was less than 29.5 MPa. In other words, this steel plate did not have excellent fracture toughness in a low-temperature sour environment.

[0162] The steel sheet of test number 1-23 had a too low Mo content. As a result, the grain boundary Mo content of this steel sheet was Mo The content of K in the steel sheet was less than 5.0 mass%, and Fn1 was less than -300. 1SSC The value was less than 29.5 MPa. In other words, this steel plate did not have excellent fracture toughness in a low-temperature sour environment.

[0163] The steel plate with test number 1-24 had too high a Mn content. 1SSCThe value was less than 29.5 MPa. In other words, this steel plate did not have excellent fracture toughness in a low-temperature sour environment.

[0164] The steel plate with test number 1-25 had too high a N content. As a result, this steel plate had 1SSC The value was less than 29.5 MPa. In other words, this steel plate did not have excellent fracture toughness in a low-temperature sour environment.

[0165] The steel plate of test number 1-26 had too high a P content. 1SSC The value was less than 29.5 MPa. In other words, this steel plate did not have excellent fracture toughness in a low-temperature sour environment. EXAMPLES

[0166] In Example 2, steel materials having a yield strength of 862 MPa or more were investigated. Specifically, 180 kg of molten steel having the chemical composition shown in Tables 4-1 and 4-2 was produced. In Table 4-2, "-" means that the content of each element is at the impurity level. Specifically, the Co content, W content, Ni content, and Cu content of Test No. 2-1 are rounded off to the nearest two decimal places and are 0%. The Nb content of Test No. 2-1 is rounded off to the nearest four decimal places and are 0%. Furthermore, the Ca content, Mg content, Zr content, and REM content of Test No. 2-1 are rounded off to the nearest five decimal places and are 0%.

[0167] [Table 4-1]

[0168] [Table 4-2]

[0169] Ingots were produced using the molten steel of test numbers 2-1 to 2-26. The produced ingots were hot rolled to produce steel plates with a plate thickness of 15 mm. The steel plates of test numbers 2-1 to 2-26 after hot rolling were allowed to cool to room temperature (25°C). The steel plates of test numbers 2-1 to 2-26 after cooling were held at the quenching temperature (920°C) for 20 minutes, and then quenched by immersing the steel plates in a water tank. At this time, the cooling rate during quenching (CR 800-500 ) was 600℃ / min. The quenching temperature and the cooling rate CR during quenching were measured using a sheathed K thermocouple inserted in the center of the thickness of the steel plate. 800-500 was measured.

[0170] After quenching, the steel plates of test numbers 2-1 to 2-26 were tempered. In this example, the tempering at high temperature is also called the first tempering, and the tempering at medium temperature is also called the second tempering. The holding temperature (°C), holding time (min), and diffusion distance (√Dt H (nm) or √Dt L (nm)) is shown in Table 5. Furthermore, the diffusion distance √Dt H , √Dt L , and the diffusion distance throughout the tempering process, √Dt, calculated from equation (8) To The thickness (nm) of the specimen is shown in Table 5. In Table 5, "-" in the "First tempering" column means that the first tempering was not performed. Similarly, in Table 5, "-" in the "Second tempering" column means that the second tempering was not performed.

[0171] [Table 5]

[0172] Here, the tempering holding temperature in this example was the temperature of the heat treatment furnace in which tempering was performed, and further, the tempering holding time in this example was the time from when the temperature of the steel plate of each test number reached a predetermined tempering temperature until it was extracted from the heat treatment furnace.

[0173] [Evaluation test] The tempered steel plates of test numbers 2-1 to 2-26 were subjected to a tensile test, a dislocation density measurement test, a grain boundary Mo content measurement test, and a DCB test, which will be described below.

[0174] [Tensile test] Tensile tests were conducted on the steel plates of test numbers 2-1 to 2-26 by the method described above. Specifically, round bar tensile test pieces with a parallel part diameter of 4 mm and a gauge length of 16 mm were prepared from the center of the plate thickness of the steel plates of test numbers 2-1 to 2-26. The axial direction of the round bar tensile test pieces was parallel to the rolling direction of the steel plates. Using the round bar test pieces of test numbers 2-1 to 2-26, tensile tests were conducted in accordance with ASTM E8 / E8M (2021) at room temperature (25°C) in air to obtain the yield strength (MPa) of the steel plates of test numbers 2-1 to 2-26. The obtained yield strengths were referred to as "yield strength σ YS The values ​​are shown in Table 6 as "(MPa)".

[0175] [Table 6]

[0176] [Dislocation density measurement test] A dislocation density measurement test was carried out for the steel plates of test numbers 2-1 to 2-26 by the method described above. Specifically, test pieces of 20 mm width × 20 mm length × 2 mm thickness were prepared from the center of the plate thickness of the steel plates of test numbers 2-1 to 2-26. The half-widths ΔK of the peaks of the (110), (211), and (220) planes were obtained by the X-ray diffraction method by the method described above. Using the obtained half-widths ΔK, the dislocation densities ρ(m -2 The obtained dislocation density was defined as the dislocation density ρ(10 14 m -2 )" in Table 6.

[0177] [Grain boundary Mo content measurement test] For the steel plates of test numbers 2-1 to 2-26, a grain boundary Mo content measurement test was carried out by the above-mentioned method. Specifically, test pieces were prepared that included the thickness t / 4 position of the steel plates of test numbers 2-1 to 2-26 in the center and had a length of 10 mm in the rolling direction, a width of 5 mm, and a thickness of 8 mm. The grain boundary Mo content (mass%) was obtained by the above-mentioned method using the prepared test pieces. The obtained grain boundary Mo content was referred to as "grain boundary Mo content γ Mo The yield strengths obtained are shown in Table 6 as "(mass%)". YS (MPa) and dislocation density ρ(m -2 ) and grain boundary Mo content γ Mo Fn1 was obtained using (mass%) and formula (1). The obtained Fn1 is shown in Table 6.

[0178] [DCB Test] The DCB test was carried out by the above-mentioned method for the steel plates of test numbers 2-1 to 2-26. Specifically, the above-mentioned DCB test pieces and wedges were prepared from the center of the plate thickness of the steel plates of test numbers 2-1 to 2-26. The prepared test pieces and wedges were used to carry out the DCB test in accordance with NACE TM0177-2016 Method D under the above-mentioned conditions. The fracture toughness values ​​K obtained by the DCB test carried out by the above-mentioned method were 1SSC The value is "K 1SSC The values ​​are shown in Table 6 as "(MPa√m)".

[0179] [Evaluation Results] With reference to Tables 4-1, 4-2, 5, and 6, the chemical compositions of the steel plates of test numbers 2-1 to 2-14 were appropriate, and the yield strength was 862 MPa or more (125 ksi or more). Mo was 5.0 mass% or more, and Fn1 was -520 or more. 1SSC The values ​​were 24.1 MPa or more. In other words, these steel plates had a yield strength of 110 ksi or more and excellent fracture toughness in a low-temperature sour environment.

[0180] On the other hand, the steel plates of test numbers 2-15 and 2-16 were not subjected to the intermediate temperature tempering process. As a result, the Fn1 of these steel plates was less than -520.1SSC The values ​​were less than 24.1 MPa. In other words, these steel plates did not have excellent fracture toughness in a low-temperature sour environment.

[0181] The steel plate of test number 2-17 had a too high holding temperature and too short holding time in the intermediate temperature tempering process. As a result, the Fn1 of this steel plate was less than -520. 1SSC The value was less than 24.1 MPa. In other words, this steel plate did not have excellent fracture toughness in a low-temperature sour environment.

[0182] The steel plates of test numbers 2-18 and 2-19 had too short a holding time in the intermediate temperature tempering process. As a result, the Fn1 of these steel plates was less than -520. 1SSC The values ​​were less than 24.1 MPa. In other words, these steel plates did not have excellent fracture toughness in a low-temperature sour environment.

[0183] The steel plate of test number 2-20 had a diffusion distance √Dt L As a result, the Fn1 of this steel plate was less than -520. 1SSC The value was less than 24.1 MPa. In other words, this steel plate did not have excellent fracture toughness in a low-temperature sour environment.

[0184] The steel plate of test number 2-21 has a diffusion distance √Dt L As a result, the Fn1 of this steel plate was less than -520. 1SSC The value was less than 24.1 MPa. In other words, this steel plate did not have excellent fracture toughness in a low-temperature sour environment.

[0185] The steel plate of test number 2-22 had too low a Cr content. 1SSC The value was less than 24.1 MPa. In other words, this steel plate did not have excellent fracture toughness in a low-temperature sour environment.

[0186] The steel sheet of test number 2-23 had a too low Mo content. As a result, the grain boundary Mo content of this steel sheet was Mo The amount of K in the steel sheet was less than 5.0 mass%, and Fn1 was less than -520. 1SSC The value was less than 24.1 MPa. In other words, this steel plate did not have excellent fracture toughness in a low-temperature sour environment.

[0187] The steel plate with test number 2-24 had too high a Mn content. 1SSC The value was less than 24.1 MPa. In other words, this steel plate did not have excellent fracture toughness in a low-temperature sour environment.

[0188] The steel plate with test number 2-25 had too high a N content. As a result, this steel plate had 1SSC The value was less than 24.1 MPa. In other words, this steel plate did not have excellent fracture toughness in a low-temperature sour environment.

[0189] The steel plate of test number 2-26 had too high a P content. 1SSC The value was less than 24.1 MPa. In other words, this steel plate did not have excellent fracture toughness in a low-temperature sour environment.

[0190] The embodiments of the present disclosure have been described above. However, the above-described embodiments are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiments, and can be implemented by appropriately modifying the above-described embodiments without departing from the spirit of the present disclosure.

Claims

1. A steel material, In mass percent, C: more than 0.20 to 0.60%, Si: 0.05-2.00%, Mn: 0.02 to 0.60%, P: 0.025% or less, S: 0.0100% or less, Al: 0.005-0.100%, Cr: 0.20-1.50%, Mo: 0.35-1.50%, V: 0.01-0.60%, Ti: 0.002 to 0.050%, B: 0.0001 to 0.0050%, N: 0.0100% or less, O: 0.0100% or less, Nb: 0 to 0.030%, Ca: 0-0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, Rare earth elements: 0 to 0.0100%, Co: 0 to 0.50%, W: 0-0.50%, Ni: 0 to 0.20%, Cu: 0 to 0.50%, and The balance is Fe and impurities, Yield strength σ YS is 758 MPa or more, In the microstructure of the steel material, the total volume fraction of tempered martensite and tempered bainite is 90% or more, In the steel material, the Mo content γ in the vicinity of the prior austenite grain boundary Mo is 5.0% by mass or more, The yield strength σ YS is less than 862 MPa, Fn1 defined by formula (1) satisfies −300 or more, The yield strength σ YS When the compressive strength is 862 MPa or more, Fn1 defined by formula (1) satisfies −520 or more; Steel material. Fn1=83×(γ) Mo -10 -7 ×√ρ)-σ YS (1) Here, γ in formula (1) Mo is substituted with the Mo content in the vicinity of the prior austenite grain boundary in mass%, and ρ in formula (1) is the dislocation density m -2 and σ in Eq. (1) is substituted. YS The yield strength in MPa is substituted for

2. The steel material according to claim 1, Nb: 0.001-0.030%, Ca: 0.0001-0.0100%, Mg: 0.0001 to 0.0100%, Zr: 0.0001 to 0.0100%, Rare earth elements: 0.0001 to 0.0100%, Co: 0.01 to 0.50%, W: 0.01-0.50%, Ni: 0.01 to 0.20%, and Cu: 0.01 to 0.50%, containing one or more elements selected from the group consisting of; Steel material.

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

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