Steel material suitable for use in sour environment

Abstract

To provide a steel material having high yield strength and superior fracture toughness in a low-temperature sour environment.SOLUTION: A steel material according to the present disclosure has a chemical composition as described in the specification, having Fn1 defined by formula (1) of 2.80 or more and Fn2 defined by formula (2) of 41.0 to 150.0, and having a yield strength of 862 MPa to less than 965 MPa. In the steel material, when NR is defined as the number ratio of precipitates with an equivalent circle diameter of 20 to 100 nm among those with an equivalent circle diameter of 20 nm or more, Fn3 defined by formula (3) is 0.74 or more. Fn1=3.3×Si-1.4×Mn+1.1×Mo (1). Fn2=390×C-57.5×Cr-21.4×Mo-114.1×V (2). Fn3=NR×(C+4×Si-1.5×Mn-0.5×Cr+0.2×Mo-0.5×V-Ti)1 / 2 (3).SELECTED DRAWING: Figure 1

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 steel materials for oil wells, such as steel pipes for oil wells. 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) steel materials for oil wells 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) and 125 ksi class (yield strength of 125 to less than 140 ksi, i.e., less than 862 to 965 MPa) steel pipes for oil wells.

[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 C 6 The proportion of type carbides is (200 / t) mass% or less when the wall thickness of the product is t (mm). Patent Document 1 describes 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 C 6 Mold precipitates are 0.1 pieces / mm 2The 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 inhibition of 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 defects, there is a concern that the microscopic defects may become the starting points of cracks, causing the cracks to propagate. On the other hand, if the propagation of cracks can be inhibited, it is believed that the steel materials will have excellent resistance to fracture (i.e., excellent fracture toughness). Therefore, there is 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: 0.35-0.50%, Si: 0.80-1.50%, Mn: 0.02 to less than 0.50% P: 0.025% or less, S: 0.0100% or less, Al: 0.005 to 0.100%, Cr: 0.20~1.50%, Mo: 0.35-3.00%, 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%, Co: 0-0.50%, W: 0 to 0.50%, Ni: 0-1.20%, Cu: 0-0.50%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, Rare earth elements: 0 to 0.0100%, and the balance being Fe and impurities, Fn1 defined by formula (1) is 2.80 or more, Fn2 defined by formula (2) is 41.0 to 150.0, The yield strength is less than 862 to 965 MPa, In the steel material, When the number ratio of precipitates with an equivalent circle diameter of 20 to 100 nm among precipitates with an equivalent circle diameter of 20 nm or more is defined as NR, Fn3 defined by formula (3) is 0.74 or more. Fn1=3.3×Si-1.4×Mn+1.1×Mo (1) Fn2=390×C-57.5×Cr-21.4×Mo-114.1×V (2) Fn3=NR×(C+4×Si-1.5×Mn-0.5×Cr+0.2×Mo-0.5×V-Ti) 1 / 2 (3) Here, the element symbols in formulas (1) to (3) are substituted with the content of the corresponding element in mass%, and NR in formula (3) is substituted with the number ratio of precipitates with an equivalent circle diameter of 20 to 100 nm among precipitates with an equivalent circle diameter of 20 nm or more. 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 Fn3 (=NR×(C+4×Si-1.5×Mn-0.5×Cr+0.2×Mo-0.5×V-Ti)1 / 2) and the fracture toughness value KISSC value, which is an index of fracture toughness in a low-temperature sour environment, in this embodiment. [Figure 2A] FIG. 2A is 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 2B] FIG. 2B 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 862 MPa or more (125 ksi or more). Therefore, the inventors first investigated, from the viewpoint of chemical composition, a steel material having a yield strength of 125 ksi or more and excellent fracture toughness in a low-temperature sour environment. As a result, the inventors found that, in mass%, C: 0.35 to 0.50%, Si: 0.80 to 1.50%, Mn: 0.02 to less than 0.50%, 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 3.00%, 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% or less, and %, Nb: 0-0.030%, Co: 0-0.50%, W: 0-0.50%, Ni: 0-1.20%, Cu: 0-0.50%, Ca: 0-0.0100%, Mg: 0-0.0100%, Zr: 0-0.0100%, rare earth elements: 0-0.0100%, and the balance being Fe and impurities, it was thought that it would be possible to obtain a steel material having a yield strength of 125 ksi or more and excellent fracture toughness in a low-temperature sour environment.

[0018] On the other hand, in the case of a steel material having the above-mentioned chemical composition and a yield strength of 125 ksi or more, there are cases where sufficient fracture toughness cannot be obtained in a low-temperature sour environment. Therefore, the present inventors further investigated means for increasing the fracture toughness in a low-temperature sour environment.

[0019] Specifically, the inventors focused on the action of each element in the chemical composition. As a result, it was found that, among the elements contained in the above-mentioned chemical composition, Si, Mn, and Mo have a significant effect on the fracture toughness in a low-temperature sour environment. As a result of detailed studies by the inventors, it was found that, in a steel material having the above-mentioned chemical composition, if Fn1 defined by the following formula (1) is 2.80 or more, the fracture toughness in a low-temperature sour environment can be increased while maintaining a yield strength of 125 ksi or more, provided that the other configurations of this embodiment are satisfied. Fn1=3.3×Si-1.4×Mn+1.1×Mo (1) Here, the content of the corresponding element is substituted for the element symbol in formula (1) in the unit of mass %.

[0020] On the other hand, even if the steel material has the above-mentioned chemical composition and Fn1 is 2.80 or more, and has a yield strength of 125 ksi or more, there are cases where the steel material does not have sufficient fracture toughness in a low-temperature sour environment. Therefore, the present inventors further investigated means for increasing the fracture toughness in a low-temperature sour environment.

[0021] Specifically, the present inventors conducted various studies focusing on precipitates in steel having the above-mentioned chemical composition. Here, coarse precipitates are likely to cause stress concentration at the interface with the base material. In particular, in a low-temperature sour environment, the decrease in fracture toughness due to coarse precipitates may be easily manifested. On the other hand, if fine precipitates are dispersed, the fracture toughness of the steel may be improved. In other words, the present inventors considered that if the proportion of fine precipitates in the precipitates is increased, the fracture toughness in a low-temperature sour environment may be improved while maintaining a yield strength of 125 ksi or more.

[0022] As a result of detailed studies by the present inventors based on the above findings, it has become clear that, rather than simply increasing the number density of fine precipitates, it is possible to achieve both a yield strength of 125 ksi or more and fracture toughness in a low-temperature sour environment by adjusting the proportion of fine precipitates in the precipitates and the element contents. Specifically, in a steel material having the above-mentioned chemical composition and satisfying Fn1 of 2.80 or more, if Fn3 defined by the following formula (3) satisfies 0.74 or more, it is possible to improve fracture toughness in a low-temperature sour environment while maintaining a yield strength of 125 ksi or more. Fn3=NR×(C+4×Si-1.5×Mn-0.5×Cr+0.2×Mo-0.5×V-Ti) 1 / 2 (3) Here, the element symbols in formula (3) are substituted with the content of the corresponding element in units of mass%, and NR in formula (3) is substituted with the number ratio of precipitates with an equivalent circle diameter of 20 to 100 nm among precipitates with an equivalent circle diameter of 20 nm or more.

[0023] Hereinafter, precipitates with a circle equivalent diameter of 20 to 100 nm are also referred to as "fine precipitates". In addition, the number ratio of fine precipitates (precipitates with a circle equivalent diameter of 20 to 100 nm) among precipitates with a circle equivalent diameter of 20 nm or more is defined as NR. By increasing the number ratio NR of fine precipitates in a steel material, not only can the fracture toughness be increased by the dispersion of fine precipitates, but also the decrease in fracture toughness due to coarse precipitates can be suppressed. Furthermore, in this embodiment, instead of simply increasing the number ratio NR of fine precipitates, the fracture toughness in a low-temperature sour environment is further increased by adjusting the relationship with the element content.

[0024] Here, Fn3 is an index of fracture toughness in a low-temperature sour environment for a steel material having the above-mentioned chemical composition. The relationship between Fn3 and fracture toughness in a low-temperature sour environment will be specifically explained below with reference to the drawings. FIG. 1 shows the relationship between Fn3 (=NR×(C+4×Si-1.5×Mn-0.5×Cr+0.2×Mo-0.5×V-Ti) in this embodiment. 1 / 2 ) and the fracture toughness value K, which is an index of fracture toughness in a low-temperature sour environment. ISSC 1 is a graph showing the relationship between Fn3, determined by a method to be described later, and the fracture toughness value K ISSC Value (MPa m 1 / 2 ) and were used to create the data.

[0025] Referring to FIG. 1, in a steel material having the above-mentioned chemical composition, Fn1 being 2.80 or more, and having a tensile strength of 125 ksi or more, if Fn3 is 0.74 or more, the fracture toughness value K ISSC Value is 22.0MPa m 1 / 2 Even in a low-temperature sour environment, the fracture toughness is stable and excellent. On the other hand, if Fn3 is less than 0.74, the fracture toughness value K ISSC Value is 22.0MPa m 1 / 2Therefore, the steel material according to the present embodiment has the above-mentioned chemical composition, Fn1 satisfies 2.80 or more, and Fn3 satisfies 0.74 or more. As a result, the steel material according to the present embodiment can achieve both a yield strength of 125 ksi or more and excellent fracture toughness in low-temperature sour environments.

[0026] Furthermore, the inventors have investigated various methods for making Fn3 0.74 or more for a steel material having the above-mentioned chemical composition and satisfying Fn1 of 2.80 or more. As a result, the inventors have found that, on the premise that other conditions of this embodiment are satisfied, Fn3 can be stably made 0.74 or more if the steel material has the above-mentioned chemical composition and further satisfies Fn2 defined by the following formula (2) of 41.0 to 150.0. Fn2=390×C-57.5×Cr-21.4×Mo-114.1×V (2) Here, the element symbols in formula (2) are substituted with the contents of the corresponding elements in units of mass %.

[0027] In short, the steel material according to this embodiment has the above-mentioned chemical composition, Fn1 satisfies 2.80 or more, Fn2 satisfies 41.0 to 150.0, and Fn3 satisfies 0.74 or more. As a result, the steel material according to this embodiment can achieve both a yield strength of 125 ksi or more and excellent fracture toughness in low-temperature sour.

[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: 0.35-0.50%, Si: 0.80-1.50%, Mn: 0.02 to less than 0.50% P: 0.025% or less, S: 0.0100% or less, Al: 0.005 to 0.100%, Cr: 0.20~1.50%, Mo: 0.35-3.00%, 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%, Co: 0-0.50%, W: 0 to 0.50%, Ni: 0-1.20%, Cu: 0-0.50%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, Rare earth elements: 0 to 0.0100%, and the balance being Fe and impurities, Fn1 defined by formula (1) is 2.80 or more, Fn2 defined by formula (2) is 41.0 to 150.0, The yield strength is less than 862 to 965 MPa, In the steel material, When the number ratio of precipitates with an equivalent circle diameter of 20 to 100 nm among precipitates with an equivalent circle diameter of 20 nm or more is defined as NR, Fn3 defined by formula (3) is 0.74 or more. Steel material. Fn1=3.3×Si-1.4×Mn+1.1×Mo (1) Fn2=390×C-57.5×Cr-21.4×Mo-114.1×V (2) Fn3=NR×(C+4×Si-1.5×Mn-0.5×Cr+0.2×Mo-0.5×V-Ti) 1 / 2 (3) Here, the element symbols in formulas (1) to (3) are substituted with the content of the corresponding element in mass%, and NR in formula (3) is substituted with the number ratio of precipitates with an equivalent circle diameter of 20 to 100 nm among precipitates with an equivalent circle diameter of 20 nm or more.

[0030] [2] The steel material according to [1], Nb: 0.001 to 0.030%, Co: 0.01 to 0.50%, W: 0.01 to 0.50%, Ni: 0.01 to 1.20%, Cu: 0.01 to 0.50%, Ca: 0.0001 to 0.0100%, Mg: 0.0001 to 0.0100%, Zr: 0.0001 to 0.0100%, and Rare earth elements: 0.0001 to 0.0100%, 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: 0.35-0.50% Carbon (C) improves the hardenability of steel and increases the strength of the steel. C also promotes the spheroidization of carbides during tempering in the manufacturing process, and increases the fracture toughness of the steel in a low-temperature sour environment. Dispersion of carbides further increases the strength of the steel. 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 fracture toughness of the steel in a low-temperature sour environment is rather reduced. Therefore, the C content is 0.35 to 0.50%. The preferred lower limit of the C content is 0.36%, more preferably 0.37%, and more preferably 0.38%. The preferred upper limit of the C content is 0.49%, more preferably 0.48%, and more preferably 0.47%.

[0036] Silicon: 0.80 to 1.50% 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 fracture toughness of the steel material in a low-temperature sour environment decreases even if the contents of other elements are within the range of this embodiment. Therefore, the Si content is 0.80 to 1.50%. The preferred lower limit of the Si content is 0.81%, more preferably 0.82%, more preferably 0.90%, more preferably 1.00%, and more preferably 1.01%. The preferred upper limit of the Si content is 1.45%, more preferably 1.40%, more preferably less than 1.36%, more preferably 1.35%, and more preferably 1.34%.

[0037] Mn: 0.02 to less than 0.50% 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 less than 0.02 to 0.50%. The preferred lower limit of the Mn content is 0.03%, more preferably 0.04%, and more preferably 0.05%. The preferred upper limit of the Mn content is 0.49%, more preferably 0.48%, more preferably 0.47%, more preferably 0.46%, 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 to the grain boundaries, and the fracture toughness of the steel material in a low-temperature sour environment will decrease. Therefore, the S content is 0.0100% or less. The preferred upper limit of the S content is 0.0075%, more preferably 0.0050%, more preferably 0.0030%, and even more preferably 0.0020%. 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, the preferred lower limit of the S content is 0.0001%, more preferably 0.0003%, and even more preferably 0.0005%.

[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 fracture toughness of the steel material in a low-temperature sour environment decreases. 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 decreases. 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 the strength of the steel. Cr also improves the tempering softening resistance of steel and enables high-temperature tempering. As a result, the fracture toughness of the 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 0.23%, more preferably 0.24%, more preferably 0.25%, more preferably 0.30%, more preferably 0.35%, and more preferably 0.40%. The preferred upper limit of the Cr content is 1.40%, more preferably 1.30%, and more preferably 1.20%.

[0042] Mo: 0.35-3.00% Molybdenum (Mo) improves the hardenability of steel material and increases its strength. 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 3.00%. 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 2.80%, more preferably 2.50%, more preferably 2.40%, and even more preferably 2.30%.

[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 0.05%. 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, even if the contents of other elements are within the range of this embodiment, coarse nitrides are generated, and the fracture toughness of the steel material in a low-temperature sour environment decreases. Therefore, the B content is 0.0001 to 0.0050%. The preferred lower limit of the B content is 0.0003%, more preferably 0.0005%, and more preferably 0.0007%. The preferred upper limit of the B content is 0.0030%, more preferably 0.0025%, 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 fracture toughness of the steel material in a low-temperature sour environment is 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 0.0003%, and even more preferably 0.0005%.

[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 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.

[0052] 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%.

[0053] 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%.

[0054] 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.

[0055] Ni: 0 to 1.20% 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. 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 1.20%. The preferred lower limit of the Ni content is more than 0%, more preferably 0.01%, more preferably 0.02%, and more preferably 0.10%. The preferred upper limit of the Ni content is 1.15%, more preferably 1.00%, more preferably 0.90%, more preferably 0.80%, and more preferably 0.60%.

[0056] 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%.

[0057] 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 fracture toughness of the steel material in a low-temperature sour environment.

[0058] 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 harmless as sulfide, and increases the fracture toughness of the steel in a low-temperature sour environment. If even a small amount of Ca is contained, the above effect can be obtained to some extent. 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 are coarsened, and the fracture toughness of the steel in a low-temperature sour environment 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%.

[0059] 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 harmless as sulfide, and increases the fracture toughness of the steel in a low-temperature sour environment. If even a small amount of Mg is contained, the above effect can be obtained to some extent. 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 will become coarse, and the fracture toughness of the steel in a low-temperature sour environment will decrease. Therefore, the Mg content is 0 to 0.0100%. The preferred 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 preferred upper limit of the Mg content is 0.0040%, more preferably 0.0030%, more preferably 0.0025%, and more preferably 0.0020%.

[0060] 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 sulfide, and increases the fracture toughness of the steel in a low-temperature sour environment. If even a small amount of Zr is contained, the above effect can be obtained to a certain 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 will become coarse, and the fracture toughness of the steel in a low-temperature sour environment will decrease. Therefore, the Zr content is 0 to 0.0100%. The preferred 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 preferred upper limit of the Zr content is 0.0040%, more preferably 0.0030%, more preferably 0.0025%, and more preferably 0.0020%.

[0061] 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 increases the fracture toughness of the steel in a low-temperature sour environment. REM further binds to P in the steel and suppresses the segregation of P at the grain boundaries. Therefore, the decrease in the fracture toughness 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 will coarsen 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 will rather decrease. 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%.

[0062] 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.

[0063] [Fn1] The steel material according to this embodiment satisfies Fn1 defined by the following formula (1) of 2.80 or more within the ranges of the contents of the elements of the steel material described above. Fn1=3.3×Si-1.4×Mn+1.1×Mo (1) Here, the content of the corresponding element is substituted for the element symbol in formula (1) in the unit of mass %.

[0064] Fn1 is an index of fracture toughness in a low-temperature sour environment. If Fn1 is too small, excellent fracture toughness cannot be obtained in a low-temperature sour environment when the steel has a 125 ksi-class yield strength. Therefore, the steel according to this embodiment has Fn1 of 2.80 or more, on the premise that the steel has the above-mentioned chemical composition. As a result, it is possible to achieve both a 125 ksi-class yield strength and fracture toughness in a low-temperature sour environment, provided that the other configurations of this embodiment are satisfied.

[0065] The lower limit of Fn1 is preferably 2.83, more preferably 2.85, more preferably 2.90, more preferably 3.00, more preferably 3.01, and more preferably 3.10. The upper limit of Fn1 is not particularly limited, but is substantially 7.73 with the above-mentioned element contents. That is, in this embodiment, Fn1 may be 2.80 to 7.73. The upper limit of Fn1 may further be 7.50, 7.30, or 6.80. Fn1 is calculated by rounding off the obtained numerical value to two decimal places.

[0066] [Fn2] The steel material according to this embodiment satisfies Fn2 defined by the following formula (2) of 41.0 to 150.0 within the ranges of the contents of the elements of the steel material described above. Fn2=390×C-57.5×Cr-21.4×Mo-114.1×V (2) Here, the element symbols in formula (2) are substituted with the contents of the corresponding elements in units of mass %.

[0067] Fn2 is an index of Fn3 in a steel material having the above-mentioned chemical composition and having Fn1 of 2.80 or more. If Fn2 is too low or too high, Fn3 may not be stable at 0.74 or more. Therefore, the steel material according to this embodiment has the above-mentioned chemical composition, and Fn2 is set to 41.0 to 150.0 on the premise that Fn1 is 2.80 or more. As a result, it is possible to achieve both a 125 ksi-class yield strength and fracture toughness in a low-temperature sour environment, provided that the other configurations of this embodiment are satisfied.

[0068] The preferred lower limit of Fn2 is 41.5, more preferably 42.0, and even more preferably 42.5. The preferred upper limit of Fn2 is 149.0, more preferably 148.0, more preferably 147.0, and even more preferably 146.0. Fn2 is calculated by rounding off the first decimal place of the obtained numerical value.

[0069] [Fn3] In the steel material according to this embodiment, when the number ratio of precipitates having an equivalent circle diameter of 20 to 100 nm among precipitates having an equivalent circle diameter of 20 nm or more is defined as NR, within the content ranges of the elements of the steel material described above, Fn3 defined by the following formula (3) satisfies 0.74 or more. Fn3=NR×(C+4×Si-1.5×Mn-0.5×Cr+0.2×Mo-0.5×V-Ti) 1 / 2 (3) Here, the element symbols in formula (3) are substituted with the content of the corresponding element in units of mass%, and NR in formula (3) is substituted with the number ratio of precipitates with an equivalent circle diameter of 20 to 100 nm among precipitates with an equivalent circle diameter of 20 nm or more.

[0070] As described above, especially in a low-temperature sour environment, the decrease in fracture toughness due to stress concentration at the interface between the coarse precipitates and the base material is likely to become evident. On the other hand, when the precipitates having a circle equivalent diameter of 20 to 100 nm are defined as "fine precipitates" in this specification, if the number ratio of fine precipitates is increased, not only can the decrease in fracture toughness due to the coarse precipitates be suppressed, but also the fracture toughness can be increased by the fine precipitates. Furthermore, in this embodiment, instead of simply increasing the number density of the fine precipitates, the ratio of the fine precipitates to the precipitates and the element content are adjusted to stably increase the fracture toughness in a low-temperature sour environment.

[0071] Specifically, the number ratio of fine precipitates (precipitates with a circle equivalent diameter of 20 to 100 nm) among precipitates with a circle equivalent diameter of 20 nm or more is defined as NR. In other words, the number ratio of fine precipitates NR means the ratio of the number of fine precipitates to the total number of precipitates with a circle equivalent diameter of 20 nm or more. In this embodiment, Fn3 is an index of fracture toughness in a low-temperature sour environment for a steel material having the above-mentioned chemical composition, Fn1 satisfying 2.80 or more, and Fn2 satisfying 41.0 to 150.0. If Fn3 is 0.74 or more, the fracture toughness value K ISSC The value is 22.0 (MPa m 1 / 2 ) or more, exhibiting excellent fracture toughness. Therefore, the steel material according to this embodiment has the above-mentioned chemical composition, and satisfies Fn1 of 2.80 or more, Fn2 of 41.0 to 150.0, and further satisfies Fn3 of 0.74 or more. As a result, the steel material according to this embodiment can achieve both a 125 ksi-class yield strength and fracture toughness in a low-temperature sour environment.

[0072] The lower limit of Fn3 is preferably 0.75, more preferably 0.76, and even more preferably 0.77. The upper limit of Fn3 is not particularly limited, but is substantially 2.78 in the content of the above elements. That is, in this embodiment, Fn3 may be 0.74 to 2.78. The upper limit of Fn3 may further be 2.50, 2.00, or 1.80. Fn3 is calculated by rounding off the obtained numerical value to two decimal places.

[0073] In this embodiment, the number ratio NR of fine precipitates is not particularly limited as long as it satisfies Fn3. NR may be, for example, 0.350 or more, 0.360 or more, 0.370 or more, or 0.375 or more. Moreover, it is preferable that the number ratio NR of fine precipitates is high. Specifically, the upper limit of the number ratio NR of fine precipitates may be, for example, 1.000, 0.900, 0.850, or 0.800. In this embodiment, the number ratio NR of fine precipitates means a value obtained by dividing the number of precipitates having a circle equivalent diameter of 20 to 100 nm in the steel material by the total number of precipitates having a circle equivalent diameter of 20 nm or more.

[0074] In this embodiment, the number ratio NR of fine precipitates can be obtained by the following method. First, a test piece is prepared from the steel material according to this embodiment. Specifically, when the steel material is a steel plate, a test piece is prepared having an observation surface 10 mm from the center of the plate thickness in the rolling direction and 10 mm in the plate thickness direction. When the steel material is a steel pipe, a test piece is prepared having an observation surface 10 mm from the center of the plate thickness in the tube axial direction and 8 mm in the wall thickness (pipe diameter) direction. When the steel material is a round bar, a test piece is prepared having an observation surface including the R / 2 position in the center, 10 mm in the axial direction, and 8 mm in the radial direction. In this specification, the R / 2 position of the round bar means the center position of the radius R in a cross section perpendicular to the axial direction of the round bar.

[0075] The observation surface of the test piece is polished to a mirror finish, and then immersed in a picral etching solution (a 2.0% picric acid-ethanol solution) for 60 seconds to reveal the structure by etching. The etched observation surface is subjected to three-dimensional roughness measurement using a scanning electron microscope (SEM) to obtain a three-dimensional roughness profile for each field of view. If the number of observation fields is three or more and the total area of ​​the observation fields is 300 μm 2 If this is the case or more, the reproducibility of the number ratio NR of fine precipitates is improved. Therefore, in this embodiment, the number of observation fields is three or more. Furthermore, the area of ​​the field is, for example, 12 μm × 9 μm = 108 μm 2(Magnification: 10,000x).

[0076] Although the number of pixels into which the field of view is divided is not particularly limited, in order to obtain stable measurement accuracy, it is preferable that one pixel is 0.020 μm×0.020 μm or less. When one pixel is 0.020 μm×0.020 μm, i.e., 20 nm×20 nm, it becomes possible to detect precipitates of 20 nm or more by three-dimensional roughness measurement. In addition, when one pixel is 0.020 μm×0.020 μm in the above-mentioned field of view area, the field of view area is divided into 270,000 pixels of 600×450.

[0077] The method for performing the three-dimensional roughness measurement is not particularly limited, and may be any well-known method. For example, in an SEM, four secondary electron detectors may be installed, and the detection results may be combined to obtain a three-dimensional roughness profile. In each observation field, the direction of the focal depth of the SEM observation is defined as the "height direction". In each observation field, a plane perpendicular to the height direction is further defined as the "observation surface". Furthermore, in the height direction, the direction from the observation surface toward the electron beam source is defined as the positive direction (the direction in which the height increases). From the three-dimensional roughness profile obtained by the above method, the area ratio Z of the steel material to the field area of ​​the observation surface at the height position h (μm) is calculated. h In this case, the resolution in the height direction is, for example, 1 nm.

[0078] Here, the minimum height h 0 and maximum height h 1 Identify the following: 0 is the corresponding area ratio Z h0 = 100.0% and Z h This means the maximum value of the height h where h = 100.0%. 1 is the corresponding area ratio Z h1 = 0.0% and Z h This refers to the minimum value of the height h where = 0.0%.

[0079] In each observation field, the horizontal axis represents the height position h (μm), and the horizontal axis represents the area ratio Zh A plot is created with the vertical axis being the percentage (%). Note that the range of the height position h is h 0 ~h 1 Let us assume that.

[0080] Next, the area ratio S (%) of precipitates in each observation field is calculated. In this embodiment, the volume ratio (%) of precipitates in the steel material is calculated and is defined as the area ratio S (%) of precipitates in each observation field. Furthermore, in this embodiment, as described above, precipitates having an equivalent circle diameter of 20 nm or more are detected. Therefore, in this embodiment, the area ratio S (%) of precipitates in each observation field means the volume ratio (%) of precipitates having an equivalent circle diameter of 20 nm or more.

[0081] As described above, most of the precipitates having a circle equivalent diameter of 20 nm or more are cementite. Furthermore, the volume fraction of cementite having a circle equivalent diameter of less than 20 nm is negligibly small. Therefore, the area fraction S (%) of precipitates in each observation field is the volume fraction V of cementite in the steel material according to this embodiment. θ From the above, in this embodiment, the area ratio S (%) of precipitates in each observation field can be approximated to the volume fraction V of cementite. θ Calculate the percentage.

[0082] Cementite volume fraction V θ The method for determining is not particularly limited and may be any known method. θ may be obtained, for example, by thermodynamic calculation. In this case, the ratio of cementite to the volume of the entire system (the whole including the parent phase, cementite, and other precipitates, inclusions, etc.) can be obtained by performing thermodynamic calculation using the chemical composition and the tempering temperature in the manufacturing process described later. Note that when performing thermodynamic calculation, it may be performed using well-known thermodynamic calculation software. In this way, the volume fraction V of cementite can be obtained by thermodynamic calculation. θ It is well within the ability of a person skilled in the art to determine (%).

[0083] Cementite volume fraction Vθ may be obtained by further capturing the extracted residue. In this case, it can be obtained by the following method. A cylindrical test piece is prepared from the steel material according to this embodiment. When the steel material is a steel plate, a cylindrical test piece is prepared from the center of the plate thickness. When the steel material is a steel pipe, a cylindrical test piece is prepared from the center of the wall thickness. When the steel material is a round steel, a cylindrical test piece is prepared from the R / 2 position. The size of the cylindrical test piece is, for example, 6 mm in diameter and 50 mm in length. The surface of the prepared cylindrical test piece is polished to about 50 μm by preliminary electrolytic polishing to obtain a new surface. The test piece with the new surface obtained is electrolyzed using an electrolyte (10% acetylacetone + 1% tetraammonium + methanol). The electrolyte after electrolysis is passed through a 0.2 μm filter to capture the residue.

[0084] The resulting residue is decomposed with acid, and ICP (inductively coupled plasma) emission spectrometry is performed to quantitatively determine the concentration of alloy elements other than carbon in the cementite in mass%. The volume fraction V of cementite is calculated from the concentration of alloy elements other than carbon in the resulting cementite and the following formula (A). θ Calculate the percentage. V θ = (sum of mole fractions of each alloying element in cementite) × (1 / 3) × (V mθ / V m ) (A)

[0085] The "molar fraction of each alloy element in the cementite" in formula (A) can be calculated by the following method. The amount of each alloy element dissolved in the cementite can be obtained by analyzing the extraction residue. The mole fraction of each alloy element in the cementite can be calculated by dividing the obtained amount of each alloy element by the total amount electrolyzed.

[0086] In addition, V in formula (A) mθ is the molar volume of cementite (m 3 / mol). Vm in formula (A) is the molar volume (m 3 / mol). Note that V mθ and V mBoth of these can be obtained using well-known thermodynamic calculation software.

[0087] As described above, in this embodiment, the volume fraction V θ The method for determining is not particularly limited, and the above-mentioned thermodynamic calculation method or the above-mentioned extraction residue capture method may be used. In addition, in the steel material according to the present embodiment having the above-mentioned chemical composition, the area ratio S of precipitates obtained by the method using thermodynamic calculation and the method using the extraction residue capture can be calculated. θ ) is almost the same. Therefore, the area fraction S (%) of precipitates in each field area can be calculated by either method.

[0088] The area ratio S (%) of the precipitates and the height h (μm) and area ratio Z (%) obtained by the above method h From the plot of area ratio Z (%) and the three-dimensional roughness profile obtained by the above-mentioned method, the circle equivalent diameter and number density of each precipitate are obtained. Specifically, they can be obtained as follows. h Determine the height at which h(%) is closest to the area ratio S(%), and t The obtained height h is defined as (μm). t From the three-dimensional roughness profile, the height h t The distribution of steel within the observation field is obtained as two-dimensional information.

[0089] The two-dimensional information of the distribution of steel in the observation field includes the area occupied by the steel and voids. In this case, the area occupied by the steel is the area occupied by the precipitates. Therefore, by analyzing the acquired two-dimensional information, the circle-equivalent diameter of each precipitate in the observation field can be obtained. In this manner, the circle-equivalent diameter of all precipitates in the observation field is obtained. From the obtained circle-equivalent diameter of each precipitate, the number of precipitates with a circle-equivalent diameter of 20 nm or more and the number of precipitates with a circle-equivalent diameter of 20 to 100 nm (fine precipitates) are counted.

[0090] The above-mentioned method is performed in each observation field, and the number of precipitates with a circle equivalent diameter of 20 nm or more and the number of fine precipitates in each observation field are counted. The ratio of the number of fine precipitates to the number of precipitates with a circle equivalent diameter of 20 nm or more in all observation fields (number ratio of fine precipitates NR) is obtained. In this embodiment, the number ratio of fine precipitates NR is obtained by rounding off the obtained value to the fourth decimal place.

[0091] [Yield strength] As described above, the steel material according to this embodiment has the above-mentioned chemical composition, Fn1 satisfies 2.80 or more, Fn2 satisfies 41.0 to 150.0, and Fn3 satisfies 0.74 or more. As a result, the steel material according to this embodiment has both a yield strength of 862 to less than 965 MPa and excellent fracture toughness in a low-temperature sour environment. In other words, the steel material according to this embodiment has a yield strength of 862 to less than 965 MPa. The yield strength in this specification means the 0.2% offset yield strength obtained in a tensile test in accordance with ASTM E8 / E8M(2021).

[0092] The yield strength of the steel material according to this embodiment can be determined by the following method. Specifically, a tensile test is performed according to a method in accordance with 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. When the steel material is a steel pipe, a round bar test piece is prepared from the center of the wall thickness. When the steel material is round steel, a round bar test piece is prepared from the R / 2 position. The axial direction of the round bar test piece is parallel to the L direction of the steel material. Here, when the steel material is a steel plate, the rolling direction of the steel plate is defined as the "L direction" (Longitudinal). When the steel material is a steel pipe, the axial direction of the steel pipe is defined as the "L direction". When the steel material is round steel, the axial direction of the round steel is defined as the "L direction". The size of the round bar test piece is, for example, 4 mm in parallel diameter and 16 mm in gauge length.

[0093] Using the prepared round bar test piece, a tensile test is performed in air at room temperature (25 ° C) in accordance with ASTM E8 / E8M (2021). The 0.2% offset yield strength obtained by the tensile test is defined as the yield strength (MPa). In this embodiment, the yield strength (MPa) is obtained by rounding off the obtained value to the first decimal place.

[0094] [Fracture toughness in low-temperature sour environments] As described above, the steel material according to the present embodiment has the above-mentioned chemical composition, Fn1 satisfies 2.80 or more, Fn2 satisfies 41.0 to 150.0, and Fn3 satisfies 0.74 or more. As a result, the steel material according to the present embodiment achieves both a yield strength of 862 to less than 965 MPa and excellent fracture toughness in a low-temperature sour environment. In the present 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 the present embodiment, having excellent fracture toughness in a low-temperature sour environment is defined as follows.

[0095] A DCB test piece shown in FIG. 2A and a wedge shown in FIG. 2B 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 3.13 mm.

[0096] Referring to FIG. 2A, 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 volume of the test bath is 1 L per test piece. Next, N is poured into the test bath. 2Gas is blown into the test bath for 3 hours, and the bath is degassed until the dissolved oxygen level in the bath falls to 20 ppb or less.

[0097] In the deaerated test bath, 3% H 2 S gas and 97% N 2 The mixed gas is blown in at a total pressure of 1 atm to make the test bath a corrosive environment. 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.

[0098] 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 (4). ISSC Value (MPa m 1 / 2 In formula (4), 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.

[0099]

number

[0100] In this embodiment, the fracture toughness value K obtained by the DCB test performed under the above conditions ISSC Value is 22.0MPa m 1 / 2 If the fracture toughness value is equal to or greater than the above, it is determined that the material has excellent fracture toughness in a low-temperature sour environment. ISSC Value (MPa m 1 / 2 ) is calculated by rounding the obtained number to one decimal place.

[0101] [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 steel material will have a yield strength of 862 to less than 965 MPa (125 ksi class) and 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 862 to less than 965 MPa (125 ksi class) 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.

[0102] 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.

[0103] After polishing the observation surface of the test piece to a mirror finish, it is 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 SEM with 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.

[0104] [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 can achieve both a 125 ksi-class yield strength and excellent fracture toughness in a low-temperature sour environment.

[0105] [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.

[0106] [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.

[0107] 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.

[0108] [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.

[0109] [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.

[0110] 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%.

[0111] 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.

[0112] In addition, when the steel material is other than a seamless steel pipe, specifically when the steel material is a round steel, an intermediate steel material is manufactured as follows. First, the material is 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 manufacture 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.

[0113] Furthermore, when the steel material is a steel plate, an intermediate steel material is manufactured as follows. First, the material is 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 rolling using a blooming mill and a continuous rolling mill to manufacture an intermediate steel material in the shape of a steel plate.

[0114] 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.

[0115] 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.

[0116] [Quenching process] In the quenching process, quenching is performed on the prepared intermediate steel material (blank pipe). 3 This means that intermediate steel material above the hardening point is rapidly cooled. The preferred hardening temperature is 800 to 1000°C. If the hardening temperature is too high, the prior γ grains become coarse, and the SSC resistance of the steel material may decrease. Therefore, the hardening temperature is preferably 800 to 1000°C.

[0117] 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.

[0118] 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.

[0119] If the cooling rate during quenching is too slow, the microstructure may not be mainly composed of martensite and bainite. In this case, the mechanical properties (yield strength of 125 ksi) specified in this embodiment are not obtained. Furthermore, excellent low-temperature toughness and excellent SSC resistance are not obtained.

[0120] 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 800-500is 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).

[0121] 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.

[0122] 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.

[0123] [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 "tempering temperature" refers to the temperature of a furnace in which the intermediate steel material is heated and held after quenching at a temperature lower than the tempering temperature. The term "tempering time" refers to the time from when the temperature of the intermediate steel material reaches a predetermined tempering temperature until it is extracted from the heat treatment furnace.

[0124] In the past, when manufacturing steel materials for oil well applications, the tempering temperature was set to 600 to 730°C in order to improve strength, corrosion resistance, and the like. Tempering at such high temperatures is prone to cause coarsening of precipitates. Therefore, in this embodiment, tempering is first performed in a normal temperature range (first tempering), and then cold working is performed to form a large number of precipitate nuclei. Then, tempering is performed at a further high temperature for a short period of time to disperse a large number of fine precipitates. As a result, the steel material according to this embodiment can increase the number ratio NR of fine precipitates and make Fn3 0.74 or more. That is, in the tempering process according to this embodiment, two-stage tempering is performed in the order of first tempering, cold working, and second tempering, and cold working is performed therebetween. Each process will be described in detail below.

[0125] [First tempering process] In the first tempering step, the quenched intermediate steel material (blank pipe) is heated from room temperature to a tempering temperature, and then held at the tempering temperature for a tempering time. In the first tempering step, the steel material is adjusted to a desired strength. Therefore, if the tempering temperature in the first tempering step is too low, the tempering is insufficient, and sufficient fracture toughness in a low-temperature sour environment cannot be obtained. On the other hand, if the tempering temperature in the first tempering step is too high, the yield strength may be less than 125 ksi. Therefore, in the first tempering step according to this embodiment, the preferred tempering temperature is 600 to less than 720°C. A more preferred lower limit of the tempering temperature in the first tempering step is 620°C, more preferably 640°C, and more preferably 660°C. A more preferred upper limit of the tempering temperature in the first tempering step is 715°C, and more preferably 710°C.

[0126] In the first tempering step, if the tempering time is too short, the tempering is insufficient, and sufficient fracture toughness in a low-temperature sour environment cannot be obtained. On the other hand, if the tempering time in the first tempering step is too long, the yield strength may become less than 125 ksi. Therefore, in the first tempering step according to this embodiment, the tempering time is preferably 10 to 180 minutes. A more preferable lower limit of the tempering time in the first tempering step is 15 minutes, and more preferably 20 minutes. A more preferable upper limit of the tempering time in the first tempering step is 120 minutes, and more preferably 90 minutes.

[0127] [Cold working process] In the cold working step, cold working is performed on the intermediate steel material (blank pipe) tempered in the first tempering step. In the cold working step, cold working is performed on the intermediate steel material, thereby introducing strain into the intermediate steel material. As a result, nucleation sites for precipitates are introduced into the intermediate steel material. Therefore, in the second tempering step described below, a large number of fine precipitates can be dispersed. The cold working can be performed by a well-known method. That is, the cold working may be cold rolling, cold drawing, or pipe expansion. The temperature of the intermediate steel material in the cold working is, for example, 0 to 250°C.

[0128] When cold rolling is performed, the preferable reduction in area is 5 to 20%. If the reduction in area is too low, the intermediate steel material is not sufficiently strained, and the nucleation sites of precipitates may not be sufficiently introduced. As a result, the number ratio NR of fine precipitates may not increase, and Fn3 may not be 0.74 or more. On the other hand, if the reduction in area is too high, the intermediate steel material is excessively strained, and recrystallization may easily occur in the second tempering process. Here, if recrystallization occurs in the second tempering process, the strength of the steel material may be too low. In addition, if the second tempering is performed under conditions that ensure the required strength, there is a possibility that dislocations will not be sufficiently reduced. In this case, recrystallization may also easily occur partially in the second tempering process. As a result, the number ratio NR of fine precipitates may not increase, and Fn3 may not be 0.74 or more.

[0129] Therefore, in this embodiment, when cold rolling is performed in the cold working step, the area reduction rate is preferably 5 to 20%. Note that, in this embodiment, the area reduction rate when cold rolling is performed is defined by the following formula (B). Area reduction rate (%) = {1-(Cross-sectional area perpendicular to the processing direction of the intermediate steel after cold working process / Cross-sectional area perpendicular to the processing direction of the intermediate steel before cold working)} x 100 (B)

[0130] [Second tempering process] In the second tempering process, the intermediate steel material (blank pipe) that has been cold worked after the first tempering process is tempered at high temperature for a short time. Tempering at high temperature for a short time causes a large number of fine precipitates to precipitate at the nucleation sites of the precipitates introduced by the cold working. Therefore, if the tempering temperature in the second tempering process is too low, the precipitation of fine precipitates in the nucleation is insufficient, the number ratio NR of fine precipitates does not increase, and Fn3 may not reach 0.74 or more. On the other hand, if the tempering temperature in the second tempering process is too high, the tempering temperature A C1 The tempering temperature may exceed this point. In this case, austenite is mixed into the microstructure of the intermediate steel material. As a result, the microstructure of the steel material after the tempering process is not mainly composed of tempered martensite and tempered bainite, and the mechanical properties specified in this embodiment are not obtained. Therefore, in the second tempering process according to this embodiment, the preferred tempering temperature is 720 to 760°C. A more preferred lower limit of the tempering temperature in the second tempering process is 721°C, and more preferably 725°C. A more preferred upper limit of the tempering temperature in the second tempering process is 755°C, and more preferably 750°C, and more preferably 740°C.

[0131] In the second tempering process, if the tempering time is too short, precipitation of fine precipitates in nucleation is insufficient, the number ratio NR of fine precipitates does not increase, and Fn3 may not reach 0.74 or more. On the other hand, if the tempering time in the second tempering process is too long, the precipitates coarsen, the number ratio NR of fine precipitates does not increase, and Fn3 may not reach 0.74 or more. Therefore, in the second tempering process according to the present embodiment, the preferable tempering time is 2 to 25 minutes. A more preferable lower limit of the tempering time in the second tempering process is 3 minutes, and more preferably 5 minutes. A more preferable upper limit of the tempering time in the second tempering process is 20 minutes, and more preferably 15 minutes.

[0132] According to the above manufacturing method, the steel material according to the present embodiment can be manufactured. In the above manufacturing method, as an example, the manufacturing method of seamless steel pipes has been described. However, the steel material according to the present embodiment may be a steel plate or other shapes. The manufacturing methods of steel plates and other shapes also include, for example, a preparation process, a quenching process, and a tempering process, similar to the above manufacturing method. However, the above manufacturing method is an example, and it may be manufactured by other manufacturing methods. Hereinafter, the present disclosure will be described more specifically by examples.

Examples

[0133] 180 kg of molten steel having the chemical compositions shown in Table 1A, Table 1B, and Table 1C was manufactured. Note that “-” in Table 1B and Table 1C 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 mean that they were 0% after rounding to the third decimal place. The Nb content of Test No. 1 means that it was 0% after rounding to the fourth decimal place. Further, the Ca content, Mg content, Zr content, and REM content of Test No. 1 mean that they were 0% after rounding to the fifth decimal place. In this example, neodymium (Nd) was used as the rare earth element (REM). Further, Fn1 obtained from the obtained chemical composition (mass%) and formula (1), and Fn2 obtained from the obtained chemical composition (mass%) and formula (2) are shown in Table 1C.

[0134] [Table 1A]

[0135] [Table 1B]

[0136] [Table 1C]

[0137] Ingots were produced using the molten steel of each test number. The produced ingots were hot rolled to produce steel plates with a plate thickness of 15 mm. After hot rolling, the steel plates of each test number were allowed to cool to room temperature (25°C). After cooling, the steel plates of each test number were held at the quenching temperature (920°C) for 20 minutes, and then quenched by immersing them 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.

[0138] After quenching, the steel plates of each test number were tempered. In the tempering, the steel plates of test numbers 1 to 26, 29 to 32, 34 and 35 were subjected to first tempering, cold working and second tempering. The steel plates of test numbers 27 and 28 were subjected to first tempering and second tempering. The steel plate of test number 33 was subjected to first tempering and cold working. In this example, cold rolling was performed as the cold working.

[0139] Table 2 shows the tempering temperature (°C) and tempering time (min) in the first tempering for the steel plate of each test number. Similarly, Table 2 shows the area reduction rate (%) of cold working for the steel plate of each test number. Table 2 shows the tempering temperature (°C) and tempering time (min) in the second tempering for the steel plate of each test number. In Table 2, "-" in the "cold working" column means that cold working was not performed. Similarly, in Table 2, "-" in the "second tempering" column means that second tempering was not performed.

[0140] [Table 2]

[0141] In this embodiment, the tempering temperature is the temperature of the heat treatment furnace in which the tempering is performed, and the tempering time is the time from when the temperature of the steel plate of each test number reaches a predetermined tempering temperature to when the steel plate is extracted from the heat treatment furnace.

[0142] [Evaluation test] The tempered steel sheets of each test number were subjected to a tensile test, a measurement test for the number ratio of fine precipitates (NR), and a DCB test, all of which are described below.

[0143] [Tensile test] A tensile test was performed on the steel plate of each test number by the method described above. Specifically, a round bar tensile test piece with a parallel part diameter of 4 mm and a gauge length of 16 mm was prepared from the center of the plate thickness of the steel plate of each test number. The axial direction of the round bar tensile test piece was parallel to the rolling direction of the steel plate. A tensile test in accordance with ASTM E8 / E8M (2021) was performed using the round bar test piece of each test number at room temperature (25°C) in air. The 0.2% offset yield strength obtained by the tensile test was obtained, and the yield strength (MPa) of each test number was obtained. The obtained yield strength is shown in Table 2 as "YS (MPa)".

[0144] [Measurement test for the number ratio of fine precipitates (NR)] For the steel plate of each test number, Fn3 was calculated from the number ratio NR of precipitates (fine precipitates) having a circle equivalent diameter of 20 to 100 nm by the above-mentioned measurement method and formula (3). The SEM used was an ERA-8900FE manufactured by ELIONIX, with an acceleration voltage of 5 kV and a working distance of 15 mm. The observation field was 12 μm × 9 μm (magnification 10,000 times), and three fields of observation were performed. The area ratio S (%) of precipitates in the observation field was calculated by the volume ratio V of cementite obtained by thermodynamic calculation using the chemical composition of the steel plate of each test number and the first and second tempering temperatures. θ The thermodynamic calculations were performed using the thermodynamic calculation software Thermo-Calc (Thermo-Calc Software, Inc., version: 2017a), and the database used was TCFE8.

[0145] The ratio of the total number of precipitates having a circle equivalent diameter of 20 to 100 nm (fine precipitates) to the total number of precipitates having a circle equivalent diameter of 20 nm or more obtained in three visual fields was calculated to obtain the number ratio NR of fine precipitates. The number ratio NR of fine precipitates obtained for the steel sheet of each test number is shown in Table 2. Furthermore, the chemical composition (mass%) of each test number, the number ratio NR of fine precipitates obtained, and Fn3 calculated from formula (3) are shown in Table 2.

[0146] [DCB Test] A DCB test was performed on the steel plate of each test number by the method described above. Specifically, the above-mentioned DCB test specimens and wedges were prepared from the center of the plate thickness of the steel plate of each test number. Using the prepared test specimens and wedges, a DCB test was performed in accordance with NACE TM0177-2016 Method D under the above-mentioned conditions. The fracture toughness value K obtained from the DCB test performed by the above-mentioned method was 1SSC The value is "K 1SSC (MPa m 1 / 2 )" in Table 2.

[0147] [Evaluation Results] With reference to Tables 1A, 1B, 1C, and 2, the chemical compositions of the steel plates of test numbers 1 to 14 were appropriate, and Fn1 satisfied 2.80 or more, and Fn2 satisfied 41.0 to 150.0. Furthermore, these steel plates had a yield strength of 862 to less than 965 MPa (125 ksi class). Furthermore, these steel plates had Fn3 of 0.74 or more. As a result, these steel plates had a fracture toughness value K ISSC Value is 22.0MPa m 1 / 2 That is, these steel plates achieved both a 125 ksi-class yield strength and excellent fracture toughness in a low-temperature sour environment.

[0148] On the other hand, the steel plates of test numbers 15 and 16 had Fn1 of less than 2.80. As a result, these steel plates had a fracture toughness value K ISSC Value is 22.0MPa m 1 / 2 and did not have excellent fracture toughness in a low-temperature sour environment.

[0149] The steel plate of test number 17 had Fn1 of less than 2.80. Furthermore, the steel plate had Fn3 of less than 0.74. As a result, the fracture toughness value K ISSC Value is 22.0MPa m 1 / 2 and did not have excellent fracture toughness in a low-temperature sour environment.

[0150] The steel plates of test numbers 18 and 19 had Fn2 of less than 41.0. These steel plates also had Fn3 of less than 0.74. As a result, these steel plates had a fracture toughness value K ISSC Value is 22.0MPa m 1 / 2 and did not have excellent fracture toughness in a low-temperature sour environment.

[0151] The steel plates of test numbers 20 and 21 had Fn2 exceeding 150.0. Furthermore, the Fn3 of these steel plates was less than 0.74. As a result, the fracture toughness value K ISSC Value is 22.0MPa m 1 / 2 and did not have excellent fracture toughness in a low-temperature sour environment.

[0152] The steel plate of test number 22 had too low a Cr content. Furthermore, the Fn3 of this steel plate was less than 0.74. As a result, the fracture toughness value K ISSC Value is 22.0MPa m 1 / 2 and did not have excellent fracture toughness in a low-temperature sour environment.

[0153] The Mo content of the steel plate of test number 23 was too low. As a result, the fracture toughness value K ISSC Value is 22.0MPa m 1 / 2 and did not have excellent fracture toughness in a low-temperature sour environment.

[0154] The steel plate with test number 24 had too high a Mn content. Furthermore, this steel plate had a Fn3 of less than 0.74. As a result, this steel plate had a fracture toughness value K ISSC Value is 22.0MPa m 1 / 2 and did not have excellent fracture toughness in a low-temperature sour environment.

[0155] The steel plate with test number 25 had too high a N content. As a result, the fracture toughness value K ISSC Value is 22.0MPa m 1 / 2 and did not have excellent fracture toughness in a low-temperature sour environment.

[0156] The steel plate with test number 26 had too high a P content. As a result, the fracture toughness value K ISSC Value is 22.0MPa m 1 / 2 and did not have excellent fracture toughness in a low-temperature sour environment.

[0157] The steel plates of test numbers 27 and 28 were not subjected to cold working in the tempering process. As a result, the Fn3 of these steel plates was less than 0.74. As a result, the fracture toughness value K ISSC Value is 22.0MPa m 1 / 2 and did not have excellent fracture toughness in a low-temperature sour environment.

[0158] The steel plates of test numbers 29 and 30 had too low a reduction in area in the cold working of the tempering process. As a result, the Fn3 of these steel plates was less than 0.74. As a result, the fracture toughness value K ISSC Value is 22.0MPa m 1 / 2 and did not have excellent fracture toughness in a low-temperature sour environment.

[0159] The steel plates of test numbers 31 and 32 had too high a reduction in area during the cold working in the tempering process. As a result, the Fn3 of these steel plates was less than 0.74. As a result, the fracture toughness value K ISSC Value is 22.0MPa m 1 / 2 and did not have excellent fracture toughness in a low-temperature sour environment.

[0160] The steel plate of test number 33 was not subjected to the second tempering in the tempering process. As a result, the Fn3 of this steel plate was less than 0.74. As a result, the fracture toughness value K ISSC Value is 22.0MPa m 1 / 2 and did not have excellent fracture toughness in a low-temperature sour environment.

[0161] The tempering temperature of the steel plates of test numbers 34 and 35 was too low in the second tempering step. As a result, the Fn3 of these steel plates was less than 0.74. As a result, the fracture toughness value K ISSC Value is 22.0MPa m 1 / 2 and did not have excellent fracture toughness in a low-temperature sour environment.

[0162] 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: 0.35-0.50%, Si: 0.80 to 1.50%, Mn: 0.02 to less than 0.50% P: 0.025% or less, S: 0.0100% or less, Al: 0.005-0.100%, Cr: 0.20-1.50%, Mo: 0.35-3.00%, 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%, Co: 0 to 0.50%, W: 0-0.50%, Ni: 0 to 1.20%, Cu: 0 to 0.50%, Ca: 0-0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, Rare earth elements: 0 to 0.0100%, and The balance is Fe and impurities, Fn1 defined by formula (1) is 2.80 or more, Fn2 defined by formula (2) satisfies 41.0 to 150.0, The yield strength is less than 862 to 965 MPa; In the steel material, When the number ratio of precipitates having an equivalent circle diameter of 20 to 100 nm among precipitates having an equivalent circle diameter of 20 nm or more is defined as NR, Fn3 defined by formula (3) is 0.74 or more; Steel material. Fn1=3.3×Si−1.4×Mn+1.1×Mo (1) Fn2=390×C-57.5×Cr-21.4×Mo-114.1×V (2) <h2 style=";text-align:left;direction:ltr">Fn3=NR×(C+4×Si-1.5×Mn-0.5×Cr+0.2×Mo-0.5×V-Ti)<h2 style=";text-align:left;direction:ltr"> 1 / 2 <h2 style=";text-align:left;direction:ltr"> (3) Here, the element symbols in formulas (1) to (3) are substituted with the contents of the corresponding elements in units of mass %, and NR in formula (3) is substituted with the number ratio of precipitates having a circle-equivalent diameter of 20 to 100 nm among precipitates having a circle-equivalent diameter of 20 nm or more.

2. The steel material according to claim 1, Nb: 0.001-0.030%, Co: 0.01 to 0.50%, W: 0.01-0.50%, Ni: 0.01 to 1.20%, Cu: 0.01 to 0.50%, Ca: 0.0001-0.0100%, Mg: 0.0001-0.0100%, Zr: 0.0001 to 0.0100%, and Rare earth elements: 0.0001 to 0.0100%, 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.

Images