steel

Abstract

Provided is a steel material that achieves both high strength at a 150 ksi grade and excellent SSC resistance. A steel material according to the present disclosure comprises, in mass%, 0.15-0.45% of C, 0.05-1.00% of Si, 0.05-1.00% of Mn, 0.030% or less of P, 0.0050% or less of S, 0.005-0.100% of Al, 0.30-1.50% of Cr, 0.40-2.00% of Mo, 0.002-0.020% of Ti, 0.002-0.100% of Nb, 0.05-0.30% of V, 0.0005-0.0040% of B, 0.0100% or less of N, and 0.0040% or less of O, with the remaining portion being Fe and impurities, and has a yield strength of greater than 1034 to 1172 MPa. In the steel material, the number density of Al oxides having a major axis that is 5.0 µm or longer is less than 30 particles / 200 mm2, and the number density of Si oxides having a major axis that is 5.0 µm or longer is no more than 5 particles / 200 mm2.

Description

[Technical Field]

[0001] The present disclosure relates to steel products. [Background technology]

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

[0003] Oil wells may also contain corrosive gases such as hydrogen sulfide gas (H2S) and carbon dioxide gas (CO2). Therefore, steel intended for use in oil wells must have not only high strength but also excellent corrosion resistance. Furthermore, oil well steel is subjected to stress during use. For this reason, sulfide stress cracking resistance (hereinafter referred to as SSC resistance) has been used as an indicator of the excellent corrosion resistance of oil well steel.

[0004] Techniques for increasing the strength and SSC resistance of steel materials have been proposed in JP 2006-28612 A (Patent Document 1), WO 2008 / 123422 A (Patent Document 2), and JP 2017-166060 A (Patent Document 3).

[0005] The steel disclosed in Patent Document 1 is a steel for steel pipes, and contains, by mass%, 0.2-0.7% C, 0.01-0.8% Si, 0.1-1.5% Mn, 0.005% or less S, 0.03% or less P, 0.0005-0.1% Al, 0.005-0.05% Ti, 0.0004-0.005% Ca, 0.007% or less N, 0.1-1.5% Cr, 0.2-1.0% Mo, and the balance being Fe and impurities. This steel further contains non-metallic inclusions containing Ca, Al, Ti, N, O, and S, with a (Ca%) / (Al%) ratio of 0.55-1.72 and a (Ca%) / (Ti%) ratio of 0.7-19. Patent Document 1 states that this steel has a high yield strength of more than 758 MPa and excellent SSC resistance.

[0006] The steel material disclosed in Patent Document 2 is a low alloy steel containing, by mass%, C: 0.10 to 0.20%, Si: 0.05 to 1.0%, Mn: 0.05 to 1.5%, Cr: 1.0 to 2.0%, Mo: 0.05 to 2.0%, Al: 0.10% or less, and Ti: 0.002 to 0.05%, and Ceq (= C + (Mn / 6) + (Cr + Mo + V) / 5) is 0.65 or more, with the balance being Fe and impurities, among which P: 0.025% or less, S: 0.010% or less, N: 0.007% or less, and B: less than 0.0003%. This steel material further contains M having a grain size of 1 μm or more. 23 C6 type precipitates 0.1 pieces / mm 2 The following is a description of the steel material: Patent Document 2 states that this steel material has a yield strength of 654 to 793 MPa and has excellent SSC resistance even in a high-pressure hydrogen sulfide environment.

[0007] The steel material disclosed in Patent Document 3 is a material for high-strength oil well steel pipes, and consists, in mass%, of C: 0.20 to 0.45%, Si: 0.05 to 0.40%, Mn: 0.3 to 0.9%, P: 0.015% or less, S: 0.005% or less, Al: 0.005 to 0.10%, N: 0.001 to 0.006%, Cr: 0.1 to 0.8%, Mo: 0.1 to 1.6%, V: 0.02 to 0.2%, Nb: 0.001 to 0.04%, B: 0.0003 to 0.0030%, O (oxygen): 0.0030% or less, and the balance being Fe and unavoidable impurities. This steel material further has a Rockwell hardness HRC that satisfies the formula (15.6 × [%C] + 29.2 ≦ HRC < 60.5 × [%C] + 31.1). Patent Document 3 describes that this steel material can provide a steel pipe having a yield strength of 758 to less than 862 MPa and excellent SSC resistance. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Laid-Open No. 2006-28612 [Patent Document 2] International Publication No. 2008 / 123422 [Patent Document 3] Japanese Patent Application Laid-Open No. 2017-166060 Summary of the Invention [Problem to be solved by the invention]

[0009] Oil well steel materials having excellent SSC resistance can be obtained according to the techniques disclosed in the above Patent Documents 1 to 3. However, oil well steel materials having excellent SSC resistance may also be obtained by techniques other than the techniques disclosed in the above Patent Documents 1 to 3.

[0010] Furthermore, in recent years, as oil well environments have become more severe, there has been a demand for even higher yield strength in oil well steel materials. Specifically, there has been a demand for oil well steel materials of the 150 ksi class (yield strength of more than 150 to 170 ksi, i.e., more than 1034 to 1172 MPa). However, Patent Documents 1 to 3 do not consider achieving both a high yield strength of the 150 ksi class and excellent SSC resistance.

[0011] An object of the present disclosure is to provide a steel material that combines high strength of 150 ksi class (over 1034 to 1172 MPa) with excellent SSC resistance. [Means for solving the problem]

[0012] The steel material according to the present disclosure is In mass%, C: 0.15~0.45%, Si: 0.05 to 1.00%, Mn: 0.05 to 1.00%, P:0.030% or less, S: 0.0050% or less, Al: 0.005 to 0.100%, Cr: 0.30~1.50%, Mo: 0.40-2.00%, Ti: 0.002 to 0.020%, Nb: 0.002 to 0.100%, V: 0.05 to 0.30%, B: 0.0005~0.0040%, N: 0.0100% or less, O: 0.0040% or less, Cu: 0-0.50% Ni: 0 to 0.50% W: 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, The yield strength is greater than 1034 to 1172 MPa, In the steel material, The Al content is 20% or more by mass, the O content is 10% or more, and the number density of Al oxides with a major axis of 5.0 μm or more is 30 / 200 mm 2 is less than In mass%, the Al content is less than 20%, the Si content is 20% or more, the O content is 10% or more, and the number density of Si oxides with a major axis of 5.0 μm or more is 5 pieces / 200 mm 2 The following is the result. [Effects of the Invention]

[0013] The steel material according to the present disclosure can achieve both high strength of 150 ksi class (over 1034 to 1172 MPa) and excellent SSC resistance. [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 1 is a diagram showing the relationship between the number density (pieces / 200 mm) of coarse Si oxides (Si oxides with a major axis of 5.0 μm or more) in this example and the number of SSC occurrences (pieces), which is an index of SSC resistance. DETAILED DESCRIPTION OF THE INVENTION

[0015] First, the inventors focused on the chemical composition and investigated how to obtain a steel material that combines 150 ksi-class yield strength with excellent SSC resistance. As a result, the inventors found a steel material containing, in mass%, C: 0.15-0.45%, Si: 0.05-1.00%, Mn: 0.05-1.00%, P: 0.030% or less, S: 0.0050% or less, Al: 0.005-0.100%, Cr: 0.30-1.50%, Mo: 0.40-2.00%, Ti: 0.002-0.020%, Nb: 0.002-0.100%, V: 0.05-0.30%, and B: 0.0005-0.100%. It was thought that if a steel material consisting of the following elements was used, N: 0.0100% or less, O: 0.0040% or less, Cu: 0-0.50%, Ni: 0-0.50%, W: 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 would be possible to achieve both a 150 ksi-class yield strength and excellent SSC resistance.

[0016] Next, the inventors investigated various methods for improving the SSC resistance of steel materials having the above-mentioned chemical composition and a 150 ksi-class yield strength. Specifically, the inventors considered that if coarse oxide-based inclusions could be reduced, it might be possible to improve SSC resistance while maintaining yield strength. Here, in steel materials having the above-mentioned chemical composition, Al oxides, mainly Al2O3, tend to coarsen. Therefore, the inventors first focused on coarse Al oxides.

[0017] As a result of investigations by the present inventors, it was found that in a steel material having the above-mentioned chemical composition and a yield strength of 150 ksi, the number density of Al oxide particles having a major axis of 5.0 μm or more was 30 particles / 200 mm 2 It has been revealed that if the Al content is less than 20%, the SSC resistance may be improved. Herein, particles having an Al content of 20% or more and an O content of 10% or more, in mass%, are also referred to as "Al oxides." Furthermore, in this specification, Al oxides having a major axis of 5.0 μm or more are also referred to as "coarse Al oxides."

[0018] Here, Al oxides are hard oxides and tend to reduce the corrosion resistance of steel materials. In particular, when the yield strength is increased to 150 ksi class, the effect of coarse Al oxides tends to become apparent, and SSC resistance tends to decrease significantly. Therefore, in the steel material according to this embodiment having the above-mentioned chemical composition and a yield strength of 150 ksi class, the number density of coarse Al oxides is set to 30 particles / 200 mm 2 Less than.

[0019] On the other hand, the number density of coarse Al oxides is 30 / 200mm 2 Even if the steel material has a yield strength of 150 ksi or less, there are cases where stable and excellent SSC resistance cannot be obtained. Therefore, the inventors have developed a steel material having the above-mentioned chemical composition and a yield strength of 150 ksi, in which the number density of coarse Al oxides is 30 particles / 200 mm 2 As a result of detailed investigations by the inventors, it was found that a steel material having the above-mentioned chemical composition, a yield strength of 150 ksi class, and a number density of coarse Al oxides of 30 particles / 200 mm 2 It became clear that for steels with a carbon content of less than 100%, if not only the coarse Al oxides in the steel but also the coarse Si oxides can be reduced, it may be possible to stably obtain excellent SSC resistance.

[0020] Herein, in this specification, particles having an Al content of less than 20%, an Si content of 20% or more, and an O content of 10% or more, in mass %, are also referred to as "Si oxides." Furthermore, in this specification, Si oxides having a major axis of 5.0 μm or more are also referred to as "coarse Si oxides." Hereinafter, particles having the above-mentioned chemical composition, a yield strength of 150 ksi, and a number density of coarse Al oxides of 30 particles / 200 mm 2 The relationship between coarse Si oxides and SSC resistance for steel materials of less than 1000 kJ / cm2 will be specifically explained with reference to the drawings.

[0021] FIG. 1 shows the number density (particles / 200 mm ) of coarse Si oxides (Si oxides with a major axis of 5.0 μm or more) in this example. 21 is a graph showing the relationship between the number of SSCs (number of SSCs) and the number of SSCs (number of SSCs), which is an index of SSC resistance. 2 For steel materials with a particle size of less than 100 mm, the number density of coarse Si oxide particles (particles / 200 mm) was determined using the method described below. 2 ) and the number of SSC occurrences (number of pieces) determined by the method described below.

[0022] Referring to FIG. 1, the aluminum alloy has the above-mentioned chemical composition and a yield strength of 150 ksi, and the number density of coarse Al oxides is 30 particles / 200 mm 2 In steel materials with a particle density of less than 5 particles / 200 mm 2 If the number of SSC occurrences was 0, the steel sheet exhibited excellent SSC resistance. Therefore, in this embodiment, the steel sheet has the above-mentioned chemical composition, a yield strength of 150 ksi, and a number density of coarse Al oxides of 30 particles / 200 mm 2 Furthermore, the number density of coarse Si oxides is set to less than 5 pieces / 200 mm 2 As a result, the steel material according to this embodiment can achieve both a 150 ksi-class yield strength and excellent SSC resistance.

[0023] The details of why reducing the number density of coarse Si oxides improves the SSC resistance of steel materials are not clear. However, the inventors speculate as follows: When manufacturing steel materials having the above-mentioned chemical composition, deoxidation is mainly carried out with aluminum (Al) in the steelmaking process. Therefore, for steel materials having the above-mentioned chemical composition, Al oxides, typified by Al2O3, have been studied, and attention has not been paid to the Si oxides, which are fewer in number. However, when the yield strength is increased to the 150 ksi class, there is a possibility that a decrease in SSC resistance will be readily apparent not only from coarse Al oxides but also from the few coarse Si oxides. Therefore, it is considered that the number density of coarse Al oxides should be reduced to 30 / 200 mm 2 Not only is the number density of coarse Si oxides reduced to less than 5 particles / 200 mm 2The present inventors speculate that by doing so, excellent SSC resistance can be stably obtained even if the steel has a yield strength of 150 ksi or more.

[0024] It is possible that the SSC resistance of steel materials is enhanced by a mechanism different from that speculated by the present inventors. However, in the case of steel materials having the above-mentioned chemical composition and a yield strength of 150 ksi class, and a number density of coarse Al oxides of 30 particles / 200 mm 2 For steel materials with a particle size of less than 5 particles / 200 mm, the number density of coarse Si oxides is set to 5 particles / 200 mm. 2 It is demonstrated by the examples described below that excellent SSC resistance can be obtained by satisfying the following conditions.

[0025] The gist of the steel material according to this embodiment, which was completed based on the above findings, is as follows.

[0026] [1] A steel material, In mass%, C: 0.15~0.45%, Si: 0.05 to 1.00%, Mn: 0.05 to 1.00%, P:0.030% or less, S: 0.0050% or less, Al: 0.005 to 0.100%, Cr: 0.30~1.50%, Mo: 0.40-2.00%, Ti: 0.002 to 0.020%, Nb: 0.002 to 0.100%, V: 0.05 to 0.30%, B: 0.0005~0.0040%, N: 0.0100% or less, O: 0.0040% or less, Cu: 0-0.50% Ni: 0 to 0.50% W: 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, The yield strength is greater than 1034 to 1172 MPa, In the steel material, The Al content is 20% or more by mass, the O content is 10% or more, and the number density of Al oxides with a major axis of 5.0 μm or more is 30 / 200 mm 2 is less than In mass%, the Al content is less than 20%, the Si content is 20% or more, the O content is 10% or more, and the number density of Si oxides with a major axis of 5.0 μm or more is 5 pieces / 200 mm 2 Below is the Steel material.

[0027] [2] [1] The steel material according to Cu: 0.01 to 0.50% Ni: 0.01 to 0.50% W: 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: containing one or more elements selected from the group consisting of 0.0001 to 0.0100%; Steel material.

[0028] [3] The steel material according to [1] or [2], The steel material is a seamless steel pipe. Steel material.

[0029] 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. Note that round bar refers to 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.

[0030] The steel material according to this embodiment will be described in detail below. Unless otherwise specified, "%" for elements means mass %.

[0031] [Chemical composition] The chemical composition of the steel material according to this embodiment contains the following elements.

[0032] C: 0.15 to 0.45% Carbon (C) improves the hardenability of steel and increases its strength. Furthermore, C promotes the spheroidization of carbides during tempering in the manufacturing process, thereby improving the SSC resistance 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 ranges of this embodiment. On the other hand, if the C content is too high, even if the contents of other elements are within the ranges of this embodiment, the amount of carbides will be too large, and the SSC resistance of the steel will decrease. Therefore, the C content is 0.15 to 0.45%. The preferred lower limit of the C content is 0.18%, more preferably 0.20%, even more preferably 0.22%, and even more preferably 0.23%. The preferred upper limit of the C content is 0.40%, more preferably 0.38%, even more preferably 0.35%, and even more preferably 0.30%.

[0033] Si: 0.05 to 1.00% Silicon (Si) deoxidizes steel. If the Si content is too low, the above effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Si content is too high, a large number of coarse Si oxides are formed, and the SSC resistance of the steel may decrease even if the contents of other elements are within the ranges of this embodiment. Therefore, the Si content is 0.05 to 1.00%. The preferred lower limit of the Si content is 0.10%, more preferably 0.15%, and even more preferably 0.20%. The preferred upper limit of the Si content is 0.85%, more preferably 0.75%, even more preferably 0.60%, even more preferably 0.50%, and even more preferably 0.40%.

[0034] Mn: 0.05 to 1.00% Manganese (Mn) deoxidizes steel. Mn also improves the hardenability of steel. If the Mn content is too low, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Mn content is too high, coarse sulfide-based inclusions are formed, reducing the SSC resistance of the steel, even if the contents of other elements are within the ranges of this embodiment. Therefore, the Mn content is 0.05 to 1.00%. The preferred lower limit of the Mn content is 0.06%, more preferably 0.08%, and even more preferably 0.10%. The preferred upper limit of the Mn content is 0.90%, more preferably 0.80%, even more preferably 0.70%, even more preferably 0.60%, and even more preferably 0.50%.

[0035] P:0.030% or less Phosphorus (P) is an impurity. That is, the lower limit of the P content is greater than 0%. If the P content is too high, even if the contents of other elements are within the ranges of this embodiment, P segregates at grain boundaries, reducing the SSC resistance of the steel material. Therefore, the P content is 0.030% or less. A preferred upper limit of the P content is 0.025%, more preferably 0.020%, even more preferably 0.015%, and even more preferably 0.010%. The P content is preferably as low as possible. However, an extreme reduction in the P content significantly increases manufacturing costs. Therefore, considering industrial production, a preferred lower limit of the P content is 0.001%, more preferably 0.002%, and even more preferably 0.003%.

[0036] S: 0.0050% or less Sulfur (S) is an impurity. That is, the lower limit of the S content is greater than 0%. If the S content is too high, even if the contents of other elements are within the ranges of this embodiment, S segregates at grain boundaries, reducing the SSC resistance of the steel material. Therefore, the S content is 0.0050% or less. A preferred upper limit of the S content is 0.0040%, more preferably 0.0032%, even more preferably 0.0030%, even more preferably 0.0020%, and even more preferably 0.0015%. The S content is preferably as low as possible. However, an extreme reduction in the S content significantly increases production costs. Therefore, considering industrial production, a preferred lower limit of the S content is 0.0001%, more preferably 0.0002%, and even more preferably 0.0003%.

[0037] Al: 0.005 to 0.100% Aluminum (Al) deoxidizes steel. If the Al content is too low, the above effect is not sufficiently obtained, even if the contents of other elements are within the ranges of this embodiment, and the SSC resistance of the steel material decreases. On the other hand, if the Al content is too high, a large number of coarse Al oxides are formed, and the SSC resistance of the steel material decreases, even if the contents of other elements are within the ranges of this embodiment. Therefore, the Al content is 0.005 to 0.100%. A preferred lower limit of the Al content is 0.010%, more preferably 0.015%, and even more preferably 0.020%. A preferred upper limit of the Al content is 0.080%, more preferably 0.060%, even more preferably 0.040%, and even more preferably 0.035%. The "Al" content in this specification refers to the content of "acid-soluble Al," i.e., "sol. Al."

[0038] Cr: 0.30~1.50% Chromium (Cr) improves the hardenability of steel materials. Cr also increases the temper softening resistance of steel materials, enabling high-temperature tempering. As a result, the SSC resistance of steel materials is improved. If the Cr content is too low, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Cr content is too high, the SSC resistance of steel materials decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the Cr content is 0.30 to 1.50%. The preferred lower limit of the Cr content is 0.35%, more preferably 0.40%, and even more preferably 0.50%. The preferred upper limit of the Cr content is 1.40%, more preferably 1.30%, even more preferably 1.20%, even more preferably 1.10%, and even more preferably 1.05%.

[0039] Mo: 0.40 to 2.00% Molybdenum (Mo) improves the hardenability of steel. Mo also increases the temper softening resistance of steel, enabling high-temperature tempering. As a result, the SSC resistance of steel is improved. If the Mo content is too low, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Mo content is too high, the above effects saturate. Therefore, the Mo content is 0.40 to 2.00%. The preferred lower limit of the Mo content is 0.45%, more preferably 0.50%, even more preferably 0.51%, even more preferably 0.55%, and even more preferably 0.60%. The preferred upper limit of the Mo content is 1.80%, more preferably 1.60%, even more preferably 1.40%, and even more preferably 1.30%.

[0040] Ti: 0.002 to 0.020% Titanium (Ti) bonds with N to form nitrides, which refine the grains of the steel material through a pinning effect. As a result, the strength of the steel material 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 ranges of this embodiment. On the other hand, if the Ti content is too high, even if the contents of other elements are within the ranges of this embodiment, the Ti nitrides become coarse, and the SSC resistance of the steel material decreases. Therefore, the Ti content is 0.002 to 0.020%. The preferred lower limit of the Ti content is 0.003%, and more preferably 0.004%. The preferred upper limit of the Ti content is 0.018%, more preferably 0.015%, even more preferably 0.010%, and even more preferably 0.008%.

[0041] Nb: 0.002 to 0.100% Niobium (Nb) combines with C and / or N to form carbides, nitrides, or carbonitrides (hereinafter referred to as "carbonitrides, etc."). Carbonitrides, etc., refine the grains of steel material through a pinning effect, thereby improving the SSC resistance of the steel material. Nb also forms fine carbides during tempering, improving the temper softening resistance of the steel material and increasing the strength of the steel material. If the Nb content is too low, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Nb content is too high, carbonitrides, etc. are formed in excess, even if the contents of other elements are within the ranges of this embodiment, and the SSC resistance of the steel material decreases. Therefore, the Nb content is 0.002 to 0.100%. A preferable lower limit of the Nb content is 0.005%, more preferably 0.010%, even more preferably 0.015%, and still more preferably 0.020%. The upper limit of the Nb content is preferably 0.080%, more preferably 0.060%, and even more preferably 0.040%.

[0042] V: 0.05 to 0.30% Vanadium (V) forms carbonitrides and the like. Carbonitrides and the like have a pinning effect, which refines the grain size of the steel material and improves the SSC resistance of the steel material. V also forms fine carbides during tempering, which increases the temper softening resistance of the steel material and increases its strength. If the V content is too low, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the V content is too high, even if the contents of other elements are within the ranges of this embodiment, excessive carbonitrides and the like are formed, and the SSC resistance of the steel material decreases. Therefore, the V content is 0.05 to 0.30%. The lower limit of the V content is preferably 0.06%, more preferably 0.07%, and even more preferably 0.08%. The upper limit of the V content is preferably 0.25%, more preferably 0.20%, and even more preferably 0.15%.

[0043] B: 0.0005 to 0.0040% Boron (B) dissolves in steel to improve the hardenability of the steel material and increase its strength. If the B content is too low, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the B content is too high, coarse nitrides are formed, even if the contents of other elements are within the ranges of this embodiment, and the SSC resistance of the steel material decreases. Therefore, the B content is 0.0005 to 0.0040%. The preferred lower limit of the B content is 0.0006%, and more preferably 0.0008%. The preferred upper limit of the B content is 0.0035%, more preferably 0.0030%, even more preferably 0.0025%, and even more preferably 0.0020%.

[0044] N: 0.0100% or less Nitrogen (N) is unavoidably contained. That is, the lower limit of the N content is greater than 0%. N combines with Ti to form nitrides, which refine the grains of the steel material through a pinning effect. As a result, the strength of the steel material is increased. However, if the N content is too high, even if the contents of other elements are within the ranges of this embodiment, coarse nitrides are formed, and the SSC resistance of the steel material is reduced. Therefore, the N content is 0.0100% or less. The preferred upper limit of the N content is 0.0080%, more preferably 0.0060%, even more preferably 0.0050%, and even more preferably 0.0040%. To more effectively obtain the above effects, the preferred lower limit of the N content is 0.0005%, more preferably 0.0010%, even more preferably 0.0015%, and even more preferably 0.0020%.

[0045] O: 0.0040% 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 ranges of this embodiment, coarse oxides are formed, and the SSC resistance of the steel material decreases. Therefore, the O content is 0.0040% or less. A preferred upper limit of the O content is 0.0035%, more preferably 0.0033%, even more preferably 0.0030%, even more preferably 0.0025%, 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 production costs. Therefore, considering industrial production, a preferred lower limit of the O content is 0.0001%, more preferably 0.0002%, and even more preferably 0.0003%.

[0046] The balance of the chemical composition of the steel material according to the present embodiment is composed of Fe and impurities. Here, the impurities refer to substances that are mixed in from raw materials such as ore and scrap, or the manufacturing environment, during industrial production of the steel material, and are acceptable within a range that does not adversely affect the steel material according to the present embodiment.

[0047] [Optional element] The chemical composition of the steel material described above may further contain one or more elements selected from the group consisting of Cu and Ni in place of a portion of Fe. All of these elements are optional elements and improve the hardenability of the steel material.

[0048] Cu: 0 to 0.50% Copper (Cu) is an optional element and may not be contained. That is, the Cu content may be 0%. When contained, Cu improves the hardenability of the steel material and increases the strength of the steel material. Even 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, the SSC resistance of the steel material will decrease even if the contents of other elements are within the ranges of this embodiment. 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%, and even more preferably 0.02%. The preferred upper limit of the Cu content is 0.35%, more preferably 0.25%, even more preferably 0.15%, even more preferably 0.10%, and even more preferably 0.05%.

[0049] Ni: 0 to 0.50% 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 and strength of the steel material. Ni also dissolves in the steel to improve the SSC resistance of the steel material. Even if even a small amount of Ni is contained, these effects can be obtained to some extent. However, if the Ni content is too high, localized corrosion is promoted and the SSC resistance of the steel material decreases, even if the contents of other elements are within the ranges of this embodiment. Therefore, the Ni content is 0 to 0.50%. The lower limit of the Ni content is preferably more than 0%, more preferably 0.01%, and even more preferably 0.02%. The upper limit of the Ni content is preferably 0.30%, more preferably 0.20%, even more preferably 0.10%, and even more preferably 0.05%.

[0050] The chemical composition of the above-mentioned steel material may further contain W instead of a part of Fe.

[0051] W: 0 to 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, it forms a protective corrosion film in a sour environment and suppresses the penetration of hydrogen into the steel material. This improves the SSC resistance of the steel material. Even if even a small amount of W is contained, the above effect can be obtained to some extent. However, if the W content is too high, even if the contents of other elements are within the ranges of this embodiment, coarse carbides will be formed in the steel material, and the SSC resistance of the steel material will decrease. 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%, even more preferably 0.03%, and even more preferably 0.05%. The preferred upper limit of the W content is less than 0.50%, even more preferably 0.48%.

[0052] The chemical composition of the steel material described above may further contain, in place of a portion 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 they neutralize S in the steel material by forming sulfides. As a result, these elements enhance the SSC resistance of the steel material.

[0053] 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 neutralizes S in the steel material as sulfides, thereby improving the SSC resistance of the steel material. Even if even a small amount of Ca is contained, the above effects can be obtained to some extent. However, if the Ca content is too high, even if the contents of other elements are within the ranges of this embodiment, oxides in the steel material will coarsen, and the SSC resistance of the steel material will decrease. 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%, even more preferably 0.0003%, and even more preferably 0.0006%. The preferred upper limit of the Ca content is 0.0040%, even more preferably 0.0025%, and even more preferably 0.0020%.

[0054] Mg: 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 neutralizes S in the steel material as sulfides, thereby improving the SSC resistance of the steel material. Even if even a small amount of Mg is contained, the above effects can be obtained to some extent. However, if the Mg content is too high, even if the contents of other elements are within the ranges of this embodiment, oxides in the steel material will coarsen, reducing the SSC resistance of the steel material. Therefore, the Mg content is 0 to 0.0100%. The lower limit of the Mg content is preferably more than 0%, more preferably 0.0001%, even more preferably 0.0003%, and even more preferably 0.0006%. The upper limit of the Mg content is preferably 0.0040%, even more preferably 0.0025%, and even more preferably 0.0020%.

[0055] 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 neutralizes S in the steel material as sulfides, thereby improving the SSC resistance of the steel material. Even if even a small amount of Zr is contained, the above effects can be obtained to some extent. However, if the Zr content is too high, even if the contents of other elements are within the ranges of this embodiment, oxides in the steel material will coarsen, and the SSC resistance of the steel material will decrease. Therefore, the Zr content is 0 to 0.0100%. The lower limit of the Zr content is preferably more than 0%, more preferably 0.0001%, even more preferably 0.0003%, and even more preferably 0.0006%. The upper limit of the Zr content is preferably 0.0040%, even more preferably 0.0025%, and even more preferably 0.0020%.

[0056] 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 neutralizes S in the steel material as sulfides, thereby improving the SSC resistance of the steel material. REM also combines with P in the steel material to suppress P segregation at grain boundaries. Therefore, a decrease in the SSC resistance of the steel material due to P segregation is suppressed. Even if even a small amount of REM is contained, the above effects can be obtained to some extent. However, if the REM content is too high, oxides in the steel material will coarsen, even if the contents of other elements are within the ranges of this embodiment, and the SSC resistance of the steel material will decrease. Therefore, the REM content is 0 to 0.0100%. The preferred lower limit of the REM content is more than 0%, more preferably 0.0001%, even more preferably 0.0003%, and even more preferably 0.0006%. The upper limit of the REM content is preferably 0.0040%, more preferably 0.0025%, and even more preferably 0.0020%.

[0057] In this specification, REM refers to one or more elements selected from the group consisting of scandium (Sc), which has atomic number 21, yttrium (Y), which has atomic number 39, and the lanthanides lanthanum (La), which has atomic number 57, to lutetium (Lu), which has atomic number 71. In addition, in this specification, the REM content refers to the total content of these elements.

[0058] [Yield strength] The steel material according to this embodiment has a yield strength of more than 1034 to 1172 MPa (more than 150 to 170 ksi). The yield strength referred to in this specification means the stress at 0.65% elongation (0.65% proof stress) obtained in a tensile test at room temperature (25°C) in accordance with ASTM E8 / E8M(2021). The steel material according to this embodiment has the above-mentioned chemical composition and satisfies the number density of coarse Al oxides and the number density of coarse Si oxides described below, and therefore has excellent SSC resistance even when the yield strength is more than 1034 to 1172 MPa.

[0059] The yield strength of the steel material according to this embodiment is determined by the following method. First, a round bar test specimen is prepared from the steel material according to this embodiment. When the steel material is a steel plate, the round bar test specimen is prepared from the center of the plate thickness. In this case, the axial direction of the round bar test specimen is parallel to the rolling direction of the steel plate. When the steel material is a steel pipe, the round bar test specimen is prepared from the center of the wall thickness. In this case, the axial direction of the round bar test specimen is parallel to the axial direction of the steel pipe. When the steel material is a round bar, the round bar test specimen is prepared from the R / 2 position. In this specification, the R / 2 position refers to the center position of the radius R in a cross section perpendicular to the axial direction of the round bar. In this case, the axial direction of the round bar test specimen is parallel to the axial direction of the round bar. The size of the round bar test specimen is, for example, 8.9 mm in parallel part diameter and 35.6 mm in gauge length. Using the prepared round bar test specimen, a tensile test is performed at room temperature (25°C) in the air according to a method in accordance with ASTM E8 / E8M (2021), and the obtained stress at 0.65% elongation (0.65% proof stress) is defined as the yield strength (MPa). In this embodiment, the yield strength (MPa) is calculated by rounding the obtained value to one decimal place.

[0060] [Number density of coarse Al oxides] The steel material according to this embodiment has the above-mentioned chemical composition and a yield strength of more than 1034 to 1172 MPa, and further has a number density of coarse Al oxides of 30 particles / 200 mm 2 As mentioned above, in this specification, particles having an Al content of 20% or more and an O content of 10% or more, by mass, are also referred to as "Al oxides." As mentioned above, in this specification, Al oxides having a major axis of 5.0 μm or more are also referred to as "coarse Al oxides." In other words, coarse Al oxides refer to particles having an Al content of 20% or more, an O content of 10% or more, and a major axis of 5.0 μm or more, by mass.

[0061] As described above, when manufacturing a steel material having the above-mentioned chemical composition, deoxidation is mainly carried out with aluminum (Al) in the steelmaking process. Therefore, a large number of Al oxides are likely to be formed in the steel material having the above-mentioned chemical composition. Furthermore, Al oxides are hard oxides and are likely to reduce the corrosion resistance of the steel material. In particular, when the steel material has a high yield strength of 150 ksi class, the effect of coarse Al oxides is likely to become apparent, and the SSC resistance is likely to be significantly reduced. Therefore, in the steel material according to this embodiment, which has the above-mentioned chemical composition and a yield strength of more than 1034 to 1172 MPa, the number density of coarse Al oxides is set to 30 particles / 200 mm 2 Less than.

[0062] In this embodiment, the preferred upper limit of the number density of coarse Al oxide particles is 28 particles / 200 mm 2 and more preferably 25 pieces / 200 mm 2 and more preferably 22 pieces / 200 mm 2 In this embodiment, the lower limit of the number density of the coarse Al oxide particles is not particularly limited, and the lower limit is 0 particles / 200 mm 2 The lower limit of the number density of the coarse Al oxides may be, for example, 5 pieces / 200 mm 2 It can be 7 pieces / 200mm 2 It can be 9 pieces / 200mm 2 The method for determining the number density of coarse Al oxide particles will be described later.

[0063] [Number density of coarse Si oxides] The steel material according to this embodiment has the above-described chemical composition, a yield strength of more than 1034 to 1172 MPa, and a number density of coarse Al oxides of 30 particles / 200 mm 2 The density of coarse Si oxides is less than 5 / 200mm 2As described above, in this specification, particles having an Al content of less than 20%, a Si content of 20% or more, and an O content of 10% or more, by mass, are also referred to as "Si oxides." As described above, in this specification, Si oxides having a major axis of 5.0 μm or more are also referred to as "coarse Si oxides." In other words, coarse Si oxides refer to particles having an Al content of less than 20%, a Si content of 20% or more, an O content of 10% or more, and a major axis of 5.0 μm or more, by mass.

[0064] As mentioned above, silicon oxides have not received much attention due to their small number. However, when the steel has a high yield strength of 150 ksi, it is possible that not only coarse aluminum oxides but also a small number of coarse silicon oxides may cause a significant decrease in SSC resistance. Therefore, the number density of coarse aluminum oxides is set to 30 / 200 mm 2 Not only is the number density of coarse Si oxides reduced to less than 5 particles / 200 mm 2 By reducing the yield strength to 150 ksi or less, it is possible to stably obtain excellent SSC resistance even when the yield strength is increased to 150 ksi class. Therefore, the steel material according to this embodiment has the above-mentioned chemical composition and a yield strength of more than 1034 to 1172 MPa, and the number density of coarse Al oxides in the steel material is 30 particles / 200 mm 2 Furthermore, the number density of coarse Si oxides is set to less than 5 pieces / 200 mm 2 The following applies.

[0065] In this embodiment, the preferred upper limit of the number density of the coarse Si oxides is 4 pieces / 200 mm 2 and more preferably 3 pieces / 200 mm 2 In this embodiment, the lower limit of the number density of the coarse Si oxides is not particularly limited, and is 0 pieces / 200 mm 2 The lower limit of the number density of the coarse Si oxides may be, for example, 1 piece / 200 mm 2 may be.

[0066] In this embodiment, the number density of coarse Al oxides and the number density of coarse Si oxides in a steel material can be determined by the following method. First, a test piece is prepared from the steel material according to this embodiment, with the observation surface being a surface including the rolling direction and the reduction direction. Specifically, when the steel material is a steel plate, a test piece is prepared from the center of the plate thickness, with the observation surface being a surface including the rolling direction and the plate thickness direction. When the steel material is a steel pipe, a test piece is prepared from the center of the wall thickness, with the observation surface being a surface including the pipe axial direction and the pipe radial direction. When the steel material is a round bar, a test piece is prepared, with the observation surface being a surface including the R / 2 position in the center and including the axial and radial directions.

[0067] The observation surface of the prepared test piece is polished to a mirror finish before measurement. The area of ​​the observation surface is not limited, but for example, 300 mm 2 The specimen is measured in a 20mm x 15mm area. The number of Si oxide particles with a major axis of 5.0 μm or greater is counted on the observation surface. Specifically, particles on the observation surface are first identified based on their contrast. Each identified particle is then subjected to elemental concentration analysis (EDS analysis). In the EDS analysis, an acceleration voltage of 20 kV is used, and the target elements are quantified as N, O, Mg, Al, Si, P, S, Ca, Ti, Cr, Mn, Fe, Cu, Zr, and Nb. Based on the EDS analysis results for each particle, if the Al content is 20% or greater by mass and the O content is 10% or greater, the particle is identified as an "Al oxide." Based on the EDS analysis results for each particle, if the Al content is less than 20% by mass, the Si content is 20% or greater, and the O content is 10% or greater, the particle is identified as a "Si oxide."

[0068] Among the Al oxides identified on the observation surface, Al oxides with a major axis of 5.0 μm or more (coarse Al oxides) are identified, and the total number of coarse Al oxides is calculated. Furthermore, among the Si oxides identified on the observation surface, Si oxides with a major axis of 5.0 μm or more (coarse Si oxides) are identified, and the total number of coarse Si oxides is calculated. The major axes of Al oxides and Si oxides can be calculated by well-known methods. In this specification, the major axes of Al oxides and Si oxides refer to the longest line segment (μm) among the line segments connecting any two points on the periphery of Al oxides and Si oxides on the observation surface.

[0069] Based on the total number of coarse Al oxides and the total area of ​​the observation surface, the number density of coarse Al oxides (pieces / 200 mm 2 Furthermore, the number density (number / 200 mm) of the coarse Si oxides was calculated based on the total number of coarse Si oxides and the total area of ​​the observation surface. 2 In this embodiment, the number density (particles / 200 mm 2 ), and the number density of coarse Si oxides (pieces / 200 mm 2 ) are all calculated by rounding off the obtained values ​​to one decimal place. The number density of the coarse Al oxides and the coarse Si oxides can be measured using a scanning electron microscope equipped with a composition analysis function (SEM-EDS device). For example, an automatic analyzer manufactured by FEI (ASPEX) under the trade name of Metals Quality Analyzer can be used as the SEM-EDS device.

[0070] [SSC resistance] The SSC resistance of the steel material according to this embodiment can be evaluated by an SSC resistance test carried out in accordance with NACE TM0177-2016 Method A. Specifically, the SSC resistance can be evaluated by the following method.

[0071] The test solution is a mixed aqueous solution (NACE solution D) of 5.0 mass % sodium chloride and 0.4 mass % sodium acetate, adjusted to pH 5.0 with hydrochloric acid. Round bar test specimens are prepared from the steel material according to this embodiment. When the steel material is a steel plate, the round bar test specimen is prepared from the center of the plate thickness. In this case, the axial direction of the round bar test specimen is parallel to the rolling direction of the steel plate. When the steel material is a steel pipe, the round bar test specimen is prepared from the center of the wall thickness. In this case, the axial direction of the round bar test specimen is parallel to the axial direction of the steel pipe. When the steel material is a round bar, the round bar test specimen is prepared from the R / 2 position. In this case, the axial direction of the round bar test specimen is parallel to the axial direction of the round bar. The size of the round bar test specimen is, for example, 6.35 mm in diameter and 25.4 mm in length of the parallel part.

[0072] A stress equivalent to 90% of the actual yield stress is applied to the prepared round bar test specimen. A test solution at 24°C is poured into a test vessel so that the stressed round bar test specimen is immersed, forming a test bath. After degassing the test bath, a mixed gas of 0.01 atm H2S gas and 0.99 atm CO2 gas is blown into the test bath to saturate it. The test bath saturated with the mixed gas is maintained at 24°C for 720 hours. In the SSC resistance test conducted under the above conditions, the steel material according to this embodiment shows no cracks after 720 hours. In this specification, "no cracks are observed" means that no cracks are observed when the test specimen is observed with the naked eye after the test.

[0073] [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 a steel material having the above-described chemical composition contains a total volume fraction of tempered martensite and tempered bainite of 90% or more, it can achieve both a 150 ksi-class yield strength and excellent SSC resistance, provided that the other configurations of this embodiment are satisfied. In other words, in this embodiment, if a steel material achieves both a 150 ksi-class yield strength and excellent SSC resistance, it is determined that the microstructure has a total volume fraction of tempered martensite and tempered bainite of 90% or more.

[0074] 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 the observation surface being a plane including the rolling direction and the plate thickness direction. When the steel material is a steel pipe, a test piece is prepared from the center of the wall thickness, with the observation surface being a plane including the pipe axial direction and the pipe radial direction. When the steel material is a round bar, a test piece is prepared with the R / 2 position in the center and the observation surface being a plane including the axial and radial directions.

[0075] 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 using a scanning electron microscope (SEM) in 10 fields of view as 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 based on contrast. The area fractions of the identified tempered martensite and tempered bainite are calculated. The method for calculating the area fractions is not particularly limited, and any known method may be used. For example, the area fractions of tempered martensite and tempered bainite can be calculated by image analysis. In this embodiment, the arithmetic mean values ​​of the area fractions of tempered martensite and tempered bainite calculated in all fields of view are defined as the volume fractions of tempered martensite and tempered bainite.

[0076] [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 as an example of a steel material according to this embodiment. The method for manufacturing a seamless steel pipe includes a step of preparing a material (steelmaking step), a step of hot-working the material to manufacture a mother pipe (hot-working 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.

[0077] [Steelmaking process] In the steelmaking process, first, molten pig iron produced by a known method is refined in a converter (primary refining). The molten steel obtained through primary refining is then subjected to secondary refining. In the secondary refining, alloy elements are added to adjust the composition, and molten steel satisfying the above-mentioned chemical composition is produced.

[0078] In the secondary refining, for example, a Ruhrstahl-Hausen (RH) vacuum degassing treatment is carried out. After that, the alloy composition is finally adjusted. In the secondary refining, a combined refining may be carried out. In this case, a refining treatment using a Ladle Furnace (LF) or a Vacuum Arc Degassing (VAD) is carried out before the RH vacuum degassing treatment.

[0079] Materials are manufactured using molten steel that has undergone secondary refinement. Specifically, cast pieces (slabs, blooms, or billets) are manufactured by continuous casting using the molten steel that has undergone secondary refinement. In the continuous casting process, molten steel is first poured from a ladle into a tundish. At this time, packing sand is usually enclosed in the nozzle of the ladle to seal the nozzle. Therefore, packing sand may be mixed in with the molten steel from the ladle to the tundish. Furthermore, when manufacturing materials having the above-mentioned chemical composition, silicon oxides may be used as packing sand. In this case, there is a concern that silicon oxides may be introduced into the manufactured materials.

[0080] Therefore, in this embodiment, the molten steel and the Si oxide are separated to prevent the Si oxide sealed in the ladle nozzle from being introduced into the tundish. The method for separating the Si oxide is not particularly limited, but the following method can be used, for example. A sloped metal plate is placed below the ladle nozzle and above the opening of the tundish. When the ladle nozzle is opened, the Si oxide is discharged first from the nozzle, followed by the molten steel. Here, the Si oxide is lighter than the molten steel. Therefore, the Si oxide discharged from the nozzle is guided along the slope of the metal plate to the outside of the opening of the tundish. The slope of the metal plate can be achieved, for example, by placing a metal plate machined into a bottomless cone shape with its apex directly below the ladle nozzle, or by other methods. Furthermore, a single metal plate can be used, or multiple metal plates can be stacked. Furthermore, the thickness of the metal plate is not particularly limited, but is, for example, approximately 1 to 10 mm.

[0081] After the Si oxides are discharged from the nozzle, the molten steel is discharged. At this time, the molten steel discharged from the nozzle is introduced into the tundish together with the metal plate through the opening. That is, in this embodiment, a part or all of the metal plate may be introduced into the tundish and mixed with the molten steel. Therefore, the metal plate in this embodiment is preferably a metal plate made of alloy elements contained in the molten steel. As a metal plate made of alloy elements contained in the molten steel, for example, an aluminum plate can be used. Note that in this specification, the aluminum plate means a metal plate made of aluminum and the remainder made of impurities.

[0082] Preferably, the metal plate is removed from below the nozzle after the Si oxides have been discharged from the nozzle but before the molten steel is discharged. In this case, it is possible to prevent Si oxides adhering to the metal plate from being mixed into the molten steel. The method for removing the metal plate from below the nozzle is not particularly limited. For example, a hole may be formed in a part of the metal plate, and the metal plate may be removed using a rod with a hook at its tip. In this case, the hook at the tip of the rod can be hooked into the hole in the metal plate and the rod can be pulled to remove the metal plate. By the above method, the Si oxides can be separated from the molten steel, and the molten steel can be introduced into the tundish. The method for separating the Si oxides from the molten steel is not limited to the above method.

[0083] Next, the prepared molten steel is cast to produce a material. The casting method is not particularly limited, but may be, for example, a continuous casting method. When producing a material by continuous casting, it is preferable to carry out the following method.

[0084] The casting speed in the continuous casting machine is preferably 1.0 to 3.0 m / min. If the casting speed is too slow, an Al oxide accumulation zone may form in the raw material. In this case, the produced steel material will contain a large amount of coarse Al oxides, and the SSC resistance of the steel material will be reduced. On the other hand, if the casting speed is too fast, the Al oxides may not float to the surface of the molten steel, and a large amount of Al oxide may remain in the raw material. In this case, the produced steel material will contain a large amount of coarse Al oxides, and the SSC resistance of the steel material will be reduced. Therefore, the casting speed in the continuous casting machine is preferably 1.0 to 3.0 m / min.

[0085] When producing a material by continuous casting, it is preferable to further electromagnetically stir the molten steel in the mold. Specifically, by performing electromagnetic stirring in the mold at a current value of 330 to 450 A, it becomes difficult for an Al oxide accumulation zone to form in the material. If the current value of electromagnetic stirring in the mold is too low, the molten steel will not be stirred sufficiently, and an Al oxide accumulation zone may form in the material. In this case, the produced steel material will contain a large amount of coarse Al oxides, and the SSC resistance of the steel material will decrease. On the other hand, if the current value of electromagnetic stirring in the mold is too high, it may place too much strain on the production equipment. Therefore, in this embodiment, it is preferable to set the current value of electromagnetic stirring in the mold to 330 to 450 A. Molten steel is cast by the above method to produce a material.

[0086] [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 hot working method is not particularly limited, and a well-known method may be used.

[0087] For example, a mother pipe may be manufactured by carrying out the Mannesmann process as the hot working. In this case, a round billet is pierced and rolled using a piercing mill. When piercing and rolling is performed, the piercing ratio is not particularly limited, but is, for example, 1.0 to 4.0. The piercing-rolled round billet is further hot-rolled using a mandrel mill, a reducer, a sizing mill, or the like to form a mother pipe. The cumulative area reduction rate in the hot working process is, for example, 20 to 70%.

[0088] 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. A mother pipe is produced through the above steps. The thickness of the mother pipe is not particularly limited, but is, for example, 9 to 60 mm.

[0089] When the steel material is round steel, the material is first heated in a heating furnace. The heating temperature is not particularly limited, but is, for example, 1100 to 1300°C. The material extracted from the heating furnace is subjected to hot working to produce an intermediate steel material having a circular cross section perpendicular to the axial direction. The hot working is, for example, blooming using a blooming mill or hot rolling using a continuous rolling mill. The continuous rolling mill has horizontal stands each having a pair of grooved rolls arranged side by side in the vertical direction, and vertical stands each having a pair of grooved rolls arranged side by side in the horizontal direction, arranged alternately.

[0090] When the steel material is a steel plate, the material is first heated in a heating furnace. The heating temperature is not particularly limited, but is, for example, 1100 to 1300°C. The material extracted from the heating furnace is hot-rolled using a blooming mill and a continuous rolling mill to produce an intermediate steel material in the shape of a steel plate.

[0091] 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 the hot working without being cooled to room temperature, or may be quenched after being reheated after the hot working.

[0092] 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, the occurrence of quench cracks in the mother pipe can be suppressed. When quenching is performed directly after hot working or after reheating, stress relief annealing (SR) may be performed after quenching and before the next heat treatment. In this case, residual stress in the mother pipe is removed.

[0093] As described above, in the hot working step, the prepared material is hot worked to produce an intermediate steel material. The quenching step will be described in detail below.

[0094] [Quenching process] In the quenching process, the prepared intermediate steel material (blank pipe) is quenched. In this specification, "quenching" means rapidly cooling the intermediate steel material at the A3 point or above. The preferred quenching temperature is 800 to 1000°C. If the quenching temperature is too high, the prior γ grains may become coarse, which may reduce the SSC resistance of the steel material. Therefore, the quenching temperature is preferably 800 to 1000°C.

[0095] In this specification, the quenching temperature corresponds to the surface temperature of the intermediate steel material measured by a thermometer installed on the outlet side of the equipment that performs the final hot working when quenching is performed directly after hot working. Furthermore, the quenching temperature corresponds to the temperature of the furnace that performs the quenching or reheating when quenching is performed after reheating or reheating after hot working.

[0096] The quenching method involves, for example, continuously cooling 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 any well-known method. Examples of continuous cooling methods include a method of immersing the mother pipe in a water bath for cooling, or a method of accelerating the cooling of the mother pipe by shower water cooling or mist cooling.

[0097] If the cooling rate during quenching is too slow, the microstructure will not be mainly composed of martensite and bainite, and the mechanical properties (yield strength of more than 1034 to 1172 MPa) specified in this embodiment will not be obtained. In this case, excellent SSC resistance will not be obtained.

[0098] Therefore, as described above, in the steel manufacturing method according to this embodiment, the intermediate steel is rapidly cooled 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 cooling rate during quenching CR 800-500 More specifically, the cooling rate during quenching, CR 800-500 is determined from the temperature measured at the location in the cross section of the intermediate steel being quenched that cools the slowest (for example, the center of the thickness of the intermediate steel when both surfaces are forcedly cooled).

[0099] 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.

[0100] Preferably, the mother pipe is heated in the austenite region multiple times and then quenched. In this case, the austenite grains before quenching are refined, thereby improving the SSC resistance of the steel material. 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.

[0101] [Tempering process] In the tempering process, the quenched mother pipe is tempered. In this specification, "tempering" refers to the process of tempering the intermediate steel material after quenching. c1 The tempering temperature corresponds to the furnace temperature when the intermediate steel material is heated and held at the tempering temperature after quenching. The tempering time refers to the time during which the intermediate steel material is held at the tempering temperature.

[0102] The tempering temperature is adjusted appropriately depending on the chemical composition of the seamless steel pipe and the yield strength to be obtained. That is, for a mother pipe having the chemical composition of this embodiment, the tempering temperature is adjusted to adjust the yield strength of the seamless steel pipe to more than 1034 and 1172 MPa. Note that it is naturally possible for a person skilled in the art to adjust the yield strength of the seamless steel pipe to more than 1034 and 1172 MPa by adjusting the tempering temperature. Specifically, in the tempering process according to this embodiment, the preferred tempering temperature is 640 to 660°C.

[0103] If the tempering time is too short, a microstructure mainly composed of tempered martensite and tempered bainite may not be obtained. On the other hand, if the tempering time is too long, the above effects will saturate. Therefore, in the tempering step of this embodiment, the tempering time is preferably 10 to 90 minutes. A more preferable lower limit of the tempering time is 15 minutes. A more preferable upper limit of the tempering time is 80 minutes.

[0104] The steel material according to this embodiment can be manufactured by the above manufacturing method. In the above manufacturing method, a method for manufacturing a seamless steel pipe has been described as an example. However, the steel material according to this embodiment may be a steel plate or other shape. Similar to the above manufacturing method, a manufacturing method for a steel plate or other shape also includes, for example, a preparation step, a quenching step, and a tempering step. Furthermore, the above manufacturing method is an example, and the steel material may be manufactured by other manufacturing methods.

[0105] The present invention will be explained in more detail below with reference to examples. [Example]

[0106] Molten steels having the chemical compositions shown in Tables 1-1 and 1-2 were produced. Note that "-" in Table 1-2 means that the content of each element was at the impurity level. Specifically, the Cu content, Ni content, and W content of Steel A were rounded to two decimal places to mean 0%. Furthermore, the Ca content, Mg content, Zr content, and rare earth element (REM) content of Steel A were rounded to five decimal places to mean 0%.

[0107] [Table 1-1]

[0108] [Table 1-2]

[0109] The molten steel was used to produce a round billet by continuous casting. During continuous casting, when molten steel was introduced from the ladle into the tundish, a metal plate processed into a cone shape without a base was placed above the opening of the tundish, with its apex positioned directly below the nozzle of the ladle. Table 2 indicates whether or not a metal plate of the above shape was placed above the opening of the tundish. Specifically, when a metal plate of the above shape was placed above the opening of the tundish, an "A" is indicated in the "Metal Plate" column of Table 2. When a metal plate of the above shape was not placed above the opening of the tundish, a "B" is indicated in the "Metal Plate" column of Table 2. The metal plate of the above shape placed above the opening of the tundish was an aluminum plate. Specifically, three 2 mm thick aluminum plates were used stacked together. When a metal plate was placed, it was removed from below the nozzle using a rod with a hook at its tip after silicon oxide was discharged from the nozzle and before the molten steel was discharged. Furthermore, the molten steel was cast into a round billet at the casting speed shown in Table 2. At this time, electromagnetic stirring was carried out in the mold at the current value shown in Table 2.

[0110] [Table 2]

[0111] The produced round billet of each test number was held at 1250°C for 1 hour, and then hot-rolled by the Mannesmann-mandrel method to produce a mother pipe (seamless steel pipe) of each test number. Furthermore, the obtained mother pipe of each test number was quenched. Specifically, the mother pipe of each test number was held at the temperature (°C) for the time (minutes) listed in the "Quenching process" column of Table 2, and then quenched by shower water cooling. Furthermore, for test number 3, after the above-mentioned quenching, it was held at 900°C for 10 minutes, and then further quenched by shower water cooling. Note that for each test number, the cooling rate during quenching, CR 800-500 The quenching rates were all within the range of 480 to 30,000°C / min. Here, the temperature (°C) in the quenching process was the temperature (°C) of the heat treatment furnace in which the mother pipe was heated. Furthermore, the quenching time (minutes) was the time (minutes) during which the mother pipe was held at the quenching temperature.

[0112] The obtained mother pipes of each test number were tempered. Specifically, the mother pipes of each test number were tempered by holding them at the temperature (°C) for the time (minutes) shown in the "Tempering step" column of Table 2. Here, the tempering temperature (°C) shown in Table 2 is the temperature (°C) of the tempering furnace in which the mother pipes were heated. Furthermore, the tempering time (minutes) shown in Table 2 is the time (minutes) for which the mother pipes were held at the tempering temperature. Through the above manufacturing process, seamless steel pipes of each test number were obtained.

[0113] [Evaluation test] The seamless steel pipes having the respective test numbers after tempering were subjected to the following tensile tests, coarse Al oxide and coarse Si oxide number density measurement tests, and SSC resistance tests.

[0114] [Tensile test] A tensile test was conducted on each seamless steel pipe to determine its yield strength. The tensile test was conducted in accordance with ASTM E8 / E8M (2021). Round bar test specimens with a parallel section diameter of 8.9 mm and a gauge length of 35.6 mm were prepared from the center of the wall thickness of each seamless steel pipe. The axial direction of the round bar test specimen was parallel to the axial direction of the seamless steel pipe. Using the prepared round bar test specimens, a tensile test was conducted at room temperature (25°C) in air to obtain the yield strength (MPa) of each seamless steel pipe. In this example, the stress at 0.65% elongation (0.65% proof stress) obtained in the tensile test was defined as the yield strength. The obtained yield strength (MPa) is shown in Table 3 as "YS (MPa)."

[0115] [Table 3]

[0116] [Measurement test of the number density of coarse Al oxides and coarse Si oxides] A number density measurement test of coarse Al oxides and coarse Si oxides was carried out on the seamless steel pipe of each test number, and the number density of Al oxides (coarse Al oxides) with a major diameter of 5.0 μm or more and Si oxides (coarse Si oxides) with a major diameter of 5.0 μm or more was determined. Using test pieces prepared from the center of the wall thickness of the seamless steel pipe of each test number, the number densities of coarse Al oxides and coarse Si oxides were determined by the above-mentioned method. The number densities of the obtained coarse Al oxides (particles / 200 mm 2 ) in Table 3. 2 The number density (pieces / 200 mm) of the obtained coarse Si oxides is shown in the column. 2 ) in Table 3. 2 ) column.

[0117] [SSC resistance test] For each seamless steel pipe with each test number, an SSC resistance test was conducted in accordance with NACE TM0177-2016 Method A to evaluate SSC resistance. Specifically, round bar test specimens with a diameter of 6.35 mm and a parallel section length of 25.4 mm were prepared from the center of the wall thickness of each seamless steel pipe with each test number. SSC resistance tests were conducted on three of the prepared test specimens. The axial direction of the test specimens was parallel to the pipe axis.

[0118] A tensile stress was applied in the axial direction to each round bar specimen. The applied stress was adjusted to 90% of the actual yield stress of each steel plate. The test solution used was a mixed aqueous solution of 5.0 mass% sodium chloride and 0.4 mass% sodium acetate (NACE solution D), adjusted to pH 5.0 with hydrochloric acid. The test solution at 24°C was poured into three test vessels to form test baths. Each of the three stressed round bar specimens was immersed in the test bath in a different test vessel. After degassing each test bath, a mixture of 0.01 atm H2S gas and 0.99 atm CO2 gas was blown into the test bath to saturate it. The test bath saturated with the mixed gas was maintained at 24°C for 720 hours.

[0119] After 720 hours of holding, the round bar test specimens of each test number were observed for the occurrence of sulfide stress cracking (SSC). Specifically, the round bar test specimens after 720 hours of holding were visually inspected. For each test number, the number of round bar test specimens in which SSC occurred out of the three round bar test specimens is shown in the "Number of SSC occurrences (pieces)" column in Table 3.

[0120] [Evaluation results] With reference to Tables 1-1, 1-2, 2, and 3, the seamless steel pipes of test numbers 1 to 12 had appropriate chemical compositions, and the manufacturing methods also satisfied the above-mentioned preferable conditions. As a result, these seamless steel pipes had a yield strength of more than 1034 to 1172 MPa, and a number density of coarse Al oxides of 30 particles / 200 mm 2 The density of coarse Si oxides is less than 5 / 200mm 2As a result, the number of SSC occurrences in these seamless steel pipes was zero in the SSC resistance test. That is, the seamless steel pipes of test numbers 1 to 12 had a yield strength of more than 1034 to 1172 MPa and excellent SSC resistance. It was determined that the total volume fraction of tempered martensite and tempered bainite in the microstructure of these seamless steel pipes was 90% or more.

[0121] On the other hand, the casting speed in the steelmaking process for the seamless steel pipes of test numbers 13 and 14 was too fast. As a result, the number density of coarse Al oxides in these seamless steel pipes was 30 particles / 200 mm. 2 As a result, these seamless steel pipes had one or more SSC occurrences in the SSC resistance test, and did not have excellent SSC resistance.

[0122] For seamless steel pipes of test numbers 15 to 17, no metal plate was used in the steelmaking process. As a result, for these seamless steel pipes, the number density of coarse Si oxides was 5 particles / 200 mm 2 As a result, these seamless steel pipes had one or more SSC occurrences in the SSC resistance test, and did not have excellent SSC resistance.

[0123] The seamless steel pipe of test number 18 had an excessively high O content. As a result, this seamless steel pipe had one or more SSC occurrences in the SSC resistance test, and did not have excellent SSC resistance.

[0124] The seamless steel pipe of test number 19 had an excessively low Mo content, and as a result, this seamless steel pipe had one or more SSC occurrences in the SSC resistance test, and did not have excellent SSC resistance.

[0125] The seamless steel pipe of test number 20 had an excessively high S content. As a result, this seamless steel pipe had one or more SSC occurrences in the SSC resistance test, and did not have excellent SSC resistance.

[0126] 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 within the scope of the present disclosure.

Claims

1. A steel material, In mass%, C: 0.15-0.45%, Si: 0.05-1.00%, Mn: 0.05-1.00%, P: 0.030% or less, S: 0.0050% or less, Al: 0.005-0.100%, Cr: 0.30-1.50%, Mo: 0.40-2.00%, Ti: 0.002 to 0.020%, Nb: 0.002-0.100%, V: 0.05-0.30%, B: 0.0005-0.0040%, N: 0.0100% or less, O: 0.0040% or less, Cu: 0 to 0.50%, Ni: 0 to 0.50%, W: 0-0.50%, Ca: 0-0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, Rare earth elements: 0 to 0.0100%, and the balance being Fe and impurities; The yield strength is greater than 1034 to 1172 MPa, In the microstructure of the steel material, the total volume fraction of tempered martensite and tempered bainite is 90% or more, In the steel material, The Al content is 20% or more, the O content is 10% or more, and the number density of Al oxide particles having a major axis of 5.0 μm or more is 30 particles / 200 mm ２ is less than In mass %, the Al content is less than 20%, the Si content is 20% or more, the O content is 10% or more, and the number density of Si oxides having a major axis of 5.0 μm or more is 5 pieces / 200 mm ２ Below is the Steel material.

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

3. The steel material according to claim 1 or claim 2, The steel material is a seamless steel pipe. Steel material.

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