MATERIAL DE AÇO

Abstract

Provided is a steel material having both high strength of at least 125 ksi and excellent low-temperature toughness. This steel material contains, 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, 0.20-2.00% of Mo, 0.002-0.030% of Ti, 0.002-0.100% of Nb, 0.0005-0.0040% of B, 0.0100% or less of N, and 0.0040% or less of O, with the remainder comprising Fe and impurities, and has a yield strength of 862-1034 MPa. In the steel material, the number density of Al oxides having a major axis of at least 5.0 μm is less than 30 counts / 200 mm2, and the number density of Si oxides having a major axis of at least 5.0 μm is 5 counts / 200 mm2 or less.

Description

1 / 57 STEEL MATERIAL TECHNICAL FIELD

[0001] The present invention relates to a steel material, and more particularly to a steel material suitable for use in oil wells. PREVIOUS ART

[0002] Due to the deepening of oil wells and gas wells (hereinafter, oil wells and gas wells are collectively referred to simply as oil wells), there is a demand to increase the strength of steel materials for oil wells typified by steel pipes for oil wells. Specifically, grade 80 ksi (yield strength is 80 to less than 95 ksi, i.e., 552 to less than 655 MPa) and grade 95 ksi (yield strength is 95 to less than 110 ksi, i.e., 655 to less than 758 MPa) oil well steel materials are in widespread use, and recently requests are also beginning to be made for grade 110 ksi (yield strength is 758 to less than 862 MPa) oil well steel materials.

[0003] Oil wells may also contain corrosive hydrogen sulfide gas (H2S) or carbon dioxide gas (CO2) or similar gases. Therefore, a steel material assumed to be used as a steel material for oil wells is also required to have excellent corrosion resistance, and not just high strength. Furthermore, in the case of a steel material for oil wells, stress is applied to the steel material during use. Therefore, sulfide stress cracking resistance (hereinafter referred to as SSC resistance) has been used as an index of excellent corrosion resistance of steel materials for oil wells.

[0004] Techniques for increasing the strength and SSC resistance of a steel material are proposed in Japanese Patent Application Publication No. 2006-28612 (Patent Document 1), International Patent Application Publication No. WO2008 / 123422 (Patent Document 2), and Publication of Petition 870250084148, dated 09 / 18 / 2025, pp. 130 / 189 2 / 57 Japanese Patent Application No. 2017-166060 (Patent Document 3).

[0005] Patent Document 1 discloses a steel material that is a pipe steel, consisting of, in % by mass, C: 0.2 to 0.7%, Si: 0.01 to 0.8%, Mn: 0.1 to 1.5%, S: 0.005% or less, P: 0.03% or less, Al: 0.0005 to 0.1%, Ti: 0.005 to 0.05%, Ca: 0.0004 to 0.005%, N: 0.007% or less, Cr: 0.1 to 1.5%, and Mo: 0.2 to 1.0%, with the remainder being Fe and impurities. In this steel material, moreover, among the non-metallic inclusions containing Ca, Al, Ti, N, O, and S, (Ca %) / (Al %) is 0.55 to 1.72 and (Ca %) / (Ti %) is 0.7 to 19. It is described in Patent Document 1 that this steel material has a high yield strength of over 758 MPa and has excellent SSC resistance.

[0006] Patent Document 2 discloses a steel material that is a low-alloy steel containing, in mass %, C: 0.10 to 0.20%, Si: 0.05 to 1.0%, Mn: 0.05 to 1.5%, Cr: 1.0 to 2.0%, Mo: 0.05 to 2.0%, Al: 0.10% or less, and Ti: 0.002 to 0.05%, with Ceq (= C+(Mn / 6)+(Cr+Mo+V) / 5) being 0.65 or more, and the remainder being Fe and impurities, in which, among the impurities, P is 0.025% or less, S is 0.010% or less, N is 0.007% or less, and B is less than 0.0003%. Furthermore, in this steel material, the number density of M23C6 type precipitates having a grain size of 1 μm or more is 0.1 / mm2 or less. It is described in Patent Document 2 that this steel material has a yield strength of 654 to 793 MPa and has excellent SSC resistance even in high-pressure hydrogen sulfide environments.

[0007] Patent Document 3 discloses a steel material that is a starting material for high-strength steel pipes for oil wells consisting of, in % by mass, C: 0.20 to 0.45%, Si: 0.05 to 0.40%, Mn: 0.3 to 0.9%, P: 0.015% or less, S: 0.005% or less, Al: 0.005 to 0.10%, N: 0.001 to 0.006%, Cr: 0.1 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%, and O (oxygen): 0.0030% or less, with the remainder being Iron and unavoidable impurities. Furthermore, in this steel material, the Rockwell hardness HRC satisfies the formula (15.6 x [%C] + 29.2 HRC < 60.5 x [%C] + 31.1). It is described in Patent Document 3 that according to this steel material, Petition 870250084148, dated 09 / 18 / 2025, pp. 131 / 189 3 / 57 a pipe having a yield strength of 758 to less than 862 MPa and excellent SSC resistance is obtained. LIST OF QUOTES PATENT DOCUMENT

[0008] Patent Document 1: Publication of Japanese Patent Application No. 2006-28612 Patent Document 2: Publication of International Patent Application No. WO2008 / 123422 Patent Document 3: Publication of Japanese Patent Application No. 2017-166060 SUMMARY OF THE INVENTION TECHNICAL PROBLEM

[0009] Furthermore, in recent years, deep wells below the sea surface are also being actively developed. Specifically, steel materials for oil wells for which use in so-called deepwater offshore oil fields that are at a water depth of 2,000 m or more is assumed, for example, and required to have a high strength of 125 ksi or more (a yield strength of 125 ksi or more, i.e., 862 MPa or more). In such deepwater offshore oil fields, moreover, the water temperature is low. Therefore, such steel materials for oil wells for which use in deepwater offshore oil fields is assumed are also required to have low-temperature toughness. However, if the yield strength of a steel material is increased, there is a concern that the low-temperature toughness of the steel material will decrease.

[0010] That is, steel materials for oil wells for which use in such harsh environments is assumed are required to achieve both a high strength of 125 ksi or more and excellent low-temperature toughness. On the other hand, in Patent Documents 1 to 3 mentioned above, no consideration is given to achieving either a high Petition 870250084148, dated 09 / 18 / 2025, pp. 132 / 189 4 / 57 resistance of 125 ksi or more, which provides excellent low-temperature toughness.

[0011] An objective of the present invention is to provide a steel material that achieves both a high strength of 125 ksi or more (862 MPa or more) and excellent low-temperature toughness. SOLUTION TO THE PROBLEM

[0012] A steel material according to the present invention consists of, in % by mass, C: 0.15 to 0.45% Yes: 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 to 1.50% Mo: 0.20 to 2.00% Ti: 0.002 to 0.030% Nb: 0.002 to 0.100% B: 0.0005 to 0.0040%, N: 0.0100% or less, O: 0.0040% or less, V: 0 to 0.30%, Cu: 0 to 0.50%, Ni: 0 to 0.50%, W: 0 to 0.50%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, and rare earth metal: 0 to 0.0100%, with the remainder being Fe and impurities; in which: Petition 870250084148, dated 09 / 18 / 2025, pp. 133 / 189 5 / 57 a yield strength is 862 to 1034 MPa, and in steel material, a number density of Al oxides in which, in % by mass, an Al content is 20% or more and an O content is 10% or more, and which have a major axis of 5.0 μm or more is less than 30 / 200 mm2, and a number density of Si oxides in which, in % by mass, an Al content is less than 20%, a Si content is 20% or more, and an O content is 10% or more, and which have a major axis of 5.0 μm or more is 5 / 200 mm2 or less. ADVANTAGEOUS EFFECTS OF THE INVENTION

[0013] The steel material according to the present invention can achieve both a high strength of 125 ksi or more (862 MPa or more) and excellent low-temperature toughness. BRIEF DESCRIPTION OF THE DRAWING

[0014] Figure 1 is a view illustrating the relationship between the number density ( / 200 mm2) of coarse Si oxides (Si oxides having a major axis of 5.0 μm or more) and a fracture appearance transition temperature (°C) which is an index of low temperature toughness in examples having a yield strength of less than 945 MPa among the present examples. Figure 2 is a view illustrating the relationship between the number density ( / 200 mm2) of coarse Si oxides (Si oxides having a major axis of 5.0 μm or more) and a fracture appearance transition temperature (°C) which is an index of low-temperature toughness in examples having a yield strength of 945 MPa or more among the present examples. DESCRIPTION OF THE MODALITIES

[0015] First, the present inventors conducted studies on obtaining a steel material that achieves both a yield strength of 125 ksi or more and excellent low-temperature toughness, which focused on chemical composition. As a result, the present inventors Petition 870250084148, dated 09 / 18 / 2025, pp. 134 / 189 6 / 57 considered that if a steel material consists of, in % by mass, C: 0.15 to 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 to 1.50%, Mo: 0.20 to 2.00%, Ti: 0.002 to 0.030%, Nb: 0.002 to 0.100%, B: 0.0005 to 0.0040%, N: 0.0100% or less, O: 0.0040% or less, V: 0 to 0.30%, Cu: 0 to 0.50%, Ni: With a composition of 0 to 0.50%, W: 0 to 0.50%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, and rare earth metal: 0 to 0.0100%, with the remainder being Fe and impurities, there is a possibility that both a yield strength of 125 ksi or more and excellent low-temperature toughness can be achieved.

[0016] Subsequently, the present inventors conducted several studies on techniques for increasing low-temperature toughness in a steel material having the chemical composition described above and a yield strength of 125 ksi or more. Specifically, the present inventors considered that if coarse oxide-based inclusions can be reduced, there is a possibility that low-temperature toughness will be increased while maintaining the yield strength. Here, in a steel material having the chemical composition described above, Al oxides that are primarily Al2O3 compounds tend to thicken. Therefore, first, the present inventors focused on coarse Al oxides.

[0017] As a result of studies conducted by the present inventors, it has been revealed that in a steel material having the chemical composition described above and a yield strength of 125 ksi or more, if the number density of Al oxides having a major axis of 5.0 μm or more is less than 30 / 200 mm2, there is a possibility that the low-temperature toughness will be increased. Here, in the present description, particles in which, by mass %, the Al content is 20% or more and the O content is 10% or more are also referred to as Al oxides. Furthermore, in the present description, 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 are prone to Petition 870250084148, dated 09 / 18 / 2025, pp. 135 / 189 7 / 57 cause a decrease in the toughness of a steel material. In particular, in a case where the yield strength is increased to 125 ksi or more, the influence of coarse Al oxides is likely to be enhanced, and the low-temperature toughness is likely to decrease markedly. Therefore, in the steel material according to the present embodiment that has the chemical composition described above and a yield strength of 125 ksi or more, the number density of coarse Al oxides should be less than 30 / 200 mm2.

[0019] On the other hand, even in the case of steel materials having the chemical composition described above and in which the number density of coarse Al oxides was less than 30 / 200 mm2, when the steel materials had a yield strength of 125 ksi or more, excellent low-temperature toughness was not stably obtained in some cases. Therefore, the present inventors investigated various techniques to stably obtain excellent low-temperature toughness in a steel material that has the chemical composition described above and a yield strength of 125 ksi or more and in which the number density of coarse Al oxides is less than 30 / 200 mm2.As a result of detailed studies conducted by the present inventors, it was revealed that in a steel material having the chemical composition described above and a yield strength of 125 ksi or more, and in which the number density of coarse Al oxides is less than 30 / 200 mm2, if coarse Si oxides in the steel material can also be reduced, and not only coarse Al oxides, there is a possibility that excellent low-temperature toughness will be stably obtained.

[0020] Here, in the present description, particles in which, in mass %, the Al content is less than 20%, the Si content is 20% or more, and the O content is 10% or more are also referred to as Si oxides. Furthermore, in the present description, Si oxides having a major axis of 5.0 μm or more are also referred to as coarse Si oxides. Hereafter, the relationship between coarse Si oxides and low-temperature toughness in a steel material having the chemical composition described above and a yield strength of 125 Petition 870250084148, dated 09 / 18 / 2025, pp. 136 / 189 8 / 57 ksi or more and in which the number density of coarse Al oxides is less than 30 / 200 mm2 is specifically described using the drawing.

[0021] Figure 1 is a view illustrating the relationship between the number density ( / 200 mm2) of coarse Si oxides (Si oxides having a major axis of 5.0 μm or more) and a fracture appearance transition temperature (°C) which is an index of low-temperature toughness in examples having a yield strength of less than 945 MPa among the present examples. Figure 1 was prepared using fracture appearance transition temperatures (°C) determined by a method described later, and a number density ( / 200 mm2) of coarse Si oxides determined by a method described later with respect to steel materials which, among examples to be described later, satisfied the chemical composition described above, had a yield strength of 862 to less than 945 MPa, and in which the number density of coarse Al oxides was less than 30 / 200 mm2.

[0022] Referring to Figure 1, in steel materials having the chemical composition described above and a yield strength of 862 to less than 945 MPa and in which the number density of coarse Al oxides was less than 30 / 200 mm2, when the number density of coarse Si oxides was 5 / 200 mm2 or less, the fracture appearance transition temperature was -50 °C or less and excellent low temperature toughness was exhibited.

[0023] Furthermore, Figure 2 is a view illustrating the relationship between the number density ( / 200 mm2) of coarse Si oxides (Si oxides having a major axis of 5.0 μm or more) and a fracture appearance transition temperature (°C) which is an index of low-temperature toughness in examples having a yield strength of 945 MPa or more among the present examples. Figure 2 was prepared using fracture appearance transition temperatures (°C) determined by a method described later, and a number density ( / 200 mm2) of coarse Si oxides determined by a method described later with respect to steel materials that, among Petition 870250084148, dated 09 / 18 / 2025, pp. 137 / 189 9 / 57 examples, to be described later, satisfied the chemical composition described above, had a yield strength of 945 to 1034 MPa, and in which the number density of coarse Al oxides was less than 30 / 200 mm2.

[0024] Referring to Figure 2, in steel materials having the chemical composition described above and a yield strength of 945 to 1034 MPa and in which the number density of coarse Al oxides was less than 30 / 200 mm2, when the number density of coarse Si oxides was 5 / 200 mm2 or less, the fracture appearance transition temperature was -40 °C or less and excellent low temperature toughness was exhibited.

[0025] Therefore, in the present embodiment, the steel material has the chemical composition described above and a yield strength of 862 to 1034 MPa, the number density of coarse Al oxides is made less than 30 / 200 mm2e, and furthermore, the number density of coarse Si oxides is made 5 / 200 mm2 or less. As a result, the steel material according to the present embodiment can achieve both a yield strength of 125 ksi or more and excellent low-temperature toughness.

[0026] The reason why the low-temperature toughness of a steel material is increased by decreasing the number density of coarse Si oxides has not been elucidated in detail. However, the present inventors assume that the reason is as follows. When producing a steel material having the chemical composition described above, during the steelmaking process, deoxidation is mainly carried out by aluminum (Al). Therefore, with regard to steel materials having the chemical composition described above, consideration has been given to Al oxides that are typified by Al2O3, and attention has not been focused on Si oxides, which are small in number. However, in the case of increasing the yield strength to 125 ksi or more, there is a possibility that a decrease in low-temperature toughness is likely to occur due to the influence not only of coarse Al oxides, but also to the influence of coarse Si oxides, which are small in number. Therefore, Petition 870250084148, dated 09 / 18 / 2025, pp. 138 / 189 10 / 57 The present inventors assume that by not only making the number density of coarse Al oxides less than 30 / 200 mm2, but also making the number density of coarse Si oxides 5 / 200 mm2 or less, excellent low-temperature toughness is stably obtained even when the steel material has a yield strength of 125 ksi or more.

[0027] Note that there is also a possibility that the low-temperature toughness of the steel material may be increased by a mechanism that is different from the mechanism supposed by the present inventors. However, the fact that in a steel material having the chemical composition described above and a yield strength of 125 ksi or more and in which the number density of coarse Al oxides is less than 30 / 200 mm2, excellent low-temperature toughness is obtained by making the number density of coarse Si oxides 5 / 200 mm2 or less has been demonstrated by examples that are described later.

[0028] The essence of the steel material according to the present embodiment, which has been completed based on the findings described above, is as follows.

[0029] [1] A steel material consisting of, in % by mass, C: 0.15 to 0.45% Yes: 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 to 1.50% Mo: 0.20 to 2.00% Ti: 0.002 to 0.030% Nb: 0.002 to 0.100% B: 0.0005 to 0.0040%, Petition 870250084148, dated 09 / 18 / 2025, pp. 139 / 189 11 / 57 N: 0.0100% or less, O: 0.0040% or less, V: 0 to 0.30%, Cu: 0 to 0.50%, Ni: 0 to 0.50%, W: 0 to 0.50%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, and rare earth metal: 0 to 0.0100%, with the remainder being Fe and impurities; in which: A yield strength is 862 to 1034 MPa, and in steel material, a number density of Al oxides in which, in mass %, an Al content is 20% or more and an O content is 10% or more, and having a major axis of 5.0 μm or more is less than 30 / 200 mm2, and a number density of Si oxides in which, in mass %, an Al content is less than 20%, a Si content is 20% or more, and an O content is 10% or more, and having a major axis of 5.0 μm or more is 5 / 200 mm2 or less.

[0030] [2] Steel material according to [1], containing one or more elements selected from a group consisting of: V: 0.01 to 0.30% 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% Petition 870250084148, dated 09 / 18 / 2025, pp. 140 / 189 12 / 57 Zr: 0.0001 to 0.0100%, and rare earth metal: 0.0001 to 0.0100%.

[0031] [3] The steel material according to [1] or [2], wherein: the steel material is a seamless steel tube.

[0032] The shape of the steel material according to the present embodiment is not particularly limited. The steel material according to the present embodiment may be a steel tube, may be a round steel bar (solid material), or may be a steel sheet. Note that the term round steel bar refers to a steel bar in which a cross-section in a direction perpendicular to the axial direction is circular. Furthermore, the steel tube may be a seamless steel tube or may be a welded steel tube.

[0033] Hereafter, the steel material according to the present embodiment is described in detail. The symbol % in relation to an element means percentage by mass unless otherwise stated.

[0034] [Chemical composition] The chemical composition of the steel material according to this embodiment contains the following elements.

[0035] C: 0.15 to 0.45% Carbon (C) increases the hardenability of steel and its strength. C also promotes the spheroidization of carbides during tempering in the production process, thus increasing the low-temperature toughness of the steel. If the C content is too low, the aforementioned beneficial effects will not 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, many carbides will be formed and the low-temperature toughness of the steel will decrease. Therefore, the C content should be... Petition 870250084148, dated 09 / 18 / 2025, pp. 141 / 189 13 / 57 0.15 to 0.45%. A preferred lower limit for C content is 0.18%, more preferably 0.20%, even more preferably 0.22%, and still more preferably 0.25%. A preferred upper limit for C content is 0.40%, more preferably 0.38%, even more preferably 0.35%, and even more preferably 0.30%.

[0036] Si: 0.05 to 1.00% Silicon (Si) deoxidizes steel. If the Si content is too low, the aforementioned advantageous effect will not 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, even if the contents of other elements are within the range of this embodiment, in some cases a large number of coarse Si oxides will form and the low-temperature toughness of the steel material will decrease. Therefore, the Si content should be 0.05 to 1.00%. A preferred lower limit of the Si content is 0.10%, more preferably 0.15%, and even more preferably 0.20%. A 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%.

[0037] Mn: 0.05 to 1.00% Manganese (Mn) deoxidizes steel. Mn also increases the hardenability of the steel material. If the Mn content is too low, the aforementioned advantageous effects will not 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, even if the contents of other elements are within the range of this embodiment, coarse sulfide-based inclusions will form and the low-temperature toughness of the steel material will decrease. Therefore, the Mn content should be 0.05 to 1.00%. A preferred lower limit for the Mn content is 0.06%, more preferably 0.08%, and even more preferably 0.10%. A preferred upper limit for the Mn content is 0.90%, more preferably 0.80%. Petition 870250084148, dated 09 / 18 / 2025, pp. 142 / 189 14 / 57 even more preferably is 0.70%, even more preferably is 0.60%, even more preferably is 0.50%, and even more preferably is 0.45%.

[0038] P: 0.030% 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 to grain boundaries and the low-temperature toughness of the steel material will decrease. Therefore, the P content should be 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, drastically reducing the P content will greatly increase the cost of production. Therefore, when considering 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: 0.0050% or less Sulfur (S) is an impurity. That is, the lower limit of the S content is more than 0%. If the S content is too high, even if the contents of other elements are within the range of this embodiment, S will segregate to grain boundaries and the low-temperature toughness of the steel material will decrease. Therefore, the S content should be 0.0050% or less. A preferred upper limit of the S content is 0.0040%, 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, drastically reducing the S content will greatly increase the cost of production. Therefore, when considering industrial production, a preferred lower limit of the S content is 0.0001%, more preferably 0.0002%, and even more preferably 0.0003%.

[0040] Al: 0.005 to 0.100% Aluminum (Al) deoxidizes steel. If the Al content is too low, Petition 870250084148, dated 09 / 18 / 2025, pp. 143 / 189 15 / 57 Even if the contents of other elements are within the range of the present embodiment, the aforementioned advantageous effect will not be sufficiently obtained, and the low-temperature toughness of the steel material will decrease. On the other hand, if the Al content is too high, even if the contents of other elements are within the range of the present embodiment, a large number of coarse Al oxides will form, and the low-temperature toughness of the steel material will decrease. Therefore, the Al content should be 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%. As used in this description, the term Al content means the content of Al soluble in acid, that is, sol. Al.

[0041] Cr: 0.30 to 1.50% Chromium (Cr) increases the hardenability of steel material. Cr also increases the resistance to softening by tempering of the steel material and thus allows tempering at high temperatures. As a result, the low-temperature toughness of the steel material increases. If the Cr content is too low, the aforementioned advantageous effects will not 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 low-temperature toughness of the steel material will decrease even if the contents of other elements are within the range of this embodiment. Therefore, the Cr content should be 0.30 to 1.50%. A preferred lower limit of the Cr content is 0.35%, more preferably 0.40%, and even more preferably 0.50%.A preferred upper limit for 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%.

[0042] Mo: 0.20 to 2.00% Molybdenum (Mo) increases the hardenability of steel material. Petition 870250084148, dated 09 / 18 / 2025, pp. 144 / 189 16 / 57 Molybdenum (Mo) also increases the resistance to softening by tempering of the steel material and thus allows tempering at high temperatures. As a result, the low-temperature toughness of the steel material increases. If the Mo content is too low, the aforementioned advantageous effects will not 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 aforementioned advantageous effect will be saturated. Therefore, the Mo content should be 0.20 to 2.00%. A preferred lower limit of the Mo content is 0.22%, more preferably 0.25%, even more preferably 0.30%, even more preferably 0.40%, even more preferably 0.45%, even more preferably 0.50%, and even more preferably 0.60%.A preferred upper limit for Mo content is 1.80%, more preferably 1.60%, even more preferably 1.40%, and still more preferably 1.30%. Note that, in a case where the yield strength of the steel material is 945 MPa or more, a preferred lower limit for Mo content is 0.40%.

[0043] Ti: 0.002 to 0.030% Titanium (Ti) combines with N to form nitrides, and refines the grains of the steel material through an anchoring effect. As a result, the strength of the steel material increases. If the Ti content is too low, the aforementioned advantageous effect will not 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, Ti nitrides will thicken and the low-temperature toughness of the steel material will decrease. Therefore, the Ti content should be 0.002 to 0.030%. A preferred lower limit for the Ti content is 0.003%, and more preferably 0.004%. A preferred upper limit for the Ti content is 0.028%, more preferably 0.025%, even more preferably 0.023%, even more preferably 0.020%, even more preferably 0.018%, even more preferably 0.015%, Petition 870250084148, dated 09 / 18 / 2025, pp. 145 / 189 17 / 57 even more preferably is 0.010%, and even more preferably is 0.008%.

[0044] Nb: 0.002 to 0.100% Niobium (Nb) combines with C and / or N to form carbides, nitrides, or carbonitrides (hereinafter also referred to as carbonitrides and the like). Carbonitrides and the like refine the grains of the steel material by anchoring, thus increasing the low-temperature toughness of the steel material. Nb also forms fine carbides during tempering, thus increasing the resistance to temper softening of the steel material and increasing the strength of the steel material. If the Nb content is too low, the aforementioned advantageous effects will not be sufficiently obtained even if the contents of other elements are within the range of this embodiment. On the other hand, if the Nb content is too high, even if the contents of other elements are within the range of this embodiment, carbonitrides and the like will form excessively and the low-temperature toughness of the steel material will decrease. Therefore, the Nb content should be between 0.002% and 0.100%.A preferred lower limit for Nb content is 0.005%, more preferably 0.010%, even more preferably 0.015%, even more preferably 0.020%, and even more preferably 0.025%. A preferred upper limit for Nb content is 0.080%, more preferably 0.060%, even more preferably 0.040%, and even more preferably 0.035%.

[0045] B: 0.0005 to 0.0040% Boron (B) dissolves in steel, thus increasing the hardenability and strength of the steel material. If the B content is too low, the aforementioned advantageous effect will not be sufficiently achieved, 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 will form, and the toughness will decrease. Petition 870250084148, dated 09 / 18 / 2025, pp. 146 / 189 18 / 57 The temperature of the steel material will decrease. Therefore, the B content should be 0.0005 to 0.0040%. A preferred lower limit for the B content is 0.0006%, more preferably 0.0008%, and even more preferably 0.0010%. A preferred upper limit for the B content is 0.0035%, more preferably 0.0030%, even more preferably 0.0025%, even more preferably 0.0020%, and even 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, thus refining the grains of the steel material by the anchoring effect. As a result, the strength of the steel material increases. However, if the N content is too high, even if the contents of other elements are within the range of this embodiment, coarse nitrides will form and the low-temperature toughness of the steel material will decrease. Therefore, the N content should be 0.0100% or less. A preferred upper limit of the N content is 0.0080%, more preferably 0.0060%, even more preferably 0.0050%, even more preferably 0.0045%, and even more preferably 0.0040%.A preferred lower limit of the N content to more effectively obtain the aforementioned beneficial effect is 0.0005%, more preferably 0.0010%, even more preferably 0.0015%, even more preferably 0.0020%, even more preferably 0.0025%, and even more preferably 0.0030%.

[0047] 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 range of this embodiment, coarse oxides will form and the low-temperature toughness of the steel material will decrease. Therefore, the O content should be 0.0040% or less. A preferred upper limit of the O content is 0.0035%, more preferably 0.0030%, even more preferably 0.0025%, and even more preferably 0.0020%. Petition 870250084148, dated 09 / 18 / 2025, pages 147 / 189 19 / 57 The oxygen content is preferably as low as possible. However, drastically reducing the oxygen content will greatly increase the cost of production. Therefore, when considering industrial production, a preferred lower limit for the oxygen content is 0.0001%, more preferably 0.0002%, and even more preferably 0.0003%.

[0048] The remainder of the chemical composition of the steel material according to the present embodiment is Fe and impurities. Here, the term impurities refers to substances that, when industrially producing the steel material, are mixed from ore or scrap that is used as raw material or from the production environment or similar, and that are permitted within a range that does not adversely affect the steel material according to the present embodiment.

[0049] [Optional elements] The chemical composition of the steel material described above may additionally contain V in place of a portion of Fe.

[0050] V: 0 to 0.30% Vanadium (V) is an optional element and does not need to be included. That is, the V content can be 0%. When included, V forms carbonitrides and similar compounds. Carbonitrides and similar compounds refine the grains of the steel material through an anchoring effect, thus increasing the low-temperature toughness of the steel material. V also forms fine carbides during tempering, thus increasing the resistance to temper softening of the steel material and increasing the strength of the steel material. If even a small amount of V is included, the aforementioned advantageous effects will be obtained to some extent. However, if the V content is too high, even if the contents of other elements are within the range of this embodiment, carbonitrides and similar compounds will form excessively, and the low-temperature toughness of the steel material will decrease. Therefore, the V content should be 0 to 0.30%.A preferred lower limit for the V content is more than 0%, more preferably 0.01%, even more preferably 0.03%, and even further. Petition 870250084148, dated 09 / 18 / 2025, pp. 148 / 189 20 / 57 preferably is 0.05%, and even more preferably is 0.08%. A preferred upper limit for the V content is 0.25%, more preferably is 0.20%, and even more preferably is 0.15%. Note that in a case where the yield strength of the steel material is 945 MPa or more, a lower limit for the V content is preferably 0.01%.

[0051] The chemical composition of the steel material described above may additionally contain one or more elements selected from the group consisting of Cu and Ni in place of a portion of Fe. Each of these elements is an optional element, and increases the hardenability of the steel material.

[0052] Cu: 0 to 0.50% Copper (Cu) is an optional element and does not need to be included. That is, the Cu content can be 0%. When included, Cu increases the hardenability of the steel material and increases the strength of the steel material. If even a small amount of Cu is included, the aforementioned advantageous effect will 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 will be too high and the low-temperature toughness of the steel material will decrease. Therefore, the Cu content should be 0 to 0.50%. A preferred lower limit of the Cu content is more than 0%, more preferably 0.01%, even more preferably 0.02%, and even more preferably 0.03%. A preferred upper limit for the Cu content is 0.35%, more preferably it is 0.25%, even more preferably it is 0.15%, even more preferably it is 0.10%, and even more preferably it is 0.05%.

[0053] Ni: 0 to 0.50% Nickel (Ni) is an optional element and does not need to be included. That is, the Ni content can be 0%. When included, Ni increases the hardenability of the steel material and increases the strength of the steel material. Ni also dissolves in steel and increases the low-temperature toughness of the steel material. If even a small amount of Ni is included, these advantageous effects are achieved. Petition 870250084148, dated 09 / 18 / 2025, pp. 149 / 189 21 / 57 will be achieved 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, localized corrosion will be promoted and the low-temperature toughness of the steel material will decrease. Therefore, the Ni content should be 0 to 0.50%. A preferred lower limit for the Ni content is more than 0%, more preferably 0.01%, and even more preferably 0.02%. A preferred upper limit for the Ni content is 0.30%, more preferably 0.20%, even more preferably 0.10%, and even more preferably 0.05%.

[0054] The chemical composition of the steel material described above may additionally contain W in place of a portion of Fe.

[0055] W: 0 to 0.50% Tungsten (W) is an optional element and does not need to be contained. That is, the W content can be 0%. When contained, in acidic environments W forms a protective corrosion coating and suppresses hydrogen penetration into the steel material. As a result, the low-temperature toughness of the steel material is increased. If even a small amount of W is contained, the aforementioned advantageous effect will be obtained to some 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 will form in the steel material, and the low-temperature toughness of the steel material will decrease. Therefore, the W content should be 0 to 0.50%. A 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%. A preferred upper limit for W content is less than 0.50%, and more preferably 0.48%.

[0056] The chemical composition of the steel material described above may additionally contain one or more elements selected from the group consisting of Ca, Mg, Zr and rare earth metal in place of a portion of Fe. Each of these elements is an optional element, and each element renders S in the steel material harmless by forming sulfides. As a result, these Petition 870250084148, dated 09 / 18 / 2025, pp. 150 / 189 22 / 57 elements increase the low-temperature toughness of the steel material.

[0057] Ca: 0 to 0.0100% Calcium (Ca) is an optional element and does not need to be included. That is, the Ca content can be 0%. When included, Ca renders sulfur in the steel material harmless by forming sulfides, thus increasing the low-temperature toughness of the steel material. If even a small amount of Ca is included, the aforementioned advantageous effect will 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, oxides in the steel material will thicken and the low-temperature toughness of the steel material will decrease. Therefore, the Ca content should be 0 to 0.0100%. A 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%. A preferred upper limit for Ca content is 0.0040%, more preferably 0.0025%, even more preferably 0.0020%, and still more preferably 0.0015%.

[0058] Mg: 0 a 0,0100% Magnesium (Mg) is an optional element and does not need to be included. That is, the Mg content can be 0%. When included, Mg renders sulfur in the steel material harmless by forming sulfides, thus increasing the low-temperature toughness of the steel material. If even a small amount of Mg is included, the aforementioned advantageous effect will 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, oxides in the steel material will thicken and the low-temperature toughness of the steel material will decrease. Therefore, the Mg content should be 0 to 0.0100%. A preferred lower limit of the Mg content is more than 0%, more preferably 0.0001%, even more preferably 0.0003%, and even more preferably 0.0006%. A preferred upper limit for Mg content is 0.0040%, more preferably 0.0025%, even more preferably 0.0020%, and still Petition 870250084148, dated 09 / 18 / 2025, pp. 151 / 189 23 / 57 more preferably is 0.0015%.

[0059] Zr: 0 to 0.0100% Zirconium (Zr) is an optional element and does not need to be contained. That is, the Zr content can be 0%. When contained, Zr renders sulfur in the steel material harmless by forming sulfides, and thus increases the low-temperature toughness of the steel material. If even a small amount of Zr is contained, the aforementioned advantageous effect will be obtained to some extent. However, if the Zr content is too high, even if the contents of other elements are within the range of this embodiment, oxides in the steel material will thicken and the low-temperature toughness of the steel material will decrease. Therefore, the Zr content should be 0 to 0.0100%. A preferred lower limit of the Zr content is more than 0%, more preferably 0.0001%, even more preferably 0.0003%, and even more preferably 0.0006%.A preferred upper limit for Zr content is 0.0040%, more preferably 0.0025%, even more preferably 0.0020%, even more preferably 0.0015%, and even more preferably 0.0010%.

[0060] Rare earth metal (REM): 0 to 0.0100% Rare earth metal (REM) is an optional element and does not need to be contained. That is, the REM content can be 0%. When contained, REM renders sulfur (S) in the steel material harmless by forming sulfides, and thus increases the low-temperature toughness of the steel material. REM also combines with phosphorus (P) in the steel material and thus suppresses P segregation to the grain boundaries. Therefore, a decrease in the low-temperature toughness of the steel material attributable to P segregation is suppressed. If even a small amount of REM is contained, the aforementioned advantageous effects will be obtained to some extent even if the contents of other elements are within the range of this embodiment. However, if the REM content is too high, even if the contents of other elements are within the range of this embodiment, oxides in the steel material will thicken and the low-temperature toughness of the steel material will decrease. Therefore, the REM content should be... Petition 870250084148, dated 09 / 18 / 2025, pp. 152 / 189 24 / 57 being 0 to 0.0100%. A 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%. A preferred upper limit of the REM content is 0.0040%, more preferably 0.0025%, even more preferably 0.0020%, and even more preferably 0.0015%.

[0061] Note that, in the present description, the term REM means one or more types of elements selected from the group consisting of scandium (Sc), which is the element with atomic number 21, yttrium (Y), which is the element with atomic number 39, and the elements from lanthanum (La), with atomic number 57, to lutetium (Lu), with atomic number 71, which are lanthanides. In the present description, the term REM content refers to the total content of these elements.

[0062] [Flow limit] The yield strength of the steel material according to the present embodiment is 862 to 1034 MPa (125 to 150 ksi). As used in the present description, in a case where the yield strength is 862 to less than 945 MPa, the term yield strength means the stress at a moment of 0.65% elongation (0.65% proof stress) obtained in a tensile test conducted at normal temperature (25°C) in accordance with ASTM E8 / E8M (2021). As used in the present description, in a case where the yield strength is 945 to 1034 MPa, the term yield strength means the stress at a moment of 0.7% total elongation (0.7% total elongation proof stress) obtained in a tensile test conducted at normal temperature (25°C) in accordance with ASTM E8 / E8M (2021).Having the chemical composition described above, and satisfying requirements relating to a numerical density of coarse Al oxides and a numerical density of coarse Si oxides to be described later, the steel material according to the present embodiment has excellent low-temperature toughness even when the yield strength of the steel material is 862 to 1034 MPa.

[0063] The yield strength of steel material according to Petition 870250084148, dated 09 / 18 / 2025, pp. 153 / 189 25 / 57 The present embodiment is determined by the following method. First, a round bar specimen is prepared from the steel material according to the present embodiment. If the steel material is a steel plate, the round bar specimen is prepared from the central portion of the thickness. In this case, the axial direction of the round bar specimen must be made a direction that is parallel to the rolling elongation direction of the steel plate. If the steel material is a steel tube, the round bar specimen is prepared from the central portion of the wall thickness. In this case, the axial direction of the round bar specimen must be made a direction that is parallel to the axial direction of the steel tube. If the steel material is a round steel bar, the round bar specimen is prepared from a position R / 2.In this description, the term position R / 2 means the central position of a radius R in a cross-section perpendicular to the axial direction of the round steel bar. In this case, the axial direction of the round bar specimen must be made a direction that is parallel to the axial direction of the round steel bar. Regarding the size of the round bar specimen, for example, the round bar specimen has a diameter of the parallel portion of 8.9 mm and a gauge length of 35.6 mm.

[0064] A tensile test is performed in the atmosphere at normal temperature (25°C) by a method in accordance with ASTM E8 / E8M (2021) using the prepared round bar specimen. When the stress obtained at a moment of 0.65% elongation (0.65% proof stress) is 862 to less than 945 MPa, the 0.65% proof stress is defined as the yield strength (MPa). When the stress obtained at a moment of 0.7% total elongation (0.7% total elongation proof stress) is 945 to 1034 MPa, the 0.7% total elongation proof stress is defined as the yield strength (MPa). Note that a value obtained by rounding decimals from the numerical value obtained is adopted as the yield strength (MPa) in this embodiment.

[0065] [Number density of coarse Al oxides] The steel material according to the present embodiment has the Petition 870250084148, dated 09 / 18 / 2025, pp. 154 / 189 26 / 57 chemical composition described above and has a yield strength of 862 to 1034 MPa and, in addition, the number density of coarse Al oxides in the steel material is less than 30 / 200 mm2. As described above, in the present description, particles in which, by mass %, the Al content is 20% or more and the O content is 10% or more are also referred to as Al oxides. Furthermore, as described above, in the present description, Al oxides having a major axis of 5.0 μm or more are also referred to as coarse Al oxides. That is, the term coarse Al oxides means particles in which, by mass %, the Al content is 20% or more and the O content is 10% or more, and which have a major axis of 5.0 μm or more.

[0066] As described above, when producing a steel material having the chemical composition described above, during the steelmaking process, deoxidation is mainly carried out by aluminum (Al). Therefore, a large number of Al oxides are prone to form in a steel material having the chemical composition described above. Furthermore, Al oxides are hard oxides, and are prone to cause a decrease in the toughness of the steel material. In particular, in a case where the steel material has a high yield strength of 125 ksi or more, the influence of coarse Al oxides is likely to be enhanced, and the low-temperature toughness is likely to decrease markedly. Therefore, in the steel material according to the present embodiment that has the chemical composition described above and has a yield strength of 862 to 1034 MPa, the number density of coarse Al oxides should be less than 30 / 200 mm2.

[0067] In the present embodiment, a preferred upper limit of the number density of coarse Al oxides is 28 / 200 mm2, more preferably it is 26 / 200 mm2, even more preferably it is 25 / 200 mm2, and even more preferably it is 22 / 200 mm2. In the present embodiment, the lower limit of the number density of coarse Al oxides is not particularly limited, and may be 0 / 200 mm2. The lower limit of the number density of coarse Al oxides, for example, may be 5 / 200 mm2, may be 7 / 200 mm2, Petition 870250084148, dated 09 / 18 / 2025, pp. 155 / 189 27 / 57 or it could be 9 / 200 mm2. A method for determining the numerical density of coarse Al oxides is described later.

[0068] [Number density of coarse Si oxides] The steel material according to the present embodiment has the chemical composition described above, has a yield strength of 862 to 1034 MPa, the number density of coarse Al oxides in the steel material is less than 30 / 200 mm2e, and, in addition, the number density of coarse Si oxides in the steel material is 5 / 200 mm2 or less. As described above, in the present description, particles in which, in mass %, the Al content is less than 20%, the Si content is 20% or more, and the O content is 10% or more are also referred to as Si oxides. Furthermore, as described above, in the present description, Si oxides having a major axis of 5.0 μm or more are also referred to as coarse Si oxides. That is, the term coarse Si oxides means particles in which, in mass %, the Al content is less than 20%, the Si content is 20% or more, and the O content is 10% or more, and which have a major axis of 5.0 μm or more.

[0069] As described above, until now, attention has not been given to Si oxides due to their small number. However, when a steel material has a high yield strength of 125 ksi or more, there is a possibility that a decrease in low-temperature toughness is likely to occur not only due to coarse Al oxides, but also due to coarse Si oxides, which are small in number. Therefore, by not only making the number density of coarse Al oxides less than 30 / 200 mm2, but also making the number density of coarse Si oxides 5 / 200 mm2 or less, there is a possibility that excellent low-temperature toughness can be stably obtained even when the yield strength is raised to 125 ksi or more.Therefore, the steel material according to the present embodiment has the chemical composition described above and a yield strength of 862 to 1034 MPa, and in the steel material the numerical density of coarse Al oxides is made less than 30 / 200 mm2e, furthermore. Petition 870250084148, dated 09 / 18 / 2025, pp. 156 / 189 28 / 57 the numerical density of coarse Si oxides is made 5 / 200 mm2 or less.

[0070] In the present embodiment, a preferred upper limit of the number density of coarse Si oxides is 4 / 200 mm2, and more preferably 3 / 200 mm2. In the present embodiment, the lower limit of the number density of coarse Si oxides is not particularly restricted, and may be 0 / 200 mm2. The lower limit of the number density of coarse Si oxides may be, for example, 1 / 200 mm2.

[0071] In the present embodiment, the number density of coarse Al oxides and the number density of coarse Si oxides in steel material can be determined by the following method. First, a test specimen in which a face including the rolling elongation direction and the rolling reduction direction is adopted as an observation surface is prepared from the steel material according to the present embodiment. Specifically, if the steel material is a steel plate, a test specimen in which a face including the rolling elongation direction and the thickness direction is adopted as the observation surface is prepared from a central portion of the thickness. If the steel material is a steel tube, a test specimen in which a face including the tube axis direction and the tube radius direction is adopted as the observation surface is prepared from a central portion of the wall thickness.If the steel material is a round steel bar, a test specimen is prepared that includes an R / 2 position at its center and in which a face including the axial and radial directions is adopted as the observation surface.

[0072] After polishing the observation surface of the prepared test specimen to obtain a mirror-like surface, the measurement is performed. Although the area of ​​the observation surface is not limited, for example, the area is set to a size of 300 mm2 (20 mm x 15 mm). On the observation surface, the number of Al oxides having a major axis of 5.0 μm or more is determined. In addition, on the observation surface, the number of Si oxides having a major axis of 5.0 μm or more is determined. Specifically, Petition 870250084148, dated 09 / 18 / 2025, pages 157 / 189 29 / 57 First, particles on the observation surface are identified based on contrast. Each of the identified particles is subjected to an element concentration analysis (EDS analysis). The EDS analysis is conducted with an accelerating voltage of 20 kV for quantification of N, O, Mg, Al, Si, P, S, Ca, Ti, Cr, Mn, Fe, Cu, Zr, and Nb as elements to be analyzed. Based on the EDS analysis result for each particle, particles in which, by mass %, the Al content is 20% or more and the O content is 10% or more are identified as Al oxides. Furthermore, based on the EDS analysis result for each particle, particles in which, by mass %, the Al content is less than 20%, the Si content is 20% or more, and the O content is 10% or more are identified as Si oxides.

[0073] Among the Al oxides identified on the observation surface, Al oxides having a major axis of 5.0 μm or more (coarse Al oxides) are identified, and the total number of coarse Al oxides is determined. Furthermore, among the Si oxides identified on the observation surface, Si oxides having a major axis of 5.0 μm or more (coarse Si oxides) are identified, and the total number of coarse Si oxides is determined. Note that the major axis of the Al oxides and Si oxides can be determined by a well-known method. Additionally, in the present description, the term major axis of the Al oxides and Si oxides means, on the observation surface, the longest line segment among line segments connecting two arbitrary points on the outer circumference of each of the Al oxides and Si oxides.

[0074] The numerical density of coarse Al oxides ( / 200 mm2) is determined based on the total number of coarse Al oxides and the gross area of ​​the observation surface. Furthermore, the numerical density of coarse Si oxides ( / 200 mm2) is determined based on the total number of coarse Si oxides and the gross area of ​​the observation surface. Note that, in this embodiment, a number obtained by rounding decimals from the relevant numerical value obtained is adopted as the numerical density of oxides. Petition 870250084148, dated 09 / 18 / 2025, pages 158 / 189 30 / 57 Coarse Al oxides ( / 200 mm2), and the numerical density of coarse Si oxides ( / 200 mm2), respectively. Furthermore, the measurement of the numerical density of coarse Al oxides and the numerical density of coarse Si oxides can be performed using an instrument in which a scanning electron microscope is provided with a composition analysis function (SEM-EDS instrument). For example, an automated analyzer with the commercial name Metals Quality Analyzer manufactured by FEI (ASPEX) can be used as the SEM-EDS instrument.

[0075] [Low temperature toughness] The steel material according to the present embodiment has the chemical composition described above and has a yield strength of 862 to 1034 MPa, and in the steel material the number density of coarse Al oxides is less than 30 / 200 mm2e, in addition, the number density of coarse Si oxides is 5 / 200 mm2 or less. As a result, the steel material according to the present embodiment has excellent low-temperature toughness even though the yield strength is 125 ksi or more. In the present embodiment, the phrase excellent low-temperature toughness is determined by a fracture appearance transition temperature obtained by conducting a Charpy impact test according to ASTM E23 (2018).

[0076] First, a full-size or sub-size V-notch test specimen is prepared in accordance with API 5CT (2019) from the steel material according to the present embodiment. Here, if the steel material is a steel plate, the rolling elongation direction of the steel plate is defined as the L (longitudinal) direction, and the width direction of the steel plate is defined as the T (transverse) direction. If the steel material is a steel pipe, the radial direction of the pipe is defined as the C direction, the axial direction of the steel pipe is defined as the L direction, and a direction perpendicular to the C direction and the L direction is defined as the T direction. If the steel material is a round steel bar, the sectional radial direction of the round steel bar is defined as the C direction, the axial direction of the bar is defined as the L direction. Petition 870250084148, dated 09 / 18 / 2025, pages 159 / 189 31 / 57 round steel is defined as direction L, and a direction perpendicular to direction C and direction L is defined as direction T.

[0077] The prepared V-notch test specimens are subjected to a Charpy impact test according to ASTM E23 (2018). Specifically, the test temperature range is -120 to 20 °C, and the test temperatures are defined in eight levels that are varied in 20 °C increments (-120 °C, -100 °C, -80 °C, -60 °C, -40 °C, -20 °C, 0 °C, and 20 °C). The Charpy impact test is performed using two test specimens for each test temperature. The brittle fracture rate (%) of the respective test specimens after testing at each temperature under the above conditions is determined. The test temperatures (°C) and the obtained brittle fracture rates (%) are plotted to obtain an approximate curve. The temperature (°C) at which the brittle fracture rate becomes 50% is determined from the approximate curve obtained, and the temperature thus determined is defined as the fracture appearance transition temperature (°C).Note that, in this embodiment, a value obtained by rounding decimals from the numerical value obtained is adopted as the fracture appearance transition temperature (°C).

[0078] In the present embodiment, excellent low-temperature toughness is defined according to a yield strength range. Specifically, in a case where the yield strength is 862 to less than 945 MPa, if the fracture appearance transition temperature is -50 °C or less, the relevant steel material is determined to have excellent low-temperature toughness. Also, in a case where the yield strength is 945 to 1034 MPa, if the fracture appearance transition temperature is -40 °C or less, the relevant steel material is determined to have excellent low-temperature toughness.

[0079] [Microstructure] In the microstructure of the steel material according to the present embodiment, the total volume ratios of tempered martensite and tempered bainite is 90% or more. The equilibrium of the microstructure is, for example, ferrite or Petition 870250084148, dated 09 / 18 / 2025, pp. 160 / 189 32 / 57 pearlite. If the total volumetric ratios of tempered martensite and tempered bainite contained in the microstructure of a steel material having the chemical composition described above is 90% or more, provided that the other requirements of the present embodiment are satisfied, the steel material can achieve both a yield strength of 125 ksi or more and excellent low-temperature toughness. That is, in the present embodiment, if the steel material achieves both a yield strength of 125 ksi or more and excellent low-temperature toughness, it is determined that the total volumetric ratios of tempered martensite and tempered bainite contained in the microstructure is 90% or more.

[0080] Note that, in the case of determining the volumetric ratios of tempered martensite and tempered bainite by observation, the volumetric ratios can be determined by the following method. First, a test specimen having an observation surface is prepared from the steel material according to the present embodiment. If the steel material is a steel plate, a test specimen in which a face including the rolling elongation direction and the thickness direction is adopted as the observation surface is prepared from a central portion of the thickness. If the steel material is a steel tube, a test specimen in which a face including the tube axis direction and the tube radius direction is adopted as the observation surface is prepared from a central portion of the wall thickness.If the steel material is a round steel bar, a test specimen is prepared that includes an R / 2 position at its center, and in which a face including the axial and radial directions is adopted as the observation surface.

[0081] After polishing the observation surface of the test specimen to obtain a mirror-like surface, the test specimen is immersed for approximately 10 seconds in a nital etching reagent to reveal the microstructure by etching. The etched observation surface is observed by means of a secondary electron image obtained using a scanning electron microscope (SEM), and the observation is performed in 10 visual fields. The area of Petition 870250084148, dated 09 / 18 / 2025, pp. 161 / 189 33 / 57 each visual field is, for example, 0.01 mm2 (1000x magnification). In each visual field, tempered martensite and tempered bainite are identified based on contrast. The area fractions of the identified tempered martensite and tempered bainite are determined. The method for determining the area fractions is not particularly limited, and a well-known method can be used. For example, the area fractions of tempered martensite and tempered bainite can be determined by image analysis. In the present embodiment, an arithmetic mean of the area fractions of tempered martensite and tempered bainite determined in all visual fields is defined as the volumetric ratio of tempered martensite and tempered bainite.

[0082] [Production method] A method for producing steel material according to the present embodiment will now be described. Hereinafter, a method for producing a seamless steel tube as an example of steel material according to the present embodiment is described. The method for producing a seamless steel tube includes a process for preparing a starting material (steelmaking process), a process for subjecting the starting material to hot working to produce a hollow shell (hot working process), and a process for subjecting the hollow shell to quenching and tempering to make a seamless steel tube (quenching and tempering process). Note that a production method according to the present embodiment is not limited to the production method described below. Each process is described in detail hereinafter.

[0083] [Steel manufacturing process] In the steelmaking process, firstly, pig iron produced by a well-known method is subjected to refining using a converter (primary refining). The molten steel that has undergone primary refining is then subjected to secondary refining. In secondary refining, alloying elements that have undergone composition adjustment are added to the molten steel to produce a molten steel that meets the described chemical composition. Petition 870250084148, dated 09 / 18 / 2025, pp. 162 / 189 34 / 57 above.

[0084] In secondary refining, for example, a vacuum degassing treatment RH (Ruhrstahl-Hausen) is carried out. After that, the final adjustment of the alloying elements is performed. In secondary refining, compound refining can be carried out. In such a case, before the vacuum degassing treatment RH, for example, a refining treatment using an LF (ladle furnace) or VAD (vacuum arc degassing) is carried out.

[0085] A starting material is produced using molten steel that has undergone secondary refining. Specifically, a casting (a plate, a block, or a billet) is produced by a continuous casting process using molten steel that has undergone secondary refining. In the continuous casting process, molten steel is first poured from a ladle into a tundish. At this point, to seal the ladle nozzle, the nozzle is usually filled with sand. Therefore, in some cases, the sand used for filling may mix with the molten steel poured from the ladle into the tundish. Furthermore, when producing a starting material having the chemical composition described above, in some cases Si oxides may be used as the sand for filling. In such a case, there is a concern that Si oxides will be introduced into the starting material produced.

[0086] Therefore, in the present embodiment, to prevent Si oxides that are filled in the ladle nozzle from being introduced into the distributor, the molten steel and the Si oxides are separated. Although the method for separating Si oxides from molten steel is not particularly limited, for example the following method can be used. An inclined metal plate is placed in a position that is below the ladle nozzle and above the distributor opening. When the ladle nozzle is opened, first, Si oxides are discharged from the nozzle, and then molten steel is discharged. Here, the Si oxides are light compared to the molten steel. Therefore, the Si oxides discharged from the nozzle are guided out of the distributor opening along the inclination of the metal plate. The inclination of the metal plate can be Petition 870250084148, dated 09 / 18 / 2025, pp. 163 / 189 35 / 57 supplied, for example, by arranging a machined metal plate in a conical shape without a bottom surface in such a way that the apex of the conical metal plate is directly below the mouth of the pan, or it may be supplied by another method. Furthermore, one metal plate may be used, or a plurality of metal plates may be stacked one on top of the other and used. Also, although not particularly limited, the thickness of the metal plate is, for example, about 1 to 10 mm.

[0087] After the Si oxides have been discharged from the nozzle, the molten steel is discharged. At this point, the molten steel discharged from the nozzle is introduced into the distributor through the opening along with the metal plate. That is, in the present embodiment, part or all of the metal plate can be introduced into the distributor and mixed with the molten steel. Therefore, the metal plate in the present embodiment is preferably a metal plate composed of an alloying element contained in the molten steel. For example, an aluminum plate can be used as a metal plate composed of an alloying element contained in the molten steel. Note that, in the present description, the term aluminum plate means a metal plate that is formed of aluminum with the balance being impurities.

[0088] Preferably, after the Si oxides have been discharged from the nozzle, the metal plate is removed from below the nozzle before discharging the molten steel. In this case, Si oxides that have adhered to the metal plate can be prevented from mixing with the molten steel. Note that a method for removing the metal plate from below the nozzle is not particularly limited, and for example a hole can be pre-formed in a part of the metal plate, and the metal plate can be removed using a rod that has a hook formed at its front end. In this case, the metal plate can be removed by hooking the hook formed at the front end of the rod into the hole in the metal plate and then pulling the rod. Using the method described above, Si oxides can be separated from the molten steel, and the molten steel can be introduced into the distributor. Note that, a Petition 870250084148, dated 09 / 18 / 2025, pp. 164 / 189 36 / 57 The method for separating Si oxides from molten steel is not limited to the method described above.

[0089] Next, the prepared molten steel is melted to produce a starting material. Although not particularly limited, the casting method is, for example, a continuous casting process. In the case of producing the starting material by a continuous casting process, preferably the casting is carried out by the following method.

[0090] Preferably, the casting speed in the continuous casting apparatus is set to 1.0 to 3.0 m / min. If the casting speed is too slow, in some cases an accumulation zone of Al oxides may form in the starting material. In such a case, a large number of coarse Al oxides will be contained in the produced steel material, and the low-temperature toughness of the steel material will decrease. On the other hand, if the casting speed is too fast, in some cases Al oxides may not be able to float to the surface, and a large number of Al oxides will remain in the starting material. In such a case, a large number of coarse Al oxides will be contained in the produced steel material, and the low-temperature toughness of the steel material will decrease. Therefore, the casting speed in the continuous casting apparatus is preferably set to 1.0 to 3.0 m / min.

[0091] Furthermore, in the case of producing the starting material by a continuous casting process, preferably the molten steel is electromagnetically stirred in the mold. Specifically, performing electromagnetic stirring in the mold with a defined current value in the range of 330 to 450 A makes it difficult for an accumulation zone of Al oxides to form in the starting material. If the current value set for electromagnetic stirring in the mold is too low, in some cases the stirring of the molten steel will be insufficient and an accumulation zone of Al oxides will form in the starting material. In such a case, a large number of coarse Al oxides will be contained in the produced steel material, and the low-temperature toughness of the steel material will decrease. On the other hand, if the current value that is set Petition 870250084148, dated 09 / 18 / 2025, pages 165 / 189 37 / 57 for electromagnetic agitation in the mold is too high, in some cases an excessive load will be applied to the production plant. Therefore, in the present embodiment, preferably the current value defined for electromagnetic agitation in the mold is defined within a range of 330 to 450 A. The molten steel is cast and a starting material is produced by the method described above.

[0092] [Hot work process] In the hot working process, the prepared starting material is subjected to hot working to produce an intermediate steel material. If the steel material is a seamless steel pipe, the intermediate steel material corresponds to a hollow shell. First, a billet is heated in a heating furnace. Although not particularly limited, the heating temperature is, for example, 1100 to 1300 °C. After the billet is extracted from the heating furnace, it is subjected to hot working to produce a hollow shell (seamless steel pipe). The hot working method is not particularly limited, and it is sufficient to use a well-known method.

[0093] For example, the Mannesmann process can be carried out as hot working to produce a hollow shell. In this case, a round billet is subjected to punch rolling using a punching machine. When carrying out punch rolling, although not particularly limited, for example, the punching ratio is 1.0 to 4.0. The round billet subjected to punch rolling is further subjected to hot rolling with a mandrel mill, a reducer, a sizing mill or similar to produce a hollow shell. The cumulative area reduction in the hot working process is, for example, 20 to 70%.

[0094] A hollow shell can be produced from the billet by another hot working method. For example, in a case where the steel material is a thick-walled steel material of short length such as a coupling, a hollow shell can be produced by forging using the Ehrhardt process or similar. A hollow shell is produced by the above process. Although not particularly limited, the wall thickness of the hollow shell is, Petition 870250084148, dated 09 / 18 / 2025, pages 166 / 189 38 / 57 for example, 9 to 60 mm.

[0095] If the steel material is a round steel bar, first, the starting material is heated in a heating furnace. Although not particularly limited, the heating temperature is, for example, 1100 to 1300 °C. The starting material extracted from the heating furnace is subjected to hot working to produce an intermediate steel material in which a cross-section perpendicular to the axial direction is a circular shape. Hot working is, for example, roughing which is carried out using a roughing mill or hot rolling which is carried out using a continuous mill. In a continuous mill, a horizontal stand having a pair of grooved rolls arranged one above the other 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.

[0096] If the steel material is a steel sheet, first, the starting material is heated in a heating furnace. Although not particularly limited, the heating temperature is, for example, 1100 to 1300 °C. The starting material extracted from the heating furnace is subjected to hot rolling using a roughing mill and a continuous mill to produce an intermediate steel material having a steel sheet shape.

[0097] Hollow shells produced by hot working can be air-cooled (as a rolled product). Hollow shells produced by hot working can be directly quenched after hot working without being cooled to normal temperature, or they can be quenched after undergoing supplementary heating (reheating) after hot working.

[0098] In the case of performing direct quenching after hot working, or performing quenching after supplementary heating, cooling can be interrupted mid-quenching or slow cooling can be performed. In this case, the occurrence of quenching cracking in the hollow shell can be suppressed. Furthermore, in the case of performing direct quenching after hot working... Petition 870250084148, dated 09 / 18 / 2025, pp. 167 / 189 39 / 57 hot, or perform quenching after supplemental heating, stress relief annealing (SR) can be performed at a time that is after quenching and before the heat treatment of the next process. In this case, residual stress in the hollow shell is eliminated.

[0099] As described above, in the hot working process, the prepared starting material is subjected to hot working to produce an intermediate steel material. Hereafter, the quenching process is described in detail.

[0100] [Tempering process] In the quenching process, the prepared intermediate steel material (hollow shell) is subjected to quenching. As used in this description, the term quenching means rapidly cooling the intermediate steel material to a temperature not lower than point A3. A preferred quenching temperature is 800 to 1000 °C. If the quenching temperature is too high, in some cases previous austenite grains will become coarse and the low-temperature toughness of the steel material will decrease. Therefore, a quenching temperature in the range of 800 to 1000 °C is preferred.

[0101] In the present description, in a case where direct quenching is performed after hot working, the term quenching temperature corresponds to the surface temperature of the intermediate steel material as measured by a thermometer placed on the exit side of the apparatus that performs the final hot working. Furthermore, in a case where quenching is performed after supplemental heating or reheating after hot working, the term quenching temperature corresponds to the temperature of the furnace that performs the supplemental heating or reheating.

[0102] The quenching method is a method that, for example, continuously cools the intermediate steel material (hollow shell) from the quenching start temperature and continuously lowers the surface temperature of the hollow shell. The method of performing continuous cooling treatment is not particularly limited, and a well-known method may be Petition 870250084148, dated 09 / 18 / 2025, pp. 168 / 189 40 / 57 used. The method of performing continuous cooling treatment is, for example, a method that cools the hollow shell by immersing the hollow shell in a water bath, or a method that cools the hollow shell in an accelerated manner by water shower cooling or mist cooling.

[0103] If the cooling rate during quenching is too slow, the microstructure will not become a microstructure that is mainly composed of martensite and bainite, and the mechanical property defined in this embodiment (yield strength of 862 to 1034 MPa) will not be obtained. In such a case, moreover, excellent low-temperature toughness will not be obtained.

[0104] Therefore, as described above, in the method for producing the steel material according to the present embodiment, the intermediate steel material is rapidly cooled during quenching. Specifically, in the quenching process, the average cooling rate when the surface temperature of the intermediate steel material (hollow shell) is within the range of 800 to 500 °C during quenching is defined as the quenching cooling rate CRsqqW. More specifically, the quenching cooling rate CR8qq-5qq is determined based on a temperature measured in a region that is cooled more slowly within a cross-section of the intermediate steel material being quenched (e.g., in the case of forced cooling of both surfaces, the cooling rate is measured in the central portion of the thickness of the intermediate steel material).

[0105] A preferred cooling rate during CR8qq-5qq quenching is 300 °C / min or more. A more preferred lower limit of the cooling rate during CR8qq-5qq quenching is 450 °C / min, and even more preferably is 600 °C / min. Although an upper limit of the cooling rate during CR8qq-5qq quenching is not particularly defined, the upper limit is, for example, 60000 °C / min.

[0106] Preferably, tempering is carried out after heating the hollow hull in the austenite zone multiple times. Petition 870250084148, dated 09 / 18 / 2025, pp. 169 / 189 41 / 57 In this case, the low-temperature toughness of the steel material increases because austenite grains are refined before quenching. Heating in the austenite zone can be repeated multiple times, performing quenching multiple times, or heating in the austenite zone can be repeated multiple times, performing normalization and quenching. Furthermore, quenching and tempering, which is described later, can be performed in combination multiple times. That is, both quenching and tempering can be performed multiple times. In such a case, the low-temperature toughness of the steel material increases even further. The tempering process is described in detail below.

[0107] [Revenue process] In the tempering process, the hollow shell in which the aforementioned quenching was performed is subjected to tempering. In this description, the term tempering means reheating the intermediate steel material after quenching to a temperature that is lower than the Ac1 point and maintaining the intermediate steel material at that temperature. Here, the term tempering temperature corresponds to the furnace temperature when the intermediate steel material after quenching is heated and held at the relevant temperature. The term tempering time means the period from when the temperature of the intermediate steel material reaches a predetermined tempering temperature until the steel material is extracted from the heat treatment furnace.

[0108] The tempering temperature is appropriately adjusted according to the chemical composition of the seamless steel tube and the yield strength to be obtained. That is, for a hollow shell having the chemical composition of the present embodiment, the tempering temperature is adjusted so as to adjust the yield strength of the seamless steel tube to within the range of 862 to 1034 MPa. Note that a person skilled in the art is fully capable of adjusting the tempering temperature so as to adjust the yield strength of the seamless steel tube to within the range of 862 to 1034 MPa. Petition 870250084148, dated 09 / 18 / 2025, pages 170 / 189 42 / 57 Specifically, in the tempering process according to the present embodiment, a preferred tempering temperature is 580 to 690 °C.

[0109] If the tempering time is too short, in some cases a microstructure that is mainly composed of tempered martensite and tempered bainite may not be obtained. On the other hand, if the tempering time is too long, the aforementioned advantageous effect will be saturated. Therefore, in the tempering process of the present embodiment, preferably the tempering time is defined within a range of 10 to 90 minutes. A more preferred lower limit of the tempering time is 15 minutes. A more preferred upper limit of the tempering time is 80 minutes.

[0110] The steel material according to the present embodiment can be produced by the production method described above. Note that, in the previous description of the production method, a method for producing a seamless steel tube was described as an example. However, the steel material according to the present embodiment can also be a steel sheet or other form. A method for producing a steel sheet or a steel material of another form also includes, for example, a preparation process, a quenching process, and a tempering process, similarly to the production method described above. Furthermore, the production method described above is an example, and the steel material can also be produced by another production method.

[0111] Hereafter, the present invention is described more specifically by way of examples. EXAMPLE 1

[0112] In Example 1, steel materials having a yield strength of 862 to less than 945 MPa were investigated. Specifically, cast steels having the chemical compositions shown in Table 1-1 and Table 1-2 were produced. Note that the symbol - in Table 1-2 means that the content of the relevant element was at the level of an impurity. Specifically, Petition 870250084148, dated 09 / 18 / 2025, pp. 171 / 189 43 / 57 means that the V content, Cu content, Ni content, and W content of steel A were each 0% when rounded to two decimal places. Additionally, it means that the Ca content, Mg content, Zr content, and rare earth metal (REM) content of steel A were each 0% when rounded to four decimal places.

[0113] [Table 1-1] TABLE 1-1 Steel Symbol Chemical Composition (unit is % by mass; equilibrium is Fe and impurities) C Si Mn PS Al Cr Mo Ti Nb BA 0.28 0.30 0.42 0.006 0.0012 0.023 1.02 0.47 0.021 0.035 0.0012 B 0.27 0.25 0.43 0.006 0.0012 0.027 0.98 0.42 0.022 0.027 0.0013 C 0.25 0.28 0.40 0.006 0.0018 0.031 1.05 0.32 0.023 0.028 0.0011 D 0.26 0.29 0.42 0.007 0.0011 0.032 1.00 0.55 0.021 0.031 0.0012 E 0.28 0.35 0.45 0.007 0.0020 0.026 0.52 0.72 0.019 0.033 0.0015 F 0.26 0.32 0.45 0.008 0.0025 0.025 0.94 0.68 0.006 0.032 0.0012 G 0.29 0.29 0.45 0.008 0.0022 0.024 0.50 0.75 0.007 0.028 0.0011 H 0.25 0.35 0.43 0.009 0.0019 0.028 1.03 0.28 0.021 0.029 0.0011 I 0.27 0.35 0.43 0.009 0.0018 0.032 1.02 0.32 0.019 0.032 0.0014 J 0.27 0.30 0.42 0.008 0.0012 0.033 0.75 0.93 0.006 0.033 0.0012 K 0.29 0.28 0.43 0.006 0.0013 0.028 0.75 1.00 0.006 0.028 0.0013 L 0.25 0.25 0.44 0.006 0.0012 0.029 0.52 0.14 0.021 0.027 0.0012 M 0.26 0.35 0.42 0.008 0.0052 0.031 0.48 0.90 0.006 0.025 0.0011

[0114] [Table 1-2] TABLE 1-2 Steel Symbol Chemical Composition (unit is % by mass; equilibrium is Fe and impurities) NOV Cu Ni W Ca Mg Zr REM A 0.0048 0.0021 - - - - - - - - B 0.0035 0.0008 - - - - - - - - C 0.0044 0.0012 - 0.03 - - - - - - D 0.0028 0.0009 - - 0.03 - - - - - E 0.0035 0.0012 - - - 0.44 - - - - F 0.0041 0.0011 0.10 - - - - - 0.0013 - G 0.0032 0.0022 - - - - 0.0012 - - - H 0.0042 0.0011 - - - - - 0.0014 - - I 0.0038 0.0009 - - - - - - - 0.0013 J 0.0043 0.0015 0.09 0.02 - - 0.0012 - - - K 0.0027 0.0054 0.09 - - - - - - - L 0.0033 0.0012 - - - - - - - - M 0.0042 0.0011 - - - - - - - -

[0115] The round billets were produced by a continuous casting process using the cast steels described above. In the process of Petition 870250084148, dated 09 / 18 / 2025, pp. 172 / 189 44 / 57 continuous casting, when pouring some of the molten steel into a tundish from a ladle, a metal plate that had been machined into a conical shape without a bottom surface was placed above the tundish opening in such a way that the apex of the conical metal plate was directly below the ladle nozzle. Whether or not a metal plate having the aforementioned shape was placed above the tundish opening is indicated in Table 2. Specifically, a case where a metal plate having the aforementioned shape was placed above the tundish opening is indicated by A in the Metal Plate column in Table 2. A case where a metal plate having the aforementioned shape was not placed above the tundish opening is indicated by B in the Metal Plate column in Table 2.Note that aluminum plates were used as metal plates having the aforementioned shape, arranged above the distributor opening. Specifically, three aluminum plates, each 2 mm thick, were stacked one on top of the other and used. Furthermore, in cases where metal plates were arranged, at a time after the Si oxides were discharged from the nozzle and before the molten steel was discharged, the metal plates were removed from under the nozzle using a rod with a hook formed at its front end. Additionally, a round billet was cast from the molten steel at the casting speed described in Table 2. Note that at this time, the interior of the mold was subjected to electromagnetic stirring at a current that was adjusted to a current value described in Table 2.

[0116] [Table 2] TABLE 2 Test Number Steel Symbol Steelmaking Process Tempering Process Annealing Process Metal Plate Casting Speed ​​(m / min) Current Value (A) Temperature (°C) Time (min) Temperature (°C) Time (min) 1 AA 1.8 400 920 10 600 30 2 AA 1.5 400 920 10 600 30 3 AA 1.2 400 920 10 600 30 4 BA 1.2 450 920 10 600 60 Petition 870250084148, dated 09 / 18 / 2025, pp. 173 / 189 45 / 57 5 CA 1.7 400 920 10 590 30 6 DA 1.3 400 920 10 600 60 7 E A 1.2 400 920 10 600 30 8 FA 2.5 400 900 10 685 30 9 GA 2.0 400 920 10 600 60 10 HA 2.0 400 900 10 590 30 11 IA 1.8 400 900 10 590 30 12 JA 1.3 400 920 10 685 30 13 AA 4.2 400 920 10 600 30 14 AA 3.2 400 920 10 600 30 15 AB 1.9 450 920 10 600 30 16 DB 2.2 450 920 10 600 30 17 EB 2.5 450 920 10 600 30 18 KA 1.2 450 10 685 30 19 LA 1.5 350 920 10 600 30 20 MA 1.5 350 920 10 600 30

[0117] Round billets produced in Tests Nos. 1 to 20 were held at 1250 °C for one hour, and then subjected to hot rolling by the Mannesmann-mandrel process to produce hollow shells (seamless steel tubes) from Tests Nos. 1 to 20. Furthermore, the hollow shells obtained from Tests Nos. 1 to 20 were subjected to quenching. Specifically, the hollow shells from Tests Nos. 1 to 20 were held at temperatures (°C) for times (min) that are each described in the Quenching Process column in Table 2, and then subjected to quenching by shower water cooling. Note that the cooling rates during CR800-500 quenching were within the range of 480 to 30000 °C / min for Tests Nos. 1 to 20. Here, the temperatures (°C) of the heat treatment furnace that heated the hollow shells were adopted as the tempering temperatures (°C) described in Table 2.Furthermore, the times (min) for which the hollow shells were held at the tempering temperatures were adopted as the tempering time (min) described in Table 2.

[0118] The hollow shells obtained from Tests Nos. 1 to 20 were subjected to tempering. Specifically, the tempering of the hollow shells from Tests Nos. 1 to 20 was carried out by maintaining the hollow shells at each temperature (°C) for each time (min), each of which is described in the Tempering Process column in Table 2. Here, the furnace temperatures (°C) of Petition 870250084148, dated 09 / 18 / 2025, pp. 174 / 189 46 / 57 tempering that heated the hollow shells were adopted as the tempering temperature (°C) described in Table 2. Furthermore, the times (min) for which the hollow shells were held at the tempering temperature were adopted as the tempering time (min) described in Table 2. The seamless steel tubes from Tests Nos. 1 to 20 were obtained by the production process described above.

[0119] [Assessment tests] The seamless steel tubes from Tests Nos. 1 to 20, after the tempering described above, were subjected to a tensile test, a test to measure the number density of coarse Al oxides and the number density of coarse Si oxides, and a Charpy impact test.

[0120] [Traction test] Seamless steel pipes from Tests Nos. 1 to 20 were subjected to a tensile test, and the yield strength was determined. The tensile test was performed in accordance with ASTM E8 / E8M (2021). Round bar specimens having a parallel portion diameter of 8.9 mm and a gauge length of 35.6 mm were prepared from the center portion of the wall thickness of the seamless steel pipes from Tests Nos. 1 to 20. The axial direction of the round bar specimen was parallel to the axial direction of the seamless steel pipe. Tensile tests were performed in the atmosphere at normal temperature (25 °C) using the prepared round bar specimens, and the yield strength (MPa) of the seamless steel pipes from Tests Nos. 1 to 20 was determined. Note that, in the present examples, the stress at a moment of 0.65% elongation (0.65% proof stress) obtained in the tensile test was defined as the yield strength.The yield strength obtained (MPa) is shown in Table 3 as YS (MPa).

[0121] [Table 3] TABLE 3 Test Number Steel Symbol YS (MPa) Coarse Al Oxides ( / 200 mm2) Coarse Si Oxides ( / 200 mm2) Fracture Appearance Transition Temperature (°C) Petition 870250084148, dated 09 / 18 / 2025, pp. 175 / 189 47 / 57 1 A 883 11 4 -56 2 A 910 12 4 -53 3 A 883 19 3 -62 4 B 889 20 2 -69 5 C 896 22 2 -54 6 D 910 24 3 -61 7 E 917 29 4 -67 8 F 903 10 2 -72 9 G 883 19 4 -57 10 H 910 23 4 -53 11 I 903 22 2 -64 12 J 889 25 3 -69 13 A 876 33 2 -40 14 A 917 40 2 -44 15 A 910 18 11 -38 16 D 917 21 7 -42 17 E 938 19 8 -44 18 K 924 28 4 -40 19 L 917 21 2 -42 20 M 910 22 3 -32

[0122] [Test for measuring the number density of coarse Al oxides and coarse Si oxides] Tests to measure the number density of coarse Al oxides and the number density of coarse Si oxides were performed on seamless steel pipes from Tests Nos. 1 to 20, and the number densities of Al oxides having a major axis of 5.0 μm or more (coarse Al oxides) and the number densities of Si oxides having a major axis of 5.0 μm or more (coarse Si oxides) were determined. The number densities of coarse Al oxides and coarse Si oxides were determined by the method described above using test specimens prepared from the central portion of the wall thickness of the seamless steel pipes from Tests Nos. 1 to 20. The number density obtained for coarse Al oxides ( / 200 mm2) is shown in the Coarse Al Oxides ( / 200 mm2) column in Table 3. The number density obtained for coarse Si oxides ( / 200 mm2) is shown in the Coarse Si Oxides ( / 200 mm2) column in Table 3.

[0123] [Charpy impact test] Charpy impact tests were performed on the steel pipes. Petition 870250084148, dated 09 / 18 / 2025, pp. 176 / 189 48 / 57 seamless tubes from Tests Nos. 1 to 20, and low-temperature toughness was evaluated. Full-size V-notch test specimens were prepared from the central portion of the wall thickness of the seamless steel tubes from Tests Nos. 1 to 20. The longitudinal direction of the test specimen was parallel to the circumferential direction of the tube. Note that the circumferential direction of the tube means a direction both perpendicular to the axial direction of the tube and to the radial direction of the tube of the seamless steel tube. The notched surface of the test specimen was perpendicular to the axial direction of the seamless steel tube. Charpy impact tests were performed according to ASTM E23 (2018) under the aforementioned conditions, and the brittle fracture rates (%) of Tests Nos. 1 to 20 were determined.The temperature (°C) at which the brittle fracture rate became 50% was determined from an approximate curve in which test temperatures (°C) and brittle fracture rates (%) were plotted, and the determined temperature (°C) was defined as the fracture appearance transition temperature (°C). The obtained fracture appearance transition temperature (°C) is shown in Table 3.

[0124] [Evaluation Results] Referring to Table 1-1, Table 1-2, Table 2, and Table 3, the chemical compositions of the seamless steel pipes from Tests Nos. 1 to 12 were appropriate, and the production methods of Tests Nos. 1 to 12 also satisfied the preferred conditions described above. As a result, for each of these seamless steel pipes, the yield strength was 862 to less than 945 MPa, the number density of coarse Al oxides was less than 30 / 200 mm2, and, in addition, the number density of coarse Si oxides was 5 / 200 mm2 or less. As a result, for each of these seamless steel pipes, the fracture appearance transition temperature in the Charpy impact test was -50 °C or less. That is, the seamless steel pipes from Tests Nos. Elements 1 to 12 had a yield strength of 862 to less than 945 MPa and had excellent low-temperature toughness. Note that it was determined that the total volumetric ratios of tempered martensite and tempered bainite in Petition 870250084148, dated 09 / 18 / 2025, pp. 177 / 189 49 / 57 microstructure of each of these seamless steel tubes was 90% or more.

[0125] On the other hand, for seamless steel tubes from Tests Nos. 13 and 14, the casting speed in the steelmaking process was very fast. As a result, the number density of coarse Al oxides in these seamless steel tubes was 30 / 200 mm2 or more. Consequently, for each of these seamless steel tubes, the fracture appearance transition temperature in the Charpy impact test was more than -50 °C, and thus these seamless steel tubes did not have excellent low-temperature toughness.

[0126] For seamless steel tubes from Tests Nos. 15 to 17, a metal plate was not used in the steelmaking process. As a result, the number density of coarse Si oxides in these seamless steel tubes was more than 5 / 200 mm2. Consequently, for each of these seamless steel tubes, the fracture appearance transition temperature in the Charpy impact test was more than -50 °C, and thus these seamless steel tubes did not have excellent low-temperature toughness.

[0127] In the seamless steel pipe from Test No. 18, the O content was very high. As a result, for this seamless steel pipe, the fracture appearance transition temperature in the Charpy impact test was more than -50 °C, and thus the seamless steel pipe did not have excellent low-temperature toughness.

[0128] In the seamless steel pipe from Test No. 19, the Mo content was very low. As a result, for this seamless steel pipe, the fracture appearance transition temperature in the Charpy impact test was more than -50 °C, and thus the seamless steel pipe did not have excellent low-temperature toughness.

[0129] In the seamless steel pipe from Test No. 20, the S content was very high. As a result, for this seamless steel pipe, the fracture appearance transition temperature in the Charpy impact test was more than -50 °C, and thus the seamless steel pipe did not have excellent Petition 870250084148, dated 09 / 18 / 2025, pp. 178 / 189 50 / 57 low temperature toughness. EXAMPLE 2

[0130] In Example 2, steel materials having a yield strength of 945 to 1034 MPa were investigated. Specifically, cast steels having the chemical compositions shown in Table 4-1 and Table 4-2 were produced. Note that the symbol - in Table 4-2 means that the content of the relevant element was at the level of an impurity. Specifically, - means that the Cu content, Ni content, and W content of steel A were each 0% when rounded to the second decimal place. Furthermore, - means that the Ca content, Mg content, Zr content, and rare earth metal (REM) content of steel A were each 0% when rounded to the fourth decimal place.

[0131] [Table 4-1] TABLE 4-1 Symbol Chemical Composition (unit is % by mass; balance is Fe and impurities) do Aço C Si Mn P S Al Cr Mo Ti Nb B N 0,29 0,25 0,45 0,006 0,0012 0,025 1,02 0,72 0,006 0,025 0,0013 O 0,28 0,29 0,41 0,008 0,0009 0,028 0,72 0,68 0,004 0,028 0,0012 P 0,29 0,31 0,11 0,006 0,0009 0,032 0,85 1,05 0,004 0,027 0,0011 Q 0,25 0,25 0,06 0,007 0,0011 0,031 0,52 1,21 0,004 0,025 0,0011 R 0,23 0,85 0,09 0,007 0,0008 0,033 0,75 0,92 0,006 0,032 0,0012 S 0,26 0,28 0,15 0,008 0,0011 0,025 1,03 0,66 0,006 0,025 0,0011 T 0,26 0,32 0,12 0,009 0,0013 0,028 1,09 0,91 0,005 0,032 0,0012 U 0,25 0,32 0,09 0,008 0,0012 0,033 0,55 0,92 0,004 0,032 0,0011 V 0,27 0,35 0,13 0,009 0,0012 0,021 0,72 0,88 0,005 0,025 0,0011 W 0,28 0,34 0,22 0,008 0,0014 0,032 0,55 1,23 0,006 0,025 0,0011 X 0,23 0,24 0,18 0,006 0,0013 0,033 0,79 0,99 0,006 0,031 0,0011 Y 0,28 0,25 0,11 0,008 0,0052 0,024 0,52 0,93 0,006 0,025 0,0013

[0132] [Tabela 4-2] TABELA 4-2 Steel Symbol Chemical Composition (unit is % by mass; balance is Fe and impurities) NOV Cu Ni W Ca Mg Zr REM N 0.0033 0.0011 0.09 - - - - - - - O 0.0035 0.0012 0.10 - - - - - - - P 0.0033 0.0025 0.09 0.03 - - - - - - Q 0.0032 0.0009 0.09 - 0.02 - - - - - R 0.0042 0.0013 0.10 - - 0.48 - - - - S 0.0032 0.0015 0.10 - - - 0.0011 - - - Petition 870250084148, dated 09 / 18 / 2025, pp. 179 / 189 51 / 57 T 0.0041 0.0013 0.11 - - - - 0.0011 - - U 0.0039 0.0012 0.09 - - - - - 0.0009 - V 0.0042 0.0013 0.12 - - - - - - 0.0013 W 0.0045 0.0011 0.10 - 0.03 0.50 0.0011 - - -

[0133] Similar to Example 1, round billets were produced by a continuous casting process using the cast steels described above. In the continuous casting process, when pouring some of the molten steel into a tundish from a ladle, a metal plate that had been machined into a conical shape without a bottom surface was placed above the tundish opening in such a way that the apex of the conical metal plate was directly below the ladle spout. Whether or not a metal plate having the aforementioned shape was placed above the tundish opening is indicated in Table 5. Specifically, a case where a metal plate having the aforementioned shape was placed above the tundish opening is indicated by A in the Metal Plate column in Table 5.A case where a metal plate having the aforementioned shape was not placed above the tundish opening is indicated by B in the Metal Plate column in Table 5. Note that aluminum plates were used as metal plates having the aforementioned shape placed above the tundish opening. Specifically, three aluminum plates, each 2 mm thick, were stacked one on top of the other and used. Furthermore, in cases where metal plates were placed, at a time after the Si oxides were discharged from the nozzle and before the molten steel was discharged, the metal plates were removed from under the nozzle using a rod that had a hook formed at its front end. Additionally, a round billet was cast from the molten steel at the casting speed described in Table 5.Note that, at this point, the interior of the mold was subjected to electromagnetic agitation with a current that was adjusted to a current value described in Table 5. Petition 870250084148, dated 09 / 18 / 2025, pages 180 / 189 52 / 57

[0134] [Table 5] TABLE 5 Test Number Steel Symbol Steelmaking Process Quenching Process Tempering Process Metal Plate Casting Speed ​​(m / min) Current Value (A) Temperature (°C) Time (min) Temperature (°C) Time (min) 21 NA 1.9 400 920 10 670 30 22 NA 1.8 400 920 10 665 30 23 NA 1.2 400 920 10 670 30 24 OA 1.1 450 920 10 670 30 25 PA 1.2 400 920 10 670 30 26 QA 1.1 400 920 10 675 60 27 RA 1.4 400 920 10 670 30 28 SA 2.7 400 920 10 670 60 29 TA 2.2 400 900 10 675 30 30 UA 1.2 400 900 10 675 30 31 VA 2.3 400 900 10 675 30 32 WA 2.5 400 920 10 670 30 33 NA 4.5 400 920 10 675 30 34 NA 4.2 400 920 10 675 30 35 NB 2.1 450 920 10 670 30 36 OB 2.2 450 900 10 665 30 37 XA 1.3 450 920 10 670 30 38 YA 1.5 350 920 10 670 30

[0135] Similar to Example 1, the round billets produced from Tests Nos. 21 to 38 were held at 1250 °C for one hour, and then subjected to hot rolling by the Mannesmann-mandrel process to produce hollow shells (seamless steel tubes) of Tests Nos. 21 to 38. Furthermore, the hollow shells obtained from Tests Nos. 21 to 38 were subjected to quenching. Specifically, the hollow shells of Tests Nos. 21 to 38 were held at temperatures (°C) for times (min) that are each described in the Quenching Process column in Table 5, and then subjected to quenching by shower water cooling. Note that the cooling rates during CR800-500 quenching were within the range of 480 to 30000 °C / min for Tests Nos. 21 to 38. Here, the temperatures (°C) of the heat treatment furnace that heated the hollow shells were adopted as the tempering temperatures (°C) described in Table 5. Additionally, the times (min) for which the hollow shells were held at the tempering temperatures... Petition 870250084148, dated 09 / 18 / 2025, pp. 181 / 189 53 / 57 were adopted as the tempering time (min) described in Table 5.

[0136] Similar to Example 1, the hollow shells obtained from Tests Nos. 21 to 38 were subjected to tempering. Specifically, the tempering of the hollow shells from Tests Nos. 21 to 38 was carried out by holding the hollow shells at each temperature (°C) for each time (min), each of which is described in the Tempering Process column in Table 5. Here, the temperatures (°C) of the tempering furnace that heated the hollow shells were adopted as the tempering temperature (°C) described in Table 5. In addition, the times (min) for which the hollow shells were held at the tempering temperature were adopted as the tempering time (min) described in Table 5. The seamless steel tubes from Tests Nos. 21 to 38 were obtained by the production process described above.

[0137] [Assessment tests] The seamless steel tubes from Tests Nos. 21 to 38, after the tempering described above, were subjected to a tensile test, a test to measure the number density of coarse Al oxides and the number density of coarse Si oxides, and a Charpy impact test.

[0138] [Traction test] Seamless steel pipes from Tests Nos. 21 to 38 were subjected to a tensile test, and the yield strength was determined. The tensile test was performed in accordance with ASTM E8 / E8M (2021). Round bar specimens having a parallel portion diameter of 8.9 mm and a gauge length of 35.6 mm were prepared from the central portion of the wall thickness of the seamless steel pipes from Tests Nos. 21 to 38. The axial direction of the round bar specimen was parallel to the axial direction of the seamless steel pipe. Tensile tests were performed in the atmosphere at normal temperature (25 °C) using the prepared round bar specimens, and the yield strength (MPa) of the seamless steel pipes from Tests Nos. 21 to 38 was determined. Note that, in the present examples, the stress at a moment of 0.7% full elongation (0.7% proof stress) Petition 870250084148, dated 09 / 18 / 2025, pp. 182 / 189 The 54 / 57 total elongation) obtained in the tensile test was defined as the yield strength. The yield strength obtained (MPa) is shown in Table 6 as YS (MPa).

[0139] [Table 6] TABLE 6 Test Number Steel Symbol YS (MPa) Coarse Al Oxides ( / 200 mm2) Coarse Si Oxides ( / 200 mm2) Fracture Appearance Transition Temperature (°C) 21 N 1016 17 4 -55 22 N 1015 19 5 -57 23 N 1005 14 4 -65 24 O 982 11 2 -58 25 P 1029 21 3 -54 26 Q 945 20 2 -55 27 R 1004 16 3 -58 28 S 977 10 4 -50 29 T 1027 10 3 -53 30 U 993 16 4 -52 31 V 1020 19 4 -51 32 W 1007 12 3 -59 33 N 1034 32 4 -38 34 N 1030 35 3 -34 35 N 1031 23 10 -35 36 O 1006 24 7 -37 37 X 991 21 3 -30 38 Y 1005 21 2 -31

[0140] [Test for measuring the number density of coarse Al oxides and coarse Si oxides] Tests to measure the number density of coarse Al oxides and the number density of coarse Si oxides were performed on seamless steel pipes from Tests Nos. 21 to 38, and the number densities of Al oxides having a major axis of 5.0 μm or more (coarse Al oxides) and the number densities of Si oxides having a major axis of 5.0 μm or more (coarse Si oxides) were determined. The number densities of coarse Al oxides and coarse Si oxides were determined by the method described above using test specimens prepared from the central portion of the wall thickness of the seamless steel pipes from Tests Nos. 21 to 38. The number density obtained for coarse Al oxides ( / 200 mm2) is shown in the column. Petition 870250084148, dated 09 / 18 / 2025, pages 183 / 189 55 / 57 Coarse Al Oxides ( / 200 mm2) in Table 6. The numerical density obtained for coarse Si oxides ( / 200 mm2) is shown in the Coarse Si Oxides ( / 200 mm2) column in Table 6.

[0141] [Charpy Impact Test] Charpy impact tests were performed on seamless steel pipes from Tests Nos. 21 to 38, and low-temperature toughness was evaluated. Full-size V-notch test specimens were prepared from the mid-wall thickness portion of the seamless steel pipes from Tests Nos. 21 to 38. The longitudinal direction of the test specimen was parallel to the circumferential direction of the pipe. Note that the circumferential direction of the pipe means a direction both perpendicular to the axial direction of the pipe and to the radial direction of the seamless steel pipe. The notched surface of the test specimen was perpendicular to the axial direction of the seamless steel pipe. Charpy impact tests were performed according to ASTM E23 (2018) under the aforementioned conditions, and the brittle fracture rates (%) of Tests Nos. 21 to 38 were determined.The temperature (°C) at which the brittle fracture rate became 50% was determined from an approximate curve in which test temperatures (°C) and brittle fracture rates (%) were plotted, and the determined temperature (°C) was defined as the fracture appearance transition temperature (°C). The obtained fracture appearance transition temperature (°C) is shown in Table 6.

[0142] [Evaluation Results] Referring to Table 4-1, Table 4-2, Table 5, and Table 6, the chemical compositions of the seamless steel pipes from Tests Nos. 21 to 32 were appropriate, and the production methods of Tests Nos. 21 to 32 also satisfied the preferred conditions described above. As a result, for each of these seamless steel pipes, the yield strength was 945 to 1034 MPa, the number density of coarse Al oxides was less than 30 / 200 mm², and, in addition, the number density of coarse Si oxides was 5 / 200 mm² or less. As a result, for each of these seamless steel pipes Petition 870250084148, dated 09 / 18 / 2025, pp. 184 / 189 56 / 57 seam, the fracture appearance transition temperature in the Charpy impact test was -40 °C or less. That is, the seamless steel tubes from Tests Nos. 21 to 32 had a yield strength of 945 to 1034 MPa and had excellent low-temperature toughness. Note that it was determined that the total volumetric ratios of tempered martensite and tempered bainite in the microstructure of each of these seamless steel tubes was 90% or more.

[0143] On the other hand, for seamless steel tubes from Tests Nos. 33 and 34, the casting speed in the steelmaking process was very fast. As a result, the number density of coarse Al oxides in these seamless steel tubes was 30 / 200 mm2 or more. Consequently, for each of these seamless steel tubes, the fracture appearance transition temperature in the Charpy impact test was more than -40 °C, and thus these seamless steel tubes did not have excellent low-temperature toughness.

[0144] For seamless steel tubes from Tests Nos. 35 and 36, a metal plate was not used in the steelmaking process. As a result, the number density of coarse Si oxides in these seamless steel tubes was more than 5 / 200 mm2. Consequently, for each of these seamless steel tubes, the fracture appearance transition temperature in the Charpy impact test was more than -40 °C, and thus these seamless steel tubes did not have excellent low-temperature toughness.

[0145] In the seamless steel pipe from Test No. 37, the O content was very high. As a result, for this seamless steel pipe, the fracture appearance transition temperature in the Charpy impact test was more than -40 °C, and thus the seamless steel pipe did not have excellent low-temperature toughness.

[0146] In the seamless steel pipe from Test No. 38, the S content was very high. As a result, for this seamless steel pipe, the fracture appearance transition temperature in the Charpy impact test was more than -40 °C, and thus the seamless steel pipe did not have excellent Petition 870250084148, dated 09 / 18 / 2025, pp. 185 / 189 57 / 57 low temperature toughness.

[0147] One embodiment of the present invention has been described above. However, the embodiment described above is merely an example of carrying out the present invention. Therefore, the present invention is not limited to the embodiment described above, and can be implemented by appropriately modifying the embodiment described above within a range that does not depart from the spirit thereof. Petition 870250084148, dated 09 / 18 / 2025, pp. 186 / 189

Claims

1 / 2 CLAIMS 1. Steel material, CHARACTERIZED to consist of, in % by mass, C: 0.15 to 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 to 1.50%, Mo: 0.20 to 2.00%, Ti: 0.002 to 0.030%, Nb: 0.002 to 0.100%, B: 0.0005 to 0.0040%, N: 0.0100% or less, O: 0.0040% or less, V: 0 to 0.30%, Cu: 0 to 0.50%, Ni: 0 to 0.50%, W: 0 to 0.50%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, and rare earth metal: 0 to 0.0100%, with the remainder being Fe and impurities; where: a yield strength is 862 to 1034 MPa, and in the steel material, a number density of Al oxides in which, in % by mass, an Al content is 20% or more and an O content is 10% or more, and which have an axis Petition 870250084148, dated 18 / 09 / 2025, p.187 / 189 2 / 2 greater than 5.0 μm or more is less than 30 / 200 mm2, and a number density of Si oxides in which, in % by mass, an Al content is less than 20%, a Si content is 20% or more, and an O content is 10% or more, and having a major axis of 5.0 μm or more is 5 / 200 mm2 or less.

2. Steel material according to claim 1, CHARACTERIZED in that it comprises one or more elements selected from a group consisting of: V: 0.01 to 0.30%, 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 metal: 0.0001 to 0.0100%.

3. Steel material according to claim 1 or claim 2, CHARACTERIZED in that: the steel material is a seamless steel tube. Petition 870250084148, dated 09 / 18 / 2025, pp. 188 / 189

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