Steel material

Abstract

Provided is steel material that achieve both a high strength of 150 ksi grade and excellent SSC resistance. A steel material according to the present disclosure consists of, in 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: 0.40 to 2.00%, Ti: 0.002 to 0.020%, Nb: 0.002 to 0.100%, V: 0.05 to 0.30%, B: 0.0005 to 0.0040%, N: 0.0100% or less, O: 0.0040% or less, with the balance being Fe and impurities; wherein a yield strength is more than 1034 to 1172 MPa, in the steel material, a number density of Al oxides which have a major axis of 5.0 µm or more is less than 30 / 200 mm2, and a number density of Si oxides which have a major axis of 5.0 µm or more is 5 / 200 mm2 or less.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a steel material.BACKGROUND ART

[0002] Due to the deepening of oil wells and gas wells (hereunder, oil wells and gas wells are collectively referred to as simply "oil wells"), there is a demand to enhance the strength of oil-well steel materials typified by oil-well steel pipes. Specifically, oil-well steel materials of 80 ksi grade (yield strength is 80 to less than 95 ksi, that is, 552 to less than 655 MPa) and 95 ksi grade (yield strength is 95 to less than 110 ksi, that is, 655 to less than 758 MPa) are in widespread use, and recently requests are also starting to be made for oil-well steel materials of 110 ksi grade (yield strength is 110 to less than 125 ksi, that is, 758 to less than 862 MPa).

[0003] Oil wells may also contain corrosive hydrogen sulfide gas (H 2 S) or carbon dioxide gas (CO 2 ) or the like. Therefore, a steel material assumed to be used as an oil-well steel material is also required to have excellent corrosion resistance, and not just high strength. Further, in the case of an oil-well steel material, stress is applied to the steel material during use. Therefore, sulfide stress cracking resistance (hereunder, referred to as "SSC resistance") has been used as an index of excellent corrosion resistance of oil-well steel materials.

[0004] Techniques for increasing the strength and SSC resistance of a steel material are proposed in Japanese Patent Application Publication No. 2006-28612 (Patent Literature 1), International Application Publication No. WO2008 / 123422 (Patent Literature 2), and Japanese Patent Application Publication No. 2017-166060 (Patent Literature 3).

[0005] Patent Literature 1 discloses a steel material that is a steel for pipes, consisting of, in mass%, C: 0.2 to 0.7%, Si: 0.01 to 0.8%, Mn: 0.1 to 1.5%, S: 0.005% or less, P: 0.03% or less, Al: 0.0005 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 balance being Fe and impurities. In this steel material, in addition, among the inclusions of non-metallic inclusions which contain 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 Literature 1 that this steel material has a high yield strength of more than 758 MPa and has excellent SSC resistance.

[0006] Patent Literature 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 balance 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%. In this steel material, in addition, the number density of M 23 C 6 -type precipitates having a grain size of 1 µm or more is 0.1 / mm 2< or less. It is described in Patent Literature 2 that this steel material has a yield strength of 654 to 793 MPa and has excellent SSC resistance even in highpressure hydrogen sulfide environments.

[0007] Patent Literature 3 discloses a steel material that is a starting material for high-strength oil-well steel pipes which consists of, in mass%, C: 0.20 to 0.45%, Si: 0.05 to 0.40%, Mn: 0.3 to 0.9%, P: 0.015% or less, S: 0.005% or less, Al: 0.005 to 0.10%, N: 0.001 to 0.006%, Cr: 0.1 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 balance being Fe and unavoidable impurities. In addition, in this steel material, the Rockwell hardness HRC satisfies the formula (15.6 × [%C] + 29.2 ≤ HRC < 60.5 × [%C] + 31.1). It is described in Patent Literature 3 that according to this steel material, a pipe having a yield strength of 758 to less than 862 MPa and excellent SSC resistance is obtained.CITATION LISTPATENT LITERATURE

[0008] Patent Literature 1: Japanese Patent Application Publication No. 2006-28612 Patent Literature 2: International Application Publication No. WO2008 / 123422 Patent Literature 3: Japanese Patent Application Publication No. 2017-166060 SUMMARY OF INVENTIONTECHNICAL PROBLEM

[0009] According to the techniques disclosed in the Patent Literatures 1 to 3, an oil-well steel material that has excellent SSC resistance can be obtained. However, other techniques than those disclosed in the Patent Literatures 1 to 3 may as well be used to obtain an oil-well steel material that has excellent SSC resistance.

[0010] In recent years, accompanying the increasing severity of oil well environments, there is a demand for a higher yield strength for oil-well steel materials. Specifically, there is a demand for oil-well steel materials of 150 ksi grade (yield strength is more than 150 to 170 ksi, that is, more than 1034 to 1172 MPa). On the other hand, in the Patent Literatures 1 to 3, no consideration is given to achieving both a high yield strength of 150 ksi grade and excellent SSC resistance.

[0011] An objective of the present disclosure is to provide a steel material that achieves both a high strength of 150 ksi grade (more than 1034 to 1172 MPa) and excellent SSC resistance.SOLUTION TO PROBLEM

[0012] A steel material according to the present disclosure consists of, in 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: 0.40 to 2.00%, Ti: 0.002 to 0.020%, Nb: 0.002 to 0.100%, V: 0.05 to 0.30%, B: 0.0005 to 0.0040%, N: 0.0100% or less, O: 0.0040% or less, 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%, rare earth metal: 0 to 0.0100%, with the balance being Fe and impurities; wherein: a yield strength is more than 1034 to 1172 MPa, and in the steel material, a number density of Al oxides in which, in mass%, a content of Al is 20% or more and a content of O is 10% or more, and which have a major axis of 5.0 µm or more is less than 30 / 200 mm 2< , and a number density of Si oxides in which, in mass%, a content of Al is less than 20%, a content of Si is 20% or more, and a content of O is 10% or more, and which have a major axis of 5.0 µm or more is 5 / 200 mm 2< or less. ADVANTAGEOUS EFFECTS OF INVENTION

[0013] A steel material according to the present disclosure can achieve both a high strength of 150 ksi grade (more than 1034 to 1172 MPa) and excellent SSC resistance.BRIEF DESCRIPTION OF DRAWING

[0014] [FIG. 1] FIG. 1 is a view illustrating the relation between a number density ( / 200 mm 2< ) of coarse Si oxides (Si oxides having a major axis of 5.0 µm or more) and the number of specimens in which SSC occurred (specimens) that is an index of SSC resistance, among the present examples.DESCRIPTION OF EMBODIMENTS

[0015] Focusing on the chemical composition, the present inventors first conducted studies with a view to obtaining a steel material that achieves both a yield strength of 150 ksi grade and excellent SSC resistance. As a result, the present inventors have considered that a steel material consisting of, in 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: 0.40 to 2.00%, Ti: 0.002 to 0.020%, Nb: 0.002 to 0.100%, V: 0.05 to 0.30%, B: 0.0005 to 0.0040%, N: 0.0100% or less, O: 0.0040% or less, 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%, rare earth metal: 0 to 0.0100%, with the balance being Fe and impurities may make it possible to achieve both a yield strength of 150 ksi grade and excellent SSC resistance.

[0016] Next, the present inventors conducted various studies regarding techniques for increasing SSC resistance in a steel material having the aforementioned chemical composition and a yield strength of 150 ksi grade. Specifically, the present inventors considered that if coarse oxide-based inclusions can be reduced, there is a possibility that SSC resistance will be increased while maintaining the yield strength. Here, in a steel material having the chemical composition described above, Al oxides that are mainly made up of Al 2 O 3 tend to coarsen. 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 aforementioned chemical composition and a yield strength of 150 ksi grade, if the number density of Al oxides having a major axis of 5.0 µm or more is less than 30 / 200 mm 2< , there is a possibility that SSC resistance will be increased. Here, in the present description, particles in which, in mass%, the content of Al is 20% or more and the content of O is 10% or more are also referred to as "Al oxides". In addition, 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 likely to cause the corrosion resistance of a steel material to decrease. In particular, in a case where the yield strength is increased to 150 ksi grade, the influence of coarse Al oxides is likely to be actualized, and the SSC resistance is likely to markedly decrease. Therefore, in the steel material according to the present embodiment which has the aforementioned chemical composition and a yield strength of 150 ksi grade, the number density of coarse Al oxides is to be less than 30 / 200 mm 2< .

[0019] On the other hand, even in the case of steel materials having the aforementioned chemical composition and in which the number density of coarse Al oxides was less than 30 / 200 mm 2< , when the steel materials had a yield strength of 150 ksi grade, excellent SSC resistance was not stably obtained in some cases. Therefore, the present inventors investigated various techniques for stably obtaining excellent SSC resistance in a steel material which has the aforementioned chemical composition and a yield strength of 150 ksi grade and in which the number density of coarse Al oxides is less than 30 / 200 mm 2< . As a result of detailed studies conducted by the present inventors, it has been revealed that in a steel material which has the aforementioned chemical composition and a yield strength of 150 ksi grade and in which the number density of coarse Al oxides is less than 30 / 200 mm 2< , if coarse Si oxides in the steel material can also be reduced, and not just coarse Al oxides, there is a possibility that excellent SSC resistance will be stably obtained.

[0020] Here, in the present description, particles in which, in mass%, the content of Al is less than 20%, the content of Si is 20% or more, and the content of O is 10% or more are also referred to as "Si oxides". In addition, in the present description, Si oxides having a major axis of 5.0 µm or more are also referred to as "coarse Si oxides". Hereunder, the relation between coarse Si oxides and SSC resistance in a steel material having the aforementioned chemical composition and a yield strength of 150 ksi grade and in which the number density of coarse Al oxides is less than 30 / 200 mm 2< is specifically described using the drawing.

[0021] FIG. 1 is a view illustrating the relation between a number density ( / 200 mm 2< ) of coarse Si oxides (Si oxides having a major axis of 5.0 µm or more) and the number of specimens in which SSC occurred (specimens) that is an index of SSC resistance, among the present examples. FIG. 1 was prepared using a number density ( / 200 mm 2< ) of coarse Si oxides determined by a method described later and the number of specimens in which SSC occurred (specimens) determined by a method described later, with respect to steel materials which, among examples to be described later, satisfied the aforementioned chemical composition, had a yield strength of 150 ksi grade, and in which the number density of coarse Al oxides was less than 30 / 200 mm 2< .

[0022] Referring to FIG. 1, in steel materials having the aforementioned chemical composition and a yield strength of 150 ksi grade and in which the number density of coarse Al oxides was less than 30 / 200 mm 2< , when the number density of coarse Si oxides was 5 / 200 mm 2< or less, the number of specimens in which SSC occurred was zero, indicating excellent SSC resistance. Therefore, in the present embodiment, a steel material has the aforementioned chemical composition, has a yield strength of 150 ksi grade, the number density of coarse Al oxides is made less than 30 / 200 mm 2< , and further the number density of coarse Si oxides is made 5 / 200 mm 2< or less. As a result, the steel material according to the present embodiment can achieve both a yield strength of 150 ksi grade and excellent SSC resistance.

[0023] The reason why the SSC resistance of a steel material is increased by lowering the number density of coarse Si oxides has not been clarified in detail. However, the present inventors surmise that the reason is as follows. When producing a steel material having the chemical composition described above, during the steelmaking process, deoxidation is performed mainly by aluminum (Al). Therefore, with regard to steel materials which have the chemical composition described above, consideration has been given to Al oxides that are typified by Al 2 O 3 , and attention has not been focused on Si oxides, which are small in number. However, in the case of increasing the yield strength to 150 ksi grade, there is a possibility that a decrease in SSC resistance is liable to be actualized due to the influence of not only coarse Al oxides, but also the influence of coarse Si oxides, which are small in number. Therefore, the present inventors surmise that by not only making the number density of coarse Al oxides less than 30 / 200 mm 2< , but also making the number density of coarse Si oxides 5 / 200 mm 2< or less, excellent SSC resistance is stably obtained even when the steel material has a yield strength of 150 ksi grade.

[0024] Note that, there is also a possibility that the SSC resistance of the steel material is increased by a mechanism which is different from the mechanism surmised by the present inventors. However, the fact that in a steel material having the chemical composition described above and a yield strength of 150 ksi grade and in which the number density of coarse Al oxides is less than 30 / 200 mm 2< , excellent SSC resistance is obtained by making the number density of coarse Si oxides 5 / 200 mm 2< or less has been demonstrated by examples that are described later.

[0025] The gist of the steel material according to the present embodiment, which has been completed based on the findings described above, is as follows. [1] A steel material consisting of, in 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: 0.40 to 2.00%, Ti: 0.002 to 0.020%, Nb: 0.002 to 0.100%, V: 0.05 to 0.30%, B: 0.0005 to 0.0040%, N: 0.0100% or less, O: 0.0040% or less, 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%, rare earth metal: 0 to 0.0100%, with the balance being Fe and impurities; wherein: a yield strength is more than 1034 to 1172 MPa, and in the steel material, a number density of Al oxides in which, in mass%, a content of Al is 20% or more and a content of O is 10% or more, and which have a major axis of 5.0 µm or more is less than 30 / 200 mm 2< , and a number density of Si oxides in which, in mass%, a content of Al is less than 20%, a content of Si is 20% or more, and a content of O is 10% or more, and which have a major axis of 5.0 µm or more is 5 / 200 mm 2< or less. [2] The steel material according to [1], containing one or more elements selected from a group consisting of: 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] The steel material according to [1] or [2], wherein: the steel material is a seamless steel pipe.

[0026] 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 pipe, may be a round steel bar (solid material), or may be a steel plate. 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 a circular shape. Further, the steel pipe may be a seamless steel pipe or may be a welded steel pipe.

[0027] Hereunder, the steel material according to the present embodiment is described in detail. The symbol "%" in relation to an element means mass percent unless otherwise stated.[Chemical composition]

[0028] The chemical composition of the steel material according to the present embodiment contains the following elements.C: 0.15 to 0.45%

[0029] Carbon (C) increases hardenability of the steel material and increases strength of the steel material. C also promotes spheroidization of carbides during tempering in the production process, and thereby increases the SSC resistance of the steel material. If the content of C is too low, the aforementioned advantageous effects will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment. On the other hand, if the content of C is too high, even if the contents of other elements are within the range of the present embodiment, there will be too many carbides formed and the SSC resistance of the steel material will decrease. Therefore, the content of C is to be 0.15 to 0.45%. A preferable lower limit of the content of C is 0.18%, more preferably is 0.20%, further preferably is 0.22%, and further preferably is 0.23%. A preferable upper limit of the content of C is 0.40%, more preferably is 0.38%, further preferably is 0.35%, and further preferably is 0.30%.Si: 0.05 to 1.00%

[0030] Silicon (Si) deoxidizes the steel. If the content of Si is too low, the aforementioned advantageous effect will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment. On the other hand, if the content of Si is too high, even if the contents of other elements are within the range of the present embodiment, in some cases a large number of coarse Si oxides will be formed and the SSC resistance of the steel material will decrease. Therefore, the content of Si is to be 0.05 to 1.00%. A preferable lower limit of the content of Si is 0.10%, more preferably is 0.15%, and further preferably is 0.20%. A preferable upper limit of the content of Si is 0.85%, more preferably is 0.75%, further preferably is 0.60%, further preferably is 0.50%, and further preferably is 0.40%.Mn: 0.05 to 1.00%

[0031] Manganese (Mn) deoxidizes the steel. Mn also increases hardenability of the steel material. If the content of Mn is too low, the aforementioned advantageous effects will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment. On the other hand, if the content of Mn is too high, even if the contents of other elements are within the range of the present embodiment, coarse sulfide-based inclusions will be formed and the SSC resistance of the steel material will decrease. Therefore, the content of Mn is to be 0.05 to 1.00%. A preferable lower limit of the content of Mn is 0.06%, more preferably is 0.08%, and further preferably is 0.10%. A preferable upper limit of the content of Mn is 0.90%, more preferably is 0.80%, further preferably is 0.70%, further preferably is 0.60%, and further preferably is 0.50%.P: 0.030% or less

[0032] Phosphorus (P) is an impurity. That is, the lower limit of the content of P is more than 0%. If the content of P is too high, even if the contents of other elements are within the range of the present embodiment, P will segregate to grain boundaries and the SSC resistance of the steel material will decrease. Therefore, the content of P is to be 0.030% or less. A preferable upper limit of the content of P is 0.025%, more preferably is 0.020%, further preferably is 0.015%, and further preferably is 0.010%. The content of P is preferably as low as possible. However, extremely reducing the content of P will greatly increase the production cost. Therefore, when taking industrial production into consideration, a preferable lower limit of the content of P is 0.001%, more preferably is 0.002%, and further preferably is 0.003%.S: 0.0050% or less

[0033] Sulfur (S) is an impurity. That is, the lower limit of the content of S is more than 0%. If the content of S is too high, even if the contents of other elements are within the range of the present embodiment, S will segregate to grain boundaries and the SSC resistance of the steel material will decrease. Therefore, the content of S is to be 0.0050% or less. A preferable upper limit of the content of S is 0.0040%, more preferably is 0.0032%, more preferably is 0.0030%, further preferably is 0.0020%, and further preferably is 0.0015%. The content of S is preferably as low as possible. However, extremely reducing the content of S will greatly increase the production cost. Therefore, when taking industrial production into consideration, a preferable lower limit of the content of S is 0.0001%, more preferably is 0.0002%, and further preferably is 0.0003%.Al: 0.005 to 0.100%

[0034] Aluminum (Al) deoxidizes the steel. If the content of Al is too low, 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 SSC resistance of the steel material will decrease. On the other hand, if the content of Al 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 be formed and the SSC resistance of the steel material will decrease. Therefore, the content of Al is to be 0.005 to 0.100%. A preferable lower limit of the content of Al is 0.010%, more preferably is 0.015%, and further preferably is 0.020%. A preferable upper limit of the content of Al is 0.080%, more preferably is 0.060%, further preferably is 0.040%, and further preferably is 0.035%. As used in the present description, the term content of "Al" means the content of "acid-soluble Al", that is, "sol. Al".Cr: 0.30 to 1.50%

[0035] Chromium (Cr) increases hardenability of the steel material. Cr also increases temper softening resistance of the steel material and thereby enables high-temperature tempering. As a result, the SSC resistance of the steel material increases. If the content of Cr is too low, the aforementioned advantageous effects will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment. On the other hand, if the content of Cr is too high, the SSC resistance of the steel material will decrease even if the contents of other elements are within the range of the present embodiment. Therefore, the content of Cr is to be 0.30 to 1.50%. A preferable lower limit of the content of Cr is 0.35%, more preferably is 0.40%, and further preferably is 0.50%. A preferable upper limit of the content of Cr is 1.40%, more preferably is 1.30%, further preferably is 1.20%, further preferably is 1.10%, and further preferably is 1.05%.Mo: 0.40 to 2.00%

[0036] Molybdenum (Mo) increases hardenability of the steel material. Mo also increases temper softening resistance of the steel material and thereby enables high-temperature tempering. As a result, the SSC resistance of the steel material increases. If the content of Mo is too low, the aforementioned advantageous effects will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment. On the other hand, if the content of Mo is too high, the aforementioned advantageous effect will be saturated. Therefore, the content of Mo is to be 0.40 to 2.00%. A preferable lower limit of the content of Mo is 0.45%, further preferably is 0.50%, further preferably is 0.51%, further preferably is 0.55%, and further preferably is 0.60%. A preferable upper limit of the content of Mo is 1.80%, more preferably is 1.60%, further preferably is 1.40%, and further preferably is 1.30%.Ti: 0.002 to 0.020%

[0037] Titanium (Ti) combines with N to form nitrides, and refines the grains of the steel material by the pinning effect. As a result, strength of the steel material increases. If the content of Ti is too low, the aforementioned advantageous effect will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment. On the other hand, if the content of Ti is too high, even if the contents of other elements are within the range of the present embodiment, Ti nitrides will coarsen and the SSC resistance of the steel material will decrease. Therefore, the content of Ti is to be 0.002 to 0.020%. A preferable lower limit of the content of Ti is 0.003%, and more preferably is 0.004%. A preferable upper limit of the content of Ti is 0.018%, further preferably is 0.015%, further preferably is 0.010%, and further preferably is 0.008%.Nb: 0.002 to 0.100%

[0038] Niobium (Nb) combines with C and / or N to form carbides, nitrides, or carbo-nitrides (hereunder, also referred to as "carbo-nitrides and the like"). The carbo-nitrides and the like refine the grains of the steel material by the pinning effect, and thereby increase the SSC resistance of the steel material. Nb also forms fine carbides during tempering and thereby increases temper softening resistance of the steel material and increases strength of the steel material. If the content of Nb is too low, the aforementioned advantageous effects will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment. On the other hand, if the content of Nb is too high, even if the contents of other elements are within the range of the present embodiment, carbo-nitrides and the like will excessively be formed and the SSC resistance of the steel material will decrease. Therefore, the content of Nb is to be 0.002 to 0.100%. A preferable lower limit of the content of Nb is 0.005%, more preferably is 0.010%, further preferably is 0.015%, and further preferably is 0.020%. A preferable upper limit of the content of Nb is 0.080%, more preferably is 0.060%, and further preferably is 0.040%.V: 0.05 to 0.30%

[0039] Vanadium (V) forms carbo-nitrides and the like. The carbo-nitrides and the like refine the grains of the steel material by the pinning effect, thereby increasing the SSC resistance of the steel material. V also forms fine carbides during tempering, and thereby increases temper softening resistance of the steel material and increases strength of the steel material. If the content of V is too low, the aforementioned advantageous effects will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment. On the other hand, if the content of V is too high, carbo-nitrides and the like will excessively be formed and the SSC resistance of a steel material will decrease, even if the contents of other elements are within the range of the present embodiment. Therefore, the content of V is to be 0.05 to 0.30%. A preferable lower limit of the content of V is 0.06%, more preferably is 0.07%, and further preferably is 0.08%. A preferable upper limit of the content of V is 0.25%, more preferably is 0.20%, and further preferably is 0.15%.B: 0.0005 to 0.0040%

[0040] Boron (B) dissolves in the steel and thereby increases hardenability of the steel material and increases strength of the steel material. If the content of B is too low, the aforementioned advantageous effect will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment. On the other hand, if the content of B is too high, even if the contents of other elements are within the range of the present embodiment, coarse nitrides will be formed and the SSC resistance of the steel material will decrease. Therefore, the content of B is to be 0.0005 to 0.0040%. A preferable lower limit of the content of B is 0.0006%, and more preferably is 0.0008%. A preferable upper limit of the content of B is 0.0035%, more preferably is 0.0030%, further preferably is 0.0025%, and further preferably is 0.0020%.N: 0.0100% or less

[0041] Nitrogen (N) is unavoidably contained. That is, the lower limit of the content of N is more than 0%. N combines with Ti to form nitrides, thereby refining the grains of the steel material by the pinning effect. As a result, strength of the steel material increases. However, if the content of N is too high, even if the contents of other elements are within the range of the present embodiment, coarse nitrides will be formed and the SSC resistance of the steel material will decrease. Therefore, the content of N is to be 0.0100% or less. A preferable upper limit of the content of N is 0.0080%, more preferably is 0.0060%, further preferably is 0.0050%, and further preferably is 0.0040%. A preferable lower limit of the content of N for more effectively obtaining the aforementioned advantageous effect is 0.0005%, more preferably is 0.0010%, further preferably is 0.0015%, and further preferably is 0.0020%.O: 0.0040% or less

[0042] Oxygen (O) is an impurity. That is, the lower limit of the content of O is more than 0%. If the content of O is too high, even if the contents of other elements are within the range of the present embodiment, coarse oxides will be formed and the SSC resistance of the steel material will decrease. Therefore, the content of O is to be 0.0040% or less. A preferable upper limit of the content of O is 0.0035%, more preferably is 0.0033%, more preferably is 0.0030%, further preferably is 0.0025%, and further preferably is 0.0020%. The content of O is preferably as low as possible. However, extremely reducing the content of O will greatly increase the production cost. Therefore, when taking industrial production into consideration, a preferable lower limit of the content of O is 0.0001%, more preferably is 0.0002%, and further preferably is 0.0003%.

[0043] The balance of the chemical composition of the steel material according to the present embodiment is Fe and impurities. Here, the term "impurities" refers to substances which, when industrially producing the steel material, are mixed in from ore or scrap that is used as the raw material or from the production environment or the like, and which are allowed within a range that does not adversely affect the steel material according to the present embodiment.[Optional elements]

[0044] The chemical composition of the steel material described above may further contain one or more elements selected from the group consisting of Cu and Ni in lieu of a part of Fe. Each of these elements is an optional element, and increases hardenability of the steel material.Cu: 0 to 0.50%

[0045] Copper (Cu) is an optional element, and does not have to be contained. That is, the content of Cu may be 0%. When contained, Cu increases hardenability of the steel material and increases strength of the steel material. If even a small amount of Cu is contained, the aforementioned advantageous effect will be obtained to a certain extent. However, if the content of Cu is too high, even if the contents of other elements are within the range of the present embodiment, the SSC resistance of the steel material will decrease. Therefore, the content of Cu is to be 0 to 0.50%. A preferable lower limit of the content of Cu is more than 0%, more preferably is 0.01%, and further preferably is 0.02%. A preferable upper limit of the content of Cu is 0.35%, more preferably is 0.25%, further preferably is 0.15%, further preferably is 0.10%, and further preferably is 0.05%.Ni: 0 to 0.50%

[0046] Nickel (Ni) is an optional element, and does not have to be contained. That is, the content of Ni may be 0%. When contained, Ni increases hardenability of the steel material and increases strength of the steel material. Ni also dissolves in the steel and increases the SSC resistance of the steel material. If even a small amount of Ni is contained, these advantageous effects will be obtained to a certain extent. However, if the content of Ni is too high, even if the contents of other elements are within the range of the present embodiment, local corrosion will be promoted and the SSC resistance of the steel material will decrease. Therefore, the content of Ni is to be 0 to 0.50%. A preferable lower limit of the content of Ni is more than 0%, more preferably is 0.01%, and further preferably is 0.02%. A preferable upper limit of the content of Ni is 0.30%, more preferably is 0.20%, further preferably is 0.10%, and further preferably is 0.05%.

[0047] The chemical composition of the steel material described above may further contain W in lieu of a part of Fe.W: 0 to 0.50%

[0048] Tungsten (W) is an optional element, and does not have to be contained. That is, the content of W may be 0%. When contained, in sour environments W forms a protective corrosion coating and suppresses penetration of hydrogen into the steel material. As a result, the SSC resistance of the steel material is increased. If even a small amount of W is contained, the aforementioned advantageous effect will be obtained to a certain extent. However, if the content of W is too high, even if the contents of other elements are within the range of the present embodiment, coarse carbides will be formed in the steel material, and the SSC resistance of the steel material will decrease. Therefore, the content of W is to be 0 to 0.50%. A preferable lower limit of the content of W is more than 0%, more preferably is 0.01%, further preferably is 0.03%, and further preferably is 0.05%. A preferable upper limit of the content of W is less than 0.50%, and more preferably is 0.48%.

[0049] The chemical composition of the steel material described above may further contain one or more elements selected from the group consisting of Ca, Mg, Zr and rare earth metal in lieu of a part 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 elements increase the SSC resistance of the steel material.Ca: 0 to 0.0 100%

[0050] Calcium (Ca) is an optional element, and does not have to be contained. That is, the content of Ca may be 0%. When contained, Ca renders S in the steel material harmless by forming sulfides, and thereby increases the SSC resistance of the steel material. If even a small amount of Ca is contained, the aforementioned advantageous effect will be obtained to a certain extent. However, if the content of Ca is too high, even if the contents of other elements are within the range of the present embodiment, oxides in the steel material will coarsen and the SSC resistance of the steel material will decrease. Therefore, the content of Ca is to be 0 to 0.0100%. A preferable lower limit of the content of Ca is more than 0%, more preferably is 0.0001%, further preferably is 0.0003%, and further preferably is 0.0006%. A preferable upper limit of the content of Ca is 0.0040%, more preferably is 0.0025%, and further preferably is 0.0020%.Mg: 0 to 0.0100%

[0051] Magnesium (Mg) is an optional element, and does not have to be contained. That is, the content of Mg may be 0%. When contained, Mg renders S in the steel material harmless by forming sulfides, and thereby increases the SSC resistance of the steel material. If even a small amount of Mg is contained, the aforementioned advantageous effect will be obtained to a certain extent. However, if the content of Mg is too high, even if the contents of other elements are within the range of the present embodiment, oxides in the steel material will coarsen and the SSC resistance of the steel material will decrease. Therefore, the content of Mg is to be 0 to 0.0100%. A preferable lower limit of the content of Mg is more than 0%, more preferably is 0.0001%, further preferably is 0.0003%, and further preferably is 0.0006%. A preferable upper limit of the content of Mg is 0.0040%, more preferably is 0.0025%, and further preferably is 0.0020%.Zr: 0 to 0.0100%

[0052] Zirconium (Zr) is an optional element, and does not have to be contained. That is, the content of Zr may be 0%. When contained, Zr renders S in the steel material harmless by forming sulfides, and thereby increases the SSC resistance of the steel material. If even a small amount of Zr is contained, the aforementioned advantageous effect will be obtained to a certain extent. However, if the content of Zr is too high, even if the contents of other elements are within the range of the present embodiment, oxides in the steel material will coarsen and the SSC resistance of the steel material will decrease. Therefore, the content of Zr is to be 0 to 0.0100%. A preferable lower limit of the content of Zr is more than 0%, more preferably is 0.0001%, further preferably is 0.0003%, and further preferably is 0.0006%. A preferable upper limit of the content of Zr is 0.0040%, more preferably is 0.0025%, and further preferably is 0.0020%.Rare earth metal (REM): 0 to 0.0100%

[0053] Rare earth metal (REM) is an optional element, and does not have to be contained. That is, the content of REM may be 0%. When contained, REM renders S in the steel material harmless by forming sulfides, and thereby increases the SSC resistance of the steel material. REM also combines with P in the steel material and thereby suppresses segregation of P to the grain boundaries. Therefore, a decrease in the SSC resistance of the steel material attributable to segregation of P is suppressed. If even a small amount of REM is contained, the aforementioned advantageous effects will be obtained to a certain extent. However, if the content of REM is too high, even if the contents of other elements are within the range of the present embodiment, oxides in the steel material will coarsen and the SSC resistance of the steel material will decrease. Therefore, the content of REM is to be 0 to 0.0100%. A preferable lower limit of the content of REM is more than 0%, more preferably is 0.0001%, further preferably is 0.0003%, and further preferably is 0.0006%. A preferable upper limit of the content of REM is 0.0040%, more preferably is 0.0025%, and further preferably is 0.0020%.

[0054] Note that, in the present description the term "REM" means one or more types of element 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 that are lanthanoids. In the present description, the term "content of REM" refers to the total content of these elements.[Yield strength]

[0055] The yield strength of the steel material according to the present embodiment is more than 1034 to 1172 MPa (more than 150 to 170 ksi). As used in the present description, the term "yield strength" means the stress at the time 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). By having the aforementioned chemical composition, and satisfying the number density of coarse Al oxides and the number density of coarse Si oxides to be described later, the steel material according to the present embodiment has excellent SSC resistance even when the yield strength is more than 1034 to 1172 MPa.

[0056] The yield strength of the steel material according to 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 center portion of the thickness. In this case, the axial direction of the round bar specimen is to be made a direction that is parallel to the rolling elongation direction of the steel plate. If the steel material is a steel pipe, the round bar specimen is prepared from the center portion of the wall thickness. In this case, the axial direction of the round bar specimen is to be made a direction that is parallel to the axial direction of the steel pipe. If the steel material is a round steel bar, the round bar specimen is prepared from an R / 2 position. In the present description, the term "R / 2 position" means the center 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 is to 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 parallel portion diameter of 8.9 mm and a gage length of 35.6 mm. A tensile test is carried out in the atmosphere at normal temperature (25°C) by a method in accordance with ASTM E8 / E8M (2021) using the prepared round bar specimen, and the obtained stress at the time of 0.65% elongation (0.65% proof stress) is defined as the yield strength (MPa). Note that, in the present embodiment, a value obtained by rounding off decimals of the obtained numerical value is adopted as the yield strength (MPa).[Number density of coarse Al oxides]

[0057] The steel material according to the present embodiment has the aforementioned chemical composition, and a yield strength of more than 1034 to 1172 MPa, with the number density of coarse Al oxides being less than 30 / 200 mm 2< . As described above, in the present description, particles in which, in mass%, the content of Al is 20% or more and the content of O is 10% or more are also referred to as "Al oxides". Further, 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, in mass%, the content of Al is 20% or more and the content of O is 10% or more, and which have a major axis of 5.0 µm or more.

[0058] As described above, when producing a steel material having the chemical composition described above, during the steelmaking process, deoxidation is performed mainly by aluminum (Al). Therefore, a large number of Al oxides are likely to be formed in a steel material having the chemical composition described above. Further, Al oxides are hard oxides, and are likely to cause the corrosion resistance of a steel material to decrease. In particular, in a case where the steel material has a high yield strength of 150 ksi grade, the influence of coarse Al oxides is likely to be actualized, and the SSC resistance is likely to markedly decrease. Therefore, in the steel material according to the present embodiment which has the aforementioned chemical composition and a yield strength of more than 1034 to 1172 MPa, the number density of coarse Al oxides is to be less than 30 / 200 mm 2< .

[0059] In the present embodiment, a preferable upper limit of the number density of coarse Al oxides is 28 / 200 mm 2< , further preferably is 25 / 200 mm 2< , and further preferably is 22 / 200 mm 2< . In the present embodiment, the lower limit of the number density of coarse Al oxides is not particularly limited, and may be 0 / 200 mm 2< . The lower limit of the number density of coarse Al oxides, for example, may be 5 / 200 mm 2< , may be 7 / 200 mm 2< , or may be 9 / 200 mm 2< . A method for determining the number density of coarse Al oxides is described later.[Number density of coarse Si oxides]

[0060] The steel material according to the present embodiment has the chemical composition described above, has a yield strength of more than 1034 to 1172 MPa, the number density of coarse Al oxides in the steel material is less than 30 / 200 mm 2< , and in addition, the number density of coarse Si oxides in the steel material is 5 / 200 mm 2< or less. As described above, in the present description, particles in which, in mass%, the content of Al is less than 20%, the content of Si is 20% or more, and the content of O is 10% or more are also referred to as "Si oxides". Further, 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 content of Al is less than 20%, the content of Si is 20% or more, and the content of O is 10% or more, and which have a major axis of 5.0 µm or more.

[0061] As described above, until now, attention has not been given to Si oxides due to the smallness of the number thereof. However, when a steel material has a high yield strength of 150 ksi grade, there is a possibility that a decrease in SSC resistance is liable to occur not only due to coarse Al oxides but also due to even coarse Si oxides, which are small in number. Therefore, by not only making the number density of coarse Al oxides less than 30 / 200 mm 2< , but also making the number density of coarse Si oxides 5 / 200 mm 2< or less, there is a possibility that excellent SSC resistance can be stably obtained even when the yield strength is raised to 150 ksi grade. Therefore, the steel material according to the present embodiment has the chemical composition described above and a yield strength of more than 1034 to 1172 MPa, and in the steel material the number density of coarse Al oxides is made less than 30 / 200 mm 2< and, in addition, the number density of coarse Si oxides is made 5 / 200 mm 2< or less.

[0062] In the present embodiment, a preferable upper limit of the number density of coarse Si oxides is 4 / 200 mm 2< , and more preferably is 3 / 200 mm 2< . In the present embodiment, the lower limit of the number density of coarse Si oxides is not particularly limited, and may be 0 / 200 mm 2< . The lower limit of the number density of coarse Si oxides may be, for example, 1 / 200 mm 2< .

[0063] In the present embodiment, the number density of coarse Al oxides and the number density of coarse Si oxides in the 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 center portion of the thickness. If the steel material is a steel pipe, a test specimen in which a face including the pipe axis direction and the pipe radius direction is adopted as the observation surface is prepared from a center portion of the wall thickness. If the steel material is a round steel bar, a test specimen which includes an R / 2 position at the center thereof and in which a face including the axial direction and the radial direction is adopted as the observation surface is prepared.

[0064] After polishing the observation surface of the prepared test specimen to obtain a mirror surface, measurement is performed. Although the area of the observation surface is not limited, for example, the area is set to a size of 300 mm 2< (20 mm × 15 mm). On the observation surface, the number of Si oxides having a major axis of 5.0 µm or more is determined. Specifically, first, particles at 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, in mass%, the content of Al is 20% or more and the content of O is 10% or more are identified as "Al oxides". In addition, based on the EDS analysis result for each particle, particles in which, in mass%, the content of Al is less than 20%, the content of Si is 20% or more, and the content of O is 10% or more are identified as "Si oxides".

[0065] Among the Al oxides identified at 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 the coarse Al oxides is determined. In addition, among the Si oxides identified at 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 the coarse Si oxides is determined. Note that, the major axis of the Al oxides and the Si oxides can be determined by a well-known method. In the present description, the term "major axis" of the Al oxides and the Si oxides means, at the observation surface, the longest line segment (µm) among line segments connecting an arbitrary two points on the outer circumference of an Al oxide and a Si oxide.

[0066] The number density of coarse Al oxides ( / 200 mm 2< ) is determined based on the total number of the coarse Al oxides and the gross area of the observation surface. In addition, the number density of coarse Si oxides ( / 200 mm 2< ) is determined based on the total number of the coarse Si oxides and the gross area of the observation surface. Note that, in the present embodiment, a number obtained by rounding off decimals of the relevant numerical value obtained is adopted as the number density of coarse Al oxides ( / 200 mm 2< ), and the number density of coarse Si oxides ( / 200 mm 2< ), respectively. Further, measurement of the number density of coarse Al oxides and the number density of coarse Si oxides can be performed using an apparatus in which a scanning electron microscope is provided with a composition analysis function (SEM-EDS apparatus). For example, an automatic analyzer having the trade name "Metals Quality Analyzer" manufactured by FEI (ASPEX) Company can be used as the SEM-EDS apparatus.[SSC resistance]

[0067] The SSC resistance of the steel material according to the present embodiment can be evaluated by an SSC resistance test conducted by a method carried out in accordance with NACE TM0177-2016 Method A. Specifically, the SSC resistance can be evaluated by the following method.

[0068] A mixed aqueous solution containing 5.0 mass% of sodium chloride and 0.4 mass% of sodium acetate that is adjusted to pH 5.0 with hydrochloric acid (NACE solution D) is employed as the test solution. 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 center portion of the thickness. In this case, the axial direction of the round bar specimen is to be made a direction that is parallel to the rolling elongation direction of the steel plate. If the steel material is a steel pipe, the round bar specimen is prepared from the center portion of the wall thickness. In this case, the axial direction of the round bar specimen is to be made a direction that is parallel to the axial direction of the steel pipe. If the steel material is a round steel bar, the round bar specimen is prepared from an R / 2 position. In this case, the axial direction of the round bar specimen is to 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 6.35 mm, and the length of a parallel portion is 25.4 mm.

[0069] A stress equivalent to 90% of the actual yield stress is applied to the prepared round bar specimen. The test solution at 24°C is poured into a test vessel in a manner so that the round bar specimen to which the stress has been applied is immersed therein, and this is adopted as a test bath. After degassing the test bath, a gaseous mixture of H 2 S gas at 0.01 atm and CO 2 gas at 0.99 atm is blown into the test bath to saturate the test bath. The test bath saturated with the gaseous mixture is held at 24°C for 720 hours. In an SSC resistance test conducted under the above conditions, cracking is not confirmed after 720 hours elapses in the steel material according to the present embodiment. Note that, in the present description, "cracking is not confirmed" means that no cracking is confirmed in observation with the naked eye of a specimen after the test.[Microstructure]

[0070] In the microstructure of the steel material according to the present embodiment, the total of the volume ratios of tempered martensite and tempered bainite is 90% or more. The balance of the microstructure is, for example, ferrite or pearlite. If the total of the volume ratios of tempered martensite and tempered bainite contained in the microstructure of a steel material having the aforementioned chemical composition is 90% or more, on the condition that the other requirements of the present embodiment are satisfied, the steel material can achieve both a yield strength of 150 ksi grade and excellent SSC resistance. That is, in the present embodiment, if the steel material achieves both a yield strength of 150 ksi grade and excellent SSC resistance, it is determined that the total of the volume ratios of tempered martensite and tempered bainite contained in the microstructure is 90% or more.

[0071] Note that, in the case of determining the volume ratios of tempered martensite and tempered bainite by observation, the volume 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 center portion of the thickness. If the steel material is a steel pipe, a test specimen in which a face including the pipe axis direction and the pipe radius direction is adopted as the observation surface is prepared from a center portion of the wall thickness. If the steel material is a round steel bar, a test specimen which includes an R / 2 position at the center thereof, and in which a face including the axial direction and the radial direction is adopted as the observation surface is prepared.

[0072] After polishing the observation surface of the test specimen to obtain a mirror surface, the test specimen is immersed for about 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 each visual field is, for example, 0.01 mm 2< (magnification of 1000×). 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 average value of the area fractions of tempered martensite and tempered bainite determined in all of the visual fields is defined as the volume ratio of tempered martensite and tempered bainite.[Production method]

[0073] A method for producing the steel material according to the present embodiment will now be described. Hereunder, a method for producing a seamless steel pipe as one example of the steel material according to the present embodiment is described. The method for producing a seamless steel pipe includes a process of preparing a starting material (steelmaking process), a process of subjecting the starting material to hot working to produce a hollow shell (hot working process), and a process of subjecting the hollow shell to quenching and tempering to make a seamless steel pipe (quenching process 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 hereunder.[Steelmaking process]

[0074] In the steelmaking process, firstly, molten iron produced by a well-known method is subjected to refining using a converter (primary refining). The molten steel that has undergone the primary refining is then subjected to secondary refining. In the secondary refining, alloy elements that were subjected to composition adjustment are added to the molten steel to thereby produce a molten steel that satisfies the chemical composition described above.

[0075] In the secondary refining, for example, an RH (Ruhrstahl-Hausen) vacuum degassing treatment is performed. Thereafter, final adjustment of the alloy elements is performed. In the secondary refining, composite refining may be performed. In such case, prior to the RH vacuum degassing treatment, for example, a refining treatment that uses an LF (ladle furnace) or VAD (vacuum arc degassing) is performed.

[0076] A starting material is produced using the molten steel that underwent the secondary refining. Specifically, a cast piece (a slab, a bloom, or a billet) is produced by a continuous casting process using the molten steel subjected to the secondary refining. In the continuous casting process, first, molten steel is poured from a ladle into a tundish. At such time, in order to seal the nozzle of the ladle, usually the nozzle is filled with sand. Therefore, in some cases, the sand for filling may get mixed in together with the molten steel poured from the ladle to the tundish. Further, 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 produced starting material.

[0077] Therefore, in the present embodiment, in order to prevent Si oxides which are filled in the nozzle of the ladle being introduced into the tundish, the molten steel and the Si oxides are separated. Although the method for separating Si oxides from the molten steel is not particularly limited, for example the following method can be used. An inclined metal plate is placed at a position that is below the nozzle of the ladle and is above the opening of the tundish. When the nozzle of the ladle is opened, first, Si oxides are discharged from the nozzle, and next molten steel is discharged. Here, the Si oxides are light in comparison to the molten steel. Therefore, the Si oxides discharged from the nozzle are guided to outside of the opening of the tundish along the inclination of the metal plate. The inclination of the metal plate may be provided, for example, by arranging a metal plate machined into a conical shape without a bottom surface in a manner so that the apex of the conically shape metal plate is directly below the nozzle of the ladle, or may be provided by the other methods. Further, one metal plate may be used, or a plurality of metal plates may be stacked on each other and used. In addition, although not particularly limited, the thickness of the metal plate is, for example, about 1 to 10 mm.

[0078] After the Si oxides have been discharged from the nozzle, the molten steel is discharged. At such time, the molten steel that is discharged from the nozzle is introduced into the tundish through the opening together with the metal plate. That is, in the present embodiment, a part or all of the metal plate may be introduced into the tundish and mixed into 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 which is formed of aluminum with the balance being impurities.

[0079] 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 adhered to the metal plate can be prevented from becoming mixed in 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 may be formed in advance in one part of the metal plate, and the metal plate may be removed using a rod that has a hook formed at the front end thereof. In this case, the metal plate can be removed by hooking the hook that is formed at the front end of the rod into the hole in the metal plate and then pulling the rod. By using the method described above, Si oxides can be separated from the molten steel, and the molten steel can be introduced into the tundish. Note that, a method for separating the Si oxides from the molten steel is not limited to the method described above.

[0080] Next, the prepared molten steel is cast 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 performed by the following method.

[0081] 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 Al oxide accumulation zone 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 SSC resistance 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 of the molten steel, 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 SSC resistance 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.

[0082] In addition, 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 current value set in the range of 330 to 450 A makes it difficult for an Al oxide accumulation zone to be formed in the starting material. If the current value that is set for the electromagnetic stirring in the mold is too low, in some cases stirring of the molten steel will be insufficient and an Al oxide accumulation zone 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 SSC resistance of the steel material will decrease. On the other hand, if the current value that is set for the electromagnetic stirring in the mold is too high, in some cases an excessive load will be applied to the production facility. Therefore, in the present embodiment, preferably the current value set for the electromagnetic stirring in the mold is set within a range of 330 to 450 A. The molten steel is cast and a starting material is produced by the above method.[Hot working process]

[0083] 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, the billet is subjected to hot working to produce a hollow shell (seamless steel pipe). The method of hot working is not particularly limited, and it suffices to use a well-known method.

[0084] For example, the Mannesmann process may be performed as hot working to produce a hollow shell. In this case, a round billet is subjected to piercing-rolling using a piercing machine. When performing piercing-rolling, although not particularly limited, for example, the piercing ratio is 1.0 to 4.0. The round billet subjected to piercing-rolling is further subjected to hot rolling with a mandrel mill, a reducer, a sizing mill or the like to produce a hollow shell. The cumulative reduction of area in the hot working process is, for example, 20 to 70%.

[0085] A hollow shell may be produced from the billet by the other hot working methods. For example, in a case where the steel material is a heavy-wall steel material of a short length such as a coupling, a hollow shell may be produced by forging by the Ehrhardt process or the like. A hollow shell is produced by the above process. Although not particularly limited, the wall thickness of the hollow shell is, for example, 9 to 60 mm.

[0086] 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. The hot working is, for example, blooming that is performed using a blooming mill or hot rolling that is performed using a continuous mill. In a continuous mill, a horizontal stand having a pair of grooved rolls arranged one on 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.

[0087] If the steel material is a steel plate, 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 blooming mill and a continuous mill to produce an intermediate steel material having a steel plate shape.

[0088] The hollow shell produced by hot working may be air-cooled (as-rolled). The hollow shell produced by hot working may be subjected to direct quenching after the hot working without being cooled to normal temperature, or may be subjected to quenching after undergoing supplementary heating (reheating) after the hot working.

[0089] In a case of performing direct quenching after the hot working, or performing quenching after supplementary heating, cooling may be stopped midway through the quenching process or slow cooling may be performed. In this case, the occurrence of quench cracking in the hollow shell can be suppressed. In addition, in the case of performing direct quenching after hot working, or performing quenching after supplementary heating, stress relief annealing (SR) may be performed at a time that is after quenching and before the heat treatment of the next process. In this case, residual stress of the hollow shell is eliminated.

[0090] As described above, in the hot working process, the prepared starting material is subjected to hot working to produce an intermediate steel material. Hereunder, the quenching process is described in detail.[Quenching process]

[0091] In the quenching process, the prepared intermediate steel material (hollow shell) is subjected to quenching. As used in the present description, the term "quenching" means rapidly cooling the intermediate steel material which is at a temperature not lower than the A 3 point. A preferable quenching temperature is 800 to 1000°C. If the quenching temperature is too high, in some cases prior-y grains will become coarse and the SSC resistance of the steel material will decrease. Therefore, a quenching temperature in the range of 800 to 1000°C is preferable.

[0092] 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 that is measured by a thermometer placed on the exit side of the apparatus that performs the final hot working. Further, in a case where quenching is performed after supplementary heating or reheating after hot working, the term "quenching temperature" corresponds to the temperature of the furnace that performs the supplementary heating or reheating.

[0093] The quenching method is a method that, for example, continuously cools the intermediate steel material (hollow shell) from the quenching starting temperature and continuously decreases the surface temperature of the hollow shell. The method of performing the continuous cooling treatment is not particularly limited, and a well-known method can be used. The method of performing the 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 shower water cooling or mist cooling.

[0094] If the cooling rate during quenching is too slow, the microstructure will not become a microstructure that is principally composed of martensite and bainite, and the mechanical property defined in the present embodiment (yield strength of more than 1034 to 1172 MPa) will not be obtained. In such case, in addition, excellent SSC resistance will not be obtained.

[0095] 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 "cooling rate during quenching CR 800-500 ". More specifically, the cooling rate during quenching CR 800-500 is determined based on a temperature measured at a region that is most slowly cooled within a cross-section of the intermediate steel material that is being quenched (for example, in the case of forcedly cooling both surfaces, the cooling rate is measured at the center portion of the thickness of the intermediate steel material).

[0096] A preferable cooling rate during quenching CR 800-500 is 300°C / min or more. A more preferable lower limit of the cooling rate during quenching CR 800-500 is 450°C / min, and further preferably is 600°C / min. Although an upper limit of the cooling rate during quenching CR 800-500 is not particularly defined, the upper limit is, for example, 60000°C / min.

[0097] Preferably, quenching is performed after performing heating of the hollow shell in the austenite zone a plurality of times. In this case, the SSC resistance of the steel material increases because austenite grains are refined prior to quenching. Heating in the austenite zone may be repeated a plurality of times by performing quenching a plurality of times, or heating in the austenite zone may be repeated a plurality of times by performing normalizing and quenching. Further, quenching and tempering that is described later may be performed in combination a plurality of times. That is, both quenching and tempering may be performed a plurality of times. In such case, the SSC resistance of the steel material increases further. The tempering process is described in detail hereunder.[Tempering process]

[0098] In the tempering process, the hollow shell on which the aforementioned quenching was performed is subjected to tempering. In the present description, the term "tempering" means reheating the intermediate steel material after quenching to a temperature that is less than the A c1 point and holding the intermediate steel material at that temperature. Here, the term "tempering temperature" corresponds to the temperature of the furnace when the intermediate steel material after quenching is heated and held at the relevant temperature. The term "tempering time" means the times for which the intermediate steel material is held at the tempering temperature.

[0099] The tempering temperature is appropriately adjusted in accordance with the chemical composition of the seamless steel pipe 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 pipe to within the range of more than 1034 to 1172 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 pipe to within the range of more than 1034 to 1172 MPa. Specifically, in the tempering process according to the present embodiment, a preferable tempering temperature is 640 to 660°C.

[0100] If the tempering time is too short, in some cases a microstructure that is principally 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 set within a range of 10 to 90 minutes. A more preferable lower limit of the tempering time is 15 minutes. A more preferable upper limit of the tempering time is 80 minutes.

[0101] The steel material according to the present embodiment can be produced by the production method described above. Note that, in the foregoing description of the production method, a method for producing a seamless steel pipe has been described as one example. However, the steel material according to the present embodiment may also be a steel plate or the other shapes. A method for producing a steel plate or a steel material of the other shapes also includes, for example, a preparation process, a quenching process, and a tempering process, similarly to the production method described above. Further, the production method described above is an example, and the steel material may also be produced by the other production methods.

[0102] Hereunder, the present invention is described more specifically by way of examples.EXAMPLE

[0103] Molten 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, "-" means that the content of Cu, the content of Ni, and the content of W of steel A were each 0% when rounded off to second decimal places. In addition, "-" means that the content of Ca, the content of Mg, the content of Zr, and the content of rare earth metal (REM) of steel A were each 0% when rounded off to fourth decimal places.[Table 1-1]

[0104] TABLE 1-1Steel SymbolChemical Composition (unit is mass%; balance is Fe and impurities)CSiMnPSAlCrMoTiNbVA0.290.300.250.0060.00120.0250.720.950.0060.0250.10B0.280.250.170.0080.00090.0280.750.830.0040.0280.10C0.290.280.200.0060.00090.0320.831.210.0040.0270.09D0.250.290.150.0070.00110.0310.700.920.0040.0250.09E0.230.350.120.0070.00080.0330.680.850.0060.0320.10F0.250.320.150.0080.00120.0330.650.920.0040.0320.09G0.260.290.450.0080.00110.0250.720.810.0060.0250.10H0.260.350.170.0090.00130.0280.741.190.0050.0320.11I0.270.350.450.0090.00120.0211.020.750.0050.0250.12J0.280.300.400.0080.00140.0320.550.680.0060.0250.10K0.230.280.180.0060.00130.0330.790.990.0060.0310.10L0.260.250.150.0060.00120.0321.030.320.0060.0150.10M0.280.350.110.0080.00520.0240.520.930.0060.0250.10 [Table 1-2]

[0105] TABLE 1-2Steel SymbolChemical Composition (unit is mass%; balance is Fe and impurities)BNOCuNiWCaMgZrREMA0.00130.00430.0010-------B0.00120.00350.0008-------C0.00110.00440.00120.02------D0.00110.00280.0009-0.03-----E0.00120.00350.0012--0.48----F0.00110.00410.0011-----0.0011-G0.00110.00320.0022---0.0012---H0.00120.00420.0011----0.0009--I0.00110.00380.0009------0.0011J0.00110.00430.0015-0.03-0.0012---K0.00110.00270.0045-------L0.00130.00330.0012-------M0.00130.00420.0011-------

[0106] Round billets were produced by a continuous casting process using the molten steels described above. In the continuous casting process, when introducing some of the molten steels into a tundish from a ladle, a metal plate which had been machined into a conical shape without a bottom surface was arranged above the opening of the tundish in a manner so that the apex of the conically shape metal plate was directly below the nozzle of the ladle. Whether or not a metal plate having the aforementioned shape was arranged above the opening of the tundish is indicated in Table 2. Specifically, a case where a metal plate having the aforementioned shape was arranged above the opening of the tundish is indicated by "A" in the column "Metal Plate" in Table 2. A case where a metal plate having the aforementioned shape was not arranged above the opening of the tundish is indicated by "B" in the column "Metal Plate" in Table 2. Note that aluminum plates were used as metal plates having the aforementioned shape arranged above the opening of the tundish. Specifically, three aluminum plates each having a thickness of 2 mm were stacked on top of each other and used. Further, in the cases where the metal plates were arranged, at a timing that was after Si oxides were discharged from the nozzle and was before the molten steel was discharged, the metal plates were removed from below the nozzle using a rod that had a hook formed at the front end thereof. In addition, a round billet was cast from the molten steel at the casting speed described in Table 2. Note that, at such time, the inside of the mold was subjected to electromagnetic stirring at a current that was set to a current value described in Table 2.[Table 2]

[0107] TABLE 2Test NumberSteel SymbolSteelmaking ProcessQuenching ProcessTempering ProcessMetal PlateCasting Speed (m / min)Current Value (A)Temperature (°C)Time (mins)Temperature (°C)Time (mins)1AA1.740092010640302AA1.640092010640303AA1.540092010645304BA1.545092010640605CA1.840092010650306DA1.140092010640607EA1.440092010645308FA2.040090010640309GA2.5400920106406010HA2.5400900106453011IA2.0400900106503012JA2.5400920106453013AA4.5400920106403014AA3.8400920106503015AB2.0450920106503016BB1.7450920106456017DB2.1400920106403018KA1.5450920106453019LA1.0350920106403020MA1.53509201064030

[0108] The produced round billets labeled with the Test Nos. were held at 1250°C for one hour, and thereafter were subjected to hot rolling by the Mannesmann-mandrel process to produce hollow shells (seamless steel pipes) labeled with the Test Nos.. In addition, the obtained hollow shells labeled with the Test Nos. were subjected to quenching. Specifically, the hollow shells labeled with the Test Nos. were held at temperatures (°C) for times (mins) which are each described in the column "Quenching Process" in Table 2, and thereafter were subjected to quenching by shower water cooling. The hollow shell of Test No. 3 was held at 900°C for 10 minutes after subjecting to the aforementioned quenching, and thereafter was subjected to quenching by shower water cooling. Note that, the cooling rates during quenching CR 800-500 were within the range of 480 to 30000°C / min for the Test Nos.. Here, the temperatures (°C) of the heat treatment furnace that heated the hollow shells were adopted as the temperatures (°C) of the quenching process. Further, the times (mins) for which the hollow shells were held at the quenching temperatures were adopted as the times (mins) of the quenching process.

[0109] The obtained hollow shells labeled with the Test Nos. were subjected to tempering. Specifically, tempering of the hollow shells labeled with the Test Nos. were carried out by holding the hollow shells at each temperature (°C) for each time (mins) which are each described in the column "Tempering Process" in Table 2. Here, the temperatures (°C) of the tempering furnace that heated the hollow shells were adopted as the temperature (°C) of the tempering described in Table 2. Further, the times (mins) for which the hollow shells were held at the tempering temperature were adopted as the tempering time (mins) described in Table 2. Seamless steel pipes of Test Nos. were obtained by the production process described above.[Evaluation tests]

[0110] The seamless steel pipes of Test Nos. after the tempering described above were subjected to a tensile test, a test to measure the number density of coarse Al oxides and number density of coarse Si oxides, and a SSC resistance test.[Tensile test]

[0111] The seamless steel pipes of Test Nos. were subjected to a tensile test, and the yield strength was determined. The tensile test was carried out in accordance with ASTM E8 / E8M (2021). Round bar specimens having a parallel portion diameter of 8.9 mm and a gage length of 35.6 mm were prepared from the center portion of the wall thickness of the seamless steel pipes of Test Nos. The axial direction of the round bar specimen was parallel to the axial direction of the seamless steel pipe. The tensile tests were carried out in the atmosphere at normal temperature (25°C) using the prepared round bar specimens, and the yield strength (MPa) of the seamless steel pipes of Test Nos. were determined. Note that, in the present examples, the stress at a time of 0.65% elongation (0.65% proof stress) obtained in the tensile test was defined as the yield strength. The obtained yield strength (MPa) is shown in Table 3 as "YS (MPa)".[Table 3]

[0112] TABLE 3Test NumberSteel SymbolYS (MPa)Coarse Al Oxides ( / 200 mm 2< )Coarse Si Oxides ( / 200 mm 2< )Number of Specimens in which SSC Occurred (Specimens)1A113815302A117218403A113111404B112410205C108218306D108925307E109615308F115811209G1076195010H1145124011I1089233012J1131262013A1145352214A1089383315A10962210216B11331012217D1131118218K1124255219L1151224320M11172323 [Test to measure number density of coarse Al oxides and coarse Si oxides]

[0113] Tests to measure the number density of coarse Al oxides and number density of coarse Si oxides were carried out on the seamless steel pipes of Test Nos., 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 a test specimens prepared from the center portion of the wall thickness of the seamless steel pipes of Test Nos.. The obtained number density of coarse Al oxides ( / 200 mm 2< ) is shown in the column "Coarse Al Oxides ( / 200 mm 2< )" in Table 3. The obtained number density of coarse Si oxides ( / 200 mm 2< ) is shown in the column "Coarse Si Oxides ( / 200 mm 2< )" in Table 3.[SSC resistance test]

[0114] The seamless steel pipes labeled with the Test Nos. were subjected to an SSC resistance test conducted by a method carried out in accordance with NACE TM0177-2016 Method A, and the SSC resistance was evaluated. Specifically, round bar specimens of 6.35 mm in diameter in which the length of a parallel portion was 25.4 mm were prepared from the center portion of the wall thickness of the seamless steel pipes labeled with the Test Nos.. The SSC resistance test was performed on three specimens among the prepared specimens. Note that the axial direction of each specimen was parallel to the pipe axis direction.

[0115] Tensile stress was applied in the axial direction of the round bar specimens labeled with the Test Nos.. At this time, the applied stress was adjusted so as to be 90% of the actual yield stress of each steel plate. A mixed aqueous solution containing 5.0 mass% of sodium chloride and 0.4 mass% of sodium acetate that was adjusted to pH 5.0 with hydrochloric acid (NACE solution D) was used as the test solution. The test solution at 24°C was poured into three test vessels, and these were adopted as test baths. Three round bar specimens to which the stress was applied were immersed individually in mutually different test vessels as the test baths. After each test bath was degassed, a gaseous mixture of H 2 S gas at 0.01 atm and CO 2 gas at 0.99 atm was blown into each test bath to saturate the test baths. The test bath saturated with the gaseous mixture is held at 24°C for 720 hours.

[0116] After being held for 720 hours, the round bar specimens labeled with the Test Nos. were observed to determine whether or not sulfide stress cracking (SSC) had occurred. Specifically, after being held for 720 hours, the round bar specimens were observed with the naked eye. For each test number, the number of round bar specimens in which SSC had occurred among the three round bar specimens is shown in the column "Number of Specimens in which SSC Occurred (Specimens)" in Table 3.[Evaluation results]

[0117] Referring to Table 1-1, Table 1-2, Table 2, and Table 3, the chemical compositions of the seamless steel pipes of Test Nos. 1 to 12 were appropriate, and the production methods of Test Nos. 1 to 12 also satisfied the preferable conditions described above. As a result, for each of these seamless steel pipes, the yield strength was more than 1034 to 1172 MPa, the number density of coarse Al oxides was less than 30 / 200 mm 2< , and the number density of coarse Si oxides was 5 / 200 mm 2< or less. As a result, for each of these seamless steel pipes, the number of specimens in which SSC occurred was zero in the SSC resistance test. That is, the seamless steel pipes of Test Nos. 1 to 12 had a yield strength of more than 1034 to 1172 MPa and had excellent SSC resistance. Note that, it was determined that the total of the volume ratios of tempered martensite and tempered bainite in the microstructure of each of these seamless steel pipes was 90% or more.

[0118] On the other hand, for the seamless steel pipes of Test Nos. 13 and 14, the casting speed in the steelmaking process was too fast. As a result, the number density of coarse Al oxides in these seamless steel pipes was 30 / 200 mm 2< or more. As a result, for these seamless steel pipes, the number of specimens in which SSC occurred was one or more in the SSC resistance test, and thus the seamless steel pipes did not have excellent SSC resistance.

[0119] For the seamless steel pipes of Test 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 pipes was more than 5 / 200 mm 2< . As a result, for these seamless steel pipes, the number of specimens in which SSC occurred was one or more in the SSC resistance test, and thus the seamless steel pipes did not have excellent SSC resistance.

[0120] In the seamless steel pipe of Test No. 18, the content of O was too high. As a result, for this seamless steel pipe, the number of specimens in which SSC occurred was one or more in the SSC resistance test, and thus the seamless steel pipe did not have excellent SSC resistance.

[0121] In the seamless steel pipe of Test No. 19, the content of Mo was too low. As a result, for this seamless steel pipe, the number of specimens in which SSC occurred was one or more in the SSC resistance test, and thus the seamless steel pipe did not have excellent SSC resistance.

[0122] In the seamless steel pipe of Test No. 20, the content of S was too high. As a result, for this seamless steel pipe, the number of specimens in which SSC occurred was one or more in the SSC resistance test, and thus the seamless steel pipe did not have excellent SSC resistance.

[0123] An embodiment of the present disclosure has been described above. However, the embodiment described above is merely an example for carrying out the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiment, and can be implemented by appropriately modifying the above-described embodiment within a range not departing from the spirit thereof.

Claims

1. A steel material consisting of, in 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: 0.40 to 2.00%, Ti: 0.002 to 0.020%, Nb: 0.002 to 0.100%, V: 0.05 to 0.30%, B: 0.0005 to 0.0040%, N: 0.0100% or less, O: 0.0040% or less, 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%, rare earth metal: 0 to 0.0100%, with the balance being Fe and impurities; wherein: a yield strength is more than 1034 to 1172 MPa, and in the steel material, a number density of Al oxides in which, in mass%, a content of Al is 20% or more and a content of O 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 mass%, a content of Al is less than 20%, a content of Si is 20% or more, and a content of O is 10% or more, and which have a major axis of 5.0 µm or more is 5 / 200 mm2 or less.

2. The steel material according to claim 1, containing one or more elements selected from a group consisting of: 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. The steel material according to claim 1 or claim 2, wherein the steel material is a seamless steel pipe.

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