MATERIAL DE AÇO

Abstract

Provided is a steel material which combines a yield strength of 80 ksi or greater (552 MPa or greater) with excellent low-temperature toughness in an extremely low temperature environment of -70°C or below. The steel material according to the present disclosure comprises, in mass %, 0.26-0.35% C, 0.10-1.00% Si, 1.00-1.40% Mn, up to 0.015% P, up to 0.0020% S, 0.15-0.50% Cr, 0.05-0.25% Mo, 0.001-0.070% sol.Al, 0.0010-0.0080% N, and Fe and impurities as the balance, and has a yield strength of 552 MPa or higher but less than 655 MPa. In this steel material according to the present disclosure, the number of carbide grains having an equivalent circle diameter of 2 μm or larger is 10 per mm2 or less and the prior-austenite grain diameter DA and the block diameter DB satisfy relationship (1).　(1): 2.5≤DA / DB≤10.0

Description

[001] The present invention relates to a steel material, and more particularly to a steel material to be used for CO2 storage technology. State of the Art

[002] An increase in the concentration of carbon dioxide (CO2) above ground is currently a global problem. Therefore, efforts to suppress CO2 emissions are ongoing. One such effort to suppress CO2 emissions that is attracting particular attention is CCUS.

[003] CCUS is an abbreviation for carbon dioxide capture, utilization and storage. That is, CCUS includes the three technologies of capturing, utilizing and storing CO2. Among these, the technology that captures CO2 emitted from an industrial facility such as a power plant or a factory and injects the CO2 into a depleted oil well to store the CO2 there is attracting attention as a technology for storing CO2.

[004] A steel material to be used for this type of CO2 storage technology is required to have a yield strength of, for example, 80 ksi or more (552 MPa or more). Steel materials having a yield strength of 80 ksi or more have already been proposed in Japanese Patent Application Publication No. 2002-115028 (Patent Literature 1) and International Application Publication No. WO2017 / 149570 (Patent Literature 2).

[005] The steel material disclosed in Patent Literature 1 is a steel pipe for oil wells, and is characterized in that a yield strength YS of the steel material is 552 MPa (80 ksi) or more, and an energy absorbed in the LE direction (J) at 0°C satisfies the inequality (E > 0.4169*YS+480). It is disclosed in Patent Literature 1 that this steel material is capable of withstanding collapse caused by strong external pressure on the outer surface, which is required as a casing pipe. Petition 870250083723, dated 09 / 17 / 2025, pp. 99 / 159 2 / 59

[006] The steel material disclosed in Patent Literature 2 is a seamless, low-alloy, high-strength steel pipe for oil wells having a composition consisting of, in % by mass, C: 0.23 to 0.27%, Si: 0.01 to 0.35%, Mn: 0.45 to 0.70%, P: 0.010% or less, S: 0.001% or less, O: 0.0015% or less, Al: 0.015 to 0.080%, Cu: 0.02 to 0.09%, Cr: 0.8 to 1.5%, Mo: 0.5 to 1.0%, Nb: 0.02 to 0.05%, B: 0.0015 to 0.0030%, Ti: 0.005 to 0.020%, and N: 0.005% or less, where the value of a ratio (Ti / N) of Ti content to N content is 3.0 to 4.0, and the remainder is Fe and unavoidable impurities. Furthermore, in this steel material, the value of a ratio (σ0.7 / σ0.4) of a stress at 0.7% strain with respect to a stress at 0.4% strain on a stress-strain curve is 1.02 or less, and the yield strength is 655 MPa or more.It is disclosed in Patent Literature 2 that according to this steel material, a seamless, low-alloy, high-strength steel pipe for oil wells that stably exhibits a high Kissc value while having a high strength of API grade T95 or higher can be provided. List of Citations Patent Literature

[007] Patent Literature 1: Publication of Japanese Patent Application No. 2002-115028 Patent Literature 2: Publication of International Patent Application No. WO2017 / 149570 Summary of the Invention Technical Problem

[008] In this connection, when injecting CO2 into a depleted oil well, in some cases the CO2 gas is compressed and pressurized to a supercritical state to increase injection efficiency. On the other hand, in the unlikely event of a CO2 leak occurring, the CO2 gas changes from a supercritical state to a gas, and the temperature of the CO2 gas drops rapidly due to the sudden drop in pressure. In such a case, the ambient temperature will fall well below normal, and there is a possibility of the temperature dropping to -70°C. Petition 870250083723, dated 09 / 17 / 2025, pp. 100 / 159 3 / 59 or less. Therefore, a steel material for which this type of technology is assumed to be applied to store carbon dioxide is required not only to have high strength, but also to have low-temperature toughness in an extremely low temperature environment of -70°C or less. However, in Patent Literature 1 and 2 mentioned above, there is no discussion of any kind regarding low-temperature toughness in an extremely low temperature environment of -70°C or less.

[009] An object of the present invention is to provide a steel material that achieves both a yield strength of 80 ksi or more (552 MPa or more) and excellent low-temperature toughness in an extremely low temperature environment of -70°C or less. Solution to the Problem

[0010] A steel material according to the present invention consists of, in % by mass, C: 0.26 to 0.35% Yes: 0.10 to 1.00% Mn: 1.00 to 1.40% P: 0.015% or less, S: 0.0020% or less, Cr: 0.15 to 0.50% Mo: 0.05 to 0.25% Al sol.: 0.001 to 0.070%, N: 0.0010 to 0.0080%, V: 0 to 0.60%, Nb: 0 to 0.03%, Ti: 0 to 0.05%, W: 0 to 0.50%, Zr: 0 to 0.0050%, Co: 0 to 0.50%, Ni: 0 to 0.30%, Petition 870250083723, dated 09 / 17 / 2025, pp. 101 / 159 4 / 59 Cu: 0 to 0.50%, Sn: 0 to 0.100%, Ca: 0 to 0.0035%, Mg: 0 to 0.0035% B: 0 to 0.0010%, rare earth metal: 0 to 0.0050%, and the remainder: Fe and impurities, where: a yield strength is 552 to less than 655 MPa; and in steel material, a number density of carbides having an equivalent circular diameter of 2 μm or more is 10 / mm2 or less, and a prior austenite grain diameter is defined as DA and a block diameter is defined as DB, DA and DB satisfy Formula (1): 2.5 < DA / DB < 10.0 (1) where, the diameter of the previous austenite grain in μm units is replaced by DA in Formula (1), and the block diameter in μm units is replaced by DB in Formula (1). Advantageous Effects of the Invention

[0011] The steel material according to the present invention can achieve both a yield strength of 80 ksi or more (552 MPa or more) and excellent low-temperature toughness in an extremely low temperature environment of -70°C or less. Description of the Modalities

[0012] The present inventors first conducted studies with respect to obtaining a steel material having a yield strength of 552 to less than 655 MPa, which is assumed to have applications in carbon dioxide storage technology. In other words, the present inventors carried out investigations and studies on techniques for obtaining a Petition 870250083723, dated 09 / 17 / 2025, pp. 102 / 159 5 / 59 yield strength of 552 to less than 655 MPa and excellent low-temperature toughness in an extremely low-temperature environment in a steel material for which application to carbon dioxide storage technology is assumed. As a result, the present inventors have obtained the following findings.

[0013] Initially, the present inventors focused on the chemical composition and conducted studies on obtaining a steel material having a yield strength of 552 to less than 655 MPa and also having excellent low-temperature toughness in an extremely low-temperature environment. As a result, the present inventors considered that if a steel material consists of, in % by mass, C: 0.26 to 0.35%, Si: 0.10 to 1.00%, Mn: 1.00 to 1.40%, P: 0.015% or less, S: 0.0020% or less, Cr: 0.15 to 0.50%, Mo: 0.05 to 0.25%, Al sol.The composition is as follows: 0.001 to 0.070%, N: 0.0010 to 0.0080%, V: 0 to 0.60%, Nb: 0 to 0.03%, Ti: 0 to 0.05%, W: 0 to 0.50%, Zr: 0 to 0.0050%, Co: 0 to 0.50%, Ni: 0 to 0.30%, Cu: 0 to 0.50%, Sn: 0 to 0.100%, Ca: 0 to 0.0035%, Mg: 0 to 0.0035%, B: 0 to 0.0010%, rare earth metal: 0 to 0.0050%, and the remainder: Fe and impurities. There is a possibility that a flow resistance of 552 to less than 655 MPa and excellent performance is possible. low-temperature toughness can be achieved in an extremely low-temperature environment.

[0014] Next, the present inventors focused on fine particles in the steel material with a view to obtaining a steel material that achieves both a yield strength of 552 to less than 655 MPa and excellent low-temperature toughness in an extremely low-temperature environment. In particular, the present inventors focused on comparatively coarse particles, and focused their attention on techniques to increase low-temperature toughness in an extremely low-temperature environment while maintaining yield strength. As a result, the present inventors obtained the following findings. Petition 870250083723, dated 09 / 17 / 2025, pp. 103 / 159 6 / 59

[0015] When attempting to obtain a yield strength of 552 to less than 655 MPa in a steel material having the chemical composition mentioned above, in some cases coarse carbides are formed. As a result of detailed studies conducted by the present inventors, it was revealed that when a large number of carbides having an equivalent circular diameter of 2 μm or more are formed in a steel material, the low-temperature toughness in an extremely low-temperature environment decreases. Specifically, in a steel material having the chemical composition mentioned above, by reducing the number density of carbides having an equivalent circular diameter of 2 μm or more to 10⁻⁶ / mm² or less, there is a possibility that both a yield strength of 552 to less than 655 MPa and low-temperature toughness in an extremely low-temperature environment can be achieved.Hereafter, in the present description, carbides having an equivalent circular diameter of 2 μm or more are also referred to as coarse carbides. Note that, as used herein, the term equivalent circular diameter means the diameter of a circle in the case where the area of ​​a precipitate observed on a visual field surface during microstructure observation is converted into a circle having the same area.

[0016] On the other hand, even when a steel material had the chemical composition mentioned above and a yield strength of 552 to less than 655 MPa and the number density of coarse carbides was reduced to 10 / mm2 or less, there were some cases in which low-temperature toughness in an extremely low-temperature environment could not be obtained. Therefore, the present inventors investigated various techniques to increase low-temperature toughness in an extremely low-temperature environment with respect to a steel material having the chemical composition mentioned above and a yield strength of 552 to less than 655 MPa and in which the number density of coarse carbides having an equivalent circular diameter of 2 μm or more is 10 / mm2 or less. Petition 870250083723, dated 09 / 17 / 2025, pp. 104 / 159 7 / 59

[0017] Specifically, the present inventors focused on the microstructure of the steel material. A technique that reduces the prior austenite grain diameter (hereinafter, the prior austenite grain diameter is also referred to as the prior γ DA grain diameter) in the microstructure of a steel material is known as a technique for increasing the low-temperature toughness of a steel material. On the other hand, in steel materials having the chemical composition mentioned above and a yield strength of 552 to less than 655 MPa in which the number density of coarse carbides was 10⁶ / mm² or less, in some cases low-temperature toughness in an extremely low temperature environment was not obtained even when the prior γ DA grain diameter was small. This point will be described specifically using a table.

[0018] [Table 1] TABLE 1 Test Number Yield Strength YS (MPa) Grain Diameter γ Previous DA (μm) Block Diameter DB (μm) Fn1 (=DA / DB) vTrs (°C) 13 599 141.6 14.5 9.8 -72 14 573 146.8 14.8 9.9 -72 49 575 128.1 10.0 12.8 -60

[0019] Table 1 is a table created by extracting some of the results from examples to be described later. The steel materials from each of Tests Nos. 13, 14, and 49 had the chemical composition mentioned above and a yield strength of 552 to less than 655 MPa, and the number density of coarse carbides in each of these steel materials was 10 / mm2 or less. Referring to Table 1, comparing Tests Nos. 13 and 14 with Test No. 49, it is found that even though Tests Nos. 13 and 14 each have a larger previous grain diameter γ DA than Test No. 49, a fracture appearance transition temperature vTrs (°C), which is an index of low-temperature toughness, is lower for Tests Nos. 13 and 14. Petition 870250083723, dated 09 / 17 / 2025, pp. 105 / 159 8 / 59 compared to Test No. 49. Note that, as described in detail later, the lower the fracture appearance transition temperature vTrs, the more excellent the low-temperature toughness of the steel material. That is, it can be confirmed that Tests Nos. 13 and 14, which have larger previous grain diameters γ DA, exhibit more excellent low-temperature toughness in an extremely low-temperature environment than Test No. 49, which has a smaller previous grain diameter γ DA.

[0020] As a result of detailed studies conducted by the present inventors, it was revealed that in a steel material having the chemical composition mentioned above and a yield strength of 552 to less than 655 MPa and in which the number density of coarse carbides is 10 / mm2 or less, not only the previous grain diameter γ DA but also the size of blocks that are the sub-microstructure influences the low-temperature toughness in an extremely low-temperature environment. Here, the microstructure of a steel material having the chemical composition mentioned above and a yield strength of 552 to less than 655 MPa and having excellent low-temperature toughness is mainly composed of quenched martensite and quenched bainite. Here, a group of laths having almost the same orientation in the martensite sub-microstructure is referred to as a martensite block.Similarly, a group of bainite laths having nearly the same orientation in the bainite submicrostructure is referred to as a bainite block. In the present description, martensite blocks and bainite blocks are also collectively referred to as blocks. Furthermore, in the present description, the average grain diameter of martensite blocks and the average grain diameter of bainite blocks are also collectively referred to as block diameter DB.

[0021] As described above, in general, the finer the previous grain diameter γ DA of a steel material, the more excellent the low-temperature toughness that the steel material will tend to exhibit. The present inventors thought that, similarly, steel materials with Petition 870250083723, dated 09 / 17 / 2025, pp. 106 / 159 9 / 59 a finer DB block diameter would exhibit more excellent low-temperature toughness. However, referring to Table 1, contrary to the present inventors' expectations, it was confirmed that Tests Nos. 13 and 14, which had not only a larger previous grain diameter γ DA but also a larger DB block diameter, exhibited more excellent low-temperature toughness in an extremely low-temperature environment than Test No. 49, which had a smaller previous grain diameter γ DA and a smaller DB block diameter.

[0022] As a result of further detailed studies conducted by the present inventors taking into account the above findings, it was revealed that in a steel material having the chemical composition mentioned above and a yield strength of 552 to less than 655 MPa and in which the number density of coarse carbides is 10 / mm2 or less, if the previous grain diameter γ DA and block diameter DB satisfy the following Formula (1), excellent low temperature toughness in an extremely low temperature environment will be obtained. 2.5 < DA / DB < 10.0 (1) where, the diameter of the previous austenite grain in μm units is replaced by DA in Formula (1), and the block diameter in μm units is replaced by DB in Formula (1).

[0023] Let Fn1 be defined as Fn1 = DA / DB. Fn1 is an index of low-temperature toughness in an extremely low-temperature environment of a steel material having the chemical composition mentioned above and a yield strength of 552 to less than 655 MPa and in which the number density of coarse carbides is 10 / mm2 or less. When Fn1 in the steel material mentioned above is greater than 10.0, the previous grain diameter γ DA is too large relative to the block diameter DB. As a result, excellent low-temperature toughness in an extremely low-temperature environment is not obtained. Furthermore, when Fn1 in the steel material mentioned above is 2.5 or greater, low-temperature toughness in an extremely low-temperature environment is stably obtained. Therefore, Petition 870250083723, dated 09 / 17 / 2025, pp. 107 / 159 10 / 59 in a steel material having the chemical composition mentioned above and a yield strength of 552 to less than 655 MPa and in which the number density of coarse carbides is 10 / mm2 or less, when Fn1 satisfies a condition of being within the range of 2.5 to 10.0, the steel material has excellent low-temperature toughness even in an extremely low-temperature environment.

[0024] Note that the reason why a steel material having the chemical composition mentioned above and a yield strength of 552 to less than 655 MPa and in which the number density of coarse carbides is 10⁻⁶ / mm² or less has excellent low-temperature toughness even in an extremely low temperature environment as a result of Fn1 satisfying a condition of being within the range of 2.5 to 10.0 has not been explained in detail. However, the fact that a steel material having the chemical composition mentioned above and a yield strength of 552 to less than 655 MPa and in which the number density of coarse carbides is 10⁻⁶ / mm² or less has excellent low-temperature toughness even in an extremely low temperature environment as a result of Fn1 satisfying a condition of being within the range of 2.5 to 10.0 has been demonstrated by examples that are described later.

[0025] The core of the steel material according to the present embodiment, which has been completed based on the findings described above, is as follows.

[0026] [1] A steel material consisting of, in % mass, C: 0.26 to 0.35% Yes: 0.10 to 1.00% Mn: 1.00 to 1.40% P: 0.015% or less, S: 0.0020% or less, Cr: 0.15 to 0.50% Mo: 0.05 to 0.25% Petition 870250083723, dated 09 / 17 / 2025, pp. 108 / 159 11 / 59 Al sol.: 0.001 to 0.070%, N: 0.0010 to 0.0080%, V: 0 to 0.60%, Nb: 0 to 0.03%, Ti: 0 to 0.05%, W: 0 to 0.50%, Zr: 0 to 0.0050%, Co: 0 to 0.50%, Ni: 0 to 0.30%, Cu: 0 to 0.50%, Sn: 0 to 0.100%, Ca: 0 to 0.0035%, Mg: 0 to 0.0035% B: 0 to 0.0010%, rare earth metal: 0 to 0.0050%, and the remainder: Fe and impurities, where: a yield strength is 552 to less than 655 MPa; and in steel material, a number density of carbides having an equivalent circular diameter of 2 μm or more is 10 / mm2 or less, and a prior austenite grain diameter is defined as DA and a block diameter is defined as DB, DA and DB satisfy Formula (1): 2.5 < DA / DB < 10.0 (1) where, the diameter of the previous austenite grain in μm units is replaced by DA in Formula (1), and the block diameter in μm units is replaced by DB in Formula (1).

[0027] [2] Steel material according to [1], containing one or more elements selected from a group consisting of: Petition 870250083723, dated 09 / 17 / 2025, pp. 109 / 159 12 / 59 V: 0.01 to 0.60% Nb: 0.01 to 0.03%, Ti: 0.01 to 0.05% W: 0.01 to 0.50% Zr: 0.0001 to 0.0050% Co: 0.01 to 0.50% Ni: 0.01 to 0.30%, Cu: 0.01 to 0.50% Sn: 0.001 to 0.100% Ca: 0.0001 to 0.0035% Mg: 0.0001 to 0.0035% B: 0.0001 to 0.0010%, and rare earth metal: 0.0001 to 0.0050%.

[0028] [3] The steel material according to [1] or [2], where: the steel material is a steel pipe.

[0029] 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.

[0030] 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. [Chemical composition]

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

[0032] C: 0.26 to 0.35% - Carbon (C) increases hardenability Petition 870250083723, dated 09 / 17 / 2025, pp. 110 / 159 13 / 59 of the steel material and increases the strength of the steel material. C also makes the block diameter DB of the steel material thinner. If the C content is too low, the advantageous effects mentioned above 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 C content is too high, even if the contents of other elements are within the range of the present embodiment, the strength of the steel material will be too high and the low-temperature toughness of the steel material will decrease. Therefore, the C content should be 0.26 to 0.35%. A preferable lower limit of the C content is 0.27%, but preferably it is 0.28%, and even more preferably it is 0.29%. A preferable upper limit of the C content is 0.34%, but preferably it is 0.33%, and even more preferably it is 0.32%.

[0033] Si: 0.10 to 1.00% - Silicon (Si) deoxidizes steel. If the Si content is too low, the advantageous effect mentioned above 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, 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 Si content should be 0.10 to 1.00%. A preferable lower limit of the Si content is 0.12%, but preferably it is 0.14%, and even more preferably it is 0.15%. A preferable upper limit of the Si content is 0.95%, but preferably it is 0.90%, and even more preferably it is 0.80%.

[0034] Mn: 1.00 to 1.40% - Manganese (Mn) increases the hardenability of steel and enhances the strength of steel. If the Mn content is too low, the aforementioned beneficial effect will not be sufficiently achieved even if the contents of other elements are within the range of this embodiment. Conversely, if the Mn content is too high, even if the contents of other elements are within the range of this embodiment, Mn will segregate to grain boundaries along with impurities such as P and S, and the low-temperature toughness of the steel will decrease. Therefore, the Mn content should be 1.00 to 1.40%. A lower limit Petition 870250083723, dated 09 / 17 / 2025, pp. 111 / 159 A preferred upper limit for Mn content is 1.02%, and more preferably it is 1.05%. A preferred upper limit for Mn content is 1.35%, and more preferably it is 1.30%.

[0035] P: 0.015% 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 0.015% or less. A preferable upper limit of the P content is 0.014%, and more preferably 0.013%. The P content is preferably as low as possible. However, reducing the P content extremely will greatly increase the cost of production. Therefore, when considering industrial production, a preferable lower limit of the P content is 0.001%, but preferably 0.002%, and even more preferably 0.003%.

[0036] S: 0.0020% 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 0.0020% or less. A preferable upper limit of the S content is 0.0019%, but preferably it is 0.0018%, and even more preferably it is 0.0016%. 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 preferable lower limit of the S content is 0.0001%, but preferably it is 0.0002%, and even more preferably it is 0.0003%.

[0037] Cr: 0.15 to 0.50% - Chromium (Cr) increases the hardenability of steel material and increases the strength of steel material. Cr also concentrates in the cementite in the steel material and thus suppresses cementite thickening. As a result, the low-temperature toughness of the steel material increases. If the Cr content is too low, the advantageous effects mentioned above will not be sufficiently obtained even if the contents of other elements are increased. Petition 870250083723, dated 09 / 17 / 2025, pp. 112 / 159 15 / 59 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 from 0.15 to 0.50%. A preferable lower limit for the Cr content is 0.18%, and more preferably 0.20%. A preferable upper limit for the Cr content is 0.45%, and more preferably 0.40%.

[0038] Mo: 0.05 to 0.25% - Molybdenum (Mo) increases the hardenability of steel material and increases the strength of steel material. If the Mo content is too low, the advantageous effect mentioned above 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, even if the contents of other elements are within the range of this embodiment, Mo carbides will form excessively and the low-temperature toughness of the steel material will decrease. Therefore, the Mo content should be 0.05 to 0.25%. A preferable lower limit of the Mo content is 0.06%, and more preferably it is 0.07%. A preferable upper limit of the Mo content is 0.24%, but preferably it is 0.20%, and even more preferably it is 0.18%.

[0039] Al sol.: 0.001 to 0.070% - Aluminum (Al) deoxidizes steel. If the Al content is too low, the advantageous effect mentioned above 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 Al content is too high, even if the contents of other elements are within the range of this embodiment, coarse oxide-based inclusions will form and the low-temperature toughness of the steel material will decrease. Therefore, the Al content should be 0.001 to 0.070%. A preferable lower limit of the Al content is 0.005%, and more preferably 0.010%. A preferable upper limit of the Al content is 0.065%, and more preferably 0.060%. As used in this description, the Al content means the acid-soluble Al content, i.e., the Al sol. content.

[0040] N: 0.0010 to 0.0080% - Nitrogen (N) forms nitrides, and refines the previous grain diameter γ DA of the steel material by the effect of Petition 870250083723, dated 09 / 17 / 2025, pp. 113 / 159 16 / 59 anchoring. As a result, the low-temperature toughness of the steel material increases. If the N content is too low, the advantageous effect mentioned above 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 N content is too high, even if the contents of other elements are within the range of the present embodiment, coarse nitrides will form and the low-temperature toughness of the steel material will decrease. Therefore, the N content should be 0.0010 to 0.0080%. A preferable lower limit of the N content is 0.0015%, but preferably it is 0.0020%, and even more preferably it is 0.0025%. A preferable upper limit of the N content is 0.0075%, and more preferably it is 0.0070%.

[0041] 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. [Optional elements]

[0042] The chemical composition of the steel material described above may additionally contain one or more elements selected from the group consisting of V, Nb, Ti, W and Zr in place of a portion of Fe. Each of these elements is an optional element, and increases the resistance to softening by tempering of the steel material and thus increases the strength of the steel material.

[0043] V: 0 to 0.60% - Vanadium (V) is an optional element and does not need to be present. That is, the V content can be 0%. When present, V forms fine carbides during tempering and thus increases the resistance to temper softening of the steel material and increases the strength of the steel material. If even a small amount of V is present, the advantageous effect mentioned above will be obtained to some extent. However, if Petition 870250083723, dated 09 / 17 / 2025, pp. 114 / 159 17 / 59 If the V 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 V content should be from 0 to 0.60%. A preferable lower limit for the V content is more than 0%, but preferably it is 0.01%, even more preferably it is 0.02%, even more preferably it is 0.04%, and even more preferably it is 0.06%. A preferable upper limit for the V content is 0.40%, but preferably it is 0.30%, and even more preferably it is 0.20%.

[0044] Nb: 0 to 0.03% - Niobium (Nb) is an optional element and does not need to be included. That is, the Nb content can be 0%. When included, Nb forms fine carbides during tempering and thus increases the temper softening resistance of the steel material and increases the strength of the steel material. If even a small amount of Nb is included, the advantageous effect mentioned above will be obtained to some extent. However, if the Nb content is too high, even if the contents of other elements are within the range of this embodiment, carbonitrides and the like will form excessively, and the low-temperature toughness and SSC strength of the steel material will decrease. Therefore, the Nb content should be 0 to 0.03%. A preferable lower limit of the Nb content is more than 0%, but preferably 0.01%, and even more preferably 0.02%. A preferred upper limit for Nb content is less than 0.03%.

[0045] Ti: 0 to 0.05% - Titanium (Ti) is an optional element and does not need to be contained. That is, the Ti content can be 0%. When contained, Ti forms fine carbides and thus increases the temper softening resistance of the steel material and increases the strength of the steel material. If even a small amount of Ti is contained, the advantageous effect mentioned above will be obtained to some extent. However, if the Ti 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 Ti content should be 0 to 0.05%. A preferable lower limit of the Ti content is more than 0%, but Petition 870250083723, dated 09 / 17 / 2025, pp. 115 / 159 18 / 59 preferably it is 0.01%, and even more preferably it is 0.02%. A preferable upper limit for the Ti content is less than 0.05%, and more preferably it is 0.04%.

[0046] W: 0 to 0.50% - Tungsten (W) is an optional element and does not need to be included. That is, the W content can be 0%. When included, W forms fine carbides during tempering and thus increases the temper softening resistance of the steel material and increases the strength of the steel material. If even a small amount of W is included, the advantageous effect mentioned above 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 and the low-temperature toughness of the steel material will decrease. Therefore, the W content should be 0 to 0.50%. A preferable lower limit of the W content is more than 0%, but preferably 0.01%, and even more preferably 0.02%. A preferred upper limit for the W content is 0.45%, and more preferably it is 0.40%.

[0047] Zr: 0 to 0.0050% - Zirconium (Zr) is an optional element and does not need to be included. That is, the Zr content can be 0%. When included, Zr forms fine carbides during tempering and thus increases the temper softening resistance of the steel material and increases the strength of the steel material. If even a small amount of Zr is included, the aforementioned advantageous effect will be achieved 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, coarse oxides will form and the low-temperature toughness of the steel material will decrease. Therefore, the Zr content should be 0 to 0.0050%. A preferred lower limit for Zr content is more than 0%, but preferably it is 0.0001%, even more preferably it is 0.0003%, even more preferably it is 0.0006%, and even more preferably it is 0.0010%.A preferred upper limit for Zr content is 0.0045%, but preferably it is 0.0040%, and even more preferably it is 0.0035%.

[0048] The chemical composition of the steel material described above Petition 870250083723, dated 09 / 17 / 2025, pp. 116 / 159 19 / 59 may additionally contain one or more types of elements selected from the group consisting of Co and Ni in place of a portion of Fe. Each of these elements is an optional element, and each element increases the hardenability of the steel material and increases the strength of the steel material.

[0049] Co: 0 to 0.50% - Cobalt (Co) is an optional element and does not need to be included. That is, the Co content can be 0%. When included, Co increases the hardenability of the steel material and increases the strength of the steel material. If even a small amount of Co is included, the advantageous effect mentioned above will be obtained to some extent. However, if the Co content is too high, even if the contents of other elements are within the range of this embodiment, the hardenability of the steel material will, conversely, decrease and the strength of the steel material will decrease. Therefore, the Co content should be 0 to 0.50%. A preferable lower limit of the Co content is more than 0%, but preferably it is 0.01%, even more preferably it is 0.02%, even more preferably it is 0.03%, and even more preferably it is 0.05%. A preferred upper limit for Co content is 0.45%, and more preferably it is 0.40%.

[0050] Ni: 0 to 0.30% - 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. If even a small amount of Ni is included, the advantageous effect will be obtained to a certain extent. However, if the Ni content is too high, even if the contents of other elements are within the range of this embodiment, localized corrosion will be promoted and the corrosion resistance of the steel material will decrease. Therefore, the Ni content should be 0 to 0.30%. A preferable lower limit of the Ni content is more than 0%, but preferably 0.01%, and even more preferably 0.02%. A preferable upper limit of the Ni content is 0.28%, and more preferably 0.25%.

[0051] The chemical composition of the steel material described above may additionally contain one or more types of elements selected from the group consisting of Cu and Sn in place of a portion of Fe. Each of these Petition 870250083723, dated 09 / 17 / 2025, pp. 117 / 159 20 / 59 elements is an optional element, and each element increases the strength 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 strength of the steel material. If even a small amount of Cu is included, the advantageous effect mentioned above 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 strength 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 preferable lower limit of the Cu content is more than 0%, but preferably it is 0.01%, even more preferably it is 0.02%, and even more preferably it is 0.05%. A preferable upper limit of the Cu content is 0.45%, but preferably it is 0.35%, and even more preferably it is 0.25%.

[0053] Sn: 0 to 0.100% - Tin (Sn) is an optional element and does not need to be included. That is, the Sn content can be 0%. When included, Sn increases the strength of the steel material. If even a small amount of Sn is included, the advantageous effect mentioned above will be obtained to some extent. However, if the Sn content is too high, even if the contents of other elements are within the range of this embodiment, the hot workability of the steel material will decrease. Therefore, the Sn content should be 0 to 0.100%. A preferable lower limit of the Sn content is more than 0%, but preferably it is 0.001%, even more preferably it is 0.002%, and even more preferably it is 0.003%. A preferred upper limit for Sn content is 0.095%, but preferably it is 0.090%, even more preferably it is 0.080%, and even more preferably it is 0.070%.

[0054] The chemical composition of the steel material described above may additionally contain one or more types of elements selected from the group consisting of Ca, Mg, B and rare earth metal in place of a portion of Fe. Each of these elements is an optional element, and each element Petition 870250083723, dated 09 / 17 / 2025, pp. 118 / 159 21 / 59 improves the hot workability of steel material.

[0055] Ca: 0 to 0.0035% - Calcium (Ca) is an optional element and does not need to be present. That is, the Ca content can be 0%. When present, Ca renders S in the steel material harmless by forming sulfides, thus improving the hot workability of the steel material. If even a small amount of Ca is present, the advantageous effect mentioned above 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, coarse oxides will form and the low-temperature toughness of the steel material will decrease. Therefore, the Ca content should be 0 to 0.0035%. A preferred lower limit for the Ca content is more than 0%, but preferably it is 0.0001%, even more preferably it is 0.0003%, even more preferably it is 0.0006%, and even more preferably it is 0.0010%. A preferred upper limit for the Ca content is 0.0033%, and more preferably it is 0.0030%.

[0056] Mg: 0 to 0.0035% - Magnesium (Mg) is an optional element and does not need to be present. That is, the Mg content can be 0%. When present, Mg renders S in the steel material harmless by forming sulfides, thus improving the hot workability of the steel material. If even a small amount of Mg is present, the aforementioned advantageous effect will be achieved 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, coarse oxides will form and the low-temperature toughness of the steel material will decrease. Therefore, the Mg content should be 0 to 0.0035%. A preferred lower limit for Mg content is more than 0%, but preferably it is 0.0001%, even more preferably it is 0.0003%, even more preferably it is 0.0006%, and even more preferably it is 0.0010%. A preferred upper limit for Mg content is 0.0033%, and even more preferably it is 0.0030%.

[0057] B: 0 to 0.0010% - Boron (B) is an optional element and does not need to be present. That is, the B content can be 0%. When present, B suppresses sulfur segregation in the steel material for grain boundaries and improves... Petition 870250083723, dated 09 / 17 / 2025, pp. 119 / 159 22 / 59 the hot workability of the steel material. If even a small amount of B is present, the advantageous effect mentioned above will be obtained to some extent. However, 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 low-temperature toughness of the steel material will decrease. Therefore, the B content should be from 0 to 0.0010%. A preferable lower limit of the B content is more than 0%, but preferably 0.0001%, and even more preferably 0.0002%. A preferable upper limit of the B content is 0.0009%, and more preferably 0.0008%.

[0058] Rare Earth Metal (REM): 0 to 0.0050% - Rare earth metal (REM) is an optional element and does not need to be present. That is, the REM content can be 0%. When present, REM renders S in the steel material harmless by forming sulfides, thus improving the hot workability of the steel material. If even a small amount of REM is present, the aforementioned advantageous effect will be achieved to some extent. However, if the REM content is too high, even if the contents of other elements are within the range of this embodiment, coarse oxides will be formed and the low-temperature toughness of the steel material will decrease. Therefore, the REM content should be 0 to 0.0050%. A preferred lower limit for REM content is more than 0%, but preferably it is 0.0001%, even more preferably it is 0.0003%, and even more preferably it is 0.0006%. A preferred upper limit for REM content is 0.0045%, and more preferably it is 0.0040%.

[0059] 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, which are lanthanides. Furthermore, in the present description, the term REM content refers to the total content of these elements. [Resistance to flow] Petition 870250083723, dated 09 / 17 / 2025, pp. 120 / 159 23 / 59

[0060] The yield strength of the steel material according to the present embodiment is 552 to less than 655 MPa (80 to less than 95 ksi). In the present description, the term yield strength means a 0.2% offset proof stress (MPa) obtained by a tensile test performed at normal temperature (24±3°C) in accordance with ASTM E8 / E8M (2021).

[0061] In the present embodiment, the yield strength of the steel material can be determined by the following method. Specifically, a tensile test is performed in accordance with ASTM E8 / E8M (2021). 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 should 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 central portion of the wall thickness. In this case, the axial direction of the round bar specimen should be made a direction that is parallel to the axis direction of the 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 must be parallel to the axial direction of the round steel bar. Note that, in the present 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. Regarding the size of the round bar specimen, for example, the round bar specimen has a diameter of the parallel portion of 6 mm and a gauge length of 30 mm. A tensile test is performed in the atmosphere at normal temperature (24±3°C) using the round bar specimen, and the 0.2% offset proof stress (MPa) obtained 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 the present embodiment. Petition 870250083723, dated 09 / 17 / 2025, pp. 121 / 159 24 / 59 [Number density of coarse carbides]

[0062] The steel material according to the present embodiment has the chemical composition mentioned above and a yield strength of 552 to less than 655 MPa; furthermore, the number density of coarse carbides (carbides having an equivalent circular diameter of 2 μm or more) in the steel material is reduced to 10 / mm2 or less. As a result, provided that the other requirements of the present embodiment are satisfied, the steel material according to the present embodiment has excellent low-temperature toughness in an extremely low-temperature environment. Note that, as described above, in the present description the term equivalent circular diameter means the diameter of a circle in the case where the area of ​​a precipitate observed on a visual field surface during microstructure observation is converted into a circle having the same area.

[0063] In a steel material with the chemical composition mentioned above, coarse carbides are prone to form. In particular, if carbides do not dissolve in a quenching process described later and remain in the steel material, the carbides that remain in the steel material are prone to thicken and become coarse carbides in a subsequent tempering process. On the other hand, if coarse carbides precipitate in an amount equivalent to a number density of more than 10 / mm2, there is a risk that the coarse carbides will cause a decrease in the low-temperature toughness of the steel material. In particular, in an extremely low-temperature environment such as an environment where the temperature is -70°C or less, the influence of coarse carbides becomes noticeably apparent.Therefore, the steel material according to the present embodiment has the chemical composition mentioned above and a yield strength of 552 to less than 655 MPa; furthermore, the numerical density of coarse carbides in the steel material is 10 / mm2 or less.

[0064] In the present modality, a preferred upper limit of Petition 870250083723, dated 09 / 17 / 2025, pp. 122 / 159 25 / 59 The numerical density of coarse carbides is 9 / mm2, but preferably it is 8 / mm2, even more preferably it is 7 / mm2, even more preferably it is 6 / mm2, and even more preferably it is 5 / mm2. Note that the lower limit of the numerical density of coarse carbides is not particularly limited, and may be 0 / mm2, or it may be 1 / mm2.

[0065] In the present embodiment, the numerical density of coarse carbides in steel material can be determined by the following method. First, a test specimen 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 the 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 the central portion of the wall thickness. If the steel material is a round steel bar, a test specimen that 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.

[0066] 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 300 mm2 (20 mm χ 15 mm). On the observation surface, the number of carbides having an equivalent circular diameter of 2 μm or more is determined. Specifically, 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) by energy-dispersive X-ray spectrometry (EDS). The EDS analysis is conducted with an accelerating voltage of 20 kV for C, N, O, Mg, Al, Si, P, S, Ca, Ti, Cr, Mn, Fe, Cu, Zr, and Nb as elements to be analyzed. With Petition 870250083723, dated 09 / 17 / 2025, pp. 123 / 159 26 / 59 Based on the results of the EDS analysis for the respective particles, those particles in which C is detected and in which the Fe content is 50% by mass or more are identified as carbides. Here, in the present description, the phrase "C is detected" means that a characteristic X-ray peak that is identified as C is confirmed. Note that confirming a characteristic X-ray peak that is identified as C is a task that those skilled in the art are fully capable of performing.

[0067] Among the carbides identified on the observation surface, carbides having an equivalent circular diameter of 2 μm or more (coarse carbides) are identified, and the total number of coarse carbides is determined. Note that the equivalent circular diameter of a carbide can be determined by a well-known method, for example, it can be determined by image analysis. The numerical density ( / mm2) of coarse carbides is determined based on the total number of coarse carbides and the total area of ​​the observation surface. Note that, in the present embodiment, a number obtained by rounding decimals of the relevant numerical value obtained is adopted as the numerical density ( / mm2) of coarse carbides. Furthermore, the measurement of the numerical density of coarse carbides can be performed using an apparatus (SEM-EDS apparatus) in which a scanning electron microscope (SEM) is provided with a composition analysis function.For example, an automated analyzer bearing the trade name Metals Quality Analyzer, manufactured by FEI (ASPEX) Company, can be used as the SEM-EDS apparatus.

[0068] [Fn1 (= DA / DB)] - The steel material according to the present embodiment has the chemical composition mentioned above and a yield strength of 552 to less than 655 MPa, and the number density of coarse carbides in the steel material is 10 / mm2 or less, furthermore, the prior austenite grain diameter DA and the block diameter DB satisfy the following Formula (1). As a result, the steel material according to the present embodiment can achieve both a yield strength of Petition 870250083723, dated 09 / 17 / 2025, pp. 124 / 159 27 / 59 552 to less than 655 MPa, which indicates excellent low-temperature toughness in an extremely low-temperature environment. 2.5 < DA / DB < 10.0 (1) where, the diameter of the previous austenite grain in μm units is replaced by DA in Formula (1), and the block diameter in μm units is replaced by DB in Formula (1).

[0069] As described above, in the present description, the grain diameter of prior austenite in the microstructure of the steel material is also referred to as the prior grain diameter γ DA. Furthermore, in the present description, the average grain diameter of martensite blocks and the average grain diameter of bainite blocks are also collectively referred to as the block diameter DB. Note that, as described above, the microstructure of the steel material according to the present embodiment is mainly composed of tempered martensite and tempered bainite. Therefore, in the present description, martensite blocks and bainite blocks are also collectively referred to as blocks.

[0070] Fn1 (= DA / DB) is an index of low-temperature toughness in an extremely low-temperature environment in a steel material having the chemical composition mentioned above and a yield strength of 552 to less than 655 MPa and in which the number density of coarse carbides is 10⁻⁶ / mm² or less. If Fn1 is greater than 10.0, excellent low-temperature toughness in an extremely low-temperature environment will not be obtained. On the other hand, in a steel material having the chemical composition mentioned above and a yield strength of 552 to less than 655 MPa and in which the number density of coarse carbides is 10⁻⁶ / mm² or less, if Fn1 is 2.5 or greater, low-temperature toughness in an extremely low-temperature environment will be stably obtained.Therefore, the steel material according to the present embodiment has the chemical composition mentioned above and a yield strength of 552 to less than 655 MPa, and the number density of coarse carbides in the steel material is 10 / mm2 or less, furthermore, Fn1 is. Petition 870250083723, dated 09 / 17 / 2025, pages 125 / 159 28 / 59 designed to fall within the range of 2.5 to 10.0.

[0071] As described above, in a steel material having the chemical composition mentioned above, in the case where carbides do not dissolve in a quenching process that is described later and remain in the steel material, the carbides that remained in the steel material are prone to thicken and become coarse carbides in a subsequent tempering process. On the other hand, when attempting to cause carbides to dissolve sufficiently in a quenching process, the previous grain diameter γ DA and / or the block diameter DB is prone to become large. In such a case, if either the previous grain diameter γ DA or the block diameter DB becomes too large, Fn1 will be less than 2.5 or will be more than 10.0. As a result, excellent low-temperature toughness in an extremely low-temperature environment will not be stably obtained.On the other hand, if carbides are dissolved in a quenching process to be described later and the number density of coarse carbides is reduced to 10 / mm2 or less, and furthermore Fn1 satisfies a condition of being within the range of 2.5 to 10.0, excellent low-temperature toughness will be stably obtained even in an extremely low-temperature environment.

[0072] Note that, in the steel material according to the present embodiment, provided that Fn1 satisfies a condition of being within the range of 2.5 to 10.0, the prior grain diameter γ DA and the block diameter DB are not particularly limited. However, in a steel material having the chemical composition mentioned above and a yield strength of 552 to less than 655 MPa and in which the number density of coarse carbides is 10 / mm2 or less, the prior grain diameter γ DA is, for example, 10.0 to 200.0 μm. In the present embodiment, a preferred upper limit of the prior grain diameter γ DA is 180.0 μm, more preferably it is 160.0 μm, and even more preferably it is 150.0 μm. In the steel material according to the present embodiment, even if the previous grain diameter γ DA is, for example, 15.0 Petition 870250083723, dated 09 / 17 / 2025, pp. 126 / 159 29 / 59 μm or more, 20.0 μm or more, 30.0 μm or more, 40.0 μm or more, or even if it is a coarse grain diameter of 50.0 μm or more, because Fn1 satisfies a condition of being within the range of 2.5 to 10.0, both a yield strength of 552 to less than 655 MPa and excellent low-temperature toughness in an extremely low-temperature environment can be obtained.

[0073] In a steel material having the chemical composition mentioned above and a yield strength of 552 to less than 655 MPa and in which the number density of coarse carbides is 10 / mm2 or less, moreover, the block diameter DB is, for example, 2.0 to 30.0 μm. In the present embodiment, a preferred upper limit of the block diameter DB is 25.0 μm, more preferably it is 20.0 μm, and even more preferably it is 15.0 μm. In the steel material according to the present embodiment, even if the block diameter DB is, for example, 5.0 μm or more, 7.0 μm or more, 8.0 μm or more, 10.0 μm or more, or even a thick diameter of 11.0 μm or more, because Fn1 satisfies a condition of being within the range of 2.5 to 10.0, both a yield strength of 552 to less than 655 MPa and excellent low-temperature toughness in an extremely low-temperature environment can be obtained.

[0074] In the present embodiment, a preferred lower bound of Fn1 is 3.0, more preferably 3.5, and even more preferably 4.0. In the present embodiment, a preferred upper bound of Fn1 is 9.8, more preferably 9.5, and even more preferably 9.0.

[0075] In the present embodiment, Fn1 can be determined by the following method. First, the previous grain diameter γ DA of the steel material according to the present embodiment is determined. Specifically, a test specimen to measure the previous grain diameter γ DA is prepared from the steel material according to the present embodiment. If the steel material is a steel plate, a test specimen is adopted in which a face including the rolling elongation direction and the thickness direction is adopted. Petition 870250083723, dated 09 / 17 / 2025, pp. 127 / 159 30 / 59 as the observation surface is prepared from the central portion of the thickness. If the steel material is a steel pipe, a test specimen in which a face including the direction of the pipe axis and the direction of the pipe radius is adopted as the observation surface is prepared from the central portion of the wall thickness. If the steel material is a round steel bar, a test specimen that includes a position R / 2 in the center thereof and in which a face including the axial direction and the radial direction is adopted as the observation surface is prepared.

[0076] After soaking the test specimen in resin and polishing the observation surface to obtain a mirror-like surface, the test specimen is immersed for approximately 60 seconds in a solution obtained by mixing an appropriate amount of a surfactant in an aqueous solution saturated with picric acid to reveal previously established γ grain boundaries by chemical etching. Although not limited, the area of ​​the observation surface is defined to, for example, 100 mm² (10 mm χ 10 mm). Observation by optical microscope is performed to determine a grain size number G based on the intercept method according to JIS G 0551 (2020). The obtained grain size number G is converted to an average grain diameter M (μm) based on the following Formulas (2) and (3). In the present embodiment, the average grain diameter M obtained by the above method is defined as the previously established γ grain diameter DA (μm).That is, in the present embodiment, the term prior austenite grain diameter DA (μm) means the average grain diameter of prior austenite grains determined based on the intercept method according to JIS G 0551 (2020). n = 2G+3(2) M = 1000 / n1 / 2(3)

[0077] Next, the DB block diameter of the steel material according to the present embodiment is determined. Specifically, a test specimen for measuring the DB block diameter is prepared from the steel material according to the present embodiment. If the steel material is Petition 870250083723, dated 09 / 17 / 2025, pages 128 / 159 31 / 59 If the steel material is a steel plate, a test specimen having an observation surface with dimensions of 25 μm x 25 μm is prepared from the central portion of the thickness. If the steel material is a steel pipe, a test specimen having an observation surface with dimensions of 25 μm x 25 μm is prepared from the central portion of the wall thickness. If the steel material is a round steel bar, a test specimen that has an observation surface with dimensions of 25 μm x 25 μm and that includes the R / 2 position at its center is prepared.

[0078] Measurement of the observation surface of the test specimen is performed by electron backscatter diffraction (EBSD). The EBSD measurement is performed with an accelerating voltage of 20 kV with respect to visual fields of 25 μm χ 25 μm at a step of 0.1 μm. The orientation of a body-centered cubic (iron) structure is identified based on a Kikuchi diffraction pattern obtained by EBSD measurement. A crystal orientation figure is determined based on the orientation of the body-centered cubic (iron) structure. From the crystal orientation figure, regions enclosed by a contour having an orientation difference of 15° or more with adjacent crystals are identified to obtain a crystal orientation map. A region enclosed by an orientation difference of 15° or more is defined as a single block.

[0079] The average grain diameter DB (μm) with respect to the defined blocks is determined based on the area of ​​each block. Specifically, the total number of blocks in the crystal orientation map mentioned above is defined as k blocks, the area of ​​each block is defined as SBi (i is a natural number less than or equal to k) (μm2), and the area of ​​the crystal orientation map is defined as ST (μm2). In this case, the average grain diameter DB (μm) of the blocks is defined by the following formula. Petition 870250083723, dated 09 / 17 / 2025, pp. 129 / 159 32 / 59 (4). is it L ISBI? SBi \ DB = > 2x. / ---x--- 4) y π ST y Where, in Formula (4), the total number (a natural number) of blocks in the crystal orientation map is replaced by k, the area (pm2) of the i-th block is replaced by SBi, pi is replaced by π, and the total area (pm2) of the crystal orientation map is replaced by ST.

[0080] Fn1 (= DA / DB) is determined based on the previous grain diameter Y DA (pm) and block diameter DB (pm) obtained. Note that, in this embodiment, a value obtained by rounding to the first decimal place of the relevant numerical value obtained is adopted as the previous grain diameter γ DA (pm). Furthermore, in this embodiment, a value obtained by rounding to the first decimal place of the relevant numerical value obtained is adopted as the block diameter DB (pm). Furthermore, in this embodiment, a value obtained by rounding to the first decimal place of the relevant numerical value obtained is adopted as Fn1. [Toughness at low temperatures]

[0081] The steel material according to the present embodiment has the chemical composition mentioned above, the number density of coarse carbides in the steel material is 10 / mm2 or less, and Fn1 in the steel material is 2.5 to 10.0. As a result, the steel material according to the present embodiment has a yield strength of 552 to less than 655 MPa, and has excellent low-temperature toughness in an extremely low-temperature environment. In the present embodiment, excellent low-temperature toughness in an extremely low-temperature environment is defined as follows.

[0082] A Charpy impact test in accordance with ASTM E23 (2018) is performed on the steel material according to the present embodiment. Petition 870250083723, dated 09 / 17 / 2025, pp. 130 / 159 33 / 59 First, a V-notch test specimen according to ASTM E23 (2018) is prepared from the steel material according to the present embodiment. Specifically, if the steel material is a steel plate, a V-notch test specimen having a notched surface perpendicular to the thickness direction, and whose longitudinal direction is parallel to the width direction, is prepared from the central portion of the thickness. If the steel material is a steel pipe, a V-notch test specimen having a notched surface perpendicular to the pipe axis direction, and whose longitudinal direction is perpendicular to both the pipe axis direction and the pipe radius direction, is prepared from the central portion of the wall thickness.If the steel material is a round steel bar, a V-notch test specimen that has a notched surface perpendicular to the axial direction, and whose longitudinal direction is perpendicular to the axial direction and to a sectional radial direction, is prepared from a position R / 2 in a cross-section perpendicular to the axial direction.

[0083] 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 percentage of brittle fracture (%) of the respective test specimens after testing at each temperature under the above conditions is determined. The test temperatures (°C) and the percentage of brittle fracture (%) obtained are plotted to obtain an approximate curve. The temperature (°C) at which the percentage of brittle fracture becomes 50% is determined from the approximate curve obtained, and the determined test temperature is defined as a fracture appearance transition temperature vTrs (°C).In this embodiment, if the fracture appearance transition temperature vTrs is -70°C or less, it is determined that the relevant steel material has... Petition 870250083723, dated 09 / 17 / 2025, pp. 131 / 159 34 / 59 Excellent low-temperature toughness in an extremely low temperature environment. [Microstructure]

[0084] As described above, the microstructure of the steel material according to the present embodiment is mainly composed of quenched martensite and quenched bainite. Specifically, in the microstructure of the steel material according to the present embodiment, the total volumetric ratios of quenched martensite and quenched bainite is 90% or more. The remainder of the microstructure is, for example, ferrite or pearlite. In a steel material having the chemical composition mentioned above and a yield strength of 552 to less than 655 MPa, and in which the number density of coarse carbides is 10 / mm2 or less and Fn1 is 2.5 to 10.0, when the steel material has a microstructure in which the total volumetric ratios of quenched martensite and quenched bainite is 90% or more, the steel material has excellent low-temperature toughness in an extremely low-temperature environment.Therefore, in the present embodiment, if a steel material having the chemical composition mentioned above and a yield strength of 552 to less than 655 MPa, and in which the number density of coarse carbides is 10 / mm2 or less and Fn1 is 2.5 to 10.0 has excellent low-temperature toughness in an extremely low-temperature environment, it is determined that the total volumetric ratios of quenched martensite and quenched bainite in the microstructure of the steel material is 90% or more.

[0085] Note that the following method can be used when determining the volumetric ratio of tempered martensite and tempered bainite by observation. First, a test specimen having a specified 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 tube Petition 870250083723, dated 09 / 17 / 2025, pp. 132 / 159 35 / 59 of steel, a test specimen in which a face including the direction of the tube axis and the direction of the tube radius 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 that 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.

[0086] 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 chemical etching reagent to reveal the microstructure by chemical etching. The etched observation surface is observed by means of a secondary electron image obtained using a SEM, and the observation is performed in 10 visual fields. The area of ​​each visual field is, for example, 0.01 mm2 (1000* 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 this 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. [Production method]

[0087] 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 hollow tube (preparation process), and a process for subjecting the hollow tube to quenching and tempering to form a seamless steel tube (quenching process and tempering process). Petition 870250083723, dated 09 / 17 / 2025, pp. 133 / 159 36 / 59 tempering). 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 hereafter. [Preparation process]

[0088] In the preparation process, an intermediate steel material having the chemical composition described above is prepared. A method for producing the intermediate steel material is not particularly limited as long as the intermediate steel material has the chemical composition described above. Here, the intermediate steel material is a sheet-shaped steel material in the case where the final product is a steel sheet, a hollow tube in the case where the final product is a steel tube, and a steel material in which a cross-section perpendicular to the axial direction is circular in the case where the final product is a round steel bar.

[0089] The preparation process may include a process of preparing a starting material (starting material preparation process), and a process of subjecting the starting material to hot working to produce an intermediate steel material (hot working process). Hereafter, a case in which the preparation process includes a starting material preparation process and a hot working process is described in detail. [Starter material preparation process]

[0090] In the process of preparing starting material, a starting material is produced using a molten steel having the chemical composition described above. The method for producing the starting material is not particularly limited, and it is sufficient to use a well-known method. Specifically, a casting (a plate, a block, or a billet) can be produced by a continuous casting process using the molten steel. An ingot can also be produced by an ingot making process using the molten steel. As needed, the plate, block, or ingot can be roughed to produce a billet. A starting material (a Petition 870250083723, dated 09 / 17 / 2025, pp. 134 / 159 37 / 59 plate, block, or billet) is produced by the above process. [Hot work process]

[0091] 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 pipe. First, a billet is heated in a heating furnace. Although not particularly limited, the heating temperature is, for example, 1100 to 1300°C. After extracting the billet from the heating furnace, the billet is subjected to hot working to produce a hollow pipe (seamless steel pipe). The hot working method is not particularly limited, and it is sufficient to use a well-known method.

[0092] For example, the Mannesmann process can be carried out as hot working to produce a hollow tube. 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 tube. The cumulative area reduction in the hot working process is, for example, 20 to 70%.

[0093] A hollow tube can be produced from the billet by performing another hot working method. For example, in the case where the steel material is a thick-walled steel material of short length such as a coupling, a hollow tube can be produced by forging using the Ehrhardt process or similar. A hollow tube is produced by the above process. Although not particularly limited, the wall thickness of the hollow tube is, for example, 9 to 60 mm.

[0094] 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, Petition 870250083723, dated 09 / 17 / 2025, pages 135 / 159 38 / 59 1100 to 1300°C. After being extracted from the heating furnace, the starting material 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 performed using a roughing mill or hot rolling performed 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.

[0095] 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. After being extracted from the heating furnace, the starting material is subjected to hot rolling using a roughing mill and a continuous mill to produce an intermediate steel material having a steel sheet shape.

[0096] Hollow tubing produced by hot working can be air-cooled (as rolled). Hollow tubing produced by hot working can be directly quenched after hot working without being cooled to normal temperature, or it can be quenched after undergoing supplementary heating (reheating) after hot working.

[0097] In the case of performing direct quenching after hot working, or performing quenching after supplementary heating, cooling can be interrupted in the middle of the quenching process or slow cooling can be performed. In this case, the occurrence of quenching cracks in the hollow tube can be suppressed. Furthermore, in the case of performing direct quenching after hot working, or performing quenching after supplementary 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 tube is eliminated. Petition 870250083723, dated 09 / 17 / 2025, pp. 136 / 159 39 / 59

[0098] As described above, an intermediate steel material is prepared in the preparation process. The intermediate steel material may be produced by the preferred process mentioned above, or it may be an intermediate steel material produced by third parties, or an intermediate steel material may be prepared that was produced in a plant other than the plant in which a quenching process and a tempering process to be described later are carried out, or that was produced in different works. Hereinafter, the quenching process is described in detail. [Tempering process]

[0099] In the quenching process, the prepared intermediate steel material (hollow tube) is subjected to quenching. As used in the present description, the term quenching means rapidly cooling the intermediate steel material to a temperature not lower than point A3. Here, in the present description, the temperature of the intermediate steel material immediately before rapid cooling during quenching is also referred to as the quenching temperature. Preferably, in the quenching process according to the present embodiment, after two-stage heating has been carried out, the intermediate steel material is rapidly cooled. That is, the quenching process according to the present embodiment includes a first heating process, a second heating process, and a rapid cooling process. Hereafter, each process is described in detail. [First heating process]

[00100] Preferably, in the first heating process according to the present embodiment, the prepared intermediate steel material (hollow tube) is heated to a heating temperature T1 (°C), and is held at the heating temperature T1 for a heating time t1 (mins). If the heating temperature T1 is too low, in some cases the austenite transformation will not be completed. In such a case, the steel material produced will not have a yield strength of 552 to less than 655 MPa. Petition 870250083723, dated 09 / 17 / 2025, pp. 137 / 159 40 / 59 On the other hand, if the heating temperature T1 is too high, in some cases the previous grain diameter γ DA will thicken and Fn1 will be too large. In such a case, the steel material produced will not exhibit excellent low-temperature toughness in an extremely low-temperature environment. Therefore, preferably the heating temperature T1 (°C) in the first heating process is set within the range of 900 to 980°C.

[00101] Furthermore, if the heating time t1 is too short, in some cases the austenite transformation will not be completed. In such a case, the steel material produced will not have a yield strength of 552 to less than 655 MPa. On the other hand, if the heating time t1 is too long, in some cases the previous grain diameter γ DA will thicken and Fn1 will be too large. In such a case, the steel material produced will not exhibit excellent low-temperature toughness in an extremely low-temperature environment. Therefore, preferably the heating time t1 (mins) in the first heating process is set within the range of 5 to 15 minutes. [Second heating process]

[00102] Preferably, in the second heating process according to the present embodiment, the intermediate steel material (hollow tube) heated in the first heating process is heated to a heating temperature T2 (°C), and is held at the heating temperature T2 for a heating time t2 (mins). If the heating temperature T2 is too low, in some cases carbides will not dissolve sufficiently. In such a case, the number density of coarse carbides in the produced steel material will be too high and the produced steel material will not exhibit excellent low-temperature toughness in an extremely low-temperature environment. On the other hand, if the heating temperature T2 is too high, in some cases the previous grain diameter γ DA will thicken and Fn1 will be too large. In such a case, the produced steel material will not exhibit excellent low-temperature toughness in an extremely low-temperature environment. Therefore, Petition 870250083723, dated 09 / 17 / 2025, pp. 138 / 159 41 / 59 Preferably the heating temperature T2 (°C) in the second heating process is set within the range of 1000 to 1100°C.

[00103] Furthermore, if the heating time t2 is too short, in some cases carbides will not dissolve sufficiently. In such a case, the number density of coarse carbides in the produced steel material will be too high, and the produced steel material will not exhibit excellent low-temperature toughness in an extremely low-temperature environment. On the other hand, if the heating time t2 is too long, in some cases the previous grain diameter γ DA will thicken, and Fn1 will be too large. In such a case, the produced steel material will not exhibit excellent low-temperature toughness in an extremely low-temperature environment. Therefore, preferably the heating time t2 (mins) in the second heating process is set within the range of 3 to 10 minutes. [Rapid cooling process]

[00104] In the rapid cooling process, the intermediate steel material (hollow tube) heated in the second heating process is rapidly cooled. In the rapid cooling process, the intermediate steel material (hollow tube) is continuously cooled to continuously decrease the surface temperature of the hollow tube. 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 tube by immersing the hollow tube in a water bath, or a method that cools the hollow tube in an accelerated manner by water shower cooling or mist cooling.

[00105] If the cooling rate during quenching is too slow, the microstructure will not become a microstructure mainly composed of quenched martensite and quenched bainite after the tempering process to be described later, and the mechanical property defined in the present embodiment will not be obtained. Here, in the rapid cooling process according to the present embodiment, the average cooling rate Petition 870250083723, dated 09 / 17 / 2025, pages 139 / 159 42 / 59 when the surface temperature of the intermediate steel material (hollow tube) is within the range of 800 to 500°C during quenching is defined as the CR800-500 quenching cooling rate. Specifically, the CR800-500 quenching cooling rate is determined based on a temperature measured in a region that is cooled most slowly within a cross-section of the intermediate steel material being quenched (e.g., in the case of forcibly cooling both surfaces, the cooling rate is measured in the central portion of the thickness of the intermediate steel material).

[00106] In the rapid cooling process according to the present embodiment, a preferred cooling rate during CR800-500 quenching is 60°C / min or more. A more preferred lower limit of the cooling rate during CR800-500 quenching is 300°C / min, and even more preferably it is 600°C / min. Although an upper limit of the cooling rate during CR800-500 quenching is not particularly defined, the upper limit is, for example, 6000°C / min. The quenching process according to the present embodiment can be carried out by the above process. Hereafter, the tempering process is described in detail. [Annealing process]

[00107] The tempering process is carried out by tempering after performing the quenching mentioned above. In the present description, the term tempering means reheating the intermediate steel material after quenching to a temperature that is equal to or less than the Ac1 point, and maintaining the intermediate steel material at that temperature. The holding temperature in the tempering process is appropriately adjusted according to the chemical composition of the steel material and the yield strength to be obtained. That is, with respect to an intermediate steel material (hollow tube) having the chemical composition of the present embodiment, the holding temperature is adjusted to adjust the yield strength of the steel material so as to be within the range of 80 to less than 95 ksi (552 to less than 655 MPa). Petition 870250083723, dated 09 / 17 / 2025, pages 140 / 159 43 / 59 Here, the holding temperature corresponds to the furnace temperature when the intermediate steel material after quenching is heated and held at the relevant temperature. The term holding time means the period of time from when the temperature of the intermediate steel material reaches a predetermined holding temperature until the steel material is extracted from the heat treatment furnace.

[00108] The holding temperature is appropriately adjusted according to the chemical composition of the steel material and the yield strength to be obtained. That is, with respect to an intermediate steel material (hollow tube) having the chemical composition of the present embodiment, the holding temperature is adjusted to adjust the yield strength of the steel material so as to be within the range of 552 to less than 655 MPa. In the tempering process according to the present embodiment, a preferred holding temperature is 640 to 720°C.

[00109] If the holding time is too short, in some cases a microstructure that is mainly composed of tempered martensite and tempered bainite will not be obtained. On the other hand, if the holding time is too long, the advantageous effect mentioned above will be saturated. Therefore, in the tempering process of the present embodiment, preferably the holding time is defined within a range of 20 to 180 minutes. A more preferable lower limit of the holding time is 30 minutes. A more preferable upper limit of the holding time is 150 minutes, and even more preferably it is 120 minutes.

[00110] The steel material according to the present embodiment can be produced by the production method described above. Note that, in the above description of the production method, a method for producing a steel pipe 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 process of Petition 870250083723, dated 09 / 17 / 2025, pp. 141 / 159 44 / 59 tempering, and a tempering process, similarly to the production method described above. Furthermore, the production method described above is an example, and steel material can also be produced by other production methods.

[00111] Hereafter, the present invention is described more specifically by way of examples. Note that the conditions adopted in the examples described herein are examples of conditions adopted to confirm the feasibility and advantageous effects of the steel material according to the present embodiment. That is, the steel material according to the present embodiment is not limited to the examples described herein. Examples

[00112] Cast steels having the chemical compositions shown in Table 2-1 and Table 2-2, each weighing 180 kg, were produced. Note that the symbol - in Table 2-1 and Table 2-2 means that the content of the corresponding element was at the level of an impurity. Specifically, the symbol - means that the V content, Nb content, Ti content, W content, Co content, Ni content, and Cu content of Test No. 1 were each 0% when rounded to the second decimal place. Additionally, the symbol - means that the Sn content of Test No. 1 was 0% when rounded to the third decimal place. Furthermore, the symbol - means that the Zr content, Ca content, Mg content, B content, and REM content of Test No. 1 were each 0% when rounded to the fourth decimal place.

[00113] [Table 2-1] TABLE 2-1 Test Number Chemical Composition (Unit % by mass; remainder is Fe and impurities) C Si Mn PS Cr Mo Al NV Nb Ti 1 0.35 0.16 1.04 0.005 0.0003 0.36 0.12 0.047 0.0036 - - - 2 0.27 0.19 1.32 0.008 0.0007 0.43 0.09 0.032 0.0076 - - - 3 0.30 0.90 1.22 0.009 0.0003 0.29 0.24 0.033 0.0069 - - - 4 0.30 0.10 1.24 0.007 0.0018 0.26 0.11 0.054 0.0050 - - - Petition 870250083723, dated 09 / 17 / 2025, pp. 142 / 159 45 / 59 5 0,29 0,45 1,39 0,007 0,0017 0,22 0,07 0,048 0,0025 - - - 6 0,34 0,25 1,08 0,009 0,0014 0,36 0,16 0,036 0,0029 - - - 7 0,27 0,26 1,39 0,013 0,0002 0,16 0,08 0,043 0,0073 - - - 8 0,29 0,31 1,25 0,007 0,0020 0,46 0,09 0,051 0,0077 - - - 9 0,29 0,20 1,19 0,003 0,0018 0,49 0,13 0,030 0,0068 - - - 10 0,27 0,10 1,34 0,007 0,0009 0,15 0,18 0,036 0,0047 - - - 11 0,31 0,20 1,34 0,007 0,0007 0,34 0,06 0,026 0,0080 - - - 12 0,33 0,41 1,07 0,003 0,0004 0,42 0,24 0,031 0,0034 - - - 13 0,27 0,26 1,39 0,003 0,0002 0,16 0,08 0,043 0,0073 - - - 14 0,26 0,33 1,28 0,003 0,0001 0,30 0,14 0,051 0,0049 - - - 15 0,34 0,39 1,15 0,006 0,0013 0,19 0,10 0,041 0,0024 0,11 - - 16 0,28 0,45 1,32 0,011 0,0011 0,49 0,12 0,054 0,0044 - 0,03 - 17 0,29 0,16 1,23 0,011 0,0006 0,37 0,07 0,048 0,0059 - - 0,02 18 0,29 0,29 1,25 0,012 0,0006 0,33 0,07 0,032 0,0025 - - - 19 0,28 0,21 1,37 0,010 0,0015 0,37 0,10 0,045 0,0052 - - - 20 0,29 0,16 1,24 0,012 0,0011 0,29 0,07 0,034 0,0050 - - - 21 0,28 0,26 1,33 0,012 0,0008 0,32 0.22 0.034 0.0037 - - - 22 0.31 0.21 1.16 0.006 0.0009 0.37 0.14 0.052 0.0053 - - - 23 0.30 0.40 1.17 0.012 0.0002 0.16 0.11 0.058 0.0066 - - - 24 0.32 0.46 1.24 0.013 0.0004 0.38 0.17 0.030 0.0063 - - - 25 0.28 0.27 1.22 0.009 0.0015 0.20 0.15 0.059 0.0015 - - - 26 0.31 0.12 1.22 0.009 0.0019 0.23 0.22 0.029 0.0041 - - - 27 0.30 0.31 1.20 0.010 0.0017 0.23 0.07 0.056 0.0056 - - - 28 0.28 0.41 1.29 0.010 0.0008 0.19 0.07 0.052 0.0062 - 0.02 0.01 29 0.28 0.28 1.36 0.007 0.0004 0.26 0.14 0.050 0.0058 0.03 - - 30 0.33 0.21 1.05 0.005 0.0013 0.36 0.15 0.040 0.0026 - - - 31 0.29 0.25 1.28 0.006 0.0005 0.34 0.21 0.059 0.0073 - - - 32 0.29 0.35 1.28 0.006 0.0018 0.34 0.21 0.033 0.0075 - - - 33 0.31 0.23 1.15 0.003 0.0019 0.17 0.19 0.001 0.0055 - - -, Petition 870250083723, dated 09 / 17 / 2025, pp. 143 / 159 46 / 59 34 0.29 0.36 1.17 0.005 0.0007 0.29 0.11 0.045 0.0068 - - - 35 0.27 0.12 1.36 0.009 0.0016 0.47 0.13 0.042 0.0055 0.50 - - 36 0.31 0.37 1.28 0.005 0.0018 0.18 0.08 0.045 0.0079 - - 0.04 37 0.29 0.43 1.22 0.009 0.0001 0.25 0.08 0.056 0.0030 - 0.02 - 38 0.33 0.29 1.11 0.007 0.0001 0.32 0.09 0.049 0.0033 - - - 39 0.24 0.30 1.39 0.011 0.0020 0.22 0.07 0.027 0.0071 - - - 40 0.27 0.18 1.69 0.004 0.0007 0.19 0.11 0.026 0.0061 - - - 41 0.27 0.29 1.31 0.021 0.0009 0.40 0.08 0.029 0.0078 - - - 42 0.29 0.16 1.23 0.011 0.0056 0.37 0.07 0.048 0.0059 - - - 43 0.35 0.12 1.37 0.005 0.0008 0.08 0.08 0.038 0.0034 - - - 44 0.27 0.14 1.09 0.004 0.0002 0.80 0.16 0.028 0.0022 - - - 45 0.28 0.27 1.11 0.009 0.0005 0.21 0.50 0.054 0.0066 - - - 46 0.31 0.45 1.15 0.010 0.0012 0.44 0.18 0.054 0.0025 - - - 47 0.32 0.31 1.17 0.007 0.0011 0.38 0.19 0.029 0.0040 - - - 48 0.28 0.24 1.16 0.011 0.0007 0.43 0.19 0.028 0.0037 - - - 49 0.33 0.16 1.35 0.004 0.0002 0.24 0.20 0.028 0.0022 - - -

[00114] [Table 2-2] TABLE 2-2 Test Number Chemical Composition (unit of % by mass; remainder is Fe and impurities) W Zr Co Ni Cu Sn Ca Mg B REM 1 - - - - - - - - - - 2 - - - - - - - - - - 3 - - - - - - - - - - 4 - - - - - - - - - - 5 - - - - - - - - - - 6 - - - - - - - - - - 7 - - - - - - - - - - 8 - - - - - - - - - - 9 - - - - - - - - - - Petition 870250083723, dated 09 / 17 / 2025, pages 144 / 159 47 / 59 10 - - - - - - - - - - 11 - - - - - - - - - - 12 - - - - - - - - - - 13 - - - - - - - - - - 14 - - - - - - - - - - 15 - - - - - - - - - - 16 - - - - - - - - - - 17 - - - - - - - - - - 18 0.28 - - - - - - - - - 19 - 0.0020 - - - - - - - - 20 - - 0.06 - - - - - - - 21 - - - 0.25 - - - - - - 22 - - - - 0.08 - - - - - 23 - - - - - 0.056 - - - - 24 - - - - - - 0.0030 - - - 25 - - - - - - - 0.0026 - - 26 - - - - - - - - 0.0010 - 27 - - - - - - - - - 0.0011 28 - - - - - - - - - - 29 - 0.0050 - - - - - - - - 30 - - 0.25 0.08 - - - - - - 31 - - 0.40 - 0.18 - - - - - 32 - - - - 0.45 0.018 - - - - 33 - - - 0.09 - - 0.0011 - - - 34 - - - - - 0.091 - - 0.0006 - 35 - - 0.20 - - - - - - - 36 - - - - - - - 0.0032 - - 37 0.41 - - - - - - - 0.0008 - 38 - - -0.14 0.03 -0.0017 0.0014 -0.0040 Petition 870250083723, dated 09 / 17 / 2025, pp. 145 / 159 48 / 59 39 - - - - - - - - - - 40 - - - - - - - - - - 41 - - - - - - - - - - 42 - - - - - - - - - - 43 - - - - - - - - - - 44 - - - - - - - - - - 45 - - - - - - - - - - 46 - - - - - - - - - - 47 - - - - - - - - - - 48 - - - - - - - - - - 49 - - - - - - - - - -

[00115] The cast steel from each test number was used to produce a round billet by a continuous casting process. The round billet produced from each test number was heated and subjected to hot working. Specifically, the round billet from each test number was subjected to hot rolling by the Mannesmann-mandrel process as hot working to produce a hollow shell (seamless steel tube) from each test number.

[00116] The hollow shell obtained from each test number was subjected to quenching and tempering. At this point, the heating in the quenching process was carried out by performing heating in two stages through a first heating process and a second heating process. Specifically, the hollow shell from each test number was subjected to a first heating process in which heating was carried out at a heating temperature T1 (°C) for a heating time t1 (min), each of which is shown in the Quenching Process column in Table 3, and then the hollow shell was subjected to a second heating process in which heating was carried out at a heating temperature T2 (°C) for a heating time t2 (min), each of which is shown in the Process column. Petition 870250083723, dated 09 / 17 / 2025, pages 146 / 159 49 / 59 of Tempering in Table 3. The heated hollow shell of each test number was tempered by water quenching. At this time, the cooling rate during the CR800-500 tempering of the hollow shell of each test number satisfied a condition of being within the range of 60 to 6000°C / min.

[00117] [Table 3] TABLE 3 Test Number Tempering Process Annealing Process Heating Temperature T1 (°C) Heating Time t1 (min) Heating Temperature T2 (°C) Heating Time t2 (min) Holding Temperature (°C) Holding Time (min) 1 950 10 1000 5 700 30 2 950 10 1000 5 700 30 3 950 10 1000 5 700 30 4 950 10 1050 5 680 30 5 950 10 1100 5 660 30 6 950 10 1000 5 700 30 7 950 10 1000 5 700 30 8 950 10 1000 5 680 30 9 950 10 1000 5 700 30 10 950 10 1000 5 680 30 11 950 10 1000 5 680 30 12 950 10 1050 5 700 30 13 950 10 1050 5 700 30 14 950 10 1050 5 700 30 15 950 10 1000 5 700 30 16 950 10 1000 5 700 30 17 950 10 1100 5 700 30 18 950 10 1000 5 700 30 19 950 10 1000 5 700 30 Petition 870250083723, dated 09 / 17 / 2025, pp. 147 / 159 50 / 59 20 950 10 1000 5 700 30 21 950 10 1000 5 700 30 22 950 10 1050 5 700 30 23 950 10 1000 5 700 30 24 950 10 1050 5 700 30 25 950 10 1000 5 700 30 26 950 10 1000 5 700 30 27 950 10 1050 5 700 30 28 950 10 1000 5 680 30 29 950 10 1000 5 680 30 30 950 10 1000 5 700 30 31 950 10 1000 5 700 30 32 950 10 1000 5 700 30 33 950 10 1000 5 700 30 34 950 10 1050 5 700 30 35 950 10 1050 5 680 30 36 950 10 1000 5 680 30 37 950 10 1000 5 680 30 38 950 10 1050 5 700 30 39 950 10 1000 5 650 30 40 950 10 1050 5 640 30 41 950 10 1050 5 660 30 42 950 10 1000 5 680 30 43 950 10 1000 5 640 30 44 950 10 1000 5 700 30 45 950 10 1000 5 700 30 46 950 10 1000 5 620 30 47 950 10 - - 680 30 48 950 10 1150 5 660 30 Petition 870250083723, dated 09 / 17 / 2025, pp. 148 / 159 51 / 59 49 950 10 1050 30 660 30

[00118] In addition, the hollow shell obtained from each test number was subjected to tempering. Specifically, the hollow shell from each test number was subjected to tempering in which the hollow shell was held at a holding temperature (°C) for a holding time (min) which are each shown in the Tempering Process column in Table 3. A seamless steel tube from each test number was obtained by the above production process. [Assessment Tests]

[00119] The seamless steel tube of each test number after the tempering described above was subjected to a tensile test, a coarse carbide number density measurement test, an Fn1 measurement test, and a Charpy impact test. [Traction Test]

[00120] The seamless steel pipe of each test number was subjected to a tensile test by a method in accordance with ASTM E8 / E8M (2021). Specifically, a round bar specimen having a parallel portion diameter of 6 mm and a gauge length of 30 mm was prepared from the center portion of the wall thickness of the seamless steel pipe of each test number. The axial direction of the round bar specimen was parallel to the axial direction of the seamless steel pipe. The tensile test was performed in the atmosphere at normal temperature (25°C) using the prepared round bar tensile test specimens, and the yield strength (MPa) of the seamless steel pipe of each test number was determined. Note that, in the present examples, the 0.2% offset proof stress (MPa) obtained in the tensile test was defined as the yield strength. The yield strength obtained from each test number is shown in Table 4 as YS (MPa).

[00121] [Table 4] Petition 870250083723, dated 09 / 17 / 2025, pp. 149 / 159 52 / 59 TABLE 4 Test Number YS (MPa) Coarse Carbon Numerical Density ( / mm2) DA (μm) DB (μm) Fn1 (DA / DB) vTrs (°C) 1 653 5 49.9 9.0 5.5 -85 2 591 0 49.1 13.9 3.5 -86 3 654 0 59.9 12.0 5.0 -71 4 644 0 67.3 12.2 5.5 -74 5 646 0 69.1 12.8 5.4 -82 6 654 4 48.4 9.5 5.1 -75 7 569 0 46.9 14.0 3.4 -81 8 640 0 46.4 12.7 3.7 -77 9 627 0 57.7 12.8 4.5 -82 10 647 0 58.1 13.6 4.3 -71 11 629 0 47.8 11.6 4.1 -96 12 652 2 63.7 9.9 6.4 -88 13 599 0 141.6 14.5 9.8 -72 14 573 0 146.8 14.8 9.9 -72 15 635 3 52.3 9.5 5.5 -89 16 612 0 52.9 12.9 4.1 -85 17 600 0 67.0 12.3 5.4 -81 18 592 0 59.9 12.8 4.7 -78 19 594 0 48.4 13.3 3.6 -88 20 593 0 46.1 12.6 3.7 -76 21 636 0 55.9 13.2 4.2 -72 22 639 1 64.6 11.3 5.7 -81 23 612 1 53.7 11.8 4.6 -70 24 647 1 61.7 10.7 5.8 -80 25 614 0 48.4 13.1 3.7 -70 Petition 870250083723, dated 09 / 17 / 2025, pp. 150 / 159 53 / 59 26 653 1 54,4 11,2 4,9 -72 27 598 0 62,9 12,0 5,2 -76 28 614 0 55,5 13,3 4,2 -72 29 641 0 55,8 12,9 4,3 -83 30 653 3 45,9 10,2 4,5 -75 31 645 0 48,4 12,5 3,9 -82 32 645 0 57,0 12,5 4,6 -79 33 647 1 51,0 11,6 4,4 -73 34 614 0 69,2 12,5 5,5 -78 35 642 0 66,1 13,5 4,9 -79 36 634 0 54,7 11,5 4,8 -86 37 632 0 59,3 12,3 4,8 -73 38 631 2 62,5 10,3 6,1 -85 39 586 0 48,2 20,5 2,4 -65 40 593 0 60,0 15,5 3,9 -65 41 590 0 60,3 13,7 4,4 -58 42 576 0 50,8 12,3 4,1 -64 43 558 4 59,0 9,1 6,5 -59 44 785 0 54,2 10,0 5,4 -55 45 585 0 53,0 11,2 4,7 -60 46 686 0 55,5 11,6 4,8 -66 47 588 11 49,4 11,7 4,2 -65 48 575 0 239,0 21,0 11,4 -55 49 575 0 128,1 10,0 12,8 -60 [Coarse Carbide Numerical Density Measurement Test]

[00122] The seamless steel tube of each test number was subjected to a coarse carbide numerical density measurement test using the method described above. Specifically, a test specimen in which one face including the tube axis direction and the tube radius direction was Petition 870250083723, dated 09 / 17 / 2025, pages 151 / 159 The observation surface adopted as the observation surface was prepared from the central portion of the wall thickness of the seamless steel tube of each test specimen. After polishing the observation surface of each of the prepared test specimens to obtain a mirror-like surface, on an observation surface with an area of ​​300 mm² (20 mm χ 15 mm), particles that were identified based on contrast by the method described above were subjected to EDS analysis by the method described above, and carbides were thus identified. Note that, in the present examples, similarly to the method described above, particles in which C was detected and in which the Fe content was 50% by mass or more were identified as carbides.

[00123] Among the carbides identified, those carbides having an equivalent circular diameter of 2 μm or more were identified using image analysis. The numerical density of coarse carbides ( / mm2) was determined based on the total number of coarse carbides and the total area of ​​the observation surface. The numerical density of coarse carbides obtained ( / mm2) for each test number is shown in Table 4. [Fn1 measurement test]

[00124] The prior grain diameter γ DA (μm), the block diameter DB (μm), and Fn1 (= DA / DB) of the seamless steel pipe of each test number were determined using the methods described above. Specifically, a test specimen to measure the prior grain diameter γ DA, in which a face including the pipe axis direction and the pipe radius direction was adopted as the observation surface, was prepared from the central portion of the wall thickness of the seamless steel pipe of each test number. After embedding each prepared test specimen in resin and polishing the observation surface to obtain a mirror surface, the test specimen was immersed for approximately 60 seconds in a solution obtained by mixing an appropriate amount of a surfactant in an aqueous solution saturated with picric acid, in order to reveal prior grain boundaries by chemical etching. An etched observation surface of 100 mm2 (10 mm χ 10 mm) was Petition 870250083723, dated 09 / 17 / 2025, pages 152 / 159 55 / 59 observed using an optical microscope, and the grain size number was determined based on the intercept method according to JIS G 0551 (2020). The determined grain size number was used to determine the grain diameter of previous austenite (previous grain diameter γ DA) by the method described above.

[00125] Next, a test specimen for measuring the DB block diameter having a 25 μm x 25 μm observation surface was prepared from the central portion of the wall thickness of the seamless steel tube of each test specimen. The observation surface of each test specimen was subjected to EBSD measurement. The EBSD measurement was performed with an accelerating voltage of 20 kV with respect to 25 μm x 25 μm visual fields in a 0.1 μm step. A crystal orientation figure was determined based on the obtained Kikuchi diffraction pattern, and regions enclosed by a contour having an orientation difference of 15° or more with adjacent crystals were identified and a crystal orientation map was obtained. A region enclosed by an orientation difference of 15° or more was defined as a single block. For the defined blocks, the method described above was used to determine the average grain diameter (block diameter DB) of the blocks.

[00126] The previous grain diameter γ DA (μm) obtained from each test number is shown in the DA (μm) column in Table 4. The block diameter DB (μm) obtained from each test number is shown in the DB (μm) column in Table 4. Fn1 (= DA / DB) was determined based on the previous grain diameter γ DA (μm) and block diameter DB (μm) obtained from each test number. The Fn1 obtained from each test number is shown in Table 4. [Charpy impact test]

[00127] The seamless steel pipe of each test number was subjected to a Charpy impact test in accordance with ASTM E23 (2018). Specifically, V-notch test specimens that had a notched surface perpendicular to the direction of the pipe axis, and whose direction Petition 870250083723, dated 09 / 17 / 2025, pages 153 / 159 56 / 59 longitudinal sections, perpendicular to the tube axis direction and the tube radius direction, were prepared from the central portion of the seamless steel tube wall thickness of each test number. A Charpy impact test according to ASTM E23 (2018) was performed on the prepared V-notch test specimens. The test temperature range was defined as 120 to 20°C, and the test temperatures were set at eight levels that 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 was performed using two test specimens for each test temperature. The percentage of brittle fracture (%) of the respective test specimens after the test at each temperature was determined.The temperature (°C) at which the percentage of brittle fracture became 50% was determined from an approximate curve obtained by plotting the test temperatures (°C) and the percentage of brittle fracture obtained (%), and the fracture appearance transition temperature vTrs (°C) was thus obtained. The fracture appearance transition temperature vTrs obtained (°C) for each test number is shown in the vTrs (°C) column in Table 4. [Evaluation Results]

[00128] Referring to Table 2-1, Table 2-2, Table 3, and Table 4, the chemical compositions of the seamless steel tubes from Tests Nos. 1 to 38 were appropriate, and the production methods of Tests Nos. 1 to 38 also satisfied the preferred conditions described in the present description. As a result, for each of these seamless steel tubes, the yield strength was 552 to less than 655 MPa, the number density of coarse carbides was 10 / mm2 or less, and Fn1 satisfied the condition of being within the range of 2.5 to 10.0. As a result, for each of these seamless steel tubes, the fracture appearance transition temperature vTrs was -70°C or less, and thus each of these seamless steel tubes had excellent low-temperature toughness even in an extremely low-temperature environment. Note that, for each of these seamless steel tubes, the total volumetric ratios of tempered martensite and tempered bainite were determined to be... Petition 870250083723, dated 09 / 17 / 2025, pp. 154 / 159 57 / 59 90% or more.

[00129] On the other hand, in the seamless steel pipe of Test No. 39, the C content was very low, and Fn1 was very low. As a result, for this seamless steel pipe the fracture appearance transition temperature vTrs was more than -70°C, and thus the seamless steel pipe did not have excellent low-temperature toughness in an extremely low-temperature environment.

[00130] In the seamless steel pipe from Test No. 40, the Mn content was very high. As a result, for this seamless steel pipe the fracture appearance transition temperature vTrs was more than -70°C, and thus the seamless steel pipe did not have excellent low-temperature toughness in an extremely low-temperature environment.

[00131] In the seamless steel pipe from Test No. 41, the P content was very high. As a result, for this seamless steel pipe the fracture appearance transition temperature vTrs was more than -70°C, and thus the seamless steel pipe did not have excellent low-temperature toughness in an extremely low-temperature environment.

[00132] In the seamless steel pipe from Test No. 42, the S content was very high. As a result, for this seamless steel pipe the fracture appearance transition temperature vTrs was more than -70°C, and thus the seamless steel pipe did not have excellent low-temperature toughness in an extremely low-temperature environment.

[00133] In the seamless steel pipe from Test No. 43, the Cr content was very low. As a result, for this seamless steel pipe the fracture appearance transition temperature vTrs was more than -70°C, and thus the seamless steel pipe did not have excellent low-temperature toughness in an extremely low-temperature environment.

[00134] In the seamless steel pipe from Test No. 44, the Cr content was very high. As a result, the yield strength of this seamless steel pipe was 655 MPa or more, and thus the desired yield strength. Petition 870250083723, dated 09 / 17 / 2025, pp. 155 / 159 58 / 59 was not obtained. Consequently, for this seamless steel pipe, the fracture appearance transition temperature vTrs was greater than -70°C, and thus the seamless steel pipe did not have excellent low-temperature toughness in an extremely low-temperature environment.

[00135] In the seamless steel pipe from Test No. 45, the Mo content was very high. As a result, for this seamless steel pipe the fracture appearance transition temperature vTrs was more than -70°C, and thus the seamless steel pipe did not have excellent low-temperature toughness in an extremely low-temperature environment.

[00136] For the seamless steel pipe of Test No. 46, the holding temperature in the tempering process was too low. As a result, the yield strength of this seamless steel pipe was 655 MPa or more, and thus the desired yield strength was not obtained. Consequently, for this seamless steel pipe the fracture appearance transition temperature vTrs was more than -70°C, and thus the seamless steel pipe did not have excellent low-temperature toughness in an extremely low-temperature environment.

[00137] For seamless steel pipe from Test No. 47, the second heating process was not carried out in the quenching process. As a result, in this seamless steel pipe the numerical density of coarse carbides was more than 10 / mm2. Consequently, for this seamless steel pipe the fracture appearance transition temperature vTrs was more than 70°C, and thus the seamless steel pipe did not have excellent low-temperature toughness in an extremely low-temperature environment.

[00138] For seamless steel pipe from Test No. 48, the heating temperature T2 in the second heating process of the quenching process was too high. As a result, in this seamless steel pipe, Fn1 was greater than 10.0. Consequently, for this seamless steel pipe, the fracture appearance transition temperature vTrs was greater than 70°C, and thus the seamless steel pipe did not have excellent low-temperature toughness. Petition 870250083723, dated 09 / 17 / 2025, pp. 156 / 159 59 / 59 temperature in an extremely low temperature environment.

[00139] For seamless steel pipe from Test No. 49, the heating time t2 in the second heating process of the quenching process was too long. As a result, in this seamless steel pipe, Fn1 was more than 10.0. Consequently, for this seamless steel pipe the fracture appearance transition temperature vTrs was more than -70°C, and thus the seamless steel pipe did not have excellent low-temperature toughness in an extremely low-temperature environment.

[00140] 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 core of the invention.

Claims

1. Steel material, CHARACTERIZED by the composition of the following by mass: C: 0.26 to 0.35%, Si: 0.10 to 1.00%, Mn: 1.00 to 1.40%, P: 0.015% or less, S: 0.0020% or less, Cr: 0.15 to 0.50%, Mo: 0.05 to 0.25%, Al sol.: 0.001 to 0.070%, N: 0.0010 to 0.0080%, V: 0 to 0.60%, Nb: 0 to 0.03%, Ti: 0 to 0.05%, W: 0 to 0.50%, Zr: 0 to 0.0050%, Co: 0 to 0.50%, Ni: 0 to 0.30%, Cu: 0 to 0.50%, Sn: 0 to 0.100%, Ca: 0 to 0.0035%, Mg: 0 to 0.0035%, B: 0 to 0.0010%, rare earth metal: 0 to 0.0050%, and the remainder: Fe and impurities, where: a yield strength is 552 to less than 655 MPa, and in the steel material, a number density of carbides having a circular diameter Petition 870250083723, dated 17 / 09 / 2025, p.158 / 159 2 / 2 equivalent of 2 μm or more is 10 / mm2 or less, and a prior austenite grain diameter is defined as DA and a block diameter is defined as DB, DA and DB satisfy Formula (1): 2.5 < DA / DB < 10.0 (1) where, the prior austenite grain diameter in μm units is replaced by DA in Formula (1), and the block diameter in μm units is replaced by DB in Formula (1).

2. Steel material, according to claim 1, CHARACTERIZED by containing one or more elements selected from a group consisting of: V: 0.01 to 0.60%, Nb: 0.01 to 0.03%, Ti: 0.01 to 0.05%, W: 0.01 to 0.50%, Zr: 0.0001 to 0.0050%, Co: 0.01 to 0.50%, Ni: 0.01 to 0.30%, Cu: 0.01 to 0.50%, Sn: 0.001 to 0.100%, Ca: 0.0001 to 0.0035%, Mg: 0.0001 to 0.0035%, B: 0.0001 to 0.0010%, and rare earth metal: 0.0001 to 0.0050%.

3. Steel material, according to claim 1 or 2, CHARACTERIZED in that: the steel material is a steel tube.