EP4696796A1 - Steel material - Google Patents

Abstract

A steel material that has 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 is provided. A steel material according to the present disclosure consists of, in 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%, sol. Al: 0.001 to 0.070%, N: 0.0010 to 0.0080%, and the balance: Fc and impurities, and has a yield strength of 552 to less than 655 MPa. Further, in the steel material according to the present disclosure, the 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 DA and a block diameter DB satisfy Formula (1). $$2.5\le \mathrm{DA}/\mathrm{DB}\le 10.0$$

Description

TECHNICAL FIELD
• The present disclosure relates to a steel material, and more particularly relates to a steel material to be used for CO₂ storage technology.
BACKGROUND ART
• An increase in the concentration of carbon dioxide (CO₂) above ground is currently a global problem. Therefore, efforts to suppress CO₂ emissions are proceeding. One such effort to suppress CO₂ emissions that is attracting particular attention is CCUS.
• 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, technology which captures CO₂ emitted from an industrial facility such as an electric power plant or a factory and injects the CO₂ into a depleted oil well to store the CO₂ therein is attracting attention as technology for storing CO₂.
• A steel material to be used for such kind of CO₂ storage technology is required to have a yield strength of, for example, 80 ksi or more (552 MPa or more). Steel materials that have 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).
• The steel material disclosed in Patent Literature 1 is an oil-well steel pipe, and is characterized in that a yield strength YS of the steel material is 552 MPa (80 ksi) or more, and an L-direction absorbed energy E (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 from the external surface, which is required as a casing pipe.
• The steel material disclosed in Patent Literature 2 is a low-alloy, high-strength seamless steel pipe for oil wells having a composition consisting of, in 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, in which the value of a ratio (Ti/N) of the content of Ti to the content of N is 3.0 to 4.0, and the balance is Fe and unavoidable impurities. In addition, in this steel material the value of a ratio (σ_0.7/σ_0.4) 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 low-alloy, high-strength seamless 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.
CITATION LIST PATENT LITERATURE
• Patent Literature 1: Japanese Patent Application Publication No. 2002-115028
• Patent Literature 2: International Application Publication No. WO2017/149570
SUMMARY OF INVENTION TECHNICAL PROBLEM
• 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 the injection efficiency. On the other hand, in the unlikely event of a CO₂ leak occurring, the CO₂ gas changes from the supercritical state into a gas, and the temperature of the CO₂ gas rapidly decreases due to the sudden drop in pressure. In such a case, the environmental temperature will drop far below normal, and there is a possibility of the temperature dropping to -70°C or less. Therefore, a steel material for which application to such kind of technology for storing carbon dioxide is assumed is required to not only have a high strength, but to also have low-temperature toughness in an extremely low temperature environment of -70°C or less. However, in the aforementioned Patent Literatures 1 and 2, there is no discussion of any kind regarding low-temperature toughness in an extremely low temperature environment of -70°C or less.
• An objective of the present disclosure 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 PROBLEM
• A steel material according to the present disclosure consists of, in 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%,
• sol. Al: 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 balance: Fe and impurities,
• wherein:
  • a yield strength is 552 to less than 655 MPa; and
  • in the 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",
  • the DA and the DB satisfy Formula (1): $$2.5\le \mathrm{DA}/\mathrm{DB}\le 10.0$$
  • where, the prior-austenite grain diameter in units of µm is substituted for DA in Formula (1), and the block diameter in units of µm is substituted for DB in Formula (1).
ADVANTAGEOUS EFFECTS OF INVENTION
• The steel material according to the present disclosure 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 EMBODIMENTS
• The present inventors firstly conducted studies with respect to obtaining a steel material having a yield strength of 552 to less than 655 MPa for which application to carbon dioxide storage technology is assumed. That is, the present inventors carried out investigations and studies regarding techniques for obtaining a 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 obtained the following findings.
• Initially, the present inventors focused on the chemical composition, and conducted studies regarding 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 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%, sol. Al: 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 balance: Fe and impurities, there is a possibility that a yield strength of 552 to less than 655 MPa and excellent low-temperature toughness in an extremely low temperature environment will be obtained.
• 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 for increasing low-temperature toughness in an extremely low temperature environment, while maintaining yield strength. As a result, the present inventors obtained the following findings.
• When it is attempted to obtain a yield strength of 552 to less than 655 MPa in a steel material having the aforementioned chemical composition, in some cases coarse carbides are formed. As the 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, low-temperature toughness in an extremely low temperature environment decreases. Specifically, in a steel material having the aforementioned chemical composition, by lowering the number density of carbides having an equivalent circular diameter of 2 µm or more to 10 /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. Hereunder, 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 a 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.
• On the other hand, even when a steel material had the aforementioned chemical composition and a yield strength of 552 to less than 655 MPa and the number density of coarse carbides was lowered to 10 /mm² or less, there were some cases where low-temperature toughness in an extremely low temperature environment could not be obtained. Therefore, the present inventors investigated various techniques for increasing low-temperature toughness in an extremely low temperature environment with respect to a steel material having the aforementioned chemical composition 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.
• Specifically, the present inventors focused on the microstructure of the steel material. A technique which reduces the prior-austenite grain diameter (hereinafter, the prior-austenite grain diameter is also referred to as "prior-y grain diameter DA") 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 aforementioned chemical composition and a yield strength of 552 to less than 655 MPa in which the number density of coarse carbides was 10 /mm² or less, in some cases low-temperature toughness in an extremely low temperature environment was not obtained even when the prior-y grain diameter DA was small. This point will be described specifically using a table.
[Table 1]
• TABLE 1

  Test Number	Yield Strength YS (MPa)	Prior-γ Grain Diameter 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

• Table 1 is a table created by extracting some of the results of examples to be described later. The steel materials of each of Test Nos. 13, 14 and 49 had the aforementioned chemical composition and a yield strength of 552 to less than 655 MPa, and the number density of coarse carbides in each of these steel material was 10 /mm² or less. Referring to Table 1, comparing Test Nos. 13 and 14 with Test No. 49, it is found that even though Test Nos. 13 and 14 each have a larger prior-y grain diameter DA than Test No. 49, a fracture appearance transition temperature vTrs (°C) which is an index of low-temperature toughness is lower for Test Nos. 13 and 14 compared to Test No. 49. Note that, as described in detail later, the lower that the fracture appearance transition temperature vTrs is, the more excellent the low-temperature toughness of the steel material is. That is, it can be confirmed that Test Nos. 13 and 14 which have the larger prior-y grain diameters DA exhibit more excellent low-temperature toughness in an extremely low temperature environment than Test No. 49 which has the smaller prior-y grain diameter DA.
• As a result of detailed studies conducted by the present inventors, it was revealed that in a steel material having the aforementioned chemical composition 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 prior-y grain diameter DA but also the size of blocks which are the sub-microstructure influences the low-temperature toughness in an extremely low temperature environment. Here, the microstructure of a steel material which has the aforementioned chemical composition and a yield strength of 552 to less than 655 MPa and has excellent low-temperature toughness is principally composed of tempered martensite and tempered bainite. Here, a group of laths having almost the same orientation in the sub-microstructure of martensite is referred to as a "martensite block". Similarly, a group of bainite laths having almost the same orientation in the sub-microstructure of bainite is referred to as a "bainite block". In the present description, martensite blocks and bainite blocks are together also referred to as "blocks". In addition, in the present description, the average grain diameter of martensite blocks and average grain diameter of bainite blocks are together also referred to as "block diameter DB".
• As described above, in general, the finer that the prior-y grain diameter DA of a steel material is, the more excellent the low-temperature toughness that the steel material exhibits will tend to be. The present inventors thought that, similarly, it is likely that steel materials with a finer block diameter DB would exhibit more excellent low-temperature toughness. However, referring to Table 1, contrary to the expectation of the present inventors, it was confirmed that Test Nos. 13 and 14 which had not only the larger prior-y grain diameter DA but also the larger block diameter DB exhibited more excellent low-temperature toughness in an extremely low temperature environment than Test No. 49 which had a smaller prior-y grain diameter DA and a smaller block diameter DB.
• As a result of further detailed studies conducted by the present inventors that took into account the above findings, it was revealed that in a steel material having the aforementioned chemical composition 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 prior-y grain diameter DA and the block diameter DB satisfy the following Formula (1), excellent low-temperature toughness in an extremely low temperature environment will be obtained. $$2.5\le \mathrm{DA}/\mathrm{DB}\le 10.0$$
• Where, the prior-austenite grain diameter in units of µm is substituted for DA in Formula (1), and the block diameter in units of µm is substituted for DB in Formula (1).
• Let Fnl be defined as Fnl = DA/DB. Fn1 is an index of the low-temperature toughness in an extremely low temperature environment of a steel material having the aforementioned chemical composition 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 aforementioned steel material is more than 10.0, the prior-y grain diameter 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. Further, when Fn1 in the aforementioned steel material is 2.5 or more, low-temperature toughness in an extremely low temperature environment is stably obtained. Therefore, in a steel material having the aforementioned chemical composition 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.
• Note that, the reason why a steel material having the aforementioned chemical composition 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 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 clarified in detail. However, the fact that a steel material having the aforementioned chemical composition 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 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.
• The gist of the steel material according to the present embodiment, which has been completed based on the findings described above, is as follows.
1. [1] A steel material consisting of, in 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%,
   • sol. Al: 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 balance: Fe and impurities,
   • wherein:
     • a yield strength is 552 to less than 655 MPa; and
     • in the 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",
     • the DA and the DB satisfy Formula (1): $$2.5\le \mathrm{DA}/\mathrm{DB}\le 10.0$$
     • where, the prior-austenite grain diameter in units of µm is substituted for DA in Formula (1), and the block diameter in units of µm is substituted for DB in Formula (1).
2. [2] The steel material according to [1], 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. [3] The steel material according to [1] or [2], wherein:
   the steel material is a steel pipe.
• The shape of the steel material according to the present embodiment is not particularly limited. The steel material according to the present embodiment may be a steel pipe, may be a round steel bar (solid material), or may be a steel plate. Note that, the term "round steel bar" refers to a steel bar in which a cross section in a direction perpendicular to the axial direction is a circular shape. Further, the steel pipe may be a seamless steel pipe or may be a welded steel pipe.
• Hereunder, the steel material according to the present embodiment is described in detail. The symbol "%" in relation to an element means mass percent unless otherwise stated.
[Chemical composition]
• The chemical composition of the steel material according to the present embodiment contains the following elements.
C: 0.26 to 0.35%
• Carbon (C) increases hardenability of the steel material and increases strength of the steel material. C also makes the block diameter DB of the steel material fine. If the content of C is too low, the aforementioned advantageous effects will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment. On the other hand, if the content of C is too high, even if the contents of other elements are within the range of the present embodiment, strength of the steel material will be too high and the low-temperature toughness of the steel material will decrease. Therefore, the content of C is to be 0.26 to 0.35%. A preferable lower limit of the content of C is 0.27%, more preferably is 0.28%, and further preferably is 0.29%. A preferable upper limit of the content of C is 0.34%, more preferably is 0.33%, and further preferably is 0.32%.
Si: 0.10 to 1.00%
• Silicon (Si) deoxidizes the steel. If the content of Si is too low, the aforementioned advantageous effect will not be sufficiently obtained even if the contents of other clements are within the range of the present embodiment. On the other hand, if the content of Si 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 the present embodiment. Therefore, the content of Si is to be 0.10 to 1.00%. A preferable lower limit of the content of Si is 0.12%, more preferably is 0.14%, and further preferably is 0.15%. A preferable upper limit of the content of Si is 0.95%, more preferably is 0.90%, and further preferably is 0.80%.
Mn: 1.00 to 1.40%
• Manganese (Mn) increases hardenability of the steel material and increases strength of the steel material. If the content of Mn is too low, the aforementioned advantageous effect will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment. On the other hand, if the content of Mn is too high, even if the contents of other elements are within the range of the present embodiment, Mn will segregate to grain boundaries together with impurities such as P and S, and the low-temperature toughness of the steel material will decrease. Therefore, the content of Mn is to be 1.00 to 1.40%. A preferable lower limit of the content of Mn is 1.02%, and more preferably is 1.05%. A preferable upper limit of the content of Mn is 1.35%, and more preferably is 1.30%.
P: 0.015% or less
• Phosphorus (P) is an impurity. That is, the lower limit of the content of P is more than 0%. If the content of P is too high, even if the contents of other elements are within the range of the present embodiment, P will segregate to grain boundaries and the low-temperature toughness of the steel material will decrease. Therefore, the content of P is to be 0.015% or less. A preferable upper limit of the content of P is 0.014%, and more preferably is 0.013%. The content of P is preferably as low as possible. However, extremely reducing the content of P will greatly increase the production cost. Therefore, when taking industrial production into consideration, a preferable lower limit of the content of P is 0.001%, more preferably is 0.002%, and further preferably is 0.003%.
S: 0.0020% or less
• Sulfur (S) is an impurity. That is, the lower limit of the content of S is more than 0%. If the content of S is too high, even if the contents of other elements are within the range of the present embodiment, S will segregate to grain boundaries and the low-temperature toughness of the steel material will decrease. Therefore, the content of S is to be 0.0020% or less. A preferable upper limit of the content of S is 0.0019%, more preferably is 0.0018%, and further preferably is 0.0016%. The content of S is preferably as low as possible. However, extremely reducing the content of S will greatly increase the production cost. Therefore, when taking industrial production into consideration, a preferable lower limit of the content of S is 0.0001%, more preferably is 0.0002%, and further preferably is 0.0003%.
Cr: 0.15 to 0.50%
• Chromium (Cr) increases hardenability of the steel material and increases strength of the steel material. Cr also concentrates in cementite in the steel material and thereby suppresses coarsening of the cementite. As a result, the low-temperature toughness of the steel material increases. If the content of Cr is too low, the aforementioned advantageous effects will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment. On the other hand, if the content of Cr is too high, the low-temperature toughness of the steel material will decrease even if the contents of other elements are within the range of the present embodiment. Therefore, the content of Cr is to be 0.15 to 0.50%. A preferable lower limit of the content of Cr is 0.18%, and more preferably is 0.20%. A preferable upper limit of the content of Cr is 0.45%, and more preferably is 0.40%.
Mo: 0.05 to 0.25%
• Molybdenum (Mo) increases hardenability of the steel material and increases strength of the steel material. If the content of Mo is too low, the aforementioned advantageous effect will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment. On the other hand, if the content of Mo is too high, even if the contents of other elements are within the range of the present embodiment, Mo carbides will excessively form and the low-temperature toughness of the steel material will decrease. Therefore, the content of Mo is to be 0.05 to 0.25%. A preferable lower limit of the content of Mo is 0.06%, and more preferably is 0.07%. A preferable upper limit of the content of Mo is 0.24%, more preferably is 0.20%, and further preferably is 0.18%.
Sol. Al: 0.001 to 0.070%
• Aluminum (Al) deoxidizes the steel. If the content of Al is too low, the aforementioned advantageous effect will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment. On the other hand, if the content of Al is too high, even if the contents of other elements are within the range of the present embodiment, coarse oxide-based inclusions will form and the low-temperature toughness of the steel material will decrease. Therefore, the content of Al is to be 0.001 to 0.070%. A preferable lower limit of the content of Al is 0.005%, and more preferably is 0.010%. A preferable upper limit of the content of Al is 0.065%, and more preferably is 0.060%. As used in the present description, the content of "Al" means the content of "acid-soluble Al", that is, the content of "sol. Al".
N: 0.0010 to 0.0080%
• Nitrogen (N) forms nitrides, and refines the prior-y grain diameter DA of the steel material by the pinning effect. As a result, the low-temperature toughness of the steel material increases. If the content of N is too low, the aforementioned advantageous effect will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment. On the other hand, if the content of N 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 content of N is to be 0.0010 to 0.0080%. A preferable lower limit of the content of N is 0.0015%, more preferably is 0.0020%, and further preferably is 0.0025%. A preferable upper limit of the content of N is 0.0075%, and more preferably is 0.0070%.
• The balance of the chemical composition of the steel material according to the present embodiment is Fe and impurities. Here, the term "impurities" refers to substances which, when industrially producing the steel material, are mixed in from ore or scrap that is used as the raw material or from the production environment or the like, and which are allowed within a range that does not adversely affect the steel material according to the present embodiment.
[Optional elements]
• The chemical composition of the steel material described above may further contain one or more elements selected from the group consisting of V, Nb, Ti, W, and Zr in lieu of a part of Fe. Each of these elements is an optional element, and increases the temper softening resistance of the steel material and thereby increases strength of the steel material.
V: 0 to 0.60%
• Vanadium (V) is an optional element, and does not have to be contained. That is, the content of V may be 0%. When contained, V forms fine carbides during tempering and thereby increases the temper softening resistance of the steel material and increases strength of the steel material. If even a small amount of V is contained, the aforementioned advantageous effect will be obtained to a certain extent. However, if the content of V 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 the present embodiment. Therefore, the content of V is to be 0 to 0.60%. A preferable lower limit of the content of V is more than 0%, more preferably is 0.01%, further preferably is 0.02%, further preferably is 0.04%, and further preferably is 0.06%. A preferable upper limit of the content of V is 0.40%, more preferably is 0.30%, and further preferably is 0.20%.
Nb: 0 to 0.03%
• Niobium (Nb) is an optional element, and does not have to be contained. That is, the content of Nb may be 0%. When contained, Nb forms fine carbides during tempering and thereby increases the temper softening resistance of the steel material and increases strength of the steel material. If even a small amount of Nb is contained, the aforementioned advantageous effect will be obtained to a certain extent. However, if the content of Nb is too high, even if the contents of other elements are within the range of the present embodiment, carbo-nitrides and the like will excessively form and the low-temperature toughness and SSC resistance of the steel material will decrease. Therefore, the content of Nb is to be 0 to 0.03%. A preferable lower limit of the content of Nb is more than 0%, more preferably is 0.01%, and further preferably is 0.02%. A preferable upper limit of the content of Nb is less than 0.03%.
Ti: 0 to 0.05%
• Titanium (Ti) is an optional element, and does not have to be contained. That is, the content of Ti may be 0%. When contained, Ti forms fine carbides and thereby increases the temper softening resistance of the steel material and increases strength of the steel material. If even a small amount of Ti is contained, the aforementioned advantageous effect will be obtained to a certain extent. However, if the content of Ti 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 content of Ti is to be 0 to 0.05%. A preferable lower limit of the content of Ti is more than 0%, more preferably is 0.01%, and further preferably is 0.02%. A preferable upper limit of the content of Ti is less than 0.05%, and more preferably is 0.04%.
W: 0 to 0.50%
• Tungsten (W) is an optional element, and does not have to be contained. That is, the content of W may be 0%. When contained, W forms fine carbides during tempering and thereby increases the temper softening resistance of the steel material and increases strength of the steel material. If even a small amount of W is contained, the aforementioned advantageous effect will be obtained to a certain extent. However, if the content of W is too high, even if the contents of other elements are within the range of the present embodiment, coarse carbides will form and the low-temperature toughness of the steel material will decrease. Therefore, the content of W is to be 0 to 0.50%. A preferable lower limit of the content of W is more than 0%, more preferably is 0.01%, and further preferably is 0.02%. A preferable upper limit of the content of W is 0.45%, and more preferably is 0.40%.
Zr: 0 to 0.0050%
• Zirconium (Zr) is an optional element, and does not have to be contained. That is, the content of Zr may be 0%. When contained, Zr forms fine carbides during tempering and thereby increases the temper softening resistance of the steel material and increases strength of the steel material. If even a small amount of Zr is contained, the aforementioned advantageous effect will be obtained to a certain extent. However, if the content of Zr is too high, even if the contents of other elements are within the range of the present embodiment, coarse oxides will form and the low-temperature toughness of the steel material will decrease. Therefore, the content of Zr is to be 0 to 0.0050%. A preferable lower limit of the content of Zr is more than 0%, more preferably is 0.0001%, further preferably is 0.0003%, further preferably is 0.0006%, and further preferably is 0.0010%. A preferable upper limit of the content of Zr is 0.0045%, more preferably is 0.0040%, and further preferably is 0.0035%.
• The chemical composition of the steel material described above may further contain one or more types of element selected from the group consisting of Co and Ni in lieu of a part of Fe. Each of these elements is an optional element, and each element increases hardenability of the steel material and increases strength of the steel material.
Co: 0 to 0.50%
• Cobalt (Co) is an optional clement, and does not have to be contained. That is, the content of Co may be 0%. When contained, Co increases hardenability of the steel material and increases strength of the steel material. If even a small amount of Co is contained, the aforementioned advantageous effect will be obtained to a certain extent. However, if the content of Co is too high, even if the contents of other elements are within the range of the present embodiment, hardenability of the steel material will, on the contrary, decrease and strength of the steel material will decrease. Therefore, the content of Co is to be 0 to 0.50%. A preferable lower limit of the content of Co is more than 0%, more preferably is 0.01%, further preferably is 0.02%, further preferably is 0.03%, and further preferably is 0.05%. A preferable upper limit of the content of Co is 0.45%, and more preferably is 0.40%.
Ni: 0 to 0.30%
• Nickel (Ni) is an optional element, and does not have to be contained. That is, the content of Ni may be 0%. When contained, Ni increases hardenability of the steel material and increases strength of the steel material. If even a small amount of Ni is contained, the advantageous effect will be obtained to a certain extent. However, if the content of Ni is too high, even if the contents of other elements are within the range of the present embodiment, localized corrosion will be promoted and corrosion resistance of the steel material will decrease. Therefore, the content of Ni is to be 0 to 0.30%. A preferable lower limit of the content of Ni is more than 0%, more preferably is 0.01%, and further preferably is 0.02%. A preferable upper limit of the content of Ni is 0.28%, and more preferably is 0.25%.
• The chemical composition of the steel material described above may further contain one or more types of element selected from the group consisting of Cu and Sn in lieu of a part of Fe. Each of these elements is an optional element, and each element increases strength of the steel material.
Cu: 0 to 0.50%
• Copper (Cu) is an optional element, and does not have to be contained. That is, the content of Cu may be 0%. When contained, Cu increases strength of the steel material. If even a small amount of Cu is contained, the aforementioned advantageous effect will be obtained to a certain extent. However, if the content of Cu is too high, even if the contents of other elements are within the range of the present embodiment, strength of the steel material will be too high and the low-temperature toughness of the steel material will decrease. Therefore, the content of Cu is to be 0 to 0.50%. A preferable lower limit of the content of Cu is more than 0%, more preferably is 0.01%, further preferably is 0.02%, and further preferably is 0.05%. A preferable upper limit of the content of Cu is 0.45%, more preferably is 0.35%, and further preferably is 0.25%.
Sn: 0 to 0.100%
• Tin (Sn) is an optional clement, and does not have to be contained. That is, the content of Sn may be 0%. When contained, Sn increases strength of the steel material. If even a small amount of Sn is contained, the aforementioned advantageous effect will be obtained to a certain extent. However, if the content of Sn is too high, even if the contents of other elements are within the range of the present embodiment, hot workability of the steel material will decrease. Therefore, the content of Sn is to be 0 to 0.100%. A preferable lower limit of the content of Sn is more than 0%, more preferably is 0.001%, further preferably is 0.002%, and further preferably is 0.003%. A preferable upper limit of the content of Sn is 0.095%, more preferably is 0.090%, further preferably is 0.080%, and further preferably is 0.070%.
• The chemical composition of the steel material described above may further contain one or more types of element selected from the group consisting of Ca, Mg, B, and rare earth metal in lieu of a part of Fe. Each of these elements is an optional element, and each element improves hot workability of the steel material.
Ca: 0 to 0.0035%
• Calcium (Ca) is an optional clement, and docs not have to be contained. That is, the content of Ca may be 0%. When contained, Ca renders S in the steel material harmless by forming sulfides, and thereby improves hot workability of the steel material. If even a small amount of Ca is contained, the aforementioned advantageous effect will be obtained to a certain extent. However, if the content of Ca is too high, even if the contents of other elements are within the range of the present embodiment, coarse oxides will form and the low-temperature toughness of the steel material will decrease. Therefore, the content of Ca is to be 0 to 0.0035%. A preferable lower limit of the content of Ca is more than 0%, more preferably is 0.0001%, further preferably is 0.0003%, further preferably is 0.0006%, and further preferably is 0.0010%. A preferable upper limit of the content of Ca is 0.0033%, and more preferably is 0.0030%.
Mg: 0 to 0.0035%
• Magnesium (Mg) is an optional clement, and does not have to be contained. That is, the content of Mg may be 0%. When contained, Mg renders S in the steel material harmless by forming sulfides, and thereby improves hot workability of the steel material. If even a small amount of Mg is contained, the aforementioned advantageous effect will be obtained to a certain extent. However, if the content of Mg is too high, even if the contents of other clements are within the range of the present embodiment, coarse oxides will form and the low-temperature toughness of the steel material will decrease. Therefore, the content of Mg is to be 0 to 0.0035%. A preferable lower limit of the content of Mg is more than 0%, more preferably is 0.0001%, further preferably is 0.0003%, further preferably is 0.0006%, and further preferably is 0.0010%. A preferable upper limit of the content of Mg is 0.0033%, and more preferably is 0.0030%.
B: 0 to 0.0010%
• Boron (B) is an optional element, and does not have to be contained. That is, the content of B may be 0%. When contained, B suppresses the segregation of S in the steel material to grain boundaries and improves hot workability of the steel material. If even a small amount of B is contained, the aforementioned advantageous effect will be obtained to a certain extent. However, if the content of B is too high, even if the contents of other elements are within the range of the present embodiment, coarse nitrides will form and the low-temperature toughness of the steel material will decrease. Therefore, the content of B is to be 0 to 0.0010%. A preferable lower limit of the content of B is more than 0%, more preferably is 0.0001%, and further preferably is 0.0002%. A preferable upper limit of the content of B is 0.0009%, and more preferably is 0.0008%.
Rare earth metal (REM): 0 to 0.0050%
• Rare earth metal (REM) is an optional element, and does not have to be contained. That is, the content of REM may be 0%. When contained, REM renders S in the steel material harmless by forming sulfides, and thereby improves hot workability of the steel material. If even a small amount of REM is contained, the aforementioned advantageous effect will be obtained to a certain extent. However, if the content of REM is too high, even if the contents of other elements are within the range of the present embodiment, coarse oxides will be formed and the low-temperature toughness of the steel material will decrease. Therefore, the content of REM is to be 0 to 0.0050%. A preferable lower limit of the content of REM is more than 0%, more preferably is 0.0001%, further preferably is 0.0003%, and further preferably is 0.0006%. A preferable upper limit of the content of REM is 0.0045%, and more preferably is 0.0040%.
• Note that, in the present description the term "REM" means one or more types of element selected from the group consisting of scandium (Sc) which is the element with atomic number 21, yttrium (Y) which is the element with atomic number 39, and the elements from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71 that arc lanthanoids. Further, in the present description, the term "content of REM" refers to the total content of these elements.
[Yield strength]
• 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 0.2% offset proof stress (MPa) obtained by a tensile test carried out at normal temperature (24±3°C) in accordance with ASTM E8/E8M (2021).
• In the present embodiment, the yield strength of the steel material can be determined by the following method. Specifically, a tensile test is carried out 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 center portion of the thickness. In this case, the axial direction of the round bar specimen is to be made a direction that is parallel to the rolling elongation direction of the steel plate. If the steel material is a steel pipe, the round bar specimen is prepared from the center portion of the wall thickness. In this case, the axial direction of the round bar specimen is to be made a direction that is parallel to the pipe axis direction. If the steel material is a round steel bar, the round bar specimen is prepared from an R/2 position. In this case, the axial direction of the round bar specimen is to be made a direction that is parallel to the axial direction of the round steel bar. Note that, in the present description, the term "R/2 position" means the center position of a radius R in a cross section perpendicular to the axial direction of the round steel bar. Regarding the size of the round bar specimen, for example, the round bar specimen has a parallel portion diameter of 6 mm and a gage length of 30 mm. A tensile test is carried out in the atmosphere at normal temperature (24±3°C) using the round bar specimen, and the obtained 0.2% offset proof stress (MPa) is defined as the yield strength (MPa). Note that, a value obtained by rounding off decimals of the obtained numerical value is adopted as the yield strength (MPa) in the present embodiment.
[Number density of coarse carbides]
• The steel material according to the present embodiment has the aforementioned chemical composition and a yield strength of 552 to less than 655 MPa, and in addition, the number density of coarse carbides (carbides having an equivalent circular diameter of 2 µm or more) in the steel material is lowered to 10 /mm² or less. As a result, on the condition 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 a 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.
• In a steel material that has the aforementioned chemical composition, coarse carbides are liable to form. In particular, in a case where carbides do not dissolve in a quenching process that is described later and remain in the steel material, the carbides which remained in the steel material are liable to coarsen 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 the low-temperature toughness of the steel material to decrease. In particular, in an extremely low temperature environment such as an environment in which the temperature is -70°C or less, the influence of coarse carbides noticeably appears. Therefore, the steel material according to the present embodiment has the aforementioned chemical composition and a yield strength of 552 to less than 655 MPa, and in addition, the number density of coarse carbides in the steel material is 10 /mm² or less.
• In the present embodiment, a preferable upper limit of the number density of coarse carbides is 9 /mm², more preferably is 8 /mm², further preferably is 7 /mm², further preferably is 6 /mm², and further preferably is 5 /mm². Note that, the lower limit of the number density of coarse carbides is not particularly limited, and may be 0 /mm², or may be 1 /mm².
• In the present embodiment, the number density of coarse carbides in the 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 center portion of the thickness. If the steel material is a steel pipe, a test specimen in which a face including the pipe axis direction and the pipe radius direction is adopted as the observation surface is prepared from the center portion of the wall thickness. If the steel material is a round steel bar, a test specimen which includes an R/2 position at the center thereof and in which a face including the axial direction and the radial direction is adopted as the observation surface is prepared.
• After polishing the observation surface of the prepared test specimen to obtain a mirror surface, measurement is performed. Although the area of the observation surface is not limited, for example, the area is 300 mm² (20 mm × 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 at the observation surface are identified based on contrast. Each of the identified particles is subjected to an clement 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. Based on the EDS analysis results for the respective particles, those particles in which C is detected and in which the content of Fe 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 which those skilled in the art are fully capable of performing.
• Among the carbides identified at the observation surface, carbides having an equivalent circular diameter of 2 µm or more (coarse carbides) are identified, and the total number of the 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 number 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 off decimals of the relevant numerical value obtained is adopted as the number density (/mm²) of coarse carbides. Further, measurement of the number 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 automatic analyzer having the trade name "Metals Quality Analyzer" manufactured by FEI (ASPEX) Company can be used as the SEM-EDS apparatus. $$\left[\mathrm{Fn}1\left(=\mathrm{DA}/\mathrm{DB}\right)\right]$$
• The steel material according to the present embodiment has the aforementioned chemical composition 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, and in addition, 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 552 to less than 655 MPa, and excellent low-temperature toughness in an extremely low temperature environment. $$2.5\le \mathrm{DA}/\mathrm{DB}\le 10.0$$
• Where, the prior-austenite grain diameter in units of µm is substituted for DA in Formula (1), and the block diameter in units of µm is substituted for DB in Formula (1).
• As described above, in the present description, the prior-austenite grain diameter in the microstructure of the steel material is also referred to as "prior-γ grain diameter DA". Further, in the present description, the average grain diameter of martensite blocks and average grain diameter of bainite blocks are together also referred to as "block diameter DB". Note that, as described above, the microstructure of the steel material according to the present embodiment is principally composed of tempered martensite and tempered bainite. Therefore, in the present description, martensite blocks and bainite blocks are together also referred to as "blocks".
• Fn1 (= DA/DB) is an index of low-temperature toughness in an extremely low temperature environment in a steel material having the aforementioned chemical composition 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 Fn1 is more 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 aforementioned chemical composition 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 Fn1 is 2.5 or more, low-temperature toughness in an extremely low temperature environment will be stably obtained. Therefore, the steel material according to the present embodiment has the aforementioned chemical composition and a yield strength of 552 to less than 655 MPa, and the number density of coarse carbides in the steel material is 10 /mm ² or less, and in addition, Fn1 is made to fall within the range of 2.5 to 10.0.
• As described above, in a steel material having the aforementioned chemical composition, in a 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 liable to coarsen and become coarse carbides in a subsequent tempering process. On the other hand, when it is attempted to cause carbides to sufficiently dissolve in a quenching process, the prior-y grain diameter DA and/or the block diameter DB is liable to become large. In such a case, if cither the prior-y grain diameter 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 dissolve in a quenching process to be described later and the number density of coarse carbides is lowered to 10 /mm² or less, and furthermore Fnl 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.
• Note that, in the steel material according to the present embodiment, as long as Fn1 satisfies a condition of being within the range of 2.5 to 10.0, the prior-y grain diameter DA and the block diameter DB are not particularly limited. However, in a steel material having the aforementioned chemical composition 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-y grain diameter DA is, for example, 10.0 to 200.0 µm. In the present embodiment, a preferable upper limit of the prior-y grain diameter DA is 180.0 µm, more preferably is 160.0 µm, and further preferably is 150.0 µm. In the steel material according to the present embodiment, even if the prior-y grain diameter DA is, for example, 15.0 µm or more, 20.0 µm or more, 30.0 µm or more, 40.0 µm or more, or even is a coarse grain diameter of 50.0 µm or more, by Fn1 satisfying 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.
• In a steel material having the aforementioned chemical composition 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, furthermore, the block diameter DB is, for example, 2.0 to 30.0 µm. In the present embodiment, a preferable upper limit of the block diameter DB is 25.0 µm, more preferably is 20.0 µm, and further preferably 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 coarse diameter of 11.0 µm or more, by Fn1 satisfying 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.
• In the present embodiment, a preferable lower limit of Fn1 is 3.0, more preferably is 3.5, and further preferably is 4.0. In the present embodiment, a preferable upper limit of Fn1 is 9.8, more preferably is 9.5, and further preferably is 9.0.
• In the present embodiment, Fn1 can be determined by the following method. First, the prior-y grain diameter DA of the steel material according to the present embodiment is determined. Specifically, a test specimen for measuring the prior-y grain diameter DA 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 the center portion of the thickness. If the steel material is a steel pipe, a test specimen in which a face including the pipe axis direction and the pipe radius direction is adopted as the observation surface is prepared from the center portion of the wall thickness. If the steel material is a round steel bar, a test specimen which includes an R/2 position at the center thereof and in which a face including the axial direction and the radial direction is adopted as the observation surface is prepared.
• After embedding the test specimen in resin and polishing the observation surface to obtain a mirror surface, the test specimen is immersed for about 60 seconds in a solution obtained by mixing an appropriate amount of a surfactant into an aqueous solution saturated with picric acid to reveal prior-y grain boundaries by etching. Although not limited, the area of the observation surface is set to, for example, 100 mm² (10 mm × 10 mm). Observation by optical microscope is performed to determine a grain size number G based on the intercept method in accordance with JIS G 0551 (2020). The obtained grain size number G is converted to an average grain diameter M (µm) based on the following Formulae (2) and (3). In the present embodiment, the average grain diameter M obtained by the above method is defined as the prior-y 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 in accordance with JIS G 0551 (2020). $$\mathrm{n}=2^{\mathrm{G}+3}$$ $$\mathrm{M}=1000/{\mathrm{n}}^{1/2}$$
• Next, the block diameter DB of the steel material according to the present embodiment is determined. Specifically, a test specimen for measuring the block diameter DB is prepared from the steel material according to the present embodiment. If the steel material is a steel plate, a test specimen having an observation surface with dimensions of 25 µm × 25 µm is prepared from the center portion of the thickness. If the steel material is a steel pipe, a test specimen having an observation surface with dimensions of 25 µm × 25 µm is prepared from the center 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 × 25 µm and that includes the R/2 position at the center thereof is prepared.
• Measurement of the observation surface of the test specimen is performed by electron backscatter diffraction (EBSD). The EBSD measurement is carried out with an accelerating voltage of 20 kV with respect to visual fields of 25 µm × 25 µm at a pitch of 0.1 µm. The orientation of a body-centered cubic structure (iron) is identified based on a Kikuchi diffraction pattern obtained by the EBSD measurement. A crystal orientation figure is determined based on the orientation of the body-centered cubic structure (iron). From the crystal orientation figure, regions surrounded by a boundary having an orientation difference of 15° or more with adjacent crystals are identified to thereby obtain a crystal orientation map. A region surrounded by an orientation difference of 15° or more is defined as a single block.
• 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 aforementioned crystal orientation map is defined as "k" blocks, the area of each block is defined as "SBi" (i is a natural number that is 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 (4). $$\mathrm{DB}={\displaystyle \sum _{\mathrm{i}=1}^{\mathrm{k}}\left(2\times \sqrt{\frac{{\mathrm{SB}}_{\mathrm{i}}}{\pi }}\times \frac{{\mathrm{SB}}_{\mathrm{i}}}{\mathrm{ST}}\right)}$$
• Where, in Formula (4), the total number (a natural number) of blocks in the crystal orientation map is substituted for k, the area (µm²) of the i-th block is substituted for SBi, the pi is substituted for π, and the total area (µm²) of the crystal orientation map is substituted for ST.
• Fn1 (= DA/DB) is determined based on the obtained prior-y grain diameter DA (µm) and block diameter DB (µm). Note that, in the present embodiment, a value obtained by rounding off to the first decimal place of the relevant obtained numerical value is adopted as the prior-y grain diameter DA (µm). Further, in the present embodiment, a value obtained by rounding off to the first decimal place of the relevant obtained numerical value is adopted as the block diameter DB (µm). Furthermore, in the present embodiment, a value obtained by rounding off to the first decimal place of the relevant obtained numerical value is adopted as Fn1.
[Low-temperature toughness]
• The steel material according to the present embodiment has the aforementioned chemical composition, 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.
• A Charpy impact test in accordance with ASTM E23 (2018) is performed on the steel material according to the present embodiment. First, a V-notch test specimen in accordance with 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 which has a notched surface perpendicular to the thickness direction, and whose longitudinal direction is parallel to the width direction is prepared from the center portion of the thickness. If the steel material is a steel pipe, a V-notch test specimen which has a notched surface perpendicular to the pipe axis direction, and whose longitudinal direction is perpendicular to the pipe axis direction and the pipe radius direction is prepared from the center portion of the wall thickness. If the steel material is a round steel bar, a V-notch test specimen which has a notched surface perpendicular to the axial direction, and whose longitudinal direction is perpendicular to the axial direction and a sectional radial direction is prepared from an R/2 position in a cross section perpendicular to the axial direction.
• The prepared V-notch test specimens are subjected to a Charpy impact test in accordance with ASTM E23 (2018). Specifically, the range of the test temperatures is -120 to 20°C, and the test temperatures are set at eight levels which are varied in increments of 20°C (-120°C, -100°C, -80°C, -60°C, -40°C, -20°C, 0°C, and 20°C). The Charpy impact test is carried out using two test specimens for each test temperature. The percent brittle fracture (%) of the respective test specimens after the test at each temperature under the above conditions is determined. The test temperatures (°C) and obtained percent brittle fracture (%) are plotted to obtain an approximated curve. The temperature (°C) at which the percent brittle fracture becomes 50% is determined from the obtained approximated curve, and the determined test temperature is defined as a fracture appearance transition temperature vTrs (°C). In the present embodiment, in a case where the fracture appearance transition temperature vTrs is -70°C or less, it is determined that the relevant steel material has excellent low-temperature toughness in an extremely low temperature environment.
[Microstructure]
• As described above, the microstructure of the steel material according to the present embodiment is principally composed of tempered martensite and tempered bainite. Specifically, in the microstructure of the steel material according to the present embodiment, the total of the volume ratios of tempered martensite and tempered bainite is 90% or more. The balance of the microstructure is, for example, ferrite or pearlite. In a steel material which has the aforementioned chemical composition 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 of the volume ratios of tempered martensite and tempered 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 which has the aforementioned chemical composition 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 of the volume ratios of tempered martensite and tempered bainite in the microstructure of the steel material is 90% or more.
• Note that, the following method can be used when determining the volume 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 center portion of the thickness. If the steel material is a steel pipe, a test specimen in which a face including the pipe axis direction and the pipe radius direction is adopted as the observation surface is prepared from a center portion of the wall thickness. If the steel material is a round steel bar, a test specimen which includes an R/2 position at the center thereof and in which a face including the axial direction and the radial direction is adopted as the observation surface is prepared.
• After polishing the observation surface of the test specimen to obtain a mirror surface, the test specimen is immersed for about 10 seconds in a nital etching reagent to reveal the microstructure by etching. The etched observation surface is observed by means of a secondary electron image obtained using an SEM, and observation is performed in 10 visual fields. The area of each visual field is, for example, 0.01 mm² (magnification of 1000×). In each visual field, tempered martensite and tempered bainite are identified based on the contrast. The area fractions of the identified tempered martensite and tempered bainite are determined. The method for determining the area fractions is not particularly limited, and a well-known method can be used. For example, the area fractions of tempered martensite and tempered bainite can be determined by image analysis. In the present embodiment, an arithmetic average value of the area fractions of tempered martensite and tempered bainite determined in all of the visual fields is defined as the volume ratio of tempered martensite and tempered bainite.
[Production method]
• A method for producing the steel material according to the present embodiment will now be described. Hereunder, a method for producing a seamless steel pipe as one example of the steel material according to the present embodiment is described. The method for producing a seamless steel pipe includes a process of preparing a hollow shell (preparation process), and a process of subjecting the hollow shell to quenching and tempering to form a seamless steel pipe (quenching process and tempering process). Note that, a production method according to the present embodiment is not limited to the production method described below. Each process is described in detail hereunder.
[Preparation process]
• 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 plate-shaped steel material in a case where the end product is a steel plate, is a hollow shell in a case where the end product is a steel pipe, and is a steel material in which a cross section perpendicular to the axial direction is a circular shape in a case where the end product is a round steel bar.
• 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). Hereunder, a case where the preparation process includes a starting material preparation process and a hot working process is described in detail.
[Starting material preparation process]
• In the starting material preparation process, 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 suffices to use a well-known method. Specifically, a cast piece (a slab, a bloom, or a billet) may be produced by a continuous casting process using the molten steel. An ingot may also be produced by an ingot-making process using the molten steel. As necessary, the slab, bloom, or ingot may be subjected to blooming to produce a billet. A starting material (a slab, a bloom, or a billet) is produced by the above process.
[Hot working process]
• In the hot working process, the prepared starting material is subjected to hot working to produce an intermediate steel material. If the steel material is a seamless steel pipe, the intermediate steel material corresponds to a hollow shell. First, a billet is heated in a heating furnace. Although not particularly limited, the heating temperature is, for example, 1100 to 1300°C. After extracting the billet from the heating furnace, the billet is subjected to hot working to produce a hollow shell (seamless steel pipe). The method of hot working is not particularly limited, and it suffices to use a well-known method.
• For example, the Mannesmann process may be performed as hot working to produce a hollow shell. In this case, a round billet is subjected to piercing-rolling using a piercing machine. When performing piercing-rolling, although not particularly limited, for example, the piercing ratio is 1.0 to 4.0. The round billet subjected to piercing-rolling is further subjected to hot rolling with a mandrel mill, a reducer, a sizing mill or the like to produce a hollow shell. The cumulative reduction of area in the hot working process is, for example, 20 to 70%.
• A hollow shell may be produced from the billet by performing another hot working method. For example, in a case where the steel material is a heavy-wall steel material of a short length such as a coupling, a hollow shell may be produced by forging by the Ehrhardt process or the like. A hollow shell is produced by the above process. Although not particularly limited, the wall thickness of the hollow shell is, for example, 9 to 60 mm.
• If the steel material is a round steel bar, first, the starting material is heated in a heating furnace. Although not particularly limited, the heating temperature is, for example, 1100 to 1300°C. 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. The hot working is, for example, blooming performed using a blooming mill or hot rolling performed using a continuous mill. In a continuous mill, a horizontal stand having a pair of grooved rolls arranged one on the other in the vertical direction, and a vertical stand having a pair of grooved rolls arranged side by side in the horizontal direction are alternately arranged.
• If the steel material is a steel plate, first, the starting material is heated in a heating furnace. Although not particularly limited, the heating temperature is, for example, 1100 to 1300°C. After being extracted from the heating furnace, the starting material is subjected to hot rolling using a blooming mill and a continuous mill to produce an intermediate steel material having a steel plate shape.
• The hollow shell produced by hot working may be air-cooled (as-rolled). The hollow shell produced by hot working may be subjected to direct quenching after the hot working without being cooled to normal temperature, or may be subjected to quenching after undergoing supplementary heating (reheating) after the hot working.
• In the case of performing direct quenching after the hot working, or performing quenching after supplementary heating, cooling may be stopped midway through the quenching process or slow cooling may be performed. In this case, the occurrence of quench cracking in the hollow shell can be suppressed. In addition, in the case of performing direct quenching after hot working, or performing quenching after supplementary heating, stress relief annealing (SR) may be performed at a time that is after quenching and before the heat treatment of the next process. In this case, residual stress of the hollow shell is eliminated.
• As described above, an intermediate steel material is prepared in the preparation process. The intermediate steel material may be produced by the aforementioned preferable process, or may be an intermediate steel material produced by a third party, or an intermediate steel material may be prepared that was produced in another factory other than the factory in which a quenching process and a tempering process to be described later are performed or that was produced at different works. Hereunder, the quenching process is described in detail.
[Quenching process]
• In the quenching process, the prepared intermediate steel material (hollow shell) is subjected to quenching. As used in the present description, the term "quenching" means rapidly cooling the intermediate steel material which is at a temperature not lower than the A₃ point. Here, in the present description, the temperature of the intermediate steel material immediately prior to rapid cooling when performing quenching is also referred to as "quenching temperature". Preferably, in the quenching process according to the present embodiment, after heating in two stages has been performed, 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. Hereunder, each process is described in detail.
[First heating process]
• Preferably, in the first heating process according to the present embodiment, the prepared intermediate steel material (hollow shell) 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 austenite transformation will not be completed. In such a case, the produced steel material will not have a yield strength of 552 to less than 655 MPa. On the other hand, if the heating temperature T1 is too high, in some cases the prior-y grain diameter DA will coarsen 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 temperature T1 (°C) in the first heating process is set within the range of 900 to 980°C.
• In addition, if the heating time t1 is too short, in some cases the transformation of austenite will not be completed. In such a case, the produced steel material 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 prior-y grain diameter DA will coarsen 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 t1 (mins) in the first heating process is set within the range of 5 to 15 minutes.
[Second heating process]
• Preferably, in the second heating process according to the present embodiment, the intermediate steel material (hollow shell) 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 sufficiently dissolve. 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 prior-y grain diameter DA will coarsen 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 temperature T2 (°C) in the second heating process is set within the range of 1000 to 1100°C.
• In addition, if the heating time t2 is too short, in some cases carbides will not sufficiently dissolve. 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 prior-y grain diameter DA will coarsen 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]
• In the rapid cooling process, the intermediate steel material (hollow shell) heated in the second heating process is rapidly cooled. In the rapid cooling process, the intermediate steel material (hollow shell) is continuously cooled to continuously decrease the surface temperature of the hollow shell. The method of performing the continuous cooling treatment is not particularly limited, and a well-known method can be used. The method of performing the continuous cooling treatment is, for example, a method that cools the hollow shell by immersing the hollow shell in a water bath, or a method that cools the hollow shell in an accelerated manner by shower water cooling or mist cooling.
• If the cooling rate during quenching is too slow, the microstructure will not become a microstructure principally composed of tempered martensite and tempered bainite after 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 when the surface temperature of the intermediate steel material (hollow shell) is within the range of 800 to 500°C during quenching is defined as "cooling rate during quenching CR_800-500". Specifically, the cooling rate during quenching CR_800-500 is determined based on a temperature measured at a region that is most slowly cooled within a cross-section of the intermediate steel material that is being quenched (for example, in the case of forcedly cooling both surfaces, the cooling rate is measured at the center portion of the thickness of the intermediate steel material).
• In the rapid cooling process according to the present embodiment, a preferable cooling rate during quenching CR_800-500 is 60°C/min or more. A more preferable lower limit of the cooling rate during quenching CR_800-500 is 300°C/min, and further preferably is 600°C/min. Although an upper limit of the cooling rate during quenching CR_800-500 is not particularly defined, the upper limit is, for example, 6000°C/min. The quenching process according to the present embodiment can be carried out by the above process. Hereunder, the tempering process is described in detail.
[Tempering process]
• The tempering process is carried out by performing tempering after performing the aforementioned quenching. 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 A_c1 point, and holding the intermediate steel material at that temperature. The holding temperature in the tempering process is appropriately adjusted in accordance with the chemical composition of the steel material and the yield strength to be obtained. That is, with respect to an intermediate steel material (hollow shell) 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). Here, the holding temperature corresponds to the temperature of the furnace when the intermediate steel material after quenching is heated and held at the relevant temperature. The term "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.
• The holding temperature is appropriately adjusted in accordance with the chemical composition of the steel material and the yield strength to be obtained. That is, with respect to an intermediate steel material (hollow shell) 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 preferable holding temperature is 640 to 720°C.
• If the holding time is too short, in some cases a microstructure that is principally composed of tempered martensite and tempered bainite will not be obtained. On the other hand, if the holding time is too long, the aforementioned advantageous effect will be saturated. Therefore, in the tempering process of the present embodiment, preferably the holding time is set 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 further preferably is 120 minutes.
• 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 has been described as one example. However, the steel material according to the present embodiment may also be a steel plate or another shape. A method for producing a steel plate or a steel material of another shape also includes, for example, a preparation process, a quenching process, and a tempering process, similarly to the production method described above. Further, the production method described above is an example, and the steel material may also be produced by other production methods.
• Hereunder, the present invention is described more specifically by way of examples. Note that, the conditions adopted in the examples described hereunder are one example of conditions adopted for confirming 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 hereunder.
EXAMPLES
• Molten steels having the chemical compositions shown in Table 2-1 and Table 2-2 and which each had a weight of 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 content of V, the content of Nb, the content of Ti, the content of W, the content of Co, the content of Ni, and the content of Cu of Test No. 1 were each 0% when rounded off to the second decimal place. Further, the symbol "-" means that the content of Sn of Test No. 1 was 0% when rounded off to the third decimal place. Furthermore, the symbol "-" means that the content Zr, the content of Ca, the content of Mg, the content of B, and the content of REM of Test No. 1 were each 0% when rounded off to the fourth decimal place.
[Table 2-1]
• TABLE 2-1

  Test Number	Chemical Composition (unit is mass%; balance is Fe and impurities)
  	C	Si	Mn	P	S	Cr	Mo	Al	N	V	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	-	-	-
  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	-	-	-
  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	-	-	-

[Table 2-2]
• TABLE 2-2

  Test Number	Chemical Composition (unit is mass%; balance is Fe and impurities)
  	W	Zr	Co	Ni	Cu	Sn	Ca	Mg	B	REM
  1	-	-	-	-	-	-	-	-	-	-
  2	-	-	-	-	-	-	-	-	-	-
  3	-	-	-	-	-	-	-	-	-	-
  4	-	-	-	-	-	-	-	-	-	-
  5	-	-	-	-	-	-	-	-	-	-
  6	-	-	-	-	-	-	-	-	-	-
  7	-	-	-	-	-	-	-	-	-	-
  8	-	-	-	-	-	-	-	-	-	-
  9	-	-	-	-	-	-	-	-	-	-
  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
  39	-	-	-	-	-	-	-	-	-	-
  40	-	-	-	-	-	-	-	-	-	-
  41	-	-	-	-	-	-	-	-	-	-
  42	-	-	-	-	-	-	-	-	-	-
  43	-	-	-	-	-	-	-	-	-	-
  44	-	-	-	-	-	-	-	-	-	-
  45	-	-	-	-	-	-	-	-	-	-
  46	-	-	-	-	-	-	-	-	-	-
  47	-	-	-	-	-	-	-	-	-	-
  48	-	-	-	-	-	-	-	-	-	-
  49	-	-	-	-	-	-	-	-	-	-

• The molten steel of each test number was used to produce a round billet by a continuous casting process. The produced round billet of each test number was heated and subjected to hot working. Specifically, the round billet of cach test number was subjected to hot rolling by the Mannesmann-mandrel process as hot working to produce a hollow shell (seamless steel pipe) of each test number.
• The obtained hollow shell of each test number was subjected to quenching and tempering. At such time, the heating in the quenching process was carried out by performing heating in two stages by means of a first heating process and a second heating process. Specifically, the hollow shell of each test number was subjected to a first heating process in which heating was performed at a heating temperature T1 (°C) for a heating time tl (mins) which are each shown in the column "Quenching Process" in Table 3, and thereafter the hollow shell was subjected to a second heating process in which heating was performed at a heating temperature T2 (°C) for a heating time t2 (mins) which are each shown in the column "Quenching Process" in Table 3. The heated hollow shell of each test number was quenched by water cooling. At such time, the cooling rate during quenching CR_800-500 of the hollow shell of each test number satisfied a condition of being within the range of 60 to 6000°C/min.
[Table 3]
• TABLE 3

  Test Number	Quenching Process				Tempering 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
  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
  49	950	10	1050	30	660	30

• In addition, the obtained hollow shell of each test number was subjected to tempering. Specifically, the hollow shell of 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 column "Tempering Process" in Table 3. A scamless steel pipe of each test number was obtained by the above production process.
[Evaluation tests]
• The seamless steel pipe of each test number after the tempering described above was subjected to a tensile test, a coarse carbides number density measurement test, an Fn1 measurement test, and a Charpy impact test.
[Tensile test]
• 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 gage 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 carried out 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 obtained yield strength of each test number is shown in Table 4 as "YS (MPa)".
[Table 4]
• TABLE 4

  Test Number	YS (MPa)	Coarse Carbides Number 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	42	-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
  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 carbides number density measurement test]
• The seamless steel pipe of each test number was subjected to a coarse carbides number density measurement test by the method described above. Specifically, a test specimen in which a face including the pipe axis direction and the pipe radius direction was adopted as the observation surface was prepared from the center portion of the wall thickness of the seamless steel pipe of each test number. After polishing the observation surface of each of the prepared test specimens to obtain a mirror surface, on an observation surface with an area of 300 mm² (20 mm × 15 mm), particles which were identified based on contrast by the method described above were subjected to EDS analysis by the method described above, and carbides were thereby identified. Note that, in the present examples, similarly to the method described above, particles in which C was detected and in which the content of Fe was 50% by mass or more were identified as carbides.
• Among the identified carbides, those carbides having an equivalent circular diameter of 2 µm or more were identified using image analysis. The number density of coarse carbides (/mm²) was determined based on the total number of coarse carbides and the total area of the observation surface. The obtained coarse carbides number density (/mm²) of each test number is shown in Table 4.
[Fn1 measurement test]
• The prior-y grain diameter DA (µm), the block diameter DB (µm), and Fn 1 (= DA/DB) of the seamless steel pipe of each test number were determined using the methods described above. Specifically, a test specimen for measuring the prior-y grain diameter 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 center 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 about 60 seconds in a solution obtained by mixing an appropriate amount of a surfactant into an aqueous solution saturated with picric acid, to thereby reveal prior-y grain boundaries by etching. An etched observation surface of 100 mm² (10 mm × 10 mm) was observed using an optical microscope, and the grain size number was determined based on the intercept method in accordance with JIS G 0551 (2020). The determined grain size number was used to determine the prior-austenite grain diameter (prior-y grain diameter DA) by the method described above.
• Next, a test specimen for measuring the block diameter DB having an observation surface of 25 µm × 25 µm was prepared from the center portion of the wall thickness of the seamless steel pipe of each test number. The observation surface of each test specimen was subjected to EBSD measurement. The EBSD measurement was carried out with an accelerating voltage of 20 kV with respect to visual fields of 25 µm × 25 µm at a pitch of 0.1 µm. A crystal orientation figure was determined based on the obtained Kikuchi diffraction pattern, and regions surrounded by a boundary having an orientation difference of 15° or more with adjacent crystals were identified and a crystal orientation map was obtained. A region surrounded 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.
• The obtained prior-y grain diameter DA (µm) of each test number is shown in the column "DA (µm)" in Table 4. The obtained block diameter DB (µm) of each test number is shown in the column "DB (µm)" in Table 4. Fn1 (= DA/DB) was determined based on the obtained prior-y grain diameter DA (µm) and block diameter DB (µm) of each test number. The obtained Fn1 of each test number is shown in Table 4.
[Charpy impact test]
• 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 which had a notched surface perpendicular to the pipe axis direction, and whose longitudinal direction was perpendicular to the pipe axis direction and the pipe radius direction were prepared from the center portion of the wall thickness of the seamless steel pipe of each test number. A Charpy impact test in accordance with ASTM E23 (2018) was carried out on the prepared V-notch test specimens. The range of the test temperatures was set to -120 to 20°C, and the test temperatures were set at eight levels which were varied in increments of 20°C (-120°C, -100°C, -80°C, -60°C, -40°C, -20°C, 0°C, and 20°C). The Charpy impact test was carried out using two test specimens for each test temperature. The percent brittle fracture (%) of the respective test specimens after the test at each temperature was determined. The temperature (°C) at which the percent brittle fracture became 50% was determined from an approximated curve obtained by plotting the test temperatures (°C) and the obtained percent brittle fracture (%), and the fracture appearance transition temperature vTrs (°C) was thereby obtained. The obtained fracture appearance transition temperature vTrs (°C) of each test number is shown in the column "vTrs (°C)" in Table 4.
[Evaluation Results]
• Referring to Table 2-1, Table 2-2, Table 3, and Table 4, the chemical compositions of the seamless steel pipes of Test Nos. 1 to 38 were appropriate, and the production methods of Test Nos. 1 to 38 also satisfied the preferred conditions described in the present description. As a result, for each of these seamless steel pipes, the yield strength was 552 to less than 655 MPa, the coarse carbides number density 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 pipes, the fracture appearance transition temperature vTrs was -70°C or less, and thus each of these seamless steel pipes had excellent low-temperature toughness even in an extremely low temperature environment. Note that, for each of these seamless steel pipes, it was determined that the total of the volume ratios of tempered martensite and tempered bainite was 90% or more.
• On the other hand, in the seamless steel pipe of Test No. 39, the content of C was too low, and Fnl was too 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.
• In the seamless steel pipe of Test No. 40, the content of Mn was too 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.
• In the seamless steel pipe of Test No. 41, the content of P was too 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.
• In the seamless steel pipe of Test No. 42, the content of S was too 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.
• In the seamless steel pipe of Test No. 43, the content of Cr was too 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.
• In the seamless steel pipe of Test No. 44, the content of Cr was too high. 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.
• In the seamless steel pipe of Test No. 45, the content of Mo was too 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.
• 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.
• For the seamless steel pipe of Test No. 47, the second heating process was not performed in the quenching process. As a result, in this seamless steel pipe the number 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.
• For the seamless steel pipe of 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 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.
• For the seamless steel pipe of 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.
• An embodiment of the present disclosure has been described above. However, the embodiment described above is merely an example for carrying out the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiment, and can be implemented by appropriately modifying the above-described embodiment within a range not departing from the gist thereof.

Claims (3)

1. A steel material consisting of, in 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%,

   sol. Al: 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 balance: Fe and impurities,

   wherein:

   a yield strength is 552 to less than 655 MPa, and

   in the steel material,

   a number density of carbides having an equivalent circular diameter of 2 µm or more is 10 /mm ² or less, and

   a prior-austenite grain diameter is defined as "DA" and a block diameter is defined as "DB",

   the DA and the DB satisfy Formula (1): $$2.5\le \mathrm{DA}/\mathrm{DB}\le 10.0$$

   where, the prior-austenite grain diameter in units of µm is substituted for DA in Formula (1), and the block diameter in units of µm is substituted for DB in Formula (1).
2. The steel material according to claim 1, 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. The steel material according to claim 1 or claim 2, wherein:
   the steel material is a steel pipe.