Steel material suitable for use in sour environment

Abstract

A steel material that has a high yield strength and that has excellent low-temperature toughness and excellent SSC resistance is provided. The steel material according to the present disclosure has the chemical composition described in the description, and a yield strength of 862 MPa or more, and satisfies Formulae (1) to (4). In the steel material, a number density NDF of precipitates having an equivalent circular diameter of 20 to 150 nm is 0.650 / µm2 or more, and a number density NDC of precipitates having an equivalent circular diameter of 250 nm or more is 0.290 / µm2 or less. 0.157×C−0.0006×Cr−0.0098×Mo−0.0482×V+0.0006 / θCr≤0.300 1+263×C−Cr−16×Mo−80×V / 98−358×C+159×Cr+15×Mo+96×V≤0.355 −9.7×Mn−104×S+0.8×Mo+0.08×Ni2−4.1×Ni−5.1×Ti≥−9.0 15.8×Si−33.8×Mn−28.8×Ni≥−51.0

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a steel material, and more particularly relates to a steel material suitable for use in a sour environment.BACKGROUND ART

[0002] Due to the deepening of oil wells and gas wells (hereunder, oil wells and gas wells are collectively referred to as "oil wells"), there is a request to enhance the strength of oil-well steel materials represented by oil-well steel pipes. Specifically, 80 ksi grade (yield strength is 80 to less than 95 ksi, that is, 552 to less than 655 MPa) and 95 ksi grade (yield strength is 95 to less than 110 ksi, that is, 655 to less than 758 MPa) oil-well steel materials are being widely utilized, and recently requests are also starting to be made for 110 ksi grade (yield strength is 110 to less than 125 ksi, that is, 758 to less than 862 MPa), 125 ksi grade (yield strength is 125 to less than 140 ksi, that is, 862 to less than 965 MPa) and 140 ksi or more (yield strength is 140 ksi or more, that is, 965 MPa or more) oil-well steel pipes.

[0003] On the other hand, in recent years, deep wells beneath the surface of the sea are also being actively developed. For example, in so-called "deep-sea offshore oil fields" that are at a water depth of 2000 m or more, the water temperature is low. Oil-well steel pipes that are used in such severe environments are required to have not only high strength, but also to have excellent low-temperature toughness. However, if the yield strength of a steel material is increased excessively, there is a concern that the low-temperature toughness of the steel material will decrease.

[0004] Furthermore, most deep wells are in a sour environment containing corrosive hydrogen sulfide. In the present description, the term "sour environment" means an acidified environment containing hydrogen sulfide. Note that, in some cases a sour environment may also contain carbon dioxide. Oil-well steel pipes for use in such sour environments are required to have not only high strength, but to also have sulfide stress cracking resistance (hereunder, referred to as "SSC resistance"). Thus, a steel material which has high strength and excellent low-temperature toughness and also has excellent SSC resistance has started to be demanded.

[0005] Technology for increasing the low-temperature toughness and SSC resistance of steel materials as typified by oil-well steel pipes is proposed in Japanese Patent Application Publication No. 2000-297344 (Patent Literature 1), Japanese Patent Application Publication No. 2001-271134 (Patent Literature 2), and International Application Publication No. WO2008 / 123422 (Patent Literature 3).

[0006] A steel for oil wells that is disclosed in Patent Literature 1 contains, in mass%, C: 0.15 to 0.3%, Cr: 0.2 to 1.5%, Mo: 0.1 to 1%, V: 0.05 to 0.3%, and Nb: 0.003 to 0.1%. In this steel for oil wells, the amount of precipitating carbides is within the range of 1.5 to 4% by mass, the proportion that MC-type carbides occupy among the amount of carbides is within the range of 5 to 45% by mass, and when the wall thickness of the product is taken as t (mm), the proportion of M 23 C 6 -type carbides is (200 / t) or less in percent by mass. It is described in Patent Literature 1 that the aforementioned steel for oil wells is excellent in toughness and SSC resistance.

[0007] A low-alloy steel material that is disclosed in Patent Literature 2 consists of, in mass%, C: 0.2 to 0.35%, Si: 0.05 to 0.5%, Mn: 0.1 to 1%, P: 0.025% or less, S: 0.01% or less, Cr: 0.1 to 1.2%, Mo: 0.1 to 1%, B: 0.0001 to 0.005%, Al: 0.005 to 0.1%, N: 0.01% or less, V: 0.05 to 0.5%, Ni: 0.1% or less, W: 1.0% or less and O: 0.01% or less, with the balance being Fe and impurities, and satisfies the formula (0.03≤Mo×V≤0.3) and the formula (0.5×Mo-V+GS / 10≥1) and has a yield strength of 1060 MPa or more. Note that, "GS" in the formula represents the ASTM grain size number of prior-austenite grains. It is described in Patent Literature 2 that the aforementioned low-alloy steel material is excellent in SSC resistance and toughness.

[0008] A low-alloy steel disclosed in Patent Literature 3 consists of, in mass%, C: 0.10 to 0.20%, Si: 0.05 to 1.0%, Mn: 0.05 to 1.5%, Cr: 1.0 to 2.0%, Mo: 0.05 to 2.0%, Al: 0.10% or less and Ti: 0.002 to 0.05%, with Ceq (= C+(Mn / 6)+(Cr+Mo+V) / 5) being 0.65 or more, and with the balance being Fe and impurities, and among the impurities the low-alloy steel contains P: 0.025% or less, S: 0.010% or less, N: 0.007% or less, and B: less than 0.0003%. In the low-alloy steel, the amount of M 23 C 6 -type precipitates having a grain size of 1 µm or more is not more than 0.1 per mm 2< . It is described in Patent Literature 3 that in the low-alloy steel, toughness is secured and SSC resistance is enhanced.CITATION LISTPATENT LITERATURE

[0009] Patent Literature 1: Japanese Patent Application Publication No. 2000-297344 Patent Literature 2: Japanese Patent Application Publication No. 2001-271134 Patent Literature 3: International Application Publication No. WO2008 / 123422 SUMMARY OF INVENTIONTECHNICAL PROBLEM

[0010] In the aforementioned Patent Literatures 1 to 3, steel materials having excellent toughness and excellent SSC resistance are proposed. However, a steel material (for example, an oil-well steel pipe) having a high yield strength, excellent low-temperature toughness, and excellent SSC resistance may be obtained by a technique other than the techniques disclosed in the aforementioned Patent Literatures 1 to 3.

[0011] An objective of the present disclosure is to provide a steel material having a high yield strength, excellent low-temperature toughness, and excellent SSC resistance.SOLUTION TO PROBLEM

[0012] A steel material according to the present disclosure consists of, in mass%, C: more than 0.20 to 0.35%, Si: 0.05 to 1.50%, Mn: 0.02 to 1.00%, P: 0.025% or less, S: 0.0100% or less, Al: 0.005 to 0.100%, Ni: more than 0.10 to 2.50%, Cr: 0.40 to 1.50%, Mo: 0.30 to 1.50%, Ti: 0.002 to 0.050%, B: 0.0001 to 0.0050%, N: 0.0100% or less, O: 0.0100% or less, V: 0 to 0.60%, Nb: 0 to 0.030%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, rare earth metal: 0 to 0.0100%, Co: 0 to 0.50%, W: 0 to 0.50%, Cu: 0 to 0.50%, and the balance being Fe and impurities, wherein: a yield strength is 862 MPa or more, a Cr concentration in precipitates having an equivalent circular diameter of 20 nm or more is defined as "θ Cr ", within ranges of contents of elements of the steel material, the contents of elements of the steel material and a Cr concentration θ Cr in precipitates having an equivalent circular diameter of 20 nm or more satisfy Formulae (1) to (4), and in the steel material: a number density NDF of precipitates having an equivalent circular diameter of 20 to 150 nm is 0.650 / µm 2< or more, and a number density NDC of precipitates having an equivalent circular diameter of 250 nm or more is 0.290 / µm 2< or less: 0.157 × C − 0.0006 × Cr − 0.0098 × Mo − 0.0482 × V + 0.0006 / θ Cr ≤ 0.300 1 + 263 × C − Cr − 16 × Mo − 80 × V / 98 − 358 × C + 159 × Cr + 15 × Mo + 96 × V ≤ 0.355 − 9.7 × Mn − 104 × S + 0.8 × Mo + 0.08 × Ni 2 − 4.1 × Ni − 5.1 × Ti ≥ − 9.0 15.8 × Si − 33.8 × Mn − 28.8 × Ni ≥ − 51.0 where, a content in units of percent by mass of a corresponding element is substituted for each symbol of an element in Formulae (1) to (4), and if a corresponding element is not contained, "0" is substituted for the symbol of the relevant element, and a Cr concentration in units of mass fraction in precipitates having an equivalent circular diameter of 20 nm or more is substituted for θ Cr in Formula (1). ADVANTAGEOUS EFFECT OF INVENTION

[0013] The steel material according to the present disclosure has a high yield strength and has excellent low-temperature toughness and excellent SSC resistance.DESCRIPTION OF EMBODIMENTS

[0014] The present inventors conducted investigations and studies regarding a method for obtaining a high yield strength, excellent low-temperature toughness, and excellent SSC resistance in a steel material that will assumedly be used in a sour environment, and obtained the following findings.

[0015] Specifically, the present inventors attempted to obtain a steel material having a yield strength of 862 MPa or more (125 ksi or more) as a high yield strength. Therefore, first, the present inventors conducted studies from the viewpoint of the chemical composition with respect to a steel material having a yield strength of 125 ksi or more, excellent low-temperature toughness and excellent SSC resistance. As a result, the present inventors found that if a Ni content is made a high content within a range of more than 0.10 to 2.50%, there is a possibility that the low-temperature toughness of the steel material can be increased.

[0016] That is, the present inventors considered that if a steel material consists of, in mass%, C: more than 0.20 to 0.35%, Si: 0.05 to 1.50%, Mn: 0.02 to 1.00%, P: 0.025% or less, S: 0.0100% or less, Al: 0.005 to 0.100%, Ni: more than 0.10 to 2.50%, Cr: 0.40 to 1.50%, Mo: 0.30 to 1.50%, Ti: 0.002 to 0.050%, B: 0.0001 to 0.0050%, N: 0.0100% or less, O: 0.0100% or less, V: 0 to 0.60%, Nb: 0 to 0.030%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, rare earth metal: 0 to 0.0100%, Co: 0 to 0.50%, W: 0 to 0.50%, and Cu: 0 to 0.50%, with the balance being Fe and impurities, there is a possibility of obtaining a steel material that has a yield strength of 125 ksi or more and has excellent low-temperature toughness and excellent SSC resistance.

[0017] On the other hand, in the case of a steel material having the aforementioned chemical composition in which the Ni content is made a high content of more than 0.10 to 2.50%, there is a concern that in a sour environment, local corrosion will be promoted and the SSC resistance of the steel material will decrease. Further, with regard to a steel material having the aforementioned chemical composition in which the Ni content is made a high content of more than 0.10 to 2.50% and the low-temperature toughness of the steel material was made high, it is preferable to further increase the low-temperature toughness. Therefore, with respect to a steel material having the aforementioned chemical composition, the present inventors conducted various studies regarding a technique for increasing the low-temperature toughness and the SSC resistance while maintaining a yield strength of 125 ksi or more. Specifically, the present inventors conducted various studies that focused on precipitates in a steel material having the aforementioned chemical composition. As a result, the present inventors discovered that if the number density of coarse precipitates is reduced and, in addition, the number density of fine precipitates is increased, there is a possibility of increasing the low-temperature toughness and the SSC resistance while maintaining a yield strength of 125 ksi or more.

[0018] In a steel material having the aforementioned chemical composition, if the amount of fine precipitates is made large, there is a possibility of increasing the low-temperature toughness of the steel material while maintaining the strength of the steel material. In addition, in a steel material having the aforementioned chemical composition, if the amount of coarse precipitates is made small, there is a possibility that a decrease in the SSC resistance of the steel material can be suppressed. In short, if a large number of fine precipitates can be caused to precipitate instead of coarse precipitates, there is a possibility that in a steel material having the aforementioned chemical composition, the low-temperature toughness and the SSC resistance of the steel material can be enhanced while maintaining a yield strength of 125 ksi or more.

[0019] As a result of further detailed studies conducted by the present inventors taking into consideration the above findings, it was clarified that in a steel material having the aforementioned chemical composition and a yield strength of 125 ksi or more, specifically, if a number density NDF of precipitates having an equivalent circular diameter of 20 to 150 nm is 0.650 / µm 2< or more and a number density NDC of precipitates having an equivalent circular diameter of 250 nm or more is 0.290 / µm 2< or less, both excellent low-temperature toughness and excellent SSC resistance can be achieved. Hereunder, in the present description, precipitates having an equivalent circular diameter of 20 to 150 nm are defined as "fine precipitates", and precipitates having an equivalent circular diameter of 250 nm or more are defined as "coarse precipitates".

[0020] In addition, the present inventors conducted various studies regarding a technique which, in a steel material having the aforementioned chemical composition and a yield strength of 125 ksi or more, makes a number density NDF of fine precipitates 0.650 / µm 2< or more, and makes a number density NDC of coarse precipitates 0.290 / µm 2< or less. As a result, the present inventors discovered that in a steel material having the aforementioned chemical composition and a yield strength of 125 ksi or more, if the chemical composition and a chromium (Cr) concentration in precipitates of the steel material satisfy Formula (1), the number density NDF of fine precipitates can be made 0.650 / µm 2< or more and, furthermore, the number density NDC of coarse precipitates can be made 0.290 / µm 2< or less. 0.157 × C − 0.0006 × Cr − 0.0098 × Mo − 0.0482 × V + 0.0006 / θ Cr ≤ 0.300

[0021] Where, a content in units of percent by mass of a corresponding element is substituted for each symbol of an element in Formula (1). If a corresponding element is not contained, "0" is substituted for the symbol of the relevant element. Further, a Cr concentration in units of mass fraction in precipitates having an equivalent circular diameter of 20 nm or more is substituted for θ Cr in Formula (1).

[0022] It is defined that Fn1 = (0.157×C-0.0006×Cr-0.0098×Mo-0.0482×V+0.0006) / θ Cr . The numerator of Fn1 is an index of the total precipitation amount of cementite. The denominator θ Cr of Fn1 is the Cr concentration (unit: mass fraction) in precipitates having an equivalent circular diameter of 20 nm or more.

[0023] Here, in a steel material having the aforementioned chemical composition, almost all of the precipitates having an equivalent circular diameter of 20 nm or more are cementite. Therefore, in a steel material having the aforementioned chemical composition, cementite is liable to coarsen due to Ostwald growth in a tempering process that is described later. During Ostwald growth, a single coarse cementite particle is formed from a plurality of fine cementite particles in the steel material. That is, if Ostwald growth of the cementite can be suppressed, there is a possibility that the number density NDF of fine precipitates can be increased and the number density NDC of coarse precipitates can be decreased.

[0024] Further, in the case of Ostwald growth, after precipitation of cementite is completed, fine cementite particles dissolve in the matrix, and comparatively large cementite particles grow further. That is, if dissolution of fine cementite particles in the matrix can be suppressed, there is a possibility that coarsening of cementite can be suppressed. On the other hand, Cr concentrates in cementite, and stabilizes the cementite. That is, it becomes difficult for cementite in which the Cr concentration is high to dissolve in the steel material. It is considered that, as a result, Ostwald growth of cementite is suppressed.

[0025] That is, the Cr concentration θ Cr in precipitates having an equivalent circular diameter of 20 nm or more that is the denominator of Fn1 is an index that indicates the degree of difficulty of Ostwald growth of cementite. The larger that the denominator (θ Cr ) of Fn1 is, the greater the possibility that the number density NDF of fine precipitates in the steel material will increase and that the number density NDC of coarse precipitates in the steel material will decrease. Further, as described above, the numerator of Fn1 is an index of the total precipitation amount of cementite. Furthermore, in a steel material having the aforementioned chemical composition, the larger the total precipitation amount of cementite is, the easier it is for coarse cementite to be formed. That is, if the numerator of Fn1 is reduced, there is a possibility that the number density NDC of coarse precipitates can be reduced.

[0026] In short, Fn1 is an index relating to the number density NDF of fine precipitates and the number density NDC of coarse precipitates in a steel material having the aforementioned chemical composition. As long as the other conditions of the present embodiment are satisfied and Fn1 is not more than 0.300, the number density NDF of fine precipitates in the steel material can be made 0.650 / µm 2< or more and the number density NDC of coarse precipitates can be made 0.290 / µm 2< or less. Therefore, in the present embodiment, Fn1 is not more than 0.300.

[0027] The present inventors also studied methods for increasing the Cr concentration θ Cr in precipitates having an equivalent circular diameter of 20 nm or more. As a result, the present inventors discovered that, on the precondition that the other conditions of the present embodiment are satisfied, if the aforementioned chemical composition also satisfies the following Formula (2), the Cr concentration θ Cr in precipitates having an equivalent circular diameter of 20 nm or more can be increased. 1 + 263 × C − Cr − 16 × Mo − 80 × V / 98 − 358 × C + 159 × Cr + 15 × Mo + 96 × V ≤ 0.355

[0028] Where, a content in units of percent by mass of a corresponding element is substituted for each symbol of an element in Formula (2). If a corresponding element is not contained, "0" is substituted for the symbol of the relevant element.

[0029] It is defined that Fn2 = (1+263×C-Cr-16×Mo-80×V) / (98-358×C+159×Cr+15×Mo+96×V). Fn2 is an index that indicates the degree to which it is difficult for Cr to concentrate in precipitates. If Fn2 is not more than 0.355, Cr concentrates sufficiently in precipitates and it is easy to suppress Ostwald growth of cementite. Therefore, in the steel material according to the present embodiment, Fn2 is not more than 0.355.

[0030] In this connection, as described above, in a steel material having the aforementioned chemical composition in which the Ni content is made a high content of more than 0.10 to 2.50%, there is a concern that in a sour environment, local corrosion will be promoted and the SSC resistance of the steel material will decrease. Therefore, the present inventors investigated techniques for stably increasing the SSC resistance in a steel material having the aforementioned chemical composition in which the Ni content is made high. As a result, the present inventors discovered that if the aforementioned chemical composition also satisfies the following Formula (3) and Formula (4), the SSC resistance of the steel material is stably increased. − 9.7 × Mn − 104 × S + 0.8 × Mo + 0.08 × Ni 2 − 4.1 × Ni − 5.1 × Ti ≥ − 9.0 15.8 × Si − 33.8 × Mn − 28.8 × Ni ≥ − 51.0

[0031] Where, a content in units of percent by mass of a corresponding element is substituted for each symbol of an element in Formula (3) and Formula (4).

[0032] Let Fn3 be defined as Fn3 = -9.7×Mn-104×S+0.8×Mo+0.08×Ni 2< -4.1×Ni-5.1×Ti. Let Fn4 be defined as Fn4 = 15.8×Si-33.8×Mn-28.8×Ni. Fn3 and Fn4 are indexes of the SSC resistance of a steel material having the aforementioned chemical composition. If Fn3 satisfies a condition of being -9.0 or more, and Fn4 satisfies a condition of being -51.0 or more, local corrosion of the steel material will be suppressed and the SSC resistance of the steel material can be stably increased. Therefore, the steel material according to the present embodiment has the aforementioned chemical composition, and in the steel material, Fn1 is not more than 0.300, Fn2 is not more than 0.355, and furthermore, Fn3 is -9.0 or more and Fn4 is - 51.0 or more.

[0033] Thus, the steel material according to the present embodiment has the aforementioned chemical composition, and in the steel material, Fn1 is not more than 0.300, Fn2 is not more than 0.355, Fn3 is -9.0 or more, and Fn4 is -51.0 or more, and the steel material has a yield strength of 862 MPa or more, and in addition, the number density NDF of fine precipitates in the steel material is 0.650 / µm 2< or more, and the number density NDC of coarse precipitates is 0.290 / µm 2< or less. As a result, the steel material according to the present embodiment has a high yield strength of 125 ksi or more (862 MPa or more) and has excellent low-temperature toughness and excellent SSC resistance.

[0034] The steel material according to the present embodiment that was completed based on the above findings has the following configuration. [1] A steel material consisting of, in mass%, C: more than 0.20 to 0.35%, Si: 0.05 to 1.50%, Mn: 0.02 to 1.00%, P: 0.025% or less, S: 0.0100% or less, Al: 0.005 to 0.100%, Ni: more than 0.10 to 2.50%, Cr: 0.40 to 1.50%, Mo: 0.30 to 1.50%, Ti: 0.002 to 0.050%, B: 0.0001 to 0.0050%, N: 0.0100% or less, O: 0.0100% or less, V: 0 to 0.60%, Nb: 0 to 0.030%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, rare earth metal: 0 to 0.0100%, Co: 0 to 0.50%, W: 0 to 0.50%, Cu: 0 to 0.50%, and the balance being Fe and impurities, wherein: a yield strength is 862 MPa or more, a Cr concentration in precipitates having an equivalent circular diameter of 20 nm or more is defined as "θ Cr ", within ranges of contents of elements of the steel material, the contents of elements of the steel material and a Cr concentration θ Cr in precipitates having an equivalent circular diameter of 20 nm or more satisfy Formulae (1) to (4), and in the steel material: a number density NDF of precipitates having an equivalent circular diameter of 20 to 150 nm is 0.650 / µm 2< or more, and a number density NDC of precipitates having an equivalent circular diameter of 250 nm or more is 0.290 / µm 2< or less. 0.157 × C − 0.0006 × Cr − 0.0098 × Mo − 0.0482 × V + 0.0006 / θ Cr ≤ 0.300 1 + 263 × C − Cr − 16 × Mo − 80 × V / 98 − 358 × C + 159 × Cr + 15 × Mo + 96 × V ≤ 0.355 − 9.7 × Mn − 104 × S + 0.8 × Mo + 0.08 × Ni 2 − 4.1 × Ni − 5.1 × Ti ≥ − 9.0 15.8 × Si − 33.8 × Mn − 28.8 × Ni ≥ − 51.0 Where, a content in units of percent by mass of a corresponding element is substituted for each symbol of an element in Formulae (1) to (4). If a corresponding element is not contained, "0" is substituted for the symbol of the relevant element. Further, a Cr concentration in units of mass fraction in precipitates having an equivalent circular diameter of 20 nm or more is substituted for θ Cr in Formula (1). [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: 0.001 to 0.030%, Ca: 0.0001 to 0.0100%, Mg: 0.0001 to 0.0100%, Zr: 0.0001 to 0.0100%, rare earth metal: 0.0001 to 0.0100%, Co: 0.01 to 0.50%, W: 0.01 to 0.50%, and Cu: 0.01 to 0.50%. [3] The steel material according to [1], wherein within ranges of contents of elements of the steel material, the contents of elements of the steel material, the number density NDF of precipitates having an equivalent circular diameter of 20 to 150 nm, and the number density NDC of precipitates having an equivalent circular diameter of 250 nm or more satisfy Formula (5). − Mn − 20 × P + 11 × Ni + Mo × NDF 2 / NDC 1 / 2 ≥ 4.0 Where, a content in units of percent by mass of a corresponding element is substituted for each symbol of an element in Formula (5). Further, a number density in units of / µm 2< of precipitates having an equivalent circular diameter of 20 to 150 nm is substituted for NDF in Formula (5). Furthermore, a number density in units of / µm 2< of precipitates having an equivalent circular diameter of 250 nm or more is substituted for NDC in Formula (5), and in a case where a number density of precipitates having an equivalent circular diameter of 20 to 150 nm is less than 0.001 / µm 2< , 0.001 is substituted for NDC. [4] The steel material according to [2], wherein: within ranges of contents of elements of the steel material, the contents of elements of the steel material, the number density NDF of precipitates having an equivalent circular diameter of 20 to 150 nm, and the number density NDC of precipitates having an equivalent circular diameter of 250 nm or more satisfy Formula (5). − Mn − 20 × P + 11 × Ni + Mo × NDF 2 / NDC 1 / 2 ≥ 4.0 Where, a content in units of percent by mass of a corresponding element is substituted for each symbol of an element in Formula (5). Further, a number density in units of / µm 2< of precipitates having an equivalent circular diameter of 20 to 150 nm is substituted for NDF in Formula (5). Furthermore, a number density in units of / µm 2< of precipitates having an equivalent circular diameter of 250 nm or more is substituted for NDC in Formula (5), and in a case where a number density of precipitates having an equivalent circular diameter of 20 to 150 nm is less than 0.001 / µm 2< , 0.001 is substituted for NDC. [5] The steel material according to any one of [1] to [4], wherein: the steel material is an oil-well steel pipe.

[0035] Note that, 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.

[0036] Hereunder, the steel material according to the present invention is described in detail.[Chemical Composition]

[0037] The chemical composition of the steel material according to the present invention contains the following elements. The symbol "%" in relation to an element means "mass percent" unless specifically stated otherwise.C: more than 0.20 to 0.35%

[0038] Carbon (C) enhances hardenability of the steel material and increases strength of the steel material. C also promotes spheroidization of carbides during tempering in the production process, and thereby enhances the SSC resistance of the steel material. If carbides are dispersed, strength of the steel material increases further. If the C content is too low, even when the contents of other elements are within the range of the present embodiment, the aforementioned effects cannot not be sufficiently obtained. On the other hand, if the C content is too high, even when the contents of other elements are within the range of the present embodiment, too many carbides will be formed and the low-temperature toughness of the steel material will decrease. In addition, if the C content is too high, quench cracking is liable to occur during quenching in the production process in some cases. Therefore, the C content is within the range of more than 0.20 to 0.35%. A preferable lower limit of the C content is 0.22%, more preferably is 0.24%, and further preferably is 0.26%. A preferable upper limit of the C content is 0.32%.Si: 0.05 to 1.50%

[0039] Silicon (Si) deoxidizes the steel. If the Si content is too low, even when the contents of other elements are within the range of the present embodiment, the aforementioned effect cannot be sufficiently obtained. On the other hand, if the Si content is too high, even when the contents of other elements are within the range of the present embodiment, the SSC resistance of the steel material decreases. Therefore, the Si content is within the range of 0.05 to 1.50%. A preferable lower limit of the Si content is 0.15%, and more preferably is 0.20%. A preferable upper limit of the Si content is 1.40%, more preferably is 1.38%, and further preferably is 1.30%.Mn: 0.02 to 1.00%

[0040] Manganese (Mn) deoxidizes the steel. Mn also enhances hardenability of the steel material and increases strength of the steel material. If the Mn content is too low, even when the contents of other elements are within the range of the present embodiment, the aforementioned effects cannot be sufficiently obtained. On the other hand, if the Mn content is too high, even when the contents of other elements are within the range of the present embodiment, Mn segregates at grain boundaries together with impurities such as P and S, and consequently the SSC resistance of the steel material decreases. Therefore, the Mn content is within a range of 0.02 to 1.00%. A preferable lower limit of the Mn content is 0.03%, and more preferably is 0.05%. A preferable upper limit of the Mn content is 0.90%, and more preferably is 0.80%.P: 0.025% or less

[0041] Phosphorous (P) is an impurity. That is, the lower limit of the P content is more than 0%. If the P content is too high, even when the contents of other elements are within the range of the present embodiment, P segregates at the grain boundaries and decreases the low-temperature toughness and the SSC resistance of the steel material. Therefore, the P content is 0.025% or less. A preferable upper limit of the P content is 0.020%, and more preferably is 0.015%. Preferably, the P content is as low as possible. However, if the P content is excessively reduced, the production cost increases significantly. Therefore, when taking industrial production into consideration, a preferable lower limit of the P content is 0.001%, more preferably is 0.002%, and further preferably is 0.003%.S: 0.0100% or less

[0042] Sulfur (S) is an impurity. That is, the lower limit of the S content is more than 0%. If the S content is too high, even when the contents of other elements are within the range of the present embodiment, S segregates at the grain boundaries and decreases the low-temperature toughness and the SSC resistance of the steel material. Therefore, the S content is 0.0100% or less. A preferable upper limit of the S content is 0.0075%, more preferably is 0.0050%, and further preferably is 0.0030%. Preferably, the S content is as low as possible. However, if the S content is excessively reduced, the production cost increases significantly. Therefore, when taking industrial production into consideration, a preferable lower limit of the S content is 0.0001%, more preferably is 0.0002%, and further preferably is 0.0003%.Al: 0.005 to 0.100%

[0043] Aluminum (Al) deoxidizes the steel material. If the Al content is too low, even when the contents of other elements are within the range of the present embodiment, the aforementioned effect cannot not be sufficiently obtained. As a result, the SSC resistance of the steel material decreases. On the other hand, if the Al content is too high, even when the contents of other elements are within the range of the present embodiment, coarse oxide-based inclusions are formed and the SSC resistance of the steel material decreases. Therefore, the Al content is within a range of 0.005 to 0.100%. A preferable lower limit of the Al content is 0.015%, and more preferably is 0.020%. A preferable upper limit of the Al content is 0.080%, and more preferably is 0.060%. In the present description, the "Al" content means "acid-soluble Al", that is, the content of "sol. Al".Ni: more than 0.10 to 2.50%

[0044] Nickel (Ni) enhances hardenability of the steel material and increases the strength of the steel material. In addition, Ni dissolves in the steel and enhances the low-temperature toughness of the steel material. If the Ni content is too low, even when the contents of other elements are within the range of the present embodiment, the aforementioned effects cannot be sufficiently obtained. On the other hand, if the Ni content is too high, even when the contents of other elements are within the range of the present embodiment, the Ni will promote local corrosion, and the SSC resistance of the steel material will decrease. Therefore, the Ni content is within the range of more than 0.10 to 2.50%. A preferable lower limit of the Ni content is 0.11%, more preferably is 0.12%, and further preferably is 0.15%. A preferable upper limit of the Ni content is 2.30%, more preferably is 2.00%, further preferably is 1.95%, and further preferably is 1.80%.Cr: 0.40 to 1.50%

[0045] Chromium (Cr) enhances hardenability of the steel material and increases strength of the steel material. Cr also concentrates in cementite in the steel material and thereby suppresses Ostwald growth of the cementite. As a result, the number density NDF of fine precipitates in the steel material increases, and the number density NDC of coarse precipitates decreases. Thus, the low-temperature toughness and the SSC resistance of the steel material are enhanced. Cr also increases the temper softening resistance of the steel material and enables high-temperature tempering. As a result, the low-temperature toughness and the SSC resistance of the steel material increase. If the Cr content is too low, even when the contents of other elements are within the range of the present embodiment, the aforementioned effects cannot not be sufficiently obtained. On the other hand, if the Cr content is too high, even when the contents of other elements are within the range of the present embodiment, the low-temperature toughness and the SSC resistance of the steel material will decrease. Therefore, the Cr content is within a range of 0.40 to 1.50%. A preferable lower limit of the Cr content is 0.45%, and more preferably is 0.50%. A preferable upper limit of the Cr content is 1.30%, and more preferably is 1.25%.Mo: 0.30 to 1.50%

[0046] Molybdenum (Mo) enhances hardenability of the steel material and increases strength of the steel material. Mo also increases the temper softening resistance of the steel material and enables high-temperature tempering. As a result, the low-temperature toughness and the SSC resistance of the steel material increase. If the Mo content is too low, even when the contents of other elements are within the range of the present embodiment, the aforementioned effects cannot not be sufficiently obtained. On the other hand, if the Mo content is too high, the aforementioned effects are saturated. Therefore, the Mo content is within a range of 0.30 to 1.50%. A preferable lower limit of the Mo content is 0.40%, more preferably is 0.50%, and further preferably is 0.55%. A preferable upper limit of the Mo content is 1.40%, more preferably is 1.30%, and further preferably is 1.25%.Ti: 0.002 to 0.050%

[0047] Titanium (Ti) combines with N to form nitrides, and thereby refines grains of the steel material by the pinning effect. As a result, strength of the steel material increases, and in addition, the low-temperature toughness and the SSC resistance of the steel material are enhanced. If the Ti content is too low, even when the contents of other elements are within the range of the present embodiment, the aforementioned effect cannot not be sufficiently obtained. On the other hand, if the Ti content is too high, even when the contents of other elements are within the range of the present embodiment, Ti nitrides coarsen and the SSC resistance of the steel material decreases. Therefore, the Ti content is within a range of 0.002 to 0.050%. A preferable lower limit of the Ti content is 0.003%, and more preferably is 0.005%. A preferable upper limit of the Ti content is 0.030%, more preferably is 0.020%, and further preferably is 0.018%.B: 0.0001 to 0.0050%

[0048] Boron (B) dissolves in the steel, enhances hardenability of the steel material and increases strength of the steel material. If the B content is too low, even when the contents of other elements are within the range of the present embodiment, the aforementioned effect cannot not be sufficiently obtained. On the other hand, if the B content is too high, even when the contents of other elements are within the range of the present embodiment, coarse nitrides form and the SSC resistance of the steel material decreases. Therefore, the B content is within a range of 0.0001 to 0.0050%. A preferable lower limit of the B content is 0.0003%, and more preferably is 0.0007%. A preferable upper limit of the B content is 0.0030%, more preferably is 0.0025%, further preferably is 0.0020%, and further preferably is 0.0015%.N: 0.0100% or less

[0049] Nitrogen (N) is unavoidably contained. That is, the lower limit of the N content is more than 0%. N combines with Ti to form nitrides, and thereby refines grains of the steel material by the pinning effect. As a result, strength of the steel material increases. However, if the N content is too high, even when the contents of other elements are within the range of the present embodiment, coarse nitrides are formed and the low-temperature toughness and the SSC resistance of the steel material decreases. Therefore, the N content is 0.0100% or less. A preferable upper limit of the N content is 0.0060%, more preferably is 0.0050%, and further preferably is 0.0045%. A preferable lower limit of the N content for more effectively obtaining the aforementioned effect is 0.0005%, more preferably is 0.0010%, further preferably is 0.0015%, and further preferably is 0.0020%.O: 0.0100% or less

[0050] Oxygen (O) is an impurity. That is, the lower limit of the O content is more than 0%. If the O content is too high, even when the contents of other elements are within the range of the present embodiment, O forms coarse oxides, and causes the low-temperature toughness and the SSC resistance of the steel material to decrease. Therefore, the O content is 0.0100% or less. A preferable upper limit of the O content is 0.0050%, more preferably is 0.0030%, and further preferably is 0.0020%. Preferably, the O content is as low as possible. However, if the O content is excessively reduced, the production cost increases significantly. Therefore, when taking industrial production into consideration, a preferable lower limit of the O content is 0.0001%, more preferably is 0.0002%, and further preferably is 0.0003%.

[0051] 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 elements which, during industrial production of the steel material, are mixed in from ore or scrap that is used as a raw material of the steel 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]

[0052] The chemical composition of the steel material described above may further contain one or more elements selected from the group consisting of V and Nb in lieu of a part of Fe. Each of these elements is an optional element, and increases the low-temperature toughness and the SSC resistance of the steel material.V: 0 to 0.60%

[0053] Vanadium (V) is an optional element, and need not be contained. That is, the V content may be 0%. If contained, V combines with C or N to form carbides, nitrides or carbo-nitrides (hereinafter, referred to as "carbo-nitrides and the like"). Carbo-nitrides and the like refine the grains of the steel material by the pinning effect, and increase the low-temperature toughness and the SSC resistance of the steel material. V also forms fine carbides during tempering to increase the temper softening resistance of the steel material and to increase strength of the steel material. If even a small amount of V is contained, the aforementioned effects can be obtained to a certain extent. However, if the V content is too high, even when the contents of other elements are within the range of the present embodiment, the low-temperature toughness of the steel material decreases. Therefore, the V content is within the range of 0 to 0.60%. A preferable lower limit of the V content 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 V content is 0.40%, more preferably is 0.30%, and further preferably is 0.20%.Nb: 0 to 0.030%

[0054] Niobium (Nb) is an optional element, and need not be contained. That is, the Nb content may be 0%. If contained, Nb forms carbo-nitrides and the like. Carbo-nitrides and the like refine the grains of the steel material by the pinning effect, and increase the low-temperature toughness and the SSC resistance of the steel material. Nb also forms fine carbides during tempering and thereby increases the temper softening resistance of the steel material and enhances strength of the steel material. If even a small amount of Nb is contained, the aforementioned effects can be obtained to a certain extent. However, if the Nb content is too high, even when the contents of other elements are within the range of the present embodiment, carbo-nitrides and the like are excessively formed and the low-temperature toughness and the SSC resistance of the steel material decrease. Therefore, the Nb content is within the range of 0 to 0.030%. A preferable lower limit of the Nb content is more than 0%, more preferably is 0.002%, further preferably is 0.003%, and further preferably is 0.007%. A preferable upper limit of the Nb content is 0.025%, more preferably is 0.020%, and further preferably is 0.015%.

[0055] The chemical composition of the steel material described above may further contain one or more elements selected from the group consisting of Ca, Mg, Zr and rare earth metal in lieu of a part of Fe. Each of these elements is an optional element, and render S in the steel material harmless by forming sulfides. As a result, these elements increase the low-temperature toughness and the SSC resistance of the steel material.Ca: 0 to 0.0100%

[0056] Calcium (Ca) is an optional element, and need not be contained. That is, the Ca content may be 0%. If contained, Ca renders S in the steel material harmless by forming sulfides, and increases the low-temperature toughness and the SSC resistance of the steel material. If even a small amount of Ca is contained, the aforementioned effect can be obtained to a certain extent. However, if the Ca content is too high, even when the contents of other elements are within the range of the present embodiment, oxides in the steel material coarsen and the low-temperature toughness and the SSC resistance of the steel material decrease. Therefore, the Ca content is within the range of 0 to 0.0100%. A preferable lower limit of the Ca content 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 Ca content is 0.0040%, more preferably is 0.0025%, further preferably is 0.0020%, and further preferably is 0.0015%.Mg: 0 to 0.0100%

[0057] Magnesium (Mg) is an optional element, and need not be contained. That is, the Mg content may be 0%. If contained, Mg renders S in the steel material harmless by forming sulfides, and increases the low-temperature toughness and the SSC resistance of the steel material. If even a small amount of Mg is contained, the aforementioned effect can be obtained to a certain extent. However, if the Mg content is too high, even when the contents of other elements are within the range of the present embodiment, oxides in the steel material coarsen and decrease the low-temperature toughness and the SSC resistance of the steel material. Therefore, the Mg content is within the range of 0 to 0.0100%. A preferable lower limit of the Mg content 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 Mg content is 0.0040%, more preferably is 0.0025%, further preferably is 0.0020%, and further preferably is 0.0015%.Zr: 0 to 0.0100%

[0058] Zirconium (Zr) is an optional element, and need not be contained. That is, the Zr content may be 0%. If contained, Zr renders S in the steel material harmless by forming sulfides, and increases the low-temperature toughness and the SSC resistance of the steel material. If even a small amount of Zr is contained, the aforementioned effect can be obtained to a certain extent. However, if the Zr content is too high, even when the contents of other elements are within the range of the present embodiment, oxides in the steel material coarsen and the low-temperature toughness and the SSC resistance of the steel material decrease. Therefore, the Zr content is within the range of 0 to 0.0100%. A preferable lower limit of the Zr content 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 Zr content is 0.0040%, more preferably is 0.0025%, and further preferably is 0.0020%.Rare earth metal (REM): 0 to 0.0100%

[0059] Rare earth metal (REM) is an optional element, and need not be contained. That is, the REM content may be 0%. If contained, the REM renders S in the steel material harmless by forming sulfides, and increases the SSC resistance of the steel material. REM also combines with P in the steel material and suppresses segregation of P at the grain boundaries. Therefore, a decrease in the low-temperature toughness and the SSC resistance of the steel material that is attributable to segregation of P is suppressed. If even a small amount of REM is contained, the aforementioned effects can be obtained to a certain extent. However, if the REM content is too high, even when the contents of other elements are within the range of the present embodiment, oxides in the steel material coarsen and the low-temperature toughness and the SSC resistance of the steel material decrease. Therefore, the REM content is within the range of 0 to 0.0100%. A preferable lower limit of the REM content 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 REM content is 0.0040%, more preferably is 0.0025%, and further preferably is 0.0020%.

[0060] Note that, in the present description the term "REM" refers to one or more types of elements selected from a group consisting of scandium (Sc) which is the element with atomic number 21, yttrium (Y) which is the element with atomic number 39, and the elements from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71 that are lanthanoids. Further, in the present description the term "REM content" refers to the total content of these elements.

[0061] The chemical composition of the steel material described above may further contain one or more elements selected from the group consisting of Co and W in lieu of a part of Fe. Each of these elements is an optional element that forms a protective corrosion coating in a sour environment and suppresses the penetration of hydrogen into the steel material. As a result, each of these elements increases the SSC resistance of the steel material.Co: 0 to 0.50%

[0062] Cobalt (Co) is an optional element, and need not be contained. That is, the Co content may be 0%. If contained, in a sour environment Co forms a protective corrosion coating and suppresses the penetration of hydrogen into the steel material. As a result, the SSC resistance of the steel material increases. If even a small amount of Co is contained, the aforementioned effect can be obtained to a certain extent. However, if the Co content is too high, even when the contents of other elements are within the range of the present embodiment, hardenability of the steel material will decrease, and strength of the steel material will decrease. Therefore, the Co content is within the range of 0 to 0.50%. A preferable lower limit of the Co content is more than 0%, more preferably is 0.02%, further preferably is 0.03%, and further preferably is 0.05%. A preferable upper limit of the Co content is 0.45%, and more preferably is 0.40%.W: 0 to 0.50%

[0063] Tungsten (W) is an optional element, and need not be contained. That is, the W content may be 0%. If contained, W forms a protective corrosion coating in a sour environment and suppresses hydrogen penetration into the steel material. As a result, the SSC resistance of the steel material increases. If even a small amount of W is contained, the aforementioned effect can be obtained to a certain extent. However, if the W content is too high, even when the contents of other elements are within the range of the present embodiment, coarse carbides form in the steel material, and the low-temperature toughness and the SSC resistance of the steel material decrease. Therefore, the W content is within the range of 0 to 0.50%. A preferable lower limit of the W content is more than 0%, more preferably is 0.02%, further preferably is 0.03%, and further preferably is 0.05%. A preferable upper limit of the W content is 0.45%, and more preferably is 0.40%.

[0064] The chemical composition of the steel material described above may further contain Cu in lieu of a part of Fe.Cu: 0 to 0.50%

[0065] Copper (Cu) is an optional element, and need not be contained. That is, the Cu content may be 0%. If contained, Cu enhances hardenability of the steel material and increases strength of the steel material. If even a small amount of Cu is contained, the aforementioned effects can be obtained to a certain extent. However, if the Cu content is too high, even when the contents of other elements are within the range of the present embodiment, hardenability of the steel material will be too high, and the SSC resistance of the steel material will decrease. Therefore, the Cu content is within the range of 0 to 0.50%. A preferable lower limit of the Cu content 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 Cu content is 0.35%, and more preferably is 0.25%.[Fn1]

[0066] In the steel material according to the present embodiment, within the ranges of the contents of elements of the steel material described above, the contents of elements of the steel material and the Cr concentration θ Cr in precipitates having an equivalent circular diameter of 20 nm or more satisfy the following Formula (1). 0.157 × C − 0.0006 × Cr − 0.0098 × Mo − 0.0482 × V + 0.0006 / θ Cr ≤ 0.300

[0067] Where, a content in units of percent by mass of a corresponding element is substituted for each symbol of an element in Formula (1). If the corresponding element is not contained, "0" is substituted for the symbol of an element. Further, the Cr concentration in units of mass fraction in precipitates having an equivalent circular diameter of 20 nm or more is substituted for θ Cr in Formula (1).

[0068] Fn1 (= (0.157×C-0.0006×Cr-0.0098×Mo-0.0482×V+0.0006) / θ Cr ) is an index relating to the number density NDF of fine precipitates and the number density NDC of coarse precipitates in a steel material having the aforementioned chemical composition. As long as the other conditions of the present embodiment are satisfied and Fn1 is not more than 0.300, the number density NDF of fine precipitates in the steel material can be made 0.650 / µm 2< or more, and the number density NDC of coarse precipitates in the steel material can be made 0.290 / µm 2< or less.

[0069] Cr concentrates in cementite and can suppress Ostwald growth of the cementite. Specifically, by concentrating in cementite, Cr can suppress dissolution of fine cementite particles in the matrix in a tempering process in a production process that is described later. As a result, Cr can suppress coarsening of cementite by Ostwald growth.

[0070] In a steel material having the aforementioned chemical composition, almost all of the precipitates having an equivalent circular diameter of 20 nm or more are cementite. On the other hand, in a steel material having the aforementioned chemical composition, there is a possibility that MC-type carbides and M 2 C-type carbides are included in precipitates having an equivalent circular diameter of less than 20 nm. Therefore, in Formula (1) of the steel material according to the present embodiment, the Cr concentration θ Cr in precipitates having an equivalent circular diameter of 20 nm or more is defined. As a result, in Formula (1) of the steel material according to the present embodiment, the Cr concentration in cementite can substantially be defined.

[0071] As described above, the Cr concentration θ Cr contained in precipitates having an equivalent circular diameter of 20 nm or more that is the denominator of Fn1 is an index that indicates the degree of difficulty of Ostwald growth of cementite. If θ Cr that is the denominator of Fn1 is increased, there is a possibility that coarsening of cementite can be suppressed, the number density NDF of fine precipitates can be increased, and the number density NDC of coarse precipitates can be decreased. Further, as described above, the numerator of Fn1 is an index of the total precipitation amount of cementite. In a steel material having the aforementioned chemical composition, the larger the total precipitation amount of cementite is, the easier it is for coarse cementite to be formed. That is, if the numerator of Fn1 is reduced, there is a possibility that the number density NDC of coarse precipitates can be decreased.

[0072] In short, as long as Fn1 is not more than 0.300, on the condition that the other requirements of the present embodiment are satisfied, the number density NDF of fine precipitates in the steel material can be made 0.650 / µm 2< or more, and the number density NDC of coarse precipitates can be made 0.290 / µm 2< or less. Therefore, in the steel material according to the present embodiment, Fn1 is not more than 0.300. A preferable upper limit of Fn1 is 0.295, more preferably is 0.290, further preferably is 0.285, further preferably is 0.280, more preferably is 0.260, and further preferably is 0.240. The lower limit of Fn1 is not particularly limited. The lower limit of Fn1 is, for example, 0. Note that, Fn1 is a value obtained by rounding off the fourth decimal place of the obtained numerical value.

[0073] The Cr concentration θ Cr contained in precipitates having an equivalent circular diameter of 20 nm or more can be determined by the following method. A micro test specimen for making an extraction replica is prepared from the steel material according to the present embodiment. If the steel material is a steel plate, the micro test specimen is prepared from a center portion of the plate thickness. If the steel material is a steel pipe, the micro test specimen is prepared from a center portion of the wall thickness. If the steel material is a round steel bar, the micro test specimen is prepared from an R / 2 position. 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.

[0074] The surface of the micro test specimen is mirror-polished, and thereafter the micro test specimen is immersed for 10 minutes in a 3% nital etching reagent to etch the surface. The etched surface is then covered with a carbon deposited film. The micro test specimen whose surface is covered with the deposited film is immersed for 20 minutes in a 5% nital etching reagent. The deposited film is peeled off from the immersed micro test specimen. The deposited film that was peeled off from the micro test specimen is cleaned with ethanol, and thereafter is scooped up with a sheet mesh and dried.

[0075] The deposited film (replica film) is observed using a transmission electron microscope (TEM). Specifically, arbitrary locations among the deposited film are specified, and observation of the specified locations is conducted using an observation magnification of ×10000 and an acceleration voltage of 200 kV. Note that, the number of locations that are specified is not particularly limited as long as the number of locations is at least three or more. Further, each visual field is, for example, 8 µm × 8 µm. Precipitates having an equivalent circular diameter of 20 nm or more are identified in each visual field to specify a total of 20 precipitate particles for the entire visual fields, and are defined as "specific precipitates". Note that the precipitates can be identified based on contrast. The equivalent circular diameter of the respective precipitates can be determined by image analysis of an observation image in TEM observation.

[0076] The specific precipitates (precipitates having an equivalent circular diameter of 20 nm or more) are subjected to point analysis by energy dispersive X-ray spectrometry (EDS). By means of the point analysis by EDS, the Cr concentration is determined in units of mass percent when taking the total of the alloying elements excluding carbon in each precipitate as 100%. The Cr concentration is determined for 20 specific precipitate particles, and the arithmetic average value of the obtained values is defined as the Cr concentration θ Cr (unit: mass fraction) in the specific precipitates. Note that, in the present embodiment, the Cr concentration θ Cr in the specific precipitates is a value obtained by rounding off the fifth decimal place of the obtained numerical value.[Fn2]

[0077] In the steel material according to the present embodiment, within the ranges of the contents of elements of the steel material described above, the contents of elements of the steel material satisfy the following Formula (2). 1 + 263 × C − Cr − 16 × Mo − 80 × V / 98 − 358 × C + 159 × Cr + 15 × Mo + 96 × V ≤ 0.355

[0078] Where, a content in mass% of a corresponding element is substituted for each symbol of an element in Formula (2). If a corresponding element is not contained, "0" is substituted for the symbol of the relevant element.

[0079] Fn2 (= (1+263×C-Cr-16×Mo-80×V) / (98-358×C+159×Cr+15×Mo+96×V)) is an index that indicates the degree to which it is difficult for Cr to concentrate in precipitates. If Fn2 is not more than 0.355, Cr concentrates sufficiently in precipitates and it is easy to cause Ostwald growth of cementite to be suppressed. Therefore, in the steel material according to the present embodiment, Fn2 is not more than 0.355.

[0080] A preferable upper limit of Fn2 is 0.350, more preferably is 0.340, further preferably is 0.330, further preferably is 0.320, more preferably is 0.310, and further preferably is 0.300. The lower limit of Fn2 is not particularly limited. The lower limit of Fn2 is, for example, 0. Note that, Fn2 is a value obtained by rounding off the fourth decimal place of the obtained numerical value.[Fn3]

[0081] In the steel material according to the present embodiment, within the ranges of the contents of elements of the steel material described above, the contents of elements of the steel material satisfy the following Formula (3). − 9.7 × Mn − 104 × S + 0.8 × Mo + 0.08 × Ni 2 − 4.1 × Ni − 5.1 × Ti ≥ − 9.0

[0082] Where, a content in percent by mass of a corresponding element is substituted for each symbol of an element in Formula (3).

[0083] Fn3 (= -9.7×Mn-104×S+0.8×Mo+0.08×Ni 2< -4.1×Ni-5.1×Ti) is an index of SSC resistance. If a condition that Fn3 is -9.0 or more is satisfied, on the condition that the other requirements of the present embodiment are satisfied, local corrosion of the steel material will be suppressed, and the SSC resistance of the steel material can be stably increased. Therefore, in the steel material according to the present embodiment, Fn3 is -9.0 or more.

[0084] A preferable lower limit of Fn3 is -8.7, and more preferably is -8.5. The upper limit of Fn3 is not particularly limited. The upper limit of Fn3 is, for example, 0.5. Note that, Fn3 is a value obtained by rounding off the second decimal place of the obtained numerical value.[Fn4]

[0085] In the steel material according to the present embodiment, within the ranges of the contents of elements of the steel material described above, the contents of elements of the steel material satisfy the following Formula (4). 15.8 × Si − 33.8 × Mn − 28.8 × Ni ≥ − 51.0

[0086] Where, a content in percent by mass of a corresponding element is substituted for each symbol of an element in Formula (4).

[0087] Similarly to Fn3, Fn4 (= 15.8×Si-33.8×Mn-28.8×Ni) is an index of SSC resistance. On the precondition that the value of Fn3 is -9.0 or more, if Fn4 is -51.0 or more, on the condition that the other requirements of the present embodiment are satisfied, local corrosion of the steel material will be suppressed, and the SSC resistance of the steel material can be stably increased. Therefore, in the steel material according to the present embodiment, Fn4 is -51.0 or more.

[0088] A preferable lower limit of Fn4 is -50.9, more preferably is -50.7, and further preferably is -50.5. The upper limit of Fn4 is not particularly limited. The upper limit of Fn4 is, for example, 20.1. Note that, Fn4 is a value obtained by rounding off the second decimal place of the obtained numerical value.[Yield strength]

[0089] The yield strength of the steel material according to the present embodiment is 862 MPa or more (125 ksi or more). As used in the present description, the term "yield strength" means 0.2% offset proof stress obtained in a tensile test in conformity with ASTM E8 / E8M (2021). Note that, an upper limit of the yield strength of the steel material according to the present embodiment is not particularly limited. Meanwhile, at least when the yield strength is within a range of 862 to 1069 MPa, it has been proved by examples described later that the steel material according to the present embodiment has excellent low-temperature toughness and excellent SSC resistance. Accordingly, the yield strength of the steel material according to the present embodiment includes at least 862 to 1069 MPa (125 to 155 ksi). In other words, the yield strength of the steel material according to the present embodiment includes at least 862 to less than 965 MPa (125 ksi grade) and 965 to 1069 MPa (140 ksi grade).

[0090] The yield strength of the steel material according to the present embodiment can be determined by the following method. Specifically, a tensile test is performed by a method in conformity with ASTM E8 / E8M (2021). A round bar test specimen is prepared from the steel material according to the present embodiment. If the steel material is a steel plate, the round bar test specimen is prepared from the center portion of the thickness. In this case, the axial direction of the round bar test specimen is made a direction parallel to the rolling elongation direction of the steel plate. If the steel material is a steel pipe, the round bar test specimen is prepared from the center portion of the wall thickness. In this case, the axial direction of the round bar test specimen is made a direction parallel to the axial direction of the steel pipe. If the steel material is a round steel bar, the round bar test specimen is prepared from an R / 2 position. In this case, the axial direction of the round bar test specimen is made a direction parallel to the axial direction of the round steel bar. Regarding the size of the round bar test specimen, for example, the round bar test specimen has a parallel portion diameter of 4 mm and a gage length of 16 mm. A tensile test is performed in the atmosphere at normal temperature (25°C) using the round bar test specimen, and obtained 0.2% offset proof stress is defined as the yield strength (MPa). Note that, the yield strength (MPa) in the present embodiment is a value obtained by rounding off the first decimal place of the obtained numerical value.[Number density of precipitates]

[0091] In the steel material according to the present embodiment, within the ranges of the contents of elements of the steel material described above, the contents of elements of the steel material and the Cr concentration θ Cr in precipitates having an equivalent circular diameter of 20 nm or more satisfy the following Formulae (1) to (4), the steel material has a yield strength of 862 MPa or more, and furthermore, the number density NDF of precipitates having an equivalent circular diameter of 20 to 150 nm is 0.650 / µm 2< or more and the number density NDC of precipitates having an equivalent circular diameter of 250 nm or more is 0.290 / µm 2< or less. As a result, the steel material according to the present embodiment has a yield strength of 125 ksi or more (862 MPa or more) and has excellent low-temperature toughness and excellent SSC resistance.

[0092] As described above, in the present description, precipitates having an equivalent circular diameter of 20 to 150 nm are defined as "fine precipitates", and precipitates having an equivalent circular diameter of 250 nm or more are defined as "coarse precipitates". Further, as described above, in a steel material having the aforementioned chemical composition, almost all of the precipitates having an equivalent circular diameter of 20 nm or more are cementite. That is, in the steel material according to the present embodiment, at the same time as causing a large number of fine cementite particles to precipitate, the precipitation of coarse cementite is suppressed. As a result, a steel material that has a yield strength of 125 ksi or more and has excellent low-temperature toughness and excellent SSC resistance is obtained.

[0093] A preferable lower limit of the number density NDF of fine precipitates is 0.700 / µm 2< , and more preferably is 0.750 / µm 2< . The upper limit of the number density NDF of fine precipitates is not particularly limited. The upper limit of the number density NDF of fine precipitates, for example, may be 20.000 / µm 2< , may be 15.000 / µm 2< , or may be 10.000 / µm 2< . A preferable upper limit of the number density NDC of coarse precipitates is 0.285 / µm 2< , more preferably is 0.280 / µm 2< , and further preferably is 0.275 / µm 2< . The lower limit of the number density NDC of coarse precipitates is not particularly limited. The lower limit of the number density NDC of coarse precipitates, for example, may be 0 / µm 2< , may be 0.001 / µm 2< , or may be 0.010 / µm 2< .

[0094] In the steel material according to the present embodiment, the number density NDF of fine precipitates and the number density NDC of coarse precipitates are determined by the following method. A test specimen is prepared from the steel material according to the present embodiment. Specifically, in a case where the steel material is a steel plate, a test specimen having an observation surface with dimensions of 10 mm in the rolling elongation direction and 10 mm in the thickness direction is prepared from a center portion of the thickness. In a case where the steel material is a steel pipe, a test specimen having an observation surface with dimensions of 10 mm in the pipe axis direction and 8 mm in the wall thickness (pipe radius) direction is prepared from a center portion of the wall thickness. In a case where the steel material is a round steel bar, a test specimen which includes an R / 2 position at the center thereof and has an observation surface with dimensions of 10 mm in the axial direction and 8 mm in the radial direction is prepared.

[0095] After polishing the observation surface of the test specimen to obtain a mirror surface, the test specimen is immersed for 60 seconds in a picral etching reagent (2.0 mass% picric acid ethanol solution), to reveal the microstructure by etching. The etched observation surface is subjected to three-dimensional roughness measurement using a scanning electron microscope (SEM) to thereby obtain a three-dimensional roughness profile of each visual field. If the number of observation visual fields is three or more visual fields and the total of the area of the observation visual fields is 300 µm 2< or more, the reproducibility in the measurement of the number densities of fine precipitates and coarse precipitates is enhanced. Therefore, in the present embodiment the number of observation visual fields is set to not less than three visual fields. In addition, the visual field area is, for example, 108 µm 2< (magnification of ×10000) that is 12 µm × 9 µm.

[0096] Although the number of pixels (picture elements) into which the visual field area is divided is not particularly limited, it is preferable to make a single pixel not more than 0.020 µm × 0.020 µm in order to obtain stable measurement accuracy. If a single pixel is 0.020 µm × 0.020 µm, that is, 20 nm × 20 nm, it is possible to detect precipitates of 20 nm or more by means of three-dimensional roughness measurement. Note that, in a case where a single pixel is set as 0.020 µm × 0.020 µm in the aforementioned visual field area, the visual field area is divided into 270000 pixels in the form of 600 × 450 pixels.

[0097] A method for performing three-dimensional roughness measurement is not particularly limited, and a well-known method can be used. For example, four secondary electron detectors may be arranged in a SEM, and a three-dimensional roughness profile may be obtained by combining the detection results of the four secondary electron detectors. In each visual field, the focal depth direction in the SEM observation is defined as "height direction". In each visual field, a plane perpendicular to the height direction is defined as "observation plane". In addition, with respect to the aforementioned height direction, the direction from the observation plane toward the electron beam source is defined as the positive direction (direction in which the height increases). An area fraction Z h (%) that the steel material occupies in the visual field area of the observation plane at a position h (µm) in the height direction is determined from a three-dimensional roughness profile obtained by the aforementioned method. At this time, the resolution in the height direction is, for example, 1 nm.

[0098] In this case, a lowest height h 0 and a highest height h 1 are identified in each visual field. The height "h 0 " means the maximum value among heights h at which Z h = 100.0% and for which a corresponding area fraction Z h0 = 100.0%. The height "h 1 " means the minimum value among heights h at which Z h = 0.0% and for which a corresponding area fraction Z h1 = 0.0%.

[0099] A plot in which the position h (µm) in the height direction is taken as the abscissa and the area fraction Z h (%) that the steel material occupies is taken as the ordinate is created with respect to the respective visual fields. At this time, the range of the positions h in the height direction is set as h 0 to h 1 .

[0100] Next, an area fraction S (%) of precipitates in each visual field is determined. In the present embodiment, the volume ratio (%) of precipitates in the steel material is determined and is taken as the area fraction S (%) of precipitates in each visual field. In addition, in the present embodiment, as described above, precipitates having an equivalent circular diameter of 20 nm or more are detected. Therefore, in the present embodiment, the area fraction S (%) of precipitates in each visual field means the volume ratio (%) of precipitates having an equivalent circular diameter of 20 nm or more.

[0101] Further, as described above, most of the precipitates having an equivalent circular diameter of 20 nm or more are cementite. In addition, among the volume ratio of the cementite, the volume ratio of cementite having an equivalent circular diameter that is less than 20 nm is small enough to be negligible. Therefore, the area fraction S (%) of precipitates in each visual field can be approximated as a volume ratio V θ (%) of cementite in the steel material according to the present embodiment. Thus, in the present embodiment, the volume ratio V θ (%) of cementite is determined as the area fraction S (%) of precipitates in each visual field.

[0102] A method for determining the volume ratio V θ of cementite is not particularly limited, and a well-known method can be used. For example, V θ may be determined by thermodynamic calculation. In this case, by performing a thermodynamic calculation using the chemical composition and a tempering temperature in a production process that is described later, the proportion that cementite occupies in the volume of the system overall (entire structure including the matrix, cementite, and other precipitates and inclusions) can be determined. Note that, in the case of performing a thermodynamic calculation, the thermodynamic calculation may be performed using well-known thermodynamic calculation software. Thus, it is sufficiently possible for a person skilled in the art to determine the volume ratio V θ (%) of cementite by thermodynamic calculation.

[0103] The volume ratio V θ of cementite may also be determined by capturing extraction residue. In this case, the volume ratio V θ of cementite can be determined by the following method. A cylindrical test specimen is prepared from the steel material according to the present embodiment. In a case where the steel material is a steel plate, the cylindrical test specimen is prepared from a center portion of the thickness. In a case where the steel material is a steel pipe, the cylindrical test specimen is prepared from a center portion of the wall thickness. In a case where the steel material is a round steel bar, the cylindrical test specimen is prepared from the R / 2 position. The size of the cylindrical test specimen is, for example, a diameter of 6 mm and a length of 50 mm. The surface of the prepared cylindrical test specimen is polished to remove about 50 µm by preliminary electropolishing to obtain a newly formed surface. The test specimen in which the newly formed surface was obtained is subjected to electrolysis using an electrolyte solution (10% acetylacetone + 1% tetra-ammonium + methanol). The electrolyte solution after electrolysis is passed through a 0.2 µm filter to capture residue.

[0104] The obtained residue is subjected to acid decomposition, and the concentrations of alloying elements excluding carbon in cementite are determined in units of percent by mass by ICP (inductively coupled plasma) emission spectrometry. The volume ratio V θ (%) of cementite is determined based on the obtained concentrations of alloying elements excluding carbon in cementite and the following Formula (A).

[0105] The "molar fractions of respective alloying elements in cementite" in Formula (A) can be determined by the following method. The amount of each alloying element dissolved in cementite can be acquired by analysis of extraction residue. The molar fractions of the respective alloying elements in the cementite can be determined by dividing the acquired amount of each alloying element by the total amount that was electrolyzed.

[0106] Further, V mθ in Formula (A) represents the molar volume (m 3< / mol) of cementite. In addition, V m in Formula (A) represents the molar volume (m 3< / mol) of the system overall (entire structure including the matrix, cementite, and other precipitates and inclusions). Note that, V mθ and V m can each be obtained by means of well-known thermodynamic calculation software.

[0107] As described above, in the present embodiment a method for determining the volume ratio V θ of cementite is not particularly limited, and the aforementioned method that utilizes thermodynamic calculation may be used or the aforementioned method that captures extraction residue may be used. Further, in the steel material according to the present embodiment having the aforementioned chemical composition, there is almost no difference between the area fraction S (that is, the volume ratio V θ of cementite) of precipitates obtained by the method that utilizes thermodynamic calculation and the area fraction S of precipitates obtained by the method that captures extraction residue. Therefore, whichever method is used, the area fraction S (%) of precipitates in each visual field area can be determined.

[0108] The equivalent circular diameter and the number density of the each precipitate are determined based on the area fraction S (%) of the precipitate that was determined, a plot of the height h (µm) and area fraction Z h (%) determined by the aforementioned method, and a three-dimensional roughness profile obtained by the aforementioned method. Specifically, the equivalent circular diameter and the number density of the respective precipitates can be determined as follows. From the aforementioned plot, a height at which the area fraction Z h (%) is closest to the area fraction S (%) is identified, and is defined as h t (µm). Based on the obtained height h t and the three-dimensional roughness profile, the distribution of the steel material in a visual field at the height h t is acquired as two-dimensional information.

[0109] A region that the steel material occupies and vacant space are included in the two-dimensional information of the distribution of the steel material in a visual field. At this time, the region that the steel material occupies is, more specifically, a region that precipitates occupy. Therefore, by analyzing the acquired two-dimensional information, the respective equivalent circular diameters of the precipitates in the visual field can be determined. In this way, the equivalent circular diameters of all of the precipitates in the visual field region are determined. Based on the equivalent circular diameters of the respective precipitates that are obtained, the number of precipitates having an equivalent circular diameter of 20 to 150 nm (fine precipitates), and the number of precipitates having an equivalent circular diameter of 250 nm or more (coarse precipitates) are counted.

[0110] The aforementioned method is performed for each observation visual field to thereby count the number of fine precipitates and the number of coarse precipitates in each observation visual field. The number density NDF of fine precipitates ( / µm 2< ) is determined using the sum of the numbers of fine precipitates in all of the observation visual fields, and the total area (µm 2< ) of the observation visual fields. Similarly, the number density NDC of coarse precipitates ( / µm 2< ) is determined using the sum of the numbers of coarse precipitates in all of the observation visual fields, and the total area (µm 2< ) of the observation visual fields. Note that, in the present embodiment, values obtained by rounding off the fourth decimal place of the obtained numerical values are adopted as the number density NDF of fine precipitates ( / µm 2< ) and the number density NDC of coarse precipitates ( / µm 2< ), respectively.[Fn5]

[0111] In the steel material according to the present embodiment, within the ranges of the contents of elements of the steel material described above, the contents of elements of the steel material, the number density NDF of precipitates having an equivalent circular diameter of 20 to 150 nm (fine precipitates), and the number density NDC of precipitates having an equivalent circular diameter of 250 nm or more (coarse precipitates) may satisfy Formula (5). In this case, in addition to having a yield strength of 125 ksi or more (862 MPa or more) and excellent SSC resistance, the steel material according to the present embodiment also has excellent low-temperature toughness. − Mn − 20 × P + 11 × Ni + Mo × NDF 2 / NDC 1 / 2 ≥ 4.0

[0112] Where, a content in units of percent by mass of a corresponding element is substituted for each symbol of an element in Formula (5). Further, a number density in units of / µm 2< of precipitates having an equivalent circular diameter of 20 to 150 nm is substituted for NDF in Formula (5). In addition, a number density in units of / µm 2< of precipitates having an equivalent circular diameter of 250 nm or more is substituted for NDC in Formula (5), and in a case where the number density of precipitates having an equivalent circular diameter of 20 to 150 nm is less than 0.001 / µm 2< , 0.001 is substituted for NDC.

[0113] Let Fn5 be defined as Fn5 = (-Mn-20×P+11×Ni+Mo)×(NDF 2< / NDC 1 / 2< ). Fn5 is an index of low-temperature toughness in a steel material that has the aforementioned chemical composition and that satisfies Formulae (1) to (4), and in which the number density NDF of fine precipitates is 0.650 / µm 2< or more and the number density NDC of coarse precipitates is 0.290 / µm 2< or less. Specifically, on the condition that the other requirements of the present embodiment are satisfied, if a condition that Fn5 is 4.0 or more is satisfied, the steel material will have even more excellent low-temperature toughness.

[0114] Accordingly, in the present embodiment, preferably Fn5 is 4.0 or more. A more preferable lower limit of Fn5 is 4.2, and further preferably is 4.3. The upper limit of Fn5 is not particularly limited, and for example is 90000.0. The upper limit of Fn5 may be 30000.0, may be 3000.0, may be 300.0, may be 200.0, or may be 150.0.

[0115] Note that, in the steel material according to the present embodiment, it is also possible for a case to occur in which the number density NDC of coarse precipitates is 0 / µm 2< , and consequently Fn5 cannot be defined. Therefore, in the present embodiment, in a case where the number density NDC of coarse precipitates is less than 0.001 / µm 2< , 0.001 is substituted for NDC in Formula (5). Note that, Fn5 is a value obtained by rounding off the second decimal place of the obtained numerical value.[Low-temperature toughness]

[0116] In the steel material according to the present embodiment, within the ranges of the contents of elements of the steel material described above, the contents of elements of the steel material and the Cr concentration θ Cr in precipitates having an equivalent circular diameter of 20 nm or more satisfy Formulae (1) to (4), the yield strength of the steel material is 862 MPa or more, and furthermore, the number density NDF of fine precipitates is 0.650 / µm 2< or more and the number density NDC of coarse precipitates is 0.290 / µm 2< or less. As a result, the steel material according to the present embodiment has a yield strength of 125 ksi or more (862 MPa or more) and has excellent low-temperature toughness and excellent SSC resistance. In the present embodiment, the low-temperature toughness of the steel material is evaluated by a Charpy impact test in conformity with JIS Z 2242 (2018). Specifically, in the present embodiment, the phrase "the steel material has excellent low-temperature toughness" is defined as follows.

[0117] First, a full-size or sub-size V-notch test specimen is prepared in conformity with API 5CT (2019) from the steel material according to the present embodiment. Here, if the steel material is a steel plate, the rolling elongation direction of the steel plate is defined as an "L direction" (longitudinal direction), and the plate width direction of the steel plate is defined as a "T direction" (transverse direction). If the steel material is a steel pipe, the radial direction of the steel pipe is defined as a "C direction", the axial direction of the steel pipe is defined as an "L direction", and a direction perpendicular to the C direction and the L direction is defined as a "T direction". If the steel material is a round steel bar, the cross-sectional radial direction of the round steel bar is defined as a "C direction", the axial direction of the round steel bar is defined as an "L direction", and a direction perpendicular to the C direction and the L direction is defined as a "T direction". For a V-notch test specimen prepared using the steel material, excellent low-temperature toughness is defined for each yield strength as described hereunder.[Low-temperature toughness when yield strength is less than 965 MPa]

[0118] A Charpy impact test in conformity with JIS Z 2242 (2018) is performed on a prepared V-notch test specimen to determine an absorbed energy vE(-80°C)(J) at - 80°C. Note that, in a case where a sub-size V-notch test specimen is used, the obtained absorbed energy is divided by a reduction factor described in API 5CT (2019) to convert the obtained absorbed energy to the absorbed energy for a full-size V-notch test specimen. Further, a value obtained by rounding off the first decimal place of the obtained numerical value is adopted as the absorbed energy vE(-80°C)(J) at -80°C. In the present embodiment, in a case where the yield strength of the steel material is less than 965 MPa, if the absorbed energy vE(-80°C) at -80°C determined by the above method is 105 J or more, it is determined that the steel material has excellent low-temperature toughness.[Low-temperature toughness when yield strength is 965 MPa or more]

[0119] A Charpy impact test in conformity with JIS Z 2242 (2018) is performed on prepared V-notch test specimen to determine an absorbed energy vE(-65°C)(J) at - 65°C. Note that, in a case where a sub-size V-notch test specimen is used, the obtained absorbed energy is divided by a reduction factor described in API 5CT (2019) to convert the obtained absorbed energy to the absorbed energy for a full-size V-notch test specimen. Further, a value obtained by rounding off the first decimal place of the obtained numerical value is adopted as the absorbed energy vE(-65°C)(J) at -65°C. In the present embodiment, in a case where the yield strength of the steel material is 965 MPa or more, if the absorbed energy vE(-65°C) at -65°C determined by the above method is 75 J or more, it is determined that the steel material has excellent low-temperature toughness.

[0120] In the present embodiment, in addition, within the ranges of the contents of elements of the steel material described above, when the contents of elements of the steel material, the number density NDF of fine precipitates, and the number density NDC of coarse precipitates satisfy Formula (5), the steel material has even more excellent low-temperature toughness. In the present embodiment, the phrase "the steel material has even more excellent low-temperature toughness" is defined as follows. In a case where the yield strength is less than 965 MPa, if the absorbed energy vE(-80°C) at -80°C determined by the aforementioned method is 115 J or more, it is determined that the steel material has even more excellent low-temperature toughness. In a case where the yield strength is 965 MPa or more, if the absorbed energy vE(-65°C) at -65°C determined by the aforementioned method is 78 J or more, it is determined that the steel material has even more excellent low-temperature toughness.[SSC resistance]

[0121] In the steel material according to the present embodiment, within the ranges of the contents of elements of the steel material described above, the contents of elements of the steel material and the Cr concentration θ Cr in precipitates having an equivalent circular diameter of 20 nm or more satisfy Formulae (1) to (4), the yield strength of the steel material is 862 MPa or more, and furthermore, the number density NDF of fine precipitates is 0.650 / µm 2< or more and the number density NDC of coarse precipitates is 0.290 / µm 2< or less. As a result, the steel material according to the present embodiment has a yield strength of 125 ksi or more and has excellent low-temperature toughness and excellent SSC resistance. In the present embodiment, the SSC resistance of the steel material is evaluated by a method in accordance with "Method A" specified in NACE TM0177-2016. Specifically, in the present embodiment, the phrase "the steel material has excellent SSC resistance" is defined as follows.

[0122] A round bar test specimen is prepared from the steel material according to the present embodiment. If the steel material is a steel plate, the round bar test specimen is prepared from the center portion of the thickness. In this case, the axial direction of the round bar test specimen is made a direction parallel to the rolling elongation direction of the steel plate. If the steel material is a steel pipe, the round bar test specimen is prepared from the center portion of the wall thickness. In this case, the axial direction of the round bar test specimen is made a direction parallel to the axial direction of the steel pipe. If the steel material is a round steel bar, the round bar test specimen is prepared from the R / 2 position. In this case, the axial direction of the round bar test specimen is made a direction parallel to the axial direction of the round steel bar. Regarding the size of the round bar test specimen, for example, the round bar test specimen has a diameter of 6.35 mm and a parallel portion length of 25.4 mm.[SSC resistance when yield strength is less than 965 MPa]

[0123] A mixed aqueous solution containing 5.0 mass% of sodium chloride and 0.5 mass% of acetic acid (NACE solution A) is employed as the test solution. The temperature of the test solution is set to 24°C. A stress equivalent to 80% of the actual yield stress (80% AYS) is applied to the round bar test specimen. The test solution at 24°C is poured into a test vessel so that the round bar test specimen to which the stress has been applied is immersed therein, and this is adopted as a test bath. After degassing the test bath, a mixed gas of H 2 S gas at 0.15 atm pressure and N 2 gas at 0.85 atm pressure is blown into the test bath and is caused to saturate in the test bath. The test bath is held at 24°C for 720 hours.

[0124] In the present embodiment, in a case where the yield strength of the steel material is less than 965 MPa, in an SSC resistance test conducted under the conditions described above, if cracking is not confirmed after 720 hours elapse, it is determined that the steel material has excellent SSC resistance. Note that, in the present description, the phrase "cracking is not confirmed" means that cracking is not confirmed in the test specimen in a case where the test specimen after the test was observed by the naked eye and by means of a projector with a magnification of ×10.[SSC resistance when yield strength is 965 MPa or more]

[0125] A mixed aqueous solution containing 5.0 mass% of sodium chloride and 0.5 mass% of acetic acid (NACE solution A) is employed as the test solution. The temperature of the test solution is set to 24°C. A stress equivalent to 85% of the actual yield stress (85% AYS) is applied to the round bar test specimen. The test solution at 24°C is poured into a test vessel so that the round bar test specimen to which the stress has been applied is immersed therein, and this is adopted as a test bath. After degassing the test bath, a mixed gas of H 2 S gas at 0.01 atm pressure and N 2 gas at 0.99 atm pressure is blown into the test bath and is caused to saturate in the test bath. The test bath is held at 24°C for 720 hours.

[0126] In the present embodiment, in a case where the yield strength of the steel material is 965 MPa or more, in an SSC resistance test conducted under the conditions described above, if cracking is not confirmed after 720 hours elapse, it is determined that the steel material has excellent SSC resistance.[Microstructure]

[0127] In the microstructure of the steel material according to the present embodiment, the total of the volume ratios of tempered martensite and tempered bainite is 90% or more. The balance of the microstructure is, for example, ferrite or pearlite. If the microstructure of a steel material having the aforementioned chemical composition contains tempered martensite and tempered bainite in an amount equivalent to a total volume ratio of 90% or more, on the condition that the other requirements according to the present embodiment are satisfied, the yield strength will be 862 MPa (125 ksi) or more, and the steel material will exhibit excellent low-temperature toughness and excellent SSC resistance in a sour environment. That is, in the present embodiment, if the steel material has a yield strength of 862 MPa (125 ksi) or more and has excellent low-temperature toughness and excellent SSC resistance, it is determined that the total of the volume ratios of tempered martensite and tempered bainite in the microstructure is 90% or more.

[0128] Note that, the following method can be adopted in the case of determining the volume ratio of tempered martensite and tempered bainite by observation. First, a test specimen having an observation surface is prepared from the steel material according to the present embodiment. In a case where 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 an observation surface is prepared from a center portion of the thickness. In a case where 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 an observation surface is prepared from a center portion of the wall thickness. In a case where 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 an observation surface is prepared.

[0129] 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 performing observation with respect to 10 visual fields by means of a SEM. The visual field area is, for example, 0.01 mm 2< (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 of the measurement of the area fractions will not be 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 performing the image analysis. In the present embodiment, the 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.[Shape of steel material]

[0130] As described above, the shape of the steel material according to the present embodiment is not particularly limited. The steel material is, for example, a steel pipe, a steel plate, or a round steel bar. In a case where the steel material is an oil-well steel pipe, a preferable wall thickness is 9 to 60 mm. More preferably, the steel material according to the present embodiment is a seamless steel pipe. In a case where the steel material according to the present embodiment is a seamless steel pipe, even if the steel material is a heavy-wall seamless steel pipe with a thickness of 15 mm or more, the steel material has a yield strength of 125 ksi or more, excellent low-temperature toughness, and excellent SSC resistance in a sour environment.[Production method]

[0131] A method for producing the steel material according to the present embodiment will now be described. The production method described hereunder is a method for producing a seamless steel pipe as one example of the steel material according to the present embodiment. 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 hereunder. Each process is described in detail hereunder.[Preparation process]

[0132] In the preparation process, an intermediate steel material having the aforementioned chemical composition is prepared. As long as the intermediate steel material has the aforementioned chemical composition, the method for producing the intermediate steel material is not particularly limited. As used here, the term "intermediate steel material" refers to a plate-shaped steel material in a case where the end product is a steel plate, refers to a hollow shell in a case where the end product is a steel pipe, and refers to 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.

[0133] The preparation process may include a process in which a starting material is prepared (starting material preparation process), and a process in which the starting material is subjected to hot working to produce an intermediate steel material (hot working process). Hereunder, a case in which the preparation process includes the starting material preparation process and the hot working process is described in detail.[Starting material preparation process]

[0134] In the starting material preparation process, a starting material is produced using molten steel having the aforementioned chemical composition. The method for producing the starting material is not particularly limited, and a well-known method can be used. Specifically, a cast piece (a slab, bloom or 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. The starting material (a slab, bloom or billet) is produced by the above described process.[Hot working process]

[0135] In the hot working process, the starting material that was prepared is subjected to hot working to produce an intermediate steel material. In a case where the steel material is a seamless steel pipe, the intermediate steel material corresponds to a hollow shell. First, the billet is heated in a heating furnace. Although the heating temperature is not particularly limited, for example, the heating temperature is within a range of 1100 to 1300°C. The billet that is extracted from the heating furnace is subjected to hot working to produce a hollow shell (seamless steel pipe). The method of performing the hot working is not particularly limited, and a well-known method can be used.

[0136] For example, the Mannesmann process is performed as the hot working to produce the hollow shell. In this case, a round billet is piercing-rolled using a piercing machine. When performing piercing-rolling, although the piercing ratio is not particularly limited, the piercing ratio is, for example, within a range of 1.0 to 4.0. The round billet that underwent piercing-rolling is further hot-rolled to form a hollow shell using a mandrel mill, a reducer, a sizing mill or the like. The cumulative reduction of area in the hot working process is, for example, 20 to 70%.

[0137] A hollow shell may also be produced from the billet by performing other hot working methods. For example, in the case of 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.

[0138] In a case where the steel material is a round steel bar, first, the starting material is heated in a heating furnace. Although not particularly limited, the heating temperature is, for example, 1100 to 1300°C. The starting material that is extracted from the heating furnace is subjected to hot working to produce an intermediate steel material in which a cross section perpendicular to the axial direction is a circular shape. The hot working is, for example, blooming 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.

[0139] In a case where the steel material is a steel plate, first, the starting material is heated in a heating furnace. Although not particularly limited, the heating temperature is, for example, 1100 to 1300°C. The starting material that is extracted from the heating furnace is subjected to hot rolling using a blooming mill and a continuous mill to produce a plate-shaped intermediate steel material.

[0140] 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 hot working without being cooled to normal temperature, or may be subjected to quenching after undergoing supplementary heating (reheating) after hot working. In a case of performing direct quenching after 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, a 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.

[0141] 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 that was produced by a third party, or an intermediate steel material that was produced in another factory other than the factory in which a quenching process and a tempering process that are described later are performed, or at a different works. The quenching process is described in detail hereunder.[Quenching process]

[0142] In the quenching process, the intermediate steel material (hollow shell) that was prepared is subjected to quenching. In the present description, the term "quenching" means rapidly cooling the intermediate steel material that is at a temperature not less than the A 3 point. A preferable quenching temperature is 800 to 1000°C. If the quenching temperature is too high, in some cases crystal grains of prior-γ grains become coarse and the SSC resistance of the steel material decreases. Therefore, a quenching temperature in the range of 800 to 1000°C is preferable.

[0143] In the present description, in a case where direct quenching is performed after hot working, the term "quenching temperature" corresponds to the surface temperature of the intermediate steel material that is measured by a thermometer placed on the exit side of the apparatus that performs the final hot working. Further, in a case where quenching is performed after supplementary heating or reheating after hot working, the term "quenching temperature" corresponds to the temperature of the furnace that performs the supplementary heating or reheating.

[0144] The quenching method, for example, continuously cools the intermediate steel material (hollow shell) from the quenching starting temperature, and continuously decreases the surface temperature of the hollow shell. The method of performing the continuous cooling treatment is not particularly limited, and a well-known method can be used. The method of performing the continuous cooling treatment is, for example, a method that cools the hollow shell by immersing the hollow shell in a water bath, or a method that cools the hollow shell in an accelerated manner by shower water cooling or mist cooling.

[0145] If the cooling rate during quenching is too slow, the microstructure may not become one that is principally composed of martensite and bainite. In this case, the mechanical properties defined in the present embodiment (a yield strength of 125 ksi or more) cannot be obtained. In this case, in addition, excellent low-temperature toughness and excellent SSC resistance are not obtained.

[0146] Therefore, as described above, in the method for producing the steel material according to the present embodiment, the intermediate steel material is rapidly cooled during quenching. Specifically, in the quenching process, the average cooling rate when the surface temperature of the intermediate steel material (hollow shell) is within the range of 800 to 500°C during quenching is defined as a cooling rate during quenching CR 800-500 . More specifically, the cooling rate during quenching CR 800-500 is determined based on a temperature that is 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).

[0147] A preferable cooling rate during quenching CR 800-500 is 300°C / min or higher. A more preferable lower limit of the cooling rate during quenching CR 800-500 is 450°C / min, and further preferably is 600°C / min. Although an upper limit of the cooling rate during quenching CR 800-500 is not particularly defined, the upper limit is for example, 60000°C / min.

[0148] Preferably, quenching is performed after performing heating of the hollow shell in the austenite zone a plurality of times. In this case, the SSC resistance of the steel material increases because austenite grains are refined prior to quenching. Heating in the austenite zone may be repeated a plurality of times by performing quenching a plurality of times, or heating in the austenite zone may be repeated a plurality of times by performing normalizing and quenching. Further, quenching and tempering that is described later may be performed in combination a plurality of times. That is, quenching and tempering may be performed a plurality of times. In this case, the SSC resistance of the steel material increases further. The tempering process is described in detail hereunder.[Tempering process]

[0149] 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 less than the A c1 point and holding the intermediate steel material at that temperature. Here, the tempering temperature corresponds to the temperature of the furnace when the intermediate steel material after quenching is heated and held at the relevant temperature. The tempering time means the period of time from the temperature of the intermediate steel material reaching a predetermined tempering temperature till the extracting from the heat treatment furnace.

[0150] As described above, in the steel material according to the present embodiment, most of the precipitates having an equivalent circular diameter of 20 nm or more are cementite. In addition, the cementite is liable to coarsen due to Ostwald growth during holding for tempering. In particular, in the case of producing a steel material that is to be used for oil wells, for the purpose of increasing the low-temperature toughness and the SSC resistance, the tempering temperature is set within the range of 600 to 730°C. In tempering at such a high temperature, there is a tendency for cementite to easily coarsen due to Ostwald growth.

[0151] Therefore, in the tempering process according to the present embodiment, tempering at a high temperature is performed for a short time period, and thereafter cold working is performed to form a large number of cementite nuclei. Thereafter, tempering is performed at a temperature which is a little lower (hereunder, also referred to as "intermediate-temperature tempering") than the temperature in the high-temperature tempering to cause the large number of cementite nuclei formed as described above to grow. As a result, a large number of fine cementite particles can be formed in the steel material according to the present embodiment. That is, in the tempering process according to the present embodiment, two-stage tempering is performed, with cold working being performed between the two tempering stages, i.e., the tempering process is carried out in the order of high-temperature tempering, cold working, and intermediate-temperature tempering. Hereunder, each process is described in detail.[High-temperature tempering process]

[0152] In the high-temperature tempering process, the intermediate steel material (hollow shell) that was subjected to quenching is heated from room temperature to the tempering temperature, and thereafter is held at the tempering temperature for the tempering time. In the high-temperature tempering process, cementite nuclei precipitate during holding at a high temperature. Therefore, coarsening of cementite can be suppressed by an intermediate-temperature tempering process that is described later.

[0153] If the tempering temperature in the high-temperature tempering process is too low, the cementite nuclei will not sufficiently precipitate during holding for tempering, and the cementite may be coarsened by the intermediate-temperature tempering process that is described later. In this case, in the steel material after the intermediate-temperature tempering process, the number density NDC of coarse precipitates will be too high and the number density NDF of fine precipitates will decrease. As a result, the low-temperature toughness and the SSC resistance of the steel material will decrease. On the other hand, if the tempering temperature in the high-temperature tempering process is too high, the tempering temperature may become higher than the A c1 point. In such a case, austenite will be mixed in the microstructure of the intermediate steel material. As a result, the microstructure of the steel material after the intermediate-temperature tempering process that is described later will not be principally composed of tempered martensite and tempered bainite, and mechanical properties defined in the present embodiment cannot be obtained.

[0154] Therefore, in the high-temperature tempering process according to the present embodiment, a preferable tempering temperature is within the range of 695 to 720°C. A more preferable lower limit of the tempering temperature in the high-temperature tempering process is 700°C. A more preferable upper limit of the tempering temperature in the high-temperature tempering process is 715°C.

[0155] If the tempering time is too short, the cementite nuclei will not sufficiently precipitate during holding for tempering, and the cementite will be coarsened by the intermediate-temperature tempering process that is described later. In this case, in the steel material after the intermediate-temperature tempering process, the number density NDC of coarse precipitates will be too high, and the number density NDF of fine precipitates will decrease. As a result, the low-temperature toughness and SSC resistance of the steel material will decrease. On the other hand, if the tempering time in the high-temperature tempering process is too long, in some cases the cementite may coarsen during holding for tempering. In this case, in the steel material after the intermediate-temperature tempering process, the number density NDC of coarse precipitates will be too high and the number density NDF of fine precipitates will decrease. As a result, the low-temperature toughness and SSC resistance of the steel material will decrease. In addition, if the tempering time is too long, in some cases the yield strength will decrease.

[0156] Therefore, in the high-temperature tempering process according to the present embodiment, a preferable tempering time is within the range of 2 to less than 20 minutes. A more preferable upper limit of the tempering time in the high-temperature tempering process is 15 minutes. A more preferable lower limit of the tempering time in the high-temperature tempering process is 3 minutes, and further preferably is 5 minutes. Hereunder, the cold working process is described in detail.[Cold working process]

[0157] In the cold working process, the intermediate steel material (hollow shell) that was held at a high temperature in the high-temperature tempering process is subjected to cold working. In the cold working process, strain is introduced into the intermediate steel material by performing cold working on the intermediate steel material. As a result, a further large number of nuclei formation sites are introduced in addition to the cementite nuclei which precipitated in the high-temperature tempering process. Therefore, coarsening of cementite can be further suppressed by an intermediate-temperature tempering process that is described later. Note that, the cold working can be performed by a well-known method. That is, the cold working may be cold rolling, may be cold drawing, or may be expanding. Further, the temperature of the intermediate steel material in the cold working is, for example, 0 to 250°C.

[0158] In the case of performing cold rolling, a preferable area reduction ratio is 5 to 20%. If the area reduction ratio is too low, in some cases strain will not sufficiently enter the intermediate steel material, and cementite nuclei formation sites will not be sufficiently introduced. In such a case, in the steel material after the intermediate-temperature tempering process, the number density NDC of coarse precipitates will be too high and the number density NDF of fine precipitates will decrease. As a result, the low-temperature toughness and the SSC resistance of the steel material will decrease. On the other hand, if the area reduction ratio is too high, in some cases too much strain will enter the intermediate steel material, and recrystallization will easily occur in the intermediate-temperature tempering process. Here, if recrystallization occurs in the intermediate-temperature tempering process, there is a possibility that the strength of the steel material will be too low. Further, if intermediate-temperature tempering is performed under conditions such that the required strength can be secured, there is a possibility that dislocations will not be sufficiently reduced. In such a case, in addition, recrystallization will be liable to occur partially in the intermediate-temperature tempering process. As a result, the number density NDC of coarse precipitates will be too high and the number density NDF of fine precipitates will decrease. As a result, the low-temperature toughness and the SSC resistance of the steel material will decrease.

[0159] Therefore, in the present embodiment, in the case of performing cold rolling in the cold working process, the area reduction ratio is preferably made 5 to 20%. Note that, in the present embodiment, the area reduction ratio in the case of performing cold rolling is defined by the following Formula (B). [Intermediate-temperature tempering process]

[0160] In the intermediate-temperature tempering process, the intermediate steel material (hollow shell) that was subjected to the high-temperature tempering process is held for a tempering time at a tempering temperature in a temperature region that is a little lower than the temperature region in the high-temperature tempering process. In the intermediate-temperature tempering process, the yield strength of the steel material is adjusted to 862 MPa or more (125 ksi or more).

[0161] If the tempering temperature in the intermediate-temperature tempering process is too low, in some cases the yield strength of the steel material after tempering will be too high. In such a case, the strength will be too high, and the low-temperature toughness and SSC resistance of the steel material may decrease. On the other hand, if the tempering temperature in the intermediate-temperature tempering process is too high, in some cases the yield strength of the steel material after tempering may become lower. As a result, a yield strength of 862 MPa or more will not be obtained. Therefore, in the intermediate-temperature tempering process according to the present embodiment, a preferable tempering temperature is within the range of 600 to 690°C. A more preferable upper limit of the tempering temperature in the intermediate-temperature tempering process is less than 690°C, and further preferably is 685°C. A more preferable lower limit of the tempering temperature in the intermediate-temperature tempering process is 620°C, and further preferably is 640°C.

[0162] If the tempering time in the intermediate-temperature tempering process is too short, in some cases the yield strength of the steel material after tempering will be too high. As a result, the strength will be too high, and the low-temperature toughness and SSC resistance of the steel material may decrease. On the other hand, if the tempering time is too long, the aforementioned effects are saturated. Accordingly, in the present embodiment, a preferable tempering time in the intermediate-temperature tempering process is within the range of 10 to 180 minutes. A more preferable upper limit of the tempering time is 120 minutes, and further preferably is 90 minutes. A more preferable lower limit of the tempering time is 15 minutes, and further preferably is 20 minutes. Note that, in a case where the steel material is a steel pipe, in comparison to other shapes, temperature variations with respect to the steel pipe are liable to occur during holding for tempering. Therefore, in a case where the steel material is a steel pipe, the tempering time is preferably set within a range of 15 to 180 minutes.

[0163] As described above, in the intermediate-temperature tempering process, the tempering temperature and tempering time are adjusted to obtain a steel material having a yield strength of 862 MPa or more. Note that, it is sufficiently possible for a person skilled in the art to obtain a steel material having a yield strength of 862 MPa or more by subjecting an intermediate steel material (hollow shell) having the chemical composition of the present embodiment to intermediate-temperature tempering in which the aforementioned tempering temperature and the aforementioned tempering time are appropriately adjusted.

[0164] The steel material according to the present embodiment can be produced by the production method that is described above. A method for producing a seamless steel pipe has been described as one example of the aforementioned production method. However, the steel material according to the present embodiment may 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. However, the aforementioned production method is one example, and the steel material according to the present embodiment may also be produced by other production methods. Hereunder, the present disclosure is described more specifically by way of examples.EXAMPLE 1

[0165] In Example 1, steel materials having a yield strength of less than 965 MPa were investigated. Specifically, molten steels of a weight of 180 kg having the chemical compositions shown in Table 1-1, Table 1-2, and Table 1-3 were produced. Fn2 that was determined based on the obtained chemical composition (mass%) and Formula (2), Fn3 that was determined based on the obtained chemical composition (mass%) and Formula (3), and Fn4 that was determined based on the obtained chemical composition (mass%) and Formula (4) are shown in Table 1-3. Note that, "-" in Table 1-2 and Table 1-3 means that the contents of the respective elements are at the level of an impurity. Specifically, "-" means that the V content, Co content, W content, and Cu content of Test Number 1-3 were 0% when rounded off to two decimal places. In addition, it means that the Nb content of Test Number 1-1 was 0% when rounded off to three decimal places. Further, it means that the Ca content, Mg content, Zr content, and REM content of Test Number 1-1 were 0% when rounded off to four decimal places.[Table 1-1]

[0166] TABLE 1-1Test NumberChemical Composition (Unit is mass%; balance is Fe and impurities)CSiMnPSAlNiCrMo1-10.250.220.060.0080.00100.0500.280.650.741-20.250.260.200.0090.00070.0270.460.760.941-30.290.220.380.0100.00080.0510.541.120.811-40.240.340.470.0100.00090.0470.770.850.611-50.280.220.430.0110.00100.0370.271.070.671-60.250.240.470.0110.00060.0380.900.681.051-70.320.290.400.0110.00100.0430.630.971.221-80.290.310.380.0090.00080.0440.450.751.211-90.300.340.400.0080.00070.0430.260.910.781-100.310.350.360.0070.00100.0410.771.031.121-110.240.220.470.0080.00090.0470.120.920.731-120.270.680.170.0120.00090.0391.770.930.681-130.260.980.310.0070.00100.0490.150.841.081-140.291.230.330.0110.00070.0351.461.040.881-150.240.460.190.0120.00100.0251.740.790.751-160.281.130.770.0090.00060.0490.180.691.091-170.290.830.610.0070.00090.0280.680.920.651-180.260.380.090.0110.00060.0530.220.911.001-190.260.450.340.0080.00080.0380.140.690.551-200.300.740.750.0100.00080.0350.790.901.131-210.290.720.610.0070.00090.0301.141.140.571-220.280.310.730.0090.00070.0301.210.871.131-230.300.780.300.0110.00100.0511.821.130.881-240.261.160.530.0070.00080.0531.660.940.581-250.270.200.790.0110.00110.0340.580.900.521-260.270.240.140.0090.00060.0341.860.770.921-270.250.500.150.0060.00060.0291.930.700.881-280.310.250.390.0080.00080.0550.370.861.041-290.240.230.370.0120.00070.0470.771.011.201-300.240.310.410.0060.00090.0540.010.820.611-310.250.350.360.0120.00070.0260.760.950.911-320.250.280.420.0070.00080.0360.030.670.621-330.310.350.460.0070.00070.0250.050.690.501-340.300.280.410.0110.00080.0350.450.140.811-350.260.270.350.0120.00060.0470.670.570.061-360.270.331.810.0080.00060.0410.661.200.651-370.290.260.460.0060.00060.0440.711.110.631-380.310.250.380.0510.00080.0430.151.050.871-390.250.300.350.0120.00070.0370.230.701.171-400.260.300.440.0060.00100.0541.180.750.811-410.250.430.310.0080.00070.0310.280.900.631-420.291.150.410.0060.00060.0300.130.801.171-430.240.540.350.0120.00060.0520.121.121.081-440.270.970.240.0060.00060.0470.441.041.07 [Table 1-2]

[0167] TABLE 1-2Test NumberChemical Composition (Unit is mass%; balance is Fe and impurities)TiBNOVNbCaMg1-10.0140.00140.00460.00140.15---1-20.0130.00140.00470.00140.08---1-30.0120.00120.00420.0007----1-40.0090.00120.00390.00080.110.014--1-50.0150.00120.00390.00160.08-0.0012-1-60.0130.00140.00430.00160.09--0.00091-70.0110.00110.00460.00060.11---1-80.0150.00140.00340.00070.13---1-90.0100.00150.00340.00060.15---1-100.0100.00150.00440.00150.11---1-110.0130.00120.00250.00060.15---1-120.0110.00110.00250.00140.12---1-130.0110.00120.00440.0008----1-140.0150.00120.00360.00150.13---1-150.0120.00130.00310.00070.15---1-160.0150.00110.00310.00180.15---1-170.0090.00130.00380.00100.15---1-180.0130.00110.00410.0009----1-190.0150.00140.00370.00060.15---1-200.0090.00120.00480.00180.12---1-210.0120.00130.00380.00100.09---1-220.0110.00140.00480.00160.09---1-230.0140.00130.00410.00170.10---1-240.0140.00130.00350.00190.13---1-250.0140.00140.00420.00150.13---1-260.0150.00130.00310.00120.15---1-270.0110.00140.00480.00080.10---1-280.0150.00120.00420.00120.12---1-290.0130.00140.00250.00130.08---1-300.0110.00110.00250.00110.140.013--1-310.0140.00110.00280.00160.08---1-320.0150.00140.00290.00150.080.010--1-330.0130.00110.00440.00180.15---1-340.0090.00120.00260.00060.14---1-350.0110.00150.00350.00110.14---1-360.0120.00130.00220.00110.13---1-370.0150.00130.04610.00100.11---1-380.0090.00110.00360.00100.14---1-390.0120.00120.00270.0015----1-400.0100.00150.00220.00190.140.009--1-410.0130.00120.00290.0010----1-420.0090.00150.00370.00110.14-0.0015-1-430.0110.00120.00430.0011----1-440.0130.00120.00470.00150.08--0.0019 [Table 1-3]

[0168] TABLE 1-3Test NumberChemical Composition (Unit is mass%; balance is Fe and impurities)Fn2Fn3Fn4ZrNdCoWCu1-1-----0.308-1.3-6.61-2-----0.295-3.2-15.91-3-----0.343-5.4-24.91-4-----0.268-7.3-32.71-5-----0.304-4.9-18.81-6-----0.298-7.5-38.01-70.0015----0.335-5.6-27.11-8--0.26--0.325-4.7-20.91-9---0.28-0.338-4.4-15.61-10----0.160.307-5.9-28.81-11-0.0019---0.215-4.6-15.91-12-----0.296-8.3-46.01-13-----0.331-2.90.71-14-----0.279-8.5-33.81-15-----0.241-8.3-49.31-16-----0.322-7.5-13.41-17-----0.328-8.3-27.11-18-----0.319-1.1-3.41-19-----0.349-3.6-8.41-20-----0.316-9.7-36.41-21-----0.310-10.2-42.11-22-----0.300-11.1-54.61-23-----0.294-9.6-50.21-24-----0.278-11.4-47.41-25-----0.318-9.8-40.21-26-----0.293-8.1-54.51-27-----0.308-8.5-52.81-28-----0.367-4.6-19.91-29-----0.189-5.9-31.01-30-----0.257-3.7-9.21-31-----0.248-6.0-28.51-32-----0.377-3.9-10.61-33-----0.521-4.4-11.51-34-----1.446-5.3-22.41-35-----0.516-6.2-26.91-36-----0.233-19.8-75.01-37-----0.300-7.0-31.91-38-----0.312-3.7-13.21-39-----0.345-3.5-13.71-40-----0.297-8.5-44.11-41-----0.346-3.8-11.71-42-----0.306-3.70.61-43-----0.222-3.1-6.81-44-----0.249-3.4-5.5

[0169] Ingots were produced using the molten steels of Test Numbers 1-1 to 1-44. The produced ingots were hot rolled to produce steel plates having a thickness of 15 mm. The steel plates of Test Numbers 1-1 to 1-44 after hot rolling were allowed to cool to bring the steel plate temperature to normal temperature (25°C). After being allowed to cool, the steel plates of Test Numbers 1-1 to 1-44 were held for 20 minutes at the quenching temperature (920°C), the steel plates were immersed in a water bath to be quenched. At this time, the cooling rate during quenching (CR 800-500 ) was 600°C / min for each test number. Note that, a type K thermocouple of a sheath type was inserted into a center portion of the thickness of the steel plate in advance, and the quenching temperature and cooling rate during quenching CR 800-500 were measured using the type K thermocouple.

[0170] After quenching, the steel plates of Test Numbers 1-1 to 1-44 were subjected to tempering. In the tempering, the steel plates of Test Numbers 1-1 to 1-28, 1-31 to 1-38, and 1-41 to 1-44 were subjected to a first tempering, cold working, and a second tempering. On the other hand, the steel plates of Test Numbers 1-29 and 1-30 were subjected to tempering one time and cold working. The steel plates of Test Numbers 1-39 and 1-40 were subjected to a first tempering and a second tempering. Note that, in the present examples, cold rolling was performed as the cold working.

[0171] For the steel plate of each test number, the tempering temperature (°C) and the tempering time (mins) in the first tempering are shown in Table 2. Similarly, for the steel plate of each test number, the area reduction ratio (%) of the cold working is shown in Table 2. For the steel plate of each test number, the tempering temperature (°C) and the tempering time (mins) in the second tempering are shown in Table 2. Note that, "-" in the "Cold Working" column in Table 2 means that cold working was not performed. Likewise, "-" in the "Second Tempering" column in Table 2 means that the second tempering was not performed.[Table 2]

[0172] TABLE 2Test NumberFirst TemperingCold WorkingSecond TemperingTempering Temperature (°C)Tempering Time (min)Area Reduction Ratio (%)Tempering Temperature (°C)Tempering Time (min)1-1705510690501-2710510680401-37051010670801-4705155690301-5715510690501-6705510690501-77051010690501-8705515690501-9705510690501-10705510690501-11705510690701-12705510690501-13705510690501-14705517690501-15705510690501-16705510690501-17705510690501-18705510680501-19705510690501-20705510690501-21705510690501-22705510690501-23705510690501-24705510690501-25705510690501-26705510690501-27705510690501-28705510690501-296906010--1-306906010--1-317003510660201-32705510680501-33705510690501-34705510690501-35705510690501-36705510690501-37705510690501-38705510690501-397055-680501-407055-690501-4170552680501-4270552690501-43705540680201-4470554069020

[0173] Here, in the present example, the tempering temperature was the temperature of the heat treatment furnace where the tempering is performed. Further, in the present example, the tempering time was taken as the period of time from the temperature of the steel plate of each test number reaching a predetermined tempering temperature till the extracting from the heat treatment furnace.[Evaluation tests]

[0174] The steel plates of Test Numbers 1-1 to 1-44 that underwent tempering were subjected to a tensile test, a test to measure the Cr concentration in precipitates having an equivalent circular diameter of 20 nm or more, a test to measure the number density of precipitates, a Charpy impact test and an SSC resistance test that are described hereunder.[Tensile test]

[0175] The steel plates of Test Numbers 1-1 to 1-44 were subjected to a tensile test according to the method described above. Specifically, round bar tensile test specimens having a parallel portion diameter of 4 mm and a gage length of 16 mm were prepared from the center portion of the thickness of the steel plates of Test Numbers 1-1 to 1-44. The axial direction of the round bar tensile test specimen was parallel to the rolling elongation direction of the steel plate. The tensile test was performed in conformity with ASTM E8 / E8M (2021) in the atmosphere at normal temperature (25°C) using the round bar test specimens of Test Numbers 1-1 to 1-44, and the yield strengths (MPa) of the steel plates of Test Numbers 1-1 to 1-44 were obtained. The obtained yield strengths are shown in Table 3 as "YS (MPa)".[Table 3]

[0176] TABLE 3Test NumberYS (MPa)θ Cr (mass fraction)Fn1Number Density of Fine Precipitates NDF ( / µm 2< )Number Density of Coarse Precipitates NDC ( / µm 2< )Fn5vE (-80°C) (J)SSC Resistance1-18850.10320.2421.1910.19411.6140E1-29270.11440.2301.3920.18225.5142E1-39540.13440.2790.7650.2756.9117E1-48740.11890.2231.4140.16441.5149E1-59050.13490.2481.0860.2137.6130E1-68820.10320.2401.1540.20430.3142E1-79200.12120.2721.0590.24117.2119E1-89010.10270.2681.0340.25012.0126E1-99040.11720.2750.9780.2535.9122E1-109060.13120.2471.1700.23125.9132E1-118810.13960.1671.6390.1679.3156E1-129070.12110.2481.1880.22858.3127E1-138700.11240.2701.1820.2386.5125E1-149360.12920.2371.1110.24740.7124E1-158810.11520.2021.3520.16487.8158E1-169270.09740.2691.1450.2385.7120E1-179260.11540.2771.2190.25621.7118E1-188910.12230.2540.7870.2693.7114E1-198890.10110.2810.7470.2811.7106E1-209260.11210.2701.1300.23823.2120NA1-219210.13230.2681.0990.22231.7123NA1-228980.11450.2501.1080.23834.0132NA1-239270.13910.2411.1230.21056.1128NA1-249260.12480.2321.1700.19855.9142NA1-258920.11310.2751.0710.24113.8124NA1-268930.10980.2391.1910.19467.8130NA1-278910.10630.2441.1140.20160.5131NA1-289110.10700.3060.3670.3241.1102NA1-298730.15450.1430.4810.3063.8101NA1-308790.12190.2060.4260.3150.184E1-318660.13290.1990.4350.3093.087NA1-329290.09680.3050.4850.2930.278E1-339080.08510.4320.4170.3150.174E1-348980.01901.7330.3300.3670.963NA1-358630.07740.4360.3800.3241.771NA1-368900.14880.1991.1510.20117.6106NA1-379050.13730.2481.2220.21325.452NA1-389120.13360.2501.0830.2222.842NA1-398800.10930.2560.4480.3211.1100NA1-408960.11910.2210.4810.3095.5102NA1-418760.12450.2660.4070.3330.998NA1-429010.11680.2350.4410.3180.7100NA1-438680.14140.1910.5090.2960.987NA1-448990.13810.2030.5120.2992.786NA [Test to measure Cr concentration in precipitates having equivalent circular diameter of 20 nm or more]

[0177] The Cr concentration in precipitates having an equivalent circular diameter of 20 nm or more in the respective steel plates of Test Numbers 1-1 to 1-44 was measured and calculated by the measurement method described above. Note that, the TEM used was JEM-2010 manufactured by JEOL Ltd., and the acceleration voltage was set to 200 kV. The Cr concentration in precipitates having an equivalent circular diameter of 20 nm or more in the steel plates of Test Numbers 1-1 to 1-44 are shown in Table 3 as "θ Cr (mass fraction)". In addition, Fn1 that was determined based on the chemical composition (mass%), θ Cr (mass fraction), and Formula (1) for each of Test Numbers 1-1 to 1-44 is shown in Table 3.[Test to measure number density of precipitates]

[0178] For the steel plates of Test Numbers 1-1 to 1-44, the number density NDF ( / µm 2< ) of precipitates having an equivalent circular diameter of 20 to 150 nm (fine precipitates) and the number density NDC ( / µm 2< ) of precipitates having an equivalent circular diameter of 250 nm or more (coarse precipitates) were calculated by the measurement method described above. Note that, the SEM used was model ERA-8900FE manufactured by ELIONIX INC., and the acceleration voltage was set to 5 kV and the working distance was set to 15 mm. The observation visual field was set to 12 µm × 9 µm (magnification of ×10000), and three visual fields were observed. The area fraction S (%) of precipitates in the observation visual field was determined as the volume ratio V θ (%) of cementite obtained by thermodynamic calculation using the chemical composition of the steel plate of each test number and the first and second tempering temperatures. Note that, thermodynamic calculation was performed using a thermodynamic calculation software named Thermo-Calc (available from Thermo-Calc Software, version 2017a), and TCFE8 was used as the database.

[0179] The number density NDF of fine precipitates ( / µm 2< ) was determined based on the sum of the numbers of fine precipitates obtained in the three visual fields, and the total area (µm 2< ) of the three visual fields. Similarly, the number density NDC of coarse precipitates ( / µm 2< ) was determined based on the sum of the numbers of coarse precipitates obtained in the three visual fields, and the total area (µm 2< ) of the three visual fields. The number density NDF of fine precipitates ( / µm 2< ) and number density NDC of coarse precipitates ( / µm 2< ) obtained for the steel plates of Test Numbers 1-1 to 1-44 are shown in Table 3. In addition, Fn5 that was determined for Test Numbers 1-1 to 1-44 based on the respective chemical compositions (mass%), the obtained number density NDF of fine precipitates ( / µm 2< ), the obtained number density NDC of coarse precipitates ( / µm 2< ), and Formula (5) is shown in Table 3.[Charpy impact test]

[0180] A Charpy impact test in conformity with JIS Z 2242 (2018) was performed on the respective steel plates of Test Numbers 1-1 to 1-44, and the low-temperature toughness was evaluated. Specifically, a full-size V-notch test specimens were prepared from the center portion of the thickness of the steel plates of Test Numbers 1-1 to 1-44. The longitudinal direction of the test specimen was parallel to the plate width direction. The notched surfaces of the test specimens were perpendicular to the rolling elongation direction of the steel plate. Five test specimens that were prepared were cooled to -80°C. A Charpy impact test in conformity with JIS Z 2242 (2018) was performed on the cooled test specimens, and the absorbed energy (J) was determined. The arithmetic average value of the absorbed energy determined for each of the five test specimens was defined as the absorbed energy vE(-80°C)(J). The obtained absorbed energy vE(-80°C)(J) for the steel plates of Test Numbers 1-1 to 1-44 is shown in Table 3.[SSC resistance test]

[0181] The SSC resistance of the respective steel plates of Test Numbers 1-1 to 1-44 was evaluated by a method performed in accordance with "Method A" specified in NACE TM0177-2016. Specifically, round bar test specimens having a diameter of 6.35 mm and a parallel portion length of 25.4 mm were prepared from the center portion of the thickness of the steel plates of Test Numbers 1-1 to 1-44. The round bar test specimen was prepared in a manner so that the axial direction thereof was parallel to the rolling elongation direction of the steel plate. Tensile stress was applied in the axial direction of the round bar test specimens of the respective test numbers. At this time, the applied stress was adjusted so as to be 80% of the actual yield stress (80% AYS) of each steel plate of the respective test numbers.

[0182] A mixed aqueous solution containing 5.0 mass% of sodium chloride and 0.5 mass% of acetic acid (NACE solution A) was used as the test solution. The test solution at 24°C was poured into three test vessels, and these were adopted as test baths. Three round bar test specimens to which the stress was applied were immersed individually in mutually different test vessels as the test baths. After each test bath was degassed, a mixed gas of H 2 S gas at 0.15 atm pressure and N 2 gas at 0.85 atm pressure was blown into the respective test baths and caused to saturate. The test baths in which the gaseous mixture was saturated were held at 24°C for 720 hours.

[0183] After being held for 720 hours, the round bar test specimens of each test number were observed to determine whether or not sulfide stress cracking (SSC) had occurred. Specifically, after being immersed for 720 hours, the round bar test specimens were observed with the naked eye and using a projector with a magnification of ×10. Steel plates for which cracking was not confirmed in all three of the round bar test specimens as the result of the observation were determined as being "E" (Excellent). On the other hand, steel plates for which cracking was confirmed in at least one round bar test specimen were determined as being "NA" (Not Acceptable).[Test results]

[0184] The test results are shown in Table 3.

[0185] Referring to Table 1-1, Table 1-2, Table 1-3, Table 2, and Table 3, for each of the steel plates of Test Numbers 1-1 to 1-19, the chemical composition was appropriate and the yield strength was 862 MPa or more (125 ksi or more). In addition, Fn1 was not more than 0.300, Fn2 was not more than 0.355, Fn3 was -9.0 or more, and Fn4 was -51.0 or more. Further, the number density NDF of fine precipitates was 0.650 / µm 2< or more, and the number density NDC of coarse precipitates was 0.290 / µm 2< or less. As a result, for these steel plates, the absorbed energy vE(-80°C) was 105 J or more, and these steel plates exhibited excellent low-temperature toughness. In addition, these steel plates exhibited excellent SSC resistance in the SSC resistance test. Note that, because these steel plates had a yield strength of 862 MPa or more and had excellent low-temperature toughness and excellent SSC resistance, it was determined that the total of the volume ratios of tempered martensite and tempered bainite was 90% or more in the microstructure of each of these steel plates.

[0186] In the steel plates of Test Numbers 1-1 to 1-17, in addition, Fn5 was 4.0 or more. As a result, in these steel plates the absorbed energy vE(-80°C) was 115 J or more, and thus these steel plates exhibited even more excellent low-temperature toughness.

[0187] On the other hand, in the steel plates of Test Numbers 1-20, 1-21, and 1-23 to 1-25, Fn3 was less than -9.0. As a result, these steel plates did not exhibit excellent SSC resistance in the SSC resistance test.

[0188] In the steel plate of Test Number 1-22, Fn3 was less than -9.0 and Fn4 was less than -51.0. As a result, this steel plate did not exhibit excellent SSC resistance in the SSC resistance test.

[0189] In the steel plates of Test Numbers 1-26 and 1-27, Fn4 was less than -51.0. As a result, these steel plates did not exhibit excellent SSC resistance in the SSC resistance test.

[0190] In the steel plate of Test Number 1-28, Fn2 was more than 0.355 and Fn1 was more than 0.300. As a result, the number density NDF of fine precipitates was less than 0.650 / µm 2< and the number density NDC of coarse precipitates was more than 0.290 / µm 2< . Consequently, in this steel plate the absorbed energy vE(-80°C) was less than 105 J, and this steel plate did not exhibit excellent low-temperature toughness. In addition, this steel plate did not exhibit excellent SSC resistance in the SSC resistance test.

[0191] The steel plate of Test Number 1-29 was not subjected to the second tempering. As a result, the number density NDF of fine precipitates was less than 0.650 / µm 2< and the number density NDC of coarse precipitates was more than 0.290 / µm 2< . Consequently, in this steel plate the absorbed energy vE(-80°C) was less than 105 J, and excellent low-temperature toughness was not exhibited. In addition, this steel plate did not exhibit excellent SSC resistance in the SSC resistance test.

[0192] In the steel plate of Test Number 1-30, the Ni content was too low. In addition, the second tempering was not performed. As a result, the number density NDF of fine precipitates was less than 0.650 / µm 2< and the number density NDC of coarse precipitates was more than 0.290 / µm 2< . Consequently, in this steel plate the absorbed energy vE(-80°C) was less than 105 J, and the steel plate did not exhibit excellent low-temperature toughness.

[0193] For the steel plate of Test Number 1-31, the tempering time of the high-temperature tempering was too long. As a result, the number density NDF of fine precipitates was less than 0.650 / µm 2< and the number density NDC of coarse precipitates was more than 0.290 / µm 2< . Consequently, in this steel plate the absorbed energy vE(-80°C) was less than 105 J, and excellent low-temperature toughness was not exhibited. In addition, this steel plate did not exhibit excellent SSC resistance in the SSC resistance test.

[0194] In the steel plates of Test Numbers 1-32 and 1-33, the Ni content was too low. In addition, Fn2 was more than 0.355, and Fn1 was more than 0.300. As a result, the number density NDF of fine precipitates was less than 0.650 / µm 2< and the number density NDC of coarse precipitates was more than 0.290 / µm 2< . Consequently, in these steel plates the absorbed energy vE(-80°C) was less than 105 J, and the steel plates did not exhibit excellent low-temperature toughness.

[0195] In the steel plate of Test Number 1-34, the Cr content was too low. In addition, Fn2 was more than 0.355, and Fn1 was more than 0.300. As a result, the number density NDF of fine precipitates was less than 0.650 / µm 2< and the number density NDC of coarse precipitates was more than 0.290 / µm 2< . Consequently, in this steel plate the absorbed energy vE(-80°C) was less than 105 J, and excellent low-temperature toughness was not exhibited. In addition, this steel plate did not exhibit excellent SSC resistance in the SSC resistance test.

[0196] In the steel plate of Test Number 1-35, the Mo content was too low. In addition, Fn2 was more than 0.355, and Fn1 was more than 0.300. As a result, the number density NDF of fine precipitates was less than 0.650 / µm 2< and the number density NDC of coarse precipitates was more than 0.290 / µm 2< . Consequently, in this steel plate the absorbed energy vE(-80°C) was less than 105 J, and excellent low-temperature toughness was not exhibited. In addition, this steel plate did not exhibit excellent SSC resistance in the SSC resistance test.

[0197] In the steel plate of Test Number 1-36, the Mn content was too high. In addition, Fn3 was less than -9.0, and Fn4 was less than -51.0. As a result, this steel plate did not exhibit excellent SSC resistance in the SSC resistance test.

[0198] In the steel plate of Test Number 1-37, the N content was too high. As a result, in this steel plate the absorbed energy vE(-80°C) was less than 105 J, and excellent low-temperature toughness was not exhibited. In addition, this steel plate did not exhibit excellent SSC resistance in the SSC resistance test.

[0199] In the steel plate of Test Number 1-38, the P content was too high. As a result, in this steel plate the absorbed energy vE(-80°C) was less than 105 J, and excellent low-temperature toughness was not exhibited. In addition, this steel plate did not exhibit excellent SSC resistance in the SSC resistance test.

[0200] The steel plates of Test Numbers 1-39 and 1-40 were not subjected to cold working between the first tempering and the second tempering. As a result, the number density NDF of fine precipitates was less than 0.650 / µm 2< and the number density NDC of coarse precipitates was more than 0.290 / µm 2< . Consequently, in these steel plates the absorbed energy vE(-80°C) was less than 105 J, and excellent low-temperature toughness was not exhibited. In addition, these steel plates did not exhibit excellent SSC resistance in the SSC resistance test.

[0201] For the steel plates of Test Numbers 1-41 and 1-42, the area reduction ratio of the cold working performed between the first tempering and the second tempering was too low. As a result, the number density NDF of fine precipitates was less than 0.650 / µm 2< and the number density NDC of coarse precipitates was more than 0.290 / µm 2< . Consequently, in these steel plates the absorbed energy vE(-80°C) was less than 105 J, and excellent low-temperature toughness was not exhibited. In addition, these steel plates did not exhibit excellent SSC resistance in the SSC resistance test.

[0202] For the steel plates of Test Numbers 1-43 and 1-44, the area reduction ratio of the cold working performed between the first tempering and the second tempering was too high. As a result, the number density NDF of fine precipitates was less than 0.650 / µm 2< and the number density NDC of coarse precipitates was more than 0.290 / µm 2< . Consequently, in these steel plates the absorbed energy vE(-80°C) was less than 105 J, and excellent low-temperature toughness was not exhibited. In addition, these steel plates did not exhibit excellent SSC resistance in the SSC resistance test.EXAMPLE 2

[0203] In Example 2, steel materials having a yield strength of 965 MPa or more were investigated. Specifically, molten steels of a weight of 180 kg having the chemical compositions shown in Table 4-1, Table 4-2, and Table 4-3 were produced. Fn2 that was determined based on the obtained chemical composition (mass%) and Formula (2), Fn3 that was determined based on the obtained chemical composition (mass%) and Formula (3), and Fn4 that was determined based on the obtained chemical composition (mass%) and Formula (4) are shown in Table 4-3. Note that, "-" in Table 4-2 and Table 4-3 means that the contents of the respective elements are at the level of an impurity. Specifically, "-" means that the V content, Co content, W content, and Cu content of Test Number 2-3 were 0% when rounded off to two decimal places. In addition, it means that the Nb content of Test Number 2-1 was 0% when rounded off to three decimal places. Further, it means that the Ca content, Mg content, Zr content, and REM content of Test Number 2-1 were 0% when rounded off to four decimal places.[Table 4-1]

[0204] TABLE 4-1Test NumberChemical Composition (Unit is mass%; balance is Fe and impurities)CSiMnPSAlNiCrMo2-10.260.270.080.0070.00090.0340.391.101.142-20.250.280.450.0100.00100.0500.160.741.032-30.260.220.360.0060.00100.0280.430.960.902-40.250.340.400.0110.00090.0450.860.581.192-50.270.250.360.0100.00090.0390.620.641.222-60.260.250.360.0080.00080.0530.720.850.962-70.280.310.420.0070.00090.0530.830.920.642-80.310.300.430.0090.00100.0460.131.241.022-90.280.280.450.0100.00100.0360.790.960.732-100.260.350.440.0100.00080.0510.411.020.612-110.290.300.220.0100.00080.0440.670.760.942-120.270.450.100.0100.00080.0491.850.691.082-130.250.600.170.0110.00090.0500.190.940.762-140.291.260.310.0070.00100.0480.740.761.162-150.300.620.130.0060.00060.0411.931.020.962-160.300.860.280.0100.00100.0321.350.921.072-170.270.780.740.0110.00100.0430.141.000.692-180.260.280.370.0080.00060.0520.150.771.142-190.260.470.130.0120.00100.0450.230.780.942-200.250.330.790.0090.00070.0530.581.030.742-210.250.310.750.0120.00090.0530.700.690.632-220.240.970.440.0120.00080.0591.760.770.552-230.290.400.630.0120.00080.0340.971.040.752-240.241.010.620.0100.00100.0251.400.710.532-250.240.640.250.0060.00090.0411.880.850.562-260.300.210.100.0120.00110.0391.801.201.012-270.310.200.320.0090.00100.0331.541.141.122-280.300.290.450.0060.00100.0450.570.821.152-290.260.220.360.0100.00060.0420.620.830.942-300.290.220.440.0110.00090.0450.040.971.032-310.300.330.420.0090.00090.0270.351.120.522-320.250.220.440.0110.00100.0310.060.550.652-330.280.280.450.0090.00060.0320.030.600.502-340.260.310.460.0060.00100.0510.460.211.152-350.250.330.350.0120.00100.0310.710.580.142-360.260.331.660.0060.00100.0260.440.930.542-370.260.240.400.0120.00090.0440.360.961.082-380.310.350.440.1670.00090.0530.450.981.102-390.301.210.250.0110.00090.0431.221.220.812-400.310.420.230.0090.00080.0541.291.120.662-410.310.530.250.0110.00080.0260.731.200.922-420.290.830.290.0120.00100.0530.490.880.632-430.250.820.420.0110.00100.0520.621.080.672-440.280.810.420.0090.00060.0460.580.851.05 [Table 4-2]

[0205] TABLE 4-2Test NumberChemical Composition (Unit is mass%; balance is Fe and impurities)TiBNOVNbCaMg2-10.0140.00110.00250.00060.13---2-20.0120.00130.00420.00140.11---2-30.0150.00120.00270.0016----2-40.0120.00130.00420.00160.130.010--2-50.0110.00140.00410.00120.12-0.0009-2-60.0140.00110.00360.00120.14--0.00102-70.0110.00140.00330.00120.13---2-80.0130.00110.00440.00090.12---2-90.0150.00110.00480.00070.14---2-100.0130.00110.00360.00180.14---2-110.0130.00110.00430.00100.15---2-120.0110.00140.00310.00090.10---2-130.0130.00110.00370.0009----2-140.0120.00120.00410.00140.11---2-150.0150.00130.00230.00070.11---2-160.0110.00140.00270.00180.13---2-170.0120.00110.00390.00100.11---2-180.0100.00140.00250.0006----2-190.0100.00140.00310.00110.08---2-200.0110.00110.00330.00160.15---2-210.0140.00150.00320.00120.13---2-220.0150.00140.00430.00110.13---2-230.0110.00140.00330.00190.12---2-240.0130.00110.00300.00130.08---2-250.0150.00130.00450.00160.13---2-260.0110.00110.00340.00180.09---2-270.0100.00130.00330.00090.08---2-280.0110.00130.00250.00120.09---2-290.0120.00150.00240.00190.15---2-300.0110.00130.00460.00180.130.011--2-310.0110.00110.00260.00100.10---2-320.0140.00140.00350.00090.090.008--2-330.0140.00140.00480.00070.12---2-340.0120.00120.00240.00090.08---2-350.0140.00140.00410.00060.13---2-360.0120.00110.00360.00070.10---2-370.0150.00130.05360.00060.08---2-380.0150.00150.00360.00160.09---2-390.0100.00110.00240.0006----2-400.0120.00140.00410.00140.090.007--2-410.0090.00130.00310.0011----2-420.0130.00130.00230.00140.15-0.0011-2-430.0110.00140.00370.0019----2-440.0130.00140.00410.00190.09--0.0010 [Table 4-3]

[0206] TABLE 4-3Test NumberChemical Composition (Unit is mass%; balance is Fe and impurities)Fn2Fn3Fn4ZrNdCoWCu2-1-----0.189-1.6-9.72-2-----0.268-4.4-15.42-3-----0.316-4.7-21.12-4-----0.280-6.5-32.92-5-----0.318-5.2-26.12-6-----0.250-5.8-29.02-70.0018----0.320-7.1-33.22-8--0.21--0.262-4.1-13.52-9---0.33-0.291-7.2-33.52-10----0.150.250-5.6-21.22-11-0.0016---0.345-4.2-22.02-12-----0.336-7.6-49.62-13-----0.317-2.0-1.72-14-----0.344-5.2-11.92-15-----0.308-8.2-50.22-16-----0.311-7.4-34.82-17-----0.282-7.4-16.72-18-----0.349-3.4-12.42-19-----0.313-1.6-3.62-20-----0.212-9.6-38.22-21-----0.325-9.8-40.62-22-----0.284-11.0-50.22-23-----0.300-9.6-42.92-24-----0.345-11.3-45.32-25-----0.261-9.6-52.52-26-----0.270-7.5-51.92-27-----0.296-8.5-52.02-28-----0.364-5.9-27.02-29-----0.251-5.4-26.52-30-----0.280-3.8-12.52-31-----0.336-5.2-19.12-32-----0.425-4.2-13.12-33-----0.503-4.2-11.72-34-----0.702-5.6-23.92-35-----0.464-6.3-27.12-36-----0.304-17.6-63.62-37-----0.247-4.7-20.12-38-----0.338-5.4-22.32-39-----0.334-6.8-24.52-40-----0.347-7.0-38.32-41-----0.348-4.8-21.12-42-----0.344-4.5-10.82-43-----0.289-6.2-19.12-44-----0.317-5.7-18.1

[0207] Ingots were produced using the molten steel of Test Numbers 2-1 to 2-44. The produced ingots were hot rolled to produce steel plates having a thickness of 15 mm. The steel plates of Test Numbers 2-1 to 2-44 after hot rolling were allowed to cool to bring the steel plate temperature to normal temperature (25°C). After being allowed to cool, the steel plates of Test Numbers 2-1 to 2-44 were held for 20 minutes at the quenching temperature (920°C), the steel plates were immersed in a water bath to be quenched. At this time, the cooling rate during quenching (CR 800-500 ) was 600°C / min for each test number. Note that, a type K thermocouple of a sheath type was inserted into a center portion of the thickness of the steel plate in advance, and the quenching temperature and cooling rate during quenching CR 800-500 were measured using the type K thermocouple.

[0208] After quenching, the steel plates of Test Numbers 2-1 to 2-44 were subjected to tempering. In the tempering, the steel plates of Test Numbers 2-1 to 2-28, 2-31 to 2-38, and 2-41 to 2-44 were subjected to a first tempering, cold working, and a second tempering. On the other hand, the steel plates of Test Numbers 2-29 and 2-30 were subjected to tempering one time and cold working. The steel plates of Test Numbers 2-39 and 2-40 were subjected to a first tempering and a second tempering. Note that, in the present examples, cold rolling was performed as the cold working.

[0209] For the steel plate of each test number, the tempering temperature (°C) and the tempering time (mins) in the first tempering are shown in Table 5. Similarly, for the steel plate of each test number, the area reduction ratio (%) of the cold working is shown in Table 5. For the steel plate of each test number, the tempering temperature (°C) and the tempering time (mins) in the second tempering are shown in Table 5. Note that, "-" in the "Cold Working" column in Table 5 means that cold working was not performed. Likewise, "-" in the "Second Tempering" column in Table 5 means that the second tempering was not performed.[Table 5]

[0210] TABLE 5Test NumberFirst TemperingCold WorkingSecond TemperingTempering Temperature (°C)Tempering Time (min)Area Reduction Ratio (%)Tempering Temperature (°C)Tempering Time (min)2-1700510670502-271055660402-36951010650702-46951515670302-5700510670502-6700510670502-77001010670502-8700510670502-9700510670502-10700510670502-11695510670702-12695517670502-13700510670502-14700510680502-15695510670502-16700510680502-17700510670502-18700510670502-19700510680502-20700510670502-21700510670502-22695510670502-23700510670502-24695510670502-25695510670502-26695510670502-27695510670502-28700510670502-296706010--2-306706010--2-316953510640202-32700510660502-33700510670502-34700510670502-35700510670502-36695510670502-37700510670502-38700510670502-397005-660502-407005-670502-4170052660502-4270052670502-43700540660202-4470054067020

[0211] Here, in the present example, the tempering temperature was the temperature of the heat treatment furnace where the tempering is performed. Further, in the present example, the tempering time was taken as the period of time from the temperature of the steel plate of each test number reaching a predetermined tempering temperature till the extracting from the heat treatment furnace.[Evaluation tests]

[0212] The steel plates of Test Numbers 2-1 to 2-44 that underwent tempering were subjected to a tensile test, a test to measure the Cr concentration in precipitates having an equivalent circular diameter of 20 nm or more, a test to measure the number density of precipitates, a Charpy impact test and an SSC resistance test that are described hereunder.[Tensile test]

[0213] The steel plates of Test Numbers 2-1 to 2-44 were subjected to a tensile test according to the method described above. Specifically, round bar tensile test specimens having a parallel portion diameter of 4 mm and a gage length of 16 mm were prepared from the center portion of the thickness of the steel plates of Test Numbers 2-1 to 2-44. The axial direction of the round bar tensile test specimen was parallel to the rolling elongation direction of the steel plate. The tensile test was performed in conformity with ASTM E8 / E8M (2021) in the atmosphere at normal temperature (25°C) using the round bar test specimens of Test Numbers 2-1 to 2-44, and the yield strengths (MPa) of the steel plates of Test Numbers 2-1 to 2-44 were obtained. The obtained yield strengths are shown in Table 6 as "YS (MPa)".[Table 6]

[0214] TABLE 6Test NumberYS (MPa)θ Cr (mass fraction)Fn1Number Density of Fine Precipitates NDF ( / µm 2< )Number Density of Coarse Precipitates NDC ( / µm 2< )Fn5vE (-65°C) (J)SSC Resistance2-19960.16310.1432.1450.12767.3104E2-210450.11440.2102.0590.17621.684E2-310550.13760.2332.0150.17949.483E2-49870.09180.2351.9070.20780.293E2-510010.09240.2690.8670.26211.085E2-69920.12170.2031.9140.19170.195E2-710030.12360.2551.2840.24430.788E2-810160.15380.2131.8860.19814.791E2-910050.12870.2341.8640.19868.590E2-109940.14360.1961.8920.19136.798E2-1110100.10720.2730.9810.25615.078E2-1210040.09850.2760.7530.26923.182E2-139670.13670.2331.6850.21615.086E2-1410280.10540.2751.2190.24126.881E2-1510190.13490.2401.1140.24754.888E2-1610150.12390.2451.1640.23143.594E2-1710160.12670.2391.2470.2104.393E2-1810100.11400.2610.7500.2782.475E2-199980.11430.2440.7900.2753.777E2-209840.15510.1602.0680.17063.897NA2-219880.09700.2781.3770.23129.084NA2-2210110.11440.2291.8460.198147.390NA2-2310100.14540.2231.8550.19881.691NA2-2410060.10360.2781.1390.23540.484NA2-259960.13050.1992.0190.173204.591NA2-2610130.15680.2091.9660.198177.889NA2-2710180.14270.2371.9540.198150.785NA2-2810120.10440.3030.4690.3122.765NA2-299950.12480.1960.5030.2993.367NA2-3010060.12660.2310.4880.3060.355E2-319320.13560.2740.4600.3021.569E2-3210430.08020.3590.4660.3060.353E2-3310020.08070.4150.4380.3270.156E2-349760.03270.8000.3890.3431.551NA2-359670.08260.3860.4290.3272.436NA2-369710.12080.2551.0960.2508.676NA2-3710000.13940.1891.3950.18819.714NA2-3810210.12430.2701.0220.2534.78NA2-3910280.14040.2780.5190.3216.566NA2-4010220.13840.2730.5340.3127.468NA2-4110290.14110.2800.4690.3333.254NA2-4210130.12600.2560.5090.3182.563NA2-439910.13970.2340.4720.3062.868NA2-4410000.12430.2370.4940.3093.061NA [Test to measure Cr concentration in precipitates having equivalent circular diameter of 20 nm or more]

[0215] The Cr concentration in precipitates having an equivalent circular diameter of 20 nm or more in the respective steel plates of Test Numbers 2-1 to 2-44 was measured and calculated by the measurement method described above. Note that, the TEM used was JEM-2010 manufactured by JEOL Ltd., and the acceleration voltage was set to 200 kV. The Cr concentration in precipitates having an equivalent circular diameter of 20 nm or more in the steel plates of Test Numbers 2-1 to 2-44 are shown in Table 6 as "θ Cr (mass fraction)". In addition, Fn1 that was determined based on the chemical composition (mass%), θ Cr (mass fraction), and Formula (1) for each of Test Numbers 2-1 to 2-44 is shown in Table 6.[Test to measure number density of precipitates]

[0216] For the steel plates of Test Numbers 2-1 to 2-44, the number density NDF ( / µm 2< ) of precipitates having an equivalent circular diameter of 20 to 150 nm (fine precipitates) and the number density NDC ( / µm 2< ) of precipitates having an equivalent circular diameter of 250 nm or more (coarse precipitates) were calculated by the measurement method described above. Note that, the SEM used was model ERA-8900FE manufactured by ELIONIX INC., and the acceleration voltage was set to 5 kV and the working distance was set to 15 mm. The observation visual field was set to 12 µm × 9 µm (magnification of ×10000), and three visual fields were observed. The area fraction S (%) of precipitates in the observation visual field was determined as the volume ratio V θ (%) of cementite obtained by thermodynamic calculation using the chemical composition of the steel plate of each test number and the first and second tempering temperatures. Note that, thermodynamic calculation was performed using a thermodynamic calculation software named Thermo-Calc (available from Thermo-Calc Software, version 2017a), and TCFE8 was used as the database.

[0217] The number density NDF of fine precipitates ( / µm 2< ) was determined based on the sum of the numbers of fine precipitates obtained in the three visual fields, and the total area (µm 2< ) of the three visual fields. Similarly, the number density NDC of coarse precipitates ( / µm 2< ) was determined based on the sum of the numbers of coarse precipitates obtained in the three visual fields, and the total area (µm 2< ) of the three visual fields. The number density NDF of fine precipitates ( / µm 2< ) and number density NDC of coarse precipitates ( / µm 2< ) obtained for the steel plates of Test Numbers 2-1 to 2-44 are shown in Table 6. In addition, Fn5 that was determined for Test Numbers 2-1 to 2-44 based on the respective chemical compositions (mass%), the obtained number density NDF of fine precipitates ( / µm 2< ), the obtained number density NDC of coarse precipitates ( / µm 2< ), and Formula (5) is shown in Table 6.[Charpy impact test]

[0218] A Charpy impact test in conformity with JIS Z 2242 (2018) was performed on the respective steel plates of Test Numbers 2-1 to 2-44, and the low-temperature toughness was evaluated. Specifically, full-size V-notch test specimens were prepared from the center portion of the thickness of the steel plates of Test Numbers 2-1 to 2-44. The longitudinal direction of the test specimen was parallel to the plate width direction. The notched surface of the test specimen was perpendicular to the rolling elongation direction of the steel plate. Five test specimens were prepared and were cooled to -65°C. A Charpy impact test in conformity with JIS Z 2242 (2018) was performed on the cooled test specimens, and the absorbed energy (J) was determined. The arithmetic average value of the absorbed energy determined for each of the five test specimens was defined as the absorbed energy vE(-65°C)(J). The obtained absorbed energy vE(-65°C)(J) for the respective steel plates of Test Numbers 2-1 to 2-44 is shown in Table 6.[SSC resistance test]

[0219] The SSC resistance of the respective steel plates of Test Numbers 2-1 to 2-44 was evaluated by a method performed in accordance with "Method A" specified in NACE TM0177-2016. Specifically, round bar test specimens having a diameter of 6.35 mm and a parallel portion length of 25.4 mm were prepared from the center portion of the thickness of the steel plates of Test Numbers 2-1 to 2-44. The round bar test specimen was prepared in a manner so that the axial direction thereof was parallel to the rolling elongation direction of the steel plate. Tensile stress was applied in the axial direction of the round bar test specimens of the respective test numbers. At this time, the applied stress was adjusted so as to be 85% of the actual yield stress (85% AYS) of each steel plate of the respective test numbers.

[0220] A mixed aqueous solution containing 5.0 mass% of sodium chloride and 0.5 mass% of acetic acid (NACE solution A) was used as the test solution. The test solution at 24°C was poured into three test vessels, and these were adopted as test baths. Three round bar test specimens to which the stress was applied were immersed individually in mutually different test vessels as the test baths. After each test bath was degassed, a mixed gas of H 2 S gas at 0.01 atm pressure and N 2 gas at 0.99 atm pressure was blown into the respective test baths and caused to saturate. The test baths in which the gaseous mixture was saturated were held at 24°C for 720 hours.

[0221] After being held for 720 hours, the round bar test specimens of each test number were observed to determine whether or not sulfide stress cracking (SSC) had occurred. Specifically, after being immersed for 720 hours, the round bar test specimens were observed with the naked eye and using a projector with a magnification of ×10. Steel plates for which cracking was not confirmed in all three of the round bar test specimens as the result of the observation were determined as being "E" (Excellent). On the other hand, steel plates for which cracking was confirmed in at least one round bar test specimen were determined as being "NA" (Not Acceptable).[Test results]

[0222] The test results are shown in Table 6.

[0223] Referring to Table 4-1, Table 4-2, Table 4-3, Table 5, and Table 6, for each of the steel plates of Test Numbers 2-1 to 2-19, the chemical composition was appropriate and the yield strength was 965 MPa or more (140 ksi or more). In addition, Fn1 was not more than 0.300, Fn2 was not more than 0.355, Fn3 was -9.0 or more, and Fn4 was -51.0 or more. Further, the number density NDF of fine precipitates was 0.650 / µm 2< or more, and the number density NDC of coarse precipitates was 0.290 / µm 2< or less. As a result, for these steel plates, the absorbed energy vE(-65°C) was 75 J or more, and these steel plates exhibited excellent low-temperature toughness. In addition, these steel plates exhibited excellent SSC resistance in the SSC resistance test. Note that, because these steel plates had a yield strength of 862 MPa or more and had excellent low-temperature toughness and excellent SSC resistance, it was determined that the total of the volume ratios of tempered martensite and tempered bainite was 90% or more in the microstructure of each of these steel plates.

[0224] In the steel plates of Test Numbers 2-1 to 2-17, in addition, Fn5 was 4.0 or more. As a result, in these steel plates the absorbed energy vE(-65°C) was 78 J or more, and thus these steel plates exhibited even more excellent low-temperature toughness.

[0225] On the other hand, in the steel plates of Test Numbers 2-20 to 2-24, Fn3 was less than -9.0. As a result, these steel plates did not exhibit excellent SSC resistance in the SSC resistance test.

[0226] In the steel plate of Test Number 2-25, Fn3 was less than -9.0 and Fn4 was less than -51.0. As a result, this steel plate did not exhibit excellent SSC resistance in the SSC resistance test.

[0227] In the steel plates of Test Numbers 2-26 and 2-27, Fn4 was less than -51.0. As a result, these steel plates did not exhibit excellent SSC resistance in the SSC resistance test.

[0228] In the steel plate of Test Number 2-28, Fn2 was more than 0.355, and Fn1 was more than 0.300. The number density NDF of fine precipitates was less than 0.650 / µm 2< and the number density NDC of coarse precipitates was more than 0.290 / µm 2< . Consequently, in this steel plate the absorbed energy vE(-65°C) was less than 75 J, and excellent low-temperature toughness was not exhibited. In addition, this steel plate did not exhibit excellent SSC resistance in the SSC resistance test.

[0229] The steel plate of Test Number 2-29 was not subjected to the second tempering. As a result, the number density NDF of fine precipitates was less than 0.650 / µm 2< and the number density NDC of coarse precipitates was more than 0.290 / µm 2< .Consequently, in this steel plate the absorbed energy vE(-65°C) was less than 75 J, and excellent low-temperature toughness was not exhibited. In addition, this steel plate did not exhibit excellent SSC resistance in the SSC resistance test.

[0230] In the steel plate of Test Number 2-30, the Ni content was too low. In addition, the second tempering was not performed. As a result, the number density NDF of fine precipitates was less than 0.650 / µm 2< and the number density NDC of coarse precipitates was more than 0.290 / µm 2< . Consequently, in this steel plate the absorbed energy vE(-65°C) was less than 75 J, and the steel plate did not exhibit excellent low-temperature toughness.

[0231] For the steel plate of Test Number 2-31, the tempering time of the high-temperature tempering was too long. As a result, the yield strength was less than 965 MPa. In addition, the number density NDF of fine precipitates was less than 0.650 / µm 2< and the number density NDC of coarse precipitates was more than 0.290 / µm 2< . Consequently, in this steel plate the absorbed energy vE(-65°C) was less than 75 J, and this steel plate did not exhibit excellent low-temperature toughness.

[0232] In the steel plates of Test Numbers 2-32 and 2-33, the Ni content was too low. In addition, Fn2 was more than 0.355, and Fn1 was more than 0.300. As a result, the number density NDF of fine precipitates was less than 0.650 / µm 2< and the number density NDC of coarse precipitates was more than 0.290 / µm 2< . Consequently, in these steel plates the absorbed energy vE(-65°C) was less than 75 J, and these steel plates did not exhibit excellent low-temperature toughness.

[0233] In the steel plate of Test Number 2-34, the Cr content was too low. In addition, Fn2 was more than 0.355, and Fn1 was more than 0.300. As a result, the number density NDF of fine precipitates was less than 0.650 / µm 2< and the number density NDC of coarse precipitates was more than 0.290 / µm 2< .Consequently, in this steel plate the absorbed energy vE(-65°C) was less than 75 J, and excellent low-temperature toughness was not exhibited. In addition, this steel plate did not exhibit excellent SSC resistance in the SSC resistance test.

[0234] In the steel plate of Test Number 2-35, the Mo content was too low. In addition, Fn2 was more than 0.355, and Fn1 was more than 0.300. As a result, the number density NDF of fine precipitates was less than 0.650 / µm 2< and the number density NDC of coarse precipitates was more than 0.290 / µm 2< . Consequently, in this steel plate the absorbed energy vE(-65°C) was less than 75 J, and excellent low-temperature toughness was not exhibited. In addition, this steel plate did not exhibit excellent SSC resistance in the SSC resistance test.

[0235] In the steel plate of Test Number 2-36, the Mn content was too high. In addition, Fn3 was less than -9.0, and Fn4 was less than -51.0. As a result, this steel plate did not exhibit excellent SSC resistance in the SSC resistance test.

[0236] In the steel plate of Test Number 2-37, the N content too high. As a result, in this steel plate the absorbed energy vE(-65°C) was less than 75 J, and excellent low-temperature toughness was not exhibited. In addition, this steel plate did not exhibit excellent SSC resistance in the SSC resistance test.

[0237] In the steel plate of Test Number 2-38, the P content was too high. As a result, in this steel plate the absorbed energy vE(-65°C) was less than 75 J, and excellent low-temperature toughness was not exhibited. In addition, this steel plate did not exhibit excellent SSC resistance in the SSC resistance test.

[0238] The steel plates of Test Numbers 2-39 and 2-40 were not subjected to cold working between the first tempering and the second tempering. As a result, the number density NDF of fine precipitates was less than 0.650 / µm 2< and the number density NDC of coarse precipitates was more than 0.290 / µm 2< . Consequently, in these steel plates the absorbed energy vE(-65°C) was less than 75 J, and excellent low-temperature toughness was not exhibited. In addition, these steel plates did not exhibit excellent SSC resistance in the SSC resistance test.

[0239] For the steel plates of Test Numbers 2-41 and 2-42, the area reduction ratio of the cold working performed between the first tempering and the second tempering was too low. As a result, the number density NDF of fine precipitates was less than 0.650 / µm 2< and the number density NDC of coarse precipitates was more than 0.290 / µm 2< . Consequently, in these steel plates the absorbed energy vE(-65°C) was less than 75 J, and excellent low-temperature toughness was not exhibited. In addition, these steel plates did not exhibit excellent SSC resistance in the SSC resistance test.

[0240] For the steel plates of Test Numbers 2-43 and 2-44, the area reduction ratio of the cold working performed between the first tempering and the second tempering was too high. As a result, the number density NDF of fine precipitates was less than 0.650 / µm 2< and the number density NDC of coarse precipitates was more than 0.290 / µm 2< . Consequently, in these steel plates the absorbed energy vE(-65°C) was less than 75 J, and excellent low-temperature toughness was not exhibited. In addition, these steel plates did not exhibit excellent SSC resistance in the SSC resistance test.

[0241] An embodiment of the present disclosure has been described above. However, the embodiment described above is merely an example for implementing the present disclosure. Accordingly, the present disclosure is not limited to the above embodiment, and the above embodiment can be appropriately modified and performed within a range that does not deviate from the gist of the present invention.

Claims

1. A steel material consisting of, in mass%, C: more than 0.20 to 0.35%, Si: 0.05 to 1.50%, Mn: 0.02 to 1.00%, P: 0.025% or less, S: 0.0100% or less, Al: 0.005 to 0.100%, Ni: more than 0.10 to 2.50%, Cr: 0.40 to 1.50%, Mo: 0.30 to 1.50%, Ti: 0.002 to 0.050%, B: 0.0001 to 0.0050%, N: 0.0100% or less, O: 0.0100% or less, V: 0 to 0.60%, Nb: 0 to 0.030%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, rare earth metal: 0 to 0.0100%, Co: 0 to 0.50%, W: 0 to 0.50%, Cu: 0 to 0.50%, and the balance being Fe and impurities, wherein: a yield strength is 862 MPa or more, a Cr concentration in precipitates having an equivalent circular diameter of 20 nm or more is defined as "θCr", within ranges of contents of elements of the steel material, the contents of elements of the steel material and a Cr concentration θCr in precipitates having an equivalent circular diameter of 20 nm or more satisfy Formulae (1) to (4), and in the steel material: a number density NDF of precipitates having an equivalent circular diameter of 20 to 150 nm is 0.650 / µm2 or more, and a number density NDC of precipitates having an equivalent circular diameter of 250 nm or more is 0.290 / µm2 or less: 0.157 × C − 0.0006 × Cr − 0.0098 × Mo − 0.0482 × V + 0.0006 / θ Cr ≤ 0.300 1 + 263 × C − Cr − 16 × Mo − 80 × V / 98 − 358 × C + 159 × Cr + 15 × Mo + 96 × V ≤ 0.355 − 9.7 × Mn − 104 × S + 0.8 × Mo + 0.08 × Ni 2 − 4.1 × Ni − 5.1 × Ti ≥ − 9.0 15.8 × Si − 33.8 × Mn − 28.8 × Ni ≥ − 51.0 where, a content in units of percent by mass of a corresponding element is substituted for each symbol of an element in Formulae (1) to (4), and if a corresponding element is not contained, "0" is substituted for the symbol of the relevant element, and a Cr concentration in units of mass fraction in precipitates having an equivalent circular diameter of 20 nm or more is substituted for θCr 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: 0.001 to 0.030%, Ca: 0.0001 to 0.0100%, Mg: 0.0001 to 0.0100%, Zr: 0.0001 to 0.0100%, rare earth metal: 0.0001 to 0.0100%, Co: 0.01 to 0.50%, W: 0.01 to 0.50%, and Cu: 0.01 to 0.50%.

3. The steel material according to claim 1, wherein within ranges of contents of elements of the steel material, the contents of elements of the steel material, the number density NDF of precipitates having an equivalent circular diameter of 20 to 150 nm, and the number density NDC of precipitates having an equivalent circular diameter of 250 nm or more satisfy Formula (5): − Mn − 20 × P + 11 × Ni + Mo × NDF 2 / NDC 1 / 2 ≥ 4.0 where, a content in units of percent by mass of a corresponding element is substituted for each symbol of an element in Formula (5), a number density in units of / µm2 of precipitates having an equivalent circular diameter of 20 to 150 nm is substituted for NDF in Formula (5), and a number density in units of / µm2 of precipitates having an equivalent circular diameter of 250 nm or more is substituted for NDC in Formula (5), and in a case where a number density of precipitates having an equivalent circular diameter of 20 to 150 nm is less than 0.001 / µm2, 0.001 is substituted for NDC.

4. The steel material according to claim 2, wherein within ranges of contents of elements of the steel material, the contents of elements of the steel material, the number density NDF of precipitates having an equivalent circular diameter of 20 to 150 nm, and the number density NDC of precipitates having an equivalent circular diameter of 250 nm or more satisfy Formula (5): − Mn − 20 × P + 11 × Ni + Mo × NDF 2 / NDC 1 / 2 ≥ 4.0 where, a content in units of percent by mass of a corresponding element is substituted for each symbol of an element in Formula (5), a number density in units of / µm2 of precipitates having an equivalent circular diameter of 20 to 150 nm is substituted for NDF in Formula (5), and a number density in units of / µm2 of precipitates having an equivalent circular diameter of 250 nm or more is substituted for NDC in Formula (5), and in a case where a number density of precipitates having an equivalent circular diameter of 20 to 150 nm is less than 0.001 / µm2, 0.001 is substituted for NDC.

5. The steel material according to any one of claim 1 to claim 4 wherein: the steel material is an oil-well steel pipe.

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