STEEL MATERIAL

Abstract

A steel material is provided that has high strength and excellent low-temperature toughness. The steel material in accordance with the present description consists of, % by mass, of C: 0.15 to 0.45%, Si: 0.05 to 1.00%, Mn: 0.05 to less than 0.80%, P: 0.030% or less, S: 0.0100% or less, Al: 0.100% or less, Cr: 0.30 to 1.50%, Mo: 0.25 to 2.00%, Ti: 0.001 to 0.015%, N: 0.0100% or less, O: 0.0050% or less, V: 0 to 0.05%, Nb: 0 to 0.010%, B: 0 to less than 0.0005%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, and rare earth metal: 0 to 0.0100%, the remainder being Fe and impurities. In the steel material, the grain size number of the pre-austenite grains is less than 7.0, formulas (1) to (4) described in the specification are satisfied, the yield strength is 896 MPa or more, and the energy absorbed at -10 °C is 95 J or more.

Description

STEEL MATERIAL FIELD OF INVENTION

[0001] The present description relates to a steel material, and more particularly relates to a steel material for use in oil wells. STATE OF THE ART

[0002] Oil wells and gas wells (hereinafter oil wells and gas wells are simply referred to as oil wells) are becoming deeper and deeper, and accordingly, there is a demand for increasing the strength of steel materials for use in oil wells, which are typified as oil well steel pipes. At present, oil well steel pipes with a yield strength of 80 ksi grade (yield strength is 80 to less than 95 ksi, i.e., 552 to less than 655 MPa) and oil well steel pipes with a yield strength of 95 ksi grade (yield strength is 95 to less than 110 ksi, i.e., 655 to less than 758 MPa) are widely used as steel materials for use in oil wells.However, recently, steel material with a yield strength of 110 ksi grade (yield strength is 110 to less than 125 ksi, i.e., 758 to less than 862 MPa), and steel material with a yield strength of 862 MPa (125 ksi) or higher, have also begun to be required.

[0003] In addition, recently, oil wells are also being developed in cold regions. Steel oil well pipes to be used in a deep well in such cold regions must have not only high strength but also excellent low-temperature toughness.

[0004] Patent Document 1 (Japanese Patent Application Publication No. 2017-2369) proposes a seamless steel pipe having a yield strength of 125 ksi or more and excellent low temperature toughness. The seamless steel tube described in Patent Document 1 contains, in mass %, C: 0.21 to 0.35%, Si: 0.10 to 0.50%, Mn: 0.05 to 1.00%, P: 0.025% or less, S: 0.010% or less, Al: 0.005 to 0.100%, N: 0.010% or less, Cr: 0.10 to 1.30%, Mo: 0.05 to 1.00%, Ti: 0.002 to 0.040%, V: 0 to 0.30%, Nb: 0 to 0.050%, and B: 0 to 0.0050%, and also contains one or two kinds of elements selected from the group consisting of Ca: 0.0010 to 0.0060% and metal Rare earth elements: 0.0010 to 0.0060%, the remainder being Fe and impurities. The grain size number of the pre-austenite grains in this seamless steel tube is 7.0 or more.Furthermore, among the sulfide-based inclusions in this seamless steel tube, the number of specific sulfide-based inclusions with a major axis of 1 pm or more is 5000 / 100 pm2 or less, and the average aspect ratio of the specific sulfide-based inclusions is 3.4 or less. Furthermore, the yield strength of this seamless steel tube is 862 MPa or more.

[0005] In the seamless steel tube described in Patent Document 1, the low temperature toughness is increased by making the grain size number of the pre-austenite grains 7.0 or more, P L07 ίΠ / ΖΖηΖ / Ε / ΥΙΛΙ and grain refining. Specifically, in the production process, the heating temperature of the starting material before hot working is set to 1100 °C or less, and the rotation speed of a cross roll during punch rolling is decreased to suppress the occurrence of heat incurred in processing during punch rolling. In this way, the grains are kept fine. In the seamless steel tube of Patent Document 1, furthermore, the low-temperature strength and toughness of the seamless steel tube are improved by grain refinement and precipitation hardening by precipitates such as Ti precipitates, V precipitates, or Nb precipitates. LIST OF REFERENCES PATENT DOCUMENT

[0006] Patent Document 1: Japanese Patent Application Publication No. 2017-2369 SUMMARY OF THE INVENTION TECHNICAL PROBLEM

[0007] However, a steel material having high strength and excellent low temperature toughness can also be obtained by a technique other than that described in Patent Document 1.

[0008] An objective of the present disclosure is to provide a steel material having a high yield strength and also excellent low temperature toughness. SOLUTION TO THE PROBLEM

[0009] The steel material according to the present description is composed, in mass %, C: 0.15 to 0.45%, Yes: 0.05 to 1.00%, Mn: 0.05 to less than 0.80%, P: 0.030% or less, S: 0.0100% or less, To: 0.100% or less, Cr: 0.30 to 1.50%, Mo: 0.25 to 2.00%, Ti: 0.001 to 0.015%, N: 0.0100% or less, O: 0.0050% or less, V: 0 to 0.05%, Nb: 0 to 0.010%, P L07 ίη / ΖΖΩΖ / Ε / ΥΙΛΙ Β: 0 to less than 0.0005%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, rare earth metal: 0 to 0.0100%, Ni: 0 to 0.50%, and Cu: 0 to 0.50%, the remainder being Fe and impurities, where: a grain size number of pre-austenite grains is less than 7.0; Under the precondition that the content of each element is within the range described above, formulas (1) to (4) are satisfied: a yield strength is equal to or greater than 896 MPa; and an absorbed energy at -10 °C is 95 J or more: {C + Mn / 5 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 + 10 x B} x (7.0 / GN)0'45> 0.678...(1) {Mn / 5.5 + 10 x Ti + 1.2 x V + 15 x Nb + 200 x B} X Ti + V + 10 X Nb > 0.015...(3) (10 x Ti + 1.2 x V + 30 x Nb) / Mo < 0.205...(4) where, a mass % content of a corresponding element is substituted for each element symbol, and the grain size number is substituted for GN. ADVANTAGEOUS EFFECTS OF THE INVENTION

[0010] The steel material according to the present disclosure has a high yield strength and also has excellent low temperature toughness. BRIEF DESCRIPTION OF THE DRAWING

[0011] Figure 1 is a schematic diagram (plan view) illustrating a rotary hearth heating oven which is an example of a continuous heating oven. DESCRIPTION OF THE MODALITIES

[0012] The present inventors carried out studies with respect to a steel material having excellent low temperature toughness while increasing the strength of the steel material. In conventional steel materials for use in oil wells, as also described in Patent Document 1, in order to obtain a high strength of 862 MPa or more (125 ksi or more), the strength of the steel material is increased by precipitation hardening of precipitates such as Ti precipitates, V precipitates, or Nb precipitates. Therefore, the present inventors initially Γ L07 ίη / 77Π7 / Ε / ΥΙΛΙ investigated means of increasing the low temperature toughness of a steel material and at the same time increasing the strength of the steel material by precipitation hardening.

[0013] Inclusions are a cause of the decrease in low-temperature toughness of a steel material. Mn, Ti, and B are inclusion-forming elements. On the other hand, as mentioned above, V precipitates and Nb precipitates increase the strength of a steel material by precipitation hardening. Therefore, the present inventors considered increasing the strength of a steel material by suppressing the formation of inclusions by decreasing the contents of Mn, Ti, and B and, furthermore, by precipitation hardening by V and Nb.

[0014] However, as a result of further research, the present inventors discovered that it is difficult to compatibly achieve both high strength and excellent low-temperature toughness simply by reducing the contents of inclusion-forming elements. Specifically, although high strength was obtained, excellent low-temperature toughness was not achieved. Therefore, the present inventors carried out further studies on the reason why low-temperature toughness cannot be sufficiently obtained. As a result, it has been revealed that the cause of the decrease in low-temperature toughness in a cold environment of less than 0 °C is not only inclusions, but also precipitates.Therefore, the present inventors considered that in a case where, as in conventional steel material, a precipitation hardening mechanism that hardens by means of precipitates is adopted as the main hardening mechanism to increase the strength of a steel material, there is a limit with respect to sufficiently increasing the low-temperature toughness in a cold environment of less than 0 °C.

[0015] Therefore, in order to compatibly achieve both high strength and excellent low-temperature toughness, the present inventors thought of adopting a hardening mechanism that hardens by improving hardenability as the main hardening mechanism of a steel material instead of the precipitation hardening mechanism adopted in conventional steel materials. In this case, from the viewpoint of chemical composition, it is preferable that the content of elements susceptible to forming inclusions and precipitates can be kept low while containing elements that increase hardenability.

[0016] Therefore, the present inventors conducted studies with respect to a chemical composition that is suitable for the aforementioned technical idea. As a result, the present inventors considered that if a steel material has a chemical composition consisting of, % by mass, C: 0.15 to 0.45%, Si: 0.05 to 1.00%, Mn: 0.05 to less than 0.80%, P: 0.030% or less, S: 0.0100% or less, Al: 0.100% or less, Cr: 0.30 to 1.50%, Mo: 0.25 to 2.00%, Ti: 0.001 to 0.015%, N: 0.0100% or less, O: 0.0050% or less, V: 0 to 0.05%, Nb: 0 to 0.010%, B: 0 to less than 0.0005%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, metal Γ L07 ίη / 77Ω7 / Β / ΥΙΛΙ of rare earth (REM): 0 to 0.0100%, Ni: 0 to 0.50%, and Cu: 0 to 0.50%, with the remainder being Fe and impurities, there is the possibility of compatibly achieving both high strength and excellent low temperature toughness.

[0017] As mentioned above, in the steel material of the present embodiment, a hardening mechanism that hardens by improving hardenability is adopted as the main hardening mechanism of the steel material. In this regard, the coarser the grains of the steel material are, the more the hardenability will increase. Therefore, instead of making the grains fine as in conventional steel materials, the present inventors conceived to make the grains in the steel material of the present embodiment coarse grains to thereby increase the strength of the steel material. As a result of studies carried out by the present inventors, the present inventors considered that when the contents of the respective elements in the chemical composition are within the ranges described above, if the grain size number of the pre-austenite grains of the steel material is less than 7.0, there is a possibility that the hardenability of the steel material can be further increased and the strength can be increased.

[0018] Therefore, taking the above-described requirements as a precondition, in order to compatibly achieve both high strength and excellent low-temperature toughness in a steel material having the above-described chemical composition, the present inventors carried out more detailed studies with respect to (1) the relationship between the hardenability-enhancing elements in the above-described chemical composition and grain size, (2) the relationship between inclusion- and precipitate-forming elements and grain size, (3) the auxiliary use of a precipitation hardening mechanism, and (4) the auxiliary use of a solid solution hardening mechanism by means of Mo.As a result, the present inventors discovered that, with the precondition that the contents of the respective elements in the chemical composition are within the ranges described above, if formulas (1) to (4) are also satisfied, it is possible to sufficiently achieve both high strength and excellent low-temperature toughness:. {C + Mn / 5 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 + 10 x B} x (7.0 / GN)0'45> 0.678...(1) {Mn / 5.5 + 10 x Ti + 1.2 x V + 15 x Nb + 200 x B} x (7.0 / GN)045< 0.240...(2) x Ti + V + 10 x Nb > 0.015...(3) (10 x Ti + 1.2 x V + 30 x Nb) / Mo < 0.205...(4) where, the mass % content of a corresponding element is replaced by each element symbol, and the grain size number of the pre-austenite grains is replaced by GN. Formulas (1) to (4) are described below. Γ L07 ίη / 77Π7 / Ε / ΥΙΛΙ

[0019] As for formula (1), F1 is defined as F1 = {C + Mn / 5 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 + 10 B} (7.0 / GN)0.45. F1 is an index of the hardenability of the steel material. As mentioned above, the hardenability-enhancing elements (hereinafter also referred to as hardenability-enhancing elements) and the grain size of the pre-austenite synergistically affect the hardenability. Each of the elements C, Mn, Cu, Ni, Cr, Mo, V, and B in F1 is a hardenability-enhancing element. In addition, the term (7.0 / GN)°4:>in F1 indicates the degree to which the grain size of the pre-austenite contributes to the hardenability.

[0020] If F1 is less than 0.678, even if the content of each element in the chemical composition is within the range of the present embodiment and formulas (2) to (4) are satisfied, the hardenability of the steel material will be insufficient. In this case, the yield strength of the steel material cannot be sufficiently increased. If the content of each element in the chemical composition of the steel material is within the range of the present embodiment and F1 is 0.678 or more, provided that the chemical composition satisfies formulas (2) to (4) described later, the strength of the steel material can be sufficiently increased. Specifically, the yield strength of the steel material can be made 896 MPa (130 ksi) or more.

[0021] As for formula (2), F2 is defined as F2 = {Mn / 5.5 + 10 x Ti + 1.2 x V + 15 x Nb + 200 x B} x (7.0 / GN)0.45. F2 is an index of the low-temperature toughness of the steel material. As mentioned above, when the content of each element in the chemical composition is within the range of the present embodiment, each of the elements Mn, Ti, V, Nb, and B is likely to form inclusions or precipitates. Hereinafter, these elements are also referred to as inclusion / precipitate-forming elements. In the case that these elements have formed inclusions and / or precipitates, if the pre-austenite grains are coarse, cracks may occur. On the other hand, if the pre-austenite grains are fine grains, crack propagation is easily suppressed.Consequently, inclusion / precipitate-forming elements and the grain size number of the pre-austenite grains synergistically affect low-temperature toughness. Each of the elements Mn, Ti, V, Nb, and B in F2 is an inclusion / precipitate-forming element. Furthermore, the term (7.0 / GN)045 in F2 indicates the extent to which the grain size of the pre-austenite contributes to low-temperature toughness.

[0022] If F2 is greater than 0.240, even if the contents of the respective elements in the chemical composition are within the ranges described above, and the chemical composition satisfies Formula (1), Formula (3), and Formula (4), inclusions and / or precipitates containing Mn, Ti, V, Nb, and B will be excessively formed in the steel material. Therefore, in a case where the yield strength of a steel material in which the content of each element in the chemical composition is within the range of the present embodiment is 896 MPa (130 ksi) or more, the low-temperature toughness of the Γ L07 ίη / 77Ω7 / Β / YILI steel material will decrease. Specifically, the absorbed energy at -10 °C will be less than 95 J. If F2 is 0.240 or less, the formation of inclusions and / or precipitates containing Mn, Ti, V, Nb and B in the steel material can be sufficiently suppressed. Therefore, under the precondition that the contents of the respective elements in the chemical composition are within the ranges described above and the chemical composition satisfies Formula (1), Formula (3) and Formula (4), excellent low-temperature toughness is also obtained, while the strength of the steel material is sufficiently increased.

[0023] As for Formula (3), F3 is defined as F3 = 10 x Ti + V + 10 x Nb. F3 is an index of a precipitation hardening mechanism which is auxiliaryly used as a hardening mechanism in a steel material in which the content of each element in the chemical composition is within the range of the present embodiment. In the steel material of the present embodiment, a hardening mechanism which improves hardenability by satisfying Formula (1) is in principle adopted as the main hardening mechanism. However, in a steel material in which the contents of the respective elements in the chemical composition are within the ranges described above, in some cases a yield strength of 896 MPa (130 ksi) or more cannot be stably obtained by only a hardenability hardening mechanism.Therefore, in the present embodiment, in addition to a hardenability hardening mechanism, a precipitation hardening mechanism that hardens by precipitating Ti, V, and Nb is auxiliarily employed. If F3 is 0.015 or more, the precipitation hardening mechanism can be auxiliarily employed in addition to the hardenability hardening mechanism. Thus, under the precondition that the content of each element in the chemical composition is within the range of the present embodiment, and the chemical composition satisfies Formula (1), Formula (2), and Formula (4), the strength of the steel material can be sufficiently increased. Specifically, the yield strength of the steel material becomes 896 MPa (130 ksi) or more.

[0024] As for formula (4), F4 is defined as F4 = (10 x Ti + 1.2 x V + 30 x Nb) / Mo. F4 is an index of the degree to which Mo contributes to improving the low-temperature toughness. As mentioned above, the strength of the steel material of the present embodiment is increased by adopting a hardening mechanism that hardens by improving the hardenability as the main hardening mechanism of the steel material. In addition, the low-temperature toughness of the steel material is increased by reducing inclusions and precipitates as much as possible. However, in some cases, a sufficiently high yield strength of 896 MPa (130 ksi) or more cannot be stably obtained by increasing the strength of the steel material solely by the hardening mechanism that hardens by improving the hardenability.Therefore, as defined in Formula (3), in addition to the hardening mechanism that hardens by improving the. Γ L07 ίΠ / ΖΖηΖ / Ε / ΥΙΛΙ hardenability, a precipitation hardening mechanism is auxiliary adopted which hardens by precipitation of Ti, V and Nb. However, when the precipitates of Ti, V and Nb increase, the low-temperature toughness of a steel material in which the contents of the respective elements in the chemical composition are within the ranges described above decreases.

[0025] In this case, Mo not only increases the strength of a steel material by improving hardenability, but also hardens the steel material by solid solution hardening. Solid solution hardening by Mo can suppress a decrease in low-temperature toughness caused by precipitates of Ti, V, and Nb. Therefore, in the present embodiment, the ratio of the Mo content to the Ti, V, and Nb content is increased. If F4 is 0.205 or less, the ratio of the Mo content to the Ti, V, and Nb content will be high. In this case, even when the precipitation hardening mechanism is auxiliarily used, a decrease in low-temperature toughness can be suppressed.Therefore, under the precondition that the content of each element in the chemical composition is within the range of the present embodiment and that formulas (1) to (3) are also satisfied, the strength of the steel material can be sufficiently increased and excellent low-temperature toughness can also be obtained.

[0026] As described above, in the steel material of the present embodiment, in order to increase not only the strength but also the low-temperature toughness, a hardening mechanism that hardens by improving hardenability is adopted instead of a precipitation hardening mechanism that conventional steel materials actively adopt as the main hardening mechanism. Furthermore, in order to make the hardening mechanism that hardens by improving hardenability act more strongly, the pre-austenite grains are intentionally made into coarse grains. Furthermore, in order to achieve both high strength and excellent low-temperature toughness, the hardenability-improving elements, the inclusion / precipitate-forming elements, and the grain size of the pre-austenite are adjusted to satisfy formulas (1) to (4).By having the configuration described above, in the steel material of the present embodiment, while the yield strength is 896 MPa or more, the absorbed energy at -10 °C can also be 95 J or more.

[0027] The steel material of the present embodiment has been completed on the basis of the technical idea described above, and is as follows.

[0028] [1] A steel material consisting of, % by mass, C: 0.15 to 0.45%, Yes: 0.05 to 1.00%, Mn: 0.05 to less than 0.80%, P L07 ίΠ / ΖΖηΖ / Ε / ΥΙΛΙ P: 0.030% or less, S: 0.0100% or less, To: 0.100% or less, Cr: 0.30 to 1.50%, Mo: 0.25 to 2.00%, Ti: 0.001 to 0.015%, N: 0.0100% or less, O: 0.0050% or less, V: 0 to 0.05%, Nb: 0 to 0.010%, B: 0 to less than 0.0005%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, rare earth metal: 0 to 0.0100%, Ni: 0 to 0.50%, and Cu: 0 to 0.50%, the remainder being Fe and impurities, where: a grain size number of the pre-austenite grains is less than 7.0 with the precondition that a content of each element is within a range described above, formulas (1) to (4) are satisfied; a yield strength is equal to or greater than 896 MPa; and an absorbed energy at -10 °C is 95 J or more: {C + Mn / 5 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 + 10 x B} x (7.0 / GN)0'45> 0.678...(1) {Mn / 5.5 + 10 x Ti + 1.2 x V + 15 x Nb + 200 x B} x (7.0 / GN)045< 0.240...(2) x Ti + V + 10 x Nb > 0.015...(3) (10 x Ti + 1.2 x V + 30 x Nb) / Mo < 0.205...(4) where, a mass % content of a corresponding element is substituted for each element symbol, and the grain size number is substituted for GN.

[0029] [2] The steel material according to [1], where: a number density of Mn sulfides with an equivalent circular diameter of 5.0 pm or more is 10 / 100 mm2 or less; and an absorbed energy at -10 °C is 100 J or more. Γ L07 ίΠ / ΖΖηΖ / Ε / ΥΙΛΙ

[0030] [3] Steel material in accordance with [1] or [2], containing one or more types of elements selected from the group consisting of: V: 0.01 to 0.05%, Nb: 0.001 to 0.010%, B: 0.0001 to less than 0.0005%, Ca: 0.0001 to 0.0100%, Mg: 0.0001 to 0.0100%, rare earth metal: 0.0001 to 0.0100%, Ni: 0.01 to 0.50%, and Cu: 0.01 to 0.50%.

[0031] [4] Steel material according to any of [1] to [3], wherein: The steel material is oil well steel pipe.

[0032] The steel material according to the present embodiment will be described in detail below. The % sign following each element means % by mass unless otherwise indicated.

[0033] [Chemical composition] The chemical composition of the steel material according to this embodiment contains the following elements.

[0034] C: 0.15 to 0.45%. Carbon (C) improves hardenability, thereby increasing the strength of the steel material. If the C content is less than 0.15%, the aforementioned effect cannot be sufficiently obtained, even if the contents of other elements are within the range of the present embodiment. On the other hand, if the C content is more than 0.45%, even if the contents of other elements are within the range of the present embodiment, the strength of the steel material will be too high, and as a result, the low-temperature toughness of the steel material will decrease. Therefore, the C content is 0.15 to 0.45%. A lower limit of the C content is preferably 0.17%, more preferably 0.20%, more preferably 0.22%, and most preferably 0.24%. An upper limit of the C content is preferably 0.40%, more preferably 0.36%, more preferably 0.34%, more preferably 0.32%, and most preferably 0.30%.

[0035] If: 0.05 to 1.00%. Silicon (Si) deoxidizes steel. If the Si content is less than 0.05%, the above-mentioned effect cannot be sufficiently obtained, even if the contents of other elements are within the Γ L07 ίΠ / ΖΖηΖ / Ε / ΥΙΛΙ range of the present embodiment. On the other hand, if the Si content is more than 1.00%, even if the contents of other elements are within the present embodiment, the low-temperature toughness of the steel material will decrease. Therefore, the Si content is 0.05 to 1.00%. A lower limit of the Si content is preferably 0.10%, more preferably 0.13%, more preferably 0.15%, more preferably 0.17%, and most preferably 0.20%. An upper limit of the Si content is preferably 0.85%, more preferably 0.70%, more preferably 0.60%, more preferably 0.50%, and most preferably 0.40%.

[0036] Mn: 0.05 to less than 0.80%. Manganese (Mn) deoxidizes steel. Mn also improves the hardenability of the steel material, thereby increasing its strength. If the Mn content is less than 0.05%, the above-mentioned effects cannot be sufficiently obtained, even if the contents of other elements are within the range of this embodiment. On the other hand, if the Mn content is 0.80% or more, Mn segregates at the grain boundaries along with impurities such as P and S. In addition, an excessively large amount of coarse Mn sulfides will form. In this case, the low-temperature toughness of the steel material will decrease even if the contents of other elements are within the range of this embodiment. Therefore, the Mn content is 0.05 to less than 0.80%. A lower limit of the Mn content is preferably 0.15%, more preferably 0.25%, more preferably 0.30%, more preferably 0.35% and most preferably 0.40%.An upper limit of the Mn content is preferably 0.79%, more preferably 0.78%, more preferably 0.75%, more preferably 0.70%, and most preferably 0.65%.

[0037] P: 0.030% or less. Phosphorus (P) is an unavoidably contained impurity. That is, the P content is greater than 0%. If the P content is greater than 0.030%, even if the contents of other elements are within the range of the present embodiment, P will segregate at the grain boundaries and the low-temperature toughness of the steel material will decrease. Therefore, the P content is 0.030% or less. An upper limit of the P content is preferably 0.025%, more preferably 0.020%, and most preferably 0.015%. It is preferable that the P content be as low as possible. However, an excessive reduction in the P content will cause a significant increase in the production cost. Therefore, considering industrial production, a lower limit of the P content is preferably 0.001% and more preferably 0.003%.

[0038] S: 0.0100% or less Sulfur (S) is an unavoidable impurity. That is, the S content is greater than 0%. If the S content is greater than 0.0100%, even if the contents of other elements are within the range of the present embodiment, S will segregate at the grain boundaries and the toughness at low temperatures will be reduced. Γ L07 ίη / 77Ω7 / Β / YILI temperature of the steel material will decrease. Therefore, the S content is 0.0100% or less. An upper limit of the S content is preferably 0.0080%, more preferably 0.0070%, more preferably 0.0060%, more preferably 0.0050%, and most preferably 0.0045%. It is preferable that the S content be as low as possible. However, an excessive reduction in the S content will cause a significant increase in the production cost. Therefore, considering industrial production, a lower limit of the S content is preferably 0.0001% and more preferably 0.0003%.

[0039] Al: 0.100% or less Aluminum (Al) is inevitably contained. That is, the Al content is greater than 0%. Al deoxidizes steel. When Al is contained even in a small amount, the above-mentioned effect will be obtained to a certain extent. However, if the Al content is greater than 0.100%, coarse oxide inclusions will form even if the contents of other elements are within the range of the present embodiment. In this case, the low-temperature toughness of the steel material will decrease. Therefore, the Al content is 0.100% or less. A lower limit of the Al content is preferably 0.001%, more preferably 0.005%, more preferably 0.010%, and most preferably 0.020%. An upper limit of the Al content is preferably 0.080%, more preferably 0.070%, more preferably 0.060%, and most preferably 0.050%.Note that the Al content mentioned in the present description means the acid-soluble Al content, i.e., Al sol.

[0040] Cr: 0.30 to 1.50%. Chromium (Cr) improves the hardenability of the steel material. Cr also increases the resistance to tempering. Therefore, Cr increases the strength of the steel material. If the Cr content is less than 0.30%, the above-mentioned effects cannot be sufficiently obtained, even if the contents of other elements are within the range of the present embodiment. On the other hand, if the Cr content is more than 1.50%, the low-temperature toughness of the steel material will decrease even if the contents of other elements are within the present embodiment. Therefore, the Cr content is 0.30 to 1.50%. A lower limit of the Cr content is preferably 0.40%, more preferably 0.45%, more preferably 0.50%, and most preferably 0.60%. An upper limit of the Cr content is preferably 1.40%, more preferably 1.30%, and most preferably 1.20%.

[0041] Mo: 0.25 to 2.00%. Molybdenum (Mo) increases the hardenability of steel material. In addition, Mo dissolves in the steel material and thus hardens it. If the steel material is hardened with dissolved Mo, the decrease in low-temperature toughness attributable to V precipitates, Nb precipitates, and Ti precipitates can be suppressed. Therefore, Mo can increase the strength of the steel material while suppressing the occurrence of a decrease in low-temperature toughness. If the Γ L07 ίΠ / ΖΖηΖ / Ε / ΥΙΛΙ Mo content is less than 0.25%, the above-mentioned effect cannot be sufficiently obtained even if the contents of other elements are within the range of the present embodiment. On the other hand, if the Mo content is more than 2.00%, even if the contents of other elements are within the range of the present embodiment, the low-temperature toughness of the steel material will, on the contrary, decrease. Therefore, the Mo content is 0.25 to 2.00%. A lower limit of the Mo content is preferably 0.28%, more preferably 0.30%, and more preferably 0.35%. An upper limit of the Mo content is preferably 1.50%, more preferably 1.25%, more preferably 1.00%, and most preferably 0.80%.

[0042] Ti: 0.001 to 0.015%. Titanium (Ti) forms precipitates (nitrides) and increases the strength of the steel material through precipitation hardening. If the Ti content is less than 0.001%, the above-mentioned effect cannot be obtained. On the other hand, if the Ti content is more than 0.015%, even if the contents of other elements are within the range of the present embodiment, coarse inclusions and an excessive amount of Ti precipitates will form. In this case, the low-temperature toughness of the steel material will noticeably decrease. Accordingly, the Ti content is 0.001 to 0.015%. A lower limit of the Ti content is preferably 0.002%, more preferably 0.003%, more preferably 0.004%, and most preferably 0.005%. An upper limit of the Ti content is preferably 0.012%, more preferably 0.010%, more preferably 0.009%, and most preferably 0.008%.

[0043] N: 0.0100% or less Nitrogen (N) is an unavoidably present impurity. That is, the N content exceeds 0%. If the N content exceeds 0.0100%, the N will form coarse nitrides even if the contents of other elements are within the range of the present embodiment. In this case, the low-temperature toughness of the steel material will decrease. Therefore, the N content is 0.0100% or less. An upper limit of the N content is preferably 0.0080%, more preferably 0.0070%, more preferably 0.0060%, and most preferably 0.0055%. It is preferable that the N content be as low as possible. However, an excessive reduction in the N content will cause a significant increase in production costs. Therefore, considering industrial production, a lower limit of the N content is preferably 0.0001% and more preferably 0.0010%.

[0044] O: 0.0050% or less Oxygen (O) is an unavoidably contained impurity. That is, the O content is greater than 0%. If the O content is greater than 0.0050%, even if the contents of other elements are within the range of the present embodiment, the O will form coarse oxides and the low-temperature toughness of the steel material will decrease. Consequently, the O content is 0.0050% or less. An upper limit of the O content is preferably 0.0040%, more preferably 0.0030% and more. Γ L07 ίη / 77Π7 / Ε / ΥΙΛΙ preferably 0.0025%. It is preferable that the O content be as low as possible. However, an excessive reduction in the O content will result in a significant increase in production costs. Therefore, considering industrial production, a lower limit of the O content is preferably 0.0001% and more preferably 0.0003%.

[0045] The remainder of the chemical composition of the steel material according to the present embodiment is Fe and impurities. Here, the term impurities refers to elements that, during the industrial production of the steel material, are mixed from ores and scrap as raw materials 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.

[0046] [Regarding optional items] [First group] The steel material of the present embodiment may further contain one or more kinds of elements selected from the group consisting of V and Nb instead of a portion of Fe. Each of these elements is an optional element and each of these elements forms precipitates and increases the strength of the steel material by precipitation hardening.

[0047] V: 0 to 0.05%. Vanadium (V) is an optional element and does not have to be contained. That is, the V content can be 0%. When contained, that is, when the V content is greater than 0%, V improves hardenability. V also forms precipitates (carbides). The V precipitates increase the strength of the steel material by precipitation hardening. However, in the chemical composition of the present embodiment, when the yield strength of the steel material is raised to 896 MPa or more (130 ksi or more), if the V content is greater than 0.05%, the V precipitates will markedly decrease the low-temperature toughness of the steel material, even if the contents of other elements are within the range of the present embodiment. Accordingly, the V content is 0 to 0.05%. A lower limit of the V content is preferably 0.01%. An upper limit of the V content is preferably 0.04%, more preferably 0.03% and more preferably 0.02%.

[0048] Nb: 0 to 0.010%. Niobium (Nb) is an optional element and does not have to be contained. That is, the Nb content can be 0%. When contained, that is, when the Nb content is greater than 0%, Nb forms precipitates (carbonitrides). The Nb precipitates increase the strength of the steel material by precipitation hardening. However, in the chemical composition of the present embodiment, when the yield strength of the steel material is raised to 896 MPa or more (130 ksi or more), if the Nb content is greater than 0.010%, the Nb precipitates will noticeably decrease the low-temperature toughness of the steel material, even if the contents of other elements are within the range of the Nb content. Γ L07 ίη / 77Π7 / Ε / ΥΙΛΙ present embodiment. Accordingly, the Nb content is 0 to 0.010%. A lower limit of the Nb content is preferably 0.001% and more preferably 0.002%. An upper limit of the Nb content is preferably 0.009% and more preferably 0.008%.

[0049] [Second group] The steel material of the present embodiment may also contain B instead of a portion of Fe.

[0050] B: 0 to less than 0.0005%. Boron (B) is an optional element and does not have to be contained. That is, the B content can be 0%. When it is contained, that is, when the B content is greater than 0%, B dissolves in the steel material and increases the hardenability of the steel material, thereby increasing the strength of the steel material. When B is contained even in a small amount, the above-mentioned effect will be obtained to a certain extent. However, in the chemical composition of the present embodiment, when the yield strength of the steel material rises to 896 MPa or more (130 ksi or more), if the B content is 0.0005% or more, even if the contents of other elements are within the range of the present embodiment, B inclusions formed in the steel material will decrease the low-temperature toughness of the steel material. Accordingly, the B content is 0 to less than 0.0005%. An upper limit of the B content is preferably 0.0004% and more preferably 0.0003%. A lower limit of the B content is preferably 0.0001%. [0051 ] [Third group]. The steel material of the present embodiment may further contain one or more kinds of elements selected from the group consisting of Ca, Mg and rare earth metal (REM) instead of a portion of Fe. Each of these elements refines Mn sulfides in the steel material and thereby increases the low temperature toughness of the steel material.

[0052] Ca: 0 to 0.0100%. Calcium (Ca) is an optional element and does not have to be contained. That is, the Ca content can be 0%. When it is contained, that is, when the Ca content is greater than 0%, Ca refines the Mn sulfides in the steel material and thus increases the low-temperature toughness of the steel material. When Ca is contained even in a small amount, the aforementioned effect will be obtained to a certain extent. However, if the Ca content is greater than 0.0100%, even if the contents of other elements are within the range of the present embodiment, the oxides in the steel material will coarsen, and the low-temperature toughness of the steel material will, on the contrary, decrease. Accordingly, the Ca content is 0 to 0.0100%. A lower limit of the Ca content is preferably 0.0001%, more preferably 0.0003%, more preferably 0.0006%, and most preferably 0.0010%. A P L07 ίη / ΖΖΩΖ / Ε / ΥΙΛΙ upper limit of the Ca content is preferably 0.0060%, more preferably 0.0050%, more preferably 0.0040%, more preferably 0.0025% and most preferably 0.0020%.

[0053] Mg: 0 to 0.0100% Magnesium (Mg) is an optional element and does not have to be contained. That is, the Mg content can be 0%. When it is contained, that is, when the Mg content is greater than 0%, Mg refines the Mn sulfides in the steel material and thus increases the low-temperature toughness of the steel material. When Mg is contained even in a small amount, the aforementioned effect will be obtained to a certain extent. However, if the Mg content is greater than 0.0100%, even if the contents of other elements are within the range of the present embodiment, the oxides in the steel material will coarsen, and the low-temperature toughness of the steel material will, conversely, decrease. Accordingly, the Mg content is 0 to 0.0100%. A lower limit of the Mg content is preferably 0.0001%, more preferably 0.0003%, more preferably 0.0006%, and most preferably 0.0010%.An upper limit of the Mg content is preferably 0.0060%, more preferably 0.0050%, more preferably 0.0040%, more preferably 0.0025%, and most preferably 0.0020%.

[0054] Rare earth metal (REM): 0 to 0.0100%. Rare earth metal (REM) is an optional element and does not have to be contained. That is, the REM content can be 0%. When it is contained, that is, when the REM content is greater than 0%, the REM refines the Mn sulfides in the steel material and thus increases the low-temperature toughness of the steel material. When REM is contained even in a small amount, the aforementioned effect will be achieved to a certain extent. However, if the REM content is greater than 0.0100%, even if the content of other elements is within the range of the present embodiment, the oxides in the steel material will coarsen, and the low-temperature toughness of the steel material will, conversely, decrease. Consequently, the REM content is 0 to 0.0100%. A lower limit of the REM content is preferably 0.0001%, more preferably 0.0003%, more preferably 0.0006%, and most preferably 0.0010%.An upper limit of the REM content is preferably 0.0060%, more preferably 0.0050%, more preferably 0.0040%, more preferably 0.0025%, and most preferably 0.0020%.

[0055] Note that in the present description the term REM means one or more types of elements selected from the group consisting of scandium (Se) which is the element with atomic number 21, yttrium (Y) which is the element with atomic number 39, and the elements from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71 which are lanthanides. In the present description the term REM content refers to the total content of these elements.

[0056] [Fourth group] Γ L07 ίη / 77Π7 / Ε / ΥΙΛΙ The chemical composition of the steel material described above may additionally contain one or more types of elements selected from the group consisting of Ni and Cu instead of a portion of Fe. Each of these elements is an optional element and increases the hardenability of the steel.

[0057] Ni: 0 to 0.50%. Nickel (Ni) is an optional element and does not have to be contained. That is, the Ni content can be 0%. When contained, that is, when the Ni content is greater than 0%, Ni increases the hardenability of the steel, thereby increasing the strength of the steel material. When Ni is contained even in a small amount, the aforementioned effect will be obtained to a certain extent. However, if the Ni content is greater than 0.50%, even if the contents of other elements are within the range of the present embodiment, Ni will promote local corrosion, and the corrosion resistance of the steel material will decrease. Accordingly, the Ni content is 0 to 0.50%. A lower limit of the Ni content is preferably 0.01% and more preferably 0.02%. An upper limit of the Ni content is preferably 0.40%, more preferably 0.30%, more preferably 0.20%, more preferably 0.10%, most preferably 0.08% and more preferably 0.06%.

[0058] Cu: 0 to 0.50%. Copper (Cu) is an optional element and does not have to be contained. That is, the Cu content can be 0%. When it is contained, that is, when the Cu content is greater than 0%, Cu increases the hardenability of the steel, thereby increasing the strength of the steel material. When Cu is contained even in a small amount, the aforementioned effect will be obtained to a certain extent. However, if the Cu content is greater than 0.50%, even if the contents of other elements are within the range of the present embodiment, the hardenability of the steel material will be too high and the low-temperature toughness of the steel material will decrease. Therefore, the Cu content is 0 to 0.50%. A lower limit of the Cu content is preferably 0.01% and more preferably 0.02%. An upper limit of the Cu content is preferably 0.40%, more preferably 0.30%, more preferably 0.20%, more preferably 0.10%, most preferably 0.0.08% and more preferably 0.06%.

[0059] [Regarding the grain size number of the pre-austenite grains] In the microstructure of the steel material according to the present embodiment, the grain size number of the pre-austenite grains is less than 7.0.

[0060] As mentioned above, in the steel material of the present embodiment, a hardening mechanism that hardens by improving hardenability is adopted as the main hardening mechanism. The coarser the grains of the steel material are, the more the hardenability of the steel material increases. In a steel material in which the content of each element in the chemical composition is within the range of the present embodiment, when the grain size number of the grains is greater than the grain size number of the grains of the steel material, the hardenability of the steel material is greater than the grain size number of the grains of the steel material. P L07 ίΠ / ΖΖηΖ / Ε / ΥΙΛΙ pre-austenite grains are 7.0 or more, sufficient hardenability cannot be obtained. In this case, the strength of the steel material will be insufficient, and the yield strength of the steel material will be less than 896 MPa (130 ksi).

[0061] If the grain size number of the pre-austenite grains is less than 7.0, with the precondition that the content of each element in the chemical composition is within the range of the present embodiment and formulas (1) to (4) are satisfied, the hardenability of the steel material will be sufficiently high. Therefore, the strength of the steel material will be sufficiently increased, specifically, the yield strength of the steel material will be 896 MPa (130 ksi) or more.

[0062] A lower limit of the grain size number of the pre-austenite grains is not particularly limited. If industrial production is taken into account, a lower limit of the grain size number of the pre-austenite grains is, for example, 2.0, or for example 2.5, or for example 3.0, or for example 3.5, or for example 4.0.

[0063] The grain size number of the pre-austenite grains in the steel material of the present embodiment can be determined by the following method. A test sample is taken from the steel material such that a cross section perpendicular to the longitudinal direction (rolling direction) of the steel material becomes the surface to be examined. If the steel material is a steel plate, the test sample is taken from a central portion of the plate thickness. If the steel material is a steel pipe, the test sample is taken from a central portion of the wall thickness. The taken test sample is embedded in resin, and the surface to be examined is mirror-polished. After the surface to be examined has been mirror-polished, the grain boundaries of the pre-austenite are revealed by the Bechet-Beaujard method, in which the surface to be examined is etched with a saturated aqueous solution of picric acid.The surface to be examined in which the previous austenite grain boundaries have been revealed is used to measure the grain size number of the previous austenite grains in accordance with ASTM E12-13.

[0064] [Regarding parameter formulas] Provided that the respective elements in the chemical composition of the steel material of the present embodiment are within the ranges of the present embodiment described above, the chemical composition also satisfies the following Formula (1) to Formula (4): {C + Mn / 5 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 + 10 x B} x (7.0 / GN)0 45> 0.678...(1) {Mn / 5.5 + 10 x Ti + 1.2 x V + 15 x Nb + 200 x B} x (7.0 / GN)0 45< 0.240...(2) 10 x Ti + V + 10 x Nb > 0.015...(3) (10 x Ti + 1.2 x V + 30 x Nb) / Mo < 0.205...(4) where, the mass % content of a corresponding element is replaced by each element symbol, and the grain size number of the pre-austenite grains is replaced by GN. P L07 ίΠ / ΖΖηΖ / Ε / ΥΙΛΙ Formulas (1) to (4) are described below.

[0065] [Regarding Formula (1)] F1 is defined as F1 = {C + Mn / 5 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 + 10 x B} x (7.0 / GN)0.45. F1 is an index of the hardenability of steel material. The hardenability-enhancing elements and the grain size of previous austenite synergistically affect hardenability. Each of the elements C, Mn, Cu, Ni, Cr, Mo, V, and B in F1 is a hardenability-enhancing element. Furthermore, the term (7.0 / GN)0.45in F1 indicates the degree to which the grain size of previous austenite contributes to hardenability.

[0066] If F1 is less than 0.678, even if the content of each element in the chemical composition is within the range of the present embodiment and formulas (2) to (4) are satisfied, the hardenability of the steel material will be insufficient. In this case, the yield strength of the steel material cannot be sufficiently increased. If the content of each element in the chemical composition of the steel material is within the range of the present embodiment and F1 is 0.678 or more, provided that the chemical composition satisfies formulas (2) to (4) described later, the strength of the steel material can be sufficiently increased. Specifically, the yield strength of the steel material can be made 896 MPa (130 ksi) or more.

[0067] A lower limit of F1 is preferably 0.680, more preferably 0.685, more preferably 0.690, and most preferably 0.695. An upper limit of F1 is not particularly limited. For example, the upper limit of F1 is 2.445. Note that, the value of F1 is a value obtained by rounding off to the fourth decimal place of an obtained value.

[0068] [Regarding Formula (2)] F2 is defined as F2 = {Mn / 5.5 + 10 x Ti + 1.2 x V + 15 x Nb + 200 x B} x (7.0 / GN)0 45. F2 is an index of the low-temperature toughness of the steel material. Mn, Ti, V, Nb, and B are inclusion / precipitate-forming elements. When the content of each element in the chemical composition is within the range of the present embodiment, these inclusion / precipitate-forming elements are likely to form inclusions (Mn inclusions, Ti inclusions, B inclusions) or precipitates (Ti precipitates, V precipitates, Nb precipitates). Specifically, Mn and B are likely to form inclusions. V and Nb may form precipitates. Ti is likely to form both inclusions and precipitates.In a case where the yield strength of a steel material in which the content of each element in the chemical composition is within the range of the present embodiment is 896 MPa (130 ksi) or more, these inclusions and precipitates together cause the low-temperature toughness of the steel material to decrease remarkably.

[0069] In addition, the size of the pre-austenite grains also affects the low-temperature toughness of the steel material. In particular, if the pre-austenite grains are coarse, they are likely to be Γ L07 ίη / 77Π7 / Ε / ΥΙΛΙ propagate cracks initiated by inclusions and precipitates. In contrast, if the pre-austenite grains are fine, crack propagation can be suppressed. Therefore, inclusions and precipitates and the size of the pre-austenite grains synergistically affect low-temperature toughness.

[0070] If F2 is greater than 0.240, even if the content of each element in the chemical composition is within the range of the present embodiment and formulas (1), (3), and (4) are satisfied, an excessive amount of inclusions and / or precipitates containing Mn, Ti, V, Nb, and B will be formed in the steel material. Alternatively, the pre-austenite grains will be too large relative to the amount of inclusions and / or precipitates formed. Therefore, in a case where the yield strength of the steel material is 896 MPa (130 ksi) or more, the low-temperature toughness of the steel material will decrease.

[0071] If F2 is 0.240 or less, in the steel material, the formation of inclusions and / or precipitates containing Mn, Ti, V, Nb and B can be sufficiently suppressed, and the size of the pre-austenite grains relative to the formed amount of inclusions and precipitates will also be suitable. Therefore, under the precondition that the content of each element in the chemical composition is within the range of the present embodiment and the chemical composition satisfies Formula (1), Formula (3) and Formula (4), even when the yield strength of the steel material is 896 MPa (130 ksi) or more, excellent low temperature toughness is obtained.

[0072] An upper limit of F2 is preferably 0.235, more preferably 0.230, more preferably 0.225, more preferably 0.220, more preferably 0.215, more preferably 0.210, and most preferably 0.200. The lower limit of F2 is not particularly limited. For example, the lower limit of F2 is 0.019. Note that, the value of F2 is a value obtained by rounding off to the fourth decimal place of an obtained value.

[0073] [With respect to formula (3)] F3 is defined as F3 = 10 x Ti + V + 10 x Nb. F3 is a precipitation hardening index which is auxiliaryly employed as a hardening mechanism in a steel material in which the content of each element in the chemical composition is within the range of the present embodiment. In the steel material of the present embodiment, as mentioned above, a hardening mechanism which hardens by improving the hardenability is adopted as the main hardening mechanism of the steel material. Specifically, by the content of each element in the chemical composition being within the range of the present embodiment and the formula (1) being satisfied, the hardenability of the steel material is increased and therefore the strength of the steel material is increased.However, in a steel material in which the content of each element in the chemical composition is within the range of the present embodiment, in some cases a high yield strength of 896 MPa (130 ksi) or more cannot be stably obtained only by a hardening mechanism that hardens by improving hardenability. P L07 ίη / ΖΖΩΖ / Ε / ΥΙΛΙ

[0074] Therefore, in the present embodiment, while a hardening mechanism that hardens by improving hardenability is adopted as the main hardening mechanism, a precipitation hardening mechanism that hardens by precipitation of Ti, V, and Nb is also employed as an auxiliary hardening mechanism. Specifically, if F3 composed of Ti, V, and Nb is less than 0.015, even if the content of each element in the chemical composition is within the range of the present embodiment and Formula (1), Formula (2), and Formula (4) are satisfied, the strength of the steel material will be insufficient. In this case, the yield strength of the steel material will be less than 896 MPa (130 ksi).

[0075] If F3 is equal to or greater than 0.015, precipitation hardening by means of Ti, V and Nb can be used auxiliarily. Therefore, under the precondition that the content of each element in the chemical composition is within the range of the present embodiment and that formulas (1), (2) and (4) are satisfied, the yield strength of the steel material will be sufficiently increased. Specifically, the yield strength of the steel material will be 896 MPa (130 ksi) or more.

[0076] A lower limit of F3 is preferably 0.020, more preferably 0.025, more preferably 0.030, more preferably 0.035, more preferably 0.040, and more preferably 0.045. The upper limit of F3 is not particularly limited. If the content of each element in the chemical composition is within the range of the present embodiment, the value of the upper limit of F3 is, for example, 0.300. The upper limit of F3 is preferably 0.290, more preferably 0.260, more preferably 0.240, and more preferably 0.220. Note that, the F3 value is a value obtained by rounding off to the fourth decimal place of an obtained value.

[0077] [Regarding Formula (4)] F4 is defined as F4 = (10 x Ti + 1.2 x V + 30 x Nb) / Mo. F4 is an index indicating the degree to which Mo contributes to improving low temperature toughness.

[0078] As mentioned above, in the steel material of the present embodiment, the low-temperature toughness of the steel material is increased by reducing inclusions and precipitates as much as possible, while the strength of the steel material is increased by adopting a hardening mechanism that hardens by improving hardenability as the main hardening mechanism of the steel material. However, in some cases, a high yield strength of 896 MPa (130 ksi) or more cannot be stably obtained by increasing the strength of the steel material solely by a hardening mechanism that hardens by improving hardenability. Therefore, a precipitation hardening mechanism that hardens by means of precipitates of Ti, V, and Nb is employed auxiliarily together with the hardening mechanism that hardens by improving hardenability. C L07 ΙΠ / ΖΖηΖ / Ε / ΥΙΙΛΙ

[0079] However, if the precipitates of Ti, V, and Nb are increased, the low-temperature toughness of the steel material in which the content of each element in the chemical composition is within the range of the present embodiment will decrease. On the other hand, Mo not only increases the strength of the steel material by improving the hardenability, but also hardens the steel material by solid solution hardening. The solid solution hardening by Mo can suppress the decrease in low-temperature toughness caused by the precipitates of Ti, V, and Nb. Therefore, in the present embodiment, the ratio of the content of Mo to the content of Ti, V, and Nb is increased. When F4 is greater than 0.205, the ratio of the content of Mo to the content of Ti, V, and Nb is low.In this case, although the yield strength of the steel material is made high enough so that the yield strength of the steel material becomes 896 MPa (130 ksi) or more by the content of each element in the chemical composition which is within the range of the present embodiment and the chemical composition satisfying the formulas (1) to (3), sufficient low temperature toughness is not obtained.

[0080] When F4 is 0.205 or less, the ratio of the Mo content to the Ti, V, and Nb content is high. In this case, under the precondition that the content of each element in the chemical composition is within the range of the present embodiment and formulas (1) to (3) are satisfied, the yield strength of the steel material is increased sufficiently so that the yield strength of the steel material is 896 MPa (130 ksi) or more, and excellent low-temperature toughness is also obtained. [0081 ] An upper limit of F4 is preferably 0.202, more preferably 0.200, more preferably 0.198, more preferably 0.195, more preferably 0.190, more preferably 0.185, more preferably 0.180, and most preferably 0.175. The lower limit of F4 is not particularly limited. If the content of each element of the chemical composition is within the range of the present embodiment, the lower limit of F4 is, for example, 0.005. The lower limit of F4 is more preferably 0.010, and more preferably 0.012. Note that, the F4 value is a value obtained by rounding off to the fourth decimal place of an obtained value.

[0082] [Regarding the yield strength of steel material] With respect to the steel material of the present embodiment composed as described above, under the precondition that the content of each element in the chemical composition is within the range of the present embodiment, the grain size number of the pre-austenite grains is less than 7.0, satisfying Formula (1) to Formula (4) the steel material has a high yield strength. Specifically, the steel material of the present embodiment has a yield strength of 896 MPa (130 ksi) or more. The term yield strength, as used in the present description, means the stress at a moment of 0.65% of total elongation (0.65% of conventional yield strength) obtained in a tensile test. Γ L07 ίΠ / ΖΖηΖ / Ε / ΥΙΛΙ

[0083] A lower limit of the yield strength is preferably 900 MPa, more preferably 910 MPa, and most preferably 920 MPa. The upper limit of the yield strength is not particularly limited. The upper limit of the yield strength, for example, is 1103 MPa (160 ksi), or for example is 1090 MPa, or for example is 1069 MPa (155 ksi).

[0084] The yield strength of the steel material of the present embodiment can be determined by the following method. A tensile test is performed by a method in accordance with ASTM E8 / E8M (2013). Specifically, a round bar sample is taken from the steel material. If the steel material is a steel plate, the round bar sample is taken from a central portion of the plate thickness. If the steel material is a steel pipe, the round bar sample is taken from a central portion of the wall thickness. For example, the diameter of the parallel portion of the round bar sample is 6.35 mm, and the length of the parallel portion is 25.4 mm. Note that the axial direction of the round bar sample is parallel to the longitudinal direction (rolling direction) of the steel material.The tensile test is carried out in the atmosphere at normal temperature (25 °C) using the round bar specimen, and the stress obtained at a moment of 0.65% total elongation (0.65% conventional yield strength) is defined as the yield strength (MPa).

[0085] [Regarding the low temperature toughness of steel material] In the steel material according to the present embodiment, under the precondition that the content of each element in the chemical composition is within the range of the present embodiment, the grain size number of the pre-austenite grains is less than 7.0, satisfying formulas (1) to (4), the above-mentioned high yield strength and excellent low-temperature toughness can be achieved. Specifically, in the steel material according to the present embodiment, the absorbed energy at -10 °C is 95 J or more. More specifically, the absorbed energy at -10 °C in accordance with ASTM E23 (2018) is 95 J or more.

[0086] A lower limit of the absorbed energy at -10 °C is preferably 96 J, more preferably 98 J, and further preferably 100 J. An upper limit of the absorbed energy at -10 °C is not particularly limited. The upper limit of the absorbed energy is, for example, 200 J, or for example is 180 J, or for example is 160 J.

[0087] The absorbed energy at -10 °C can be determined by the following method. The steel material of the present embodiment is subjected to a Charpy impact test according to ASTM E23 (2018) to evaluate the low temperature toughness. Specifically, V-notched test specimens are taken from the steel material. If the steel material is a steel plate, the V-notched test specimens are taken from a central portion of the plate thickness. If the steel material is a steel pipe, the V-notched test specimens are taken from a central portion of the wall thickness. The V-notched test specimens are prepared in accordance with API specification 5CT (10th edition). Γ L07 ίΠ / ΖΖηΖ / Ε / ΥΙΛΙ Charpy impact test is carried out at -10 °C in accordance with ASTM E23 (2018), using three of the V-notched specimens as a set to measure the absorbed energy. In case the absorbed energy is measured using undersized specimens, the obtained absorbed energy is divided by a reduction factor described in API 5CT specification (10th edition) in order to convert the obtained value into the absorbed energy for the full-size specimens. The arithmetic mean value of the absorbed energy values ​​of the three V-notched test specimens is defined as the absorbed energy E (J) at -10 °C. Note that the absorbed energy E (J) at -10 °C is a value obtained by rounding the first decimal place of a numerical value obtained.

[0088] [Microstructure] The microstructure of the steel material according to the present embodiment is mainly composed of martensite and / or bainite. More specifically, in the microstructure, the total area fraction of martensite and bainite is 90% or more. The remainder of the microstructure is composed of, for example, ferrite and / or pearlite. Although in some cases the balance of the microstructure may also include retained austenite in addition to ferrite and / or pearlite, the area of ​​retained austenite is insignificant compared to the area of ​​martensite, bainite, ferrite, and pearlite. If the total area fraction of martensite and bainite in the microstructure of the steel material having the chemical composition described above is 90% or more, provided that the other requirements of the present embodiment are met, the yield strength of the steel material will be 896 MPa or more (130 ksi or more).That is, in the present embodiment, if the contents of the respective elements in the chemical composition are within the ranges described above, the grain size number of the pre-austenite grains is less than 7.0, formulas (1) to (4) are satisfied, and the yield strength of the steel material is 896 MPa or more, it can be determined that the total area fraction of martensite and bainite in the microstructure is 90% or more.

[0089] When determining the total area fraction of martensite and bainite through observation, the total area fraction can be determined by the following method. If the steel material is a steel plate, a test sample having an observation surface including the rolling direction and the thickness direction is taken from a central portion of the thickness of the plate. If the steel material is a steel pipe, a test sample having an observation surface including the pipe axis direction and the wall thickness (pipe diameter) direction is taken from a central portion of the wall thickness.

[0090] After the observation surface of the test sample is polished to a mirror finish, the test sample is immersed for 10 seconds in a nital etching reagent to reveal the microstructure by etching. A scanning electron microscope (SEM) is used to observe 10 visual fields of the etched observation surface by a secondary electron image. The area of ​​the visual field is, for example, 0.01 mm2 (magnification of 1000x). In each field Γ L07 ίη / 77Ω7 / Β / ΥΙΛΙ Visually, martensite and bainite are identified based on contrast. Those skilled in the art can easily distinguish martensite and bainite from other phases (ferrite and pearlite) based on contrast. Note that, in the present embodiment, since the total area fraction of martensite and bainite is to be determined, it is not necessary to distinguish between martensite and bainite.

[0091] The total area fraction (%) of the identified martensite and bainite is then determined. In the present embodiment, the arithmetic mean value of the total area fraction (%) of martensite and bainite determined in all fields of view is defined as the total area fraction (%) of martensite and bainite.

[0092] [Preferable number density of Mn sulfides] In the present embodiment, a Mn sulfide is defined as follows. In a case where all elements (but excluding C) detected in an element concentration analysis performed by energy-dispersive X-ray spectrometry (hereinafter also referred to as EDS) are quantified, a Mn sulfide is defined as an inclusion in which, in mass %, a Mn content of 20% or more and an S content of 10% or more are detected. Furthermore, in the present embodiment, a Mn sulfide having an equivalent circular diameter of 5.0 pm or more is defined as a coarse Mn sulfide.

[0093] In the steel material of the present embodiment, under the precondition that the content of each element in the chemical composition is within the range of the present embodiment, the grain size number of the pre-austenite grains is less than 7.0, and formulas (1) to (4) are met, a preferable number density of coarse Mn sulfides is 10 / 100 mm2 or less. In this case, while the yield strength of the steel material is 896 MPa or more, the absorbed energy at -10 °C may be 100 J or more.

[0094] The number density of coarse Mn sulfides can be determined by the following method. Specifically, if the steel material is a steel plate, a test sample is taken from a central portion of the plate thickness. If the steel material is a steel pipe, a test sample is taken from a central portion of the wall thickness. If the steel material is a steel plate, the taken test sample is embedded in resin so that a face of the test sample including the rolling direction and the thickness direction becomes the observation surface. If the steel material is a steel pipe, the taken test sample is embedded in resin so that a face of the test sample including the pipe axis direction and the wall thickness (pipe diameter) direction becomes the observation surface.The observation surface of the resin-embedded test specimen is then polished. Ten arbitrary fields of view are observed on the observation surface after polishing. The area of ​​each field of view is set, for example, to 100 mm2. Γ L07 ίη / 77Π7 / Ε / ΥΙΛΙ

[0095] In each field of view, the number of Mn sulfides having an equivalent circular diameter of 5.0 pm or more is determined. Specifically, the inclusions in each field of view are identified based on contrast. Each of the identified inclusions is subjected to an element concentration analysis (EDS analysis). When all detected elements are quantified (however, excluding C), inclusions in which, in % by mass, a Mn content of 20% or more and an S content of 10% or more are detected are specified as Mn sulfides.

[0096] Among the specified Mn sulfides in the 10 fields of view, the total number of Mn sulfides having an equivalent circular diameter of 5.0 pm or more (coarse Mn sulfides) is determined. The number density of the coarse Mn sulfides ( / 100 mm2) is determined based on the total number of coarse Mn sulfides and the total area of ​​the 10 fields of view. In the present embodiment, in determining the number density of coarse Mn sulfides ( / 100 mm2), the first decimal place of an obtained numerical value is rounded off. Note that the measurement of the number density of the coarse Mn sulfides can also be performed using an apparatus in which a scanning electron microscope is provided with a composition analysis function (SEM-EDS apparatus).

[0097] A preferable upper limit of the number density of coarse Mn sulfides is 9 / 100 mm2 and more preferably 8 / 100 mm2.

[0098] [Shape of steel material] The form of the steel material according to the present embodiment is not particularly limited. The steel material is, for example, a steel pipe or a steel plate. The steel material is, for example, an oil well steel pipe. The oil well steel pipe is, for example, a casing pipe, a drill pipe, or the like used for drilling an oil well or a gas well, gathering crude oil or natural gas, and the like.

[0099] The oil well steel pipe may be a welded steel pipe or it may be a seamless steel pipe. Preferably, the steel material of the present embodiment is a seamless oil well steel pipe. The term "seamless oil well steel pipe" means an oil well steel pipe that is a seamless steel pipe.

[0100] [Production method] A method for producing the steel material according to the present embodiment will now be described. In the following description, a method for producing a steel pipe is described as an example of the steel material according to the present embodiment. However, a method for producing the steel material according to the present embodiment is not limited to the production method described below. That is, the production method is not particularly limited and may be a different production method as long as a composite steel material can be produced as described above. Γ L07 ίη / 77Π7 / Ε / ΥΙΛΙ

[0101] An example of the method for producing the steel material according to the present embodiment includes a process of preparing a starting material (starting material preparation process), a process of heating the prepared starting material (heating process), a process of subjecting the heated starting material to hot working (hot working process), and a process of subjecting the steel material after hot working to quenching and tempering (heat treatment process). Each process is described in detail below.

[0102] [Starting material preparation process] In the raw material preparation process, molten steel in which the content of each element in the chemical composition is within the range of the present embodiment, and which satisfies Formula (1) to Formula (4) when converted into a steel material, is produced by a well-known steelmaking method. A casting is produced by a continuous casting process using the produced molten steel. In this case, the casting is a plate, a billet, or a bar. Instead of the casting, an ingot may be produced by an ingot making process using the aforementioned molten steel. As needed, the plate, billet, or ingot may be subjected to hot working to produce a bar. The starting material (plate, billet, or bar) is produced by the production process described above.

[0103] [Heating process] In the heating process, the raw material prepared in the starting material preparation process is loaded into a continuous heating furnace and heated. The heating furnace may be a rotary hearth heating furnace or a walking beam heating furnace. In the following description, the use of a rotary hearth heating furnace is described as an example of a continuous heating furnace.

[0104] Figure 1 is a schematic diagram (plan view) illustrating a rotary hearth heating furnace which is an example of a continuous heating furnace. Referring to Figure 1, a heating furnace 10 includes a furnace main body 13 having a charging port 11 and a withdrawal port 12. A bar B1 as a starting material or raw material that is the object to be heated is charged into the heating furnace 10 from the charging port 11. In Figure 1, the bar B1 is heated while moving inside the heating furnace 10. In Figure 1, the bar B1 charged at the charging port 11 moves in a clockwise direction. When the bar B1 that is heated while moving reaches the withdrawal port 12, the bar B1 is drawn out to the outside from the withdrawal port 12.

[0105] The main body of the oven 13 is divided into a preheating zone Z1, a heating zone Z2, and a temperature homogenizing zone Z3 in that order in the direction from the charging port 11 to the extraction port 12. The preheating zone Z1 is a zone having the charging port 11, and is the zone in which the temperature inside the oven is lowest among the three zones. Γ L07 ίη / 77Π7 / Ε / ΥΙΛΙ (preheating zone Zl, heating zone Z2, and temperature homogenization zone Z3). The heating zone Z2 is a zone arranged between the preheating zone Zl and the temperature homogenization zone Z3. The temperature homogenization zone Z3 is a zone following the heating zone Z2, and has the exhaust port 12 at its rear end. The heating zone Z2 and the temperature homogenization zone Z3 are maintained at approximately the same temperature. Specifically, although the temperature in the temperature homogenization zone Z3 is somewhat higher than the temperature in the heating zone Z2, the temperature difference between the temperature homogenization zone Z3 and the heating zone Z2 is 20 °C or less. One or more burners are provided in each of the zones. In each zone, the temperature is adjusted by means of the burners.

[0106] In the present embodiment, the temperature and residence time in the preheating zone Zl, the heating zone Z2 and the temperature homogenization zone Z3 are as follows.

[0107] [Preheating zone Z1] In the preheating zone Zl, a furnace temperature TI is 820 to 1300 °C, and the temperature in the preheating zone Zl is set lower than a furnace temperature T2 in the heating zone Z2 and the temperature homogenization zone Z3. In addition, a residence time ti of the raw material in the preheating zone Zl is set to 45 minutes or more. The term residence time ti means a time (minutes) from when the raw material enters the preheating zone Zl from the charging port 11 until the raw material is discharged into the heating zone Z2. The preheating zone Zl mainly plays the role of increasing the temperature of the raw material that is at normal temperature. Preferably, the residence time ti in the preheating zone Zl is set to 50 minutes or more, and more preferably, 55 minutes or more.The upper limit of the residence time ti is not particularly limited. However, considering productivity, a preferable upper limit of the residence time ti is 300 minutes.

[0108] [Heating zone Z2 and temperature homogenization zone Z3] In the heating zone Z2 and the temperature homogenization zone Z3, the oven interior temperature T2 is set between 1100 and 1380 °C, and the temperature in the heating zone Z2 and the temperature homogenization zone Z3 is set at a temperature higher than the oven interior temperature in the preheating zone Z1. Here, an arithmetic mean value of an oven interior temperature in the heating zone Z2 and an oven interior temperature in the temperature homogenization zone Z3 is adopted as the oven interior temperature T2. In addition, a total residence time t2 (minutes) in the heating zone Z2 and the temperature homogenization zone Z3 is set to 50 minutes or more and is more preferably set to 100 minutes. P L07 ίη / 77Ω7 / Β / YILI in 55 minutes or more. Here, the term total residence time t2 means a time (minutes) from when the raw material enters the heating zone Z2 until the raw material is discharged to the outside from the extraction port 12. An upper limit of the total residence time t2 is not particularly limited. However, in consideration of productivity, a preferable upper limit of the total residence time t2 is 600 minutes.

[0109] [Preferred conditions in the heating zone Z2 and in the temperature homogenization zone Z3] Preferably, the temperature in the furnace T2 and the total residence time t2 in the heating zone Z2 and the temperature homogenization zone Z3 satisfy the following Formula (A): 1420 < (t2 / 60)°5x (T2 + 273)...(A) where, in Formula (A), the total residence time t2 (minutes) of the starting material is replaced by t2, and the temperature in the furnace T2 (°C) is replaced by T2.

[0110] FA is defined asFA = (t2 / 60)°5x (T2 + 273). If FA is 1420 or more, the Mn contained in the starting material will sufficiently diffuse into the overall starting material. In this case, some of the Mn sulfides in the starting material will dissolve. Furthermore, the formation of coarse Mn sulfides will be suppressed. As a result, the number density of Mn sulfides having an equivalent circular diameter of 5 pm or more will be 10 / 100 mm2 or less.

[0111] A lower limit of FA is preferably 1500, more preferably 1550, more preferably 1600, more preferably 1650, and more preferably 1700. An upper limit of FA is not particularly limited. However, taking into account the load of the facility and the specific productivity of the production, the upper limit of FA is preferably 4500, more preferably 4400, more preferably 4300, and more preferably 4200.

[0112] Note that, a lower limit of the total oven time in the preheating zone Z1, the heating zone Z2 and the temperature homogenization zone Z3 is preferably 95 minutes, more preferably 120 minutes, more preferably 140 minutes, more preferably 150 minutes and most preferably 160 minutes. An upper limit of the total oven time is preferably 900 minutes, more preferably 800 minutes and most preferably 750 minutes.

[0113] Note that, in Figure 1, the preheating zone Z1, the heating zone Z2 and the temperature homogenization zone Z3 are equally divided within the main body of the furnace 13. However, the preheating zone Z1, the heating zone Z2 and the temperature homogenization zone Z3 do not have to be equally divided.

[0114] [Hot working process] In the hot working process, the raw material heated under the conditions mentioned above by the heating process is subjected to hot working. If the final product is a steel pipe, the Γ L07 ίη / 77Ω7 / Β / YILI heated stock material is hot worked to produce an intermediate steel material (hollow shell). For example, hot rolling by the Mannesmann mandrel process is performed as hot working to produce a hollow shell. In this case, the bar is subjected to punch rolling by a punching machine. When punch rolling is performed, although not particularly limited, the punch ratio is, for example, 1.0 to 4.0. After punch rolling, the bar is rolled by mandrel rolling mills. In addition, as required, the bar after rolling is subjected to diameter adjustment rolling using a reducer or a finishing mill. A hollow shell is produced by the above process.

[0115] Hot extrusion can be performed as hot working. For example, the Ugine-Sejoumet process or the Ehrhardt drawbench process can be used to produce a hollow shell.

[0116] [Preferred working time] Preferably, the working time in the hot working process according to the present embodiment is 15 minutes or less. Here, the term "working time" (minutes) means the time from the time the starting material is removed from the heating furnace until the hot working is completed. If the working time is 15 minutes or less, provided that the aforementioned Formula (A) is met, the coarse growth of Mn sulfides and the formation of new Mn sulfides during hot working can be suppressed. As a result, the number density of Mn sulfides having an equivalent circular diameter of 5 pm or more will be 10 / 100 mm2 or less.

[0117] A more preferable upper limit of the working time is 14 minutes and more preferably 13 minutes. A lower limit of the working time is not particularly limited, and for example is 5 minutes.

[0118] [Heat treatment process] In the heat treatment process, the intermediate steel material (hollow shell) after hot working is subjected to a quenching process and annealing process.

[0119] [Tempering process] In the hardening process, either in-line or off-line hardening is performed. In this case, in-line hardening is a treatment in which direct hardening is performed after hot working without cooling the intermediate steel material (hollow shell) produced by hot working to normal temperature, or in which hardening is performed after subjecting the intermediate steel material (hollow shell) to supplementary heating (reheating) at a time after hot working and before cooling the intermediate steel material (hollow shell) to normal temperature. In the case of in-line hardening, hardening may be performed immediately after hot working. Γ L07 ίη / 77Ω7 / Β / YILI hot working on the production line. On the other hand, the treatment in which the intermediate steel material (hollow shell) after hot working is cooled to normal temperature and then subjected to hardening using a heat treatment furnace is called off-line hardening. The following describes on-line hardening and off-line hardening.

[0120] [Temple online] The quenching temperature in in-line quenching is 800 to 1100 °C. As used herein, 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 measured by a thermometer placed on the outlet side of the apparatus that performs the final hot working. In the case where quenching is performed by a supplementary heating furnace or a heat treatment furnace after hot working, the quenching temperature corresponds to the temperature of the supplementary heating furnace or the heat treatment furnace.

[0121] As mentioned above, the in-line quenching may be performed by rapidly cooling the intermediate steel material that is at a temperature of 800 to 1100 °C after hot working. Alternatively, the intermediate steel material that is in a state after hot working and before being cooled to the normal temperature (intermediate steel material whose temperature is 400 °C or more) may be heated to 800 to 1100 °C by using a supplementary heating furnace or a heat treatment furnace installed on the production line, and then rapidly cooled. An upper limit of the quenching temperature in the in-line quenching is preferably 1050 °C, more preferably 1000 °C, and more preferably 980 °C. A lower limit of the quenching temperature in the in-line quenching is preferably 850 °C, and more preferably 900 °C.

[0122] In the case of performing in-line tempering using a supplementary heating furnace or a heat treatment furnace after hot working, the holding time at the tempering temperature is, for example, 5 to 45 minutes.

[0123] The quenching method is, for example, a method that rapidly cools the hollow shell from the quenching temperature. It is sufficient that the rapid cooling method is a known method. The rapid cooling method is, for example, a method in which the hollow shell is cooled by immersing it in a water bath or a method in which the hollow shell is cooled by water bath cooling or mist cooling.

[0124] [Temple offline] The tempering temperature in off-line tempering is 930 to 1100°C. The holding time at tempering temperature is 10 to 125 minutes.

[0125] When quenching is performed off-line, the grains are refined by reverse transformation. Therefore, if the quenching temperature is too low, even if the content of each element in the Γ L07 ίΠ / ΖΖηΖ / Ε / ΥΙΛΙ chemical composition is within the range of the present embodiment, in some cases the grain size number of the pre-austenite grains will be 7.0 or more. If the quenching temperature is 930 to 1100 °C and the holding time at the quenching temperature is 10 to 125 minutes, the austenite grains can be made coarse during quenching. As a result, under the precondition that a requirement relating to the holding time to be described later is met, the grain size number of the pre-austenite grains can be made less than 7.0. A lower limit of the quenching temperature in the off-line quenching is preferably 940 °C and more preferably 950 °C. An upper limit of the quenching temperature in the off-line quenching is preferably 1050 °C.

[0126] [Tempering process] In the annealing process, the intermediate steel material undergoes tempering after the quenching process. In this embodiment, precipitates that contribute to precipitation hardening are formed in the steel material during the annealing process. In this way, in addition to a hardening mechanism that improves hardenability, a precipitation hardening mechanism is used as an auxiliary to sufficiently increase the strength of the steel material. Specifically, the yield strength of the steel material is 896 MPa (130 ksi) or more. In addition, by adopting appropriate annealing conditions, deformation in the steel material is reduced and low-temperature toughness is increased. The absorbed energy E (J) at -10 °C is 95 J or more.

[0127] Specifically, in the annealing process, a tempering parameter TMP defined by the following formula is made to fall within the range of 17000 to 17950. TMP = (annealing temperature (°C) + 273) x (20 + log (holding time (minutes) / 60))

[0128] If the annealing parameter TMP is less than 17000, the annealing effect will not be sufficiently obtained, and the deformation introduced into the steel material in the quenching process will not be sufficiently removed. In this case, even if the content of each element in the chemical composition is within the range of the present embodiment, the grain size number of the pre-austenite grains is less than 7.0, and formulas (1) to (4) are satisfied, the absorbed energy E (J) at -10 °C will be less than 95 J. On the other hand, if the annealing parameter TMP is greater than 17950, sufficient strength cannot be obtained. Consequently, even if the content of each element in the chemical composition is within the range of the present embodiment, the grain size number of the pre-austenite grains is less than 7.0, and formulas (1) to (4) are satisfied, the yield strength will be less than 896 MPa (130 ksi).

[0129] If the tempering parameter TMP is 17000 to 17950, the excessive deformation introduced during quenching can be adequately eliminated while precipitates contributing to precipitation hardening are adequately formed. As a result, with the precondition that the content of each element in the chemical composition is within the range of the present embodiment, P L07 ίΠ / ΖΖηΖ / Ε / ΥΙΛΙ the grain size number of the pre-austenite grains is less than 7.0, and the formulas (1) to (4) are met, sufficiently high strength is obtained, and excellent low-temperature toughness is also obtained. Specifically, the yield strength of the steel material will be 896 MPa (130 ksi) or more, and the absorbed energy E (J) at -10 °C will be 95 J or more.

[0130] The annealing temperature in the annealing process is 600 to 720 °C. In addition, the holding time at the annealing temperature is 10 to 90 minutes. That is, in the annealing process, the annealing temperature is set to 600 to 720 °C, the holding time at the annealing temperature is set to 10 to 90 minutes, and in addition, the annealing parameter TMP is 17000 to 17950.

[0131] A lower limit of the annealing temperature is preferably 605 °C and more preferably 610 °C. An upper limit of the annealing temperature is preferably 700 °C, more preferably 680 °C and more preferably 660 °C. A lower limit of the annealing parameter TMP is preferably 17050, more preferably 17100 and more preferably 17130. An upper limit of the annealing parameter TMP is preferably 17940, more preferably 17920 and more preferably 17910.

[0132] The steel material according to the present embodiment can be produced by the above production process. Note that, in the production method described above, a method for producing a steel pipe is described as an example. However, the steel material according to the present embodiment may also be a steel plate. Similar to the production method described above, an exemplary method for producing a steel plate also includes, for example, a raw material preparation process, a heating process, a hot working process, and a heat treatment process. Note that, even when another production method is employed, the production method is not particularly limited as long as a composite steel material as described above can be produced. EXAMPLE

[0133] The advantageous effects of the steel material of the present embodiment will be described more specifically by way of an example. The conditions adopted in the following example are an example of the conditions used to confirm the feasibility and advantageous effects of the steel material of the present embodiment. Accordingly, the steel material of the present embodiment is not limited to this example of the conditions.

[0134] Cast steels were produced with the chemical compositions shown in Table 1. Note that a blank field in Table 1 means that the corresponding element was not contained. For example, in the case of test No. 1, with respect to the V content, the blank field means that, as a result of rounding off to the third decimal place, the V content was 0 %. Furthermore, with respect to the Nb content, the blank field means that, as a result of rounding off to the fourth decimal place, the Nb content was 0 %. The same applies to the contents of the other elements. Γ L07 ίΠ / ΖΖηΖ / Ε / ΥΙΛΙ

[0135] Table 1 P L07 ίη / ΖΖΩΖ / Ε / ΥΙΛΙ MX / a / 2022 / 012813 un m TABLE 1 MX / a / 2022 / 012813

[0136] The above-mentioned cast steels were used to produce bars by a continuous casting process. The produced bar of each test number was heated in a rotary hearth continuous heating furnace. The furnace temperature TI and the residence time ti in the preheating zone Zl, the furnace temperature T2 and the total residence time t2 in the heating zone Z2 and in the temperature homogenization zone Z3, the FA value and the time in the heating furnace (the period of time from when the bar was charged into the charging port 11 of the preheating zone Zl until the bar was discharged from the withdrawal port 12 of the temperature homogenization zone Z3) were as indicated in the columns Temperature TI (°C), Residence time ti (minutes), Temperature T2 (°C), Total residence time t2 (minutes), FA and Time in heating furnace (minutes), respectively, in Table 2.In addition, the work time for each test number was as shown in the Work Time (minutes) column in Table 2. Specifically, <15 in the Work Time (minutes) column indicates that the work time was 15 minutes or less. Additionally, >15 indicates that the work time was greater than 15 minutes.

[0137] Table 2 Γ L07 ίη / 77Π7 / Ε / ΥΙΛΙ MX / a / 2022 / 012813 m TABLE 2 Comments Inventive example | Inventive example | Inventive example | Inventive example | Inventive example | Inventive example | Inventive example I Inventive example | Inventive example | Inventive example | Inventive example | Inventive example | Inventive example | Inventive example | Inventive example | Inventive example | Inventive example | Inventive example | Inventive example | Inventive example | Comparative example | Comparative example | Comparative example | Comparative example | Comparative example | Comparative example | Comparative example | Comparative example | Absorbed energy (J) 8 5 O ggg CO LO CO $ o co co g CO OJ o LO 2 2 g LO ay LO 03 g 03 CM 03 co 03 o 03 CC CC CM co YS (ksi) 8 8 co co o co 8 8 LO 5 co LO co 5 o ·*· 03 03 03 03 CM CM CM CM S LO g LO LO YS (MPa) 03 to ay ay ay to ay ay co ay I 959 I ay o 8 ay 903 | 938 | 8 a> ay l 899 | ay 3 03 8 ay σ> CM ayCM 03 03 03 CM 03 LO CO 8 co 8 03 g co 8 co BB Number density of coarse Mn sulfides ( / 100 mmj u-> <o LO LO to co ay r- OI co co co LO OJ LO -r 03 03 CO $2 r- co 03 r- LO - CC LO £ 0.203 0.032 0.088 0.105 0.135 0.105 0.079 0.135 0.167 0.167 0.167 0.184 0.167 0.150 0.150 0.180 0.200 0.180 0.190 0.176 0.154 0.154 0.173 0.173 0.173 0.178 0.178 0.200 0.200 0.096 0.020 0.233 0.233 ¡2 0.120 0.020 0.050 090 0 0¿00 0.060 0.050 0.070 080'0 080 0 080 0 060Ό 080 0 0.090 0.090 060'0 080'0 060Ό 5 0.090 080Ό 080Ό 0.090 0.090 060'0 080'0 080Ό 00L0 0.100 0.040 οωο «7» 5 CM 0.212 0.160 0.168 0.180 0.172 0.156 0.161 0.197 0.197 0.197 0.203 0.197 0.212 0.213 0.200 0.191 0.197 0.228 0.220 0.190 06 LO 0.203 0.203 0.203 0.194 0.194 0.263 0.265 0.155 0.115 0.217 0.220 lC 0.733 0.801 0.798 | 0.778 0.768 0.798 0.814 0.732 0.731 | 0.731 0.731 0.719 | 0.731 0.782 0.787 0.719 θ 0.707 0.753 0.706 | 0.709 H° 0.723 0.719 0.719 0.673 | 0.676 0.769 0.779 | 0.737 669'0 0.682 | I 969'0 Número de tamaño deGrain and previous CD cq LO LO cq <ri s co LO s £ 2 <D co oq cq co LO co co ay LO 03 LO 03 co LO cq 03 co LO LO 5 2 Proceso de tratamiento térmico Revenido TMP 17901 17493 17802 17802 17802 17802 17802 I 17550 17690 17128 17889 17802 17141 17889 17141 17889 17141 17141 17889 17848 17532 17532 17532 17532 17532 17889 17889 17889 17889 17889 17889 17889 17889 Tiempo (minutos) g 8 o g g g g LO 8 LO LO g 8 S 8 8 CJ θ 8 LO θ θ 8 8 8 s 8 8 8 8 Temperatura m 8 o LO 5 8 to LO 8 8 LO 8 ιο OJ CO co LO to LO 8 LO CM to LO 8 OJ CO LO CO lo CM CO LO LO LO CO LO co CM CO LO co co co LO CM CO LO CM to § LO CM to LO co CM to LO CM to 8 8 LO CM to Temple Tiempo (minutos) g g g g g g g g g g g g g 2 O O g g g g g g g g g g g g g Temperatura (°C) 8 S s 03 σ> ss § 03 § 03 o> 03 § 03 03 § § § sg 1 g 1 g 03 S 03 03 03 § 03 03 Tipo de temple CC Φ Φ CZ Φ c Φ tZ Φ c Φ tz φ c Φ co φ c Φ re φ c Φ CC Φ c Φ ce φ c Φ ce φ c Φ ce φ c Φ ce φ c Φ ce φ c Φ ce φ c Φ CC Φ c Φ ce φ c Φ ce φ c Φ ce φ c Φ ce φ c Φ ce φ o Φ CCΦ Φ outline outline outline outline outline outline outline outline CC Φ Φ CC Φ c Φ CC Φ C Φ tz Φ c Φ CC Φ c Φ CC Φ c Φ C Φ CC Φ C Φ Working time (Minutes LOVI) VI vi LO VI vi LO vi in ​​vi LO VI LO VI LO VI vi LO VI LO vi LO VI VI LO VI LO VI VI VI VI VI LO LO VI VI VI vi m VI LO LO VI Bar heating process | Time in the heating oven (minutes) cm ggggggggsgg co 03 8 g CM g 8 CM g § S to 8 CM gg 03 g B heating 'perature mogenization 2376 2212 | 2376 3054 3054 2035 4195 1940 2403 | ¿8tzZ 2326 1966 1429 1406 2154 2227 8 1940 1759 2212 2126 2240 | Total residence time 12 (minutes) g 8 gg § § S g 8 LO g 8 g LO sggggi 8 LO OJ gggggg LO Zone ce i + Zone of ho temí Temperature T2 (°C) 1230 | 1290 | 1270 | 1270 | 1300 | 1270 | I 1250 I 1230 | 1250 | 1250 | 1230 | 1230 | 1230 | 1270 | 1270 | 1230 | CM 1230 | 1300 | 1300 | 1250 | 1250 | 1220 | ON 03 1250 |1240 | 1240 | 1230 | 1250 | 1220 | 1230 | 1230 | 1220 | Preheating zone Residence Time ti (minutes) ggggg § go> gggggg § g CM ggg § gg § § gggg co Temperature T1 (°C) 1230 | g 1220 | 1220 | 1220 | g I 1200 I g 1200 | g 1220 | 1050 | co g 1240 | CM gg CM g 1230 | 1200 | gg 1200 | 1200 | g 1 1200 | gg 1220 | 1200 | Test No. - co IO co CO σ> og co g lo CO £ co ay g c7¡ gg LO g co CM 03 CM g 8 8 MX / a / 2022 / 012813 Os ce

[0138] Each bar, once heated, was subjected to hot rolling by the Mannesmann mandrel process (hot working) to produce a hollow shell (seamless steel tube) of each test number.

[0139] The hollow shell produced from each test number was subjected to either on-line quenching or off-line quenching. In the case of on-line quenching (described as On-line in the Quenching Type column of Table 2), the hollow shell after hot working was not cooled to normal temperature, and instead the hollow shell after hot working which was at a temperature of 400 °C or more was charged into a supplementary heating furnace. The hollow shell was held for a holding time (minutes) indicated in the Time (minutes) column at a quenching temperature (°C) indicated in the Temperature (°C) column of the Quenching column of Table 2, and then quenched with water. On the other hand, in the case of off-line quenching (described as Off-line in the Quenching Type column of Table 2), the hollow shell after hot working was cooled to normal temperature.After cooling, the hollow shell was placed in a heat treatment furnace. The hollow shell was held for the holding time (minutes) shown in the Time (minutes) column at the quench temperature (°C) shown in the Temperature (°C) column of the Quench column in Table 2, and then quenched with water.

[0140] After quenching, the hollow shell of each test number was subjected to annealing. Specifically, the hollow shell of each test number was subjected to annealing in which the hollow shell was held for an annealing time (minutes) shown in the Time (minutes) column at an annealing temperature (°C) shown in the Temperature (°C) column of the Tempering column in Table 2. Note that the annealing parameter TMP (= (annealing temperature (°C) + 273) x (20 + log (annealing time (minutes) / 60 ))) is shown in the TMP column of Table 2.

[0141] Note that, in the present example, the temperature of the supplementary heating furnace or heat treatment furnace used for heating in hardening was taken as the hardening temperature (°C). In addition, the temperature of the heat treatment furnace used for tempering was taken as the tempering temperature (°C).

[0142] Seamless steel pipes which were steel materials were produced by the above production process.

[0143] [Assessment tests] The steel material (seamless steel tube) of each test number was subjected to the following evaluation tests.

[0144] [Microstructure observation test] The microstructure of the steel material (seamless steel tube) of each test number was observed by the following method, and the total area fraction (%) of martensite and bainite was determined. P L07 ίΠ / ΖΖηΖ / Ε / ΥΙΛΙ A test sample with an observation surface including the tube axis direction and the wall thickness direction (tube diameter) was taken from a central portion of the wall thickness of the steel material. After the observation surface of the test sample was polished to a mirror finish, the test sample was immersed for 10 seconds in a nital etchant to reveal the microstructure by etching. Using a SEM, 10 fields of view of the etched observation surface were observed in a secondary electron image. The field of view area was set at 0.01 mm2 (magnification of 1000x). In each field of view, martensite and bainite were identified based on contrast, and the total area fraction (%) of the identified martensite and bainite was determined.The arithmetic mean of the total area fraction (%) of martensite and bainite determined across the 10 fields of view was defined as the total area fraction (%) of martensite and bainite. The measurement results showed that in each test series, the total area fraction of martensite and bainite was 90% or more.

[0145] [Test to determine the grain size number of pre-austenite grains] The grain size number of the pre-austenite grains of the steel material (seamless steel tube) of each test number was determined by the following method. A test sample was taken from a central portion of the wall thickness of the steel material (seamless steel tube) such that a cross-section perpendicular to the longitudinal direction (rolling direction) of the steel material became the surface to be examined. The taken test sample was embedded in resin, and the surface to be examined was mirror-polished. After mirror-polishing the surface to be examined, the grain boundaries of the pre-austenite were revealed by the Bechet-Beaujard method, in which the surface to be examined was etched with a saturated aqueous solution of picric acid. The grain size number of the pre-austenite grains was measured in accordance with ASTM E12-13.The obtained grain size number is shown in the column Previous γ Grain Size Number in Table 2. Note that, the F1 to F4 values ​​of each test number are shown in the columns Fl to F4 immediately to the right of the column Previous γ Grain Size Number in Table 2.

[0146] [Test for measuring the number density of Mn sulfides]. The number density ( / 100 mm2) of Mn sulfides in the steel material of each test number was determined by the following method. A test sample was taken from a central portion of the wall thickness of the steel material (seamless steel pipe). The taken test sample was embedded in resin so that a face of the test sample that included the direction of the pipe axis and the direction of the wall thickness (pipe diameter) became the observation surface. The observation surface of the resin-embedded test sample was polished. 10 arbitrary visual fields were observed on the observation surface after polishing. The area of ​​each visual field was set at 100 mm2. The Mn sulfides in each visual field were identified by the method described Γ L07 ίη / 77Π7 / Ε / ΥΙΛΙ previously. The total number of Mn sulfides with an equivalent circular diameter of 5.0 pm or more (coarse Mn sulfides) among the Mn sulfides identified in the 10 fields of view was determined. The number density of coarse Mn sulfides ( / 100 mm2) was determined from the total number of coarse Mn sulfides determined and the total area of ​​the 10 fields of view. The obtained number density of coarse Mn sulfides is shown in the column Coarse Mn sulfide number density ( / 100 mm2) in Table 2.

[0147] [Yield limit measurement test] The yield strength of the steel material of each test number was determined by the following method. A tensile test was performed using a method in accordance with ASTM E8 / E8M (2013). A round bar sample was taken from a central portion of the wall thickness of the steel material (seamless steel pipe) of each test number. The size of the round bar sample was as follows: the diameter of the parallel portion was 6.35 mm, and the length of the parallel portion was 25.4 mm. The axial direction of the round bar sample was parallel to the longitudinal direction (rolling direction) of the steel material (seamless steel pipe). A tensile test was performed in an atmosphere at normal temperature (25 °C) on the round bar sample, and the stress obtained at a moment of 0.65% of total elongation was defined as the yield strength (MPa).The obtained yield strength (MPa) is shown in column YS (MPa) of Table 2, and the yield strength (ksi) is shown in column YS (ksi) of Table 2.

[0148] [Low Temperature Toughness Evaluation Test] The absorbed energy at -10 °C of the steel material of each test number was determined by the following method. The steel material of each test number was subjected to a Charpy impact test in accordance with ASTM E23 (2018). Specifically, in accordance with API Specification 5CT (10th Edition), full-size V-notched test specimens were taken from a central portion of the wall thickness of the steel material (seamless steel pipe) of each test number. The longitudinal direction of each V-notched test specimen was made perpendicular to the longitudinal direction (rolling direction) of the steel material (seamless steel pipe). The V-notched test specimens were prepared in accordance with ASTM E23 (2018). The Charpy impact test was conducted at -10 °C in accordance with ASTM E23 (2018) using three of the V-notched specimens as a set to measure the absorbed energy.The arithmetic mean value of the absorbed energy of the three test samples was defined as the absorbed energy (J) at -10 °C. The obtained absorbed energy is shown in the Absorbed Energy (J) column of Table 2.

[0149] [Evaluation results]. Referring to Table 1 and Table 2, the content of each element in the chemical compositions of test numbers 1 to 25 was appropriate. In addition, the grain size number of the Γ L07 ίΠ / ΖΖηΖ / Ε / ΥΙΛΙ of previous austenite grains was less than 7.0. In addition, F1 to F4 satisfied formulas (1) to (4). As a result, sufficiently high strength and excellent low-temperature toughness were obtained. Specifically, the yield strength was 896 MPa (130 ksi) or more, and the absorbed energy at -10 °C was 95 J or more.

[0150] In addition, in tests No. 1 to 23, in the heating process, the FA in the heating zone Z2 and in the temperature homogenization zone Z3 was 1420 or more and the working time was 15 minutes or less. On the other hand, in test No. 24, the FA was less than 1420. In test No. 25, the working time was more than 15 minutes. Therefore, in Tests No. 1 to 23, the number density of Mn sulfides was 10 / 100 mm2 or less, and the number density of Mn sulfides was lower than the number density in Tests No. 24 and 25. Consequently, the absorbed energy at -10 °C in Tests No. 1 to 23 was 100 J or more, which was even higher than the absorbed energy in Tests No. 24 and 25.

[0151] On the other hand, in tests No. 26 and 27, F1 did not satisfy formula (1). Therefore, the strength was low. Specifically, the yield strength was less than 896 MPa (130 ksi).

[0152] In tests No. 28 and 29, F2 did not satisfy formula (2). Therefore, the low-temperature toughness was low. Specifically, the absorbed energy at -10 °C was less than 95 J.

[0153] In Test No. 30, the chemical composition did not contain Ti. Therefore, the strength was low. Specifically, the yield strength was less than 896 MPa (130 ksi).

[0154] In Test No. 31, F3 did not satisfy formula (3). Therefore, the strength was low. Specifically, the yield strength was less than 896 MPa (130 ksi).

[0155] In tests No. 32 and 33, F4 did not satisfy formula (4). Therefore, the low-temperature toughness was low. Specifically, the absorbed energy at -10 °C was less than 95 J.

[0156] In Test No. 34, the Mn content was too high. Therefore, the low-temperature toughness was low. Specifically, the absorbed energy at -10°C was less than 95 J.

[0157] In tests 35 to 37, the Mn content was too high. In addition, the V content was too high. Therefore, the low-temperature toughness was low. Specifically, the absorbed energy at 10 °C was less than 95 J.

[0158] In Tests No. 38 and 41, F2 did not satisfy Formula (2), and F4 did not satisfy Formula (4). Therefore, the low-temperature toughness was low. Specifically, the absorbed energy at -10 °C was less than 95 J.

[0159] In Test No. 39, the Ti content was too high. Therefore, the low-temperature toughness was low. Specifically, the absorbed energy at -10°C was less than 95 J.

[0160] In Test No. 40, the B content was too high. Therefore, the low-temperature toughness was low. Specifically, the absorbed energy at -10°C was less than 95 J. Γ L07 ίΠ / ΖΖηΖ / Ε / ΥΙΛΙ

[0161] In tests No. 42 and 43, the tempering parameter TMP was too low. Therefore, the low-temperature toughness was low. Specifically, the absorbed energy at -10 °C was less than 95 J.

[0162] In Test No. 44, the tempering parameter TMP was too high. Therefore, the strength was low. Specifically, the yield strength was less than 896 MPa (130 ksi).

[0163] In Tests 45 and 46, although the quenching was performed off-line, the quenching temperature was lower than 930 °C. Therefore, the grain size number of the pre-austenite grains was 7.0 or more, and F1 did not satisfy Formula (1). Consequently, the strength was low. Specifically, the yield strength was lower than 896 MPa (130 ksi).

[0164] An embodiment of the present invention has been described above. However, the above-described embodiment is merely an example for carrying out the present invention. Therefore, the present invention is not limited to the application described above, and can be practiced by suitably modifying the application described above within a range not departing from the scope thereof.

Claims

1. A steel material consisting of, % by mass: C: 0.15 to 0.45%, Si: 0.05 to 1.00%, Mn: 0.05 to less than 0.80%, P: 0.030% or less, S: 0.0100% or less, Al: 0.100% or less, Cr: 0.30 to 1.50%, Mo: 0.25 to 2.00%, Ti: 0.001 to 0.015%, N: 0.0100% or less, O: 0.0050% or less, V: 0 to 0.05%, Nb: 0 to 0.010%, B: 0 to less than 0.0005%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, metal of rare earths: 0 to 0.0100%, Ni: 0 to 0.50%, and Cu: 0 to 0.50%, the remainder being Fe and impurities, wherein: a grain size number of the previous austenite grains is less than 7.0; with the prior condition that the content of each element is within a range described above, Formulas (1) to (4) are satisfied; a yield strength is 896 MPa or more; and an energy absorbed at -10 °C is 95 J or more: {C + Mn / 5 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 + 10 x B} x (7.0 / GN)0·45 > 0.678...(1) {Mn / 5.5 + 10 x Ti + 1.2 x V + 15 x Nb + 200 x B} (7.0 / GN)045 < 0.240...(2) (10 x Ti + V + 10 x Nb > 0.015...(3) (10 x Ti + 1.2 x V + 30 x Nb) / Mo < 0.205...(4) P L07 ίη / ZZOZ / E / YILI where, a mass % content of a corresponding element is replaced by each element symbol, and the grain size number is replaced by GN.

2. The steel material according to claim 1, wherein a number density of Mn sulfides having an equivalent circular diameter of 5.0 pm or more is 10 pieces / 100 mm2 or less; and an energy absorbed at -10 °C is 100 J or more.

3. The steel material according to claim 1, containing one or more types of elements selected from the group consisting of: V: 0.01 to 0.05%, Nb: 0.001 to 0.010%, B: 0.0001 to less than 0.0005%, Ca: 0.0001 to 0.0100%, Mg: 0.0001 to 0.0100%, rare earth metal: 0.0001 to 0.0100%, Ni: 0.01 to 0.50%, and Cu: 0.01 to 0.50%.

4. The steel material according to claim 2, containing one or more types of elements selected from the group consisting of: V: 0.01 to 0.05%, Nb: 0.001 to 0.010%, B: 0.0001 to less than 0.0005%, Ca: 0.0001 to 0.0100%, Mg: 0.0001 to 0.0100%, rare earth metal: 0.0001 to 0.0100%, Ni: 0.01 to 0.50%, and Cu: 0.01 to 0.50%.

5. The steel material according to any of claims 1 to 4, wherein: the steel material is a steel pipe for oil wells.