Steel material

Abstract

The present invention provides a steel material which has high strength and excellent low temperature toughness in an extremely low temperature environment at -80°C or lower. A steel material according to the present disclosure comprises 0.09% or less of C, 0.60% or less of Si, 1.0-2.5% of Mn, 0.05% or less of P, 0.005% or less of S, 0.005-0.050% of Ti, 0.005-0.100% of Al, 0.0050-0.0150% of N, 0.0050% or less of O, and the balance made up of Fe and impurities. With respect to the steel material, Fn1 that is defined by formula (1) is 5.0 or more, Fn2 that is defined by formula (2) is 1.80 or less, and the yield strength is 448 MPa or more. The number density of Ti-containing particles in the steel material is 0.8 × 1018 particles / mm3 or more, and among the Ti-containing particles, the number ratio of Ti-containing particles that have an equivalent circle diameter of 60 nm or less is 75% or more.　(1): Fn1 = Ti × N × 105, (2): Fn2 = Ti / N

Description

steel material

[0001] This disclosure relates to steel products, and more particularly to steel products suitable for use in cryogenic environments.

[0002] A pipeline is a system installed on land or the seabed that transports natural gas, crude oil, etc. Pipelines laid on the seabed are made up of multiple steel pipes (line pipes). Pipelines laid on the seabed are further subjected to high pressure from the production fluids that flow through them. Pipelines are also subjected to repeated strain from waves and seawater pressure from the outside. For this reason, the steel pipes (line pipes) that make up pipelines are required to have high strength and excellent toughness.

[0003] To date, techniques for increasing the strength and toughness of steel materials for line pipes have been proposed in Japanese Patent Laid-Open No. 2010-174343 (Patent Document 1), Japanese Patent Laid-Open No. 2015-190042 (Patent Document 2), and International Publication No. 2016 / 056216 (Patent Document 3).

[0004] The steel material proposed in Patent Document 1 is a thick, high-tensile, hot-rolled steel sheet containing, by mass%, 0.02 to 0.25% C, 1.0% or less Si, 0.3 to 2.3% Mn, 0.03% or less P, 0.03% or less S, 0.1% or less Al, 0.03 to 0.25% Nb, and 0.001 to 0.10% Ti, with Nb, Ti, and C contained so as to satisfy the formula ((Ti + Nb / 2) / C<4), with the balance consisting of Fe and impurities. This steel material further has a structure in which the structure at a position 1 mm from the surface in the plate thickness direction is a single phase consisting of a bainite phase or a bainitic ferrite phase, and the ratio of the length of grain boundary cementite to the total grain boundary length is 10% or less. Patent Document 1 discloses that this steel material can ensure high strength and excellent low-temperature toughness.

[0005] The steel material proposed in Patent Document 2 is a steel plate for high-strength line pipe, and contains, in mass%, C: 0.02 to 0.20%, Si: 0.02 to 0.50%, Mn: 0.6 to 2.0%, P: 0.02% or less, S: 0.01% or less, Al: 0.010 to 0.080%, Nb: 0.002 to 0.060%, Ti: 0.003 to 0.030%, Ca: 0.0003 to 0.0060%, N: 0.0010 to 0.010%, REM: 0.0001 to 0.0300%, and Zr: 0.0001 to 0.0200%, with the balance being Fe and impurities. This steel further has an average grain size of 10 μm or less at the t / 4 (t: plate thickness) position, and a separation index SI measured from the fracture surface of a Charpy test piece at a specified temperature of 0.30 mm / mm 2 The following is the description: Patent Document 2 discloses that this steel material has high strength and can ensure a high limit CTOD value even when separation occurs.

[0006] The steel material proposed in Patent Document 3 is a steel plate for line pipe, which contains, in mass%, C: 0.02 to 0.10%, Si: 0.01 to 0.50%, Mn: 0.10 to 1.0%, P: 0.015% or less, S: 0.0020% or less, Ca: 0.0002 to 0.0050%, Nb: 0.03 to 0.15%, Ti: 0.002 to 0.070%, Al: 0.002 to 0.080%, and N: 0.001 to 0.008%, in a range in which the CP (= 4.46 [C] + 2.37 [Mn] / 6 + 22.36 [P]) value is 0.85 or less and the formula (0.8≦[Mn] / [Nb] ≦ 25) is satisfied, with the balance consisting of Fe and impurities. This steel further has a structure mainly composed of bainite. Patent Document 3 discloses that this steel has high strength, high toughness, and excellent HIC resistance.

[0007] JP 2010-174343 A JP 2015-190042 A International Publication No. 2016 / 056216

[0008] In recent years, the rise in carbon dioxide (CO2) concentrations on land has become a global problem. Therefore, efforts to curb CO2 emissions have been underway. Among these efforts to curb CO2 emissions, CCUS has been attracting particular attention. CCUS is an abbreviation for Carbon Dioxide Capture, Utilization, and Storage. That is, CCUS includes three technologies: CO2 capture, utilization, and storage. Among these, a CO2 storage technology that has attracted attention is one that captures CO2 emitted from industrial facilities such as power plants and factories and injects and stores the CO2 into depleted oil wells.

[0009] On the other hand, when injecting CO2 into depleted oil wells, the CO2 gas may be compressed and pressurized to a supercritical state to increase the injection efficiency. If CO2 leaks, the CO2 gas will transition from a supercritical state to a gaseous state, and its temperature will drop rapidly due to a sudden drop in pressure. In this case, the ambient temperature may fall far below normal, possibly even dropping to -80°C or below. Therefore, steel materials intended for application to such CO2 storage technologies are required to have not only high strength but also low-temperature toughness in cryogenic environments of -80°C or below. However, Patent Documents 1 to 3 do not consider low-temperature toughness in cryogenic environments of -80°C or below.

[0010] An object of the present disclosure is to provide a steel material having high strength and excellent low-temperature toughness in an extremely low-temperature environment of −80° C. or lower.

[0011] The steel material according to the present disclosure has, in mass %, C: 0.09% or less, Si: 0.60% or less, Mn: 1.0 to 2.5%, P: 0.05% or less, S: 0.005% or less, Ti: 0.005 to 0.050%, Al: 0.005 to 0.100%, N: 0.0050 to 0.0150%, O: 0.0050% or less, Cu: 0 to 0.50%, Ni: 0 to 0.50%, Cr: 0 to 0.50%, Mo: 0 to 0.50%, Co: 0 to 0.25%, V: 0 to 0.10%, Nb: 0 to 0.040%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, B: 0 to less than 0.0005%, rare earth elements: 0 to less than 0.0005%, Sn: 0 to 0.015%, As: 0 to 0.015%, Sb: 0 to 0.015%, Bi: 0 to 0.015%, and the balance: Fe and impurities, wherein Fn1 defined by formula (1) is 5.0 or more, Fn2 defined by formula (2) is 1.80 or less, the yield strength is 448 MPa or more, and in the steel material, the number density of Ti-containing particles is 0.8×10 18 pieces / mm 3 and the proportion of Ti-containing particles having an equivalent circle diameter of 60 nm or less among the Ti-containing particles is 75% or more. Fn1=Ti×N×10 5 (1) Fn2=Ti / N (2) Here, the element symbols in formulas (1) and (2) are substituted with the contents of the corresponding elements in units of mass %.

[0012] The steel material according to the present disclosure has high strength and excellent low-temperature toughness in cryogenic environments of −80° C. or below.

[0013] FIG. 1 is a graph showing the relationship between the percentage (%) of fine Ti-containing particles (Ti-containing particles having an equivalent circle diameter of 60 nm or less) and the fracture appearance transition temperature (° C.), which is an index of low-temperature toughness.

[0014] First, the inventors considered obtaining a steel material having a high strength, i.e., a yield strength of 448 MPa or more. Next, the inventors investigated, from the standpoint of chemical composition, steel materials having a yield strength of 448 MPa or more and excellent low-temperature toughness in an extremely low-temperature environment of -80°C or less. As a result, the following contents were found in mass %: C: 0.09% or less, Si: 0.60% or less, Mn: 1.0 to 2.5%, P: 0.05% or less, S: 0.005% or less, Ti: 0.005 to 0.050%, Al: 0.005 to 0.100%, N: 0.0050 to 0.0150%, O: 0.0050% or less, Cu: 0 to 0.50%, Ni: 0 to 0.50%, Cr: 0 to 0.50%, Mo: 0 to 0.50%, Co: 0 to 0.25%, V: 0 to 0.10%, Nb: 0 The present inventors considered that a steel material having a chemical composition consisting of: Cr: 0 to 0.040%, Ca: 0 to 0.0100%, Mg: 0 to less than 0.0005%, rare earth elements: 0 to less than 0.0005%, Sn: 0 to 0.015%, As: 0 to 0.015%, Sb: 0 to 0.015%, Bi: 0 to 0.015%, and the balance: Fe and impurities, may be able to achieve both a yield strength of 448 MPa or more and excellent low-temperature toughness in an extremely low-temperature environment.

[0015] Next, the inventors investigated various methods for improving the low-temperature toughness of steel having the above-mentioned chemical composition. Specifically, the inventors focused on precipitates and inclusions in the steel and investigated methods for improving the low-temperature toughness of the steel. As a result, it was found that the low-temperature toughness of steel having the above-mentioned chemical composition can be improved if a large number of Ti-containing particles can be dispersed. In this specification, "Ti-containing particles" means particles having a Ti content of 50% or more by mass, as specified by the method described below.

[0016] Here, in a steel material having the above-mentioned chemical composition, most of the Ti-containing particles are Ti nitrides. Ti nitrides are finely dispersed in the steel material and refine the crystal grains of the steel material by a pinning effect. As a result, it is possible to improve the low-temperature toughness of a steel material having the above-mentioned chemical composition. Furthermore, in a steel material having the above-mentioned chemical composition, the finer the Ti-containing particles, the higher the number density of the Ti-containing particles. Therefore, the inventors have conducted detailed studies on the number density of Ti-containing particles to effectively obtain the effect of refining the crystal grains. As a result, it was found that the number density of Ti-containing particles is 0.8 × 10 18 pieces / mm 3 It has become clear that if the above conditions are met, the low temperature toughness of the steel material can be improved.

[0017] Furthermore, the inventors adjusted the chemical composition to reduce the number density of Ti-containing particles (Ti nitrides) to 0.8 × 10 18 pieces / mm 3 As a result of detailed studies by the present inventors, it has become clear that the number density of Ti-containing particles can be increased by adjusting the contents of Ti and N. Specifically, in the steel material according to this embodiment, Fn1 defined by the following formula (1) is set to 5.0 or more, and Fn2 defined by the following formula (2) is set to 1.80 or less. As a result, in the steel material according to this embodiment, the number density of Ti-containing particles in the steel material is set to 0.8 × 10 18 pieces / mm 3 Fn1=Ti×N×10 5 (1) Fn2=Ti / N (2) Here, the element symbols in formulas (1) and (2) are substituted with the contents of the corresponding elements in units of mass %.

[0018] Fn1 (=Ti×N×10 5 ) is an index of the driving force for precipitation of Ti nitrides. The higher Fn1 is, the higher the driving force for precipitation of Ti nitrides is, and more Ti nitride nuclei are formed during precipitation. The more Ti nitride precipitation nuclei there are, the higher the number density of Ti nitrides is. Therefore, in the steel material according to this embodiment, Fn1 is set to 5.0 or more. As a result, provided that the other configurations of this embodiment are satisfied, the number density of Ti-containing particles in the steel material is set to 0.8 × 10 18 pieces / mm 3It can be increased even further.

[0019] Fn2 (=Ti / N) is an index showing the rate of Ostwald ripening of Ti nitrides. In steel materials having the above-mentioned chemical composition, Ti nitrides precipitated or crystallized during solidification may coarsen due to Ostwald ripening during subsequent heating in hot working or the like. Here, in Ostwald ripening, one coarse Ti nitride is formed from multiple Ti nitrides. In other words, the more difficult Ostwald ripening is, the more likely it is that the number density of Ti-containing particles will increase.

[0020] Here, the smaller Fn2 is, the slower the rate of Ostwald ripening is. In other words, the smaller Fn2 is, the less likely Ti nitrides are to become coarse due to Ostwald ripening, even when the same heat treatment is performed. As a result, the number density of Ti nitrides increases in the manufactured steel material. Therefore, in the steel material according to this embodiment, Fn2 is set to 1.80 or less. As a result, the number density of Ti-containing particles in the steel material is set to 0.8 × 10, provided that the other configurations of this embodiment are satisfied. 18 pieces / mm 3 It can be increased even further.

[0021] On the other hand, as a result of detailed investigations by the present inventors, it has been found that the steel material has the above-mentioned chemical composition, Fn1 is 5.0 or more, Fn2 is 1.80 or less, and the number density of Ti-containing particles in the steel material is 0.8 × 10 18 pieces / mm 3 Even if the steel has a yield strength of 448 MPa or more, there are cases where excellent low-temperature toughness cannot be obtained in a cryogenic environment of −80° C. or less. Therefore, the present inventors further investigated a method for improving the low-temperature toughness of the above-mentioned steel in a cryogenic environment of −80° C. or less.

[0022] It has been thought that fine Ti nitrides have almost no effect on the low-temperature toughness of steel materials. However, in an extremely low-temperature environment such as -80°C, the size of Ti nitrides may have a significant effect. In other words, the present inventors have considered that the low-temperature toughness of steel materials may be improved even in an extremely low-temperature environment of -80°C by controlling the size of Ti nitrides rather than simply increasing the number density of Ti nitrides.

[0023] Specifically, the inventors have focused on the distribution of the equivalent circle diameters of Ti-containing particles and investigated various methods for improving low-temperature toughness in a cryogenic environment of -80°C or below. As a result, the inventors have found that if the number ratio of Ti-containing particles having an equivalent circle diameter of 60 nm or less among the Ti-containing particles is 75% or more, the low-temperature toughness of a steel material can be improved even in a cryogenic environment of -80°C. Hereinafter, Ti-containing particles having an equivalent circle diameter of 60 nm or less will also be referred to as "fine Ti-containing particles." Furthermore, the number ratio of fine Ti-containing particles among the Ti-containing particles will also be simply referred to as "number ratio of fine Ti-containing particles." Below, the relationship between the number ratio (%) of fine Ti-containing particles and low-temperature toughness will be specifically explained using drawings.

[0024] Fig. 1 is a diagram showing the relationship between the number ratio (%) of fine Ti-containing particles (Ti-containing particles having an equivalent circle diameter of 60 nm or less) and the fracture appearance transition temperature (°C), which is an index of low-temperature toughness. Fig. 1 shows a steel material having the above-mentioned chemical composition, Fn1 of 5.0 or more, Fn2 of 1.80 or less, and a number density of Ti-containing particles in the steel material of 0.8 × 10 18 pieces / mm 3 The above was done using the percentage (%) of fine Ti-containing particles determined by the method described later and the fracture appearance transition temperature (°C) determined by the method described later for steel materials having a yield strength of 448 MPa or more.

[0025] 1 , the steel material has the above-mentioned chemical composition, Fn1 is 5.0 or more, Fn2 is 1.80 or less, and the number density of Ti-containing particles in the steel material is 0.8×10 18 pieces / mm 3 In a steel material having a yield strength of 448 MPa or more, if the number ratio of fine Ti-containing particles is 75% or more, the fracture appearance transition temperature becomes −80° C. or less, and it can be confirmed that the steel material has excellent low-temperature toughness in an extremely low-temperature environment of −80° C. or less. Therefore, the steel material according to this embodiment has the above-mentioned chemical composition, Fn1 is 5.0 or more, Fn2 is 1.80 or less, the yield strength is 448 MPa or more, and the number density of Ti-containing particles in the steel material is 0.8×10 18 pieces / mm 3Furthermore, the proportion of Ti-containing particles having an equivalent circle diameter of 60 nm or less among the Ti-containing particles is 75% or more. As a result, the steel material according to this embodiment has high strength and excellent low-temperature toughness in an extremely low-temperature environment of −80° C. or less.

[0026] The gist of the steel material according to this embodiment, which was completed based on the above findings, is as follows.

[0027] [1] In mass%, C: 0.09% or less, Si: 0.60% or less, Mn: 1.0 to 2.5%, P: 0.05% or less, S: 0.005% or less, Ti: 0.005 to 0.050%, Al: 0.005 to 0.100%, N: 0.0050 to 0.0150%, O: 0.0050% or less, Cu: 0 to 0.50%, Ni: 0 to 0.50%, Cr: 0 to 0.50%, Mo: 0 to 0.50%, Co: 0 to 0.25%, V: 0 to 0.10%, Nb: 0 to 0.040%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, B: 0 to less than 0.0005%, rare earth elements: 0 to less than 0.0005%, Sn: 0 to 0.015%, As: 0 to 0.015%, Sb: 0 to 0.015%, Bi: 0 to 0.015%, and the balance: Fe and impurities, wherein Fn1 defined by formula (1) is 5.0 or more, Fn2 defined by formula (2) is 1.80 or less, the yield strength is 448 MPa or more, and in the steel material, the number density of Ti-containing particles is 0.8×10 18 pieces / mm 3 and the proportion of Ti-containing particles having an equivalent circle diameter of 60 nm or less is 75% or more. Fn1=Ti×N×10 5 (1) Fn2=Ti / N (2) Here, the element symbols in formulas (1) and (2) are substituted with the contents of the corresponding elements in units of mass %.

[0028] [2] The steel material according to [1], comprising Cu: 0.01 to 0.50%, Ni: 0.01 to 0.50%, Cr: 0.01 to 0.50%, Mo: 0.01 to 0.50%, Co: 0.01 to 0.25%, V: 0.01 to 0.10%, Nb: 0.001 to 0.040%, Ca: 0.0001 to 0.0100%, Mg: 0.0001 to 0.0100%, B: 0.0001 to less than 0.0005%, rare earth elements: 0.0001 to less than 0.0005%, Sn: 0.001 to 0.015%, As: 0.001 to 0.015%, A steel material containing one or more elements selected from the group consisting of Sb: 0.001 to 0.015% and Bi: 0.001 to 0.015%.

[0029] [3] The steel material according to [2], containing Nb: 0.010 to 0.040%, wherein the number density of Nb-containing particles in the steel material is 4.4 × 10 18 pieces / mm 3 That's it, steel.

[0030] [4] The steel material according to any one of [1] to [3], wherein the steel material is a seamless steel pipe.

[0031] The shape of the steel material according to this embodiment is not particularly limited. The steel material according to this embodiment may be a steel pipe, a round bar (solid material), or a steel plate. The round bar refers to a steel bar having a circular cross section perpendicular to the axial direction. The steel pipe may be a seamless steel pipe or a welded steel pipe.

[0032] The steel material according to this embodiment will be described in detail below.

[0033] [Chemical Composition] The chemical composition of the steel material according to this embodiment contains the following elements: "%" relating to elements means mass % unless otherwise specified.

[0034] C: 0.09% or less Carbon (C) is inevitably contained. That is, the lower limit of the C content is more than 0%. C improves the hardenability of the steel material and increases the strength of the steel material. On the other hand, if the C content is too high, even if the contents of other elements are within the ranges of this embodiment, the strength of the steel material becomes too high and the low-temperature toughness of the steel material decreases. Therefore, the C content is 0.09% or less. A preferred lower limit of the C content is 0.01%, and more preferably 0.02%. A preferred upper limit of the C content is 0.08%, and more preferably 0.07%.

[0035] Si: 0.60% or less Silicon (Si) is inevitably contained. That is, the lower limit of the Si content is more than 0%. Si deoxidizes steel. On the other hand, if the Si content is too high, the hot workability of the steel material will deteriorate even if the contents of other elements are within the ranges of this embodiment. Therefore, the Si content is 0.60% or less. A preferred upper limit of the Si content is 0.58%, more preferably 0.55%, and even more preferably 0.45%. A preferred lower limit of the Si content to more effectively obtain the above effects is 0.01%, more preferably 0.02%, and even more preferably 0.04%.

[0036] Mn: 1.0 to 2.5% Manganese (Mn) improves the hardenability of steel and increases its strength. Mn also improves the hot workability of steel. If the Mn content is too low, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Mn content is too high, the hardenability of the steel becomes too high, and the low-temperature toughness of the steel decreases, even if the contents of other elements are within the ranges of this embodiment. Therefore, the Mn content is 1.0 to 2.5%. A preferred lower limit of the Mn content is 1.1%, more preferably 1.2%, and even more preferably 1.3%. A preferred upper limit of the Mn content is 2.3%, more preferably 2.0%, and even more preferably 1.9%.

[0037] P: 0.05% or less Phosphorus (P) is an unavoidable impurity. That is, the lower limit of the P content is greater than 0%. If the P content is too high, even if the contents of other elements are within the ranges of this embodiment, P segregates at grain boundaries, reducing the low-temperature toughness of the steel material. Therefore, the P content is 0.05% or less. A preferred upper limit of the P content is 0.04%, more preferably 0.03%, and even more preferably 0.02%. The P content should be as low as possible. However, an extreme reduction in the P content significantly increases manufacturing costs. Therefore, considering industrial production, a preferred lower limit of the P content is 0.01%.

[0038] S: 0.005% or less Sulfur (S) is an unavoidable impurity. That is, the lower limit of the S content is more than 0%. If the S content is too high, even if the contents of other elements are within the ranges of this embodiment, coarse sulfides are formed in the steel material, and the low-temperature toughness of the steel material decreases. Therefore, the S content is 0.005% or less. A preferred upper limit of the S content is 0.004%, more preferably 0.003%. The S content should preferably be as low as possible. However, an extreme reduction in the S content significantly increases manufacturing costs. Therefore, considering industrial production, a preferred lower limit of the S content is 0.001%.

[0039] Ti: 0.005 to 0.050% Titanium (Ti) combines with nitrogen to form Ti-containing particles, thereby improving the low-temperature toughness of steel materials in cryogenic environments. If the Ti content is too low, the above effect cannot be sufficiently achieved, even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Ti content is too high, even if the contents of other elements are within the ranges of this embodiment, the Ti-containing particles become coarse, thereby reducing the low-temperature toughness of steel materials in cryogenic environments. Therefore, the Ti content is 0.005 to 0.050%. The preferred lower limit of the Ti content is more than 0.005%, more preferably 0.006%, and even more preferably 0.007%. The preferred upper limit of the Ti content is 0.040%, more preferably 0.035%, even more preferably 0.030%, and even more preferably 0.028%.

[0040] Al: 0.005 to 0.100% Aluminum (Al) deoxidizes steel. If the Al content is too low, the above effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Al content is too high, coarse oxide-based inclusions are formed, reducing the low-temperature toughness of the steel material even if the contents of other elements are within the ranges of this embodiment. Therefore, the Al content is 0.005 to 0.100%. A preferred lower limit of the Al content is 0.010%, more preferably 0.015%, and even more preferably 0.020%. A preferred upper limit of the Al content is 0.090%, more preferably 0.080%, and even more preferably 0.070%. As used herein, the "Al" content refers to the content of "acid-soluble Al," that is, "sol. Al."

[0041] N: 0.0050 to 0.0150% Nitrogen (N) combines with Ti to form Ti-containing particles, thereby improving the low-temperature toughness of the steel in a cryogenic environment. If the N content is too low, the above effect cannot be fully achieved, even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the N content is too high, the amount of N dissolved in the steel increases, even if the contents of other elements are within the ranges of this embodiment, thereby reducing the low-temperature toughness of the steel in a cryogenic environment. Therefore, the N content is 0.0050 to 0.0150%. The preferred lower limit of the N content is 0.0053%, more preferably 0.0055%, and even more preferably 0.0060%. The preferred upper limit of the N content is 0.0148%, more preferably 0.0146%, and even more preferably 0.0140%.

[0042] O: 0.0050% or less Oxygen (O) is an unavoidable impurity. That is, the lower limit of the O content is greater than 0%. If the O content is too high, oxides are formed, even if the contents of other elements are within the ranges of this embodiment, and the low-temperature toughness of the steel material decreases. Therefore, the O content is 0.0050% or less. A preferred upper limit of the O content is 0.0048%, more preferably 0.0045%, and even more preferably 0.0040%. The O content is preferably as low as possible. However, an extreme reduction in the O content increases manufacturing costs. Therefore, considering industrial production, a preferred lower limit of the O content is 0.0001%, more preferably 0.0003%, and even more preferably 0.0005%.

[0043] The balance of the chemical composition of the steel material according to the present embodiment is composed of Fe and impurities. Here, the impurities in the chemical composition refer to substances that are mixed in from raw materials such as ore and scrap, or the manufacturing environment, during industrial production of the steel material, and are acceptable within a range that does not adversely affect the steel material according to the present embodiment.

[0044] [Optional Elements] The chemical composition of the steel material according to this embodiment may further contain, in place of a portion of Fe, one or more elements selected from the group consisting of Cu, Ni, Cr, Mo, and Co. All of these elements are optional elements, and improve the hardenability and strength of the steel material.

[0045] Cu: 0 to 0.50% Copper (Cu) is an optional element and does not necessarily need to be contained. That is, the Cu content may be 0%. When contained, Cu improves the hardenability of the steel material and increases its strength. Even if even a small amount of Cu is contained, the above effects can be obtained to some extent. However, if the Cu content is too high, even if the contents of other elements are within the ranges of this 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%. The preferred lower limit of the Cu content is more than 0%, more preferably 0.01%, even more preferably 0.03%, and even more preferably 0.05%. The preferred upper limit of the Cu content is 0.48%, even more preferably 0.46%.

[0046] Ni: 0 to 0.50% Nickel (Ni) is an optional element and does not necessarily need to be contained. That is, the Ni content may be 0%. When contained, Ni improves the hardenability of the steel material and increases its strength. Even if even a small amount of Ni is contained, the above effects can be obtained to some extent. However, if the Ni content is too high, even if the contents of other elements are within the ranges of this embodiment, the hardenability of the steel material becomes too high and the low-temperature toughness of the steel material decreases. Therefore, the Ni content is 0 to 0.50%. The preferred lower limit of the Ni content is more than 0%, more preferably 0.01%, even more preferably 0.03%, and even more preferably 0.05%. The preferred upper limit of the Ni content is 0.48%, even more preferably 0.46%.

[0047] Cr: 0 to 0.50% Chromium (Cr) is an optional element and does not necessarily need to be contained. That is, the Cr content may be 0%. When contained, Cr improves the hardenability of the steel material and increases its strength. Even if even a small amount of Cr is contained, the above effects can be obtained to some extent. However, if the Cr content is too high, even if the contents of other elements are within the ranges of this embodiment, the hardenability of the steel material will be too high and the low-temperature toughness of the steel material will decrease. Therefore, the Cr content is 0 to 0.50%. The preferred lower limit of the Cr content is more than 0%, more preferably 0.01%, even more preferably 0.03%, even more preferably 0.05%, and even more preferably 0.08%. The preferred upper limit of the Cr content is 0.48%, even more preferably 0.46%.

[0048] Mo: 0 to 0.50% Molybdenum (Mo) is an optional element and does not necessarily need to be contained. That is, the Mo content may be 0%. When contained, Mo improves the hardenability of the steel material and increases its strength. Even if even a small amount of Mo is contained, the above effects can be obtained to some extent. However, if the Mo content is too high, even if the contents of other elements are within the ranges of this embodiment, the hardenability of the steel material will be too high and the low-temperature toughness of the steel material will decrease. Therefore, the Mo content is 0 to 0.50%. The preferred lower limit of the Mo content is more than 0%, more preferably 0.01%, even more preferably 0.03%, and even more preferably 0.05%. The preferred upper limit of the Mo content is 0.48%, even more preferably 0.46%.

[0049] Co: 0 to 0.25% Cobalt (Co) is an optional element and does not necessarily need to be contained. That is, the Co content may be 0%. When contained, Co improves the hardenability of the steel material and increases its strength. Even if even a small amount of Co is contained, the above effects can be obtained to some extent. However, if the Co content is too high, even if the contents of other elements are within the ranges of this embodiment, the hardenability of the steel material becomes too high and the low-temperature toughness of the steel material decreases. Therefore, the Co content is 0 to 0.25%. The preferred lower limit of the Co content is more than 0%, more preferably 0.01%, even more preferably 0.03%, and even more preferably 0.05%. The preferred upper limit of the Co content is 0.22%, even more preferably 0.20%.

[0050] The chemical composition of the steel material according to this embodiment may further contain one or more elements selected from the group consisting of V and Nb in place of a portion of Fe. All of these elements are optional elements, and form carbides or the like in the steel material to increase the strength of the steel material.

[0051] V: 0 to 0.10% Vanadium (V) is an optional element and does not necessarily need to be contained. That is, the V content may be 0%. When contained, V forms carbides and the like to increase the strength of the steel material. Even if even a small amount of V is contained, the above effect can be obtained to some extent. However, if the V content is too high, even if the contents of other elements are within the ranges of this embodiment, excessive carbides and the like will be formed, reducing the corrosion resistance of the steel material. Therefore, the V content is 0 to 0.10%. The preferred lower limit of the V content is more than 0%, more preferably 0.01%, and even more preferably 0.02%. The preferred upper limit of the V content is 0.09%, more preferably 0.08%, and even more preferably 0.07%.

[0052] Nb: 0 to 0.040% Niobium (Nb) is an optional element and does not necessarily need to be contained. That is, the Nb content may be 0%. When contained, Nb forms carbides and the like to increase the strength of the steel material. Even if even a small amount of Nb is contained, the above effect can be obtained to some extent. However, if the Nb content is too high, even if the contents of other elements are within the ranges of this embodiment, excessive carbides and the like will be formed, and the corrosion resistance of the steel material will decrease. Therefore, the Nb content is 0 to 0.040%. The preferred lower limit of the Nb content is more than 0%, more preferably 0.001%, even more preferably 0.002%, and even more preferably 0.005%.

[0053] In this embodiment, it is more preferable that the Nb content be 0.010 to 0.040%. In this case, provided that the other configurations of this embodiment are satisfied, the number density of Nb-containing particles described later is stably 4.4 × 10 18 pieces / mm 3 Therefore, in the chemical composition of the steel material according to this embodiment, the lower limit of the Nb content is more preferably 0.010%, even more preferably 0.012%, and even more preferably 0.015%. The upper limit of the Nb content is more preferably 0.039%, even more preferably 0.038%, even more preferably 0.035%, and even more preferably 0.030%.

[0054] The chemical composition of the steel material according to this embodiment may further contain one or more elements selected from the group consisting of Ca and Mg in place of a portion of Fe. These elements are optional elements and enhance the low-temperature toughness of the steel material.

[0055] Ca: 0 to 0.0100% Calcium (Ca) is an optional element and does not necessarily need to be contained. That is, the Ca content may be 0%. When contained, Ca renders S in the steel harmless as sulfides, thereby improving the low-temperature toughness of the steel. Even if even a small amount of Ca is contained, the above effect can be achieved to some extent. However, if the Ca content is too high, even if the contents of other elements are within the ranges of this embodiment, coarse inclusions will form, reducing the low-temperature toughness of the steel. Therefore, the Ca content is 0 to 0.0100%. The preferred lower limit of the Ca content is over 0%, more preferably 0.0001%, even more preferably 0.0003%, even more preferably 0.0005%, even more preferably 0.0006%, even more preferably 0.0008%, and even more preferably 0.0009%. The preferred upper limit of the Ca content is 0.0090%, even more preferably 0.0080%.

[0056] Mg: 0 to 0.0100% Magnesium (Mg) is an optional element and does not necessarily need to be contained. That is, the Mg content may be 0%. When contained, Mg renders S in the steel harmless as sulfides, thereby improving the low-temperature toughness of the steel. Even if even a small amount of Mg is contained, the above effect can be achieved to some extent. However, if the Mg content is too high, even if the contents of other elements are within the ranges of this embodiment, coarse inclusions will form, reducing the low-temperature toughness of the steel. Therefore, the Mg content is 0 to 0.0100%. The preferred lower limit of the Mg content is greater than 0%, more preferably 0.0001%, even more preferably 0.0003%, even more preferably 0.0005%, even more preferably 0.0006%, even more preferably 0.0008%, and even more preferably 0.0009%. The preferred upper limit of the Mg content is 0.0090%, even more preferably 0.0080%.

[0057] The chemical composition of the steel material according to this embodiment may further contain, in place of a portion of Fe, one or more elements selected from the group consisting of B and rare earth elements. All of these elements are optional elements and improve the hot workability of the steel material.

[0058] B: 0 to less than 0.0005% Boron (B) is an optional element and does not necessarily need to be contained. That is, the B content may be 0%. When contained, B suppresses the segregation of S to grain boundaries in the steel material, improving the hot workability of the steel material. Even if even a small amount of B is contained, the above effect can be obtained to some extent. However, if the B content is too high, even if the contents of other elements are within the ranges of this embodiment, coarse nitrides are formed, reducing the low-temperature toughness of the steel material. Therefore, the B content is 0 to less than 0.0005%. The preferred lower limit of the B content is more than 0%, and more preferably 0.0001%. The preferred upper limit of the B content is 0.0004%, and more preferably 0.0003%.

[0059] Rare earth elements (REM): 0 to less than 0.0005% Rare earth elements (REM) are optional elements and do not necessarily need to be contained. That is, the REM content may be 0%. When contained, REM renders S in the steel harmless as sulfides, improving the hot workability of the steel. Even if even a small amount of REM is contained, the above effects can be obtained to some extent, even if the contents of other elements are within the ranges of this embodiment. However, if the REM content is too high, coarse oxides are formed, even if the contents of other elements are within the ranges of this embodiment, and the low-temperature toughness of the steel decreases. Therefore, the REM content is 0 to less than 0.0005%. A preferred lower limit of the REM content is more than 0%, more preferably 0.0001%. A preferred upper limit of the REM content is 0.0004%, more preferably 0.0003%.

[0060] In this specification, REM refers to one or more elements selected from the group consisting of scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanoids lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. In addition, in this specification, the REM content refers to the total content of these elements.

[0061] The chemical composition of the steel material according to this embodiment may further contain one or more elements selected from the group consisting of Sn, As, Sb, and Bi in place of a portion of Fe. All of these elements are optional elements, and enhance the corrosion resistance of the steel material.

[0062] Sn: 0 to 0.015% Tin (Sn) is an optional element and does not necessarily need to be contained. That is, the Sn content may be 0%. When contained, Sn enhances the corrosion resistance of the steel material. Even if even a small amount of Sn is contained, the above effect can be obtained to some extent. However, if the Sn content is too high, the hot workability of the steel material will decrease even if the contents of other elements are within the ranges of this embodiment. Therefore, the Sn content is 0 to 0.015%. The preferred lower limit of the Sn content is more than 0%, more preferably 0.001%, and even more preferably 0.002%. The preferred upper limit of the Sn content is 0.010%, more preferably 0.008%, and even more preferably 0.005%.

[0063] As: 0 to 0.015% Arsenic (As) is an optional element and does not necessarily need to be contained. That is, the As content may be 0%. When contained, As enhances the corrosion resistance of the steel material. Even if even a small amount of As is contained, the above effect can be obtained to some extent. However, if the As content is too high, the hot workability of the steel material will deteriorate even if the contents of other elements are within the ranges of this embodiment. Therefore, the As content is 0 to 0.015%. The preferred lower limit of the As content is more than 0%, more preferably 0.001%, and even more preferably 0.002%. The preferred upper limit of the As content is 0.010%, more preferably 0.008%, and even more preferably 0.005%.

[0064] Sb: 0 to 0.015% Antimony (Sb) is an optional element and does not necessarily need to be contained. That is, the Sb content may be 0%. When contained, Sb enhances the corrosion resistance of the steel material. Even if even a small amount of Sb is contained, the above effect can be obtained to some extent. However, if the Sb content is too high, the hot workability of the steel material will deteriorate even if the contents of other elements are within the ranges of this embodiment. Therefore, the Sb content is 0 to 0.015%. The preferred lower limit of the Sb content is more than 0%, more preferably 0.001%, and even more preferably 0.002%. The preferred upper limit of the Sb content is 0.010%, more preferably 0.008%, and even more preferably 0.005%.

[0065] Bi: 0 to 0.015% Bismuth (Bi) is an optional element and does not necessarily need to be contained. That is, the Bi content may be 0%. When contained, Bi enhances the corrosion resistance of the steel material. Even if even a small amount of Bi is contained, the above effect can be obtained to some extent. However, if the Bi content is too high, the hot workability of the steel material will decrease even if the contents of other elements are within the ranges of this embodiment. Therefore, the Bi content is 0 to 0.015%. The preferred lower limit of the Bi content is more than 0%, more preferably 0.001%, and even more preferably 0.002%. The preferred upper limit of the Bi content is 0.010%, more preferably 0.008%, and even more preferably 0.005%.

[0066] [Fn1] The steel material according to this embodiment has the above-described chemical composition, and has Fn1 defined by the following formula (1) of 5.0 or more: Fn1=Ti×N×10 5 (1) Here, the element symbols in formula (1) are substituted with the contents of the corresponding elements in units of mass %.

[0067] Fn1 (=Ti×N×10 5) is an index of the driving force for precipitation of Ti nitrides. The higher Fn1 is, the higher the driving force for precipitation of Ti nitrides is, and more Ti nitride nuclei are formed during precipitation. The more Ti nitride precipitation nuclei there are, the higher the number density of Ti nitrides is. Therefore, in the steel material according to this embodiment, Fn1 is set to 5.0 or more. As a result, provided that the other configurations of this embodiment are satisfied, the number density of Ti-containing particles in the steel material is set to 0.8 × 10 18 pieces / mm 3 It can be increased even further.

[0068] The lower limit of Fn1 is preferably 5.1, and more preferably 5.3. The upper limit of Fn1 is not particularly limited, but may be, for example, 50.0. The upper limit of Fn1 may be 40.0. Fn1 is calculated by rounding the obtained numerical value to one decimal place.

[0069] [Fn2] The steel material according to this embodiment has the above-described chemical composition, Fn1 is 5.0 or more, and Fn2 defined by the following formula (2) is 1.80 or less: Fn2=Ti / N (2) Here, the element symbols in formula (2) are substituted with the contents of the corresponding elements in units of mass%.

[0070] Fn2 (=Ti / N) is an index showing the rate of Ostwald ripening of Ti nitrides. The smaller Fn2, the slower the rate of Ostwald ripening. In other words, the smaller Fn2, the less likely Ti nitrides are to become coarse due to Ostwald ripening, even when the same heat treatment is performed. As a result, the number density of Ti nitrides increases in the manufactured steel material. Therefore, in the steel material according to this embodiment, Fn2 is set to 1.80 or less. As a result, the number density of Ti-containing particles in the steel material is set to 0.8 × 10, provided that the other configurations of this embodiment are satisfied. 18 pieces / mm 3 It can be increased even further.

[0071] The upper limit of Fn2 is preferably 1.79, and more preferably 1.78. The lower limit of Fn2 is not particularly limited, but is substantially 0.33. The lower limit of Fn2 may be 0.35. Fn2 is calculated by rounding the obtained value to two decimal places.

[0072] [Yield Strength] The yield strength of the steel material according to this embodiment is 448 MPa or more (65 ksi or more). The upper limit of the yield strength of the steel material according to this embodiment is not particularly limited, but is, for example, 586 MPa (85 ksi). In other words, the yield strength of the steel material according to this embodiment may be 448 to 586 MPa. A preferred lower limit of the yield strength is 450 MPa, more preferably 455 MPa, and even more preferably 460 MPa. The upper limit of the yield strength may be 552 MPa (80 ksi) or 517 MPa (75 ksi).

[0073] In this embodiment, the tensile strength and yield ratio of the steel material are not particularly limited. The tensile strength of the steel material is, for example, 483 to 689 MPa. The yield ratio of the steel material is, for example, 0.75 or more. If the yield ratio of the steel material is 0.75 or more, the low-temperature toughness of the steel material can be stably increased. Therefore, in this embodiment, the yield ratio is preferably 0.75 to 1.00. The yield ratio (%) can be determined as the ratio of the yield strength to the tensile strength.

[0074] In this embodiment, the yield strength, tensile strength, and yield ratio of a steel material are determined by the following method. Specifically, a tensile test is performed according to ASTM E8 / E8M (2022). A test specimen is prepared from the steel material according to this embodiment. When the steel material is a steel plate, a tensile test specimen is prepared from the center of the plate thickness. In this case, the longitudinal direction of the tensile test specimen is parallel to the rolling direction of the steel plate. When the steel material is a steel pipe, a tensile test specimen is prepared from the center of the wall thickness. In this case, the longitudinal direction of the tensile test specimen is parallel to the axial direction of the steel pipe. When the steel material is a round bar, a tensile test specimen is prepared from the R / 2 position. In this specification, the R / 2 position of the round bar means the center position of the radius R in a cross section perpendicular to the axial direction of the round bar. In this case, the longitudinal direction of the tensile test specimen is parallel to the axial direction of the round bar.

[0075] The tensile test specimen is, for example, a round bar test specimen with a parallel portion diameter of 12.7 mm and a gauge length of 50.8 mm. Depending on the thickness of the steel material, round bar test specimens with smaller diameters or arc-shaped test specimens may also be used. A tensile test is performed using the prepared tensile test specimen at room temperature (25°C) in air, and the resulting 0.5% total elongation yield strength is defined as the yield strength (MPa). The maximum stress during uniform elongation is defined as the tensile strength (MPa). The ratio of the yield strength to the tensile strength is defined as the yield ratio. The yield strength (MPa) is calculated by rounding the obtained value to one decimal place. The tensile strength (MPa) is calculated by rounding the obtained value to one decimal place. The yield ratio is calculated by rounding the obtained value to two decimal places.

[0076] [Ti-containing particles] The steel material according to this embodiment has the above-described chemical composition, Fn1 is 5.0 or more, Fn2 is 1.80 or less, and the number density of Ti-containing particles in the steel material is 0.8 × 10 18 pieces / mm 3 The Ti-containing particles have a circle-equivalent diameter of 60 nm or less, and the number ratio of Ti-containing particles having a circle-equivalent diameter of 60 nm or less is 75% or more. As described above, in this specification, "Ti-containing particles" means particles having a Ti content, determined by the method described below, of 50% by mass or more.

[0077] As described above, in this specification, Ti-containing particles having an equivalent circle diameter of 60 nm or less are also referred to as "fine Ti-containing particles." Furthermore, in this embodiment, due to limitations of the measuring device, the equivalent circle diameter of the Ti-containing particles is 10 nm or more. Therefore, in the steel material according to this embodiment, the proportion of Ti-containing particles having an equivalent circle diameter of 10 to 60 nm (fine Ti-containing particles) among the Ti-containing particles having an equivalent circle diameter of 10 nm or more is 75% or more. In this embodiment, the upper limit of the equivalent circle diameter of the Ti-containing particles is not particularly limited, but is, for example, 500 nm. The upper limit of the equivalent circle diameter of the Ti-containing particles may be 400 nm or 300 nm.

[0078] As described above, the Ti-containing particles are finely dispersed in the steel material, and the grains of the steel material are refined by the pinning effect. As a result, the low-temperature toughness of the steel material is improved. In this embodiment, the number density of the Ti-containing particles in the steel material is set to 0.8 × 1018 pieces / mm 3 Not only is the number of Ti-containing fine particles increased to 75% or more, but the number of Ti-containing fine particles is also increased to 75% or more. As a result, excellent low-temperature toughness can be obtained even in an extremely low-temperature environment of -80°C or less.

[0079] In this embodiment, the preferred lower limit of the number density of Ti-containing particles is 0.9 × 10 18 pieces / mm 3 and more preferably 1.0 × 10 18 pieces / mm 3 and more preferably 1.2 × 10 18 pieces / mm 3 In this embodiment, the upper limit of the number density of the Ti-containing particles is not particularly limited, but is, for example, 200.0 × 10 18 pieces / mm 3 150.0×10 18 pieces / mm 3 120.0×10 18 pieces / mm 3 may be.

[0080] In this embodiment, the preferred lower limit of the number ratio of the fine Ti-containing particles is 76%, and more preferably 77%. In this embodiment, a higher number ratio of the fine Ti-containing particles is preferred. In other words, the upper limit of the number ratio of the fine Ti-containing particles may be 100%.

[0081] In this embodiment, the number density of Ti-containing particles is determined by the following method. A thin film test piece for observing Ti-containing particles is prepared from the steel material according to this embodiment. When the steel material is a steel plate, the thin film test piece is prepared from the center of the plate thickness. When the steel material is a steel pipe, the thin film test piece is prepared from the center of the wall thickness. When the steel material is a round bar, the thin film test piece is prepared from the R / 2 position. The thin film test piece is prepared by electrolytic polishing using the Twin Jet method. The size of the thin film test piece is not particularly limited as long as it can provide the observation field described below.

[0082] Arbitrary observation fields are identified from the observation surface of the obtained thin film test piece. The number of observation fields is preferably 10 or more. The identified observation fields are subjected to structure observation using a transmission electron microscope (hereinafter also referred to as "TEM"). Here, the area of ​​each observation field is 500 nm × 500 nm. The structure observation of each identified observation field is performed at an acceleration voltage of 200 kV under diffraction conditions suitable for particle observation (for example, 011 incidence with a diffraction vector g = 11-1).

[0083] Particles are identified in each observation field. The particles can be identified from the contrast. The equivalent circle diameter of each particle can be determined by image analysis of the observed image in TEM observation. In this embodiment, the equivalent circle diameter of the particles to be identified is not particularly limited. However, when observed under the above conditions, the equivalent circle diameter of the particles is, for example, 10 nm or more.

[0084] The identified particles are subjected to point analysis by energy dispersive X-ray spectrometry (EDS). The content of elements contained in each particle is determined by EDS point analysis. In EDS point analysis, the acceleration voltage is set to 200 kV, and the target elements are quantified as Ti, V, Cr, Mn, Fe, Ni, Mo, and Nb. Based on the EDS analysis results for each particle, particles having a Ti content of 50% or more by mass are identified as "Ti-containing particles."

[0085] The total number of Ti-containing particles identified in each observation field is counted. Furthermore, the volume (mm 3 The thickness of the observation field can be determined from the total integrated intensity of the electron energy loss intensity spectrum (EELS) and the integrated intensity of the zero-loss spectrum for the thin film specimen. The number density (particles / mm 3 In this embodiment, the number density (particles / mm 3 ) when calculating the unit is 1018 pieces / mm 3 The number density is calculated by using exponential notation and the mantissa is rounded off to the nearest tenth.

[0086] Furthermore, among the Ti-containing particles identified in each observation field, the number of Ti-containing particles (fine Ti-containing particles) having an equivalent circle diameter of 60 nm or less is counted. The number ratio (%) of fine Ti-containing particles is calculated from the total number of fine Ti-containing particles and the total number of Ti-containing particles. In this specification, the number ratio of fine Ti-containing particles is calculated by rounding the obtained percentage to one decimal place.

[0087] Here, in EDS point analysis of steel material having the above-mentioned chemical composition, accurate quantification of the nitrogen (N) content is often difficult with current technology. Therefore, in this embodiment, particles having a Ti content of 50 mass % or more are specified as Ti-containing particles. That is, the Ti-containing particles according to this embodiment may also include Ti compounds other than Ti nitrides. However, in steel material having the above-mentioned chemical composition, the amount of Ti compounds other than Ti nitrides (Ti carbides, Ti oxides, etc.) in the Ti-containing particles is negligibly small. That is, in this embodiment, the number density (particles / mm 3 ) is essentially the number density of Ti nitrides (pieces / mm 3 ) is equivalent to

[0088] [Nb-containing particles] Preferably, the steel material according to this embodiment has the above-described chemical composition, and further has an Nb content of 0.010 to 0.040%, Fn1 is 5.0 or more, Fn2 is 1.80 or less, and the number density of Ti-containing particles in the steel material is 0.8 × 10 18 pieces / mm 3 or more, the number ratio of Ti-containing particles having an equivalent circle diameter of 60 nm or less among the Ti-containing particles is 75% or more, and further, the number density of Nb-containing particles in the steel material is 4.4 × 10 18 pieces / mm 3 In this specification, the term "Nb-containing particles" refers to particles having an Nb content, determined by the method described below, of 50% by mass or more.

[0089] In the steel material according to this embodiment having the above-described chemical composition, most of the Nb-containing particles are either Nb carbide, Nb nitride, or Nb carbonitride. Hereinafter, in this specification, Nb carbide, Nb nitride, and Nb carbonitride are collectively referred to as "Nb carbonitride, etc." Nb carbonitride, etc. are finely dispersed in the steel material, and refine the crystal grains of the steel material by a pinning effect. Therefore, in the steel material according to this embodiment, the number density of the Nb-containing particles is further set to 4.4 × 10 18 pieces / mm 3 Therefore, the steel material according to this embodiment has an Nb content of 0.010 to 0.040%, and the number density of Nb-containing particles in the steel material is 4.4 × 10 or more, provided that the other configurations of this embodiment are satisfied. 18 pieces / mm 3 That's all.

[0090] In this embodiment, the size of the Nb-containing particles is not particularly limited. However, when the above-described chemical composition is used and the Nb content is 0.010 to 0.040%, the size of the Nb-containing particles is, for example, 10 to 500 nm in equivalent circle diameter. Preferably, the size of the Nb-containing particles is 10 to 200 nm in equivalent circle diameter.

[0091] In this embodiment, the preferred lower limit of the number density of Nb-containing particles is 4.5 × 10 18 pieces / mm 3 and more preferably 4.8 × 10 18 pieces / mm 3 and more preferably 5.0 × 10 18 pieces / mm 3 In this embodiment, the upper limit of the number density of the Nb-containing particles is not particularly limited, but is, for example, 100.0 × 10 18 pieces / mm 3 90.0×10 18 pieces / mm 3 80.0×10 18 pieces / mm 3 may be.

[0092] In this embodiment, the number density of Nb-containing particles is determined by the following method. Similar to the method for determining the number density of Ti-containing particles described above, a thin film test piece is prepared from the steel material according to this embodiment, and structural observation is performed using a TEM. Here, the conditions for structural observation are also similar to those for the method for determining the number density of Ti-containing particles described above. Furthermore, similar to the method for determining the number density of Ti-containing particles described above, particles are identified from the contrast. Furthermore, similar to the method for determining the number density of Ti-containing particles described above, EDS point analysis is performed on the identified particles to determine the content of elements contained in each particle. Based on the results of EDS analysis for each particle, particles having an Nb content of 50% or more by mass are identified as "Nb-containing particles."

[0093] The total number of Nb-containing particles identified in each observation field is counted to determine the density. Furthermore, similarly to the method for determining the number density of Ti-containing particles described above, the total volume (mm 3 Based on the total number of Nb-containing particles obtained and the total volume of the observation field, the number density of Nb-containing particles (particles / mm 3 In this embodiment, the number density of Nb-containing particles (particles / mm 3 ) when calculating the unit is 10 18 pieces / mm 3 The number density is calculated by using exponential notation and the mantissa is rounded off to the nearest tenth.

[0094] As mentioned above, in EDS point analysis of steel material having the above-mentioned chemical composition, accurate quantification of the nitrogen (N) content is often difficult with current technology. Furthermore, accurate quantification of the carbon (C) content is also often difficult. Therefore, in this embodiment, particles having an Nb content of 50 mass% or more are specified as Nb-containing particles. That is, the Nb-containing particles according to this embodiment may also include Nb compounds other than Nb carbonitrides and the like. However, in steel material having the above-mentioned chemical composition, the amount of Nb compounds other than Nb carbonitrides and the like in the Nb-containing particles is negligibly small. That is, in this embodiment, the number density (particles / mm 3 ) is essentially the number density (pieces / mm3 ) is equivalent to

[0095] [Low temperature toughness] The steel material according to this embodiment has the above-described chemical composition, Fn1 is 5.0 or more, Fn2 is 1.80 or less, and the number density of Ti-containing particles in the steel material is 0.8 × 10 18 pieces / mm 3 The proportion of Ti-containing particles having an equivalent circle diameter of 60 nm or less is 75% or more. As a result, the steel material according to this embodiment has a yield strength of 448 MPa or more and excellent low-temperature toughness in a cryogenic environment. In this embodiment, excellent low-temperature toughness in a cryogenic environment is defined as follows.

[0096] A Charpy impact test in accordance with ASTM E23 (2018) is performed on the steel material according to this embodiment. First, a V-notch test specimen is prepared from the steel material according to this embodiment in accordance with ASTM E23 (2018). Specifically, when the steel material is a steel plate, a V-notch test specimen is prepared from the center of the plate thickness, having a notch surface perpendicular to the plate thickness direction and a longitudinal direction parallel to the plate width direction. When the steel material is a steel pipe, a V-notch test specimen is prepared from the center of the wall thickness, having a notch surface perpendicular to the pipe axial direction and a longitudinal direction perpendicular to the pipe axial direction and the pipe radial direction. When the steel material is a round bar, a V-notch test specimen is prepared from the R / 2 position on a cross section perpendicular to the axial direction, having a notch surface perpendicular to the axial direction and a longitudinal direction perpendicular to the axial direction and the cross-sectional radial direction.

[0097] The prepared V-notch test specimens are subjected to a Charpy impact test in accordance with ASTM E23 (2018). Specifically, the test temperature is set to six levels (-120°C, -100°C, -80°C, -60°C, -40°C, and -20°C) that are changed in 20°C increments within the range of -120 to -20°C. The Charpy impact test is performed using two test specimens for each test temperature. Under the above conditions, the brittle fracture surface ratio (%) of the test specimens after testing at each temperature is determined. An approximation curve is obtained by plotting the test temperature (°C) and the obtained brittle fracture surface ratio (%). From the obtained approximation curve, the temperature (°C) at which the brittle fracture surface ratio becomes 50% is determined and defined as the fracture transition temperature vTrs (°C). In this embodiment, if the fracture transition temperature vTrs is -80°C or lower, it is determined that the specimen has excellent low-temperature toughness in a cryogenic environment.

[0098] The steel material according to this embodiment further satisfies the other configurations of this embodiment, and has an Nb content of 0.010 to 0.040%, and the number density of Nb-containing particles in the steel material is 4.4 × 10 18 pieces / mm 3 In this embodiment, if the fracture appearance transition temperature vTrs obtained by the above method is −122° C. or lower, the steel material according to this embodiment is determined to have even better low-temperature toughness in an extremely low-temperature environment.

[0099] [Shape of Steel Material] As described above, the shape of the steel material according to this embodiment is not particularly limited. Preferably, the steel material according to this embodiment is a seamless steel pipe. When the steel material according to this embodiment is a seamless steel pipe, even if the wall thickness is 5 mm or more, it has a yield strength of 448 MPa or more and excellent low-temperature toughness in an extremely low-temperature environment.

[0100] [Microstructure] Preferably, the microstructure of the steel material according to this embodiment is mainly composed of tempered bainite. More specifically, the volume fraction of tempered bainite in the microstructure of the steel material according to this embodiment is 90% or more. The remainder of the microstructure is, for example, ferrite or pearlite. Note that the microstructure of the steel material according to this embodiment may contain minute amounts of precipitates, inclusions, etc. in addition to tempered bainite, ferrite, and pearlite. However, in the microstructure of the steel material according to this embodiment, the volume fraction of precipitates, inclusions, etc. is negligibly small compared to the volume fractions of tempered bainite, ferrite, and pearlite.

[0101] When the volume fraction of tempered bainite is determined by observation, it can be determined by the following method. First, a test piece for microstructure observation is prepared from the steel material according to this embodiment. When the steel material is a steel plate, a test piece is prepared with an observation surface being a plane including the rolling direction and the plate thickness direction from the center of the plate thickness. When the steel material is a steel pipe, a test piece is cut out with an observation surface being a plane including the pipe axial direction and the pipe radial direction from the center of the wall thickness. When the steel material is a round bar, a test piece is cut out with an observation surface being a plane including the R / 2 position in the center and including the axial direction.

[0102] The observation surface of the test piece is polished to a mirror finish, and then immersed in a nital etching solution for about 10 seconds to reveal the structure by etching. The etched observation surface is observed in 10 fields of view as secondary electron images using a scanning electron microscope (SEM). The field area is, for example, 0.01 mm. 2 (Magnification: 1000x). In each field of view, tempered bainite is identified from the contrast. The area fraction of the identified tempered bainite is determined. The method for determining the area fraction is not particularly limited, and any well-known method may be used. For example, the area fraction of tempered bainite can be determined by image analysis. In this embodiment, the arithmetic mean value of the area fractions of tempered bainite determined in all fields of view is defined as the volume fraction of tempered bainite.

[0103] [Manufacturing method] A method for manufacturing a steel material according to this embodiment will be described below. A method for manufacturing a seamless steel pipe will be described below as an example of a steel material according to this embodiment. The method for manufacturing a seamless steel pipe includes a step of preparing a mother pipe (preparation step) and a step of subjecting the mother pipe to heat treatment to produce a seamless steel pipe (heat treatment step). Note that the manufacturing method according to this embodiment is not limited to the manufacturing method described below. Each step will be described in detail below.

[0104] [Preparation step] In the preparation step, an intermediate steel material having the above-mentioned chemical composition is prepared. As long as the intermediate steel material has the above-mentioned chemical composition, there is no particular limitation on the method for manufacturing the intermediate steel material. The intermediate steel material referred to here is a plate-shaped steel material if the final product is a steel plate, a mother pipe if the final product is a steel pipe, or a steel bar having a circular cross section perpendicular to the axial direction if the final product is a round steel bar.

[0105] The preparation step may include a step of preparing a material (material preparation step) and a step of hot working the material to produce an intermediate steel material (hot working step). Below, the case where the material preparation step and the hot working step are included will be described in detail.

[0106] [Material Preparation Step] In the material preparation step, a material is produced using molten steel having the above-described chemical composition. The method for producing the material is not particularly limited and may be a well-known method. Specifically, a cast piece (slab, bloom, or billet) may be produced using the molten steel by a continuous casting method. An ingot may be produced using the molten steel by an ingot casting method. If necessary, the slab, bloom, or ingot may be subjected to blooming to produce a billet. The material (slab, bloom, or billet) is produced through the above steps.

[0107] [Hot working process] In the hot working process, a prepared raw material is hot worked to produce an intermediate steel material. When the steel material is a seamless steel pipe, the intermediate steel material corresponds to a mother pipe. First, a billet is heated in a heating furnace. The billet extracted from the heating furnace is hot worked to produce a mother pipe (seamless steel pipe). At this time, the heating temperature of the billet is preferably 1100 to 1200°C, and the heating time is preferably 300 to 450 minutes. Here, the heating temperature corresponds to the temperature of the heat treatment furnace when the raw material is heated and held. The heating time means the time from when the raw material is charged into the heat treatment furnace to when it is extracted from the heat treatment furnace.

[0108] If the heating temperature is too high, Ti nitrides in the material may undergo Ostwald ripening, resulting in a decrease in the number density of Ti-containing particles in the steel. On the other hand, if the heating temperature is too low, the billet may not be heated sufficiently, placing too much strain on the hot working equipment. Therefore, in the hot working step according to this embodiment, the heating temperature is preferably set to 1100 to 1200°C.

[0109] If the heating time is too long, Ti nitrides in the material may undergo Ostwald ripening, resulting in a decrease in the number density of Ti-containing particles in the steel. On the other hand, if the heating time is too short, the billet may not be heated sufficiently, placing too much strain on the hot working equipment. Therefore, in the hot working step according to this embodiment, the heating time is preferably set to 300 to 450 minutes.

[0110] The raw material extracted from the heating furnace is hot worked to produce an intermediate steel material. Although the hot working method is not particularly limited, when the intermediate steel material is a mother pipe, it is preferable to carry out the Mannesmann process to produce the mother pipe. In this case, the round billet is pierced and rolled using a piercer. When piercing and rolling is carried out, the piercing ratio is not particularly limited, but is, for example, 1.0 to 4.0. The round billet that has been pierced and rolled is further elongated using an elongating mill (e.g., a mandrel mill or an elongator), and then sizing rolled using a sizing mill (e.g., a sizer or a stretch reducer).

[0111] In the hot working step of this embodiment, the area reduction rate Rs in sizing rolling is preferably 5% or more. Sizing rolling is performed in a region where the temperature of the material is lower than that in piercing rolling and elongation rolling. Specifically, in the hot working step of this embodiment, sizing rolling is performed at a material temperature of greater than 900°C to 1050°C. Therefore, in sizing rolling performed in this temperature range, strain that serves as nuclei for precipitation of Ti nitrides is easily introduced into the material. Therefore, if the area reduction rate Rs in sizing rolling is increased, the strain introduced during rolling increases, and the amount of Ti nitrides precipitated in the Ti nitride precipitation treatment step described below increases. Therefore, in this embodiment, the area reduction rate Rs in sizing rolling is preferably 5% or more. The upper limit of the area reduction rate Rs in sizing rolling is not particularly limited, but is, for example, 20%. The area reduction rate Rs in sizing rolling is defined by the following formula: Area reduction rate in sizing rolling Rs (%)=100×{1−(cross-sectional area perpendicular to the rolling direction of the material after sizing rolling / cross-sectional area perpendicular to the rolling direction of the material before sizing rolling)}

[0112] The intermediate steel material produced by hot working may be air-cooled (as-rolled), or may be quenched directly after hot working without being cooled to room temperature, or may be reheated after hot working and then quenched. The heat treatment process will be described in detail below.

[0113] [Heat Treatment Step] The heat treatment step according to this embodiment preferably includes a step of precipitating Ti nitrides in the intermediate steel material (Ti nitride precipitation treatment step), a step of quenching the intermediate steel material (quenching step), and a step of tempering the quenched intermediate steel material (tempering step). Each step will be described in detail below.

[0114] [Ti nitride precipitation treatment step] Preferably, the heat treatment step according to this embodiment includes a Ti nitride precipitation treatment step. In the Ti nitride precipitation treatment step, heat treatment is performed on the intermediate steel material to precipitate Ti nitrides. As described above, in the hot working step according to this embodiment, the cross-sectional area reduction rate Rs in sizing rolling is set to 5% or more. As a result, strain that becomes nuclei for precipitation of Ti nitrides is introduced into the intermediate steel material. Therefore, in the Ti nitride precipitation treatment step according to this embodiment, the holding temperature is set to Ar3 It is preferable to carry out heat treatment at a temperature of 1000 to 900°C for a holding time of 5 to 15 minutes. As a result, very fine Ti nitrides are precipitated, and the number density of Ti-containing particles in the produced steel increases, and further, the number ratio of fine Ti-containing particles increases. Here, the holding temperature corresponds to the temperature of the heat treatment furnace when the intermediate steel is heated and held. The holding time means the time from when the temperature of the intermediate steel reaches the predetermined holding temperature to when it is extracted from the heat treatment furnace.

[0115] If the holding temperature in the Ti nitride precipitation treatment step is too low, fine Ti nitrides may not be sufficiently precipitated. If the holding temperature in the Ti nitride precipitation treatment step is too high, fine Ti nitrides may not be sufficiently precipitated. In these cases, the precipitation of fine Ti nitrides is insufficient, and the number density of Ti-containing particles in the manufactured steel material decreases, and / or the number ratio of fine Ti-containing particles decreases. Therefore, in the Ti nitride precipitation treatment step according to this embodiment, the holding temperature is set to A r3 The temperature is preferably from 100°C to 900°C.

[0116] If the holding time in the Ti nitride precipitation treatment step is too short, fine Ti nitrides may not be sufficiently precipitated. If the holding time in the Ti nitride precipitation treatment step is too long, fine Ti nitrides may not be sufficiently precipitated. In these cases, the precipitation of fine Ti nitrides is insufficient, and the number density of Ti-containing particles and / or the number ratio of fine Ti-containing particles in the produced steel material decreases. Therefore, in the Ti nitride precipitation treatment step according to this embodiment, the holding time is preferably set to 5 to 15 minutes.

[0117] The Ti nitride precipitation treatment process according to this embodiment further comprises: setting the holding temperature to A r3 Preferably, the heat treatment is performed at a temperature of 1000 to 850°C for a holding time of 10 minutes or more. r3 The temperature range from 0°C to 850°C is a temperature range in which the precipitation of Nb carbonitrides and the like is promoted. r3If the temperature is maintained at a temperature between 0.15 and 850°C for 10 minutes or more, a large number of Nb carbonitrides and the like are precipitated, and the number density of Nb-containing particles can be increased in the manufactured steel material. r3 In the heat treatment with the temperature held at 500°C to 900°C for 5 to 15 minutes, r3 Preferably, the temperature is 100° C. to 850° C. and the holding time is 10 minutes or more.

[0118] In the Ti nitride precipitation treatment step according to this embodiment, the heat treatment may be performed in one stage or in two stages. r3 After the heat treatment at 850 to 850° C., the heat treatment may be performed at 850 to 900° C. For example, after the heat treatment at 850 to 900° C., r3 In this case, the heat treatment may be performed at a temperature of from 0°C to 850°C. r3 The effects of the Ti nitride precipitation treatment step described above can be obtained by holding the steel sheet at a temperature of 100°C to 900°C for 5 to 15 minutes. In this embodiment, it is preferable to carry out the Ti nitride precipitation treatment step and the quenching step described below consecutively.

[0119] [Quenching Process] In the quenching process, the intermediate steel material that has undergone the Ti nitride precipitation treatment process is quenched. In this specification, "quenching" refers to rapidly cooling the intermediate steel material at or above the A3 point. The preferred quenching temperature is 920 to 1000°C. If the quenching temperature is too high, prior austenite grains may become coarse, preventing excellent low-temperature toughness in a cryogenic environment. On the other hand, if the quenching temperature is too low, the desired strength may not be obtained. Therefore, the quenching temperature is preferably 920 to 1000°C. The quenching time is not particularly limited, but is, for example, 10 to 60 minutes. Here, the quenching temperature corresponds to the temperature of the heat treatment furnace when the intermediate steel material is heated and maintained. The quenching time refers to the time from when the temperature of the intermediate steel material reaches the predetermined quenching temperature to when it is extracted from the heat treatment furnace.

[0120] The quenching method is, for example, to continuously cool the intermediate steel material from the quenching start temperature, thereby continuously lowering the surface temperature of the intermediate steel material. The method of continuous cooling is not particularly limited, and any well-known method may be used. For example, the continuous cooling method is a method of cooling the intermediate steel material by immersing it in a water tank, or a method of accelerated cooling the intermediate steel material by shower water cooling or mist cooling. The tempering process will be described in detail below.

[0121] [Tempering step] In the tempering step, the intermediate steel material that has been subjected to the above-mentioned quenching is tempered. c1 The tempering temperature corresponds to the temperature of the heat treatment furnace when the intermediate steel material is heated and held at a temperature below the tempering point after quenching. The tempering time refers to the time from when the temperature of the intermediate steel material reaches the predetermined tempering temperature until it is extracted from the heat treatment furnace.

[0122] The tempering temperature is adjusted appropriately depending on the chemical composition of the steel and the yield strength to be obtained. That is, the tempering temperature is adjusted for an intermediate steel having the chemical composition of this embodiment to adjust the yield strength of the steel to 448 MPa or more. In the tempering process of this embodiment, the preferred tempering temperature is 500 to 700°C. Furthermore, in the tempering process of this embodiment, the preferred tempering time is 5 to 300 minutes.

[0123] The steel material according to this embodiment can be manufactured by the above-described manufacturing method. However, as mentioned above, the above-described manufacturing method is only an example, and the steel material may be manufactured by other manufacturing methods. Hereinafter, the present invention will be described in more detail with reference to examples.

[0124] Molten steels having the chemical compositions shown in Tables 1A and 1B were produced. Note that "-" in Tables 1A and 1B indicates that the content of each element was at the impurity level. Specifically, the Cu content, Ni content, Cr content, Mo content, Co content, and V content of Test No. 1 were rounded to two decimal places to mean 0%. The Ti content of Test No. 31 and the Nb content, Sn content, As content, Sb content, and Bi content of Test No. 1 were rounded to four decimal places to mean 0%. The Ca content, Mg content, B content, and REM content of Test No. 1 were rounded to five decimal places to mean 0%.

[0125]

[0126]

[0127] The molten steel of each test number was used to produce a round billet by continuous casting. The round billet of each test number was heated in a heating furnace and hot-worked. The heating temperature (°C) and heating time (minutes) in the heating before hot-working were as shown in Table 2A.

[0128]

[0129] The heated round billet was subjected to piercing, elongation, and sizing. The area reduction rate Rs (%) of the sizing is shown in the "Rs (%)" column of the "Hot working" column in Table 2A. Furthermore, the area reduction rate A obtained for the blank pipe of each test number was r3 The points are listed in Table 2A under "A r3 The temperature is shown in the "(℃)" column.

[0130] The produced mother pipes were subjected to a Ti nitride precipitation treatment, quenching, and tempering. Specifically, the mother pipes of each test number were subjected to a Ti nitride precipitation treatment at a holding temperature (°C) for a holding time (minutes) listed in the "TiN precipitation treatment" column of Table 2B. Note that for test numbers 9, 10, and 13, a two-stage heat treatment was performed as the Ti nitride precipitation treatment. Specifically, the heat treatment was performed at a holding temperature (°C) for a holding time (minutes) listed in the left column of the "TiN precipitation treatment" column of Table 2B, followed by a heat treatment at a holding temperature (°C) for a holding time (minutes) listed in the right column of the "TiN precipitation treatment" column of Table 2B. Note that for each test number except for test numbers 9, 10, and 13, a single-stage heat treatment was performed as the Ti nitride precipitation treatment.

[0131]

[0132] Next, the mother pipe of each test number was quenched by holding it at the quenching temperature (°C) for the quenching time (minutes) shown in the "Quenching" column of Table 2B, followed by rapid cooling. Furthermore, the mother pipe of each test number was tempered by holding it at the tempering temperature (°C) for the tempering time (minutes) shown in the "Tempering" column of Table 2B. Through the above manufacturing process, seamless steel pipes of each test number were obtained.

[0133] From the chemical composition of the seamless steel pipe of each test number and the above formula (1), Fn1 (= Ti × N × 10 5 ) was obtained. Fn2 (= Ti / N) was obtained from the chemical composition of the seamless steel pipe of each test number and the above formula (2). The obtained Fn1 and Fn2 for each test number are shown in Table 3.

[0134]

[0135] [Evaluation Tests] The seamless steel pipes with each test number were subjected to a tensile test, a Ti-containing particle observation test, an Nb-containing particle observation test, and a Charpy impact test, which will be described below.

[0136] [Tensile Test] A tensile test was performed on each seamless steel pipe according to ASTM E8 / E8M (2022). Specifically, a round bar test piece with a parallel diameter of 12.7 mm and a gauge length of 50.8 mm was prepared as a tensile test piece from the center of the wall thickness of each seamless steel pipe according to ASTM E8 / E8M (2022). The longitudinal direction of the tensile test piece was parallel to the axial direction of the steel pipe. Using the prepared tensile test piece, a tensile test was performed in air at room temperature (25°C) according to ASTM E8 / E8M (2022). The obtained 0.5% total elongation proof stress was defined as the yield strength (MPa). The maximum stress during uniform elongation obtained in the same tensile test was defined as the tensile strength (MPa). The ratio of the yield strength to the tensile strength was defined as the yield ratio. For the seamless steel pipe of each test number, the obtained yield strength (MPa) is shown as "YS (MPa)", the tensile strength is shown as "TS (MPa)", and the yield ratio is shown as "YR".

[0137] [Ti-containing particle observation test] A Ti-containing particle observation test was carried out on the seamless steel pipe of each test number to measure the number density of Ti-containing particles (particles / mm 3 ) and the number ratio (%) of fine Ti-containing particles were determined. Specifically, thin film test pieces were prepared from the central part of the wall thickness of the seamless steel pipe of each test number. Using the prepared test pieces of each test number, TEM observation was performed using the method described above, and particles were identified from the contrast. The number of observation fields in the TEM observation was 10, and the area of ​​each observation field was 500 nm × 500 nm. EDS point analysis was performed on each identified particle to identify particles with a Ti content of 50 mass% or more. Furthermore, the circle equivalent diameter was determined for each identified Ti-containing particle. Furthermore, the thickness of the test piece was determined using the method described above, and the total volume of the observation field was calculated. Based on the total number of identified Ti-containing particles and the total volume of the observation field, the number density of the Ti-containing particles (particles / mm 3 ) was determined. Furthermore, the percentage (%) of Ti-containing particles having a circle-equivalent diameter of 60 nm or less was determined. The number density (particles / mm 3 ) in Table 3. 18 pieces / mm 3The percentage of the number of fine Ti-containing particles obtained for each test number is shown in "Number percentage of fine TiN (%)" in Table 3.

[0138] [Nb-containing particle observation test] A Nb-containing particle observation test was carried out on the seamless steel pipe of each test number, and the number density of Nb-containing particles (particles / mm 3 ) was determined. Specifically, similarly to the Ti-containing particle observation test, thin film test pieces were prepared from the central part of the wall thickness of the seamless steel pipe of each test number. Using the prepared test pieces of each test number, TEM observation was performed by the above-mentioned method, and particles were identified from the contrast. The number of observation fields in the TEM observation was 10, and the area of ​​each observation field was 500 nm × 500 nm. EDS point analysis was performed on each identified particle to identify particles with an Nb content of 50 mass% or more. Furthermore, the thickness of the test piece was determined by the above-mentioned method, and the total volume of the observation field was calculated. Based on the total number of identified Nb-containing particles and the total volume of the observation field, the number density of the Nb-containing particles (particles / mm 3 The number density (particles / mm) of the Nb-containing particles obtained for each test number was determined. 3 ) in Table 3. 18 pieces / mm 3 ) shown below.

[0139] [Charpy Impact Test] A Charpy impact test in accordance with ASTM E23 (2018) was conducted on each seamless steel pipe of each test number. Specifically, a V-notch test specimen was prepared from the center of the wall thickness of each seamless steel pipe of each test number, with a notch surface perpendicular to the pipe axis direction and a longitudinal direction perpendicular to the pipe axis direction and the pipe diameter direction. A Charpy impact test was conducted on the prepared V-notch test specimen in accordance with ASTM E23 (2018). The test temperature was set to six levels (-120°C, -100°C, -80°C, -60°C, -40°C, and -20°C) in 20°C increments within the range of -120 to -20°C. Two test specimens were used for each test temperature to conduct the Charpy impact test. The brittle fracture surface ratio (%) of the test specimen after the test at each temperature was determined. The temperature (°C) at which the brittle fracture rate became 50% was determined from an approximate curve obtained by plotting the test temperature (°C) versus the brittle fracture rate (%), and the fracture transition temperature vTrs (°C) was obtained. The fracture transition temperature vTrs (°C) for each test number is shown in the "vTrs (°C)" column of Table 3.

[0140] [Evaluation Results] With reference to Tables 1A, 1B, 2A, 2B, and 3, the seamless steel pipes of test numbers 1 to 28 had appropriate chemical compositions, Fn1 was 5.0 or more, Fn2 was 1.80 or less, and they were manufactured by the preferred manufacturing method described in the specification. As a result, these seamless steel pipes had a yield strength of 448 MPa or more and a number density of Ti-containing particles of 0.8 × 10 18 pieces / mm 3 The number ratio of fine Ti-containing particles was 75% or more. As a result, these seamless steel pipes had a fracture appearance transition temperature vTrs of -80°C or less. In other words, these seamless steel pipes had high strength and excellent low-temperature toughness in an extremely low temperature environment of -80°C.

[0141] Furthermore, the seamless steel pipes of test numbers 10 to 13 have an Nb content of 0.010 to 0.040%, and the number density of Nb-containing particles is 4.4 × 10 18 pieces / mm 3 As a result, these seamless steel pipes had a fracture appearance transition temperature vTrs of -122°C or lower. In other words, these seamless steel pipes had even better low-temperature toughness in an extremely low-temperature environment of -80°C.

[0142] On the other hand, the seamless steel pipe of test number 29 had an excessively high Ti content and an excessively high Fn2. As a result, the number density of Ti-containing particles in this seamless steel pipe was 0.8 × 10 18 pieces / mm 3 The number ratio of fine Ti-containing particles was less than 75%. As a result, the seamless steel pipe had a fracture appearance transition temperature vTrs exceeding −80° C., and did not have excellent low-temperature toughness in an extremely low-temperature environment.

[0143] The seamless steel pipe of test number 30 had an excessively low Ti content. As a result, the number density of Ti-containing particles in this seamless steel pipe was 0.8 × 10 18 pieces / mm 3 As a result, this seamless steel pipe had a fracture appearance transition temperature vTrs exceeding −80° C., and did not have excellent low-temperature toughness in an extremely low-temperature environment.

[0144] The seamless steel pipe of test number 31 had an excessively low Ti content and an excessively low Fn1. As a result, the number density of Ti-containing particles in this seamless steel pipe was 0.8 × 10 18 pieces / mm 3 As a result, this seamless steel pipe had a fracture appearance transition temperature vTrs exceeding −80° C., and did not have excellent low-temperature toughness in an extremely low-temperature environment.

[0145] The seamless steel pipe of test number 32 had an excessively high N content. As a result, this seamless steel pipe had a fracture appearance transition temperature vTrs exceeding −80° C., and did not have excellent low-temperature toughness in an extremely low-temperature environment.

[0146] The seamless steel pipe of test number 33 had an N content that was too low and Fn2 that was too high. As a result, the number density of Ti-containing particles in this seamless steel pipe was 0.8 × 10 18 pieces / mm 3 As a result, this seamless steel pipe had a fracture appearance transition temperature vTrs exceeding −80° C., and did not have excellent low-temperature toughness in an extremely low-temperature environment.

[0147] The seamless steel pipes of test numbers 34 to 36 had too low Fn1. As a result, the number density of Ti-containing particles in these seamless steel pipes was 0.8 × 10 18 pieces / mm3 As a result, these seamless steel pipes had a fracture appearance transition temperature vTrs exceeding −80° C., and did not have excellent low-temperature toughness in an extremely low-temperature environment.

[0148] The seamless steel pipes of test numbers 37 to 39 had too high Fn2. As a result, the number density of Ti-containing particles in these seamless steel pipes was 0.8 × 10 18 pieces / mm 3 As a result, these seamless steel pipes had a fracture appearance transition temperature vTrs exceeding −80° C., and did not have excellent low-temperature toughness in an extremely low-temperature environment.

[0149] The seamless steel pipe of test number 40 was heated at an excessively high temperature during hot working. As a result, the number density of Ti-containing particles in this seamless steel pipe was 0.8 × 10 18 pieces / mm 3 As a result, this seamless steel pipe had a fracture appearance transition temperature vTrs exceeding −80° C., and did not have excellent low-temperature toughness in an extremely low-temperature environment.

[0150] The seamless steel pipe of test number 41 had a heating time during hot working that was too long. As a result, the number density of Ti-containing particles in this seamless steel pipe was 0.8 × 10 18 pieces / mm 3 As a result, this seamless steel pipe had a fracture appearance transition temperature vTrs exceeding −80° C., and did not have excellent low-temperature toughness in an extremely low-temperature environment.

[0151] The seamless steel pipe of test number 42 had an area reduction rate Rs in sizing rolling during hot working that was too low. As a result, the number density of Ti-containing particles in this seamless steel pipe was 0.8 × 10 18 pieces / mm 3 The number ratio of fine Ti-containing particles was less than 75%. As a result, the seamless steel pipe had a fracture appearance transition temperature vTrs exceeding −80° C., and did not have excellent low-temperature toughness in an extremely low-temperature environment.

[0152] The seamless steel pipe of test number 43 had a Ti nitride precipitation treatment temperature that was too low. As a result, the number density of Ti-containing particles in this seamless steel pipe was 0.8 × 10 18 pieces / mm 3The number ratio of fine Ti-containing particles was less than 75%. As a result, the seamless steel pipe had a fracture appearance transition temperature vTrs exceeding −80° C., and did not have excellent low-temperature toughness in an extremely low-temperature environment.

[0153] The seamless steel pipe of test number 44 was subjected to a Ti nitride precipitation treatment at a holding temperature that was too high. As a result, the number density of Ti-containing particles in this seamless steel pipe was 0.8 × 10 18 pieces / mm 3 The number ratio of fine Ti-containing particles was less than 75%. As a result, the seamless steel pipe had a fracture appearance transition temperature vTrs exceeding −80° C., and did not have excellent low-temperature toughness in an extremely low-temperature environment.

[0154] The seamless steel pipe of test number 45 was not subjected to Ti nitride precipitation treatment. As a result, the number density of Ti-containing particles in this seamless steel pipe was 0.8 × 10 18 pieces / mm 3 The number ratio of fine Ti-containing particles was less than 75%. As a result, the seamless steel pipe had a fracture appearance transition temperature vTrs exceeding −80° C., and did not have excellent low-temperature toughness in an extremely low-temperature environment.

[0155] The seamless steel pipe of test number 46 had a Ti nitride precipitation treatment holding time that was too long. As a result, the number density of Ti-containing particles in this seamless steel pipe was 0.8 × 10 18 pieces / mm 3 The number ratio of fine Ti-containing particles was less than 75%. As a result, the seamless steel pipe had a fracture appearance transition temperature vTrs exceeding −80° C., and did not have excellent low-temperature toughness in an extremely low-temperature environment.

[0156] The seamless steel pipe of test number 47 was quenched at an excessively low temperature, resulting in a yield strength of less than 448 MPa, which was not sufficient to meet the desired strength.

[0157] The seamless steel pipe of test number 48 had an excessively low reduction in area Rs during sizing during hot working. As a result, the number ratio of fine Ti-containing particles in this seamless steel pipe was less than 75%. As a result, the fracture appearance transition temperature vTrs of this seamless steel pipe exceeded -80°C, and the seamless steel pipe did not have excellent low-temperature toughness in an extremely low-temperature environment.

[0158] The seamless steel pipe of test number 49 had a Ti nitride precipitation treatment temperature that was too low. As a result, the number ratio of fine Ti-containing particles in this seamless steel pipe was less than 75%. As a result, the fracture appearance transition temperature (vTrs) of this seamless steel pipe exceeded -80°C, and the seamless steel pipe did not have excellent low-temperature toughness in an extremely low-temperature environment.

[0159] The seamless steel pipe of test number 50 had a Ti nitride precipitation treatment temperature that was too high. As a result, the number ratio of fine Ti-containing particles in this seamless steel pipe was less than 75%. As a result, the fracture appearance transition temperature (vTrs) of this seamless steel pipe exceeded -80°C, and the seamless steel pipe did not have excellent low-temperature toughness in an extremely low-temperature environment.

[0160] The seamless steel pipe of test number 51 had a holding time of the Ti nitride precipitation treatment that was too short. As a result, the number ratio of fine Ti-containing particles in this seamless steel pipe was less than 75%. As a result, the fracture appearance transition temperature (vTrs) of this seamless steel pipe exceeded -80°C, and the seamless steel pipe did not have excellent low-temperature toughness in an extremely low-temperature environment.

[0161] The seamless steel pipe of test number 52 had a Ti nitride precipitation treatment holding time that was too long. As a result, the number ratio of fine Ti-containing particles in this seamless steel pipe was less than 75%. As a result, the fracture appearance transition temperature (vTrs) of this seamless steel pipe exceeded -80°C, and the seamless steel pipe did not have excellent low-temperature toughness in an extremely low-temperature environment.

[0162] The embodiments of the present disclosure have been described above. However, the above-described embodiments are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiments, and can be implemented by appropriately modifying the above-described embodiments within the scope of the present disclosure.

Claims

1. In mass%, C: 0.09% or less, Si: 0.60% or less, Mn: 1.0 to 2.5%, P: 0.05% or less, S: 0.005% or less, Ti: 0.005 to 0.050%, Al: 0.005 to 0.100%, N: 0.0050 to 0.0150%, O: 0.0050% or less, Cu: 0 to 0.50%, Ni: 0 to 0.50%, Cr: 0 to 0.50%, Mo: 0 to 0.50%, Co: 0 to 0.25%, V: 0 to 0.10%, Nb: 0 to 0.040%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, B: 0 to less than 0.0005%, rare earth elements: 0 to less than 0.0005%, Sn: 0 to 0.015%, As: 0 to 0.015%, Sb: 0 to 0.015%, Bi: 0 to 0.015%, and the balance: Fe and impurities, wherein Fn1 defined by formula (1) is 5.0 or more, Fn2 defined by formula (2) is 1.80 or less, the yield strength is 448 MPa or more, and in the steel material, the number density of Ti-containing particles is 0.8×10 18 pieces / mm 3 and the proportion of Ti-containing particles having an equivalent circle diameter of 60 nm or less is 75% or more. Fn1=Ti×N×10 5 (1) Fn2=Ti / N (2) Here, the element symbols in formulas (1) and (2) are substituted with the contents of the corresponding elements in units of mass %.

2. The steel material according to claim 1, comprising: Cu: 0.01 to 0.50%, Ni: 0.01 to 0.50%, Cr: 0.01 to 0.50%, Mo: 0.01 to 0.50%, Co: 0.01 to 0.25%, V: 0.01 to 0.10%, Nb: 0.001 to 0.040%, Ca: 0.0001 to 0.0100%, Mg: 0.0001 to less than 0.0005%, rare earth elements: 0.0001 to less than 0.0005%, Sn: 0.001 to 0.015%, As: 0.001 to 0.015%, A steel material containing one or more elements selected from the group consisting of: Sb: 0.001 to 0.015%; and Bi: 0.001 to 0.015%.

3. A steel material according to claim 2, containing Nb: 0.010 to 0.040%, wherein the number density of Nb-containing particles in the steel material is 4.4 × 10 18 pieces / mm 3 That's it, steel.

4. A steel material according to any one of claims 1 to 3, wherein the steel material is a seamless steel pipe.