Steel material

Abstract

Provided is a steel material providing high strength and excellent low-temperature toughness in a component for a machine structure. The steel material according to the present disclosure has a chemical composition containing, in terms of mass%, 0.20%-0.48% of C, 0.20%-1.30% of Si, 0.80%-2.00% of Mn, 0.050% or less of P, 0.010%-0.090% of S, 0.05%-0.50% of Cr, 0.05%-0.30% of V, 0.0001%-0.0055% of Ti, 0.005%-0.050% of Al, 0.003%-0.030% of N, 0.0001%-0.0050% of Ca, and 0.0030% or less of O, with the balance being Fe and impurities, wherein: the number density ND of coarse inclusions having an equivalent circle diameter of 3.0 μm or more is 0.70 or less per 1 mm2; and the number ratio NR of coarse specific inclusions in which the Mn content, the Al content, the Ti content, and the V content in terms of mass% satisfy formula (1) among the coarse inclusions is 75% or less.　(1): (Mn + Al) / (Ti + V) < 0.30

Description

steel material

[0001] The present disclosure relates to a steel material, and more particularly to a steel material that can be used as a raw material for machine structural parts manufactured by hot forging.

[0002] Carbon steel materials for machine structures and alloy steel materials for machine structures are used for machine structural parts, such as automotive parts, industrial machinery parts, and construction machinery parts. These steel materials are usually manufactured into machine structural parts by hot forging. Machine structural parts are required to have high strength.

[0003] Among mechanical structural parts, those typified by automobile suspension parts such as front axles, tie rod arms, knuckles, and knuckle arms are sometimes used in cold climates, and therefore, these mechanical structural parts are required to have not only high strength but also excellent low-temperature toughness.

[0004] Furthermore, when manufacturing machine structural parts using steel as a raw material, in the conventional manufacturing process, a thermal refining treatment (quenching and tempering) is performed after hot forging. However, in order to reduce manufacturing costs, omission of the thermal refining treatment has recently been considered. Therefore, even machine structural parts manufactured without the thermal refining treatment are required to have high strength and excellent low-temperature toughness.

[0005] A non-heat treated steel material having excellent strength and low temperature toughness is proposed in Japanese Patent Laid-Open Publication No. 8-3680 (Patent Document 1).

[0006] The hot forging steel disclosed in Patent Document 1 contains, by weight, C: 0.20 to 0.40%, Si: 0.05 to 0.50%, Mn: 0.80 to 2.00%, P: 0.018% or less, S: 0.030% or less, Cr: 0.30 to 1.50%, Mo: 0.05 to 0.50%, Al: 0.002 to 0.060%, V: 0.05 to 0.50%, and N: 0. and optionally one or two of Pb: 0.05-0.30% and Ca: 0.0005-0.01%, with Ti(%) + Nb(%) ≦ 0.01%, Mo(%) + V(%) ≧ 0.20(%), 1.8Mn(%) + Cr(%) + 0.5Mo(%) ≦ 20C(%), and the balance consisting of Fe and impurity elements. Patent Document 1 describes that by adjusting the chemical composition, high strength and excellent low-temperature toughness can be obtained even without thermal refining treatment.

[0007] Japanese Patent Application Publication No. 8-3680

[0008] However, by using means other than that described in Patent Document 1, high strength and excellent low-temperature toughness may be obtained in machine structural parts even when the thermal refining treatment is omitted.

[0009] An object of the present invention is to provide a steel material which, when used as a material for mechanical structural parts, can provide high strength and excellent low-temperature toughness in the mechanical structural parts even if thermal refining treatment is omitted in the manufacturing process of the mechanical structural parts.

[0010] The steel material of the present disclosure contains, in mass %, C: 0.20 to 0.48%, Si: 0.20 to 1.30%, Mn: 0.80 to 2.00%, P: 0.050% or less, S: 0.010 to 0.090%, Cr: 0.05 to 0.50%, V: 0.05 to 0.30%, Ti: 0.0001 to 0.0055%, Al: 0.005 to 0.050%, N: 0.003 to 0.030%, Ca: 0.0001 to 0.0050%, O: 0.0030% or less, C The alloy contains u: 0-0.40%, Ni: 0-0.30%, Mo: 0-0.20%, Nb: 0-0.050%, Zr: 0-0.050%, B: 0-0.0050%, Mg: 0-0.0050%, Bi: 0-0.050%, Sn: 0-0.100%, Sb: 0-0.050%, As: 0-0.050%, Se: 0-0.100%, Te: 0-0.050%, and Pb: 0-0.09%, with the balance being Fe and impurities. The number density ND of coarse inclusions having a circle equivalent diameter of 3.0 μm or more is 0.70 pieces / mm ２ The ratio NR of the number of specific coarse inclusions whose Mn content, Al content, Ti content, and V content in mass % satisfy the formula (1) is 75% or less: (Mn+Al) / (Ti+V)<0.30 (1)

[0011] When the steel material of the present disclosure is used as a material for machine structural parts, the machine structural parts can have high strength and excellent low-temperature toughness even if the thermal refining treatment is omitted in the manufacturing process of the machine structural parts.

[0012] Fig. 1 is a schematic diagram for explaining measurement points for carrying out EDS point analysis of coarse inclusions in a method for measuring the number ratio of specific coarse inclusions. Fig. 2 is a cross-sectional view of a tundish.

[0013] The present inventors first investigated the chemical composition of a steel material that, when used as a material for machine structural parts, increases the strength of the machine structural parts, and as a result, the present inventors found a steel material containing, in mass %, C: 0.20 to 0.48%, Si: 0.20 to 1.30%, Mn: 0.80 to 2.00%, P: 0.050% or less, S: 0.010 to 0.090%, Cr: 0.05 to 0.50%, V: 0.05 to 0.30%, Ti: 0.0001 to 0.0055%, Al: 0.005 to 0.050%, N: 0.003 to 0.030%, Ca: 0.0001 to 0.0050%, O: 0.0030% or less, Cu: 0 to 0.40%, Ni: 0 to 0.30%, and Mo: 0 to 0.40%. The inventors believed that a steel material having a chemical composition containing 0.20%, Nb: 0-0.050%, Zr: 0-0.050%, B: 0-0.0050%, Mg: 0-0.0050%, Bi: 0-0.050%, Sn: 0-0.100%, Sb: 0-0.050%, As: 0-0.050%, Se: 0-0.100%, Te: 0-0.050%, Pb: 0-0.09%, with the balance being Fe and impurities, would increase the strength of machine structural parts manufactured using this steel material. Furthermore, with the above chemical composition, the Ti content is kept as low as possible, and therefore the inventors believed that excellent low-temperature toughness would also be obtained in machine structural parts.

[0014] However, even if a steel material satisfies the above chemical composition, there are cases in which sufficient low-temperature toughness is not obtained when it is used as a machine structural part. Therefore, the present inventors further investigated means for improving the low-temperature toughness of machine structural parts made from a steel material satisfying the above chemical composition.

[0015] Here, the inventors focused on inclusions in steel materials. Coarse inclusions become the starting point of cracks in low-temperature environments such as cold regions. Therefore, the inventors attempted to improve low-temperature toughness by reducing the number density of coarse inclusions in steel materials as much as possible. As a result of their investigation, they found that in the case of steel materials that satisfy the above-mentioned chemical composition, the number density ND of coarse inclusions having a circle-equivalent diameter of 3.0 μm or more is 0.70 pieces / mm ２ The present inventors have found that if the temperature is below this range, the low temperature toughness of machine structural parts will be improved.

[0016] However, even if the number density ND of the coarse inclusions is reduced, there are still cases where sufficient low-temperature toughness cannot be obtained. Therefore, the present inventors conducted further studies. As a result, the present inventors have come to the following findings.

[0017] The main inclusions that can be contained in the steel material having the above chemical composition are Al ２ O ３ These inclusions are Al oxides typified by AlS, Mn sulfides typified by MnS, nitrides of Ti and / or V (hereinafter referred to as TiV nitrides) and composite inclusions thereof. Among these inclusions, composite inclusions of Al oxides and / or Mn sulfides and TiV nitrides are the coarsest inclusions. Therefore, if the number density ND of the coarse inclusions is 0.70 pieces / mm ２ Even if the above-mentioned composite inclusions account for a high proportion of the coarse inclusions, the coarse inclusions become the starting points for cracks, and sufficient low-temperature toughness cannot be obtained.

[0018] Based on the above-mentioned investigation results, the present inventors have conducted investigations focusing on the chemical composition of coarse inclusions remaining in steel materials, and have obtained the following findings.

[0019] F1 is defined as follows: F1 = (Mn + Al) / (Ti + V) Here, Mn, Al, Ti, and V in F1 are substituted with the Mn content, Al content, Ti content, and V content (%) in the coarse inclusion in mass %, when the chemical composition of the coarse inclusion is taken as 100% in mass %. Inclusions with F1 of less than 0.30 are referred to as "specific inclusions" in this specification. Specific inclusions correspond to the above-mentioned composite inclusions.

[0020] The number density ND of coarse inclusions having an equivalent circle diameter of 3.0 μm or more in the steel material is 0.70 pieces / mm ２or less, and the number ratio NR of coarse inclusions having an F1 of less than 0.30 (hereinafter referred to as "specific coarse inclusions") among the coarse inclusions is 75% or less, the number ratio of specific coarse inclusions corresponding to composite inclusions among the coarse inclusions is sufficiently small. As a result, it is possible to sufficiently prevent the coarse inclusions remaining in the steel from becoming crack initiation points in low-temperature environments. As a result, when the steel is used as a material for machine structural parts, even if thermal refining treatment is omitted in the manufacturing process of the machine structural parts, the machine structural parts can achieve high strength and excellent low-temperature toughness.

[0021] The steel material according to this embodiment, which has been completed based on the above technical concept, has the following configuration.

[0022] The steel material of the first configuration has, in mass%, C: 0.20 to 0.48%, Si: 0.20 to 1.30%, Mn: 0.80 to 2.00%, P: 0.050% or less, S: 0.010 to 0.090%, Cr: 0.05 to 0.50%, V: 0.05 to 0.30%, Ti: 0.0001 to 0.0055%, Al: 0.005 to 0.050%, N: 0.003 to 0.030%, Ca: 0.0001 to 0.0050%, O: 0.0030% or less, The alloy contains Cu: 0-0.40%, Ni: 0-0.30%, Mo: 0-0.20%, Nb: 0-0.050%, Zr: 0-0.050%, B: 0-0.0050%, Mg: 0-0.0050%, Bi: 0-0.050%, Sn: 0-0.100%, Sb: 0-0.050%, As: 0-0.050%, Se: 0-0.100%, Te: 0-0.050%, and Pb: 0-0.09%, with the balance being Fe and impurities. The number density ND of coarse inclusions having a circle equivalent diameter of 3.0 μm or more is 0.70 pieces / mm ２ The ratio NR of the number of specific coarse inclusions whose Mn content, Al content, Ti content, and V content in mass % satisfy the formula (1) is 75% or less: (Mn+Al) / (Ti+V)<0.30 (1)

[0023] The steel material of the second configuration is the steel material of the first configuration, and contains, in mass%, Cu: 0.01 to 0.40%, Ni: 0.01 to 0.30%, Mo: 0.01 to 0.20%, Nb: 0.001 to 0.050%, Zr: 0.001 to 0.050%, B: 0.0001 to 0.0050%, Mg: 0.0001 to 0.0050% , Bi: 0.001 to 0.050%, Sn: 0.001 to 0.100%, Sb: 0.001 to 0.050%, As: 0.001 to 0.050%, Se: 0.001 to 0.100%, Te: 0.001 to 0.050%, and Pb: 0.01 to 0.09%.

[0024] The steel material of this embodiment will be described below. In the following description, "%" for elements means mass % unless otherwise specified.

[0025] [Features of the Steel Material of the Present Embodiment] The steel material of the present embodiment satisfies the following features 1 to 3. (Feature 1) The chemical composition is, in mass%, C: 0.20 to 0.48%, Si: 0.20 to 1.30%, Mn: 0.80 to 2.00%, P: 0.050% or less, S: 0.010 to 0.090%, Cr: 0.05 to 0.50%, V: 0.05 to 0.30%, Ti: 0.0001 to 0.0055%, Al: 0.005 to 0.050%, N: 0.003 to 0.030%, Ca: 0.0001 to 0.0050%, and O: 0.0030% or less. (Feature 2) The number density ND of coarse inclusions having a circle equivalent diameter of 3.0 μm or more is 0.70 pieces / mm ２ (Feature 3) Of the coarse inclusions, the number ratio NR of coarse specific inclusions whose Mn content, Al content, Ti content, and V content in mass% satisfy the formula (1) is 75% or less: (Mn+Al) / (Ti+V)<0.30 (1) Features 1 to 3 will be described below.

[0026] [(Feature 1) Chemical Composition] The chemical composition of the steel material of this embodiment contains the following elements.

[0027] C: 0.20 to 0.48% Carbon (C) increases the strength of machine structural components manufactured using steel as a raw material. If the C content is less than 0.20%, 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 C content exceeds 0.48%, the strength of the machine structural component increases excessively. In this case, the low-temperature toughness of the machine structural component decreases, even if the contents of other elements are within the ranges of this embodiment. Therefore, the C content is 0.20 to 0.48%. The preferred lower limit of the C content is 0.21%, more preferably 0.22%, even more preferably 0.24%, even more preferably 0.26%, and even more preferably 0.28%. The preferred upper limit of the C content is 0.45%, even more preferably 0.40%, and even more preferably 0.38%.

[0028] Si: 0.20 to 1.30% Silicon (Si) deoxidizes steel during the steelmaking process. Si also increases the strength of machine structural components manufactured using steel as a raw material. If the Si content is less than 0.20%, 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 Si content exceeds 1.30%, the strength of the machine structural component increases excessively. In this case, the low-temperature toughness of the machine structural component decreases, even if the contents of other elements are within the ranges of this embodiment. Therefore, the Si content is 0.20 to 1.30%. The preferred lower limit of the Si content is 0.30%, more preferably 0.40%, even more preferably 0.45%, even more preferably 0.50%, even more preferably 0.55%, and even more preferably 0.60%. The preferred upper limit of the Si content is 1.20%, even more preferably 1.10%, even more preferably 1.00%, and even more preferably 0.90%.

[0029] Mn: 0.80 to 2.00% Manganese (Mn) deoxidizes steel during the steelmaking process. Mn also increases the strength of machine structural components manufactured using the steel material. If the Mn content is less than 0.80%, 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 Mn content exceeds 2.00%, the strength of the machine structural component increases excessively. In this case, the low-temperature toughness of the machine structural component decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the Mn content is 0.80 to 2.00%. The preferred lower limit of the Mn content is 0.90%, more preferably 1.00%, even more preferably 1.10%, even more preferably 1.20%, even more preferably 1.30%, and even more preferably 1.40%. The upper limit of the Mn content is preferably 1.90%, more preferably 1.80%, even more preferably 1.70%, even more preferably 1.60%, and still more preferably 1.50%.

[0030] P: 0.050% or less Phosphorus (P) is an unavoidable impurity. In other words, the P content is greater than 0%. P segregates at grain boundaries and reduces the hot workability of steel. P also reduces the low-temperature toughness of machine structural components. Therefore, the P content is 0.050% or less. The lower the P content, the better. However, excessive reduction in the P content increases manufacturing costs. Therefore, considering normal industrial production, the preferred lower limit of the P content is 0.001%, more preferably 0.002%, and even more preferably 0.005%. The preferred upper limit of the P content is 0.045%, more preferably 0.040%, even more preferably 0.035%, even more preferably 0.030%, even more preferably 0.025%, and even more preferably 0.020%.

[0031] S: 0.010 to 0.090% Sulfur (S) forms sulfides and improves the machinability of steel. If the S content is less than 0.010%, 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 S content exceeds 0.090%, S segregates excessively at grain boundaries. In this case, the low-temperature toughness of machine structural components decreases, even if the contents of other elements are within the ranges of this embodiment. Therefore, the S content is 0.010 to 0.090%. The preferred lower limit of the S content is 0.015%, more preferably 0.020%, even more preferably 0.025%, even more preferably 0.030%, and even more preferably 0.035%. The preferred upper limit of the S content is 0.080%, even more preferably 0.070%, even more preferably 0.065%, and even more preferably 0.060%.

[0032] Cr: 0.05 to 0.50% Chromium (Cr) improves the hardenability of steel. Therefore, the strength of machine structural components manufactured using steel is increased. If the Cr content is less than 0.05%, 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 Cr content exceeds 0.50%, the steel cannot achieve sufficient machinability, even if the contents of other elements are within the ranges of this embodiment. Therefore, the Cr content is 0.05 to 0.50%. The preferred lower limit of the Cr content is 0.07%, more preferably 0.10%, even more preferably 0.15%, and even more preferably 0.20%. The preferred upper limit of the Cr content is 0.45%, even more preferably 0.40%, even more preferably 0.35%, and even more preferably 0.30%.

[0033] V: 0.05 to 0.30% Vanadium (V) precipitates in the ferrite of steel during the manufacturing process of machine structural components made from steel. This increases the hardness of the ferrite in the steel. As a result, the strength of the machine structural component is increased. If the V content is less than 0.05%, 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 V content exceeds 0.30%, the strength of the machine structural component becomes excessively high. In this case, the low-temperature toughness of the machine structural component decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the V content is 0.05 to 0.30%. The preferred lower limit of the V content is 0.07%, more preferably 0.10%, and even more preferably 0.15%. The preferred upper limit of the V content is 0.28%, more preferably 0.26%, even more preferably 0.24%, even more preferably 0.22%, and even more preferably 0.20%.

[0034] Ti: 0.0001 to 0.0055% Titanium (Ti) forms precipitates (carbides and / or carbonitrides). These precipitates refine the grains of the steel material through a pinning effect, thereby improving the low-temperature toughness of the machine structural component. If the Ti content is less than 0.0001%, the above effect cannot be fully achieved. On the other hand, if the Ti content exceeds 0.0055%, excessive coarse inclusions are formed. In this case, even if the contents of other elements are within the ranges of this embodiment, the low-temperature toughness of the machine structural component will decrease. Therefore, the Ti content is 0.0001 to 0.0055%. The preferred lower limit of the Ti content is 0.0005%, more preferably 0.0010%, even more preferably 0.0015%, and even more preferably 0.0020%. The upper limit of the Ti content is preferably 0.0050%, more preferably 0.0045%, even more preferably 0.0040%, and still more preferably 0.0035%.

[0035] Al: 0.005 to 0.050% Aluminum (Al) deoxidizes steel. If the Al content is less than 0.005%, 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 exceeds 0.050%, excessive coarse inclusions are formed. In this case, the low-temperature toughness of the machine structural component decreases, even if the contents of other elements are within the ranges of this embodiment. Therefore, the Al content is 0.005 to 0.050%. The preferred lower limit of the Al content is 0.010%, more preferably 0.015%, and even more preferably 0.020%. The preferred upper limit of the Al content is 0.045%, more preferably 0.040%, and even more preferably 0.035%.

[0036] N: 0.003 to 0.030% Nitrogen (N) forms nitrides and / or carbonitrides during the manufacturing process of machine structural components made from steel, thereby precipitation-strengthening the steel. As a result, the strength of the machine structural components is increased. If the N content is less than 0.003%, 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 N content exceeds 0.030%, the hot workability of the steel deteriorates even if the contents of other elements are within the ranges of this embodiment. Therefore, the N content is 0.003 to 0.030%. The preferred lower limit of the N content is 0.005%, and more preferably 0.010%. The preferred upper limit of the N content is 0.025%, more preferably 0.020%, even more preferably 0.018%, and even more preferably 0.015%.

[0037] Ca: 0.0001 to 0.0050% Calcium (Ca) improves the machinability of steel. If the Ca content is less than 0.0001%, 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 Ca content exceeds 0.0050%, excessive coarse oxides are formed. In this case, the low-temperature toughness of the machine structural component decreases, even if the contents of other elements are within the ranges of this embodiment. Therefore, the Ca content is 0.0001 to 0.0050%. The preferred lower limit of the Ca content is 0.0002%, and more preferably 0.0003%. The preferred upper limit of the Ca content is 0.0045%, more preferably 0.0040%, even more preferably 0.0035%, even more preferably 0.0030%, even more preferably 0.0020%, even more preferably 0.0015%, and even more preferably 0.0010%.

[0038] O: 0.0030% or less Oxygen (O) is an unavoidable impurity. In other words, the O content is greater than 0%. O forms oxides and reduces the low-temperature toughness of machine structural components. Therefore, the O content is 0.0030% or less. The O content is preferably as low as possible. However, excessive reduction of the O content increases manufacturing costs. Therefore, considering normal industrial production, the preferred lower limit of the O content is 0.0001%, more preferably 0.0002%, and even more preferably 0.0005%. The preferred upper limit of the O content is 0.0025%, more preferably 0.0020%, even more preferably 0.0018%, and even more preferably 0.0015%.

[0039] The balance of the chemical composition of the steel material according to this embodiment is composed of Fe and impurities. Here, the impurities 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 this embodiment.

[0040] [Optional Elements] The chemical composition of the steel material of this embodiment may further contain, in place of a portion of Fe, one or more elements selected from the group consisting of Cu: 0-0.40%, Ni: 0-0.30%, Mo: 0-0.20%, Nb: 0-0.050%, Zr: 0-0.050%, B: 0-0.0050%, Mg: 0-0.0050%, Bi: 0-0.050%, Sn: 0-0.100%, Sb: 0-0.050%, As: 0-0.050%, Se: 0-0.100%, Te: 0-0.050%, and Pb: 0-0.09%. All of these elements are optional elements. These optional elements will be described below.

[0041] [First group: Cu, Ni, Mo, Nb, Zr, and B] The chemical composition of the steel material of the present embodiment may further contain, in place of a portion of Fe, one or more elements selected from the group consisting of Cu, Ni, Mo, Nb, Zr, and B. All of these elements are optional elements, and all of them increase the strength of machine structural parts manufactured using the steel material as a raw material.

[0042] Cu: 0 to 0.40% Copper (Cu) is an optional element and does not necessarily need to be contained. In other words, the Cu content may be 0%. When contained, that is, when the Cu content exceeds 0%, Cu increases the strength of machine structural components. Even if even a small amount of Cu is contained, the above effect can be obtained to some extent. However, if the Cu content exceeds 0.40%, 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 Cu content is 0 to 0.40%. The preferred lower limit of the Cu content is 0.01%, and more preferably 0.02%. The preferred upper limit of the Cu content is 0.30%, more preferably 0.20%, even more preferably 0.10%, and even more preferably 0.05%.

[0043] Ni: 0 to 0.30% Nickel (Ni) is an optional element and does not necessarily need to be contained. In other words, the Ni content may be 0%. When contained, that is, when the Ni content exceeds 0%, Ni increases the strength of machine structural components. Even if even a small amount of Ni is contained, the above effect can be obtained to some extent. However, if the Ni content exceeds 0.30%, the hardness of the steel material becomes excessively high even if the contents of other elements are within the ranges of this embodiment. In this case, the hot workability of the steel material decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the Ni content is 0 to 0.30%. The preferred lower limit of the Ni content is 0.01%, and more preferably 0.02%. The preferred upper limit of the Ni content is 0.25%, more preferably 0.20%, and even more preferably 0.15%.

[0044] Mo: 0 to 0.20% Molybdenum (Mo) is an optional element and does not necessarily need to be contained. In other words, the Mo content may be 0%. When contained, that is, when the Mo content exceeds 0%, Mo increases the strength of machine structural parts. Even if even a small amount of Mo is contained, the above effect can be obtained to some extent. However, if the Mo content exceeds 0.20%, the hardness of the steel material becomes excessively high. In this case, the hot workability of the steel material decreases. Therefore, the Mo content is 0 to 0.20%. The preferred lower limit of the Mo content is 0.01%, more preferably 0.03%, and even more preferably 0.05%. The preferred upper limit of the Mo content is 0.18%, more preferably 0.16%, and even more preferably 0.14%.

[0045] Nb: 0 to 0.050% Niobium (Nb) is an optional element and does not necessarily need to be contained. That is, the Nb content may be 0%. When Nb is contained, that is, when the Nb content exceeds 0%, Nb enhances the strength of machine structural components through precipitation strengthening. Nb also refines the grains of the steel through its pinning effect. Therefore, the low-temperature toughness of machine structural components is improved. Even if even a small amount of Nb is contained, the above effects can be achieved to some extent. However, if the Nb content exceeds 0.050%, the hardness of the steel becomes excessively high. In this case, even if the contents of other elements are within the ranges of this embodiment, the hot workability of the steel deteriorates. Therefore, the Nb content is 0 to 0.050%. The preferred lower limit of the Nb content is 0.001%, more preferably 0.005%, and even more preferably 0.010%. The upper limit of the Nb content is preferably 0.045%, more preferably 0.040%, even more preferably 0.035%, and still more preferably 0.025%.

[0046] Zr: 0 to 0.050% Zirconium (Zr) is an optional element and does not necessarily need to be contained. That is, the Zr content may be 0%. When contained, that is, when the Zr content exceeds 0%, Zr enhances the strength of machine structural components through precipitation strengthening. Zr also refines the grain size of the steel through its pinning effect. This improves the low-temperature toughness of machine structural components. Even if even a small amount of Zr is contained, the above effects can be achieved to some extent. However, if the Zr content exceeds 0.050%, the hardness of the steel becomes excessively high. In this case, even if the contents of other elements are within the ranges of this embodiment, the hot workability of the steel deteriorates. Therefore, the Zr content is 0 to 0.050%. The preferred lower limit of the Zr content is 0.001%, more preferably 0.003%, and even more preferably 0.005%. The upper limit of the Zr content is preferably 0.045%, more preferably 0.040%, even more preferably 0.035%, and still more preferably 0.030%.

[0047] B: 0 to 0.0050% Boron (B) is an optional element and does not necessarily need to be contained. In other words, the B content may be 0%. When contained, that is, when the B content exceeds 0%, B segregates at grain boundaries to increase grain boundary strength, thereby improving the strength and low-temperature toughness of machine structural components. Even if even a small amount of B is contained, the above effects can be obtained to some extent. However, if the B content exceeds 0.0050%, 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 B content is 0 to 0.0050%. The preferred lower limit of the B content is 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%. The preferred upper limit of the B content is 0.0040%, more preferably 0.0035%, even more preferably 0.0030%, and even more preferably 0.0020%.

[0048] [Second Group: Mg, Bi, Sn, Sb, As, Se, Te, and Pb] The chemical composition of the steel material of this embodiment may further contain one or more elements selected from the group consisting of Mg, Bi, Sn, Sb, As, Se, Te, and Pb in place of a portion of Fe. All of these elements are optional elements, and all of them improve the machinability of the steel material.

[0049] Mg: 0 to 0.0050% Magnesium (Mg) is an optional element and does not necessarily need to be present. In other words, the Mg content may be 0%. When present, that is, when the Mg content exceeds 0%, Mg improves the machinability of the steel material. Even if even a small amount of Mg is present, the above effect can be achieved to some extent. However, if the Mg content exceeds 0.0050%, Mg forms coarse oxides even if the contents of other elements are within the ranges of this embodiment. The coarse oxides reduce the low-temperature toughness of machine structural components manufactured using the steel material. Therefore, the Mg content is 0 to 0.0050%. The preferred lower limit of the Mg content is 0.0001%, more preferably 0.0003%, and even more preferably 0.0005%. The preferred upper limit of the Mg content is 0.0040%, more preferably 0.0035%, even more preferably 0.0030%, and even more preferably 0.0020%.

[0050] Bi: 0 to 0.050% Bismuth (Bi) is an optional element and does not necessarily need to be contained. That is, the Bi content may be 0%. When Bi is contained, that is, when the Bi content exceeds 0%, Bi generates Bi particles in the matrix, embrittling the steel. This improves the machinability of the steel. Even if even a small amount of Bi is contained, the above effect can be achieved to some extent. However, if the Bi content exceeds 0.050%, excessive Bi particles will be generated even if the contents of other elements are within the ranges of this embodiment. In this case, the hot workability of the steel will be reduced. Therefore, the Bi content is 0 to 0.050%. The preferred lower limit of the Bi content is 0.001%, more preferably 0.003%, and even more preferably 0.005%. The preferred upper limit of the Bi content is 0.040%, more preferably 0.030%, even more preferably 0.020%, and even more preferably 0.015%.

[0051] Sn: 0 to 0.100% Tin (Sn) is an optional element and does not necessarily need to be contained. That is, the Sn content may be 0%. If Sn is contained, that is, if the Sn content exceeds 0%, Sn segregates at the interface between the matrix and inclusions, embrittling the steel. This improves the machinability of the steel. Even if even a small amount of Sn is contained, the above effect can be achieved to some extent. However, if the Sn content exceeds 0.100%, even if the contents of other elements are within the ranges of this embodiment, excessive Sn segregation occurs. In this case, the hot workability of the steel deteriorates. Therefore, the Sn content is 0 to 0.100%. The preferred lower limit of the Sn content is 0.001%, more preferably 0.005%, and even more preferably 0.010%. The preferred upper limit of the Sn content is 0.050%, more preferably 0.045%, even more preferably 0.040%, and even more preferably 0.030%.

[0052] Sb: 0 to 0.050% Antimony (Sb) is an optional element and does not necessarily need to be contained. That is, the Sb content may be 0%. When contained, that is, when the Sb content exceeds 0%, Sb segregates at the interface between the matrix and inclusions, embrittling the steel. This improves the machinability of the steel. Even if even a small amount of Sb is contained, the above effect can be achieved to some extent. However, if the Sb content exceeds 0.050%, even if the contents of other elements are within the ranges of this embodiment, excessive Sb segregation occurs. In this case, the hot workability of the steel deteriorates. Therefore, the Sb content is 0 to 0.050%. The preferred lower limit of the Sb content is 0.001%, more preferably 0.005%, and even more preferably 0.010%. The preferred upper limit of the Sb content is 0.040%, more preferably 0.030%, and even more preferably 0.020%.

[0053] As: 0 to 0.050% Arsenic (As) is an optional element and does not necessarily need to be contained. In other words, the As content may be 0%. If arsenic is contained, that is, if the As content exceeds 0%, As segregates at the interface between the matrix and inclusions, embrittling the steel. This improves the machinability of the steel. Even if even a small amount of As is contained, the above effect can be achieved to some extent. However, if the As content exceeds 0.050%, excessive As segregation occurs even when the contents of other elements are within the ranges of this embodiment. In this case, the hot workability of the steel deteriorates. Therefore, the As content is 0 to 0.050%. The preferred lower limit of the As content is 0.001%, more preferably 0.005%, and even more preferably 0.010%. The preferred upper limit of the As content is 0.040%, more preferably 0.030%, and even more preferably 0.020%.

[0054] Se: 0 to 0.100% Selenium (Se) is an optional element and does not necessarily need to be contained. That is, the Se content may be 0%. When contained, that is, when Se exceeds 0%, Se segregates at the interface between the matrix and inclusions, embrittling the steel. This improves the machinability of the steel. Even if even a small amount of Se is contained, the above effect can be achieved to some extent. However, if the Se content exceeds 0.100%, the hot workability of the steel deteriorates even if the contents of other elements are within the ranges of this embodiment. Therefore, the Se content is 0 to 0.100%. The preferred lower limit of the Se content is 0.001%, more preferably 0.005%, and even more preferably 0.010%. The preferred upper limit of the Se content is 0.090%, more preferably 0.080%, even more preferably 0.070%, even more preferably 0.060%, and even more preferably 0.050%.

[0055] Te: 0 to 0.050% Tellurium (Te) is an optional element and does not necessarily need to be contained. That is, the Te content may be 0%. When contained, that is, when the Te content exceeds 0%, Te improves the machinability of the steel material. Even if even a small amount of Te is contained, the above effect can be obtained to some extent. However, if the Te content exceeds 0.050%, 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 Te content is 0 to 0.050%. The preferred lower limit of the Te content is 0.001%, more preferably 0.003%, and even more preferably 0.010%. The preferred upper limit of the Te content is 0.040%, more preferably 0.030%, even more preferably 0.025%, and even more preferably 0.020%.

[0056] Pb: 0 to 0.09% Lead (Pb) is an optional element and does not necessarily need to be contained. That is, the Pb content may be 0%. If Pb is contained, that is, if the Pb content exceeds 0%, Pb generates Pb particles in the steel material, embrittling the steel material. This improves the machinability of the steel material. Even if even a small amount of Pb is contained, the above effect can be achieved to some extent. However, if the Pb content exceeds 0.09%, excessive Pb particles will be generated even if the contents of other elements are within the ranges of this embodiment. In this case, the hot workability of the steel material will be reduced. Therefore, the Pb content is 0 to 0.09%. The preferred lower limit of the Pb content is 0.01%, more preferably 0.02%, and even more preferably 0.03%. The preferred upper limit of the Pb content is 0.08%, more preferably 0.07%, even more preferably 0.06%, and even more preferably 0.05%.

[0057] [(Feature 2) Number density of coarse inclusions ND (pieces / mm ２ In the steel material of this embodiment, the number density ND of coarse inclusions having a circle equivalent diameter of 3.0 μm or more is 0.70 pieces / mm ２ Here, the circle equivalent diameter means the diameter (μm) of a circle having the same area as the area of ​​the inclusion.

[0058] In the steel material of this embodiment, the number density ND of coarse inclusions having a circle equivalent diameter of 3.0 μm or more is 0.70 pieces / mm ２ The number density ND of the coarse inclusions is suppressed to 0.70 pieces / mm or less. Coarse inclusions can become the starting point of cracks in a low-temperature environment. In this embodiment, the number density ND of the coarse inclusions is suppressed to 0.70 pieces / mm ２ By keeping the content below this level, the occurrence of cracks in low-temperature environments can be suppressed, and as a result, the low-temperature toughness of machine structural parts manufactured using the steel material can be significantly improved.

[0059] The preferred upper limit of the number density ND is 0.65 pieces / mm ２ and more preferably 0.60 pieces / mm ２ and more preferably 0.55 pieces / mm ２ It is preferable that the number density ND is as small as possible. However, an excessive reduction in the number density ND increases the manufacturing cost. Therefore, in consideration of normal industrial production, the preferable lower limit of the number density ND is 0.10 pieces / mm ２ and more preferably 0.20 pieces / mm ２ and more preferably 0.30 pieces / mm ２ and more preferably 0.40 pieces / mm ２ is.

[0060] [Method for measuring the number density ND of coarse inclusions] The number density ND of specific inclusions is measured by the following method. From a steel material having a circular cross section perpendicular to the axial direction, one test piece having a cross section perpendicular to the axial direction of the steel material is taken. The cross section is mirror-polished. The mirror-polished cross section is observed using a scanning electron microscope (SEM-EDS) equipped with a composition analysis function. When the cross section is circular with a radius R (mm), the region to be observed is the region of the cross section from a depth position of 0.50R to a depth position of 0.90R, starting from the surface of the steel material.

[0061] A plurality of observation fields for SEM observation are selected from the target region. The selected plurality of observation fields are observed at a magnification of 750 times to generate an SEM image (backscattered electron image). ２The size and number of observation fields are selected so that the total area of ​​the observation target region (region from the 0.50R depth position to the 0.90R depth position) in one cross section is 260 mm ２ If the total area of ​​the cross section of the second test piece is less than 260 mm, prepare the cross section of the second test piece so that the total area of ​​the cross section of the first test piece and the total area of ​​the cross section of the first test piece are 260 mm ２ One or more observation fields are selected from the target region of the second cross section so that: Note that the SEM-EDS is equipped with a backscattered electron detector, and the electron gun is a tungsten filament.

[0062] In the SEM image, particles are identified based on contrast. The circle-equivalent diameter (μm) of the identified particles is calculated based on their area. The circle-equivalent diameter can be calculated using well-known image processing techniques. Of the identified particles, particles having a circle-equivalent diameter of 3.0 μm or more are recognized as coarse inclusions.

[0063] The total number of coarse inclusions in the observation field is counted. The total number of coarse inclusions and the total area of ​​the observation field (260 mm ２ ) and the number density ND (pieces / mm ２ The number density ND is calculated by rounding the obtained value to two decimal places.

[0064] [(Feature 3) Number Proportion NR (%) of Coarse Specific Inclusions] Furthermore, in the steel material of this embodiment, the number proportion NR of coarse specific inclusions, of which the equivalent circle diameter is 3.0 μm or more, whose Mn content, Al content, Ti content, and V content in mass % satisfy the formula (1) is 75% or less: (Mn + Al) / (Ti + V)<0.30 (1) Here, Mn, Al, Ti, and V in formula (1) are substituted with the Mn content, Al content, Ti content, and V content in mass % of the coarse inclusions, when the chemical composition of the coarse inclusions is taken as 100% in mass%.

[0065] The coarse specific inclusions, in which F1 (= (Mn + Al) / (Ti + V)) satisfies the formula (1), correspond to composite inclusions of Al oxides and / or Mn sulfides and TiV nitrides. The size of the composite inclusions is coarse compared to Al oxides, Mn sulfides, and TiV nitrides. Therefore, if the number density ND of the coarse inclusions is 0.70 pieces / mm ２ Even if the number of specific coarse inclusions is less than this, if the proportion of the number of specific coarse inclusions among the coarse inclusions is high, the coarse inclusions in the steel material are likely to become the starting point of cracks in a low-temperature environment.

[0066] Therefore, in this embodiment, the number density ND of the coarse inclusions is set to 0.70 pieces / mm ２ or less, and the number ratio NR of the specific coarse inclusions that satisfy formula (1) among the coarse inclusions is set to 75% or less. In this case, the coarse inclusions remaining in the steel material can be sufficiently prevented from becoming crack initiation points in low-temperature environments. As a result, the low-temperature toughness of machine structural parts manufactured using the steel material can be significantly improved.

[0067] The upper limit of the number ratio NR is preferably 73%, more preferably 71%, and even more preferably 69%. The lower limit of the number ratio NR is not particularly limited. However, excessive reduction of the number ratio NR increases manufacturing costs. Therefore, in the case of normal industrial production, the lower limit of the number ratio NR is, for example, 30%, for example, 40%, or for example, 50%.

[0068] [Method for measuring the number ratio NR of specific coarse inclusions] The number ratio NR (%) of specific coarse inclusions is determined by the following method. ２ Coarse inclusions are identified in all observation fields. Each identified coarse inclusion is subjected to element concentration analysis using energy dispersive X-ray spectroscopy (EDS) to identify the specific coarse inclusion. In the EDS analysis (element concentration analysis), the acceleration voltage is 20 kV, the emission current is 21 μA, and the quantified elements are Si, Mn, S, Cr, Ti, V, Cu, Ni, Ca, N, O, and Al.

[0069] As shown in Figure 1, one line segment L connecting two points P1 and P2 on the outer edge of the identified coarse inclusion 10 is selected, which is larger than the circle-equivalent diameter of the coarse inclusion. The line segment L is divided into three equal sections S1 to S3, and the center positions CP1 to CP3 of each section S1 to S3 are used as measurement points. In other words, there are three measurement points (CP1 to CP3). The EDS analysis time is set so that the X-ray count is 2000 Counts or more.

[0070] At each of the measurement points CP1 to CP3, the Mn content, Al content, Ti content, and V content are determined in mass%, assuming that the chemical composition of the coarse inclusion is 100% by mass (that is, assuming that the total content of the quantified elements Si, Mn, S, Cr, Ti, V, Cu, Ni, Ca, N, O, and Al is 100% by mass). The arithmetic average of the Mn contents obtained at the three measurement points is taken as the Mn content (mass%) of the coarse inclusion. The Mn content is the value obtained by rounding the obtained arithmetic average value to one decimal place. The arithmetic average of the Al contents obtained at the three measurement points is taken as the Al content (mass%) of the coarse inclusion. The Al content is the value obtained by rounding the obtained arithmetic average value to one decimal place. The arithmetic average of the Ti contents obtained at the three measurement points CP1 to CP3 is taken as the Ti content (mass%) of the coarse inclusion. The Ti content is determined by rounding the obtained arithmetic mean value to one decimal place. The arithmetic mean of the V content obtained at the three measurement points is determined to be the V content (mass%) of the coarse inclusion. The V content is determined by rounding the obtained arithmetic mean value to one decimal place. Based on the obtained Mn content, Al content, Ti content, and V content, coarse inclusions whose F1 satisfies formula (1) are recognized as "specific coarse inclusions." Note that F1 is determined by rounding the obtained value to one decimal place.

[0071] Total area is 260 mm ２ The number ratio NR (%) of the coarse inclusions is calculated based on the total number of the coarse inclusions in all the observation fields and the total number of the coarse inclusions. The number ratio NR is an integer obtained by rounding the obtained value to one decimal place.

[0072] [Effects of the Steel Material of the Present Embodiment] As described above, the steel material of the present embodiment satisfies Features 1 to 3. Therefore, when manufacturing a machine structural component using the steel material of the present embodiment as a raw material, even if thermal refining treatment (quenching and tempering) is omitted, the machine structural component can have high strength and, further, excellent low-temperature toughness.

[0073] [Microstructure of the Steel Material of the Present Embodiment] The microstructure of the steel material of the present embodiment is not particularly limited. The steel material of the present embodiment is prepared by subjecting the steel material to A ｃ３ The steel material is heated to a temperature equal to or higher than the transformation point. This heating transforms the microstructure of the steel material to austenite. Therefore, the microstructure of the steel material of this embodiment is not particularly limited. For example, in the microstructure of the steel material of this embodiment, in the R / 2 part, the total area ratio of ferrite and pearlite is 80% or more, and the remainder is bainite or martensite. However, the microstructure of the steel material of this embodiment is not particularly limited to the microstructure described above. Here, the R / 2 part refers to the center part of the radius in a cross section (circular cross section) perpendicular to the axial direction of the steel material.

[0074] [Preferred Uses and Shapes of the Steel Material of the Present Embodiment] The steel material of the present embodiment can be widely applied, for example, as a material for machine structural parts. The steel material of the present embodiment is particularly suitable for machine structural part applications, since when used as a material for machine structural parts, it can achieve both high strength and excellent low-temperature toughness even if quenching and tempering are omitted in the manufacturing process of the machine structural parts. Such machine structural parts are, for example, automobile suspension parts such as front axles, tie rod arms, knuckles, and knuckle arms.

[0075] The steel material in this embodiment is a steel material having a circular cross section perpendicular to the axial direction, and is, for example, a steel bar or a wire rod.

[0076] [Manufacturing Method] An example of a method for manufacturing a steel material according to this embodiment will be described. A steel material satisfying Features 1 to 3 may be manufactured by a manufacturing method other than the manufacturing method described below. However, the manufacturing method described below is a preferred example of a method for manufacturing a steel material according to this embodiment.

[0077] An example of the method for producing wire rod according to this embodiment includes the following steps: (Step 1) Steelmaking step (Step 2) Blooming step (Step 3) Finish rolling step Each step will be described below.

[0078] [(Process 1) Steelmaking Process] In the steelmaking process, primary refining and secondary refining are performed on molten steel having a chemical composition that satisfies Feature 1. When the primary refining is performed in an electric furnace, the following primary refining and secondary refining are performed in the steelmaking process.

[0079] Primary refining is carried out using a converter or electric furnace. The molten steel after primary refining is tapped and received in a ladle. After receiving the steel, secondary refining is carried out on the molten steel in the ladle. In the secondary refining, first, refining is carried out in the atmosphere using an LF (Ladle Furnace). After the LF treatment, a vacuum degassing treatment is carried out. The vacuum degassing treatment is, for example, a VD treatment (Vacuum Degasser) and / or an RH treatment (Ruhrstahl Heraeus).

[0080] After secondary refining, the molten steel in the ladle is transferred to a tundish and continuous casting is carried out to produce a bloom. At this time, the following conditions are satisfied in the tundish: (Condition 1) The molten steel temperature T1 in the tundish is maintained at 1520 to 1570°C. (Condition 2) In the tundish, lower weirs and upper weirs are alternately arranged in this order along the path from the pouring nozzle of the ladle to the submerged entry nozzle, with two or more lower weirs and two or more upper weirs arranged along the path. Conditions 1 and 2 are explained below.

[0081] [Condition 1: Regarding the molten steel temperature T1 in the tundish] If the molten steel temperature T1 in the tundish is too high, the aggregation of Al oxides (alumina) and Mn sulfides present in the molten steel is suppressed. In this case, Al oxides and Mn sulfides are less likely to float and remain in the molten steel before flowing into the submerged entry nozzle. As a result, composite inclusions (specific inclusions) with TiV nitrides are formed around Al oxides and Mn sulfides as nuclei, and coarse specific inclusions are likely to be formed. Therefore, the number ratio NR of the coarse specific inclusions increases. On the other hand, if the molten steel temperature T1 in the tundish is too low, Mn sulfides are excessively crystallized. Therefore, the number density ND of the coarse inclusions becomes excessive. Furthermore, after the molten steel flows into the submerged entry nozzle, composite inclusions (specific inclusions) with TiV nitrides are likely to be formed around Mn sulfides as nuclei, and coarse specific inclusions are likely to be formed. Therefore, the number ratio NR of large specific inclusions becomes high. Therefore, the molten steel temperature T1 is set to 1520 to 1570°C. The molten steel temperature T1 is measured by a thermocouple disposed near the injection hole of the submerged nozzle of the tundish.

[0082] [(Condition 2) Number of Lower Weirs and Upper Weirs in a Tundish] Fig. 2 is a cross-sectional view of a tundish. Referring to Fig. 2, in a tundish 1, lower weirs 5 and upper weirs 6 are alternately arranged in this order along a path from the pouring nozzle 3 of a ladle 2 to the submerged nozzle 4, with two or more lower weirs 5 and two or more upper weirs arranged along the path. Here, the lower weirs 5 are weirs extending upward from the bottom 1A of the tundish 1. The upper weirs 6 are weirs extending downward from the lid 1B of the tundish 1. When molten steel is poured into the tundish 1, the molten steel flows above the lower weirs 5 and below the upper weirs 6 along the path from the pouring nozzle 3 to the submerged nozzle 4. More specifically, when the molten steel approaches the lower weir 5, it rises and passes above the lower weir 5. When the molten steel approaches the upper weir 6, it descends and passes below the upper weir 6.

[0083] If upward and downward flows occur in the molten steel along the path from the pouring nozzle 3 of the ladle to the submerged nozzle 4 in the tundish 1, the Al oxides and Mn sulfides in the molten steel have a greater chance of floating to the surface of the molten steel, making them more likely to be absorbed into the slag on the surface of the molten steel. As described above, the lower weir 5 creates an upward flow in the molten steel. The upper weir 6 creates a downward flow in the molten steel. Therefore, by alternately arranging the lower weirs 5 and the upper weirs 6 along the path from the pouring nozzle 3 to the submerged nozzle 4 and by providing a greater number of lower weirs 5 and upper weirs 6, more upward and downward flows can be formed in the molten steel.

[0084] When the number of lower weirs 5 is less than two or the number of upper weirs 6 is less than two, the number of ascending and descending flows in the molten steel in the tundish 1 is insufficient. In this case, Al oxides and Mn sulfides in the molten steel are not sufficiently floated and separated, and the molten steel flows into the submerged nozzle 4 with a large amount of Al oxide and Mn sulfide remaining. As a result, composite inclusions (specific inclusions) with TiV are formed using Al oxides and Mn sulfides as nuclei, and coarse specific inclusions are likely to be formed. Therefore, the number ratio NR of coarse specific inclusions increases.

[0085] Therefore, on the path from the injection nozzle 3 of the ladle 2 to the submerged nozzle 4, the lower weirs 5 and upper weirs 6 are arranged alternately along the path in the order of lower weirs 5 and upper weirs 6, and the number of lower weirs 5 arranged on the path is two or more, and the number of upper weirs 6 is two or more.

[0086] If the number of lower weirs 5 and the number of upper weirs 6 arranged on the path are too large, the travel time of the molten steel until it reaches the submerged nozzle 4 becomes long, which may result in an excessive drop in the molten steel temperature T1. Therefore, the preferred upper limit for the number of lower weirs 5 is four, and the preferred upper limit for the number of upper weirs 6 is four.

[0087] The molten steel flowing into the submerged nozzle 4 is used to produce a bloom by continuous casting.

[0088] [(Step 2) Blooming Step] In the blooming step, blooming is performed on the bloom produced in the steelmaking step to produce a billet. In the blooming step, the material is first heated in a heating furnace by a well-known method. The heating temperature is not particularly limited. Any well-known temperature will suffice. The heating temperature is, for example, 1200 to 1300°C.

[0089] The heated material is hot rolled (rough rolled) using a blooming mill, or a blooming mill and a continuous rolling mill, to produce billets. Specifically, the heated material is reverse rolled using the blooming mill to produce billets. If a well-known continuous rolling mill is located downstream of the blooming mill, the billets after blooming may be further subjected to tandem rolling using the continuous rolling mill to produce smaller billets.

[0090] [(Step 3) Finish Rolling Step] In the finish rolling step, the billet produced in the blooming step is finish rolled to produce a steel material (steel bar or wire rod). The finish rolling step includes the following steps. In the finish rolling step, first, the billet produced in the blooming step is heated using a heating furnace. The heating temperature is, for example, 1000 to 1150°C. The heated billet is then finish rolled using a continuous rolling mill to produce a steel material (steel bar). The finish rolling temperature (temperature at the end of finish rolling) is, for example, 800 to 1000°C.

[0091] The steel material of this embodiment is manufactured by the above manufacturing process.

[0092] [Method of Manufacturing Machine Structural Components] Machine structural components are manufactured using the steel material of the present embodiment as a raw material. Methods of manufacturing machine structural components are well known, and are, for example, as follows.

[0093] The steel material of this embodiment is hot worked to produce an intermediate product in the shape of a machine structural part (e.g., a crankshaft). The hot working is, for example, hot forging. The hot forging temperature is, for example, 1100 to 1300°C. The produced intermediate product is allowed to cool in the atmosphere. After cooling, the intermediate product is cut to a predetermined shape.

[0094] Machine structural components are manufactured through the above steps. When machine structural components are manufactured using the steel material of this embodiment as a raw material, thermal refining treatment (quenching and tempering) can be omitted. Even if quenching and tempering are omitted, the manufactured machine structural components can obtain high strength and excellent low-temperature toughness. Note that when machine structural components are manufactured using the steel material of this embodiment as a raw material, thermal refining treatment may be performed.

[0095] The effects of the steel material of this embodiment will be explained more specifically using examples. The conditions in the following examples are one example of conditions adopted to confirm the feasibility and effects of the steel material of this embodiment. Therefore, the steel material of this embodiment is not limited to this one example of conditions.

[0096] Steel materials having the chemical compositions shown in Table 1 (Tables 1A and 1B) were produced.

[0097]

[0098]

[0099] Specifically, primary refining was performed on the molten steel using a 270-ton converter. The molten steel was then subjected to secondary refining using LF treatment and RH treatment. The molten steel after secondary refining was transferred to a tundish, and blooms were produced by continuous casting. The molten steel temperature T1 in the tundish is shown in the "Molten steel temperature T1 (°C)" column in Table 2. The numbers of lower and upper weirs in the path from the pouring nozzle of the ladle to the submerged entry nozzle in the tundish used are shown in the "Number of lower weirs" and "Number of upper weirs" columns in Table 2.

[0100]

[0101] The produced blooms were subjected to a blooming process to produce billets. The heating temperature in the blooming process was 1200 to 1300°C. The produced billets were then subjected to finish rolling. The heating temperature in the finish rolling was 1000 to 1150°C. The finish rolling temperature was 800 to 1000°C. Through the above production process, a steel bar (steel material) having a diameter of 80 mm and a circular cross section perpendicular to the axial direction was produced.

[0102] [Evaluation Tests] The following evaluation tests were carried out on the steel material with each test number: (Test 1) Measurement test of number density of coarse inclusions (ND) (Test 2) Measurement test of number ratio of specific coarse inclusions (NR) (Test 3) Measurement test of yield strength of simulated machine structural parts (Test 4) Evaluation test of low-temperature toughness of simulated machine structural parts Each evaluation test will be explained below.

[0103] [(Test 1) Measurement of the number density ND of coarse inclusions] For the steel material of each test number, the number density ND (number / mm ２ The results are shown in Table 2 as "Number density ND (pieces / mm ２ ) column.

[0104] [(Test 2) Measurement test of the number ratio NR of coarse specific inclusions] For the steel material of each test number, the number ratio NR (%) of coarse specific inclusions was determined by the method described above in [Method for measuring the number ratio NR of coarse specific inclusions]. The obtained results are shown in the "Number ratio NR (%)" column in Table 2.

[0105] [(Test 3) Yield Strength Measurement Test of Simulated Machine Structural Parts] The steel material of each test number was heated at a heating temperature of 1200°C for a holding time of 15 minutes. Then, hot forging was performed to produce simulated machine structural parts, which were steel bars with a diameter of 50 mm. The simulated machine structural parts after hot forging were allowed to cool.

[0106] A No. 4 test piece specified in JIS Z 2241:2022 was prepared as a tensile test piece from the simulated mechanical structural part. The central axis of the tensile test piece was coaxial with the central axis of the simulated mechanical structural part. Using the prepared tensile test piece, a tensile test was performed in air at room temperature in accordance with JIS Z 2241:2022 to obtain the yield strength (MPa). The yield strength was defined as 0.2% proof stress. The obtained yield strength is shown in the "Yield strength (MPa)" column in Table 2.

[0107] [(Test 4) Low-Temperature Toughness Evaluation Test of Simulated Machine Structural Parts] The steel material of each test number was heated at a heating temperature of 1200°C for a holding time of 15 minutes. Then, hot forging was performed to produce simulated machine structural parts, which were steel bars with a diameter of 50 mm. The simulated machine structural parts after hot forging were allowed to cool.

[0108] A JIS No. 3 U-notch Charpy test specimen specified in JIS Z2242:2023 was prepared from the simulated mechanical structural part. The test specimen size was half size, 5 mm x 10 mm x 55 mm, and the longitudinal direction of the test specimen was parallel to the longitudinal direction of the simulated mechanical structural part. Using the test specimen, a Charpy impact test was carried out in accordance with JIS Z2242:2018. Specifically, a Charpy impact test was carried out in the atmosphere using the test specimen held at -50°C, and the Charpy impact value (J / cm ２ The results are shown in Table 2 as "Charpy impact value (J / cm)". ２ ) column.

[0109] [Evaluation Results] Referring to Tables 1 and 2, test numbers 1 to 40 satisfied features 1 to 3. Therefore, the yield strength of the simulated machine structural parts was 600 MPa or more, and high strength was obtained. Furthermore, the Charpy impact value at -50°C was 20 (J / cm ２ ) or more, and excellent low-temperature toughness was obtained.

[0110] On the other hand, in test numbers 41 and 42, the Ti content was too high. Therefore, the number density ND of the coarse inclusions was excessive, and the number ratio NR of the coarse specific inclusions was high. As a result, the Charpy impact value of the simulated machine structural part at -50°C was 20 (J / cm ２ ) and sufficient low-temperature toughness was not obtained.

[0111] In test numbers 43 and 44, the molten steel temperature T1 in the tundish was too high. Therefore, although the number density ND of the coarse inclusions was appropriate, the number ratio NR of the specific coarse inclusions was high. As a result, the Charpy impact value of the simulated machine structural part at -50°C was 20 (J / cm ２ ) and sufficient low-temperature toughness was not obtained.

[0112] In test numbers 45 and 46, the molten steel temperature T1 in the tundish was too low. Therefore, the number density ND of the coarse inclusions was excessive, and the number ratio NR of the specific coarse inclusions was high. As a result, the Charpy impact value of the simulated machine structural part at −50°C was 20 (J / cm ２ ) and sufficient low-temperature toughness was not obtained.

[0113] In test numbers 47 and 48, the number of lower gates in the tundish was small. Therefore, although the number density ND of the coarse inclusions was appropriate, the number ratio NR of the specific coarse inclusions was high. As a result, the Charpy impact value of the simulated machine structural part at -50°C was 20 (J / cm ２ ) and sufficient low-temperature toughness was not obtained.

[0114] In test numbers 49 and 50, the number of upper dams in the tundish was small. Therefore, although the number density ND of the coarse inclusions was appropriate, the number ratio NR of the specific coarse inclusions was high. As a result, the Charpy impact value of the simulated machine structural part at -50°C was 20 (J / cm ２ ) and sufficient low-temperature toughness was not obtained.

[0115] The embodiments of the present invention have been described above. However, the above-described embodiments are merely examples for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiments, and the above-described embodiments can be appropriately modified and carried out without departing from the spirit of the present invention.

Claims

1. In mass%, C: 0.20 to 0.48%, Si: 0.20 to 1.30%, Mn: 0.80 to 2.00%, P: 0.050% or less, S: 0.010 to 0.090%, Cr: 0.05 to 0.50%, V: 0.05 to 0.30%, Ti: 0.0001 to 0.0055%, Al: 0.005 to 0.050%, N: 0.003 to 0.030%, Ca: 0.0001 to 0.0050%, O: 0.0030% or less, Cu: 0 to 0.40%, Ni: 0 to 0.30%, Mo: 0 to 0.20%, Nb: 0 to 0.050%, Zr: 0 to 0.050%, B: 0 to 0.0050%, Mg: 0 to 0.0050%, Bi: 0 to 0.050%, Sn: 0 to 0.100%, Sb: 0 to 0.050%, As: 0 to 0.050%, Se: 0 to 0.100%, Te: 0 to 0.050%, and Pb: 0 to 0.09%, with the balance being Fe and impurities, and the number density ND of coarse inclusions having an equivalent circle diameter of 3.0 μm or more is 0.70 pieces / mm ２ A steel material, wherein the number ratio NR of the coarse specific inclusions whose Mn content, Al content, Ti content and V content in mass % satisfy the formula (1) is 75% or less, (Mn + Al) / (Ti + V) < 0.30 (1) 2. The steel material according to claim 1, comprising, in mass%, Cu: 0.01 to 0.40%, Ni: 0.01 to 0.30%, Mo: 0.01 to 0.20%, Nb: 0.001 to 0.050%, Zr: 0.001 to 0.050%, B: 0.0001 to 0.0050%, Mg: 0.0001 to 0.0050%, Bi: 0.001 to 0.050%, Sn: 0.001 to 0.100%, Sb: 0.001 to 0.050%, As: 0.001 to 0.050%, Se: 0.001 to 0.100%, Te: 0.001 to 0.050%, and Pb: 0.01 to 0.09%.

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