steel

Abstract

The present invention provides a steel material that provides high strength and excellent low-temperature toughness for machine structural components. The steel material of this disclosure has a chemical composition, by mass%, of C: 0.20-0.48%, Si: 0.20-1.30%, Mn: 0.80-2.00%, P: 0.050% or less, S: 0.010-0.090%, Cr: 0.05-0.50%, V: 0.05-0.30%, Ti: 0.0001-0.0055%, Al: 0.005-0.050%, N: 0.003-0.030%, Ca: 0.0001-0.0050%, and O: 0.0030% or less, with the remainder being Fe and impurities, and has a number density ND of coarse inclusions with an equivalent circle diameter of 3.0 μm or more of 0.70 inclusions / mm². 2 The following conditions must be met: Among the coarse inclusions, the number percentage NR of coarse specific inclusions that satisfy formula (1) in terms of mass %, such as Mn content, Al content, Ti content, and V content, is 75% or less. (Mn+Al) / (Ti+V)<0.30 (1)

Description

[Technical Field]

[0001] This disclosure relates to steel materials, and more particularly to steel materials that can be used as materials for machine structural components manufactured by hot forging. [Background technology]

[0002] Structural components such as automotive parts, industrial machinery parts, and construction machinery parts utilize structural carbon steel and structural alloy steel. These steels are typically manufactured into structural components through hot forging. High strength is required for structural components.

[0003] Among mechanical structural components, those represented by automotive suspension parts such as front axles, tie rod arms, knuckles, and knuckle arms are sometimes used in cold climates. Therefore, these mechanical structural components require not only high strength but also excellent low-temperature toughness.

[0004] Furthermore, when manufacturing structural components for machinery using steel as the raw material, conventional manufacturing processes involve performing heat treatment (quenching and tempering) after hot forging. However, in order to reduce manufacturing costs, the omission of heat treatment is being considered. Therefore, even structural components for machinery manufactured without heat treatment are required to have high strength and excellent low-temperature toughness.

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

[0006] The hot forging steel disclosed in Patent Document 1 has the following weight ratios: C: 0.20-0.40%, Si: 0.05-0.50%, Mn: 0.80-2.00%, P: 0.018% or less, S: 0.030% or less, Cr: 0.30-1.50%, Mo: 0.05-0.50%, Al: 0.002-0.060%, V: 0.05-0.50%, and N: 0. It contains 0.08-0.020% of Pb and, if necessary, 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 remainder consists of Fe and impurity elements. Patent Document 1 states that by adjusting the chemical composition, high strength and excellent low-temperature toughness can be obtained even without tempering treatment. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Application Publication No. 8-3680 [Overview of the project] [Problems that the invention aims to solve]

[0008] However, high strength and excellent low-temperature toughness may be obtained in mechanical structural components even when the tempering treatment is omitted by means other than those described in Patent Document 1.

[0009] The object of the present invention is to provide a steel material that, when used as a material for mechanical structural parts, can provide high strength and excellent low-temperature toughness in mechanical structural parts even if heat treatment is omitted in the manufacturing process of the mechanical structural parts. [Means for solving the problem]

[0010] The steel material of the present disclosure contains, by 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%, and the balance consists of Fe and impurities. The number density ND of coarse inclusions having an equivalent circle diameter of 3.0 μm or more is 2 0.70 pieces / mm or less, and among the coarse inclusions, the number ratio NR of coarse specific inclusions whose Mn content, Al content, Ti content, and V content by mass% satisfy formula (1) is 75% or less. (Mn + Al) / (Ti + V) < 0.30 (1)

Advantages of the Invention

[0011] In the steel material of the present disclosure, when used as a material for mechanical structure parts, high strength and excellent low-temperature toughness can be obtained in the mechanical structure parts even if the quenching and tempering treatment is omitted in the manufacturing process of the mechanical structure parts.

Brief Description of the Drawings

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

Embodiments for Carrying Out the Invention

[0013] The inventors first investigated the chemical composition of steel materials that, when used as materials for machine structural parts, increase the strength of those parts. As a result, the inventors found that, in mass%, C: 0.20~0.48%, Si: 0.20~1.30%, Mn: 0.80~2.00%, P: 0.050% or less, S: 0.010~0.090%, Cr: 0.05~0.50%, V: 0.05~0.30%, Ti: 0.0001~0.0055%, Al: 0.005~0.050%, N: 0.003~0.030%, Ca: 0.0001~0.0050%, O: 0.0030% or less, Cu: 0~0.40%, Ni: 0~0.30%, Mo: 0~ We hypothesized that if a steel material has a chemical composition containing 0.20% of the following elements: 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 remainder being Fe and impurities, then using this steel material to manufacture structural components for machinery would result in increased strength. Furthermore, we hypothesized that, given the extremely low Ti content in the above chemical composition, excellent low-temperature toughness could also be obtained in the structural components.

[0014] However, even steel materials that satisfy the above chemical composition sometimes fail to provide sufficient low-temperature toughness when used as mechanical structural components. Therefore, the inventors further investigated means to improve the low-temperature toughness of mechanical structural components made from steel materials that satisfy the above chemical composition.

[0015] Here, the inventors focused on inclusions in steel materials. Coarse inclusions can become the starting point for cracks in low-temperature environments such as cold regions. Therefore, the inventors attempted to improve low-temperature toughness by minimizing the number density of coarse inclusions in steel materials. As a result of their investigation, they found that for steel materials satisfying the above chemical composition, the number density ND of coarse inclusions with an equivalent circle diameter of 3.0 μm or more is 0.70 inclusions / mm². 2 The inventors have found that the low-temperature toughness of mechanical structural components is increased if the following conditions are met.

[0016] However, even when the number density ND of coarse inclusions was reduced, sufficient low-temperature toughness was still sometimes not obtained. Therefore, the inventors conducted further investigations. As a result, the inventors obtained the following findings.

[0017] The main inclusions that may be contained in steel materials having the above-mentioned chemical composition are Al oxides represented by Al2O3, Mn sulfides represented by MnS, Ti and / or V nitrides (hereinafter referred to as TiV nitrides in this specification), and composite inclusions thereof. Of these inclusions, composite inclusions of Al oxides and / or Mn sulfides and TiV nitrides are the coarsest inclusions. Therefore, assuming a number density ND of coarse inclusions of 0.70 pieces / mm³ 2 Even if the following conditions are met, if the proportion of the aforementioned composite inclusions among the coarse inclusions is high, these coarse inclusions will become the starting point for cracking, and sufficient low-temperature toughness cannot be obtained.

[0018] Based on the above findings, the inventors focused on the chemical composition of coarse inclusions remaining in the steel material and conducted an investigation. As a result, they obtained the following findings.

[0019] F1 is defined as follows: F1 = (Mn + Al) / (Ti + V) Here, the values ​​for Mn, Al, Ti, and V in F1 are substituted with the mass percentages of Mn, Al, Ti, and V in the coarse inclusions, assuming that the chemical composition of the coarse inclusions is 100% by mass. In this specification, inclusions with an F1 value of less than 0.30 are referred to as "specific inclusions." Specific inclusions correspond to the composite inclusions described above.

[0020] The number density (ND) of coarse inclusions with an equivalent circular diameter of 3.0 μm or larger in the steel material is 0.70 pieces / mm². 2If the following conditions are met, and the proportion NR of coarse inclusions with an F1 of less than 0.30 (hereinafter referred to as "specific coarse inclusions") is 75% or less, then the proportion of specific coarse inclusions corresponding to composite inclusions among the coarse inclusions is sufficiently small. As a result, it is possible to sufficiently suppress the coarse inclusions remaining in the steel from becoming the initiation point of cracks in low-temperature environments. Consequently, when the steel is used as a material for machine structural parts, even if heat treatment is omitted in the manufacturing process of the machine structural parts, high strength and excellent low-temperature toughness can be obtained in the machine structural parts.

[0021] Based on the above technical concept, the steel material according to this embodiment has the following configuration.

[0022] The first composition of steel is as follows (by mass%): C: 0.20~0.48%, Si: 0.20~1.30%, Mn: 0.80~2.00%, P: 0.050% or less, S: 0.010~0.090%, Cr: 0.05~0.50%, V: 0.05~0.30%, Ti: 0.0001~0.0055%, Al: 0.005~0.050%, N: 0.003~0.030%, Ca: 0.0001~0.0050%, O: 0.0030% or less. It 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 remainder being Fe and impurities. The number density ND of coarse inclusions with an equivalent circle diameter of 3.0 μm or more is 0.70 pieces / mm 2 The following conditions must be met: among the coarse inclusions, the number percentage NR of coarse specific inclusions whose mass % Mn content, Al content, Ti content, and V content satisfy formula (1) is 75% or less. (Mn+Al) / (Ti+V)<0.30 (1)

[0023] The second compositional steel is the same as the first compositional steel, and contains, by mass%, one or more elements selected from the group consisting of Cu: 0.01-0.40%, Ni: 0.01-0.30%, Mo: 0.01-0.20%, Nb: 0.001-0.050%, Zr: 0.001-0.050%, B: 0.0001-0.0050%, Mg: 0.0001-0.0050%, Bi: 0.001-0.050%, Sn: 0.001-0.100%, Sb: 0.001-0.050%, As: 0.001-0.050%, Se: 0.001-0.100%, Te: 0.001-0.050%, and Pb: 0.01-0.09%.

[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 this embodiment] The steel material of this embodiment satisfies the following features 1 to 3. (Feature 1) The chemical composition, in mass%, is as follows: C: 0.20-0.48%, Si: 0.20-1.30%, Mn: 0.80-2.00%, P: 0.050% or less, S: 0.010-0.090%, Cr: 0.05-0.50%, V: 0.05-0.30%, Ti: 0.0001-0.0055%, Al: 0.005-0.050%, N: 0.003-0.030%, Ca: 0.0001-0.0050%, O: 0.0030% or less. It 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 remainder being Fe and impurities. (Feature 2) The number density (ND) of coarse inclusions with an equivalent circular diameter of 3.0 μm or larger is 0.70 pieces / mm². 2 The following applies: (Feature 3) Of the coarse inclusions, the number percentage NR of coarse specific inclusions whose mass % Mn content, Al content, Ti content, and V content satisfy formula (1) is 75% or less. (Mn+Al) / (Ti+V)<0.30 (1) Features 1 to 3 are explained below.

[0026] [(Feature 1) Regarding chemical composition] The chemical composition of the steel material in this embodiment contains the following elements:

[0027] C: 0.20~0.48% Carbon (C) increases the strength of machine structural components manufactured from steel. If the C content is less than 0.20%, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the carbon content exceeds 0.48%, the strength of the mechanical structural component becomes excessively high. In this case, even if the content of other elements is within the range of this embodiment, the low-temperature toughness of the mechanical structural component decreases. Therefore, the C content is 0.20-0.48%. The preferred lower limit of the C content is 0.21%, more preferably 0.22%, more preferably 0.24%, more preferably 0.26%, and more preferably 0.28%. The preferred upper limit for the C content is 0.45%, more preferably 0.40%, and even more preferably 0.38%.

[0028] Si: 0.20~1.30% Silicon (Si) deoxidizes steel during the steelmaking process. Si further enhances the strength of machine structural components manufactured using steel as a material. If the Si content is less than 0.20%, the above effects cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Si content exceeds 1.30%, the strength of the mechanical structural component becomes excessively high. In this case, even if the content of other elements is within the range of this embodiment, the low-temperature toughness of the mechanical structural component decreases. Therefore, the Si content is 0.20-1.30%. The preferred lower limit of the Si content is 0.30%, more preferably 0.40%, more preferably 0.45%, more preferably 0.50%, more preferably 0.55%, and more preferably 0.60%. The preferred upper limit for the Si content is 1.20%, more preferably 1.10%, even more preferably 1.00%, and even more preferably 0.90%.

[0029] Mn: 0.80~2.00% Manganese (Mn) deoxidizes steel during the steelmaking process. Mn further enhances the strength of machine structural components manufactured using steel as a raw material. If the Mn content is less than 0.80%, the above effects cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Mn content exceeds 2.00%, the strength of the mechanical structural component becomes excessively high. In this case, even if the content of other elements is within the range of this embodiment, the low-temperature toughness of the mechanical structural component decreases. Therefore, the Mn content is 0.80-2.00%. The preferred lower limit of the Mn content is 0.90%, more preferably 1.00%, more preferably 1.10%, more preferably 1.20%, more preferably 1.30%, and more preferably 1.40%. The preferred upper limit for the Mn content is 1.90%, more preferably 1.80%, more preferably 1.70%, more preferably 1.60%, and 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, reducing the hot workability of steel. Furthermore, P reduces the low-temperature toughness of mechanical structural components. Therefore, the P content is 0.050% or less. A low phosphorus (P) content is preferable. However, excessive reduction of 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 for the P content is 0.045%, more preferably 0.040%, more preferably 0.035%, more preferably 0.030%, more preferably 0.025%, and more preferably 0.020%.

[0031] S: 0.010~0.090% Sulfur (S) generates sulfides, which improve the machinability of steel. If the S content is less than 0.010%, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the sulfur content exceeds 0.090%, sulfur will excessively segregate at the grain boundaries. In this case, even if the content of other elements is within the range of this embodiment, the low-temperature toughness of the mechanical structural component will decrease. Therefore, the sulfur content is 0.010 to 0.090%. The preferred lower limit of the S content is 0.015%, more preferably 0.020%, more preferably 0.025%, more preferably 0.030%, and more preferably 0.035%. The preferred upper limit for the S content is 0.080%, more preferably 0.070%, even more preferably 0.065%, and even more preferably 0.060%.

[0032] Cr: 0.05~0.50% Chromium (Cr) enhances the hardenability of steel. Therefore, the strength of machine structural components manufactured from steel is increased. If the Cr content is less than 0.05%, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Cr content exceeds 0.50%, sufficient machinability cannot be obtained in the steel material, even if the content of other elements is within the range of this embodiment. Therefore, the Cr content is 0.05-0.50%. The preferred lower limit for 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 for the Cr content is 0.45%, more preferably 0.40%, even more preferably 0.35%, and even more preferably 0.30%.

[0033] V: 0.05~0.30% Vanadium (V) precipitates as a V precipitate in ferrite within steel during the manufacturing process of mechanical structural parts made from steel. This increases the hardness of the ferrite in the steel. As a result, the strength of the mechanical structural parts increases. If the V content is less than 0.05%, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the V content exceeds 0.30%, the strength of the mechanical structural component becomes excessively high. In this case, even if the content of other elements is within the range of this embodiment, the low-temperature toughness of the mechanical structural component decreases. Therefore, the V content is 0.05-0.30%. The preferred lower limit for the V content is 0.07%, more preferably 0.10%, and even more preferably 0.15%. The preferred upper limit for the V content is 0.28%, more preferably 0.26%, more preferably 0.24%, more preferably 0.22%, and more preferably 0.20%.

[0034] Ti: 0.0001~0.0055% Titanium (Ti) forms precipitates (carbides and / or carbonitrides). These precipitates refine the grain size of the steel material through a pinning effect. This increases the low-temperature toughness of mechanical structural components. If the Ti content is less than 0.0001%, the above effect cannot be fully obtained. On the other hand, if the Ti content exceeds 0.0055%, coarse inclusions will be excessively generated. In this case, even if the content of other elements is within the range of this embodiment, the low-temperature toughness of the mechanical structural component will decrease. Therefore, the Ti content is between 0.0001% and 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 preferred upper limit for the Ti content is 0.0050%, more preferably 0.0045%, even more preferably 0.0040%, and even more preferably 0.0035%.

[0035] Al: 0.005~0.050% Aluminum (Al) deoxidizes steel. If the Al content is less than 0.005%, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Al content exceeds 0.050%, coarse inclusions are excessively generated. In this case, even if the content of other elements is within the range of this embodiment, the low-temperature toughness of the mechanical structural component will decrease. Therefore, the Al content is 0.005-0.050%. The preferred lower limit for the Al content is 0.010%, more preferably 0.015%, and even more preferably 0.020%. The preferred upper limit for the Al content is 0.045%, more preferably 0.040%, and even more preferably 0.035%.

[0036] N: 0.003~0.030% Nitrogen (N) strengthens steel structural components by forming nitrides and / or carbonitrides during the manufacturing process, thereby increasing the strength of the structural components. If the N content is less than 0.003%, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the N content exceeds 0.030%, the hot workability of the steel material will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the N content is 0.003 to 0.030%. The preferred lower limit for the N content is 0.005%, and more preferably 0.010%. The preferred upper limit for the N content is 0.025%, more preferably 0.020%, even more preferably 0.018%, and even more preferably 0.015%.

[0037] Ca: 0.0001~0.0050% Calcium (Ca) improves the machinability of steel materials. If the Ca content is less than 0.0001%, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Ca content exceeds 0.0050%, coarse oxides are produced in excess. In this case, even if the content of other elements is within the range of this embodiment, the low-temperature toughness of the mechanical structural component will decrease. Therefore, the Ca content is between 0.0001% and 0.0050%. The preferred lower limit for the Ca content is 0.0002%, and more preferably 0.0003%. The preferred upper limit for the Ca content is 0.0045%, more preferably 0.0040%, more preferably 0.0035%, more preferably 0.0030%, more preferably 0.0020%, more preferably 0.0015%, and more preferably 0.0010%.

[0038] O: 0.0030% or less Oxygen (O) is an unavoidable impurity. That is, the O content is greater than 0%. O forms oxides, which reduce the low-temperature toughness of mechanical structural components. Therefore, the O content is 0.0030% or less. A low oxygen content is preferable. However, excessive reduction of the oxygen content increases manufacturing costs. Therefore, considering normal industrial production, the preferred lower limit of the oxygen content is 0.0001%, more preferably 0.0002%, and even more preferably 0.0005%. The preferred upper limit for the O content is 0.0025%, more preferably 0.0020%, even more preferably 0.0018%, and even more preferably 0.0015%.

[0039] The remainder of the chemical composition of the steel material according to this embodiment consists of Fe and impurities. Here, impurities refer to substances that are mixed in from the raw materials such as ore, scrap, or the manufacturing environment during the industrial production of steel material, and are acceptable as long as they do not adversely affect the steel material according to this embodiment.

[0040] [Optional Elements] The chemical composition of the steel material in 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 arbitrary elements. These arbitrary elements will be described below.

[0041] [Group 1: Cu, Ni, Mo, Nb, Zr, and B] The chemical composition of the steel material in this embodiment may further contain one or more elements selected from the group consisting of Cu, Ni, Mo, Nb, Zr, and B in place of a portion of Fe. All of these elements are arbitrary and all of them increase the strength of machine structural parts manufactured using the steel material.

[0042] Cu: 0~0.40% Copper (Cu) is an optional element and does not need to be included. In other words, the Cu content may be 0%. When present, i.e., when the Cu content is greater than 0%, Cu increases the strength of mechanical structural components. Even a small amount of Cu will provide some degree of this effect. However, if the Cu content exceeds 0.40%, the hot workability of the steel material will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the copper content is 0-0.40%. The preferred lower limit for the Cu content is 0.01%, and more preferably 0.02%. The preferred upper limit for the Cu content is 0.30%, more preferably 0.20%, even more preferably 0.10%, and even more preferably 0.05%.

[0043] Ni: 0~0.30% Nickel (Ni) is an optional element and does not need to be included. In other words, the Ni content may be 0%. When present, i.e., when the Ni content is greater than 0%, Ni increases the strength of mechanical structural components. Even a small amount of Ni will provide some degree of this effect. However, if the Ni content exceeds 0.30%, the hardness of the steel becomes excessively high, even if the content of other elements is within the range of this embodiment. In this case, the hot workability of the steel decreases, even if the content of other elements is within the range of this embodiment. Therefore, the Ni content is 0-0.30%. The preferred lower limit for the Ni content is 0.01%, and more preferably 0.02%. The preferred upper limit for the Ni content is 0.25%, more preferably 0.20%, and even more preferably 0.15%.

[0044] Mo: 0~0.20% Molybdenum (Mo) is an optional element and does not need to be included. In other words, the Mo content may be 0%. When present, i.e., when the Mo content is greater than 0%, Mo increases the strength of mechanical structural components. Even a small amount of Mo will provide some degree of the above effect. However, if the Mo content exceeds 0.20%, the hardness of the steel becomes excessively high. In this case, the hot workability of the steel decreases. Therefore, the Mo content is 0-0.20%. The preferred lower limit for the Mo content is 0.01%, more preferably 0.03%, and even more preferably 0.05%. The preferred upper limit for the Mo content is 0.18%, more preferably 0.16%, and even more preferably 0.14%.

[0045] Nb: 0~0.050% Niobium (Nb) is an optional element and does not need to be included. In other words, the Nb content may be 0%. When Nb is present, i.e., when the Nb content is greater than 0%, Nb enhances the strength of mechanical structural components through precipitation strengthening. Furthermore, Nb refines the grain size of the steel through a pinning effect. As a result, the low-temperature toughness of mechanical structural components is increased. Even a small amount of Nb can provide some of the above effects. However, if the Nb content exceeds 0.050%, the hardness of the steel becomes excessively high. In this case, even if the content of other elements is within the range of this embodiment, the hot workability of the steel decreases. Therefore, the Nb content is 0-0.050%. The preferred lower limit of the Nb content is 0.001%, more preferably 0.005%, and even more preferably 0.010%. The preferred upper limit for the Nb content is 0.045%, more preferably 0.040%, even more preferably 0.035%, and even more preferably 0.025%.

[0046] Zr: 0~0.050% Zirconium (Zr) is an optional element and does not need to be included. In other words, the Zr content may be 0%. When Zr is present, i.e., when the Zr content is greater than 0%, Zr enhances the strength of mechanical structural components through precipitation strengthening. Furthermore, Zr refines the grain size of the steel through a pinning effect. As a result, the low-temperature toughness of mechanical structural components is increased. Even a small amount of Zr will provide some of the above effects. However, if the Zr content exceeds 0.050%, the hardness of the steel becomes excessively high. In this case, even if the content of other elements is within the range of this embodiment, the hot workability of the steel decreases. Therefore, the Zr content is 0-0.050%. The preferred lower limit for the Zr content is 0.001%, more preferably 0.003%, and even more preferably 0.005%. The preferred upper limit for the Zr content is 0.045%, more preferably 0.040%, even more preferably 0.035%, and even more preferably 0.030%.

[0047] B: 0~0.0050% Boron (B) is an optional element and does not need to be included. In other words, the B content may be 0%. If present, i.e., if the B content is greater than 0%, B segregates at the grain boundaries, increasing grain boundary strength and improving the strength and low-temperature toughness of mechanical structural components. Even a small amount of B present will provide some degree of the above effect. However, if the B content exceeds 0.0050%, the hot workability of the steel material will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the B content is 0-0.0050%. The preferred lower limit for the B content is 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%. The preferred upper limit for the B content is 0.0040%, more preferably 0.0035%, even more preferably 0.0030%, and even more preferably 0.0020%.

[0048] [Group 2: Mg, Bi, Sn, Sb, As, Se, Te, and Pb] The chemical composition of the steel material in this embodiment may further contain, in place of some of the Fe, one or more elements selected from the group consisting of Mg, Bi, Sn, Sb, As, Se, Te, and Pb. All of these elements are arbitrary and all enhance the machinability of the steel material.

[0049] Mg: 0~0.0050% Magnesium (Mg) is an optional element and does not need to be included. In other words, the Mg content may be 0%. When present, i.e., when the Mg content is greater than 0%, Mg improves the machinability of steel. Even a small amount of Mg will provide some degree of this effect. However, if the Mg content exceeds 0.0050%, the Mg will form coarse oxides, even if the content of other elements is within the range of this embodiment. These coarse oxides reduce the low-temperature toughness of machine structural components manufactured from steel. Therefore, the Mg content is 0-0.0050%. 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 for the Mg content is 0.0040%, more preferably 0.0035%, even more preferably 0.0030%, and even more preferably 0.0020%.

[0050] Bi: 0~0.050% Bismuth (Bi) is an optional element and does not need to be included. In other words, the Bi content may be 0%. When Bi is present, that is, when the Bi content is greater than 0%, Bi generates Bi particles in the matrix phase, making the steel brittle. As a result, the machinability of the steel increases. Even if only a small amount of Bi is present, the above effect can be obtained to some extent. However, if the Bi content exceeds 0.050%, excessive Bi particles will be generated, even if the content of other elements is within the range of this embodiment. In this case, the hot workability of the steel material will decrease. Therefore, the Bi content is 0-0.050%. 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 for the Bi content is 0.040%, more preferably 0.030%, even more preferably 0.020%, and even more preferably 0.015%.

[0051] Sn: 0~0.100% Tin (Sn) is an optional element and does not need to be included. In other words, the Sn content may be 0%. If sn is present, that is, if the sn content is greater than 0%, the sn segregates at the interface between the matrix and the inclusions, embrittles the steel. This increases the machinability of the steel. Even a small amount of sn can provide the above effect to some extent. However, if the Sn content exceeds 0.100%, excessive segregation of Sn will occur, even if the content of other elements is within the range of this embodiment. In this case, the hot workability of the steel material will decrease. Therefore, the Sn content is 0-0.100%. The preferred lower limit for the Sn content is 0.001%, more preferably 0.005%, and even more preferably 0.010%. The preferred upper limit for the Sn content is 0.050%, more preferably 0.045%, even more preferably 0.040%, and even more preferably 0.030%.

[0052] Sb: 0~0.050% Antimony (Sb) is an optional element and does not need to be included. In other words, the Sb content may be 0%. If Sb is present, that is, if the Sb content is greater than 0%, Sb segregates at the interface between the matrix phase and the inclusions, embrittles the steel. As a result, the machinability of the steel increases. Even if only a small amount of Sb is present, the above effect can be obtained to some extent. However, if the Sb content exceeds 0.050%, Sb will segregate excessively, even if the content of other elements is within the range of this embodiment. In this case, the hot workability of the steel material will decrease. Therefore, the Sb content is 0-0.050%. The preferred lower limit for the Sb content is 0.001%, more preferably 0.005%, and even more preferably 0.010%. The preferred upper limit for the Sb content is 0.040%, more preferably 0.030%, and even more preferably 0.020%.

[0053] As: 0~0.050% Arsenic (As) is an optional element and may not be present. In other words, the As content may be 0%. When As is present, that is, when the As content is greater than 0%, As segregates at the interface between the matrix phase and the inclusions, embrittles the steel. This increases the machinability of the steel. Even a small amount of As can provide the above effect to some extent. However, if the As content exceeds 0.050%, excessive segregation of As will occur, even if the content of other elements is within the range of this embodiment. In this case, the hot workability of the steel material will decrease. Therefore, the As content is 0-0.050%. The preferred lower limit for the As content is 0.001%, more preferably 0.005%, and even more preferably 0.010%. The preferred upper limit for the As content is 0.040%, more preferably 0.030%, and even more preferably 0.020%.

[0054] Se: 0~0.100% Selenium (Se) is an optional element and does not need to be present. In other words, the Se content may be 0%. When Se is present, i.e., when Se is greater than 0%, Se segregates at the interface between the matrix phase and the inclusions, embrittles the steel. This increases the machinability of the steel. Even a small amount of Se present can provide the above effect to some extent. However, if the Se content exceeds 0.100%, the hot workability of the steel material will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the Se content is 0-0.100%. 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 for the Se content is 0.090%, more preferably 0.080%, more preferably 0.070%, more preferably 0.060%, and more preferably 0.050%.

[0055] Te: 0~0.050% Tellurium (Te) is an optional element and does not need to be included. In other words, the Te content may be 0%. When present, i.e., when the Te content is greater than 0%, Te improves the machinability of steel. Even a small amount of Te will provide some degree of the above effect. However, if the Te content exceeds 0.050%, the hot workability of the steel material will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the Te content is 0-0.050%. The preferred lower limit for the Te content is 0.001%, more preferably 0.003%, and even more preferably 0.010%. The preferred upper limit for the Te content is 0.040%, more preferably 0.030%, more preferably 0.025%, and still more preferably 0.020%.

[0056] Pb: 0~0.09% Lead (Pb) is an optional element and does not need to be included. In other words, the Pb content may be 0%. If Pb is present, that is, if the Pb content is greater than 0%, Pb will generate Pb particles in the steel, making the steel brittle. As a result, the machinability of the steel will improve. Even if only a small amount of Pb is present, the above effect can be obtained to some extent. However, if the Pb content exceeds 0.09%, excessive Pb particles will be generated, even if the content of other elements is within the range of this embodiment. In this case, the hot workability of the steel material will decrease. Therefore, the Pb content is 0-0.09%. The preferable lower limit of the Pb content is 0.01%, more preferably 0.02%, and even more preferably 0.03%. The preferable 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) Regarding the number density ND (number / mm 2 ) of coarse inclusions] In the steel material of the present embodiment, further, the number density ND of coarse inclusions having an equivalent circle diameter of 3.0 μm or more is 0.70 number / mm 2 or less. Here, the equivalent circle diameter means the diameter (μm) of a circle having the same area as the area of the inclusion.

[0058] In the steel material of the present embodiment, the number density ND of coarse inclusions having an equivalent circle diameter of 3.0 μm or more is 0.70 number / mm 2 or less. Coarse inclusions can be the starting point of cracks in a low-temperature environment. In the present embodiment, by suppressing the number density ND of coarse inclusions to 0.70 number / mm 2 or less, the occurrence of cracks in a low-temperature environment is suppressed. As a result, the low-temperature toughness of mechanical structure parts manufactured using the steel material can be significantly increased.

[0059] The preferable upper limit of the number density ND is 0.65 number / mm 2 and more preferably 0.60 number / mm 2 and even more preferably 0.55 number / mm 2 and even more preferably 0.50 number / mm It is preferable that the number density ND is as small as possible. However, excessive reduction of the number density ND increases the manufacturing cost. Therefore, considering normal industrial production, the preferable lower limit of the number density ND is 0.10 number / mm 2 and more preferably 0.20 number / mm 2 and even more preferably 0.30 number / mm 2 and even more preferably 0.40 number / mm 2 and even more preferably 0.50 number / mm

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

[0061] Multiple observation fields are selected from the target area for SEM observation. The selected multiple observation fields are observed at 750x magnification to generate SEM images (backscattered electron images). The total area of ​​the multiple observation fields is 260 mm². 2 Select the size and number of observation fields accordingly. Note that the total area of ​​the observation region in one cross-section (the region from 0.50R depth position to 0.90R depth position) is 260 mm². 2 If it is less than 260 mm², prepare the above cross-section of the second test specimen and the total area with the observation field of view of the first test specimen. 2 Select one or more observation fields from the target region of the second cross-section to achieve this. Note that the SEM-EDS is equipped with a backscattered electron detector, and the electron gun is a tungsten filament.

[0062] In SEM images, particles are identified based on contrast. The equivalent circle diameter (μm) of the identified particles is determined based on their area. The equivalent circle diameter can be determined using well-known image processing techniques. Among the identified particles, those with an equivalent circle diameter of 3.0 μm or larger are classified as coarse inclusions.

[0063] Count the total number of coarse inclusions within the observation field. The total number of coarse inclusions and the total area of ​​the observation field (260 mm²) are then compared. 2 Based on this, the number density of coarse inclusions is calculated as ND (pieces / mm²). 2 ) is calculated. The number density ND is the value obtained by rounding the third decimal place of the result to the second decimal place.

[0064] [(Feature 3) Regarding the number ratio NR (%) of coarse specific inclusions] In the steel material of this embodiment, the number percentage NR of coarse inclusions having an equivalent circular diameter of 3.0 μm or more that satisfy formula (1) in terms of mass%, is 75% or less. (Mn+Al) / (Ti+V)<0.30 (1) Here, in equation (1), the values ​​of Mn, Al, Ti, and V are substituted for the mass percentage content of Mn, Al, Ti, and V in the coarse inclusions, assuming that the chemical composition of the coarse inclusions is 100% by mass.

[0065] Coarse specific inclusions where F1 (=(Mn+Al) / (Ti+V)) satisfies equation (1) correspond to composite inclusions of Al oxide and / or Mn sulfide and TiV nitride. The size of these composite inclusions is coarser compared to Al oxide, Mn sulfide, and TiV nitride. Therefore, assuming the number density ND of the coarse inclusions is 0.70 pieces / mm³ 2 Even if the following conditions are not met, if the proportion of coarse, specific coarse inclusions among the coarse inclusions is high, the coarse inclusions in the steel are more likely to become the starting point for cracks in low-temperature environments.

[0066] Therefore, in this embodiment, the number density ND of coarse inclusions is set to 0.70 pieces / mm². 2 The following conditions shall be met, and the proportion NR of coarse specific inclusions that satisfy formula (1) among the coarse inclusions shall be 75% or less. In this case, it is possible to sufficiently suppress the coarse inclusions remaining in the steel from becoming the initiation point of cracks in low-temperature environments. As a result, the low-temperature toughness of machine structural parts manufactured using steel as the material can be significantly improved.

[0067] The preferred upper limit for the number ratio NR is 73%, more preferably 71%, and even more preferably 69%. There is no particular lower limit to the number percentage NR. However, excessive reduction of the number percentage NR increases manufacturing costs. Therefore, in the case of normal industrial production, the lower limit of the number percentage NR is, for example, 30%, 40%, or 50%.

[0068] [Method for measuring the number ratio NR of coarse specific inclusions] The percentage of large, specific inclusions, NR(%), is determined by the following method. The total area observed using the method described in the above-mentioned [Method for measuring the number density ND of coarse inclusions] was 260 mm². 2 Identify coarse inclusions within the entire field of view. For each identified coarse inclusion, elemental concentration analysis is performed using energy-dispersive X-ray spectroscopy (EDS) to identify specific coarse inclusions. In the EDS analysis (elemental concentration analysis), the acceleration voltage is set to 20 kV, the emission current to 21 μA, and the elements to be quantified are Si, Mn, S, Cr, Ti, V, Cu, Ni, Ca, N, O, and Al.

[0069] As shown in Figure 1, one line segment L is selected from the line segments L connecting two points P1 and P2 on the outer edge of the identified coarse inclusion 10, and that is larger than the equivalent circular diameter of the coarse inclusion. This line segment L is divided into three equal parts S1 to S3, and the central 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 measurement point CP1 to CP3, the Mn content, Al content, Ti content, and V content are determined in mass percent, assuming the chemical composition of the coarse inclusion is 100% by mass (i.e., the sum of the content of the quantitative elements Si, Mn, S, Cr, Ti, V, Cu, Ni, Ca, N, O, and Al is 100% by mass). The arithmetic mean of the Mn content obtained at the three measurement points is taken as the Mn content (mass %) of the coarse inclusion. The Mn content is rounded to the first decimal place by rounding the second decimal place of the obtained arithmetic mean. The arithmetic mean of the Al content obtained at the three measurement points is taken as the Al content (mass %) of the coarse inclusion. The Al content is rounded to the first decimal place by rounding the second decimal place of the obtained arithmetic mean. The arithmetic mean of the Ti content obtained at the three measurement points CP1 to CP3 is taken as the Ti content (mass %) of the coarse inclusion. The Ti content is rounded to the first decimal place by rounding the second decimal place of the obtained arithmetic mean. The V content (mass%) of the coarse inclusion is taken as the arithmetic mean of the V content obtained at the three measurement points. The V content is rounded to the first decimal place by rounding the second decimal place of the obtained arithmetic mean. Based on the obtained Mn content, Al content, Ti content, and V content, coarse inclusions where F1 satisfies equation (1) are identified as "coarse specific inclusions". Note that F1 is rounded to the second decimal place by rounding the third decimal place of the obtained value.

[0071] Total area is 260mm 2 The total number of coarse specific inclusions in all observed fields and the total number of coarse specific inclusions are used to determine the percentage NR (%) of coarse specific inclusions. The percentage NR is rounded to the first decimal place of the obtained value.

[0072] [Effects of the steel material in this embodiment] As described above, the steel material of this embodiment satisfies features 1 to 3. Therefore, when manufacturing machine structural parts using the steel material of this embodiment, even if heat treatment (quenching and tempering) is omitted, high strength and excellent low-temperature toughness can be obtained in the machine structural parts.

[0073] [Regarding the microstructure of the steel material of this embodiment] The microstructure of the steel material in this embodiment is not particularly limited. The steel material in this embodiment is A before hot forging in the manufacturing process of machine structural parts. c3 The material is heated above its transformation point. This heating causes the microstructure of the steel to transform into austenite. Therefore, the microstructure of the steel in this embodiment is not particularly limited. For example, in the R / 2 portion of the steel, 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 in this embodiment is not particularly limited to the above-described microstructure. Here, the R / 2 portion refers to the central part of the radius in a cross-section (circular cross-section) perpendicular to the axial direction of the steel.

[0074] [Preferred uses and shapes of the steel material of this embodiment] The steel material of this embodiment can be widely applied, for example, as a material for mechanical structural parts. In particular, when used as a material for mechanical structural parts, the steel material of this embodiment is suitable for applications in mechanical structural parts because it can achieve both high strength and excellent low-temperature toughness even if quenching and tempering are omitted in the manufacturing process of the mechanical structural parts. Such mechanical structural parts include, 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 with a circular cross-section perpendicular to the axial direction, such as a steel bar or wire.

[0076] [Manufacturing method] An example of a method for manufacturing steel materials according to this embodiment will be described. Steel materials satisfying features 1 to 3 may be manufactured by methods other than those described below. However, the manufacturing methods described below are preferred examples of the method for manufacturing steel materials according to this embodiment.

[0077] An example of the wire manufacturing method of this embodiment includes the following steps. (Process 1) Steelmaking process (Process 2) Blooming rolling process (Process 3) Finish rolling process The following describes each step.

[0078] [(Process 1) Steelmaking Process] In the steelmaking process, primary and secondary refining are performed on molten steel having a chemical composition that satisfies characteristic 1. When primary refining is performed in an electric furnace, the following primary and secondary refining processes are carried out 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 ladle, secondary refining is carried out on the molten steel in the ladle. In the secondary refining, refining in the atmosphere is first carried out using an LF (Ladle Furnace). After the LF treatment, vacuum degassing is carried out. Vacuum degassing treatments include, for example, VD treatment (Vacuum Degasser) and / or RH treatment (Ruhrstahl Heraeus).

[0080] The molten steel in the ladle after secondary refining is transferred to a tundish, and continuous casting is carried out to produce bloom. At this time, the following conditions are met in the tundish. (Condition 1) Maintain the molten steel temperature T1 in the tundish at 1520-1570°C. (Condition 2) Within the tundish, in the path from the pouring nozzle to the immersion nozzle of the ladle, lower weirs and upper weirs are arranged alternately along the path, in the order of lower weirs and upper weirs, with at least two lower weirs and at least two upper weirs arranged along the aforementioned 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 to the surface and remain in the molten steel as they flow into the immersion nozzle. As a result, composite inclusions (specific inclusions) with TiV nitrides are formed using Al oxides and Mn sulfides as nuclei, and coarse specific inclusions are more likely to form. Therefore, the number ratio NR of coarse specific inclusions increases. On the other hand, if the molten steel temperature T1 in the tundish is too low, Mn sulfides crystallize in excess. As a result, the number density ND of coarse inclusions becomes excessive. Furthermore, after the molten steel flows into the immersion nozzle, composite inclusions (specific inclusions) with TiV nitrides are formed using Mn sulfides as nuclei, and coarse specific inclusions are easily formed. As a result, the number ratio NR of coarse specific inclusions becomes high. Therefore, the molten steel temperature T1 is set to 1520-1570°C. The molten steel temperature T1 is measured using a thermocouple placed near the injection hole of the immersion nozzle in the tundish.

[0082] [(Condition 2) Regarding the number of lower and upper weirs within the tundish] Figure 2 is a cross-sectional view of the tundish. Referring to Figure 2, within the tundish 1, in the path from the injection nozzle 3 of the ladle 2 to the immersion nozzle 4, the lower weir 5 and upper weir 6 are arranged alternately along the path in the order of lower weir 5 and upper weir 6, with at least two lower weirs and at least two upper weirs arranged along the path. Here, the lower weir 5 is a weir extending upward from the bottom 1A of the tundish 1. The upper weir 6 is a weir extending downward from the lid 1B of the tundish 1. When molten steel is injected into the tundish 1, the molten steel flows above the lower weir 5 and below the upper weir 6 in the path from the injection nozzle 3 to the immersion nozzle 4. More specifically, as the molten steel approaches the lower weir 5, it rises and flows over the lower weir 5. As the molten steel approaches the upper weir 6, it descends and flows below the upper weir 6.

[0083] In the path from the injection nozzle 3 to the immersion nozzle 4 of the ladle within the tundish 1, when upward and downward flows occur in the molten steel, the opportunities for Al oxides and Mn sulfides in the molten steel to float to the surface increase, and the Al oxides and Mn sulfides are more easily absorbed by 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 arranging the lower weirs 5 and upper weirs 6 alternately along the path from the injection nozzle 3 to the immersion nozzle 4, and by increasing the number of lower weirs 5 and upper weirs 6, more upward and downward flows can be formed in the molten steel.

[0084] If the number of lower weirs 5 is less than 2, or the number of upper weirs 6 is less than 2, there is an insufficient number of upward and downward flows in the molten steel within the tundish 1. In this case, Al oxides and Mn sulfides in the molten steel are not sufficiently separated by flotation, and a large amount of Al oxides and Mn sulfides remain in the molten steel as it flows into the immersion nozzle 4. As a result, composite inclusions (specific inclusions) with TiV are formed using Al oxides and Mn sulfides as nuclei, and coarse specific inclusions are easily formed. Therefore, the proportion NR of coarse specific inclusions increases.

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

[0086] Furthermore, if the number of lower weirs 5 and upper weirs 6 placed along the path is too large, the travel time of the molten steel to the immersion nozzle 4 will increase. As a result, the molten steel temperature T1 may drop excessively. Therefore, the preferred upper limit for the number of lower weirs 5 is 4, and the preferred upper limit for the number of upper weirs 6 is 4.

[0087] A bloom is produced by continuous casting using molten steel flowing into the immersion nozzle 4.

[0088] [(Process 2) Blossom rolling process] In the bract rolling process, billets are produced by bract rolling the bloom manufactured in the steelmaking process. In the bract rolling process, the material is first heated in a heating furnace in a well-known manner. The heating temperature is not particularly limited; any known temperature is sufficient. For example, the heating temperature is 1200-1300°C.

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

[0090] [(Process 3) Finish Rolling Process] In the finishing rolling process, the billets produced in the bract rolling process are subjected to finishing rolling to produce steel materials (bars or wires). The finishing rolling process includes the following steps: First, the billets produced in the bract rolling process are heated using a heating furnace. The heating temperature is, for example, 1000 to 1150°C. After heating, the billets are subjected to finishing rolling using a continuous rolling mill to produce steel materials (bars). The finishing rolling temperature (temperature at the end of finishing rolling) is, for example, 800 to 1000°C.

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

[0092] [Regarding the manufacturing method of mechanical structural parts] The structural components of the machine are manufactured using the steel material of the above-described embodiment as the raw material. The method for manufacturing structural components of the machine is well known and is as follows, for example.

[0093] In this embodiment, the steel material 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 manufactured intermediate product is allowed to cool in the atmosphere. After cooling, the intermediate product is subjected to machining to cut it into a predetermined shape.

[0094] Through the above process, machine structural parts are manufactured. When manufacturing machine structural parts using the steel material of this embodiment as the raw material, the heat treatment (quenching and tempering) can be omitted. Even without quenching and tempering, high strength and excellent low-temperature toughness can be obtained in the manufactured machine structural parts. However, when manufacturing machine structural parts using the steel material of this embodiment as the raw material, heat treatment may be performed. [Examples]

[0095] The effects of the steel material of this embodiment will be further explained in detail by the following examples. The conditions in the following examples are just 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 (Table 1A and Table 1B) were manufactured.

[0097] [Table 1A]

[0098] [Table 1B]

[0099] Specifically, primary refining was performed on the molten steel using a 270-ton converter. Subsequently, secondary refining was performed on the molten steel using LF and RH treatments. The molten steel after secondary refining was transferred to a tundish, and bloom was produced by continuous casting. The molten steel temperature T1 in the tundish is shown in the "Molten Steel Temperature T1 (°C)" column of Table 2. In addition, the number of lower and upper weirs in the path from the pouring nozzle to the immersion nozzle of the ladle in the tundish used is shown in the "Number of Lower Weirs" and "Number of Upper Weirs" columns of Table 2.

[0100] [Table 2]

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

[0102] [Evaluation Test] The following evaluation tests were conducted on the steel materials for each test number. (Test 1) Measurement test of the number density (ND) of coarse inclusions (Test 2) Measurement test of the number of specific coarse inclusions (NR) (Test 3) Yield strength measurement test of simulated mechanical structural components (Test 4) Low-temperature toughness evaluation test of simulated mechanical structural components The following describes each evaluation test.

[0103] [(Test 1) Measurement test of the number density (ND) of coarse inclusions] For each test number of steel material, the number density ND of coarse inclusions (pieces / mm²) was determined based on the method described in the above-mentioned [Method for measuring the number density ND of coarse inclusions]. 2 The number density ND (pieces / mm²) was calculated. The results obtained are shown in Table 2. 2 This is shown in the ")" column.

[0104] [(Test 2) Measurement test of the number of specific coarse inclusions (NR)] For each steel sample with a given test number, the percentage of coarse specific inclusions (NR) was determined using the method described in the above-mentioned [Method for Measuring the Percentage of Coarse Specific Inclusions NR]. The results are shown in the "Percentage of Coarse Specific Inclusions NR (%)" column of Table 2.

[0105] [(Test 3) Yield strength measurement test of simulated mechanical structural components] The steel samples for each test number were heated to a temperature of 1200°C for a holding time of 15 minutes. Afterward, hot forging was performed to produce 50mm diameter steel bars, which are simulated mechanical structural components. The simulated mechanical structural components were then allowed to cool after hot forging.

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

[0107] [(Test 4) Low-temperature toughness evaluation test of simulated mechanical structural components] The steel samples for each test number were heated to a temperature of 1200°C for a holding time of 15 minutes. Afterward, hot forging was performed to produce 50mm diameter steel bars, which are simulated mechanical structural components. The simulated mechanical structural components were then allowed to cool after hot forging.

[0108] A JIS No. 3 U-notch Charpy test specimen, as specified in JIS Z2242:2023, was prepared from a simulated mechanical structural component. The specimen size was half-size, 5 mm × 10 mm × 55 mm, and the longitudinal direction of the specimen was parallel to the longitudinal direction of the simulated mechanical structural component. A Charpy impact test was performed using the specimen in accordance with JIS Z2242:2018. Specifically, the Charpy impact test was performed in air using a specimen held at -50°C to obtain the Charpy impact value (J / cm²). 2 The obtained results are shown in Table 2 as "Charpy impact value (J / cm²)". 2 This is shown in the ")" 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 mechanical structural components was 600 MPa or higher, demonstrating high strength. Furthermore, the Charpy impact value at -50°C was 20 J / cm². 2 ) and excellent low-temperature toughness was obtained.

[0110] On the other hand, in test numbers 41 and 42, the Ti content was too high. As a result, the number density ND of coarse inclusions was excessive, and the number proportion NR of specific coarse inclusions was high. Consequently, the Charpy impact value of the simulated mechanical structural component at -50°C was 20 (J / cm²). 2 The value was less than ), and sufficient low-temperature toughness could not be obtained.

[0111] In tests 43 and 44, the molten steel temperature T1 in the tundish was too high. Therefore, although the number density ND of coarse inclusions was appropriate, the number proportion NR of specific coarse inclusions was high. As a result, the Charpy impact value of the simulated machine structural component at -50°C was 20 (J / cm²). 2 The value was less than ), and sufficient low-temperature toughness could not be obtained.

[0112] In tests 45 and 46, the molten steel temperature T1 in the tundish was too low. As a result, the number density ND of coarse inclusions was excessive, and the number proportion NR of specific coarse inclusions was high. Consequently, the Charpy impact value of the simulated machine structural component at -50°C was 20 (J / cm²).2 The value was less than ), and sufficient low-temperature toughness could not be obtained.

[0113] In tests 47 and 48, the number of lower weirs in the tundish was small. Therefore, although the number density ND of coarse inclusions was appropriate, the number proportion NR of specific coarse inclusions was high. As a result, the Charpy impact value of the simulated mechanical structural component at -50°C was 20 (J / cm²). 2 The value was less than ), and sufficient low-temperature toughness could not be obtained.

[0114] In tests 49 and 50, the number of upper weirs within the tundish was small. Therefore, although the number density ND of coarse inclusions was appropriate, the number proportion NR of coarse specific inclusions was high. As a result, the Charpy impact value of the simulated mechanical structural component at -50°C was 20 (J / cm²). 2 The value was less than ), and sufficient low-temperature toughness could not be obtained.

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

Claims

1. In mass percent, C: 0.20-0.48%, Si: 0.20-1.30%, Mn: 0.80-2.00%, P: 0.050% or less, S: 0.010-0.090%, Cr: 0.05-0.50%, V: 0.05-0.30%, Ti: 0.0001 to 0.0055%, Al: 0.005-0.050%, N: 0.003-0.030%, Ca: 0.0001-0.0050%, 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: 0-0.100%, Sb: 0 to 0.050%, As: 0 to 0.050%, Se: 0 to 0.100%, Te: 0-0.050%, and, It contains Pb: 0-0.09%, The remainder consists of Fe and impurities. The number density (ND) of coarse inclusions with an equivalent circular diameter of 3.0 μm or larger is 0.70 inclusions / mm². ２ The following: Of the aforementioned coarse inclusions, the number percentage NR of coarse specific inclusions whose mass percent Mn content, Al content, Ti content, and V content satisfy formula (1) is 75% or less. Steel bars or wire rods. (Mn+Al) / (Ti+V)<0.30 (1)

2. A steel bar or wire rod according to claim 1, In mass percent, Cu: 0.01-0.40%, Ni: 0.01 to 0.30%, Mo: 0.01-0.20%, 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: 0.001-0.050%, 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, Contains one or more Pb selected from the group consisting of Pb: 0.01 to 0.09%. Steel bars or wire rods.

Images