Steel materials and carburized steel parts

Abstract

The steel material disclosed herein has a chemical composition of, in mass%, C: 0.10-0.30%, Si: 0.05-1.50%, Mn: 0.40-2.00%, P: 0.025% or less, S: 0.001-0.050%, Cr: 0.05-2.30%, Al: 0.100% or less, N: 0.030% or less, O: 0.0003-0.0050%, Nb: 0.020-0.080%, with the remainder being Fe and impurities. In a cross section parallel to the rolling direction, the number density of specific composite precipitates is 0.010-1.000 particles / mm³. 2 The specific composite precipitate is 10-50 μm in size. 2 Nb precipitates having an area of ​​, and positioned within the maximum length Lmax of the Nb precipitates, with a length of 1-80 μm. 2 It contains Mn sulfide having an area of ​​,

Description

[Technical Field]

[0001] This disclosure relates to steel materials and carburized steel parts, and more particularly to steel materials and carburized steel parts applicable as materials for carburized steel parts, which are carburized parts. [Background technology]

[0002] Structural components used in industrial machinery and automotive power units and power transmission mechanisms are subjected to bending stress during use due to loads such as impacts and sliding. Therefore, high bending fatigue strength is required for structural components used in these applications.

[0003] To obtain high bending fatigue strength, carburized steel parts are sometimes used as structural components for machinery. Carburized steel parts are manufactured, for example, in the following process: After spheroidizing annealing of the steel material, a raw material is formed by cold working (cold forging). The raw material is then subjected to carburizing treatment (gas carburizing or vacuum carburizing). After carburizing, the raw material is machined to achieve the final shape. Carburized steel parts are manufactured through the above process. Carburized steel parts have a carburized hardened layer on the surface. The carburized hardened layer has a higher carbon concentration than the carbon concentration in the core. Therefore, the hardness of the carburized hardened layer is higher than that of the core. As a result, excellent bending fatigue strength can be obtained in carburized steel parts.

[0004] In the carburizing process during the manufacturing of the above-mentioned carburizing machine structural parts, the steel material (raw material) is A c3 Heating is performed above the transformation point. At this time, the austenite grains may become coarser. If quenching is performed with coarse austenite grains, the prior austenite grains will become coarse. Coarse prior austenite grains reduce the bending fatigue strength of the carburized steel parts.

[0005] To suppress the coarsening of prior austenite grains due to carburizing, there is a method of performing a normalizing treatment after cold forging but before carburizing. However, with the recent demand for carbon neutrality, there is a need to omit the normalizing treatment. Therefore, even when the normalizing treatment is omitted after cold forging, there is a need for steel materials with excellent resistance to grain coarsening (suppression of grain coarsening of prior austenite grains).

[0006] Japanese Patent Publication No. 2011-157597 (Patent Document 1), Japanese Patent Publication No. 2012-229475 (Patent Document 2), and Japanese Patent Publication No. 2012-158827 (Patent Document 3) propose a technique to suppress the coarsening of prior austenite crystal grains during carburizing, even without performing a normalizing treatment after cold forging and before carburizing.

[0007] The hot-rolled steel bar described in Patent Document 1 has a chemical composition in mass%, containing C: 0.1-0.3%, Si: 0.05-1.0%, Mn: 0.4-2.0%, S: 0.005-0.05%, Cr: 0.5-2.0%, Al: 0.01-0.06%, N: 0.005-0.025%, and Nb: 0.02-0.08%, with the remainder being Fe and impurities, and the amounts of P, Ti, and O (oxygen) in the impurities being P: 0.025% or less, Ti: 0.003% or less, and O (oxygen): 0.002% or less, respectively. In the region from the surface to 1 / 5 of the radius and in the region from the center to 1 / 5 of the radius of the steel bar, the amount of Al precipitated as AlN and AlN-Nb(CN) is 0.010% or less, the amount of Nb precipitated as Nb(CN) and AlN-Nb(CN) is 0.020% or less, and the total number density of AlN, Nb(CN), and AlN-Nb(CN) particles with a diameter of 100 nm or more is 50 particles / 100 μm. 2 The following is described in Patent Document 1: The metal structure has a ferrite-bainite area ratio of 80% or more, a bainite area ratio of 30-70%, and an average ferrite grain size of 15-40 μm. This suppresses the coarsening of austenite grains.

[0008] The hot-rolled bar steel described in Patent Document 2 contains, in mass %, C: 0.1 to 0.3%, Si: 0.05 to 1.0%, Mn: 0.4 to 2.0%, S: 0.003 to 0.05%, Cr: 0.5 to 3.0%, N: 0.010 to 0.025%, and Al: 0.02 to 0.05%, with the balance being Fe and impurities. The amounts of P, Ti, and O (oxygen) in the impurities are each P: 0.025% or less, Ti: 0.003% or less, and O (oxygen): 0.002% or less. The metallographic structure of the bar steel consists of a ferrite-bainite structure or a ferrite-bainite-perlite structure, the bainite structure fraction exceeds 70%, and the average grain size of the ferrite is 40 μm or less. In the region from the surface of the bar steel to 1 / 5 of the radius and the region from the center to 1 / 5 of the radius, the amount of Al precipitated as AlN is 0.005% or less, and the number density of AlN with a diameter of 100 nm or more is 5 pieces / 100 μm 2 or less. Patent Document 2 describes that this can suppress the coarsening of austenite grains.

[0009] The hot-worked steel material for surface hardening described in Patent Document 3 contains, in mass %, C: 0.10 to 0.30%, Si: 0.50% or less, Mn: 0.15 to 1.5%, P: 0.04% or less, S: 0.005 to 0.07%, Cr: 0.7 to 3.0%, Al: 0.01 to 0.05%, N: 0.007 to 0.030%, Nb: 0.02 to 0.07%, and H: 0.00004% or less, with the balance being Fe and impurities. The ratio of Nb precipitated as Nb(C, N) among the Nb in the steel is 85% or more, and the number density of Nb(C, N) with a diameter of 100 nm or more is 5 pieces / 100 μm 2 or less, and the standard deviation of the ferrite crystal grain size is 0.15 or less. Patent Document 3 describes that this can suppress the coarsening of austenite grains.

Prior Art Documents

Patent Documents

[0010]

Patent Document 1

[0011] However, when manufacturing carburized steel parts using steel as a material by means other than those described in Patent Documents 1 to 3, excellent grain coarseness resistance and excellent bending fatigue strength may be obtained. Furthermore, in the cold forging process of the above-mentioned manufacturing process for carburized steel parts, excellent cold workability of the steel material is also required.

[0012] The purpose of this disclosure is to provide a steel material and a carburized steel part that exhibit excellent cold workability and excellent resistance to grain coarseness, and further exhibit excellent bending fatigue strength when subjected to carburizing treatment. [Means for solving the problem]

[0013] The steel materials disclosed herein have a chemical composition in mass percent of: C: 0.10-0.30%, Si: 0.05-1.50%, Mn: 0.40-2.00%, P: 0.025% or less, S: 0.001-0.050%, Cr: 0.05-2.30%, Al: 0.100% or less, N: 0.030% or less, O: 0.0003-0.0050%, Nb: 0.020-0.080%, Mo: 0-0.40%, Ni: 0-0.30%, B: 0 It contains approximately 0.005% of the following elements: Cu: 0-0.40%, W: 0-0.50%, Ti: 0-0.10%, V: 0-0.15%, Zr: 0-0.10%, Ca: 0-0.0050%, Mg: 0-0.0050%, rare earth elements: 0-0.010%, Sn: 0-0.100%, Pb: 0-0.30%, Bi: 0-0.30%, Sb: 0-0.015%, and Te: 0-0.20%, with the remainder being Fe and impurities. In a cross-section parallel to the rolling direction, the number density of specific composite precipitates is 0.010-1.000 particles / mm³. 2 The specific composite precipitate is 10-50 μm in size. 2Nb precipitates having an area of, and Mn sulfides having an area of 1 to 80 μm disposed at positions within the maximum length Lmax of the Nb precipitates from the Nb precipitates, are included. 2 Mn sulfides having an area of, are included.

[0014] The carburized steel part of the present disclosure includes a carburized hardened layer formed on the surface layer and a core part inside the carburized hardened layer. The chemical composition of the core part, in mass%, is C: 0.10 to 0.30%, Si: 0.05 to 1.50%, Mn: 0.40 to 2.00%, P: 0.025% or less, S: 0.001 to 0.050%, Cr: 0.05 to 2.30%, Al: 0.100% or less, N: 0.030% or less, O: 0.0003 to 0.0050%, Nb: 0.020 to 0.080%, Mo: 0 to 0.40%, Ni: 0 to 0.30%, B: 0 to 0.005%, Cu: 0 to 0.40%, W: 0 to 0.50%, Ti: 0 to 0.10%, V: 0 to 0.15%, Zr: 0 to 0.10%, Ca: 0 to 0.0050%, Mg: 0 to 0.0050%, rare earth elements: 0 to 0.010%, Sn: 0 to 0.100%, Pb: 0 to 0.30%, Bi: 0 to 0.30%, Sb: 0 to 0.015%, and Te: 0 to 0.20%, and the balance consists of Fe and impurities. In a cross-section parallel to the rolling direction, the number density of specific composite precipitates is 0.010 to 1.000 per mm 2 and the specific composite precipitate has an area of 10 to 50 μm 2 Nb precipitates having an area of, and Mn sulfides having an area of 1 to 80 μm disposed at positions within the maximum length Lmax of the Nb precipitates from the Nb precipitates, are included. 2 Mn sulfides having an area of, are included.

Advantages of the Invention

[0015] In the steel material of the present disclosure, excellent cold workability and excellent grain coarsening resistance can be obtained. Further, when carburizing treatment is performed, excellent bending fatigue strength can be obtained. The carburized steel part of the present disclosure can obtain excellent bending fatigue strength.

Brief Description of the Drawings

[0016] [Figure 1]Figure 1 is a schematic diagram illustrating the sampling locations for test specimens used to measure the number density of specific composite precipitates. [Figure 2A] Figure 2A is a schematic diagram illustrating the method for identifying and counting specific complex precipitates. [Figure 2B] Figure 2B is another schematic diagram that differs from Figure 2A. [Figure 2C] Figure 2C is another schematic diagram that differs from Figures 2A and 2B. [Figure 2D] Figure 2D is a different schematic diagram from Figures 2A to 2C. [Figure 3] Figure 3 is a side view of the test specimen used in the cold workability evaluation test. [Figure 4] Figure 4 shows an example of an approximate straight line in a graph illustrating the relationship between cumulative failure probability and critical compression ratio. [Figure 5] Figure 5 is a side view of the Ono-type rotary bending fatigue test specimen used in bending fatigue strength evaluation tests. [Modes for carrying out the invention]

[0017] The inventors investigated, from the perspective of chemical composition, a steel material that exhibits excellent cold workability and excellent resistance to grain coarseness, and furthermore, excellent bending fatigue strength when subjected to carburizing treatment. As a result, in mass%, C: 0.10~0.30%, Si: 0.05~1.50%, Mn: 0.40~2.00%, P: 0.025% or less, S: 0.001~0.050%, Cr: 0.05~2.30%, Al: 0.100% or less, N: 0.030% or less, O: 0.0003~0.0050%, Nb: 0.020~0.080%, Mo: 0~0.40%, Ni: 0~0.30%, B: 0~0.005%, Cu: 0~0.40%, W: 0~0 We considered that the above-mentioned effects could potentially be obtained with steel having a chemical composition containing 0.50%, Ti: 0-0.10%, V: 0-0.15%, Zr: 0-0.10%, Ca: 0-0.0050%, Mg: 0-0.0050%, rare earth elements: 0-0.010%, Sn: 0-0.100%, Pb: 0-0.30%, Bi: 0-0.30%, Sb: 0-0.015%, and Te: 0-0.20%, with the remainder being Fe and impurities.

[0018] Therefore, the inventors further investigated means to improve the cold workability and grain coarseness resistance of steel materials having the above chemical composition from the perspective of microstructure. As a result, the inventors obtained the following findings.

[0019] 10-50 μm 2 Nb precipitates with a large surface area (hereinafter also referred to as coarse Nb precipitates) exhibit a high degree of lattice mismatch with the matrix phase. The higher the degree of lattice mismatch, the more difficult it becomes for dislocations to overcome the precipitate. Therefore, coarse Nb precipitates exert a high pinning force.

[0020] However, when coarse Nb precipitates exist individually in the steel material, they tend to dissolve during the heating process of the carburizing treatment in the manufacturing process of carburized steel parts made from steel, causing the size of the Nb precipitates to decrease. In this case, the pinning force of the Nb precipitates decreases. Therefore, abnormal grain growth is more likely to occur during the heating process of the carburizing treatment, and the prior austenite grains on the surface of the steel material tend to become coarser. In other words, the resistance to grain coarsening tends to decrease.

[0021] On the other hand, when Mn sulfides are present near coarse Nb precipitates, the coarse Nb precipitates are difficult to dissolve. The reason for this is not entirely clear, but the following reasons are possible. In this specification, coarse Nb precipitates and Mn sulfides that are in contact with each other, or coarse Nb precipitates and Mn sulfides that are within close proximity to each other, are referred to as "specific composite precipitates." When Mn sulfides are in contact with coarse Nb precipitates, or when Mn sulfides are present in the vicinity of coarse Nb precipitates, the interfacial energy of the specific composite precipitates decreases, and the specific composite precipitates become more stable. Therefore, it is considered that the coarse Nb precipitates in the specific composite precipitates become less likely to dissolve.

[0022] As described above, the formation of specific composite precipitates in steel significantly enhances the pinning effect and improves resistance to grain coarseness. On the other hand, excessive formation of specific composite precipitates increases the likelihood that these precipitates will become crack initiation points during bending fatigue and cold working. Therefore, adjusting the number density of specific composite precipitates to an appropriate range is effective in improving resistance to grain coarseness and cold workability.

[0023] Based on the above findings, the number density of specific composite precipitates was found to be between 0.010 and 1.000 particles / mm³. 2 Therefore, the inventors have discovered that it is possible to improve cold workability while enhancing resistance to grain coarseness, and furthermore, when carburizing treatment is applied, excellent bending fatigue strength can be obtained, thus completing the present invention.

[0024] Based on the above technical concept, the steel material of this embodiment has the following configuration.

[0025] The first form of steel has a chemical composition in mass percent of: C: 0.10~0.30%, Si: 0.05~1.50%, Mn: 0.40~2.00%, P: 0.025% or less, S: 0.001~0.050%, Cr: 0.05~2.30%, Al: 0.100% or less, N: 0.030% or less, O: 0.0003~0.0050%, Nb: 0.020~0.080%, Mo: 0~0.40%, Ni: 0~0.30%, B: It contains 0-0.005% of the following elements: Cu: 0-0.40%, W: 0-0.50%, Ti: 0-0.10%, V: 0-0.15%, Zr: 0-0.10%, Ca: 0-0.0050%, Mg: 0-0.0050%, rare earth elements: 0-0.010%, Sn: 0-0.100%, Pb: 0-0.30%, Bi: 0-0.30%, Sb: 0-0.015%, and Te: 0-0.20%, with the remainder being Fe and impurities. In a cross-section parallel to the rolling direction, the number density of specific composite precipitates is 0.010-1.000 particles / mm³. 2 The specific composite precipitate is 10-50 μm in size. 2 Nb precipitates having an area of ​​, and positioned within the maximum length Lmax of the Nb precipitates, with a length of 1-80 μm. 2 It contains Mn sulfide having an area of ​​,

[0026] The second form of steel is the same as the first form of steel, with a chemical composition in mass percent of: Mo: 0.01-0.40%, Ni: 0.01-0.30%, B: 0.001-0.005%, Cu: 0.01-0.40%, W: 0.01-0.50%, Ti: 0.01-0.10%, V: 0.01-0.15%, Zr: 0.01-0.10%, Ca: 0 It contains one or more elements selected from the group consisting of 0.0001-0.0050%, Mg: 0.0001-0.0050%, rare earth elements: 0.001-0.010%, Sn: 0.001-0.100%, Pb: 0.01-0.30%, Bi: 0.01-0.30%, Sb: 0.001-0.015%, and Te: 0.01-0.20%.

[0027] The third form of steel is the first or second form of steel, wherein the number ratio of Nb coarse specific composite precipitates, which are specific composite precipitates in which the area of ​​Nb precipitates is larger than the area of ​​Mn sulfide, to the total number of specific composite precipitates is 55.0% or more.

[0028] The first form of the carburized steel part comprises a carburized hardened layer formed on the surface and a core portion located inside the carburized hardened layer. The chemical composition of the core portion is, in mass%, C: 0.10-0.30%, Si: 0.05-1.50%, Mn: 0.40-2.00%, P: 0.025% or less, S: 0.001-0.050%, Cr: 0.05-2.30%, Al: 0.100% or less, N: 0.030% or less, O: 0.0003-0.0050%, Nb: 0.020-0.080%, Mo: 0-0.40%, Ni: 0-0.30%, B: 0-0. It contains 0.005%, Cu: 0-0.40%, W: 0-0.50%, Ti: 0-0.10%, V: 0-0.15%, Zr: 0-0.10%, Ca: 0-0.0050%, Mg: 0-0.0050%, rare earth elements: 0-0.010%, Sn: 0-0.100%, Pb: 0-0.30%, Bi: 0-0.30%, Sb: 0-0.015%, and Te: 0-0.20%, with the remainder being Fe and impurities. In a cross-section parallel to the rolling direction, the number density of specific composite precipitates is 0.010-1.000 particles / mm³. 2 The specific composite precipitate is 10-50 μm in size. 2 Nb precipitates having an area of ​​, and positioned within the maximum length Lmax of the Nb precipitates, with a length of 1-80 μm. 2 It contains Mn sulfide having an area of ​​,

[0029] The second form of the carburized steel part is the same as the first form of the carburized steel part, with a chemical composition in mass% of: Mo: 0.01-0.40%, Ni: 0.01-0.30%, B: 0.001-0.005%, Cu: 0.01-0.40%, W: 0.01-0.50%, Ti: 0.01-0.10%, V: 0.01-0.15%, Zr: 0.01-0.10%, C It contains one or more elements selected from the group consisting of a: 0.0001-0.0050%, Mg: 0.0001-0.0050%, rare earth elements: 0.001-0.010%, Sn: 0.001-0.100%, Pb: 0.01-0.30%, Bi: 0.01-0.30%, Sb: 0.001-0.015%, and Te: 0.01-0.20%.

[0030] The third form of a carburized steel part is a carburized steel part of the first or second form in which the proportion of Nb coarse specific composite precipitates, which are specific composite precipitates in which the area of ​​Nb precipitates is larger than the area of ​​Mn sulfide, to the total number of specific composite precipitates is 55.0% or more.

[0031] The steel material of this embodiment will be described in detail below. Unless otherwise specified, the "%" in relation to elements refers to mass percentage.

[0032] [Features of the steel material of this embodiment] The steel material of this embodiment satisfies the following features 1 and 2. (Feature 1) The chemical composition, in mass%, is as follows: C: 0.10-0.30%, Si: 0.05-1.50%, Mn: 0.40-2.00%, P: 0.025% or less, S: 0.001-0.050%, Cr: 0.05-2.30%, Al: 0.100% or less, N: 0.030% or less, O: 0.0003-0.0050%, Nb: 0.020-0.080%, Mo: 0-0.40%, Ni: 0-0.30%, B: 0-0.0 It contains 0.5%, Cu: 0-0.40%, W: 0-0.50%, Ti: 0-0.10%, V: 0-0.15%, Zr: 0-0.10%, Ca: 0-0.0050%, Mg: 0-0.0050%, rare earth elements: 0-0.010%, Sn: 0-0.100%, Pb: 0-0.30%, Bi: 0-0.30%, Sb: 0-0.015%, and Te: 0-0.20%, with the remainder being Fe and impurities. (Feature 2) In a cross-section parallel to the rolling direction, the number density of specific composite precipitates is 0.010 to 1.000 particles / mm³. 2 The specific composite precipitate is 10-50 μm in size. 2 Nb precipitates having an area of ​​, and positioned within the maximum length Lmax of the Nb precipitates, with a length of 1-80 μm. 2 It contains Mn sulfide having an area of ​​[value]. The following describes each of its features.

[0033] [(Feature 1) Regarding chemical composition] The chemical composition of the steel material in this embodiment contains the following elements. Unless otherwise specified, percentages in relation to the chemical composition refer to mass percentages.

[0034] C: 0.10~0.30% Carbon (C) enhances the core strength and bending fatigue strength of mechanical structural components manufactured from steel. If the C content is less than 0.10%, 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 carbon content exceeds 0.30%, the strength of the steel becomes excessively high. In this case, even if the content of other elements is within the range of this embodiment, the cold workability (cold forging properties) of the steel decreases. Therefore, the C content is 0.10-0.30%. The preferred lower limit of the C content is 0.12%, more preferably 0.15%, even more preferably 0.16%, and even more preferably 0.17%. The preferred upper limit for the C content is 0.28%, more preferably 0.25%, more preferably 0.23%, more preferably 0.22%, and more preferably 0.18%.

[0035] Si: 0.05~1.50% Silicon (Si) enhances the hardenability of steel. Therefore, the strength of mechanical structural components manufactured from steel increases, and their bending fatigue strength improves. If the Si content is less than 0.05%, 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 1.50%, the strength of the steel becomes excessively high. In this case, even if the content of other elements is within the range of this embodiment, the cold workability (cold forging properties) of the steel decreases. Therefore, the Si content is 0.05 to 1.50%. The preferred lower limit for the Si content is 0.06%, more preferably 0.10%, and even more preferably 0.15%. The preferred upper limit for the Si content is 1.20%, more preferably 1.00%, more preferably 0.80%, more preferably 0.50%, and more preferably 0.35%.

[0036] Mn: 0.40~2.00% Manganese (Mn) enhances the hardenability and tempering softening resistance of steel. Therefore, the strength and fatigue strength of machine structural components manufactured from steel are increased. Furthermore, Mn forms specific composite precipitates as Mn sulfides, improving grain coarseness resistance. If the Mn content is less than 0.40%, 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 steel becomes excessively high. In this case, even if the content of other elements is within the range of this embodiment, the cold workability (cold forging properties) of the steel decreases. Therefore, the Mn content is 0.40-2.00%. The preferred lower limit of the Mn content is 0.50%, more preferably 0.60%, even more preferably 0.65%, and even more preferably 0.80%. The preferred upper limit for the Mn content is 1.50%, more preferably 1.20%, even more preferably 0.90%, and even more preferably 0.85%.

[0037] P:0.025% or less Phosphorus (P) is an unavoidable impurity. In other words, the P content is greater than 0%. If the P content exceeds 0.025%, P 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 fatigue strength of the mechanical structural component will decrease. Therefore, the P content is 0.025% 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 0.003%. The preferred upper limit for the P content is 0.020%, more preferably 0.015%, and even more preferably 0.010%.

[0038] S: 0.001~0.050% Sulfur (S) combines with manganese (Mn) to form Mn sulfides. Therefore, the S content is 0.001% or more. On the other hand, if the sulfur content exceeds 0.050%, coarse manganese sulfides are produced in excess. As a result, even if the content of other elements is within the range of this embodiment, the fatigue strength of the mechanical structural components decreases. Therefore, the sulfur content is between 0.001% and 0.050%. The preferred lower limit for the S content is 0.002%, more preferably 0.003%, and even more preferably 0.005%. The preferred upper limit for the S content is 0.040%, more preferably 0.030%, even more preferably 0.020%, and even more preferably 0.015%.

[0039] Cr: 0.05~2.30% Chromium (Cr) enhances the hardenability and tempering resistance of steel. Therefore, the strength and fatigue strength of machine structural components manufactured from steel are increased. If the Cr content is less than 0.05%, 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 Cr content exceeds 2.30%, the strength of the steel becomes excessively high. In this case, even if the content of other elements is within the range of this embodiment, the cold workability (cold forging properties) of the steel decreases. Therefore, the Cr content is between 0.05% and 2.30%. The preferred lower limit of the Cr content is 0.10%, more preferably 0.20%, more preferably 0.30%, more preferably 0.55%, more preferably 0.80%, more preferably 0.85%, and more preferably 1.15%. The preferred upper limit for the Cr content is 2.00%, more preferably 1.90%, more preferably 1.50%, and still more preferably 1.25%.

[0040] Al: 0.100% or less Aluminum (Al) is inevitably present. That is, the Al content is greater than 0%. Al deoxidizes steel during the steelmaking process. Even a small amount of Al will provide some degree of this effect. On the other hand, if the Al content exceeds 0.100%, coarse oxides are formed. In this case, even if the content of other elements is within the range of this embodiment, the fatigue strength of the machine structural parts manufactured using steel as the material will decrease. Therefore, the Al content is 0.100% or less. The preferred lower limit of the Al content is 0.001%, more preferably 0.002%, more preferably 0.005%, more preferably 0.020%, and more preferably 0.025%. The preferred upper limit for the Al content is 0.080%, more preferably 0.060%, even more preferably 0.040%, and even more preferably 0.030%.

[0041] N: 0.030% or less Nitrogen (N) is inevitably present; that is, the N content is greater than 0%. N forms precipitates, increasing the fatigue strength of mechanical structural components. Even a small amount of N will provide some degree of this effect. 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 0.030% or less. The preferred lower limit of the N content is 0.001%, more preferably 0.005%, more preferably 0.010%, more preferably 0.014%, and still more preferably 0.020%. The preferred upper limit for the N content is 0.026%, more preferably 0.025%, and even more preferably 0.023%.

[0042] O: 0.0003~0.0050% Oxygen (O) generates fine oxides. These fine oxides act as nuclei for coarse Nb precipitates, promoting their formation. This can increase the number density of specific composite precipitates. If the O content is less than 0.0003%, the above effect cannot be fully obtained. On the other hand, if the O content exceeds 0.0050%, coarse oxides will be formed even if the content of other elements is within the range of this embodiment. In this case, the fatigue strength of the mechanical structural component will decrease. Therefore, the O content is between 0.0003% and 0.0050%. 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 0.0005%, more preferably 0.0007%, and even more preferably 0.0009%. The preferred upper limit for the O content is 0.0040%, more preferably 0.0030%, even more preferably 0.0025%, and even more preferably 0.0020%.

[0043] Nb: 0.020~0.080% Niobium (Nb) forms specific composite precipitates as Nb precipitates, and during heating in the carburizing process, it suppresses abnormal grain growth in the steel material through a pinning effect. This increases the fatigue strength of machine structural components. If the Nb content is less than 0.020%, 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 Nb content exceeds 0.080%, excessively coarse Nb precipitates will be formed. In this case, even if the content of other elements is within the range of this embodiment, the fatigue strength of the mechanical structural component will decrease. Therefore, the Nb content is 0.020-0.080%. The preferred lower limit of the Nb content is 0.025%, more preferably 0.030%, even more preferably 0.035%, and even more preferably 0.040%. The preferred upper limit for the Nb content is 0.075%, more preferably 0.070%, even more preferably 0.065%, and even more preferably 0.060%.

[0044] The remainder of the chemical composition of the steel material in this embodiment contains Fe and impurities. Preferably, the remainder of the chemical composition of the steel material in this embodiment consists of Fe and impurities. Here, impurities in the chemical composition refer to substances that are mixed in from raw materials such as ore, scrap, or the manufacturing environment during the industrial production of the steel material, and are acceptable within a range that does not adversely affect the steel material in this embodiment.

[0045] [About Optional Elements] The steel material of this embodiment may further contain, in place of some of the Fe, one or more elements selected from the group consisting of Mo: 0-0.40%, Ni: 0-0.30%, B: 0-0.005%, Cu: 0-0.40%, W: 0-0.50%, Ti: 0-0.10%, V: 0-0.15%, Zr: 0-0.10%, Ca: 0-0.0050%, Mg: 0-0.0050%, rare earth elements: 0-0.010%, Sn: 0-0.100%, Pb: 0-0.30%, Bi: 0-0.30%, Sb: 0-0.015%, and Te: 0-0.20%. Any of these elements are arbitrary. Each element will be described below.

[0046] [Group 1 (Mo, Ni, B, Cu, and W)] The steel material of this embodiment may further contain one or more elements selected from the group consisting of Mo, Ni, B, Cu, and W in place of a portion of Fe. These elements are arbitrary and all increase the fatigue strength of machine structural parts made from steel. Each element will be described below.

[0047] Mo: 0~0.40% 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 enhances the hardenability and tempering softening resistance of the steel. As a result, the fatigue strength of machine structural components increases. Even a small amount of Mo can provide some of the above effects. However, if the Mo content exceeds 0.40%, the strength of the steel becomes excessively high. In this case, even if the content of other elements is within the range of this embodiment, the cold workability (cold forging properties) of the steel decreases. Therefore, the Mo content is 0-0.40%. The preferred lower limit of the Mo content is 0.01%, more preferably 0.03%, more preferably 0.05%, more preferably 0.10%, and more preferably 0.15%. The preferred upper limit for the Mo content is 0.38%, more preferably 0.35%, more preferably 0.30%, more preferably 0.25%, and still more preferably 0.20%.

[0048] 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 enhances the hardenability of steel. As a result, the fatigue strength of mechanical structural components increases. Even a small amount of Ni is sufficient to achieve the above effect to some extent. However, if the Ni content exceeds 0.30%, the strength of the steel becomes excessively high. In this case, even if the content of other elements is within the range of this embodiment, the cold workability (cold forging properties) of the steel decreases. Therefore, the Ni content is 0-0.30%. The preferred lower limit of the Ni content is 0.01%, more preferably 0.02%, more preferably 0.03%, more preferably 0.05%, and more preferably 0.10%. The preferred upper limit for the Ni content is 0.25%, more preferably 0.20%, and even more preferably 0.15%.

[0049] B: 0~0.005% Boron (B) is an optional element and does not need to be included. In other words, the B content may be 0%. When present, i.e., when the B content is greater than 0%, B enhances the hardenability of the steel. As a result, the fatigue strength of machine structural components increases. Even a small amount of B present will provide some degree of the above effect. However, if the B content exceeds 0.005%, the strength of the steel becomes excessively high. In this case, even if the content of other elements is within the range of this embodiment, the cold workability (cold forging properties) of the steel will decrease. Therefore, the B content is 0-0.005%. The preferred lower limit for the B content is 0.001%, and more preferably 0.002%. The preferred upper limit for the B content is 0.004%, and more preferably 0.003%.

[0050] 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 enhances the hardenability of the steel. As a result, the fatigue strength of mechanical structural components increases. Even a small amount of Cu can provide some degree of the above effect. However, if the Cu content exceeds 0.40%, the strength of the steel becomes excessively high. In this case, even if the content of other elements is within the range of this embodiment, the cold workability of the steel decreases. Therefore, the copper content is 0-0.40%. The preferred lower limit for the Cu content is 0.01%, more preferably 0.05%, and even more preferably 0.10%. The preferred upper limit for the Cu content is 0.35%, more preferably 0.30%, and even more preferably 0.25%.

[0051] W: 0~0.50% Tungsten (W) is an optional element and does not need to be included. In other words, the W content may be 0%. When present, i.e., when the W content is 0%, W increases the strength of the steel. As a result, the fatigue strength of machine structural components increases. Even if only a small amount of W is present, the above effect can be obtained to some extent. However, if the W content exceeds 0.50%, the strength of the steel becomes excessively high. In this case, even if the content of other elements is within the range of this embodiment, the cold workability (cold forging properties) of the steel decreases. Therefore, the W content is 0-0.50%. The preferred lower limit of the W content is 0.01%, more preferably 0.05%, and even more preferably 0.10%. The preferred upper limit for the W content is 0.45%, more preferably 0.40%, and even more preferably 0.35%.

[0052] [Group 2 (Ti, V, and Zr)] The steel material of this embodiment may further contain one or more elements selected from the group consisting of Ti, V, and Zr in place of a portion of Fe. All of these elements form precipitates and suppress grain coarsening through a pinning effect.

[0053] Ti: 0~0.10% Titanium (Ti) is an optional element and does not need to be included. In other words, the Ti content may be 0%. If Ti is present, that is, if the Ti content is greater than 0%, Ti forms precipitates, and the pinning effect suppresses the coarsening of the prior austenite grains. Even if only a small amount of Ti is present, the above effect can be obtained to some extent. However, if the Ti content exceeds 0.10%, the strength of the steel becomes excessively high. Therefore, even if the content of other elements is within the range of this embodiment, the cold workability (cold forging properties) of the steel decreases. Therefore, the Ti content is 0-0.10%. The preferred lower limit for the Ti content is 0.01%, more preferably 0.02%, and even more preferably 0.03%. The preferred upper limit for the Ti content is 0.09%, more preferably 0.08%, and even more preferably 0.07%.

[0054] V: 0~0.15% Vanadium (V) is an optional element and does not need to be present. In other words, the V content may be 0%. If V is present, that is, if the V content is greater than 0%, V forms precipitates, and the pinning effect suppresses the coarsening of the prior austenite grains. Even if only a small amount of V is present, the above effect can be obtained to some extent. However, if the V content exceeds 0.15%, V combines with N to form VN, reducing the amount of dissolved N. As a result, the amount of Nb precipitate produced decreases. Therefore, the V content is 0-0.15%. The preferred lower limit for the V content is 0.01%, more preferably 0.02%, and even more preferably 0.03%. The preferred upper limit for the V content is 0.12%, more preferably 0.10%, and even more preferably 0.07%.

[0055] Zr: 0~0.10% Zirconium (Zr) is an optional element and does not need to be included. In other words, the Zr content may be 0%. If Zr is present, that is, if the Zr content is greater than 0%, Zr forms precipitates, and the pinning effect suppresses the coarsening of the prior austenite grains. Even if only a small amount of Zr is present, the above effect can be obtained to some extent. However, if the Zr content exceeds 0.10%, the strength of the steel becomes excessively high. As a result, even if the content of other elements is within the range of this embodiment, the cold workability (cold forging properties) of the steel decreases. Therefore, the Zr content is 0-0.10%. The preferred lower limit for the Zr content is 0.01%, more preferably 0.02%, and even more preferably 0.03%. The preferred upper limit for the Zr content is 0.09%, more preferably 0.08%, and even more preferably 0.07%.

[0056] [Group 3 (Ca, Mg, and rare earth elements)] The steel material of this embodiment may further contain one or more elements selected from the group consisting of Ca, Mg, and rare earth elements (REM). All of these elements enhance the cold workability (cold forging properties) of the steel material.

[0057] Ca: 0~0.0050% Calcium (Ca) is an optional element and does not need to be included. In other words, the Ca content may be 0%. When calcium is present, i.e., when the calcium content is greater than 0%, calcium promotes the spheroidization of sulfides, thereby improving the cold workability (cold forgeability) of the steel. Even a small amount of calcium can provide some degree of this effect. However, if the Ca content exceeds 0.0050%, coarse oxides will be formed. In this case, even if the content of other elements is within the range of this embodiment, the fatigue strength of the mechanical structural component will decrease. Therefore, the Ca content is 0-0.0050%. The preferred lower limit for the Ca content is 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%. The preferred upper limit for the Ca content is 0.0045%, more preferably 0.0040%, and even more preferably 0.0035%.

[0058] 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 Mg is present, that is, when the Mg content is greater than 0%, Mg promotes the spheroidization of sulfides, thereby improving the cold workability (cold forgeability) of the steel. Even a small amount of Mg can provide some degree of this effect. However, if the Mg content exceeds 0.0050%, coarse oxides will be formed. In this case, even if the content of other elements is within the range of this embodiment, the fatigue strength of the mechanical structural component will decrease. Therefore, the Mg content is 0-0.0050%. The preferred lower limit of the Mg content is 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%. The preferred upper limit for the Mg content is 0.0045%, more preferably 0.0040%, and even more preferably 0.0035%.

[0059] Rare earth elements: 0~0.010% Rare earth elements (REMs) are optional elements and do not need to be included. In other words, the REM content may be 0%. If REM is present, i.e., if the REM content is greater than 0%, REM promotes the spheroidization of sulfides, thereby improving the cold workability of steel. Even a small amount of REM can provide some degree of the above effect. However, if the REM content exceeds 0.010%, coarse oxides are formed. In this case, even if the content of other elements is within the range of this embodiment, the fatigue strength of the mechanical structural component will decrease. Therefore, the REM content is 0-0.010%. The preferred lower limit for the REM content is 0.001%, more preferably 0.002%, and even more preferably 0.003%. The preferred upper limit for the REM content is 0.009%, more preferably 0.008%, and even more preferably 0.007%.

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

[0061] [Group 4 (Sn)] The steel material of this embodiment may further contain Sn instead of some of the Fe.

[0062] 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%. When present, i.e., when the Sn content is greater than 0%, Sn enhances the corrosion resistance of steel. Even a small amount of Sn present will provide some degree of the above effect. However, if the Sn 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 Sn content is 0-0.100%. The preferred lower limit for the Sn content is 0.001%, more preferably 0.002%, and even more preferably 0.005%. The preferred upper limit for the Sn content is 0.090%, more preferably 0.080%, even more preferably 0.060%, and even more preferably 0.040%.

[0063] [Group 5 (Pb, Bi, Sb, and Te)] The steel material of this embodiment may further contain one or more elements selected from the group consisting of Pb, Bi, Sb, and Te in place of a portion of Fe. All of these elements enhance the machinability of the steel material.

[0064] Pb: 0~0.30% Lead (Pb) is an optional element and does not need to be included. In other words, the Pb content may be 0%. When Pb is present, that is, when the Pb content is greater than 0%, Pb improves the machinability of steel. 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 0.30%, the fatigue strength of the mechanical structural component will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the Pb content is 0-0.30%. The preferred lower limit for the Pb content is 0.01%, more preferably 0.03%, and even more preferably 0.05%. The preferred upper limit for the Pb content is 0.25%, more preferably 0.20%, even more preferably 0.15%, and even more preferably 0.09%.

[0065] Bi: 0~0.30% Bismuth (Bi) is an optional element and does not need to be included. In other words, the Bi content may be 0%. When present, i.e., when the Bi content is greater than 0%, Bi improves the machinability of steel. Even a small amount of Bi will provide some degree of the above effect. However, if the Bi content exceeds 0.30%, the fatigue strength of the mechanical structural component will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the Bi content is 0-0.30%. The preferred lower limit for the Bi content is 0.01%, more preferably 0.03%, and even more preferably 0.05%. The preferred upper limit for the Bi content is 0.25%, more preferably 0.20%, even more preferably 0.15%, and even more preferably 0.10%.

[0066] Sb: 0~0.015% Antimony (Sb) is an optional element and does not need to be included. In other words, the Sb content may be 0%. When present, i.e., when the Sb content is greater than 0%, Sb improves the machinability of steel. Even a small amount of Sb will provide some degree of the above effect. However, if the Sb content exceeds 0.015%, the fatigue strength of the mechanical structural component will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the Sb content is 0-0.015%. The preferred lower limit for the Sb content is 0.001%, more preferably 0.003%, and even more preferably 0.005%. The preferred upper limit for the Sb content is 0.012%, more preferably 0.010%, and even more preferably 0.008%.

[0067] Te: 0~0.20% 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 0.20%, the fatigue strength of the mechanical structural component will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the Te content is 0-0.20%. The preferred lower limit for the Te content is 0.01%, more preferably 0.03%, and even more preferably 0.05%. The preferred upper limit for the Te content is 0.18%, more preferably 0.16%, even more preferably 0.14%, and even more preferably 0.12%.

[0068] [(Feature 2: Regarding the number density ND of specific composite precipitates)] In the steel material of this embodiment, the number density ND of specific composite precipitates in a cross-section parallel to the rolling direction (longitudinal direction) of the steel material is 0.010 to 1.000 particles / mm². 2 Here, the specific composite precipitate is 10-50 μm. 2 Nb precipitates having an area of ​​, and positioned within the maximum length Lmax of the Nb precipitates, with a length of 1-80 μm. 2 It contains Mn sulfide having an area of ​​[value].

[0069] 10-50 μm 2 Nb precipitates with a large surface area (hereinafter also referred to as coarse Nb precipitates) exhibit a high degree of lattice mismatch with respect to the matrix phase. Therefore, specific composite precipitates containing coarse Nb precipitates exhibit high pinning force. As a result, abnormal grain growth of austenite grains in the steel can be suppressed during heating in the carburizing process during the manufacturing of carburized steel parts made from steel. Furthermore, when coarse Nb precipitates are present in the steel as specific composite precipitates, the coarse Nb precipitates are less likely to dissolve during heating in the carburizing process. Therefore, specific composite precipitates significantly suppress abnormal grain growth of austenite grains during heating in the carburizing process.

[0070] The number density (ND) of the specific composite precipitate is 0.010 particles / mm³. 2If the number density ND of the specific composite precipitates is less than 1,000 particles / mm³, the amount of specific composite precipitates in the steel is excessively low. In this case, sufficient pinning effect is not achieved during heating in the carburizing treatment. As a result, the resistance to grain coarseness decreases. On the other hand, if the number density ND of the specific composite precipitates is 1,000 particles / mm³, then the number density ND is 1,000 particles / mm³. 2 If it exceeds a certain value, the number density ND of specific composite precipitates in the steel becomes excessive. In this case, the excess specific composite precipitates are more likely to become crack initiation points, and the cold workability of the steel decreases.

[0071] Therefore, the number density ND of specific composite precipitates is 0.010 to 1.000 particles / mm³. 2 That is the case.

[0072] The preferred lower limit for the number density ND of specific composite precipitates is 0.012 particles / mm³. 2 And more preferably 0.015 pieces / mm 2 And more preferably 0.017 pieces / mm 2 And more preferably 0.020 pieces / mm 2 That is the case. The preferred upper limit for the number density ND of specific composite precipitates is 0.900 particles / mm³. 2 And more preferably 0.850 pieces / mm 2 And more preferably 0.800 pieces / mm 2 And more preferably 0.750 pieces / mm 2 That is the case.

[0073] [Preferred form of specific complex precipitates] Among the specified composite precipitates described above, those in which the area of ​​coarse Nb precipitates is larger than the area of ​​Mn sulfide are defined as "coarse Nb specified composite precipitates." Preferably, among the specified composite precipitates described above, the number ratio RA of coarse Nb specified composite precipitates to the specified composite precipitates is 55.0% or more.

[0074] In specific composite precipitates, if the area of ​​coarse Nb precipitates is larger than the area of ​​Mn sulfide precipitates, the interfacial energy of the specific composite precipitate becomes even lower, further stabilizing it. Therefore, the dissolution of coarse Nb precipitates during heating in the carburizing treatment can be further suppressed. As a result, the resistance to grain coarsening is further enhanced.

[0075] The preferred lower limit of the number ratio RA of Nb coarse specific complex precipitates relative to the specific complex precipitates is 60.0%, more preferably 65.0%, and even more preferably 70.0%.

[0076] [Method for measuring the number density of specific complex precipitates] The method for measuring the number density of specific composite precipitates is as follows: As shown in Figure 1, a test specimen is taken with a cross-section parallel to the rolling direction (axial direction) and containing the central axis of the steel material as the observation surface. The size of the observation surface of the test specimen is not particularly limited, as long as it includes a rectangular observation area 50 that is 10 mm in the rolling direction (longitudinal direction) and 6 mm in the radial direction of the steel material. The observation area 50 is set so that its center position C50 is the R / 2 position of the steel material in the radial direction. Here, the R / 2 position refers to the center position of the radius (R) in the radial direction of the steel material, as shown in Figure 1.

[0077] The test specimen is embedded in resin, and the observation surface is mirror-polished. Using a scanning electron microscope (SEM-EDS) equipped with compositional analysis capabilities, the observation area (10mm x 6mm = 60mm) within the mirror-polished observation surface is analyzed at 500x magnification. 2 The entire surface of the ) is observed. The observation area is divided into multiple observation fields, and each observation field is observed at a magnification of 500x to obtain a backscattered electron image. In the backscattered electron images of all observation fields, particles (deposits or inclusions) are identified based on contrast. The area of ​​the identified particles is determined. The area of ​​the particles is determined by known image processing methods.

[0078] Furthermore, the area is 1 μm 2For each of the identified particles, elemental concentration analysis will be performed using EDS (energy-dispersive X-ray spectroscopy) attached to a SEM to identify Nb precipitates and Mn sulfides. In the EDS analysis (elemental concentration analysis), the acceleration voltage will be set to 20kV, and the elements to be quantified will be N, O, Na, Mg, Al, Si, S, Cl, K, Ca, Ti, Cr, Mn, Nb, Zr, Ce, La, C, and Fe. The EDS analysis time will be 0.5 seconds.

[0079] In the EDS analysis results for each particle, if, when the total mass of the elements excluding C and Fe from the above-mentioned quantitative elements is taken as 100%, the Mn content is 10% or more and the S content is 5% or more by mass%, then the particle is identified as a Mn sulfide.

[0080] In the EDS analysis results of particles, if the Nb content is 30% or more by mass when the chemical composition of the particles is set to 100% by mass, then the particles are identified as Nb precipitates.

[0081] Of the identified Nb precipitates, those with an area of ​​10-50 μm 2 The Nb precipitates are identified as "coarse Nb precipitates." The maximum length Lmax of each identified coarse Nb precipitate is determined by the following method: Two straight lines are placed tangent to the outer edge of the coarse Nb precipitate. These two lines are parallel to each other. The maximum distance between the two lines is defined as the maximum length Lmax (μm).

[0082] Figures 2A to 2D are schematic diagrams illustrating the method for identifying and counting specific composite precipitates. Referring to Figure 2A, in the coarse Nb precipitate 10 identified within the observation field, a position (within region 20) within the maximum length Lmax from the coarse Nb precipitate 10 is measured from 1 to 80 μm. 2 If one or more Mn sulfides having an area of ​​the specified size are present, the coarse Nb precipitate 10 and one or more Mn sulfides within the maximum length Lmax are identified as specific composite precipitates. In Figure 2A, 1 to 80 μm 2Mn sulfides 11-13, each having an area of ​​10-50 μm, are arranged within the maximum length Lmax (within region 20) of the coarse Nb precipitate 10. Therefore, in this case, the area is 10-50 μm. 2 It is determined that the coarse Nb precipitate 10 and Mn sulfides 11-13 constitute a single specific composite precipitate. Furthermore, if at least a portion of the Mn sulfide is included in region 20, as in Mn sulfide 11 in Figure 2A, it is determined that the Mn sulfide 11 is located within the maximum length Lmax from the coarse Nb precipitate 10. On the other hand, Mn sulfide 14 is located outside the range of region 20 with a maximum length Lmax from the coarse Nb precipitate 10. Therefore, Mn sulfide 14 does not constitute a specific composite precipitate.

[0083] Furthermore, Figure 2B shows coarse Nb precipitates 10 and 1-80 μm 2 The Mn sulfide 15, which has a certain surface area, is in contact with each other. In this case as well, the Mn sulfide 15 is located within the maximum length Lmax (within region 20) from the coarse Nb precipitate 10. Therefore, the coarse Nb precipitate 10 and the Mn sulfide 15 constitute one specific composite precipitate.

[0084] In Figure 2C, coarse Nb precipitates 10A and 10B are 1-80 μm in size. 2 It is in contact with one Mn sulfide 16 having an area of ​​. In other words, the Mn sulfide 16 is located within the maximum length Lmax (within region 20) from the coarse Nb precipitate 10A, and the Mn sulfide 16 is further located within the maximum length Lmax (within region 20) from the coarse Nb precipitate 10B. Therefore, the coarse Nb precipitate 10A and the Mn sulfide 16 constitute one specific composite precipitate, and further, the coarse Nb precipitate 10B and the Mn sulfide 16 constitute one specific composite precipitate. In other words, in Figure 2C, two specific composite precipitates are counted.

[0085] In Figure 2D, 1–80 μm 2One Mn sulfide 17 with an area of ​​is located within the maximum length Lmax (within region 20) of the coarse Nb precipitate 10C, although it is not in contact with the coarse Nb precipitate 10C. Furthermore, the Mn sulfide 17 is located within the maximum length Lmax (within region 20) of the coarse Nb precipitate 10D, although it is not in contact with the coarse Nb precipitate 10D. Therefore, the coarse Nb precipitate 10C and the Mn sulfide 17 constitute one specific composite precipitate, and furthermore, the coarse Nb precipitate 10D and the Mn sulfide 17 constitute one specific composite precipitate. In other words, in Figure 2D, two specific composite precipitates are counted.

[0086] The number of specific complex precipitates identified across the entire observation area and the area of ​​the observation area (60 mm²). 2 Based on this, the number density of specific composite precipitates ND (pieces / mm³) 2 The number density ND of the specific composite precipitate is calculated by rounding the fourth decimal place of the obtained value to the third decimal place.

[0087] [Method for measuring the number ratio RA of Nb coarse specific complex precipitates relative to specific complex precipitates] The number ratio RA of Nb coarse specific complex precipitates relative to specific complex precipitates is determined by the following method. In the above-described method for measuring the number density of specific composite precipitates, among the specific composite precipitates identified throughout the entire observation area, those in which the area of ​​coarse Nb precipitates is larger than the area of ​​Mn sulfides are designated as Nb coarse specific composite precipitates. Furthermore, as shown in Figure 2A, if a specific composite precipitate comprises one coarse Nb precipitate and multiple Mn sulfides, and the area of ​​the coarse Nb precipitate is larger than the total area of ​​the multiple Mn sulfides (in the case of Figure 2A, the total area of ​​Mn sulfides 11-13), then the specific composite precipitate is designated as Nb coarse specific composite precipitate. The number of identified Nb coarse specific complex precipitates is counted. Based on the number of specific complex precipitates in the entire observation area and the number of Nb coarse specific complex precipitates, the percentage (RA) of Nb coarse specific complex precipitates relative to the number of specific complex precipitates is calculated. The percentage RA is rounded to the first decimal place by rounding the second decimal place of the obtained value.

[0088] [Regarding the effects of the steel material in this embodiment] The steel material of this embodiment satisfies features 1 and 2. Therefore, excellent cold workability is obtained, and furthermore, excellent fatigue strength can be obtained in machine structural parts made from this steel material.

[0089] [Regarding the shape of the steel material in this embodiment] The shape of the steel material in this embodiment is not particularly limited. The steel material is, for example, a bar or wire with a circular cross-section perpendicular to the rolling direction (longitudinal direction). The shape of the steel material in this embodiment is not limited to steel bars or wire rods. The steel material in this embodiment may be annealed material produced by annealing steel bars or wire rods. Furthermore, it may be raw material produced by forging steel bars or wire rods.

[0090] [Regarding the microstructure of the steel material of this embodiment] The microstructure of the steel material in this embodiment is not particularly limited. When the steel material in this embodiment is used as a material for machine structural parts, the steel material may undergo heating during spheroidizing annealing before cold forging or heating during surface hardening heat treatments such as carburizing. c1 The steel is heated above its transformation point (austenite temperature range or two-phase range). Therefore, the microstructure of the steel changes during each manufacturing process of the machine structural parts. Consequently, the microstructure of the steel is not particularly limited.

[0091] [Regarding the manufacturing method of the steel material of this embodiment] An example of a method for manufacturing the steel material of this embodiment will be described. The steel material having the above-described structure may be manufactured by methods other than the manufacturing method described below. However, the manufacturing method described below is a preferred example of a method for manufacturing the steel material of this embodiment.

[0092] An example of a method for manufacturing steel materials according to this embodiment includes the following steps. (Process 1) Refining process (Process 2) Continuous casting process (Process 3) Hot working process The following describes each step.

[0093] [(Process 1) Refining Process] In the refining process, molten steel having a chemical composition satisfying characteristic 1 described above is produced. The refining method is not particularly limited, and any well-known method may be used. For example, molten iron produced by a well-known method is subjected to refining in a converter (primary refining). Molten steel tapped from the converter is subjected to a well-known secondary refining. Through the above process, molten steel having a chemical composition satisfying characteristic 1 is produced.

[0094] [(Process 2) Continuous casting process] In the continuous casting process, bloom is produced using the molten steel manufactured in the refining process via the continuous casting method. The conditions for the continuous casting process will be described later.

[0095] [(Process 3) Hot working process] In the hot working process, the bloom is hot-worked to produce steel materials. The hot working process includes the bract rolling process and the product rolling process.

[0096] In the bract rolling process, the bloom is first heated in a heating furnace. After heating, the bloom is hot-rolled (bract rolling) using a bract mill to produce a billet. Furthermore, the billet is hot-rolled using a continuous rolling mill located downstream of the bract mill to further reduce its size. The continuous rolling mill is equipped with multiple rolling stands arranged in a row. The billet produced in the bract rolling process is cooled to room temperature (releasing) before the product rolling process. The conditions in the bract rolling process will be described later.

[0097] In the product rolling process, the steel material of this embodiment is manufactured by hot rolling (product rolling) of the billet. Specifically, the billet after the bract rolling process is heated using a heating furnace. The heating temperature is, for example, 1000 to 1250°C. Hot rolling (product rolling) is then performed on the heated billet using a continuous rolling mill. The continuous rolling mill is equipped with multiple rolling stands arranged in a row. The steel material of this embodiment is manufactured through the above manufacturing process. As described above, the manufactured steel material is, for example, a steel bar or wire rod.

[0098] [Regarding the manufacturing conditions in the manufacturing method of this embodiment] The above manufacturing method satisfies the following conditions: (Condition 1) In the continuous casting process, the high-temperature cooling rate CR1 in the high-temperature range where the bloom surface temperature is 1500-1350°C is set to 3-500°C / min. (Condition 2) In the continuous casting process, the low-temperature cooling rate CR2 in the low-temperature range of 1000-500°C for the bloom surface temperature is set to 0.5-5.0°C / min. (Condition 3) The heating temperature T1 during the bloc rolling process in the hot working process is set to be between 1300°C and 1350°C. The following explains each condition.

[0099] [Condition 1: Regarding the high-temperature cooling rate CR1] In the continuous casting process, the high-temperature cooling rate CR1 is set to 3-500°C / min in the high-temperature range where the bloom surface temperature is 1500-1350°C. At high temperatures, Mn sulfides are formed in the bloom. If the high-temperature cooling rate CR1 is less than 3°C / min, the cooling rate of the bloom at high temperatures during the continuous casting process is too slow. In this case, an excess of coarse Mn sulfides is formed. As a result, an excess of specific complex precipitates is formed.

[0100] On the other hand, if the high-temperature cooling rate CR1 exceeds 500°C / min, the Mn sulfide is less likely to coarseen, and the area is 1 μm. 2 Numerous minute Mn sulfides less than 1 / 3 are formed. In this case, the number density ND of the specific composite precipitate becomes excessively low. Therefore, the high-temperature cooling rate CR1 should be set to 3-500°C / min.

[0101] In a continuous casting apparatus used in a continuous casting process, the surface temperature of the bloom during continuous casting is measured along the longitudinal direction of the strand. Based on the measured surface temperature, the time it takes for the bloom surface temperature to decrease from 1500°C to 1350°C is determined. Based on the obtained time, the high-temperature cooling rate CR1 (°C / min) is determined.

[0102] [Regarding the adjustment method for the high-temperature cooling rate CR1] To adjust the high-temperature cooling rate CR1 to 3-500°C / min, for example, the following method can be used: Water-cool the bloom surface in the high-temperature range during the continuous casting process. For example, the high-temperature cooling rate CR1 can be adjusted by adjusting the number and flow rate of the water-cooling nozzles.

[0103] [Condition 2: Regarding the low-temperature cooling rate CR2] In the continuous casting process, the low-temperature cooling rate CR2 in the low-temperature range of 1000-500°C for the bloom surface temperature is set to 0.5-5.0°C / min. The low-temperature range is the temperature range after the bloom has been cut by a torch cutting device. Coarse Nb precipitates are formed in the low-temperature range. If the low-temperature cooling rate CR2 is less than 0.5°C / min, the Nb precipitates become too coarse. In this case, the number density ND of specific composite precipitates becomes excessively low.

[0104] On the other hand, if the low-temperature cooling rate CR2 exceeds 5.0°C / min, the resulting Nb precipitates become too fine. In this case, the number density ND of the specific composite precipitate becomes excessively low. Therefore, the low-temperature cooling rate CR2 should be set to 0.5 to 5.0°C / min.

[0105] Preferably, the low-temperature cooling rate CR2 is 4.5°C / min or less. In this case, the number ratio RA of coarse Nb specific composite precipitates relative to specific composite precipitates increases further.

[0106] The low-temperature cooling rate CR2 is determined by the following method: After cutting with a torch cutting device, the surface temperature of the bloom is measured using a thermometer such as a radiation thermometer or thermocouple, and the time it takes for the bloom surface temperature to decrease from 1000°C to 500°C is determined. Based on the obtained time, the low-temperature cooling rate CR2 (°C / min) is calculated.

[0107] [Regarding the adjustment method for the low-temperature cooling rate CR2] To adjust the low-temperature cooling rate CR2 to 0.5-5.0°C / min, for example, the following method can be used: In the continuous casting process, the manufactured bloom is cut with a torch cutting device. The cut blooms are stacked in multiple layers. The low-temperature cooling rate CR2 can be adjusted by adjusting the number of blooms to be stacked, the number of stacking layers, the spacing between adjacent blooms, etc.

[0108] [Condition 3: Regarding the heating temperature T1 in the bloc rolling process] The heating temperature T1 in the bract rolling process affects the growth of Nb precipitates in specific composite precipitates. If the heating temperature T1 is 1300°C or lower, the Nb precipitates will grow further, resulting in the excessive generation of coarse Nb precipitates. In this case, the number density ND of the specific composite precipitates will be excessive. On the other hand, if the heating temperature T1 exceeds 1350°C, coarse Nb precipitates become more easily dissolved. As a result, the number density ND of specific composite precipitates becomes excessively low. Therefore, the heating temperature T1 is between 1300°C and 1350°C. Note that the heating temperature T1 is determined by taking furnace temperature data measured at one-minute intervals after the steel material is placed in the heating furnace, and using the first measurement time of the three consecutive furnace temperature data when the sample standard deviation of the three consecutive furnace temperature data sets first falls below 30°C as the reference time. The heating temperature T1 (°C) is the arithmetic mean of all furnace temperature data from the reference time to the measurement time immediately before the steel material is removed from the heating furnace. The heating temperature T1 (°C) is an integer value obtained by rounding the first decimal place of the obtained arithmetic mean.

[0109] [Carburized steel parts of this embodiment] The carburized steel component of this embodiment is made from the steel material described above. The carburized steel component comprises a carburized hardened layer formed on the surface and a core portion located inside the carburized hardened layer. The carbon content of the carburized hardened layer is higher than that of the core portion. Therefore, a person skilled in the art can easily distinguish between the carburized hardened layer and the core portion.

[0110] The chemical composition of the core is as follows (by mass%): C: 0.10-0.30%, Si: 0.05-1.50%, Mn: 0.40-2.00%, P: 0.025% or less, S: 0.001-0.050%, Cr: 0.05-2.30%, Al: 0.100% or less, N: 0.030% or less, O: 0.0003-0.0050%, Nb: 0.020-0.080%, Mo: 0-0.40%, Ni: 0-0.30%, B: 0-0. It contains 0.05%, Cu: 0-0.40%, W: 0-0.50%, Ti: 0-0.10%, V: 0-0.15%, Zr: 0-0.10%, Ca: 0-0.0050%, Mg: 0-0.0050%, rare earth elements: 0-0.010%, Sn: 0-0.100%, Pb: 0-0.30%, Bi: 0-0.30%, Sb: 0-0.015%, and Te: 0-0.20%, with the remainder being Fe and impurities. In other words, the chemical composition of the core is the same as the chemical composition of the steel material in this embodiment.

[0111] In a cross-section of a carburized steel part parallel to the rolling direction, the number density of specific composite precipitates is 0.010 to 1.000 particles / mm³. 2 The specific complex precipitates are 10-50 μm in size. 2 Nb precipitates having an area of ​​, and positioned within the maximum length Lmax of the Nb precipitates, with a length of 1-80 μm. 2 It contains Mn sulfide having an area of ​​[value].

[0112] Here, the rolling direction of the carburized steel part refers to the direction in which the metal flow of the carburized steel part is elongated.

[0113] Preferably, among the specified composite precipitates, the number ratio of Nb coarse specified composite precipitates, which are specified composite precipitates in which the area of ​​Nb precipitates is larger than the area of ​​Mn sulfide, is 55.0% or more of the total specified composite precipitates.

[0114] [Manufacturing method for carburized steel parts] The carburized steel parts of this embodiment are manufactured using the steel material of this embodiment by a well-known method. For example, the steel material is subjected to spheroidizing annealing, and then cold-worked (cold forging) to form a raw shape. The raw shape is then subjected to carburizing treatment (gas carburizing or vacuum carburizing). After carburizing, the raw shape is machined to achieve the final shape. The carburized steel parts of this embodiment are manufactured through the above steps. [Examples]

[0115] 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.

[0116] Steel materials having the chemical compositions shown in Table 1 (Tables 1A to 1D) were manufactured.

[0117] [Table 1A]

[0118] [Table 1B]

[0119] [Table 1C]

[0120] [Table 1D]

[0121] Specifically, blooms were produced using the molten steel manufactured by continuous casting. Furthermore, billets were produced by bloc rolling of the manufactured slabs. The high-temperature cooling rate CR1 (Condition 1, in °C / min), low-temperature cooling rate CR2 (Condition 2, in °C / min), and heating temperature T1 (Condition 3, in °C) during the manufacturing process are shown in Table 2 (Tables 2A and 2B).

[0122] [Table 2A]

[0123] [Table 2B]

[0124] Product rolling was performed on the manufactured billets. The heating temperature during the product rolling process was 1000-1250°C. Through the above process, steel bars (steel material) with a diameter of 45 mm were manufactured.

[0125] [About the evaluation test] The following evaluation tests were performed on the steel materials of each test number produced. (Test 1) Measurement test of the number density ND of specific composite precipitates (Test 2) Cold workability evaluation test (Test 3) Evaluation test for resistance to coarse grain formation (Test 4) Bending fatigue strength evaluation test The following describes each test.

[0126] [(Test 1) Measurement test of the number density ND of specific composite precipitates] Based on the method described in the above-mentioned [Method for Measuring the Number Density of Specific Composite Precipitates], the number density of specific composite precipitates of the steel material for each test number is calculated as ND (pieces / mm²). 2 The number density ND (pieces / mm³) of the obtained specific composite precipitate was determined. 2 The results are shown in Table 2 (Tables 2A and 2B).

[0127] Furthermore, based on the method described in [Method for measuring the number ratio RA of Nb coarse specific composite precipitates relative to specific composite precipitates] above, the number ratio RA (%) of Nb coarse specific composite precipitates relative to specific composite precipitates for each test number of steel material was determined. The obtained number ratios RA (%) are shown in Table 2 (Table 2A and Table 2B).

[0128] [(Test 2) Cold workability evaluation test] The following limit compression tests were conducted to evaluate the cold workability of the steel materials for each test number. Specifically, spheroidizing annealing was performed on the steel bars (steel bars) for each test number. In spheroidizing annealing, the steel was held at 760°C for 4 hours. After that, the steel was slowly cooled at 0.25°C / min until the steel temperature reached 660°C. Then, the steel was allowed to cool to room temperature (25°C).

[0129] Twenty-five specimens 30, as shown in Figure 3, were taken from the steel materials of each test number after spheroidizing annealing. The diameter D30 of the specimens was 14 mm, and the length L30 was 21 mm. The longitudinal direction of the specimens 30 was parallel to the longitudinal direction of the steel materials (steel bars) of each test number. The central axis of the specimens 30 corresponded to the R / 2 position of the steel materials (steel bars) of each test number. A single notch 31 extending in the longitudinal direction was formed on the circumferential surface of the specimens. The length of the notch 31 was 21 mm, the notch angle A30 was 30°, the notch depth V30 was 0.8 mm, and the radius of curvature R30 at the tip of the notch was 0.15 mm.

[0130] Twenty-five test specimens were subjected to cold compression using a 500-ton hydraulic press, and the limit compressibility (%) of the steel material for each test number was determined based on the following steps.

[0131] (Step 1) The 25 test specimens are divided into groups of 5 (Groups 1 to 5). (Step 2) Compression tests (cold compression) using the hydraulic press described above are performed on the five test specimens of the first group. The compression tests are performed at room temperature in air. Cold compression is performed at a strain rate of 10⁻¹ / second. A constrained die is used on the test specimens during cold compression. A constrained die is a pressure plate with concentric grooves formed on it, also called a constrained pressure plate. During cold compression, the hydraulic press is stopped every 2% increase in compression ratio to check whether microcracks of 0.5 mm or more have formed near the notches of each test specimen. Cracks are observed visually and using a magnifying glass with a magnification of 5x to check for the presence of microcracks.

[0132] (Step 3) When checking for the presence of microcracks, if a microcrack of 0.5 mm or larger is found near the notch in one or more of the five test specimens, the number of test specimens with microcracks is counted. For example, if a microcrack is found in one of the five test specimens, the compression ratio at that time is defined as the critical compression ratio (%) when the cumulative failure probability reaches 20%. If microcracks are found in two of the five test specimens, the compression ratio at that time is defined as the critical compression ratio (%) when the cumulative failure probability reaches 40%. Here, the cumulative failure probability refers to the proportion of test specimens in which microcracks were found out of the five test specimens.

[0133] (Step 4) After removing the specimens in which microcracks have occurred, the remaining specimens are subjected to further cold compression. As described above, for every 2% increase in the compression ratio during cold compression, the hydraulic press is stopped and checked to see if microcracks of 0.5 mm or more have occurred near the notch of each specimen. If microcracks are found, the cumulative failure probability is calculated according to the specimen in which the microcracks were found, and the compression ratio at that time is set as the critical compression ratio (%) for that cumulative failure probability. The cumulative failure probability and critical compression ratio (%) are calculated until the cumulative failure probability reaches 100%.

[0134] (Step 5) After the cumulative failure probability reaches 100%, the obtained cumulative failure probability and critical compressibility are plotted on a graph with the cumulative failure probability (%) on the vertical axis and the critical compressibility (%) on the horizontal axis. Linear approximation is performed based on the plot to obtain an approximate line showing the relationship between the cumulative failure probability and the critical compressibility. The approximate line is obtained by the least squares method. Figure 4 shows an example of an approximate line in the graph showing the relationship between the cumulative failure probability and the critical compressibility. Referring to Figure 4, for example, if cold compression is performed using 5 test pieces and the critical compressibility at a cumulative failure probability of 20% is X1%, at a cumulative failure probability of 40% is X2%, at a cumulative failure probability of 60% is X3%, at a cumulative failure probability of 80% is X4%, and at a cumulative failure probability of 100% is X5%, then the approximate line L1 is obtained for these data by the least squares method. Based on the obtained approximate line, the critical compressibility (%) at a cumulative failure probability of 50% is determined for the first group.

[0135] (Step 6) For each of the remaining groups 2 through 5, the critical compression ratio (%) at which the cumulative corruption probability reaches 50% is determined using the same method as described for group 1 above. (Step 7) The arithmetic mean of the five calculated critical compressibility ratios shall be taken as the critical compressibility ratio (%) of the steel material for the given test number. The critical compressibility ratio shall be an integer value obtained by rounding the obtained arithmetic mean to the nearest tenth.

[0136] Based on the obtained critical compression ratio (%), the following evaluation was performed. A: The maximum compression ratio is 56% or higher. B: The limiting compression ratio is 51-55%. C: The maximum compression ratio is 46-50%. X: The maximum compression ratio is 45% or less. A rating of A to C indicates that excellent cold workability was achieved. On the other hand, a rating of X indicates that sufficient cold workability was not achieved.

[0137] [(Test 3) Evaluation test for resistance to coarse granulation] The following grain coarseness resistance evaluation tests were performed on the steel materials for each test number. First, spheroidizing annealing was performed on the steel bars (steel bars) for each test number. In spheroidizing annealing, the steel bars were held at 760°C for 4 hours. After that, the steel bars were slowly cooled at 0.25°C / second until the steel bar temperature reached 660°C. Then, the steel bars were allowed to cool to room temperature (25°C).

[0138] Test specimens were taken from steel materials cooled to room temperature, and cold forging was performed on these specimens. In the cold forging process, cylindrical specimens with a diameter of 14 mm and a length of 21 mm were taken from the R / 2 position of the steel material. The central axis of the cylindrical specimen coincided with the R / 2 position of the steel material. The longitudinal direction of the cylindrical specimen was parallel to the longitudinal direction (rolling direction) of the steel material. Upsetting forging was performed on the taken cylindrical specimens at room temperature. Specifically, the cylindrical specimens were cold-compressed by 70% in the longitudinal direction.

[0139] Vacuum carburizing was performed on cylindrical test specimens after cold forging. In the vacuum carburizing process, first, the cylindrical test specimens were heated to 1000°C under reduced pressure of 100 Pa and soaked at 1000°C for 60 minutes. Then, a carburizing process was carried out for 30 minutes, followed by a diffusion process for 76 minutes. After that, the cylindrical test specimens were cooled to 880°C and held at 880°C for 30 minutes. Subsequently, oil quenching was performed by immersion in oil at 80°C. Tempering was then performed on the quenched cylindrical test specimens. The tempering temperature was 180°C and the holding time was 120 minutes.

[0140] The resistance to grain coarseness was evaluated for cylindrical specimens after vacuum carburizing treatment using the following method. A cylindrical specimen was cut along a cross-section parallel to its central axis, and an observation specimen including the cut surface was taken. In the observation specimen, the cut surface was used as the observation surface. The observation surface corresponded to a cross-section parallel to the rolling direction of the steel material.

[0141] The observation surface was mirror-polished. After mirror polishing, the observation surface was etched with a mixed solution of picric acid and ethanol (4 g of picric acid per 100 ml of alcohol) to reveal the prior austenite grain boundaries.

[0142] The entire observation surface was observed at 100x magnification using an optical microscope, and a photographic image of the entire observation surface was obtained. The equivalent circle diameter (μm) of each prior austenite grain in the photographic image was determined. The obtained equivalent circle diameter was considered to be the grain size (μm) of the prior austenite. The average grain size (μm) corresponding to grain size number 4 as described in JIS G 0551 (2020) was compared with the grain size (μm) of each prior austenite grain as described above, and the number of prior austenite grains with a grain size larger than the average grain size corresponding to grain size number 4 (i.e., the number of prior austenite grains with grain size numbers of 4 or less) was counted. Based on the obtained number of prior austenite grains with grain size numbers of 4 or less, the following evaluation was performed. A: The number of old austenite grains with a grain size number of 4 or less is 0. B: The number of old austenite grains with a grain size number of 4 or less is 1 to 3. C: The number of old austenite grains with a grain size number of 4 or less is between 4 and 8. The number of old austenite grains with a grain size number of D:4 or lower is 9 to 10. ×: The number of old austenite grains with a grain size number of 4 or less is 11 or more. For ratings A through D, it was determined that excellent resistance to granulation coarseness was achieved. On the other hand, for rating X, it was determined that sufficient resistance to granulation coarseness was not achieved.

[0143] [(Test 4) Bending fatigue strength evaluation test] The bending fatigue strength of the steel material for each test number was determined using the following method. Multiple Ono-type rotary bending fatigue test specimens 40, conforming to JIS Z 2274 (1978), were taken from the R / 2 position of the steel bar for each test number, as shown in Figure 5. In the Ono-type rotary bending fatigue test specimens, the diameter D40 of the parallel section was set to 10 mm, the length L40 of the parallel section to 20.67 mm, and the radius SR40 of the shoulder section to 24 mm. Furthermore, a semicircular notch 42 with a depth of 1 mm and a notch bottom radius R40 of 1.0 mm was formed at the center of the longitudinal direction of the parallel section. The central axis of the Ono-type rotary bending fatigue test specimen 40 was parallel to the longitudinal direction (rolling direction) of the steel bar.

[0144] A simulated carburized steel part was manufactured by performing the same vacuum carburizing treatment as in [(Test 3) Grain Coarseness Resistance Evaluation Test] on Ono-type rotary bending fatigue test specimen 40. Specifically, the test specimen was heated to 1000°C under reduced pressure of 100 Pa and soaked at 1000°C for 60 minutes. Then, a carburizing process was carried out for 30 minutes, followed by a diffusion process for 76 minutes. After that, the test specimen was cooled to 880°C and held at 880°C for 30 minutes. Then, oil quenching was performed by immersion in oil at 80°C. Tempering was performed on the quenched test specimen. The tempering temperature was 180°C and the holding time was 120 minutes.

[0145] Using simulated carburized steel parts (Ono-type rotary bending fatigue test specimen 40) after vacuum carburization treatment, an Ono-type rotary bending fatigue test was conducted at room temperature and in air, in accordance with JIS Z 2274 (2011). The rotation speed was set to 3000 rpm. Three test specimens were used for each stress condition in the Ono-type rotary bending fatigue test. All three test specimens underwent a cycle count of 1 × 10⁻⁶. 7 The highest stress at which fracture did not occur up to a certain number of cycles was defined as the bending fatigue strength (MPa). Based on the obtained bending fatigue strength, the following evaluation was performed. A: Bending fatigue strength of 550 MPa or more B: Bending fatigue strength of 500-549 MPa C: Bending fatigue strength of 450-499 MPa ×: Bending fatigue strength is 449 MPa or less In the case of evaluations A to C, it was determined that excellent bending fatigue strength was obtained when manufacturing carburized steel parts using steel as the raw material. In the case of evaluation X, it was determined that sufficient bending fatigue strength was not obtained.

[0146] [Evaluation Results] The evaluation results are shown in Table 2 (Tables 2A and 2B). Referring to Tables 1 and 2, the steel materials for test numbers 1 to 51 satisfied features 1 and 2. Therefore, these test numbers showed excellent cold workability and excellent resistance to grain coarseness. Furthermore, the simulated carburized steel parts (Ono-type rotary bending fatigue test specimens) manufactured using the steel materials showed excellent bending fatigue strength.

[0147] Furthermore, in tests 1-38, 40-46, and 48-51, among the test numbers 1-51, the number ratio RA of Nb coarse specific composite precipitates (where the area of ​​Nb precipitates is larger than the area of ​​Mn sulfide) relative to the specific composite precipitates was 55.0% or higher. Therefore, compared to test numbers 39 and 47, where the number ratio RA was less than 55.0%, the resistance to coarsening was even better.

[0148] On the other hand, in test number 52, the high-temperature cooling rate CR1 was too slow. As a result, the number density ND of the specific composite precipitate was 1,000 particles / mm³. 2 It exceeded that limit. As a result, sufficient cold workability could not be obtained.

[0149] In test number 53, the high-temperature cooling rate CR1 was too fast. As a result, the number density ND of the specific composite precipitate was 0.010 particles / mm³. 2 It was less than [a certain value]. As a result, sufficient resistance to coarse granulation could not be obtained.

[0150] In test number 54, the low-temperature cooling rate CR2 was too slow. As a result, the number density ND of the specific composite precipitate was 0.010 particles / mm³. 2 It was less than [a certain value]. As a result, sufficient resistance to coarse granulation could not be obtained.

[0151] In test number 55, the low-temperature cooling rate CR2 was too fast. As a result, the number density ND of the specific composite precipitate was 0.010 particles / mm³. 2 It was less than [a certain value]. As a result, sufficient resistance to coarse granulation could not be obtained.

[0152] In test number 56, the heating temperature T1 during the bloc rolling process was too low. As a result, the number density ND of the specific composite precipitate was 1,000 particles / mm³. 2 It exceeded that limit. As a result, sufficient cold workability could not be obtained.

[0153] In test number 57, the heating temperature T1 during the bloc rolling process was too high. As a result, the number density ND of the specific composite precipitate was 0.010 particles / mm³. 2It was less than [a certain value]. As a result, sufficient resistance to coarse granulation could not be obtained.

[0154] In test number 58, the oxygen content of the steel was too low. As a result, the number density ND of the specific composite precipitate was 0.010 particles / mm³. 2 It was less than [a certain value]. As a result, sufficient resistance to coarse granulation could not be obtained.

[0155] The embodiments of this disclosure have been described above. However, the embodiments described above are merely examples for implementing this disclosure. Therefore, this disclosure is not limited to the embodiments described above, and the embodiments described above can be modified as appropriate without departing from the spirit of this disclosure.

Claims

1. The chemical composition is expressed in mass percent. C: 0.10-0.30%, Si: 0.05-1.50%, Mn: 0.40-2.00%, P: 0.025% or less, S: 0.001-0.050%, Cr: 0.05-2.30%, Al: 0.100% or less, N: 0.030% or less, O: 0.0003 to 0.0050%, Nb: 0.020-0.080%, Mo: 0 to 0.40%, Ni: 0 to 0.30%, B: 0 to 0.005%, Cu: 0 to 0.40%, W: 0-0.50%, Ti: 0 to 0.10%, V: 0 to 0.15%, Zr: 0 to 0.10%, Ca: 0-0.0050%, Mg: 0 to 0.0050%, Rare earth elements: 0 to 0.010%, Sn: 0-0.100%, Pb: 0 to 0.30%, Bi: 0-0.30%, Sb: 0-0.015%, and, It contains Te: 0-0.20%, The remainder consists of Fe and impurities. In a cross-section parallel to the rolling direction, The number density of specific composite precipitates is 0.010 to 1.000 particles / mm³. ２ And, The aforementioned specific complex precipitate is 10–50 μm ２ Nb precipitate having the area of, It is positioned within the maximum length Lmax of the Nb precipitate, and is 1 to 80 μm in size. ２ A Mn sulfide having the area of, including, Steel material.

2. The steel material according to claim 1, The aforementioned chemical composition is, in mass%, Mo: 0.01-0.40%, Ni: 0.01 to 0.30%, B: 0.001-0.005%, Cu: 0.01-0.40%, W: 0.01-0.50%, Ti: 0.01 to 0.10%, V: 0.01-0.15%, Zr: 0.01 to 0.10%, Ca: 0.0001-0.0050%, Mg: 0.0001 to 0.0050%, Rare earth elements: 0.001 to 0.010%, Sn: 0.001 to 0.100%, Pb: 0.01 to 0.30%, Bi: 0.01-0.30%, Sb: 0.001–0.015%, and, Contains one or more selected from the group consisting of Te: 0.01 to 0.20%. Steel material.

3. A steel material according to claim 1 or claim 2, Among the specified composite precipitates, the number ratio of Nb coarse specified composite precipitates, in which the area of ​​the Nb precipitate is larger than the area of ​​the Mn sulfide, to the specified composite precipitates is 55.0% or more. Steel material.

4. The carburized hardened layer formed on the surface, It comprises a core portion located inside the aforementioned carburized hardened layer, The chemical composition of the core is, in mass%, C: 0.10-0.30%, Si: 0.05-1.50%, Mn: 0.40-2.00%, P: 0.025% or less, S: 0.001-0.050%, Cr: 0.05-2.30%, Al: 0.100% or less, N: 0.030% or less, O: 0.0003 to 0.0050%, Nb: 0.020-0.080%, Mo: 0 to 0.40%, Ni: 0 to 0.30%, B: 0 to 0.005%, Cu: 0 to 0.40%, W: 0-0.50%, Ti: 0 to 0.10%, V: 0 to 0.15%, Zr: 0 to 0.10%, Ca: 0-0.0050%, Mg: 0 to 0.0050%, Rare earth elements: 0 to 0.010%, Sn: 0-0.100%, Pb: 0 to 0.30%, Bi: 0-0.30%, Sb: 0-0.015%, and, It contains Te: 0-0.20%, The remainder consists of Fe and impurities. In a cross-section parallel to the rolling direction, The number density of specific composite precipitates is 0.010 to 1.000 particles / mm³. ２ And, The aforementioned specific complex precipitate is 10–50 μm ２ Nb precipitate having the area of, It is positioned within the maximum length Lmax of the Nb precipitate, and is 1 to 80 μm in size. ２ A Mn sulfide having the area of, including, Carburized steel parts.

5. A carburized steel part according to claim 4, The aforementioned chemical composition is, in mass%, Mo: 0.01-0.40%, Ni: 0.01 to 0.30%, B: 0.001-0.005%, Cu: 0.01-0.40%, W: 0.01-0.50%, Ti: 0.01 to 0.10%, V: 0.01-0.15%, Zr: 0.01 to 0.10%, Ca: 0.0001-0.0050%, Mg: 0.0001 to 0.0050%, Rare earth elements: 0.001 to 0.010%, Sn: 0.001 to 0.100%, Pb: 0.01 to 0.30%, Bi: 0.01-0.30%, Sb: 0.001–0.015%, and, Contains one or more selected from the group consisting of Te: 0.01 to 0.20%. Carburized steel parts.

6. A carburized steel part according to claim 4 or claim 5, Among the specified composite precipitates, the number ratio of Nb coarse specified composite precipitates, in which the area of ​​the Nb precipitate is larger than the area of ​​the Mn sulfide, to the specified composite precipitates is 55.0% or more. Carburized steel parts.

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