Component for machine construction

Abstract

To provide a component for a machine construction having a sufficient strength toughness balance, and suppressed in heat treatment strain.SOLUTION: A component for a machine construction of the present embodiment includes, by mass%, 0.31-0.65% of C, 0.01-0.73% of Si, 0.65-1.72% of Mn, 0.015% or less of P, 0.025% or less of S, 0.50% or less of Cr, 0.21% or less of Mo, 0.005-0.100% of Al, 0.0020-0.0300% of N, 0.0015% or less of O, and a balance of Fe with impurities, where the DA defined by the formula (1A) is 133 or more, and satisfies a formula (2A). DA=8.59×√C×(1+0.64×Si)×(1+4.1×Mn)×(1+2.33×Cr)×(1+3.14×Mo) (1A). 40.0×C+10.0×Si+30.0×Mn-10.0×Cr-0.5×Mo+200.0×N-0.8×DA≥-62.0 (2A).SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present disclosure relates to machine structural components. [Background technology]

[0002] Machine structural parts are used in automobiles, construction vehicles, mining machinery, etc. Examples of machine structural parts include gears, shafts, bearing parts, etc. In recent years, with the trend toward electrification of automobiles and other vehicles, machine structural parts have become increasingly lightweight and compact. Therefore, machine structural parts are required to have an excellent balance of strength and toughness.

[0003] Machine structural parts with an excellent balance of strength and toughness are manufactured using, for example, an alloy steel for machine structural use, such as SCM440 specified in JIS G 4053 (2016), in the following manufacturing process. The steel material is subjected to hot working (hot forging). Then, cutting is performed as necessary to manufacture an intermediate product of the desired shape. The intermediate product is subjected to heat treatment to adjust the hardness and microstructure of the intermediate product. The heat treatment is, for example, a quenching and tempering treatment. The quenching performed in the quenching and tempering treatment is, for example, through quenching (full quenching). Through the above manufacturing process, machine structural parts with an excellent balance of strength and toughness are manufactured.

[0004] However, when quenching and tempering treatment is performed, machine structural parts are prone to deformation. In this specification, the deformation that occurs in machine structural parts due to the influence of heat during quenching and tempering treatment is referred to as heat treatment distortion. Heat treatment distortion distorts the shape of the machine structural parts. The distortion of the shape of the machine structural parts causes noise and vibration when an automobile or the like is operating.

[0005] In conventional automobiles and other vehicles powered by internal combustion engines, the noise emitted by the engines was louder than the noise emitted by mechanical structural parts such as gears, shafts, and bearings. Therefore, the noise of mechanical structural parts has not received much attention. However, with the recent trend toward electrification of automobiles and other vehicles, the noise emitted from the power source has been significantly reduced, and as a result, the noise emitted by mechanical structural parts such as gears has become more noticeable. Therefore, there has been a recent demand for mechanical structural parts that can suppress heat treatment distortion during quenching and tempering.

[0006] A technique for suppressing heat treatment distortion is proposed in International Publication No. 2014 / 038548 (Patent Document 1).

[0007] The steel material for machine structural use disclosed in Patent Document 1 contains, by mass%, 0.20 to 0.30% C, 0.10 to 1.50% Si, 0.10 to 1.20% Mn, 0.030% or less P, 0.030% or less S, 1.30 to 2.50% Cr, 0.30% or less Cu, 0.008 to 0.300% Al, 0.0030% or less O, 0.0020 to 0.0300% N, and the balance being Fe and unavoidable impurities. This steel material for machine structural use has a martensitic transformation start temperature (Ms point) of 460°C or less. Furthermore, when the hardness measured by the Jominy end-quenching method at a position 1.5 mm from the quenched end of a steel material is defined as J1.5, the hardness at a position 9 mm from the quenched end as J9, and the hardness at a position 11 mm from the quenched end as J11, (J9 / J1.5) is 0.70 to 0.85, and (J11 / J1.5) is 0.67 to 0.78. Patent Document 1 states that in this steel material for machine structural use, heat treatment deformation can be suppressed by adjusting the Ms point and hardness. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] International Publication No. 2014 / 038548 Summary of the Invention [Problem to be solved by the invention]

[0009] However, machine structural components in which heat treatment distortion is suppressed may be obtained by means other than using the steel material disclosed in Patent Document 1 as a raw material.

[0010] An object of the present disclosure is to provide a machine structural component that has a sufficient balance of strength and toughness and in which heat treatment distortion is suppressed. [Means for solving the problem]

[0011] The machine structural component of the present disclosure comprises: A hardened layer is formed on at least the surface layer. The prior austenite grain size number in the surface layer is less than 11.0, The chemical composition of the machine structural part is, in mass%, C: 0.31-0.65%, Si: 0.01 to 0.73%, Mn: 0.65 to 1.72% P: 0.015% or less, S: 0.025% or less, Cr: 0.50% or less, Mo: 0.21% or less Al: 0.005 to 0.100%, N: 0.0020 to 0.0300%, and O: 0.0015% or less, the balance being Fe and impurities, DA defined by formula (1A) is 133 or more, Satisfies equation (2A). DA=8.59×√C×(1+0.64×Si)×(1+4.1×Mn)×(1+2.33×Cr)×(1+3.14×Mo) (1A) 40.0×C+10.0×Si+30.0×Mn-10.0×Cr-0.5×Mo+200.0×N-0.8×DA≧-62.0 (2A) Here, the content of the corresponding element in mass % is substituted for each element symbol in formula (1A) and formula (2A).

[0012] The machine structural component of the present disclosure comprises: A hardened layer is formed on at least the surface layer. The prior austenite grain size number in the surface layer is less than 11.0, The chemical composition of the machine structural part is, in mass%, C: 0.31-0.65%, Si: 0.01 to 0.73%, Mn: 0.65 to 1.72% P: 0.015% or less, S: 0.025% or less, Cr: 0.50% or less, Mo: 0.21% or less Al: 0.005 to 0.100%, N: 0.0020 to 0.0300%, and O: 0.0015% or less, Further, it contains one or more selected from the group consisting of Groups 1 to 4, the balance being Fe and impurities, DB defined by formula (1B) is 133 or more, Satisfies equation (2B). [Group 1] Cu: 0.30% or less, and Ni: 0.30% or less, one or more selected from the group consisting of [Group 2] V: 0.20% or less, Nb: 0.100% or less, Ti: 0.100% or less, and W: 0.20% or less, one or more selected from the group consisting of [Group 3] Ca: 0.0050% or less, and Mg: 0.010% or less, one or more selected from the group consisting of [Group 4] Te: 0.050% or less, Bi: 0.100% or less, Pb: 0.09% or less, Sn: 0.050% or less, and Sb: 0.050% or less, one or more selected from the group consisting of DB=8.59×√C×(1+0.64×Si)×(1+4.1×Mn)×(1+2.33×Cr)×(1+3.14×Mo)×(1+0.35×Cu)×(1+0.36×Ni) (1B) 40.0×C+10.0×Si+30.0×Mn-10.0×Cr-0.5×Mo+200.0×N-0.8×DB≧-62.0 (2B) Here, the content of the corresponding element in mass % is substituted for each element symbol in formula (1B) and formula (2B), and when an element is not contained, "0" is substituted for the corresponding element symbol. [Effects of the Invention]

[0013] The machine structural component of the present disclosure has a sufficient balance of strength and toughness, and heat treatment strain is suppressed. [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 1 is a perspective view of a test piece for evaluating heat treatment distortion used in a heat treatment distortion evaluation test. [Figure 2] FIG. 2 is a view showing a surface (measurement surface) perpendicular to the longitudinal direction of the test piece for evaluating heat treatment distortion shown in FIG. [Figure 3] FIG. 3 is a schematic diagram in which a circle is approximated by the least squares method based on the measurement points in FIG. [Figure 4] FIG. 4 is a schematic diagram for explaining a method for determining the amount of bending based on the center positions obtained on the three measurement surfaces in the heat treatment distortion evaluation test. DETAILED DESCRIPTION OF THE INVENTION

[0015] The present inventors first investigated, from the standpoint of chemical composition, machine structural parts that can achieve a sufficient strength-toughness balance equal to or greater than that of machine structural parts manufactured by quenching and tempering alloy steel materials for machine structures, such as SCM440. As a result, it was thought that a sufficient balance of strength and toughness could be obtained with a machine structural part containing, by mass%, C: 0.31 to 0.65%, Si: 0.01 to 0.73%, Mn: 0.65 to 1.72%, P: 0.015% or less, S: 0.025% or less, Cr: 0.50% or less, Mo: 0.21% or less, Al: 0.005 to 0.100%, N: 0.0020 to 0.0300%, and O: 0.0015% or less, with the balance being Fe and impurities, and having a DA, which is an index of hardenability, defined by formula (1A), of 133 or more, or a machine structural part containing, instead of a portion of the Fe in the above chemical composition, one or more elements selected from the group consisting of the above-mentioned first to fourth groups, and having a DB, which is an index of hardenability, defined by formula (1B), of 133 or more. DA=8.59×√C×(1+0.64×Si)×(1+4.1×Mn)×(1+2.33×Cr)×(1+3.14×Mo) (1A) DB=8.59×√C×(1+0.64×Si)×(1+4.1×Mn)×(1+2.33×Cr)×(1+3.14×Mo)×(1+0.35×Cu)×(1+0.36×Ni) (1B) Here, the content of the corresponding element in mass % is substituted for each element symbol in formula (1A) and formula (1B), and when an element is not contained, "0" is substituted for the corresponding element symbol.

[0016] Therefore, the present inventors further investigated techniques capable of suppressing heat treatment distortion in machine structural parts having the above-mentioned chemical composition.

[0017] First, the present inventors attempted to reduce heat treatment strain by lowering the Ms point, as in Patent Document 1. However, in machine structural components having the above-described chemical composition, there were cases where heat treatment strain could not be sufficiently suppressed even when the contents of each element were adjusted to lower the Ms point.

[0018] Therefore, the present inventors attempted to suppress the heat treatment distortion from a different perspective, rather than suppressing the heat treatment distortion by lowering the Ms point.

[0019] First, the inventors investigated the main factors of heat treatment strain that occurs during quenching and tempering. As a result of the investigation, it was found that, among the various strains that constitute the heat treatment strain that occurs during quenching and tempering, the main factors are transformation plastic strain and transformation expansive strain. Therefore, the inventors thought that if the transformation plastic strain and transformation expansive strain could be reduced, the heat treatment strain could be suppressed.

[0020] The transformation plastic strain ε is expressed by the following formula, where K is the transformation plasticity coefficient and σ is the stress applied by the transformation during quenching and tempering. Note that the formula below shows the transformation plastic strain ε after complete transformation (i.e., after 100% of the phase transformation has progressed). ε=Kσ

[0021] Therefore, if the transformation plasticity coefficient can be reduced, the transformation plastic strain will be reduced, and as a result, the heat treatment strain will also be suppressed. Therefore, the present inventors have investigated means for reducing the transformation plasticity coefficient from the viewpoint of chemical composition. As a result, the present inventors have obtained the following findings.

[0022] In the above-mentioned chemical composition, C, Si, Mn, and N have the effect of decreasing the transformation plasticity coefficient. On the other hand, Cr and Mo have the effect of increasing the transformation plasticity coefficient. Therefore, by appropriately adjusting the contents of C, Si, Mn, and N and the contents of Cr and Mo within the above-mentioned chemical composition range, the transformation plasticity coefficient can be decreased.

[0023] Transformation expansion strain is caused by volume expansion due to martensitic transformation during quenching. Therefore, by suppressing excessive increases in DA and DB, which are indicators of the hardenability of machine structural parts, while maintaining DA and DB at 133 or higher, transformation expansion strain can be reduced without impairing the strength-toughness balance. As a result, heat treatment strain is suppressed.

[0024] Based on the above findings, the inventors investigated the relationship between the contents of C, Si, Mn, N, Cr, and Mo, the hardenability parameters DA or DB, and heat treatment distortion. As a result, the inventors found that if the chemical composition consists of essential elements, formula (2A) is satisfied, and if the chemical composition contains essential elements and optional elements, formula (2B) is satisfied, heat treatment distortion can be suppressed. 40.0×C+10.0×Si+30.0×Mn-10.0×Cr-0.5×Mo+200.0×N-0.8×DA≧-62.0 (2A) 40.0×C+10.0×Si+30.0×Mn-10.0×Cr-0.5×Mo+200.0×N-0.8×DB≧-62.0 (2B) Here, the content of the corresponding element in mass % is substituted for each element symbol in formula (2A) and formula (2B).

[0025] The machine structural component of this embodiment has been completed based on the above technical concept, and has the following configuration.

[0026] The machine structural component of the first configuration comprises: A hardened layer is formed on at least the surface layer. The prior austenite grain size number in the surface layer is less than 11.0, The chemical composition of the machine structural part is, in mass%, C: 0.31-0.65%, Si: 0.01 to 0.73%, Mn: 0.65 to 1.72% P: 0.015% or less, S: 0.025% or less, Cr: 0.50% or less, Mo: 0.21% or less Al: 0.005 to 0.100%, N: 0.0020 to 0.0300%, and O: 0.0015% or less, the balance being Fe and impurities, DA defined by formula (1A) is 133 or more, Satisfies equation (2A). DA=8.59×√C×(1+0.64×Si)×(1+4.1×Mn)×(1+2.33×Cr)×(1+3.14×Mo) (1A) 40.0×C+10.0×Si+30.0×Mn-10.0×Cr-0.5×Mo+200.0×N-0.8×DA≧-62.0 (2A) Here, the content of the corresponding element in mass % is substituted for each element symbol in formula (1A) and formula (2A).

[0027] The second configuration of the machine structural component is A hardened layer is formed on at least the surface layer. The prior austenite grain size number in the surface layer is less than 11.0, The chemical composition of the machine structural part is, in mass%, C: 0.31-0.65%, Si: 0.01 to 0.73%, Mn: 0.65 to 1.72% P: 0.015% or less, S: 0.025% or less, Cr: 0.50% or less, Mo: 0.21% or less Al: 0.005 to 0.100%, N: 0.0020 to 0.0300%, and O: 0.0015% or less, Further, it contains one or more selected from the group consisting of Groups 1 to 4, the balance being Fe and impurities, DB defined by formula (1B) is 133 or more, Satisfies equation (2B). [Group 1] Cu: 0.30% or less, and Ni: 0.30% or less, one or more selected from the group consisting of [Group 2] V: 0.20% or less, Nb: 0.100% or less, Ti: 0.100% or less, and W: 0.20% or less, one or more selected from the group consisting of [Group 3] Ca: 0.0050% or less, and Mg: 0.010% or less, one or more selected from the group consisting of [Group 4] Te: 0.050% or less, Bi: 0.100% or less, Pb: 0.09% or less, Sn: 0.050% or less, and Sb: 0.050% or less, one or more selected from the group consisting of DB=8.59×√C×(1+0.64×Si)×(1+4.1×Mn)×(1+2.33×Cr)×(1+3.14×Mo)×(1+0.35×Cu)×(1+0.36×Ni) (1B) 40.0×C+10.0×Si+30.0×Mn-10.0×Cr-0.5×Mo+200.0×N-0.8×DB≧-62.0 (2B) Here, the content of the corresponding element in mass % is substituted for each element symbol in formula (1B) and formula (2B), and when an element is not contained, "0" is substituted for the corresponding element symbol.

[0028] The third configuration of mechanical structural parts is: A machine structural component having a second configuration, The chemical composition includes the first group.

[0029] The fourth configuration of mechanical structural parts is: A machine structural part having the second or third configuration, The chemical composition includes the second group.

[0030] The fifth component of the mechanical structural parts is: A machine structural component having any one of the second to fourth configurations, The chemical composition includes the third group.

[0031] The sixth component of the mechanical structural parts is: A machine structural part having any one of the second to fifth configurations, The chemical composition includes the fourth group.

[0032] The machine structural component according to this embodiment will be described in detail below. Note that "%" for elements means mass % unless otherwise specified.

[0033] [Configuration of the machine structural component of this embodiment] The term "machine structural parts" as used herein refers to parts manufactured by quenching and tempering steel materials, such as gears, shafts, and bearings.

[0034] The mechanical structural component of this embodiment has a quench-hardened layer at least on the surface layer. The prior austenite grain size number in the surface layer is less than 11.0. In this specification, the surface layer of the mechanical structural component refers to a region extending from the surface of the mechanical structural component to a depth of 100 μm. The mechanical structural component of this embodiment may have a quench-hardened layer formed in a region deeper than the surface layer.

[0035] The quench-hardened layer is a layer hardened by quenching and is mainly composed of martensite. The quench-hardened layer has a Vickers hardness of 500 HV or more according to JIS Z 2244-1 (2020). When the Vickers hardness at a depth of 100 μm from the surface of a mechanical structural part is 500 HV or more, the mechanical structural part is determined to have a quench-hardened layer at least on the surface.

[0036] The lower limit of the prior austenite grain size number in the surface layer is not particularly limited as long as it is less than 11.0. Considering normal industrial production, the lower limit of the prior austenite grain size number in the surface layer is, for example, 5.0. The upper limit of the prior austenite grain size number in the surface layer is preferably 10.9, more preferably 10.6, and even more preferably 10.3.

[0037] [Vickers hardness measurement method] The Vickers hardness at a depth of 100 μm from the surface of a machine structural part is determined by the following method. A test piece is taken with a cross section perpendicular to any surface of the mechanical structural component as the measurement surface. The size of the measurement surface is not particularly limited as long as it includes a position 100 μm deep from the surface of the mechanical structural component. The measurement surface is mirror-polished. After mirror-polishing, the Vickers hardness is determined at five arbitrary points 100 μm from the surface using a Vickers hardness test in accordance with JIS Z 2244-1 (2020). The test force is 2.9 N. The arithmetic mean of the Vickers hardness values ​​obtained at the five points is calculated. The obtained arithmetic mean value is the Vickers hardness at a position 100 μm deep from the surface of the mechanical structural component.

[0038] [Method for measuring prior austenite grain size number in the surface layer] The prior austenite grain size number in the surface layer is determined by the following method. A test piece is taken from a machine structural component, with a cross section perpendicular to the surface as the measurement surface. The measurement surface includes an observation area measuring 100 μm x 100 μm. The center of the observation area corresponds to a depth of 100 μm from the surface of the machine structural component. The measurement surface is mirror-polished. The mirror-polished measurement surface is etched using a saturated aqueous solution of picric acid to reveal the prior austenite grain boundaries. Three fields of view, centered at a depth of 100 μm from the surface, are observed using an optical microscope at 1000x magnification. The prior austenite grain size number for each field is then obtained in accordance with JIS G 0551 (2020). The arithmetic mean of the three prior austenite grain size numbers is calculated. The obtained arithmetic mean value is the prior austenite grain size number for the surface layer of the machine structural component. The prior austenite grain size number in the surface layer is the value obtained by rounding off the obtained value to one decimal place.

[0039] [Features of the machine structural component of this embodiment] The machine structural component of this embodiment includes the following features. (Feature 1) The chemical composition, in mass%, is C: 0.31-0.65%, Si: 0.01-0.73%, Mn: 0.65-1.72%, P: 0.015% or less, S: 0.025% or less, Cr: 0.50% or less, Mo: 0.21% or less, Al: 0.005-0.100%, N: 0.0020-0.0300%, O: 0.0015% or less, Cu: 0-0.30%, Ni: 0- 0.30%, V: 0-0.20%, Nb: 0-0.100%, Ti: 0-0.100%, W: 0-0.20%, Ca: 0-0.0050%, Mg: 0-0.010%, Te: 0-0.050%, Bi: 0-0.100%, Pb: 0-0.09%, Sn: 0-0.050%, and Sb: 0-0.050%, with the remainder being Fe and impurities. (Feature 2) When the chemical composition consists of essential elements, DA defined by formula (1A) is 133 or more, and when the chemical composition contains essential elements and optional elements, DB defined by formula (1B) is 133 or more. DA=8.59×√C×(1+0.64×Si)×(1+4.1×Mn)×(1+2.33×Cr)×(1+3.14×Mo) (1A) DB=8.59×√C×(1+0.64×Si)×(1+4.1×Mn)×(1+2.33×Cr)×(1+3.14×Mo)×(1+0.35×Cu)×(1+0.36×Ni) (1B) Here, the content of the corresponding element in mass % is substituted for each element symbol in formula (1A) and formula (1B), and when an element is not contained, "0" is substituted for the corresponding element symbol. (Feature 3) When the chemical composition consists of essential elements, it satisfies formula (2A), and when the chemical composition contains essential elements and optional elements, it satisfies formula (2B). 40.0×C+10.0×Si+30.0×Mn-10.0×Cr-0.5×Mo+200.0×N-0.8×DA≧-62.0 (2A) 40.0×C+10.0×Si+30.0×Mn-10.0×Cr-0.5×Mo+200.0×N-0.8×DB≧-62.0 (2B) Here, the content of the corresponding element in mass % is substituted for each element symbol in formula (2A) and formula (2B). Features 1 to 3 will be explained below.

[0040] [(Feature 1) Chemical composition] The chemical composition of the machine structural component of this embodiment contains the following elements.

[0041] C: 0.31 to 0.65% Carbon (C) improves hardenability and increases the strength of machine structural parts. If the C content is 0.31% or more, these effects can be sufficiently obtained. On the other hand, if the C content is 0.65% or less, the formation of coarse precipitates is suppressed, thereby improving the toughness of machine structural parts. Therefore, the C content is 0.31 to 0.65%. The lower limit of the C content is preferably 0.33%, more preferably 0.37%, and even more preferably 0.40%. The upper limit of the C content is preferably 0.62%, more preferably 0.59%, even more preferably 0.56%, even more preferably 0.53%, and even more preferably 0.50%.

[0042] Si: 0.01 to 0.73% Silicon (Si) improves hardenability and increases the strength of machine structural components. Si also increases the temper softening resistance of machine structural components. If the Si content is less than 0.01%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Si content exceeds 0.73%, the grain boundary oxide layer after quenching and tempering becomes excessively deep, and in this case, the toughness of the machine structural part decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the Si content is 0.01 to 0.73%. The lower limit of the Si content is preferably 0.05%, more preferably 0.10%, and even more preferably 0.20%. The upper limit of the Si content is preferably 0.70%, more preferably 0.65%, and even more preferably 0.60%.

[0043] Mn: 0.65 to 1.72% Manganese (Mn) improves hardenability and increases the strength of machine structural parts. Mn also suppresses heat treatment distortion. If the Mn content is less than 0.65%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Mn content exceeds 1.72%, the formation of Mn oxides in the surface layer is promoted during quenching and tempering. In this case, the grain boundary oxide layer of the machine structural component becomes excessively deep. Therefore, even if the contents of other elements are within the ranges of this embodiment, the toughness of the machine structural component decreases. Therefore, the Mn content is 0.65 to 1.72%. The lower limit of the Mn content is preferably 0.70%, more preferably 0.80%, and even more preferably 0.90%. The upper limit of the Mn content is preferably 1.65%, more preferably 1.60%, even more preferably 1.55%, and still more preferably 1.50%.

[0044] P:0.015% or less Phosphorus (P) is an impurity. If the P content exceeds 0.015%, P segregates excessively at grain boundaries, reducing grain boundary strength. As a result, the toughness of the machine structural component decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the P content is 0.015% or less. The P content is preferably as low as possible. However, excessive reduction in the P content increases production costs. Therefore, in consideration of normal industrial production, the lower limit of the P content is preferably more than 0%, more preferably 0.001%, even more preferably 0.002%, and even more preferably 0.003%. The upper limit of the P content is preferably 0.012%, more preferably 0.010%, and even more preferably 0.008%.

[0045] S: 0.025% or less Sulfur (S) is an impurity. If the S content exceeds 0.025%, coarse sulfide-based inclusions are formed. Therefore, even if the contents of other elements are within the ranges of this embodiment, the toughness of the machine structural part is reduced. Therefore, the S content is 0.025% or less. The S content is preferably as low as possible. However, excessive reduction in the S content increases production costs. Therefore, in consideration of normal industrial production, the lower limit of the S content is preferably more than 0%, more preferably 0.001%, and even more preferably 0.002%. The upper limit of the S content is preferably 0.020%, more preferably 0.010%, and even more preferably 0.005%.

[0046] Cr:0.50% or less Chromium (Cr) improves hardenability and increases the strength of machine structural parts. Even if the content of Cr is small, the above effects can be obtained to some extent. On the other hand, Cr increases the transformation plasticity coefficient. If the Cr content exceeds 0.50%, even if the contents of other elements are within the ranges of this embodiment, the transformation plasticity coefficient increases excessively, and the heat treatment strain during quenching and tempering increases. Therefore, the Cr content is 0.50% or less. The lower limit of the Cr content is preferably more than 0%, more preferably 0.01%, even more preferably 0.05%, and still more preferably 0.10%. The upper limit of the Cr content is preferably 0.47%, more preferably 0.44%, even more preferably 0.41%, even more preferably 0.38%, even more preferably 0.35%, even more preferably 0.32%, and even more preferably 0.29%.

[0047] Mo: 0.21% or less Molybdenum (Mo) improves hardenability and increases the strength of machine structural parts. Even if the content of Mo is small, the above effects can be obtained to some extent. On the other hand, if the Mo content exceeds 0.21%, the hot workability of the steel material used to make machine structural parts will decrease even if the contents of other elements are within the ranges of this embodiment. Therefore, the Mo content is 0.21% or less. The lower limit of the Mo content is preferably more than 0%, more preferably 0.01%, even more preferably 0.03%, and still more preferably 0.05%. The upper limit of the Mo content is preferably 0.18%, more preferably 0.15%, and even more preferably 0.12%.

[0048] Al: 0.005 to 0.100% Aluminum (Al) combines with N to form AlN, which has a pinning effect that suppresses grain coarsening during heating in quenching and tempering. This improves the toughness of machine structural parts. If the Al content is less than 0.005%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Al content exceeds 0.100%, even if the contents of other elements are within the ranges of this embodiment, coarse clustered oxides are formed, and the coarse clustered oxides reduce the toughness of the machine structural part. Therefore, the Al content is 0.005 to 0.100%. The lower limit of the Al content is preferably 0.008%, and more preferably 0.010%. The upper limit of the Al content is preferably 0.080%, more preferably 0.070%, and even more preferably 0.060%.

[0049] N: 0.0020~0.0300% Nitrogen (N) reduces the transformation plasticity coefficient, thereby suppressing heat treatment strain during quenching and tempering. If the N content is less than 0.0020%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the N content exceeds 0.0300%, the hot workability of the steel material used to make machine structural parts will decrease even if the contents of other elements are within the ranges of this embodiment. Therefore, the N content is 0.0020 to 0.0300%. The lower limit of the N content is preferably 0.0021%, and more preferably 0.0022%. The upper limit of the N content is preferably 0.0280%, more preferably 0.0250%, and even more preferably 0.0200%.

[0050] O: 0.0015% or less Oxygen (O) is an impurity. O combines with other elements to form coarse oxide-based inclusions. The coarse oxide-based inclusions reduce the toughness of machine structural parts. If the O content exceeds 0.0015%, the toughness of the machine structural parts will be significantly reduced even if the contents of other elements are within the ranges of this embodiment. Therefore, the O content is 0.0015% or less. The O content is preferably as low as possible. However, excessive reduction of the O content increases production costs. Therefore, in consideration of normal industrial production, the lower limit of the O content is preferably more than 0%, more preferably 0.0001%, even more preferably 0.0003%, and even more preferably 0.0005%. The upper limit of the O content is preferably 0.0013% or less, more preferably 0.0011%, and even more preferably 0.0009%.

[0051] The balance of the chemical composition of the machine structural component of this embodiment is Fe and impurities. Here, the impurities in the chemical composition refer to substances that are mixed in from raw materials such as ore and scrap, or the manufacturing environment, during industrial production of the machine structural component, and are acceptable within a range that does not adversely affect the machine structural component of this embodiment.

[0052] [About optional elements] The chemical composition of the machine structural component of this embodiment may further contain one or more elements selected from the group consisting of first to fourth groups in place of a portion of Fe. [Group 1] Cu: 0.30% or less, and Ni: 0.30% or less, one or more selected from the group consisting of [Group 2] V: 0.20% or less, Nb: 0.100% or less, Ti: 0.100% or less, and W: 0.20% or less, one or more selected from the group consisting of [Group 3] Ca: 0.0050% or less, and Mg: 0.010% or less, one or more selected from the group consisting of [Group 4] Te: 0.050% or less, Bi: 0.100% or less, Pb: 0.09% or less, Sn: 0.050% or less, and Sb: 0.050% or less, one or more selected from the group consisting of Each of the elements in the first to fourth groups will be explained below.

[0053] [Group 1: Cu and Ni] The chemical composition of the machine structural component of this embodiment may further contain elements of Group 1 in place of a portion of Fe. These elements are optional elements, and all of them improve the hardenability of the machine structural component. Each element of Group 1 will be described below.

[0054] Cu: 0.30% or less Copper (Cu) is an optional element and may not be contained, that is, the Cu content may be 0%. When contained, that is, when the Cu content is more than 0%, Cu improves the hardenability and increases the strength of machine structural parts. Even if even a small amount of Cu is contained, the above effects can be obtained to some extent. However, if the Cu content exceeds 0.30%, the hardness of the steel material used for machine structural parts becomes excessively high, and therefore the machinability of the steel material decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the Cu content is 0 to 0.30%, and when Cu is contained, the Cu content is 0.30% or less. The lower limit of the Cu content is preferably 0.01%, more preferably 0.02%, and even more preferably 0.05%. The upper limit of the Cu content is preferably 0.20%, more preferably 0.10%, and even more preferably 0.05%.

[0055] Ni: 0.30% or less Nickel (Ni) is an optional element and may not be contained, that is, the Ni content may be 0%. When contained, that is, when the Ni content is more than 0%, Ni improves hardenability and increases the strength of machine structural parts. Even if even a small amount of Ni is contained, the above effects can be obtained to some extent. However, if the Ni content exceeds 0.30%, the hardness of the steel material used for machine structural parts becomes excessively high, and therefore the machinability of the steel material decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the Ni content is 0 to 0.30%, and when Ni is contained, the Ni content is 0.30% or less. The lower limit of the Ni content is preferably 0.01%, more preferably 0.02%, and even more preferably 0.05%. The upper limit of the Ni content is preferably 0.20%, more preferably 0.12%, even more preferably 0.10%, and still more preferably 0.05%.

[0056] [Group 2: V, Nb, Ti, and W] The chemical composition of the machine structural component of this embodiment may further contain elements of Group 2 in place of a portion of Fe. These elements are optional elements, and all of them form precipitates to increase the strength of the machine structural component. Each element of Group 2 will be described below.

[0057] V:0.20% or less Vanadium (V) is an optional element and may not be contained, that is, the V content may be 0%. When V is contained, that is, when the V content exceeds 0%, V forms V precipitates such as V carbides and V carbonitrides. The V precipitates have a pinning effect, which suppresses grain coarsening during quenching and tempering treatments. This increases the strength of machine structural parts. Even if even a small amount of V is contained, the above effects can be obtained to some extent. However, if the V content exceeds 0.20%, the hardness of the steel material used for machine structural parts becomes excessively high, and therefore the machinability of the steel material decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the V content is 0 to 0.20%, and when V is contained, the V content is 0.20% or less. The lower limit of the V content is preferably 0.01%, more preferably 0.03%, even more preferably 0.05%, and still more preferably 0.07%. The upper limit of the V content is preferably 0.17%, more preferably 0.15%, even more preferably 0.12%, and still more preferably 0.08%.

[0058] Nb: 0.100% or less Niobium (Nb) is an optional element and may not be contained, that is, the Nb content may be 0%. When Nb is contained, that is, when the Nb content exceeds 0%, Nb forms Nb precipitates such as Nb carbides and Nb carbonitrides. The Nb precipitates have a pinning effect, which suppresses the coarsening of crystal grains during quenching and tempering. This increases the strength of machine structural parts. Even if even a small amount of Nb is contained, the above effects can be obtained to some extent. However, if the Nb content exceeds 0.100%, the Nb precipitates become coarse. In this case, the coarsening of crystal grains during quenching and tempering cannot be sufficiently suppressed. Therefore, even if the contents of other elements are within the ranges of this embodiment, the toughness of the machine structural part decreases. Therefore, the Nb content is 0 to 0.100%, and when Nb is contained, the Nb content is 0.100% or less. The lower limit of the Nb content is preferably 0.001%, more preferably 0.010%, even more preferably 0.030%, and still more preferably 0.050%. The upper limit of the Nb content is preferably 0.090%, more preferably 0.075%, and even more preferably 0.060%.

[0059] Ti:0.100% or less Titanium (Ti) is an optional element and may not be contained, that is, the Ti content may be 0%. When Ti is contained, that is, when the Ti content exceeds 0%, Ti forms Ti precipitates such as Ti carbides and Ti carbonitrides. The Ti precipitates have a pinning effect, which suppresses the coarsening of crystal grains during quenching and tempering. This increases the strength of machine structural parts. Even if Ti is contained even a small amount, the above effects can be obtained to some extent. However, if the Ti content exceeds 0.100%, the Ti precipitates become coarse. In this case, the coarsening of crystal grains during quenching and tempering cannot be sufficiently suppressed. Therefore, even if the contents of other elements are within the ranges of this embodiment, the toughness of the machine structural part decreases. Therefore, the Ti content is 0 to 0.100%, and when Ti is contained, the Ti content is 0.100% or less. The lower limit of the Ti content is preferably 0.001%, more preferably 0.010%, even more preferably 0.030%, and still more preferably 0.050%. The upper limit of the Ti content is preferably 0.090%, more preferably 0.075%, and even more preferably 0.060%.

[0060] W: 0.20% or less Tungsten (W) is an optional element and may not be contained, that is, the W content may be 0%. When W is contained, that is, when the W content exceeds 0%, W forms W precipitates such as W carbides and W carbonitrides. The W precipitates have a pinning effect, which suppresses the coarsening of crystal grains during quenching and tempering. This increases the strength of machine structural parts. Even if even a small amount of W is contained, the above effects can be obtained to some extent. However, if the W content exceeds 0.20%, the W precipitates become coarse. In this case, the coarsening of grains during quenching and tempering cannot be sufficiently suppressed. Therefore, even if the contents of other elements are within the ranges of this embodiment, the toughness of the machine structural part decreases. Therefore, the W content is 0 to 0.20%, and when W is contained, the W content is 0.20% or less. The lower limit of the W content is preferably 0.01%, more preferably 0.05%, and even more preferably 0.10%. The upper limit of the W content is preferably 0.18%, more preferably 0.15%, and even more preferably 0.12%.

[0061] [Group 3: Ca and Mg] The chemical composition of the machine structural component of this embodiment may further contain elements of Group 3 in place of a portion of Fe. These elements are optional elements, and all of them refine and spheroidize sulfides to improve the toughness of the machine structural component. Each element of Group 3 will be described below.

[0062] Ca: 0.0050% or less Calcium (Ca) is an optional element and may not be contained, that is, the Ca content may be 0%. When contained, that is, when the Ca content is more than 0%, Ca refines sulfides. Furthermore, Ca promotes the spheroidization of sulfides. This increases the toughness of machine structural parts. Even if even a small amount of Ca is contained, the above effects can be obtained to some extent. However, if the Ca content exceeds 0.0050%, the formation of coarse Ca oxides is promoted, and in this case, the toughness of the machine structural part decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the Ca content is 0 to 0.0050%, and when Ca is contained, the Ca content is 0.0050% or less. The lower limit of the Ca content is preferably 0.0001%, more preferably 0.0005%, even more preferably 0.0010%, and still more preferably 0.0015%. The upper limit of the Ca content is preferably 0.0045%, more preferably 0.0040%, and even more preferably 0.0030%.

[0063] Mg: 0.010% or less Magnesium (Mg) is an optional element and may not be contained, that is, the Mg content may be 0%. When Mg is contained, that is, when the Mg content is more than 0%, Mg refines sulfides. Furthermore, Mg promotes the spheroidization of sulfides. This increases the toughness of machine structural parts. Even if even a small amount of Mg is contained, the above effects can be obtained to some extent. However, if the Mg content exceeds 0.010%, the formation of coarse Mg oxides is promoted, and in this case, the toughness of the machine structural part decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the Mg content is 0 to 0.010%, and when Mg is contained, the Mg content is 0.010% or less. The lower limit of the Mg content is preferably 0.001%, more preferably 0.003%, and even more preferably 0.005%. The upper limit of the Mg content is preferably 0.008%, more preferably 0.006%, and even more preferably 0.005%.

[0064] [Group 4: Te, Bi, Pb, Sn, and Sb] The chemical composition of the machine structural component of this embodiment may further contain elements of Group 4 in place of a portion of Fe. These elements are optional elements, and all of them improve the machinability of the steel material that is the raw material for the machine structural component. Each element of Group 4 will be described below.

[0065] Te: 0.050% or less Tellurium (Te) is an optional element and may not be contained, that is, the Te content may be 0%. When contained, that is, when the Te content is more than 0%, Te improves the machinability of the steel material used to make machine structural parts. Even if even a small amount of Te is contained, the above effect can be obtained to some extent. However, if the Te content exceeds 0.050%, the hot workability of the steel material used to make machine structural parts will decrease even if the contents of other elements are within the ranges of this embodiment. Therefore, the Te content is 0 to 0.050%, and when Te is contained, the Te content is 0.050% or less. The lower limit of the Te content is preferably 0.001%, more preferably 0.003%, and even more preferably 0.010%. The upper limit of the Te content is preferably 0.045%, more preferably 0.040%, even more preferably 0.035%, and still more preferably 0.030%.

[0066] Bi:0.100% or less Bismuth (Bi) is an optional element and may not be contained, that is, the Bi content may be 0%. When contained, that is, when the Bi content is more than 0%, Bi improves the machinability of the steel material used to make machine structural parts. Even if even a small amount of Bi is contained, the above effect can be obtained to some extent. However, if the Bi content exceeds 0.100%, the hot workability of the steel material used to make machine structural parts will decrease even if the contents of other elements are within the ranges of this embodiment. Therefore, the Bi content is 0 to 0.100%, and when Bi is contained, the Bi content is 0.100% or less. The lower limit of the Bi content is preferably 0.001%, more preferably 0.002%, even more preferably 0.005%, and still more preferably 0.010%. The upper limit of the Bi content is preferably 0.090%, more preferably 0.080%, even more preferably 0.070%, even more preferably 0.060%, and still more preferably 0.050%.

[0067] Pb: 0.09% or less Lead (Pb) is an optional element and may not be contained, that is, the Pb content may be 0%. When Pb is contained, that is, when the Pb content is more than 0%, Pb improves the machinability of the steel material used to make machine structural parts. Even if even a small amount of Pb is contained, the above effect can be obtained to some extent. However, if the Pb content exceeds 0.09%, the hot workability of the steel material used to make machine structural parts will decrease even if the contents of other elements are within the ranges of this embodiment. Therefore, the Pb content is 0 to 0.09%, and if Pb is contained, the Pb content is 0.09% or less. The lower limit of the Pb content is preferably 0.01%, more preferably 0.02%, and even more preferably 0.03%. The upper limit of the Pb content is preferably 0.08%, more preferably 0.07%, even more preferably 0.06%, and still more preferably 0.05%.

[0068] Sn: 0.050% or less Tin (Sn) is an optional element and may not be contained, that is, the Sn content may be 0%. When contained, that is, when the Sn content is more than 0%, Sn improves the machinability of the steel material used to make machine structural parts. Even if even a small amount of Sn is contained, the above effect can be obtained to some extent. However, if the Sn content exceeds 0.050%, the hot workability of the steel material used to make machine structural parts will decrease even if the contents of other elements are within the ranges of this embodiment. Therefore, the Sn content is 0 to 0.050%, and when Sn is contained, the Sn content is 0.050% or less. The lower limit of the Sn content is preferably 0.001%, more preferably 0.010%, and even more preferably 0.020%. The upper limit of the Sn content is preferably 0.045%, more preferably 0.040%, even more preferably 0.035%, and still more preferably 0.030%.

[0069] Sb: 0.050% or less Antimony (Sb) is an optional element and may not be contained, that is, the Sb content may be 0%. When Sb is contained, that is, when the Sb content is more than 0%, Sb improves the machinability of the steel material that is used as the raw material for machine structural parts. Even if even a small amount of Sb is contained, the above effect can be obtained to some extent. However, if the Sb content exceeds 0.050%, the hot workability of the steel material used to make machine structural parts will decrease even if the contents of other elements are within the ranges of this embodiment. Therefore, the Sb content is 0 to 0.050%, and when Sb is contained, the Sb content is 0.050% or less. The lower limit of the Sb content is preferably 0.001%, more preferably 0.010%, and even more preferably 0.020%. The upper limit of the Sb content is preferably 0.045%, more preferably 0.040%, even more preferably 0.035%, and still more preferably 0.030%.

[0070] [(Feature 2) Regarding DA defined by formula (1A) and DB defined by formula (1B)] Furthermore, in the mechanical structural component of this embodiment, when the chemical composition consists of essential elements, DA defined by formula (1A) is 133 or more, and when the chemical composition contains essential elements and optional elements, DB defined by formula (1B) is 133 or more. DA=8.59×√C×(1+0.64×Si)×(1+4.1×Mn)×(1+2.33×Cr)×(1+3.14×Mo) (1A) DB=8.59×√C×(1+0.64×Si)×(1+4.1×Mn)×(1+2.33×Cr)×(1+3.14×Mo)×(1+0.35×Cu)×(1+0.36×Ni) (1B) Here, the content of the corresponding element in mass % is substituted for each element symbol in formula (1A) and formula (1B), and when an element is not contained, "0" is substituted for the corresponding element symbol.

[0071] DA defined by formula (1A) and DB defined by formula (1B) are indicators of the hardenability of a machine structural part. Even if a machine structural part satisfies Feature 1, if DA or DB is less than 133, martensitic transformation does not proceed sufficiently during hardening. In this case, a sufficient balance of strength and toughness cannot be obtained.

[0072] If DA or DB is 133 or more, the hardenability of machine structural parts is sufficiently ensured, and therefore a sufficient balance of strength and toughness is obtained.

[0073] The lower limits of DA and DB are preferably 137, more preferably 138, even more preferably 140, even more preferably 150, even more preferably 161, even more preferably 170, and even more preferably 180. The upper limits of DA and DB are not particularly limited. When the machine structural part satisfies Features 1 and 3, the upper limits of DA and DB are, for example, 185. DA and DB shall be integer values ​​obtained by rounding off the obtained numerical values ​​to the nearest tenth.

[0074] [(Feature 3) Regarding Formula (2A) and Formula (2B)] Furthermore, in the machine structural component of this embodiment, when the chemical composition is made of essential elements, formula (2A) is satisfied, and when the chemical composition contains essential elements and optional elements, formula (2B) is satisfied. 40.0×C+10.0×Si+30.0×Mn-10.0×Cr-0.5×Mo+200.0×N-0.8×DA≧-62.0 (2A) 40.0×C+10.0×Si+30.0×Mn-10.0×Cr-0.5×Mo+200.0×N-0.8×DB≧-62.0 (2B) Here, the content of the corresponding element in mass % is substituted for each element symbol in formula (2A) and formula (2B).

[0075] FA and FB are defined as follows: FA=40.0×C+10.0×Si+30.0×Mn-10.0×Cr-0.5×Mo+200.0×N-0.8×DA FB=40.0×C+10.0×Si+30.0×Mn-10.0×Cr-0.5×Mo+200.0×N-0.8×DB FA and FB are indicators of the degree of suppression of heat treatment strain caused by quenching and tempering. As mentioned above, if the transformation plasticity coefficient decreases, heat treatment strain is suppressed. Of the elements mentioned above in the chemical composition, C, Si, Mn, Cr, Mo, and N affect the transformation plasticity coefficient. Specifically, C, Si, Mn, and N have the effect of lowering the transformation plasticity coefficient. On the other hand, Cr and Mo have the effect of increasing the transformation plasticity coefficient. FA and FB have different coefficients depending on the degree of influence each element has on the transformation plasticity coefficient. In addition, heat treatment strain is also suppressed by a decrease in the hardenability of machine structural parts. Therefore, if DA and DB, which are indicators of hardenability, decrease, FA and FB increase.

[0076] If FA or FB is −62.0 or more, the transformation plastic strain and transformation expansion strain, which are the main causes of heat treatment strain, can be sufficiently reduced, and therefore the heat treatment strain during quenching and tempering can be suppressed.

[0077] The preferred lower limits of FA and FB are -61.0, more preferably -60.5, even more preferably -60.0, even more preferably -59.0, even more preferably -50.0, even more preferably -40.0, even more preferably -30.0, and even more preferably -20.0. The upper limits of FA and FB are not particularly limited. When the machine structural part satisfies Features 1 and 2, the upper limits of FA and FB are, for example, −15.7. FA and FB shall be values ​​up to one decimal place obtained by rounding off the obtained number to one decimal place.

[0078] [Effects of the machine structural component of this embodiment] The machine structural component of this embodiment satisfies Features 1 to 3. Therefore, the machine structural component of this embodiment has a sufficient balance of strength and toughness, and heat treatment distortion is suppressed. Therefore, it is suitable for gears, shafts, bearing parts, etc. used in automobiles, construction vehicles, mining machines, etc.

[0079] [Mechanical manufacturing method for machine structural parts] An example of a manufacturing method for a machine structural component of this embodiment will be described. The manufacturing method for a machine structural component described below is one example for manufacturing the machine structural component of this embodiment. Therefore, a machine structural component having the above-described configuration may be manufactured by a manufacturing method other than the manufacturing method described below. However, the manufacturing method described below is a preferred example of a manufacturing method for a machine structural component of this embodiment.

[0080] An example of a method for manufacturing a machine structural component according to this embodiment includes the following steps. (Process 1) Steel material preparation process (Process 2) Hot processing process (Process 3) Heat treatment process Each step will be described below.

[0081] [(Process 1) Steel material preparation process] In the material preparation step, a steel material that will be used as the material for the machine structural component of this embodiment is prepared. Specifically, molten steel having a chemical composition that satisfies Features 1 to 3 is produced. The refining method is not particularly limited, and any known method may be used. For example, molten pig iron produced by a known method is subjected to refining (primary refining) in a converter. The molten steel tapped from the converter is subjected to known secondary refining. In the secondary refining, alloy elements are added to the molten steel to adjust the composition, and molten steel having a chemical composition that satisfies Features 1 to 3 is produced.

[0082] The molten steel produced by the above-described refining method is used to produce a material by a known casting method. For example, an ingot may be produced by an ingot casting method using the molten steel. Alternatively, a bloom may be produced by a continuous casting method using the molten steel. The produced material (ingot or bloom) is subjected to known hot working to produce a steel material of a predetermined shape. For example, hot forging may be performed as the hot working. Alternatively, finish rolling using a continuous rolling mill may be performed after blooming or hot forging. The continuous rolling mill has a plurality of rolling stands arranged in a row. The heating temperature during hot working is not particularly limited, but is, for example, 1000 to 1300°C. The shape of the steel material produced in this way is, for example, a steel bar.

[0083] [(Process 2) Hot processing process] In the hot working step, the produced steel material is subjected to hot working to produce an intermediate product having a predetermined shape. The hot working is, for example, hot forging. The heating temperature in the hot working step is, for example, 1000 to 1300°C. The intermediate product after hot working is cooled to room temperature. If necessary, cutting may be performed on the intermediate product after hot working. In other words, cutting is an optional step.

[0084] [(Process 3) Heat treatment process] In the heat treatment process, the intermediate product is subjected to quenching and tempering. The quenching method is, for example, deep quenching. Specifically, the intermediate product is carried into a heat treatment furnace and c3The intermediate product is heated and held at a temperature equal to or higher than the quench point. The holding time is 15 to 180 minutes. The intermediate product is removed from the heat treatment furnace and quenched. The quenching method is not particularly limited, but examples include water quenching and oil quenching. In tempering, for example, the quenched intermediate product is held at 150 to 200°C for a predetermined time. By carrying out the quenching and tempering treatment under these conditions, a quench-hardened layer is formed at least on the surface layer. In addition, the prior austenite grain size number in the surface layer becomes less than 11.0.

[0085] The machine structural component of this embodiment is manufactured through the above steps. Note that the steel material may be subjected to normalizing treatment or spheroidizing annealing treatment as needed. [Example]

[0086] Steel materials with each test number were manufactured as materials for machine structural parts having the chemical compositions shown in Tables 1A and 1B. The steel material with test number REF was a reference steel material and had a chemical composition equivalent to SCM440 specified in JIS G 4053 (2016).

[0087] [Table 1A]

[0088] [Table 1B]

[0089] Molten steel was used to produce blooms by continuous casting. The produced blooms were heated and subjected to blooming and subsequent continuous rolling to produce 160 mm x 160 mm billets. The heating temperature of the slab during blooming was 1000°C to 1300°C. The produced billets were then heated again and hot rolled to produce steel products (steel bars) with a diameter of 50 mm. The heating temperature of the billets at this time was 1000°C to 1300°C. The produced steel products were air-cooled to room temperature. Steel products with a diameter of 50 mm and each test number were produced using the above production process.

[0090] [About the evaluation test] The manufactured steel materials were subjected to the following evaluation tests. (Test 1) Prior austenite grain size number measurement test (Test 2) Strength-toughness balance evaluation test (Test 3) Heat treatment distortion evaluation test Tests 1 to 3 will be explained below.

[0091] [(Test 1) Prior austenite grain size number measurement test] Simulated machine structural components were manufactured from the steel material of each test number. Specifically, the steel material of each test number was heated at a heating temperature of 1200°C for a holding time of 30 minutes. Then, hot working (hot forging) was performed at a finishing temperature of 950°C or higher to manufacture steel bars (intermediate products) with a diameter of 35 mm. The 35 mm diameter steel bars were subjected to quenching and tempering treatment to manufacture simulated machine structural components of each test number.

[0092] The quenching and tempering process was carried out as follows. The intermediate products of each test number were subjected to deep quenching. The quenching temperature was 870°C, and the holding time at the quenching temperature was 30 minutes. After deep quenching, the products were cooled in oil at 130°C. After oil cooling, the intermediate products were tempered at a tempering temperature of 180°C for a holding time of 120 minutes.

[0093] Using the above manufacturing process, cylindrical machine structural component simulants with a diameter of 35 mm and each test number were manufactured. The Vickers hardness at a depth of 100 μm from the surface of each machine structural component simulant with each test number was determined using the method described above in [Vickers Hardness Measurement Method]. As a result, the Vickers hardness at a depth of 100 μm from the surface of each machine structural component simulant with each test number was 500 HV or higher. Therefore, each machine structural component simulant with each test number had a quench-hardened layer at least on the surface.

[0094] The prior austenite grain size number in the surface layer of the machine structural component simulant of each test number was determined based on the method described in "Method for measuring prior austenite grain size number in surface layer." The results are shown in the "Prior austenite grain size number" column in Table 2. As shown in Table 2, the prior austenite grain size number in the surface layer of the machine structural component simulant of each test number was less than 11.0.

[0095] [Table 2]

[0096] [(Test 2) Strength-toughness balance evaluation test] Simulated machine structural components of each test number were manufactured from the steel material of each test number under the same conditions as in Test 1. The strength-toughness balance of the simulated machine structural components of each test number was evaluated by conducting the following test. (Test 2A) Tensile test (Test 2B) Charpy impact test Test 2A and Test 2B will be described below.

[0097] [(Test 2A) Tensile test] From the machine structural component simulants of each test number, No. 4 test pieces specified in JIS Z 2241 (2011) were prepared as tensile test pieces. The central axis of the tensile test piece corresponded to the central axis of the machine structural component simulants. Using the prepared tensile test pieces, tensile tests in accordance with JIS Z 2241 (2011) were conducted in air at room temperature (25°C), and the tensile strength (MPa) of the machine structural component simulants of each test number was obtained.

[0098] [(Test 2B) Charpy impact test] V-notch test specimens were prepared from the R / 2 position of the machine structural component simulant for each test number. The R / 2 position corresponds to the center position of the radius R connecting the surface and the central axis of the machine structural component simulant. The size of the V-notch test specimen was 10 mm × 10 mm × 55 mm. The longitudinal direction of the V-notch test specimen was parallel to the central axis of the machine structural component simulant. A V-notch was formed at the center of one of the surfaces parallel to the longitudinal direction of the V-notch test specimen, in a direction perpendicular to the longitudinal direction. The V-notch had a depth of 2 mm, a V-notch angle of 45°, and a V-notch tip radius of 0.25 mm. Using the prepared V-notch test specimens, Charpy impact tests were conducted in air at room temperature (25°C) in accordance with JIS Z 2242 (2018) to obtain the absorbed energy (J) of the machine structural component simulant for each test number.

[0099] Based on the tensile strength of the machine structural part simulant of each test number obtained in Test 2A and the absorbed energy of the machine structural part simulant of each test number obtained in Test 2B, the strength-toughness balance ratio based on the machine structural part simulant of Test No. REF was calculated using the following formula. Strength-toughness balance ratio = (tensile strength of the test number (MPa) × absorbed energy of the test number (J)) / (tensile strength of test number REF (MPa) × absorbed energy of test number REF (J)) If the strength-toughness balance ratio was 1.0 or more, it was determined that a sufficient strength-toughness balance was obtained (indicated by "〇" in the "Strength-toughness balance" column in Table 2). On the other hand, if the strength-toughness balance ratio was less than 1.0, it was determined that a sufficient strength-toughness balance was not obtained (indicated by "×" in the "Strength-toughness balance" column in Table 2).

[0100] [(Test 3) Heat treatment distortion evaluation test] The degree of suppression of heat treatment strain in the machine structural parts of each test number was determined using the following method. First, a heat treatment strain evaluation test piece simulating the machine structural part was prepared from the steel material (steel bar with a diameter of 50 mm) of each test number. The shape of the heat treatment strain evaluation test piece is shown in Figure 1. The numbers in Figure 1 represent dimensions (unit: mm). "φ" in Figure 1 refers to the diameter. In the following explanation, we will assume that the heat treatment strain evaluation test piece is placed in a three-dimensional Cartesian coordinate system (xyz coordinate system). The longitudinal direction of the heat treatment strain evaluation test piece is defined as the y direction, the height direction of the heat treatment strain evaluation test piece is defined as the z direction, and the direction perpendicular to the z and y directions is defined as the x direction.

[0101] The steel material for each test number was heated at a heating temperature of 1200°C for a holding time of 30 minutes. It was then hot-worked (hot-forged) at a finishing temperature of 950°C or higher to produce a 25 mm diameter steel bar. The 25 mm diameter steel bar was machined (cut) to produce an intermediate heat treatment strain evaluation specimen with a diameter of 10 mm and a length of 100 mm. A groove with a depth of 2.5 mm, a width of 4 mm, and a length of 100 mm was machined (cut) into the side of the intermediate to produce the heat treatment strain evaluation specimen shown in Figure 1.

[0102] The straightness of the test piece for evaluating heat treatment distortion before quenching and tempering was measured by the following method. Referring to FIG. 1, the outer circumferential shape of the test specimen was measured on a plane (a plane on the x-z plane) perpendicular to the longitudinal direction (y direction) at measurement positions A1 and A2 at both ends in the longitudinal direction and at measurement position A3, which was the center position in the longitudinal direction. Specifically, a CNC three-dimensional measuring machine (product name: Crystal-Apex) manufactured by Mitutoyo Corporation was used as the three-dimensional measuring machine. As shown in FIG. 2, the test specimen for evaluating heat treatment strain was positioned so that the grooves of the test specimen were positioned downward on a plane perpendicular to the longitudinal direction of the test specimen (hereinafter referred to as the measurement plane). With the CNC three-dimensional measuring machine, the coordinates (x, y, z) of measurement points P1 to P7 at 45° intervals around the outer periphery were measured at each of measurement positions A1 to A3, with the apex of the measurement plane set to 0°. Note that the 180° position, when the apex of the measurement plane is set to 0°, corresponds to the grooves, so the coordinates at the 180° position were not measured. That is, the coordinates were determined at seven measurement points P1 to P7 on the outer periphery of each measurement surface.

[0103] As shown in Figure 3, the coordinates of the seven measurement points P1 to P7 obtained on each measurement surface were used to approximate a circle C0 using the least squares method. From the obtained circle C0, the three-dimensional coordinates (x, y, z) of the center position CP of the circle C0 were calculated. In the following explanation, the center position of the circle C0 at measurement position A1 is defined as CP1. Similarly, the center position of the circle C0 at measurement position A2 is defined as CP2, and the center position of the circle C0 at measurement position A3 is defined as CP3.

[0104] As shown in Fig. 4, based on the coordinates of the obtained three center positions CP1 to CP3, the distance D in the x direction between the line segment SG connecting the center positions CP1 and CP2 and the center position CP3 was calculated, and the calculated distance D was defined as the amount of bending D. The amount of bending D is an index that indicates the degree of bending of the test piece.

[0105] After determining the amount of bending D of the heat treatment strain evaluation test piece before quenching and tempering, the heat treatment strain evaluation test piece was subjected to quenching and tempering under the same conditions as in Test 1. The amount of bending D of the heat treatment strain evaluation test piece after quenching and tempering was determined using the same method as for the heat treatment strain evaluation test piece before quenching and tempering. The difference value ΔD between the amount of bending D of the heat treatment strain evaluation test piece after quenching and tempering and the amount of bending D of the heat treatment strain evaluation test piece before quenching and tempering was determined. The obtained difference value ΔD is an index that indicates the amount of heat treatment strain introduced by quenching and tempering.

[0106] Using the difference value ΔD obtained for the reference test piece with test number REF and the difference value ΔD for each test number, the bending amount ratio of the heat treatment strain evaluation test piece with each test number was calculated according to the following formula. Bending ratio = difference value ΔD of the test number / difference value ΔD of test number REF If the obtained bending ratio was 0.8 or less, it was determined that the heat treatment distortion was sufficiently suppressed (indicated by "○" in the "Heat Treatment Distortion" column in Table 2). On the other hand, if the bending ratio was more than 0.8, it was determined that the heat treatment distortion was not sufficiently suppressed (indicated by "×" in the "Heat Treatment Distortion" column in Table 2).

[0107] [Test Results] The test results are shown in Table 2. Referring to Table 1 (Table 1A and Table 1B) and Table 2, the machine structural parts with test numbers 1 to 17 satisfied Features 1 to 3. Therefore, a sufficient balance of strength and toughness was obtained. Furthermore, heat treatment strain was sufficiently suppressed.

[0108] On the other hand, in test number 18, the DA defined by formula (1A) was too low, and therefore a sufficient balance of strength and toughness was not obtained.

[0109] In test number 19, DB defined by formula (1B) was too low, and therefore a sufficient balance of strength and toughness was not obtained.

[0110] In test number 20, the FA was too low, so the heat treatment distortion was not sufficiently suppressed.

[0111] In test number 21, FB was too low, so the heat treatment distortion was not sufficiently suppressed.

[0112] The embodiments of the present disclosure have been described above. However, the above-described embodiments are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiments, and can be implemented by appropriately modifying the above-described embodiments within the scope of the present disclosure.

Claims

1. A machine structural part, A hardened layer is formed on at least the surface layer. The prior austenite grain size number in the surface layer is less than 11.0, The chemical composition of the machine structural part is, in mass%, C: 0.31-0.65%, Si: 0.01-0.73%, Mn: 0.65-1.72%, P: 0.015% or less, S: 0.025% or less, Cr: 0.50% or less, Mo: 0.21% or less, Al: 0.005-0.100%, N: 0.0020 to 0.0300%, and O: 0.0015% or less, the balance being Fe and impurities; DA defined by formula (1A) is 133 or more, Satisfies formula (2A), Mechanical structural parts. DA=8.59×√C×(1+0.64×Si)×(1+4.1×Mn)×(1+2.33×Cr)×(1+3.14×Mo) (1A) 40.0×C+10.0×Si+30.0×Mn-10.0×Cr-0.5×Mo+200.0×N-0.8×DA≧-62.0 (2A) Here, the content of the corresponding element in mass % is substituted for each element symbol in formula (1A) and formula (2A).

2. A machine structural part, A hardened layer is formed on at least the surface layer. The prior austenite grain size number in the surface layer is less than 11.0, The chemical composition of the machine structural part is, in mass%, C: 0.31-0.65%, Si: 0.01-0.73%, Mn: 0.65-1.72%, P: 0.015% or less, S: 0.025% or less, Cr: 0.50% or less, Mo: 0.21% or less, Al: 0.005-0.100%, N: 0.0020 to 0.0300%, and O: 0.0015% or less, Further, it contains one or more selected from the group consisting of Groups 1 to 4, the balance being Fe and impurities; DB defined by formula (1B) is 133 or more, Satisfies formula (2B), Mechanical structural parts. [Group 1] Cu: 0.30% or less, and Ni: 0.30% or less, one or more selected from the group consisting of [Group 2] V: 0.20% or less, Nb: 0.100% or less, Ti: 0.100% or less, and W: 0.20% or less, one or more selected from the group consisting of [Group 3] Ca: 0.0050% or less, and Mg: 0.010% or less, one or more selected from the group consisting of [Group 4] Te: 0.050% or less, Bi: 0.100% or less, Pb: 0.09% or less, Sn: 0.050% or less, and Sb: 0.050% or less, one or more selected from the group consisting of DB=8.59×√C×(1+0.64×Si)×(1+4.1×Mn)×(1+2.33×Cr)×(1+3.14×Mo)×(1+0.35×Cu)×(1+0.36×Ni) (1B) 40.0×C+10.0×Si+30.0×Mn-10.0×Cr-0.5×Mo+200.0×N-0.8×DB≧-62.0 (2B) Here, the content of the corresponding element in mass % is substituted for each element symbol in formula (1B) and formula (2B), and when an element is not contained, "0" is substituted for the corresponding element symbol.

3. The machine structural component according to claim 2, the chemical composition contains the first group; Mechanical structural parts.

4. The machine structural component according to claim 2, the chemical composition contains the second group; Mechanical structural parts.

5. The machine structural component according to claim 2, The chemical composition contains the third group. Mechanical structural parts.

6. The machine structural component according to claim 2, The chemical composition contains the fourth group. Mechanical structural parts.

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