Steel-toothed car

Abstract

Provide a steel gear having excellent pitching strength. 【Solution means】The steel gear (1) of this embodiment includes a carburized hardened layer formed on the surface layer of the tooth surface of a plurality of teeth (GT), and in mass %, C: 0.15 to 0.30%, Si: 0.70 to 2.00%, Mn: 0.45 to 1.00%, P: 0.035% or less, S: 0.035% or less, Cr: 0.05 to 2.00%, Mo: 0.05 to 0.60%, Al: 0.005 to 0.050%, and N: 0.0020 to 0.0200%, and the balance is composed of Fe and impurities, and the compressive residual stress σ in the tooth profile direction on the tooth surface P (MPa) and the compressive residual stress σ in the tooth rib direction on the tooth surface T (MPa) satisfy the formulas (1) and (2). σ P +σ T ≧2400 (1) |σ P -σ T |≦250 (2)

Description

Technical Field

[0001] The present disclosure relates to a steel gear, and more particularly to a steel gear including a carburized hardened layer.

Background Art

[0002] Steel gears are used as components of power units such as automobile engines and motors, for example. Many of these steel gears are made of steel materials.

[0003] In the steel gears used for the above applications, the tooth surfaces slide against each other at a high surface pressure when the gears mesh. Therefore, surface fatigue failure called pitting occurs by continuously operating for a long time. Therefore, high pitting strength is required for steel gears.

[0004] As a general means for increasing the pitting strength of a steel gear, carburizing treatment may be performed on the steel gear. In this specification, carburizing treatment includes carbonitriding treatment. A carburized hardened layer is formed on the surface layer of the tooth surface of the carburized steel gear. The carburized hardened layer increases the hardness of the surface layer of the tooth surface. As a result, the pitting strength of the steel gear increases.

[0005] In recent years, with the increase in output and the reduction in size and weight of automobile parts and the like, further pitting strength has been required for steel gears. A technique for increasing the pitting strength of a carburized steel gear has been proposed, for example, in Japanese Patent Laid-Open No. 10-259470 (Patent Document 1).

[0006] The steel gear disclosed in Patent Document 1 contains C: 0.10~0.30%, Si: 0.50~1.50%, Mn: 0.30~1.00%, Cr: 0.50~2.00%, and Mo: 0.50% or less, and satisfies 1.5 ≤ 3 × Si(%) - Mn(%) + Cr(%) / 4 + Mo(%), with the remainder being Fe and unavoidable impurities, and has a carburized layer with a C concentration of 0.65% or more and a retained austenite content of 25% or less. Furthermore, this steel gear has a carburized abnormal layer consisting of an incomplete quenched structure on the outer layer of the carburized layer, with a maximum depth of 5~40 μm, and the area occupied by the carburized abnormal layer in the cross-section from the maximum depth to the surface is 70% or more. Patent Document 1 states that this improves the pitting resistance (pitting strength) of the steel gear. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Application Publication No. 10-259470 [Overview of the project] [Problems that the invention aims to solve]

[0008] However, the pitting strength of the steel gear may be increased by means other than those specified in Patent Document 1.

[0009] The purpose of this disclosure is to provide a steel gear having excellent pitting strength. [Means for solving the problem]

[0010] The steel gears described herein are A steel gear having multiple teeth, A carburized hardened layer formed on the surface of the tooth surface, The system comprises a core portion which is the part other than the aforementioned carburized hardened layer, The chemical composition of the core is, in mass%, C: 0.15~0.30%, Si: 0.70~2.00%, Mn: 0.45~1.00%, P: 0.035% or less, S: 0.035% or less, Cr: 0.05~2.00%, Mo: 0.05~0.60%, Al: 0.005~0.050%, and, It contains N: 0.0020 to 0.0200%, The remainder consists of Fe and impurities. Compressive residual stress σ in the tooth profile direction on the tooth surface P (MPa) and the compressive residual stress σ in the tooth trace direction on the tooth surface T (MPa) satisfies equations (1) and (2). σ P +σ T ≥2400 (1) |σ P -σ T |≦250 (2)

[0011] The steel gears described herein are A steel gear having multiple teeth, A carburized hardened layer formed on the surface of the tooth surface, The system comprises a core portion which is the part other than the aforementioned carburized hardened layer, The chemical composition of the core is, in mass%, C: 0.15~0.30%, Si: 0.70~2.00%, Mn: 0.45~1.00%, P: 0.035% or less, S: 0.035% or less, Cr: 0.05~2.00%, Mo: 0.05~0.60%, Al: 0.005~0.050%, and, It contains N: 0.0020 to 0.0200%, Furthermore, it contains one or more selected from the group consisting of Group 1 and Group 2, The remainder consists of Fe and impurities. Compressive residual stress σ in the tooth profile direction on the tooth surface P(MPa) and the compressive residual stress σ in the tooth flank direction on the tooth surface T (MPa) satisfy equations (1) and (2). [Group 1] Cu: 0.40% or less, Ni: 0.30% or less, V: 0.50% or less, Ti: 0.050% or less, Nb: 0.050% or less, and B: 0.0040% or less, one or more selected from the group consisting of [Group 2] Bi: 0.020% or less, Sn: 0.100% or less, As: 0.050% or less, Sb: 0.050% or less, Pb: 0.060% or less, Ca: 0.0020% or less, and Mg: 0.0050% or less, one or more selected from the group consisting of σ P +σ T ≧2400 (1) |σ P -σ T |≦250 (2)

Advantages of the Invention

[0012] In the steel gear according to the present disclosure, excellent pitching strength can be obtained.

Brief Description of the Drawings

[0013] [Figure 1] Figure 1 is a perspective view of the steel gear of the present embodiment. [Figure 2] Figure 2 is a perspective view of the teeth of the steel gear of Figure 1. [Figure 3] Figure 3 is a schematic diagram of shot peening treatment implemented in an example of the manufacturing method of the steel gear of the present embodiment. [Figure 4] Figure 4 is a side view of a small roller test piece for a pitching strength evaluation test. [Figure 5]Figure 5 is a schematic diagram of the roller pitting test performed in the pitting strength evaluation test. [Figure 6] Figure 6 is a front view of the large roller test specimen shown in Figure 5. [Modes for carrying out the invention]

[0014] The inventors investigated means to increase the pitting strength of steel gears. As a result, they obtained the following findings.

[0015] As described above, a useful method for increasing the pitting strength of steel gears is to perform a carburizing treatment on the steel gears to form a carburized hardened layer on the surface of the tooth surface. The carburized hardened layer has a higher solid solution carbon concentration compared to the core portion, which is the part of the gear other than the carburized hardened layer. Therefore, the carburized hardened layer is harder than the core portion. As a result, the pitting strength of the steel gear is increased.

[0016] On the other hand, when steel gears are used and their tooth surfaces slide against each other under high pressure, the tooth surfaces may generate heat. In this case, the carbon (C) that was dissolved in the carburized hardened layer formed on the surface of the tooth surface precipitates as carbides. As a result, the hardness of the carburized hardened layer decreases, which promotes pitting. In other words, by increasing the tempering softening resistance of the steel gear, the softening of the carburized hardened layer due to heat generated during sliding can be suppressed, and the occurrence of pitting can also be suppressed. Si is known as an element that increases tempering softening resistance. That is, by increasing the Si content in the chemical composition of a steel gear with a carburized hardened layer, the pitting strength of the steel gear can be improved.

[0017] Based on the above findings, the inventors investigated the chemical composition of the core of a steel gear having a carburized hardened layer on the surface of the tooth surface. As a result, the chemical composition of the core was found to be as follows (by mass%): C: 0.15~0.30%, Si: 0.70~2.00%, Mn: 0.45~1.00%, P: 0.035% or less, S: 0.035% or less, Cr: 0.05~2.00%, Mo: 0.05~0.60%, Al: 0.005~0.050%, N: 0.0020~0.0200%, Cu: 0~0.40%, Ni: 0~0.30%, V: 0~0.50% We hypothesized that a steel gear containing Ti: 0-0.050%, Nb: 0-0.050%, B: 0-0.0040%, Bi: 0-0.020%, Sn: 0-0.100%, As: 0-0.050%, Sb: 0-0.050%, Pb: 0-0.060%, Ca: 0-0.0020%, and Mg: 0-0.0050%, with the remainder being Fe and impurities, would provide excellent pitting strength.

[0018] However, even steel gears having a carburized hardened layer and a core satisfying the above-mentioned chemical composition sometimes failed to achieve excellent pitting strength. Therefore, the inventors focused on the compressive residual stress applied to the tooth surface in order to improve the pitting strength of steel gears.

[0019] The greater the compressive residual stress applied to the tooth surface, the higher the hardness of the tooth surface. Furthermore, the compressive residual stress applied to the tooth surface suppresses the propagation of cracks that cause pitting. As a result, the pitting strength of steel gears increases.

[0020] The direction of compressive residual stress on a tooth surface can be decomposed into the tooth profile direction and the tooth trace direction. Conventionally, among the compressive residual stresses on the tooth surface, particular emphasis has been placed on increasing the compressive residual stress in the tooth profile direction. This is because the component of shear stress acting on the tooth surface of a sliding steel gear is larger in the tooth profile direction than in the tooth trace direction. On the other hand, pitting is a delamination damage that occurs along the tooth surface and generally spreads not only in the tooth profile direction but also in the tooth trace direction. In other words, cracks that cause pitting propagate not only in the tooth profile direction but also in the tooth trace direction. Therefore, the inventors of this invention considered that by sufficiently increasing not only the compressive residual stress in the tooth profile direction but also the compressive residual stress in the tooth trace direction on the tooth surface, the propagation of cracks that cause pitting can be further suppressed.

[0021] Furthermore, during the use of steel gears, the compressive residual stress on the tooth surface is released by heat generated during sliding and gradually decreases. The inventors' research revealed that the smaller the difference between the compressive residual stress in the tooth profile direction and the compressive residual stress in the tooth trace direction, the more suppressed the release of compressive residual stress on the tooth surface of a steel gear during use. While the reason for this is unclear, it is thought to be because the movement of dislocations is inhibited compared to cases where the direction of compressive residual stress is biased in a particular direction.

[0022] Based on the above findings, the inventors further investigated the relationship between the compressive residual stress in the tooth profile direction and the compressive residual stress in the tooth trace direction on the tooth surface and the pitting strength. As a result, the compressive residual stress in the tooth profile direction on the tooth surface (MPa) was found to be σ P And the compressive residual stress (MPa) in the tooth trace direction on the tooth surface is σ T When defined as such, the inventors have found that excellent pitting strength can be obtained if the following equations (1) and (2) are satisfied. σ P +σ T ≥2400 (1) |σ P -σ T |≦250 (2)

[0023] The steel gear of this embodiment was completed based on the above technical concept and has the following configuration. Note that the above mechanism is a hypothesis. Therefore, it is possible that the steel gear of this embodiment may achieve superior pitting strength through a different mechanism. However, it has been proven in the later examples that a steel gear having the following configuration can achieve superior pitting strength.

[0024] The steel gear of the first configuration is A steel gear having multiple teeth, A carburized hardened layer formed on the surface of the tooth surface, The system comprises a core portion which is the part other than the aforementioned carburized hardened layer, The chemical composition of the core is, in mass%, C: 0.15~0.30%, Si: 0.70~2.00%, Mn: 0.45~1.00%, P: 0.035% or less, S: 0.035% or less, Cr: 0.05~2.00%, Mo: 0.05~0.60%, Al: 0.005~0.050%, and, It contains N: 0.0020 to 0.0200%, The remainder consists of Fe and impurities. Compressive residual stress σ in the tooth profile direction on the tooth surface P (MPa) and the compressive residual stress σ in the tooth trace direction on the tooth surface T (MPa) satisfies equations (1) and (2). σ P +σ T ≥2400 (1) |σ P -σ T |≦250 (2)

[0025] The steel gear of the second configuration is A steel gear having multiple teeth, A carburized hardened layer formed on the surface of the tooth surface, The system comprises a core portion which is the part other than the aforementioned carburized hardened layer, The chemical composition of the core is, in mass%, C: 0.15~0.30%, Si: 0.70~2.00%, Mn: 0.45~1.00%, P: 0.035% or less, S: 0.035% or less, Cr: 0.05~2.00%, Mo: 0.05~0.60%, Al: 0.005~0.050%, and, It contains N: 0.0020 to 0.0200%, Furthermore, it contains one or more selected from the group consisting of Group 1 and Group 2, The remainder consists of Fe and impurities. Compressive residual stress σ in the tooth profile direction on the tooth surface P (MPa) and the compressive residual stress σ in the tooth trace direction on the tooth surface T (MPa) satisfies equations (1) and (2). [Group 1] Cu: 0.40% or less, Ni: 0.30% or less, V: 0.50% or less, Ti: 0.050% or less, Nb: 0.050% or less, and, B: Select one or more from the group consisting of 0.0040% or less. [Group 2] Bi: 0.020% or less, Sn: 0.100% or less, As: 0.050% or less, Sb: 0.050% or less, Pb: 0.060% or less, Ca: 0.0020% or less, One or more selected from the group consisting of Mg: 0.0050% or less. σ P +σ T ≥2400 (1) |σ P -σ T |≦250 (2)

[0026] The third configuration of the steel gear is, A steel gear of the second configuration, It contains the aforementioned group 1.

[0027] The fourth configuration of the steel gear is, A steel gear having a second or third configuration, It contains the aforementioned second group.

[0028] The steel gear of this embodiment will be described in detail below.

[0029] [Structure of steel gears] Figure 1 is a perspective view of the steel gear of this embodiment. Referring to Figure 1, the steel gear 1 comprises a plurality of teeth GT. The plurality of teeth GT are arranged around a rotation axis C which corresponds to the central axis of the steel gear 1. The steel gear 1 is a spur gear.

[0030] Figure 2 is an enlarged perspective view of the teeth GT of the steel gear 1 of this embodiment. Referring to Figure 2, in the steel gear 1 of this embodiment, the direction P, which is parallel to the tooth surface 11 and perpendicular to the rotation axis C, is defined as the tooth profile direction. The direction T, which is parallel to the tooth surface 11 and parallel to the rotation axis C, is defined as the tooth trace direction.

[0031] [Carburized hardened layer] The steel gear of this embodiment further includes a carburized hardened layer and a core. In other words, the steel gear is subjected to a carburizing treatment. In this specification, the region from the tooth surface of the steel gear to a depth of 50 μm is defined as the surface layer. The carburized hardened layer is a layer that has hardened due to the penetration of carbon by the carburizing treatment and is formed in the surface layer. Note that the carburized hardened layer may be formed in parts other than the surface layer, as long as it is formed in at least the surface layer. Here, if the carbon concentration of the surface layer is 0.75 to 1.10% by mass, it is determined that the carburized hardened layer is formed in the surface layer. The core is the part of the steel gear other than the carburized hardened layer.

[0032] [Method for measuring the carbon concentration in the surface layer] The carbon concentration in the surface layer is determined using an electron probe microanalyzer (EPMA) in the following way. A test specimen is taken from a steel gear, with a cross-section parallel to the depth direction from the tooth surface serving as the observation surface. The observation surface includes a depth of 50 μm from the tooth surface. Linear analysis of the carbon concentration (mass%) is performed on the observation surface of the obtained test specimen using EPMA. Specifically, measurements are taken at 5 μm intervals in the depth direction from a depth of 5 μm to 50 μm from the tooth surface on the observation surface. The arithmetic mean of the carbon concentration (mass%) at a total of 10 measurement points is defined as the carbon concentration (mass%) of the surface layer. For EPMA, the acceleration voltage is set to 15.0 kV, the irradiation current to 100 nA, and the beam diameter to 1 μm.

[0033] [Microstructure of the surface] The surface microstructure consists mainly of martensite. Here, "mainly of martensite" means that the area ratio of martensite is 85% or more. Other microstructures in the surface besides martensite include, for example, pearlite and retained austenite.

[0034] [Method for measuring martensite area ratio] The area ratio of martensite on the surface of the steel gear in this embodiment is determined by the following method. First, the area ratio of pearlite is determined using a Field Emission Scanning Electron Microscope (FE-SEM). Specifically, a specimen is taken from a steel gear with a cross-section parallel to the depth direction from the tooth surface as the observation surface. The observation surface includes five 45 μm × 30 μm observation areas contained in the surface layer. After polishing the observation surface, it is etched at room temperature for 10 seconds using 3% nitric acid alcohol (Nital etching solution). On the etched observation surface, the five observation areas (45 μm × 30 μm) are observed with the FE-SEM and secondary electron images are taken. The observation magnification is set to 2000x. At 2000x observation, pearlite is observed as a phase with a lamellar structure. On the other hand, lamellar structure is not observed in martensite. Therefore, based on the morphology of the microstructure, pearlite in the observation field can be easily identified. The area ratio of perlite is defined as the area ratio calculated based on the total area of ​​perlite identified in all observation areas and the total area of ​​all observation areas.

[0035] Next, the area fraction of retained austenite is determined by X-ray diffraction. Specifically, the tooth surface is electropolished to expose a depth of 50 μm from the tooth surface. X-ray diffraction is performed at the exposed 50 μm depth as the measurement point, and the integrated intensity of the diffraction peaks between the (211) plane of the α-phase bcc structure and the (220) plane of the γ-phase fcc structure is measured. For the X-ray diffraction measurement, an X-ray diffractometer manufactured by Rigaku Corporation (product name: AutoMATE) is used, with a Cr tube as the light source, a tube voltage of 40 kV, a tube current of 40 mA, and a collimator diameter of 1.0 mm. The measurement range for the bcc structure is 146.20 to 166.70°, and the measurement range for the fcc structure is 118.20 to 138.70°, with a measurement interval of 0.10° and an oscillation width of 2° in both cases. Furthermore, the smoothing points are set to 13, the smoothing method to a weighted average, and the background points to 5 on the low-angle side and 5 on the high-angle side. LP correction is performed using the filter method, and the linear absorption coefficient is set to 850.4 / cm. Based on the measurement results, the retained austenite volume fraction Vγ (%) is calculated using the following formula. Vγ=100 / (0.36746×(((Iα / 90) / (Iγ×200))+1)) Here, Iα is the integrated intensity of the (211) plane of the α phase, and Iγ is the integrated intensity of the (220) plane of the γ phase.

[0036] The volume fraction Vγ (%) of retained austenite obtained by the above-described X-ray diffraction method is considered to be the area fraction (%) of retained austenite. Then, the area fraction of martensite in the surface layer is calculated using the following formula. The area ratio of martensite on the surface = 100 - (area ratio of perlite + area ratio of retained austenite)

[0037] [Features of the steel gear of this embodiment] The steel gear of this embodiment has the following features: (Feature 1) The chemical composition of the core is as follows (in mass%): C: 0.15-0.30%, Si: 0.70-2.00%, Mn: 0.45-1.00%, P: 0.035% or less, S: 0.035% or less, Cr: 0.05-2.00%, Mo: 0.05-0.60%, Al: 0.005-0.050%, N: 0.0020-0.0200%, Cu: 0-0.40%, Ni: 0-0.3% It contains 0%, V: 0-0.50%, Ti: 0-0.050%, Nb: 0-0.050%, B: 0-0.0040%, Bi: 0-0.020%, Sn: 0-0.100%, As: 0-0.050%, Sb: 0-0.050%, Pb: 0-0.060%, Ca: 0-0.0020%, and Mg: 0-0.0050%, with the remainder being Fe and impurities. (Feature 2) Compressive residual stress σ in the tooth profile direction on the tooth surface P (MPa) and compressive residual stress σ in the tooth trace direction on the tooth surface T (MPa) satisfies equations (1) and (2). σ P +σ T ≥2400 (1) |σ P -σ T |≦250 (2) Features 1 and 2 are described below.

[0038] [(Feature 1) Chemical Composition] The chemical composition of the core of the steel gear in this embodiment contains the following elements:

[0039] C: 0.15~0.30% Carbon (C) enhances the hardenability of steel gears. As a result, the pitting strength of steel gears increases. If the C content is less than 0.15%, the above effect is not sufficiently obtained. On the other hand, if the carbon content exceeds 0.30%, the steel used as the material for the gears becomes excessively hard. As a result, its machinability decreases. Therefore, the C content is 0.15-0.30%. The preferred lower limit for the C content is 0.17%, more preferably 0.19%, and even more preferably 0.21%. The preferred upper limit for the C content is 0.28%, more preferably 0.26%, and even more preferably 0.24%.

[0040] Si: 0.70~2.00% Silicon (Si) increases the tempering softening resistance of steel gears. Therefore, it can suppress the softening of the carburized hardened layer due to heat generated during tooth surface sliding. As a result, the pitting strength of the steel gear is increased. If the Si content is less than 0.70%, the above effect cannot be fully obtained. On the other hand, if the Si content exceeds 2.00%, the steel material used for the gears becomes excessively hard. As a result, its machinability decreases. Therefore, the Si content is 0.70-2.00%. The preferred lower limit for the Si content is 0.72%, more preferably 0.75%, and even more preferably 0.77%. The preferred upper limit for the Si content is 1.90%, more preferably 1.80%, and even more preferably 1.70%.

[0041] Mn: 0.45~1.00% Manganese (Mn) enhances the hardenability of steel gears. As a result, the pitting strength of steel gears increases. If the Mn content is less than 0.45%, the above effect is not sufficiently obtained. On the other hand, if the Mn content exceeds 1.00%, the steel used as the material for the gears becomes excessively hard. As a result, its machinability decreases. Therefore, the Mn content is 0.45-1.00%. The preferred lower limit for the Mn content is 0.47%, more preferably 0.49%, and even more preferably 0.51%. The preferred upper limit for the Mn content is 0.95%, more preferably 0.90%, and even more preferably 0.85%.

[0042] P:0.035% or less Phosphorus (P) is an impurity. If the P content exceeds 0.035%, P will excessively segregate at the grain boundaries, reducing the grain boundary strength. As a result, the pitting strength of the steel gear will decrease. Therefore, the P content is 0.035% or less. A low phosphorus (P) content is preferable. However, excessively reducing the P content increases manufacturing costs. Therefore, considering normal industrial production, the preferred lower limit for the P content is greater than 0%, more preferably 0.001%, even more preferably 0.002%, and even more preferably 0.003%. The preferred upper limit for the P content is 0.030%, more preferably 0.025%, and even more preferably 0.020%.

[0043] S: 0.035% or less Sulfur (S) is an impurity. S combines with Mn to form MnS. If the S content exceeds 0.035%, the MnS molecules become coarser. As a result, the pitting strength of the steel gear decreases. Therefore, the sulfur content is 0.035% or less. A low sulfur (S) content is preferable. However, excessively reducing the S content increases manufacturing costs. Therefore, considering normal industrial production, the preferred lower limit of the S content is greater than 0%, more preferably 0.001%, even more preferably 0.002%, and even more preferably 0.003%. The preferred upper limit for the S content is 0.030%, more preferably 0.025%, and even more preferably 0.020%.

[0044] Cr: 0.05~2.00% Chromium (Cr) enhances the hardenability of steel gears. As a result, the pitting strength of steel gears increases. If the Cr content is less than 0.05%, the above effect is not sufficiently obtained. On the other hand, if the chromium content exceeds 2.00%, the steel used as the material for the gears becomes excessively hard. As a result, its machinability decreases. Therefore, the Cr content is between 0.05% and 2.00%. The preferred lower limit for the Cr content is 0.10%, more preferably 0.15%, and even more preferably 0.20%. The preferred upper limit for the Cr content is 1.80%, more preferably 1.60%, and even more preferably 1.40%.

[0045] Mo: 0.05~0.60% Molybdenum (Mo) enhances the hardenability of steel gears. As a result, the pitting strength of steel gears increases. If the Mo content is less than 0.05%, the above effect cannot be fully obtained. On the other hand, if the Mo content exceeds 0.60%, the steel used as the material for the gears becomes excessively hard. As a result, its machinability decreases. Therefore, the Mo content is 0.05-0.60%. The preferred lower limit for the Mo content is 0.10%, more preferably 0.15%, and even more preferably 0.20%. The preferred upper limit for the Mo content is 0.55%, more preferably 0.50%, and even more preferably 0.45%.

[0046] Al: 0.005~0.050% Aluminum (Al) deoxidizes steel. If the Al content is less than 0.005%, the above effect will not be fully obtained. On the other hand, if the Al content exceeds 0.050%, clustered, coarse oxides are formed. As a result, the pitting strength of the steel gear decreases. Therefore, the Al content is 0.005-0.050%. The preferred lower limit for the Al content is 0.010%, more preferably 0.015%, and even more preferably 0.020%. The preferred upper limit for the Al content is 0.048%, more preferably 0.045%, and even more preferably 0.040%.

[0047] N: 0.0020~0.0200% Nitrogen (N) increases the hardness of the tooth surface through solid solution strengthening. As a result, the pitting strength of steel gears increases. If the N content is less than 0.0020%, the above effect cannot be fully obtained. On the other hand, if the N content exceeds 0.0200%, coarse nitrides are formed. As a result, the pitting strength of the steel gear decreases. Therefore, the N content is between 0.0020% and 0.0200%. The preferred lower limit for the N content is 0.0030%, more preferably 0.0040%, and even more preferably 0.0050%. The preferred upper limit for the N content is 0.0180%, more preferably 0.0170%, and even more preferably 0.0160%.

[0048] The remainder of the chemical composition of the core of the steel gear 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 steel, and are permissible within a range that does not adversely affect the steel gear in this embodiment.

[0049] [About Optional Elements] The chemical composition of the core of the steel gear in this embodiment may further include one or more elements selected from the groups consisting of Group 1 and Group 2, instead of a portion of Fe. [Group 1] Cu: 0.40% or less, Ni: 0.30% or less, V: 0.50% or less, Ti: 0.050% or less, Nb: 0.050% or less, and, B: Select one or more from the group consisting of 0.0040% or less. [Group 2] Bi: 0.020% or less, Sn: 0.100% or less, As: 0.050% or less, Sb: 0.050% or less, Pb: 0.060% or less, Ca: 0.0020% or less, One or more selected from the group consisting of Mg: 0.0050% or less.

[0050] [Group 1: Cu, Ni, V, Ti, Nb and B] The chemical composition of the core of the steel gear in this embodiment may further include the elements of the first group described above, instead of a portion of the Fe. These elements are arbitrary and all enhance the pitting strength of the steel gear. The elements of the first group will be described below.

[0051] Cu: 0.40% or less 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 improves the hardenability of steel gears. As a result, the pitting strength of steel gears increases. Even a small amount of Cu content will provide some degree of the above effect. On the other hand, if the copper content exceeds 0.40%, the steel used as the material for the gears becomes excessively hard. As a result, its machinability decreases. Therefore, the Cu content is 0-0.40%, and if present, the Cu content is 0.40% or less. The preferred lower limit for the Cu content is 0.01%, more preferably 0.03%, and even more preferably 0.05%. The preferred upper limit for the Cu content is 0.35%, more preferably 0.30%, and even more preferably 0.25%.

[0052] Ni: 0.30% or less 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 improves the hardenability of steel gears. As a result, the pitting strength of steel gears increases. Even a small amount of Ni is sufficient to achieve the above effect to some extent. On the other hand, if the nickel content exceeds 0.30%, the steel used as the material for the steel gears becomes excessively hard. As a result, its machinability decreases. Therefore, the Ni content is 0-0.30%, and if present, the Ni content is 0.30% or less. The preferred lower limit for the Ni content is 0.01%, more preferably 0.03%, and even more preferably 0.05%. The preferred upper limit for the Ni content is 0.25%, more preferably 0.20%, and even more preferably 0.15%.

[0053] V:0.50% or less 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 will form precipitates, and the pinning effect will suppress grain coarsening during carburizing. As a result, the pitting strength of the steel gear will increase. Even if only a small amount of V is present, the above effect can be obtained to some extent. On the other hand, if the V content exceeds 0.50%, the V precipitates become coarser. In this case, the coarsening of the crystal grains during the carburizing treatment cannot be sufficiently suppressed. As a result, the pitting strength of the steel gear decreases. Therefore, the V content is 0-0.50%, and if present, the V content is 0.50% or less. The preferred lower limit of the V content is 0.01%, more preferably 0.05%, and even more preferably 0.10%. The preferred upper limit for the V content is 0.40%, more preferably 0.30%, and even more preferably 0.20%.

[0054] Ti: 0.050% or less 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, i.e., if the Ti content is greater than 0%, Ti will form precipitates, and the pinning effect will suppress grain coarsening during carburizing. As a result, the pitting strength of the steel gear will increase. Even if only a small amount of Ti is present, the above effect can be obtained to some extent. On the other hand, if the Ti content exceeds 0.050%, the Ti precipitates become coarser. In this case, the coarsening of the crystal grains during the carburizing process cannot be sufficiently suppressed. As a result, the pitting strength of the steel gear decreases. Therefore, the Ti content is between 0 and 0.050%, and if present, the Ti content is 0.050% or less. The preferred lower limit for the Ti content is 0.001%, more preferably 0.003%, and even more preferably 0.005%. The preferred upper limit for the Ti content is 0.045%, more preferably 0.040%, and even more preferably 0.035%.

[0055] Nb: 0.050% or less Niobium (Nb) is an optional element and does not need to be included. In other words, the Nb content may be 0%. If Nb is present, i.e., if the Nb content is greater than 0%, Nb will form precipitates, and the pinning effect will suppress grain coarsening during carburizing. As a result, the pitting strength of the steel gear will increase. Even if only a small amount of Nb is present, the above effect can be obtained to some extent. On the other hand, if the Nb content exceeds 0.050%, the Nb precipitates become coarser. In this case, the coarsening of the crystal grains during the carburizing treatment cannot be sufficiently suppressed. As a result, the pitting strength of the steel gear decreases. Therefore, the Nb content is 0-0.050%, and if present, the Nb content is 0.050% or less. The preferred lower limit for the Nb content is 0.001%, more preferably 0.003%, and even more preferably 0.005%. The preferred upper limit for the Nb content is 0.045%, more preferably 0.040%, and even more preferably 0.035%.

[0056] B: 0.0040% or less Boron (B) is an optional element and does not need to be included. In other words, the B content may be 0%. If B is present, that is, if the B content is greater than 0%, B improves the hardenability of steel gears. As a result, the pitting strength of steel gears increases. Even if only a small amount of B is present, the above effect can be obtained to some extent. On the other hand, if the B content exceeds 0.0040%, the above effect will saturate. Therefore, the B content is between 0 and 0.0040%, and if present, the B content is 0.0040% or less. The preferred lower limit for the B content is 0.0001%, more preferably 0.0003%, and even more preferably 0.0005%. The preferred upper limit for the B content is 0.0035%, more preferably 0.0030%, and even more preferably 0.0025%.

[0057] [Group 2: Bi, Sn, As, Sb, Pb, Ca, and Mg] The chemical composition of the core of the steel gear in this embodiment may further include the elements of the second group described above, in place of some of the Fe. These elements are arbitrary and all improve the machinability of the steel material used to make the steel gear. The elements of the second group will be described below.

[0058] Bi:0.020% or less Bismuth (Bi) is an optional element and does not need to be included. In other words, the Bi content may be 0%. If present, i.e., if the Bi content is greater than 0%, it improves the machinability of the steel material used for steel gears. Even if only a small amount of Bi is present, the above effect can be obtained to some extent. On the other hand, if the Bi content exceeds 0.020%, the hot workability of the steel material used for steel gears decreases. Therefore, the Bi content is 0-0.020%, and if present, the Bi content is 0.020% or less. The preferred lower limit of the Bi content is 0.001%, more preferably 0.003%, and even more preferably 0.005%. The preferred upper limit for the Bi content is 0.018%, more preferably 0.016%, and even more preferably 0.014%.

[0059] Sn: 0.100% or less Tin (Sn) is an optional element and does not need to be included. In other words, the Sn content may be 0%. If present, i.e., if the Sn content is greater than 0%, it improves the machinability of the steel material used for steel gears. Even if only a small amount of Sn is present, the above effect can be obtained to some extent. On the other hand, if the Sn content exceeds 0.100%, the hot workability of the steel material used for steel gears decreases. Therefore, the Sn content is between 0 and 0.100%, and if present, the Sn content is 0.100% or less. The preferred lower limit for the Sn content is 0.001%, more preferably 0.003%, and even more preferably 0.005%. The preferred upper limit for the Sn content is 0.090%, more preferably 0.080%, and even more preferably 0.070%.

[0060] As: 0.050% or less Arsenic (As) is an optional element and does not need to be included. In other words, the As content may be 0%. When present, i.e., when the As content is greater than 0%, it improves the machinability of the steel material used for steel gears. Even a small amount of As can provide some of the above effect. On the other hand, if the As content exceeds 0.050%, the hot workability of the steel material used for steel gears decreases. Therefore, the As content is 0-0.050%, and if present, the As content is 0.050% or less. The preferred lower limit for the As content is 0.001%, more preferably 0.005%, and even more preferably 0.010%. The preferred upper limit for the As content is 0.045%, more preferably 0.040%, and even more preferably 0.035%.

[0061] Sb: 0.050% or less Antimony (Sb) is an optional element and does not need to be included. In other words, the Sb content may be 0%. If present, i.e., if the Sb content is greater than 0%, it improves the machinability of the steel material used for steel gears. Even if only a small amount of Sb is present, the above effect can be obtained to some extent. On the other hand, if the Sb content exceeds 0.050%, the hot workability of the steel material used for steel gears decreases. Therefore, the Sb content is between 0 and 0.050%, and if present, the Sb content is 0.050% or less. The preferred lower limit for the Sb content is 0.001%, more preferably 0.005%, and even more preferably 0.010%. The preferred upper limit for the Sb content is 0.045%, more preferably 0.040%, and even more preferably 0.035%.

[0062] Pb:0.060% or less Lead (Pb) is an optional element and does not need to be included. In other words, the Pb content may be 0%. If Pb is present, that is, if the Pb content is greater than 0%, it improves the machinability of the steel material used for steel gears. Even if only a small amount of Pb is present, the above effect can be obtained to some extent. On the other hand, if the Pb content exceeds 0.060%, the hot workability of the steel material used for steel gears decreases. Therefore, the Pb content is 0-0.060%, and if present, the Pb content is 0.060% or less. The preferred lower limit of the Pb content is 0.001%, more preferably 0.005%, and even more preferably 0.010%. The preferred upper limit for the Pb content is 0.055%, more preferably 0.050%, and even more preferably 0.045%.

[0063] Ca:0.0020% or less 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 controls the form of sulfides and improves the machinability of the steel used as the material for steel gears. Even a small amount of calcium can provide some degree of the above effect. On the other hand, if the Ca content exceeds 0.0020%, the above effect saturates. Therefore, the Ca content is between 0 and 0.0020%, and if present, the Ca content is 0.0020% or less. 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 0.0018%, more preferably 0.0015%, and even more preferably 0.0013%.

[0064] Mg: 0.0050% or less Magnesium (Mg) is an optional element and does not need to be included. In other words, the Mg content may be 0%. When present, i.e., when the Mg content is greater than 0%, Mg controls the form of sulfides and improves the machinability of the steel used as the material for steel gears. Even a small amount of Mg can provide some degree of the above effect. On the other hand, if the Mg content exceeds 0.0050%, the above effect saturates. Therefore, the Mg content is between 0 and 0.0050%, and if present, the Mg content is 0.0050% or less. 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%.

[0065] [(Feature 2) Compressive residual stress on the tooth surface] In the steel gear of this embodiment, the compressive residual stress σ in the tooth profile direction on the tooth surface is further... P (MPa) and compressive residual stress σ in the tooth trace direction on the tooth surface T (MPa) satisfies equations (1) and (2). σ P +σ T ≥2400 (1) |σ P -σ T |≦250 (2)

[0066] [Formula (1)] Fn1 is defined as follows: Fn1=σ P +σ T Fn1 is the left-hand side of equation (1), where σ is the compressive residual stress in the tooth profile direction on the tooth surface. P (MPa) and compressive residual stress σ in the tooth trace direction on the tooth surface T It is expressed as the sum of (MPa). In this specification, σ P σ represents the absolute value of the compressive residual stress in the tooth profile direction and is expressed as a positive value. T This represents the absolute value of the compressive residual stress in the tooth path direction and is expressed as a positive value.

[0067] As described above, pitting is a type of delamination damage that occurs along the tooth surface. Pitting is caused by the propagation of initial cracks that form on the surface due to fatigue, in the direction of the tooth profile and tooth trace. Therefore, from the viewpoint of suppressing the propagation of cracks that cause pitting, it is preferable to increase not only the compressive residual stress in a specific direction such as the direction of the tooth profile or the tooth trace, but also the compressive residual stress in both the direction of the tooth profile and the direction of the tooth trace.

[0068] Assuming that Fn1 is 2400 or greater, and that equation (2) above is satisfied, the compressive residual stress σ in the tooth profile direction on the tooth surface is... P (MPa) and compressive residual stress σ in the tooth trace direction on the tooth surface T Both (MPa) and are sufficiently large. In this case, both the propagation of cracks that cause pitting in the tooth profile direction and the propagation in the tooth trace direction can be suppressed. As a result, assuming that Feature 1 is satisfied, the steel gear of this embodiment can obtain excellent pitting strength.

[0069] A preferred lower limit for Fn1 is 2450, more preferably 2500, and even more preferably 2550. There is no particular upper limit to Fn1, but considering normal industrial production, it is, for example, 2800.

[0070] [Formula (2)] Fn2 is defined as follows: Fn2=|σ P -σ T | Fn2 is the left-hand side of equation (2), where σ is the compressive residual stress in the tooth profile direction on the tooth surface. P (MPa) and compressive residual stress σ in the tooth trace direction on the tooth surface T It is expressed as the absolute value of the difference from (MPa).

[0071] As described above, during the use of steel gears, the compressive residual stress on the tooth surface is released by heat generated during sliding and gradually decreases. The greater the difference between the compressive residual stress in the tooth profile direction and the compressive residual stress in the tooth trace direction, the easier it is for the compressive residual stress on the tooth surface to be released. If the compressive residual stress on the tooth surface is released and decreases during the use of steel gears, the hardness of the tooth surface decreases, and crack propagation cannot be adequately suppressed. As a result, the occurrence of pitting is promoted.

[0072] In other words, if the difference between the compressive residual stress in the tooth profile direction and the compressive residual stress in the tooth trace direction on the tooth surface is small, the release of compressive residual stress on the tooth surface of a steel gear in use can be suppressed. If Fn2 is 250 or less, the compressive residual stress σ in the tooth profile direction on the tooth surface P (MPa) and compressive residual stress σ in the tooth trace direction on the tooth surface T The difference from (MPa) is sufficiently small. In this case, the release of compressive residual stress during use of the steel gear can be suppressed. As a result, assuming that Feature 1 and the above-mentioned equation (1) are satisfied, the steel gear of this embodiment can obtain excellent pitting strength.

[0073] The lower limit of Fn2 is not particularly limited, but is preferably 0, and considering normal industrial production, it is, for example, 50. A preferred upper limit for Fn2 is 240, more preferably 220, and even more preferably 200.

[0074] [σ P and σ T [Measurement Method] Compressive residual stress σ in the tooth profile direction on the tooth surface P (MPa) and compressive residual stress σ in the tooth trace direction on the tooth surface T (MPa) is determined by the following method. Four test specimens are taken from the steel gear, each containing a tooth surface as the measurement surface. The test specimens are taken from four teeth closest to any four points on the pitch circle of the steel gear, arranged at 90° intervals around the gear's axis of rotation. In other words, when viewing the steel gear from the direction of the rotation axis, one test specimen is taken from each of the four teeth closest to the 3 o'clock, 6 o'clock, 9 o'clock, and 12 o'clock positions. The center of the measurement surface is on the pitch circle of the steel gear and corresponds to the center of the tooth width direction on the tooth surface. Residual stress is measured on the measurement surface using X-ray diffraction (tilting method) with a microfocus X-ray stress analyzer. Specifically, the residual stress in the tooth profile direction and the residual stress in the tooth trace direction are measured, with the center of the measurement surface as the measurement point. For example, the AutoMATE microfocus X-ray stress analyzer manufactured by Rigaku Corporation is used. A Cr tube is used as the light source. The tube voltage is 40kV, the tube current is 40mA, and the collimator diameter is 1.0mm. The analysis method for compressive residual stress in the tooth profile direction is the lateral tilt method, and the measurement mode is the constant ψ0 method. The measurement range is 146.50° to 166.70°, the measurement interval is 0.1°, and the oscillation amplitude is 2°. The ψ angles are 0.0°, 20.7°, 30.0°, 37.8°, and 45.0°. The analysis method for compressive residual stress in the tooth trace direction is the parallel tilt method, and the measurement mode is the constant ψ0 method. The measurement range is 146.50° to 166.70°, the measurement interval is 0.1°, and the oscillation amplitude is 2°. The ψ angles are 0.0°, 15.0°, 30.0°, and 40.0°. For compressive residual stress in all directions, the stress constant is -318.0MPa, and the peak search method is the midpoint of the half-width method. Furthermore, the smoothing points will be 13, the smoothing method will be a weighted average, and the background points will be 5 on the low-angle side and 5 on the high-angle side. LP correction will be performed using the filter method, and the linear absorption coefficient will be 850.4 / cm.

[0075] The arithmetic mean of the absolute values ​​of the compressive residual stress (MPa) in the tooth profile direction obtained from four test specimens is used to define the compressive residual stress σ in the tooth profile direction on the tooth surface. P (MPa) is defined as the compressive residual stress σ in the tooth trace direction. The arithmetic mean of the absolute values ​​of the compressive residual stress (MPa) in the tooth trace direction obtained from four test specimens is defined as the compressive residual stress σ in the tooth trace direction on the tooth surface. T(MPa) is defined as follows. P and σ T This value is an integer obtained by rounding the first decimal place of the obtained value.

[0076] [Effects of the steel gear of this embodiment] The steel gear of this embodiment satisfies features 1 and 2. Therefore, the steel gear of this embodiment provides excellent pitting strength.

[0077] [Shape of steel gears] Figure 1 shows a spur gear as an example of a steel gear of this embodiment. However, the steel gear of this embodiment is not limited to a spur gear. The steel gear of this embodiment may be, for example, a helical gear, a screw gear, or a spigot gear, in which the tooth traces intersect with respect to the axis of rotation C. The steel gear of this embodiment may also be a bevel gear in which the teeth are arranged on a conical surface. Examples of bevel gears include straight-hinged bevel gears, helical bevel gears, and hypoid gears. The steel gear may also be an internal-hinged gear.

[0078] [Manufacturing method for steel gears] An example of a manufacturing method for the steel gear of this embodiment will be described. The manufacturing method for the steel gear described below is just one example for manufacturing the steel gear of this embodiment. Therefore, the steel gear having the above-described configuration may be manufactured by a manufacturing method other than the one described below. However, the manufacturing method described below is a preferred example of a manufacturing method for the steel gear of this embodiment.

[0079] An example of a method for manufacturing steel gears according to this embodiment includes the following steps. (Process 1) Shape processing process (Process 2) Heat treatment process (Step 3) Shot Peening Process (Process 4) Polishing process The following describes each step.

[0080] [(Process 1) Shape processing process] In the shaping process, the steel material that will become the steel gear is processed to create a shape that closely resembles the final steel gear product.

[0081] The processing method for the steel material may be any well-known method. For example, the steel material may be hot-worked to form a predetermined shape. Examples of hot-working methods include hot forging and hot rolling. The steel material may also be cold-worked to form a predetermined shape. Examples of cold-working methods include cold forging. The steel material may also be machined to form a predetermined shape. The steel material may be hot-worked or cold-worked, and then further machined to form a predetermined shape.

[0082] [(Step 2) Heat Treatment Process] In the heat treatment process, the processed steel material is subjected to heat treatment to produce an intermediate product having a carburized hardened layer on its surface. Specifically, the processed steel material is subjected to vacuum carburizing and tempering. In this example of a manufacturing method, the vacuum carburizing process includes a vacuum carburizing step and a quenching step. In other words, the heat treatment process includes the following steps. (Step 21) Vacuum carburizing process (Process 22) Hardening process (Step 23) Tempering process The following describes each step.

[0083] [(Step 21) Vacuum carburizing process] The vacuum carburizing process further includes the following steps: (Step 211) Heating step (Step 212) Carburizing process (Step 213) Diffusion process

[0084] [(Step 211) Heating step] In the heating process, the steel material charged into the heat treatment furnace is heated to its carburizing temperature. The carburizing temperature in the heating process is, for example, 930 to 1100°C. In the heating process, the furnace is further evacuated or depressurized. For example, the pressure inside the furnace is reduced to 1 kPa or less.

[0085] [(Step 212) Carburizing process] The carburizing process is carried out after the heating process. In the carburizing process, a hydrocarbon gas is introduced into the furnace under vacuum or reduced pressure, and the steel material is held at the carburizing temperature for a predetermined time. The gas introduced in the carburizing process is not particularly limited as long as it is a hydrocarbon gas. Examples of hydrocarbon gases include acetylene and propane. The holding time at the carburizing temperature is not particularly limited. For example, the holding time at the carburizing temperature is 5 to 120 minutes.

[0086] [(Step 213) Diffusion Step] The diffusion process is carried out after the carburizing process. In the diffusion process, the material is held at the carburizing temperature for a predetermined time without introducing hydrocarbon gases into the furnace. The pressure inside the furnace during the diffusion process may be the same as that during the carburizing process. Alternatively, the pressure inside the furnace during the diffusion process may be reduced to a lower level than that during the carburizing process (e.g., 1 kPa or less) to remove residual gases from the carburizing process. The holding time at the carburizing temperature during the diffusion process is not particularly limited, but is, for example, 5 to 150 minutes.

[0087] The carburizing and diffusion processes may be performed once each, or they may be performed multiple times alternately.

[0088] [(Process 22) Hardening Process] The quenching process is carried out after the vacuum carburizing process. In the quenching process, the steel material after the vacuum carburizing process is subjected to a well-known quenching method. Specifically, the steel material, which has been heated and held at the carburizing temperature in the diffusion process, is rapidly cooled. The rapid cooling method is, for example, oil cooling or water cooling. The temperature of the cooling medium, oil or water, is, for example, room temperature to 160°C. In addition, sub-zero treatment may be performed as needed.

[0089] [(Step 23) Tempering process] The tempering process is performed after the quenching process. In the tempering process, the steel material is tempered after the quenching process. Specifically, the quenched steel material is heated to a tempering temperature T (°C) and held for a predetermined time. The holding time is, for example, 60 to 150 minutes.

[0090] In this example of a manufacturing method, a shot peening process is performed to impart compressive residual stress to the tooth surface, satisfying feature 2. In the shot peening process, shot particles are impacted onto the tooth surface to impart compressive residual stress. In addition, during the shot peening process, retained austenite contained in the surface layer is transformed into martensite by processing induction. When retained austenite is transformed into martensite, the volume of the crystal grains expands. As a result, further compressive residual stress is imparted to the surface layer, including the tooth surface.

[0091] If a large compressive residual stress is already present on the tooth surface before the shot peening process, work-induced martensitic transformation accompanied by volume expansion is suppressed. As a result, the compressive residual stress applied to the tooth surface by the shot peening process is actually reduced. If the tempering temperature T in the tempering process is less than 200°C, the compressive residual stress applied to the tooth surface up to the tempering process is not sufficiently released during the tempering process. In this case, the compressive residual stress on the tooth surface before the shot peening process is too high. Therefore, work-induced martensitic transformation in the shot peening process is excessively suppressed. As a result, the compressive residual stress applied to the tooth surface in the shot peening process is too low, and the manufactured steel gear cannot satisfy equation (1) of characteristic 2.

[0092] On the other hand, if the tempering temperature T exceeds 250°C, the compressive residual stress applied to the tooth surface up to the tempering process is excessively released during the tempering process. In this case, the compressive residual stress on the tooth surface before the shot peening process is too low. Therefore, even after the shot peening process is performed, the magnitude of the compressive residual stress on the tooth surface is insufficient. As a result, the manufactured steel gear cannot satisfy equation (1) of characteristic 2.

[0093] If the tempering temperature T is 200-250°C, the manufactured steel gear can satisfy characteristic 2, provided that the shot peening process satisfies the conditions described below. Therefore, the tempering temperature T in the tempering process is set to 200-250°C.

[0094] [(Step 3) Shot Peening Process] In the shot peening process, the tooth surface of the intermediate product after the heat treatment process is subjected to shot peening. As described above, by performing the shot peening process, compressive residual stress can be imparted to the tooth surface.

[0095] In the shot peening process, shot particles are projected from two different projection directions D1 and D2. Figure 3 is a schematic diagram of the shot peening process viewed from the side. In the shot peening process, the intermediate product 1' is held by a holding part (not shown) and rotates around the rotation axis C' of the intermediate product 1'. Shot particles are projected from a projection nozzle (not shown) in projection direction D1 onto at least the tooth surface of the rotating intermediate product 1'. The projection direction D1 is perpendicular to the rotation axis C'. Figure 3 is a schematic diagram when the projection direction D1 is parallel to the plane of the paper. After projecting shot particles from projection direction D1, shot particles are projected from projection direction D2 onto at least the tooth surface of the rotating intermediate product 1'. The projection direction D2 is a direction that makes an angle θ(°) with the projection direction D1 in a plane that includes the projection direction D1 and is parallel to the rotation axis C'.

[0096] By projecting shot particles from two different projection directions D1 and D2, the difference between the compressive residual stress in the tooth profile direction and the tooth trace direction applied to the tooth surface by shot peening can be reduced compared to projecting from a single direction. However, if the angle θ between projection direction D1 and projection direction D2 is less than 35°, the difference between the compressive residual stress in the tooth profile direction and the tooth trace direction applied to the tooth surface by shot peening cannot be sufficiently reduced. As a result, the manufactured steel gear cannot satisfy equation (2) of feature 2.

[0097] On the other hand, if the angle θ between projection direction D1 and projection direction D2 is 35° or greater, the manufactured steel gear can satisfy characteristic 2, provided that the tempering process satisfies the above conditions. Therefore, the angle θ between projection direction D1 and projection direction D2 is set to 35° or greater. There is no particular upper limit to the angle θ between projection direction D1 and projection direction D2, but it is, for example, 65°.

[0098] In the shot peening process described above, conditions other than the projection direction are not particularly limited. For example, in the shot peening process, shot particles with a diameter of 0.8 mm or less are used, the projection pressure is set to 0.2 MPa or higher, and the coverage is set to 300% or higher.

[0099] [(Step 4) Polishing step] In the polishing process, the tooth surface of the intermediate product after the shot peening process is polished. In the polishing process, approximately 50 μm of the tooth surface of the intermediate product is ground down to prepare the surface. The method of polishing is not particularly limited and can be carried out using a well-known method. For example, the polishing process can be carried out by pressing a high-speed rotating grinding wheel against the tooth surface using a gear grinding machine.

[0100] The steel gear of this embodiment is manufactured by the manufacturing method described above. The above manufacturing method is just one example of a manufacturing method for the steel gear of this embodiment. Therefore, the steel gear of this embodiment may be manufactured by other manufacturing methods as long as it satisfies the above-described features. [Examples]

[0101] The effects of one embodiment of the steel gear of this embodiment will be further described in detail below with reference to examples.

[0102] Steel materials having the chemical compositions shown in Tables 1A and 1B were prepared.

[0103] [Table 1A]

[0104] [Table 1B]

[0105] Multiple small roller test pieces 1a, as shown in Figure 4, were manufactured from the steel material of each test number. The numbers in Figure 4 represent dimensions (in mm). "φ" in Figure 4 means diameter. The small roller test piece 1a is equipped with an evaluation surface 11a, which is the outer circumferential surface of the large diameter portion. Here, the evaluation surface 11a corresponds to the tooth surface of a spur gear. The circumferential direction of the evaluation surface 11a corresponds to the tooth profile direction of the spur gear. The axial direction of the evaluation surface 11a corresponds to the tooth trace direction of the spur gear.

[0106] Specifically, intermediate steel materials with the shapes shown in Figure 4 were manufactured by processing the steel materials of each test number. A vacuum carburizing process was performed on the intermediate steel materials of each test number that were manufactured. First, the intermediate steel materials were placed in a heat treatment furnace and heated to 950°C for 60 minutes. After that, the pressure inside the furnace was reduced to 1 kPa. After the pressure reduction, acetylene gas was introduced into the furnace and the material was held at the carburizing temperature of 950°C for 60 minutes. After that, with the introduction of acetylene gas stopped, the material was held at 950°C for 25 minutes under reduced pressure. After that, it was held at 850°C for 30 minutes.

[0107] After the vacuum carburizing process, a quenching process was carried out. In the quenching process, the intermediate steel material was removed from the furnace and oil-cooled using 60°C oil.

[0108] A tempering process was performed on the intermediate steel material after the quenching process. In the tempering process, the intermediate steel material was heated to a tempering temperature T (°C) and held for 120 minutes. After that, the intermediate steel material was allowed to cool in the atmosphere. The tempering temperature T (°C) for each test number is shown in Table 2. Through the above process, an intermediate product having a carburized and hardened layer was manufactured.

[0109] [Table 2]

[0110] Shot peening was performed on the evaluation surface of the intermediate product for each test number. Specifically, shot particles were projected onto the evaluation surface of the intermediate product, which rotates around its axis of rotation, from a projection direction D1 perpendicular to the axis of rotation. Subsequently, shot particles were projected onto the rotating evaluation surface of the intermediate product from a projection direction D2 that included projection direction D1 and was parallel to the axis of rotation, forming an angle θ(°) with projection direction D1. The angle θ(°) for each test number is shown in Table 2. An air nozzle type shot peening machine was used for the shot peening process. Furthermore, the shot peening process was performed using round-cut wire with an average particle size of 0.4 to 0.8 mm and a hardness of 800 HV as the shot particles, with a projection pressure of 0.2 MPa and a coverage of 300% or more.

[0111] Polishing was performed on the evaluation surfaces of intermediate samples for each test number after shot peening. In the polishing process, a high-speed rotating grinding wheel was pressed against the tooth surface. The polishing was performed so that the arithmetic mean roughness Ra of the tooth surface, as specified in JIS B 0601 (2013), was 0.8 μm or less, and the grinding depth was 50 μm.

[0112] The small roller test pieces 1a for each test number were manufactured using the above manufacturing process.

[0113] Furthermore, the C concentration in the region from the evaluation surface 11a to a depth of 50 μm (hereinafter also referred to as the "surface layer of the evaluation surface 11a") of each test number's small roller test specimen 1a was measured based on the method described in [Method for measuring the C concentration of the surface layer] above. As a result, the C concentration of the surface layer of the evaluation surface 11a was 0.75 to 1.10% by mass for all test number's small roller test specimen 1a. In other words, a carburized hardened layer was formed on the surface layer of the evaluation surface 11a for all test number's small roller test specimen 1a. In addition, the area ratio of martensite on the surface layer of the evaluation surface 11a of each test number's small roller test specimen 1a was measured based on the method described in [Method for measuring the area ratio of martensite] above. As a result, the area ratio of martensite on the surface layer of the evaluation surface 11a was 85% or more for all test number's small roller test specimen 1a.

[0114] [About the evaluation test] The following evaluation tests were performed on the manufactured small roller test piece 1a. (Test 1) σ P and σ T Measurement test (Test 2) Pitching strength evaluation test The following describes Exam 1 and Exam 2.

[0115] [(Test 1) σ P and σ T [Measurement test] σ in small roller test piece 1a for each test number P and σ T This was determined by the following method. Four test specimens were taken from the small roller test specimen 1a, each containing the evaluation surface 11a as the measurement surface. The central position of the measurement surface of the four test specimens was the axial center of the evaluation surface 11a, corresponding to any four points arranged at 90° intervals around the rotation axis of the small roller test specimen 1a. [σ P and σ T Based on the method described in [Measurement Method], the circumferential and axial residual stresses were measured at the center of each measurement surface. The arithmetic mean of the absolute values ​​of the circumferential compressive residual stresses obtained from the four specimens was calculated as σ P (MPa) was assumed. The arithmetic mean of the absolute values ​​of the axial compressive residual stress obtained from the four test specimens was used as σ. T (MPa) was assumed. The obtained results are shown in Table 2 as "σ P (MPa)" column and "σ T It was recorded in the "(MPa)" column.

[0116] obtained σ P and σ T Fn1 and Fn2 were calculated from the above. The results obtained are recorded in the "Fn1" and "Fn2" columns in Table 2.

[0117] [(Test 2) Pitching Strength Evaluation Test] The pitting strength of each small roller test specimen 1a was determined by roller pitting tests. Figure 5 is a schematic diagram of the roller pitting test. As shown in Figure 5, the small roller test specimen 1a was rotated while pressing the large roller test specimen 2 against the evaluation surface 11a of the small roller test specimen 1a. The large roller test specimen 2 had the shape shown in Figure 6. The numbers in Figure 6 indicate dimensions (in mm). "R300" in Figure 6 indicates that the radius of curvature of the outer surface was 300 mm.

[0118] Large roller test specimen 2 was manufactured by the following method. An intermediate product having the shape shown in Figure 6 was prepared by processing a steel material having a chemical composition equivalent to bearing steel SUJ2 specified in JIS G 4805 (2019). The intermediate product was subjected to a quenching treatment. Specifically, the processed intermediate product was placed in a heat treatment furnace and heated to 830°C. After heating, the intermediate product was held at 830°C for 60 minutes in an atmosphere with a carbon potential Cp of 0.6%. After heating and holding, the intermediate product was oil-cooled using oil at 60°C. Tempering treatment was performed on the intermediate product after the quenching treatment. In the tempering treatment, the intermediate product was held at 180°C for 120 minutes. After that, the intermediate product was allowed to cool in the air. Surface polishing to a grinding depth of 50 μm was performed on the intermediate product after the tempering treatment. Large roller test specimen 2 was manufactured by the above process.

[0119] A roller pitting test was conducted using the manufactured large roller test piece 2 and the small roller test piece 1a for each test number. Specifically, the large roller test piece 2 was rotated while being pressed against the evaluation surface 11a of the small roller test piece 1a for each test number with various surface pressures. At this time, the rotation speed of the small roller test piece 1a for each test number was 2000 rpm. The peripheral speed direction of both roller test pieces at the contact point was set to the same direction, and the slip ratio was set to -40% (the peripheral speed at the contact point of the large roller test piece 2 was 40% greater than that of the small roller test piece 1a). ATF (automatic transmission fluid) was supplied as a lubricant to the contact point between the large roller test piece 2 and the evaluation surface 11a of the small roller test piece 1a for each test number. The oil temperature of the ATF was 120°C, and the oil flow rate was 1.5 L / min. The number of test cycles was limited to 10 million (1.0 × 10⁻⁶). 7The number of tests was set to 10. Based on the above conditions, a roller pitting test was conducted, and the evaluation surface 11a of the small roller test piece 1a for each test number was visually observed after the test. The maximum surface pressure at which no pitting (fatigue peeling) was observed on the evaluation surface 11a was set to 10. 7 Durability Strength was defined as the number of cycles.

[0120] 10 obtained 7 If the pitting strength was 3000 MPa or higher, it was evaluated as "E (Excellent)," and it was determined that excellent pitting strength had been obtained (indicated as "E" in the "Pitching Strength" column in Table 2). On the other hand, the obtained 10 7 If the pitting strength was less than 3000 MPa, the evaluation was set to "B (Bad)," indicating that excellent pitting strength was not achieved (indicated as "B" in the "Pitching Strength" column in Table 2).

[0121] [Test Results] Referring to Tables 1A, 1B, and 2, the small roller test specimens 1a, which simulated flat steel gears for test numbers 1 to 17, met features 1 and 2. Therefore, excellent pitting strength was obtained.

[0122] On the other hand, in tests 18 and 19, the Si content in the chemical composition of the core was too low. As a result, excellent pitting strength could not be obtained.

[0123] In tests 20 and 21, the tempering temperature T (°C) during the tempering process was too low. Therefore, Fn1 was too low. As a result, excellent pitting strength could not be obtained.

[0124] In tests 22 and 23, the tempering temperature T (°C) during the tempering process was too high. Consequently, Fn1 was too low. As a result, excellent pitting strength could not be obtained.

[0125] In tests 24 and 25, the angle θ(°) during the shot peening process was too small. As a result, Fn2 was too high. Consequently, excellent pitting strength could not be obtained.

[0126] 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. [Explanation of Symbols]

[0127] 1: Steel gear GT: Teeth

Claims

1. A steel gear having multiple teeth, A carburized hardened layer formed on the surface of the tooth surface, The system comprises a core portion which is the part other than the aforementioned carburized hardened layer, The chemical composition of the core is, in mass%, C: 0.15-0.30%, Si: 0.70-2.00%, Mn: 0.45-1.00%, P: 0.035% or less, S: 0.035% or less, Cr: 0.05-2.00%, Mo: 0.05-0.60%, Al: 0.005-0.050%, and, N: Contains 0.0020 to 0.0200%, The remainder consists of Fe and impurities. Compressive residual stress σ in the tooth profile direction on the tooth surface Ｐ (MPa) and the compressive residual stress σ in the tooth trace direction on the tooth surface. Ｔ (MPa) and satisfy equations (1) and (2), steel gears. s Ｐ +s Ｔ ≧2400 (1) |s Ｐ -s Ｔ |≦250 (2)

2. A steel gear having multiple teeth, A carburized hardened layer formed on the surface of the tooth surface, The system comprises a core portion which is the part other than the aforementioned carburized hardened layer, The chemical composition of the core is, in mass%, C: 0.15-0.30%, Si: 0.70-2.00%, Mn: 0.45-1.00%, P: 0.035% or less, S: 0.035% or less, Cr: 0.05-2.00%, Mo: 0.05-0.60%, Al: 0.005-0.050%, and, N: Contains 0.0020 to 0.0200%, Furthermore, it contains one or more selected from the group consisting of Group 1 and Group 2, The remainder consists of Fe and impurities. Compressive residual stress σ in the tooth profile direction on the tooth surface Ｐ (MPa) and the compressive residual stress σ in the tooth trace direction on the tooth surface. Ｔ (MPa) and satisfy equations (1) and (2), steel gears. [Group 1] Cu: 0.40% or less, Ni: 0.30% or less, V: 0.50% or less, Ti: 0.050% or less, Nb: 0.050% or less, B: One or more selected from the group consisting of 0.0040% or less. [Group 2] Bi: 0.020% or less, Sn: 0.100% or less, As: 0.050% or less, Sb: 0.050% or less, Pb: 0.060% or less, Ca: 0.0020% or less, One or more selected from the group consisting of Mg: 0.0050% or less. s Ｐ +s Ｔ ≧2400 (1) |s Ｐ -s Ｔ |≦250 (2)

3. A steel gear according to claim 2, The above group 1 contains, steel gears.

4. A steel gear according to claim 2, The following include the second group: steel gears.

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