Gear treated with vacuum carburization treatment, engine unit having gear treated with vacuum carburization treatment, and saddle-riding type vehicle having gear treated with vacuum carburization treatment

Abstract

To provide a gear treated with vacuum carburization treatment with an improved productivity as well as with an improved abrasion resistance and a rolling fatigue strength while reducing an emission amount of carbon dioxide in production.SOLUTION: A gear treated with vacuum carburization treatment is (a) configured by a specification material or a standard material containing at least one of Cr, Mo, Si, and Ni, and Mn in a region 10 μm or less deep in a direction vertical to a surface of a gear 1, and is (b) configured to be in a state where a Mn concentration is maintained in a region deeper than 10 μm and shallower than 15 μm, and is used in a state where a gear surface of the gear after the vacuum carburization treatment is not grounded nor polished.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a gear that has been subjected to vacuum carburizing treatment, an engine unit having a gear that has been subjected to vacuum carburizing treatment, and a straddle-type vehicle having a gear that has been subjected to vacuum carburizing treatment. [Background technology]

[0002] A typical gas carburizing process is carried out at atmospheric pressure and high temperatures, which makes the surface of the treated member susceptible to oxidation. It is known that such surface oxidation reduces the hardenability of the surface layer due to depletion of alloying elements, thereby deteriorating the strength of the member. Methods for improving the deterioration of member strength include the application of vacuum carburizing and the application of high-strength steel in which elements that promote oxidation are reduced and non-oxidizing elements are added (Patent Document 1: paragraph

[0003] reference).

[0003] Carburizing processes include gas carburizing and vacuum carburizing. Vacuum carburizing has the following advantages over gas carburizing: With vacuum carburizing, the carburizing temperature can be increased, making it possible to obtain carburized parts with a specified carbon concentration in a short time. Furthermore, vacuum carburizing can suppress grain boundary oxidation that occurs during carburizing, making it easier to obtain carburized parts with high rolling fatigue strength. Furthermore, with vacuum carburizing, the process is performed under reduced pressure, so the amount of atmospheric gas is small and there is no need to burn exhaust gases, making it possible to reduce carbon dioxide emissions (see Patent Document 2, paragraph

[0002] ).

[0004] Patent Document 3 describes an example in which vacuum carburizing was performed on a disk-shaped steel material containing Mn. From Figure 5 of Patent Document 3, it is presumed that the disk-shaped steel material is a gear. In the method for carburizing steel described in Patent Document 3, sublimation of Mn from the steel material is suppressed while the temperature is rising in a carburizing furnace, and adhesion of Mn to the high-frequency coil in the carburizing furnace is suppressed. By suppressing adhesion of Mn to the high-frequency coil, the steel material in the carburizing furnace is stably heated to a temperature equal to or higher than the A3 transformation point at which the steel material transforms into austenite, which is necessary for hardening, and the desired carburizing quality is obtained (see Patent Document 3: paragraphs

[0013] to

[0019] and Figure 5).

[0005] Patent Document 4 proposes steel gears, gear steel, and a manufacturing method for the steel gears used as parts for motorcycles and the like. The proposed steel gears, gear steel, and manufacturing method for the steel gears have little heat treatment distortion during surface hardening by carbonitriding, ensuring excellent dimensional accuracy of the gear parts, while also providing the strength originally required for gears, particularly excellent properties against tooth flaking damage in the gear surface layer. The technology in Patent Document 4 reduces dependency on the type of carburizing treatment and the manufacturing method by devising the composition of the gear steel (Patent Document 4:

[0006] reference). [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2003-193128 [Patent Document 2] Japanese Patent Application Laid-Open No. 2016-194156 [Patent Document 3] Japanese Patent Application Laid-Open No. 2015-183226 [Patent Document 4] Japanese Patent Application Laid-Open No. 2006-328484 Summary of the Invention [Problem to be solved by the invention]

[0007] To achieve carbon neutrality, reductions in carbon dioxide emissions are required. For example, in the manufacturing process of gears used in straddle-type vehicles such as motorcycles, gas carburizing is generally performed on gears made from readily available standard or standard materials. Therefore, in order to reduce carbon dioxide emissions, it is conceivable to adopt vacuum carburizing in the manufacturing process of gears used in motorcycles. The adoption of vacuum carburizing is expected to improve the wear resistance and rolling fatigue strength of gears while reducing carbon dioxide emissions.

[0008] Furthermore, when adopting vacuum carburizing, it is possible to adopt gears made of readily available standard or standard materials used in gas carburizing. This is expected to maintain productivity even when using vacuum carburizing compared to when using gears made of specialized materials that are difficult to obtain. Furthermore, if the grinding and polishing of the gear tooth surfaces, which is performed after vacuum carburizing, can be eliminated, productivity can be improved even when using vacuum carburizing. Furthermore, by adopting gears with improved wear resistance and rolling fatigue strength while reducing carbon dioxide emissions in an engine unit that uses the same lubricant for both the engine body and the transmission, carbon dioxide emissions during the manufacture of the engine unit can be reduced, while preventing contamination of the lubricant by wear debris and reducing the impact of the wear debris on the engine body. Furthermore, by adopting gears with improved wear resistance and rolling fatigue strength while reducing carbon dioxide emissions in a saddle-type vehicle, it is expected to reduce carbon dioxide emissions during the manufacture of the saddle-type vehicle, while preventing an increase in gear backlash due to wear and reducing noise changes caused by increased backlash.

[0009] An object of the present invention is to provide a gear that has been subjected to vacuum carburizing treatment, which has improved wear resistance and rolling fatigue strength while reducing carbon dioxide emissions during production, and which also improves productivity. The present invention aims to provide an engine unit having gears that have been subjected to vacuum carburizing treatment, which reduces carbon dioxide emissions during manufacturing, suppresses contamination of lubricating oil by wear debris, and reduces the impact of wear debris on the engine body. The present invention aims to provide a saddle-type vehicle having gears that have been subjected to vacuum carburizing treatment, which reduces carbon dioxide emissions during manufacturing while suppressing an increase in gear backlash due to wear and suppressing changes in sound caused by an increase in backlash. [Means for solving the problem]

[0010] In order to apply vacuum carburizing to the manufacturing process of gears or shafts used in motorcycles, we performed vacuum carburizing on test pieces made of standard materials used in gas carburizing, and conducted tests on wear resistance and rolling fatigue strength assuming use in motorcycles. However, we found that the number of cycles was sometimes lower than that of gas carburized parts (see Comparative Examples 1 and 2 in Figure 1 [2]).

[0011] To compare the rolling fatigue strength of gas-carburized and vacuum-carburized test specimens, the number of cycles until pitting occurred was measured. In the Weibull distribution by the median rank method, the number of cycles at the same cumulative failure probability for each test specimen was smaller for the vacuum-carburized test specimens than for the gas-carburized test specimens (see Comparative Examples 1 and 2 in Figure 1[1]). In other words, it was found that the rolling fatigue strength of the vacuum-carburized test specimens was lower than that of the gas-carburized test specimens.

[0012] To investigate the cause of the difference in rolling fatigue strength between the gas-carburized and vacuum-carburized test specimens, the wear volume of each test specimen was measured. The number of cycles at the same wear depth for each test specimen was smaller for the vacuum-carburized test specimens than for the gas-carburized test specimens (see Comparative Examples 1 and 2 in Figure 1 [2]). In other words, it was found that the wear resistance of the vacuum-carburized test specimens was lower than that of the gas-carburized test specimens.

[0013] These phenomena were inferred as follows: In test specimens that had been vacuum carburized to improve rolling fatigue strength, low-hardness areas, so-called incompletely hardened structures, were sometimes scattered irregularly on the surface. When incompletely hardened structures were scattered irregularly on the surface of the test specimen, wear progressed from the incompletely hardened structures with low hardness due to engagement with other components. It was inferred that pitting was induced in the test specimens with advanced wear, resulting in a decrease in rolling fatigue strength.

[0014] Therefore, to investigate the cause of the irregular distribution of incompletely hardened structures, the inventors performed vacuum carburizing treatment under multiple conditions using test pieces composed of several types of standard and standard materials with different compositional materials. Furthermore, the inventors evaluated the surface condition of each test piece, focusing on the incompletely hardened structure. The results indicated that the degree of vacuum is thought to have an effect, but that the evaluation is largely dependent on the material (see Figure 1 [3]). In other words, even for standard and standard materials, the evaluation results of the incompletely hardened structure differ depending on the degree of vacuum and the content of alloying elements. Note that the materials rated GOOD in Figure 1 [3] do not specifically refer to the materials corresponding to the present invention.

[0015] The inventors have conducted detailed studies on the incompletely hardened structure that is irregularly scattered on the tooth surface of gears made from standard or standard materials. "JIS G4052:2016 Structural Steels with Guaranteed Hardenability (H Steels) Table 2 - Chemical Composition" primarily lists the proportions of alloying elements in structural steels with guaranteed hardenability used for machine structures. Therefore, structural steels with guaranteed hardenability, including SMn443H and SCM420H, ensure hardenability by adding alloying elements such as Si, Mn, Ni, Cr, and Mo. The alloying contents and hardenability multipliers of elements Ni, Si, Cr, Mo, and Mn, which affect hardenability, differ between standard and standard materials (see Figure 1[4]). Additionally, the contents of elements Ni, Si, Cr, Mo, and Mn, which affect hardenability, differ between multiple types of standard materials and standard materials (see Figure 1[5]). Then, focusing on Mn, the element that has the greatest effect on hardenability, the Mn concentration (weight percent) was measured at any depth from the surface after vacuum carburizing for several standard materials and standard materials (see Figure 1 [6]).

[0016] Among the standard materials SMn443H, SCM420H, and SNCM220H, which were vacuum-carburized at a vacuum of 20 Pa, and the standard material KKG8 from the KKG (registered trademark)-T series of high-toughness gear steel (Kobe Steel, Ltd.) (Kobe Steel, Ltd.: High-toughness gear steel KKG-T series, reference URL: https: / / www.kobelco.co.jp / products / download / steel―aluminum / files / sb028.pdf), SMn443H, which has the highest Mn concentration, was rated BAD due to the significant decrease in Mn concentration, which is likely to affect the hardenability of the test specimens. On the other hand, SCM420H, SNCM220H, and KKG8 showed similar decreases in Mn concentration. SCM420H had low amounts of Ni and Mo added, which is likely to affect hardenability due to the Mn, which was rated BAD. SNCM220H has a higher Ni content than SCM420H, so it appears that Ni, Cr, and Mo compensate for the sublimated Mn. KKG8 has a higher Mo content than SCM420H, so it appears that Cr and Mo compensate for the sublimated Mn. This is also confirmed by the fact that Ni, Cr, and Mo do not sufficiently compensate for the sublimated Mn in SMn443H, which is essentially free of Ni, Cr, and Mo. From the above, it can be inferred that alloy elements such as Ni, Cr, and Mo compensate for the reduced hardenability caused by Mn sublimation. In other words, it can be inferred that the hardenability of the standard and reference materials does not depend solely on the Mn concentration. Note that Figures 1[6] and [7] do not specify the materials corresponding to the present invention.

[0017] Furthermore, SCM420H, which was vacuum-carburized at a vacuum of 20 Pa, was rated BAD due to the significant decrease in Mn concentration, which was likely to affect the hardenability of the test specimen. On the other hand, SCM420H, which was vacuum-carburized at vacuums of 500 Pa and 1500 Pa, was rated GOOD due to the gradual decrease in Mn concentration, which was unlikely to affect the hardenability of the test specimen (see Figure 1 [7]). Thus, SCM420H (1500 Pa, 500 Pa), SNCM220H, and KKG8, which received a GOOD rating, maintained their Mn concentration in the region deeper than 10 μm and shallower than 15 μm in depth in the direction perpendicular to the vacuum-carburized surface.

[0018] In statistics, the coefficient of variation (coefficient of variation = standard deviation / average value x 100) is used as an index to evaluate the dispersion and variability of data. When the coefficient of variation is less than 10 percent, the data is evaluated as being statistically stable with little variation. In particular, in quality control in manufacturing industries, when the coefficient of variation is less than 5 percent, the data is evaluated as being statistically stable with little variation. In addition, in linear regression analysis using the least squares method, the coefficient of determination (R 2 ) is used. If the coefficient of determination is less than 0.3, the regression model is evaluated as having weak statistical explanatory power. In addition, in the linear regression analysis using the least squares method, the p-value is used as an index to evaluate whether the effect of the depth (independent variable) on the Mn concentration (dependent variable) is significant. If the p-value is greater than 0.05, the effect of the depth on the Mn concentration is evaluated as not being statistically significant.

[0019] In Figure 1[7], the coefficient of variation for SCM420H (1500 Pa) between the depth of 5 μm and the depth of 15 μm is 4.06 percent, and the coefficient of variation for SCM420H (500 Pa) is 3.33 percent. In other words, the Mn concentrations of SCM420H (1500 Pa) and SCM420H (500 Pa) between the depth of 5 μm and the depth of 15 μm are within a range that is evaluated as statistically stable with little variation.

[0020] In Figure 1[7], the coefficient of determination for SCM420H (1500 Pa) between the depths of 15 μm and 5 μm is 0.00086 and 0.00043, respectively. Furthermore, the p-value for SCM420H (1500 Pa) between the depths of 15 μm and 5 μm is 0.4725 and 0.8733, respectively. Therefore, the regression models for SCM420H (1500 Pa) and SCM420H (500 Pa) between the depths of 15 μm and 5 μm do not statistically adequately represent the relationship between the Mn concentration and the depth from the surface, and it is therefore determined that the effect of the depth on the Mn concentration is not significant. In other words, it is evaluated that the Mn concentrations of SCM420H (1500 Pa) and SCM420H (500 Pa) do not show a significant decreasing or increasing trend from the depth of 15 μm to the depth of 5 μm.

[0021] In addition to Mn, SCM420H also contains small amounts of Ni, Cr, and Mo. These findings confirm that, in standard and standard materials containing Ni, Si, Cr, Mo, and Mn, elements that affect hardenability, Ni, Cr, and Mo can compensate for the decrease in hardenability due to Mn sublimation, provided the Mn concentration is maintained in the region deeper than 10 μm and shallower than 15 μm in depth perpendicular to the vacuum-carburized surface. The contribution of changes in the amount of Si has not been confirmed in this test data. However, as shown in Figure 1 [4] and [5], the hardenability multiplier of Si is greater than that of Ni. Therefore, it is clear that similar properties can be obtained with Si.

[0022] On the other hand, even if the Mn concentration in the region deeper than 10 μm and shallower than 15 μm is maintained, the decrease in hardenability due to Mn sublimation cannot be compensated for unless elements that affect hardenability are contained. For example, SMn443H(20 Pa), which is substantially free of elements that affect hardenability, maintains the Mn concentration in the region deeper than 10 μm and shallower than 15 μm. However, in SMn443H(20 Pa), the decrease in hardenability due to Mn sublimation in the region shallower than 5 μm is not compensated for by the elements that affect hardenability, and therefore, is rated as BAD.

[0023] Based on detailed consideration of the above evaluation results, the inventors came up with the technical idea of ​​adjusting the dispersion of elements that affect hardenability so that at least one of Si, Ni, Cr, and Mo can compensate for the decrease in hardenability caused by the sublimation of Mn in a region of a gear that has been vacuum-carburized and is located to a depth of 10 μm or less in the direction perpendicular to the surface.

[0024] Therefore, we conducted wear resistance and rolling fatigue strength tests on the following vacuum-carburized test specimens, assuming use in motorcycles. The vacuum-carburized test specimens (a) were composed of standard and standard materials containing at least one of Cr, Mo, Si, and Ni, and Mn, in a region 10 μm or less deep in the direction perpendicular to the vacuum-carburized surface, and (b) were configured so that the Mn concentration was maintained in a region deeper than 10 μm and shallower than 15 μm deep in the direction perpendicular to the surface. Furthermore, the test was conducted without grinding or polishing the surface of the vacuum-carburized test specimens. The results confirmed that the number of cycles was equivalent to that of gas-carburized parts, confirming significantly improved wear resistance (see Figure 1 [2], present invention).

[0025] A gear according to one embodiment of the present invention may have the following configuration (1): (1) A gear that has been subjected to vacuum carburizing. A vacuum carburized gear is (a) made of a standard or standard material containing Mn and at least one of Cr, Mo, Si, and Ni in a region of the gear that is 10 μm or less in depth in a direction perpendicular to the surface, and (b) configured so that the Mn concentration is maintained in a region that is deeper than 10 μm and shallower than 15 μm in depth in a direction perpendicular to the surface, and the gear tooth surface is used without being ground or polished after vacuum carburizing.

[0026] This gear is vacuum-carburized rather than gas-carburized. This reduces carbon dioxide emissions during manufacturing. Test results show that the Mn concentration is maintained in a region deeper than 10 μm and shallower than 15 μm. The reduced hardenability due to Mn sublimation is compensated for by the addition of at least one of Si, Ni, Cr, and Mo. This improves the wear resistance and rolling fatigue strength of the vacuum-carburized gear, even if the tooth flanks are not ground or polished after vacuum carburization. Furthermore, the vacuum-carburized gear can be used without grinding or polishing the tooth flanks after vacuum carburization. This eliminates the need for grinding or polishing the tooth flanks of the gear, which has improved hardness and wear resistance after vacuum carburization, thereby improving productivity. The gear may also be shot-peened to further improve the rolling fatigue strength and bending fatigue strength according to required specifications.

[0027] Note that "at least one of Cr, Mo, Si, and Ni" means that two of Cr, Mo, Si, and Ni may be included. "at least one of Cr, Mo, Si, and Ni" means that three of Cr, Mo, Si, and Ni may be included. "at least one of Cr, Mo, Si, and Ni" means that all of Cr, Mo, Si, and Ni may be included.

[0028] A gear according to one embodiment of the present invention may have the following configuration (2) in addition to the configuration (1): (2) A gear that has been subjected to vacuum carburizing treatment is configured such that (c) the Mn concentration is maintained in a region that is deeper than 8 μm and not more than 10 μm deep in a direction perpendicular to the surface of the gear.

[0029] In a gear having this configuration, the Mn concentration is adjusted in (b) a region adjacent to the region having a depth perpendicular to the surface that is deeper than 10 μm but shallower than 15 μm, and (c) a region closer to the surface than the region having a depth of 8 μm but 10 μm or less. Therefore, as shown in test results, in a gear having the above configuration that has been subjected to vacuum carburization and in which the Mn concentration is maintained in the region having a depth of 8 μm but 10 μm or less, the decrease in hardenability due to Mn sublimation is compensated for by at least one element of Si, Ni, Cr, and Mo. Therefore, the wear resistance and fatigue strength of the gear can be further improved even when the tooth surface is not ground or polished after vacuum carburization.

[0030] A gear according to one embodiment of the present invention may have the following configuration (3) in addition to the configuration (1) or (2): A gear that has been subjected to vacuum carburizing treatment is configured such that (d) the Mn concentration is maintained in a region having a depth of more than 5 μm and not more than 8 μm in a direction perpendicular to the surface of the gear.

[0031] In a gear having this configuration, the Mn concentration is adjusted in a region (c) adjacent to a region deeper than 8 μm but shallower than 10 μm in depth in the direction perpendicular to the surface, and in a region (d) closer to the surface than the region (c), but deeper than 5 μm and not deeper than 8 μm. Therefore, as shown in test results, in a gear having the above configuration and having been subjected to vacuum carburization, the Mn concentration in the region deeper than 5 μm and not deeper than 8 μm is maintained. The decrease in hardenability due to Mn sublimation is compensated for by at least one element of Si, Ni, Cr, and Mo. Therefore, the wear resistance and rolling fatigue strength of the gear having been subjected to vacuum carburization and having the Mn concentration maintained in the region deeper than 5 μm and not deeper than 8 μm can be further improved.

[0032] A gear according to one embodiment of the present invention may have the following configuration (4) in addition to the configuration (1), (2), or (3). (4) In a region of the gear having a depth of 10 μm or less in a direction perpendicular to the surface thereof, the amounts of Si, Cr, Mo, Ni, and Mn may be the following weight percents: Si is greater than 0.35. Cr is greater than 0.35. Mo is greater than 0.25. Ni is greater than 0.25. Mn is greater than 0.60.

[0033] The amount of Si that can compensate for the decrease in hardenability caused by sublimation of Mn during vacuum carburizing in the region of the gear with a depth of 10 μm or less in the direction perpendicular to the surface can be considered from the test results and evaluation results. Regarding Si, the evaluation results for SMn443H (vacuum degree 20 Pa) and SCM420H (vacuum degree 20 Pa) indicate that a Si concentration of 0.15 to 0.35 weight percent is unlikely to be sufficient to compensate for the decrease in hardenability. Therefore, the amount of Si that can compensate is considered to be greater than 0.35 weight percent.

[0034] Regarding Mo, the evaluation results for SCM420H (vacuum degree 20 Pa) suggest that a Mo concentration of 0.15 to 0.25 weight percent is unlikely to be sufficient to compensate for the decrease in hardenability. Therefore, it is believed that an amount of Mo that can compensate is an amount greater than 0.25 weight percent.

[0035] Regarding Cr, the evaluation results for SMn443H (vacuum degree 20 Pa) suggest that a Cr concentration of 0.35 weight percent or less is unlikely to be sufficient to compensate for the decrease in hardenability. Therefore, it is believed that an amount of Cr that is sufficient to compensate is an amount greater than 0.35 weight percent.

[0036] Regarding Ni, the evaluation results for SMn443H (vacuum degree 20 Pa) suggest that a Ni concentration of 0.25 weight percent or less is unlikely to be sufficient to compensate for the decrease in hardenability. Therefore, an amount of Ni sufficient to compensate for the decrease in hardenability is considered to be an amount greater than 0.25 weight percent.

[0037] Regarding Mn, the amount that can be compensated for based on the evaluation results for SNCM220H (vacuum degree 20 Pa) is considered to be a Mn concentration of 0.60 weight percent or more.

[0038] Actual standard materials and reference materials contain multiple elements, including Si, Ni, Cr, and Mo, making it difficult to determine the effect of individual element content on hardenability. Whether the hardenability reduced by Mn sublimation is compensated for by at least one of Si, Ni, Cr, and Mo to a degree that results in satisfactory hardening (carburization) can be determined, for example, based on the amount of incompletely hardened structure per unit area (see Figure 1 [3]). When the amount of incompletely hardened structure per unit area in a vacuum-carburized target component is less than the reference value, it is considered that the alloying elements contained in the target component compensate for the hardenability reduced by Mn sublimation to a degree that results in satisfactory hardening. In other words, the types and amounts of alloying elements contained in a target component in which the amount of incompletely hardened structure per unit area after vacuum-carburizing is less than the reference value are the combination and amounts of alloying elements necessary to compensate for the hardenability reduced by Mn sublimation to a degree that results in satisfactory hardening.

[0039] It is generally accepted that the sublimation rate of manganese in properly vacuum-carburized steel is 0.1 to 0.3 weight percent. The sublimation rate can be calculated based on the manganese vapor pressure. The Clausius-Clapeyron equation, which calculates the manganese vapor pressure, and the Knudsen-Langmuir equation, which calculates the mass flow rate (the mass passing vertically through a cross section of a unit area in a unit time) from the manganese vapor pressure, are well-known technical formulas related to manganese sublimation. The manganese sublimation rate can be calculated using the Clausius-Clapeyron and Knudsen-Langmuir equations.

[0040] The vapor pressure of Mn at a vacuum carburizing temperature of 1000°C is 3.42 Pa according to the Clausius-Clapeyron equation. For example, if the mass is 1.2 kg and the surface area is 400 cm 2 When a gear is vacuum carburized at 1000°C for 5 hours, the sublimation rate of Mn is 0.19 weight percent, based on the Knudsen-Langmuir equation, assuming a Mn vapor pressure of 3.42 Pa. Furthermore, as shown in Figure 1 [7], the sublimation rate of Mn on the surface of the GOOD-rated "SCM420H (1500 Pa)" and "SCM420H (500 Pa)" steels is approximately 0.2 to 0.3 weight percent. Thus, based on common technical knowledge, calculations, and measurements, the sublimation rate of Mn in properly vacuum-carburized steel is 0.1 to 0.3 weight percent.

[0041] Additionally, the Grossmann formula is known as common technical knowledge as a basic formula for quantifying the hardenability of steel materials. The Grossmann formula calculates the diameter D at which 50% of the central structure of a round bar of a metal material becomes martensite when the metal material is hardened, based on the concentration of alloying elements contained in the metal material. The larger the diameter D, the higher the hardenability. In other words, the hardenability of the metal material can be evaluated by the diameter D. Below is an example of calculation using the coefficients of each element experimentally determined by Hollomon & Jaffe. D=D1×(1+0.64Si)×(1+4.10Mn)×(1+2.83P)×(1-0.62S)×(1+2.33Cr)×(1+0.52Ni)×(1+3.14Mo)×(1+0.27Cu)×(1+1.5(0.90-B)) D: assumed critical diameter, D1: critical diameter taking grain size into consideration (applied critical diameter 10.95 for crystal grain size 4), C, Si, Mn, P, S, Cr, Ni, Mo, Cu, B: weight percentage of each element

[0042] As shown in Figure 9, the estimated critical diameters D based on the concentrations of alloying elements in SCM420H, SMn443H, SNCM220H, and KKG8, which were vacuum-carburized at a vacuum of 20 Pa, and the surface carbon concentration (0.7 percent) and manganese concentration (see Figure 1[6]), are as follows: SCM420H (BAD rating): 100.13, SMn443H (BAD rating): 25.66, SNCM220H (GOOD rating): 117.31, and KKG8 (GOOD rating): 256.28. Therefore, it can be inferred that the threshold for assessing whether the hardenability reduced by the sublimation of manganese during vacuum carburizing is compensated for to an acceptable level by the alloying elements is greater than the estimated critical diameter D = 100.13 for SCM420H (BAD rating) and less than the estimated critical diameter D = 117.31 for SNCM220H (GOOD rating). In other words, when the assumed critical diameter D calculated from the concentrations of the alloying elements using the Grossmann equation is equal to or greater than the threshold value, it can be evaluated that the alloying elements have compensated for the hardenability reduction caused by Mn sublimation to a level that leaves no problems in the hardened state. From the above, the contents of Si, Mn, Ni, Cr, Mo, etc. necessary to compensate for the hardenability reduction caused by Mn sublimation can be calculated. Based on the above common general technical knowledge, the technical concept of the present invention, and the evaluation results, the amounts of alloying elements that compensate for the hardenability reduction caused by Mn sublimation during vacuum carburizing to a level that leaves no problems in the hardened state in the region of the gear to a depth of 10 μm or less in the direction perpendicular to the surface of the gear may be defined or interpreted as follows:

[0043] The present invention is based on the technical concept of adjusting the dispersion state of elements that affect hardenability so that, in a region of 10 μm or less in depth perpendicular to the surface of the vacuum-carburized gear, the alloying elements compensate for the decrease in hardenability caused by the sublimation of Mn due to vacuum carburization to an extent that does not affect the hardenability. Test results have shown that the wear resistance and rolling fatigue strength of the vacuum-carburized gear can be improved. Therefore, in this specification, the amount of alloying elements that compensates for the decrease in hardenability caused by the sublimation of Mn due to vacuum carburization to an extent that does not affect the hardenability in a region of 10 μm or less in depth perpendicular to the surface of the gear may be defined as follows, based on evaluation results:

[0044] For Si, the amount of hardenability compensation to the extent that there are no problems with the hardening state may be defined as greater than 0.35 weight percent of the standard material or standard material. For Si, when expressed as 0.15 to 0.35, or 0.15 or less, or 0.35 or less, it may be defined as an amount that does not compensate for the hardenability to the extent that there are no problems with the hardening state. For Ni, the amount of hardenability compensation to the extent that there are no problems with the hardening state may be defined as greater than 0.25 weight percent of the standard material or standard material. For Ni, when expressed as 0.25 or less, it may be defined as an amount that does not compensate for the hardenability to the extent that there are no problems with the hardening state. For Cr, the amount of hardenability compensation to the extent that there are no problems with the hardening state may be defined as greater than 0.35 weight percent of the standard material or standard material. For Cr, when expressed as 0.35 or less, it may be defined as an amount that does not compensate for the hardenability to the extent that there are no problems with the hardening state. For Mo, the amount of hardenability that compensates for the hardenability to an extent that leaves no problems may be defined as greater than 0.25 weight percent of the standard material or reference material. For Mo, when expressed as 0.15 to 0.2, 0.25 or less, it may be defined as not being an amount that compensates for the hardenability to an extent that leaves no problems. For Mn, the amount of hardenability that compensates for the hardenability to an extent that leaves no problems may be defined as 0.60 or more weight percent of the standard material or reference material.

[0045] An engine unit according to one embodiment of the present invention may include an engine body, a transmission that changes the output rotation speed of the engine body, and a gear that has been subjected to the vacuum carburizing treatment according to any one of the configurations (1), (2), (3), and (4) and that is lubricated with a lubricating oil that lubricates both the engine body and the transmission.

[0046] The gears of configuration (1) are vacuum-carburized rather than gas-carburized. Therefore, carbon dioxide emissions during manufacturing can be reduced. Test results show that vacuum-carburized gears have improved wear resistance and rolling fatigue strength. The vacuum-carburized gears are used without grinding or polishing their tooth surfaces after vacuum carburizing. This eliminates the need to grind or polish the tooth surfaces of the gears, which have improved hardness and wear resistance, thereby improving productivity. Furthermore, in an engine unit of this configuration, the vacuum-carburized gears are lubricated with a lubricating oil that lubricates both the engine body and the transmission. This reduces carbon dioxide emissions during manufacturing of the engine unit, while suppressing contamination of the lubricating oil by wear debris and reducing the impact of wear debris on the engine body. The issue of wear debris from the transmission affecting the engine body is a unique issue for engine units, in which the transmission gears are lubricated with a lubricating oil that lubricates both the engine body and the transmission.

[0047] A saddle-type vehicle according to one embodiment of the present invention may have gears that have been subjected to the vacuum carburizing treatment according to any one of the configurations (1), (2), (3), and (4) meshed with each other.

[0048] The gears of the configuration (1) are vacuum-carburized rather than gas-carburized. Therefore, carbon dioxide emissions during manufacturing can be reduced. Test results show that the wear resistance and rolling fatigue strength of the vacuum-carburized gears can be improved. The vacuum-carburized gears are used without grinding or polishing their tooth surfaces after vacuum carburizing. This eliminates the need to grind or polish the tooth surfaces of the gears, which have improved hardness and wear resistance, thereby improving productivity. Furthermore, a saddle-type vehicle of this configuration has gears that have been vacuum-carburized in mesh with each other. This reduces carbon dioxide emissions during manufacturing of the saddle-type vehicle while suppressing an increase in the amount of backlash of the gears due to wear and suppressing changes in sound caused by an increase in backlash. Unlike automobiles, the rider of a saddle-type vehicle is not enclosed in a cabin. Therefore, depending on the driving conditions, the sound of the gears is easily heard by the rider. This is a problem unique to saddle-type vehicles.

[0049] The present invention is based on the technical concept of adjusting the dispersion of elements that affect hardenability so that at least one of Si, Ni, Cr, and Mo compensates for the decrease in hardenability caused by the sublimation of Mn in a region of a gear that has been vacuum-carburized to a depth of 10 μm or less in the direction perpendicular to the surface.

[0050] Patent Documents 1, 2, 3, and 4 neither disclose nor suggest the relationship between the depth in the direction perpendicular to the surface and the Mn concentration. Patent Documents 1, 2, 3, and 4 also neither disclose nor suggest the idea of ​​adjusting the dispersion state of elements that affect hardenability so as to compensate for the hardenability that decreases due to Mn sublimation. Therefore, it is difficult to derive the present invention from Patent Documents 1, 2, 3, and 4.

[0051] [Standard materials] In this specification, standard materials refer to materials specified in material standards that stipulate the physical properties, mechanical properties, composition, shape, etc. of industrial materials such as metals. Material standards include, for example, industry standards such as the Japanese Industrial Standards (JIS), regional standards, national standards, and international standards (ISO). The component values ​​(weight percent) of alloying elements such as Mn, Cr, Mo, Si, and Ni, which are minor components contained in standard steel materials, are specified within certain ranges. Standard materials are easily available due to their large distribution volume.

[0052] [Standard material] In this specification, a standard material refers to a material with a composition independently defined by a material manufacturer or the like. Standard materials are materials with compositions different from standard materials, including materials offered by each manufacturer as standard products. The component values ​​(weight percent) of the main alloying elements of standard materials, such as Mn, Cr, Mo, Si, and Ni, fall within a certain range defined by the manufacturer. Standard materials are readily available due to their large distribution volume.

[0053] [Alloy element] In this specification, alloying elements refer to elements added to a metallic material primarily composed of a given metallic element to impart certain properties to the metallic material. The alloying elements are added to the metallic material in a predetermined weight percentage. Examples of alloying elements include C, Si, Mn, P, S, Cr, Mo, Ni, B, and Ti. A gear subjected to vacuum carburizing may be made of a standard or standard material containing at least one of Cr, Mo, Si, and Ni and Mn. Cr, Mo, Si, and Ni improve hardenability. Therefore, in metallic materials containing alloying elements such as Cr, Mo, Si, and Ni, at least a portion of the reduced hardenability due to the sublimation of Mn is compensated for by the alloying elements. A gear subjected to vacuum carburizing may also contain alloying elements other than Cr, Mo, Si, and Ni that improve hardenability. A gear subjected to vacuum carburizing may also contain alloying elements other than Cr, Mo, Si, and Ni that improve hardenability. A gear subjected to vacuum carburizing may also contain, for example, B, Ti, etc.

[0054] [Vacuum carburizing treatment] In this specification, vacuum carburizing refers to a process in which a target component to be carburized is heated with a hydrocarbon gas (typically acetylene) in a carburizing furnace reduced to 10,000 Pa or less, thereby penetrating and diffusing carbon (C) from the surface of the target component. Vacuum carburizing emits less carbon dioxide than gas carburizing, in which heating is performed in a carburizing furnace filled with carbon monoxide. In vacuum carburizing, the Mn concentration of the gear may be adjusted by adjusting the degree of vacuum. In vacuum carburizing, the Mn concentration of the gear may be adjusted by adjusting the carburizing temperature. In vacuum carburizing, the Mn concentration of the gear may be adjusted by adjusting the degree of vacuum and the carburizing temperature. In vacuum carburizing, the Mn concentration of the gear may be adjusted by adjusting other process control conditions.

[0055] [Vacuum degree] In this specification, the degree of vacuum refers to the pressure of a space filled with gas at a pressure lower than atmospheric pressure under standard conditions. The degree of vacuum is classified according to the range of pressure. A low vacuum refers to a pressure of, for example, 100 Pa or more. A medium vacuum refers to a pressure of, for example, 0.1 Pa or more and less than 100 Pa. A high vacuum refers to a pressure of less than 0.1 Pa.

[0056] [Hardenability] In this specification, hardenability refers to the ease with which a steel material hardens by quenching, and refers to the relationship between the hardness of the steel material hardened by quenching and the depth perpendicular to the surface of the steel material. A steel material with good hardenability is, for example, a material that hardens deeper from the surface than other steel materials when quenched under the same conditions, or a material that hardens more than other steel materials.

[0057] [Incomplete hardening area] In this specification, the incompletely hardened region refers to a region where the proportion of martensite transformed is low due to a decrease in the concentration of alloying elements such as C, Mn, Ni, Mo, Cr, and Si that improve the hardenability of steel. For example, a region where martensite transformation is not possible and a structure such as troostite or pearlite is formed is an incompletely hardened region.

[0058] [Mn concentration is maintained] In this specification, "the Mn concentration is maintained" means that, in a region of any depth in the direction perpendicular to the surface of the gear, statistical fluctuations in the Mn concentration are small and it is evaluated that there is no significant decreasing trend as the depth becomes shallower. Statistical evaluations include, for example, the coefficient of variation (standard deviation / average value x 100), which is an index for evaluating the dispersion and variability of data, and the coefficient of determination (R 2 ) is based on the p-value, which is an index for evaluating whether the effect of the depth (independent variable) on the Mn concentration (dependent variable) is significant in a linear regression analysis using the least squares method. The coefficient of variation is an index for statistically evaluating whether the Mn concentration is maintained in the depth region. The coefficient of determination and the p-value are indexes for statistically evaluating whether the Mn concentration shows a significant decreasing or increasing trend in the depth region. When the coefficient of variation is 5% or less, it is evaluated that the fluctuation of the Mn concentration in the depth region is statistically small and stable. When the coefficient of determination is 0.3 or less, it is evaluated that the explanatory power of the regression model showing the relationship between the depth and the Mn concentration is weak. When the p-value is 0.1 or more, it is evaluated that the depth does not have a significant effect on the Mn concentration.

[0059] [gear] In this specification, the gears may be gears with parallel axes. The gears may be gears with parallel axes, such as spur gears, helical gears, internal gears, racks, etc. The gears may be gears with intersecting axes. The gears may be gears with intersecting axes, such as straight bevel gears, spiral bevel gears, helical bevel gears, etc. The gears may be gears with intersecting axes. The gears may be gears with intersecting axes, such as worm gears, hypoid gears, etc. The gears may have involute teeth. The gears may be gears whose teeth have an involute curve shape. The gears may have cycloid teeth. The gears may be gears whose teeth have a cycloid curve shape.

[0060] [Tooth surface] In this specification, the term "tooth flank" refers to the surface of the teeth of one gear that comes into contact with the teeth of the other gear when one gear transmits a rotational force to the other gear. When one gear transmits a rotational force to the other gear, the tooth flank of the one gear slides radially on the tooth flank of the other gear while being pressed against the tooth flank of the other gear.

[0061] [Depth perpendicular to the gear surface] In this specification, the "depth perpendicular to the gear surface" refers to the depth perpendicular to the tooth flank or side surface of the gear that is not machined after vacuum carburizing, excluding the shaft insertion portion that is machined. In this specification, the "depth perpendicular to the gear surface" defines the range of the gear that indicates the state of vacuum carburizing. For example, even if a vacuum-carburized gear is assembled and used in a transmission, the state of vacuum carburizing of the tooth flank of the gear before use can be determined from the state of vacuum carburizing within the range defined by the depth perpendicular to the side surface that is not machined after vacuum carburizing. The "side surface" of a gear refers to the surface visible when the gear is viewed along the gear's rotation axis, but is not the tooth surface. Therefore, the "side surface" of a gear includes the surface visible when the gear is viewed along the gear's rotation axis, but is not in contact with the teeth of other gears. For example, the "side surface" of a spur gear refers to the surface visible when the gear is viewed along the gear's rotation axis. For example, the side surface of a cylindrical worm includes the axial end surface of the cylindrical worm that is visible when the cylindrical worm is viewed in the direction of its rotation axis and the tooth surfaces of the cylindrical worm that do not come into contact with the teeth of a worm wheel. For example, the side surface of a helical gear includes the surface of the helical gear that is visible when the helical gear is viewed in the direction of its rotation axis and the tooth surfaces of the helical gear that do not come into contact with the teeth of other helical gears.

[0062] [Grinding or polishing gears] In this specification, grinding or polishing a gear means cutting or polishing the tooth flanks by machining. For example, wearing or scraping the tooth flanks of a gear due to the use of the gear in a transmission is not included in grinding or polishing the tooth flanks of the gear.

[0063] In this specification, the vacuum-carburized gear may be used in products other than engine units and saddle-type vehicles, such as outboard motors. The vacuum-carburized gear may constitute at least a part of a power transmission mechanism to which driving force from a drive source of a saddle-type vehicle is transmitted. The drive source may be an engine. The drive source may be an electric motor. The power transmission mechanism may include output members such as tires and a propeller. The power transmission mechanism may be a power transmission mechanism to a generator in a series hybrid in which driving force is not output to tires and a propeller. The term "saddle-type vehicle" refers to a vehicle in which a rider sits astride a saddle. Examples of saddle-type vehicles include motorcycles, motorcycles, and all-terrain vehicles (ATVs). Examples of saddle-type vehicles include one-wheel vehicles, two-wheel vehicles, three-wheel vehicles, and four-wheel vehicles.

[0064] The terminology used in this specification is used for the purpose of defining particular embodiments only, and is not intended to limit the invention.

[0065] As used herein, "at least one of," "any one of," and / or" includes all combinations of one or more of the associated listed members.

[0066] As used herein, the use of "including," "comprising," or "having" and variations thereof identify the presence of stated features, steps, operations, elements, components, and / or equivalents thereof, but may include one or more of the steps, operations, elements, components, and / or groups thereof.

[0067] As used herein, the terms "attached," "connected," "coupled," and / or their equivalents are used broadly to encompass both "direct and indirect" attachments, connections, and couplings. Furthermore, "connected" and "coupled" are not limited to physical or mechanical connections or couplings, but can also include direct or indirect electrical connections or couplings.

[0068] Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs.

[0069] Terms defined in commonly used dictionaries should be construed to have a meaning consistent with the relevant art and meaning in the context of this disclosure, and should not be construed in an idealized or overly formal sense unless expressly defined herein.

[0070] It will be understood that in describing the present invention, several techniques and processes are disclosed, each of which has distinct advantages and can be used in conjunction with one or more, or in some cases all, of the other disclosed techniques.

[0071] Thus, for the sake of clarity, the description of the present invention refrains from unnecessarily repeating every possible combination of the individual steps, but the specification and claims should be read with the understanding that all such combinations are within the scope of the present invention.

[0072] In this specification, embodiments of a gear, an engine unit, and a saddle-ride type vehicle according to the present invention will be described.

[0073] In the following description, numerous specific examples are set forth to provide a thorough understanding of the present invention. However, it will be apparent to one skilled in the art that the present invention may be practiced without these specific examples.

[0074] Accordingly, the following disclosure is to be considered as illustrative of the present invention and is not intended to limit the invention to the specific embodiments illustrated by the following drawings or description. [Effects of the Invention]

[0075] According to one embodiment of the present invention, it is possible to provide a gear that has been subjected to a vacuum carburizing process, which reduces carbon dioxide emissions during production, improves wear resistance and rolling fatigue strength, and improves productivity. According to one embodiment of the present invention, it is possible to provide an engine unit having gears that have been subjected to vacuum carburizing treatment, which reduces carbon dioxide emissions during manufacturing while suppressing contamination of lubricating oil by wear debris and reducing the impact of wear debris on the engine body. According to one embodiment of the present invention, it is possible to provide an engine unit having gears that have been subjected to vacuum carburizing treatment, which reduces carbon dioxide emissions during manufacturing while suppressing an increase in gear backlash due to wear and suppressing changes in sound caused by an increase in backlash. [Brief explanation of the drawings]

[0076] [Figure 1] FIG. 1 is a graph and table according to an embodiment of the present invention. [Figure 2] FIG. 2 is a graph showing the results of the rolling fatigue test. [Figure 3] FIG. 3 is a graph showing the relationship between the rolling fatigue test results and the amount of wear. [Figure 4] FIG. 4 is a table showing the evaluation results of the incompletely hardened structure. [Figure 5] FIG. 5 is a graph showing alloy content and hardenability multiple. [Figure 6] Figure 6 is a table showing the alloy content for each standard material and standard material. [Figure 7] FIG. 7 is a graph showing the Mn concentration at different depths from the surface for different standard materials. [Figure 8]FIG. 8 is a graph showing the Mn concentration at different depths from the surface for different degrees of vacuum. [Figure 9] FIG. 9 is a table showing the contents of alloy elements in the test pieces that were subjected to vacuum carburizing treatment. [Figure 10] FIG. 10 is a flow chart showing the steps of a vacuum carburization process according to an embodiment of the present invention. [Figure 11] FIG. 11 is a graph showing changes in the treatment temperature in the vacuum carburization treatment according to the embodiment of the present invention. [Figure 12] FIG. 12 is a schematic diagram of a gear according to an embodiment of the present invention. [Figure 13] FIG. 13 is a schematic diagram of an engine unit according to an embodiment of the present invention. [Figure 14] FIG. 14 is a schematic diagram of a saddle-ride type vehicle according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0077] Each embodiment will be described below with reference to the drawings. In each drawing, the same parts are designated by the same reference numerals, and the description of the same parts will not be repeated. Note that the dimensions of the components in each drawing do not faithfully represent the actual dimensions of the components and the dimensional ratios of the components.

[0078] [Embodiment] <Overall structure> A gear 1, an engine unit 10, and a saddle-ride type vehicle 20 according to an embodiment of the present invention will be described with reference to Figures 12 to 14. Figure 12 is a schematic diagram of the gear 1 according to an embodiment of the present invention. Figure 13 is a schematic diagram of the engine unit 10 according to an embodiment of the present invention. Figure 14 is a schematic diagram of the saddle-ride type vehicle 20 according to an embodiment of the present invention.

[0079] The gear 1 shown in FIG. 12 has been subjected to vacuum carburizing. The engine unit 10 shown in FIG. 13 has an engine body 11, a transmission 12, and a gear 1 that has been subjected to vacuum carburizing and is lubricated with a lubricating oil that lubricates both the engine body 11 and the transmission 12. The saddle-type vehicle 20 shown in FIG. 13 has gears 1 that have been subjected to vacuum carburizing, in a state where they mesh with each other. In this embodiment, the saddle-type vehicle 20 has an engine unit 10 including the gears 1. The gear 1, engine unit 10, and saddle-type vehicle 20 according to the embodiment of the present invention are embodiments that fulfill the present invention, which were derived from consideration of the test results and evaluation results shown in the graphs and tables of FIGS. 1 to 11. Note that descriptions related to the above considerations are omitted from the following description to avoid duplication.

[0080] FIG. 1 shows graphs and tables according to an embodiment of the present invention. [1] in FIG. 1 is the same as FIG. 2. [2] in FIG. 1 is the same as FIG. 3. [3] in FIG. 1 is the same as FIG. 4. [4] in FIG. 1 is the same as FIG. 5. [5] in FIG. 1 is the same as FIG. 6. [6] in FIG. 1 is the same as FIG. 7. [7] in FIG. 1 is the same as FIG. 8.

[0081] 2 and 3 are graphs showing the results of wear resistance and fatigue tests. Comparative Example 1 shows data for a test specimen subjected to gas carburizing treatment. Comparative Example 2 shows data for a test specimen subjected to vacuum carburizing treatment at a medium vacuum of 30 Pa. The data for the present invention shows data for a test specimen subjected to vacuum carburizing treatment at 1500 Pa. All of the test specimens are standard materials. The test specimens are SCM420H. The tests were conducted using a roller pitching tester. The test conditions were set to achieve wear resistance and rolling fatigue strength expected for use in motorcycles. The test conditions were set to approximate the meshing conditions of gear 1 (see FIG. 12). It was confirmed that Comparative Example 2 had the same wear depth as Comparative Example 1, but was inferior in terms of the number of cycles. It was confirmed that the present invention had the same number of cycles as Comparative Example 1, but had improved wear resistance.

[0082] Figure 4 is a table showing the results of an evaluation of the surface condition of multiple vacuum-carburized test specimens, focusing on the incompletely hardened structure. The evaluation results were obtained by visually inspecting the cross-sectional structure of each test specimen. Test specimens with scattered incompletely hardened structures greater than the reference value were rated as "BAD," while test specimens with scattered incompletely hardened structures less than the reference value were rated as "GOOD." The evaluation results for SCM420H confirmed that adjusting the degree of vacuum can suppress the incompletely hardened structure. Furthermore, the evaluation results for SCM420H, SMn443H, and SNCM220H confirmed that the evaluation results for incompletely hardened structures differ depending on the alloy element content, even for standard and standard materials. While the evaluation was based on visual inspection of specific cross-sectional structures and is not rigorous, it is sufficient to grasp the trends.

[0083] The results shown in Figure 4 confirm that the cause of the irregular distribution of incompletely hardened structures in vacuum carburizing treatment is largely dependent on the degree of vacuum and the content of alloying elements in the material. In other words, Figure 4 shows that the evaluation results of incompletely hardened structures differ depending on the degree of vacuum and the content of alloying elements, even for standard and standard materials. Note that the evaluation in Figure 4 does not specify the materials that correspond to the present invention.

[0084] Figure 5 is a graph showing the alloy content and hardenability factor. As shown in Figure 4, the hardenability for each concentration (weight percent) of the alloying elements Mn, Cr, Mo, Si, and Ni increases in the order Mn, Mo, Cr, Si, and Ni.

[0085] Figure 6 is a table showing the alloying element contents (weight percent) of standard and reference materials. The contents of elements that affect hardenability, Ni, Si, Cr, Mo, and Mn, differ among the multiple standard and reference materials.

[0086] Figure 7 is a graph showing the Mn concentration at depth perpendicular to the surface for standard and standard materials. Focusing on Mn, the element that has the greatest impact on hardenability, we measured the Mn concentration at depth perpendicular to the surface for several standard and standard materials after vacuum carburizing. Among SMn443H, SCM420H, and SNCM220H, which underwent vacuum carburizing at a vacuum of 20 Pa, SMn443H, which had the highest Mn concentration, experienced a significant decrease in Mn concentration, which likely affected the hardenability of the test specimens, and was therefore rated as BAD. Meanwhile, SCM420H, SNCM220H, and KKG8 all showed similar decreases in Mn concentration. SCM420H, with its low Ni and Mo content, was rated as BAD due to the high likelihood that Mn would affect hardenability. This confirms that the hardenability of standard and standard materials does not simply depend on Mn concentration. The Mn concentration in SNCM220H decreases significantly around 10 μm and 4 μm. The Mn concentration in SCM420H also decreases significantly around 6 μm. This is the grain boundary. Unlike the regular atomic arrangement inside crystal grains, the grain boundary has a structure in which the atomic arrangement is disordered. Therefore, atoms within the grain boundary can easily diffuse, and a significant decrease in the Mn concentration due to sublimation is observed.

[0087] Fig. 7 focuses on Mn, the element that has the greatest effect on hardenability, and shows the results of measuring the Mn concentration (weight percent) at any depth perpendicular to the surface for several standard materials with different Mn concentrations and a standard material after vacuum carburization at the same vacuum level. From the results shown in Figs. 6 and 7, it can be inferred that alloy elements such as Ni, Cr, and Mo compensate for the decrease in hardenability due to Mn sublimation. In other words, this shows that the hardenability of the standard and standard materials is independent of the Mn concentration. Note that the evaluation in Fig. 7 does not specify the materials that correspond to the present invention.

[0088] Figure 8 is a graph showing the Mn concentration at different depths perpendicular to the surface after vacuum carburizing at different vacuum levels. Focusing on Mn, the element that has the greatest impact on hardenability, the depth and Mn concentration were measured. Specifically, the graph shows the depth and Mn concentration for SCM420H test pieces at different vacuum levels. Note that the Mn concentration of SCM420H vacuum-carburized at a vacuum level of 20 Pa significantly decreases around 6 μm. This is the grain boundary. Unlike the orderly atomic arrangement within crystal grains, the atomic arrangement within the grain boundary is disordered. Therefore, atoms within the grain boundary can easily diffuse, resulting in a significant decrease in the Mn concentration due to sublimation.

[0089] It was confirmed that the test specimens rated GOOD were configured in a state where the Mn concentration was maintained without a decreasing trend in the depth region deeper than 10 μm and shallower than 15 μm. It was confirmed that the test specimens rated BAD were configured in a state where the Mn concentration was not maintained but tended to decrease from the depth of 15 μm to a depth of 10 μm. It was confirmed that the test specimens rated GOOD were configured in a state where the Mn concentration was maintained without a decreasing trend in the depth region deeper than 8 μm and equal to or less than 10 μm. It was confirmed that the test specimens rated BAD were configured in a state where the Mn concentration was not maintained but tended to decrease from the depth region 10 μm to 8 μm. It was confirmed that the test specimens rated GOOD were configured in a state where the Mn concentration was maintained without a decreasing trend in the depth region deeper than 5 μm and equal to or less than 8 μm.

[0090] Fig. 10 is a flowchart showing the steps of the vacuum carburization treatment according to an embodiment of the present invention. Fig. 11 is a graph showing the change in treatment temperature in the vacuum carburization treatment according to an embodiment of the present invention.

[0091] The evacuation step S1 is a step of creating a vacuum inside the carburizing furnace. In the evacuation step S1, the carburizing furnace containing the gear 1 (see FIG. 12) is depressurized to a predetermined vacuum level by a vacuum pump until time t1. The heating step S2 is a step of raising the temperature inside the carburizing furnace, which has been depressurized to the predetermined vacuum level, to a predetermined carburizing temperature Tm1. In the heating step S2, the carburizing furnace is heated to the carburizing temperature Tm1 by a heater between time t1 and time t2. The carburizing temperature Tm1 is, for example, 930°C. The carburizing furnace is then maintained at the carburizing temperature Tm1 from time t2 to time t3 so that the gear 1 contained therein is uniformly heated to the carburizing temperature Tm1. The structure of the gear 1 is transformed to austenite by heating it to the carburizing temperature Tm1.

[0092] The carburizing step S3 is a step of impregnating and diffusing carbon (C) into the surface of the gear 1, including the tooth flank 2a of the tooth 2. In the carburizing step S3, a hydrocarbon gas is supplied into the carburizing furnace. In the carburizing furnace, carbon is generated by thermal decomposition of the hydrocarbon gas. Between time t3 and time t4, the carbon penetrates from the surface of the gear 1, including the tooth flank 2a, into the interior of the gear 1. The C impregnated into the interior of the gear 1 diffuses from the surface toward the interior of the gear 1. The degree of vacuum may be sufficient to suppress the formation of an oxide film on the surface of the gear 1. In the vacuum step S1, the temperature-raising step S2, and the carburizing step S3, oxygen is evacuated from the carburizing furnace to suppress the formation of an oxide film and intergranular oxidation on the surface of the gear 1. In the carburizing step S3, the carbon required for complete hardening is impregnated from the surface of the gear 1.

[0093] The quenching step S4 increases the surface hardness of the carburized gear 1. In the quenching step S4, the temperature of the structure of the gear 1, which has been transformed into austenite, is adjusted to the quenching temperature Tm2, ​​and then the structure is cooled, for example, by oil cooling. This transforms the structure of the gear 1 from austenite to martensite, the hardness of which corresponds to the amount of carburization. The quenching temperature Tm2 is, for example, 850°C.

[0094] In the above-described embodiment, the vacuum carburizing treatment involves reducing the pressure inside the carburizing furnace in the vacuum step S1, and then increasing the temperature inside the carburizing furnace in the temperature-raising step S2. However, the vacuum carburizing treatment may also involve increasing the temperature in the temperature-raising step, and then reducing the pressure in the vacuum step.

[0095] In the above-described embodiment, the vacuum carburization treatment involves raising the temperature to the carburization temperature Tm1 in the temperature-raising step S2, and then impregnating C in the carburization step S3. However, the vacuum carburization treatment may involve raising the temperature to a temperature different from the carburization temperature in the temperature-raising step, and then adjusting the temperature to the carburization temperature in the carburization step.

[0096] In the above-described embodiment, the Mn concentration of the gear 1 is adjusted by adjusting the degree of vacuum during the vacuum carburization treatment. However, the gear may be configured to adjust the sublimation of Mn by adjusting the carburization temperature in addition to the degree of vacuum during the vacuum carburization treatment. [Explanation of symbols]

[0097] 1. Gears 2 teeth 2a Tooth surface 10 Engine Unit 11 Engine body 12. Transmission 20 Saddle-type vehicle t1, t2, t3, t4, t5 time Tm1 Carburizing temperature Tm2 Quenching temperature

Claims

1. A gear that has been subjected to vacuum carburizing treatment, The gear is (a) The gear is made of a standard or standard material containing Mn and at least one of Cr, Mo, Si, and Ni in a region of 10 μm or less in depth in a direction perpendicular to the surface of the gear, and (b) the Mn concentration is maintained in a region of more than 10 μm and less than 15 μm in depth in a direction perpendicular to the surface, and the gear is used in a state where the tooth surface of the gear is not ground or polished after the vacuum carburizing treatment. Gears that have undergone vacuum carburizing treatment.

2. A gear that has been subjected to the vacuum carburizing treatment according to claim 1, The gear is (c) A gear that has been subjected to vacuum carburizing treatment and is configured so that the Mn concentration is maintained in a region that is deeper than 8 μm and not more than 10 μm deep in a direction perpendicular to the surface of the gear.

3. A gear that has been subjected to the vacuum carburizing treatment according to claim 2, The gear is (d) the Mn concentration is maintained in a region having a depth of more than 5 μm and not more than 8 μm in a direction perpendicular to the surface of the gear; Gears that have undergone vacuum carburizing treatment.

4. A gear that has been subjected to the vacuum carburizing treatment according to claim 1, A gear that has been subjected to vacuum carburizing treatment, wherein the amounts of Si, Cr, Mo, Ni, and Mn in a region having a depth of 10 μm or less in a direction perpendicular to the surface of the gear are the following weight percents: Si: more than 0.35 Cr: more than 0.35 Mo: more than 0.25 Ni: more than 0.25 Mn: more than 0.60

5. The engine body and a transmission that changes the output rotation speed of the engine body; and a gear subjected to vacuum carburizing treatment according to claim 1 or 2, which is lubricated with a lubricating oil that lubricates both the engine body and the transmission. Engine unit.

6. A straddle-type vehicle having gears that have been subjected to the vacuum carburizing treatment according to claim 1 or 2 in a state of meshing with each other.

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