Vacuum carburized gear, engine unit having same, and saddle-type vehicle

Abstract

The invention relates to a vacuum carburized gear, an engine unit having the same, and a saddle type vehicle. The present teachings are directed to providing a vacuum carburized gear in which wear resistance, rolling contact fatigue strength and productivity of the gear are improved while reducing carbon dioxide emissions during manufacturing. The vacuum carburized gear (1) is configured as follows: (a) the gear is made of a standard material or a standard material containing Mn and Mn and at least one of Cr, Mo, Si or Ni in a region having a depth of 10 [mu] m or less in a direction perpendicular to the surface of the gear (1); and (b) the Mn concentration is maintained in a region having a depth greater than 10 [mu] m and less than 15 [mu] m in the direction perpendicular to the surface. The gear 1 is used without the vacuum carburized tooth flank thereof being subjected to grinding or polishing.

Description

TECHNICAL FIELD

[0001] The present teachings relate to a gear treated by vacuum carburization (vacuum carburized gear), an engine unit having a gear treated by vacuum carburization, and a saddle-riding type vehicle having a gear treated by vacuum carburization. BACKGROUND

[0002] Generally, gas carburization treatment is performed at high temperature under atmospheric pressure, which makes it easy for the surface of a workpiece to be processed to be oxidized. It is known that such surface oxidation reduces the hardenability of the surface layer and reduces the strength of the workpiece due to the loss of alloying elements. As a method of mitigating the deterioration of the strength of the workpiece, vacuum carburization treatment is employed, or a high-strength steel having reduced oxidation-promoting elements and added non-oxidizing elements is employed (see Patent Document 1, paragraph

[0003] ).

[0003] Carburization treatment includes, for example, gas carburization treatment and vacuum carburization treatment. Vacuum carburization treatment has the effects described below compared to gas carburization treatment. Vacuum carburization treatment allows a higher carburization temperature, so that a carburized part having a predetermined carbon concentration can be obtained in a shorter time. Further, vacuum carburization treatment can suppress internal oxidation that occurs during carburization treatment, so that a carburized part having high rolling contact fatigue strength can be more easily obtained. Further, in vacuum carburization treatment, the treatment is performed under reduced pressure, which requires a reduction in the amount of atmospheric gas and does not require the combustion of exhaust gas, so that carbon dioxide emissions can be reduced (see Patent Document 2, paragraph

[0002] ).

[0004] Patent Document 3 describes an example in which vacuum carburization treatment is applied to a disc-shaped steel containing Mn. From Patent Document 3, it is understood that the disc-shaped steel is a gear. Figure 5 It can be inferred that the disc-shaped steel is a gear. The steel carburization method described in Patent Document 3 suppresses the sublimation of Mn from the steel during heating in a carburization furnace, and also suppresses the attachment of Mn to a high-frequency coil in the carburization furnace. By suppressing the attachment of Mn to the high-frequency coil, the steel in the carburization furnace is stably heated to a temperature above the A3 transformation temperature at which the steel transforms into austenite required for quenching, so that a desired carburization quality is achieved (see Patent Document 3, paragraphs

[0013] to

[0019] and Figure 5 ).

[0005] Patent Document 4 proposes a steel gear, a gear steel, and a manufacturing method of a steel gear for use as a part of a motorcycle or the like. It proposes a steel gear, a gear steel, and a manufacturing method of a steel gear in which the gear exhibits reduced heat treatment distortion during a surface hardening treatment by carbonitriding and ensures excellent dimensional accuracy of the gear part while also providing the strength required of the gear as such, particularly exhibiting superior properties of resistance to flaking damage of the tooth surface in the surface layer of the gear. The technology of Patent Document 4 reduces the dependency on the type of carburizing treatment and the manufacturing method by adjusting the composition of the gear steel (see Patent Document 4, paragraph

[0006] ).

[0006] List of Citations

[0007] Patent Documents

[0008] Patent Document 1: Japanese Patent Laid-Open No. 2003-193128

[0009] Patent Document 2: Japanese Patent Laid-Open No. 2016-194156

[0010] Patent Document 3: Japanese Patent Laid-Open No. 2015-183226

[0011] Patent Document 4: Japanese Patent Laid-Open No. 2006-328484 SUMMARY

[0012] TECHNICAL PROBLEM

[0013] In order to achieve carbon neutrality, it is necessary to reduce carbon dioxide emissions. For example, in the manufacturing process of a gear used in a straddle-type vehicle such as a motorcycle, a gas carburizing treatment is generally applied to a gear made of a readily available, conforming material or a standard material. In order to reduce carbon dioxide emissions, it can be considered to adopt a vacuum carburizing treatment in the manufacturing process of a gear used in a motorcycle. By adopting a vacuum carburizing treatment, it is expected that the wear resistance and rolling contact fatigue strength of the gear can be improved while reducing carbon dioxide emissions.

[0014] When vacuum carburizing is employed, a gear made of a material conforming to a specification or a standard material that is easily available and used in gas carburizing can be considered. This method is expected to maintain productivity even when vacuum carburizing is employed, compared to a gear made of a special material that is difficult to obtain. Furthermore, if grinding and polishing of the gear tooth surface can be omitted after vacuum carburizing, productivity is expected to be improved even when vacuum carburizing is employed. Furthermore, by employing a gear having improved wear resistance and rolling contact fatigue strength and reduced carbon dioxide emissions in an engine unit that uses the same lubricating oil for engine body lubrication and transmission lubrication, it is expected that contamination of the lubricating oil by wear debris will be suppressed, thereby reducing the influence of wear debris on the engine body, while reducing the amount of carbon dioxide emitted during engine unit manufacturing. By employing a gear having improved wear resistance and rolling contact fatigue strength and reduced carbon dioxide emissions in a saddle-type vehicle, it is expected that an increase in gear backlash due to wear will be suppressed, thereby reducing sound variation caused by an increase in backlash, while reducing the amount of carbon dioxide emitted during saddle-type vehicle manufacturing.

[0015] An object of the present teachings is to provide a vacuum carburized gear in which wear resistance, rolling contact fatigue strength, and productivity of the gear are improved, while reducing carbon dioxide emissions during manufacturing.

[0016] Another object of the present teachings is to provide an engine unit having a vacuum carburized gear in which contamination of lubricating oil by wear debris from the gear is suppressed, thereby reducing the influence of wear debris on the engine body, while reducing carbon dioxide emissions during manufacturing.

[0017] Still another object of the present teachings is to provide a saddle-type vehicle having a vacuum carburized gear in which an increase in gear backlash due to wear of the gear is suppressed, thereby suppressing sound variation caused by an increase in backlash, while reducing carbon dioxide emissions during manufacturing.

[0018] Solution to the problem

[0019] In order to employ vacuum carburizing in the manufacturing process of a gear or shaft for use in a motorcycle, test pieces made of a material conforming to a specification employed in gas carburizing were subjected to vacuum carburizing. Assuming that these test pieces are used in a motorcycle, tests were performed on wear resistance and rolling contact fatigue strength. However, this showed that there was a case where the number of cycles was reduced compared to a gas carburized part (see Comparative Examples 1 and 2 in [2]). Figure 1 [2]).

[0020] To compare the rolling contact fatigue strength between the gas carburized specimen and the vacuum carburized specimen, the number of cycles until pitting corrosion occurred was measured. In the Weibull distribution based on the median rank method, the number of cycles at the same cumulative failure probability for each test was less for the vacuum carburized specimen than for the gas carburized specimen (see Table 1). In other words, this shows that the rolling contact fatigue strength of the vacuum carburized specimen is lower than that of the gas carburized specimen. Figure 1 [1] Comparative Examples 1 and 2 in Table 1 and Table 2). In other words, this shows that the rolling contact fatigue strength of the vacuum carburized specimen is lower than that of the gas carburized specimen.

[0021] To investigate the reason for the difference in the rolling contact fatigue strength between the gas carburized specimen and the vacuum carburized specimen, the amount of wear of each specimen was measured. The number of cycles at the same wear depth for each test was less for the vacuum carburized specimen than for the gas carburized specimen (see Table 2). In other words, this shows that the wear resistance of the vacuum carburized specimen is lower than that of the gas carburized specimen. Figure 1

[0022] These phenomena were inferred in the following manner. In a specimen subjected to vacuum carburizing treatment to improve the rolling contact fatigue strength, there are cases where a region having a lower hardness (referred to as an incomplete quenching structure) is irregularly dispersed on the surface. When the incomplete quenching structure is irregularly dispersed on the surface of the specimen, the incomplete quenching structure having a lower hardness can cause wear due to engagement with other members. Therefore, it can be inferred that, in a specimen in which wear has progressed, pitting corrosion is caused, thereby leading to a decrease in the rolling contact fatigue strength.

[0023] Therefore, in order to investigate the reason for the irregular dispersion of the incomplete quenching structure, the inventors of the present teaching used specimens made of a plurality of types of conforming materials and standard materials having different compositions, and subjected them to vacuum carburizing treatment under a plurality of conditions. In addition, the inventors evaluated the surface layer condition of each specimen, focusing on the incomplete quenching structure. The results showed that, while the vacuum level was considered to have some influence, the irregularly dispersed incomplete quenching structure largely depended on the material (see Table 3). In other words, for both the conforming materials and the standard materials, the evaluation results of the incomplete quenching structure varied depending on the vacuum level and the content of its alloying elements. Note that, Figure 1 [3] the material evaluated as GOOD in Table 3 is not intended to identify a material corresponding to the present teaching. Figure 1 [3] the material evaluated as GOOD in Table 3 is not intended to identify a material corresponding to the present teaching.

[0024] In this context, the inventors of the present teaching have made a detailed examination of the incomplete quenching structure irregularly dispersed on the gear tooth surface of a conforming material or a standard material. “JIS G4052:2016 Hardenable Structural Steel (H-Steel) Table 2 - Chemical Composition” specifies the proportions of alloying elements in structural steels having guaranteed hardenability, which are mainly used for mechanical structures (see Table 4). Note that, Figure 1 ​[5]). Thus, in the quenched and tempered structural steels including SMn443H and SCM420H, the hardenability is guaranteed by adding alloying elements such as Si, Mn, Ni, Cr, and Mo. The alloy contents of the elements Ni, Si, Cr, Mo, and Mn, which affect the hardenability, and their respective multiples are different from each other (see Figure 1 [4]). Furthermore, in the various types of conforming materials and standard materials, the contents of each of the alloying elements Ni, Si, Cr, Mo, and Mn, which affect the hardenability, are different (see Figure 1 [5]). Focusing on the alloying element Mn, which has the greatest effect on the hardenability, the Mn concentration (in weight percent) at any depth from the surface after the vacuum carburizing treatment was measured for a variety of conforming materials and standard materials (see Figure 1 [6]).

[0025] The conforming materials SMn443H, SCM420H, and SNCM220H, and the standard material KKG8 of the high-toughness gear steel KKG (registered trademark) -T series of 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) were all vacuum carburized at a vacuum level of 20 Pa, and SMn443H, which has the highest Mn concentration, was evaluated as BAD because it exhibited a significant reduction in the Mn concentration, which can affect the hardenability of the sample. On the other hand, the Mn concentrations of SCM420H, SNCM220H, and KKG8 also exhibited similar reductions. SCM420H was evaluated as BAD because it contains small amounts of added Ni and Mo, making it possible for Mn to affect the hardenability. In comparison with SCM420H, SNCM220H has a higher content of Ni, which indicates that the sublimated Mn is compensated for by Ni, Cr, and Mo. In comparison with SCM420H, KKG8 has a higher content of Mo, which indicates that the sublimated Mn is compensated for by Cr and Mo. This can also be confirmed by the fact that in SMn443H, the sublimated Mn is not sufficiently compensated for by Ni, Cr, Mo, since it contains essentially no Ni, Cr, or Mo. From the above, it can be inferred that the alloying elements such as Ni, Cr, and Mo compensate for the reduction in hardenability caused by the sublimation of Mn. In other words, it is inferred that the hardenability of the conforming and standard materials is not affected only by the Mn concentration. Note that, Figure 1 [6] and 1[7] are not intended to identify materials corresponding to the present teachings.

[0026] The SCM420H specimen subjected to the vacuum carburizing process at the vacuum level of 20 Pa was evaluated as BAD because the Mn concentration thereof was significantly reduced, which can affect the hardenability of the specimen. On the other hand, the SCM420H specimens subjected to the vacuum carburizing process at the vacuum levels of 500 Pa and 1500 Pa, respectively, were evaluated as GOOD because they exhibited a gradual reduction in the Mn concentration, which is less likely to affect the hardenability of the specimen (see Table 1). Figure 1 [7]). Thus, in the SCM420H (1500 Pa, 500 Pa), SNCM220H, and KKG8, the Mn concentration was maintained in the region having a depth of greater than 10 μm and less than 15 μm in the direction perpendicular to the vacuum carburized surface.

[0027] In statistics, the coefficient of variation (coefficient of variation = standard deviation / average x 100) is used as an index for evaluating the dispersion or variability of data. When the coefficient of variation is less than 10%, the data is evaluated as statistically stable with less variability. In particular, in quality control in manufacturing and similar industries, data with a coefficient of variation of less than 5% is evaluated as statistically stable with less variability. Furthermore, in linear regression analysis using the least squares method, the coefficient of determination (R 2 ) is used as an index for evaluating the explanatory power of a regression model. When R 2 is less than 0.3, the regression model is evaluated as statistically weak in explanatory power. Furthermore, in linear regression analysis using the least squares method, the p-value is used as an index for evaluating whether the effect of depth (independent variable) on the Mn concentration (dependent variable) is significant. When the p-value is greater than 0.05, the effect of depth on the Mn concentration is evaluated as statistically insignificant.

[0028] In Figure 1 [7], the coefficient of variation of the SCM420H (1500 Pa) and the SCM42H (500 Pa) was 4.06% and 3.33%, respectively, in the region having a depth of from 5 μm to 15 μm. In other words, the Mn concentration of the SCM420H (1500 Pa) and the SCM42H (500 Pa) fell within a range evaluated as statistically stable with less variability in the region having a depth of from 5 μm to 15 μm.

[0029] In Figure 1[7] In the region from the 15 μm depth to the 5 μm depth, the coefficient of determination of SCM420H (1500 Pa) and SCM420H (500 Pa) were 0.00086 and 0.00043, respectively. In addition, in the region from the 15 μm depth to the 5 μm depth, the p-value of SCM420H (1500 Pa) and SCM42H (500 Pa) were 0.4725 and 0.8733, respectively. Therefore, in the region from the 15 μm depth to the 5 μm depth, in the regression model for SCM420H (1500 Pa) and SCM420H (500 Pa), it was assessed that the relationship between the Mn concentration and the depth from the surface was not statistically well supported, and thus the effect of the depth on the Mn concentration was assessed as not significant. In other words, in the region from the 15 μm depth to the 5 μm depth, the Mn concentration of SCM420H (1500 Pa) and SCM42H (500 Pa) was assessed as not having any significant downward or upward trend.

[0030] SCM420H contains Ni, Cr, and Mo in addition to Mn; however, the amount of Ni, Cr, and Mo is small. These results confirm that, in the case where the in-spec material and the standard material containing Ni, Si, Cr, Mo, and Mn, which are alloying elements that affect hardenability, are configured so that the Mn concentration is maintained in the region having a depth greater than 10 μm and less than 15 μm in the direction perpendicular to the vacuum carburized surface, the decrease in hardenability due to the sublimation of Mn is compensated for by Ni, Cr, and Mo. Note that, in the present test data, the contribution of the change in the amount of Si has not been confirmed. However, as Figure 1 [4] and 1 [5], the multiple of Si is greater than the multiple of Ni. Therefore, it is clear that similar performance can also be obtained by Si.

[0031] On the other hand, even in the case where the Mn concentration is maintained in the region having a depth greater than 10 μm and less than 15 μm, the decrease in hardenability due to the sublimation of Mn cannot be compensated for in the absence of alloying elements that affect hardenability. For example, in SMn443H (20 Pa), which contains substantially no alloying elements that affect hardenability, the Mn concentration is maintained in the region having a depth greater than 10 μm and less than 15 μm. However, in SMn443H (20 Pa), in the region having a depth less than 5 μm, the decrease in hardenability due to the sublimation of Mn is not compensated for by the alloying elements that affect hardenability, which results in the BAD assessment.

[0032] Based on a detailed examination of the above evaluation results, the inventors conceived a technical idea of adjusting the dispersion state of an alloy element that affects hardenability so as to compensate for a decrease in hardenability caused by Mn sublimation with at least one of the alloy elements Si, Ni, Cr, or Mo in a region 10 pm or less in depth in a direction normal to the surface of the gear subjected to vacuum carburizing.

[0033] Therefore, wear resistance and rolling contact fatigue strength tests were performed on a vacuum carburized test piece, as described below, assuming its use in a motorcycle. The vacuum carburized test piece was configured as follows: (a) it was made of a specification-compliant material and a standard material that contains Mn and at least one of Cr, Mo, Si, or Ni in a region 10 pm or less in depth in a direction normal to the vacuum carburized surface; and (b) in a region greater than 10 pm and less than 15 pm in depth in a direction normal to the surface, the Mn concentration was maintained. Furthermore, the tests were performed without polishing or grinding the surface of the vacuum carburized test piece. The results confirmed that the number of cycles was comparable to that of a gas carburized part, and the wear resistance was significantly improved (see Figure 1 [2], the present teaching).

[0034] A gear according to one embodiment of the present teaching can have the following configuration (1). (1) The gear is subjected to vacuum carburizing, wherein the vacuum carburized gear is configured as follows: (a) the gear is made of a specification-compliant material or a standard material that contains Mn and at least one of Cr, Mo, Si, or Ni in a region 10 pm or less in depth in a direction normal to the surface of the gear; and (b) in a region greater than 10 pm and less than 15 pm in depth in a direction normal to the surface, the Mn concentration is maintained, and the vacuum carburized tooth surface of the gear is not polished or ground.

[0035] The gear having such a configuration is subjected to vacuum carburizing treatment instead of gas carburizing treatment. Thus, carbon dioxide emission during manufacture can be reduced. As shown in the test results, in the gear having the above configuration, which is subjected to vacuum carburizing treatment and in which the Mn concentration is maintained in a region having a depth greater than 10 μm and less than 15 μm, the decrease in hardenability due to sublimation of Mn is compensated for by at least one of the alloying elements Si, Ni, Cr, or Mo. Thus, in the vacuum carburized gear having the above configuration, the wear resistance and the rolling contact fatigue strength can be improved without the need for grinding or polishing the vacuum carburized tooth surface thereof. Furthermore, the vacuum carburized gear can be used without the need for grinding or polishing the vacuum carburized tooth surface thereof. Thus, the vacuum carburized tooth surface of the gear having improved hardness and wear resistance is not subjected to grinding or polishing, thereby ensuring an improvement in productivity. Furthermore, depending on the required specifications, the tooth surface of the gear can be subjected to shot peening to further improve the rolling contact fatigue strength and the bending fatigue strength.

[0036] "at least one of Cr, Mo, Si, or Ni" means that two of Cr, Mo, Si, or Ni can be included. "at least one of Cr, Mo, Si, or Ni" means that three of Cr, Mo, Si, or Ni can be included. "at least one of Cr, Mo, Si, or Ni" means that all of Cr, Mo, Si, and Ni can be included.

[0037] In addition to the above configuration (1), the gear according to one embodiment of the present teaching can have the following configuration (2). (2) The vacuum carburized gear is configured as follows: (c) in a region having a depth greater than 8 μm and less than or equal to 10 μm in a direction perpendicular to the surface of the gear, the Mn concentration is maintained.

[0038] In the gear having such a configuration, the Mn concentration in a region having a depth greater than 8 μm and less than or equal to 10 μm in a direction perpendicular to the surface (feature (c)) is adjusted, where the region is adjacent to the region having a depth greater than 10 μm and less than 15 μm described in feature (b) and is closer to the surface than the region in feature (b). Thus, as shown in the test results, in the gear having the above configuration, which is subjected to vacuum carburizing treatment and in which the Mn concentration is maintained in a region having a depth greater than 8 μm and less than or equal to 10 μm, the decrease in hardenability due to sublimation of Mn is compensated for by at least one of the alloying elements Si, Ni, Cr, or Mo. This allows further improvement in the wear resistance and the rolling contact fatigue strength of the gear without the need for grinding or polishing the vacuum carburized tooth surface thereof.

[0039] In addition to the above-described configurations (1) or (2), the gear according to one embodiment of the present teaching can further have the following configuration (3). (3) The vacuum carburized gear is configured as follows: (d) the Mn concentration is maintained in a region having a depth greater than 5 μm and less than or equal to 8 μm in a direction perpendicular to the surface of the gear.

[0040] In the gear having such a configuration, the Mn concentration in a region having a depth greater than 5 μm and less than or equal to 8 μm in a direction perpendicular to the surface (feature (d)) is adjusted, wherein the region is adjacent to the region having a depth greater than 8 μm and less than or equal to 10 μm described in feature (c) and is closer to the surface than the region in feature (c). Thus, as shown in the test results, in the vacuum carburized gear having the above-described configuration and having the Mn concentration maintained in a region having a depth greater than 5 μm and less than or equal to 8 μm, the decrease in hardenability caused by the sublimation of Mn is compensated by at least one of the alloying elements Si, Ni, Cr or Mo. Thus, in the vacuum carburized gear having the Mn concentration maintained in a region having a depth greater than 5 μm and less than or equal to 8 μm, its wear resistance and rolling contact fatigue strength can be further improved.

[0041] In addition to configurations (1), (2) or (3), the gear according to one embodiment of the present teaching can further have the following configuration (4). (4) In a region having a depth of 10 μm or less in a direction perpendicular to the surface of the gear, the amounts (in weight percent) of Si, Cr, Mo, Ni and Mn are as follows: Si is greater than 0.35; Cr is greater than 0.35; Mo is greater than 0.25; Ni is greater than 0.25; and Mn is greater than 0.60.

[0042] According to the test results and evaluation results in the present study, in a region having a depth of 10 μm or less in a direction perpendicular to the surface of the gear, the amount sufficient to compensate for the decrease in hardenability caused by the sublimation of Mn by vacuum carburizing treatment is examined. For Si, according to the evaluation results of SMn443H (vacuum level: 20 Pa) and SCM420H (vacuum level: 20 Pa), a Si concentration of 0.15 to 0.35 weight percent is unlikely to compensate for the decrease in hardenability. Thus, the Si amount sufficient to compensate for the decrease in hardenability is considered to be greater than 0.35 weight percent.

[0043] For Mo, according to the evaluation results of SCM420H (vacuum level: 20 Pa), a Mo concentration of 0.15 to 0.25 weight percent is unlikely to compensate for the decrease in hardenability. Thus, the Mo amount sufficient to compensate for the decrease in hardenability is considered to be greater than 0.25 weight percent.

[0044] For Cr, according to the evaluation results of SMn443H (vacuum level: 20 Pa), a Cr concentration of 0.35 weight percent or less is unlikely to compensate for the decrease in hardenability. Therefore, the amount of Cr sufficient to compensate for the decrease in hardenability is considered to be greater than 0.35 weight percent.

[0045] For Ni, according to the evaluation results of SMn443H (vacuum level: 20 Pa), a Ni concentration of 0.25 weight percent or less is unlikely to compensate for the decrease in hardenability. Therefore, the amount of Ni sufficient to compensate for the decrease in hardenability is considered to be greater than 0.25 weight percent.

[0046] For Mn, according to the evaluation results of SNCM220H (vacuum level: 20 Pa), the amount of Mn sufficient to compensate for the decrease in hardenability is considered to be 0.60 weight percent or more.

[0047] Actual in-spec materials and standard materials contain multiple alloying elements among Si, Ni, Cr, and Mo, and therefore it is difficult to determine the effect on hardenability based on the content of a single alloying element. For example, it is determined whether the decrease in hardenability caused by the sublimation of Mn is sufficiently compensated for by at least one of Si, Ni, Cr, or Mo based on the amount of incomplete quench structure per unit area to achieve an acceptable quench (carburizing) state (see the Figure 1 [3]) When the amount of incomplete quench structure per unit area on the vacuum carburized workpiece is less than the reference value, it is considered that the decrease in hardenability caused by the sublimation of Mn is sufficiently compensated for by the alloying elements contained in the workpiece to achieve an acceptable quench state. In other words, the type and amount of alloying elements contained in the workpiece having an amount of incomplete quench structure less than the reference value after vacuum carburizing treatment are the combination and amount of alloying elements required to sufficiently compensate for the decrease in hardenability caused by the sublimation of Mn to achieve an acceptable quench state.

[0048] As publicly known technical knowledge, the sublimation ratio of Mn in a steel appropriately vacuum carburized is 0.1 to 0.3 weight percent. The sublimation ratio of Mn can be calculated from the vapor pressure of Mn. Equations related to the sublimation of Mn, i.e., the Clausius-Clapeyron equation for determining the vapor pressure of Mn and the Knudsen- Langmuir equation for determining the mass flux (i.e., mass per unit time through a unit cross-sectional area perpendicular to the flow) from the vapor pressure of Mn, are referred to as publicly known technical knowledge. The sublimation ratio of Mn can be calculated with the Clausius-Clapeyron and Knudsen-Langmuir equations.

[0049] At a vacuum carburizing treatment temperature of 1000°C, the vapor pressure of Mn is 3.42 Pa as determined from the Clausius-Clapeyron equation. For a mass of 1.2 kg, a surface area of 400 cm 2of 0.19 weight percent, based on a Mn vapor pressure of 3.42 Pa, as determined according to the Knudsen- Langmuir equation. As shown in FIG. 6, the Mn sublimation ratio on the surface of the SCM420H (1500 Pa) and the SCM42H (500 Pa) evaluated as GOOD was about 0.2 to 0.3 weight percent. Thus, the Mn sublimation ratio of a properly vacuum carburized steel is 0.1 to 0.3 weight percent, from the knowledge of the art, the calculation results, and the measurement results. Figure 9 [7], the Mn sublimation ratios on the surfaces of the SCM420H (1500 Pa) and the SCM42H (500 Pa) evaluated as GOOD were both about 0.2 to 0.3 weight percent. Thus, the Mn sublimation ratio of a properly vacuum carburized steel is 0.1 to 0.3 weight percent, from the knowledge of the art, the calculation results, and the measurement results.

[0050] Grossman equation, which is known as the knowledge of the art, is a basic equation for quantifying hardenability of a steel. The Grossman equation calculates a diameter D at which a 50% martensite phase transformation occurs in the center of a round bar made of a metallic material, based on concentrations of alloying elements contained in the metallic material. The larger the diameter D, the higher the hardenability is indicated. In other words, the hardenability of the metallic material can be evaluated using the diameter D. The following is an example calculation using coefficients for each alloying element, which are determined by Hollomon and Jaffe through experiments:

[0051] D = D1 x (1 + 0.64 Si) x (1 + 4.10 Mn) x (1 + 2.83 P) x (1 - 0.62 S) x (1 + 2.33 Cr) x (1 + 0.52 Ni) x (1 + 3.14 Mo) x (1 + 0.27 Cu) x (1 + 1.5 (0.90 - B)),

[0052] where D is a hypothetical critical diameter, D1 is a critical diameter taking into account a particle size (for a case where the grain size is 4, a critical diameter of 10.95 is applied), and C, Si, Mn, P, S, Cr, Ni, Mo, Cu, and B represent weight percentages of respective alloying elements.

[0053] As shown in FIG. 6, the Mn sublimation ratio on the surface of the SCM420H (1500 Pa) and the SCM42H (500 Pa) evaluated as GOOD was about 0.2 to 0.3 weight percent. Thus, the Mn sublimation ratio of a properly vacuum carburized steel is 0.1 to 0.3 weight percent, from the knowledge of the art, the calculation results, and the measurement results. Figure 7 As shown in FIG. 6, the Mn sublimation ratio on the surface of the SCM420H (1500 Pa) and the SCM42H (500 Pa) evaluated as GOOD was about 0.2 to 0.3 weight percent. Thus, the Mn sublimation ratio of a properly vacuum carburized steel is 0.1 to 0.3 weight percent, from the knowledge of the art, the calculation results, and the measurement results. Figure 1) are assumed to be as follows: 100.13 (BAD evaluation) for SCM420H; 25.66 (BAD evaluation) for SMn443H; 117.31 (GOOD evaluation) for SNCM220H; and 256.28 (GOOD evaluation) for KKG8. Thus, the threshold of the assumed critical diameter D for evaluating whether the decrease in hardenability caused by sublimation of Mn by vacuum carburizing treatment is sufficiently compensated by the alloying elements to achieve an acceptable quenched state can be inferred to fall within a range greater than the assumed critical diameter D (BAD evaluation) of 100.13 for SCM420H and less than or equal to the assumed critical diameter D (GOOD evaluation) of 117.31 for SNCM220H. In other words, when the assumed critical diameter D calculated based on the concentrations of the alloying elements using the Grossman equation is greater than the threshold, it can be evaluated that the decrease in hardenability caused by sublimation of Mn is sufficiently compensated by the alloying elements to achieve an acceptable quenched state. In this way, the contents of the alloying elements (such as Si, Mn, Ni, Cr, and Mo) required to compensate for the decrease in hardenability caused by sublimation of Mn can be calculated. According to the above-mentioned known technical knowledge, based on the technical idea and evaluation results of the present teachings, within a region of a depth of 10 μm or less in a direction normal to the surface of the gear, the content of the alloying elements sufficient to compensate for the decrease in hardenability caused by sublimation of Mn by vacuum carburizing treatment to achieve an acceptable quenched state can be defined or understood in the following manner.

[0054] The present teachings are based on the technical idea of adjusting the distribution state of the alloying elements that affect hardenability so that, within a region of a depth of 10 μm or less in a direction normal to the surface of the vacuum carburized gear, the decrease in hardenability caused by sublimation of Mn is sufficiently compensated by at least one of the alloying elements Si, Ni, Cr, or Mo to achieve an acceptable quenched state. As shown by the test results, in the vacuum carburized gear, its wear resistance and rolling contact fatigue strength can be improved. Thus, in the present specification, within a region of a depth of 10 μm or less in a direction normal to the surface of the gear, the content of the alloying elements sufficient to compensate for the decrease in hardenability caused by sublimation of Mn by vacuum carburizing treatment to achieve an acceptable quenched state can be defined based on the evaluation results in the following manner.

[0055] For Si, an amount sufficient to compensate for the reduction in hardenability to achieve an acceptable as-hardened condition can be defined as greater than 0.35 weight percent in a conforming or standard material. For Si, if specified as 0.15 to 0.35 weight percent, 0.15 weight percent or less, or 0.35 weight percent or less, it can be defined as insufficient to compensate for the reduction in hardenability to achieve an acceptable as-hardened condition. For Ni, an amount sufficient to compensate for the reduction in hardenability to achieve an acceptable as-hardened condition can be defined as greater than 0.25 weight percent in a conforming or standard material. For Ni, if specified as 0.25 weight percent or less, it can be defined as insufficient to compensate for the reduction in hardenability to achieve an acceptable as-hardened condition. For Cr, an amount sufficient to compensate for the reduction in hardenability to achieve an acceptable as-hardened condition can be defined as greater than 0.35 weight percent in a conforming or standard material. For Cr, if specified as 0.35 weight percent or less, it can be defined as insufficient to compensate for the reduction in hardenability to achieve an acceptable as-hardened condition. For Mo, an amount sufficient to compensate for the reduction in hardenability to achieve an acceptable as-hardened condition can be defined as greater than 0.25 weight percent in a conforming or standard material. For Mo, if specified as 0.15 to 0.2 weight percent, or 0.25 weight percent or less, it can be defined as insufficient to compensate for the reduction in hardenability to achieve an acceptable as-hardened condition. For Mn, an amount sufficient to compensate for the reduction in hardenability to achieve an acceptable as-hardened condition can be defined as 0.60 weight percent or greater in a conforming or standard material.

[0056] An engine unit according to one embodiment of the present teachings can include an engine body, a transmission configured to change an output rotational speed of the engine body, and a vacuum carburized gear according to any one of configurations (1), (2), (3), and (4), the gear being lubricated using lubricating oil that lubricates both the engine body and the transmission.

[0057] The gear configured as [1] is subjected to vacuum carburizing instead of gas carburizing. Thus, carbon dioxide emission during manufacturing can be reduced. As shown in the test results, in the vacuum carburized gear, the wear resistance and the rolling contact fatigue strength can be improved. The vacuum carburized gear is used without grinding or polishing the vacuum carburized tooth surface. Thus, the tooth surface of the gear having improved hardness and wear resistance does not need to be ground or polished, thereby ensuring an improvement in productivity. Further, in the engine unit having this configuration, the vacuum carburized gear is lubricated using lubricating oil that lubricates both the engine main body and the transmission. Thus, contamination of the lubricating oil with wear debris can be suppressed, thereby reducing the influence of the wear debris on the engine main body, while reducing the amount of carbon dioxide emitted during manufacturing of the engine unit. The challenge of wear debris from the transmission affecting the engine main body is unique to the engine unit in which the gear of the transmission is lubricated with lubricating oil that lubricates both the engine main body and the transmission.

[0058] The saddle-type vehicle according to one embodiment of the present teachings can include the vacuum carburized gear according to any one of configurations (1), (2), (3), and (4) in a meshed state with each other.

[0059] The gear having configuration (1) is subjected to vacuum carburizing instead of gas carburizing. Thus, carbon dioxide emission during manufacturing can be reduced. As shown in the test results, in the vacuum carburized gear, the wear resistance and the rolling contact fatigue strength can be improved. In the vacuum carburized gear, the vacuum carburized tooth surface does not need to be ground or polished. Further, the vacuum carburized gear is used without grinding or polishing the tooth surface. Thus, the tooth surface of the gear having improved hardness and wear resistance does not need to be ground or polished, thereby ensuring an improvement in productivity. Further, the saddle-type vehicle having this configuration can include the vacuum carburized gears in a meshed state with each other. Thus, an increase in the gear clearance due to wear can be suppressed, thereby suppressing a change in sound caused by the increase in the clearance, while reducing the amount of carbon dioxide emitted during manufacturing of the saddle-type vehicle. Unlike an automobile, the saddle-type vehicle does not have a cabin that encloses the rider. Thus, depending on the driving conditions, the rider is more likely to hear the sound of the gears. This is a challenge unique to the saddle-type vehicle.

[0060] The present teachings are based on the technical idea of adjusting the distribution state of alloy elements that affect the hardenability so as to compensate for the decrease in the hardenability due to the sublimation of Mn with at least one of the alloy elements Si, Ni, Cr, or Mo in a region 10 pm or less in depth in the direction normal to the surface of the vacuum carburized gear.

[0061] Patent Documents 1, 2, 3, and 4 do not disclose or suggest any relationship between the depth in the direction perpendicular to the surface and the Mn concentration. Patent Documents 1, 2, 3, and 4 also do not disclose or suggest any concept of adjusting the distribution state of alloying elements that affect the hardenability to compensate for the decrease in hardenability caused by the sublimation of Mn. Thus, it is difficult to derive the present teachings from Patent Documents 1, 2, 3, and 4.

[0062] [Material in conformity with a specification]

[0063] A material in conformity with a specification herein refers to a material defined in a material standard that specifies physical properties, mechanical properties, composition, shape, or other characteristics of an industrial material such as a metal. Material standards include, for example, industry association standards, regional standards, and national standards such as Japanese Industrial Standards (JIS), and international standards (ISO). The composition values (weight percentages) of alloying elements such as Mn, Cr, Mo, Si, and Ni, which are secondary components included in a steel material in conformity with a specification, are limited within a certain range. A material in conformity with a specification is easily available due to a large amount of circulation.

[0064] [Standard material]

[0065] A standard material herein refers to a material having a composition independently defined by a material manufacturer or the like. A standard material includes a material that is different in composition from a material in conformity with a specification and is provided as a standard product by each manufacturer. The composition values (weight percentages) of primary alloying elements such as Mn, Cr, Mo, Si, and Ni in a standard material fall within a certain range specified by the manufacturer. A standard material is easily available due to a large amount of circulation.

[0066] [Alloying element]

[0067] An alloying element herein refers to an element that is added to a metal material having any metal element as a primary component to make the metal material have certain properties. An alloying element is added to a metal material in a predetermined weight percentage. Alloying elements include, for example, C, Si, Mn, P, S, Cr, Mo, Ni, B, and Ti. A vacuum carburized gear can be made of a material in conformity with a specification or a standard material that contains Mn and contains at least one of Cr, Mo, Si, or Ni. The alloying elements Cr, Mo, Si, and Ni improve the hardenability. Thus, in a metal material containing alloying elements such as Cr, Mo, Si, and Ni, at least a part of the decrease in hardenability caused by the sublimation of Mn is compensated for by these alloying elements. A vacuum carburized gear can contain alloying elements other than Cr, Mo, Si, and Ni that improve the hardenability. A vacuum carburized gear can contain, for example, B, Ti, or other alloying elements.

[0068] [Vacuum carburizing treatment]

[0069] The vacuum carburizing treatment herein refers to a treatment in which a workpiece to be subjected to carburizing treatment and a hydrocarbon-based gas (representative example: acetylene) are heated in a carburizing furnace that is reduced to 10,000 Pa or less, thereby allowing carbon (C) to permeate and diffuse to the surface of the workpiece. The vacuum carburizing treatment has a lower carbon dioxide emission amount than a gas carburizing treatment in which a carburizing furnace is heated while being filled with carbon monoxide. In the vacuum carburizing treatment, the Mn concentration of the gear can be adjusted by adjusting the vacuum level. In the vacuum carburizing treatment, the Mn concentration of the gear can be adjusted by adjusting the carburizing temperature. In the vacuum carburizing treatment, the Mn concentration of the gear can be adjusted by adjusting the vacuum level and the carburizing temperature. In the vacuum carburizing treatment, the Mn concentration of the gear can be adjusted by adjusting other process control conditions.

[0070] [Vacuum level]

[0071] The vacuum level herein refers to the pressure in a space filled with a gas having a pressure lower than atmospheric pressure under standard conditions. The vacuum level is classified according to the pressure range. A low vacuum level refers to a pressure of, for example, 100 Pa or more. A medium vacuum level refers to a pressure of, for example, 0.1 Pa or more and less than 100 Pa. A high vacuum level refers to a pressure of less than 0.1 Pa.

[0072] [Quenching property]

[0073] The quenching property herein indicates the degree of ease of hardening by quenching, and refers to the relationship between the hardness of a quenched steel and the depth of hardening in a direction perpendicular to the surface of the steel. For example, under the same quenching conditions, a steel having a good quenching property is a material that is hardened deeper from the surface of the steel than other steels, or a material that has a higher hardness than other steels.

[0074] [Incomplete quenching region]

[0075] The incomplete quenching region herein refers to a region in which the proportion of martensite transformation is low due to a decrease in the concentration of alloying elements (such as C, Mn, Ni, Mo, Cr, and Si) that improve the quenching property of the steel. For example, a region that fails to transform into martensite but forms structures such as troostite and pearlite is referred to as an incomplete quenching region.

[0076] [Mn concentration is maintained]

[0077] The "Mn concentration is maintained" herein refers to a state in which the Mn concentration is evaluated to show a small variation statistically and does not have any significant downward trend as the depth decreases, within a region having an arbitrary depth in a direction perpendicular to the surface of the gear. The statistical evaluation is, for example, based on a coefficient of variation (coefficient of variation = standard deviation / average x 100) as an index for evaluating the dispersion or variability of data, on a determination coefficient (R2) as an index for evaluating the explanatory power of a regression model in a linear regression analysis using the least squares method, on a p-value as an index for evaluating whether the influence 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 within a region having different depths. The determination coefficient and the p-value are indices for statistically evaluating whether the Mn concentration within a region having different depths shows any significant downward or upward trend. When the coefficient of variation is less than or equal to 5%, the Mn concentration is evaluated to be statistically stable with a small variation. When the determination coefficient is less than or equal to 0.3, the regression model showing the relationship between the depth and the Mn concentration is evaluated to have a weak explanatory power. When the p-value is greater than or equal to 0.1, the influence of the depth on the Mn concentration is evaluated to be insignificant.

[0078] [Gear]

[0079] The gear herein can be a parallel-axes gear. The gear can have parallel axes and can be a spur gear, a helical gear, an internal gear, a rack, or the like. The gear can be an intersecting-axes gear. The gear can have intersecting axes and can be a bevel gear, a spiral bevel gear, a skew bevel gear, or the like. The gear can be a helical gear. The gear can have non-parallel, non-intersecting axes and can be a worm gear, a hypoid gear, or the like. The gear can be a involute gear. The gear can have an involute curve of tooth profile. The gear can be a cycloid gear. The gear can have a cycloid curve of tooth profile.

[0080] [Flank]

[0081] The flank herein refers to a surface located on a tooth of one gear and contacting a tooth of another gear when the two gears are engaged and one gear is transmitting rotational force to the other gear. The flank of one gear slides radially on the flank of the other gear while pressing against the flank of the other gear when one gear is transmitting rotational force to the other gear.

[0082] [Depth in a direction perpendicular to the surface of the gear]

[0083] The depth in the direction perpendicular to the surface of the gear in this context refers to the depth in the direction perpendicular to the tooth surface or the side surface of the gear after the vacuum carburizing treatment without machining, excluding the machined shaft insertion portion. In this specification, the depth in the direction perpendicular to the surface of the gear defines the range of the region indicating the vacuum carburizing state in the gear. For example, even in a state where the vacuum carburized gear is installed in the transmission and in use, the vacuum carburizing state of the tooth surface of the gear can be determined before use by checking the vacuum carburizing state in the range defined by the depth in the direction perpendicular to the side surface of the gear after the vacuum carburizing treatment without machining. The side surface of the gear is the surface visible when the gear is viewed in the direction of its rotational axis, and does not include the tooth surface. Therefore, the side surface of the gear includes the surface visible when the gear is viewed in the direction of its rotational axis, and in the surface of the gear tooth, the side surface is the region not in contact with the tooth of another gear. For example, the side surface of a spur gear is the surface visible when the spur gear is viewed in the direction of its rotational axis. For example, the side surface of a cylindrical worm includes the axial end surface of the cylindrical worm visible when the cylindrical worm is viewed in the direction of its rotational axis, and the region in the surface of the cylindrical worm tooth not in contact with the tooth of a worm wheel. For example, the side surface of a helical gear includes the surface of the helical gear visible when the helical gear is viewed in the direction of its rotational axis, and the region in the surface of the helical gear tooth not in contact with the tooth of another helical gear.

[0084] [Grinding or polishing of the gear]

[0085] The grinding or polishing of the gear in this context refers to grinding of the tooth surface by machining, or polishing of the tooth surface by machining. For example, the wear or abrasion of the tooth surface of the gear caused by mounting the gear in the transmission and using it is different from the grinding or polishing of the tooth surface of the gear.

[0086] The vacuum carburized gear (vacuum carburized gear) in this context can be used for products other than engine units and saddle-type vehicles, such as outboard motors. The vacuum carburized gear can constitute at least a part of a power transmission mechanism to which driving force is transmitted from a driving source of a saddle-type vehicle. The driving source can be an engine. The driving source can be an electric motor. The power transmission mechanism can include output members such as tires and propellers. The power transmission mechanism can be configured to transmit power to a generator in a series hybrid system without outputting driving force to the tires and propellers. The saddle-type vehicle refers to a vehicle on which a rider straddles a saddle. The saddle-type vehicle includes, for example, a motorcycle, a motor tricycle, and an ATV (All Terrain Vehicle). The saddle-type vehicle includes, for example, a single-wheeled vehicle, a two-wheeled vehicle, a three-wheeled vehicle, and a four-wheeled vehicle.

[0087] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the present application.

[0088] As used herein, the terms "at least one," "any one," or "and / or" include any and all combinations of one or more of the associated listed items.

[0089] It should be further understood that the terms "includes," "comprising," or "having" and variations thereof used in this specification specify the presence of stated features, steps, operations, elements, components, and / or their equivalents, but do not preclude the presence or addition of one or more steps, operations, elements, components, and / or combinations thereof.

[0090] It should be further understood that the terms "mounted," "connected," "coupled," and / or their equivalents are used broadly to encompass direct and indirect mounting, connecting, and coupling. Furthermore, "connected" and "coupled" are not limited to physical or mechanical connections or couplings, but may also include direct or indirect electrical connections or couplings.

[0091] 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.

[0092] It should also be understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning consistent with their meaning in the context of the relevant art and this disclosure and will not be interpreted in an idealized or overly formal sense unless expressly defined herein.

[0093] In describing the present invention, it should be understood that many techniques and steps are disclosed. Each of these techniques and steps has its own benefits and each can also be used in combination with one or more, or in some cases, all other disclosed techniques.

[0094] Therefore, for the sake of clarity, this description will avoid repeating every possible combination of the steps in an unnecessary manner. However, when reading the description and claims, it should be understood that such combinations are fully within the scope of the present invention.

[0095] Here, embodiments of a gear, an engine unit, and a saddle-ride type vehicle according to the present teachings will be described.

[0096] In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, it is apparent to one skilled in the art that the present invention can be practiced without these specific details.

[0097] This disclosure should be considered as an exemplification of the invention and is not intended to limit the invention to the specific embodiments shown in the following figures or described.

[0098] Advantageous Effects of the Invention

[0099] According to one embodiment of the present teachings, a vacuum carburized gear can be provided in which wear resistance, rolling contact fatigue strength, and productivity of the gear are improved while reducing carbon dioxide emissions during manufacturing.

[0100] According to one embodiment of the present teachings, an engine unit having a vacuum carburized gear can be provided in which contamination of lubricating oil by wear debris from the gear is suppressed, thereby reducing the influence of the wear debris on the engine body, while reducing carbon dioxide emissions during manufacturing.

[0101] According to one embodiment of the present teachings, a saddle-type vehicle having a vacuum carburized gear can be provided in which an increase in gear backlash due to wear of the gear is suppressed, thereby suppressing a change in sound caused by the increased backlash, while reducing carbon dioxide emissions during manufacturing. BRIEF DESCRIPTION OF DRAWINGS

[0102] [ Figure 1 ] Figure 2 are charts and tables according to embodiments of the present teachings.

[0103] [ Figure 2 ] Figure 3 is a chart showing results of a rolling contact fatigue strength test.

[0104] [ Figure 3 ] Figure 4 is a chart showing a relationship between results of a rolling contact fatigue test and an amount of wear.

[0105] [ Figure 4 ] Figure 5 is a table showing evaluation results of an incomplete quench structure.

[0106] [ Figure 5 ] Figure 6 is a chart showing alloy content and a multiple.

[0107] [ Figure 6 ] Figure 7 is a table showing alloy content for each of a specification-compliant material and a standard material.

[0108] [ Figure 7 ] Figure 8 is a chart showing Mn concentration at different depths from a surface for each of a specification-compliant material and a standard material.

[0109] [ Figure 8 ] Figure 9 is a chart showing Mn concentration at different depths from a surface at a vacuum level.

[0110] [ Figure 9 ]Figure 10 is a table showing the alloy element content of a vacuum carburized sample.

[0111] [ Figure 10 ] Figure 11 is a flowchart showing a process of a vacuum carburizing treatment according to an embodiment of the present teachings.

[0112] [ Figure 11 ] Figure 12 is a graph showing a change in treatment temperature in a vacuum carburizing treatment according to an embodiment of the present teachings.

[0113] [ Figure 12 ] Figure 13 is a schematic view of a gear according to an embodiment of the present teachings.

[0114] [ Figure 13 ] Figure 14 is a schematic view of an engine unit according to an embodiment of the present teachings.

[0115] [ Figure 14 ] Figures 12 to 14 is a schematic view of a saddle-type vehicle according to an embodiment of the present teachings. DETAILED DESCRIPTION

[0116] Embodiments will be described below with reference to the accompanying drawings. In the drawings, the same or corresponding portions are denoted by the same reference signs, and descriptions thereof will not be repeated. The dimensions of the components in the drawings do not strictly represent the actual dimensions of the components, the dimensional proportions of the components, and the like.

[0117] <EMBODIMENT>

[0118] <OVERALL CONFIGURATION>

[0119] REFERENCE Figure 12 A gear 1, an engine unit 10, and a saddle-type vehicle 20 according to embodiments of the present teachings will be described. Figure 13 is a schematic view of a gear 1 according to an embodiment of the present teachings.

[0120] Figure 14 is a schematic view of an engine unit 10 according to an embodiment of the present teachings. Figure 12 is a schematic view of a saddle-type vehicle 20 according to an embodiment of the present teachings.

[0121] Figure 13 The gear 1 shown has been subjected to a vacuum carburizing treatment. Figure 14 The engine unit 10 shown has an engine body 11, a transmission 12, and a gear 1 that has been subjected to a vacuum carburizing treatment and is lubricated using lubricating oil that lubricates both the engine body 11 and the transmission 12. Figures 1 to 11The saddle-type vehicle 20 shown has the vacuum carburized gear 1 in mesh with each other. In the present embodiment, the saddle-type vehicle 20 has an engine unit 10 including the gear 1. The gear 1, the engine unit 10, and the saddle-type vehicle 20 according to the embodiment of the present teaching represent a fulfillment of the embodiment of the present teaching, which is made by checking the test results and evaluation results shown in the charts and tables as Figure 1 described above are omitted in the following description to avoid repetition.

[0122] Figure 1 are charts and tables according to the embodiment of the present teaching. Figure 2 [1] is the same as Figure 1 [2]. Figure 3 [2] is the same as Figure 1 [3]. Figure 4 [3] is the same as Figure 1 [4]. Figure 5 [4] is the same as Figure 1 [5]. Figure 6 [5] is the same as Figure 1 [6]. Figure 7 [6] is the same as Figure 1 [7]. Figure 8 [7] is the same as Figure 2 [8].

[0123] Figure 3 and Figure 12 are charts showing the results of the wear resistance and fatigue tests. Comparative Example 1 is data of a test specimen subjected to gas carburizing. Comparative Example 2 is data of a test specimen subjected to vacuum carburizing at a medium vacuum level of 30 Pa. The present teaching is data of a test specimen subjected to vacuum carburizing at 1500 Pa. The test specimens are all materials conforming to the specifications. The test specimens are SCM420H. These tests were performed using a roller block tester. The test conditions were set to satisfy the wear resistance and the rolling contact fatigue strength, assuming that they are used in a motorcycle. In the test conditions, the meshing conditions (see Figure 4 ) of the gear 1 were set to be substantially the same in the comparative examples and the present teaching. It can be confirmed that Comparative Example 2 has a comparable wear depth but is inferior in the number of cycles compared to Comparative Example 1. It can also be confirmed that the present teaching shows a comparable number of cycles and an improvement in wear resistance compared to Comparative Example 1.

[0124] Figure 4is a table showing the evaluation results of the surface layer conditions of a plurality of vacuum carburized test pieces, with emphasis on the dispersed unquenched structure. The evaluation results are based on visual observation of the cross-sectional structure of each test piece. Test pieces having a dispersed unquenched structure equal to or greater than the reference value were evaluated as BAD, while test pieces having a dispersed unquenched structure less than the reference value were evaluated as GOOD. The evaluation results of SCM420H demonstrate that unquenched structure can be suppressed by adjusting the vacuum level. Furthermore, the evaluation results of SCM420H, SMn443H, SNCM220H, and KKG8 demonstrate that the evaluation results of unquenched structure differ depending on the contents of their alloying elements, regardless of whether the material conforms to the specification or the standard. Although this is an evaluation based on visual observation of a specific cross-sectional structure, rather than a highly strict evaluation, it is still sufficient to grasp the trend.

[0125] The results shown in Figure 4 demonstrate that the cause of the irregularly dispersed unquenched structure in the vacuum carburizing process largely depends on the influence of both the vacuum level and the contents of the alloying elements contained in the material. In other words, Figure 4 it is shown that the evaluation results of unquenched structure differ depending on the vacuum level and the contents of its alloying elements, regardless of whether the material conforms to the specification or the standard. Note that Figure 5 the evaluation shown in is not intended to identify the material corresponding to the present teaching.

[0126] Figure 5 is a graph showing the alloy contents and the multiples. As shown in Figure 6 , the hardenability of the alloying elements Mn, Cr, Mo, Si, and Ni is highest at the respective concentrations (weight percent) in the order of Mn, then Mo, then Cr, Si, and finally Ni.

[0127] Figure 7 is a table showing the contents of the alloying elements (weight percent) in each of the specification-conforming materials and the standard materials. The contents of the alloying elements Ni, Si, Cr, Mo, and Mn, which affect the hardenability, differ among the various types of specification-conforming materials and standard materials.

[0128] Figure 7is a graph showing the Mn concentration at different depths in the direction normal to the surface for each of the conforming and standard materials. Focusing on the alloying element Mn, which has the most impact on hardenability, the Mn concentration at different depths in the direction normal to the surface after vacuum carburizing was measured for a variety of conforming and standard materials. Among SMn443H, SCM420H, SNCM220H, and KKG8, which were vacuum carburized at a vacuum level of 20 Pa, SMn443H, which had the highest Mn concentration, was evaluated as BAD because it exhibited a significant decrease in Mn concentration, which can affect the hardenability of the specimen. On the other hand, SCM420H, SNCM220H, and KKG8 also exhibited similar decreases in Mn concentration. SCM420H was evaluated as BAD because it has low Ni and Mo contents, so Mn can affect the hardenability. This confirms that the hardenability of the conforming and standard materials can not be affected only by the Mn concentration. The Mn concentration in SNCM220H significantly decreased at about 10 pm and about 4 pm. The Mn concentration in SCM420H also significantly decreased at about 6 pm. These regions correspond to grain boundaries. Unlike grains, which have a regular arrangement of atoms, grain boundaries have a structure with disordered atomic arrangements. Therefore, atoms within the grain boundaries easily diffuse, resulting in a significant decrease in the Mn concentration due to sublimation.

[0129] Figure 6 shows the measured results of the Mn concentration (wt%) at any depth in the direction normal to the surface after vacuum carburizing at the same vacuum level for a variety of conforming and standard materials with different Mn concentrations, focusing on the alloying element Mn, which has the most impact on hardenability. From the results shown in Figure 7 and Figure 7 It can be inferred from the results shown in that the decrease in hardenability due to sublimation of Mn is compensated for by alloying elements such as Ni, Cr, and Mo. In other words, the hardenability of the conforming and standard materials is not related to the Mn concentration. Note that Figure 8 The evaluation shown in is not intended to identify materials corresponding to the present teachings.

[0130] Figure 10is a graph showing the Mn concentration at different depths in the direction perpendicular to the surface after the vacuum carburizing process at different vacuum level. The alloying element Mn which has the most impact on the hardenability is focused on and the Mn concentration at different depths is measured. Specifically, the graph shows the Mn concentration at different depths of the specimen of SCM420H at different vacuum level. The Mn concentration in the specimen of SCM420H subjected to the vacuum carburizing process at the vacuum level of 20 Pa is significantly reduced at approximately 6 pm. These regions correspond to the grain boundaries. Unlike the grains which have a regular arrangement of atoms, the grain boundaries have a structure of disordered arrangement of atoms. Therefore, the atoms within the grain boundaries easily diffuse, resulting in a significant reduction in the Mn concentration due to sublimation.

[0131] It can be confirmed that the specimen evaluated as GOOD is configured such that the Mn concentration is maintained without a downward trend in the region having a depth greater than 10 pm and less than 15 pm. It can be confirmed that the specimen evaluated as BAD is configured such that the Mn concentration has a downward trend instead of being maintained in the region from the depth of 15 pm to the depth of 10 pm. It can be confirmed that the specimen evaluated as GOOD is configured such that the Mn concentration is maintained without a downward trend in the region having a depth greater than 8 pm and less than or equal to 10 pm. It can be confirmed that the specimen evaluated as BAD is configured such that the Mn concentration has a downward trend instead of being maintained in the region from the depth of 10 pm to the depth of 8 pm. It can be confirmed that the specimen evaluated as GOOD is configured such that the Mn concentration is maintained without a downward trend in the region having a depth greater than 5 pm and less than or equal to 8 pm.

[0132] Figure 11 is a flowchart showing a process of a vacuum carburizing process according to an embodiment of the present teaching. Figure 12 is a graph showing a change in the process temperature in the vacuum carburizing process according to an embodiment of the present teaching.

[0133] The vacuum step S1 is a process for generating a vacuum in the carburizing furnace. In the vacuum step S1, the carburizing furnace (see ​ ) containing the gear 1 is depressurized to a predetermined vacuum level by a vacuum pump for a period of time up to tl. The heating step S2 is a process for heating the inside of the carburizing furnace, which has been depressurized to a predetermined vacuum level, to a predetermined carburizing temperature Tml. In the heating step S2, the carburizing furnace is heated to the carburizing temperature Tml by a heater between the time tl and the time t2. For example, the carburizing temperature Tml is 930°C. Further, the carburizing furnace is maintained at the carburizing temperature Tml from the time t2 to the time t3 to ensure that the gear 1 contained therein is uniformly heated to the carburizing temperature Tml. By heating the gear 1 to the carburizing temperature Tml, its structure is transformed into austenite.

[0134] The carburizing step S3 is a process of infiltrating and diffusing carbon (C) into the surface of the gear 1 including the tooth surface 2a of the tooth 2. In the carburizing step S3, a hydrocarbon-based gas is supplied into the carburizing furnace. Inside the carburizing furnace, the carbon is generated by thermal decomposition of the hydrocarbon-based gas. Between the time t3 and the time t4, the carbon is infiltrated into the gear 1 from the surface of the gear 1 including the tooth surface 2a. Once infiltrated into the gear 1, the carbon diffuses from the surface to the inside of the gear 1. The vacuum level can be set to a level that suppresses the formation of an oxide film on the surface of the gear 1. During the vacuum step S1, the heating step S2, and the carburizing step S3, the oxygen in the carburizing furnace is exhausted, thereby suppressing the formation of an oxide film on the surface of the gear 1 and internal oxidation. In the carburizing step S3, the carbon required for complete quenching is infiltrated from the surface of the gear 1.

[0135] The quenching step S4 is a process of increasing the surface hardness of the carburized gear 1. In the quenching step S4, after adjusting the temperature of the structure to the quenching temperature Tm2, the structure of the gear 1 that has been transformed into austenite is cooled, for example, by oil cooling. As a result, the structure of the gear 1 is transformed from austenite to martensite having a hardness corresponding to the carburized amount. For example, the quenching temperature Tm2 is 850°C.

[0136] In the above-described embodiment, the vacuum carburizing process includes reducing the pressure of the carburizing furnace in the vacuum step S1, and then heating the inside of the furnace in the heating step S2. Alternatively, in the vacuum carburizing process, the heating step can be performed first to heat the inside of the carburizing furnace, and then the vacuum step can be performed to reduce the pressure of the furnace.

[0137] In the above-described embodiment, the vacuum carburizing process includes heating the inside of the furnace to the carburizing temperature Tm1 in the heating step S2, and then infiltrating the carbon in the carburizing step S3. Alternatively, in the vacuum carburizing process, the inside of the furnace can be heated to a temperature different from the carburizing temperature in the heating step, and then adjusted to the carburizing temperature in the carburizing step.

[0138] In the above-described embodiment, the concentration of Mn in the gear 1 is adjusted by adjusting the vacuum level in the vacuum carburizing process. Alternatively, the gear can be configured such that, in the vacuum carburizing process, in addition to the vacuum level, the sublimation of Mn can be adjusted by adjusting the carburizing temperature.

[0139] List of Reference Signs

[0140] 1 gear

[0141] 2 tooth

[0142] 2a tooth surface

[0143] 10 engine unit

[0144] 11 engine body

[0145] 12 transmission

[0146] 20 saddle-type vehicle

[0147] t1, t2, t3, t4, t5 time

[0148] Tm1 carburizing temperature

[0149] Tm2 quenching temperature

Claims

1. A vacuum carburized gear wherein, The gear is configured as follows: (a) the gear is made of a material that meets a specification or a standard material that contains Mn and at least one of Cr, Mo, Si, or Ni in a region of 10 pm or less in depth in a direction perpendicular to a surface of the gear; and (b) in a region of greater than 10 pm and less than 15 pm in depth in the direction perpendicular to the surface, the Mn concentration is maintained, and The gear is used without subjecting its vacuum carburized tooth surface to grinding or polishing.

2. The vacuum carburized gear according to claim 1, further configured as follows: (c) in a region of greater than 8 pm and less than or equal to 10 pm in depth in the direction perpendicular to the surface of the gear, the Mn concentration is maintained.

3. The vacuum carburized gear according to claim 2, further configured as follows: (d) in a region of greater than 5 pm and less than or equal to 8 pm in depth in the direction perpendicular to the surface of the gear, the Mn concentration is maintained.

4. The vacuum carburized gear of claim 1, wherein, In a region of 10 pm or less in depth in the direction perpendicular to the surface of the gear, the amount of each of Si, Cr, Mo, Ni, and Mn is as follows in weight percentage: Si: greater than 0.35; Cr: greater than 0.35; Mo: greater than 0.25; Ni: greater than 0.25; Mn: greater than 0.

60.

5. An engine unit comprising: an engine body; a transmission configured to change an output rotational speed of the engine body; and a vacuum carburized gear according to claim 1 or 2, the gear being lubricated with lubricating oil that lubricates both the engine body and the transmission.

6. A saddle-type vehicle comprising a vacuum carburized gear according to claim 1 or 2, the gears being in mesh with each other. ​

Images