Gear subjected to vacuum carburizing treatment, engine unit having gear subjected to vacuum carburizing treatment, and saddle-riding type vehicle having gear subjected to vacuum carburizing treatment

Abstract

The purpose of the present invention is to provide a gear which is subjected to vacuum carburizing treatment, and which has improved abrasion resistance and rolling fatigue strength while having reduced carbon dioxide emissions during manufacture, and improved productivity. A gear 1 subjected to vacuum carburizing treatment is (a) formed of a standard material or a normal material containing Mn and at least one of Cr, Mo, Si, and Ni in amounts that compensate for a decrease in hardenability caused by sublimation of Mn through vacuum carburizing treatment, in a region where the depth in a direction perpendicular to the surface of the gear 1 is 10 μｍ or less, and (b) formed in a state in which the Mn concentration is maintained without exhibiting a decreasing trend in a region where the depth in the direction perpendicular to the surface is larger than 10 μｍ but smaller than 15 μｍ. The gear 1 is used in a state in which the tooth surface of the gear 1 is not ground or polished.

Description

Vacuum carburized gear, engine unit having vacuum carburized gear, and straddle-type vehicle having vacuum carburized gear

[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.

[0002] A typical gas carburizing process is performed at high temperatures under atmospheric pressure, which makes the surface of the treated component 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 component's strength. Methods for improving component strength include the application of vacuum carburizing and the use of high-strength steel in which elements that promote oxidation are reduced and non-oxidizing elements are added (see paragraph

[0003] of Patent Document 1).

[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 predetermined 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 gas, thereby reducing carbon dioxide emissions (see Patent Document 2, paragraph

[0002] ).

[0004] Patent Document 2 describes an example in which vacuum carburizing was performed on a disk-shaped steel material containing Mn. From FIG. 5 of Patent Document 2, it is presumed that the disk-shaped steel material is a gear. The carburizing method for steel described in Patent Document 2 suppresses sublimation of Mn from the steel material while the temperature is rising in a carburizing furnace, and also suppresses adhesion of Mn to a high-frequency coil in the carburizing furnace. 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 FIG. 5).

[0005] Patent Document 4 proposes steel gears and gear steel for use as parts for motorcycles and the like, and a manufacturing method for the steel gears. The proposed steel gears and gear steel, and a manufacturing method for the steel gears, exhibit minimal heat treatment distortion during surface hardening by carbonitriding, ensuring excellent dimensional accuracy of the gear components, while also providing the strength inherently required for gears, particularly excellent resistance to tooth flaking damage in the gear surface. 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 (see Patent Document 4:

[0006] ).

[0006] JP 2003-193128 A JP 2016-194156 A JP 2015-183226 A JP 2006-328484 A

[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] The present invention aims to provide vacuum-carburized gears that have improved wear resistance and rolling fatigue strength while reducing carbon dioxide emissions during manufacturing and that also improve productivity.The present invention aims to provide an engine unit having vacuum-carburized gears that have reduced 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.The present invention aims to provide a saddle-type vehicle having vacuum-carburized gears that have reduced 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.

[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 then 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] In order 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, in order to investigate the cause of the irregular distribution of incompletely hardened structures, the inventors performed vacuum carburizing treatment under multiple conditions using test pieces made of several types of standard and standard materials with different compositions. Furthermore, the inventors evaluated the condition of the surface layer of each test piece, focusing on the incompletely hardened structure. As a result, it was found that the degree of vacuum is thought to have an effect, but that it is also largely dependent on the material (see Figure 1 [3]).

[0015] The inventors conducted a detailed study of the incompletely hardened structure irregularly scattered on the tooth surface of gears made from standard or standard materials. The alloy contents and hardenability multipliers of elements Ni, Si, Cr, Mo, and Mn, which affect hardenability, differ between the 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 standard and standard materials (see Figure 1[5]). Focusing on Mn, the element that most affects hardenability, the inventors measured the Mn concentration (weight percent) at a given depth from the surface after vacuum carburizing for multiple standard and standard materials (see Figure 1[6]).

[0016] Of the standard materials SMn443H, SCM420H, and SNCM220H, which were subjected to vacuum carburizing at a vacuum of 20 Pa, and the standard material KKG8 from the KKG (registered trademark)-T series of high toughness gear steels (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 because the decrease in Mn concentration was significant and was likely to affect the hardenability of the test specimen. On the other hand, the decrease in Mn concentration was similar for SCM420H, SNCM220H, and KKG8. SCM420H was rated BAD because the amount of Ni and Mo added was small and Mn was likely to affect hardenability. SNCM220H has a higher Ni content than SCM420H, so it seems that Ni, Cr, and Mo compensate for the sublimated Mn. KKG8 has a higher Mo content than SCM420H, so it seems that Cr and Mo compensate for the sublimated Mn. This can also be confirmed by the fact that Ni, Cr, and Mo do not sufficiently compensate for the sublimated Mn in SMn443H, which is substantially free of Ni, Cr, and Mo.

[0017] Furthermore, SCM420H vacuum-carburized at 20 Pa was rated "BAD" due to a significant decrease in Mn concentration, which likely affected the hardenability of the test specimen. On the other hand, SCM420H vacuum-carburized at 500 Pa and 1500 Pa was rated "GOOD" due to a gradual decrease in Mn concentration, which was unlikely to affect the hardenability of the test specimen (see Figure 1 [7]). SCM420H contains small amounts of Ni, Cr, and Mo in addition to Mn. These findings confirm that, for standard and reference materials containing Ni, Si, Cr, Mo, and Mn, elements that affect hardenability, Ni, Cr, and Mo can compensate for the sublimation of Mn if the Mn concentration is maintained within a depth of 10 to 15 μm 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 factor of Si is larger than that of Ni, so it is clear that similar properties can be obtained with Si.

[0018] 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 element 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 and is located to a depth of 10 μm or less in the direction perpendicular to the surface.

[0019] Therefore, we conducted wear resistance and rolling fatigue strength tests on vacuum-carburized test specimens as described below, assuming use in motorcycles. The vacuum-carburized test specimens were (a) formed from standard and standard materials containing Mn and at least one of Cr, Mo, Si, and Ni in amounts sufficient to compensate for the reduced hardenability caused by the sublimation of Mn during vacuum carburizing in a region of 10 μm or less in depth perpendicular to the surface, and (b) formed so that the Mn concentration remained constant without any tendency to decrease in a region of 10 μm to 15 μm in depth perpendicular to the surface. Furthermore, the tests were conducted without grinding or polishing the surfaces of the vacuum-carburized test specimens. The results confirmed that the number of cycles was comparable to that of gas-carburized parts, demonstrating significantly improved wear resistance (see Figure 1 [2], present invention).

[0020] 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 carburization. The vacuum carburized gear (a) is formed from a standard or standard material containing Mn and at least one of Cr, Mo, Si, and Ni in an amount sufficient to compensate for the decrease in hardenability caused by sublimation of Mn during vacuum carburization in a region of the gear to a depth of 10 μm or less in a direction perpendicular to the surface of the gear, and (b) is formed such that the Mn concentration is maintained without any tendency to decrease in a region of the gear to a depth of more than 10 μm but less than 15 μm in a direction perpendicular to the surface, and the gear tooth surface is used without being ground or polished.

[0021] This gear is vacuum-carburized rather than gas-carburized. This reduces carbon dioxide emissions during manufacturing. As shown in test results, the gear is vacuum-carburized to maintain the Mn concentration in the region greater than 10 μm and smaller than 15 μm without any decreasing tendency. The reduction in hardenability due to Mn sublimation is compensated for by the addition of at least one element selected from Si, Ni, Cr, and Mo. This improves the wear resistance and rolling fatigue strength of the vacuum-carburized gear, even when its tooth flanks are not ground or polished. Furthermore, the vacuum-carburized gear is used without grinding or polishing its tooth flanks. This eliminates the need for grinding or polishing the tooth flanks, improving productivity. The gear may also be shot-peened to further improve its rolling fatigue strength and bending fatigue strength, depending on the required specifications.

[0022] 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.

[0023] A gear according to one embodiment of the present invention may have the following feature (2) in addition to the feature (1): (2) A gear that has been subjected to vacuum carburizing is formed such that (c) the Mn concentration is maintained without decreasing in a region having a depth of more than 8 μm and not more than 10 μm in a direction perpendicular to the surface of the gear.

[0024] In a gear having this configuration, the Mn concentration is adjusted in a region (b) adjacent to a region having a depth of more than 10 μm but less than 15 μm in the direction perpendicular to the surface, and in a region (c) closer to the surface than the region having a depth of more than 8 μm but less than 10 μm. Therefore, as shown in the test results, a gear having the above configuration, which has been subjected to vacuum carburization so that the Mn concentration in the region having a depth of more than 8 μm but less than 10 μm does not show a decreasing trend, has its hardenability reduced by Mn sublimation 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 without grinding or polishing the tooth surfaces.

[0025] A gear according to one embodiment of the present invention may have the following feature (3) in addition to the feature (1) or the feature (2): (d) a gear that has been subjected to vacuum carburizing is formed in a state in which the Mn concentration is maintained without showing a decreasing tendency 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.

[0026] In a gear having this configuration, the Mn concentration is adjusted in a region (c) adjacent to a region having a depth of more than 8 μm but less than 10 μm in the direction perpendicular to the surface, and in a region (d) closer to the surface than the region having a depth of more than 5 μm but less than 8 μm. Therefore, as shown in test results, in a gear having the above configuration, which has been vacuum-carburized so that the Mn concentration in the region having a depth of more than 5 μm but less than 8 μm does not decrease, 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 vacuum-carburized so that the Mn concentration in the region having a depth of more than 5 μm but less than 8 μm does not decrease can be further improved.

[0027] 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, the amounts of Si, Cr, Mo, Ni, and Mn sufficient to compensate for the decrease in hardenability caused by sublimation of Mn due to vacuum carburization 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; and Mn is greater than 0.60.

[0028] 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 conducted this time. 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.

[0029] 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.

[0030] Regarding Cr, from the evaluation results of SMn443H (vacuum degree 20 Pa), it is difficult to consider that a Cr concentration of 0.35 weight percent or less is an amount sufficient to compensate for the decrease in hardenability. Therefore, regarding Cr, an amount sufficient to compensate is considered to be an amount greater than 0.35 weight percent.

[0031] Regarding Ni, from the evaluation results of SMn443H (vacuum degree 20 Pa), it is unlikely that a Ni concentration of 0.25 weight percent or less is sufficient to compensate for the decrease in hardenability. Therefore, regarding Ni, an amount sufficient to compensate for the decrease in hardenability is considered to be an amount of more than 0.25 weight percent.

[0032] Regarding Mn, the evaluation results for SNCM220H (vacuum degree 20 Pa) suggest that the amount of Mn that compensates for this is a Mn concentration of 0.60 weight percent or more. Note that actual standard and reference materials contain multiple elements, including Si, Ni, Cr, and Mo, making it difficult to determine the effect of each element on hardenability. However, based on the technical concept and evaluation results of the present invention, the amount that compensates for the decrease in hardenability caused by Mn sublimation during vacuum carburizing in the region of the gear to a depth of 10 μm or less in the direction perpendicular to the surface may be defined or interpreted as follows:

[0033] The present invention is based on the technical idea of ​​adjusting the dispersion state 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 10 μm or less in depth perpendicular to the surface of the vacuum-carburized gear. As shown in test results, the wear resistance and rolling fatigue strength of the vacuum-carburized gear can be improved. Therefore, in this specification, the amount that compensates for the decrease in hardenability caused by the sublimation of Mn in vacuum carburization 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:

[0034] For Si, the amount of hardenability compensation may be defined as greater than 0.35 weight percent of the specified material or standard material. For Si, if it is expressed as 0.15 to 0.35, or 0.15 or less, or 0.35 or less, it may be defined as not being an amount of hardenability compensation. For Ni, it may be defined as greater than 0.25 weight percent of the specified material or standard material. For Ni, if it is expressed as 0.25 or less, it may be defined as not being an amount of hardenability compensation. For Cr, it may be defined as greater than 0.35 weight percent of the specified material or standard material. For Cr, if it is expressed as 0.35 or less, it may be defined as not being an amount of hardenability compensation. For Mo, it may be defined as greater than 0.25 weight percent of the specified material or standard material. For Mo, a value of 0.15 to 0.2, or 0.25 or less may be defined as not being an amount that compensates for hardenability, and for Mn, a compensated amount may be defined as 0.60 or more in weight percent of the specified or standard material.

[0035] 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 vacuum carburizing treatment according to any one of the configurations (1), (2), (3), and (4) and that is lubricated with lubricating oil that lubricates both the engine body and the transmission.

[0036] 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 vacuum-carburized gears have improved wear resistance and rolling fatigue strength. The vacuum-carburized gears are used without grinding or polishing their tooth surfaces. This eliminates the need for grinding or polishing the tooth surfaces of the gears, 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.

[0037] A saddle-type vehicle according to an embodiment of the present invention may have gears that have been subjected to the vacuum carburizing treatment according to any one of the above configurations (1), (2), (3), and (4) in a state where they mesh with each other.

[0038] The gears of the configuration (1) are subjected to vacuum carburizing rather than gas carburizing. 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. This eliminates the need for grinding or polishing the tooth surfaces of the gears, 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.

[0039] 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 10 μm or less in depth perpendicular to the surface of a vacuum-carburized gear.

[0040] Patent Documents 1, 2, 3, and 4 do not disclose or 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 do not disclose or 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.

[0041] [Standard Material] In this specification, standard material means a material specified in a material standard that defines the physical properties, mechanical properties, composition, shape, etc. of an industrial material such as a metal. 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 alloy elements such as Mn, Cr, Mo, Si, and Ni, which are minor components contained in standard steel materials, are specified within a certain range. Standard materials are easily available due to their large distribution volume.

[0042] [Standard Material] In this specification, a standard material refers to a material with a composition independently defined by a material manufacturer or the like. A standard material is a material with a composition different from that of a standard material, and includes materials provided by each manufacturer as a standard product. The component values ​​(weight percent) of the main alloy elements such as Mn, Cr, Mo, Si, and Ni of the standard material fall within a certain range defined by the manufacturer. Standard materials are readily available due to their large distribution volume.

[0043] [Alloying Elements] In this specification, the term "alloying elements" refers 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 the 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 material containing at least one of Cr, Mo, Si, and Ni and Mn. A gear subjected to vacuum carburizing may also contain alloying elements other than Cr, Mo, Si, and Ni to improve hardenability. A gear subjected to vacuum carburizing may also contain, for example, B, Ti, etc.

[0044] [Vacuum Carburizing Treatment] In this specification, vacuum carburizing treatment refers to a treatment 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. The amount of carbon dioxide emitted in vacuum carburizing treatment is less than that in gas carburizing treatment, in which heating is performed in a carburizing furnace filled with carbon monoxide. In vacuum carburizing treatment, the Mn concentration of the gear may be adjusted by adjusting the degree of vacuum. In vacuum carburizing treatment, the Mn concentration of the gear may be adjusted by adjusting the carburizing temperature. In vacuum carburizing treatment, the Mn concentration of the gear may be adjusted by adjusting the degree of vacuum and the carburizing temperature. In vacuum carburizing treatment, the Mn concentration of the gear may be adjusted by adjusting other process control conditions.

[0045] [Degree of vacuum] In this specification, degree of vacuum refers to the pressure of a space filled with gas at a pressure lower than atmospheric pressure under standard conditions. The smaller the number, the lower the 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.

[0046] [Hardenability] In this specification, hardenability refers to the ease of hardening by quenching, and refers to the relationship between the hardness and depth of a steel material hardened by quenching. A steel material with good hardenability is, for example, a material that hardens from the surface to a deeper depth than other steel materials when quenched under the same conditions, or a material that hardens harder than other steel materials.

[0047] [Incompletely hardened region] 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 alloy 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.

[0048] [Gears] In this specification, gears may be gears with parallel axes. Gears may be gears with parallel axes, such as spur gears, helical gears, internal gears, and racks. Gears may be gears with intersecting axes. Gears may be gears with intersecting axes, such as straight bevel gears, spiral bevel gears, and helical bevel gears. Gears may be gears with non-intersecting axes. Gears may be gears with non-intersecting axes, such as worm gears and hypoid gears. Gears may have involute teeth. Gears may have teeth with an involute curve shape. Gears may have cycloid teeth. Gears may have teeth with a cycloid curve shape.

[0049] [Tooth flank] In this specification, the term "tooth flank" refers to the surface of two meshing gears that comes into contact with one gear when transmitting 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.

[0050] In this specification, a gear that has undergone vacuum carburizing treatment may be used in products other than engine units and saddle-riding vehicles, for example, in outboard motors. A gear that has undergone vacuum carburizing treatment may constitute at least a part of a power transmission mechanism that transmits driving force from a drive source of a saddle-riding vehicle. 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. A saddle-riding vehicle refers to a vehicle in which a rider sits astride a saddle. Examples of saddle-riding vehicles include motorcycles, motorcycle tricycles, and ATVs (All Terrain Vehicles). Examples of saddle-riding vehicles include one-wheeled vehicles, two-wheeled vehicles, three-wheeled vehicles, and four-wheeled vehicles.

[0051] The terminology used in this specification is used for the purpose of defining particular embodiments only, and is not intended to limit the invention.

[0052] As used herein, "at least one of," "any one of," and / or" includes all combinations of one or more of the associated listed members.

[0053] In this specification, 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.

[0054] 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.

[0055] 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.

[0056] 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.

[0057] 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.

[0058] 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.

[0059] In this specification, embodiments of a gear, an engine unit, and a saddle-ride type vehicle according to the present invention will be described.

[0060] 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.

[0061] 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.

[0062] According to one embodiment of the present invention, it is possible to provide gears that have been subjected to vacuum carburizing treatment, which improve wear resistance and rolling fatigue strength while reducing carbon dioxide emissions during manufacture and also improve 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 manufacture and inhibits contamination of lubricating oil by wear debris, thereby 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 manufacture and inhibits an increase in gear backlash due to wear, thereby suppressing changes in sound due to an increase in backlash.

[0063] FIG. 1 shows graphs and tables according to an embodiment of the present invention. FIG. 2 shows graphs illustrating rolling fatigue test results. FIG. 3 shows graphs illustrating the relationship between rolling fatigue test results and wear loss. FIG. 4 shows a table illustrating evaluation results of incompletely hardened structures. FIG. 5 shows graphs illustrating alloy content and hardenability multiplier. FIG. 6 shows a table illustrating alloy content for standard materials and standard materials. FIG. 7 shows graphs illustrating Mn concentration at depth from the surface for standard materials. FIG. 8 shows graphs illustrating Mn concentration at depth from the surface for different vacuum levels. FIG. 9 shows a flowchart illustrating steps of a vacuum carburizing treatment according to an embodiment of the present invention. FIG. 10 shows graphs illustrating changes in treatment temperature during vacuum carburizing treatment according to an embodiment of the present invention. FIG. 11 shows a schematic diagram of a gear according to an embodiment of the present invention. FIG. 12 shows a schematic diagram of an engine unit according to an embodiment of the present invention. FIG. 13 shows a schematic diagram of a saddle-type vehicle according to an embodiment of the present invention.

[0064] 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.

[0065] [Embodiment] <Overall Configuration> 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 11 to 13. Figure 11 is a schematic diagram of the gear 1 according to an embodiment of the present invention. Figure 12 is a schematic diagram of the engine unit 10 according to an embodiment of the present invention. Figure 13 is a schematic diagram of the saddle-ride type vehicle 20 according to an embodiment of the present invention.

[0066] The gear 1 shown in FIG. 11 has been subjected to vacuum carburizing. The engine unit 10 shown in FIG. 12 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. 12 has gears 1 that have been subjected to vacuum carburizing and are in mesh with each other. In this embodiment, the saddle-type vehicle 20 has an engine unit 10 including the gear 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 10. Note that descriptions related to the above considerations are omitted from the following description to avoid duplication.

[0067] 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.

[0068] 2 and 3 are graphs showing the results of wear resistance and fatigue tests. Comparative Example 1 shows data for test specimens subjected to gas carburizing treatment. Comparative Example 2 shows data for test specimens subjected to vacuum carburizing treatment at a medium vacuum of 30 Pa. The present invention shows data for test specimens subjected to vacuum carburizing treatment at 1500 Pa. All of the test specimens were standard materials. The test specimens were SCM420H. The tests were conducted using a roller pitting 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. 11 ). 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 achieved the same number of cycles and improved wear resistance compared to Comparative Example 1.

[0069] Figure 4 is a table showing the results of evaluation of the surface condition of multiple vacuum-carburized test specimens, focusing on incompletely hardened structures. The evaluation results were obtained by visually observing 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 incompletely hardened structures. 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 observation of specific cross-sectional structures and is not rigorous, it is sufficient to grasp the trends.

[0070] Figure 5 is a graph showing the alloy content and hardenability multiplier. 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.

[0071] 6 is a table showing the alloying element contents (weight percent) of the standard and reference materials. The standard and reference materials have different contents of elements that affect hardenability: Ni, Si, Cr, Mo, and Mn.

[0072] Figure 7 is a graph showing the Mn concentration at different depths from 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 different depths from the surface after vacuum carburizing for several standard and standard materials. Among SMn443H, SCM420H, and SNCM220H, which were vacuum-carburized at a vacuum of 20 Pa, SMn443H, which had the highest Mn concentration, was rated BAD due to a significant decrease in Mn concentration, which was likely to affect the hardenability of the test specimens. On the other hand, SCM420H, SNCM220H, and KKG8 all showed similar decreases in Mn concentration. SCM420H, which had low Ni and Mo contents, was rated BAD due to a 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 of SNCM220H decreases significantly around 10 μm and 4 μm. The Mn concentration of SCM420H also decreases significantly around 6 μm. These areas are grain boundaries. Unlike the interior of crystal grains, which have a regular atomic arrangement, the interior of grain boundaries has a structure in which the atomic arrangement is disordered. Therefore, atoms within the grain boundaries can easily diffuse, and a significant decrease in the Mn concentration due to sublimation is observed.

[0073] Figure 8 is a graph showing the Mn concentration at different depths from the surface for different vacuum levels. Focusing on Mn, the element that has the greatest impact on hardenability, we measured the Mn concentration versus depth from the surface after vacuum carburization. Specifically, this graph shows the Mn concentration versus depth from the surface after vacuum carburization for SCM420H test pieces at different vacuum levels. The Mn concentration of SCM420H also significantly decreases around 6 μm. This area 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 Mn concentration due to sublimation.

[0074] It was confirmed that the test specimens rated GOOD were formed in a state where the Mn concentration was maintained without a decreasing trend in the region where the depth in the direction perpendicular to the surface was greater than 10 μm and less than 15 μm. It was confirmed that the test specimens rated BAD were formed in a state where the Mn concentration was not maintained but tended to decrease in the region where the depth in the direction perpendicular to the surface was greater than 10 μm and less than 15 μm. It was confirmed that the test specimens rated GOOD were formed in a state where the Mn concentration was maintained without a decreasing trend in the region where the depth in the direction perpendicular to the surface was greater than 8 μm and less than 10 μm. It was confirmed that the test specimens rated BAD were formed in a state where the Mn concentration was not maintained but tended to decrease in the region where the depth in the direction perpendicular to the surface was greater than 8 μm and less than 10 μm. It was confirmed that the test specimens rated GOOD were formed in a state where the Mn concentration was maintained without a decreasing trend in the region where the depth in the direction perpendicular to the surface was greater than 5 μm and less than 8 μm.

[0075] Fig. 9 is a flowchart showing the steps of the vacuum carburization process according to an embodiment of the present invention, and Fig. 10 is a graph showing the change in processing temperature in the vacuum carburization process according to an embodiment of the present invention.

[0076] The evacuation step S1 is a step of creating a vacuum inside the carburizing furnace. In this step, the carburizing furnace containing the gear 1 (see FIG. 11 ) is depressurized to a 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 a vacuum level, to a predetermined carburizing temperature Tm1. In this step, 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.

[0077] 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 impregnates 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 a degree of vacuum that suppresses the formation of an oxide film on the surface of the gear 1. In 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.

[0078] The quenching step S4 is a step for increasing 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 a quenching temperature Tm2, ​​and then the structure is cooled, for example, by oil cooling. As a result, the structure of the gear 1 is transformed from austenite to martensite, the hardness of which corresponds to the amount of carburization. The quenching temperature Tm2 is, for example, 850°C.

[0079] 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 involve increasing the temperature in the temperature-raising step, and then reducing the pressure in the vacuum step.

[0080] 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 the material with 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.

[0081] 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.

[0082] REFERENCE SIGNS LIST 1 gear 2 tooth 2a tooth surface 10 engine unit 11 engine body 12 transmission 20 straddle-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, wherein the gear is (a) made of a standard or standard material that contains Mn and at least one of Cr, Mo, Si and Ni in an amount sufficient to compensate for the reduced hardenability caused by the sublimation of Mn by the vacuum carburizing in an area of ​​10 μm or less in depth in the direction perpendicular to the surface of the gear, and (b) formed in a state in which the Mn concentration does not show a decreasing tendency and is maintained in an area of ​​more than 10 μm but less than 15 μm in depth in the direction perpendicular to the surface, and the gear is used without having its tooth surface ground or polished.

2. A gear that has been subjected to vacuum carburizing treatment as set forth in claim 1, wherein: (c) the gear has been subjected to vacuum carburizing treatment so that the Mn concentration is maintained without showing any tendency to decrease in an area of ​​the gear that is greater than 8 μm and not greater than 10 μm deep in a direction perpendicular to the surface of the gear.

3. A gear that has been subjected to vacuum carburizing treatment as set forth in claim 2, wherein the gear is formed in such a state that (d) the Mn concentration is maintained without showing any tendency to decrease 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.

4. A gear that has been subjected to vacuum carburizing treatment as set forth in any one of claims 1 to 3, wherein the amounts of Si, Cr, Mo, Ni and Mn in an area of ​​10 μm or less in depth in the direction perpendicular to the gear surface, sufficient to compensate for the decrease in hardenability caused by the sublimation of Mn during the vacuum carburizing treatment, are as follows in weight percent: 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. An engine unit comprising: an engine body; a transmission that changes the output speed of the engine body; and a gear that has been subjected to vacuum carburizing treatment according to any one of claims 1 to 4 and that is lubricated with lubricating oil that lubricates both the engine body and the transmission.

6. A straddle-type vehicle having gears that have been subjected to the vacuum carburizing treatment according to any one of claims 1 to 4 in a state where they mesh with each other.

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