Gear manufacturing method and gear

Abstract

To provide a gear manufacturing method and gear capable of effectively improving the strength of a gear.SOLUTION: A manufacturing method of a gear has the steps of producing an austenitized steel by heating a raw material steel as an austenitic state and forming the tooth surface on the austenitized steel by rolling and then quenching to below the martensitic transformation temperature. The raw material steel comprises a component composition of 0.75-1.10 mass% of carbon, 1.60-2.50 mass% of silicon, 0.20-1.50 mass% of manganese, 0.005-0.025 mass% of sulfur, 1.60-3.00 mass% of chromium, 0.10-0.60 mass% of molybdenum, 0.005-0.100 mass% of aluminum, 0.010-0.025 mass% of nitrogen, and the remainder of iron and impurities.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for manufacturing a gear and a gear. [Background technology]

[0002] In recent years, there has been a demand for smaller and lighter automotive parts in order to improve fuel efficiency and reduce costs. On the other hand, as the power output of engines mounted on automobiles increases, there is a strong demand for high-strength automotive parts that can withstand the load. Therefore, improvements have been made in materials and surface treatments to improve the strength of mechanical parts, including automotive parts.

[0003] For example, Patent Document 1 discloses a toroidal type continuously variable speed power roller and its manufacturing method, in which the outer ring (or inner ring) material of the power roller is carburized or carbonitrided, a bearing groove is formed by hot rolling using rollers or balls, the roller is quenched and tempered, and then the roller is ground and superfinished. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2000-234658 Summary of the Invention [Problem to be solved by the invention]

[0005] However, the method described in Patent Document 1 has a problem in that even if carburizing or carbonitriding treatment and hot rolling, which are effective in improving the strength of steel, are performed, the effect of improving the strength of the manufactured power roller is low.

[0006] The present invention has been made to solve these problems, and has as its object to provide a method for manufacturing a gear that can effectively improve the strength of the gear.

[0007] Another object of the present invention is to provide a gear having excellent strength. [Means for solving the problem]

[0008] A method for manufacturing a gear according to one embodiment includes the steps of heating a raw steel material to bring the raw steel material into an austenitic state to produce austenitized steel material, and forming tooth surfaces on the austenitized steel material by rolling, and then quenching the austenitized steel material to a temperature below the martensitic transformation temperature. The raw steel material contains 0.75 to 1.10 mass% carbon, 1.60 to 2.50 mass% silicon, 0.20 to 1.50 mass% manganese, 0.005 to 0.025 mass% sulfur, 1.60 to 3.00 mass% chromium, 0.10 to 0.60 mass% molybdenum, 0.005 to 0.100 mass% aluminum, and 0.010 to 0.025 mass% nitrogen, with the remainder being iron and impurities.

[0009] In one embodiment of the gear, the raw steel contains 0.75 to 1.10 mass% carbon, 1.60 to 2.50 mass% silicon, 0.20 to 1.50 mass% manganese, 0.005 to 0.025 mass% sulfur, 1.60 to 3.00 mass% chromium, 0.10 to 0.60 mass% molybdenum, 0.005 to 0.100 mass% aluminum, and 0.010 to 0.025 mass% nitrogen, with the balance being iron and impurities. The gear has a composition in which the average aspect ratio of prior austenite grains in the range of 300 μm from the surface of the tooth flank and tooth base is 8 or more, the Vickers hardness of the tooth flank when tempered at 300°C is 850 HV or more, and the maximum compressive residual stress in the range of 300 μm from the surface of the tooth base is 1500 MPa or more. [Effects of the Invention]

[0010] The present invention provides a gear manufacturing method that can effectively improve the strength of the gear, and also provides a gear with excellent strength. [Brief explanation of the drawings]

[0011] [Figure 1] 10 is a flowchart showing a method for manufacturing a gear in a comparative example and a method for manufacturing a gear in accordance with the first embodiment. [Figure 2] FIG. 3 is a diagram illustrating a rolling and quenching step in the gear manufacturing method according to the first embodiment. [Figure 3] FIG. 3 is an explanatory diagram showing the thermal history of carburizing and quenching and the thermal history of rolling and quenching according to the first embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0012] Embodiment 1 Hereinafter, embodiments of the present invention will be described with reference to the drawings. However, the present invention is not limited to the following embodiments. In addition, the following description and drawings have been simplified as appropriate for clarity of explanation.

[0013] First, the chemical composition of the raw steel material used in the manufacturing method for a gear according to this embodiment will be described. The raw steel material contains 0.75 to 1.10 mass% carbon. Carbon is an important element that significantly affects the strength of a gear. To ensure sufficient strength after rolling, the carbon content must be 0.75 mass% or more, and preferably 0.80 mass% or more. On the other hand, if the carbon content exceeds 1.10 mass%, the ductility and toughness of the steel decrease, resulting in a decrease in workability. Therefore, the carbon content must be 1.10 mass% or less, and preferably 1.05 mass% or less.

[0014] The silicon content is set to 1.60 to 2.50 mass%. Silicon is a useful element that improves the tempering softening resistance of steel and suppresses softening of steel with increasing temperature. The hardness of steel is improved by cold working after quenching. In particular, when a large amount of silicon is contained, tempering after cold working has the effect of significantly suppressing softening of steel. To achieve this effect, the silicon content must be 1.60 mass% or more, and preferably 1.80 mass% or more. On the other hand, if the silicon content is excessive, not only will the ductility and toughness of the steel decrease, resulting in a decrease in workability, but the effect of significantly suppressing softening of steel will saturate and no effect commensurate with the silicon content can be expected. Therefore, the silicon content is set to 2.50 mass% or less.

[0015] Furthermore, the manganese content is set to 0.20 to 1.50 mass%. Manganese is an element that has the effect of increasing the hardenability of steel. To obtain this effect, the manganese content is set to 0.20 mass% or more. On the other hand, if the manganese content exceeds 1.50 mass%, the work hardenability becomes excessive and workability decreases, so the manganese content is set to 1.50 mass% or less.

[0016] The sulfur content is set to 0.005 to 0.025% by mass. Sulfur is an element that has the effect of improving the machinability of steel. To achieve this effect, the sulfur content is set to 0.005% by mass or more. On the other hand, if the sulfur content is excessive, a large amount of manganese sulfide is generated, which reduces ductility. Therefore, the sulfur content is set to 0.025% by mass or less.

[0017] Furthermore, the chromium content is set to 1.60 to 3.00 mass%. Chromium is a useful element that has the effect of improving the hardenability and temper softening resistance of steel. To obtain this effect, the chromium content is set to 1.60 mass% or more. However, if the chromium content is excessive, the effect of improving temper softening resistance saturates, while the hardenability becomes too high. Therefore, the chromium content is set to 3.00 mass% or less.

[0018] Furthermore, the molybdenum content is set to 0.10 to 0.60 mass%. Molybdenum is a useful element that has the effect of improving the hardenability and temper softening resistance of steel. To obtain this effect, the molybdenum content is set to 0.10 mass% or more. However, if the molybdenum content exceeds 0.60 mass%, the effect of improving the hardenability and temper softening resistance of steel saturates, while workability decreases. Therefore, the molybdenum content is set to 0.60 mass% or less.

[0019] The aluminum content is set to 0.005 to 0.100 mass%. Aluminum has a deoxidizing effect. Furthermore, aluminum combines with nitrogen during heat treatment to form aluminum nitride, which prevents coarsening of austenite grains and increases toughness. To achieve this effect, the aluminum content is set to 0.005 mass% or more. On the other hand, if the aluminum content exceeds 0.100 mass%, the cleanliness of the steel decreases and the effect of increasing toughness saturates. Therefore, the aluminum content is set to 0.100 mass% or less.

[0020] In addition, nitrogen 0.010 Nitrogen is bound to aluminum to form aluminum nitride, which prevents the coarsening of austenite grains and increases toughness. To achieve this effect, the nitrogen content is 0.010 On the other hand, if the nitrogen content exceeds 0.0250 mass%, the toughness-enhancing effect saturates, so the nitrogen content is set to 0.0250 mass% or less.

[0021] The remainder of the components contained in the raw steel are iron and impurities. The content of phosphorus, one of the impurities, is set to 0.030% by mass or less. Because phosphorus segregates at grain boundaries and reduces grain boundary strength, the phosphorus content should be as low as possible. Therefore, the phosphorus content is set to 0.030% by mass or less. The term "impurities" refers to components that are mixed in from the ore or scrap used as raw steel materials, the manufacturing process environment, etc., and are not intentionally contained in the raw steel. The above is a suitable component composition for the raw steel used in the manufacturing method of a gear according to this embodiment.

[0022] In the gear manufacturing method according to this embodiment, raw steel having the above-described chemical composition is used as the base material for the gear. The gear manufacturing process will now be described with reference to FIG. 1. FIG. 1 is a flowchart showing a gear manufacturing method according to a comparative example and a gear manufacturing method according to the first embodiment. First, the gear manufacturing method according to the comparative example shown in FIG. 1 is a method for forming tooth flanks by cutting, and includes a carburizing process. As shown in FIG. 1, the gear manufacturing method according to the comparative example includes the following steps S1 to S7.

[0023] In the hot forging process of step S1, the prepared raw steel is hot forged to obtain a roughly shaped forged product. In the normalizing process of step S2, the forged product is normalized to obtain a normalized product with a uniform steel structure. In the blanking process of step S3, a blank product is obtained with a shaft hole formed in the center of the normalized product. In the hobbing process of step S4, the blank product is cut to form the gear tooth surface to obtain a hobbed product. In the carburizing process of step S5, carbon is diffused and penetrated into the surface of the hobbed product to obtain a carburized product. In the shot process of step S6, compressive residual stress is generated in the tooth surface and surface layer of the tooth root of the carburized product by shot peening to obtain a post-processed product. In the gear grinding process of step S7, the tooth surface of the post-processed product is smoothed to obtain a gear.

[0024] The above steps are described in detail below. In the hot forging step of step S1, raw steel is prepared as a gear material. The prepared raw steel is heated, for example, to 1200±30°C, and then hot forged to obtain a preformed forged product. The forged product obtained by this step has, for example, a substantially cylindrical shape. More specifically, it is composed of a large-diameter portion having a cylindrical shape with a central portion in the axial direction of the cylinder having a larger diameter than the other portions, and small-diameter portions having cylindrical shape with diameters smaller than the large-diameter portion at both ends excluding the large-diameter portion in the axial direction. The forged product obtained by hot forging has fiber flow formed inside the forged product that follows the shape of the forged product, thereby increasing the strength and toughness of the steel.

[0025] In the normalizing process of step S2, the forged product obtained in step S1 is normalized by holding it at 900±20°C for 60 minutes and then air-cooling it to obtain a normalized product. This process refines the steel's crystal grains, homogenizes the crystal structure, improves machinability, and removes residual stress.

[0026] In the blanking process of step S3, a shaft hole is drilled using a lathe in the center of the normalized product obtained in step S2 to obtain a blank product. In the hobbing process of step S4, the blank product obtained in step S3 is cut using a hobbing machine to obtain a hobbed product in which the tooth surface of the blank product is formed into a gear shape.

[0027] The carburizing process in step S5 uses gas carburizing. The hob product obtained in step S4 is heated and held at 950°C with a carbon potential of 0.8% by mass, then cooled to 845°C while maintaining the carbon potential at 0.8% by mass, and oil quenched. The product is then tempered at 150°C for 60 minutes. The conditions for the carburizing process are not particularly limited, and any known or arbitrary conditions may be used. The carburizing process described above results in a carburized product with hardened carburized portions. The carburizing process improves the hardness of the material surface.

[0028] In the shot process of step S6, the carburized product obtained in step S5 is subjected to shot peening to obtain a post-machined product. In shot peening, countless shots are projected onto the carburized product, generating compressive residual stress in the surface layers of the tooth flanks and tooth root, thereby increasing the fatigue strength of the tooth flanks and tooth root. In step S7, the post-machined product obtained in step S6 is subjected to a gear grinding process in which the tooth flanks of the post-machined product are smoothed by grinding or other methods to obtain a gear.

[0029] Next, the gear manufacturing method according to the first embodiment shown in Fig. 1 is a manufacturing method of a gear in which the tooth flanks are formed by rolling, and does not require carburizing treatment during the process. As shown in Fig. 1, the gear manufacturing method according to the first embodiment includes the following steps S11 to S16.

[0030] In the hot forging process of step S11, hot forging is performed on the prepared raw steel material to obtain a roughly shaped forged product. In the spheroidizing annealing process of step S12, the forged product is spheroidized to obtain an annealed product with improved plastic workability and toughness. In the blanking process of step S13, a blank product with a shaft hole formed in the center of the annealed product is obtained. In the rolling quenching process of step S14, the blank product is heated to an austenitic state, and in this state, the tooth flank is formed by rolling, and then rapidly cooled below the martensitic transformation temperature to obtain a rolled and quenched product. In the shot peening process of step S15, compressive residual stress is generated in the tooth flank and tooth root surface layer of the rolled and quenched product to obtain a post-processed product. In the gear grinding process of step S16, a gear with smooth tooth flanks is obtained.

[0031] The above steps are described in detail below. The hot forging step in step S11 is the same as step S1 in the gear manufacturing method of the comparative example, except that the raw steel material used is different. As described above, the raw steel material used has a chemical composition suitable for the gear manufacturing method of this embodiment. That is, the raw steel material contains 0.75 to 1.10 mass% carbon, 1.60 to 2.50 mass% silicon, 0.20 to 1.50 mass% manganese, 0.005 to 0.025 mass% sulfur, 1.60 to 3.00 mass% chromium, 0.10 to 0.60 mass% molybdenum, 0.005 to 0.100 mass% aluminum, 0.010 to 0.025 mass% nitrogen, and the balance being iron and impurities. In a forged product obtained by hot forging, fiber flow is formed inside the forged product along its shape, which increases the strength and toughness of the steel.

[0032] In the spheroidizing annealing step of step S12, the forged product obtained in step S11 is heated to, for example, 820±10°C, then slowly cooled to 700±10°C over 10 hours, and then air-cooled to perform spheroidizing annealing, thereby obtaining an annealed product. The conditions for the spheroidizing annealing step are not particularly limited, and any known or arbitrary conditions that can spheroidize the cementite in the steel may be used. This step homogenizes the crystalline structure of the steel and also spheroidizes the cementite in the steel. Steel that has been subjected to spheroidizing annealing becomes softer and has improved workability.

[0033] In the blanking process of step S13, a shaft hole is drilled using a lathe in the center of the annealed product obtained in step S12 to obtain a blank. In the rolling quenching process of step S14, the blank obtained in step S13 is heated to a temperature of, for example, 1000±25°C in 30 seconds and then held for 5 seconds so that its metal structure becomes austenitic. This results in an austenitic steel. Note that the transformation point at which the metal structure becomes austenitic varies depending on the carbon content of the raw steel. Therefore, the heating temperature and holding time are not particularly limited, and appropriate conditions can be selected depending on the carbon content of the raw steel.

[0034] The resulting austenitic steel is then placed in a rolling device, where plastic working is performed using a rolling die to form the tooth flank of the blank into a gear shape. An example of a rolling device used in the rolling and quenching step of step S14 will now be described with reference to Figure 2. Figure 2 is a diagram illustrating the rolling and quenching step in the gear manufacturing method according to the first embodiment.

[0035] First, as shown in steps S101 and S102 of FIG. 2, the rolling device 110 has a pair of opposing rolling dies 111 and 112 and a support tool 113. The rolling dies 111 and 112 have a substantially cylindrical shape. The rolling dies 111 and 112 have rolling surfaces on their outer circumferential surfaces for forming tooth profiles on the tooth surfaces of the molding material. The rolling dies 111 and 112 are supported by the rolling device 110 so as to be rotatable about the central axes of the rolling dies 111 and 112. In the rolling device 110, the rolling dies 111 and 112 are arranged a predetermined distance apart so that the outer circumferential surface of the rolling die 111 faces the outer circumferential surface of the rolling die 112. The support tool 113 has a rod-shaped shaft at its tip and supports the molding material.

[0036] The blank obtained by steps S11 to S13 described above can be used as the molding material to be molded using such a rolling device 110. As shown in step S101 of Fig. 2, the blank W1 has a generally cylindrical shape composed of a large-diameter portion W2 having a cylindrical shape with a central portion in the axial direction of the cylinder having a larger diameter than the other portions, and a small-diameter portion W3 having a cylindrical shape with diameters smaller than the large-diameter portion W2 at both ends in the axial direction excluding the large-diameter portion W2. The blank W1 has an axial hole W4 formed in the center of the cylinder.

[0037] The shaft portion of a support tool 113 of the rolling device 110 is inserted into the shaft hole W4. As a result, the blank W1 is placed between the rolling dies 111 and 112 of the rolling device 110. With the shaft portion of the support tool 113 inserted into the shaft hole W4, the blank W1 is supported by the support tool 113 and is rotatable around the central axis of the blank W1.

[0038] Then, as shown in step S101, with the blank W1 placed in the rolling device 110, the rolling dies 111 and 112, which rotate in the same direction, move toward the blank W1. As a result, the rolling dies 111 and 112 come into contact with the outer peripheral surface of the large diameter portion W2 so as to press against the outer peripheral surface. The rolling dies 111 and 112 and the blank W1 rotate while pressing against the outer peripheral surface of the large diameter portion W2.

[0039] By performing the above-described forming operation, as shown in step S102, the outer peripheral surface of the large diameter portion W2 is plastically deformed to obtain a rolled product W10 having the groove portion W5 formed therein. Furthermore, the rolled product W10 formed into the desired shape is water quenched from a temperature range of 780 to 950°C, and is rapidly cooled to 50°C or less in, for example, 10 seconds. As a result, the outer peripheral surface of the large diameter portion W2 is rapidly cooled, and the metal structure in the surface layer of the outer peripheral surface is transformed into a martensitic structure.

[0040] If the temperature at the start of water quenching is below 780°C, the steel is quenched without austenite, resulting in no martensite structure. If the temperature at the start of water quenching is above 950°C, the austenite structure will be quenched with an excessive carbon concentration, resulting in a large amount of untransformed austenite remaining in the steel even after subsequent cold working. This results in a decrease in the 300°C temper hardness (Vickers hardness of the tooth surface when a gear is tempered at 300°C). Therefore, it is preferable to start water quenching at a temperature between 780 and 950°C.

[0041] After water quenching, the workpiece is tempered, for example, at 150°C for 1 hour. Through the above rolling and quenching process, in step S14, the shape of the outer peripheral surface of the rolling dies 111, 112 is transferred to the outer peripheral surface of the large diameter portion W2, and a molded product with tooth surfaces formed into a gear shape is obtained. For example, a high-frequency induction heating device can be used for heating in this rolling and quenching process.

[0042] In this way, in the rolling process, the tooth flanks are formed by the material flow on the surface of the forming material, so the tooth flanks of the formed product are densified and have improved strength. Furthermore, rapid cooling transforms the metal structure into a martensite structure, improving the hardness of the surface layer of the tooth flanks of the formed product. The formed product obtained through steps S11 to S14 preferably has a tempered hardness of 850 HV or more at 300°C.

[0043] The shot process in step S15, in which the molded product is subjected to post-processing, is the same as step S6 in the gear manufacturing method of the comparative example. In step S15, the molded product obtained in step S14 is shot peened to obtain a post-processed product. In shot peening, countless shots are projected onto the molded product, generating compressive residual stress in the surface layers of the tooth flanks and tooth roots of the molded product, thereby increasing the fatigue strength of the tooth flanks and tooth roots. The surface hardness of the post-processed product is preferably 950 HV or higher, and more preferably 1050 HV or higher.

[0044] The post-processing method is not limited to shot peening, but may also be roller burnishing, cold rolling, or the like. Any cold processing method that can introduce compressive residual stress into the surface layer of the formed product and minimizes surface roughness is acceptable. Then, in step S16, the post-processed product obtained in step S15 is subjected to a gear grinding process in which the tooth flanks of the post-processed product are smoothed by grinding or the like, thereby obtaining a gear.

[0045] In the gear manufacturing method of the comparative example, the tooth surface is formed by cutting (tooth cutting), so the raw steel material must have good machinability. Therefore, the composition of the raw steel material must be reduced to prevent a decrease in machinability by reducing the content of elements such as carbon and silicon, which are useful for improving the strength of steel. In other words, in gear cutting, low-carbon steel is used as the raw steel material, and carburizing is performed in a subsequent process. Furthermore, when carburizing steel, the problem of poor carburization occurs with steels with a high silicon content, so the silicon content of the raw steel material must be reduced. Thus, in the gear manufacturing method of the comparative example, it is difficult to use a raw steel material with high hardness, from the standpoint of tool breakage and tool life.

[0046] Furthermore, in the gear manufacturing method of the comparative example, the fiber flow formed in the hot forging process in step S1 is cut by the cutting process performed in the hobbing process in step S4, and therefore, the improved strength and toughness resulting from the fiber flow cannot be expected in the manufactured gear.

[0047] On the other hand, the gear manufacturing method according to this embodiment uses rolling to form the tooth surface, which is advantageous in that it can process raw steel with high hardness and high contents of elements such as carbon and silicon. Furthermore, if the raw steel is high-carbon steel, carburizing is not required, so the silicon content of the raw steel can be increased compared to cutting. Increasing the silicon content of the raw steel improves the surface fatigue strength of the gear. Therefore, improved strength of the manufactured gear can be expected.

[0048] Furthermore, in the gear manufacturing method according to this embodiment, there is no step involving cutting of the fiber flow lines after the fiber flow lines are formed in the hot forging step of step S11, so the manufactured gear can be expected to have improved strength and toughness due to the fiber flow lines.

[0049] Furthermore, in the gear manufacturing method according to this embodiment, dislocations accumulate in the crystals of the steel material during rolling, resulting in work hardening of the steel material surface. This method is characterized by the ability to obtain the effect of strain. Here, the introduction of the effect of strain in this embodiment will be described with reference to FIG. 3 . FIG. 3 is an explanatory diagram showing the thermal history of carburizing and quenching and the thermal history of rolling and quenching according to the first embodiment. The thermal history of carburizing and quenching in FIG. 3 shows a manufacturing process in which a shape is imparted to a forming material by plastic processing, such as cold forging, and the effect of strain is then introduced, followed by carburizing and quenching to impart strength. In this case, the steel material hardened by the effect of strain introduced during the cold forging period 201 is heated at the carburizing temperature during the carburizing period 202, and then quenched during the quenching period 203. Carburizing is performed to improve the strength of the steel material, but the heating temperature during carburizing is high enough to eliminate dislocations, so the effect of strain is lost during the carburizing period 202. Therefore, the effect of strain cannot be obtained in products manufactured by such a manufacturing process.

[0050] On the other hand, the thermal history of rolling and quenching according to the first embodiment of FIG. 3 shows a manufacturing process in which the rolling process imparts shape and strength to the forming material while also introducing the effect of strain. In this case, the steel material hardened by the effect of strain introduced during the rolling period 212 is then quenched during the quenching period 213. This manufacturing process eliminates the need for heating to eliminate the effect of strain. Therefore, the state in which the effect of strain has been introduced can be maintained even after quenching is complete. This improves the strength of the resulting gear.

[0051] Hereinafter, a description will be given of a gear manufacturing method of a comparative example or a gear manufacturing example manufactured based on the gear manufacturing method according to the first embodiment with reference to Tables 1 and 2. Table 1 shows the chemical compositions of raw steel materials. Of raw steel materials A to J shown in Table 1, raw steel materials A to F, I, and J have chemical compositions within the range suitable for the gear manufacturing method according to the present embodiment. Raw steel materials G and H are raw steel materials that do not satisfy the suitable chemical compositions. Specifically, raw steel material G has a low silicon content, and raw steel material H has a low carbon content.

[0052] [Table 1]

[0053] [Table 2]

[0054] Table 2 shows examples of gear manufacturing. Among the categories shown in Table 2, Examples 1 to 6 are examples of gear manufacturing produced by the gear manufacturing method according to this embodiment. Reference Examples 7 to 10 are examples of gear manufacturing that do not satisfy the conditions of the gear manufacturing method according to this embodiment. In Tables 1 and 2, data outside the range of this embodiment is underlined.

[0055] In the following description, the manufacturing methods are classified by the method of forming the tooth surface, and the manufacturing method of the gear of the comparative example (steps S1 to S7) may be referred to as the hobbing method, and the manufacturing method of the gear according to the first embodiment (steps S11 to S16) may be referred to as the rolling method. Furthermore, the average aspect ratio of the prior austenite grains, the 300°C tempered hardness, and the maximum compressive residual stress were measured for each manufactured gear. Each measurement method will now be described.

[0056] The average aspect ratio of prior austenite grains was measured by first cutting the gear perpendicular to the tooth flank, polishing the cut surface, and then corroding it to reveal the prior austenite grains. This was used as the specimen to be observed. The cut surface of the specimen to be observed was then observed under an optical microscope. Furthermore, 100 prior austenite grains were randomly selected within a 300 μm range from the surface of the tooth flank and root, and the aspect ratio of each prior austenite grain was calculated by dividing the major axis by the minor axis. The average aspect ratio of each prior austenite grain was calculated from the aspect ratios of the calculated prior austenite grains.

[0057] In a gear, high stress acts on the tooth flank and within 300 μm from the surface of the tooth base, and therefore, in order to increase the strength of the gear, this range must be strengthened. The larger the average aspect ratio of the prior austenite grains, the more the effect of strain remains in the product after quenching (the rolled and quenched product in this embodiment). If the average aspect ratio of the prior austenite grains within 300 μm from the surface of the tooth flank and base of the gear is less than 8, the effect of strain is considered to be insufficient. To ensure the effect of strain, it is preferable that the average aspect ratio of the prior austenite grains within 300 μm from the surface of the tooth flank and base of the gear be 8 or more.

[0058] To measure the 300°C tempered hardness, first, the gear is held at 300°C for one hour and then allowed to cool. The gear is then cut on a plane perpendicular to the tooth flank and the cut surface is polished. This is the test sample. Furthermore, the Vickers hardness of the cut surface of the test sample is measured at a position 50 μm from the surface in accordance with the Vickers hardness test specified in JIS Z 2244:2009. The test load is 300 gf. The Vickers hardness is then measured at five points using the same procedure, and the average value is taken as the 300°C tempered hardness of the tooth flank.

[0059] The maximum compressive residual stress was measured in a range of 300 μm from the surface of the gear tooth base using an X-ray stress measurement method that utilizes X-ray diffraction, and the maximum value was taken as the maximum compressive residual stress.

[0060] Next, details of the manufacturing examples of the gears shown in Table 2 will be described. In Example 1 shown in Table 2, gears were manufactured by rolling from Steel A shown in Table 1. The average aspect ratio of prior austenite grains of the obtained gears was 8.5, the hardness after tempering at 300°C was 860 HV, and the maximum compressive residual stress was 1520 MPa.

[0061] In Example 2 shown in Table 2, a gear was manufactured by rolling from Steel B shown in Table 1. The average aspect ratio of the prior austenite grains of the obtained gear was 9.0, the hardness after tempering at 300°C was 910 HV, and the maximum compressive residual stress was 1546 MPa.

[0062] In Example 3 shown in Table 2, gears were manufactured by rolling from Steel C shown in Table 1. The average aspect ratio of prior austenite grains of the obtained gears was 10.8, the hardness after tempering at 300°C was 968 HV, and the maximum compressive residual stress was 1598 MPa.

[0063] In Example 4 shown in Table 2, a gear was manufactured by rolling from Steel D shown in Table 1. The average aspect ratio of prior austenite grains of the obtained gear was 8.7, the hardness after tempering at 300°C was 898 HV, and the maximum compressive residual stress was 1532 MPa.

[0064] In Example 5 shown in Table 2, a gear was manufactured by rolling from Steel E shown in Table 1. The average aspect ratio of the prior austenite grains of the obtained gear was 9.7, the hardness after tempering at 300°C was 930 HV, and the maximum compressive residual stress was 1566 MPa.

[0065] In Example 6 shown in Table 2, a gear was manufactured by rolling from Steel F shown in Table 1. The average aspect ratio of the prior austenite grains of the obtained gear was 9.9, the hardness after tempering at 300°C was 932 HV, and the maximum compressive residual stress was 1573 MPa.

[0066] In Reference Example 7 shown in Table 2, a gear was manufactured by rolling from Steel G shown in Table 1. The average aspect ratio of the prior austenite grains of the obtained gear was 8.8, the hardness after tempering at 300°C was 755 HV, and the maximum compressive residual stress was 1297 MPa.

[0067] In Reference Example 8 shown in Table 2, a gear was manufactured by rolling from Steel H shown in Table 1. The average aspect ratio of the prior austenite grains of the obtained gear was 9.5, the hardness after tempering at 300°C was 769 HV, and the maximum compressive residual stress was 1302 MPa.

[0068] In Reference Example 9 shown in Table 2, a gear was manufactured by a rolling method from Steel I shown in Table 1. However, in Reference Example 9, the shot process of step S15 was omitted. The average aspect ratio of the prior austenite grains of the obtained gear was 8.9, the 300°C tempered hardness was 746 HV, and the maximum compressive residual stress was 182 MPa.

[0069] In Reference Example 10 shown in Table 2, the steel shown in Table 1 J Gears were manufactured from the steel by the hob method. The average aspect ratio of the prior austenite grains of the obtained gears was 1.8, the hardness after tempering at 300°C was 790 HV, and the maximum compressive residual stress was 1508 MPa.

[0070] As is clear from these results, the gears of Examples 1 to 6 manufactured by the gear manufacturing method according to the first embodiment have an average aspect ratio of prior austenite grains of 8.5 or more, a 300°C tempered hardness of 860 HV or more, and a maximum compressive residual stress of 1520 MPa. End On the other hand, in Reference Examples 7 to 10, two of the three data items, namely, the prior austenite aspect ratio, the 300°C tempered hardness, and the maximum compressive residual stress, did not satisfy the specifications of Examples 1 to 6.

[0071] Furthermore, as suggested by the above manufacturing example, the gear manufacturing method according to the first embodiment allows the use of raw steel material with high carbon and silicon contents. High carbon content raw steel material allows the omission of the carburizing process, thereby enabling the use of high silicon materials. Furthermore, the high silicon content raw steel material improves the surface fatigue strength of the gear.

[0072] Furthermore, the gear manufacturing method according to the first embodiment can produce a gear in which the average aspect ratio of prior austenite grains at 300 μm from the surface of the tooth flank and tooth base is 8 or more. In the gear manufacturing method according to the first embodiment, the forming material is given shape and strength by rolling, and then quenched. In other words, since no heating that would eliminate the effect of strain introduced by rolling is involved, the manufactured gear can retain the effect of the strain. Furthermore, since no cutting is involved, fiber flow remains in the manufactured gear. This improves the strength and toughness of the gear.

[0073] Furthermore, according to the gear manufacturing method of the first embodiment, the tooth flanks are formed by rolling, which allows the raw steel material to contain an increased amount of silicon, which contributes to improving temper softening resistance. Furthermore, the manufactured gears benefit from the effect of strain, further improving temper softening resistance. These factors make it possible to obtain gears with a tooth flank hardness of 850 HV or more after tempering at 300°C, and also improve the gear's surface fatigue strength.

[0074] Furthermore, by performing post-processing such as shot peening, the gear manufacturing method according to the first embodiment can produce gears with a maximum compressive residual stress of 1500 MPa or more within a range of 300 μm from the surface of the gear tooth base. The compressive residual stress improves the gear tooth base bending fatigue strength by suppressing the generation and propagation of cracks.

[0075] The gear manufacturing method according to this embodiment has the above-mentioned effects. It was confirmed that the gears of Examples 1 to 6 manufactured according to this embodiment had superior strength to the gears of Reference Examples 7 to 10. The gear manufacturing method according to this embodiment can effectively improve the strength of the gear by using the component composition of the raw steel material, rolling processing, and heat treatment, which are effective in improving the strength of the gear. [Explanation of symbols]

[0076] 110 Rolling equipment 111 Rolling dies 112 Rolling dies 113 Supports 201 Cold forging period 202 Carburizing treatment period 203, 213 Quenching period 212 Rolling Period W1 Blank W2 Large diameter part W3 Small diameter section W4 shaft hole W5 Groove W10 rolled products

Claims

1. heating a raw steel material to bring the raw steel material into an austenitic state to produce an austenitized steel material; forming tooth surfaces of the austenitic steel material by rolling, and then quenching the austenitic steel material to a martensitic transformation temperature or lower by water quenching at a quenching temperature of 780 to 950°C; and a step of subjecting the molded product obtained by the quenching step to cold working capable of introducing compressive residual stress into the tooth flanks and the surface layer of the tooth base of the molded product, The raw steel material has a component composition containing 0.75 to 1.10 mass% carbon, 1.60 to 2.50 mass% silicon, 0.20 to 1.50 mass% manganese, 0.005 to 0.025 mass% sulfur, 1.60 to 3.00 mass% chromium, 0.10 to 0.60 mass% molybdenum, 0.005 to 0.100 mass% aluminum, and 0.010 to 0.025 mass% nitrogen, with the balance being iron and impurities; The average aspect ratio of prior austenite grains within a range of 300 μm from the surface layer of the tooth surface and the tooth base is 8 or more, The Vickers hardness of the tooth surface when tempered at 300°C is 850 HV or more, A method for manufacturing a gear in which the maximum compressive residual stress within a range of 300 μm from the surface of the tooth base is 1500 MPa or more.

2. The raw steel material has a component composition containing 0.75 to 1.10 mass% carbon, 1.60 to 2.50 mass% silicon, 0.20 to 1.50 mass% manganese, 0.005 to 0.025 mass% sulfur, 1.60 to 3.00 mass% chromium, 0.10 to 0.60 mass% molybdenum, 0.005 to 0.100 mass% aluminum, and 0.010 to 0.025 mass% nitrogen, with the balance being iron and impurities; The average aspect ratio of prior austenite grains within a range of 300 μm from the surface layer of the tooth surface and the tooth base is 8 or more, The Vickers hardness of the tooth surface when tempered at 300°C is 850 HV or more, A gear having a maximum compressive residual stress of 1500 MPa or more within a range of 300 μm from the surface of the tooth base.

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