JP2023079577A - Gear manufacturing method and gear - Google Patents

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 The present invention relates to a gear manufacturing method and a gear.

2. Description of the Related Art In recent years, in order to reduce the fuel consumption and cost of automobiles, there has been a demand for smaller and lighter automobile parts. On the other hand, with the increasing output of engines mounted on automobiles, there is a strong demand for high-strength automobile parts that can withstand the load. Therefore, in order to improve the strength of mechanical parts such as automobile parts, improvements in materials, surface treatments, and the like are being made.

For example, in Patent Document 1, after carburizing or carbonitriding the outer ring (or inner ring) material of a power roller, bearing grooves are formed by hot rolling using rollers or balls, and quenching and tempering are performed. Later, a toroidal continuously variable speed power roller for superfinishing grinding and a method of manufacturing the same are disclosed.

JP-A-2000-234658

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

SUMMARY OF THE INVENTION It is an object of the present invention to provide a gear manufacturing method capable of effectively improving the strength of gears.

Furthermore, another object of the present invention is to provide a gear having excellent strength.

A method for manufacturing a gear according to one embodiment includes the steps of heating a raw steel material to convert the raw steel material into an austenite state to generate an austenitized steel material, and forming a tooth flank on the austenitized steel material by rolling. , and a step of quenching below the martensite transformation temperature, and the raw steel material contains 0.75 to 1.10% by mass of carbon, 1.60 to 2.50% by mass of silicon, and 0.5% by mass of manganese. 20 to 1.50% by mass, 0.005 to 0.025% by mass of sulfur, 1.60 to 3.00% by mass of chromium, 0.10 to 0.60% by mass of molybdenum, and aluminum It contains 0.005 to 0.100% by mass, 0.010 to 0.025% by mass of nitrogen, and the balance is iron and impurities.

Further, in the gear according to one embodiment, the raw steel material contains 0.75 to 1.10% by mass of carbon, 1.60 to 2.50% by mass of silicon, and 0.20 to 1.50% by mass of manganese. 0.005-0.025% by weight sulfur, 1.60-3.00% by weight chromium, 0.10-0.60% by weight molybdenum, and 0.005-0.00% aluminum .100% by mass and 0.010 to 0.025% by mass of nitrogen, the balance having a component composition of iron and impurities, prior austenite grains in the range of 300 μm from the surface layer of the tooth surface and tooth root has an average aspect ratio of 8 or more, a Vickers hardness of the tooth surface when tempered at 300° C. is 850 HV or more, and a maximum compressive residual stress is 1500 MPa or more in a range of 300 μm from the surface layer of the tooth root.

ADVANTAGE OF THE INVENTION By this invention, the manufacturing method of the gear which can improve the intensity|strength of a gear effectively can be provided. Moreover, according to the present invention, it is possible to provide a gear having excellent strength.

5 is a flow chart showing a gear manufacturing method of a comparative example and a gear manufacturing method according to the first embodiment; FIG. 4 is a diagram illustrating a rolling quenching process in the gear manufacturing method according to the first embodiment; FIG. 5 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;

Embodiment 1
BEST MODE FOR CARRYING OUT THE INVENTION 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. Also, for clarity of explanation, the following description and drawings are simplified as appropriate.

First, the chemical composition of the raw material steel used in the gear manufacturing method according to the present embodiment will be described. The carbon content in the raw material steel is 0.75 to 1.10% by mass. Carbon is an important element that greatly affects the strength of gears. In order to ensure sufficient strength after rolling, the carbon content should be 0.75% by mass or more, preferably 0.80% by mass or more. On the other hand, when the carbon content exceeds 1.10% by mass, the ductility and toughness of the steel are lowered, and the workability is lowered. Therefore, the carbon content should be 1.10% by mass or less, preferably 1.05% by mass or less.

Also, the silicon content is 1.60 to 2.50% by mass. Silicon is a useful element that improves the resistance to temper softening of steel and suppresses the softening of steel that accompanies temperature rise. Hardness of steel is improved by subjecting it to cold working after quenching. In particular, when a large amount of silicon is contained, it has the effect of remarkably suppressing the softening of steel due to tempering performed after cold working. In order to obtain this effect, the silicon content should be 1.60% by mass or more, preferably 1.80% by mass or more. On the other hand, if the silicon content is excessive, not only will the ductility and toughness of the steel decrease, the workability will decrease, but the effect of significantly suppressing the softening of the steel will be saturated, and the silicon content will be saturated. You can't expect the effect to match the amount. Therefore, the content of silicon is set to 2.50% by mass or less.

Also, manganese is 0.20 to 1.50% by mass. Manganese is an element that has the effect of increasing the hardenability of steel. In order to obtain this effect, the content of manganese is set to 0.20% by mass or more. On the other hand, if the manganese content exceeds 1.50% by mass, the work hardening property becomes excessively large and the workability is lowered.

Also, sulfur is 0.005 to 0.025% by mass. Sulfur is an element that has the effect of improving the machinability of steel. In order to obtain this effect, the sulfur content is set to 0.005% by mass or more. On the other hand, when the sulfur content is excessive, the ductility decreases due to the large amount of manganese sulfide produced. Therefore, the sulfur content is set to 0.025% by mass or less.

Also, chromium is 1.60 to 3.00% by mass. Chromium is a useful element that has the effect of improving the hardenability and temper softening resistance of steel. In order to obtain this effect, the content of chromium is set to 1.60% by mass or more. However, if the chromium content is excessive, the effect of improving the temper softening resistance is saturated, but the hardenability becomes too high. Therefore, the chromium content is set to 3.00% by mass or less.

Also, molybdenum is 0.10 to 0.60% by mass. Molybdenum is a useful element that has the effect of improving the hardenability and temper softening resistance of steel. In order to obtain this effect, the content of molybdenum is set to 0.10% by mass or more. However, when the molybdenum content exceeds 0.60% by mass, the effect of improving the hardenability and temper softening resistance of steel saturates, but workability decreases. Therefore, the content of molybdenum is set to 0.60% by mass or less.

In addition, aluminum should be 0.005 to 0.100% by mass. Aluminum has a deoxidizing action. In addition, aluminum combines with nitrogen to form aluminum nitride during heat treatment, thereby preventing austenite grains from coarsening and increasing toughness. In order to obtain this effect, the content of aluminum is set to 0.005% by mass or more. On the other hand, if the aluminum content exceeds 0.100% by mass, the cleanliness of the steel is lowered and the effect of increasing the toughness is saturated. Therefore, the content of aluminum is set to 0.100% by mass or less.

Also, nitrogen is 0.0010 to 0.0250% by mass. Nitrogen combines with aluminum to form aluminum nitride, thereby preventing coarsening of austenite grains and increasing toughness. To obtain this effect, the nitrogen content is set to 0.0010% by mass or more. On the other hand, if the nitrogen content exceeds 0.0250% by mass, the effect of increasing the toughness is saturated, so the nitrogen content is made 0.0250% by mass or less.

The rest of the components contained in the raw steel material are iron and impurities. Phosphorus, which is one of the impurities, should be 0.030% by mass or less. Phosphorus segregates at grain boundaries and lowers the grain boundary strength, so the phosphorus content should be as low as possible. Therefore, the phosphorus content is set to 0.030% by mass or less. The term "impurity" refers to a component that is mixed in from the ore or scrap used as a raw material for steel, or from the environment of the manufacturing process, etc., and is not a component that is intentionally included in the raw steel material. The above is the preferred chemical composition of the raw material steel used in the gear manufacturing method according to the present embodiment.

In the gear manufacturing method according to the present embodiment, the raw steel material having the above composition is used as the base material of the gear. Therefore, the gear manufacturing process will be described with reference to FIG. FIG. 1 is a flow chart showing a gear manufacturing method of a comparative example and a gear manufacturing method according to a first embodiment. First, the gear manufacturing method of the comparative example shown in FIG. 1 is a gear manufacturing method in which tooth flanks are formed by cutting, and includes a carburizing process. As shown in FIG. 1, the gear manufacturing method of the comparative example has the following steps S1 to S7.

In the hot forging process of step S1, hot forging is performed on the prepared raw material steel to obtain a rough-shaped forged product. In the normalizing step of step S2, the forged product is normalized to obtain a normalized product with a homogenized steel structure. In the blanking process of step S3, a blank product is obtained in which a shaft hole is formed in the central part of the normalized product. In the hobbing step of step S4, a hobbing product is obtained by cutting the blank product to form the tooth flanks of the gear. In the carburizing process of step S5, a carburized product is obtained by diffusing carbon on the surface of the hob product. In the shot process of step S6, a post-processed product is obtained by shot peening to generate compressive residual stress in the surface layer of the tooth flank and tooth root of the carburized product. In the gear grinding step of step S7, a gear having smoothed tooth flanks is obtained as a post-processed product.

The above steps are described in detail below. In the hot forging process of step S1, a raw steel material is prepared as a gear material. The prepared raw material steel material is heated to, for example, 1200±30° C., and subjected to hot forging to obtain a preformed forged product. A forged product obtained by this process has, for example, a substantially cylindrical shape. More specifically, the large-diameter portion has a cylindrical shape in which the central portion in the axial direction of the cylinder has a larger diameter than the other portions, and the diameter of both end portions excluding the large-diameter portion in the axial direction is larger than that of the large-diameter portion. and a small-diameter portion having a cylindrical shape with a small diameter. A forged product obtained by hot forging has a fiber flow formed inside the forged product along the shape of the forged product, thereby enhancing the strength and toughness of the steel.

In the normalizing process of step S2, the forged product obtained in step S1 is held at 900±20° C. for 60 minutes and then air-cooled for normalizing to obtain a normalized product. This process can refine the grains of the steel, homogenize the crystal structure, improve machinability, and remove residual stress.

In the blanking process of step S3, a blank is obtained by boring a shaft hole in the central portion of the normalized product obtained in step S2 using a lathe. In the hobbing step of step S4, the blank obtained in step S3 is cut using a hobbing machine to obtain a hobbing product in which the tooth surface of the blank is formed into a gear shape.

The gas carburizing method is used in the carburizing process of step S5. The hob product obtained in step S4 is heated and held at 950°C with a carbon potential of 0.8% by mass, and then cooled to 845°C with the carbon potential of 0.8% by mass, followed by oil quenching. Further, tempering is performed at 150° C. for 60 minutes. The conditions for the carburizing process are not particularly limited, and known or arbitrary conditions may be used. By the carburizing treatment described above, a carburized product in which the carburized portion is hardened is obtained. Carburizing increases the surface hardness of the material.

In the shot process of step S6, a post-processed product is obtained by subjecting the carburized product obtained in step S5 to shot peening. In shot peening, by projecting countless shots onto a carburized product, compressive residual stress is generated in the surface layer of the tooth flank and tooth root, and the fatigue strength of the tooth flank and tooth root can be increased. In step S7, a gear is obtained by subjecting the post-processed product obtained in step S6 to a gear grinding step for smoothing the tooth flanks of the post-processed product by polishing or the like.

Next, the gear manufacturing method according to the first embodiment shown in FIG. 1 is a gear manufacturing method in which 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.

In the hot forging process of step S11, hot forging is performed on the prepared raw material steel to obtain a rough-shaped forged product. In the spheroidizing annealing step of step S12, the forged product is subjected to spheroidizing annealing to obtain an annealed product with improved plastic workability and toughness. In the blanking process of step S13, a blank product is obtained in which a shaft hole is formed in the central part of the annealed product. In the rolling and quenching step of step S14, the blank 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. get In the shot process of step S15, shot peening is used to generate compressive residual stress in the surface layer portions of the tooth flank and tooth root of the rolled and hardened product to obtain a post-processed product. In the gear grinding step of step S16, a gear having smoothed tooth flanks is obtained as a post-processed product.

The above steps are described in detail below. The hot forging process of step S11 is the same as step S1 in the gear manufacturing method of the comparative example, except that the raw material steel used is different. As the raw material steel, as described above, the raw material steel having a chemical composition suitable for the gear manufacturing method according to the present embodiment is used. That is, the raw steel material contains 0.75 to 1.10% by mass of carbon, 1.60 to 2.50% by mass of silicon, 0.20 to 1.50% by mass of manganese, and 0.005% by mass of sulfur. ~0.025% by mass, 1.60-3.00% by mass of chromium, 0.10-0.60% by mass of molybdenum, 0.005-0.100% by mass of aluminum, and 0% by mass of nitrogen .010 to 0.025% by mass, and the balance has a component composition of iron and impurities. A forged product obtained by hot forging has a fiber flow formed inside the forged product along the shape of the forged product, thereby enhancing the strength and toughness of the steel.

In the spheroidizing annealing step of step S12, the forged product obtained in step S11 is heated to, for example, 820±10° C., slowly cooled to 700° C.±10° C. over 10 hours, and then air-cooled to form a spherical shape. Annealing is performed to obtain 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. By this step, the crystal structure of the steel can be homogenized and the cementite in the steel can be spheroidized. Spheroidized steel softens and improves workability.

In the blanking step of step S13, a blank is obtained by boring a shaft hole in the center of the annealed product obtained in step S12 using a lathe. In the rolling and quenching step of step S14, the blank obtained in step S13 is heated to a temperature of, for example, 1000±25° C. in 30 seconds so that the metal structure is in an austenitic state, and then heated for 5 seconds. Hold. Thereby, an austenitized steel material is obtained. Note that the transformation point at which the metal structure becomes austenite varies depending on the carbon content of the raw material steel. Therefore, the heating temperature and holding time are not particularly limited, and appropriate conditions can be selected according to the carbon content of the raw material steel.

Subsequently, the obtained austenitized steel material is placed in a rolling device, and plastic working is performed using a rolling die to form the tooth flank of the blank into a gear shape. Here, with reference to FIG. 2, an example of a rolling apparatus used in the rolling and quenching process of step S14 will be described. FIG. 2 is a diagram illustrating a rolling quenching step in the gear manufacturing method according to the first embodiment.

First, as shown in steps S101 and S102 of FIG. 2 , the rolling device 110 has a pair of rolling dies 111 and 112 facing each other and a support 113 . The rolling dies 111 and 112 have a substantially cylindrical shape. The rolling dies 111 and 112 have rolling surfaces on their outer peripheral surfaces for forming tooth profiles on the tooth surfaces of the molding material. Also, 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 at a predetermined distance such that the outer peripheral surface of the rolling die 111 and the outer peripheral surface of the rolling die 112 face each other. The support 113 has a rod-shaped shaft at its tip and supports the molding material.

As a molding material to be molded using such a rolling apparatus 110, blank products obtained in steps S11 to S13 can be used. As shown in step S101 in FIG. 2, the blank W1 has a large diameter portion W2 having a cylindrical shape in which the central portion in the axial direction of the cylinder has a larger diameter than the other portions, and the large diameter portion W2 in the axial direction is excluded. and a small-diameter portion W3 having a cylindrical shape in which the diameter of both end portions is smaller than that of the large-diameter portion W2. A shaft hole W4 is formed in the center of the cylinder in the blank W1.

A shaft portion of a support 113 of the rolling device 110 is inserted into the shaft hole W4. Thereby, the blank W<b>1 is installed between the rolling die 111 and the rolling die 112 of the rolling device 110 . With the shaft portion of the support 113 inserted into the shaft hole W4, the blank W1 is supported by the support 113 and is rotatable about the central axis of the blank W1.

Then, as shown in step S101, with the blank W1 placed on the rolling device 110, the rolling dies 111 and 112 rotating 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 the outer peripheral surface. The rolling dies 111 and 112 and the blank W1 rotate while pressing the outer peripheral surface of the large diameter portion W2.

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 the rolled product W10 in which the groove portion W5 is formed. Further, the rolled product W10 molded into a desired shape is subjected to water quenching from a temperature range of 780 to 950° C., and then rapidly cooled to 50° C. or less for 10 seconds, for example. As a result, the outer peripheral surface of the large diameter portion W2 is rapidly cooled, so that the metal structure in the surface layer portion of the outer peripheral surface transforms into a martensitic structure.

If the temperature at the start of water quenching is less than 780° C., the steel is quenched in a state where no austenitic structure exists, so a martensitic structure cannot be obtained. If the temperature at the start of water quenching exceeds 950°C, the austenite structure is quenched with an excessive carbon concentration, and even if cold working is performed in the subsequent process, a large amount of untransformed austenite structure remains in the steel. end up This causes a decrease in 300° C. tempering hardness (Vickers hardness of the tooth surface when the gear is tempered at 300° C.). Therefore, it is preferable to start water quenching from a temperature range of 780-950°C.

After water quenching, it is tempered, for example, at 150° C. for 1 hour. By the above-described rolling and quenching process, in step S14, the shape of the outer peripheral surface of the rolling dies 111 and 112 is transferred to the outer peripheral surface of the large diameter portion W2, and a molded product having gear-shaped tooth flanks is obtained. A high-frequency induction heating device, for example, can be used for heating in this rolling and quenching step.

In this way, in the rolling process, the tooth flanks are formed by material flow on the surface of the molding material, so that the tooth flanks in the molded product are densified and the strength is improved. Further, rapid cooling transforms the metal structure into a martensitic structure, so that the hardness of the surface layer of the tooth flank in the molded product is improved. It is preferable that the molded product obtained through steps S11 to S14 has a hardness tempered at 300° C. of 850 HV or more.

The shot process of step S15 for post-processing the molded product is the same as step S6 in the gear manufacturing method of the comparative example. In step S15, a post-processed product is obtained by subjecting the molded product obtained in step S14 to shot peening. In shot peening, by projecting countless shots onto the molded product, compressive residual stress is generated in the surface layer of the tooth flank and tooth root of the molded product, and the fatigue strength of the tooth flank and tooth root can be increased. The surface hardness of the post-processed product is preferably 950 HV or more, more preferably 1050 HV or more.

The method of post-processing is not limited to shot peening, and may be roller burnishing, cold rolling, or the like. Any cold working method that can introduce compressive residual stress into the surface layer of the molded article and that does not increase the surface roughness as much as possible is not limited to these. Then, in step S16, the post-processed product obtained in step S15 is subjected to a gear grinding step for smoothing the tooth flanks of the post-processed product by polishing or the like, thereby obtaining a gear.

By the way, in the gear manufacturing method of the comparative example, since the tooth flank is formed by cutting (tooth cutting), the raw material steel is required to have good machinability. Therefore, it is necessary to reduce the content of elements such as carbon and silicon, which are useful for improving the strength of the steel, in the chemical composition of the raw steel material to prevent deterioration of machinability. That is, in the gear cutting of gears, carburizing treatment is performed in a post-process along with the use of low-carbon steel as a raw material steel. Further, when carburizing treatment is performed, the carburization property of steel with a high silicon content is deteriorated. Therefore, it is necessary to reduce the silicon content in the raw material steel. As described above, in the gear manufacturing method of the comparative example, it is difficult to use high-hardness raw material steel from the viewpoint of tool breakage and tool life.

Further, in the gear manufacturing method of the comparative example, the fiber flow formed in the hot forging process of step S1 is cut by cutting performed in the hobbing process of step S4. Therefore, the manufactured gear cannot be expected to have improved strength and toughness due to fiber flow.

On the other hand, in the gear manufacturing method according to the present embodiment, since the method of forming the tooth flanks is rolling, it is possible to process a high-hardness raw material steel material with a high content of elements such as carbon and silicon. have. Also, if the raw material steel is high-carbon steel, there is no need to perform carburizing treatment, so the silicon content in the raw material steel can be increased compared to cutting. As the silicon content in the raw material steel increases, the contact fatigue strength of the gear improves. Therefore, an improvement in the strength of the manufactured gear can be expected.

Further, in the gear manufacturing method according to the present embodiment, there is no process involving cutting of the fiber flow after the fiber flow is formed in the hot forging process of step S11. Therefore, the manufactured gear can be expected to have improved strength and toughness due to fiber flow.

Furthermore, according to the gear manufacturing method according to the present embodiment, dislocations are accumulated in the crystals of the steel material due to the rolling process, and work hardening occurs on the surface of the steel material. It has the feature that such a distortion effect can be obtained. The introduction of the distortion effect in this embodiment will now be described with reference to FIG. 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 heat history of carburizing and quenching shown in FIG. A manufacturing process is shown. In this case, the steel material hardened by the strain effect introduced during the cold forging period 201 is heated at the carburizing temperature during the carburizing process period 202 and then quenched during the quenching process period 203 . The carburizing process is performed to improve the strength of the steel material, but since the heating temperature of the carburizing process is high enough to eliminate dislocations, the strain effect disappears during the carburizing process period 202. Therefore, products manufactured by such a manufacturing process do not have the effect of distortion.

On the other hand, the heat history of rolling and quenching according to the first embodiment in FIG. 3 shows the manufacturing process of imparting shape and strength to the molding material by rolling and introducing the effect of distortion. In this case, the steel material hardened by the strain effect introduced during the rolling period 212 is subjected to quenching during the quenching period 213 . According to such a manufacturing process, it is not necessary to perform heating to eliminate the effect of distortion. Therefore, the strain effect can be maintained even after the hardening is completed. Therefore, the strength of the obtained gear is improved.

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

Table 2 shows examples of manufacturing gears. Among the categories shown in Table 2, Examples 1 to 6 are manufacturing examples of gears manufactured by the gear manufacturing method according to the present embodiment. Reference Examples 7 to 10 are gear manufacturing examples in which the conditions of the gear manufacturing method according to the present embodiment are not satisfied. In Tables 1 and 2, data outside the scope of the present embodiment are underlined.

Further, in the following description, each manufacturing method is classified according to the tooth surface forming method, the gear manufacturing method (steps S1 to S7) of the comparative example is referred to as a hobbing method, and the gear manufacturing method according to the first embodiment ( Steps S11 to S16) may be referred to as a rolling method. Further, for each of the manufactured gears, the average aspect ratio of prior austenite grains, hardness tempered at 300° C., and maximum compressive residual stress were investigated. Each measurement method will be explained.

The method for measuring the average aspect ratio of the prior austenite grains is as follows: First, the gear is cut along a plane perpendicular to the tooth surface of the gear, and after the cut surface is polished, the prior austenite grains are revealed by corrosion. This sample is used as a sample to be observed. Next, the cut surface of the sample to be observed is observed with an optical microscope. Furthermore, 100 prior austenite grains were randomly extracted in a range of 300 μm from the tooth flank and tooth root surface layer, and the value obtained by dividing the major axis by the minor axis was calculated as the aspect ratio of each prior austenite grain. An average value was calculated from the calculated aspect ratios of the respective prior austenite grains, and was taken as the average aspect ratio of the prior austenite grains.

In a gear, since high stress acts in a range of 300 μm from the surface layer of the tooth flank and tooth root, it is necessary to strengthen this range in order to increase the strength of the gear. The larger the average aspect ratio of the prior austenite grains, the more the strain effect remains in the product after quenching (rolled and quenched product in the present embodiment). If the average aspect ratio of the prior austenite grains at 300 μm from the surface layer of the tooth flank and tooth root of the gear is less than 8, the strain effect is considered to be insufficient. In order to reliably obtain the effect of distortion, the average aspect ratio of the prior austenite grains in the range of 300 μm from the surface layer of the tooth flank and tooth root of the gear is preferably 8 or more.

The 300° C. tempering hardness is measured by holding the gear at 300° C. for 1 hour and then allowing it to cool. After that, the gear is cut along a plane perpendicular to the tooth surface, and the cut surface is polished. This is used as a sample to be measured. Furthermore, according to the Vickers hardness test specified in JIS Z 2244:2009, the Vickers hardness is measured at a position 50 μm from the surface of the cut surface of the sample to be measured. The test load shall be 300 gf. Then, the Vickers hardness was measured at 5 points according to the above procedure, and the average value was taken as the 300° C. tempered hardness of the tooth surface.

As for the maximum compressive residual stress, the compressive residual stress was measured in a range of 300 μm from the tooth root surface layer of the gear using an X-ray stress measurement method using X-ray diffraction, and the maximum value was taken as the maximum compressive residual stress.

Next, the details of the manufacturing example of the gear shown in Table 2 will be described. In Example 1 shown in Table 2, gears were manufactured from steel A shown in Table 1 by rolling. The obtained gear had an average aspect ratio of prior austenite grains of 8.5, a hardness tempered at 300° C. of 860 HV, and a maximum compressive residual stress of 1520 MPa.

In Example 2 shown in Table 2, gears were manufactured from Steel B shown in Table 1 by rolling. The obtained gear had an average aspect ratio of prior austenite grains of 9.0, a hardness tempered at 300° C. of 910 HV, and a maximum compressive residual stress of 1546 MPa.

In Example 3 shown in Table 2, gears were manufactured from Steel C shown in Table 1 by rolling. The obtained gear had an average aspect ratio of prior austenite grains of 10.8, a hardness tempered at 300° C. of 968 HV, and a maximum compressive residual stress of 1598 MPa.

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

In Example 5 shown in Table 2, gears were manufactured from Steel E shown in Table 1 by rolling. The obtained gear had an average aspect ratio of prior austenite grains of 9.7, a hardness tempered at 300° C. of 930 HV, and a maximum compressive residual stress of 1566 MPa.

In Example 6 shown in Table 2, a gear was manufactured from steel F shown in Table 1 by rolling. The obtained gear had an average aspect ratio of prior austenite grains of 9.9, a hardness tempered at 300° C. of 932 HV, and a maximum compressive residual stress of 1573 MPa.

In Reference Example 7 shown in Table 2, gears were manufactured from Steel G shown in Table 1 by rolling. The obtained gear had an average aspect ratio of prior austenite grains of 8.8, a hardness tempered at 300° C. of 755 HV, and a maximum compressive residual stress of 1297 MPa.

In Reference Example 8 shown in Table 2, a gear was manufactured from steel H shown in Table 1 by rolling. The obtained gear had an average aspect ratio of prior austenite grains of 9.5, a hardness tempered at 300° C. of 769 HV, and a maximum compressive residual stress of 1302 MPa.

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

In Reference Example 10 shown in Table 2, a gear was manufactured from Steel I shown in Table 1 by the hobbing method. The obtained gear had an average aspect ratio of prior austenite grains of 1.8, a hardness tempered at 300° C. of 790 HV, and a maximum compressive residual stress of 1508 MPa.

As is clear from these results, in the gears of Examples 1 to 6 manufactured based on the gear manufacturing method according to Embodiment 1, the average aspect ratio of the prior austenite grains was 8.5 or more, and the tempering at 300 ° C. A hardness of 860 HV or more and a maximum compressive residual stress of 1520 MPa or less were obtained. On the other hand, in Reference Examples 7 to 10, two of the three data of the prior austenite aspect ratio, the 300° C. tempering hardness, and the maximum compressive residual stress did not satisfy the specifications of Examples 1 to 6. rice field.

In addition, as suggested by the above manufacturing examples, according to the gear manufacturing method according to the first embodiment, it is possible to use a raw steel material with a high carbon and silicon content. Increasing the carbon content of the raw material steel makes it possible to omit the carburizing process, which makes it possible to use high-silicon materials. Further, the contact fatigue strength of the gear is improved by increasing the silicon content of the raw material steel.

Furthermore, according to the gear manufacturing method according to the first embodiment, it is possible to obtain a gear in which the average aspect ratio of the prior austenite grains at 300 μm from the tooth flank and tooth root surface layer is 8 or more. In the gear manufacturing method according to the first embodiment, quenching is performed after giving shape and strength to the molding material by rolling. That is, the manufactured gear can obtain the effect of strain because it is not accompanied by heating that would eliminate the effect of strain introduced by the rolling process. In addition, since cutting is not involved, fiber flow remains in the manufactured gear. Therefore, the strength and toughness of the gear are improved.

Furthermore, according to the gear manufacturing method according to the first embodiment, since the tooth flanks are formed by rolling, it is possible to increase the content of silicon, which contributes to the improvement of temper softening resistance, in the raw material steel used. In addition, the gears produced have the effect of strain, which further improves the resistance to temper softening. As a result, it is possible to obtain a gear whose tooth flank has a hardness tempered at 300° C. of 850 HV or more, and the surface fatigue strength of the gear is improved.

Furthermore, according to the gear manufacturing method according to the first embodiment, by performing post-processing such as shot peening, it is possible to obtain a gear having a maximum compressive residual stress of 1500 MPa or more in a range of 300 μm from the surface layer of the tooth root of the gear. is possible. Compressive residual stress improves the root bending fatigue strength of gears by suppressing the occurrence and propagation of cracks.

The gear manufacturing method according to the present embodiment has the above effects. Further, it was confirmed that the gears of Examples 1 to 6 manufactured according to the gear manufacturing method according to the present embodiment were superior in strength to the gears of Reference Examples 7 to 10. The method for manufacturing a gear according to the present embodiment can effectively improve the strength of the gear through the chemical composition of the raw material steel material, rolling, and heat treatment that are effective in improving the strength of the gear.

110 rolling device 111 rolling die 112 rolling die 113 support tool 201 cold forging period 202 carburizing treatment period 203, 213 quenching period 212 rolling period W1 blank W2 large diameter portion W3 small diameter portion W4 shaft hole W5 groove portion W10 rolling fake

Claims (2)

a step of heating a raw material steel material to bring the raw material steel material into an austenite state to produce an austenitized steel material;
a step of forming a tooth flank on the austenitized steel material by rolling, and then rapidly cooling the austenitic steel material to a martensite transformation temperature or less;
The raw steel material contains 0.75 to 1.10% by mass of carbon, 1.60 to 2.50% by mass of silicon, 0.20 to 1.50% by mass of manganese, and 0.005 to 0.005% of sulfur. 0.025% by weight, 1.60-3.00% by weight of chromium, 0.10-0.60% by weight of molybdenum, 0.005-0.100% by weight of aluminum, and 0.00% by weight of nitrogen. 010 to 0.025% by mass, the balance being iron and impurities. The raw steel material contains 0.75 to 1.10% by mass of carbon, 1.60 to 2.50% by mass of silicon, 0.20 to 1.50% by mass of manganese, and 0.005 to 0.005% of sulfur. 0.025 wt%, 1.60-3.00 wt% chromium, 0.10-0.60 wt% molybdenum, 0.005-0.100 wt% aluminum, 0.010 wt% nitrogen ~ 0.025% by mass, the balance having a component composition of iron and impurities,
The average aspect ratio of the prior austenite grains in the range of 300 μm from the surface layer of the tooth surface and tooth root is 8 or more,
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 in a range of 300 μm from the surface layer of the tooth root.

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