JP2012017499A - Gear with excellent fatigue resistance and method of manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a gear with high surface pressure fatigue resistance which is required for an automobile, a variety of industrial equipment and the like, and a method of manufacturing the same.SOLUTION: The gear is manufactured by shaping steel having a specific component into a gear shape by forging or machining after forging, and carburizing, quenching and tempering the steel followed by shot peening. In the gear, a carburization surface layer: residual γ structure from the surface of the gear to 30 μm depth (vol.%) is 25% or more and 58% or less, and is 7% or less after shot peening with the remainder having martensite structure; the crystal particle size in the carburization surface layer is 8.5 or more; the compression residual stress of the tooth surface and tooth root surface after shot peening is 1,500 MPa or more, the surface hardness thereof is HV850 or more; and a parameter expression having a residual γ quantity, tooth surface Vickers hardness and tooth surface residual stress as factors is satisfied.

Description

  The present invention relates to a gear having a high surface pressure fatigue strength used for automobiles and various industrial equipment and a method for manufacturing the same, and more particularly to a gear suitable for a final gear.

  In recent years, gears used in automobiles and the like have been required to be reduced in size with the reduction in weight of the vehicle body to save energy. On the other hand, the load has increased due to the higher output of the engine. There is a need for improved durability against fracture and tooth surface fatigue failure.

  Conventionally, gears were formed using case-hardened steel such as JIS SCM420H, SCM822H, etc., and surface treatment such as carburization etc. has been used for automobiles, etc., but since it can not withstand use under high stress, change of steel Research and development to improve tooth root bending fatigue strength and pitting resistance by changing the heat treatment method and work hardening of the surface have been promoted.

  For example, in Patent Document 1, by reducing Si in steel and controlling Mn, Cr, Mo, Ni, the grain boundary oxide layer on the surface after carburizing heat treatment is reduced, and the occurrence of cracks is reduced. A method is described in which the generation of incompletely hardened layer is suppressed, the reduction in surface hardness is suppressed, the fatigue strength is increased, and the extension of MnS that promotes the generation and propagation of cracks by addition of Ca is described. .

  Patent Document 2 describes that the temper softening resistance is increased by using a steel material to which Si is added in an amount of 0.25 to 1.50 mass%.

Japanese Patent Publication No. 07-122118 Japanese Patent No. 2945714

  However, according to Patent Document 1, when Si is reduced, it is possible to reduce the grain boundary oxide layer and the incomplete quenching layer and suppress the occurrence of bending fatigue cracks at the tooth root, but conversely the reduction in temper softening resistance. The starting point of fracture shifts from the tooth base to the tooth surface side, it becomes impossible to suppress temper softening due to frictional heat on the tooth surface, and the surface becomes soft and pitching tends to occur.

  In Patent Literature 2, Si or the like is added to increase the temper softening resistance, and the carburizing method is limited to vacuum carburizing or plasma carburizing to suppress the progress of grain boundary oxidation. It is not effective for mass production because of increased manufacturing costs.

  An object of the present invention is to solve the above-described problems and provide a gear having excellent fatigue characteristics and a manufacturing method capable of mass production.

In the present invention, in order to solve the above-mentioned problems, intensive research has been conducted and the following has been found.
1. It is effective to increase the surface hardness and the surface compressive residual stress in order to suppress the destruction due to the pitching of the tooth surface. Furthermore, it is possible to further extend the service life by suppressing the characteristic deterioration caused by the low-temperature tempering of the tooth surface when the gear is driven.
2. As the surface hardness is higher, the occurrence of cracks in surface fatigue is suppressed, but the hardness after carburization is limited. Therefore, the surface hardness can be drastically improved by performing the shot peening treatment after appropriately depositing the retained austenite on the carburized surface.
3. The surface compressive residual stress can suppress the crack propagation from the part where the pitting occurs and can extend the life. The maximum value is closer to the surface layer, and the higher the better.
4). The addition of Si in steel increases not only the temper softening resistance and reduces the decrease in hardness, but also suppresses the decrease in compressive residual stress on the surface, thereby delaying the generation and propagation of cracks in the tooth surface when the gear is driven.
5. The surface oxidation due to the addition of Si increases, but the progress in the depth direction is delayed because the oxidation proceeds at the grain boundaries on the surface and in the whole area within the grains. Also, the surface oxide layer is peeled off by shot peening, so there are fewer surface defects than steel where only the oxidation along the grain boundary proceeds, and it is also possible to prevent the occurrence of pitting on the tooth surface. It is also effective in preventing cracking.
6). The reduction in the surface hardness and the reduction in the compressive residual stress when the gear is driven are closely related to the characteristics after tempering at 300 ° C. for 1 hour.
This invention is made | formed based on the above knowledge, ie, (1) component composition is C: 0.15-0.35 mass%, Si: 0.6-1.1 mass%, Mn: 0.3 -1.3 mass%, S: 0.006 mass% or more, Cr: 0.7-1.7 mass%, Mo: 0.4 mass% or less, Al: 0.005-0.050 mass%, N: 0.0040- 0.0200 mass%, balance Fe and inevitable impurities steel material is forged or after forging and machined into a gear shape, then carburized, quenched and carburized, nitrocarburized, quenched and tempered, then shot peened A gear to be manufactured,
The residual austenite structure from the surface to the 30 μm depth position after carburizing and tempering or carburizing and nitriding quenching and tempering is 25% by volume or more and 58% by volume or less, and 7% by volume or less after shot peening. It has a martensitic structure, the grain size of the carburized tempered or carburized nitrocarburized quenched and tempered region is 8.5 or more, and the compressive residual stress on the tooth surface and root surface after shot peening is 1500 MPa. Thus, a gear having excellent fatigue strength, characterized in that the hardness of the tooth surface and the tooth root surface is Vickers of 850 or more and further satisfies the following formula (1).
(1.96γR _CS −0.4γR _SS + HV _CS + σR _SS ) − (HV _{300 ° C.} + σR _{300 ° C.} ) ≦ 640
... (1)
Where γRcs: residual austenite of the carburized surface layer portion, γRss: residual austenite after shot peening of the carburized surface layer portion, HVcs: tooth surface Vickers hardness after carburizing or carburizing, σR _SS : tooth surface residual stress after shot peening (MPa), HV _{300 ° C .} : tooth surface Vickers hardness after tempering at 300 ° C., σR _{300 ° C .} : tooth surface compressive residual stress after tempering at 300 ° C. (MPa)
(2) As a component composition of steel materials, Nb: 0.010-0.060mass%, V: 0.03-0.20mass%, Ti: 0.005-0.200mass% of 1 or more types are contained further. A gear having excellent fatigue strength as described in (1).
(3) The gear having excellent fatigue strength according to (1) or (2), further including B: 0.0005 to 0.0050 mass% as a component composition of the steel material.
(4) A steel material having the composition described in any one of (1) to (3) is subjected to cold forging, or machining after hot forging, machining after cold forging, or after warm forging. After making into a gear shape by any of machining, carburizing treatment or carburizing nitriding and diffusion treatment is performed at 900 to 980 ° C, and then furnace cooling to 780 to 880 ° C, followed by oil at 60 to 130 ° C A method for producing a gear excellent in fatigue strength, characterized in that after tempering and quenching, tempering is performed by reheating to 150 to 220 ° C., and shot peening is further performed on the tooth surface of the gear.
(5) The method for producing a gear excellent in fatigue strength according to (4), wherein the shot peening is performed using particles having a particle size of 0.05 to 0.1 mmΦ and a hardness of 700 HV or more.
(6) The shot peening is performed using grains having a particle diameter of 0.4 to 1.2 mmΦ and a hardness of 700 HV or more, and then performed again using grains having a hardness of 0.05 to 0.1 mmΦ and a hardness of 700 HV or more. (4) The manufacturing method of the gear excellent in fatigue strength characterized by the above-mentioned.
(7) The shot peening is performed by mixing grains having a particle diameter of 0.4 to 1.2 mmΦ and a hardness of 700 HV or more and grains having a hardness of 0.05 to 0.1 mmΦ and a hardness of 700 HV or more. (4) The manufacturing method of the gear excellent in the fatigue strength of description.

  According to the present invention, a gear excellent in fatigue strength suitable for use in automobiles, industrial machines and the like can be obtained, which is extremely useful industrially.

The figure explaining the carburizing and tempering process conditions used for the Example.

In the present invention, the composition of steel as a raw material, the microstructure and hardness of the surface layer of the gear, and the magnitude of compressive residual stress on the tooth surface and the surface of the tooth root are defined. Each limitation reason is described below.
[Ingredient composition]
C: 0.15-0.35 mass%
C is necessary for securing the strength, and determines the internal hardness after carburizing and tempering. If it is less than 0.15 mass%, the internal hardness is lowered, the bending stress at the tooth root is lowered, and the strength as a gear cannot be secured. On the other hand, if it exceeds 0.35 mass%, the toughness inside the gear deteriorates, and cracking occurs early due to bending stress at the tooth root, resulting in a short life, so 0.15 to 0.35 mass%. To do.

Si: 0.6-1.1 mass%
Si is an element effective for increasing the temper softening resistance. When the softening resistance is low, the tooth surface layer is tempered and softened by the friction between the gears when the gear is driven, and the surface fatigue characteristics are deteriorated. The effect is effective at 0.6 mass% or more.

  Si also has the effect of raising the transformation point. In order to obtain a martensitic structure partially containing retained austenite, it is necessary to heat it to the transformation point or more once during carburizing and quenching to obtain an austenitic structure. However, if it exceeds 1.1 mass%, it is heated only below the transformation point at the time of carburizing, and ferrite is generated inside the carburized portion, so that the bending fatigue strength at the tooth base is lowered. -1.1 mass%.

Mn: 0.3 to 1.3 mass%
Mn is an element that enhances hardenability. In order to ensure hardenability, it is 0.3 mass% or more. On the other hand, if added in excess of 1.3 mass%, the hardenability becomes excessive, the amount of retained austenite becomes excessive, the hardness of the carburized portion is lowered, and the fatigue characteristics are lowered. 3 mass%.

S: 0.006 mass% or more S combines with Mn to form MnS and improves machinability. If the amount is less than 0.006 mass%, the amount of MnS produced is so small that the effect cannot be sufficiently obtained.

Cr: 0.7-1.7 mass%
Cr is an element improving the temper softening resistance as well as a hardenability improving element. Addition of 0.7 mass% or more is necessary to exert both performances, but if it exceeds 1.7 mass%, the effect of increasing the softening resistance is saturated. Further, since the hardenability becomes too high, the toughness inside the gear deteriorates, the fatigue crack progresses quickly, and the bending fatigue strength decreases, so 0.7 to 1.7 mass%.

Mo: 0.4 mass% or less Mo is an element that improves hardenability. However, if it exceeds 0.4 mass%, the hardenability is too high, and cracking is likely to occur. 4 mass% or less.

Al: 0.005 to 0.050 mass%
Al is an element effective for deoxidation, and the effect is exhibited by addition of 0.005 mass% or more. Further, when added up to 0.050 mass%, it combines with N to produce AlN, and has a function of suppressing coarsening of crystal grains. If it exceeds 0.050 mass%, coarse grains are generated and fatigue cracks are likely to progress, and the bending fatigue strength and the surface fatigue strength are reduced. Therefore, the content is set to 0.005 to 0.050 mass%.

N: 0.0040-0.0200 mass%
N combines with Al to produce AlN, which suppresses coarsening of crystal grains and improves fatigue strength. In order to obtain the effect, 0.0040 mass% or more is necessary, but if it exceeds 0.0200 mass%, the effect is not only saturated, but also defects such as blowholes are generated inside, leading to a decrease in bending fatigue strength. Therefore, it is set as 0.0040-0.0200 mass%.

  The above is the basic component composition of the present invention, but when further improving the characteristics, one or more of Nb, V, Ti and B are added.

V: 0.030-0.20 mass%
V improves the hardenability and increases the temper softening resistance like Si and Cr. In addition, carbonitride is formed to suppress coarsening of crystal grains. In order to exert such an effect, addition of 0.030 mass% or more is necessary, but even if added over 0.20 mass%, it is saturated, a sufficient effect cannot be obtained, and the manufacturing cost Therefore, when added, the content is set to 0.030 to 0.20 mass%.

Nb: 0.010 to 0.060 mass%
Nb refines crystal grains by forming carbonitride and improves the bending fatigue strength of the tooth root. The effect of refining crystal grains increases at 0.010 mass% or more. On the other hand, even if added over 0.060 mass%, the effect becomes saturated, so it is set to 0.010 to 0.060 mass%.

Ti: 0.005 to 0.200 mass%
Ti produces a fine Ti compound to reduce crystal grains after forging and increase the strength. The effect increases at 0.005 mass% or more. On the other hand, if added over 0.200 mass%, the Ti precipitate becomes coarse, and the life becomes lower as a starting point of fatigue failure. 0.005 to 0.200 mass%.

B: 0.0005 to 0.0050 mass%
B is effective in increasing the hardenability. The effect is obtained at 0.0005 mass% or more, and when it exceeds 0.0050 mass%, saturation occurs. Therefore, when added, the content is set to 0.0005 to 0.0050 mass%.
[Microstructure and hardness]
The crystal grain size as the microstructure of the carburized surface layer of the gear is specified to be 8.5 or more. It is preferable to increase the strength in order to suppress the occurrence of cracks in the tooth root and tooth surface and the progress of the cracks that have occurred. The smaller the crystal grain, the higher the strength. Therefore, the crystal grain size is set to 8.5 or more. The carburized surface layer portion is a carburized quenching tempered or carburized nitrocarburized quenched and tempered region up to a depth of 30 μm from the gear surface.

  Further, the amount of residual γ (also referred to as austenite) in the carburized surface layer after carburizing is set to 25 to 58% by volume. Residual austenite is soft by itself, but its hardness can be increased by the process-induced transformation of shot peening. In order to obtain a surface hardness of HV850 or more that prevents initial cracks from occurring after shot peening, the amount of retained austenite in the carburized surface layer after carburizing is set to 25 to 58% by volume. In the present invention, the hardness (HV) is defined as a value obtained with a load of 10 kg.

  If it is less than 25% by volume, the increase in hardness is small even if shot peening is performed under various conditions, and it is difficult to obtain an improvement in surface hardness. On the other hand, if it is higher than 58% by volume, the retained austenite is present after the shot peening, and the hardness is not improved.

  Furthermore, the amount of retained austenite in the carburized surface layer after shot peening is set to 7% by volume or less. If a large amount of retained austenite is present after shot peening, the unevenness of the tooth surface after driving becomes intense, fine pitching occurs early, and the time to final breakage is shortened.

  The hardness of the tooth surface and the root surface after shot peening is 850 HV or more. The initial cracks at the tooth surface and the root that lead to the occurrence of pitching are less likely to occur as the hardness increases. In order to suppress initial cracks, the hardness of the tooth surface and root after shot peening is 850 HV or more.

[Compressive residual stress on tooth surface and root surface]
The compressive residual stress of the tooth surface and root after shot peening is 1500 MPa or more. It is possible to further improve the life by suppressing the occurrence of initial cracks that lead to pitching with the above-mentioned definition of hardness and further suppressing the crack propagation. In order to suppress the progress of cracks, it is desirable to apply compressive residual stress. In order to obtain a long life, the compressive residual stress of the tooth surface after shot peening is limited to 1500 MPa or more.

(1.96γR _CS −0.4γR _SS + HV _CS + σR _SS ) − (HV _{300 ° C.} + σR _{300 ° C.} ) ≦ 640 (1)
Where γRcs: residual austenite of the carburized surface layer portion, γRss: residual austenite after shot peening of the carburized surface layer portion, HVcs: tooth surface Vickers hardness after carburizing or carburizing, σR _SS : tooth surface residual stress after shot peening (MPa), HV _{300 ° C .} : tooth surface Vickers hardness after tempering at 300 ° C., σR _{300 ° C .} : tooth surface compressive residual stress (MPa) after tempering at 300 ° C.

  This parameter formula is defined in order to improve the surface fatigue strength of the tooth surface. The tooth surface hardness after shot peening, the tooth surface compressive residual stress, the residual austenite amount, and the characteristics after tempering of the tooth surface are 300. It consists of the Vickers hardness (also referred to as tooth surface Vickers hardness) of the tooth surface after tempering at 1 ° C. for 1 hour. As described above, the decrease in the surface hardness and the decrease in the compressive residual stress when the gear is driven are closely correlated with the characteristics after tempering at 300 ° C. for 1 hour. When this parameter formula is satisfied, the surface fatigue strength is improved.

  The gear according to the present invention is manufactured as follows. A steel material having the above composition is formed into a desired gear shape by cold forging. Alternatively, a desired gear shape is formed by machining after hot forging, cold forging, or warm forging. Hot forging is performed at 1000 to 1300 ° C, and warm forging is performed at 400 to 950 ° C.

  The member processed into a gear shape is then subjected to carburizing treatment or carburizing / nitrogenizing and diffusion treatment at 900 to 980 ° C., and then furnace-cooled to 780 to 880 ° C. and then poured into oil at 60 to 130 ° C. After tempering, tempering is performed by reheating to 150 to 220 ° C.

  Finally, shot peening is performed on the tooth surface of the gear. (1) Shot peening using particles having a particle size of 0.05 to 0.1 mmΦ and a hardness of 700 HV or more, and (2) particle size of 0.4 to 1 Shot peening performed using particles having a hardness of 700 HV or more having a hardness of 2 mmΦ, and then again using particles having a hardness of 700 to HV having a particle size of 0.05 to 0.1 mmΦ, and (3) a particle size of 0.4 Any one of shot peening is performed by mixing grains having a hardness of 700 HV or higher with a diameter of ˜1.2 mmΦ and grains having a hardness of 700 HV or higher with a hardness of 0.05 to 0.1 mmΦ. As the shot peening, the fatigue characteristics are improved in the order of (1) to (3), but the proper selection is made in consideration of the manufacturing cost. Hereafter, the effect of this invention is shown concretely by an Example.

  Steels having chemical components shown in Table 1 were melted. No. shown in the table. Nos. 1 to 25 are developed steels, which are within the range of the composition of the present invention. Nos. 26 to 44 are out of the component range of the present invention and are comparative examples. No. Nos. 45 and 46 are conventional steels. 45 is JIS SCM420H, No. 45. 46 is an SCM822H standard material.

  The ingots of the present invention steel, comparative steel, and conventional steel that were melted were prepared into a round bar steel having a diameter of 70 mm by hot rolling, and the obtained round bar steel was subjected to a normalization treatment.

  A plurality of helical gears having a module 2.5 and a pitch diameter of 80 mm were formed by hot-forging and machining the round bar after the normalizing treatment. The carburizing and tempering treatment shown in FIG. 1 was performed on the helical gear.

  One of the plurality was used for measuring the amount of retained austenite on the tooth surface. Further, the surface of the remaining gear is subjected to shot peening with an arc height of 0.4 mmA or more using a steel ball having a particle diameter of 0.05 to 1.0 mm, and one of them is used to retain residual austenite on the tooth surface. The amount, grain boundary oxide layer depth, surface hardness, effective hardened layer depth investigation, carburized former austenite grain size, and tooth surface residual stress were investigated. The other one was used, and after tempering at 300 ° C. for 1 hour, the surface hardness (tooth surface Vickers hardness after tempering at 300 ° C.) and the residual stress on the tooth surface were investigated. Further, a gear fatigue test was performed using the remaining gears.

  No. For the 25 gears, in addition to the above, separately prepared gears were prepared, and the carburizing time at the time of carburizing, the ratio of the diffusion time, and the process of changing those times were implemented in addition to the above four types. Three things were prepared and the same investigation was conducted. Details of each investigation method will be described below.

[Effective hardened layer depth, structure observation, surface hardness]
The hardness distribution of the shot peened tooth cross section was measured, and the depth at which 550 HV was obtained from the tooth surface in terms of Vickers hardness was determined as the effective hardened layer depth. Moreover, the grain boundary oxide layer depth was investigated by the prior austenite grain size and structure observation. The surface hardness was an average value of 10 points of the Vickers hardness (load 10 kg) on the tooth surface.

[Residual γ (austenite) amount]
The amount of residual γ up to a position 30 μm below the surface was measured. Electrolytic polishing was used for polishing the measurement surface, and an X-ray diffractometer was used for measurement.

[Residual stress]
For the measurement, the same X-ray diffractometer as that for measuring the amount of retained austenite was used.

[Gear fatigue test]
Using a power circulation type gear fatigue tester, transaxle oil at 80 ° C. is used for lubrication, a predetermined torque is applied, and the rotational speed is 3000 r. p. The test was conducted at m. At that time, the torque reaching up to 10 million repetitions was determined as fatigue strength.

  Table 2 shows the test results. No. in the table. No. in Table 1. Refers to the same thing. From Table 2, the following matters are clear.

  Invented steel No. For 1 to 25, high gear fatigue strength was obtained. In contrast, no. No. 26 has a C% lower than the scope of the invention. Therefore, the internal hardness after carburizing, quenching and tempering was low, the amount of retained austenite was less than the range of the present invention, and the surface layer hardness after shot peening was also lower than the range of the present invention. As a result, the surface hardness after tempering at 300 ° C. also decreased, the tooth root was easily broken and the tooth surface was pitched, and the gear fatigue strength was reduced.

  No. 27 has a higher C content than the scope of the invention. Therefore, the amount of residual γ after carburizing is too large, and since there is a lot of retained austenite even after shot peening, high surface hardness is not obtained. Therefore, equation (1) is not satisfied and pitching is not achieved. It became easy to happen. In addition, the internal toughness was insufficient, and the destruction at the root portion was likely to occur. Therefore, the gear fatigue strength decreased.

  No. In No. 28, the Si amount is less than the range of the present invention, so that the grain boundary oxide layer is deepened. The tempering hardness at 300 ° C. and the residual stress are low, and as a result, the formula (1) is not satisfied. Due to these effects, pitching is likely to occur, and the gear fatigue strength is reduced.

  No. 29 has a larger Si content than the range of the present invention. As a result, ferrite was generated inside, bending fatigue failure at the tooth root was likely to occur, and the gear fatigue strength was reduced.

  No. No. 30 has a Mn content lower than the range of the present invention. For this reason, the hardenability is too low, and the effective hardened layer depth is too shallow, so that bending fatigue failure at the root tends to occur, and the gear fatigue strength is reduced.

  No. No. 31 has a Mn amount higher than the range of the present invention. For this reason, the amount of retained austenite after carburizing was too large, so that the surface hardness was low, the equation (1) was not satisfied, pitching was likely to occur, and the gear fatigue strength was reduced.

  No. 32 has a small amount of S. Therefore, the processing accuracy was poor in the specimen processing, and the fatigue test was not possible due to the excessive vibration.

  No. No. 33 has a small amount of Cr, and the hardenability is too low. For this reason, the hardened layer depth becomes too shallow and breakage at the tooth base is likely to occur, and pitching is also likely to occur, resulting in a reduction in gear fatigue strength.

  No. Since the amount of Cr is too high, the internal hardness is too high. As a result, the toughness of the tooth root deteriorates and the bending breakage easily occurs, and the fatigue strength decreases.

  No. 35 has a high Mo content. Therefore, the hardenability was too high, and cracking occurred during carburizing and quenching, so that a fatigue test could not be performed.

  No. In No. 36, since the Al amount was too low, destruction at the root and tooth surface due to inclusion starting points was likely to occur, and the gear fatigue strength was reduced.

  No. In No. 37, since the amount of Al was too high, the austenite grains in the carburized portion were coarsened, and breakage at the tooth root and tooth surface was likely to occur, resulting in a reduction in gear fatigue strength.

  No. No. 38 was added with a V amount lower than the range of the invention, so that the hardenability was not improved and the crystal grains were coarsened. Therefore, bending breakage at the tooth root and pitching at the tooth surface are likely to occur, and the gear fatigue strength is reduced.

  No. No. 39 has a Nb content less than the range of the present invention. As a result, the crystal grains became coarse, bending breakage at the tooth roots easily occurred, and the gear fatigue strength decreased.

  No. No. 40 has a Ti amount less than the range of the present invention. For this reason, the crystal grains are large, bending breakage at the tooth root is likely to occur, and the gear fatigue strength is reduced.

  No. 41 has too much Ti amount. Therefore, destruction at the Ti-based inclusion starting point occurred frequently. For this reason, bending breakage at the tooth root and pitching at the tooth surface are likely to occur, and the gear fatigue strength is reduced.

  No. 42 is too low in N content. As a result, coarsening of the crystal grains occurred, bending breakage at the tooth roots easily occurred, and the gear fatigue strength decreased.

  No. 43 is too high in N content. Therefore, many fractures occurred at the starting point of internal cracks, and the gear fatigue strength decreased.

  No. No. 44 has too low B amount. Therefore, hardenability is insufficient and the effective hardened layer depth is too shallow. As a result, fracture due to tooth root bending stress easily occurred and the gear fatigue strength decreased.

  No. In Nos. 45 and 46, since the Si amount is lower than the range of the present invention, the tempering hardness at 300 ° C. and the residual stress are low. Therefore, the formula (1) is not satisfied, and pitching is likely to occur. Further, since the surface oxidation proceeded deeply along the grain boundary, it was easy to break due to the root bending stress, and the gear fatigue strength decreased.

  No. 25-2 is within the component range of the present invention, but the amount of retained austenite after carburizing is too high. Therefore, even after shot peening, the amount of retained austenite is too large, the surface hardness is low, and pitching occurs. The gear fatigue strength decreased.

  No. 25-3 is also within the range of the composition of the present invention. However, since the processing-induced transformation of residual austenite by shot peening is insufficient and there is a lot of residual austenite after shot peening, surface hardness is low and pitching occurs. It became easier and the gear fatigue strength decreased.

  No. 25-4 is also within the range of the present invention component, but the surface hardness is reduced by carburizing conditions. Therefore, pitching is likely to occur, and the gear fatigue strength is reduced.

  No. Although 25-5 is also within the range of the present invention, the application of residual stress by shot peening is small. Therefore, pitching is likely to occur, and the gear fatigue strength is reduced.

Claims (7)

Component composition is C: 0.15-0.35 mass%, Si: 0.6-1.1 mass%, Mn: 0.3-1.3 mass%, S: 0.006 mass% or more, Cr: 0.7 ~ 1.7 mass%, Mo: 0.4 mass% or less, Al: 0.005-0.050 mass%, N: 0.0040-0.0200 mass%, balance Fe and inevitable impurities steel material by forging, or After forging, after making into a gear shape by machining, carburizing quenching tempering or carburizing nitrocarburizing quenching and tempering, then shot peening to produce the gear,
The residual austenite structure from the surface to the 30 μm depth position after carburizing and tempering or carburizing and nitriding quenching and tempering is 25% by volume or more and 58% by volume or less, and 7% by volume or less after shot peening. It has a martensitic structure, the grain size of the carburized tempered or carburized nitrocarburized quenched and tempered region is 8.5 or more, and the compressive residual stress on the tooth surface and root surface after shot peening is 1500 MPa. Thus, a gear having excellent fatigue strength, characterized in that the hardness of the tooth surface and the tooth root surface is Vickers of 850 or more and further satisfies the following formula (1).
(1.96γR _CS −0.4γR _SS + HV _CS + σR _SS ) − (HV _{300 ° C.} + σR _{300 ° C.} ) ≦ 640
... (1)
Where γRcs: residual austenite of the carburized surface layer portion, γRss: residual austenite after shot peening of the carburized surface layer portion, HVcs: tooth surface Vickers hardness after carburizing or carburizing, σR _SS : tooth surface residual stress after shot peening (MPa), HV _{300 ° C .} : tooth surface Vickers hardness after tempering at 300 ° C., σR _{300 ° C .} : tooth surface compressive residual stress (MPa) after tempering at 300 ° C.   As a component composition of the steel material, Nb: 0.010 to 0.060 mass%, V: 0.03 to 0.20 mass%, and Ti: 0.005 to 0.200 mass% are further contained. The gear having excellent fatigue strength according to claim 1.   The gear having excellent fatigue strength according to claim 1 or 2, further comprising B: 0.0005 to 0.0050 mass% as a component composition of the steel material.   The steel material having the composition according to claim 1 is any one of cold forging, machining after hot forging, machining after cold forging, or machining after warm forging. After making into a gear shape, perform carburizing treatment, or carburizing and nitriding and diffusion treatment at 900 to 980 ° C., then cool to 780 to 880 ° C., then put into oil at 60 to 130 ° C. and quenching Thereafter, the method for producing a gear excellent in fatigue strength, characterized by re-heating to 150 to 220 ° C. and tempering, and further performing shot peening on the tooth surface of the gear.   5. The method for producing a gear with excellent fatigue strength according to claim 4, wherein the shot peening is performed using particles having a particle size of 0.05 to 0.1 mmΦ and a hardness of 700 HV or more.   The shot peening is performed using particles having a particle diameter of 0.4 to 1.2 mmΦ and a hardness of 700 HV or more, and then performed again using particles having a hardness of 0.05 to 0.1 mmΦ and a hardness of 700 HV or more. The manufacturing method of the gear excellent in the fatigue strength of Claim 4.   5. The shot peening is performed by mixing grains having a particle diameter of 0.4 to 1.2 mmΦ and a hardness of 700 HV or more and grains having a hardness of 0.05 to 0.1 mmΦ and a hardness of 700 HV or more. The manufacturing method of the gear excellent in the fatigue strength of description.

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