JP2020050935A - Manufacturing method of gear parts - Google Patents

Abstract

To provide a technology capable of acquiring both an enough amount of carbide and an enough amount of residual austenite.SOLUTION: A manufacturing method of gear parts includes a heating step and a cooling step. In the heating step, gear parts made of steel is heated at a peak temperature of 750°C or higher and 950°C or lower. In the cooling step, the non-acute angle part of the gear parts heated in the heating step is cooled at a first cooling rate and the acute angle part of the gear parts is cooled at a second cooling rate lower than the first cooling rate. A larger amount of residual austenite is thus obtained in the acute angle part than in the non-acute angle part.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a method for manufacturing a gear component.

  There is known a technique of increasing the amount of retained austenite in a part of a gear component where toughness is desired to be ensured compared to other parts. Specifically, a technique is known in which, after carburizing and quenching an entire part, induction hardening is performed only on a portion where toughness is to be ensured, thereby increasing the amount of retained austenite in a portion where toughness is to be ensured (Patent Document 1). 1, 2).

JP 2007-182607 A JP 2007-182609 A

As in Patent Literatures 1 and 2, when heating to 800 to 1000 ° C. in induction hardening, the amount of solid solution carbon increases while the amount of carbide decreases. Here, the carbide is a factor that inhibits softening due to heat generation during tempering or use of parts. Therefore, in Patent Literatures 1 and 2, there is a problem that softening cannot be suppressed due to a decrease in the amount of carbide, resulting in a decrease in fatigue resistance.
The present invention has been made in view of the above problems, and has as its object to provide a technique capable of achieving both the amount of carbide and the amount of retained austenite.

  In order to achieve the above object, a method of manufacturing a gear part according to the present invention includes a heating step of heating a gear part made of a steel material at a peak temperature of 750 ° C. or more and 950 ° C. or less, and a gear heated in the heating step. The amount of retained austenite at the sharp corners is reduced by cooling the non-sharp corners of the parts at a first cooling rate and cooling the sharp corners of the gear parts at a second cooling rate lower than the first cooling rate. And a cooling step of making it larger than the acute angle portion.

  In the configuration of the present invention described above, it is known that when the cooling rate when cooling austenite is reduced, transformation to martensite is suppressed and the amount of retained austenite increases. Therefore, the amount of retained austenite in the acute angle portion can be made larger than that in the non-acute angle portion by reducing the cooling rate of the acute angle portion in the cooling step. In addition, by suppressing the peak temperature in the heating step to 950 ° C. or lower, solid solution of carbon constituting carbide can be suppressed, and the amount of carbide can be maintained. Therefore, the amount of carbide and the amount of retained austenite can be compatible at the acute angle portion of the gear part, and both wear resistance and toughness can be compatible.

FIG. 1A is a side view of a gear component, and FIG. 1B is a schematic diagram of a heat treatment. FIG. 2A is a schematic sectional view of an induction heating device, and FIG. 2B is a schematic sectional view of a cooling device. FIG. 3 is a graph of the amount of retained austenite. 4A and 4B are schematic sectional views of an induction heating device for tempering. FIG. 5A is a graph of residual stress, and FIG. 5B is a graph of crack generation energy.

Here, embodiments of the present invention will be described in the following order.
(1) Principle and embodiment of the present invention:
(2) Other embodiments:

(1) Principle and embodiment of the present invention:
FIG. 1A is a side view of the gear component 1 manufactured by the manufacturing method of the present invention. The gear component 1 has a cylindrical shape, and has teeth formed on a side surface. The gear component 1 is a member that rotates around a rotation axis Z indicated by a broken line, and is used, for example, as a planetary gear of a vehicle automatic transmission. The use and function of the gear component 1 are not particularly limited.

  As shown in FIG. 1A, surfaces of both ends of the gear component 1 in the axial direction of the rotation axis Z are defined as an acute angle portion A, and a surface of a portion sandwiched between the acute angle portions A is defined as a non-acute angle portion B. Further, the axial direction of the gear component 1 means the direction of the central axis of the rotation axis Z. The acute angle portion A is a portion within 10% of the entire length from the axial end face of the gear component 1. The teeth (tooth width) of the gear component 1 are formed in the entire area in the axial direction, and the end surfaces of the teeth are included in the acute angle portion A. Although the gear part 1 whose tooth width direction coincides with the axial direction has been illustrated, the gear part 1 may be a helical gear whose tooth width direction is inclined with respect to the axial direction. The gear component 1 is formed by, for example, JIS-SCM420.

  FIG. 1B is a schematic diagram illustrating each step of the present embodiment and the amount of retained austenite. In the present embodiment, a heating step, a cooling step, and a tempering step are performed. In performing the heating step, gear cutting and carburizing slow cooling of the gear component 1 are performed in advance. In carburizing, the target carbon concentration was 1.35%, the temperature was 1000 ° C., and the carburizing and diffusion time was 4 hours. After carburizing, slow cooling is performed in nitrogen gas or inert gas. By performing carburizing slow cooling, a carbide in which iron and carbon are combined can be formed on the surface of the gear component 1.

  Next, a heating step and a cooling step are sequentially performed. The heating step and the cooling step are so-called induction hardening. The heating step is a step of heating the gear part 1 made of steel at a peak temperature of 750 ° C. or more and 950 ° C. or less. Specifically, in the heating step, the gear component 1 is held at a peak temperature of 820 ° C. for 30 minutes. In the heating step, the entire surface of the gear component 1 whose carbon concentration is increased by carburization is heated, and an austenite phase is formed. The peak temperature in the heating step may be low enough to maintain iron and carbon carbide, and may be 950 ° C. or less. If the peak temperature of the heating step is too high, the carbon constituting the carbide will be dissolved in the matrix, and as a result, the wear resistance will be reduced. The peak temperature in the heating step may be high enough to austenitize the surface of the gear component 1, and may be 750 ° C. or higher.

  FIG. 2A shows a cross-sectional structure of the induction heating device 10 used in the heating step. The induction heating device 10 includes a cylindrical induction coil 12. The induction coil 12 forms a cylindrical heating space 11 therein, and the gear component 1 heated in the heating step is introduced into the heating space 11. The gear component 1 is sandwiched and held by a pair of jigs 13 from both sides in the axial direction. The gear component 1 is held in the heating space 11 such that the central axis C (dashed line) of the gear component 1 and the heating space 11 coincide. By supplying a high-frequency current to the induction coil 12, the entire surface of the gear component 1 in the axial direction can be heated. As a result, both the acute angle portion A and the non- acute angle portion B are heated by the induction heating device 10.

  In the cooling step, the hardness of the surface of the gear component 1 is ensured by rapidly cooling the austenite phase and transforming it into martensite. As shown in FIG. 1B, in the cooling step, the non-acute angle portion B is rapidly cooled at the first cooling speed V1 (the magnitude of the inclination of the broken line), and the acute angle portion A is cooled at the second cooling speed lower than the first cooling speed V1. Rapid cooling at V2 (the magnitude of the slope of the solid line). Thereby, the amount of retained austenite at the acute angle portion A can be made larger than the amount of retained austenite at the non-acute angle portion B. For example, as shown in the lower graph of FIG. 1B, the amount of retained austenite on the surface of the acute angle portion A after the cooling step may be 35% by volume, and the amount of retained austenite on the surface of the non-acute angle portion B may be 10% by volume.

  FIG. 2B shows a cross-sectional structure of the cooling device 20 used in the cooling step. The cooling device 20 includes a cylindrical water cooling wall 22. The water-cooling wall surface 22 forms a cylindrical water-cooling space 21 therein, and the gear component 1 heated in the heating step is introduced into the water-cooling space 21. The gear component 1 is sandwiched and held by a pair of jigs 23 from both sides in the axial direction. The gear component 1 is held in the water cooling space 21 so that the central axis C of the gear component 1 and the water cooling space 21 coincide. The direction of the central axis C is a vertical direction.

  The cooling device 20 has a structure that is rotationally symmetric about the central axis C. Four refrigerant chambers P1 to P4 are provided along the outer side surface of the water cooling wall 22. The refrigerant chambers P1 to P4 are formed in an annular shape, and water as a refrigerant is supplied to an internal hollow space. Water is supplied to each of the refrigerant chambers P1 to P4 by a small pump (not shown), and the water pressure in the refrigerant chambers P1 to P4 can be individually adjusted.

  A number of nozzles are formed on the water-cooling wall surface 22, and water in the refrigerant chambers P <b> 1 to P <b> 4 is jetted toward the water-cooling space inside the water-cooling wall surface 22 via the nozzles. Water injected through the nozzle of the water-cooling wall surface 22 and directed toward the water-cooling space is indicated by gray arrows. Of the nozzles on the water cooling wall 22, the nozzles communicating with the first and second refrigerant chambers P1 and P2 counted from the top are inclined downward by 10 degrees with respect to the horizontal direction. The nozzle of the water-cooling wall surface 22 that communicates with the third refrigerant chamber P3 counted from the top is in the horizontal direction. The nozzle of the water-cooled wall surface 22 communicating with the refrigerant chamber P4 disposed at the lowest position is inclined upward by 10 degrees with respect to the horizontal direction.

  The water pressure in the refrigerant chambers P1, P4 is adjusted so that water is jetted at a flow rate of 10 L / min from each nozzle of the water cooling wall surface 22 communicating with the uppermost and lowermost refrigerant chambers P1, P4 (light gray). I have. On the other hand, the water in the refrigerant chambers P2 and P3 is so sprayed at a flow rate of 40 L / min from each nozzle of the water-cooling wall surface 22 communicating with the second and third refrigerant chambers P2 and P3 (dark gray) counted from the top. Water pressure is adjusted.

  Here, the portion cooled by the water injected from the nozzle of the water cooling wall surface 22 communicating with the refrigerant chambers P1 and P4 is the acute angle portion A in FIG. 1A. That is, the portion cooled by the water injected at a flow rate of 10 L / min becomes the acute angle portion A. On the other hand, the part cooled by the water injected from the nozzle of the water-cooling wall surface 22 communicating with the refrigerant chambers P2 and P3 is the non-acute angle part B in FIG. 1A. That is, the portion cooled by the water injected at a flow rate of 40 L / min becomes the non-acute angle portion B.

  As described above, since the flow rate of the supplied refrigerant is smaller at the acute angle portion A than at the non- acute angle portion B, as shown in FIG. 1B, the first cooling speed V1 of the non-acute angle portion B (the magnitude of the inclination of the broken line) The second cooling rate V2 (the magnitude of the slope of the solid line) of the acute angle portion A is smaller than that. As a result, the amount of retained austenite at the acute angle portion A can be 35% by volume, and the amount of retained austenite at the non-acute angle portion B can be 10% by volume.

  FIG. 3 is a graph showing the relationship between the cooling rate in the cooling step and the amount of retained austenite. The horizontal axis in the figure shows the amount of solid solution carbon, and the vertical axis shows the volume fraction of retained austenite. The amount of solute carbon means the mass concentration of carbon dissolved in austenite before martensitic transformation immediately before the cooling step. In FIG. 3, the volume fraction of retained austenite when the cooling rate is 750 ° C./sec, 1250 ° C./sec, and 1750 ° C./sec is indicated by a broken line, a dashed-dotted line, and a solid line, respectively.

  As shown in FIG. 3, there was a tendency that the lower the cooling rate, the larger the amount of retained austenite, and the faster the cooling rate, the smaller the amount of retained austenite. In the present embodiment, immediately before the cooling step, it can be estimated that about 0.78% by mass of carbon is dissolved in austenite before the martensitic transformation. Therefore, by setting the first cooling rate V1 to 1750 ° C./sec, the amount of retained austenite in the non-acute angle portion B can be reduced to 10% by volume. On the other hand, by setting the second cooling rate V2 to 750 ° C./sec, the amount of retained austenite in the acute angle portion A can be 35% by volume.

  The cooling device 20 is not limited to the device shown in FIG. 2B as long as the cooling speed of the acute angle portion A can be made slower than that of the non-acute angle portion B. For example, the cooling device 20 may be a device that cools the gear component 1 in a state where the gear component 1 is held such that the central axis C is horizontal. Further, the temperature and concentration of the refrigerant supplied to the refrigerant chambers P1 to P4 may be different.

  Next, tempering is performed. Tempering is a process in which a part of the retained austenite in the acute angle portion A is transformed into martensite by tempering the acute angle portion A after the cooling step. In the tempering, the acute angle portion A of the gear component 1 is held at a peak temperature of 180 ° C. for 45 to 75 minutes (for example, 60 minutes). The peak temperature in the heating step may be a temperature at which a part of retained austenite can be transformed into martensite, and may be 150 ° C. or more. The peak temperature of the heating step may be low enough to maintain the already formed martensite, and may be 200 ° C. or less. After the tempering heating, the gear component 1 is air-cooled. As a result, 5% by volume of the retained austenite at the acute angle portion A can be transformed into martensite, and the amount of retained austenite can be reduced to 30% by volume.

  FIG. 4A shows a cross-sectional structure of an induction heating device 30 used for tempering. The induction heating device 30 includes a pair of upper and lower induction coils 32 formed in a cylindrical shape. Each of the induction coils 32 forms a cylindrical heating space 31 inside. The gear component 1 heated in the tempering is introduced into the induction heating device 30 so that the acute angle portion A exists at a position close to the heating space 31. The gear component 1 is sandwiched and held by a pair of jigs 33 from both sides in the axial direction. The gear component 1 is held in the induction heating device 30 so that the gear component 1 and the central axis C (dashed line) of the heating space 31 match. By supplying a high-frequency current to each of the induction coils 32, the acute angle portion A of the gear component 1 can be heated. As shown in FIG. 4B, tempering may be performed using an induction heating device 40 that heats the acute angle portion A by a magnetic field coil 42 formed so as to contact the acute angle portion A.

  In the configuration of the present embodiment described above, the amount of retained austenite in the acute angle portion A can be made larger than that in the non-acute angle portion B by reducing the cooling rate of the acute angle portion A in the cooling step. Further, by suppressing the peak temperature in the heating step to 950 ° C. or less, the solid solution of carbon constituting the carbide is suppressed, and the amount of the carbide is maintained such that the area ratio on the surface becomes 5 to 20%. be able to. Therefore, the amount of carbide and the amount of retained austenite can be compatible at the acute angle portion of the gear part, and both wear resistance and toughness can be compatible.

  Further, after the cooling step, a part of the retained austenite of the acute angle portion A is transformed into martensite by tempering the acute angle portion A, so that the volume of the transformed portion can be increased, and the internal stress of compression can be reduced. Can be left.

  FIG. 5A is a graph showing the relationship between the presence or absence of tempering and residual stress. In the figure, the vertical axis indicates residual stress, and the horizontal axis indicates volume fraction of retained austenite. Negative residual stress means that compressive residual stress exists, and means that the smaller the residual stress, the greater the compressive residual stress. In FIG. 5A, a black square and a solid line indicate residual stress when tempering is not performed, and a white square and a broken line indicate residual stress when tempering is performed. As shown in FIG. 5A, by performing tempering, the residual stress of compression can be increased.

  As described above, by leaving the internal stress of the compression, a reaction force of the force for expanding the crack can be applied, and the possibility of the occurrence of the crack in the acute angle portion A can be reduced. FIG. 5B is a graph showing the relationship between the presence or absence of tempering and the difficulty of crack generation. The vertical axis of the figure shows the crack generation energy, and the horizontal axis shows the volume fraction of retained austenite. The crack generation energy means the energy required until a crack is generated in the Charpy impact test, and the larger the crack generation energy, the harder the crack is generated. In FIG. 5B, a black square and a solid line indicate crack generation energy when tempering was not performed, and a white square and broken line indicate crack generation energy when tempering was performed. As shown in FIG. 5B, by performing tempering, it is possible to make it difficult to generate cracks.

(2) Other embodiments:
It should be noted that the present invention is not limited to the above-described embodiment, but can be variously modified based on the gist of the present invention, and they are not excluded from the scope of the present invention. For example, the heating step may be performed after the shape of the gear component 1 (the acute angle portion A, the non-sharp angle portion B) is formed to some extent, and the preceding step of the heating step may not be carburizing slow cooling. Further, the amount of retained austenite is merely an example, and may be appropriately adjusted according to the toughness and crack resistance required of the gear part 1. Further, the cooling speed of the gear component 1 is not limited to being adjusted in two stages, and the gear component 1 may be cooled at a cooling speed of three or more stages.

  In the present invention, it is desirable that the steel material constituting the gear component contains an alloy element other than C that forms carbide, and that C contains only 0.05 to 2.00% by mass. In addition, carburization may be performed as a pre-process of the present invention. Specifically, the steel material may contain Mo by 0.15 to 0.35% by mass, or may contain Cr by 0.5 to 2.0% by mass. The heating step is a step of performing quenching. Therefore, the lower limit of the peak temperature in the heating step may be a temperature equal to or higher than the lower limit (750 ° C.) of the temperature at which the structure of the gear component can be temporarily changed to the austenite phase. On the other hand, the upper limit of the peak temperature in the heating step may be a temperature that is equal to or lower than the upper limit (950 ° C.) of the temperature at which the amount of carbon constituting carbide can be reduced.

  By making at least the second cooling rate lower than the first cooling rate, the amount of retained austenite at the acute angle portion can be made larger than at the non-acute angle portion. Specifically, the second cooling rate may be 0.25 to 0.75 times the first cooling rate. Further, the first cooling rate may be 1250 to 1750 ° C / sec, and the second cooling rate may be 750 to 1250 ° C / sec. Furthermore, the first cooling rate and the second cooling rate may be cooling rates within a cooling temperature range of 1000 to 400 ° C.

  Further, after the cooling step, a part of the retained austenite in the acute angle portion may be transformed into martensite by tempering the acute angle portion. By transforming part of the retained austenite at the acute angle portion into martensite, the volume of the transformed portion can be increased, and the internal stress of compression can be left. By leaving the internal stress of the compression, a reaction force of a force for expanding the crack can be applied, and the possibility of the occurrence of a crack at an acute angle portion can be reduced. However, it is not essential to temper the sharp corner after the cooling step, and the tempering may be omitted.

  In the cooling step, the flow rate of the refrigerant supplied to the acute angle portion may be smaller than the flow rate of the refrigerant supplied to the non-acute angle portion. Thereby, the cooling rate at the acute angle part can be made slower than the cooling rate at the non-acute angle part. However, the cooling rate does not necessarily have to be adjusted by the flow rate of the refrigerant, and the cooling rate may be adjusted by the temperature and concentration of the refrigerant.

  Furthermore, the acute angle may be the end of the tooth in the direction of the axis of rotation of the gear component. This makes it possible to achieve both wear resistance and toughness of the end of the tooth in the direction of the rotation axis.

Reference Signs List 1 gear part, 10 induction heating device, 11 heating space, 12 induction coil, 13 jig, 20 cooling device, 21 water cooling space, 22 water cooling wall surface, 23 jig, 30 induction heating Apparatus, 31: heating space, 32: induction coil, 33: jig, 40: induction heating device, 42: magnetic field coil, A: acute angle portion, B: non-acute angle portion, C: central axis, P1 to P4: refrigerant chamber , P4: refrigerant chamber, V1: first cooling speed, V2: second cooling speed, Z: rotating shaft

Claims (4)

A heating step of heating a gear part formed of steel at a peak temperature of 750 ° C. or more and 950 ° C. or less;
Cooling the non-sharp part of the gear component heated in the heating step at a first cooling rate, and cooling the sharp part of the gear component at a second cooling rate lower than the first cooling rate. By the cooling step of increasing the amount of retained austenite in the acute angle portion than in the non-acute angle portion,
A method for manufacturing a gear component including: After the cooling step, a part of the retained austenite of the acute angle portion is transformed into martensite by tempering the acute angle portion,
A method for manufacturing the gear component according to claim 1. In the cooling step, the flow rate of the refrigerant supplied to the acute angle portion is smaller than the flow rate of the refrigerant supplied to the non-acute angle portion,
A method for manufacturing a gear component according to claim 1. The acute angle portion is an end of a tooth in a direction of a rotation axis of the gear component,
A method for manufacturing a gear component according to any one of claims 1 to 3.