JP4501573B2 - Gear and gear manufacturing method - Google Patents

Description

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

Conventionally, steel gears used for transmission gears and the like of automobiles, as described in Patent Document 1, for example, are subjected to carburizing and nitrogen quenching treatment on the tooth surfaces, and nitrogen is contained in steel constituting the gears. Is trying to invade. This is because when the gear is used, the tooth surface temperature rises and the vicinity of the tooth surface of the gear is softened to easily cause pitching and seizure. Therefore, nitrogen is penetrated into the vicinity of the tooth surface to reduce the softening resistance near the tooth surface. It is for raising. Pitching resistance and seizure resistance can be improved by such a carburizing and nitrogen quenching treatment. As described above, when the pitting resistance is improved by the carburizing and nitriding quenching process, the shape of the gear is deformed or distorted when the gear is quenched. Therefore, if it is used as a gear in this state, a tooth contact failure of the gear pair may occur, noise may be generated, and a gear contact may occur. When the gears come into contact with each other, the stress tends to concentrate at that portion, so that the tooth surfaces of the gears are peeled off or the gear teeth themselves are destroyed. Therefore, the gear accuracy is usually corrected by grinding the gear after the carburizing and nitriding quenching process.
Japanese Patent No. 0387427

  However, if the carburizing and nitriding quenching process is performed to improve the strength in the vicinity of the tooth surface of the gear, grinding for improving the dimensional accuracy of the gear becomes difficult, and expensive tools are required for processing. Necessary. Alternatively, in the carburizing and nitriding quenching process, it is necessary to infiltrate carbon and nitrogen deeply in advance in consideration of the grinding allowance for grinding in the grinding process, which wastes process time and materials.

  An object of the present invention is to provide a gear capable of ensuring dimensional accuracy more easily at a lower cost and a manufacturing method for manufacturing such a gear.

Moreover, in order to solve the said subject, the manufacturing method of the gear of this invention contains the carbonized gas after the gear processing process which processes the steel material used as a gear, and obtains a gear of a predetermined shape, and a gear processing process. A first quenching step of quenching the gear by quenching the gear into a martensite region after raising the temperature to the austenite region in an atmosphere or an atmosphere containing a carbonized gas and ammonia gas; A heat treatment step including at least a carburizing / nitrogen-quenching step of quenching after raising the temperature to an austenite region in an atmosphere containing a carbonized gas and an ammonia gas, and the carburizing / nitrogen-quenching step includes a tooth surface of the gear. The amount of retained austenite at the outermost surface containing 40% to 80% by volume, the carbon content from 0.75% to 1% by mass, and the nitrogen content from 0.5 to 0 Characterized in that it is intended to 7 mass%.

  Hereinafter, in this specification, the region from the tooth surface to the depth at which the X-ray penetrates when the outermost surface of the tooth surface is irradiated is referred to as “the vicinity of the tooth surface”. Here, since the penetration depth of X-rays into steel (depth at which the intensity of irradiated X-rays attenuates to 5%) is about 10 μm, in this specification, “near tooth surface” is defined from the tooth surface. It is defined as a region up to a depth of 10 μm.

  On the other hand, the region from the tooth surface to the depth at which carbon and nitrogen diffuse by carburizing and nitriding quenching is referred to as the “surface portion” of the gear in this specification. Specifically, although it depends on the processing time of the carburizing and nitriding quenching process, a region from the tooth surface to a depth of 0.15 to 0.25 mm can be set as the surface portion.

  When the conventional carburizing and nitriding quenching treatment is applied to the tooth surface of the gear, it was basically done for the purpose of improving the strength of the tooth surface, so the amount of retained austenite at the surface is usually suppressed. Met. Specifically, the amount of retained austenite in the vicinity of the tooth surface was normally suppressed to around 20% by volume. This is a well-known fact that the hardness decreases as the amount of retained austenite increases. Therefore, when the amount of retained austenite near the tooth surface of the gear is more than about 20% by volume, the amount of retained austenite at the surface portion of the gear is also increased. This is because the hardness of the surface portion is reduced due to the increase, and bending fatigue strength, tensile strength, and pitting resistance strength are considered to be inferior.

  However, as a result of intensive studies by the present inventors, it has been found that the pitting resistance of the gear does not deteriorate even if the amount of retained austenite in the surface portion of the gear is relatively large. That is, in the case of a steel gear that has been subjected to carburizing and nitriding and quenching as in the gear of the present invention, the pitting resistance does not deteriorate even if the amount of retained austenite on the surface portion is larger than before. I understood. This is because the carbonitriding and quenching treatment forms carbonitrides in the grain boundaries of the steel or in the grains due to the carbon and nitrogen that have infiltrated the surface including the tooth surface, so that the strength of the surface is sufficiently high. This is presumed to be maintained. Specifically, in the measurement result when X-ray diffraction measurement was performed by irradiating the outermost surface of the tooth surface, it was found that the amount of retained austenite may be 40 to 80% by volume. Moreover, since the penetration depth of X-rays into steel was about 10 μm, it was found that the amount of retained austenite in the region from the tooth surface to the depth of 10 μm may be 40 to 80% by volume. If the amount of retained austenite in the vicinity of the tooth surface is large to this extent, it is considered that the amount of retained austenite in the surface portion subjected to the carburizing and nitriding quenching process is relatively large.

  As described above, the amount of retained austenite in the vicinity of the tooth surface is set to 40 to 80% by volume, and the amount of retained austenite in the surface portion is increased, so that the processing for improving the dimensional accuracy after quenching becomes easy or unnecessary. It becomes. Here, the processing for improving the dimensional accuracy is easy. When the amount of retained austenite in the vicinity of the tooth surface is 40 to 80% by volume, the hardness of the surface portion becomes relatively low. This means that the surface portion can be processed without using it. On the other hand, the fact that processing for improving dimensional accuracy is not required means that the tooth contact between gears can be improved by using the gears after carburizing and nitriding and quenching as they are as gear pairs. It is. In other words, by engaging the other gears with the gears after the carburizing and quenching quenching treatment and sliding the gear pair, the surface portion including the tooth surface is deformed at the initial sliding of the gear pair. Or, since the tooth contact of the gear is improved by wearing, processing for improving the dimensional accuracy becomes unnecessary.

  As described above, when using the gear after carburizing and nitriding and quenching as it is, the tooth contact of the gear is improved by the initial sliding of the gear pair, so no special processing is performed to improve dimensional accuracy. However, it is also possible to suppress tooth destruction caused by noise reduction in the gear and stress concentration due to contact with the gear.

  Furthermore, even if the amount of retained austenite in the surface portion is increased, the tooth surface is stressed at the stage of using the gear, and as a result, the retained austenite is transformed into martensite by the processing-induced transformation of the retained austenite. It is also inferred that the hardness of the steel increases with the use of gears. Thus, it is considered that the tooth contact of the gear is improved with the use of the gear, the strength after the tooth contact is improved can be sufficiently maintained, and the pitting resistance can be secured.

  Here, if the amount of retained austenite in the vicinity of the tooth surface is less than 40% by volume, the surface portion is not sufficiently softened, so that grinding is necessary or the gear may be used without grinding after quenching. This is not preferable because the effect of improving tooth contact is not exhibited and noise and contact with each other are caused. On the other hand, if the amount of retained austenite in the vicinity of the tooth surface exceeds 80% by volume, a large amount of nitrogen needs to be infiltrated into the surface portion. However, if the amount of intruding nitrogen is excessive, voids are formed in the tooth surface. Defects called are generated, and the strength in the vicinity of the tooth surface and in the surface portion is reduced. Therefore, if the gear is used as it is, it causes pitching, tooth surface separation, and the like, so that grinding is required again, and the effect of the present invention cannot be obtained.

  Furthermore, as described above, in the gear of the present invention, the Vickers hardness profile at the outermost surface of the tooth surface is Hv450 to 650 in the Vickers hardness profile along the depth direction from the tooth surface, and the depth from the tooth surface. There is a maximum value of Vickers hardness in the region of 0.15 to 0.25 mm, and the Vickers hardness decreases from the maximum value toward the inside of the gear. This means that the surface portion of the gear that has been subjected to carburizing and nitriding and quenching is softened. The Vickers hardness at the outermost surface of the tooth surface being Hv 450 to 650 corresponds to the amount of retained austenite in the vicinity of the tooth surface being 40 to 80% by volume. In order to make the Vickers hardness of the outermost surface of the tooth surface lower than Hv450, it is necessary to increase the amount of retained austenite, and it is necessary to penetrate a large amount of nitrogen, which is an austenite stabilizing element, into the surface portion near the tooth surface. However, if the amount of nitrogen penetration is too large, voids are generated on the tooth surface as described above, which is not preferable. On the other hand, if the Vickers hardness exceeds Hv650, even if the gear is used as it is after the carburizing and quenching quenching process, the effect of improving the tooth contact cannot be obtained, and grinding or the like is required. Can't get.

  In addition, in the Vickers hardness profile, if the maximum value of the Vickers hardness does not exist in a region having a depth of 0.15 to 0.25 mm from the tooth surface, the surface portion including the tooth surface of the gear is sufficiently carburized and nitrocarburized. It has not been put in, and the strength at the surface portion including the tooth surface cannot be sufficiently maintained.

  Next, in the gear manufacturing method of the present invention, in the heat treatment step, a carburizing / nitrogen quenching step is adopted, and the atmosphere in which the carburizing / nitrogen quenching step is performed is performed in an atmosphere in which nitrogen easily enters steel. As a result, nitrogen, which is an austenite stabilizing element, is allowed to actively penetrate the surface of the gear to increase the ratio of residual austenite in the surface, and a large amount of carbonitride is formed in the crystal grain boundaries and grains in the surface. can do. Thus, by increasing the ratio of retained austenite in the surface portion while forming carbonitrides on the surface portion by carburizing and nitriding quenching treatment, it is possible to sufficiently improve the pitting resistance and wear resistance on the tooth surface of the gear. While maintaining, the hardness of the surface portion can be reduced. That is, it is possible to manufacture a gear having good pitting resistance and easy or unnecessary processing for improving dimensional accuracy.

  Further, in the heat treatment step, before the carburizing and nitriding quenching treatment, the gear is heated to an austenite region in an atmosphere containing a carbonized gas or an atmosphere containing a carbonized gas and an ammonia gas, and then martensite. By quenching to the site region and performing the first quenching step of quenching the gear, a larger amount of finer carbonitride can be formed in the crystal grain boundaries or grains, This further contributes to improving the strength of the surface portion. That is, in the first quenching step, high-density dislocations and carbonitrides are formed in the martensite at the surface portion. However, during the reheating in the carburizing and nitriding quenching step, these high-density dislocations and Numerous austenite grains are formed with carbonitrides serving as nuclei. Therefore, a large number of fine carbonitrides are formed in the crystal grain boundaries or grains due to carbon and nitrogen that have entered from the atmosphere containing the carbonized gas and ammonia.

  Furthermore, in the first quenching step, the strength of the surface portion can be further improved by setting the atmosphere to an atmosphere containing carbonized gas and ammonia. In other words, the amount of carbonitride formed in the first quenching step is larger than the amount of carbonitride formed in the first quenching step as compared with the case of carburizing in an atmosphere containing only the carbonized gas. This is because it becomes finer and more are formed.

  Further, in the gear manufacturing method of the present invention, after the step of preparing a molding gear that slides in a pair with the gear and the heat treatment step, the gear and the molding gear are engaged to slide. If the process is further included, the dimensional accuracy of the gear can be more easily improved without performing cutting or the like.

  Hereinafter, embodiments of a gear and a method for manufacturing a gear according to the present invention will be described with reference to the accompanying drawings. FIG. 1 is a schematic diagram for explaining a gear according to the present embodiment. The gear 10 has a plurality of teeth 12 and a surface portion 16 including a tooth surface 14. Here, the surface portion 16 is a region where carbon and nitrogen have penetrated by the carburizing and nitriding quenching process, and can be, for example, a region from the tooth surface 14 of the gear 10 to a depth of 0.2 mm. . A region deeper than the surface portion 16 from the tooth surface 12 is a center portion 18.

  Here, alloy steel can be adopted as the material of the gear 10 of the present embodiment. In particular, an alloy steel containing Ni, Cr, Mo or the like can be employed. In the gear 10 of this embodiment, the content of the constituent elements in the central portion 18 is, for example, as follows. That is, the content of chromium (Cr) is 0.7 to 1.1% by mass, the content of molybdenum (Mo) is 0.3 to 0.55% by mass, and carbon (C) Content is 0.17 to 0.23 mass%, and the balance is iron (Fe). Furthermore, you may make it contain nickel (Ni) as another additive addition element. When adding Ni, content of nickel (Ni) shall be 0.8-0.11 mass%. Furthermore, Mn, Nb, etc. can also be added as other additive elements.

  Here, if the carbon content is less than 0.17% by mass, the hardenability of the steel is inferior and the hardness after quenching becomes insufficient, such being undesirable. On the other hand, if the carbon content exceeds 0.23% by mass, the internal hardness after quenching becomes too high, which may cause a decrease in strength.

  The gear 10 of the present embodiment is such that the surface portion 16 including the tooth surface 14 is subjected to carburizing and nitriding and quenching. The surface portion 16 is carburized and nitrocarburized, and martensite is generated. However, not all regions of the surface portion 16 are martensitic transformed. In particular, in the gear 10 of the present embodiment, the ratio of retained austenite is 40 to 80% by volume in the vicinity of the tooth surface 14 of the gear 10.

  The proportion of retained austenite in the vicinity of the tooth surface 14 of the gear 10 can be known by performing X-ray diffraction measurement by irradiating the tooth surface 14 with X-rays. Specifically, it can be obtained by determining the ratio between the integrated intensity of the peak related to martensite and the integrated intensity of the peak related to austenite in the X-ray diffraction pattern obtained by irradiating the tooth surface 14 with X-rays. it can.

  A large number of carbonitrides are formed on the surface portion 16. The strength at the surface portion 16 of the gear is sufficiently maintained by the carbonitride. Specifically, the number of carbonitrides in a circle having a diameter of 20 μm (hereinafter, the unit is referred to as “pieces / φ20 μm”) is 5 to 60 pieces / φ20 μm in the cross-sectional observation image on the surface portion 16. Is good. If the number of carbonitrides is less than 5 / φ20 μm, the effect of strengthening the precipitation of carbonitrides on the surface portion 16 may be insufficient. On the other hand, when the number of carbonitrides exceeds 60 pieces / φ20 μm, there is a problem that the amount of Cr dissolved is reduced and an incompletely hardened layer is formed in the vicinity of the surface.

  In such a gear 10, a Vickers hardness profile indicating a distribution of Vickers hardness along the depth direction from the tooth surface 14 is as shown in FIG. As can be seen from FIG. 2, the Vickers hardness profile increases with increasing depth from the outermost surface side of the tooth surface 14 of the gear 10, and the depth from the outermost surface of the tooth surface 14 is 0.15 to 0. 0. The Vickers hardness becomes the maximum value in the region of 25 mm, and the Vickers hardness gradually decreases toward the inside of the gear 10. In particular, in the gear 10 according to the present embodiment, the Vickers hardness on the outermost surface of the tooth surface 14 is set to Hv 450 to 650. Moreover, the maximum value of the Vickers hardness in a region having a depth of 0.15 to 0.25 mm from the outermost surface of the tooth surface 14 is set to Hv 720 to 850. As described above, since the Vickers hardness on the outermost surface of the tooth surface 14 is relatively low, the processing for further improving the dimensional accuracy of the gear 10 becomes easy or unnecessary. Furthermore, when such a gear 10 is used, a part of the surface 16 including the tooth surface 14 is deformed or worn at an early stage of sliding of the gear pair, so that the tooth contact of the gear pair is improved. If the tooth contact is improved, it is possible to reduce noise and to prevent the teeth 12 from being broken due to contact with each other.

  Next, the manufacturing method of the gear of this embodiment is demonstrated. The manufacturing method of the gear according to the present embodiment can be broadly divided into a gear processing step for obtaining a gear 10 having a predetermined shape by processing a steel material to be the gear 10, and a heat treatment for subjecting the gear 10 to a carburizing / nitrogen quenching process. It has a process. Further, after the heat treatment step, a step of improving the dimensional accuracy of the gear 10 after the carburizing and nitriding quenching treatment may be performed.

  In the gear machining step, the steel material is machined into a gear shape by, for example, normal forging, punching, cutting, or the like. The steel material supplied to the gear machining step is the same as the material of the gear 10 in FIG. In particular, the annealed ferrite state can be suitably used. This is because roughing such as punching and cutting is performed relatively easily.

  The processed gear obtained in the gear processing step (hereinafter referred to as the processed gear) is processed to a dimension substantially equivalent to the final product shape, but the gear size changes due to the heat treatment step described later. It is better to work in anticipation of dimensional changes. In particular, it is desirable to process the steel material so that the shape of the gear after the heat treatment step becomes the final product shape.

  The processed gear obtained by the gear machining step as described above is then heat-treated in the heat treatment step. FIG. 3 shows a heat pattern of the heat treatment step in the gear manufacturing method of the present embodiment. First, as shown in FIG. 3, the heat treatment process according to the present embodiment is roughly composed of four steps T1 to T4. In particular, the heat treatment process of the present embodiment includes a first quenching step and a carburizing and nitrogen quenching step. In the process T1, the processed gear is held in an atmosphere containing a carbonized gas or an atmosphere containing a carbonized gas and ammonia, and the temperature of the processed gear is raised to 900 to 950 ° C. Carburizing or carbonitriding is performed on the surface, and carbon (and nitrogen) is diffused to the surface including the tooth surface of the gear after processing. Here, in the atmosphere containing the carbonized gas or the atmosphere containing the carbonized gas and ammonia, the carbon potential (CP), which is the ratio of carbon in the atmosphere, is set to 0.75 to 1% by mass, and the nitrogen in the atmosphere It is an atmosphere in which the nitrogen potential (NP) as a ratio is 0 to 0.7 mass%. Moreover, the temperature at which the post-processed gear is held is a temperature at which the steel material constituting the post-processed gear becomes austenite.

  Before the heat treatment step, the total amount of carbon contained in the processed gear is, for example, 0.17 to 0.23% by mass, and the amount of nitrogen is, for example, 0 to 0.05% by mass. By T1, the carbon content at a depth of 0.2 mm from the surface of the processed gear is, for example, 0.75 to 1% by mass, and the nitrogen content is 0 to 0.7% by mass. This process T1 is performed for several hours at the above temperature. By being held for this time, a predetermined amount of carbon is carburized on the surface portion of the processed gear, and the steel material of the processed gear is transformed into austenite.

  Next, in process T2, carburization is performed by lowering the temperature of the processed gear to 820 to 870 ° C. in the above atmosphere. This process T2 is performed for about 20 to 60 minutes. This is to reduce distortion after quenching.

  Then, after the processing T2 as described above, the processed gear is immersed in oil at about 120 to 140 ° C., and the processed gear is rapidly cooled. These processing T1, processing T2 and quenching processing in which the gears are quenched after the processing by this rapid cooling are the first quenching processing according to the present invention.

  In the gear manufacturing method according to the present embodiment, a carburizing / nitrogen quenching step is performed on the post-processed gear that has been quenched as described above, followed by carburizing / nitrogenizing treatment. . Specifically, as shown in process T3 of FIG. 3, the post-processed gear after the first quenching process is held in an atmosphere containing a carbonized gas and ammonia, and the temperature of the post-processed gear is set to 800. Raise the temperature to ˜850 ° C. And after the temperature of the processed gear is raised to such a temperature, it is held for 20 to 60 minutes. Thereby, the carburizing and nitriding treatment can be performed on the post-processed gear after the first quenching step. Here, the atmosphere containing the carbonized gas and ammonia is a carbon potential (CP) that is a ratio of carbon contained in the atmosphere is 0.75 to 1% by mass, and is a ratio of nitrogen contained in the atmosphere. The atmosphere has a nitrogen potential (NP) of 0.5 to 0.7% by mass.

  The temperature at which the carburizing and nitriding treatment is performed is a temperature at which the steel material constituting the post-processing gear becomes austenite. Further, it is set so that the hardenability of the steel material and the hardness inside the processed gear can be sufficiently secured.

  After the carburizing and nitriding treatment in the process T3 as described above, the processed gear that has been subjected to the carburizing and nitriding treatment is immersed in oil having a temperature of 120 to 140 ° C. to rapidly cool the processed gear. As a result, the processed gear is quenched. This process T3 and the subsequent quenching process are the carburizing and nitriding quenching steps.

  In the processed gear after the carburizing and nitrogen quenching step, the carbon content in the vicinity of the surface of the processed gear is 0.75 to 1% by mass, and the nitrogen content is 0.5 to 0.00. The amount is 7% by mass, and the amount of carbon and nitrogen decrease as it goes inside.

  After the process T3 as described above, a tempering process of the process T4 is performed. The tempering process is performed by holding the gear after machining for several hours at a temperature of 120 to 200 ° C. After this tempering treatment, the ratio of retained austenite on the tooth surface of the processed gear is 40 to 80% by volume, and the gear 10 according to the present invention is manufactured.

  You may perform the process of improving the following dimensional accuracy with respect to the gear 10 produced by the above heat processing processes. In this embodiment, the process of improving dimensional accuracy is performed as follows. That is, a forming gear that slides in pairs with the gear 10 is prepared, and the gear 10 and the forming gear are engaged and slid, and the tooth surface of the gear 10 is moved by the tooth surface of the forming gear. By deforming, the dimensional accuracy of the gear is improved. Since the residual austenite amount in the vicinity of the tooth surface 14 of the gear 10 is 40 to 80% by volume by the heat treatment step, the tooth surface 14 of the gear 10 is included by sliding the gear 10 and the forming gear. A portion of the surface portion 16 that becomes inferior in contact with the forming gear is deformed or worn, so that no incomplete contact is finally caused. As a result, the dimensional accuracy of the gear 10 is improved, and the reduction of noise and the destruction of the teeth 12 due to a single contact are suppressed.

  Hereinafter, in order to demonstrate the effect of the present invention, the following experiment was conducted. First, an alloy having a Ni content of 0.96% by mass, a Cr content of 0.73% by mass, a Mo content of 0.47% by mass, a C content of 0.22% by mass, and the balance being Fe. Steel was adopted and the drive gear (gear) of the transmission gear was processed from the steel material. The drive gear was subjected to carburizing and nitriding under various conditions based on the heat treatment method of the embodiment described above. In the first quenching step, the carbon potential of the atmosphere holding the processed gear was set to 0.80% by mass. On the other hand, in the carburizing and nitrogen quenching step, the carbon potential and the nitrogen potential in the atmosphere holding the drive gear are variously changed to perform the carburizing and nitrogen quenching treatment, and the ratio of residual austenite in the surface portion 16 is determined. A plurality of different drive gears were produced. In the carburizing and nitriding quenching step, the temperature for holding the drive gear was 800 to 850 ° C., and the holding time was 60 minutes.

  X-ray diffraction measurement was performed on the tooth surfaces of the sun gears produced as described above, and the proportion of austenite in the vicinity of the tooth surfaces was measured. More specifically, the X-ray diffraction measurement was performed with a micro X-ray stress measurement device (PSPC-RSF, manufactured by Rigaku Corporation). Further, the Vickers hardness on the tooth surface of the drive gear was measured by a micro Vickers hardness tester.

  Further, the tooth surface of the drive gear after the carburizing and nitriding and quenching treatment was visually observed to evaluate whether or not a void was formed on the tooth surface. Those in which the formation of voids was recognized as defective without performing the following durability test.

In addition, the driven gears used as a pair with the drive gears are engaged with these drive gears, and both gears are slid, and are slid at a torque of 600 N · m for a predetermined time to endure the gears. A test was conducted. The material of the driven gear is the same as that of the drive gear. After such an endurance test, the presence or absence of pitting on the tooth surface of the sun gear was visually observed and evaluated for pitting resistance. Pitching resistance was evaluated as ◯ when no pitting was observed, Δ when pitting was observed but no problem in strength, and x when tooth surface peeling or destruction occurred. In addition, what evaluates as (circle) or (triangle | delta) can be used as a product without a problem. Furthermore, the tooth surface of the driving gear was visually observed, and it was evaluated whether the contact of the tooth surface of each gear was improved by the area of the tooth contact mark between the tooth surface of the driving gear and the tooth surface of the driven gear. . More specifically, when the tooth surface of the drive gear is observed and a metallic luster part is formed on the entire tooth surface, it is assumed that the tooth contact is improved (with improvement), and the carburizing nitrocarburizing is performed on the tooth surface. When the surface portion as it was after the insertion treatment remained, it was evaluated that the tooth contact of the tooth surface was not improved (no improvement). The results are shown in Table 1.

  As shown in Table 1, the drive gear is heated to the austenite region in an atmosphere having a carbon potential (CP) of 0.75% by mass or more and a nitrogen potential (NP) of 0.52% by mass or more, and carburizing. By performing nitrogen quenching, the austenite ratio in the vicinity of the tooth surface of the drive gear can be set to 40 to 80% by volume. 4, FIG. 5 and FIG. It is a cross-sectional observation image orthogonal to the tooth surfaces of 17, 12, and 3. Each observation image has a vertical width of 0.18 mm and a horizontal width of 0.24 mm. In the observed image, the portion projected in white is retained austenite, and the portion projected in black is martensite. Compared with the sample shown in FIG. 6, in the samples shown in FIGS. 4 and 5, the ratio of austenite in the region from the tooth surface to a predetermined depth (for example, 0.1 to 0.2 mm) is more in the interior. It is very large compared with the ratio of austenite, and it can be seen that austenite is mainly generated on the surface portion.

  Further, as in the comparative example of Table 1, when the ratio of austenite in the vicinity of the tooth surface is less than 40% by volume, the Vickers hardness on the surface exceeds Hv650, and the hardness of the surface portion becomes too high, so that the drive gear Since the tooth contact is not improved even if it slides in mesh with the driven gear, it can be said that a process for improving the dimensional accuracy of the gear such as grinding is required after the carburizing and nitrocarburizing treatment. On the other hand, when the ratio of austenite in the vicinity of the tooth surface exceeds 80% by volume, it can be seen that voids are observed on the tooth surface of the drive gear. This is presumed to be the result of a large amount of nitrogen, which is an austenite stabilizing element, entering the surface portion. A gear in which such a void is formed cannot be used because it is certainly inferior in durability. Or the process of removing a void is needed.

  On the other hand, it can be seen that the sun gear of this example in which the ratio of retained austenite in the vicinity of the tooth surface is 40 to 80% by volume has good pitting resistance and also improved tooth contact. This is because the retained austenite in the surface part is relatively large, and the surface part softens even though the strength of the surface part is sufficiently maintained because a large amount of carbonitride is formed on the surface part. It is inferred that this is the cause.

  As can be seen from the above embodiments, the gear according to the present invention is excellent in pitting resistance and easy or unnecessary to improve the dimensional accuracy after quenching. Therefore, the dimensional accuracy of the gear can be ensured more easily at a lower cost.

Sectional drawing which shows the outline | summary of the gearwheel concerning this invention. The figure explaining the heat treatment process in the manufacturing method of the gear concerning the present invention. The figure which shows the Vickers hardness profile which follows a depth direction from the tooth surface of the gearwheel concerning this invention. The cross-sectional observation image of the tooth surface vicinity of the gear in an Example. FIG. 5 is a cross-sectional observation image of the vicinity of a tooth surface of a gear in an embodiment different from FIG. 4. The cross-sectional observation image of the tooth surface vicinity of the gear in a comparative example.

Explanation of symbols

10 Gear 12 Tooth part 14 Tooth surface 16 Surface part 18 Center part

Claims (1)

A gear processing step of obtaining a gear having a predetermined shape by processing a steel material to be a gear;
After the gear machining step, the gear is rapidly cooled to a martensite region after being heated to an austenite region in an atmosphere containing a carbonized gas or an atmosphere containing a carbonized gas and ammonia gas, A first quenching step of quenching the gear;
A heat treatment step including at least a carburizing and nitrocarburizing step of quenching the gear after raising the temperature to an austenite region in an atmosphere containing a carbonized gas and an ammonia gas,
In the carburizing and nitrogen quenching step, the amount of retained austenite on the outermost surface including the tooth surface of the gear is 40% to 80% by volume, the carbon content is 0.75% to 1% by mass, and the nitrogen content is 0.00. A method for producing a gear, characterized in that the ratio is 5 to 0.7% by mass.

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