JP2005048292A - Carburizing and quenching method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a carburizing and quenching method by which carbon steel having an austenite grain size of ≥#10 can be obtained though the steel is a non-forged article of the stock made of low carbon steel. <P>SOLUTION: The carburizing and quenching method comprises: a first stage where the stock of a power transmission member made of low carbon steel, and in which tooth parts are formed at equal intervals is subjected to carburizing treatment so as to be heated to an austenitizing temperature or higher and to control the concentration of carbon in the surface to 0.6 to 0.9%; a second stage where, after the first stage, cooling is performed to less than the austenitizing temperature; a third stage where the carburized part in the surface of the stock and the inside of the stock cooled in the second stage are rapidly heated just above the austenitizing temperature; and a fourth stage where, successively to the third stage, quenching treatment is performed. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a carburizing and quenching method suitable for application to a power transmission member such as a gear.

  In general, case-hardened steel (low carbon steel) is used as a gear for pinion of a differential gear of an automobile, and its surface is subjected to gas carburizing quenching or plasma carburizing quenching to improve fatigue resistance and wear resistance. It is used above.

In addition, as a surface effect treatment method for gears and the like that replaces the carburizing and tempering treatment, a high-frequency contour quenching method for medium and high carbon steel as disclosed in Patent Document 1, for example, has been proposed. In this method, a material made of medium and high carbon steel is heated to be austenitized, plastically processed at least in a warm region and cooled, and then the material is preheated at or below the austenitizing temperature, and thereafter Then, after rapidly reheating to just above the austenitizing temperature to austenite the structure, quenching and tempering treatment is performed.
JP-A-6-116628

  By the way, in the usual gas carburizing quenching or plasma carburizing quenching, since the workpiece is heated to a high temperature for a long time during carburizing, impurities such as phosphorus and sulfur are segregated at the grain boundaries, and carbides are precipitated to form grains. The strength of the field is reduced.

  Therefore, cracks are likely to start from the grain boundary at the edge of the tooth bottom (shown by A in FIG. 1) where high bending stress is applied, and the crack propagation is fast, resulting in insufficient static fracture strength and impact strength. There was a problem to do. In order to alleviate this problem, there is a method of reducing the carburizing depth. However, when the carburizing depth is reduced, the spalling strength also decreases on the tooth contact surface (shown by B in FIG. 1) where high surface pressure is applied. There is a problem.

  On the other hand, in the high-frequency contour quenching method for medium and high carbon steel, the heating time of the workpiece is extremely short, so the problem as in the carburizing treatment is solved. Since the machinability is inferior to that of low carbon steel, the cost of the cutting tool is high, and the core portion of the gear is made of medium / high carbon steel, so that the toughness of the core portion is low.

  Furthermore, in a gear, it is desirable in terms of strength that the austenite crystal grain size is fine at both the root portion and the tooth contact surface, but the conventional method has a fine austenite crystal grain size of # 10 or more except for forged products. Carbon steel made of various crystal grains was not obtained.

  In view of the above-mentioned problems, the present invention is a non-forged product made of a low-carbon steel, while preventing segregation of PS or carbide that lowers the grain boundary grain size, and austenite crystal grain size of # 10 or more. It aims at providing the carburizing and quenching method which can obtain steel.

In the carburizing and quenching method according to the present invention, the material of the power transmission member made of low carbon steel and having tooth portions formed at equal intervals is set at an austenitizing temperature or higher (Ac ₃ or more points for hypoeutectoid carburizing, Accm for hypereutectoid carburizing). 1st step which heats to above point, see FIG. 2 and performs carburizing treatment to surface carbon concentration of 0.6-0.9%, and cools to below austenitizing temperature after said first step (quenching) Alternatively, a tempering treatment may be performed.) The second step, the third step of rapidly heating the surface carburized portion of the material cooled in the second step and the inside of the material immediately above the austenitizing temperature, and the third step And a fourth step of performing a quenching process.

  If the surface carbon concentration during the carburizing process is less than 0.6%, the surface hardness cannot be secured, and if it exceeds 1.2%, a net-like carbide precipitates at the grain boundary, so that 0.6 to 1.2%. In particular, it is desirable to set the surface carbon concentration in the vicinity of the eutectoid point S (0.6 to 0.9%).

  Furthermore, it is desirable to supply a coolant to the surface of the material during the rapid heating to cool the surface of the material to a temperature lower than the eutectoid temperature (723 ° C.), for example.

  Specifically, examples of the power transmission member include gears, sprockets, splines, and the like, and case-hardened steel is used as a material. As a carburizing treatment means, gas carburizing or plasma carburizing is appropriately used, and plasma carburizing is desirable from the viewpoint of carburizing efficiency and carburizing uniformity. However, since plasma carburizing is performed in a vacuum, cost and mass productivity are pointed out. Therefore, gas carburization is desirable.

  The heating during the quenching process is desirably induction quenching.

  Moreover, while performing the said carburizing process by a plasma carburizing process, the raw material of the said power transmission member may be heated so that the heating temperature of a tooth bottom part may become higher than the heating temperature of a tooth contact surface at the time of the said hardening. The heating at the time of quenching is desirably set so that the heating temperature at the bottom of the tooth becomes higher than the heating temperature at the tooth contact surface from temperature rise to soaking and main heating. It is desirable to decarburize the outermost surface portion of the carburized layer and lower the carbon concentration of the outermost surface portion from the carbon concentration below it. In particular, it is preferable that the carbon concentration of the outermost surface portion of the carburized layer in the tooth bottom portion is lower than the carbon concentration below it. And it is desirable that the heating of the quenching is also an induction quenching.

  In carburized materials, it is thought that segregation of carbon grain boundaries occurs during carburizing, and film-like cementite is present, reducing the grain boundary strength. However, as in the carburizing and quenching method according to the present invention, For example, since it is rapidly heated above the austenitizing temperature by high-frequency heating, a new grain boundary is formed by recrystallization, the grain boundary strength is improved, and austenite grain size is reduced (austenite grain size is # 10 or more). Since it is realizable, static fracture strength and impact strength can be improved without reducing the spalling strength.

  Further, in the conventional gas carburizing, the target was a surface carbon concentration of 0.9 to 1.0%. However, when the target is set near the eutectoid point, the strength improvement is about 19% compared with the carburized and quenched product. Even when compared under the same induction hardening conditions, an improvement in strength of 5 to 19% was confirmed.

In the case of hypoeutectoid carburization, when the rapid heating temperature is less than Ac ₃ point, ferrite precipitates inside the material and the strength is lowered. In the case of hypereutectoid carburization, if the rapid heating temperature is less than Accm point, the surface carburized layer is formed. Cementite precipitates and the strength decreases, and when the rapid heating temperature exceeds 1100 ° C., austenite crystal grains grow and the strength decreases. In the method of the present invention, the rapid heating temperature is set immediately above the austenitizing temperature. In addition, since the surface carburized portion and the inside of the material are both heated to the austenitizing temperature or more by the rapid heating, the austenite crystal grains can be refined and the inside of the material can be quenched. It has the effect of increasing the hardness inside the material.

  Furthermore, in the above rapid heating, if the hardness is increased by quenching to the inside of the material, the entire surface is heated, so that the surface of the material becomes a high temperature for a relatively long time, the austenite crystal grains grow too much, and the toughness decreases. However, in the method of the present invention, in the course of the rapid heating, a coolant is supplied to the surface of the material, and the surface of the material is cooled to a temperature lower than the eutectoid temperature (723 ° C.), for example. While maintaining the internal temperature at or above the austenitizing temperature, it is possible to prevent the surface from becoming hot for a long time, and as a result, the austenite crystal grains on the surface of the material are refined (even if it is a non-forged product, an austenite crystal) The particle size is # 10 or more) and the material is quenched to improve the toughness.

  In addition, when applying the method of the present invention to a power transmission member such as a gear, at the time of quenching after plasma carburization, the heating temperature of the tooth bottom is set to be higher than the heating temperature of the tooth contact surface. More carbides precipitated along the grain boundaries in the carburized layer at the bottom of the tooth are dissolved in the matrix, and the carbon concentration in that part increases, so the amount of retained austenite after quenching is relatively low at the bottom of the tooth. Become more. Further, since the heating temperature of the tooth bottom portion is set higher than the tooth contact surface, the time during which the tooth bottom portion is cooled from the heating temperature to the martensite transformation temperature becomes longer, which also increases the amount of retained austenite. Cause.

  An appropriate amount of retained austenite in the carburized layer at the bottom of the tooth has an effect of improving the bending fracture strength of the edge portion, and as a guideline, it preferably remains about 25 to 35% in terms of area ratio. If the amount of retained austenite is less than this, the toughness of the structure itself will be insufficient, and there will be many carbides that cannot be dissolved in the matrix as a starting point of fracture. There arises a problem that the strength is lowered and the edge portion is easily deformed. The amount of retained austenite can be adjusted as appropriate according to the heating temperature during quenching and the holding time in the high temperature range (the longer the holding time in the high temperature range, the more solid carbide dissolves and the more the amount of retained austenite increases).

  On the other hand, since the heating temperature at the tooth contact surface is set lower than that at the bottom of the tooth, the amount of solid solution of carbide in the parent phase is small and the carbon concentration in that part is not as high as that at the bottom of the tooth. The amount is relatively small. Similarly, since the heating temperature of the tooth contact surface is set lower than the bottom of the tooth, the time during which the tooth contact surface is cooled from the heating temperature to the martensite transformation temperature is shortened, and this also reduces the amount of retained austenite. Causes to decrease.

  In the carburized layer on the tooth contact surface, in order to ensure the spalling strength, the amount of retained austenite is preferably small and a structure in which a predetermined amount of carbide is dispersed in a granular form, and as a guide, retained austenite having an area ratio of 5 to 15%. It is good to make it quantity. If the amount of retained austenite is less than this, it means that there is a high possibility that the carbides precipitated during carburizing will hardly dissolve and remain in a net form. This means that the amount of precipitated carbide is reduced, and both cause a decrease in the spalling strength.

  When a normal quenching method for uniformly heating the tooth contact surface and the tooth bottom portion is applied, the amount of retained austenite at the tooth bottom portion is generally lower than that at the tooth contact surface, contrary to the present invention. In other words, when carburizing a member such as a gear, the carbon concentration of the carburized layer is relatively high on the tooth contact surface that is close to a simple planar shape, and in a portion of the shape such as the inside of the tooth bottom, This is because the carbon concentration tends to be relatively low.

  The heating at the time of quenching is set so that the heating temperature of the bottom of the tooth becomes higher than the heating temperature of the tooth contact surface from the temperature rise to the soaking and the main heating. The temperature is kept high for a long time, and has an action of promoting solid solution of carbide in the carburized layer at the bottom of the tooth.

  Usually, the outermost surface portion of the carburized layer tends to be excessively carburized, and the carbon concentration of the outermost surface portion after carburizing is higher than the carbon concentration below it, but it is retained in a high temperature range during quenching. When it is carried out for a certain long time, the outermost surface portion of the carburized layer is decarburized, and grain boundary embrittlement is less likely to occur as the carbon concentration decreases in that portion. Therefore, when the decarburization of the outermost surface portion is advanced, the heating condition is set so that the holding time in the high temperature region becomes long.

  When the heating at the time of quenching is set so that the heating temperature of the tooth bottom portion is higher than the heating temperature of the tooth contact surface from the temperature rise to the soaking and the main heating, the tooth bottom portion is set to a higher temperature. It will be held for a long time, and in particular, the carbon concentration is greatly reduced at the outermost surface portion of the bottom carburized layer, and the fracture strength of the edge portion of the bottom is improved. Decarburization is preferably performed such that the carbon concentration in the outermost surface portion from the surface to 20 μm is less than the eutectoid point, particularly about 0.6 to 0.75%.

  As specific heating means for making the heating temperature of the tooth bottom portion higher than the heating temperature of the contact surface, well-known high-frequency induction heating is preferable for the amount of quenching, and adjusting the frequency, output, etc. Thus, the heating temperature setting for each region can be easily realized with one coil. In high-frequency induction heating, the surface of the member rises to a predetermined temperature in a relatively short time and causes recrystallization. As a result, the crystal grains are refined and segregated at the grain boundaries before heating. The impurities are dissolved in the grains, and the carbides are also dissolved in the grains, but the parts that are not dissolved are divided and become granular.

  According to the present invention, after the carburizing treatment, rapid heating is performed immediately above the austenitizing temperature, so that a new grain boundary is formed by recrystallization, the grain boundary strength is improved, and austenite grain size is reduced (austenite grain size). Is # 10 or higher). As a result, the static fracture strength and the impact resistance strength can be improved without reducing the spalling strength.

  Further, since both the surface carburized portion of the material and the inside of the material are heated to the austenitizing temperature or higher, the inside of the material is quenched, and the hardness inside the material is increased.

  Furthermore, since the surface carbon concentration in the carburizing treatment is set in the vicinity of the eutectoid point (0.6 to 0.9%), the strength can be improved as compared with carburized and quenched products.

  Examples of the present invention will be described below.

  The materials used were C: 0.20%, Si: 0.08%, Mn: 0.75%, P: 0.016%, S: 0.026%, Cr: 1.02%, Mo: 0 .42%, Al: 0.024%, non-forged product made of case-hardened steel material with the remaining Fe, but it is better to add Nb as a grain refinement element. Then, in order to avoid the influence of variation factors (tooth contact state, damage to other parts) in the actual machine, a test piece having dimensions as shown in FIG. 3 is evaluated by a basic test (three-point bending test). It was.

  First, the test specimen is subjected to gas carburization treatment and quenching at a temperature equal to or higher than the austenitizing temperature. The carburization depth is set to 1.2 mm in order to ensure the spalling strength, and the surface carbon concentration is 0.7% below the eutectoid point (condition a in FIG. 4), which is the same as in conventional carburizing and quenching. The target was 0% (condition b in FIG. 4).

  Next, induction hardening was performed, but a heat pattern as shown in FIG. 5 was set in order to change the surface and inside of the material to a quenched structure and change the austenite grain size of the surface. Table 1 below shows the temperature measurement results obtained by adjusting the output and time by preparing a 400 kHz machine and a 10 kHz machine.

  With respect to the test piece subjected to the carburizing treatment and induction hardening in this way, the breaking load was measured using a three-point bending tester shown in FIG. The test conditions are shown in Table 2 below.

  Table 3 below shows the investigation results of the metallurgical characteristics of the test pieces for each combination of the carburizing treatment and the induction hardening and the average breaking load in the three-point bending test (N = 3).

  (1) Basic test results: FIG. 7 shows the relationship between the austenite grain size and the three-point bending fracture load. An improvement in breaking load of up to 50% over the conventional carburized and quenched product was observed.

  (2) Results of carburizing condition examination: By setting the surface carbon concentration from 1% to 0.7% below the eutectoid point at which cementite that causes grain boundary embrittlement does not precipitate, the carburized and quenched product has a strength of about 19%. It was confirmed that the strength was improved by 5 to 19% even under the same high frequency conditions.

  (3) Induction quenching examination results: Fig. 8 shows the relationship between the maximum induction heating temperature and the austenite grain size. Even in short-time heating, the coarsening of crystal grains is inevitable as the heating temperature rises. However, when the heating temperature is set in the temperature range immediately above the austenitizing temperature, the austenite grain size, which is difficult to be carburized and quenched, is # 10 or more. Crystal grain refinement became possible. And as for this test piece, a core part (material inside) consists of low carbon steel, and the surface part (surface carburized part) has the carbon concentration similar to medium and high carbon steel by carburizing process.

  Next, a method in the case of subjecting the workpiece W (pinion) as shown in FIG. 1 to carburizing and then induction hardening will be described.

Since induction hardening is rapid heating for a short time, the austenite crystal grains are fine. However, the surface of the workpiece W is relatively comparative because not only the surface but also the entire surface is heated for those requiring hardness to the core. It becomes a high temperature for a long time, the crystal grain grows too much, and the toughness decreases. Therefore, in this embodiment, by adding a surface quenching step with gas (Ar, N ₂ ) between the preheating step and the main heating step, the core portion is just above the austenitizing temperature as shown in FIG. While maintaining the temperature, the surface austenite crystal grains are miniaturized. Thus, surface crystal grains having an austenite grain size of # 10 or more are obtained while quenching to the core. And as for this workpiece | work W, a core part consists of low carbon steel, and the surface part has the carbon concentration similar to medium and high carbon steel by carburizing process.

  FIG. 10 is a diagram schematically showing an induction hardening apparatus in that case. A nozzle holder 4 having a number of cooling gas injection nozzles 3 as shown in FIG. The insulating ceramic materials 5 and 5 are attached. Then, induction hardening is performed according to the heat pattern of FIG. 9 while rotating the workpiece W around its axis.

  As described above, in this example, since rapid heating to the austenitizing temperature or higher by high-frequency heating after carburizing treatment, a new grain boundary is formed by recrystallization, the grain boundary strength is improved, and the fine grains of austenite crystal grains (Austenite grain size # 10 or more) can be realized, so that the static fracture strength and impact strength can be improved without lowering the spalling strength.

  The present embodiment relates to a carburizing and quenching method based on plasma carburizing.

  The workpiece used in this example is C: 0.18%, Si: 0.09%, Mn: 0.69%, P: 0.006%, S: 0.021%, Cr: 1.02% , Mo: 0.39%, Al: 0.35%, Nb: 0.035%, differential gear pinion (outer diameter 41mmφ, height 17.6mm, hole diameter 15mm, made of case-hardened steel material of Fe Fig. 1). A static carburization test and a spalling test were performed on a gas carburized and quenched steel as a conventional example and a plasma carburized and induction hardened steel as an example.

  The above-mentioned conventional gas carburizing and quenching targets a surface carbon concentration of 0.9%, (1) gas carburizing treatment at 920 ° C. for 5 hours, and (2) oil quenching at 120 ° C. after holding at 860 ° C. for 1 hour. It consists of the following steps: (3) reheating at 180 ° C. and tempering for 2 hours.

On the other hand, in the above example, plasma carburization is similarly targeted for a surface carbon concentration of 0.9%, and (1) a workpiece is placed in a vacuum furnace and soaked in a vacuum at 1000 ° C. for 10 minutes, (2) H ₂ gas was introduced into the vacuum furnace, the furnace pressure was adjusted to 2 Torr, glow discharge was performed under the conditions of 400 V and 1.5 A, cleanup treatment for 20 minutes, (3) H ₂ gas was extracted and C ₃ H ₈ Introduce gas, adjust furnace pressure to 3 Torr, glow discharge under conditions of 360V, 2A, carburize for 50 minutes, (4) vacuum in the furnace and diffuse for 72 minutes, followed by slow cooling After cooling, induction surface quenching was performed, and finally tempering was performed at 180 ° C. for 2 hours.

  In the induction hardening according to the embodiment, as shown in FIG. 12, a work W is sandwiched between a rotatable jig 11 connected to a motor (not shown) and a driven rotating jig 12, and a high frequency coil is provided at the outer peripheral position thereof. No. 13 was placed and heated for a total of 42 seconds under the conditions shown in Table 4 below. After heating, oil at 80 ° C. was sprayed for 35 seconds and cooled. Further, in order to check the temperature of the surface of the workpiece W, a thermocouple 14 is attached to each part A to E (see FIG. 13) of the surface of the workpiece W, and the detected value is recorded on the pen recorder through the slip ring 15 and the fixed support portion 16. I was able to do it. The surface portion A of the workpiece W is the heel side bottom of the pinion gear (corresponding to A in FIG. 1), B is the pitch surface (corresponding to B in FIG. 1), C is the center of the bottom, D is the toe side bottom, E is a tooth tip.

  As shown in FIG. 14 showing the temperature measurement results, the maximum heating temperature of the tooth bottom part is high, and particularly in the heel side tooth bottom part A where a high bending stress is applied to the edge part, compared to the pitch surface B, residual heat, soaking, It is heated to a high temperature throughout the heating. For reference, the relationship between the temperature of the heel side tooth bottom portion A and time and the relationship between the temperature of the pitch surface B and time are shown in a simplified manner as shown in FIG.

  In the static fracture test, the workpiece W is incorporated in the differential gear unit 21 (see FIG. 16), the output shafts 22 and 23 are fixed, the gear case 24 is rotated and twisted, and the torque when the workpiece breaks is measured. Therefore, as shown in Table 5 below, in the examples, a higher static fracture strength (average value of all three) was obtained compared to the conventional example. This is because, in the example, the bottom of the tooth, particularly the heel side bottom A, is maintained at a high temperature for a long time, so that the carbide dissolves, the amount of retained austenite in that portion is large, the carbide is reduced, and the toughness of the edge portion. This is thought to be because the propagation of cracks is delayed and the propagation of cracks is delayed, and the reason is that the carbon concentration in the outermost surface portion is lowered and the starting point of cracks is reduced, as will be described later. The area ratio of retained austenite at the heel side tooth bottom A was 30% on the average, and the average for pitch surface B was about 10%.

  In the spalling test, similarly, the work W is incorporated into a gear unit of a differential gear, this is connected to the engine via the transmission, one output shaft of the unit is fixed, the input rotation speed to the transmission is 262 rpm, and the input torque is The measurement was performed under the conditions of 117 to 123 N · m, the rotation speed of the other output shaft of the unit was 50 rpm, and the output shaft torque was 459 to 471 N · m. And the vibration of the unit was always detected, the number of cycles at that time was calculated from the time until the vibration exceeded a certain reference value, and this was used as the spalling life. As shown in Table 5, in the example, compared to the conventional example, a spalling life (both average values of both) is obtained.

  Further, in this example, when the relationship between the depth from the surface of the root A and the carbon concentration was examined, as shown in FIG. 17A, the carbon concentration in the outermost surface portion from the surface to around 50 μm decreased. is doing. This is because, in this example, in order to promote solid solution of carbides at the bottom of the tooth, it was kept at a high temperature for a long time, which is unusual for heating during induction hardening, so that decarburization from the surface proceeds. is there.

  As a comparative example, instead of the high-frequency heating conditions of the examples, high-frequency heating was performed under conditions of a frequency of 8.2 kHz, an output of 40 kW, and a heating time of 9 seconds (surface quenching conditions normally applied to this kind of pinion). Then, since the time kept at high temperature is shortened, as shown in FIG. 7B, decarburization from the surface has not occurred.

  According to the present embodiment, when the power transmission member such as a gear is subjected to surface quenching, the toughness is increased at the bottom of the tooth by setting the heating temperature at the bottom of the tooth to be higher than the heating temperature of the tooth contact surface. The breaking strength of the edge portion can be improved, while the spalling strength can be improved on the tooth contact surface.

  Also, when decarburizing the outermost surface portion of the carburized layer by continuing high-temperature heating during surface quenching for a certain length of time, decarburization tends to proceed especially at the bottom of the tooth maintained at a high temperature, and the fracture strength of the edge portion is increased. Can be improved.

Cross section of differential gear pinion Steel transformation diagram Front view and side view of test piece in Example 1 The figure which shows the heat pattern of the carburizing quenching The figure which shows the heat pattern of the same induction hardening Illustration of the same three-point bending tester Figure showing the relationship between the austenite grain size and the three-point bending fracture load Diagram showing the relationship between the same high-frequency heating temperature and austenite grain size The figure which shows the heat pattern of induction hardening using the induction hardening apparatus of FIG. The figure which shows schematically the induction hardening apparatus used in Example 1 Top view of the nozzle holder The figure which shows schematically the induction hardening test apparatus used in Example 2. Explanatory drawing of each measurement part of pinion of differential gear The graph which shows the relationship between the heating temperature by high frequency heating in Example 2, and time. Explanatory drawing which simplified the graph of FIG. Schematic diagram of static fracture test equipment Diagram showing surface carbon concentration after induction hardening

Explanation of symbols

DESCRIPTION OF SYMBOLS 1, 2 Heating high frequency coil 3 Cooling gas injection nozzle 4 Nozzle holder 5 Insulating ceramic material 11, 12 Jig 13 holding work 13 High frequency coil 14 Thermocouple 15 Slip ring 21 Differential gear unit 22, 23 Output shaft 24 Gear case W for differential gear Workpiece (differential gear pinion)

Claims (6)

A first step of performing a carburizing process in which the material of the power transmission member made of low-carbon steel and having tooth portions formed at equal intervals is heated to an austenitizing temperature or higher so that the surface carbon concentration becomes 0.6 to 0.9%. ,
After the first step, a second step of cooling below the austenitizing temperature,
A third step of rapidly heating the surface carburized portion of the material cooled in the second step and the inside of the material immediately above the austenitizing temperature;
A carburizing and quenching method comprising: a fourth step of performing a quenching process subsequent to the third step.   The carburizing and quenching method according to claim 1, wherein a coolant is supplied to the surface of the material during the rapid heating.   2. The carburizing and quenching according to claim 1, wherein the carburizing treatment is performed by plasma carburizing treatment, and at the time of quenching, the raw material is heated so that a heating temperature of a tooth bottom portion is higher than a heating temperature of a tooth contact surface. Method.   4. The carburization according to claim 3, wherein the heating at the time of quenching is set so that the heating temperature of the tooth bottom portion is higher than the heating temperature of the tooth contact surface from temperature rise to soaking and main heating. Quenching method.   The carburizing and quenching method according to claim 3 or 4, wherein the outermost surface portion of the carburized layer is decarburized by heating at the time of quenching, and the carbon concentration of the outermost surface portion is lowered from the carbon concentration below the carbon surface. 6. The carburizing and quenching method according to claim 5, wherein the outermost surface portion of the carburized layer in the tooth bottom portion is decarburized by heating at the time of quenching, and the carbon concentration of the outermost surface portion is made lower than the carbon concentration below it. .

Images