JPH06116628A - Manufacture of contour quenching parts - Google Patents

Abstract

PURPOSE:To economically manufacture a parts having high strength and high precision by heating a medium or high carbon steel blank, austenizing, plastic- working, reheating at just above the austenizing temp. after once cooling, quenching and tempering it. CONSTITUTION:The blank composed of the medium or high carbon steel is heated and austenized. This blank is subjected to plastic working at least in the warm temp. zone and precisely formed to a desired shape of the parts having matrix structure accomulating plastic strain. Successively, this worked blank is cooled to the pearlite transformation temp. or lower to precipitate carbide. Thereafter, this blank is preheated to the austenizing temp. or lower to remove the residual working strain and also, adjusted to the structure being apt to be austenized. This preheating is desirable to execute at 550-720 deg.C for 5sec-3min. Successively, the preheated blank is immediately cooled after rapidly heating to just above the austenizing temp. By this method, the contour quenching to only the surface along the contour of the blank is executed and further, the tempering treatment is executed to improve the toughness.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a manufacturing method for manufacturing parts such as gears used in automobiles and the like with high strength, high accuracy and economically.

[0002]

2. Description of the Related Art Conventionally, as a manufacturing technology for gears for automobiles, case hardening steel is blank-cut, gear-cut, and after shaving, the tooth surface is finished with high accuracy of JIS class 3 or so, and then carburized and tempered. A method of performing surface hardening is generally used (for example, the Iron and Steel Institute of Japan, heat treatment of steel, 5th edition, Maruzen, 1968). However, the biggest problem with this method is
At a high temperature of 30 to 950 ° C. and for a long time of about 4 hours, the crystal grains are coarsened to 20 to 30 μm, and the presence of an abnormal carburized layer reduces the strength. There was Further, as described above, even if the surface is pre-finished with high accuracy in advance, the dimensional accuracy of the finished product will decrease due to the subsequent high temperature and long time carburizing treatment, and moreover there is a large space for installing the carburizing treatment furnace. There were problems such as being required.

Therefore, a high-frequency contour quenching method for medium and high carbon steels has been studied as a surface hardening treatment method for gears and the like, which is an alternative to the carburizing and quenching treatments (for example, heat treatment of steel by the Japan Iron and Steel Institute). , Revised 5th Edition, Maruzen, 1
968). According to this method, there is an advantage that the treatment can be performed in a short time of about 1 minute and the space of the production facility can be reduced as compared with the carburizing treatment. However, since this method is premised on the treatment after gear cutting, it is necessary to use a blank of an annealed structure mainly composed of a ferrite structure,
In order to austenite the structure by heating for a short time of about 1 second, it is necessary to raise the temperature to 950 ° C. or higher, so that there is a problem that the crystal grains become coarse and the strength decreases. Further, it was also difficult to perform contour hardening at a depth of about 1 mm along the shape of the sample.

In order to improve this, JP-A-57-207
119, Japanese Patent Laid-Open No. 61-56242 proposes a high-frequency contour hardening method by raising the temperature in two steps. According to this method, the structure austenitized by the first heating is cooled to form the pearlite structure. Therefore, even if the heating is performed for a short time of about 1 second, the austenite treatment is 950 by the second heating.
It can be performed at a low temperature of about ℃ or less. Therefore, the above problem that contour hardening is difficult is improved. However, since this method uses two-stage heating, the problem of crystal grain coarsening due to the use of the blank of the annealed structure mainly composed of the ferrite structure cannot be solved, and therefore,
It has a big problem in strength that it has lower toughness than carburized and hardened steel, and also has a problem that strain becomes large due to two heat histories, and the heating and quenching process becomes complicated. There was a drawback that the problem of. Further, the present inventors have proposed a method for producing a tough part by refining the crystal grains of the hardened part in which thermo-mechanical treatment and reheat quenching are combined (JP-A-64-52018). However, although this method could be applied to the overall quenching of machine parts, contour quenching of gears etc. was difficult. That is, it is difficult to perform a treatment at a low temperature of 950 ° C. or lower for a short time of about 1 second, and it is difficult to carry out contour hardening at a depth of about 1 mm along the shape of a gear or the like. Therefore, high compressive stress cannot be applied to the surface, and a component having high bending fatigue strength cannot be obtained.

[0005]

SUMMARY OF THE INVENTION The present invention has been made in view of the problems of the above-mentioned prior art, and its object is ordinary carburization in a process including high-frequency contour hardening of medium / high carbon steel. It is an object of the present invention to provide a method for economically manufacturing a component having higher strength and higher precision than that obtained by a process such as quenching.

The inventors of the present invention repeated the detailed examination of the problems of each of the above methods, and conducted tests and studies, and as a result, found the facts described below. That is, when the material is subjected to thermomechanical treatment to form a fine pearlite structure and then preheated at an austenitizing temperature or lower, at a low temperature of 950 ° C. or lower, which was impossible without the preheat treatment, In addition, processing such as a short time of about 1 second is possible, contour hardening with a depth of about 1 mm along the shape of gears etc. can be obtained, and extremely fine parts with crystal grains at the root of 5 μm can be obtained. did it. The present invention is based on such a fact.

[0007]

A method for manufacturing a contour-hardened part according to the present invention comprises an austenitizing step of heating a material made of a medium / high carbon steel to austenite, and at least a warm region of the austenitized material. In the above, the plastic working process is performed to form a desired part shape, the cooling process is performed to cool the plastic processed material to a temperature below which pearlite starts to be transformed, and the cooled material is heated to an austenitizing temperature or lower. And a preheating step for rapidly heating the preheated material to just above the austenitizing temperature, austenitizing the structure, and then immediately cooling, only on the surface along the contour of the material. It is characterized by comprising a step of performing contour hardening and a tempering step of performing tempering treatment on the material subjected to the contour hardening.

[0008]

The operation of the present invention having the above construction is as follows. First, in the austenitizing process, a material made of medium-high carbon steel is austenitized. Next, in the plastic working step, the material is plastically worked at least in the warm region, whereby it is precisely formed into a desired part shape having a matrix structure in which plastic strain is accumulated. Then
In the cooling step, the austenite having plastic strain in the matrix structure is transformed into fine pearlite, and carbide is precipitated. Next, in the preheating step, the residual processing strain is removed and the structure is adjusted to austenite.

Next, in the contour hardening step, first, rapid heating is performed to austenite the structure of a predetermined depth along the contour of the part shape, thereby transforming into a fine and uniform austenite equiaxed crystal. Next, before it grows, and after the carbides precipitated in the cooling step are sufficiently decomposed and eliminated, the material is immediately cooled, so that the surface along the contour of the material has a quenched structure having a fine prior austenite grain size. Becomes In this step, although the reason is not clear so far, by providing the preheating step,
For the first time, contour quenching with a depth of about 1 mm along the shape of gears, etc., which was previously impossible, became possible. Furthermore, by tempering, the toughness of martensite can be improved.

[0010]

According to the method for manufacturing a contour-hardened part of the present invention, it is possible to obtain a part having higher strength and higher precision than in the case of the ordinary carburizing and hardening process. Further, as a carburizing process and a gear cutting process which conventionally require a large equipment space can be omitted, a small space production process can be provided.

Further, because of high productivity by plastic working in hot and warm regions and in-line heat treatment instead of carburization, the production time can be greatly shortened.

[0012]

【Example】

(First Specific Example) This specific example specifically describes a method for manufacturing a contour-hardened part according to the present invention.
The medium-high carbon steel used in the method of the present specific example preferably contains 0.45 to 0.7% by weight of C. If it is less than 0.45%, the quenching hardness is lowered, and the austenitizing temperature is also increased, which is not preferable, and if it is more than 0.7%, quenching cracks are likely to occur or the amount of precipitated carbides is increased. However, it adversely affects austenitization and lowers toughness, which is not preferable.

Further, it is desirable that the content of Mn is less than 0.5% in order to easily obtain the contour quenching state without quenching the inside of the sample, that is, to reduce the quenching property.

Further, in order to improve fatigue strength,
It is preferable to keep the contained Si, P, S, and O at low values. That is, the contents of these elements are Si:
Less than 0.35%, P: less than 0.015%, S: 0.01
Less than 5% and O: less than 0.0015% are desirable.

Further, Nb, V, and
One or more kinds of Ti and the like may be added. In this case, the amount of C contained is set to a high value of about 0.6 to 1%, the carbide is finely dispersed, and the C concentration as a matrix is set to about 0.5 to 0.6%. desirable.

When contour hardening is performed for a short time, for example, for a heating time of about 0.1 to 0.2 seconds in order to reduce the hardening depth, elements such as Cr and Mo are added to the steel. Hardenability may be improved.

Next, FIG. 1 shows the method according to the present invention schematically developed on the temperature-time axis as an example for each step. In the figure, (a) is an austenitizing process, (b) is a plastic working process, (c) is a cooling process,
(D) is the preheating process, (e) is the contour hardening process, (f)
Indicates the tempering process (hereinafter the same).

In the step of heating the material to austenite, an electric furnace or a high frequency induction heating device is used for heating, and the heating temperature is preferably 800 to 1300 ° C. In the case of heating by an electric furnace, if the holding time of 20 to 30 minutes is taken, the austenitizing temperature is 800 to 90.
It may be about 0 ° C. Further, in the case of heating by a high frequency induction heating device, only a necessary portion can be locally heated in a few seconds to 30 seconds, but the ultimate temperature is 1000 to 1000 for austenitization.
It is desirable to set the temperature to about 1300 ° C.

Next, in the step of performing plastic working in the warm region, the working temperature is 600 to 900 ° C., and a supercooled austenite state and a partial ferrite transformation occur. At this time, since austenite is not recrystallized, a large strain is accumulated. Furthermore, when the process is performed at a temperature of about 650 ° C., there is little oxidation even in the atmosphere, and precise processing can be performed. Further, when performing the plastic working in the hot region before performing the plastic working in the warm region, the working temperature is as high as 950 to 1300 ° C., and therefore the material can be largely deformed in advance.

Next, in the cooling step, the raw material is cooled to a temperature at which the pearlite transformation of the raw material starts, that is, 600 to 500 ° C. or lower.

Next, in the preheating step, the material is preheated below the austenitizing temperature. Desirably, the temperature is maintained for a few seconds to a few minutes in a temperature range from just below the austenite transformation start temperature to about 500 ° C. depending on the size of the steel material and the quenching depth. Further, as long as it is within the temperature / time range, it is not always necessary to keep the temperature constant during preheating, and it may be changed under the condition that the same effect can be obtained. For example, after raising the temperature to 700 ° C., allowing it to cool for about 20 seconds and cooling to 600 ° C., and then performing contour heating, after repeating the heating up to 700 ° C. as an upper limit during this cooling, contour quenching is performed. May be. Next, in the contour quenching treatment step, the preheated material is rapidly heated in a contour along the shape of the part, and then rapidly cooled to perform contour quenching. The contour hardening is preferably performed by means of induction hardening or the like.

The rapid heating time is preferably about 0.1 to 1 second in consideration of heat conduction in the steel. If it exceeds 1 second, especially for relatively small parts such as gears for automobiles, the hardening pattern will be close to that of overall quenching, which is inappropriate.
If it is less than 0.1 seconds, the formed cured layer will be too thin, and a high-output and large-scale device for heating is required, which is not preferable. In this heating time zone, the temperature range is set to 7 so that the austenite crystal grains are not coarsened.
70-1100 degreeC is desirable. However, when the heating temperature is as high as 950 ° C. or higher, as shown in FIG. 2, the heating time is preferably 0.6 seconds or less because of the refinement of crystal grains.

The means for quenching after heating is not particularly limited, and a water-soluble quenching agent or a quenching agent such as quenching oil is sprayed on the surface, or heated parts are rapidly charged into the quenching agent. The method etc. can be used.

Next, in the tempering step, the toughness is improved by decomposing the high carbon martensite in which carbon is excessively solid-dissolved into the low carbon martensite and the carbide by quenching. In case of heating by electric furnace, 30 in the temperature range of 120-200 ℃
It is desirable to perform the treatment for a minute to 2 hours. Processing temperature is 1
If the temperature is lower than 20 ° C, the carbon remains in an excessively solid-solved state and the toughness is low, which is not preferable. Also, the processing temperature is 2
If the temperature exceeds 00 ° C, the decomposition of martensite proceeds and the hardness sharply decreases, which is not preferable. The processing time is determined by the thickness and weight of the processed product. If the treatment time is too short, the interior will not be sufficiently tempered. Also, if it is too long, it is not economical. Therefore, the processing time is preferably 30 minutes to 2 hours. When a high-frequency heating device is used, the conditions shift to a short time side at a higher temperature than this.

(Second Embodiment) In this embodiment, in the method for manufacturing a high-strength and high-precision contour-quenched part of the present invention, the preheating temperature range is 550 to 720 ° C. and the treatment time is 5 hours.
It is characterized in that the condition is from 3 seconds to 3 minutes. If the preheating temperature is lower than 550 ° C, it takes a long time to remove the residual working strain, which is not preferable. On the other hand, if the temperature exceeds 720 ° C., austenite is generated and the crystal grains are coarsened, which affects contour quenching, which is not preferable.

On the other hand, the processing time is determined by the relationship with the processing temperature. Generally, the higher the processing temperature, the shorter the processing time, and conversely, the lower the processing time, the longer the processing time. Therefore, the processing temperature is 7
When the temperature is 20 ° C., the treatment time may be about 5 seconds, and when it is too long, the productivity is lowered and, structurally, agglomeration of carbides and transformation to ferrite proceed, and a short time is required in the next contour hardening step. It is unfavorable since it inhibits austenitization at 3, so the maximum time is 3 minutes. Desirably, heating at 700 ° C. for about 30 seconds immediately below the austenite transformation start point is preferable. In this case, since the temperature range is effective for removing the residual machining strain, the effect on the contour heating for a short time is greatest. These conditions are determined by the heating means used or the production tact. By setting the temperature and time for the preheating treatment within the above ranges, the temperature and time for contour hardening can be set to a low temperature and a short time as shown in FIG.

(Third embodiment) This embodiment is a method for manufacturing a high-strength and highly accurate contour-quenched part of the present invention, after contour-quenching and tempering, and / or at least after plastic working in a warm zone. The finished product is finished. As described above, after the contour quenching and tempering treatment, or at least after the plastic working in the warm region, the finished product is subjected to the finish working to obtain a highly precise contour-quenched part having further excellent dimensional accuracy. You can Further, since the present invention can significantly reduce the production time due to the in-line heat treatment, even when the process that ensures high accuracy is introduced by introducing the tooth polishing in the final step after the heat treatment, the conventional carburization is performed. It exhibits excellent effects such as maintaining cost advantage over the method.

(Embodiment 1) The present invention will be described below with reference to embodiments.

As a raw material, C is 0.63%, Si is 0.28%, Mn is 0.38%, P is 0.012%, and S is 0. 002%, C
Using carbon steel containing 0.04% of u, 0.11% of Cr and the balance of Fe, the carbon steel was processed by rolling to form a ring gear. The material is a ring having an outer diameter of 181 mm, an inner diameter of 108 mm, and a width of 19 mm. The outer peripheral portion of this ring material was heated to 1200 ° C. for 30 seconds using a high frequency induction heating device to austenite the structure of the material. Next, the outer peripheral portion of this austenitized ring material is rolled with a gear-shaped roller to form a module 2.
4, a ring gear having 65 teeth, a twist angle of 30 °, an outer diameter of 184 mm, an inner diameter of 108 mm and a width of 21 mm was formed. At this time, the processing start temperature is 1050 ° C and the processing completion temperature is about 6
The temperature was 50 ° C., and rolling was continuously performed during this period. The roller pushing speed was 1 mm per revolution of the material, and the processing time was 6 seconds.

After the rolling process was completed, the molded product was allowed to cool to 25 ° C. Then, after performing a preheating step of heating to 720 ° C. and allowing to cool to 650 ° C. for 10 seconds, 9 seconds were applied for 9 seconds.
After rapidly reheating to 00 ° C. to austenite the structure of the outer peripheral portion along the contour of the material, a step of contour quenching was immediately performed with a water-soluble quenching agent.

Then, a tempering process was performed at 160 ° C. for 120 minutes.

The surface finish of the obtained ring gear was smooth and had no defects such as cracks at all, and could be polished as it was.

The hardness distribution of the tooth portion of the ring gear manufactured by the above method was measured. A micro Vickers hardness meter was used for the measurement, and the cross section perpendicular to the tooth was measured as the measurement surface. The load applied to the hardness meter was 500 gf. The results are shown in Fig. 3.
The horizontal axis represents the distance from the tooth surface, and the vertical axis represents the Vickers hardness. From this result, it can be seen that the hardness is high near the surface of the tooth tip and the tooth root, and the quenching is performed along the contour of the tooth. In particular, the hardening depth at the root portion, which is most important in terms of strength, shows an appropriate value of 1 mm. Although such contour hardening has not been possible in the past, it became possible only by introducing a preheating treatment. Further, in the structure observation of the cross section of the same sample at a right angle to the tooth, the prior austenite crystal grain size at the tooth root was 5 μm.

Next, the residual stress on the tooth root surface was measured by X-ray. As a result, the residual stress value of the dedendum is about 70 kgf / mm ² on the compressed side, was higher sufficiently compressive side of the stress value 0~20kgf / mm ² tensile in the case of performing normal carburizing quenching.

(Example 2) As shown in FIG. 4, a gear was manufactured by rolling in a hot / warm zone and cooled to room temperature. The materials and manufacturing conditions used are the same as in Example 1. After cooling, a finishing process was performed by shaving to improve the tooth surface accuracy to JIS class 3. After that, even if preheating and contour quenching were performed, the dimensional accuracy did not deteriorate as in the case of carburizing and quenching used at present, and a gear with JIS class 3 maintained was obtained.

(Example 3) A material made of steel having the same composition as in Example 1 except that it contained 0.6% of C was prepared. From the material, a square bar shape (height 12 mm, width 10 mm, V-notch (radius 1 mm, depth 1.9 mm, angle 60 °) in the center,
A test piece having a length of 90 mm was manufactured. Next, as shown in FIG. 5, this test piece was subjected to austenitizing treatment at 850 ° C. for 20 minutes, then subjected to 50% compression processing at 730 ° C. in the middle of air cooling using a knuckle joint press, and air-cooled to room temperature. This was subjected to a preheating step at 700 ° C. for 30 seconds, followed by an induction contour hardening step at an ultimate temperature of 840 ° C. for a heating time of 1 second, and tempering was performed to obtain a notch test piece shown in FIG. The bottom of the V notch is ground and then shot peened.

The hardness distribution of this notched test piece was measured. The measurement conditions are the same as in Example 1. The measurement result is shown in FIG. 7. From this result, the carburizing and quenching (H
v: It can be seen that a hardness distribution comparable to that of the outer peripheral portion 780 to the inner 350) is obtained. Also, the former austenite crystal grain size of the hardened layer was about 5 μm. When the contour hardening process was performed on the test piece that was not subjected to warm working, ferrite remained in an area ratio of about several percent, and uniform and sufficient surface hardness could not be obtained.

Next, the bending fatigue strength was measured using this notch test piece. Schenk type bending fatigue tester is used for measurement, complete swing (stress ratio 0), repetition rate 1800
The measurement was performed under the condition of rpm. The results are shown in Fig. 8. The horizontal axis represents the number of load cycles and the vertical axis represents the nominal total stress amplitude at the bottom of the V-notch. In the same figure, as a comparative example, the bending fatigue strength measurement results of the carburized and quenched test pieces are also shown.
Compared with the case of carburizing and quenching, the test piece of this example is 10
It can be seen that the fatigue strength is high at about%.

Next, an impact test was conducted using test pieces having various heating temperatures in the contour hardening process. The test was performed using a Charpy impact tester. The test results are shown in FIG. The horizontal axis represents the contour hardening temperature, and the vertical axis represents the impact value. From this result, the impact value is higher as the contour hardening temperature is lower, and becomes lower as the contour hardening temperature rises,
It can be seen that up to about 950 ° C, it is at least equivalent to the carburized and quenched product.

Next, a static bending strength test was carried out using the test pieces with various contour hardening temperatures. For the test, a three-point bending test was performed using a universal material testing machine under the conditions of a load speed of 2 mm / min and a distance between fulcrums of 50 mm. The result is shown in Figure 1.
It shows in 0. The horizontal axis represents the contour hardening temperature, and the vertical axis represents the bending rupture stress. As a comparative example, the figure also shows the normal static bending strength level of the carburized and hardened material and the static bending strength test result when 0.6% C steel was hardened by the conventional induction hardening method. The test piece of this example has a strength about 30% higher than the normal static bending strength level of the carburized and hardened material, and the static bending when 0.6% C steel is hardened by the conventional induction hardening method. It can be seen that it is about 2.5 times higher than the strength.

[Brief description of drawings]

FIG. 1 is an explanatory view schematically showing a process of the present invention.

FIG. 2 is a diagram showing a range of a quenching temperature and a temperature rising time of contour quenching according to the present invention.

FIG. 3 is a diagram showing a result of measuring a hardness distribution of a gear that has been subjected to the treatment according to the present invention in the first embodiment.

FIG. 4 is an explanatory view schematically showing the process of Example 2.

FIG. 5 is an explanatory view schematically showing the process of Example 3.

FIG. 6 is a diagram showing the geometrical dimensions of the test piece of Example 3;

FIG. 7 is a diagram showing the results of measuring the hardness distribution of a test piece that has been treated according to the present invention in Example 3.

FIG. 8 is a diagram showing the results of measurement of bending fatigue strength of a test piece in Example 3.

FIG. 9 is a diagram showing a result of an impact test of a test piece in Example 3.

FIG. 10 is a view showing a static bending strength test result of a test piece in Example 3.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Atsushi Danno, Nagakute-cho, Aichi-gun, Aichi-gun, Nagatake 1 1st at 41 Yokomichi Toyota Central Research Institute Co., Ltd. 1 at 41 Chuo-dori, Toyota Central Research Institute Co., Ltd. (72) Inventor Masasumi Onishi 1 Toyota Town, Toyota City, Aichi Prefecture Toyota Automobile Co., Ltd. (72) Noritaka Miyamoto 1 Toyota Town, Toyota City, Aichi Prefecture Inside Toyota Motor Co., Ltd. (72) Inventor Mineo Ogino 1 Toyota Town, Toyota City, Aichi Prefecture Inside Toyota Motor Co., Ltd.

Claims (3)

[Claims] 1. An austenitizing step of heating a material made of medium-high carbon steel to austenite, and a plasticizing step of plastically working the austenitized material in at least a warm region to obtain a desired part shape. A cooling step of cooling the plastically worked material below a temperature at which it transforms into pearlite, a preheating step of heating the cooled material below an austenitizing temperature, and a preheating treatment of the preheated material. Rapid reheating to just above the austenitizing temperature, austenitizing the structure, then immediately cooling, and a contour hardening step of carrying out contour hardening only on the surface along the contour of the material, and performing the contour hardening And a tempering step of subjecting the material to a tempering process, and a method for manufacturing a contour-quenched part. 2. The method for manufacturing a contour-hardened part according to claim 1, wherein the preheating treatment is performed under the conditions of a temperature range of 550 to 720 ° C. and a treatment time of 5 seconds to 3 minutes. 3. The method for producing a contour-quenched part according to claim 1, wherein the workpiece is finished after the contour-quenching treatment and / or after the plastic working in at least the warm region.