JPH0663080B2 - Manufacturing method of carburized parts having fine grain structure - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention relates to a method for producing a carburized part in which carburizing treatment and warm forging are combined, and in particular, a carburized part excellent in strength and toughness with a small grain size. The manufacturing method which can be manufactured.

(Prior Art) Although there are many kinds of parts (hereinafter referred to as carburized parts) used after being carburized, transportation machines, shafts of construction industrial machines, gears, etc. are typical examples. Since these parts are required to have fatigue strength, wear resistance, and pitting resistance, they are carburized. The general manufacturing process for manufacturing such carburized parts is as follows. That is,
As shown in FIG. 1, carburizing steel, which is a raw material, is cut into a predetermined size, heated to a temperature of approximately 600 to 900 ° C., and warm forged,
It has a predetermined shape. Then, after performing necessary machining, it is heated to a temperature of Ac ₃ or higher to carry out a carburizing treatment, and is quenched and tempered. The warm forging in this step can be more precisely formed than the hot forging, and can be processed with a load smaller than that of the cold forging, and thus has been widely adopted in recent years for the production of mass-produced parts as described above.

In the above conventional process, the carburizing process is performed in the final step after machining. Since the temperature of the carburizing treatment is Ac ₃ point or higher, usually around 920 ° C., coarsening of the austenite crystal grains here is unavoidable. In particular, the parts that have undergone warm forging prior to the carburizing treatment show significant grain coarsening during the carburizing treatment. When the austenite crystal grains become coarse, the crystal grains after carburizing and tempering become large, and the toughness and static strength decrease. Further, there is a problem that the heat treatment deforms so much that the shape of the product cannot be maintained.

As a method for preventing the coarsening of crystal grains during carburizing of warm forged parts, from the viewpoint of improving the material composition, JP-A-60-
As disclosed in JP 159155 and JP 60-262941
A method of adding appropriate amounts of Al, N, and Nb to promote formation of fine precipitates and prevent coarsening of crystal grains is disclosed in JP-B-62-6617 from the viewpoint of improvement of treatment method. A method has been proposed in which an intermediate heat treatment is performed before various carburizing treatments. Among these, in the method of improving the material composition, a slight variation in the composition greatly affects the crystal grain coarsening behavior, and a stable fine grain structure may not always be obtained. Further, in the method of Japanese Patent Publication No. 62-6617, the number of steps of intermediate heat treatment before carburizing is increased, which complicates the manufacturing process and increases the production cost.

(Problems to be Solved by the Invention) The present invention not only prevents the coarsening of the crystal grains of the final carburized product while actively utilizing the advantage of warm forging, but also positively refines the crystal grains to improve the product quality. It aims to improve mechanical properties and provides a new method therefor.

(Means for Solving Problems) According to the invention of the present application, “a steel material for carburizing parts is carburized and cooled to room temperature, and then heated to a temperature range of Ac ₁ point or more and 950 ° C. or less, A method for producing a carburized component having a fine grain structure characterized by holding for 15 minutes or less including the temperature rise time, forging into a desired shape in this temperature range, and directly quenching ”.

Here, the above-mentioned "material steel for carburized parts" is a steel obtained by a so-called melting process and is a material steel in which crystal grains are coarsened by heating to a carburizing temperature, and it is excluded by the powder metallurgy method. To do.

FIG. 2 is a heat pattern for explaining the method of the present invention. The features of the method of the present invention become clear by comparing with the conventional method shown in FIG. That is, the biggest difference between the present invention and the conventional method is the order of carburizing and warm forging.

In the conventional process shown in FIG. 1, the carburizing treatment is performed after the warm forging, so that the crystal grains are inevitably coarsened during the carburizing treatment. And the effect is carried to the product after quenching and tempering, which impairs its mechanical properties. Despite these problems, carburizing was carried out after warm forging because when the carburized material was warm forged, surface cracks tended to occur and the forging load was high. Because there was an idea.

The present inventor has reexamined the above-mentioned conventional common sense, and even if the carburizing treatment is performed before the warm forging, there is no problem in the process, and if the conditions of each process are appropriately selected, the product machine I learned that it can greatly improve the physical properties.

In the process of the present invention shown in FIG. 2, even if the austenite crystal grains become coarse during the carburizing treatment, the crystal grains become fine due to the effect of recrystallization due to the processing in the next warm forging step. By quenching from this state, a fine martensite structure can be obtained, and a product having excellent mechanical properties described below can be obtained.

Each step of the method of the present invention will be described in detail below.

The material is general carburizing steel. The steel type should be selected according to the type of final product and the properties required for it. This is prepared into a predetermined size and used as a material (blank). This material is first subjected to a carburizing process. The carburizing process itself may be performed by a conventional method. For example, a carburizing treatment by a batch type gas carburizing furnace or a continuous carburizing treatment by a continuous gas carburizing furnace in which a tray on which a material is placed moves in the furnace is desirable.

The carburized material is once cooled to room temperature, but the cooling method is not particularly limited.

Next, the material is heated to a temperature range from Ac ₁ point to 950 ° C. This heating is for making the temperature suitable for forging and for properly performing quenching after forging. In order to obtain a highly accurate dimensional shape with a low load, which is an advantage of warm forging, it is necessary to forge within the above temperature range. It is natural that the lower the temperature, the larger the forging load, but especially in the case of a material that is carburized before forging and has a high carbon content in the surface layer, Ac ₁
When the temperature is lower than the point, the forging load increases remarkably, and the superiority to cold forging is lost. Furthermore, the heating temperature is Ar ₁
Below the point, the martensite structure cannot be obtained by quenching after forging, and the product cannot have the necessary hardness. On the other hand, if the temperature is higher than 950 ° C, the scale formation on the surface becomes serious and the dimensional accuracy of the product deteriorates, and eventually the advantage of warm forging over hot forging is lost. Further, if the heating temperature here is too high, the diffusion of carbon that has penetrated into the surface layer portion due to the carburization in the previous step becomes severe, and the surface decarburization phenomenon becomes remarkable and the surface hardness after quenching decreases. For these reasons, the heating temperature is in the range of Ac ₁ point to 950 ° C.

The above-mentioned problems of scale formation, carbon internal diffusion, and surface decarburization are affected not only by heating temperature but also by time.
In consideration of this point, the present invention limits the time from the start of temperature increase to the start of forging, that is, the total time of temperature increase and heat retention within 15 minutes. Within this range, the above-mentioned adverse effects are almost negligible.

As for warm forging, in addition to closed forging for forming a complicated shape, various methods such as free upsetting, forward extrusion and backward extrusion can be selected depending on the processing purpose. This forging process has the original purpose of forming into a predetermined product shape and the purpose of refining the product structure (crystal grains). The austenite grains coarsened in the carburizing process are recrystallized and finely divided by the warm forging. The recrystallization temperature of austenite differs slightly depending on the steel type, but if it is a general carburizing steel, Ac ₁
Partial recrystallization occurs just above the point.

If the warm forging temperature is between Ac ₃ point and Ac ₁ point, the central part of the material has two phases of austenite and ferrite, but since the carbon content in the surface layer is high due to carburization, It is one phase of austenite. If quenching is performed from this state, the surface layer portion has a perfect martensite structure and sufficient hardness is secured. Since the central part is quenched from the two-phase state of austenite and ferrite, it does not become a martensite single layer, but the crystal grains are finer than the surface layer part. In a normal carburized component, it is not necessary to form a martensite single layer up to the center, but if hardness at the center is required, consideration should be given to raising the forging end temperature, that is, the quenching temperature.

Consider increasing the quenching temperature.

If the temperature of warm forging becomes lower than Ac ₁ point, austenite is completely decomposed even in the surface layer and pearlite and bainite are formed. Even if this is hardened, the martensite structure does not occur and the desired hardness is obtained. Can't get

The reason for quenching immediately after the completion of warm forging is to rapidly cool fine recrystallized austenite to generate fine martensite. As is clear from FIG. 1, in the past, quenching was directly linked to carburizing treatment and was rapidly cooled from the carburizing treatment end temperature. In this process, the austenite coarsened during the carburizing process is transformed, so that the martensite structure obtained is also coarse. On the other hand, according to the method of the present invention, even if the austenite grains become coarse during the carburizing treatment, they can be finely grained in the subsequent forging step. This means that the carburizing temperature, which has been conventionally suppressed by fear of coarsening of crystal grains, may be increased. If the carburizing temperature is raised, the diffusion rate of carbon is increased and the processing time can be shortened.However, in the method of the present invention, since forging is performed after the carburizing treatment, the carburizing depth at the material stage and that of the product are Not necessarily the same. In addition, the final machining process to finish the final product shape is also performed, where a part of the carburized layer may be removed. The carburizing conditions, carburizing depth, etc. must be determined in consideration of such circumstances.

The device for carrying out the method of the present invention does not require any special device other than the forging device provided with the cooling tank for directly quenching the forged product after the forging is completed. It does not matter if the carburizing process and the warm forging are performed on different lines.

Hereinafter, the effects of the present invention will be specifically described with reference to examples.

(Example) A 44 mmφ x 27.5 mm forging blank was machined from SCM420 hot rolled steel bar (50 mmφ) having the composition shown in Table 1, and a carbon potential of 0.
Carburizing treatment was performed at 950 ° C. for 2 hours in an atmosphere of 85%, and the mixture was cooled to room temperature by air cooling. Graphite lubrication is applied to this carburized blank and it is heated to 50 ° C in the range of 650 to 1050 ° C by high frequency heating.
Heated to each temperature in the interval. The temperature raising time was 1 to 2 minutes, and the holding (soaking) time was 2 minutes.

The blank at each temperature was forged into a spur gear having the specifications shown in Table 2 by using a forging machine capable of measuring the forging load. After the forging was completed, it was immersed for 5 seconds in quenching oil at 120 ° C. For some carburizing blanks, the heating temperature is 900 ° C.
Forging was performed while keeping the temperature constant and changing the total time of temperature rise and soaking between 2 and 20 minutes.

After the gear manufactured in the above process was tempered at 170 ° C for 1 hour, the grain size (according to JIS) of the former austenite grains in the root surface layer and the center of the root shown in Fig. 3 and the root surface layer Hardness distribution was measured. The effective hardened layer depth (distance from the surface equivalent to Hv550) of each gear is 0.55 ~
It was within the range of 0.58 mm.

Then, after each gear was shot-blasted to remove the scale, the three-tooth bridging tooth thickness was measured at 18 points using a tooth thickness micrometer by the method shown in FIG. 10, and the average value was calculated. In addition, the surface roughness of the tooth flank center (measurement distance 2m
m) was also measured.

Further, with respect to these gears, a static root breakage test by the method shown in FIG. 11 and an impact test by the tester shown in FIG. 12 which is a modified Charpy impact tester were carried out.

On the other hand, for comparison, a similar gear was manufactured by the conventional method. The conditions are as follows.

A graphite lubricated blank was placed in a high frequency furnace as in the example.
After heating to each temperature between 650 and 1050 ° C and forging at each temperature to form a spur gear, machining (tooth surface polishing), carburization, quenching, and tempering were performed. The conditions of each step at this time are all the same as the above-mentioned conditions except that the carburizing time was 1.75 hours so that the effective hardening depth was 0.555 to 0.58 mm. However, needless to say, here, the carburizing and forging steps are opposite to those of the method of the present invention, and the quenching is performed after carburizing. The same measurement as described above was performed for the thus obtained one.

Hereinafter, the test results will be described with reference to the accompanying drawings.

FIG. 4 shows the grain sizes of the former austenite grains in the tooth root surface layer portion and the tooth root center portion of the gears produced by the method of the present invention and the conventional method, arranged by forging temperature. In the conventional method, the particle size number is small (that is, coarse particles) in every part. All of the particles produced by the method of the present invention are fine grains of 8.5 or more, and especially those forged at 850 to 950 ° C. have ultrafine grains of 11 or more. The forged austenite grain size cannot be measured because the one forged at 700 ° C or less is forged with a ferrite-pearlite structure and quenched from there. In addition, the mark * in the figure indicates that only the martensite portion was measured because it has two phases of ferrite-martensite.

FIG. 5 shows the relationship between the forging temperature and the forging load. Here, the method of the present invention and the conventional method show the same tendency. That is, the method of the present invention in which forging is performed after the carburizing treatment does not significantly increase the forging load as compared with the conventional method.
However, the forging load sharply increases at temperatures lower than the Ac ₁ point. From the results shown in FIG. 4 above, it is understood that it is important to carry out forging with Ac ₁ point or more.

Figure 6 shows the surface hardness after scale removal (0.025m from the surface
It is the relationship between hardness at the m position) and the forging temperature. If the forging temperature becomes too high, the carbon that has permeated by the carburizing process diffuses inside and the hardness after quenching becomes low.

FIG. 7 shows the results of measuring the denture thickness and surface roughness. When the forging temperature exceeds 950 ° C, the deviation of the denture thickness from the target value (7.63 mm in this case) becomes large, and the surface becomes rough. This is because the scale formation on the blank surface becomes severe and this is removed by shot blasting, which eventually deteriorates the dimensional accuracy of the product.

From the results shown in FIGS. 6 and 7, it is clear that the upper limit of the forging temperature should be 950 ° C.

8 and 9 show the sum of the temperature raising time and the soaking time,
False tooth thickness, surface roughness and surface hardness (0.025m from the surface
hardness at m position). It is clear that a long time of more than 15 minutes should be avoided for the same reason that the forging temperature is too high.

FIGS. 13 and 14 show the gears obtained by the method of the present invention and the conventional method, in which the static breakage load and the impact absorption energy are arranged at the forging temperature. It is clearly superior to.

(Effects of the Invention) As is clear from the test results of the examples, according to the present invention, a carburized component having a fine grain structure that cannot be obtained by the conventional method can be obtained. The carburized component having a fine martensite structure obtained by the method of the present invention is equivalent in surface hardness to that obtained by the conventional method, and far exceeds static strength and toughness.

[Brief description of drawings]

FIG. 1 is a heat pattern diagram showing a conventional warm forging-carburizing method, FIG. 2 is a heat pattern diagram showing the method of the present invention, and FIG. 3 is a crystal grain size of gears manufactured in Examples. The figure which shows the measurement position of hardness, FIGS. 4-9 is a graph which shows various test results similarly, FIG. 10 is a measuring method of a bristle tooth thickness, FIG. 11 is a root fracture test method, FIG. FIG. 12 is a diagram explaining the impact test method of the gear, and FIGS. 13 and 14 are graphs showing the results of the breakage test and the impact test of the gear obtained by the method of the present invention and the conventional method, respectively. is there.

Claims (1)

[Claims] 1. A material steel for carburized parts is carburized, cooled to room temperature, and then heated to a temperature range of ₁ Ac or higher and 950 ° C. or lower, and the time is 15 minutes or less including this temperature rise time. A method for producing a carburized component having a fine grain structure, which is characterized by holding, forging to a desired shape in this temperature range, and directly quenching.