JPH0663079B2 - Method and apparatus for manufacturing carburized parts having fine grain structure - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for manufacturing a carburized part, particularly a method for manufacturing a carburized part having a small crystal grain size and excellent strength and toughness, and An apparatus for performing the method.

(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. The present invention aims to improve mechanical properties and provides a new method and device therefor.

(Means for Solving Problems) The first invention of the present application is that "a material steel for carburizing parts is subjected to a carburizing treatment in a temperature range of Ac ₃ points or higher, and subsequently cooled to a temperature range of Ar ₁ point or higher. A method for producing a carburized part having a fine grain structure, which comprises forging into a desired shape in a temperature range and then 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, in the method of the present invention, first, carburization is performed in a temperature range of Ac ₃ or higher, and warm forging is performed in a temperature range of Ar ₁ or higher in the cooling process from that temperature. Furthermore, after warm forging, quenching is performed directly from a temperature of Ar ₁ point or higher.

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. On the other hand, 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 finer 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.

The carburizing treatment is usually performed in an austenite region having a high carbon solubility, that is, at Ac ₃ or higher. The carburizing process itself in the present invention may be a conventionally known process. A practical method suitable for mass production is a continuous carburizing process using a continuous gas carburizing furnace in which trays containing materials move in the furnace.

After carburizing, warm forging is performed in the temperature range of Ar ₁ point or higher. 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 the austenite grains differs slightly depending on the steel type, but in the case of general carburizing steel, partial recrystallization occurs even just above Ar ₁ point.

If the warm forging temperature is a temperature between Ac ₃ point and Ar ₁ point, the central part of the material has two phases of austenite and ferrite, but the surface layer part has a high carbon content 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.

If the temperature of warm forging falls below the Ar ₁ point, austenite will be completely decomposed even in the surface layer and pearlite and bainite will be 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 comparison between FIG. 1 and FIG. 2, what was conventionally cooled once to room temperature after warm forging, reheated after machining, carburized and quenched,
In the method of the present invention, since quenching is performed directly from the temperature of warm forging, the effect of reducing energy cost is great.

Further, according to the method of the present invention, even if the austenite grains are coarsened during the carburizing treatment, the austenite grains can be refined in the next step, so that the carburizing treatment temperature can be made higher than the conventional temperature of 920 to 930 ° C. If the carburizing temperature is increased, the diffusion rate of carbon is increased and the processing time can be shortened.

Another advantage of shifting from the carburizing step to the continuous forging step in the method of the present invention is that if the step of temporarily cooling after carburizing and reheating for forging is adopted, the inside of the carburized layer is reheated. It is possible to prevent deterioration of surface hardness and dimensional accuracy of the product without carbon diffusion, decarburization of the surface layer and generation of surface scale.

However, in the method of the present invention, since forging is performed after the carburizing treatment, the carburizing depth at the material stage does not always match that of the product. 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 second invention of the present application is "a carburizing furnace for carburizing a material steel for carburizing parts in a temperature range of Ac ₃ or more, and Ar ₁ in a cooling process which is installed in the vicinity of the rear of the carburizing furnace. A carburized part characterized by having a forging device for forging the carburized raw material steel in the temperature range above the point, and a cooling tank for directly quenching the forged product after the forging is attached to the forging device. Manufacturing equipment ”

FIG. 3 is a perspective view illustrating the device of the second invention. The carburizing furnace 1 is preferably a continuous carburizing furnace in which a material steel (blank) prepared in a predetermined size is continuously carburized while moving through the steel. A warm forging device 4 is provided behind the carburizing furnace. The carburizing furnace 1 and the warm forging device 4 are connected by the transfer device 2, but are installed as close to each other as possible in order to reduce the temperature drop between them.
It is recommended to attach a heat insulation cover to the carrier. In addition, the temperature measuring device 3 is provided near the entrance of the warm forging device 4,
It is desirable to constantly measure the temperature of the material so that the forging is done at the proper temperature.

As the warm forging in the present invention, not only closed forging but also various forging methods such as free upsetting, forward extrusion and backward extrusion can be adopted. Therefore, it goes without saying that the model of the forging device 4 shown in FIG. 3 can be changed according to the forging method adopted.

Immediately after the forging device 4, a cooling tank 5 for quenching is placed. This is because after the warm forging is finished, the material is immediately put into the cooling tank and quenched from a temperature of ₁ point or more of Ar. After quenching, the continuous tempering furnace 6 performs necessary tempering treatment.
Although not shown, the tempered product is subjected to final finishing such as shaving and polishing with an appropriate finishing device.

What is important in the equipment shown in FIG. 3 is that the carburizing furnace 1, forging machine 4 and cooling tank 5 are directly connected in one line. Conventionally, a warm forging device equipped with a blank heating device was usually placed separately from the carburizing line. With the conventional method, after warm forging, once cooled to room temperature,
If the carburizing process is performed by reheating, it is not necessary to directly connect the forging device and the carburizing furnace. However, as described above, the method of the present invention is characterized in that carburizing, warm forging, and quenching are performed in a continuous process. Therefore, it is desirable that the equipment for that purpose be as shown in FIG.

Hereinafter, the effects of the present invention will be specifically shown by 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 carbon potential was obtained by using a dropping carburizing furnace.
Carburizing was performed at 950 ° C. for 2 hours in an atmosphere of 0.85%. After that, 900 ℃, 850 ℃, 800 ℃, 750 ℃, 700 ℃, 650
After cooling to each temperature of ° C and holding for 15 minutes, it was taken out of the carburizing furnace and immediately forged into spur gears. The specifications of the spur gear are the second
It is as shown in the table. After forging, it was immersed for 5 seconds in a quenching oil of 120 ° C. for quenching, and then tempered at 170 ° C. for 1 hour.

Regarding the spur gear obtained as described above, as shown in FIG. 4, the crystal grain size of the former austenite grains in the tooth root central portion and the tooth root surface layer portion, the structure and hardness distribution of the tooth root surface layer portion, and the tooth root central portion Hardness was measured.

Further, after machining (tooth surface grinding) to make a product gear, a static root breakage test was conducted by the method shown in FIG. 5 to measure the breakage load. Further, an impact test was carried out using a tester shown in FIG. 6 which is a modification of the Charpy impact tester. The results of these measurements are shown in Table 3.

The conventional method in Table 3 means that after forging at each temperature to form a spur gear, machining (tooth surface polishing), carburization,
It has gone through each process of quenching and tempering. The carburizing conditions at this time are all the same as the above-mentioned conditions, except that the effective hardening depth (distance from the surface corresponding to Hv550) was set to 1.75 hours in order to match the one according to the method of the present invention. . The same measurement as above was performed for the thus obtained one.

As shown in Table 3, in the gear obtained by the method of the present invention, the former austenite grain size of the root surface layer portion and the root central portion is
It is extremely fine with a size of 10.8 or more, and as a result, the tooth root breakage load is 6500 kg or more and the strength is high and the impact absorption energy is 98 kgf
Has high toughness of m or more. On the other hand, with the conventional method, the coarsening of crystal grains is remarkable, the breaking load is only 4000 kg or so, and the impact absorption energy is 40 kg.
Remarkably low below f ・ m.

Even if the same steps as the method of the present invention are taken, the forging temperature is Ar
_{When the} temperature is lower than ₁ point (about 727 ° C for the steel described in Table 1), the surface layer structure becomes pearlite as shown as a comparative example, and the tooth surface hardness (hardness at a position of 0.03 mm from the surface), The hardness of the center of the tooth root is extremely low, and the breaking load and the impact absorption energy are also low.

(Effects of the invention) As is clear from the results of the examples, according to the present invention, it is possible to efficiently manufacture a carburized component having a high static strength. Therefore, it is particularly required to improve quality and reduce cost at the same time. The method and apparatus of the present invention can greatly contribute to the processing of mass-produced parts.

[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 perspective view illustrating an example of the apparatus of the present invention. FIG. 4 and FIG. 4 to FIG. 6 are views for explaining the method for measuring the product characteristics in the examples.

Claims (2)

[Claims] 1. A steel material for carburizing parts is carburized in a temperature range of Ac ₃ points or higher, continuously cooled to a temperature range of Ar ₁ point or higher, forged into a desired shape in this temperature range, and then directly quenched. A method of manufacturing a carburized component having a fine grain structure, comprising: 2. A carburizing furnace for carburizing a material steel for carburizing parts in a temperature range of Ac ₃ or higher, and a carburizing furnace installed close to the rear of the carburizing furnace and having Ar ₁ or higher in the subsequent cooling process. An apparatus for producing carburized parts, characterized by having a forging device for forging raw steel that has been carburized in a temperature range, and a cooling tank for directly quenching the forged product after the forging is attached to the forging device. .