JPH0680164B2 - Sintered forged product manufacturing method - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing a sintered forged product capable of improving mechanical properties. INDUSTRIAL APPLICABILITY The present invention can be used for manufacturing automobile parts such as connecting rods, gears, hubs and joint cups, or metal parts such as office machine parts and agricultural machine parts. [Prior Art] In recent years, a sintering forging method has been attracting attention as a method for producing a resource-saving and energy-saving type metal molded product. This sinter forged product is manufactured by compression molding and sintering metal powder containing iron as the main component to obtain a preformed product, and forging the obtained preformed product at high temperature. It consists of a forging process to obtain a sintered forged product. When manufactured by such a sintering forging method, it is possible to obtain a product having higher strength than a product manufactured by a normal powder metallurgy method. [Problems to be Solved by the Invention] In the above-described manufacturing method, a product having higher strength can be obtained as compared with a usual powder metallurgy method. However, in the above-mentioned manufacturing method, the forging die directly contacts the surface portion of the preform in the forging step. Therefore, the surface of the preform, which is heated to a high temperature during sintering, is rapidly cooled by the forging die,
Therefore, abnormal tissue is likely to occur on the surface portion. Further, linear defects are likely to occur on the surface portion during forging. In addition, since the molded and sintered product has many pores, if it is taken out of the sintering furnace into the atmosphere, it may be decarburized to the inside considerably before being forged after the furnace is discharged, and even the forged body still has pores. Therefore, decarburization progresses during standing and cooling after forging. Therefore, a decarburized layer may occur on the surface. Further, due to the above-mentioned circumstances, etc., although the sintered forged product has a higher strength than the ordinary powder metallurgy method, the mechanical properties are not sufficiently satisfactory for use in products requiring higher strength. It wasn't something to do. The present invention has been made to solve the above-mentioned problems of the conventional technology. [Means for Solving the Problems] As a result of intensive studies by the present inventor, the preform formed by compression molding and sintering was forged at high temperature to form a sintered forged product, Holding the sintered forged product at a temperature of 800 ° C to 1150 ° C for 50 seconds or more and less than 30 minutes reduces or eliminates the various defects described above, thereby improving the mechanical properties of the sintered forged product. I found that. The present invention is based on this finding. That is, the method for producing a sintered forged product of the present invention comprises a forming and sintering step of forming a preformed product by performing compression molding and sintering on a metal powder containing iron as a main component, and a preformed product. Forging process to form a sintered forged product by forging under high temperature, reheating process for heating and holding the sintered forged product at a temperature of 800 to 1150 ° C for 50 seconds or more and less than 30 minutes, and the sintered forged product heated and held. And a cooling step for cooling. Hereinafter, the manufacturing method of the present invention will be described in detail for each step. (Molding and Sintering Step) In the molding and sintering step, a preform is formed by performing compression molding and sintering on metal powder containing iron as a main component. The metal powder is not particularly limited, and a conventional metal powder raw material containing iron as a main component, which is used in a normal sintered forged product, can be used. More specifically, the metal powder having a composition of 0.5 to 5.0% by weight of copper, 0.3 to 0.8% of carbon or graphite, unavoidable impurities, and balance iron can be used. In this case, a mixed powder composed of iron powder, copper powder and graphite powder can be used as the raw material for the sintered metal powder. Here, copper forms a solid solution in iron at the time of heating for sintering and forging, and acts as a solid solution strengthening, and partly precipitates at the time of cooling to improve hardness and strength of the base steel. Also, graphite and carbon are elements that increase the hardness and strength of mother iron,
With a small content, the strength improving effect cannot be sufficiently achieved,
On the other hand, if the amount is too large, cementite precipitates in the matrix to lower the strength, and the machinability deteriorates significantly. Lubricants and the like correspond to the inevitable impurities. Examples of the lubricant include zinc stearate and the like. In addition, for example, additives such as P (phosphorus) and B (boron) are included as elements for promoting sintering, or S (sulfur) is added to improve machinability, and MnS is added.
The powder may be mixed and used. As the above-mentioned metal powder, it is preferable to use one having good compression moldability and sinterability, and generally, ore reduced powder, mill scale reduced powder,
It is appropriately selected from atomized powder, electrolytic powder and the like according to the type of the sintered forged product. In the molding and sintering step, known compression molding means and sintering means that have been conventionally used can be used. In this case usually
After compression molding the above-mentioned metal powder to form a green compact,
The green compact is sintered to form a preform, but in some cases, a hot press method in which compression molding and sintering are performed simultaneously may be used. It is also preferable to increase the surface density of the green compact by blasting or the like during the green compact formation. This is because the number of pores on the surface is reduced. (Forging Step) In the forging step, the preform is forged at a high temperature to form a forged article as in the conventional case. As a result, the above-mentioned preform becomes a densified sintered forged product. The temperature at which forging is performed is appropriately selected depending on the type of sintered forged product and the composition of the metal powder, but is generally 1000 ° C. or higher. The atmosphere for forging is generally in the air, but may be in an inert gas if necessary. The pressing force and the number of times of forging in the forging step are appropriately selected according to factors such as the shape of the sintered forged product and the composition of the metal powder. The pressing force is generally 60 kg to 100 kg per square millimeter, and the number of times of forging is generally once. However, the values are not limited to the above values. As the forging method, appropriate means such as known die forging, swaging forging or free forging can be adopted depending on the processed shape of the preform and the purpose of use. (Reheating Step) The reheating step is a step which characterizes the present invention. In the reheating step, the sintered forged product is heated to a temperature of 800 to 1150 ° C in 50 seconds or more for 3 seconds.
It means a step of heating and holding for less than 0 minutes. When the sintered forged product subjected to the reheating step has an A3 transformation point, the reheating step is preferably performed at the A3 transformation point to 1150 ° C. The reason why the temperature in the reheating step is 800 to 1150 ° C as described above is mainly because the degree of coarsening of crystal grains becomes large when the temperature exceeds 1150 ° C, and the effect of improving mechanical properties is less than 800 ° C. Because. If a decarburized layer is formed on the surface of the preform, the reheating step is performed in an atmosphere that can carburize the preform, for example, a hydrocarbon gas such as propane or butane, or a gas obtained by sintering charcoal. It is preferable. In this case, it is preferable to heat and hold for 50 seconds to less than 30 minutes. Especially, about 1 to 30 minutes is more preferable.
The main reason is that carburization tends to be insufficient in less than 1 minute, and the carburizing effect becomes saturated in more than 30 minutes. If the main purpose is to improve the tensile strength of the preform, the reheating step can be performed in the atmosphere. However, in this case, it is preferable to shorten the heating and holding time. For example, about 50 seconds to 15 minutes is preferable. The main reason for this is that if the heating and holding time is long, the degree of oxidation of the surface portion of the sintered forged product increases due to heating in the atmosphere. From the viewpoint of energy saving, it is preferable to perform the reheating step immediately after the forging step is completed. However, in some cases, the sintered forged product that has undergone the forging process is allowed to cool to a predetermined temperature, for example, room temperature, and then the sintered forged product is 800 to 115
It is also possible to heat to a temperature of 0 ° C. and carry out a reheating step there. In this case, a large number of sintered forged products that have undergone the forging process can be put together in a heating furnace, and thus the reheating process can be performed collectively, which is suitable for mass production of sintered forged products. . (Cooling Step) In the cooling step, the sintered forged product heated and held in the reheating step is cooled. In this cooling step, known means such as cooling by cooling, air cooling, water spray cooling and the like can be appropriately selected according to the shape and size of the forged product. Metal powder 0.5-5.0% copper 0.3-0.8% carbon or graphite,
Inevitable impurities, balance iron composition, and reheating process
When the A3 was performed in the temperature range of lower than the transformation point, cooling step, it is carried out at an average cooling rate of 2 to 5 ° C. / sec in a temperature range up to at least the A3 transformation point (more specifically Ar ₃ transformation point) Is preferred. It has been confirmed by an experiment that this is effective in increasing the strength of the sintered forged product. By the way, metal powder is 0.5-5.0% by weight.
In the case of the composition of copper, 0.3 to 0.8% of carbon or graphite, unavoidable impurities, and the balance of iron, the microstructure of the manufactured sintered forged product is generally composed mainly of ferrite and pearlite. Become. This structure is structurally different from sorbite, which is known as a quenched and tempered structure, or tallstite (tempered martensite).
In the present invention, generally, the quenching and tempering process is not performed on the sintered forged product after the cooling process. [Example 1] To -80 mesh atomized pure iron powder, -200 mesh electrolytic copper powder and scaly graphite powder were added, whereby a composition of 0.6% graphite-2% copper-balance iron in weight% was obtained. It was blended so that Then, zinc stearate powder was added as a lubricant in an amount of 0.8% of the entire mixed powder, and the mixture was mixed for 30 minutes by a dry type mixer. Next, using a hydraulic press machine, a green compact is formed with a pressure of 5 tons per square centimeter.
Heat sintering was carried out at 1120 ° C. for 5 minutes in an endothermic gas commonly known as P × gas, thereby performing a molding and sintering step. Immediately after the molding and sintering process was completed, hot forging was carried out at a surface pressure of 8 tons per square centimeter, whereby the forging process was carried out to form a test piece. Then, it was air-cooled to 550 ° C. The forging process was performed in the atmosphere in the temperature range of about 1000 to 1090 ° C. The shape of the test piece of this example is a plate-like test piece having a side surface shown in FIG. 1 and having a chuck portion and a test portion. The overall length L is 100 mm and the width B of the chuck portion is 24 mm. The width b of the test part is 10 mm and the length P of the test part is
Is 20 mm, the radius R of the fillet connecting the chuck part and the test part is 18 mm, and the thickness is 5.0 mm. The above test pieces are formed in three types (NO.1 to NO.3), the NO.1 test piece at 900 ° C, the NO.2 test piece at 1000 ° C, and the NO.3 test piece at 1100 ° C. Each was heated and held in the atmosphere for 1 minute, and the reheating step was performed. After finishing the reheating process, test pieces of NO.1 to NO.3
Air-cooling was performed to 550 ° C. at a cooling rate of ˜5 ° C./sec, and a cooling step was performed. [Example 2] In this example, the forming and sintering process, the forging process, the cooling process, and the shape and size of the test piece are basically the same as those in the first embodiment.
However, the reheating process is different. That is, three kinds of 5.0 mm-thick test pieces (NO.11 to NO.13) were formed by the same forming / sintering step and forging step as in Example 1, and NO.11
No.12 test piece at 900 ℃, NO.12 test piece at 1000 ℃, NO.13
Each test piece was heated and held at 1100 ° C. for 1 minute, and then each test piece was heated and held at 900 ° C. for 15 minutes, thereby performing the reheating step. In this example, the reheating step is performed by C · P =
It was performed in an endothermic gas atmosphere of 0.60 to 0.65%. In this respect,
This is different from the case of Example 1 in which the reheating step is performed in the atmosphere. After the reheating step was completed, it was air-cooled to 500 ° C at a cooling rate of 2 to 5 ° C / sec, and the cooling step was performed. [Test and Evaluation] In order to investigate the effect of the reheating step which characterizes the above-described examples, the test pieces of Example 1 (NO. 1 to NO. 3) and the test pieces of Example 2 (NO. 11 to NO. .13), a tensile test and a plate bending fatigue test were performed on the black skin. In addition, in the second embodiment
A plate bending fatigue test was conducted on the NO.1 test piece with the black skin as it was. Here, the tensile test was specifically carried out at a tensile speed of 5 mm / min with a universal testing machine of 10 Ton. Further, the plate bending fatigue test was specifically carried out using a Schenk type double swing flat bending fatigue tester with a stress repetition rate of 3000 cpm. A comparative example (NO.20) was also formed and tested in the same manner. In the comparative example, the forming and sintering process is performed in the same manner as in Examples 1 and 2,
The forging process and the cooling process were performed, but the reheating process was not performed. The test results of the tensile test are shown in FIGS. 2 and 3. The test results of the plate bending fatigue test are shown in FIG. Regarding tensile strength, as shown in Fig. 2, a comparative example (NO.2
In the case of 0), it was 70 kg / mm ² . In this respect, (N
In the case of O.1 to NO.3), it was as large as 78 to 85 kg / mm ² . Particularly, in the case of Example 1, the heating temperature in the reheating step is 900
It can be seen that the tensile strength increases as the temperature increases to ℃, 1000 ℃, 1100 ℃. In addition, as shown in FIG.
In the case of (NO.11 to NO.13), the tensile strength is 84 to 88kg / mm ²
As in the case of Example 1, the number of comparisons is considerably higher than that of the comparison example. The elongation was about 4% in the case of the comparative example (NO.20). In this respect, in the case of Example 1 (NO.1 to NO.3)
It is about 8.3 to 10.8%, which is a considerable increase compared to the comparative example. In the case of Example 2 (NO.11 to NO.13), 7.5 to 9.0%
It has increased considerably as well as the degree. The fatigue limit was about 27 kg / mm ^{2 in} the case of the comparative example, as shown in FIG. In this respect, in the case of the second embodiment,
It was about 34 kg / mm ^2, which was a considerable increase compared to the comparative example. From the above test results, it can be understood that it is extremely effective to perform the reheating step on the sintered forged product after the forging step in order to improve the mechanical properties such as tensile strength, elongation and fatigue limit. It is presumed that the reason why the excellent effects described above are obtained in the present embodiment is due to the following reasons (1) and (2). That is, (1) In this example and the comparative example, linear defects are formed in the surface layer portion of the test piece during the forging step. However, in this embodiment, the linear defects are reduced or disappear by diffusion bonding in the reheating process. (2) In this example and the comparative example, an abnormal structure (ferrite spherical cementite) is generated on the surface due to mold cooling generated during the forging process. However, in this embodiment, the reheating process results in a normal ferrite + pearlite structure. This (1)
Due to the reason (2), it is presumed that elongation and tensile strength commensurate with the internal hardness can be obtained. In particular, if the reheating step is performed in a carburizing endothermic gas, it is presumed that the surface decarburized layer is carburized to improve not only tensile properties but also fatigue strength. [Effects of the Invention] According to the method for manufacturing a sintered forged product of the present invention, the mechanical properties such as tensile strength, elongation, and fatigue limit are clearly determined from the test results of Examples 1 and 2 described above. It can be improved as compared with the conventional sintered forged product.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side view of a test piece. FIG. 2 is a graph showing the relationship between the tensile strength and the heating temperature in the reheating step and the relationship between the elongation and the heating temperature in the reheating step in Example 1.
FIG. 3 is a graph showing the relationship between the tensile strength and the heating temperature in the reheating step and the relationship between the elongation and the heating temperature in the reheating step in Example 2. FIG. 4 is a graph showing the fatigue limit of Comparative Example and Example 2.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Hiroshi Hamamoto             Aichi Prefecture Nagachite Town Aichi District             Ground 1 Toyota Central Research Institute Co., Ltd. (72) Inventor Mikio Kondo             Aichi Prefecture Nagachite Town Aichi District             Ground 1 Toyota Central Research Institute Co., Ltd.              (56) Reference JP-A-55-41969 (JP, A)               JP 55-38983 (JP, A)

Claims (1)

Claim: What is claimed is: 1. A molding and sintering step of forming a preformed product by performing compression molding and sintering on a metal powder containing iron as a main component, and the preformed product at a high temperature. A forging step of forging under to form a sintered forged product, a reheating step of heating and holding the sintered forged product at a temperature of 800 to 1150 ° C. for 50 seconds or more and less than 30 minutes, and the sintering held by heating A method for manufacturing a sintered forged product, comprising a cooling step of cooling the forged product. 2. The manufacturing method according to claim 1, wherein the reheating step is performed in a carburizable atmosphere or air. 3. The metal powder comprises 0.5 to 5.0% by weight of copper and 0.1% by weight.
The manufacturing method according to claim 1, which has a composition of 3 to 0.8% of carbon or graphite, inevitable impurities, and the balance iron. 4. The manufacturing method according to claim 1, wherein the cooling step is performed at an average cooling rate of 2 to 5 ° C./sec at least in a temperature range up to the A3 transformation point. 6. The sintered forged product that has undergone the cooling step has a tensile strength of 75 kg or more per 1 square mil meter and an elongation of 7
% Or more and the fatigue limit is 30 per square millimeter.
The manufacturing method according to claim 1, wherein the manufacturing method is kg or more.