JPS61117203A - Production of sinter forged parts - Google Patents

Abstract

PURPOSE:To produce sinter-forged parts having excellent mechanical properties by subjecting metallic powder consisting essentially of iron to compression molding and sintering, forging the resulted preform at a high temp. then heating and holding the forged parts for a prescribed period to and at an adequate temp. and cooling the parts. CONSTITUTION:The metallic powder consisting essentially of Fe and having the compsn. contg. 0.5-5.0wt% Cu, 0.3-0.8% C or graphite and unavoidable impurities and the balance Fe is subjected to compression molding and sintering to form the preform. The preform is then forged at a high temp. of about >=1,000 deg.C and under about 60-100kg/mm<2> pressurizing force to form the sinter- forged parts. The sinter-forged parts are subjected to the reheating treatment by heating and holding the same for 50sec-30min to and at 800-1,150 deg.C, more preferably 900-1,000 deg.C in a carburizable atmosphere or the atm. air and thereafter the parts are cooled at an average cooling rate of 2-5 deg.C/sec in the temp. region down to at least the A3 transformation point. The sinter forged parts having the excellent mechanical properties of >=75kg/mm<2> tensile strength, >=7% elongation and >=30kg/mm<2> fatigue limit are thus obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a method for manufacturing a sintered forged product that can improve mechanical properties. The present invention can be used, for example, to manufacture automobile parts such as connecting rods, gears, hubs, and joint cups, or metal parts such as office machine parts and agricultural machinery parts.

[Prior Art J] In recent years, sintering and forging has attracted attention as a method for manufacturing resource-saving and energy-saving metal molded products. The manufacturing method of this sintered forged product consists of a forming and sintering process in which a metal powder whose main component is iron is compression molded and sintered to obtain a preform, and the obtained preform is forged at high temperature. It consists of a forging process to obtain a sintered forged product. If manufactured using such a sinter-forging method, a product with greater strength can be obtained compared to products manufactured using normal powder metallurgy.

[Problems to be Solved by the Invention] In the above-described manufacturing method, a product with higher strength can be obtained compared to the usual powder metallurgy method. However, in the above manufacturing method, the forging mold directly contacts the surface portion of the preform during the forging process. Therefore, the surface of the preform, which is heated to a high temperature during sintering, is suddenly cooled in the forging die.
Therefore, abnormal tissue is likely to occur on the surface. Furthermore, linear defects are likely to occur on the surface portion during forging. In addition, molded and sintered products have many pores, so if they are taken out of the sintering furnace into the atmosphere, they may be decarburized to the inside by the time they are forged. Because it has. Decarburization progresses during forging and cooling. Therefore, there is a possibility that a decarburized layer will form on the surface.

In addition, due to the above-mentioned circumstances, although sintered forged products have greater strength than those made using normal powder metallurgy, their mechanical properties are not necessarily sufficient for use in products that require higher strength. It wasn't something.

The present invention has been made in order to solve the problems of the above-mentioned conventional techniques.

"Means for Solving the Problems" As a result of extensive research, the inventor of the present invention has found that after forming a preformed product by compression molding and sintering, and forming a sintered forged product by forging the preformed product at high temperature, , the sintered forged product is heated to 800°C to 115°C.
It has been discovered that the various defects described above are reduced or eliminated by holding the product at a temperature of 0°C for a predetermined period of time, thereby improving the mechanical properties of the sintered forged product. The present invention has been made based on this discovery.

That is, the method for manufacturing a sintered forged product of the present invention includes a shaping and sintering step of forming a preform by compression molding and sintering a metal powder whose main component is iron; a forging step in which a sintered forged product is formed by forging under a-temperature; a reheating step in which the sintered forged product is heated and held at a temperature of 800 to 1150°C for a predetermined time; and a sintered forged product that is heated and held. The method is characterized by comprising a cooling step of cooling the product.

Hereinafter, each step of the manufacturing method of the present invention will be explained in detail.

<Molding and Sintering Process> In the forming and sintering process, a preformed product is formed by compressing and sintering metal powder whose main component is iron.

The metal powder is not particularly limited, and conventional metal powder raw materials containing iron as a main component, which are used for ordinary sintered forged products, can be used.

More specifically, the metal powder has a Φ amount of 0.5 to 5°0%.
of copper, 0.3 to 0.8% carbon or graphite, unavoidable impurities, balance iron can be used.

In this case, a mixed powder consisting of iron powder, copper powder, and graphite powder can be used as the sintered metal powder raw material. Here, copper dissolves in solid solution in the iron during heating for sintering and forging and acts as solid solution strengthening, and partially precipitates during the cold section to improve the hardness and strength of the base iron. In addition, graphite and carbon are also elements that increase the hardness and strength of magnetic iron, but if they are contained in a small amount, the strength improvement effect can be sufficiently achieved, but if the content is too large, cementite will precipitate in the matrix. Strength decreases and stiffness significantly deteriorates.

Note that unavoidable impurities include lubricants, etc. Examples of lubricants include zinc stearate. In addition, P(
Additives such as phosphorus), B (ho 1li), etc.
Alternatively, it may contain S (sulfur), which improves machinability, or may be mixed with MnS powder. It is preferable to use the above-mentioned metal powder that has good compression moldability and sinterability, and generally, depending on the type of sintered forged product, from reduced ore powder, mill scale reduced powder, atomized powder, electrolytic powder, etc. Select as appropriate.

In the shaping and sintering step, conventionally known compression molding means and sintering means can be used.

In this case, the above-mentioned metal powder is usually compression-molded to form a green compact, and then this green compact is sintered to form a preformed product, but in some cases, compression-molding and sintering are performed simultaneously. A hot press method may also be used. It is also preferable to increase the density of the surface of the green compact by a plasting treatment or the like when compacting the green compact. This is because there are fewer pores on the surface.

(Forging process) In the forging process, the preform is forged at high temperature to form a forged product, as in the past. As a result, the above-mentioned preformed product has a%t! ! ! It becomes a highly 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 air, but it can also be an inert gas atmosphere if necessary. #
The pressing force and the number of forgings in the first selection step are appropriately selected depending on factors such as the shape of the sintered forged product and the composition of the metal powder. Pressure force is generally 60 kg per square millimeter.
100ka, and the number of forgings is generally one. However, it is not limited to the above values. As the forging method, appropriate means such as known die forging, swaging forging, or free forging can be used depending on the processed shape of the preform and the purpose of use.

(Reheating process) The reheating process is a process that characterizes the present invention.

The reheating step means a step of heating and holding the sintered forged product at a temperature of 800 to 1150° C. for a predetermined period of time.

When the sintered forged product to be subjected to the reheating process has an A3 transformation point, it is preferable to perform the reheating process at a temperature between the A3 transformation point and 1150°C. The reason why the temperature in the reheating step is 800 to 1150°C as mentioned above is mainly due to 11
When the temperature exceeds 50℃, the degree of coarsening of crystal grains increases,
Further, if the temperature is lower than 800°C, the effect of improving mechanical properties is small.

If a decarburized layer has formed on the surface of the preform, the reheating step is performed in an atmosphere that will carburize the preform, such as a hydrocarbon gas such as propane or butane, or a gas made by sintering charcoal. It is preferable.

In this case, it is preferable to heat and hold for about 50 seconds to 40 minutes. Among them, vi' and a for 1 to 30 minutes are more preferable. The main reason for this is that carburizing tends to be insufficient if the carburizing time is less than 1 minute, and if the carburizing effect exceeds 30 minutes, the carburizing effect becomes saturated.

Note that 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 holding time. For example, about 50 seconds to 15 minutes is preferable.

The main reason for this is that if the heating 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, after the sintered forged product that has completed the forging process is allowed to cool to a predetermined temperature, for example, room temperature, the sintered forged product is
It is also possible to heat to a temperature of 1150° C. and carry out a reheating step there.

In this case, #1I3i! A large number of sintered forged products that have gone through the I process can be charged into a heating furnace all at once, and the reheating process can then be performed all at once, which is suitable for mass producing sintered forged products.

(Cooling process) In the cooling process, the sintered forged product heated and held in the reheating process is cooled. In this cooling step, depending on the shape, size, etc. of the forged product, air cooling, wind cooling, water spray cooling, and other known means can be selected as appropriate.

The metal powder has a composition of 0.5 to 5.0% copper, 0.3 to 0.8% carbon or graphite, unavoidable impurities, and the balance iron, and the reheating process is performed in a temperature range above the A3 transformation point. In this case, the cooling step is preferably carried out at a temperature of 2 to 5 degrees Celsius in the temperature range up to at least the A3 transformation point (more specifically, the Ar3 transformation point). It has been confirmed through experiments that this method is effective in increasing the strength of sintered forged products. By the way, metal powder is 0% by weight.
．． If the composition is 5 to 5.0% copper, 0.3 to 0.8% carbon or graphite, unavoidable impurities, and the balance iron, the microscopic structure of the manufactured sintered forged product is generally It is mainly composed of ferrite and pearlite. This structure is structurally different from sorbite, which is known as a quenched and tempered structure, or thollstone (restored martensite). In the present invention, the sintered forged product that has undergone the cooling process is generally not subjected to quenching and tempering treatment. However, in some cases, if it is desired to further improve the mechanical properties of the sintered forged product, quenching and tempering may be performed as necessary.

[Example 1] -200 meshes of electrolytic copper powder and flaky graphite powder were added to 180 meshes of atomized pure iron powder, resulting in a composition of 0.6% graphite, 2% copper, and the balance iron in weight percent. It was blended to achieve the following composition. Then, 0.8% of the total mixed powder was added with zinc stearate powder as a lubricant.
The powder was mixed using a minute-minute mixer. Next, a compact is formed using a hydraulic press machine with a pressure of 5 tons per square centimeter, and then heated at 1120°C for 5 minutes, commonly known as RX.
The molding and sintering process was carried out by heating and sintering in an endothermic gas known as gas.

Immediately after completing the shaping and sintering process, hot forging was carried out under a surface pressure of 8 tons per square centimeter, thereby carrying out the forging process to form a test piece.

Thereafter, it was air cooled to 550°C. Note that the forging process was performed in the atmosphere at a concentration range of about 1000 to 1090°C.

The shape of the test piece in this example is a plate-like test piece with side surfaces as shown in Figure 1, and a chuck part and a test part.The overall length L is 100 mm, and the width B of the chuck part is 24 mm. mm, the width of the test section is 10 mm,
The length P of the test section is 20 mm, the radius R of the fillet connecting the chuck section and the test section is 18 mm, and the thickness is 5.0 mm.

Three types of the above test pieces (No. 1 to No. 3) were formed, and N
091 test piece at 900℃, NO62 test piece at 10
At 00℃, test pieces No. and 3 were heated to 1100℃, respectively.
The mixture was heated and maintained in the atmosphere for a minute, and then a reheating step was performed.

After finishing the reheating process, NO, 1 to No.

3 each test piece from 2 to b A cooling step was performed by air cooling to 50°C.

[Example 2] In this example, the shaping and sintering process, the forging process, the cooling process, and the shape and dimensions of the test piece are basically the same as in Example 1.

However, the reheating process is different. That is, by performing the same forming and sintering process and forging process as in Example 1, three types of thickness 5.0
Millimeter test pieces (No. 11 to No. 13) were formed, and the N0011 test piece was heated at 900°C to No. 1-2.
test piece No. 13 at 110°C.
Each test piece was heated and held at 0°C for 1 minute, and then each test piece was heated and held at 900°C for 15 minutes, thereby performing a reheating process. In this example, the reheating step is C.P-0,60
It was carried out in an endothermic gas atmosphere of ~0.65%. In this point,
This differs from the case of Example 1 in which the reheating step was performed in the atmosphere. After the reheating step was completed, the sample was air cooled to 2 to 0° C. for a cooling step.

[Test and Evaluation] In order to investigate the effect of the reheating process that characterizes the above-mentioned Examples, test pieces of Example 1 (No. 1 to No. 3) and Example 2 (No. 11 to No. 3) were tested. Regarding .13), a tensile test and a plate bending fatigue test were conducted with the black skin intact. In addition, a plate bending fatigue test was conducted on the N011 test piece of Example 2 with the black skin intact. Specifically, the tensile test is performed using a 10Ton universal testing machine at 5 ram/s.
It was carried out at a tensile speed of in. In addition, the plate bending fatigue test was specifically conducted using a 4K (1-rigid Sienck-type bidirectional plane bending fatigue test J11 machine) at a stress repetition rate of 3000 cps.

A comparative fff (No. 20> was also formed and tested in the same manner.

In the comparative example, the molding and sintering process, forging process, and cooling process were performed in the same manner as in Examples 1 and 2, but the reheating process was not performed.

The test results of the tensile test are shown in FIGS. 2 and 3.

Figure 4 shows the test results of the plate bending fatigue test.

Regarding tensile strength, as shown in Figure 2, comparative example (No.
, 20), it was 7Qkg/sm2. In this respect, in the case of (No, 1 to No, 3) in Example 1, 78 to 8
It was large at 5k (J/II').

In particular, in the case of Example 1, the heating temperature in the reheating step was 9
It can be seen that the tensile strength increases as the temperature increases from 00°C to 1000°C to 1100°C. Moreover, as shown in FIG. 3, in the case of Example 2<No, 11-N0913), the tensile strength is 84-88 kg/l! 11112, which is considerably increased compared to the comparative example as in the case of Example 1.

Regarding elongation, in the case of comparative example (No. 20), it is 4%.
It was about. In this respect, Example 1 (No.

In the case of No. 1 to No. 3 and No. 3, it is about 8.3 to 10.8%, which is a considerable increase compared to the comparative example. Moreover, Example 2 (No.
, 11~No, 13)+7), 7°5~9.
This is a significant increase, as it was around 0%.

As for the fatigue limit, as shown in FIG. 4, in the case of the comparative example, it was about 27 kg/+uu. In this respect, in the case of Example 2, it was approximately 34 kg/s1, which was considerably increased compared to the comparative example.

From the above test results, it can be understood that in order to improve mechanical properties such as tensile strength, elongation, and fatigue limit, it is extremely effective to perform a reheating process on a sintered forged product that has completed the forging process. It is presumed that the reason why the excellent effects described above are not achieved in this example is due to the following reasons (1) and (2). That is, (1) In the present example and comparative example, linear defects are formed in the surface layer portion of the test piece during the forging process. However, in this embodiment, the linear defects are reduced or eliminated by wide bonding in the reheating step.

(2) In the present example and comparative example, an abnormal group m (ferrite spherical cementite) is formed on the surface part due to mold cooling that occurs during the forging process. However, in this example, a normal ferrite+pearlite structure is formed by the reheating process. This (1)
It is presumed that elongation and tensile strength commensurate with the internal hardness can be obtained due to the reason (2). In particular, if the reheating step is performed in an endothermic gas that can be carburized, it is estimated that the surface decarburized layer will be carburized, thereby improving not only the tensile properties but also the fatigue strength.

[Effect of the invention 1] According to the method for manufacturing a sintered forged product of the present invention, mechanical properties such as tensile strength, elongation, and fatigue limit can be improved, as is clear from the test results of Examples 1 and 2 described above. It can be improved compared to conventional sintered forged products.

[Brief explanation of the drawing]

FIG. 1 is a side view of the test piece. FIG. 2 is a graph showing the relationship between tensile strength and heating temperature in the reheating step and the relationship between elongation and heating temperature in the reheating step in Example 1. FIG. 3 is a graph showing the relationship between tensile strength and heating temperature in the reheating step and the relationship between elongation and heating temperature in the reheating step in Example 2. FIG. 4 is a graph showing the fatigue limits of Comparative Example and Example 2. Patent applicant: Toyota Motor Corporation Toyota Central Research Institute Co., Ltd. Agent Patent attorney: Hirodo Okawa Patent attorney: Shudo Fujitani Patent attorney: Akio Maruyama (Figure 1, Figure 4, several repetitions)

Claims (6)

[Claims] (1) A molding and sintering process in which a preformed product is formed by compression molding and sintering metal powder whose main component is iron, and a sintering forging process in which the preformed product is forged at a high temperature. A forging process that forms a product, a reheating process that heats and holds the sintered forged product at a temperature of 800 to 1150°C for a predetermined time, and a cooling process that cools the heated and held sintered forged product. A method for manufacturing a sintered forged product. (2) The manufacturing method according to claim 1, wherein the reheating step is performed in a carburizing atmosphere or in the air. (3) The manufacturing method according to claim 1, wherein the reheating step is performed in a temperature range of 900 to 1000°C for 50 seconds to 30 minutes. (4) The metal powder contains 0.5 to 5.0% copper by weight, 0
．． The manufacturing method according to claim 1, having a composition of 3 to 0.8% carbon or graphite, unavoidable impurities, and the balance iron. (5) The manufacturing method according to claim 1, wherein the cooling step is carried out at an average cooling rate of 2 to 5° C./second in the temperature range up to at least the A3 transformation point. (6) Sintered forged products that have gone through the cooling process have a tensile strength of 75 kg or more per square millimeter 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 amount is more than 1 kg.