JP6810883B2 - Iron-carbon sintered member and its manufacturing method - Google Patents

Description

The present invention relates to a sintering member that sinters the green compact obtained by filling the cavity of the stamp with the raw material powder and compression molding with an upper and lower punch, and a method for producing the same. In particular, the raw material powder is iron powder and graphite powder. The present invention relates to an iron-carbon-based sintered member using the raw material powder to which the above is added and a method for producing the same.

In the production of the sintered machine parts by the powder metallurgy method, as shown in FIG. 3, the raw material powder is filled in the cavity formed by the mold hole 11 of the stamp 10 and the lower punch 20, and then the raw material powder is referred to as the lower punch 20. This is performed by compression molding with the upper punch 30 to prepare a molded product (so-called stamping method), and heating the obtained molded product in a sintering furnace to sinter it. Such an embossing method has the advantages that in addition to being able to form sintered machine parts into a near-net shape, it is possible to produce a large number of products of the same shape once the embossing is made, and the manufacturing cost is low. Because it has, it is used in various fields. In particular, structural machine parts are cheaper than other manufacturing methods, so the application of sintered machine parts by the powder metallurgy method is advancing.

As the sintering material for structural machine parts, a mixed powder obtained by adding copper powder and graphite powder to iron powder as a raw material powder is used, and the green compact obtained by compression molding this is the melting point of copper. The SMF4 type iron-copper-carbon-based sintered material (Fe-0.2 to 1% by mass Cu-1 to 5% by mass C) specified in JIS standard Z2550, which has been sintered above, is most widely applied. Has been done.

In iron-copper-carbon based sintered materials, copper generated by melting during sintering promotes sintering, and copper diffuses quickly into iron, so it becomes a liquid phase and the surface of iron powder is exposed. As a result of the copper covering being uniformly diffused to the iron base to strengthen the solid solution of the iron base and the copper dissolved in the iron base improving the hardenability of the iron base, it precipitates in the cooling process of the sintering process. Since pearlite is made fine and works to strengthen the iron matrix, even an iron-based sintered material having pores can obtain mechanical strength equivalent to that of carbon steel. (Non-Patent Document 1, FIG. 37 on page 59, etc.) Therefore, it is widely applied to structural mechanical parts. (Non-Patent Document 2, page 237, etc.)

In comparison, the SMF3 type iron-carbon sintered material (Fe-0.2 to 0.8% by mass C) specified in JIS standard Z2550 is SMF4 type iron-copper-carbon type. The mechanical strength is low compared to the sintered material of. (Non-patent Document 1, 56 p. 33 etc.) Therefore, has remained for application to mechanical strength is relatively requested not part products.

Author: Wakabayashi ▲ Hikari ▼ "New Edition Powder Metallurgy", Technical Shoin Co., Ltd., published on June 10, 1976 "Powder Powder Metallurgy Handbook" edited by Japan Powder Metallurgy Association, published on November 10, 2010

Although it is an iron-copper-carbon-based sintered material whose application is expanding to structural machine parts, the cost of copper bullion has been rising in recent years, and the raw material cost tends to increase year by year. On the other hand, further cost reduction is required for structural mechanical parts, and it is difficult to add the increase in raw material cost to the manufacturing cost.

Therefore, if an iron-carbon-based sintered member using an inexpensive iron-carbon-based sintered material that does not use copper as a raw material can be made stronger than the iron-copper-carbon-based sintered material, a conventional method can be used. By applying a high-strength iron-carbon-based sintered member instead of the iron-copper-carbon-based sintered material, structural mechanical parts can be provided at low cost.

From this, the present invention provides an iron-carbon-based sintered member having higher strength than the iron-copper-carbon-based sintered material by using an inexpensive iron-carbon-based sintered material that does not use copper as a raw material. The purpose is to do. Another object of the present invention is to provide a manufacturing method capable of manufacturing such an iron-carbon-based sintered member at a lower cost.

As a result of diligent studies, the present inventors have conducted diligent studies, and found that when the density of the iron-carbon-based sintered material is increased, iron-carbon-based sintering is performed rather than iron-copper-carbon-based sintered material above a certain density region. We have obtained a previously unpredictable finding that the mechanical strength of the binder is higher.

The iron-carbon-based sintered member of the present invention is based on this finding, and specifically comprises C: 0.2 to 1.0% by mass, the balance: Fe and unavoidable impurities, and the sintered body density is high. It is characterized in that the base structure of the iron-carbon-based sintered member is martensite and the tensile strength is 1300 MPa or more at 7.4 Mg / m ³ or more and 7.53 Mg / m ³ or less .

In the iron-carbon-based sintered member of the present invention, the maximum pore diameter of the pores of the iron -carbon- based sintered member is preferably 50 μm or less. Further, when the iron base is martensite, the tensile strength is 1400 MPa or more, which is more preferable.

Further, the method for producing the iron-carbon-based sintered member of the present invention relates to the above-mentioned preferable method for producing the iron-carbon-based sintered member, and specifically, graphite powder: 0.3 to 1.2 mass. % And the raw material powder whose balance is iron powder is filled in the cavity of the stamping die and compression-molded with an upper and lower punch to obtain a green compact having a density of 7.45 Mg / m ³ or more. It is characterized by comprising a sintering step of obtaining a sintered body by sintering the powder at 1000 to 1300 ° C.

In the method for producing an iron-carbon-based sintered member, it is preferable that the cavity of the stamping die is coated with a molding lubricant, and the raw material powder contains 0.3% by mass or less of the molding lubricant. preferable.

The iron-carbon-based sintered member of the present invention does not contain copper, for which the cost of bare metal has been increasing in recent years, the raw material cost is low, and the mechanical strength is higher than that of the iron-copper-carbon-based sintered member. Therefore, it is possible to provide an inexpensive iron-carbon-based sintered member in place of the iron-copper-carbon-based sintered member, which is a special effect.

It is a graph which shows the relationship between the sintered body density, and the tensile strength of an iron-carbon-based sintered member and an iron-copper-carbon-based sintered member. It is a drawing substitute photograph which shows the pore distribution of the iron-carbon-based sintered member and the iron-copper-carbon-based sintered member having a sintered body density of 7.5 Mg / m ³ . It is a figure which shows the outline of the molding apparatus in the molding process by a stamping method.

1, C: 0.8 wt% and iron balance being Fe and No避不pure product - and heat treatment of the carbon-based sintered member, C: 0.8 mass%, Cu: 1.5 wt% and It is a graph which shows the relationship between the sintered body density and the tensile strength of the heat-treated body of the iron-copper-carbon system sintered member whose balance is made of Fe and unavoidable impurities.

From FIG. 1, both the iron-carbon-based sintered member and the iron-copper-carbon-based sintered member tend to increase the tensile strength as the sintered body density increases. Further, in the region of the sintered body density of 6.8 to 7.2 Mg / m ³ used in general structural machine parts, the iron-copper-carbon system is affected by the effect of promoting sintering by copper and strengthening the solid solution of the matrix. The mechanical strength of the sintered member is higher. Conventionally, based on the results of this density region, the iron-copper-carbon-based sintered member is considered to have higher mechanical strength, and the iron-copper-carbon-based sintered member has been applied to various structural parts. ..

However, the rate of increase in tensile strength due to the increase in the density of the iron-carbon-based sintered member is larger than the rate of increase in tensile strength due to the increase in the density of the iron-copper-carbon-based sintered member, and the sintered body density is 7. In the region of .4 Mg / m ³ or more, the tensile strength of the iron-carbon-based sintered member is conversely higher than the tensile strength of the iron-copper-carbon-based sintered member. The result is that if the sintered body density is 7.4 Mg / m ³ or more, the iron-carbon-based sintered member can have a higher strength than the iron-copper-carbon-based sintered member. is there. For this reason, in the iron-carbon-based sintered member of the present invention, the sintered body density is set to 7.4 Mg / m ³ or more, preferably 7.5 Mg / m ³ or more.

When the present inventors examined this reversal phenomenon of tensile strength, it was considered that the cause was that the shape of the pores had a great influence. That is, the iron-copper-carbon-based sintered member sintered at a temperature equal to or higher than the melting point of copper using a mixed powder obtained by adding graphite powder and copper powder to iron powder as a raw material powder has copper in the sintering process. The portion where the copper powder was present remains as an outflow hole when it melts into a liquid phase. Since such coarse pores have a distorted shape and are difficult to be spheroidized by sintering, they tend to be a starting point of fracture when tensile stress is applied. Therefore, in the conventional region of the sintered body density of 6.8 to 7.2 Mg / m ³ containing a large amount of pores, the tensile strength is high due to the influence of the promotion of sintering by copper and the solid solution strengthening of the matrix. However, in a high-density region where the sintered body density is 7.4 Mg / m ³ or more, the effect of pores becomes greater than the effect of promoting sintering and strengthening the solid solution of the matrix, resulting in an increase in the sintered body density. It is considered that the rate of increase in tensile strength that accompanies it is small.

On the other hand, the iron-carbon-based sintered member sintered by using a mixed powder obtained by adding graphite powder to iron powder as a raw material powder does not have the effects of promoting sintering and strengthening the solid solution of the matrix. In a high-density region where the sintered body density is 7.4 Mg / m ³ or more, the amount of pores is small, the size of the pores is also small, and coarse pores such as outflow pores of copper powder are not included. It is considered that the tensile strength is higher than that of the iron-copper-carbon-based sintered member.

FIG. 2 shows the pore distribution of the iron-carbon-based sintered member and the iron-copper-carbon-based sintered member having a sintered body density of 7.5 Mg / m ³ . In the iron-copper-carbon-based sintered member, coarse pores that are thought to have existed in the copper powder were observed, and the coarse pores had an irregular shape, and stress was concentrated and fracture proceeded. It has an easy shape. On the other hand, the pores of the iron-carbon-based sintered member are all small, and since the size is small, spheroidization progresses and the shape is such that stress is difficult to concentrate.

The iron-carbon-based sintered member has a pore size of 50 μm or less as a maximum particle size in a high-density region such as a sintered body density of 7.4 Mg / m ³ or more. Such small pores become spheroidized during sintering and become a relatively rounded shape, making it difficult for stress to concentrate.

The iron-carbon sintered member shall consist of C: 0.2 to 1.0% by mass, the balance: Fe, and unavoidable impurities. If the amount of C is less than 0.2% by mass, the strengthening of the iron base becomes insufficient and the mechanical strength becomes low. On the other hand, when the amount of C exceeds 1.0% by mass, cementite is deposited in the iron matrix, the iron-carbon sintered member becomes brittle, and the mechanical strength also decreases. .. The amount of C in the iron-carbon sintered member is preferably in the range of 0.3 to 0.7% by mass.

The iron-carbon-based sintered member may be used as a sintered body, but it is preferable to perform quenching treatment and tempering treatment to martensite the matrix structure because the mechanical strength of the iron matrix increases. If the sintered body density of the iron-carbon-based sintered member is in a high-density region such as 7.4 Mg / m ³ or more and the iron base of the iron-carbon-based sintered member is martensite, iron-copper- It exhibits a tensile strength of 1400 MPa or more, which is higher than that of the carbon-based sintered member.

For the iron-carbon-based sintered member having a sintered body density of 7.4 Mg / m ³ or more as described above, a raw material powder obtained by adding graphite powder: 0.4 to 1.2% by mass to iron powder and mixing it is used. It can be obtained by performing molding, sintering, or the like using a conventionally known high-density method. For example, even if it is manufactured by a method such as a 2P-2S (Double Pressing-Double Sintering) method in which the raw material powder is pre-molded, temporarily sintered, and then the recompressed body is main-sintered, or a warm forming method. Good. However, the 2P-2S method is not preferable because the labor for molding and sintering is doubled and the manufacturing cost is remarkably increased, and the warm molding method is also used for molding press devices and raw material powder filling devices such as hoppers and feeders. A heating device is required, which is also not preferable because it leads to an increase in manufacturing cost.

On the other hand, the high-pressure molding method in which a raw material powder obtained by adding graphite powder to iron powder and mixing it is cold-molded under high pressure to obtain a green compact having a high green compact density is a special additional device. This is preferable because the molding press device that has been used conventionally can be used as it is. Specifically, a raw material powder consisting of graphite powder: 0.4 to 1.2% by mass and iron powder as the balance is filled in the cavity of the stamping die and compression-molded with an upper and lower punch to 7.4 Mg / m ³ or more. The powder powder has a density of. Such a green compact density can be obtained by molding at a molding pressure of 750 to 1200 MPa, which is much higher than the conventional general molding pressure. The obtained green compact is sintered at 1000 to 1300 ° C. in a non-oxidizing gas atmosphere as in the conventional case. By molding and sintering in this way, it is composed of C: 0.2 to 1.0% by mass, the balance: Fe and unavoidable impurities, and is an iron-carbon system with a sintered body density of 7.4 Mg / m ³ or more. A sintered member can be obtained.

In the above molding step, since the compaction molding is performed at a higher molding pressure than in the conventional case, a large extraction pressure is required when extracting the green compact after the compaction molding from the cavity of the stamping die, and at the time of extraction. There is a risk that high-density green compact will adhere to the cavity of the stamp and it will not be possible to perform good extraction. Therefore, in the above molding process, instead of the cemented carbide stamp that is usually used as the stamp, a coated stamp in which the surface of the mold hole is coated with TiN, TiCN, etc. having a small friction coefficient, a ceramic stamp, or a cermet stamp is used. Is preferable.

When a normal cemented carbide stamp is used, a coating of molding lubricant is formed on the surface of the mold hole of the stamp, and then the raw material powder is filled in the cavity coated with the molding lubricant to perform compaction molding. At the time of extracting the green compact, the molding lubricant coated on the surface of the mold hole reduces friction and facilitates the extraction of the green compact, which is preferable.

If the raw material powder contains a large amount of molding lubricant, the density of the sintered body obtained will be reduced by that amount because the molding lubricant is decomposed and separated at the time of dewaxing in the sintering step. Further, the molding lubricant decomposed and gasified at the time of dewaxing contaminates the surface of the iron powder and hinders the progress of sintering. This contamination is not a problem because the gasified molding lubricant is easily released when sintering the green compact in the normal region of 6.8 to 7.2 Mg / m ³ , but the green compact used in the present invention is not a problem. Since the body has a high density of 7.4 Mg / m ³ or more, the decomposed and gasified molding lubricant is difficult to escape, and remains to hinder the sinterability. Therefore, it is preferable that the raw material powder obtained by adding and mixing graphite powder to iron powder does not contain a molding lubricant.

On the other hand, if the amount of the molding lubricant added to the raw material powder is small, it contributes to the improvement of the powder density of the powder compact because it promotes densification due to slippage between the powders during powder molding. Therefore, if the amount of the molding lubricant added is 0.3% by mass or less with respect to the raw material powder, the molding lubricant may be added to the raw material powder. The amount of the molding lubricant added is preferably 0.05% by mass or more in order to better obtain the effect of promoting densification by slipping between the powders.

[First Example]
Atmel 250M manufactured by Kobe Steel Co., Ltd. was prepared as an iron powder, CE-15 manufactured by Fukuda Metal Foil Powder Industry Co., Ltd. as a copper powder, and SW1651 manufactured by Asbery Carbon Co., Ltd. as a graphite powder.

As the iron-carbon-based sintered member, a raw material obtained by adding 0.8% by mass of graphite powder to iron powder and mixing it is used, and a mold in which zinc stearate is applied as a molding lubricant to the mold holes is used to control the molding pressure. Instead, it was molded into a columnar shape with a width of 10 mm, a length of 60 mm, and a height of 10 mm, and the obtained molded product sample was sintered in a non-oxidizing atmosphere at 1120 ° C., and the obtained sintered body sample was subjected to a non-oxidizing atmosphere. After heating to medium 850 ° C. and quenching in oil, tempering was performed at 180 ° C. to prepare sintered samples of sample numbers 01 to 09 . The overall composition of sample numbers 01 to 09 thus obtained was C: 0.6% by mass and the balance was Fe and unavoidable impurities (impurities: 0.4% by mass or less).

As the iron-copper-carbon-based sintered member, a raw material obtained by adding 1.5% by mass of copper powder and 0.8% by mass of graphite powder to iron powder and mixing them is used, as in the case of the above-mentioned iron-carbon-based sintered member. In the same manner, sintered samples of sample numbers 10 to 17 were prepared. The overall composition of sample numbers 10 to 17 thus obtained is Cu: 1.5% by mass, C: 0.6% by mass, and the balance is Fe and unavoidable impurities (impurities: 0.4% by mass or less). there were.

The densities of the molded product samples and sintered body samples of sample numbers 01 to 17 described above were measured by the method specified in JIS Z2505. In addition, the sintered body sample was machined into the shape of a tensile test piece, and a tensile test was performed. Further, the sintered sample was cut, the cross section was mirror-polished, and the state of pores was observed with an optical microscope to confirm the presence or absence of pores having a maximum particle size of 50 μm or more. These results are also shown in Table 1.

From the results in Table 1 and the figure, the relationship between the sintered body density and the tensile strength of the iron-carbon-based sintered member (sample numbers 01 to 09) and the iron-copper-carbon-based sintered member (sample numbers 10 to 17). Is shown in FIG.

From Table 1 and FIG. 1, for both the iron-carbon-based sintered member and the iron-copper-carbon-based sintered member, when the molding pressure is increased, the molded body density increases and the sintered body density increases. Along with this, the tensile strength also tends to increase. However, in the iron-copper-carbon-based sintered member, when the sintered body density is 7.2 Mg / m ³ or more, the rate of increase in tensile strength with the increase in density is small. On the other hand, the iron-carbon-based sintered member shows a tendency that the tensile strength increases sharply from 7.2 Mg / m ³ to 7.4 Mg / m ³ in the sintered body density. At 7.4 Mg / m ³ or more, the tensile strength is higher than that of the iron-copper-carbon-based sintered member.

FIG. 2 shows the pore distribution of the iron-carbon sintered member sample (sample number 08 ) and the iron-copper-carbon sintered member sample (sample number 17 ) having a sintered body density of approximately 7.5 Mg / m ^3. ) Shows the pore distribution.

From FIG. 2, in the sample of the iron-copper-carbon-based sintered member, coarse and irregularly shaped pores of 50 μm or more, which are considered to be traces of the outflow of copper powder, are observed. On the other hand, in the sample of the iron-carbon-based sintered member, no pores of 50 μm or more were observed, and the shape of the pores also showed a rounded circular shape.

From these facts, in the iron-copper-carbon-based sintered member, in the high-density region where the sintered body density is 7.4 Mg / m ³ or more, the residual holes formed by the outflow of copper powder become the starting point of fracture. Therefore, it is considered that the tensile strength does not increase for the improvement of the density. On the other hand, in the iron-carbon-based sintered member, spheroidization of pores is promoted in a high-density region of 7.4 Mg / m ³ or more, so that stress is less likely to be concentrated and iron-copper-carbon-based firing is performed. It is considered to exhibit higher tensile strength than the connecting member.

From the above, the iron-carbon-based sintered member exhibits higher tensile strength than the iron-copper-carbon-based sintered member in a high-density region where the sintered body density is 7.4 Mg / m ³ or more, and 1400 MPa. It was confirmed that the above high tensile strength was exhibited. Further, in order to set the sintered body density of the iron-carbon-based sintered member to 7.4 Mg / m ³ or more, the molding pressure is set to 790 MPa or more and the molded body density is set to 7.45 Mg / m ³ or more. It was confirmed that it should be sintered.

[Second Example]
Using the iron powder and graphite powder used in the first example, using the raw materials added and mixed at the addition ratio shown in Table 2, the molding pressure was set to 1000 MPa, and other than that, molding and firing were performed in the same manner as in the first example. After knotting, the obtained sintered body sample was quenched and tempered to prepare sintered body samples of sample numbers 18 to 27. A tensile test was carried out on these sintered body samples in the same manner as in the first example, and the presence or absence of pores having a maximum particle size of 50 μm or more was confirmed. These results are also shown in Table 2. In Table 2, the values of sample number 07 of the first example are also shown.

From Table 2, since the sample of sample No. 18 in which graphite powder is not added to the iron powder does not contain C, the iron matrix is not strengthened and the tensile strength is low. Further, in the sample of sample No. 19 in which graphite powder is added to iron powder and the C content of the iron-carbon sintered member is 0.1% by mass, the iron matrix is strengthened by C and the tensile strength is improved. However, the tensile strength is less than 1000 MPa. On the other hand, in the sample of sample No. 20 in which the amount of C is 0.2% by mass, the iron matrix is sufficiently strengthened by C, and the tensile strength exceeds 1000 MPa. Further, when the amount of graphite powder added to the iron powder is increased to increase the amount of C in the iron-carbon-based sintered member, the tensile strength of the sample (sample number 23) in which the amount of C is 0.5% by mass is increased. Is the maximum, and when the amount of C is further increased, the tensile strength tends to decrease. Then, in the sample of sample number 27 in which the amount of C exceeds 1.0% by mass, the tensile strength is lower than 1000 MPa.

This is because the iron matrix is strengthened by C after sintering by adding the graphite powder to the raw material powder, so that the mechanical strength of the iron matrix increases, but the graphite powder is added to the raw material powder. As a result, the compressibility of the raw material powder decreases, and the graphite powder in the raw material powder diffuses as C to the iron matrix during sintering. Therefore, when the amount of the graphite powder added increases, the pores that accompany the disappearance of the graphite powder increase. Since the amount increases and the mechanical strength of the sintered member decreases, the mechanical strength becomes maximum when the amount of C is about 0.5% by mass, and when it exceeds this, the amount of pores increases due to the disappearance of the graphite powder, resulting in tension. It is thought that the strength will decrease.

Samples with a C amount in the range of 0.3 to 0.7% by mass (Sample Nos. 07 , 21 to 24) have a good balance between the strengthening of the iron matrix by C and the increase in the amount of pores due to the disappearance of graphite powder. An extremely high tensile strength of 1200 MPa or more is obtained.

From the above, the C amount of the iron-carbon-based sintered member is 0.2 to 1.0% by mass, and the tensile strength is 1000 MPa or more, which is higher mechanical strength than the general iron-copper-carbon-based sintered member. It was confirmed that it can be used. Further, in order to make the C amount of the iron-carbon-based sintered member 0.2 to 1.0% by mass, the addition amount of the graphite powder to the raw material powder should be 0.4 to 1.2% by mass. It was confirmed that it was good.

Since the iron-carbon-based sintered member of the present invention exhibits higher mechanical strength than the iron-copper-carbon-based sintered member, it is inexpensive and can be used as an iron-based sintered member for structural mechanical parts. It is suitable.

Claims (5)

C: 0.4 to 0.6 % by mass, balance: Fe and unavoidable impurities, sintered body density is 7.4 Mg / m ³ or more, 7.53 Mg / m ³ or less, and iron-carbon-based sintered member. An iron-carbon-based sintered member characterized by having a base structure of martensite and a tensile strength of 1300 MPa or more. Wherein the iron - carbon sintered member - in the pores with carbon-based sintered member, the iron according to claim 1, the maximum pore diameter is equal to or is 50μm or less. Graphite powder: A raw material powder of 0.4 to 1.2% by mass, the balance of which is iron powder, is filled in the cavity of the stamping die and compression-molded with upper and lower punches to produce a powder with a density of 7.45 Mg / m ³ or more. The molding process to obtain the body and
A sintering step of sintering the green compact at 1000 to 1300 ° C. to obtain a sintered body,
A quenching treatment and a tempering treatment step of performing a quenching treatment and a tempering treatment on the sintered body to make the base structure martensite.
A method for producing an iron-carbon-based sintered member, which comprises the above. The method for producing an iron-carbon-based sintered member according to claim 3 , wherein the cavity of the stamp is coated with a molding lubricant. The method for producing an iron-carbon-based sintered member according to claim 3 or 4 , wherein the raw material powder contains 0.3% by mass or less of a molding lubricant.

Images