IRON-BASE POWDER MIXTURE FOR POWDER METALLURGY HAVING EXCELLENT FLUIDITY AND MOLDABILITY AND PROCESS FOR PREPARING THE SAME

Abstract: There is provided an iron-based powder composition for powder metallurgy excellent in flowability and compactibility. The iron-based powder composition for powder metallurgy is produced in accordance with a method comprising the steps of: adding to iron-based and alloying powders, for a primary mixing, a surface treatment agent, and in addition, for a secondary mixing, a fatty acid amide and at least one lubricant, wherein the lubricant has a melting point higher than that of the fatty acid amide and is selected from the group comprising, a thermoplastic resin, a thermoplastic elastomer, and inorganic or organic compounds having layered crystal structure; heating and stirring up a mixture after the secondary mixing at a temperature over a melting point of the fatty acid amide to melt the fatty acid amide; cooling, while mixing, the mixture subjected to the heating and stirring process so that the alloying powder and a lubricant having a melting point higher than the fatty acid amide adhere to a surface of the iron base powder subjected to the surface treatment by an adhesive force of the melt; and adding at time of the cooling, for a tertiary mixing, a metallic soap and at least one type selected from a group comprising thermoplastic resin or thermoplastic elastomer powders and inorganic or organic compounds having layered crystal structure. The mixture is heated to about 423K and loaded into a die for compaction.

Description: Technical Field The present invention relates to iron-based powder compositions for powder metallurgy in which lubricant, graphite powder, copper powder and the like are added and mixed beforehand, and more particularly to an iron-based powder composition for powder metallurgy which in normal handling undergoes little segregation of the additive materials and dust generation and has excellent flowability and compactibility in a wide temperature range over the order of room temperature to 473K. Background Art Hitherto, iron-based powder compositions for powder metallurgy have been produced by a mixing method in which alloying powders such as copper, graphite, and iron phosphide powders, are mixed with an iron powder, and according to the necessity, in addition to the powders for improving the machinability, a lubricant such as zinc stearate, aluminium stearate, and lead stearate is mixed. Such a lubricant has been adopted in view of homogeneous mixing with the metal powder, easy decomposition and removability at the time of sintering. Recently, as the requirement of higher strength for sintering manufactures has increased, as disclosed in Japanese Patent Application Laid Open Gazette (Kokai) Hei.2-156002, Japanese Patent Publication (Kokoku) Hei.7-103404, U.S. Patent No. 5,256,185 and U.S. Patent No. 5,368,630, there has been proposed a warm compaction technology which permits higher density and higher strength of compacts by performing compaction while the metal powders are heated. It is considered for the lubricant used in such a compaction pracedure, that lubricity at the time of heating is important as well as homogeneous mixing with the metal powder, easy decomposition and removability at the time of sintering. Specifically, the mixing of a plurality of lubricants having mutually different melting points with metal powders serves, at the time of the warm compaction, to melt part of the lubricants, to uniformly spread the lubricants between the iron and/or alloying metal particles, and to decrease the frictional resistances between the particles and between the compact and the dies, so that compactibility is improved. However, such a metal powder composition involves the following drawbacks. Firstly the raw material mixture undergoes segregation. Regarding the segregation, since the metal powder composition contains powders having different sizes, shapes and densities, segregation occurs readily during transport after mixing and upon charging the powder composition into hoppers, or upon discharging the powder composition from the hoppers or during molding treatments. For example, it is well known that segregation of a mixture of iron-based powder and graphite powder occurs within a transport vehicle owing to vibrations during trucking, so that the graphite powder rises to the top. It is also known, in the case of graphite charged into a hopper, that the concentration of graphite powder differs at the beginning, middle, and end of the discharging operation from the hopper owing to segregation within the hopper. These segregations cause fluctuations in the composition of products of the powder metallurgy; fluctuations in dimensional changes and strength become large, and these cause the production of inferior products. Secondly the flow rate of the powder composition increases as a result of the increased specific surface area of the mixture, since graphite and other powders are fine powders. Such an increase in flow rate is disadvantageous because it decreases the production speed of green compacts by decreasing the charging speed of the powder composition into the die cavities for compaction. As technologies for preventing segregation of such a powder composition, there are known methods based on selection of an appropriate binder as disclosed in Japanese Patent Application Laid Open Gazette (Kokai) Sho.56-136901 and Japanese Patent Application Laid Open Gazette (Kokai) Sho.58-28321. However, these methods involve the drawback that if the quantity of binder added is increased so that segregation of the powder composition is sufficiently improved, the flow rate of the powder composition is increased. The present inventors proposed, in Japanese Patent Application Laid Open Gazette (Kokai) Hei.1-165701 and Japanese Patent Application Laid Open Gazette (Kokai) Hei.2-47201, methods in which a melt composed of the combination of an oil and a metal soap or wax, melted together is selected as a binder. These methods make it possible to sufficiently reduce segregation of the powder composition and dust generation, and also to improve the flowability. However, these methods involve the problem that the flowability of the powder composition varies with the passage of time because of the means for preventing segregation as mentioned above. Hence, the present inventors developed a method in which a melt composed of the combination of a high-melting point oil and a metal soap, melted together is selected as a binder, as proposed in Japanese Patent Application Laid Open Gazette (Kokai) Hei.2-57602. According to this method, the melt has a small change of elapse, and the change of elapse of flow rate of the powder composition is reduced. However, this method involves another drawback in that the apparent density of the powder composition varies; since a high-melting point saturated fatty acid in the solid state and a metal soap are mixed with iron-based powders at room temperature. In order to solve this problem, the present inventors proposed, in Japanese Patent Application Laid Open Gazette (Kokai) Hei.3-162502, a method in which after a surface of the iron-based powder is coated with a fatty acid, an additive material is adhered to the surface of the iron-based powder by means of a melted-together binder composed of a fatty acid and a metal soap, and further a metal soap is added to the outer surface of the iron-based powder. US-A-5, 135, 566 discloses an iron base powder mixture for powder metallurgy, comprising a ferrous powder, an alloying powder and a melted-together binder composed of an oil and a metal soap or wax. Disclosure of the Invention The problems of segregation and dust generation have been considerably solved in accordance with technologies disclosed in Japanese Patent Application Laid Open Gazette (Kokai) Hei.2-57602 and Japanese Patent Application Laid Open Gazette (Kokai) Hei.3-162502. However, it is still insufficient as to the flowability, particularly, at the time of heating during so-called warm compaction in which the powder compositions are heated up to about 423K and charged into a heated die cavity to be molded. Also according to the methods disclosed in Japanese Patent Application Laid Open Gazette (Kokai) Hei.3-162502, Japanese Patent Application Laid Open Gazette (Kokai) Hei.7-103404, U.S. Patent No. 5,256,185 and U.S. Patent No. 5,368,630, in which compactibility during warm compaction is improved, it is difficult to provide the excellent flowability during the warm compaction of the powder composition, since the low-melting point lubricant component forms a liquid cross-linking among the particles. Inferior flowability causes not only a hindrance in the productivity of the green compact as mentioned above, but also fluctuations in density distribution of the green compact because of disunity when charging into dies for compaction. This causes fluctuations in the properties of the sintered body. The first object of the present invention is to provide an iron-based powder composition for powder metallurgy having excellent flowability at not only room temperature but also during warm compaction, and is also to provide a method of producing the composition. Technologies concerning the warm compaction disclosed in Japanese Patent Application Laid Open Gazette (Kokai) Hei. 3-162502 contribute to the production of an iron-based powder compact having high density and high strength, but involve the drawback that an ejection force at the time of compaction is high. Thus, there are problems in that defects occur on the surface of the compact, and the lifetime of the compacting dies is decreased. The second object of the present invention is to provide an iron-based powder composition for powder metallurgy improved in compactibility, which is capable of reducing the ejection force at the time of compaction at room temperature and during warm compaction, and is also to provide a method of producing the composition., First, in order to solve the first object of the present invention, the present inventors studied the case where the flow rate of metal powders mixed with organic compounds such as a lubricant and the like is extremely increased as compared with metal powders mixed with no such organic compound. As a result, the present inventors noticed that the reason why the flow rate is increased is that the frictional resistances between the iron and/or alloying particles and adhesion between the iron or alloying particles and the organic compound is increased, and they thus examined how the frictional resistances and the adhesion can be decreased. The present inventors found that treating or coating the surfaces of the iron and optionally also the alloying powders with a certain type of organic compound i.e. one which is chemically stable until a high temperature range (about 473K), results in the frictional resistances between iron-based and/or alloying particles being reduced, and further that selecting the surface potential of the surfaces of the iron-based and/or alloying particles so as to approach the surface potential of the organic compound (except for the surface treatment agent) so as to suppress contact-charging between the iron-based or alloying particles and the organic compound at the time of mixing, inhibits adhesions of particles due to electrostatic force. Further, in order to improve the compactibility, the present inventors studied the effect of various solid-state lubricants, and found that inorganic or organic compounds having layered crystal structure, during room temperature and warm compactions, and thermoplastic resins or elastomers which undergo plastic deformation at a temperature above 373K, during warm compaction, serve to reduce the ejection force at the time of compaction so that the compactibility can be improved. Furthermore, the present inventors also found that coating the surfaces of iron-based and optionally also alloying particles with a surface treatment agent for improving the flow rate serves secondarily to reduce the ejection force at the time of compaction so that the compactibility can be improved. The present invention relates to iron-based powder composition for powder metallurgy according to claim 1 which have excellent flowability and compactibility properties and to a method of producing the composition according to claim 16 characterized in that the iron-based powder composition contains an iron-based powder, an alloying powder, a binder and a lubricant; at least the iron-based powder is coated with a surface treatment agent; and as the lubricant, there are included inorganic or organic compounds having a layered crystal structure, or a thermoplastic resin or an elastomer. The surface treatment agent is one or more types selected from among organoalkoxysilane or organosilazane compounds, a titanate coupling agent, a fluorine-containing silicon silane coupling agent. It is preferable that the inorganic compound having the layered crystal structure is one selected from among graphite, carbon fluoride and MoS 2 . Further, it preferable that the organic compound having the layered crystal structure is melamine-cyanuric acid adduct or N-alkylasparatic acid- β-alkylester. It is preferable that the thermoplastic resin is any one selected from among polystyrene, nylon, polyethylene and fluorine-contained resin, and has a particle diameter of 30 µm or less. It is preferable that the thermoplastic elastomer (TPE) is one selected from among a styrene block copolymer (SBC), a thermoplastic elastomer olefin (TEO), a thermoplastic elastomer polyamide (TPAE) and a silicone elastomer, and has a particle diameter of 30 µm or less. These iron-based powder composition can be produced as follows. That is, there is provided a method of producing an iron-based powder composition, comprising the steps of: coating at least the iron-based powder with a surface treatment agent at room temperature; adding to the iron-based powder subjected to a surface treatment and an alloying powder, for a primary mixing, a fatty acid amide binder and at least one lubricant, wherein the lubricant has a melting point higher than that of the fatty acid amide and is selected from the group comprising, a thermoplastic resin, a thermoplastic elastomer, and inorganic or organic compounds having a layered crystal structure; heating and stirring the composition produced by the primary mixing at a temperature above the melting point of the fatty acid amide to melt the fatty acid amide; mixing and cooling the mixture subjected to the heating and stirring process so that the alloying powder and the lubricant having a melting point higher than the fatty acid amide adhere to the surface of the iron-based powder coated with to the surface treatment agent by the adhesive force of the melt; and adding at the time of the cooling, for a secondary mixing, a metallic soap and at least one lubricant selected from the group comprising thermoplastic resin or thermoplastic elastomer powders and inorganic or organic compounds having a layered crystal structure. In an alternative embodiment, the iron-based powder composition may be produced by the method of Claim 16. It is acceptable that the surface treatment mentioned above may be carried out after the primary mixing. That is, there is provided a method of producing an iron-based powder composition, comprising the steps of: adding to the iron-based powder, for a primary mixing, a fatty acid amide binder and at least one lubricant, wherein the lubricant has a melting point higher than that of the fatty acid amide and is selected from the group comprising, a thermoplastic resin, a thermoplastic elastomer, and inorganic or organic compounds having a layered crystal structure; heating and stirring the composition obtained by the primary mixing at a temperature above the melting point of the fatty acid amide to melt the fatty acid amide; cooling the composition subjected to the heating and stirring process so that the alloying powder and the lubricant having a melting point higher than the fatty acid amide adhere to the surface of the iron-based powder coated with the surface treatment agent by the adhesive force of the melt, the surface treatment agent being added and mixed at a temperature of not less than 373K and not more than the melting point of the fatty acid amide; and adding at the time of cooling, for a secondary mixing, metallic soap and at least one lubricant selected from the group comprising thermoplastic resin or thermoplastic elastomer powders and inorganic or organic compounds having layered crystal structures. In this case, the surface treatment agent is one or more compounds selected from the group composed of organoalkysilane or organosilazane compounds, a titanate-containing coupling agent, a fluorine-containing silicon silane coupling agent. Including at least a copper powder or a cuprous oxide powder in the alloying powder contained in the iron-based powder composition according to the present invention makes it possible to increase the strength of the resultant sintered body. The use of a melt of one type of fatty acid amide, a partial melt of two or more types of fatty acid amide having mutually different melting points, or a melted-together binder composed of a fatty acid amide and a metallic soap, as the binder contained in the iron-based powder composition according to the present invention, may effectively prevent segregation and dust generation in and by the iron-based powder composition, and in addition improve the flowability. As the amide-containing binder, a fatty acid, bisamide such as N,N'-Ethylenebis(stearamide) is particularly preferable. Best Mode for Carrying Out the Invention Hereinafter, the present invention will be described in technical concept and effect. As mentioned above, the flowability of iron-based and alloying powders mixed with an organic compound such as a lubricant and the like is extremely decreased as compared with iron-based and alloying powders mixed with no organic compound. The reason why the flow rate is decreased is that frictional resistances between the iron-based and alloying powders and adhesions between the iron-based or alloying powders and the organic compound are increased. Thus, there is provided a countermeasure where surfaces of the iron-based and/or alloying powders are treated (coated) with a certain type of organic compound, so that the frictional resistances between the iron-based and alloying powders are reduced, and further the surface potential of the surfaces of the iron-based and alloying powders is selected to approach the surface potential of the organic compound (except for the surface treatment agent) so as to suppress contact-charging between the hereto-particles at the time of mixing, thereby prohibiting adhesion of particles due to electrostatic force. Thus, it is possible to improve the flowability of the mixed powders by a compound effect of both. Specifically, it is possible to ensure the stable flowability over a temperature range from room temperature to 473K so that the technology can be applied to warm compaction. Next, there will be described in more detail the reason why the flowability is improved over the broad temperature range by coating surfaces of at least the iron-based powder with an organoalkoxy or organosilazane compound, a titanate-containing coupling agent, a fluorine-containing silicon silane coupling agent . Here, organosilicon compounds are restricted to organoalkoxysilane, organosilazane. The above-mentioned surface treatment agents have a lubricating function owing to their bulky molecular structure and in addition they are chemically stable in high temperature regions as compared with fatty acids, mineral oils and the like. Thus, those surface treatment agents exhibit a lubricating function over a broad temperature range from room temperature to about 473K. Particularly, organoalkoxysilane, organosilazane and titanate coupling agent or fluorine-containing silicon silane coupling agents perform a surface treatment by chemical bonding of an organic compound on surfaces of at least the iron-based powder through the condensation reaction of a hydroxyl group existing on the surfaces of the iron-based powder with a functional group containing N or O combining with Si or Ti, in molecules of the surface treatment agents. These surface treatment agents do not come off or flow out from the surfaces of the particles even at high temperature, and thus bring a remarkable effect of surface treatment at high temperature. The organoalkoxysilane may have non-substituted or substituted organic groups and can be represented by the structural formulas R n Si (OR') 4-n (n = 1,2,3; R = organic group; R' = alkyl group) and (n = 1,2,3; R = organic group; R= alkyl group; X = substituent), respectively. As the substituent (X) of the substituted organic group, any one of an acrylic group, an epoxy group and an amino group is suitable. It is acceptable that mixed substituent may be present except for mixtures of epoxy groups and amino groups since they react with one another and undergo change of properties. Organosilazane is a general term for compounds representend by the structural formulas R n Si (NH 2 ) 4-n (n = 1,2,3), (R 3 Si) 2 NH, and R 3 Si - NH - Si - (R' 2 SiNH) n - Si - R" 3 (n≧ 1). While the organosilazane is not particularly restricted, polyorganosilazanes as represented by the above-noted third structure formula is particularly effective in improving the flowability. Incidentally, it is preferable that the number of alkoxy groups (OR') of the organoalkoxysilane is small. Of the organoalkoxysilanes having non-substituted organic groups, methyl trimethoxy silane, phenyl trimethoxy silane and diphenyl methoxy silane are especially effective in improving the flowability. Of the organoalkoxysilanes having substituted organic groups, as organoalkoxysilane substituted with an acrylic group, γ-methacryloxypropyl trimethoxy silane is especially effective in improving the flowability; as organoalkoxysilane substituted with an epoxy group, γ-glycidoxypropyl trimethoxy silane can be exemplified; and as organoalkoxysilane substituted with an amino group, γ-aminopropyl trimethoxy silane can be exemplified. With regard to organoalkoxysilanes having non-substituted or substituted organic groups, there are also available those in which part of the hydrogen of the organic group R in the above-noted structure formulas is replaced by fluorine (it happens that an organoalkoxysilane, in which part of hydrogen in the organic group R is replaced by fluorine, is classified as a fluorine-contained silicon silane coupling agent). As titanate coupling agent, isopropyltriisostearoyl titanate is suitable. In iron powder mixtures having stable flowability over a broad temperature range from room temperature to about 473K, it is preferable that, for the binder. For adhesion of iron-based and alloying powders, there are used two or more types of wax each having mutually different melting points, especially, partial melts of amide lubricant. A method in which a melted-together compound composed of a fatty acid and a metallic soap is used, which is disclosed in Japanese Patent Application Laid Open Gazette (Kokai) Hei.3-162502 by the present inventor, is optimum since melts coat the whole of the additive particles by capillarity so as to tightly adhere them to the iron-based powder. Two or more types of wax each having mutually different melting point and partial melts of amid lubricant are preferred for the same reason. The metallic soap to be used is melted with a low melting point material so that the flow rate at higher temperatures is increased. Consequently, it is desired that the melting point is not less than at least 423 K. Next, there will be described the reasons why the ejection force at the time of compaction is reduced, so that the compactibility is improved, by mixing an inorganic or organic compound having a layered crystal structure with iron-based and alloying powders. With regard to the lubricating effect of compounds having a layered crystal structure, there are several theories. In the case of the present invention, however, it is considered that the above-mentioned materials, which undergo shearing stress at the time of compaction, are easily subjected to cleavage along the crystal surface, and thus this causes a reduction of frictional resistances between the particles inside the compact, or easy sliding between the compact and the die walls. It is acceptable that the inorganic organic compound having-a layered crystal structure is any one selected from among graphite, MoS 2 , and carbon fluoride. The more fine is the size of the particles, the more effective is the reduction of the ejection force. As the organic compound having a layered crystal structure, melamine-cyanuric acid adduct compound (MCA) or N-alkylasparatic acid - β - alkylester is suitable. Next, there will be described the reasons why the ejection force at the time of compaction, particularly, at the time of warm compaction is reduced by mixing thermoplastic resin or thermoplastic elastomer with the iron-based and alloying powders. An aspect of the thermoplastic resin resides in the fact that as the temperature rises the yield stress decreases, and as a result, it is easily deformed with low pressure. During warm compaction in which a particle-like thermoplastic resin is mixed with iron-based and alloying powder and is heated for compaction, particles of the thermoplastic resin will easily undergo plastic deformation between the iron-based and/or alloying particles or between compacted particles and the die walls, and as a result, frictional resistances between mutually contacted surfaces are decreased. The thermoplastic elastomer implies a material having the multi-phase texture of a thermoplastic resin (hard phase) and a polymer having a rubber structure (soft phase). An aspect of the thermoplastic elastomer resides in the fact that as the temperature rises the yield stress of the thermoplastic resin soft phase decreases, and as a result, it is easily deformed with low pressure. Accordingly, the effect of the case in which a particle-like thermoplastic elastomer is mixed with iron-based and alloying powder and is subjected to a warm compaction process is the same as for the above-mentioned thermoplastic resin. As the thermoplastic resin, particles of polystyrene, nylon, polyethylene or fluorine-containing resin are suitable. As the thermoplastic elastomer, in the form of the soft phase, styrene resin, olefin resin, polyamide resin or silicone resin is suitable, and particularly, styrene-acryl and styrene-butadiene copolymers. The size of the particles of the thermoplastic resin or elastomer is suitably 30µm or less, and desirably 5µm -20 µm. When the size of the particles of the thermoplastic resin or elastomer is over 30 µm, it will prevent particles of the resin or elastomer from being sufficiently dispersed among the metal particles. Thus, the lubricating effect cannot be expected. As specific producing methods, embodiments will be exemplarily shown hereinafter. In the following, the examples using mineral oil or silicon oil as surface treatment agent are herein described for comparative purposes only. (Embodiment 1) Various types of organoalkoxy silane, organosilazane and a coupling agent were melted in ethanol, and silicone oil and mineral oil were diluted with xylene. These were sprayed on iron powder for powder metallurgy having a mean particle diameter of 78 µm, or native graphite having a mean particle diameter of 23 µm or less, or copper powder having a mean particle diameter of 25 µm or less, by a suitable amount as indicated in Table 1, and mixed up with a high speed mixer at 1000 rpm for one minute. Thereafter, the solvents were removed by a vacuum dryer and the powders, were heated for about one hour at about 373K. This process is referred to as preliminary treatment A1. Table 1 shows the types and loadings of the surface treatment agents loaded in the preliminary treatment A1. The symbols set forth in the columns for the surface treatment agents in Table 1 are the same as those shown in Table 14. Iron powder for powder metallurgy having a mean particle diameter of 78 µm, which has undergone the preliminary treatment A1, native graphite having a mean particle diameter of 23 µm or less, which has undergone the preliminary treatment A1, or which has not undergone the preliminary treatment A1, and copper powder having a mean particle diameter of 25 µm or less, which has undergone the preliminary treatment A1, or which has not undergone the preliminary treatment A1, were mixed as indicated in Table 1 After this, 0.2% by weight stearamide and 0.2% by weight N, N'-ethylenebis (stearamide) were added, and mixed and heated at 383K. These were then further mixed and cooled below 358K. Then, 0.2% by weight stearamide stearate and 0.2% by weight zinc stearate were added and mixed up homogeneously, after which the mixture was discharged from the mixer. (Practical examples 1-9) For a comparison, iron powder for powder metallurgy having a mean particle diameter of 78 µm, native graphite having a mean particle diameter of 23 µm or less, and copper powder having a mean particle diameter of 25 µm or less, which have not undergone the preliminary treatment A1, were used and mixed in a similar fashion to that of the above-mentioned embodiment 1, thereby obtaining a mixed powder (comparative example 1). 100 g of each of the mixed powders of Table 1 were separately discharged through an orifice having an emission hole of 5 mm in diameter, and the discharge time at room temperature was measured. The results are shown in Table 1. As is apparent from a comparison of comparative example 1 with practical examples 1-9, the flowability of the mixed powders was dramatically improved in the case where treatment with the surface treatment agents was carried out. (Embodiment 2) Iron powder for powder metallurgy having a mean particle diameter of 78 µm, native graphite having a mean particle diameter of 23 µm or less, and copper powder having a mean particle diameter of 25 µm or less were mixed, and various types of organoalkoxysilane, organosilazane, a coupling agent, silicone oil or mineral oil were sprayed on the mixture in a suitable amount as indicated in Table 2, and mixed up in a high speed mixer at 1000 rpm for one minute. Thereafter, 0.1% by weight oleic acid and 0.3% by weight zinc stearate were added, and mixed and heated at 383K. After this, the mixtures were cooled below 358K. The above-mentioned process where various types of organoalkoxysilane, organosilazane, a coupling agent, silicone oil or mineral oil were sprayed on the powders by a suitable amount, and mixed up in a high speed mixer at 1000 rpm for one minute is referred to as preliminary treatment B1. Table 2 shows the types and loadings of the surface treatment agents loaded in the preliminary treatment B1. The symbols set forth in the column for the surface treatment agents in Table 2 are the same as those shown in Table 14. Then, 0.4% by weight zinc stearate was added in each case and mixed up homogeneously, after which the mixture was discharged from the mixer. (Practical examples 10-15) For a comparison, iron powder for powder metallurgy having a mean particle diameter of 78 µm, native graphite having a mean particle diameter of 23 µm or less, and copper powder having a mean particle diameter of 25 µm or less were mixed, and further mixed in a similar fashion to that of the above-mentioned embodiment 2 without practicing the preliminary treatment B1, thereby obtaining a mixed powder (comparative example 2). 100 g of each mixed powder of Table 2 were separately discharged through an orifice having an emission hole of 5 mm in diameter, and the discharge time at room temperature was measured. The result is shown in Table 2. As is apparent from a comparison of comparative example 2 with practical examples 10-15, the flowability of the mixed powders was dramatically improved in the case where treatment was carried out with the surface treatment agents. (Embodiment 3) 0.2% by weight stearamide and 0.2% by weight N, N'-Ethylenebis (stearamide) were added to mixtures of iron powder for powder metallurgy having a mean particle diameter of 78 µm, native graphite having a mean particle diameter of 23 µm or less, and copper powder having a mean particle diameter of 25 µm or less as indicated in Table 3 and mixed and heated at 383K. After this, further various types of organoalkoxysilane, organosilazane, a coupling agent, silicone oil or mineral oil were sprayed on each mixture by a suitable amount as indicated in Table 3, and mixed up in a high speed mixer at 1000 rpm for one minute. Thereafter, the mixtures were cooled below 358K. The process where various types of organoalkoxysilane, organosilazane, a coupling agent, silicone oil or mineral oil were sprayed on the mixtures by a suitable amount, and mixed up in a high speed mixer at 1000 rpm for one minute is referred to as preliminary treatment C1. Table 3 shows the types and loadings of the surface treatment agents loaded in the preliminary treatment C1. The symbols set forth in the column for the surface treatment agents in Table 3 are the same as those shown in Table 14. Then, 0.2% by weight stearamide and 0.4% by weight zinc stearate were added in each case and mixed up homogeneously, after which the mixture was discharged from the mixer. (Practical examples 16-20) For a comparison, iron powder for powder metallurgy having a mean particle diameter of 78 µm, native graphite having a mean particle diameter of 23 µm or less, and copper powder having a mean particle diameter of 25 µm or less were used, and mixed in a similar fashion to that of the above-mentioned embodiment 3 without practicing the preliminary treatment C1, thereby obtaining a mixed powder (comparative example 3). 100 g of each of the mixed powders of Table 3 were separately discharged through an orifice having an emission hole of 5 mm in diameter, and the discharge time was measured at room temperature. The results are shown in Table 3. As is apparent from a comparison of comparative example 3 with practical examples 16-20, the flowability of the mixed powders was dramatically improved in the case where treatment was carried out with the surface treatment agents. (Embodiment 4) Various types of organoalkoxysilane, organosilazane and a coupling agent were diluted with ethanol, and silicone oil and mineral oil were diluted with xylene. These were sprayed on partially alloyed steel powder for powder metallurgy having a mean particle diameter of 80 µm, and/or native graphite having a mean particle diameter of 23 µm, by a suitable amount as indicated in Tables 4-1 and 4-2 and mixed up with a high speed mixer at 1000 rpm for one minute. Thereafter, the solvents were removed by a vacuum dryer and the powders were, heated for about one hour at about 373K. This process is referred to as preliminary treatment A2. Tables 4-1 and 4-2 show the types and loadings of the surface treatment agents loaded in the preliminary treatment A2. The symbols set forth in the column for the surface treatment agents in Tables 4-1 and 4-2 are the same as those shown in Table 14. Partially alloyed steel powder for powder metallurgy having a mean particle diameter of 78 µm, which has undergone the preliminary treatment A2, and native graphite having a mean particle diameter of 23 µm or less, which has undergone the preliminary treatment A2, or which has not undergone the preliminary treatment A2, were mixed up with one another as shown in Tables 4-1 and 4-2. After this, 0.1% by weight stearamide and 0.2% by weight ethylenebis (stearamide) and 0.1 % by weight lithium stearate were added in each case, and mixed and heated at 433K. These were further mixed and cooled below 358K. Then, 0.4% by weight lithium stearate was added in each case and mixed up homogeneously, after which the mixture was discharged from the mixer. (Practical examples 21-24) For a comparison, alloyed steel powder for powder metallurgy having a mean particle diameter of 80 µm, and native graphite having a mean particle diameter of 23 µm or less, which had not undergone the preliminary treatment A2, were used and mixed in a similar fashion to that of the above-mentioned embodiment 4, thereby obtaining a mixed powder (comparative example 4). 100g of each mixed powder of Tables 4-1 and 4-2 were separately discharged through an orifice having an emission hole of 5 mm in diameter, and the discharge time was measured at room temperature. The results are shown in Tables 4-1 and 4-2. As is apparent from a comparison of comparative example 4 with practical examples 21-24 the flowability of the mixed powders was dramatically improved in the case where treatment was carried out using the surface treatment agents. (Embodiment 5) Partially alloyed steel powder for powder metallurgy having a mean particle diameter of 80 µm, and native graphite having a mean particle diameter of 23 µm or less, were mixed, and various types of organoalkoxysilane, organosilazane, a coupling agent, silicone oil or mineral oil were sprayed on the mixture by a suitable amount as indicated in Tables 5-1 and 5-2, and mixed up in a high speed mixer at 1000 rpm for one minute. Thereafter, 0.2% by weight stearamide and 0.2% by weight ethylenebis (stearamide) were added, and mixed and heated at 433K. After this, the mixtures were cooled below 358K. The above-mentioned process where various types of organoalkoxysilane, organosilazane, a coupling agent, silicone oil or mineral oil were sprayed on the mixture in a suitable amount, and mixed up in a high speed mixer at 1000 rpm for one minute is referred to as preliminary treatment B2. Tables 5-1 and 5-2 show the types and amounts of the surface treatment agents added in the preliminary treatment B2. The symbols set forth in the column for the surface treatment agents in Tables 5-1 and 5-2 are the same as those shown in Table 14. Then, 0.4% by weight lithium hydroxy stearate was added in each case and mixed up homogeneously, after which the mixture was discharged from the mixer. (Practical examples 25-28). For a comparison, partially alloyed steel powder for powder metallurgy having a mean particle diameter of 80 µm, and native graphite having a mean particle diameter of 23 µm or less were mixed, and further mixed in a similar fashion to that of the above-mentioned embodiment 5 without practicing the preliminary treatment B2, thereby obtaining a mixed powder (comparative example 5). 100 g of the mixed powders of Tables 5-1 and 5-2 were separately discharged through an orifice having an emission hole of 5 mm in diameter, and the discharge time was measured at respective temperature from 293K to 413K. The results are shown in Tables 5-1 and 5-2. As is apparent from a comparison of comparative example 5 with practical examples 25-28, the flowability of the mixed powders was dramatically improved in the case where treatment was carried out with the surface treatment agents. (Embodiment 6) 0.2% by weight stearamide and 0.2% by weight ethylenebis (stearamide) were added to mixtures of partially alloyed steel powder for powder metallurgy having a mean particle diameter of 80 µm, and native graphite having a mean particle diameter of 23 µm or less, and mixed and heated at 433K. Thereafter, the mixtures were cooled to about 383K. After this, various types of organoalkoxysilane, organosilazane, a coupling agent, silicone oil or mineral oil were sprayed on the mixture by a suitable amount as indicated in Table 6, and mixed up in a high speed mixer at 1000 rpm for one minute. Thereafter, the mixtures were cooled below 358K. This process is referred to as preliminary treatment C2. Table 6 shows the types and loadings of the surface treatment agents loaded in the preliminary treatment C2. The symbols set forth in the column for the surface treatment agents in Table 6 are the same as those shown in Table 14. Then, 0.4% by weight lithium hydroxy stearate was added in each case and mixed up homogeneously, after which the mixture was discharged from the mixer. (Practical examples 29-31). 100g of each of the mixed powders of Table 6 were separately discharged through an orifice having an emission hole of 5 mm in diameter, and the discharge time was measured at room temperature. The results are shown in Table 6. As is apparent from a comparison of comparative example 5 with practical examples 29-31 the flowability of the mixed powders was dramatically improved in the case where treatment was carried out with the surface treatment agents. (Embodiment 7) Various types of organoalkoxysilane, organosilazane and a coupling agent were diluted with ethanol, and silicone oil and mineral oil were diluted with xylene. These were sprayed on partially alloyed steel powder for powder metallurgy having a mean particle diameter of 80 µm, or native graphite having a mean particle diameter of 23 µm or less, by a suitable amount as indicated in Tables 7-1 and 7-2, and mixed up with a high speed mixer at 1000 rpm for one minute. Thereafter, the solvents were removed by a vacuum dryer and the powders were heated for one hour at about 373K. This process is referred to as preliminary treatment A2. Tables 7 -1 and 7-2 show the types and loadings of the surface treatment agents loaded in the preliminary treatment A2. The symbols set forth in the column for the surface treatment agents in Tables 7-1 and 7-2 are the same as those shown in Table 14. Partially alloyed steel powder for powder metallurgy having a mean particle diameter of 80 µm, which has undergone the preliminary treatment A2, or which has not undergone the preliminary treatment A2, and native graphite having a mean particle diameter of 23 µm or less, which has undergone the preliminary treatment A2, or which has not undergone the preliminary treatment A2, were mixed up with one another as indicated in Tables 7-1 and 7-2. After this, 0.1% by weight stearamide, 0.2% by weight ethylenebis (stearamide) and 0.1% by weight of any one of thermoplastic resin, thermoplastic elastomer and compounds having layered crystal structure were added, and mixed and heated at 433K. These were further mixed and cooled below 358K. In this case, the types of the added materials and their amounts are shown in Tables 7-1 and 7-2. The symbols set forth in the column for the names of the materials in Tables 7-1 and 7-2 are the same as those shown in Table 15. Then, 0.2% by weight of at least one material selected from among lithium stearate, lithium hydroxy stearate and calcium laurate was added in each case and mixed up homogeneously, after which the mixture was discharged from the mixer (Practical examples 32-36). The names of the added materials are shown in Tables 14 and 15. 100g of each of the mixed powders of Tables 7-1 and 7-2 were separately discharged through an orifice having an emission hole of 5 mm in diameter, and the discharge time was measured at respective temperatures from 293K to 413K. Further, each mixed powder was heated to 423K to form a tablet 11 mm in diameter using a pressure of 686 MPa. The ejection force and the green compact density at the time of compaction were measured in each case. The results are shown in Tables 7-1 and 7-2. As is apparent from a comparison of comparative example 6 with practical examples 32-36 the flowability of the mixed powders at the respective temperatures was dramatically improved in the case where treatment was practiced with the surface treatment agents. Further, as is apparent from a comparison of comparative example 6 with practical examples 32-36, the green compact density is improved, and the ejection force is decreased. Thus the compactibility was improved in the case where thermoplastic resin, thermoplastic elastomer or a compound having a layered crystal structure was added in addition to the treatment with the surface treatment agents. (Embodiment 8) Partially alloyed steel powder for powder metallurgy having a mean particle diameter of 80 u m, and native graphite having a mean particle diameter of 23 µm or less, were mixed, and various types of organoalkoxysilane, organosilazane, a coupling agent, silicone oil or mineral oil were sprayed on the mixture in a suitable amount as indicated in Tables 8-1 and 8-2, and mixed up in a high speed mixer at 1000 rpm for one minute. Thereafter, 0.2% by weight stearamide, 0.2% by weight ethylenebis (stearamide) and 0.1% by weight of any one of thermoplastic resin, thermoplastic elastomer and compounds having a layered crystal structure were added as indicated in Tables 8-1 and 8-2 and mixed and heated at 433K. After this, the mixtures were further mixed and cooled below 358K. The above-mentioned process where various types of organoalkoxysilane, organosilazane, a coupling agent, silicone oil or mineral oil were sprayed on the mixture in a suitable amount, and mixed up in a high speed mixer at 1000 rpm for one minute is referred to as preliminary treatment B2. Tables 8-1 and 8-2 show the types and loadings of the surface treatment agents loaded in the preliminary treatment B2, and the amount of the thermoplastic resin, thermoplastic elastomer or compound having a layered crystal structure. The symbols set forth in the column for the surface treatment agents in Tables 8-1 and 8-2 are the same as those shown in Table 14. The symbols set forth in the column for thermoplastic resin, thermoplastic elastomer or compounds having layered crystal structure in Tables 8-1 and 8-2 are the same as those shown in Table 15. Then, 0.2% by weight of at least one material selected from lithium stearate, lithium hydroxy stearate and calcium laurate was added and mixed up homogeneously, after which the mixture was discharged from the mixer (Practical examples 37-40). The names of the added materials are shown in tables 14 and 15. 100g of each of the mixed powders of Tables 8-1 and 8-2 were separately discharged through an orifice having an emission hole of 5 mm in diameter, and the discharge time was measured at the respective temperatures from 293K to 413K. Further, each mixed powder was heated to 150 °C to form a tablet of 11 mm in diameter at a pressure of 686 MPa, and the ejection force and the green compact density at the time of compaction were measured. The results are shown in Tables 8-1 and 8-2. As is apparent from a comparison of comparative example 6 with practical examples 37-40, the flowability of the mixed powders at the respective temperatures was dramatically improved in the case where treatment was practiced with the surface treatment agents. Further, as is apparent from a comparison of comparative example 6 with practical examples 37-40, the green compact density is improved, and the ejection force is decreased, i.e. the compactibility was improved in the case where thermoplastic resin, thermoplastic elastomer or a compound having a layered crystal structure was added and in addition treatment was practiced with the surface treatment agents. (Embodiment 9) 0.2% by weight stearamide, 0.2% by weight ethylenebis (stearamide) and 0.1% by weight of any one of thermoplastic resin, thermoplastic elastomer and compounds having a layered crystal structure were added to mixtures of partially alloyed steel powder for powder metallurgy having a mean particle diameter of 80 µm, and native graphite having a mean particle diameter of 23 µm or less, and mixed and heated at 433K. Thereafter the mixtures were cooled to about 383K. After this, various types of organoalkoxysilane, organosilazane, a coupling agent, silicone oil or mineral oil were sprayed on to the mixture in a suitable amount as indicated in Tables 9-1 and 9-2, and mixed up with a high speed mixer at 1000 rpm for one minute. Thereafter, the mixtures were cooled below 358K. The above-mentioned process where various types of organoalkoxysilane, organosilazane, a coupling agent, silicone oil or mineral oil were sprayed on the mixture in a suitable amount, and mixed up using a high speed mixer at 1000 rpm for one minute is referred to as preliminary treatment C2. Tables 9-1 and 9-2 show the types and loadings of the surface treatment agents loaded in the preliminary treatment C2, and of the thermoplastic resin, thermoplastic elastomer or compounds having layered crystal structure. The symbols set forth in the column for the surface treatment agents in Tables 9-1 and 9-2 are the same as those shown in Table 14. The symbols set forth in the column for thermoplastic resin, thermoplastic elastomer or compounds having layered crystal structure in Tables 9-1 and 9-2 are the same as those shown in Table 15 and its footnotes. Then, 0.4% by weight lithium hydroxy stearate was added in each case and mixed up homogeneously, after which the mixture was discharged from the mixer. (Practical examples 41-45) 100 g of each mixed powder of Tables 9-1 and 9-2 were separately discharged through an orifice having an emission hole of 5 mm in diameter, and the discharge time was measured at the respective temperatures from 293K to 413K. Further, each mixed powder was heated to 423K to form a tablet 11 mm in diameter using a pressure of 686 MPa, and the ejection force and green compact density at the time of compaction were measured. The results are shown in Tables 9-1 and 9-2. As is apparent from a comparison of comparative example 6 with practical examples 41-45, the flowability of the mixed powders at the respective temperatures was dramatically improved in the case where treatment was carried out with the surface treatment agents. Further, as is apparent from a comparison of comparative example 6 with practical examples 41-45, the green compact density is improved, and the ejection force is decreased. Thus the compactibility was improved in the case where thermoplastic resin, thermoplastic elastomer or a compound having a layered crystal structure was added and in addition treatment was carried out with the surface treatment agents. (Embodiment 10) Various types of organoalkoxysilane, organosilazane silane and coupling agent were diluted with ethanol, and silicone oil and mineral oil were diluted with xylene. These were sprayed on partially alloyed steel powder for powder metallurgy having a mean particle diameter of 80 µm, or native graphite having a mean particle diameter of 23 µm or less, in a suitable amount as indicated in Tables 10-1 and 10-2 and mixed up with a high speed mixer at 1000 rpm for one minute. Thereafter, the solvents were removed by a vacuum dryer the mixtures were heated for one hour at about 373K. This process is referred to as preliminary treatment A2. Tables 10-1 and 10-2 show the types and amounts of the surface treatment agents loaded in the preliminary treatment A2. The symbols set forth in the column for the surface treatment agents in Tables 10-1 and 10-2 are the same as those shown in Table 14. Partially alloyed steel powder for powder metallurgy having a mean particle diameter of 80 µm, which has undergone the preliminary treatment A2, and native graphite having a mean particle diameter of 23 µm or less, which has undergone the preliminary treatment A2, or which has not undergone the preliminary treatment A2, were mixed up with one another as indicated in Tables 10-1 and 10-2. After this, 0.1% by weight stearamide, 0.2% by weight ethylenebis (stearamide) and 0.1% by weight of any one of thermoplastic resin, thermoplastic elastomer and compounds having a layered crystal structure were added, and mixed and heated at 433K. These were further mixed and cooled below 358K. In this case, the types and amounts of the loaded thermoplastic resin, thermoplastic elastomer or compounds having a layered crystal structure are shown in Tables 10-1 and 10-2. The symbols set forth in the column for thermoplastic resin, thermoplastic elastomer or compounds having a layered crystal structure shown in Tables 10-1 and 10-2 are the same as those shown in Table 15. Then, 0.2% by weight of at least one material selected from among lithium stearate, lithium hydroxy stearate and calcium laurate was added in each case and mixed up homogeneously, after which the mixture was discharged from the mixer (Practical examples 46-49). In this case, the names of the loaded materials are shown in Tables 14 and 15. 100 g of each mixed powder of Tables 10-2 and 10-2 were separately discharged through an orifice having an emission hole of 5 mm in diameter, and the discharge time was measured at respective temperatures from 293K to 413K. Further, each mixed powder was heated to 423K to form a tablet 11 mm in diameter using a pressure of 686 MPa, and the ejection force and the green compact density at the time of compaction were measured. The results are shown in Tables 10-1 and 10-2. As is apparent from a comparison of comparative example 6 with practical examples 46-49, the flowability of the mixed powders at the respective temperatures was dramatically improved in the case where treatment was carried out with the surface treatment agents. Further, as is apparent from a comparison of comparative example 6 with practical examples 46-49, the green compact density is improved, and the ejection force is decreased. Thus the compactibility was improved in the case where thermoplastic resin, thermoplastic elastomer or a compound having a layered crystal structure was added and in addition treatment was practiced with the surface treatment agents. (Embodiment 11) Partially alloyed steel powder for powder metallurgy having a mean particle diameter of 80 µm, and native graphite having a mean particle diameter of 23 µm or less, were mixed, and various types of organoalkoxysilane, organosilazane, a coupling agent, silicone oil or mineral oil were sprayed on the mixture in a suitable amount as indicated in Tables 11-1 and 11-2 and mixed up in a high speed mixer at 1000 rpm for one minute. Thereafter, 0.2% by weight stearamide and 0.2% by weight ethylenebis (stearamide) were added, and mixed and heated at 433K. After this, the mixtures were further mixed and cooled to 85 °C (358K). The above-mentioned process where various types of organoalkoxysilane, organosilazane, a coupling a gent, silicone oil or mineral oil were sprayed on the mixture in a suitable amount, and mixed up using a high speed mixer at 1000 rpm for one minute is referred to as preliminary treatment B2. Tables 11-1 and 11-2 show the types and loadings of the surface treatment agents loaded in the preliminary treatment B2. The symbols set forth in the column for the surface treatment agents in Tables 11-1 and 11-2 are the same as those shown in Table 14. Then, 0.1% by weight lithium stearate and 0.2% by weight at least one of thermoplastic resin, thermoplastic elastomer and compounds having a layered crystal structure were added in each case and mixed up homogeneously, after which the mixture was discharged from the mixer (Practical examples 50-53). In this case, the names of the added materials and amounts are shown in tables 11-1 and 11-2. The symbols set forth in the column for thermoplastic resin, thermoplastic elastomer or compounds having a layered crystal structure shown in Tables 11-1 and 11-2 are the same as those shown in Table 15. 100 g of each mixed powder of Tables 11-1 and 11-2 were separately discharged through an orifice having an emission hole of 5 mm in diameter, and the discharge time was measured at the respective temperatures from 293K to 413K. Further, each mixed powder was heated to 423K to form a tablet 11 mm in diameter using a pressure of 686 MPa, and the ejection force and the green compact density at the time of compaction were measured. The results are shown in Tables 11-1 and 11-2. As is apparent from a comparison of comparative example 6 with practical examples 50-53, the flowability of the mixed powders at the respective temperatures was dramatically improved in the case where treatment was practiced with the surface treatment agents. Further, as is apparent from a comparison of comparative example 6 with practical examples 50-53, the green compact density is improved, and the ejection force is decreased. Thus the compactibility was improved in the case where thermoplastic resin, thermoplastic elastomer or a compound having a layered crystal structure was added and in addition treatment was practiced with the surface treatment agents. (Embodiment 12) 0.2% by weight stearamide and 0.2% by weight ethylenebis (stearamide) were added to the mixtures of partially alloyed steel powder for powder metallurgy having a mean particle diameter of 80 µm, and native graphite having a mean particle diameter of 23 µm or less as indicated in Table 12; and mixed and heated at 433K. Thereafter the mixtures were cooled to about 383K. After this, various types of organoalkoxysilane, organosilazane, a coupling agent, silicone oil or mineral oil were sprayed on the mixture in a suitable amount as indicated in Table 12; and mixed up using a high speed mixer at 1000 rpm for one minute. Thereafter, the mixtures were cooled below 358K. The above-mentioned process where various types of organoalkoxysilane, organosilazane, a coupling agent, silicone oil or mineral oil were sprayed on the mixture in a suitable amount, and mixed up using a high speed mixer at 1000 rpm for one minute is referred to as preliminary treatment C2. Table 12 shows the types and amounts of surface treatment agents added in the preliminary treatment C2. The symbols set forth in the column for the surface treatment agents in Table 12 are the same as those shown in Table 14. Then, 0.1% by weight lithium stearate and 0.2% by weight of at least one of thermoplastic resin, thermoplastic elastomer and a compound having a layered crystal structure were added in each case and mixed up homogeneously, after which the mixture was discharged from the mixer (Practical examples 54-56). In this case, the names of the added materials and the amounts are shown in Table 12. The symbols set forth in the column for thermoplastic resin, thermoplastic elastomer or compounds having a layered crystal structure shown in Table 12 are the same as those shown in Table 15. 100 g of each mixed powder of Table 12 were separately discharged through an orifice having an emission hole of 5 mm in diameter, and the discharge time was measured at the respective temperatures from 293K to 413K. Further, each mixed powders was heated to 423K to form a tablet 11 mm in diameter at a pressure of 686 MPa, and the ejection force and the green compact density at the time of compaction were measured. The results are shown in Table 12. As is apparent from a comparison of comparative example 6 with practical examples 54-56, the flowability of the mixed powders at the respective temperatures was dramatically improved in the case where treatment was practiced with the surface treatment agents. Further, as apparent from a comparison of comparative example 6 with practical examples 54-56, the green compact density was improved, and the ejection force was decreased. Thus the compactibility was improved in the case where thermoplastic resin, thermoplastic elastomer or a compound having a layered crystal structure was added and in addition treatment was practiced with the surface treatment agents. (Embodiment 13) 0.2% by weight stearamide and 0.2% by weight ethylenebis (stearamide) were added to mixtures of partially alloyed steel powder for powder metallurgy having a mean particle diameter of 80 µm, and native graphite having a mean particle diameter of 23 µm or less as indicated in Tables 13-1 and 13-2, and mixed and heated at 433K. Thereafter the mixtures were cooled to about 383K. After this, various types of organoalkoxysilane, organosilazane, a coupling agent, silicone oil or mineral oil were sprayed on the mixture in a suitable amount as indicated in Tables 13-1 and 13-2, and mixed up with a high speed mixer at 1000 rpm for one minute. Thereafter, the mixtures were cooled below 358K. The above-mentioned process where various types of organoalkoxysilane, organosilazane, a coupling agent, silicone oil or mineral oil were sprayed on the mixture in a suitable amount, and mixed up using a high speed mixer at 1000 rpm for one minute is referred to as preliminary treatment C2. Tables 13-1 and 13-2 show the types and loadings of the surface treatment agents loaded in the preliminary treatment C2. The symbols set forth in the column for the surface treatment agents in Tables 13-1 and 13-2 are the same as those shown in Table 14. Then, 0.1% by weight lithium stearate and 0.2% by weight at least one of thermoplastic resin, thermoplastic elastomer and a compound having a layered crystal structure were added in each case and mixed up homogeneously, after which the mixture was discharged from the mixer (Practical examples 57-60). In this case, the names of the added materials and their amounts are shown in Tables 13-1 and 13-2. The symbols set forth in the column for thermoplastic resin, thermoplastic elastomer or compounds having a layered crystal structure shown in Tables 13-1 and 13-2 are the same as those shown in Table 15. 100 g of each mixed powder of Tables 13-1 and 13-2 were separately discharged through an orifice having an emission hole of 5 mm in diameter, and the discharge time was measured at the respective temperatures from 293K to 413K. Further, each mixed powder was heated to 423K to form a tablet 11 mm in diameter using a pressure of 686 MPa, and the ejection force and the green compact density at the time of compaction were measured. The results are shown in Tables 13-1 and 13-2. As is apparent from a comparison of comparative example 6 with practical examples 57-60, the flowability of the mixed powders at the respective temperatures was dramatically improved in the case where treatment is carried out with the surface treatment agents. Further, as is apparent from a comparison of comparative example 6 with practical examples 57-60, the green compact density is improved and the ejection force is decreased. Thus the compactibility has been improved in the case where thermoplastic resin, thermoplastic elastomer or a compound having a layered crystal structure was added and in addition treatment was practiced with the surface treatment agents. Industrial Applicability The present invention is suitably applicable to iron-based powder compositions for powder metallurgy in which lubricant, graphite powder, copper powder and the like are added and mixed. The iron-based powder composition for powder metallurgy in normal handling undergoes little segregation and dust generation and has stable flowability and excellent compactibility in a wide temperature range over the order of room temperature to 473K, and particularly, has excellent warm compactibility properties.

Claims: An iron-based powder composition for powder metallurgy having excellent flowability and compactibility even when subjected to warm compaction and comprising an iron-based powder, an alloying powder, a lubricant and a binder characterised in that at least the iron-based powder is coated with a surface treatment agent consisting of at least one of organoalkoxysilane, organosilazane, titanate-containing coupling agent or fluorine-containing silicon silane coupling agent, so as to reduce the frictional resistance and adhesion between the iron-based powder particles and between the iron-based powder particles and other powder particles at up to 473 K. An iron-based powder composition for powder metallurgyaccording to claim 1, wherein the surface treatment agent is an organoalkoxysilane or an organosilazane. An iron-based powder composition for powder metallurgy according to claim 2, wherein an organic group of said organoalkoxysilane is substituted with an acrylic group, an epoxy group or an amino group. An iron based powder composition for powder metallurgy according to claim 2, wherein said organosilazane is a polyorganosilazane. An iron-based powder composition for powder metallurgy according to claim 1, wherein said surface treatment agent is a titanate-containing coupling agent or a fluorine-containing silicon silane coupling agent. An iron-based composition for powder metallurgy according to any preceding claim, wherein said lubricant is an inorganic or organic compound having a layered crystal structure. An iron-based powder composition for powder metallurgy according to claim 6, wherein said inorganic compound having a layered crystal structure is graphite, carbon fluoride or MoS 2 . An iron-based powder composition for powder metallurgy according to claim 6, wherein said organic compound having a layered crystal structure is a melamine-cyanuric acid adduct or N-alkylasparatic acid-β-alkylester. An iron-based powder composition for powder metallurgy according to any one of claims 1 to 5, wherein said lubricant is a thermoplastic resin. An iron-based powder composition for powder metallurgy according to claim 9, wherein said thermoplastic resin is polystyrene, nylon, polyethylene or a fluorine-containing resin, each having a particle diameter of 30 µm or less. An iron-based powder composition for powder metallurgy according to any one of claim 1 to 5 wherein said lubricant is a thermoplastic elastomer having a particle diameter of 30 µm or less. An iron-based powder composition for powder metallurgy according to claim 11, wherein said thermoplastic elastomer is a styrene block copolymer (SBC), a thermoplastic elastomer olefin (TEO), a thermoplastic elastomer polyamide (TPAE) or a silicone elastomer. An iron-based powder composition for powder metallurgy according to any one of claims 1 to 5, wherein said lubricant is a metallic soap having a melting point of not less than 423 K. An iron-based powder composition for powder metallurgy according to any preceding claim wherein said binder is a fatty acid amide. An iron-based powder composition for powder metallurgy according to claim 14, wherein said fatty acid amide is a fatty acid monoamide and/or a fatty acid bisamide. A method of producing an iron-based powder composition for powder metallurgy having excellent flowability and compactibility, which method comprises the steps of: (a) subjecting at least an iron-basedpowder and an alloying powder to a mixing at room temperature with a fatty acid amide binder and at least one lubricant, wherein the lubricant has a melting point higher than that of the fatty acid amide and is selected from the group comprising a thermoplastic resin, a thermoplastic elastomer, and an inorganic or organic compound having a layered crystal structure; (b) heating and stirring up the mixture resulting from the mixing in step (a) at a temperature above the melting point of the fatty acid amide to melt the fatty acid amide; (c) coolingwhilst mixing, the mixture subjected to the above heating and stirring process so that the alloying powder and the lubricant adhere to the surface of the iron-based powder by the adhesive force of the melt; and (d) adding at the time of cooling, for a next mixing, a metal soap and at least one lubricant selected from the group comprising a thermoplastic resin powder, a thermoplastic elastomer powder, an inorganic or organic compound having a layered crystal structure, wherein at least the surface of the iron-based powder in step (c) is coated with a surface treatment agent consisting of at least one of organoalkoxysilane, organosilazane, titanate containing coupling agent or fluorine-containing silicon silane coupling agent, so as to reduce the frictional resistance between the iron-based powder particles and between the iron-based powder particles and other powder particles even when the composition is subjected to warm compaction up to about 473 K, by any one of the following steps: (1) coating at least the iron-based powder and optionally the alloying powder with the surface treatment agent before step (a), (2) subjecting the iron-based powder and the alloying powder to a mixing at room temperature with the surface treatment agent before step (a) to cause the surface treatment agent to combine with the iron-based and alloying particles during step (b), and (3) adding and mixing the surface treating agent at a temperature of not less than 373 K and not more than the melting point of the fatty acid amide during cooling in step (c). A method according to claim 16 wherein the coating of the surface agent in step (1) is conducted by mixing the surface agent with the iron-based powder and optionally the alloying powder by a high speed mixer at 1000 rpm for one minute. A method according to claim 16 wherein the mixing step in step (2) or (3) is conducted by a high speed mixer at 1000 rpm for one minute.

Inventors: OGURA KUNIAKI, OZAKI YUKIKO, UESOSONO SATOSHI, OGURA, KUNIAKI, OZAKI, YUKIKO, UESOSONO, SATOSHI
Applicants: JFE STEEL CORP, JFE STEEL CORPORATION
Application year: 1997
Main classification: B22F1
IPC codes: C22C33, B22F1