LUBRICATING AGENT FOR MOLD AT ELEVATED TEMPERATURE AND METHOD FOR PRODUCING HIGH DENSITY IRON-BASED SINTERED COMPACT

Abstract: A process for producing a high-density iron-based green compact is provided which can form a green compact with a high density. Also provided is a process for producing a sintered compact from the green compact. A specified combination lubricant is applied to the surface of a die for pressure compaction by electrical charging, which lubricant is composed of a lubricant having a higher melting point than a preset compaction temperature, and a lubricant having a lower melting point than the compaction temperature. A heated iron-based powder mixture is filled into the die, followed by pressure compaction, whereby a green compact is formed. The green compact can be sintered to provide a sintered compact. The powder mixture comprises an iron-based powder, a powder compaction lubricant and a graphite powder, wherein the powder compaction lubricant comprises a lubricant having a lower melting point than the compaction temperature and in a content from 10 to 75% by mass, and a lubricant having a higher melting point than the compaction temperature and as the balance, and the content of the graphite powder is less than 0.5% by mass based on the total amount of the iron-based powder mixture.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lubricated die comprising a die lubricant as defined in the preamble of claim 1 and to a process for the production of a high-density iron-based green compact as defined in the preamble of claim 5. A die and a method according to the preamble of claims 1 and 5, respectively, are known from US-A-5 682 591. 2. Description of the Related Art In general, a powdered iron-based green compact for powder metallurgy is produced by filling an iron-based powder mixture into a die. The powder mixture is generally derived by mixing an iron-based powder with alloying powders such as copper powder, graphite powder and the like and further with lubricants such as zinc stearate, lead stearate and the like, and then by subjecting the iron-based powder mixture to. The resultant green compact usually has a density in the range from 6.6 to 7.1 Mg/m 3 . Such a green compact is further sintered to obtain a sintered compact which, where desired, is sized or cut into a powder metallurgical product. Where great strength is required, carburizing heat treatment or brightening heat treatment is in some instances performed after completion of the sintering. The above powder metallurgy permits components parts of complicated shapes to be formed with high dimensional accuracy and in near net structure, significantly saving the cost of cutting work as contrasted to conventional production methods. With regard to powder metallurgical iron products, a keen demand has recently been made for more higher dimensional accuracy to omit cutting work and to save production cost, and also for more greater strength to make components parts small in size and light in weight. In order to give greater strength to a powder metallurgical product (a sintered compact), it is beneficial to form high-density sintered compacts from an iron-based green compact which has been produced to have a high density. As the density of a sintered compact increases, the number of voids in the compact decreases so that the component part is obtainable with improved mechanical properties such as tensile strength, impact value, fatigue strength and the like. As warm compaction techniques evolved to form a high-density iron-based green compact, there have been proposed a double molding-double sintering method in which an iron-based powder mixture is pressed and sintered in the usual manner, followed by repeated pressing and sintering, and a sinter forging method in which single pressing and single sintering are performed, followed by hot forging. Moreover, warm compaction techniques are known in which metal powder is compacteds with heat as disclosed for instance in Japanese Unexamined Patent Application Publication No. 2-156002, Japanese Examined Patent Application Publication No. 7-103404, U.S. Patent No. 5,256,185 and U.S. Patent No. 5,368,630. Such a warm compaction techniques are designed to melt and disperse a lubricant partly or wholly between the metallic particles, thereby reducing the frictional resistance between the metallic particles and the frictional resistance between the green compact and an associated die, so that improved compressibility is attained. The compaction technique noted here is thought to be most advantageous in view of possible cost savings amongst the methods previously mentioned for the production of high-density iron-based green compacts. A green compact of about 7.30 mg/m 3 in density can be obtained by the above compaction technique when an iron-based powder mixture is compacted at a pressure of 7 t/cm 2 and at a temperature of 150 C, which powder mixture is derived by mixing a partially alloyed iron powder of a Fe-4Ni- 0.5Mo-1.5Cu with 0.5% by mass of graphite and 0.6% by mass of lubricant. However, according to the warm compaction techniques of the above-cited publications, i.e., Japanese Unexamined Patent Application Publication No. 2-156002, Japanese Examined Patent Application Publication No. 7-103404, U.S. Patent No. 5,256,185 and U.S. Patent No. 5,368,630, the problem arises that an iron-based powder mixture is less fluid and hence less productive, the resultant green compact is irregular in respect of densities, and the resultant sintered compact is unfavorably variable in respect of physical properties. Another drawback is that a high force must be applied to draw the green compact from the corresponding mold with consequent marred surface of the product and shortened lifetime of the die. In these warm compaction techniques, a lubricant is also contained in an iron-based powder mixture so as to reduce the resistance between the metallic particles and the resistance between the green compact and the associated mold, thereby providing improved conpressibility. During warm compaction, the lubricant is partly or wholly melted and then pushed to locate adjacent to the surface of the green compact. Upon subsequent sintering, the lubricant gets thermally decomposed or volatilized and hence escapes from the green compact, leaving coarse voids near to the surface of the sintered compact. This poses the problem that the sintered compact results in insufficient mechanical strength. To cope with this problem, Japanese Unexamined Patent Application Publication No. 8-100203 discloses that when room temperature compaction or warm compaction is effected, the content of a lubricant to be incorporated in an iron-based powder mixture is decreased by coating the surface of a die with an electrical charged lubricant powder such that a high-density green compact is produced. In this technique, however, the coating lubricant is susceptible to morphological changes at around its melting point since it is of a single nature so that the lubricating action is largely variable. This has the drawback that the compaction temperature range depends restrictedly upon the melting point of the coating lubricant. Also defectively, even if the content of the lubricant in the powder mixture can be decreased with the coating lubricant applied on to the old surface, the content of the former lubricant may be too low which is dependent upon the lubricant components to be incorporated in the powder mixture. In this instance, the former lubricant does not exhibit lubrication, failing to enhance the density of a pressurized powder. US-A-5 682 591 discloses a lubricant for powder metallurgy to be applied to the die walls by electrostatically charging. The compaction of the metal powder may be conducted with any known process including warm and cold pressing. This prior art document also discloses that all lubricants may be used as a single component lubricant or as a mixture of lubricants thus resulting in a mixture of lubricants having different melting points. As examples of lubricants, the prior art document discloses metal stearates, ethylene-bis stearamide, polyolefin- and polyethylene-based fatty acids, soaps, molybdenium sulfite, natural and synthetic waxes and the like. From the viewpoints of great strength and cost saving of automotive parts, there has been a need for the development of a process capable of producing an iron-based green compact with a higher density but by single compaction. SUMMARY OF THE INVENTION In order to eliminate the foregoing problems of the conventional art, a first object of the present invention is to provide a lubricated die capable of producing a high-density iron-based green compact which permits a high-density green compact to be formed with a density of 7.4 Mg/m 3 or above and by single pressing when warm pressure compaction is effected as to an iron-based powder mixture derived by mixing a partially alloyed iron powder of, for example, a Fe-4Ni-0.5Mo-1.5Cu composition, with 0.5% by mass of a graphite powder. A second object of the invention is to provide a process for producing a high-density iron-based sintered compact which permits a high-density sintered compact to be formed by sintering such an iron-based green compact. To achieve the above objects by utilizing a warm compaction technique and a die lubrication compaction technique, the present inventors have conducted intensive researches on various lubricants for mold lubrication and various formulations of iron-based powder mixtures containing lubricants. It has now been found that the force for drawing an iron-based green compact from the corresponding mold can be effectively lessened by the use of a certain specific combination lubricant as a lubricant for mold lubrication. This combination lubricant is composed in a suitable ratio of a lubricant having a lower melting point than a preset compaction temperature and a lubricant having a higher melting point than the compaction temperature and can be applied to the surface of a preheated die by electrical charging. The present invention has been made on the bass of the aforesaid finding and further supporting studies. More specifically, according to the first aspect of the present invention, there is provided a lubricated die as defined in claim 1. Preferred embodiments of the inventive die are set out in the claims 2 to 4. According to another aspect of the present invention, there is provided a process as defined in claim 5. Preferred embodiments of the inventive process are defined in claims 6 through 10. The above and other objects, features and advantages of the present invention will become manifest upon reading of the following detailed description. DESCRIPTION OF THE PREFERRED EMBODIMENTS In the practice of the present invention, a heated iron-based powder mixture is filled in a die and then molded by pressure compaction at a preset compaction temperature, whereby an iron-green compact is obtained. In the invention, a die to be used is preheated at a suitable temperature. The preheating temperature is not particularly restricted so long as an iron-based powder mixture can be maintained at a preset compaction temperature. The preheating temperature is set to be preferably higher than the compaction temperature by 20 to 60 C. An electrically charged lubricant for mold lubrication is introduced into a preheated die to apply the lubricant to the surface of the mold by electrical charging. Desirably, the lubricant (solid powder) for mold lubrication is placed in a die lubricating system (for example, Die Wall Lubricant System manufactured by Gasbarre Co.) where electrical charging is performed by means of contact charging between the solid lubricant particles and the inner wall of the system. The electrically charged lubricant is jetted into the mold and applied to the mold surface by electrical charging. The amount of the lubricant to be applied to the mold surface by electrical charging is set preferably in the range from 5 to 100 g/m 2 . Amounts less than 5 g/m 2 result in insufficient lubricating action, calling for a high force to draw the resultant green compact from the mold. Amounts more than 100 g/m 2 cause the lubricant to remain on the product surface, making the product unsightly in appearance. The die lubricant for warm compaction with die lubrication is used in electrically charged relation to the surface of a preheated die when a powdered material is compacted by pressure compaction. This lubricant is a mixture of a lubricant having a higher melting point than a preset compaction temperature and in a content from 0.5 to 80% by mass, and a lubricant having a lower melting point than the compaction temperature and as the balance. The preset compaction temperature used herein denotes a temperature as measured on the mold surface at the time pressure compaction is carried out. The higher-melting lubricant is present in a solid state in the die lubricant for warm compaction with die lubrication at the time compaction is effected, and it behaves like a solid lubricant that acts as "a roller" within a die, consequently lessening the force for drawing a green compact from the mold. Moreover, such higher-melting lubricant has a role to prevent a completely or partially molten lubricant (a lower-melting lubricant to be described later) from getting migrated within the mold, decreasing the frictional resistance between the green compact and the mold surface so that the force for product drawing is prevented from being unfavorably increased. If the content of the higher-melting lubricant is less than 0.5% by mass, the lower-melting lubricant becomes relatively abundant. This causes a large amount of a molten lubricant that migrates within a die and does not distribute uniformly on the surface of the mold, increasing the frictional resistance between the green compact and the mold surface and hence failing to lessen the force for product drawing to a sufficient extent. Conversely, if the content of the higher-melting lubricant is more than 80% by mass, a lubricant not subject to melting in a die is too large in amount for uniform distribution on the die surface. This is responsible for diminished mold lubrication and hence for increased force for product drawing. Hence, the content of the higher-melting lubricant present in the die lubricant for warm compaction with die lubrication should be within the range from 0.5 to 80% by mass. The lubricant for mold lubrication contains, in addition to the above-specified higher-melting lubricant, a lubricant having a lower melting point than the preset compaction temperature. This lower-melting lubricant melts completely or partially at the compaction temperature and presents a grease-like state on the surface of a die, exerting a beneficial effect on lessening the force for drawing a green compact from the mold. The higher-melting lubricant is preferably at least one selected from the group consisting of metallic soap, thermoplastic resin, thermoplastic elastomer, and an organic or inorganic lubricant having a lamellar crystal structure. Suitable examples are chosen from the following lubricants depending upon the compaction temperature used. As the metallic soap, zinc stearate, lithium stearate, lithium hydroxystearate or the like is preferred. As the thermoplastic resin, polystyrene, polyamide, fluorine resin or the like is preferred. As the thermoplastic elastomer, polystyrene elastomer, polyamide elastomer or the like is preferred. The inorganic lubricant of a lamellar crystal structure is graphite, MoS 2 or carbon fluoride, and finer particle sizes are more effective in lessening the force for product drawing. The organic lubricant of a lamellar crystal structure is melamine-cyanuric acid adduct (MCA) or N-alkyl aspartate- -alkyl ester. Meanwhile, the lower-melting lubricant is desired to be a lubricant that melts completely or partially at the compaction temperature and tends to applied to the surface of a die at a low melting point by electrical charging. This lower-melting lubricant is preferably at least one selected from the group consisting of metallic soap, amide wax, polyethylene, and an eutectic mixture of at least two members thereof. Suitable examples are chosen from the following lubricants depending upon the compaction temperature used. As the metallic soap, zinc stearate or calcium stearate is preferred. As the amide wax, ethylene bis-stearoamide, monoamide stearate or the like is preferred. As the eutectic mixture, ethylene bis-stearoamide-polyethylene eutectic, ethylene bis-stearoamide-zinc stearate eutectic, ethylene bis-stearoamide-calcium stearate eutectic is preferred. Subsequently, a heated iron-based powder mixture is charged into a die electrically charged with a lubricant for mold lubrication, followed by pressure compaction, whereby a green compact is obtained. The iron-based powder mixture is heated preferably at from 70 to 200 °C. Lower temperatures than 70 °C result in an iron powder having increased yield stress and hence lead to a green compact having decreased density. Inversely, higher temperatures than 200°C show no appreciable rise in density, arousing a fear of an iron powder getting oxidized. Thus, the temperature at which the iron-based powder mixture is heated should be set within the range from 70 to 200 °C. The iron-based powder mixture is derived by mixing an iron-based powder with a lubricant (a powder compaction lubricant) or an alloying powder. The method of mixing the iron-based powder with the compaction lubricant or the alloying powder is not particularly restrictive, but any known method is suitably useful. In the case where the iron-based powder is mixed with the alloying powder, it is desired that after completion of primary mixing in which the iron-based powder and alloying powder are mixed with a part of the powder compaction lubricant, secondary mixing be performed in which the resultant mixture is stirred with heat at a higher temperature than the melting point of at least one of the aforesaid lubricant in order to melt the one lubricant, and the mixture having been melted is cooled with stirring to thereby apply the one lubricant to the surface of the iron-based powder mixture so that the alloying powder is bonded, followed by mixing of the balance of the powder compaction lubricant. The iron-based powder that comes to use in the present invention is selected from among pure iron powders such as an atomized iron powder, a reduced iron powder or the like, a partially diffusively alloyed steel powder, a completely alloyed steel powder, and a mixed powder thereof. The content of the powder compaction lubricant in the iron-based powder mixture is set preferably in the range from 0.05 to 0.40% by mass based on the total amount of the iron-based powder mixture. Contents less than 0.05% by mass make the resultant iron-based powder mixture less fluid and fail to apply the lubricant uniformly to the surface of a die, producing a green compact having decreased density. Conversely, contents more than 0.40% by mass suffer high voiding after sintering and give a sintered compact having decreased density. The powder compaction lubricant contained in the iron-based powder mixture is a mixed lubricant obtained by mixing a lubricant having a lower melting point than the preset compaction temperature and a lubricant having a higher melting point than the compaction temperature. The content of the lower-melting lubricant in the powder compaction lubricant is in the range from 10 to 75% by mass, whereas the content of the higher-melting lubricant is in the range from 25 to 90% by mass as the balance. The lower-melting lubricant is effective in that it melts during pressure compaction, penetrates in between the iron-based particles by capillary action, disperses uniformly in the particles, reduces particle-to-particle contact resistance and facilitates reorientation of iron-based particles, thus accelerating the enhancement of product density. If the content of the lower-melting lubricant is less than 10% by mass, such lubricant fails to disperse uniformly in the iron-based particles and suffers poor density of a green compact. If the content of the lower-melting lubricant is more than 75% by mass, a molten lubricant is squeezed toward the surface of a die as the density of a green compact is increased so that passages are provided on the product surface for the molten lubricant to escape out of the product. The passages cause many coarse voids on the product surface, giving insufficient strength to the resultant sintered compact. The higher-melting lubricant contained in the iron-based powder mixture is present in a solid state at the time compaction is effected. This lubricant acts as "a roller" on the surface protrusions of iron-based particles where it repels a molten lubricant, promoting particle reorientation and enhancing product density. The higher-melting lubricant contained in the powder compaction lubricant for the iron-based powder mixture is preferably at least one selected from the group consisting of metallic soap, thermoplastic resin, thermoplastic elastomer, and an organic or inorganic lubricant having a lamellar crystal structure. Suitable examples are chosen from the following lubricants depending upon the compaction temperature used. As the metallic soap, zinc stearate, lithium stearate, lithium hydroxystearate or the like is preferred. As the thermoplastic resin, polystyrene, polyamide, fluorine resin or the like is preferred. As the thermoplastic elastomer, polyethylene elastomer, polyamide elastomer or the like is preferred. As the inorganic lubricant of a lamellar crystal structure, graphite, MoS 2 or carbon fluoride is preferred, and finer particle sizes are more effective for lessening the force for drawing a green compact from a die. As the organic lubricant of a lamellar crystal structure, melamine-cyanuric acid adduct (MCA) or N-alkyl aspartate- -alkyl ester is preferred. The lower-melting lubricant contained in the powder compaction lubricant for the iron-based powder mixture is preferably at least one selected from the group consisting of metallic soap, amide wax, polyethylene, and an eutectic mixture of at least two members thereof. Suitable examples are chosen from the following lubricants depending upon the compaction temperature used. As the metallic soap, zinc stearate, calcium stearate or the like is preferred. As the amide wax, ethylene bis-stearoamide, monoamide stearate or the like is preferred. As the eutectic mixture, ethylene bis-stearoamide-polyethylene eutectic, ethylene bis-stearoamide-zinc stearate eutectic, ethylene bis-stearoamide-calcium stearate eutectic or the like is preferred. Though dependent upon the compaction temperature used, some of these lower-melting lubricants may be utilized as higher-melting lubricants. Graphite can be used as an alloying powder in the iron-based powder mixture. This graphite powder is effective to reinforce a sintered compact to be produced, but too high a content is liable to decrease product density largely. Hence, the content of graphite should be set to be less than 0.5% by mass based on the total amount of the iron-based powder mixture. In the present invention, the high-density iron-based green compact formed by the above-specified production process can be further sintered, whereby a high-density iron-based sintered compact is obtained. Here, any conventional sintering method may be suitably used without limitation. Sinter hardening may also be used by which rapid cooling is effected after sintering to enhance product strength. The present invention may be more fully understood with reference to the following examples. Example 1 A partially alloyed steel powder of a Fe-4Ni-0.5Mo-1.5Cu composition derived by diffusively bonding Ni, Mo and Cu to a pure atomized iron powder was used as an iron-based powder. Iron-based powder mixtures were prepared by mixing this alloyed steel powder with 0.5% by mass of a graphite powder and various lubricants shown in Table 1. The mixing was effected with heat and by use of a high-speed mixer. First, a die for compacting was preheated at each of the temperatures listed in Table 1. A die lubricant for warm compaction with die electrically charged by a die lubricating system (manufactured by Gasbarre Co.) was jetted into the die and applied to the mold die surface by means of electrical charging. The die lubricant for was prepared by choosing a lower-melting lubricant and a higher-melting lubricant from among the lubricants shown in Table 2, and then by formulating both lubricants as shown in Table 1. The temperature measured on the mold surface was taken as a pressure compaction temperature. Subsequently, the mold thus treated was filled with a heated iron-based powder mixture, followed by pressure compaction, whereby a rectangular green compact with a size of 10 × 10 × 55 mm was produced. The pressure loading was 686 MPa, and other pressure compaction conditions were as listed in Table 1. A powder compaction lubricant contained in the iron-based powder mixture was prepared by choosing a lower-melting lubricant and a higher-melting lubricant from among the lubricants listed in Table 2, and then by formulating both lubricants as shown in Table 1. As a conventional example, a similar rectangular green compact (Green compact No. 38) was formed in the same manner as in Example 1 except that a die was not coated with a die lubricant for warm compaction with die. After completion of the compaction, the force was measured which was required for the green compact to be drawn from the mold. With regard to each green compact thus formed, the density was determined by Archimedes' principle. The principle noted here denotes a method by which the density of a test specimen, each green compact in this case, is determined by measuring the volume of the product after immersion in ethyl alcohol. Additionally, visual inspection was made of the appearance of the green compact to find faults such as marring, breakage and the like. The green compact was centrally cut, embedded in resin and then abraded, followed by examination of voiding in section on a light microscope. The drawing force, density, appearance and sectional structure of the green compact are tabulated in Table 1. All the green compacts representing this invention show as low a drawing force after compaction as 20 MPa or below and as high a density as 7.4 Mg/m 3 or above. Furthermore, these products are free of surface oxidation due to heating as well as faults such as marring, breakage and the like. The sectional structures are normal with the absence of coarse voids. The comparative and conventional examples that fall outside the scope of the invention revealed a high drawing force exceeding 20 MPa, a low density of less than 7.35 Mg/m 3 , or coarse voids near to the sectional surface of the green compact. Advantageously, the present invention can form a high-density green compact which exhibits superior appearance and sectional structure and low drawing force. Example 2 The following six different powders were used as iron-based powders; namely (1) a partially alloyed steel powder a of a Fe-4Ni-0.5Mo-1.5Cu composition derived by diffusively bonding Ni, Mo and Cu to a pure atomized iron powder, (2) a partially alloyed steel powder b of a Fe-2Ni-1Mo composition derived by diffusively bonding Ni and Mo to a pure atomized iron powder, (3) a prealloyed steel powder c of a Fe-3Cr-0.3Mo-0.3V composition derived by prealloying Cr, Mo and V, (4) a prealloyed steel powder d of a Fe-1Cr-0.3Mo-0.3V composition derived by prealloying Cr, Mo and V, (5) an atomized iron powder e , and (6) a reduced iron powder f . The atomized iron powder denotes an iron-based powder resulting from atomization of molten steel with high-pressure water, and the reduced iron powder denotes an iron-based powder resulting from reduction of iron oxide. The partially alloyed steel powder a , partially alloyed steel powder b , prealloyed steel powder c , prealloyed steel powder d , atomized iron powder e and reduced iron powder f were each mixed with graphite in the contents shown in Table 3 and with the lubricants shown in Table 3, whereby iron-based powder mixtures were prepared. The mixing was effected with heat and by use of a high-speed mixer. In case of the atomized iron powder e and reduced iron powder f , 0.8% by mass of graphite and 2.0% by mass of a Cu powder were mixed. The content of graphite is by a mass ratio relative to the total amount of iron-based powder and graphite, or of iron-based powder, graphite and alloy powder. First, a pressure compaction die was preheated at each of the temperatures listed in Table 3. A die lubricant for warm compaction with die electrically charged by a die lubricating system (manufactured by Gasbarre Co.) was jetted into the mold and applied to the mold surface by means of electrical charging. The die lubricant for warm compaction with die lubrication was prepared by choosing a lower-melting lubricant and a higher-melting lubricant from among the lubricants shown in Table 2, and then by formulating both lubricants as shown in Table 3. The temperature measured on the mold surface was taken as a pressure compaction temperature. Secondly, the mold thus treated was filled with a heated iron-based powder mixture, followed by pressure compaction, whereby a rectangular green compact with a size of 10 × 10 × 55 mm was produced. The pressure loading was 686 MPa, and other pressure compaction conditions were as listed in Table 3. A powder compaction lubricant contained in the iron-based powder mixture was prepared by choosing a lower-melting lubricant and a higher-melting lubricant from among the lubricants listed in Table 2, and then by formulating both lubricants as shown in Table 3. With regard to each iron-based green compact thus obtained, the density was determined by Archimedes' principle as in Example 1. Subsequently, the iron-based green compact was sintered in a N 2 -10%H 2 atmosphere and at 1,130 C for 20 minutes, whereby an iron-based sintered compact was formed. The density of the sintered compact was determined by Archimedes' principle. This product was then machined to obtain a sample in the shape of a small round rod dimensioned to be 5 mm in parallel plane diameter and 15 mm in length. The sample used to measure tensile strength. Similar rectangular green compacts were formed in the same manner as in Example 2 except that a die was not coated with a die lubricant for warm compaction with die. Each green compact was further sintered as in Example 2 to form an iron-based sintered compact which was taken as a conventional example. The test results are tabulated in Table 3. The present invention provides high density and great tensile strength in contrast to the conventional examples (Sintered compacts Nos. 2 to 12). Example 3 A partially alloyed steel powder of a Fe-4Ni-0.5Mo-1.5Cu composition derived by diffusively bonding Ni, Mo and Cu to a pure atomized iron powder was used as an iron-based powder. Iron-based powder mixtures were prepared by mixing this alloyed steel powder with 0.2% by mass of a graphite powder and various lubricants shown in Table 3. The mixing was effected with heat and by use of a high-speed mixer. First, a pressure compaction die was preheated at each of the temperatures listed in Table 4. A die lubricant for warm compaction with die electrically charged by a die lubricating system (manufactured by Gasbarre Co.) was jetted into the die and applied to the die surface by means of electrical charging. The die lubricant for warm compaction with die lubrication was prepared by choosing a lower-melting lubricant and a higher-melting lubricant from among the lubricants shown in Table 2, and then by formulating both lubricants as shown in Table 4. The temperature measured on the die surface was taken as a pressure compaction temperature. Subsequently, the mold thus treated was filled with a heated iron-based powder mixture, followed by pressure compaction, whereby a rectangular green compact with a size of 10 × 10 × 55 mm was produced. The pressure loading was 686 MPa, and other pressure compaction conditions were as listed in Table 4. A powder compaction lubricant contained in the iron-based powder mixture was prepared by choosing a lower-melting lubricant and a higher-melting lubricant from among the lubricants listed in Table 2, and then by formulating both lubricants as shown in Table 4. As a conventional example, a similar rectangular green compact (Green compact No. 38) was formed in the same manner as in Example 4 except that a die was not coated with a die lubricant for warm compaction with die. After completion of the compaction, the ejection force was measured. With regard to each of the resultant green compacts, the density was determined by Archimedes' principle. Visual inspection was then made of the appearance of the green compact to find faults such as marring, breakage and the like. The green compact was centrally cut, embedded in resin and then abraded, followed by examination of voiding in section on a light microscope. The drawing force, density, appearance and sectional structure of the green compact are tabulated in Table 4. All the green compacts according to this invention show as low a drawing force after compaction as 20 MPa or below and as high a density as 7.43 Mg/m 3 or above. In addition, each such product causes neither surface oxidation resulting from heating nor faults such as marring, breakage and the like. The sectional structure is normal with the absence of coarse voids. The comparative and conventional examples that depart from the scope of the invention suffered a high drawing force exceeding 20 MPa, a low density of less than 7.39 Mg/m 3 , or coarse voids near to the sectional surface of the green compact. The present invention is highly advantageous in that a high-density green compact is obtainable with superior appearance and sectional structure as well as low drawing force. Example 4 The following two different powders were used as iron-based powders; namely (1) a partially alloyed steel powder a of a Fe-4Ni-0.5Mo-1.5Cu composition derived by diffusively bonding Ni, Mo and Cu to a pure atomized iron powder, and (2) a prealloyed steel powder b of a Fe-3Cr-0.3Mo-0.3V composition derived by prealloying Cr, Mo and V. The partially alloyed steel powder a , and prealloyed steel powder b were mixed with graphite in the contents shown in Table 5 and the lubricants shown in Table 5, whereby iron-based powder mixtures were prepared. The mixing was effected with heat and by use of a high-speed mixer. The content of graphite is by a mass ratio relative to the total amount of the iron-based powder mixture. First, a die was preheated at each of the temperatures listed in Table 5. A die lubricant for warm compaction with die electrically charged by a die lubricating system (manufactured by Gasbarre Co.) was jetted into the die and applied to the die surface by means of electrical charging. The die lubricant for warm compaction with die lubrication was prepared by choosing a lower-melting lubricant and a higher-melting lubricant from among the lubricants shown in Table 2, and then by formulating both lubricants as shown in Table 5. The temperature measured on the mold surface was taken as a pressure compaction temperature. Secondly, the die thus treated was filled with a heated iron-based powder mixture, followed by pressure compaction, whereby a rectangular green compact with a size of 10 x 10 x 55 mm was produced. The pressure loading was 686 MPa, and other pressure compaction conditions were as listed in Table 5. A powder compaction lubricant contained in the iron-based powder mixture was prepared by choosing a lower-melting lubricant and a higher-melting lubricant from among the lubricants listed in Table 2, and then by formulating both lubricants a lubricants as shown in Table 5. With regard to each iron-based green compact thus obtained, the density was determined by Archimedes' principle as in Example 1. Subsequently, the iron-based green compact was sintered in a N 2 -10%H 2 atmosphere and at 1,130 C for 20 minutes, whereby an iron-based sintered compact was formed. The density of the resultant sintered compact was determined by Archimedes' principle. The test results are tabulated in Table 5. The examples of the invention provides high densities. As stated above, the present invention is significantly advantageous in that a high-density green compact can be produced with superior appearance and sectional structure and by single compaction. Drawing of the product from the associated mold is possible at a low force with a prolonged lifetime of the die. Also notably, a high-density sintered compact is easy to produce.

Claims: A lubricated die comprising a die lubricant for warm compaction with die lubrication, comprising a mixture of lubricants, said die lubricant for warm compaction with die lubrication being applied to the surface of a die at warm compaction temperature by means of electrical charging, characterized in that said mixture comprises a lubricant in a solid state and a lubricant in a molten state. The lubricated die according to claim 1, characterized in that said lubricant in a solid state is in a content from 0.5 to 80% by mass, and said lubricant in a molten state is contained as the balance. The lubricated die according to claim 1 or 2, wherein the solid state lubricant is at least one selected from the group consisting of metallic soap, thermoplastic resin, thermoplastic elastomer, and an organic or inorganic lubricant having a lamellar crystal structure. The lubricated die according to claim 1 or 2, wherein the molten state lubricant is at least one selected from the group consisting of metallic soap, amide wax, polyethylene, and an eutectic mixture of at least two members thereof. A process for the production of a high-density iron-based green compact, comprising the steps of: preheating a die at a selected temperature; applying a die lubricant for warm compaction with die lubrication to a surface of the die by means of electrical charging; filling a heated iron-based powder mixture in the die; and then subjecting the powder mixture to pressure compaction at a preset compaction temperature, wherein the lubricant for warm compaction with die lubrication comprises a lubricant having a higher melting point than the compaction temperature and in a content from 0.5 to 80% by mass, and a lubricant having a lower melting point than the compaction temperature and as the balance; and the iron-based powder mixture comprises an iron-based powder and a powder compaction lubricant, the powder compaction lubricant comprising a lubricant having a lower melting point tha n the compaction temperature and in a content from 10 to 75% by mass based on the total amount of the powder compaction lubricant, and a lubricant having a higher melting point than the compaction temperature and as the balance The process according to claim 5, characterized in that the iron-based powder mixture comprises an iron-based powder, a powder compaction lubricant and a graphite powder, said graphite powder being added in a content less than 0.5% by mass based on the total amount of the iron-based powder mixture. The process according to one of claims 5 and 6, wherein the higher-melting die lubricant contained in said die lubricant and/or said powder compaction lubricant are at least one selected from the group consisting of metallic soap, thermoplastic resin, thermoplastic elastomer, and an organic or inorganic lubricant having a lamellar crystal structure. The process according to one of claims 5 and 6, wherein the lower melting lubricant contained in said die lubricant and/or said powder compaction lubricant are at least one selected from the group consisting of metallic soap, amide wax, polyethylene, and an eutectic mixture of at least two members thereof. The process according to any one of the claims 5 to 8, wherein the lubricant for warm compaction lubrication is added in a content from 0.05 to 0.40% by mass. A process for the production of a high-density iron-based sintered compact, comprising the step of sintering the high-density iron-based green compact after producing it by the process according to any of claims 5 to 9 , thereby forming a sintered compact.

Inventors: OZAKI UKIKO, UENOSONO SATOSHI, UNAMI SHIGERU, OZAKI, UKIKO, UENOSONO, SATOSHI, UNAMI, SHIGERU, OZAKI, Ukiko, Technical Research Laboratories, Uenosono, Satoshi, Technical Research Laboratories, UNAMI, Shigeru, Technical Research Laboratories
Applicants: JFE STEEL CORP, JFE STEEL CORPORATION
Application year: 2000
Main classification: B22F1
IPC codes: B22F3