JP2007031841A - Iron-based powder mixture for warm die lubricating compaction - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an iron-based powder mixture for warm die lubricating compaction used for manufacturing a green compact of iron-based powder. <P>SOLUTION: An iron-based powder mixture containing an iron-base powder and a compaction lubricant in which a lubricant having a low melting point not higher than a prescribed compaction temperature comprises 10 to 75 mass% of the total amount of lubricant and the balance is composed of a lubricant having a melting point higher than the prescribed compaction temperature by 0.05 to 0.40 mass% is heated. A lubricant, which consists of lubricant having a melting point higher than the compaction temperature and the balance lubricant having a melting point lower than the compaction temperature, is allowed to adhere to the surface of a die by electrification. After the die is filled with the heated iron-based powder mixture, the powder mixture is compacted at prescribed temperature into a green compact. In this way, the high density green compact can be obtained by a single compaction. Further, the green compact may be sintered into a high density sintered compact. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an iron-based powder mixture for powder metallurgy and an iron-based powder mixture used for the production of an iron-based sintered body, and in particular, used for producing a high-density iron-based powder molded body by warm forming. It relates to the improvement of lubricant.

The iron-based powder compact for powder metallurgy is filled with iron-based powder mixture in which iron-based powder is mixed with alloy powder such as copper powder and graphite powder, and lubricant such as zinc stearate and lead stearate. After that, it is generally produced by pressure molding. The density of the molded body is generally 6.6 to 7.1 Mg / m ³ .
These iron-based powder compacts are further subjected to a sintering treatment to obtain sintered bodies, and further subjected to sizing and cutting as necessary to obtain powder metallurgy products. When higher strength is required, carburizing heat treatment or bright heat treatment may be performed after sintering.

By this powder metallurgy technology, it becomes possible to produce a part having a complicated shape with high dimensional accuracy in a near net shape, and the cutting cost can be greatly reduced as compared with the conventional manufacturing method.
Further, recently, there has been a strong demand for iron-based powder metallurgy products to further increase the dimensional accuracy for cost reduction by omitting cutting and to increase the strength for reducing the size and weight of parts.
For increasing the strength of powder metallurgy products (sintered parts), it is effective to increase the density of the sintered parts by increasing the density of the compact. The higher the density of the sintered part, the fewer the voids in the part, and the mechanical properties such as tensile strength, impact value and fatigue strength are improved.

As a molding method that enables the densification of iron-based powder compacts, the two-molding twice-sintering method in which the iron-based powder mixture is subjected to normal molding and sintering, followed by repeated molding and sintering. In addition, a sintering forging method in which hot forging is performed after one-time molding and single sintering has been proposed.
Further, for example, Patent Document 1, Patent Document 2, Patent Document 3, and Patent Document 4 disclose warm forming techniques for forming metal powder while heating. This warm molding technology melts part or all of the lubricant during warm molding to uniformly disperse the lubricant between the powder particles, reducing the frictional resistance between the particles and between the compact and the mold. Among the above-described methods for producing a high-density molded body, it is considered that it is most advantageous in terms of cost. According to this warm forming technology, a Fe-4Ni-0.5Mo-1.5Cu-based partially alloyed iron powder was mixed with an iron-based powder mixture containing 0.5% by mass of graphite and 0.6% by mass of lubricant at 150 ° C for 7t. When molded at a pressure of / cm ² (686 MPa), a molded body of about 7.30 Mg / m ³ is obtained.

However, in the techniques described in Patent Document 1, Patent Document 2, Patent Document 3, and Patent Document 4, the fluidity of the powder mixture is insufficient, the productivity is lowered, and the density of the molded body varies, There are problems that the characteristics of the sintered body fluctuate, and further, there are problems such as high extraction force during molding, generation of wrinkles on the surface of the molded body, and short life of the mold.
Further, in these warm molding techniques, a lubricant is contained in the iron-based powder mixture for the purpose of reducing the frictional resistance between the particles and between the molded body and the mold and improving the moldability. At the time of warm forming, a part or all of it is melted and extruded near the surface of the molded body, and then heated and decomposed or evaporated by the sintering process to dissipate from the molded body. Form holes. Therefore, there is a problem that the mechanical strength of the sintered body is lowered.

In order to solve this problem, Patent Document 5 discloses that, in normal temperature or warm molding, a charged lubricant powder is applied to the mold surface to reduce the amount of lubricant in the iron-based powder mixture, and the high density. A technique for forming a molded body is disclosed.
JP-A-2-156002 Japanese Patent Publication No.7-103404 USP No. 5,256,185 USP No. 5,368,630 JP-A-8-100203

  However, in the method described in Patent Document 5, since the type of lubricant to be applied is a simple substance, the form of the lubricant changes before and after its melting point, and the lubricating function changes significantly. For this reason, there has been a problem that the molding temperature range is limited by the melting point of the lubricant. Furthermore, even if a mold lubricant is applied to the mold surface and the amount of lubricant in the iron-based powder mixture is reduced, depending on the components of the lubricant to be mixed, the lubrication effect is lost due to the reduction in the amount, and the dust density increases. There is also a problem that it cannot be realized.

In addition, from the viewpoint of increasing the strength of automobile parts and from the viewpoint of cost, a method for producing a high-density iron-based powder molded body that can obtain a further high-density molded body by a single molding. Development was desired.
The present invention advantageously solves the above-mentioned problems of the prior art. For example, an iron-based powder mixture obtained by mixing 0.5% by weight of graphite powder with partially alloyed iron powder having a composition of Fe-4Ni-0.5Mo-1.5Cu is heated. Proposes an iron-based powder mixture that can be used for the production of high-density iron-based powder compacts that can be obtained by single molding of high-density compacts of 7.4 Mg / m ³ or more when subjected to hot pressing. The purpose is to do.

  In order to achieve the above-mentioned problems by utilizing the warm molding technique and the mold lubrication molding technique, the present inventors have conducted intensive studies on the blending of the lubricant for the mold lubrication and the iron-based powder mixed powder. It was. As a result, in order to reduce the extraction force, as a lubricant for mold lubrication that can be adhered to the preheated mold surface by electrification, a lubricant having a low melting point below a predetermined pressure molding temperature and its temperature It was found that a lubricant obtained by mixing a lubricant having a higher melting point with an appropriate blend is good.

The present invention has been completed by further study based on the above findings.
That is, the first present invention is an iron-based powder mixture containing an iron-based powder and a powder molding lubricant, wherein the powder molding lubricant is 10 to 75% by mass of the total amount of the powder molding lubricant. A lubricant having a melting point lower than the pressure molding temperature, and the balance is a lubricant having a melting point higher than the pressure molding temperature, and the content of the powder molding lubricant is 0.05 to 0.40. It is an iron-based powder mixture for warm mold lubrication molding, characterized in that it is mass%.

    According to the present invention, a high-density molded body can be easily obtained by one press molding.

  According to the present invention, it is possible to easily produce a high-density molded body having good appearance properties and cross-sectional properties by one molding, and has a low extraction force after molding, thereby extending the life of the mold. In addition, there is a remarkable industrial effect that a sintered body with higher density can be easily obtained.

In the present invention, the mold is filled with a heated iron-based powder mixture, and then pressure-molded at a predetermined temperature to obtain an iron-based powder molded body.
In the present invention, the mold used for molding is preheated to a predetermined temperature in advance. The preheating temperature of the mold is not particularly limited as long as the iron-based powder mixture can be maintained at a predetermined pressure molding temperature, but is 20 to 60 ° C. higher than the predetermined pressure molding temperature. Is desirable.

A charged mold lubricant is introduced into a preheated mold and charged and adhered to the mold surface. It is preferable that the lubricant for mold lubrication (solid powder) is charged in a mold lubrication apparatus (for example, Die Wall Lubricant System manufactured by Gasbarre) and charged by contact charging between the lubricant (solid) powder and the inner wall of the apparatus. . The charged mold-lubricating lubricant is introduced into the mold by spraying, and is charged and adhered to the mold surface. The adhesion amount of the lubricant for mold lubrication to be charged and adhered to the mold surface is preferably 5 to 100 g / m ² . When the adhesion amount is less than 5 g / m ² , the lubrication effect is insufficient and the extraction force after molding becomes high, and when it exceeds 100 g / m ² , the lubricant remains on the surface of the molded body, resulting in poor appearance of the molded body.

  The lubricant for warm mold lubrication used by charging and adhering to the mold surface when the powder is pressure-molded with a preheated mold is a lubricant having a melting point higher than a predetermined pressure molding temperature of 0.5 to A mixed lubricant comprising 80% by mass, the balance being a lubricant having a low melting point below the predetermined pressure molding temperature. The predetermined pressure molding temperature in the present invention refers to the temperature on the mold surface during pressure molding.

  A lubricant having a melting point higher than a predetermined pressure molding temperature is unmelted in the mold at the time of molding, and acts as a solid lubricant like a “roller” in the mold to reduce the extraction force. In addition, the molten or partially melted lubricant (lubricant having a melting point lower than the predetermined pressure molding temperature) is prevented from moving in the mold, and the frictional resistance between the molded body and the mold surface is reduced. Thus, it has a role of preventing an increase in extraction force.

  When the content of the lubricant having a melting point higher than a predetermined pressure molding temperature is less than 0.5% by mass, the lubricant having a melting point lower than the pressure molding temperature is increased, and the amount of the lubricant to be melted is increased. The lubricant moves and does not have a uniform distribution on the mold surface, and the frictional resistance between the molded body and the mold surface increases, and the effect of reducing the extraction force is small. On the other hand, if it exceeds 80% by mass, the amount of the lubricant that does not melt in the mold becomes excessive, the distribution of the lubricant on the mold surface becomes uneven, the mold lubrication is insufficient, and the extraction force increases. For this reason, the blending amount of the lubricant having a melting point higher than the predetermined pressure molding temperature in the warm mold lubricant lubricant is limited to the range of 0.5 to 80% by mass.

The balance in the lubricant for mold lubrication is a lubricant having a low melting point equal to or lower than a predetermined pressure molding temperature. A lubricant having a low melting point below a predetermined pressure molding temperature is melted or partially melted at the pressure molding temperature, becomes a grease-like state on the mold surface, and has the effect of reducing the extraction force. ing.
Lubricants having a melting point higher than a predetermined pressure molding temperature in the lubricant for warm mold lubrication are metal soaps, thermoplastic resins, thermoplastic elastomers, inorganic lubricants having a layered crystal structure, or organic lubricants. It is preferable to use one or more selected from among them. Depending on the predetermined pressure molding temperature, the lubricant can be appropriately selected from the following lubricants.

As the metal soap, lithium stearate, lithium hydroxystearate and the like are preferable. Further, as the thermoplastic resin, polystyrene, polyamide, fluororesin and the like are suitable. As the thermoplastic elastomer, polystyrene elastomer, polyamide elastomer and the like are suitable. The inorganic lubricant having a layered crystal structure may be any of graphite, MoS ₂ , and fluorocarbon, and the finer the particle size, the more effective the reduction of the extraction force. As the organic lubricant having a layered crystal structure, any of melamine-cyanuric acid adduct (MCA) and N-alkylaspartic acid-β-alkyl ester can be used.

  On the other hand, a lubricant having a low melting point equal to or lower than a predetermined pressure molding temperature in the lubricant for warm mold lubrication is charged with a low melting point that melts or partially melts on the mold surface at the predetermined pressure molding temperature. It is desirable to use an easy lubricant. Such a lubricant is preferably one or more selected from metal soap, amide wax, polyethylene, and two or more co-melts thereof. Depending on the temperature of the predetermined pressure molding, it can be selected from the following lubricants. Zinc stearate and calcium stearate are suitable as the metal soap, ethylene bisstearamide, stearic acid monoamide and the like are suitable as the amide wax, and ethylene bisstearamide and polyethylene as the co-melt. Of these, a co-melt of ethylene bis-stearamide and zinc stearate, and a co-melt of ethylene bis-stearamide and calcium stearate are preferred.

Next, a heated iron-based powder mixture is charged into a mold charged with a lubricant for mold lubrication and is pressure-molded to obtain a molded body.
The heating temperature of the iron-based powder mixture is preferably 70 to 200 ° C. If heating temperature is less than 70 degreeC, the yield stress of iron powder is high and the density of a molded object falls. On the other hand, even if the heating temperature exceeds 200 ° C, there is no substantial increase in density, and there is a concern about oxidation of iron powder. Therefore, the heating temperature of the iron-based powder mixture is preferably in the range of 70 to 200 ° C. .

  The iron-based powder mixture is obtained by mixing a lubricant (powder-forming lubricant) or an alloy powder with iron-based powder. The mixing of the iron-based powder and the powder molding lubricant or the alloy powder is not particularly limited, and any known mixing method can be suitably used. In particular, when mixing the powder for alloying with the iron-based powder, in order to avoid segregation of the contained powder, after adding 1 part of the powder-forming lubricant to the iron-based powder and alloying powder, Further, the powder molding lubricant is stirred while being heated to the melting point of at least one of the lubricants, to melt at least one lubricant among the powder molding lubricants, and the melted mixture is stirred. A mixing method is preferred in which the alloy powder is adhered by cooling and fixing the molten lubricant to the surface of the iron-based powder, and then the remainder of the powder molding lubricant is added and secondarily mixed.

The iron-based powder in the present invention is preferably pure iron powder such as atomized iron powder or reduced iron powder, partially diffusion alloyed steel powder, fully alloyed steel powder, or a mixed powder thereof.
The content of the powder molding lubricant contained in the iron-based powder mixture is preferably 0.05 to 0.40 mass% with respect to the entire iron-based powder mixture. When the content of the powder molding lubricant is less than 0.05% by mass, the fluidity of the iron-based mixed powder is poor and the mold surface is not uniformly filled, so that the density of the compact is lowered. On the other hand, when the content of the powder molding lubricant exceeds 0.40% by mass, the porosity after sintering becomes high and the density of the compact is lowered.

  The powder molding lubricant contained in the iron-based powder mixture is a mixed lubricant comprising a lubricant having a low melting point lower than a predetermined pressure molding temperature and a lubricant having a melting point higher than a predetermined pressure molding temperature. And The content of the lubricant having a low melting point equal to or lower than the predetermined pressure molding temperature is 10 to 75 mass% of the total amount of the powder molding lubricant contained, and the remaining 25 to 90 mass% is the predetermined pressure molding. The lubricant has a melting point higher than the temperature. A lubricant having a low melting point below a predetermined pressure molding temperature melts during pressure molding, penetrates between the powder particles by capillary force, and is uniformly dispersed inside the powder particles, thereby improving the contact resistance between the particles. It has the effect of reducing and promoting particle rearrangement to promote densification of the compact. When the content of the lubricant having a low melting point below the predetermined pressure molding temperature is less than 10% by mass, the lubricant is not evenly dispersed inside the powder particles, and the density of the compact is lowered. On the other hand, if it exceeds 75% by mass, the molten lubricant is squeezed out to the surface of the molded body as the density of the molded body increases, and a relief passage for the lubricant is formed on the surface. New pores are formed, resulting in a decrease in strength of the sintered member.

A lubricant having a melting point higher than a predetermined pressure molding temperature contained in the iron-based powder mixture is present as a solid at the time of molding, and is formed at the convex portion on the surface of the iron-based powder particles where the molten lubricant is repelled. ”To promote particle rearrangement and increase the density of the compact.
Among the powder molding lubricants contained in the iron-based powder mixture, the lubricants having a melting point higher than the predetermined pressure molding temperature include metal soaps, thermoplastic resins, thermoplastic elastomers, and inorganic having a layered crystal structure. Or it is preferable to use 1 type, or 2 or more types chosen from organic lubricants. Depending on the predetermined pressure molding temperature, the lubricant can be appropriately selected from the following lubricants.

As the metal soap, lithium stearate, lithium hydroxystearate and the like are preferable. Further, as the thermoplastic resin, polystyrene, polyamide, fluororesin and the like are suitable. As the thermoplastic elastomer, polystyrene elastomer, polyamide elastomer and the like are suitable. The inorganic lubricant having a layered crystal structure may be any of graphite, MoS ₂ , and fluorocarbon, and the finer the particle size, the more effective the reduction of the extraction force. As the organic lubricant having a layered crystal structure, any of melamine-cyanuric acid adduct (MCA) and N-alkylaspartic acid-β-alkyl ester can be used.

Among the powder molding lubricants contained in the iron-based powder mixture, the lubricant having a low melting point below the predetermined pressure molding temperature includes metal soap, amide wax, polyethylene, and at least two of these It is preferable to use 1 type (s) or 2 or more types selected from these co-melts. Depending on the predetermined pressure molding temperature, the lubricant can be appropriately selected from the following lubricants.
As the metal soap, zinc stearate, calcium stearate and the like are preferable. As the amide wax, ethylene bisstearamide, stearic acid monoamide, or the like is suitable. As the co-melt, a co-melt of ethylene bis-stearamide and polyethylene, a co-melt of ethylene bis-stearamide and zinc stearate, a co-melt of ethylene bis-stearamide and calcium stearate, and the like are suitable. Depending on the molding temperature, a part of these lubricants can be used as a lubricant having a melting point higher than the pressure molding temperature.

The graphite contained as the alloy powder in the iron-based powder mixture has an effect of strengthening the sintered body. If the content of graphite is small, the effect of strengthening the sintered body is not sufficient. On the other hand, if the content is too large, pro-eutectoid cementite precipitates and the strength decreases. Therefore, the graphite contained in the iron-based powder mixture is preferably 0.5 to 1.2% by mass with respect to the total amount of the iron-based powder mixture.
In the present invention, the high-density iron-based powder compact obtained by the above-described manufacturing method is subjected to a sintering treatment to obtain a high-density iron-based sintered body. The sintering treatment in the present invention is not particularly limited, and any known sintering treatment method can be suitably used. Moreover, the method (sinter hardening) which raises an intensity | strength rapidly by sintering after sintering can also be used.

Example 1
As the iron-based powder, a partially alloyed steel powder having a composition of Fe-4Ni-0.5Mo-1.5Cu in which Ni, Mo, and Cu were diffused and adhered to atomized pure iron powder was used. To this partially alloyed steel powder, 0.5% by mass of graphite powder and various lubricants shown in Table 1 were mixed by a heat mixing method using a high-speed mixer to obtain an iron-based powder mixture.
First, the mold for pressure molding is preheated to the temperature shown in Table 1, and a warm mold lubricant lubricant charged by using a mold lubricator (manufactured by Gasbarre) is sprayed into the mold. And charged on the mold surface. The warm mold lubricant is selected from the various lubricants shown in Table 2, and includes a lubricant having a melting point lower than the pressure molding temperature and a lubricant having a melting point higher than the pressure molding temperature. What was mixed as shown in Table 1 was used. The temperature of the mold surface was measured and used as the pressure molding temperature.

Next, the mold thus treated was filled with a heated iron-based powder mixture, and then subjected to pressure molding to obtain a 10 × 10 × 55 mm rectangular compact. The applied pressure was 7 t / cm ² (686 MPa). The pressure molding conditions are shown in Table 1. The powder molding lubricant contained in the iron-based powder mixture is selected from the various lubricants shown in Table 2, and has a low melting point below the pressure molding temperature and a melting point higher than the pressure molding temperature. A lubricant is mixed as shown in Table 1.

In addition, as a conventional example, an example in which a mold not coated with a lubricant for mold lubrication is filled with a heated iron-based powder mixture and subjected to pressure molding to form a similar rectangular parallelepiped (molding) Body No.38).
After molding, the extraction force when the molded body was extracted was measured.
Moreover, the density was measured by Archimedes method about these molded objects. The Archimedes method is a method for measuring the density by immersing a molded body, which is an object to be measured, in ethanol and measuring the volume. Furthermore, the external appearance of these compacts was visually observed to investigate the presence of defects such as wrinkles and cracks. Further, these molded bodies were cut at the center, embedded in a resin and polished, and the presence or absence of pores in the cross section was observed with an optical microscope.

  Table 1 shows the results regarding the drawing force, the density of the molded body, the appearance of the molded body, and the properties of the cross section of the molded body.

In all of the examples of the present invention, the extraction force after molding is as low as 20 MPa or less, and the molded product has a high density of 7.4 Mg / m ³ or more. Further, the molded body did not have defects such as wrinkles and cracks as well as surface oxidation by heating. Moreover, the cross-sectional property of the molded body was normal, and no coarse pores were observed.
In the comparative example and the conventional example that are out of the scope of the present invention, the extraction force is higher than 20 MPa, the density is low as less than 7.35 Mg / m ³ , or coarse pores are observed near the surface of the cross section of the molded body. It was.

According to the present invention, there is an effect that it is possible to form a high-density molded body having both good appearance properties and cross-sectional properties with low extraction force.
(Example 2)
As iron-based powders, (1) partially alloyed steel powder a of Fe-4Ni-0.5Mo-1.5Cu composition in which Ni, Mo and Cu are diffused and attached to atomized pure iron powder, (2) atomized pure iron powder Fe-2Ni-1Mo composition partially alloyed steel powder b in which Fe, Ni and Mo are diffused and adhered, (3) Pre-alloy steel powder of Fe-3Cr-0.3Mo-0.3V composition pre-alloyed with Cr, Mo, V c, (4) Pre-alloyed steel powder d having a composition of Fe-1Cr-0.3Mo-0.3V pre-alloyed with Cr, Mo, V, (5) atomized iron powder e, and (6) reduced iron powder f were used. Atomized iron powder is an iron-based powder obtained by spraying molten steel with high-pressure water, and reduced iron powder is an iron-based powder obtained by reducing iron oxide.

  Each of these partially alloyed steel powder a, partially alloyed steel powder b, pre-alloyed steel powder c, pre-alloyed steel powder d, atomized iron powder e, reduced iron powder f, The various lubricants shown were mixed by a heat mixing method using a high-speed mixer to obtain an iron-based powder mixture. In the case of atomized iron powder e and reduced iron powder f, 2.0% by mass of Cu powder was mixed in addition to 0.8% by mass of graphite. The content of graphite is a mass ratio with respect to the total amount of iron-based powder and graphite or further alloy powder.

  First, the mold for pressure molding is preheated to the temperature shown in Table 3, and a warm mold lubricant lubricant charged by using a mold lubricator (manufactured by Gasbarre) is sprayed into the mold. And charged on the mold surface. The warm mold lubricant is selected from the various lubricants shown in Table 2, and includes a lubricant having a melting point lower than the pressure molding temperature and a lubricant having a melting point higher than the pressure molding temperature. A mixture as shown in Table 3 was used. The temperature of the mold surface was measured and used as the pressure molding temperature.

Next, the mold thus treated was filled with a heated iron-based powder mixture, and then subjected to pressure molding to obtain a 10 × 10 × 55 mm rectangular compact. The applied pressure was 7 t / cm ² (686 MPa). Table 3 shows the pressure molding conditions. The powder molding lubricant contained in the iron-based powder mixture is selected from the various lubricants shown in Table 2, and has a low melting point below the pressure molding temperature and a melting point higher than the pressure molding temperature. A lubricant is mixed as shown in Table 3.

The density of these iron-based powder compacts was measured by the Archimedes method in the same manner as in Example 1.
Next, these iron-based powder compacts were subjected to sintering treatment at 1130 ° C. for 20 minutes in an N ₂ -10% H ₂ atmosphere to obtain iron-based sintered bodies. About the obtained iron-based sintered body, first, the density was measured by the Archimedes method. Further, from these iron-based sintered bodies, small round bar test pieces having a parallel part diameter of 5 mm and a length of 15 mm were sampled by machining, a tensile test was performed, and a tensile strength was measured.

In addition, a mold not coated with a lubricant for mold lubrication is filled with a heated iron-based powder mixture, pressure-molded, and formed into a similar rectangular parallelepiped molded body, and further subjected to a sintering treatment to obtain an iron-based sintered body and This example is a conventional example.
The results are shown in Table 3.

  Compared to the conventional example (sintered body No. 2-12) in which the mold lubrication is not performed, the example of the present invention has a high density and has a high tensile strength.

Claims (1)

  An iron-based powder mixture containing an iron-based powder and a powder molding lubricant, wherein the powder molding lubricant has a pressure molding temperature of 10 to 75% by mass with respect to the total amount of the powder molding lubricant. It includes a lubricant having the following low melting point, the balance is a lubricant having a melting point higher than the pressure molding temperature, and the content of the powder molding lubricant is 0.05 to 0.40% by mass An iron-based powder mixture for warm mold lubrication molding.