Iron-based mixed powder for powder metallurgy and iron-based green compact

Abstract

The present invention provides an iron-based mixed powder for powder metallurgy, which contains an iron-based powder and one or more binders that are selected from the group consisting of oleic acid amide, erucic acid amide, lauric acid amide, palmitic acid amide, stearic acid amide, behenic acid amide, and ethylene bis-oleic acid amide, have a melting point of 60°C to 120°C inclusive, and adhere to the surface of the iron-based powder, wherein the content of the binders with respect to the mass that is obtained by subtracting the mass of the binders from the total mass of the iron-based mixed powder for powder metallurgy is 0.01 mass% to 1.0 mass% inclusive. According to the present invention, the powder compact strength of an iron-based green compact that is obtained by pressure molding the iron-based mixed powder for powder metallurgy is increased, and damage to the iron-based green compact in green processing can be prevented. In addition, the energy consumption required for heating and mixing of an iron-based mixed powder for powder metallurgy can be reduced.

Description

Iron-based mixed powders and iron-based compacts for powder metallurgy

[0001] The present invention relates to an iron-based mixed powder for powder metallurgy and an iron-based green compact.

[0002] Iron-based mixed powder for powder metallurgy is a metal powder raw material mainly composed of iron-based powder. By compacting the iron-based mixed powder for powder metallurgy using a die, an iron-based green compact having a shape close to the final shape can be obtained. By sintering the obtained iron-based green compact at high temperature, sintered machine parts made of iron-based sintered compacts having complex shapes can be manufactured without using cutting processes.

[0003] When sintered machine parts have complex shapes that are difficult to form by pressure molding, such as side holes or undercuts, or when high dimensional accuracy is required for sintered machine parts, machining such as cutting may be required for the sintered iron-based sintered body. However, iron-based sintered bodies have higher mechanical strength than general iron-based structural materials due to their fine crystal grain size. Furthermore, their high porosity increases cutting resistance and frictional heat, which increases the surface temperature of cutting tools. This increases the wear of cutting tools, leading to more frequent replacement, and as a result, increases manufacturing costs. Therefore, as a solution to the above problem, so-called green machining, in which a pre-sintered iron-based powder compact (green) is subjected to cutting before sintering and then sintered, has attracted attention. Because green compacts have lower mechanical strength than iron-based sintered bodies, green machining can significantly reduce the total machining time.

[0004] On the other hand, alloying powders are sometimes added to iron-based powder mixtures for powder metallurgy to enhance mechanical strength, improve machinability, and the like. The alloying powders added to or mixed with iron-based powder mixtures for powder metallurgy generally have finer particle sizes than the iron-based powders. This can lead to non-uniform composition of the sintered compact and the generation of dust, which can worsen the working environment. To solve these problems associated with mixed powders, techniques have been developed for bonding the iron-based powder and the alloying powder using a binder. For example, Patent Document 1 describes an invention of an iron-based powder mixture for powder metallurgy in which a mixture of the iron-based powder, the alloying powder, and a binder made of a specific organic compound is heated, stirred while partially melting the binder, and then cooled, thereby adhering the alloying powder to the surface of the iron-based powder.

[0005] Japanese Patent Application Publication No. 5-148505

[0006] Generally, green compacts before sintering are brittle and have difficulty maintaining their shape during green processing. Green compacts before sintering cannot withstand the clamping stress they receive when fixed to a jig for processing or the local stress they receive from a cutting tool, and they easily break. Furthermore, green compacts are prone to chipping during handling. Therefore, it is desirable to increase the strength of green compacts so that they can withstand green processing and handling. The same problem exists with the iron-based green compacts formed using the iron-based mixed powder for powder metallurgy described in Patent Document 1.

[0007] The present invention has been made in view of the above-mentioned problems, and has as its object to prevent damage to an iron-based powder compact during green processing by increasing the green strength of the iron-based powder compact obtained by compacting an iron-based mixed powder for powder metallurgy.

[0008] As a result of extensive research, the present inventors have found that the above object can be achieved by the following constitution, and have completed the present invention.

[0009] The gist and configuration of the present invention are as follows.

[0010] [1] An iron-based mixed powder for powder metallurgy, comprising: an iron-based powder; and one or more binders selected from the group consisting of oleic acid amide, erucic acid amide, lauric acid amide, palmitic acid amide, stearic acid amide, behenic acid amide, and ethylene bisoleic acid amide, the binder having a melting point of 60°C or higher and 120°C or lower, the binder adhering to the surface of the iron-based powder; wherein the content of the binder is 0.01% by mass or higher and 1.0% by mass or lower relative to the total mass of the iron-based mixed powder for powder metallurgy minus the mass of the binder.

[0011] [2] The iron-based mixed powder for powder metallurgy according to the above [1], further comprising an alloying powder that is bound to the surface of the iron-based powder by the binder.

[0012] [3] The iron-based mixed powder for powder metallurgy according to the above [1] or [2], wherein the iron-based powder is at least one selected from reduced iron powder and atomized iron powder.

[0013] [4] The iron-based mixed powder for powder metallurgy according to any one of the above [1] to [3], further containing a lubricant present in a state free from the iron-based powder.

[0014] [5] The iron-based mixed powder for powder metallurgy according to any one of [1] to [4] above, wherein a green strength measured by a transverse rupture test specified in Japanese Industrial Standard JIS Z 2511:2006 using a test piece obtained by molding the iron-based mixed powder for powder metallurgy in accordance with the same standard is 10.0 MPa or more.

[0015] [5] An iron-based powder compact made from the iron-based mixed powder for powder metallurgy according to any one of [1] to [5] above.

[0016] According to the present invention, the green strength of the iron-based powder compact obtained by pressure molding can be increased compared to the prior art. As a result, green processing can be easily performed while maintaining the shape of the iron-based powder compact, which can significantly reduce the total time and cost required for processing the final product, the sintered machine part. Furthermore, by including a binder with a lower melting point than conventional binders, the amount of energy consumed for heating and mixing the iron-based mixed powder for powder metallurgy can be reduced.

[0017] Hereinafter, embodiments for carrying out the present invention will be described in detail. Note that the following description shows examples of preferred embodiments of the present invention, and the embodiments for carrying out the present invention are not limited to these embodiments.

[0018] [Iron-based mixed powder for powder metallurgy] In one embodiment, the present invention relates to an iron-based mixed powder for powder metallurgy, which contains an iron-based powder and one or more binders selected from the group consisting of oleic acid amide, erucic acid amide, lauric acid amide, palmitic acid amide, stearic acid amide, behenic acid amide, and ethylene bisoleic acid amide, the binder having a melting point of 60°C or higher and 120°C or lower and adhering to the surface of the iron-based powder, wherein the binder content is 0.01% by mass or higher and 1.0% by mass or lower relative to the total mass of the iron-based mixed powder for powder metallurgy minus the mass of the binder. As described above, by compacting the iron-based mixed powder for powder metallurgy according to the present invention using a mold, an iron-based green compact having a shape close to the final shape can be obtained. By sintering the obtained iron-based green compact at high temperature, a sintered machine part made of an iron-based sintered body having a complex shape can be manufactured without using cutting.

[0019] (a) Iron-based powder The iron-based mixed powder for powder metallurgy according to the present invention contains an iron-based powder. In this specification, "iron-based powder" refers to a metal powder containing 50% by mass or more of Fe. As the iron-based powder, any iron-based powder containing 50% by mass or more of Fe can be used. The iron-based powders that can be used in the present invention are broadly divided into two types: iron powder and alloy steel powder.

[0020] Iron powder is a powder containing Fe and unavoidable impurities, and is generally referred to as pure iron powder in the technical field. The total amount of unavoidable impurities contained in the iron-based powder is preferably 3.0 mass% or less. Preferred contents of representative unavoidable impurities are C: 0.05 mass% or less, Si: 0.10 mass% or less, Mn: 0.50 mass% or less, P: 0.03 mass% or less, S: 0.03 mass% or less, O: 0.50 mass% or less, and N: 0.1 mass% or less. Of these, the Mn content is the content allowable when Mn is not added as an alloy element, as described below.

[0021] The alloy steel powder is a powder containing, in addition to Fe, one or more alloying elements selected from, for example, Cr, Mn, Ni, Mo, V, Cu, Nb, etc., and the above-mentioned inevitable impurities. Examples of alloy steel powder that can be used include pre-alloyed steel powder or fully alloyed steel powder in which alloying elements are pre-alloyed during melting, partially diffused alloyed steel powder in which alloying elements are partially diffused into iron powder, and hybrid steel powder in which alloying elements are further partially diffused into pre-alloyed steel powder.

[0022] The mass ratio of the iron-based powder to the total mass of the iron-based mixed powder for powder metallurgy minus the mass of the binder (described later) is not particularly limited. The mass ratio of the iron-based powder is preferably 86 mass% or more. If the mass ratio of the iron-based powder is 86 mass% or more, the mechanical strength of the iron-based sintered body is increased. The mass ratio of the iron-based powder is more preferably 90 mass% or more.

[0023] The iron-based powder contained in the iron-based mixed powder for powder metallurgy according to the present invention may be produced by any known method. When classified according to the production method, the iron-based powder may be, for example, reduced iron powder, atomized iron powder, or a mixture thereof. Reduced iron powder is an iron-based powder produced by reducing iron oxide. Atomized iron powder is an iron-based powder produced by water atomization or gas atomization. Furthermore, powder in which alloy elements are diffused and attached to the surface of reduced iron powder or atomized iron powder can also be used as the iron-based powder.

[0024] In a preferred embodiment, the iron-based powder is at least one selected from reduced iron powder and atomized iron powder. Both reduced iron powder and atomized iron powder are iron-based powders that are widely available on the market, and therefore can be easily procured for use in producing the iron-based mixed powder for powder metallurgy according to the present invention.

[0025] The average particle size of the iron-based powder is not particularly limited and may be any particle size. However, if the particle size of the iron-based powder is too small, dust is likely to be generated, making handling difficult. From the viewpoint of ease of handling, the average particle size of the iron-based powder is preferably 1 μm or more, and more preferably 10 μm or more. On the other hand, if the particle size of the iron-based powder is too large, it may have an adverse effect on the strength of the sintered body. Therefore, from the viewpoint of improving the strength of the sintered body, the average particle size of the iron-based powder is preferably 200 μm or less. In this specification, the median diameter D in the particle size distribution based on weight of the iron-based powder is used as a numerical value representing the average particle size of the iron-based powder. ５０ The particle size distribution of the iron-based powder on a weight basis is measured in accordance with the method specified in Japanese Industrial Standard JIS Z 8815-1994, "General Rules for Sieving Test Methods."

[0026] (b) Binder The iron-based mixed powder for powder metallurgy according to the present invention contains one or more binders having a melting point of 60°C or more and 120°C or less, which are adhered to the surface of the iron-based powder. The binder in the present invention is an organic compound that exists in a state of being adhered to the surface of the iron-based powder. The mixture of the iron-based powder and the binder is heated to a temperature equal to or higher than the melting point while being stirred, and then cooled, whereby the binder can be adhered to the surface of the iron-based powder.

[0027] The binder contained in the iron-based mixed powder for powder metallurgy according to the prior art has been one with a relatively high melting point exceeding 120°C. This is because it was thought that the use of a binder with a low melting point could have some adverse effect on the compaction of an iron-based compact using the iron-based mixed powder for powder metallurgy. However, the inventors' investigations have revealed that even when a low-melting-point binder with a melting point of 120°C or less is used, not only can the iron-based compact be compacted without any problems, but the strength of the iron-based compact is significantly increased compared to the prior art. Therefore, unlike the prior art, the present invention uses a binder with a melting point of 120°C or less. If a binder with a melting point lower than 60°C is used, the iron-based mixed powder for powder metallurgy is prone to agglomeration and the flowability is reduced, so the melting point of the binder is set to 60°C or higher.

[0028] Although the reasons for the improved green strength of an iron-based compact obtained by compacting an iron-based mixed powder for powder metallurgy containing a binder with a lower melting point than that of conventional techniques are not fully understood, the following is likely to be the reason: In an iron-based compact, bonding between iron-based powder particles occurs due to plastic deformation and mechanical bonding caused by direct contact between convex portions on the surface of the iron-based powder. In conventional techniques, much of the binder adhering to the surface of the iron-based powder remains at the convex portions even after the heat-melting process described below, preventing direct contact between the convex portions. In the iron-based mixed powder for powder metallurgy according to the present invention, the binder has a low melting point, so the binder is fully molten during the heat-melting process, reducing surface tension and improving wettability. Therefore, the liquid binder does not remain at the convex portions but flows toward the more stable concave portions. As a result, direct contact between the iron-based powder particles at the convex portions is possible, which is thought to improve the green strength of the iron-based compact.

[0029] In the iron-based mixed powder for powder metallurgy according to the present invention, the binder is one or more binders selected from the group consisting of oleic acid amide, erucic acid amide, lauric acid amide, palmitic acid amide, stearic acid amide, behenic acid amide, and ethylene bisoleic acid amide. These compounds all have melting points of 60°C or higher and 120°C or lower. Their inclusion in the iron-based mixed powder for powder metallurgy improves the density and strength of the iron-based compact. The melting point of an organic compound decreases as the molecular weight of the compound decreases. Furthermore, compounds with double bonds, such as oleic acid amide and erucic acid amide, tend to have lower melting points than compounds without double bonds. The binder is preferably in a powder form before heat melting to ensure uniform mixing.

[0030] In a preferred embodiment, the melting point of the binder is less than 80°C. Compared to binders with melting points of 80°C or more and 120°C or less, an iron-based green compact made from an iron-based mixed powder for powder metallurgy containing a binder with a melting point of less than 80°C has further improved green compact strength. In this preferred embodiment, when two or more compounds are used as binders, it is sufficient that at least one of the compounds has a melting point of less than 80°C. In a more preferred embodiment, the melting point of the binder is 70°C or more. If the melting point of the binder is 70°C or more, the flowability of the iron-based mixed powder for powder metallurgy is further improved.

[0031] In the iron-based powder mixture for powder metallurgy according to the present invention, the binder content is 0.01% by mass or more and 1.0% by mass or less, relative to the total mass of the iron-based powder mixture minus the binder content. When the iron-based powder mixture for powder metallurgy further contains an alloying powder (described below), a binder content of 0.01% by mass or more prevents segregation by adhering the alloying powder to the surface of the iron-based powder via the binder, thereby further improving the properties of the sintered compact. Powders in which the alloying powder and other components are adhered to the surface of the iron-based powder via the binder are referred to as segregation-prevented powders. The binder content is preferably 0.02% by mass or more, more preferably 0.05% by mass or more. A binder content of 1.0% by mass or less prevents a decrease in the density of the iron-based compact due to the inclusion of an excess binder and ensures that the direct contact between the iron-based powder particles is not impeded. The binder content is preferably 0.5% by mass or less, more preferably 0.3% by mass or less.

[0032] (c) Alloying Powder In a preferred embodiment, the iron-based mixed powder for powder metallurgy according to the present invention further contains an alloying powder that is bound to the iron-based powder by a binder. When the iron-based mixed powder for powder metallurgy containing the alloying powder is sintered, the alloying elements dissolve in the iron to form a solid alloy, thereby improving the strength of the final sintered body. The alloying powder is not particularly limited, and any powder containing components that can become alloy components can be used. For example, the alloying powder can be a powder containing one or more elements selected from the group consisting of C, Cu, Ni, Mo, Mn, Cr, V, and Si. When C is used as an alloying component, graphite powder is preferably used as the alloying powder.

[0033] The particle size of the alloying powder is not particularly limited, and powder of any size can be used. However, if the particle size of the alloying powder is excessively small, the alloying powder may aggregate in the iron-based powder mixture for powder metallurgy, which may cause problems in handling. Therefore, from the viewpoint of ease of handling, the average particle size of the alloying powder is preferably 1 μm or more. On the other hand, if the particle size of the alloying powder is excessively large, it may have a negative effect on the strength of the sintered body. Therefore, from the viewpoint of improving the strength of the sintered body, the average particle size is preferably 100 μm or less. In this specification, the median diameter D in the volume-based particle size distribution of the alloying powder is used as a numerical value representing the average particle size of the alloying powder. ５０ The particle size distribution of the alloy powder on a volume basis can be measured using a laser diffraction / scattering particle size distribution analyzer.

[0034] The ratio of the total mass of the alloying powders to the total mass of the iron-based mixed powder for powder metallurgy minus the mass of the binder is not particularly limited, but if a lower limit value for the ratio of the iron-based powder is set in a preferred embodiment, the upper limit value for the ratio of the total mass of the alloying powders will naturally be set accordingly.

[0035] (d) Lubricant In a preferred embodiment, the iron-based mixed powder for powder metallurgy according to the present invention further contains a lubricant that exists in a state free from the iron-based powder. The lubricant in the present invention is an organic compound that does not adhere to the iron-based powder but exists in a state free from the iron-based powder. The lubricant is a solid lubricant that is solid at room temperature and is preferably in the form of powder. The lubricant is dispersed in a state free from the iron-based powder. The lubricant can be dispersed in a state free from the iron-based powder by stirring the mixture of the iron-based powder and the lubricant at room temperature without heating.

[0036] When an iron-based mixed powder for powder metallurgy, obtained by mixing an iron-based powder with a lubricant, is compacted using a die, the lubricant acts to reduce sliding friction between the iron-based powder particles. This allows the density of the iron-based compact to be increased. The lubricant also reduces sliding friction between the side walls of the die and the iron-based compact. This reduces the ejection pressure of the iron-based compact and prevents damage to the side surfaces of the iron-based compact due to friction with the side walls of the die.

[0037] In the iron-based powder mixture for powder metallurgy according to the present invention, the binder and the lubricant are distinguished by their respective forms of existence. The binder adheres to the surface of the iron-based powder, while the lubricant exists in a free state without adhering to the surface of the iron-based powder. An organic compound may function as either a binder or a lubricant depending on its form of existence. When an organic compound is used as a binder, the same organic compound may also be used as a lubricant. By sieving an iron-based powder mixture for powder metallurgy containing both a binder and a lubricant through a sieve with a mesh size larger than that of the lubricant but smaller than that of the iron-based powder, the lubricant alone can be separated from the iron-based powder mixture for powder metallurgy. By using such a method, the lubricant contained in the iron-based powder mixture for powder metallurgy can be analyzed to identify and measure the organic compounds contained therein.

[0038] In a preferred embodiment in which the iron-based mixed powder for powder metallurgy according to the present invention further contains an alloying powder, the lubricant present in a free state from the iron-based powder acts to reduce sliding friction between the iron-based mixed powder for powder metallurgy, which is made of iron-based powder having alloying powder bonded to its surface, and between the iron-based compact and the side wall of the die. This allows the density of the iron-based compact to be increased. Furthermore, the ejection pressure of the iron-based compact is reduced, and damage to the side surface of the iron-based compact due to friction with the side wall of the die can be prevented.

[0039] The lubricant used in the preferred embodiment may be any solid lubricant with excellent lubricity. Specifically, for example, metal soaps such as zinc stearate and lithium stearate, and fatty acid amides such as stearic acid amide and ethylene bisstearic acid amide can be suitably used as the lubricant. Among these, stearic acid amide is an organic compound that can also be used as a binder in the present invention.

[0040] In a preferred embodiment, the lubricant content is 0.02% by mass or more and 1.5% by mass or less, relative to the total mass of the iron-based mixed powder for powder metallurgy minus the mass of the binder and the mass of the lubricant. A lubricant content of 0.02% by mass or more can increase the density of the iron-based compact by reducing sliding friction between the iron-based powder particles. The lubricant content is more preferably 0.05% by mass or more, and even more preferably 0.1% by mass or more. A lubricant content of 1.5% by mass or less can prevent a decrease in the density of the iron-based compact due to the inclusion of an excess binder. The lubricant content is more preferably 1.2% by mass or less, and even more preferably 1.0% by mass or less. The combined content of the binder and lubricant is preferably 2.0% by mass or less, and more preferably 1.5% by mass or less.

[0041] (e) Green Strength In a preferred embodiment, a test piece obtained by molding an iron-based mixed powder for powder metallurgy in accordance with the provisions of Japanese Industrial Standard JIS Z 2511:2006 ("Metal Powders - Method for Measuring Green Strength by Transverse Rupture Test") has a green strength of 10.0 MPa or more, as measured by a transverse rupture test specified in the same standard. A preferred green strength of the test piece is 11.5 MPa or more. A test piece obtained by molding an iron-based mixed powder for powder metallurgy according to the present invention, which contains an iron-based powder and one or more binders having a melting point of 60°C or more and 120°C or less and adhering to the surface of the iron-based powder, can achieve a strength of 10.0 MPa or more. When the melting point of the binder is less than 80°C, the strength of the test piece reaches 11.5 MPa or more.

[0042] The preparation of test specimens and measurement of green compact strength in the transverse rupture test specified in Japanese Industrial Standard JIS Z 2511:2006 are carried out according to the following procedure. First, an appropriate amount of iron-based powder mixture for powder metallurgy is filled into a mold having a cavity at least 30 mm long and 10 to 13 mm wide, and the iron-based powder mixture for powder metallurgy is compression-molded using a punch to obtain a test specimen. The molding pressure is 500 MPa or more. Next, the test specimen is placed on two support rollers arranged in parallel with a span L shorter than the length of the test specimen. A compressive load is applied to the center of the span L using the loading rollers, and the load P at which the test specimen breaks is recorded. Finally, the strength S of the iron-based green compact is calculated using the following formula (1): where t is the thickness of the test specimen and w is the width of the test specimen.

[0043]

[0044] The Japanese Industrial Standard JIS Z 2511:2006 is a standard established based on ISO 3995:1985, "Metallic powders - Determination of green strength by transverse rupture of rectangular compacts." The content of the Japanese Industrial Standard JIS Z 2511:2006 standard is almost the same as the content of the above ISO standard.

[0045] [Iron-based Compact] In another embodiment, the present invention relates to an iron-based compact made from the iron-based mixed powder for powder metallurgy according to the present invention. As described above, the iron-based compact made from the iron-based mixed powder for powder metallurgy according to the present invention has improved compact strength compared to iron-based compacts made from conventional iron-based mixed powders for powder metallurgy. Therefore, it is not easily broken even after green processing, maintains its original shape, and is less likely to chip during handling. This reduces the defect rate and also significantly reduces the total time and cost required for processing compared to finishing a final product of the same shape by processing only an iron-based sintered body. The type of green processing is not particularly limited and includes all types of processing, such as cutting, drilling, and polishing.

[0046] [Method for Producing Iron-Based Mixed Powder for Powder Metallurgy] In another embodiment, the present invention provides a method for producing an iron-based mixed powder for powder metallurgy, which comprises mixing an iron-based powder with one or more binders having a melting point of 60°C or higher and 120°C or lower to form a mixture, stirring the mixture while heating it to a temperature equal to or higher than the melting point of the binder, and then cooling the mixture. By heating the mixture to a temperature equal to or higher than the melting point of the binder, the binder melts and becomes liquid. By stirring the mixture in this state, the surface of the iron-based powder is coated with the molten binder. Thereafter, by gradually cooling the mixture while continuing stirring, the liquid binder solidifies on the surface of the iron-based powder. As a result, an iron-based mixed powder for powder metallurgy can be produced in which the binder adheres to the surface of the iron-based powder.

[0047] The means for mixing the iron-based powder and binder to form a mixture and the means for stirring the mixture while heating are not particularly limited, and various known mixers can be used. Examples of mixers that can be used include high-speed bottom-stirring mixers, tilted rotating pan mixers, rotary hoe mixers, and conical planetary screw mixers. Mixers equipped with a propeller that rotates at high speed are more preferred because they can coat the iron-based powder with the binder and bond it to the alloy powder while preventing agglomeration of the iron-based mixed powder for powder metallurgy. If the mixer is equipped with a means for heating the contents, mixing at room temperature and stirring while heating can be performed continuously using the same mixer.

[0048] The temperature to which the mixture is heated after mixing is a temperature equal to or higher than the melting point of the binder. If the heating temperature is a temperature equal to or higher than the melting point plus 10°C, the surface tension of the binder in a liquid state decreases, allowing it to quickly wet and spread over the surface of the iron-based powder. Therefore, it is preferable that the temperature to which the mixture is heated is a temperature equal to or higher than the melting point of the binder plus 10°C. Furthermore, if the heating temperature is a temperature equal to or lower than the melting point plus 100°C, it is possible to prevent a decrease in the binding function of the binder due to thermal decomposition. Therefore, it is preferable that the temperature to which the mixture is heated is a temperature equal to or higher than the melting point of the binder plus 100°C. When two or more binders are used as binders, it is preferable that the temperature to which the mixture is heated is a temperature equal to or higher than the lowest melting point of the binders plus 10°C and lower than the highest melting point.

[0049] In a preferred embodiment, the alloying powder is further mixed with the binder, thereby bonding the alloying powder to the surface of the iron-based powder. This makes it possible to suppress component segregation and dust generation in the iron-based mixed powder for powder metallurgy. In this case, the order in which the iron-based powder, binder, and alloying powder are mixed can be any order. For example, they may all be mixed from the beginning and stirred while heated. Alternatively, the iron-based powder and binder may be mixed and stirred while heated, and then cooled, and the alloying powder may be mixed with the binder and stirred again while heated.

[0050] As described above, the binder contained in the iron-based mixed powder for powder metallurgy according to the prior art has a relatively high melting point of over 120° C. This means that the temperature at which the mixture of the iron-based powder and the binder is heated must also exceed 120° C., which poses a problem of increased energy consumption for heating. In contrast, the method for producing an iron-based mixed powder for powder metallurgy according to the present invention uses a binder with a melting point of 60° C. or more and 120° C. or less, making it possible to reduce the amount of energy consumed for heating the mixture compared to the prior art.

[0051] In addition, when a binder with a melting point of 60°C or higher and 120°C or lower is used, the viscosity of the binder is expected to be lower under heating temperatures than conventional binders with melting points above 120°C. Therefore, when bonding an alloy powder to an iron-based powder using a binder, the bonding of the iron-based powder and the alloy powder can be completed in a shorter time than with conventional binders, thereby reducing the energy consumption required for mixing the mixture. Therefore, according to the present invention, by using a binder with a melting point of 60°C or higher and 120°C or lower, not only can the green strength of the iron-based powder compact be increased, but the energy consumption required for heating and mixing the mixture can also be reduced.

[0052] In a preferred embodiment, a lubricant is further mixed into an iron-based mixed powder for powder metallurgy, which contains an iron-based powder and a binder, and preferably further contains an alloying powder. This can improve the moldability of the iron-based mixed powder for powder metallurgy. In this case, the lubricant is preferably mixed at a temperature lower than the melting point of the binder so that the binder adhering to the surface of the iron-based powder does not melt.

[0053] [Method for manufacturing an iron-based compact] In another embodiment, the present invention is a method for manufacturing an iron-based compact, in which pressure molding is performed using as a raw material the iron-based mixed powder for powder metallurgy manufactured by the above-mentioned manufacturing method. As described above, the binder mixed with the iron-based mixed powder for powder metallurgy according to the present invention has a melting point of 60° C. or more and 120° C. or less. This makes it possible to improve the green strength of the iron-based compact obtained by the present invention compared to the green strength of an iron-based compact manufactured using a conventional iron-based mixed powder for powder metallurgy as a raw material.

[0054] The pressure molding method is not particularly limited, and a commonly used method can be used. Examples of pressure molding methods include filling a powder metallurgy mixed powder into a mold and then pressure molding. The pressure for pressure molding is not particularly limited, but can be, for example, 400 MPa or more and 1000 MPa or less. The iron-based green compact obtained by the production method according to the present invention has high green strength, so that even before sintering, it can be subjected to green processing such as cutting while suppressing damage, and can be handled.

[0055] [Iron-based sintered body and sintered mechanical part] In another embodiment, the present invention is an invention of an iron-based sintered body using the above-mentioned mixed powder for powder metallurgy as a raw material, or an invention of a sintered mechanical part using the iron-based sintered body. The manufacturing method of the iron-based sintered body is not particularly limited, but it can usually be manufactured by sintering an iron-based green compact manufactured by the above-mentioned manufacturing method.

[0056] As described above, the iron-based sintered body and sintered mechanical component according to the present invention are not easily broken and maintain their original shape even when green processing is performed on the iron-based powder compact stage before sintering. This makes it possible to significantly reduce the total time required for processing compared to finishing a final product of the same shape by processing only the iron-based sintered body. In particular, when manufacturing an iron-based sintered body and a sintered mechanical component with a shape that is difficult to provide by pressure molding, such as a lateral hole perpendicular to the pressure molding direction, the present invention is extremely effective in reducing processing costs.

[0057] The sintering method is not particularly limited, and can be carried out by a conventional method. The sintering temperature can be 1100°C or higher, preferably 1120°C or higher, from the viewpoint of sufficiently progressing sintering. On the other hand, the higher the sintering temperature, the more uniform the distribution of alloy elements in the sintered body becomes, so the upper limit of the sintering temperature is not particularly limited, but from the viewpoint of suppressing production costs, it is preferably 1250°C or lower, more preferably 1180°C or lower.

[0058] Atomized iron powder was used as the iron-based powder, and electrolytic copper powder and graphite powder were used as the alloy powder. JIP 301A manufactured by JFE Steel Corporation was used as the atomized iron powder. Graphite powder with a specific surface area of ​​2.3 m ２ / g natural graphite powder was used. The ratio of electrolytic copper powder was 2.0 mass%, the ratio of graphite powder was 0.8 mass%, and the remainder was atomized iron powder. Thirteen types of mixtures were prepared by adding binders shown in Examples 1 to 13, each having a melting point of 60°C or higher and 120°C or lower, as shown in Table 1. The amount of binder added shown in Table 1 is expressed as a mass percentage relative to the total of the iron-based powder and alloy powder. In the mixtures of Examples 11 to 13, two or three types of binders were added in the amounts shown in Table 1. Similarly, three types of mixtures were prepared by adding binders shown in Comparative Examples 1 to 3, each having a melting point above 120°C, as shown in Table 1.

[0059] Next, these 16 types of mixtures were individually placed in a mixer, and the mixtures were mixed while being heated to a temperature between 10°C and 100°C above the melting point of the binder shown in Table 1, and then cooled. However, for the mixtures of Invention Examples 11 to 13, the mixtures were mixed while being heated to a temperature between 10°C and the highest melting point of the binders contained in the mixture, and then cooled.

[0060] Next, to the mixtures that had been mixed under heating and cooled, 0.25 mass % of stearic acid amide, 0.25 mass % of ethylene bisstearic acid amide, and 0.1 mass % of zinc stearate, expressed as mass percentages relative to the total mass of the iron-based powder and the alloy powder, were added as lubricants that were present in a state free from the iron-based powder, and the mixtures were mixed at room temperature to obtain 16 types of iron-based mixed powders for powder metallurgy.

[0061] Next, according to the method specified in Japanese Industrial Standard JIS Z 2511:2006, 15 g of the obtained iron-based mixed powder for powder metallurgy was filled into a mold having a length of 35 mm and a width of 10 mm, and a pressure of 686 MPa was applied at room temperature to prepare a test piece. The obtained test piece was subjected to a bending test specified in the same standard to measure the green strength. The measurement results are shown in Table 1.

[0062]

[0063] According to Table 1, the iron-based mixed powders for powder metallurgy of Examples 1 to 13, which contain one or more binders with a melting point of 120° C. or lower, all have test pieces with a green strength exceeding 10.0 MPa, indicating that these test pieces have better green strength than the iron-based mixed powders for powder metallurgy of Comparative Examples 1 to 3, which contain binders with a melting point higher than 120° C. In particular, the iron-based mixed powders for powder metallurgy of Examples 1 and 8 to 12, which contain oleic acid amide with a melting point of 76° C., all have test pieces with a green strength exceeding 11.5 MPa, indicating that these test pieces are superior to the other examples.

Claims

1. An iron-based mixed powder for powder metallurgy containing an iron-based powder and one or more binders selected from the group consisting of oleic acid amide, erucic acid amide, lauric acid amide, palmitic acid amide, stearic acid amide, behenic acid amide, and ethylene bisoleic acid amide, the binder having a melting point of 60°C or higher and 120°C or lower and adhering to the surface of the iron-based powder, wherein the content of the binder relative to the total mass of the iron-based mixed powder for powder metallurgy minus the mass of the binder is 0.01% by mass or higher and 1.0% by mass or lower.

2. The iron-based mixed powder for powder metallurgy according to claim 1, further comprising an alloying powder bound to the surface of said iron-based powder by said binder.

3. The iron-based mixed powder for powder metallurgy according to claim 1 or 2, wherein the iron-based powder is at least one selected from the group consisting of reduced iron powder and atomized iron powder.

4. The iron-based mixed powder for powder metallurgy according to any one of claims 1 to 3, further comprising a lubricant present in a state separated from the iron-based powder.

5. The iron-based mixed powder for powder metallurgy according to any one of claims 1 to 4, wherein a test piece obtained by molding the iron-based mixed powder for powder metallurgy in accordance with Japanese Industrial Standard JIS Z 2511:2006 has a green strength of 10.0 MPa or more, as measured by a flexural test specified in said standard.

6. An iron-based powder compact made from the iron-based mixed powder for powder metallurgy according to any one of claims 1 to 5.

Images