WO2014103287A1 - 粉末冶金用鉄基粉末 - Google Patents
粉末冶金用鉄基粉末 Download PDFInfo
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- WO2014103287A1 WO2014103287A1 PCT/JP2013/007553 JP2013007553W WO2014103287A1 WO 2014103287 A1 WO2014103287 A1 WO 2014103287A1 JP 2013007553 W JP2013007553 W JP 2013007553W WO 2014103287 A1 WO2014103287 A1 WO 2014103287A1
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- powder
- iron
- binder
- carbon black
- iron powder
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C33/00—Making ferrous alloys
- C22C33/02—Making ferrous alloys by powder metallurgy
- C22C33/0207—Using a mixture of prealloyed powders or a master alloy
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B22—CASTING; POWDER METALLURGY
- B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
- B22F1/00—Metallic powder; Treatment of metallic powder, e.g. to facilitate working or to improve properties
- B22F1/05—Metallic powder characterised by the size or surface area of the particles
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B22—CASTING; POWDER METALLURGY
- B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
- B22F1/00—Metallic powder; Treatment of metallic powder, e.g. to facilitate working or to improve properties
- B22F1/10—Metallic powder containing lubricating or binding agents; Metallic powder containing organic material
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B22—CASTING; POWDER METALLURGY
- B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
- B22F1/00—Metallic powder; Treatment of metallic powder, e.g. to facilitate working or to improve properties
- B22F1/10—Metallic powder containing lubricating or binding agents; Metallic powder containing organic material
- B22F1/102—Metallic powder coated with organic material
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B22—CASTING; POWDER METALLURGY
- B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
- B22F1/00—Metallic powder; Treatment of metallic powder, e.g. to facilitate working or to improve properties
- B22F1/10—Metallic powder containing lubricating or binding agents; Metallic powder containing organic material
- B22F1/108—Mixtures obtained by warm mixing
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B22—CASTING; POWDER METALLURGY
- B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
- B22F1/00—Metallic powder; Treatment of metallic powder, e.g. to facilitate working or to improve properties
- B22F1/16—Metallic particles coated with a non-metal
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B22—CASTING; POWDER METALLURGY
- B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
- B22F1/00—Metallic powder; Treatment of metallic powder, e.g. to facilitate working or to improve properties
- B22F1/17—Metallic particles coated with metal
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
- Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
- Y10T428/00—Stock material or miscellaneous articles
- Y10T428/29—Coated or structually defined flake, particle, cell, strand, strand portion, rod, filament, macroscopic fiber or mass thereof
- Y10T428/2982—Particulate matter [e.g., sphere, flake, etc.]
- Y10T428/2991—Coated
Definitions
- the present invention relates to an iron-based powder that is suitable for use in powder metallurgy and has an excellent ability to prevent segregation.
- powder metallurgy technology can produce machine parts with complex shapes with extremely high dimensional accuracy, it is possible to significantly reduce the manufacturing costs of the machine parts. Therefore, various machine parts manufactured by applying the powder metallurgy technique are used in various fields. Furthermore, recently, there is an increasing demand for downsizing or weight reduction of machine parts, and various powders for powder metallurgy for producing machine parts having a small size, light weight and sufficient strength have been studied.
- Patent Documents 1, 2, and 3 disclose raw material powder for powder metallurgy in which an alloy powder is adhered to the surface of iron powder or alloy steel powder.
- Such iron-based powders (hereinafter referred to as iron-based powders) are usually sub-material powders (eg, copper powder, graphite powder, iron phosphide powder, manganese sulfide powder) and lubricants (eg, zinc stearate). , Aluminum stearate, etc.) are added, and the resulting mixed powder is used for the manufacture of machine parts.
- sub-material powders eg, copper powder, graphite powder, iron phosphide powder, manganese sulfide powder
- lubricants eg, zinc stearate
- the fluidity of the mixed powder is not uniform because the iron-based powder, the auxiliary raw material powder, and the lubricant have different properties (shape, particle size, etc.). That is, (a) Under the influence of vibration or dropping during the transportation of the mixed powder to the storage hopper, the iron-based powder, the auxiliary raw material powder, and the lubricant are locally unevenly distributed. (b) Since a relatively large gap occurs between the particles of the mixed powder charged into the hopper, the apparent density of the mixed powder is reduced. (c) While the apparent density of the mixed powder deposited on the lower part of the hopper increases with time (i.e., under the influence of gravity), the upper apparent density is stored in a low state. In the lower part, there was a problem that the apparent density of the mixed powder was not uniform. That is, in the prior art, when mixed powder is used, it is extremely difficult to manufacture a large amount of mechanical parts having uniform strength.
- Patent Document 4 discloses an iron-based powder mainly composed of iron powder having a particle size in a predetermined range.
- iron powder that falls outside the specified range cannot be used, so not only the yield of iron powder is reduced, but also the iron-based powder is uniformly and sufficiently filled in a thin cavity such as a gear edge. It is difficult.
- Patent Document 5 discloses that 0.005 to 2% by weight of SiO 2 having a particle size of less than 40 nm is contained to improve fluidity during warm forming.
- this technique has a problem that silicon dioxide remains during sintering and inhibits the sintering between the iron powder particles, so that the strength of the obtained sintered body is reduced.
- Patent Document 6 discloses a method for increasing the fluidity of a powder metallurgy composition containing iron or an iron-based metal powder, a lubricant and / or a binder. Shows a method of adding 0.001 to 0.2% by weight of carbon black having a specific surface area of less than 200 nm and a specific surface area of greater than 100 m 2 / g.
- Patent Document 7 discloses an iron-based powder metallurgy composition containing iron powder or iron-based powder and a particulate composite lubricant, wherein the composite lubricant is carbon on a solid organic lubricant material.
- a composition is shown that contains particles having a core containing the organic lubricating material to which the microparticles are deposited. This is a technology that mixes carbon powder finely attached to the surface of the lubricant in advance with the iron powder before mixing the iron powder and the lubricant. By doing this, the fluidity is excellent and the lubricant It is intended to prevent aggregation between each other.
- Patent Document 8 discloses that fluidity improving particles containing 50 to 100% by mass of carbon black are passed through a binder having a penetration of 0.05 to 2 mm on the surface of iron powder.
- An iron-base powder for powder metallurgy is shown in which the coverage of iron powder by the binder is 10% to 50% and the coverage of the binder by the flowability improving particles is 50% or more. Yes.
- Patent Document 7 needs to provide a step of adhering carbon fine particles to the lubricant surface in advance, which is inefficient and at the same time has a density difference from iron powder.
- the problem of powder segregation has not been solved.
- Patent Document 8 uses a powder having general lubricity as a binder, when the coverage of the iron powder surface by this binder is 50% or less, the iron powder If it is molded as it is, it will be seized on the mold, the output will be high, or the molded body may be abnormal in appearance or damaged in some cases.
- Patent Document 8 and the like contain not only a lubricant as a binder but also a so-called free lubricant that is not bound to iron powder and contains about 0.1 to 1.0%. Yes.
- these lubricants are newly added and mixed after the segregation preventing treatment with the binder. However, at this time, if the mixing temperature is too high, the lubricants may aggregate to produce abnormal aggregated particles.
- the present invention has been developed in view of the above-mentioned present situation, and by effectively preventing the aggregation of the lubricant, it has excellent fluidity and can be uniformly filled even in a thin-walled cavity, while being molded.
- An object of the present invention is to provide an iron-base powder for powder metallurgy that can suppress the subsequent extraction force, does not cause poor appearance of the molded body and the sintered body, and does not decrease the strength of the sintered body.
- the iron powder or alloy steel powder used as the material of the iron-based powder includes atomized iron powder, reduced iron powder, etc. depending on the production method. In these classifications, iron powder is used in a broad sense including alloy steel powder. It has been.
- segregation prevention treatment for powder metallurgy is performed by combining iron powder and alloying components such as graphite, copper and Ni powder, and cutting improvers such as MnS, CaF 2 and talc. Are mixed together, and this auxiliary material is adhered to the surface of the iron powder with this binder.
- a resin such as cellulose ester resin or a material having lubricity is selected as the binder.
- the purpose of this is to reduce friction between particles, improve fluidity, apparent density, and compressibility during molding, and reduce friction with the mold surface during molding to improve compressibility and extractability. It also has a purpose. However, for the latter purpose, it is only necessary to have the lubricity of the iron powder in the part in contact with the mold. Does not contribute.
- a lubricant in order to improve the lubricity with the mold surface, there is a method of adding a lubricant separately from the binder.
- a lubricant added in this manner is called a free lubricant.
- Free lubricant is generally wax or metal soap powder, and has a specific gravity difference from iron powder. Therefore, even when it is in a mixed state with iron powder, it is discharged from the mixture and filled on the mold surface when filling the mold. Easy to adhere.
- the conventional segregation preventing iron powder has a lubricant used as a binder and a free lubricant powder added and mixed separately, and is added in an amount of about 0.4 to 1.5% by mass as a whole.
- the binder is often used in the range of about 0.1 to 0.6% by mass
- the free lubricant is often used in the range of about 0.2 to 1% by mass.
- the free lubricant has an average particle diameter of 5 to 40 ⁇ m and a relatively low melting point, and its melting point is relatively low. Therefore, the particles are likely to aggregate and may produce aggregated particles during mixing. Many.
- the gist of the present invention is as follows. 1. Either the alloying component and the cutting improver, or both the alloying component and the cutting improving agent are adhered to the surface of the iron powder for powder metallurgy by a binder having a melting point of 150 ° C. or lower. An iron-based powder for powder metallurgy having carbon black adhered and a free binder of 0.02% by mass or less.
- a powder that uniformly fills a thin-walled cavity and keeps the output power after molding low does not cause poor appearance of the molded body and the sintered body, and does not decrease the strength of the sintered body.
- An iron-based powder for metallurgy can be obtained.
- a high-speed mixer which is a kind of mechanical stirring mixer, iron powder, various alloying components such as graphite, Cu powder and Ni powder, and machinability such as MnS powder, CaF 2 powder and talc
- a high-speed mixer which is a kind of mechanical stirring mixer, iron powder, various alloying components such as graphite, Cu powder and Ni powder, and machinability such as MnS powder, CaF 2 powder and talc
- iron-based powder for powder metallurgy which is prepared by heating and mixing an improver and the like together with a binder and further adding a lubricant for ensuring moldability, when adding and mixing the binder and lubricant, It is characterized in that it is produced by adding and mixing a binder and carbon black instead of adding a binder and a lubricant.
- the iron powder for powder metallurgy according to the present invention has either an alloying component and a cutting improver, or both an alloying component and a cutting improver attached to the surface of the binder by the binder, and further to the binder surface. It is characteristic that carbon black adheres.
- a schematic diagram of the iron-based powder used in the present invention is shown in FIG. In the figure, 1 is iron powder, 2 is an alloying component (graphite), 3 is an alloying component (copper powder), and 4 is a binder. Therefore, in the present invention, carbon black (not shown) adheres to the surface of the binder 4 shown in FIG.
- the binder has a melting point of 150 ° C. or less. These are the same as those conventionally used as binders or lubricants. However, in the present invention, by limiting the melting point, the following step of adding and mixing carbon black is realized and released. It has an unprecedented feature of reducing the binder.
- the binder is melted once by heat mixing to uniformly wet individual iron powder particles and alloying components, and then cooled and solidified, and fixed to the iron powder surface. If the temperature exceeds 150 ° C. and is too high, it takes time for subsequent cooling, and it is not efficient for the present invention having a step of adding and mixing the fluidity improving particles, and carbon black is embedded in the binder layer. It is because it becomes easy. On the other hand, if it is 150 ° C. or less, heating and cooling mixing can be performed in about 1 hour per cycle. Therefore, it is important that the melting point of the binder used here is 150 ° C. or less. In addition, although the minimum of melting
- the binder can be any one that is heated and melted, or heated and solidified, but it must have lubricity after solidification. The reason is to reduce the frictional force between the powder particles, improve the fluidity of the powder, and promote the rearrangement of particles in the initial stage of molding.
- one of fatty acid, fatty acid amide, fatty acid bisamide, and metal soap, or a mixture thereof is preferable.
- Amide wax, polyamide, polyethylene, polyethylene oxide, and the like can also be used.
- zinc stearate, lithium stearate, calcium stearate, stearic acid monoamide, and ethylene bisstearamide are preferred.
- These binders may be used alone or in combination of two or more.
- the carbon black used here is used in toners and paints, and the specific surface area is desirably 50 m 2 / g or more and 100 m 2 / g or less. This is because when the specific surface area is less than 50 m 2 / g, the particle size is large, and accordingly, it is necessary to increase the amount of addition in order to coat the binder surface, and the compressibility of the mixed powder tends to deteriorate. On the other hand, when the specific surface area is more than 100 m 2 / g, the dimensions fluctuate at the time of sintering, and the mechanical properties deteriorate. Therefore, the specific surface area of carbon black is desirably 50 m 2 / g or more and 100 m 2 / g or less.
- the method for measuring the specific surface area of carbon black preferably follows the BET method (JIS K 6217).
- the average particle size of carbon black is not particularly limited, but is preferably in the range of 5 to 500 nm. If the average particle size of the carbon black is less than 5 nm, there is a possibility that the surface of the iron powder is uneven or buried in the lubricant present on the iron powder surface. In addition, these fine particles are present in an aggregated state, but if it is too fine, it is not preferable because the aggregate remains attached to the iron powder surface. On the other hand, when the average particle diameter of carbon black exceeds 500 nm, the curvature of the unevenness present on the surface of the iron powder from the beginning becomes the same, and it makes no sense to bother these particles. For these reasons, the average particle size of the fluidity improving particles is preferably in the range of 5 to 500 nm.
- the average particle diameter of carbon black is an arithmetic average diameter obtained by observing the carbon black particles with an electron microscope.
- the amount of carbon black added is less than 0.01 parts by mass with respect to 100 parts by mass of iron powder, the coverage of the binder surface may be insufficient, and the effect of improving fluidity is hardly seen.
- the added amount exceeds 3 parts by mass, the free powder increases, and at the same time, when molded at the same pressure, the density of the green compact decreases and the strength of the sintered body decreases, which is not preferable. . Therefore, the amount of carbon black added is preferably in the range of 0.01 to 3 parts by mass with respect to 100 parts by mass of iron powder.
- the present invention is characterized in that the coverage of the iron powder surface with the binder is 30% to 100%, preferably 50% to 100% of the iron powder surface area. If the coverage is less than 30%, alloying components and the like cannot be sufficiently adhered to the iron powder surface. If it is less than 50%, the function as a lubricant may not be sufficiently exhibited. For this reason, the coverage of the iron powder surface by the binder is 30% or more, preferably 40% or more, more preferably 50% or more. On the other hand, the upper limit may be 100%.
- auxiliary materials such as alloying components
- the binder even if these components are heated and mixed and cooled and solidified, not all of the binder adheres to the iron powder surface. Then, the free binder generated at that time causes the auxiliary raw material graphite to aggregate or the free binders to aggregate. Furthermore, the free binder remaining without adhering to the surface of the iron powder not only adversely affects the fluidity but also causes a poor appearance of the molded body and the sintered body in some cases.
- the present invention takes the following method.
- a mixer for fixing the binder and a mixer for adding carbon black are separated. It is preferable to arrange a mixer for heating and mixing at the upper part and a mixer for mixing carbon black at the lower part.
- the mixer for fixing the binder is preferably a mixer capable of heating and cooling and having a relatively strong shearing force, such as a Henschel mixer.
- a mixer capable of heating and cooling and having a relatively strong shearing force such as a Henschel mixer.
- the iron powder, the binder, and the auxiliary material are sufficiently heated and mixed above the melting point of the binder, and then cooled to the melting point of the binder or less.
- the dust is mainly a light component in the mixture, and includes iron powder fine particles, a binder, and the like. It is preferable to collect the dust because the residual binder can be removed.
- the above-mentioned predetermined amount of iron powder is charged into a high-speed mixer that is a first mixer, and an alloy component such as graphite and Cu powder and a binder are added thereto. After adding these raw materials, heating and mixing are started.
- the rotational speed of the rotary blade in the high-speed mixer varies depending on the size of the mixing tank and the shape of the rotary blade, but is generally about 1 to 10 m / s at the peripheral speed of the tip of the rotary blade. Heat and mix until the temperature in the mixing tank is equal to or higher than the melting point of the binder, and mix at a temperature equal to or higher than the melting point for about 1 to 30 minutes. After mixing these raw materials sufficiently, the inside of the mixing tank is cooled. While the binder is solidified during the cooling process, auxiliary materials such as alloy components adhere to the surface of the iron powder.
- the binder is solidified, and then the carbon black does not get into the binder, or the binder and the carbon black do not form agglomerated particles. is there.
- the cooling temperature is considered based on the binder having the lowest melting point.
- a dust collection port is provided near the discharge port to collect light components including residual binder and fine powder.
- a sieve with an opening of about 60 mesh may be provided directly under the discharge port, and the dust generated there may be collected.
- carbon black is added after the binder is solidified and free components are removed. These particles are added after the binder is solidified with a particle size of about 25 to 80 nm. However, since these particles have a very small particle size, they adhere to the iron powder surface by van der Waals force or electrostatic force.
- heating mixing and mixing with carbon black can be performed with one mixer.
- the mixed powder is once discharged after heating and mixing.
- a dust collector is brought close to the discharge port to remove light components such as residual binder.
- a sieve of about 60 mesh can be provided at the outlet, and the dust can be collected by discharging the sieve.
- the method of removing completely the component which has not adhered to iron powder by magnetic selection or wind magnetic selection can be taken.
- the coverage of the binder with carbon black adhered to the binder surface is preferably 30% or more of the binder adhesion area.
- the binder fixed on the surface of the iron powder can reduce the friction between the particles, but the interparticle attractive force and the adhesion force are increased. Therefore, in order to obtain iron powder having a really good flow, it is desirable to cover the surface of the binder with fine particles or the like to reduce the adhesion between the binders.
- Carbon black is suitable for the coating of the binder, and when the coverage is less than 30% of the adhesion area of the binder, it is not very effective in reducing the adhesion force, so it is preferable to be 30% or more.
- the upper limit of the coverage of carbon black is not limited, and may be the entire area of the binder, that is, 100%.
- the specific surface area of the iron powder (iron powder for powder metallurgy) used in the present invention is preferably 0.01 to 0.1 m 2 / g. If the specific surface area of the iron powder is less than 0.01 m 2 / g, the strength of the compact or sintered body will decrease, while if the specific surface area of the iron powder exceeds 0.1 m 2 / g, the surface of the iron powder will be covered. This is because it is necessary to increase the amount of the binder.
- the method for measuring the specific surface area of the iron powder preferably follows the BET method.
- the iron-based powder for powder metallurgy in the present invention combines an iron alloy with an alloying component such as graphite powder and copper powder and / or a cutting improver such as MnS, CaF 2 , enstatite and steatite.
- an alloying component such as graphite powder and copper powder
- a cutting improver such as MnS, CaF 2 , enstatite and steatite.
- the specific surface area of the iron-based powder for powder metallurgy is an important judgment material in determining the appropriate adhesion state and the amount of free carbon black. That is, when carbon black is not sufficiently adhered and is in a free state, the specific surface area of the mixed powder (iron-based powder for powder metallurgy) is increased, while if the adhesion is sufficient, the specific surface area is decreased. Moreover, when it adheres excessively and it gets into a binder, the specific surface area of mixed powder becomes still smaller. Thus, by examining the specific surface area of the iron-based powder for powder metallurgy, it is possible to determine whether the carbon black is attached or not.
- the specific surface area of the iron-base powder for powder metallurgy according to the present invention is preferably 0.05 to 0.5 m 2 / g. If the specific surface area is less than 0.05 m 2 / g, the carbon black may be squeezed into the binder to attach a necessary and sufficient amount to ensure fluidity on the iron powder (binder). Can not. On the other hand, if it exceeds 0.5 m 2 / g, the amount of carbon black in the free state not adhering to the iron powder increases, which hinders the flow of the iron powder.
- the method for measuring the specific surface area of the iron-base powder for powder metallurgy preferably follows the BET method.
- iron powder, Cu powder and graphite powder are used as alloying components, stearamide, erucamide, zinc stearate and ethylene bisstearamide (EBS) are added as binders, and Henschel type
- EBS ethylene bisstearamide
- Henschel type The mixture was heated and mixed with a high-speed mixer, cooled to 80 ° C., and then put into a Nauter mixer. At that time, dust was collected at the high-speed mixer outlet. Next, carbon black was added and mixed under the conditions shown in Table 1.
- the filling property of the iron-based powder obtained by the above procedure was evaluated with a filling tester shown in FIG. Specifically, the evaluation was performed by filling the iron-based powder 6 in the cavity 5 having a length of 20 mm, a depth of 40 mm, and a width of 5 mm.
- the filling density after filling (filling mass / cavity volume) expressed as a percentage of the apparent density before filling is the filling rate (100% filling rate means complete filling), and the same test is repeated 10 times
- the filling variation was expressed as a 100 fraction obtained by dividing the (maximum value)-(minimum value) of the filling rate by the average value of 10 filling rates.
- a 5 mm thick tensile test piece (based on JPMA M 04-1992 No. 2 test piece) and a 10 mm thick impact test piece (compliant with JPMA M 05-1992) were molded. After molding at a pressure of 686 MPa, sintering was performed at 1130 ° C.
- test piece for 20 min in an RX atmosphere to prepare a test piece.
- the tensile strength and impact value were determined using such test pieces (JPMA: conforming to Japanese Powder Metallurgy Industry Standard, test temperature: room temperature).
- Inventive Examples 1 to 8 in Table 2 show the test results.
- three cylindrical tablets having an outer diameter of 11.3 mm ⁇ ⁇ height: 11 mmh were formed, and the surface was visually observed for foreign matters (black spots) of 0.3 mm or more. In the observation, if there was no black spot: ⁇ (good), and even one: x (defect).
- Comparative Example 1 As shown in Table 2, the appearance of Comparative Example 1 was poor. In Comparative Example 2, the filling variation was large, and the appearance was poor. In Comparative Example 3, the filling variation was small, but the appearance was poor, and the strength of the sintered body was lower than that of Comparative Example 1.
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Abstract
Description
すなわち、
(a)混合粉末を貯蔵用のホッパーへ輸送する途中の振動や落下の影響を受けて、鉄基粉末や副原料粉末、潤滑剤が局部的に偏って分布する、
(b)ホッパーに投入された混合粉末の粒子間に比較的大きい隙間が生じるので、混合粉末の見掛け密度が低下する、
(c)ホッパーの下部に堆積した混合粉末の見掛け密度が時間の経過とともに(すなわち重力の影響を受けて)上昇する一方で、上部の見掛け密度は低い状態で貯蔵されるので、ホッパーの上部と下部では混合粉末の見掛け密度が不均一になる
という問題が生じていた。
すなわち、従来技術では、混合粉末を用いた場合、均一な強度を有する機械部品を大量に製造することは極めて困難であった。
そこで、特許文献4には、所定の範囲の粒径を有する鉄粉を主体とする鉄基粉末が開示されている。しかしながら、この技術では、規定された範囲を外れる鉄粉を使用できないので鉄粉の歩留りが低下するばかりでなく、歯車刃先のような薄肉のキャビティーに鉄基粉末を均一かつ十分に充満させることは困難である。
しかしながら、この時、混合温度が高すぎたりすると、潤滑剤同士が凝集して異常な凝集粒を生じる場合がある。このような凝集粒が混入した粉末で成形を行うと、成形体表面に外観異常を生じるだけでなく、焼結時の脱ロウによって、この部分の潤滑剤が抜け、空洞を生じる場合がある。これら空洞は、焼結体表面に存在すると外観不良となって、場合によっては強度低下をも招来してしまう。
上記のようにカーボンブラックの取り扱いには、その特性をよく理解した上で、使用量、使用方法に注意する必要がある。
なお、鉄基粉末の素材となる鉄粉あるいは合金鋼粉は、その製法に応じてアトマイズ鉄粉,還元鉄粉等があり、これらの分類において鉄粉は、合金鋼粉を含む広い意味で用いられている。
そこで、発明者らは、この遊離潤滑剤を低減する方策につき鋭意検討した。その結果、遊離潤滑剤を効果的に低減する方策に想到し、本発明を完成させた。
1.粉末冶金用鉄粉の表面に、合金化成分および切削改善剤のいずれか、または合金化成分および切削改善剤の両方が、融点が150℃以下であるバインダーによって付着し、さらに、そのバインダー表面にカーボンブラックが付着し、かつ遊離バインダーが0.02質量%以下である粉末冶金用鉄基粉末。
本発明では、機械撹拌式混合機の一種である高速ミキサーを用いて、鉄粉と、黒鉛、Cu粉およびNi粉等の各種合金化成分、ならびにMnS粉、CaF2粉およびタルク等の切削性改善剤等を結合剤とともに加熱混合し、さらに成形性を確保するための潤滑剤を添加して作製する粉末冶金用鉄基粉末の製造過程で、結合剤および潤滑剤を添加混合する際に、結合剤および潤滑剤を入れないで、代わりに、バインダーおよびカーボンブラックを添加混合して製造するところに特徴を有している。すなわち、本発明に従う粉末冶金用鉄粉は、その表面に、合金化成分および切削改善剤のいずれか、または合金化成分および切削改善剤の両方が、バインダーによって付着し、さらに、そのバインダー表面にカーボンブラックが付着していることが特徴である。本発明に用いる鉄基粉末の模式図を図1に示す。なお、図中、1は鉄粉、2は合金化成分(黒鉛)、3は合金化成分(銅粉)および4はバインダーである。
従って、本発明では、上記図1のバインダー4の表面にカーボンブラック(図示せず)が付着するのである。
カーボンブラックの平均粒径が5nm未満では、鉄粉表面の凹凸や鉄粉表面に存在する潤滑剤中に埋没する可能性がある。また、これらの微粒子は凝集して存在するが、細か過ぎると凝集体のまま鉄粉表面に付着することになって好ましくない。一方、カーボンブラックの平均粒径が500nmを超えると、初めから鉄粉表面に存在する凹凸の曲率と同じになり、わざわざこれらの粒子を付着させる意味がなくなる。これらの理由から、流動性改善粒子の平均粒径は5~500nmの範囲とすることが好ましい。
なお、カーボンブラックの平均粒径は、カーボンブラック粒子を電子顕微鏡で観察して求めた算術平均径である。
すなわち、カーボンブラックを添加する効果として、鉄粉表面に細かな凹凸を設けることで、粒子間の接触面積を減少し、粒子間の付着力を下げるという効果も考えられる。さらに、鉄粉表面にあるバインダー同士の付着を妨げる効果もある。
被覆率が、30%未満では、合金化成分などを鉄粉表面に十分付着することができない。また、50%未満では、潤滑剤としての機能が充分発揮できない場合がある。このため、バインダーによる鉄粉表面の被覆率は、30%以上であって、好ましくは40%以上、より好ましくは50%以上である。一方、上限は100%で良い。
第一の混合機である高速ミキサーに、前記した所定量の鉄粉を装入し、ここに黒鉛やCu粉等の合金成分と、バインダーを添加する。これらの原料を投入した後、加熱混合を開始する。高速ミキサーにおける回転翼の回転数は、その混合槽の大きさ、回転翼の形状によって異なるが、一般には回転翼先端の周速で1~10m/s程度である。混合槽内の温度がバインダーの融点以上になるまで加熱混合し、融点以上の温度で1~30分程度混合する。これらの原料を十分混合した後、混合槽内を冷却する。冷却過程でバインダーが固化するが、その際、合金成分等の副原料は鉄粉の表面に付着する。
鉄粉表面に固定されたバインダーは、先にも述べたとおり、粒子間の摩擦を低減することはできるが、粒子間引力、付着力は大きくなる。従って、真に流れのよい鉄粉とするためには、このバインダーの表面を微粒子などで覆い、バインダー同士の付着力を低減することが望ましい。
カーボンブラックは、上記バインダーの被覆に適しており、その被覆率が、バインダーの付着面積の30%未満の場合、付着力低減にあまり効果がないため、30%以上とすることが好ましい。なお、カーボンブラックの被覆率の上限に限定はなく、バインダーの付着面積の全て、すなわち、100%であっても良い。
このように、粉末冶金用鉄基粉末の比表面積を調べることで、カーボンブラックの付着状況の良し悪しを判断することができる。
上記比表面積が0.05m2/g未満であると、カーボンブラックは、バインダー内にもぐりこむなどして、鉄粉(結合剤)上に流動性を確保するための必要十分な量を付着させることができない。一方、0.5m2/gを超える場合には、鉄粉に付着していない遊離状態のカーボンブラックが多くなって、それが鉄粉の流動を妨げるものとなるからである。なお、本発明において、粉末冶金用鉄基粉末の比表面積の測定方法は、BET法に従うことが好ましい。
上記により得られた粉末1kgを磁選し、得られた非磁性物(尾鉱)を水中に入れ、沈降しないものを回収、乾燥後、質量測定し、元の粉末質量に対する百分率で表したものを遊離バインダー量とした。
なお、外観は、外径:11.3mmΦ×高さ:11mmhの円筒形タブレットを3個成形し、その表面に、0.3mm以上の異物(黒色斑点)がないかを目視観察した。その観察で、黒色斑点が1個もなければ:○(良)、1個でもあれば:×(不良)と評価した。
2 合金化成分(黒鉛)
3 合金化成分(銅粉)
4 バインダー
5 キャビティー
6 試験鉄粉
7 粉箱
8 移動方向
Claims (7)
- 粉末冶金用鉄粉の表面に、合金化成分および切削改善剤のいずれか、または合金化成分および切削改善剤の両方が、融点が150℃以下であるバインダーによって付着し、さらに、そのバインダー表面にカーボンブラックが付着し、かつ遊離バインダーが0.02質量%以下である粉末冶金用鉄基粉末。
- 前記バインダーによる鉄粉表面に対する被覆率が、鉄粉表面積の30%から100%である請求項1に記載の粉末冶金用鉄基粉末。
- 前記バインダーが、脂肪酸、脂肪酸アミド、脂肪酸ビスアミドおよび金属石鹸のうちの1種、もしくはそれらの混合物である請求項1または2に記載の粉末冶金用鉄基粉末。
- 前記カーボンブラックによる前記バインダー付着面に対する被覆率が、バインダー付着面積の30%以上である請求項1~3のいずれかに記載の粉末冶金用鉄基粉末。
- 前記カーボンブラックの比表面積が、50~100m2/gの範囲である請求項1~4のいずれかに記載の粉末冶金用鉄基粉末。
- 前記鉄粉の比表面積が0.01~0.1m2/gの範囲である請求項1~5のいずれかに記載の粉末冶金用鉄基粉末。
- 前記粉末冶金用鉄基粉末の比表面積が0.05~0.5m2/gの範囲である請求項1~6のいずれかに記載の粉末冶金用鉄基粉末。
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