Specialized metallurgical processes – compositions for use therei – Compositions – Loose particulate mixture containing metal particles
Reexamination Certificate
2002-04-16
2003-10-28
Mai, Ngoclan (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Compositions
Loose particulate mixture containing metal particles
Reexamination Certificate
active
06638335
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to highly compressible iron powder that is suitable for manufacturing electric and mechanical parts that require high magnetism and/or high mechanical strength by powder metallurgy.
2. Description of the Related Art
Powder metallurgy allows production of metallic parts having complicated shapes by near net shape forming and is widely used in production of various parts. Near net shape forming can readily produce target shapes without additional machining.
In powder metallurgy, metal powder such as iron powder having a desired particle size distribution is prepared by controlling the atomizing conditions for molten metal or the reduction conditions for metal oxide as a low material or by classifying powder particles through sieves. The controlled powder is mixed with a lubricant and another metal powder (or other metal powders) for forming an alloy, if necessary. The metal powder or metal powder mixture is compacted in a die and the resulting green compact is sintered or treated with heat to form a part. Alternatively, the metal powder or metal powder mixture is mixed with a binder such as resin and the mixture is compacted in a die.
Such powder metallurgy is employed in production of mechanical parts for use in vehicles and soft magnetic parts such as transformer cores and noise filter cores for eliminating noise in electronic circuits. Higher density is required to maintain high mechanical strength for mechanical parts and high permeability for magnetic parts. High compressibility must be required of iron powder to increase the density of the parts.
For example, Japanese Examined Patent Application Publication No. 8-921 (hereinafter referred to as JP-B2-8-921) discloses an iron powder having the following particle size distribution: On the bases of mass percent of fractions after sieve classification using sieves defined in Japanese Industrial Standard (JIS) Z 8801 (Ed. 1984), the iron powder contains 5% or less of −60/+83-mesh particles that pass through a sieve having a nominal opening of 250 &mgr;m and do not pass through a sieve having a nominal opening of 165 &mgr;m, 4% and more to 10% or less of −83/+100-mesh particles that pass through a sieve having a nominal opening of 165 &mgr;m and do not pass through a sieve having a nominal opening of 150 &mgr;m, 10% and more to 25% or less of −100/+140-mesh particles that pass through a sieve having the nominal opening of 150 &mgr;m and do not pass through a sieve having a nominal opening of 106 &mgr;m and 10% and more to 30% or less particles that pass through a 330-mesh sieve having a nominal opening of 45 &mgr;m. Furthermore, the crystal grain size in iron particles of the particle size of −60/+200-mesh that pass through a sieve having the nominal opening of 250 &mgr;m and do not pass through a sieve having a nominal opening of 75 &mgr;m grows large by the grain size number of 6.0 or less according to a method for measuring a ferrite particle size defined in JIS G 0552 (Ed. 1977). According to JP-B2-8-921, high-density parts are obtained from such a pure iron powder.
The resulting iron powder is compounded with 0.75% zinc stearate as a powder metallurgy lubricant and the resulting compound is compacted under a compacting pressure of 490 MPa. However, the density of the green compact is 7.08 to 7.12 g/cm
3
(7.08 to 7.12 Mg/m
3
). When this pure iron powder is used in magnetic parts such as magnetic cores, the parts do not have satisfactorily high flux density and permeability. Accordingly, the green density is still insufficient.
Nowadays, iron powder metallurgy parts must have higher strength to reduce the volume and weight of mechanical parts for vehicles. In general powder metallurgy, high-strength parts are produced by a double-press double-sintering method including a first compaction and sintering step and a second compaction and sintering step. Alternatively, the high-strength parts are produced by a sinter forging process including a compaction and sintering step and a hot forging step. Unfortunately, these processes increase production costs.
It would, therefore, be advantageous to provide a highly compressible iron powder suitable for production of magnetic parts having excellent magnetic characteristics and mechanical parts having high mechanical strength.
SUMMARY OF THE INVENTION
We have discovered that a highly compressible iron powder can be obtained by controlling the particle sizes of iron powder and by softening coarse iron particles. We have further discovered that a green density higher than 7.20 Mg/m
3
can be attained by using this iron powder in a one-stage compaction process substantially at room temperature and about 490 MPa.
According to a first aspect of the invention, a highly compressible iron powder for powder metallurgy comprises, on the basis of mass percent of fractions after sieve classification using sieves defined in Japanese Industrial Standard (JIS) Z 8801-1:00 Edition 2000), substantially 0% particles that do not pass through a sieve having a nominal opening of 1 mm; more than 0% to about 45% or less particles that pass through a sieve having a nominal opening of 1 mm and do not pass through a sieve having a nominal opening of 250 &mgr;m; about 30% and more to about 65% or less particles that pass through a sieve having a nominal opening of 250 &mgr;m and do not pass through a sieve having a nominal opening of 180 &mgr;m; about 4% and more to about 20% or less particles that pass through a sieve having a nominal opening of 180 &mgr;m and do not pass through a sieve having a nominal opening of 150 &mgr;m; and about 0% and more to about 10% or less particles that pass through a sieve having a nominal opening of 150 &mgr;m, wherein the Vickers microhardness of the particles that do not pass through a sieve having a nominal opening of 150 &mgr;m is at most about 110. The iron powder does not substantially contain particles that do not pass through a sieve having a nominal opening of 1 mm.
According to a second aspect of the invention, a highly compressible iron powder for powder metallurgy comprises, on the basis of mass percent of fractions after sieve classification using sieves defined in Japanese Industrial Standard (JIS) Z 8801-1:00 Edition 2000), substantially 0% particles that do not pass through a sieve having a nominal opening of 1 mm; more than 0.0% to about 2% or less particles that pass through a sieve having a nominal opening of 1 mm and do not pass through a sieve having a nominal opening of 180 &mgr;m; about 30% and more to about 70% or less particles that pass through a sieve having a nominal opening of 180 &mgr;m and do not pass through a sieve having a nominal opening of 150 &mgr;m; and about 20% and more to about 60% or less particles that pass through a sieve having a nominal opening of 150 &mgr;m, wherein the Vickers microhardness of the particles that do not pass through a sieve having a nominal opening of 150 &mgr;m is at most about 110. Also, the iron powder does not substantially contain particles that do not pass through a sieve having a nominal opening of 1 mm.
Preferably, the impurity contents in the iron powder, on the basis of mass percent, are: C≦ about 0.1%, Si≦ about 0.1%, Mn≦ about 0.5%, P< about 0.02%, S≦ about 0.01%, O≦ about 1%, and N≦ about 0.01%. More preferably, the impurity contents in the iron powder, on the basis of mass percent, are: C≦ about 0.005%, Si≦ about 0.01%, Mn≦ about 0.05%, P≦ about 0.01%, S≦ about 0.01%, O≦ about 0.10%, and N≦ about 0.003%.
Preferably, the iron powder is formed by a water atomizing process.
The highly compressible iron powder according to the invention is suitable for production of magnetic parts having high magnetism and mechanical parts having high mechanical strength.
DETAILED DESCRIPTION
The particle size distribution of iron powder in the invention is based on the mass percent of fractions after sieve classification using sieves defined in JIS Z 8801-1:00
Nakamura Naomichi
Ozaki Yukiko
Ueta Masateru
Kawasaki Steel Corporation
Mai Ngoclan
Piper Rudnick LLP
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