Metal treatment – Process of modifying or maintaining internal physical... – Magnetic materials
Reexamination Certificate
2002-08-05
2004-02-24
Sheehan, John (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Magnetic materials
C148S102000, C148S538000, C148S540000, C164S463000, C164S474000, C164S475000, C164S477000, C164S479000, C164S488000, C164S489000, C164S490000
Reexamination Certificate
active
06695929
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a method of making a material alloy for an iron-based rare earth magnet, for use in, for example, motors and actuators of various types.
BACKGROUND ART
Recently, it has become more and more necessary to further improve the performance of, and further reduce the size and weight of, consumer electronic appliances, office automation appliances and various other types of electric equipment. For these purposes, a permanent magnet for use in each of these appliances is required to maximize its performance to weight ratio when operated as a magnetic circuit. For example, a permanent magnet with a remanence B
r
of 0.5 T or more is now in high demand. Hard ferrite magnets have been used widely because magnets of this type are relatively inexpensive. However, the hard ferrite magnets cannot achieve the high remanence B
r
of 0.5 T or more.
An Sm—Co type magnet, produced by a powder metallurgical process, is currently known as a typical permanent magnet that achieves the high remanence B
r
of 0.5 T or more. However, the Sm—Co type magnet is expensive, because Sm and Co are both expensive materials.
Examples of other high-remanence magnets include an Nd—Fe—B type sintered magnet produced by a powder metallurgical process and an Nd—Fe—B type rapidly solidified magnet produced by a melt quenching process. An Nd—Fe—B type sintered magnet is disclosed in Japanese Laid-Open Publication No. 59-46008, for example, and an Nd—Fe—B type rapidly solidified magnet is disclosed in Japanese Laid-Open Publication No. 60-9852, for instance. The Nd—Fe—B type sintered magnet is mainly composed of relatively inexpensive Fe (typically at about 60 wt % to about 70 wt % of the total weight), and is much less expensive than the Sm—Co type magnet. Nevertheless, it is still expensive to produce the Nd—Fe—B type magnet. This is partly because huge equipment and a great number of manufacturing and processing steps are required to separate and purify, or to obtain by reduction reaction, Nd, which usually accounts for about 10 at % to about 15 at % of the magnet. Also, a sintered compact should be further processed. Furthermore, a powder metallurgical process normally requires a relatively large number of manufacturing and processing steps by its nature.
Compared to an Nd—Fe—B type sintered magnet formed by a powder metallurgical process, an Nd—Fe—B type rapidly solidified magnet can be produced at a lower cost by a melt quenching process. This is because an Nd—Fe—B type rapidly solidified magnet can be produced through relatively simple process steps of melting, melt quenching and heat treating. However, to obtain a permanent magnet in bulk by a melt quenching process, a bonded magnet should be formed by compounding a magnet powder, made from a rapidly solidified alloy, with a resin binder. Accordingly, the magnet powder normally accounts for at most about 80 volume % of the molded bonded magnet. Also, a rapidly solidified alloy, formed by a melt quenching process, is magnetically isotropic.
As for an Nd—Fe—B type rapidly solidified magnet, an alternative magnet material was proposed by R. Coehoorn et al., in J. de Phys, C8, 1998, pp. 669-670. The Coehoorn material has a composition including a rare earth element at a relatively low mole fraction (i.e., around Nd
3.8
Fe
77.2
B
19
, where the subscripts are indicated in atomic percentages); and an Fe
3
B phase as its main phase. This permanent magnet material is obtained by heating and crystallizing an amorphous alloy that has been prepared by a melt quenching process. Also, the crystallized material has a metastable structure in which soft magnetic Fe
3
B and hard magnetic Nd
2
Fe
14
B phases coexist and in which crystal grains of very small sizes (i.e., on the order of several nanometers) are distributed finely and uniformly as a composite of these two crystalline phases. For that reason, a magnet made from such a material is called a “nanocomposite magnet”.
It has been proposed that various metal elements be added to the material alloy of a nanocomposite magnet to improve the magnetic properties thereof. See, for example, PCT International Publication No. WO 003/03403 and W. C. Chan et. al., “The Effects of Refractory Metals on the Magnetic Properties of &agr;-Fe/R
2
Fe
14
B-type Nanocomposites”, IEEE Trans. Magn. No.5, INTERMAG. 99, Kyongiu, Korea, pp. 3265-3267, 1999.
However, in producing a nanocomposite magnet by a melt quenching process, the microcrystalline structure of a rapidly solidified alloy is seriously affected by how a melt of a material alloy contact the surface of a chill roller, and the resultant magnet properties may sometimes deteriorate. Particularly when a nanocomposite magnet was be made by a strip casting process, the present inventors found it very difficult to obtain a rapidly solidified alloy having the desired micro crystalline structure uniformly and with good reproducibility. Specifically, when a melt of the material alloy was fed onto a chill roller by way of some guide such as a shoot, an oxide film was easily formed on the surface of the melt on the shoot. In that case, the melt flow was obstructed by the oxide film, and the rapid cooling process could not be carried out uniformly enough.
DISCLOSURE OF INVENTION
In order to overcome the problems described above, preferred embodiments of the present invention provide (1) a method of making an iron-based rare earth magnet material alloy that has a uniform microcrystalline structure required for a high-performance nanocomposite magnet by performing the manufacturing processing step of rapidly cooling and solidifying a molten alloy using a chill roller, constantly and with good reproducibility, and (2) a method for producing a permanent magnet by using the iron-based rare earth magnet material alloy.
According to a preferred embodiment of the present invention, a method of making a material alloy for an iron-based rare earth magnet includes the step of preparing a melt of an iron-based rare earth material alloy having a composition represented by the general formula (Fe
1-m
T
m
)
100-x-y-z
Q
x
R
y
M
z
. In this formula, T is at least one element selected from the group consisting of Co and Ni; Q is at least one element selected from the group consisting of B and C; R is at least one element selected from the group consisting of Y (yttrium) and the rare earth elements; and M is at least one element selected from the group consisting of Al, Si, Ti, V, Cr, Mn, Cu, Zn, Ga, Zr, Nb, Mo, Ag, Hf, Ta, W, Pt, Au and Pb. The mole fractions x, y, z and m satisfy the inequalities of: 10 at %≦x≦30 at %; 2 at %≦y<10 at %; 0 at %≦z≦10 at %; and 0≦m≦0.5, respectively. The method further includes the steps of feeding the melt of the material alloy onto a guide and forming a flow of the melt on the guide for tranfer to a chill roller so as to move the melt onto a region where the melt comes into contact with the chill roller; and rapidly cooling the melt using the chill roller to make a rapidly solidified alloy. The method further includes the step of controlling an oxygen concentration of the melt yet to be fed onto the guide such that the oxygen concentration is about 3,000 ppm or less in mass percentage.
In one preferred embodiment of the present invention, the method further includes the step of controlling a kinematic viscosity of the melt yet to be fed onto the guide such that the kinematic viscosity is about 5×10
−6
m
2
/sec or less.
In another preferred embodiment of the present invention, the rapid cooling step includes the step of using, as the guide, a shoot that controls the flow of at least a portion of the melt running down toward the surface of the chill roller rotating to bring the melt into contact with the surface of the chill roller such that the melt has a predetermined width in an axial direction of the chill roller. The shoot is disposed near the chill roller and includes a melt drain that has the predetermined width in the axial direction of the chill roller. The rapid cooling step further inclu
Hirosawa Satoshi
Kanekiyo Hirokazu
Keating & Bennett LLP
Sheehan John
Sumitomo Special Co., Ltd.
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