Metal treatment – Process of modifying or maintaining internal physical... – Magnetic materials
Reexamination Certificate
2000-06-19
2002-09-03
Sheehan, John (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Magnetic materials
C419S012000, C148S302000, C075S255000
Reexamination Certificate
active
06444048
ABSTRACT:
TECHNICAL FIELD
The present invention relates to an alloy used for producing a high-performance R—T—B sintered magnet and a method for producing the sintered magnet. More particularly, the present invention relates to a raw-material alloy, which is used for producing a high coercive-force R—T—B sintered magnet which is used, in turn, mainly for a motor or the like, and it relates to a method for producing such sintered alloy.
BACKGROUND TECHNIQUE
The R—T—B sintered magnet, in which R is at least one rare-earth element including Y, and T is Fe or a transition element, a part of which may be replaced with one of or both Co and Ni, is a representative high-performance magnet. The R—T—B sintered magnet is indispensable functional material, which supports miniaturizing, weight reducing and performance-enhancing of the magnet-utilizing parts. The R—T—B sintered magnet is applied in a broad field, such as electronics manufacture, various motors for OA, FA, diagnosis apparatuses for medical use and the like. Recently, the R—T—B sintered magnet is used in various motors for automobiles.
The R—T—B sintered magnet consists of a ferromagnetic. R
2
T
14
B phase, on which the magnetism is based, R-rich-phase (the non-magnetic phase with high concentration of a rare-earth element such as Nd), and B-rich phase (the B-rich non-magnetic phase, for example, Nd
1.1
FeB
4
phase in a case where R is Nd).
The raw-material alloy, which is used for producing the R—T—B sintered alloy, also usually consists of the R
2
T
14
B phase, the R-rich phase and the B-rich phase. Among these phases, the R-rich phase plays the role of supporting the liquid-phase sintering. This phase plays, therefore, an important role of enhancing the characteristics of the sintered magnet and is, hence, indispensable. Since the R-rich phase is easily oxidizable, it is oxidized in the production steps of the sintered magnet. The R content of the sintered alloy is considerably more than that of the R
2
T
14
B, which is 11.8 at %. This enables an effective R-rich phase, even after the oxidation, to remain at a certain greater level during the sintering.
On the other hand, the volume fraction of the R
2
T
14
B phase, i.e., the ferromagnetic phase, must be increased as the performance of the sintered magnet is more enhanced. This, in turn, leads to decrease in the volume fraction of R-rich phase. Therefore, when the raw-material alloy is cast by the block mold casting method, the R-rich phase detrimentally disperses in the ingot and becomes locally insufficient. When the raw material powder crushed from such ingot is used for the sintered magnet, it is difficult to obtain satisfactory magnetic properties.
Meanwhile, the dendritic &agr; Fe phase is more likely to form in the alloy which has a higher composition of the R
2
T
14
B phase volume fraction. This &agr; Fe phase dedrimentally impairs the crushability of the raw-material alloy, so that the composition of the crushed powder varies. The magnetic properties of the sintered magnet are lowered and increasingly disperse as well. A considerable amount of the &agr; Fe phase can be diminished by means of heat treating the raw material in inert gas, such as Ar, or under vacuum at 1000° C. or higher for an extended time. However, upon application of this heat treatment, since the dispersion of the R-rich phase is impaired, the magnetic properties cannot be improved.
Such problems are involved in the production of a high-performance sintered magnet. A strip casting method is proposed to solve these problems (for example, Japanese Unexamined Patent Publication (kokai) No. 5-22488 and Japanese Unexamined Patent Publication (kokai) 5-295490). This method resides in the production of an alloy by means of feeding melt on the surface of a rotary roll, while the circumferential speed of the roll and melt-feeding amount are controlled to produce a thin strip alloy having from approximately 0.1 to 0.5 mm of average thickness. Therefore, this method enables a higher cooling speed in solidification than in the conventional block-mold casting method. It is, therefore, possible to finely disperse the R-rich phase and to suppress formation of the dendritic &agr; Fe phase in the alloy produced. By means of this method, no dendritic &agr; Fe phase can be formed in the alloy, for example in the case of Nd-Fe-B based alloy, as long-as Nd content is up to approximately 28.5% by weight.
Meanwhile, a two-alloy mixing method has been proposed (for example Japanese Unexamined Patent Publication (kokai) No. 4-338607), such that an R—T—B alloy with a low R content (hereinafter referred to as “the main-phase alloy”) and an R—T or R—T—B alloy with a high R content (hereinafter referred to as “the boundary-phase alloy”) are prepared separately, and are mixed to produce a sintered magnet. By means of adding Co to the boundary-phase alloy, chemically stable R
3
(Fe.Co) is formed and suppresses oxidation of the boundary-phase alloy during the production of a sintered magnet (Japanese Unexamined Patent Publication (kokai) No. 7-283016).
When the fine powder of the R—T—B alloy is slightly surface-oxidized, it is not suddenly oxidized even on exposure to ambient air, and can, therefore, be shaped under a magnetic field in ambient air. Fine pulverization is usually carried out in the production of a sintered magnet. A jet mill is used, for example, for the fine pulverization in the inert-gas atmosphere, in which trace amount of oxygen is incorporated. The thus produced fine powder of from 4000 to 10000 ppm of oxygen concentration can be shaped under the magnetic field in ambient air.
However, the permissible oxygen concentration to avoid lowering the magnet properties is lower in a high-performance sintered magnet with lower R content and hence, a less R-rich phase. It is, therefore, impossible to oxidize the surface of fine powder as described above, since the less R-rich phase must be effectively utilized. The shaping under the magnetic field must be carried out, while taking such measures as mounting the entire metal die in a glove box, establishing the protective gas atmosphere of N
2
and Ar in the glove box, and carrying out the magnetic-field shaping in the glove box. In the other steps, the causes of the oxidation must be eliminated as much as possible. The cost is accordingly increased.
Meanwhile, it is necessary to suppress the grain size to approximately 10 to 30 &mgr;m so as not to decrease the coercive force and squareness ratio of the sintered magnet. However, when the oxygen concentration of the sintered magnet is suppressed to an extremely low level, abnormal growth of crystal grains is likely to occur during the sintering, occasionally up to approximately 1 mm.
SUMMARY OF INVENTION
The present inventors considered a raw-material alloy, which is difficult to be oxidized and to undergo the abnormal growth of crystal grains in the production process of the sintered magnet, and which is used for producing a high-performance R—Fe—B sintered magnet. They also considered a method for producing said sintered magnet. More particularly, the present inventors considered a raw-material alloy, which is used for producing a high coercive-force rare earth sintered magnet which is used, in turn, mainly for a motor or the like. They also considered a producing method of such sintered magnet. As a result, the discovery was made that, when the sintered magnet is produced by a two-alloy mixing method, specifically when the main-phase alloy with an R component less than that of R
2
T
14
B and the boundary-phase alloy are mixed, only slight oxidation occurs during the production process of the sintered magnet, and no abnormal growth of crystal grains occurs during the sintering. Based on this discovery, the present invention was attained.
Namely, the present invention provides a raw-material alloy used for producing an R—T—B sintered magnet consisting of R
2
T
14
B, in which R is at least one rare-earth element including Y, and T is Fe, a part of which may be replaced with one of or both Co and Ni, and B is B (boron), a part
Hasegawa Hiroshi
Hirose Yoichi
Armstrong Westerman & Hattori, LLP
Sheehan John
Showa Denko K.K.
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