Metal treatment – Process of modifying or maintaining internal physical... – Magnetic materials
Reexamination Certificate
2002-07-09
2003-03-25
Sheehan, John P (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Magnetic materials
C209S008000, C209S214000, C209S215000, C241S024160, C241S079100, C419S030000, C419S033000
Reexamination Certificate
active
06537385
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to methods for manufacturing R—Fe—B type rare earth magnets, alloy powder for such magnets, and magnets produced by such methods.
Rare earth sintered magnets are produced by pulverizing an alloy for rare earth magnets to form alloy powder, compacting the alloy powder, and subjecting the alloy powder to sintering and aging. Presently, as the rare earth sintered magnets, two types of magnets, that is, samarium-cobalt magnets and neodymium-iron-boron magnets, are extensively used in various fields. In particular, neodymium-iron-boron magnets (hereinafter, referred to as “R—Fe—B type magnets”, where R is any rare earth element and/or Y, Fe is iron, and B is boron), which exhibit the highest magnetic energy product among a variety of magnets and have a comparatively low cost, have been vigorously applied to various types of electronic equipment. Note that a transition metal element such as Co may substitute for part of Fe and C may substitute for part of B.
Powder of the material alloy for R—Fe—B type rare earth magnets may be produced by a method including a first pulverization process for coarsely pulverizing the material alloy and a second pulverization process for finely pulverizing the material alloy. In general, in the first pulverization process, the material alloy is coarsely pulverized to a size of the order of several hundred micrometers or less using a hydrogen embrittlement apparatus. In the second pulverization process, the coarsely pulverized alloy (coarsely pulverized powder) is finely pulverized to an average particle size of the order of several micrometers with a jet mill or the like.
The material alloy can be produced by methods largely classified into two types. The first type is an ingot casting method where a molten alloy is poured into a mold and cooled comparatively slowly. The second type is a rapid cooling method, typified by a strip casting method and a centrifugal casting method, where a molten material alloy is put into contact with a single chill roll, twin chill rolls, a rotary chill disk, a rotary cylindrical chill mold, or the like, to be rapidly cooled thereby producing a solidified alloy thinner than an ingot cast alloy.
In the rapid cooling method, the molten alloy is cooled at a rate in the range between 102
2
° C./sec and 10
4
° C./sec. The resultant alloy produced by the rapid cooling method has a thickness in the range between 0.03 mm and 10 mm. The molten alloy starts solidifying at the face that comes into contact with a chill roll. From the roll contact face, crystal grows in the thickness direction into the shape of pillars or needles. The resultant rapidly solidified alloy therefore has a fine crystal structure including portions of a R
2
T
14
B crystal phase having a size in the range between 0.1 &mgr;m and 100 &mgr;m in the minor-axis direction and in the range between 5 &mgr;m and 500 &mgr;m in the major-axis direction, and portions of an R-rich phase dispersed at grain boundaries of the R
2
T
14
B crystal phase portions. The R-rich phase is a nonmagnetic phase in which the concentration of any rare earth element R is relatively high, and has a thickness (which corresponds to the width of the grain boundaries) of 10 &mgr;m or less.
Because the rapidly solidified alloy is cooled in a relatively short time compared with an ingot alloy produced by a conventional ingot casting method, the alloy has a fine structure and is small in grain size. In addition, with finely dispersed crystal grains, the area of grain boundaries is wide, and thus the R-rich phase spreads thinly over the grain boundaries. This results in good dispersion of the R-rich phase.
When a rare earth alloy (especially a rapidly solidified alloy) is coarsely pulverized in a hydrogen embrittlement process where the rare earth alloy first occludes hydrogen (this way of pulverization is herein called “hydrogen pulverization”), the R-rich phase portions existing at grain boundaries react with hydrogen and expand. This tends to cause the alloy to crack from the R-rich phase portions (grain boundary portions). Therefore, the R-rich phase tends to be exposed on the surfaces of particles of the rare earth alloy powder obtained by the hydrogen pulverization. In addition, in the case of a rapidly solidified alloy, where the R-rich phase portions are fine and highly dispersed, the R-rich phase particularly tends to be exposed on the surfaces of the hydrogen-pulverized powder.
According to experiments performed by the present inventors, when the coarsely pulverized powder in the above state is finely pulverized by a jet mill or the like, R-rich super-fine powder (fine powder having a particle size of 1 &mgr;m or less) is produced. Such R-rich super-fine powder particles oxidize very easily compared with other powder particles (having a relatively large particle size) that contain a relatively smaller amount of R. Therefore, if a sintered magnet is produced from the resultant finely pulverized powder without removing the R-rich super-fine powder, oxidation of the rare earth element vigorously proceeds during the manufacturing process steps. The rare earth element R is thus consumed for reacting with oxygen, and as a result, the production amount of the R
2
T
14
B crystal phase as the major phase decreases. This results in reducing the coercive force and remanent flux density of the resultant magnet and deteriorating the squareness of the demagnetization curve, which is the second quadrant curve of the hysteresis loop.
In order to prevent oxidation of the R-rich finely pulverized powder, the entire process from pulverizing through sintering may ideally be performed in an inert atmosphere. It is however very difficult to realize this in a mass-production scale in production facilities.
There is proposed a method for solving the above problem, where fine pulverization is performed in an inert atmosphere containing a trace amount of oxygen, to intentionally coat the surfaces of finely pulverized powder particles with a thin oxide film to thereby suppress fast oxidation of the powder particles in the atmosphere.
However, the present inventors have found that the above method fails to sufficiently improve the final magnet properties and maintain the properties at the highest level, as long as the finely pulverized powder contains R-rich super-fine powder in a percentage equal to or more than a predetermined value.
An object of the present invention is to provide alloy powder for R—Fe—B type rare earth magnets capable of sufficiently improving and stabilizing the magnet properties.
Another object of the present invention is to provide alloy powder for R—Fe—B type rare earth magnets capable of sufficiently improving the final magnet properties and maintaining the properties at the highest level even when a material alloy including an R-rich phase is used and such a material alloy is coarsely pulverized by the hydrogen pulverization method.
SUMMARY OF THE INVENTION
The method for manufacturing alloy powder for R—Fe—B type rare earth magnets of the present invention includes a first pulverization step of coarsely pulverizing a material alloy for rare earth magnets and a second pulverization step of finely pulverizing the material alloy, wherein the first pulverization step comprises a step of pulverizing the material alloy by a hydrogen pulverization method, and the second pulverization step comprises a step of removing at least part of fine powder having a particle size of 1.0 &mgr;m or less to adjust the particle quantity of the fine powder having a particle size of 1.0 &mgr;m or less to 10% or less of the particle quantity of the entire powder.
In a preferred embodiment, the average concentration of the rare earth element contained in the fine powder having a particle size of 1.0 &mgr;m or less is greater than the average concentration of the rare earth element contained in the entire powder.
Alternatively, the method for manufacturing alloy powder for R—Fe—B type rare earth magnets of the present invention includes a first pulverizati
Ishigaki Naoyuki
Okayama Katsumi
Okumura Shuhei
Costellia Jeffrey L.
Nixon & Peabody LLP
Sheehan John P
Sumitomo Special Metals Co. Ltd.
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