Metal treatment – Process of modifying or maintaining internal physical... – With casting or solidifying from melt
Reexamination Certificate
2002-04-26
2004-09-28
Sheehan, John P (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
With casting or solidifying from melt
C148S101000, C164S114000, C164S116000, C164S286000, C164S298000
Reexamination Certificate
active
06797081
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a casting method which employs rapid solidification of metal, rare-earth metal, high-melting-point metal, nonmetal or the like, as well as to a casting apparatus and a cast alloy.
BACKGROUND ART
In recent years, as peripheral equipment for personal computers—such as HDDs (hard disk drives)—AV equipment, household electric equipment, and the like have become lightweight, compact, and of higher performance, demand for sintered rare-earth magnets represented by Nd-based (neodymium-based) magnets has sharply increased. Typical alloys for such magnets are the Nd—Fe—B-type alloys, which additionally contain iron and boron and are typified by a composition Nd
2
Fe
14
B.
In many of these rare-earth magnets, in order to improve magnetic properties, to enhance economical efficiency through effective use of rare-earth elements, which are limited resources, and to enhance use-related properties (such as heat resistance and corrosion resistance), Dy (dysprosium), Pr (praseodymium), or similar rare-earth elements are incorporated so as to substitute for some portion of Nd, and Co, Al, Cu, or like elements are incorporated so as to substitute for some portion of Fe.
When there is no particular reason to limit a rare-earth element contained in rare-earth magnets including those mentioned above to Nd, the rare-earth magnets are collectively referred to as R-T-B-type magnets (R: rare-earth element; T: transition metal element).
Generally, all industrially produced R-T-B-type magnets contain R in an amount slightly exceeding the stoichiometric amount for the composition R
2
T
14
B. Thus, in a magnet alloy ingot, a phase which contains a rare-earth element(s), represented by R, at high concentration (hereinafter called the R-rich phase) is generated.
The R-rich phase is known to play the following important roles in R-T-B-type magnets.
(1) Since melting point of the R-rich phase is low, the phase becomes a liquid phase during sintering in a magnet production step, thereby contributing to achievement of high density of the resultant magnet and thus to improvement in remanence.
(2) The R-rich phase functions to smoothen grain boundaries, thereby reducing the number of nucleation sites in a reversed magnetic domain. Moreover, being nonmagnetic, the R-rich phase magnetically insulates the main phase, thereby enhancing the coercivity.
(3) Since the R-rich phase expands through absorption of hydrogen, this feature is utilized for decrepitating an ingot into pieces. Specifically, the R-rich phase is caused to absorb hydrogen so as to expand. As a result, cracks are generated within an alloy ingot, thereby decrepitating the ingot into pieces. The R-rich phase serves as a starting point of so-called hydrogen decrepitation.
In recent years, R-T-B-type magnets of improved magnetic characteristics, particularly R-T-B-type magnets of enhanced maximum magnetic energy product (BHmax), have been developed. In order to obtain such a high-performance magnet, the percentage of the R
2
T
14
B phase (hereinafter called the T1 phase), which produces magnetism, must be increased, and the R-rich phase must be reduced. In order to fulfill these needs, the total rare-earth element content (hereinafter called the TRE content) must be reduced so as to attain a near stoichiometric composition.
In such a case, the following problems that affect magnetic properties of the produced magnets are involved in alloy production steps and magnet production steps.
First, in melting and casting of an alloy; for example, a ternary alloy of Nd—Fe—B, the T1 phase forms through peritectic reaction between a primary &ggr;Fe phase and a liquid phase. Thus, as the TRE content (the total R content) decreases, an &agr;Fe phase, which is a transformed form of &ggr;Fe, tends to form. The &agr;Fe phase appears in the form of dendrites and extends three-dimensionally within the alloy, thereby significantly deteriorating crushability of the alloy in the magnet production step.
Second, when the TRE content is decreased, the percentage of the existing R-rich phase decreases. Thus, the aforementioned effects exerted by the R-rich phase; i.e., achievement of high density of the resultant magnet and enhancement in coercivity to a magnet, cannot be expected.
In order to solve the above problems, a strip casting process (SC process) has been developed (see, for example, Japanese Patent Application Laid-Open (kokai) Nos. 5-222488 and 5-295490). According to the SC process, a molten alloy is poured onto a water-cooled rotating roller of copper through a tundish and solidifies upon contact with the roll, so as to continuously produce a strip-like ingot. Subsequently, the strip-like ingot is crushed coarsely, and ultimately into flakes.
When an R-T-B-type rare-earth magnet alloy is cast by the SC process, very thin flakes, each having a thickness of about 0.2 mm to 0.4 mm, can be obtained, and therefore, cooling for solidification can be high. Thus, the molten metal can be cooled below a co-existence region of a liquid phase and &ggr;Fe. That is, the T1 phase forms directly without formation of &ggr;Fe. For example, a ternary alloy of Nd—Fe—B can be cast without formation of dendritic &agr;Fe while the Nd content ranges down to about 12.7 at. % (28.5% by mass), at which Nd content a high-performance magnet of 400 kJ/m
3
or higher can be produced. (Y. Hirose, H. Hasegawa, S. Sasaki and M. Sagawa, Proceedings of the 15th International Workshop on Rare-Earth Magnets and Their Applications, Volume 1, pages 77-86, 30 Aug.-3 Sep. 1998, Dresden, Germany).
Because of high rate of solidification, an alloy cast by the SC process has a relatively small crystal grain size of 20 &mgr;m to 30 &mgr;m as measured along the short axis.
FIG. 7
schematically shows a cross-sectional structure of an R-T-B rare-earth alloy cast by the SC process and having an R content of 11.8 at. % (26.5% by mass) or more. In
FIG. 7
, the bottom surface (called the mold contact surface) is the surface of an ingot in contact with a mold, and the top surface (called the free surface) is opposite the mold contact surface.
Excess R over the stoichiometric amount in the composition R
2
T
14
B is diffused out from the solidification interface during solidification, thereby generating lamellar R-rich phases
30
arranged at intervals of 3 &mgr;m to 10 &mgr;m. The R-rich phases
30
form on the grain boundaries
28
of and within a crystal grain
29
. As compared with a conventional alloy cast by means of a book-mold, the R-rich phases
30
are distributed finely and uniformly. Thus, crushability during hydrogen decrepitation is significantly improved, such that pulverized particles attain a size which is a fraction of the crystal grain size. That is, it is possible to obtain a powder constituted solely by single-crystal particles. A region denoted by reference numeral
32
is the T1 phase.
A powder consisting of single-crystal particles facilitates, in a later step of compaction in a magnetic field, formation of a compact which is oriented in the direction of the C-axis, which serves as an easy-magnetization axis.
However, mere mechanical pulverization disintegration without involvement of hydrogen decrepitation causes cracking to propagate through grains (i.e., penetrating grains) in the form of cleavage fracture without utilization of the R-rich phases generated on grain boundaries and within grains. As a result, among pulverized particles, an increased number of particles come to have crystal grain boundaries
28
, or in other words, are not single crystal particles. Accordingly, the degree of alignment drops at the time of compaction in a magnetic field, causing an impairment in magnetization and a decrease in magnetic energy product after sintering.
The present inventors devised another rapid solidification process and an apparatus therefor (Japanese Patent Application Laid-Open (kokai) Nos. 08-13078 and 08-332557). Specifically, a molten material is introduced into a rotating mold via a box-like tundish, which is disposed in a reciprocative manne
Hasegawa Hiroshi
Hirose Yoichi
Hosono Uremu
Utsunomiya Masahide
Sheehan John P
Showa Denko K.K.
Sughrue & Mion, PLLC
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