R-T-B-based magnets, which have the maximum magnetic energy product in permanent magnets, are used for HD (hard disks), MRI (magnetic resonance imaging methods), various motors, etc. because they have high characteristics. In recent years, since saving energy has been increasingly demanded, in addition to an improvement of heat resistance of the R-T-B-based magnets, the use of R-T-B magnets as motors including a motor for vehicles has increased.
Since the main components of the R-T-B-based magnets are Nd, Fe, and B, the R-T-B magnets are also collectively called Nd—Fe—B magnets.
In the R-T-B magnets, R means Nd, a part of which is replaced with at least one rare-earth element, such as Pr, Dy, and Tb, in particular, often Nd, a part of which is replaced with at least one of rare-earth elements including Y. T means an alloy of Fe as an essential component, and Co, Ni, etc. B is boron, and may be partially substituted with C or N.
Other elements such as Cu, Al, Ti, V, Cr, Ga, Mn, Nb, Ta, Mo, W, Ca, Sn, Zr, and Hf may be added to the R-T-B-based alloys, singly or in combination of two or more species.
R-T-B-based alloys, which are R-T-B magnets, contain a ferromagnetic phase, R2T14B crystals, which contribute to magnetization, as the main phase, and a nonmagnetic R-rich phase having a low melting point and containing a non-magnetic rare-earth element at high concentration.
Since the R-T-B-based alloy is an active metallic material, the alloy is generally melted and cast in a vacuum or under an inert gas. When a sintered magnet is obtained from casting a R-T-B-based alloy ingot by a powder metallurgy method, in general, an alloy ingot is crushed to obtain alloy powder having a particle size of about 3 μm (as measured by means of FSSS (Fisher Sub-Sieve Sizer)), the powder is subjected to pressing in a magnetic field, the obtained compact is sintered in a sintering furnace at about 1,000 to 1,100° C., the sintered product is heated, mechanically processed, and plated for corrosion prevention, and a sintered magnet is obtained.
The R-rich phase plays the following important roles in the R-T-B-based sintered magnet.
(1) Since the R-rich phase has a low melting point, the phase liquefies during sintering, thereby contributing to achievement of high remanence, leading to improved magnetization.
(2) The R-rich phase functions to smoothen grain boundaries, thereby reducing the number of nucleation sites of reversed magnetic domains, thereby enhancing the coercive force.
(3) The R-rich phase magnetically insulates the main phase, thereby enhancing the coercive force.
When the distribution of the R-rich phase in a cast magnet is inferior, sintering may be partially defective, and magnetic properties may be decreased. Therefore, it is important to disperse uniformly the R-rich phase into the cast magnet. The distribution of the R-rich phase depends greatly on the microstructure of raw material, an R-T-B-based alloy.
Another problem involved in casting of the R-T-B-based alloy is that α-Fe is formed in the cast alloy. The α-Fe has deformability, and remains in a crusher, without being crushed. Due to this, α-Fe not only deteriorates crushing efficiency during the crushing of the alloy, but also changes the composition before and after crushing, and greatly affects the particle distribution. In addition, if α-Fe remains even after sintering, magnetic characteristics of the sintered product are deteriorated.
In order to solve the above problems caused by formation of α-Fe in the R-T-B-based alloy, a strip casting method (abbreviated as SC method), in which an alloy ingot is cast with a higher cooling rate has been developed, and employed in actual production steps.
In the SC method, an alloy is rapidly solidified by pouring a molten alloy onto a rotating copper roller, the inside of which is cooled by water, to cast a strip having a thickness of about 0.1 to about 1 mm. During casting, the molten alloy is supercooled to the formation temperature of R2T14B or less, which is the main phase. Therefore, it is possible to form directly R2T14B from the molten alloy. Due to this, it is possible to prevent the formation of α-Fe.
In addition, since the crystalline structure of the alloy is minutely dispersed, it is possible to form an alloy having a structure in which an R-rich phase is finely dispersed. The R-rich phase reacts with hydrogen in a hydrogen atmosphere, expands, and forms brittle hydride (hydrogen decrepitation step). It is possible to generate fine cracks using the R-rich phase. When an alloy is finely crushed after the hydrogen decrepitation step, since the alloy is broken due to a lot of fine cracks, which are formed by the hydrogenation, crushability of the alloy is excellent.
As explained above, since the R-rich phase is minutely dispersed in the alloy ingot produced through the SC method, dispersion of R-rich phase in the product obtained by crushing and sintering the alloy also becomes satisfactory. Thereby, it is possible to improve magnetic properties of the obtained magnet (For example, Patent Document No. 1)
In addition, the alloy flakes, which are cast by the SC method, have superior uniformity of microstructure. The uniformity in microstructure can be evaluated based on a crystal grain size and the dispersion state of the R-rich phase. In alloy flakes formed by the SC method, chill crystals sometimes generate on a side which contacts with a cast roller (abbreviated as “cast surface side” below). Therefore, it is possible to obtain a reasonably fine and uniform microstructure by rapid solidification.
As explained above, the R-T-B-based alloy obtained by the SC method has a finely dispersed R-rich phase, and the formation of α-Fe is also prevented. Therefore, when a sintered magnet is obtained, uniformity of the R-rich phase in the final magnet product is improved, and crushing and adverse effects due to α-Fe can be prevented. In this way, the R-T-B-based alloy ingot obtained by the SC method has superior microstructure for producing sintered magnets.    Patent Document No. 1: Japanese Unexamined Patent Application, First Publication No. H5-222488