Large Improvement in Magnetic Properties of Hot-Deformed HREE-Free Nd–Fe–B Magnets Using the Rapid Heating Technique of the Powder

Nd-Fe-B magnets for electric vehicles are required to have high coercivity, and heavy rare earth elements are added. However, it is necessary to reduce the heavy rare earth element that is a scarce resource. Decreasing the grain size is an effective method of increasing H
_cj
, and hot-deformed magnets which are consisted of rapidly quenched powders are known to have fine grains. This paper focused on coarse grains observed in the microstructure of hot-deformed magnets. Measuring the local magnetic properties by XMCD, it was shown that the coarse grain areas had lower magnetic properties than those of the fine grain areas. The coarsening was located at the powder interface. Therefore, the crystal growth behavior in the powder was investigated in detail. Due to the Fe concentrated in the powder along the wheel side of the melt-spun, Nd
₂
O
₃
and α-Fe were grown as the primary crystals. It is considered that the coarsening of hot-deformed magnets originates in those heterogeneous crystallizations. In order to prevent the coarsening, this paper focused on the heating rate of heat treatment of powders. The coarsening was prevented under the conditions, over 400 °C/min of heating rate and over 600 °C of heating temperature. The rapid heating of powders using the free-fall-type furnace prevented the coarsening in hot-deformed magnets. As a result of decreasing the coarse grains, H
_cj
was increased from 1584 to 1614 kA/m and M
_r
was increased from 1.33 to 1.40 T.