1. Field of the Invention
The present invention relates to an R-T-B-based rare earth sintered magnet and an alloy for an R-T-B-based rare earth sintered magnet.
Priority is claimed on Japanese Patent Application No. 2015-051353, filed on Mar. 13, 2015 and Japanese Patent Application No. 2015-236922, filed on Dec. 3, 2015 the content of which is incorporated herein by reference.
2. Description of Related Art
In the related art, an R-T-B-based rare earth sintered magnet (hereinafter, in some cases, abbreviated as the “R-T-B-based magnet”) was used in motors such as a voice coil motor in a hard disc drive and engine motors for hybrid vehicles or electrical vehicles.
An R-T-B-based magnet is obtained by shaping and sintering R-T-B-based alloy powder including Nd, Fe, and B as main components. Generally, in the R-T-B-based alloy, R represents Nd or Nd, some of which is substituted with other rare earth elements such as Pr, Dy, and Tb. T represents Fe or Fe, some of which is substituted with other transition metals such as Co and Ni. B represents boron, some of which can be substituted with C or N.
The structure of an ordinary R-T-B-based magnet is made up of, mainly, the main phase and an R-rich phase. The main phase is constituted with R2T14B. The R-rich phase is present in a grain boundary of the main phase and has a higher concentration of Nd than the main phase. The R-rich phase is also referred to as a grain boundary phase.
The composition of the R-T-B-based alloy is generally set so that, in order to increase the proportion of the main phase in the structure of the R-T-B-based magnet, the ratio between Nd, Fe, and B approximates to that of R2T14B as much as possible (for example, refer to Masato Sagawa, Permanent Magnet—Material Science and Application, second impression of the first edition published on Nov. 30, 2008, pp. 256 to 261).
In addition, an R-T-B-based magnet used in a motor for vehicles is exposed to a high temperature in the motor, and thus a high coercive force (Hcj) is required.
As a technique for improving the coercive force of an R-T-B-based magnet, there is a technique in which the R in the R-T-B-based alloy is substituted from Nd to Dy. However, Dy is an eccentrically located resource and has a limited production, and thus the supply of Dy is unstable. Therefore, studies are underway regarding a technique for improving the coercive force of the R-T-B-based magnet without increasing the amount of Dy in the R-T-B-based alloy.
The present inventors studied the composition of the R-T-B-based alloy and, consequently, found that the coercive force improves when the concentration of a specific B is lower than that in the R-T-B-based alloy in the related art. In addition, the present inventors successfully developed an R-T-B-based alloy with which an R-T-B-based magnet having a high coercive force can be obtained even when the amount of Dy is zero or extremely low (for example, refer to Japanese Unexamined Patent Application, First Publication No. 2013-216965).
An R-T-B-based magnet manufactured using the R-T-B-based alloy developed by the present inventors includes a main phase made of R2T14B and a grain boundary phase including a larger amount of R than the main phase. In the R-T-B-based magnet, as the grain boundary phase, a grain boundary phase (transition metal-rich phase) having a lower concentration of rare earth elements and a higher concentration of transition metal elements than the grain boundary phase of the related art is included as well as a grain boundary phase (R-rich phase) having a high concentration of rare earth elements, which is known in the related art. The transition metal-rich phase is a phase capable of imparting a coercive force, and an R-T-B-based magnet in which the transition metal-rich phase is present in the grain boundary phase is a revolutionary technique that renders obsolete the conventional wisdom of the related art.