1. Field of the Invention
The present invention relates to a rare-earth magnet, in particular relates to a starting alloy for an R-T-B system sintered magnet comprising a rare-earth element (R), one or more transition metal elements (T) essentially comprising Fe, or Fe and Co, and boron (B) as the main components.
2. Description of the Related Art
An R-T-B system sintered magnet has advantages of excellent magnetic properties, and relatively low cost because Nd as the main component is an abundant resource. It is produced by powder metallurgy comprising the following main steps. First, a starting alloy is prepared by melting a given composition of the components; the alloy is crushed to a given particle size; and the resulting powder is compacted into a shape in a magnetic field, and sintered and thermally treated.
The starting alloy is frequently produced by strip casting, where it is quenched on the rotating rolls. When the starting alloy produced by strip casting is treated for hydrogen crushing, hydrogen absorption time including that for activation treatment, crushing time and crushability in the presence of hydrogen largely disperse lot by lot, as disclosed by Japanese Patent Laid-Open No. 11-50110. This document discusses that the dispersions are caused by the following phenomenon. An R-T-B system alloy is mainly composed of R2Fe14B as the main phase and grain boundary phase (R-rich phase) both having a high affinity for oxygen, with the result that an oxide film and/or slug is formed on the contact surface on the roll even when it is melted and solidified in an Ar gas atmosphere, for example, in the strip casting to retard adsorption of hydrogen molecules on the alloy base.
Japanese Patent Laid-Open No. 11-50110 proposes acid treatment to remove the oxide film and/or slug on the starting alloy surfaces, to greatly improve hydrogen absorption efficiency of a starting alloy produced by strip casting (hereinafter sometimes referred to as SC alloy).