R-t-b sintered magnet and process for producing the same

1 . An R-T-B based sintered magnet having a composition comprising:

27.0 mass % to 32.0 mass % of R, which is at least one of Nd, Pr, Dy and Tb and which always includes either Nd or Pr;

63.0 mass % to 72.5 mass % of T, which always includes Fe and up to 50% of which is replaceable with Co;

0.01 mass % to 0.08 mass % of Ga; and

0.85 mass % to 0.98 mass % of B,

wherein the magnet comprises a main phase with a tetragonal R 2 T 14 B type crystal structure, which accounts for at least 90% of the overall volume of the magnet, but includes substantially no R 1.1 Fe 4 B 4 phases.

2 . The R-T-B based sintered magnet of claim 1 , further comprising at most 2.0 mass % of M, which is at least one element selected from the group consisting of Al, Si, Ti, V, Cr, Mn, Ni, Cu, Zn, Zr, Nb, Mo, In, Sn, Hf, Ta and W.

3 . (canceled)

4 . The R-T-B based sintered magnet of claim 1 or 2 , having an oxygen concentration of at most 0.5 mass %, a nitrogen concentration of at most 0.2 mass %, and a hydrogen concentration of at most 0.01 mass %.

5 . A method for producing an R-T-B based sintered magnet, the method comprising the steps of:

preparing a powder of an alloy that has a composition comprising 27.0 mass % to 32.0 mass % of R (which is at least one of Nd, Pr, Dy and Tb and which always includes either Nd or Pr), 63.0 mass % to 72.5 mass % of T (which always includes Fe and up to 50% of which is replaceable with Co), 0.01 mass % to 0.08 mass % of Ga and 0.85 mass % to 0.98 mass % of B;

compacting and sintering the alloy powder, thereby making a sintered magnet; and

subjecting the sintered magnet to a heat treatment at a temperature of 400° C. to 600° C.

6 . The method of claim 5 , wherein the step of preparing the alloy powder includes the steps of:

preparing a melt of the alloy;

rapidly cooling and solidifying the melt of the alloy by a strip casting process, thereby making a rapidly solidified alloy; and

pulverizing the rapidly solidified alloy.