R-T-B-type sintered magnet and method for production thereof

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

27.3 mass % to 29.5 mass % of R, which is at least one of the rare-earth elements that include Y and of which at least 50 mass % is Pr and/or Nd;

0.92 mass % to 1 mass % of B;

0.05 mass % to 0.3 mass % of Cu;

at most 0.5 mass % (including 0 mass %) of M, which is one, two, or more elements that are selected from the group consisting of Al, Ti, V, Cr, Mn, Ni, Zn, Ga, Zr, Nb, Mo, Ag, In, Sn, Hf, Ta, W, Au, Pb and Bi; and

T as the balance, which is Fe with or without Co and of which at least 50 mass % is Fe, and

having an oxygen content of 0.02 mass % to 0.2 mass %,

wherein the main phase of the sintered magnet is an R 2 T 14 B type compound, and

wherein the crystal grain size of the main phase is represented by an equivalent circle diameter of 8 μm or less and wherein crystal grains with equivalent circle diameters of 4 μm or less account for at least 80% of the overall area of the main phase.

2. A method for producing an R-T-B based sintered magnet having a composition comprising:

27.3 mass % to 29.5 mass % of R, which is at least one of the rare-earth elements that include Y and of which at least 50 mass % is Pr and/or Nd;

0.92 mass % to 1 mass % of B;

0.05 mass % to 0.3 mass % of Cu;

at most 0.5 mass % (including 0 mass %) of M, which is one, two, or more elements that are selected from the group consisting of Al, Ti, V, Cr, Mn, Ni, Zn, Ga, Zr, Nb, Mo, Ag, In, Sn, Hf, Ta, W, Au, Pb and Bi; and

T as the balance, which is Fe with or without Co and of which at least 50 mass % is Fe, and

having an oxygen content of 0.02 mass % to 0.2 mass %,

wherein the method comprises the steps of:

providing, as a material alloy, a strip cast alloy that comprises R-rich phases in which an average interval between the R-rich phases is 4 μm or less;

exposing the material alloy to a hydrogen atmosphere, thereby getting the material alloy decrepitated and obtaining a coarse powder;

obtaining a fine powder by finely pulverizing the coarse powder so that the fine powder has a particle size represented by a D50 of 3 μm or less as measured by dry jet dispersion laser diffraction analysis and has an oxygen content of 0.2 mass % or less;

obtaining a compact by performing a press compaction process on the fine powder under a magnetic field; and

sintering the compact by keeping the compact heated to a temperature of 850° C. to 1,000° C. for 4 to 48 hours.

3. The method of claim 2 , wherein the step of obtaining the compact includes mixing the fine powder and a saturated hydrocarbon based organic solvent together to obtain a slurry of the fine powder, and

wherein the press compaction process is performed on the slurry of the fine powder.

4. The method of claim 2 , wherein the step of obtaining the fine powder includes getting the fine pulverization process done by a jet pulverizer using helium or argon gas.

5. The method of claim 4 , wherein the step of obtaining the fine powder includes achieving a target particle size by using a classifier that is connected to the pulverizer.