Method for manufacturing R-T-B based sintered magnet

1. A method for manufacturing an R-T-B based sintered magnet, which comprises:

1) a step of preparing an R-T-B based sintered magnet material by sintering a molded body at a temperature of 1,000° C. or higher and 1,100° C. or lower, and then performing (condition b) below:

(Condition b) temperature dropping to 500° C. at 10° C./min or less after performing a first heat treatment of holding at a first heat treatment temperature of 800° C. or higher and 950° C. or lower, the R-T-B based sintered magnet material comprising:

27.5% by mass or more and 34.0% by mass or less of R,

(R being at least one element of rare earth elements and indispensably containing Nd);

0.85% by mass or more and 0.93% by mass or less of B,

0.20% by mass or more and 0.70% by mass or less of Ga,

0.05% by mass or more and 0.50% by mass or less of Cu, and

0.05% by mass or more and 0.50% by mass or less of Al, with the balance being T (T being Fe and Co, and 90% or more of T in terms of a mass ratio being Fe) and inevitable impurities, the R-T-B based sintered magnet material satisfying inequality expressions (1) and (2) below:

[T]−72.3[B]>0  (1)

([T]−72.3[B])/55.85<13[Ga]/69.72  (2)

where [T] is a T content in % by mass, [B] is a B content in % by mass, and [Ga] is a Ga content in % by mass; and

2) a heat treatment step of performing a second heat treatment by heating the R-T-B based sintered magnet material to a second heat treatment temperature of 650° C. or higher and 750° C. or lower, after performing (condition b) without cooling to a temperature of lower than 400° C., and then cooling the R-T-B based sintered magnet material to 400° C. at 5° C./min or more.

2. The method for manufacturing an R-T-B based sintered magnet according to claim 1 , wherein, in the step 2), the R-T-B based sintered magnet material is cooled from the second heat treatment temperature to 400° C. at 15° C./min or more.

3. The method for manufacturing an R-T-B based sintered magnet according to claim 1 , wherein, in the step 2), the R-T-B based sintered magnet material is cooled from the second heat treatment temperature to 400° C. at 50° C./min or more.

4. The method for manufacturing an R-T-B based sintered magnet according to claim 1 , wherein the R-T-B based sintered magnet material includes 1.0% by mass or more and 10% by mass or less of Dy and/or Tb.

5. The method for manufacturing an R-T-B based sintered magnet according to claim 1 , wherein, in the step 1) (condition b), after sintering and cooling to a temperature lower than the first heat treatment temperature, the first heat treatment is performed by heating to the first heat treatment temperature.

6. The method for manufacturing an R-T-B based sintered magnet according to claim 1 , wherein, in the step 1) (condition b), after sintering and cooling to the first heat treatment temperature, the first heat treatment is performed.

7. The method for manufacturing an R-T-B based sintered magnet according to claim 1 , which comprises a low-temperature heat treatment step of heating the R-T-B based sintered magnet after the step 2) to a low-temperature heat treatment temperature of 360° C. or higher and 460° C. or lower.

8. The method for manufacturing an R-T-B based sintered magnet according to claim 2 , wherein the R-T-B based sintered magnet material includes 1.0% by mass or more and 10% by mass or less of Dy and/or Tb.

9. The method for manufacturing an R-T-B based sintered magnet according to claim 3 , wherein the R-T-B based sintered magnet material includes 1.0% by mass or more and 10% by mass or less of Dy and/or Tb.