Method for manufacturing R-T-B based sintered magnet, and R-T-B based sintered magnet
A method for manufacturing an R-T-B based sintered magnet according the present disclosure comprises: a step for preparing a coarse ground powder which is made from an alloy for R-T-B based sintered magnets and which has an average particle size of 10-500 μm; a step for obtaining a fine powder having an average particle size of 2.0-4.5 μm, by feeding the coarse ground powder to a jet mill device that has a grinding chamber filled with inert gas and grinding the coarse ground powder; and a step for producing a sintered body of the fine powder, wherein the inert gas has been humidified, and the oxygen content of the R-T-B based sintered magnet is 1000-3500 ppm by mass.
1. A method for producing a sintered R-T-B based magnet, R is a rare-earth element and contains at least one selected from the group consisting of Nd, Pr and Ce with no exception, and T is at least one transition metal and contains Fe with no exception, the method comprising the steps of:
preparing a coarse-pulverized powder of an alloy for the sintered R-T-B based magnet, the coarse-pulverized powder having an average particle size not shorter than 10 μm and not longer than 500 μm;
supplying the coarse-pulverized powder to a jet mill machine including a pulverization chamber filled with inert gas and pulverizing the coarse-pulverized powder to obtain a fine-pulverized powder having an average particle size not shorter than 2.0 μm and not longer than 4.5 μm; and
forming a sintered body of the fine-pulverized powder, wherein
the inert gas is supplied with moisture at a rate of not lower than 0.5 g and not higher than 6.0 g per 1 kg of the coarse-pulverized powder, and
the sintered R-T-B based magnet contains oxygen at a content not lower than 1000 ppm by mass and not higher than 3500 ppm by mass.
2. The method for producing a sintered R-T-B based magnet of claim 1 , wherein the sintered R-T-B based magnet contains R at a content not higher than 31% by mass.
3. The method for producing the sintered R-T-B based magnet of claim 2 , wherein, the content of R is measured by Inductively Coupled Plasma Optical Emission Spectroscopy.
4. The method for producing a sintered R-T-B based magnet of claim 1 , wherein the inert gas is nitrogen gas.
5. The method for producing a sintered R-T-B based magnet of claim 1 , further comprising a diffusion step of diffusing a heavy rare-earth element RH, RH is at least one of Tb, Dy and Ho, from a surface to an interior of the sintered body.
6. The method for producing a sintered R-T-B based magnet of claim 1 , wherein the step of forming the sintered body of the fine-pulverized powder includes the steps of:
forming a powder compact of the fine-pulverized powder by magnetic field wet press, or magnetic field press in an inert gas atmosphere, and
sintering the powder compact.
7. The method for producing a sintered R-T-B based magnet of claim 1 , wherein in the step performed to obtain the fine-pulverized powder, the fine-pulverized powder has an average particle size not shorter than 2.0 μm and not longer than 3.5 μm.
8. The method for producing the sintered R-T-B based magnet of claim 1 , wherein a dew point of the inert gas is in a range that is not lower than −55° C. and not higher than −30° C.
9. The method for producing the sintered R-T-B based magnet of claim 1 , wherein the average particle size of the coarse-pulverized powder and the average particle size of the fine-pulverized powder are each measured by an airflow-dispersion laser diffraction method conformed to JIS Z 8825:2013 revised edition, under conditions of a dispersion pressure of 4 bar, a measurement range of R2, and a calculation mode of HRLD.
10. The method for producing the sintered R-T-B based magnet of claim 1 , wherein amounts of oxygen, nitrogen, and carbon are measured by use of a gas analyzer, respectively by a gas fusion-infrared absorption method, a gas fusion-infrared absorption method, and a combustion-infrared absorption method.