Manufacturing method for rotor core and manufacturing method for motor core

Abstract

A manufacturing method for a rotor core included in a rotor of a motor punches an electromagnetic steel sheet includes punching a plurality of plates for rotor core from an electromagnetic steel sheet; producing a rotor-core precursor by stacking up the plates for rotor core; manufacturing a rotor core by annealing an outer circumferential region of the rotor-core precursor at a first predetermined temperature, and annealing an inner circumferential region of the rotor-core precursor at a second predetermined temperature; the first predetermined temperature being a temperature at which grain growth of crystals of the electromagnetic steel sheet is promoted; and the second predetermined temperature being a temperature at which grain growth of the crystals of the electromagnetic steel sheet is not promoted.

What is claimed is: 
     
         1 . A rotor core comprising:
 a stacked body formed of a plurality of plates;   an outer circumferential region positioned at an outer circumferential side of the stacked body, and an inner circumferential region positioned at an inner circumferential side of the stacked body;   a magnet slot provided in the outer circumferential region;   wherein the outer circumferential region is an annular region extending from an outer circumferential contour of the rotor core in a radially inward direction to a predetermined position between the outer circumferential contour and a center portion of the magnet slot; and   an average grain size of crystals in the outer circumferential region is larger than an average grain size of crystals in the inner circumferential region.   
     
     
         2 . The rotor core according to  claim 1 , wherein the average grain size of the crystals in the outer circumferential region is 50 μm or more, and the average grain size of the crystals in the inner circumferential region is less than 50 μm. 
     
     
         3 . The rotor core according to  claim 1 , wherein the outer circumferential region is a surface layer of the rotor core. 
     
     
         4 . The rotor core according to  claim 1 , wherein a yield strength of the inner circumferential region is larger than a yield strength of the outer circumferential region by an amount of 70 MPa to 120 MPa. 
     
     
         5 . The rotor core according to  claim 1 , wherein the outer circumferential contour has a circular shape. 
     
     
         6 . The rotor core according to  claim 1 , wherein the inner circumferential region extends in a radially outward direction from an inner circumferential contour of the rotor core to the predetermined position located between the outer circumferential contour and the center portion of the magnet slot. 
     
     
         7 . The rotor core according to  claim 1 , wherein a portion of the magnet slot is provided in the outer circumferential region and a portion of the magnet slot is provided in the inner circumferential region.