To provide a steel sheet for the rotor iron core of an IMP motor which has satisfactory punching workability, has high yield strength when being made into a rotor iron core, and also has excellent magnetic properties.
The steel sheet is obtained by subjecting a hot rolled steel sheet having a componential composition comprising, by mass, 0.05 to 0.35% C, 0.05 to 1.0% Si, 0.2 to 1.5% Mn, ≤0.05 P, ≤0.02% S, 0.005 to 2.95% acid soluble Al and also Si+Al: ≤3.0%, and the balance Fe with inevitable impurities to cold rolling for one time or for two or more times including intermediate annealing into a prescribed sheet thickness, and subjecting the steel sheet to three step annealing including the first heat treatment of holding the steel sheet in the temperature range of Ac1-50°C to <Ac1 for ≥0.5 hr, the second heat treatment of holding the steel sheet in the temperature range of Ac1 to Ac1+100°C for 0.5 to 20 hr and the third heat treatment of holding the steel sheet in the temperature range of Ar1 to 80°C to Ar1 for 2 to 60 hr, and in which the cooling velocity from the heat treatment temperature in the second step to the heat treatment temperature in the third step is controlled to 5 to 30°C/h.
FUJIWARA SUSUMU
Hidetoshi Furukawa
Suzuki Kenchi
Kajinami order
Kazuhiro Oyaku
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