Copper Bearing High Strength Cold Rolled Steel Sheets with Superior Drawability

Copper and phosphorus bearing high strength cold rolled steel sheets with superior drawability have been developed. The copper and phosphorus contents of these steels are 0.6% and 0.08% respectively. The tensile strengths of the steels are 500–550 MPa and the r values are as high as 1.5–1.7. The excellent r values of these steels are obtained by the precipitation treatment of 650° C × 10 hrs before cold rolling. This high r value is considered to be caused by the copper precipitates occured during this heat treatment. Both silicon and manganese strengthened copper bearing steels are investigated. High-silicon steels have better elongation than high-manganese steels. As to the r value, however, the latter are superior. In order to obtain excellent r value, high reduction in cold rolling and slow heating in annealing are required in addition to the precipitation treatment before cold rolling. The results of the trial production at Kobe Steel's Kakogawa Works showed good agreement with those of labolatory study.