| 摘要 |
A high-strength cold rolled steel sheet having excellent deep drawability and bake hardenability, which has a tensile strength of not less than 440 MPa, an average r-value of not less than 1.20 and a bake hardening value of not less than 40 MPa, is obtained by subjecting a steel raw material having a chemical composition comprising C: 0.010-0.06 mass%, Si: more than 0.5 mass% but not more than 1.5 mass%, Mn: 1.0-3.0 mass%, Nb: 0.010-0.090 mass%, Ti: 0.015-0.15 mass% and satisfying (Nb/93)/(C/12) < 0.20 to hot rolling, cold rolling and then to annealing comprising steps of heating to a temperature of 800-900°C while a temperature region of 700-800°C is an average heating rate of less than 3°C/s, and soaking and thereafter cooling at a rate of not less than 5°C/s from the soaking temperature to a cooling stop temperature of not higher than 500°C to thereby form a microstructure comprising ferrite phase with an area ratio of not less than 70% and martensite phase with an area ratio of not less than 3%. |
