HIGH-STRENGTH COLD-ROLLED STEEL SHEET HAVING EXCELLENT DEEP-DRAWABILITY AND BAKE HARDENABILITY, AND METHOD FOR MANUFACTURING SAME.

摘要

The present invention provides a high-strength cold-rolled steel sheet having excellent deep-drawability and bake hardenability, a tensile strength of 440 MPa or greater, an average r value of 1.20 or greater, and a bake hardening value of 40 MPa or greater by a process in which a steel raw material containing, in terms of mass%, 0.010 to 0.06% C, between 0.5% and 1.5% Si, 1.0 to 3.0% Mn, 0.010 to 0.090% Nb, and 0.015 to 0.15% Ti, and having a component composition satisfying conditions of (Nb/93)/(C/12) 0.20 and a solid-solution C content of 0.005 to 0.025%, is hot rolled and cold rolled, then annealed by heating from a temperature of 700 to 800°C to a temperature of 800 to 900°C at an average temperature increase rate of less than 3°C/s, soaking, and then cooling at 5°C/s or faster from the soaking temperature to a cooling stop temperature of 500°C or below to obtain a formation which includes a ferrite phase having an area ratio of 70% or greater and a martensite phase having an area ratio of 3% or greater.