PURPOSE: To manufacture a high tensile steel sheet excellent in formability and capable of being remarkably hardened by means of subsequent heat treatment by subjecting a slab of low carbon steel containing specific amounts of Ti, Cu, etc., to hot rolling under specific conditions and to winding, applying descaling and cold rolling to the above plate, and then continuously annealing the resulting cold rolled sheet.
CONSTITUTION: A steel slab having a composition which contains, by weight, <0.0080% C, <0.5% Si, <0.25% Mn, <0.05% P, <0.020% S, <0.10% sol. Al, 0.01-0.20% Ti, <0.0050% N, and 0.5-3.0% Cu and further contains, if necessary, 0.0003-0.0020% B and [in which Ti≥4(C+12/14N)] is satisfied is heated up to ≤1100°C, hot-rolled at a temp. of the Ar3 transformation point of the steel or above, and wound up. Subsequently, the above hot rolled plate is descaled by means of acid pickling, worked into a sheet stock by means of cold rolling, and continuously annealed at 750-900°C in succession. By this method, the high tensile steel sheet in which small amounts of C and N are fixed by Ti in the form of TiC and TiN, respectively, and, by the addition of Cu, strength is improved and superior formability is provided and, further, hardness can be improved by means of heat treatment after forming can be manufactured.
OKAMOTO ATSUKI
MIZUI NAOMITSU
