HIGH STRENGTH COLD ROLLED STEEL SHEET WITH SUPERIOR FORMABILITY AND WELDABILITY, AND MANUFACTURING METHOD THEREFOR

摘要

A high strength cold-rolled steel sheet, which simultaneously provides a tensile strength of 70 to 90 kgf/mm2 grade and is easily formed, thereby increasing an impact energy absorption in case of collision and enhancing the safety of automobiles, and its manufacturing method. The high strength cold-rolled steel sheet with excellent formability and weldability, comprises, in terms of percent by weight, 0.15 to 0.25% C, 0.5 to 1.5% Si, 1.0 to 2.0% Mn, 0.25% or less P, 0.020% or less S, 0.015 to 0.050% Al, 0.008 to 0.026% N, balance Fe and incidental impurities while satisfying a condition of 1.2<= Si[%]+50/8P[%]<= 2.0. The method comprises the steps of hot rolling a steel having the above composition using a conventional method, cold rolling the hot-rolled steel sheet using a conventional method to form a cold-rolled steel sheet; continuous annealing the cold-rolled steel sheet at a temperature range satisfying a condition of 563+651C[%]+42Si[%]+18Mn[%]<= annealing temperature [°C]<= 850; and quenching the continuous annealed steel sheet at a rate of 20 to 100°C/s to an austempering initiating temperature ranging from 400 to 450°C and then cooling to a temperature ranging from 350 to 400°C .