High strength cold rolled steel sheet with superior formability and weldability, and manufacturing method therefor

Abstract

A high strength cold-rolled steel sheet, which simultaneously provides a tensile strength of 70 to 90 kgf/mm 2 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 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, cold rolling the hot-rolled steel sheet to form a cold-rolled steel sheet; continuous annealing the cold-rolled steel sheet at a temperature range satisfying a condition of 563+651C[%]+42 Si[%]+ 18 Mn[%]≦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.

1 . A high strength cold-rolled steel sheet with excellent formability and weldability, comprising, 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.  
     
     
         2 . The high strength cold-rolled steel sheet as set forth in  claim 1 , wherein the content of Si is set to 0.5 to 1.2%.  
     
     
         3 . A method for manufacturing a high strength cold-rolled steel sheet, comprising the steps of: 
 preparing a steel comprising, 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;    hot rolling the steel using a conventional method and then 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% and then cooling to a temperature ranging from 350 to 400° C.    
     
     
         4 . The method as set forth in  claim 3 , wherein the content of Si is set to 0.5 to 1.2%.  
     
     
         5 . The method as set forth in  claim 3 , comprising reheating the steel to a temperature range of 1,050 to 1,300° C., finishing hot rolling the reheated steel at a temperature range of 890 to 940° C., coiling the hot-rolled steel sheet at a temperature range of 600 to 700° C., pickling the coiled hot-rolled steel sheet and cold rolling the pickled steel sheet at a thickness reduction rate of 40 to 70% to form a cold-rolled steel sheet, prior to the continuous annealing.