High-strength cold-rolled steel sheet having excellent bending workability and manufacturing method therefor

Abstract

Provided is a method of manufacturing a cold-rolled steel sheet. The method includes: cold-rolling a steel material; applying nickel (Ni) powder on a surface of the cold-rolled steel material in a coating amount of 300 mg/m 2 ; heating the steel material to completely transform the steel material to austenite; slowly cooling the heated steel material at a cooling rate of 5-12° C./s to a slow cooling termination temperature of 630-670° C., and maintaining the steel material at the slow cooling termination temperature for 10-90 seconds; rapidly cooling the slowly cooled and maintained steel material at a cooling rate of 7-30° C./s to a temperature range of a martensitic transformation termination temperature or higher and a martensitic transformation initiation temperature or lower; and maintaining the rapidly cooled steel material at a temperature higher than the martensitic transformation initiation temperature and the bainite transformation initiation temperature or lower for 300-600 seconds and partitioning the steel material.

What is claimed is: 
     
         1 . A method of manufacturing a cold-rolled steel sheet, the method comprising:
 cold-rolling a steel material including, by weight %, 0.13-0.25% of carbon (C), 1.0-2.0% of silicon (Si), 1.5-3.0% of manganese (Mn), 0.08-1.5% of aluminum (Al)+chromium (Cr)+molybdenum (Mo), 0.1% or less of phosphorus (P), 0.01% or less of sulfur(S), 0.01% or less of nitrogen (N), and a balance of Fe and inevitable impurities, and applying nickel (Ni) powder on a surface of the cold-rolled steel material in a coating amount of 300 mg/m 2 ;   heating the steel material to completely transform the steel material to austenite;   slowly cooling the heated steel material at a cooling rate of 5-12° C./s to a slow cooling termination temperature of 630-670° C., and maintaining the steel material at the slow cooling termination temperature for 10-90 seconds;   rapidly cooling the slowly cooled and maintained steel material at a cooling rate of 7-30° C./s to a temperature range of a martensitic transformation termination temperature (Mf) or higher and a martensitic transformation initiation temperature (Ms) or lower; and   maintaining the rapidly cooled steel material at a temperature higher than the martensitic transformation initiation temperature (Ms) and the bainite transformation initiation temperature (Bs) or lower for 300-600 seconds and partitioning the steel material.   
     
     
         2 . The method of  claim 1 , wherein the steel material further includes, by weight %, one or more of 0.001-0.005% of boron (B) and 0.005-0.04% of titanium (Ti). 
     
     
         3 . The method of  claim 1 , wherein aluminum (Al) is included in the steel material in an amount of 0.01-0.09 weight %. 
     
     
         4 . The method of  claim 1 , wherein chromium (Cr) is included in the steel material in an amount of 0.01-0.7 weight %. 
     
     
         5 . The method of  claim 1 , wherein molybdenum (Mo) is included in the steel material in an amount of 0.02-0.08 weight %. 
     
     
         6 . The method of  claim 1 , wherein an alloyed hot-dip galvanized layer is formed on the surface of the cold-rolled steel sheet.