Ultrahigh-strength steel sheet having excellent yield ratio, and manufacturing method therefor

Abstract

One aspect of the present invention relates to an ultrahigh-strength steel sheet having an excellent yield ratio, comprising, by wt %, 0.3-0.5% of C, 2.0% (excluding 0%) of Si, 3.0-6.5% of Mn, 0.02% or less of P, 0.01% or less of S, 0.01-3.0% of Al, 0.02% or less (excluding 0%) of N, and the balance of Fe and other inevitable impurities, and a microstructure comprises 5-30% of remaining austenite by area fraction and comprises 5% or less of secondary martensite.

What is claimed: 
     
         1 . A manufacturing method of a steel sheet, comprising steps of:
 heating a steel slab including 0.3 to 0.5 wt % of carbon (C), 2.0 wt % of (Si) (excluding 0%), 3.0 to 6.5 wt % of manganese (Mn), 0.02 wt % or less of phosphorus (P), 0.01 wt % or less of sulfur (S), 0.01 to 3.0 wt % of aluminum (Al), 0.02 wt % or less of nitrogen (N) (excluding 0%), and a remainder of Fe and other unavoidable impurities to a temperature of 1000 to 1250° C.;   performing hot-rolling on the heated steel sheet such that a temperature on a finish rolling exit side is 500 to 950° C. to obtain a hot-rolled steel sheet;   winding the hot-rolled steel sheet at a temperature of 750° C. or lower;   performing cold-rolling the wound hot-rolled steel sheet at a reduction ratio of 30 to 80° C. to obtain a cold-rolled steel sheet;   annealing the cold-rolled steel sheet in a temperature range of 750 to 950° C.;   cooling the annealed cold-rolled steel sheet to a cooling termination temperature of Mf to Ms−90° C.; and   heat-treating the cooled cold-rolled steel sheet at a temperature of Ms+100° C. or higher for 250 seconds or longer.   
     
     
         2 . The manufacturing method of the steel sheet of  claim 1 , further comprising a step of heat-treating the hot-rolled steel sheet wound before cold-rolling after the winding step at a temperature of 800° C. or lower for 30 minutes or longer. 
     
     
         3 . The manufacturing method of the steel sheet of  claim 1 , wherein the Ms temperature is obtained by the following Relational Expression 1 
       
         
           
             
               
                 
                   
                     Ms 
                     = 
                     
                       547.6 
                       - 
                       
                         596.9 
                            
                         C 
                       
                       - 
                       
                         27.4 
                            
                         Mn 
                       
                       - 
                       
                         13.1 
                            
                         Si 
                       
                       - 
                       
                         17.7 
                            
                         Cr 
                       
                       + 
                       
                         8.8 
                            
                         Al 
                       
                     
                   
                 
                 
                   
                     Relational 
                     ⁢ 
                         
                     Expression 
                     ⁢ 
                         
                     1 
                   
                 
               
             
           
         
         where each element symbol is a value representing a content of each element in weight %, and a unit of Ms is in ° C., while when an element was not included, it was calculated as 0. 
       
     
     
         4 . The manufacturing method of the steel sheet of  claim 1 , wherein the steel slab further comprises one or more of 1.5 wt % or less of Cr (excluding 0%), 0.005 to 0.5 wt % of Ti, 0.005 to 0.5 wt % of Nb, 0.005 to 0.5 wt % of V, and 0.05 to 0.3 wt % of Mo. 
     
     
         5 . The manufacturing method of the steel sheet of  claim 1 , further comprising a step of immersing the cold-rolled steel sheet heat-treated after the heat-treating step in a zinc plating bath to form a hot-dip galvanized layer. 
     
     
         6 . The manufacturing method of the steel sheet of  claim 1 , further comprising a step of alloying heat-treating the cold-rolled steel sheet having the hot-dip galvanized layer thereon to form an alloyed hot-dip galvanized layer. 
     
     
         7 . A steel sheet manufactured by the method of  claim 1 .