Ultra-high strength steel sheet having excellent bendability, and manufacturing method therefor

Abstract

The present invention relates to a steel sheet suitable for automobile chassis members, etc., and, more particularly, to an ultra-high strength steel sheet having excellent bendability, and a manufacturing method therefor.

1 . An ultra-high strength steel sheet having excellent bendability, comprising: by wt %, carbon (C): 0.1 to 0.3%, manganese (Mn): 1.0 to 2.3%, silicon (Si): 0.05 to 1.0%, phosphorus (P): 0.1% or less (excluding 0%), sulfur (S): 0.03% or less (excluding 0%), aluminum (Al): 0.01 to 0.5%, two or more types of chromium (Cr): 0.01 to 0.2%, molybdenum (Mo): 0.01 to 0.2% and boron (B): 0.005% or less, one or more types of titanium (Ti): 0.1% or less and niobium (Nb): 0.1% or less, and a balance of Fe and unavoidable impurity elements, and satisfying the following relational expression 1,
 wherein a microstructure comprises, in area fraction, 99% or more of martensite and/or tempered martensite phases,   
       
         
           
             
               
                 
                   
                     
                       
                         0 
                         . 
                         1 
                       
                       ⁢ 
                       2 
                     
                     ≤ 
                     
                       
                         
                           Ceq 
                           ⁢ 
                           
                             1 
                             2 
                           
                         
                         + 
                         
                           Ceq 
                           ⁢ 
                           
                             2 
                             2 
                           
                         
                       
                       2 
                     
                     ≤ 
                     
                       
                         0 
                         . 
                         2 
                       
                       ⁢ 
                       8 
                     
                   
                 
                 
                   
                     < 
                     Relational 
                     ⁢ 
                         
                     Expression 
                     ⁢ 
                         
                     1 
                     > 
                   
                 
               
             
           
         
         where Ceq1=C+(Mn/20)+(Si/30)+(2P)+(4S), and Ceq2=C+(Mn/6)+(Si/30)+(Cr+Mo+V+Nb)/5+(Cu+Ni)/15). 
       
     
     
         2 . The ultra-high strength steel sheet having excellent bendability of  claim 1 , wherein the steel sheet has a C content ratio (average C content of region A/C content of steel sheet) of 0.6 or less in region A of 1 to 3 μm in a surface reference thickness direction as compared to the C content. 
     
     
         3 . The ultra-high strength steel sheet having excellent bendability of  claim 1 , wherein the steel sheet has a C content ratio (average C content of region B/C content of steel sheet) of 0.9 or less in region B of 0.2 to 30 μm in a surface reference thickness direction as compared to the C content. 
     
     
         4 . The ultra-high strength steel sheet having excellent bendability of  claim 1 , wherein the steel sheet includes 1% or less (including 0%) of a ferrite phase and/or a bainite phase. 
     
     
         5 . The ultra-high strength steel sheet having excellent bendability of  claim 1 , wherein in the steel sheet, a microstructure of a surface layer, a region ranging from a minimum of 50 μm to a maximum of 70 μm in a thickness direction from a surface thereof, comprises tempered martensite with an area fraction of 70% or less (excluding 0%) and one or more types of ferrite and bainite as a balance. 
     
     
         6 . The ultra-high strength steel sheet having excellent bendability of  claim 1 , wherein the steel sheet has tensile strength of 1300 MPa or more, a yield ratio of 0.72 or more, and a bending property (R/t) of 3 or less. 
     
     
         7 . The ultra-high strength steel sheet having excellent bendability of  claim 1 , wherein the steel sheet satisfies the following relational expression 2,
   (Tensile Strength (TS)/Maximum Bending Angle)≤25.  <Relational expression 2>
   
     
     
         8 . A manufacturing method for an ultra-high strength steel sheet having excellent bendability, comprising:
 heating a steel slab including, by wt %, carbon (C): 0.1 to 0.3%, manganese (Mn): 1.0 to 2.3%, silicon (Si): 0.05 to 1.0%, phosphorus (P): 0.1% or less (excluding 0%), sulfur (S): 0.03% or less (excluding 0%), aluminum (Al): 0.01 to 0.5%, two or more types of chromium (Cr): 0.01 to 0.2%, molybdenum (Mo): 0.01 to 0.2% and boron (B): 0.005% or less, one or more types of titanium (Ti): 0.1% or less and niobium (Nb): 0.1% or less, and a balance of Fe and unavoidable impurity elements, and satisfying the following relational expression 1, in a temperature range of 1100 to 1300° C.;   manufacturing a hot-rolled steel sheet by finish hot rolling the reheated steel slab at Ar3 or higher;   coiling the hot-rolled steel sheet at a temperature of 700° C. or less;   manufacturing a cold-rolled steel sheet by cold rolling the coiled hot-rolled steel sheet with a total reduction ratio of 30 to 80%;   continuously annealing the cold-rolled steel sheet for seconds or more at Ac3 or higher;   performing primary cooling at an average cooling rate of 1 to 10° C./s to a temperature range of 550 to 750° C. after the continuous annealing;   performing secondary cooling at an average cooling rate of 20 to 80° C./s to a temperature of Ms−190° C. or less after the performing primary cooling; and   performing reheating after the secondary cooling and then performing an over-aging treatment,   wherein the reheating and over-aging include heating to a temperature range that satisfies the following relational expression 3,   
       
         
           
             
               
                 
                   
                     
                       
                         0 
                         . 
                         1 
                       
                       ⁢ 
                       2 
                     
                     ≤ 
                     
                       
                         
                           Ceq 
                           ⁢ 
                           
                             1 
                             2 
                           
                         
                         + 
                         
                           Ceq 
                           ⁢ 
                           
                             2 
                             2 
                           
                         
                       
                       2 
                     
                     ≤ 
                     
                       
                         0 
                         . 
                         2 
                       
                       ⁢ 
                       8 
                     
                   
                 
                 
                   
                     < 
                     Relational 
                     ⁢ 
                         
                     Expression 
                     ⁢ 
                         
                     1 
                     > 
                   
                 
               
             
           
         
         (where Ceq1=C+(Mn/20)+(Si/30)+(2P)+(4S), Ceq2=C+(Mn/6)+(Si/30)+(Cr+Mo+V+Nb)/5+(Cu+Ni)/15)
   CT2+30° C.≤ A ≤270° C.  <Relational Expression 3>
 
 
         where CT2 denotes a secondary cooling end temperature (° C.), and A means a reheating and over-aging temperature (° C.). 
       
     
     
         9 . The manufacturing method for an ultra-high strength steel sheet having excellent bendability of  claim 8 , wherein the continuous annealing is performed in a continuous annealing furnace with a dew point temperature of 0 to 20° C. 
     
     
         10 . The manufacturing method for an ultra-high strength steel sheet having excellent bendability of  claim 8 , wherein the over-aging treatment is performed for 1 to 20 minutes.