Ultrahigh-strength steel sheet with excellent bendability and stretch flangeability, and manufacturing method therefor

Abstract

Provided is an ultrahigh-strength steel sheet with excellent bendability and stretch flangeability, and a manufacturing method therefor and, more specifically, to: a steel sheet having excellent bendability and stretch flangeability by using rapid low-temperature tempering, and having a high yield ratio and ultrahigh strength; and a manufacturing method therefor.

1 . An ultrahigh strength steel sheet comprising, by weight: carbon (C): 0.12 to 0.4%, silicon (Si): 0.5% or less (excluding 0%), manganese (Mn): 2.5 to 4.0%, phosphorous (P): 0.03% or less (excluding 0%), sulfur (S): 0.012% or less (excluding 0%), aluminum (Al): 0.1% or less (excluding 0%), chromium (Cr): 1% or less (excluding 0%), titanium (Ti): 48/14×[N]˜0.1%, niobium (Nb): 0.1% or less (excluding 0%), boron (B): 0.005% or less (excluding 0%), nitrogen (N): 0.01% or less (excluding 0%), with balance of Fe and other impurities,
 wherein the steel sheet comprises, as a microstructure, by area, martensite: 90% or more, and a sum of ferrite and bainite: 10% or less, 
 wherein a value of M defined by Relational expression 1 below satisfies a range of 100 to 500.
     M=P   size   ×P   number ×[C] 0.5 ×[Mn] 2 ×[S]  [Relational expression 1]
 
 
 In the above Relational expression 1, P size  is an average diameter of inclusions having a diameter of 1 μm or more, and P number  is an average number of inclusions having a diameter of 1 μm or more. [C] and [Mn] are average weight % contents of elements in parentheses in the steel sheet, respectively, and [S] is an average ppm content of element in parentheses in the steel sheet. 
 
     
     
         2 . The ultrahigh strength steel sheet of  claim 1 , wherein a yield strength is 1140 to 1500 MPa, and a tensile strength is 1470 to 1700 MPa. 
     
     
         3 . The ultrahigh strength steel sheet of  claim 2 , wherein a yield ratio is 0.8 or more. 
     
     
         4 . The ultrahigh strength steel sheet of  claim 1 , wherein stretch flangeability HER is 25% or more, and bendability R/t is 4 or less. 
     
     
         5 . A manufacturing method of an ultrahigh strength steel sheet, comprising operations of:
 preparing a steel sheet, which comprises, by weight: carbon (C): 0.12 to 0.4%, silicon (Si): 0.5% or less (excluding 0%), manganese (Mn): 2.5 to 4.0%, phosphorous (P): 0.03% or less (excluding 0%), sulfur (S): 0.012% or less (excluding 0%), aluminum (Al): 0.1% or less (excluding 0%), chromium (Cr): 1% or less (excluding 0%), titanium (Ti): 48/14×[N]˜0.1%, niobium (Nb): 0.1% or less (excluding 0%), boron (B): 0.005% or less (excluding 0%), nitrogen (N): 0.01% or less (excluding 0%), with a balance of Fe and other impurities, and consists of a microstructure comprising, by area, martensite: 90% or more, and a sum of ferrite and bainite: 10% or less; and   performing tempering on the steel sheet,   wherein a value of P defined by relational expression 2 below satisfies a range of 1.5 to 77.0,   
       
         
           
             
               
                 
                   
                     P 
                     = 
                     
                       
                         
                           exp 
                           ⁡ 
                           ( 
                           
                             
                               T 
                               + 
                               273.15 
                             
                             40 
                           
                           ) 
                         
                         10000 
                       
                       × 
                       
                         [ 
                         
                           
                             log 
                             ⁡ 
                             ( 
                             
                               t 
                               eff 
                             
                             ) 
                           
                           - 
                           0.25 
                         
                         ] 
                       
                     
                   
                 
                 
                   
                     [ 
                     
                       Relational 
                       ⁢ 
                           
                       expression 
                       ⁢ 
                           
                       2 
                     
                     ] 
                   
                 
               
             
           
         
         In the above Relational expression 2, T is a maximum temperature of tempering, and a unit thereof is ° C. In addition, t eff  is an effective heat treatment time, and a unit thereof is seconds. 
       
     
     
         6 . The manufacturing method of an ultrahigh strength steel sheet of  claim 5 , wherein the T satisfies a range of 100 to 300° C. 
     
     
         7 . The manufacturing method of an ultrahigh strength steel sheet of  claim 5 , wherein the t eff satisfies a range of 1 to 120 seconds. 
     
     
         8 . The manufacturing method of an ultrahigh strength steel sheet of  claim 5 , wherein the following Relational expression 3 is satisfied,
   5≤t total ≤120   [Relational expression 3]
   In the above Relational expression 3, t total  is a total heat treatment time of tempering, and a unit thereof is seconds.   
     
     
         9 . The manufacturing method of an ultrahigh strength steel sheet of  claim 5 , wherein the following Relational expression 4 is satisfied,
   1≤t heat ≤119   [Relational expression 4]
   In the above Relational expression 4, t heat  is a heating time of tempering, and a unit thereof is seconds.   
     
     
         10 . The manufacturing method of an ultrahigh strength steel sheet of  claim 5 , wherein the following Relational expression 5 is satisfied,
   1≤t hold ≤119   [Relational expression 5]
   In the above Relational expression 5, t hold  is a holding time of tempering, and a unit thereof is seconds.