Ultra-high strength cold-rolled steel sheet having excellent elongation and manufacturing method thereof

Abstract

The present invention relates to an ultra-high strength cold-rolled steel sheet and a manufacturing method thereof and, more specifically, to a cold-rolled steel sheet and a manufacturing method thereof, wherein the cold-rolled steel sheet has a tensile strength of 1.5 GPa and exhibits an excellent elongation rate, and as such, can be suitably used for cold stamping.

1 . A cold-rolled steel sheet comprising, by weight: 0.15 to 0.3% of C, 0.1 to 1.5% of Si, 2.5 to 5.0% of Mn, 0.1% or less (excluding 0%) of P, 0.03% or less (excluding 0%) of S, 0.01 to 0.1% of Al, 0.01% or less (excluding 0%) of N, and 0.005% or less (excluding 0%) of B, with a remainder of Fe and other inevitable impurities, and
 comprising, by area: 0.5 to 20% of residual austenite and 80 to 99.5% of martensite, as a microstructure,   wherein the cold-rolled steel sheet satisfies the following Relation 1-1:   
       
         
           
             
               
                 
                   
                     5 
                     ≤ 
                     
                       
                         AF 
                         Mn_s 
                       
                       
                         AF 
                         Mn_h 
                       
                     
                     ≤ 
                     25 
                   
                 
                 
                   
                     [ 
                     
                       Relation 
                       ⁢ 
                           
                       1 
                       - 
                       1 
                     
                     ] 
                   
                 
               
             
           
         
         wherein AF Mn_s  is an area fraction of low-Mn crystal grains having a Mn content in the martensite of more than 2% and less than 5% and a unit thereof is areas, and AF Mn_h  is an area fraction of high-Mn crystal grains having a Mn content in the martensite of 5% or more and a unit thereof is area %. 
       
     
     
         2 . The cold-rolled steel sheet of  claim 1 , wherein the cold-rolled steel sheet satisfies the following Relation 1-2: 
       
         
           
             
               
                 
                   
                     
                       Δσ 
                       TS 
                     
                     = 
                     
                       
                         
                           σ 
                           
                             TS_Mn 
                             ⁢ 
                             _h 
                           
                         
                         - 
                         
                           σ 
                           
                             TS_Mn 
                             ⁢ 
                             _s 
                           
                         
                       
                       ≥ 
                       
                         600 
                         ⁢ 
                             
                         MPa 
                       
                     
                   
                 
                 
                   
                     [ 
                     
                       Relation 
                       ⁢ 
                           
                       1 
                       - 
                       2 
                     
                     ] 
                   
                 
               
             
           
         
       
       wherein Δσ TS  is a difference between a tensile strength average value (σ TS_Mn_h ) Of high-Mn crystal grains having a Mn content in the martensite of 5% or more and a tensile strength average value (O TSMn_s ) of low-Mn crystal grains having a Mn content in the martensite of more than 2% and less than 5%, and a unit thereof is MPa. 
     
     
         3 . The cold-rolled steel sheet of  claim 1 , further comprising: one or more selected from 0.1% or less (including 0%) of Cr and 0.1% or less (including 0%) of Mo. 
     
     
         4 . The cold-rolled steel sheet of  claim 1 , wherein the cold-rolled steel sheet has a tensile strength of 1500 MPa or more and a total elongation rate of 10% or more. 
     
     
         5 . The cold-rolled steel sheet of  claim 1 , wherein the cold-rolled steel sheet has a yield strength of 1000 MPa or more. 
     
     
         6 . A manufacturing method of a cold-rolled steel sheet, the method comprising:
 reheating a steel slab having a composition including, by weight: 0.15 to 0.3% of C, 0.1 to 1.5% of Si, 2.5 to 5.0% of Mn, 0.1% or less (excluding 0%) of P, 0.03% or less (excluding 0%) of S, 0.01 to 0.1% of Al, 0.01% or less (excluding 0%) of N, and 0.005% or less (excluding 0%) of B with a remainder of Fe and other inevitable impurities at 1100 to 1300° C.;   hot rolling the reheated slab at 800 to 1000° C. to obtain a hot-rolled steel sheet;   coilcoiling the hot-rolled steel sheet at 400 to 700° C.;   cold rolling the coiled hot-rolled steel sheet at a reduction rate of 20 to 75% to obtain a cold-rolled steel sheet;   first annealing of heating the cold-rolled steel sheet in a range of 600 to 700° C. and maintaining the cold-rolled steel sheet for 2 to 24 hours;   first cooling of cooling the first annealed cold-rolled steel sheet at an average cooling rate of 30° C./s or more;   second annealing of heating the first cooled cold-rolled steel sheet to a temperature higher than an austenite single-phase region at an average heating rate of 30° C./s or more; and   second cooling the second annealed cold-rolled steel sheet at an average cooling rate of 30° C./s or more.   
     
     
         7 . The manufacturing method of a cold-rolled steel sheet of  claim 6 , wherein the first annealing is performed at an average heating rate of 30° C./s or more. 
     
     
         8 . The manufacturing method of a cold-rolled steel sheet of  claim 6 , wherein the cold-rolled steel sheet satisfies the following Relation 2: 
       
         
           
             
               
                 
                   
                     1. 
                     ≤ 
                     
                       TH 
                       ⁢ 
                       1 
                       / 
                       TH 
                       ⁢ 
                       2 
                     
                     ≤ 
                     1.5 
                   
                 
                 
                   
                     [ 
                     
                       Relation 
                       ⁢ 
                           
                       2 
                     
                     ] 
                   
                 
               
             
           
         
         wherein TH1 is a highest temperature on a surface of the cold-rolled steel sheet in the first annealing, and TH2 is a highest temperature on a surface of the cold-rolled steel sheet in the second annealing.