Cold-rolled steel sheet having excellent bendability and hole expandability and method for manufacturing same

Abstract

Provided is a method of manufacturing a cold-rolled steel sheet. The method includes: hot-rolling a steel slab under a finish rolling outlet temperature condition of Ar3 to Ar3+50° C. after reheating the steel slab; coiling the hot-rolled steel sheet at a temperature of 600° C. to 750° C.; by cold-rolling the hot-rolled steel sheet at a reduction rate of 40% to 70%; and overaging the cold-rolled steel sheet after continuous annealing, primary cooling at a cooling rate of 1 to 10° C./sec to 650° C. to 700° C., and secondary cooling at a cooling rate of 5 to 20° C./sec to a temperature of Ms−100° C. to Ms° C. The steel slab includes, by wt %, 0.03 to 0.07 of carbon, 2.0 to 3.0 of manganese, 0.01 to 0.10 of soluble aluminum, 0.3 to 1.2 of chromium, 0.03 to 0.08 of titanium, 0.01 to 0.05 of niobium, 0.0010 to 0.0050 of boron, 0.001 to 0.10 of phosphorous.

1 . A method of manufacturing a cold-rolled steel sheet having excellent bendability and hole expandability, the method comprising:
 obtaining a hot-rolled steel sheet by hot-rolling a steel slab under a finish rolling outlet temperature condition of Ar3 to Ar3+50° C. after reheating the steel slab including 0.03 wt % to 0.07 wt % of carbon (C), 0.3 wt % or less of silicon (Si) (including 0 wt %), 2.0 wt % to 3.0 wt % of manganese (Mn), 0.01 wt % to 0.10 wt % of soluble aluminum (Sol.Al), 0.3 wt % to 1.2 wt % of chromium (Cr), 0.03 wt % to 0.08 wt % of titanium (Ti), 0.01 wt % to 0.05 wt % of niobium (Nb), 0.0010 wt % to 0.0050 wt % of boron (B), 0.001 wt % to 0.10 wt % of phosphorous (P), 0.010 wt % or less of sulfur(S) (including 0 wt %), 0.010 wt % or less of nitrogen (N) (including 0 wt %), and the balance being Fe and other impurities;   coiling the hot-rolled steel sheet at a temperature in a range of 600° C. to 750° C.;   obtaining a cold-rolled steel sheet by cold-rolling the hot-rolled steel sheet at a cold-reduction rate of 40% to 70%; and   overaging treating the cold-rolled steel sheet after performing continuous annealing, primary cooling at a cooling rate of 1° C./sec to 10° C./sec to 650° C. to 700° C., and then secondary cooling at a cooling rate of 5° C./sec to 20° C./sec to a temperature section of Ms−100° C. to Ms° C.,   wherein Ac 3 , an annealing temperature, Ms, and a secondary cooling finish temperature are satisfied with Relational Expression (1),   
       
         
           
             
               
                 
                   
                     0.9 
                     ≤ 
                     
                       
                         0.055 
                         B 
                       
                       - 
                       
                         
                           0 
                           . 
                           0 
                         
                         ⁢ 
                         7 
                         ⁢ 
                         A 
                       
                     
                     ≤ 
                     
                       2 
                       . 
                       8 
                     
                   
                 
                 
                   
                     [ 
                     
                       Relational 
                       ⁢ 
                           
                       Expression 
                       ⁢ 
                           
                       1 
                     
                     ] 
                   
                 
               
             
           
         
         where A: Ac 3 —Annealing Temperature and B: Ms—Secondary Cooling finish Temperature. 
       
     
     
         2 . The method of  claim 1 , wherein the steel sheet has a microstructure including 75% or more to less than 87% by area of a transformed structure and 13% to 25% by area of ferrite, the transformed structure includes martensite and bainite, the martensite has an average particle diameter of 2 μm or less, the bainite has an average particle diameter of 3 μm or less, the bainite fraction of 3 μm or more is 5% or less, and the interphase hardness ratio is 1.4 or less. 
     
     
         3 . The method of  claim 2 , wherein the transformed structure has a fraction of 83% to 87% by area. 
     
     
         4 . The method of  claim 2 , wherein the steel sheet includes a precipitate with 10 nm or less, provided as 150 precipitates/μm 2  or more. 
     
     
         5 . The method of  claim 2 , wherein the transformed structure has a hardness value (Hv) of 310 or more. 
     
     
         6 . The method of  claim 2 , wherein the steel sheet has tensile strength of 780 MPa or more, yield strength of 650 MPa or more, elongation of 12% or more, R/t of 0.5 or less, a HER of 65% or more, and a yield ratio of 0.8 or more.