Cold-rolled steel sheet and plated steel sheet having excellent bake hardenability and room-temperature aging resistance and method of manufacturing same

Abstract

Provided is a steel sheet having properties particularly suitable as a material of automotive external panels because bake hardenability and room-temperature aging resistance are excellent, and a method of manufacturing the steel sheet.

1 . A cold-rolled steel sheet having excellent bake hardenability and room-temperature aging resistance, the cold-rolled steel sheet comprising, in percentage by weight,
 C: 0.002˜0.015%, Mn: 1.5˜3.0%, P: 0.03% or less, S: 0.01% or less, N: 0.01% or less, sol.Al: 0.02˜0.06%, Cr: 1.2% or less (excluding 0%), and a balance of Fe and unavoidable impurities,   and comprising ferrite, which is a matrix structure, and a balance of a hard structure as a microstructure,   wherein a hard structure ratio V of a grain boundary triple point defined by the following Equation 1 is 70% or more,
     V (%)={ Vtp /( Vgb+Vtp )}×100  [Equation 1]
 
   in Equation 1, Vgb is the number of hard structures observed at a ferrite grain boundary in an observation region and Vtp is the number of hard structures observed at a ferrite grain boundary triple point in the observation region.   
     
     
         2 . The cold-rolled steel sheet of  claim 1 , wherein a fraction of the ferrite is 95% or more in area percentage, and
 the hard structure includes martensite.   
     
     
         3 . The cold-rolled steel sheet of  claim 1 , wherein Hel defined by the following Equation 2 satisfies a range of 1.2˜2.5,
   Hel=[C]+0.5*[Mn]+0.75*[Cr]  [Equation 2]
 
 in Equation 2, [C], [Mn], and [Cr] are contents (percentages by weight) of C, Mn, and Cr, respectively. 
 
     
     
         4 . The cold-rolled steel sheet of  claim 1 , further comprising silicon (Si) of 0.1% or less (0% included) in percentage by weight. 
     
     
         5 . The cold-rolled steel sheet of  claim 1 , wherein a bake hardening amount is 30 MPa or more (BH, tension test after heat treatment at 170° C. for 20 minutes) and yield point elongation is 0.2% or less (YP-El, tension test after heat treatment at 100° C. for 1 hour). 
     
     
         6 . (canceled) 
     
     
         7 . A method of manufacturing a cold-rolled steel sheet having excellent bake hardenability and room-temperature aging resistance, the method comprising:
 heating a slab including, in percentage by weight, C: 0.002˜0.015%, Mn: 1.5˜3.0%, P: 0.03% or less, S: 0.01% or less, N: 0.01% or less, sol.A1: 0.02˜0.06%, Cr: 1.2% or less (excluding 0%), and a balance of Fe and unavoidable impurities;   providing a hot-rolled steel sheet by hot-rolling the slab;   coiling the hot-rolled steel sheet;   
       providing a cold-rolled steel sheet by cold-rolling the hot-rolled steel sheet; and
 continuously annealing the cold-rolled steel sheet, 
 wherein the continuous annealing increases temperature up to a range of (Ac1+5° C.)˜(Ac3−20° C.) at a temperature increasing speed of 1˜10° C./s and then maintains the temperature for 30˜240 seconds. 
 
     
     
         8 . The method of  claim 7 , wherein the slab is configured such that Hel defined by the following Equation 2 satisfies a range of 1.2˜2.5,
   Hel=[C]+0.5*[Mn]+0.75*[Cr]  [Equation 2]
 
 in Equation 2, [C], [Mn], and [Cr] are contents (percentages by weight) of C, Mn, and Cr, respectively. 
 
     
     
         9 . The method of  claim 7 , wherein the slab further includes silicon (Si) of 0.1% or less (0% included) in percentage by weight. 
     
     
         10 . The method of  claim 7 , wherein heating temperature of the slab is 1100˜1300° C.,
 finish rolling temperature of the hot rolling is 880° C. or more, 
 the coiling temperature is 400˜700° C., and 
 a reduction ratio of the cold rolling is 50˜90%. 
 
     
     
         11 . (canceled)