Cold-rolled steel sheet and method for manufacturing the same

Abstract

A cold-rolled steel sheet has a partially recrystallized grain structure with a degree of unrecrystallization of 25% to 90% and a Rockwell hardness HRB of 83 or more, the cold-rolled steel sheet containing 0.01% to 0.15% C, 0.03% or less Si, 0.10% to 0.70% Mn, 0.025% or less P, 0.025% or less S, 0.01% to 0.05% Al, and 0.008% or less N on a mass basis, the remainder being Fe and unavoidable impurities, wherein the mean diameter of ferrite is 2 to 10 μm and these components satisfy Formula (1): (C %)+0.15×(Mn %)+0.85×(P %)≧0.21, wherein (M %) represents the content (mass percent) of an element M.

1 . A cold-rolled steel sheet having a partially recrystallized grain structure with a degree of unrecrystallization of 25% to 90% and a Rockwell hardness HRB of 83 or more, the cold-rolled steel sheet containing:
 0.01% to 0.15% C;   0.03% or less Si;   0.10% to 0.70% Mn;   0.025% or less P;   0.025% or less S;   0.01% to 0.05% Al; and   0.008% or less N on a mass basis, the remainder being Fe and unavoidable impurities,   wherein the mean diameter of ferrite is 2 to 10 μm and these components satisfy Formula (I):
   (C %)+0.15×(Mn %)+0.85×(P %)≧0.21  (1)
 
   
       where (M %) represents the content (mass percent) of an element M. 
     
     
         2 . A method for manufacturing a cold-rolled steel sheet comprising:
 hot-rolling a slab at a finishing temperature not lower than the Ar 3  transformation point to form a hot-rolled sheet;   coiling the hot-rolled sheet at a coiling temperature of 580° C. to 750° C.;   pickling the hot-rolled sheet;   cold-rolling the hot-rolled sheet at a rolling reduction of 65% or more to form a cold-rolled sheet; and   annealing the cold-rolled sheet at a temperature of 680° C. or lower by continuously annealing,   wherein the slab contains 0.01% to 0.15% C, 0.03% or less Si, 0.10% to 0.70% Mn, 0.025% or less P, 0.025% or less S, 0.01% to 0.05% Al, and 0.008% or less N on a mass basis, the remainder being Fe and unavoidable impurities, and these components satisfy Formula (1):
   (C %)+0.15×(Mn %)+0.85×(P %)≧0.21  (1)
 
   
       where (M %) represents the content (mass percent) of an element M.