High-strengh steel sheet and method sheet for manufacturing the same

Abstract

A high-strength steel sheet having good formability and methods for manufacturing. The high-strength steel sheet has a chemical composition including, in mass %, C: 0.010% to 0.080%, Si: 0.05% or less, Mn: 0.10% to 0.70%, P: 0.03% or less, S: 0.020% or less, Al: 0.005% to 0.070% and N: 0.0120% to 0.0180%, the balance being Fe and inevitable impurities. A content of nitrogen in the form of solute nitrogen is 0.0100% or more, average ferrite grain size of the steel sheet is 7.0 μm or less, the density of dislocations at a depth of 1/4 sheet thickness from the surface of the steel sheet is 4.0×10 14 m −2 to 2.0×10 15 m −2 , and the tensile strength and the elongation in the transverse direction, perpendicular to the rolling direction, after aging treatment is 530 MPa or more and 7% or more respectively.

1 . A high-strength steel sheet having a chemical composition comprising:
 C: 0.010% to 0.080%, by mass°  9 :,,   Si: 0.05% or less, by mass %;   Mn: 0.10% to 0.70%, by mass %;   P: 0.03% or less, by mass %;   S: 0.020% or less, by mass %;   Al: 0.005% to 0.070%, by mass %; and   N: 0.0120% to 0.0180%, by mass %;   the balance being Fe and inevitable impurities,   wherein:
 a content of nitrogen in the form of solute nitrogen is 0.0100% or more, 
 an average ferrite grain size of the steel sheet is 7.0 μm or less, 
 a density of dislocations at a depth of ¼ sheet thickness from the surface of the steel sheet is 4.0×10 14  m −2  to 2.0×10 15  m −2 , 
 a tensile strength in a transverse direction, perpendicular to a rolling direction, after aging treatment of the steel sheet is 530 MPa or more, and 
 an elongation in the transverse direction after the aging treatment is 7% or more. 
   
     
     
         2 . A method for manufacturing the high-strength steel sheet described in  claim 1 , the method comprising:
 a hot rolling step of (i) heating a slab at a heating temperature of 1180° C. or more, (ii) rolling the slab with a hot rolling finish temperature of 820 to 900° C. to produce a hot-rolled steel sheet, and (iii) coiling the hot-rolled steel sheet at a coiling temperature of 640° C. or less,   a primary cold rolling step of (i) pickling the hot-rolled steel sheet and (ii) cold rolling the hot-rolled steel sheet with a rolling reduction of 85% or more,   an annealing step of annealing the primarily cold-rolled steel sheet at 620° C. to 690° C., and   a secondary cold rolling step of secondarily cold rolling the annealed steel sheet with a rolling reduction of 8 to 20%.   
     
     
         3 . The high-strength steel sheet according to  claim 1 , wherein the chemical composition comprises Al from 0.005% to 0.020%, by mass %. 
     
     
         4 . The high-strength steel sheet according to  claim 1 , wherein the chemical composition comprises N from more than 0.0130% to 0.0180%, by mass %, 
     
     
         5 . The high-strength steel sheet according to  claim 1 , wherein the content of nitrogen in the form of solute nitrogen is 0.0120% or more. 
     
     
         6 . The high-strength steel sheet according to  claim 1 , wherein the chemical composition comprises Mn from more than 0.50% to 0.70%, by mass %. 
     
     
         7 . The method according to  claim 2 , wherein, during the hot rolling step, the coiling temperature is 600° C. or less. 
     
     
         8 . The method according to  claim 2 , wherein, during the annealing step, annealing the primarily cold-rolled steel sheet at 620° C. to 680° C.