High-strength and thick steel sheet having excellent hole expandability and ductility, and manufacturing method therefor

Abstract

The present invention relates to steel suitable as a material for an automotive structural member and particularly, to a high-strength and thick steel sheet having a low yield ratio and high strength, excellent formability and collision resistance due to excellent hole expandability through improved ductility, and a method thereof.

1 . A high-strength thick steel sheet having excellent hole expandability and ductility comprising, by weight:
 0.05 to 0.12% of carbon (C), 2.0 to 3.0% of manganese (Mn), 0.5% or less (excluding 0%) of silicon (Si), 1.0% or less (excluding 0%) of chromium (Cr), 0.1% or less (excluding 0%) of niobium (Nb), 0.1% or less (excluding 0%) of titanium (Ti), 0.003% or less (excluding 0%) of boron (B), 0.02 to 0.05% of aluminum (sol.Al), 0.05% or less (excluding 0%) of phosphorus (P), 0.01% or less (excluding 0%) of sulfur (S), and 0.01% or less (excluding 0%) of nitrogen (N), with a remainder of Fe and other unavoidable impurities,   wherein the steel sheet includes, by area fraction, 10 to 30% of ferrite, 10 to 25% of recrystallized ferrite bridge, and 20 to 30% of bainite, and residual martensite as a microstructure.   
     
     
         2 . The high-strength thick steel sheet having excellent hole expandability and ductility of  claim 1 , wherein the recrystallized ferrite bridge has an average equivalent circular diameter of 1 to 6 μm. 
     
     
         3 . The high-strength thick steel sheet having excellent hole expandability and ductility of  claim 1 , wherein the steel sheet has a martensite phase in an area fraction of 15% or more. 
     
     
         4 . The high-strength thick steel sheet having excellent hole expandability and ductility of  claim 1 , wherein the steel sheet has a residual austenite phase in an area fraction of 3% or less (including 0%). 
     
     
         5 . The high-strength thick steel sheet having excellent hole expandability and ductility of  claim 1 , wherein the steel sheet has a tensile strength of 980 MPa or more, a yield strength of 550 to 700 MPa, and a total elongation of 14% or more. 
     
     
         6 . The high-strength thick steel sheet having excellent hole expandability and ductility of  claim 1 , wherein the steel sheet has a hole expansion ratio (HER) of 30% or more. 
     
     
         7 . The high-strength thick steel sheet having excellent hole expandability and ductility of  claim 1 , wherein the steel sheet has a thickness of 1 to 3 mm. 
     
     
         8 . A method for manufacturing a high-strength thick steel sheet having excellent hole expandability and ductility comprising by weight:
 preparing a steel slab including by weight, 0.05 to 0.12% of carbon (C), 2.0 to 3.0% of manganese (Mn), 0.5% or less (excluding 0%) of silicon (Si), 1.0% or less (excluding 0%) of chromium (Cr), 0.1% or less (excluding 0%) of niobium (Nb), 0.1% or less (excluding 0%) of titanium (Ti), 0.003% or less (excluding 0%) of boron (B), 0.02 to 0.05% of aluminum (sol.Al), 0.05% or less (excluding 0%) of phosphorus (P), 0.01% or less (excluding 0%) of sulfur (S), and 0.01% or less (excluding 0%) of nitrogen (N), with a remainder of Fe and other unavoidable impurities;   heating the steel slab at a temperature within a range of 1100 to 1300° C.;   hot rolling the heated steel slab to manufacture a hot-rolled steel sheet;   coiling the hot-rolled steel sheet at a temperature within a range of 400 to 700° C.;   cooling the hot-rolled steel sheet to room temperature after the coiling;   cold rolling the cooled hot-rolled steel sheet at a reduction rate of 55 to 80% to manufacture a cold-rolled steel sheet;   continuously annealing the cold-rolled steel sheet;   performing primary cooling to a temperature within a range of 650 to 700° C. at an average cooling rate of 1 to 10° C./s after the continuous annealing; and   performing secondary cooling to a temperature within a range of 450 to 500° C. at an average cooling rate of 5 to 50° C./s, after the primary cooling,   wherein the continuous annealing is performed in an equipment equipped with a heating zone, a soaking zone, and a cooling zone, and the cold-rolled steel sheet passes through a recrystallization zone in which the cold-rolled steel sheet is maintained at a temperature within a range of 600 to 700° C. for 1 to 3 minutes when the temperature of the cold-rolled steel sheet is raised to a temperature within a range of the heating zone.   
     
     
         9 . The method for manufacturing a high-strength thick steel sheet having excellent hole expandability and ductility of  claim 8 , wherein the hot rolling is performed wherein temperature range of an outlet is Ar3 to 1000° C. 
     
     
         10 . The method for manufacturing a high-strength thick steel sheet having excellent hole expandability and ductility of  claim 8 , wherein cooling after the coiling is performed at a cooling rate of 0.1° C./s or less (excluding 0° C./s). 
     
     
         11 . The method for manufacturing a high-strength thick steel sheet having excellent hole expandability and ductility of  claim 8 , wherein the heating zone and the soaking zone are controlled to a temperature within a range of 790 to 850° C. 
     
     
         12 . The method for manufacturing a high-strength thick steel sheet having excellent hole expandability and ductility of  claim 8 , further comprising:
 performing an overaging treatment after the secondary cooling,   wherein the overaging treatment is performed for 200 to 800 seconds.