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

Abstract

Provided is a steel suitable as a material for automobiles and, specifically, to a high-strength steel sheet having excellent hole expandability and ductility, and a manufacturing method therefor. The high-strength steel sheet of the present invention has a microstructure comprising a hard phase and a soft phase, wherein a martensite phase, which is the hard phase, is evenly distributed in a recrystallized ferrite matrix through optimized cold-rolling and annealing processes, and a nonequilibrium (quasi-equilibrium) ferrite phase is introduced at the interface between the hard phase and the soft phase so as to increase the crack resistance during processing.

1 . A high-strength steel sheet having excellent hole expandability, the high-strength steel sheet comprising:
 by wt %, carbon (C): 0.05 to 0.12%, manganese (Mn): 2.5 to 3.0%, silicon (Si): 1.2% or less (excluding 0%), chromium (Cr): 0.1% or less (excluding 0%), molybdenum (Mo): 0.1% or less (excluding 0%), niobium (Nb): 0.1% or less (excluding 0%), titanium (Ti): 0.1% or less (excluding 0%), boron (B): 0.002% or less (excluding 0%), aluminum (sol.Al): 0.02 to 0.05%, phosphorus (P): 0.05% or less (excluding 0%), sulfur (S): 0.01% or less (excluding 0%), nitrogen (N): 0.01% or less (excluding 0%), iron (Fe) and other unavoidable impurities,   wherein a microstructure comprises ferrite of 20 to 30% by area fraction, non-equilibrium (quasi-equilibrium) ferrite of 5 to 15% by area fraction, and residual martensite.   
     
     
         2 . The high-strength steel sheet of  claim 1 , wherein the steel sheet includes a martensite phase of 50% or more by area fraction. 
     
     
         3 . The high-strength steel sheet of  claim 1 , wherein the steel sheet further comprises a retained austenite phase of 3% or less (including 0%) by area fraction. 
     
     
         4 . The high-strength steel sheet of  claim 1 , wherein the steel sheet has a tensile strength of 1100 MPa or more, a yield strength of 550 to 700 MPa, and a total elongation of 12% or more. 
     
     
         5 . The high-strength steel sheet of  claim 1 , wherein the steel sheet has a hole expansion rate (HER) of 25% or more. 
     
     
         6 . A method of manufacturing a high-strength steel sheet having excellent hole expandability, the method comprising:
 preparing a steel slab including, by wt %, carbon (C): 0.05 to 0.12%, manganese (Mn): 2.5 to 3.0%, silicon (Si): 1.2% or less (excluding 0%), chromium (Cr): 0.1% or less (excluding 0%), molybdenum (Mo): 0.1% or less (excluding 0%), niobium (Nb): 0.1% or less (excluding 0%), titanium (Ti): 0.1% or less (excluding 0%), boron (B): 0.002% or less (excluding 0%), aluminum (sol.Al): 0.02 to 0.05%, phosphorus (P): 0.05% or less (excluding 0%), sulfur (S): 0.01% or less (excluding 0%), nitrogen (N): 0.01% or less (excluding 0%), iron (Fe) and other unavoidable impurities,   heating the steel slab in a temperature 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 in a temperature 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 to manufacture a cold-rolled steel sheet;   continuously annealing the cold-rolled steel sheet;   primarily cooling the cold-rolled steel sheet at an average cooling rate of 1 to 10° C./s to a temperature range of 570 to 630° C. after the continuous annealing; and   secondarily cooling the cooled cold-rolled steel sheet at an average cooling rate of 5 to 50° C. to a temperature range of 300 to 400° C., after the primary cooling,   wherein the continuous annealing is performed in a facility equipped with a heating zone, a soaking zone, and a cooling zone, and the heating zone and the soaking zone are controlled in a temperature range of 810 to 850° C.   
     
     
         7 . The method of  claim 6 , wherein the hot-rolling is finish hot-rolling at an exit side temperature of Ar3 or more to 1000° C. or less. 
     
     
         8 . The method of  claim 6 , wherein the cooling after the coiling is performed at a cooling rate of 0.1° C./s or less (excluding 0° C.). 
     
     
         9 . The method of  claim 6 , wherein the cold-rolling is performed in one stand, and a total reduction ratio is 55 to 70%. 
     
     
         10 . The method of  claim 6 , further comprising:
 performing overaging treatment after the secondary cooling,   wherein the overaging treatment is performed for 200 to 800 seconds.