Ultra high strength steel sheet having excellent ductility and method for manufacturing thereof

Abstract

Provided is an ultra-high strength steel sheet having excellent ductility including, 0.1 to 0.2% of carbon (C), 0.1 to 1.0% of silicon (Si), 2.0 to 3.0% of manganese (Mn), 1.0% or less of aluminum (Al), 1.0% or less of chromium (Cr), 0.5% or less of molybdenum (Mo), 0.1% or less of titanium (Ti), 0.1% or less of niobium (Nb), 0.1% or less of antimony (Sb), 0.05% or less of phosphorus (P), 0.02% or less of sulfur (S), 0.02% or less of nitrogen (N), and a balance of Fe and unavoidable impurities. The steel sheet satisfies the following: 1110[C]+41.5[Si]+575[Mn]−1092[Al]−3590[Nb]−5181[Ti]+258[Cr]+664 [Mo]≥1380; 2853 [C]+95 [Si]+309 [Mn]−153 [Al]+4661 [Nb]−780[Ti]+210[Cr]+457[Mo]≥1300; and −29[C]+0.6[Si]−7.3[Mn]+7.8[Al]−145.2[Nb]+62.6[Ti]−3.3[Cr]−2.2[Mo]≥−24.

1 . An ultra-high strength steel sheet having excellent ductility comprising, by wt %, 0.1 to 0.2% of carbon (C), 0.1 to 1.0% of silicon (Si), 2.0 to 3.0% of manganese (Mn), 1.0% or less (excluding 0%) of aluminum (Al), 1.0% or less of chromium (Cr), 0.5% or less of molybdenum (Mo), 0.1% or less of titanium (Ti), 0.1% or less of niobium (Nb), 0.1% or less (excluding 0%) of antimony (Sb), 0.05% or less of phosphorus (P), 0.02% or less of sulfur (S), 0.02% or less of nitrogen (N), and a balance of Fe and unavoidable impurities,
 wherein the ultra-high strength steel sheet satisfies the following Relational Expressions 1 to 3:
   1110[C]+41.5[Si]+575[Mn]−1092[Al]−3590[Nb]−5181[Ti]+258[Cr]+664[Mo]≥1380  [Relational Expression 1]
 
   2853[C]+95[Si]+309[Mn]−153[Al]+4661[Nb]−780[Ti]+210[Cr]+457[Mo]≥1300  [Relational Expression 2]
 
   −29[C]+0.6[Si]−7.3[Mn]+7.8[Al]−145.2[Nb]+62.6[Ti]−3.3[Cr]−2.2[Mo]≥−24  [Relational Expression 3]
 
   (in Relational Expressions 1 to 3, each element represents a weight content).   
     
     
         2 . The ultra-high strength steel sheet having excellent ductility of  claim 1 , wherein the steel sheet contains, in terms of an area fraction, 3 to 20% of ferrite, 1 to 10% of retained-austenite, 1 to 30% of bainite, 30 to 70% of tempered martensite, and a balance of fresh martensite, as a microstructure. 
     
     
         3 . The ultra-high strength steel sheet having excellent ductility of  claim 2 , wherein the steel sheet contains a fresh martensite phase in an area fraction of 3% or more. 
     
     
         4 . The ultra-high strength steel sheet having excellent ductility of  claim 1 , wherein the steel sheet has a yield strength of 700 MPa or more, a tensile strength of 980 MPa or more, and an elongation of 13% or more. 
     
     
         5 . The ultra-high strength steel sheet having excellent ductility of  claim 1 , wherein the steel sheet is any one of a cold-rolled steel sheet, a hot-dip galvanized steel sheet, and an hot-dip galva-annealed steel sheet. 
     
     
         6 . A method for manufacturing an ultra-high strength steel sheet having excellent ductility, the method comprising:
 preparing a steel slab containing, by wt %, 0.1 to 0.2% of carbon (C), 0.1 to 1.0% of silicon (Si), 2.0 to 3.0% of manganese (Mn), 1.0% or less (excluding 0%) of aluminum (Al), 1.0% or less of chromium (Cr), 0.5% or less of molybdenum (Mo), 0.1% or less of titanium (Ti), 0.1% or less of niobium (Nb), 0.1% or less (excluding 0%) of antimony (Sb), 0.05% or less of phosphorus (P), 0.02% or less of sulfur (S), 0.02% or less of nitrogen (N), and a balance of Fe and unavoidable impurities, and satisfying the following Relational Expressions 1 to 3;   heating the steel slab to a temperature within a range of 1,050 to 1,300° C.;   hot rolling the heated steel slab at a temperature within a range of 800 to 1,000° C. to manufacture a hot-rolled steel sheet;   coiling the hot-rolled steel sheet at a temperature within a range of 400 to 700° C.;   cold rolling the coiled hot-rolled steel sheet at a total reduction ratio of 20 to 70% to manufacture a cold-rolled steel sheet;   annealing the cold-rolled steel sheet at a temperature within a range of 800 to 900° C.;   cooling the continuously annealed cold-rolled steel sheet to a temperature range of 250 to 400° C.; and   reheating and maintaining the cooled cold-rolled steel sheet,   wherein the reheating and maintaining are performed at a temperature within a range of the cooled temperature+50° C. or higher to the cooled temperature+200° C. or lower for 0.1 to 60 minutes:
   1110[C]+41.5[Si]+575[Mn]−1092[Al]−3590[Nb]−5181[Ti]+258[Cr]+664[Mo]≥1380  [Relational Expression 1]
 
   2853[C]+95[Si]+309[Mn]−153[Al]+4661[Nb]−780[Ti]+210[Cr]+457[Mo]≥1300  [Relational Expression 2]
 
   −29[C]+0.6[Si]−7.3[Mn]+7.8[Al]−145.2[Nb]+62.6[Ti]−3.3[Cr]−2.2[Mo]≥−24  [Relational Expression 3]
 
   (in Relational Expressions 1 to 3, each element represents a weight content).   
     
     
         7 . The method for manufacturing an ultra-high strength steel sheet having excellent ductility of  claim 6 , wherein the cooling of the cold-rolled steel sheet is performed at a cooling rate of 2 to 50° C./s. 
     
     
         8 . The method for manufacturing an ultra-high strength steel sheet having excellent ductility of  claim 6 , further comprising, before the reheating of the cooled cold-rolled steel sheet, maintaining the cooled cold-rolled steel sheet in the cooled temperature range for 0.1 to 60 minutes. 
     
     
         9 . The method for manufacturing an ultra-high strength steel sheet having excellent ductility of  claim 6 , further comprising, after the reheating and maintaining, performing hot-dip galvanizing. 
     
     
         10 . The method for manufacturing an ultra-high strength steel sheet having excellent ductility of  claim 6 , further comprising, after the hot-dip galvanizing, performing an alloying heat treatment.