High-strength steel sheet having excellent ductility and low-temperature toughness and method for manufacturing thereof

Abstract

The present invention relates to a structural steel sheet suitable for ships or steel structures and, more particularly, to a high-strength steel sheet having excellent ductility and low-temperature toughness and a method for manufacturing same.

1 . A high-strength steel sheet having excellent ductility and low-temperature toughness, comprising: by wt %, 0.05 to 0.12% of carbon (C), 0.2 to 0.5% of silicon (Si), 1.2 to 1.8% of manganese (Mn), 0.012% or less of phosphorus (P), 0.005% or less of sulfur (S), 0.01 to 0.06% of aluminum (Al), 0.005 to 0.02% of titanium (Ti), 0.01 to 0.03% of niobium (Nb), 0.002 to 0.006% of nitrogen (N), 0.5% or less of nickel (Ni), the balance Fe, and inevitable impurities,
 wherein the steel comprises polygonal ferrite having an average grain size (circle equivalent diameter) of 2 to 8 μm as a main phase and pearlite and bainite as a second phase in a microstructure, and has a thickness of 8 to 15 mm.   
     
     
         2 . The high-strength steel sheet of  claim 1 , wherein in the second phase, the pearlite is contained in an area fraction of 5 to 25%, and the bainite is contained in an area fraction of 2% or less (including 0%). 
     
     
         3 . The high-strength steel sheet of  claim 1 , wherein the steel sheet has a yield strength of 355 MPa or more, a tensile strength of 490 MPa or more, and an elongation of 30% or more. 
     
     
         4 . The high-strength steel sheet of  claim 1 , wherein the steel sheet has an impact toughness of 100 J or more at −40° C. 
     
     
         5 . A method for manufacturing of a high-strength steel sheet having excellent ductility and low-temperature toughness, comprising:
 heating a steel slab in a temperature range of 1100 to 1200° C., the steel slab comprising, by wt %, 0.05 to 0.12% of carbon (C), 0.2 to 0.5% of silicon (Si), 1.2 to 1.8% of manganese (Mn), 0.012% or less of phosphorus (P), 0.005% or less of sulfur (S), 0.01 to 0.06% of aluminum (Al), 0.005 to 0.02% of titanium (Ti), 0.01 to 0.03% of niobium (Nb), 0.002 to 0.006% of nitrogen (N), 0.5% or less of nickel (Ni), the balance Fe, and inevitable impurities;   manufacturing the heated steel slab into a hot-rolled steel plate by rough rolling and finish rolling the heated steel slab; and   cooling the hot-rolled steel plate,   wherein the finish rolling is performed in a temperature range of Ar3+70° C. to Ar3+170° C. and has a thickness of 8 to 15 mm.   
     
     
         6 . The method of  claim 5 , wherein the finish rolling is performed such that a cumulative reduction ratio is 60 to 90%. 
     
     
         7 . The method of  claim 5 , wherein the cooling is air-cooled up to room temperature.