Low-yield-ratio type high-strength steel, and manufacturing method therefor

Abstract

A low-yield-ratio type high-strength steel includes 0.02-0.11 wt % of carbon (C), 0.1-0.5 wt % of silicon (Si), 1.5-2.5 wt % of manganese (Mn), 0.01-0.06 wt % of aluminum (Al), 0.1-0.6 wt % of nickel (Ni), 0.01-0.03 wt % of titanium (Ti), 0.005-0.08 wt % of niobium (Nb), 0.1-0.5 wt % of chromium (Cr), 0.01 wt % or less of phosphorus (P) (excluding 0 wt %), 0.01 wt % or less of sulfur (S) (excluding 0 wt %), 5-30 weight ppm of boron (B), 20-70 weight ppm of nitrogen (N), 50 weight ppm or less of calcium (Ca) (excluding 0 weight ppm), 5-50 weight ppm or less of tin (Sn) (excluding 0 weight ppm), and a remainder thereof, being iron (Fe), and other inevitable impurities.

1 . Low-yield-ratio type, high-strength steel comprising:
 0.02 to 0.11 wt % of carbon (C);   0.1 to 0.5 wt % of silicon (Si);   1.5 to 2.5 wt % of manganese (Mn);   0.01 to 0.06 wt % of aluminum (Al);   0.1 to 0.6 wt % of nickel (Ni);   0.01 to 0.03 wt % of titanium (Ti);   0.005 to 0.08 wt % of niobium (Nb);   0.1 to 0.5 wt % of chromium (Cr);   0.01 wt % or less of phosphorus (P);   0.01 wt % or less of sulfur (S);   5 to 30 wt ppm of boron (B);   20 to 70 wt ppm of nitrogen (N);   50 wt ppm or less of calcium (Ca) (excluding 0);   5 to 50 wt ppm or less of tin (Sn);   iron (Fe) as a remainder thereof; and   other inevitable impurities.   
     
     
         2 . The low-yield-ratio type, high-strength steel of  claim 1 , wherein the steel further includes one or more of 0.1 to 0.5 wt % of copper (Cu), 0.15 to 0.3 wt % of molybdenum (Mo), and 0.005 to 0.3 wt % of vanadium (V). 
     
     
         3 . The low-yield-ratio type, high-strength steel of  claim 1 , wherein a refined structure of the steel includes bainitic ferrite and granular bainite in a primary phase and includes M-A in a secondary phase. 
     
     
         4 . The low-yield-ratio type, high-strength steel of  claim 3 , wherein the bainitic ferrite has an area fraction of 80 to 95%, the granular bainite has an area fraction of 5 to 20%, and the M-A has an area fraction of 3% or less (including 0%). 
     
     
         5 . The low-yield-ratio type, high-strength steel of  claim 1 , wherein PImax. (111)/PImax. (100) as a ratio of pole intensity (PImax.) of (100) and (111) crystalline surfaces of the steel is 1.0 or more or 1.8 or less (where the PImax. (111) is pole intensity of the (111) crystalline surface and the PImax. (100) is pole intensity of the (100) crystalline surface). 
     
     
         6 . The low-yield-ratio type, high-strength steel of  claim 1 , wherein the steel has a yield ratio of 0.85 or less and has tensile strength of 800 MPa or more. 
     
     
         7 . The low-yield-ratio type, high-strength steel of  claim 1 , wherein the steel has a thickness of 60 mm or less. 
     
     
         8 . A method of manufacturing low-yield-ratio type, high-strength steel, the method comprising:
 heating a slab including 0.02 to 0.11 wt % of carbon (C), 0.1 to 0.5 wt % of silicon (Si), 1.5 to 2.5 wt % of manganese (Mn), 0.01 to 0.06 wt % of aluminum (Al), 0.1 to 0.6 wt % of nickel (Ni), 0.01 to 0.03 wt % of titanium (Ti), 0.005 to 0.08 wt % of niobium (Nb), 0.1 to 0.5 wt % of chromium (Cr), 0.01 wt % or less of phosphorus (P), 0.01 wt % or less of sulfur (S), 5 to 30 wt ppm of boron (B), 20 to 70 wt ppm of nitrogen (N), 50 wt ppm or less of calcium (Ca) (excluding 0), 5 to 50 wt ppm or less of tin (Sn), iron (Fe) as a remainder thereof, and other inevitable impurities, at 1050 to 1250° C.;   rough-rolling the heated slab at 950 to 1050° C. to acquire a bar;   hot-rolling the bar at final rolling temperature of 700 to 950° C. to acquire hot rolled steel; and   cooling the hot rolled steel at cooling speed of 25 to 50° C./s up to cooling termination temperature of Bs or less.   
     
     
         9 . The method of  claim 8 , wherein the slab further includes one or more of 0.1 to 0.5 wt % of copper (Cu), 0.15 to 0.3 wt % of molybdenum (Mo), and 0.005 to 0.3 wt % of vanadium (V). 
     
     
         10 . The method of  claim 8 , wherein the hot-rolling is performed at a reduction ratio of 50 to 80%.