METHOD FOR MANUFACTURING HIGH STRENGTH/TOUGHNESS BULLETPROOF STEEL HAVING SUPERIOR INTERIOR MICROSTRUCTURE

PURPOSE: To manufacture high strength/toughness bulletproof steel having superior surface and interior quality without slab bending or nozzle clogging, metal elements such as Ti and Nb are added in steelmaking process instead of V. In addition, vacuum degassing is carried out at lower than 2 torr for 20min to lower hydrogen content up to a level that does not cause internal crack. CONSTITUTION: The manufacture method of high strength/toughness bulletproof steel comprises the steps: (i) continuous cast of a molten steel comprising C 0.13-0.18wt.%, Si 0.20-0.40wt.%, Mn 0.50-0.80wt.%, P 0.05wt.% or less, S 0.005wt.% or less, sol-Al 0.025wt.% or less, Ni 3.5-4.5wt.%, Cr 0.8-1.2wt.%, Mo 0.4-0.6wt.%, Nb 0.03-0.06wt.%, Ti 0.005-0.020wt.%, a balance of Fe at a cast rate 0.95 to 1.0m/min while maintaining overheat rate of the molten steel within a range of 18-23deg.C; (ii) regulating an amount of cooling water to be 0.4-0.45L/kg in the second cooling zone of continuous casting process.