METHOD FOR MANUFACTURING STEEL IN WHICH TOUGHNESS AT LOW TEMPERATURE REQUIRED IN ULTRA-COLD AREA IS GREATLY IMPROVED WHILE MAINTAINING 60 KGF/MM¬2 OR MORE TENSILE STRENGTH BY MEANS OF CONTROLLING ALLOY COMPOSITION, HOT ROLLING AND COOLING CONDITIONS OF STEEL PROPERLY

摘要

PURPOSE: To provide a method for manufacturing steel in which toughness at low temperature has 300 J/cm or more Charpy impact energy at -20 deg.C, and tensile strength is 60 kgf/mm¬2 by controlling an alloy composition to a proper range and properly controlling hot rolling conditions and cooling conditions of steel. CONSTITUTION: The method comprises a hot rolling step of heating a steel slab consisting of 0.05 to 0.09 wt.% of C, 0.2 to 0.5 wt.% of Si, 1.2 to 2.0 wt.% of Mn, 0.02 to 0.06 wt.% of Al, 0.01 to 0.03 wt.% of Ti, 0.04 to 0.06 wt.% of Nb, 0.05 to 0.07 wt.% of V, 0.001 to 0.002 wt.% of B, 0.002 wt.% or less of S, 0.02 wt.% or less of P, 10 to 100 ppm of N, 0.01 to 0.05 wt.% of O, 10 to 100 ppm of Ca, a balance of Fe and other inevitable impurities in a temperature range of 1,050 to 1,300 deg.C, hot rolling the heated steel slab in a recrystallization range, and finishing hot rolling in a temperature range of Ar330 deg.C by hot rolling the hot rolled steel to a rolling reduction range of 60 to 80% in a non-recrystallization range that is the following recrystallization temperature(R) or less: R(deg.C)=887+464C+(6645Nb-64(Nb)¬1/2)+(732V-230(V)¬1/2)+890Ti+363Al-357Si(deg.C); and a cooling step of air cooling the water cooled steel to an ordinary temperature after water cooling the hot rolled steel to a temperature range of 400 to 600 deg.C in a cooling rate of 10 to 20 deg.C/second.