| 摘要 |
PURPOSE: A method for manufacturing ultra high tensile steel is provided to sharply improving hardness and toughness which existing hardness (HRc) 45 class steel has, thereby expanding the application range of the steel by low temperature tempering after adding small amount of Nb which can obtain effect of reduction in non-recrystallization zone by lowering temperature of non-recrystallization, controlled rolling and direct quenching. CONSTITUTION: The method for manufacturing ultra high tensile steel comprises the steps of sufficiently heating a steel slab comprising 0.25 to 0.40 wt.% of C, 1.5 wt.% or less of Mn, 0.1 to 0.3 wt.% of Si, 2.0 to 3.5 wt.% of Ni, 0.1 to 1.5 wt.% of Cr, 0.20 to 0.35 wt.% of Mo, 0.035 wt.% or less of Nb, 0.035 wt.% or less of Ti and a balance of Fe and other inevitable impurities at a temperature of 1150 to 1300 deg.C, controlled rolling the heated steel slab so that an accumulated reduction ratio is more than 40 to 75% in the non-recrystallization temperature zone, water cooling the controlled rolled steel slab to an ordinary temperature in a cooling rate of 5 to 50 deg.C/sec by initiating cooling within 30 seconds, and tempering the water cooled steel slab at a temperature of 180 to 250 deg.C.
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