| 摘要 |
Provided are a steel to which large quantities of carbon and silicon are added to prevent deterioration of plating property and weldability, and which has high strength as well as excellent formability by obtaining tensile strength of 120 kgf/mm^2 grade or more and yield ratio of 0.6 or less, and a method for manufacturing the same. A method for manufacturing an ultra high strength steel of 120 kgf/mm^2 grade having excellent formability comprises: a step of reheating a steel slab having a composition comprising 0.10 to 0.15 wt.% of C, 0.7 to 1.7 wt.% of Si, 4.0 to 7.0 wt.% of Mn, 0.03 wt.% or less of P, 0.005 wt.% or less of S, 0.05 to 0.15 wt.% of Al, 0.003 wt.% or less of N, 0.0001 to 0.002 wt.% of B, 0.01 to 0.05 wt.% of Mo and the balance being Fe and other inevitable impurities at a temperature of 1,200 to 1,250 deg.C for 60 to 180 minutes; a step of hot finish rolling the reheated slab at a temperature of Ar3 to Ar3+100 deg.C, and coiling the hot finish rolled steel sheet at a temperature of 560 to 680 deg.C; and a step of cold rolling the coiled hot rolled steel sheet to a reduction ratio of 15 to 75%, batch annealing the cold rolled steel sheet at a temperature of 620 to 650 deg.C for 1 to 12 hours, and air cooling the batch annealed steel sheet.
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