| 摘要 |
PURPOSE: A method for manufacturing high strength steel sheet with a yield strength of 120 kgf/mm¬2 for use in armor plate is provided, which is characterized in that Nb and B are added to conventional high strength steel sheet and than it is directly quenched so that manufacture cost is drastically save. CONSTITUTION: The method for manufacturing high strength steel sheet for use in armor plate includes the steps of hot rolling a steel slab comprising C 0.17 to 0.23 wt.%, 0.4 wt.% or less of Mn, 0.1 wt.% or less of Si, Ni 8.5 to 9.5 wt.%, Co 3.5 to 4.5 wt.%, Cr 0.5 to 1.0 wt.%, Mo 0.8 to 1.2 wt.%, V 0.08 to 0.12 wt.%, 0.01 wt.% or less of P, 0.01 wt.% or less of S, B 0.0007 to 0.0020 wt.%, Nb 0.01 to 0.02 wt.%, a balance of Fe and incidental impurities at higher than 1150 to 1300°C; hot rolling the steel slab in a temperature range of 750 to 980°C at a reduction ratio of 5 to 30 % per pass; water cooling the hot rolled steel sheet at a cooling rate of 5 to 50°C/sec to room temperature within 30 seconds after hot rolling; solid solution treatment at a temperature range of 820 to 860°C; oil quenching the steel sheet to room temperature; aging the steel sheet in a temperature range of 550 to 580 deg.C for more than 1 hr; and water cooling.
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