| 摘要 |
<p>A high yield ratio type ultra high strength steel sheet having excellent bendability manufactured by controlling alloy components and/or cooling conditions, thereby controlling microstructure of the steel sheet is provided, and a method for manufacturing a hot dip galvanized steel sheet using the same is provided. A high yield ratio type ultra high strength steel sheet having excellent bendability contains, by weight percent, 0.1 to 0.2% of C, 0.1 to 1.0% of Si, 2.0 to 3.0% of Mn, 0.01 to 0.05% of Ti, 0.01 to 0.05% of Ti, 0.01 to 0.05% of Nb, 5 to 20 ppm of B and the balance being Fe and other inevitable impurities, and microstructure of the steel sheet comprises at least 70% of tempered martensite and 30% or below of bainite and ferrite. A method for manufacturing a high yield ratio type ultra high strength hot dip galvanized steel sheet having excellent bendability comprises: annealing a steel sheet comprising, by weight percent, 0.1 to 0.2% of C, 0.1 to 1.0% of Si, 2.0 to 3.0% of Mn, 0.01 to 0.05% of Ti, 0.01 to 0.05% of Ti, 0.01 to 0.05% of Nb, 5 to 20 ppm of B, and the balance being Fe and other inevitable impurities at 790 to 850 deg.C; rapidly cooling the steel sheet to 300 deg.C or below that is lower than a martensite transformation initiating temperature by maintaining the hydrogen concentration in a furnace in a range of 40 to 80%; and galvanizing the rapidly-cooled steel sheet at a steel sheet inlet temperature of a galvanizing plating bath in a range of 480 to 520 deg.C.</p> |
