| 摘要 |
PURPOSE: A method for manufacturing a bake-hardenable cold rolled steel sheet with superior formability for use in automobile outer panels is provided, in which by adding a specific amount of Nb acting as carbonitride, excellent bake-hardenability can be secured even if conducting slow cooling after annealing. The bake-hardenable cold rolled steel sheet of the present invention has a bake-hardenability of 3 to 6 kgf/mm¬2, average 'r' value of higher than 2.0, and an elongation rate of higher than 38%. CONSTITUTION: The method includes the steps of soaking a steel slab comprising C 0.0018-0.0025 wt.%, Si 0.02 wt.% or less, Mn 0.07-0.2 wt.%, P 0.05-0.08 wt.%, S 0.008 wt.% or less, Sol.Al 0.02-0.06 wt.%, N 0.0030 wt.% or less, Nb 0.008-0.012 wt.%, a balance of Fe and other inevitable impurities at higher than 1200°C; finish hot rolling the steel slab in the temperature range of 900 to 950°C; coiling the hot rolled steel sheet in the temperature range of 700 to 750°C; cold rolling the hot coil at a reduction ratio of 75 to 80%; continuous annealing the cold rolled steel sheet in the temperature range of 850 to 870°C; temper rolling the annealed cold-rolled steel sheet at a reduction ratio of 1.3 to 2.0%.
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