| 摘要 |
PURPOSE: Provided is a method for manufacturing organic film coated cold rolled steel sheets having superior formability and bake hardenability where a trace amount of B acting as an element for accelerating penetration of solid solution into the Al-killed steel containing a trace amount of Nb acting as a powerful carbiding/nitriding forming element is added and also Zn-Ni electroplating and chromate treatment are performed. CONSTITUTION: The organic film coated cold rolled steel sheet is manufactured by homogenizing an ultra low carbon/nitrogen Al-killed steel sheet comprising C 0.0015-0.0030wt.%, Si 0.2wt.% or less, Mn 0.07-0.4wt.%, P 0.008-0.09wt.%, S 0.008-0.015wt.%, sol-Al 0.02-0.08wt.%, N 0.0030wt.% or less, B 0.0003-0.0010wt.%, Nb 0.002-0.01wt.% at over 1200deg.C where the atomic ratio of Nb/C is 0.5 or less; finish hot rolling the Al-killed steel sheet in the temperature range of 900 to 950deg.C; hot coiling it in the temperature range of 700 to 750deg.C; cold rolling it at a reduction ratio of 75%; continuous annealing the cold rolled steel sheet in the temperature range of 800 to 860deg.C; temper rolling it at a reduction ratio of 1.0 to 2.0%; electroplating the cold rolled steel sheet in Zn-Ni basin; treating it with chromate; applying an organic film thereon; and then baking it in the temperature range of 140 to 200deg.C.
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