| 摘要 |
PROBLEM TO BE SOLVED: To produce a galvannealed steel sheet using a hot rolled steel sheet obtd. by an ordinary process as a stock, producible without being undergone pickling and also without increasing the operation and equipment costs and moreover having excellent powdering resistance and plating adhesion equal to those of a plated steel sheet produced with pickling undergone. SOLUTION: This galvannealed steel sheet is produced by subjecting a hot rolled steel sheet subjected to no pickling to heat treatment in an atmosphere of a reducing gas in a continuous hot dip plating line and thereafter subjecting the steel sheet to galvannealing treatment. In this case, the surface of ferrite is applied with an unreduced scale layer having 0,3 to 3.5μm average thickness, and the upper layer is applied with a galvannealing layer. It has excellent powdering resistance equal to that of a plated steel sheet produced with pickling undergone owing to the remaining unreduced scale layer, and furthermore, by setting an upper limit in the unreduced scale layer thickness, good plating adhesion can be obtd. as well.
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