Patent ID: 8303739

Claim:
A method for producing a high-strength hot-dip galvannealed steel sheet, the method comprising, sequentially: pickling a high-strength steel sheet in an aqueous solution of hydrochloric acid, the high-strength steel sheet including a pickled base steel sheet; drying the pickled high-strength steel sheet; optionally rinsing the dried steel sheet; further pickling the dried high-strength steel sheet in an aqueous solution of sulfuric acid; preplating the high-strength steel sheet with between 0.2 g/m 2 and 2.0 g/m 2 of Ni; heating the high-strength steel sheet in a nonoxidizing atmosphere to a temperature between about 430° C. and about 500° C. with a heating rate of more than 30° C./sec, in a non-oxidizing or reducing atmosphere; hot-dip plating the high-strength steel sheet in a hot-dip galvanizing bath containing between 0.05% and 0.2% of Al; alloying the high-strength steel sheet by rapidly heating the plated steel sheet to a temperature of about 470 to about 550° C. with a heating rate of more than 20° C./sec; cooling the alloyed steel sheet by one of: (i) cooling without soaking; or (ii) holding the steel sheet for soaking for less than about 15 seconds; and further cooling the steel sheet to produce the high-strength hot-dip galvannealed steel sheet.