Patent ID: 8617324
Filing Date: 2013-12-31
Classification: C23C

Abstract:
1. A method for producing a high-strength hot-dip galvannealed steel sheet containing C: 0.02-0.2%, Si: 0.2-3%, Mn: 0.15-2.5% and a balance of Fe and unavoidable impurities, the method comprising sequentially: pickling a steel sheet comprising at least one of (i) a pickled hot rolled steel sheet, or (ii) an annealed and pickled cold rolled steel sheet in an aqueous solution of hydrochloric acid; rinsing the steel sheet in a rinsing water having a pH of less than 6; preplating the steel sheet with between about 0.2 g/m2 and about 2.0 g/m2 of Ni without drying the steel sheet after the steel sheet is rinsed; heating the steel sheet in at least one of a nonoxidizing atmosphere or a reducing atmosphere to a temperature of about 430° C. to about 500° C. at a heating rate of more than 30° C/sec; hot-dip plating the steel sheet in a galvanizing bath comprising between about 0.05% and about 0.2% of Al; wiping the steel sheet; immediately rapidly heating the plated steel sheet for alloying to a temperature between about 470° C. and about 550° C. at a rate of more than 20° C/sec; and cooling the alloyed steel sheet by at least one of (i) cooling without soaking or (ii) holding the steel sheet for soaking for less than 15 seconds and then further cooling the steel sheet to produce-the high-strength hot-dip galvannealed steel sheet.