Interfacial layer development in hot-dip galvanneal coatings on interstitial free (IF) steel

Abstract

During the annealing of hot-dip galvanized coatings on interstitial free (IF) steel, an interfacial layer was found to develop and grow at the steel/coating interface. The interfacial layer followed a three-step growth process in which there was initial rapid growth up to a thickness of approximately 1.0 µm, followed by a period of essentially no growth with continued zinc and iron interdiffusion into the coating, and finally renewed growth at long time (60 second) anneals. The interfacial layer did not inhibit zinc and iron interdiffusion or the development of the Fe-Zn alloy layer. Coating iron content increased rapidly before the interfacial layer grew to a thickness of 1.0 µm, however, once the coating reached a total iron content in excess of 11.0 wt pct, interfacial layer growth became active and coating iron content increased only slightly with continued annealing. Although powdering of the coating as evaluated by a 60 deg bend test was generally found to increase with an increase in interfacial layer thickness, particularly in excess of 1.0 µm, no definitive relationship between interfacial layer thickness and powdering was found. The thickness of this interfacial layer, however, can be used as an indicator of formability performance.