INTERFACIAL LAYER DEVELOPMENT IN HOT-DIP GALVANNEAL COATINGS ON INTERSTITIAL-FREE (IF) STEEL

During the annealing of hot-dip galvanized coatings on interstitial fr ee (IF) steel, an interfacial layer was found to develop and grow at t he steel/coating interface. The interfacial layer followed a three-ste p growth process in which there was initial rapid growth up to a thick ness of approximately 1.0 mu m, 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 inter facial layer did not inhibit zinc and iron interdiffusion or the devel opment of the Fe-Zn alloy layer. Coating iron content increased rapidl y before the interfacial layer grew to a thickness of 1.0 mu m, howeve r, once the coating reached a total iron content in excess of 11.0 wt pct, interfacial layer growth became active and coating iron content i ncreased 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, particularl y in excess of 1.0 mu m, no definitive relationship between interfacia l layer thickness and powdering was found. The thickness of this inter facial layer, however, can be used as an indicator of formability perf ormance.