THE EFFECT OF STEEL CHEMISTRY ON THE FORMATION OF FE-ZN INTERMETALLIC COMPOUNDS OF GALVANNEAL-COATED STEEL SHEETS

The effects of steel chemistry on the formation of Fe-Zn intermetallic compounds in the galvanneal coatings have been investigated by examining the microstructure of galvanneal coat ings on extra-low-carbon (ELC) steel, interstitial-free (IF) steel, and interstitial-free rephosphorized (IFP) steel. The layer structure of the coatings was revealed by chemical etching. Phases present in each layer were then identified using electron diffraction in transmission electron microscopy (TEM). A two-layer structure one consisting of the delta phase with a small fraction of the zeta phase dispersed on the surface and Gamma phases and another consisting of the delta and Gamma(1) phases, was observed in the ELC sample and the IFP sample, respectively. A three-layer structure consisting of the delta, Gamma(1) + delta, and Gamma phases was observed in the IF sample. The presence of C in the steel substrate retarded the alloying between Fe and Zn; while P in the steel favored the formation of the Gamma(1) phase over the Gamma phase by its surface segregation in the steel substrate. The orientation relationship between coating and substrate was also studied by electron diffraction. Three alpha-Fe/Gamma orientation relationships were frequently observed.