PHASE EVOLUTION IN GALVANNEAL COATINGS ON STEEL SHEETS

The evolution of the phases in galvanneal coatings on IF and IFP steel sheets is studied by combining optical microscopy (OM), scanning electron microscopy (SEM) with X-ray microanalyses (EDS) and transmission electron microscopy (TEM). The early stage of galvannealing is dominated by the formation of intermetallic compounds, particularly delta phase by constitutional supercooling except the region near the steel substrate where rapid formation of a thin alloy layer, the nature of which depends strongly on surface segregation of minor elements such as phosphorus in a steel substrate. The Gamma phase, which shows a well-defined columnar structure, grows at the expense of the Gamma(1) phase. The thickness of the Gamma(1) phase decreases, while maintaining the total thickness of (Gamma+Gamma(1)) constant, similar to 2 mu m, during galvannealing. Three different grain morphologies of delta phase have been observed. The surface segregation of phosphorus in steel was found to promote the nucleation of the Gamma(1) phase at the solid-liquid interface in the very beginning of galvannealing. The formation of the Gamma phase was retarded accordingly. The hot-dip simulator was found to reproduce the microstructures of galvanneal coatings obtained by the in-line production and to facilitate the investigation of the phase evolution during hot-dip galvanizing and galvannealing.