Formation and growth of iron-zinc intermetallics during annealing treatment of galvanized steel

The development of desired microstructure of galvannealed sheet requires in-depth understanding of the formation and growth kinetics of various Fe-Zn intermetallic phases in the coating during post-treatment after hot dipping of the strip in liquid zinc alloy bath. Keeping this in view, annealing treatment of as-received commercial galvanized sheets, obtained from low (0.135 wt.% Al) and high (0.2 wt.% Al) aluminium containing zinc bath, is carried out to examine the microstructure of annealed sheet with varying strip annealing duration at a fixed galvannealing temperature. The effect of the bath aluminium concentration on the formation of the inhibition layer and on the formation mechanisms of various Fe-Zn intermetallics during subsequent galvannealing (GA) treatment is investigated. Strip annealing simulation is performed using Gleeble® 3800 thermo-mechanical simulator. A detailed characterization of the annealed specimens is carried out to study the galvannealed microstructure produced with varying annealing parameters for both the galvanized specimens obtained from commercial liquid zinc alloy bath with varying aluminium content. A systematic study of annealing treatment of galvanized sheet is performed to enhance the understanding of the role of reacted aluminium present at the interface of the substrate and coating in controlling the nucleation and growth kinetics of Fe-Zn phases to obtain the optimum galvannealed structure. It is observed that high aluminium galvanized bath hinders the formation of the desired sequence of Fe-Zn intermetallics in the coating during annealing treatment. However, galvannealed coating formed using low Al-containing zinc bath shows the desired sequence of various Fe-Zn intermetallics with favourable galvannealed microstructure mainly consisted of compact delta (d) phase, particularly after annealing for 20s at 470°C.