Evolution of microstructures of galvanised and galvannealed coatings formed in 0·2 wt-% aluminium–zinc bath

Abstract

The present investigation examines the evolution of the microstructures of galvanised steel during the galvannealing process with a special reference to the formation and breakdown behaviour of aluminium rich inhibition layer at the substrate/coating interface. The interstitial free steel was galvanised in a molten zinc bath with 0·2 wt-% aluminium content. The bath temperature and strip entry temperatures were 460 and 480°C respectively. Aluminium of 0·2 wt-% was chosen to retard the formation of iron–zinc intermetallic compounds in the coating. The as dipped galvanised interstitial free steel was annealed above the melting point of zinc in a salt bath at 480°C for a period ranging from 1 to 60 s. After annealing, the as galvanised coatings exhibited microstructural changes as a function of test time. The present work highlights the possible mechanisms of nucleation of iron–zinc intermetallic compounds during galvannealing treatment of steel galvanised in aluminium containing zinc bath.