The surface and sub-surface structure of continuously annealed sheet steel: Implications for galvanizing and galvannealing

Abstract

The gas atmosphere in continuous annealing and galvanizing lines alters both composition and micro-structure of the surface and sub-surface of sheet steel. The formation and the morphology of the surface oxides of alloying elements in High-strength Interstitial-free, Dual Phase and Transformation-Induced Plasticity steels is strongly influenced by the furnace dew point, and the presence of specific film-forming oxides may result in surface defects and bare areas on galvanized sheet products. The present contribution reviews the progress made recently in understanding the selective formation of surface and subsurface oxides during annealing in hot dip galvanizing and conventional continuous annealing lines. The mechanisms involved reveal the fact that the surface and subsurface composition and microstructure influence galvanized sheet product surface quality.