MORPHOLOGY DEVELOPMENT IN HOT-DIP GALVANNEAL COATINGS

Hot-dip galvanized drawing quality special killed (DQSK) steel and titanium stabilized interstitial free (IF) steel substrates were annealed under varying temperature and time conditions in order to characterize the coating structure development which occurs during the annealing portion of the galvannealing process. Through the use of light optical microscopy, the coating morphology development (Fe-Zn alloy layer growth) observed in cross section on both substrates was defined in three distinct stages. The three characteristic microstructures were classified as type 0 (underalloyed), type 1 (marginally alloyed), and type 2 (overalloyed) morphologies. The morphology transitions were quantitatively defined by total iron content in the coating and by the thickness of an interfacial Fe-Zn gamma phase layer. The DQSK steel coating type 1 to type 2 morphology transition occurred at an iron content of 9 to 10 wt pct. For the titanium IF material, the same type 1 to type 2 morphology transition occurred at an iron content of 10.5 to 11.5 wt pct and at an interfacial layer thickness of approximately 1.0 mum. An increased amount of aluminum in the galvanizing bath delayed the alloying reaction during galvannealing for both substrates. The overall inhibition effect of aluminum was less pronounced on the titanium stabilized IF material, indicating that its coating alloying kinetics were not as significantly influenced by bath aluminum content.