MORPHOLOGY DEVELOPMENT IN HOT-DIP GALVANNEAL COATINGS

Hot-dip galvanized drawing quality special killed (DQSK) steel and tit anium stabilized interstitial free (IF) steel substrates were annealed under varying temperature and time conditions in order to characteriz e 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 gro wth) observed in cross section on both substrates was defined in three distinct stages. The three characteristic microstructures were classi fied as type 0 (underalloyed), type 1 (marginally alloyed), and type 2 (overalloyed) morphologies. The morphology transitions were quantitat ively defined by total iron content in the coating and by the thicknes s of an interfacial Fe-Zn gamma phase layer. The DQSK steel coating ty pe 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 p ct and at an interfacial layer thickness of approximately 1.0 mum. An increased amount of aluminum in the galvanizing bath delayed the alloy ing reaction during galvannealing for both substrates. The overall inh ibition effect of aluminum was less pronounced on the titanium stabili zed IF material, indicating that its coating alloying kinetics were no t as significantly influenced by bath aluminum content.