ALLOYING REACTIONS IN HOT-DIP GALVANIZING AND GALVANNEALING PROCESSES

The iron-zinc alloying reactions which take place during hot dip galva nizing and galvannealing processes were investigated, with special ref erence to the formation and growth behavior of iron-zinc intermetallic compounds. The SEM observation and X-ray diffraction analysis of the intermetallic compounds which had formed on laboratory galvanized Ti s tabilized interstitial-free steel and on low carbon aluminum killed st eel sheets showed that there were two types of elementary iron-zinc al loying reactions; one was the formation of zeta and delta(1) crystals and another was so-called outburst reaction. The zeta crystal nucleate d on the Fe-Al intermetallic compound which had been formed on the ste el surface, and grew into the melt as a single phase crystal. Simultan eously, the outburst structure was generated as a result of the direct reaction between iron and molten zinc, and grew as a multiple phase s tructure (zeta, delta(1) and Gamma phases). The zeta phase was generat ed at the isothermal galvannealing temperature of less than 773 K, and it was explained in terms of the iron-zinc equilibrium phase diagram (peritectic temperature of zeta phase). The effects of solute element in steel on the thickness of Gamma phase were explained by its suppres sion effect toward the outburst reaction.