ALLOYING REACTIONS AND COATING MICROSTRUC TURE IN CONTINUOUS GALVANIZING AND GALVANNEALING PROCESS

The iron-zinc alloying reactions which take place during hot dip galva nizing and galvannealing processes were investigated with special refe rence to the formation and growth behavior of Fe-Zn intermetallic comp ounds. The SEM observation and X-ray diffraction analysis of the inter metallic compounds which had formed on laboratory galvanized Ti stabil ized interstitial free steel and low carbon Al killed steel sheets sho wed that there were two types of elementary Fe-Zn alloying reactions ; one was the formation of zeta or delta1 crystals and another was so-c alled outburst reaction. The zeta crystals nucleated on the Fe-Al inte rmetallic compound which had been formed on the steel surface and grew into the melt as a single phase crystal, while the outburst structure generated as a result of direct reaction between iron and molten zinc and grew as a multi-phase structure (zeta, delta1 and GAMMA phases ). The zeta-phase was generated only at the isothermal galvannealing tem perature of below 773K, and it was explained in terms of the Zn-Fe equ ilibrium phase diagram (peritectic temperature of zeta-phase). The eff ect of solute element in steel on the thickness of GAMMA-phase was exp lained by its suppression effect toward the outburst reaction.