ALLOYING REACTIONS AND COATING MICROSTRUCTURE IN CONTINUOUS GALVANIZING AND GALVANNEALING PROCESS

The iron-zinc alloying reactions which take place during hot dip galvanizing and galvannealing processes were investigated with special reference to the formation and growth behavior of Fe-Zn intermetallic compounds. The SEM observation and X-ray diffraction analysis of the intermetallic compounds which had formed on laboratory galvanized Ti stabilized interstitial free steel and low carbon Al killed steel sheets showed that there were two types of elementary Fe-Zn alloying reactions ; one was the formation of zeta or delta1 crystals and another was so-called outburst reaction. The zeta crystals nucleated on the Fe-Al intermetallic 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 temperature of below 773K, and it was explained in terms of the Zn-Fe equilibrium phase diagram (peritectic temperature of zeta-phase). The effect of solute element in steel on the thickness of GAMMA-phase was explained by its suppression effect toward the outburst reaction.