EFFECTS OF METASTABLE DROSS PARTICLES FORMED IN MOLTEN ZINC BATH AND PREDICTION OF EFFECTIVE ALUMINUM DURING GALVANNEALING PROCESSES(38)

摘要

Minimizing the defects caused from dross particles precipitated as Fe-Al-Zn intermetalliccompounds in molten zinc bath is the key issue during galvannealing processes to produceautomotive galvannealed steel sheets for exposed panels. Controlling the aluminum concentrationand temperature in the molten zinc bath with a very small fluctuation can minimize the drossformation and also reduce the defects caused by these particles. The existence of the metastabledross particles formed at the very initial stage in molten zinc bath reported previously was used toidentify their morphologies, chemistries and crystallography etc. The particles examined containedaluminum contents of 15 to 80 wt %, and the values of aluminum contents decreased as the sizes ofparticles increased. As a result of crystallographic analysis, a metastable state of dross particles wasfound tu consist of a mixture comprised of crystalline phase of FeZn10 called "delta phase? and highaluminum containing amorphous phase layer covering the whole surface of the crystalline phase. Inthis study, the "meta Q? phase is defined as a quantity of metastable dross particles, and the quantityof "meta Q? can vary with fluctuating temperature and iron concentration during the remelting andgenerating the dross particles and represents an indicating parameter of changing soluble aluminumin molten zinc bath. This concept was utilized to predict the effective aluminum in molten zinc bathwith representing the kinetics of the formation and growth of the dross particles.