12th International Conference on Zinc & Zinc Alloy Coated Steel Sheet - GALVATECH 2021

“Dross” is solid intermetallic compound generated in molten zinc baths of CGL, and it can be the cause of surface defects of hot dip galvanized steel sheets. Their behavior in the molten zinc such as thermochemical stability is well investigated before, but its crystallization or growth behavior is not understood yet because their reaction occurs in high-temperature and opaque molten zinc. In this study we tried in-situ observation of crystallization and growth behavior of Fe-Zn intermetallic compounds (“bottom dross”). X-ray transmission imaging has been tried using SPring-8 BL20XU. From the X-ray upstream, X-ray shutter and slit, sample stage with heater in a vacuum chamber, and high resolution X-ray image detector, are equipped. The selected X-ray energy is 25keV and the pixel resolution of the detector is 0.5um, time resolution is 2 fps. Zn-0.10wt%Al-0.10wt%Fe alloy (sample A) and Zn-0.10wt%Al-0.15wt%Fe alloy (sample B) were selected in order to observe crystallization and growth behavior of Fe-Zn intermetallic compounds. The Zn alloy ingot was cut and polished into the 100um thickness thin specimen. The specimen was set on the sample stages and melt homogeneously in the low pressure environment by heating up to 600ºC. Then specimen was cooled by various rates. The X-ray transmission imaging has been recorded during the cooling process. Fe-Zn intermetallic compounds has higher mass density and X-ray absorption coefficient than molten zinc, they can be observed as darker contrast crystals in the zinc matrix. Their growth behavior by time can be observed. In sample A, their nucleation frequency is not changed by changing cooling rate, but their nucleation frequency is accelerated and its crystal size gets smaller by faster cooling rate in sample B. This technique can be a strong method to understand the behavior of dross particles in the molten zinc baths. We are going to discuss about the relation between the published thermochemical stability data and observed crystallization or growth behavior. Acknowledgment: These experimental results were obtained from the use of Spring-8 industrial application No.2018A1551, 2018B1587 and 2019A1627.