2021 Volume 61 Issue 8 Pages 2264-2273
It is known that Al added to the Zn coating layer of hot-dip galvanized steel sheets (HDG) diffuses to the surface at room temperature and forms Al-based oxides in air. In order to understand the diffusion behavior of Al in Zn coating layer, this study investigated the effect of the aging temperature on the segregation behavior of Al-based oxides in HDG with and without temper rolling (skinpass rolling) using a material with a Zn coating weight of about 56 g/m2 with an Al content of approximately 0.20 mass%. The specimens were aged at −15, 5, 20, 38, 100 or 200°C in air after production, and the surface and cross sections were observed and analyzed by XRF, SEM-EDX, EBSD and TEM. As a result, up to the aging temperature of 38°C, the amount of Al-based oxides increased linearly to the square root of aging time, suggesting that the formation rate is determined by the diffusion of Al in Zn coating layer in this temperature range. However, this linear relationship did not hold at aging temperatures above 100°C. In addition, in the temper-rolled HDG, the formation rate of Al-based oxides is larger than that without temper rolling up to the aging temperature of 38°C, and then decreased drastically at aging temperatures above 100°C. The segregation behavior of Al-based oxides is discussed in view of the diffusion behavior of Al and the changes in the macrostructure of the Zn coating layer during the aging after production.