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

Hot-dip galvanized steel sheets (HDG) are one of the popular automotive materials because it is easier to satisfy both high corrosion resistance and low manufacturing cost due to the ease of controlling the thickness of the Zn coating layer than other Zn based coating. Generally, less than 1 mass% of Al is added to molten Zn bath in the HDG manufacturing process in order to decrease dross and to prevent Zn from alloying with the substrate steel. Although the amount of added Al is small, it has been reported that this Al segregates on HDG surface as oxides with aging time increasing. Hoshino et al. have investigated the effect of these Al-based oxides on the friction coefficient and reported that the friction coefficient of HDG decreased due to the existence of the Al-based oxides on the surface. However the segregation mechanism of Al-based oxides on HDG surface has not been clarified yet. In order to understand the segregation mechanism of Al-based oxides on the surface of HDG, HDG with and without temper rolling aged in air at 20ºC and in liquid-nitrogen were investigated, in this study. Conventional HDG with and without temper rolling, which have Zn coating weight of 55-57 g/m2 including 0.19-0.20 mass% of Al, were used as test specimens. The surface and cross-sections of HDG after aging were observed and analyzed with XRF, SEM-EDX and EBSD. As results, it was found that the velocity of the Al-based oxides segregation on the surface of HDG with temper rolling was much larger than that of HDG without temper rolling owing to the difference of the area where the Al-based oxides can form. It was also found that the Zn crystals in Zn coating layer was refined by the contact with the temper roll due to recrystallization, and some grains remained including strain inside. This could promote the velocity of the Al-based oxides segregation because Al diffusion path to segregate on the surface as oxides such as the grain boundaries and dislocations in the grains were increased. From the cross-sectional observation and analysis, it was suggested that the Al oversaturated in the Zn coating layer due to low solubility of Al in Zn. This could cause the fast diffusion of Al in Zn even at room temperature.