Segregation Mechanism of Al-based Oxides on Surface of Zn-0.2mass%Al Hot-dip Galvanized Steel Sheets

It is known that the Al added to the Zn coating layer of hot-dip galvanized steel sheets (HDG) segregates on the surface of temper-rolled HDG as Al-based oxides with increasing aging time in air at room temperature. In this study, the surfaces of Zn-0.2mass%Al HDG with and without temper rolling were investigated to clarify the segregation mechanism. Specimens with a Zn coating weight of 55–57 g/m2 including 0.19–0.20 mass% of Al were used. The specimens were aged in air at 20°C or held in liquid nitrogen, and the surface and cross sections of the specimens were then observed and analyzed by XRF, SEM-EDX and EBSD. As a result, it was found that the velocity of Al-based oxide segregation on the surface of the temper-rolled HDG was much higher than that of the HDG without temper rolling. This was attributed to the difference in the area where formation of Al-based oxides was possible. It was also found that the Zn crystal grains in the coating layer were refined by recrystallization due to contact with the temper roll, resulting in an increased number of grain boundaries that can serve as Al diffusion paths. Some unrecrystallized grains also remained after temper rolling and could increase the number of formation sites for Al-based oxides, as they contain numerous dislocations that can serve as Al diffusion paths. These two different formation sites could lead to difference in the segregation rates observed in this study.