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

Abstract

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/m 2 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.

Diffusion Behavior of Al in Zn Coating Layer of Zn-0.2mass%Al Hot-dip Galvanized Steel Sheets with and without Temper Rolling during Aging after Production

“…[16][17][18][19][20] As a result, the Zn coating layer contains a small amount of Al, and it is thought that the Al in the Zn coating layer diffuses from the coating layer to the sheet surface via the grain boundaries of the Zn as aging time increases, and then forms Al-based oxides when exposed to air. 21) Since the solubility of Al in solid Zn is known to be quite small, 22,23) the Al in the Zn coating layer is supersaturated. This is thought to be one reason why Al diffuses rapidly at room temperature.…”

“…This is thought to be one reason why Al diffuses rapidly at room temperature. 21) HDG is usually temper rolled (skinpass rolled) after hot-dip galvanizing to reduce yield elongation. The effect of temper rolling on the formation behavior of Al-based oxides has also been investigated, and it has been reported that temper rolling accelerates the formation of Al-based oxides per unit area because the density of diffusion paths of Al in the Zn coating layer is increased by temper rolling.…”

“…The effect of temper rolling on the formation behavior of Al-based oxides has also been investigated, and it has been reported that temper rolling accelerates the formation of Al-based oxides per unit area because the density of diffusion paths of Al in the Zn coating layer is increased by temper rolling. 21) Contact with the temper roll causes dislocations in the Zn coating layer, and then induces recrystallization of the Zn even at room temperature due to the low beginning temperature of recrystallization of Zn alloys. 21) As a result, the density of the grain boundaries is increased by contact with the temper roll, although some grains remain unrecrystallized and include diffusion paths such as dislocations.…”

“…21) Contact with the temper roll causes dislocations in the Zn coating layer, and then induces recrystallization of the Zn even at room temperature due to the low beginning temperature of recrystallization of Zn alloys. 21) As a result, the density of the grain boundaries is increased by contact with the temper roll, although some grains remain unrecrystallized and include diffusion paths such as dislocations. 21) Consequently, it is thought that the rate of Al-based oxide formation is accelerated due to an increase in the formation site of Al-based oxides on the surface.…”

“…21) As a result, the density of the grain boundaries is increased by contact with the temper roll, although some grains remain unrecrystallized and include diffusion paths such as dislocations. 21) Consequently, it is thought that the rate of Al-based oxide formation is accelerated due to an increase in the formation site of Al-based oxides on the surface. 21) As mentioned above, control of Al-based oxides is very important for obtaining stable HDG quality, particularly frictional properties.…”