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.

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The effect of the Al-based oxide layer which segregates on the surface of hot-dip galvanized steel sheets (GI) with aging on the frictional properties of the GI was investigated.Conventional GI with the Zn coating weight of 67 g/m 2 including 0.36 mass% Al were used as test specimens. It was found that an Al-based oxide layer grew on the GI surface with aging after production, and the friction coefficient tended to decrease due to the existence of these Al-based oxides. However, this tendency was clearer under the sliding conditions of shorter tool length and higher contact pressure than of longer tool length and lower contact pressure.In order to understand this behavior, surface observation and analysis of both the test specimens and the tools after sliding were carried out by SEM, EDX and EPMA. Both Al-based oxides and metallic Zn were detected as adhered materials on the tool surface after sliding, and the surfaces of adhered materials were covered with Al-based oxides. This suggests that the adhered materials on the tool have the effects of reducing the adhesion force between the metallic Zn of the GI and the tool and reducing the tool roughness. These effects led to a lower friction coefficient because both shearing and plowing resistance were decreased. In addition, the area on the tool which were covered by the adhered materials depended on the tool length. This is thought to be the reason why the effect of the Al-based oxide layer depended on the sliding conditions.

The effect of the Al oxide layer which segregated on the surface of hot dip galvanized steel sheet (GI) with aging on the frictional properties of the GI was investigated. Conventional GI with the Zn coating weight of 67 g/m 2 including 0.36 mass% Al were used as test specimens. It was found that an Al oxide layer grew on the GI surface with aging after production, and the friction coefficient tended to decrease due to the existence of this Al oxide. However, the tendency was clearer under the sliding conditions with shorter tool length and higher contact pressure than with longer tool length and lower contact pressure. In order to clarify the mechanism, surface observation and analysis of both the test specimens and the tools after sliding were carried out by SEM, EDX and EPMA. Both Al oxide and metallic Zn were detected as adhesions on the tool surface after sliding, and the surface of the adhesions was covered with Al oxide. It is thought that the adhesions on the tool had the effects of reducing the stickiness between metallic Zn of GI and the tool, and reducing the tool roughness. These effects led to a lower friction coefficient because both shearing and plowing resistance were decreased. In addition, the area on the tool which was covered by the adhesions depended on the tool length. This is thought to be the reason why the effect of the Al oxide layer on the friction coefficient depended on the sliding conditions.

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