Arihiko Morita

The spot weldability of vapor zinc deposited steel sheets.

The spot weldability of zinc vapor-deposited steel sheets (VZ) and galvannealed steel sheets (VZA) was investigated. Weldability lobes of VZA are approximately equal to those of hot-dip galvannealed sheets, and broader than those of VZ or hot-dip galvanized sheets. Electrode life depends on both welding conditions and surface conditions, but is independent of both the coating process, (i.e., zinc vapor deposition, hot-dip galvanizing, electro-galvanizing) and the alloying process. Electrode life increases with increasing welding current and electrode tip diameter, but decreases with increasing welding time and coating weight. Electrode life in the welding of VZA is 1.4-1.6 times longer than in the case of VZ. Γ phase (Fe3Zn7), γ phase (Cu5Zn8) and β phase (CuZn) are observed at the electrode faces for welding both VZ and VZA, γ phase is so brittle that its growth rate affects electrode wear. A high rate of growth of γ phase and a narrow weld current range both contribute to short electrode life when welding VZ and heavy coated VZA.

Effect on drawability of the Fe content of the deposited layer in zinc vapor deposited and galvannealed steel sheet.

The drawability of zinc vapor deposited and galvannealed steel sheet, and the powdering behavior of the deposited layer during drawing were investigated. Heat treatment to form an alloy layer may be conducted either at higher temperatures (450-550°C) for short periods or at lower temperatures (280-340°C) for longer periods. The iron content of the deposited layer varied with heat treatment conditions from 9 to 15wt. %. When the ζ phase remains in the surface layer of the deposit, the degree of powdering decreases and drawability declines, but when the phase disappears, the layer peels easily and drawability improves. This may result from a decrease in the friction coefficient during drawing due to peeled powders. The ζ phase tends to remain in the deposited layer, when treatment is at lower temperatures, and little powdering occurs in these cases even though the iron content of the deposited layer is much higher than in the other cases.