Solidification structure of coating layer in hot-dip Zn-11%Al-3%Mg-0.2%Si coated steel sheet

The solidification structure of the coating layer in hot-dip Zn-11%Al-3/%Mg-0.2/%Si coated steel sheet was studied via metallographic examinations together with the calculation of a phase diagram based on Thermo-Calc. The solidification structure observed, which exhibited a combination of the Zn/Al/MgZn2 ternary eutectic structure, the primary Al phase and the MgZn2 phase, turned out to be different from the predicted one under an equilibrium state in the sense that MgZn2 instead of Mg2Zn11 was observed under the present condition. Excluding the Mg2Zn11 phase from the equilibrium phase diagram, the metastable phase diagram was calculated and excellent agreement obtained between the calculation and the experiment in terms of the solidification structure of the coating layer. Consequently MgZn2 is considered to form easily as the metastable structure known as the Laves phase, because the high cooling rate associated with the present experiment does not provide any potential for peritectic-eutectic reactions, which usually occur in the equilibrium state. Furthermore, MgZn2, which has a C14 type Laves structure and a high rate of nucleation in the liquid phase, is considered to cause the preferential Zn/Al/MgZn2 ternary eutectic reactions.