Solidification of AZ31 magnesium alloy plate in a horizontal continuous casting process

Magnesium plate of 120 mm width and 30 mm thickness was produced by horizontal continuous casting, and its microstructures were analyzed on the surface and in the plate interior. The defect formation on the surface and subsurface, such as side cracks, buckling, pores and segregation, was very sensitive to the casting condition and processing parameters. Especially, a secondary cooling was effective in refining the microstructure and controlling the grain size distribution across the plate thickness, which means that the cooling rate and curvature of the solid/liquid interface in the mold changed. According to AES analysis on the surface, the thickness of the oxide layer was not more than 0.3 μm. However, the cyclic marks on the surface were obvious, because the plate were drawn by four-step cycle. In the case of several defects, they caused side cracks during rolling and survived even after rolling. Thus, it was concluded that pretreatment of the as-cast plate, such as homogenization and surface machining, is necessary to produce sound thin sheet by hot rolling.