The present paper outlines twin roll casting followed by hot rolling to produce magnesium alloy sheet as an alternative to classical production via DC casting and hot rolling or continuous casting. Results and evaluations from trials with semi-scale twin roll cast and hot rolled AZ31B are presented. Strip of 4.5mm thickness and up to 650mm width has been cast at casting speeds between 1.0 m/min and 2.0 m/min. After the twin roll casting, the sheets were hot rolled down to 1.5mm final gauge on a semiscale hot mill. Rolling temperature was chosen between 350 C and 400 C deg. In the as-cast TRC condition a typical grain size of 200-400 micrometer can be achieved without any added grain refinement; additionally, a relatively small cell (sub-grain) size of 6-7 micrometer was measured. This grain structure is in contrast to samples from DC cast slabs showing a grain size of 1-2 mm when no grain refinement is used. After preheating prior to hot rolling, a significant grain refinement was noted, indicating a severe plastic deformation within the roll/casting gap and subsequent recrystallization. After hot rolling with several passes and reheating steps, samples were again taken for metallographic investigation. Gauges of 3mm, 2.5mm and 1.5mm were successively examined. After rolling to the intermediate gauges of 3.0mm and 2.5mm, the deformation energy was insufficient for complete recrystallization during subsequent annealing. Thus, it can be concluded that a relative reduction of >60% is necessary to achieve a homogeneous fine-grained microstructure. At 1.5mm a mean grain size of about 10 micrometer can be achieved, which is a distinctive feature in comparison with sheet produced via the classical route of DC casting and hot rolling, where grain sizes of 20-50 micrometer are common. After standard tensile testing of specimens, the yield strength reaches values in the final gauge (1.5mm) of 215MPa and 235MPa in the longitudinal and transverse directions. For the surface roughness (Ra), values of about 1 micron for the as-cast condition and typically 0.3 and 0.2 micron for the hot rolled condition in the transverse and longitudinal directions, respectively, were determined.
An efficient route to magnesium alloy sheet: Twin roll casting and hot rolling
2005
11 Seiten, 7 Bilder, 6 Quellen
Conference paper
English
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