Dual-phase (DP) advanced high-strength steels (AHSSs) are widely used in the automotive industry due to their excellent combination of strength, ductility, and work hardening properties. However, defects occurring during processing make these ferrous alloys expensive. Toward this ends, high-temperature tensile tests using a Gleeble thermomechanical simulator have been conducted to determine the stress/strain behaviour at temperatures between 1250 to 1480 C in order to quantify the tensile strength and ductility. The results of both as-cast and transfer-bar material will be presented as well as three different sample geometries in order to better understand the effects of starting microstructure, thermal gradient, and tress/strain distribution on the reproducibility of high temperature properties. Optical and scanning electron microscopy are then performed to further elucidate the structure/property relationships. The results show that the presence of preexisted prorosities in the as-cast structure decreases the high-temperature strength of the material, while the transfer-bar samples show lower ductility at ultra-high temperatures, (T 1450 C), due to their severe susceptibility to melting. In terms of the two mentioned thermomechanical characteristics, voids nucleation, growth, and coalescence initiated with porosity clustering are the main mechanisms behind the lower strength of the as-cast samples, whilst tearing apart of the melt plays an important role to drastically drop the ductility of transfer-bars at mentioned temperature interval. Moreover, the long-gauge-length (LGL) geometry proposes better reproducibility of data compared with the other geometries. This is attributed to a suitable combination between low stress localization and high thermal gradient during the Gleeble testing that provides a condition in which the samples experience sharp localized necking right on the hot-spot zone. The obtained data can be used as part of multi-physics process and microstructure continuous casting models. / Thesis / Master of Applied Science (MASc)

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