Optimizing Microstructure and Property by Ausforming in a Medium-carbon Bainitic Steel

The transformation behavior and microstructure in a medium-carbon bainitic steel were investigated by combination of metallography and dilatometry. The fine micro-structural units of carbide-free bainite in non-ausformed and ausformed materials were measured by a transmission electron microscope. Mechanical stabilization of austenite in deformed material and its effect on property were analyzed by nanoindentation and tensile tests. Ausforming with a strain of 0.2 at 573 K can not only accelerate bainite transformation, but also improve the comprehensive properties. The strength and ductility of nanostructured bainitic steel can be simultaneously enhanced by ausforming, which should be attributed to the refinement of bainite and the enhanced volume fraction of retained austenite. Compared to the non-deformed material, the mechanical stabilization of austenite can be optimized by ausforming, resulting in good transformation-induced plasticity effects. Also a very important advantage was that, the bainite transformation time could be minimized into practical scale by prior ausforming compared to traditional low-temperature austempering.