EFFECTS OF COMPOSITION, TEMPERATURE AND STRAIN RATE ON THE MECHANICAL BEHAVIOR OF HIGH-ALLOYED MANGANESE STEELS

Due to the high manganese content and additions of aluminum, silicon, and possibly other alloying elements, TWIP steels possess a stacking fault energy that favors twinning as the predominant deformation mechanism together with ordinary dislocation gliding [1]. Among all common deformation and/or transformation processes, twinning has the most beneficial effect on the strain hardening rate of austenitic steels. TWIP steels start to yield at relatively low stress levels but exhibit moderate-to-high tensile strengths combined with extremely high total elongations, making the steels very suitable for forming applications while still leaving good strength and energy absorption potential to the manufactured parts. Because of these properties, TWIP steels have become very attractive especially for the automotive industry, where increasing safety requirements and the need for weight reduction of the car body are motivating the search for new structural materials. At the same time, good formability allows considerable cost reductions during the manufacturing process. Besides the chemical composition, also temperature affects the stacking fault energy, and therefore a change in the governing deformation process occurs with the changing deformation temperature of TWIP steels. At very low temperatures, a change from the twinning transformation process to partial martensitic phase transformation can be observed, while at high temperatures, dislocation glide becomes the dominant (and sole) deformation mechanism. At the same time, strain hardening and deformation mechanisms in TWIP steels usually depend also on strain rate, which is an important aspect for the automotive safety. In order to better understand the dependence of the mechanical behavior of TWIP steels on composition, temperature, and strain rate, three high-alloyed manganese steel grades were mechanically tested at various strain rates and temperatures. In this paper, we present the results of these measurements together with a discussion of the deformation mechanisms of TWIP steels in various conditions