The influence of silicon additions on the deformation behavior of austenite-ferrite duplex medium manganese steels

The present study systematically investigated the effect of Si additions from 0 to 3 wt.% on the deformation mechanisms of a 0.2C-10Mn-3Al medium Mn steel. Two austenite-ferrite duplex microstructures, characterized by the different austenite characteristics (i.e. fraction and stability), were produced for different Si-alloyed steels by intercritical annealing and subsequently subjected to tensile testing. The influence of Si content on the activation and kinetics of transformation-induced plasticity (TRIP) effect, the formation of deformation twins, as well as the strain partitioning between phase constituents during deformation was studied in detail. For the low austenite fraction (∼30%) duplex structure with a high austenite stability (low Ms temperature), the tensile strength was only slightly changed with Si levels, whereas the uniform elongation was significantly influenced, with first an increase followed by a decreasing trend with higher Si contents. This was attributed to changes in strain partitioning between austenite and ferrite and to different extent of deformation twinning in austenite, which were highly dependent on the solute Si contents and the related microstructural changes. It was found that strain partitioning in this type of microstructure can result in a substantially high strain hardening rate and high uniform elongation. Conversely, for the samples with a high austenite fraction (∼45%) and low stability, the tensile properties were insensitive to the Si content, being mainly controlled by the TRIP effect. In this structure, a higher fraction of martensite was formed at the beginning of the plastic deformation, which decreased the strain partitioning between austenite and ferrite. The TRIP effect after the Lüders strain is believed to be the main factor influencing the strain hardening rate of this type of structure.