Austenite Stability in Multi-Phase Ultrafine-Grained 6 pct Mn Transformation-Induced Plasticity Steel

摘要

The austenite stability in multi-phase ultra-fine grained 6 pct Mn transformation-induced plasticity steel was investigated by means of TEM-EDS and dilatometry. The size of the ultrafine austenite islands and the partitioning of Mn to austenite during intercritical annealing were the two main contributions to the austenite stability. The partitioning of Mn to ultra-fine austenite grain took place during the intercritical annealing of the cold-rolled martensitic microstructure in which no prior partitioning of alloying elements had occurred. The calculated volume contraction related to the austenite formation and the associated alloying element partitioning were used to verify the Mn partitioning during the intercritical annealing. Austenite grains over one micron in size readily transformed to martensite during cooling to room temperature. The amount of partitioned Mn in austenite reached its equilibrium value at the intercritical temperature. Mechanical stabilization of the austenite was not a factor contributing to the austenite stability due to the very low dislocation density of the austenite grains.