Effect of the heat treatment on the microstructure and mechanical properties of medium-Mn-steels

The heat treatment parameters, whether in the form of an ART-(austenite-reversed-transformation)-cycle or a simpler direct batch annealing cycle, have a substantial effect on the microstructure and mechanical properties of Medium-Mn-Steels. After the austenitizing step of the ART-process the microstructure is transformed into lath martensite, whereby the transformation behavior of the steel determines the process window for the quenching step and the potential process route. During intercritical annealing the microstructure is partially austenitized and stabilized by C and Mn partitioning from the bcc to the fcc phase, leading to a final microstructure of tempered martensite with large amounts of finely distributed metastable retained austenite. Depending on the intercritical annealing temperature, the stability of the retained austenite can be altered to a large extend, influencing the TRIP-(transformation-induced-plasticity)-behavior of the steel significantly. As a result of the deformation behavior, the mechanical properties can be adjusted from high strength with very high ductility to very high strength with reasonable ductility. The presentation will show the dependency of the microstructural characteristics and the material behavior on the heat treatment parameters for alloy compositions with different manganese contents.