Phase transformation behavior of microalloyed medium-manganese steels

Abstract

Medium-manganese (medium-Mn) steels are promising candidates for advanced high-strength steel (AHSS) applications due to their ability to achieve an exceptional combination of strength and ductility through transformation-induced plasticity (TRIP) and twinning-induced plasticity (TWIP) mechanisms. However, the fundamental understanding of how microalloying influences phase transformation behavior, microstructural evolution, and mechanical response remains limited, particularly regarding solute partitioning, precipitation, and grain boundary phenomena. Therefore, this work systematically investigates the role of microalloying in medium-Mn steels, with particular emphasis on vanadium (V) and molybdenum (Mo) additions.

A series of steels were produced through laboratory-scale casting with controlled compositional variations (0-2 wt.% V and selected Mo additions). The experimental program combined scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), energy-dispersive spectroscopy (EDS), X-ray diffraction (XRD), mechanical testing, and thermodynamic calculations to evaluate phase constitution, crystallographic characteristics, mechanical response, phase stability, and solute partitioning behavior.

Following casting, vanadium microalloying significantly influenced microsegregation, prior austenite grain structure, and phase constitution. Increasing vanadium content promoted grain refinement and altered retained austenite distribution through solute partitioning and carbide formation. During intercritical annealing, austenite reversion was governed by the interplay among thermodynamic driving forces, diffusion kinetics, and precipitation behavior. Austenite nucleation occurred preferentially at high-angle grain boundaries due to enhanced solute segregation, while growth was controlled by grain boundary, dislocation (pipe), and bulk manganese diffusion. Vanadium carbide formation reduced carbon availability, increasing the nucleation barrier and decreasing density of active nucleation sites.

Significant δ-ferrite formation was observed at high vanadium additions. The retention of δ-ferrite was found to be influenced not only by vanadium content but also by solidification conditions and segregation behavior. Furthermore, δ-ferrite constrained martensitic transformation, resulting in variant selection.

Building upon these findings, molybdenum microalloying was explored as a preliminary investigation for future work. Molybdenum delayed martensitic transformation, refined prior austenite grains, reduced microstructural banding, and promoted a more uniform retained austenite distribution. In summary, this work provides fundamental insights into the effects of vanadium and molybdenum microalloying on phase transformation behavior in medium-Mn steels through solute partitioning, diffusion, and precipitation phenomena.