5th International High Manganese Steel Conference - HMNS 2022

Application of the double soaking treatment to medium-Manganese (Mn) steels has been shown to result in impressive combinations of tensile strength and uniform elongation. The two-step double soaking heat treatment is intended to produce an austenite-martensite microstructure by intercritical annealing followed by secondary soaking. The intercritical annealing step serves to partition Mn from ferrite to newly formed austenite, while the secondary soaking step is intended to transform the remaining Mn-lean ferrite into austenite (and subsequently into martensite upon quenching). If Mn is not appreciably redistributed during the secondary soaking step, the Mn-rich austenite (primary austenite) may be retained, while the Mn-lean austenite (secondary austenite) will transform to martensite upon quenching.

The present work discusses dilatometry results from a Fe-7Mn steel with an ultra-low residual carbon (C) concentration. These results indicate that a heterogeneous Mn distribution may be maintained in a fully austenitic microstructure during secondary soaking, and provide the basis for calculations of the Mn concentrations associated with the primary and secondary austenite. MICRESS® simulations were employed to help understand the evolution of microstructure during the multi-step heat treatment. These simulations corroborate the dilatometry results and provide further insight on the effects of the secondary soaking temperature on the ferrite-to-austenite transformation and Mn redistribution during secondary soaking. Finally, energy-dispersive X-ray spectroscopy results collected from a Fe-0.2C-4.5Mn steel using scanning transmission electron microscopy are presented, and demonstrate that a heterogeneous Mn distribution can also be maintained in a similar carbon-bearing steel after double soaking.