A new combinatorial processing route to achieve an ultrafine-grained, multiphase microstructure in a medium Mn steel

A new combination of factors enhancing the stabilization of austenite, including preexisted austenite among quenched martensite, prior deformation, and partitioning at high temperatures is employed to create a multi-component refined microstructure in a medium Mn steel (Fe-4Mn-0.31C-2Ni-0.5Al-0.2Mo, wt.%). The microstructure evolution and phase fraction during the processing are systematically investigated using various characterization methods. The microstructure of the specimen after 0.4 strain deformation of 73% martensite-27% austenite at 250 degrees C and subsequent partition-annealing at 600 degrees C for 20 min was composed of several phases including tempered martensite, fresh martensite, pearlite, 10% of retained austenite (RA) and undissolved cementite. By increasing the annealing temperature, the pearlitic transformation was suppressed, whereas recrystallization of the deformed martensite and carbide dissolution occurred following annealing at 650 degrees C for 20 min resulting in an ultrafine-grained microstructure composed of equiaxed ferrite, 32% RA along with some fresh martensite during final cooling and few carbide precipitates. The results demonstrate that the combinatorial approach accelerated partitioning of alloying elements from martensite and carbides to largely pre-existing austenite is responsible for the improved austenite stabilization during intercritical annealing of the deformed dual-phase specimens. However, competitive processes are also enhanced so that the RA content is not increased by deformation. (C) 2021 The Author(s). Published by Elsevier B.V.

Automated summary

A new combination of factors is employed to enhance the stabilization of austenite in a medium Mn steel, resulting in a refined multi-component microstructure. The microstructure evolution and phase fraction during processing are systematically investigated, revealing a combination of phases including martensite, pearlite, retained austenite, and carbide precipitates. By optimizing annealing parameters, an ultrafine-grained microstructure is achieved with equiaxed ferrite, retained austenite, and some fresh martensite.