Mechanism of Dynamic Strain-Induced Ferrite Transformation in a 3Mn-0.2C Medium Mn Steel

Medium Mn steels (MMSs) have Mn contents of 3%-12% (mass fraction), and have been energetically investigated as the most promising candidates of the third-generation advanced high-strength steel. Their phase transformations and microstructures during various heat treatments and thermomechanical processes have received wide attention with the purpose to achieve an optimal balance of cost-efficient alloying compositions and mechanical properties. The aim of this work is to investigate the microstructural behavior of deformation-induced ferrite transformation (DIFT), starting from austenite, which occurs in MMS. Then, improved understandings of the formation of ultrafine ferrite via the DIFT and conservation of this microstructure during the post-deformation period can be obtained. For this purpose, a 3Mn-0.2C MMS with lower contents of alloying elements was selected. Microstructures and alloying element distributions of the thermomechanically processed samples were analyzed via EBSD and EPMA. The results showed that the DIFT occurred in the thermomechanically processed 3Mn-0.2C MMS in the alpha + gamma region. Characteristic multiphase microstructures consisting isolated martensite and fine-grained equiaxed ferrite concomitant with fine islands of retained austenite dispersed between ferrite grains can be obtained. During the DIFT, the enhanced nucleation of ferrite at alpha/gamma interfaces can not only increase the ferrite nucleation density but also facilitate extensive impingement among the neighboring grains. Formation of ultrafine ferrite via the DIFT in MMS can be interpreted in terms of unsaturated nucleation and limited growth. In addition, partitioning of Mn between the ultrafine ferrite and austenite is accelerated during the DIFT such that a large number of Mn-enriched fine islands of austenite are left untransformed at the alpha/alpha grain boundaries or at triple junctions. These islands of austenite are considered to play critical roles not only for obtaining retained austenite at room temperature but also for conserving the ultrafine microstructure of the DIFT during the post-deformation processing.

Automated summary

This study aims to investigate the microstructural behavior of deformation-induced ferrite transformation (DIFT) in Medium Mn steels (MMSs) in order to understand the formation and post-deformation conservation of ultrafine ferrite. The results showed that DIFT occurs in the alpha+gamma region of thermomechanically processed MMSs, resulting in characteristic multiphase microstructures that consist of isolated martensite and fine-grained equiaxed ferrite with islands of retained austenite dispersed between ferrite grains. Enhanced ferrite nucleation at alpha/gamma interfaces during DIFT increases nucleation density and facilitates impingement among neighboring grains. The partitioning of Mn between ultrafine ferrite and austenite is accelerated during DIFT, leaving numerous Mn-enriched islands of austenite untransformed at grain boundaries or triple junctions, which play critical roles in retaining austenite at room temperature and conserving the ultrafine microstructure of DIFT during post-deformation processing.