PSI - Issue 69

R. Surki Aliabad et al. / Procedia Structural Integrity 69 (2025) 69–75

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1. Introduction Medium manganese steels (MMnS), containing 4-12 wt% manganese, have emerged as promising candidates for the third generation of advanced high-strength steels (AHSS). These alloys offer an excellent combination of high strength, large ductility, and cost-effectiveness, making them suitable for diverse industrial applications [1]. Among these, MMnS with 3-10 wt% Mn and 0.05-0.3 wt% C tend to exhibit almost fully martensitic microstructures with a small fraction of retained austenite (RA) after hot rolling, owing to their remarkable hardenability [2]. By carefully controlling thermomechanical processing, these steels can be widely tailored to develop a multi-phase microstructure, ensuring a desired RA distribution and stability. This flexibility facilitates the customization of mechanical properties across a broad spectrum, as highlighted by the recent review article [3]. However, tailoring MMnS microstructures to achieve desired mechanical properties poses significant challenges. A comprehensive understanding of the thermodynamic and kinetic factors influencing phase transformations, particularly during heating stage and intercritical annealing treatment (IAT), is critical. For example, manganese segregation to lattice defects and carbide precipitation significantly affect the thermodynamic driving force for austenite nucleation [4]. The competition between carbide formation and martensite-to-austenite reversion adds further complexity to the microstructural evolution [5]. However, this competition is not always observed. For instance, after hot rolling, MMnS may undergo auto-tempering during slow cooling resulted in (partially) tempered martensitic microstructure [2]. Similarly, tempering prior to IAT [6] results in a tempered martensitic structure, where carbides, particularly cementite, serve as the primary nucleation sites for new austenite formation during IAT. Recent multi-stage processing of MMnS has leveraged Mn-rich cementite particles to improve mechanical properties by serving as nucleation sites for austenite reversion [7]. The resulting austenite is finer and more Mn enriched than that found in conventionally treated alloys [8]. Moreover, Enomoto and Hayashi found that austenite is more likely to nucleate on cementite at prior austenite grain boundaries and martensite packet boundaries than at inter lath boundaries [9]. Additionally, larger cementite particles would be more effective nucleation sites, as noted by Zhang et al. [10]. An additional complexity in this intricate microstructure is the redistribution of carbon during carbides formation. Zhao et. al [11] observed that tempering at 500°C for 60 minutes applied to an intercritically annealed microstructure led to reduction in RA volume fraction from 18.5% to 12% due to RA decomposition, accompanied by an increase in carbon content from 0.1% to 0.8% by weight in remaining RA. Recent advancements have incorporated in-situ high-energy X-ray diffraction (HEXRD) techniques to gain deeper insights into microstructural changes during heating and IAT of MMnS. For instance, Muller et al. investigated the evolution of microstructure during heating and holding of IAT, starting from a martensitic microstructure with 2 vol.% RA. Their study revealed that new film-like austenite forms rather than the preservation of existing RA [12]. Similarly, Hu et al. [13] observed RA decomposition and cementite precipitation during heating, followed by an increase in austenite fraction during isothermal holding. Mehrabi et al. [14] monitored austenite fraction and lattice parameters throughout the annealing process under varying temperature and time conditions. Despite extensive research on phase transformations during the isothermal holding stage of IAT and the resulting final microstructures [15–18], limited attention has been given to the heating stage prior to reaching the intercritical annealing temperature. This critical stage involves possible complex microstructural changes, including carbon redistribution, carbide formation, and the decomposition or preservation of existing austenite, which significantly influence the initial conditions for subsequent phase transformations. Therefore, this work focuses on examining the microstructural evolution and carbon redistribution occurring during the heating stage prior to IAT. This is crucial for accurately characterizing the initial microstructure at the onset of IAT, including carbide formation, carbon partitioning between RA and the surrounding matrix, and the preservation of existing austenite as nucleation sites. 2. Materials and method The steel material used in this study, with the nominal composition of Fe-0.40C-1Si-6Mn-2Al-0.05Nb (wt.%), was produced in a vacuum induction furnace. After casting, the material was subjected to austenitization at 1200 °C for 2 hours, followed by hot rolling with a 90% thickness reduction to achieve a final thickness of 4 mm. The finish rolling was conducted at 950 °C, and the material was subsequently cooled in ambient air.