Modeling retained austenite in Q&P steels accounting for the bainitic transformation and correction of its mismatch on optimal conditions

Abstract

Modeling retained austenite in quenching and partitioning (Q&P) steels remains a challenge, and the conventional 'constrained carbon equilibrium' (CCE) model fails to predict the optimal condition for achieving the maximal amount of retained austenite in various systems, which impedes the optimization of the Q&P process. One of the main limitations is that the possible decomposition of austenite to bainite during partitioning is completely ignored by the essential assumptions of the Q&P process and hence the associated CCE model. In this study, a CCET model that combines the conventional CCE model with the T0 model for the bainitic transformation and incorporates the effect of the isothermal bainitic transformation to describe the austenite stability during the Q&P process has been proposed. A detailed comparison between the experimental observation and model predictions demonstrated that the retained austenite could be better described by the CCET model, including its carbon content. The current model therefore provides a more accurate approach for tailoring the amount and stability of retained austenite after the Q&P processing of Fe-C-Mn-Si steels.

Fundamentals and application of solid-state phase transformations for advanced high strength steels containing metastable retained austenite

“…In addition to experimental investigations, bainite formation during the Q&P process was also analysed based on diffusionless [168,261,264,265] or diffusional theories [266]. HajyAkbary et al [168] studied the influence of carbon partitioning from martensite into austenite on bainite formation in an Fe-0.3C-3.5 Mn-1.6Si steel.…”

Effect of Step Quenching Heat Treatments on the Kinetics of Ferrite Formation and Quenching & Partitioning Modeling for a Commercial C-Mn-Si Steel

“…Aiming to simulate and to calculate the optimized parameters for SQ-Q&P (step quenching combined with quenching and partitioning heat treatments) for the studied steel, thermodynamic simulations were performed using: the MatCalc software, provided with the mc_fe 2.060 database; the CCE model, proposed by Speer et al 17 ; and CCET model, proposed by Li et al 25 . For the CCET model to be used, considering the chemical composition of studied alloy, the T 0 curve 32 was raised in the MatCalc software.…”

“…Again, by the K-M equation, the final retained austenite fraction was determined after final quenching. In addition to this, the CCET model 25 considers the decomposition of the untransformed austenite into bainite (without carbide precipitation) according to the T 0 curve 32 , during the partitioning stage. Hence, it was possible to predict the martensite, bainite and retained austenite fractions in the final microstructure.…”

“…From the CCE model, Li et al 25 proposed an alternative one for low carbon steels with Si and Mn additions, but now considering the possible occurrence of isothermal bainite formation during the partitioning stage. The model, named CCET, considers that the carbon partition from martensite to austenite is completed in the first seconds of the partitioning step, with the CCE model being applicable until that moment.…”

“…Moreover, the kinetics of ferritic grain growth as a time function was modeled by the Zener's law (1949) 24 . Using the MatCalc software and thermodynamic models 17,25 applied to the quenching and partitioning concept, Q&P cycles considering previous SQ heat treatments (SQ-Q&P) were thermodynamically simulated and the final phase fractions were predicted for optimized heat treatment conditions.…”