Characterization of Nb-Si-doped low-carbon steel treated by quenching and partitioning: Thermic treatment in two stages supported by computational thermodynamical simulation and controlled sample dimensions

Quench and partition steels find wide use in the automotive industry because of their high capability of energy absorption. Industrial demands have prompted the expansion of this research field because of the influence these materials have on components that can absorb high energy of impact to reduce passenger damage, for example. The partition process's difficulties lie mainly in controlling the thermodynamics and the kinetics of the phase transformation. Both affect achieving adequate austenite retention and optimal mechanical properties. Many researchers have attempted to increase these materials' energy absorption efficiency by incorporating micro -alloying elements that control phase transformation during the partitioning process, typically done in three steps. However, no research has been carried out on this topic using Nb and Si microalloying on low-carbon steels in two stages. Therefore, an alloy was designed and modelled with mechanical reinforcement by precipitation and transformation-induced plasticity (TRIP), doping the steel with Nb and Si in a two-stage quenching and parti-tioning process. Then, steel samples were fabricated to validate the model. There were two groups of samples with different dimensions to evaluate the sensitivity of austenite retention concerning the sample thickness. The main results showed that 10.75% of retained austenite allows an energy absorption of 30.55 GPa% with a two -stage quenching and partitioning heat treatment. Sample thickness influences austenite retention due to diffusion kinetics during the partitioning process. Finally, virtual tests quantified the unit strain energy absorption of the retained austenite at 1.9 mJ at 25 degrees C.