This work presents insights into the manganese influence on the driving force and bainite transformation kinetics. Three different medium-Mn steels were subjected to theoretical calculations and dilatometric study in order to determine the Mn impact on bainite formation. The theoretical approach shows that the increase of manganese leads to a lower bainite fraction formed during the isothermal stage. This implicates the carbon enrichment of the austenite during thermal treatment. The less bainite is formed, the higher is the fraction of residual austenite which enrichment of carbon is globally low. Meanwhile, the manganese influences the incubation and transformation time. As the manganese content increases, the incubation period and formation time of bainite are longer because the chemical driving force essential to start and complete austenite into bainite transformation decreases. This was proved by theoretical calculations and dilatometric analysis, which show that even a small increase in manganese content leads to a longer time necessary to occur the bainitic transformation. For the steel containing 5% manganese, the driving force was too small that the transformation could occur even after 3 h. Additionally, the XRD analysis was conducted to determine the retained austenite fraction and its carbon enrichment. These results were compared with the theoretical values to determine the accuracy of the applied model.

The objective of this study was to analyze the thermodynamic feasibility of forming nanobainite in Al-alloyed medium-Mn steels through intercritical annealing (IA) and subsequent heat treatments. The research aimed to determine the influence of IA temperature and Mn content on the stability of austenite, the M_s temperature, and the resulting bainite plate thickness (BPT). Our findings indicate that the IA temperature range of 780–860 °C effectively decreased the M_s temperature, facilitating the formation of nanobainite. The results demonstrated that a higher Mn content increases an austenite fraction during IA, thus enhancing the potential for nanobainite formation. For the 3MnNb steel, the IA temperature of 860°C was sufficient to achieve bainitic plates thinner than 100 nm, whereas the 4MnNb steel required lower IA temperatures due to its higher Mn content. The transformation kinetics was found to be faster in 3MnNb steel, with a complete transformation time of 300 min, compared to approximately 600 min for the 4MnNb steel. Dilatometric analysis confirmed that the real austenite fractions were approximately 20% higher than the ones predicted by thermodynamic simulations, indicating potential limitations of the commercial software in accurate predicting the experimental conditions. The obtained results validate the proposed heat treatment strategy for achieving nanobainitic structures in medium-Mn steels.

The fundamentals of discontinuous precipitation (DP) reaction modelling using a cellular automata (CA) method are presented. In the proposed CA model, cell states, internal variables, equations, and transition rules were defined to predict the manner of mass transport during DP reaction and to relate changes in the microstructure with corresponding changes in chemical composition. Furthermore, the concept of digital material representation (DMR) was introduced into the CA model, which allowed schematic images of the microstructure to be used as starting structures in the modelling of the DP reaction. The preliminary assumptions adopted in the proposed CA model for the DP reaction were verified by numerical simulations of the growth of discontinuous precipitates at a steady-state at the example of Al-22 at.% Zn alloy. The outcomes achieved from the CA simulations were presented in a different form than that most commonly used previously (single concentration profiles), namely as the 2D maps showing changes in Zn content accompanying the successive stages of growth of discontinuous precipitates. The model used for the description of the solute diffusion along of the reaction front (RF) allowed two-dimensional systems at the nano-scale to be treated within a reasonable simulation time. The obtained results indicate that the developed CA model was able to realistically simulate the DP reaction, which was confirmed by the visualisation of migrating RFs together with associated chemical composition changes in the microstructure.