Revealing the Conditions of Bainitic Transformation in Quenching and Partitioning Steels

Isothermal decomposition of austenite during the partitioning process of quenching and partitioning (Q&P) steels is normally ignored but may play an important role in determining the condition of retained austenite in the final microstructure and hence the mechanical properties of the steel. To provide a theoretical basis and practical guideline for controlling the isothermal bainitic transformation and optimizing the competition with other transformation mechanisms, the isothermal bainitic transformation was systematically investigated and the associated time-temperature-transformation diagrams were constructed for 0.2C-1.60/2.82/3.92Mn-1.6Si steels during one-step and two-step quenching and partitioning processes. The influences of quenching temperatures, partitioning temperatures, and Mn concentrations on the bainitic transformation window were quantitatively analyzed by dilatometry and electron microscopy. The similarities and differences between one-step and two-step quenching and partitioning processes were discussed. The results reveal the conditions of isothermal bainitic transformation when subjected to different Q&P heat treatment scenarios and justify the relative importance of these conditions in determining the final microstructure with respect to the industrial practices of hot rolled and continuous annealing processes. The bainitic transformation process window obtained by this work could provide direct guidelines in optimizing the compositions and heat treatment parameters of Q&P steels.