Isothermal heat treatments below the martensite start temperature are carried out in a 0.25C-1.4Mn-1.4Si-0.32Mo steel (in wt%). The microstructural observation reveals a combination of tempered lath and plate martensite, accompanied by bainite and retained austenite. The temperature variation of the austenitic carbon content under different constrained equilibrium models is calculated and compared with experimental measurements. Neither of the existing models seems to predict the behavior of the measured austenitic carbon content in the temperature range studied. The results suggest that the diffusional formation of bainite governs the carbon content of austenite at temperatures nearby the M_S.

This study focuses on the effect of non-conventional annealing strategies on the micro-structure and related mechanical properties of austempered steels. Multistep thermo-cycling (TC) and ultrafast heating (UFH) annealing were carried out and compared with the outcome obtained from a conventionally annealed (CA) 0.3C-2Mn-1.5Si steel. After the annealing path, steel samples were fast cooled and isothermally treated at 400 °C employing the same parameters. It was found that TC and UFH strategies produce an equivalent level of microstructural refinement. Neverthe-less, the obtained microstructure via TC has not led to an improvement in the mechanical properties in comparison with the CA steel. On the other hand, the steel grade produced via a combination of ultrafast heating annealing and austempering exhibits enhanced ductility without decreasing the strength level with respect to TC and CA, giving the best strength–ductility balance among the studied steels. The outstanding mechanical response exhibited by the UFH steel is related to the formation of heterogeneous distribution of ferrite, bainite and retained austenite in proportions 0.09–0.78–0.14. The microstructural formation after UFH is discussed in terms of chemical hetero-geneities in the parent austenite.

The microstructure and mechanical properties of an Fe-0.24C-1.4Mn-1.4Si steel were investigated after combining ultrafast heating (UFH) at a heating rate of 500 °C/s followed by fast cooling to room temperature (DQ) or quenching and partitioning processes (Q&P). Two peak temperatures were studied, annealing into the intercritical range and above the A_C3 temperature. After ultrafast heating and quenching, the resulting microstructures revealed that intercritical annealing led to the formation of a banded ferritic-martensitic microstructure. On the other hand, heating above the intercritical range led to an even distribution of allotriomorphic ferrite grains upon fast cooling and a complex phase microstructure, consisting mainly of martensite, was produced. Q&P steel grades exhibit an enhanced mechanical behavior compared to their DQ counterparts, where yield strength, uniform elongation, and total elongation increased after partitioning at 400 °C. The ultimate tensile strength of the Q&P steels decreased compared to the DQ steels annealed at the same peak temperature. However, the final strength-ductility balance of the studied Q&P steels was superior to the DQ steel grades. Moreover, considerable strength and improved ductility were obtained through the combination of peak annealing above the A_C3 temperature followed by Q&P. These results are attributed to an interplay between a sustainable TRIP effect and effective strain-stress partitioning among the microconstituents resulted after the Q&P process.