The behaviour of transformation-induced plasticity steels subjected to combined thermomechanical loading is studied at the microscale by means of numerical simulations. The microstructure is composed of an austenitic phase that may deform plastically and/or transform into martensite, and a ferritic phase that may deform plastically. The micromechanical models capturing these effects are derived from a thermodynamical framework, which has been extended from previous work in order to adequately account for the thermal contributions to the kinematics and the Helmholtz energy. The models are used in numerical simulations on a polycrystalline sample composed of an aggregate of multiple austenitic and ferritic grains of various orientations. The thermomechanical response of the sample is studied under (i) isothermal straining at different temperatures above the martensitic start temperature, and (ii) under different paths of straining and cooling to temperatures below the martensitic start temperature. The first type of analysis shows that at lower temperatures the transformation mechanism is more dominant than the plasticity mechanism, whereas the converse occurs at higher temperatures. The second type of analysis illustrates that, in comparison to a benchmark initially stress-free sample at room temperature, the transformation rate under straining is higher when performed on a pre-cooled sample, but the transformation rate under cooling is lower when carried out on a pre-strained sample. The results of this analysis indicate that, for optimizing the formability of this class of steels, it is recommended to make a judicious choice regarding the thermomechanical loading parameters during manufacturing processes.