The current main challenge of the automotive industry is to reduce the fuel consumption of vehicles by increasing the efficiency of their structural parts. It requires the development of new steels that combine the necessary ductility for the forming operations and high strength levels. This unique combination of properties depends on specific microstructures and particular strengthening mechanisms like the TRIP effect. This study deals with the influence of the hot rolling parameters on the microstructure and mechanical properties of 0.4C-1.5Mn-1.5Si steels with or without Nb microaddition. Deformation under the T-nr temperature leads to a refinement of the microstructure consisting of retained austenite and bainite and to an enhancement of the mechanical properties. Furthermore, the deformation leads to a larger amount of retained austenite and to intragranularly nucleated bainitic ferrite. The effects of the Nb addition and of the deformation temperature are demonstrated.