Experimental simulations of thermomechanical processing (TM) using press forging of Si-Mn TRIP (transformation induced plasticity) steel were performed. In order to rationalize the retained austenite (RA) volume fraction, six TM schedules were employed at experiment where different austenite conditioning was considered. The various multiphase structure characteristics were obtained after of TM processing, with different volume fraction of ferrite, bainite and RA. The modification of structural characteristics influenced the mechanical properties of TRIP steel. The present work also focuses on monitoring of RA transformation during mechanical incremental straining using in situ neutron diffraction technique. This method is used to characterize the kinetics of RA transformation and its stability during the straining.