This work provides quantitative experimental evaluation and new insights into the coupling mechanisms between the deformation of austenite and its transformation into ferrite. A Fe-0.16C-1.4Mn steel after different torsion ratios in the austenitic range is considered. Interrupted isothermal holdings at different transformation temperatures are used to follow ferrite transformation from a large austenitic grain size. Metallographical observations show intragranular nucleation near austenite grain boundaries and inside deformation bands on top of classical ferrite at grain boundaries. A kinetic model for phase transformation is proposed. It accounts for two contributions to the kinetics of ferrite transformation: 1) austenite grain shape modification due to deformation and 2) formation of two types of ferrite, one at grain boundaries, and one inside the grains induced by plasticity. This model describes correctly our experimental kinetics and can be applied to continuous cooling.