The kinetics of reversed austenite formation in 301 stainless steel and its effect on the deformation of an automobile front bumper beam are studied by using modeling approaches at different length scales. The diffusion-controlled reversed austenite formation is studied by using the Johnson–Mehl–Avrami–Kolmogorov (JMAK) model, based on the experimental data. The model can be used to predict the volume fraction of reversed austenite in a temperature range of 650–750 °C. A three-dimensional elastoplastic phase-field model is used to study the diffusionless shear-type reversed austenite formation in 301 steel at 760 °C. The phase-field simulations show that reversion initiates at martensite lath boundaries and proceeds inwards of laths due to the high driving force at such high temperature. The effect of reversed austenite (RA) and martensite on the deformation of a bumper beam subjected to front and side impacts is studied by using finite element (FE) analysis. The FE simulations show that the presence of reversed austenite and martensite increased the critical speed at which the beam yielded and failed. RA fraction also affects the performance of the bumper beam.