A physically-based material model has been developed to describe the austenite formation in a manganese-boron steel during heating in hot stamping processes. The equations were formulated based on three austenite formation mechanisms: nucleation, growth and impingement. It is able to characterise the phase transformation process under both non-isothermal and isothermal conditions, where the effects of heating rate and soaking temperature on the austenite formation have been rationalised. Heat treatment tests of the manganese-boron steel were performed on a Gleeble 3800 subjected to various heating conditions (heating rate: 1 K/s − 25 K/s, soaking temperature: 1023 K − 1273 K). The dimensional changes of specimens associated with the phase transformation, which was measured using a high resolution dilatometer, has been quantitatively related to the volume fraction of austenite formation. The experimental data was used to calibrate and validate the equations. Good agreement between the experimental and predicted results has been obtained. Further analysis has been made to illustrate the significance of the model in applications.