The formation of austenite in manganese-boron steels during selective heat treatment has great significance in the application of innovative hot stamping processes. Heat treatment tests were designed according to the thermal cycle of industrial heating and hot stamping processes and were conducted on a Gleeble 3800 thermomechanical testing system. Specimens were subjected to non-isothermal (heating rates: 1 K/s–25 K/s) and isothermal (soaking temperatures: 1023 K–1173K) temperature profiles. A high-resolution dilatometer was employed to detect the dimensional change of the specimens associated with austenitization. The dilatometric measurement was quantitatively related to the volume fraction of austenite. By analysing the evolution curves of austenite fraction, the effects of heating rate and temperature on the progress of austenite formation under both non-isothermal and isothermal conditions were investigated and characterised, improving the current understanding of the mechanisms that control austenite formation in manganese-boron steels.