In order to avoid the appearance of soft particles composed of ferrite or pearlite in the actual production of new bainitic steel, the phase transformation behaviour and bainite formation kinetics were investigated by DIL805A dilatometer, optical microscopy, scanning electron microscopy and Vickers-durometer. The results show that the soft particles cannot appear when the cooling rate exceeds 0.025 K s −1 , and this condition can be ensured by direct spray cooling in production. The local activation energy decreases with increasing transformed bainite volume fraction ( f b ), and the average energy is about 136.7 kJ mol −1 . The local Avrami exponent mainly lies between 0.5 and 3 in a wide f b range, indicating that the dominating mechanism of bainite formation is two-dimensional and one-dimensional growth.