Abstract: To understand the knowledge of liner thermal expansion and thermal expansion coefficient of Q450NQR1 casting steel, dilatometry experiments were performed at two different cooling rates of 5℃·min^-1 and 20℃·min^-1. A mathematical model was developed to quantitatively extract the kinetic information of austenite transformation based on the concept of average atomic volume. The model was verified by microstructure examinations, indicating that the model was appropriate to describe the behavior of austenite transformation and could be applied to multi-phase transformations during steel continuous casting. The influence of cooling rate on the austenite transformation was discussed with the model. it is found that, with increasing cooling rate, two peaks associated to the precipitation of ferrite and pearlite on the thermal expansion coefficient curve shifted towards a lower temperature region with higher values. As the cooling rate increases from 5℃·min^-1 to 20℃·min^-1, the initial temperatures of ferrite and pearlite precipitation decrease by 32℃ and 37℃, and the final volume fractions of ferrite and pearlite gradually change from 0.894 and 0.106 to 0.945 and 0.055, respectively.