Abstract: First, strip cast samples of high strength microalloyed steel with sub-rapid solidification characteristics were prepared by simulated strip casting technique. Next, the isothermal growth of austenite grain during the reheating treatment of strip casts was observed in situ through confocal laser scanning microscope (CLSM). The results indicated that the time exponent of grains growth suddenly rise when the isothermal temperature higher than 1000°C. And the activation energy for austenite grain growth were calculated to be 538.0 kJ/mol in the high temperature region (above 1000°C) and 693.2 kJ/mol in the low temperature region (below 1000°C), respectively. Then, the kinetics model of austenite isothermal growth was established, which can predict the austenite grain size during isothermal hold very well. Besides, high density of second phase particles with small size was found during the isothermal hold at the low temperature region, leading to the refinement of austenite grain. After isothermal hold at different temperature for 1800 s, the bainite transformation in microalloyed steel strip was also observed in situ during the continuous cooling process. And growth rates of bainite plates with different nucleation positions and different prior austenite grain size (PAGS) were calculated. It was indicated that the growth rate of the bainite plate is not only related to the nucleation position but also to the PAGS.