Abstract: The common Q345B steel plate was upgraded to Q420D steel plate by thermomechanical control process (TMCP). The microstructure and mechanical properties of the tested steel were studied by different heat rolling processes. The results show that the strength and toughness of the tested steels all could meet the requirement of GB/T 1591—2008. The microstructure at the quarter of thickness was significantly refiner than that at center of thickness. When the starting rolling temperature was 780 ℃ and the finishing rolling temperature was 790 ℃, the microstructure of the tested steel was finest. The absorbed energies for cracks stable extensive stage and for cracking preventing decreased gradually with the decrease of the impact test temperature, and the impact energy decreased.