Abstract: The controlled rolling and controlled cooling test was conducted on V-N microalloying Q550D high strength steel plate. The microstructures at different positions along the thickness of the steel were studied and the mechanical properties were tested. The results showed that the microstructure of V-N microalloying Q550D plate is composed of polygonal and acicular ferrites. From the surface to the center of the test sample， the average grain size increases， while the mass fraction of acicular ferrite decreases. It was also observed that the size of (Ti，V)N precipitates is 20~30nm and the size of V(C，N) precipitates is smaller than 10nm， and both precipitates distribute dispersedly in the matrix of polygonal and acicular ferrites. The yield strength， tensile strength， elongation and impact work at -20℃ of test steel were 651MPa， 733MPa， 18% and 170J， respectively. The fine grain strengthening， precipitation strengthening， dislocation strengthening， solid solution strengthening and acicular ferrite strengthening were the major mechanisms of strengthening. Meanwhile， the role of fine grain， low C component and the formation of acicular ferrite were the major mechanisms of toughening.

Key words: Q550D, V-N microalloying, mid-thick plate, strengthening mechanism, microstructure