Influence of vanadium on the microstructure refinement of high strength automobile beam steel

The effects of vanadium on the microstructure refinement of automobile beam steel with a yield strength of 700 MPa were investigated by Gleeble-3500 Thermo Simulator, scanning electron microscopy, transmission electron microscopy and electron back-scattered diffraction. The results show that a mixture structure of acicular ferrite and granular bainite can be obtained in a cooling rate range of 2 to 7℃·s^-1. It is recognized that the volume fraction of granular bainite increases and the microstructure can be refined by adding vanadium. After the addition of vanadium, the size of M/A islands decreases obviously. Compared to the V-free steel, high angle grain boundaries in the V-bearing steel increases by 18.2%, indicating a potential improvement in toughness. No VC precipitation is observed in the tested steel due to low carbon content, so it can be deduced that V in the solid solution can promote the bainite transformation and refine the microstructure.