The effects of process parameters on microstructural evolution and grain refinement are determined in a Nb-Ti microalloyed high strength dual phase steel. With the increase of cooling rate, final microstructures change from a mixture of acicular ferrite (AF)+martensite/retained austenite (M/A) to conventional bainite (CB)+M/A. Accordingly, the morphology of M/A constituent changes from an equiaxed island in AF to an elongated interlath in CB. The length and width of bainite packets become smaller with the increase of cooling rate and the decrease of deformation temperature. The length of individual bainitic ferrite plates within the packets become smaller with the increase of cooling rate and the decrease of deformation temperature, whereas the thickness of them varies slightly with them. The optimized relaxing time on grain refinement is 60 s. The reheating temperature, reduction ratio and interrupt temperature has no obvious effect on the formation of dual phase of acicular ferrite or bainite and M/A.