Microstructure refinement of high strength low carbon bainitic steel

The ultra-fine bainitic microstructure in a high strength low carbon bainitic Mn-Nb-B steel has been obtained by a newly developing relaxation-precipitation-controll phase transformation (RPC) processing. In a pan-cake like prior-austenite grain, the microstructure consists of lath bainite, and some amount of abnormal granular bainites or/and acicular ferrites. The laths are 3-5 mum in length and less than 1 mum in width, the packet size of lath bainite is refined to 3-5 mum. In spite of refining the effective grain, the morphologies of bainite and retained austenite are different in RPC processed steel comparing with the commonly CR or RQ-T processing. The relaxation time after finishing rolling also influences the refinement of the microstructure. The yield strength of RPC processed steel is increased by about 30% comparing with the RQ-T processed steel with the same composition. The refining mechanism by RPC process is the formation of polygonal dislocation substructure pined by strain induced precipitates during relaxation, and the substructure would be benefit of nucleating bainite and acicular ferrite or limiting the growth of bainitic lath.