Influence of Cooling Rate on Microstructure Evolution in a Low Carbon High Strength Microalloyed Dual Phase Steel

摘要

The effect of continuous cooling rate on transformed microstructures is studied on a low carbon Nb-Ti microalloyed bainite/martensite dual phase steel. A fine-grained dual phase steel with a predominant conventional bainite (CB) or acicular ferrite (AF) and a small volume fraction of martensite/retained austenite (M/A) are obtained in a wide range of cooling rates. The morphology of M/A constituent changes from an elongated interlath in CB to an equiaxed island in AF. The length and width of bainite packets become smaller with the increase of cooling rate. The length of individual bainitic plates becomes smaller with the increase of cooling rate, whereas the thickness of them just varies slightly with cooling rate. The grain refinement of mixed microstructures is likely to be attributed to the formation of acicular ferrite and the solute drag effect of Mn.