Development of ultra-fine grained dual phase microalloyed steels through severe cold rolling and intercritical annealing

A Nb-microalloyed structural steel with ferrite-pearlite microstructure was subjected to cold rolling and intercritical annealing to produce ultra-fine grained dual phase microstructure. Optical and transmission electron microscopy techniques were employed to characterise the microstructure. Initial results showed that the intercritical annealing (at 790°C for 90s) of samples rolled to a true strain of 2.4 resulted in a significant grain refinement from the average initial grain size of 20 μm to 1–2 microns. The microstructure primarily consisted of UFG ferrite matrix with homogeneously distributed islands of plate martensite with volume fraction of 27%.