Microstructure Characteristics in Continuous Cooling Transformation of an Nb Microalloyed Steel

Deformation dilatometry has been used to simulate controlled hot rolling followed by controlled cooling of a Nb microalloyed pipeline steels, the microstructure and transformation characteristics in the steel and the effect of deformation on transformation are studied. According to the results of both dilatometry measurements and microstructure observations, the continuous cooling transformation curves (CCT) of the tested steels are constructed. The results show that Nb content and deformation enhance the formation of acicular ferrite; the microstructure of the steel range from PF, QF to AF with increasing of cooling rates from 0.5 to 50 degrees C/s in a two stages controlled rolling and the microstructure revolution is sensitive to cooling rates when it is lower than 5 degrees C/s, however, when the cooling rate increasing further, the microstructure didn't change very much but M/A constituents in matrix is refined and dispersed.