Effect of controlled rolling and cooling on continuous cooling transformation of high Nb containing microalloyed steel

Process of controlled hot rolling followed by controlled cooling of a Nb microalloyed pipeline steel was simulated on a Gleeble-1500D simulator and dilatometry measurements,microstructure and phase transformation characteristics of the steel and the effect of deformation on transformation were studied.According to the results of both dilatometry measurements and microstructure observations,the continuous cooling transformation curves(CCT) of the tested steel are constructed.The results show that Nb content and deformation enhance the formation of acicular ferrite,the microstructure of the steel ranges from PF,QF to AF with increasing of cooling rates from 0.5 to 50 ℃/s after two stage controlled rolling and the microstructure revolution is sensitive to cooling rates when it is lower than 5 ℃/s,however,when the cooling rate increases further,the microstructure does not change very much but M/A constituents in matrix is refined and dispersed.