Continuous cooling transformation and microstructure of high strength pipeline steel

Continuous cooling transformation(CCT)behaviors of X80 and X100 low-carbon microalloyed pipeline steels were investigated under the undeformed and deformed conditions,respectively.The corresponding CCT diagrams were constructed by using a Gleeble-2000 thermomechanical simulator,and microstructure of the steels at different cooling rates and deformation conditions was observed by OM,SEM and TEM.The results show that polygonal ferrite and pearlite can be formed at low cooling rates,massive ferrite,granular ferrite,acicular ferrite and bainitic ferrite can be obtained with increase of the cooling rate.Furthermore,hot deformation can strongly promote the formation of acicular ferrite,that is,the transformation temperature of acicular ferrite is increased by 50-100 ℃,this leads to CCT curves move towards the top left corner,which results in grain refining and more random orientation.