Effect of two-stage controlled cooling on the microstructure and properties of Mo-containing X80 high-deformability pipeline steel

A Mo-containing X80 high-deformability pipeline steel was processed by TMCP rolling and two-stage controlled cooling in laboratory.The effects of cooling conditions on the microstructure and properties of the steel were investigated by scanning electron microscopy(SEM) and transmission electron microscopy(TEM).It is found that the microstructure of the steel processed by two-stage controlled cooling is ferrite-bainite dual phase.As the start temperature in accelerated cooling comes down,both the strength and yield ratio decrease and the uniform elongation(uEl) increases with the ferritic volume fraction increasing.As the finish temperature in accelerated cooling becomes lower,the amount of martensite-austenite(M/A) constituents in bainite decreases and the M/A islands are much finer and more uniformly dispersed.The strength changes little but the uEl increases significantly.It is considered that uEl is closely related to M/A constituents in bainite when the ferritic volume fraction is constant.Finely and uniformly dispersed M/A islands can increase the value of uEl by increasing the work hardening rate and delaying the onset of necking.When the accelerated cooling starts at 690℃ and finishes at 450℃,the ferritic volume fraction is about 23%,the grain size is approximately 5 μm,and the average grain size of M/A islands is approximately 1 μm.The uniform microstructure provides good combination of strength-ductility properties.