Microstructure of Nb-bearing pipeline steel with improved property applying ultrafast cooling process

X70 pipe line steel with high mechanical strength and high ductility can be achieved through hot rolling and ultrafast cooling process at a low cost. A large amount of nano-sized precipitates and dislocations are identified near the surface layer of the steel applying ultrafast cooling, which contribute immeasurably to the improvement of mechanical properties. In addition, high cooling rate decreases the mean distance of pearlite laminae, and massive ferrite grains characterized by curved grain boundaries and high dislocation density are also transformed as one of the typical microstructures in the samples applying ultrafast cooling process. The comprehensive property of Nb-bearing pipe line steel processed with ultrafast cooling is corporately improved through dislocation and precipitation strengthening, while the matrix microstructures are mostly massive ferrite characterized by sharp-edged curved grain boundary.