Effect of cooling media on microstructure and mechanical properties of Nb/Ti containing microalloyed steel

A microalloyed steel, industrially produced X60 grade of linepipe steel, has been processed by a single pass hot rolling with 30% reduction followed by cooling under different conditions to understand the microstructural evolution and the ensuing mechanical properties. The imperative is to comprehend the formation of various microstructural constituents with cooling rate and correlate them with the achievable mechanical properties commensurate with higher grades of linepipe steels. The microstructural content showed a continuous change from polygonal ferrite, acicular ferrite, bainitic ferrite to martensite with cooling in still air, forced air, oil and water, respectively. The tensile fracture surfaces indicated a transition from ductile to brittle mode of fracture with increasing cooling rate. It is concluded that the given steel composition can be tailored to higher grades for linepipe applications by suitably modifying the thermo-mechanical process route.