Strength, strain capacity and toughness of five dual-phase pipeline steels

The effect of microstructures on strength, strain capacity and low temperature toughness of a micro-alloyed pipeline steel was elucidated. Five various dual-phase microstructures, namely, acicular ferrite and a small amount of (around 2 vol.%) polygonal ferrite (AF + PF), polygonal ferrite and bainite (PF + B), polygonal ferrite and martensite/austenite islands (PF + M/A), polygonal ferrite and martensite (PF + M) and elongated polygonal ferrite and martensite (ePF + M), have been studied. Experimental results show that AF + PF microstructure has high yield strength and excellent low temperature toughness, whereas its yield ratio is the highest. Polygonal ferrite-based dual-phase steels, PF + B, PF + M/A and PF + M microstructures show better strain capacity and low temperature toughness. The strain capacity and low temperature toughness of ePF + M microstructure are the worst due to its high strength. The relationship between microstructure, strength, strain capacity and toughness has been established. Based on the results, the optimum microstructure for a better combination of strength, strain capacity and toughness is suggested to be the one having appropriate polygonal ferrite as second phase in an acicular ferrite matrix.