Microstructure and Mechanical Properties of X80/X100 High Deformability Pipeline Steel

In the present study, X80 and X100 grade high deformability pipeline steels have been processed by using TMCP and followed two-stage cooling process. The microstructures of the X80HD (HD, high deformability) and X100HD steels were both characterized by ferrite-bainite dual phase. The grains sizes of ferrite were mostly less than 5 mu m and the volume fractions were about 20 similar to 25% in X80HD and 10-15% in X100HD steel. The bainite structure in X80HD steel was granular bainite (GB); while in X100HD steel large amounts of lath bainite (LB) were also formed besides GB, and bainite grains were much finer. Ferrite-bainite dual phase microstructure has large strain hardenability that resulting high strength and high deformability combination. Both the steels exhibit high strength/toughness in transverse direction and high deformability in longitudinal direction. The X100HD steel with more volume of LB and less volume of PF has higher strength but lower deformability than that of X80HD steel.