Effect of volume fraction of bainite on microstructure and mechanical properties of X80 pipeline steel with excellent deformability

Pipeline steel with excellent deformability and dual-phase microstructure consisting of ferrite and bainite was obtained by inter-critical accelerated cooling. Dual-phase microstructure was adopted to obtain high deformation capacity, enabling the prevention of structural failure during geological disasters such as landslides and earthquakes. The effect of the volume fraction of bainite on the mechanical properties of the dual-phase pipeline steel was investigated by microscopic analysis and testing of mechanical properties. Results indicate that both the yield strength and ultimate tensile strength of the steel increases almost linearly with the increasing volume fraction of bainite, whilst ductility, work hardening exponent, and impact absorption energy decrease. At a 50% volume fraction of bainite, the yield strength, yield strength/tensile strength ratio (Y/T), work hardening exponent, uniform elongation, and impact absorption energy of X80 dual-phase pipeline steel are 665 MPa, 0.8, 0.12, 8%, and 245J, respectively. At this stage, the microstructure of the dual-phase pipeline steel consists mainly of small bainitic ferrite, supplemented by small polygonal ferrite with high-density dislocation. This dual-phase microstructure endows the pipeline steel with excellent strength and toughness, as well as good deformation characteristics. (C) 2011 Elsevier B.V. All rights reserved.