Dual-phase microstructure is an essential measure for improving deformability of steels. Ferrite-bainite or bainite-MA microstructure control is practically applied for the high strain linepipes. In order to explore the optimum microstructure that enhances deformability such as strain hardenability and uniform elongation, micromechanical investigations were conducted. Microscopic deformation behavior of dual-phase steel was modeled by an axisymmetric unit cell model based on a regular array of second-phase particles arranged on a BCC lattice. Finite element analyses were carried out to investigate the macroscopic and microscopic response of unit cells with the morphological features based on actual steels. Macroscopic deformation behavior of ferrite-bainite steels calculated by the unit cell model showed good agreement with experimental results. In addition, the effect of bainite volume fraction on Y/T and n-value was well simulated by the unit cell model. Microscopic investigation on the unit cells revealed that significant strain concentration existed in the ferrite phase near the ferrite/bainite boundary for ferrite-16% and 35% bainite steel, which had higher experimental n-value. However, further increased bainite volume fraction caused plastic deformation of bainite phase and the strain concentration was decreased. Therefore, high level of microscopic strain concentration can be a driving force for developing strained region inside the soft phase, which results in higher deformability. Effect of bainite morphology on the strain hardening behavior of ferrite-bainite steels and relation of microscopic deformation behavior and macroscopic strain hardening behavior were discussed based on analytical study using the unit cell model.
Linepipes installed in seismic or permafrost regions must have sufficient resistance against buckling or weld fracture caused by large ground deformation of buried pipeline. In order to design the pipelines installed in seismic or permafrost regions, new design methodology, so called "strain-based design", has been developed (Denys et al., 2000; Wang et al., 2002, 2007; Ishikawa et al., 2004; Suzuki et al., 2002, 2004).
