The demand for safety and reliability in pipelines has been increasing steadily. Dual-phase steels, especially with a bainite matrix and a well-dispersed martensite-austenite constituent (MA), provide ingredients necessary for the improvement of the yield ratio and toughness. To design alloy elements and ensure that dual-phase steels have the required mechanical properties, an understanding of the governing microscopic deformation mechanisms is essential. For this purpose, multi-constitutive crystal plasticity simulation coupled with local strain/stress partitioning, ductile damage and transformation-induced plasticity evolution was employed. Microstructural cell responses were captured by fast Fourier transform crystal plasticity analysis. Representative microstructural patches with the same high spatial resolution as those obtained by electron backscatter diffraction (EBSD) tomography provide new insights into the deformation mechanism in dual-phase microstructures, especially regarding the effects of the matrix and secondary phase distribution on the strain, ductile damage and transformation localization behavior.

Duplex stainless steel has excellent strength and corrosion resistance, and it is widely used such, for example, in plant applications. The microduplex structure affects not only the physical properties but also the deformation behavior. The fracture considered to be caused by the peeling duplex interface has been confirmed in various experiments and parts manufacturing factories, but there are few detailed references to the morphology of the microduplex structure. In this study, we examined the relationship between the morphology of the microduplex structure and the deformation behavior by the shearing test with load, horizontal or vertical to the stretching direction of the austenite phase. The latter load divided the duplex interface and delayed the fracture. The results showed that the microduplex structure acts as resistance to the load orthogonal to itself.

The effects of the roller tip radius of curvature on defect formation in the nosing of a SUS409 tube were investigated using finite element simulations. For a roller tip radius of curvature of 6.0 mm, a large inwardly convex bending deformation occurs and the tube end becomes wrinkled. A smaller roller tip radius of curvature is associated with a larger axial elongation and a smaller wall thickness ratio. The wall thickness ratio is smallest when the tip curvature radius is 8.0 mm, where cracks occur. The damage value obtained from the ductile fracture criterion becomes maximum at the tapered corner. That maximum value decreases as the roller tip radius of curvature increases.