Electron back scatter diffraction analysis for inhomogeneous deformation in a ferrite-martensite dual phase steel

Synopsis : Substructures in a ferrite-martensite dual phase steel have been investigated by using a SEM-EBSD technique. Particular attention has been paid on the inhomogeneous deformation developed in the ferrite phase during a tensile deformation, i.e., a kind of deformation bands which enhance work-hardening and uniform elongation. Effects of small angle boundaries (SAB) included in ferrite grains on the stress-strain curve have also been studied. Two kinds of a Cr-added low carbon dual phase steel were prepared by different heat treatments. They have almost the same microstructures except the density of SAB in the ferrite phase. The specimen with the lower density of SAB exhibited lower yield strength and larger uniform elongation. SEM-EBSD observation demonstrated that small angle lattice bending due to deformation bands were developed in ferrite phase with tensile strain, indicating a kind of grain subdivision in ferrite grains. Such kind of grain subdivision was commonly observed in large ferrite grains, and it was enhanced in the area neighboring to the hard phase of martensite. It should be noted that mechanical properties of dual phase steels are influenced by such substructures in ferrite grains.