フェライト-ベイナイト複相鋼の延性破壊挙動に及ぼす局所変形組織の影響

Microscopic deformation and fracture behavior of a ferrite-bainite dual phase steel was investigated by the micro-grid method and FE analysis to understand the inherent conditions of plastic instability and ductile fracture. The micro-grid method, which the microscopic strain is measured by the displacement of grids with 500 nm intervals drawn on the specimen surface, clearly revealed that the shear deformation along the lath structure in the bainite phase was seen before reaching the maximum loading point. Then, voids were observed in the ferrite phase adjacent to the ferrite/bainite boundary, where showing higher strain concentration. From the FE analysis with the model simulating actual ferrite-bainite microstructure, stress distribution was seen in the bainite phase, and high stressed regions could cause the shear deformation of the bainite phase. The local shear deformation in the bainite phase decreased strain hardenability and triggered the macroscopic plastic instability. It is considered that the macroscopic plastic instability accelerates the strain localization, and promotes the void nucleation and growth. Ductile fracture path was also visualized by the micro-grids in the ferrite phase along the shear deformation bands which is connecting the high strain regions. Development of shear deformation bands inside the ferrite phase was well simulated with the FE analysis, same as the development of high stressed region in the bainite phase in the early stage. It can be stated, therefore, that plastic instability and ductile fracture of dual phase steel is a structure dependent phenomenon which is strongly controlled by the morphologies of each constituent phases.