Development of a constitutive model for microforming of 304 stainless steel based on strain path effects

This study systematically investigates the influence of size effect and the strain path effects on the deformation behavior of SUS304 stainless steel. Furthermore, a macro-micro constitutive model that incorporates the size effect is developed by introducing the ratio of plate thickness to average grain size, which serves as a key scale parameter. Based on two-step uniaxial tensile test of SUS304 stainless steel, the flow stress-strain curves under different strain paths and scale parameters were obtained, facilitating a systematic study on the effects of size and strain path on the material's deformation behavior. It was found that both the material flow stress and the hardening rate decrease with an increase in the strain path angle, defined as the angle relative to the pre-stretching direction. By performing Electron Backscatter Diffraction (EBSD) on specimens before and after deformation, the influence of the strain path effects on the mechanical properties and microstructure at the microscale was revealed. This insight led to the development of a mixed constitutive equation for microforming by incorporating the Hall-Petch relation, which accounts for both the size effect and the strain path. The hybrid constitutive equation for micro-plastic forming materials, accounting for both size effects and strain path changes (SPC), was successfully solved.