Prediction of strain path and forming limit curve of AHSS by incorporating microstructure evolution

Austenitic stainless steel is known as second-generation advanced high strength steel (AHSS) due to its very high strength and elongation. The stability of austenite phase under deformation at room temperature depends on alloying elements such as Ni and Mn. Low-Ni austenitic stainless steel undergoes phase transformation, austenite to martensite, during plastic deformation. The phase transformation and crystallographic texture development are strain and strain path dependent. Consequently, material properties especially work hardening exponent (n) and normal anisotropy (r) are altered during plastic deformation. To understand the effect of dynamic mechanical properties on the formability of low-Ni austenitic stainless steels, limiting dome height (LDH) tests were performed. Strain path diagrams (SPD) and forming limit diagrams (FLD) were plotted for this material. Finite element (FE) analyses were carried out with constant and varying (with strains and strain paths) n and r values. The work hardening exponent was calculated for uniaxial strain path and correlated with in-grain misorientation and martensite phase fraction at different strains. This relationship is further extended to estimate the work hardening parameter for other strain paths. Strain path-dependent instantaneous r values were determined by using bulk texture analysis. The strain path-dependent n and r values were incorporated during FE simulation which remarkably improved the SPD and FLD predictability of low-Ni austenitic stainless steel.