Local and Global Stress–Strain Behaviors of Transformation-Induced Plasticity Steel Using the Combined Nanoindentation and Finite Element Analysis Method

Transformation-induced plasticity (TRIP) steels have excellent strain hardening exponents and resistibility against tensile necking using the strain-induced martensite formation that occurs as a result of the plastic deformation and strain on the retained austenite phase. Detailed studies on the microstructures and local mechanical properties, as well as global mechanical properties, are necessary in order to thoroughly understand the properties of TRIP steels with multiple phases of ferrite, bainite, retained austenite, and martensite. However, methods for investigating the local properties of the various phases of the TRIP steel are limited due to the very complicated and fine microstructures present in TRIP steel. In this study, the experimental and numerical methods, i.e., the experimental nanoindenting results and the theoretical finite element analyses, were combined in order to extract the local stress–strain curves of each phase. The local stress–strain curves were in good agreement with the values presented in the literature. In particular, the global plastic stress–strain behavior of the TRIP steel was predicted using the multiple phase unit cell finite element analysis, and this demonstrated the validity of the obtained properties of each local phase. The method of extracting the local stress–strain curves from the nanoindenting curves and predicting the global stress–strain behavior assists in clarifying the smart design of multi-phase steels.