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The present study was undertaken to provide a more comprehensive understanding of the deformation modes of advanced fully austenitic steels exhibiting an enhanced combination of strength and ductility. For this purpose, a new plasticity, called microband induced plasticity (MBIP), was introduced. In addition, the origin of its superb combination of strength and ductility over the well-known transformation induced plasticity (TRIP) and twin induced plasticity (TWIP) was elucidated. With the aids of previously developed models, we focused on predicting the transitions among TRIP, TWIP, and MBIP, primarily in terms of the stacking fault energy. The analysis revealed that the TRIP-TWIP transition can be reasonably predicted by the energy balance for FCC austenite -HCP ε martensite transformation. The TWIP-MBIP transition can be addressed by the critical stress for mechanical twinning, which causes the infinite divergence of the Shockley partials. Lastly, the TWIP-MBIP transition model was validated by comparing it with the experimental data.

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The effect of sheet thickness on shear deformation and texture for the ferrite rolling has been studied.The shear deformation was estimated from the distorted shape of inserted wire before rolling and FEM simulation. The deformed shape of inserted wires and FEM results showed that the shear strain decreased with increasing initial sheet thickness. The measuredand simulated textures also showed that the Goss component changedto Dillamore componentas the thickness increased, From rolling simulation of 20'1-reduction, the minimumshear deformation was found to exist for the given roll bite geometry and friction coefficient.

In order to measure the flow curves of steel sheets at high temperatures, which are dependent on strain and strain rate as well as temperature and temperature history, a tensile test machine and specimens were newly developed in this work. Besides, an indirect method to characterize mechanical properties at high temperatures was developed by combining experiments and its numerical analysis, in which temperature history were also accounted for. Ultimately, a modified Johnson-Cook type hardening law, accounting for the dependence of hardening behavior with deterioration on strain rate as well as temperature, was successfully developed covering both pre-and post-ultimate tensile strength ranges for a hot press forming steel sheet. The calibrated hardening law obtained based on the inverse characterization method was then applied and validated for hot press forming of a 2-D mini-bumper as for distributions of temperature history, thickness and hardness considering the continuous cooling transformation diagram. The results showed reasonably good agreement with experiments

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