Mechanical Characterization and Simulation of TWIP Steel Under Plastic Deformation

Abstract

Recently, the importance of environmental problems such as greenhouse effect has emerged as a big issue. To improve these problems, demand for steel that has advanced mechanical properties such as high strength and ductility occurs. Among various kinds of steels, TWIP steels receive attention as a next generation steel. TWIP steels have the advantages of strong work hardening, high strength and good ductility due to Twinning Induced Plasticity in deformation process. Likely TWIP steel, twin boundaries induced by deformation are strong barriers to dislocation motion and thus improve the strength and ductility of metals. It is known that twin boundaries are of very low energy and are stable. The aim of this thesis is to investigate the twin boundaries effect in TWIP steel on the deformation behavior during plastic deformation using the finite element method. The stress-strain curves of experiment and simulation are in good agreement. The simulation results show that stacking fault energy (SFE) affect strain hardening during the tensile test and thickness change during the cup drawing test. Therefore, it was concluded that strain hardening and thickness change of TWIP steel is related to friction stress (σ_0) and constant k. The thesis is concerned with large plastic deformation behavior of TWIP steel under HPT. A high-manganese twinning-induced plasticity (TWIP) steel is processed by high-pressure torsion (HPT) for up to 1 turn under 6 GPa Pressure. The HPT-processed TWIP steels exhibit a homogeneous microstructure with a peak hardness of Hv 550. Deformation twinning is developed significantly in the early stage of the shear deformation. The strength of the HPT-processed TWIP steel significantly increased due to the accumulation of dislocations, but elongation dramatically decreased due to a lack of dislocations available for plastic deformations. An analysis of the evolution of strength by imposed large strain under high pressure suggests that strain hardening due to dislocation and twinning is exhausted in the early stages of the HPT process According to the Hall-Petch relation, Mechanical properties of steels were affected by grain size. Mechanical and microstructural behaviors of the high-pressure torsion processed twinning-induced plasticity (TWIP) steels are investigated. Initial coarse grained TWIP steel (ICG-TWIP) presents frequent deformation twinning and martensitic transformation during high-pressure torsion (HPT), while initial fine grained TWIP steel (IFG-TWIP) exhibits less deformation twinning and dislocation-based deformation. This implies that the initial grain size can affect the deformation mechanism during the HPT processes and mechanical properties of severely deformed TWIP steels, contrary to the common belief that responses to the severe plastic deformation of metallic materials are not affected by initial grain size.