Deformation Characteristics And Microstructure Evolution Of As-cast Fe-21Mn-0.7c-0.1Si TWIP Steel

The overall objective of this thesis was to investigate the cold and hot compression and microstructures of the as-cast high-Mn TWIP steel.The cold compression was performed using a servo automatic press machine at different ratios ranging from10/12 to 4/12.The microstructure and mechanical properties of both as-cold deformed and annealed samples were examined.The hot compression was tested using a Gleeble?3500 thermal-mechanical simulator under deformation conditions(temperatures ranging from 950 to 1100°C and strain rates ranging from 0.01 to 5 s^-1).The hot deformation behavior was determined.The constitutive equation incorporating the Zener–Hollomon parameter was established and the activation energy（Q）was calculated in the whole range of deformation conditions.The processing map was constructed using the dynamic material model（DMM）to determine the regions of deformation stability and instability.The microstructure examinations and the Vickers hardness were measured to reflect the changes in the local mechanical properties before and after hot deformation.The microstructures of both undeformed and deformed samples were analyzed using a metallographic microscope（MM）and scanning electron microscope（SEM）.The cold deformation resulted in significant strengthening due to the occurrence of deformation twins.The hardness increased from 142.16 to 352.4 HV1 after the compression ratio of 4/12.At the temperature below 900℃,the microstructure developed the recovery after subsequent annealing treatment,whereas at the temperature above 900℃,the recrystallization occurred,which led to the refinement of the microstructure.The recrystallized grain was decreased from 7.3 to 1.2μm with a reduction in the deformation ratio from 10/12 to 4/12,at the annealing temperature of 1100℃.The fine-grained microstructures exhibited remarkable improvements in mechanical properties.The hot deformation results showed that the flow curve stress increased while the deformation temperature decreased and the strain rate increased.Therefore,the flow curves exhibited yield-point-elongation（YPE）and dynamic recrystallization（DRX）as the main softening mechanism.The activation energy（Q）was calculated to be394,975 k J.mol^-1 at the strain rate of 0.7.The processing maps displayed two peak values for power dissipation efficiency,which were in the deformation temperature range of 1046-1055℃ and strain rate of 0.01 s^-1 and 5 s^-1,respectively.The flow instability region was characterized by adiabatic shear bands,flow localization and cracks.The microstructure analysis revealed that the DRX grains were twinned,formed through the bulging mechanism of nucleation at the serrated grain boundaries accompanied by twin boundary migration.The twin boundaries contributed significantly to the growth of the DRX.The ideal processing parameters were found to be at 1100℃/0.1 s^-1 with the power dissipation efficiency reaching 32%,which gave a fine and uniform microstructure.The average hardness value was estimated to be extended from 142.16 to 208.48 HV1.The above results contributed to provide a deep understanding of the mechanism of cold and hot deformation processing.Recrystallization is the mechanism responsible for the refinement of the microstructure and the enhancement of the mechanical properties of the as-cast high Mn TWIP steel.