Study On Macroscopic And Microscopic Dynamic Deformation Behavior Of TRIP780 High Strength Steel

TRIP steel is a new type steel, and is characterized by its Transform Induced Plasticity property. Usually, TRIP steel has a multi-phase micro structure under room temperature,which consists of ferrite(F), bainite(B) and retained austenite(Ar). The retained austenite is unstable, and will transform into martensite under certain plastic deformation conditions. The new-born martenite has a better hardening property comparing to the former austenite, which increases the deformation capability and contributes to plasticity property of the material.This thesis focused on mechanical properties of the TRIP780 steel produced by Baoshan Steel Company. Experiments were carried out based on both macroscopic and microscopic methods. The dynamic mechanical properties of TRIP780 steel were studied through the study on tensile tests, metallographic microstructure observation, and XRD measurement. A strain rate dependent material constitutive equation was developed and introduced into the numerical simulation of dynamic deformation during impact processes of high strength steel components. Firstly, the static uni-axial tensile experiment and the dynamic uni-axial tensile experiment under four different tensile speeds (0.15m/s,4m/s,10m/s and 15m/s) were carried out, then the stress-strain curves under various strain rates were obtained. Through the analysis on the microstructure near to the fracture surface, the Transform Induced Plasticity effect was investigated. Then, through the XRD experiment, the major factors which have impacts on the transformation of retained austenite were discussed. Johnson-Cook model was employed to describe the rate-dependent deformation behavior of the TRIP steel. Based on the macroscopic and microscopic research of dynamic deformation of TRIP780, a strain rate dependent material constitutive model was developed. The constitutive model was verified by simulation of the uni-axial tensile experiment. Finally, the crash process of a column and B-pillar made of high strength steel were carried out by introducing the developed constitutive model into ABAQUS. The mechanical behavior of TRIP steel during the crash processes was studied and compared with other two steels, especially the capability of crash energy absorbtion.The research results show that, TRIP steel exhibits both high strength and high plasticity deformation properties. The rate-dependent hardening property and energy absorbtion potential enable TRIP steel a good choice in vehicle manufacturing. The developed strain rate dependent constitutive model can predict the dynamic deformation of the TRIP steel well.