Research On The Dynamic Deformation Behavior Of DP780 High Strength Steel

The dual phase (DP) steel is a kind of high strength steel which is produced by heat treatment processing in some critical region or special rolling stage of low carbon steel or low alloy steel. The microstructure of DP steel consists of two phases. One is ferrite, a kind of soft phase, which can provide good formability; the other is martensite, a kind of hard phase, which can improve the strength. Due to coexistence of the two phases, the high strength steel has great advantages in the strength, ductility and work hardening capability. The high strength and strain rate dependent hardening properties make DP steel have good performance in large deformation. The research on the dynamic deformation theory and application of DP steel has drawn more and more attention in auto industry.DP780 is a typical one of dual phase steel, with an outstanding yield and tensile strength, and an obvious strain rate dependent capability. This thesis focuses on the research of the dynamic deformation behavior of DP780 steel based on the static and dynamic uni-axial tensile experiments, and then a material constitutive equation and corresponding user material subroutine are developed and applied in numerical simulation.In this thesis, at first the standard static uni-axial tensile experiment and the dynamic uni-axial tensile experiment under four different tensile speeds (0.1m/s,2m/s,10m/s and 15m/s) are carried out, the stress-strain curves under various strain rates are obtained. Combining the analysis on the microstructure near to the fracture location, the basic deformation mechanism is investigated. Then the Johnson-Cook constitutive model is used to describe the deformation behavior. After study on the influence of strain rate, a new constitutive equation based on description of the impact velocity is developed, and it could perfectly predict the evolution trend of strain rate. Consequently, based on the commercial finite element software ABAQUS and FORTRAN language, a user material subroutine is developed and embedded into the ABAQUS. The validation of the user material subroutine is verified by simulation of the benchmark problems. Finally the crush process of a square column is simulated and the influence of the strain rate sensitive and isotropic hardeding coefficients on the simulation process is investigated. The results show that the high strength and the special rate hardening properties make the DP780 steel a very good choice for the auto industry. By applying the new constitutive model based on the impacting velocity into the user material subroutines, a perfectly matched result is obtained, comparing to the experiments. The new constitutive model will contribute to the improvement in the accuracy of autobody crash simulation.