Research On The Plastic Deformation Behaviors Of Typical High Strength Steel Under Pre-strain And High Strain Rate

In order to achieve global energy saving and emission reduction,lightweight materials represented by aluminum alloys,magnesium alloys,new composite materials and advanced high-strength steels have become more and more widely used in the field of automobile manufacturing.The advanced high-strength steel has high comprehensive mechanical properties and is an excellent structural member material.It can effectively realize the lightweight of the vehicle while ensuring the safety of the vehicle body.However,the forming range is generally narrow under room temperature conditions,and the forming performance is not good.Due to its high yield and high tensile strength,advanced high-strength steel will bring many problems such as high residual stress of stamping parts,large rebound of parts,difficulty in forming complex parts,and so on.The processing cost of forming high-strength steel has increased,which has become a bottleneck for its widespread application.How to improve the forming performance of advanced high-strength steel is one of the hot issues that need to be solved in the current car lightweighting process.Developing new technology to improve the forming performance of advanced highstrength steel is an effective way to expand its application range on automobile bodies.It is found that high strain rate composite forming process can effectively improve the forming properties of high strength steel sheets at room temperature,but the strain path and strain rate change during the composite forming process.The influence mechanism of the high strength steel forming process is still unclear.In this paper,two kinds of advanced high-strength steel DP590 and B280 VK sheets are taken as the research objects.Through the combination of experiment and simulation,the plastic deformation behavior of typical advanced high-strength steel under pre-strain high strain rate is studied systematically.Quasi-static tensile test,biaxial tensile test,Hopkinson tensile bar test,electromagnetic forming experiment and composite electromagnetic forming experiment under room temperature conditions,combined with ANSYS LS-Dyna nonlinear finite element analysis software electromagnetic field simulation calculation,test analysis A series of experimental and simulation results were obtained for the deformation behavior of two advanced high-strength steels under different pre-strain and strain rates.The quasi-static uniaxial tensile tests of the sheets at room temperature were designed and carried out,and the basic mechanical properties were obtained.The methods for measuring the strain of metal sheet by electrochemical corrosion grid printing on the surface of the metal sheet and the GMASystem grid strain measurement analysis system were described.Uniaxial tensile specimens with different uniaxial prestrain were obtained by a universal materials testing machine.Combined with ANSYS 15.0 LS-Dyna finite element simulation analysis,the optimized biaxial tensile specimen size was obtained.The shape and size of the single-arm 9 parallel-slots specimen was determined,and the biaxial tensile experiment was carried out by a biaxial tensile testing machine.The stress-strain curves of advanced high-strength steel DP590 and B280 VK sheets for biaxial tensile were obtained.The Hopkinson tensile bar experiments for advanced high-strength steel DP590 and B280 VK were designed.Several kinds of specimens with different shape and size were analyzed and compared,and the method of glue bonding was adopted to complete the dynamic tensile test.The stress-strain curves of advanced high-strength steel DP590 and B280 VK under pre-strain and high strain rate were obtained.It is found that advanced high-strength steel DP590 and B280 VK are both sensitive to strain rate,and the flow stress increases with the increase of strain rate.The Johnson-Cook constitutive models with different pre-strain were established.In the dynamic mechanical properties of the steels,as the pre-strain increases,the value of the material hardening index n increases,while the material strain rate sensitivity coefficient C decreases.Work hardening and the increased strain rates of the material can result in an increase for the material flow stress.The molds and tooling required for the electromagnetic forming limit test of advanced high-strength steel DP590 and B280 VK sheets were designed.And the specific method of electromagnetic forming experiment for advanced high-strength steel was introduced.The forming limit diagrams of advanced high strength steel DP590 and B280 VK under different process conditions were established.The forming limit of steel sheets were improved by electromagnetic forming compared with quasistatic condition.And advanced high-strength steel DP590 and B280 VK shown strain rate sensitivity.The composite electromagnetic forming process can improve the forming limit of steel sheets,and the advanced high-strength steels were still sensitive to strain rate with pre-strain.With the increase of uniaxial tensile pre-strain,the forming limit of composite electromagnetic forming conditions first increases and then decreases.And the composite electromagnetic forming limit of high strength steel is improved in the range of 0-2.5% in the biaxial tensile pre-strain.It was found that the high strain rate process and the composite forming process could refine the grain size of high strength steel,make the material deformation more uniform,and improve the material formability through the microscopic metallography analysis.The electromagnetic forming process for advanced high-strength steel DP590 and B280 VK sheets were carried out by the ANSYS 15.0 and HyperWorks 2017 software simulation analysis with the Johnson-Cook material constitutive model established by Hopkinson tensile bar test based on the flat spiral coil and driver piece.The current density of the flat spiral coil,the current density of the T2 Cu driver piece,the Lorentz force vector,the velocity vector distribution on the T2 Cu driver piece were obtained.During the electromagnetic forming process,the force generated by the flat spiral coil is not uniform,and the Lorentz force is the largest at 1/2 center of the driver piece,and gradually decreases in the radial direction.Comparing the deformation of the simulated specimen with the experimental specimen,it was found that the simulation results were in good agreement with the experimental results,then the accuracy of the established Johnson-Cook material constitutive model was verified.The bulging height of high strength steel increases with the increase of discharge voltage by electromagnetic forming.The increase of uniaxial pre-strain in a certain range can improve the formability of high strength steel,but when the pre-strain increases to a certain value,there will be a turning point.The biaxial pre-strain can be used to improve the forming properties of high strength steel in the range of 0-2.5%.