Investigation On Deformation Behavior And Microstructure Evolution Mechanism Of TRIP Seamless Steel Tube In Hydroforming Process

As the development of automobile industry and the increase of vehicle population,how to effectively reduce the energy consumption of automobile and vehicle exhaust caused an extensive concern of the whole society.The lightweight technology is a significant measure to reduce emission and energy conservation,which mainly consists of material lightweight and structural lightweight.In the field of material lightweight,with the development of material technology,TRIP steeld have been widely applied in the field of auto parts manufacturing in recent years because of their high strength and high plasticity.In the field of structural lightweight,it is a very effective method of using the hollow parts instead of the solid ones.Therefore,tube hydroforming technology which is an advanced manufacturing technology as characterized by lightweight and integration has been paid more and more attention in automotive design field.Based on this background,this research combines material lightweight technology and structure lightweight technology.The TRIP steel is introduced into hydroforming manufacture field.The axial-compression bulge tube and T-shape tube were successfully manufactured.Macro deformation behavior and microstructure evolution of different sampling positions in these tubes during the hydroforming process were also studied systematically.The main contents are as follows.(1)Using varieties of metallographic observation methods,the microstructure and volume fraction of retained austenite in different sampling positions of the experimental tube were analyzed.TEM was also used to observe the microscopic morphology of TRIP steel tubes in axial-compression hydroforming.(2)The cracking mechanism in hydraulic bulging experiments with different parameters were studied.The failure mode of the TRIP steel tube in hydroforming process was obtained.(3)Based on the plastic mechanic knowledge,the change of stress state of the tube in hydroforming process were analyzed using the analytical method.The stress and strain distribution was also analyzed at different parts of the tube during the hydroforming process.(4)The stress and strain distribution in T-forming process was studied with FEM simulation.By analyzing the macro deformation behavior and microstructure of T-shape TRIP steel tube combined with the numerical simulation results,the relationship between the transformation rate of retained austenite and strain was studied.