Research On Hot Gas Forming Technology For Ultra-High Strength Thin-Walled Steel Tube

Lightweighting is an important way to save energy and reduce emissions in the automotive industry.The main problems faced by automotive lightweighting are cost and safety,the use of high-strength steel can solve these problems well.In order to solve the forming problems of ultra-high strength steel tubes,a new technology was explored in this paper,which was named hot gas forming technology for ultra-high strength thinwalled steel tube.The principle of this technology is that the thin-walled tube billets of ultra-high strength steel are heated to fully austenitized temperature range(850～1000℃),then the high pressure nitrogen is used as bulging medium to make the high temperature steel tubes formed by hot gas forming.At the same time,the forming parts are quenched rapidly in the die.Finally,the strength of more than 1500MPa is obtained.The technology can not only reduce the weight of automobiles,but also enhance the safety of drivers.The main contents and results are as follows:(1)In this paper,an experimental equipment for thin-walled tube was developed,which was suitable for the hot gas forming for ultra-high strength steel.A high-pressure rapid intake system was designed to solve the problem of the inflation rate in the hot gas forming.A triple sealed structure was designed to solve the sealing problem.And a cooling system with two cooling modes(high pressure nitrogen cooling and mold contact cooling)was designed to ensure the realization of Quenching function.(2)The main failure forms of 22MnB5 welded tubes were analyzed and the results show that there are three kinds of failure modes:wrinkle,rupture and insufficiently bulging.The influence of heating temperature,insulation time,bulging pressure and other process parameters on hot gas forming for 22MnB5 welded tubes were also investigated.(3)The original microstructure of tube is ferrite and pearlite,and the microstructure of the main forming area transforms into martensite structure after forming.The martensite of the weld area is finer than the martensite of the other area,and the Vickers hardness is also larger.(4)The thickness of the weld center in the main forming area of the bulging part is higher than the circumferential average wall thickness and the axial thickness of the weld region is more stable.The fracture position of 22MnB5 welded pipe appears at the junction of the weld zone and the base area,the fracture is ductile fracture.