Resistance Spot Welding Of A Press Hardened 22MnB5 Steel:Process And Properties

In order to ensure energy conservation, environmental protection and security, lightweight of automobile has become the research direction to the vehicles, and the development of high strength automobile steel has become an inevitable trend. However, the cold forming of high strength steel is very difficult, and it’s very easy to cause crack and springback, which affects the shape and dimensional stability of the molding member. Then hot stamping technology appears, it can effectively solve the contradiction between good formability and high strength. Currently speaking, resistance spot welding is the main welding technology used in the production of automobile. The final microstructure of hot stamping steel is whole martensite after forming and hardening. Compared with ordinary steel, the physical metallurgy mechanism of resistance spot welding of hot stamping steel is greatly different. There are many other problems during the welding process, such as shrinkage, incomplete penetration, softening of heat affected zone and so on, and microstructure and properties also have great effects on weldability of hot stamping material. Therefore, this paper intends to study the microstructure of hot stamping steel, weldability, and physical metallurgy mechanism of resistance spot welding of hot stamping steel.In this paper, the microstructure and properties, phase transformation behavior of 22MnB5 hot stamping steel were studied. On this basis, the weldability of hot stamping steel was discussed by adjusting welding process parameters and researching the mechanical properties, nugget diameter, hardness and microstructure of welding joints. The main research results are shown as follows:(1)The static continuous cooling transformation of hot stamping steel are studied. The fully austenitization temperature(Ac3) is 785 ℃. The critical cooling rate to obtain full martensite is 20℃/s. In order to ensure the final microstructure is martensite, the cooling and quenching rate must be ensured in practical hot stamping process. At the same time, to avoid the ferrite precipitation, Ensure performance hot stamping parts, the time interval between sheet out of furnace and being hot-formed should be as short as possible. The experimental materials for resistance spot welding in this research have been press hardened in a flat die with pressure in a hot stamping pilot line. Fully martensitic microstructure and 1380MPa ultimate tensile strength have been achieved in the press-hardened materials.(2) Acceptable welds properties have been achieved with optimized welding parameters:welding current:9.5～10.5kA, welding duration:20～35cyc and electrode force:8.0-9.5kN. In this conditions, all the failure mode are plug out. The welds have performed tensile shear strength of 20～26kN, cross tensile strength of 7.0-8.5kN(the ratio of CTS/TSS:-0.31).(3) The microstructure in both the heat affected zone and fusion zone are tempered martensite. The martensite in HAZ was heated up and got tempered due to the welding heat, whose hardness decreased to 435-440HV10 compared with 498HV10 in base metal and improved the toughness of welds. The martensite in fusion zone formed from fast solidification with hardness of 488HV10.