Study On Microstructure, Properties, And Resistance Spot Welding Of A Hot Stamping Steel

In order to ensure energy conservation, environmental protection and security, lightweight of automobile has become the research direction to the new generation of 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 affect the stability of shape and dimension of formed parts. 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. Currently, physical metallurgy mechanism of resistance spot welding of hot stamping steel is still unclear. Therefore, this paper intends to study the microstructure and properties of hot stamping steel, and process, weldability, and physical metallurgy mechanism of resistance spot welding of hot stamping steel.In this paper, the initial microstructure and properties, phase transformation behavior and the incubation period of ferrite transformation of 22MnB hot stamping steel were studied. This thesis also focused on influences of heating temperature, holding time and die temperature on microstructure and properties of hot stamping steel. By simulating the process of hot stamping in semi-industrial production line, a hot stamping steel with ultimate tensile strength of 1590MPa was developed successfully. 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) Through study on the annealing process, it is found that annealing temperature has a greater effect on recovery and recrystallization of cold-rolled hot stamping steel. When the annealing temperature is 600 ?, the cold-rolled hot stamping steel is experiencing the transition from recovery to recrystallization.(2) The static continuous cooling transformation and isothermal transformation of hot stamping steel are studied. The critical cooling rate of whole martensite transformation to the hot stamping steel 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, the time interval between sheet out of furnace and being hot-formed should be as short as possible.(3) The effects of parameters of hot stamping process on microstructure and properties of hot stamping steel are investigated. When the heating temperature in practical hot forming process is 925?, the microstructure of hot stamping steel is martensite. Holding time should not be too long, or it will easily lead to coarsening, increasing production cycles and reducing production efficiency. In order to guarantee the mechanical properties of hot stamping steel, the die temperature must be low.(4) The hot stamping steel has good weldability, and the nugget diameter and tensile shear strength both meet production requirements. When the welds are welded with optimal process parameters, the failure mode of welding joints is nugget pullout. The whole welding joint is divided into base metal, heat affected zone and nugget zone, and there is a softening zone in HAZ, which is a narrow annular region. The softening zone's hardness is obviously lower than others, and it contains ferrite, which both reduce strength and hardness. The existence of softening zone is the main reason to the reduction of cross-tensile strength, and it can be improved by taking definite measures.