| With the advent of modern standards such as environmental protection,energy conservation,and safety,the automotive industry is beginning to move toward lightweight and inexpensive.This means that steel must have better formability,impact energy absorption,dent resistance and good weldability while effectively increasing the strength.As a dual-phase steel with a very high proportion of automotive body materials,achieving the above performance indicators has become the current mainstream research content.Therefore,this paper systematically studied the microstructure evolution,heat treatment process,forming properties and welding performance of DP780 dual-phase steel.The main work contents and research results of this paper are as follows:(1)The equilibrium phase diagram and the volume fraction of each phase with temperature of the tested steel were calculated by Thermo-calc.The phase transition point and the continuous cooling transformation behavior of the tested steel were measured by DIL805 A/D phase change instrument,and the static CCT curve of the tested steel was drawn.(2)The investigation of ferrite recrystallization process of tested steel at different heating rates found that the increase of heating rate promoted the recrystallization of ferrite,but there was a lag period,that is,the recrystallization process was delayed.At the same time,the interaction mechanism between ferrite recrystallization and austenite transformation at different heating rates shows that the nucleation and growth of austenite hinders the ferrite recrystallization process at lower heating rate,but there is still have enough driving force to complete recrystallization in a short time.At higher heating rate,the longer diffusion of C atoms is limited by time and the recrystallization of ferrite is suppressed more seriously.The carbide particles distributed in the grain boundary provide a more favorable position for austenite nucleation,the spacing between the two bands is smaller,and the band structure is heavier.The austenite forming kinetics further confirms the above viewpoint,and the uniformity of the structure is remarkably increased after the pre-recrystallization treatment.(3)The continuous annealing process of dual-phase steel was simulated on the multi-function continuous annealing testing machine in the laboratory.The effects of annealing temperature,slow cooling temperature,overaging temperature and overaging time on the microstructure and mechanical properties of the tested steel were investigated.The optimum annealing process parameters of the tested steel:annealing temperature is 790?,slow cooling temperature is 630?,overaging temperature is 300?,and the overaging time is 300s.The optimized mechanical property could be obtained at the optimum annealing parameters.The tensile strength,elongation,and product of strong and elongation reached 875 MPa,21.70%and 18988 MPa%,respectively.(4)The basic forming experiments were carried out on DP590 and DP780 respectively.The combination of cupping experiment and conical cup experiment showed that the DP780's cupping value and relative conical value were inferior to DP590 due to the higher strength level.From the hole expanding performance,it can be seen that the DP780's reaming rate decreases with the increase of control load and punch speed.(5)Experimental comparisons were made on the mechanical properties,microstructure,interface properties and failure modes of the spot welded joints from the aspects of welding current,welding time and electrode pressure.The final experimental results show that the optimum spot welding process parameters are welding current is 7.5kA,welding time is 25cyc,electrode pressure is 4kN,at this time the experimental steel has the best comprehensive performance,the nugget diameter is 7.01mm,and the tensile shear force is 25.74kN. |
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