The Simulation On Weld Microstructure Of High Strength Steel And The Study On Its Impact Properties

High strength steel, ultrahigh-strength steel is excellent automotive steel meeting social need of high service performance, high security and low emissions. However, with a lot of promotion of strength and plasticity(product of tensile strength and elongation), connection problems among steels become more and more, the resistance spot welding is one of the most traditional and the most widely used technique, heat affected zone is the main venerable area of cracks creating and extension, the quality of heat affected zone will decide the security of auto joins. For better research the relationship of heat affected zone microstructure and properties of the resistance spot welding, simulated the heat affected zone by different heat treatment methods in this paper, compared to the heat affected zone microstructure at constant setting condition using thermal mechanical simulator, heat treatment can achieve partial enlargement of heat affected zone, and study the uniform single microstructure and mechanical properties.The research material is WHT1300 HF hot forming of automotive steel provided by Wuhan Iron & steel Group. The characteristics of simulated organization of heat affection zone by different heat treatment have been investigated by using the optical microscopy(OM), scanning electron microscopy(SEM), Vickers indenter(HV-1000) combined with Salt spray corrosion resistance test, impact test, through the research the following results are obtained in the paper:(1)Through JMatPro software simulated phase transformation point and phase transformation process of plate under the ideal state, figured up the A3、A1、Ms and Mf transformation points and temperature is 809℃、707℃、403℃、293℃ respectively.(2)Microstructure contains martensite, retained austenite as well as ferrite at Q&P treatment, with increasing of quenching temperature and partitioning time, hardness trend falling, microhardness is very close at 250℃ and 300℃ quenching temperature for 60 s. The microstructure consists of martensite, ferrite, dispersive fine bainite and carbide particles along grain boundary with austenitizing 10 minutes at 850℃ then quenching and tempering 1 hour at 350℃ and 650℃, martensite begins to decompose at 650℃, and carbide particles increase, microhardness(202) is no different with matrix structure.(3)Simulated organization appeared rust, rust layers falling off, air bubble and color change in neutral salt fog environment, by unit weight loss and corrosion rate calculation, welding process to a certain extent, have reduced the salt spray corrosion resistance of material, the structure austenitizing 10 mins at 850℃, then quenching to 300℃ for 90 s, has the best density of the corrosion layer and corrosion resistance, salt spray corrosion resistance performance of specimen at 350℃ tempering 1 hour is better than specimen at 650℃ tempering 1 hour.(4)Impact test to a large extent inspect the mechanical properties of welding spot, impact properties of sample by quenching and tempering is weaker than the sample by Q&P process. The results show it has the best impact performance at 300℃ hold 120 s, at the limited experimental data, low quenching temperature or short time of carbon distribution in Q&P process all make the impact properties of sample at low level.