Study On Tensile And Fatigue Behavior Of TRIP800 Steel Plate And Its Resistance Spot Welding Joint

TRIP (Transformation Induced Plasticity) steel has been paied more attention as a newly developed automotive steel, due to good combination of strength and toughness. However, it has not been really applied in automobile industry because of insufficient research on the fatigue behavior and weldability of TRIP steel. Therefore, in this thesis, by using OM (Optical Microscope), SEM (Scanning Electron Microscope), TEM (Transmission Electron Microscope), microhardness tester and XRD (X-Ray Diffraction), the microstructure as well as quality, morphology and distribution of TRIP800 steel sheet and its spot welded joint are investigated. The TRIP effect and deformation mechanisms during tension and fatigue are discussed.The result of OM microstructure observation indicates that TRIP800 steel consists of ferrite, bainite and retained austenite, with the corresponding volume fraction of 60.27%,29.18% and 10.58%, respectively. The experimental results of tensile test show that the TRIP800 steel sheet has good strength and toughness including YS (Yield Strength) of 573.69MPa, TS (Tensile Strength) of 805.32MPa and EL (Elongation) of 25.70%, which is attributed to the TRIP effect of retained austenite during tensile process. It is shown from XRD observation that the volume fraction of retained austenite around tensile fracture region is lower than that before tension. The closer to the tensile fracture surface, the less volume fraction of retained austenite be. Fracture morphology of the TRIP steel is a fine dimple pattern and fracture surface. The ferrite grains are deformed along the tensile direction by observation under SEM. TEM observation shows the existence of film retained austenite in bainite.The constant stress amplitude fatigue test demonstrates that the fatigue strength of TRIP800 steel sheet is 560MPa, which is close to yield stress. The high fatigue resistance is a result of increase of both ductility and strength initiated by TRIP effect, and inhibition of fatigue crack propagation by retained austenite. The constant strain amplitude fatigue test shows the fatigue strength remains almost unchanged first with the increasing cycles when the strain amplitude value is constant, then decreases dramatically and finally rupture. The fatigue cycles increase when the strain amplitude value decreases. It indicates from the results of XRD that the volume fraction of retained austenite increases with increasing distance from the fractured surface. However, the volume fraction of retained austenite reduces at the same position of fractured surface for different samples when the strain amplitude value decreases. SEM observation indicates that fatigue cracks initiate at the surface of sample. Fatigue striations and a few tyre-like patterns are observed in the crack propagation region. Also, the fatigue fracture surface has a quasi-cleavage feature.TRIP800 steel sheet weldment after resistance spot welding includes fusion zone, heat affected zone and base metal. The fusion zone mainly contains coarse martensites. The heat affected zone consists of ferrite, martensite and tempered sorbite. And there is no significant microstructure variation in base metal. The microhardness test shows that the average hardness of base metal is 270HV. The hardness tends to grow from base metal to heat affected zone, reaches the maximum hardness of 528HV in the region of heat affected zone with fine lath martensites, and decreases to 470HV remaining constant in the fusion zone. The fatigue strength of the TRIP800 weldment is approximately only 56MPa, down 90% from the TRIP800 steel sheet, which leads to an obvious fatigue property deterioration. The fractograph of fatigue fracture surface displays with a quasi-cleavage mode. It is clear that fatigue cracks initiate and propagate in HAZ, which are attributed to the significant microstructure change without TRIP effect of retained austenite in HAZ.