In this paper, a medium-frequency inverter spot welder was used for resistance spot-welding experiments on 980 MPa grade cold-rolled δ-TRIP(Transformation-induced Plasticity) steel. The effects of the tempering process on the morphology, microstructure, element distribution, and properties of spot-welded joints were studied by Scanning Electron Microscope (SEM), Transmission Electron Microscopy (TEM), and Electron-Probe MicroAnalysis (EPMA). The microstructure of the nugget zone obtained by single-pulse process was δ ferrite, lath martensite, and twin martensite. After adding tempering under the single-pulse process, the microstructure was δ ferrite and lath martensite. However, the morphology of the microstructure was still dendritic, which remained unchanged. The tensile shear failure of spot-welded joints under the two processes was an interface failure, and the fractures were cleavage fractures. After adding tempering, the interface fracture surface presents two kinds of fracture characteristics: the outer cracks’ growth direction was consistent with the columnar crystal growth direction, and the inner crystal cracks occurred in the nugget core and finally grew along the columnar grain boundary. Due to the significant hardness difference between δ ferrite (283 HV) and martensite (533 HV), the low-strength δ ferrite phase was the main channel of crack propagation. After adding tempering, the hardness distribution of the spot-welded joints was more uniform and the tensile shear force increased (7.4 kN→8.5 kN).

Focused on the common problem of easy buckling of specimens in the tension-compression axial stress control fatigue test of automobile sheet steel, a stress fatigue test method for thickness less than 2.5 mm was proposed. For two kinds of automobile sheet materials with different strength levels and thicknesses, fatigue specimens with different widths of test section, parallel lengths, and fillet radii are designed. Without the anti-buckling device, high-cycle fatigue tests with a stress ratio Rs = −1 at different stress levels were carried out, and the shape and size of the fatigue specimens of automobile sheet materials were determined by the number of fatigue life cycles of specimens with different sizes under the same stress cycle form. The results show that for automobile sheet steel with a thickness of 1.0–2.5 mm, the recommended sizes of the uniform-gauge fatigue specimens are width of test section w = (1–3) t, parallel length Lp = (1–3) w, and fillet radius r = (3–10) t. For automobile sheet steel with a thickness less than 1 mm, because the time-displacement curve jitters during the test, it is recommended to install an anti-buckling device, specifically the anti-buckling device with mortise and tenon structure by self-design and with a flattop structure recommended by ISO 1099-2017, Metallic Materials—Fatigue Testing—Axial Force-Controlled Method.