To investigate the effect of welding speed on the out-of-plane deformation of Q420 low-alloy high-strength steel thin plates, this study employed a three-dimensional digital image correlation system to monitor the deformation dynamically during TIG welding and cooling. Unlike existing studies that mostly focus on post-weld residual deformation or a single welding stage, this study, under a fixed current of 36 A and arc voltage of 14 V, sets welding speeds ranging from 4.5 to 11.8 mm/s, and for the first time systematically reveals the complete evolution path of Q420 thin plate (2 mm) welding deformation, which includes “thermal expansion—instability mutation—elastic rebound—residual stabilization”. The results show that the welding speed is significantly negatively correlated with the out-of-plane deformation. Although low-speed welding has a high peak plastic strain, the final residual strain is almost completely released; while high-speed welding has a low peak strain but retains a relatively high residual strain. This abnormal phenomenon reveals the deep mechanism that the accumulation and release of plastic strain are asymmetrically regulated by the welding speed. These findings support process optimization for high-strength steel thin plates.

Q420 high-strength steel exhibits pronounced anisotropy due to its rolling process, and conventional uniaxial tensile testing is incapable of acquiring strain field evolution information during the local necking stage. In this study, quasi-static uniaxial tensile tests were conducted on Q420 cold-rolled high-strength steel sheets at six orientations (0°, 15°, 30°, 45°, 60°, and 90°) using Digital Image Correlation (DIC) technology. The evolution of the strain field and the corresponding stress–strain responses at different orientations were systematically investigated. The results show that the DIC technique effectively captured the full-field strain evolution of the specimens from uniform deformation to local necking and final fracture in all directions. Taking the 0° direction as an example, the local maximum engineering strain prior to fracture reached 35.866%, whereas the average fracture strain within the gauge section was only approximately 22.5%, corresponding to a ratio of approximately 1.6 and clearly demonstrating the severe strain concentration within the necking zone. The stress–strain curves corresponding to different rolling directions exhibited pronounced anisotropy. The tensile strength was highest in the 90° direction and lowest in the 0° direction; however, the 0° direction exhibited the best ductility, whereas the 45° direction showed the poorest ductility. Among the six orientations, the midpoint transverse engineering strain exhibited the largest absolute value in the 45° direction, further indicating that this orientation is the most susceptible to plastic instability. In this work, DIC-based full-field measurement was combined with multi-directional tensile testing to quantitatively characterize the relationship between local strain concentration and anisotropy. The findings provide high-precision experimental data for the calibration of anisotropic constitutive models and the optimization of forming processes.