To investigate the micromechanical fracture behavior of high-strength steel, an integrated experimental and numerical study was conducted on Q460C steel and its welded joints, with specimens extracted from the base metal, weld metal, and the heat-affected zone (HAZ). Eighteen smooth round bars were tested under monotonic and cyclic loading to analyze mechanical performance and stress–strain curves. A constitutive model was developed based on the experimental results and numerical simulations. Additionally, eighteen notched round bars with three different notch sizes and three different zones were tested under monotonic loading, and thirty-six notched round bars with three different notch sizes, three different zones, and two loading protocols were tested under cyclic loading. The stress-modified critical strain model (SMCS) and void growth model (VGM) were calibrated and validated using the test results. The study reveals that the HAZ is more susceptible to cracking under cyclic loading. A positive correlation between toughness parameters and plasticity was discovered. The validated VGM and SMCS provide a reliable tool for predicting ductile fracture in Q460C steel and its welds, offering significant insights for the design and safety assessment of high-strength steel structures.