Abstract
<title>Abstract</title> <p>To understand the fundamental fracture mechanisms of advanced high-strength steels during complex loading, this study investigates the intrinsic synergistic effects of grain size and multi-step stress states. TRIP800 steel specimens with controlled average grain sizes (0.9 to 25.3 µm) were prepared to isolate grain size as an independent experimental variable. Systematic characterizations, including micrographic observation, micro-hardness mapping, and hole expansion tests, were conducted to decouple the influence of microstructural features from macroscopic process parameters. The results demonstrate that grain coarsening significantly increases the burnish zone proportion (from 9.9% to 61.1%) and reduces damage zone width. Further mechanism analysis reveals that the "delayed strain hardening" characteristic of coarser grains works synergistically with the compressive-shear stress state induced by multi-step blanking to postpone void nucleation and fracture initiation. Specifically, the redistribution of stress triaxiality paths effectively suppresses damage accumulation. This work provides a theoretical framework for understanding the microstructural dependence of sheared edge failure, offering essential scientific data for the future development of localized property modification technologies in high-strength materials.</p>