Hole expansion behavior of hot-rolled steel related to punching morphology considering banded microstructure

The hole expansion behavior serves as a key metric for assessing the formability of automotive structural components containing holes. With the growing emphasis on decarbonization in automotive manufacturing, hot-rolled steels have gained attention as promising substitutes for cold-rolled counterparts. Owing to their thermomechanical processing characteristics, hot-rolled steels frequently exhibit banded microstructures oriented along the rolling direction; however, the effect of these banded structures on hole expansion behavior remains insufficiently understood. In the present work, two hot-rolled steels with distinct banded morphologies were investigated under punching conditions in accordance with ISO 16630. The study focused on the evolution of microstructural features within the shear-affected zone (SAZ) and at the hole-edge surface before and after hole expansion. The findings indicate that the banded arrangement facilitates circumferential crack propagation along the hole edge, thereby improving hole expansion performance. This advantageous effect is particularly evident when the fracture zone constitutes a large fraction of the hole-edge surface, promoting stress relief through the generation of cross cracks. In contrast, when the fracture zone is limited, fracture initiation in the burr zone at the hole periphery leads to visible hole expansion cracks, contributing to the safe identification of failure during service. Additionally, the matrix hardness and strength were found to play a decisive role in governing hole expansion behavior, with a positive correlation established between edge hardening induced by punching and the hole expansion ratio.