New approach to hole-expansion ratio in complex phase and martensitic steels: Understanding the role of punching damage

Although advanced high-strength steel (AHSS) demonstrates excellent uniaxial tensile formability, its application in automotive parts is limited due to inferior flange formability. Despite extensive efforts to enhance the flange formability of AHSS, our understanding of it remains limited. In this study, we propose a novel approach to enhance the hole expansion ratio (HER), which is an indicator of flange formability. Firstly, we correct the existing misconception that the HER does not correlate with uniaxial tensile formability. It was verified through the artificial neural network technique that the uniaxial tensile formability has a strong correlation only with the HER in holes fabricated by electrical discharge machining. However, in the case of punched holes, minimizing the damage accumulated during punching is more important than enhancing uniaxial tensile formability. As an example, the MT steel used in this study showed a decrease in uniaxial tensile formability due to tempering embrittlement after heat treatment. However, this microstructural characteristic effectively reduced punching damage, resulting in a nearly twofold improvement in the HER of punched holes. Consequently, the approach to improving the HER should differ depending on the hole fabrication method. Specifically, to increase the HER of a punched hole, it is crucial to design the material with microstructural characteristics that minimize punching damage.