Enhancing stretch-flangeability in high-strength hot-rolled automotive steel: the role of interface affected zone in optimized microstructure

Advanced high-strength steels (AHSS) used in automotive components demand excellent stretch-flangeability, quantified by the hole expansion ratio (HER). This study investigates the influence of microstructure—specifically ferrite + bainite, ferrite, and ferrite + pearlite—on HER at comparable strength levels in AHSS. The ferrite + bainite structure exhibits superior stretch-flangeability performance than the ferrite and ferrite + pearlite, attributed to the optimized ferrite + bainite structure which featuring a gradient strain layer characterized as interface affected zone (IAZ). This structure promotes stretch-flangeability by facilitating a smooth hardness transition between soft domain (ferrite) and hard domain (bainite). Such transition improves the uniformity of stress distribution. Furthermore, a low coiling temperature of 550 °C refines both the effective grain size and the size of nano-precipitates at grain boundaries, thereby further reducing stress concentration. These findings highlight the potential of introducing gradient distributed IAZ in optimizing AHSS stretch-flangeability for automotive applications.