Investigation of Necking and Fracture Strain Limits of Medium-Mn Steel Sheet Under Hot Stamping Conditions

Medium-Mn (MMn) steels with low austenitisation temperatures have recently attracted much research attention because of their suitability for hot stamping to save cost and improve productivity. However, research on their formability under conditions that mimic industrial hot stamping is still lacking. In this study, an innovative method for biaxial testing, which includes a cruciform specimen design, is used on a Gleeble 3800 simulator to obtain the forming limit curves (FLCs) and fracture forming limit curves (FFLCs) of a representative MMn (8Mn) steel. The localised necking strain and fracture strain are measured using a newly developed spatio-temporal method. The FLCs and FFLCs are constructed using strain limits in different loading states, e.g. uniaxial, plane strain and equi-biaxial stretching. The results show that the loading state conditions are acceptably simulated using the biaxial test system, and the FLCs and FFLCs are well-constructed, displaying a V shape. The data will contribute to the design of forming process. In addition, the forming strain limits of the MMn steel are much higher than those of a conventional boron steel (22MnB5) under the same hot stamping conditions, suggesting that the MMn steel has great potential and benefit as an alternative in hot stamping.