Influence of processing factors on the sheared-edge formability of vanadium bearing dual-phase steels produced using continuous galvanizing line simulations

Dual-phase steels are commercially attractive since they offer good formability at a high strength and reasonable cost. In addition to high strength, these steels also exhibit high "n-values" and high uniform and total tensile elongations. However, one limitation in the application of high-strength dual-phase steels is the fairly limited ductility observed during sheared-edge stretching or punched hole expansion. Therefore, successful high-strength dual-phase steels must have controlled microstructures in order to achieve the desired performance. The failure mechanism observed during stretching of the sheared edge is closely related to the microstructures (i.e., amount, hardness, size and distribution of the different phases and microconstituents), and the cracks were found to initiate predominately at the interphase interfaces separating the ferrite and the martensite. In the present research, the influence of three main variables on the hole expansion ratio (HER) behavior was explored: the metallurgical condition going into the continuous galvanizing line (CGL), different post-anneal CG line thermal processing paths and the influence of vanadium. Depending on conditions, all three variables can influence the sheared-edge ductility as determined in the hole expansion test. Suggestions for improving the sheared-edge ductility while maintaining adequate strength are presented.