Investigation of the critical factors controlling sheared edge stretching of ultra-high strength dual-phase steels

The present study aimed to explore dual-phase (DP) steels with a good combination of high strength and reasonable global ductility (i.e., total elongation and general stretch formability) and local ductility (i.e., sheared edge ductility or hole expansion ratio). Therefore, a series of ultra-high strength dual-phase steels were designed, melted, rolled, annealed and formed. These steels contained various aluminum additions and vanadium contents and were processed with different coiling temperatures and continuous galvanizing line (CGL) thermal path simulations conducted using a Gleeble 3800 system. The microstructures, tensile properties and hole expansion behaviors of all candidate DP steels were determined and compared. The microstructural and damage evolutions in the process of both hole punching and hole expansion were examined. The results indicated that hole expansion ratios of DP steels could be correlated well with (i) the burnished-to-fracture zone ratios in shear surfaces after hole punching, (ii) the values of reduction in area of tensile specimens after fracturing, and (iii) nanohardness difference between soft ferrite and hard constituents. The micro-voids and micro-cracks introduced by hole punching acted as crack initiation sites, which severely affected the subsequent hole expansion process. Therefore, better sheared edge ductility may benefit from microstructures that retard the crack propagation or void growth and coalescence during hole expansion.