Investigation of the Edge Crack Sensitivity of Cold Rolled Hot-Dip Galvanized DP780 Steels

In this work, the edge crack sensitivities of two cold rolled hot-dip galvanized DP780 steels with similar chemical compositions were investigated. Using a bulging test device and the GOM-ARAMIS system, the strain distribution contour and different points of strain evolution affected by edge crack sensitivity were obtained. Based on the strain localization theory and strain evolution characteristics, the edge crack sensitivity was evaluated according to the order of necking and edge cracking during the bulging process. Edge-milled and blanking punched specimens were prepared to verify the edge crack sensitivity. The edge crack sensitivity was proved to exist, and milled-edge specimens eliminated its influence. Although they had similar mechanical properties, two hot-dip galvanized DP780 steels presented different edge crack sensitivities. The #1 DP780 steel presented a clearer edge crack sensitivity than #2 steel. The microstructural influences on the materials’ mechanical properties and edge crack sensitivity were investigated and discussed. It was found that the added Nb+Ti elements in #2 steel improved the phase interface bonding strength and further lowered the edge crack sensitivity.