Investigation of effects of processing on stretch-flangeability of the ultra-high strength, vanadium-bearing dual-phase steels

Abstract

The purpose of this paper was to gain a better insight into the sheared-edge ductility of high strength dual-phase steels for the purpose of helping improve the hole expansion behavior of such steels. A candidate dual-phase (DP) steel in this study was designed and processed with distinct coiling temperatures after conventional finish rolling, and then further processed with two continuous galvanizing line (CGL) simulations. Two CGL simulations with slightly different thermal paths were conducted on a Gleeble 3800 machine; one was to replicate standard galvanizing (GI), where the intercritically annealed steels were cooled to the zinc pot temperature, and the other one was a supercooling process (SC) where the intercritically annealed steels were first cooled to the M90 temperature of the intercritically formed austenite, then up-quenched to the zinc pot temperature. The findings indicated that the combination of a low coiling temperature and GI anneal can obtain the microstructures characterized by a high-volume-percentage of martensite and fine-grained ferrite with a high tensile strength (UTS) of 1092.8 MPa and a good total elongation (TE) of 20.8%. A large amount of fresh martensite was replaced by tempered martensite caused by the changes in CGL thermal paths from GI to SC anneals, resulting in the considerable increase in sheared-edge ductility properties, although with a loss of UTS. Additionally, the damages caused by hole punching were examined by electron backscattered diffraction (EBSD) - kernel average misorientation (KAM) and nanoindentation technologies. These results showed that the micro-voids or micro-cracks in the initial punched hole surfaces and plastic internal strains near the initial punched hole sheared edges introduced by hole punching will severely influence stretch-flangeability.