Back-annealing of hot dip galvanised strip steel microalloyed with vanadium

Abstract

The steel market increasingly demands higher strength thinner products with reduced weight and decreased cost, in particular galvanised products for the roofing, automotive and solar industries, which are in high demand. A viable option to meet this demand is via back-annealing, also known as partial or recovery annealing, which consists of annealing cold-worked material at low temperatures. Back-annealing aims to only utilise the recovery process, i.e. to only restore a fraction of the elongation whilst retaining most of the strength obtained from cold-rolling. The targeted specification is EN 10346 S700GD, i.e. the desired properties are in the region of 800 MPa tensile strength and 10% total elongation. Therefore, the purpose of this study was to examine the viability of using V-N microalloying for high-strength recovery annealed steel. In this work, back-annealing (recovery) was applied to heavily cold-worked low C-microalloyed V-N steels in an attempt to achieve a yield strength of 700 MPa and a minimum A80 elongation of 10%. Laboratory simulations of annealing during hot dip galvanising (HDG) were employed to compare recovery and recrystallisation kinetics of V-N, Nb-Ti and plain low C steel. Based on these results, industrial slabs were subjected to conventional hot rolling and back-annealing cycles during HDG. Precipitation in both V-N and Nb-Ti steels extended the back-annealing window by about 50 °C compared to low C steel. Initial hardening due to precipitation during annealing was observed in V-N and Nb-Ti steels. A temperature-time parameter M was used to compare the results. The Mrs (recrystallisation start) in both V-N and Nb-Ti steels was 20.1 compared to 18 in plain low C-Mn. The EN 10346 S700GD specification was achieved by subjecting low C-microalloyed V-N steel to low finishing rolling temperatures and coiling temperatures followed by back-annealing of the heavily cold-worked strip at an M value to prevent full recrystallisation. The V-N steel showed improved stability during annealing when compared to Nb-Ti, due to the unstable nature of the NbC precipitation during the back-annealing process. In other words, the V-N steel was found to be friendlier in terms of microstructural control in order to achieve consistent mechanical properties.