Controlling microalloy interactions on precipitation, hot ductility, and microstructure -Mechanical property relationships /by Madhuri Varadarajan.
"One of the main problems faced in the continuous casting of micro-alloy steels is the formation of transverse cracks. Transverse cracks are surface, or near-surface cracks formed perpendicular to the casting direction. The research focuses on using laboratory hot tensile tests methods to determine the low ductility ranges in high strength steel grades with different micro-alloy additions of titanium, niobium, and vanadium. The hot ductility of commercially produced as-cast slab and beam blank samples was evaluated using two experimental methods: tensile testing utilizing a servo-hydraulic load frame with a resistance furnace and thermomechanical testing using rapid Joule heating. The tests were performed at a 3 x 10⁻³/s strain rate in a temperature window of 650℃ - 950℃ to mimic industrial unbending temperatures during the continuous casting. A ductility trough with a minimum percentage reduction in area (% RA) was observed closer to the Ar₃ transformation temperature of the alloys. The ductility drop at this temperature is likely related to the formation of a thin layer of ferrite film along the austenite grain boundaries resulting in minimum ductility and intergranular failure. Both test methods showed similar low ductility trends, but the upper and lower edges of the ductility trough temperature range differed between the two test methods. The differences are attributed to the heating and cooling rates of the two test methods. Future studies are required to perform in-situ-based deformation tests with the aim of directly observing transverse crack formation during solidification and cooling"--Abstract, page iv.