Influence of Aluminium on Castability of V-Microalloyed Steels

Abstract

The influence of aluminium content on the hot ductility behaviour in V-N steels was investigated. Cylindrical specimens were subjected to thermal cycles and strain rates approximating those experienced by a conventional slab surface during continuous casting. The resulting microstructures were examined using light optical and electron microscopy and correlated with measured reduction-in-area (RA) values, calculated precipitate chemistries and volume fractions, as well as the flow stress behaviour. It was found that removal of aluminium significantly improved the hot ductility. However, increasing the total [V][N] product in Al-free steels reduces RA. Poor hot ductility is caused by low austenite grain boundary mobility characterized by high work hardening rates. The fracture mode in brittle specimens is intergranular along thin ferrite films. AlN appears to inhibit austenite grain boundary mobility in V-high N steels when the cooling rate and strain rate are both very slow as experienced during unbending. SEM analysis of fracture surfaces revealed the presence of MnS-AlN particles in microvoids. TEM-EDAX spectra showed that the larger particles observed in the Al-containing steels are mostly a constitution of duplex/triplex grain boundary precipitates, i.e., MnS-AlN and V(C,N). Conversely, good ductility in austenite is associated with high grain boundary mobility that produces fine, recrystallised grains and subsequent dimple fracture after plastic tensile stress.