The effect of microalloy precipitates on final product mechanical properties

Niobium, titanium and vanadium are added to high strength low alloy (HSLA) steels to provide grain boundary pinning, and strengthening, precipitates to help produce the desired fine-grained microstructure. Niobium and titanium segregate strongly between the liquid and solid phase during steel solidification, with vanadium and aluminium showing much more limited segregation behaviour. Hence, macro- and micro-segregation of mobium and titanium occurs during thick slab casting resulting in a spatial distribution of precipitates and solute element. This precipitate spatial distribution has significant effects on grain size development during processing (slab solidification, reheating and rolling) and can result in a bimodal grain size distribution (abnormally large grains in a matrix of smaller grains). This paper describes, for HSLA steels containing niobium, levels between approximately 0.02 and 0.06 wt%, grain size development during slab solidification, reheating and rolling with particular emphasis on the effect of micro-segregation. The reheat conditions leading to a significantly bimodal grain size distribution are included. The effect of the microalloying precipitates, either directly or via their influence on grain size distributions, on the mechanical properties (strength and toughness) is also discussed.