The hot direct rolling of thin slab cast microalloyed, low-carbon steels was simulated on a laboratory scale. The carbon content of the steels ranged from 0.026 to 0.11 wt%. In steels microalloyed with niobium, a significant loss of yield and tensile strength occurred when the carbon content exceeded approximately 0.07 wt%. Large, eutectic NbCN precipitates, which formed during solidification, were observed at higher carbon contents. It was concluded that when the carbon content of the steel exceeded a critical level, segregation of niobium to the liquid phase during solidification of the castings resulted in the presence of the eutectic NbCN. This effectively removed a significant fraction of the niobium from being able to participate in precipitation strengthening. The addition of up to 0.09 wt% V, and of V plus up to 0.025 wt% Ti, to steel containing 0.04 wt% Nb failed to increase yield or tensile strength.