The microstructure and corresponding tensile properties of both plain and Nb-microalloyed grades of ultrathin cast strip (UCS) low alloy steel produced using the CASTRIP�� process were studied. Both as-cast and hot-rolled strip cast steels with various levels of Nb microalloying were manufactured and investigated in this study. Hot rolling had little effect on the yield strength of Nb microalloyed UCS specimens for a given chemical composition, but resulted in a slightly finer microstructure. The effect of Nb microalloying was significant, this is attributable to the promotion of finer, tougher austenite transformation products such as bainite and acicular ferrite at the expense of large polygonal ferrite grains. A fine dispersion of Nb solute clusters was observed in all Nb-containing steels following hot rolling, it is suggested that this also contributes to the observed strengthening. �� 2011 The Minerals, Metals & Materials Society and ASM International.