This study investigates the effect of N diffusion on a Nb-microalloyed steel made by twin roll casting at 525�� C in a KNO3 salt bath. Nitriding up to 4 h increases the yield strength of the steel by ���50% with only a small drop in ductility, while 6 hours of nitriding causes brittle fracture. The improved mechanical performance after 4 hours of nitriding is thought to be a combined effect of solid solution strengthening of N diffusion and dispersion strengthening from extremely fine Nb-rich precipitates. Coarse features along grain boundaries consistently observed in steel nitrided for 6 hours are considered to be responsible for brittle fracture in samples nitrided for longer. �� (2010) Trans Tech Publications.