Density-functional theory has been used to study the segregation of iron at Sigma 3-(110) and Sigma 5-(001) twist grain boundaries. We find both grain boundaries bind interstitial iron by less than 0.4 eV, and modify the equilibrium spin state. Although interstitial Fe binds relatively weakly at fully bonded grain boundaries, it is more strongly trapped by vacancies at the grain boundary, and perhaps more critically, by Fe already trapped there. We conclude that precipitation of Fe at grain boundaries is energetically favourable, even in the absence of direct chemical bonding afforded by the substitution of Si by iron.