Mill logs obtained from the Hylsa CSp (TM) (thin slab casting/direct rolling - TSC/DR) mill were examined so that the mean flow stresses at each pass were calculated using the Sims equation modified to take into account the forward slip ratio, the redundant strain and the work roll flattening. The mean flow stresses were then compared to predicted values obtained from a model. The microstructures during the CSP process were predicted by a mathematical model which was initially derived for conventional slab/roughing mill/hot strip mill (HSM) processing route. The adapted model takes into account the deformation of the as-cast structure in the finishing CSP mill, by using particular microstructural equations to calculate the softening kinetics and grain sizes. The main metallurgical features such as the occurrence of Nb(C,N) precipitation, the softening mechanism which takes place (static or metadynamic recrystallization) as well as the strain accumulation between passes were calculated. The mean-flow-stress results obtained from the calculations are in good agreement with the mill data. The present analysis indicates that it is possible to produce fine-grained microalloyed steels with homogeneous microstructure in thin slab casting/direct rolling processing.