Thin slab rolling of Nb microalloyed steels belongs to a new generation of thermomechanical treatments, which shows some singularities in comparison to tradictional processes. This paper analyses the influence of some rolling parameters and Nb content in the austenite evolution during rolling in order to obtain the best austenite conditioning prior to transformation. The results obtained, with the help of a previoulsy developed model, indicate that Nb solute drag effect is a very significat mechanism acting upon the austenite grain size evolution during the overall rolling process.