In the thermo-mechanical processing of niobium microalloyed steels, the strain induced precipitation of Nb(C,N) in austenite impedes the mobility of austenite grain boundaries during recrystallization, leading to the well-known pancaked austenite structure, which is critical to the achievement of the fine ferrite grain size. However, retention of some Nb in solid solution in austenite upon completion rolling in desirable. Solute niobium suppresses the austenite to ferrite transformation, prompting the formation of a bainitic or acicular microstructure, contributing to transformation hardening. As well, solute Nb is available to precipitate in the ferrite/bainite during and subsequent to transformation and thus offers a substantial precipitation strengthening potential. Both factors enhance the strength of steel. A detailed study has been conducted using dilatometry to establish the effect of soluble Nb on austenite transformation kinetics. Transformation hardening as indicated by the hardness of the specimen and the microstructure is found to correlate well with the magnitude of undercooling with the transformation. While solute niobium and cooling rate increase the undercooling and therefore promote transformation hardening, deformation prior to transformation decreases the undercooling as solute niobium is depleted through precipitation. However, strengthening by precipitation of NbC partly compensates for the loss of transformation hardening through depletion of Nb and C from the matrix during precipitate growth. The effect of solute niobium on transformation hardening is evaluated from laboratory hot rolling trials and the results are used to rationalize the mechanical properties of linepipe alloys (0.04C, 1.60 Mn, 0.07 Nb, and 0.015 Ti) processed in the laboratory and actual mill production runs.