The microstructural evolution during continuous cooling has been investigated in an ultralow-C and high-Nb containing steel and compared to that of a traditional Nb-Mo pipeline steel. The deformation promotes the formation of fine-grained quasi-polygonal and acicular ferrite in coarse grain sized austenite. Lowering the austenite grain size leads to a loss in hardenability of austenite despite the fact that grain sizes of the final microstructure are refined. The high-Nb and Nb-Mo bearing materials have the nearly same effect on lowering the onset temperatures of transformation, but the former is somewhat faster in the progress of transformation due to an additional work-hardening effect. Thus, to obtain sufficient amounts of ultrafine-grained acicular ferrite, cooling rate must be increased to suppress the formation of high-temperature transformed products in high-Nb materials.