Effects of niobium content and cooling rate on ferrite and bainite start temperatures (Ar-3, Bs) and microstructural features have been studied in niobium. bearing ultralow carbon microalloyed steels. The Ar-3 and Bs temperatures decrease as niobium content or cooling rate is increased. The dependence of Ar-3 on cooling rate is greater than that of Bs in all niobium. contents. The bainitic ferrite laths become longer and narrower with increasing niobium content and cooling rate, and niobium also shows a tendency to decrease polygonal ferrite grain size.