Effect of initial microstructure on ultrafine grain formation through warm deformation in medium-carbon steels

► We obtain a sub-micrometer grain-sized ferrite structure through simple compression in medium carbon steels. ► We prepare four different starting microstructures for warm compression. ► Lath martensite is the most effective for obtaining uniform ultrafine grained structure. ► This refinement is due to both a kind of fine-grained structure and high dislocation density in the initial microstructure. The microstructural change caused by one-pass warm deformation in medium-carbon steels with various initial microstructures has been examined. Fine equiaxed ferrite grains having a mean diameter of 0.5 μm surrounded by high angle boundaries are formed by dynamic recrystallization when the initial microstructure is lath martensite. Although ferrite–pearlite, spheroidized structure and upper bainite also have fine initial microstructures, uniform fine ferrite grains are not obtained. The microstructures have been characterized in terms of grain boundary misorientation, grain size and dislocation density.