Stress-strain behavior and microstructural evolution of ultra-high carbon Fe-C-Cr-V-Mo steel subjected to hot deformation

Abstract

Numerous processes have been applied to refine the microstructure and the precipitation of carbides in ultra-high carbon steel (UHCS). However, the evolution of microstructure and carbide precipitation in UHCSs during the hot deformation compression process remains poorly understood. The present study addresses this issue by applying hot deformation compression to Fe-C-Cr-V-Mo UHCS specimens at temperatures of 950–1150 °C under strain rates of 0.01–5 s−1, and evaluating the resulting flow curves and microstructures of the specimens after cooling in air. High deformation temperature and high strain rate conditions are observed to generate flow curve stress peaks, which result in dynamic recrystallization. In addition, dislocation cell formation and substructure are observed at deformation temperatures of 950 °C and 1050 °C, respectively, under a strain rate of 1 s−1. The recrystallized grain structure fraction is found to be positively correlated with the deformation temperature at strain rates less than 5 s−1, and a fully recrystallized grain structure is not obtained at a strain rate of 5 s−1 even at a temperature of 1150 °C. Moreover, the concentration of primary M7C3 carbides is observed to decrease with increasing deformation temperature, and fine precipitated M23C6 particles are observed in the matrix during hot deformation. An increased concentration of precipitated M23C6 particles is also observed in the specimen microstructure at a deformation temperature of 1150 °C and a strain rate of 5 s−1. The recrystallization process is also observed to vary according to M23C6 particle size, where large M23C6 particles are observed to stimulate the formation of recrystallization nuclei and fine M23C6 particles delay or hinder the recrystallization process.