THERMOMECHANICAL PROCESSING OF MICROALLOYED STEELS: EXPERIMENTS AND MODELLING

Abstract

Recovery, recrystallization, grain growth and precipitation constitute the fundamentals of thermomechanical controlled processing (TMCP) of microalloyed steels. In-depth understanding of these phenomena is indeed needed. In this work, the individual components and some of the potential mutual interactions have been investigated deliberately. The effect of alloying elements of Mn, Si, and Al on recovery and recrystallization has been systematically studied by conducting the stress relaxation tests on binary Fe-0.1%C and ternary Fe-0.1%C-X alloys. The effect of temperature on recovery kinetics was also investigated. The effects were considered by fitting the recovery model through the activation volume term. Higher temperature or lower solute content will accelerate the recovery process and then facilitate the onset of recrystallization. NbC precipitation behavior has been investigated using a nickel-based model alloy, having samples deformed at both room temperature and elevated temperature and subjected to annealing at 700℃ for different times, in order to elucidate the stages of nucleation, growth and coarsening for precipitation. The microstructures preserved by water quenching were examined using transmission electron microscopy (with both metal foil and carbon replica specimens). Results from mechanical response and microstructural evolution are linked and discussed. The precipitate number density and size evolution show good agreements with predictions from a classical strain-induced precipitation model. The in-situ laser-ultrasonics measurement of C-Mn steels provides a unique way to evaluate grain size evolution during TMCP, for different strains of 0.15, 0.25 and 0.35, at 950℃ and 1050℃. Effects of temperature and strain on recovery, recrystallization and grain growth have been covered and elucidated. Higher strains facilitate the onset of recrystallization and grain size refinement. However, higher temperatures only shorten the onset of recrystallization but lead to larger grain size. The effect of microalloying element of Nb on softening kinetics was also investigated by comparing C-Mn/C-Mn-Nb steels at the same conditions. The solute drag effect of Nb can be seen by the onset-delays of recrystallization and larger grain sizes. The laser-ultrasonics results can match well with stress relaxation measurements. The in-situ grain size evolution data has given the possibility to develop robust thermomechanical processing (TMP) models combining deformation, recovery, precipitation, recrystallization and grain growth. The application and validation of the TMP models have been attempted and remain ongoing.