MICROMECHANICAL BEHAVIOUR OF MICROALLOYED STEELS BY HOT DEFORMATION STUDY

Abstract

The hot deformation process of a material comprises of an important step in metal working. It accounts for the microstructural evolution through process control, finally resulting in the desired mechanical properties for technical functionality. The aim of the project work is to focus on the dynamic recrystallization behaviour during hot deformation through microstructural evolution of microalloyed steel. The hot deformation study involves process control parameters, namely temperature, strain and strain rate. Together they play an important role in the dynamic recrystallization, yielding fine grain structure through dislocation annihilation and recovery. In order to facilitate the process, the presence of the alloying elements and second phase particles come under scrutiny. The hot deformation behaviour of microalloyed steel was conducted on a Gleeble 3800 thermo-mechanical simulator by hot isothermal compression. The deformation temperature was varied between 850°C and 1050°C based on the microstructural constituents and the secondphase particles (e.g. precipitate) evaluated by SGTE database of the Thermo-Calc study. Hereby keeping the temperature as constant, and altering the strain rate from 0.001/s to 0.1/s resulted in deformations between 20%, 30%, 40%, and 50% i.e. equivalent to the true strain for compression of -0.22, -0.35, -0.51, -0.69. The samples prior to deformation and after deformation were characterized by light optical microscopy for microstructural features of the grain size. Transmission electron microscopy (TEM) was performed to resolve finer details of the microstructure, illustrating shear band formations, precipitate dispersions and dislocation annihilation. Electron back scattered diffraction (EBSD) was performed to observe microstructural changes by hot deformation, such as the sub-grain formation and the misorientation angles of the grains, confirming dynamic recrystallization. The activation energy calculated from the peak stress and temperature data of the thermo mechanical study is 411 KJ/mol, in accordance with the previous investigation performed by various authors claiming climb as the rate controlling parameters.