عنوان مقاله [English]
In the present research, a warm torsion testing technique was carried out on low carbon Nb-Ti microalloyed steel. The schedule
was conducted in a two phase $left( alpha +gamma right) $ region. The effect of the deformation strain rate on the mechanism of dynamic softening and grain refinement of ferrite during the warm deformation was investigated. The physical processes that occur during deformation were studied by analyzing the true stress-true strain flow curves of ferrite. Examination of the microstructural evolution was carried out by optical microscopy. The mechanism of the equiaxed fine ferrite grains (EFG) formation was investigated by means of high resolution electron back scattered diffraction (EBSD) measurements. This was carried out using a Field Emission Gun Leo Scanning Electron Microscope. Microstructural studies show that equiaxed fine ferrite grains are produced during deformation within the inter-critical $left( alpha +gamma right) $ region. The EBSD results show that, as the strain rate increases, the percentage of high angle boundaries increases and that of low angle boundaries decreases. It was considered that the dynamical formation of new fine grains was caused by continuous dynamic recrystallization (CDRX) of ferrite. With increasing strain rate, the EFG size of the ferrite reduces and its volume fraction increases. These implied that increasing the strain rate has a positive effect on the CDRX process of ferrite.