The microstructural evolution and mechanical properties of ultrafine grained interstitial free steel fabricated by multipass differential speed rolling (DSR) technique were investigated. A series of DSR operations were performed up to four passes under a roll speed ratio of 1 : 4 for lower and upper rolls. Microstructural observations using a transmission electron microscope showed that the deformed microstructure comprised the severely deformed ferrite grains with a mean size of y0?7 mm having a relatively high dislocation density. In addition, the nearly equiaxed grains were effectively formed due to the sample rotation of 180u around the transverse axis between each passage where the macroscopic shearing was reversed. Electron backscattered diffraction analysis revealed an increase in the fraction of the high angle grain boundaries in the severely deformed samples with increasing number of DSR operations. The mechanical properties were also examined utilising microhardness and tensile tests, and these were discussed in relation to the shearing and microstructural characteristics.
The present work deals with the spheroidisation behaviour of pearlitic cementite in 1·0 wt-% carbon steel subjected to equal channel angular extrusion followed by annealing. The influences of intense strain and annealing temperature on the morphological change of cementite were investigated. Intense strain even by single equal channel angular extrusion was sufficiently beneficial to accelerate spheroidisation kinetics upon annealing, resulting in reducing annealing time required for achieving fully globular cementites with uniform distribution. An optimum condition to obtain the fully spheroidised microstructure of 1·0 wt-% carbon steel was suggested. In addition, the correlation between processing variables and spheroidisation of 1·0 wt-% carbon steel was explored based on microstructural evolution.
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