Microstructure evolution and mechanical properties of ultrafine grained IF steel via multipass differential speed rolling

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 ∼0·7 μm having a relatively high dislocation density. In addition, the nearly equiaxed grains were effectively formed due to the sample rotation of 180° 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.