Grain Subdivision Mechanism For Constructing Lamellar Microstructure In Cold-Rolled Ultra-Low Carbon Steel

To clarify the formation process of a lamellar microstructure during rolling, we employ a method for following the crystal orientation distribution in the same region before and after the rolling process, in addition, the local strain distribution owing to the formation of the rolled structure was investigated by measuring the amount of strain in the grain from cold rolling using nano-order fine markers applying a focused ion beam (FIB). During rolling at a 50% to 70% reduction in thickness, different crystal rotations were observed in the initial grains. A trace analysis of the slip band suggested that the crystal rotation in different directions was caused by the activation of different slip systems in each region. The distribution of the equivalent plastic strain from a 60% to 70% reduction owing to a grain subdivision was examined and compared with the change in crystallographic orientation, suggesting that non-uniform slippage deviating from the Taylor model occurred in the regions where a significant orientation change occurred. Furthermore, the strain distribution measured by the marker showed that tensile deformation caused by rolling tended to be suppressed compared to compressive deformation near the grain boundaries inclined toward the rolling direction. These results suggest that the local crystal rotation inside the grain, which leads to a grain subdivision during rolling, is caused by a deviation of the strain tensor inside the grain from the ideal rolling state.