Effect of Solute Carbon Content on Microstructures of Cold-rolled Ferritic Steel -The Same Area Analyses by Using TEM and SEM-EBSD-

Synopsis : Deformation microstructures developed in cold-rolled ultra low carbon (ULC) steel as well as those in low carbon (LC) steel have been investigated by using TEM and SEM-EBSD techniques. Particular attention has been paid to the effect of solute carbon on the development of those microstructures. Dislocation structures characteristic to the preferred orientations such as y-fiber (ND//<111>) and a-fiber (RD//<011>) have been revealed by the same area observation employing the above two techniques. TEM images of dislocation cell boundaries observed in ULC are sharper than those in LC structures. Images of dislocation line segments were separately distinguished in cell structures in ULC, while in LC they were indistinguishable because of high density of dislocations. This indicates that dislocation density increases with increasing the amount of solute carbon, which was confirmed also by XRD measurement. In grains of ND//<111>, fine microbands and/or shear bands (SBs) were developed while in RD//<011>grains such remarkable inhomoge-neous microstructures were not observed, which suggests that work-hardening in ND//<111>grains is more prominent than that in the other preferred orientations. In {111}<211>grains of LC steel, the same kinds of shear bands as observed in Fe-Si steels were formed as the most characteristic microstructure, where elongated fine-grained structures with the orientation scattering of 35° between the {111}<211>and {110}<001>Goss orientation were found.