Origin of Fine Ferrite Grain Structure of Sheet Steel Hot-rolled in Ferrite Region without Lubrication

Publisher Website
Google Scholar
Abstract
The structural variation in the thickness direction of ultra-low C sheet steel rolled in the ferrite region without lubrication is analyzed with EBSD technique as well as conventional measuring method. (1) A nonuniform microstructure through the thickness is formed due to the additional shear strain introduced by frictional force between the rolls and the material, and the fine grains with the size of 1.0 to 1.5 microns are observed near the surface where the shear strain is maximum. Since the dislocation density of these grains is relatively low and these grains are surrounded by high-angle boundaries having a misorientation angle over 15°, these grains might be recrystallized ones. (2) The microstructure observed in the etched steel with SEM are changed continuously from the deformed grains to the subgrains and the recrystallized grains with increasing shear strain. On the other hand, the size of the subgrains consisting of substructure is 0.5 to 1.0 microns by means of EBSD, and it is independent of shear strain. (3) The misorientation angle between the adjacent subgrains is increased with increasing shear strain, and high-angle boundaries are formed near the surface. (4) These experimental results suggest that the fine recrystallized grains might have formed in such a way that the subgrains are rotated continuously and the misorientation angle between the adjacent subgrains is increased over 15° with increasing shear strain, and the rearrangement of dislocations occurs during and/or after rolling, resulting in the formation of recrystallized grains.