Origin of Fine Ferrite grain Structure of Sheet Steel Hot-roll in Ferrite Region without Lubrication

摘要

The structrual variation in the thicknessdirection of ultra-low C sheet steel rolled in theferrite region without lubrication is analyzed withEBSD technique as well as conventional measuringmethod.(1)A nonuniform microstructure through thethickness is formed due to the additional shear strainintroduced by frictional force between the rolls andthe material, and the fine grains with the size of 1.0to1.5 microns are observed near the surface where theshear strain is maximum. Since the dislocation densityof these grains is relatively low and these grains aresurrounded by high-angle boundaries having amisorientation angle over 15, these grains might berecrystallized ones.(2)The microstructure observed inthe etched steel with SEM are changed continuously fromthe deformed grains to the subgrains and therecrystallized grains with increasing shear strains.On the other hand , the size of the subgrains consistingof substructure is 0.5 to 1.0 microns by means of EBSD,and it is independent of shear strain.(3)Themisorientation angle between the adjacent subgrains isincreased with increasing shear strain, and high-angleboundaries are formed near the surface.(4)Theseexperimental results suggest that the finerecrystallized grain might have formed in such a waythat the subgrains are rotated continuously and themisorientation angle between the adjacent subgrains isincreased over 15 with increasing shear strain, and therearrangement of dislocations occurs during and/orafter rolling, resulting in the formation of recrystallized grains.