Study On Microstructural Evolution Of Ultra-low Carbon Steel During Rolling Process

As a basis steel of IF steel and silicon steel, study on microstructural evolution of ultra-low carbon steel during rolling process will be important guiding significance of design and improvement for both steels. In this thesis, a ultra-low carbon steel was cold-rolled under various rolling parameters. The texture and microstructural evolution of ultra-low carbon steel during rolling process were investigated by SEM, TEM, EBSD methods. The research results supplied a theoretical basis for the development and research on IF steel and silicon steel.The obtained results show as follows:the rolling texture of ultra-low carbon steel changed after different process. Heat rolling textures in ultra-low carbon steel at730℃～815℃were rotated Cube with peak at {001}<110>. It showed none obvious texture orientation for ultra-low carbon steel heat treated at950℃and1000℃. The cold rolling reduction of60%deformation led to the formation of γ texture with peak at (111)[011] and{111}<112>, α texture with peak at {112}<110> and (111)[110]. The grain size before cold rolling and rolling temperature didn’t influence the final type and intensity of cold rolling texture obviously.The rolling microstructures of ultra-low carbon steel have changed complexly under different cold rolling reduction. One or two sets of straight visible microbands occurred under rolling reduction of20%, none shear bands. The straight strip shear bands with obvious characteristics occurred within certain grains under rolling reduction of40%, and microbands were sheared into folding line style, their width decreased, many IDB formed in them. The boundaries of shear bands were curved and microband boundaries were fuzzy under rolling reduction of60%. Under the same deformation, cold rolling substructure of small-size grains distributed more evenly. It was easy to form dislocation cell structures at rolling temperature of300℃.The shear bands originated at grain boundaries of cold rolled ultra-low carbon steel, and sheared grain boundaries to form step grain boundaries or crenellated grain boundaries. Fish-bone microstructures or "S-style" shear structures occurred in grains under rolling reduction of60%. The boundary misorientation of microbands increased with the arising of shear bands.In the ultra-low carbon steel under rolling reduction of60%, shear bands have been formed in α, γ and near rotated Goss orientation grains. In α orientation grains, matrix orientation rotated around the specific axis about50°to form shear bands. In γ and near rotated Goss orientation grains, matrix orientation rotated around the [110] axis about30～55°to form shear bands. The misorientation between shear band edge and original grain was15～40°, the misorientation of microcells in grain interior was less than5°. The microstructures like "micro-shear structure" and "micrograin" occurred in shear bands, their boundaries misorientation was5～40°. Large-sized grains were easy to produce wide-sized shear bands. When the shear band was wider, the change of orientation was more obvious.