Effects of aluminum content on cracking phenomenon occurring during cold rolling of three ferrite-based lightweight steel

The effects of Al content on cracking phenomena occurring during cold rolling of ferritic lightweight steels were investigated in relation to microstructural modification including κ-carbides. Three steels were fabricated, varying the Al content between 4 and 6 wt.%, and the center and edge areas of steel sheets containing 6 wt.% Al were cracked during cold rolling. The three steels were composed of ferrite grains and κ-carbides in a banded shape, and the overall volume fraction of κ-carbide increased with increasing Al content. The shape of lamellar κ-carbides inside κ-carbide bands was changed from short and thin to long and thick. Microstructural observation of the deformed region of fractured tensile specimens revealed that deformation bands were homogeneously formed in wide areas of ferrite matrix in the steels containing 4–5 wt.% Al, and κ-carbide bands and boundary κ-carbides were hardly cracked. In the steel containing 6 wt.% Al, however, microcracks were initiated at grain boundary κ-carbides and long lamellar κ-carbides inside κ-carbide bands. They led to the center or edge cracking during cold rolling. To prevent or minimize cracking, it was necessary to avoid the lengthening or thickening of lamellar κ-carbides. Therefore, it was recommended that the steels should be rapidly cooled from the finish rolling temperature to the coiling temperature through the formation temperature of κ-carbide.