Microstructural Analysis of Cracking Phenomenon Occurring During Cold Rolling of (0.1 similar to 0.7)C-3Mn-5Al Lightweight Steels

An investigation was conducted into the cracking phenomenon occurring during the cold rolling of lightweight steel plates. Four steels of varying C contents were fabricated and steel plates containing C contents of 0.5 wt% or higher were cracked during the initial stage of the cold rolling. The steels were basically composed of ferrite grains and -carbides in a band shape, but the volume fraction and thickness of kappa-carbide band increased as the C content increased. 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 C contents of 0.3 wt% or lower, while kappa-carbide bands were hardly deformed or cracked. In the steels containing high C contents of 0.5 wt% or higher, on the other hand, microcracks were initiated mostly at fine proeutectoid ferrite located within kappa-carbide bands, and were grown further to coalesce with other microcracks to form long cracks. To prevent the cracking, thus, the proeutectoid ferrite should be minimized by the hot rolling in the (alpha+gamma) two phase region. As practical methods, the content of C below 0.5% or Al above 5% was suggested to expand the (alpha+gamma) phase region.