Rolling sheet material can result in edge cracking due to overworking between anneals, exacerbated by the poor-quality surface on the edges.
As we work a metal, work hardening results in a loss of further ductility and, ultimately, failure of the material by cracking. The metal should be sufficiently annealed during working steps to allow further work.
Preventing cracking
As soon as cracking is seen at the edges, the sheet should be trimmed to prevent cracks from propagating into the center of the sheet.
Fins and laps can arise during rod rolling, which can open up as cracks at later stages:
Fins form when too much material is pushed into the rolling groove, so the excess metal is squeezed sideways.
Laps form when newly formed fins are rolled into the rod.
To prevent lap formation, use a smaller reduction on each pass and rotate the rod through 90˚ between successive passes.