Cold rolling is a metallurgical process in which the metal is passed through a pair of rolls and the temperature of the metal is below its recrystallization temperature. This hardens the metal as the crystals are being stretched out. After a few times rolling the metal should be annealed by heating it above the recrystallization temperature. This should be done regularly as otherwise the metal sheet cracks due to brittleness. Most non ferrous metals are rolled cold to make sheet. However, steel is usually rolled hot unless it is very thin sheet.