GRAPHITE POLLUTION MECHANISM AT THE STEEL SURFACE ON ANNEALING OF COLD-ROLLED OPEN-COIL

On open-coil annealing of cold rolled steel sheet in HNX gas, the gas composition in the furnace are analyzed. On heating, CO, CO2 and hydrocarbons are generated in the atmosphere by combustion and vaporization of the lubricant on the sheet surface. In order to simulate the open-coil annealing, a steel sheet is annealed in CO-CO2-8%H2-N2. Then, the graphite and cementite are formed on the sheet surface. The graphite pollution on the steel surface can be reproduced by the simulative annealing, because the morphology of graphite by the simulative annealing is same as the formed by the open-coil annealing. According to the addition of CO2 in CO-8%H2-N2 atmosphere, the graphite increases incipiently and then decreases, and more addition than the critical CO2 content, which is affected with CO2/CO, suppresses the formation of graphite perfectly. From these results, the graphite on steel surface by annealing in HNX gas atmosphere are formed by the decomposition of CO, and the lubricant for cold rolling is one of the carbon source.