Control of Carbon and Oxygen in Iron Melt by CH4 or CO2 under Controlled Atmosphere

Abstract

Kinetics of carburization and/or oxidation of molten iron under reduced pressure by injecting CH4 and/or CO2 gas were investigated in order to control carbon and oxygen content dissolved in ultra-low carbon steel during vacuum treatment. The major decarburization reaction of C + O = CO(g) during vacuum treatment reduces the amount of carbon and oxygen according to the stoichiometry of the reaction (d(wt%O)/d(wt%C) = 4/3). It is strongly desirable, however, to be able to control carbon and oxygen independently so that the optimum final composition can be obtained in a smooth way. In the present study, the control of carbon and oxygen content in the melt at 1873K was attempted by injecting CH4 and CO2 gas, respectively, under reduced pressure. Partial pressures of these gases were controlled by diluting with argon gas. Under the reduced pressure without injecting CH4 or CO2 gases, the expected stoichiometry of the reaction were confirmed (d(wt%O)/d(wt%C) = 4/3). While CH4 was injected, an additional reaction of CH4(g) + O(ad) = CO(g) + 2H2(g) occurred, therefore further deoxidation in the molten iron could be achieved (d(wt%O)/d(wt%C) > 4/3). When CO2 was injected, a different additional reaction of CO2(g) = CO(g) + O occurred, therefore oxygen content in the molten iron increased. Based on experimental data in the present study, it was confirmed that decarburization and deoxidation reactions in melt are cooperated by the stoichiometric relationship and the independent control of carbon and oxygen in the melt is possible by this approach. Rates of decarburization and deoxidation in the present study were analyzed, and a possibility of applying this approach to practical operation was discussed.