Mechanisms and Modeling of Liquid Steel Decarburization below 10 ppm Carbon

Abstract

Steel grades for packaging or automotive industry applications require ultra low carbon levels and carbon contents below 10 ppm have to be obtained in the industrial decarburization reactor (tank degasser or vacuum circulation degasser, known as RH). In the ultra low carbon range (below 10 ppm in the vacuum vessel), the decarburization kinetics cannot be obtained from the analysis of metal samples but can be deduced from continuous exhaust gas analysis and flow rate measurements. This was done on an industrial RH-OB degasser and a reliable and precise measurement of reaction rates in the ultra low carbon range was obtained. The rate was evaluated by an overall volumetric coefficient of decarburization ak⁄S. A model of decarburization kinetics in vacuum vessel has been developed at IRSID based on a physical description of three different reaction sites: spontaneous CO boiling in metal layer just beneath bath surface where carbon and oxygen contents are in excess with respect to equilibrium CO partial pressure, decarburization by purging gas in zones where metal and injected gas bubbles are in contact and decarburization of metal drops projected in vacuum vessel by bubble bursting. The experimental results indicate that CO boiling is responsible for the largest part of decarburization, but, the decarburization of metal drops projected in vacuum vessel is dominating below 10 ppm. The dynamic modeling of the process describes simultaneously the evolution of vessel pressure, oxygen and carbon contents. It explains with very good accuracy the experimental results and is used to define optimal process parameters for industrial treatments.