Effect of Refining Conditions for Ultra Low Carbon Steel on Decarburization Reaction in RH Degasser.

On the basis of a mixed-control mechanisminvolving the mass transfer of carbon and oxygen in the molten steel in the vacuum vessel and the transport of carbon and oxygen by the circulation flow, a new reaction model for RH decarburization has been developed to clarify the effect of oxygen concentration on the decarburization rate.The model can also predict the effect of the KTBmethod, in which oxygen gas is blown onto the molten steel in the vacuum vessel, on the decarburization rate.A significant dependenceof the decarburization rate in the RH experiments on the oxygen concentration in the ladle.OL, has been found in the range of the carbon concentration in the ladle, CL, of more than 200ppm.In the range CL the decarburization rate is apparently proportional to the carbon concentration, hence its dependence on OL is small.The model analysis predicts the dependenceof the decarburization rate on OL, however, there is some difference between the calculated decarburization rate and the observed rate.This is presumedto be cavsed by an unidentified supply of oxygen to the molten steel in the vacuum vessel, which may result from the slag being drawn into the vacuum vessel and the leakage of atmospheric air.Volumetric coefficients of the mass transfer of carbon in the vacuum vessel of the RH, akc, was also investigated in the ultra low carbon range of less than 50ppm using recent data, and was shownto have a strong dependence on the carbon concentration and the circulating flow rate, O, KEYWORDS: secondary steelmaking; RH degassing; decarburization; ultra low carbon steel, l .IntroductionTo meet increasing demand for cold rolled steel sheets with improved mechanical properties, and to cope with the changefrom the batch-type to continuous annealing, the production of ultra low carbon steel is increasing.1)With the increased demand for ultra low carbon steel, it became essential to establish and effective refining process for ultra low carbon steel, with a carbon concentration of less than 20 ppm.