Prediction of Reoxidation Inclusion Composition in Casting of Steel

A model is developed to calculate the composition of reoxidation inclusions that form during pouring of steel castings. The software package Thermo-Calc is used to obtain the inclusion phase fractions and compositions as a function of the temperature and oxygen content of the steel. Oxygen is assumed to be continually absorbed by the steel until the liquidus temperature is reached. Both lever rule and Scheil-type analyses are performed. The model is applied to reoxidation of two carbon steels, one low-alloy steel and one high-alloy steel. The effects of variations in the steel composition and the oxygen absorption rate on the inclusion composition are investigated in a parametric study. The mass fraction of absorbed oxygen is determined by matching predicted with previously measured reoxidation inclusion compositions for the various steels. Good agreement is obtained for most phases present in the inclusions. Interestingly, the agreement in the inclusion compositions occurs for all steel grades when the percentage of absorbed oxygen is equal to 0.9 wt pct. This value is explained using a separate model for the rate of oxygen absorption at the steel-atmosphere interface. Various scenarios are outlined that allow for the 0.9 wt pct of absorbed oxygen to be achieved. The model is then used to calculate the amount of alloy elements consumed and inclusions formed as a function of the oxygen boundary layer thickness in the atmosphere and the integrated free surface area of the liquid steel during pouring. It is found that for unprotected liquid steel transfer operations, such as tapping and ladle filling, the integrated free surface area and exposure time product can reach values of the order of 100 m s per ton of steel, and that the air-to-steel volume ratio during pouring can be as large as 40. It is concluded that, in order to create a comprehensive tool for simulating reoxidation formation, more detailed models are needed for the external oxygen transfer in the atmosphere, the flow of the liquid steel during pouring, and the internal transport and reactions of chemical species in the steel.