Thermodynamic Basis of Isothermal Carbothermic Reduction of Oxide in Liquid Steel for Simultaneous Analysis of Soluble/Insoluble Oxygen Contents in the Steel Specimens

Abstract

Oxygen in liquid steel exists as soluble O (chemically dissolved in the steel matrix) and insoluble O (physically dispersed as the oxide inclusion). To measure the soluble O content ([S.O]) and the insoluble O content ([I.O]), both in the context of gas fusion analysis technique such as inert gas fusion infrared absorption method, it is necessary to melt the specimen under a C saturation condition in a “two-stage” isothermal heating mode. The first stage is carried out at a low temperature (TL), where only soluble O is extracted as CO(g). The second stage is subsequently carried out at a high temperature (TH), where the oxide inclusion is reduced by a carbothermic reaction. Therefore, it is necessary to identify the carbothermic reduction temperature of the oxide inclusion in the liquid steel at the C saturation (\( T_{{{\text{M}}_{x} {\text{O}}_{y} }}^{{{\text{Fe}}}} \)). A series of CALPHAD-type thermodynamic calculations were carried out to estimate the temperature of each oxide inclusion (Al2O3, FetO, SiO2, CaO, and MgO). It was found that the temperature depends on the metal content ([pct M]) and total pressure (P) inside the gas fusion chamber: increasing [pct M] or P increases \( T_{{{\text{M}}_{x} {\text{O}}_{y} }}^{{{\text{Fe}}}} \). The proposed method may be applied for the steel systems containing those inclusions except for FetO. In the case of Al2O3 inclusion containing steel ([pct Al] = 0.04), for example, \( T_{{{\text{Al}}_{2} {\text{O}}_{3} }}^{{{\text{Fe}}}} \), lies in the range of 1791 K (1518 °C) to 1838 K (1565 °C) depending on P inside the gas fusion chamber. Possible range of the \( T_{{{\text{M}}_{x} {\text{O}}_{y}}}^{{{\text{Fe}}}}\) is reported.