Oxide stability diagram of liquid steels - Construction and utilization

Abstract

Oxide stability diagrams are often used to figure out types of oxide inclusions (i.e., phase) in liquid steels. Such diagram is useful for quick identification of the inclusions by concentrations of deoxidizing components. Obviously, deoxidation of the liquid steel to form the inclusions takes place rapidly. Therefore, thermodynamic principles are applied in the construction of the diagram. To construct it in the correct manner, and to use it in more versatile manner, available methods were reviewed, and possible limits are discussed. It is pointed out that an oxide stability diagram is a projection of several iso-oxygen phase diagram sections, each of which should obey the phase diagram rule and should represent accurate thermodynamic equilibria. In this regard, a CALPHAD method is preferred to conventionally used methods that 1) stable oxide phases are naturally taken into account, 2) activity of oxide component even in solution phase (e.g., liquid oxide) is not assumed to be a constant, and 3) iso-oxygen contours are calculated efficiently as Zero-Phase Fraction (ZPF) line of several oxide phases, which are in equilibrium with the liquid steel. Rule of the phase diagram construction is automatically obeyed in the CALPHAD method. Interpretation and utilization of the oxide stability diagram are discussed. It is proposed that concentration axes be extended to a lower concentration where liquid oxide (rich in Fe oxide) forms. It is also explained how oxide stability diagram can be used to read deoxidation/reoxidation path of liquid steels. Finally, consideration of C in a liquid steel is emphasized.