The databases integrated in FactSage include the largest available thermodynamic database for oxide systems, that can be used for modelling liquid metal-slag equilibria. As an example it is shown here, how Calcium treatment of steels can be modelled on a thermodynamic basis.
The basic system of this example is the Al2O3-CaO system. The phase diagram calculated using FactSage is shown in the following figure.
Figure 1: Al2O3 – CaO phase diagram
With changing the ratio between CaO and Al2O3, the liquidus temperature changes drastically, with a minimum around 50 wt% of CaO and Al2O3. This composition is therefore desirable for metallurgical slags and called the “slag window”. While this phase diagram can easily be found online, it is of limited use for metallurgical slag processing since it does not take into account that there are further elements involved in the process: If this slag is supposed to be used for steelmaking, there is obviously also iron, but other elements will also be present, one example is sulfur. In the following figure a phase diagram is shown for the Fe-S-Al-Ca-O2-system at 1600°C, where the sulfur content is fixed at 50 ppm and the aluminum content is fixed at 400 ppm. On the x-Axis, the Ca content is shown while on the y-axis the oxygen partial pressure is plotted, which is a convenient observable during steel treatment.
Figure 2: Phase diagram in a five component system.
At the left, there is single liquid metal phase. This might seem desirable at first glance, however, it is evident that this implies that all the sulfur is dissolved in the liquid metal! If calcium is added to desulfurize the steel three different regions can be distinguished: At low oxygen partial pressure solid CaS precipitates. High oxygen partial pressures lead to solid aluminate formation. Both solid precipates are undesirable since they are detrimental for the processing, leaving a narrow oxygen partial pressure window around 3*10-15 atm (≈ log pO2 = -14.5) where the liquid metal is in equilibrium only with a liquid slag.
The sulfur content in liquid steel as function of CaSi addition to 1 tonne of steel containing 6 ppm O, 30 ppm S, 500 ppm Al and 100 ppm Al2O3 can be calculated convieniently using the Equilib module. The resulting stable phases and impurity concentrations are shown in the following diagrams:
Figure 3: Inclusion management in steel using FactSage calculations.
It can be seen that there is a window where only liquid slag and steel are present for CaSi additions between 70 and 110 grams per tonne of steel. For lower CaSi additions, solid Al2O3 or aluminates are formed while larger CaSi additions lead to precipitation of CaO and CaS. The associated S, O and Ca impurity concentrations in the liquid steel are shown in the upper right.
Using the Integrated Thermodynamic Databank advantage, the metallurgist can now proceed adding other elements to find the processing conditions that result in acceptable impurity concentrations.
Check out the next application example about solid-state processing of an aluminum alloy or read further information about the Phase Diagram and Equilib module that were used for this application example!