Simulation of Non-Metallic Inclusions Formation during Liquid Steel Reoxidizing

Alexander Alexeenko The results of simulation of reoxidation inclusions formation in Si- and Al-killed steel grades were presented by Alexander Alexeenko (Lasmet Co. - Laboratory of Special Metallurgy) at the International Symposium on Liquid Metal Processing and Casting (Santa Fe, New Mexico USA, September 20-23, 2009). By the use of the simulation authors have found an explanation for high Mn inclusions formation in Si- and Al-killed steels. The developed method can be used for investigation of inclusions formation in various liquid steels and alloys under any conditions.
You can find the Abstract and Full Presentation at section “Library”.
The Presentation Summary:
Liquid metals reoxidation during casting has a negative effect on the quality of ingots, billets or slabs. Solid reoxidizing products can clog nozzles to the point where casting speed can fall to cause a full pouring break. Furthermore, reoxidation increases the metal contamination by oxide inclusions. Coarse inclusions can provoke surface defects during rolling and stretch pressing.
When oxide inclusions are detected near defects it is always necessary to define the cause of the inclusion-formation (e.g. whether it is a product of reoxidation, trapping of tundish slag or mold flux, etc.).
It is known that reoxidation inclusions are often coarse and contain high amount of manganese.
High manganese content is typical for these inclusions even in case when they are formed in Si- or Al-killed steels.
It is very interesting because simple thermodynamic calculation shows that these steels must not contain such inclusions. The ordinary thermodynamic approach doesn’t explain this phenomenon.
Our goal was to investigate the inclusion formation process during casting of Si- and Al-killed steels and understand how the high manganese inclusions appear into the melts.
For this purpose we have used computer simulation and SEM approaches.
It is known that interaction between liquid metal droplets and atmosphere during casting leads to oxidation of the droplets entirely or partially [H.- U. Lindenberg and H. Vorwerk ].
When these iron oxide droplets and skins fall to the metal pool they are transformed by interaction with deoxidizers which exist in the metal.
For creation of the model of FeO particles transformation we assumed that:
- Molten steel and oxide inclusions tend to equilibrium state.
- All elements are allocated uniformly throughout the melt bulk.
- Inclusions are liquid and spherical.
- The rate determining step of inclusions transformation is mass transfer in metal.
Mass transfer depends on difference in components concentrations in volume and near the inclusion boundary.
Those boundary concentrations are completely determined at any moment by the following conditions:
1. They are in equilibrium with inclusions (because chemical reactions don’t control the process).
2. The flows of all components are in balance with oxygen flow (condition of quasi-stationarity of the process).
The simulation of FeO particle transformation in Si-killed steel (wt. pct: 0.09 C, 0.55 Si, 1.2 Mn) gave the next results. At the beginning of the transformation iron is being reduced from the oxide phase by silicon and manganese. And only after some decrease of the iron oxide fraction, a reduction of manganese by silicon must begin.
But the rise of SiO2 fraction in liquid inclusion must be stopped around 50 wt. pct. value because it is the point of supersaturation of SiO2 in the MnO-SiO2 system.
If further increase of SiO2 in the solution occurs, the process of solid cristobalite formation in liquid oxide matrix must begin.
However, the phase formation needs an additional energy.
If there is not enough energy in the system, non-equilibrium manganese silicates must remain in the metal.
In other cases, cristobalite is formed inside the liquid matrix.
These conclusions correlate well with the experimental results and provide an explanation for the genesis of manganese silicates in Si-killed steels.

Manganese silicates in low carbon Si-killed steel
Manganese Silicates

The simulation of FeO particle transformation in low Si LCAK-steel (wt. pct: 0.01 Si, 0.04 Al, 0.2 Mn) showed that in this case initially iron is being reduced from the oxide phase generally by manganese and aluminum. The MnO fraction increases significantly.
Because of this transformation sequence, the conditions for precipitation of galaxite and hercynite solutions as well as the corundum crystals appear.
These conclusions correlate well with SEM results for inclusions with high manganese content which were found in low silicon LCAK-steel.

Reoxidation inclusions in low silicon LCAK-steel
Reoxidation inclusions

The simulation of FeO particle transformation in LCAK-steel with 0.2 wt. pct. Si content demonstrated that under these conditions initially iron is being reduced from the oxide phase generally by silicon and manganese. Then reduction of manganese by silicon and aluminum begins in spite of a high aluminum concentration in the steel.
The SiO2 fraction in the inclusion increases to about 80 wt. pct and only then, does the aluminum begin to reduce silicon from the inclusion.
Using both an our simulation results and the ternary phase diagram allows one to conclude that mullite and phase on the basis of Al2O3-SiO2-MnO system must form the reoxidation inclusions in LCAK-steel with 0.2 wt. pct. Si. It correlates well with the experimental results.

Computed trajectory of inclusions composition alteration on Al2O3-SiO2-MnO phase diagram


Conclusions:

- The process of inclusion formation during Si- and Al-killed steel reoxidation was investigated by both computer simulation and SEM analysis.

- The simulation results correlate well with the analysis of real inclusions.

- By the use of the simulation we have found an explanation for high Mn inclusions formation in Si- and Al-killed steels.

- The developed method can be used for investigation of inclusions formation in various liquid steels and alloys under any conditions.

You can find the Full Presentation at section “Library”.