Computer Simulation of Oxide Inclusions Transformation during Secondary Steelmaking and Casting

Alexander AlexeenkoThe software for simulation of liquid oxide inclusions transformation during secondary steelmaking and casting was presented by Alexander Alexeenko (Lasmet Co. - Laboratory of Special Metallurgy) at steelmaking session of the Asia Steel International Conference 2009 (Busan, Korea, May 24-27, 2009).
The Presentation Summary:
Some steel grades, e.g. rail, cord, deep drawing, pipe steels, etc. are strictly controlled on non-metal inclusions characteristics (composition, size, quantity per unit). It is necessary to guarantee a good workability during manufacturing and high mechanical properties of products. The oxide inclusion control pursues the tasks: a) to limit maximum inclusion size (it may vary from 15 to 100 µm for different steel grades) and b) to ensure presence of microinclusions of required types only and restricted in number per unit.
Macroinclusions observed in crude steel as a rule are products of microinclusions coagulation or reoxidation. Also they may have exogenous nature, i.e. refractory or slag particles. Large deoxidation products are successfully removed during secondary steelmaking operations, therefore they are usually absent. Measures to decrease macroinclusions contamination of continuous casting billets or slabs are well known and at the mainly they are realized on continuous casting stage. Microinclusions have another origin than macroinclusions. They nucleate and grow in the melt during secondary treatment and casting or during crystallization process. Composition of microinclusions usually corresponds to that of metal. To manufacture steel with non-metallic inclusions of specified characteristics, it is necessary to provide control of inclusions forming and transformation processes during secondary steelmaking. Such control is very important for preparation of “clean” steel free from detrimental non-metallic inclusions before the ladle is sent for casting. It is obvious that control of inclusions composition requires control of technological factors which influence on inclusions behavior. They are: modes of heating, slag forming, and changing chemical composition of the melt. Control of inclusions composition is especially important in aluminum non-killed high quality steel grades. The chemical interaction of oxide inclusions with liquid aluminum non-killed steel has one peculiarity. It is a considerable variation of inclusions composition due to small changes in amounts of high oxygen affinity elements or changing of oxygen activity in the melt. Besides, for aluminum non-killed steel grades (cord steel, rail steel, wheel and tire steel, etc.) it is important that a certain chemical composition of metal before casting would ensure minimal amount of crystallization inclusions, their composition and dispersion being optimal.

The goal of this work was to create thermodynamic based model and software for simulation of non-metallic inclusions transformation during secondary steelmaking and casting, as a result of changing of melt chemical composition, temperature and pressure. Herein, transformation means changes of composition and total mass of inclusions.

For creation of the model 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 concept may be written as the following equations system.
Solution of the system gives momentary flows of the components.
It allows to compute changes of components shares in inclusions.
Current metal composition is calculated on the basis of material balance conservation in inclusions-metal system.

Developed software consumes the following input data: temperature, pressure, chemical composition and mass of melt and of inclusions formed before (or slag emulsified).
After reading these input parameters the software iteratively computes redistribution of elements between inclusions and metal in direction to equilibrium.
Simultaneously thermodynamic possibility of competitive reactions is tested. And increment of such phases as sulfides and CO is also computed.
When the average inclusions chemical composition is computed, the software finds solid phases in liquid oxide inclusions. To get this effect a database is used which was developed on basis of oxide phases diagrams.
For illustration “how it works” we demonstrated results of the simulation in cases of pressure drop, temperature reduce, and Ti addition.
Initial conditions for these variants were uniform:
- Temperature: 1600o C; pressure: 1 atm;
- Metal composition (wt. pct): 0.160 C, 0.54 Mn, 0.177 Si, 0.00028 Al, 0.0004 Ti, 0.0001 Ca, 0.0001 Mg, 0.004 N, and 0.0120 Ofree;
- Inclusions composition (wt. pct): 43 SiO2, 17 Al2O3, 27 CaO, 5 MgO, 1.5 Ti2O3, 5 MnO, 1.5 FeO;
- Inclusions mass: 0.1 g per 1 kg of steel.
Simulation of liquid inclusion transformation after pressure dropped to 0.1 atm
Simulation of liquid inclusion transformation after pressure dropped to 0.1 atm
Vacuum–carbon deoxidation (free oxygen decreased from initial value 120 to 18 ppm) results in a partial dissolving of oxide inclusions. Elements with smaller affinity to oxygen are recovered predominantly.
Simulation of liquid inclusion transformation after temperature dropped to 1550o C
Simulation of liquid inclusion transformation after temperature dropped
The temperature decrease results in some drift of deoxidation reactions to field of lower equilibrium oxygen, and excessive oxygen is connected by deoxidizers.
In case of considered conditions the most of excessive oxygen combines with silicon, and SiO2 share in oxide phase increases. Inclusions mass grows appreciably.
Simulation of liquid inclusion transformation after Ti addition
Simulation of liquid inclusion transformation after Ti addition
The results of simulation after Ti addition up to 0.015 wt. pct. are demonstrated here.
We can see that Ti2O3 share in oxide phase rises to 34 wt. pct, and inclusions mass grows 1.5 times.
We’ve compared computed results with analyses of real inclusions in steel probes.
The probes were sampled on different melts from ladles during secondary steelmaking and from tundish during casting.
Simultaneously oxygen activity in melt and temperature were measured.
Composition of non-metallic inclusions was analyzed with scanning electron microscope JEOL JSM-646OLV at South Ural State University (Chelyabinsk, Russia).
Comparison of the analyses with values computed demonstrates that developed software computes inclusions composition correctly enough.
Today we are looking for further test and development of the software.
But already now it is evidently that to get correct results during secondary steelmaking in on-line mode the software should have valid input data.
For the purpose of determination and control of current metal conditions during secondary steelmaking we are planning to use our GIBBS® melt control system.
Secondary_steelmaking.jpg

The GIBBS® system is based on thermodynamic and kinetic approaches.
The system has demonstrated ability to precisely predict and control of melt chemistry and temperature during secondary steelmaking.
It allows to supply the “Inclusions” software with valid information about current metal conditions.
GIBBS® system also is able to compute inclusions remained or emulsified slag.
Summary and future prospects
- The software for simulation of liquid oxide inclusions transformation was developed and tested.
- More testing is on the way to ensure software correctness for wide group of steel grades.
- Application of the “Inclusions” software in combination with GIBBS® melt control system would allow to facilitate prediction and control of inclusions formed during secondary steelmaking and casting.

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