Text of Simulation of non metallic inclusions formation during liquid steel reoxidizing
1. September 20-23, 2009 Santa Fe, New Mexico Simulation of Non-Metallic Inclusions Formation During Liquid Steel Reoxidizing Alexander Alexeenko and Elena Baybekova Lasmet Co. (Laboratory of Special Metallurgy Co.)
2. Introduction Liquid metals reoxidation during casting has a negative effect on the quality of ingots, billets or slabs. Products of the reoxidation clog nozzles and affect casting parameters. Reoxidation increases the metal contamination by oxide inclusions. Coarse reoxidation inclusions can provoke surface defects during rolling and stretch pressing.
3. High Mn is a typical sign of reoxidation inclusions It is also 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 doesnt explain this phenomenon.
4. Goal 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.
5. Steps of reoxidation inclusions formation Interaction between liquid metal droplets and atmosphere during casting leads to oxidation of the droplets entirely or partially.  When these iron oxide droplets and skins fall to the metal pool they are transformed by interaction with deoxidizers which exist in the metal. 1. H.- U. Lindenberg and H. Vorwerk
6. Model assumptions 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.
7. Model concept 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 dont control the process). 2. The flows of all components are in balance with oxygen flow (condition of quasi-stationarity of the process).
8. Model formalization The concept may be written as the following equations system. Solution of the system gives momentary flows of the components. It allows the program to compute changes of components fractions in liquid inclusions. Current metal composition is calculated on the basis of material balance conservation in inclusions-metal system.
9. The simulation of FeO particle transformation in Si-killed steel (wt. pct: 0.09 C, 0.55 Si, 1.2 Mn) 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. Area where solid phase precipitation begins
10. The trajectory on MnO-SiO2 phase diagram 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.
11. Manganese silicates in low carbon Si-killed steel These conclusions correlate well with the experimental results and provide an explanation for the genesis of manganese silicates in Si- killed steels.
12. The simulation of FeO particle transformation in low Si LCAK-steel (wt. pct: 0.01 Si, 0.04 Al, 0.2 Mn) 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. Area where solid phase precipitation begins
13. The precipitation regions on the ternary diagrams MnO-Al2O3-SiO2 FeO-Al2O3-SiO2 Red circles are the precipitation regions (based on computed results) [Si] = 0.01 wt. pct.
14. Reoxidation inclusions in low silicon LCAK-steel At the beginning of the transformation At the end of the transformation 1 20 FeO, 80 MnO; 2 15 FeO, 28 MnO, 57 Al2O3; 3 9 FeO, 56 MnO, 25 SiO2, 10 Al2O3 1 Fe; 2 galaxite (MnO.Al2O3); 3 36 Al2O3, 31 SiO2, 33 MnO; 4 Al2O3 cover 1 2 3 1 3 2 4 Our conclusions correlate well with SEM results for inclusions with high manganese content which were found in low silicon LCAK-steel.
15. Reoxidation inclusions in low silicon LCAK-steel Galaxite-hercynite grains Alumina cover Phase on basis of Al2O3SiO2MnO system Alumina grains Matrix (wt. pct.): 36 Al2O3, 31 SiO2, 33 MnO Fe Al O Mn Al Si
16. The simulation of FeO particle transformation in LCAK-steel with 0.2 wt. pct. Si content 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.
17. Trajectory on Al2O3-SiO2-MnO phase diagram 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. Red arrow is a computed trajectory of inclusions composition alteration. Red dots correspond to reoxidation inclusions revealed. [Si]=0.2 wt. pct.
18. Reoxidation inclusions in LCAK-steel with 0.2 wt. pct. Si content a) (wt. pct.): 41 MnO, 39 SiO2, 20 Al2O3 a) b) b) (wt. pct.): 43 MnO, 48 SiO2, 4 Al2O3, 5 FeO
19. Superimposition of real inclusions compositions on the computed diagram (LCAK-steel, [Si]=0.2%) Here we superimposed the experimental data on the computed diagram so that dots of SiO2 percentage were put on SiO2 theoretical line. We can see that compositions of real reoxidation inclusions correlate qualitatively with the simulated ones.
20. Conclusions 1. The process of inclusion formation during Si- and Al-killed steel reoxidation was investigated by both computer simulation and SEM analysis. 2. The simulation results correlate well with the analysis of real inclusions. 3. By the use of the simulation we have found an explanation for high Mn inclusions formation in Si- and Al- killed steels. 4. The developed method can be used for investigation of inclusions formation in various liquid steels and alloys under any conditions.
21. Thank you!
22. Appendix. About rate determining step For detection of the rate determining step we compare diffusion flows of some component R through oxide inclusion and metal diffusion layers: