Processes of inclusions formation during steel reoxidizing were investigated by computer simulation and SEM analysis of oxide inclusions in steel samples. The thermodynamic-based model of interaction between oxide inclusions and liquid metal in the line of equilibrium state and program for computation of inclusions transformation are developed. For Al- and Si-killed steels trajectories of change of inclusions chemical composition from initial FexO phase formed during reoxidation to final inclusions oxide phases were computed. Those finals phases are: heterogeneous inclusions (grains of hard spinals solution |MnO.Al2O3,FeO.Al2O3| + interlayers from phase based on MnO–SiO2–Al2O3 system, and Al2O3 cover) in LCAK-steel with Si content 0.01 wt. pct; hard inclusions based on Al2O3–MnO–SiO2 system in LCAK-steel with Si content 0.2 wt. pct, manganese silicates in medium carbon Si-killed steel. Computer simulation of inclusion transformation in LCAK-steel showed that Si significantly increases a time of transformation from initial liquid phase FexO to hard phases. It explains the well-known fact that LCAK-steels with Si > 0.1 wt. pct has better castability than low silicon ones.

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
doesn’t 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. [1]
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 don’t 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
Al2O3–SiO2–MnO 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: