simulation of non metallic inclusions formation during liquid steel reoxidizing
Post on 22-Jan-2018
879 Views
Preview:
TRANSCRIPT
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.)
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.
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 approachdoesn’t explain this phenomenon.
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.
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
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.
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).
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.
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
The trajectory on MnO-SiO2 phase diagram
If further increase of SiO2 in the solution occurs, the process of solid cristobaliteformation 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.
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.
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
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.
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.
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
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.
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.
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
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.
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.
Thank you!
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:
top related