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: