steel making: lecture deoxidation

MY: 301 Steel Making Processes

Dr.R.H Tupkary, An Introduction to Modern Steel Making (Chapter No 04 page nos: 58-64)

Deoxidation of Steel: (Why Deoxidized)

Steel making is carried out under oxidizing condition, since oxygen is

bound to dissolve in it.

The solubility of Oxygen in liquid steel is 0.23% at 16000C and rise

0.48% at 18000C, but in solid it is only 0.003%

OR

The solubility of Oxygen in liquid steel is 0.16%, but in solid it is only

0.003% www.steeluniversity.org (Referance)

Therefore, steps have to be taken to reduce the excess oxygen

content (deoxidized) of the steel before it solidifies (casting) in order to

prevent blowholes formation during casting or large quantities of FeO

being precipitated.

The removal of residual oxygen content of refined steel is known as

deoxidation or killing of steel. it can be done as precipitation

deoxidation:

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Precipitation Deoxidation:

The principle behind precipitation deoxidation is to use elements (called Deoxidizers) having higher affinity for oxygen than iron are added to the melt.

It leads to the formation of oxide Product as

[X] + [O] (XO) any phase

This method is generally adopted as it is very effective in decreasing the oxygen content of steel.

The choice of a deoxidizer depends on a number of factors.

1. Those elements are preferred which are more stable and have highest possible deoxidizing ability.

2. Those elements to be preferred that form low melting oxides (passing rapidly into the slag)

3. Those elements to be used that form oxides that have lowest possible densities (come out from melt and join slag phase)

4. Insoluble in melt (steel) and do not revert (go) back to melt.

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Thermodynamics of Deoxidation:

Thermodynamically the best deoxidizers should have the minimum %

[O] in equilibrium with its own minimum contact in steel.

Use of Ellingham Diagram to help you decide which is suitable and

cheapest.

It indicates that Al, SI, Mn and C are commonly used deoxidizers for

being reasonably cheap.

It may be noted that Mn, Si and C are elements oxidized earlier as

impurities from iron and the same element are used later to remove

excess oxygen from the refined steel.

AL, Si and Mn are the most deoxidizers used in steel making. the

chemical reactions associated are.

2[Al] +3[O] (Al2O3)

[Si] + 2[O] (SiO2)

[Mn] + [O] (MnO)

Elements like Zr,Ti,B,V etc. may be used but these are costlier than

common deoxidezers.

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These element must be in the range of chemical specifications at the

end of deoxidation.

Effectiveness of Deoxidizers

Equilibrium concentration of oxygen under different concentration of

Mn. Si and Al.

Target dissolved oxygen/ppm

Equilibrium Concentration/% Temperature/0C

Mn Si Al

1000 0.5 0.003 4.5 x 10-6

1560100 5 0.3 0.00014

10 - - 0.0045

Al is more powerful deoxidizers than Si and Mn. Al will also reduce Si

and Mn oxidize if these are present in the refractories. Therefore for

maximum and predictable deoxidation by Al, fireclay refractories

(which are Si rich) should not be used.

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CALCULATING AL ADDITIONS

The weight percentage of aluminum required for deoxidation is therefore:

When calculating the total aluminum addition required, this value must be added to the aim (or residual) Al composition of steel

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ExampleA 250 tonne of steel having an oxygen content of 450 ppm is to be Al-deoxidized at tap. Assuming an Al recovery rate of 60 % and an aim Al composition of 0.04 %, calculate the amount of 98 % Al alloy addition that is required.

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Kinetics of deoxidation: Kinetically the deoxidizers should be quick in action to obtain its high

percentage utilization for deoxidation reaction.

Such data however largely unknown and thermodynamically rather

than kinetics consideration dictate the choice of a deoxidizers.

It is important that the oxide product (particles) of deoxidation

reaction should not remain in steel.

These particles are known as non-metallic inclusions which impair the

mechanical properties also this can cause nozzle blockage during

casting.

The mechanical properties vary with number, size, shape, distribution

and composition of these inclusions.

The kinetics of deoxidation reaction is not so much important as is the

kinetics of elimination of the product of deoxidation produce

relatively cleaner steel.

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Alumina Modification /Ca Treatment

The removal of oxides from liquid to the slag is most effective if the

oxide products are liquid.

This is because liquid particle are less dense and can combine to form

large particles, which will float to the surface.

Since the melting point of alumina is 20380C any particle will be solid at

typical casting temperature. This can cause nozzle blockage during

casting with a consequential reduction in casting speed and possibly

the formation of large aggregated alumina clusters in the solidified

product.

Calcium treatment is carried out to produce liquid calcium aluminates,

which reduce nozzle blockage.

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steel making: lecture deoxidation

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