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1. 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 16000
C and rise
0.48% at 18000
C, 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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2. MY: 301 Steel Making Processes
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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3. MY: 301 Steel Making Processes
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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4. MY: 301 Steel Making Processes
• 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/0
C
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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5. MY: 301 Steel Making Processes
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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6. MY: 301 Steel Making Processes
Example
A 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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7. MY: 301 Steel Making Processes
• 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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8. MY: 301 Steel Making Processes
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 20380
C 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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