2. DEOXIDATION OF STEEL
• Oxygen is bound to dissolve in iron melt as steel
making is carried out under oxidising conditions.
• The oxygen content in iron varies inversely with the
impurity contents, particularly with carbon.
• As refining progresses, the oxygen content in the melt
increases.
• At the end of refining, a considerable amount of
oxygen ( 0.05-0.10% ) is left in liquid steel.
• The solid solubility of oxygen in pure iron is only
0.003%.
3. • If a steel with 0.05% oxygen is cast, the excess
oxygen is evolved in the form of gases leading to
blowholes and non metallic inclusions .
• The removal of residual oxygen content of refined
steel is known as deoxidation or killing of steel.
• The procedure involves adding materials with a high
affinity for oxygen, the oxides of which are either
gaseous or readily form slag.
• Deoxidation can be carried out either by single
element such as Si, Al, Mn or by mixture of elements
such as Si + Mn, Ca−Si−Al etc.
• It is termed as precipitation deoxidation.
4. THERMODYNAMICS OF DEOXIDATION
• Simple deoxidation can be represented by
• If deoxidation product is pure then activity of =1
and if elements are in dilute solution ,
• Where is deoxidation constant .
)(][][ baOMObMa
baOM
m
b
o
a
m kWW ][][
mk
5. YTXKm /log
where, X and Y are constants and T is temperature.
Comparison of deoxidising powers of various elements at 1600 degree C.
6. KINETICS OF DEOXIDATION
Kinetics of deoxidation :
• It consists of dissolution of deoxidisers into molten
steel, chemical reaction between dissolved oxygen
and the deoxidising element, nucleation and initial
growth of the deoxidation product.
• The process is fast and gets completed in a minute or
two.
Kinetics of elimination of deoxidation products :
• It consists of further growth of deoxidation products
by agglomeration and their elimination from liquid
steel.
• This is a slow process.
7. MECHANISM OF DEOXIDATION
• Dissolution and homogenization of the deoxidiser in
the steel melt so as to make the deoxidation reaction
move in the direction of oxide formation.
• Formation of critical nuclei of the deoxidation product
in a homogeneous medium since it involves formation
of a new phase.
• Progress of deoxidation resulting in growth of the
reaction products.
• Separation of product of the deoxidization reaction by
way of their floatation from the steel melt to improve
cleanliness.
8. DEOXIDATION PRACTICE
• At the end of refining, steel bath can be deoxidised
either inside the furnace or while being tapped in a
ladle.
• During tapping, bath is stirred due to potential
energy but this subsides towards the end. Hence,
bath stirring is important.
• Deoxidation products are lighter than steel; hence
they move up.
• During their movement, they may collide with one
another. Stirring of melt may help floating of de
oxidation products.
9. • Degree of stirring in the melt is important. Vigorous
stirring may not be of much help since deoxidation
product may be circulated in the liquid.
• For the removal of deoxidation product, equally
important is the design of synthetic slag to absorb
the deoxidation product.
• The required amounts of deoxidizers, are weighed
and kept ready in bags for charging during tapping.
• The first bag is not thrown in ladle until it is one-
third full.
• All the additions must be over by the time two-thirds
of the ladle is full.
10. • Steel is held in the ladle for nearly 10-20 minutes
after tapping is over.
• By this time the products of deoxidation are
expected to rise and stratify at the surface of the
metal.
• The tapping temperature should be high enough to
allow the additions and holding period and still be
left with adequate superheat to obtain good
castings.
11. DEOXIDISERS
Aluminium:
• It is a very effective deoxidiser which is used in most
steelmaking operations.
• It is used in the form of rods, pellets, powders etc;
• Usually the aluminum deoxidation is carried out in
the ladle.
• It is an alloying addition in heat resistant steels.
)(32 32OAlOAl
13. Silicon :
• It is used as a primary deoxidising agent in a furnace
where a reducing slag is to be made.
• It is used in the form of Ferro-silicon.
• Silicon is used as alloying element for better
strength, hardenability and electrical properties.
22 SiOOSi
14. Manganese:
• It is a weaker deoxidiser than silicon.
• As an alloying element it gives strength and
toughness.
• It is used in the form Ferro-manganese of various
grades.
)(MnOOMn
15. • Aluminium is a very effective since is far
more stable oxide than , MnO , etc.
• But is solid even at steelmaking temperatures
and hence cannot be used alone to deoxidise steel
completely.
• It is generally used along with Mn and Si when
aluminium has a chance to join the thin liquid slag
product of deoxidation.
• B, Ti, Zr are also effective deoxidisers. They are not
used primarily because of their high cost.
32OAl
2SiO
32OAl
16. INCLUSION CONTROL
• Inclusion formation in steels is in a way unavoidable.
• Attempts should be made to minimise them.
• The strategy is to modify the inclusions such that
they become less harmful.
• This process of inclusion modification is being called
inclusion engineering.
• The general approach is to convert the inclusions into
globular form so that they would not act as stress
raisers during rolling.
17. Bottom stirring by argon:
• The bubbles of argon rise from the bottom of ladle
and pick up the floating non-metallic particles and
are assimilated by the slag.
• This can achieve considerable cleanliness.
• Gentle stirring helps decantation of inclusions.
Calcium Injection:
• Much cleaner steels can be obtained through calcium
injection.
• It is either pure Ca metal or as Ca-Fe-Al alloy with
32% Ca and 40% Al in powder form.
• Calcium must injected as deep as possible in the
steel melt to improve its efficiency.
18. • When Calcium is introduced, it reacts with sulphides
and silicate inclusions to form complex sulphide-
oxide, which is molten at steel making temperatures.
• The inclusions are modified step by step and by
assimilation of Cao in particles.
• It converts some of the solid alumina into liquid
alumina particles and the others will be semi-solid or
even solid state.
• Calcium also combines with other oxide inclusions to
form liquid phase inclusions.
• These liquid phase inclusions then surround the
solid alumina and form inclusions with solid core and
liquid periphery.
32OAl
19. • In this process, all or part of the suspended alumina
are encircled by this liquid.
20. • The size of the original alumina inclusion increases
slightly when it gets enveloped by the liquid phase.
• It helps to move the particles upward.
• Calcium injection thus changes the morphology of
sulphide and alumina inclusions and help them rise
through the melt depth to join the slag at the surface
and thereby produce relatively much cleaner steels.
