DEOXIDATION OF STEEL AND INCLUSION CONTROL

BATCH - 7

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%.

• 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.

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

mb

oa

m kWW ][][

mk

YTXKm /log where, X and Y are constants and T is temperature.

Comparison of deoxidising powers of various elements at 1600 degree C.

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.

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.

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.

• 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.

• 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.

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

Al-O Equilibrium at Various Temperatures

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

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

• 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.

32OAl2SiO

32OAl

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.

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.

• 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

• In this process, all or part of the suspended alumina are encircled by this liquid.

• 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.

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