Deoxidation of Liquid Steel

Before we know deoxidation mainly steel making is two type
1. Chemical and physical operation
2. Transport operation

● After process of primary steel making at BOF or EAF, there are lots of amount of oxygen present in liquid steel in the form of FeO. Which is to be removed by adding Al, FeSi, FeMn mainly. This is called  deoxidation of steel. For better deoxidation and secondary treatment of steel, is required a high basic and fresh slag (which is not a carry over slag from furnace) is required to give a protective layer for liquid steel from atmospheric reactions and losses of hea, also it helps in the inclusions separated from the steel bath.

Deoxidizer and it's Practice :

● For better Deoxidation stirring is essential beacuse it determine to better mass transport of a elements from one point to the another, which is good for oxide free steelmaking. During the tapping of metal in to ladle we added the deoxidizer or ferroalliy as per required continuously by which oxygen from the bath drive out, is called deoxidation of steel. The commonly used of deoxidizer are Al, FeSi, FeMn or SiMn etc, among them 'Al' have high affinity towards the oxygen and removes oxygen from the bath after that Si, Mn.

● All the deoxidizer are in room temperature before used and they added to steel ladle then their temperature usually grow up and melt in steel. After melting it dissolve in the bath and react with dissolve 'O' that present in bath.

For exam :
1. Al (solid) ----------> Al (liquid)
2. Al (liquid) --------> [Al] (dissolve) then
[Al]  +  [O]     --------> (Al2O3)
According to this dissolve reaction is

x[M]  +  y[O]  ----------> (MxOy)  slag phase.

● This is appropriate for all deoxidizer that used for detoxidation for steel, which is forward reaction. Where (MxOy) is the oxide slag phase. But in case of some deoxidation, this is the reaction of reversible in an equilibrium state.

● There are two types of deoxidizer used in steelmaking that is
Class 1 type : Primary deoxidizer (which have lower melting point from steel bath temperature)
So, in class 1 type deoxidizer the material are melts first, then it dissolve in steel and react with oxygen. This is called melt transport phenomenon.
Ex : FeSi, FeMn, SiMn, Al .             Tm1 < T (bath at 1600℃)

Class 2 type : Micro alloys (which have more melting point from steel bath temperature)
But, in class 2 type or Micro alloys the materials are doesn't melts because of high melting temperature rather then steel. It directly dissolve in steel which is a mass transport phenomenon.
Ex : Mo, Ti, Nb, V, B, Ni etc.            Tm2 > T (bath at 1600℃)

● According to elingram diagram 'C' also good tendency to deoxidized the melt but it is not preferred because of certain grade of steel are not allowed 'C' content in steel or in lower side,  so C is not recommended for deoxidiser.

● Due to the lighter weight then steel the primary material like FeSi, FeMn, FeSi, Al are added in the tapping where the liquid steel pinning to the ladle surface by which more chances to penetrate or downward motion to the steel bath and better results for deoxidation. As the deoxidizer elements get more downward to the bath they melt and dissolve with oxygen and slag formed, which is goes up. Due to the 'Ar' stirring the inclusion or formed slag are float at above the bath surface and continuously mass transport of elements is to be done.

● In under equilibrium condition the thermodynamics process is :

Steel deoxidation equilibrium equation
Steel deoxidation equilibrium

Activity of material used for deoxidation is inversely proportional to the rate of activity of oxygen in an equilibrium condition

Ex :  Aluminum required for 50ppm = 0.02%, but 10ppm = 0.05%.

● The deoxidizer have two requirement stage :
1. Stoichiometric required (According to theoretical)
2. Thermodynamics required (calculated through equilibrium)

According to material used deoxidation is two type :
1. Simple deoxidation ( Al, FeSi )
2. Complex deoxidation ( FeMn, SiMn )

● As final stage the deoxidation product have may be in solid, liquid or gaseous.

1. If the product are in gases and the bath slag is in liquid condition then it evaporate to the atmosphere easily.
2. If the product are in liquid form then, one or more liquid phases are contaminate with each other then due to lighter then steel it comes in slag at the above of steel surface.

3. If the product are in solid form then the inclusion are not combined each other and it present in liquid steel. Then we stirring mildly the steel bath by which the lighter solid inclusion are goes up slowly and contaminate with the liquid slag which is at the above of steel surface.

References :
1. Modern Steel Making :  Dr R.H. Tupkary and V.R. Tupkary.
2. Ironmaking and Steelmaking Theory an d Practice : A. Ghosh and A. Chatterjee
3. Steel Making : A.K.Chakravorty

4. Sr. Professors from IIT's & BPUT. (INDIA)

Author :
Subir Kumar Sahu

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