Degassing of Liquid Steel

Principle :

During steelmaking gases like oxygen, hydrogen and nitrogen dissolve in steel. The term degassing is employed to removed nitrogen and hydrogen from steel. Dissolve oxygen from steel melt cannot be removed as molecular oxygen, by which removal of oxygen is term deoxidation.

● Both nitrogen and hydrogen impair the mechanical properties of steel. The maximum solubility of nitrogen in liquid steel is 450ppm and less then 10ppm at room temperature. During solidification excess nitrogen is rejected which may from either blow holes or nitrides. Excess nitrogen causes embrittlement of heat affected zone of welded steels and impair cold formability.

● Hydrogen in steel impair steel properties. Solubility of hydrogen in steel is low at ambient temperature. Excess hydrogen is required during solidification and results in pinhole formation and porosity in steel. Few ppm of hydrogen causes blistering and loss of tensile ductility. Thus both are necessary to removed.
Rh degassing
Rh degassing

[H]  =  1/2{H2}
[N]  =  1/2{N2}
[Wt% N]  =  (P. 1/2N2 × Kn) / fN
[Wt% H]  =  (P. 1/2H2 × Kh) /fH

fH and fN are activity coefficient of nitrogen and hydrogen in steel. PN2 and PH2 are equilibrium partial pressure of nitrogen and hydrogen in steel.

Side Reaction :

During vaccum degassing nitride and oxide inclusion can decompose according to 

Al N  =  [Al]  +  [N]
Application of vacuum nitrogen which favours decomposition of Al N oxide inclusion can react with carbon,

SiO2 + C  =  {Si} + 2{CO}
Lowering of CO pressure favours the forward reaction. Reaction between lining and carbon of liquid steel or decomposition of lining may occurs

SiO2 + C = {Si} + 2{CO}
MgO = Mg(g) + [O]
CaO = Ca(g) + [O]
MgO + C = Mg(g) + {CO}

● SiO2 is a high reactive gas at the steel making temperature. All reaction generate one or more gaseous species, hence lowering of pressure promotes the occurrence of side reaction Mg and Ca are stable gases at steel making temperature. Volatilization of elements of high vapour pressure may occurs. Mn and Fe have vapour pressure and their losses occurs during vacuum treatment. Losses of Aluminum and Silicon is negligible.

Fluid flow in Degassing :

Degassing can be either by placing ladle, containing steel under vacuum or recirculation of molten steel in vacuum.

● In ladle degassing the effectiveness of degassing decreases from top to bottom of the molten steel bath. Bottom layers of steel are very much less affected by vaccum, since these layer are under the influence of ferrostatic pressure due to column of liquid steel. Hence bath agitation would help exposing the entire content of molten steel to the vacuum.

● Argon bubbling during degassing of molten steel leads to massive volumetric expansion of bubbles due to temperature. More over rising gas bubble absorbed dissolved gases. Radial expansion of gas bubbles in vacuum in between 10t/min to 100t/min.

The process of degassing are three types
1. Ladle degassing
2. Steam degassing
3. Circulation degassing.

All the process are carried out in ladle.

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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