Side reactions

During vacuum degassing the following reaction may occur. Nitride and oxide inclusions can decompose according to

 

Application of vacuum decreases nitrogen which favors decomposition of . Oxide inclusions can react with C

 

Lowering of  pressure favours the forward reaction. Reaction between lining and carbon of liquid steel or decomposition of lining may occur:

 

 

 

Note that  is a highly reactive gas at the steelmaking temperature. All reactions generate one or more gaseous species, hence lowering of pressure promotes the occurrence of side reactions. Mg and Ca are stable gases at steelmaking temperatures.

Volatilization of elements of high vapor pressure may occur.  and  have high vapor pressures and their losses occur during vacuum treatment. Loss of and  is negligible.

General considerations

1. The desorption of gases is a gas/ metal interfacial reaction.  The atomic nitrogen from the molten steel has to diffuse at the gas/metal interface, where it is converted to molecular nitrogen which can then be desorbed. The effectiveness of vacuum treatment increases with increase in surface area of liquid exposed to vacuum. The increased surface area of molten steel exposed to vacuum e.g. in the form of a thin stream or gas induced stirring will accelerate the degassing process.
   
   
2. Temperature of molten steel drops during vacuum treatment. More is the surface area of stream exposed to vacuum higher will be the temperature drop. To compensate for the loss of heat, the following alternative may be considered.
   
   

   Tapping of steel at a higher temperature. This requires increased heat load in BOF/EAF
   
   

   Additional heating during vacuum treatment.
   
   
3. The degassing time must be kept to minimum.
   
   
4. The degree of degassing increases with the degree of vacuum. Vacuum of the order of 1mm or even less than  1mm Hg (1mmHg=1torr) is employed in the practice. Vacuum pumping capacity should be adequate.

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