Composition and temperature during the blow

Typical variation of composition of metal with the blowing time is shown in figure 12.1

The following observations can be made:

• Impurities like C, Si, Mn begin to oxidize simultaneously. Si and Mn oxidize faster relative to C. Also Fe oxidizes to FeO. Rate of carbon removal is low in the beginning.
  
  
• Formation of slag begins with the oxidation of  and
  
  
• Dissolution of lime increases during the blow. In the initial periods FeO helps lime dissolution.
  
  
• Formation of basic and limy slag promotes removal of P. It may be noted that once slag is formed both C and P removal occur simultaneously.
  
  
• In the initial stages, carbon removal rate is kept lower than P removal since P removal is favoured at lower temperatures. If carbon is removed at a faster rate in the beginning bath temperature would increase which impedes dephosphorization. Once phosphorus removal is complete, carbon removal rate can be increased.
  
  
• Note that Mn content of metal decreases initially but at later periods of blow Mn content of bath increases.  This is due to the onset of the following reaction:

 In the later stages of the blow bath temperature increases due to decrease in carbon content and at the sametime FeO content of slag decreases. Both conditions are responsible for increase in Mn content of the bath. To overcome, sometimes iron ore additions are made to increase the FeO content of slag to adjust the Mn content of steel.

• Temperature of the bath increases continuously.