Module 2: Modern Steelmaking Practice
  Lecture12: Converter Steelmaking Practice & combined blowing
 


Developments in Top blown steelmaking practice


The most important development in top blown steelmaking practice is the simultaneous gas stirring of the bath form the bottom of the converter. This has resulted in combination blowing processes. These processes differ in terms of bottom gas rate, number and arrangement of bottom tuyeres and type of bottom injection elements i.e. porous plugs or tuyeres and whether inert gas or oxidizing gas is used.

All processes which use top blowing of oxygen and bottom stirring by inert gas is known as bath stirred top blown processes. In another type, oxygen is blown form top and bottom and is called top and bottom blowing processes.

Concept of bottom stirring in top blowing

In pure top blown steelmaking, bath agitation is very weak particularly during the initial and final stages of the blow. In the initial stages Si and Mn removal delays carbon removal whereas in the final stages carbon removal rate decreases. Figure 12.3 shows decarburization rate Vs time, typically observed in top blown steelmaking practice.

Figure 12.3:

Rate of carbon removal as a function of time of blow

In the hatched regions, CO evolution in the bath is very low in pure top blown steelmaking;  jet could not produce adequate bath stirring. Evolution of CO is the principle cause of bath agitation. Both in initial periods (silicon oxidation period) and in final periods (where rate of carbon removal is mass transfer controlled), evolution of CO is low. Slag analysis reveals higher rate of oxidation of Fe to FeO in both the periods which is due to weak stirring in the bath. In main part of the blow higher carbon removal rate produces higher amount of CO and produces enough bath stirring. It is considered appropriate to introduce bottom stirring gas in a top blowing converter to stir the bath.