Introduction

Steelmaking in BOF is very fast. It takes approximately 15 to 20 minutes for oxygen blowing and 50 to 60 minutes to tap molten steel. Liquid steel at turndown results from several non linear interconnected complex processes like  gas/ liquid metal reaction as dependent on oxygen availability, slag/metal reaction as dependent on physic-chemical properties of slag and faster reaction rate induced by three phase dispersion (CO+ slag +metal droplets).

Control of the above processes is needed so that for a given input of hot metal, scrap, flux and oxygen flow rate, steel of the desired chemistry is produced with minimum loss of iron in slag

How are the above processes controlled?

Rates of gas/metal, slag/metal and gas/slag/metal droplet reaction are controlled by lance profile, oxygen flow rate and bottom stirring rate, it is required to raise or lower the lance distance for a given oxygen flow rate and bottom stirring rate so that steel of desired composition and tapping temperature can be obtained within the stipulated blowing time. For this purpose we need to develop process models which can describe the process quantitatively. These models must be supported by the data for accurate predictions for the future requirements.

Process Control Models    
   
One of the objectives of the process control models is to predict turndown composition and temperature of steel so that unnecessary blowing of oxygen is not required. For a long time human expertise was the control tool. The operator used to deliver instructions to exercise the process control. These controls were human specific.

Developments in computer has resulted into development of sophisticated models, like

   a)   Static model

   b)   Semi-dynamic model

   c)   Dynamic model