Secondary Steelmaking

The major steelmaking processes like Open heart, LD, OBM, Electric Arc etc are known as primary steelmaking processes wherein major portion of the total refining or melting is carried out. But, still the liquid steel cannot achieved cleanliness, tighter grain size control, hadenability. Thus the final refining and finishing is carried out in secondary steelmaking process. Basically it resorted to achieve :

1. Improvement in quality
2. Improvement in production rate
3. Decrease in energy consumption
4. Use of relatively cheaper grade or alternative raw materials
5. Use of alternative sources of energy
6. Higher recovery of alloying elements.

Categories of Secondary Steelmaking :

1. Stirring Treatment
2. Synthetic Slag refining with stirring
3. Vacuum Treatments
4. Decarburization Technique
5. Injection Metallurgy
6. Plunging Techniques
7. Post-solidification Treatment.

● Each one of these processes may be capable of achieving more than one goals of refining and finishing. Where many of these processes are carried out in a steel transfer ladle car or in a simple ladle like furnace which is known as Ladle Furnace(LF). So secondary steelmaking mainly popular as a ladle metallurgy.

● The addition of secondary unit in a way forms a duplex process. The principle of duplexing is to shift the major part of the lengthy refining, alloying, deoxidation which are also desulphurisation operation to the unit operated at the times under vaccum.

Stirring Techniques :

Stirring a steel heat in the transfer ladle is the simplest kind of secondary treatment of steel. It aims :
1. Homogenisation of temperature and composition
2. Minor alloying for close control of chemistry of the bath
3. Cleanliness improvement.
Ladle stirring - porous plug of steel ladle
Ladle stirring through bottom porous plug

● Stirring is caused by bubbling inert gas like argon in the refined steel bath. This has a quenching effect as well. The rising bubbles tend to lift up the non metallic inclusions, due to its surface tension effect and these join the slag layer invariably present at the surface of the bath. As the bubbling has chilling effect on the bath it must be used sparing to achieve most of the beneficial effect.
Steel ladle porous plug
Porous plugs

● Gas is bubbled either through a refractory protected Lance insert in the bath from the top or porous plug which is made of magnesia or alumina, fitted at the bottom of the ladle. The plug porosity must be maintained around 30 to 35% as it tends to get clogged by the slag towards the end of the teeming.

Synthetic Slag Refining with Stirring :

● It involves the use of specially prepared slag to act as a sink for the non metallic inclusions. These slags are essentially non oxidising in nature and used in effecting desulphurisation as well. An unstirred slag metal system is most ineffective in carrying out any interaction between slag and metal. Hence argon stirring through porous plug provided at the ladle bottom, is employed to cleaning of the bath and desulphurisation.

● In the sealed argon bubbling, known as SAB process, a slag of the composition CaO : SiO2 : Al2O3 = 40 : 40 : 20 is put on the surface of the bath in the transfer ladle. An immersion box is used to dip just below the slag layer. It is located vertically above where the porous plug is fitted at the bottom. This prevents the metal from getting exposed to atmosphere due to the rising bubbles.

● A similar process using a capped ladle is known as capped argon bubbling or CAB for short. Both of these processes have been developed and practiced by the NSC, Japan. They claim that these are the simplest and very cheap secondary refining processes. The advantages of stirring the bath are all available in this as well.

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

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