Secondary Metallurgy - what does it mean?

The conventional practice of steel making is characterised by producing the required steel grade in one single operational unit. The consequences are long melting times, extended refining periods and high stresses on the steelmaking equipment itself.

Using  modern technology the steel production is divided into a primary and a secondary step which makes steelmaking much more economical. The primary step comprises the production of a basic melt. After oxidation of carbon and phosphorus, the melt is tapped directly into a ladle. The temperature of the melt is kept at a low level which prolongs the life cycle of the refractory lining and increases productivity.
Secondary metallurgy takes place exclusively in the ladle and comprises all further measures required to produce a high-grade steel.

Functions of the equipment for secondary metallurgy

The necessary functions of the equipment used for secondary metallurgical treatments are as follows:

• introduction of inert stirring gas to homogenise the ladle content
• accurate addition of alloying elements, achieving high alloy yield and close chemical composition tolerances
• vacuum treatment to remove hydrogen, oxygen, nitrogen and last, but not least, carbon
• injection of powder materials, particularly for desulphurisation purposes
• feeding of solid or cored wires containing various powdered materials for micro-alloying and inclusion shape control
• heating of the melt with electrical energy through graphite electrodes or, alternatively, with chemical energy using the exothermic reaction of aluminium and oxygen.

Vacuum treatment - a special application of secondary metallurgy

Pressure dependent reactions are the key to remove certain undesired elements in steel. Such vacuum treatment results in removal of elements like carbon, oxygen, hydrogen or nitrogen from the steel.

For these reactions to take place, the thermodynamic equilibrium, the inert gas flow rate and the transfer kinetics from the liquid to the gaseous phase are factors which play an important role.

SMS Mevac has applied these principles to units of industrial scale, tailor-made for the wide range of customer requirements, utilizing interfacing modules with proven design.

Advantages and economy

• increases in productivity of the steel plant
• shortening of the tap to tap time of the melting unit
• savings on primary energy and utilities
• use of low cost charge material as well as ferro alloys
• consistent steel quality from heat to heat
• production of ultra clean steel
• development of new steel qualities
• low maintenance cost
• improved co-ordination of all units involved in the production sequence
• better working conditions for the operating personnel.