Thursday, April 1, 2010

Carbon Steel Fining

Fining is the process of removing traces of the so-called iron companions P, Si, Mn and reducing the carbon content to the desired level.

Small quantities of sulphur and phosphorus are heated in crucible. The sulphur combines with atmosphere oxygen burning with a blue flame to produce SO2 (liquid odor). The yellow phosphorus ignites when heated even slightly (caution). A white phoporus pentoxide (P2O3) is produced.

Steel is the result of chemical changes in pig iron when it is heated to temperature above 1600oC. The carbon is released as carbon dioxide in the process, and the sulphur and phoporus absorbed in the slag.

If air or oxygen is passed through the melt, phosphorus is oxidised to phosphorus pentoxide, sulphur to sulphur dioxide, silicon to silicon dioxide, manganese to manganese oxide and carbon to carbon dioxide.

Oxygen Conversion Process
The best known is the LD-process, named alter the Austrian steel works in Linz Donawitz.

About 70% of all steel used is produced by the oxygen conversion process, since it is more economical than the Siemen Martin (open hearth) process.

The LD-converter is charged with molten pig iron, sponge iron, scrap and additions.

Oxygen is blown through a water-cooled lance (steel pipe) on to the melt, at an overpressure of around 12 bar. The oxidation of carbon and iron companions generates a considerable quantity of heat, which sets the melt into violent motion. The excess heat is neutralized by adding cold scrap. Lime is added to bind the oxidised iron companions, such as manganese, silicon, phosphorus and sulphur into slag.

Alloying elements are added to improve the quality at the end of the lining process or while tapping. The steel thus produced is known as basic oxygen steel.

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