The L-D Process of Steel Making

The latest development in steel making process is the L-D Process. The name of this process comes from the initials of two separate plants in Austria, at Linz and Donawitz. The local Austrian ore is two low in Phosphorous to enable the air-blown basic Bessemer method to be used. Since air , a mixture of nitrogen and oxygen , is used , the resulting steel contains nitrogen which makes the steel liable to brittleness under certain conditions. Further the bulk of nitrogen which is not dissolved carries away so much heat that only a metal at high phosphorous content will generate enough heat to give the required temperature of the liquid steel. The remedy has been to replace the air blast by oxygen or a gas mixture containing no nitrogen.

The L-D process consists of blowing a jet of almost pure oxygen about 99.5 per cent at high pressure and travelling at supersonic speed through a water cooled lance on to the surface of molten iron held in a converter lined with basic refractories such as dolomite or magnesite. The tip of the lance is within about 1200mm of the surface of the bath. The blowing of oxygen at supersonic speed produces intense heat (about 2500 *C to 3000 *C ) and reduces having time which is 18 to 20 per cent of the original. The latter is achieved by increasing the proportion of the steel scrap in the charge up to 30 to 33 per cent instead of usual 5 to 8 per cent.

These advantages have won the L-D process general recognition. It is expected that over 20 million tones of steel will be produced by this process annually in the near future. The capacity of L-D converters is steadily increasing, the most commonly used size being 100 tons per heat. In modern combined blow techniques , oxygen is blown simultaneously from top as well as through bottom of the furnace. Output increases to 400 tons per heat.

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