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Steel in flux: casting the steel
The still-liquid steel produced in secondary metallurgy is cast and solidified for subsequent further processing. About 96 per cent of the liquid steel in Germany now undergoes a continuous casting process – this figure is about 93 per cent worldwide. The rest is cast in so-called ingots. In material flow terms, casting operations in an iron and steel mill are located after the steelworks and before further processing operations, the rolling mill or forging plant.
In continuous casting the liquid steel is transported from the pouring ladle to a water-cooled mould via distribution troughs. The shape of the mould determines the format of the strand. As a result of its liquid core, the strand must be cooled with water or a water-air mixture until it has solidified completely. The strand is supported by rollers in order to prevent any bursting out of the strand shell. The intensive cooling leads to an evenly solidified structure with favourable technological properties. After hardening, the strand can be divided into the desired lengths by means of a cutting torch or shears.
In ingot casting the casting moulds (described as ingot moulds) are filled with liquid steel via a fireproof system of channels either, most commonly, from below or, in rare cases, from above. The steel solidifies in the moulds. After the steel has solidified, the moulds are stripped (i.e. separated from the solidified block) using a crane. The steel can then be further processed in a rolling mill or forging plant. Common formats are round, square or polygonal blocks. Block weight can vary between 50 kg and 600 tonnes.
Ingot casting is mainly used for special steels in small quantities, semi-finished products for profiles with large cross-sections (for the forging industry), slabs for thick sheets, roller bearing and tool steels, fully austenitic steels and steel grades that are not sufficiently mastered in continuous casting.
Unlike ingot casting, continuous casting technology is designed for mass production. The possibility of sequential casting (the casting of several ladles in succession using a set of moulds) permits maximum plant capacity utilisation.
On the track of the perfect steel
In addition to a wide range of customer-specific steel grades, the European steel industry produces about 2,500 standardised steels. The compositions of about 100 steel grades are adapted to the increasing requirements every year. Up to 30 completely new steel grades are developed annually. Whereby – from the targeted adjustment of steel compositions, through the control of production processes, to proving that customer specifications have been met – the use of analytical processes plays a major role.
In system alliances with suppliers, scientific institutes and customers, the collaborating partners exploit shared knowledge and their differing competences and continuously expand their expertise. Germany has a unique steel research and development network. Whereby technical colleges and universities play just as important a role as institutes, e.g. the Max Planck Institutes or the Fraunhofer Society.