Continuous casting is a process in which molten steel at 1,600°C is converted into slabs of manageable size. The ladle with molten steel is placed in a holder. From the ladle, the steel is tapped through a nozzle into the tundish. The tundish is an intermediate vessel designed to maintain a constant level and allows for flying ladle changes during the course of casting in a continuous process.
Continuous casting takes place through a water-cooled mould that is open at the top and bottom.
A casting powder is used, so that the steel will slide smoothly through the mould. Intensive water cooling of the mould side plates immediately gives the hot melt a hard shell of solidified steel. The cooled steel shrinks in volume as it is withdrawn from the underside of the mould in a long strand. The strand is continuously cooled on its arc-shaped path down to the cutting station.
At this stage, the steel is still hot and glowing, but is sufficiently solid to enable the strand to be cut with movable oxygen lances into pieces up to 11 meters long.
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Slabs can vary in length, width and thickness depending on the SSAB plant in which they are produced and the customer specifications. A common slab weighs about 25 tonnes. The steel from one ladle is sufficient for a handful of slabs and is the smallest quantity of a given grade of steel that SSAB can produce.
The slabs we cast in Sweden are thicker than those cast in the USA, where we can cast thinner and wider slabs. Casting thinner slabs has energy benefits, since the slabs do not need to be rolled as many times to reach the desired thickness.
Every slab has an identity number and is a carefully recorded individual intended for a certain end product at a certain customer. Samples of the steel are taken throughout the production chain and finally also from the slab. All slabs are inspected and some aftertreatment may be necessary.
The slabs are cooled in different ways. Certain sensitive steel grades require slow cooling at a uniform rate, and the slabs are left to cool under special hoods.
The metallurgy processes take about 12 hours from the time that the blast furnace is charged with pellets and the iron has passed through the steelworks, to the time when slabs are produced and ready for rolling into high strength steel plate.
Steel commuter train
The “Steel commuter train” is a train system in Sweden that runs from Luleå and Oxelösund to Borlänge, where the rolling mills receive the steel slabs from both SSAB metallurgical plants. In addition, Oxelösund has its own rolling mill. In the USA, both steelworks have integrated metallurgical plants with their own rolling mills.