How a Steel Mill Continuous Caster Works
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Background
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Continuous casting works as a steady-state process. This means that, essentially, the state of the metal (steel in this case) at any juncture of production will always be the same no matter when it is observed. This is accomplished using a continuous caster. There are a variety of caster designs, but most steel mills use a curved casting machine. This means the steel leaves the mold and must be curved or bent into a flat piece. The continuous casting process can be used to produce a variety of thicknesses and shapes, including rectangular, circular and I-shaped (also called dog-bone). While the precise process varies among designs, most continuous casters follow a process similar to what follows.
Production
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The process begins when molten steel is fed from a ladle into a tundish. The tundish is an intermediate holding container lined with refractory bricks that simultaneously holds molten steel and feeds it into the mold. A layer of slag on top of the molten steel is used to protect the metal from oxygen exposure. It is standard for a ceramic nozzle to feed the molten steel from the ladle to underneath the slag layer. The flow of molten steel between the tundish and the mold is controlled by a slide gate. The molds are made of copper and water cooled. As the molten steel enters the mold, it partially solidifies on the outside to form a shell. It is common for oil to be inserted between the mold and the shell to improve the surface production and to protect the metal from oxygen damage. The mold is moved up and down (oscillated) to stop the shell from adhering to the mold.
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The shell is drawn from the bottom of the mold by drive rolls. These are set to a constant speed that is (ideally) the same as the rate of the metal flow. When the metal reaches the rolls, it is referred to as the strand. The rolls themselves are carefully spaced to minimize the potential for bulging in the strand from ferrostatic pressure (pressure put on the shell by the molten steel core that is still solidifying). As the strand moves along the rolls, it is sprayed with water at a predetermined rate to assist in solidifying the core. In some systems, electromagnetic stirrers are used to keep the molten core of the strand properly mixed during the cooling process. When the strand has been sufficiently cooled, it is cut. This is done with an oxyacetylene torch.
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