The Process
Continuous casting is the process whereby molten metal is solidified into a "semifinished" billet, bloom, slab or beam blank. Prior to the introduction of continuous casting in the 1950s, steel was poured into stationary moulds to form "ingots". Since then, "continuous casting" has evolved to achieve improved yield, quality, productivity and cost efficiency. Nowadays, continuous casting is the predominant way by which steel is produced in the world. Continuous casting is used to solidify most of the 750 million tons of steel, 20 million tons of aluminum, and many tons of other alloys produced in the world every year
In the continuous casting process, illustrated in Figure 1, molten metal is poured from the ladle into the tundish and then through a submerged entry nozzle into a mould cavity. The mould is water-cooled so that enough heat is extracted to solidify a shell of sufficient thickness. The shell is withdrawn from the bottom of the mould at a "casting speed" that matches the inflow of metal, so that the process ideally operates at steady state. Below the mould, water is sprayed to further extract heat from the strand surface, and the strand eventually becomes fully solid when it reaches the ''metallurgical length''.
Solidification begins in the mould, and continues through the different zones of cooling while the strand is continuously withdrawn at the casting speed. Finally, the solidified strand is straightened, cut, and then discharged for intermediate storage or hot charged for finished rolling.
To start a cast, the bottom of the mould is sealed by a steel dummy bar. This bar prevents liquid metal from flowing out of the mould and the solidifying shell until a fully solidified strand section is obtained. The liquid poured into the mould is partially' solidified in the mould, producing a strand with a solid outer shell and a liquid core. In this primary cooling area, once the steel shell has a sufficient thickness, the partially solidified strand will be withdrawn out of the mould along with the dummy bar at the casting speed. Liquid metal continues to pour into the mould to replenish the withdrawn metal at an equal rate. Upon exiting the mould, the strand enters a roller containment section and secondary cooling chamber in which the solidifying strand is sprayed with water, or a combination of water and air (referred to as "air-mist") to promote solidification. Once the strand is fully solidified and has passed through the straightened, the dummy bar is disconnected, removed and stored.
Heat Transfer in Continuous Casting
By its nature, continuous casting is primarily a heat-extraction process. The conversion molten metal into a solid semi-finished shape involves the removal of the following forms of heat:
These heats are extracted by a combination of the following heat-transfer mechanisms:
Also not less important is heat transfer before the molten metal is poured into the mould. or instance, in the casting of steel, heat transfer is important before the steel enters the mould because control of superheat in the molten steel is vital to the attainment of a predominantly equiaxed structure and good internal quality. Thus, conduction of heat into ladle and tundish linings, the preheat of these vessels, convection of the molten steel and heat losses to the surroundings also play an important role in continuous casting.
Because heat transfer is the major phenomenon occurring in continuous casting, it is also the limiting factor in the operation of a casting machine. The distance from the meniscus to the cut-off stand should be greater than the metallurgical length, which is dependent on the rate of heat conduction through the solid shell and of heat extraction from the outside surface, in order to avoid cutting into a liquid core. Thus, the casting speed must be limited to allow sufficient time for the heat of solidification to be extracted from the strand.
Heat transfer not only limits maximum productivity but also profoundly influences cast quality, particularly with respect to the formation of surface and internal cracks. In part, this is because metals expand and contract during periods of heating or cooling. That is, sudden changes in he temperature gradient through the solid shell, resulting from abrupt changes in surface heat extraction, causes differential thermal expansion and the generation of tensile strains. Depending on the magnitude of the strain relative to the strain-to-fracture of the metal and the proximity' of the strain to the solidification front, cracks may form in the solid shell. The rate of heat extraction also influences the ability of the shell to withstand the bulging force due to the ferrostatic pressure owing to the effect of temperature on the mechanical properties of the metal. Therefore, heat transfer analysis of the continuous casting process should not be overlooked in the design and operation of a continuous casting machine. 1.Thomas, B. G., “Continuous Casting: Modeling”, The Encyclopedia of Advanced Materials (Dantzig, J., Greenwell, A., Michalczyk, J., eds.), Pergamon Elsevier Science Ltd., Oxford, UK, Vol. 2, 1999 2.Carslaw, M. S. and Jaeger, C. J., Conduction of Heat in Solids, Oxford University Press, Oxford, 1973 3. Rubinsky, B. and Cravalho, E. G., “Analysis for the temperature distribution during the thawing of a frozen biological organ”, Heat transfer – San Diego 1979, A.I.Ch. E. sympl. Ser, 75 (189), 81-88 (979) 4. Fic, A., Nowak, A. J., and Bialecki, R., “Heat Transfer Analysis of the Continuous Casting Process by the Front Tacking BEM”, Engineering Analysis with Boundary Elements, Vol. 24, p. 215-223, 2000 5. Choudhary, K. and Mazumdar, Dipak, “Mathematical modeling of fluid flow, heat transfer and solidification phenomena in continuous casting of steel”, Steel Research, 66, No. 5, 1995
- superheat from the liquid entering the mould from the tundish.
- the latent heat of fusion at the solidification front as liquid is transformed solid, and finally
- the sensible heat (cooling below the solidus temperature) from the solid shell
- convection in the liquid pool.
- heat conduction down temperature gradients in the solid shell from the solidification front to the colder outside surface of the cast
- external heat transfer by radiation, conduction and convection to surroundings.
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