Continuous Casting : In essential, the process consists of pouring molten metal into a short vertical metal die or mould (at a controlled rate), which is open at both ends, cooling the melt rapidly and withdrawing the solidified product in a continuous length from the bottom of the mould at a rate consistent with that of pouring as shown in figure.
The process is used for producing blooms, billets and slabs for rolling structural shapes. The process (a German invention perfected in the early 1960's) allows steel produces to by pass the costly process of pouring molten metal (steel) into ingots and then reheating them for milling (roll) and finishing. This is cheaper than rolling from ingots. Any shape of uniform cross-section (solid or hollow) such as : round, rectangular, square, hexagonal, gear toothed and many other forms can be produced by the process. The process is mainly employed for copper, brass, bronze and aluminium and also increasingly with C.I. and steel. Again, the process to quite a considerable extent, has replaced the batch methods of producing ingot castings.
Since the mould is an open cylinder of the required cross-section, a metal block must be placed at the lower end of the mould to support the initial metal until it has solidified. The object of the process must be to achieve such a steady state of thermal equilibrium that the casting solidifies before leaving the mould. The mould is cooled by circulating water around it. The solidification zone is localized in the water cooled mould when casting non-ferrous metals. However, ferrous metals conduct heat comparatively slowly, freeze only skin deep while in the mould. They are further cooled with the water sprays throughout after leaving the mould. The solidified casting is withdrawn gradually from the bottom end of the mould at a controlled speed by the withdrawing rolls, typically at speeds of 25 mm/s.
Theoretically, the process may go on almost indefinitely. In that case, the required lengths of billet, bloom or slab are cut up during their movement with a flying saw or torch. In the "semi continuous casting process", the required lengths of billets, blooms and slabs or ingots are obtained by successively pouring to remove the casting. Large diameter and long iron water pipes with flanged ends are also cast by the semi-continuous process with a separate charge of molten metal for each pipe.
Moulds are made of graphite or copper (particularly for casting of steels due to its high-heat conductivity). To prevent the adhesion and welding of the melt to the mould, its surface is given a coating of ceramic powder for steel castings or of a lubricating type parting compound often containing graphite or molybdenum disulphide. Some systems employ vibrating or reciprocating mould to keep the casting from sticking.
Continuous casting process has many advantages : High quality metal can be obtained because it is protected from contamination while melting and being poured. Its yield in rolled shapes is about 10% more as compared to that from ingots. The physical properties and surface qualities are qualities are comparable to those obtained in other permanent mould processes. Ease of automation, Reduced energy consumption, Fuel economy, Less installation and operating costs and safety. Also, increased productive capacity, improved product structure and reduced waste metal.
Fig. shows the principle of continuous casting of steel. The system contains water cooling mould as well as secondary cooling zone where the spray of water brings the ingot to complete solidification. As it passes through the withdrawal-roll set, the ingot enters the carriage of a cutting device moving together with the hot ingot. A gas cutter cuts off the ingots into billets. Roller conveyers then carry the billets for rolling.