Continuous casting is a refinement of the casting process for the continuous, high-volume production of metal sections with a constant cross-section. It allows lower-cost production of metal sections with better quality, due to finer control through automation of the casting process. Steel is the metal with the largest tonnage cast by this process, although aluminium and copper are also continuously cast. One half of a bronze mold for casting a socketed spear head dated to the period 1400-1000 BC. This article is about the manufacturing process. ... The old steel cable of a colliery winding tower Steel is a metal alloy whose major component is iron, with carbon being the primary alloying material. ... General Name, Symbol, Number aluminium, Al, 13 Chemical series poor metals Group, Period, Block 13, 3, p Appearance silvery Atomic mass 26. ... General Name, Symbol, Number copper, Cu, 29 Chemical series transition metals Group, Period, Block 11, 4, d Appearance metallic brown Atomic mass 63. ...

Equipment and Process

Continuous casting machine; molten metal is indicated by the hatched area

Molten metal (known as hot metal in industry) is tapped into the ladle from furnaces. After undergoing any ladle treatments, such as alloying or degassing, the ladle is transported to the top of the casting machine. Usually, the ladle sits in a slot on a rotating turret on top of the casting machine; one ladle is 'on tap' (feeding the casting machine) while the other is made ready, and is switched to the tapping position once the first ladle is empty.

From the ladle, the hot metal is transferred via a refractory shroud (pipe) to a holding bath called a tundish. The tundish allows a reservoir of metal to feed the casting machine while ladles are switched, thus acting as a buffer of hot metal. The tundish also helps any turbulence in the metal to smooth out - depending on temperature and composition, the metal can have a consistency ranging from similar to water to similar to honey - and also allows any slag present in the metal to float to the top of the molten pool. Although not shown in the image above, the tundish may also contain dams to control the flow of metal. The term refractory can refer to multiple things: A refractory clergyman is one who refused to swear an oath to the French Revolution-era French state under the Civil Constitution of the Clergy. ... For the Transformers character, see Slag (Transformer). ...

Metal is drained from the tundish through another shroud into the top of an open-base copper mould. The depth of the mould can range from 0.5m to 2m, depending on the casting speed and strand size. The mould is water-cooled and oscillates vertically to prevent the metal sticking to the mould walls. Powder can also be added to the metal in the mould to prevent sticking, and to trap any slag particles - including oxide particles or scale - that may still be present in the metal and bring them to the top of the pool to form a floating layer of slag. Often, the shroud is set so the hot metal exits it below surface of the slag layer in the mould. In some cases, shrouds may not be used between tundish and mould. Moldy cream cheese Molds (British English: moulds) are various fungi that cover surfaces as fluffy mycelium and usually produce masses of asexual, sometimes sexual spores. ...

In the mould, a thin layer of metal next to the mould walls solidifies before the metal section, now called a strand, exits the base of the mould into a spray-chamber; the bulk of metal within the walls of the strand is still molten. The strand is immediately supported by closely-spaced, watercooled rollers; these act to support the walls of the strand against the ferrostatic pressure (compare hydrostatic pressure) of the still-solidifying liquid within the strand. To increase the rate of solidification, the strand is also sprayed with large amounts of water as it passes through the spray-chamber. Final solidification of the strand may take place after the strand has exited the spray-chamber.
It is here that the design of continuous casting machines may vary. The image above shows a 'horizontal' casting machine; vertical configurations are also used. In a horizontal casting machine, the strand exits the mould vertically and as it travels through the spray-chamber, the rollers gradually curve the strand towards the horizontal. In a vertical casting machine, the strand stays vertical as it passes through the spray-chamber. Hydrostatic pressure is the pressure exerted by a fluid due to its weight. ...

After exiting the spray-chamber, the strand passes through straightening rolls (if cast on a horizontal machine) and withdrawal rolls. There may be a hot rolling stand after withdrawal, in order to take advantage of the metal's hot condition to pre-shape the final strand. Finally, the strand is cut into predetermined lengths by mechanical shears or by travelling oxyacetylene torches, is marked for identification and then taken away to a stockpile or (usually) to the next forming process. Sections that can be continuously cast include strip (a few millimetres thick by ~5m wide), blooms (630mm wide, 400mm thick, 5-6m long), billets (90 - 160mm square cross-section, 12m long) and slabs (1.25m wide, 230mm thick, 12m long). For billets, several strands may be fed from the same tundish. Hot rolling is a metallurgical process in which the metal is passed through a pair of rolls and the temperature of the metal is above its recrystallization temperature. ...

Startup, Control of the Process and Problems

Starting a continuous casting machine involves placing a dummy bar (essentially a metal beam) up through the spray chamber to close off the base of the mould. Metal is poured into the mould and withdrawn with the dummy bar once it solidifies.

Many continuous casting operations are now fully computer-controlled. Several electromagnetic and thermal sensors in the ladle shroud, tundish and mould sense the metal level, flow rate and temperature of the hot metal, and set the rate of strand withdrawal via speed control of the withdrawal rolls. Flow rate of hot metal through the shrouds is controlled by slide gates at the tops of the shrouds. The computer can also set the mould oscillation rate and the rate of mould powder feed, as well as the mould cooling rate (through control of the water flow).

While the large amount of automation helps produce castings with no shrinkage and little segregation, continuous casting is of no use if the metal is not clean beforehand, or becomes 'dirty' during the casting process. One of the main methods through which hot metal may become dirty is by oxidation, which occurs rapidly at molten metal temperatures (up to 1700˚C). To prevent oxidation, the metal is isolated from the atmosphere as much as possible. This is why shrouds are used for metal transfer, and why powder is introduced to sit on top of the mould pool. Often the tundish metal pool is also covered in a slag layer.

A major problem that may occur in continuous casting is breakout. This is when the thin shell of the strand breaks, allowing the still-molten metal inside the strand to spill out and foul the machine, requiring an expensive shutdown. Often, breakout is due to too high a withdrawal rate, as the shell has not had the time to solidify to the required thickness, or the metal is too hot, which means that final solidification takes place below the straightening rolls and the strand breaks due to stresses applied during straightening.

References

• Mechanical Engineer's Reference Book, 12th Edition. Edited by E.H. Smith. Published by Elsevier, Amsterdam, 1998.
• T Frederick Walters, Fundamentals of Manufacturing for Engineers. Taylor and Francis, London, 2001
• Section sizes from the Bluescope Steel website