Continuous casting

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 the inherently lower costs of continuous, standardised production of a product, as well as providing increased control over the process through automation. Steel is the metal with the largest tonnage cast by this process, although aluminium and copper are also continuously cast.

Incidentally, Sir Henry Bessemer - of Bessemer converter fame - received a patent in 1857 for casting metal between two contra-rotating rollers. The basic outline of this system has recently been implemented today in the casting of steel strip.

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Equipment and process

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 at the casting machine; one ladle is 'on cast' (feeding the casting machine) while the other is made ready, and is switched to the casting 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, as well as smoothing out flow, regulating metal feed to the molds and cleaning the metal (see below).

Metal is drained from the tundish through another shroud into the top of an open-base copper mold. The depth of the mold can range from 0.5 m to 2 m, depending on the casting speed and strand size. The mold is water-cooled and oscillates vertically (or in a near vertical curved path) to prevent the metal sticking to the mold walls. A lubricant (powder or liquid) can also be added to the metal in the mold 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 mold. In some cases, shrouds may not be used between tundish and mold. Some continuous casting layouts feed several molds from the same tundish.

In the mold, a thin shell of metal next to the mold walls solidifies before the metal section, now called a strand, exits the base of the mold 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 'curved apron' casting machine; vertical configurations are also used. In a curved apron casting machine, the strand exits the mold vertically (or on a near vertical curved path) 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. Molds in a curved apron casting machine can be straight or curved, depending on the basic design of the machine.

In a true "Horizontal Casting Machine" (image is not available) the mold axis is horizontal and the flow of steel is horizontal from liquid to thin shell to solid (no bending). In this type of machine, either strand oscillation or mold oscillation is used to prevent sticking in the mold.

After exiting the spray-chamber, the strand passes through straightening rolls (if cast on other than a vertical 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.

Casting machines for aluminium and copper

Aluminium and copper can be cast horizontally and can be more easily cast into near net shape, especially strip, due to their lower melting temperatures.

Range of continuously cast sections

• Casting machines are designated to be billet, bloom or slab casters.
• Slab casters tend to cast sections with an aspect ratio that is much wider than it is thick.
  • Conventional slabs lie in the range 100-1600 mm wide by 180-250 mm thick and up to 12 m long with conventional casting speeds of up to 1.4 m/minute.
  • Wider slabs are available up to 3200 mm wide.
  • Thin slabs: 1680 x 50 mm, 12 m long
• Conventional bloom casters cast sections above 200 mm x 200 mm e.g. the Aldwarke Bloom caster in Rotherham, UK casts sections of 480 x 360 mm. The bloom length can vary from 4 m to 10 m
• Billet casters cast smaller section sizes, such as below 200 mm square, with lengths up to 12 m long.
• Rounds: either 500 mm or 140 mm in diameter
• Conventional beam blanks: look similar to I-beams in cross-section; 1048 x 450 mm or 438 x 381 mm overall
• Near net shape beam blanks: 850 x 250 mm overall
• Strip: 2-5 mm thick by 760-1330 mm wide

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. It is extremely important that the metal supply afterwards be guaranteed to avoid unnecessary shutdowns and restarts. This requires the meltshop, including ladle furnaces (if any) to keep tight control on the temperature of the metal, which can vary dramatically with alloying additions, slag cover and deslagging, and the preheating of the ladle before it accepts metal, among other parameters. However, if the caster has multiple strands, one or more strands may be shut down (effectively slowing the casting rate) to accommodate upstream delays.

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 or weight, 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). Computer control also allows vital casting data to be repeated to other manufacturing centres (particularly the melt shop), allowing their work rates to be adjusted to avoid 'overflow' or 'underrun' of product.

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); inclusions of gas, slag or undissolved alloys may also be present. To prevent oxidation, the metal is isolated from the atmosphere as much as possible. To achieve this, exposed metal surfaces are covered - by the shrouds, or in the case of the ladle, tundish and mould, by synthetic slag. In the tundish, any inclusions - gas bubbles, other slag or oxides, or undissolved alloys - may also be trapped in the 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 well below the straightening rolls and the strand breaks due to stresses applied during straightening. Breakout can also occur if solidifying steel sticks to the mould surface, causing a tear in the shell of the strand. If the incoming metal is overheated, it is preferable to stop the caster than to risk a breakout. Another problem that may occur is a carbon boil - oxygen dissolved in the steel reacts with also-present carbon to generate bubbles of carbon monoxide. As the term boil suggests, this reaction is extremely fast and violent, generating large amounts of hot gas, and is especially dangerous if it occurs in the confined spaces of a casting machine. Oxygen can be removed through the addition of aluminium to the steel, which reacts to form aluminium oxide.

Computational fluid dynamics and other fluid flow techniques are being used extensively in the design of new continuous casting operations, especially in the tundish, to ensure that inclusions and turbulence are removed from the hot metal, yet ensure that all the metal reaches the mould before it cools too much. Slight adjustments to the flow conditions within the tundish or the mould can mean the difference between high and low rejection rates of the product.

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 and from the AISI's website on continuous casting