Continuous casting


Continuous casting, also called strand casting, is the process whereby molten metal is solidified into a "semifinished" billet, bloom, or slab for subsequent rolling in the finishing mills. Prior to the introduction of continuous casting in the 1950s, steel was poured into stationary molds to form ingots. Since then, continuous casting has evolved to achieve improved yield, quality, productivity, and cost efficiency. 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. This process is used most frequently to cast steel. Aluminium and copper are also continuously cast.
Sir Henry Bessemer, of Bessemer converter fame, received a patent in 1857 for casting metal between two counter-rotating rollers. The basic outline of this system has recently been implemented in the casting of steel strip.

Equipment and process

Steel

Molten metal is tapped into the ladle from furnaces. After undergoing any ladle treatments, such as alloying and degassing, and arriving at the correct temperature, 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 in the "on-cast" position while the other is made ready in the "off-cast" position, and is switched to the casting position when the first ladle is empty.
From the ladle, the hot metal is transferred via a refractory shroud 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.
Usually a disposable working lining refractory known as "tundish boards" is used.
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, depending on the casting speed and section size. The mold is water-cooled to solidify the hot metal directly in contact with it; this is the primary cooling process. It also oscillates vertically to prevent the metal sticking to the mold walls. A lubricant is added to the metal in the mold to prevent sticking, and to trap any slag particles—including oxide particles or scale—that may be present in the metal and bring them to the top of the pool to form a floating layer of slag. The shroud is set so the hot metal exits it below the surface of the slag layer in the mold and is thus called a submerged entry nozzle. In some cases, shrouds may not be used between tundish and mold ; in this case, interchangeable metering nozzles in the base of the tundish direct the metal into the molds. 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 center, and then the molded metal, now called a strand, exits the base of the mold into a spray chamber. The bulk of the metal within the walls of the strand is still molten. The strand is immediately supported by closely spaced, water-cooled rollers which support the walls of the strand against the ferrostatic pressure of the still-solidifying liquid within the strand. To increase the rate of solidification, the strand is sprayed with large amounts of water as it passes through the spray-chamber; this is the secondary cooling process. 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. This describes a "curved-apron" casting machine; vertical configurations are also used. In a curved-apron casting machine, the strand exits the mold 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. 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, the mold axis is horizontal and the flow of steel is horizontal from liquid to thin shell to solid. In this type of machine, either strand or mold oscillation is used to prevent sticking in the mold.
After exiting the spray-chamber, the strand passes through straightening rolls and withdrawal rolls. There may be a hot-rolling stand after withdrawal 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 is taken either to a stockpile or to the next forming process.
In many cases, the strand may continue through additional rollers and other mechanisms which may flatten, roll, or extrude the metal into its final shape.
Developments since the mid 1980s reduced the thicknesses that can be cast, initially to transfer bars of ~50mm thickness, also called thin slabs, and then more recently down to thin strip castings of 2mm thickness.

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, slab, or strip casters.
  • Slab casters tend to cast sections that are much wider than 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; however, slab widths and casting speeds are currently increasing.
  • *Wider slabs are available up to 3250×150 mm.
  • *Thick slabs are available up to 2200×450 mm at a specific steel facility, generically ranging typically from 200mm to 300mm.
  • *Thin slabs : 1680×50 mm at a specific facility, generically ranging from 40mm to 110mm thick depending on an individual machine's design.
  • Conventional bloom casters cast sections above 200×200 mm. The bloom length can vary from 4 to 10 m.
  • Billet casters cast smaller section sizes, such as below 200 mm square, with lengths up to 12 m long. Cast speeds can reach up to 4 m/minute.
  • Rounds: either 500 mm or 140 mm in diameter.
  • Conventional beam blanks: look similar to I-beams in cross-section; 1048×450 mm or 438×381 mm overall.
  • Near net shape beam blanks: 850×250 mm overall.
  • Strip: 2–5 mm thick by 760–1330 mm wide.

    Startup and control of the process

Starting a continuous casting machine involves placing a dummy bar up through the spray chamber to close off the base of the mold. Metal is poured into the mold 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, known as turnarounds. Each time the caster stops and restarts, a new tundish is required, as any uncast metal in the tundish cannot be drained and instead freezes into a "skull". Avoiding turnarounds requires the melt shop, including ladle furnaces, to keep tight control on the temperature of the metal, which can vary dramatically with alloying additions, slag cover, deslagging, and the preheating of the ladle before it accepts metal, among other parameters. However, the cast rate may be lowered by reducing the amount of metal in the tundish, or if the caster has multiple strands, one or more strands may be shut down to accommodate upstream delays. Turnarounds may be scheduled into a production sequence if the tundish temperature becomes too high after a certain number of heats or the service lifetime of a non-replaceable component is reached.
Many continuous-casting operations are now fully computer-controlled. Several electromagnetic, thermal, or radiation sensors at the ladle shroud, tundish, and mold sense the metal level or weight, flow rate, and temperature of the hot metal, and the programmable logic controller can set the rate of strand withdrawal via speed control of the withdrawal rolls. The flow of metal into the molds can be controlled via three methods:
  • By stopper rods that descend through the tundish
  • By slide gates at the top of the mold shrouds
  • If the metal is open-poured, then the metal flow into the molds is controlled solely by the internal diameter of the metering nozzles. These nozzles are usually interchangeable.
Overall casting speed can be adjusted by altering the amount of metal in the tundish, via the ladle slide gate. The PLC can also set the mold oscillation rate and the rate of mold powder feed, as well as the flow of water in the cooling sprays within the strand. Computer control also allows vital casting data to be transmitted to other manufacturing centres, allowing their work rates to be adjusted to avoid "overflow" or "underrun" of product.

Problems

Contamination by oxygen

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 ; 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 liquid-metal surfaces are covered – by the shrouds, or in the case of the ladle, tundish, and mold, by synthetic slag. In the tundish, any inclusions that are less dense than the liquid metal – gas bubbles, other slag or oxides, or undissolved alloys – may also float to the surface and be trapped in the slag layer. While the tundish and mold fill for the first time at the start of a casting run, the liquid is badly contaminated with oxygen, and the first items produced are typically quarantined or diverted to customers who do not require top-quality material.
Upcasting solves this problem by forming a continuous product from a metal seed.