Descriptions of Continuous casting

Continuous casting also known as continuous casting, is a process in which molten metal solidifies into "semi-finished" billets, blooms or slabs for subsequent rolling in finishing mills.Before the introduction of continuous casting in the 1950s, steel was poured into stationary molds to form ingots.Since then, "continuous casting" has continued to evolve to improve output, quality, productivity and cost efficiency.It allows better quality metal extrusions to be produced at lower cost due to the inherent lower costs of continuous, standardized production of products and the increased control over the process provided through automation.The process is most commonly used for casting steel (in terms of cast tonnage). Aluminum and copper are also continuously cast.Sir Henry Bessemer, famous for the Bessemer converter, patented the casting of metal between two counter-rotating rollers in 1857.The basic outlines of the system have recently been implemented in today's strip casting.

Equipment and process

Steel:

Molten metal flows from a furnace into a ladle.After any ladle treatment such as alloying and degassing and reaching the correct temperature, the ladle is transported to the top of the continuous casting machine.Typically, the ladle is located in a trough in the rotating turret of the casting machine.One ladle is in the "casting" position (feeding the pouring machine), while the other is ready in the "stopping" position and switched to the pouring position when the first ladle is empty.From the ladle, the molten iron is transferred through a refractory shroud (pipe) into a holding tank called a tundish.The tundish allows the metal container to feed the casting machine during ladle changes, thereby acting as a buffer for the molten iron and smoothing the flow, regulating metal feed to the mold and cleaning the metal.Commonly used disposable service lining refractories are called "tundish plates".

Metal is discharged from the tundish through another shroud to the top of the open-bottomed copper mold.Mold depths range from 0.5 to 2 meters (20 to 79 inches), depending on casting speed and section size.The mold is water-cooled so that the molten iron in direct contact with it solidifies; this is the primary cooling process.It also oscillates vertically (or in a nearly vertical curved path) to prevent the metal from sticking to the mold walls.Lubricants (powders or liquids that melt when in contact with the metal) are added to the metal in the mold to prevent sticking and to capture any slag particles (including oxide particles or scale) that may be present in the metal and bring them in A layer of scum forms on top of the pool.The shroud is arranged so that the molten iron is discharged from below the surface of the slag layer in the mold, hence the name submerged entry nozzle (SEN).In some cases, no shroud may be used between the tundish and mold ("open pour" casting); in such cases, interchangeable metering nozzles at the bottom of the tundish direct the metal into the mold.Some continuous casting layouts feed multiple molds from the same tundish.

In the mold, a thin shell of metal near the mold walls solidifies before the center, and the molded metal (now called strands) exits the bottom of the mold into the spray chamber.Most of the metal in the strand wall is still molten.The strand is immediately supported by closely spaced water-cooled rollers that support the strand walls against the ferrostatic pressure (compare hydrostatic pressure) of liquid still solidifying within the strand.In order to increase the solidification rate, the steel strand is sprayed with a large amount of water as it passes through the spray chamber; this is the secondary cooling process.Final solidification of the wire may occur after the wire has exited the spray chamber.It is here that the design of the caster may vary.This describes a "curved apron" casting machine; a vertical configuration was also used.In a curved apron caster, the strand leaves the mold vertically (or along a nearly vertical curved path), and as it passes through the shower chamber, rollers gradually bend the strand horizontally.In a vertical casting machine, the strand remains vertical as it passes through the spray chamber.The 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 molten steel from liquid to thin shell to solid is horizontal (without bending).In this type of machine, strands or mold swings are used to prevent sticking in the mold.After leaving the spray booth, the cast strand passes through straightening rolls (if cast on a non-vertical machine) and stripping rolls.After withdrawal there may be a hot rolling stand to take advantage of the hot state of the metal to preform the final strand.Finally, the strands are cut to predetermined lengths with mechanical shears or a mobile oxyacetylene torch and marked for identification before being sent to stock or to the next forming process.In many cases, the strands may continue through additional rollers and other mechanisms that may flatten, roll, or extrude the metal into its final shape.Developments since the mid-1980s have reduced the thickness that can be cast, initially by transferring about 50mm thick rebar, also known as thin slab and then more recently to thin strip castings of 2mm thickness.