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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. This 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.The basic outlines of the system have recently been implemented in today's strip casting.
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 reservoir to feed the casting machine during ladle changes,thereby acting as a buffer for the molten iron,as well as smoothing the flow,regulating metal feed to the mold and cleaning the metal (see below).Commonly used disposable service lining refractories are known as "tundish plates".Metal is discharged from the tundish through another shroud to the top of the open-bottomed copper mold.Molds range in depth 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 that melt when in contact with metal,or liquid) 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 to the top of the pool to form a slag scum.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 close to the mold wall solidifies before the middle section (now called the strand) exits the bottom of the mold and enters the spray chamber.Most of the metal inside the walls of the strand is still molten.
The strands are immediately supported by closely spaced water-cooled rollers that support the walls of the strand against the ferrostatic pressure (compare hydrostatic pressure) of the 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 strand may occur after the strand leaves 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,a strand or mold swing is 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 strand is cut to predetermined lengths using 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 rods,also known as thin slabs,and then more recently to 2mm thick thin strip castings.
Continuous Casting Profile Range
The caster is specified as a billet, bloom, slab or strip caster.
Slab casters tend to cast sections that are much wider than thick.
Conventional slabs range in width from 100-1600mm, thickness from 180-250mm,lengths up to 12m,and traditional casting speeds up to 1.4m/min; however,slab widths and casting speeds are currently increasing.
Wider sheets up to 3250×150 mm available.
Plates up to 2200 x 450mm are available from selected mills, typically in the range of 200mm to 300mm.
Thin slabs (mild steel): 1680 x 50mm in specific facilities, typically between 40mm and 110mm thick, depending on individual machine design.
Traditional bloom casters cast cross-sections exceeding 200 x 200 mm. Flowering length can vary from 4 to 10 m.
The continuous billet caster can cast smaller cross-sections,eg less than 200mm², and lengths up to 12m.Casting speeds up to 4 m/min.
Bullets:500mm or 140mm in diameter.
Conventional beam blanks: cross-section similar to I-beams; 1048×450mm or overall 438×381mm.
Near net shape beam blank: overall 850×250 mm.
Steel strip: 2-5mm thick,760-1330mm wide.
Twin-belt continuous casting
Twin-strip casting is a continuous casting process that produces high volumes of continuous metal rod or strip with a constant rectangular cross-section.Twin belt casting uses a moving mold consisting of parallel carbon steel belts as the top and bottom casting surfaces to maintain tension.A chain of rectangular steel or copper blocks moves with the belt and is spaced according to the desired casting width to form the sides of the mould.Molten metal is introduced from the tundish into the twin-belt caster through nozzles located between the casting belts.The metal is cooled by direct contact with the belt, which in turn is cooled by high pressure circulating water.Various coatings can be applied to strip casting surfaces to provide the desired mold interface properties and prevent sticking.
The cast metal from the twin-belt caster is synchronized with the hot rolling mill and fed directly into the hot rolling mill.Combining casting and rolling operations results in significant energy and cost savings compared to other casting processes that include intermediate casting and reheating steps.Metals cast by twin-belt casters: copper (rod,strip,anode),aluminum (strip),zinc (strip),lead (strip).Productivity and speed: twin-belt continuous casting up to 60 tons per hour and 14 meters per minute.Twin-strip casting is a near-net-shape casting process that significantly reduces the need for secondary rolling or forming operations. For example,when casting copper anode plates,the billet is not rolled but directly sheared into different anode plates.
The cooling strip is usually made of mild steel and is held under tension within the casting machine to ensure flatness and precision.As the "cold" strip enters the mold area,it is heated in the casting area and is subjected to powerful forces caused by thermal expansion.When casting wide strips,these forces must be controlled to eliminate buckling and thermal deformation of the strip at the mold entry.These forces can be controlled by preheating the belt before entering the mold or by magnetically stabilizing the belt after it enters the mold.Strip preheating:For wide strip casting, a strip preheating system can be used to heat the strip to 150 °C or higher immediately before it enters the mold,thereby reducing the effects of cold forming.Induction heating coils are available across the width to preheat each belt.In addition to preventing heat distortion,the high preheat temperature is used to eliminate any moisture present on the belt surface.
Magnetic Stabilization:When casting wide strips, the tendency to localized thermal deformation can be prevented by using high-strength tape support rolls in the mold area.The moving belt is secured to support rollers by magnetized rotating fins that keep the belt in-plane.In a twin-belt caster, the molten metal gradually solidifies on the mold surfaces as it passes through the mold area,with pools of molten metal between the solidified outer surfaces.Belt coating, texture and gas layer modifications are used to fine-tune the rate of heat transfer from the cast metal to the belt. Through-thickness solidification can occur as early as 30% of the pass through the die for thin strip, and as far as 2 m beyond the die exit, or even more,for larger bars requiring outlet water spray cooling and roll support.