Process Description :
Molten metal (known as hot metal in industry) 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 '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.



10.Meniscus 11.Withdrawal unit 12. Slab

A. Liquid Steel B. Hardened Steel C. Slag D. Water-cooled Copper plate E. Heat-proof Rollers
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.

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 section 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 and is thus called a submerged entry nozzle (SEN). 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 moulds (a distance of about 30 cm). 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, water cooled 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. This describes 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", 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 either taken to a stockpile or the next forming process.

In many cases the strand may continue through additional rollers and other mechanisms which might flatten, roll or extrude the metal into its final shape.

Equipment :
The main components of a continuous casting system are shown in the figure. Liquid metal is poured from the ladle into a reservoir and the casting is bent from a vertical to a horizontal plane in straightening rolls and finally cut into the appropriate lengths for secondary rolling.



Cross Section of a Slab Caster:-

Electromagnetic Stirring
Electric induction stirring is used in many continuous casting machines to stir the molten interior of the cast as it leaves the mold. This stirring improves the crystalline properties of the cast and allows a faster casting speed by speeding solidification.

Gas Cutting
Continuous cast sections are cut to length using gas cutting torches. These torches typically use acetylene, propane, or MAPP fuel along with oxygen to cut the metal. Pressurized natural gas can also be used for cutting in steel mills and in other steel fabrication operations, resulting in operating savings and greater operational safety.

Combustion Technology:
Air/natural gas burners are used for tundish and nozzle preheating applications. Tundish heating during casting is achieved using some type of electric heating. Induction heating is most common, replacing most resistance heaters that were originally used. Development work is underway on arc plasma heaters that offer better control.