Billet
Billet machines, which cast section sizes up to approximately 5 inches square, are multi?strand machines that are widely used in the mini?mill sector of the industry but only to a relatively limited extent in fully integrated plants. This has occurred because of practical considerations which are related to the heat size, casting rate per strand (tons/minute) and casting time. In general, casting times are limited to approximately one hour for each heat because of heat losses in the ladle. It is practical, for example, to cast a 50 net?ton heat on a 2?strand machine or a 100 net?ton heat on a 4?strand machine. However, the number of strands required for casting heat sizes in excess of 200 net tons, which are common in integrated steel plants, becomes impractical.
Bloom
Bloom casters have been more widely installed by integrated plants because the casting rate for the larger section size is higher than for billet sizes and, consequently, larger heat sizes can be cast with relatively fewer strands. Bloom section sizes cast can vary, for example, from 7 in. sq., cast on a 6?strand machine from 150 net?ton heats, up to as large as 14.6 in. x 23.6 in., cast on a 3?strand machine from 180 net?ton heats.
Round
The installation of machines for casting rounds, principally for seamless tube production, has been relatively slow. Although a 4?strand caster was installed, for example to produce 125 and 210?mm (4.9 and 8.3 in.) diameter rounds from a 30 metric?ton (33 net ton) heat in 1965, potential surface cracking problems delayed the introduction of round casting. Some modifications were made later for an existing 6?strand billet/bloom caster, for example, to produce 152?mm (6?in.) diameter rounds in 1980s. The installations of the modified caster included the 640,000 metric tons (700,000 net tons) per year U.S. Steel machine at Lorain, which is a 6?strand caster producing up to 232?mm (9 and 1/4 in.) diameter rounds.
Slab
There are a large number of slab casters throughout the world which, although operated principally in integrated steel plants, are also used for producing stainless and specialty steel. These machines are generally high production units with rated annual capacities of up to 1.4 million metric tons (1.5 million net tons) and above. They are usually either single or twin?strand machines casting large heat sizes. A wide variety of low carbon, low alloy, alloy and stainless steel grades are cast for sheet, strip, plate and specialty applications. Successful examples of the slab casters are the CSP technology developed by then SMS, and ISP from then Mannesmann Demag, both in Germany. (The two companies have merged as SMS Demag). Since late 1980s and early 1990 the CSP and ISP have become very popular.
Beam Blank and Special Shapes
Beam blanks are cast to be subsequently rolled into I beams. Other special shapes are also cast to produce near-net shapes for various final products. Though the casters for beam blank, etc., was introduced pretty early in the industry, the intensive development of this technology was performed in the late 80s and early 90s, for example in then Mannesmann Demag and SMS. The new technology has been developed to cast beam blank with thinner web, such as 50 mm and less.
Development and Principal Types of Casting Machines
One of the major objectives in the design of continuous casting machines has been to reduce the capital cost of the installation while at the same time maintaining or improving the quality of the cast product. This objective has been achieved by a progressive reduction in the height of the machine which has resulted in a reduction in the size of the supporting structure, building height and foundation. It has led to the development of five principal types of casting machines which are essentially applicable to all section shapes cast whether billets, blooms, slabs, etc. Chronologically, these types, illustrated schematically in Fig. 1, are:
1) Vertical machine with a straight mold and cutoff in the vertical position.
2) Vertical machine with a straight mold, single-point bending and straightening.
3) Vertical machine with a straight mold, progressive bending and straightening.
4) Bow type machine with curved mold and straightening.
5) Bow type machine with curved mold and progressive straightening.
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Billet sections are self?supporting in the secondary cooling zone, while slabs are usually not. Generally, billet casters have tended to be simple in design, with open?pouring streams, limited automatic controls, and no roll support in the secondary cooling zone. Conversely, slab casters are complex and use the total range of subsystems such as total stream shrouding, computer controls, and total roll containment throughout the machine. Bloom casters are intermediate between these two extremes.
Major Components of a Continuous Casting Machine
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Tundish
There are many types and shapes of tundish. One common tundish design for multistrand billet and bloom casters is a trough shape with a pouring box offset at the midpoint; for slab casters the tundish is a short box or tub shape. The pouring stream from the ladle is directed downward to a position in the tundish bottom which is protected with a wearresistant pouring pad. This position is usually as far as possible from the tundish nozzle to minimize turbuletice. In other locations, the tundish is lined with refractory bricks or boards. Weirs and dams are used as flow?coritrot devices which both increase the residence time as well as reduce the detrimental effects of turbulence on the metal surface, the metal streams entering the mold and dead zones. '
Tundish Nozzles
Two basic types of tundish nozzles are used: (1) a metering or open nozzle and; (2) a stopper rod?controlled nozzle. Metering nozzles, a simpler system, have been generally employed in billet and small bloom casters, producing silicon?killed steels. Metal discharge rate is controlled by the bore of the nozzle and the ferrostatic pressure (metal height in the tundish) above the nozzle. Different bores are selected depending on the section size cast and casting speed required. Stopper rod?controlled nozzles are used for casting slabs and large sections when aluminum?killed steels are produced. In this application, metal discharge rate through the nozzle is controlled manually or automatically by the setting of the stopper head in relation to the nozzle opening. Originally, over?sized nozzles were used for casting aluminum?killed steels: as alumina buildup occurred, the stopper head was raised to compensate for a reduction in flow rate.
Modern developments in deoxidation practice together with the use of argon bubbling through the stopper head and nozzle units have minimized the alumina buildup problem. Another development in controlling metal flow from the tundish is the application of slide gate systems which are similar to those employed on ladles. These gate systems can also provide the capability for changing nozzles during casting as well as changing nozzle size.
Mold and Mode Design
The mold is constructed as an open-ended box structure which contains an inner lining fabricated from a copper alloy which serves as the interface with the steel being cast and provides the desired shape to the cast section. The liner is rigidly connected to an outer steel supporting structure.
There are two types of mold designs; tubular molds and plate molds. Tubular molds conventionally consist of a one?piece copper lining that usually has relatively thin walls and is restricted to smaller billet and bloom casters. Plate molds consist of a 4?piece copper lining attached to steel plates. In some plate mold designs opposite pair of plates can be adjusted in position to provide different section sizes. For example, slab width can be changed by positioning the narrow?face plates, and the slab thickness changed by altering the size of the narrow?face plates. The plate mold is inherently more adaptable than the fixed?configuration, tubular mold. In addition to permitting size changes, changes can also be made to the mold taper (to compensate for different shrinkage characteristics of different steel grades) as well as ease of fabrication and reconditioning.
Although the material of construction of the inner lining is usually a high purity cold?rolled copper, copper with small amounts of silver is commonly used to obtain increased elevated?temperature strength. The working surface of the liner is often plated with chromium or nickel to provide a harder working surface and also to avoid copper pickup on the surface of the cast strand.
During the casting operation, the copper liner is subjected to distortion (a change in the internal dimensions of the mold). It is caused mainly by mold wear and mold deformation due to thermal and mechanical strains. For example, one type of distortion produces a reverse taper caused by mold wear at the exit end of the mold, which can adversely affect product quality. Deformation due to thermal strains is particularly important. Two common causes are thermal expansion due to non-uniform heating of the mold wall, and restraint of the free expansion of the copper liner by the mold-support system. The resulting thermal strains and stresses may be sufficient to cause yielding and permanent deformation, especially at the meniscus level where the yield strength of the copper is reduced because the highest temperatures in the mold are encountered at this position.