Continuous casting machines are currently widely used in steel mills in various forms and for different purposes. The names of continuous casting machines are also inconsistent. Here we introduce the classification method of continuous casting machines according to general habits: (1) Classification by the appearance of continuous casting machines: vertical continuous casting machine, vertical bending continuous casting machine, arc continuous casting machine, ultra-low head (elliptical) continuous casting machine, horizontal continuous casting machine, wheel continuous casting machine, etc. (2) Classification by the cross section of the cast billet: – Billet continuous casting machine: billets with a cross section less than or equal to 150×150mm are called small billets, while billets with a cross section greater than 150×150mm are called large billets. In the continuous casting of small billets, 120×120mm is used as a dividing line. For billets above this value, submerged nozzles and protective slag casting are used, while for billets below this value, open casting or gas protection casting is used. Rectangular billet casting machines with a ratio of the long side to the wide side less than 3 are also called billet continuous casting machines. —Slab continuous casting machine: The cross section of the billet is rectangular. Its width-to-thickness ratio is generally above 3.
—Round billet continuous casting machine: The cross section of the billet is circular, with a diameter of Φ60 to Φ400 mm.
—Special-shaped billet continuous casting machine: Casts special-shaped billets such as I-beams.
—Slab and square billet continuous casting machine: A single casting machine can cast both slabs and square billets.
(3) Classification by casting speed: High-speed continuous casting machine and low-speed continuous casting machine. The main difference between them is that at high casting speed, the billet is straightened with the liquid core, while at low casting speed, the billet is straightened with full solidification.
(4) Classification by static pressure head of molten steel: The high-head continuous casting machine has a larger static pressure, such as vertical and vertical bending continuous casting machines. The low-head continuous casting machine has a smaller static pressure, such as arc-shaped, elliptical, and horizontal continuous casting machines.
Number of Continuous Casting Machines, Machines, and Streams
In continuous casting, any continuous casting equipment that uses a common ladle to simultaneously cast one or more strands of ingots is called a continuous casting machine. A continuous casting machine can consist of multiple units or a single unit. A unit is a set of continuous casting equipment with independent transmission and operating systems within a continuous casting machine, capable of continuing to operate even if another unit fails. The total number of strands a continuous casting machine can simultaneously cast is called the stream number. A continuous casting machine with only one unit and capable of casting only one strand is called a single machine. A machine capable of casting two or more strands simultaneously is called a single machine with multiple streams. A continuous casting machine with multiple units that can each cast multiple strands is called a multi-machine, multi-stream system.
Compared to multiple units, a single machine with multiple streams offers lighter equipment weight and lower investment. However, a single stream failure in a single machine with multiple streams can cause the machine to stop production, and production operations and coordination between streams can be difficult. In recent years, billets have been cast in up to eight strands, with most using two to four. Slabs are cast in up to four strands, with most using one to two.
Components and Plant Layout of Continuous Casting Equipment
Continuous steel casting equipment primarily consists of nine components: a ladle (steel ladle) turret, a ladle, a tundish (tank), a mold (primary cooling), a mold vibrating mechanism, a secondary cooling system, a straightening system, a cutting system, and a billet removal system.
Currently, continuous steel casting plants primarily have two layouts: horizontal and vertical. A horizontal layout aligns the centerline of the continuous casting machine perpendicular to the plant’s columns, while a vertical layout aligns the centerline parallel to the columns. The horizontal layout is suitable for large-scale plants with multiple continuous casting machines, while the vertical layout is often used in the renovation of older plants or when the number of continuous casting machines is small.
Continuous Steel Casting Operation
As molten steel from the primary steelmaking furnace (converter, electric furnace, or open-hearth furnace) is poured into the ladle, it undergoes simultaneous deoxidation and alloying. The molten steel is then transported to the ladle refining station, where its temperature and composition are adjusted before being sent to the continuous casting platform. Because direct ladle pouring makes it difficult to control the molten steel flow rate, a tundish is used to temporarily store the molten steel before pouring it into the crystallizer. The molten steel rapidly cools in the water-cooled crystallizer, forming a strand with a solidified shell on the outside and unsolidified molten steel inside. The solidified shell at the lower exit of the crystallizer should be thick enough to prevent the molten steel from leaking out (this leakage is called “leaking”). As the pulling rollers slowly pull the strand with a liquid core out of the crystallizer, the molten steel in the tundish is continuously injected into the crystallizer. This produces a very long strand with a liquid core. The strand with a liquid core is forced to cool by water spray in the secondary cooling zone. When it is pulled to the specified position, the interior of the strand is completely solidified. The strand is then cut to the specified size and sent to the discharging device for subsequent processing.
Types of Continuous Steel Casting Machines
Currently, there are five types of continuous casting machines used worldwide: vertical, vertical-bend, curved, elliptical, and horizontal.
In vertical continuous casting machines, pouring, crystallization, solidification, secondary cooling, and cutting processes are all performed sequentially on a vertical line. Vertical-bend casting machines start vertically, then bend the slab 90° into a horizontal position after solidification, before cutting and transporting it. These machines are lower in height than vertical casting machines. Curved continuous casting machines pour molten steel into a curved mold, which then travels along a curved track, passing through a quarter arc, before discharging the slab horizontally. These machines are even lower in height than vertical-bend casting machines. Further improvements based on curved continuous casting machines have led to the development of elliptical continuous casting machines. Horizontal continuous casting machines are currently under development.
According to incomplete statistics, vertical types account for 17% of the continuous casting machines currently built worldwide, vertical-bend types for 21%, curved types for 55%, and other types for 7%. Currently, curved types are the most common type of newly built continuous casting machines.
Features of a Vertical Continuous Caster
A vertical continuous casting machine arranges all key equipment, from the tundish to the cutting station, in a vertical line. The entire machine stands on the workshop floor or in a deep underground pit. This type of continuous casting machine occupies a small footprint and is compact. High-temperature ingots exhibit minimal bending and deformation, and exhibit minimal surface and internal cracks. Inclusions in the unsolidified liquid within the ingot float easily, resulting in relatively clean molten steel. The secondary cooling system and pinch rollers are simple and easy to maintain. However, the disadvantages of vertical continuous casting machines include: The machine body is 20 to 40 meters high, requiring elevated plant buildings or deep pits, resulting in high capital costs. Because the casting machine cannot be raised excessively, casting speeds are limited, resulting in low productivity. The high static pressure of the molten steel can easily cause the solidified ingot shell to bulge (called “bellying”), which can deteriorate ingot quality.
Features of Vertical Bending Continuous Casting Machines
Compared to vertical casting machines, vertical bending continuous casting machines offer a lower machine height, saving investment. Horizontal casting facilitates machine lengthening and enables high-speed casting. Inclusions in the unsolidified molten steel within the ingot float easily, ensuring uniform inclusion distribution. However, a disadvantage is that the ingot undergoes repeated bending and straightening, which can lead to internal cracks. While the casting machine height is reduced, capital costs remain high.
There are two types of vertical bending continuous casting machines: one with a vertical section below the mold. When the ingot is fully or nearly fully solidified, it bends into a curved section before being straightened. This is primarily suitable for small square billets. The other type features a straight mold with a vertical section (2-3 meters), which bends at multiple points with a liquid core or gradually bends into a curved section before being straightened. The advantages of this type of continuous casting machine are: 1) The vertical section allows inclusions in the liquid phase cavity to float freely; 2) The strand is bent at multiple points or gradually, avoiding localized concentrations of bending deformation stress and effectively reducing crack formation; 3) The bending with a liquid core increases casting speed and improves casting machine productivity. Currently, to address the quality issues of inclusion accumulation within the arc of the strand cast by arc-type continuous casting machines, there is an increasing trend in the construction of vertical-bend continuous casting machines.
Features of Arc-Type Continuous Casting Machines
While vertical and vertical-bend continuous casting machines have straight molds, arc-type continuous casting machines use curved molds with a specific radius of curvature. The mold, secondary cooling system, and straightening device are all arranged at a quarter of the radius of a circle with a specific radius. The strand is curved during solidification in the mold. The strand with the liquid core is pulled out of the mold and continues along a curved track, where it continues to be cooled by water sprays. Solidification is completed at the quarter of the circle, and then it is straightened and pulled out for the cutting station. The height of a curved continuous casting machine is only one-third that of a vertical one, resulting in low construction costs, low static pressure on the molten steel, minimal bulging of the strand between the rollers, and high strand quality. Extending the machine is also relatively easy, enabling high-speed casting and high productivity. However, the disadvantages of curved continuous casting machines include: the tendency to cause internal cracks due to the bending and straightening of the strand; uneven distribution of inclusions within the strand, with concentrations of inclusions on the inner curve; and the increased complexity of the equipment, making maintenance more difficult.
Despite its shortcomings, the curved continuous casting machine remains the most widely used type of machine in steel mills worldwide, thanks to technological advances in both equipment and processes.
Features of an Ultra-Low-Head Continuous Caster (Oval Continuous Caster)
An ultra-low-head continuous casting machine is one that is lower than a typical continuous casting machine. Consequently, its liquid core pressure is also naturally low, hence the name. At present, there is no unified definition standard in various countries. Concast uses the ratio of the casting machine height h and the billet thickness H as an indicator for definition: h/H>50 high-pressure head continuous casting machine h/H=40～50 normal-pressure head continuous casting machine h/H=25～40 low-pressure head continuous casting machine h/H<25 ultra-low-head continuous casting machine The advantages of ultra-low-head continuous casting machine are: (1) Due to the low hydraulic head of the steel liquid, the static pressure of the molten steel is small, and the bulging deformation is small. According to calculations, when the radius of the continuous casting machine is reduced from R=10.5m to R=3m, the strain caused by the bulging deformation in the two-phase zone of the billet is only less than 10% of the original, which improves the billet quality. Due to the small bulging deformation, the clamping roller structure in the secondary cooling zone is simplified; (2) The main equipment of the continuous casting machine is placed in a horizontal position, which is easy to maintain; (3) The equipment is light in weight, which reduces the cost. However, due to the low pressure head, it is not conducive to the floating of inclusions. Therefore, measures should be taken to purify the molten steel and protect the pouring. This type of machine is suitable for retrofitting older plants. Chinese companies such as Tianjin Iron and Steel, Handan Iron and Steel, and Maanshan Iron and Steel have all installed this ultra-low head continuous casting machine, achieving excellent production results.
Features of a horizontal continuous casting machine
The tundish, mold, secondary cooling system, and billet drawing unit of a horizontal continuous casting machine are all arranged horizontally and in a straight line on the ground. Molten steel is poured into the tundish, which is connected to the inlet of the mold. The molten steel in the tundish flows through a separation ring at the connection between the mold and the tundish and enters the water-cooled mold, where it solidifies into a shell of a predetermined shape. The strand with the liquid core is then pulled out of the mold, fully solidified after secondary cooling, and then sent to the cutting station to be cut into lengths.
The horizontal continuous casting machine is low in height and can be installed within existing plant buildings, significantly reducing capital costs. The equipment is simple, lightweight, and easy to maintain. The sealed connection between the tundish and mold prevents secondary oxidation, ensuring high steel purity. It is easy to operate, eliminating the need for mold level control, mold protection slag, and nozzle clogging. Straightening of the strands is unnecessary, allowing casting of crack-sensitive alloy steels.
These advantages make the horizontal continuous casting machine particularly suitable for electric furnace steelmaking plants with a wide variety of alloy steels and small batch sizes. There are three major technical keys to the development of horizontal continuous casting: the first is the material, life and cost of the separation ring; the second is the coating and lubrication of the crystallizer; and the third is a casting mechanism that meets the requirements of the pull-stop-reverse casting method. In recent years, breakthroughs have been made in these three key technologies. At present, there are horizontal continuous casting machines for industrial production in Germany, Japan and the United States. Ma Steel and other units in my country have conducted development tests of horizontal continuous casting machines. Hengyang Steel Pipe Plant in my country has achieved full continuous casting with a horizontal continuous casting machine. In the next few years, horizontal continuous casting technology will have a significant development in small and medium-sized steel plants in my country.
Centrifugal rotary continuous casting machine
This type of machine was studied in the 1950s. The first centrifugal rotary vertical continuous casting machine was put into production in France in 1969. This casting machine can only cast round billets and is also called a rotary round billet continuous casting machine. The characteristics of this type of machine are: (1) During the casting process, the crystallizer rotates at a speed of 30 to 150 r/min. The molten steel generates centrifugal force, while the inclusions generate centripetal force, which causes the inclusions to float up and be removed, thereby improving the cleanliness of the steel. (2) Due to the rotation, the molten steel washes away the dendrites at the solidification front, expanding the equiaxed crystal zone in the center of the ingot and reducing segregation and porosity. (3) The round ingot has precise geometric dimensions and good surface quality. Therefore, the round ingot produced by the rotary continuous casting machine can be directly sent to the pipe rolling mill without surface cleaning. However, the disadvantages of this type of machine are: (1) The casting machine structure is complex, and the equipment and maintenance costs are high. (2) The casting machine is tall, requiring a tall factory building, which requires a large investment. (3) It can only cast round ingots, and the production flexibility is limited. (4) Because it is vertical, the increase in casting speed is limited, so this type of machine has not been widely developed.
Wheel-Belt Continuous Caster
The wheel-belt continuous casting machine consists of a wheel with a crystallization trough around it and an endless steel belt. The crystallization trough is made of a heat-fatigue-resistant copper alloy. The steel belt seals the trapezoidal opening, forming the crystallization chamber. Molten steel begins solidifying after being poured from the tundish into the crystallization chamber, moving synchronously with the rotating crystallization wheel. Near the wheel’s undercut point, the initially solidified billet shell is straightened and released from the crystallization wheel. It then enters the secondary cooling zone for cooling and is finally cut into finished billets of a specified length.
The wheel can be positioned vertically or horizontally. Its advantages include: 1) high speed, with casting speeds reaching 5-10 m/min, making it easy to integrate continuous casting and rolling with a rolling mill. 2) low caster height, simplified equipment, and low investment. 3) no mold vibration, resulting in excellent surface quality and no vibration marks on the cast billet surface. Wheel-belt continuous casting machines are widely used for casting non-ferrous metals, but their use for casting steel billets is still under trial.
Features of Thin Slab Continuous Casting Machines
A thin slab continuous casting machine is one that can cast slabs with a thickness of 20 to 50 mm and a width of 900 to 1600 mm.
This type of continuous casting machine, built by Nucor Corporation in the United States, can cast slabs with a thickness of 50 mm and a width of 1295 to 1560 mm, with an annual production capacity of 1 million tons. The machine is a vertical bending type. The mold is straight, with a funnel-shaped top opening of 170 mm for the insertion of the water nozzle. Its longitudinal section consists of two fold lines. The mold is long, 1100 mm in length, and the distance between the two narrow sides can be adjusted to accommodate thin slabs of varying widths.
Below the mold is a 700 mm long cooling grid to support the thin shell of the newly drawn slab. Three sets of nip rollers are installed below the grid. The first set consists of segmented rollers with a relatively small roller diameter and roller pitch. The total length of the three nip rollers is 4600 mm. After being fully cooled in the secondary cooling zone, the slab is formed into an arc of R = 3m by hydraulic bending rollers, then pulled out of the slab by a four-roll straightening machine and finally cut into finished slabs of a specified length by mechanical shears.
The mold uses a four-eccentric oscillator mechanism, operating sinusoidally with an amplitude of ±3mm. The frequency varies with the casting speed, i.e., ƒ = 60V min-1, where V is the casting speed in m/min-1. The maximum frequency is 400 min-1. For example, at a casting speed of V = 4 m/min, ƒ = 240 min-1.
The advantages of thin slab continuous casting machines lie in their simplified production process, lightweight equipment, reduced capital investment, energy savings, and lower production costs.
How many steel grades and slab cross-sections can continuous casting machines cast?
According to statistics, continuous casting machines can currently cast over 130 steel grades and over 500 different steel grades. These include plain carbon steel, low-alloy steel, stainless steel, silicon steel, high-speed steel, bearing steel, and high-temperature alloys. Approximately 63% of these are carbon steel, and 37% are alloy steel and stainless steel.
Continuous casting can produce the following cross-sections: billets (small squares): with cross-sections less than 150×150mm, with the smallest cross-section being 50×50mm; blooms (large squares): with cross-sections greater than 150×150mm, with the largest being 450×450mm; slabs (rectangular billets): with cross-sections ranging from 50×108mm to 300-250×2500-3000mm; round billets: with diameters ranging from Φ50mm to Φ450mm; and special-shaped billets, such as I-beams and hollow round billets. .
Principles to be considered when matching continuous casting machines with steelmaking furnaces
Continuous casting machines can be used in conjunction with open-hearth furnaces, electric furnaces, and converters. The following points must be noted:
(1) Multi-furnace continuous casting: Due to the long smelting time of open-hearth furnaces, continuous casting machines can generally only cast from a single furnace. In conjunction with converters and electric furnaces, in order to fully utilize the production capacity of the continuous casting machine, improve the molten steel yield, and reduce costs, multi-furnace continuous casting must be achieved.
(2) The casting time of the continuous casting machine should be well coordinated with the steelmaking furnace tapping cycle.
(3) The production capacity of the continuous casting machine should be appropriately greater than the production capacity of the steelmaking furnace to meet the production potential of the smelting equipment. Generally, there should be a 10-20% surplus.
(4) Select appropriate off-furnace refining equipment that meets product quality requirements to ensure the quality of the continuous casting molten steel.
Principles to be considered in the coordination between continuous casting machine and rolling mill
The following points should be considered in the coordination between continuous casting machine and rolling mill:
(1) Steel type: Currently, there are more than 100 types of steel that can be continuously cast. The next processing step should be selected based on the product quality.
(2) Billet cross section: The size and shape of the billet cross section are mainly determined by the capacity of the rolling mill and the quality requirements of the rolling mill products. The cross section produced by the continuous casting machine should be adapted to the billet cross section required by the rolling mill. The small square billet continuous casting machine mainly supplies 650, 500 or 400 type rolling mills to produce profiles and wire rods. Generally, it is a single-fired material. Its typical billet cross section is: 650 rolling mill, 140-220mm billet, billet fixed length 2.8-3.2m; 500/350 rolling mill, 120-140mm billet, fixed length 1.4-1.6m; 400/300 rolling mill, 90-120mm billet, fixed length 1.4-1.6m. The cross section of the ingot is larger than 160mm square billet, which is mainly used to produce steel sections such as rails, I-beams, channel steels, etc. Continuous casting slabs are mainly used to produce medium and thick plates or hot-rolled strips in thick plate mills or wide strip mills. (3) Compression ratio: Generally speaking, a smaller cross section of the ingot can reduce the number of heating times and rolling passes, improve the productivity of the rolling mill and save energy. However, the cross section should meet a certain compression ratio to obtain qualified product quality. The so-called compression ratio refers to the ratio of the cross-sectional area of the ingot to the cross-sectional area of the rolled material. In order to ensure that the internal structure of the rolled material is dense and has good mechanical properties, different products require different compression ratios. When the compression ratio is 3, the mechanical properties required by general products can be achieved; when the compression ratio is 4, the product obtains good physical properties and can make the central structure uniform; when the compression ratio is 5 to 8, the columnar crystal structure of the rolled material can be destroyed, the steel structure can be homogenized, and good mechanical and physical properties can be obtained.
(4) Shape tolerance: For square billets, the shape must be regular, with the deviation of the two diagonals of the cross section less than 2.5-3%, and the deviation of the cross section corners within 90 ± 2°. If the billet is severely diamond-shaped, it may cause steel accumulation when pushing the billet in the heating furnace, or it may be difficult for the billet to bite into the die during rolling. For slabs, severe bulging should be avoided. The maximum curvature of the billet is 10 mm/m.
Principles to be considered in selecting continuous casting machines
When designing a steel plant, the first thing to consider is the selection of a continuous casting machine. A suitable machine model should be determined based on the product plan, product quality, and the need to reduce construction investment. The appropriate machine model creates conditions for future production management and high-yield and high-quality continuous casting machines. The basic principles for selecting continuous casting machines are:
The production capacity of the continuous casting machine should match the production capacity of the steelmaking furnace. In order to maximize the production potential of steelmaking, the production capacity of the continuous casting machine should be 10-20% surplus.
Meet the requirements of steel grade and rolling section specifications. According to the statistical data of production practice, the vertical, vertical bending and arc continuous casting machines can meet the requirements of casting ordinary carbon steel, low alloy steel, alloy steel and cross-sections of different specifications, but the arc continuous casting machine is the most widely used (accounting for more than 60% of the continuous casting machines built).
Meeting product quality requirements. The quality of the ingot mainly includes two aspects: one is the cracks and segregation inside the ingot, and the other is the purity of the ingot. The former is determined by the static pressure of the molten steel causing the ingot to bulge, bend and straighten. In the order of vertical → vertical bending → arc → elliptical → horizontal, the height of the casting machine decreases, and the crack tendency also decreases accordingly. The latter is related to the floating of inclusions in the liquid phase cavity of the crystallizer. In the order of vertical → horizontal, the lowering of the casting machine height gradually increases the accumulation of inclusions in the ingot. In the comprehensive evaluation of the above two contradictory influencing factors, the arc continuous casting machine is still the most ideal model. In addition, the use of measures such as refining outside the furnace and non-oxidizing casting can fully meet the requirements of product purity. In order to completely solve the problem of arc inclusion accumulation in the arc continuous casting machine, there is a new development trend to build a vertical bending continuous casting machine with a straight crystallizer and a vertical section (2-3m).
Reduce construction investment and equipment manufacturing costs. There is no unified standard for the selection of machine models. It is necessary to combine the factory product plan and comprehensively consider the aspects of productivity, product quality, casting machine height, investment, etc. The construction of the continuous casting machine should be regarded as a systematic project. Upstream of the continuous casting machine, it is necessary to consider that the steelmaking system should provide qualified quality molten steel for continuous casting (such as refining outside the furnace). Downstream of the continuous casting machine, it is necessary to consider the possibility of hot delivery, hot loading, direct rolling of the casting and the quality assurance system of the rolled product. In the continuous casting machine itself, the equipment, process and control system for producing defect-free castings should be considered.
