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Dr. Dmitri Kopeliovich
Continuous casting methods using traveling “endless” molds (rolls, belts, wheels) are characterized by zero relative movement between the mold and casting surfaces. Strips and slabs cast by such methods have low defect surface. The castings may be coiled and further processed (rolled) without surface machining in contrast to those cast in stationary molds (eg. Horizontal continuous casting in graphite mold).
Most moving molds provide high cooling rate due to very small air gap between the mold and casting surfaces. Effective heat transfer through metallic water-cooled mold also favors fast Solidification.
Methods of continuous casting in moving mold:
In a single roll caster liquid metal is supplied to the surface of a water-cooled rotating roll. The melt contacting with the roll surface cools down and solidifies forming a thin continuous strip.
The advantages of the single roll continuous casting method are:
Single roll caster provides very high cooling rate.
Linear speed of the roll surface is 50 - 400 f/min (15-120 m/min).
The method is used for casting thin-gauge (0.04-0.12” / 1-3 mm) aluminum strips. Low variation of casting thickness is achieved by uniform cooling of the roll surface.
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In twin-roll casting process a melt is fed through a ceramic nozzle into the gap between two rotating water-cooled rolls. The melt cools down and solidifies between the rolls. Additionally the solid strip exerts hot rolling with the thickness reduction of about 5-20%.
There are three possible configuration of twin-roll casters: vertical downward casting, vertical upward casting and horizontal casting (see the picture below).
The most popular configuration now is horizontal and 15º tilt back. The roll dimensions may vary in wide ranges: diameter 12-60” (300-1500 mm), width 6-86” (150-2184 mm). The water-cooled roll shells are made of steel, copper or copper-nickel alloys.
Sticking between the roll and the solidified strip is prevented by means of a parting agent applied to the roll surface. The agent contains Graphite or magnesium hydroxide.
Twin-roll casting process provides extremely high cooling rate up to 1000ºF/s (550ºC/s). Fast cooling allows to reach high casting rate, which is 3-125 ft/min (0.9-38 m/min) for Aluminum alloys and 15-1500 ft/min (5-460 m/min) for steels.
Thickness of the cast strips may be as low as 0.004” (0.1 mm). Maximum thickness reaches 0.2” (5 mm) for steels and 0.4” (10 mm) for aluminum. Typical gauge of twin-roll cast strips is about 0.1” (2.5 mm).
Twin-roll casters are used for casting alloys with relatively low temperature interval of solidification: aluminum and its alloys containing max. 2.5% Mg (Wrought aluminum alloys (1xxx), Wrought aluminum-manganese alloys (3xxx), Wrought aluminum-magnesium alloys (5xxx)), Stainless steels, special steels, Carbon steels.
Twin-roll casters are also used for continuous casting of aluminum based bearing alloys.
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In twin-belt casting process a melt is fed through a ceramic nozzle into the gap between two rotating thin belts held in tension. The belts are cooled by water jets from the side opposite to the surface contacting with the casting. The melt cools down and solidifies between the belts. Width of the cast slab is determined by two chains of steel blocks traveling with the belts and preventing side leakage of the non-solidified melt.
The cast slab/strip is pulled out by a withdrawal unit synchronized with the drive of the belts.
Most twin-belt casters use carbon steel belts of about 0.06” (1.5 mm) thick. The max. width of the cast slabs is 79” (2000 mm). The strip/slab thickness is within the range 0.1-1.3” (2.5-33 mm). The typical casting thickness is 0.5-1” (12.5-25 mm).
The max. twin-belt casting speed is 30 ft/min (9 m/min).
The slab leaving the twin-belt caster is still hot and may be hot rolled in-line to a gauge 0.1-0.2” (2.5-5 mm) and then coiled.
Twin-belt casters are generally used for casting aluminum alloys containing up to 3% of Mg: Wrought aluminum alloys (1xxx), Wrought aluminum-copper alloys (2xxx), Wrought aluminum-manganese alloys (3xxx), Wrought aluminum-magnesium alloys (5xxx)), Wrought aluminum-magnesium-silicon alloys (6xxx).
Twin-belt caster is successfully used for continuous casting of aluminum based engine bearing alloys. The belts are supported and chilled by two water-cooled copper plate.
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In a block caster a molten metal is fed through a ceramic nozzle into the space formed by two chains of steel blocks traveling like caterpillar tracks. The blocks are cooled on their way back by chillers. The melt cools down and solidifies in the space between the steel blocks. The blocks are adjacent to each other in order to prevent penetration of liquid metal into the gap between them.
The cast slab/strip is pulled out by a withdrawal unit synchronized with the drive of the blocks.
The max. width of the cast slabs is 69” (1750 mm). The slab gauge is within the range 0.4-1.6” (10-40 mm).
The casting rate is 1.6-33 ft/min (0.5-10 m/min).
The technology may be used for most Aluminum alloys containing up to 5% of Mg. Cooling rate is controllable by the blocks temperature and the casting speed. The method allows to cast alloys susceptible to cracks caused by fast cooling.
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Rotary caster technology combines twin-roll and twin-belt casting methods. A rotary caster consists of a steel wheel (diameter up to 102” (2600 mm)) with a peripheral groove and an endless steel belt closing the groove and forming the casting space. The groove has triangular, trapezoidal rectangular cross-section. The wheel and the belt rotate with the linear speed of about 45 ft/min (14 m/min). The belt is cooled by water jets.
When a melt is poured into the space between the groove and the belt it cools down and solidifies. The slab/strip/bar leaving the caster is still hot and may be hot rolled in-line to and then coiled.
The wheel diameter is up to 102” (2600 mm).
Aluminum alloys (Wrought aluminum alloys (1xxx), Wrought aluminum-manganese alloys (3xxx), Wrought aluminum-magnesium-silicon alloys (6xxx)) and Copper alloys for electrical applications (wires, conductors, bus bars) are cast by the rotary method.
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