Continuous Casting
What is a continuous casting process?
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| (a) Continuous Casting Process |
The continuous casting process consists of producing casting continuously with an infinite length by solidifying the casting progressively from the sides of the casting starting from the copper mould towards the centre of the casting until the cutting operation.
Continuous casting has the flexibility of moulding casting according to size, shape, complexity, and length infinitely by solidifying the casting along the producing line with fans, water and other cooling system elements.
This flexibility of continuous production is lacking in other casting processes such as die casting, investment casting, sand casting, centrifugal casting process, slush casting, gravity die casting or permanent mould casting.
Due to the production setup of manufacturing casting continuously this process gives higher production volume for specific shapes than any other casting process including the true centrifugal casting process.
This process is better than sand casting when it comes to surface finish, hotspots, surface defects, dross, porosity formation, inclusions, casting yield, dirt, shrinkage cavities and length of the casting, and flexibility in producing casting sections such as (T, C and L).
This process is called continuous casting because products are manufactured continuously with a faster cooling and production rate as compared to permanent mould casting, sand casting, investment casting, centrifugal casting, die casting and shell casting.
Continuous casting machines are highly productive and fully automated and can mass produce steel bloom, billets and slabs from 100mm to 250mm in size and 5m to 15m in radius.
Continuous casting (CC) is also called concasting and products are manufactured on continuous casting machines.
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Continuous Casting Types
This method is explained below in detail with a diagram and step-by-step process.
- Vertical Continuous Casting Process
- Horizontal Continuous Casting Process
- Curved Continuous Casting Process
- Strip Continuous Casting Process
- Upward Continuous Casting Process
Vertical Continuous Casting Process
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| (b) Vertical Continuous Casting Process |
A vertical continuous casting process is a downward continuous casting process used to produce straight vertical casting.
This process is referred to as the semi-continuous casting and direct chill continuous casting process used to produce cast iron, steel, copper and aluminium slabs and blooms of 300mm to 1500mm in size replacing ingot casting production.
Casting up to 1,00,000 tonnes/year can be easily produced using this process. The step-by-step process with a diagram for producing vertical continuous casting is as follows:
Step 1: Melting Ingots And Pouring Molten Metal In The Mould
Molten metal is melted in the furnace and poured through a ladle in the tundish. Tundish is the vessel where molten metal is stored temporarily which has an outlet that allows molten metal to flow in the copper-based mould which has a starter bar in it.
Casting powder is added to the molten metal to reduce the friction between the metal, mould and rollers giving a smooth surface finish, reducing the sticking of metal, reducing clogging, and bottlenecking and eliminating crack formation on the casting surface.
Mould can be coated with ceramic powder to avoid metal adhesion. Metal adhesion is where molten metal sticks to the mould.
Any impurities and slag get filtered at this point. An inert gas such as argon is blown while pouring molten metal into Tundish. This gas does not allow oxygen to react with molten metal.
Step 2: Starter Bar And Primary Solidification Process
After metal passes through the tundish and starts flowing over the copper mould metal comes in contact with the starter bar. Metal is first allowed to solidify along the starter bar and solidified casting is allowed to ascend downward vertically.
When the mould is filled starter bar or dummy bar is withdrawn from the mould and molten metal is allowed to cool that is flowing out of mould.
The starter bar is used as a supporting segment where molten metal starts solidification first and is then guided by the rollers for secondary solidification.
Solidification of molten metal takes place due to the mould getting cooled because of the cooling water sections in the copper mould.
The rate of solidification, pouring rate and flow rate should match the feed rate of the casting coming out from the copper mould.
Solidification takes place from the outer section of casting moving towards the center. Due to the directional solidification of casting, casting comes out to be of fine structure and homogeneous.
Water-cooled casting gives uniform fine-grain structure as compared to air-cooled casting gives coarse-grain structure to the final casting products.
Segregation during this process is less as high-quality dense molten metal accumulates together producing high-quality casting products.
A monoblock stopper is used to stop the molten metal outlet from the tundish to the copper mould stopping the casting process in case the casting output is not desired.
Step 3: Secondary Solidification And Cutting Operation
Once the casting is solidified and moves towards the ground vertically the casting is cooled through air, jet water and cut in the required shape. Very long casting is balanced, straightened and supported with the side rollers.
A contactless laser slab measurement system is used to reduce the error related to the measurement of the slab length that occurs due to improper function of the encoder as the casting slab slippage between the supporting rollers wheels due to dirt between rollers, wear and tear of the roller, variation in the size of the rollers and improper mechanical calibration.
This non-contact laser system gives accurate casting to be cut by a saw cutter or flame-cutting machine.
Horizontal Continuous Casting Process
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| (c) Horizontal Continuous Casting Process |
The horizontal continuous casting process consists of producing casting horizontally by applying pressure to the molten metal in the die horizontally along the straight line and allowing the casting to cool on the roller conveyors.
The dummy starter bar used in this process comes in contact with the metal and is one to guide the casting out of the mould moving ahead towards the cooling station which consists of water and air spray depending upon the grain structure requirement of the final casting.
Hydraulic oscillators are used to oscillate the casting output from the mould and stabilize the casting without getting clogged.
Solidification of casting happens along the edges and sides before the casting is completely moves forward. This gives a supporting structure to the casting as casting is rolled on the conveyors.
Further, the secondary solidification process starts when casting is uniformly solidified and cooled with air and water to achieve dense, uni-axial uniform distribution of grain structure to manufacture high-quality casting in the foundry.
The disadvantage of this process is that the floor space required for the casting process is more for the larger length casting sizes.
Better control and requirement of the time is more as the casting output from the mould needs to be controlled for the casting to solidify.
This process replaces traditional colled rolling steel production where ingots are rolled between the rollers reducing in length to produce slabs.
Instead, steel ingots are melted instead of being cold pressed and solidified horizontally to produce continuous casting and cut on the cutting station according to the requirement of the customer.
Strip Continious Casting Process
What is the strip continuous casting process, explain its working with a diagram?
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| (d) Strip Continuous Casting Process |
As molten metal comes in contact with the interior of the mould it starts solidifying and metal is pushed down the mould opening.
A stopper mechanism is used in this process to suddenly stop the molten metal feed in the copper mould in case any problem is faced during the casting process which controls and stops the flow of the molten metal in the mould.
The solidified metal is rolled out between the twin rollers which are of larger size than the regular rolling strips as we discussed above in the article.
These twin rollers have a larger surface area and less gap between them to press the metal and reduce its thickness.
Straightening rollers and withdrawal rollers are used to straighten the casting strip and withdraw
There are supporting rollers, clamps and withdrawal rollers to support the thin strip coming from the twin rollers and pass it conveniently on the conveyors.
All rollers are used in the process to control the output feed, feed rate, velocity, stop and start of the strip production.
Cutting stations are employed to cut the continuous strips according to the requirement based on the length in batches.
This process is best for producing rolled-out thin-strip low-carbon steel products with high yield as compared to any other process such as cold and hot rolling processes.
Upward Continuous Casting Process
The upward continuous casting process consists of melting required metal into an electric furnace. This process is widely used for producing small strands, pipes, sheets, metal rods, wires, plates and tubes made from gold, silver, copper, brass and bronze wires.
Furnaces such as the electric furnace, oxygen furnace or electric furnace is used for this process. This process starts with melting ingots and heating the molten metal to the required temperature standards.
Oxides, slag, dirt and impurities are removed from the surface of the metal. Degrassing is done to improve the quality of the final casting.
Required molten metal powders are added to the molten metal to improve the mechanical properties of the final casting and reduce oxidation.
An initial crystallizer cylinder is used to solidify the metal by inserting an initial starter bar/rod in the molten metal and pulling it upwards. While this happens molten metal in this process starts cooling in the crystallizer cylinder forming continuous casting.
The floor space required for this process is smaller, is fully automated, has a higher production rate and drawing speed and uses servo motors for the continuous casting of products.
Directional solidification and shrinkage are effectively controlled in this process.
Continuous casting upward machines are fully automated and computerized monitoring controlled to look for any change in temperature, feed rate, casting defects, solidification rate, cooling system function, friction between mould and molten metal, porosity analysis, uneven solidification, oxide formation, slag, starter bar feed rate, cooling jacket function, mould temperature and flow rate.
Curved Continuous Casting Process
The curved continuous casting process is initiated by melting the ingots according to the customer's requirements.
To increase the properties of the slab fluxes, minerals such as magnesium oxide, silica, sodium oxides, aluminium oxides and calcium oxides are added before the casting process starts.
These minerals, flux and molten mould powders are mixed with the molten metal through an immersion nozzle that is present in the mould.
Moulds are lubricated before pouring molten metal from the tundish to the mould to reduce the friction between mould and molten metal reducing bottlenecking of the solidified casting.
Molten metal once melted is poured into the tundish which is a storage vessel which regulates the flow of the molten metal to the copper-based mould through the submerged entry nozzle (SEN).
The flow rate of the molten metal is controlled by the stopper mechanism in case any problem occurs during the casting process.
Once molten metal starts solidifying the mould outlet with the help of a starter bar the metal is allowed to pass through the outlet of the mould. The mould used in this process is water-cooled and made from copper.
The oscillation mould table is placed below the copper-cooled mould to ensure the smooth flow of the continuous castings.
In his process, the solidified continuous castings are rolled over between the rollers to bend and straighten them continuously.
The casting needs to go through primary and secondary solidification and cooling processes.
The solidification process starts with the water-cooled system, water is prayed on this casting continuously until the casting solidifies fully before the slab-cutting process starts.
If this is done with multiple rollers in multiple steps increasing the end, straightening the casting chances and risk of cracking of the casting products reduces over some time.
This process has replaced the traditional rolling process for producing billets, slabs and blooms as this process is economical, fast and highly productive.
This process is widely used in producing casting steel slabs and blooms, almost 90% of the steel produced is manufactured by this casting process.
Complex high tolerance lengthier quality slabs can be manufactured using this method.
What are defects that are caused in continuous casting products during the continuous casting process?
Process parameters of continuous casting are molten metal pouring rate, flow rate, solidification rate, mould oscillation rate, operation temperature, molten metal temperature, pouring temperature, thermal conductivity during the process, density, temperature of mould and casting ascending rate.
Early and uneven solidification of the molten metal can cause deep marks, breaking of the casting and longitudinal cracks.
Mould and molten metal have a thin lubrication between them which controls the surface defect of the final casting. For molten metal with lower temperature viscosity reduces over time making lubricating the die mould an important step in reducing the sticking of molten metal to the mould surface.
A defect such as a slab breakdown due to the solidified shell sticking to the mould surface occurs when the weak stressed casting surface edge that is solidified is pushed downwards but at the same time is stuck to the mould surface of the mould.
This defect is also caused when the mould oscillation is not adequate.
Uniform cooling, directional solidification, proper lubrication, viscosity, surface tension of molten metal and proper use of mould powder result in defect-free uniform casting.
Non-uniform secondary cooling can lead to cracks, shrinkage, cavities, blowholes, and diagonal cracks due to formation of the uneven thermal stress in the casting slabs.
To perform uniform cooling continuous casting nozzles are used having hexagonal spray, cone spray, oval spray and round spray nozzle in the casting process.
What materials are used for producing dies and what properties should continuous casting process dies have?
Dies used in the continuous casting process are made from brass, tool steel, carbon steel, stainless steel, cupro nickel, tin, bronze, copper, aluminium bronze alloy material and special alloys.
The die used in this process should have the following properties:
- Smooth surface quality.
- Chemical resistance.
- Low thermal expansion.
- Thermal conductivity.
- High dimensional accuracy.
- Thermal shock properties.
- Consistent uniform material.
- High-quality machinability and workability.
- Better mechanical properties such as hardness, strength and ductility.
- Die lubrication properties.
Des available here are from 150mm diameter to 1500mm diameter for circular casting. For block casting having square surfaces 150mm to 1000mm sizes are available.
Continuous Casting Process Vs Extrusion Process
| Continuous Casting | Extrusion Process |
|---|---|
| 1) Continuous casting produces longer and bigger castings than the extrusion casting process. | 1) The extrusion process produced smaller castings than continuous casting. |
| 2) The plunger and shaft mechanism is not used in this process, instead molten metal is poured from the mould with a starter bar and solidified as it ascends with gravitational force. | 2) In the extrusion process molten metal is stored in the cylinder and the piston is used to push the molten metal out of the die against the gravitational force. |
| 3) Limited casting shapes such as I,T,C,L,J and Y sections can be produced using this process. | 3) Multiple casting sections such as circular, billets, rectangular, hexagonal, pentagon, trapezoid and star shape castings can be manufactured using extrusion. |
| 4) There are no limitations over the cross-section area of the casting. | 4) Cross section area of the casting has its own limitations. |
| 4) The process is continuous and best for mass production of casting unless and until molten metal is poured into the mould continuously. | 4) The process is not as continuous as continuous casting because molten metal needs to be poured into the plunger or cylinder first and then pressure needs to be applied gradually to push the molten metal out of the die. |
| 5) The process is continuous and best for mass production of casting unless and until molten metal is poured into the mould continuously. | 5) The process is not as continuous as continuous casting because molten metal needs to be poured in the plunger or cylinder first and then pressure needs to be applied gradually to push the molten metal out of the die. |
Continuous Casting Vs Direct Rolling
| Continuous Casting | Rolling Process |
|---|---|
| 1) Continuous casting is a process where ingots are melted and converted into molten metal to shape casting in an open mould cavity. | 1) The rolling process is an ingot thickness reduction process in which the size of the ingot is reduced through passing rollers making it thinner. |
| 2) The continuous casting process is not a forming process but a casting process done in the foundry. | 2) Rolling is a mechanical forming process not done in the foundry. |
| 3) The mechanical properties of continuous casting are less superior to the rolling process. | 3) The mechanical properties of these castings are superior to continuous castings. |
| 4) Continous casting can produce larger, heavier and bigger cross-section castings. | 4) There are limitations over the length, weight, and cross-section of the casting on the upper side of the dimensions. |
| 5) Delicate and intricate cross-sections are not possible in this CC process. | 5) Delicate, intricate cross-sections of casting are possible in this CC process. |
Continuous Casting Application
This process is used to produce longer-length rods, strands, billets, tubes, sheet casting, plates, wires, thin slabs and blooms.
The material used for producing continuous casting products and components is made from silver, steel, cold-forming steel, zinc, brass, lead, cast iron, ductile iron, coper-zinc, copper-iron, copper-tin, copper-bronze, nickel, carbon steel, tubular steel, ball-bearing steel, gold, bronze, platinum, aluminium and engineering precision material.
Components and products manufactured from this process are mentioned below in the article.
- Jewellery strands made from gold, platinum and silver material.
- Copper, bronze, nickel and aluminium with upward continuous casting.
- Beam and casting blanks.
- Bars and pipes.
- Strips and channels.
- I-section, L-section, C-section, and T-section cast beams.
- Cables.
- Billet and Bloom.
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Plates and blanks.
- Alloy wires.
- Metal profiles.
- Wire bars.
- Sheet metal.
- Dog bones.
- Metal cakes.
- Rail section.
Continuous Casting Advantages
- Wastage of material is lower in this process resulting in higher yield as there is the absence of a riser.
- The cost, time, money, material, energy and resources required are less for continuous large versatile consistence quality casting products.
- Best for slabs, blooms, stands and billets as compared to rolling or any other forming process.
- An uninterrupted 24-hour production cycle can be achieved using continuous casting machines for high-quality casting output.
- The environmental impact of continuous casting is less for producing casting from stainless steel and cast iron.
- Single and multiple-strand castings can be manufactured.
- Solidification of the slabs is quicker reducing many solidification defects.
- Floor space for producing casting is less as compared to the production rate of the process.
- Casting at higher speed is possible due to the development of more sophisticated and advanced process monitoring systems such as tundish level, pouring rate, molten metal level and temperature level detection.
- Simple bending, progressive bending, vertical casting, upward casting, downward casting, curved bending, straightening, progressive straightening and cutting casting in the required length of final casting are possible.
- This process replaces the traditional process of producing billets and sections of steel slabs where ingots were first produced and then cold pressed between the rollers forcefully to manufacture casting with different sections and shapes.
- Continuous casting takes less foundry floor space as compared to the ingot production process.
- As this process is not sand casting, casting has a better surface finish and fewer surface defects than the sand mould as die moulds are used in this process.
- The life of the continuous casting moulds and casting tools is higher as compared to die casting as the moulds are not subjected to high pressure and forces during the casting operation.
- The requirement of machining operations, deburring and other secondary operations is the least required in this process.
- This process produced near-net shape clean casting components with minimal waste and less scrap.
- Continuous casting simulation software can be used that give a detailed analysis of various process parameters before starting the production on the foundry shop floor.
Continuous Casting Disadvantages
- This process is limited to specific shapes, casting such as engines, gears, impellers, and turbine blades cannot be manufactured by this process.
- This process is not suitable for foundries with smaller production capacities, the initial investment cost of this process is very high as continuous casting machines and process is automated.
- The process of mould production, conveyor system, monitoring system, and casting defect-detected system is complex and requires a skilled trained operator to learn and operate the concast machine control panel.
- Floor space for horizontal continuous casting machine and vertical height for vertical continuous casting machine requirement is high as compared to sand casting and die casting machines.
- The inclusion of slag, oxides, impurities, defects in the rollers, non-calibrated monitoring ladle, molten metal and temperature system can cause downtime of the foundry.
- Suitable for mass production of casting sections such as C, T, L and I which are simple in design and sections that do not have internal cavities.
- There is no need to cut a complex gating system in this casting process as we see in the sand casting process.
Continuous Casting Process Summary And Conclusion
This process is best for producing infinite solid strands, slabs, plates, boom and billets casting with numerous size variations and is better than ingot casting using a highly automated controlled system.
Continuous casting machines are best for producing casting with a simple design such as circular shapes, rectangles, round, hexagonals, blanks, ingots, tubes, octagonals, squares and rectangles.
This process is cheaper, faster, economical, safer, optimised, simple, well-automated and compact than the rolling and ingot production process for producing steel casting.
This process can produce aluminium, cast iron, copper, spring steel and alloy casting blooms at the lowest cost with reduced machining giving higher life to the moulds and tools used in this process.
The modern continuous casting process is equipped with automatic sensory, alarm and visual detection systems to detect any ladle damage during pouring operation, pouring rate detection, slag detection system in the molten metal, detection of cracks/damage on the slab, tundish and mould level detection system reducing any safety concerns in the foundry.
This system implementation starts reducing rejection of casting, improving the quality of casting, increasing the production rate and providing a safe environment for the casting operators on the foundry floor.
The solidification time required for producing continuous casting slabs is lesser reducing resources, environmental waste, immesion and the cost of manufacturing components.
This results in high-quality steel continuous casting production in the foundry from a permanent non-expandable moulding process.