Continuous casting

1. Continuous Casting
Prof. Dr. Kerem Altuğ GÜLER

2. Introduction
The advantages of continuous casting in primary metals
production have been recognized for more than a
century. In recent decades, a dramatic growth of this
processing technology has been realized in both ferrous
and nonferrous metal production. The principal
advantages of continuous casting are a substantial
increase in yield, a more uniform product, energy
savings, and higher manpower productivity. These
advantages and the ease of integration into metals
production systems have led to the wide application of
continuous casting processes.

3. Historical Aspects
One of the earliest references to continuous casting is a
patent granted in 1840 to George Sellers, who had
developed a machine for continuously casting lead pipe.
There is some indication that this process had been
underway before Sellers' patent, which was directed
toward improvement of this continuous casting process.
The first work on continuous casting of steel was by Sir
Henry Bessemer, who patented a process for
"manufacture of continuous sheets of iron and steel" in
1846 and made plant trials on continuous casting of
steel in the 1890s.

4. Although continuous casting had its start before the
beginning of the 20th century, it was not until the mid-1930s
in Germany that commercial production of continuously cast
brass billets was introduced. Sigfried Junghans, an active
inventor of casting technology, provided many improvements
in the process, in particular the introduction of the oscillating-
mould system to prevent the casting from sticking to the
mould. Further development of the process for the casting of
nonferrous metals continued, including the installation of
processing units in North America. Mould lubrication in the
form of oil, or more recently, low-melting slag powders, was
introduced. Taper of the mould to compensate for metal
shrinkage on solidification provided improved heat transfer
and, more importantly, fewer cracks.

5. In 1935, a plant with casting rolls for continuous
production of brass plates was operated at Scovill
Manufacturing in the United States and the Vereinigte
Leichtmetallwerke in 1936 started a semi-continuous
casting machine for aluminium alloys. Immediately
after World War II, commercial development of
continuous casting of steel began in earnest, with pilot
plants at Babcock and Wilcox Company (United States),
Low Moor (Great Britain), Amagasaki (Japan),
Eisenwerk Breitenfeld (Austria), BISRA (Great Britain),
and Allegheny Ludlum Corporation (United States).
These were followed by production plants for casting
billets in the West and stainless slabs in the Soviet
Union and Canada (the latter at Atlas Steels).

6. Nonferrous Continuous Casting
The early development of continuous casting processes
occurred to a large extent at production installations for
nonferrous alloys.
Direct-Chill Casting
The principal casting process for light metals is the direct-
chill process. The vertical direct-chill casting process was
patented by Alcoa in 1942, and is shown schematically in
its present form in figure below . The process can directly
prepare billets for extrusion blocks for rolling, and sheet
for fabrication, thus eliminating intermediate mechanical
working processes by casting near-net shapes.

7. Conventional vertical direct-chill casting arrangement

10. Air-slip DC casting technique

11. Most direct-chill casting capacity is of the vertical type
for semi-continuous casting but more importance is
being assumed by the continuous horizontal direct-chill
casting process. The section sizes in which aluminium
alloys are cast range from 1.5 × 0.5 m blocks for rolling
to 5 to 30 mm thick by 2 m wide for plate and strip.
There is considerable economic advantage in wide strip
casting.

12. Horizontal direct-chill casting system

14. Vk : Katılaşma hızı, Vç : Çekme hızı

15. Sıvı gölcük derinliği döküm özelliklerini ve yapıyı önemli
derecede belirleyen faktörlerden biridir. Derinlik azaldıkça
segregasyon azalır, katılaşma hızının kesit boyunca değişimi azalır.
İstenen bu derinliğin olabilecek minimum değerde tutulmasıdır.

16. Twin-Roll Casting
The major barrier to the widespread use of aluminium
sheets in high volume is its high cost of production.
Production of aluminium sheets by continuous casting
route rather than the conventional direct chill (DC) casting
and hot mill method offers an opportunity to substantially
reduce the cost. Because of the economics and
metallurgical advantages offered by twin-roll casting (TRC)
process, it has become widely popular in aluminium
industries. Currently TRC process offers distinct advantage
in lowering of greenhouse emissions. The capital
investment for TRC is significantly lower than the
conventional DC casting and hot rolling process. TRC
requires low energy consumption, limited space
requirement, and it also offers possibilities of easy
diversification.

17. It is pertinent to note that industrial TRC of aluminium is focused
mainly on horizontal or near horizontal technology. In TRC, the
thickness of the processed sheet is generally between 3 and 7 mm,
although less than 3mm thickness is possible at higher casting speed.

18. In general metallurgical terms, TRC is expected to have
refined microstructure, fine intermetallic particles, and
increased solid solubility, which is advantageous for
mechanical properties. Despite the above-mentioned
advantages, the aluminium industry is still looking for
improving the economic advantage in terms of
productivity to remain competitive with other
processes and emerging flat roll products. The quality
has been assessed for limited alloy compositions but
not universal across the wide spectrum of compositions
of Al alloys. Like, for alloys with narrow solidification
range, TRC has been successful, however processing is
still restricted for few wide freezing range alloys. There
are also issues of performance of products made from
TRC strips due to centerline segregation.

23. Ingot casting machines

24. Continuous Casting of Steel
The purpose of continuous casting is to bypass
conventional ingot casting and to cast to a form that is
directly rollable on finishing mills. The use of this
process should result in improvement in yield, surface
condition, and internal quality of product when
compared to ingot-made material. A diagram showing
the main components of a continuous casting machine
is presented in figure. Molten steel in a ladle is
delivered to a reservoir above the continuous casting
machine called a tundish. The flow of steel from the
tundish into one or more open-ended, water-cooled
copper moulds is controlled by a stopper rod-nozzle or
a slide gate valve arrangement. Withdrawn at the same
rate that molten steel is added to the mould.

25. Main components of a continuous casting
strand
Principal types of continuous casting. V, vertical; VB,
vertical with bending; VPB, vertical with progressive
bending; CAS, circular arc with straight mould; CAS,
circular arc with curved mould; PBC, progressive
bending with curved mould; H, horizontal.

26. Continuous casting involves the following sequence of
operations:
• Delivery of liquid metal to the casting strand.
• Flow of metal through a distributor (tundish) into the
casting mould.
• Formation of the cast section in a water-cooled copper
mould.
• Continuous withdrawal of the casting from the mould.
• Further heat removal to solidify the liquid core from the
casting by water spraying beyond the mould.
• Cutting to length and removing the cast sections.
To initiate a cast, a starter or dummy bar, is inserted into
the mould and sealed so that the initial flow of steel is
contained in the mould and a solid skin is formed. After the
mould has been filled to the desired height, the dummy bar
is gradually removed.

29. Evolution of hot steel strip production technology: İngot casting, CCC, TSC and
strip casting technology.

30. Principles of Twin-roll Casting of Steels
Unlike TRC of aluminium sheet, the development of this
process in the steel industry was terminated in the
1940’s due to difficulties with roll wear, low productivity,
poor ascast strip quality, and variable solidification
structure and mechanical properties. Responsible factors
for the delay in commercializing TRC of steel included the
high melting point and density, the complexity of the Fe–
Fe3C phase diagram and low thermal conductivity
compared with aluminium. Consequently, significant
engineering advances in direct strip casting (DSC) were
essential before the Bessemer’s idea could be
commercialized.

31. During the last two decades, an extensive research and
development program in the area of TRC of steels is
carried out to: (i) generate a better understanding of
early solidification; (ii) develop procedures for uniform
delivery of molten metal; (iii) devise methods for
controlling the melt pool edge; (iv) control roll
distortion, and (v) understand the interactions between
molten steel and refractory materials. In March 2000,
the US steel company Nucor, joined BHP (Australian
steel company) and IHI Japanese plant manufacture to
establish Castrip Limited Liability Company (Castrip LLC)
as the first license of the TRC technology. In 2005,
Castrip had the capability of producing over 500 000
Mt p.a. of strip-cast steel in a range of grades for
various markets.

32. (a) Bessemer’s twin-roll caster design conceived in 1846 and patented in
1857 and (b) a twin-roll caster for steel, patented in 1865.

33. CASTRIP process schematic.

34. The steel strip formation via TRC is an extremely fast
process in which solidification time of the outer shell with a
thickness of ~ 0.2 mm is in the order of ~ 0.02 s. The
complete solidification of the steel strip takes place in less
than a second. The roll sleeve material for TRC of steels is
mainly high conductivity copper alloy. To control the heat
transfer during casting, the roll surface may be textured.
Roll surface texturing is achieved by a range of methods
such as knurling, chemical etching, electric discharge
machining, and laser ablation or shot blasting. Moreover,
the nozzle is made by a thermally resistant ceramic material
such as alumina-graphite refractory.

35. Video links
• https://www.youtube.com/watch?v=k-Gqwajh4OU
• https://www.youtube.com/watch?v=rXkwdgxFgj0
• https://www.youtube.com/watch?v=vkbJo8VTyOw
• https://www.youtube.com/watch?v=HvouMeeJFzk
• https://www.youtube.com/watch?v=bWSGNkZVWCA
• https://www.youtube.com/watch?v=SkIzCnkm4mk
• https://www.youtube.com/watch?v=U9kqpn_vk7g
• https://www.youtube.com/watch?v=uc6uyBkxrvQ
• https://www.youtube.com/watch?v=d-72gc6I-_E
• https://www.youtube.com/watch?v=QYeQ0hHIQ0Y
• https://www.youtube.com/watch?v=4oMAjMCgeQY