Continuous Casting Equipment
Continuous Casting Machines
Casting machines can be classified into several main groups depending
on the section shape produced: billet, bloom, round, slab
and beam blank. In some cases, overlaps occur where the molds on a particular
machine can be changed to cast other shapes; for example, billets or blooms,
blooms or small slabs, and blooms or rounds. In addition, machines exist where
special shapes, such as rectangles and dogbone
structural sections can be cast as well as billets or blooms.
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. See the
feature paper Modern Strip Production Technologies CSP and ISP in
the Flat
Rolling category of the Tech Resource section.
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. See also our tech papers in the Shape
Rolling category of the Tech Resources section.
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.
Fig. 1: Development of casting machines [1]
The choice between these types of casting machines depends
on a complex optimization of the specific facility requirements for caster
productivity, product quality and machine complexity, and cost. With the
introduction of the newer designs there has been an increasing adoption of the
bow‑type machines with curved molds for slab casters and to a lesser
extent for billet and bloom machines. Curved machines are usually simpler to
build (i.e., lower cost) and maintain than vertical with bending machines, as
the bender is eliminated. However, for some grades of steel, for example, plate
grades, quality and casting speed limitations were previously more restrictive
on these curved machines. Recently, technical developments such as
"clean" steel practices and electromagnetic stirring have been
applied to curved machines to overcome these restrictions. In general, the
complexity of the casting process and machine varies greatly between the type
of product being cast (e.g., billet, bloom, or slab). This is due both to the thermomechanical characteristics of these cast sections, and
to the different applications of the cast product.
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
Major components of a continuous casting machine are
illustrated in Fig. 2. Liquid steel flows from the ladle into a cast container
called tundish, and from the tundish
into the mold for heat extraction. After the cooling with the mold (called
primary cooling), the steel is under secondary cooling conducted with water
sprays. In the following section of this paper the primary components of a cast
machine is introduced.
Fig. 2: Major components of a continuous casting machine [1]
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.
References
[1] The making, Shaping and Treating of Steel, 1985, US
Steel.
[2] The making, Shaping and Treating of Steel, 2002, AISE
Steel Foundation.
