Continuous Casting Practices
STEELMAKING PROCESS
In general, the
requirements to the steelmaking practices for continuously cast
steels are higher than those to the ingot steels. Firstly, the
tapping temperature should be higher and tolerance limit of
temperature should be closer. Secondly, the steel must be more fully
deoxidized.
Temperature control
is more critical than in ingot production. The tapping temperature
is generally higher to compensate for heat losses associated with
the increased transfer time to a caster. On the other hand, the
temperature must be maintained within closer limits. If the
temperature is too high, there will be "breakouts"; and if the
temperature is too low, premature freezing in the tundish nozzles
will occur. Casting temperature can also affect the crystallization
structure of the cast product. Optimum structures are developed with
low superheats that should be uniform throughout the entire cast.
One common practice employed to reach the uniform temperature is to
stir the metal in the ladle by the injection of a small quantity of
argon into the liquid steel.
Continuously cast
steel is also required to be fully deoxidized (killed) to prevent
the formation of blowholes or pinholes at or close to the surface of
the cast product. Those blowholes or pinholes may cause seams in
subsequent rolling operations. There are generally two practices for
the deoxidation, depending on the grade of steel and product
applications: (1) silicon deoxidation with a small addition of
aluminum for coarse grain steels; and (2) aluminum deoxidation for
fine grained steel. Silicon‑killed steels are easier to cast than
aluminum killed steels. This is because deposits of alumina in the
tundish nozzle, which cause nozzle blockage, are avoided. Normally,
a ladle (or Ladle Metallurgical Furnace LMF) refining practice prior
to casting is employed. In certain situation when still higher steel
quality is expected, a vacuum degas (VDG) process is used. So the
most common practice used in steel making plants is: EAF/BOF + LMF
(+ VDG) + CAS.
LIQUID METAL FLOW
CONTROL
Tundish application
In the continuous
casting process, the liquid steel is transferred at first from a
ladle into the tundish. Then the liquid steel is distributed into
different strand of molds, through a nozzle for each strand. The
tundish is essentially a rectangular box with nozzles located along
the bottom. The tundish makes it possible:
-
to reach stability of the metal
streams entering the casting mold, and in turn, to achieve a
constant casting speed.
-
to cast a sequence of heats
-
to change over the empty ladle for a
full ladle without interrupting the flow of metal in the molds
-
to make a mixed grade with steel from
two different grade from different heats, if needed
-
to provide possibility to prevent
inclusions and slag from entering tundish and thus slipping into
mold
Tundishes are
usually preheated prior to casting to minimize heat losses from the
liquid steel during the initial stages of casting and thus avoid
metal freezing, particularly in the critical nozzle areas. Tundish
covers are also used to reduce radiant heat losses throughout the
casting operation.
Liquid
Metal Shrouding
In open stream
casting the liquid metal flows directly, through the air, from the
ladle to the tundish or from the tundish to the mold. Under these
conditions the unprotected metal stream picks up oxygen (and some
nitrogen) from the air and deleterious inclusions are formed in the
liquid steel. These inclusions are transferred into the casting mold
where they are either retained within the cast section or float to
the surface of the liquid steel. Those present on the liquid steel
surface are subsequently trapped in, the solidifying shell and
either result in surface defects on the product in rolling or a
catastrophic break in the shell below the mold. In addition to the
direct formation of inclusions in the exposed steel stream, air
entrained in the stream can also react with liquid steel both in the
mold and tundish.
To avoid these
problems shrouded‑stream casting is employed. Emphasis was first
placed on shrouding the metal stream between the tundish and mold
because of severity of the problem. However, ladle to tundish stream
shrouding is now widely employed, especially in slab casting of
aluminum‑killed steels where the prevention of alumina inclusions is
of paramount importance. There are two basic types of shrouding with
numerous variations and combinations: (1) gas shrouding; and (2)
refractory tube shrouding.
Gas shrouding is
frequently used in casting small sections on billet machines (i.e.
4‑in. sq.) because of operating difficulties experienced with
refractory tubes: there is insufficient space to introduce a tube
without encountering metal freezing between the mold wall and tube.
There is a variety
of designs including: the Pollard steel tube shroud in which gas is
introduced at the mid point of the tube at low velocity and exits
between the tube and nozzle, and between the tube and mold; complete
enclosure between the tundish and mold using a flexible coupling;
truncated pyramidal enclosures; and a liquid nitrogen curtain (Fig.
1). Nitrogen or argon is used as the protection gas. Gas shrouding
alone is not commonly used for preventing oxidation of the ladle to
tundish stream. However, one design in use employs a circular ring
which is attached to the ladle at one end and is sealed by a sand
seal at the other end when the ladle is lowered toward the tundish
thus forming an enclosed box: the box is then pressurized with
argon.
Refractory tube
shrouds are commonly used for casting aluminum‑killed steel. They
are used both between the ladle and tundish, and tundish and mold.
One end of the tube is attached to the ladle (or tundish) with the
other end immersed in the steel when the tundish for mold) is filled
with metal. Refractory tubes are usually made of fused silica or
alumina graphite.
Fig. 1:
Configurations for shrouding from ladle to mold [1].
The mechanical
design of the refractory tube is important, especially at the exit
end that is immersed in the steel. One type is a straight‑through
design. Another type, generally used in the mold, has a multi‑port
(opening) design, such as a bifurcated tube with the bottom of the
tube closed and two side openings located near the bottom of the
tube. This type of shroud avoids deep penetration of the pouring
stream into the creator of the solidifying strand and modifies the
flow pattern in the mold. Thus, the inclusions in the pouring stream
are not entrapped in the solidifying section but rise to the surface
of the liquid metal and are removed with the slag formed by the mold
powder.
In many plants, the
design of the shroud attachment includes the capability for
replacing a worn shroud so that along sequence of heats can be cast
without interruption.
At some plants,
argon is introduced into the refractory tube to avoid aspiration of
air through pores and joints that is caused by the venturi effect of
a moving metal stream.
MOLD AND HEAT
TRANSFER
The primary
function of the mold system is to contain and start solidification
of the liquid steel to achieve the following goals:
-
shape (overall configuration and shell
thickness)
-
temperature distribution
-
internal and surface quality (i.e.
structure, chemical uniformity together with an absence of cracks,
porosity and non‑metallic inclusions)
Besides the heat
transfer ability, the high-temperature strength and resistant
against mold wear and mold deformation is also extremely important.
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.
SECONDARY COOLING,
STRAND CONTAINMENT AND WITHDRAWAL
In modern slab
casting machines, secondary cooling, strand containment and
withdrawal form a closely integrated and interlocked system
that also includes strand bending and straightening. In the older
designs of billet and bloom casting machines, there was a greater
functional as well as physical separation of the components of this
part of the casting operation. For the purposes of this discussion
the concepts employed in the design and operation of modern slab
casting machines will be considered.
Secondary cooling
and the containment / withdrawal system extends from the bottom of
the mold through complete solidification of the strand to the cutoff
operations. The system is designed to produce a final cast section
that has the proper shape, and internal and surface quality. To
accomplish these results the solidifying section leaving the mold is
cooled in a series of spray zones and contained and withdrawn by a
series of roll assemblies until the solidified cast section reaches
the cut‑off machine and horizontal runout table.
Secondary
Cooling ‑The secondary cooling system is
normally divided into a series of zones to control the cooling rate
as the strand progresses through the machine. This system,
conventionally, consists of water sprays that are directed at the
strand surface through openings between the containment rolls.
Recently, air-water "mist" sprays (discussed later) have been
employed which provide more uniform cooling.
Heat Transfer in
Secondary Cooling‑The main heat‑transfer functions of the
spray‑water system are to provide:
- The proper
amount of water to obtain complete solidification under the
constraints of the casting operation, i.e., steel grade, casting
speed, etc.
- The capability
to regulate the thermal conditions of the strand from below the
mold to the cut‑off operation, i.e. strand surface temperature and
thermal gradients in the strand.
- Auxiliary
functions such as cooling of the containment rolls.
It is necessary to
control both the temperature levels and thermal gradients in the
strand to avoid the occurrence of surface and internal defects such
as improper shape and cracks. At high temperature, the strength
properties of the steel shell play a critical role in the ability of
the shell to withstand the external and internal forces that are
imposed by the casting operation. The primary forces are those
exerted by the ferrostatic pressure of the liquid core and the
traction of the withdrawal operation. In particular, the ductility
of steel close to the solidus temperature is low and the shell is
susceptible to crack formation‑ It is important to control
temperature gradients because thermal strains can be caused which
exceed the strength of the steel resulting in cracks. Excessive
thermal strains result from changes in the heat‑extraction rate by
either over‑ or under‑cooling. The latter conditions can occur by
reheating, which is produced when spray cooling is terminated
improperly and the strand reheats by heat transfer from the interior
with an increase in temperature before decaying by radiation heat
transfer to the environment. Under these conditions, excessive
strains and cracks can result. This effect can be reduced by
extending and varying the water‑spray cooling operation to provide a
smooth transition with the radiation cooling area.
Thus, in the design
of a secondary cooling system, the thermal conditions along the
strand must be established which satisfy the product integrity and
quality. For example, the surface temperatures along the strand are
specified. They are generally in the range of 1200° to 700°C (2190°
to 1290°F). Based on this information the cooling rates along the
strand are determined from heat‑transfer equations. Important
parameters in these calculations include the convection heat
transfer coefficient of the water sprays and the water flux (the
amount of water per unit area of surface contact). The type of spray
nozzle, nozzle position with respect to the strand surface, number
of nozzles and water pressure are selected to provide the required
water flux and distribution throughout the secondary cooling sector.
Multiple nozzles are typically used at each level along the strand
that has an overlapping pattern.
Generally a series
of cooling zones is established along the strand, each of which has
the same nozzle configurations and heat‑transfer characteristics.
Since the required cooling rates decrease along the length of the
strand, its water flux in successive zones decreases.
During operation,
changes in the water flux are made to compensate for changes in
casting conditions such as casting speed, strand surface
temperature, cooling‑water temperature and steel grade.
The spray‑water
system is typically a recirculating system.
Strand
Containment ‑The strand is contained by a
series of retaining rolls which extend across its two opposite faces
of the cast sections in a horizontal direction: edge rolls may also
be positioned across the other pair of faces in a direction
perpendicular to the casting direction to further enhance
containment. The basic functions of the mechanical strand
containment and withdrawal equipment, which forms an integral part
of its secondary cooling system, are: (1) to support and guide the
strand from the mold exit to the cut‑off operations; and (2) to
drive the strand at a controlled speed through the caster. In both
of these functions, the final objective is to minimize the
mechanical stress and strains incurred during the process.
For illustrative
purposes, a casting machine design that consists of a vertical
discharge from the mold to horizontal delivery prior to the cut‑off
operations is discussed. In this typical case there is a series of
rolls or guides arranged vertically below the mold, followed by a
series of rolls arranged in a curve (which provides a transition to
the horizontal) and a series of rolls in a horizontal plane before
the cut‑off equipment. Each series of rolls may be segmented and
contain different diameter rolls and roll spacings to meet the
conditions existing at that location.
Strand support
involves the restraint of the solidifying steel shape that consists
of a solid steel shell with a liquid core. The ferrostatic pressure,
created by the height of liquid steel present, tends to bulge the
steel especially in the upper levels just below the mold where the
solidified shell thickness is small (Fig. 2). Bulging at this
location would not only cause product defects such as internal
cracks but also cause a skin rupture and a breakout. Bulging is
controlled by an appropriate roll spacing that, in general, is
closest just below the mold and progressively increases in the lower
levels of the machine as the skin thickness increases. All four
faces of the strand are usually supported below the mold with only
two faces supported at the lower levels. In addition to ferrostatic
pressure and skin thickness, roll spacing is also based on strand
surface temperature and the grade of steel cast.
Fig. 2. Stresses in the solidifying skin due to ferrostatic pressure
[1]
Strand Bending and
Straightening‑In addition to contain the strand, the series of rolls
that guide the strand through a prescribed arc from the vertical to
the horizontal plane must be strong enough to withstand the bending
reaction forces. During bending, the outer radius of the solid shell
is placed in tension and the inner radius in compression. The
resulting strain, which is a function of the radius of the arc and
the strength of the particular grade of steel being cast, can be
critical; excessive strain in the outer radius will result in metal
failure and surface defects (cracks). To minimize the occurrence of
surface defects but, at the same time, maintain a minimum effective
arc radius, triple‑point bending has recently been introduced (i.e.,
three arcs, with progressively smaller radii).
A multi‑roll
straightner is installed following the completion of bending which,
as the name implies, straightens the strand and completes the
transition from the vertical to horizontal phase. During
straightening the strand is "unbent" which reverses the tension
and compression forces in the horizontal faces of the strand.
Strand
Withdrawal ‑The strand is drawn through
the different parts of the casting machine by drive rolls which can
be located in the vertical, curved and horizontal roll sections.
This multiple drive‑roll system is designed, wherever possible, to
produce compression forces in the surface of the strand to enhance
the surface quality. Thus, the objective is to "push" the strand
through the casting machine, as opposed to "pilling" the strand with
the attendant tensile stresses that tend to produce surface defects.
In addition, the use of multiple sets of drive rolls distributes the
required traction force along the length of the strand and
consequently reduces the deleterious effects of tensile forces. The
proper placing of drive rolls can also reduce adverse bending and
straightening strains by exerting an offsetting compression force,
i.e., by placing drive rolls before a set of bending rolls. In all
cases, the pressure exerted by the drive rolls to grip the strand
must not be excessive; excessive pressure will deform the shape of
the section being cast.
Following
straightening, the strand is conveyed on roller tables to a cut off
machine where the section is cut to the desired length. There are
two types of cut‑off machines: oxygen torches and mechanical shears.
Oxygen torches are employed for large sections such as slabs and
blooms. Billets are either cut by torches or shears. The cast
product is then either grouped or transported directly to the
finishing mills or, in the case of billets, to cooling beds which
are predominantly of the walking beam type to maintain product
straightness.
References
[1] The making, Shaping and Treating of Steel, 1985, US Steel.
[2] The making, Shaping and Treating of Steel, 2002, AISE Steel
Foundation.
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