In continuous casting the
transformation of molten metal into solid the control of
steel flow rates is accomplished by the tundish. This
refractory-lined distributor, is placed over the mould,
receives molten steel from the furnace ladle, and acts as a
buffer vessel between the steel ladle and the mould.
The tundishs key control
parameter consists in matching the flow of liquid steel into
the mould with the withdrawal speed of the strand out of the
mould (see figure), and this is where refractories
have their key application.
The tundishs main duty has
always been the continuous production of steel in the
continuous casting process, serving the purpose of a reservoir.
But over the years, the device has been assigned further tasks,
and has undergone sophisticated change.
Today, the tundish is multitasking.
In addition to its original function, the device now has to
separate non-metallic inclusions, calm the flow, and balance
the temperature to prevent the reoxidation of the steel.
As a consequence, the tundish has
increasingly gained importance from being a mere reservoir to
being viewed as a metallurgical vessel, becoming
one of the most important areas of refractory application.
As one of the biggest single cost
centres in the continuous casting process, tundish refractories
mainly comprise andalusite and magnesia, in addition to
aluminas, alumina carbon, zircon silicates and low cement
Tundish with permanent lining before spraying (left)
and with the wear
lining above after spraying (right)
Lining and dams
The tundish has to resist
corrosion, thermal shock, and high temperatures ranging between
1,520º-1,600ºC. The most important tundish components
for refractory applications are the permanent lining, the wear
lining and dams including the weir and the baffle, since
they contain the most refractory materials.
The permanent lining, primarily a safety lining and the base of
the wear lining, is used for embedding well blocks and nozzles
as well as lock nozzle sets and the submerged entry nozzles
(SEN). The requirements placed on the permanent lining include
high thermal shock resistance, high chemical resistance to
ladle slag, tundish powder and steel melts, low sintering
tendency with the wear lining, and high mechanical stress
(tilting/removal of skull).
It is possible to brick or
cast monolithic permanent lining the permanent lining
which is mainly made of bauxite or andalusite based LCC,
microsilica, and high alumina cement (4-6%).
Usually the quantity of andalusite
can range around 40-50%. A monolithic permanent lining is
distinguished by the following characteristics: lining without
any joints, thus easier to remove skull, high strength and
stability, high thermal shock resistance, shorter time required
for lining work and easy repair. The permanent lining can be
used in 500 to 1,000 heats and therefore has to be changed once
or twice a year.
Wear lining. The
wear lining is a temporary lining located above the permanent
lining. There is a host of different tundish lining practices
which can be categorised into five major types: bricked,
gunnable, board, sprayable and dry.
The wear lining is mainly made of
magnesia, with olivine or dunite. The average ratio is 3:1 but
the MgO content can vary from 60% to 100% for high quality
steels with olivine increasing from 0% to over 40%.
The thickness of the wear lining
depends on two things. First, the number of ladles in the
sequence: the more ladles, the more centimetres of lining. For
instance, in an average plant, for a sequences of up to 12
ladles, wear thickness will be 3-4cm. Second, it also depends
on the steel and slag activity against the magnesia refractory.
It is used in three to 50 heats a few hours of
baffle. As shown on the schematic, these different
types of dams are used to control the flow and purify the steel
before it goes to the mould as steel at about 1,550ºC has
similar flow ability than water. The flow from the tundish to
the slab must be stable.
These dams have a similar
composition to the permanent lining, being mainly made of
andalusite based LCC. Like the permanent lining, they are
supposed to last between 500 and 1,000 heats.
Simplified tundish showing use of
As explained above,
andalusite or bauxite magnesia and olivine or
dunite - are the main refractories raw materials used in the
tundish. Except for olivine, their prices have fluctuated
significantly since the beginning of the recession, therefore
impacting refractory material prices. An industry source told
IM that after reaching prices at
$400-750/tonne FOB last year, dead burned magnesia prices used
for refractories dropped to around $300-342/tonne FOB at the
beginning of 2010. As a consequence, refractory prices
decreased in parallel, dropping by about 12% during the same
But the trend seems to have made a
U-turn since then. Magnesia prices are now starting to
increase again, explained Jean Martin, marketing director
of French refractories producer Daussan Group, who expects the
impact on the market to appear next summer. Therefore, it is
quite likely that prices of related refractories will augment
during H2 2010.
The magnesia/olivine or dunite
balance of the wear linings composition usually depends
on the type of steel which is made. Until now, the trend was to
use up to 85-100% MgO with less dunite or olivine for high
quality steel such as tool steel, diminishing to up to 60% MgO
for a lower quality such as flat steel.
However, the subject is still a hot
debate in the industry. According to some recent studies, the
MgO content could be reduced in profit of a higher content of
olivine or dunite - with no effect on the end
products quality. A source of worries for magnesite
producers, it comes as good news for refractories producers as
olivine prices, mainly sourced in Norway - are at present
reported at 110/tonne ($150/tonne), allowing them to
reduce their production costs.
Technically, using bauxite or
andalusite for the permanent lining and the dams in the tundish
does not make a huge difference. In this application, the
properties are quite similar and the decision to use one or the
other will mainly depend on the raw material cost. According to
various sources in the industry, bauxite prices (Guyana,
refractory grade) are at present flirting with $600/tonne while
andalusite is about $570/tonne FOB.
Depending on the size of the
tundish, about 3-7 tonnes/tundish of andalusite is needed. It
will therefore come as no surprise that refractories
manufacturers very often decide to choose andalusite over
bauxite. As bauxite prices are quite high at the moment,
we are going back to andalusite, commented Martin.
For the long term, the future of
these refractory materials seems to be bright as the steel
industry is on a firm road to recovery. Steel is
performing fairly well. Most of the plants are working at
utilisation rates between 70 to 90%, commented a source
from the refractory industry.
Some steel producers already report
full sales for H1 2010, but that may not be the trend for H2.
The recovery, slow and laborious, is not expected before early
2011. That will depend as well on raw material price
control, which is not easy with other competing markets
increasing with almost double digit growth, added the
In the short term, low demand is expected from steel end
markets from H2 2010, which if compounded by a forecast
increase in MgO prices may result in increased refractories
prices within the next six months.