Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Impact Pad for a Tundish
This invention relates to the handling of molten metal, especially steel. In
particular, the invention relates to a tundish impact pad, i.e. an article
formed from
refractory material to be placed on the floor of a tundish to receive an
incoming
stream of molten metal poured into the tundish from a ladle. The invention has
particular utility in the continuous casting of steel.
International Patent Application No. WO 96/14951 (FOSECO) discloses a
tundish impact pad comprising a body of refractory material capable of
withstanding
contact with molten steel in a tundish. The impact pad body comprises a base
having an impact surface, an outer sidewall extending upwardly from the impact
surface, and a top surface connected to the sidewall and defining an opening
therein. The top surface has an inner annular portion substantially parallel
to the
impact surface, and there is a substantially right angled corner between the
sidewall
and the impact surface, and also between the sidewall and the inner annular
portion
of the top surface. This impact pad provides increased residence time of the
molten
steel in the tundish, which is important to ensure that unwanted inclusions
have
sufficient time to float to the surface of the steel and to be removed.
International Patent Application No. WO 97/37799 (FOSECO) discloses a
tundish impact pad comprising a body of refractory material capable of
withstanding
contact with molten steel in a tundish. The impact pad body comprises a base
having an impact surface for molten steel, an outer sidewall extending
upwardly
from the impact surface and extending around the base to completely enclose
it. An
annular body portion connected to the sidewall provides a top surface
substantially
parallel to the impact surface and defines an opening into which molten steel
can be
poured, the lower surface of the annular body portion and the inner face of
the
sidewall defining a recess having an undercut portion extending continuously
around
and above the impact surface. A portion of the top surface is at a lower level
than
the remainder of the top surface and the recess beneath the portion of the top
surface is of smaller cross-section than the remainder of the recess. This
impact
pad is particularly useful for improving the flow characteristics in elongate
tundishes
in which the molten steel is poured into the tundish at an end of the tundish
and the
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or each outlet for the steel is at the opposite end of the tundish. The impact
pad is
oriented such that the lower level portion of the end wall is adjacent to the
end wall
next to the incoming stream of molten steel, so that the steel rebounding from
the
pad preferentially flows towards this end wall. This has the effect of
markedly
reducing the surface turbulence created in the tundish, and generally
improving the
flow paths, resulting in the production of cleaner steel.
It will be appreciated that the process of designing a new tundish impact pad
which meets particular pre-determined criteria is extremely complex, since
changing
one aspect of the design of an impact pad generally has unforeseen
ramifications on
the flow dynamics of the entire tundish system. The present inventors have
identified a need for a new tundish impact pad which causes the molten metal
rebounding from the impact surface to be directed generally symmetrically
about a
longitudinal axis of the tundish. This is particularly important, for example,
for
tundishes in which the or each outlet is located on the longitudinal axis of
the
tundish, or where two or more outlets are arranged symmetrically about this
axis.
Furthermore, it is common for the incoming stream of molten metal (the "ladle
stream") to be "off-center", i.e. not located on the longitudinal axis of the
tundish.
This commonly occurs because the ladle shroud (the pipe through which in-
coming
metal flows from the ladle into the tundish) normally moves together with a
sliding
gate valve which controls the flow of the metal from the ladle, and since the
valve is
frequently only partially open in order to limit the flow rate, the ladle
shroud is
frequently off-centre with respect to the tundish.
According to a first aspect of the present invention, a tundish impact pad
formed from refractory material is provided, the impact pad comprising a base
having an impact surface which in use faces upwardly to receive molten metal
poured onto the impact pad, and a wall extending upwardly from the base around
at
least part of the periphery of the impact surface, the wall including an
overhang
projecting over a peripheral region of the impact surface, the overhang
including a
plurality of protrusions which project further over the impact surface than
the
remainder of the overhang.
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The wall preferably extends around the entire periphery of the impact surface,
fully enclosing the impact surface.
According to a second aspect, the invention provides a tundish containing an
impact pad according to the first aspect of the invention. The tundish is
preferably
elongate in shape (i.e. in plan view), it preferably has outlets near each
longitudinal
end, and in use the incoming stream of molten metal, and thus the impact pad,
are
preferably located substantially in the longitudinal centre of the tundish.
The tundish
preferably includes at least one dam extending across its width between the
impact
pad and an outlet. The top surface of the dam preferably has a central portion
(i.e.
central with respect to the longitudinal axis of the tundish) which is lower
than
peripheral portions on each side of it. This generally aids in directing the
flow of the
molten metal towards the longitudinal axis of the tundish.
The inventors of the present invention have found that the provision of a
plurality of protrusions which project further over the impact surface than
the
remainder of the overhang of the peripheral wall generally causes the molten
metal
rebounding from the impact surface to be directed substantially towards the
centre
of the impact pad. This generally has the effect of causing substantially
symmetrical
flow of the molten metal out of the impact pad, even if the metal flowing into
the pad
onto the impact surface is "off-centre", i.e. spaced a distance from the
centre of the
pad. Such substantially symmetrical flow out of the impact pad generally
causes
symmetrical flow throughout the tundish, resulting in the reduction of so-
called "dead
zones" where the rate of flow of the molten metal is relatively low (causing
poor
replenishment of fresh metal and the possibility of solidification of the
metal) and
generally improving the consistency of residence time in the tundish
experienced by
the molten metal.
The impact surface of the pad preferably has at least two corners (e.g. four
corners) over which a respective protrusion projects. Advantageously, at least
one
(and preferably all) of the corners may be formed at the convergence of
substantially
straight sides of the impact surface. More preferably, the impact surface may
be
substantially polygonal, most preferably substantially quadrangular,
especially
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substantially rectangular or square. A protrusion preferably projects over
each
corner of a polygonal impact surface. Additionally or alternatively, at least
one
protrusion (preferably two protrusions or more) may extend along at least most
of
the length of a respective side of the impact surface, and may even extend
along
substantially the entire length of a respective side of the impact surface.
In some preferred embodiments, the distance by which the overhang
extending between two said protrusions projects over the impact surface is
substantially uniform along at least most of its length. The overhang
extending
between two protrusions may advantageously be substantially straight along at
least
most of its length. Additionally or alternatively, however, for one or more of
the
overhangs) extending between two protrusions, the distance by which the
overhang
projects over the impact surface may be substantially non-uniform along at
least
most of its length, for example one or more of the overhangs) extending
between
two protrusions may be curved in plan view, for example such that the or each
overhang projects further over the impact surface at the centre of its length
than it
does at each end, or alternatively such that it projects further at each end
than it
does at its centre. It will be appreciated that the shape and size of each
overhang
between protrusions, and/or the shape and size of each protrusion, may be
varied
according to each particular circumstance, e.g. depending upon the shape
and/or
size of the tundish, and/or the amount by which the incoming stream of molten
metal
is off-centre, or according to any other particular requirements of the user.
The maximum distance by which the or each protrusion of the impact pad
projects over the impact surface is preferably at least 110%, more preferably
at least
120%, even more preferably at least 140%, especially at least 150% of the
maximum distance by which the remainder of the overhang projects over the
impact
surface. The or each protrusion of the impact pad projects over the impact
surface
by a maximum distance preferably of no more than 600%, more preferably no more
than 500%, especially no more than 450% of the maximum distance by which the
remainder of the overhang projects over the impact surface.
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Advantageously, the height of the overhang above the impact surface may be
non-uniform along its length. For embodiments of the invention having at least
three
protrusions, the height of the overhang extending between two of the
protrusions is
preferably different to that of the overhang extending between one of the two
protrusions and another protrusion, or between two other protrusions.
Advantageously, at least one portion of the overhang which is lower than that
of
another portion of the overhang may project a greater distance over the impact
surface than does the other, higher, portion of the overhang. This has been
found to
cause molten metal rebounding from the impact surface and impinging on such
lower overhangs to be re-directed back towards the centre of the impact pad
generally more forcefully than the metal impinging on the higher overhangs.
This in
turn appears to contribute to causing the metal to flow symmetrically away
from the
impact pad. (However, no limitation on the interpretation of the invention
should be
construed from the postulated explanations stated herein of the functioning of
its
various aspects.)
In some preferred embodiments, the peripheral region of the impact surface
comprises a trough extending around at least part of a central region of the
impact
surface. For embodiments in which the impact surface has corners, the trough
is
preferably wider at the corners than elsewhere. For example, the shape of the
trough (in plan view) may substantially mirror that of the overhang of the
wall.
The invention will now be described, by way of example, with reference to the
accompanying drawings, of which:
Figure 1 shows, in isometric projection, a tundish impact pad according to the
invention;
Figure 2 shows, in isometric projection, a longitudinal cross-section of the
tundish impact pad shown in Figure 1;
Figure 3 is a schematic representation of the asymmetric flow of molten metal
in an elongate tundish from an off-centre ladle-shroud;
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Figure 4 is a schematic representation of the symmetric flow of molten metal
in an elongate tundish from an off-centre ladle-shroud using an impact pad and
novel dams according to the invention; and
Figure 5 shows a dam as shown schematically in Figure 4.
Figure 1 shows a tundish impact pad 1 according to the invention, comprising
a base 3 having an impact surface 5 which in use faces upwardly to receive
molten
metal poured onto the impact pad, and a wall 7 extending upwardly from the
base
around the periphery of the impact surface, the wall including an overhang 9
projecting over a peripheral region 11 of the impact surface, the overhang
including
a plurality of protrusions 13 which project further over the impact surface
than the
remainder of the overhang.
The impact pad 1 is rectangular in plan view, both the base 3 and the impact
surface 5 being rectangular in shape. Each of the protrusions 13 is located at
one of
the four corners of the pad. Along each of the longer sides of the rectangular
pad,
between the protrusions 13, the overhang 9a is relatively small, and is
substantially
straight and uniform. Along each of the shorter sides of the pad, between the
protrusions 13, the overhang 9b is relatively large, and is curved in plan
view.
Furthermore, the overhangs 9b are at a lower level above the impact surface
than
are the overhangs 9a.
The peripheral region 11 of the impact surface comprises a trough, the shape
of which substantially mirrors that of the entire overhang (9a, 9b, 13).
Figure 2 shows a longitudinal cross-section of the impact pad of Figure 1.
The extent of the overhangs 9b is clearly visible, as is the depth of the
trough 11. It
is also clear that the shorter walls 7b are perpendicular to the impact
surface 5, and
the overhangs 9b are perpendicular to the walls 7b. The longer walls 7a are,
however, inclined to the perpendicular, i.e. they slope outwardly from the
impact
surface to the overhangs 9a.
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Figure 3 is a schematic representation of the asymmetric flow of molten metal
in an elongate tundish from an off-centre ladle-shroud. In each of parts (a) -
(c), the
tundish is shown in plan view, with the incoming ladle-stream of molten metal
represented by a circle in the longitudinal centre of the tundish but off-
centre in a
transverse direction, i.e. closer to one longitudinal sidewall of the tundish
than the
other. The tundish also contains a pair of dams 15 (containing holes to permit
flow-through of the molten metal) on either longitudinal side of the tundish,
and
outlets indicated by circles at each end of the tundish.
The arrows indicate the flow paths of the molten metal; .in part (a), the
initial
flow paths of metal entering the tundish are shown. (According to normal
working
practice, the tundish is already full of molten metal flowing through it - the
flow paths
shown merely indicate the predominant flow paths at various distances from the
tundish.) Even at this early stage it can be seen that because the ladle-
stream is
off-centre, the flow is asymmetric with respect to the longitudinal axis of
the tundish.
In part (b), the molten metal has begun to flow through the dams, and the
asymmetry of the flow paths is even more apparent. In part (c), the molten
metal
has reached the outlets. Indicated by the numeral 17 are so-called "dead-
zones"
where there is little or no flow of the molten metal during these steady-state
conditions. The presence of dead zones in a tundish is a major problem, since
the
metal cools in these regions and may start to solidify; they also cause
variations in
the composition of the metal flowing out of the tundish.
Figure 4 shows how the flow paths of the molten metal are changed by the
use of an impact pad 1 according to the invention. Also present in the tundish
of
Figure 4 are dams 19 located closer to the outlets than the dams of Figure 3.
These
dams have a central portion 21 of their top surface which is lower than the
peripheral
portions 23 on either side of it.
As in Figure 3, the in-coming ladle-stream of Figure 4 is once more off-
centre,
but this time the presence of an impact pad according to the invention causes
the
flow paths of the molten metal leaving the impact pad to be more symmetrical.
By
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the time that the molten metal has reached the outlets the flow paths are
substantially symmetrical, and no dead-zones are present.
Figure 5 shows a front view and a cross-sectional view of a dam 19 (shown
only schematically in Figure 4). The dam has a bottom surface 25 and side
surfaces
27 shaped and sized to fit tightly in a tundish across its width. Its top
surface has a
central portion 21 which is lower than peripheral portions 23 on either side
of it. This
has the effect of channelling molten metal flowing over the dam along a
generally
axial flow path in the tundish. It is therefore complementary to the impact
pad, in
causing symmetrical and/or axial flow of molten metal in the tundish. The dam
also
has a plurality of holes 29 extending through it, to allow molten metal to
flow through
the dam in addition to the molten metal which can flow over the dam. The holes
29
are inclined upwards in the direction of flow of the molten metal, so as to
impart an
upwardly directed flow path to the metal flowing through the holes. The angle
of
inclination of the holes is preferably greater for the lower holes and
preferably
decreases on rising up the dam. As indicated in the drawing, the lowest holes
preferably have an angle of inclination of 40°, reducing to 30°
and then 20°
respectively for the higher holes. The dimensions shown are in millimetres.
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