Note: Descriptions are shown in the official language in which they were submitted.
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ii32
TUNDISH PLATE FOR STREAM S~APE CONTROL
~ n .... . . _
This invention relates generally to the casting
industry, and has to do particularly with an improvement
applicable to tundishes for use in the continuous casting
of steelO
BACKGROUND OF THIS INVENTION
.
In the continuous casting of steel, especially
billet casting, the shape of the tundish stream has a
strong effect on the surface quality of the billet, the
internal quality of the billet, and caster performance.
A tundish stream that swirls or breaks up into droplets
between the tundish and the mold will entrain more air
and lead to increased slag patches on the surface of the
billet and inclusions in the interior of the billet.
Either of these could cause increased quality rejects.
In addition, increased slag quantities on the billets
leads to higher breakout frequency. The slag patches
reduc~ heat transfer in the mold, causing thinner shells
which are unable to support the weight of the liquid
steel core. Such breakouts reduce productivity and
increase maintenance costs.
In addition, a wildly swirling tundish stream
can cause further casting difficulties, particularly on
smaller billet sizes. The metal can hit either the mold
wall or the top of the mold and solidify, requiring
termination of the cast, and again resulting in reduced
productivity and increased maintenance costs. Swirling
of the stream can cccur under a variety of operating
conditions. It is strongly affected by any action which
causes mo~ement in the liquid above the tundish nozzle
For example, when the ladle nozzle is opened fully to
bring up the level in the tundish, the tundish streams
swirl appreciabl~, especially the stream closest to the
steel entry point in the tundish. This swirling tends to
be worse when the liquid level in the tundish is higherO
Another problem in continuous casting is the
entrainment of slag. At lower tundish levels, especially
under very quiet tundish conditions (the same type that
gives very good stream shape), vortexes form above the
nozzle and entrain slag into the casting stream. This
can give either large surface slag patches on the
billets, large internal inclusions, or in some cases a
change in steel chemistry by reversion of elements in the
slag ~e.g., MnO reverting to Mn in the steel).
Ceramic shrouds have been developed in order to
1~ contain the metal stream to ensure that it hits the
desired area in the mold. However, ceramic shrouds have
several disadvantages. They do not allow access to the
casting stream (for shut off or lancing in the case of
blockage), they are difficult to install, their use is
restricted to the larg~r billet sizes (because of
clearance between outside and the mold wall), and special
startup practices are often required to clear the skull
that forms on the outside and can bridge the gap between
shroud and mold wall. In addition, such shrouds make it
25 more difficult to control the level of steel in the mold.
Other systems are available to prevent or
restrict stream oxidation. ~11 of these involve the use
of an inert gas around the casting stream. All reqllire
equipment fitted on the mold or tundish to feed or
30 contain the gas around the tundish stream and, as such,
interfere to some extent with access to the stream.
~lso, because gas is involved, air flow in this area
(often used to remove fumes) must be controlled.
Devices are known which reduce vortexing and
35 slag entrainment, these being gas-purged porous nozzles.
However, their use tends to produce worse stream shape.
GENER~L D~SCRIPTION OF THIS INVENTIO~
In view of the foregoing discussionl it is
apparent that it would be desirable to achieve a
streamlined shape for a -~undish stream, without swirllng
or break-up/ while at the same time reducing vortexing
and slag entrainment in the tundish. By reducing swirl
and break-up, stream oxidation will be restricted.
Tt is therefore an aspect of this invention to
provide a device for use with a tundish, and a process
utilizing this device, which will produce a tight, smooth
tundish stream that enters the mold in a small area and
reduces breakout fre~uency in the billet.
A further aspect of this invention is to obtain
a minimum of reoxidation in the stream or by air carried
into the steel where the stream hits.
A still further aspect of this invention is to
reduce the amount of slag entrained in the casting stream
hy vortexing in the tundish.
In general terms, this invention consists in
providing a plate with a number of holes or perforations,
the plate being positioned in the path of the molten
metal as it moves from the tundish to the nozzle at the
bottom of a nozzle well. Preferably, the plate is
located at the top of each nozzle well.
From tests reported below in this
specification, it can be concluded genèrally that the
plate acts as a kind of barrier separating the liquid
conditions in the tundish from those in the well. Its
presence inhibits the formation of a tundish vortex under
quiet tundish conditions, and shields the well from
disturbance under agitated tundish conditions. As the
liquid metal passes through the holes of the plate, some
slight agitation immediately below the plate can be
expected to occur, but such agitation appears to settle
into laminar flow as the liquid nears the nozzle.
More particularly, this invention provides a
continuous casting process for metal, including several
steps. The molten m~tal is teemed into a tundish, and
then passed into a nozzle well and toward a nozzle at the
bottom of the well~ During its passage from the tundish
to the nozzle, the molten metal is passed through a
perforated plate located above the nozzle, thereby to
promote laminar flow below the plate. Finally, Erom the
well, the molten metal is passed through the nozzle to
form a stream entering a contlnuous casting ~old.
This invention further provides a perforated
plate for use with an apparatus for the continuous
casting of metal. The apparatus comprises a tundish
having at least one nozzle well with a nozzle at the
bottom of the well, and a continuous casting mold into
which molten metal can stream from the nozzle. The
perforated plate is positioned in the pa-th of molten
metal passing from the tundish to the nozzle, but is
spaced above the nozzle, whereby the molten metal must
pass through the plate perforations to reach the nozzle.
In a preferred embodiment, the plate has at
least six holes and is located substantially at the top
of the well. The plate must retain its strength and
erosion resistance at molten metal temperatures.
GENERAL DESCRIPTION OF THE DR~WINGS
__
- Five embodiments of this invention are
illustrated in the accompanying drawings, in which like
numerals denote like parts throughout the several views,
and in which:
Figure 1 is a sectional view through a tundish
and continuous casting mold with the plate of this
25 invention in place;
Figure la is a partial sectional view through a
tundish without a plate, showing a vortex in the liquid
steel;
Figures 2 through 6 show five preferred
30 embodiments of this invention;
Figures 7 through 9 show configurations which
were also tested; and
Figure 10 is a section through a conventional
wafer nozzle.
35 DETAILED DESCRIPTIOM OF THE DRAWINGS
Attention is first directed to Figure 1, which
shows a tundish 10 having side walls 12, end walls 13 and
14, a bottom wall 16, a well 18, and a nozzle 20 at the
bottom of the well 18. The tundish contains molten steel
(or other molten metal) 22 up to a level identified by
the numeral 24, and a layer of slag 26 i9 located above
the level 24. In some situations, the slag layer may be
thin ox non-e~istent. The inside wall 29 of the well 18
defines a well chamber through which the molten metal
passes in moving from the tundish to the nozzle 20.
Located below the well ]8 is a con-tinuous
casting mold 30 having cooling chamber 32 for cooling
water or the like. As can be seen in Figure l, a stream
33 carries molten steel dcwnwardly from the nozzle 20 to
the mold 30.
In the continuous casting process, the billet
36 is formed on a continuous basis, and continuously
moves downwardly from the mold 30. As it does so, it
gradually solidifies from the outside inwardly, so that
the solidified side "wall'~ of the billet gradually
thickens as it moves downward from the mold 30. The
gradually thickening wall is identified by the numeral 38
in Figure l.
2~ Dissolved in the steel are elements
(deoxidizers) designed to combine with the oxygen which
comes out of soiution as the steel solidifies. With a
swirling or broken up stream, these elements will combine
with the oxygen in the air that is either around the
stream or entrained in the mold as the stream
penetrates, leading to excessive quantities of slag
(e.g., MnO, SiO2, etc.). This slag can either float to
the top of the steel in the mold and form a layer 40 or
be entrapped iIl the solidifying steel shell. The layer
of slag on the top of the steel can lead to either
breakout problems or surface quality problems as it
solidifies against the continuous casting mold. The slag
entrapped in the solidifying steel shell leads to
worsened internal quality.
Attention is now directed to Figure la, which
illustrates two of the typical problems encountered with
the continuous casting process for steel. In Figure la,
it is seen that the stream 33a tends to flare outwardly
as it descends, thus entraining air into the sleel and
leading to the problems discussed earlier.
Within the tundish 10 in Figure la, the steel
22 has begun ko form a vorte~ 42 above the nozzle 20, and
as a result of ~his vortex slag in the slag layer 26 is
being drawn downwardly and into the steel strearn 33a.
It should be pointed out that these two
problems, while illustrated simultaneously in Figure la,
do not necessarily occur together in prior art processes.
Returning to Figure 1, it will be seen that, in
accordance with this invention, a plate 44 having a
plurality of holes 46 is provided in the path or steel
moving from the tundish to the nozzle 20~ More
particularly, the plate 4~ is located substantially flush
with the bottom of the tundish 10, and at the top of the
chamber defined by the internal wall 29 of the tundish
well 18.
TEST DAT~
In order to evaluate the following plate
conditions with a view to determining the characteristics
o~ an optimum perforated plate, a program of water model
simulation tests was carried out at the University of
Alberta. It is well understood that water model
simulations can provide useful data from the evaluation
of systems designed for other liquids. By using a full
size model, the Reynolds number and Froud number are
substantially the same for the two systems. The key term
in these expressions is kinematic viscosity (absolute
viscosity , density). The kinematic viscosity of steel
and water is about the same. Since the viscosity of most
liquid metals is close to that of liquid st~el, any
metals whose density is similar to steel (e.g., copper,
tin, zinc, etc.) should also show the improvement noted
for wate~.
Figures 2 through 9 illustrate various plate
configurations which were tested. The main plate
characteristics under investigation were the following:
lo Plate Location
~. Plate Thickness
3O Hole diameters
4i Nu~ber of Holes
5. Hole arrays
Some of the testlng involved wafer noz~les~ for
which a word of explanation is in order.
Attempts in the past to use strong deoxidizers
such as aluminum have led to nozzle blockage problems. A
wafer nozzle (seen in axial section in Figure 10) will
pour aluminum deoxidized steels but gives much poorer
stream shape than regular nozzles. This leads to a
worsened condition with respect to slag formation because
of the high affinity of aluminum for oxygen. It was
decided to test a wafer nozzle in combination with
different hole arrays, hole diameters and plate
thicknesses for a perforated plate, to determine whether
the combination could improve the flow characteristics
out of a wafer nozzle.
` In selecting which plate configurations were to
be evaluated, certain factors were kept in mind.
Firstly, shop practice required the plates to have a
centre hole which is 1 1/8 inch diameter or larger.
Also, because of possible stren~th limitatior.s in the
proposed plate material, as few holes as possible should
be used. The spacing of holes could well be critical to
the life of the perforated plate in practice. In
addition, the plate would be more likely to be 1~ inch
thick in practice, especially if it were intended to
extend the existing fibre liner board over th~ nozzle
well. Finally, it was decided to evaluate the
performance of the plate when located some distance down
into the nozzle well, as well as at the top of the well.
In the material below, each of the investigated
characteristics are dealt with in separate sections, and
at the end of each, appropriate conclusions are drawn.
A. Effect of Plate Location in the No _ le_30x
For these tes-ts it was necessary to reduce the
number or holes in the plates so that, in all plate
locations, all holes would function.
Four different hole arrays were evaluated, and
the plates were placed both flush with the bottom of the
tundish, and about 4 inches (10 cm) below the bottom of
the tundish. The observations follow.
a~ Fou_ 1 inch _oles, ~ inch plate
It was observed that the four hole confiyuration
yielded ~very poor flow conditions downstream of the
nozzle. Conditions were worse at a tundish head of
42 cm, and were also worse when the plate was placed
4 inches closer to the nozzle. The hole array was
that shown in Figure 7.
b) Five 1 inch holes, ~_ inch plate
The hole array for this test was that shown in Figure
8. For this test, it was observed that the flow was
quite erratic with the flow conditions worse at a
tundish head of 42 cm, and also worse with the plate
placed 4 inches down into the nozzle well.
c) Six I inch holes with hole plugged, ~" plate
This test used the pla~e shown in Figure 9. The
plate had a circular six hole array, with the centre
hole plugged. It was observed that the flow
downstream of the nozzle was poorer with the plate
placed 4 inches closer to the nozzle, and also that
the flow was more erratic at a tundish head of 42 cm.
In comparing the two different five hole arrays lone
including a centre hole), it would appear that, with
the plates flush with the bottom of the tundish, the
flow conditions were somewhat improved with the array
having no hole in the centre. A second comparison
was made between a six hole array as illustrated in
Figure and the six hole array illustrated in Figure
3~ Here again it was seen that the plate with no
centre hole showed some improvement in downstream
flow conditions.
d) Si~ l inch holes and eEfect of plate location
It was noted that the downstream flow conditions
worsened when the plate was placed closer to the
nozzle. However, the si~ hole plate exhibited better
downstream flow conditions than the five hol.e plate,
although none of these flow conditions could be
considered ideal, nor were they as good as were
achieved wi.th the use of more holes.
Conclusions
The placing of the perforated plate in the nozzle
well below the bottom of the tundish tended to
produce poor downstream flow conditions out of the
nozzle~
Four, five and even sl~ hole plate configurations
produced poorer downstream flow conditions than
plates with a greater number of holes.
For a five hole plate, the downstream flow conditions
were somewhat improved if there was no centre hole in
the array. Also, for six l-inch holes, the
downstream flow conditions were better when the plate
had no centre hole. It would be expected that the
presence or absence of the centre hole would have
less of an effect as the number of holes in the plate
increased.
B. Effect of Plate Thickness
For this evaluation, plate thicknesses of 2,
1~, 1 and ~ inches were tested using 6, 7 and 9 one-inch
diameter holes. These plate configurations are shown in
Figures 3, 4 and 5.
a~ Six hole plate, circular array, no centre hole
At a 16 cm tundish head, the flow characteristics of
the stream out of the nozzle all appeared about the
same regardless of which plate thickness was used.
The streams appeared to be somewhat ragged but none
was wildly splaying. The density and penetration of
the air buhbles all appeared to be the same. Thus~
at a 16 cm tundish head the effect of plate thickness
in the range from 0~5 inches to 2 inches appeared to
be insignificant.
At the 42 cm tundlsh head~ there was an increase in
erratic flow patterns for all pla-te thicknesses
evaluated with perhaps less turbulence noted for the
~ and l inch plates than for the other two thicker
plates. Changing the plate thickness did not seem to
alter the depth of bubble penetration into the mold
box.
b) Se~en hole plate, circular array, with centre hole
Using a l~ cm tundish head, the flow out of the
nozæles for plate thicknesses of l, l~ and 2 inches
was more laminar in appearance than observed with the
use of ~ inch plate. The depth of bubble penetration
appeared to be least with the l~ inch plate, but was
also most dense for this plate.
With a 42 cm tundish head, there did not seem to be
any difference in flow conditions regardless of which
plate thickness was used.
c) Nirle hole plate, square array with centre hole
At a l~ cm tundish head the nine hole plate array
yielded improved downstream flow conditions over
those obser~ed for the six or seven hole arrays
evaluated above. For the nine hole array (Figure 5),
-the l~ inch plate appeared to give the least density
o bubbles in the mold box. The laminar appearance
of the streams out of the nozzle was unaffected by
plate thickness in the range of O.S inches to
inches.
At a 42 cm tundish head, there was noted a slightly
more laminar-like stream with a l inch plate than
with the others, but generally all streams appeared
to be tight and laminar in appearance. The depth of
penetration of bubbles in the mold box was about the
same for all plate thicknesses with possibly less
density of bubbles observed with the use of a 1~ inch
plate thickness.
l l.
Conclusions
Generally, the plate thickness appeared to have less
influence on the downstream flow characteristlcs than
did the number of holes used in the array. The
results appear to indicate that a nine hole array is
clearly superior to the seven or six hole arrays
evaluated.
The density of bubbles in the mold bo~es was less at
a 16 cm tundish head than at a 42 cm tundish head.
The efect of plate thickness, although difficult to
evaluate, showed that a ~ inch plate produced poorer
results at a 16 cm tundish head for a seven hole
plate array, and 1~ inch and 2 inch plates produced
slightly poorer stream shapes with a six hole array
at a ~2 cm tundish head. It would appear that the
inch plate may not be suitable for all hole arrays
whereas the 1~ inch plate may be close to or at the
optimum thickness for a number of different hole
arrayso
C. Effect of Hole Diameter
Two different hole arrays, seven and ninej were
used to evaluate the effects of hole diame-ters of 1, 1
and 1~ inches. All plates were 1~ inch thick.
a) Seven hole circular array
At a 16 cm tundish head, the stream shape out of the
nozzle was better for the 1~ or 1~ inch diameier
holes than for the l-inch hole size. However, the
penetration of air bubbles in the mold boxes was
least for the one-inch holes.
For a tundish head of ~2 cm, the stream shape was
best for the 1~ inch diameter holes. Also, the depth
of penetration was least with the plate having 1
inch holes. The plate configuration is shown in
Figure 4.
b) Nine hole square array
At a 16 cm tundish head, the str~am shape for the
different hole diameters was about the same with
12
possibly more laminar flow ~or the l~ or 1~ inch
holes than for the 1 inch holes. The penetration of
air bubbles in the mold boxes was least for the
l-inch diameter holes and most for the 1~ inch
diameter holes.
At a 42 cm tundish head, the stream shape was good
for all three hole sizes with slightly better results
for the l~ inch hole size. The density of
bubbles in the mold boxes was about the same for all
three hole sizes evaluated.
Conclusions
Generally it appears that while hole size a~fected
the stream flow conditions out of the nozzle, the
effect was not a pronounced one. ~ole diameter
appeared to be a stronger factor with the seven hole
array than with the nine hole array. This would be
expected in that the percentage increase in hole area
would be greater with fewer holes in the plate.
Again~ it appeared that the nine hole array produced
more laminar flow conditions than did the seven hole
array. The plate with 1~ inch holes appeared to give
good downstream flow characteristics.
D. Tests with a Wafer Nozzle
Wafer nozzles are well known in the art of
steel making, and a typical cross section of a wafer
nozzle is shown in Figure 10.
Previous to the making of this invention, a
wafer nozzle was tested in a continuous casting process.
30 However, it was found that the flow out of the wafer
nozzle, although 22% reduced compared with that of the
regular nozzle, had wildly erratic flow characteristics.
Because the wafer nozzle has advantages when used in
conjunction with a deoxidation practice, it was desirable
35 to evaluate some of the "better" perforated plates to see
whether they could be used to improve the downstream flow
conditions out of a wafer nozzle. Three different hole
arrays and three hole diameters were evaluated. The 16
l3
hole pla~e was ~ inch ~hick, whereas the other plates
were l~ inch thick.
a) Sixteen l-inch diameter hole square array
A sixteen one-inch diame-ter hole array as seen in
Figure 6 was evaluated with a wafer nozzle and
compared with a regular nozzle. At both a 16 and a
~ cm tundish head, the flows out of the nozzles were
tight and laminar in appearance. However, the
density of air bubbles in -the mold box was
considerably less for the
wafer nozzle than for the regular nozzle. It has
been noted that for the same tundish head, the flow
out of the wafer nozzle was about 22~ less than out
of a regular nozzle. Therefore a more valid
comparison, although still only approximate~ would be
to compare the flow out of the wafer nozzle at a head
of 42 cm to the flow out of a regular nozzle at 16
cm. When this was done, it was still evident that
there was less density of bubbles in the mold box
with the wafer nozzle than with the regular noæzle.
b) Nine l-inch hole square array
The use of the nine hole plate as seen in Figure 5
improved the flow out of the wafer nozzle. In
comparing the wafer nozzle with the regular nozzle,
it should be remembered that there was ?2~ less flow
out of the wafer nozzle than out of the regular
nozzle for the same tundish head. At a 42 cm tundish
head there was more penetration in the mold box than
at the 16 cm tundish head. ~ven accounting for the
differences in flow rates it appeared there was less
density of bubbles in the mold box with a wafer
nozzle than with a regular nozzle.
c) Seven hole arrav - effect of different hole diameters
~t a 16 cm tundish head, increasing the hole diameter
from one to l~ to 1~ inches reduced the density but
increased the depth of bubble penetration in the mold
box. The flow out of the nozzles appeared to be
about the same for all hole sizes evaluated.
~5~
14
With the tundish at 42 cm the flows out of the
nozzles were about equal or all hole diameters but
the density of bubbles in the mold box was least for
the 1~ inch holes. The depth of penetration of
bubbles in the mold box was greatest with the 1~ inch
diameter holes.
Conclusions
Probably more than with the regular nozzle, the
merits of a nine hole array over that of the seven
hole array were observed in this set of tests~ The
nine hole array in combination with a wafer nozzle
produced much less density of air bubbles in the mold
box than with the seven hole array. The 1~ and 1
inch hole diameter appeaxed to yield laminar flow
lS conditions downstream of the nozzle.
Preferred Materials
Tests with plates fabricated from tundish liner
boards (a silica refractory material called Profax by
its manufacturer) have withstood erosion in the
tundish at the steel plant~ Any refractories
commonly used in steel teeming systems (e.g. alumina,
zirconiaj magnesia, alumina-graphite) could be used.
