So, having said so much about ladle processing.
Let me now draw a full-fledged general ladle metallurgical circuitry as well as a specialized
ladle treatment circuitry. So, first we start with a BOF and an EAF.
- Basic Oxygen Furnace or Electric Arc Furnace steelmaking - So, this is the primary steelmaking;
followed by that, we have tapping operation, and this is tapping and we have ladle treatment,
and basically in this ladle treatment, we are talking about argon stirring, and what
we are doing in this ladle following tapping is nothing but deoxidation, may be we are
doing aluminum wire feeding or bulk deoxidation. So, once deoxidation is over, the next general
treatment is the LF. So, in LF, we have argon rinsing or argon stirring, which is
always there, and then, in LF, we have alloying; we have arcing and we have temperature control.
Basically, we have just composition and control or enhance the melt temperature, and following
this, we have teeming and this teeming is teeming into tundish. So, this is the most
common ladle metallurgy steelmaking circuitry or the secondary steelmaking circuitry.
But we must understand that we have an option here; that means, after ladle metallurgy,
ladle furnace, we need not go for teeming, we can have here. What is known as the VD
operation? - Vacuum Degassing - and then, after VD, we can take the ladle either here
or teeming or we can take it to holding as well as calcium treatment. So, that is also
possible, and then, after this, we again go here, and then, finally, this is teeming into
tundish and continuous casting. So, these are optional treatments. We can
have vacuum degassing as well as holding and calcium treatment. They are not for every
grade of steel. So, therefore, I have left it on the right hand side and this is the
straightforward ladle metallurgy circuitry, and these are the expensive or more exotic
ladle metallurgy circuitry for enhanced composition and control of a steel.
Now, during this process, if I now wish to summarize that how does really the temperature
of the melt changes, which is an important aspect of ladle metallurgy steelmaking operation.
So, you will have time here, and as I said in the beginning that, the entire treatment
operation can really go up, you know, from 0 which represents the tapping from the BOF
and this could be something about 80, 90 or 100 minutes. So, I will write it like this
80, and so, this is a total span of ladle metallurgy steelmaking operation where we
can take all these things into account. So, we have temperature here, and if you tap,
for example, the melt at 1630 degree centigrade by the time, the tapping is over, so, 15 minutes,
or so, the temperatures drops down drastically and say about up to the value of 15. How much
temperature can be dropped during the taping operation and initial period of holding of
metal in the ladle filling? So, something around 80 to 90 degrees of drop in temperature
can really takes place. So, if you start from 1630 degree centigrade, so, this essentially
implies that we will get something like 1550 degree centigrade or something like that,
which essentially tells us that there is about 80 degree drop in temperature. And then, we have, as I have mentioned in
the previous lecture, that we have various endothermic and exothermic processes, and
also, we supply heat in the ladle furnace. Now, the temperature gradually goes up to
compensate, and ultimately, as it is plot, so, something like I would say 1585, which
may be our desired casting temperature. So, let us just mention it 1585; so, that is the
temperature we are talking about here, and then, we are transporting the ladle basically
to continuous casting or this is the period the ladle is in tundish, so, tundish teeming.
So, this is the duration something like 10 to 15 minutes of time the tapping and holding
in ladle, and then, this represents the duration of ladle metallurgy steelmaking including
physical transport. So, this is the desired temperature at which
the molten steel is to be delivered into the continuous casting dish that is tundish. Although
we have poured molten steel at 1600 degree centigrade and you have heated in between
through ladle furnace, we see that by the time in about 80 minutes of time, the material
is delivered not at 1630 degree centigrade but marginally higher than 1550, which may
be about 1560-1570 degree centigrade as I have tried to indicate in this particular
figure. There are two other processes which are of
course directly not relevant in the context of discussion of secondary steelmaking, but
they are very important and similar process like the ladle metallurgy steelmaking although
used in some different context. For example, in stainless steelmaking which you will be
doing separately, we have processes like AOD and VAD. These processes resemble very similarly
with the ladle metallurgy steelmaking operation. So, I thought that maybe I should discuss
them here and they are not absolutely out of context.
So, AOD essentially represents Argon, Oxygen, Decarburization and VAD essentially represents,
this is not VAD, sorry, this is VOD. - Vacuum Oxygen Decarburization – So, this process
is not carried out under vacuum; this process is carried out under vacuum. In both the processes,
oxygen are supplied and the processes are essentially de-essentially implies decarburization
processes. And as I have mentioned that we have carbon
oxygen reaction and this carbon oxygen reaction can be shifted; the equilibrium can be shifted
to the right provided. We can increase the oxygen partial pressure, and also, if we decrease
the pressure of C O, in that case, the reaction will be moving in the forward direction or
we can drive the reaction preferentially in the forward direction. So, in the vacuum oxygen decarburization process
let me now draw quickly the figure. So, argon, if we look at argon oxygen decarburization
process, it is again carried out in a vessel, which is similar to our oxygen steelmaking
vessel. Only difference this that we have a side blown nozzle through which argon gases
is going to be supplied. So, we have a side blown nozzle and we have a supersonic lance
and where the metal here, and through this, the oxygen argon bubble rises, produces stirring,
and through this, the oxygen is supplied, and as a result of which, what happens is
this melt which contains relatively high carbon, the carbon oxygen reaction takes place and
that is how the oxygen gets into the melt and removes carbon; so, the carbon oxygen
reaction takes place. So, it is a high carbon melt basically. High
carbon melt is initially subjected in the converter to oxygen and combination of argon.
Initially, In initial versions of AOD, both argon and oxygen are introduced through the
side blown lance, but today, oxygen is introduced from a separate lance. Initially at a supersonic
speed, when there are lot of carbon present in the bath, but finally, towards the end
of the process, when the carbon concentration drops down as more and more carbon monoxide
forms; the flow rate of oxygen is gradually decreased.
Now, in stainless steel making, may be you know already that we require a very high temperature
in order to produce a low carbon melt. So, therefore, we want that the excess heat is
to be evolved. So, the supersonic oxygen gas which also does some amount post combustion.
So, one sort of, one heat is that carbon will initially oxidize. Under steelmaking condition,
we know that in the melt particularly, we can form carbon, oxygen preferentially carbon
monoxide. Carbon dioxide is not thermodynamically stable, but towards when the gas rises upward
because of its thermal buoyancy, what happens is towards the mouth the temperature gets
smaller, and if you have a high partial pressure of oxygen, we can oxidize some of the carbon
monoxide to carbon dioxide and some additional heat or chemical heat contained in carbon
monoxide can be released within the furnace. If that heat can be directed to the melt,
in that case, the melt temperature can be increased substantially high, which will be
conducive to making of low carbon grade of steel, and low carbon means we are talking
about stainless steel which has an extremely small content of carbon, because carbon is
not tolerated in the presence of high chromium, because chromium carbide forms and it leads
to grain boundary deposition and grain boundary crank. So, coming back to this particular issue that
if we blow in large oxygen from the top, in that case, there is excess of oxygen particularly
during the blowing period and this excess oxygen may combine with carbon monoxide releasing
additional heat and this phenomena in EAF as well as in BOF as also in AOD converters
is basically to harness the chemical heat of carbon monoxide so that the poisonous carbon
monoxide is not released outside and also the chemical heat of carbon monoxide is released
within the vessel and thereby which is called post combustion, and thereby, somehow direct
the heat towards the melt; the melt temperature can be substantially increased.
You must understand that post combustion is going to be meaningful towards the initial
part of the blowing when there are lot of carbon monoxide evolution and the carbon concentration
is very large. VOD process is similar to, we have a ladle here, so, we draw a ladle,
and then, we have oxygen from the top and then we have a melt here. We introduce argon
from the bottom and then the entire thing is going to be sitting on a platform here,
and this is the oxygen. Then, the entire thing is under vacuum.
So, basically, the same process we have metal here. We have argon gas which is injected
from the bottom as usual, because the gas has to dissolve into the metal and this dissolution
is a mass transfer control process, and oxygen, the dissolved oxygen has to go everywhere
to find out carbon. So, thereby, for you require some stirring,
top lances as you must be knowing that it is not conducive to producing extensive amount
of stirring here. Bottom blowing of gas is far more effective for producing good amount
of stirring. So, therefore, argon is introduced here, which produces lot of stirring. Oxygen
gas preferentially dissolved and this bath contains carbon or a high carbon melt and
then the carbon oxygen reaction takes place, and because this is entire container is under
vacuum; it is under sealed environment that this reaction goes on. Therefore, this equation which I have written
here under reduced carbon monoxide partial pressure or reduced pressure within the system,
because whatever carbon monoxide is forming, may be you can draw here a line that through
this or suction is going to be applied and all carbon monoxide is going to be taken out,
and as a result of which, this is maintained at a very low pressure, and under low pressure,
this equation will be driven towards the forward reaction.
So, this much for ladle metallurgy, we will now move on to the tundish metallurgy, the
continuous casting, and teeming into the tundish is what we are going to discuss continuous
casting will be discussed later on. So, having done all sorts of operations in terms composition
adjustment, in terms cleanliness adjustment through injection metallurgy, in terms of
gas control through vacuum degassing techniques, we are now set to cast the molten steel through
continuous casting. Today as you all know, ingot casting has been phased out because
of techno-economic significance. We all know that the rate at which BOF will be turning
out hot metal; it cannot be matched by ingot casting.
So, we require a downstream process which can match with the higher rate of production
of BOF. Here, we are talking of 2 kg of decarburization per second and this is an enormous rate of
steel production. So, we must be having a very quick method of casting of molten metal
and that is why continuous casting has become very popular; it can really match the productivity
of the BOF and ingot casting is no match here. Now, let me first draw a ladle tundish mold
assembly which is the essence ladle tundish mold assembly. I will introduce make a small
drawing and then introduce some of the terminology. So, the ladle, which contains now molten metal.
At this particular stage, when we take the molten metal ladle for continuous casting,
although the porous plug is still attached to the ladle, we must indicate that at this
particular stage, there is no argon injection. So, argon injection is stopped. When we take
the ladle to the continuous casting web, the plug is nearly the connection here pours is
disconnected from the plug. As you know, we repair the ladle or take the ladle after all
these operations to the continuous casting bay.
So, there is no argon rinsing now. The material is sitting steel is here in the ladle itself,
and then, as I have mentioned to you that we have a slide gate arrangement here, and
through this slide gate arrangement, I am going to explain that slide gate little later,
and through this slide gate, we have what is known as is the shroud which is typically
attached and this shroud basically protects the stream of molten metal that flows out
of the ladle. So, from the ladle, the material now flows
into tundish which typically you can assume that it is like a distributor of molten metal
and we have strands here, and this is continuous casting mold. So, the molten metal enters
here and then the continuous casting product is drawn like
this. So, this is the water cooled mold, I will
draw like this. Then you can better understand it is the water cooled copper mold. This is
the solidified casting; this is the tundish, and as you can see that metal which is there
in the ladle, it first comes into the tundish and this is the shroud, and what is the purpose
of shroud? The purpose of shroud is to protect the molten stream. Otherwise, what is going
to happen? This molten stream, if it remains exposed, there is no shroud; this is going
to interact with my surroundings. I have oxygen and nitrogen and there is going to be continuous
transfer of oxygen and nitrogen. So, the shroud is going to be submerged up
to a certain depth in order to ensure that molten steel has no chance to come in contact
with the surrounding here. So, we have to prevent; this is the transfer operation. Whatever
we have been talking here, in the ladle metallurgy steelmaking, the material was always sitting
in the tundish. So, this is a chemical processing operation, a physical processing operation
to some extent also inclusion removal, etcetera. But now, we are talking about transfer operation,
teeming operations, tapping operations are transfer operations, and these transfer operations
are extremely important in steel making circuitry. We would like to see that the quality of steel
which we have achieved during the processing steps must not be compromised or deteriorated
during the subsequent transfer operation. They are extremely critical as far as the
quality of steel is concerned. We must understand there is a phenomenon which
you will come across and is called an air ingression. If there are leakages, when the
shroud is going to be attached to the bottom of the ladle, in that case, there may be some
pours and holes here through which air maybe drawn into and that phenomena is termed as
the air ingression which drastically reduces the quality of steel. Now, what is there within the shroud? And
the shroud is filled up with inert gases basically. So, this tube which is attached to the bottom
of the ladle, so thus, through the slide gate the slide gates can be moved like this way
and the openings maybe made bigger or smaller, and thereby, the flow rate can be controlled
and the flow rate is typically is not controlled in the industry. Manually it is basically
done through a control algorithm which takes into account the melt depth and the diameter
how much is this to be opened in order to pump in molten metal into the tundish at a
constant rate. In the tundish, these are called the strand
of the tundish. Again, we have a slide gates are here and we do not have shrouds here.
What enters here is called the SEN - Submerged Entry Nozzle. So, these are the submerged
entry nozzle and this is my slide gate, and the purpose of slide gate is to regulate the
flow. In many tundishes, you have seen steel plants; you may have visited steel plants.
It is not necessary all steel plants will have slide gate, but it is a modern technology
necessitates that good quality of steel can be made with slide gate arrangements, but
there is another arrangement also which is called a stopper rod arrangement. This is
the stopper rod; this also does the flow regulation. And this stopper rod if it is raised, in that
case, this opening is going to be bigger; more metal is going to flow out of the tundish,
and this, but it is kept down in contact with the surface, then this strand is closed and
this strand is closed, and therefore, no material flows out. So, by controlling the movement
of the stopper rod up and down, we will be able to control the flow of material into
the mold itself. So, ladle tundish mold this is the arrangement
and we are talking of a height of three floors. The ladle is sitting something around 15 to
20 meters above the ground level. This billets are going to be discharge or the slag is going
to be discharged at the floor level. So, above the continuous casting machine and this is
a curved mold. So, you require an enormous amount of space. The mold may be sitting you
know about 6 to 7 meters or 8 meters high, and then, the billet comes out, it bends and
it is discharged horizontally. So, from this point onwards to this point, we are talking
about more than 20-30 meters of height. It looks like a three storey building; actually
it is a steel plant. Now, the tundish is a refractory lined vessel.
We are storing molten metal here which is understood that we have to have refractory
line. The tundish will also perform just like the ladle for a some amount of heats only.
This one tundish will not work forever; it can take about 16 or 17 ladles, maybe 20 ladles,
sometimes maybe 8 ladles depending on the condition of the plant that we are talking
about. The ladle is, one ladle is brought here, and
then, as soon as this ladle is emptied, the fresh ladle comes maybe with the same composition
or different composition, but the tundish can still go on. So, we can go on ladle after
ladle and the tundish can work for about 16 or 17 ladles or 20 ladles as I have mentioned
depending on the quality of construction and the refractory bricks with which the tundish
is made. So, it is a steel lined vessel. Basically
same type of refractory’s which are there in ladles are used, and the thickness of the
lining is also almost same like the thickness of lining in the ladle; so, there is not much.
Holding and transfer vessels have similar types of characteristics. Magnesia bricks,
basically magnesite, magnesia, tar dolomite lined bricks, they are basically used in ladles
and tundishes. As you can see here that, we have one shroud
entering the tundish, but the shroud distributes the molten metal into two different molds.
Therefore, traditionally we visualize tundish as a distributor of molten metal into continuous
casting molds. I have shown only two molds; it is not necessary that we have only two
molds. There are to be multiple number of molds actually. Let me draw a figure and then
explain it in the contest of billet or bloom casting. This is a drawing. Two strands I
have shown here basically, essentially. This is a slab casting arrangement, but if you
have billet and bloom casting arrangements, we can have multiple or more number of strands
as I may explain. So, as we know that steel plants produces
what is known as long products or flat products. Long products basically are your rails, beam
blanks which are used in buildings, wires and rods, and on the other hand, flats products
are basically slabs and strips. Now, we can have long products are basically
billets or blooms, as we all know, billets and blooms, while the flat product is slab.
Now, the slabs have massive cross sectional area, the size, how much we can be talking
about 30 centimeter or 20 centimeter thick slab with the width of the slab maybe 1.5
to 1.8 meters. On the another hand, billets could be 8 inch by 8 inch; blooms could be
something like 20 inch by 20 inch. So, therefore, it is understood that if we
are talking of a casting rate, that plant demands that we have a casting rate of about
through one continuous casting 8 tonnes per minute for example. In that case, it is understood
that if you have a billet cast at tundish, if the tundish is producing billet or casting
billet, billet casting tundish, you will have more number of strands.
So, we can have one strand here, second strand, third strand, forth strand, fifth strand,
sixth strand, because billets, billet cross section as I have mentioned, billet cross
section could be about 8.5 to 8.5 inches something like this. On the other hand, we can say that
if you convert it, say 20 centimeter into 20 centimeter, maybe I will say 10 centimeter
is a better size 10 into 10 centimeter. That is the cross sectional area of a billet. On
the other hand, if you talk of slab, for example, the slab could have thickness of about 200
mm and this could be about 1500 to 1800 mm. So, here, I am saying this is equal to 100
mm and this is also equal to 100 mm. So, the flow rate for a given casting speed, the flow
of amount of molten metal in a slab caster tundish is going to be enormous. So, for example, in industry, we can see that
we can have 3.5 tonnes per minute. That is the rate of casting in a conventional steel
plant or steel mill. So, we are casting through two strands about
7 tons, and a same plant producing billet now would be using strand number 1 2 3 4 5
6, and in each of which, maybe we will have casting something like 1.33 ton per minute.
So, 6 multiplied by 1.33 tonnes is roughly about 1.16. I would say this becomes equal
to 7 ton per minute and this is going to be equivalent to 3.5 tons per minute.
So, therefore, if you are producing small cross section jobs like billets, we have to
have a multiple strand tundish; so, it is not necessary that we have always two strand
tundish. It will depend on what kind of a plant. We are talking about billet casters
will have, for example, 6 or 8 strand tundishes, multiple number of strands are there. In bloom,
which could be about 300 by 300 mm, 300 by 300 mm. So, that is a bloom cast, and in bloom,
we can basically have not 6, but about 3 strands, sometimes about 2 strand also it is possible
for smaller mills such that the cross section is bigger. So, as the cross section becomes
bigger, the number of strand decreases. That is the thumb rule you can apply.
So, in continuous casting therefore, tundish is going to feed. A number of molds and how
many numbers of molds are going to there. That will dictated by what sort of a product.
We are talking about billet bloom slab and so on. The tundish basically has lot of furnitures
in it, and what are the furnitures in tundish or these are also called flow modifiers. I have drawn a tundish which is a bare tundish.
There is nothing inside the tundish. The material is coming from the ladle; it is flowing out.
So, that is how the materials goes and flows out of the strand. This is not desirable in
tundish as I am going to explain to you. So, therefore, inside of tundish will contains
some furnitures and this will also vary from one tundish to another. Also I have drawn a tundish which basically
looks like a rectangular shaped tundish. If you look at the cross section and the cross
section for the top view of the tundish will look something like this. So, it is a rectangular
shaped tundish, but it is not necessary that all the tundishes are going to be always rectangular
cross section. So, we have various shapes of tundishes; we
have many numbers of strands. We have different kinds of geometries and this gives us lot
of possible combinations to play with in order to regulate the fluid flow in the tundish
itself. How the material comes and how the material flows out of the tundish into the
mold that can be governed by various types of flow control devices or flow modifiers
as well as tundish geometry and so on. I can show that beyond rectangular tundish,
we can have what is known as a delta shaped tundish. A delta shaped tundish will look
something like this. And this is the region where the cross essentially tells us that
where the shroud enters the tundish. So, that is the region where shroud enters, and this
is the 0 are nothing but strands. So, this shows a 4-strand delta shaped tundish. We
can have T shaped tundish also. This is a 3-strand T shaped tundish.
So, most of the industries either uses T delta or a rectangular shaped tundish, of course,
some minor variations could be there, but in general, these are the three types of tundishes
which are commonly seen in steel industries. So, 3 strand T- shaped tundish.
What are the furnitures that we are talking about in tundish? Various kinds of furnitures
are possible. I am going to explain and show, place them in the tundish and then you will
be able to understand. Number 1 is Turbo stop this is common number
2 Dams and number 3. These are the 3 essential or the basic types of furnitures which are
normally introduced in the tundish. What is the objective of introducing such furnitures
in the tundish? Let me just explain this to you. For example,
if you imagine that the material coming from the ladle, it is falling through a great height;
so, it is coming into the tundish with an enormous velocity. If it is coming with an
enormous velocity, it comes and strikes at the bottom. Typically, at the bottom where
we may have always what is known as an impact pad, because here, otherwise, the refractive
wear is going to be maximum. So, if you look at this figure, it is that
once clear that this material which is coming at a very high rate. The velocity with which
it impinges on the tundish is also extremely high 8 meters per second, 7 meter per second,
so on depending on the rate of casting. So, we have to have in order to prolong the life
of the tundish, because at no point, the refractory can get weaker and this is the region object
impacts the material from ladle comes in. So, we require what is known as a pouring
pad. So, this is nothing but a reinforcement by refractory so that the wear of tundish
refractory is not significant. Now the fluid comes or the molten steel comes
impinges on the surface, and then, it spreads radially and it comes here and flows out through
this. If it happens like this, then what happens is that we call that it is going to be short
circuiting. The flow is coming impinging on the pouring pad and straight forward getting
into the mold or through the strand without spending much time in the tundish itself.
Now, for example, we are casting at a constant rate. The tundish as I will demonstrate to
you except for the initial period of filling, when the ladle is opened for the first time
or towards the final stage when there is no more ladle sitting at the top the level of
the tundish really changes, but otherwise, the tundish is continuously operated at a
constant height and this height ensures that we have a constant casting rate. Therefore,
the flow rate of metal which comes from the ladle is exactly equal to the flow rate of
the metal which leaves out of tundish. So, Q 1 plus Q 2 is equal to Q input. That condition
is met under steady state casting condition or throughout the casting region except for
the initial period of tundish filling and the final stages of tundish draining.
So, if you look at this, the volume of the tundish to which it is going to be filled
up. Finally, the steady state volume, and if you divided it by the volumetric flow rate,
then this gives us a times scale. So, volume divided by volumetric flow rate, the dimension
of this is going to be time and this time is known as tau, which is nothing but the
residence time and this residence time is called the Nominal Residence time.
So, the material which comes in here has a potential to spend so much of time depending
on how much of volume we can have at the volumetric flow rate. So, for example, we may be casting
at 7 tons per minute and this tundish may have volume of 28 tons or volume corresponding
to 28 tons. So, I can say that for a 28 ton tundish and 3 tons per minute, we are talking
about something like 9 minutes. So, you can take because the density is same
in both the expressions. So, 3 tonnes is the mass flow rate and 28 ton is the capacity.
So, therefore, the residence time in terms of the masses can also be found out; 28 ton
is the volume of the tundish; 3 ton is the volumetric flow rate into the tundish or if
you make it say 3.5, 3.5, then I can say it to be 7. So, let us say this is 3.5 tonnes
we are drawing through here; 3.5 tonnes we are drawing through here. Therefore, 7 tonnes
per minute is the material is inlet flow rate. Therefore, we can say that it is not 9; in
this particular case, its value is 4 minutes. So, the reactors has a potential to hold molten
material at least for 4 minutes of time. Now, if the material comes here and then it is
goes out like this, it is spending virtually no time in the tundish itself. So, therefore
we say it is short circuiting, it is by passing the tundish volume, and tundish processing
as I have mentioned, the tundish need not be today looked as solely as a distributor
of molten metal. Particularly when we are talking of quality improvement, we are talking
about inclusions removal the enhancement of steel quality as well as minor adjustment
of composition and control of temperature also in the tundish itself. So, tundish should
not be looked like an ideal reactor; it is a very vital processing reactor, but the quality
of steel can be significantly influenced. But if such a thing happens in tundish that
the material comes without spending much time, it goes into the mold, then you have no scope
to maneuver with the quality of molten metal in the tundish itself. So, as a result of
which, what we say? We want material to spend longer amount of time; therefore, we apply
flow modifiers. For example, if I put a dam here, just hypothetically
speaking, on this particular side, in that case, you can see that this stream is not
going to come directly, because the dam would not allow it to come. The dam will direct
it to flow upward, and in that case, what happens? The fluid will flow something like
this and then come back in this particular fashion so that the residence time of the
material… So, in the in the absence of this dam, there would have been short circuit,
but placing the dam, I have been able to eliminate short circuiting completely, allowing material
to spend longer amount of time, and within this time, now I can adjust the, I can do
something. I have some time available in my hand and I could do some modifications. I
can improve certain aspects of steel composition and quality by taking advantage of this particular
characteristics of the flow itself. So, to control or to alter the flow field
or flow of liquid in the tundish or melt in the tundish, favorably various flow modifiers
are introduced. We must say also note here that the tundish is going to be always covered
with a slag, which is known as the tundish slag. So, this is the tundish slag, and basically,
this is based on rice-husk; this serves as a source for absorption of the inclusions.
So, if we can create, if we can spend liquid allow, force liquid steel to spend some more
time in the tundish itself, so, that particular time lighter inclusions will tend to float,
and once they float, they can be absorbed by this slag which is always there. So, the
slag is freshly prepared, it is not the ladle slag, but the tundish slag is made out of
rice-husk and different chemicals in order to create a conducive environment for the
absorption of the harmful not metallic inclusion. Tundishes are also going to be covered with
the refectory, because tundish, if we are talking of slab casting tundishes, what would
be the length of this tundish? We are talking about 6 to 7 meters long. So, we can imagine
that the surface area is going to be enormous, and if the surface area is enormous, there
is going to be tremendous amount of heat loss from the tundish itself.
We also have seen that why do not you have the strands so close to each other, because
if we have close to each other, in that case, they will not be spending much time; bulk
of the reactor will remain idle, material will enter through this and then flow out
through the strands if they are located here. So, the strands are to be placed far away
and this necessitates that the exposed surface area of tundish is going to be therefore very
large. Tundish is a shallow but long reactor. So, what tundish if you would note has come
from stables where horses are used to be fed. If you have seen that there are 6 or 7 horses
and there is a tundish shaped vessel, where you know food materials are kept and all the
horses then pick up their food with their mouth from the tundish itself. So, the shape of their food bowl in stables
are similar to the shape of tundish and that is why I think that the name tundish has come
in the steelmaking literature because of resemblance with the platter that the horses use in stable.
So, there because of this large surface area, we have to have tundish covers also and this
tundish covers really prevents lot of heat. So, not much heat can go out of the tundish,
the wall reflects the heat back, and as a result of which, even though the surface area
may be very large, the drop in temperature of the molten metal at this particular location
and at this particular location is barely 5 to 7 degrees of centigrade.
So, if it is 1660 here, it could be 1653 here, 1654 here, and if you are operating with tundish
cover, in that case, that will be much more pronounced temperature drop. I would like
to also mention that ladles; in many steel plants, ladles are also, in all ladle metallurgy
operation, ladles are also operated with physical covers and this physical cover really cuts
the radiative heat losses from the surface of the melt. So, now, we are very conscious about energy
because of cost of reduction of specific energy consumption, because where from the energy
comes eventually? They come from the fossil fuels and which has tremendous influence on
the environment. So, we have to be very careful not to waste energy but to minimize the consumption
of energy, and therefore, operating ladles and tundish with cover has assumed all the
more importance in the current scenario. The material comes from the ladle into the
tundish at a very high speed. So, we can except that significant amount of turbulence is going
to be prevalent in this particular region. So, this is the region in the tundish where
we are going to except lot of turbulence, and why lot of turbulence? Because the velocity
is large; the size of the reactor is big, and as I have mentioned that the kinematic
viscosity of steel is very, steel is a very flowable liquid just like water. Water and
steel have the same kinematic viscosity; so, the flow is going to be extremely turbulent
here. And turbulence is no good as far as removal
of inclusion is concerned. So, we want that the turbulence is to be reduced with significant
extent. We also want that the flow need not be going downward, because inclusions travel
in which direction, inclusions will travel towards the upward direction. So, I should
create a flow which itself is directed upward; so, thereby, helping the inclusions, you guys
go head, you know. I am giving you some assistance with the upwardly directed flow, and as a
result of which, the inclusions will have a greater tendency to move towards the upper
surface. So, for example, this dam would be very good
in comparison to no dam situation, because the dam is deflecting the flow and directing
it in a direction which is directed towards the where? These are slags it is directed
towards the slag, and therefore, this sort of a dam, placement of a dam is going to create
a surface directed flow, which will be very good for inclusion float up also.
So, turbulence is not very good here and we must also know that we have lot of slag here.
So, if we have lot of turbulence activities here, what is going to be happened? This area
is going to blow opened up because of the tremendous amount of turbulence present. So,
therefore, we can see that this lot of reoxidation, we are always worried about reoxidation in
ladle in tundish, and reoxidation I have categorically mentioned that it spoils the quality of steel.
So, the slag layer need not be broken at any stage and it is here that immediately above
the impinging jet from the ladle. We have a possibility for breaking of the slag layer
by atmospheric oxygen and nitrogen can go into melt. So, the submergence of the shroud into the
melt, the geometry of this particular, you know, the promo defects in this particular
regions are very important in order to control turbulence in order to prevent any extensive
rupture of the slag layer or even as such slag metal interaction. If too much of an
activity here, then what is going to happen? We have slag here and that slag particles
maybe entrained into the melt, and as a result of which, what happens? This area from the
top will look open. So, if you draw the cross sectional top view,
for example, the top view will then look like, I will just draw a section of it to give you
an idea. So, we have, this is the shroud region and we have everywhere slag, and basically,
that is what I am talking about that this region is going to be exposed; there is no
slag here, and because of high flow rate of molten steel, because of no way to damp the
turbulence itself. The slag may be, there may be lot of activity as the slag droplets
maybe entrained. These droplets may be entrained here, and these droplets, if there are no
dams, etcetera, maybe entrained also. So, slag emulsification can take place because
of the turbulence, we can have rupture of the slag layer, whereby reoxidation can take
place, and therefore, it is necessary today to restrict turbulence here and that is why
we have structures which are placed or furnitures which are placed below the nozzle, which are
known as turbo stop or this is also known as pouring box.
This pouring box is going to be placed immediately below the
shroud and the objective of the pouring box is to dampen turbulence or restrict the turbulence
within this particular region itself, do not let it go to the other side, because you would
like to have very weak kind of a flow here which will be conducive. We do not want any
significant amount of turbulence or high velocity. In the bulk of the tundish, it has to be restricted
in this part of the tundish, and therefore, we say, we can apply a pouring box here.
A dam is what I have shown here, then we can have weir also here, and somewhere, you can
say that weir is something like this, and as a result of which, what happened is we
have, for example, the flow which comes like this, which, you know, goes like this there
is no scope for the slag to be entrained here. So, the presence of a weir actually prevents
very little amount of slag entrainment. So, this is the dam and this is the weir.
So, dam and weir are typically applied in combination and the objective of dam is to
produce a surface directed flow. The objective of weir is to not allow any slag material
to be moving along with the melt. The slag will not be moving with this fluid or melt
and that is why this dam and the weir are applied in conjunction.
Now, many times today, as I said that, all these things are not going to be applied in
conjunction. If I have a turbo stop or a pouring box here below stream, in that case, the dam
and the weir may not be necessary, because turbo stop is going to restrict turbulence,
and thereby, there will be as such no slag entrainment and the turbo stop will also create
a vertically directed flow. For example, a geometry of, you can imagine
that is with the base of the tundish is like this, a turbo stop will look something like
this. That is the way the turbo stop really looks like. So, it is a section; so, this
is the shroud. This is the pouring box or the turbo stop, and now, this is a refractory
line; this is actually made out of castables, very strong structures. So, this is the box.
So, the material is now discharged not within the tundish but within the box, and the material
comes here and then it goes out something like this; it goes out something like this
and it also creates some amount of vertically, you know, upwardly directed fluid motion and
which is created by the dam itself. So, the turbo stop, actually the placement
of a turbo stop can create surface directed flow. It can prevent also entrainment of the
slag, because the flow, you know the upward direction flow here, the flow is moving up;
the flow is moving down here. So, the flow can be significantly reduced, and as a result
of which, the entrainment of slag can also be or the mixing of the slag or opening of
this area can also be substantially eliminated. So, when you have a turbo stop, it is not
necessary to have dam and the weir, but we must understand that if we do not have, for
example, a near strand dam in most of the cases. Let me draw a separate figure; illustrate
this concept. So, this is the central line and this is half of the turbo stop. I am showing
you this is the turbo stop and this is my strand here and this is the shroud and this
is my center line. So, the material, this is my free surface. The material goes like
this and we have often in here strand dam. The objective of this dam again is to prolong
the residence time. You see the material come here and it will not be able to flow through.
Even that is going to approach like this, it cannot go. Then, again, it will going to
rebound striking the dam, but the most important function of the dam in industrial tundish
is to prevent strand freezing. Because when the first metal comes in from
the ladle, this metal is not very hot, because it is sitting at the bottom of the tundish
and through where the heat losses taking place. So, it is relatively cold, and then, as the
material in the core of the ladle will flow out that metal, is really much more hotter.
So, this material as it comes and as it fills up this cavity and then it flows, trickles
here the first metal. So, the metal trickles and gradually flows here; so, the first metal
is going to be subjected to a very high level of radiation losses. So, by the time it really
reaches here, what can happen is that material can freeze and block this nozzle.
So, we would like some material to pile up here. If this dam is there, in that case,
material will not flow but the material will keep on piling here lot of material and there
will be no freezing, and then, at one point of time, the material will start to flow.
This is the initial stage of tundish filling; that is what I am trying to say.
So, the near strand dam has no function once the tundish starts to work. This has a function
only in the initial stages when we do not want any strand freezing because of substantial
amount of heat loss from the initial amount of metal, which enters the tundish. We can now talk about the distribution of
temperature a little bit; distribution of heat in tundish. It is often required that
if the temperature drop of this material is significant, suppose if you are operating
with not a thick slag, if you are operating with no tundish cover, in that case, there
can be significant amount of heat loss, and in that case, in tundish, it may be necessary
that the temperature of the metal is heated up. So, in that particular case, how to heat
the molten metal? How to supply heat to molten metal?
We have plasma heaters for example, and through these plasma heaters, there can be one plasma
heater here, second plasma heater here, third plasma heater here and fourth plasma heater
here, and these plasma heaters from the top are going to supply heat to molten metal,
and thereby, the material in the tundish can be heated up.
We must understand that if you want to remove inclusions from the tundish, we require a
very quiescent flow; we require a upwardly directed flow, we do not want too much of
turbulence. On the other hand, if you want that we have to supply heat and that heat
is distributed all through the molten metal uniformly, in that case, there has to be a
good amount of mixing in the system or not a weak flow or not a surface directed flow
but, you know, a very well mixed kind of a flow has to be there, which will take the
heat from this surface to the bulk of the liquid and distribute it there.
So, the conditions, therefore, we learned from this that the conditions which are conducive
to inclusion floatation. The fluid flow conditions which are conducive to inclusion floatation
are not same as the conditions which are required to improve or enhance the temperature of molten
steel. So, enhancement of temperature or enhancement of material mixing, if you are adding some
alloying additions to distribute it in the system itself, then we will require a high
velocity; we will require high mixing conditions prevalent.
On the other hand, if you want to separate out or remove inclusions, we do not want that
high mixing flow. We require what is known as a flock flow or a very weak kind of a flow
which will be conducive to inclusion floatation. Indeed if you have flock flow and surface
directed flow, that tundish is ideal as far as removal of inclusion is concerned.
