Let's start here. So we had already given an introduction about these, the hotstroke MEL. OK. And we were going through a number of configurations.

Let's review configuration one again, and a slightly different layout here. You start with two furnaces, which are called walking beam furnaces in this case.

And then you have, this is a slightly different line. It's got three roughing MELs in succession. Many hotstroke MEL configurations have a single roughing MEL that works as a reversible MEL

to do the rolling. Some MELs have roughing MELs in succession. That makes the line very long. And of course, there are additional investments

in terms of the building, but also additional investment in terms of the MEL stands. And then you have many configuration where the transfer bar is just exposed to the air.

You have also configurations where the transfer bar can be covered, either passively covered or actively covered. We'll see what that means.

Crop shear, descaler, finisher. And then measuring here of the thickness, temperature of the hotcoming strip, laminar cooling in the run out table before the three coilers. And you can see this is a long line.

This is an alternative here, an alternative configuration of a standard MEL, which has the advantage of being shorter. And how do you reduce the length of the MEL?

Well, one of the reasons why the hot strip MEL is so long is because of the length of the transfer bar. The transfer bar, as I told you, is 100 meters. So you've got to have room for the transfer bar. If you use coil box technologies,

you achieve two things. First of all, you make the line shorter because you coil the transfer bar. Yes, you don't lay it all out, yes? And so you achieve two things.

First of all, you don't have a long transfer bar. And the second thing is you minimize heat losses of the transfer bar. And that's also a positive. Although the line is shorter, the investments for the coil box are very important. So it ends up being a costly solution.

But anyway, this is the coil box here. And we'll be talking about this in more detail. But you can see, again, furnaces, descaling, reversing rougher coil box, crop shear, descaling, edging,

and finishing mill cooling in the run out table, and then two or three coilers. OK? So that's some features here.

So let's look at the different parts quickly. So the reheating furnace. Reheating furnace, why do we need this? Well, we need to start at high temperatures to do the hot deformation. And the choice of temperature is basically

based on metallurgical considerations. You want to have a homogeneous austenite. And you want to be able to do the processing. So this is about, as you go through the line, you get pretty much a linear decrease in temperature

of the material. Yes? And you want to make sure that you end at the right temperature, at the finishing temperature, called the finishing temperature.

And that temperature should be in the homogeneous austenite range. So that you can carry out the cooling and the transformation in the run out table and coiling. In the coil. OK, so let's start the types of reheating furnaces

for strip products. You have what we call walking beam furnaces, pusher type furnaces, and tunnel furnaces. And tunnel furnaces is a type of furnace that's used in many mills, like a CSP line, which

will have separate session on that, so many mills. So what you usually have are walking beam or pusher beam furnaces. And the current technology is walking beam furnaces, so that you avoid damaging the slabs.

The furnace, what can we say about the heating zones? Well, the slabs, remember, they'll be a very high temperature. And so you have to put them on something.

And well, you basically put them on what we call skids. And those are water, or rather, steam cooled rails that are internally cooled. Because they're cooler, they will leave marks

on the slab, which we call skid marks. These marks can be very pronounced if you have a pusher type, a pusher type reheating furnace. Because in a pusher type furnace, you basically push the slab through the furnace

over these rails. And you leave a trace on the slabs. And we'll see that even though this looks like it can't possibly be a big effect,

it actually has a big effect on the product. These furnaces are heated with heavy oil, or with LPG, because they are working around the clock. And so you choose the least expensive heating method.

And natural gas is much cheaper than electricity, so you usually use LPG. It has three zones, preheating to 600, heating to the high temperature, and then a soaking,

where you homogenize the slab temperature. So this furnace is at ambient. So you've got a lot of thermal oxidation. And you form scale.

And that means a loss in yield, because you've just made steel, and now you're turning it into iron oxide. So it will cost you in terms of yield. It will cost you in terms of having to remove this oxide, and then to recycle it in the process.

So the scale that you form, how much of it, what type, et cetera, is related to the steel composition. And that you can't really change very much. You will form, of course, more scale at higher temperatures. So that's why you don't have extremely high temperatures.

You prefer to work at slightly lower temperatures. The heating time, yes, longer you leave the slab at high temperature, the more scale you will form. And the atmosphere of the furnace will also determine how much scale you form.

So you don't want to have your burners work with a very high oxygen excess. Say a few more words about this. So pusher type and walking beams. You see here, I think, yes, this

is the exit of these furnaces. You can see the slab coming onto the rolls here. So what is important is that you achieve temperature homogeneity. So you have these huge blocks of steels weighing up to 20 tons.

They have to be homogeneously heated to about 1,250 or slightly more. You charge them at ambient temperature,

and they usually stay in the furnace for 2 and 1 half hours. That's a typical residency time. That is necessary, yes, not for metallurgical reasons. Because one of the things metallurgically that you want

is to, for instance, dissolve all the precipitates. You don't need 2 and 1 half hours to dissolve most precipitates. That is much faster. The reason why you stay there is to achieve a homogeneous temperature. And because these blocks, these slabs are so massive.

And this is a typical temperature range, 1,200 to 1,288. So it's about 1,200 to 1,300 degrees C. And almost universally, people use 1,250. And I want to point this out that, again, sometimes it's

interesting from a metallurgical point of view, physical metallurgical point of view, to use a lower reheating temperature. You may have reasons, for instance, to not dissolve a precipitate, or for other reasons.

Or because you know that if you don't reheat at these high temperatures, you may be able to have smaller grain sizes. But you cannot do this. These furnaces are run at a stable temperature.

So you cannot turn the knob and say, well, this slab will be 100 degrees lower. And this slab, you cannot do this. So this is not, it is a laboratory circumstance, easy parameter to change. In industrial circumstances, very difficult to change.

And it would be almost impossible, as a matter of fact. So of course, because it takes two and a half hour for a slab to heat up, that means that the capacity of a hot strip mill is dependent on how much slabs you can heat up per hour.

And a typical furnace will be able to process about 300 tons of steel per hour. So usually, the slab charging is at ambient temperature.

But there are some slabs which are very sensitive to cracking. And those ones will be charged, for instance, will come straight from the steel plant to be reheated at a higher temperature. For instance, that applies typically

to high silicon grades, which tend to be very brittle. So this is a cross section here of a typical furnace. You see it's got different zones. And this is a pusher.

That means that the slabs are brought in here. And there is a mechanism that just pushes the new slab in, and out comes the one that's in the dropout zone. So you have preheating, heating,

and then homogenizing parts of the furnace. Of course, because you heat here, there's a lot of waste heat. And that is recuperated in a recuperator. So typically, you have some sizes here, about 30 meters.

A convection zone, that's the first zone here. A heating zone of nine meter, and an equalizing or soaking zone of eight meters. So in total, about a good 30 meters. So these are large units.

This is a walking beam furnace. So there, you basically have a mechanism whereby you move the slabs through the furnace

with a walking beam system. So you have a slab here. And it rests on these skids, yes, on skids. And then there is a mechanism here, yes,

that goes like this, and goes up, yes, picks up the slab, and moves it ahead, and puts it down again. So the slabs are not pushed against each other.

And they are not, so they don't touch each other. And they're not pushed across the support skids. So there is less damage to the surface of the slabs.

More expensive, of course, because you need to have this mechanical system in operation at high temperature. Let's have a look at what's happening inside a furnace. So we've got our slabs here, very high temperature.

So we have refractories on the walls of our furnace. And we burn heavy oil or gas. Usually, it's gas, natural gas. And natural gas, the main constituents is methane.

And we burn it with air. So that's the oxidizing. So what happens, we have methane plus air, plus nitrogen, of course, in the air. That gives me CO2. That's the heat, yes, water vapor, nitrogen, and the same nitrogen we had originally, plus heat.

So the relative amount of oxygen to methane is important, because you can have a flame that is neutral. You could have a flame that's oxidizing, strongly

oxidizing, or a flame that's reducing. So when you have the exact ratio, such as an example here, you will have 10 volumes of air for one volume of gas. And the flame temperature here

is close to 2,000 degrees. It's very hot in these furnaces, right? So 2,000 degrees. If you have an excess air, for instance, 12 volumes of air for one volume of gas, the flame temperature will be lower, because some of the air, some of the oxygen,

is not used to produce heat, yes? You can also have an excess natural gas. And then your flame is slightly reducing.

For instance, eight volumes of air, one volume of gas, the flame temperature will be lower. Why would you do this? Well, for instance, if you have a problem of excessive scale formation, OK? Right, so how does the heat, is the heat transferred?

You don't actually directly, you don't actually direct the burners on the slab, yes? You don't do that, right? So the heating is indirect.

So you get radiation, mainly, from the walls, refractory walls of your furnace, yes? Convection, yes? And radiation from the flame, yes? OK, that's how you heat up the slabs. You don't have the burner actually burning onto,

directed to the slabs. So the actual transfer to steel of the energy is about 70%. And you have a considerable amount of waste, heat,

that's lost to the stack, about 22%. The reason why the slabs stay in this very hot atmosphere for so long is basically to homogenize the temperature. So if you look here, this is in the furnace.

So you start with really cold slabs, yes? And they heat up to about 1250 at the exit. The slab temperature increases gradually. But the wall and the gas temperature in the furnace

are very high, OK? And as I said, the flame itself can be close to 2,000 degrees C. So what are the energetics about reheating furnace?

So you enter with 100% energy, you go through the reheating furnace. The actual yield is about 52%. I just said it was about 70%.

The reason is because we can recuperate some of the heat that's not used. So we do have a way to increase this. But the direct transfer is about 52%.

Heat loss is to exhaust gas 20%. We lose some to cooling water and to heat that's transferred to the walls of the furnace, about 5%. This here, where does this heat loss come from, cooling water? Well, mainly from the skids.

The skids, we need to protect them from excessively high temperatures. So they are water cooled. Actually, they're steam cooled. So that's where this 10% comes from. OK. So let's say a few things, important things, about what happens metallurgically to the product

as you heat up a slab. Well, the most important things that happens to the microstructure is very strong coarsening of the grain size. So this shows, however, that the coarsening

is very much a function of the type of steel that you process. And in particular, the reason is because you have particular precipitates

in the microstructure. So let's look, for instance, at what happens to a carbon manganese steel that is being reheated. And here it's shown as a function of the temperature.

You see carbon manganese steel, there's this red line here. You see that the temperature increases. We're in this range, as the temperature increases, the grain size increases. We're typically in this temperature range, 1,200 to 1,300. And you see that the corresponding grain size is of the order of close to 300, if not more, yes?

If you have grades that contain small precipitates, such as vanadium carbonitrate, aluminum nitrate, nitrite,

excuse me, niobium carbide, what you see is that the grain size will stay small, as long as the precipitates are present. But once you cross the solubility temperature,

the precipitates will go fully into solution. And they will not be there to prevent grain growth. So when vanadium goes into solution in a vanadium microalloyed steel,

suddenly the grain growth starts. In an aluminum nitrite containing steel, once the aluminum nitrite is in solution, the grain growth will start. Niobium carbide goes in solution.

Again, grain growth will start. And you see this usually happens for normal steel compositions below 1,200. So when you reheat slabs, most of the time, whether or not you have these precipitates

at this type of temperature, they'll be in solution. And you'll have very coarse grains. Is that true for all the precipitates? Well, there's one precipitate in particular that doesn't go into solution, and that's titanium nitrite.

And that is what you see here, titanium. If you have a steel that is microalloyed with titanium, that titanium will form extremely stable titanium

nitrite with a very low solubility. So the solubility of titanium nitrite is very low. In other words, to dissolve titanium nitrite, you need very, very high temperatures,

temperatures that are even higher than reheating temperatures. So titanium alloyed grades will usually not show excessive grain growth. OK? And in certain cases, we use this. We use this phenomena that titanium microalloying allows

us to refine the grain even at the level of the reheating furnace. Let's see in detail what we have here.

So this coarsening, of course, coarsening of grains is a dynamic phenomenon, right? It takes time, yes? And so it's dependent on the temperature. It's depending on the time. And of course, it's depending on the specific composition,

yes? For instance, in the case of niobium carbide, it depends on how much niobium carbide you have. In other words, what is the concentration of niobium? And this is illustrated here. So the coarsening, the type, and the amount of precipitate

is important. So we have a carbon manganese steel now, yes? And it contains niobium. We have three. One contains 100 ppm. One contains 500 ppm.

And one contains about 1,000 ppm. And you can see, of course, that if you have only 100 ppm of niobium carbide, that niobium carbide will be dissolved, yes, at lower temperatures, yes? You remember, why is that?

Because so this is niobium, carbon. This is your solubility line, yes? So all the compositions that are below the solubility line, say, for temperature 1, all the compositions

that are below this line will be in solution, yes? So if I have three compositions, let's put one here, three compositions, this one has lower niobium and carbon.

This one has more niobium and carbon. This one has the largest amount. So this composition will be in solution at T1. But these two, there will still be a certain amount of niobium precipitated. So the amount of niobium carbide, of niobium,

is important. And so you can see here, if you have 500 ppm of niobium, the coarsening will only start at 1,100. If you have 1,000, the coarsening will start at temperatures slightly below 1,200.

Again, 1,000 ppm of niobium, for your information, is an excessive, is a relatively large amount of niobium. Usually in microalloyed steels, we have about 500 ppm, 400 to 500 ppm.

So that means that in a niobium microalloyed steels, which when they are reheated, the niobium is fully in solution and will have coarse grains.

So the composition is important in terms of the coarsening, but also the temperature and the time. So what you see here is the grain size again. But now it's plotted as a function of the time.

And there are three curves. There is 950, 1,050, and 1,150. So you see that the grain size, if you reheat to 950,

nothing much happens. The grain size coarsens a little bit. We have a typical reheating times 2 and 1.5 hours on this scale. So you don't keep slabs in for 10 hours.

But let's have a look at what happens at 1,050. At 1,050, the grain size increases slightly. You have to imagine, of course, that the precipitates

don't go suddenly into solution. They need some time to dissolve. So between 6 to 10 hours, you see that the grain size increases. Not suddenly, just gradually to 1,050.

So that means that if we have a reheating time of 2 and 1.5 hours, the grain size here will still be small if we reheat to 1,050. Now that same steel, if we reheat at 1,150,

we see that the coarsening starts within half an hour. You see here the grain size already starts. So that means that this particular steel here,

which contains aluminum nitride, precipitates because we put them 2 and 1.5 hours in the slab reheating. The coarsening will already have started long before that. And so again, grain sizes 200, 300 microns

will be achieved during the slab reheating. So that's one important thing that happens to your material, very coarse grains, 300 microns.

What are typical grain sizes that we want to have in practice? There are very few, very, very few steels where we have grain sizes that are large.

Typically, it would be electrical steels. There you need grain sizes which can be as high as 100, 150 microns. Most of the production will be less than 20 microns.

So that's a considerable reduction of grain size that you will need to achieve as you process the steel. So that's one thing that happens. The other thing that happens, of course, and we talked about this, is scale formation. Now in the hot strip mill, you form scale. In the reheating furnace and in the rolling section.

The scale you form in the reheating furnace, we call primary scale. The scale you form in the hot strip mill outside the reheating furnace, you call secondary. OK.

So it's a whole science formation of scale on steels. And it's pretty complex. And the reason is because the oxide layer, the scale layer, is complex. And we have typically three types of iron oxide compounds,

hematite, magnetite, and wustite. In addition, of course, most of the alloying elements are also oxidized. So that adds to the complexity of the types of oxides

you have. And there may be microstructural features such as blow holes in your oxides, et cetera. The Fe2O3 is very dense and has good thermal conductivity. Fe3O4 is porous, poor thermal conductivity.

And FeO is also porous and also poor thermal conductivity and actually has an insulation almost. This oxide easily delaminate, so come off the surface.

And well, if you are familiar with thermal oxidation, there are classical theories for the growth of these oxides. For instance, one of these theories states that the thickness or the weight of the oxide

is proportional to the square root of the time. Not very useful in practice to study this oxidation. We form typically two to three millimeters of scale.

So that's a considerable amount. In the secondary scale, it's about 10 times thinner, 0.1 to 0.25 millimeters. Yes? And so there will be a loss, iron loss, due to scale formation. And that's of the order of $50,000 to $100,000 per year.

So that is not a huge amount of money for a steel company, but it is appreciable still. Right.

So the scale thickness will be a function of the temperature. Now, we can't do much about this, right? Because you have your reheating temperature. The time exposed to these high temperature, again, you can't do much about it. We know that it will be two and a half hours,

that the slab will have to reside at high temperature. But what it's also a function of is the air excess in the furnace, the oxygen excess in the furnace. So typically here, the scale formation, the amount of scale in pounds per square feet.

And one pound per square feet is about almost five kilograms per square meter. So as a function of the temperature here, excuse me, as a function of the time. So you can see here, these are typical times

for the reheating time. So we will have about, in this particular case, 12, 18. We look at about two and a half kilograms

per square meter of scale. If the temperature is increased or you increase the excess oxygen, you will, of course, make more oxides at the surface.

What else happens to our slab? Well, the slab is put on a skid, a skid pipe. So this is, for instance, the top of what's inside a furnace.

You can see these are the skid pipes here. Skid pipes are water cooled. So around this contact point, you'll have slightly lower temperatures. And of course, the skid pipes, the distance

between the skid pipes may seem like not very important thing. But actually, the skid pipes will determine how short the maximum or the minimum length of slabs that you can process.

You cannot process a slab that will fall in between the skids, right? Obviously. And of course, the size of the furnace will determine how long the slabs can be. Some practical things. Now, the skid pipe marks will leave

what we call a thermal profile on the slab. And that's why we use isolated skid pipes, yes? But still, we get thermal marking of the slab.

And this is really interesting. This thermal profile stays on the material, yes? This is an example here of the thermal profile that you get from a slab on the bar.

So the material has been rolled and is a bar now. You see the thermal profile. And after the finisher, you can still see the same oscillation, the same thermal profile

due to these skid pipes, yes? OK? If you look at this, you'll say, well, what are we looking at? Well, this is in Fahrenheit, so I would have to recalculate. And Fahrenheit tends to be 20.

What I want to say is that here you see it's about 50 degrees Fahrenheit. In Celsius, it's much smaller. But at this stage, you'll have to forgive me. But I didn't. So it's a number, a few tenths of a degree that you oscillate.

And it means that the properties of the steel vary also with this oscillation, yes? And this brings us back to what we were discussing yesterday is when your temperature changes,

the properties of the material change, yes? And when the properties of the material change, your strip thickness will change, right? And it's one of the reasons why it's absolutely essential to have gauge control on melts

because temperature differences would always result in gauge inhomogeneity. And of course, not to speak about possible metallurgical problems.

Important things in the slab reheating, yes? Burnt steel surfaces, yes? Slabs should never be exposed directly to hot flame, yes? Very silly, yes? Because why is that? Because the flame is very hot, 2,000 degrees, right?

So what happens? The oxide, yes? This is the iron oxygen phase diagram. You see here, you see wustite and magnetite and hematite

here, OK? You see that at about slightly less than 1,400 degrees C, our oxides start to become liquid, yes? You don't want to have liquid scale, yes?

Because liquid scale gives you a crazed, a non-flat, bumpy surface appearance which looks, well, crazed or burned. So this is a no-no. Never melt the scale that's important.

OK, so slab comes out. You've got to hot roll it, yes? What are you going to do? First things first, you have to take care of the descaling. You cannot roll material with two or three millimeters

of scale on top of it, yes? So the first things you do in any type, whether it's strip or wire or bar products

at high temperatures before the roughing, you always descale, right? Always descale. So what do you do in the descaling? You remove the oxide layers which are formed by thermal corrosion. Of course, if you don't do that,

you will have poor surface quality. You have to remove primary scale when you come out of the reheating furnace, yes? And you do this, it's a thick oxide, right? So you start by, and although the addition of it is not very good, you have to make sure

it's fully removed, so you start with a vertical scale breaker, VSB, and by, sometimes you can also give it a horizontal, pass through a horizontal scale breaker. And then it's removed basically by high pressure water, yes?

2,000 to 6,000 PSI, and I will give you some metric numbers in a moment. Secondary scale is formed during the rolling itself, yes? And that is removed only with high pressure water, yes?

And that's essentially before the finishing mill. These layers are less thick, and they're easier to remove. Finally, when we coil the material, we will also form scale in the coiled material.

That scale will also need to be removed, but we'll do that in a separate unit, which is called the pickling line. Yes? OK. And so we'll talk about pickling separately

in a moment. So this is a vertical scale breaker. It removes primary scale. So what you see, this is the slab here. And here you see a roll on the side, yes? And there's one on the left and one on the right. And it basically rolls the sides of the slab,

and it basically cracks the scale to make it easier to be removed. In addition, this vertical scale breaker is also an edger. An edger, as you may remember, allows

you to slightly change the width of your slab before you do the rough rolling. Well, there's not much to see to the high pressure descaler.

This is seen from the descaler from outside. Obviously, everything looks very rusty. And of these units, because there's lots of iron oxide, it's one of the reasons why hot strip mills are not very nice to visit.

There's always a lot of very fine iron oxide flying around. And you see, so you have headers. That's where the high pressure water comes from. You have pinch rolls. That's to avoid that a lot of the descaling water

is sprayed everywhere. And you also have to descale on top and at the bottom. So you have here the descale headers. And so if you look at the inside, so that's from the outside, you don't see much.

You just see the hot strip coming in and out. Yes? So you have top sprays here, top sprays that spray on the top and on the bottom of the strip. And the water is one of the reasons why you use a box.

You have to recuperate the water, because water costs money. And you recycle this water, yes? There are developments in the area of descaling.

Why is that? Well, the trend is in descaling is to use less water, because the treatment of descaler water is expensive.

You need to remove the particles. You need to recycle this. And the smaller the volume is that you have to treat the less expensive. So you want to have very efficient descaling. That's one thing. The other thing is you want to have the space where

you do the descaling confined, make it small. So in particular, in mini-mills, they use rotary descalers.

And instead of having stationary sprays, you have sprays that rotate at very high speeds and very high water pressures. They're very compact, again. You can see here, this is a unit before it gets mounted.

And you have two of these rotary arms on top and the bottom, and left and right. Very high descale efficiencies and low temperature losses are the advantage

of this technology. Again, you don't tend to see them in conventional hot strip mills. You tend to see them in compact strip mills. So where do we do the descaling? Well, everywhere where we start to roll.

So you have your scale breaker. And you descale before the roughing mill. And you descale before the finishing mills. And there may be descalers in between the passes.

So what does 2,000 PSR mean? It's around 120 bars. These descalers, these are some parameters, descaler parameters before the roughing and before the descalers before the finishing stands.

And there may be inter-stand descalers. And this is what they look like, these conventional. So you've got a header and then smaller tubes with spray nozzles at the end.

Good. Now about the descaling. Some people think that you need a lot of water to descale. And the water's got to be cold. And that is not right.

The descaling is based on pressure. So there's no effect from thermal shock, that you use cold water and spray on red hot scale.

And so the thermal shock will result in scale removal. That's not how it works. So the descaling force developed by the descaler nozzle depend on two factors, the pressure and the amount of water.

But if you have high pressure, you can reduce the amount of water and have the same efficiency. Effectiveness of the descaling depends on many things, nozzle angle, distance to strip, strip speed, lead angle, offset angle, et cetera.

And the impact force is a parameter that's often used, the force divided by the width and length of the impact area. So we'll see that the impact, these nozzles, it's not a round spray. It's like a knife-like spray, a line-type spray.

And you see here that the effectiveness of removing the scale is a function of the pressure. So if you have high pressures, yes, you don't need so much water. And that's very advantageous.

Actually, there's also a good reason to avoid using too much water. It's because the more water you use, the more you cool your strip. So you're basically wasting heat at the same time. So that's why pressure is the most of such importance.

So how does the descaling mechanism actually work? It's not due to thermal shock from cold water on the strip. It's due to the impingement force that breaks the scale steel bond. If the impingement force, and certainly

when the water volume is too high, you get undesirable strip cooling. And this will affect strip hardness and the gauge. So what you basically do is you

let the high pressure water remove the scale basically by brute force, by breaking the bond between the scale and the metal. And that's why the geometry of the water spray

is so important. So the descaling will work best when it works on the cracked scale. So that's why the descaler works by breaking the oxide

metal bond and lift off the scale. So in the impact area, the water spray must get under the scale and apply enough force on the scale. So that's why we have these oxide breakers ahead of the descaler. So you can have horizontal roll or the vertical etchers.

And they're basically there to put cracks in the scale. So this is a nice schematic here and a nice figure of these nozzles. You can see the nozzles are the spray pattern.

You see here is a linear spray, a high pressure linear spray. And this is seen from the side. Again, a descaler looks like a pretty dirty box

and not have to do too much with it. But it is essential that it is working correctly. If the distance between the nozzle and the strip is not good, if the angle is not perfect,

this angle here, beta angle, is not 7 and 1 half degrees, you will have very poor performance. So descaling may not look like a very important thing, but it's important to have the parameters right

and to have the setup of the descaler very nice. So this is an example here of typical ranges. For instance, for the distance here, this distance here is in the range of 6 to 14 inch.

And so that would be about 10 centimeters to 20 centimeters. The nozzles here are, this is high tech. You cannot, I remember visiting a company in India

and they had changed this into something they had bought in some supermarket. So this is not a garden spray thing. This is really high tech to achieve the right spray

pattern. So you have to buy this from specialized companies and you cannot just replace them and this can have major influence. And I say this because some people don't know that for most people,

a nozzle is a nozzle, right? And it's something you, it's a garden hose nozzle. They're all the same. It's not like this. And so to be aware of this, certainly with the descaling that the right nozzles are used.

So we come to the roughing. The material has been descaled. You go into the roughing mill. So first of all, your strip has a certain width, yes? Your mill has a certain position. You have to make.

sure that the strip goes right through the mill in the correct position. So you have centering guides, centering, you can see them here. They will make sure that your strip, at that time the slab, goes through the middle

of the line. So there are a number of things that happen also, as I said the vertical edger, we talked about it to say it can work as a scale breaker, but it also acts to change the width of the

slab. The width itself is established at the caster. And the edger can control, give some variation in the product width.

You cannot give large amounts of reduction. The reason is because you will form a dog bone. A dog bone, so if you want to change the width here by passing, so these are rolls,

by passing your slab through the rolls of the edger, you can only do a small amount

of reduction. If you do too much reduction, you will have this shape. And your slab will look like a dog bone. And so there is a limit to how much you can reduce the width.

In particular, this can generate not only problems on the sides, but also at the ends of the bar, so after you roll the slab, giving rise to tongs and fish tails.

We will talk about this in a moment. Of course, the position of the vertical edger is right before the roughing mill.

This is an example here of this edger. It's basically a mill where the rolls are vertical, so it can be usually in the front of the rougher, it can be attached to the rougher, or it can be a standalone unit.

And typical edging capacity is two inches. So two inches, that's about five centimeters. That's about the amount of change you can give to the width, so it's not very much. And this is an edger here.

So what you usually have with the edger, this is seen from the top, from the top view. So the edger changes, for instance, the width of the slab by a centimeter or two.

And then you always pass through a horizontal mill. And the reason is to flatten this, and so it will give you some recovery of the width also.

I'll show this in more detail in a moment here. So the width control, and in particular these effects here, are important because they result in this, if it's too important, you get tongue effects and fishtail effects.

See, when you roll this, so say you roll this, and here it's slightly thicker.

At the ends, this slightly thicker part will be flattened, and this will give you this shape. And depending on the profile you have, you can also have a tongue shape.

But fishtails are very common. So it basically depends on the amount of deformation you have given. So that's been studied. These tongues and fishtails in the bar.

So these tongues and fishtail refer to the bar, so after the roughing. But the reason why they occur is because the poor, basically poor control of the edger.

So what you want is to have a square end. Why do you want a square end? Because you don't want to start rolling with a strip or a bar that's not straight.

You have to start rolling with a bar that's a straight line. And you also don't want to have this, because you need to scrap it. So I told you this is where you can get your samples.

So as researchers, you actually don't mind these things. But for the process people, you want to avoid it. So that's basically what happens.

So as I said, at the end of the slabs here, the edging will result in the free flow of the material in the rolling direction, rather than a spread in the thickness and the formation of this dog bone shape. So depending on the relative change in width and height,

you can have tongue shape or fishtails. And they need to be removed. So you'll have to cut this off. And that means you will have material loss. So what is this delta w?

Delta w is the difference in the width you realized and the difference in thickness that you realized in the edger. So normally, you want to have, this is your slab, yes? You want it to, when you squash it,

you want it to deform homogeneously into a thicker slab that's not as narrow. However, depending on the relative amount of thickness change and width change, you will have fishtails or tongs on your bar.

So this is, in more detail, this vertical edger. So you basically have these grooved rolls. They are here. You see them here, yes?

And of course, you need to position them. So you have four cylinders that press. And everything is horizontal. And they're driven. So you need a motor, spindle, a motor here, a gearbox,

a spindle that goes from the gearbox to the roll. You can see here, this is the edger here. There are new technologies where

instead of using rolls to change the width, you forge, you use forging. You actually forge the slab, yes? And those are called sizing presses, yes?

They're a considerable cost, but they're worth it because you minimize fishtailings, yes? And it allows you to get more variety in widths, yes?

And that's important because the inventory, the slab widths that you have to produce, you can do with only four to five slab widths to cover all the widths you need to produce, right?

So that simplifies the processing, right? And so again, you don't have as much loss. You don't need to cut off material so much to remove the fishtails or the tongs.

It's costly. And of course, that is an issue. But what you do is basically, instead of rolling the edges, you forge the material. Now the problem is when you forge, the easy thing with rolls is that the product can move.

When you forge, at the moment you hit the material, the product moves, moves through the line. So that's why this sizing press has to be a flying press.

So it's got to hit the material and move with it, yes? So the product continues to move, yes? And it basically, as I said, it's a forging operation, yes?

So you work with an anvil. And the anvil is shaped, has a particular profile, yes? And every time the anvil hits the slab, it will reduce the width of the slab

and increase the thickness, of course. And it does that because it's not so localized, yes? You don't get this dog bone effect.

So this is an example here. I don't know how well you can see, but it's basically a flying press. The units can go up to 2,000 millimeters. So what's important here is the width reductions.

So it's 30 centimeters, yes? You remember when we were talking about the conventional edger? We were talking about inches, 5 centimeters. Here it's 30 centimeters, right?

So we're talking about big difference, OK? OK, and then we come to the roughing mill. And the roughing mill is usually a two or a four high

stand. And it's basically used as a reversing mill. So the slab goes through and then is rolled back and forth

through the roll gap till you achieve the thickness that you want. This is an example here. And you can see here that this is the roughing mill.

And in front of it, you have the vertical edger, right? So depending on the choices that people make when they design the lines, yes, there are many varieties of choices that people make. You can have the edger very close to the roughing mill

or not. The number of passes is always uneven, of course, because you need to move in a certain direction, yes? And here you have some parameters here about diameters of the roll.

The barrel length means the total length of the roll, rather than the actual product, yes? So what do we do in the roughing mill? We prepare the slab to make a bar, which

will be finish rolled. So we get a thickness and a width reduction. And we are usually going from, as I said here, from 25 centimeters, about 25 centimeters, to 25 millimeters.

So that's a considerable reduction. And it's 25 millimeters to 38 millimeters. That will go into the finishing mill. That's the typical entry thickness. The width reductions that we can give typically

are of the order of, in the edger, five centimeters, so two inches. That's for a standard vertical edger. If you use a flying sizing press, you can go up to 30. So what we also need to do, keep in mind when

you do the roughing mill, is the temperature control. So we need to deliver the bar to the finishing mill at the right temperature, which is about 1,050 degrees C. And remember, the material may be very heavy.

It is relatively soft at this temperature, so we want to avoid surface defect. Scale removal, of course, happens before the roughing, stable positioning, and then minimizing hook and camber.

That is the general shape of the strip. If your mill is not well aligned, your strip can move this way. Or it can have this shape or that shape. So that's what we mean with hook and camber.

And of course, that's a nightmare when that happens. And we'll talk about this later on when we talk about profile of strip and flatness aspects. So usually, roughing mills are two high or four high types

of mills. This is an example where a two high mill, you can see the slab passing through the mill. Here is the four high, so you have two work rolls and two backup rolls. And here you have an example of types of rolls.

And for different conventional hot strip mills, I guess what is important for you is the diameter of the rolls and things like that.

We'll talk in a moment about the fact that these rolls operate at high temperatures. So they're subject to wear, other wear. And so you need to replace them.

And the work rolls that are direct contact with the sheet need to be replaced every about 2,500 tons. So if you say that we produce 20 tons coils,

yes, that means that about every 1,000 coils

you need to make a replacement. Did I get this right? Oh, no. Excuse me.

It looked a little bit too much here, just 25. Yeah, 2,500, it looked too much. That's more reasonable. So every about 100, 150 coils, you need to replace the work rolls. So that's actually rather frequent.

The backup rolls too, these can stay about 1,000 tons. So every 25,000 to 30,000 tons of steel processed, they need to be replaced. Actually, we don't replace them.

We reprocess the surface. We rework the surface. We remove the damage. So you can use the same roll for a while, but not forever. Right.

So now we're out of the roughing mill. We made a bar. And we're going to transfer this bar to the finisher mill, to the finisher. So let's first look at temperatures again. This is a more detailed view of what the temperature

variations look like. Let's look at what is the temperature in the center of our material. So that would be this middle gray line. So I see the temperature reducing.

And I'm passing through the seven roughing passes. And when I come out of the roughing passes, my temperature should be at around 1,100 so that at 1,050 I can start the rolling. Now what's funny, you'll notice here,

is that the temperature actually increases. Why does the temperature increase? The temperature increases because I'm deforming the material. And so I'm adding energy. And it heats up the material. However, the surface, oh sorry,

I'm looking at this is the strip middle. This is between the surface. So the strip middle is in red here. I'm sorry about this. But anyway, the message is the same. Inside the material, you basically heat up

every time you deform. The surface, it's at the surface, not the story. Because there, you are touching the cold roll surface. And so you get, every time you roll, you get a dip, a big dip in the temperature

of the surface of the strip. But because the center is warm, you reheat the center. You reheat the surface as you go. You see also that between the steps, the rolling passes,

there's a considerable amount of time, 20, 30 seconds. Yes? And if you compare now to the finishing passes, there is much faster succession, because they're tandem. But you have the same story. Every time you touch the roll, the surface

will dip considerably. However, in the center, you get heating. The deformation gives you heating. So the important message about this figure here is that you have inhomogeneity in deformation.

You also have strain rates that can be inhomogeneous. We have continuous temperature changes. And we also have a temperature inhomogeneity.

So we need to have very strict temperature control. That's the best thing to achieve. Because if we do that, we can get the right microstructure and the properties. And there are different ways in which we do this in the hot strip mill.

And in the hot strip mill, because we do deformation and a thermal cycle, we talk about thermal mechanical processing. And so there are basically three simple ways. In actuality, it's a little bit more complex. And we'll talk about this as we go. So you can do what we call a high temperature rolling.

Basically, everything is in austenitic. And you always fully recrystallize the grains. These recrystallized grains may coarsen between the deformation steps. And eventually, you get, if you're working with constructional steels, relatively coarse ferrite, pearlite microstructure.

And the way you control the grain size is by this recrystallization. You can reduce the grain size. You can do normalizing rolling. There you are. You try to stay close to the finishing rolling,

close to the transformation. Yes? So that the austenite is fully recrystallized, but it's a smaller austenite grain. It's not as coarse. And then you have many types of different ways to do thermal mechanical processing. And it's very important for plate steels.

There, we are doing the finishing either very close to the transformation or sometimes even in the two-phase region, yes? The austenite grains may not recrystallize.

And that happens when we add these micro alloying additions, such as niobium, yes? And we obtain an extremely fine microstructure. And that's the realm of the real thermal mechanical processing. And this is very often combined with what

we call accelerated cooling. So you do not get ferrite plus pearlite microstructure, but a bainitic microstructure, or in some cases, martensitic microstructures. And so this is kind of the range of temperatures

where you would work for high temperature normalizing. And so usually, you're in the normalizing range or in the thermal mechanical range, OK? Temperature variations.

So we already saw that in function of the thickness, there is a temperature variation. But there is also a temperature variation along the length of your material, yes? And so this is a picture that you have to imagine. This is very long. It's 500 meters, right? But the image has been compressed, right?

So it's compressed. And you basically look at the temperature distribution of a strip, yes, along its length and also along the width. So what do you see? The head is 870.

So this is what comes out of the finisher. And this is what comes out of the finisher at the end. It's 810. So there's 60 degree difference. That's one thing. The other thing you can see is that, well, the edges

are black. So that means the edges are cooler than the rest of the material. So in other words, we have not only temperature variations in the thickness, but we have temperature variations in the width and in the length directions.

And we're talking about, here we talk about 60 degrees, considerable amounts of temperature difference. So we will try to minimize these differences, yes?

And in particular, we'll do that at the level of the transfer bar. Of course, achieving high levels of temperature homogeneity implies that you need additional equipment, yes?

When you need equipment, additional equipment, it means that investments, operation costs, and maintenance costs. So you always have to balance this with the type of products that you make, yes? So let's have a look at these solutions. Well, one of the solution is a transfer table with a cover.

So you see here the bar coming out of what looks like a tunnel. It's actually basically a cover, yes? A steel construction with some refractories, yes?

So that the radiant heat, you don't lose so much heat. So this is an insulated, basically, cover, yes? You can have active covers. And these active covers actually have burners, yes?

So they're able to avoid heat loss. So that, in principle, should give you constant and uniform bar temperature and may also work as a buffer. What's a buffer?

Well, it's a place where you keep the bar for a while, yes? And if you have an active cover, you can keep it longer because there's no risk for, less risk for large temperature losses, OK? Again, typical temperatures for the start of the rolling

is 1,050, 1,090 here, OK? And the new technologies are the coil box technologies. Because the coil box technologies are really, really good in homogenizing the temperature.

And the reason is very simple. If I have a bar that's 100 meter long, yes? That's 23 millimeters thick and 1,600 millimeters wide, yes? I have a surface exposed of 325 square meters, yes?

That's a lot of surface where you can lose heat, yes? If I take the same bar and I roll it into a coil, the total surface is 11 square meters, yes? So the heat loss will be minimized. And on top of that, because I have a massive coil,

I will have less temperature in homogeneities, yes? And this is shown here, for instance. You have constant temperatures. And so when you roll, your temperatures don't change. So you don't have to correct for the fact

that the material becomes harder. So for instance, if you have the conventional rolling and you measure the power, the rolling power, as a function of the time, yes? You have to increase, steadily increase, the rolling load.

Because your material becomes stronger and stronger, yes? If you have a coil box, the temperature is much flatter, yes? And you don't have to increase the rolling load. There's no difference in rolling load,

less difference in the rolling load from the start to the end of the coil, of the bar, rather, yes? And this is a temperature measurement here, yes? Coming out of the coil box into the finisher. So the temperature should be at around, well,

here in this particular case, around 1,050, yes? You can see that what comes out of the coil box, the temperature difference is about 12 degrees, yes? In the case of the conventional, 80 degrees. The example I showed was 870 on one side, 810 on the other side

was about 60 degrees. So that's now reduced from more than 50 degrees to less to about 10 degrees, yes? So very stable rolling conditions, yes? And of course, everything translates into more homogeneous microstructure, OK?

So this is basically what a coil box looks like. It's positioned between the roughing and the finishing stand, conserves the transfer bar temperature, and you get a homogeneous transfer bar temperature.

It looks basically like a coiler, yes? You basically coil it, yes? And there are, by the way, this is a slide about this, there are, for certain, high quality grades,

mostly in the stainless steel area, yes? You will have not only coil boxes, but actually coil box furnaces, yes? Where you can actually reheat the bar, the coil bar,

to a specific temperature, yes? It's a very important investment, of course, and you only do this for really high end products, yes? Where you don't want to make too much scrap and have losses, high value added products. This is an example of what this coil box

furnace would look like, looks like, OK? So the big advantage, an additional advantage of the coil box is that you have a shorter line also.

So note that these units consist of a unit that coils the bar and a unit that uncoils it.

So it's not, there's always one unit that makes the coil and one unit that uncoils it. So you can coil, be coiling on one end and decoiling on the other end, yes?

Otherwise, you would keep the finisher doing nothing while you're coiling this guy, OK? So there is an increased productivity.

Now, there are some interesting, so what you usually, the original classic mandrel, excuse me, classical coil box has a mandrel, yes? So a mandrel is basically this axis, this bar here

around which you form the coil, yes? Now, of course, this metal here, this mandrel here, is cooled, yes?

So that means that when you use a mandrel, the inner wraps are actually not at the right temperature. They're cold, colder, yes? So in order to avoid this, the latest coil box technology is called mandrel-less, yes?

And so you wrap the coil without a central mandrel. Now, the big advantage of having a mandrel is that it's very simple then to transport the coiled bar from the coiling position

to the uncoiling position, because then this bar just goes from this position to this position and you're in business. If you don't have a mandrel, you need a more complicated system to transfer the coil from the coiling position

to the uncoiling position. So it kind of gets rolled over these rolls here. But the big advantage is that you have absolute perfect homogeneity in the temperature.

You can have a look at this pictural here that shows you how the bar is transferred. It's done very carefully because you don't want to damage the coil and it has to remain in nice circular shape. You don't want to.

Now, certain people are very concerned about the fact that the edges of the strip lose a lot of heat and are cold.

And for products where it's important that there is no edge cooling, there is a possibility to mount induction heaters on the line. And basically, this induction heating, these inductors

produce eddy currents in the strip edge. And these currents will heat up the strip edge. So this is what it looks like from the outside. There's no strip going in. This is a edge heater in operation.

It's basically non-contact heating method. And this is how it works. You basically have a magnet and a coil. And you generate eddy currents in the strip as it passes.

The heating is very fast, so there's no problem with the fact that the strip is moving by. So these are some parameters here.

And you can also reheat considerable thickness of material. So we've got this bar. It's at the right temperature and the right thickness.

So the entry thickness about 25 millimeters. The temperature about 1,050 degrees C. And you're ready to go into the fishing mill. The first thing you do is the bar will pass through the shears. The shears remove the fishtails or the tongs.

So you arrive at the finishing mill with a straight edge. This is an example of the equipment. The shear is called a crop shear.

And it can be either a flying shear or a drum shear. The reason is that the shearing operations are made. You don't want to stop the bar to shear it. You crop the bar as it goes.

So you have to have a flying shear that goes down with the bar and then cuts it as it comes by. That's why you have a flying shear or a drum shear.

So the knives are on the drum. And as the strip passes, it gets cut. So the bar is not stationary. Again, why do we do this? Because we want continuous operation. We don't want temperature losses, et cetera.

Again, this is a very simple thing. But it's very important. It's very important to have the right sheared profile. Because if your profile is not correct, you may be damaging the rolls.

So the sheared profile looks like this. I don't know if you've ever done shearing in the laboratory, but usually you use kind of punches. And you have terrible edge lips that stick out

on which you can cut yourself. Well, in shearing, you don't want this kind of profile. You don't want to have that kind of profile. So shearing, very important.

And this is an example here of a crop shear that are mounted on drums. This is a slightly different system. Again, the entry equipment will involve a guide. Now you're seeing the guide in the direction

of where the strip would move. So you have a width adjuster and a position adjuster. So because you want to have the bar move in the correct direction towards the finishing mill.

So what do we want to do in the finishing mill? Require the finishing temperature. Obtain the thickness, of course. Maintain the width.

Achieve the profile. What do we mean with profile? Basically, the shape of the strip. And we want to avoid cobbles. Cobbles are this kind of things. Yes? That is a problem during the rolling.

This is a particular type of cobble that occurs when you're rolling thinner hot strip. But you can also have cobbles between two stands, where the material, where there is a problem with tension

control. And then you accumulate a strip between two stands. Yes? And it means that you get 300, 400 meters of red hot metal between two stands. And it's total disaster, you can imagine, because that has to be removed.

And this also has to be removed. So rolling stability is important. OK, and there are lots of improvements in technologies nowadays. Hydraulic gauge adjustments, roll banding, CVC technologies

to control the shape of the strip. I will talk about this later in separate session. And crossed rolls, and then hydraulic loopers. Those are type of loopers which control the tension between the stands.

An example of this cobble here, where does this come from? Well, when the strip exits the last stand,

there's basically nothing holding onto it. And it will have to travel, the front end will have to travel from the finisher to the coiler on its own. Of course, it will pass the cooling section. But basically, nothing is holding it.

It's got to go straight to the coiler. If you have thinner strip, there is a possibility that the strip will move, will start to fly, basically. Will start to basically float in the air.

And it will come back down. And so you will hit the rollers, it will rebound, and it will start forming waves. The head kind of flies, waves.

And then by the time, the disturbance can be so important that by the time you arrive at the coilers, the place where you have to engage the strip is missed. And then you get this.

The material comes out 10 meters per second, and it's nothing to pick it up, and it's a disaster. So it's very important to know that this is very much function of the strip thickness. So you get very high waves of instability waves

when you make thin strip. Definitely one of the reasons why people in the hot strip mill do not like to make very thin products, because there's no basic metallurgical reason.

Well, but there are two other reasons, is that the risks of this happening, and the fact that their productivity goes down. So cobbles. So a finishing mill, in general, will look like this.

You have stands in tandem, and the strip goes through each stand. And then finally, to the coiler. Of course, the temperature is reduced as you go through each stand.

This is a typical view here of the temperature distribution in the thickness of the strip as you roll it. So what do we see? First look at the middle. Again, the middle, every time you roll,

increases the temperature. That's because you dump energy in your material, and most of it is turned into heat. At the surface, it's an entirely different story. There we have the effect of, for instance, here. What is this? Here you have temperature loss at the surface,

but no temperature increase in the middle. That's because we are descaling. We're descaling, and that we do with water. Every time you pass through the roll bite, we touch the cold surface, colder surface,

of the work rolls. And so you get temperature drops. And in between the rolls, you also have interstand sprays for a variety of reasons. So the strain rates in strip production

are pretty important. What is also important are the interpass times. So the interpass times in strip production, so the strip is usually processed

between 1,000 and 900 degrees C. And the interpass times can be, you can see here, less than one second.

And the reason is, and that is very important, because that means that we can have very optimal thermal mechanical processing. We can accumulate deformation at high temperature and achieve grain refinement. That's very important.

In the case of plate material, that is much more difficult. Because in the case of plate processing, we usually work with reversing mills, because plates are thick. We have reversing mills.

And our interpass times are usually at least 10 seconds. Plate goes back and forth in the reversing mill. So it's more difficult to achieve thermal mechanical processing on plate products than it is on strip products. And we'll come back to that when we talk about plate

products and how they're produced. The hot strip mills, the finishers are usually four high type of mills.

The diameter of the rolls is typically around 700 millimeters. The gap setting nowadays in modern facilities, hydraulic automatic gauge control. And this is a typical values of the reductions

that you get in a seven stand mill. So you start by relatively high reductions, yes, as you remember. And then as you go through the line, you reduce the amount of reduction, yes.

So what happens here is also interesting, is the speed of the strip coming in and coming out is very different. In fact, you have a speed of about 2.5 meters

per second coming in and a speed that can be 20 meters per second coming out. So very large differences in speed, yes. And so let me say a few words about that.

It means that the end of your strip and the start of your strip goes through a different thermal cycle. The end of the strip has a lot more time to cool.

Yes. So even if your strip was perfectly homogeneous in the bar, yes, and even if your strip was

everything went perfect, still then, even if the cooling rate was the same everywhere, still then the end would always be cooler than the start because it's got to wait, yes, it's got to wait until all the material that goes before

is processed, yes. So it takes more time for the end pieces to go through the line, basically. So it'd always be cooler. So how do we work with this? Well, there are ways in which finishing mails

can address this problem. And that's called speed up, yes. What basically happens is that the mail rolls not with a stable rolling speed but with an accelerating rolling

speed. So you start to roll at relatively low speeds and then you speed up the rolling, yes, so that you compensate for this time difference, yes. And again, minimize temperature differences.

And so of course, you can only do this if you have very clever models, mechanical and thermal models, for your process, OK. And again, it's something you don't see when you visit a hot strip mail, the fact

that there's a lot of intelligence needed to achieve, that you can use to achieve high product homogeneity and as a consequence, high product quality. So basically, we've already talked about the basic structure of these mails.

You have a mill housing. You have two work rolls and you have a top and a bottom. Backup rolls, yes. On the top, you can have screw down or hydraulic cylinders to control the pressure, the load on the rolls

and the roll gap, yes. And on the bottom here, for instance, for this particular configuration, you have a load cell. Now, then you have, so this is the basics, yes.

Very often, you have systems here that will make sure that the rolls, that the strip profile is controlled, OK.

So what do I mean? When you're rolling strip, you have to imagine these are your work rolls and here is your strip, yes. And you remember the pressure is applied on the chocks

and in the chocks, you have the bearings, yes, the bearings like this, and then your roll, yes. Because of this, there is a slight bending of these rolls,

yes. And when the roll bends, that means that the material actually is not flat, but it has a shape, a profile, yes. And of course, you want to control this in today,

nowadays, very large. This is called crown, yes, or it's a profile defect, as we say. So you need to control this, yes. And there are different ways in which you can do this. But one of the ways in which you can do this

is by compensating the bending that's given this way by having, for instance, this is one of the ways, the system that will cause bending in the other way, that

will bend the cylinders in the other directions, yes. And so these bending cylinders are mounted in a specific case in what we call Mae West blocks,

yes. And so these basically hold the bending cylinders, OK. There's a lot of equipment associated with a mill stand,

yes. Just to give you an idea of what you typically have on a finishing mill, you have cooling equipment. And there may be lubrication equipment. For instance, here, I want to point out

a system here, which is called roll thermal crown control, yes. And these are special units that control what's called the thermal crown, yes.

What is thermal crown? At the same time as you're rolling and you're bending this, what happens? You're rolling a very hot piece of material, yes. What happens when you heat?

material, it expands. So what does it mean? It's on top of having this bending, this part here of the strip, excuse me, of the roll, will have a different diameter

because it's warmer. It'll be whatever the heating is. And the expansion can be different depending on if this is, for instance, a hot spot, a hotter part. For some reason, it's hotter than the diameter

will be different. And so you will also have what's called thermal crown. So your profile will not only have a profile like this, but it will also have slight differences, which are due to thermal variations on the roll surface.

So how do you address this? Well, you address this, for instance, by having roll thermal crown control. So what is this? If I look sideways, it's basically a header, yes,

with lots of, let me put it this way, with lots of nozzles, yes, lots of nozzles. And each nozzle controls the temperature of a segment of the roll, yes? And so if you need more cooling there,

there will be more water coming out of that particular nozzle, yes? And that way, you can achieve or minimize the effect of thermal crown. Very important, again, to achieve quality.

There are systems to blow oxides away. You may want to cool the strip in between the interstand. Of course, one of the things you absolutely need to do is also cool the surface of your roll.

So there are also just cooling systems so for the backup roll and for the work roll, you'll have cooling systems, yeah? So what happens with this interaction

between the hot strip and the work roll, for instance? So when they come into contact, before they come into contact, there's some radiation heat to the surface of the roll. And then in contact, there is heat transfer.

And then again, radiation heat transfer at the exit. You use cooling heaters before and after the rolling to make sure that the roll doesn't heat up, yes?

So the temperature variation at the surface of the roll is quite considerable, yes?

So the surface gets heated by the hot strip and then gets cooled by the thermal spray. So let's have a look at point one, two, three, four, and five, yes? So what does the temperature look like at the surface? So when you go from point one to point two,

the temperature raises, yes? 500 degrees, yes? And then as we come out, four, the temperature drops. And as we come to five, you come to the cooling spray header, the temperature is back to normal.

And that is within less than two seconds, right? Below the surface, 25 millimeters below the surface, so about an inch below the surface, you get a temperature increase of about 100. Five centimeters below the surface, there's not so much effect.

So you get a lot of thermal stresses at the surface, and a lot of thermal fatigue, yes? So what is the result of that? Is damage to the surface, damage to the surface. So we need, again, that's one of the reasons

in addition to just wear, this thermal fatigue requires us to change the roles so frequently. This is in between the interstance, yes? So you have, so the strip goes from one out of one

melt into the next one. And the strip tension is controlled by what we call a looper. Let's have a look into more details, yes?

So we have some cooling of the strip in between the mill, the two mill stands, yes? And we have this thing called cobble guard, OK? The, what do we have here?

Cooling of the cooling headers here, cooling headers that spray the water on the work rolls here, yes? And the other thing that's important between those two

mill stands is the looper. And the looper is basically the mechanism that controls the tension between those two successive mill stands, yes? Basically consists of a roll, yes?

And there is, with a motor here, it is a servo motor. It's pressed against the strip, yes? So any time, so you don't want, in the mill, there are velocity differences, yes?

Velocity difference. So the exit, in perfect conditions, the exit velocity is equal to the entry velocity, right? Then the mass is, you have mass balance, yes?

But any change in this will cause either an increase in tension when you take in too much more than what comes out, or you will get a decrease in tension when the velocity in is much smaller, yes?

And you can get cobbles, yes? So the mechanism that controls, makes sure the tension stays the same is this looper roll, OK? Before I finish, I'll finish in two, three minutes now.

I just want to finish with the finishing mill, yes? About these roll changes. So we need to replace them, because there's a lot of wear, and they usually,

you don't throw them away, no, you recycle them. You transfer them to grinding shop, yes? And the backup rolls, several days to several weeks, they will be replaced. Work rolls, we grind them much more frequently, yes? And so they are replaced every few hours or so, or few days.

It depends on how much production you have. And finishing mills, I think, as I said, 2,500 tons of steels, 70,000 for the backup rolls, yes? And of course, when you replace the rolls,

you cannot roll, right? OK, so that has to be minimized. So you have 10 to 15 minutes time to replace the rolls, yes? And then you bring them to the grinding shop. So usually, they're set up. They're ready there. And if you visit, if you've ever visited a hot strip mill, you will see that in front of every mill stand,

there is a pair of rolls ready to replace the used rolls, yes? And they're usually pre-mounted in the chocks. So you just slide them in, and it's quick. So in the grinding shops, these rolls will then go.

They will be ground. You will take out, remove the damaged layers on special CNC machines, yes? And it's a lot of work. Again, it's 24. I don't know if it's 24 hours a day. But you process typically about hundreds of rolls, yes?

And it takes you about 30, 20 to 30 minutes to process them. And how often can you reuse this? Well, about 100 times. After that, you've removed either the diameter is changed too much so that the diameter has become

too small, right? Or you have removed too much of the hard surface layer, the very hard surface layer. So about 100 times.

So for instance, if you have about 700 millimeter diameter roll, it will be replaced when it's 630 millimeters, yes? So this is an example here. This is the roll. It's mounted on a CNC machine.

So you remove the worn surface with this grinding wheel, yes? It's a grinding wheel, OK? And you remove of the order of half a millimeter to a few millimeters.

It depends very much on the amount of damage on the roll, yes? And here, you see work rolls being replaced. And these are backup rolls being replaced. And there are a variety of schemes where you do single work rolls.

Most of the times, you do work roll pre-packaged. And you can replace them very quickly. OK? Right, so I'll stop here because I'm already over time. And I guess the main thing we'll do on Monday

at 4 o'clock, yes?