>> In this lesson, we will discuss about casting and rolling as a final part of steel processing. After steel is produced, the steel is delivered to casting plant. The steel is poured into ladle here, and the other parts of the casting machines as steel is solidified. Then, it's produced as semi-final product. This semi-final product is moving to rolling plant and this is produced in form of final product. Now let's get in casting part. The liquid steel is prepared, then liquid steel is delivered to casting plant, and liquid steel is contained in a ladle. Then, this liquid steel is transferred into a tundish mold, and strand, then this is finally solidified. The first, liquid steel is delivered to, that's called tundish, here. And between ladle and tundish, liquid steel passes through a route called shroud nozzle. The tundish distribute liquid steel into two molds. And each of them is called strand. Between tundish and mold, liquid steel is passing through a route called submerged entry nozzle. When liquid steel is delivered into a mold, then steel starts to solidify. That solidified part of steel is moving through the strand and it is cut by torch, then it becomes a semi-final product. There are different kinds of semi-final product. First is called Slab, second is called bloom, the third one is called billett. This slide shows several- different kinds of semi final products. The most typical is called slab. Slab is, kind of thick plates shown here. It is used for producing sheet, plate, pipe and tube. This kind of semi-final product is called Bloom. This looks a long bar-type and it is used to produce structural part, or rail, rod and pipe. There are similar kinds of semi-final products called billet. It's also looks like a long bar-type but cross section area of billet is less than that of bloom. This is produced by continuous casting process. In old times when continuous casting technology is not prepared, most of their product is cast in ingot. And the product of cast, the name of this type of process is called the ingot slab. But in these days, most modern steel makers employs continuous casting process. Continuous casting process is really continuous by delivering liquid steel from steel-making plant. But from this part, delivering molten steel into tundish mold and strand and producing the semi-final product are all continuous. So this is called continuous casting and it actually increases productivity of steel product. The casting system is composed of several different parts: The first, ladle, this contain liquid steel you'll rarely find. Second, tundish. This is kind of intermediate steel vessel in order to control casting speed and distribute liquid steel into different strands. Liquid steel is passing through shroud nozzle. And third one, one of the most important part, mold. This actually set the shape of this semi-final product by solidifications. Once steel starts to solidify inside mold, then this is withdrawn to this side and passed through several rollers and it becomes solidified semi-final product. And at certain lengths, it is cut by torch then it becomes slab, billet or bloom. Now, let's look at the functions of several different parts. The first, tundish. Right inside figure shows the side view of tundish. Liquid steel is delivers from ladle to tundish by shroud nozzle. Then, the steel passes through this kind of route. And finally it lifts out of tundish through-hole. The reason why this kind of complicated route is followed is because, to suppress the complex flow inside liquid steel in tundish. And also, the nominator inclusions stayed in liquid steel should be floated up toward the top out of liquid steel, which is covered by tundish flux. Once liquid steel arrives near the hole, then this lifts out of tundish. And the flow rate of liquid steel is controlled either by this type of equipment called stopper, or this type of equipment called slide gate. The flow rate is articulated to casting speed. Then the function of this tundish is, it distribute liquid steel into different strands and also, it controls steel flow inside the tundish and it makes very even flow into the mold. And during passing through this tundish, inclusions residing in the liquid steel are float up then it is removed into tundish flux which covers liquid steel here. And sometimes, if necessary, there are some, the injection of alloy [inaudible] calcium. Then, what is the requirement for the tundish? The first, the inclusions should be removed as much as possible while steel is passing through this tundish. In note to that, the flow pattern of liquid steel must permit the much of steel to move close to the surface of liquid steel. This is why we have this kind of complicated steel route inside and it also ensures same length of time of steel flowing to each strand. And it controls flow rate of liquid steel, in other words casting speed. And it minimize the turbulence, complicated flow of liquid steel coming from ladle. And this liquid steel is covered by tundish flux, here. This tundish flux should not entrapped into this hole, so it means that soft tundish should be secured not to allow the vortex of slag. Now, let's take casting mold. Once liquid steel lifts out of tundish, then it pass through a route called submerged entry nozzle. This nozzle has the port, about here. And depending on port angle and port height, the liquid steel flows either this direction or this directions. Once liquid steel touches the wall of copper mold here, then, because they start to solidify from the surface. Of course, this copper mold is cooled by cooling water. Between tundish mold slide gate is shown here or the other type, the stopper, controlled flow rate of steel and it controlled casting speed. And during the casting, this liquid steel should not be exposed to air. So, we add some metal called mold flux. Mold flux covers liquid steel and it distribute in form of powder and mold flux is really doing important role in the casting. Since mold flux is added in form of powder, it forms the powder layer and sintered layer by observing heat from liquid steel. And eventually, it melts when the mold flux liquid steel. And one of the function of this mold powder is lubricating between the steel and the mold to not allow sticking of liquid steel to the mold. This mold is always oscillating. Now, let's talk about mold flux. Mold flux is similar to the slag which we have discussed so far. But it is typically composed of this kind of components, typically molten oxide with some amount of fluoride. Also, it contains a certain amount of carbon powder to control melting rate of this mold powder. When it is applied to mold, it gradually sintered and melt. Mold flux has several functions shown here. The first function of mold flux is thermal insulation of liquid steel here. So it actually cut the heat released to this section. And second, it actually prevents the oxidation of liquid steel by the oxygen in the air. The third one, if the steel still contains certain amount of inclusions, then the inclusions are brought up and it is to be absorbed by the mold flux. This allows the cleanness of steel. And the next one, if mold flux enters between copper mold and steel, then it also solidifies. The fraction of crystallized mold flux and glass mold flux controls the heat transfer rate between mold and liquid steel. And the last one, the mold flux here works as a lubricate between copper mold and solidifying shell. When solidifying shell with liquid steel leaves out of mold, then it enters into strand, composed of several different lobes and some spraying part. So the steel is solidified again here, this is called second degree. The solidified shell lose its heat by the spray cooling here and various micro structures developed inside this solidified part. This is quite important part in steel production that, how micro structure of steel is develop here determines the property of the same final product. If the steel contains less amount of carbon or high amount of carbon, then the controlling, solidification is relative to each. But if the steel contains some middle age of carbon then solidification involves pearlitic transformation then it becomes, it makes it quite difficult for solidification of steel slab. And there are several different kinds of strand. The first one here is vertical type. After passing the mold, the solidified part of steel slab moves straight or downward, this is called vertical type. But it requires very tall lengths of the equipment. So it cost a lot of the insulation cost. So sometimes, this vertical type of the strand is changed to cover type or vertical and bending type. Once the steel is fully solidified then the steel become fully solid product. Then, since this is continuous process after a certain length, this semi-final product is cut by torch and it becomes a slab, bloom or billet. Then it is delivered to the next part called rolling plant. In the rolling plant, it undergoes several different type of rolling and it becomes the final product of steel plant; coil, plate, or weir.
