[MUSIC] In previous lesson, we have learned how liquid steel is made from hot metal, which was produced from blast furnace process. In this lesson, we'll discuss about new steel making, in which is called secondary refining. Which steel produced from furnace, but delicate steel is further cleaned and be transformed into a molten form for the next process, casting. And also in this lesson, we'll discuss the other type of steelmaking route, which is called process. And we will also discuss about different type of steel making process for stainless steel. This figure shows process overview of whole steelmaking plant. And we have covered the raw material, iron making, and hot metal treatment, and steel making so far. In this lesson, we'll discuss about steel, the secondary refining process, which is about here. Actually, steel produced from this converter or furnace is delivered into the secondary refining plant. And liquid steel produced from this furnace is treated, then the silicon steel will be transferred into the casting plant. Let's get the purpose of the secondary refining process. The figure shown on the right-hand side shows the chemistry of steel, depending on its process. The vertical axis shows the progress of process which has been treated so far. And the horizontal axis shows the chemistry or chemical composition of the steel which has been so far. First, this is an ore which contains lots of oxygen inside, suppose we have every ore, then it contains over 30% of oxygen inside. Then it undergoes Ironmaking mode, then it is transformed into hot metal. It's oxygen contents has been lowered significantly, so almost there's no oxygen remaining in hot metal. However, the carbon concentration in hot metal increases up to 4 or 4.5%. Then this hard metal is treated through the steelmaking route, using basic oxygen furnace. The hard metal is now transformed into the steel, its carbon concentration becomes very, very low. However, at the same time, the oxygen concentration in liquid steel increases about a few hundredth ppm. When this liquid steel is made, the steel cannot be used directly. It's necessary to treat this liquid steel one more time, through secondary refining process. The purpose of doing secondary refining process is first to decrease the oxygen concentration, which has been increased up to a few hundredth ppm, typically 5 to 6 hundredth ppm. We need to decrease oxygen first. Then during the decreasing oxygen, we face new method called inclusions, which is small particle of oxide. We call this as include, this should be decreased, or they should be modified. And also, we need to remove another impurities inside liquid steel. And we need to also change concentration of liquid steel, by adding alloy elements. So the topic shown here is the issues, what should be done during that kind of refining process? First, liquid steel is tapped from converter, then the liquid steel may be reheated, which is suitable for the casting process. And during reheating process, deoxidation inside liquid steel may be carried out. And at the same time, we need to refine liquid steel in terms of chemistry. So we need to deoxidize liquid steel, and we also need to desulfurize liquid steel. And as a result of the oxidation process, once inclusions are formed, then this liquid inclusion should be removed or modified. And some other type of impurity like nitrogen or hydrogen may enter into liquid steel during tapping process. This is a gaseous element, and this should be removed from liquid steel through degassing process. And depending on steel product types, certain type of alloying elements must be added into liquid steel, this is alloying. Once chemical compositions and temperatures are well adjusted, then this is sent to tundish for continuous casting process, which will be discussed in the next lesson. In this slide, there are various types of the equipment which is used for refining liquid steel. The purpose of the refining is shown here, we may need to alloy liquid steel. We may need to degas liquid steel, to take nitrogen or hydrogen out of liquid steel. And during the second refining process, temperature of liquid steel gradually decreases, if this is too low, then it is not good for casting process. So sometimes, we need to reheat liquid steel in order to have suitable temperature for casting. And also, during the oxidation process, no metal oxide inclusions have formed. Then this inclusion needs to be removed or modified. All the facilities shown in this slide are used for this kind of the process. In the middle, we see two types of facilities, the left one is called ladle arc furnace, or simply we call it LF, right hand side we call RH, degasser. The LF furnace is used to reheat, temperature of liquid steel, this is electrode, and it causes an electric arc. The arcing is done at the surface of liquid steel, then this part of liquid steel gets enough energy to increase temperature. Then, by stirring liquid steel, using our gas bubble, liquid steel is well stated and temperatures are evenly distributed. On the right-hand side, we see RH degasser. This has multiple purposes, but most important one is degassing. The upper part, this is used to make the inside chamber to be vacuumed, so this is going to vacuum pump, and liquid steel is separated through this route. And gaseous elements, inert gas like nitrogen or hydrogen are evacuated into this chamber. And more importantly, If we like to decrease carbon level inside the liquid steel, then carbon may form the CO gas, and this is also evacuated. And this facility also is used to saturate this liquid steel, so liquid steel is. The other type of the facilities, this is called powder injection. The lens is put inside liquid steel, and since this lens has many, many holes, so it is so argon gas and some flux are added into liquid steel, and well state. The shows wire feeder, certain type of alloying elements or some other flux is provided though wire injection. And it is delivered in the middle of liquid steel, or even bottom of liquid steel, then this is wire mixed by this alloying gas. Right hand side, this is called composition adjustment by sealed air bubbling, oxygen blowing, simply called CAS-OB. We have certain type of hopper, and the oxygen is blowing here, and also aluminum is added here. So in this place, aluminum linked with the oxygen gas that it forms alumina with a lot of heat. So this is another type of heating facility inside, then again, this liquid steel is well by our gas, so temperature is evenly distributed. The last one is called vacuum tank degasser, or VTD. The function of this VTD is very similar to RH. The ladle containing liquid steel is put inside a vacuum tank, and the gaseous element is well evacuated. Then about the refining topics, let's look at two important ones. First one is the inclusions and deoxidation, second one is degassing. Deoxidation is one of the most important parts in refining liquid steel. Once liquid steel is tapped from BOF, it contains significant amount of oxygen, about 500 ppm. If this liquid steel is cast directly, then oxygen solubility in solid steel is quite low. Most of this oxygen will involve in forming of CO gas, by forming CO gas through this reaction. But since steel is solidified, the CO gas cannot escape out of steel, so it stays inside the solid steel product. This is called pinhole, this is defect, therefore we must decrease oxygen level before the steel is cast. There are several types of deoxidation methods. We may deoxidize liquid steel by adding alloy that forms Mn oxide. Or we may deoxidize liquid steel using Ferrosilicon, the silicon forms SiO2. Or silicon linked with oxygen and magnesium inside of liquid steel, then it forms magnesium silicate oxide, which is typically liquid at steel making temperatures. But most typically we use aluminum to deoxidize liquid steel. It is a very strong deoxidizer, and it's also chip. By adding aluminum, we can form alumina, then oxygen level in liquid steel is decreased, very low level after just few ppm, this is quite effective. However, there's some price to pay using aluminum. It forms alumina as a result of the deoxidation process, that encourages significant trouble during the process, and even for the product. It stays in liquid steel, and if the liquid steel is cast, then this inclusion stays in steel product, it becomes defect. It may appear on the surface of steel product, then, this steel product becomes very, very, less useful. If this inclusion is stays in y product or within y product, then it cause breakage. Inclusion forms crack inside the steel product, so this inclusion should be removed during steel making process. However, removal of inclusion perfectly is almost impossible. Size of inclusions are very, very small, even below micrometer. So if it is not possible to remove all inclusions, then we may try to modify property of inclusions, not to be so harmful to steel product and process. The other important topic of secondary refining process is degassing. There are three elements to be removed in degassing process. The first two are nitrogen and hydrogen, which may enter into liquid steel from air, and the third one is carbon remaining in liquid steel. In RH degasser, what is done is, liquid steel is separated through this route and this side is evacuated, so pressure inside the chamber becomes low. Then nitrogen or hydrogen may be easily evaporated, and carbon with oxygen, then it becomes CO, then CO is also evaporated. How liquid steel is separated here. In this part, there are two routes for liquid steel, and in one route, argon gas is supplied. And then, apparent density, in the left-hand side becomes lower than right-hand side. And since this is part evacuate, the lighter part of liquid steel goes up, and this moves through this route. In this way, liquid steel is well circulated here. During this circulation, gaseous elements will be easily evacuated here, and also if necessary, the oxygen is applied here with aluminum. Then this aluminum also links with oxygen to form alumina, but also it causes a lot of heat. So this facility is also used to reheat the liquid steel, it has variety of functions.
