There are some different route to produce steel, and this slide shows the whole iron and steel making route. And we have iron making route where some raw materials like iron ore and the coke are charged into Blast Furnace, then this produce hot metal, this hot metal is water injected into Basic Oxygen Furnace to make a steel. Once steel is made, then it undergoes casting process, rolling process, and it becomes the final product. And these products are used everywhere in the world. Once these steel products are used, then they are supposed to be recycled. So all the used steel products are collected back to the iron and steel making plant, now we call this scrap. That some of scrap may be used in the Basic Oxygen Furnace as a coolant and iron source, which is discussed in previous lesson. However, there are some other routes which effectively use the scrap. This is called Electric Arc Furnace process. In Electric Arc Furnace, raw material for iron is not ore or coke, but it just directly uses scrap. Once scrap is made from Electric Arc Furnace, then it undergoes, secondary fine process and casting process. Now, let's look at the graph right inside, this shows crude steel production by different process. In old times, the report was called the Open Hearth Furnace. This was about 30% in 1974. But in these days, we do not use this, the old process, and or Blast Furnace and fluid process. This is the main steel production work, we simply call it Oxygen Blown Converter process here. You might notice that, this Electric Furnace process, which uses scrap, it is gradually increasing, because there are accumulated scraps. We need to effectively use it. So Electric Arc Furnace process is also very important to understand. This slide shows the basic structure of Electric Arc Furnace process. In the middle part, we have furnace shell and this furnace is covered by water-cooled roof. Through the top position of this loop, we have electrode which is immersed into Electric Arc Furnace, and this actually delivers heat to melt to scrap, and to produce liquid steel. Once liquid steel is produced, then this liquid steel is tapped into ladle. Once liquid steel is made from this Electric Arc Furnace process, then liquid steel is tapped into teaming ladle. Now, this shows schematic operation of EAF process. Once scrap is collected and delivered into the steel making plant, then this scrap is charged into Electric Arc Furnace, then this roof with electrode covers Electric Arc Furnace. Then this is melt, the Electric Arcing is done. The arc produce a lot of energy to melt scrap. However, just getting arc is not enough to melt the whole amount of scrap, so there is additional processes done, by injecting carbon_oxygen. How this works, which should be discussed in next slide. Once liquid steel, the scrap, completely melts, then the liquid steel is tapped into this teaming ladle. Then delivered to secondary fine process. Now, why do we inject carbon and oxygen inside? If you look at the inside Electric Arc Furnace, at first there huge amount of scrap stacked, and electrode is loaded and it starts to deliver electric energy. Then between electrode and scrap there's a huge amount of arc generated. Because of this arc, there is a lot of heat energy obtained, and this heat energy is used to scrap. But this arcing appears in the local positions. The heat energy generated by this arc is not effectively used to melt whole amount of scrap. The heat energy just leaves from the arc and the scrap regions. So, in order to catch this heat effectively, what is done is, the oxygen is injected through this lens. That oxygen interacts with already molten steel, then it forms FeO. It becomes slag. Then carbon is injected through the other lens. The carbon interacts with FeO, then it easily uses FeO to form CO, and Fe is reverted into liquid steel. This forms the CO, can form the foamy slag here. So the slag becomes very foamy. The arc generated in this part can effectively stay inside this foam slag, then heat is effectively delivered to liquid steel. This is why we inject oxygen and carbon in Electric Arc Furnace process. But let's look at the raw material used in Electric Arc Furnace process. Of course, the most important raw material is scrap. This is recycled scrap, obtained from in-house or the purchased scrap. In-house scrap means, the some part of steel product which has not been used inside the plant. Purchased scrap means, this is used everywhere in the world, like car, or bridge, or some other places, then it's collected again and delivered into the steel plant. This is main source of iron, in Electric Arc Furnace process. There are some other raw materials like, ferroalloys, to adjust combustion of liquid steel, and the carbon which is discussed in previous slide, and oxygen gas. And like other, the Combustion Route, we may need to add lime or dolomite as a flux to make slag. Of these raw materials, the scrap is of course most important. If we just use in-house scrap, that it may be okay, because the in-house scrap is almost very clean and almost pure iron, iron and alloys. However, if we purchased scrap from the scrap market, then it contains separate amounts of other elements inside scrap. Among these elements, some elements are not well removed and are not required during Steel Making process. This is called Tramp Elements. Typical tramp elements are shown in this table. Representative of tramp elements are copper and tin. Copper is introduced from electric motor which is used in various cars, and tin is introduced from can which we use in daily life. But why copper and tin are the important tramp elements to be removed? Once copper remains in liquid steel, and if steel is cast, that it causes serious trouble. Copper will penetrate into grain boundary and it melts even at lower temperature, than it cause the failure of steel product. Shown in the left hand side, it shows the hot bending test of steel product. At low copper concentration chance of crack is very, very low. However, if copper concentration increase 0.1-0.2 weight percent, then it starts to make a crack. And tin itself, it actually gives some synergetic effect with copper. So adding that small amount of tin into copper contents here causes even serious trouble. At lower temperature, having 0.3% copper may not give some harmful effect. However, adding just small amount of tin, 0.03% tin, causes the penetration of liquid tramp element in the grain boundaries, it causes trouble. Therefore, when we want to use a scrap, in EAF process, or even in the BOF process, how effectively copper and tin and some other tramp elements can be removed, this is a big issue. We need to develop a very effective way to remove these tramp elements from scrap.
