Last lecture, we have learned about ironmaking process. In ironmaking process, raw materials like iron ore is produced to liquid iron. We call that liquid iron as pig iron or hot metal. The hot metal is not directly used to produce final product, but hot metal needs to be further transformed into liquid steel because hot metal contains a number of impurities inside [inaudible] carbon. So in this lecture, we will learn about the steelmaking. This lecture is divided in two parts. The first, hot metal pretreatment, and second, BOF steelmaking. This figure shows the order process of homes to the productions. In the last lecture, we have learned about ironmaking site. That is ironmaking. The hot metal is produced from this blust furnace, then this hot metal is tapped into torpedo car, then this is moved to steelmaking blust. So, all of this part is steelmaking plant. The hot metal is first pretreated. This is hot metal pretreatment, then it is charged into a vessel called converter. In this converter, hot metal is converted into a steel. The role of steelmaking process is actually to convert hot metal to steel. Hot metal contains a number of impurities like carbon, silicon, and phosphorus, sulfur. In particular, the carbon concentration of hot metal is quite high, it's up to four percent. This is almost carbon saturated liquid iron. Through steelmaking process, this carbon and some other impurities like silicon and phosphorus are removed by injecting oxygen gas onto this liquid hot metal. So all these element, carbon, silicon, and phosphorus are oxidized and they are removed. Then [inaudible] , we can produce liquid steel in which carbon content is quite low. Then let's think about, why we need to remove these kind of elements or impurities? After hot metal is produced, there are five main elements. Carbon, sulfur, silicon, phosphorus and manganese. Now, let's look at what is typical role of these elements and why it need to be removed. The first, carbon. If there is carbon inside iron or steel, then it actually increases the strength and hardness. So it actually good for mechanical properties of the steel. On the other hand, it makes steel quite brittle. So, this is why we do not use pig iron directly to produce the steel product. We need to decrease common contents up to lower amount to allow the steel product quite machinable and malleable. Second, sulfur. When sulfur excite in steel, then it actually encourages machinability of steel product. This is necessary property. However, the sulfur tend to segregate on the grain boundaries of steel. So, sometimes, it causes hot shortness. Hot shortness means, sulfur is binding to Fe, so it forms iron sulfide, which is quite low melting temperature. So, during the processing of the steel, it melts along the grain boundary, then it caused the failure of steel product. So it also needs to be reduced. The third one is silicon. Silicon increases strengths of steel product. This is good fact. However, it lowers toughness and also formability. Phosphorus. Phosphorus also encourages machinability of steel product. So, this is good side, but it lowers impact toughness. So, it need to be decreased to certain limit. And there's manganese. Manganese may be considered as alloy element. So, it is necessary to stay inside steel. But sometimes, it gives very bad oxidation resistance. So depending on steel product, manganese needs to be removed in some cases. So, this is why these five elements are to be removed during the steelmaking process. Then, now, let's look at the removalisation of each element. In this table, we have desulfurization, desiliconization, dephosphorization, and decarburization. The four reactions. And what is the character of these reactions and how this is effectively removed during steelmaking process? The first, sulfur. Sulfur is removed into slag in form of sulfide ions. In this reactions, oxygen is not necessary. On the other elements like silicon, phosphorus and carbon, they all need to react with oxygen, then forms silica and ions or phosphate and ions. Or in case of carbon, it forms carbon monoxide gas. So, conditions to remove these elements are different. In case of sulfur, oxygen should not appear. However, for silicon, phosphorus, and carbon, oxygen is necessary. So, this is why the process condition for desulfurization is in reducing condition. On the other hand, removal condition of silicon, phosphorus, and carbon is under oxidizing condition. The process should be distinctive. Second. Sulfur, silicon, and phosphorus. They are removed into slag phase. The sulfur becomes sulfide and ions. Silicon becomes silicate and ions, and phosphorus becomes phosphate and ions. They are removed into slag. So, slag is necessary. On the other hand, carbon is not removed into slag phase but it reacts with oxygen to form gas, carbon monoxide gas. So, the reaction to removal is different. So, in case of desulfurization, desiliconization, and dephosphorization, they are much easily removed when liquid iron contains high amount of carbon. In high carbon liquid iron, they are easily removed. So, this kind of reactions actually prefer high carbon concentration. And since these are removed into slag phase, slag should have a certain amout of light to take sulfur, silicon, and phosphorus and ions into slag. So, reaction is divided in this way that the first under reducing condition when the sulfur is removed into slag phase. Second, [inaudible]. So, oxygen is introduced into the system. Silicon and phosphorus are removed into slag but still, we have high carbon content. Once they are removed, then, addition of oxygen is introduced into the system to take carbon out of hot metal to form carbon monoxide gas. Now, this shows the process of your whole steelmaking process. From [inaudible] , hot metal is transferred from ironmaking plant to steelmaking plant through this torpedo car. This torpedo car delivers hot metal to steelmaking plant. Then, it his tapped into ladle. The first desulfurization is preliminary carried out under existing conditions, and then this processed hot metal is charged into converter shown here. Then, oxygen is introduced first to remove silicon and phosphorus from hot metal under oxidized conditions. When certain amount of silicon and phosphorus are removed from hot metal, slag contains the silicon and phosphorus as impurities. So this slag is now removed. After that, the oxygen is supplied again to decarburize the hot metal. Now, carbon concentration of hot metal is lowered and hot metal is eventually converted into liquid steel. This is steelmaking process. Once liquid steel is produced, then combustion of liquid steel is fine adjusted. This is called secondary steelmaking process.
