Now let's look at sequence of the steelmaking processes. The hot metal was well treated to remove sulfur, silicon, and phosphorus this will be decarbonized. This is supposed to be put into converter but before hot metal is charged it into converter, we use the scrap first. Scrap is first charged into converter the reason why we use a steel scrap is in several forth. The first scrap itself is good ion source. This was already well treated this required good steel source. Second, during the steelmaking reaction the temperature of this converter encourages enormously high sometimes we may need cool down this whole process. The steel scrap charges from ambient temperature, so it could observe heat inside converter. So it works as the coolant. Third one using this hot metal which contains carbon means during the steelmaking reaction, it involved carbon monoxide gas. This is unwanted in terms of environmental issues, but steel scrap contains very little amount of carbon so it does not emit carbon, so this is also good for environment reasons and fourth using steel scrap means steel is well recycled. This is very good effect of using steel so this is why use steel scrap. Steel scrap is first charged into converter then hot metal is charged over this scrap, then this converter is tilted back. Once converter is tilted back in this way, then oxygen lance is lowered from upper portion to inside this converter. During reaction, temperature inside this converter becomes very high so this lance must be protected by water-cooled lance. This lance has several holes to deliver oxygen gas, the blowing time and blowing rates for oxygen is not constant during reaction, but it varies, so this steelmaking reactions is very dynamic. First when lance is lowered then it's position is relatively high then first oxygen gas is blown very softly. So first it forms very oxidic condition on the surface of hot metal so slag oxidation occur, then the next batch the lance is lowered a little more and oxygen blowing a bit more to form the slag. First this slag is many composed of the iron oxide and silicon oxide, and of course to form more effective slag we add some flux like lime, or dolomite. What is dolomite or lime, will be discussed later. Then the oxygen lance is lowered a bit more and major reaction start the oxygen is blown at high speed, the speed and position of this lance is compromised between high-carbon removal rate but the slag formation. Why? During the oxygen blowing, if the lance position is too high or if oxygen which is delivered, is injected very slow, then the oxygen is delivered just on the surface of this hot metal. Then, at the surface silicon and carbon are oxidized, but it's yield is concerned that Fe becomes oxidized. So in this case the slag is all oxidized and having high iron oxide concentration slag. This lowers the yield of steelmaking and also, it increases the slag volume if there are too much slag inside this vessel then sometimes this slag is going out. This is called slopping and the injected oxygen is not effectively used to decrease carbon content. So this actually lowers the carbon removal rate. On the other hand, if the oxygen lance is lowered too much, then it could increase rate of carburization. However, the tip of this lance is too close to liquid iron or liquid steel so often cause spitting of metal droplets. The metal droplets go everywhere. This is all unwanted situation. So lance position, the blowing rate or injected rate of oxygen should be compromised during reaction and during oxygen injection. Some amount of flux is added, this is to make slag more effectively used during steelmaking reactions. During the reactions, this slag should be used to control the chemistry of slag and to absorb some impurities like sulfur or phosphorus. For that purpose the main the flux which is use, is lime. Lime is the major element, major component in the steelmaking slags. This is effectively used to take the phosphorus also sulfur. the other flux is the dolomite. Dolomite is providing magnesium oxide into slag. The reason adding this magnesium oxide containing flux is the refractory composing this the converter is easily eroded by steelmaking slags. Steelmaking slag contains such amount of iron oxide this easily eroded the refractor wall. The refractory wall is actually composed of magnesium oxide, so in order to treat chemical erosion, the magnesium oxide is intentional added to steelmaking slag to protect refractory wall and in order to improve the fluidity of slag. Sometimes calcium fluoride or fluorspar is added but due to environmental issue nowadays using calcium fluoride is quite limited and as I told you before during the reaction the temperature increases enormously if temperature increases up to a level of which we want, then it's quite successful cases. However, sometimes, before the converter reaction is finished, the temperature of liquid steel goes too high. In such event we need to put some coolant to decrease the temperature inside reaction vessels, the typical coolant is scrap which we already mentioned and iron ore is also add, iron ore provides iron, but it also works as very effective coolant. This is more efficient coolant than scrap because iron ore is added to this vessel, the iron ore is reduced by carbon, then this reduction absorbs a lot of heat, so this is very effective coolant and some other coolant like the produced pellets or limestone which is actually flux but this also lowers the temperature inside. Now let's get detailed over refined reactions inside, the graph shown here shows concentration change of these five elements carbon sulfur, phosphorus, manganese and silicon as a function of reaction time. Among these five elements, silicon is most attractive element to oxygen, so when oxygen blowing is start silicon is the first element to oxidize. Silicon concentration quickly decreases, then it becomes silicon oxide formed slag with added lime and dolomite lime and iron oxide so it is a form of slag. How much silicon is oxidized? this is very important because amount of silicon oxide is related to heat generated by this reaction and temperature inside this vessel. And second, oxidized amount of silicon is directly related to amount of slag formed inside control of silicon level during the steelmaking reaction is very important. Second phosphorus. This is also an element to be removed during the steelmaking reactions. Oxidation of phosphorus is favored under oxidizing conditions so when slag has high iron oxide content, the phosphorus level could lowered easily and if slag has more lime, high CaO ratio, the phosphorus oxidation or dephosphorization is taking place easily. So at first when oxygen is provided to the system. The phosphorus level quickly decreases during the reactions the phosphorus level in the hot metal increases back. This is due to consumption of FetO in the slag by carbon. Due to decarburized reactions so during the steelmaking reactions phosphorus level slightly goes up. But later when carbon level decreases further then the available iron oxides can be used to take phosphorus into slag so this is typical change of phosphorus. Manganese looks very similar to the phosphorous it will catch phosphorus more effectively. Opposite, flux like lime should be added to the system and next the very important relation carbon oxidation or decarburization. In decarburization we may look at this carbon content change in three different steps. The first step, the carburization rate is not as fast as that of silicon. This is because provided oxygen is first used to take silicon out of hot metal so during this step the decarburizing rate is not fast enough. However once silicon level becomes very low then supplied oxygen is effectively used to take carbon, so this is a main decarburization step. Decarburization becomes very fast. However at the later stage of oxygen blowing decarburizing rate beco mes slow again, this is because carbon concentration in liquid steel becomes very low, and carbon mass transfer in liquid steel, actually dominates decarburizing rate. This is why decarburizing rate at the last stage of blowing becomes slow back again and in case of sulfur level does not change that much. This is because sulfur is removed under reducing condition not oxidation condition. So this is why sulfur methods does not change that much so it means desulfurization should be carried out in advance to steelmaking reaction. This is why we have hot metal pre-treatment. So in this way, concentration of all the impurities could be controlled during steelmaking reactions.
