The coke predoxidation process and the princible for recuding aluminium consumption

Steel is an essential material for all human kind. It is fundamental to aspect our lives. Every part of our life, we built on it. Despite being very essential, we have to pay some worth. Recently, in the world, all the materials for making steel such as raw materials, fluxes, deoxidants, scraps and gases have a cost. And the cost of that material has been rising day by day. Deoxidant and alloying materials come third after hot metal and the scrap costs. Deoxidation is the method of removing oxygen from molten metal and it is the last part of the steelmaking process. In the Basic Oxygen Furnace (BOF) steel bath contains 400 to 800 ppm activity of oxygen because of the using oxygen for refining, reducing anti purity elements such as carbon, silicon, phosphorous, sulphur etc… Solubility of oxygen in steel is negligibly small. During solidification of molten steel, excess oxygen is rejected by the solidifying steel. Solubility of oxygen in liquid steel is 0.23 % at 1700 deg C. It decreases during cooling down process and then drops sharply during the solidification of liquid steel reaching 0.003 % in solid steel. The excess oxygen liberated from the solid solution oxidizes the components of steel such as C, Fe, and alloying elements resulting into blowholes and non metallic inclusions entrapped within the cast steel structure. Both blowholes and inclusions have considerable effect on the mechanical properties and impact adversely the steel quality. In order to prevent oxidizing of steel components during solidification the oxygen content of liquid steel need to be reduced. This is done by deoxidation of steel which is a steel making technological operation, in which concentration (activity) of oxygen dissolved in liquid steel is reduced to a required level. In addition to production of sound steel by eliminating blowholes and minimizing of non metallic inclusions, deoxidation is also employed for grain size control to enhance the toughness of the steel. Several strategies have been developed for deoxidation of steel. This may be accomplished by adding metallic deoxidizing agents to the liquid steel either before or after it is tapped, or by vacuum treatment, in which carbon dissolved in the steel is the deoxidizer. Besides deoxidation by metallic deoxidizers and deoxidation by vacuum, one more method of deoxidation namely diffusion deoxidation is sometimes employed. The process involves adding materials with a high affinity for oxygen, the oxides of which are either gaseous or readily form slags. Deoxidation is carried out during tapping by adding into the tap-ladle appropriate amount. There are primarily three elements used in steel deoxidation of steel. These are manganese (Mn), silicon (Si) and aluminum (Al). Manganese and silicon are added in the form of either high carbon or low carbon ferro alloys or as silico-manganese (Si-Mn) alloy. Aluminum which is added for deoxidation is having a purity level of around 98 %. Sometimes calcium (Ca) is also used for deoxidation. Researchers, engineers have been studying on cost saving in steel making and they found several saving such as reducing deoxidant materials by using appropriate amounts of coke instead before feeding aluminium during the tapping of the steel to the steel ladle from the convertor. So, in this study, the aim was to reduce the cost of primary steel deoxidation by substitution of aluminium primary deoxidation with a practice that uses carbon. The cost of carbon is four times lower than that of aluminium. The application of carbon would directly result in raw material cost savings. Investigations were conducted at Erdemir Steel plant on 7112K steel grade. In this study below issues were examined; ➤ Cost saving with using coke in the deoxidation part ➤ Ideal material adding time ➤ Steel cleanliness In the deoxidation, the feeding time of the deoxidant into the ladle is very important as well as the type and the amount of the deoxidant material. Whole heats of the year were examined and the calculation of the feeding time was calculated via regression formulas. So, ideal feeding time defined into the steel making process model. Due to the nature of steel making process, it is impossible to produce inlusion free steel. Based on this fact, the investigations on clean steel are concentrated on the reduction of inclusions and conversion of inclusions into harmless state by modification. The definition of steel cleanness is dependent upon the final product applications. Non-metallic inclusions become important when they are responsible for producing defects during steel process or in the final product application. Aluminum-killed steels contain Al2O3 and MnS inclusions. Alumina inclusions are formed during deoxidation or reoxidation in steel making and continuous casting process, and particularly harmful because they tend to clog the refractory nozzles, show dendritic fracture during hot rolling. During solidification of steel, Mn and S are rejected from the solidifying dendrites, causing an increase in their concentration in the remaining liquid, which finally leads to MnS precipitation in the interdendritic spaces towards the end of solidification. MnS inclusions are readily deformable at hot rolling temperatures, thus gets elongated to long stringers, causing anisotropy in mechanical properties of hot rolled steel products. Both Al2O3 and MnS inclusions have adverse effects on final product mechanical and physical properties causing cracking, fracture, tearing, surface defects etc. during forming and welding. In addition to all of these studies, the coke which is fed as predeoxidation, inclusion analysis was investigated whether the effect of steel cleanliness. According to the results, obtained from the examination of the coke used and unused, there were no differences in the inclusion analysis. Analyses are similar to each other. Departing from this, coke using as predeoxidation, there was not the negative impact of steel cleanliness. When coke is added to the steel, it will react with oxygen, sulfur and modify the sulfide and Al2O3 inclusions. Finally, this study showed us that carbon deoxidation is useful application for cost saving. Carbon primary deoxidation has been implemented at Erdemir Steel Plant on a large range of steel grades, ranging from ultra high carbon (> 0.60% C) to ultra low carbon (< 0.03% C) grades. With using this kind of cost saving method in steel making, a net saving between 400.000 $ and 800.000 $ was achieved.