Alüminyum tüketimini azaltmak için kok ile ön deoksidasyon işlemi ve prensibi
The coke predoxidation process and the princible for recuding aluminium consumption
Yazar: YILMAZ TETİK
Danışman: PROF. DR. MUSTAFA KELAMİ ŞEŞEN
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Metalurji ve Malzeme Mühendisliği Ana Bilim Dalı / Üretim Metalurjisi ve Teknolojileri Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2015
Konu: Metalurji Mühendisliği = Metallurgical Engineering

Özet:
Çelik insanoğlu için vazgeçilmezdir. Hayatımızın her alanını ona göre inşa etmekteyiz. Bu vazgeçilmeze sahip olabilmemiz için bir paha ödememiz gerekmektedir. Dünyadaki çelik üretiminde, hammaddeler, fluks malzemeleri, deoksidantlar, hurda ve gaz maliyetleri günden güne artmaktadır. Maliyet kalemlerinden olan alaşımlama ve deoksidant malzemeleri maliyetleri, sıvı ham demir ve hurda maliyetlerinden sonra gelmektedir.
Deoksidasyon, çelik üretimin son aşamasıdır ve ergimiş metalden oksijeni alma metodudur. Bazik oksijen fırınlarında, istenmeyen safsızlık elementlerinin rafine edilmesi için oksijen kullanımı nedeniyle, çelik banyosu üfleme sonunda 400 -800 ppm arası oksijen ihtiva eder. Deoksidasyon prosesi, oksijene afinitesi yüksek olan metallerin istenilen miktardaki ferromangan, ferrosilis, alüminyum veya özel deoksidantların sıvı çelik içerisine beslenmesiyle olur. Çelik banyosundaki çözünmüş oksijen, beslenecek deoksidant malzemelerinin besleme miktarında ve çelik temizliğinde etkin rol oynar.
Araştırmacılar, mühendisler, her geçen gün, çelik üretimindeki maliyetleri düşürmeye yönelik çalışmalar yapmaktadır. Bu çalışmalardan biri de, BOF prosesinin ürünü olan sıvı çeliğe, potaya döküm esnasında alüminyumdan önce belirli miktarda kok beslenmesiyle, deoksidant ve alaşımlama için kullanılan alüminyumundan tasarruf etmek için yapılan öndeoksidasyon işlemidir.
Bu çalışmada, alüminyum deoksidant malzemenin maliyetinin, döküm esnasında alüminyumdan önce kok beslenmesiyle öndeoksidasyon yapılmasıyla azaltılmasını amaçlamıştır. Erdemir çelik üretim tesisinde, yapılan denemelerle çalışma yapılmıştır. Bu çalışmada;
➤ Kok beslemesi yapılarak Alüminyum tasarrufu
➤ İdeal deoksidant malzeme besleme zamanı
➤ Kok beslenmesinin çelik temizliğine olan etkisi
incelenmiştir.
Deoksidasyonda, deoksidant malzemenin cinsi, miktarının yanısıra potaya beslenme zamanı da çok önemlidir. İdeal deoksidant malzeme besleme zamanı olarak da,  yıl bazında tüm dökümlerin döküm süreleri regresyon denklemleri kullanılarak tahmini döküm süresi hesaplanmıştır. Bu sayede, en verimli olacak şekilde deoksidant malzemelerin potaya besleme zamanları da belirlenmiştir. Bu hesap, çelik üretim sistemi modeline monte edilerek, sürekli hale getirilmiştir.
Tüm bu çalışmaların yanında, öndeoksidasyon olarak beslenen kokun inklüzyon analizleri yapılarak, çelik temizliğine etkisi olup olmadığı incelenmiştir. Yapılan incelemelerden alınan sonuçlara göre kok kullanılan ve kullanılmayan dökümlerdeki çeliklerin inklüzyon analizlerinde farklılık çıkmamıştır. Analizler birbirlerine yakın çıkmıştır. Bundan yola çıkarak, kok öndeoksidasyonun ile çelik temizliğine negatif etkisi tespit edilmemiştir.
Sonuç olarak, konvertörden sıvı çeliğin potaya dökümü esnasında alüminyumdan önce kok beslenmesinin maliyet düşürücü bir uygulama olduğu gösterilmiştir. Karbon öndeoksidasyonu analiz aralıkları ultra-yüksek karbon (> 0.60% C) 'dan ultra düşük karbon (< 0.03% C) çelik kalitelerine de uygulanmaya başlanmıştır. Bu maliyet düşürücü uygulama ile yılda net tasarruf 400.000$  800.000$ arası olabileceği anlaşılmıştır.

Abstract:
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.