Sürekli Döküm Slab Üretimindeki Tandişlerde Çarpma Havuzunun Etkisinin İncelenmesi

Abstract

andiş, çelik üretimde kullanılan çelik potaları ile slab kalıbı arasında bulunan ve sıvı çeliğin kalıba dökülmesinde kullanılan hazne olup aynı zamanda kalıp içerisine sıvı çeliğin dağıtımını sağlar. Tandiş, çeliğin mikro ve makro inklüzyonlardan arındırılması, sıcaklık ve kompozisyon kontrolünün artırılması ile temiz çelik üretmek amacıyla kullanılır. Bunun için de tandişte, sıvı çeliğin homojen dağılmasına yarayan ve ayrıca inklüzyonların yüzdürülmesini sağlayarak temiz çelik üretimine katkı sağlayan refrakter çarpma havuzları kullanılmaktadır. Bu amaçla Vesuvius firmasından temin edilen Turbostop adıyla da anılan hazır tandiş çarpma havuzları İsdemir slab tandişlerde kullanılmaktadır. Tandiş çarpma havuzu kullanımıyla tandişteki sıvı çeliğin akışının düzenlenip sıvı çeliğin homojen hale getirilmesi, havadan azot kapmalarının azaltılması ve böylece tandişteki inklüzyon sayısını azaltarak temiz çelik üretimine katkı sağlanması amaçlanmaktadır. Dünyada da tandiş tasarımlarında yaygın şekilde kullanılan, fiilen çalışan tandişlerin ölçeklendirilip laboratuvar koşullarında şeffaf tanklarda sıvı çelik su ile cürufun ise boyar madde ile simülasyon analizleri yapılmaktadır. İsdemir’de kullanılan slab tandişlere göre ölçeklendirilen tandiş çarpma havuzların, tandiş su modelleme başarılı sonuçlanmıştır. Tandiş su modelleme uygulaması sonucunda fiili ortamda incelenmesi amacıyla tandiş çarpma havuzu denemeleri yapılmıştır. Deneme süresince ilk olarak sıvı çeliğin pota metalurji istasyonundan kalıba dökülmesine kadar olan sürede havadan azot kapmaları incelenmiştir. İnceleme sonucuna göre azot kapmaların azalması ile ilişkilendirilebilecek veri elde edilememiştir. İkinci olarak tandiş çarpma havuzunun 35 ara vermeksizin ard arda dökümlerde yapısını koruyarak tandişteki homojen yapıya etkisi incelenmiştir. Döküm sonrası incelemelerde yapısını koruduğu gözlenmiştir. Üçüncü olarak üretilen çeşitli kalitelerdeki slabların sıcak haddeleme sonrasındaki bobinlerde makro inklüzyon sayısı analizi yapılmıştır. Devam eden süreçte ve aynı koşullarda analizde kıyaslama yapılabilmesi için tandiş çarpma havuzu kullanılmayan tandişlerde dökümler de değerlendirilmeye alınmıştır. ASTM E45 standardına göre bobinlerden makro inklüzyon numuneleri alınarak analizleri yapılmıştır. Analizlerin sonucunda bobin numunelerindeki makro inklüzyon çeşidi ve sayısına bakıldığında tandiş çarpma havuzu kullanılan ve kullanılmayan slab tandişlerde makro inklüzyonların azaldığına dair herhangi bir veriye ulaşılamamıştır. Ayrıca makro inklüzyon incelemesi sonrasında seçilen numunelerde elektron mikroskobu ile yapılan EDS analizinde inklüzyonların sülfit ve alumina oranlarının yüksek olduğu görülmüştür. Sonuç olarak 12 adet denemesi yapılan tandiş çarpma havuzu ile üretilen slablarda havadan azot kapma sayısında azalma ve haddelenen bobinlerde makro inklüzyon sayısında azalma ile ilgili olarak kıyaslanabilir bir fark görülmemiştir.

Steel cleanliness is an important factor of steel quality and the demand for cleaner steels increases every year. The so-called clean steel generally is the steel in which the content of impurity elements, such as phosphorus, sulphur, total oxygen, nitrogen, hydrogen (including carbon sometimes) and inclusions are very low. The improvement of steel cleanliness has therefore become a more and more important subject in the development of ferrous metallurgical technology, and also an important task for the iron and steel producers. The demand for better mechanical properties of steels was urging steel producers to improve cleanliness of their final products. In order to obtain the satisfactory cleanliness of steel it is necessary to control and improve a wide range of operating practices throughout the steelmaking processes like deoxidant and alloy additions, secondary metallurgy treatments, shrouding systems and casting practice. Continuous casting transforms molten metal into solid on a continuous basis and includes a variety of important commercial processes. These processes are the most efficient way to solidify large volumes of metal into simple shapes for subsequent processing. Continuous casting is distinguished from other solidification processes by its steady state nature, relative to an outside observer in a laboratory frame of reference. The molten metal solidifies against the mold walls while it is simultaneously withdrawn from the bottom of the mold at a rate which maintains the solid / liquid interface at a constant position with time. The process works best when all of its aspects operate in this steady-state manner. Molten metal, from some nearby source, is poured into a tundish. Tundish is a reservuar at steelmaking process that located between ladle and slab mold and keeps pouring molten steel to mold as well deliver molten steel into mold. This particular casting operation uses the force of gravity to fill the mold and to help move along the continuous metal casting. It is the job of the tundish to keep the mold filled to the right level throughout the manufacturing operation. Since the metal casting is constantly moving through the mold, the tundish must always be supplying the mold with more molten metal to compensate. During the transfer of metal through the tundish, molten steel interacts with refractories, slag, and atmosphere. Thus, proper design and operation of a tundish are important for delivering steel of correct composition, quality, and temperature. Tundish is used for producing clean steel with decontaminating macro and micro inclusions in steel and improving control of temperature and composition. The main causes for inclusion formation and contamination of the melt include reoxidation of the melt by air and carried over oxidizing ladle slag, any metal splash are heavily reoxidized by the ambient air, entrainment of tundish and ladle slag, and emulsification of these slags into the melt. These inclusions should be floated out of the melt during its flow through the tundish before being teemed into the mold. the mold entrance may be filled with an inert gas such as argon. Both the reoxidation and the slag entrainment generate harmful macro oxide inclusions. Deoxidized melt is commonly refined in the ladle to trim temperature and chemical composition and to remove deoxidation products. Ladle refining usually removes most of the large indigenous inclusions, leaving only a small amount of inclusions of up to 20-50 μm in diameter suspended in the melt. Inclusions greater than about 50 μm, often of exogenous origin, may be called macro inclusions. Inclusions smaller than about 50 μm, mostly of indigenous origin, and small agglomerates of the indigenous inclusions may be called micro inclusions. In tundishes, refractory impact pads are used to produce clean steel with dispersing molten steel homogeneously and float inclusions. Ready to use tundish impact pads are also known as Turbostops. Designed to redirect steel upward and outward for enhanced steel residence time in the tundish and prevent short circuiting to the closest strand. Also used for optimized drainage and yield enhancement for multiple radical grade change sequences. The tundish impact pad includes a base having a base surface with a conical impact surface area establishing an apex, a sidewall, and a top wall extending inwardly relative to the sidewall to terminate at an inner edge establishing a mouth opening spaced above and centered relative to the apex. The top wall includes a lip sloping radially inwardly and downwardly towards the conical impact surface. The top wall of the tundish impact pad features a lower surface that, collectively with the base surface and a continuous inner surface of the sidewall, establish a continuous annular chamber configured to reduce turbulence of an incoming ladle stream of molten liquid steel. The tundish impact pad is made of a high alumina material suitable for the intended use in a caster tundish for molten steel processing. High alumina material have high impact and abrasion resistance, high hot strength and refractoriness, and good castability. It has simulated melt flow and inclusion removal aspects by physical models before actually using the design in industrial production. In physical modeling, a low temperature aqueous analog, generally water, is used to represent molten metal in a tundish. Water flow in a transparent model tundish can be used to observe melt flow physically taking place in an actual tundish. In physical modeling, 1/3 reduced scale tundish model may be designed based on appropriate Froude similarity criteria in which the flow of molten metal is simulated by the flow of water. Isdemir supplies slab turbostops from Vesuvius Refractory Company. The supplied tundish impact pads are used for controling molten steel flow and homogeneous liquid steel formation in order to produce clean steel. By controlling the molten steel flow the vortex formation can be decreased and these will cause decreased nitrogen pick up from air and reduced inclusions. For demonstrating molten steel behaviour at tundishes water modelling is used. At this model in a transparent vessel molten steel is simulated with water and slag is simulated with dye. By scaling actual tundish at labaratory condition we can predict slag and steel behaviour in a tundish. According to water modelling test result, fastest transition at ladle change, less surface turbulances at ladle change and supresses splashing at start of casting has occur. Water modelling test results at tundishes that were using turbostops for İsdemir was resulted successfuly. After successful results, it has been decided to use tundish impact pad at actual condition. 12 tundish impact pad was ordered for trial. Measurement was made with OES 4460 optical emission spectrum which measure spectrumof frequencies of electromagnetic radiation emitted due to an atom or molecule making a transition from a high energy state to a lower energy state. They were measured from ladle furnace and tundish mold chemical compositions. 15 tundish impact pad specimens were aligned ASTM A53 10 Grade A steel chemical composition and they all were in range of chemical composition. At first trial in elapsed time between ladle furnace station until the pouring period nitrogen pick up of molten steel is analysed. 12 tundish impact pad was used for trial. But one of them couldn’t be used because of operational condition. Measurement was made LECO TC500 which measure oxjgen and nitrogen pick up amount in tundish using infrared light. Nitrogen pick up measurement was taken from ladle furnace and tundish mold. As a result of nitrogen pick up measurement there wasn’t any difference between mold nitrogen pick up amount and ladle furnace nitrogen amount. The nitrogen pick up analyse result did not show any significant change associated with tundish impact pad useage. At second trial the durability of tundish impact pads is examined after 35 heats. 12 tundish impact pad was used for trial. But one of them couldn’t be used because of operational condition and one of them couldn’t be used due to tandish impact pas was worn off by calcium added steel melt rapidly. After 35 heats it has been monitored that tundish impact pad is still preserved its’ original form and there is no significant wearage. After 35 heats traditional tundish impact pad was floating on melting steel at tundish and for this reason wasn’t preserved its’ original form. At third trial, macro inclusion severity is investigated for different quality slabs after coil production. Along trial ASTM A53 10 Grade A steel was used due to its quality steel rather than the appearance of the mechanical properties is important because it is used in tinned sheet production. Macro inclusions after acid lowers product quality and galvanizing processes become visible and in some cases for examination was made of the steel due to be scrapped. Measurement was made Parysetc which measure defecting images by image merging and by using the extended dynamic range based on the superior image quality captured from the sensor hardware. When macro inclusion on coil was observed they was left off and sent to labaratory. They were measured with optical miscroscope which analyze area via Clemex Cir software. They were analysed 4 tundishes without impact pad and 11 tundishes with impact pad. As a result of analyse most of macro inclusions were D type globular inclusion like calcium alumina slicate they werent observed any decreasing amount of macro inclusion.Tundishes without impact pads compared with used ones at continuing process in same condition. Macro inclusion samples taken from coils were analysed according to ASTM E45 standarts. Analyse results showed similar inclusion content results between tundish impact pads used tundishes and unused ones. In addition, after macro inclusion test sulfide and alumina substance in inclusion severity is measured with SEM EDS analysis. Measurement was made JEOL JSM 6510 scanning electron microscobe which produces images of a sample by scanning it with a focused beam of electrons and Energy-dispersive X-ray spectroscopy (EDS) that a method used to determine the energy spectrum of X-ray radiation. High level of sulfide and alumina is dedected. After different trials no substantial data was achieved that showing decreasing nitrogen pick up from air and macro inclusion severity. As a result they were used 12 tundish impact pad for trial. Firstly there was no any substanial data was achieved that showing decreasing nitrogen pick up from air. Secondly After 35 heats tundish impact pad is still preserved its’ original form and there is no significant wearage. Thirdly there was no any substanial data was achieved that showing decreasing amount of macro inclusion on coils. At last they were observed that most of inclusion were D type globular inclusions.