Sanayi tav fırınlarında çelik yüzey oksidasyonu ve verimliliğe etkileri

Abstract

Bu çalışmada çelik yarımamüllerin tav fırınlarında tavlanmalan sırasında fırın atmosferinin oksitleyici etkisinden dolayı, çelik yüzeyinde oluşan oksit tabakasını minimum seviyede tutabilmek için oksidasyon hızının kontrol edilebilme koşullarının incelenmesi amaçlanmıştır. Bu çalışma ile tav fırınlarında doğalgazın fuel-oil yerine daha ekonomik olarak kullanılabileceği gösterilmiştir. Bu amaçla yapılan deneylerle, çelik kütük örneklerinden hazırlanan numunelerin doğalgaz ve fuel-oil ile çalışan tav fırınlarında oksitlenme koşulları incelenmiştir. Deneyler endüstri tav fırın koşullarında gerçekleştirilmiş ve deneylerde küp şeklinde numuneler kullanılmıştır. Endüstri şartlarında gerçekleştirilen deneylerde, doğalgazın tav fırınında çelik yüzey oksidasyonu açısından daha verimli olduğu görülmüştür. Çelik yüzey oksidasyonuna etki eden parametrelerin kontrolü için yapılan deneyler sonucunda, yanma hava miktarının % 5 olduğu 1050°C tavlama sıcaklığında minimum oksit tabakası oluşumu görülmüştür. Tav fırınında yüzey oksidasyonu kinetik açıdan parabolik hız kanununa uygun olarak geliştiği, oksidasyon hızının zamanla azaldığı tesbit edilmiştir. Tav sıcaklık artışının oksit miktarında ve hızında artış getirdiği görülmüştür. Düşük karbonlu çeliklerin, yüksek karbonlu çeliklere göre daha fazla oksitlendikleri tesbit edilmiştir. Çelik tavlama sıcaklığı ve süresi arttıkça, tufal bileşiminde wustit (FeO) miktarının arttığı, buna karşı manyetit (Fe304) ve hematit (Fe203) miktarlarının azaldığı tesbit edilmiş, 1200°C tav sıcaklığı ve 180 dakika tav süresi sonunda çelik oksit tabakasının % 92 FeO, %4 Fe304 ve %4 Fe203'den oluştuğu görülmüştür. Tufalleşme hızı azalırken, metal kaybının azaldığı fakat buna karşılık dekarbürize tabaka kalınlığının arttığı tesbit edilmiştir. Oksit tabaka kalınlık artışının, tav fınn kapasitesinde düşme ve buna bağlı olarak tavlama için daha fazla enerji ihtiyacı gerektirdiği görülmüştür. Fazla hava miktarı arttıkça, daha kırılgan ve metalden kolayca ayrılan oksit tabakası tesbit edilmiştir.

Scale formation during the reheating of steel occurs for the simple reason that the furnace atmosphere has sufficient oxidizing potential to promote the formation of an oxide scale layer. For a similar reason, decarburized of the outer skin of the steel may also occur. Scale formation lowers the efficiency of steel reheating furnaces. Scale is a measure of product yield loss during reheating. In our country, the weight loss in reheating operations varies from 1 to 5 %. In 1993, Rolled steel production in Türkiye was 10 million tons, the average 3 % scale loss represents 300,000 tons of steel. The minimization of scale formation during reheating of steel semi products is important goal of the researchers. There are 3 basic factor that affect the type and the rate of scale formation on any given steel (slab,billet,bloom and ingot) surface and they are temperature, time and atmosphere. The rate of scale formation increases as the surface temperature of the steel increases. The time required to properly heat a steel has an effect on scale formation, the rate of scaling is very rapid when steel is first exposed to sufficient temperature, but this rate of formation decreases the longer the steel remains in the furnace. This occurs because ; 1) The scale acts as an insulating layer for the steel and lowers the temperature of scale/metal surface, thus lowering the scaling rate. 2) The increasing thickness of the scale layer slows the oxygen diffusion to the scale/metal surface, which ultimately lowers the rate of scaling to a negligible level. VII- The composition of the furnace atmosphere is the third basic factor that affects the rate and type of scale formation. Scale formed in an oxidizing atmosphere or with excess air is loose and easily removed. At high temperatures, three oxides of iron are stable and in the scale these form parallel layers, arranged in order of oxygen content. The innermost layer with the lowest oxygen content is wustite, (FeO) the intermediate layer is magnetite, (Fe304) and the outer layer is hematite, (Fe203). The oxidation of steels is far more complex than that of pure iron. The complications arise from the fact that different elements with different properties are interacting simultaneously. It than becomes difficult to give the cause and effect of the behaviour of an element in the alloy. This complexity is a result of some, or all, of the following 1)The different affinities of alloying elements for oxygen. 2)The different mobilities of metal ions in the oxide phases. 3)The different diffusivities of different metals in the alloy. 4)lnternal oxidation of one or more alloying elements. 5)A solid solubility between oxides may exist. Scale growth in oxygen or air is controlled by the diffusion of iron through wustite and is slowed down by the appearance of gaps in wustite which disrupt the outward diffusion of iron. In this situation, the rate of absorption of oxygen into the scale may become slower than the rate of arrival of iron by outward diffusion through the scale. This will affect both the kinetics of scale growth and the structure. When a phase - boundary reaction, such as the adsorption of oxygen at the outer surface, is rate - controlling, the rate of scale growth is independent of thickness, and the amount of scale formed is directly proportional to the time. As the scale thickens the increase in thickness leads to a decrease in the diffusion gradient, this leads to a slow changeover from a linear rate, in which diffusion occurs between boundary conditions that are varying, and finally to parabolic scaling - vih - The formation of scale in the reheat furnace is detrimental to the furnace operation for the following reasons: 1)Scale tends to fall and get accumulated on the bottom zone of the hearths which may cause burner flame deflection in the pusher furnace. 2)Overheating scale of the steel acts as insulation when trying to get heat into the center of the steel. 3)Excessive scale would cause the rolled defects in the product creating a quality problem. 4)Excessive scale formation causes an unnecessary yield loss. 5)lncreased maintenance caused by scale formation results in reduced furnace availability and loss production. 6)Reduced heat transfer results in longer heating times and reduced furnace production. In the present work, the minimization of scale formation during the reheating of steel and optimization conditions in the reheating were investigated. The steel billet samples with three different grades were used ( St 37.2, St 44.2 and St 60.2 ). The natural gas used was that distributed from Luleburgaz-Hamidabad. This gas had a calorific value of 8,600 Kcal/Nm3. Experiments were carried out in the following groups in order to investigate the minimization conditions of the surface oxidation of the steel in the reheating furnace used natural gas and fuel-oil as energy sources. The first group of experiments were carried out to determine the effect of temperature, time and excess air on the surface oxidation of the three different steel grade as weight gain, gm/cm2. IX- The second group of experiments were carried out to determine the effect temperature, time and scale thickness on the scale chemical composition. The third group of experiments were carried out to investigate the effect of surface oxidation of steel on the productivity of the reheating furnace and rolling mill. In the experiments, 4 different temperatures were used such as 1050,1100,1150 and 1200° C. The reheating time was in the range 50- 300 minutes. Steel surface oxidized layers analyzed by means of wet chemical analysis, and examined by scanning electron microscopy, optic microscopy and also x-ray diffraction studies was carried out. The following results were obtained from the experiments: 1) The minimum surface oxidation of the steel occurred in the reheating furnace used natural gas. 2) When more excess air present inside the furnace, the greater is the oxidation of the steel and hence the greater is the scale formation. 3) The chemical composition of steel effects the scaling behavior and the increase in the carbon content decreases the surface oxidation. The carbon diffuses to the oxide-metal interface where it forms carbon monoxide CO as a result of chemical reaction with FeO. In the presence of the carbonmonoxide at the oxide -metal interface, the gap formation between scale and metal enhances and the scale adhesion reduces. 4)The increase in reheating temperature enhances the scaling of the steel. 5) The surface oxidation of the steel obeys the parabolic rate law within 50-300 minute heating time. And the oxidation showed a gas - phase transport and a gap formation mechanism behavior. -x- 6)The scale polished cross section shows three parallel layers, a typical thick porous wustite scale layer followed by magnetite and the outer hematite layer contained fine cavities. 7)As the heating time and temperature increases, the wustite amount of the scale increases but reversely magnetite and hematite decreases. At 1200°C heating temperature and 180 minutes heating time the scale composition was contained 92 % FeO, 4 % Fe303 and 4 % Fe203. 8)lt was also observed that the wustite amount increases with unit weight increase of oxidized layer. 9)The decarburized experiments showed that reducing the scaling rate reduces metal wastage but increases the observed depth of decar- burization. 10) The rate of steel surface oxidation accelerates more rapidly with increasing excess air and the effect of steel quality and type of energy used in the reheating furnaces comes later 11)The experiments showed that the oxidized layer amount effects the productivity of the reheating furnace and rolling mill. 12)lt was observed that the scale formed in an oxidizing atmosphere or with excess air is loose and easily removed.