In situ Observation of Calcium Oxide Treatment of Inclusions in Molten Steel by Confocal Microscopy
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The demand for clean steel production is increasing in engineering industries to produce ultra-clean steels with superior mechanical and performance properties for various structural applications. Non-metallic inclusions in steel are harmful as they cause nozzle clogging during steel processing and also deteriorate the mechanical and performance properties of the final product.[1–3] During secondary steel making process, steel is de-oxidized with
BHARAT KHURANA and GOUR GOPAL ROY are with the Department of Metallurgical Engineering, Indian Institute of Technology, Kharagpur, West Bengal, 721302, India. STEPHEN SPOONER and PRAKASH SRIRANGAM are with WMG, University of Warwick, Coventry, Warwickshire, CV4 7AL, UK. M.B.V. RAO is with the R&D Department, Visakhapatnam Steel Plant, Visakhapatnam, Andhra Pradesh, 530031, India. Contact e-mail: [email protected] Manuscript submitted 13 September, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS B
aluminum which results in the formation of alumina inclusions in steel. While small inclusions are less harmful, large inclusions can often be detrimental. They are sites for crack initiation. They also hinder performance in final products through reduced mechanical strength, toughness, corrosion resistance, and surface quality. In general, inclusions are removed by absorption into ladle slag during secondary steel making process. Since removal of all inclusions is not possible, residual inclusions are treated with calcium wire injection in molten steel which results in transforming solid alumina inclusions to liquid calcium aluminate inclusions, which helps in preventing the nozzle clogging.[4–6] Further, the mechanical properties of calcium-treated steel are improved in comparison to non-treated steels. Understanding inclusions morphology, distribution, and their agglomeration behavior is essential to further improve steel cleanliness and mechanical properties of the steel. However, several research studies were performed in the past to understand the effect of calcium treatment on morphological changes of oxide inclusions in steels, but still there is lack of information on understanding of clustering behavior and kinetics agglomeration of inclusions before and after calcium treatment.[6–8] Since steel is a high-temperature material, studying the clustering behavior of non-metallic inclusions in molten steel is a very challenging task. Confocal microscopy is an excellent technique which enables in situ observation of phase transformations at high temperatures in metallic alloys such steels, titanium, and nickel alloys.[9–11] HT-CSLM has been used by several researchers to investigate the agglomeration behavior of alumina inclusions in molten steel.[12–15] However, all these studies involved performing two experiments using two different samples collected from steel plants; one sample after aluminum killing of steel and the other after calcium treatment of aluminum killed steel. For the first time, we present in situ observation of calcium treatment of inclusions in liquid s
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