Characterization of the Three-Dimensional Morphology and Formation Mechanism of Inclusions in Linepipe Steels
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THE transportation of oil and natural gas through pipelines requires a good low-temperature impact resistance property of linepipe steels, especially those used in cold areas, and low hydrogen-induced cracking and high strength properties as well. The non-metallic inclusions have a great influence on the properties mentioned above.[1–4] Aluminum is usually used as a deoxidizer for pipeline steels so that the primary oxidation products, such as Al2O3 inclusions, are generated. It was reported that the solid Al2O3 inclusions can attach to the submerged entry nozzle (SEN) during continuous casting and may block the path of the molten steel resulting in early termination of casting.[5–7] Calcium treatment is one of the approaches to prevent nozzle clogging and to improve the continuity and efficiency of continuous casting. Grotts[8] reported that calcium treatment can also enhance the mechanical properties of steel, such as
XUEWEI ZHANG and YANG LIU, Ph.D Candidates, LIFENG ZHANG, Professor, and WEN YANG and YI WANG, Lecturers, are with the School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 100083, People s Republic of China. Contact e-mail: [email protected] YUANCHI DONG, Professor, is with the School of Metallurgical Engineering, An-hui University of Technology, Maanshan 243002, An-hui Province, People s Republic of China. Manuscript submitted March 19, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS B
toughness and ductility. Bielefeldt et al.[9,10] studied the characteristics of CaO-Al2O3 inclusions in linepipe steels. Previous investigations[11,12] have been carried out to modify the type of CaO-Al2O3-SiO2 inclusions into low-melting calcium aluminates in the ternary phase diagram, which has a certain deformability during rolling process and can reduce the adverse effect of inclusions on the properties of steel. Wang et al.[13] found severe stringer-shaped B-type inclusions in X80 linepipe steel plates, which are detrimental to the mechanical properties. They further proposed to remove large-sized inclusions as much as possible before calcium treatment. So that the subsequent calcium treatment was efficient to modify small alumina inclusions and calcium aluminates to small solid CaO-CaS-type inclusions, by which the long stringer inclusions in the plate were diminished. Holappa et al. and Verma et al.[14,15] reported that through calcium treatment, calcium aluminates were formed from the original alumina inclusions by the wire-feeding process and the calcium-rich phase was formed and had a high sulfide capacity so that the majority of the sulfur in steel reacted with calcium-rich phase at the outside of inclusion and formed an inner calcium aluminate core wrapped by CaS phase. Some researchers[16–18] reported that the ratio of calcium and sulfur ([Ca]/[S]) in molten steel was applied to change MnS into CaS in order to prevent the hydrogen-induced cracking. Furthermore, [Ca]/[S] could also indirectly reflect the modification degree of sulfide,
where the ratio with
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