Transformation of Inclusions in Pipeline Steels During Solidification and Cooling
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High strength and toughness, good welding performance, and corrosion resistance are essential properties for pipeline steels. As a typical Al-killed, Ca-treated steel, pipeline steel generally has non-metallic inclusions such as the MgO-CaO-Al2O3-CaS system. Inclusions, especially the large ones with string shapes, are extremely detrimental to the performance of pipeline steel as they result in anisotropy and induce cracks. Different inclusions have been controlled, such as liquid calcium aluminate,[1,2] CaO-CaS type[3] and Al2O3-CaS type,[4,5] to reduce their detrimental effects. The formation and evolution of inclusions after Ca treatment in molten steel have also been investigated.[3–10] Inclusions are controlled mainly by adjusting the compositions of steel and slag, which are performed at
WEN YANG and LIFENG ZHANG are with the School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing (USTB), Beijing 100083, P.R. China. Contact e-mail: [email protected] CHANGBO GUO is with the School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing (USTB) and also with the Qingdao Iron and Steel Group Co. Ltd., Qingdao 266000, P.R. China. CHAO LI is with the R&D Institute, HBIS Group Co. Ltd., Shijiazhuang 050023, Hebei, P.R. China. Manuscript submitted February 19, 2017. METALLURGICAL AND MATERIALS TRANSACTIONS B
the steelmaking temperature. However, since the equilibrium constant is closely related to the temperature, the equilibrium between steel and inclusions moves when temperature decreases, resulting in a change in the inclusion compositions.[11] For practical applications, we should also control the composition of inclusions in slabs. Thus, it is necessary to understand the characteristics of transformation of inclusions during the cooling process for better inclusion controlling. In the current work, the transformation phenomenon of inclusions in pipeline steel during the cooling and solidification of steel from tundish to slab was presented through plant trials and includes thermodynamic considerations. The basic composition of the pipeline steel investigated in this work is listed in (Table I). Plant trials for the production of pipeline steels followed the route ‘‘knotted reactor (KR) pretreatment fi basic oxygen furnace (BOF) fi ladle furnace (LF) refining fi Ca treatment fi slab continuous casting (CC)’’ were performed in 300 t ladles with magnesia carbon slag lining. Highly basic, reductive slag was introduced in LF refining for desulphurization. After refining, calcium treatment was carried out by feeding pure Ca cored wires followed by soft blowing. Subsequently, the molten steels were casted into slabs with thickness of 230 mm via an 80 t tundish lined with MgO-based refractory material. Two experimental heats with different Ca contents were considered. Molten steel samples were taken from tundish after two thirds of the molten steel in the ladle being casted, and then quenched in water. After casting, steel samples were taken from th
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