Formation Mechanism of CaS-Al 2 O 3 Inclusions in Low Sulfur Al-Killed Steel After Calcium Treatment
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INTRODUCTION
CALCIUM is added to Al-killed steel for modifying solid alumina inclusions. Through proper calcium treatment, Al2O3 can be converted into low melting point calcium aluminates.[1–5] There are two methods for predicting the composition of inclusions after calcium treatment: One involves monitoring of the total calcium (T.Ca) and acid-soluble aluminum ([Al]s) contents of the steel,[6,7] e.g., the alumina inclusions are converted into liquid calcium aluminates when T.Ca/[Al]s is larger than 0.09. The other involves monitoring of the T.Ca and total oxygen (T.O) contents of the steel,[8,9] and a T.Ca/ T.O of 0.91 to 1.25 has been proposed for ensuring good modification results. However, calcium aluminate inclusions adversely affect steel properties in the case of some steel grades, e.g., API X80 pipeline steel. This is because they can aggregate to larger inclusions of 10 to 20 lm in continuous casting and can be deformed into stringershaped inclusions in steel plates during rolling due to their good deformability,[10,11] which results in the amount of B-type inclusions in rolled steel plates being higher than the standard value. Thus, inclusions with the high melting point and poor deformability are favorable in these rolled steel plates, and some studies[10,11] found that the severities of B-type inclusions can be lowered from ‘‘2.0’’ to ‘‘0’’ (ASTM E45 standard) when the inclusions found in the steel plates were all JIANFEI XU, Ph.D. Student, FUXIANG HUANG, Doctor, Lecturer, and XINHUA WANG, Doctor, Professor, are with the School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, 30 Xueyuan Road, Haidian District, Beijing 100083, P.R. China. Contact e-mail: [email protected] Manuscript submitted on November 19, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS B
composed of the high melting point CaO-CaS system. However, CaO-CaS inclusions were obtained in steel with a high T.Ca content,[12,13] which increases the cost of production. Meanwhile, it is found from the CaO-Al2O3-CaS ternary phase diagram that CaS-Al2O3 also has a high melting point and a poor deformability compared with CaO-CaS, and a new strategy for controlling the formation of CaS-Al2O3 inclusions in rolled steel plates was adopted. However, the formation of CaS-Al2O3 inclusions after calcium treatment has seldom been reported. Verma and Ren et al.[14–17] found that CaS-Al2O3 inclusions were formed initially after calcium addition, and then, they converted to calcium aluminates. Based on this, they proposed that CaS-Al2O3 inclusions are the intermediate products in the formation of calcium aluminates via calcium addition. However, the influence of T.O content on the modification of Al2O3 inclusions by calcium treatment was not evaluated in their experiments, and hence, the formation of CaS-Al2O3 inclusions after calcium treatment was not adequately clarified. In this study, the laboratory experiments of alumina inclusions modified by calcium treatment in Al-killed steel were performed and the characteri
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