Influence of Calcium Addition on Inclusions in LCAK Steel with Ultralow Sulfur Content
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INTRODUCTION
ALUMINUM is always used as deoxidizer for strong affinity for oxygen and low cost. The deoxidation product alumina, however, has been proven to be harmful to the steelmaking process and final product, such as submerged entry nozzle (SEN) clogging,[1] sliver in deep drawing sheet,[2] fatigue breakage in bearing steel,[3] hydrogen induced cracking (HIC) in the pipeline,[4] and so on. Calcium treatment is effective to modify Al2O3 into low-melting calcium aluminates,[5–7] which can effectively prevent nozzle clogging. However, liquid calcium aluminates are not always good. Recently, Wang et al.[8] found severe stringer-shaped B type inclusions in API X80 linepipe steel plates, which are detrimental to the mechanical properties. They assumed that low-melting CaOAl2O3 of 10 to 20 lm in cast slabs can be deformed into stringer-shaped B type inclusions longer than 150 to 350 lm. The big CaO-Al2O3 inclusions are from (1) aggregation of small ones after the secondary refining and calcium treatment and (2) large calcium aluminates wrapped by CaS, CaO, or CaO-CaS, which are generated by calcium treatment. They further proposed removing as much as possible inclusions (especially large-sized inclusions) before calcium treatment. With this strategy, liquid calcium aluminates are changed to solid CaO-CaS-type inclusions, and this issue is improved. The mechanism of inclusion variation, however, had not been addressed. GUANGWEI YANG, DIE YANG and XIN HAO, Ph.D. Candidates, XINHUA WANG, Professor, FUXIANG HUANG, Lecturer, and PENGYUAN WEI, Master Candidate, are with the School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, No. 30, Xueyuan Road, Haidian District, Beijing 100083, P.R. China. Contact e-mail: ustb_jack@ hotmail.com Manuscript submitted April 22, 2014. Article published online September 30, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS B
During past decades, intensive efforts regarding calcium treatment have been carried out and were focused mainly on how to change Al2O3 into low-melting calcium aluminates. Few reports were published on how to change Al2O3 into CaO and CaS inclusions. Lu and Irons[9] studied inclusion modification during and after calcium injection in AISI 1045 steel. They found CaO-CaS type inclusions in their experiments. However, the carbon and sulfur contents in steel are high, 0.45 pct and 0.003 to 0.02 pct, respectively. Numata et al.[10] investigated the calcium consumption and calcium addition pattern on the change of inclusion composition. They found CaO-CaStype inclusions in steel with 0.0015 pct sulfur content after calcium addition. The mechanisms of the change of CaO and CaS content in the inclusions during and after calcium addition were found. But they did not give the suitable steel composition for CaO-CaS formation. In the current article, experiments with different calcium addition were carried out to investigate the variation of inclusion composition in low carbon Alkilled (LCAK) steel with ultralow sulfur content. Al2O3 in the incl
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