《Table 1.Average steel composition and temperature at the end of the RH refinement process》

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《Novel mechanism for the modification of Al_2O_3-based inclusions in ultra-low carbon Al-killed steel considering the effects of magnesium and calcium》

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Note:a[O]is oxygen activity of oxygen in liquid steel.

Industrial trials were conducted on ultra-low-carbon Al-killed steel coil in a 120-t ladle in a steel plant in China.Two routes were performed to evaluate the different behaviors of inclusions with different Ca and Mg contents during the secondary refining process.The first route（Type A）followed the sequences of basic oxygen furnace（BOF）–ladle furnace（LF）–Ruhrstahl-Hereaeus（RH）–continuous casting（CC），whereas the second route（Type B）followed the sequences of BOF–RH–CC.The main difference between the two routes is that Ca treatment was performed at the end of the RH refinement process in the first route，whereas Ca treatment was not performed in the second route.Correspondingly，liquid steel treated through the first route had higher Ca（20–50 ppm）and Mg（10–20 ppm）contents，whereas that treated through the second route had considerably lower Ca and Mg contents，as shown in Table 1.Table2 lists the slag composition，which was computed as the average of five heats，at the end of the two RH refinings.Highly basic refined slags were used in the two secondary refining routes.However，the slag used for the second route had lower basicity and higher oxidizing component（Fe O+Mn O）content.Steel samples were taken out after calcium wire feeding at the end of the RH refinement process（approximately five minutes after Ca treatment），as well as in the tundish.The activities of dissolved oxygen in liquid steel and steel temperatures were measured with an oxygen probe and a sensor.Steel and slag samples were then prepared for chemical analysis.The composition of the steel samples was analyzed through inductively coupled plasma–atomic emission spectrometry and with a carbon/sulfur analyzer.The slag composition was analyzed using an X-ray fluorescence spectrometer.The morphology，size，and composition of inclusions in cross-sectioned steel samples were detected and analyzed through scanning electron microscopy（SEM）and energy dispersive spectroscopy（EDS）.Each route was measured five times.Approximately 20 inclusions were examined in accordance with a designed direction per sample.Fact Sage 6.4 was employed to calculate the component activities in liquid inclusions for thermodynamic analysis.