Inclusions in steel samples collected 0.5, 2, 5, 10 and 20 min after calcium addition were investigated in Low Carbon Aluminum Killed steel (LCAK steel) with ultra low sulfur content (0.04-0.065% Al, 9 ppm S). It's found that inclusions change from CaO to CaO-CaS and finally to CaO-Al2O3-CaS with time. Al2O3 in inclusions linearly decreases by increasing T.Ca/T.O of the steel and disappears when T.Ca/T.O exceeds 3. CaS/CaO of the inclusions linearly increases by increasing S/T.O of the steel. Contents of CaO, Al2O3 and CaS can be estimated through T.Ca, S and T.O. CaO-CaS clusters are found at 5 min. The mechanism of inclusion evolution and formation of CaO-CaS clusters are discussed.

Inclusion morphology, composition and number at different times of RH vacuum treatment were investigated on API‐X70 pipeline steel with ASPEX. It is found that the inclusions are mainly globular CaO–Al2O3–MgO. The inclusion number decreases with time during RH treatment and the removal ratio can reach 72% after 28 min treatment. The initial number of inclusion plays a vital role in final inclusion number, so efforts must be made to eliminate the inclusion before RH treatment to get a highly clean steel. Non‐liquid inclusions including C3A (C is CaO and A is Al2O3) and Al2O3 · MgO decrease much more quickly and thoroughly than liquid ones, C12A7. This can be explained by the fact that non‐liquid inclusions have a much higher contact angle and interfacial energy but a much lower work of adhesion than liquid ones between inclusion and steel melt.

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Reoxidation usually happens during steelmaking, especially in tundish. Severe reoxidation contaminates the steel cleanliness, and detrimentally affects the steel quality and performance. The purpose of this study is to highlight the adverse effect of reoxidation in tundish on non-metallic inclusion in liquid steel and the end product for Ca-treated Al-killed steel. Inclusions of RH refining, tundish, slab, and plate were investigated on API-X70 pipeline steel. To get a more reliable result, automatic scanning electron microscopy ASPEX PSEM EXPLORER was employed to analyze the inclusion with a much larger scan area. It was found that reoxidation in tundish converted the well-modified liquid calcium aluminates into solid Al 2 O 3 -rich inclusions, which are more likely to cause nozzle clogging than pure Al 2 O 3 , and the inclusion number dramatically increased form 2.8 mm À2 after RH treatment to 13.5 mm À2 in tundish, further decreased to about 6.0 mm À2 in slab and plate. It indicated that reoxidation in tundish put the calcium treatment and strong capacity of inclusion removal during RH treatment in vain. The ratio of liquid inclusions was proportional to average CaO/ Al 2 O 3 in inclusions. Big aggregates and stringer shaped inclusions with CaO/Al 2 O 3 locating between 0.08 and 0.13 were found at the 1/2 and 1/4 slab and plate thickness. The stringer shaped inclusions in the plate were the product of big aggregates in the slab after rolling, and made the plate fail to pass ultrasonic testing.

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