Blast furnace (BF) dust can be recycled as one of excellent secondary sources, in view of the valuable amounts of iron and carbon contents, and the in-flight reduction technology is considered to allow processing the fine metallurgical dusts efficiently. In this work, the reduction behavior of BF dust particles during in-flight process was studied using a high-temperature drop-tube furnace in laboratory scale. The effects of temperature and gas composition on the reduction degree were remarkable. Based on the morphological observation and phase diagram analysis, the product layer of the reduced BF dust particle was confirmed in molten state at 1673–1723 K. Kinetic analysis showed that the gas-molten particle reduction during in-flight process was a mixed control of the chemical reaction and diffusion of Fe3+ and O2– ions through the molten product layer, and the corresponding chemical reaction constant (k r) and the diffusion coefficient (D) was also obtained.

The effects of oxygen, magnesium, and sulfur content in Al-killed tinplate steel on the "liquid zone" of inclusion modification by calcium treatment were clarified through industrial experiments and thermodynamic calculation, and the characteristics of inclusions modification were studied. The results show that the inclusions in molten steel before calcium treatment are mainly Al 2 O 3 inclusions.The inclusions in molten steel are CaO•Al 2 O 3 after calcium treatment with a holding time of 10min, while the inclusions are mainly 3CaO•Al 2 O 3 with a holding time of 30min. And 12CaO•7Al 2 O 3 inclusions are observed in molten steel when T.O content increases to 40ppm after calcium treatment with a holding time of 30min. As the increase of T.O content from 10ppm to 40ppm, the difference between the upper and lower limits of the critical calcium content corresponding to the "liquid zone" increases from 5ppm to 17ppm. The increase of T.O content in molten steel will enlarge the "liquid zone" range of inclusion modification by calcium treatment, and increase the critical calcium content.With the increase of magnesium content in molten steel, the liquid phase ratio of inclusions modification by calcium treatment decreases. To obtain the liquid phase ratio of inclusions at least 50% in molten steel, not only the calcium content in steel should be strictly controlled, but also the magnesium content in steel should not be larger than 15ppm. With the increase of sulfur content in molten steel, the "liquid zone" range of inclusion modification by calcium treatment becomes narrow.

The characteristics of inclusions including composition, morphology, number, and size in tinplate steel were studied by industrial experiments and thermodynamic calculations during the RH refining process. The results indicated that two types of Al 2 O 3 inclusions including cluster and single-particle are generated at first after Al addition. With the slag-metal and refractory-metal reactions, Al 2 O 3 inclusions, CaO•Al 2 O 3 inclusions, MgO•Al 2 O 3 spinel inclusions, and CaO-MgO-Al 2 O 3 ternary system inclusions are found in the middle of RH refining. Only single-particle Al 2 O 3 , CaO•Al 2 O 3 inclusions with high melting point, and CaO-MgO-Al 2 O 3 ternary system inclusions are found at the end of RH refining. From Al addition to the end of RH refining, the total number of inclusions showed a decreasing trend and the proportion of the number density decreased by 70%. About 62% of inclusions are smaller than 10 μm at the end of RH refining, which are difficult to be removed from the liquid steel. The mass transfer of Ca from the refining slag to the liquid steel has a significant effect on the content of [Ca] in liquid steel. Al 2 O 3 inclusions generated in liquid steel can only be modified to CaO•Al 2 O 3 inclusions in the present RH refining time. Aiming to generate 12CaO•7Al 2 O 3 inclusions quickly, moderate calcium treatment as a supplementary measure for refining slag is recommended to modify inclusions during the RH refining process.

The three-dimensional morphology, size, content and composition of large-size inclusions extracted by large sample electrolysis from RH refining to hot rolling were investigated during the tinplate steel industrial test without calcium treatment. The results showed that the large-size inclusions in the RH refining process are Al 2 O 3 inclusions and incompletely modified CaO•2Al 2 O 3 inclusions, while those in tundish are

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