Industrial investigation and application on process of Ca increase by ferrosilicon alloy in CAS refining process

To address the challenges associated with conventional Ca treatment during CAS（composition adjustment by sealed argon bubbling） refining, such as low Ca yield, secondary oxidation of molten steel, long treatment period, and high production costs, and meantime to achieve the goal of reducing Ca treatment, a process that eliminated the need for traditional Ca treatment through Ca increase by ferrosilicon alloy was developed. Industrial trials were conducted on the high-silicon weathering steel SPAH（steel plate for atmospheric corrosion resistance-high strength）to investigate the influence of different Ca increase methods on molten steel cleanliness, inclusion evolution, and continuous casting castability during CAS refining process, and the mechanism of melting and inclusion generation of ferrosilicon was analyzed. The results indicate that during Ca treatment process, the average composition of inclusions in the tundish is 75. 5%Al₂O₃-10. 8%CaO-13. 7%CaS. In contrast, for Ca increase by ferrosilicon alloy, the average inclusion composition in the tundish is 84. 3%Al₂O₃-5. 2%CaO-10. 5%CaS. The types of inclusions on the hot rolling sheet under the two processes are similar, predominantly consisting of Al₂O₃-CaOCaS. The number densities of inclusions lager than 5 μm are 1. 19 mm^-2 and 0. 99 mm^-2 for the Ca treatment process and Ca increase by ferrosilicon alloy, respectively. No large-sized inclusions are generated by adding ferrosilicon to the molten steel. Compared with traditional Ca treatment, the number density, average diameter, maximum diameter of inclusions as well as the molten steel cleanliness under the Ca increase by ferrosilicon alloy show less significant difference. The gradual release of residual Ca from the ferrosilicon prevents explosive oxidation, increasing the Ca yield from 12. 5% to 45. 1%. Furthermore, combining alloying and Ca addition into a single step by this process shortens the CAS refining period from 30 min to 22 min, and the fluctuation of stopper rod position and mold liquid level in continuous casting process is stable, so that the nozzle is continuously cast for more than 440 min without replacing. The Ca increase by ferrosilicon alloy can effectively replace the traditional Ca treatment, which provides both theoretical foundation and industrial solution for reducing Ca usage in aluminum-killed steels.