Effect of the moisture content of protective materials on the surface quality of continuous-cast semifinished products and pollution of the air by fluorides

When semifinished products are cast on a continuous caster, the metal is protected by slag-forming mixtures (SFMs) that contain fluorine and have different moisture contents. The specialized firm "Metallurgika" (Germany) makes protective materials for continuous casters operating in different countries and does not allow adsorbed moisture in these products. The finished products are sealed in waterproof packages. The concentration of water of crystallization in the materials at 600 deg C is limited to 0.8%. Other specialized companies such as "Vonderwerk" (Austria) limit the content of adsorbed moisture to 1% (at 105 deg C). The steel cast on the continuous casters at some metallurgical plants is protected by SFMs made by the plants themselves. They are prepared by a simplified method that allows contents of adsorbed moisture up to 0.5-1%. The finished powdered materials are stored in open boxes or waterproof paper bags. In this article, we examine the consequences of using moist SFMs made by a simplified (economical) technology. Statistical analysis of the surface quality of blooms from more than 2000 heats cast on continuous casters during "dry" and "wet" months of the year showed that the average number of heats with a scale rating of four was roughly twice as great in the second case. Thus, the savings realized by making SFMs with a high moisture content are obtained at the expense of an increase in the volume of cast blooms and rolled products that require conditioning. The concentration of hydrogen fluoride in the air in the working zone of caster operators (this zone is 2-m high and 6 m in length and width) was calculated for the most unfavorable conditions, when the plenum ventilation system was not operating. It was established that the concentration of hydrogen fluoride in the air of the working zone of the operators was 0.36 and 3.05 mg /m exp 3 when "dry" and "moist" SFMs were used. Thus, the use of moist (adsorbed water content up to 1%) fluorine-bearing SFMs to protect the metal on continuous casters makes it more difficult to cast semifinished products without external defects and sharply increases pollution of the air by hydrogen fluoride in the working zone of the operators.