The processes and products of modern steelmaking are placing increased demands on the steelmaker for continuous improvement of steel cleanliness. It is important that the size, morphology and distribution of inclusions, particularly oxides be controlled and minimized in order to meet increased quality standards.

In addition, particularly due to the advent of thin slab casting, controlled and consistent calcium treatment products, processes and parameters are necessary to maintain good caster quality and throughput with minimal cost and time expenditure.

The steel industry has long been aware of the beneficial aspects of calcium treatment of molten steel. In addition to desulfurization via injection of calcium silicon powder, calcium’s benefits as an inclusion modifier are well known. By reacting with sulfur, alumina and oxygen, steel cleanliness is enhanced and the remaining inclusions are present in less deleterious forms.

Manganese sulfides which typically form elongated stringers are transformed to calcium sulfide, and more importantly for castability, high melting oxides are transformed into lower melting temperature calcium aluminates.

Alumina, the primary deoxidation product in aluminum and aluminum/silicon killed steels has a melting temperature of over 2000°C, well above steelmaking temperatures. These inclusions deposit on the tundish nozzles and SEN tubes and form a buildup of solid alumina interspersed with steel droplets. Eventually one of two things occurs. Either the nozzle becomes totally clogged leading to the end of the cast, or chunks of solid alumina are flushed through the nozzle into the mold where they become defects in the finished product. This alumina build up can be eliminated via the addition of calcium. The addition of calcium modifies solid alumina inclusions to a form that is liquid at steelmaking temperatures.

Although this phenomenon is also present in bloom, billet and conventional slab casters, unobstructed steel flow is particularly critical in thin slab casting where the combination of high casting speeds, high refractory area and relatively low volumes of steel in the mold increase the sensitivity to alumina buildup.

Poor mold level control has a negative effect on breakout frequency, slag entrapment and other quality parameters.

Improvements in machine and nozzle design continue to enhance steel flow characteristics, but due to the continued need for improved steel quality, calcium treatment, usually via cored wire injection, is required for virtually all thin slab cast steel. This requirement is not unique to any particular machine design.

Global experience on a variety of casting machine designs, including SMS-CSP, Danieli, Voest, MDH-ISP and Sumitomo demonstrates the need for consistent calcium treatment.

Through an appropriate Ca treatment the Al2O3 inclusions (TLiquidus = 2050 C) can be transformed into 7CaO. Al2O3 (Tliquidus = 1415 C). Compared to Al2O3 inclusions, these are liquid during casting temperatures. As such no clogging occurs. Moreover, these inlusions are globular in shape compared Al2O3 inclusions.,