Production of hot strip by the conventional continuous casting route requires reheating and roughing of continuously cast slabs (150-250mm) by 2-high roughing mills into sheet bar (30-60mm) followed by hot rolling of the sheet bar by a tandem strip mill or steckel mill into strip of the aimed thickness. Recent thin slab casting technology has made it possible to eliminate the roughing mills and reduce the number of roll stands in the tandem strip mill. If the steel melt is directly cast into strip, as originally conceptualized by Henry Bessemer in 1856, substantial reductions in investment and operational costs would be expected. This idea has attracted the attention of a number of investigators, but without any success in industrialization.

In the last decade, however, attempts were made in Japan, Europe, and the U.S.A. at the pilot plant scale to overcome the inherent difficulties of sustaining mold containment, controlling the melt meniscus level in the mold containment, adjusting the strip tension and roll gap, and improving the profile and thickness of the strip. Successful performance has been reported in several of the pilot plant investigations on a few tons per cast basis. Nippon Steel Corporation has announced the first commercialization of the twin-drum type strip caster, which is to come onstream in 1997, and will be used to cast austenitic stainless steel strip 2.0-5.0mm in thickness and 760-1,330mm in width at the 60 tons per cast scale.

A schematic of the twin-drum type strip caster is shown in the figure. This process casts steel from the ladle to the tundish and from the tundish via nozzle into a mold containment comprising twin-drum rolls and side dams. The drum rolls are made of steel, water cooled inside, and ceramic coated outside for heat transfer control and lubrication. The side dams are made of ceramics and preheated to prevent the formation of a steel shell.

The productivity of a twin drum type strip caster is given in the figure in comparison with that of a thin slab caster and conventional slab caster. At present, the strip caster can only produce 0.5 million ton/year, and variable cost of strip casting is not substantially lower due to the cost of the nozzle, side dams and rolls. Accordingly, the strip caster is not in a position to replace existing conventional casters for casting commodity carbon steels in mass, but is rather suited to cast speciality steels in small lots.

The through-thickness structure of the cast strip is largely the same as the surface layer of conventional continuous cast slabs, i.e., chilled crystals at the surface are followed by fine columnar dendrites, but the central equiaxed zone is very thin and center segregation much less.

Some grades of steels require texture control by applying reduction in excess of a threshold value which cannot be met if the starting material thickness is as thin as the cast strip thickness. Commodity carbon steels might fall in this category. Further exploration of such relations as those between the reduction ratio, texture, and properties is, therefore, needed before extensive use of strip casting will be possible.