Improvements & Innovations in the Continuous Casting Process at POSCO

Abstract

Continuous casting is the essential process converting liquid steel to solid in the form of slabs or billets/blooms in the steel plant. The economy and quality of the steel products are greatly dependent on how successfully the continuous casting is performed. New technologies have been actively developed in the process during the last decades in order to increase the productivity and, therefore, to decrease the operational cost. Since its first commissioning of a slab caster in 1976, POSCO has constructed a number of continuous slab, bloom and billet casters including a thin slab caster not only for plain carbon steels but for stainless steels. Through the operation of various types of continuous casters for more than 30 years so far, POSCO has steadily developed fundamental technologies and operational know-how and achieved the equipment innovations to improve the surface and internal qualities of cast products as well as to extend the productivity of continuous casters. Furthermore, POSCO has deepened the basic understanding on the solidification phenomena of liquid steel and also accumulated the engineering backgrounds to design the most optimal continuous casters. It has also devised the indispensable and auxiliary equipments and the key technologies to control the process precisely and efficiently in order to guarantee the quality and productivity. An innovative technology under development is the POCAST process, where controlled amount of the pre-molten mold flux instead of conventional powder mold flux is continuously fed into free surface of molten steel through the plunger-type feeding system from the flux melting furnace. In order to prevent the molten flux from freezing at the meniscus, a reflective insulation cover is installed, leading to the suppression of thermal radiation from the molten steel and flux. It is generally understood that, as casting speed increases, the occurrence of breakout increases since mold lubrication becomes insufficient due to the lack of mold flux flow from the meniscus into the solid shell/mold boundary. However, by utilizing the especially composition controlled pre-molten flux, it becomes possible to eliminate the formation of slag bear in the mold. Therefore, the mold flux consumption rate is increased even at the reduced oscillation rate & stroke and more importantly, the mold flux infiltration becomes more uniform throughout the boundary between the mold and the solidified shell. This consequently results in drastic reduction of the formation and depth of the oscillation mark and the occurrence of surface hooks without increasing the possibility of breakout, as has been proved in the casting trials carried out with the 10 ton pilot slab caster in Pohang. A key trend in the development of the continuous casting process is to reduce the thickness of cast products. Examples include thin slab casting and strip casting. In the thin slab casting process, a major drawback is the relatively low casting speed and, as a result, the inefficient equipment layout in the plant where two casters are connected to a hot rolling unit. The drawback could be resolved if the casting speed exceeds a certain limit. At the high casting speed, the productivity of casting becomes equivalent to that of hot rolling, and the thin slab casting plant is to be designed so that one strand