The productivity and yield that are so important for operating a continuous caster can be markedly improved by casting many heats continuously without interrupting casting. This is called continuous-continuous casting or sequence casting, and has the advantage of eliminating the need for preparations for starting casting. Consequently, productivity is increased and the amount of the cast strand which must be cropped at the initial and final casting positions due to poor quality is decreased. Techniques have been developed for sequence casting, which allow the mold width to be changed and different steel grades to be cast without interrupting casting operations. These allow strands of different width and grade to be cast continuously without interruption. Submerged entry nozzles wear and become clogged as throughput of the melt increases; therefore, methods have been developed for the quick, automatic exchange of submerged entry nozzles without suspending the casting operation. As one extremely serious practical problem, in "breakout", the solidified shell grows unevenly, the thinner portion of the shell ruptures, and the molten steel leaks from the mold, requiring a full stop of the line. Thermal monitoring techniques for predicting breakout are used at many casters. Productivity can be improved by raising the casting speed, as well as by improving the operating rate. Progress in techniques and equipment has now enabled a casting speed of 1.5-2.8 m/min in continuous slab casters, which corresponds to a production capacity of 5 ton/min per strand. Thus, approximately 3.6 million ton/year can be produced with a 2-strand continuous caster. The sequence of the casting operation starts with inserting the dummy bar into the mold to seal the bottom end. Molten steel is then poured into the mold from the tundish while taking great care to prevent contact with the air. The withdrawal of the cast strand is started by pulling the dummy bar downward. The molten steel flowing into the mold is rapidly cooled and forms a thin solidified shell composed of fine granular crystals on the surface and an array of fine columnar dendrites inside. The solidified shell becomes thicker due to the growth of columnar dendrites as it descends through the mold. A lime silicate flux is added to the molten steel surface in the mold to prevent heat loss from the molten steel surface and absorb nonmetallic inclusions as they surface. This flux also infiltrates between the mold and the cast strand, and provides lubrication which also prevents sticking of the cast strand to the mold during the oscillation of the mold. At the same time, the layer of mold flux between the steel and mold reduces heat transfer and avoids a rapid decrease in the temperature and resulting deformation and crack formation of the strand. Surface defects are formed on the cast strand when the level of the steel bath fluctuates in the mold. The level is therefore measured with a sensor and kept as constant as possible by controlling the flow rate of molten steel from the tundish. Electromagnetic braking of the melt flow in the mold is now a representative technique for meniscus level control. The cast strand, which still contains unsolidified molten steel, exits the mold and is withdrawn downward while being supported by a group of rolls and water-cooled with the sprays. During this process, columnar dendrites continue to grow, and equiaxed dendrites are finally formed to complete solidification. At this time, the solidified shell is subjected to high thermal strain, shrinkage, and transformation caused by cooling, and to ferrostatic pressure. Since the hot solidified shell is substantially lower in strength and toughness, the cast strand is susceptible to surface and internal cracks. Consequently, during spraying the cooling pattern is carefully controlled to prevent the growth of cracks due to strain while ensuring solidification by cooling. This pattern control involves controlling the intensity of the water-mist spray along the widthwise and drawing direction of the cast strand as required by the steel grade. Reduction is then applied to the cast strand at the crater bottom to reduce center segregation. After cutting to length with gas torches, the cast piece, or slab, is delivered to the hot-rolling process. As the productivity of the cast strand has increased and defects have decreased to the extent that no off-line surface conditioning by scarfing and grinding is required, hot-charge rolling and hot direct rolling have been widely adopted. In hot-charge rolling, the hot slab is charged into the reheating furnace, but is rolled without substantial reheating, while hot direct rolling is performed immediately after casting, omitting the reheating process completely.