【正文】 nd near the mould lies between 20 and 26, depending on the operating conditions。 for the secondary coolingarea the figure is 29 33. The thickness of the solidified strand shell on leaving the mould is about 8 10% of the strandthickness, depending on casting rate. A secondary coolingarea under the mould speeds up pletion of the solidification process. The coolant usually is water but water / air mixture or pressed air is also sometimes used. The secondary cooling area is divided into several zones to suit coolant flow rates. The necessary quantity of water is sprayed over the entire strand by spraybars. The ferrostatic pressure may be so high in relation to the strand crosssection and the casting rate that the strand has to be supported to prevent buckling. The equipment for this is expensive in plants producing blooms and especially slabs. Process Control For productivity and quality reasons there is a trend in modern steelmaking to transfer timeconsuming operations, such as temperature adjustment, deoxidation and alloying, from the furnace to the ladle treatment stations. These treatments are particularly important where the continuous casting process is involved because temperature and position must closely be controlled. The temperature control of molten steel as it enters the mould needs to be more accurate in the continuous casting process than in conventional casting. Too high a superheat can cause breakouts or a dendritic structure, which is often associated with poor internal quality. On the other hand, too low a temperature may cause casting difficulties due to nozzle clogging and result in dirty steel. The steel temperature in the tundish normally lies between 5 and 20℃ above the liquids for slab casting and between 5 and 50℃ for billet or bloom casting. This differential depends on steel grade and, for example, is about 45t for stainless steel slab casting from small furnaces. In order to keep the steel temperature within the prescribed limits during the whole cast, temperature uniformity in the ladle is of paramount importance. Stirring is required before casting in order to destroy any temperature variations in the ladle, and rinsing is sometimes used. The heat is flushed with either nitrogen or argon, injected by means of a porous plug at the bottom of the ladle or through a hollow stopper rod at a separate rinsing station. Control of chemical position can be performed during vacuum or rinsing treatments. On the basis of the analysis of a sample or of an electromotive force oxygen activity measurement made after homogeneity of the metal is attained, trimming additions can be calculated to ensure correct deoxidation. The best way to introduce trim deoxidants is at a high velocity (powder injection with inert gas, wire feeding or bullet shooting) while stirring the bath. Decreasing the need for alloys by careful exclusion of furnace slag from the ladle simplifies trimming. Vacuum treatment is versatile and useful to achieve for good ladle metallurgy. Lowpressure treatment, however, is the only way to remove hydrogen before casting or to decarburize to extremely low levels. Mouldlevel control The most vital part of the control of a continuous casting machine is to ensure that the withdrawal of the cast and the partiallycooled billet is such as to keep the liquid level in the mould constant (within a few centimeters). This is done in two ways. (1) The tundish is weighed and the rate of feed to the tundish from the ladle varied automatically to keep the total tundish weight constant. In this way the rate of feed from the tundish is constant. (2) The rate of withdrawal of the partially cooled billet is controlled so as to keep the level of liquid steel in the mould roughly constant. In the early days of continuous casting the level of the top of the liquid steel in the caster was maintained constant by an operator viewing it and adjusting the tundish stopper accordingly. It is now normal to have a means of finding the level using a measuring instrument and automatically adjusting the level. The table below lists several ways in which the level is detected. Two of them, the gammaray (radioactive) and the infrared methods will be described in detail. The operation is selfevident from this diagram. The infrared device was developed in order to avoid the use of powerful radioactive isotopes. The detector views the junction of the metal level with the back wall of the mould. As the metal level rises within the field of view more radiation is received by the single photocell and an increased output is obtained. Special provisions are made to pensate for interruption of the view of the metal. The photocell unit receives the infrared radiation and provides an electrical signal to the control unit, which is in turn connected to the operator39。s unit and the castingmachine drives. The operator can select automatic or manual control and he receives indication of the operating rod from signal lamps. The radiation emitted from the liquid steel is collimated through a slotted mask and then focused on to a photo detector by a cylindrical lens. The light is filtered to eliminate radiation below a wavelength of 1 mm, so reducing interference from ambient light and oil flames. The entire system is duplicated within the had with two detectors and two fit beams normally arranged to view either side of the steel stream. It is possible to adjust the spacing between the two areas seen by the photocells by changing the slot spacing in the mask. There are three photo detectors fitted for each channel: the first measures the metal level using the beam described above。 the second receives no light and enables temperature drift pensation。 and the third looks through the slot at a small region above the normal metal level and between the main beam and the metal stream. Its purpose is to detect the metal stream if it wanders from a central position and is in danger of in