The Development of Continuous Casting连铸翻译.doc

The Development of Continuous Casting【连铸】Continuous Casting From the Making, Shaping and Treating of Steel by William，McGrawHill Companies, Inc., 2002 The Development of Continuous Casting Continuous casting was developed very rapidly after the Second World War. Steel-producers arc today generally convinced that continuous casting is at least as economical as ingot production and can match the quality of the latter across much of the production spectrum for high-quality steels. Continual development of the technique aimed at improved steel characteristics is leading to increasing adoption of the process in works producing special high-grade steels. The reasons for continuous-casting systems are: (1) lower investment outlay compared with that for a blooming train (mini-steelworks); (2) about 10% more productivity than with conventional ingot-casting; (3) high degree of consistency of steel composition along the whole length of the strand; better core quality, especially with flat strands; high inherent surface quality, leading to savings on an otherwise expensive surfacing process; (4) high degree of automation; (5) friendlier to the environment; (6) better working conditions. Types of Installation The first continuous-casting plants were aligned vertically; however, with larger cross-sections, increasing strand-length, and, above all, with increasing pouring-rates this type of construction leads to unreasonable building-heights. These factors also lead to a considerable increase in the length of the liquid phase which has metallurgical effects. The length of the liquid phase in a continuously-cast strand is determined by the following formula: L=D2/4x2Vc Where D =strand thickness (mm) x = solidification characteristic (mm / min1/2) These values amount to 2633 for the whole cooling length. Vc = casting rate (m /min) Efforts to reduce building-height first led to continuous-casting systems in which molten metal passed into a vertical mould and solidified completely before being bent or where the strand has been in the liquid phase and later to the bow-type installation which has a curved mould and is the system most used today. Vertical systems and those in which the strand is bent when completely solidified have long straight liquid phases and can lead to unacceptably high capital outlay. However, these systems have metallurgical advantages from the point of view of maintenance. A vertical system in which the strand is bent while still in the liquid phase has the advantage that the building need not be as tall as when the strand is bent after solidification; however, the liquid-phase bending system requires higher initial outlay and greater maintenance costs. The bow-type system represents a compromise between the costs of capital outlay and of maintenance and what can be achieved metallurgic ally. Continuous-casting is suitable for the production of almost any cross-section imaginable; square, rectangular, polygonal, round, and oval sections are all available. There are also some instances of preliminary sections for tubes and slabs, blooms, and billets. Sections with a breadth /thickness ratio greater than 1.6 are normally described as slabs. Billet-machines produce square or nearly-square, round, or polygonal cross-sections up to 160mm across. Larger sections and those with a breadth /thickness ratio less than 1.6 are cast in bloom-machines. Billets nowadays normally produced in this way range from 80 x80 to 300 x300 mm, and slabs are 50 - 350mm thick and 300 - 2500 mm wide. Continuous-casting output-rates have risen sharply, especially in the last few years. This is essentially because of increase in the breadth of the strand and in casting rate. The following outputs have been exceeded per section per minute: slabs 5 tones blooms 1 tones billets 350 kg Finally, we should mention horizontal continuous-casting systems which are already used for non-ferrous metals and cast iron and which are being further developed for steel. R. Thieimann and R. Steffen have produced a comprehensive report about the state of development of horizontal continuous-casting systems for producing billets from unalloyed and alloy steels. Horizontal continuous-casting systems have three important advantages over conventional continuous-casting system: (1) low height and cost of building; (2) simple means of protecting the melt against reoxidatioin; (3) no strand deformation because the ferrostatic pressure is much lower. Casting Technique Molten steel is poured from a casting ladle via a tundish into an open water-cooled copper mould. At first the bottom of the mould is closed off by a starting-bar, which then leads transport of the hot strand from the mould into the continuous withdrawing rolls. The strand, which starts to solidify in the mould, passes through a cooling system before it finally reaches the withdrawing rolls, whereupon the hot strand takes over transport. The starting-bar is separated from the hot strand before or after it reaches the parting device. The latter, which may either be a flame-cutter or hot shears, moves at the same rate as the hot strand and cuts it into the lengths required. The purpose of the tundish is to feed a defined quantity of molten steel into one or more moulds. This can be done by using nozzles controlled by stoppers, slide-gates, or other means. The tundish may initially be cold, warm, or hot according to the nature of its refractory lining. Where difficult steels are processed the pouring stream is protected against oxidation between the submerged boxes. The mould not only forms the strand section but also extracts a defined quantity of heat, so that the strand shell is strong enough for transport by the time it reaches the mould-outlet. The mould may be made from copper tube or hard enable copper alloy, depending on the shape and size of the strand to be cast. As a rule, tubular moulds tire used for smaller sections. The interior surface of the mould may be coated with chronic or molybdenum to reduce wear and to suit heat-transfer from the alloy being cast. The mould is tapered to match steel-shrinkage and casting-rate and the type of steel concerned. Moulds used today range from 400 to 1200 mm in length overall, but their usual length is between 700 and 800 mm. The problem of steel adhering to the mould-sides is usually countered by oscillating the mould sinusoidally relative to the strand and by adding lubricant (oil or casting flux in an attempt to cut friction between the mould and the steel. The lubricant, particularly casting-flux, has an additional metallurgical function. The choice of lubricant depends on the qualities required and the casting conditions; it is particularly important that casting-flux should be chosen to match the quality-programme precisely. The level of steel in the mould may be controlled manually or by an automatic system. Either method may be used to keep the level constant or to match the incoming molten steel, i. e. to accommodate variations in casting rate. Manual control is affected via the stopper in the tundish or by varying the output rate. An automatic control system may meter radioactivity or infrared radiation or measure temperature via a probe in the mould wall to determine the steel-level and compensate any changes by actuating the stopper-mechanism (for constant pouring rate) or controlling the speed of the withdrawing rolls (varying casting rate). The type of starting-bar used for continuous-casting depends on the type of installation. Rigid starting-bars can be used in vertical systems, while articulated dummy bars or flexible strip have to be used in bowed installations. The starting bar can be connected to the hot strand in different ways, one is by welding the fluid steel using a jointing element (flat slab, screw, or fragment of rail) which is soluble in the starting-bar; another is by casting the connector in a specially shaped head in the dummy bar in a way that enables it to be released by unlatching. The thickness of the solidified strand shell on leaving the mould depends first of all on how long the steel is in contact with the mould, but it also depends on the specific thermal conductivity of the mould and on the amount of superheat that steel has when it enters the mould. It can be determined with fair accuracy using the following parabolic formula: C=x. T where C is the thickness of the strand shell (mm) x is the solidification characteristic (mm/min1/2) t is the solidification time (min) The solidification characteristic in and near the mould lies between 20 and 26, depending on the operating conditions; for the secondary cooling-area the figure is 29 -33. The thickness of the solidified strand shell on leaving the mould is about 8 10% of the strand-thickness, depending on casting rate. A secondary cooling-area under the mould speeds up completion of the solidification process. The coolant usually is water but a water / air mixture or compressed 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 spray-bars. The ferrostatic pressure may be so high in relation to the strand cross-section 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 time-consuming 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 composition 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 composition 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. Low-pressure treatment, however, is the only way to remove hydrogen before casting or to decarburize to extremely low levels. Mould-level control The most vital part of the control of a continuous casting machine is to ensure that the withdrawal of the cast and the partially-cooled 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 gamma-ray (radioactive) and the infrared methods will be described in detail. The operation is self-evident 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 compensate 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 operators unit and the casting-machine 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 compensation; 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 interfering with the main beam. The balance between the two main beams and the threshold level of the stream detectors can be adjusted with small potentiometers mounted in the back of the unit. The level signal detected by each channel is fed, after temperature compensation, to a simple circuit which selects the largest signal. Thus the unit always controls on the higher of the two level signals. If the stream-sensing photocell sees that the teeming stream is moving towards the detection beam it blocks the signal and the unit switches to control on the other channel. There is an additional feature that if both channels are blocked together, for example by a fan-shaped metal stream, the unit switches to a memory, equivalent to the fast detected metal level, and prevents a sudden loss of control. As the memory decays the metal level gradually drops allowing the operator ample time to intervene. The unit gives a smooth transition from manual to automatic control by preventing automatic operation if there would be a large jump in withdrawal speed at changeover. It does not provide bumpless transfer when changing from automatic to manual. There is also protection against changing to automatic when there is a cable fault. The control system receives the chosen level signal and, following proportional and integral action, outputs a voltage signal directly to the withdrawal drive unit. The drive creates a withdrawal speed proportional to this voltage signal. Benefits of Continuous Casting Sequence of Operations-prior to the development of continuous casting, ingots provided the only starting material in wrought-steel products. The typical sequence of operations from the steelmaking furnace to the rolling mills was: (1) Tapping liquid steel into ingot molds. (2) Transferring ladle to pouring platform and teeming liquid steel into ingot molds. (3) Transferring filled molds to stripping area for ingot removal. (4) Transferring and charging ingots into soaking pits and heating to rolling temperature. (5) Removal of heated ingots from soaking pits and transfer to primary mill for rolling into semi-finished shapes. (6) Transferring semi-finished shapes to subsequent rolling mills. Using continuous casting, the following much shorter sequence of operations is required: (1) Tapping liquid steel from a steelmaking furnace into a ladle. (2) Transferring the ladle to a casting platform and continuously casting liquid steel into semifinished shapes. (3) Transferring the semi-finished shapes to rolling mills. The benefits derived from the shorter sequence of operations provided the main impetus for the adoption of continuously casting; increased yield; improved product quality; energy savings; less pollution; and reduced costs. Yield Increased yield from liquid steel in the ladle to the semi-finished rolled shape results from a reduction in scrap generation in three areas: the primary rolling mill; the pouring operation; and ingot heating. The major contribution to the improved yield is the absence of crop losses corresponding to the ingot top and bottom location when an ingot is rolled in the primary mill. Reduction in yield losses associated with the pouring operation includes short ingots, ingot butts and general pit scrap. Scaling losses associated with ing