The Development of Continuous Casting【连铸】
Continuous Casting
From the Making, Shaping and Treating of Steel by William，McGraw—Hill 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:
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(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 otherwi expensive surfacing process;
(4) high degree of automation;上海对外贸易学院研究生部
(5) friendlier to the environment;
(6) better working conditions.
Types of Installation
The first continuous-casting plants were aligned vertically; however, with larger cross-ctions, increasing strand-length, and, above all, with increasing pouring-rates this type of construction leads to unreasonable building-heights. The factors also lead to a considerable increa in the length of the liquid pha which has metallurgical effects. The length of the liquid pha in a continuously-cast strand is determined by the following formula:
L=D2/4x2Vc
Where D =strand thickness (mm)
decent什么意思x = solidification characteristic (mm / min1/2)
The values amount to 26~33 for the whole cooling length.
Vc = casting rate (m /min)
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Efforts to reduce building-height first led to continuous-casting systems in which molten metal pasd into a vertical mould and solidified completely before being bent or where the strand has been in the liquid pha and later to the bow-type installation which has a curved mould and is the system most ud today. Vertical systems and tho in which the strand is bent when completely solidified have long straight liquid phas and can lead to unacceptably high capital outlay.
However, the 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 pha has the advantage that the building need not be as tall as when the strand is bent after solidification; however, the liquid-pha bending system requires higher initial outlay and greater maintenance costs. The bow-type system reprents a compromi 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-ction imaginable; square, rectangular, polygonal, round, and oval ctions are all available. There are also some instances of preliminary ctions 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-ctions up to 160mm across. Larger ctions and tho 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 rin sharply, especially in the last few years. This is esntially becau of increa in the breadth of the strand and in casting rate. The following outputs have been exceeded per ction per minute:
slabs 5 tones
华氏温度计blooms 1 tones
billets 350 kg
Finally, we should mention horizontal continuous-casting systems which are already ud for non-fer
rous 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 becau 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 clod 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, pass through a cooling system before it finally reaches the withdrawing rolls, whereupon the hot strand takes over transport. The starting-bar is parated from 口才与交际
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 purpo 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 procesd the pouring stream is protected against oxidation between the submerged boxes. The mould not only forms the strand ction 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 ud for smaller ctions. 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 ud 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 chon to match the quality-programme precily.
The level of steel in the mould may be controlled manually or by an automatic system. Either method may be ud 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 ud for continuous-casting depends on the type of installation. Rigid starting-bars can be ud in vertical systems, while articulated dummy bars or flexible strip have to be ud 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 relead by unlatching.
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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 condary 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 cas
ting rate. A condary cooling-area under the mould speeds up completion of the solidification process. The coolant usually is water but a water / air mixture or compresd air is also sometimes ud. The condary cooling area is divided into veral 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-ction 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. The treatments are particularly important where the continuous casting process is involved becau temperature and composition must cloly 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 cau breakouts or a dendritic structure, which is often associated with poor internal quality. On the other hand, too lo
上海交大昂立>w开头的英文名w a temperature may cau 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 ud. 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 parate rinsing station.
smoke freeControl 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 uful 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
