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1、An introduction to Continuous Casting,Contents,Introduction Solidification of Steel Description of the continuous casting process Defects of the slab, billet and bloom,Introduction,Continuous casting (CC) of steel has a relatively short history of only about 50 years. The CC ratio for the world stee
2、l industry, now approaching 90% of crude steel output, attained a mere 4% in 1970.,Continuous casting has emerged as one of the great technological developments of the twentieth century, replacing ingot casting and blooming operations for the production of semi-finished shapes: slabs, blooms and bil
3、lets. The process has been adopted worldwide by the steel industry in the latter half of the twentieth century owing to its inherent advantages of low cost, high yield, flexibility of operation, and the ability to achieve a high quality cast product.,Evolution of world steel production and share of
4、continuous casting,Characteristic caster types and design height,Solidification Structures,There is an outer chill zone comprising fine, randomly-oriented grains; an intermediate columnar zone comprising elongated, oriented grains; and a central equiaxed zone, again comprising randomly-oriented grai
5、ns. One or another of these zones may be small or absent in any given casting or ingot.,Sketch of ingot grain structure, showing chill, columnar and equiaxed zones,Illustration of dendrites: (a) 3-dimensional view; (b) typical columnar dendrites during solidification of an alloy against a cold chill
6、 wall; and (c) equiaxed dendritic solidification.,Grain Size,The final solidification grain size is determined by: - nucleation frequency, - grain survival during growth, and - multiplication of grains during solidification.,Nucleation theory for both very clean, isolated liquid metals (homogeneous
7、nucleation) and for metals in contact with substrates such as inclusions (heterogeneous nucleation)has been well developed over the last 50 years. The greater the cooling rate, the greater the nucleation frequency, one may expect that the greater the number of grains. It is often, but not always, th
8、e case that faster cooling rates result in finer grain size.,At the very first stages of solidification, dendrites are fragile and arms are easily broken off. The process is favored by vigorous convection and by low metal superheat ahead of the solidifying front (so that the separated dendrite arms
9、do not remelt). Electromagnetic stirring of continuous castings is now widely employed to minimize the columnar zone and to refine the grain size of the equiaxed zone.,The magnitude of electromagnetic force and the fraction solid to which the electromagnetic force is applied are controlled in magnet
10、ic stirring of continuous castings in order to maximize the grain-refining effect. While the extent of equiaxed zone in casts is certainly increased and thereby the centerline segregation is reduced by the electromagnetic stirring, the formation of semimacro segregation around the equiaxed grains.,I
11、nitial formation of equiaxed grains is into an undercooled melt. The solid/liquid interface has the highest temperature.,Microsegregation,Segregation of alloy elements in castings and ingots is termed microsegregation when it takes place over distances the order of the dendrite arm spacing (typicall
12、y 50400 microns). Microsegregation results because the solid forming is different in composition from the liquid from which it forms (generally less rich in solute). The excess solute is thereby rejected into the liquid, into the narrow spaces between the dendrite arms.,Model of solidification for t
13、he liquid-solid zone: (a) dendritic structure,From the point of view of reducing microsegregation by subsequent thermal treatment, the spacing of the segregate is of prime importance; as noted above, that spacing is essentially the dendrite arm spacing. Dendrite arm spacing depends solely on cooling
14、 rate, as noted earlier. It is determined by a phenomenon known as coarsening.,Inclusion Formation,Inclusion formation during solidification of ferrous alloys can often be fully described by the basic solidification model, simply viewing the inclusion as an additional phase in a multicomponent syste
15、m that solidifies with negligible undercooling or kinetic limitation to growth.,Description of the continuous casting process,Schematic illustration of continuous casting process,(Neely,1989),The major components of the casting system,Ladle, Tundish, Mold, Secondary cooling, and Steel properties-tem
16、perature, composition and cleanliness (influence the quality of the molten steel and its castability).,Ladle,Regarding the ladle, it traditionally has been employed as a transfer vessel, moving heats of steel weighing 20 to 350 tonnes from the steelmaking furnace to the continuous casting machine. However, increasingly the ladle is being used as a reactor in ladle furnaces or ladle-treatment stations, installed between the steelmaking furnace and the caster. In these operations, the c
