Continuous Steel Casting专业英语学习.ppt

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 steel 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 billets. 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 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 grains. 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 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 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, the 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 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 magnetic 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. Initial 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 (typically 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 the 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 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 system 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-temperature, 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 composition and temperature of the steel can be adjusted to meet final specifications. In this way, the productivity of the steelmaking furnace can be increased, since its primary functions are reduced to melting scrap and decarburization. In transferring steel from the steelmaking furnace to the caster, a major problem is oxygen absorption from the air, furnace slag and the ladle refractory lining. Slide-gate valves have been attached to the tap hole of the steelmaking furnace to shut off the flow of oxidized furnace slag into the ladle at the end of the tapping operation. The surface of the steel in the ladle is covered with a synthetic slag, again to prevent oxygen absorption from the air and also to absorb nonmetallic inclusions and to minimize heat losses. In some operations the ladle is covered with a lid. Finally, when located over the casting machine, the ladle is usually equipped with a refractory pouring tube to prevent oxygen pickup as the steel is poured into the tundish. Flow of steel from the ladle to the tundish is controlled with a slide-gate valve, and in some operations the weight of the ladle is continuously measured with load cells. Tundish The tundish acts as a distributor, discharging steel to the several strands that normally comprise a casting machine. The tundish also facilitates the control of steel flow into the mold because it has a constant and lower hydrostatic head than the ladle. This is particularly important during the start-up of the caster, since the tundish can be filled with steel to its normal steady- state level before pouring into the molds commences. Another important function that influences the cleanliness of the cast product is the float-out of inclusions. Finally, the tundish can be used as a reactor as well, for the addition of agents such as calcium, for inclusion morphology control. Mold The “heart” of the caster, The primary heat-extraction device whose functions are to extract superheat from the liquid steel, to grow a solid shell of sufficient thickness, to contain the liquid pool below the mold without breakouts and to support the shell during its initial growth. The design and operation of the mold, which governs heat extraction, profoundly affects surface and internal quality. Schematic of phenomena in the mold region of a steel slab caster Mold powder When continuous casting was introduced, lubrication and heat transfer between the steel shell and the mold were provided by the use of oils like rape seed oil. Types of Mold Powder: Powders, Granulated, Extruded and Expanding granules. The functions of the mold powder to protect the steel meniscus from oxidation. to provide thermal insulation to prevent solidification of the steel surface. to absorb inclusions into the molten slag pool. to lubricate the strand. The mold may be straight or have a curvature of 4 to 15 m in radius. Curved molds and Straight molds. In any event, steel leaving a straight mold is normally bent gradually to a horizontal orientation, again to reduce the plant height. However, in some casting operations, the mold and sub-mold region are straight so that the cast product is not subjected to mechanical bending forces that may generate cracks. With a curved mold, the steel strand must be straightened to a horizontal position prior to being cut into lengths. Primary and Secondary Cooling Control The most critical component of a continuous casting machine is the mold. It is in the mold that the initial shell formation occurs, and sufficient shell growth and cooling must occur to allow the shell to pass into the roll containment without rupturing. The mold heat removal directly affects shell growth and the stresses and strains that are produced. Stresses and strains must be below limits that would permit the formation of micro cracks, which could be the beginning of larger cracks caused by additional stresses and strains in the secondary cooling, bending and straightening zones. Flow of the liquid steel and liquid slag properties must be such that oxide inclusions are not trapped near the surface of the shell in the meniscus region. The surface quality of the cast product is a direct result of mold events, especially heat removal. Primary Cooling Control Solidification occurs in the mushy zone, where the heat of fusion is removed. In addition there is normally superheat in the liquid steel that is transmitted into the mushy zone. Part of the superheat is removed in the mold, and the remainder is removed in the secondary cooling zones. The percent of the mold heat load contributed by the superheat is dependent on the mold surface area/volume ratio and the degree of superheat. The effective mold cooling is the energy that passes through the surface of the solid steel shell, the liquid and solid slag films, the copper mold wall and into the cooling water to be removed from the mold. Slag film is solid on the copper side and liquid on the steel side in the upper part of the mold. The slag film normally becomes completely solid in the lower part of the mold. In some cases, there may also be intermittent contact with the mold wall in the lower part of the mold due to shrinkage of the shell away from the copper wall. The intermittent contact would create air gaps, causing additional thermal resistance. Control Factors Speed Mold Powder Lubricant Oscillation Taper Cooling Water Velocity Cooling Water Temperature Metal Level Superheat Steel Composition Mold Condition SEN Design Secondary Cooling Control A significant part of the heat removal in the secondary cooling zone is independent of the casting process. Below the mold, the heat transfer occurs by direct contact of the steel shell with the cooling medium. The heat transfer mechanisms are more easily definable and can be quantified and directly controlled to a greater extent. After the strand leaves the mold, it must be cooled until it is completely solidified and can be cut for further processing. For a slab machine, the large section size requires that the strand be contained until the solidification is essentially complete to prevent the shell from bulging outward due to the ferrostatic pressure. Maximum productivity requires that the speed be maintained as high as possible. This means that the strand secondary cooling also must be kept at a high level to speed up the solidification rate and produce a completely solidified strand before the strand is cut. As the speed is increased, the shell thickness leaving the mold is reduced, and the risk of breakouts in the region just below the mold increases. Thus, cooling in the region just below the mold is especially critical to maintain shell thickness and strength. Cooling of the strand until the point of final solidification is also critical to preventing both surface and internal cracks from forming due to excessive stresses and strains. Typical cracks observed in a continuously cast slab. Legend: Internal cracks 1. midway 2. triple-point 3. centerline 4. diagonal 5. straightening/bending 6. pinch roll Surface cracks 7. longitudinal, mid-face 8. longitudinal, corner 9. transverse, mid-face 10. transverse, corner 11. star Classifications and Appearance of Surface Defects longitudinal facial or corner cracks, transverse facial or corner cracks, star cracks and hot shortness, longitudinal and transverse depressions, deep oscillation marks, bleeding, inclusion clusters, slag patches or entrapped slags/scums, and gas holes, including blowholes and pinholes. Longitudinal Cracks The initial solidification of the steel shell in the CC mold takes place just below the meniscus of the melt. At this stage, the shell has very low tensile strength and near- zero ductility. Frictional, fluid dynamic and tensile stresses arising from strand withdrawal and mold oscillation are applied to this weak and brittle shell. Also, volume contraction upon solidification and thermal stresses upon cooling impose additional stresses on the shell. When the sum of these stresses or strains exceeds the tensile strength or ductility of the shell, cracks occur. The occurrence of the cracks is enhanced by uneven growth along the mold perimeter of the initial shell caused by meniscus fluctuation and submeniscus flow turbulence. Many factors contribute to crack formation to a different degree, causing a variety of cracks. Longitudinal Facial Cracks In the mid-1970s, while CC technology was still in the early stages of development, the occurrence of large cracks extending a few meters long and 1030 mm deep was not uncommon. This particular crack formed in the withdrawal direction on the mid-face of a 1500-mm-wide CC slab. Today, however, because of technologys maturation, such cracks are seen much less frequently unless a serious failure in operation and/or maintenance occurs. Such cracks are observed mostly during transient periods of casting operation. Today, typical longitudinal facial cracks may be classified into two types. Type-1 longitudinal facial cracks are above 100 mm long and less than a few millimeters deep and can be removed by off-line conditioning. The cracks occasionally accompany depression. Primary dendrite arms are observed along the cracks. Type-2 cracks are much shorter (less than 2030 mm) and shallower (1 mm or less) facial cracks formed in the withdrawal direction. They are not limited to the mid-face but can be scattered or clustered at mid- to quarter-width of the wide face. The occurrence of these cracks became evident in the late 1970s to early 1980s, when high-speed casting was practiced for better productivity, even on crack-sensitive peritectic grades. Critical casting speeds for peritectic grades in conventional and thin-slab casters today are around 2 m/min and 4 m/min, respectively, even after all available measures are taken, thereby imposing a productivity bottleneck. Thermal and mechanical factors that have been known to increase the occurrence of the cracks are related to CC machines and their operation as follows: 1. Shallow mold taper. 2. Mold with large width/thickness ratio or large width. 3. Deviation from unity of the ratio of cooling water flow rate per unit of surface area for the wide face to that of the narrow face. 4. Abrupt change in the meniscus level in the mold. 5. Excessively cool meniscus temperature during transient periods of casting. 6. Turbulent and/or asymmetric biased flow of the melt out of the SEN into the mold. 7. Irregular mold oscillation. 8. Intensive spray cooling below the mold. Otherwise, most influential to the occurrence of the cracks are: 9. Peritectic steels, and 10. Mold flux properties. Among them, 15, 7 and 8 have been more or less resolved; hence, the influence of 6, 9 and 11 has become more important in parallel to the progress of high-throughput/high-speed casting. Countermeasures to prevent the cracks, therefore, are the following: 1. Minimize the turbulence of metal flow, particularly submeniscus flow, by optimizing metal transfer operations and tundish design during transient periods and by using an electromagnetic flow controller for high-throughput casting. 2. Minimize meniscus fluctuation by utilizing integrated control of metal transfer from the ladle to the tundish to the mold and by reducing nozzle accretion with cleaner melts and less reoxidation during casting. 3. Minimize excessive temperature drops during transient operations by optimizing ladle and tundish preheating, ladle stirring, metal transfer operations, and with tundish heating. 4. Reduce heat transfer in the mold by the use of a slightly basic mold flux having a higher crystallizing temperature that provides the flux film with a rough surface at the mold/film boundary. The surface roughness (micro air gap) and crystallized film reduce, respectively, the conduction and radiation heat transfer from the shell to the heat transfer and, hence, uniform growth of the shell results, thereby decreasing excessive development of localized heating and resultant shell stresses. 5. Use a mold made of a heat-resistant alloy that has low thermal conductivity. So far, low thermal conduction has been synonymous with inferior heat resistance that causes sticker breakouts and intolerable wear. 6. Implementation of an air-mist spray system for the secondary cooling is effective for avoiding nonuniform and excessive cooling below the mold. Longitudinal Corner Cracks longitudinal corner cracks occurs in the vicinity of the corner of billets and blooms in the withdrawal direction, often along the junction between the corner and face. Slabs also experience this type of crack in high- speed casting of peritectic grades. Features of the cracks are the same as those for the longitudinal facial cracks. Conditioning is unavoidable. Heavy cracks can cause breakouts. The mechanism for the occurrence of longitudinal corner cracks is largely the same