Continuous Casting At Atlas Steels Ltd.
by L. F. Banrhardt G. E. Stock, and W. U. Porter A recent 12-ton experimental heat being poured. The machine operator is shown controlling withdrawal rate.
ORTH America's first commercial continuous N steel casting machine is located at Atlas Steels Ltd., WeIland, Ont. The machine is adjacent to the
melt shop and ladles of molten metal are transported to it by transfer car from the pouring pit area, via a turntable. An 85-ton overhead crane raises the ladle to the casting floor from the transfer car. The casting floor, 31 ft above ground level, contains an oscillating mold of the Rossi-Junghans type, machine operation controls, water valve controls, and instruments to record machine speed, cast bar temperatures, water flow, and water temperatures. Bottom pour ladles, fitted with harness and stopper rod, are now used, with cost of ladle bricking lower and steel cleanliness better than with lip pour ladles. Ladles currently in use are of 30 to 35-ton and 10 to 12-ton capacities, for production and development heats respectively. A spill-over ladle is installed behind the tundish to take an overflow, in the rare event of a running nozzle. Between ladle and mold, a refractory-lined tundish is employed to supply a clean, low velocity, turbulence-free stream of metal at the desired rate of flow. Magnesia and stabilized zirconia nozzles are used, usually single taper, round-shouldered design. The tundish is preheated by gas burners to approximately 2400°F and any fragments of spalled refractory cement are cleaned out shortly before casting. Protective propane atmospheres are maintained on and around exposed metal surfaces and stream L. F. BARNHARDT, G. E. STOCK, and W. U. PORTER are Melting Superintendent, General Foreman, and Development Metallurgist, respectively, at Atlas Steels Ltd., Weiland, ant., Canada. 1050-JOURNAL OF METALS, AUGUST 1957
to prevent oxidation of metal and smoky combustion of the rapeseed oil mold lubricant. The molds are solid castings, 20 in. long, of electrolytic copper, the walls being aproximately 6 in. thick. Vertical cooling ducts contain restrictor rods which confine the cooling water to a 1/16 in. annulus. Water flow varies up to 250 gpm, according to mold size. The mold is mounted on a table which oscillates during casting, about % in . downward at the rate of withdrawal of the bar, then upward at three times this rate. A starting bar, supported by two sets of pinch rolls, forms a bottom for the mold. The first metal cast freezes around one or more large starting bolts inserted in the top of the bar. The pinch rolls are set in motion and withdraw the bar from the mold together with the newly formed casting. Immediately below the mold, the casting, consisting of a thin shell with molten center, is subjected to cooling from mold sprays which effectively minimize the possibility of the liquid center remelting the comparatively thin shell and breaking out. Below the mold sprays, the bar enters a roller apron, inside the spray chamber, which gives it mechanical support and further cools it by means of spray nozzles mounted on vertical water pipes opposite the spaces between rolls. Spray water volume can be varied between 30 and 980 gpm, depending on grade, size, and speed of casting. From the spray chamber, the bar travels through the pinch rolls, which are set at a predetermined pressure sufficient to support its weight. The speed of rotation of the rolls is the operating speed of the machine, and is synchronized with mold oscillation. Cutting is done by oxyacetylene powder torches (see figure). Complete dependability of the cutting
View of the cast bar and roller apron immediately below the mold.
Dual torch system for cutt ing castings. This photo shows the twin torches which move downward with the casting and cut it into desired lengths. They automatically clamp on, cut and release from the bar in proper time and sequence.
operation is vital; many problems respecting powder flow, constant speed of torch travel, fast cutting, and spitting back, had to be overcome. A pneumatically operated lowering bracket lowers the cut bar, maximum 16 ft long, to the discharge carriage, which is pulled on inclined tracks from a 20 ft pit. At ground level, a pneumatic pusher then moves the bar from the carriage on to a drag chain conveyor which drops it into a cradle. Hence, the bars, now around 1500° to 1700°F, are taken by crane directly to the soaking pits, or stacked nearby. The cycle is entirely automatic.
finement presents casting difficulties ; aluminum contents in excess of 0.02 pet cause skulling in the tundish nozzle. Deoxidation is largely accomplished without the use of aluminum, and residual contents are normally too low to cause trouble. Vanadium has been used on certain grades for grain refinement; where aluminum is desired, it is fed (as wire) into the tundish stream. A lip-pour tundish is being designed to facilitate casting metal with an aluminum content higher than can be practicably added as wire. An occasional recurrent nuisance with all grades is the presence of small subsurface gas holes, presumed to be from hydrogen. They are most troublesome with austenitic stainless steels, necessitating additional grinding and reduced yields. In most grades, they appear to weld up with rolling. Their precise source has not, at present, been ascertained, but precautions are taken in melting to ensure the lowest possible hydrogen level in the steel. It is possible that contamination can result from some factor or factors in the casting process itself. Good cleanliness is usual with continuously cast steels. Such non-metallic inclusions as are encountered are more evenly distributed and less deleterious than is the case with conventionally cast steels. Central porosity is nonexistent or negligible, depending on the grade and its other quality requirements, particularly surface. Surfaces, generally, are good . The principal contributory factors involved are mold lubrication, inert atmosphere protection, metal temperature, and casting speed. Within limits, increasing the casting speed-the other factors being under proper control -gives optimum surface quality, though usually at some cost to center quality.
Casting problems Precise temperature control is avitally important factor. Accurate furnace tapping temperatures are ensured by scrupulous maintenance of quick immersion thermocouples. Temperatures at start of casting range approximately from 2700° to 2900°F, depending on grade of steel, and are kept as low as practicable; this is necessary to avoid serious stopper erosion and ensure good billet quality. Control of metal temperature in the ladle within ±10°F has been found necessary. Small adjustments, only occasionally necessary, are made at the casting floor by argon injection. Ladles have an additional brick insulation course. Dolomite is shovelled on top of the slag for extra insulation and to cool the slag in contact with the rod. Circular tundishes are being developed for initial ease of bricking, easier shell maintenance, and to eradicate the necessity for a total re-brick after every heat. The nozzle designs employed were adopted only after extensive testing. Use of aluminum for deoxidation and grain re-
AUGUST 1957, JOURNAL OF METALS-1051
