Strand Casting is the process whereby molten metal is solidified into a "semi finished" billet, bloom, or slab for subsequent rolling in the finishing mills.
History: Prior to the introduction of continuous casting in the 1950s, steel was poured into stationary molds to form "ingots". Since then, "continuous casting" has evolved to achieve improved yield, quality, productivity and cost efficiency. Sir Henry Bessemer, of Bessemer converter fame, received a patent in 1857 for casting metal between two contra-rotating rollers. The basic outline of this system has recently been implemented today in the casting of steel strip.
Strand Casting Advantages: It allows lower-cost production of metal sections with better quality, due to the inherently lower costs of continuous, standardized production of a product, as well as providing increased control over the process through automation. This process is used most frequently to cast Steel (in terms of tonnage cast). Aluminum and copper are also continuously cast. Aluminum and copper are casted this way.
Strand Casting Process: Molten metal (known as hot metal in industry) is tapped into the ladle from furnaces. After undergoing alloying and degassing, and arriving at the correct temperature, the ladle is transported to the top of the casting machine. From the ladle, the hot metal is transferred via a refractory shroud (pipe) to a holding bath called a tundish. Metal is drained from the tundish through another shroud into the top of an open-base copper mold. The mold is water-cooled. In the mold, a thin shell of metal next to the mold walls solidifies before the metal section, now called a strand, exits the base of the mold into a spray-chamber; the bulk of metal within the walls of the strand is still molten.
Final solidification of the strand may take place after the strand has exited the spray-chamber. After exiting the spray-chamber, the strand passes through straightening rolls (if cast on other than a vertical machine) and withdrawal rolls. There may be a hot rolling stand after withdrawal, in order to take advantage of the metal's hot condition to pre-shape the final strand. Finally, the strand is cut into predetermined lengths by mechanical shears or by traveling oxyacetylene torches, is marked for identification and either taken to a stockpile or the next forming process. In many cases the strand may continue through additional rollers and other mechanisms which might flatten, roll or extrude the metal into its final shape.
Applications of Strand Casting: Long shapes of simple cross section (such as round, square, and hexagonal rods) can be produced by continuous casting, which is done in a short, bottomless, water-cooled metal mold. The casting is continuously withdrawn from the bottom of the mold; because the mold is water cooled, cooling rate is very high. As a result of continuous feeding, castings generally are free of porosity. In most instances, however, the same product can be made by extrusion at approximately the same cost and with better properties, and thus use of continuous casting is limited. The largest application of continuous casting is production of ingot for rolling, extrusion, or forging. |
