Vacuum Casting and Continuous Casting
Vacuum casting is similar China CNC machining to Low Pressure Die Casting in that a permanent mould is linkedto a crucible of molten metal by a riser tube, but instead of pressure being applied to themolten metal, a vacuum is created in the mould cavity, thus drawing the metal up into the cavity.
Generally the starting point of any structural steel product is the ingot which is subsequently rolled through number of mills before a final product. However, the wide adoption of continuous casting has changed that scenario by directly casting slabs,billets and blooms without going through the rolling process. This process is fast and also economical.
Continuous casting is the process whereby molten steel is solidified into a semifinished billets, blooms, China CNC machining or slabs for subsequent rolling in finishing mills.
In continuous casting, liquid steel is transferred in a ladle to the casting machine. When the casting operation starts, the sliding shutter at the bottom of the ladle is opened and the steel flows at a controlled rate into the tundish and from the tundish into one or more molds The liquid steel is poured into a double walled, bottomless water cooled mould where a solid skin is quickly formed and a semi-finished skin energy form the open mould bottom. The skin formed in the mould is about 10 to 25 mm in thickness and is further solidified by intensive cooling with water spays as casting moves downwards. About 55 percent of the world's liquid steel production is solidified in continuous casting processes, the most widely used of which feeds liquid steel continuously into a short, water-cooled vertical copper mold and, at the same time, continuously withdraws the frozen shell, including the liquid steel it contains.
Fig. 11.9 Schematic of continuous casting process
A typical arrangement of continuous casting plant is shown schematically in Fig. 11.9. The molten steel is collected in a ladle and kept over a refractory lined intermediate pouring vessel named tundish. The steel is taken poured into water cooled vertical cooper moulds which are 450 to 750 mm long. Before starting the casting a dummy starter bar is kept in the moulds bottom as shown in Fig.11.9. After starting the casting process as the metal level rises in the mould to a desirable height, the starter bar is withdrawn at a rate equal to the steel pouring rate. The initial metal freezes onto the starter bar as well as the periphery of the mould. This solidified shell supports the liquid steel as it moves downwards. This steel shell is mechanically supported (rollers) as it moves down through the secondary cooling zone where water is spried onto the shell surface to complete the solidification process. After the casting is completely solidified, it is cut to the desired lengths by a suitable cutoff apparatus.
To appreciate fully the substantial benefits of continuous casting it is necessary to review some aspects of modern steelmaking and the older process of ingot casting.
In both processes, molten steel (usually called a “heat”) is prepared or an electric furnace. The in an oxygen furnace, an open-hearth furnace molten steel is next transferred in a ladle to either a nearby ingot or continuous casting facility.
Various types of ingots are prepared, in both processes, depending on the size and shape of the final steel products to be manufactured. China CNC machiningThree types of ingots are made: billets, blooms and slabs. Slabs are used to make plate and other flat products. Billets and blooms are used to make structural shapes, round products and tubes.