Views: 2 Author: Site Editor Publish Time: 2023-05-23 Origin: Site
Starting the caster requires passing a dummy bar (essentially a curved metal beam) up through the spray booth to seal off the bottom of the mold.The metal is poured into molds and removed with dummy rods once solidified.It is extremely important to secure the supply of metal afterwards to avoid unnecessary downtime and restarts (known as "turnarounds").Every time the casting machine is stopped and restarted, a new tundish is required because any uncast metal in the tundish cannot be drained but instead freezes into a "skull".Avoiding turnaround requires tight control of the melt shop, including the ladle furnace (if present), over the temperature of the metal, which can vary dramatically with alloy addition, slag covering and deslagging, and preheating of the ladle before receiving the metal, among others parameter.However, pour rates can be reduced by reducing the amount of metal in the tundish (although this increases wear on the tundish), or if the caster has multiple streams, one or more streams can be shut down to accommodate upstream delays.Turnaround can be scheduled into the production sequence if the tundish temperature becomes too high after a certain number of heat-ups or if the service life of a non-replaceable component (i.e. submerged nozzle (SEN) in a thin slab caster) is shortened.
Many continuous casting operations are now fully computer controlled.Multiple electromagnetic, thermal or radiation sensors on ladle shroud, tundish and mold to detect metal level or weight, flow and hot metal temperature, Programmable Logic Controller (PLC) to set pull speed with speed control to exit coil.The flow of metal into the mold can be controlled in three ways:
By lowering the stopper rod through the tundish,
Through a slide gate on top of the mold shield,
If the metal is poured open, the flow of metal into the mold is controlled only by the inside diameter of the metering nozzle.These nozzles are usually interchangeable.The overall casting speed can be adjusted by changing the amount of metal in the tundish through the ladle sliding door.The PLC can also set the mold oscillation rate and mold flux feed rate, as well as the water flow in the cooling spray within the strand.Computer control also allows important casting data to be transmitted to other manufacturing centers (especially steelmaking furnaces), allowing their work rates to be adjusted to avoid product "overfill" or "underload".
Contamination by oxygen
While extensive automation helps produce castings with no shrinkage and little segregation, continuous casting is useless if the metal is not clean beforehand, or becomes "dirty" during casting.One of the main ways molten iron becomes dirty is through oxidation, which occurs rapidly at molten metal temperatures (up to 1700 °C for steel); gas inclusions, slag or undissolved alloys may also be present.To prevent oxidation, the metal is kept as far as possible from the atmosphere.To achieve this, the exposed liquid metal surfaces are covered by shrouds or, in the case of ladles, tundishes and moulds, by synthetic slag. In the tundish, any inclusions that are less dense than the liquid metal bubbles, other slag or oxides, or undissolved alloys may also float to the surface and become trapped in the slag layer.Although tundishes and molds are first filled at the start of a casting run, the liquid is heavily contaminated with oxygen and the first products produced are often quarantined or transferred to customers who do not require high-quality material.Upcasting solves this problem by forming a continuous product from a metal (such as copper or silver) seed (such as a metal rod).