Metallurgy

Casting / Mould Powder

Mould Powder have a major role in the continuous casting of steel. The powder is fed onto the top of the molten metal surface, whereupon they form first a sinter layer, then a mushy layer, and eventually forms a liquid flux pool. Liquid slag from the molten pool infiltrates into the mould/strand channel and lubricates the newly-formed steel shell. However, most of the first liquid entering the channel freezes against the water-cooled, copper mould and forms a glassy, solid slag film (2 mm thick). A thin liquid slag film (0.1 mm thick) moves with the steel shell and provides liquid lubrication to the shell. In time, the glassy slag may partially crystallize. The solid slag layer is usually considered to stay attached to the mould wall or, if it does move, it must be much slower than the velocity of the shell. The mould is oscillated to prevent the shell from sticking to the mould. The horizontal heat transfer is controlled by both the thickness and the nature of the solid slag layer. Thus, in summary, the liquid slag layer controls the lubrication and the solid slag layer controls the horizontal heat transfer.

The role of the slag in continuous casting is to:

  •  Protect the meniscus of the steel from oxidation.
  •  Provide thermal insulation to prevent the steel surface from freezing.
  •  Provide liquid slag to lubricate the strand.
  •  Provide the optimum level of horizontal heat transfer for the steel grade being cast.
  •  Absorb inclusions from the steel.

All of these functions are important but in routine practice it is the lubrication and the horizontal heat transfer, that are the most important. The principal factors affecting flux performance are: Casting conditions (casting speed, Vc, oscillation characteristics)

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