Mold
Chill molds are reusable forms for casting ( chill casting process ) of metals and alloys . A mold can be closed at the bottom for ingot casting or open at the bottom for continuous casting .
Ingot molds
A permanent mold for ingot casting consists of a geometrically simple casting mold, usually made of nodular cast iron , which can be reused. The sides are usually bevelled (conical) so that the block can be easily removed from the mold after 1 to 30 hours of solidification . These molds are used for:
- Large forging blanks with a block weight of over 10 tons.
- Alloys which are difficult to cast or which are sensitive to segregation and which cannot be produced homogeneously in continuous casting .
- Intermediate blocks as electrodes for ESR processes. These molds have no conicity.
Almost exclusively, molds are used for rising ingot casting , which are arranged on a clamping plate for this purpose . Historically, the conicity and the ratio of height H to diameter D (H / D ratio) were used to characterize molds and adjusted as part of process optimization:
- Conicity: A slight conicity is required by the subsequent process such as forging or rolling. On the other hand, a higher conicity improves the solidification process in the central axis and reduces defects found there such as micro-cavities or non-metallic inclusions.
- H / D ratio: Depends on the surface to volume ratio. The solidification time largely depends on this. If the solidification time is too short, the course of the solidification front in the core can be unfavorable. Too long a solidification time occupies the casting area and reduces the maximum possible production output.
The following geometries are mainly used:
- Slab with a rectangular cross-section, the length being significantly greater than the width.
- Square, almost square cross-section. Depending on the shape of a bevel, it is also called an octagon. For both rolling mills and forges.
- Polygonal as a regular n-corner with rounded corners.
- Round for further use as an ESR electrode; usually without conicity.
Corners are always avoided and replaced by roundings or chamfers, since too small radii lead to cracks when exposed to temperature changes.
Flat side surfaces are often cambered: The surface is curved inwards or outwards. With increasing use, non-cambered surfaces tend to bulge into the volume. This crowning leads to the typical shape of the polygonal blocks.
When the casting is falling, metal splashes cause surface defects.
Molds in continuous casting
A mold for continuous casting, on the other hand, consists of water-cooled copper plates (mold plate ) or copper pipe (mold tube). Copper is used because it can dissipate the heat of the steel to be cast much better than z. B. steel.
In continuous casting, slabs for sheet metal products and billets with round and square formats for wire, bar and profile products are cast.
In continuous casting, a distinction is made between the narrow and the broad sides. The narrow sides are adjustable for the purpose of adjusting the width of the slabs to be cast. The broad sides, hydraulically or mechanically adjusted by spring force, are relaxed for this purpose in order to enable the narrow sides to be adjusted. The narrow sides can therefore be adjusted during the casting operation. After the adjustment to the desired new width, the original clamping force is restored.
The narrow sides are conical , that is, they are further apart at the top than at the bottom. This is necessary because the liquid steel in the mold is cooled so much that it forms a thin shell and contracts in the process. If the narrow sides were not tapered, the strand would no longer be guided in the lower area of the mold and heat would no longer be possible. The still very thin shell would melt and tear again due to the liquid core. In this case, the liquid core of the strand would break through the strand shell and flow into the casting arch, thus causing the plant to be shut down for hours to days. The taper of the narrow sides depends mainly on the format width and the steel quality to be cast. To put it simply, the following applies: the wider the format to be cast, the greater the narrow side taper is set.
The mold oscillates vertically during casting . This is necessary so that the strand shell that forms does not stick to the copper plates. The cast slabs therefore have so-called oscillation marks - especially on the narrow sides.
In order to prevent sticking in the mold, casting powder (release agent) is applied to the steel bath in the mold. This casting powder contains substances that prevent the strand from sticking to the copper plates. Due to the oscillation movement, the melted casting powder gets between the strand shell and the copper plates. The casting powder also has the function of isolating the liquid steel from the ambient air, as it would react with the air and thus affect the steel quality.
The slabs that can be cast vary in width and thickness. Conventional molds are typically used to produce slabs 200 to 400 mm thick and up to 2000 mm wide. Since the early 1990s, thin slab molds have also been used, with which slabs 45 to 90 mm thick and up to 2000 mm wide are cast. There are also special constructions, for example for slabs up to 3300 mm wide and 150 mm thick.
To improve the product quality, some molds are equipped with electromagnetic brakes, and more rarely with electromagnetic stirrers. These consist of coils with which magnetic fields are generated that influence the flow conditions within the steel bath in the mold. These devices can either be permanently installed on the mold or they can be devices that are only moved hydraulically to the mold after the mold has been inserted into the casting machine.
The molds for continuous billet casting are not adjustable, but have to be exchanged when changing formats. The molds are symmetrical and therefore conical on all sides. The billet formats vary from 130 to 240 mm square and 150 to 300 mm round. There are also intermediate formats for slab casting.
Beam blank formats (H format) are also used for casting for beams for both plates and tubes.
