GB2112135A

GB2112135A - Indicating molten metal levels

Info

Publication number: GB2112135A
Application number: GB08232744A
Authority: GB
Inventors: David George Goodrich
Original assignee: Kaiser Aluminum and Chemical Corp
Current assignee: Kaiser Aluminum and Chemical Corp
Priority date: 1981-12-10
Filing date: 1982-11-17
Publication date: 1983-07-13
Also published as: KR840002675A; SE458641B; AU550044B2; FR2517996A1; SE8207039L; IT8224682A0; JPS58105017A; JPH0259406B2; PT75944A; BR8207113A; NL8204770A; DE8233113U1; DE3243650A1; ES278758Y; CA1200976A; SE8207039D0; ES278758U; IT1207968B; PT75944B; FR2517996B1

Abstract

A float (10) for sensing a molten metal level (28), comprises an upper section (11) which has an essentially flat under surface (12) adapted to rest on the molten metal surface and a lower section (13) adapted to be submerged beneath the level (28) of the molten metal. The area of the flat under surface (12) is at least 10% of the area of an image of the float (10) projected downwardly. The volume of the lower section (13) of the float (10) submerged beneath the level (28) of the molten metal displaces a volume of molten metal essentially equal in weight to the weight of the float (10) and any vertical force applied to the float (10) by attachments thereto. Preferably, the area of the flat undersurface (12) is at least 25% of the area of the image projected downwardly; the float (10) may be formed from a lightweight refractory material. <IMAGE>

Description

SPECIFICATION Molten metal float This invention relates to an improved float for accurately sensing the level of a molten metal surface. The float is particularly suitable in molten metal level control systems for vertical DC (direct chill) casting and vertical EM (electromagnetic) casting of metals such as aluminum, magnesium and their alloys.

DC casting generally comprises directing molten metal into the feed end of a water cooled, open ended tubular mold and withdrawing solidified or partially solidified metal out of the discharge end of the mold. Coolant, usually water, is directed onto the surfaces of the metal exiting the discharge end of the mold and this application of coolant effects most of the solidification. At the start of the cast, when molten metal is first introduced into the mold, the discharge end thereof is blocked off with a downwardly moveable bottom block which supports the ingot or billet in its downward descent during casting.

EM casting is very similar to DC casting except that instead of a mold controlling the shape of the molten metal until it solidifies, the shape of the molten metal is controlled by the pressure developed by an electromagnetic field which is generated by an annular inductor surrounding the molten metal. In the EM casting process essentially all solidification is effected by the application of coolant to the metal surface at the discharge end of the inductor.

Controlling the molten metal level within the metal shaping means, whether it be a mold or an electromagnetic field is very important in vertical continuous casting processes. For accurate head control it has been found most advantageous to utilize a float device which is operatively connected to a level sensor such as the linear displacement transducer described in copending Application SN 266,788, filed May26, 1981. In the process described and claimed in the above application the signal from the transducer is used to regulate the flow of molten metal to the mold or inductor which thereby controls the level of molten metal.

However, in the past accurate positioning of the float has been difficult due to the variability of the molten metal meniscus. For example, if a float is pushed down into the molten metal it will frequently not return to the same precise position when the added thrust is removed and the float is allowed to rise by its natural boyancy. Similar results occur when the float is partially raised out of the molten metal. Because of this inability of the float to return to the same position each time, accurate molten metal level control has been difficult when the float is used to sense the metal level. This is particularly critical with EM casting because slight molten metal head changes can significantly change the dimensions of the ingot or billet being cast. Changes in the surface tension of molten metal due to changes in temperature, composition and the like can aggravate this problem.

It is against this background that the present invention was developed.

This invention relates to an improved molten metal float which when positioned on a molten metal surface consistently has the same relative position with respect to the surface of the molten metal.

In accordance with this invention the float is provided with an upper portion which has an essentially flat lower surface adapted to rest on a molten metal surface and a lower section comprising an element which projects from the flat areas of the upper portion of the float into the body of the molten metal. The area of the essentially flat surface of the upper portion in contact with the molten metal should be at least 10%, preferably at least 25% of the area of the float facing the molten metal surface. The element projecting into the molten metal displaces an amount of metal having a weight essentially equal to the total weight of the float and any force applied to the float by attachments thereto.

In a preferred embodiment the float is used as a skimming device to prevent the incorporation of oxides or dross into the metal being cast. In this case the float is generally annular or ringlike in nature so that when molten metal is introduced into the inner portion of the float the oxides which are on the surface or which rise to the surface during casting are contained by the float. In this preferred embodiment, the element projecting into the molten metal ensures that the oxides or dross do not escape from the float during casting. To effectively skim, the lower section should project at least 0.5 inch preferably at least 1.0 inch into the molten metal. Although the shape of the element projecting into the molten metal is not cirtical, excessive projections, e.g. 3 inches or more, into the molten metal are not desirable.

Reference is made to the drawings which illustrate embodiments of the invention. Figure 1 is a perspective view partially in section of a preferred float of the invention. Figure 2 is a cross sectional view in elevation of the flat of the invention installed in an elctromagnetic casting assembly. Figures 3 and 4 represent other embodiments of the invention. In the drawings all corresponding parts are numbered the same.

Figure 1 shows a perspective view partially in section of a preferred float 10 which comprises an upper section or collar 11 with an essentially flat lower surface 1 2 and a lower section or projecting element 13. The volume of projecting element 13 is equal to the volume of molten metal it displaces so that the flat surface 12 of collar 11 rests on the molten metal surface. The weight of the metal displaced by element 1 3 should be equal to the weight of the float and any force applied to the float by attachments thereto. The area of the flat surface 1 2 must be at least 10%, preferably at least 25%, of the total area of the float projected downwardly onto the molten metal surface.

Figure 2 illustrates a preferred float 10 in association with an EM casting assembly 20, which comprises an inductor 21, a water jacket 22 and a refractory down spout 23 which is adapted to feed molten metal to the interior or inner space of the annular inductor 21. A bottom block 24 is also associated with the assembly 20 and at the start of the cast is positioned within the inductor to vertically support the molten metal therein while the electromagnetically induced pressure against the molten metal controls the lateral spread of the metal until it solidifies into its final shape. The inductor 21 is provided with a plurality of holes or conduits 25 through which water from the water jacket passes for the application thereof onto the ingot or billet which exits from the discharge end of the inductor.The float 11 is provided with rods 26 and 27 to support the float during noncasting periods when no molten metal is within the inductor 20.

Preferably, one of the rods 26 or 27 is operatively connected to a linear displacement transducer (not shown) or its equivalent which will generate a signal representing the molten metal level 28 on which the float 10 rests. The signal from the transducer can be used to control the flow of molten metal from a source such as a trough through a refractory lined down spout 23. (See such use described in copending application of Takeda et al, SN 266,798, filed May 1 981.) In Figures 3 and 4 alternative float designs are provided wherein the relative positions of the flat bottomed collars 11 and the projecting elements 1 3 have been changed in comparison with the embodiment shown in Figure 1.

The float is made from a material which has a density much less than the density of the molten metal and which is sufficiently resistant to the molten metal and the surrounding harsh environment to provide a reasonably long service life. Suitable materials of construction include lightweight refractories such as a fibrous magnesium silicate, glass rock foam, foamed refractories and the like. A preferred material is fibrous magnesium silicate sold under the trademark Marinite.

It is obvious that various modifications and improvements can be made to the invention without departing from the spirit of the invention and the scope of the appended claims.

Claims

1. A molten metal float for sensing a molten metal level comprising an upper section which has an essentially flat under surface adapted to rest on the molten metal surface and a lower section adapted to be submerged beneath the level of the molten metal, the area of the flat under surface being at least 10% of the area of an image of the float projected downwardly and the volume of the lower section of the float submerged beneath the level of the molten metal displacing a volume of molten metal essentially equal in weight to the weight of the float and any vertical force applied to the float by attachments thereto.

2. The float of claim 1 wherein the area of the fiat area is at least 25% of the area of the image projected downwardly.

3. The float of claim 1 wherein the float is formed from a lightweight refractory material.

4. The float of claim 1 wherein the lower section thereof projects into molten metal at least 0.5 inch but less than 3 inches from the flat surface of the upper section.

5. The float of claim 4 wherein the lower section projects at least 1.0 inch from the flat surface of the upper section.

6. An annular or ring shaped molten metal float and skimmer comprising an upper section which has a flat under surface adapted to rest on a molten metal surface and a lower section adapted to be submerged beneath the level of the molten metal, the area of the flat under surface being at least 1 0% of the area of an image of the float projected downwardly and the volume of the lower section of the float submerged beneath the level of the molten metal displacing a volume of molten metal essentially equal in weight to the weight of the float and any vertical force applied to the float by attachments thereto.

7. A molten metal float for sensing a molten metal level in electromagnetic casting, substantially as hereinbefore described with reference to the accompanying drawing.

8. An electromagnetic casting assembly incorporating a molten metal float, for sensing a molten metal level, as claimed in any preceding claim.