WO1996036449A1

WO1996036449A1 - A method of continuously casting a metal

Info

Publication number: WO1996036449A1
Application number: PCT/SE1996/000651
Authority: WO
Inventors: Lars Gunnar Johansson; Göran Carlsson
Original assignee: Mefos, Stiftelsen För Metallurgisk Forskning
Priority date: 1995-05-19
Filing date: 1996-05-16
Publication date: 1996-11-21
Also published as: AU5786396A; SE9501866D0; SE9501866L

Abstract

In continuous casting, the mould (11) has a heat insulating cover (12) and molten steel and liquid synthetic slag are supplied to the mould through tubes (14 and 13 resp.) that extend through the cover. The positions of the surface (21) of the steel and the surface (20) of the slag are continuously measured by means of a radar (15) or any other radiation based measuring device and the supply of steel and slag is controlled so that the surfaces are constantly on the same predetermined levels in the mould and so that the slag surface is at a distance from the cover. The space between the slag surface and the cover is flushed with a protective gas, for example argon.

Description

A method of continuously casting a metal.

This invention relates to a method of continuously casting a metal in a mould.

Conventionally, in the continuous casting of steel, a water cooled, oscillating mould of copper is used. A casting powder is used as a lubricant. It is supplied as a powder directly onto the metal surface in the mould. The powder against the metal surface melts to form a molten slag and in the steady state, there will normally be a 10-15 mm thick layer of molten slag floating on the molten metal. Above this layer, there will be a sintered semi-solid layer of casting powder and on top there will be a porous layer of heat insulating comparatively cold casting powder. A molten slag layer which is normally about 0.25 mm will be drawn down along the walls of the mould and form a lubricant layer between the mould and the metal. This consumption of casting powder is compensated for by an intermittent supply of casting powder to the mould.

The casting powder serves many purposes, Besides being a lubricant, it serves as a collector of nonmetallic

inclusions in the molten metal, as a thermal insulator of the metal surface, as a dampener αf surface movement, and as a barrier between the molten metal and the atmosphere. As can be readily understood, even a superior casting powder cannot be perfect for all these purposes and this is true particularly when the mould has a large relation between its width and its breadth or when there is a large variation in the velocity of the metal close to the surface so that the heat transmitted to the slag varies. When casting steel with extra low carbon or ultra low carbon content, the carbon content of the casting powder desirable for controlling the melting properties of the powder may be detrimental to the steel. It is an object of the invention to permit for a more controlled and stable continuous casting process, a higher process capability and a higher quality of the surface of the cast product ( eg a slab, a billet or a bloom). This is accomplished generally by the separation of the various functions in the way defined in the claim so that the process can be controlled better and can reach its steady state faster when the casting is started.

The invention will be described with reference to the accompanying drawing which shows schematically a portion of a mould for continuous casting. The copper walls of the water-cooled oscillating mould have been given reference numeral 11. The mould 11 has a thermal insulating cover 12 through which a slag supplying tube 13 and a steel supplying tube 14 extend. A radiation based distance measuring

instrument 15 that measures reflected radiation is arranged above the cover 12 and measures through the cover. It transmits electromagnetic signals and receives the signals reflected in the surfaces and calculates the distance to the surfaces, i.e. the positions of the surfaces. Casting powder is melted to a synthetic liquid slag in a non-illustrated furnace and is then supplied through the tube 13 and forms a layer of liquid slag 16 that floats on the molten steel 22. The slag is drawn down along the solidified steel shell as a result of the pumping effect of the oscillating mould. The oscillation of the mould is indicated by the arrow 25 whereas the steady downward movement of the steel shell 17 is indicated by the arrow 26. The non-illustrated means for oscillating the mould is conventional. As the slag

solidifies against the walls of the mould 11, the thickness of the layer 16 of liquid slag decreases gradually. Outside the liquid slag 16, there is a layer 18 of crystalline slag and outside this layer 18 there is a layer 19 of glossy slag.

With the measuring instrument 15, one can detect the position of the surface 20 of the slag as well as the surface 21 of the steel, that is, the interface between the steel and the slag. Both these surfaces 20,21 are distinct since the slag layer is completely liquid. Thus, the supply of molten steel can be controlled so that the surface 21 of the molten steel can be held at a predetermined position with a minimum of variation. The suppply of liquid slag can also be controlled so that the slag thickness can be held constant with a minimum of variation and at a distance from the cover 12. These supply controls can be carried out manually or automatically in response to the actual

positions of the surfaces 20,21. The space between the cover 12 and the slag is preferably filled or flushed with a protective gas, for example argon. The conditions of the casting will thus be very stable and the capability of the process will be better than it will be in conventional continuous casting. Also when casting very thin slabs, one will have a stable liquid slag layer with a defined

thickness adjacent all parts of the walls 11 of the mould and the slag layer will be less sensitive to the surface waves and movements that may occur due to the supply of the steel. The risk of surface defects as well as the risk of slag inclusions are therefore reduced.

The distance measuring instrument 15 may use radar or microwaves. It can for example principally be of the kind described in WO 94/18549 or be a conventional radar

equipment.

Claims

Claim

A method of continuously casting a metal in a mould, comprising

providing a heat insulating cover (12) on the mould, supplying liquid metal and liquid slag to the mould, using a radiation based equipment (15) to determine the positions of the metal surface and the slag surface, and controlling the supply of metal and slag such that the slag will cover the surface of the metal and the slag surface will be at a distance from the cover.