Method and apparatus for stirring and treating continuous and semi continuous metal casting

Abstract

A method and apparatus for stirring and treating continuous and semi-continuous casting of metallic rods (12). The apparatus comprising: a stand (30) for suspending at least one electric arc electrode (24) over the upper surface of a metallic casting (14) after or during pouring, at least one electrode (24) for forming a moving electric arc (38) over the upper surface of the mettalic casting being cast, a second electrode, being the liquid metal (14), for completion of an electric circuit including said electric arc (38) and the liquid metal (14); and control means (34), connected between said apparatus and a power supply (36) and arranged to supply power to the plasma arc electrodes (14, 24), for monitoring the parameters of the electric arc (38). The invention also provides a method for casting the metallic rods (12) by using the above described apparatus.

Description

FIELD AND BACKGROUND OF THE INVENTION

The present invention relates to metal continuous and semi continuous castings. More particularly, the invention provides an improved method and apparatus for stirring and treating continuous and semi-continuous casting to produce a high-quality cast of any desired length and cross section composed of a ferrous or of a non-ferrous metal.

Continuous and Semi-continuous casting are the most commonly used methods for casting and producing semi-finished products such as plats, tubs, shits, etc. While continuous casting is the main method for casting ferrous alloys i.e. low carbon steel, stainless steel, etc., semi-continuous casting is the preferred method for casting non-ferros alloys such as aluminium and copper alloys. Both methods based on continuously pouring metal from a reservoir mostly known as the tundish or the distributor, to a cooled open mold. The mold is open both for metal entrance (usually from the top) and for metal exit (usually from the bottom). Unlike other casting methods the metal does not solidifies completely in the mold, but rather receives an initial solidifying shell and continues to cools outside the mold most commonly by water jets. The cast produced by continuous or semi-continuous casting such as billet, bloom, bar, slab etc, hereandafter defined as “rod” is advanced by means of rollers, gravitation, piston etc. the rod undergo further plastic deformation for producing the semi-finished products, mentioned above. The main difference between continuous and semi-continuous casting regards the length been cast. In continuous casting the total length can be hundreds of meters in a continuous process. At the end of the casting the rods, are cut from the casting. While in semi-continuous casting the length is constant in the size of several meters, and each billet is cast in a single process.

A further mentions of continuous casting will refer to continuous as well as semi continuous casting.

A further effect in metal alloy casting is the appearance of dendrites during cooling, these being formed during solidification as various points in the mass take up a lattice tree like structure. During the formation of dendrites, alloying elements, such as C, Cr or Ni are pushed outwardly to form a crystal grain boundary, these form later a site for the initiation of cracks in a finished component. A concentration of these alloying elements is referred to as segregation, which can to some extent be combated by lower pouring temperatures.

Impurities from the ladle or the casting powder can form inclusions and further reduce the mechanical properties of the products.

Gases, from the atmosphere or other sources are also present in the liquid metal, these being the main cause of casting porosity.

Applying roller pressure to the rod during continuous casting is proposed by Fukuoka et al. in Japanese Patent no. JP56050705A2. Pressure is said to prevent the generation of a crack on the bottom side of the casting groove. The roller is located at the point where the bent ingot is straightened. Obviously this process is of no help in improving the microstructure of the metal.

In U.S. Pat. No. 4,756,749 Praitoni et al describe and claim a process for the continuous casting of steel from a tundish having several casting spouts. While in the tundish the molten steel is subjected to further heating, which in claim 5 is a transferred-arc plasma torch. This patent is for heating the metal in the tundish, while the present invention is for treating the molten metal in the mold during the solidification process.

Lowry et al in U.S. Pat. No. 4,770,724 describe an unusual continuous casting method for metals which claims to eliminate voids and flaws and to produce a dense homogeneous product. This is achieved by forcing the metal to flow upwards, against gravity, by means of an electromagnetic field which also provides containment forces. The method disclosed includes electromagnetic stirring of the molten metal; this aspect of the process is in common industrial use, and is known to improve homogeneity and produce a fine grain structure.

However electromagnetic stirring consumes much electric power, cannot be applied to very large castings and has limited influence on ingots having aspect ratios (width/breadth ratio) in excess of 1.8

In U.S. Pat. No. 5,963,579 Henryon discloses a method of heating a molten metal in a continuous casting tundish, using a plasma torch and a special tundish. Other prior-art casting systems are referred to in this specification which also apply a plasma arc to metal held in the tundish. As with the Praitoni disclosure, such heating and/or stirring does not effect the solidification process in the mold.

PCT/IL97/00023 describes an electric arc which could be used in the present aparatus.

OBJECTS OF THE INVENTION

It is therefore one of the objects of the present invention to obviate the disadvantages of prior art for stirring and treating continuous casting rods and to provide an improved method and an apparatus for producing better quality rods, without the need to consume great quantities of electric power.

It is a further object of the present invention to provide an apparatus that will stir the metal during the solidification process and thus, increase homogeneity, decrease gas porosity and break up dendrites into small pieces and thereby reduce the grain size of the finished rod.

Yet a further object of the invention is to improve the production of a continuous rods having aspect ratios exceeding 1.8.

SUMMARY OF THE INVENTION

The present invention achieves the above objects by providing a stirring and treating continuous casting apparatus for the production of improved-quality rods from molten metal, including rod cross-sections having a aspect ratio exceeding 1.8, the apparatus comprising:

• • a) a stand for suspending at least one electric arc electrode over the upper surface of a metallic casting after or during pouring;
  • b) at least one electrode for forming a moving electric arc over the upper surface of said metallic casting being cast:
  • c) a second electrode, being the liquid metal, for completion of an electric circuit including said electric arc and the liquid metal; and
  • d) control(s) mean(s) connected between said apparatus and a power supply and arranged to supply power to the plasma arc electrode(s) for monitoring the parameters of the electric arc.

The invention also provides a method for improving quality, such as: reducing voids, inclusions, porosity, dendrite and grain size in metallic continuous rod casting and for improving homogeneity therein, the method comprising

• • step a) continuously pouring the liquid metal into a mold for casting rods.
  • step b) providing at least one first electric arc electrode and positioning same slightly above the upper surface of the molten metal;
  • step c) providing a second electrode in electrical contact with a section of the mold being electrically conductive, and applying an electric current to said electrodes to form an arc between said first electrode and said upper surface; and
  • step d) continuously moving said electric arc(s) over said upper surface.

Yet further embodiments of the invention will be described hereinafter.

It will thus be realized that the novel device of the present invention improves rod quality by stirring the liquid metal in the mold. The electric power required for this purpose is moderate. For example, in a test carried out in producing a semi-continuous slab, aspect ratio 2, made of copper nickel alloy, the plasma-arc consumed only 2 kWh per ton.

It is to be stressed that the method and apparatus of the present invention have been tested in practice, and castings produced by the new method have been examined and compared to the same castings produced by conventional methods. Test results, also to be found in the description, leave no doubt as to the improved quality achieved by use of apparatus constructed according to the present invention.

SHORT DESCRIPTION OF THE DRAWINGS

The invention will now be described further with reference to the accompanying drawings, which represent by example preferred embodiments of the invention.

Structural details are shown only as far as necessary for a fundamental understanding thereof. The described examples, together with the drawings, will make apparent to those skilled in the art how further forms of the invention may be realized.

In the drawings:

FIG. 1 is a partially section of a preferred embodiment of the apparatus according to the invention;

FIG. 2 is a detail section of the first electrode suspended over the rod;

FIG. 3 is a partially sectional view of an embodiment of the apparatus wherein a second electric circuit is provided to create a magnetic field urging said plasma arc towards the center of the upper surface.

FIG. 4 is a detail sectional view of an embodiment with a refractory guard ring arranged to prevent solid impurities such as casting powder from reaching the upper surface under the plasma arc;

FIG. 5 is a detail sectional view of an embodiment arranged to provide a stream of inert gas for preventing oxidation of the molten metal;

FIG. 6 is a photographic view of a section of a slab cast by conventional methods; and

FIGS. 7 a and b are photographic views of a section of a slab cast from the same material using the method and the apparatus of the present invention.

FULL DESCRIPTION OF THE INVENTION

There is seen in FIGS. 1 and 2 a continuous casting apparatus 10 for the production of improved-quality rods 12 from molten metal 14.

A water-cooled mold 16 has a cavity open at its upper end 20 to receive molten metal 14 and open at its lower end 22 to release the partially solidified rod 12 being formed. The mold 16 is electrically conductive, and functions as one of the components of a electric circuit.

A first plasma arc electrode 24 is positioned so that its lower face 26 thereof is disposed in proximity to the upper surface 28 of the molten metal 14 having been poured into the mold 16.

A stand 30 is provided for supporting the first plasma arc electrode 24 above the upper surface 28, leaving a gap of between 2-20 mm between the first electrode 24 and the upper surface 28 of the molten metal 14.

A second plasma arc electrode 32 is attached to the electrically conducting mold 16 or to the metal 12 or 14.

Control means 34 are connected between the apparatus 10 and power supply source 36. The control means 34 controls the electric circuit parameters, thus enabling an operator to increase or decrease power in accordance with the needs of the process. The plasma arc 38 is formed between the upper surface 28 of the molten metal 14 and the lower face 26 of the first electrode 24. The plasma arc 38 completes an electric circuit, and causes stirring of the molten metal 14 in the mold 16.

The rod 12 continuous to grow in length as long as required and as long as further molten metal 14 is available from the tundish 46 through the nozzle 48.

With reference to the rest of the figures, similar reference numerals have been used to identify similar parts.

Referring now to FIG. 3, there is seen a continuous casting apparatus further provided with a second electric circuit 68, requiring less power than the first electric circuit, is provided to create a magnetic field urging the plasma arc 38 (see FIG. 2) towards the center of the upper surface 28 (see FIG. 2) of the molten metal 14.

The second electric circuit 68 passes through the molten metal held in the tundish 46 and through the nozzle 48 and reaches the molten metal 14 in the mold 16.

FIG. 4 shows a detail of a continuous casting apparatus 10, further provided with a refractory guard ring 54 arranged to prevent solid impurities such as casting powder 56 from the upper surface 28 under the plasma arc 38; The refractory guard ring 54 submerged to a level below the upper surface 28 of the molten metal 14 contained in the mold 16.

FIG. 5 shows a detail of a continuous casting apparatus 10, further provided with a stream of inert gas 76 device for preventing oxidation of the molten metal 14; The first electrode 24 is hollow and a jet tube 78 is provided for directing a stream of an inert gas 76, The gas jet 78 impacts the upper surface 28 of the molten metal 14 of the rod 12 in the mold 16. The inert gas prevents oxidation of the molten metal forming the upper surface 28, and removes therefrom any solid impurities, particularly any casting powder if present.

Any combination of the embodiments described and illustrated in FIG. 3, FIG. 4 and FIG. 5 can be applied.

FIG. 6 is a photograph 90 of a copper-nickel slab section of 400*200 mm cast by conventional semi-continuous casting.

FIGS. 7 a and b are photographs of the same alloy been treated by the method and apparatus of the present invention. The improved macrostructure is evident.

The scope of the described invention is intended to include all embodiments coming within the meaning of the following claims. The foregoing examples illustrate useful forms of the invention, but are not to be considered as limiting its scope, as those skilled in the art will readily be aware that additional variants and modifications of the invention can be formulated without departing from the meaning of the following claims.

Claims

1. A continuous and semi-continuous casting apparatus for the production of improved-quality rods, including rod cross-sections having a aspect ratio exceeding 1.8, the apparatus comprising: a) a stand for suspending at least one electric arc electrode over the upper surface of the molten metal casting after or during pouring; b) at least one electrode for forming a moving electric arc over the upper surface of said metallic casting being cast: c) a second electrode, being the liquid metal, for completion of an electric circuit including said electric arc and the liquid metal; and d) control(s) mean(s) connected between said apparatus and a power supply and arranged to supply power to the plasma arc electrode(s) and monitoring the parameters of the electric arc.

2. The continuous and semi continuous casting apparatus as claimed in claim 1, wherein a second electric circuit is provided to create a magnetic field urging said plasma arc towards the center of said upper surface of said molten metal, by said second electric circuit passes through molten metal held in a reservior and reaches said metallic component of said mold through a molten metal column formed in a feed tube.

3. The continuous and semi continuous casting apparatus as claimed in claim 1, wherein said first electrode is hollow and means are provided for directing a stream of an inert gas, there through to impact said upper surface for preventing oxidation of the molten metal forming said upper surface and for the removal therefrom of solid impurities and the removal of casting powder if present.

4. The continuous and semi continuous casting apparatus as claimed in claim 1, further including a refractory guard ring slightly deeped in the surface of said rod to maintain exclusion of solid impurities such as casting powder from said upper surface under the plasma arc, and to electrically insulate an upper portion of said mold from said plasma arc.

5. A continuous and semi continuous casting apparatus substantially as described hereinbefore and with reference to the accompanying drawings.

6. A method for continuous and semi continuous metal casting for improving quality of metallic continuous rods, said method comprising; step a) continuously pouring the liquid metal into a mold for casting rods. step b) providing at least one first electric arc electrode and positioning same slightly above the upper surface of the molten metal; step c) providing a second electrode in electrical contact with a section of the mold being electrically conductive, and applying an electric current to said electrodes to form an arc between said first electrode and said upper surface; and step d) continuously moving said electric arc(s) over said upper surface.

7. A continuous or semi continuous casting method as claimed in claim 6 including the use of casting powder by: step a) continuously pouring a liquid metal into a mold and applying casting powder to the rod; step b) removing the casting powder from the arc working zone. One of the ways to remove the casting powder is by blasting an inert gas such as argon over said upper surface; step c) preventing the return of said casting powder by placing at list one refractory guard ring on said upper surface; step d) providing at least one plasma arc electrode and positioning same slightly above said upper surface of the molten metal, inside each protecting ring; step f) providing a second electrode in electrical contact with a section of the mold being electrically conductive, and applying an electric current to said electrodes to form an arc between said first electrode and said upper surface; and step e) continuously moving said electric arc(s) over said upper surface.

8. A continuous or semi continuous casting method as claimed in claim 6, wherein said electric current applied to said electrodes to form a plasma is a DC current.

9. A continuous casting or semi continuous method, substantially as described hereinbefore and with reference to the accompanying drawings.