GB2040751A

GB2040751A - Salt mixture and process for the casting of copper and copper alloys

Info

Publication number: GB2040751A
Application number: GB7943729A
Authority: GB
Original assignee: Wieland Werke AG
Current assignee: Wieland Werke AG
Priority date: 1979-01-18
Filing date: 1979-12-19
Publication date: 1980-09-03
Also published as: DE2901838C2; FR2446688A1; GB2040751B; SE8000177L; FR2446688B1; DE2901838B1

Abstract

The mixture contains a strong oxidising agent so that an oxidised layer is provided on the surface of the casting. The mixture acts as a lubricant between the casting the mould and overcomes the problem caused by blow holes on the outer shell of the casting while it is still liquid. Various salt mixtures are described which promote flaking off of the glass/salt film immediately below the mould.

Description

SPECIFICATION Salt mixture and process for the continuous casting of copper and copper alloys This invention relates to a salt mixture when used as a separating agent and lubricant in the continuous casting of copper and copper alloys, in particular although not exclusively to zinc-containing copper alloys.

In the continuous casting of copper and copper alloys, and particularly brass alloys, it is generally known to use carbon in the form of carbon black or graphite as a separating agent and lubricant.

In the casting of zinc-containing copper alloys, it has also been known for some time to employ other inorganic materials, namely salt mixtures or so-called "glassy melts" (for example borate glasses), as separating agents and lubricants. These glassy materials are sprinkled onto the bath level of the continuous casting mould used, are fused there and cover the wall of the mould with a film of a significant thickness, which separates the casting from the wall of the mould and possesses good lubricant properties at the interface with the casting. The glassy film restricts the vaporisation of zinc, which is very extensive when zinc-containing copper alloys are cast with a carbon black coverage, but it still permits a significant vaporisation of zinc from the surface of the casting. The zinc condenses on the cold wall of the mould and acts as an adhesion promoter for the salt film (compare, for example, "Stranggiessen", Deutsche Gesellschaft fur Metallkunde 1978, page 97-103).

In spite of the advantages of the glassy film, which have been described, however, the disadvantageous so-called "blow holes" still occur in the casting. "Blow holes" are understood as defects, the primary cause of which is to be ascribed to the action of a gas pressure (bubbles) on the outer shell of the casting while it is still liquid. Worm-like pipes of a few millimeters' length form in the front of the growing crystals, which pipes are filled with salt in the present case and cannot even be removed, for example, by machining of the cast bolts.

Hitherto it was not possible to employ salt mixtures of this type with zinc-free copper alloys, since there were no adhesion promoters for the salt film present.

It is an object of this invention to provide a salt mixture for the continuous casting of copper and copper alloys, by means of which the formation of "blow holes" in the casting is substantially avoided and by means of which an adhesion promoter for the salt film is produced on the wall of the mould is also provided.

According to this invention, there is provided a salt mixture when used as a separating agent and lubricant in the continuous casting of copper and copper alloys, wherein the salt mixture contains a strong oxidising agent.

"Strong" oxidising agents are understood in this context as chemical substances which, in the temperature range used, give in the salt mixture an oxygen activity which is sufficient for the formation of oxide layers of copper and the usual main alloy constituents (in particular zinc, tin and nickel) in an adequate thickness on the solidifying melt.

The oxide film built up from oxides of copper and its alloy constituents has a thickness in the lim range.

The oxide film forming on the surface of the casting acts in particular as a gas barrier and prevents "blow holes". However, when metal moulds (in particular copper moulds) are used, the copper is oxidised so that the copper oxide acts as an adhesion promoter and the salt film acting as a separating agent and lubricant is more firmly retained in the mould.

It is known from German Patent Specification 622,182 to use salt mixtures together with oxidising agents in the cleaning or degassing of alloys, in particular those smelted from scrap metal. In that said Patent, however, the oxygen introduced with the oxidising agent must stoichiometrically correspond to the ratio of the impurities dissolved in the bath so that oxidation of the metal to be cleaned is thus to be avoided. At the same time, the action of the oxidising agent in the salt mixture must be assisted by the flame which is necessary for heating the melt in the furnace.

In this invention, the following can be used as the oxidising agent in an advantageous manner: Nitrates of the alkali metals or alkaline earth metals (for example KNO3, Ca(NO3)2), peroxides (for example Na2O2), metal oxides (for example MnO2, PbO2, Pb304) and also persulphates (for example Na2S208).

It is preferable to select a proportion of the oxidising agent in the salt mixture of between 1 and 10% by weight.

Usually the salt film solidifies on the casting emerging from the mould. In many cases, however, it is a requirement that the continuously cast bolts or plates, which are to be processed further, must not have a salt crust. For example, bolts consisting of simple brass materials are not machined, but a clean surface of the casting is a prerequisite for perfect flow behaviour in the extruder and it is therefore necessary completely to avoid a salt crust.

in a special embodiment of the invention, the salt mixture contains a substance which promotes crystallisation, preferably calcium oxide. In this way, the production of solid glassy crusts on the cast bolts is suppressed and a crystalline lattice structure is forced to form in the salt layer. The crystallisation is associated with the formation of micro-cracks which promote flaking-off of the film or its dissolution by the cooling water used in continuous casting. The proportion of this substance in the salt mixture is advantageously 1 to 10% by weight.

In a further preferred embodiment of the invention, the salt mixture, in order to avoid a salt film on the solidified casting, contains components of such a type that a miscibility gap arises in the solid state, two or more phases being formed. The sodium chloride/potassium chloride (NaCI/KC) system is preferred (concerning the formation of the miscibility gap, compare, for example, M.K. Reser (Editor): "Phase Diagrams for Ceramists", 1964, in particular page 376).

According to a feature of this invention a process for the continuous casting of copper and copper alloys, in particular zinc-containing copper alloys, is provided wherein a salt mixture according to the invention is added as a separating agent and lubricant to the metal melt wherein the oxidising agent is added in a concentration of 0.5 to 30 g/m2 of surface area of the casting, preferably 1 to 10 g/m2 of surface area of the casting.

In a preferred embodiment of the process according to the invention, sufficient oxidising agent is added for a visible oxide film to form on the casting which is to be poured.

The invention will now be described with reference to the following illustrative examples. Substances of the following composition were used in the continuous casting of different copper alloys in sliding moulds which were independent of the furnace. Perfect surfaces of the casting were obtained in all cases Example I. When continuously casting tin bronze bolts which were to be machined before further processing, salt mixtures without those additives which promote flaking-off of the glass film were used, for example: A) 60% by weight of Na4P207 (sodium diphosphate) 30% by weight of Na2SO4 (sodium sulphate) 5% by weigh.. of CaSO4 (calcium sulphate) 2% by weigh of KCI (potassium chloride) 2% by weight of Na2CO3 (sodium carbonate) 1% by weight of KNO3 (potassium nitrate) b) 90% by weight of Na2B407 (sodium tetraborate) 7% by weight of Na2CO3 (sodium carbonate) 3% by weight of MnO2 (manganese dioxide) c) 90% by weight of K2O.4SiO2 (potassium silicate) 7% by weight of Na4P207 (sodium diphosphate) 3 Ó by weight of MnO2 (manganese dioxide) Example 2. When continuously casting brass bolts which were not to be machined further before subsequent processing in the extruder, salt mixtures with NaCI and KCI or CaO were used, which favour flaking-off of the salt film, for example: a) 60% by weight of Na2B407 (sodium tetraborate) 15% by weight of KCI (potassium chloride) 10% by weight of NaCI (sodium chloride) 4% by weight of CaSO4 (calcium sulphate) 3 zÓ by weight of K2CO3 (potassium carbonate) 3% by weight of Na2CO3 (sodium carbonate) 30ib by weight of Li2CO3 (lithium carbonate) 2% by weight of Na2O2 (sodium peroxide) (b) 50% by weight of Na2B407 (sodium tetraborate) 18% by weight of NaCI (sodium chloride) 189b by weight of KCI (potassium chloride) 6% by weight of Na2CO3 (sodium carbonate) 3% by weight of CaO (calcium oxide) 5% by weight of KNO3 (potassium nitrate) The conditions described are illustrated in more detail by reference to the drawings in which Figure 1 shows a longitudinal section through a continuous casting installation known in the art and Figure 2 shows, on an enlarged scale, the casting!mould transition in accordance with the process of the invention.

In Figure 1, a water-cooled continuous casting mould 1 surrounds a metal casting 2 on which is forming a solid salt film 3, the liquid salt film 4 being located between the solid salt film 3 and the metal casting 2. As can be seen, "blow holes" 5 filled with salt appear in the metal casting 2. The salt film has substantially remained on the solidified metal casting 2 and has not flaked off.

As shown in Figure 2, an oxide layer 6 additionally forms in accordance with this invention, between the metal casting 2 and the liquid salt film 4, which oxide layer acts as a gas barrier so that the disadvantageous blow holes are avoided. Oxide islands 7 on the wall of the mould 1, from which act as an adhesion promoter for the solid salt film 3.

Claims

1. Salt mixture when used as a separating agent and lubricant in the continuous casting of copper and copper alloys, wherein the salt mixture contains a strong oxidising agent.

2. Salt mixture according to Claim 1,wherein it contains nitrates of the alkali metals or alkaline earth metals as the oxidising agent.

3. Salt mixture according to Claim 1, wherein it contains peroxides as the oxidising agent.

4. Salt mixture according to Claim 1, wherein it contains metal oxides as the oxidising agent.

5. Salt mixture according to Claim 1, wherein it contains persulphates as the oxidising agent.

6. Salt mixture according to any of Claims 1 to 5, wherein the proportion of the oxidising agent is 1 to 10% by weight.

7. Salt mixture according to any preceding Claim, wherein it contains a substance which promotes crystallisation.

8. Salt mixture according to Claim 7, wherein it contains calcium oxide.

9. Salt mixture according to Claim 7 or 8, wherein the proportion of the substance which promotes crystallisation is 1 to 10% by weight.

10. Salt mixture according to any preceding Claim, wherein it contains components which in the solid state form a miscibility gap.

11. Salt mixture according to Claim 10, wherein it contains the sodium chloride/potassium chloride system known per se.

12. Process for the continuous casting of copper and copper alloys, wherein a salt mixture according to Claims 1 to 11 is added as a separating agent and lubricant, and the oxidising agent is added in a concentration of 0.5 to 30 g/m2 of surface area of the casting.

13. Process according to Claim 12, wherein the concentration of the oxidising agent is 1 to 10 g/m2 of the surface area of the casting.

14. Process according to Claim 12 or 13, wherein sufficient oxidising agent is added for a visible oxide film to form on the casting which is to be poured.

15. A salt mixture substantially as herein described with reference to the examples.

16. A process for the continuous casting of copper and copper alloys substantially as herein described with reference to the examples and accompanying drawings.