GB2110713A - Mould release composition - Google Patents

Abstract

The present invention relates to compositions and methods for coating moulds used in processing of molten metals. The compositions comprising a first finely divided refractory mineral component, and a second phosphate radical and chromium radical containing material(s) component. The second component being adapted to maintain the two components in a heat-resistant bound relationship, or usuable on its own.

Description

SPECIFICATION Mould release composition The present invention relates to compositions for coating moulds used in processing of molten metals as well as to moulds coated with said compositions and to coating methods related thereto.

Moulds may be formed of cast-iron, direct metal from a blast furnace, or formed from sand bonded together with bentonite or other chemical binder. In the ensuing description, particular reference is made to the teeming of molten steel into cast-iron moulds sitting on a cast-iron stool and to the problems associated therewith. However it is to be understood that the invention is of wider application being appropriate to metal casting generally-and to use in relation to sand formed moulds and cores (also discussed hereafter).

It has been the practice in the processing of molten metals to apply a coating to the surface of moulds prior to pouring metal into the moulds. This practice has been followed extensively in the steel industry, where cast-iron ingot moulds are normally coated prior to the forming of steel ingots. The functions of mould coatings used in the processing of molten steel are to reduce mould wear and to afford a good parting plane, thus protecting the surface qualities of the moulded metal.

In the teeming of molten steel into a cast-iron mould and stool assembly, damage such as erosion and grain growth causes the development of microscopic cracks in the surface of the cast iron mould allowing the ingot or billet to firmly secure itself to the stool and faces of the mould. Accordingly a stool/mould coating conventionally is employed to fill the voids in the stooi/mould faces thus allowing easy removal of the resultant ingot or billet from the cast-iron mould and stool assembly.

In the case of sand moulds, sand, commonly silica sand, is normally mixed with a chemical binder and catalyst, moulded around a pattern and cured into the desired shape. Alternatively sand can be bonded with bentonite and rammed around a pattern to form the desired shape.

Such sand moulds have inherent porosity which prevents their effective use without a mould coating since otherwise metal penetration into the sand voids would occur thereby producing a sand-metal conglomerate firmly adhering to the surface of a casting. Removal of the conglomerate by shot blasting and other suitable tools is often a necessary but expensive step. Various forms of mould coating have been developed for such sand moulds.

Bitumenous tar, charcoal and talc combinations were amongst the early materials used in mould coatings; however, these were discarded because of the expense and difficulty involved in applying them to moulds.

More recent coating materials comprise a range of refractory mineral fillers, binders, carrier liquid, suspension of gelling agents and anti-fermentation agents. However, the disadvantage of most of these known coatings is their tendency to cause non-metallic inclusion in the molten metal. This occurs whenever the mould coating fails to adhere firmly to the stool, mould or core surface. A common reason for this occurrence is the destruction of the binder by radiated or convected heat from the poured metal before the mould cavity is filled with molten metal, allowing the molten metal to penetrate the residual mineral.

A further and major disadvantage of presently known coatings becomes evident when the poured ingot is removed from the mould. With the destruction of the binder by heat, the mould coating is broken down to its basic components. Thus the finely divided refractory material tends to become suspended in the surrounding air by convection currents.

The present invention seeks to overcome or substantially alleviate the above disadvantages.

By the invention there is provided improved: (1) Mould coating compositions; (2) Methods of mould coating; (3) Moulds coated with the foregoing compositions or by the foregoing methods.

In one broad aspect, the invention provides a composition adapted to be used for the protective coating of metallic or other moulds of the type wherein casting or other processing operations are carried out, the said composition comprising a first finely divided refractory mineral component and, in association therewith, a second component, adapted to maintain the two components in heat-resistant bound relationship, the said second component comprising at least one material which contain(s) phosphate radical(s) and chromium radical(s) and/or at least one material from which such radicals can be jointly and/or separately derived, the composition being such that, when it is applied to a mould surface and the mould is subjected to the conditions under which a said casting or the like operation is carried out, the second component will be effective to maintain the said bound relationship whilst itself functioning as a surface protecting refractory material.

The invention also provides a method for the production of a composition which is adapted to be used for the protective coating of metallic or other moulds of the type wherein casting or other processing operations are carried out, which comprises compounding together, as by aqueous slurrying, a first and second component as above defined.

In a further aspect, there is provided by the invention, a method for protectively coating a surface of a metallic or other mould of the type wherein casting or like operations are carried out, wherein the said surface has applied thereto a composition as above defined, or a second component of the type defined.

Finally, the invention also provides a mould adapted to be used for casting or other processing operations, such as an ingot mould for the casting of steel ingots, wherein the or each operative surface of the mould is coated with a composition as above defined, or with a said second component of the type defined.

The mould coating composition of the present invention is characterized by the following advantageous properties.

(1) Ease of application.

(2) An ability to cure or dry-out to form a smooth skin which adheres firmly, filling any defects in the stool, mould or core surface.

(3) It is resistant to high temperatures and remains effective whilst molten metal, typically at between 1500 C and 1 700on is poured into the mould cavity, and until the ingot is removed from the mould.

(4) It functions to prevent what is known in the art as "burn-on" (a metal-sand conglomerate crust which forms on sand moulds) by acting to prevent penetration of the molten metal into the voids around the sand grains or into other apertures on the mould surface.

(5) Being in direct contact with the metal, it minimizes adverse effects on the characteristics and quality of a produced ingot casting.

(6) Erosion of a stool surface is substantially reduced when the composition of the invention is applied.

(7) Finally, it is economically viable in terms of costs of materials, labour and equipment for application.

The invention will now be described with sequential reference to preferred sub-generic features, specific numerical examples, accompanying illustrative drawings, and a comparative test involving a typical compound of the present invention and a compound in accordance with the prior art. It is therefore to be understood that such ensuing description is not to be limitatively construed.

The second component of the coating composition, as mentioned above, is most preferably aluminochromophosphate, which may be employed in the amount of 1 to 100% by weight from which range it will be understood that, as specified in the above definitions of the methods and the mould construction provided in accordance with the several aspects of the invention, the second component of the composition is also able to exert a surface protecting/refractory function on its own (the extent to which it may be called upon to do so will generally be governed by economic considerations).In conjunction therewith and/or in lieu thereof pro- vided that chromium- and phosphate- radicals are both present or derivable aluminochromo- phosphate may be augmented, or substituted by, other chromium- and phosphate-radical containing components, such as mono aluminium ortho phosphate, chromium phosphate, phosphoric acid, chromic acid and the chromium oxides. Of these, mono aluminium ortho phosphate, in conjunction with aluminochromophosphate and in a range of 1-70% by weight or more preferably 1-30% by weight, is effective.

The first component, which functions as an inert refractory filler material in a manner known in the art and reduces the cost of the mould coating composition which at the present time would otherwise be rendered less economically attractive if the second component were to be used on its own is preferably silica flour or pyrophyllite. Other preferred first components can be selected from graphite, zircon, mica, talc, calcined magnesite, calcined alumina, various alumina silicates, chamotte, olivine and chromite. The first component is normally present in an amount up to 75% by weight.

In a further preferred aspect of the invention the finely divided refractory mineral filler (first component) is used in an amount ranging from 10-70% by weight. Relately the aluminochromophosphate (second component) is used in an amount ranging from 1-30% by weight most preferably 5-15% by weight, with the balance of the composition being water.

In addition to the refractory mineral filler and the chromium-radical containing and phosphateradical containing material, the composition preferably contains at least one suspension agent.

Such agent may be Xanthan gum, a Xanthomonas colloid or an alginate-and may be present in a preferred amount ranging from 0.1 to 2.0% by weight, more preferred 0.1-0.5% by weight. An anti4ermentation agent, for example sodium pentachlorophenate, may also be utilized in a preferred amount'ranging from 0.001-0.03% by weight, more preferred 0.01-0.02% by weight. The function of these agents is set forth hereafter.

As indicated above, the second component functions to protect the surface to which it, or the composition incorporating it, is appled. Whilst not wishing to be tied to, or bound by, any particular theoretical explanation, we believe that the second component, on heating, cures to form a chromium-phosphate radical complex which acts to bind the other component(s) and enhance the refractory/protective nature of the composition. In the particular case of aluminochromophosphate curing (drying) takes place at a low temperature of about 1 20 C thereby eliminating the need to separately heat the moulds in order to form an effective mould coating.

The mould coating composition of the present invention may thus be conveniently applied to the mould before use, without interrupting the production cycle.

Compositions of the present invention may be prepared by slurrying the first and second components with water. To assist in mixing the solid and liquid components a suspension agent, selected from those indicated above, is desirably added before the refractory mineral filler component is incorporated. As some suspension agents to not gel unless the pH of the solution is adjusted (methyl cellulose is sensitive to pH whilst Xanthan gum requires no adjustment to the pH in order to gel) such adjustment, as necessary, can be conventionally carried out. The compositions of the present invention can also be augmented by the addition thereto of one or more anti-fermentation agents as indicated above. Such agents function to prevent chemical degradation of the composition during long term storage.

The compositions of the present invention can be easily applied, by any suitable means, to the selected surfaces of ingot and other moulds.

The composition, and its method of manufacture, is now described with reference to the following specific examples.

Example 1 Silica quartz was treated to a milling operation to produce silica flour, in size approximately 75 microns.

In a 6,000 litre tank, 2045 kg of water was added together with 14 kg of Xanthan gum. The solution was stirred at high speed (200 RPM) bntil gelation, at which time 300 kg each of aluminochromophosphate and Mono Aluminium ortho phosphate were added and dispersed by stirring. 3330 kg of silica flour (75 microns) was added and the resultant mixture was blended to a smooth consistancy. Finally 600 gms of sodium penta-chlo-phenate was added.

The resulting composition has the following measured physical properties: Viscosity 2600 cps pH 1.8 Density 1.65 g/cc Example 2 A similar procedure to Example 1 above, was used with the following constituents changed: Mono Aluminium Ortho Phosphate nil Aluminochromophosphate 600 kg Example 3 Again a similar procedure to Example 1 above, was used with the following constituents changed.

Silica flour replaced by Pyrophyllite 3000 kg Mono Aluminium Ortho Phosphate nil Aluminochromophosphate 600 kg Finally, the invention will now be further illustrated by reference to the accompanying drawings and a series of comparative tests in which moulds coated in accordance with the invention are compared with moulds coated in accordance with the prior art. In relation to these tests, the criteria in respect of which comparisons are made (viz average application rate etc) are among those which are most important in practice.

In the accompanying drawings: Figure 1 represents a vertical cutaway view of a typical "bottle top" ingot mould, used with compositions of the present invention.

Figure 2 represents a vertical cutaway view of a typical "big-end down" ingot mould used with prior art compositions, showing erosion of the stool.

The ingot moulds are designed generally by the numeral 1. The mould comprises a stool 2 upon which sits a mould designated generally by numeral 3.

In Fig. 1, the inner walls 4, the top of the mould, the upper surface 5 of the mould and the inner surface 6 of the stool are coated with a composition of the present invention.

In Fig. 2 can be seen the erosion cavity 7 caused by repeated teeming of molten metal into the mould. The larger the erosion cavity, the greater is the likelihood of the stool sticking to the cooling ingot necessitating physicai removal of the stool. In prior art production a steel plate indicated by the dotted line 8 can be placed over the erosion cavity to prevent such "sticking" and consequent damage to the surface of the ingot. It has been found that it is not necessary to use said steel plate (8) with mould coatings of the present invention.

Whilst Fig. 1 illustrates a "bottle top" mould, it is to be understood that the invention is equally applicable to other shapes of moulds and also to sand moulds.

As a comparison with known compositions a test was made involving production runs over a period of twenty-eight (28) days. During this test run 292,600 tonnes of crude steel were produced STABLE , composition a known composition of example 2 of silica flour and colloidal silica as binder Average application rate 0.23 kg/tonne 0.35 kg/tonne Overall average stickerrate 6.8% 7.4% Range of reported sticker rates per 24 hr of production 0-16% 0-50% Expected repeated use of stool before patching 40 17-32 It should be noted that during the test, no protective, steel plate was used over the erosion cavity.

From the above comparative table it will be seen that the composition of the invention effects a significant improvement in respect of each of the criteria assessed. For the present composition it will therefore be seen that a substantial step forward is the art has been achieved.

Claims (14)

CLAIMS The claims defining the invention are as follows:

1. A composition adapted to be used for the protective coating of metallic or other moulds of the type wherein casting or other processing operations are carried out, the said composition comprising a first finely divided refractory mineral component and, in association therewith, a second component, adapted to maintain the two components in heat-resistant bound relationship, said second component comprising at least one material which contain(s) phosphate radical(s) and chromium radical(s) and/or at least one material from which, such radical(s) can be jointly and/or separately derived, a composition being such that, when it is applied to a mould surface and the mould is subjected to the conditions under which a said casting or the like operation is carried out, the second component will be effective to maintain the said bound relationship whilst itself functioning as a surface protecting material.

2. A composition adapted to be used for the protective coating of metallic or other moulds of the type wherein casting or other processing operations are carried out, the said compositions comprising a first finely divided refractory mineral component and, in association therewith, a second component adapted to maintain the two components in heat-resistant bound relationship, the second component comprising aluminochromophosphate, or a functional equivalent thereof more particularly monoaluminiumorthophosphate.

3. A composition as claimed in Claim 1 or 2, said composition being in the form of an aqueous slurry and further optionally comprising at least one suspension agent.

4. A composition as claimed in any one of the preceding claims wherein the finely divided refractory mineral is present in an amount up to 75% by weight, more preferably 10 to 70% by weight.

5. A composition as claimed in claim 2, or either of claims 3 or 4 where appendant thereto, wherein the or each phosphate is present in an amount of 1 to 70% by weight, more preferably 1 to 30% by weight.

6. A composition as claimed in any one of the preceding claims wherein the finely divided refractory mineral is silica flour or pyrophyllite.

7. A composition as claimed in any one of the preceding claims, wherein such composition also includes at least one anti-fermenatation agent.

8. A composition as claimed in claim 1 or 2 and substantially as herein described with reference to any one of the foregoing specific examples thereof.

9. A method for the production of a composition which is adapted to be used for the protective coating of metallic or other moulds of the type wherein casting or other processing operations are carried out, which comprises compounding together, as by aqueous slurrying, a first and second component as recited in claims 1 or 2.

10. A method as claimed in claim 9, substantially as herein described with reference to any one of the foregoing examples thereof.

11. A composition produced by the method of claim 9 or 1 0.

1 2. A method for protectively coating a surface of a metallic or other mould of the type wherein casting or like operations are carried out, wherein the said surface has applied thereto a composition as claimed in any one of claims 1 to 8 and 11.

1 3. A method for protectively coating a surface of a metallic or other mould of the type wherein casting or like operations are carried out, wherein the said surface has applied thereto a second component of the type recited in claims 1 or 2.

14. A method as claimed in claim 12 or 13 substantially as herein described.

1 5. A mould adapted to be used for casting or other processing operations wherein the or each operative surface is coated with a composition as claimed in any one of claims 1 to 8 and 11 or with a second component as recited in claims 1 or 2.

1 6. An ingot mould, adapted to be used for the casting of steel ingots, wherein the or each operative surface is coated with a composition as claimed in any one of claims 1 to 8 and 11 or with a second component as recited in claim 1 or 2.

1 7. A composition as claimed in any one of the preceding claims which remains bonded together even after removal of the ingot or casting.