LU84293A1 - COMPOSITIONS FOR COATING MOLDS FOR THE TREATMENT OF MOLTEN METALS, METHODS FOR APPLYING SUCH COMPOSITIONS, AND COATED MOLDS - Google Patents

Description

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? * The invention relates to compositions for coating molds for treating molten metals, as well as molds coated with such compositions and methods for their application.

Such molds may be formed from cast iron, a metal directly obtained from blast furnaces, or they may be formed from sand, the grains of which are bonded together with bentonite or other chemical binders.

In the following description, particular reference is made to the casting of molten steel into cast iron molds resting on a cast iron base and to the problems associated therewith. It should however be borne in mind that the scope of the invention is broader and that it applies equally well to the casting of metals in general, and to its implementation in the case of molds and cores formed of sand (as described below).

In the past, for the treatment of molten metals, it has been common practice to apply a coating to the surface of the molds before pouring metal into these molds. This practice has been systematically followed in the steel industry, where cast iron ingot molds are normally coated before the casting of steel ingots. The functions of the mold coatings used in the treatment of molten steel are to reduce the wear of the molds and to establish a good mold release plan, thereby protecting the surface qualities of the molded metal.

30 When casting molten steel into an assembly comprising a base and a cast-iron mold, damage such as erosion and grain growth causes microscopic cracks to appear in the cast-iron mold having for »3

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consequently a strong adhesion of the ingot or the hillette to the walls of the base and of the mold. Consequently, a coating is applied in a conventional manner on the base and the mold in order to fill the voids on their faces, which allows easy removal of the ingot or billet from the assembly which constitutes the mold and its base.

In the case of sand molds, sand, and commonly silica sand, is normally mixed with a chemical binder and a catalyst, is molded around a template and is hardened into a body having the desired shape.

Alternatively, sand can be bound with ben-v tonite and packed around a template to form a body of the desired profile. Such sand molds are, by their very nature, porous, and their porosity prohibits their use without the prior application of a mold coating, otherwise penetration of the metal into the voids of the sand would cause strong adhesion of the mold to the surface of the cast part, the elimination of an agglomerated crust of sand and metal on the surface of the part requires the implementation of a costly sanding or peeling operation to the tool. Various forms of mold coating have been developed for such sand molds.

Combinations of bituminous tar, charcoal and talc are among the first materials used to form mold coatings; they were, however, excluded because their application on the molds is „an expensive and difficult operation.

. Newer coating materials include a wide variety of refractory mineral fillers, binders, carrier liquid, gelling or suspending agents, and agents for avoiding fermentation. However, the disadvantage of most of these known coatings is their tendency to. cause the formation of non-metallic inclusions in the cast metal. This occurs whenever the mold coating does not adhere securely to the surface of the base, mold or core. A usual reason for this accident ♦ 4 * *

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is the destruction of the binder as a result of the transmi-J heat by radiation or convection from the molten metal before the mold cavity is filled with the molten metal, which allows penetration of the molten metal into the substance residual mineral.

Another and significant disadvantage of the coatings presently known is clearly highlighted during the release of the ingot. As a result of the destruction of the binder by heat, the coating of the mold is decomposed into its constituent ingredients. Consequently, finely divided refractory material tends to become suspended in the surrounding air as a result of the presence of convection currents.

The present invention aims to eliminate or considerably reduce the aforementioned drawbacks. the invention provides remarkable improvements: (1) to compositions for coating molds; (2) methods for applying mold coatings; (5) to molds coated with the above compositions or by carrying out the above methods.

In one of its broadest aspects, the invention relates to a composition which can be used for the application of a protective coating on metal molds or very of the type in which casting or other operations are carried out. metal treatments, said composition comprising a first finely divided refractory mineral component and, in association with said first component, a second component capable of maintaining the two components 50 in a heat-resistant bonding relationship, said second component comprising at least a substance which contains phosphate radicals and chromium radicals and / or at least one substance from which such radicals can be jointly and / or separately derived, the composition being such that, when applied to the surface of a mold and when the mold is subjected to the conditions under which said casting operations or the like are carried out, the second component sant 4 5 * f "is effective in maintaining said mutual bonding relationship while operating itself as a refractory material protecting the surface.

The invention also relates to a process for the production of a composition which can be used for constituting the protective coating of metal molds or of other materials, molds of the type in which casting operations or other processing operations metals are carried out, which method consists in mixing together, in the form for example of an aqueous suspension, a first component and a second component as defined above.

In another aspect, the invention relates to a method for applying a protective coating to a surface of a metal or other material mold, mold of the type in which casting operations or other operations metal treatment are carried out, which method consists in applying, to said surface, a composition as defined above, or a second component 20 of the defined type.

Finally, the invention also relates to a mold usable for casting or for other metal processing operations, such as a mold called ingot mold for the casting of steel ingots, which method consists in coating the working surface or each of the working surfaces of the mold with a composition as defined above, or with a second component of the type defined above.

The composition according to the invention, for coating molds, is characterized by the following advantageous properties: (1) Ease of application.

(2) Ability to harden or dry so as to form a smooth skin which adheres strongly and fills in any defects which may be present in the surface of the base, of the mold or of the core.

(3) It is resistant to high temperatures and remains effective while molten metal, typically at a temperature between 1500 ° C and 1700 ° C, is 6 * τ * poured into the mold cavity, and up to unmolding of the ingot.

(4) It functions in such a way as to prevent "burning" (crust constituted by a conglomerate of metal and sand which forms on sand molds) by opposing the penetration of the molten metal into the voids around grains of sand or into other openings on the surface of the mold.

(5) Being in direct contact with the metal, it minimizes the unfavorable effects on the characteristics and the quality of the part finally obtained, such as an ingot.

1 (6) The erosion of the surface of a base is considerably reduced when the composition according to the invention is applied to it.

(7) Finally, it is economically advantageous with regard to the costs of materials, labor and equipment for its application.

The invention is hereinafter described with reference to preferred features of these various aspects, to working examples, to the appended drawings, and to a comparative test relating to a typical composition of the present invention and to a composition established according to the principles of the prior art. It should not be overlooked that this description does not limit the wider scope of the present invention.

Fig. 1, of the accompanying drawings, shows in vertical section an ingot mold with an upper opening of the "bottle neck" type, of the type used with compositions according to the present invention.

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Fig. 2 shows in vertical section an ingot mold of the type with a large lower base used with compositions of the prior art, and shows the erosion of the base.

According to the invention, the second component of the coating composition, as mentioned above, is most advantageously an aluminochromophospha- / te which can be used in a proportion by weight ranging from Η ? Λ from 1 to 100% on the basis of which it should not be forgotten that, as is specified in the above definitions of the methods and particularities of construction of the molds according to the various aspects 5 of the invention , the second component of the composition is also capable of exerting by itself a protective function by the establishment of a refractory lining (up to a degree whose importance can be dictated by economic considerations). In conjunction with and / or in place of this substan-10 ce, provided that chromium and phosphate radicals are simultaneously present or can be derived therefrom, an aluminochromophosphate may be assisted, or 'replaced, by other components containing radicals chromium, and phosphate, such as mono-alumi-nium orthophosphate, chromium phosphate, phosphoric acid, chromic acid and chromium oxides. Among these products, mono-aluminum orthophosphate, together with an alumino-chromophosphate and in a proportion of 1 to. 70% by weight, or more advantageously from 1 to 30% by weight, is effective.

The first component, which functions as an inert refractory charge, in a manner well known in the art, and which lowers the cost of the coating composition for molds to the point of making it economically attractive ( which would not be the case if said second component were used as the sole refractory material) is preferably pyrophyllite or fine powder silica. Other preferred first components can be chosen from graphite, zirconia, mica, talc, calcined magnesia, calcined alumina, various alumina silicates, chamotte, olivine and chromite. The first component is normally present in a proportion of up to 75% by weight.

According to another preferred aspect of the invention, the finely divided refractory mineral filler (first component) 35 is used in a proportion of between 10 and 70% by weight. Therefore, aluminochromophosphate (second component) is used in an amount of from 1 to 30% by weight, and most advantageously from 5 '. "" To 15 by weight, the rest of the composition being water.

In addition to the refractory mineral filler and the substance containing chromium radicals and containing phosphate radicals, the composition preferably contains at least one suspending agent. Such an agent can be a Xanthan gum, a Xanthomonas colloid or an algin, and it can be present in a preferred proportion ranging from .0.1 to $ 2.0 by weight, and more advantageously from 0. 1 to 0.5 by weight. It is also possible to use an agent opposing fermentations, for example sodium penta-chlorophenate, used in a preferred proportion of from 0.001 to 0.03 by weight, and more advantageously from 0.01 to 0. .02 by weight. The function of these agents is indicated below.

As already indicated above, the second component operates so as to protect the surface to which it is applied, or to which the composition in which it is incorporated is applied. Although it is desired not to be bound by any particular theoretical explanation, it may however be indicated that, in all likelihood, the second component, upon heating, hardens to form a compound containing chromium and phosphate radicals which acts so as to bind the other (or the other) component (s) and to improve the refractory and protective nature of the composition. In the particular case of an aluminochromophosphate, hardening (or drying) takes place at a low temperature of around 120 ° C., which eliminates the need to heat the molds separately in order to form an effective coating on their working surface. The composition for coating molds according to the present invention can therefore be suitably applied to the mold before use, without interrupting the production cycle.

Compositions according to the present invention can be prepared by suspending the first component and the second component in suspension with water. To facilitate the formation of the mixture of solid and liquid components, it is advantageous to add a suspending agent, Ύ 3 chosen from those indicated above, before the incorporation of the refractory mineral filler. Since some suspending agents do not form a gel unless the pH of the solution is properly adjusted (methyl cellulose is pH sensitive while Xanthan gum does not require pH adjustment to gel ), such an adjustment, when it appears necessary, can be carried out in a conventional manner, the compositions according to the present invention can also be improved by adding one or more agents making it possible to combat fermentations and chosen from those which have been specified above. Such agents work to prevent chemical degradation of the composition when it is to be stored for a period of a long time. The compositions according to the invention can be easily applied, by any suitable means, to the chosen surfaces of molds of the ligotière type or of any other type.

the composition, as well as its method of manufacture, is described with the aid of the following particular examples, which of course are in no way limiting.

Example 1

A quartz type silica is transformed by grinding into a fine powder whose particles measure approximately 75 microns.

25 In a tank with a capacity of 6000 liters, 2045 kg of water and 14 kg of Xanthan gum are placed together. The solution is stirred at high speed (200 rpm) until gelling and 300 kg of aluminochromophosphate and 300 kg of mono-aluminum orthophosphate are then added, which is dispersed by stirring. 3330 kg of fine silica powder (75 microns) are added and the resulting mixture is homogenized until a smooth and uniform consistency is obtained. Finally, 600 grams of sodium pentachlorophenate are added, the measurement of the physical properties of the resulting composition gives the following results:

Viscosity 2600 centipoise pH 1.8

Density 1.65 g / cin ^ // 0 v »«

Example 2

A procedure similar to that of Example 1 above is used, but by changing the proportions of constituents as follows: 5 mono-aluminum orthophosphate none aluminochromophosphate 600 kg

Example 5

A procedure similar to that of Example 1 above is again used, but by changing the pro-10 portions of constituents as follows: fine silica powder replaced by pyrophyllite 3000 kg mono-aluminum orthophosphate none aluminochromophosphate 600 kg Finally, the invention is further illustrated with reference to the accompanying drawings and to a series of comparative tests in which molds coated in accordance with the invention are compared to molds coated in accordance with the prior art. With respect to these tests, the criteria on the basis of which the comparisons are made (i.e. the average application rate, etc.) are chosen from among those which are most important in practice.

In fig. 1 and 2 of the accompanying drawings, the ingot molds are generally designated by the reference 1. The mold comprises a base 2 on which rests a mold body proper 3 ·

In the case of the mold shown in fig. 1, the interior walls, the top of the mold, the top surface 5 of the mold and the interior surface 6 of the base are coated with a composition according to the present invention.

In fig. 2, we can see a cavity 7 due to erosion caused by repeated flows of molten metal in the mold. The larger such a cavity formed by erosion, the greater the probability of adhesion between the base and the ingot during cooling, which requires physical separation of the base. According to the prior art, a steel plate represented by the dotted line 8 can be placed above the formed cavity.

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eroded to prevent such adhesion and the consequent damage to the surface of the ingot. It has been found that it is not necessary to use such a steel plate 8 when applying mold coatings according to the present invention.

Although fig. 1 illustrates the case of a mold with an upper part in the form of a "bottle neck", it should not be forgotten that the invention is also applicable to other forms of mold and, also, to molds in sand.

By way of comparison with known compositions, a test is carried out relating to an extended production campaign over a period of twenty-eight (28) days. During this test campaign, 292,600 tonnes of crude steel were produced. The results shown in Table I below are obtained.

Table I

Composition

Known composition of powder of Example 2 and colloidal silica as binder

Average application rate 0.25 kg / tonne 0.55 kg / tonne

Overall average adhesion rate $ 6.8 $ 7.4 25 Adhesion rate interval reported per 24 h of production 0-16 $ 0-50 $

Number of repeated uses of the base expected before reconditioning 40 17-32 30 It should be noted that, during the test, no protective steel plate was used above the cavity formed by erosion .

From the examination of the Comparative Table above, it appears that the composition according to the invention makes it possible to obtain a very appreciable improvement for each of the criteria selected. The composition according to the invention therefore makes it possible to achieve substantial progress in the technical field in question.

Claims (14)

1. A composition which can be used for the application of a protective coating on metallic or other molds of the type in which casting operations or other metal treatments are carried out, said composition comprising a first finely divided refractory mineral component and, in combination with said first component, a second component capable of maintaining the two components in a heat-resistant bonding relationship, said second component comprising at least one substance which contains phosphate radicals and chromium radicals and / or at least a substance from which such radicals can be jointly and / or separately derived, the composition being such that, when applied to the surface of a mold and when the mold is subjected to the conditions under which said casting operations or the like are carried out, the second component is effective in maintaining said mutual bonding relationship while functioning itself as a material 20 protecting the surface. 2. Composition usable for the application of a protective coating on metal or other molds of the type in which casting operations or other metal treatments are carried out, said composition comprising a first finely divided refractory mineral component and, in association with said first component, a second component capable of maintaining the two components in> a heat-resistant bonding mutual relationship, the second component comprising an aluminochromophosphate 30 or a functional equivalent of such a substance, and more particularly mono-aluminum orthophosphate. 3. Composition according to claim 1 or 2, characterized in that it is in the form of an aqueous suspension and in that it further comprises at least one suspending agent. 4. Composition according to any one of the preceding claims, characterized in that the finely divided refractory mineral substance is present in a pro- "" "portion by weight up to 75 and preferably from 10 to 70 5. Composition according to any one of claims 2 to 4, characterized in that the phosphate or each phosphate is present in a proportion by weight of between e · 1 to 70 fof and more advantageously from 1 to 30 6. Composition according to any one of the preceding claims, characterized in that the finely divided refractory mineral substance is a fine powder of silica or pyrophyllite. 7. Composition according to any one of the preceding claims, characterized in that it also comprises at least one agent intended to combat fermentations. 8. Process for the production of a composition useful for the application of a protective coating on metal molds or in other materials, molds of the type in which casting operations or other metal treatment operations are carried out, which method comprises mixing together, as by aqueous suspension, a first component and a second component as defined in claim 1 or 2. 9 · Composition characterized in that it has been produced by implementing a process according to claim 8. 10. Method for applying a protective coating to a surface of a metal mold or the like of the type in which casting operations or the like are carried out, which method is characterized in that one applies, on said surface, a composition according to any one of claims 1 to 7 and 9 · 11. A method for applying a protective coating to a surface of a metallic mold or the like of which casting operations or the like are carried out, which method is characterized in that said coating is applied surface, a second component of the type specified in claim 1 or 2. * AH & '* " 12. Mold usable for carrying out casting operations or other treatment operations, characterized in that the working surface or each working surface is coated with a composition according to any one of claims 1 to 7 or 9 or with a second component as specified in claim 1 or 2. 13 · Ingot mold, usable for the casting of steel ingots, characterized in that the working surface or each working surface is coated with a composition according to any one of Claims 1 to 7 or 9 or with a second component as specified in claim 1 or 2. ‘14. Composition according to any one of the preceding claims, characterized in that the coating 15 remains bonded even after removal from the ingot or of the casting.