EP0007282A1 - Process for manufacturing composite strips by continuous casting and the strips obtained by the process - Google Patents

Abstract

Process for plating a strip of a metal having a higher melting point and mechanical strength than aluminum on at least one face of an aluminum strip by continuous casting between cylinders by means of a nozzle, the strip to be plated being interposed between the cylinder or cylinders and the cast aluminum.

The strip of plating metal is in contact with the cylinder at least on the portion between the generator closest to the end of the nozzle and that located in the plane of the axes of the cylinders. The clearance between the nozzle and the plating strip applied against the cylinder is less than 1 mm and the elongation of the plating metal is greater than 1% and less than that of solidified aluminum.

Application to the manufacture of aluminum - copper or aluminum - stainless steel composite strips.

Description

The present invention relates to a process for manufacturing composite strips consisting of an aluminum core coated on one side or on both sides with a metal having a melting point and a mechanical resistance substantially higher than aluminum, by continuous casting. between aluminum cylinders, with interposition between the working cylinder (s) and the liquid aluminum with a metal plating strip.

The invention also relates to the composite strips obtained by such a method.

In all that follows, “aluminum” will denote either a pure aluminum or an aluminum-based alloy.

French Patent No. 1,364,758 issued May 19, 1964 describes in principle a continuous casting process in which the still liquid metal is introduced between two cooled cylinders and where this liquid metal is interposed with the cylinders of working a strip of plating metal which is continuously driven with the rolled metal and is thus pressed thereon.

The main purpose of this process was to tackle an aluminum blank whose surface had, due to the low rate of wrought, a fairly coarse texture, an aluminum strip having an improved corrosion resistance and a better surface finish or more suitable, in particular, for anodic oxidation treatments or of shine.

However, the indications given in this patent do not allow plating with a metal having characteristics clearly different from aluminum, for example a plating of stainless steel, which give sufficient adhesion so that the composite strip can be further processed. .

Furthermore, various methods of continuously casting aluminum between cylinders are known. Thus, French patent n ° 1,198,006 issued on June 8, 1959, as well as its certificate of addition n ° 74,839, describe a device comprising a feed tank which receives the molten metal from a casting furnace and a flattened profile nozzle, intended to distribute the metal along a strip of given width. The end of the nozzle is engaged between two parallel cylinders spaced from each other. The molten metal coming out of the nozzle cools and solidifies on contact with the cooled cylinders and is entrained in the form of a strip which is subjected, due to the curvature of the cylinders, to a certain pressure. When it has crossed the plane of the axes of the rolls, the strip is wound on a winder.

This process makes it possible to produce aluminum strips in a range of thicknesses from 1.5 mm to 20 mm thick under advantageous economic conditions.

The present invention relates to the application of this continuous casting process to obtain, at the outlet of the cylinders, plated strips comprising an aluminum core of thickness between 1.5 mm and 20 mm and a cladding on one or more two faces of a metal having a melting point and a mechanical resistance substantially higher than aluminum, with a thickness of between 0.1 and 1 mm, so as to obtain excellent adhesion of the two components.

The invention makes it possible to obtain composite strips capable of undergoing further processing, for example by stamping, under good conditions, the manufacturing cost of these strips being very clearly competitive compared to the other plating processes.

• a) la bande de métal de placage est au contact du cylindre au moins sur la portion comprise entre la génératrice la plus proche de l'extrémité de la busette et le plan des axes des cylindres et, de préférence, sur un arc de contact supérieur à 30°.
• b) le jeu entre la busette et le cylindre entraînant le métal de placage n'excède pas de plus de 1 mm l'épaisseur de la bande de placage.
• c) la vitesse de coulée est telle que l'allongement du métal de placage est supérieur à 1 % tout en restant inférieur à celui de l'aluminium une fois solidifié.

The method according to the invention consists of the combination of the following characteristics:

• a) the plating metal strip is in contact with the cylinder at least on the portion between the generator closest to the end of the nozzle and the plane of the axes of the cylinders and, preferably, on an upper contact arc at 30 °.
• b) the clearance between the nozzle and the cylinder carrying the plating metal does not exceed more than 1 mm the thickness of the plating strip.
• c) the casting speed is such that the elongation of the plating metal is greater than 1% while remaining less than that of aluminum once it has solidified.

The plating metal can be, for example, copper or stainless steel. In the case of stainless steel, the elongation of the strip is preferably between 3 and 10 X.

The Applicant has found, on the other hand, that the specific casting conditions ensuring excellent adhesion of the plating components, one could associate a particular metallurgical structure of the solidified aluminum layer at the outlet of the cylinders. In fact, this layer has, under these conditions, in longitudinal section, solidification dendrites in the form of fish bones, oriented in the opposite direction to the advance of the casting, the mean direction of which forms, with the plane of the strip, an angle less than 85 ° and preferably less than 75 °. This structure appears very clearly when, for example, the surface of the section is treated with a usual reagent for macrography;

When a plating of the same type and of the same thickness is carried out on the two faces of the strip, it goes without saying that the conditions of heat exchange between the strip and the two rolls are almost identical. In this case, the dendrites are therefore approximately symmetrical with respect to the median plane of the aluminum layer. This is obviously no longer the case when the plating is done on one side only. In this case, the alignment of the vertices of the dendrites no longer coincides with the median plane but is shifted towards the plated face.

It has been found that, under the casting conditions defined above and leading to good adhesion of the components, the offset from the median plane should not exceed 10% of the half thickness of the aluminum layer.

• - figure 1, un schéma général du dispositif de coulée continue permettant la fabrication de la bande composite selon l'invention,
• - figure 2, une section longitudinale d'une bande composite avec placage sur une face faisant apparaître les dendrites de solidification.

The invention will be better understood from the detailed description given with regard to the appended figures which represent:

• FIG. 1, a general diagram of the continuous casting device allowing the manufacture of the composite strip according to the invention,
• - Figure 2, a longitudinal section of a composite strip with plating on one side showing the solidification dendrites.

FIG. 1 represents a pouring device comprising a chute (1) containing the liquid aluminum (2) provided with a nozzle (3) and a float (4) making it possible to maintain a constant level of metal in the tank. 'feed (5). This is in communication with a nozzle (6) having a tapered end which delivers the molten metal through its nozzle (7) over a width equal to that desired for the strip, between the two cylinders (8) and (9). A strip of plating metal (10) is unwound so as to come into contact with the cylinder (9) with a contact arc (11) at least equal to that included between the generator (12) closest to the end of the nozzle and the plane (13) of the axes of the cylinders. It is interesting that the contact arc is large enough so as to allow the heating of the plating strip (10) and a desorption of the surface moisture.

To promote the plating operation, the surface of the strip is prepared by known methods such as degreasing, pickling, brushing, abrasion. Thus, the strip (10) can be, before its introduction into contact with the aluminum, previously degreased and abraded on the face which will be coated with aluminum, for example by brushing, so as to give it a roughness of the around 0.1 to a few microns CLA approximately. It has been found that it is desirable that the abrasion operation be done just before plating rather than doing it in a prior operation. On the other hand, the other face in contact with the cylinder must preferably remain smooth so as not to deteriorate the heat exchange.

To obtain good contact with the cylinder (9), the strip (10) can of course be subjected to traction, for example 5 to 100 MPa.

The clearance between the end of the nozzle and the cylinder (9) coated with the strip (10) must be just sufficient to avoid excessive friction which would risk damaging the nozzle and a scraping of the lubricant onto the uncoated cylinder. It must be limited to approximately 1 mm in order to avoid instability of the alumina meniscus which forms in this game, which would be detrimental to adhesion.

The limitation of this play is obtained by the prestressing of the cage of the casting rolling mill to a force value close to that obtained in operation which counterbalances the play due to the yielding.

entre la sortie de la busette et la sortie des cylindres est généralement compris entre 10 et 40 %. La bande de placage est légérement laminée en passant entre les cylindres. Une condition pour obtenir un placage correct est que l'allongement de la bande de placage soit supérieur à 1 % tout en restant inférieur à l'allongement de l'aluminium entre la fin de la solidification et la sortie des cylindres.When passing between the cylinders, the aluminum gradually solidifies. If the solidification is completed before it leaves the cylinders to a thickness E, it further undergoes rolling which reduces its thickness to e. The reduction rate

between the outlet of the nozzle and the outlet of the cylinders is generally between 10 and 40%. The veneer strip is lightly laminated passing between the cylinders. A condition for obtaining correct cladding is that the elongation of the cladding strip is greater than 1% while remaining less than the elongation of the aluminum between the end of solidification and the exit of the cylinders.

As for the speed of advance, it plays an important role.

The theoretical maximum speed of a continuous casting between cylinders is that for which the most forward point of the solidification front lies in the vicinity of the plane of the axes of the cylinders.

In practice, in an unplated aluminum casting, one operates at a speed slightly lower than this theoretical speed. When introducing a plating strip, it was found that it was necessary, in order to obtain good plating, to further limit the speed so as to maintain a sufficient reduction rate of the base metal. If we want to go faster, we see that the cladding strip, especially when it is thin, tends to wrinkle probably because, aluminum being solidified only in the vicinity of the median plane, this strip is less well applied against the cylinder.

The cooling of the composite strip at the outlet of the cylinders is done just to look. Contrary to the teaching of French patent n ° 1 364 758, it is not advantageous, in the case of stainless steel plating, taking into account the different coefficients of expansion of aluminum and steel, to cool quickly , which would increase the stresses at the interface between the two metals. On the contrary, it is better for the cooling to be slow in the temperature range from 450 to 250 ° C. approximately so that the expansions are absorbed by the creep of the aluminum. We can do this without fear of possible appearance of fragile intermetallic phases because we are in a temperature zone where these phases are unlikely to form quickly. Cooling should be faster at the start if the cladding metal is a copper alloy.

It is advantageous to make, after the strip has cooled, a rolling pass which improves the adhesion of the plating and gives the aluminum wrought metal properties which are generally better than those of foundry metal.

Obviously, it is not advantageous to harden the plating metal too much in the case where the composite strip must be subsequently transformed, by stamping for example.

However, the process making it possible to obtain fairly thin composite strips between 1.5 and 6 mm, unlike the processes of the prior art, the cast strip can be used directly without rolling.

A method has been described above for plating a strip on one face of the cast aluminum strip. If we want to obtain an aluminum strip plated on both sides, we unroll on the cylinder (8) a metal strip under the same conditions as the strip (10) on the cylinder (9). In this case, the casting speed is slightly lower than it would be for single-sided plating. This process applies regardless of the inclination of the axis of the cylinders relative to the vertical.

FIG. 2 shows the appearance of a longitudinal section of an aluminum-stainless steel composite strip after pickling with the macrography fluorochloronial reagent. We see appear, in the aluminum layer, solidification dendrites in the form of fishbones, the mean direction of which forms, with the median plane of the strip, an angle less than 75 °. Due to the asymmetry of the coating, the line of the vertices of the dendrites is slightly offset from the side of the veneer, but with an eccentricity of less than 20%.

The method according to the invention makes it possible to obtain aluminum-metal composite strips with a metal having a melting point and a mechanical resistance substantially higher than aluminum and, more particularly, aluminum-copper or aluminum-steel composite strips stainless can be used in particular for the manufacture of cooking utensils or heat exchangers and this under particularly advantageous economic conditions. The adhesion between the components is excellent.

The invention will also be illustrated by nonlimiting exemplary embodiments:

Example 1:

Austenitic stainless steel according to the designation AISI. 304 0.3 mm thick is plated on 1050 aluminum 10 mm thick by continuous casting between 600 mm diameter cylinders. After degreasing, the stainless steel is brushed with a wire brush so as to obtain a roughness of the order of 1 micron CLA, then placed in the interval of about 1 mm between the cylinder and the end of the nozzle. aluminum feed.

The stainless steel strip wraps the cylinder at an angle of approximately 90 °.

During the plating operation, the stainless steel strip undergoes an elongation of approximately 8%. The eccentricity of the aluminum solidification ridge is approximately 5%, the angle of the dendrites is approximately 15 ° with the median plane.

The adhesion of the products obtained is good.

Example 2:

0.4 mm thick AISI 434 ferritic stainless steel is plated on 1050 aluminum 2.6 mm thick by continuous casting between cylinders 600 mm in diameter. After degreasing, the stainless steel is brushed with a wire brush so as to obtain a roughness of the order of 1 micron CLA, then is placed in the interval of about 0.6 mm between the cylinder and the end of the aluminum supply nozzle.

The stainless steel sheet wraps the cylinder at an angle of approximately 90 °.

During the plating operation, the stainless steel sheet undergoes an elongation of approximately 3%. The offset of the aluminum solidification backbone is 5%; the angle of the dendrites is 60 ° with the median plane.

The adhesion of the products obtained is good. After cold rolling with a reduction of 30%, the product can be drawn.

Example 3:

Copper 0.5 mm thick is plated on 1050 aluminum 6 mm thick by continuous casting between cylinders 600 mm in diameter. After degreasing, the copper is brushed with a wire brush so as to obtain a roughness of 1.5 microns CLA, then placed in the interval of approximately 1 mm between the cylinder and the end of the supply nozzle of the aluminum.

The copper foil wraps the cylinder at an angle of about 30 °.

During the plating operation, the copper foil undergoes an elongation of approximately 10.5%. The eccentricity of the aluminum solidification backbone is approximately 1%; the angle of the dendrites is of the order of 30 °.

The adhesion of the products obtained allows cold re-rolling by successive passes up to a thickness of 3.25 mm, ie a reduction rate of 45%.

Claims (10)

1 ° / Method of plating a strip of thickness between 0.1 and 1 mm of a metal having a mechanical resistance and a melting point substantially higher than aluminum on at least one face of a strip of aluminum or aluminum alloy with a thickness between 1.5 and 20 mm by continuous casting of aluminum between spaced parallel cylinders, by means of a flattened profile nozzle of width equal to the strip to flow, placed in the vicinity of the cylinders, the strip to be plated being interposed between the cylinder or cylinders and the liquid aluminum, characterized in that a) the strip of plating metal is in contact with the cylinder on at least the portion between the generator closest to the end of the nozzle and the generator located in the plane of the axes of the cylinders and, preferably, on a contact arc greater than 30 °, b) the clearance between the nozzle and the cylinder carrying the plating metal is less than the thickness of the plating strip increased by 1 mm, c) the casting speed is such that the elongation of the plating metal is greater than 1% and less than that of aluminum once it has solidified. 2 ° / A method according to claim 1, characterized in that the plating strip is abraded, for example by brushing, on the face to adhere to the aluminum before introduction into the casting zone. 3 ° / A method according to claim 2, characterized in that the brushing operation immediately precedes the casting. 4 ° / A method according to any one of claims 1 to 3, characterized in that the composite strip is cold rolled after cooling. 5 ° / A method according to any one of claims 1 to 4, characterized in that the plating metal is a copper alloy. 6 ° / A method according to any one of claims 1 to 4, characterized in that the plating metal is stainless steel 7 ° / A method according to claim 6, characterized in that the elongation stainless steel between the casting rolls is between 3 and 10 Z. 8 ° / composite strip made of a thickness of cast aluminum between 1.5 and 20 mm and a cladding on at least one face, of thickness between 0.1 and 1 mm of a metal having a mechanical resistance and a significantly higher melting point than aluminum, characterized in that the longitudinal section of the aluminum strip has dendrites of solidification in the form of fish bones whose mean direction relative to the plane of the band is less than 85 ° and preferably 75 °. 9 ° / composite strip according to claim 8, comprising a single plated face, characterized in that the vertices of the solidification dendrites are distant from the median plane of the aluminum layer by less than 10 Z of the half-thickness of this layer . 10 ° / composite strip according to one of claims 8 and 9, characterized in that its thickness is between 1.5 and 6 mm.

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