EP0064468B1 - Process for manufacturing foils consisting of hypoeutectic aluminium-iron alloys - Google Patents

Abstract

1. A process for the production of sheets of hypoeutectic aluminium-iron alloy by the addition of a fining agent to the metal bath, semicontinuous casting in an ingot mould or continuous casting between cylinders, cold rolling and annealing at from 380 degrees C to 430 degrees C, characterised in that the fining agent is added in an amount such that it generates a cast structure wherein the grains are of a maximum size of less than 100 mu m and the spacing between dendrite arms is from 8 to 30 mu m, and that the rolling operation is performed using a thickness reduction ratio of from 98 to 99.8% so as to develop a final semirecrystallised structure after annealing.

Description

The present invention relates to a process for manufacturing sheets of hypoeutectic aluminum-iron alloys, intended more particularly for the production of light dishes, flexible pipes, plugs and for all applications requiring said sheet of mechanical characteristics which favor stamping. , folding and stiffness.

The term “hypoeutectic aluminum-iron alloys” is understood here to mean those which contain less than 1.7% of iron.

Such alloys are known in particular from French patent FR no. 1,438,096 which protects a product suitable for the production of metal sheets with a thickness of less than 76 μm, characterized in that it contains from 0.6 to 1.2 % iron, the rest being aluminum. Such a sheet is obtained by conventional semi-continuous casting followed by cold rolling and has interesting mechanical characteristics, namely a tensile breaking strength of the order of 80 MPa and an elongation of 7%.

Another French patent, no. 2 291 285, also claims a process for the production of sheets of alloys containing between 1.1 and 2.5% of iron. This process uses the casting machines with cylinders of the HUNTER type , rolling and final annealing. However, his teaching is rather focused on compositions close to 1.7% iron, that is to say eutectics, which lead to homogeneous solid phases, made up of fine particles of 'intermetallic dispensed throughout the material.

The Applicant has directed its research towards alloys with a composition which is quite far from eutectic and contains at most 1.3% iron. Those skilled in the art know that, under these conditions, solidification begins with the appearance of so-called primary aluminum crystals in the form of dentrites and ends with an eutectic deposit composed of intermetallics of At _n Fe which separates the grains from each other. This leads to a heterogeneous structure quite different from the homogeneous structure obtained in the case of a eutectic composition.

dans laquelle E représente l'épaisseur avant laminage et e, l'épaisseur finale.In the field of these hypoeutectic aluminum-iron alloys, the French patent, No. 1,438,096, shows by means of curves, the variations in mechanical characteristics, such as the tensile breaking strength and the elongation, in as a function of the rate of reduction in thickness which the metal undergoes during rolling. This rate, also called rate of work hardening, corresponding to the formula

in which E represents the thickness before rolling and e, the final thickness.

It can be seen on these curves that the hypoeutectic Al ,, Fe has a significantly better elongation characteristic than that of conventional alloys 1100 and 5005. This property means that, as the patent teaches: »the alloy adapts better to thin sheet manufacturing operations and return the superior product obtained for packaging applications, for which better mechanical strength would be ineffective if it were accompanied by a significant loss of elongation. "

It is, in fact, mainly the search for an optimum compromise between these two characteristics which guides those skilled in the art in the manufacture of sheets intended for packaging and it is with this in mind that the Applicant has sought and found a way to improve said compromise, and thus give the product obtained qualities favoring stamping, folding and rigidity.

This means consists in a process aiming to develop in the metallic sheet a final structure called "semi recrystallized".

It is known, in fact, that during cold rolling in particular, the metal hardens or better hardens, that is to say that it gradually loses all capacity for plastic elongation while acquiring greater strength. It can then be heated to a temperature in the region of 400 ° C. and thus carry out a so-called recrystallization annealing, an operation during which the mechanical characteristics change considerably and are oriented towards a reduction in the breaking load and an increase in elongation. Admittedly, this significantly improves the elongation, but more often than not, the resistance has become insufficient.

This is why, the Applicant has thought that in seeking to limit this recrystallization, it was possible to produce an intermediate structure, which would simultaneously develop the advantages of the good strength of the work-hardened state and of the appropriate elongation of the recrystallized state thus concretizing the obtaining of the desired compromise.

To achieve this, it has developed a process in which the metal bath is treated with a refiner, it is poured semi-continuously in an ingot mold or continuously between cylinders, it is cold rolled and annealed between 380 ° C and 430 ° C, this process being characterized in that the refiner is added in such a quantity that it generates a pouring structure in which the grains have a maximum dimension less than 100 μm and the spacing between dendrite arms is between 8 and 30 μm and in that the rolling takes place according to a thickness reduction rate of between 98 and 99.8%.

Thus, the method according to the invention consists in implementing the conventional steps in metallurgy of refining the alloy in the liquid state, of casting, of rolling, of final annealing. It can be noted, however, that the process applies indifferently to a product cast in semi- continuous by means of ingot molds or type 3C cylinder casting machines, provided that the refining and work hardening parameters are adapted to the type of casting so as to lead to the same final structure.

The process is first characterized by the addition of a refiner in quantity such that it generates a casting structure in which the grains have a maximum dimension of less than 100 μm. Conventionally, grains with dimensions close to 200 μm are obtained by casting. However, experience has shown that with such a size, the alloy behaves too heterogeneously during rolling and that shear bands are created favorable to the development of a total recrystallization, which goes to the 'against obtaining a semi-recrystallized structure.

However, the search for too small a grain size is also not interesting because it leads to a too uniform and regular distribution of the eutectic and results in a product which does not recrystallize.

This is why, it is refined so as to have grains of dimensions between 10 and 50 μm. This refining can be obtained by means of any agent generally used in aluminum metallurgy and, in particular, products based on boron and titanium such as, for example, AT5B.

With the same aim of avoiding this homogeneity of structure, it is obvious, for a person skilled in the art, to use alloy compositions moving as far as possible from the eutectic zone, but nevertheless retaining sufficient iron to benefit from the particular properties of this element and, in particular, from its action as an agent for blocking recrystallization. Thus, a content of between 1.1 and 1.3% of iron is preferred.

Always with the same concern for finding a heterogeneous structure, the alloy is avoided from being poured into a cylinder machine of the HUNTER type because the solidification speed is too high and leads to dendrites which are too fine less than 5 μm. On the other hand, the 3C machine proves to be particularly advantageous in the process of the invention because it gives rise to the formation of dendrites which are still sufficiently large so that zones with and without eutectic can be clearly distinguished on the structure.

The method is also characterized in that the cold rolling, applied directly to the casting structure, is carried out according to a thickness reduction rate of between 98 and 99.8%.

It is understood, of course, by cold rolling, a rolling carried out at a temperature below the recrystallization temperature.

This choice results from the fact that, when this rate is too low, of the order of 96% for example, the work hardening is insufficient and, consequently, the metal will recrystallize easily and completely during the annealing. On the contrary, if the work hardening is too high, branches of dendrites have become so fine that recrystallization cannot occur and the structure only evolves by restoration.

Between these two hardening rates, a limited number of regions can recrystallize and form recrystallized grains isolated in a restored material and these grains cannot develop and invade the entire structure due to the presence of eutectic particles. These rates were chosen so as to subsequently form a semi-recrystallized structure in which the recrystallized volume represents between 10 and 30% of the total volume, distribution most compatible with the desired goal.

Finally, the process also includes an annealing between 380 and 430 ° C. after rolling at a temperature rise rate and of a duration such that the desired semi-recrystallized structure is developed and which is then composed of recrystallized grains of diameter less than 30 µm and generally between 5 and 15 µm, isolated in a restored matrix with sub-grains of 1 to 2 µm.

The semi-recrystallized structure thus obtained seems essentially due to a dispersion in the distribution of the distances between the intermetallic particles allowing both recrystallization when this distance is large and blocking of recrystallization when this distance is small.

The small volume occupied by the recrystallized grains seems essential because it brings to the alloy a notable improvement in ductility while the structure of the sub-grains still allows a high elastic limit to be retained.

This combination gives the alloy sheet an advantageous compromise between its characteristics, which is particularly favorable for subsequent processing operations such as stamping for example.

caractérisée par une strucutre dans laquelle 10 à 30% du volume est à l'état recristallisé sous forme de grains supérieurs à 5 ym et inférieurs à 30 pm, et le reste est à l'état restauré de manière à obtenir un compromis avantageux entre la charge de rupture à la traction et l'allongement, et se situant pour la première caractéristique mécanique entre 120 et 140 MPa, et pour la deuxième, au-dessus de 17%.The invention also relates to the alloy sheet obtained by the process having a thickness of between 50 and 250 μm, a composition such as:

characterized by a structure in which 10 to 30% of the volume is in the recrystallized state in the form of grains greater than 5 μm and less than 30 μm, and the rest is in the restored state so as to obtain an advantageous compromise between the tensile strength and elongation, and for the first mechanical characteristic being between 120 and 140 MPa, and for the second, above 17%.

The invention can be illustrated by means of the following examples which give the results of mechanical characteristics obtained with sheets of 100 μm thickness, produced according to the claimed process.

Example 1

a été affiné au moyen d'un alliage-mère d'AT5B à raison de 3 kg par tonne d'alliage, puis coulé en continu au moyen d'une lingotière sans fond sous forme d'une plaque dont la section a pour dimensions 38 x 7 cm avec une vitesse de 8 cm/ minute.An alloy of composition:

was refined using an AT5B mother alloy at a rate of 3 kg per tonne of alloy, then continuously cast using a bottomless ingot mold in the form of a plate whose section has the dimensions 38 x 7 cm with a speed of 8 cm / minute.

The structure of the cast product was composed of primary aluminum grains having a maximum dimension less than 150 µm separated from each other by eutectic zones of AI _n Fe.

The plate was cold rolled at a thickness reduction rate of 99.8% so as to give a sheet 100 μm thick which was annealed at 420 ° C for 30 hours.

compris de valeurs particulièrement à l'élaboration de plats obtenus par emboutissage.The final structure of this sheet had 30% of its volume in the recrystallized state and its mechanical characteristics were as follows:

understood of values particularly in the preparation of dishes obtained by stamping.

Example 2

a été affiné au moyen d'un alliage-mère d'ATSB à raison de 2 kg par tonne d'alliage, puis coulé en continu au moyen d'une machine à cylindres refroidis du type 3C sous forme d'une tôle d'épaisseur 0,8 cm avec une vitesse de 100 cm/minute.An alloy of composition:

was refined using an ATSB master alloy at a rate of 2 kg per tonne of alloy, then continuously cast using a type 3C cooled cylinder machine in the form of a thick sheet 0.8 cm with a speed of 100 cm / minute.

The structure of the cast product included primary aluminum grains of dimensions between 30 and 80 µm separated from each other by eutectic zones of Al _n Fe.

The sheet was cold rolled at a thickness reduction rate of 99% so as to form a sheet of thickness 80 μm which was annealed at 400 ° C for 30 hours.

caractéristiques qui ont permis d'utiliser cette feuille à la confection de plats emboutis, avec un taux de rebut pratiquement nul.The sheet had a final structure recrystallized at 20% and its mechanical characteristics were as follows:

characteristics which made it possible to use this sheet for the preparation of deep-drawn dishes, with a practically zero rate of reject.

The present invention finds its application in the aluminum industry whenever it is desired to obtain sheets with a thickness of between 50 and 150 μm having a couple of optimum values with regard to the tensile breaking load. and elongation.

Claims (2)

1. A process for the production of sheets of hypoeutectic aluminium-iron alloy by the addition of a fining agent to the metal bath, semi- continuous casting in an ingot mould or continuous casting between cylinders, cold rolling and annealing at from 380°C to 430°C, characterised in that the fining agent is added in an amount such that it generates a cast structure wherein the grains are of a maximum size of less than 100 µm and the spacing between dendrite arms is from 8 to 30 µm, and that the rolling operation is performed using a thickness reduction ratio of from 98 to 99.8% so as to develop a final semi- recrystallised structure after annealing.

2. A sheet of hypoeutectic aluminium-iron alloy produced in acordance with the process of claim 1, being from 50 to 250 µm in thickness and being of a composition such that:

characterised by a structure in which from 10 to 30% of the volume is in the recrystallised state in the form of grains which are larger than 5 µm and smaller than 30 µm, and the balance is in the restored state so as to achieve an optimum compromise between the tensile breaking stress and elongation, the value of the first characteristic being from 120 to 140 MPa and the value of the second characteristic being higher than 17%.