EP0660882B1 - Method for manufacturing a thin sheet for producing canning components - Google Patents

Description

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a manufacturing process by casting continuous between cylinders of a thin sheet of suitable aluminum alloy for the production of components for beverage or use boxes food.

It is known, for example, to manufacture covers intended to be tightly associated with box bodies and thus constitute packaging for food in liquid or solid state.

These covers are obtained by cutting from an alloy sheet aluminum discs which may be fitted with opening devices fixed either by riveting or by gluing.

In order to be suitable for these mechanical operations as well as for stresses resulting from the manipulations and pressures exerted on the inside of boxes with certain foods such as drinks carbonated, said sheets must have both a suitability for suitable deformation and a sufficient elastic limit.

In addition, as these leaves must resist the corrosive effects of the atmosphere as with those of the contained products, it is essential to cover them with protective agents such as varnishes, for example, this which requires an aptitude for the adhesion of said sheets with respect to of said coatings.

STATE OF THE ART

The German document DE 3247698 (Alusuisse) teaches a method of manufacture of a strip intended for the manufacture of box covers aluminum alloy from a continuous casting machine characterized in that an alloy containing by weight 0.15 to 0.50% of Si is poured, 0.3 to 0.8% Fe, 0.05 to 0.25% Cu, 0.5 to 1% Mn, 2.5 to 3.5% Mg and up to 0.20% of Ti between two cooled casting rolls constituting a casting gap of 5 to 10 mm and that the resulting strip is cold rolled to a final thickness of 0.4 to 0.2 mm.

According to this process, to obtain an elastic limit of 321 MPa and a 7.7% elongation, it is necessary to resort after rolling the strip cast to a thickness of 1.9 mm at intermediate annealing consisting of reheating the strip to 380 ° C. and keeping it for 2 hours at this temperature then to also make a final annealing softening by heating at 205 ° for 8 minutes before varnishing.

Thus, in addition to the energy required to pass, according to the example, from a thickness e1 = 6.5 mm up to a thickness e2 = 0.3 mm, reduction corresponding to a rolling rate: (e1 - e2) / e1 x 100 = 95.4%, this process also involves two two-stage reheating operations different from rolling.

The document JP 04276047 (Sky Aluminum) describes a process for obtaining hard sheets of aluminum alloy for the production of covers of boxes, method comprising a thin strip casting to a thickness less than 15 mm, with a cooling rate greater than 50 ° C / sec; the sheet obtained is submitted immediately or after rolling cold to a first intermediate annealing, to cold rolling with a reduction rate from 30 to 85%, at the second intermediate annealing and finally in the final cold rolling with a reduction rate higher than 30%, this the latter can be followed by a final annealing.

The alloy has the following composition:
Mg: 1.2 to 3%, Cu: 0.05 to 0.5%, Mn: 0.5 to 2%, Fe: 0.1 to 0.7%, Si: 0.1 to 0.5% , the balance being Al.

A 6 mm thick sheet was obtained according to this process. having an elastic limit, at 45 ° relative to the direction of rolling, from 305 to 310 N / mm2.

Document EP 99739 (Continental) describes a process for obtaining a strip of Al alloy suitable for drawing and drawing, for example for obtaining boxes. It includes continuous strip casting thickness less than 2.54 cm, preferably between 6 and 12 mm, heating to 510-620 °, followed by cold rolling with reduction at least 25% thick, annealed, second cold rolled with a thickness reduction of at least 10%, a heating of recrystallization and final cold rolling.

Strips 12.1 mm thick and of different compositions were obtained and treated according to the method described; the final products obtained have the following characteristics (tab. XIX) Elastic limit from 280 to 294 MPa Breaking load from 291 to 308 MPa Elongation 2.2 to 2.5%

Document US 4411707 (Coors) describes a process for obtaining tapes suitable for the manufacture of covers. It includes continuous casting a strip of thickness between 6 and 7 mm, this strip undergoing during solidification a reduction of not more than 25%, then rolling to cold with a reduction of at least 60%, annealing at 440 - 483 ° C, a cold rolling at least 80% to the final thickness.

The breaking strength obtained is 272 MPa, the elastic limit of 245 MPa and the elongation of 4.1%.

We see that all these processes using alloy compositions Various describe at least one intermediate annealing during rolling at cold, which complicates the implementation and increases the cost.

PURPOSE OF THE INVENTION

The object of the invention is, with at least equal properties, to reduce the rolling rate and eliminating the intermediate annealing steps when cold rolling, which simplifies the process and makes it more economic.

OBJECT OF THE INVENTION

The invention consists of a method of manufacturing a sheet of aluminum alloy intended for the housing which has for composition by weight: Mg between 1 and 4%, Mn between O and 1.6%, balance Al with its unavoidable impurities and optionally additions of Cu and / or Cr. According to claim 1 said sheet is obtained by casting said alloy in the liquid state between two cylinders in the form of a strip having a thickness at most equal to 4 mm, followed by at least one heat treatment at a temperature between 400 and 580 ° C so that the sheet is at least partially recrystallized, from cold rolling up to a final thickness of less than 0.3 mm and optionally of a coating operation.

Thus, the subject of the invention is a process first characterized by the casting a strip between two cylinders at a lower thickness or equal to 4 mm so that to reach the thickness of a cover of box to manufacture, the rolling rate is less than 95%; this avoids the recourse to intermediate annealing between the rolling passes, which is the case as soon as the thickness is greater than 4 mm, as has been seen upper.

By casting between cylinders is meant a continuous casting of strips in which the liquid metal is introduced, using a nozzle refractory, between two cooled cylinders, between which it solidifies.

An example of this type of casting is the "3C" casting developed by PECHINEY, the principle of which is described in French patent no. 1,198,006.

This invention is made possible by the use of the ranges of specific concentrations, cited above, of the various elements of the alloy constituting the sheet; it provides properties improved, in particular increased mechanical characteristics.

In addition if we exceed 4 mm thick, we obtain an anisotropy plastic too high, which results in the manufacture of cover for dimensional irregularities; especially the edge developed cover, which will be used for crimping, may not respond to the specifications and lead to rejects.

Furthermore, pouring at a thickness of less than 4 mm is favorable with regard to the quality of the tape, in particular as regards the segregations which are much reduced if not absent hence a improved formability and obtaining productivity in the vicinity of its optimum.

There is however no point in pouring to a thickness of less than 1 mm because the work hardening of the strip due to rolling then proves to be insufficient and the mechanical resistance of the strip becomes too low for a application to lids.

Another characteristic of the invention is the obtaining of a structure partially recrystallized (for example greater than about 50%) or completely, after heat treatment between 400 and 580 ° C of the strip after casting. This recrystallization of the metal is necessary to obtaining excellent formability of the alloy.

This operation can be carried out discontinuously on the wound strip or in the process either on the strip leaving continuously from the casting machine, or on a strip previously wound after casting.
The duration of the heat treatment and the temperature depend on the rate of temperature rise.
When batch processing is carried out, the heating rate is generally between 20 and 200 ° C / h.
On the other hand, in the parade, the heating rate is at least 3000 ° C / h.
The parade treatment also offers particular advantages for alloys containing less than about 0.75% of Mn. Indeed, it leads to a recrystallization with fine and isotropic grains of dimensions less than 40 micrometers while the batch annealing gives grains of dimensions between 200 and 50 micrometers; this has the effect of improving the formability of the sheet.

The parade treatment is preferably obtained by heating in a induction oven or in a hot air circulating oven but, all other means of processing the parade of a strip can be envisaged.

But, the best results are obtained by following this parade processing by batch processing on a reel in the conditions indicated above.

On the other hand, for alloys containing more than about 0.75% of Mn, it generally suffices to carry out batch processing on a reel of preferably a parade treatment (at the end of casting or on a reel).

After heat treatment, the strip is cold rolled to the final thickness and the sheet obtained can be covered with a plastic material intended to protect it from the environment. It can be, for example, a varnishing on both sides with a varnish which is then dried by heating at a temperature between 200 and 280 ° C.

To obtain covers having suitable mechanical and formability characteristics, the process still needs to be applied to a well-defined range of alloys.
These alloys must contain between 1.0 and 4% of magnesium by weight because beyond the maximum claimed, there may be segregation which adversely affects formability; on the other hand, a content of less than 1% leads to insufficient mechanical strength.

This magnesium is preferably combined with manganese in proportion by weight up to 1.6%. A content higher than the maximum value prevents proper recrystallization during annealing and causes the appearance of large grains harmful to mechanical properties.

However, it is particularly advantageous to have a presence of magnesium and manganese satisfying the condition: (3 Mn% + 2 Mg%) greater than or equal to 6% and less than or equal to 9%, so to obtain the best compromise between mechanical strength and formability.

Preferably, the magnesium content is less than 3.2% but, the best results are obtained for an Mg content of less than 2.8%; indeed, this reduces the risks of segregation during casting, associated with high Mg contents.

The presence of Mn makes it possible to limit the content of Mg and therefore to decrease the risk of segregation; it is advantageously greater than 0.4% about.

Furthermore, the addition of a small amount of copper less than or equal to 0.4% and preferably less than 0.2% and / or the addition of chromium up to about 0.2% makes it possible to improve the mechanical strength of the alloy.
The content of these elements is limited because in too large a quantity they limit the ductility of the metal and therefore its formability.

As for silicon and iron, these are mainly impurities whose presence is a function of the quality of the aluminum used.

Preferably, the silicon is less than 0.3% or better than 0.2% and the iron at 0.5% or better at 0.3%.

Indeed, the silicon leads after casting or after heat treatment by maturation to the formation of intermetallic precipitates of Mg2Si which limit the formability of the alloy.
As for iron, it gives rise to the formation of eutectic precipitates during casting and therefore of segregations which are also harmful to ductility.

APPLICATION EXAMPLES

The invention will be better understood using the application examples following, not limiting.

Three types of alloys A, B and C have been used, their composition by weight: Alloy Mg% Mn% Fe% Yes% Cu% AT 3.20 0.40 0.20 0.05 0.20 B 2.50 0.75 0.20 0.05 0.20 VS 1.5 1.4 0.19 0.05 0.20

These alloys were subjected during their development to a treatment refining by adding an aluminum alloy containing titanium and AT5B type boron introduced into the liquid metal either directly into the production furnace either by progressive fusion of a wire upstream of the oven.

I) Recuit au défilé de la bande sortant de la machine de coulée dans un four où de l'air chaud est soufflé de manière que la bande soit portée à 440°C pour les alliages A et B et 500° pour l'alliage C et soit maintenue à cette température pendant 30 secondes. La bande est ensuite refroidie à 300°C puis bobinée.

II) Recuit discontinu de la bande bobinée dans un four où le métal subit un réchauffage à 440°C pour les alliages A et B et 500°C pour l'alliage C et un maintien à cette température pendant 10 heures.

III) Le recuit I est suivi du recuit II.

Said alloys were poured between two cylinders in the form of strips of thickness 2.8 mm at the speed of 3 m / min.
These bands were subjected to heat treatments of three types:

I) Annealing on passing the strip leaving the casting machine in an oven where hot air is blown so that the strip is brought to 440 ° C for alloys A and B and 500 ° for alloy C and be kept at this temperature for 30 seconds. The strip is then cooled to 300 ° C and then wound.

II) Discontinuous annealing of the wound strip in an oven where the metal is reheated to 440 ° C for alloys A and B and 500 ° C for alloy C and maintained at this temperature for 10 hours.

III) Annealing I is followed by annealing II.

The annealed strip is then subjected to 6 rolling passes without annealing intermediate to bring it in the form of a thick sheet final of 270 micrometers.

Said sheet is then degreased, subjected to a chemical treatment conversion to be then varnished on both sides.

• limite d'élasticité : R 0,2% mesurée après recuit des vernis et dans le sens long.
• indice de formabilité Ericksen suivant la norme française NF A03 - 652.
• délaminage du vernis (mesure effectuée après incision du métal et pasteurisation de la feuille à 75°C durant 30 min. dans l'eau déminéralisée).

The following measurements were then carried out on the sheet obtained:

• yield strength: R 0.2% measured after annealing of the varnishes and in the long direction.
• Ericksen formability index according to French standard NF A03 - 652.
• delamination of the varnish (measurement carried out after incision of the metal and pasteurization of the sheet at 75 ° C for 30 min. in demineralized water).

The results obtained, which refer to alloys A, B or C heat treated according to I, II or III, appear in the following table: Reference Yield strength R0.2% (MPa) Ericksen index (mm) Varnish delamination (mm) HAVE 330 4.2 0.5 AII 325 4.5 0.4 AIII 328 4.9 0.4 BI 321 4.3 0.5 BIII 331 5.0 0.4 ITC 338 5.0 0.4

Knowing that the characteristics necessary to obtain covers suitable are a yield strength greater than 320 MPa, an index Ericksen greater than 4 and a delamination of the varnish less than 0.6 mm, it can be seen that the objectives are achieved by the process according to the invention, in particular in the case where a treatment is carried out type II or III thermal.

We see that the best results are obtained with BIII and CII, corresponding to respective heat treatments on the one hand at parade on tape leaving the casting followed by a discontinuous treatment, on the other hand discontinuously.

Claims (10)

1. Process for producing an aluminium alloy sheet intended for can manufacture composed of, by weight, between 1 and 4% of Mg, between 0 and 1,6% of Mn, optionally Cu <0, 4% and Cr < 0,2%, remainder Al with its inevitable impureties, by continuous twin-roll casting and cold rolling at a final thickness less than 0,3 mm, characterised in that :

   the strip is cast at a thickness < 4mm

   the cast strip is submitted, before cold rolling, to an annealing between 400 and 580°C so that the strip is at least partially recrystallised

   the cold rolling of the at least partially recrystallised strip is performed without intermediate annealing.
2. Process according to claim 1, characterised in that, in the presence of manganese, the magnesium satisfies the condition : 3 Mn% + 2 Mg% higher than or equal to 6% and lower than or equal to 9%.
3. Process according to claim 1, characterised in that the magnesium content of the alloy is less than 3.2% (by weight).
4. Process according to claim 3, characterized in that the magnesium content is less than 2.8% (by weight).
5. Process according to claim 1, characterized in that the Mn content is higher than 0.4%.
6. Process according to claim 1, characterised in that the heat treatment is carried out discontinuously on the strip wound in the form of a coil at a heating rate of between 20 and 200°C/h.
7. Process according to claim 6, characterised in that the discontinuous treatment on a coil is applied to alloys containing more than about 0.75% of Mn.
8. Process according to claim 1, characterised in that the heat treatment is carried out continuously on the strip at a heating rate higher than 3000°C/h.
9. Process according to claims 6 and 8, characterised in that the heat treatment is carried out successively continuously then discontinuously.
10. Process according to claim 9, characterised in than the heat treament is carried out continuously then discontinuously on a coil in the case of alloys containing less than 0.75% of Mn.