EP0292411A1 - Aluminium alloy for thin sheets suitable for making lids and bodies of cans, and process for producing said sheets - Google Patents

Abstract

The invention relates to an aluminum alloy for thin sheets suitable for obtaining lids and bodies of boxes and to a process for manufacturing said sheets. The alloy contains by weight percent: 0.8 to 1 , 8 manganese 1 to 2 silicon 0.7 to 3 magnesium less than 0.7 iron less than 0.5 copper less than 0.5 chromium balance aluminum The process includes shaping and heat treatment of said alloy consisting in casting it directly into strips with a thickness of between 4 and 20 mm, in heating said strips between 500 and 620 ° C for 2 to 20 hours, in cold rolling the strips thus treated until 1 'final thickness including an intermediate heating step between 500 and 600 ° C for 0.5 to 10 minutes followed by quenching in air. Due to the use of a single alloy, the invention allows to more easily solve the problem of recycling boxes after use.

Description

The present invention relates to an aluminum alloy for thin sheets suitable for obtaining lids and bodies of boxes intended to contain food or carbonated drinks and to a process for manufacturing said sheets.

Aluminum alloys are now widely used for the manufacture of boxes intended to contain both solid and liquid food as well as carbonated drinks.
These boxes are made up of two parts: a body with an integrated bottom, the side wall of which can be printed or not, on which is attached a cover provided with an easy opening system, in particular in the case of boxes for carbonated drinks.
The covers are generally obtained by cutting a thin sheet of thickness between 200 and 400 μm and the bodies of neighboring thickness are either by stamping or by stamping followed by re-drawing.

It is understood that, depending on the content of the box, its use for a body or a cover, the shaping process used, the presence or absence of an easy-opening system or printing, the sheet should have characteristics adapted to each of these peculiarities.

Thus, the covers with easy opening must have a mechanical resistance higher than that of the other covers so as not to tear during their use; the printed or re-stretched bodies must have a relatively low rate of horns to avoid any deformation either of the preprinted characters or of the upper part of the body which receives the cover; bodies subjected to re-stretching should not seize on contact with the tooling to avoid the appearance of scratches or even breaks.

Faced with so many imperatives, those skilled in the art turned to the use of sheets made from alloys of different composition.

Thus, in the field of boxes intended for food, generally used:
- for covers and unprinted bodies of sheets 230 µm thick in alloy 5052 in the H28 state according to the standards of the Aluminum Association, that is to say having for composition by weight percent:
Si 0.25 - Fe 0.40 - Cu 0.10 - Mn 0.10 - Mg 2.2-2.8 - Cr 0.15-0.35 - Zn 0.10- other 0.15 - balance Al .
- for the bodies of printed boxes of sheets of the same thickness, in the same alloy but in the H24 state.

In the field of cans intended for carbonated drinks, generally:
- for the bodies of boxes obtained by stamping-re-drawing of sheets of thickness 330 μm in alloy 3004 in the H19 state according to the standards of the Aluminum Association, that is to say having for composition by weight percent :
If 0.30 - Fe 0.7 - Cu 0.25 - Mn 1.0-1.5 - Mg 0.8-1.3 - Zn 0.25 - other 0.15 - balance Al.
- for the covers of sheet metal boxes 300 µm thick in alloy 5182 in the H19 state according to the standards of the Aluminum Association, that is to say having for composition by weight percent:
Si 0.20 - Fe 0.35 - Cu 0.15 - Mn 0.20-0.50 - Mg 4.0-5.0 - Cr 0.10 - Zn 0.25 - others 0.15 - balance Al .

From this inventory, it is deduced that, in particular in the field of cans for carbonated drinks, lids and bodies have very different compositions in particular as regards their magnesium and manganese content. This requires different production lines to develop them and thus strike their cost price. But to these disadvantages is grafted the problem of recycling boxes after use: in fact, given the growing share taken by aluminum alloys on the box market, we realized the significant savings which would result if we recovered said boxes instead of putting them in the landfill.
But, like the bodies, which are practically inseparable from the covers, an economic recycling involves the remelting of the whole box. Hence the obtaining of an alloy of intermediate composition between that of the body and the cover which must therefore be divided into two fractions, each of which is then returned to the title by adding pure aluminum and elements d alloys.

It therefore seems more advantageous from the point of view of recycling to deal with a single type of alloy. But, it is still necessary that this alloy meets all the constraints imposed on the sheets either by their destination: food or carbonated drinks, or by their shape: body or lids, or by their method of production: stamping or stamping-stretching or still by some of their peculiarities such as easy opening systems on lids or the ability to properly develop preprinted characters or patterns.

It is in this direction that the plaintiff has embarked. This enabled him to find an alloy composition such that it led by strip casting followed by a set of shaping operations and heat treatments judiciously chosen to obtain sheets having characteristics capable of satisfying to the various constraints to which they will be subjected.

• a) on prépare une masse fondue d'alliage d'aluminium qui, outre les impuretés habituelles, contient comme constituants principaux 0,4 à 1,0 % de manganèse et 1,3 à 2,5 % de magnésium, la teneur totale en magnésium et manganèse étant de 2,0 à 3,3 % et le rapport du magnésium au manganèse étant 1,4:1 à 4,4:1.
• b) on coule en continu la masse fondue en une bande au moyen d'une machine à couler les bandes.
• c) on lamine en continu à chaud la bande coulée à la vitesse de coulée avec une réduction d'au moins 70 %, la température de début de laminage à chaud étant comprise entre 300°C et la température de solidus de l'al­liage et la température à la fin du laminage étant d'au moins 280°C.
• d) la bande laminée à chaud est enroulée à chaud et on la laisse refroidir à l'air calme jusqu'à la température ambiante environ et,
• e) la bande laminée à chaud refroidie est laminée à froid jusqu'à l'épais­seur finale".

Admittedly, the plaintiff was not the first to have solved the problem in this way. Mention may be made, for example, of French patent No. 2,432,556 which teaches "a process for the production of a strip of aluminum alloy suitable for the manufacture of bodies and lids of boxes by stamping and stretching, characterized in than :

• a) a molten mass of aluminum alloy is prepared which, in addition to the usual impurities, contains as main constituents 0.4 to 1.0% of manganese and 1.3 to 2.5% of magnesium, the total content of magnesium and manganese being from 2.0 to 3.3% and the ratio of magnesium to manganese being 1.4: 1 to 4.4: 1.
• b) the melt is continuously poured into a strip by means of a strip casting machine.
• c) the continuous strip is hot rolled at the casting speed with a reduction of at least 70%, the temperature of the start of hot rolling being between 300 ° C. and the solidus temperature of the alloy, and the temperature at the end of the rolling being at least 280 ° C.
• d) the hot-rolled strip is rolled up hot and allowed to cool in still air to approximately room temperature and,
• e) the cooled hot rolled strip is cold rolled to the final thickness ".

In this patent, which also uses strip casting, a single alloy and a single production process are used for the can bodies and for the lids, except for cold rolling since a harder strip is required. for covers.

Under these conditions, the sheets obtained have the mechanical characteristics with respect to sheet metal for a body 0.2% elastic limit of 250 to 310 MPa, a tensile strength of 260 to 320 MPa and an elongation at break of 1 to 8%; for cover sheets, the values of these characteristics are 310-370 MPa, 320-380 MPa and 1 to 5% respectively.

In the present invention, the applicant has aimed to increase these characteristics, in particular in the case of sheets for covers. It achieved this by moving towards an alloy more charged with silicon and characterized in that it contains by weight percent: 0.8 ≦ manganese ≦ 1.8, 1 <silicon ≦ 2, 0.7 ≦ magnesium ≦ 3, iron <0.5, copper <0.5, chromium <0.5, aluminum balance.
The higher silicon content, in combination with magnesium, promotes the formation of Mg₂Si which acts as a hardening agent.
Even elsewhere, the presence of an amount of manganese on average greater than in the previous patent has the effect of greatly reducing the seizure phenomena during the re-stretching of the can bodies.

• a) on prépare une masse fondue d'alliage qui, outre les impuretés habituel­les, contient comme éléments principaux en poids pour-cent : 0,8 ≦ manganèse ≦ 1,8, 1 < silicium ≦ 2, 0,7 ≦ magnésium ≦ 3, fer < 0,7, cuivre < 0,5, chrome < 0,5
• b) on coule en continu ladite masse en bandes d'épaisseur comprise entre 4 et 20 mm
• c) on chauffe la bande coulée entre 500 et 620°C pendant 2 à 20 heures
• d) on lamine à froid la bande qui a été chauffée jusqu'à une épaisseur in­termédiaire
• e) on chauffe la bande amincie entre 500 et 600°C pendant 0,5 à 10 minutes puis trempe à l'air
• f) on lamine à froid jusqu'à l'épaisseur finale de la tôle fabriquée.

The invention also relates to a method of manufacturing said sheets. This process includes a set of operations relating to the preparation, shaping and thermal treatments and are characterized in that :

• a) an alloy melt is prepared which, in addition to the usual impurities, contains as main elements by weight percent: 0.8 ≦ manganese ≦ 1.8, 1 <silicon ≦ 2, 0.7 ≦ magnesium ≦ 3 , iron <0.7, copper <0.5, chromium <0.5
• b) continuously pouring said mass into strips of thickness between 4 and 20 mm
• c) the cast strip is heated to between 500 and 620 ° C for 2 to 20 hours
• d) the strip which has been heated to an intermediate thickness is cold rolled
• e) the thinned strip is heated to between 500 and 600 ° C. for 0.5 to 10 minutes and then quenched in air
• f) cold rolled to the final thickness of the sheet produced.

Thus, the method consists in developing a melt of given composition and in pouring it into strips for example in a cylinder machine which, thanks to the high cooling rates, makes it possible to keep in solid solution a lot of Mg, Si and Mn, which then makes it easier to dissolve. Preferably, the strip has a thickness of between 6 and 12 mm.

After casting, the strip is heated to between 500 and 620 ° C for 2 to 20 hours so as to homogenize the metal.
Then, after cold rolling to an intermediate thickness, a solution treatment is carried out at a temperature between 500 and 600 ° C. for 0.5 to 10 minutes followed by air quenching allowing characteristics to be obtained superior to those of conventional alloys.
Preferably, this dissolution is carried out between 530 and 580 ° C for 1 to 2 minutes.
Then laminated to the final thickness and optionally reheats the sheet to 200-220 ° C for 5 to 15 minutes so as to cook the varnishes. It is possible to carry out an annealing of 30 min to 2 hours between 100 and 250 ° C. at a thickness intermediate between the thickness of dissolution - hardening and the final thickness.

These operations are applicable to the manufacture of sheets suitable for all types of lids and box bodies described above.

It may be noted that, unlike French patent 2,432,556, all of the rolling operations take place cold and that the dissolution takes place at a temperature between 500 and 600 ° C. for 30 seconds to 10 minutes then that in the patent, it is partially hot rolled and that the solution temperature is between 350 and 500 ° C. for a maximum duration of 90 seconds.

Depending on the particular use that will be made of the sheets, one can modulate the operating conditions described above and possibly introduce additional operations to optimize the process.

The invention can be illustrated using the following example of application:

• a) on a coulé en une bande de 7,5 mm d'épaisseur, l'alliage de composition suivante en poids pour-cent :
        Mg : 0,80
        Mn : 1,08
        Si : 1,25
        Fe : 0,40
• b) on a chauffé la bande coulée pendant 6 heures à 540°C
• c) on a laminé à froid la bande jusqu'à une épaisseur de 1,5 mm
• d) on a chauffé la bande amincie pendant 5 min à 560°C et trempé à l'air
• e) on a laminé à froid jusqu'à l'épaisseur finale de 0,33 mm.

To make lids for cans intended to contain carbonated drinks:

• a) a 7.5 mm thick strip was poured, the alloy of the following composition by weight percent:
  Mg: 0.80
  Mn: 1.08
  If: 1.25
  Fe: 0.40
• b) the cast strip was heated for 6 hours at 540 ° C.
• c) the strip is cold rolled to a thickness of 1.5 mm
• d) the thinned strip was heated for 5 min at 560 ° C. and air quenched
• e) cold rolled to the final thickness of 0.33 mm.

Under these conditions, the characteristics of the sheet obtained were as follows:
RO, 2 = 395 MPa
Rm = 410 MPa
A% = 4.

By comparison with the values given in French patent n ° 2 432 556 and cited above, namely R 0.2 = 370 MPa and Rm = 380 MPa, we can see a great improvement.

Claims (6)

1. Aluminum alloy for the manufacture of thin sheets suitable for obtaining covers and bodies of boxes, characterized in that it contains by weight percent:
0.8 ≦ manganese ≦ 1.8
1 <silicon ≦ 2
0.7 ≦ magnesium ≦ 3
iron <0.7
copper <0.5
chromium <0.5
balance: aluminum. 2. A method of manufacturing thin sheets suitable for obtaining lids and bodies of boxes intended to contain food or carbonated drinks comprising operations of preparation, shaping and heat treatment, characterized in that: a) an alloy melt is prepared which, in addition to the usual impurities, contains as main elements by weight percent: 0.8 ≦ manganese ≦ 1.8, 1 <silicon ≦ 2, 0.7 ≦ magnesium ≦ 3 , iron <0.7, copper <0.5 chromium <0.5 b) continuously pouring said mass into strips of thickness between 4 and 20 mm c) the cast strip is heated to between 500 and 620 ° C for 2 to 20 hours d) the strip which has been heated to an intermediate thickness is cold rolled e) the thinned strip is heated to between 500 and 600 ° C. for 0.5 to 10 minutes and then quenched in air f) cold rolled to the final thickness of the sheet produced. 3. Method according to claim 2 characterized in that it flows in strips of thickness between 6 and 12 mm. 4. Method according to claim 2 characterized in that the thinned strip is heated between 530 and 580 ° C for 1 to 2 minutes. 5. Method according to claim 2, characterized in that during cold rolling to the final thickness, is introduced between two passes an income of 30 min to 2 hours between 100 and 250 ° C. 6. Method according to claim 2 characterized in that the final sheet is heated 5 to 15 minutes between 200 and 220 ° C.