EP1443123B1 - Aluminum alloy for the production of can end sheet for cans - Google Patents

Description

The invention relates to a method for producing cover strip and a cover strip for aluminum alloy cans.

Such alloys are known in the art in a variety of configurations. For example, it is known to make a lid tape for aluminum alloy cans AA 5082 or AA 5812.

The requirements for a cover tape for cans, especially beverage cans is mainly to ensure a high internal pressure stability. Such a high internal pressure stability is necessary in order to withstand the internal pressure approximately at a pasteurization of the food in the sealed can or the resulting from the carbon dioxide content of internal pressure of a canned product, usually a beverage.

Such an aluminum alloy for the production of cover tape for cans, for example, from US 5,469,912 known.

The requirement for cover tape for cans currently go that a further improved internal pressure stability is required. This applies initially with regard to an improvement of the time behavior of the internal pressure stability. In known aluminum alloys for the production of cover tape for cans, the internal pressure stability decreases with increasing storage of cans comparatively strong - one speaks of the so-called aging. This behavior is undesirable because it may cause the lids of cans to age no longer withstand the internal pressure of the packaged products. With regard to the material costs, the requirements are parallel to ensure a constant internal pressure stability at reduced material thickness. This requirement is directly related to the reduction of so-called aging.

Metal physically, hidden behind the drop in internal pressure stability after the lid is made, is the process of recovery of the material in the areas that have been severely deformed during manufacture of the can body and lid. Under the influence of temperature, this effect is exacerbated in that the waste takes place within shorter times and the final level of the internal pressure stability is lower. For this reason, the use of PVC-free paints in the coating of the cover strip has so far been problematic because they require higher baking temperatures during drying and thus cause a greater decrease in strength.

Responsible for a high internal pressure _{stability of} the cover _strip is in particular the yield strength R _{p0.2 of} the cover _strip at 45 ° to the rolling direction. This is in generally significantly lower than the yield strength below 0 ° or 90 ° to the rolling direction.

In order to achieve maximum internal pressure _{stability of} the cover _strip , therefore, the yield strength R _{p0.2 of} the cover _strip should assume a maximum value at 45 ° to the rolling direction, even after the paint has been _baked on.

Finally, under the impact of high recycling rates required, it is necessary to provide an aluminum alloy for the production of lidding tape for cans, which allows the use of recycled material. This is particularly problematic in view of the higher alloy proportions of silicon and iron in the aluminum alloys for the can bodies, since the can bodies are not separated from the can lids during recycling.

From the European patent application EP 0 506 100 A1 as well as from the Japanese Patent Application JP-A-2000-8133 Both aluminum alloys for a can lid tape and methods for its production are known. The US 4,277,280 also describes the removal of sodium and calcium from the primary aluminum.

Based on the above embodiments, the present invention has for its object to propose a method for producing cover tape for cans with an improved microstructure and a corresponding lid tape for cans.

The above object is achieved by a method having the features of claim 1.

The aluminum alloy for the production of cover tape for cans has the following proportions by weight in parts by weight: 0.05% ≤ Si ≤ 0.2% 0.2% ≤ Fe ≤ 0.4% 0.05% ≤ Cu ≤ 0.15% 0.4% ≤ Mn ≤ 0.8% 4.7% ≤ mg ≤ 5.4% 0.0% ≤ Cr ≤ 0.1% 0.0% ≤ Zn ≤ 0.25% 0.0% ≤ Ti ≤ 0.15% Ca ≤ 4 ppm N / A ≤ 4 ppm other in total max. 0.15%, individually
Max. 0.05%, balance Al.

The aluminum alloy ensures improved internal pressure stability with regard to the aforementioned aspects by combining two strategies. The first strategy is to achieve high internal pressure stability of the lid by increasing the yield strength of the painted material to increase the overall level of strength. On the other hand, the second strategy is to influence the recovery behavior of the painted material at the same yield strength by reducing the drop in internal pressure stability as time passes after the lid is made. The described first teaching of the invention adopts both strategies by adopting the essential alloying elements Mn and Mg be coordinated. Therefore, Mg essentially increases the strength of the material while at the same time improving the formability and thus implements the first strategy. With the increased addition of Mg, however, an increased recovery and thus an increased aging effect is accompanied at the same time. To compensate for this according to the second strategy, strength enhancing Mn and small amounts of Cu are also added, both of which dampen recovery.

The upper limits of Fe and Si are necessary to ensure the formability of the covers made from the aluminum alloy of the present invention. The lower limits of Fe and Si ensure the possibility of using recycled material for the aluminum alloy according to the first teaching of the invention.

In addition, it is ensured by the low Na and Ca contents of the cover strip that during hot rolling maximum deformation levels can be realized without inter-crystalline cracks in the structure of the cover strip arise. Due to the achievable degrees of deformation precipitation phases in the cover band can be optimally broken, so that a homogeneous deformation can be achieved during the subsequent cold rolling while largely avoiding deformation inhomogeneities. The homogeneous deformation achieved in this way sets an improved crystallographic texture of the cold-rolled cover strip, and at the same time a diffuse dislocation structure, without a pronounced substructure.

As a result, the yield strength R _p0.2 at 45 ° to the rolling direction after cold rolling can be significantly increased, especially after baking of the paint.

Furthermore, the aluminum alloy can be recycled particularly well by limiting the boron content only by an upper limit of B = 10 ppm.

Due to the low boron content of the aluminum alloy, it is permissible for the actual boron content to be adjusted by the amount of recycled material used and not controlled by the addition of titanium boride wire, as in conventional aluminum alloys.

As a result of the ratio between the proportions of Mg and Mn being greater than or equal to 7 and less than or equal to 11, the softening is significantly reduced as desired.

In order to ensure a sufficient formability of the lid tape for cans to a lid, it is advantageous if the sum of the alloying proportions Mn, Fe and Si does not exceed 1.25 weight percent.

Alternatives to the aluminum alloy are conceivable in the field of high-strength alloys, but here it is necessary to work regularly with modified production paths during cold rolling to set the high strength. For example, there are multi-stand cold rolling mills or rolling mills working with oil / water emulsions are required.

In the method according to the invention, the starting material used is the aluminum alloy described. From the aluminum alloy, an ingot is poured, homogenized, hot rolled and then cold rolled on a single or zweigerüstigen cold roll. The cold-rolled strip is painted and dried in a continuous oven.

The method for the production of cover strip according to the teachings of the invention can be implemented cost-effectively from the plant side and is simultaneously adapted to the aluminum alloy according to the invention. In this case, the addition of Mg and Mn is adjusted so that the introduced in the process for the production of cover tape for cans according to the teachings of the invention solidification is little lowered by Entfestigungsvorgänge. This thermal stabilization against softening is primarily caused by Mn being in solution in the aluminum matrix. Particularly important for the coordination of the method for producing cover tape for cans according to the teachings of the invention with the aluminum alloy used is, on the one hand, cold rolling on a single or two-stand cold roll On the other hand, avoiding deformation inhomogeneities in cold rolling through optimal processing parameters in upstream operations.

The deformation inhomogeneities occurring during cold rolling can preferably be minimized by homogenizing the billet after casting at 480 ° C. to 530 ° C., reversibly hot rolling to a thickness of 30-50 mm and then hot rolling on a multi-stand, in particular 3 or 4-stand tandem relay.

By homogenization at 480 ° C to 530 ° C, the cast iron stresses of the billet are first degraded. By means of the subsequent reversing hot rolling to a thickness of 30-50 mm, the billet is prepared for hot rolling in the multi-stand tandem scale. Due to the multi-stand tandem scale, maximum forming levels can be achieved during hot rolling so that coarse precipitation phases are optimally broken up.

A hot strip outlet temperature of more than 300 ° C after tandem hot rolling is necessary to achieve complete softening of the hot strip. This is accompanied by the setting of a crystallographic cube texture, which in turn produces a high yield strength below 45 ° in the cold-rolled and painted strip.

If the cover band is finally painted with a polyvinyl chloride (PVC) -free varnish, then the Environmental compatibility and recyclability of the cover strip are further increased. Despite the higher baking temperatures necessary for the drying of the PVC-free coatings, the lid tape produced according to the invention has an increased proof stress even after the paint drying and thus a sufficient internal pressure stability.

According to one teaching of the present invention, the above-mentioned object is achieved by a lid tape for cans, which is produced by the method according to the invention. As already stated, a lid tape for cans produced by the method according to the invention has an improved internal pressure stability even after the paint has been baked on.

If the cover strip is produced from a hot strip with a fine-grained microstructure, the microstructure having a mean grain size of less than or equal to 15 μm, an increase in the yield strength R _{p0.2 of} the cover _strip at 45 ° to the rolling direction due to homogeneous deformation while largely avoiding deformation inhomogeneities be achieved during cold rolling, so that the cover tape for cans is also suitable for painting with PVC-free paints.

There are now a variety of ways to design and develop the aluminum alloy for the production of cover tape for cans and the method for producing cover tape for cans according to the teachings of the invention. For example, reference is made here On the one hand to the claims subordinate to claim 1 on the other hand to the description of an embodiment of a method for producing cover tape for cans according to the teachings of the invention in conjunction with the drawing.

In the drawing, the single figure shows schematically a manufacturing path for realizing an embodiment of a method for producing cover tape for cans according to the teachings of the invention.

The manufacturing path shown in the single figure detects in a first step the ingot casting 1 of an aluminum alloy according to the teachings of the invention. The billets cast in the DC process are then homogenized in a homogenization stage 2 and hot rolled on a tandem mill 3. Alternatively, the ingot can first be reversibly rolled to a thickness of 30-50 mm on a reversing stand and the hot rolling then continued on a multi-stand, in particular 3- or 4-stand tandem scale. Following the hot rolling on the tandem mill 3, the cold rolling takes place on a one- or two-stand cold roll 4. This cold rolling essentially determines the strength of the material by the introduced cold deformation.

In order to achieve a homogeneous deformation or solidification during cold rolling of the cover strip, it is therefore necessary that the hot strip has an optimal crystallographic texture. By the maximum Forming degrees during hot rolling, the optimized texture of the cover strip is guaranteed, so that while avoiding deformation inhomogeneities in cold rolling an increased yield strength, in particular at 45 ° to the rolling direction can be achieved.

During cold rolling, the material heats up again by introduced Umformwärme, resulting in a softening in the wound state of the tape. Subsequently, the tape is painted on a continuous furnace painting line 5 and dried, the material undergoes a further heat treatment, which in turn leads to a softening or recovery.

However, due to the increased yield strength of the cover strip produced by the method according to the invention, the softening during cold rolling and during the drying of the lacquer does not affect the applicability of the cover strip for cans. This is especially true when using elevated bake temperatures, which are required when using PVC-free paints.

Claims (6)

1. Method for producing a lid strip for cans, wherein an ingot is cast from an aluminium alloy, after casting at 480°C to 530°C the ingot is homogenized and hot-rolled, the hot strip is then cold-rolled on a one- or two-stand cold roller, the cold-rolled strip is painted and dried in a continuous furnace, characterised in that an aluminium alloy is used with the following alloy proportions in percentages by weight: 0.05% ≤ Si ≤ 0.2% 0.2% ≤ Fe ≤ 0.4% 0.05% ≤ Cu ≤ 0.15% 0.4% ≤ Mn ≤ 0.8% 4.7% ≤ Mg ≤ 5.4% 0.0% ≤ Cr ≤ 0.1% 0.0% ≤ Zn ≤ 0.25% 0.0% ≤ Ti ≤ 0.15% Ca ≤ 4 ppm Na ≤ 4 ppm others in total 0.15% max., individually 0.05% max., remainder Al,
   the ingot is hot-rolled in a reversing manner to a thickness of 30 to 50 mm and hot-rolling is then continued on a multi-stand tandem mill.
2. Method according to claim 1, characterised in that the aluminium alloy comprises an alloy proportion of B ≤ 10 ppm.
3. Method according to claim 1 or 2, characterised in that the ratio between the alloy proportions of Mg and Mn is greater than or equal to 7 and less than or equal to 11.
4. The method according to any one of claims 1 to 3, characterised in that the total of the alloy proportions of Mn, Fe and Si does not exceed 1.25 percent by weight.
5. Method according to any one of claims 1 to 4, characterised in that after hot-rolling the hot strip outlet temperature is greater than 300°C.
6. Method according to any one of claims 1 to 5, characterised in that the lid strip is painted with a polyvinylchloride (PVC)-free paint.

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