EP0592267B1 - Metal can with closure of the easy-to-open type by tearing along a line of weakness and steel sheet for its manufacture - Google Patents

Abstract

The packaging according to the invention is of the type in which the lid is to be opened by breaking a line of lowest strength arranged in a part of the wall made of steel sheet. The steel of which the sheet consists has a carbon content of between 0.02 and 0.08 % by weight and a boron content of between 0.002 and 0.004 % by weight, the steel also containing nitrogen, the ratio of the boron content to the nitrogen content being higher than 0.8 and the remainder being chiefly iron and residual elements.

Description

The present invention relates to packaging of opening type by breaking a line of least resistance so-called easy opening box.

We know such packaging which are by example intended to contain a drink, preserves or others products.

They have a cover in which is arranged a line of least resistance, for example a incision, delimiting an opening panel.

An actuating ring is fixed on the panel opening using a rivet.

The opening is caused by an effort of traction on the actuation ring so as to break the metal at the incision, the opening panel possibly including a connection zone with the cover so as not to detach completely from this latest.

Such packaging can be produced in aluminum but they are then relatively expensive.

When made from steel, for reduce their cost, a sheet made up is commonly used standard low carbon steel, annealed continuous at approximately 680 ° C and tinned.

• limite d'élasticité (Re) en sens travers de l'ordre de 415 MPa,
• charge à la rupture (Rm) en sens travers de l'ordre de 430 MPa.

The mechanical characteristics of such a tin-plated steel are as follows:

• elastic limit (Re) in transverse direction of the order of 415 MPa,
• load at break (Rm) in cross direction of around 430 MPa.

The steel used for the realization of such packaging must allow on the one hand to form the rivet of fixing the opening ring and, on the other hand, a breaking of the line of least resistance easily without having to exert too much effort.

For this, the breaking load Rm must not be too high because otherwise the opening effort to be exerted to break the line of least resistance will have to be important.

Likewise the elastic limit ratio Re on breaking load Rm must be close to 1 so that the steel is sufficiently weakened to cause a easy opening.

However, this Re / Rm ratio should not be too high, i.e. too close to 1, because in this case, the steel being very weakened, it becomes almost impossible to form the rivet, this operation requiring deep drawing of the steel of the packaging lid.

Now the standard steel commonly used, the yield strength Re is approximately 415 MPa and the breaking load Rm of around 430 MPa results in a Re / Rm ratio of the order of 0.96.

An easy opening lid made with a such steel has a high opening force, due the value of Rm and a residual thickness at the level relatively large incision.

In addition, the yield point is important, which causes the formation of worms visible when the packaging is stamped, detrimental to the aesthetic appearance of the latter.

To remedy these drawbacks, the invention aims to provide an opening type package by breaking a line of least resistance, for which the overall effort required for opening is reduced, while having an improved appearance after stamping packaging, and good training ability rivet.

To this end, the invention relates to packaging metallic type of opening by rupture of a line of least resistance arranged in a part of sheet steel wall, characterized in that steel constituting the sheet has a carbon content of between 0.02 and 0.08% by weight and a boron content included between 0.002 and 0.004% by weight, the steel also containing of nitrogen, the ratio of boron content to nitrogen content being greater than 0.8, the remainder being iron and residual impurities, said sheet being subjected to continuous rolling followed by annealing on base carried out for 8 to 10 hours at a temperature between 550 and 650 ° C approximately, then subjected to a cold re-rolling with an elongation rate included between 5 and 10%.

• la composition pondérale de l'acier constituant la tôle est de préférence la suivante, en millième de pour cent :
  • carbone : 50
  • manganèse : 200
  • phosphore : 10
  • azote : 3,5
  • aluminium : 30
  • silicium : 5
  • bore : 3
  • le reste étant du fer et des impuretés résiduelles.

According to other characteristics:

• the weight composition of the steel constituting the sheet is preferably as follows, in thousandths of a percent:
  • carbon: 50
  • manganese: 200
  • phosphorus: 10
  • nitrogen: 3.5
  • aluminum: 30
  • silicon: 5
  • boron: 3
  • the remainder being iron and residual impurities.

The invention also relates to a sheet steel intended for making a packaging and characterized in that the ratio of its elastic limit on its breaking load is between 0.87 and 0.96, preferably between 0.91 and 0.92.

The invention will be better understood on reading the detailed description which will follow, given as example.

Reducing the effort required to open involves the use of a steel with a limit relatively low elasticity, which further facilitates the winding of the opening panel after breaking of the incision, and by a high Re / Rm ratio.

On the contrary, the good aptitude for training the rivet requires the use of a steel having a ratio Low Re / Rm.

The appearance of the packaging and its resistance corrosion are improved by the use of a sheet having an equiaxed fine-grained structure and not exhibiting no yield strength plateau.

• l'utilisation d'une tôle constituée d'un acier ayant une composition déterminée,
• l'adaptation d'un procédé de fabrication de tôle comportant un recuit sur base et un relaminage à froid.

Obtaining the required mechanical characteristics and the equiaxial microstructure are the result of:

• the use of a sheet made of steel having a determined composition,
• the adaptation of a sheet metal manufacturing process comprising annealing on base and cold re-rolling.

More precisely the packaging according to the invention is made from a steel sheet of which the carbon content is between 0.02 and 0.08% by weight and boron content is 0.002 and 0.004% by weight, the steel also containing nitrogen in a amount less than 0.005%, the ratio of the content of boron on the nitrogen content being greater than 0.8.

A carbon content of less than 0.02% is difficult to achieve with production processes conventional and can lead to mechanical resistance insufficient cover.

If the carbon content was greater than 0.08% by weight, the breaking load Rm of the sheet intended to carry out the packaging would be too important.

It has been found that to obtain a sheet allowing to produce packaging according to the invention, including the structure is equiaxial, the ratio of the boron content on the nitrogen content must be greater than 0.8, boron being limited to 0.004% because, for a higher content, has an increase in the mechanical characteristics of the sheet metal and we see a preferential oxidation appear in side of the product, which is unacceptable from the point of view aesthetic.

• carbone : 50
• manganèse : 200
• phosphore : 10
• azote : 3,5
• aluminium : 30
• silicium : 5
• bore : 3
• le reste étant du fer et des impuretés résiduelles.

The weight composition of the steel constituting the sheet intended for packaging according to the invention is preferably as follows, in thousandths of a percent:

• carbon: 50
• manganese: 200
• phosphorus: 10
• nitrogen: 3.5
• aluminum: 30
• silicon: 5
• boron: 3
• the remainder being iron and residual impurities.

The process used to make the suitable sheet for packaging according to the invention, consists in that after continuous rolling of the steel described above, the sheet is subjected to an annealing on base carried out during 8 at 10 a.m. at a temperature between 550 and 650 ° C approximately, the rise in temperature from the temperature ambient being carried out in approximately 20 hours. The sheet is then subjected to a cold re-rolling with a rate elongation between 5 and 10%.

• Re = 310 MPa environ,
• Rm = 340 MPa environ.

The combination of the steel composition and the suitable manufacturing process makes it possible to obtain the following mechanical characteristics:

• Re = 310 MPa approximately,
• Rm = 340 MPa approximately.

The ratio of the elastic limit to the breaking load is between 0.91 and 0.92.

This produces a package whose opening is fairly comfortable and has good training ability of the rivet.

On the other hand, such steel allows to support significant indentation before unacceptable damage, that is, the incision can be deeper, thereby allowing an effort of openness lesser.

Indeed, in the case of low carbon steel standard, the remaining thickness at the bottom of the incision should not not be less than 70 microns under penalty of damage unacceptable, while this remaining thickness in bottom of incision can go down to 50 microns in the case of the steel according to the invention.

This packaging made from a sheet metal fine-grained equiaxed structure avoids, during training of the rivet, the appearance of deformation roughness due to subsidence, which is detrimental to the appearance of the packaging and the holding of any varnish.

The presence of boron also makes it possible to brake the coalescence of the cementite which results in a greater regularity of effort to be provided during opening the package.

Claims (5)

1. Metallic container of the type which is opened by tearing along a line of least resistance provided in a part of the sheet steel wall, characterised in that the steel which forms the sheet has a carbon content lying between 0.02 and 0.08% by weight and a boron content of between 0.002 and 0.004% by weight, the steel also containing nitrogen, the ratio of the boron content to the nitrogen content being greater than 0.8, the balance being iron and residual impurities, the said sheet being subjected to continuous lamination followed by basic annealing carried out over 8 to 10 hours at a temperature lying between about 550 and 650°C, then subjected to cold relamination with an elongation rate between 5 and 10%.
2. Container according to claim 1, characterised in that the composition by weight of the steel which forms the sheet is preferably the following, in thousandths of a percent:

   carbon: 50

   manganese : 200

   phosphorus : 10

   nitrogen : 3.5

   aluminium : 30

   silicon : 5

   boron : 3

   the balance being iron and residual impurities.
3. Container according to claim 1, characterised in that the rise in temperature in the basic annealing in relation to the ambient temperature is carried out over about 20 hours.
4. Sheet steel for a container according to any one of claims 1 to 3, characterised in that the ratio of its elastic limit to its rupture strength lies between 0.87 and 0.96.
5. Sheet steel according to claim 4, characterised in that the ratio of its elastic limit to its rupture strength lies between 0.91 and 0.92.