FR2846340A1 - Fabrication of an improved aluminium alloy strip for the production of high quality culinary utensils - Google Patents

Abstract

The fabrication of aluminium alloy strip, for the fabrication of culinary utensils, comprises: (a) the continuous casting of strip with a thickness of 4 to 12 mm in an aluminium alloy; (b) the cold rolling of the cast strip with a reduction rate of between 10 and 60%; (c) the annealing of the rolled strip at a temperature between 320 and 100c. An Independent claim is also included for aluminium alloy strip fabricated by this method and having a limit of elasticity greater than 40 MPa, a rupture strength greater than 120 MPa, an elongation greater than 23% and a surface grain size of less than 100 mum.

Description

Process for manufacturing aluminum alloy strips for utensils

  Field of the Invention The invention relates to the continuous casting of aluminum alloy strips used in the manufacture of culinary utensils such as stoves, frying pans, pans, etc., which may be coated, on at least one face, of a

  anti-adhesive layer of polytetrafluoroethylene (PTFE) or an enamel layer.

  State of the art The use of PTFE as a non-stick coating for culinary utensils made of aluminum alloys is known from patents FR 1119221 and FR 1120749, filed in 1954 by Ms Wamant and Grégoire. The second of these patents describes acid treatment, particularly with hydrochloric acid, of the aluminum alloy surface to improve the adhesion of the PTFE layer to this surface. Patent FR 1137972, filed in 1955 by the same applicants, describes the deposit of a

  layer of PTFE on one or both sides of aluminum alloy blanks, for example discs, these blanks being subsequently converted into cooking utensils by stamping or embossing. The aluminum alloys most commonly used are low-alloy alloys, for example 1200 alloy according to the NF EN 57325 3 standard of October 1994.

  Aluminum alloy cooking utensils coated inside a layer of PTFE and outside an enamel layer have also been manufactured for many years. The alloy used must be compatible on the one hand with the surface treatment prior to PTFE coating, and on the other hand the enamel baking at a temperature of the order of 560 ° C. AlSi alloys (4000 series according to the designation of the Aluminum Association) have been used for this purpose, for example the composition alloy 4006 (% by weight): Si: 0.8 - 1.2 Fe: 0.5 - 0.8 Cu <0.05 Mn <0.03 Mg <0.01 Cr <0.20 or alloy 4007, corresponding to patent FR 2392130, deposited in 1977 by Forges de Crans, composition (% by weight ): Si: 1 - 1.5 Fe + Ni + Co: 0.3 -2 Cr: 0.05 - 0.2 Cu <0.2 Mg <0.01 Patent FR 2420577, filed in 1978 by Cégédur and Forges de Crans concerns an aluminum alloy for enameling of composition (% by weight): Mn: 0.9-1.5 Si: 1 - 1.5 Fe + Ni + Co: 0.8-2 Cr: 0 , 05-0.2 Cu <0.2 Mg <0.2

  The problem of the use of this type of alloy for the enameling of cooking utensils is well explained in the article by M. Deleuze and D. Marchive: "The new Io new alloys of wrought 4006 and 4007" Review of the Aluminum, June 1980.

  SEB patent application WO 01/51678 relates to the use of the same alloy 4007 for making culinary articles of which at least one of the surfaces is covered with a release coating, the other being able to be either

  covered with anti-adhesive, enamelled.

  The production of these strips is usually done by semi-continuous casting of plates, hot rolling, then cold, the strip obtained is usually cut into discs used as starting blanks for stamping cookware. It is also possible to use a continuous strip casting process. Thus, the patent application WO 98/52707 in the name of the Applicant 20 describes a method for manufacturing aluminum alloy strips containing (by weight) at least one Fe element (from 0.15 to 1.5 %) or Mn (from 0.35 to 1.9%) with Fe + Mn <2.5%, and optionally Si (<0.8%), Mg (<0.2% and preferably <0.05). %), Cu (<0.2% and preferably <0.1%), Cr (<0.2% and preferably <0.02%) or Zn (<0.2% and preferably <0, 1%), by continuous casting between rolls at a thickness between 1 and 5 mm, optionally followed by cold rolling, the force applied to the casting rolls, expressed in t per meter of bandwidth, being less than at 300 + 2000 / e, where e is the thickness of the band expressed in mm. These strips can be used for the manufacture of cooking utensils Problem posed The properties required of aluminum strips for the manufacture of cooking utensils are multiple. They must have sufficient mechanical strength

  so as to limit the thickness used, and therefore the weight of metal. The stamping must be done without difficulty, which assumes good formability and low anisotropy, measured by a rate of horns as low as possible. It is necessary to obtain a good surface appearance, and in particular to avoid the appearance of defects such as the lineage during shaping or the "orange peel" on the surface of the metal, due to the formation of large grains during heat treatments.

  The continuous casting between rolls of thin strips between 1 and 5 mm, as described in the patent application WO 98/52707 mentioned above, makes it possible to obtain strips that can be used directly in the raw casting state. which do not recrystallize at the usual enamelling and polymerization temperatures of the

  PTFE, which avoids the orange peel.

  Because of the wide variety of metal thicknesses used in the manufacture of the various cookware, it is not always possible to use raw cast metal, which would necessitate changing the casting thickness too frequently by one new setting of the continuous casting machine, which is always a

  delicate operation. It is therefore necessary to use a constant casting thickness, and to bring the band to the desired thickness by cold rolling. This rolling leads to a hardening of the band, which requires annealing to obtain sufficient formability. Such an annealing most often leads to a magnification of the surface grain, and therefore to the appearance of orange peel.

  The object of the invention is thus to allow the manufacture by continuous casting, under acceptable industrial conditions, of aluminum alloy strips in the annealed state having all the properties required for the manufacture of utensils.

culinary high quality.

  OBJECT OF THE INVENTION The subject of the invention is a process for manufacturing aluminum alloy strips intended for the manufacture of cooking utensils, comprising: continuous casting of strips of thickness between 4 and 12 mm in alloy of composition (% by weight): 0.8 <Si <2.5 0.5 <Fe <1.5 Mn <1 Mg <1 Cu <0.6 Cr <0.1 Fe + Mn <1.5 others elements' 0.1 each and <0.3 in total, remains aluminum, - the cold rolling of the cast strip with a reduction rate of between 10 and 60%,

  - The annealing of the rolled strip at a temperature between 320 and 5000C.

Description of the invention

  In order to best satisfy the requirements of the manufacture of culinary utensils, particularly the surface appearance, the invention combines a particular composition range of the alloy used and a suitable manufacturing range of the strips obtained by continuous casting. It is based on the observation that, surprisingly, the particular microstructure of the alloys in the composition field according to the invention makes it possible to avoid the appearance of large grains on the surface during the final annealing, while ensuring a high mechanical strength. and a low rate

from horns to stamping.

  The invention relates to alloys essentially based on iron and silicon, in a quantity sufficient to form the casting of AlFeSi particles, which influence

  favorably on the control of recrystallization at annealing.

  It is necessary to have a sufficient iron content> 0.5%, since this element has a favorable influence on the mechanical strength. On the other hand, iron in solid solution has the disadvantage of retarding the recrystallization at annealing, which increases the risk of abnormal growth of the grains on the surface. It is therefore preferable to rush it to maximum casting. One way to achieve this is to add sufficient silicon, above 0.8%, to promote the precipitation of iron in the form of AlFeSi phases. Under these conditions, a particular distribution of the AlFeSi precipitates is obtained, especially at the surface of the strip. In addition, the use of continuous casting leads to the appearance of finer AlFeSi phases and better

  dispersed only in a classical casting, which is also favorable.

  In order to promote the appearance of the AlFeSi phases at the expense of the silicon-free iron phases, it is desirable that the silicon content be greater than the iron content, i.e. Fe / Si <1.

  Alloys containing from 0.7 to 1.3% iron and from 0.8 to 1.5% silicon have a small solidification range, i.e., a small difference between the start and end temperature. end of the solidification, which limits, during the continuous casting, the appearance of a central segregation, likely to form lineage

  when formatting disks.

  It is important to limit the manganese content to 1%, and the sum of iron and manganese contents to 1.5%, because manganese has the effect of pushing the recrystallization to higher temperatures during annealing.

  For alloys intended for the manufacture of enamelled utensils, the magnesium content is maintained below 1%, and preferably below 0.1%, because this element is detrimental to the adhesion of certain enamels. The copper content is less than 0.6% to comply with the regulations laid down in the European standard lo EN 602 relating to aluminum alloys intended for food contact. The presence of anti-recrystallizing elements such as chromium must also be

  limited to obtain a good recrystallization annealing.

  A particularly suitable composition domain is the following:

  If: 0.8 - 1.5 Fe: 0.7 - 1.3 Mn <0.1 Cu <0.1 Mg <0.1 Fe / Si <15 and, preferably, Si: 1.0 - 1.3 and Fe: 0.9 - 1.2.

  The strips according to the invention are cast by continuous casting, in particular by casting between two cooled rolls. The casting thickness must be greater than 4 mm because, in the absence of a particular setting of the casting conditions, the reduction of the casting thickness results in greater anisotropy of the strip. The cast strip is then brought to the cold rolling use thickness. To control the recrystallization, it is indeed necessary to have some hardening of the band, with a thickness reduction of at least 10%. The reduction ratio is defined by the ratio (ej - ef) / e1 in which es is the initial thickness and ef

the final thickness.

  In fact, if the recrystallization annealing is carried out directly on the cast strip, it can be seen that it can not be recrystallized to the core, and that a heterogeneous granular structure unfavorable to formability is obtained with large grains. surface. On the other hand, if the reduction rate becomes too high and exceeds 60%, the anisotropy after annealing becomes too high. Depending on the final thickness required, it is therefore necessary to adopt a casting thickness such as the reduction ratio.

  rolling remains between 10 and 60%.

  The cold-rolled strip is then annealed at a temperature between 320 and

  500'C so as to obtain a totally recrystallized structure.

  The strips according to the invention have a breaking strength Rm> 120 MPa, a yield strength R o, 2> 40 MPa and an elongation A> 23%. The level of horns, measured on specimens machined according to standard NF-EN 1669 with a drawing ratio of 1.9, is less than 4. No large grains are observed on the surface, the grain size varying little in size. depending on the annealing temperature and remaining below 100 Jlm, which allows a greater flexibility in the implementation

  industrial heat treatment.

  All of these properties make the belts according to the invention particularly suitable for the manufacture of culinary utensils, in particular top-of-the-range enamelled articles coated with PTFE.

Example

  EXAMPLE 1 Effect of the Iron and Silicon Content and the Work Hardening Rate Two alloys A (alloy type 8011) and B (alloy according to the invention) were prepared, the composition of which is shown in table 1. The 8011 alloy has a silicon content

  lower than that of the alloy according to the invention, and the Fe / Si ratio is equal to 1.

Table 1 If Fe Mn Cu Mg

A 0.6 0.6 <0.1 <0.1 <0.05

B 1.2 1.1 <0.1 <0.1 <0.05

  Both 6.1 mm thick strips were cast for these two alloys by means of a continuous casting machine between two rolls. These strips were cold-rolled, partly up to 4 mm (35% reduction rate), partly up to 3 mm (51% reduction rate) and partly to 2 mm (reduction ratio). 65%),

then annealed in batch at 4500C.

  The mechanical cross-machine characteristics were measured on the annealed strips: yield strength Ro, 2, tensile strength Rm and elongation at break A, as well as the level of horns t, according to standard NF EN 1669, and the grain size at the surface by image analysis. The results are shown in Table 2:

Table 2

  TR = 35% Ro, 2 (MPa) Rm (MPa) A (%) t, (%) Grains (, um)

A 35 114 25 3.4 250

B 41 127 24 3.3 80

TR = 51%

A 38 112 24 3.6 300

B 45 130 24 3.6 80

TR = 65%

A 38 115 22 4.5 320

B 46 129 21 4.3 85

  It is found that the yield strength and the breaking strength of the alloy B according to the invention are greater than those of the alloy A practically without prejudice to

  lengthening, and that the rate of horns is slightly reduced.

  3 mm thickness samples were also measured with a horn level of 3.6 for both alloy A and alloy B. It can be seen that the increase in the reduction rate leads to a rate increase. horns after annealing, which however remains less than 4%. For a reduction rate of 65%, the level of horns after annealing exceeds 4% for alloys A and B. It is also noted that the samples of thickness 4, 3 and 2 mm alloy A have a number of coarse grains of several hundred ptm in the surface layer of depth 100 μm, while those of alloy B have only small grains throughout the thickness. It is also observed that the density of the precipitates present in the annealed strips of alloy A is lower than that measured in the alloy strips B. Thus, the alloy strips B satisfy all the requirements of mechanical strength, formability, anisotropy, surface condition and suitability for PTFE enamelling and curing processes required for the manufacture of articles

culinary high quality.

  EXAMPLE 2 Effect of Manganese Content A high manganese C alloy was prepared whose composition is shown in Table 3: Table 3 c if 0.3 Fe 0.4 Mn 1.5 Cu 0.1 Mg <0.1 The alloy was cast in a 6 mm thick strip on a continuous casting machine between two rolls. The strip was then cold rolled to 4mm (33% reduction rate), then annealed in batch at 5000C. Indeed, because of the high manganese content of the alloy, it is necessary to increase the annealing temperature to

  to obtain a complete recrystallization.

  The granular structure obtained is very heterogeneous, with grains of 1 mm on the surface. The rate of horns measured after annealing is 6.4%. These bands are

  unfit for the manufacture of cooking utensils.

Claims (10)

claims   1. A method of manufacturing aluminum alloy strips for the manufacture of cooking utensils, comprising: - the continuous casting of strips of thickness between 4 and 12 mm alloy composition (% by weight): 0, 8 <If <2.5 0.5 <Fe <1.5 Mn <1 Mg <1 Cu <0.6 Cr <0.1 Fe + Mn <1.5 other elements <0.1 each and <0, 3 in total, remains aluminum, 1o - the cold rolling of the cast strip with a reduction rate of between 10 and 60%,   - The annealing of the rolled strip at a temperature between 320 and 5000C.   2. Method according to claim 1, characterized in that the silicon content of the alloy is between 0.8 and 1.5%.   3. Method according to claim 2, characterized in that the silicon content of   the alloy is between 1.0 and 1.3%.   4. Method according to one of claims 1 to 3, characterized in that the content of   Iron alloy is between 0.7 and 1.3%.   5. Method according to claim 4, characterized in that the iron content of the alloy is between 0.9 and 1.2%. 25   6. Method according to one of claims 1 to 5, characterized in that the report   Fe / Si iron and silicon contents is less than 1.   7. Method according to one of claims 1 to 6, characterized in that the manganese content of the alloy is less than 0.1%.   8. Method according to one of claims 1 to 7, characterized in that the content of   Copper alloy is less than 0.1%.   9. Aluminum alloy strips manufactured by a process according to one of   Claims 1 to 8, characterized by a yield strength RO, 2> 40 MPa, a tensile strength Rm> 120 MPa, an elongation A> 23%, and a   horns according to standard NF-EN 1669 <4. Tapes according to claim 9, characterized in that the grain size in   surface is less than 100 p.rm.