EP1056560B1 - Method for making aluminium alloy strips with high surface homogeneity - Google Patents

Abstract

A process for manufacturing a strip of aluminum alloy having high surface homogeneity with an upper side showing after a 1 mum thick sulphur anodic treatment, an optical roughness value S<SUB>N </SUB>measured on three 5 cm longitudinal sections and three 5 cm transverse sections, such that its mean variation on each section, defined by the ratio: (Maximum S<SUB>N</SUB>-minimum S<SUB>N</SUB>)/Mean S<SUB>N </SUB>is less than 20%, and the difference DeltaS<SUB>N</SUB>=S<SUB>N </SUB>max -S<SUB>N </SUB>min is less than 20. The strip is manufactured by continuous casting between two cooled rolls from a casting tank containing the liquid metal connected to an injector, and feeding the liquid metal into the gap between the two rolls. The injector has an upper lip recessed by at least 2 mm with reference to a lower lip of the injector.

Description

Field of the invention

The invention relates to aluminum alloy strips having a large surface homogeneity, intended for applications requiring good quality particular optical properties, such as reflectors or anodized sheets for building and decoration. It also concerns a process for producing these strips by continuous casting between rolls.

State of the art

The continuous casting of strips between cooled rolls ("twin-roll casting") is widely used for several decades for the production of sheets thin or rolled current aluminum alloys. It consists, as indicated in the basic patent FR 1198006 filed in 1958 by Pechiney, to introduce the metal liquid, stored in a feed tray, in the interval between two cooled horizontal cylinders rotating in opposite directions, using an injector. The metal solidifies as a continuous band while undergoing a reduction thickness due to the pressure of the cylinders. These continuous casting machines are very often used to produce strips of thickness between 5 and 12 mm. In their most recent versions, such as the Pechiney JUMBO 3 CM ® casting Rhenalu, they also make it possible to cast thinner strips of thickness less than 5 mm as described, for example, in FR 2737430.

The bands from these casting machines are very rarely used raw casting. They generally undergo a first cold rolling sequence and for some applications a second finishing cold rolling sequence, possibly with particular cylinders.

These usual machines for continuous casting between rolls make it possible to obtain bands of homogeneous appearance, but for very demanding applications in surface condition associated with a surface treatment of the strip likely to reveal existing surface defects, or create them from heterogeneities metallurgical products, for example anodizing, chemical brightening or electroplating, stripping, chemical satin-coating, cataphoresis or lacquering, bands resulting from a continuous casting between cylinders do not currently have a quality sufficient surface area. The upper face of the cast strip is most often wavelets ("ripples"), which are in the form of lines perpendicular to the direction of casting (cross-long direction), and whose origin would be the oscillation of the meniscus of liquid metal during casting. After anodizing, these ripples become visible as parallel streaks; it is a visual defect manifested by a difference of gray levels with a step of the order of one to a few (for example ten) millimeters.

Another surface defect that is usually observed is scratches mechanical parallel to the long direction of the band; it is a roughness defect. The face inferior to the quality of a usual mill finish.

To these two types of surface defects are occasionally added stripes accidental, which are not specific to the continuous casting technique between cylinders.

Improvement of the surface appearance of the strips obtained by continuous casting corresponds to a long-felt need, and a number of solutions have been proposed.

By way of example, US Pat. No. 4,461,552 describes a method for the treatment of liquid metal starting with the injection into the liquid metal of a gas containing chlorine, followed by the passage of the liquid metal through a series of chambers of coalescence, and ended with filtration, thus allowing the rate to be of inclusions in the liquid metal, leading to the improvement of the surface appearance 5086 and 5182 alloy webbing for computer disks. However, industrialists are now seeking to minimize the use of chlorinated gases.

German Patent Application DE 2443068 of 1974 discloses a machine for continuous casting between steel belts which aims to improve the surface appearance of strips of aluminum or aluminum alloy so as to be able to produce strips for decorative anodizing. The technical solution proposed in this application for patent can not be applied to continuous casting between cylinders, for three reasons:

The surface quality of a band obtained by continuous casting between belts is inherently worse than continuous casting between cylinders, which is probably due to the vibrations of the belts. The conditions of solidification of the metal are totally different, since in the case of continuous casting between straps, the solidification of the center of the strip is downstream of the plane of the axes of the cylinders close to the injector, while in the case of continuous casting between cylinders, it is upstream of this axis. And finally, the casting continues between belts do not produce thin strips less than 5 mm, and therefore, the air gap between the cylinders is smaller in a machine continuous casting between cylinders.

British Patent Application GB 2198976 discloses an injector device dissymmetrical to increase the casting speed and therefore the yield industrial of a machine for continuous casting between rolls; the document does not the state of improvement of the surface of the products obtained, which was not the subject of the invention he describes.

US Pat. No. 5,350,010 seeks to optimize the surface quality of tapes for the production of offset printing plates by the end the grain size of the final product, which implies the respect of a certain composition of the metal and the control of certain process parameters, which occur downstream of the continuous casting, such as the reduction rate per cold rolling pass. knowing that the surface defects present on the raw strip of casting will not disappear usually not during the cold rolling operations that follow, this approach, according to the opinion of the plaintiff, does not seem to attack the source of the defects but seeks to minimize only their consequences on the finished product.

By following a similar technical approach, patent application EP 0821074 also discloses a method of converting a strip obtained by casting continuous between cylinders, allowing the manufacture of offset plates.

The applicant notes that none of these approaches makes it possible to obtain surfaces that meet the requirements of high optical and visual homogeneity directly by continuous casting between rolls, possibly followed by one or several cold rolling passes.

Object of the invention

The object of the present invention is to obtain, by continuous casting between rolls, strips of aluminum alloys having, on at least one side, a large surface homogeneity and usable for applications that were not open until now. It should be noted that the vast majority of these applications require a very homogeneous surface condition only for one of the two faces. It is therefore not a problem that the present invention does not improve the surface state dramatically as for one of the two faces.

a) Les bandes brutes de coulée présentent à la surface (hormis les rayures mécaniques accidentelles visibles à l'oeil nu) de leur face supérieure, après création par anodisation sulfurique d'une couche d'oxyde anodique d'une épaisseur de 1 µm, un indice de rugosité optique S_N, mesuré sur trois profils de longueur 5 cm dans le sens long et trois profils de longueur 5 cm dans le sens travers, tel que sa variation moyenne sur chaque profil, définie par le rapport: (SN maximum - SN minimum)/SN moyen est inférieure à 20%, et la différence: ΔS_N = S_N max - S_N min est inférieure à 20.

b) Les bandes après laminage à froid à une épaisseur comprise entre 4 et 0,1 mm, de préférence entre 2 et 0,1 mm, présentent à la surface de leur face supérieure un indice de rugosité optique S_N, mesuré dans les mêmes conditions, sur un échantillon ayant subi un traitement de décapage basique sur 10 µm, puis d'anodisation sulfurique conduisant à la création d'une couche d'oxyde anodique d'une épaisseur de 1 µm, tel que sa variation moyenne sur chaque profil est inférieure à 20% et ΔS_N est inférieur à 12.

c) Les bandes ayant subi un premier laminage à froid suivi d'un laminage de finition à une rugosité R_a < 0,2 µm et brillantées électrolytiquement présentent à la surface de leur face supérieure, après création par anodisation sulfurique d'une couche d'oxyde anodique d'une épaisseur de 1 µm, un indice de rugosité optique S_N tel que sa variation moyenne sur chaque profil est inférieure à 20% et ΔS_N est inférieur à 3,5, et même à 0,5.

The strips obtained according to the process of the invention, continuously cast between two rolls, of thickness less than 12 mm, preferably less than 5 mm, have an upper face whose surface state can be characterized in three stages of different manufacturing, corresponding to three types of more or less elaborate industrial products, on samples having undergone a particular preparation, representing a typical industrial surface treatment which reveals the surface defects:

a) The raw tapes have on the surface (apart from mechanical scratches visible to the naked eye) of their upper face, after creation by sulfuric anodization of a layer of anodic oxide with a thickness of 1 μm, an optical roughness index S _N , measured on three profiles of length 5 cm in the long direction and three profiles of length 5 cm in the transverse direction, such as its average variation on each profile, defined by the ratio: (S NOT maximum - S NOT minimum) / S NOT way is less than 20%, and the difference: ΔS _N = S _N max - S _N min is less than 20.

b) The strips after cold rolling to a thickness between 4 and 0.1 mm, preferably between 2 and 0.1 mm, have on the surface of their upper face an index of optical roughness S _N , measured in the same conditions, on a sample which has undergone a basic pickling treatment on 10 μm, then of sulfuric anodization leading to the creation of an anodic oxide layer with a thickness of 1 μm, such that its average variation on each profile is less than 20% and ΔS _N is less than 12.

c) The bands which underwent a first cold rolling followed by a finish rolling at a roughness R _a <0.2 μm and electrolytically shiny have on the surface of their upper face, after creation by sulfuric anodizing a layer of anodic oxide with a thickness of 1 micron, an index of optical roughness S _N such that its average variation on each profile is less than 20% and ΔS _N is less than 3.5, and even 0.5.

The strips obtained by the method according to the invention also have a surface homogeneity such as the value of the asymmetry of the distribution of the 2D roughness amplitude (parameter Sk or skewness), measured by a technique based on an optical scanner. described below, is between -0.2 and +0.3, and preferably between -0.1 and +0.2. The 3D roughness (parameter E _k ), determined by a mechanical probe parallel to the rolling direction according to a method described below, is less than 15, and preferably 8

The subject of the invention is a process for manufacturing alloy strips of aluminum by continuous casting between two cooled rolls, from a container of casting containing the liquid metal connected to an injector, having an upper lip and a lower lip, bringing the liquid metal into the gap between the two cylinders, in which the upper lip of the injector is recessed by at least 2 mm, and preferably at least 5 mm, with respect to the lower lip.

The height of liquid metal in the tundish, measured from the median plane of casting, is maintained less than 30 mm, preferably less than 25 mm.

Description of figures

FIG. 1 represents a cross-section through a plane perpendicular to the axis of cylinders of a continuous casting machine between rolls, according to the invention.

FIG. 2 represents an exemplary recording of the optical roughness index S _N along a measurement profile for a band according to the invention of example 1.

FIG. 3 represents an exemplary recording of the optical roughness index S _N along a measurement profile for a band of the prior art of example 1.

Description of the invention

The surface homogeneity of the upper face of the strips is assessed under two different aspects: the presence of roughness defects (parallel mechanical scratches in the long sense), and the presence of gray level oscillations (striations perpendicular to long sense).

For the characterization of gray level oscillations, three different techniques and complementary have been used by the plaintiff:

The optical roughness value S _N was measured using a RM 400 surface optical measuring system from RODENSTOCK. This apparatus defines and measures S _N between 4 and 100, for surface roughness between 5 and 2000 nm. It is based on the principle of diffusion of radiation by a rough surface. The surface to be evaluated receives a beam of infrared rays, a part of which is rediffused, the angular distribution of the scattered rays depending on the morphology of the surface.

The index S _N is measured continuously over profiles of length 5 cm by scanning a beam with a diameter of 0.5 mm, and on each sample 3 profiles in the long direction and 3 profiles in the cross direction taken in the same area with a diameter of about 10 mm. For each profile, aberrant isolated peaks are eliminated due to accidental mechanical scratches visible to the naked eye, distinguishing them from wavelets to be characterized.

The maximum value, the minimum value and the average value of S _N , the difference ΔS _N between the maximum value and the minimum value, as well as the variation, are determined from the recorded curve, as indicated in FIGS. 2 and 3. of this index, defined by the ratio: ΔS _N / S _N average. The 6 differences and variations corresponding to the 3 long and 3 cross measurements are averaged, giving the average difference and variation.

Another way to characterize the gray level oscillations is the determination of the parameter L * a * b *. This measurement was carried out using a Minolta ChromaMeter CR-221 colorimeter, with a measurement area of 3 mm diameter, a 45 ° illumination angle and a 0 ° observation angle. The lighting was provided by a pulsed xenon lamp. The reference spectrum was the CIE spectrum D ₆₅ .

The parameter L * a * b * was calculated according to ASTM D2244-89, § 6.2. The samples were characterized by the average value of L * parameters; and by the standard deviation on this parameter. For each sample, twenty measurements following a generator perpendicular to the oscillations were made. This allows with a unilateral risk of 0.05 to differentiate between two sets of measures including the ratio of standard deviation is 2.17. The Applicant has found that this test can reproduce visual grading of samples, except for samples showing a step oscillation much lower than the diameter of the measuring area.

A third way to characterize the gray level oscillations is the use a scanner to obtain gray level mapping, followed by the analysis rugosimetric in two dimensions by numerical methods known to man business.

A metal sheet with a minimum size of 14 cm x 14 cm is placed, preferably size close to the DIN A4 format, to be characterized on the scanner plate. We pose next to it, flat, a calibrated grayscale grid that is used for each measurement as an internal reference. The tests were carried out with a reference grid marketed by Kodak; this grid presents twenty gray ranges of progression 0.10 included between a 0.00 white density beach and a beach practically black with a density of 1.90. It has been helpful to close the white beach of 0.00 density so as not to saturate the detection system. A scanner type UMAX has been used. This device provides a resolution of 150 dpi (digit per inch) x 150 dpi with 256 gray levels. It was connected to a microcomputer that recorded the digitized image in grayscale. It has proven useful to perform first a calibration scan with the calibrated grid alone, in order to allow the device to select, in automatic mode, the appropriate contrast. Then, the digitization is done with the sample and the reference grid. AT condition of always using a calibrated reference grid, the person skilled in the art is able to use other models of performance scanners at least comparable.

Using a software (in this case Spyglass Transform 3.02 ©), the area to be studied was selected interactively; we eliminate the reference grid and the edges of the remaining image, typically 1 to 3 cm on each edge, and if necessary, peripherals showing sails or spots not representative of the oscillations of the gray level to characterize. The useful area thus obtained must have a minimum size 12 cm x 12 cm.

The roughness profile is then analyzed in two dimensions on seven lines parallel to the rolling direction (that is, perpendicular to the gray level oscillations) at least 100 mm long, randomly selected from the usable area. If the baseline this profile shows a drift (especially due to a lack of illumination of the scanner), it is then necessary to straighten it in a way that does not affect the roughness itself. This correction can be performed by the box method known from the person skilled in the art, the size of the box (box size) being adjusted so as to reproduce at best the step and the amplitude of the profile.

où

et où l_m correspond à la ligne moyenne, sachant que pour une distribution gaussienne idéale, Sk =0.The four roughness parameters are then calculated, known to those skilled in the art (see NF E 05-15 standard "Surface Condition of Products - Prescriptions - 1 ° General - Terminology - Definitions" of September 1984): the gap mean absolute Ra, the root mean square deviation Rq, the maximum roughness Rt, and the asymmetry of the amplitude distribution (skewness) Sk defined as

or

where l _m corresponds to the mean line, knowing that for an ideal Gaussian distribution, Sk = 0.

Applicant has observed that for samples of homogeneous surface, the value Sk is between - 0.2 and + 0.3. To be suitable for example in the manufacture of optical reflectors, a value Sk between -0.1 and +0.2 is preferred. The non-homogeneous surface samples, corresponding to the prior art, have a value Sk less than - 0.4. By way of example, the applicant has found values between -0.45 and -1.38 for alloy samples 8011 obtained by the continuous casting process between rolls according to the prior art.

où N est le nombre de points en x, M le nombre de points en y, Zo l'altitude moyenne sur la surface d'observation selon

For the characterization of roughness defects, a method of mechanical roughness measurement in three dimensions was used. The sample is placed on a TIXY 200 cross table. The probe (model FRW 750 from Mahr Mesures), with a radius of curvature of 5 μm, was conditioned by a Perthometer PRK device from the company Mahr Mesures. The data was recorded using an analog-to-digital conversion card in a microcomputer. The dimension of the measuring field was 20 mm x 20 mm, with a pitch of 40 μm in x and y. The theoretical resolution in depth was given by the maximum amplitude of the probe (100 μm) and the characteristics of the microprocessor (16 bit), about 0.024 μm. The three-dimensional roughness was calculated with software provided by Saphir according to the following equations:

where N is the number of points in x, M the number of points in y, Zo the average altitude on the observation surface according to

The parameter Ek, sometimes called kurtosis in English, characterizes the flattening of the distribution; it takes the value of 3 for an ideal Gaussian.

All these measurements can be made on the upper face, that is to say the face having have been in contact with the upper cylinder, outside areas that show accidents such as scratches due to handling or stains, for castings, simply cold-rolled strip or rolled strip cold then having undergone finishing passes with shiny cylinders (polished). For be representative of the intended application, the measurements are all performed on samples treated by sulfuric anodizing under the following conditions: concentration of sulfuric acid 200 g / l, temperature 20 ° C voltage 15 V. This treatment leads to an oxide layer thickness of 1 μm. It is eventually preceded by a preliminary basic etching (for example, at a temperature of 60 ° C and during 7 minutes in a 50 g / l bath of ALUMINUX 138, commercial product at soda base).

For the bands produced by continuous casting between rolls of the prior art, the average variation of S _N is greater than 50%, both for the raw tapes and for the cold-rolled tapes. For the strips obtained by the method according to the invention, the average variation is less than 20% in all cases. The difference ΔS _N is less than 20 for the raw tapes, and less than 12 for the cold-rolled tapes to a thickness between 4 and 0.1 mm, and underwent anodizing before basic etching. It is less than 3.5, and even often 0.5 for bands which have undergone a final cold rolling called "brilliant", that is to say leading to a roughness R _a less than 0.2 microns, then electrolytically brightened before anodization of 1 μm.

Surprisingly, it has been found that the homogeneity of the surface of the the upper face of the continuously cast cast-rolled strips is clearly improved by a slight modification of the casting machine shown schematically in Figure 1.

The machine comprises a pouring pan (1) fed with liquid aluminum alloy and connected to an injector (2), consisting of a lower lip (3) and an upper lip (4), bringing the liquid metal into the gap between the two cylinders (5) and (6) turning in the opposite direction. The strip (7) is solidified on the other side of the gap between cylinders. The modification according to the invention consists in using an injector having an upper lip (4) set back a distance (d) from the lower lip (3). This shrinkage (d) is at least 2 mm and preferably at least 5 mm. For avoid that this provision causes an excessive influx of liquid metal in the interval between the two cylinders, it is desirable to reduce the pressure metallostatic, that is to say the height of metal, in the tundish (1), measured at from the median casting plane, less than 30 mm, preferably less than 25 mm, and especially since the withdrawal (d) is more important. The removal of the lip The upper part of the injector also allows a finer positioning of the injector. which avoids accidental friction on the surface of the cylinders, and thus improves indirectly the surface condition of the underside of the cast strip. With a metal height less than 25 mm and removal of the upper lip of the injector of at least 25 mm, the applicant has succeeded in producing brilliant products in alloys of the 1000 and 8000 series finished by rolling with polished cylinders (rolling called "skin pass") which had the same usage properties as the known products, developed by the more expensive route of semi-continuous casting and hot rolling. These products could be used for the manufacture of flat, folded or pressings.

The invention is applicable to all aluminum alloys capable of being cast in continuous between cylinders. For example, the plaintiff obtained good results with certain alloys from the 3000 series and with some low Al-Mg alloys magnesium content such as the 5005.

The invention is particularly interesting for AlFeSi alloys of the 1000 series and 8000, containing from 0.01 to 2% by weight of iron, and from 0.1 to 2% of silicon. Indeed, these alloys, when they are continuously cast between cylinders, have mechanical characteristics significantly higher than those obtained by casting traditional and hot rolling, which facilitates their rolling "brilliant". One of the reasons for the higher mechanical strength of the strips obtained by casting continuous for this type of alloy is that the amount of iron in solid solution in aluminum is higher. For an alloy containing more than 0.01% (100 ppm) of iron, the amount of iron in solid solution is greater than 50 ppm + 0.03 x (Fe content in ppm). Another advantage of having a high level of iron in solid solution is, for a given iron content, to decrease the intermetallic compounds to iron, whose surface presence is a source of optical defects. For the same reasons nature, the invention is also particularly interesting for low-alloys Mg (Mg <1.5%).

In addition, a grain size is obtained on the surface, defined as the width average grain surface, measured perpendicular to the direction of rolling by image analysis, less than 20 μm, and often 15 μm, both on casting only on cold-rolled strips, which reduces some of the defects look like lineage. This characteristic of the bands according to the invention is also favorable for later formatting, for example by stamping.

Example 1

An alloy EN AW-1085 (according to the standard NF EN 573-3) of composition (% by weight) was prepared: Si = 0.040 Fe = 0.038 Cu = 0.0017 Mn = 0.0022 Mg = 0.0032 Zn = 0.002 Ti = 0.02
with the addition of 3 kg / t of titanium-boron refining. The metal was treated with argon in a Pechiney Rhenalu Alpur ® casting ladle and then cast continuously on a JUMBO 3 CM ® cylinder casting machine from Pechiney Rhenalu.

The diameter of the cylinders was 1150 mm, with an air gap between the two cylinders 2.3 mm. The Styrite ® brand ceramic injector featured a lip 7 mm lower than the lower lip, and was powered by a casting tray with a liquid metal height of about 18 mm. The casting was made at a width of 1370 mm, a cast strip thickness of 3.6 mm, a speed casting speed of 1.6 m / min and an inter-roll force of 800 t / m bandwidth. The The strip was then cold rolled to a thickness of 0.4 mm.

In addition, strips of the same composition have been prepared by the usual method. vertical semi-continuous casting, hot rolling of trays then cold rolling up to the same thickness of 0.4 mm at two different work-hardening rates.

The mechanical characteristics of the bands, namely the breaking strength R _m (in MPa), the yield strength R _0.2 (in MPa), the elongation (in%) and the hardening rate n (in %). The results are given in Table 1 and show that for this alloy 1085, according to the invention, values of Rm> 165 MPa, R _0.2 > 160 MPa and A> 6% are obtained. Origin R _m (MPa) R _0.2 (MPa) AT (%) not (%) continuous casting JUMBO 3CM 173 167 7.7 80 semi-continuous casting. 155 148 7 93 semi-continuous casting. 165 158 6.2 96

It can be seen that continuous casting leads to both greater mechanical strength with a lower work hardening rate, thus facilitating shiny rolling, as well as than a better elongation for easier shaping.

It was also found that the size of the grains on the surface, determined by analysis image, was 7 μm for the bands according to the invention and 80 μm for the bands from vertical semi-continuous casting.

We have also prepared bands of the same composition always with the casting continuous between JUMBO 3CM ® cylinders of Péchiney Rhénalu but with an injector of the prior art without removal of the upper lip. These bands followed up to thickness of 0.4 mm the same process as the strips according to the invention.

The strips according to the invention and the strips resulting from a continuous casting between cylinders with a prior art injector were then subjected to two finishing passes with shiny cylinders of 0.4 mm up to 0.35 mm. After electrolytic brightening and then anodizing with sulfuric acid of a thickness of 1 micron, optical properties of the strips were measured by the RM 400 system of RODENSTOCK Company. The results are given in Table 2. S _N max S _N min S _N max - S _N min S _N max -S _N min / S _N average Strips according to the invention 7.4 6.4 1 14% Strips according to the prior art 11 7 4 50%

Records of the roughness index on the strips are given in fig. 2 and 3.

Example 2

An EN AW-1070A alloy (according to standard NF EN 573-3) was prepared: Si = 0.06 Fe = 0.12 Ti = 0.015 with addition of 1.5 kg / t of titanium-boron refining. Metal was cast continuously on the same JUMBO 3 CM ® cylinder casting machine of Péchiney Rhénalu than that of example 1.

The injector, also in Styrite ® brand ceramic, had a lip upper back 10 mm from the lower lip and was powered by a casting tray with a liquid metal height of about 18 mm. The width of band was 1370 mm, the thickness of the casting band 3 mm, the casting speed of 2 m / min and the effort between cylinders of 900 t per meter of bandwidth.

The strips thus cast were then cold-rolled to a thickness of 0.8 mm and then underwent two rolling passes with cylinders shining up to 0.5 mm. Samples of tapes were taken as and when tapes first castings at 3 mm, then strips after cold rolling of 0.8 mm, and finally of strips after "glossy" lamination at 0.5 mm.

Samples of 3 mm cast tapes were treated by anodizing sulfuric acid with a thickness of 1 μm. Strip samples after cold rolling at 0.8 mm have undergone a basic etching on 10 μm then a sulfuric anodization with a thickness of 1 μm. The strip samples after glossy lamination to 0.5 mm have successively undergone electrolytic brightening and sulfuric anodizing of thickness 1 μm.

We have also prepared tapes of the same composition always with casting continuous between JUMBO 3CM ® cylinders of Péchiney Rhénalu but with an injector of the prior art without removal of the upper lip. Bands followed up to 0.5 mm the same process as the bands following the invention and have suffered, like these, an electrolytic brilliance and a sulfuric anodization of thickness 1 .mu.m.

The optical properties of all the samples were measured by the RM 400 system of RODENSTOCK Company. The results are given in Table 3. Thickness in mm S _N max S _n min S _n max - S _n min S _n max- S _n min / S _n medium Raw casting strips, anodized, according to the invention 3 53 44 9 19% Striped stripes, anodised, according to the invention 0.8 36 32 4 11% "Glossy" laminated strips, anodized, according to the invention 0.5 10 8.5 1.5 17% "Glossy" laminated strips, anodized, according to prior art 0.5 19 13 6 37%

In addition, the following bands were measured on the belts according to the invention 12 μm while bands of the same composition, having undergone the same cold rolling ranges but from the conventional process (semi-continuous casting vertical then hot rolling trays) have grain sizes in surface of the order of 70 microns.

Example 3

By continuous casting between rolls, seven different castings were made with the following characteristics: reference Alloy Fe and Si content casting speed casting thickness R1 8011 Fe 0.60; If 0.75 0.96 m / min 7.0 mm R3 1050 Fe 0.20; If 0.14 3.0 mm R4 1235 0.37 Fe; If 0.14 2.5 m / min 2.7 mm R5 1085 see example 1 R6 8011 Fe 0.70, Si 0.73 1.6 m / min 3.1 mm R7 1085 see example 1

The casting R5 corresponds to that of Example 1, carried out with the casting process continuous between cylinders according to the invention.

The casting R7 corresponds to that of Example 1, carried out according to the prior art with a traditional injector.

The other pours were made with a similar continuous casting machine to that described in Example 1.

The results of the optical and rugosimetric characterizations, after stripping and anodizing of the raw casting tapes, were as follows: Characterization of gray level oscillations Reference Measure L * a * b * The average Standard deviation on L * medium visual ranking (1: the best 7: the least good) R1 74.74 0.26 7 R2 75.47 0.31 3 R3 77.98 0.45 4 R4 79.42 0.20 2 R5 82.50 0.23 1 R6 74.54 0.46 5 R7 74.99 0.77 6 Characterization of roughness defects by 3D roughness measurement ref. Ra μm Rq μm Sk Ek R1 2,622 4,401 -4.124 35.830 R3 2,111 2,745 - 0, 784 4,640 R4 3,158 4,060 - 0.643 4,364 R5 2,905 3,661 - 0.614 3,542 R6 1,759 2,316 - 0.856 6,168 R7 2,681 4,033 - 2,678 27.002

We find that the standard deviation on the average L * parameter corresponds to the response except for sample R1 for which the oscillation step of the level of Gray is too small in relation to the diameter of the area of an individual measurement. On the In industrial terms, samples R1 and R7 are not usable for applications because their surface condition is aesthetically and optically unacceptable.

The Applicant has found that in 3D rugosimetry, only Sk and Ek parameters discriminate products obtained by the process of products that are unfit for demanding applications in surface appearance.

• la valeur de Sk en rugosimétrie 3D selon la procédure décrite soit supérieure à -2,0 et préférentiellement supérieure à -1,0 ;
• la valeur Ek en rugosimétrie 3D doit être inférieure à 15 et préférentiellement inférieure à 8.

The Applicant has also found that to have a surface that allows the applications to be used, one or both of the following conditions must be met:

• the Sk value in 3D rugosimetry according to the procedure described is greater than -2.0 and preferably greater than -1.0;
• the Ek value in 3D rugosimetry must be less than 15 and preferably less than 8.

The Applicant has observed that it is preferable that in addition to one or both conditions just mentioned, the surface also has a standard deviation on the average L * value less than 0.5 and preferably less than 0.3.

Claims (8)

1. Process for manufacturing an aluminium alloy strip with a better surface uniformity on its upper face than on its lower face, by continuous casting between two cooled cylinders (5) and (6), from a pouring bath (1) containing the liquid metal connected to an injector (2) composed of a lower lip (3) and an upper lip (4), carrying the liquid metal into the gap between the two cylinders, characterised in that the upper lip (3) of the injector (2) is set back at least 2 mm from the lower lip (4).
2. Process according to claim 2, characterised in that the upper lip (3) of the injector (2) is set back at least 5 mm from the lower lip (4).
3. Process according to either claim 1 or 2, characterised in that the depth of liquid metal in the pouring bath (1) measured from the median casting plane is less than 30 mm.
4. Process according to claim 8, characterised in that the depth of liquid metal in the pouring bath (1) measured from the median pouring plane is less than 25 mm.
5. Use of a strip obtained by a process according to any one of claims 1 to 4, for manufacturing of anodised and possibly enamelled plates for building.
6. Use of a strip obtained by a process according to any one of claims 1 to 4 for manufacturing of stamped parts.
7. Use of a strip obtained by a process according to any one of claims 1 to 4 for manufacturing of optical reflectors.
8. 1085 alloy strip produced by continuous casting between cylinders according to one of claims 1 to 4, characterised in that after one or several cold rolling passes with a total work hardening ratio of less than 85%, it has one of the following groups of properties:

   a) Rm > 165 MPa and A > 6%,

   b) R_0.2 > 160 MPa and A > 6%.

Images