BEVERAGE CAN BODY SHEET FROM CONTINUOUSLY CAST ALUMINUM ALLOY

Abstract

A continuous casting method and apparatus are described that generate alpha-phase Al(Fe,Mn)Si intermetallic particles on the surface of an aluminum alloy strip to facilitate the manufacture of beverage cans. The alpha phase particle size is generated so that subsequent rolling will ensure the particles will provide galling-free operation in the ironing stages of the can body making process. The control of alpha-particle size upon the surface layers of the strip ensures galling-free operation.

Claims

1. An aluminum alloy sheet product, suitable for the production of cans, made by continuous casting followed by rolling, annealing and cold rolling to final gauge wherein intermetallic particles of alpha phase Al(Fe, Mn)Si of diameters between 0.2 to 35 m are present on at least one surface of the cast strip such that during the ironing steps of the can making process the product is substantially free of galling defects.

2. A method of making the product of claim 1 by continuous casting on molds with surface textures such as grooves or dimples or protrusions so as to produce intermetallic particles of alpha phase Al(Fe, Mn)Si on at least one surface of the cast strip.

3. The method of claim 2 wherein the casting mold surfaces have dimples or protrusions of diameter between 0.3 to 0.7 mm and depth or height between 0.01 to 0.2 mm with an area coverage in the range of 5% to 64%.

4. The method of claim 2 where the mold surface grooves or dimples or protrusions are produced by shot peening, laser shot blasting, grit blasting, electro-discharge texturing, laser ablation, photoelectric etching, cutting, knurling, roll forming or embossing.

5. The method of claim 2 wherein the casting mold is characterized by longitudinal grooves of widths between 0.04 to 0.50 mm and depths between 0.040 to 0.40 mm and groove area coverage between 2% to 75%.

6. The rolled sheet product of claim 1 in which the intermetallic particles of alpha phase Al(Fe,Mn)Si are in the diameter range of 0.2 to 15 m with a median diameter between 2 and 7 um on at least one surface of the sheet.

7. The cast strip product of claim 1 in which the surface protrusions due to mold grooves, or mold dimples/protrusions, is 20 m or less, or 30 m or less, respectively.

8. A method of producing an aluminum alloy strip having intermetallic particles of alpha phase Al(Fe, Mn)Si by rapidly heating the strip surface to a temperature near the solidus temperature so as to coarsen the alpha phase Al(Fe, Mn)Si to a median diameter greater than 2 m.

9. The method of claim 8 where at least one surface develops a matte finish indicating the presence of large alpha phase Al(Fe,Mn)Si particles.

10. The method of claim 8 wherein the surface reaches a temperature at which the surface develops a matte finish with a maximum average strip temperature 10 C. or more below the solidus temperature.

11. The method of claim 8 wherein the surface develops a matte finish with maximum average strip temperature 20 C. or more below the solidus temperature.

12. An aluminum alloy strip according to claim 1 that is rolled to finish gauge for making beverage cans that are substantially free of galling.

Description

BRIEF DESCRIPTION OF FIGURES

[0016] FIG. 1 shows sub-micron particles of alpha-Al(Fe,Mn)Si on the surface of 3003 strip made in an experimental high-speed twin roll caster (from reference 2).

[0017] FIG. 2 shows large dendritic particles on the surface of AA5050 alloy strip produced in a commercial Hunter-Douglas block caster (from reference 3).

[0018] FIG. 3 is a schematic representation of rapidly heating the surface of the as-cast strip

[0019] FIG. 4 shows the suspension of molten metal over a groove in the mold.

[0020] FIG. 5 shows the depth of molten metal penetration as a function of groove width on the mold for three metallostatic head heights.

[0021] FIG. 6 is a plot showing the useful ranges of groove width and groove area percentage.

[0022] FIG. 7 shows the depth of molten metal penetration as a function of dimple diameter on the mold for three metallostatic head heights.

[0023] FIG. 8 is a plot showing the range of dimple diameters and dimple area percentage suitable for generating large intermetallic particles on the surface.

[0024] FIG. 9 shows dendrites of alpha phase Al.sub.12(Fe,Mn)Si on the surface of AA3304 alloy cast on a substrate textured with protrusions of about 300 m projected diameter.

DETAILED DESCRIPTION OF THE INVENTION

Example 1

[0025] Particles on the surface of a continuously cast aluminum alloy strip may be seen in scanning electron microscope (SEM) micrographs. The micrograph in FIG. 1 is typical for the process and shows sub-micron particles which are too small to be effective as scrubbing agents during ironing (image from reference 2). FIG. 2 shows the surface of an AA5050 strip produced by the Hunter-Douglas (block caster) cc method (see reference 3). The strip was 25 mm thick and 230 mm wide. It shows a dense network of IM particles on the surface. The IM particles vary in size, the largest being about 35 m. These are therefore much coarser than the average 1.7 m IM normally found on the surface of cc aluminum strip cast by other commercial methods. Energy Dispersive X-Ray (EDX) examination shows these dendritic IM particles to be quaternary AlFeMnSi phases, alpha phase Al(Fe,Mn)Si. They contain 2-8% Si, 1-3% Mn and 10-20% Fe by weight. IM particles in the main alloying elements Fe, Si and Mn near the surface reach many times the bulk content in the alloy. Depth profiles of the sample show that this enrichment near the surface is confined to a depth of no more than 1 m (reference 3). The large IM particles are thus present only on the surface of the as-cast strip. Some or all of these dendritic IM particles would break up during the subsequent rolling operations and be present in the final gauge sheet in the median size range of 2-7 m that is preferred for imparting galling resistance. It is noted that these particles are not a product of liquation or exudation which would run tens of micron deep. Rather, they are formed from molten aluminum that was suspended between the high points on the mold surface. Without physical contact this resulted in slower solidification than that in contact areas. Therefore, it is possible to control the size of surface IM particles by proper selection of the mold surface texture and the bulk chemistry of the alloy.

TABLE-US-00001 TABLE 1 Chemical Composition of the Aluminum Alloy AA5050 (reference 3) Element Si Fe Cu Mn Mg Cr Zn Ti Pb Be Na AA5050 (wt %) 0.28 0.8 0.12 0.33 1.20 0.05 0.07 0.03 0.01 0.0006 0.003

Example 2

[0026] It is also possible to modify the size of intermetallic particles on the surface of cc strip after casting. This can be done by heating the surface layers close to the solidus temperature of the alloy to grow the smaller alpha-phase particles by diffusion into larger particles at the surface. Consider a strip or slab that has been continuously cast and optionally rolled. The bulk temperature of said strip or slab could be close to the melting point if it has recently been cast. More specifically, the bulk strip or slab temperature could be within 50 C. of the solidus temperature. Alternatively, the bulk strip or slab temperature may be close to room temperature if it has been rolled and allowed to cool. By heating the surfaces of the strip sufficiently rapidly, it is possible to raise the surface temperatures of the strip close to the solidus temperature, while the average bulk temperature of the strip or slab remains below the solidus temperature. It is important that the bulk temperature remains below the solidus, so that the strip or slab does not fall apart due to bulk melting. Once the heating medium is removed, the strip or slab equilibrates to a new average temperature that is higher than the initial average temperature, but lower than the solidus temperature. During this heating, beneficially large intermetallic alpha phase Al(Fe, Mn)Si particles form, and are dispersed over the surface of the strip or slab. Subsequent rolling operations, especially hot rolling, preserve the beneficial particles in the proper size range at the surface, and they are thus available to scrub the dies during the ironing step of beverage can production.

[0027] Heating of the surface close to the melting temperature, while the average temperature of the strip or slab remains below melting, requires a high heat transfer method. A method and apparatus for rapidly heating the surface of the strip or slab is infra-red radiation (IR). FIG. 3 illustrates the concept for the case of a strip running through a high heat transfer infrared radiation heater.

[0028] In the figure, an infra-red radiation source [2] is placed proximate to the as-cast strip or slab surface [1]. Only the top half of the strip or slab is shown, but it is understood that the same method and apparatus can be simultaneously applied on the bottom half of the strip. The strip is travelling out of the caster at speed v, with average bulk temperature Tc, thickness t and the length of the infrared radiation source is L. The infrared radiation source has a temperature Tr. After equilibration, the average strip or slab temperature is Tf. The time, , spent heating up in the radiation zone is:

[00001]$\begin{array}{cc}+L/v& \mathrm{Eqn}\left(1\right)\end{array}$

[0029] Heat is transferred to the strip by radiation. A simple approximation of the heat transferred to the strip may be obtained using the equation for two parallel plates:

[00002]$\begin{array}{cc}Q={}_{\mathrm{SB}}\frac{{\mathrm{Tr}}^4-{\mathrm{Tc}}^4}{\frac{1}{r}+\frac{1}{c}-1}& \mathrm{Eqn}\left(2\right)\end{array}$ [0030] where Q=heat transfer [W/m.sup.2], .sub.SB=5.6710.sup.8 W/m.sup.2 K.sup.4 (the Stefan-Boltzmann constant), r and c are the emissivities of the radiation source and the as-cast strip or slab, respectively. Energy imparted to the strip or slab, E, is approximately:

[00003]$\begin{array}{cc}E=Q**L*W& \mathrm{Eqn}\left(3\right)\end{array}$ [0031] where W is the width of the irradiated strip or slab. The mass of strip or slab in the irradiated area is:

[00004]$\begin{array}{cc}M=*L*W*t/2& \mathrm{Eqn}\left(4\right)\end{array}$ [0032] where is the material density of the strip or slab. The final average temperature of the strip or slab can be approximated as:

[00005]$\begin{array}{cc}\mathrm{Tf}=\mathrm{Tc}+\frac{E}{\mathrm{Cp}*M}& \mathrm{Eqn}\left(5\right)\end{array}$ [0033] where Cp is the specific heat capacity of the strip or slab.

[0034] For the present invention, Tf must always be below the solidus temperature of the strip or slab. To achieve this, either the irradiation length L or the speed v may be controlled. It is noteworthy that irradiation temperature Tr and thickness h may also be controlled to regulate the final temperature, but from a practical perspective, these two variables are difficult to alter and are therefore normally held constant.

[0035] An important feature of surface heating and the growth of large intermetallic alpha phase Al(Fe,Mn)Si particles is an abrupt change of appearance of the strip surfaces. In particular, the strip surfaces change from a shiny appearance to a dull or matte appearance. This indicates that the growth of the IM has occurred. Temperatures close to the solidus facilitate the rapid growth of beneficial large intermetallic alpha phase Al(Fe,Mn)Si particles. The dull or matte surface appearance is an important indicator of the success of the process. It causes the surface emissivity to jump from around 0.1 for the rolled shiny surface to a level of up to 0.9 for the dull surface. This is visible to the naked eye and can be measured by a pyrometer. Under those conditions, the radiation heat flux at the surface increases by about the same ratio. This may be understood from Eqn. 2 by taking r=1 for the ideal radiation source and noting that the heat flux then becomes proportional to strip surface emissivity c. This factor enables the strip to reach around the solidus temperature at the surface and generate the large alpha-phase particles without breaking the strip as the bulk of the strip is still at a lower temperature.

[0036] This concept is demonstrated by the following example. An infrared radiation (IR) source was used to heat aluminum alloy strip from room temperature. The alloy was in the 3xxx family, more specifically the alloy was close to AA3105 composition. The strip had been continuously cast at high speed and rolled. Initially, the strip speed was set high to ensure that the final average temperature was well below the solidus temperature of the strip. The strip speed was then reduced gradually until the surface appearance of the strip became dull, at which point the speed was held constant and the product was made at that operating point. Table 2 below shows the operating conditions:

TABLE-US-00002 TABLE 2 Operating Conditions of Infrared Heating Aluminum Strip to Achieve Matte Surface MATERIAL Symbol Units Value ALLOY AA 3105 Heat capacity Cp J/kg K 1004 Density kg/m.sup.3 2700 Thickness* t m 0.00083 Width* W m 0.165 Strip emissivity c 0.1 IR radiator emissivity r 0.95 IR radiator temperature* Tr C 1000 IR radiator length* L m 7.6 Initial speed* m/s 0.254 Final speed* v m/s 0.167 Initial strip temperature* Tc C 13 Final strip temperature* Tf C 510 Solidus temperature C 633 Initial surface appearance Shiny Final surface appearance Matte Dwell time in furnace s 45 *measurements

[0037] Using the equations 1-5 above, the calculated final strip temperature is 519 C. This is reasonably close to the measured final strip temperature of 510 C., the difference being attributable to simplifying approximations used in the equations, and measurement errors. The measured surface temperature of 510 C. was thus below the equilibrium solidus temperature of 633 C., but sufficiently high to promote the changes in alpha-phase size and morphology.

[0038] The strip produced under these operating conditions was cold rolled to finish gauge and beverage cans were made from it. The drawn and ironed cans were free of galling after the production of more than 100,000 cans.

Example 3

[0039] Consider the casting of molten aluminum on a mold surface textured with straight grooves arranged in the casting direction of width w in the form of rectangular or hemi-cylindrical or wedge-shaped channels separated by land areas between them. This is illustrated in FIG. 3 for a wedge-shaped groove. Molten metal makes contact with the land area and penetrates into the grooves by a distance h. The surface of the solidified strip therefore shows protrusions over the grooves and relatively flat areas over the lands. Under these conditions, solidification of the metal suspending into the grooves is momentarily delayed compared to the parts solidifying at faster rates in contact with the lands. This delay allows the molten metal in suspension to take up solidification shrinkage of the newly-formed solid. The molten metal in suspension does this by pulling itself up over the grooves. As a result, the protrusion of the metal in the lands from the surface of the solidified strip is smaller than the penetration of the molten metal into the grooves.

[0040] Solidification on a mold with a groove pattern promotes the formation of large IM particles over the grooves where solidification is delayed and takes place at a substantially slower rate. The effect is restricted to the surface of the strip because as solidification progresses, differences between the groove area and land area are reduced and eventually become diffuse.

[0041] On the surface of the strip, two families of IM particles are observed. One family is in the land areas that are in contact with the mold and experience high solidification rates. This family has round IM, less than 1.5 m or finer, as is well known from experience with cc. An example of such fine IM particles is shown in FIG. 1. Such fine particles do not provide the scrubbing effect needed during the ironing stage of can-making. The second family forms over the grooves in dendritic form and in sizes up to 15 m, such as those illustrated in FIG. 2. These large IM fracture during subsequent rolling and reach an average size of about 3 m required for the scrubbing action and thus avoidance of galling during the ironing stage of can manufacture.

[0042] Estimates of the penetration depth are used to design the width range and depth for groove patterns. Consider the ideal case of a column of molten metal suspending into a groove of width w, FIG. 4. The depth of the groove and the angle of the wedge are designed so as to prevent molten metal making contact with the walls and the root of the groove, respectively. Under these conditions, the radius of curvature, r, of the suspension can be calculated from the pressure of the molten metal and its surface tension using equation 6 below. If the pressure inside the molten metal is entirely due to metallostatic head, then for a cylindrical suspension

[00006]$\begin{array}{cc}p=gH=/r& \mathrm{Eqn}\left(6\right)\end{array}$ [0043] where p=head pressure, =density of molten aluminum, g=gravitational acceleration (9.8 m s.sup.2), H=metallostatic head, =surface tension (a property of the molten aluminum alloy) and r is the radius of the cylindrical bubble over the groove. The depth of penetration h is calculated from the geometry of the bubble using:

[00007]$\begin{array}{cc}h=r\left(1-\cos \left(\right)\right)& \mathrm{Eqn}\left(7\right)\end{array}$

where w=2 r sin() [0044] and is one half of the subtended angle as shown in FIG. 3. This value of h applies to the suspension of the metal in molten state. It is recognized that the actual penetration (observed as protrusion on the strip surface) will be somewhat less than this due to the pull up during solidification caused by shrinkage.

[0045] Calculations for molten aluminum with a nominal density of 2300 kg/m.sup.3 and surface tension =0.86 J/m.sup.2 are shown in Table 3. It is clear that the higher the head level H and the wider the grooves, the deeper is the penetration h of the molten metal. An upper limit of h=30 m may typically be placed on the penetration depth h since deeper penetration results in a very rough surface that is aesthetically undesirable even after rolling to final gauge.

TABLE-US-00003 TABLE 3 Effect of Groove Width on Penetration of Molten Metal head (mm) 25.4 38 51 P (Pa) 573 859 1145 r (mm) 1.50 r (mm) 1.00 r (mm) 0.75 groove width h h h mm sin() m sin() m sin() m 0.10 0.033 1 0.050 1 0.067 2 0.20 0.067 3 0.100 5 0.133 7 0.30 0.100 8 0.150 11 0.200 15 0.40 0.133 13 0.200 20 0.266 27 0.50 0.166 21 0.250 32 0.333 43 0.60 0.200 30 0.300 46 0.399 63 0.70 0.233 41 0.350 63 0.466 87 0.80 0.266 54 0.399 83 0.533 115 0.90 0.300 69 0.449 107 0.599 150 1.00 0.333 86 0.499 134 0.666 191

[0046] FIG. 5 illustrates the relationship between groove width w and depth of penetration h, for various head heights H. Head heights H between 25 mm and 50 mm are considered practical by those skilled in the art of horizontal continuous casting. It is noted that the maximum groove width w depends on the operating metallostatic head and varies from about 0.4 to 0.6 mm. It is also clear that narrower grooves are preferred if the metal protrusion into the grooves is to be kept lower.

[0047] The depth of the grooves needed for efficient functioning may be understood as follows. Shallow grooves will not function because the depth of the grooves must be substantially greater than the calculated suspension depth to prevent contact with the walls. There also needs to be enough opening left in the grooves to allow the escape of gases (typically air) to prevent surface defects in the strip. Referring to Table 3, if the grooves were cylindrical, the molten metal would fill the grooves to about 30 m depth at a metal head of 50 mm. On the basis of this observation, for triangular grooves as shown in FIG. 3 a minimum groove depth of 40 m is recommended. However, for practical reasons an even deeper pattern needs to be applied since the casting surface is subject to wear and the groove depth is thus reduced during use.

[0048] Acceptable useful groove width and area percentage are shown in FIG. 6. A freshly textured mold surface would have the largest recommended groove width (about 0.5 mm) and the highest groove area percentage (75%). As the mold surface wears, the grooves become smaller and groove area would also be reduced until the texture loses functionality.

[0049] One other parameter for this family of textures is the depth of the grooves. A minimum 40 m depth is required for proper functionality, as discussed above. Grooves can be created in a deterministic fashion using laser ablation, photoelectric etching, cutting, knurling or embossing. The upper limit for groove depth is also dictated by practical considerations such as the action of the grooves as potential stress risers and their influence on strength of the mold. A groove depth of up to 0.40 mm is considered suitable for a newly textured surface so that when the grooves stop functioning at .sup.40 m depth, the land percentage would still be within the preferred range.

Example 4

[0050] Another form of surface roughness suitable for suspending the molten metal is depressions or dimples on the surface of the casting mold. The metal suspends into the dimples from the rims, equivalent to lands of the groove pattern. Such patterns can be created by many methods such as shot peening, grit blasting, electro-discharge texturing, cutting, laser ablation, photoelectric etching or embossing. The suitable size of such dimples can be calculated assuming the indentation of the dimple is a circle of radius R. The molten metal is modelled as a bubble of radius r above the dimple. This radius r and the depth of penetration h are calculated from Eqn. (8) and (9), respectively.

[00008]$\begin{array}{cc}p=gh=2/r& \mathrm{Eqn}.\left(8\right)\end{array}$$\begin{array}{cc}h=r\left(1-\cos \left(\right)\right)& \mathrm{Eqn}.\left(9\right)\end{array}$ [0051] where represents the half-angle subtended over the dimple as in FIG. 3. The results of these calculations are shown in Table 4 for several dimple diameters (d=2R) and metallostatic head. Dimple diameters up to 1 mm can be used while the depth of penetration is still kept below 30 m for the nominal metallostatic head of 38 mm. This calculation shows that the diameter of the dimples can be larger than the width of the grooves described in example 3 and still provide for suspension of the molten metal in the desired range. In practice, however, such large dimples are not preferred because the molten metal solidifying over the dimple is likely to develop surface cracks along a diameter as those areas are the last to solidify. This is due to the constraint from the edges of the dimples where the metal solidifies earlier in contact with the mold. Therefore, dimple dimensions similar to the width of the grooves described above (less than 0.7 mm) will be more useful. Dimple diameters applied to the casting mold surface should therefore be less than this maximum.

[0052] FIG. 5 shows the predicted depth of suspension on a dimple pattern for operation at three levels of metal head. It is seen that for any given dimple opening, the higher the metal head, the deeper is the predicted penetration. Dimple diameters up to 0.7 mm are seen to be suitable for keeping the suspension depth to a maximum of 20 m. This lower value of suspension is needed for the dimpled surfaces to avoid lubricant trapping in the depressions of the cast strip surface during subsequent rolling steps. For operation at 50 mm metallostatic head, a value of 0.2 mm is recommended for the lower end of the dimple diameter for effective suspension of the molten metal and the desired delay in solidification over dimples.

[0053] Surface coverage of dimples, measured as area percentage, can vary over a wide range. With freshly textured mold surfaces, the high end of the dimpled area coverage and dimple size are recommended. This is because of the wear of the mold surface during casting as a result of which both the dimple diameters and the dimple area percentage will get smaller. With high coverage and wide dimples, the textures will last longer before they lose their functionality. It is recognized that processes such as shot peening create a range of dimple sizes, but not a single size. Aluminum allows a large dimple diameter up to 0.7 mm. Dimple coverage of 64% is recommended. These recommendations are shown in FIG. 8. A freshly textured mold surface would have the largest recommended dimple diameter (0.7 mm) and the highest coverage (64%). As the mold surface wears, the dimples would become smaller and coverage would also be reduced until the texture loses functionality. One other parameter for this family of textures is the depth of the dimples. For this 10-100 m is recommended. Similar dimples can be created in a deterministic fashion using electro-discharge texturing, laser ablation, photoelectric etching, cutting, knurling or embossing.

[0054] In Table 4, the depth of penetration of molten aluminum into a mold surface with dimples (H=metallostatic head, h=penetration, d=dimple diameter, r=radius of the molten metal bubble above the dimple, =subtended angle; FIG. 3 may be used to illustrate this for dimples as well as grooves) are calculated.

TABLE-US-00004 TABLE 4 Effect of Dimple Diameter on Penetration of Molten Metal H(mm) 25.4 38 51 r (mm) 3.00 r (mm) 2.00 r (mm) 1.50 dimple d h h h mm sin() m sin() m sin() m 0.20 0.033 1.66 0.050 2.50 0.067 3.33 0.30 0.050 3.75 0.075 5.62 0.100 7.51 0.40 0.067 6.66 0.100 10.01 0.133 13.37 0.50 0.083 10.42 0.125 15.66 0.166 20.95 0.60 0.100 15.02 0.150 22.60 0.200 30.26 0.70 0.117 20.46 0.175 30.82 0.233 41.34 0.80 0.133 26.75 0.200 40.35 0.266 54.24 0.90 0.150 33.89 0.225 51.21 0.300 68.99 1.00 0.166 41.90 0.250 63.41 0.333 85.66

Example 5

[0055] Another surface texture particularly advantageous is discrete protrusions on the mold surface. Such protrusions can be in the form of hemispheres or cylindrical rods with a hemispherical cap. The dendritic IM particles of alpha phase Al(Fe,Mn)Si family grown on such a surface is shown in FIG. 9 for alloy AA3304 with individual IM particles reaching up to 35 m size. Such large IM particles break down in rolling operations to reach a size suitable for the scrubbing action needed during the ironing steps. It is considered that similar size and height ranges would apply to such protrusions as those described in Example 4 above for depressions. It is noted, as above, that these large IM particles are confined to the surface of the strip while IM size inside the strip thickness remains fine.