Method of manufacturing sheets for aluminum cans

Abstract

A method for producing aluminum can sheets comprises the following steps: providing a body made of an aluminum alloy of type AA3004, AA3104, or another aluminum alloy suitable for manufacturing aluminum can sheets; heating the body to a homogenizing temperature; hot-rolling said body in a hot rolling mill to produce a hot-rolled sheet, said hot-rolled sheet exiting the hot rolling mill at a hot-rolling exit temperature with a hot-rolling exit gauge, wherein the hot-rolling exit temperature is selected to substantially avoid recrystallization of the hot-rolled sheet;cold-roll the hot-rolled sheet in a cold rolling mill to apply a cold reduction to produce a cold-rolled sheet with a cold-roller exit gauge smaller than the hot-roller exit gauge; annealing the cold-rolled sheet in a selected intermediate temperature range to allow recrystallization of the cold-rolled sheet to obtain a recrystallized annealed sheet; cold-rolling the recrystallized annealed sheet to apply a cold reduction to produce a cold-rolled sheet with a final gauge.

Description

Method of manufacturing sheets for aluminum cans Technical field and state of the art

[0001] The present invention relates to a method for producing aluminum can sheets.

[0002] When aluminum can sheet is formed into cup-shaped articles, a phenomenon known as "ear formation" is common. Ear formation appears as a wavy appearance around the upper edge of the formed cup. These protruding, wave-like portions, also known as "ears," are formed during the deep-drawing process in cup manufacturing and represent an undesirable feature of the article. In aluminum can body stock (ABS), the cup is subsequently ironed into multiple rings, which can accentuate the wavy ears. Excessive ear formation can create problems with cup transport, as well as insufficient trimming after ironing, cut ears, and jams in the trimmer. These artifacts are undesirable in aluminum can manufacturing.In this way, the aim is to minimize ear formation to avoid these problems and to increase the quality of the cup.

[0003] It is known that raw materials for can bodies such as AA3004, AA3104 or other aluminum alloys are basically suitable for manufacturing aluminum can sheets with low ear formation characteristics provided a suitable manufacturing process can be established.

[0004] There is a well-established process in the aluminum industry for producing aluminum strips suitable for can-bodied goods. This process involves hot-rolling an aluminum ingot through a roughing mill and then through a multi-stand hot rolling mill, typically exiting at a high temperature to ensure the material is fully recrystallized through a self-annealing process. This well-known method produces a finished product with low ear formation and desirable mechanical characteristics. However, the installation and operation of such a continuous hot rolling mill requires a significant capital investment.

[0005] In the past, modifications have already been proposed to produce can sheets with commercially acceptable ear-forming characteristics from a single-holder reversible laminator (see, for example, U.S. Patents 5,362,340 and 5,362,341). According to the U.S. method 5.362.340, an aluminum alloy ingot is provided and heated to a temperature between approximately 527 °C and 571 °C. The ingot is then hot-rolled on a single-stand reversing mill to produce an intermediate-gauge sheet. The intermediate-gauge sheet, which is either self-annealed or batch-annealed, is then cold-rolled to produce a final-gauge aluminum can sheet that has low-wear characteristics. Relatively low-temperature homogenization (527 °C to 571 °C) is applied to prevent uncontrolled recrystallization during hot rolling on the single-stand reversing mill.

[0006] U.S. Patent Application 2002 / 0062889 A1 discloses a process and installation for producing hot-rolled aluminum strips for can manufacturing. The installation includes a reverse roughing stage for the hot feed material, immediately followed by final strip rolling, which is then heat-treated from coils. During the final finishing roll rolling steps, recrystallization of the rolled material is suppressed by controlled temperature management of the hot strip. In the embodiment, the temperature is maintained within the non-critical range of 260°C to 280°C to prevent recrystallization. Recrystallization occurs only outside the rolling mill. To this end, the hot-rolled material is transferred to a continuous furnace immediately after final rolling.Direct transfer offers the advantage that a furnace used for recrystallization only needs to apply a relatively small temperature difference (e.g., approximately 40°C - 60°C) between the rolling temperature and the recrystallization temperature, thus achieving a favorable energy balance.

[0007] The international patent application published as WO 2015 / 140833 A1 describes aluminum alloy sheets with a low wear rate suitable for manufacturing aluminum can bodies. The alloys mentioned for this purpose include A3004 and A3104 type alloys. A preferred process includes the steps of ingot melting, ingot homogenization, hot rolling, primary cold rolling, intermediate annealing, and secondary cold rolling. The hot rolling step is divided into two separate steps, namely, the "hot roughing step" and the "hot finishing step." In the hot finishing step, the final temperature is preferably between 330 °C and 380 °C. It is noted that the driving force for recrystallization is insufficient if the final temperature is below 330 °C.

[0008] The international patent application published as WO 2018 / 034960 A1 discloses anodized aluminum alloy sheets and, in particular, dark gray anodized aluminum alloy sheets. Exemplary methods for preparing an aluminum sheet comprising dispersoids are described.In some examples, the method comprises melting an aluminum alloy to form an ingot; homogenizing the ingot to form a homogenized ingot; hot rolling the homogenized ingot to produce a hot-rolled intermediate product; cold rolling the hot-rolled intermediate to produce a cold-rolled intermediate product; inter-annealing the cold-rolled intermediate to produce an inter-annealed product; cold rolling the inter-annealed product to produce a cold-rolled sheet; and annealing the cold-rolled sheet to form an annealed sheet comprising dispersoids, wherein the alloy is an alloy of the 2xxx, 3xxx, 5xxx, or 7xxx series. Production processes for aluminum can sheets are not disclosed.

[0009] WO 2016 / 115120 A1, corresponding to EP 3245309 A1, discloses novel processes for increasing productivity in a continuous solution annealing and heat-treating line for heat-treatable aluminum sheet products for the automotive industry with high T4 fire resistance and post-painting properties, and reduced waviness. As a non-limiting example, the described processes can be used in the automotive industry. The described heat-treatable alloys and processes can also be applied to the marine, aerospace, and transportation industries.

[0010] Document CN 106676440 A discloses a process heat treatment technology for anodic oxidation hard aluminum alloy. The process heat treatment technology comprises the following steps: performing a homogenization treatment on an anodic oxidation hard aluminum alloy ingot at a temperature of 490 °C; performing hot rolling: after completing the homogenization treatment, starting rolling at a temperature of 440-460 °C and controlling the final rolling temperature to be 250-270 °C; performing a natural aging treatment: resting naturally for 48 hours after completing the hot rolling; and after completing the natural aging treatment, performing cold rolling, cleaning, and annealing, where the annealing temperature is 320 °C.The process can meet the requirements of anodic oxidation treatment of various products, and if the technology is used to treat common 1090 aluminum alloy, high forming capacity and high brightness can be achieved that are equal to those of similar foreign products that use high industrial purity aluminum as the base material. Summary of the invention

[0011] An objective of the invention is to provide a method for producing aluminum can sheets suitable for making aluminum cans, wherein the aluminum sheet exhibits favorable ear-forming characteristics after a deep drawing stage and further enables the production of stable cans with a thin wall thickness.

[0012] This objective is achieved by a method comprising the features according to claim 1. Preferred embodiments are defined in the dependent claims.

[0013] According to the method for producing aluminum can sheets, a body (also called an ingot) made of an aluminum alloy is provided. The aluminum alloy is selected so that it is suitable for manufacturing aluminum can sheets. Specifically, the aluminum alloy is of type AA3004, AA3104, or another aluminum alloy suitable for manufacturing aluminum can sheets, for example, alloy AA3204.

[0014] Typical requirements for aluminum alloys suitable for manufacturing aluminum can sheets are described, for example, in the article "AlMn1Mg1 for Beverage Cans" by J. Hirsch in: "Virtual Fabrication of Aluminium Products" Wiley-VCH 2006 (ISBN: 3-527-31363-X), chapter I-4. In general, the material must provide an optimal combination of strength and sufficient forming properties. In the case of aluminum, strength is achieved by combining the addition of a suitable alloying element to achieve the best solid solution hardening (e.g., by Mg and Mn) and pre-forming (i.e., highly rolled sheets). Furthermore, the strength must remain sufficiently high even after subsequent paint firing cycles. Good formability is achieved by an optimal combination of alloying elements for good work hardening (Mg) with some particle strengthening effects (Mn).The latter also maintains homogeneous deformation and even provides a cleaning effect on the dies, preventing the accumulation of harmful oxide and wear. Consequently, the common aluminum alloys used for can body production are AlMg1Mn1 = EN-AW 3004 and AlMg1Mn1 (Cu) = EN-AW 3104, which best meet the strength and formability requirements of cans.

[0015] In the claimed method, aluminum alloys comprising the following chemical compositions are used (all numbers are weight percent): 0.0-0.60 wt% Si (silicon), preferably 0.1-0.5 wt% Si; 0.1-0.80 wt% Fe (iron), preferably 0.2-0.70 wt% Fe; 0.7-1.50 wt% Mn (manganese), preferably 0.8-1.40 wt% Mn; 0.8-1.50 wt% Mg (magnesium), preferably 0.9-1.30 wt% Mg; 0.05-0.25% by weight of Cu (Copper), preferably 0.1-0.25% by weight of Cu; up to 0.10% by weight of Ti (Titanium); up to 0.25% by weight of Zn (Zinc); and up to 0.15% by weight of impurities, preferably each of the impurities with less than 0.05% by weight; the remainder being Al (Aluminum).

[0016] Furthermore, many aluminum alloys optimized for other purposes are not considered suitable for manufacturing aluminum can sheets in the context of this application.These include, for example, 1XXX series alloys (essentially pure aluminum with a minimum aluminum content of 99% by weight), 2XXX series alloys alloyed with copper as the basic alloying element and capable of precipitation hardening to strengths comparable to steel, 4XXX series alloys alloyed with silicon as the basic alloying element, 5XXX series alloys alloyed with magnesium as the basic alloying element to provide excellent corrosion resistance, 6XXX series alloys alloyed with magnesium and silicon as basic alloying elements, 7XXX series alloys alloyed with zinc as the basic alloying element and capable of precipitation hardening, or 8XXX series alloys that are alloyed with other elements not included in other series, such as aluminum and lithium alloys.

[0017] In general, a person skilled in the art knows the chemical compositions of AA3004, AA3104, AA3204 or other aluminum alloys suitable for making foil for aluminum cans, as well as other aluminum alloys, and are available, for example, in the technical data sheets of the Aluminum Association.

[0018] The body may be made of cast aluminum, which has subsequently been descaled to obtain a body suitable for further processing. The body is heated to a homogenization temperature. The main purpose of this heating step is to homogenize the material. Homogenization temperatures may be in the range of approximately 500 °C to approximately 600 °C, depending, for example, on the desired temperature for the next process step. The body may then be cooled to temperatures suitable for hot rolling.

[0019] In a subsequent step, the body is rolled in a hot rolling mill to produce a hot sheet. The hot sheet exiting the hot rolling mill does so at a hot-rolling exit temperature. The hot rolling step produces a hot sheet that has a hot-rolling exit gauge, which is the thickness of the rolled aluminum sheet after hot rolling. In the hot rolling step, temperature control is implemented so that the hot-rolling exit temperature is selected to substantially prevent recrystallization of the hot-rolled sheet. In the context of this application, the term "recrystallization" refers to a process whereby deformed grains in a metallic body are replaced by a new set of grains that are essentially free of defects and nucleate and grow until the original grains have been completely consumed.Recrystallization reduces the strength and hardness of the material while simultaneously increasing its ductility. In the present process, the exit temperature of the hot rolling mill is selected so that the sheet exiting the hot rolling mill exhibits a high density of defects, such as dislocations, etc., and relatively high strength and hardness, while at the same time its ductility can be relatively low.

[0020] As a guide, substantially unrecrystallized sheet after hot rolling may exhibit a tensile strength in the range of 190 MPa to 240 MPa, for example, whereas the same material would show significantly lower tensile strength values ​​in a recrystallized state, for example, down to around 150 MPa for the fully recrystallized material. Hardness values ​​can be determined by the Vickers hardness test and then expressed as the Vickers pyramid number (HV) in MPa (or N / mm²). Hardness can also be approximated from ultimate tensile strength (UTS) values ​​using the well-known relationship for aluminum alloys: UTS ~ 3*HV.

[0021] In a subsequent step, the hot-rolled sheet is cold-rolled in a cold rolling mill. The purpose of this process step is to achieve cold reduction, meaning that the gauge (or thickness) of the sheet is reduced even further. Cold reduction is performed to produce a cold-rolled sheet that has a cold-rolled output gauge that is smaller than the hot-rolled output gauge. Cold rolling follows the hot-rolling step, after the sheet has been cooled to temperatures of approximately 100 °C or lower, e.g., such as 50 °C to 60 °C.

[0022] The cold-rolled sheet (having the output gauge from cold rolling) is then transferred to an annealing furnace over an intermediate temperature range, with temperatures selected to allow recrystallization of the cold-rolled sheet. The annealing step results in a recrystallized sheet having the output gauge from cold rolling. The microstructure of the recrystallized sheet typically exhibits a new set of relatively defect-free grains that replace the defective microstructure obtained by cold rolling. In some embodiments, tensile strength values ​​may be in the range, for example, from 150 MPa to approximately 200 MPa.

[0023] In a subsequent step, the recrystallized sheet is cold rolled to apply a cold reduction in order to produce a cold rolled sheet with a final gauge smaller than the output gauge of the cold rolling.

[0024] In developing a new process, the inventors have identified certain shortcomings of conventional methods and now propose a new way to economically produce aluminum can sheets while avoiding the deficiencies of the prior art. For example, in studying the process described in U.S. Patent 5,362,340, it has been found that the relatively low-temperature homogenization treatment, in combination with the chemical composition of the aluminum alloy, could produce a strong cubic texture after annealing (either self-annealing or batch annealing on the hot-roller exit gauge), which, in some cases, the cold-rolling process following annealing cannot balance. This can result in aluminum can sheets with 0° / 90° ear formation or very low 45° ear formation.This ear-forming characteristic can produce, during subsequent stretching and ironing processes, cans with pinched ears at 0° / 180° with respect to the rolling direction, as well as increased can tearing and low performance for can manufacturers.

[0025] Furthermore, some limitations of single-stand reversible rolling mills can cause problems in conventional processes. The output gauge of hot-rolled material from a single-stand reversible rolling mill can typically vary to values ​​of approximately 2.0 mm. Producing a lower output gauge from a single-stand reversible rolling mill is generally difficult and may not be feasible due to the challenges in controlling the crown, wedge, and flatness of the sheet. On the other hand, the trend among can manufacturers is to reduce the thickness of the sheet for cans, a trend also known as "down-gauging."If a thinner end product with similar ear-forming properties and strength to current standard thicknesses is desired, the same total cold reduction applied to the material after intermediate hot-annealed gauge thickness (either self-annealed or batch annealed) must be maintained. Achieving this would require reducing the hot-roller output gauge to significantly less than 2 mm. The new process substantially avoids these problems identified in conventional processes.

[0026] The process according to the above formulation of the invention introduces a cold rolling step interposed between the preceding hot rolling step and the subsequent intermediate annealing step. The new sequence of steps has at least two significant effects. The first effect can be understood by considering the final product; the second effect can be understood by considering the thermomechanical process itself.

[0027] It has been found that the final product generally exhibits relatively low ear formation values. The resulting ears are most pronounced at approximately 45° (relative to the rolling direction). This ear orientation is usually preferable from the end customer's perspective, i.e., from the can manufacturer's point of view. The new method generally avoids or reduces the high 0° / 90° ears that are undesirable from the can manufacturer's perspective and that are most likely to be obtained with the process described in the prior art, for example, in U.S. Patent 5,362,340.From a metallurgical standpoint, it is believed that cold reduction introduced after hot rolling, performed on a material that is essentially not recrystallized, can enhance the particle-stimulated nucleation (pSn) mechanism, which reduces the density of the cubic texture that the material will have after intermediate annealing. The inferior cubic texture after annealing will result in ear formation that tends toward 45° instead of 0 / 90° with respect to the final product.

[0028] Regarding the second effect (on the capability of the thermomechanical process), it is observed that the final strength of the material and the ear formation depend largely on the amount of cold working after the intermediate annealing on the hot gauge. For example, if, in a current conventional process, a material with a final gauge of 0.26 mm is produced, the intermediate annealing can be performed on a gauge of approximately 2 mm. Therefore, the total cold reduction is approximately 87%. Now consider a case where the end customer requests a final gauge of 0.24 mm. To produce the same ear formation and the same properties, it would be necessary to perform the intermediate annealing on approximately 1.85 mm. This relatively small thickness often cannot be satisfactorily achieved on a single-stand reversing rolling mill due to flatness and thickness range limitations. These limitations do not exist in the new method.Applying the new method allows the producer to create thicker material from hot rolling (e.g., approximately 2.5 mm), perform a slight cold reduction to the required intermediate annealing gauge (1.85 mm in this hypothetical example), and anneale the sheet at this intermediate gauge to fully soften the material before cold rolling it to the final gauge. In other words, some limitations of using a single-stand reversing mill as a hot rolling mill no longer restrict the overall process capabilities. If a single-stand reversing mill is used as a hot rolling mill, the method can also significantly increase the single-stand hot rolling mill's output, as it produces a thicker gauge.

[0029] From another perspective, the advantages of the new process result, at least in part, from the fact that cold rolling is carried out in two separate steps. The first cold rolling step is performed after hot rolling and before intermediate annealing (on unrecrystallized material), and the second cold rolling step is performed after recrystallizing annealing (at intermediate temperature) on recrystallizing material. As a result, preferable ear-forming characteristics and strength, as well as smaller final gauges, can be obtained, even when hot rolling is performed on a single-stand reversing mill.

[0030] Considering the advantages of the process described above, in a preferred embodiment of the process and installation, a single-stand reversing rolling mill is used as the hot rolling mill. While a tandem rolling mill can be used instead of a single-stand reversing rolling mill to perform the hot rolling step, the use of a single-stand reversing rolling mill is typically much less expensive, so that the final product can be manufactured economically.

[0031] In some preferred embodiments, the single-stand reversing rolling mill is used in two different modes of operation, wherein a first mode of operation includes one or more flat passes and a second mode of operation, used after the first mode of operation, includes one or more winding passes that produce wound sheets having the output gauge of the hot rolling mill.

[0032] The hot rolling step shall be carried out in such a way as to substantially prevent recrystallization of the hot-rolled sheet. Therefore, the exit temperature of the hot rolling process is selected to be below 290°C. In preferred processes, the exit temperature of the hot rolling process is in the range of approximately 200°C to approximately 280°C. For aluminum alloys of type AA3004, AA3104, or other aluminum alloys suitable for manufacturing sheets for aluminum cans, these temperatures are generally adequate to prevent recrystallization altogether, thus enhancing the advantages of the overall process. The correct temperatures to completely prevent recrystallization can be selected depending on the alloy type and may differ from one alloy to another.

[0033] When designing the cold rolling stage, it has been found that a cold reduction of between 5% and 70% is preferably applied in the cold rolling mill that rolls the hot-rolled sheet. Cold reductions in this range are particularly capable of enhancing particle-stimulated nucleation (PSN), which is believed to reduce the cubic texture density in the annealed material.

[0034] The cold rolling step can be performed at least in the final rolling passes to obtain coils of cold-rolled sheet on the single-stand reversing mill. In this case, it may be preferable for the annealing of the cold-rolled sheet to be carried out in a batch furnace. Alternatively, a continuous furnace can be used for the annealing step in the intermediate temperature range to obtain the recrystallized sheet.

[0035] Given that the overall process allows for high degrees of total reduction, a total reduction of more than 70% is applied to the aluminum sheet between the hot-roller exit gauge and the final gauge. The total reduction can be 80% or more, or even 85% or more. This is due in part to the fact that the cold rolling to reduce the gauge is performed in two steps rather than one.

[0036] The disclosure also relates to an installation for producing aluminum can sheets, which is configured to execute the method according to the invention. Brief description of the drawings

[0037] An embodiment of the invention will now be described in detail with reference to the drawings. Fig. 1 It shows a schematic drawing of a part of an installation configured to manufacture aluminum can sheets suitable for making cup-shaped articles; Fig. 2 It shows a diagram illustrating the relationship between the degree of recrystallization of the sheet material after the initial hot rolling step and the quantity and type of ears after applying cold reduction to the final gauge; and Fig. 3 It shows a diagram illustrating the influence of cold reduction before intermediate annealing and the effect on the type and degree of ear formation after cold reduction to final thickness. Detailed description of embodiments of the invention

[0038] Among the main requirements for aluminum can body sheets are sufficiently high strength and formability (including limited ear formation). High strength is needed to achieve sufficient structural stability and prevent bulging of the can base (inversion of concavity) under high internal pressure. High strength is also needed to obtain stable cans with very thin walls after flattening. Good formability is required because the material undergoes intensive forming operations. Anisotropic material flow due to the sheet texture, which is controlled by balancing the hot-rolled cube and the cold-rolled texture, always results in an uneven can rim during deep drawing and flattening operations. This irregularity is also known as "ears."Uneven cup edges are detrimental to the transport of can bodies or affect the entire process when the ears are stretched and cut during ironing, causing machine downtime and reducing efficiency.

[0039] The embodiments of the invention are able to address both requirements satisfactorily using an economically viable production process.

[0040] Figure 1 shows a schematic drawing of a portion of a 100 installation configured to manufacture aluminum can sheets suitable for producing cup-shaped articles. The schematic figure shows only some of the devices used in the production route.

[0041] The production facility typically includes casting devices to produce large cast ingots from molten aluminum alloy. The cast ingots are usually composed of coarse grains with a dendritic structure and random texture. Precipitates containing aluminum and other components, such as Fe, Mn, and Si, are typically distributed non-homogeneously within the cast ingot.

[0042] In a subsequent step, the molten ingots are homogenized in a homogenizing furnace (also called a preheating furnace, not shown in Fig. 1). The homogenizing treatment is usually accompanied by characteristic changes in solute content and precipitation microstructure that subsequently affect recrystallization, grain size, and texture during sheet production.

[0043] The homogenized ingots are then transferred to the hot rolling stage. In the preferred installation, a single-stand reversing mill 120 is used for hot rolling. The single-stand reversing mill 120 is capable of operating in two different modes, shown separately in schematic figure 1. In the first operating mode, HR-FP (shown on the left side of the single-stand reversing mill 120), the incoming ingots are reduced in thickness using several flat passes where the material is wound back and forth without wrapping around either side of the rolls.In a second HR-CP mode of operation, shown on the right side of the drawing depicting the single-stand reversible rolling mill 120, CR winding reels are used on each side of the MS rolling stand to wind the SH sheet between winding passes performed in mutually opposite rolling directions. In any winding pass, one of the reels functions as an unwinding reel, providing an incoming strip to the rolling space formed in the rolling stand. The other reel is used as a tensioning reel, winding the outgoing strip after the rolling path. Since single-stand reversible rolling mills are generally known in the art, a detailed description is not considered necessary in this application.

[0044] Next, the hot-rolled material is transferred, after cooling, in coil form to a cold-rolling stage 130 arranged downstream of the hot-rolling stage in the material flow direction. The cold-roller could be single-stand (as shown) or multi-stand.

[0045] A batch furnace 140 is arranged downstream of the cold rolling stage 130. The batch furnace is configured to receive multiple CL coils after cold rolling and to perform an intermediate annealing of the cold material to achieve complete recrystallization of the sheet material.

[0046] An additional cold rolling stage 150 is arranged downstream of the intermediate annealing batch furnace 140 to apply cold rolling to the recrystallized material to obtain cold rolled material with the desired final gauge for further processing steps, such as an H1X material or, more specifically, an H19 material. The cold rolling mill 150 comprises a single support in the embodiment of Figure 1. An example process for producing aluminum can sheets in the installation 100 was carried out as follows.

[0047] In a preparatory step, an aluminum alloy was melted to form a cast piece and subsequently peeled to obtain a cast and peeled aluminum alloy body suitable for further processing. This body is hereafter also referred to as an ingot. The aluminum alloy may be a can body material such as, for example, AA3004, AA3104, or another aluminum alloy basically suitable for making aluminum can sheets.

[0048] The aluminum alloy used in example processes comprised 0.30 wt% Si, 0.50 wt% Fe, 0.95 wt% Mn, 1.10 wt% Mg, 0.20 wt% Cu, less than 0.05 wt% Ti, less than 0.10 wt% Zn; and up to 0.15 wt% impurities, preferably each impurity being less than 0.05 wt%, the remainder being Al.

[0049] After smelting and descaling, the ingot was homogenized at approximately 50-595 °C with a soaking time, e.g., of 5 to 20 hours, followed by cooling the ingot to approximately 49-530 °C.

[0050] The homogenized ingot (aluminum body) was then transferred to the hot rolling mill without significant intermediate cooling, so that hot rolling of the ingot began at approximately this temperature, i.e., approximately 49–530°C. In this installation configuration, a single-stand reversible rolling mill 120 was used as the hot rolling mill.

[0051] Several flat passes were made, up to a caliber of between 25 and 45 mm. The ingot temperature after the last flat pass was between approximately 290 and 350°C. The number of flat passes can vary, for example, from 15 to 50.

[0052] After the flat passes, the material thickness was further reduced by hot rolling on the same single-stand reversing 120 rolling mill, with the difference that the material was wound after each pass (winding passes). The number of winding passes ranged from 2 to 8.

[0053] The thickness of the material after the last winding pass was approximately 1.7 mm to approximately 5 mm. In the experiments presented here, the exit temperature of the material after hot rolling, i.e., the exit temperature of the Threx hot rolling mill, was low enough to ensure the absence of recrystallization.

[0054] Normally, the exit temperature of the hot rolling mill was in the range of approximately 200 °C to approximately 340 °C and preferably between approximately 220 °C and approximately 280 °C. The reduction of each winding pass was between 20 and 70%.

[0055] The hot-rolled material was cooled and then transferred to a cold rolling mill.

[0056] A cold reduction of 5% to 70% was applied to the material in the cold rolling mill directly onto the hot strip of non-recrystallized material.

[0057] The cold-rolled sheet was then transferred in coil form to a batch furnace 140 for intermediate annealing. An intermediate annealing step was then applied to the cold-rolled sheet. The annealing temperatures and times were selected to allow the annealed material to fully recrystallize and develop a tough cubic texture. A typical annealing temperature range is 280 °C to 450 °C with a holding time of 1 to 12 hours.

[0058] The recrystallized annealed sheet was then cold-rolled to apply a suitable cold reduction to produce a cold-rolled sheet with a final gauge. Preferably, cold rolling with a reduction of 70% to 95% was applied to the recrystallized sheet, giving the material the required strength and balancing the cubic texture with the rolled texture. In the case of recrystallization (partial or total) within the thickness of the hot strip (either by self-annealing or after discontinuous annealing), the cubic texture developed after annealing was weak, and the final product had a high degree of 45° ear formation.

[0059] With the method described above, the unrecrystallized hot strip undergoes relatively low cold reduction, and then an intermediate annealing is applied to the material to make it completely soft. With this method, there is an intermediate reduction of the annealed thickness with cold rolling without deterioration of the tough cubic texture after annealing.

[0060] The combination of low cold reduction with the non-recrystallized structure immediately after hot rolling and discontinuous annealing to produce fully recrystallized material could also be applied to the conventional method of producing can bodies using a tandem hot rolling mill. In other words, in an alternative embodiment, a tandem hot rolling mill can be used instead of a single-stand reversible rolling mill to perform the hot rolling step preceding the cold rolling step.

[0061] The following explains some characteristic aspects of the new and beneficial process in relation to the schematic diagrams in Figures 2 and 3. Figure 2 schematically illustrates the technical connection between the degree of recrystallization of the sheet material after the initial hot rolling step and the quantity and type of ears after cold reduction to the final gauge. Figure 3 illustrates the importance of the cold reduction step before intermediate annealing and its effect on the type and degree of ear formation after cold reduction to the final thickness.

[0062] In each diagram in Figs. 2 and 3, the X-axis represents the degree of cold reduction (in percent) applied after intermediate annealing. In other words, the X-axis represents the amount of cold reduction achieved in the cold rolling mill 150 located downstream of the intermediate annealing furnace 140. The Y-axis represents the type and amount of ear formation (in percent). The area above the reference line BL corresponds to ear formation of 0–90°, while the area below the reference line BL represents ear formation of 45°. The absolute distance of a data point from the reference line in the Y direction of the diagram represents the amount or intensity of the corresponding ear formation, meaning that a point on the reference line BL corresponds to a sheet with no ears at all.The curves in the diagram represent general trends established in a large number of experiments. The schematic box plots of BP in Fig. 3 indicate that the trends represented by the lines are considered significant.

[0063] Fig. 2 basically illustrates the importance of the requirement that the hot rolling exit temperature be selected so as to avoid as much as possible any recrystallization of the hot rolled sheet.

[0064] The solid line represents a case where the rolled sheet is substantially unrecrystallized after the hot rolling operation is completed. This is an embodiment of the claimed invention. For comparison, the lower curve (dashed line) represents reference cases where the sheets partially recrystallized after the hot rolling step is completed, which, in other words, means that recrystallization was not sufficiently prevented in the reference processes presented. The solid line shows that there is a high degree of ear formation at 0–90° in the fully recrystallized material after the intermediate annealing and before the cold reduction begins (with a cold reduction value of 0%).As cold reduction increases, the degree of ear formation from 0-90° decreases continuously, so that shortly before reaching the final thickness (at the highest point of cold reduction), no ear formation is discernible (a continuous curve crosses the reference line). In the final product, after full cold reduction of the sheet, a certain amount of 45° ear formation can be discerned, but the degree of ear formation is low in absolute terms.

[0065] Conversely, when the material exhibits a significant amount of recrystallization after the hot rolling step is completed (dashed line), the degree of ear formation at 0-90° is less than in the embodiments of the invention. As the cold reduction increases, the degree of ear formation at 0-90° decreases and would disappear entirely with cold reduction insufficient to obtain the final thinner gauge. As the amount of cold reduction is increased to obtain the final thinner gauge, the character of the ear formation changes from 0-90° to predominantly 45°, and the amount of 45° ear formation increases to a much higher level in absolute terms than in the material according to the claimed process (solid line).This shows that the degree of recrystallization after the hot rolling step has a significant influence on the amount and character of ear formation in the final product.

[0066] The diagram in Fig. 3 can be read in a similar way. The diagram illustrates the importance of the cold reduction step applied before immediate annealing. In the diagram, the upper curve (dashed line) corresponds to a case where no cold reduction was applied before annealing. This could be a process similar to those described in the prior art mentioned at the beginning of this application. A high degree of ear formation at 0–90° is present immediately after the intermediate annealing. When the material is finally cold-rolled to the final gauge (maximum amount of cold reduction), there is almost no or very little ear formation in the final product. If a certain amount of 45° ears is present, the absolute amount is small.

[0067] In contrast, the dotted line below the dashed line represents processes according to embodiments of the invention in which cold reduction is applied prior to intermediate annealing in a cold rolling mill that rolls the (essentially non-recrystallized) material coming out of the hot-rolled state before the material is transferred to intermediate annealing. Initially, before applying the cold reduction, the number of 0-90° ears is lower than in the case where no cold reduction is applied prior to annealing. Once the sheet thickness is reduced to the final gauge (with maximum cold reduction), there is a significant number of 45° ears, which is a desirable property for many can manufacturers working with very thin aluminum sheet.

[0068] The disclosure of this patent application also relates to a method for making an aluminum can comprising the steps of the method for producing aluminum can sheets, wherein the cold-rolled sheet with the final gauge is formed into a cup-shaped article suitable for making an aluminum can.

Claims

1. A method for producing aluminum can sheets comprising: providing a body made of an aluminum alloy of type AA3004, AA3104, or another suitable aluminum alloy for manufacturing aluminum can sheets, wherein the aluminum alloy comprises 0.0-0.60 wt% Si; 0.1-0.80 wt% Fe; 0.7-1.50 wt% manganese; 0.8-1.50 wt% magnesium; 0.0-0.25 wt% Cu; up to 0.10 wt% Ti; up to 0.25 wt% Zn; and up to 0.15 wt% impurities, the remainder being Al; heating the body to a homogenization temperature; hot rolling said body in a hot rolling mill to produce a hot rolled sheet, wherein said hot rolled sheet exits the hot rolling mill at a hot rolling exit temperature with a hot rolling mill exit gauge,wherein the hot rolling exit temperature is selected to be less than 290°C in order to substantially avoid recrystallization of the hot rolled sheet; cold rolling the hot rolled sheet in a cold rolling mill to apply a cold reduction in order to produce a cold rolled sheet with a cold rolling exit gauge smaller than the hot rolling exit gauge; annealing the cold rolled sheet in an intermediate temperature range selected to permit recrystallization of the cold rolled sheet in order to obtain a recrystallized annealed sheet; cold rolling the recrystallized annealed sheet to apply a cold reduction to produce a cold rolled sheet with a final gauge.

2. Method according to claim 1,wherein the aluminum alloy comprises 0.1-0.5 wt% Si; 0.2-0.70 wt% Fe; 0.8-1.40 wt% manganese; 0.9-1.30 wt% magnesium; 0.1-0.25 wt% Cu; up to 0.15 wt% impurities, each impurity comprising less than 0.05 wt%; the remainder being Al.

3. Method according to claim 1, wherein a single-support reversible rolling mill is used as a hot rolling mill.

4. A method according to claim 3, wherein the single-stand reversible rolling mill is used in two different operating modes, wherein a first operating mode includes one or more flat passes and a second operating mode, used after the first operating mode, includes one or more winding passes that produce wound sheets having the output gauge of the hot rolling mill.

5. A method according to any one of the preceding claims,wherein the exit temperature of the hot-rolled sheet is in the range of approximately 200°C to approximately 280°C.

6. The method according to any one of the preceding claims, wherein a cold reduction of between 5% and 70% is applied in the cold rolling mill that rolls the hot-rolled sheet.

7. The method according to any one of the preceding claims, wherein the annealing of the cold-rolled sheet is carried out in a batch furnace.

8. The method according to any one of the preceding claims, wherein a total reduction of more than 70% is applied to the aluminum sheet between the exit gauge of the hot-rolled sheet and the final gauge.

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